Ion-exchange membrane electrodialytic salt production using brine discharged from a reverse osmosis seawater desalination plant

Journal of Membrane Science 222 (2003) 71–86

Ion-exchange membrane electrodialytic salt production using brine discharged from a reverse osmosis seawater desalination plant Yoshinobu Tanaka a,∗ , Reo Ehara b , Sigeru Itoi c , Totaro Goto d a

IEM Research, 1-46-3 Kamiya, Ushiku-shi, Ibaraki 300-1216, Japan IEM Research, 4-39-4 Highland, Yokosuka-shi, Kanagawa 239-0833, Japan c IEM Research, 2-32-20 Katakura, Kanagawa-ku, Yokohama-shi, Kanagawa 221-0861, Japan Water Re-use Promotion Center, 3-5-4 Nihonbashi-Ningyo-Cho, Chuo-ku, Tokyo 103-0013, Japan b

d

Received 5 December 2002; accepted 10 May 2003

Abstract Operating parameters of an ion-exchange membrane electrodialytic salt manufacturing plant (NaCl production capacity: 200,000 t per year) using brine discharged from a reverse osmosis (RO) seawater desalination plant are discussed. The results were compared with the data obtained from a salt manufacturing plant using seawater. The specifications of the electrodialyzer are: the thickness of the desalting cell, 0.05 cm; the flow-pass length in a desalting cell, 2 m; effective membrane area, 2 m2 ; overall osmotic coefficient of a membrane pair, 30 cm4 /(eq. h); and solution velocity at the inlets of desalting cells, 5 cm/s. The electrolyte concentration at the inlets of desalting cells was set at 1.5 eq./dm3 , which is consistent with the electrolyte concentration of brine discharged from a reverse osmosis seawater desalination plant. The energy consumed in the salt manufacturing process was assumed to be supplied by a simultaneous heat-generating electric power unit using a back-pressure turbine. The number of evaporators (evaporation pans) was selected to minimize the electric power shortfall of the salt manufacturing process but to be larger than zero. The electric power shortage was assumed to be made up by purchased electric power, which is generated by a condensing turbine. The energy consumption in a salt manufacturing process was obtained by adding the generation energy in the back-pressure turbine, the evaporation energy in the No. 1 evaporator in multiple-effect evaporators, the condensing energy in the heater in the No. 1 evaporator and purchased energy. The energy consumption in a salt manufacturing process using the brine discharged from a reverse osmosis seawater desalinating plant was 80% of the energy consumption in the process using seawater. The optimum current density at which the energy consumption is minimized was 3 A/dm2 for both electrodialyses of brine discharged from the reverse osmosis desalination plant and of seawater. © 2003 Elsevier B.V. All rights reserved. Keywords: Ion-exchange membranes; Electrodialysis; Reverse osmosis; Salt manufacturing; Electric power generation

1. Introduction Concentrated brine is discharged from the reverse osmosis (RO) seawater desalination process. It seems advantageous to use this brine as raw material for salt production, but this idea has not been realized to date because of economic difficulties. In 1969, the Govern∗

Corresponding author.

ment Chemical Industrial Research Institute of Japan initiated the research on “Seawater desalination and by-product utilization” [1]. In this research, seawater was desalinated by a flash evaporator. Discharged brine was concentrated by ion-exchange membrane electrodialysis (ED). The concentrated solution was introduced into a diaphragm electrolytic bath, and chlorine and a cathode solution were produced. Finally, the cathode solution was separated into sodium

0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0376-7388(03)00217-5

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hydroxide and potassium hydroxide. After the termination of this research, reverse osmosis seawater desalination technology was developed, and electrolysis technology using fluorocarbon ion-exchange membranes was advanced. Now, the results of this research are expected to be extended by applying these technologies. The 21st century is called “the century of water” because of the coming water crisis due to the population increase and environmental destruction. The fresh water from rivers, lakes and ground water totals only 0.01% of total water resources on earth. To secure adequate water resources, effective and cyclic utilization of water are available, but at the same time, desalination of seawater, which accounts for 97.5% of total water resources seems to be realistic. Recently, reverse osmosis seawater desalination technology has been further developed; that is to say, the electrolyte concentration in discharged brine has reached values of 2.5 times seawater because of increased water withdrawal (50–60%). Here, in some part of the world, salt can be manufactured by means of ion-exchange membrane electrodialysis process to which concentrated brine discharged from a reverse osmosis seawater desalination plant is supplied. Requirements to realize such an RO–ED combined process are estimated to be as follows [2]. (1) Fresh water is insufficient and seawater desalination is in operating. (2) Industry is in developing and the consumption of chlorine and sodium hydroxide is expected. (3) There is no natural salt or salt is not produced for industrial use. Regions satisfied the requirements above are considered to be Middle and Near East, Taiwan, Singapore, China, Korea, Southeast Asia, etc. In this paper, we discuss the operating performance and energy consumption in a salt manufacturing plant using an ion-exchange membrane electrodialyzer to which discharged brine from a reverse osmosis plant is supplied. This study was developed in the workshop organized in the Society of Seawater Science, Japan, for investigating the reverse osmosis–electrodialysis combined system [3].

2. Salt manufacturing process The salt manufacturing process is illustrated in the dotted frame in Fig. 1.

Discharged brine (electrolyte concentration: 1.5 eq./ dm3 ) from a reverse osmosis 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 vacuum evaporator, 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. Any electric power shortfall in the salt manufacturing process is made up by buying public electric power.

