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Chapter 1 Electrodialysis
Chapter 1
Electrodialysis 1.1.
OVERVIEW OF TECHNOLOGY
Industrial application of ion exchange membranes started at first in the field of electrodialysis (ED) (cf. Preface) and it induced the development of the fundamental theory. This fact is easily understandable from those phenomena explained in Fundamentals, which are described by taking the ED into account. The development of the fundamental theory led to further development of the ED technology. After that ion exchange membrane technology developed in the succeeding technology such as electrodialysis reversal (EDR), bipolar membrane electrodialysis (BP), electrodeionization (EDI), electrolysis (EL), fuel cell (FC) etc. describing in the succeeding chapters. Looking over these historical details, we notice that the ED becomes the fundamental technology and it is applied to the succeeding technologies based on the ion exchange membranes. In this chapter, we discuss the main subjects such as the structure of electrodialyzer, ED process, practical application of ED etc.
1.2.
ELECTRODIALYZER
1.2.1
Structure of an Electrodialyzer The basic structure of the vertical sheet-flow type module consists of stacks in which cation exchange membranes, anion exchange membranes, gaskets (desalting cells and concentrating cells) are arranged alternately (Fig. 1.1). Fastening frames are put on both outsides of the stack which is fastened up together through cross bars setting in the frames. The deformation of the membranes is prevented by regulating hydrostatic pressure in the fastening frames. Inlet manifold slots and outlet manifold slots are prepared at the bottoms and heads of the gaskets, respectively. Spacers are incorporated with the gaskets to prevent the contact of cation exchange membranes with anion exchange membranes. Many stacks are arranged through the fastening frames. Electrode cells are put on both ends of the electrodialyzer, which are fastened by a press putting on the outsides of electrode cells (Fig. 1.2). An electrolyte solution to be desalinated is supplied from solution feeding frames to entrance manifolds, flows through entrance slots, current passing portions and exit slots, and discharged from exit manifolds to the outside of the stack (Figs. 1.1 and 1.2). A concentrated solution is usually supplied to concentrating cells in a circulating flow system, and discharged to the outside of the stack through an overflow extracting system. DOI: 10.1016/S0927-5193(07)12015-5
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k
e
l
g
k
e
h
l
−
+
l
j
f d
a
i
j
b
c
f
Figure 1.1 Structure of a stack (filter-press type). a, Desalting cell; b, concentrating cell; c, manifold; d, slot; e, fastening frame; f, feeding frame; g, cation exchange membrane; h, anion exchange membrane; I, spacer; j, feeding solution; k, desalted solution; l, concentrated solution (Azechi, 1980).
Fastening frame Anode chamber
Feeding frame
Feeding frame
Press (fix)
Press (move)
Stack
Figure 1.2
Cathode chamber
Stack
Filter-press type electrodialyzer (Azechi, 1980).
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323
Effective membrane area is in the range from less than 0.5 m2 to about maximum 2 m2. In order to reduce energy consumption, it is desirable to decrease the electric resistance of the membrane and gasket thickness. Gasket material is selected from synthesized rubber, polyethylene, polypropylene, polyvinyl chloride and ethylene–vinyl acetate copolymer etc. The spacer is usually incorporated with the gasket and a solution flows dispersing along the spacer net. 1.2.2
Parts of an Electrodialyzer The electrodialyzer is composed of the parts as follows (Urabe and Doi, 1978). 1.2.2.1 Fastening Frame Maximum 2000 pairs of membranes are arranged between electrodes in an electrodialyzer, so as to let disassembling and assembling works be easy. The membrane array is divided further into several stacks consisting of 50–400 pairs. Fastening frames are fixed by bolts on both ends of the stack. The fastening frame is usually served as a solution feeding frame, so that a desalting and a concentrating solution are supplied to each gasket cell from the feeding frame incorporated in every stack. Material of the fastening frame is selected from polyvinyl chloride, polypropylene and rubber-lining iron etc. 1.2.2.2 Solution Feeding Frame A solution feeding frame is integrated for feeding solutions to each desalting and concentrating cell. Manifold holes are prepared at corresponding positions of the holes fitted in the gasket. Solutions are usually supplied through the manifolds to each stack, but as the case may be supplied to each plural stack. 1.2.2.3 Gasket The shape of the gasket is presented in Fig. 1.3. A solution is supplied from the inlet manifold put at the bottom, flows through the slot and is fed into the current passing portion. Then the solution is discharged through the outlet slot to the manifold fitted to the head. The gasket has the following functions: (1) prevents solution leakage from the inside to the outside of the electrodialyzer, (2) adjusts the distance between a cation exchange membrane and an anion exchange membrane, (3) prevents solution leakage between a desalting cell and a concentrating cell occurring at slot sections. In order to prevent the solution leakage, it is desirable to adopt a soft material for the gasket. On the other hand, it is desirable to adopt a hard and stable material to avoid dimension changes during long-term operation. The material of the gasket is selected from rubber, ethylene–vinyl acetate copolymer, polyvinyl chloride, polyethylene etc. The thickness of the gasket is in the range of 0.5–2.0 mm.
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Ion Exchange Membranes: Fundamentals and Applications
Manifold
Gasket Spacer
Slot Manifold
Figure 1.3
Gasket (Urabe and Doi, 1978). Deformation of a membrane
Membrane Gasket
Slot
Figure 1.4
Deformation of an ion exchange membrane (Urabe and Doi, 1978).
(a)
Figure 1.5
(b)
(c)
Structure of slots (Urabe and Doi, 1978).
1.2.2.4 Slot It is important to reduce the inside solution leakage (cf. Section 12.2 in Fundamentals), which arises through pinholes and cracks in the membranes or through gaps due to the membrane deformation at the slot as shown in Fig. 1.4. In order to prevent these troubles, a lot of devices are proposed as exemplified in Fig. 1.5 in which (a) decrease the width of the slot, (b) bend the slot, (c) insert the support in the slot.
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Electrodialysis
(a) Expanded PVC
(c) Diagonal net
Figure 1.6
(b) Wave porous plate
(d) Mikoshiro texture
(e) Honeycomb net
Structure of spacers (Urabe and Doi, 1978).
1.2.2.5 Spacer The function of a spacer is to keep the distance between the membranes. In addition, the spacer increases the limiting current density due to solution disturbance (cf. Sections 10.2 and 10.3 in Fundamentals). The spacer is selected taking account of the requirement such as; (1) low friction head loss, (2) low electric current screening effect, (3) easy air discharge, (4) less blocking of flowpass caused by the precipitation of fine particles suspended in a feeding solution. The structures of a spacer are classified in Fig. 1.6 as (a) expanded polyvinyl chloride, (b) wave porous plate, (c) diagonal net, (d) mikosiro texture and (e) honeycomb net.
1.2.2.6 Electrode and Electrode Chamber Platinum plated titanium, graphite or magnetite is used for anode material and stainless or iron is used for cathode material. The shape of electrodes is classified into net, bar and flat. A partition is inserted between an electrode chamber and a stack for preventing the mixing of solutions. In an anode chamber, oxidizing substances such as chlorine gas evolve. An ion exchange membrane is easily deteriorated by contact with the oxidizing substances, so it is necessary to use two sheets of partitions and put a buffer chamber between the two partitions. Material of the partition is an ion exchange membrane, an asbestos sheet or a battery partition.
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Ion Exchange Membranes: Fundamentals and Applications
An acid solution is added into a cathode solution and the electrodialyzer is operated under controlling pH of the cathode solution for preventing the precipitation of magnesium hydroxides in the cathode chamber. A feeding solution or a concentrated solution is supplied into the electrode chamber. The concentration of oxidizing substances in the anode solution is reduced by adding sodium sulfite or sodium thiosulfate into the solution being discharged. Sometimes, a sodium sulfate solution is supplied to an anode and a cathode chamber, achieving the neutralization by mixing the effluent of both chambers. 1.2.2.7 Press An oil pressure press is usually used adjusting the pressure to be 5–10 kg cm2. 1.2.3
Requirements for Improving the Performance of an Electrodialyzer In order to improve the performance of an electrodialyzer, membrane characteristics should be naturally improved. At the same time, the circumstances in an electrodialyzer in which the membranes work should be better. Here, we describe the definite problems lowering the circumstances in an electrodialyzer and requirements for improving the circumstances and performance of an electrodialyzer (Urabe et al., 1978). 1.2.3.1 Solution Velocity Distribution between Desalting Cells In an electrodialyzer, ion exchange membranes and desalting and concentrating cells are arranged alternately and a solution is supplied into desalting cells. In this flow system, the solution velocity distribution in desalting cells does not become uniform. This phenomenon causes the concentration distribution and current density distribution in the electrodialyzer, and gives rise to the decrease of the limiting current density of the electrodialyzer (cf. Sections 9.1, 11.6 and 11.7 in Fundamentals). In order to operate the elctrodialyzer stably, it becomes necessary to make the solution velocities between the desalting cells uniform. 1.2.3.2 Solution Leakage in an Electrodialyzer The dimensions of all parts of an electrodialyzer are not always consistent with the values in the specifications. Small pinholes can open in an electrodialyzer because the strength of ion exchange membranes is relatively low. Gaps may occur between the materials composing the electrodialyzer in the assembly works of an electrodialyzer. If a pressure difference between the desalting cells and concentrating cells exists in these circumstances, solutions leak through the membranes and lower the performance of the electrodialyzer (cf. Section 12.2 in Fundamentals). In order to avoid these troubles, we have to remove the pinholes and gaps in the electrodialyzer and control the pressure difference between desalting cells and concentrating cells.
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327
1.2.3.3 Distance between the Membranes Decrease of the distance between the membranes brings about the decrease of electrical resistance and energy consumption. On the other hand, it brings about the increase of friction loss of solution flow, blocking of the materials suspended in a feeding solution and the increase of pumping motive power. Accordingly, it becomes necessary to realize the optimum distance between the membranes. The optimum distance is decided further taking account of electric resistance of ion exchange membranes and that of electrolyte solution in desalting and concentrating cells. 1.2.3.4 Spacer Main functions of a spacer in to create space between a cation exchange membrane and an anion exchange membrane. When solution velocity and the Reynolds number are decreased, hydrodynamic pattern exhibits laminar flow, which means that disturbing effect of the spacer is low. In order to increase the limiting current density, turbulent flow should be induced by increasing the Reynolds number (cf. Sections 10.3.4 and 10.5 in Fundamentals). 1.2.3.5 Electric Current Leakage A part of an electric current flows through slots and manifolds causing ineffective current leakage. Current leakage is increased by the increases of the numbers of cell pairs integrated in a stack and the increase of sectional area of slots and manifolds (cf. Section 12.1 in Fundamentals). These events, however, related with the solution velocity distribution between the cells described in Section 1.2.3.1. 1.2.3.6 Simplicity of Structure of an Electrodialyzer Disassembling and assembling work is peculiar characteristics in operating an electrodialyzer (cf. Section 1.5.3 in Applications). Excellent durability of ion exchange membranes is owing to careful treatment in this work. So, the simplicity of the structure is a requirement for performing this work. 1.3.
ELECTRODIALYSIS PROCESS
The ED process had been explained in detail in several articles (Mintz, 1963; Shaffer and Mintz, 1966; Itoi et al., 1978; Yawataya, 1986; Tanaka, 1993). The following is overall description of the process. 1.3.1
One-Pass Flow Process An electrolyte solution is fed to an electrodialyzer and desalted solution is discharged to the outside of the process (Fig. 1.7). When the concentration of the feeding solution is invariable, the performance of the system becomes stable. Joining the process in Fig. 1.7 to the succeeding process, a one-pass flow
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Ion Exchange Membranes: Fundamentals and Applications
Concentrated discharge
Feeding solution
Concentrating cell Desalted solution Ion-exchange membrane
Desalting cell Feeding solution
Figure 1.7
Pump
One-pass flow process (Tanaka, 1993).
C out b
a
l
C-dC C
x+dx x
C in 0
Figure 1.8
Electrolyte concentration change in a desalting cell (Tanaka, 1993).
multiple continuous system suitable for a large-scale plant is formed. The desalination in Fig. 1.7 is proceeded as below. In the desalting cell shown in Fig. 1.8, a, b and l are the flow-pass depth (distance between membranes), the flow-pass width and the flow-pass length, respectively. Cin and Cout are the electrolyte concentration at the inlet and the outlet of the desalting cell. Passing an electric current across the membranes, ions in the desalting cell are transferred toward the concentrating cell. Assuming the transport of water to be negligible across the membranes under an electric current passing, the material balance between at x and x+dx in Fig. 1.8 is
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329
indicated by the following equation, including current density i, linear velocity in a desalting cell u, current efficiency Z and electrolyte concentration at x distant from the inlet of a desalting cell C. 1 Z i dC ¼ dx (1.1) C FC Voltage applied to a membrane pair (cell voltage) consists of a membrane potential and Ohmic loss of a cation exchange membrane, an anion exchange membrane, a desalting cell and a concentrating cell. In the desalting process, Ohmic loss of a desalting cell iRde (Rde: electric resistance of desalting cell) is dominant in the cell voltage and voltage difference between electrodes is independent of x (cf. Section 9.1 in Fundamentals). Accordingly, iRde in the desalting cell is estimated to be invariable in the range of x ¼ 0l in Fig. 1.8. Further, Rde is inversely proportional to C, so that i/C is assumed to be nearly constant and we can integrate Eq. (1.1) as follows: Z Z C out 1 Z i l dx ¼ dx (1.2) ua C FC 0 C in au
Solving Eq. (1.2) C out lZ i ¼ exp aFu C C in Desalting ratio a is defined as follows: C out lZ i ¼ 1 exp a¼1 aFu C C in
(1.3)
(1.4)
¯ instead of i/C (Yawataya, 1986) in Eq. Here, we can adopt the average value ¯i=C (1.4) as follows: ¯i i 2¯i ¼ ¼ ¯ C C C in þ C out
(1.5)
From Eqs. (1.4) and (1.5) i 2¯i ¼ C C in ð2 aÞ Substituting Eq. (1.6) into Eq. (1.4) 2Z¯i a ¼ 1 exp aF C in ðu=lÞð2 aÞ
(1.6)
(1.7)
a is calculated using Eq. (1.7), assuming a ¼ 0.1 cm, Cin ¼ 0.05 eq dm3, Z ¼ 0.90, and plotted against u/l taking ¯i as parameter (Fig. 1.9). l vs. u is computed setting a ¼ 0.90 (Fig. 1.10) indicating u is proportional to l. In a practical-scale sheet-flow electrodialyzer, flow-pass length l is from less than
330
Ion Exchange Membranes: Fundamentals and Applications
1.0 0.9
2.5
0.8
2.0
0.7
1.5
0.6 0.5
1.0
0.4 0.3
i =0.5
A / dm 2
0.2 0.1 0.0 0.00
Figure 1.9 ratio.
0.01
0.02
0.03 0.04 u/l(s-1)
0.05
0.06
Influence of linear velocity, flow-pass length and current density to desalting
40 35
u (cm s-1)
30
2.
25
5 0
2.
20 1.5
15 1.0
10
2
A / dm
i =0.5 5 0 0
Figure 1.10
1
2
3
4
5 6 l (m)
7
8
9
10
Flow-pass length and linear velocity in a desalting cell.
331
Electrodialysis
1–2 m, and linear velocities in desalting cells u are 3–10 cm s1. In a tortoise-flow type electrodialyzer, however, l and u are larger than those in the sheet-flow type (cf. Section 2.2 in Application). 1.3.2
Batch Process In Fig. 1.11, the feeding solution is prepared in the circulation tank at first. Next, open valve V1, close valve V2 and circulate the solution between the tank and the electrodialyzer. The solution is electrodialyzed applying constant voltage until electrolyte concentration of a desalted solution attains a definite value, and then the desalted solution is discharged. The process mentioned above is repeated periodically. This system is usually adopted in a small-scale plant and the desalination is proceeded as follows. A definite volume V of electrolyte solution is assumed to be circulated between the circulating tank and the electrodialyzer and it is electrodialyzed applying a constant voltage between the electrodes. An electric current I and electrolyte concentration C of the solution decrease with elapsed time t, but I/C does not change with t, because Ohmic loss IRdil is dominant in a cell voltage and roughly inversely proportional to C. Setting numbers of cell pairs integrated in the electrodialyzer M, the electrolyte concentration in a feeding solution CF and in a desalted product solution CP, and an operating time in an unit batch cycle t, and assuming the transport of water to be negligible across Concentrated discharge
Feeding solution
Feeding solution
V1 Desalted solution V2
Circulation tank
Figure 1.11
Batch process (Tanaka, 1993).
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Ion Exchange Membranes: Fundamentals and Applications
the membranes under an electric current passing, the material balance in a desalting cell in this batch system (Fig. 1.11) is expressed by the following equation. Z Z CP dC MZ 1 t ¼ V dt (1.8) F C 0 CF C Integrating Eq. (1.8) CP MZ I t ¼ exp FV C CF
(1.9)
Expressing an electric current I as I ¼ bli and the solution volume V as V ¼ QPt, (Qp is product solution volume, water transport across the membranes is assumed to be neglected) in Eq. (1.9), the desalting ratio a in the batch system is introduced as follows: CF MblZ i a¼1 ¼ 1 exp (1.10) FQP C CP Accordingly, numbers of cell pairs M integrated in the electrodialyzer is M¼
FQP lnðC F =C P Þ blZði=CÞ
(1.11)
Here, we estimate M in the following case: QP ¼ 1 m3 h1 ¼ 106/3600 cm3 s1, CF/CP ¼ 10, a ¼ 1CP/CF ¼ 0.90, b ¼ 50 cm, l ¼ 50 cm, Z ¼ 0.90, ¯i ¼ 0:01 A cm2 ; CF ¼ 5 105 eq cm3, F ¼ 96,500 C eq1. From Eq. (1.6), i/C is calculated as: i 2¯i ¼ ¼ 364 C C in ð2 aÞ Substituting these values into Eq. (1.11), we obtain M ¼ 75 pairs. 1.3.3
Partially Circulation (Feed and Bleed) Process An electrodialyzer is operated at constant current density supplying a definite amount of solution and circulating a part of feeding solution (Fig. 1.12). Joining Fig. 1.12 to the succeeding process, a multiple partially circulation system (Fig. 1.13) suitable for a middle-scale plant is formed. The desalination in Fig. 1.13 is achieved as below. Assuming the linear velocity in desalting cells u, current efficiency Z and i/C to be constant in each electrodialyzer, Cout/Cin in each electrodialyzer is expressed using Eq. (1.3) as follows: ðC out Þ1 ðC out Þ2 ðC out Þn C out ¼ ¼ ¼ ¼ ¼b ðC in Þ1 ðC in Þ2 ðC in Þn C in
(1.12)
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Electrodialysis
Concentrated discharge
Feeding solution
Desalted solution
Feeding solution
Figure 1.12
Partially circulation process (Tanaka, 1993).
QF=Q QR (Cin)1
Figure 1.13
QR-Q
QR-Q
QR-Q
(Cout)1
(Cout)2
(Cout)n
Q
1
QR
Q
2
QR (Cin)n
(Cin)2
QP=Q
n
CP=(Cout)n
QR
QR
QR
(Cout)1
(Cout)2
(Cout)n
Multiple partially circulation process (Tanaka, 1993).
From the material balance in Fig. 1.13 C F QF þ ðC out Þ1 ðQR QÞ ¼ ðC in Þ1 QR ðC out Þ1 Q þ ðC out Þ2 ðQR QÞ ¼ ðC in Þ2 QR .. .
(1.13)
ðC out Þn1 Q þ ðC out Þn ðQR QÞ ¼ ðC in Þn QR Substituting Eq. (1.12) to Eq. (1.13) C F QF ¼ ðC in Þ1 fQR bðQR QÞg ðC in Þ2 fQR bðQR QÞg ðC in Þ1 ¼ bQ .. . ðC in Þn ðC in Þn1 ¼ fQR bðQR QÞg bQ
(1.14)
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Ion Exchange Membranes: Fundamentals and Applications
Putting Eq. (1.14) together C F QF ¼
ðC in Þn fQR bðQR QÞgn ðbQÞn1
(1.15)
Substituting QF ¼ QP ¼ Q, CP ¼ (Cout)n (water transport across the membranes is neglected) and Eq. (1.12) into Eq. (1.15), and rearranging the equation n CF QR C in ¼ 1 þ1 (1.16) CP QP C out Desalting ratio a of the process is introduced as follows from Eq. (1.16). n CP QR C in a¼1 ¼1 1 þ1 (1.17) CF QP C out In one-path flow system, QR ¼ QP holds, so Eq. (1.17) becomes CF C in n ¼ CP C out
(1.18)
A large-scale plant is realized by assembling a multi-stage multiple partially circulation process arranging in each stage N units of electrodialyzer incorporated with M cell pairs as indicated in Fig. 1.14. The amount of solution QR circulating in each stage is QR ¼ abuMN
(1.19)
N
N
N
2
2
2
1
1
1
QR (Cin)1 QF=Q
QR (Cout)1 (Cin)2 Q
QR (Cout)2 (Cin)N
Q
Q
CF
CP=(Cout)N QR-Q
Figure 1.14
(Cout)N
QR-Q
QR-Q
Multi-stage multiple partially circulation process (Tanaka, 1993).
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Electrodialysis
Table 1.1 process
Arrangement of electrodialyzers in a multi-stage multiple partially circulation
n
QR (cm3 s1)
1 2 3 4 5 6
514,530 123,617 65,999 44,494 33,438 26,744
N
nN
25.7 6.2 3.3 2.2 1.7 1.3
1 26 26 33 42 52 61
We estimate the numbers and arrangement of electrodialyzer in a multi-stage multiple partially circulation process (Fig. 1.14) putting the following parameters: a ¼ 0.1 cm, b ¼ 100 cm, l ¼ 100 cm, u ¼ 5 cm s1, i/C ¼ 364 A cm eq 1, CF/CP ¼ 10, QP ¼ 200 m3 h1 ¼ 200 106/3600 cm3 s1, Z ¼ 0.9, M ¼ 400 pairs, F ¼ 96,500 Ceq1. At first, Cin/Cout is computed as follows: C in lZ i 100 0:90 364 ¼ exp ¼ exp ¼ 1:9718 aFu C 0:10 96500 5 C out Using Eq. (1.16), CF/CP is n n CF QR C in QR ð1:9718 1Þ þ 1 ¼ 1 þ1 ¼ CP QP C out 200 106 =3600 ¼ 10 Accordingly, QR is expressed as follows: QR ¼ ð101=n 1Þ
200 106 =3600 cm3 s1 0:9718
(1)
From Eq. (1.19), numbers of cell pairs per unit stage are MN ¼ 400N ¼
QR QR ¼ abu 0:1 100 5
So, we have N as follows: N¼
QR 20000
(2)
Changing the values of n, QR, N and nM are computed as indicated in Table 1.1 using Eqs. (1) and (2).
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Ion Exchange Membranes: Fundamentals and Applications
C′′
Concentrated
q′′
Desalted solution
q 0 C′out q′
C′in q ′ Feeding solution
Figure 1.15
C0 q0
Concentration or separation process.
1.3.4
Concentration Process Fig. 1.15 gives a single-stage concentration unit process. The output of a multi-stage multiple process X is expressed by the following equation. i X¼ blZMnN (1.20) F Here, we calculate numbers of electrodialyzers in the process for concentrating seawater by ED and crystallizing NaCl by evaporation. Putting as NaCl output in the evaporation process: 200,000 t y1, operating time of electrodialyzers: 8000 h y1, NaCl yield rate in the evaporation process: 0.97 and NaCl molecular weight: 58.5, we obtain: X¼
200000 ¼ 122:38 eq=s 8000 3600 0:97 58:5
Substituting X ¼ 122.38 eq s1, i ¼ 0.03 A cm2, b ¼ 100 cm, l ¼ 100 cm, e ¼ 0.92, Z ¼ 0.90, M ¼ 2000 pairs and F ¼ 96,500 C eq1 into Eq. (1.20), nN ¼ 23.8 is obtained. Accordingly, the numbers of the electrodialyzers in this multi-stage multiple processes are known to be 24. Electrolyte concentration in a concentrated solution C 00 is expressed by the following equation introduced from the overall mass transport equation (cf. Section 6.1 in Fundamentals). qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 A2 þ 4rB A C 00 ¼ 2r (1.21) A ¼ fi þ m rC 0 B ¼ li þ mC 0
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Electrodialysis
6
5
C" (eq dm-3)
4 C' (eq dm-3) 1 3 5 7 9
3
2
1
0
0
1
2
3
4
5
6
7
8
9
10
i (A dm-2)
Figure 1.16
Dependence of C00 on i and C0 .
