Sublayer structure and reflection coefficient and their effects on concentration polarization and membrane performance in FO processes

Journal of Membrane Science 376 (2011) 214–224

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Sublayer structure and reflection coefficient and their effects on concentration polarization and membrane performance in FO processes Jincai Su, Tai-Shung Chung ∗ Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore

a r t i c l e

i n f o

Article history: Received 19 February 2011 Received in revised form 13 April 2011 Accepted 15 April 2011 Available online 22 April 2011 Keywords: Cellulose acetate Forward osmosis Sublayer structure Water flux Hollow fiber membrane

a b s t r a c t We have demonstrated through experimental and theoretical study that membrane sublayer structure has significant influence on the performance in forward osmosis (FO) processes. Cellulose acetate (CA) hollow fiber membranes with different sublayer structures have been fabricated by varying the bore fluid composition during dry-jet wet spinning and then characterized in terms of pore structure of the selective layer, porosity of the sublayer, pure water permeability (PWP) coefficient, salt permeability coefficient and salt rejection. The water flux and reverse salt flux of these membranes are evaluated in the pressureretarded osmosis (PRO) mode and FO mode. It is observed that varying the sublayer structure does not have apparent influence on the FO performance under the PRO mode but has significant influence on the performance under the FO mode. The different performance under different operation modes is resulted from more severe concentration polarization (CP) in the FO mode. The characteristics of the membrane sublayer structure are the origin of CP phenomenon in the FO mode. Through NF tests and FO tests with low draw solution concentrations on one of the membranes, the reflection coefficients ( 0 ) are determined as 0.77 and 0.99 for NaCl and MgCl2 draw solutes, respectively. A low reflection coefficient indicates a low effective driving force in FO processes resulted from serious leakage of draw solutes from the draw solution to the feed. Thus, the reflection coefficient cannot be assumed to be equal to 1 and its significance must be considered when modeling FO performance using NaCl as the draw solute. The theoretical study reveals that the sublayer porosity, tortuosity and thickness have great impact on membrane performance when the draw solution flows against the sublayer. The desired FO membrane should have no sublayer. Since the phase inversed membrane may always have a sublayer, the preferred sublayer has high porosity, low tortuosity and small thickness. However, FO membranes for water reuse may have less structure requirements than those for seawater desalination. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In pressure-driven membrane separation processes such as reverse osmosis (RO) and nanofiltration (NF), concentration polarization (CP) is defined as the increase in solute concentration at the membrane surface due to the accumulation of solutes that are rejected and it would result in a significant drop in water flux [1,2]. CP is reversible and may be effectively reduced by increasing shear rate and turbulence of the flow, pulsation or ultrasound. For the forward osmosis (FO) process, CP also exists but works in a different way. It is named as external concentration polarization (ECP) if CP occurs at the membrane surface or internal concentration polarization (ICP) if CP occurs within the membrane sublayer. Since the feed is always concentrated and the draw solution is always diluted in the FO process, there exists concentrative CP and dilutive CP at

∗ Corresponding author. Tel.: +65 6516 6645; fax: +65 6779 1936. E-mail address: [email protected] (T.-S. Chung). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.04.031

the feed and draw solution sides, respectively. FO membranes are mostly asymmetric with the feed and the draw solution flowing co-currently or counter-currently along different sides of the membrane. When the feed flows against the membrane selective layer and the draw solution flows against the sublayer, namely FO mode, there exists concentrative ECP at the feed side and dilutive ICP at the draw solution side (Fig. 1 right). Similarly, there will be concentrative ICP and dilutive ECP when the feed solution flows against the sublayer while the draw solution flows against the selective layer, i.e., pressure-retarded osmosis (PRO) mode (Fig. 1 left). The adjustment of velocity may largely suppress the influence of ECP [2]. However, the influence of ICP cannot be effectively reduced because ICP occurs and exists within the porous sublayer instead of at membrane surface [3]. The impact of membrane sublayer has been studied in gas separation and pervaporation processes. Either the selectivity or the permeation rate was found to be significantly affected by the sublayer [4–8]. Loeb et al. and Masse et al. reported that membrane sublayer had certain effect on the performance of RO membranes

J. Su, T.-S. Chung / Journal of Membrane Science 376 (2011) 214–224

215

Fig. 1. Illustration of driving force for (a) the PRO mode and (b) the FO mode in FO processes.

[9,10]. Currently, intensive studies are being conducted on the improvement of FO membrane selective layer [11–17]. It was observed that water flux dropped significantly when changing the feed to a 3.5 wt.% NaCl solution [11]. The ICP phenomenon occurred within the membrane sublayer should be account for this drastic reduction in water flux. Even so, not much attention has been given to this area. A very important parameter for osmotic processes is the reflection coefficient ( 0 ). It is related to the efficiency of solute rejection and has been considered as a quantitative indicator of the real membrane departing from an ideal semipermeable membrane [18,19].  0 is equal to 1 when the membrane completely rejects the solutes while it is zero if the membrane allows all the solutes freely permeate through. In almost all FO studies,  0 is often assumed to be unity due to the high rejection achieved by the membranes [20–22]. Nonetheless, solute permeability coefficient, which is a quantitative indicator of the amount of draw solutes reversely permeating through the membrane from the draw solution to the feed, is nonzero and is a very important parameter and widely used in the theoretical analyses [20–22]. Assuming  0 to be unity seems to be somewhat contradictory. Even though a membrane may show very high rejections to certain draw solutes, the value of  0 should be between 0 and 1 because draw solutes cannot be completely excluded. The reverse permeation of draw solutes decreases the osmotic pressure difference (driving force) across the membrane and results in a lower water flux in FO processes. Unfortunately, the importance of  0 has been overlooked in most FO studies. In FO processes, the sublayer also has great impact on membrane performance as evidenced by experimental observation that the water flux for the FO mode (i.e., when the draw solution flows against the sublayer) is much lower than that for the PRO mode (i.e., when the draw solution flows against the selective layer) [14–17,23,24]. For FO membranes, the influence of the sublayer is mainly represented by the existence of ICP whenever the draw solution or the feed flows against the sublayer. The objectives of this study are to investigate the fundamentals of ICP and explore the science bridging the sublayer structure and its effects on water flux under hydraulic and osmosis testing modes. Using cellulose acetate (CA) as the raw material, hollow fiber FO membranes with