3. Performance of the ion-exchange membrane electrodialyzer 3.1. Characteristics of a membrane pair Commercially available ion-exchange membranes (Membrane I, II or III) were integrated in an experimental scale electrodialyzer (effective membrane area: 9 cm2 , distance between the membranes: 0.075 cm, number of desalting cells: 5, number of concentrating cells: 4). Artificially prepared seawater changing the level of electrolyte concentration and temperature was supplied to desalting cells at a linear velocity of 4 cm/s and was electrodialyzed using Ag–AgCl electrodes. After the electrolyte concentration of a concentrated solution reached a constant maximum, it was sampled and analyzed. The quantities of the ions m and the solutions q transported across a pair of ion-exchange membranes, electrolyte concentrations in desalting cells C and in concentrating cells C were measured. The experiment mentioned above was repeated changing current density i step-by-step. When an electrodialyzer is assembled carefully, a solution leakage between a desalting cell and a concentrating cell becomes minimal. In this case, m and q are expressed by the overall mass transport equation; Eqs. (1) and (2) [4]: m = λi − µ(C − C )

(1)

q = φi + ρ(C − C )

(2)

Y. Tanaka et al. / Journal of Membrane Science 222 (2003) 71–86

73

Fig. 1. RO–ED combined salt manufacturing process.

where λ, µ, φ and ρ are the overall transport number, the overall permeability coefficient, the overall electro-osmotic coefficient and the overall osmotic coefficient, respectively. These parameters indicate the membrane pair characteristics rather than the membrane characteristics. The term “overall” means that the coefficients are the sum of the contributions of cation- and anion-exchange membranes. It means also that the coefficients are the sum of the contributions of many kinds of ions dissolved in an electrolyte solution. λi and µ(C − C ) in Eq. (1) stand for the electro-migration and diffusion in the membrane pair, respectively. φi and ρ(C − C ) in Eq. (2) correspond to electro-osmosis and osmosis in the membrane pair, respectively. Plotting m/i and q/i against (C − C )/ i produced linear lines. λ, µ, φ and ρ were obtained from the intercepts and the gradients of the lines. Based on sixty three times of electrodialytic experiments described above, λ, µ

and φ could be expressed by the functions of ρ as [5]: λ = 3.333 × 10−2 − 4.263 × 10−6 ρ

(3)

µ = 9.745 × 10−5 ρ + 1.611 × 10−6 ρ2 − 3.585 × 10−10 ρ3

(4)

φ = 1.255ρ0.5 − 7.709 × 10−2 ρ + 1.154 × 10−3 ρ1.5 (5) Eqs. (3)–(5) indicate that ρ represents all of the membrane pair characteristics. ρ observed in this experiment was distributed within the range of 20–120 cm4 /(eq. h), but it was found that ρ has a value within the range of 30–50 cm4 /(eq. h) in electrodialysis of normal-temperature seawater or brine discharged from a reverse osmosis seawater desalination plant.

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3.2. Electrolyte concentration in desalting cells and concentrating cells An electrolyte solution is assumed to be supplied to a practical-scale electrodialyzer and electrodialyzed with a one-pass flow system. The average electrolyte concentration at the outlets of all desalting cells in a  is shown by Eq. (6): stack Cout  Cout

(ηl/aF)(I/S)  = Cin − u¯

(6)

 is the electrolyte concentration at the inlets where Cin of the desalting cells, a the distance between membranes in the desalting cells, l is the flow-pass length in the desalting cells, F the Faraday constant, I/S the average current density in an electrodialyzer, I the electric current, S the membrane area, and u¯ is the average solution velocity in the desalting cells. The average electrolyte concentration in desalting cells C is the average of the values at the inlets and outlets of the desalting cells, as shown in Eq. (7).

C =

 Cin

 + Cout

2

(7)

An electrolyte solution is assumed to be supplied to concentrating cells by a circulating system. The electrolyte concentration in the concentrating cells C is introduced from Eqs. (1) and (2) with C = m/q as Eqs. (8)–(10) [4]. C is assumed to be invariable in all concentrating cells.
A2 + 4ρB − A  C = (8) 2ρ I A = φ + µ − ρ C S B=λ

I + µ C S

(9) (10)

by Eqs. (11) and (12) [5]:  0.5   1 1 − 1.644 rNa = 0.8220 + 0.7286 i i  1.5  2 1 1 + 1.308 − 0.3573 i i

(11)

 0.5   1 1 − 0.03424 i i  1.5  2 1 1 + 0.02328 − 0.005919 (12) i i

rCl = 0.9911 + 0.02249

An rNa versus rK line, an rNa versus rMg line and an rNa versus rCa line are described by Eqs. (13)–(15) [5]. An rCl versus rSO4 line is shown in Eq. (16): rK = 0.00585 + 0.02268rNa

(13)

rMg = 0.7736 − 0.7958rNa

(14)

rCa = 0.1925 − 0.1910rNa

(15)

rSO4 = 1 − rCl

(16)

The concentration (eq./cm3 ) of ion i in a concentrated solution Ci is expressed by Eq. (17): Ci = ri C

(17)

 , and electrolytes The concentrations of NaCl, CNaCl  C in a concentrated solution, the NaCl purity θ of the concentrated solution, and the quantities of ions m and NaCl, mNaCl , transported across a membrane pair in an electrodialyzer are described by Eqs. (18)–(22):   (g/cm3 ) = 58.443 CNa CNaCl (eq./cm3 )

(18)

C (g/cm3 ) = 57.87C (eq./cm3 )