The overall transport number l, the overall solute permeability m and the overall electro-osmotic permeability f are expressed by the following empirical equations of the overall hydraulic conductivity r (cf. Section 6.1 in Fundamentals). l ¼ l1 þ l2r m ¼ mr
l 1 ¼ 9:208 106
l 2 ¼ 1:914 103
(1)
m ¼ 2:005 104
f ¼ n1 r0:2 þ n2 r
n1 ¼ 3:768 103
(2) n2 ¼ 1:019 102 2
4
(3) 1
1
Here, we calculate l, m and f by substituting r ¼ 1 10 cm eq s into Eqs. (1)–(3) as; l ¼ 9.399 106 eq C1, m ¼ 2.005 106 cm s1, 3 3 1 00 f ¼ 1.398 10 cm C . Dependence of C on i for this membrane pairs is computed as shown in Fig. 1.16 by substituting current density i, electrolyte concentration in a feeding solution C0 , l, m, f and r into Eq. (1.21). 1.3.5
Separation Process An ion exchange membrane shows the permselectivity between ions having the same charged sign, which is defined by the permselectivity coefficient Eq. (1.22) for ion A against ion B, T BA (cf. Section 3.2 in Fundamentals). T BA ¼
ðC 00B =C 00A Þ ðC 0B =C 0A Þ
(1.22)
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Ion Exchange Membranes: Fundamentals and Applications
C 00i ; is concentration (eq dm3) of ion i in a concentrating cell and it is assumed to be invariable in the cell. Concentration of ion i in a desalting cell C 0i is the average of the values at the inlet and the outlet as follow: 1 C 0i ¼ ðC 0i;in þ C 0i;out Þ 2
(1.23)
C 0i;in and C 0i;out are concentrations of ion i at the inlet and outlet of a desalting cell. Ion A is separated from ion B by applying the permselectability of the membrane in the separation process indicated in Fig. 1.15. Here, we define the separation factor of ion B against ion A, in a desalted 0 00 solution S AB and in a concentrated solution S AB by the following equations. 0
S AB ¼
00
S AB ¼
ðC 0B;out =C 0A;out Þ ðC 0B =C 0A Þ ðC 00B =C 00A Þ ðC 0B =C 0A Þ
(1.24)
(1.25)
C 0A ; C 0B : Concentration of ion A and ion B in a feeding solution. Desalting ratio of ion i (ai) in Fig. 1.15 is defined as: C i;out ¼ ð1 ai ÞC 0i
(1.26)
The material balance of ion i in Fig. 1.15 is shown by the following equations assuming q0 q00 hold. C 0i q0 ¼ C 0i;out q0 þ C 00i q00
(1.27)
C 0i;in q0 ¼ C 0i;out q0 þ C 00i q00
(1.28)
q0 is the amount of feeding solution to the process, q0 the amount of a desalting solution being supplied to the electrodialyzer and q00 the amount of a concentrating solution flowing out from the electrodialyzer. Following equations are introduced from Eqs. (1.22)–(1.28): 00
S AB ¼
00
1 aB ð1 ðq0 =2q0 ÞÞ 1 aA ð1 ðq0 =2q0 ÞÞ 0
S AB ¼ SAB T BA ¼
1 aB ð1 ðq0 =2q0 ÞÞ B T 1 aA ð1 ðq0 =2q0 ÞÞ A
(1.29)
(1.30)
When q0 q0 hold, Eqs. (1.29) and (1.30) become 0
S AB ¼
1 1 aA ð1 T BA Þ
(1.31)
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Electrodialysis
00
0
S AB ¼ SAB T BA ¼
T BA 1 aA ð1 T BA Þ
(1.32)
In Eqs. (1.31) and (1.32), the following phenomena are found: when T BA 41 when T BA ¼ 1 T BA o1
when
0
00
SAB o1
S AB oT BA
0
00
SAB ¼ 1 0
S AB ¼ T BA
(1.33)
00
S AB 41
S AB 4T BA
Equation (1.33) means that the separatability is inferior to the permselectability. Further, we find the following events in Eqs. (1.31) and (1.32): 0
lim S AB ¼ 1
aA !0
00
lim S AB ¼ T BA
aA !0 0
0
lim SAB ¼
aA !1
1 T BA
(1.34)
00
lim SAB ¼ 1
(1.35)
aA !1
00
aA vs. SAB and SAB is computed using Eqs. (1.31) and (1.32) and shown in Fig. 1.17, in which the relationships in Eqs. (1.33)–(1.35) are confirmed. 102
101 10
B
TA
S ′AB S′′AB
4
.01 =0
0.4
1
10
0.4 4 10
10-1
0. 01 10-2 0.0
0.2
0.4
A
0.6
0.8
1.0
Figure 1.17 Relationship between permselectivity coefficient, desalting ratio and sep0 00 aration factor. Open, SAB ; filled, S AB :
340
1.4.
Ion Exchange Membranes: Fundamentals and Applications
ENERGY CONSUMPTION AND OPTIMUM CURRENT DENSITY
In multi-stage multiple partially circulation system (Fig. 1.14), rectifiers are assumed to be put in each stage, in which an electric current I is supplied to electrodialyzers through a parallel circuit. I is expressed as follows: I (1.36) ZMN ¼ QRfðC in Þn ðC out Þn g F Voltage V applied to an electrodialyzer is indicated by the following equation putting the voltage in the electrode cell as VP. V ¼ MV cell þ V P
(1.37)
Vcell is cell voltage as follows (cf. Section 13.2 in Fundamentals): V cell ¼ IðRK þ RA þ Rdil þ Rconc Þ þ E M
(1.38)
RK and RA are electric resistance of a cation and an anion exchange membrane. Rdil and Rconc are electric resistance of a desalting and a concentrating cell. RM is membrane potential. A reasonable estimate of the optimum current density is introduced by assuming that the major costs are divided into three categories: those directly proportional to current density, those inversely proportional to current density, and those independent of current density (Eq. (1.39)). b (1.39) Z ¼ ai þ þ c i where Z is total cost, and a, b, c are the relative proportionality constants. The optimum current density iopt is defined by the minimum of Eq. (1.39), so it is introduced by differentiating Eq. (8.37); dZ/di ¼ 0 and expressed as (Leitz, 1986): 1=2 P (1.40) iopt ¼ QR P is depreciation cost ($/m2s), Q the energy cost ($/Ws) and R the cell pair electric resistance (O m2). 1.5. 1.5.1
SURROUNDING TECHNOLOGY
Filtration of a Feeding Solution Fine materials such as sand, clay, iron components, humus soil and miscellaneous inorganic and organic colloid are usually suspended in a raw feeding solution. In order to avoid invasion of these materials into an electrodialyzer, a feeding solution is filtrated using sand, fibers or cohesive agent and turbidity of the solution is decreased less than 0.1–0.2 ppm. First of all, the following valveless sand filter is broadly applicable (Tsunoda, 1994). In Fig. 1.18, a raw feeding
341
Electrodialysis
H3 10 9
H2
6
8
5 H1
7 11
2
1
3 4
Figure 1.18 Valve-less filter. 1, Feeding solution inlet; 2, filtrating chamber; 3, sand filter; 4, collecting chamber; 5, filtrate flow out pipe; 6, filtrate outlet; 7, connecting duct; 8, washing chamber; 9, siphon pipe; 10, siphon breaker; 11, control valve (Tsunoda, 1994).
solution is supplied into the filtrating chamber, filtrated through the sand filter and collected in the collecting chamber. The filtrate flows out through the flowout pipe and supplied to an electrodialyzer at solution level H2. At the same time, a part of filtrate flows through the connecting pipe into the washing chamber put on the filtrating chamber and is accumulated there at H2. Proceeding with the operation, solution level in the siphon pipe goes up with the increase of flow resistance in the sand filter. When the level surpasses H3, the solutions in the filtrating chamber, the collecting chamber and the washing tank are discharged to the outside of the process at one stroke through the siphon pipe due to the siphon function. In this instance, sand is washed and the level in the washing tank goes down to H1.
1.5.2 Scale Trouble Prevention 1.5.2.1 Acid Dosage Total carbonic acid dissolved in brine is equilibrated to CO2 gas (3 104 atm.) in air. Total carbonic acid concentration in seawater is 2–5 102 mol dm3 at pH ¼ 7.0 –7.4. 15–30% of this total value is carbonic acid molecules (CO2 and H2CO3), and the remainders are ionic carbonic acid 2 consisting of HCO 3 (more than 95%) and CO3 (less than 5%). In a concen trating cell in an electrodialyzer HCO3 ions decompose as follows and combine
342
Ion Exchange Membranes: Fundamentals and Applications
with Ca2+ ions to form CaCO3 precipitation. 2 2HCO 3 3CO3 þ H2 O þ CO2 "
(1)
2þ CO2 ! CaCO3 (2) 3 þ Ca In order to avoid CaCO3 precipitation, HCO3 ions are decomposed into CO2 gas by dosing with an HCl or a H2SO4 solution into concentrating cells. þ (3) HCO 3 þ H ! CO2 " þH2 O
1.5.2.2 Precipitation Controlling Agent Dosage Small amount of precipitation controlling agents such as condensed sodium phosphate Na2[Na4(PO3)6] are dosed into a concentrating cell, resulting with the absorption of crystalline nuclei to the agents and dissolution of CaSO4 or CaCO3 due to the following chelate reaction. Na2 ½Na4 ðPO3 Þ6 þ CaX ! Na2 ½Na2 CaðPO3 Þ6 þ Na2 X CaX : CaSO4 or CaCO3
(4)
Carboxyl methyl cellulose (CMC) or poly-acrylic acid is also available instead of condensed sodium phosphate. 1.5.3
Disassembling and Assembling Works In spite of the filtration described in Section 1.5.1, a very small quantity of fine particles passes through a filter and invades into an electrodialyzer. Sometimes, fine organisms pass through the filter and breed in an electrodialyzer. The fine particles are adhered on the surface of membranes and spacers in desalting cells (cf. Section 14.2.2 in Fundamentals), causing the increase of flow resistance of a solution in the desalting cell with acceleration of concentration polarization on the membrane surface (cf. Section 14.2.1 in Fundamentals). In order to avoid these troubles, an electrodialyzer is usually disassembled and washed periodically. The disassembling and washing process is generally as follows (Tanaka, 1987): (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
Electric current interruption Solution feeding interruption Solution discharge Stack extraction Stack disassembling Membrane surface washing Desalting cell, concentrating cell and spacer washing Stack assembling Leak test Integrating stack into an electrodialyzer Solution feeding Electric current passing.
343
Electrodialysis
1.6.
PRACTICE
1.6.1
Potable Water Production from Brackish Water Residents in an isolated island suffer from serious water shortage because of their high dependence on rainwater. In order to solve this problem, Asahi Chemical Co. constructed the ED plant (2500 m3/day) illustrated in Fig. 1.19 in 1990 in Ohshima island, Tokyo (Fukuhara et al., 1993). Ohshima Town supplies potable water to the residents even now. The specifications of this plant are shown in Table 1.2. Raw brackish water pumped up from wells is fed to the suspended solid filter (SSF), and then it is supplied to the first stage electrodialyzer unit (EDUA1 and EDU-B1) through the first stage desalination tank (DST-1). A part of the filtrated solution is supplied to the concentrated solution tank (CST). The desalted solution in DST-1 is further desalted at the second stage electrodialyzer unit (EDU-A2 and EDU-B2) to be about 400 ppm, and then supplied to the water cleaning tank (WCT).
ACS-A FS-A CS-A ACS-B FS-B CS-B
DS-A 1
+ EDU-A 1 − + EDU-A 2 −
P
P
P
P
P
P
DS-B 1 CCS-A DS-A 2 DS-B 2 CCS-B
+ EDU-B 1 − + EDU-B 2 −
P
P
P
P
P
P Wells
ST
SSF
CST
AT
EST
DST-1
DST-2 PWT
WCT
DR
Figure 1.19 Saline water desalting process (Ohshima island). ACS, Anode compartment solution; AT, acid tank; CCS, cathode compartment solution; CS, concentrated solution; CST, concentrated solution tank; DR, distributing reservoir; DS, desalinated solution; DST, desalination tank; EDU, electrodialyzer unit; EST, electrode solution tank; FS, frame solution; PWT, product water tank; SSF, suspended solid filter; ST, stock tank; WCT, water cleaning tank (Fukuhara et al., 1993).
344 Table 1.2
Ion Exchange Membranes: Fundamentals and Applications
Specifications of the electrodialysis plant
Site
Ohshima, Tokyo
Capacity, product water
2500 m3/day, from 1200 ppm TDS raw water 1000 m3/day, from 3000 ppm TDS raw water 4 brackish wells TDS: 450 ppm or less Chloride ions: 150 ppm or less Asahi Chemical SS-0 1780 m3 (318 pairs/stack, 4 stack) Dilution compartment: 0.5 mm Concentration compartment: 0.5 mm Dual-train, two steps per line automatic, continuous
Raw water source Product water Electrodialyzers Conductive area Thickness Mode of operation Source: Fukuhara et al. (1993).
Table 1.3
Operational results
Production rate Product water Electric consumption
2610 m3/day, from 1200 ppm TDS raw water TDS: 440 ppm Chloride ions: 145 ppm pH: 6.8 0.8 kWh m3
Source: Fukuhara et al. (1993).
Organic fouling due to organic acid including in raw brackish water is avoided by integrating anti-fouling anion exchange membranes Aciplex A-201 into the electrodialyzer instead of standard type Aciplex A-101 membranes (cf. Section 14.3.2 in Fundamentals). Operational results are shown in Table 1.3. Analysis of raw and product water are shown in Table 1.4. The values listed in both tables meet the potable water standard established by the Ministry of Welfare, Japan. Actual operating cost is determined based on the plant operation in a year as indicated in Table 1.5. 1.6.2 Electrodialysis Desalination System Powered by Photo-Voltaic Power Generation Babcock-Hitachi K.K. and Hitachi Ltd. established ED system combined with sunlight photo-voltaic power generation in Oshima island and at Sakiyama, Nagasaki (Inoue and Kuroda, 1993). In this system, the electrodialyzer was operated using direct current power generator using solar batteries and produced potable water. Fig. 1.20 gives the system in Oshima island, showing a two-stage seawater desalting process for producing 103/day of potable water. In the first stage, if it is fine, seawater (35,000 mg l1-TDS) is desalted during the daytime to obtain an
345
Electrodialysis
Table 1.4
Analysis of raw and product water Raw Watera
Product Water
Visual Inspection
Colorless, Transparent
Colorless, Transparent
Turbidity (1) Color (1) pH Electrical conductivity (mS cm1) Total hardness (CaCO3, ppm) Calcium hardness (CaCO3, ppm) Evaporation residue (ppm) Silica (SiO2, ppm) Chloride ion (Cl, ppm) Sulfuric ion (SO4, ppm) Hydrocarbonate ion (HCO3, ppm) Calcium ion (Ca, ppm) Potassium ion (K, ppm) Magnesium ion (Mg, ppm) Sodium ion (Na, ppm)
o1.0 o1.0 7.2 2500 685 406 1631 43 632 262 210 51 17 48 368
o1.0 o1.0 7.3 650 83 33 418 42 131 40 82 14 3 12 94
Source: Fukuhara et al. (1993). a From single well.
Table 1.5
Operating cost Units
Volume of product water Raw water TDS Water recovery Electric consumption Stering agents Membrane replacement Total
m3/day mg l1 % yen m3 yen m3 yen m3 yen m3
Projected
Actual
1950 1600 80.0 27.82 0.38 11.80 40.00
1853 1250 86.5 10.04 3.73 13.77
Source: Fukuhara et al. (1993).
intermediately desalted solution (5000 mg l1-TDS) and at the same time, electric power surpluses are stored in batteries (STB). In the second stage, the intermediately desalted solution is further desalted to obtain potable water (400 mg l1-TDS) in the rainy or cloudy daytime or in the nighttime using the stored electric power. Fig. 1.21 shows the system in Sakiyama for producing 200 m3/day of potable water from brackish water. In this system, electric consumption for pumping up raw brackish water is larger. So, in the fine daytime, raw brackish water (1500 mg l1-TDS) is pumped up and is desalted to obtain potable water (400 mg l1-TDS). At the same time, brackish water and electricity are stored in
346
Ion Exchange Membranes: Fundamentals and Applications
SOB t
CBO
STB
DAT FIL −
DST
+
IST
FST SWP Sea
DSP
ED
CSPCST
PWT Brine
Product water
Sea
Figure 1.20 Seawater desalination process powered by solar generation (Oshima island). CBO, Control board; CSP, concentrated solution pump; CST, concentrated solution tank; DAT, direct/altering current transducer; DSP, desalted solution pump; DST, desalted solution tank; ED, electrodialyzer; FIL, filter; FST, filtrated solution tank; IST, intermediately concentrated solution tank; PWT, product water tank; SOB, solar battery; STB, storage battery; SWP, seawater pump (Inoue and Kuroda, 1993).
the fine daytime for operating the electrodialyzer in the rainy or cloudy daytime and in the nighttime. The specifications of both systems are shown in Table 1.6. An electric power consumption pattern of the Oshima plant is illustrated in Fig. 1.22. In a batch desalting system, electric power consumption is large at beginning and it decreases with elapsed time. So, when a large amount of electric power is generated in the fine daytime, seawater is electrodialyzed to obtain intermediately desalted solution, which is temporarily stored in a tank (highconcentration operation). At the same time, electric power surpluses are stored in STB. In the cloudy or rainy daytime and in the nighttime, the stored solution is further desalted to obtain potable water (low-concentration operation) passing the stored electric power. Fig. 1.23 shows an electric power consumption pattern of the Sakiyama plant, which is automatically controlled with a sunlight photo-voltaic power generation system, a storage battery system and an ED system. An electric power generated in the solar system is directly consumed in the electrodialyzer, and the electric power surpluses charge the battery. Operating performance of both plants are indicated in Table 1.7.
347
Electrodialysis
SOB t CBO
STB
Alternating current ACL
DAT
FST to CST
DCL
Direct current
ECM
FIL
from FST −
PWT
+
DST CST
ED WEL SWP Saline water
DSP
Product water
CSP Sea
Figure 1.21 Saline water desalination process powered by solar generation (Sakiyama). ACL, Altering current load; CBO, control board; CSP, concentrated solution pump; CST, concentrated solution tank; DAT, direct/altering current transducer; DCL, direct current load; DSP, desalted solution pump; DST, desalted solution tank; ECM, electroconductivity meter; ED, electrodialyzer; FIL, filter; FST, filtrated solution tank; PWT, product water tank; SOB, solar battery; STB, storage battery; SWP, seawater pump; WEL, well (Inoue and Kuroda, 1993).
1.6.3 Electrodialytic Recovery of Wastewater from a Metal Surface Treatment Process Electrodialysis is applicable to recovering wastewater and nickel in a metal surface treatment process. The process was developed by Asahi Glass Co. as follows (Itoi et al., 1986). A nickel plating process includes several rinsing processes. In this process, Ni concentration in the effluent from the first rinsing stage is high. So that the effluent is usually returned to the electro-plating bath and the effluent from the final rinsing stage is discharged. The ED process is designed to collect Ni ions in the first rinsing stage and return them to the electro-plating bath for the purpose of increasing recovery ratio of Ni and decreasing Ni content in the waste from the final rinsing stage. Fig. 1.24 is a continuous process operating in the car components manufacturing factory. The process includes two stages of rinsing baths and one unit
348
Ion Exchange Membranes: Fundamentals and Applications
Table 1.6
Specifications of the desalting process
Solar generator Type Capacity Module number Storage battery Type Capacity Voltage Electrodialyzer Capacity Raw water Type Gasket dimension Effective membrane area Distance between membranes Number of cell pair System control
Oshima
Sakiyama
Silicon single crystal 25 kW 64 W 390 module
Silicon single crystal 65 kW 47 W 1380 module
Lead storage battery 115 kWh 96 V
Lead storage battery 230 kWh 192 V
10 m3/day Seawater (35,000 mg l1-TDS) Filter-press 185 2000 mm 0.238 m2
200 m3/day Saline water (1500 mg l1TDS) Filter-press 370 2000 mm 0.476 m2
0.8 mm
0.8 mm
250 3 modes operation 1. High-concentration operation 2. Low-concentration operation 3. Stand by operation
600 4 modes operation 1. Pump up+desalting operation 2. Pump up operation 3. Desalting operation 4. Stand by operation
Source: Inoue and Kuroda (1993). Electrodialysis
Solar battery output
Electric current (A)
Intermediately concentrated solution production
Potable water production
Auxiliary machine 6
8
10
12
14
16 Time
18
20
22
24
Figure 1.22 Electric power consumption pattern of the seawater desalination plant (Oshima island) (Inoue and Kuroda, 1993).
349
Electrodialysis
Solar battery output
Electric current (A)
Pump up current
Electrodialysis current Auxiliary machine current
Desalination + Pump up
Desalination
6
8
10
Stand by
12 14 Time
16
18
20
Figure 1.23 Electric power consumption pattern of saline water desalination plant (Sakiyama) (Inoue and Kuroda, 1993).
Table 1.7
Operating performance of the electrodialyzer Oshima
2
Sunlight quantity (kWh m ) Generation quantity (kWh/day) Water production (m3/day) Electric power consumption (kWh m3)
Sakiyama
1986
1987
1988
1900
1901
3.60 49.3 3.45 14.3
3.73 50.6 3.44 14.7
3.86 57.0 4.04 14.1
4.08 162 229 0.71
3.60 145 194 0.75
Source: Inoue and Kuroda (1993).
of electrodialyzer, which is designed to maintain the Ni concentration in the first stage rinsing bath to about 5 g l1 when the concentration in the Ni plating bath is about 84 g l1. The specifications of the electrodialyzer are presented in Table 1.8. The limiting current density for NiSO4 or NiCl2 is extremely low comparing to that for Na2SO4 or NaCl. So, it is important to control the current density to prevent the precipitation of Ni(OH)2 caused by water dissociation. Further, the organic substances in the solution added into the electro-plating bath possibly give rise to generate the organic fouling of anion exchange
350
Ion Exchange Membranes: Fundamentals and Applications
Chemical dosing
* EPB
1st RIT
2nd RIT CXC
AXC
PRT +
−
ED
*
FIL 1st RIW
DIL
CON
ELR
2nd RIW
DEW
Figure 1.24 Electrodialysis Ni2+ electro-plating wastewater recovery process. AXC, Anion exchange column; CXC, cation exchange column; CON, concentrate; DEW, demineralyzed water; DIL, diluate; ED, electrodialyzer; ELR, electrode rinse; EPB, electro plating bath; FIL, filter; PRT, pretreatment; RIT, rinse tank; RIW, rinse waste (Itoi et al., 1986). Table 1.8
Specifications of the electrodialysis plant
Electrodialyzer Ion exchange membrane Size of membrane Effective membrane area Number of membrane pairs Distance between membranes Flow velocity Current density Maximum voltage
Model DU-111 Selemion CMV/AMV 0.49 0.98 m 0.336 m2 40 pairs 2.0 mm 3.0 cm s1 1.0 A dm2 50 V
Source: Itoi et al. (1986).
membranes. So, it is necessary to remove the organic substances by means of a special pretreatment. Material balance of Ni2+ ions and water in the process is indicated in Fig. 1.25 showing that the recovery ratio is larger than 90% and a diluted solution is perfectly recycled to the first stage rinsing bath. Current efficiency and electric power of ED were, respectively, greater than 90% and 2 kWh kg1 Ni ion. Cost estimation is presented in Table 1.9.
351
Electrodialysis
Take out from plating bath 13 l/h (Ni2+ 84.2 g/l)
1 st stage Rinsing tank
Take out to 2nd stage 13 l/h (Ni2+ 5g/l)
Feed to ED 3 m3/h (Ni2+ 5g/l) Concentrated stream To plating bath 12.4 l/h (Ni2+ 83 g/l)
Figure 1.25
Electrodialyzer
Diluate stream 3 m3/h Ni2+ 4.65 g/l
Ni2+ and water balance in the electrodialysis process (Itoi et al., 1986).