different sublayer structures are fabricated. To correlate the performance of these membranes under hydraulic and osmosis pressures, the as-prepared membranes are firstly characterized by NF experiments for their pure water permeability (PWP), salt rejection and pore structural characteristics. Then the relationship between the sublayer morphology and the water flux under osmotic pressures are investigated with the aid of those characteristic parameters obtained under hydraulic pressures. Reflection coefficient is for the first time introduced into the study of FO membranes. Its value is determined through NF and FO experiments and is used to characterize the as-prepared FO membranes. A theoretical study on the water flux is also carried out by varying the thickness, porosity and tortuosity of the membrane sublayer. 2. Theoretical 2.1. Concentration polarization In FO processes, the feed and the draw solution flow separately at different sides of the membrane as shown in Fig. 1. For both solutions, the concentrations at the solution–membrane interfaces are different from those of the bulk due to dilution or concentration effects. As a result, the effective driving force (osmotic pressure difference) across the membrane is reduced, i.e., 2 − 3 < 1 − 4 . The water flux Jw is described as [25]: Jw = A(0  − P)

(1)

where A is the water permeability coefficient,  0 is the reflection coefficient,  is the difference in the osmotic pressure across the membrane and P is the difference in the hydraulic pressure across the membrane. In case the draw solution flows against the sublayer (FO mode), a dilutive ICP phenomenon occurs at the draw solution side. As a result, the actual concentration of the draw solution at the sublayer-selective layer interface is lower than that of the bulk and can be expressed as follows [20]: 2 = exp(−Jw K) 1

(2)

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Here, K is the solute resistivity for the diffusion within the porous sublayer defined as [20]:

where Ds is the solute diffusion coefficient while ı,  and ε are the thickness, tortuosity and porosity of the sublayer, respectively. In the PRO mode, dilutive ECP occurs and the draw solution concentration at the membrane surface (C2 ) is much lower than that of the bulk (C1 ) [3,20]:

unreasonable to assume  0 to be unity. In reality, a FO membrane should have different  0 values for draw solutes with different sizes or valences. Including  0 in these equations would precisely describe the importance of solute permeation and its influence on effective driving force and the water flux. The reflection coefficient (Eq. (6)) is derived from the pore flow model and is introduced into Eq. (9), which is derived based on the solution diffusion mechanism [28]. There may be a better way to include the reflection coefficient in water flux modeling and further investigation is to be conducted in future work.

2 Jw = exp − 1 k

3. Experimental

K=

ı Ds ε

(3)





(4)

k is the mass transfer coefficient and has an expression of [26]: ShDs k= dh

3.1. Chemicals (5)

where Sh is the Sherwood number and dh is the hydraulic diameter. 2.2. Determination of the reflection coefficient and the water permeability coefficient The reflection coefficient can be obtained in two ways [27]:

 P  ∞

0 = −

∞

Jw =0

=

 J w



A∞

(6)

P ∞ =0

In the PRO mode, the water flux is described as: Jw = A(0 2–3 − P)

(7)

The reverse solute flux refers to the amount of draw solute reversely leaking to the feed and is described as [28]: −Js = B(C2 − C3 )

(8)

where B is the solute permeability coefficient (L m−2

h−1 ). The neg-

ative sign for the solute flux indicates that the flow of draw solutes is opposite to that of the water flow. The reflection coefficient is not included in Eq. (8) since the solute permeability coefficient and the reflection coefficient represents the same membrane property, i.e., deviating from ideal semipermeable membrane due to draw solute leakage, in different ways. The value of A can be determined through RO tests using DI water as the feed. Considering the FO process with no pressure difference across the membrane (P = 0), Eq. (7) becomes: Jw = A0 2–3 = A0 (2 − 3 )

(9)

For the porous sublayer, the reverse solute flux can also be written as [29]: −Js = Ds ε

dC(x) − Jw C(x) dx

(10)

where Ds is the diffusion coefficient of the draw solute and ε is the porosity of the sublayer. Combining Eqs. (8) and (10) gives B(C2 − C3 ) = Ds ε

dC(x) − Jw C(x) dx

(11)

The solution of the above equation for the PRO mode is [9] Jw =

1 K

ln

B + A0 2 − Jw B + A0 4

(12)