(19)

θ=

 CNaCl (g/cm3 ) C (g/cm3 )

(20)

3.3. Ionic constituents in concentrated solutions

m (kg/m2 h) = C (kg/m3 ) q (m/h)

(21)

In the experiment cited in Section 3.1, the concentrated solutions were sampled and their ionic constituents were analyzed. The equivalent ratios of Na+ ions and Cl− ions to total ions, rNa and rCl , were plotted against 1/i. Referring to the plots in this experiment and data obtained from practical-scale electrodialyzers [6], we derived the line that is expressed

mNaCl (kg/m2 h) = m (kg/m2 h) θ

(22)

3.4. Current density distribution in an electrodialyzer The electrodes in an ion-exchange membrane electrodialyzer are conductors, and their electrical resistance and ohmic loss are negligible compared to the

Y. Tanaka et al. / Journal of Membrane Science 222 (2003) 71–86

values between the electrodes. Accordingly, the voltage difference between the electrodes at the entrances of desalting cells Vin is equal to the value at the exits Vout : Vin = Vout

(23)

Vin = A1 iin + A2

(24)

Vout = B1 iout + B2

(25)

 A1 = (ϕin + ϕin,K + ϕin,A + ϕ )N       RT γ C A2 = 2(¯tK + ¯tA − 1) N ln  C F γin in

B1 =



(26) (27)



 Yj ϕout,j + Yj ϕout,K,j  + Yj ϕout,A,j + ψ N

(28)

   RT  γ  C B2 = 2(¯tK + ¯tA − 1) Yj ln   F γout,j Cout,j 

(29) Here, the frequency distribution of the solution velocity ratio ξ[= (u − u)/ ¯ u, ¯ u : solution velocity in a desalting cell, u: ¯ average solution velocity in all desalting cells in a stack] is assumed to take a normal distribution [7]. Yj is the number of desalting cells in group j within the range of ξj − "ξj < ξj < ξj + "ξj , "ξj is half of ξ value range of desalting cells in group j. iin (iout ) is the current density at the  (ϕ ) is the elecinlets (outlets) of desalting cells, ϕin trical resistance of solutions at the inlets of desalting  cells (in concentrating cells), ϕout,j the electrical resistance of solutions at the outlets of desalting cells in group j, ϕin,K (ϕin,A ) the electrical resistance of cation-(anion-)exchange membranes at the inlets of desalting cells, ϕout,K,j (ϕout,A,j ) the electrical resistance of cation-(anion-)exchange membranes at the  (γ  ) the elecoutlets of desalting cells in group j, γin trolyte activity coefficient at the inlets of desalting  cells (in concentrating cells), γout,j the electrolyte activity coefficient at the outlets of desalting cells in  (C  ) the electrolyte concentration at the group j, Cin  inlets of desalting cells (in concentrating cells), Cout,j the electrolyte concentration at the outlets of desalting cells in group j, ¯tK (¯tA ) the transport number of

75

cation-(anion-)exchange membranes, N the number of desalting cells in a stack, R the gas constant, T the absolute temperature, and F the Faraday constant.  , ϕ and ϕ in Eqs. (26) and (28) are expressed ϕin out by Eqs. (30)–(32) [5]: a × 10−3  (1 − ε)κin

 = ϕin  ϕout =

ϕ =

a × 10−3  (1 − ε)κout

a × 10−3 (1 − ε)κ

(30)

(31) (32)

where a is the distance between the membranes, ε the  , κ and κ current screening ratio of a spacer, and κin out are specific conductivity of solutions at the inlet and outlet of a desalting cell and in a concentrating cell, respectively. The relationship between the electrolyte concentration C of solutions (artificially prepared sea , κ , κ ) is expressed by Eq. (33) [5]: water) and κ(κin out κ(mS/cm) = 1.644 C−2.168 × 10−3 C2 −1.662 × 10−6 C3 (C : g electrolyte/1000 g solution)

(33)

The electrical resistance of an ion-exchange membrane is usually evaluated by alternating current resistance ϕalt , which is measured in a 25 ◦ C, 0.5 eq./dm3 NaCl solution under an altering current of 1000 Hz. The relationship between overall osmotic coefficient ρ of an ion-exchange membrane pair and ϕalt [= (ϕalt,K + ϕalt,A )/2] is indicated by Eq. (34) [5]: ϕalt = 18.64 ρ−0.5 − 14.77ρ−1 + 3.377 ρ−1.5

(34)

In an electrodialyzer, a direct current is applied to the membranes. The electrical resistance in this circumstance is direct current resistance, ϕdir , which differs from ϕalt . In order to clarify the relationship between ϕdir and ϕalt , an ion-exchange membrane was placed in a two-cell electrodialyzer, and diluted seawater (specific conductivity κde (mS/cm)) was supplied to the cells [8]. The electrical resistance of the mem∗ (' cm 2 )] was measured at 25 ◦ C while brane [ϕdir passing a direct current. Next, diluted seawater was supplied to the desalting side cell and concentrated seawater (specific conductivity, κcon (mS/cm)) was supplied to the concentrating side cell of the electrodialyzer as described above. The electrical resistance

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of the membrane was measured at 25 ◦ C by applying a direct current and subtracting the effect of the mem∗ brane potential. The relationship between κde and ϕdir was obtained and is shown in Eq. (35) [5]: log

∗ ϕdir

ϕalt

= 1.087 − 1.138 log κde + 0.2961(log κde )2 (35)