Table 1.9 Cost estimation of electrodialytic recovery of rinsing waste in the nickel electroplating process Nickel recovery rate (as NiSO4 6H2O) Electricity consumption (as NiSO4 6H2O)
3460 kg month1 0.7 kWh kg1
Equipment installation cost Electricity unit cost Purchasing price of nickel salt (as NiSO4 6H2O)
15 million yen1 10 yen kWh1 360 yen kg1
Profit of Nickel salt recovery 360 yen kg1 3460 kg month1 ¼ Runnig cost Electricity 10 yen kWh1 0.7 kWh kg1 3460 kg month1 ¼ Maintenance and consumable item (annually 3%) 15,000,000 0.03/12 month ¼ Amortization (7 years) 15,000,000 yen (7 year 12 month)1 ¼ Interest (annually 9%) 15,000,000 yen 0.09/12 month ¼ Running cost total Profit
1,245,600 yen month1 24,220 yen month1 37,500 yen month1 178,570 yen month1 112,500 yen month1 352,790 yen month1 892,810 yen month1
Source: Itoi et al. (1986).
1.6.4
Reuse of Wastewater by Electrodialytic Treatment Ion exchange membrane ED technology is applicable to wastewater treatment for realizing resource saving and protection of environment. Tokuyama Inc. developed the technology to establish a closed system by means of
352
Ion Exchange Membranes: Fundamentals and Applications
Process
HCl, H2SO4
Acid waste
NaOH
Neutralization
Flocculant
Clarifier
Filter
Electrodialysis
Desalted solution
Concentrated solution
Traider receiving Designed process is surrounded by the dotted line.
Figure 1.26
Electrodialytic treatment process of industrial waste (Matsunaga, 1986).
Solution Circulating solution Concentrated solution
Na Ca 0.255N 13 ppm 3.870 220
Cl SO4 0.168 N 0.087 N 3.329 0.541
to process
ED
from clarifier
SAF MIT
DST
CHF
HCl
1 CST
2
Concentrated solution 65.4 l /h
Figure 1.27 Electrodialytic treatment process of industrial waste. CHF, Check filter; CST, concentrated solution tank; DST, desalted solution tank; ED, electrodialyzer; MIT, middle tank; SAF, sand filter (Matsunaga, 1986).
Electrodialysis
353
electrodialytic reuse of wastewater in a plating process as shown in Figs. 1.26 and 1.27 (Matsunaga, 1986).
Typical components of a raw feeding solution in this system. pH: 9.5, NaCl: 17.6 g l1, Na2SO4: 7.1 g l1, Ca: 48 mg l1, SS: 20 mg l1. Main component: Fe(OH)3. The others: Fe, Cu, Zn, P etc. Conditions of electrodialytic treatment. Quantity of a desalted solution: 10 ton/month. Degree of desalination: maximum. Degree of concentration: maximum. Temperature: normal. Operating time: 24 h/day, 30 days/month, 12 months/year. Operating system: partially circulation. Ion exchange membrane: Neocepta C5S-8 T (monovalent cation selectively permeable cation exchange membrane) and Neocepta ACH-45 T (anion exchange membrane). Operating results. Current density: 3.0 A dm2. Cell voltage: 0.5 V/pair (18–20 1C). Electrolyte concentration in a desalted solution: Cde,Na ¼ 0.25–0.32 eq dm3, Cde, Ca ¼ 13–15 mg l1. Electrolyte concentration in a concentrated solution Ccon,Na ¼ 3.80– 4.02 eq dm3, Ccon,Ca ¼ 220–240 mg l1. Quantity of a concentrated solution: 60–66 l h1 (1.44–1.58 m3/day). Quantity of a desalted solution 10.9– 12.1 ton/month. Running cost. Electric power: 262 kWh/day 15 yen/kWh ¼ 3930 yen/day Labor: 0.2 people/day 15,000 yen/people ¼ 3,000 yen/day The others (Ion exchange membrane, HCl, electrode, filter): 1330 yen/day Total: 8260 yen/day.
1.6.5 Simultaneous Treatment of Wastewater by Electrodialysis and Reverse Osmosis Yunichica Ltd. developed a treatment process of wastewater including toxic metallic ions discharged from a semi-conducting material manufacturing process (Ishibashi, 1986). The process is illustrated in Fig. 1.28 consisting from the following system. 1.6.5.1 High-Concentration System HS gas dissolving in the wastewater is deaerated and neutralized adding NaOH. Fine particles suspending in the wastewater are removed using a microosmosis filter. Then, a clear salt solution is supplied to an ion exchange membrane ED unit and concentrated to 3.5–4.0 M. The concentrated solution is evaporated using a vacuum evaporator (EV) to obtain Na2SO4 and NaCl crystals, which are reused after re-purification. The desalted solution obtained from the ED unit is further desalted using a reverse osmosis (RO) unit. Concentrated
354
Ion Exchange Membranes: Fundamentals and Applications
Concentrated solution
High concentration system
Low concentration system
TDS 36g/l
TDS 240g/l
Separated salt 570 kg/day
EV
TDS 1.5g/l
ED
RO TDS 0.1g/l
Polishing
IX
Pure water Heigh pure water
Process ED: Electrodialysis RO:Reverse osmosis IX: Ion exchange EV:Evaporation
Figure 1.28 Simultaneous treatment process of wastewater by electrodialysis and reverse osmosis (Ishibashi, 1986).
solution from the RO unit is returned to the ED unit. The specifications of both units are as follows:
Electrodialysis unit Tokuyama TS-25-160 type integrated with Neocepta C66-5T/AFS Batch wise operating system Desalting performance 18 m3/day Concentration of a desalted solution 1.5 g l1 Reverse osmosis unit Middle pressure hollow fiber B-9 Desalting performance 15 m3/day Concentration of a desalted solution 100 ms
1.6.5.2 Low-Concentration System After deaeration, the solution is filtered passing through a carbon filter and neutralized using weak basic ion exchange resins. Then the solution is supplied to a cation exchange and an anion exchange column to obtain pure water. The pure water is fed to the semi-conducting material manufacturing process
355
Electrodialysis
Anion exchange layer
Mn+
+
H+ Anode
Cation exchange membrane
+ + + + + +
− − − − − −
− − − − −
− H+ Cathode
Figure 1.29 H+ ion permeable cation exchange membrane. Mn+, n valent metalic ion (Katayama, 2004).
directly or via a polishing column filled up with cation and anion exchange resins and nonionic resins. 1.6.6
Electrodialytic Recovery of Acid Acid is recovered from a waste acid applying a H+ ion permselective cation exchange membrane placed an anion exchange layer on a cation exchange membrane as illustrated in Fig. 1.29. Here, multivalent cations Mn+ do not pass through the membrane due to repulsive effects between the cations and the anion exchange layer. The concept mentioned above is applicable to recover an acid from an aqueous solution dissolving for instance Fe(NO3)3 with HNO3 as shown in Fig. 1.30. In this system, cation exchange membranes correspond to the membrane illustrated in Fig. 1.29, and they permeate H+ ions selectively and do not permeate Fe3+ ions. On the other hand, anion exchange membrane permeate NO ions selectively rather than H+ ions. Consequently, HNO3 is recovered in the concentrating chamber and Fe(NO3)3 is remained in the desalting chamber. Table 1.10 shows the material balance in an electrodialyzer (effective membrane area: 90 m2) developed by Tokuyama Inc., which treated a wasted acid solution (0.54 M acid solution including H2SO4 and HCl with 0.30 M of Al3+ ions) generated in an etching process of aluminum products (Katayama, 2004). Acid concentration in a recovered acid is 1.90 M including 0.01 M of Al3+ ions. Acid recovering ratio and leak ratio are, respectively, 88% and 0.6%. Energy consumption and current efficiency are, respectively, 12 kW and 55%. 1.6.7
Seawater Concentration for Salt Production Electrodialysis is applied to concentrating seawater for producing salt in Japan. The seawater concentrating process is illustrated in Fig. 1.31 (Tomita, 1995). Seawater pumped up from sea is filtered and supplied to the electrodialyzer via the filtered solution tank. Concentrated seawater is circulated
356
Ion Exchange Membranes: Fundamentals and Applications
Recovered acid HNO3 De-acidified solution Fe(NO3)3 C
A NO3−
+
H+
H+
C NO3−
A NO3−
H+
C NO3−
H+
−
H+
Fe3+
Fe3+ Anode
Cathode Water Waste acid HNO3 Fe(NO3)3
Figure 1.30 Acid recovery by means of ion exchange membrane electrodialysis. C, H+ ion permselective cation exchange membrane; A, H+ ion low-permselective anion exchange membrane (Katayama, 2004).
Table 1.10
Material balance in an electrodialyzer for acid recovering
Solution
Waste acid (before electrodialysis) Deacid solution (after electrodialysis) Water Recovered acid
Solution Quantity (l h1)
Composition H+ (M)
Al3+ (M)
SO2 4 (M)
Cl (M)
742
0.54
0.30
0.65
0.16
671
0.07
0.33
0.50
0.08
115 186
0 1.90
0 0.01
0 0.79
0 0.35
Source: Katayama (2004).
between the concentrated seawater tank and the electrodialyzer, and its gain is supplied to an evaporating process to obtain salt crystals. CaCO3 scale precipitation in concentrating cells is prevented by adding hydrochloric acid to the 2 concentrated seawater to decompose HCO 3 and CO3 ions (cf. Section 1.5.2 in Applications). A part of filtrated seawater is supplied to anode chambers. Titanium is adopted as the anode material. In order to avoid membrane destruction due to Cl2 and HClO generated by an anode reaction, a perfluorinated ion exchange membrane is integrated between the cathode chamber and the
357
Electrodialysis
C
9 10
+
−
7
6
8
2
3
4
5
1
Figure 1.31 Electrodialytic seawater concentration process. 1, Diluted seawater tank; 2, cathode solution tank; 3, concentrated seawater tank; 4, washing solution tank; 5, HCl tank; 6, filtrated seawater; 7, concentrated seawater output; 8, HCl; 9, electrodialyzer; 10, anode solution (Tomita, 1995).
adjacent stack. The wasted solution from the anode chamber is mixed with the filtrated seawater to suppress the growth of microorganisms in seawater. Cathode material is plated with Pt. An HCl solution is supplied to the cathode chamber to neutralize OH ions generated by the cathode reaction. A washing system is provided for washing the inside of desalting cells by acid or chemical reagents and dissolving adhered substances (cf. Section 14.2.3 in Fundamentals). When the turbidity of raw seawater is 2 ppm, it is decreased to about 0.05 ppm by filtering through two-stage sand filters (cf. Section 1.5.1 in Applications). In spite of such filtration, fine particles pass through the filter and invade into the electrodialyzer and precipitate on the membrane surfaces. Fe(OH)3 components precipitated on the membrane possibly give rise to water dissociation (cf. Sections 8.8.4 and 8.9 in Fundamentals). Sometimes, fine organisms pass through the filter and breed in the electrodialyzer. These troubles are avoided by disassembling and washing the electrodialyzer at the interval of 4–6 months (cf. Section 1.5.3 in Applications). In order to increase current efficiency and avoid CaSO4 scale precipitation, membrane surfaces are treated to give monovalent ion permselectivity (cf. Section 3.7 in Fundamentals).
358
Ion Exchange Membranes: Fundamentals and Applications
Figure 1.32 Composition of an electrodialyzer (Tokuyama). 1, Fastening bolt; 2, fastening and feeding frame; 3, concentrated seawater inlet; 4, diluted seawater inlet; 5, diluted seawater outlet; 6, concentrating cell; 7, desalting cell; 8, desalting cell manifold; 9, concentrated seawater outlet; 10, cation exchange membrane; 11, anion exchange membrane; 12, concentrating cell manifold (Tomita, 1995).
Compositions of electrodialyzers developed by Tokuyama Inc., Asahi Glass Co. and Asahi Chemical Co. are illustrated in Figs. 1.32–1.34 (Tomita, 1995). Typical performance of an electrodialyzer is exemplified in Table 1.11 (Tanaka, 1991). 1.6.8 Salt Production Using Brine Discharged from a Reverse Osmosis Seawater Desalination Plant Concentrated brine is discharged from a RO seawater desalination process. It seems advantageous to use this brine as raw material for salt production.
Electrodialysis
359
Figure 1.33 Composition of an electrodialyzer (Asahi Glass Co.). 1, Cathode chamber; 2, anode chamber; 3, cathode plate; 4, anode plate; 5, intermediate frame; 6, desalting cell; 7, concentrating cell; 8, cation exchange membrane; 9, anion exchange membrane; 10, packing cell frame; 11, intermediate packing; 12, blind cell frame (Tomita, 1995).
Figure 1.34 Composition of an electrodialyzer (Asahi Chemical Co.). 1, Diluted seawater; 2, special gasket; 3, concentrating cell; 4, turn-Buckle; 5, desalting cell; 6, concentrated seawater; 7, fastening frame; 8, cation exchange membrane; 9, anion exchange membrane (Tomita, 1995).
360
Ion Exchange Membranes: Fundamentals and Applications
Table 1.11 Performance of an electrodialyzer for concentrating seawater (plant A, 1987) Current density (A dm2) Temperature (1C) Cl current efficiency (%) Na current efficiency (%) Energy consumption (kWh t1 NaCl) Concentrated solution puritya (%) Constitution of concentrated solution NaCl (g dm3) Cl (eq dm3) SO4 (eq dm3) Ca (eq dm3) Mg (eq dm3) K (eq dm3) Na (eq dm3)
2.66 25.3 88.6 80.8 178.6 90.90 190.6 3.568 0.015 0.064 0.169 0.095 3.256
Source: Tanaka (1991). a NaCl(g)/Total electrolyte (g).
The operating performance and energy consumption in a salt manufacturing plant were investigated for an ion exchange membrane ED system to which discharged brine from a RO plant is supplied as follows (Tanaka et al., 2003). The salt manufacturing process (NaCl production capacity: 200,000 ton/year) is illustrated in the dotted frame in Fig. 1.35. Discharged brine (electrolyte concentration: 1.5 eq dm3) from a RO plant is assumed to be supplied to an ion exchange membrane electrodialyzer. The concentrated solution obtained from the electrodialyzer is supplied to a multiple-effect EV, in which salt is crystallized. The salt obtained from the evaporator is supplied to an ion exchange membrane electrolytic bath, in which sodium hydroxide and chlorine are produced. Energy consumption in the salt manufacturing process is assumed to be supplied by a simultaneous heat-generating electric power unit consisting of a boiler and a back-pressure turbine. Fig. 1.36 shows the flows of electricity and steam in a salt manufacturing plant. Boiler steam is introduced to a turbine and generates electricity, which is distributed to electrodialyzers, etc. The back-pressure of a back-pressure turbine is supplied to a heater in a No. 1 evaporator in multiple-effect evaporators. Evaporated steam in a No. 1 evaporator is supplied in turn to the following evaporators. Pressure and temperature of boiler steam are set as 6 Mpa and 4801C in this study. The temperature difference between heating steam and evaporated steam is fixed to 201C at each evaporator. The number of evaporator is kept to a minimum, but the quantity of electricity does not exceed the electric power consumption in this salt manufacturing plant. An electric power shortfall is assumed to be made up by purchased electric power, which is generated by a condensing turbine.
361
Electrodialysis
Seawater 0.6 eq/dm3
Desalted solution
Discharged brine C′in=1.5 eq/dm3 RO C′′ C′out ED Cl2 T
B
EV NaOH NaCl crystals IM
Salt manufacturing process H2O
Figure 1.35 RO-ED combined salt manufacturing process. RO, Reverse osmosis; ED, electrodialyzer; EV, evaporation; IM, ion exchange membrane electrolysis; B, boiler; T, turbine. Diagonals in RO, ED and IM unit box represent the membranes (Tanaka et al., 2003).
The energy required for producing salt F is plotted against current density I/S in both cases of RO discharged brine ED and seawater ED. The plots are shown in Fig. 1.37, which indicates that the energy consumption in a salt manufacturing process using RO discharged brine is 80% of the energy consumption in the process using seawater. The optimum current density at which the energy consumption required in the salt manufacturing process being minimized is 3 A dm2 for both RO discharged brine ED and seawater ED. 1.6.9
Desalination of Amino Acid and Amino Acidic Seasonings Amino acid is an amphoteric electrolyte, and its behavior is different from that of usual electrolytes. Itoi and Utsunomiya (1965) electrodialyzed aqueous solutions of methionine and glysine containing sodium formate as follows; (a) methionine 25 g l1, HCOONa 20 g l1. (b) glysine 70 g l1, HCOONa 20 g l1. The solution was supplied to the electrodialyzer (membrane pairs: 9, effective membrane area: 209 cm2) incorporated with Selemion CSG and AST. Changing pH and maintaining cell voltage at 15 V (average current density: nearly 1 A dm2), amino acid permeation ratio (the ratio of amino acid
362
Ion Exchange Membranes: Fundamentals and Applications
Purchsed electric power
∆G
Ggen+∆G=Etotal × PNaCl RO
H*S*
Discharged brine 1.5 eq/dm3solution
Generated electric power Ggen B
ED
BPT
W = w × PNacl 70°C
50°C
30°C
St
Qcond EV NO1
EV NO2
EV NO3
Tcond=90°C Hcond Scond
Figure 1.36 Energy flow in a salt manufacturing process. B, Boiler; BPT, back-pressure turbine; EV, evaporator; ED, electrodialyzer; RO, reverse osmosis unit (Tanaka et al., 2003).
transported across a membrane pair against that dissolving in a feeding solution) was measured and shown in Fig. 1.38, indicating that the amino acid permeation ratio becomes minimum near the isoelectric point of amino acid. Changing current density and keeping pH near the isoelectric point of the amino acid, the amino acid permeability (quantity ratio of amino acid transported across a membrane pair against electricity) was measured and shown in Fig. 1.39. Inspecting Fig. 1.39 and the limiting current density measured for an HCOONa solution, it is concluded that the amino acid permeability becomes minimum by applying the limiting current density. Concentration changes of NaCl and essence in the batch system ED of soy sauce are indicated in Tables 1.12 and 1.13.
363 60
1.2
50
1.0
40
0.8
30
0.6
20
0.4
10
0.2
0 0
1
2
3
4 5 6 I/S (A/dm2)
7
8
9
(RO discharged brine electrodialysis)/ (seawater electrodialysis)
(103M cal/h)
Electrodialysis
0.0 10
: RO discharged brine electrodialysis : Seawater electrodialysis : RO discharged brine electrodialysis / Seawater electrodialysis
Figure 1.37
1.6.10
Energy consumption in a salt manufacturing plant (Tanaka et al., 2003).
Desalination of Natural Essences In an extraction process of natural essences, several kinds of salt, acid and alkali are added. Accordingly salt is obtained as a by-product in a final stage. Salt content in the natural essences is expected to be controlled for maintaining a taste and human health. Tokuyama Inc. developed the following desalination processes of natural essences (Ideue, 1986; Yamamoto, 1993). An ED system was set up using ion exchange membranes met the food hygiene standard established by the Ministry of Welfare, Japan, and incorporated with polyvinyl chloride or polypropylene materials suitable for food production. It was further necessary to pay attention to prevent solution stagnation and cultural contamination. Further cleaning in place (CIP) was necessary to operate the apparatus stably. The electrodialyzer was operated in a batch system using ED system indicated in Fig. 1.40, in which a conductivity control indicator was set at an exit of the electrodialyzer for detecting the concentration of a desalted solution. The solution feed and solution discharge were automatically controlled by switch valves operating with the conductivity control indicator. Natural essences include meat essences, seafood essences, flesh essences etc. They were extracted with the aid of a NaCl solution. Salt added in the extraction process was desalinated by means of ED mentioned above. Constituent changes in desalination of extracted meat essences, fish essences and fruit
364
Ion Exchange Membranes: Fundamentals and Applications
80
Cell voltage : 15 V
Glycine
60
ionin
e
50 40
Meth
Amino acid permentation ratio (%)
70
30 20 10 0
0
2
4
6
8
10
12
14
pH Glicine 70 g/l, HCOONa 20 g/l Methionine 25 g/l, HCOONa 20 g/l
Figure 1.38 Relationship between solution pH and amino acid permeation ratio through ion exchange membranes (Itoi and Utsunomiya, 1965).
flesh essences are shown in Table 1.14. Specifications of an electrodialyzer for seafood essences are shown in Table 1.15. 1.6.11 Electrodialysis of Milk and Whey 1.6.11.1 Composition of Milk and Whey (Ideue, 1986; Tomita et al., 1986) Whey is obtained as a by-product of a cheese production process. The output of whey amounts to nine times of that of cheese, so it is an important subject in the dairy industry to utilize the whey effectively. The effective utilization of permeates derived from an ultra-filtration process of milk and whey is also a big problem in the dairy industry. The composition of milk, whey and ultrafiltration permeates is indicated in Table 1.16. Ash content in dry matter of the whey and permeate is so high that it is expected to reduce their ash content by ED. Powdered milk for baby rising is prepared using cow milk as main raw materials. The constituents of breast milk and cow milk are not same as shown in Table 1.17. Total ash and casein compositions of cow milk are, respectively, 3.4 and 4.4 times of those of the breast milk. Accordingly, baby rising powdered
365
Electrodialysis
3.0 pH: 6.3-6.9
ine yc
2.0
Gl
1.5
nin
e
1.0
et
hio
Amino acid permeability (g/Ah)
2.5
0
M
0.5
0
0.5
1.0
1.5
2.0
Average current density (A/dm2) Glycine 70 g/l, HCOONa 20 g/l Methionine 25 g/l, HCOONa 20 g/l
Figure 1.39 Relationship between current density and transport rate of amino acid (Itoi and Utsunomiya, 1965).
Table 1.12
Electrodialytic demineralization of soy sauce Start
Solution quantity (l) NaCl concentration (g l1) Essence concentration (g l1) pH Density Current efficiency (%)
11.6 191 163 4.7 1.181
End 31 191 4.7 1.103 90
Note: Effective membrane area: 209 cm2; Numbers of membranes: 10 pairs; Current density: 3.5 A dm2; Applied voltage: 25–58 V. Source: Itoi (1983).
366
Ion Exchange Membranes: Fundamentals and Applications
Table 1.13
Concentration changes of components in electrodialysis of soy sauce
Component
Ratio (Before ED/After ED)
NaCl Total amino acid Glutamic acid Aspartic acid Lysine Leucine Isoleucine Alanine Phenylalanine Valine Total nitrogen Essence part
0.800 0.94 0.93 0.95 0.98 0.98 0.98 0.96 0.87 0.95 0.96 1.009
Source: Itoi (1983).
milk is prepared by adding whey to cow milk. Before this mixing operation, however, it is necessary to extract excessive ash from the milk or whey by ED. 1.6.11.2 Limiting Current Density in Electrodialysis of Milk and Whey (Nagasawa et al., 1973; Nagasawa et al., 1974) In ED of a dairy product solution, protein is condensed and attached on the desalting surface of an anion exchange membrane at above limiting current density due to occurrence of water dissociation. At the same time, insoluble salt such as calcium phosphate etc. precipitates on the concentrating surface of the anion exchange membrane. Accordingly, it is important to increase the limiting current density to operate a practical electrodialyzer stably. Nagasawa et al. evaluated the limiting current density as follows. In ED of a skim milk solution, limiting current density was evaluated based on the relationship between current density and voltage. The relationship between conductivity of the skim milk solution k and the limiting current density ilim was expressed by ilim ¼ 1.08 103k. (i/k)lim was known to be proportional to the 0.6th power of solution velocity of skim milk V. (i/k)lim/V0.6 vs. temperature T and dry matter content of skim milk S is given in Fig. 1.41 which indicates that (i/k)lim/V0.6 decreases with the increase of T and S. Flow length and distance between the membranes did not influence to ilim. From the investigation mentioned above, the following limiting current density equation was introduced as 0:1
ilim ¼ QS1=6 ð1:01665S ÞT V 0:6 k
(1)
Q is a constant. S, T and V can be voluntarily fixed, so the above equation is expressed by the following equation. ilim ¼ Rk
(2)
VI
Electrodialysis
AI
8 CCI
10
_
+
7 9
F1
F1
LA P1
LC
1
2
P1
P1
F1
F1
P1
6 LA
3
4 3
5 3
367
Figure 1.40 Electrodialysis process of natural essences. 1, Raw solution tank; 2, desalted solution tank; 3, drain; 4, concentrated solution tank; 5, concentrated waste solution; 6, electrode solution tank; 7, water supply; 8, ammeter; 9, electrodialyzer; 10, desalted solution; Vi, voltage indicator; Ai, ampere indicator; Fi, flow indicator; Pi, pressure indicator; CCi, conductivity control indicator, LC, level control; LA, level alarm (Yamamoto, 1993).