For the FO mode, the water flux has an expression of [9] Jw =



The polymer used for the fabrication of cellulose acetate FO hollow fiber membranes was CA-398-30 provided by Eastman Chemicals. Having high glass transition temperatures and being able to produce tough and hard films [30], this material has been extensively used for RO membranes and recently studied for FO applications [24]. A mixture of acetone and formamide was used as the solvent. A mixture of N-methyl-2-pyrrolidone (NMP) and deionized (DI) water was used as the bore fluid for spinning. Acetone (99.5%), formamide (99.5%), NMP (99.5%), NaCl (99.5%) and MgCl2 ·6H2 O (99%) were all purchased from Merck, Germany. The solutes used for the pore structural characterization were ethylene glycol (99.8%), diethylene glycol (99%), triethylene glycol (99%) and glucose (99.5%) provided by Sigma Aldrich. Industrial grade glycerol was purchased from Aik Moh Pains & Chemicals Pte. Ltd. (Singapore). DI water was obtained from a PURELAB (Elga, UK) system.

1 B + A0 1 ln K B + Jw + A0 3



(13)

It should be noted that Eqs. (12) and (13) are different from those developed by Loeb et al. [9] and used by some recent studies [20–22] because the reflection coefficient is included. Since the permeability coefficient (B) for draw solutes really exists, it is

3.2. Fabrication of cellulose acetate FO hollow fiber membranes The CA powders were dried overnight at 70 ◦ C in a vacuum oven to remove the moisture. The dope solution for spinning was prepared by dissolving a certain amount of CA powders in the mixture of acetone and formamide at room temperature. The weight percentages of CA, acetone and formamide in the dope solution were 25, 45 and 30, respectively. To avoid the evaporation of acetone, the mixing and dissolving were conducted in a 500 mL blue-cap bottle sealed with para-film. The blue-cap bottle was mounted on a rotator (STR4, Stuart) and was rotated for several days until a homogeneous solution was obtained. The CA hollow fiber membranes were fabricated through a dry-jet wet-spinning process. A detailed description of the spinning system was given elsewhere [31]. Three groups of hollow fiber membranes were fabricated with different sublayer structures through varying the concentration of NMP in the bore fluid. The detailed spinning conditions are summarized in Table 1. The as-spun fibers were immersed in water for several days to remove the residual solvents. After phase inversion, the hollow fiber membranes were annealed in a water bath at a temperature of 85 ◦ C for 10 min and the annealed samples were designated as SUB-90, SUB-70 and SUB-50, respectively. For module fabrication, the membranes were soaked in a 50 wt.% glycerol aqueous solution for 2 days and thoroughly air-dried at room temperature. 20 pieces of hollow fibers with a length around 30 cm each were bundled into a ˚3/8 in. perfluoroalkoxy tubing and the two ends were sealed with epoxy resin to assemble the membrane module. 3.3. Characterizations Some annealed hollow fibers were freeze-dried overnight using a freeze-dryer (Modulyod, Thermo Electron Cor.) at −50 ◦ C. The morphology of hollow fibers was inspected using Field Emission Scanning Electronic Microscopy (FESEM, JOEL JSM-6700).

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217

Table 1 Spinning conditions of the cellulose acetate FO hollow fiber membranes. Sample ID

SUB-90

SUB-70

SUB-50

Dope solution (wt.%)

CA(398-30)/acetone/formamide (25/45/30 wt.%) 0.12 90/10 wt.% NMP/Water 25 ◦ C 0.06 0.05 Water, 25 ◦ C 60–70% 0.12

CA(398-30)/acetone/formamide (25/45/30 wt.%) 0.12 70/30 wt.% NMP/water 25 ◦ C 0.06 0.05 Water, 25 ◦ C 60–70% 0.12

CA(398-30)/acetone/formamide (25/45/30 wt.%) 0.12 50/50 wt.% NMP/water 25 ◦ C 0.06 0.05 Water, 25 ◦ C 60–70% 0.12

Dope flow rate (L h−1 ) Bore fluid, temperature ( ◦ C) Bore flow rate (L h−1 ) Length of air gap (m) External coagulant, temperature (◦ C) Spinning humidity (%) Take up speed (m s−1 )

The membrane modules were firstly subjected to the measurement of PWP using a lab-scale membrane filtration set-up described elsewhere [23]. Subsequently, the membrane modules were subjected to the 0.02 wt.% neutral solutes (ethylene glycol, diethylene glycol, triethylene glycol and glucose) and 0.001 M salts (NaCl and MgCl2 ) separation tests with different feed solutions flowing against the membrane outer surface. The permeate was collected from the lumen side of the membrane module. All nanofiltration experiments were carried out at a transmembrane pressure of 1.0 bar at room temperature. The concentrations of each solute in the feed solution and in the permeate were determined using a Total Organic Carbon Analyzer (TOC-VCSH , Shimadzu, Japan). The concentrations of each salt in the feed solution and in the permeate were measured using an electric conductivity meter (Lab 960, Schott). The measured concentrations of the feed, Cf , and the permeate, Cp , were used to calculate the effective solute rejection coefficient RE (%):



RE =

Cp 1− Cf



× 100%

(14)

The mean pore radius and the probability density function curve of the pore radius distribution were obtained from the rejections to the neutral solutes. A detailed description of the pore structural characterization was given elsewhere [24]. The porosity, ε, of hollow fiber membranes was calculated from the apparent density, a , and the true density, t , through the following equation: ε=