The relationship between κcon /κde and ϕdir obtained as shown in Eq. (36) [5]:   ϕdir κcon = 1.000 − 0.1359 log ∗ ϕdir κde

/ϕ∗

dir

was

 Vcell =

l

+ a3

 x 2 l

ζin = ζout =

iin I/S iout I/S

Z2 =

−(A2 − B2 ) A1

(44)

3.5. Energy consumption Cell voltage Vcell applied to a membrane pair at the inlets and the outlet of a desalting cell is introduced from Eqs. (24)–(29) as:  Vcell = (ϕin + ϕin,K + ϕin,A + ϕ )iin       RT γ C ¯ ¯ + 2(tK + tA − 1) ln  C F γin in



Yj ϕout,K,j +



Yj ϕout,A,j

(39)

At the same time, the coefficients a1 , a2 and a3 in Eq. (37) are calculated using following equations

(45)

  + ϕiout

  Yj ln(γ  C /γout,j Cout,j )}

N

(38)

(42) (43)

{2(¯tK + ¯tA − 1)(RT/F)

where l is the flow-pass length in a desalting cell. By an applying three-dimensional simultaneous equation to Eq. (37), the inlet current density nonuniformity coefficient ζ in and the outlet current density non-uniformity coefficient ζ out defined by Eqs. (38) and (39) are calculated [9,10].

(41)

B1 A1

 + Yj ϕout,j

(37)

(40)

Z1 =

N +

x

  I + Z2 a1 = Z1 ζout S

  I a2 = 2 {3 − (2Z1 + 1)ζout } − 2Z2 S  

I a3 = −3 {2 − (Z1 + 1)ζout } − Z2 S

(36)

ϕdir corresponds to ϕin,K , ϕin,A , ϕout,K and ϕout,A in Eqs. (26) and (28), and is estimated from ϕalt , using ϕdir /ϕalt obtained by multiplying Eq. (35) by Eq. (36). It should be noted that ϕdir includes the effect of concentration polarization (the effect of electrical resistance and concentration membrane potential) in a boundary layer formed on the desalting surface of an ion-exchange membrane. The current density i at x distance from the inlet of a desalting cell is approximated by the following quadratic equation:

i = a1 + a2

[9,10]:

iout (46)

where iin and iout is given as (I/S) ζ in [Eq. (38)] and (I/S)ζ out [Eq. (39)], respectively, and ζ in and ζ out are calculated using the equations described in Section 3.4. The electrodialytic energy consumption required to obtain 1 t of NaCl, Eelectro , is obtained from Vcell and the quantity of NaCl transported across a membrane pair, mNaCl , as follows: Eelectro (kWh/t NaCl) =

I Vcell S mNaCl

(47)

3.6. Limiting current density When current density reaches the limit of an ion-exchange membrane ilim at the outlet of a desalting cell in which velocity and electrolyte concentration are the least, the average current density applied

Y. Tanaka et al. / Journal of Membrane Science 222 (2003) 71–86

to an electrodialyzer is defined as its limiting current density (I/S)lim . ilim is expressed by the function of solution velocity uout and electrolyte concentration  Cout [12] at the outlet of the desalting cell in which the solution velocity is the least [10]:

77

(48)

to an electrodialyzer is defined as the saturation current density of the electrodialyzer (I/S)sat . Substituting  instead of C and i Cin sat instead of I/S and setting ∗ in Eqs. (8)–(10), we obtain Eq. (52) [4,10]: C = Csat    )(µ + C ∗ ρ) (C∗ − Cin I isat sat = (52) = sat ∗ φ)ζ S sat ζin (λ − Csat in

where r, s and t are constants. We have here the flow velocity ratio ξ at the outlets of the desalting cells:

∗ in Eq. (52) was expressed by the following equaCsat tion [4,10], based on a evaporation experiment [11]:

 ilim = r(uout )s (Cout )t

ξ=

uout − u¯ out u¯ out

(49)

where uout is the solution velocity at the outlet of a desalting cell, and u¯ out is the average solution velocity at the outlets of all the desalting cells in a stack. The frequency distribution of ξ is equated with the normal distribution [7]. The minimum of ξ may be equated with −3σ, where σ is the standard deviation of the normal distribution, and Eq. (49) becomes: uout = u¯ out (1 − 3 σ) Using Eqs. (39), (48) and (50):    )t (1 − 3 σ)s r(u¯ out )s (Cout I = S lim ζout

(50)

(51)

(I/S)lim is determined using Eq. (51), and considering the material balance of ions in the desalting cell in which solution velocity is the least, and considering the material balance of a solution between this desalting cell and adjacent concentrating cells [10]. 3.7. Saturation current density The solutions in the concentrating and desalting cells flow upward and parallel to each other, so the current density and electrolyte concentration of the solutions are maximized at the entrance of the desalting cells. Accordingly, the electrolyte concentration C∗ of the solutions moving from the desalting cells to the concentrating cells also should become maximal at the entrances of the concentrating cells. When C∗ reaches ∗ , NaCl precipitates on the saturated concentration Csat the membrane surface at the entrances of the concentrating cells. In this circumstance, the current density applied to the inlets of desalting and concentrating cells is defined as the saturation current density of the membrane isat , and the average current density applied