368 Table 1.14
Ion Exchange Membranes: Fundamentals and Applications
Desalination of fish essences
Conductivity Viscosity Ash Na K Ca Mg Cl
Raw Essence
Desalted Essence
Desalting Ratio
15.4 mS cm1 4 cp
2.74 mS cm1 6.4 cp
82.2%
2975 ppm 241 ppm 38 ppm 45 ppm 3299 ppm
560 ppm 19 ppm 2.4 ppm 1.9 ppm 583 ppm
81.2% 92.1% 93.7% 95.8% 82.3%
Source: Yamamoto (1993).
Table 1.15
Specifications of an electrodialyzer for desalinating seafood essences
Requirement Raw solution NaCl Protein Specific gravity Viscosity pH Treating amount
14 g l1 100 g l1 1.05 4.0 cp 5.5 0.5 t h1
Specifications Effective membrane area Number of membrane pair Ion exchange membrane Model Operating system
256 m2 200 pairs Neocepta CM-1, AM-1 TS-25-200 Automatic batch system
Running cost
1517 yen t1
Source: Yamamoto (1993).
Table 1.16
Typical composition of milk and UF-permeate Fat (%)
Cow milk Cheese whey Permeate derived from UF of milk
3.3 0.3 0
Source: Tomita et al. (1986).
Protein (%) 2.9 0.7 0.2
Lactose (%)
Ash (%)
4.5 4.5 4.4
0.7 0.6 0.5
Water (%) 88.6 93.7 94.8
Ash/Dry Matter (%) 6.1 9.5 9.6
369
Electrodialysis
Table 1.17
Comparison of composition between breast milk and cow milk Water (%)
Breast milk Cow milk
88.0 88.6
Ash (%)
Whey Protein (%)
0.2 0.7
0.68 0.69
Casein (%) 0.42 2.21
Fat (%) 3.5 3.3
Lactose (%) 7.2 4.5
Source: Tomita et al. (1986).
(i/)lim/V0.6 x 102
10
5
2 0 S%
10 : 0.3,
20 30 Temperature (°C) : 8.3,
: 16,
40
: 25
Figure 1.41 Effects of temperature and total solid content on (i/k)lim/V0.6 (Nagasawa et al., 1974).
R is a constant. We know ilim from k, and it was confirmed that the electrodialyzer is operated stably at under ilim estimated from Eq. (2). 1.6.11.3 Ion Exchange Membrane (Okada et al., 1975) for the Demineralization of Milk or Whey In the demineralization of skim milk or whey including rich proteinaceous materials or other organic matters, conventional membranes applied to the treatment of saline water are not feasible because the life span of the membranes is shortened and the performance is deteriorated. In order to make membranes applicable to the demineralization process of milk or whey, the investigation was performed to give the following characteristics to the membranes. Anti-Organic Fouling The specific conductance of various membranes k was determined in a demineralization experiment of Gouda cheese whey. The membranes are, Aciplex K-101 (conventional cation exchange membrane, Asahi Chemical Co.), A-101 (conventional anion exchange membrane) and A-201 and A-211 (both are
370
Ion Exchange Membranes: Fundamentals and Applications
Remaining specific conductance(%)
100 K-101
80 A-211 A-201
60 40 20 0
A-101 0
2000 4000 6000 Operating time (hr)
8000
Figure 1.42 Membrane conductivity changes with operating time (Okada et al., 1975).
newly developed anion exchange membranes). k of K-101 is not changed largely with running time as indicated in Fig. 1.42. k of A-101 shows very rapid decrease. However, k of A-201 and A-211 membranes does not change significantly for 6000 h. A-201 and A-211 membranes show excellent anti-fouling (cf. Section 14.3.2 in Fundamentals). Anti-Alkaline Circumstance Organic fouling due to deposition of proteinaceous materials or fatty substances from the dairy products is cleaned by washing in an alkaline detergent by means of CIP method. Exposure of the membrane to OH ions generated from water dissociation vitiates its performance. Therefore, life span of the membrane is shortened without the durability against a basic solution in contact. Fig. 1.43 shows the durability evaluated by measuring specific conductance k of the membranes immersed in a 1% NaOH solution. k of conventional cation exchange membrane K-101 does not change, however, that of anion exchange membrane A-101 decreases remarkably with immersing duration. On the other hand, decrease of k is suppressed for anti-organic fouling anion exchange membrane A-201 and A-211. Particularly, A-211 membrane exhibits excellent anti-alkaline performance. Permeability to Larger Organic Ions of an Anion Exchange Membrane An anion exchange membrane which does not permeate larger organic anions causes pH lowering in demineralization of dairy products. This is because OH ions caused by water dissociation generated on a desalting surface of an anion exchange membrane permeate the membrane instead of the larger organic anions toward a concentrating cell. This phenomenon induces pH increase in the concentrating cell and gives rise to the precipitation of inorganic salts such as Ca3(PO4)2, CaCO3, CaSO4 etc. On the other hand, pH in the desalting cell is
371
Electrodialysis
100
K-101 A-211
Remaining specific conductance(%)
90 80 70 A-201
60 50 40 30 20 10
A-101 0
0
100
200 300 400 Immersing duration (hr)
500
Figure 1.43 Membrane conductivity changes with time in alkaline circumstances. 1 % NaOH at 371C (Okada et al., 1975).
estimated to be lowered because H+ ions are remained in the desalting cell. Fig. 1.44 shows pH changes upon 90% ash reduction of Gouda cheese whey using three types of anion exchange membranes. A-211 membrane scarcely brings about the decrease in pH. This is because pore radius in A-211 membrane is large enough (cf. Section 14.3.2.1 in Fundamentals), so that organic acids such as citric acid, lactic acid, amino acid as well as phosphoric acid easily permeate the membrane and that water dissociation does not easily occur. From the experiment described here, it is concluded that A-211 is the most suitable anion exchange membrane. CIP operations of an electrodialyzer incorporated with A-211 membranes realize 90% ash reduction and life span extension.
1.6.11.4 Electrodialysis System (Okada et al., 1975) for the Demineralization of Milk or Whey Fig. 1.45 illustrates the four stack ED process MED SV1/2 4-4 developed by Morinaga Milk Industry Co. The specifications and operating conditions of this system are shown, respectively, in Tables 1.18 and 1.19. In Fig. 1.45, the
372
Ion Exchange Membranes: Fundamentals and Applications
A-211 7
A-201 (and A-101)
pH
6
5
4
Figure 1.44
0
10
20
30 40 50 60 70 Relative deashing rate [%]
80
90
pH changes of whey in demineralization (Okada et al., 1975).
pump P1 feeds the whey to the balance tank WB-1. Succeedingly, the whey is supplied to the 1st stack of the ED system, demineralized to some extent and overflows from WB-1 to WB-2. The whey in WB-2 circulates through pump P3 and second stack, then part of which overflows into WB-3. In the similar way, the whey flows via P4, third stack, WB-4, fourth stack, WB-5, and finally demineralized whey flows out via P6 and supplied to the succeeding process such as pasteurization, evaporation and drying. At each balance tank, the conductivity of the whey is monitored to control the desalting rate. The feed rate of the whey to this system is automatically controlled by means of conductivity measurement in the effluent of demineralizing stream. 1.6.12
Desalination of Sugar Liquor The sugar manufacturing system is classified into the cane sugar–fine sugar manufacturing system and the beet sugar manufacturing system. The raw material for the former is sugar canes and that for the latter is beet sugar. In the sugar manufacturing process, the greater part of organic nonsugar components in raw sugar is removed by means of defecation, carbonation, adsorption (bone char, active carbon, ion exchanger etc.). In these treatments, inorganic components are not removed and transferred to an evaporation process, in which residual organic nonsugar components are separated from sugar crystals and remained in molasses. In the course of repeating evaporation and separation, the inorganic and organic nonsugar components are gradually accumulated in the
CB-5
CB-4
P11
for anode frame (one pass & waste)
P10
I1
V1
CB-3
P9
Con
Con
1st
2nd
DC Dil
CB-2
I2
Dil
V2
V3 DC DC
Water
CB-1
P8
I3
Electrodialysis
Va1
H2SO4
P7
Con
Con
3rd
4th
I4
V4 Rinse solution for fastening frame & cathode frame
DC Dil
Dil P12
P2 Whey
P4
P3
P5
P1
WB-1
WB-2
WB-3
WB-4
PB-5
Demineralized whey P6
1st : Stack - 1 2nd : '' -2
DC : Rectifier
3rd :
''
-3
I1-4 : Amperage meter
4th :
''
-4
Con. : Concentrating compartments Dil. : Diluting compartments
V1-4 : Voltage meter
373
Figure 1.45 Flow diagram of Morinaga continuous electrodialysis process (Okada et al., 1975).
374 Table 1.18
Ion Exchange Membranes: Fundamentals and Applications
Specifications of MED SV1/2 4-4
1. Center-press 2. Stack 3. Electrode 4. Cell pair 5. Distance between membranes 6. Effective membrane area 7. Spacer 8. No. of channel 9. Width of the channel 10. Length of the channel
1 4 4 pairs 150 cell pairs/stack 0.75 mm 50 dm2/cell Sheet-flow type 1 500 mm 1000 mm
Source: Okada et al. (1975).
Table 1.19
Operating conditions of demineralization of whey (Morinaga ED system)
1. Quality of whey to be treated 1.1 Total solids 1.2 Ash content 1.3 Specific conductance 1.4 Sediment test (200 ml, 1000 G)
20% 1.60% 0.013 S cm1 less than 0.1 ml
2. Operating conditions 2.1 Level of applied d-current density 2.2 Linear velocity in the compartment 2.3 Operating temperature
3000 ka (mA cm2) 12 cm s1 201C
Source: Okada et al. (1975). k: Specific conductance.
a
molasses and finally they are discharged to the outside of the system as waste molasses. The waste molasses includes considerable amount of sugar components, so it is utilized as raw materials for fermentation or animal food, however, its economical value is extremely low comparing to that of sugar itself. Because of the background described above, the technology development was expected for preventing sugar component transfer to waste molasses and increasing sugar recovering ratio. Further desalting technology by means of ion exchange membrane ED came to be attracted because the sugar recovering ratio is influenced by residual inorganic components in syrup. Application of ED in sugar manufacturing industry was investigated from the latter half of 1950s. However, it was difficult to put this program into practice. This is because water dissociation and organic fouling are apt to occur on anion exchange membranes (cf. Section 14.3 in Fundamentals). In order to avoid these troubles, Taito Co. and Asahi Chemical Co. developed the technology using neutral membranes consist of polyvinyl alcohol instead of anion exchange membranes (Sugiyama et al., 1982; Kokubu et al., 1983). The
375
Electrodialysis
advantages and disadvantages of this method (Transport depletion method) are as follows. 1. Advantages (a) Neutral membranes do not deteriorate due to organic fouling. (b) Sugar does not decompose, because pH decrease caused by water dissociation does not occur on the neutral membrane. (c) Current density can be increased, because water dissociation does not occur. 2. Disadvantages (a) Removing efficiency of anions is low. (b) Current efficiency Z is low, because transport number of anions of a neutral membrane ¯tA A0:5: If we assume the transport number of a cation exchange membrane ¯tK ¼ 1:0; Z ¼ ¯tK þ ¯tA 1 ¼ 0:5: In the first stage in a sugar manufacturing process, raw molasses (original syrup) is treated to remove organic materials and evaporated to obtain A sugar Table 1.20
Effect of electrodialysis on pretreatment for molasses
Desalting ratio (%) Current efficiency (%) Current density (A dm2) Molasses volume (l) Molasses concentration (oBx) Molasses purity (%) pH
Ash (% on solid) CaO MgO K2O Cl SiO2 SO3 P2O5 CO2 Sulfate ash
I
II
66.18 43.55 3.04
63.60 34.01 3.07
III 2.58 9.54 0.03
Start 10.00 51.35
End 9.76 46.85
Difference D 0.24 D 4.50
Start 10.00 50.45
End 9.76 46.45
Difference D 0.24 D 4.00
0.00 D 0.50
51.70 6.35
59.01 6.35
7.31 0.00
52.66 6.40
58.45 6.40
5.79 0.00
1.52 0.00
Start
End
Desalting ratio
Start
End
Desalting ratio
0.19 0.74 5.47 3.99 0.41 0.69 0.18 0.19 12.42
0.06 0.33 1.59 0.15 0.33 0.41 0.19 0.14 4.73
68.42 55.41 70.93 96.24 19.51 40.58 D 5.56 26.32 61.92
0.22 0.72 5.98 3.65 0.44 1.22 0.21
0.13 0.42 2.01 0.19 0.41 1.00 0.18
40.91 41.67 66.39 94.79 6.82 18.03 14.29
27.51 13.74 4.54 1.45 12.69 22.55 D 19.84
12.78
5.19
59.39
2.53
Note: I: Duple-stage centrifuging after CaCl2 adding; II: Duple-stage centrifuging without CaCl2 adding. Source: Kokubu et al. (1983).
376
Ion Exchange Membranes: Fundamentals and Applications
2nd boiling
B sugar
B molasses
Ca(OH)2
CaCl2
Mixing Water
Steam Heating
1st centrifuging
1st sludge
1st centrifuged molasses Steam
Water 2nd centrifuging 3rd centrifuging 2nd sludge
2nd centrifuged molasses
Separated molasses Final sludge Refrigerator Seawater Discharged water
Electrodialyzer
Water
Desalted molasses (C molasses)
3rd boiling
Figure 1.46
C sugar
Desalination of B molasses (Kokubu et al., 1983).
and A molasses. In the next stage, B sugar and B molasses are obtained from A molasses through the similar process. Finally, C sugar and C molasses are obtained from B molasses. Table 1.20 shows ED experiment of B molasses, which is centrifuged two times after adding CaCl2 (Case I) and without adding CaCl2 (Case II). The experiment indicates that desalting ratio and current efficiency in Case I are increased comparing those in Case II. This is because the minerals such as CaO, SiO2, SO3, P2O5 etc. are removed by the CaCl2 treatment in Case I. Based on the experiment described above, Taito Co. designed the ED treatment process of B molasses as shown in Fig. 1.46. In this process, B
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Table 1.21
Bacterial strains and media for cell count
Strains
Media (Agar)
Staphylococcus aureus Salmonella heidelberg Escherichia coli K-12 Pseudomonas aeruginosa Proteus vulgaris Klebsiella pneumoniae Bacillus subtilis
Mannitol salt Deoxycholate Deoxycholate Deoxycholate Deoxycholate Deoxycholate Nutrient
Source: Sato (1989).
+
C
I
C
A
II
III
FS
−
A
IV
V
FS
Anode
Cathode
P
P
P
AS
BSS
CS
Figure 1.47 Schematic diagram of an electrodialytic disinfection diagram. C, Cation exchange membrane (Selemion CMV); A, anion exchange membrane (Selemion AMV); CS, cathode solution; AS, anode solution; BSS, bacteria cell suspending solution; distance between the membranes, 1 cm; membrane area, 18.4 cm2 (Sato, 1989).
molasses is treated at first by defecation mixing with Ca(OH)2 and CaCl2. After heating and two stages of centrifugation, the second centrifuging molasses is supplied to the electrodialyzer via the refrigerator to obtain desalted molasses (C molasses). C sugar is crystallized in the evaporation of C molasses. By integrating the ED step mentioned above in the sugar manufacturing process, it became possible to obtain D sugar from C molasses.
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Ion Exchange Membranes: Fundamentals and Applications
100 90
Cell viability (%)
80 70 60 50 40 30 20 10 0
0.54
0.81
1.08
1.35
1.65
Current density (A/dm2) : S. aur.,
: Sal. heid,
: K. pneu.,
: B. sub (spore).
: E. coli,
:Ps. aerug.,
:Pr. vulg.,
Figure 1.48 Relation between bacteria cell viability and current density. Flow rate, 3 cm3/min; duration time, 60 min (Sato, 1989).
1.6.13
Electrodialytic Disinfection Sato et al. (1984) investigated water disinfection by means of ion exchange membrane ED. The merits of this method are as follows. (1) (2) (3)
The operation is proceeded at normal temperature. Comparing to chlorine disinfection, the electrodialytic disinfection is more powerful and proceeded during shorter time. The process is not harmful to human body.
Bacterial strains in Table 1.21 were cultivated at 371C for 18 h. The cultivated solution suspending 108 cells cm3 of bacteria cells was supplied into the desalting chamber (chamber III) in an ED system in Fig. 1.47 and electrodialyzed for 60 min. Viability of the cells is plotted against current densities and shown in Fig. 1.48. Here, limiting current density is 0.81 A dm2. Bacteria viability is seen to be decreased with the increase of current density in a range of over limiting current densities and becomes zero at 1.63 A dm2. Electron
379
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E. coli cell Water dissociation
OH
H
+
H+
Anode
Cathode
Anion exchange membrane
Figure 1.49
Desalting chamber
Cation exchange membrane
Mechanism of disinfection in electrodialysis (Sato, 1989).
microscope observation revealed that bacteria cell conformation is shrank under applying over limiting current density. The mechanism of electrodialytic disinfection in this study is estimated as follows. In Fig. 1.49, Escherichia coli cells are suspended in a solution in the desalting chamber (Chamber III). Passing over limiting current in this system, the electrolyte concentration in the desalting chamber is decreased and electric resistance of the solution is increased. In this situation, water dissociation is generated on the anion exchange membrane (cf. Section 8.8.1 in Fundamentals) and H+ ion concentration in the desalting chamber is increased. An E. coli cell is an electron conducting substance, so H+ ions pass through the E. coli cells. This phenomenon is similar to that in electrodeionization (cf. Chapter 4, Fig. 4.8 in Application). E. coli cells are estimated to be destroyed by H+ ions passing through the cells. The investigation described here should be analyzed from the biological effects of alkaline electrodialyzed water at the cellular level (Takahashi, 2006; Kikuno, 2006). REFERENCES Azechi, S., 1980, Electrodialyzer, Bull. Soc. Sea Water Sci., Jpn., 34(2), 77–83. Fukuhara, K., Hamada, M., Azuma, I., 1993, Efficient desalination of brackish water by electrodialysis, Industrial Application of Ion Exchange Membranes, vol. 2. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 159–167. Ideue, K., 1986, Desalination with ion exchange membranes in food industry, Food Dev., 21(7), 54–59.
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Inoue, S., Kuroda, O., 1993, Electrodialysis desalination system powered by photovoltaic power generation, Industrial Application of Ion Exchange Membranes, vol. 2. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 151–157. Ishibashi, T., 1986, Simultaneous treatment of waste water by electrodialysis and reverse osmosis, Industrial Application of Ion Exchange Membranes, vol. 1. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 177–180. Itoi, S., 1983, Electrodialytic demineralization of soy sauce and amino acid seasonings, Food Industry and Membrane Utilization, Saiwai Shobo Inc., Tokyo, pp. 157–162. Itoi, S., Komori, R., Terada, Y., Hazama, Y., 1978, Basis of electrodialyzer design and cost estimation, Ind. Water, No. 239, pp. 29–40. Itoi, S., Nakamura, K., Kawahara, T., 1986, Electrodialytic recovery of waste water from metal surface treatment process, Industrial Application of Ion Exchange Membranes, vol. 1. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 206–211. Itoi, S., Utsunomiya, T., 1965, Electrodialysis of aqueous solution of amino acid containing electrolyte by ion exchange membrane, Asahi Glass Res. Report, 15, 171–178. Katayama, S., 2004, Waste water treatment and recovering of useful substances, In: Seno, M., Tanioka, A., Itoi, S., Yamauchi, A., Yoshida, S. (Eds.), Functions and Applications of Ion Exchange Membranes, Industrial Publishing & Consulting Inc., Tokyo, Japan, pp. 151–169. Kikuno, R., 2006, Microbicidal effect of strong alkaline electrolyzed water, 2005 Alkaline Electrodialyzed Water Symposium, Tokyo, Kitasato University, September 3, 2006. Kokubu, T., Yamauchi, T., Miyagi, S., 1983, Application of electrodialysis in sugar industry, Food Ind., 9, 26–31. Leitz, F. B., 1986, Measurements and control in electrodialysics, Desalination, 381–401, presented at the International Congress on Desalination and Water Re-use, Tokyo (1977). Matsunaga, Y., 1986, Reuse of waste water by electrodialytic treatment, Industrial Application of Ion Exchange Membranes, vol. 1. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 188–196. Mintz, M. S., 1963, Criteria for economic optimization are presented in the form of comparative performance equations for various methods of operation, Ind. Eng. Chem., 55, 19–28. Nagasawa, T., Okonogi, S., Tomita, M., Tamura, Y., Mizota, T., 1973, Demineralization of skim milk by means of electrodialysis with ion permselective membrane, I. Relationship between limiting current density and specific conductivity of demineralizing solution, Jap. J. Zootech. Sci., 44(8), 426–431. Nagasawa, T., Okonogi, S., Tomita, M., Tamura, Y., Mizota, T., 1974, Demineralization of skim milk by means of electrodialysis with ion selective membrane, II. The effects of linear velocity, temperature and total solid content etc. on the limiting current density, Jap. J. Zootech. Sci., 45(11), 578–584. Okada, K., Tomita, M., Tamura, Y., 1975, Electrodialysis in the treatment of dairy products, Symposium ‘‘Separation processes by membranes, ion-exchange and freezeconcentration in food industry’’, Paris, March 13–14. Sato, T., 1989, Investigation on water disinfection applied ion exchange membrane electrodialysis and production of pyrogen-free water, Thesis, Yokohama National University, Yokohama, Japan.
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Sato, T., Tanaka, T., Suzuki, T., 1984, Disinfection of a colon bacillus by means of ion exchange membrane electrodialysis, J. Electrochem. Jpn., 52, 239–243. Shaffer, L. H., Mintz, M. S., 1966, Electrodialysis, In: Spiegler, K. S. (Ed.), Principles of desalination, Academic Press, New York, London, pp. 200–289. Sugiyama, M., Takatori, Y., Touyama, R., Nakamura, A., Yamauchi, T., Kawate, H., Yamaguchi, A., 1982, On the desalination process of sugar solutions by electrodialysis using neutral membranes, J. Sugar Refining Technology, 30, 26–32. Takahashi, R., 2006, Basic analysis of biological effects of potable alkaline electrodialyzed water at the cellular level, 2005 Alkaline Electrodialyzed Water Symposium, Tokyo, Kitasato University, September 3, 2006. Tanaka, Y., 1987, Concentration of seawater using electrodialysis, In: Kawasaki, J., Kunime, T., Sakai, K., Hakuta, T. (Eds.), Membrane Separation Technology Hand Book, Science Forum, Tokyo, pp. 211–215. Tanaka, Y., 1991, Membrane separation, In: Seno, M., Abe, M., Suzuki, T. (Eds.), Ion Exchange, Kodansya Scientific Co., Tokyo, pp. 211–227. Tanaka, Y., 1993, Electrodialysis, In: Sakai, K. (Ed.), Theory and Design of Membrane Separation Process, Industrial Publishing & Consulting Inc., Tokyo, pp. 69–107. Tanaka, Y., Ehara, R., Itoi, S., Goto, T., 2003, Ion exchange membrane electrodialytic salt production using brine discharged from a reverse osmosis seawater desalination plant, J. Membr. Sci., 222, 71–86. Tomita, A., 1995, Electrodialyzer, In: Ogata, N. (Ed.), Engineering in Salt Manufacturing, vol. 2, Electrodialysis, Japan Salt Industry Foundation, Tokyo, pp. 85–101. Tomita, M., Tamura, Y., Mizota, T., 1986, Electrodialysis of milk and whey, Industrial Application of Ion Exchange Membranes, vol. 1. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 147–158. Tsunoda, S., 1994, Present status and latest trends of deep bed filtration, Bull. Soc. Sea Water Sci., Jpn., 48, 27–37. Urabe, S., Doi, K., 1978, Electrodialyzer, Ind. Water, 239, 24–28. Yamamoto, Y., 1993, Desalination of natural essences by electrodialysis, Industrial application of ion exchange membranes, vol. 2. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 181–188. Yawataya, T., 1986, Ion Exchange Membranes for Engineers, Kyoritsu Shuppan Co. Ltd., Tokyo, pp. 94–98.
Electrodialysis 1.1.
