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Membrane with integrated spacer
Journal of Membrane Science 360 (2010) 185–189
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Membrane with integrated spacer J. Balster a,1 , D.F. Stamatialis a,b,∗ , M. Wessling a a
Institute of Mechanics, Process and Control – Twente (IMPACT), The Netherlands MIRA Institute for Biomedical Technology and Technical Medicine, University of Twente, Membrane Technology Group, Faculty of Science and Technology, P.O. Box 217, 7500 AE Enschede, The Netherlands b
a r t i c l e
i n f o
Article history: Received 1 February 2010 Received in revised form 26 April 2010 Accepted 2 May 2010 Available online 10 May 2010 Keywords: Membrane with integrated spacer Concentration polarisation Limiting current Mass transport Electrodialysis
a b s t r a c t Many membrane processes are severely influenced by concentration polarisation. Turbulence promoting spacers placed in between the membranes can reduce the diffusional resistance of concentration polarisation by inducing additional mixing. Electrodialysis (ED) used for desalination suffers from concentration polarisation in particular. Using spacers there leads to higher cell resistance, and therefore to higher power consumption, because of the induction of the spacer shadow effect: ions do not migrate in areas where the spacer is located. The use of ion-conductive spacers can reduce this spacer shadow effect, however the spacers are still rather thick and the cell resistance stays high. This work tries to overcome the disadvantages of thick spacers while keeping the beneficial effect of turbulence promotion. We present the preparation and characterisation of a novel cation exchange membrane where the spacer is formed directly on the membrane surface and is hence integral part of the membrane. This membrane with integrated spacer is formed by surface patterning of a drying polymer solution in contact with a regular membrane spacer where capillary forces pull the solution towards the spacer strands where they solidify. Peeling of the original spacer leaves the membrane with the spacer topology integrated in its surface. Characterisation of this novel membrane in an ED process improves process hydrodynamics while avoiding the resistance increase corresponding to the shadow effect of a non-conductive spacer. Having a spacer directly on the membrane surface has also the advantage of a much simpler membrane module assembly since the spacer becomes superfluous. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Membrane-based separation applications have been rapidly gaining practical relevance of the past decades. However, many membrane processes are severely influenced by concentration polarisation. At the membrane surface, concentration gradients evolve mostly at the feed side due to an imbalance of membrane resistance and hydrodynamic boundary layer resistance. In membrane processes such as pervaporation, vapour permeation and ultrafiltration, concentration polarisation is observed by the fact that the transmembrane flux does not continue to increase with increasing driving force (transmembrane pressure), but it reaches a limiting flux (Jlim ). The boundary layer resistance approaches values equal to the membrane resistance or even higher and hence
∗ Corresponding author at: MIRA Institute for Biomedical Technology and Technical Medicine, University of Twente, Membrane Technology Group, Faculty of Science and Technology, P.O. Box 217, 7500 AE Enschede, The Netherlands. Tel.: +31 53 4894675; fax: +31 53 4894611. E-mail address: [email protected] (D.F. Stamatialis). 1 Parker Filtration & Separation B.V., P.O. Box 258, 4870 AG Etten-Leur, The Netherlands. 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.05.011
it contributes to the overall resistance [1]. Nowadays, in these processes, additional mixing is induced by turbulence promoting spacers placed in between the membranes or using air sparging [2–7]. Recently novel spacers having a multilayer structure (sandwich of spacers containing a bigger spacer in the middle with two thin outside spacers) were developed [8–10]. These spacers result in 30% higher Sherwood numbers at the same cross flow power consumption in comparison to optimal non-woven net spacers [8]. Electrodialysis (ED) used for desalination suffers from concentration polarisation in particular. There, the operating current density is limited by concentration polarisation. The limiting flux is now called limiting current density (ilim ) and the voltage drop across the membrane is a measure of the driving force [11–13]. Concentration polarisation in ED is caused by differences between ion transport number in the electrolyte solution and the ion exchange membrane. To get the maximum ion flux per unit membrane area it is desirable to operate at the highest possible current density. The consequences of concentration polarisation in ED are twofold. The difference in transport number leads to depletion of salt ions on the dilute side of the membrane, whereas at the same time the concentration near the membrane on the concentrate side increases (Fig. 1a) [11,14]. If in the dilute channel the salt concentration at the membrane surface is reduced to zero there are
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J. Balster et al. / Journal of Membrane Science 360 (2010) 185–189
rial would lead to further improvements [19–21] since it overcomes the spacer shadow effect but still the cell resistance is rather high. In this work, we present the preparation and characterisation of a novel cation exchange membrane where the spacer is formed directly on the membrane surface [22]. We coin this new membrane architecture ‘membrane with integrated spacer’ indicating their integration into one entity making the regular spacer superfluous. Our results show that this novel membrane improves process hydrodynamics while avoiding the resistance increase corresponding to the shadow effect of a non-conductive spacer. 2. Experimental 2.1. Materials Sulfonated poly(ether ether ketone), S-PEEK, was prepared by sulphonation of PEEK 450PF from Victrex as described in Ref. [23]. Blends of S-PEEK with poly(ether sulfone) (PES) were prepared by adding the desired amount of polymers to N-methyl pyrrolidinone (NMP). 2.2. Ion exchange films
Fig. 1. (a) Schematic illustration of the concentration polarisation in electrodialysis. (b) Typical current density–voltage drop curve for an ion exchange membrane showing the Ohmic, the plateau and the overlimiting current region, taken from Ref. [1].