1−

a t



× 100%

(15)

where the apparent density was determined by the ratio of the overall weight to the overall volume of these fibers. The true density was measured using an automatic gas pycnometer (ULTRAPYC 1200e, Quantachrome). Using the membrane SUB-90 as an example, the value of pure water permeability coefficient (A) in Eq. (7) was also determined through NF experiments. Using DI water as the feed, the water flux was measured at transmembrane pressures ranging from 0.14 to 1.03 bar. The value of A can be determined from the straight line by plotting the water flux versus transmembrane pressure. To determine the salt permeability coefficient (B), the rejections to 0.001 M NaCl and 0.001 M MgCl2 solutions were measured at pressures of 0.50–1.75 bar. 3.4. Performance in the FO process The performance of CA hollow fiber membranes was tested using a lab-scale FO set-up [24]. Each membrane module had a filtration area of about 80 cm2 . Three batches of FO tests were carried out. The first batch of FO tests was conducted on the SUB-90 membrane in the PRO mode with 0.01–0.06 M NaCl and 0.005–0.03 M MgCl2 draw solutions in order to determine the reflection coefficients ( 0 ). The purpose of using low draw solution concentrations

was to minimize the influence of ECP. The second batch of FO tests was conducted with all the three groups of membranes using 0.5–2.0 M MgCl2 draw solutions and DI water feed. The third batch of FO tests was also conducted on the SUB-90 membrane with 2.0 M MgCl2 draw solution and 0.59 M (3.5 wt.%) NaCl feed solution. For all FO tests, the draw solution and the feed were counter-currently circulated in corresponding flow channels. The flow rate at the shell side was 18 L h−1 while the flow rate at the lumen side was 1.62 L h−1 in order to assure the pressure at the lumen side lower than 0.14 bar. The amount of water permeating from the feed to the draw solution over 0.5 h was measured using a digital balance. The water permeation flux, Jw (L m−2 h−2 or LMH), was then determined based on the weight change of the feed and the effective membrane area as follows: Jw =

m 1 t Am

(16)

where Am (m2 ) is the effective membrane area and m (kg) is the weight of water permeated from the feed to the draw solution over a predetermined time t (h) during FO tests. Since salts are the draw solutes in this study, the term “reverse salt flux” instead of reverse solute flux is used. The value of Js was determined from the increase in the feed conductivity: Js =

[(Ct Vt ) − (C0 V0 )] Mw 1 Am t

(17)

where C0 (M) and V0 (L) are the initial salt concentration and the initial volume of the feed, respectively, while Ct (M) and Vt (L) are the salt concentration and the volume of the feed over a predetermined time t (h), respectively, during FO tests. 4. Results and discussion 4.1. Morphology of the membranes Fig. 2(a) shows the sublayer morphology of the three groups of hollow fiber membranes. It is very evident that with decreasing NMP content in bore fluid from 90 wt.% to 70 wt.% and 50 wt.%, the sublayer becomes less porous. The corresponding membrane thickness decreases from 118 ␮m to 105 and 94 ␮m, respectively. Fig. 2(b) shows the cross-section, inner surface and sublayer of the SUB-90 membrane. Clearly, the inner surface and the sublayer are highly porous, so they may create very little resistance for water permeation. Such highly porous structure is induced by the delayed phase inversion during the spinning because the bore fluid (NMP/water mixture) contains 90 wt.% NMP [32–35]. The selective outer skin is very dense as verified by the FESEM picture at a higher magnification. It should be noted that the cracks at the outer surface are resulted from burning during FESEM focusing. Since water is used as the external coagulant in the spinning, phase inversion occurs immediately once the nascent fiber enters the coagulant. In consequence, a very dense outer skin is formed. This outer skin would act as a semi-permeable barrier, allowing water to permeate through while partially rejecting other components.

218

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Fig. 2. Morphology of the CA hollow fiber membranes: (a) inner surface of the SUB-90, SUB-70 and SUB-50 membranes; (b) cross section, inner surface and outer surface of the SUB-90 membrane.

line is obtained by plotting the water flux versus transmembrane pressure. The slope of this line gives the value of PWP. These membranes show rejections of 81.59%, 83.68% and 87.22%, respectively, to NaCl and 74.16%, 79.6% and 86.37%, respectively, to MgCl2 . It should be noted that all the three membranes show lower rejections to MgCl2 than NaCl. Donnan exclusion caused by the negative membrane charge in dilute salt solutions may account for this phenomenon [36,37]. The mean pore radii of the SUB-90, SUB-70 and SUB-50 membranes are 0.33, 0.32 and 0.32 nm, respectively, and the corresponding MWCO are 254, 237 and 237 Da, respectively. Correspondingly, the sublayer porosities are 71.70%, 68.27% and 65.55%, respectively. Also, the three membranes nearly have identical pore size distribution curves (Fig. 4). It seems that changing the bore fluid composition does not apparently affect the pore size distribution of the pores within the membrane selective layer.

1.2

Water flux (LMH)

1.0 0.8 0.6 0.4 0.2 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

ΔP (bar) Fig. 3. Water flux of the SUB-90 membrane under different transmembrane pressures.