OVERVIEW OF TECHNOLOGY
Industrial application of ion exchange membranes started at first in the field of electrodialysis (ED) (cf. Preface) and it induced the development of the fundamental theory. This fact is easily understandable from those phenomena explained in Fundamentals, which are described by taking the ED into account. The development of the fundamental theory led to further development of the ED technology. After that ion exchange membrane technology developed in the succeeding technology such as electrodialysis reversal (EDR), bipolar membrane electrodialysis (BP), electrodeionization (EDI), electrolysis (EL), fuel cell (FC) etc. describing in the succeeding chapters. Looking over these historical details, we notice that the ED becomes the fundamental technology and it is applied to the succeeding technologies based on the ion exchange membranes. In this chapter, we discuss the main subjects such as the structure of electrodialyzer, ED process, practical application of ED etc.
1.2.
ELECTRODIALYZER
1.2.1
Structure of an Electrodialyzer The basic structure of the vertical sheet-flow type module consists of stacks in which cation exchange membranes, anion exchange membranes, gaskets (desalting cells and concentrating cells) are arranged alternately (Fig. 1.1). Fastening frames are put on both outsides of the stack which is fastened up together through cross bars setting in the frames. The deformation of the membranes is prevented by regulating hydrostatic pressure in the fastening frames. Inlet manifold slots and outlet manifold slots are prepared at the bottoms and heads of the gaskets, respectively. Spacers are incorporated with the gaskets to prevent the contact of cation exchange membranes with anion exchange membranes. Many stacks are arranged through the fastening frames. Electrode cells are put on both ends of the electrodialyzer, which are fastened by a press putting on the outsides of electrode cells (Fig. 1.2). An electrolyte solution to be desalinated is supplied from solution feeding frames to entrance manifolds, flows through entrance slots, current passing portions and exit slots, and discharged from exit manifolds to the outside of the stack (Figs. 1.1 and 1.2). A concentrated solution is usually supplied to concentrating cells in a circulating flow system, and discharged to the outside of the stack through an overflow extracting system. DOI: 10.1016/S0927-5193(07)12015-5
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Ion Exchange Membranes: Fundamentals and Applications
k
e
l
g
k
e
h
l
−
+
l
j
f d
a
i
j
b
c
f
Figure 1.1 Structure of a stack (filter-press type). a, Desalting cell; b, concentrating cell; c, manifold; d, slot; e, fastening frame; f, feeding frame; g, cation exchange membrane; h, anion exchange membrane; I, spacer; j, feeding solution; k, desalted solution; l, concentrated solution (Azechi, 1980).
Fastening frame Anode chamber
Feeding frame
Feeding frame
Press (fix)
Press (move)
Stack
Figure 1.2
Cathode chamber
Stack
Filter-press type electrodialyzer (Azechi, 1980).
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323
Effective membrane area is in the range from less than 0.5 m2 to about maximum 2 m2. In order to reduce energy consumption, it is desirable to decrease the electric resistance of the membrane and gasket thickness. Gasket material is selected from synthesized rubber, polyethylene, polypropylene, polyvinyl chloride and ethylene–vinyl acetate copolymer etc. The spacer is usually incorporated with the gasket and a solution flows dispersing along the spacer net. 1.2.2
Parts of an Electrodialyzer The electrodialyzer is composed of the parts as follows (Urabe and Doi, 1978). 1.2.2.1 Fastening Frame Maximum 2000 pairs of membranes are arranged between electrodes in an electrodialyzer, so as to let disassembling and assembling works be easy. The membrane array is divided further into several stacks consisting of 50–400 pairs. Fastening frames are fixed by bolts on both ends of the stack. The fastening frame is usually served as a solution feeding frame, so that a desalting and a concentrating solution are supplied to each gasket cell from the feeding frame incorporated in every stack. Material of the fastening frame is selected from polyvinyl chloride, polypropylene and rubber-lining iron etc. 1.2.2.2 Solution Feeding Frame A solution feeding frame is integrated for feeding solutions to each desalting and concentrating cell. Manifold holes are prepared at corresponding positions of the holes fitted in the gasket. Solutions are usually supplied through the manifolds to each stack, but as the case may be supplied to each plural stack. 1.2.2.3 Gasket The shape of the gasket is presented in Fig. 1.3. A solution is supplied from the inlet manifold put at the bottom, flows through the slot and is fed into the current passing portion. Then the solution is discharged through the outlet slot to the manifold fitted to the head. The gasket has the following functions: (1) prevents solution leakage from the inside to the outside of the electrodialyzer, (2) adjusts the distance between a cation exchange membrane and an anion exchange membrane, (3) prevents solution leakage between a desalting cell and a concentrating cell occurring at slot sections. In order to prevent the solution leakage, it is desirable to adopt a soft material for the gasket. On the other hand, it is desirable to adopt a hard and stable material to avoid dimension changes during long-term operation. The material of the gasket is selected from rubber, ethylene–vinyl acetate copolymer, polyvinyl chloride, polyethylene etc. The thickness of the gasket is in the range of 0.5–2.0 mm.
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Ion Exchange Membranes: Fundamentals and Applications
Manifold
Gasket Spacer
Slot Manifold
Figure 1.3
Gasket (Urabe and Doi, 1978). Deformation of a membrane
Membrane Gasket
Slot
Figure 1.4
Deformation of an ion exchange membrane (Urabe and Doi, 1978).
(a)
Figure 1.5
(b)
(c)
Structure of slots (Urabe and Doi, 1978).
1.2.2.4 Slot It is important to reduce the inside solution leakage (cf. Section 12.2 in Fundamentals), which arises through pinholes and cracks in the membranes or through gaps due to the membrane deformation at the slot as shown in Fig. 1.4. In order to prevent these troubles, a lot of devices are proposed as exemplified in Fig. 1.5 in which (a) decrease the width of the slot, (b) bend the slot, (c) insert the support in the slot.
325
Electrodialysis
(a) Expanded PVC
(c) Diagonal net
Figure 1.6
(b) Wave porous plate
(d) Mikoshiro texture
(e) Honeycomb net
Structure of spacers (Urabe and Doi, 1978).
1.2.2.5 Spacer The function of a spacer is to keep the distance between the membranes. In addition, the spacer increases the limiting current density due to solution disturbance (cf. Sections 10.2 and 10.3 in Fundamentals). The spacer is selected taking account of the requirement such as; (1) low friction head loss, (2) low electric current screening effect, (3) easy air discharge, (4) less blocking of flowpass caused by the precipitation of fine particles suspended in a feeding solution. The structures of a spacer are classified in Fig. 1.6 as (a) expanded polyvinyl chloride, (b) wave porous plate, (c) diagonal net, (d) mikosiro texture and (e) honeycomb net.
1.2.2.6 Electrode and Electrode Chamber Platinum plated titanium, graphite or magnetite is used for anode material and stainless or iron is used for cathode material. The shape of electrodes is classified into net, bar and flat. A partition is inserted between an electrode chamber and a stack for preventing the mixing of solutions. In an anode chamber, oxidizing substances such as chlorine gas evolve. An ion exchange membrane is easily deteriorated by contact with the oxidizing substances, so it is necessary to use two sheets of partitions and put a buffer chamber between the two partitions. Material of the partition is an ion exchange membrane, an asbestos sheet or a battery partition.
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Ion Exchange Membranes: Fundamentals and Applications
An acid solution is added into a cathode solution and the electrodialyzer is operated under controlling pH of the cathode solution for preventing the precipitation of magnesium hydroxides in the cathode chamber. A feeding solution or a concentrated solution is supplied into the electrode chamber. The concentration of oxidizing substances in the anode solution is reduced by adding sodium sulfite or sodium thiosulfate into the solution being discharged. Sometimes, a sodium sulfate solution is supplied to an anode and a cathode chamber, achieving the neutralization by mixing the effluent of both chambers. 1.2.2.7 Press An oil pressure press is usually used adjusting the pressure to be 5–10 kg cm2. 1.2.3
Requirements for Improving the Performance of an Electrodialyzer In order to improve the performance of an electrodialyzer, membrane characteristics should be naturally improved. At the same time, the circumstances in an electrodialyzer in which the membranes work should be better. Here, we describe the definite problems lowering the circumstances in an electrodialyzer and requirements for improving the circumstances and performance of an electrodialyzer (Urabe et al., 1978). 1.2.3.1 Solution Velocity Distribution between Desalting Cells In an electrodialyzer, ion exchange membranes and desalting and concentrating cells are arranged alternately and a solution is supplied into desalting cells. In this flow system, the solution velocity distribution in desalting cells does not become uniform. This phenomenon causes the concentration distribution and current density distribution in the electrodialyzer, and gives rise to the decrease of the limiting current density of the electrodialyzer (cf. Sections 9.1, 11.6 and 11.7 in Fundamentals). In order to operate the elctrodialyzer stably, it becomes necessary to make the solution velocities between the desalting cells uniform. 1.2.3.2 Solution Leakage in an Electrodialyzer The dimensions of all parts of an electrodialyzer are not always consistent with the values in the specifications. Small pinholes can open in an electrodialyzer because the strength of ion exchange membranes is relatively low. Gaps may occur between the materials composing the electrodialyzer in the assembly works of an electrodialyzer. If a pressure difference between the desalting cells and concentrating cells exists in these circumstances, solutions leak through the membranes and lower the performance of the electrodialyzer (cf. Section 12.2 in Fundamentals). In order to avoid these troubles, we have to remove the pinholes and gaps in the electrodialyzer and control the pressure difference between desalting cells and concentrating cells.
Electrodialysis
327
1.2.3.3 Distance between the Membranes Decrease of the distance between the membranes brings about the decrease of electrical resistance and energy consumption. On the other hand, it brings about the increase of friction loss of solution flow, blocking of the materials suspended in a feeding solution and the increase of pumping motive power. Accordingly, it becomes necessary to realize the optimum distance between the membranes. The optimum distance is decided further taking account of electric resistance of ion exchange membranes and that of electrolyte solution in desalting and concentrating cells. 1.2.3.4 Spacer Main functions of a spacer in to create space between a cation exchange membrane and an anion exchange membrane. When solution velocity and the Reynolds number are decreased, hydrodynamic pattern exhibits laminar flow, which means that disturbing effect of the spacer is low. In order to increase the limiting current density, turbulent flow should be induced by increasing the Reynolds number (cf. Sections 10.3.4 and 10.5 in Fundamentals). 1.2.3.5 Electric Current Leakage A part of an electric current flows through slots and manifolds causing ineffective current leakage. Current leakage is increased by the increases of the numbers of cell pairs integrated in a stack and the increase of sectional area of slots and manifolds (cf. Section 12.1 in Fundamentals). These events, however, related with the solution velocity distribution between the cells described in Section 1.2.3.1. 1.2.3.6 Simplicity of Structure of an Electrodialyzer Disassembling and assembling work is peculiar characteristics in operating an electrodialyzer (cf. Section 1.5.3 in Applications). Excellent durability of ion exchange membranes is owing to careful treatment in this work. So, the simplicity of the structure is a requirement for performing this work. 1.3.
ELECTRODIALYSIS PROCESS
The ED process had been explained in detail in several articles (Mintz, 1963; Shaffer and Mintz, 1966; Itoi et al., 1978; Yawataya, 1986; Tanaka, 1993). The following is overall description of the process. 1.3.1
One-Pass Flow Process An electrolyte solution is fed to an electrodialyzer and desalted solution is discharged to the outside of the process (Fig. 1.7). When the concentration of the feeding solution is invariable, the performance of the system becomes stable. Joining the process in Fig. 1.7 to the succeeding process, a one-pass flow
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Ion Exchange Membranes: Fundamentals and Applications
Concentrated discharge
Feeding solution
Concentrating cell Desalted solution Ion-exchange membrane
Desalting cell Feeding solution
Figure 1.7
Pump
One-pass flow process (Tanaka, 1993).
C out b
a
l
C-dC C
x+dx x
C in 0
Figure 1.8
Electrolyte concentration change in a desalting cell (Tanaka, 1993).
multiple continuous system suitable for a large-scale plant is formed. The desalination in Fig. 1.7 is proceeded as below. In the desalting cell shown in Fig. 1.8, a, b and l are the flow-pass depth (distance between membranes), the flow-pass width and the flow-pass length, respectively. Cin and Cout are the electrolyte concentration at the inlet and the outlet of the desalting cell. Passing an electric current across the membranes, ions in the desalting cell are transferred toward the concentrating cell. Assuming the transport of water to be negligible across the membranes under an electric current passing, the material balance between at x and x+dx in Fig. 1.8 is
Electrodialysis
329
indicated by the following equation, including current density i, linear velocity in a desalting cell u, current efficiency Z and electrolyte concentration at x distant from the inlet of a desalting cell C. 1 Z i dC ¼ dx (1.1) C FC Voltage applied to a membrane pair (cell voltage) consists of a membrane potential and Ohmic loss of a cation exchange membrane, an anion exchange membrane, a desalting cell and a concentrating cell. In the desalting process, Ohmic loss of a desalting cell iRde (Rde: electric resistance of desalting cell) is dominant in the cell voltage and voltage difference between electrodes is independent of x (cf. Section 9.1 in Fundamentals). Accordingly, iRde in the desalting cell is estimated to be invariable in the range of x ¼ 0l in Fig. 1.8. Further, Rde is inversely proportional to C, so that i/C is assumed to be nearly constant and we can integrate Eq. (1.1) as follows: Z Z C out 1 Z i l dx ¼ dx (1.2) ua C FC 0 C in au
Solving Eq. (1.2) C out lZ i ¼ exp aFu C C in Desalting ratio a is defined as follows: C out lZ i ¼ 1 exp a¼1 aFu C C in
(1.3)
(1.4)
¯ instead of i/C (Yawataya, 1986) in Eq. Here, we can adopt the average value ¯i=C (1.4) as follows: ¯i i 2¯i ¼ ¼ ¯ C C C in þ C out
(1.5)
From Eqs. (1.4) and (1.5) i 2¯i ¼ C C in ð2 aÞ Substituting Eq. (1.6) into Eq. (1.4) 2Z¯i a ¼ 1 exp aF C in ðu=lÞð2 aÞ
(1.6)
(1.7)
a is calculated using Eq. (1.7), assuming a ¼ 0.1 cm, Cin ¼ 0.05 eq dm3, Z ¼ 0.90, and plotted against u/l taking ¯i as parameter (Fig. 1.9). l vs. u is computed setting a ¼ 0.90 (Fig. 1.10) indicating u is proportional to l. In a practical-scale sheet-flow electrodialyzer, flow-pass length l is from less than
330
Ion Exchange Membranes: Fundamentals and Applications
1.0 0.9
2.5
0.8
2.0
0.7
1.5
0.6 0.5
1.0
0.4 0.3
i =0.5
A / dm 2
0.2 0.1 0.0 0.00
Figure 1.9 ratio.
0.01
0.02
0.03 0.04 u/l(s-1)
0.05
0.06
Influence of linear velocity, flow-pass length and current density to desalting
40 35
u (cm s-1)
30
2.
25
5 0
2.
20 1.5
15 1.0
10
2
A / dm
i =0.5 5 0 0
Figure 1.10
1
2
3
4
5 6 l (m)
7
8
9
10
Flow-pass length and linear velocity in a desalting cell.
331
Electrodialysis
1–2 m, and linear velocities in desalting cells u are 3–10 cm s1. In a tortoise-flow type electrodialyzer, however, l and u are larger than those in the sheet-flow type (cf. Section 2.2 in Application). 1.3.2
Batch Process In Fig. 1.11, the feeding solution is prepared in the circulation tank at first. Next, open valve V1, close valve V2 and circulate the solution between the tank and the electrodialyzer. The solution is electrodialyzed applying constant voltage until electrolyte concentration of a desalted solution attains a definite value, and then the desalted solution is discharged. The process mentioned above is repeated periodically. This system is usually adopted in a small-scale plant and the desalination is proceeded as follows. A definite volume V of electrolyte solution is assumed to be circulated between the circulating tank and the electrodialyzer and it is electrodialyzed applying a constant voltage between the electrodes. An electric current I and electrolyte concentration C of the solution decrease with elapsed time t, but I/C does not change with t, because Ohmic loss IRdil is dominant in a cell voltage and roughly inversely proportional to C. Setting numbers of cell pairs integrated in the electrodialyzer M, the electrolyte concentration in a feeding solution CF and in a desalted product solution CP, and an operating time in an unit batch cycle t, and assuming the transport of water to be negligible across Concentrated discharge
Feeding solution
Feeding solution
V1 Desalted solution V2
Circulation tank
Figure 1.11
Batch process (Tanaka, 1993).
332
Ion Exchange Membranes: Fundamentals and Applications
the membranes under an electric current passing, the material balance in a desalting cell in this batch system (Fig. 1.11) is expressed by the following equation. Z Z CP dC MZ 1 t ¼ V dt (1.8) F C 0 CF C Integrating Eq. (1.8) CP MZ I t ¼ exp FV C CF
(1.9)
Expressing an electric current I as I ¼ bli and the solution volume V as V ¼ QPt, (Qp is product solution volume, water transport across the membranes is assumed to be neglected) in Eq. (1.9), the desalting ratio a in the batch system is introduced as follows: CF MblZ i a¼1 ¼ 1 exp (1.10) FQP C CP Accordingly, numbers of cell pairs M integrated in the electrodialyzer is M¼
FQP lnðC F =C P Þ blZði=CÞ
(1.11)
Here, we estimate M in the following case: QP ¼ 1 m3 h1 ¼ 106/3600 cm3 s1, CF/CP ¼ 10, a ¼ 1CP/CF ¼ 0.90, b ¼ 50 cm, l ¼ 50 cm, Z ¼ 0.90, ¯i ¼ 0:01 A cm2 ; CF ¼ 5 105 eq cm3, F ¼ 96,500 C eq1. From Eq. (1.6), i/C is calculated as: i 2¯i ¼ ¼ 364 C C in ð2 aÞ Substituting these values into Eq. (1.11), we obtain M ¼ 75 pairs. 1.3.3
Partially Circulation (Feed and Bleed) Process An electrodialyzer is operated at constant current density supplying a definite amount of solution and circulating a part of feeding solution (Fig. 1.12). Joining Fig. 1.12 to the succeeding process, a multiple partially circulation system (Fig. 1.13) suitable for a middle-scale plant is formed. The desalination in Fig. 1.13 is achieved as below. Assuming the linear velocity in desalting cells u, current efficiency Z and i/C to be constant in each electrodialyzer, Cout/Cin in each electrodialyzer is expressed using Eq. (1.3) as follows: ðC out Þ1 ðC out Þ2 ðC out Þn C out ¼ ¼ ¼ ¼ ¼b ðC in Þ1 ðC in Þ2 ðC in Þn C in
(1.12)
333
Electrodialysis
Concentrated discharge
Feeding solution
Desalted solution
Feeding solution
Figure 1.12
Partially circulation process (Tanaka, 1993).
QF=Q QR (Cin)1
Figure 1.13
QR-Q
QR-Q
QR-Q
(Cout)1
(Cout)2
(Cout)n
Q
1
QR
Q
2
QR (Cin)n
(Cin)2
QP=Q
n
CP=(Cout)n
QR
QR
QR
(Cout)1
(Cout)2
(Cout)n
Multiple partially circulation process (Tanaka, 1993).
From the material balance in Fig. 1.13 C F QF þ ðC out Þ1 ðQR QÞ ¼ ðC in Þ1 QR ðC out Þ1 Q þ ðC out Þ2 ðQR QÞ ¼ ðC in Þ2 QR .. .
(1.13)
ðC out Þn1 Q þ ðC out Þn ðQR QÞ ¼ ðC in Þn QR Substituting Eq. (1.12) to Eq. (1.13) C F QF ¼ ðC in Þ1 fQR bðQR QÞg ðC in Þ2 fQR bðQR QÞg ðC in Þ1 ¼ bQ .. . ðC in Þn ðC in Þn1 ¼ fQR bðQR QÞg bQ
(1.14)
334
Ion Exchange Membranes: Fundamentals and Applications
Putting Eq. (1.14) together C F QF ¼
ðC in Þn fQR bðQR QÞgn ðbQÞn1
(1.15)
Substituting QF ¼ QP ¼ Q, CP ¼ (Cout)n (water transport across the membranes is neglected) and Eq. (1.12) into Eq. (1.15), and rearranging the equation n CF QR C in ¼ 1 þ1 (1.16) CP QP C out Desalting ratio a of the process is introduced as follows from Eq. (1.16). n CP QR C in a¼1 ¼1 1 þ1 (1.17) CF QP C out In one-path flow system, QR ¼ QP holds, so Eq. (1.17) becomes CF C in n ¼ CP C out
(1.18)
A large-scale plant is realized by assembling a multi-stage multiple partially circulation process arranging in each stage N units of electrodialyzer incorporated with M cell pairs as indicated in Fig. 1.14. The amount of solution QR circulating in each stage is QR ¼ abuMN
(1.19)
N
N
N
2
2
2
1
1
1
QR (Cin)1 QF=Q
QR (Cout)1 (Cin)2 Q
QR (Cout)2 (Cin)N
Q
Q
CF
CP=(Cout)N QR-Q
Figure 1.14
(Cout)N
QR-Q
QR-Q
Multi-stage multiple partially circulation process (Tanaka, 1993).
335
Electrodialysis
Table 1.1 process
Arrangement of electrodialyzers in a multi-stage multiple partially circulation
n
QR (cm3 s1)
1 2 3 4 5 6
514,530 123,617 65,999 44,494 33,438 26,744
N
nN
25.7 6.2 3.3 2.2 1.7 1.3
1 26 26 33 42 52 61
We estimate the numbers and arrangement of electrodialyzer in a multi-stage multiple partially circulation process (Fig. 1.14) putting the following parameters: a ¼ 0.1 cm, b ¼ 100 cm, l ¼ 100 cm, u ¼ 5 cm s1, i/C ¼ 364 A cm eq 1, CF/CP ¼ 10, QP ¼ 200 m3 h1 ¼ 200 106/3600 cm3 s1, Z ¼ 0.9, M ¼ 400 pairs, F ¼ 96,500 Ceq1. At first, Cin/Cout is computed as follows: C in lZ i 100 0:90 364 ¼ exp ¼ exp ¼ 1:9718 aFu C 0:10 96500 5 C out Using Eq. (1.16), CF/CP is n n CF QR C in QR ð1:9718 1Þ þ 1 ¼ 1 þ1 ¼ CP QP C out 200 106 =3600 ¼ 10 Accordingly, QR is expressed as follows: QR ¼ ð101=n 1Þ
200 106 =3600 cm3 s1 0:9718
(1)
From Eq. (1.19), numbers of cell pairs per unit stage are MN ¼ 400N ¼
QR QR ¼ abu 0:1 100 5
So, we have N as follows: N¼
QR 20000
(2)
Changing the values of n, QR, N and nM are computed as indicated in Table 1.1 using Eqs. (1) and (2).
336
Ion Exchange Membranes: Fundamentals and Applications
C′′
Concentrated
q′′
Desalted solution
q 0 C′out q′
C′in q ′ Feeding solution
Figure 1.15
C0 q0
Concentration or separation process.
1.3.4
Concentration Process Fig. 1.15 gives a single-stage concentration unit process. The output of a multi-stage multiple process X is expressed by the following equation. i X¼ blZMnN (1.20) F Here, we calculate numbers of electrodialyzers in the process for concentrating seawater by ED and crystallizing NaCl by evaporation. Putting as NaCl output in the evaporation process: 200,000 t y1, operating time of electrodialyzers: 8000 h y1, NaCl yield rate in the evaporation process: 0.97 and NaCl molecular weight: 58.5, we obtain: X¼
200000 ¼ 122:38 eq=s 8000 3600 0:97 58:5
Substituting X ¼ 122.38 eq s1, i ¼ 0.03 A cm2, b ¼ 100 cm, l ¼ 100 cm, e ¼ 0.92, Z ¼ 0.90, M ¼ 2000 pairs and F ¼ 96,500 C eq1 into Eq. (1.20), nN ¼ 23.8 is obtained. Accordingly, the numbers of the electrodialyzers in this multi-stage multiple processes are known to be 24. Electrolyte concentration in a concentrated solution C 00 is expressed by the following equation introduced from the overall mass transport equation (cf. Section 6.1 in Fundamentals). qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 A2 þ 4rB A C 00 ¼ 2r (1.21) A ¼ fi þ m rC 0 B ¼ li þ mC 0
337
Electrodialysis
6
5
C" (eq dm-3)
4 C' (eq dm-3) 1 3 5 7 9
3
2
1
0
0
1
2
3
4
5
6
7
8
9
10
i (A dm-2)
Figure 1.16
Dependence of C00 on i and C0 .