To prepare S-PEEK and S-PEEK/PES films, the polymers were added in the desired amount to NMP (20 wt% polymer in solution). After a minimum of 24 h of stirring the polymer solution was filtered through a 25-m metal filter. The films were prepared by the evaporation technique [24,25]. The solutions were cast on glass plates with a 0.5-mm casting knife. The films were dried for 1 week in N2 atmosphere at 40–80 ◦ C, then immersed in water and subsequently dried under vacuum at 30 ◦ C for 1 week and stored in 0.5 M NaCl. 2.3. Capillary force induced surface structuring
no more extra salt ions available to carry the electrical current and the limiting current density ilim is reached (Fig. 1b). Further increase of the driving force (V across the membrane and boundary layer) does not increase the ionic flux. The plateau region of the current–voltage curve has been reached. At some critical voltage drop, transport occurs again: the overlimiting current sets in. Its nature has been disputed for some time and the reader is referred to [15–18] for details. When, due to concentration polarisation, the salt concentration in the concentrate channel exceeds the solubility limits of the solution constituents, precipitation may occur, resulting in an additional electrical resistance and eventually in membrane damage. Generally, every ED unit installed in the world is designed following a simple rule: the maximum current flowing through a stack must be 80% of the limiting current density at the lowest concentration in the diluate, resulting in high membrane areas for the removal of salt in very dilute solutions [19,20]. The fluid flow in ED modules mainly takes place in flat channels with rectangular cross-sections, where the membranes form the walls and the channels are filled with spacer material separating the membranes. The spacer design itself has great effect on the process costs and has been investigated intensively in recent years [3,8–10]. The spacers act as mechanical stabilisers for the channel geometry and promote turbulence, which reduces the polarisation phenomena near the membranes, reducing the laminar boundary layer and increasing the mass transfer coefficient [2]. As mentioned before, another way of promoting turbulence in the channel is air sparging. Recent research showed the positive effect of the use of air sparging in an ED process [7]. However, the use of spacers or the use of air bubbles as turbulence promoters leads to an increase cell resistance and therefore to a higher power consumption of the separation process. Instead of using a non-conductive spacer the use of conductive spacer mate-
The “capillary force induced surface structuring” is a technique to replicate a spacer structure on a membrane surface. The polymeric membranes were prepared in two steps [22,26]. 1st step: the membrane itself was created. The S-PEEK/PES blend was cast on glass plate with a 0.5-mm casting knife. The film was dried for 1 week in N2 atmosphere at 40–80 ◦ C. 2nd step: the spacer structure was created on the membrane surface by casting a second polymer layer of S-PEEK/PES with a 0.3-mm casting knife onto the membrane layer created in step 1 and pressing the commercial spacer with the desired structure onto this layer (Fig. 2a). The capillary forces drag the polymer under the spacer fibres, leading to structure replication. After solvent evaporation (1 week in N2 atmosphere at 40–80 ◦ C) the spacer can be removed from the membrane by immersion into a water bath. The membrane was placed in a vacuum oven at 30 ◦ C until a constant weight was reached. The dry membrane was stored in 0.5 M NaCl solution. Here, for the preparation of the spacer structure, we used the same material as for the membrane fabrication. In principle any conductive polymer soluble in the same solvent as the membrane material could be used too. This would guarantee optimal adhesion of the spacer structure onto the membrane. Besides, using different casting thicknesses in the 2nd step, one would be able to vary the height of the replicated spacer structure. 2.4. Characterisation of the ion exchange membrane layers The tailor made ion exchange membrane layers were characterised by measurements of the ion exchange capacity (IEC), water uptake (w), permselectivity (P) and electrical resistance (R). These properties were used to calculate the membrane sulphonation degree (SD), the specific membrane conductivity (Cond), and
J. Balster et al. / Journal of Membrane Science 360 (2010) 185–189
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Fig. 2. (a) Illustration of the capillary force induced spacer replication. (b) Representation of the spacer structure (the spacer height is 1.1 mm) used for the preparation of a membrane with integrated spacer. (c) Photos of a membrane with integrated spacer, showing the surface structure including zoom into two different membrane parts.