4.3. FO performance

4.2. Membrane characterization

Fig. 5 shows the water flux of the SUB-90 membrane under FO tests using dilute draw solutions. The purpose of using dilute draw solutions is to reduce the influence of CP. Plotting the water flux against the osmotic pressure difference across the membrane gives a straight line. Based on Eq. (9), the slope of this line is exactly the product of pure water permeability coefficient (A) and the reflection coefficient ( 0 ). As a result, the reflection coefficients of this membrane can be determined as 0.77 and 0.99 for NaCl and MgCl2 draw solutes, respectively. The low reflection coefficient for NaCl

Table 2 gives the results obtained from NF tests. Under transmembrane pressures ranging from 0.14 to 1.03 bar, the PWP of SUB-90, SUB-70 and SUB-50 membranes are determined as 0.97, 0.96 and 0.90 LMH/bar, respectively. As an example, Fig. 3 gives the NF data based on the SUB-90 membrane. Using DI water as the feed solution, the water flux is proportional to the transmembrane pressure applied to the membrane. Therefore, a straight Table 2 Characterization results of SUB-90, SUB-70 and SUB-50 membranes. Membrane

OD/ID/thickness (␮m)

A @ 1 bar (LMH/bar)

rp (nm)

MWCO (Da)

RE to NaCl @ 1 bar (%)

RE to MgCl2 @ 1 bar (%)

Porosity (ε) (%)

SUB-90 SUB-70 SUB-50

682/447/118 655/445/105 633/445/94

0.97 0.96 0.90

0.33 0.32 0.32

254 237 237

81.59 83.68 87.22

74.16 80.80 86.37

71.70 68.27 65.55

J. Su, T.-S. Chung / Journal of Membrane Science 376 (2011) 214–224

SUB-90 SUB-70 SUB-50

3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.00

0.15

0.30

0.45

0.60

0.75

Pore radius (nm) Fig. 4. Pore size distribution curves of the SUB-90, SUB-70 and SUB-50 membranes.

2.1 Draw solution NaCl MgCl2

1.5 1.2 0.9 0.6 0.3 0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

∆π (bar ) Fig. 5. Water flux of the SUB-90 membrane using dilute draw solutions.

implies that this membrane is not a good semipermeable membrane using NaCl draw solutions [18,19]. Since a reflection of 0.99 is obtained for MgCl2 , the subsequent FO tests and theoretical study are based on MgCl2 draw solutions. The performance of the SUB-90, SUB-70 and SUB-50 membranes using 0.5–2.0 M MgCl2 draw solutions and DI water feed is shown in Fig. 6. It is evident that all membranes exhibit higher water fluxes in the PRO mode than those in the FO mode. The reverse salt fluxes for both operation modes are very low. With increasing draw solution concentration, the initial osmotic pressure gradient increases, and

Osmotic pressure (bar)

Osmotic pressure (bar)

Waater flux (LM MH)

40

92.5

165.2

256.5

Water flux Reverse salt flux SUB-90 SUB-90 SUB-70 SUB-70 SUB-50 SUB-50

5

4

30

3

20

2

10

1

0 0.0

0.5

1.0

1.5

2.0

0 2.5

12

Reverse salt flux (gMH)

50

38.3

Waater flux (LM MH)

Water flux (LMH)

1.8

the observed water flux in the PRO mode increases significantly. However, the water flux in the FO mode increases with increasing draw solution concentration from 0.5 M to 1.0 M but does not change much with further increase in the draw solution concentration. As shown in Fig. 1, dilutive ICP occurs when running the draw solution against the porous sublayer (lumen side of the hollow fiber membrane) and dilutive ECP occurs when running the draw solution against the selective layer (shell side of the hollow fiber membrane). The dilution effect is more serious in the FO mode than in the PRO mode because the dilution effect in the former case occurs within the membrane sublayer while in the latter case the dilution effect occurs at the membrane surface. Also, the flow rate at the lumen side is 0.14 m s−1 , which is lower than that at the shell side (0.20 m s−1 ). Consequently, the effective driving force in the PRO mode is higher than that in the FO mode under the same draw solution and feed concentrations. Under both membrane orientations, a higher draw solution concentration tends to create a higher water flux and thus more severe CP, i.e., greater dilution effect, occurs. The situation is worse in the FO mode since CP exists within the porous membrane sublayer. In the FO mode, the transfer of draw solutes from the bulk to the draw solution within the sublayer is through diffusion. Since the draw solute supplementation through diffusion is slower than the dilution, the draw solution concentration at the interface of the selective layer and the sublayer is much lower than that of the bulk and the difference becomes more significant at higher bulk concentrations. That is to say, the effective driving force does not increase proportionally to the increase in the bulk concentration. This explains why the water flux does not increase significantly when the draw solution concentration is higher than 1.0 M (Fig. 6(b)). In order to find out whether flow rate affects the performance in the FO mode, the water flux was tested based on 0.5 M MgCl2 draw solution at flow rates of 1.6, 3.0, 4.5, and 6.0 L h−1 , respectively. Fig. 7 shows that increasing flow rate from 1.6 L h−1 to 3.0 and 4.5 L h−1 results in slightly enhanced water flux but further increase in flow rate does not affect water flux. This indicates that the flow rate of the draw solution has certain effect on the performance although ICP always exists. When the flow rate is low, the replenishment of the diluted draw solution by a fresh draw solution is slow. Therefore, the concentration of draw solution at the interface of the membrane and the sublayer remains low. Increasing flow rate of the draw solution helps to keep a comparatively higher draw solution concentration at the interface. This explains the slightly enhanced water flux at 3.0 and 4.5 L h−1 . When the flow rate exceeds certain level, its effect on enhancing water flux is neg-

9

38.3

92.5

165.2

256.5

Water flux Reverse salt flux SUB-90 SUB-90 SUB-70 SUB-70 SUB-50 SUB-50

6.0

4.5

6

3.0

3

15 1.5

0 0.0

0.5

1.0

1.5

MgCl 2 concentration (M)

MgCl 2 concentration (M)

(a) PRO mode

(b) FO mode

2.0

Reverrse salt flux (gMH)

-1

Probability density function (nm )

3.5

219

0.0 2.5

Fig. 6. Performance of the SUB-90, SUB-70 and SUB-50 membranes in PRO and FO modes. Draw solution: 0.5–2.0 M MgCl2 , feed: DI water.