The overall transport number l, the overall solute permeability m and the overall electro-osmotic permeability f are expressed by the following empirical equations of the overall hydraulic conductivity r (cf. Section 6.1 in Fundamentals). l ¼ l1 þ l2r m ¼ mr
l 1 ¼ 9:208 106
l 2 ¼ 1:914 103
(1)
m ¼ 2:005 104
f ¼ n1 r0:2 þ n2 r
n1 ¼ 3:768 103
(2) n2 ¼ 1:019 102 2
4
(3) 1
1
Here, we calculate l, m and f by substituting r ¼ 1 10 cm eq s into Eqs. (1)–(3) as; l ¼ 9.399 106 eq C1, m ¼ 2.005 106 cm s1, 3 3 1 00 f ¼ 1.398 10 cm C . Dependence of C on i for this membrane pairs is computed as shown in Fig. 1.16 by substituting current density i, electrolyte concentration in a feeding solution C0 , l, m, f and r into Eq. (1.21). 1.3.5
Separation Process An ion exchange membrane shows the permselectivity between ions having the same charged sign, which is defined by the permselectivity coefficient Eq. (1.22) for ion A against ion B, T BA (cf. Section 3.2 in Fundamentals). T BA ¼
ðC 00B =C 00A Þ ðC 0B =C 0A Þ
(1.22)
338
Ion Exchange Membranes: Fundamentals and Applications
C 00i ; is concentration (eq dm3) of ion i in a concentrating cell and it is assumed to be invariable in the cell. Concentration of ion i in a desalting cell C 0i is the average of the values at the inlet and the outlet as follow: 1 C 0i ¼ ðC 0i;in þ C 0i;out Þ 2
(1.23)
C 0i;in and C 0i;out are concentrations of ion i at the inlet and outlet of a desalting cell. Ion A is separated from ion B by applying the permselectability of the membrane in the separation process indicated in Fig. 1.15. Here, we define the separation factor of ion B against ion A, in a desalted 0 00 solution S AB and in a concentrated solution S AB by the following equations. 0
S AB ¼
00
S AB ¼
ðC 0B;out =C 0A;out Þ ðC 0B =C 0A Þ ðC 00B =C 00A Þ ðC 0B =C 0A Þ
(1.24)
(1.25)
C 0A ; C 0B : Concentration of ion A and ion B in a feeding solution. Desalting ratio of ion i (ai) in Fig. 1.15 is defined as: C i;out ¼ ð1 ai ÞC 0i
(1.26)
The material balance of ion i in Fig. 1.15 is shown by the following equations assuming q0 q00 hold. C 0i q0 ¼ C 0i;out q0 þ C 00i q00
(1.27)
C 0i;in q0 ¼ C 0i;out q0 þ C 00i q00
(1.28)
q0 is the amount of feeding solution to the process, q0 the amount of a desalting solution being supplied to the electrodialyzer and q00 the amount of a concentrating solution flowing out from the electrodialyzer. Following equations are introduced from Eqs. (1.22)–(1.28): 00
S AB ¼
00
1 aB ð1 ðq0 =2q0 ÞÞ 1 aA ð1 ðq0 =2q0 ÞÞ 0
S AB ¼ SAB T BA ¼
1 aB ð1 ðq0 =2q0 ÞÞ B T 1 aA ð1 ðq0 =2q0 ÞÞ A
(1.29)
(1.30)
When q0 q0 hold, Eqs. (1.29) and (1.30) become 0
S AB ¼
1 1 aA ð1 T BA Þ
(1.31)
339
Electrodialysis
00
0
S AB ¼ SAB T BA ¼
T BA 1 aA ð1 T BA Þ
(1.32)
In Eqs. (1.31) and (1.32), the following phenomena are found: when T BA 41 when T BA ¼ 1 T BA o1
when
0
00
SAB o1
S AB oT BA
0
00
SAB ¼ 1 0
S AB ¼ T BA
(1.33)
00
S AB 41
S AB 4T BA
Equation (1.33) means that the separatability is inferior to the permselectability. Further, we find the following events in Eqs. (1.31) and (1.32): 0
lim S AB ¼ 1
aA !0
00
lim S AB ¼ T BA
aA !0 0
0
lim SAB ¼
aA !1
1 T BA
(1.34)
00
lim SAB ¼ 1
(1.35)
aA !1
00
aA vs. SAB and SAB is computed using Eqs. (1.31) and (1.32) and shown in Fig. 1.17, in which the relationships in Eqs. (1.33)–(1.35) are confirmed. 102
101 10
B
TA
S ′AB S′′AB
4
.01 =0
0.4
1
10
0.4 4 10
10-1
0. 01 10-2 0.0
0.2
0.4
A
0.6
0.8
1.0
Figure 1.17 Relationship between permselectivity coefficient, desalting ratio and sep0 00 aration factor. Open, SAB ; filled, S AB :
340
1.4.
Ion Exchange Membranes: Fundamentals and Applications
ENERGY CONSUMPTION AND OPTIMUM CURRENT DENSITY
In multi-stage multiple partially circulation system (Fig. 1.14), rectifiers are assumed to be put in each stage, in which an electric current I is supplied to electrodialyzers through a parallel circuit. I is expressed as follows: I (1.36) ZMN ¼ QRfðC in Þn ðC out Þn g F Voltage V applied to an electrodialyzer is indicated by the following equation putting the voltage in the electrode cell as VP. V ¼ MV cell þ V P
(1.37)
Vcell is cell voltage as follows (cf. Section 13.2 in Fundamentals): V cell ¼ IðRK þ RA þ Rdil þ Rconc Þ þ E M
(1.38)
RK and RA are electric resistance of a cation and an anion exchange membrane. Rdil and Rconc are electric resistance of a desalting and a concentrating cell. RM is membrane potential. A reasonable estimate of the optimum current density is introduced by assuming that the major costs are divided into three categories: those directly proportional to current density, those inversely proportional to current density, and those independent of current density (Eq. (1.39)). b (1.39) Z ¼ ai þ þ c i where Z is total cost, and a, b, c are the relative proportionality constants. The optimum current density iopt is defined by the minimum of Eq. (1.39), so it is introduced by differentiating Eq. (8.37); dZ/di ¼ 0 and expressed as (Leitz, 1986): 1=2 P (1.40) iopt ¼ QR P is depreciation cost ($/m2s), Q the energy cost ($/Ws) and R the cell pair electric resistance (O m2). 1.5. 1.5.1
SURROUNDING TECHNOLOGY
Filtration of a Feeding Solution Fine materials such as sand, clay, iron components, humus soil and miscellaneous inorganic and organic colloid are usually suspended in a raw feeding solution. In order to avoid invasion of these materials into an electrodialyzer, a feeding solution is filtrated using sand, fibers or cohesive agent and turbidity of the solution is decreased less than 0.1–0.2 ppm. First of all, the following valveless sand filter is broadly applicable (Tsunoda, 1994). In Fig. 1.18, a raw feeding
341
Electrodialysis
H3 10 9
H2
6
8
5 H1
7 11
2
1
3 4
Figure 1.18 Valve-less filter. 1, Feeding solution inlet; 2, filtrating chamber; 3, sand filter; 4, collecting chamber; 5, filtrate flow out pipe; 6, filtrate outlet; 7, connecting duct; 8, washing chamber; 9, siphon pipe; 10, siphon breaker; 11, control valve (Tsunoda, 1994).
solution is supplied into the filtrating chamber, filtrated through the sand filter and collected in the collecting chamber. The filtrate flows out through the flowout pipe and supplied to an electrodialyzer at solution level H2. At the same time, a part of filtrate flows through the connecting pipe into the washing chamber put on the filtrating chamber and is accumulated there at H2. Proceeding with the operation, solution level in the siphon pipe goes up with the increase of flow resistance in the sand filter. When the level surpasses H3, the solutions in the filtrating chamber, the collecting chamber and the washing tank are discharged to the outside of the process at one stroke through the siphon pipe due to the siphon function. In this instance, sand is washed and the level in the washing tank goes down to H1.
1.5.2 Scale Trouble Prevention 1.5.2.1 Acid Dosage Total carbonic acid dissolved in brine is equilibrated to CO2 gas (3 104 atm.) in air. Total carbonic acid concentration in seawater is 2–5 102 mol dm3 at pH ¼ 7.0 –7.4. 15–30% of this total value is carbonic acid molecules (CO2 and H2CO3), and the remainders are ionic carbonic acid 2 consisting of HCO 3 (more than 95%) and CO3 (less than 5%). In a concen trating cell in an electrodialyzer HCO3 ions decompose as follows and combine
342
Ion Exchange Membranes: Fundamentals and Applications
with Ca2+ ions to form CaCO3 precipitation. 2 2HCO 3 3CO3 þ H2 O þ CO2 "
(1)
2þ CO2 ! CaCO3 (2) 3 þ Ca In order to avoid CaCO3 precipitation, HCO3 ions are decomposed into CO2 gas by dosing with an HCl or a H2SO4 solution into concentrating cells. þ (3) HCO 3 þ H ! CO2 " þH2 O
1.5.2.2 Precipitation Controlling Agent Dosage Small amount of precipitation controlling agents such as condensed sodium phosphate Na2[Na4(PO3)6] are dosed into a concentrating cell, resulting with the absorption of crystalline nuclei to the agents and dissolution of CaSO4 or CaCO3 due to the following chelate reaction. Na2 ½Na4 ðPO3 Þ6 þ CaX ! Na2 ½Na2 CaðPO3 Þ6 þ Na2 X CaX : CaSO4 or CaCO3
(4)
Carboxyl methyl cellulose (CMC) or poly-acrylic acid is also available instead of condensed sodium phosphate. 1.5.3
Disassembling and Assembling Works In spite of the filtration described in Section 1.5.1, a very small quantity of fine particles passes through a filter and invades into an electrodialyzer. Sometimes, fine organisms pass through the filter and breed in an electrodialyzer. The fine particles are adhered on the surface of membranes and spacers in desalting cells (cf. Section 14.2.2 in Fundamentals), causing the increase of flow resistance of a solution in the desalting cell with acceleration of concentration polarization on the membrane surface (cf. Section 14.2.1 in Fundamentals). In order to avoid these troubles, an electrodialyzer is usually disassembled and washed periodically. The disassembling and washing process is generally as follows (Tanaka, 1987): (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
Electric current interruption Solution feeding interruption Solution discharge Stack extraction Stack disassembling Membrane surface washing Desalting cell, concentrating cell and spacer washing Stack assembling Leak test Integrating stack into an electrodialyzer Solution feeding Electric current passing.
343
Electrodialysis
1.6.
PRACTICE
1.6.1
Potable Water Production from Brackish Water Residents in an isolated island suffer from serious water shortage because of their high dependence on rainwater. In order to solve this problem, Asahi Chemical Co. constructed the ED plant (2500 m3/day) illustrated in Fig. 1.19 in 1990 in Ohshima island, Tokyo (Fukuhara et al., 1993). Ohshima Town supplies potable water to the residents even now. The specifications of this plant are shown in Table 1.2. Raw brackish water pumped up from wells is fed to the suspended solid filter (SSF), and then it is supplied to the first stage electrodialyzer unit (EDUA1 and EDU-B1) through the first stage desalination tank (DST-1). A part of the filtrated solution is supplied to the concentrated solution tank (CST). The desalted solution in DST-1 is further desalted at the second stage electrodialyzer unit (EDU-A2 and EDU-B2) to be about 400 ppm, and then supplied to the water cleaning tank (WCT).
ACS-A FS-A CS-A ACS-B FS-B CS-B
DS-A 1
+ EDU-A 1 − + EDU-A 2 −
P
P
P
P
P
P
DS-B 1 CCS-A DS-A 2 DS-B 2 CCS-B
+ EDU-B 1 − + EDU-B 2 −
P
P
P
P
P
P Wells
ST
SSF
CST
AT
EST
DST-1
DST-2 PWT
WCT
DR
Figure 1.19 Saline water desalting process (Ohshima island). ACS, Anode compartment solution; AT, acid tank; CCS, cathode compartment solution; CS, concentrated solution; CST, concentrated solution tank; DR, distributing reservoir; DS, desalinated solution; DST, desalination tank; EDU, electrodialyzer unit; EST, electrode solution tank; FS, frame solution; PWT, product water tank; SSF, suspended solid filter; ST, stock tank; WCT, water cleaning tank (Fukuhara et al., 1993).
344 Table 1.2
Ion Exchange Membranes: Fundamentals and Applications
Specifications of the electrodialysis plant
Site
Ohshima, Tokyo
Capacity, product water
2500 m3/day, from 1200 ppm TDS raw water 1000 m3/day, from 3000 ppm TDS raw water 4 brackish wells TDS: 450 ppm or less Chloride ions: 150 ppm or less Asahi Chemical SS-0 1780 m3 (318 pairs/stack, 4 stack) Dilution compartment: 0.5 mm Concentration compartment: 0.5 mm Dual-train, two steps per line automatic, continuous
Raw water source Product water Electrodialyzers Conductive area Thickness Mode of operation Source: Fukuhara et al. (1993).
Table 1.3
Operational results
Production rate Product water Electric consumption
2610 m3/day, from 1200 ppm TDS raw water TDS: 440 ppm Chloride ions: 145 ppm pH: 6.8 0.8 kWh m3
Source: Fukuhara et al. (1993).
Organic fouling due to organic acid including in raw brackish water is avoided by integrating anti-fouling anion exchange membranes Aciplex A-201 into the electrodialyzer instead of standard type Aciplex A-101 membranes (cf. Section 14.3.2 in Fundamentals). Operational results are shown in Table 1.3. Analysis of raw and product water are shown in Table 1.4. The values listed in both tables meet the potable water standard established by the Ministry of Welfare, Japan. Actual operating cost is determined based on the plant operation in a year as indicated in Table 1.5. 1.6.2 Electrodialysis Desalination System Powered by Photo-Voltaic Power Generation Babcock-Hitachi K.K. and Hitachi Ltd. established ED system combined with sunlight photo-voltaic power generation in Oshima island and at Sakiyama, Nagasaki (Inoue and Kuroda, 1993). In this system, the electrodialyzer was operated using direct current power generator using solar batteries and produced potable water. Fig. 1.20 gives the system in Oshima island, showing a two-stage seawater desalting process for producing 103/day of potable water. In the first stage, if it is fine, seawater (35,000 mg l1-TDS) is desalted during the daytime to obtain an
345
Electrodialysis
Table 1.4
Analysis of raw and product water Raw Watera
Product Water
Visual Inspection
Colorless, Transparent
Colorless, Transparent
Turbidity (1) Color (1) pH Electrical conductivity (mS cm1) Total hardness (CaCO3, ppm) Calcium hardness (CaCO3, ppm) Evaporation residue (ppm) Silica (SiO2, ppm) Chloride ion (Cl, ppm) Sulfuric ion (SO4, ppm) Hydrocarbonate ion (HCO3, ppm) Calcium ion (Ca, ppm) Potassium ion (K, ppm) Magnesium ion (Mg, ppm) Sodium ion (Na, ppm)
o1.0 o1.0 7.2 2500 685 406 1631 43 632 262 210 51 17 48 368
o1.0 o1.0 7.3 650 83 33 418 42 131 40 82 14 3 12 94
Source: Fukuhara et al. (1993). a From single well.
Table 1.5
Operating cost Units
Volume of product water Raw water TDS Water recovery Electric consumption Stering agents Membrane replacement Total
m3/day mg l1 % yen m3 yen m3 yen m3 yen m3
Projected
Actual
1950 1600 80.0 27.82 0.38 11.80 40.00
1853 1250 86.5 10.04 3.73 13.77
Source: Fukuhara et al. (1993).
intermediately desalted solution (5000 mg l1-TDS) and at the same time, electric power surpluses are stored in batteries (STB). In the second stage, the intermediately desalted solution is further desalted to obtain potable water (400 mg l1-TDS) in the rainy or cloudy daytime or in the nighttime using the stored electric power. Fig. 1.21 shows the system in Sakiyama for producing 200 m3/day of potable water from brackish water. In this system, electric consumption for pumping up raw brackish water is larger. So, in the fine daytime, raw brackish water (1500 mg l1-TDS) is pumped up and is desalted to obtain potable water (400 mg l1-TDS). At the same time, brackish water and electricity are stored in
346
Ion Exchange Membranes: Fundamentals and Applications
SOB t
CBO
STB
DAT FIL −
DST
+
IST
FST SWP Sea
DSP
ED
CSPCST
PWT Brine
Product water
Sea
Figure 1.20 Seawater desalination process powered by solar generation (Oshima island). CBO, Control board; CSP, concentrated solution pump; CST, concentrated solution tank; DAT, direct/altering current transducer; DSP, desalted solution pump; DST, desalted solution tank; ED, electrodialyzer; FIL, filter; FST, filtrated solution tank; IST, intermediately concentrated solution tank; PWT, product water tank; SOB, solar battery; STB, storage battery; SWP, seawater pump (Inoue and Kuroda, 1993).
the fine daytime for operating the electrodialyzer in the rainy or cloudy daytime and in the nighttime. The specifications of both systems are shown in Table 1.6. An electric power consumption pattern of the Oshima plant is illustrated in Fig. 1.22. In a batch desalting system, electric power consumption is large at beginning and it decreases with elapsed time. So, when a large amount of electric power is generated in the fine daytime, seawater is electrodialyzed to obtain intermediately desalted solution, which is temporarily stored in a tank (highconcentration operation). At the same time, electric power surpluses are stored in STB. In the cloudy or rainy daytime and in the nighttime, the stored solution is further desalted to obtain potable water (low-concentration operation) passing the stored electric power. Fig. 1.23 shows an electric power consumption pattern of the Sakiyama plant, which is automatically controlled with a sunlight photo-voltaic power generation system, a storage battery system and an ED system. An electric power generated in the solar system is directly consumed in the electrodialyzer, and the electric power surpluses charge the battery. Operating performance of both plants are indicated in Table 1.7.
347
Electrodialysis
SOB t CBO
STB
Alternating current ACL
DAT
FST to CST
DCL
Direct current
ECM
FIL
from FST −
PWT
+
DST CST
ED WEL SWP Saline water
DSP
Product water
CSP Sea
Figure 1.21 Saline water desalination process powered by solar generation (Sakiyama). ACL, Altering current load; CBO, control board; CSP, concentrated solution pump; CST, concentrated solution tank; DAT, direct/altering current transducer; DCL, direct current load; DSP, desalted solution pump; DST, desalted solution tank; ECM, electroconductivity meter; ED, electrodialyzer; FIL, filter; FST, filtrated solution tank; PWT, product water tank; SOB, solar battery; STB, storage battery; SWP, seawater pump; WEL, well (Inoue and Kuroda, 1993).
1.6.3 Electrodialytic Recovery of Wastewater from a Metal Surface Treatment Process Electrodialysis is applicable to recovering wastewater and nickel in a metal surface treatment process. The process was developed by Asahi Glass Co. as follows (Itoi et al., 1986). A nickel plating process includes several rinsing processes. In this process, Ni concentration in the effluent from the first rinsing stage is high. So that the effluent is usually returned to the electro-plating bath and the effluent from the final rinsing stage is discharged. The ED process is designed to collect Ni ions in the first rinsing stage and return them to the electro-plating bath for the purpose of increasing recovery ratio of Ni and decreasing Ni content in the waste from the final rinsing stage. Fig. 1.24 is a continuous process operating in the car components manufacturing factory. The process includes two stages of rinsing baths and one unit
348
Ion Exchange Membranes: Fundamentals and Applications
Table 1.6
Specifications of the desalting process
Solar generator Type Capacity Module number Storage battery Type Capacity Voltage Electrodialyzer Capacity Raw water Type Gasket dimension Effective membrane area Distance between membranes Number of cell pair System control
Oshima
Sakiyama
Silicon single crystal 25 kW 64 W 390 module
Silicon single crystal 65 kW 47 W 1380 module
Lead storage battery 115 kWh 96 V
Lead storage battery 230 kWh 192 V
10 m3/day Seawater (35,000 mg l1-TDS) Filter-press 185 2000 mm 0.238 m2
200 m3/day Saline water (1500 mg l1TDS) Filter-press 370 2000 mm 0.476 m2
0.8 mm
0.8 mm
250 3 modes operation 1. High-concentration operation 2. Low-concentration operation 3. Stand by operation
600 4 modes operation 1. Pump up+desalting operation 2. Pump up operation 3. Desalting operation 4. Stand by operation
Source: Inoue and Kuroda (1993). Electrodialysis
Solar battery output
Electric current (A)
Intermediately concentrated solution production
Potable water production
Auxiliary machine 6
8
10
12
14
16 Time
18
20
22
24
Figure 1.22 Electric power consumption pattern of the seawater desalination plant (Oshima island) (Inoue and Kuroda, 1993).
349
Electrodialysis
Solar battery output
Electric current (A)
Pump up current
Electrodialysis current Auxiliary machine current
Desalination + Pump up
Desalination
6
8
10
Stand by
12 14 Time
16
18
20
Figure 1.23 Electric power consumption pattern of saline water desalination plant (Sakiyama) (Inoue and Kuroda, 1993).
Table 1.7
Operating performance of the electrodialyzer Oshima
2
Sunlight quantity (kWh m ) Generation quantity (kWh/day) Water production (m3/day) Electric power consumption (kWh m3)
Sakiyama
1986
1987
1988
1900
1901
3.60 49.3 3.45 14.3
3.73 50.6 3.44 14.7
3.86 57.0 4.04 14.1
4.08 162 229 0.71
3.60 145 194 0.75
Source: Inoue and Kuroda (1993).
of electrodialyzer, which is designed to maintain the Ni concentration in the first stage rinsing bath to about 5 g l1 when the concentration in the Ni plating bath is about 84 g l1. The specifications of the electrodialyzer are presented in Table 1.8. The limiting current density for NiSO4 or NiCl2 is extremely low comparing to that for Na2SO4 or NaCl. So, it is important to control the current density to prevent the precipitation of Ni(OH)2 caused by water dissociation. Further, the organic substances in the solution added into the electro-plating bath possibly give rise to generate the organic fouling of anion exchange
350
Ion Exchange Membranes: Fundamentals and Applications
Chemical dosing
* EPB
1st RIT
2nd RIT CXC
AXC
PRT +
−
ED
*
FIL 1st RIW
DIL
CON
ELR
2nd RIW
DEW
Figure 1.24 Electrodialysis Ni2+ electro-plating wastewater recovery process. AXC, Anion exchange column; CXC, cation exchange column; CON, concentrate; DEW, demineralyzed water; DIL, diluate; ED, electrodialyzer; ELR, electrode rinse; EPB, electro plating bath; FIL, filter; PRT, pretreatment; RIT, rinse tank; RIW, rinse waste (Itoi et al., 1986). Table 1.8
Specifications of the electrodialysis plant
Electrodialyzer Ion exchange membrane Size of membrane Effective membrane area Number of membrane pairs Distance between membranes Flow velocity Current density Maximum voltage
Model DU-111 Selemion CMV/AMV 0.49 0.98 m 0.336 m2 40 pairs 2.0 mm 3.0 cm s1 1.0 A dm2 50 V
Source: Itoi et al. (1986).
membranes. So, it is necessary to remove the organic substances by means of a special pretreatment. Material balance of Ni2+ ions and water in the process is indicated in Fig. 1.25 showing that the recovery ratio is larger than 90% and a diluted solution is perfectly recycled to the first stage rinsing bath. Current efficiency and electric power of ED were, respectively, greater than 90% and 2 kWh kg1 Ni ion. Cost estimation is presented in Table 1.9.
351
Electrodialysis
Take out from plating bath 13 l/h (Ni2+ 84.2 g/l)
1 st stage Rinsing tank
Take out to 2nd stage 13 l/h (Ni2+ 5g/l)
Feed to ED 3 m3/h (Ni2+ 5g/l) Concentrated stream To plating bath 12.4 l/h (Ni2+ 83 g/l)
Figure 1.25
Electrodialyzer
Diluate stream 3 m3/h Ni2+ 4.65 g/l
Ni2+ and water balance in the electrodialysis process (Itoi et al., 1986).