the charge density (cchar ) of the membranes (more details in Refs. [23,24]). The membrane was visualised by Optical (Zeiss Axiovert 40 MAT) Microscope. 2.5. Measurement of the limiting current density The influence of the spacer structured membrane surface on the mass transfer was evaluated by limiting current density measure-
ments at different flow velocities in a simplified bipolar membrane ED system, consisting of a single dilution and concentration channel pair. The ED system is a semi-automated lab scale system used in batch recycle mode. To be comparable to industrial ED application, a commercial plate and frame membrane module for small pilot applications was chosen for this research. The membrane module (FuMATech, Germany) has a membrane area of 100 cm2 (10 cm × 10 cm) for each membrane (see more details in Ref. [27]).
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Table 1 Properties of the S-PEEK/PES cation exchange film. Membrane
SD S-PEEK
dwet [m]
Cond [mS/cm]
IEC [mol/kgdry ]
w [kgwater /kgdry ]
cchar [mol/L]
P [%]
60% S-PEEK 40% PES
80%
80
1.9
1.3
0.34
3.8
96
For the tests, the commercial anion exchange membrane, AMX, and cation exchange membrane, CMX (Astom, Japan) were used. Current–voltage curves were obtained under direct current with the stack configuration shown in Fig. 3. The determination of limiting current density of the central membrane is described in detail in Ref. [10]. In the compartments 1 and 6, a 0.5 M Na2 SO4 solution was applied as electrolyte. In the compartments 2 and 5, 0.5 M NaCl was applied as shielding solution, and in the compartments 3 and 4, 0.1 M NaCl was used as measurement solution. The physical properties of a 0.1 M NaCl solution are: = 1088 kg/m3 ; = 1 mPa s; bl D = 1.48 × 10−9 m2 /s; tNa + = 0.39 [28]. 3. Results and discussion Table 1 presents the properties of the S-PEEK/PES membranes (without spacer structure, prepared as described in Section 2.2). The characteristics are comparable with commercially available cation exchange membranes [23]. Fig. 2b shows the spacer and its geometrical properties used in Section 2.3 to prepare the membrane with integrated spacer from the S-PEEK/PES blend. The spacer has a rectangular symmetrical structure with a flow attack angle of 45◦ . The use of this spacer for the spacer replication technique [22] resulted in a membrane shown in Fig. 2c. The spacer structure formed is a direct copy of the used spacer. The channels present in the structure itself show that the polymeric solution, in which the spacer was pressed for replication, was dragged under the filaments of the spacer due to capillary forces. Therefore a membrane with conductive spacer on the surface made of the same material was produced in an easy way. In this research it is shown, that the evaporation technique results in a direct copy of the spacer filaments. However, we have also used this method for the preparation of porous membranes through a phase inversion process. A negative copy of the spacer structure could be achieved by direct phase inversion in a nonsolvent bath after pressing the spacer into the liquid polymeric film. By applying longer times after pressing the spacer structure into the liquid polymeric film before phase inversion, also a direct copy of the spacer structure on the surface of the produced membranes can be achieved. Therefore the spacer replication technique can be used for all flat sheet membranes prepared by solvent evaporation or phase inversion [22].
Fig. 3. Schematic of the membrane arrangement in the electrodialysis stack taken from Ref. [10].