220

J. Su, T.-S. Chung / Journal of Membrane Science 376 (2011) 214–224

9

3

6

2

3

1

0

2

4

6

8

Reverse salt flux (gMH)

Water flux (LMH)

Water flux Reverse salt flux

0

Table 3 Diffusion coefficients of draw solutes at different concentrations.a

4

12

MgCl2 (25 ◦ C) NaCl (25 ◦ C) Sucrose (30 ◦ C)

ligible because it no longer influences the dilution effect within the porous sublayer because ICP is dominant. It should be noted that the porosity as well as the membrane thickness are changed by varying bore fluid composition during spinning (Table 2). As a result, the PWP and rejections to NaCl and MgCl2 also change. However, the SUB-90, SUB-70 and SUB-50 membranes show quite similar performance in the PRO mode, indicating that slight variations in porosity and membrane thickness have very little impact on the PRO performance. On the contrary, the variation of sublayer porosity shows significant influence on the FO performance (Fig. 6). Clearly, a high porosity is advantageous for the enhanced diffusion of draw solutes as well as water. Although the SUB-50 membrane has the thinnest sublayer, it shows the lowest water flux. This indicates that sublayer porosity has greater impact than sublayer thickness. With 0.59 M (3.5 wt.%) NaCl feed and 2.0 M MgCl2 draw solution, the SUB-90 membrane achieves water fluxes of 9.98 LMH in the PRO mode and 1.54 LMH in the FO mode. Apparently, the water fluxes in both modes are much lower than those obtained when using DI water as the feed. Fig. 8 shows the weight loss of the feed solution as a function of time. One interesting observation is that in the beginning of the test the weight change in the feed is higher in the PRO mode but is lower in the FO mode. This is because ECP takes effect only after the formation of boundary layer in the PRO mode and the draw solution concentration within the sublayer in the FO mode needs sometime to reach its highest level with draw solutes diffusing into the sublayer. The lower water flux in the FO modes further proves the more severe influence of ICP than ECP.

50 Draw solution: 2M MgCl 2

Weigh loss of feed (g)

Feed: 0.59M (3.5 wt.%) %) NaCl PRO mode FO mode

30 20 10 0 0.2

0.3

0.4

0.5

0.6

Time (h) Fig. 8. Desalination test through the SUB-90 membrane in the PRO mode.

Salt rejection (%)

80

Fig. 7. Effect of draw solution flow rate on the performance of the SUB-90 membrane in the FO mode. Draw solution: 0.5 M MgCl2 , feed: DI water.

0.1

2.5 M

1.06 1.47 0.19

1.08 1.54 0.0015

100

0

Flow rate of draw solution (L h )

0.0

0.5 M

a The diffusion coefficients for MgCl2 and NaCl were from Ref. [43] and the diffusion coefficients for sucrose were from Ref. [44].

-11

40

Diffusion coefficient (×10−9 m2 s−1 )

Draw solutes

0.001M NaCl 0.001M MgCl 2

60

40

20

0 0.0

0.5

1.0

1.5

2.0

ΔP-σΔπ (bar) Fig. 9. Salt rejection of the SUB-90 membrane versus transmembrane pressure.

4.4. Modeling of the sublayer structure versus the performance In order to understand the impact of the membrane sublayer structure on the performance, modeling work is conducted based on the SUB-90 membrane in the FO mode using Eq. (13). The diffusion coefficients for different draw solutes are given in Table 3. The results from NF experiments carried out using 0.001 M NaCl or 0.001 M MgCl2 solutions are shown in Fig. 9. The salt permeability coefficient B is determined by a linear fitting of the results shown in Fig. 9 using Eq. (18) [38]: 1 1 − RE B = RE (P − 0 )A

(18)

Using this method, the SUB-90 membrane has B values of 0.19 LMH (or 5.28 × 10−8 m s−1 ) for MgCl2 and 0.15 LMH (or 4.17 × 10−8 m s−1 ) for NaCl, respectively. The value of K is determined as 0.19 (LMH)−1 (or 6.84 × 105 s m−1 ) by regression of experimental water flux (Fig. 6) through Eq. (12). With the sublayer thickness, porosity and diffusion coefficient of MgCl2 draw solutes, the tortuosity within the SUB-90 membrane sublayer is determined as 4.5. Based on the properties of the SUB-90 membrane, the water flux in the FO mode is calculated by varying one of the three parameters, i.e., thickness, porosity and tortuosity, of the sublayer using Eq. (13). Fig. 10 shows that an increase in sublayer thickness results in a decrease in water flux. The impact is greater when the concentration of the draw solution increases because the diffusion paths for water and draw solutes are lengthened. Consequently, the actual draw solution concentration at the interface between the selective layer and the sublayer is much lower than that of the bulk at the lumen side. This is quite different from pressure-driven membranes such as RO and NF processes in which only feed solution is involved. It is also observed that the water flux is more sensitive to the variation of the sublayer thickness when the sublayer thickness is low. To enhance the water flux, the sublayer thickness should be eliminated or as thin as possible if the necessary mechanical strength is guaranteed [39].