Table 1.9 Cost estimation of electrodialytic recovery of rinsing waste in the nickel electroplating process Nickel recovery rate (as NiSO4 6H2O) Electricity consumption (as NiSO4 6H2O)
3460 kg month1 0.7 kWh kg1
Equipment installation cost Electricity unit cost Purchasing price of nickel salt (as NiSO4 6H2O)
15 million yen1 10 yen kWh1 360 yen kg1
Profit of Nickel salt recovery 360 yen kg1 3460 kg month1 ¼ Runnig cost Electricity 10 yen kWh1 0.7 kWh kg1 3460 kg month1 ¼ Maintenance and consumable item (annually 3%) 15,000,000 0.03/12 month ¼ Amortization (7 years) 15,000,000 yen (7 year 12 month)1 ¼ Interest (annually 9%) 15,000,000 yen 0.09/12 month ¼ Running cost total Profit
1,245,600 yen month1 24,220 yen month1 37,500 yen month1 178,570 yen month1 112,500 yen month1 352,790 yen month1 892,810 yen month1
Source: Itoi et al. (1986).
1.6.4
Reuse of Wastewater by Electrodialytic Treatment Ion exchange membrane ED technology is applicable to wastewater treatment for realizing resource saving and protection of environment. Tokuyama Inc. developed the technology to establish a closed system by means of
352
Ion Exchange Membranes: Fundamentals and Applications
Process
HCl, H2SO4
Acid waste
NaOH
Neutralization
Flocculant
Clarifier
Filter
Electrodialysis
Desalted solution
Concentrated solution
Traider receiving Designed process is surrounded by the dotted line.
Figure 1.26
Electrodialytic treatment process of industrial waste (Matsunaga, 1986).
Solution Circulating solution Concentrated solution
Na Ca 0.255N 13 ppm 3.870 220
Cl SO4 0.168 N 0.087 N 3.329 0.541
to process
ED
from clarifier
SAF MIT
DST
CHF
HCl
1 CST
2
Concentrated solution 65.4 l /h
Figure 1.27 Electrodialytic treatment process of industrial waste. CHF, Check filter; CST, concentrated solution tank; DST, desalted solution tank; ED, electrodialyzer; MIT, middle tank; SAF, sand filter (Matsunaga, 1986).
Electrodialysis
353
electrodialytic reuse of wastewater in a plating process as shown in Figs. 1.26 and 1.27 (Matsunaga, 1986).
Typical components of a raw feeding solution in this system. pH: 9.5, NaCl: 17.6 g l1, Na2SO4: 7.1 g l1, Ca: 48 mg l1, SS: 20 mg l1. Main component: Fe(OH)3. The others: Fe, Cu, Zn, P etc. Conditions of electrodialytic treatment. Quantity of a desalted solution: 10 ton/month. Degree of desalination: maximum. Degree of concentration: maximum. Temperature: normal. Operating time: 24 h/day, 30 days/month, 12 months/year. Operating system: partially circulation. Ion exchange membrane: Neocepta C5S-8 T (monovalent cation selectively permeable cation exchange membrane) and Neocepta ACH-45 T (anion exchange membrane). Operating results. Current density: 3.0 A dm2. Cell voltage: 0.5 V/pair (18–20 1C). Electrolyte concentration in a desalted solution: Cde,Na ¼ 0.25–0.32 eq dm3, Cde, Ca ¼ 13–15 mg l1. Electrolyte concentration in a concentrated solution Ccon,Na ¼ 3.80– 4.02 eq dm3, Ccon,Ca ¼ 220–240 mg l1. Quantity of a concentrated solution: 60–66 l h1 (1.44–1.58 m3/day). Quantity of a desalted solution 10.9– 12.1 ton/month. Running cost. Electric power: 262 kWh/day 15 yen/kWh ¼ 3930 yen/day Labor: 0.2 people/day 15,000 yen/people ¼ 3,000 yen/day The others (Ion exchange membrane, HCl, electrode, filter): 1330 yen/day Total: 8260 yen/day.
1.6.5 Simultaneous Treatment of Wastewater by Electrodialysis and Reverse Osmosis Yunichica Ltd. developed a treatment process of wastewater including toxic metallic ions discharged from a semi-conducting material manufacturing process (Ishibashi, 1986). The process is illustrated in Fig. 1.28 consisting from the following system. 1.6.5.1 High-Concentration System HS gas dissolving in the wastewater is deaerated and neutralized adding NaOH. Fine particles suspending in the wastewater are removed using a microosmosis filter. Then, a clear salt solution is supplied to an ion exchange membrane ED unit and concentrated to 3.5–4.0 M. The concentrated solution is evaporated using a vacuum evaporator (EV) to obtain Na2SO4 and NaCl crystals, which are reused after re-purification. The desalted solution obtained from the ED unit is further desalted using a reverse osmosis (RO) unit. Concentrated
354
Ion Exchange Membranes: Fundamentals and Applications
Concentrated solution
High concentration system
Low concentration system
TDS 36g/l
TDS 240g/l
Separated salt 570 kg/day
EV
TDS 1.5g/l
ED
RO TDS 0.1g/l
Polishing
IX
Pure water Heigh pure water
Process ED: Electrodialysis RO:Reverse osmosis IX: Ion exchange EV:Evaporation
Figure 1.28 Simultaneous treatment process of wastewater by electrodialysis and reverse osmosis (Ishibashi, 1986).
solution from the RO unit is returned to the ED unit. The specifications of both units are as follows:
Electrodialysis unit Tokuyama TS-25-160 type integrated with Neocepta C66-5T/AFS Batch wise operating system Desalting performance 18 m3/day Concentration of a desalted solution 1.5 g l1 Reverse osmosis unit Middle pressure hollow fiber B-9 Desalting performance 15 m3/day Concentration of a desalted solution 100 ms
1.6.5.2 Low-Concentration System After deaeration, the solution is filtered passing through a carbon filter and neutralized using weak basic ion exchange resins. Then the solution is supplied to a cation exchange and an anion exchange column to obtain pure water. The pure water is fed to the semi-conducting material manufacturing process
355
Electrodialysis
Anion exchange layer
Mn+
+
H+ Anode
Cation exchange membrane
+ + + + + +
− − − − − −
− − − − −
− H+ Cathode
Figure 1.29 H+ ion permeable cation exchange membrane. Mn+, n valent metalic ion (Katayama, 2004).
directly or via a polishing column filled up with cation and anion exchange resins and nonionic resins. 1.6.6
Electrodialytic Recovery of Acid Acid is recovered from a waste acid applying a H+ ion permselective cation exchange membrane placed an anion exchange layer on a cation exchange membrane as illustrated in Fig. 1.29. Here, multivalent cations Mn+ do not pass through the membrane due to repulsive effects between the cations and the anion exchange layer. The concept mentioned above is applicable to recover an acid from an aqueous solution dissolving for instance Fe(NO3)3 with HNO3 as shown in Fig. 1.30. In this system, cation exchange membranes correspond to the membrane illustrated in Fig. 1.29, and they permeate H+ ions selectively and do not permeate Fe3+ ions. On the other hand, anion exchange membrane permeate NO ions selectively rather than H+ ions. Consequently, HNO3 is recovered in the concentrating chamber and Fe(NO3)3 is remained in the desalting chamber. Table 1.10 shows the material balance in an electrodialyzer (effective membrane area: 90 m2) developed by Tokuyama Inc., which treated a wasted acid solution (0.54 M acid solution including H2SO4 and HCl with 0.30 M of Al3+ ions) generated in an etching process of aluminum products (Katayama, 2004). Acid concentration in a recovered acid is 1.90 M including 0.01 M of Al3+ ions. Acid recovering ratio and leak ratio are, respectively, 88% and 0.6%. Energy consumption and current efficiency are, respectively, 12 kW and 55%. 1.6.7
Seawater Concentration for Salt Production Electrodialysis is applied to concentrating seawater for producing salt in Japan. The seawater concentrating process is illustrated in Fig. 1.31 (Tomita, 1995). Seawater pumped up from sea is filtered and supplied to the electrodialyzer via the filtered solution tank. Concentrated seawater is circulated
356
Ion Exchange Membranes: Fundamentals and Applications
Recovered acid HNO3 De-acidified solution Fe(NO3)3 C
A NO3−
+
H+
H+
C NO3−
A NO3−
H+
C NO3−
H+
−
H+
Fe3+
Fe3+ Anode
Cathode Water Waste acid HNO3 Fe(NO3)3
Figure 1.30 Acid recovery by means of ion exchange membrane electrodialysis. C, H+ ion permselective cation exchange membrane; A, H+ ion low-permselective anion exchange membrane (Katayama, 2004).
Table 1.10
Material balance in an electrodialyzer for acid recovering
Solution
Waste acid (before electrodialysis) Deacid solution (after electrodialysis) Water Recovered acid
Solution Quantity (l h1)
Composition H+ (M)
Al3+ (M)
SO2 4 (M)
Cl (M)
742
0.54
0.30
0.65
0.16
671
0.07
0.33
0.50
0.08
115 186
0 1.90
0 0.01
0 0.79
0 0.35
Source: Katayama (2004).
between the concentrated seawater tank and the electrodialyzer, and its gain is supplied to an evaporating process to obtain salt crystals. CaCO3 scale precipitation in concentrating cells is prevented by adding hydrochloric acid to the 2 concentrated seawater to decompose HCO 3 and CO3 ions (cf. Section 1.5.2 in Applications). A part of filtrated seawater is supplied to anode chambers. Titanium is adopted as the anode material. In order to avoid membrane destruction due to Cl2 and HClO generated by an anode reaction, a perfluorinated ion exchange membrane is integrated between the cathode chamber and the
357
Electrodialysis
C
9 10
+
−
7
6
8
2
3
4
5
1
Figure 1.31 Electrodialytic seawater concentration process. 1, Diluted seawater tank; 2, cathode solution tank; 3, concentrated seawater tank; 4, washing solution tank; 5, HCl tank; 6, filtrated seawater; 7, concentrated seawater output; 8, HCl; 9, electrodialyzer; 10, anode solution (Tomita, 1995).
adjacent stack. The wasted solution from the anode chamber is mixed with the filtrated seawater to suppress the growth of microorganisms in seawater. Cathode material is plated with Pt. An HCl solution is supplied to the cathode chamber to neutralize OH ions generated by the cathode reaction. A washing system is provided for washing the inside of desalting cells by acid or chemical reagents and dissolving adhered substances (cf. Section 14.2.3 in Fundamentals). When the turbidity of raw seawater is 2 ppm, it is decreased to about 0.05 ppm by filtering through two-stage sand filters (cf. Section 1.5.1 in Applications). In spite of such filtration, fine particles pass through the filter and invade into the electrodialyzer and precipitate on the membrane surfaces. Fe(OH)3 components precipitated on the membrane possibly give rise to water dissociation (cf. Sections 8.8.4 and 8.9 in Fundamentals). Sometimes, fine organisms pass through the filter and breed in the electrodialyzer. These troubles are avoided by disassembling and washing the electrodialyzer at the interval of 4–6 months (cf. Section 1.5.3 in Applications). In order to increase current efficiency and avoid CaSO4 scale precipitation, membrane surfaces are treated to give monovalent ion permselectivity (cf. Section 3.7 in Fundamentals).
358
Ion Exchange Membranes: Fundamentals and Applications
Figure 1.32 Composition of an electrodialyzer (Tokuyama). 1, Fastening bolt; 2, fastening and feeding frame; 3, concentrated seawater inlet; 4, diluted seawater inlet; 5, diluted seawater outlet; 6, concentrating cell; 7, desalting cell; 8, desalting cell manifold; 9, concentrated seawater outlet; 10, cation exchange membrane; 11, anion exchange membrane; 12, concentrating cell manifold (Tomita, 1995).
Compositions of electrodialyzers developed by Tokuyama Inc., Asahi Glass Co. and Asahi Chemical Co. are illustrated in Figs. 1.32–1.34 (Tomita, 1995). Typical performance of an electrodialyzer is exemplified in Table 1.11 (Tanaka, 1991). 1.6.8 Salt Production Using Brine Discharged from a Reverse Osmosis Seawater Desalination Plant Concentrated brine is discharged from a RO seawater desalination process. It seems advantageous to use this brine as raw material for salt production.
Electrodialysis
359
Figure 1.33 Composition of an electrodialyzer (Asahi Glass Co.). 1, Cathode chamber; 2, anode chamber; 3, cathode plate; 4, anode plate; 5, intermediate frame; 6, desalting cell; 7, concentrating cell; 8, cation exchange membrane; 9, anion exchange membrane; 10, packing cell frame; 11, intermediate packing; 12, blind cell frame (Tomita, 1995).
Figure 1.34 Composition of an electrodialyzer (Asahi Chemical Co.). 1, Diluted seawater; 2, special gasket; 3, concentrating cell; 4, turn-Buckle; 5, desalting cell; 6, concentrated seawater; 7, fastening frame; 8, cation exchange membrane; 9, anion exchange membrane (Tomita, 1995).
360
Ion Exchange Membranes: Fundamentals and Applications
Table 1.11 Performance of an electrodialyzer for concentrating seawater (plant A, 1987) Current density (A dm2) Temperature (1C) Cl current efficiency (%) Na current efficiency (%) Energy consumption (kWh t1 NaCl) Concentrated solution puritya (%) Constitution of concentrated solution NaCl (g dm3) Cl (eq dm3) SO4 (eq dm3) Ca (eq dm3) Mg (eq dm3) K (eq dm3) Na (eq dm3)
2.66 25.3 88.6 80.8 178.6 90.90 190.6 3.568 0.015 0.064 0.169 0.095 3.256
Source: Tanaka (1991). a NaCl(g)/Total electrolyte (g).
The operating performance and energy consumption in a salt manufacturing plant were investigated for an ion exchange membrane ED system to which discharged brine from a RO plant is supplied as follows (Tanaka et al., 2003). The salt manufacturing process (NaCl production capacity: 200,000 ton/year) is illustrated in the dotted frame in Fig. 1.35. Discharged brine (electrolyte concentration: 1.5 eq dm3) from a RO plant is assumed to be supplied to an ion exchange membrane electrodialyzer. The concentrated solution obtained from the electrodialyzer is supplied to a multiple-effect EV, in which salt is crystallized. The salt obtained from the evaporator is supplied to an ion exchange membrane electrolytic bath, in which sodium hydroxide and chlorine are produced. Energy consumption in the salt manufacturing process is assumed to be supplied by a simultaneous heat-generating electric power unit consisting of a boiler and a back-pressure turbine. Fig. 1.36 shows the flows of electricity and steam in a salt manufacturing plant. Boiler steam is introduced to a turbine and generates electricity, which is distributed to electrodialyzers, etc. The back-pressure of a back-pressure turbine is supplied to a heater in a No. 1 evaporator in multiple-effect evaporators. Evaporated steam in a No. 1 evaporator is supplied in turn to the following evaporators. Pressure and temperature of boiler steam are set as 6 Mpa and 4801C in this study. The temperature difference between heating steam and evaporated steam is fixed to 201C at each evaporator. The number of evaporator is kept to a minimum, but the quantity of electricity does not exceed the electric power consumption in this salt manufacturing plant. An electric power shortfall is assumed to be made up by purchased electric power, which is generated by a condensing turbine.
361
Electrodialysis
Seawater 0.6 eq/dm3
Desalted solution
Discharged brine C′in=1.5 eq/dm3 RO C′′ C′out ED Cl2 T
B
EV NaOH NaCl crystals IM
Salt manufacturing process H2O
Figure 1.35 RO-ED combined salt manufacturing process. RO, Reverse osmosis; ED, electrodialyzer; EV, evaporation; IM, ion exchange membrane electrolysis; B, boiler; T, turbine. Diagonals in RO, ED and IM unit box represent the membranes (Tanaka et al., 2003).
The energy required for producing salt F is plotted against current density I/S in both cases of RO discharged brine ED and seawater ED. The plots are shown in Fig. 1.37, which indicates that the energy consumption in a salt manufacturing process using RO discharged brine is 80% of the energy consumption in the process using seawater. The optimum current density at which the energy consumption required in the salt manufacturing process being minimized is 3 A dm2 for both RO discharged brine ED and seawater ED. 1.6.9
Desalination of Amino Acid and Amino Acidic Seasonings Amino acid is an amphoteric electrolyte, and its behavior is different from that of usual electrolytes. Itoi and Utsunomiya (1965) electrodialyzed aqueous solutions of methionine and glysine containing sodium formate as follows; (a) methionine 25 g l1, HCOONa 20 g l1. (b) glysine 70 g l1, HCOONa 20 g l1. The solution was supplied to the electrodialyzer (membrane pairs: 9, effective membrane area: 209 cm2) incorporated with Selemion CSG and AST. Changing pH and maintaining cell voltage at 15 V (average current density: nearly 1 A dm2), amino acid permeation ratio (the ratio of amino acid
362
Ion Exchange Membranes: Fundamentals and Applications
Purchsed electric power
∆G
Ggen+∆G=Etotal × PNaCl RO
H*S*
Discharged brine 1.5 eq/dm3solution
Generated electric power Ggen B
ED
BPT
W = w × PNacl 70°C
50°C
30°C
St
Qcond EV NO1
EV NO2
EV NO3
Tcond=90°C Hcond Scond
Figure 1.36 Energy flow in a salt manufacturing process. B, Boiler; BPT, back-pressure turbine; EV, evaporator; ED, electrodialyzer; RO, reverse osmosis unit (Tanaka et al., 2003).
transported across a membrane pair against that dissolving in a feeding solution) was measured and shown in Fig. 1.38, indicating that the amino acid permeation ratio becomes minimum near the isoelectric point of amino acid. Changing current density and keeping pH near the isoelectric point of the amino acid, the amino acid permeability (quantity ratio of amino acid transported across a membrane pair against electricity) was measured and shown in Fig. 1.39. Inspecting Fig. 1.39 and the limiting current density measured for an HCOONa solution, it is concluded that the amino acid permeability becomes minimum by applying the limiting current density. Concentration changes of NaCl and essence in the batch system ED of soy sauce are indicated in Tables 1.12 and 1.13.
363 60
1.2
50
1.0
40
0.8
30
0.6
20
0.4
10
0.2
0 0
1
2
3
4 5 6 I/S (A/dm2)
7
8
9
(RO discharged brine electrodialysis)/ (seawater electrodialysis)
(103M cal/h)
Electrodialysis
0.0 10
: RO discharged brine electrodialysis : Seawater electrodialysis : RO discharged brine electrodialysis / Seawater electrodialysis
Figure 1.37
1.6.10
Energy consumption in a salt manufacturing plant (Tanaka et al., 2003).
Desalination of Natural Essences In an extraction process of natural essences, several kinds of salt, acid and alkali are added. Accordingly salt is obtained as a by-product in a final stage. Salt content in the natural essences is expected to be controlled for maintaining a taste and human health. Tokuyama Inc. developed the following desalination processes of natural essences (Ideue, 1986; Yamamoto, 1993). An ED system was set up using ion exchange membranes met the food hygiene standard established by the Ministry of Welfare, Japan, and incorporated with polyvinyl chloride or polypropylene materials suitable for food production. It was further necessary to pay attention to prevent solution stagnation and cultural contamination. Further cleaning in place (CIP) was necessary to operate the apparatus stably. The electrodialyzer was operated in a batch system using ED system indicated in Fig. 1.40, in which a conductivity control indicator was set at an exit of the electrodialyzer for detecting the concentration of a desalted solution. The solution feed and solution discharge were automatically controlled by switch valves operating with the conductivity control indicator. Natural essences include meat essences, seafood essences, flesh essences etc. They were extracted with the aid of a NaCl solution. Salt added in the extraction process was desalinated by means of ED mentioned above. Constituent changes in desalination of extracted meat essences, fish essences and fruit
364
Ion Exchange Membranes: Fundamentals and Applications
80
Cell voltage : 15 V
Glycine
60
ionin
e
50 40
Meth
Amino acid permentation ratio (%)
70
30 20 10 0
0
2
4
6
8
10
12
14
pH Glicine 70 g/l, HCOONa 20 g/l Methionine 25 g/l, HCOONa 20 g/l
Figure 1.38 Relationship between solution pH and amino acid permeation ratio through ion exchange membranes (Itoi and Utsunomiya, 1965).
flesh essences are shown in Table 1.14. Specifications of an electrodialyzer for seafood essences are shown in Table 1.15. 1.6.11 Electrodialysis of Milk and Whey 1.6.11.1 Composition of Milk and Whey (Ideue, 1986; Tomita et al., 1986) Whey is obtained as a by-product of a cheese production process. The output of whey amounts to nine times of that of cheese, so it is an important subject in the dairy industry to utilize the whey effectively. The effective utilization of permeates derived from an ultra-filtration process of milk and whey is also a big problem in the dairy industry. The composition of milk, whey and ultrafiltration permeates is indicated in Table 1.16. Ash content in dry matter of the whey and permeate is so high that it is expected to reduce their ash content by ED. Powdered milk for baby rising is prepared using cow milk as main raw materials. The constituents of breast milk and cow milk are not same as shown in Table 1.17. Total ash and casein compositions of cow milk are, respectively, 3.4 and 4.4 times of those of the breast milk. Accordingly, baby rising powdered
365
Electrodialysis
3.0 pH: 6.3-6.9
ine yc
2.0
Gl
1.5
nin
e
1.0
et
hio
Amino acid permeability (g/Ah)
2.5
0
M
0.5
0
0.5
1.0
1.5
2.0
Average current density (A/dm2) Glycine 70 g/l, HCOONa 20 g/l Methionine 25 g/l, HCOONa 20 g/l
Figure 1.39 Relationship between current density and transport rate of amino acid (Itoi and Utsunomiya, 1965).
Table 1.12
Electrodialytic demineralization of soy sauce Start
Solution quantity (l) NaCl concentration (g l1) Essence concentration (g l1) pH Density Current efficiency (%)
11.6 191 163 4.7 1.181
End 31 191 4.7 1.103 90
Note: Effective membrane area: 209 cm2; Numbers of membranes: 10 pairs; Current density: 3.5 A dm2; Applied voltage: 25–58 V. Source: Itoi (1983).
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Ion Exchange Membranes: Fundamentals and Applications
Table 1.13
Concentration changes of components in electrodialysis of soy sauce
Component
Ratio (Before ED/After ED)
NaCl Total amino acid Glutamic acid Aspartic acid Lysine Leucine Isoleucine Alanine Phenylalanine Valine Total nitrogen Essence part
0.800 0.94 0.93 0.95 0.98 0.98 0.98 0.96 0.87 0.95 0.96 1.009
Source: Itoi (1983).
milk is prepared by adding whey to cow milk. Before this mixing operation, however, it is necessary to extract excessive ash from the milk or whey by ED. 1.6.11.2 Limiting Current Density in Electrodialysis of Milk and Whey (Nagasawa et al., 1973; Nagasawa et al., 1974) In ED of a dairy product solution, protein is condensed and attached on the desalting surface of an anion exchange membrane at above limiting current density due to occurrence of water dissociation. At the same time, insoluble salt such as calcium phosphate etc. precipitates on the concentrating surface of the anion exchange membrane. Accordingly, it is important to increase the limiting current density to operate a practical electrodialyzer stably. Nagasawa et al. evaluated the limiting current density as follows. In ED of a skim milk solution, limiting current density was evaluated based on the relationship between current density and voltage. The relationship between conductivity of the skim milk solution k and the limiting current density ilim was expressed by ilim ¼ 1.08 103k. (i/k)lim was known to be proportional to the 0.6th power of solution velocity of skim milk V. (i/k)lim/V0.6 vs. temperature T and dry matter content of skim milk S is given in Fig. 1.41 which indicates that (i/k)lim/V0.6 decreases with the increase of T and S. Flow length and distance between the membranes did not influence to ilim. From the investigation mentioned above, the following limiting current density equation was introduced as 0:1
ilim ¼ QS1=6 ð1:01665S ÞT V 0:6 k
(1)
Q is a constant. S, T and V can be voluntarily fixed, so the above equation is expressed by the following equation. ilim ¼ Rk
(2)
VI
Electrodialysis
AI
8 CCI
10
_
+
7 9
F1
F1
LA P1
LC
1
2
P1
P1
F1
F1
P1
6 LA
3
4 3
5 3
367
Figure 1.40 Electrodialysis process of natural essences. 1, Raw solution tank; 2, desalted solution tank; 3, drain; 4, concentrated solution tank; 5, concentrated waste solution; 6, electrode solution tank; 7, water supply; 8, ammeter; 9, electrodialyzer; 10, desalted solution; Vi, voltage indicator; Ai, ampere indicator; Fi, flow indicator; Pi, pressure indicator; CCi, conductivity control indicator, LC, level control; LA, level alarm (Yamamoto, 1993).