∗ Csat = 4.068 × 10−3 + 1.355 × 10−3 rNa

+ 1.107 × 10−6 T

(53)

where rNa is indicated in Eq. (11). T is temperature (◦ C). 3.8. Electrodialysis of brine discharged from a reverse osmosis plant and the estimation of its performance Brine discharged from a reverse osmosis plant is assumed to be supplied to a salt manufacturing plant (NaCl production capacity: 200,000 t per year, operating time: 8000 h per year). The brine is supplied to electrodialyzers with one-pass flow and electrolyzed applying constant current density. The performance of electrodialyzers is evaluated using the equations described in Sections 3.1–3.5, by changing the level of current density. The performance of the electrodialyzer for electrodialyzing seawater is also evaluated for comparison with data for electrodialyzing the brine. The specifications and operating conditions of the electrodialyzer for electrodialyzing RO discharged brine are assumed to be as follows: Membrane area S = 2 m2 ; flow-pass length l = 2 m; Flow-pass width b = 1 m; distance between the membranes a = 0.05 cm; overall osmotic efficient of an ion-exchange membrane pair ρ = 30 cm4 /(eq. h); electrolyte concentration at the inlets of desalting cells  (=electrolyte concentration of brine discharged Cin from a RO plant) = 1.5 eq./dm3 ; solution velocity at the inlets of desalting cells u¯ in = 5 cm/s; standard deviation of the normal distribution of the flow velocity ratio ξ in desalting cells σ = 0.1; current screening ratio of a spacer ε = 0.1. The specifications and operating conditions of the electrodialyzers for electrodialyzing seawater was the

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Y. Tanaka et al. / Journal of Membrane Science 222 (2003) 71–86

Fig. 2. Electrolyte concentration of a concentrated solution in an electrodialyzer.

same as those for electrodialyzing RO discharged brine, excepting the electrolyte concentration at the  = 0.6 eq./dm3 . inlets of desalting cells: Cin Limiting current density and saturation current density are calculated using the equations described in Sections 3.6 and 3.7. The results are (I/S)sat = 7.83 A/dm2 < (I/S)lim for RO discharged brine electrodialysis, and (I/S)lim = 5.08 A/dm2 < (I/S)sat for seawater electrodialysis. Therefore, we discuss the

Fig. 3. Quantity of NaCl transported across a membrane pair and numbers of membrane pairs in the salt manufacturing plant.

Fig. 4. Current efficiency in an electrodialyzer.

phenomena in this study at the current density of I/S = 1, 2, 3, 4, 5, 6, 7, 7.83 A/dm2 for RO discharged brine electrodialysis, and I/S = 1, 2, 3, 4, 5, 5.08 A/dm2 for seawater electrodialysis. Fig. 2 shows electrolyte and NaCl concentrations  in concentrated solutions, C and CNaCl . Fig. 3 shows NaCl transported across a membrane pair mNaCl and the number of membrane pairs N used in this salt manufacturing plant. N decreases with the increase of I/S,

Fig. 5. Quantity of a concentrated solution across a membrane pair and production capacity of a concentrated solution from the salt manufacturing plant.

Y. Tanaka et al. / Journal of Membrane Science 222 (2003) 71–86



79



N (power consumption for N0 circulating a desalting and a concentrating solution)   N + 20 (power consumption for N0 supplying and filtering seawater)   W + 40 (power consumption in the W0

evaporation process)

+ 30

× 1.15 (coefficient of a salt manufacturing plant : power consumption of boiler, generator and anti-pollution) Fig. 6. Cell voltage, electrodialytic energy consumption and total energy consumption of the salt manufacturing plant.

because the NaCl production capacity of this salt manufacturing plant PNaCl is set as 200,000 t/8000 h = 25 t/h. Fig. 4 shows current efficiency η [=mF/(I/S)]. Fig. 5 shows quantity of a concentrated solution transported across a membrane pair q, and the production capacity of a concentrated solution in the salt manufacturing plant Q. Fig. 6 shows cell voltage Vcell and electrodialytic energy consumption Eelectro . They increase linearly with the increase of current density.

4. Performance of a salt manufacturing plant 4.1. Electrical power consumption in a salt manufacturing plant Total electrical power consumption Etotal in a salt manufacturing plant using seawater is approximated by Eq. (54) [13]. I/S is applied current density, N is the number of membrane pairs and W is the quantity of evaporated steam: Etotal

  N (I/S) = Eelectro + 20 (I/S)0 N0 (electrodialytic consumption in electrode chambers, feeding solution chambers and washing chambers)

(54)

where (I/S)0 , N0 and W0 are the values taken as the standard in this study when seawater is electrodialyzed at 3 A/dm2 . Total electric power consumption Etotal in the salt manufacturing plant using brine discharged from a RO plant is also approximated by Eq. (55):

  N (I/S) Etotal = Eelectro + 20 N0 (I/S) 0   κRO discharge × (electrodialytic κseawater consumption in electrode chambers, feeding solution chambers and washing chambers)   N + 30 (power consumption for N0 circulating a desalting and a concentrating solution)   N + 10 (power consumption for N0 supplying and filtering RO discharged brine)   W + 40 (power consumption of W0

evaporation process) × 1.15(coefficient of a salt manufacturing plant : power consumption of boiler generator and anti-pollution) (55) However, in a seawater electrodialysis system, seawater (specific electrical conductivity κseawater = 53.35 mS/cm) is assumed to be supplied to electrode

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Y. Tanaka et al. / Journal of Membrane Science 222 (2003) 71–86

chambers, feeding solution chambers and washing chambers. In an RO discharged brine electrodialysis system, on the other hand, RO discharged brine (κRO discharge = 119.63 mS/cm) is assumed to be supplied to electrode chambers, feeding solution chambers and washing chambers. Furthermore, feeding pumps and filters are simplified in a RO discharged brine electrodialysis system, so that power consumption in this system is put as 10 kWh/t NaCl. Substituting Eelectro , I/S, N and W (Sections 3.5, 3.8 and 4.2) into Eqs. (54) and (55), Etotal is computed and I/S versus Etotal lines are shown in Fig. 6. Etotal has minimal value because N/N0 increases at low current density. 4.2. Quantity of steam evaporated in evaporators Fig. 7. Steam evaporated from evaporators.