J. Su, T.-S. Chung / Journal of Membrane Science 376 (2011) 214–224

221

0.8

120

Water flux (LMH)

Draw solution 0.5M MgCl2

MgCl2 concentration 0.005M 0.02M 0.05M 0.2M 0.5M 1.0M 1.5M 2.0M 2.5M

90

2.5M MgCl2

0.6

0.5M NaCl 2.5M NaCl 0.5M Sucrose 2.5M Sucrose

0.4

60

0.2 30

0

Water flux/osmotic pressure (LMH/bar)

150

0.0 0

50

100

150

200

0

50

100

150

200

Thickness of sublayer (μm) Fig. 10. Effect of the sublayer thickness on water flux in the FO mode (calculation based on the SUB-90 membrane).

Higher sublayer porosity secures more diffusion channels for water as well as draw solutes. Thus, the water flux increases with increasing sublayer porosity (Fig. 11). If the concentration of the draw solution is low (e.g., ≤0.5 M), the impact of the porosity becomes less significant after the porosity reaches 70%. In the case of applying FO for water reuse, a low driving force (low draw solution concentration) may be preferred due to the low osmotic pressure of the feed. Thus, the sublayer porosity has less significant influence once the porosity reaches 70%. For desalination which needs a higher osmotic driving force (higher draw solution concentration), the sublayer porosity is more important since it still has significant effect on water flux even when the porosity exceeds 70%. From the point of membrane formation, normal casting or single-layer spinning may not achieve very high sublayer porosity. Flat sheet composite membranes casting from low dope polymer concentrations and hollow fiber membranes spun from dual-layer

co-extrusion may have greater sublayer porosity. These types of membranes may be useful for the FO membrane development with or without thin film polymerization. Tortuosity also has significant influence on the FO performance, i.e., low tortuosity being favorable for enhancing water flux (Fig. 12). Its impact becomes greater with increasing draw solution concentration. When using draw solutions with low concentrations (e.g., ≤0.5 M), the impact of tortuosity is not so vivid. Therefore, tortuosity may be not so harmful in the case of applying FO for water reuse. However, tortuosity is more important for desalination which needs a high draw solution concentration (high osmotic driving force). The sublayer is believed to create less resistance for the diffusion of water and draw solutes when the tortuosity is low. From Fig. 12, membrane sublayer contains straight pores that are parallel to each other or large amount fingerlike macrovoids are desirable. However, these structures may not

25 MgCl 2 concentration

20

Water flux (LMH)

Draw solution 0.5M MgCl 2

0.005M 0.02M 0.05M 0 2M 0.2M 0.5M 1.0M 1.5M 2.0M 2 5M 2.5M

15

2.5M MgCl 2

0.4

0.5M NaCl 2.5M NaCl 0.5M Sucrose 2.5M Sucrose

0.3

10

0.2

5

0.1

0

0.0 0

20

40

60

80

100

0

20

40

60

80

Water flux/osmotic pressure (LMH/bar)

0.5

100

Porosity of sublayer (%) Fig. 11. Effect of the sublayer porosity on water flux in the FO mode (calculation based on the SUB-90 membrane).

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MgCl 2 concentration 0.005M 0.02M 0.05M 0.2M 0.5M 1.0M 1.5M 2.0M 2.5M

Water flux (LMH)

40

30

Draw solution 0.5M MgCl 2 2.5M MgCl 2 0.5M NaCl 2.5M NaCl 0 5M S 0.5M Sucrose 2.5M Sucrose

0.45

0.30

20 0.15 10

0

Water flux/osmotic pressure (LMH/bar)

0.60

50

0.00 1

2

3

4

5

6

7

8

0

2

4

6

8

10

Tortuosity within sublayer Fig. 12. Effect of the sublayer tortuosity on water flux in the FO mode (calculation based on the SUB-90 membrane).

be mostly preferable because the existence of large amount of wide pores or macrovoids would greatly decrease the mechanical strength of the membrane. Furthermore, the stability of the membrane would gradually decline with frequent back washing during a long term operation or vibration mode. For comparison, the water fluxes normalized by osmotic pressure based on 0.5 M and 2.5 M draw solutions with different draw solutes, i.e., MgCl2 , NaCl and sucrose, are also calculated and included in Figs. 10–12. Under both draw solution concentrations, the FO performance of the three draw solutes is in the order of NaCl > MgCl2 > sucrose at the same sublayer thickness, porosity or tortuosity. Interestingly, this order is in consistent with the order

of diffusion coefficient. Therefore, the sublayer structure exhibits magnifying impact if the draw solute has a smaller diffusion coefficient. The dependency of ICP on the diffusion coefficient was also reported by Achilli et al. [45]. It should be noted that sucrose is almost totally rejected by the SUB-90 membrane in NF tests (data not shown here), thus sucrose draw solutes should have a B value of 0. Theoretically, no leakage of sucrose draw solutes from the draw solution to the feed secures higher effective driving force. However, the diffusion coefficient of sucrose molecules and the viscosity of sucrose solutions are much higher than the other two. Therefore, the influence of ICP is more severe when using sucrose draw solutions although the leakage of sucrose draw solutes can be ignored.