368 Table 1.14
Ion Exchange Membranes: Fundamentals and Applications
Desalination of fish essences
Conductivity Viscosity Ash Na K Ca Mg Cl
Raw Essence
Desalted Essence
Desalting Ratio
15.4 mS cm1 4 cp
2.74 mS cm1 6.4 cp
82.2%
2975 ppm 241 ppm 38 ppm 45 ppm 3299 ppm
560 ppm 19 ppm 2.4 ppm 1.9 ppm 583 ppm
81.2% 92.1% 93.7% 95.8% 82.3%
Source: Yamamoto (1993).
Table 1.15
Specifications of an electrodialyzer for desalinating seafood essences
Requirement Raw solution NaCl Protein Specific gravity Viscosity pH Treating amount
14 g l1 100 g l1 1.05 4.0 cp 5.5 0.5 t h1
Specifications Effective membrane area Number of membrane pair Ion exchange membrane Model Operating system
256 m2 200 pairs Neocepta CM-1, AM-1 TS-25-200 Automatic batch system
Running cost
1517 yen t1
Source: Yamamoto (1993).
Table 1.16
Typical composition of milk and UF-permeate Fat (%)
Cow milk Cheese whey Permeate derived from UF of milk
3.3 0.3 0
Source: Tomita et al. (1986).
Protein (%) 2.9 0.7 0.2
Lactose (%)
Ash (%)
4.5 4.5 4.4
0.7 0.6 0.5
Water (%) 88.6 93.7 94.8
Ash/Dry Matter (%) 6.1 9.5 9.6
369
Electrodialysis
Table 1.17
Comparison of composition between breast milk and cow milk Water (%)
Breast milk Cow milk
88.0 88.6
Ash (%)
Whey Protein (%)
0.2 0.7
0.68 0.69
Casein (%) 0.42 2.21
Fat (%) 3.5 3.3
Lactose (%) 7.2 4.5
Source: Tomita et al. (1986).
(i/)lim/V0.6 x 102
10
5
2 0 S%
10 : 0.3,
20 30 Temperature (°C) : 8.3,
: 16,
40
: 25
Figure 1.41 Effects of temperature and total solid content on (i/k)lim/V0.6 (Nagasawa et al., 1974).
R is a constant. We know ilim from k, and it was confirmed that the electrodialyzer is operated stably at under ilim estimated from Eq. (2). 1.6.11.3 Ion Exchange Membrane (Okada et al., 1975) for the Demineralization of Milk or Whey In the demineralization of skim milk or whey including rich proteinaceous materials or other organic matters, conventional membranes applied to the treatment of saline water are not feasible because the life span of the membranes is shortened and the performance is deteriorated. In order to make membranes applicable to the demineralization process of milk or whey, the investigation was performed to give the following characteristics to the membranes. Anti-Organic Fouling The specific conductance of various membranes k was determined in a demineralization experiment of Gouda cheese whey. The membranes are, Aciplex K-101 (conventional cation exchange membrane, Asahi Chemical Co.), A-101 (conventional anion exchange membrane) and A-201 and A-211 (both are
370
Ion Exchange Membranes: Fundamentals and Applications
Remaining specific conductance(%)
100 K-101
80 A-211 A-201
60 40 20 0
A-101 0
2000 4000 6000 Operating time (hr)
8000
Figure 1.42 Membrane conductivity changes with operating time (Okada et al., 1975).
newly developed anion exchange membranes). k of K-101 is not changed largely with running time as indicated in Fig. 1.42. k of A-101 shows very rapid decrease. However, k of A-201 and A-211 membranes does not change significantly for 6000 h. A-201 and A-211 membranes show excellent anti-fouling (cf. Section 14.3.2 in Fundamentals). Anti-Alkaline Circumstance Organic fouling due to deposition of proteinaceous materials or fatty substances from the dairy products is cleaned by washing in an alkaline detergent by means of CIP method. Exposure of the membrane to OH ions generated from water dissociation vitiates its performance. Therefore, life span of the membrane is shortened without the durability against a basic solution in contact. Fig. 1.43 shows the durability evaluated by measuring specific conductance k of the membranes immersed in a 1% NaOH solution. k of conventional cation exchange membrane K-101 does not change, however, that of anion exchange membrane A-101 decreases remarkably with immersing duration. On the other hand, decrease of k is suppressed for anti-organic fouling anion exchange membrane A-201 and A-211. Particularly, A-211 membrane exhibits excellent anti-alkaline performance. Permeability to Larger Organic Ions of an Anion Exchange Membrane An anion exchange membrane which does not permeate larger organic anions causes pH lowering in demineralization of dairy products. This is because OH ions caused by water dissociation generated on a desalting surface of an anion exchange membrane permeate the membrane instead of the larger organic anions toward a concentrating cell. This phenomenon induces pH increase in the concentrating cell and gives rise to the precipitation of inorganic salts such as Ca3(PO4)2, CaCO3, CaSO4 etc. On the other hand, pH in the desalting cell is
371
Electrodialysis
100
K-101 A-211
Remaining specific conductance(%)
90 80 70 A-201
60 50 40 30 20 10
A-101 0
0
100
200 300 400 Immersing duration (hr)
500
Figure 1.43 Membrane conductivity changes with time in alkaline circumstances. 1 % NaOH at 371C (Okada et al., 1975).
estimated to be lowered because H+ ions are remained in the desalting cell. Fig. 1.44 shows pH changes upon 90% ash reduction of Gouda cheese whey using three types of anion exchange membranes. A-211 membrane scarcely brings about the decrease in pH. This is because pore radius in A-211 membrane is large enough (cf. Section 14.3.2.1 in Fundamentals), so that organic acids such as citric acid, lactic acid, amino acid as well as phosphoric acid easily permeate the membrane and that water dissociation does not easily occur. From the experiment described here, it is concluded that A-211 is the most suitable anion exchange membrane. CIP operations of an electrodialyzer incorporated with A-211 membranes realize 90% ash reduction and life span extension.
1.6.11.4 Electrodialysis System (Okada et al., 1975) for the Demineralization of Milk or Whey Fig. 1.45 illustrates the four stack ED process MED SV1/2 4-4 developed by Morinaga Milk Industry Co. The specifications and operating conditions of this system are shown, respectively, in Tables 1.18 and 1.19. In Fig. 1.45, the
372
Ion Exchange Membranes: Fundamentals and Applications
A-211 7
A-201 (and A-101)
pH
6
5
4
Figure 1.44
0
10
20
30 40 50 60 70 Relative deashing rate [%]
80
90
pH changes of whey in demineralization (Okada et al., 1975).
pump P1 feeds the whey to the balance tank WB-1. Succeedingly, the whey is supplied to the 1st stack of the ED system, demineralized to some extent and overflows from WB-1 to WB-2. The whey in WB-2 circulates through pump P3 and second stack, then part of which overflows into WB-3. In the similar way, the whey flows via P4, third stack, WB-4, fourth stack, WB-5, and finally demineralized whey flows out via P6 and supplied to the succeeding process such as pasteurization, evaporation and drying. At each balance tank, the conductivity of the whey is monitored to control the desalting rate. The feed rate of the whey to this system is automatically controlled by means of conductivity measurement in the effluent of demineralizing stream. 1.6.12
Desalination of Sugar Liquor The sugar manufacturing system is classified into the cane sugar–fine sugar manufacturing system and the beet sugar manufacturing system. The raw material for the former is sugar canes and that for the latter is beet sugar. In the sugar manufacturing process, the greater part of organic nonsugar components in raw sugar is removed by means of defecation, carbonation, adsorption (bone char, active carbon, ion exchanger etc.). In these treatments, inorganic components are not removed and transferred to an evaporation process, in which residual organic nonsugar components are separated from sugar crystals and remained in molasses. In the course of repeating evaporation and separation, the inorganic and organic nonsugar components are gradually accumulated in the
CB-5
CB-4
P11
for anode frame (one pass & waste)
P10
I1
V1
CB-3
P9
Con
Con
1st
2nd
DC Dil
CB-2
I2
Dil
V2
V3 DC DC
Water
CB-1
P8
I3
Electrodialysis
Va1
H2SO4
P7
Con
Con
3rd
4th
I4
V4 Rinse solution for fastening frame & cathode frame
DC Dil
Dil P12
P2 Whey
P4
P3
P5
P1
WB-1
WB-2
WB-3
WB-4
PB-5
Demineralized whey P6
1st : Stack - 1 2nd : '' -2
DC : Rectifier
3rd :
''
-3
I1-4 : Amperage meter
4th :
''
-4
Con. : Concentrating compartments Dil. : Diluting compartments
V1-4 : Voltage meter
373
Figure 1.45 Flow diagram of Morinaga continuous electrodialysis process (Okada et al., 1975).
374 Table 1.18
Ion Exchange Membranes: Fundamentals and Applications
Specifications of MED SV1/2 4-4
1. Center-press 2. Stack 3. Electrode 4. Cell pair 5. Distance between membranes 6. Effective membrane area 7. Spacer 8. No. of channel 9. Width of the channel 10. Length of the channel
1 4 4 pairs 150 cell pairs/stack 0.75 mm 50 dm2/cell Sheet-flow type 1 500 mm 1000 mm
Source: Okada et al. (1975).
Table 1.19
Operating conditions of demineralization of whey (Morinaga ED system)
1. Quality of whey to be treated 1.1 Total solids 1.2 Ash content 1.3 Specific conductance 1.4 Sediment test (200 ml, 1000 G)
20% 1.60% 0.013 S cm1 less than 0.1 ml
2. Operating conditions 2.1 Level of applied d-current density 2.2 Linear velocity in the compartment 2.3 Operating temperature
3000 ka (mA cm2) 12 cm s1 201C
Source: Okada et al. (1975). k: Specific conductance.
a
molasses and finally they are discharged to the outside of the system as waste molasses. The waste molasses includes considerable amount of sugar components, so it is utilized as raw materials for fermentation or animal food, however, its economical value is extremely low comparing to that of sugar itself. Because of the background described above, the technology development was expected for preventing sugar component transfer to waste molasses and increasing sugar recovering ratio. Further desalting technology by means of ion exchange membrane ED came to be attracted because the sugar recovering ratio is influenced by residual inorganic components in syrup. Application of ED in sugar manufacturing industry was investigated from the latter half of 1950s. However, it was difficult to put this program into practice. This is because water dissociation and organic fouling are apt to occur on anion exchange membranes (cf. Section 14.3 in Fundamentals). In order to avoid these troubles, Taito Co. and Asahi Chemical Co. developed the technology using neutral membranes consist of polyvinyl alcohol instead of anion exchange membranes (Sugiyama et al., 1982; Kokubu et al., 1983). The
375
Electrodialysis
advantages and disadvantages of this method (Transport depletion method) are as follows. 1. Advantages (a) Neutral membranes do not deteriorate due to organic fouling. (b) Sugar does not decompose, because pH decrease caused by water dissociation does not occur on the neutral membrane. (c) Current density can be increased, because water dissociation does not occur. 2. Disadvantages (a) Removing efficiency of anions is low. (b) Current efficiency Z is low, because transport number of anions of a neutral membrane ¯tA A0:5: If we assume the transport number of a cation exchange membrane ¯tK ¼ 1:0; Z ¼ ¯tK þ ¯tA 1 ¼ 0:5: In the first stage in a sugar manufacturing process, raw molasses (original syrup) is treated to remove organic materials and evaporated to obtain A sugar Table 1.20
Effect of electrodialysis on pretreatment for molasses
Desalting ratio (%) Current efficiency (%) Current density (A dm2) Molasses volume (l) Molasses concentration (oBx) Molasses purity (%) pH
Ash (% on solid) CaO MgO K2O Cl SiO2 SO3 P2O5 CO2 Sulfate ash
I
II
66.18 43.55 3.04
63.60 34.01 3.07
III 2.58 9.54 0.03
Start 10.00 51.35
End 9.76 46.85
Difference D 0.24 D 4.50
Start 10.00 50.45
End 9.76 46.45
Difference D 0.24 D 4.00
0.00 D 0.50
51.70 6.35
59.01 6.35
7.31 0.00
52.66 6.40
58.45 6.40
5.79 0.00
1.52 0.00
Start
End
Desalting ratio
Start
End
Desalting ratio
0.19 0.74 5.47 3.99 0.41 0.69 0.18 0.19 12.42
0.06 0.33 1.59 0.15 0.33 0.41 0.19 0.14 4.73
68.42 55.41 70.93 96.24 19.51 40.58 D 5.56 26.32 61.92
0.22 0.72 5.98 3.65 0.44 1.22 0.21
0.13 0.42 2.01 0.19 0.41 1.00 0.18
40.91 41.67 66.39 94.79 6.82 18.03 14.29
27.51 13.74 4.54 1.45 12.69 22.55 D 19.84
12.78
5.19
59.39
2.53
Note: I: Duple-stage centrifuging after CaCl2 adding; II: Duple-stage centrifuging without CaCl2 adding. Source: Kokubu et al. (1983).
376
Ion Exchange Membranes: Fundamentals and Applications
2nd boiling
B sugar
B molasses
Ca(OH)2
CaCl2
Mixing Water
Steam Heating
1st centrifuging
1st sludge
1st centrifuged molasses Steam
Water 2nd centrifuging 3rd centrifuging 2nd sludge
2nd centrifuged molasses
Separated molasses Final sludge Refrigerator Seawater Discharged water
Electrodialyzer
Water
Desalted molasses (C molasses)
3rd boiling
Figure 1.46
C sugar
Desalination of B molasses (Kokubu et al., 1983).
and A molasses. In the next stage, B sugar and B molasses are obtained from A molasses through the similar process. Finally, C sugar and C molasses are obtained from B molasses. Table 1.20 shows ED experiment of B molasses, which is centrifuged two times after adding CaCl2 (Case I) and without adding CaCl2 (Case II). The experiment indicates that desalting ratio and current efficiency in Case I are increased comparing those in Case II. This is because the minerals such as CaO, SiO2, SO3, P2O5 etc. are removed by the CaCl2 treatment in Case I. Based on the experiment described above, Taito Co. designed the ED treatment process of B molasses as shown in Fig. 1.46. In this process, B
377
Electrodialysis
Table 1.21
Bacterial strains and media for cell count
Strains
Media (Agar)
Staphylococcus aureus Salmonella heidelberg Escherichia coli K-12 Pseudomonas aeruginosa Proteus vulgaris Klebsiella pneumoniae Bacillus subtilis
Mannitol salt Deoxycholate Deoxycholate Deoxycholate Deoxycholate Deoxycholate Nutrient
Source: Sato (1989).
+
C
I
C
A
II
III
FS
−
A
IV
V
FS
Anode
Cathode
P
P
P
AS
BSS
CS
Figure 1.47 Schematic diagram of an electrodialytic disinfection diagram. C, Cation exchange membrane (Selemion CMV); A, anion exchange membrane (Selemion AMV); CS, cathode solution; AS, anode solution; BSS, bacteria cell suspending solution; distance between the membranes, 1 cm; membrane area, 18.4 cm2 (Sato, 1989).
molasses is treated at first by defecation mixing with Ca(OH)2 and CaCl2. After heating and two stages of centrifugation, the second centrifuging molasses is supplied to the electrodialyzer via the refrigerator to obtain desalted molasses (C molasses). C sugar is crystallized in the evaporation of C molasses. By integrating the ED step mentioned above in the sugar manufacturing process, it became possible to obtain D sugar from C molasses.
378
Ion Exchange Membranes: Fundamentals and Applications
100 90
Cell viability (%)
80 70 60 50 40 30 20 10 0
0.54
0.81
1.08
1.35
1.65
Current density (A/dm2) : S. aur.,
: Sal. heid,
: K. pneu.,
: B. sub (spore).
: E. coli,
:Ps. aerug.,
:Pr. vulg.,
Figure 1.48 Relation between bacteria cell viability and current density. Flow rate, 3 cm3/min; duration time, 60 min (Sato, 1989).
1.6.13
Electrodialytic Disinfection Sato et al. (1984) investigated water disinfection by means of ion exchange membrane ED. The merits of this method are as follows. (1) (2) (3)
The operation is proceeded at normal temperature. Comparing to chlorine disinfection, the electrodialytic disinfection is more powerful and proceeded during shorter time. The process is not harmful to human body.
Bacterial strains in Table 1.21 were cultivated at 371C for 18 h. The cultivated solution suspending 108 cells cm3 of bacteria cells was supplied into the desalting chamber (chamber III) in an ED system in Fig. 1.47 and electrodialyzed for 60 min. Viability of the cells is plotted against current densities and shown in Fig. 1.48. Here, limiting current density is 0.81 A dm2. Bacteria viability is seen to be decreased with the increase of current density in a range of over limiting current densities and becomes zero at 1.63 A dm2. Electron
379
Electrodialysis
E. coli cell Water dissociation
OH
H
+
H+
Anode
Cathode
Anion exchange membrane
Figure 1.49
Desalting chamber
Cation exchange membrane
Mechanism of disinfection in electrodialysis (Sato, 1989).
microscope observation revealed that bacteria cell conformation is shrank under applying over limiting current density. The mechanism of electrodialytic disinfection in this study is estimated as follows. In Fig. 1.49, Escherichia coli cells are suspended in a solution in the desalting chamber (Chamber III). Passing over limiting current in this system, the electrolyte concentration in the desalting chamber is decreased and electric resistance of the solution is increased. In this situation, water dissociation is generated on the anion exchange membrane (cf. Section 8.8.1 in Fundamentals) and H+ ion concentration in the desalting chamber is increased. An E. coli cell is an electron conducting substance, so H+ ions pass through the E. coli cells. This phenomenon is similar to that in electrodeionization (cf. Chapter 4, Fig. 4.8 in Application). E. coli cells are estimated to be destroyed by H+ ions passing through the cells. The investigation described here should be analyzed from the biological effects of alkaline electrodialyzed water at the cellular level (Takahashi, 2006; Kikuno, 2006). REFERENCES Azechi, S., 1980, Electrodialyzer, Bull. Soc. Sea Water Sci., Jpn., 34(2), 77–83. Fukuhara, K., Hamada, M., Azuma, I., 1993, Efficient desalination of brackish water by electrodialysis, Industrial Application of Ion Exchange Membranes, vol. 2. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 159–167. Ideue, K., 1986, Desalination with ion exchange membranes in food industry, Food Dev., 21(7), 54–59.
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Ion Exchange Membranes: Fundamentals and Applications
Inoue, S., Kuroda, O., 1993, Electrodialysis desalination system powered by photovoltaic power generation, Industrial Application of Ion Exchange Membranes, vol. 2. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 151–157. Ishibashi, T., 1986, Simultaneous treatment of waste water by electrodialysis and reverse osmosis, Industrial Application of Ion Exchange Membranes, vol. 1. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 177–180. Itoi, S., 1983, Electrodialytic demineralization of soy sauce and amino acid seasonings, Food Industry and Membrane Utilization, Saiwai Shobo Inc., Tokyo, pp. 157–162. Itoi, S., Komori, R., Terada, Y., Hazama, Y., 1978, Basis of electrodialyzer design and cost estimation, Ind. Water, No. 239, pp. 29–40. Itoi, S., Nakamura, K., Kawahara, T., 1986, Electrodialytic recovery of waste water from metal surface treatment process, Industrial Application of Ion Exchange Membranes, vol. 1. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 206–211. Itoi, S., Utsunomiya, T., 1965, Electrodialysis of aqueous solution of amino acid containing electrolyte by ion exchange membrane, Asahi Glass Res. Report, 15, 171–178. Katayama, S., 2004, Waste water treatment and recovering of useful substances, In: Seno, M., Tanioka, A., Itoi, S., Yamauchi, A., Yoshida, S. (Eds.), Functions and Applications of Ion Exchange Membranes, Industrial Publishing & Consulting Inc., Tokyo, Japan, pp. 151–169. Kikuno, R., 2006, Microbicidal effect of strong alkaline electrolyzed water, 2005 Alkaline Electrodialyzed Water Symposium, Tokyo, Kitasato University, September 3, 2006. Kokubu, T., Yamauchi, T., Miyagi, S., 1983, Application of electrodialysis in sugar industry, Food Ind., 9, 26–31. Leitz, F. B., 1986, Measurements and control in electrodialysics, Desalination, 381–401, presented at the International Congress on Desalination and Water Re-use, Tokyo (1977). Matsunaga, Y., 1986, Reuse of waste water by electrodialytic treatment, Industrial Application of Ion Exchange Membranes, vol. 1. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 188–196. Mintz, M. S., 1963, Criteria for economic optimization are presented in the form of comparative performance equations for various methods of operation, Ind. Eng. Chem., 55, 19–28. Nagasawa, T., Okonogi, S., Tomita, M., Tamura, Y., Mizota, T., 1973, Demineralization of skim milk by means of electrodialysis with ion permselective membrane, I. Relationship between limiting current density and specific conductivity of demineralizing solution, Jap. J. Zootech. Sci., 44(8), 426–431. Nagasawa, T., Okonogi, S., Tomita, M., Tamura, Y., Mizota, T., 1974, Demineralization of skim milk by means of electrodialysis with ion selective membrane, II. The effects of linear velocity, temperature and total solid content etc. on the limiting current density, Jap. J. Zootech. Sci., 45(11), 578–584. Okada, K., Tomita, M., Tamura, Y., 1975, Electrodialysis in the treatment of dairy products, Symposium ‘‘Separation processes by membranes, ion-exchange and freezeconcentration in food industry’’, Paris, March 13–14. Sato, T., 1989, Investigation on water disinfection applied ion exchange membrane electrodialysis and production of pyrogen-free water, Thesis, Yokohama National University, Yokohama, Japan.
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Sato, T., Tanaka, T., Suzuki, T., 1984, Disinfection of a colon bacillus by means of ion exchange membrane electrodialysis, J. Electrochem. Jpn., 52, 239–243. Shaffer, L. H., Mintz, M. S., 1966, Electrodialysis, In: Spiegler, K. S. (Ed.), Principles of desalination, Academic Press, New York, London, pp. 200–289. Sugiyama, M., Takatori, Y., Touyama, R., Nakamura, A., Yamauchi, T., Kawate, H., Yamaguchi, A., 1982, On the desalination process of sugar solutions by electrodialysis using neutral membranes, J. Sugar Refining Technology, 30, 26–32. Takahashi, R., 2006, Basic analysis of biological effects of potable alkaline electrodialyzed water at the cellular level, 2005 Alkaline Electrodialyzed Water Symposium, Tokyo, Kitasato University, September 3, 2006. Tanaka, Y., 1987, Concentration of seawater using electrodialysis, In: Kawasaki, J., Kunime, T., Sakai, K., Hakuta, T. (Eds.), Membrane Separation Technology Hand Book, Science Forum, Tokyo, pp. 211–215. Tanaka, Y., 1991, Membrane separation, In: Seno, M., Abe, M., Suzuki, T. (Eds.), Ion Exchange, Kodansya Scientific Co., Tokyo, pp. 211–227. Tanaka, Y., 1993, Electrodialysis, In: Sakai, K. (Ed.), Theory and Design of Membrane Separation Process, Industrial Publishing & Consulting Inc., Tokyo, pp. 69–107. Tanaka, Y., Ehara, R., Itoi, S., Goto, T., 2003, Ion exchange membrane electrodialytic salt production using brine discharged from a reverse osmosis seawater desalination plant, J. Membr. Sci., 222, 71–86. Tomita, A., 1995, Electrodialyzer, In: Ogata, N. (Ed.), Engineering in Salt Manufacturing, vol. 2, Electrodialysis, Japan Salt Industry Foundation, Tokyo, pp. 85–101. Tomita, M., Tamura, Y., Mizota, T., 1986, Electrodialysis of milk and whey, Industrial Application of Ion Exchange Membranes, vol. 1. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 147–158. Tsunoda, S., 1994, Present status and latest trends of deep bed filtration, Bull. Soc. Sea Water Sci., Jpn., 48, 27–37. Urabe, S., Doi, K., 1978, Electrodialyzer, Ind. Water, 239, 24–28. Yamamoto, Y., 1993, Desalination of natural essences by electrodialysis, Industrial application of ion exchange membranes, vol. 2. Research group of electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan, pp. 181–188. Yawataya, T., 1986, Ion Exchange Membranes for Engineers, Kyoritsu Shuppan Co. Ltd., Tokyo, pp. 94–98.