Electrolyte solutions concentrated by an ionexchange membrane electrodialyzer were artificially prepared by changing the level of NaCl purity. These solutions were further concentrated while boiling and stirring on an electric range. In the course of this evaporation experiment, the electrolyte concentration in the solution, the quantity of evaporated steam and the quantity of suspended crystals were measured [11]. Based on the experimental results described above, the relationship between the electrolyte concentration in a concentrating cell C , NaCl purity in a concentrated solution θ (g NaCl/g electrolyte Eq. (20)) and the evaporated steam to obtain 1 t of NaCl w [14] was expressed by the following equation: w (t H2 O/t NaCl)   0.8850 = (71.4294 − 12.3289C0.5 θy + 0.88725C − 3.0193 × 10−2 C1.5 + 3.9999 × 10−4 C2 )

(56)

where y (=0.95) is a yield rate in an evaporator. The quantity of steam evaporated in the evaporators W is expressed as: W (t H2 O/h) = wP(t NaCl/h)

(57)

w and W are calculated using Eqs. (56) and (57) using the data obtained in Section 3.8, and are shown in Fig. 7.

4.3. Number of evaporators, quantity of electricity generated by a back-pressure turbine and the quantity of purchased electricity Fig. 8 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 an No. 1 evaporator in multiple-effect evaporators. Evaporated steam in an No. 1 evaporator is supplied in turn to the following evaporators. Pressure and temperature of boiler steam are set as 6 Mpa and 480 ◦ C in this study. The temperature difference between heating steam and evaporated steam is fixed to 20 ◦ C at each evaporator. The number of evaporators is kept to a minimum, but the quantity of electricity generated does not exceed the electric power consumption in this salt manufacturing plant. An electric power shortfall in this plant is assumed to be made up by purchased electric power, which is generated by a condensing turbine. The number of evaporators and electric power shortfall (quantity of purchased electric power) are computed based on the approximated concept as follows.The generation energy of the back-pressure turbine Egen is expressed by Eq. (58): Egen = H ∗ − Hcond − Tcond (S ∗ − Scond )

(58)

Y. Tanaka et al. / Journal of Membrane Science 222 (2003) 71–86

81

Fig. 8. Energy flow in the salt manufacturing process.

where H∗ is boiler steam enthalpy. Hcond , Scond and Tcond are condensate enthalpy, condensate entropy and condensate temperature in the heater in the No. 1 evaporator, respectively. The evaporation multiple of evaporators e is: r(1 − rn ) e= (59) 1−r where n is the number of evaporators, r an evaporation multiple for an unit evaporator. Based on the operating result of salt manufacturing plants, r = 0.94 is obtained [13], so we have Eq. (60) instead of Eq. (59):   47 e= (1 − 0.94n ) (60) 3

St =

Wv eΛcond

(62)

where W is the quantity of steam evaporated in all evaporators and v is the latent heat in evaporation. The electric power generated by the back-pressure turbine Ggen is: Ggen =

St Egen ηturbine ηgene 0.86

(63)

The steam condensing heat in the heater in the No. 1 evaporator Λcond is:

where ηgen is generator efficiency (=0.9). An electric power shortfall in this salt production process "G is expressed by the difference between electric power consumption in the salt manufacturing process Etotal PNaCl and Ggen . The number of evaporators n is selected as ∆G is kept to a minimum but is larger than zero:

Λcond = Egen (1 − ηturbine ) + Tcond (S ∗ − Scond ) (61)

"G = Etotal PNaCl − Ggen > 0 and minimum

where ηturbine (=0.9) is turbine efficiency. The quantity of steam St supplied to the heater in the No. 1 evaporator is:

n, Tcond , St, Egen , Eback , Ggen and ∆G are calculated using Eqs. (58), (62)-(64) and (66), and are shown in Table 1.

(64)

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Table 1 Number of evaporators. Condensing temperature and steam supplied to a heater in a No. 1 evaporator. Generation energy and back-pressure energy of back-pressure turbine. Electric power generated by a back-pressure turbine. Electrical shortfall in the salt manufacturing plant Tcond (◦ C)

St (t/h)

Egen (kcal/kg)

Eback (kcal/kg)

Ggen (kW)

"G (kW)

RO discharged brine electrodialysis 1 5 130 2 5 130 3 4 110 4 4 110 5 3 90 6 3 90 7 3 90 7.83 (I/S)sat 3 90

35.87 28.25 30.88 28.28 35.43 33.79 32.55 31.75

176.37 176.37 201.69 201.69 228.04 228.04 228.04 228.04

137.58 137.58 112.26 112.26 85.91 85.91 85.91 85.91

5959 4692 5865 5371 7610 7257 6991 6818

1441 1716 905 2099 700 1970 3196 4189

Seawater electrodialysis 1 5 2 4 3 4 4 3 5 3 5.08 (I/S)lim 3

41.91 39.45 33.76 40.72 37.89 37.73

176.37 201.69 201.69 228.04 228.04 228.04

137.58 112.26 112.26 85.91 85.91 85.91

6962 7494 6413 8746 8139 8104

2688 696 2139 719 2604 2756

I/S (A/dm2 )

n

130 110 110 90 90 90

PNaCl = 200, 000 t/8000 h = 25 t/h, n: number of evaporators, Tcond : condensing temperature in a heater in a No. 1 evaporator, St: steam supplied to a heater in a No. 1 evaporator, Egen : generation energy of a back-pressure turbine, Eback : back-pressure energy of a back-pressure turbine, Ggen : electric power generated by a back-pressure turbine, "G: electrical shortage in the salt manufacturing plant, (I/S)sat : saturation current density, (I/S)lim : limiting current density.