Fig. 13. Desirable membrane structure for FO processes prepared by the current phase inversion and/or interfacial polymerization processes [13–16,40–42].

J. Su, T.-S. Chung / Journal of Membrane Science 376 (2011) 214–224

Based on the modeling results, a desirable FO membrane should contain no sublayer. However, at the present it is hard to eliminate the sublayer of FO membranes prepared by phase inversion and/or interfacial polymerization processes. Under this constrain, Fig. 13 illustrates the relatively suitable sublayer structure. It should have a thin 3-dimentional fully porous and sponge-like sublayer to provide necessary robust mechanical strength without creating much resistance for water permeation. A sublayer consists of plenty of uniform and porous macrovoids may be one of the choices. However, its long term stability under frequent backwash and chemical cleans needs to be investigated. 5. Conclusions We have experimentally and theoretically studied the effect of membrane sublayer structure on the FO performance. Three groups of hollow fiber FO membranes have been fabricated by varying the bore fluid composition during spinning. The resultant membranes have different sublayer structures and show much lower water fluxes in the FO mode than the PRO mode due to more severe concentration polarization. Different in the sublayer structure, the three groups of membranes show very close water flux and reverse salt flux in the PRO mode but quite different performance in the FO mode. This is an indication that the sublayer structure has significant impact under the FO mode. With decreasing porosity and increasing tortuosity of the sublayer, water flux decreases. Lower porosity or higher tortuosity means a less and lengthened diffusion path for water and draw solutes. Thus, the dilution effect of the draw solution within the sublayer is more severe, i.e., more severe ICP. Increasing the draw solution flow rate is favorable for encouraging the turbulence and enhances water flux in the FO mode. However, the enhancement in water flux is negligible when the flow rate reaches certain level because ICP always exists and is dominant. Modeling results show that increasing porosity, decreasing thickness and decreasing tortuosity of the membrane sublayer are favorable for enhancing the water flux. The desired FO membrane should contain no sublayer. A very thin and strong but highly porous sublayer may be the preferred structure for FO membranes made by the current phase inversion and/or interfacial polymerization processes. In addition, we have found that one of the membranes studied has reflection coefficients ( 0 ) of 0.77 and 0.99 for NaCl and MgCl2 draw solutes, respectively. Since a low reflection coefficient indicates a low effective driving force in FO processes resulting from serious leakage of draw solutes from the draw solution to the feed, the reflection coefficient cannot be assumed to be equal to 1 and its significance must be considered when studying FO performance using NaCl as the draw solute. Acknowledgements The authors would like to thank Environmental and Water Industry Development Council (EWI), Singapore, National University of Singapore (NUS), and Eastman Chemical Company, USA, for funding this research with Grant Numbers of MEWR 651-06-158, R279-000-271-272 and R-279-000-315-597. Special thanks are due to Dr. May May Teoh, Ms. Sing Yee Nah, Mr. Zhi Chen and Ms. Zhengjun Tang for their valuable assistance.

Nomenclature A Am B

pure water permeability coefficient (L m−2 h−1 ) effective membrane surface area (m2 ) salt permeability coefficient (L m−2 h−1 )

223

solute concentration (mol L−1 ) initial salt concentration of the feed (mol L−1 ) solute concentration in the feed solution (mol L−1 ) solute concentration in the permeate (mol L−1 ) salt concentration in the feed after the operation time interval (mol L−1 ) C2 concentration of the draw solution at the membrane surface or the interface between the membrane selective layer and the sublayer (mol L−1 ) C3 concentration of the feed solution at the membrane surface or the interface between the membrane selective layer and the sublayer (mol L−1 ) Ds solute diffusion coefficient (m2 s−1 ) hydraulic diameter (m) dh Js reverse solute leakage (g m−2 h−1 ) Jw product water flux (L m−2 h−1 ) k mass transfer coefficient (m s−1 ) K solute resistivity for diffusion within porous sublayer (m2 h L−1 ) m permeation water weight gain (kg) Mw molecular weight of the draw solute (g mol−1 ) MWCO molecular weight cut off (g mol−1 ) P, P∞ pressure difference of bulk solutions in the FO process (bar) rp pore radius (nm) RE effective solute rejection coefficient (%) Sh Sherwood number t the operation time interval (h) V0 initial volume of the feed (L) Vt volume of the feed after the operation time interval (L) C C0 Cf Cp Ct

Greek ı ε 1 2

3

4 2–3 2–3 ∞ a t 0 

thickness of the membrane sublayer (␮m) porosity of the membrane sublayer (%) osmotic pressure of the draw solution in the bulk (bar) osmotic pressure of the draw solution at the membrane surface or the interface between the membrane selective layer and the sublayer (bar) osmotic pressure of the feed solution at the membrane surface or the interface between the membrane selective layer and the sublayer (bar) osmotic pressure of the feed in the bulk (bar) osmotic pressure difference in the FO process (bar) effective osmotic pressure difference in the FO process (bar) osmotic pressure difference of bulk solutions in the FO process (bar) apparent density of the membrane (g cm−3 ) true density of the membrane (g cm−3 ) reflection coefficient tortuosity

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