4.4. Energy consumption in the salt manufacturing process An electric power shortfall in the salt manufacturing plant is compensated for by a condensing turbine. If we assume that the back-pressure turbine in Fig. 8 is replaced by a condensing turbine, generation energy of the condensing turbine Econdens is expressed as: Econdens = H ∗ − H0 − T0 (S ∗ − S0 )

(65)

where H0 , S0 and T0 (=303.15 K) are standard enthalpy, standard entropy and standard temperature, respectively. The back-pressure energy of the back-pressure turbine Eback is: Eback = Econdens − Egen

(66)

The enthalpy difference of steam generated from a boiler "H∗ is: "H ∗ = H ∗ − H0

(67)

Here, we depict Econdens , Egen and Eback in the enthalpy–entropy diagram, Fig. 9. Using the energy indicated above and in Eqs. (68) and (69), we divide

"H∗ into the generation enthalpy difference "Hgen and back-pressure enthalpy difference ∆Hback in back-pressure [13]:   Egen "Hgen = (68) "H ∗ Econdens   Eback "Hback = "H ∗ (69) Econdens Here, the condensate enthalpy difference in the heater in the No. 1 evaporator ∆Hcond is expressed by Eq. (70): "Hcond = Hcond − H0 = Tcond (Scond − S0 )

(70)

where Hcond and Scond are the condensate enthalpy and condensate entropy in the heater in the No. 1 evaporator. The evaporation enthalpy difference in the steam in the No. 1 evaporator ∆Hevap is obtained by subtracting ∆Hcond from ∆Hback : "Hevap = "Hback − "Hcond

(71)

The electric power shortage in the salt manufacturing plant ∆G is considered in this study to be generated by

Y. Tanaka et al. / Journal of Membrane Science 222 (2003) 71–86

83

Fig. 9. Enthalpy–entropy diagram.

using a condensing turbine, so that equivalent quantity of steam "St for generating ∆G is expressed as follows: "G × 0.86 "St = (72) Econdens ηturbine ηgen

current density I/S in the electrodialyzers in the case of RO discharged brine electrodialysis, and are shown in Fig. 10. We understand how n is determined from the changes of Φgen and Φ"G by inspecting Fig. 10.

Finally, using St and "St, the generation energy in the back-pressure turbine Φgen , evaporation energy in the evaporators Φevap , condensing energy in the heater Φcond and purchased electricity energy Φ"G are calculated by the following equations: Φgen = "Hgen St

(73)

Φevap = "Hevap St

(74)

Φcond = "Hcond St

(75)

∗

Φ"G = "H "St

(76)

The energy required for producing salt Φ is obtained by summing up Eqs. (73)–(76) as: Φ = Φgen + Φevap + Φcond + Φ" G

(77)

Φgen , Φevap , Φcond , Φ"G and Φ are calculated using Eqs. (67)–(77) with Eq. (62). They are plotted against

Fig. 10. Breakdown of energy consumption in the salt manufacturing plant.

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quired in the salt manufacturing process is minimized is 3 A/dm2 for both RO discharged brine electrodialysis and seawater electrodialysis. In spite of computation based on the approximated concept in this study, the conclusion mentioned above is estimated to be reasonable.

5. Conclusion

Fig. 11. Energy consumption in the salt manufacturing plant.

Φ is plotted against I/S in both cases of RO discharged brine electrodialysis and seawater electrodialysis. The plots are shown in Fig. 11, 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 re-

The energy consumption in a salt manufacturing process using the brine discharged from a reverse osmosis seawater desalinating plant is 80% of the energy consumption in the process using seawater. The optimum current density at which the energy consumption is minimized is 3 A/dm2 for both electrodialyses of brine discharged from the reverse osmosis desalination plant and of seawater.

Acknowledgements We wish to thank Ms. N. Nakai and K. Suzuki, Central Research Institute, Japan Tobacco & Salt Public Corporation, for assistance in the experimental.

Nomenclature a a1 , a2 , a3 b C e E Eelectro Etotal F "G G "H H i I I/S l m

distance between the membranes in a desalting cell (cm) coefficients of current density distribution equation in an electrodialyzer in Eq. (37) flow-pass width in a desalting cell (cm) electrolyte concentration (eq/cm3 ) evaporation multiple of evaporators energy (kcal/kg) electrodialytic energy consumption to obtain 1 t of NaCl (kWh/t NaCl) total energy consumption to obtain 1 t of NaCl (kWh/t NaCl) Faraday constant (A/eq.) electric power shortage in a salt manufacturing plant = purchased electric power (kW) generated electric power (kW) enthalpy difference (kcal/kg) enthalpy (kcal/kg) current density (A/cm2 ) electrical current (A) average current density of an electrodialyzer (A/cm2 ) flow-pass length in a desalting cell (cm) quantity of ions transported across a membrane pair (eq/(cm2 h))

Y. Tanaka et al. / Journal of Membrane Science 222 (2003) 71–86

n N P q Q r r, s, t rA R "St S S St ¯t T u u¯ V Vcell w W x y Y

number of evaporators (unit) number of membrane pairs (pair) production capacity of electrolytes of a salt manufacturing plant (t/h) quantity of solutions transported across a membrane pair (cm3 /(cm2 h)) production capacity of concentrated solutions of a salt manufacturing plant (m3 /h) evaporation multiple per unit evaporator constants of limiting current density equation of an ion-exchange membrane in Eq. (48) equivalent ratio of ions A to total ions in a solution concentrated by an electrodialyzer gas constant (VA/(K mol)) equivalent quantity of steam for generating ∆G (t/h) membrane area (cm2 ) entropy (kcal/(kg K)) quantity of steam supplied to the heater in an No. 1 evaporator (t/h) transport number of an ion-exchange membrane temperature (◦ C, K) solution velocity in a desalting cell (cm/s) average solution velocity in all desalting cells in a stack (cm/s) voltage difference between electrodes (V) cell voltage (V/membrane pair) evaporated steam to obtain 1 t of NaCl (t H2 O/t NaCl) quantity of steam evaporated in all evaporators (t H2 O/h) distance from the inlet of a desalting cell (cm) yield rate in an evaporator frequency of solution velocity ratio ξ in a stack

Greek letters γ ε ζ η ηgene ηturbine θ κ λ Λ µ v ξ ρ σ φ Φ ϕ

activity coefficient electrical current screening ratio of a spacer current density non-uniformity coefficient in Eqs. (38) and (39) current efficiency generator efficiency turbine efficiency NaCl purity (g NaCl/g electrolyte) electrical specific conductivity (mS/cm) overall transport number of an ion-exchange membrane pair (eq/Ah) quantity of heat (kcal/kg) overall permeability coefficient of an ion-exchange membrane pair (cm/h) steam latent heat in evaporation (kcal/kg) solution velocity ratio in desalting cells [(u − u)/ ¯ u] ¯ overall osmotic coefficient of an ion-exchange membrane pair (cm4 /(eq. h)) standard deviation of the distribution of solution velocity ratio ξ in desalting cells overall electro-osmotic coefficient of an ion-exchange membrane pair (cm3 /Ah) quantity of energy consumption (Mcal/h) electrical resistance (' cm2 )

85

86

Subscripts A alt back con cond condense de dir evap gen in K lim memb o out sat "G

Y. Tanaka et al. / Journal of Membrane Science 222 (2003) 71–86

anion-exchange membrane alternating current back-pressure of back-pressure turbine concentrated solution in an electrodialyzer condensate from a heater in a No. 1 evaporator condensing turbine desalted solution in an electrodialyzer direct current evaporated steam from an No. 1 evaporator generated electric power by a back-pressure turbine inlet of a desalting cell in an electrodialyzer cation-exchange membrane limiting current density ion-exchange membrane standard outlet of a desalting cell in an electrodialyzer saturation current density purchased electricity = electric power shortfall

Superscripts 



∗ ∗

desalting cell in an electrodialyzer concentrating cell in an electrodialyzer concentrated solution moving across an ion-exchange membrane in an electrodialyzer steam supplied from a boiler

References [1] S. Ishizaka, Seawater desalination and by-product utilization, Soda and Chlorine 20 (1969) 1. [2] T. Goto, Combination system of seawater desalination and sodium chloride electrolysis, in: Proceedings of the 51st Annual Meeting of the Society of Seawater Science, Japan, 2000, p. 57. [3] Y. Tanaka, R. Ehara, S. Itoi, T. Goto, Ion-exchange membrane electrodialytic salt production using brine discharged from a reverse osmosis seawater desalination plant, in: Proceedings of the 52nd Annual Meeting of the Seawater Science, Japan, 2001, p. 23. [4] Y. Tanaka, Regularity in ion-exchange membrane characteristics and concentration of seawater, J. Membr. Sci. 163 (1999) 277. [5] Y. Tanaka, Mass transfer and energy consumption in ionexchange membrane electrodialysis of seawater, J. Membr. Sci. 215 (2003) 265. [6] Salt Production Technical Report, Japan Tobacco & Salt Public Corporation, 1977. [7] Y. Tanaka, M. Iwahashi, M. Kogure, Distribution of electrodialytic condition in an electrodialyzer and limiting current density, J. Membr. Sci. 92 (1994) 217.

[8] S. Itoi, private communication. [9] Y. Tanaka, Current density distribution and limiting current density in ion-exchange membrane electrodialysis, J. Membr. Sci. 173 (2000) 179. [10] Y. Tanaka, Current density distribution, limiting current density and saturation current density in ion-exchange membrane electrodialyzer, J. Membr. Sci. 210 (2002) 65. [11] T. Mastuo, A. Takeda, K. Hiroi, S. Saito, Concentration of brine of high sodium chloride content with ion exchange membrane method, Bull. Soc. Sea Water Sci. Jpn. 22 (1968) 307. [12] Y. Tanaka, Concentration polarization in ion-exchange membrane–the events arising in flowing solution in a desalting cell, J. Membr. Sci. 216 (2003) 149. [13] Feasibility Study to Produce Salt for Industrial Use, Research Group on Electrodialysis and Membrane Separation Technology, Society of Seawater Science, Japan, 1994. [14] Numerical Table for Salt Production by Ion-Exchange Membrane Electrodialysis, Japan Tobacco & Salt Public Corporation, 1977.