In order to test the influence of the membrane with integrated spacer in ED process, the limiting current densities of the flat side and the structured side of the same membrane facing the anode were investigated (Fig. 4) at different flow rates of 40–80 L/h (corresponding to 3.7–7.4 cm/s). The measured limiting current densities and therefore the corresponding mass transfer coefficient of the membrane with integrated spacer side facing the anode are significantly higher in comparison to the flat side facing the anode. Already at a flow rate of 40 L/h, the limiting current density is more than 30% increased compared to the flat membrane surface. At higher flow rate, the difference in limiting current density compared to the flat surface seems to increase further. In earlier work the influence of novel spacer structures on the limiting current density in a comparable system was evaluated [10]. At low flow velocities, the limiting current density of the single spacer system reported there is comparable with that of a membrane with integrated spacer reported here in a spacer free cell. Only at higher flow velocities, the spacer system caused increase in mass transfer. In [10], we also showed the importance of having a multilayer spacer system containing a big middle spacer to divert the flow from the bulk to the channel walls, and two thin net spacers close to the membrane surface. Using two “membranes with integrated spacer” (as developed in this work) we in fact need only one middle spacer to achieve similar mass transfer enhancement. A comparison of the resistance of the flat side of the membrane with the side containing the spacer structure (measured in a 0.1 M NaCl solution with a six-compartment cell as described in Refs. [23,24]) shows a resistance decrease of 3 cm2 when the side with the spacer faces the anode. Our results suggest that the membrane with integrated spacer combines the enhanced hydrodynamics and the corresponding increase in mass transfer due to the use of a spacer system, while avoiding the resistance increase corresponding to the shadow effect of a non-conductive spacer. The increased surface area and height of a spacer containing membrane leads even to a decrease in total cell resistance.
Fig. 4. Dependence of the limiting current density on the liquid flow rate. Comparison of flat membrane with a membrane with integrated spacer.
J. Balster et al. / Journal of Membrane Science 360 (2010) 185–189
4. Conclusions and outlook This work presented the preparation of a membrane with integrated spacer and its application in an ED process. This novel membrane improves the hydrodynamics of the process and leads even to a decrease in process resistance. In electro-deionisation processes, these membranes are expected to lead to a tremendous decrease of resistance. The space between the membranes in the dilution compartment, which gives the main resistance, would be filled with conducting material. Besides, the membrane module can be made in such way that the next membrane with integrated spacer with the opposite charge would be in direct contact with the other leading to water splitting inside this compartment. Currently this is achieved by applying ion exchange particles. In recent articles [8–10] the advantages of using threedimensional spacer system (sandwich of two small outside spacers close to the membrane surface and a bigger spacer in the middle) was discussed. Using the membrane with integrated spacer (producing the small spacers on the membrane surface) the same positive effect can probably be achieved using only one, the middle spacer. This is also expected to have a significant improvement on concentration polarisation resulting in decrease of resistance compared to the existing systems. Generally the advantage of applying the spacer directly on the membrane surface will lead to a much simpler membrane module production like for example the preparation of spiral wound modules. Acknowledgement This CW/STW project is financially supported by the Netherlands Organisation for Scientific Research (NWO). List of symbols cb concentration in the bulk [mol/L] charge density [mol/L] cchar cm concentration at the membrane surface [mol/L] Cond conductivity [mS/cm] dF diameter of the spacer filament D diffusion coefficient [m2 /s] h height of the spacer [m] i current density [A/m2 ] ilim limiting current density [A/m2 ] IEC ion exchange capacity [mol/kgdry ] Jlim limiting flux [mol/(m2 s)] l distance between the spacer filaments [m] P permselectivity [%] R area resistance [ cm2 ] SD sulphonation degree [%] U voltage drop [V] w water uptake [kgwater /kgdry ] CE cation exchange ED electrodialysis NMP N-methyl-2-pyrrolidinone PEEK poly(ether ether ketone) PES poly(ether sulphone) S-PEEK sulphonated poly(ether ether ketone) Greek letters ␣ flow attack angle [◦ ] ˇ angle between crossing filaments [◦ ]
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References [1] R. Ibanez, D.F. Stamatialis, M. Wessling, Role of membrane surface in concentration polarization at cation exchange membranes, Journal of Membrane Science 239 (2004) 119–128. [2] S. Geissler, H. Heits, U. Werner, Description of fluid flow through spacers in flat channel filtration systems, Filtration & Separation 32 (1995) 538–544. [3] J.-M. Chiapello, M. Bernard, Improved spacer design and cost reduction in an electrodialysis system, Journal of Membrane Science 80 (1993) 251–256. [4] Z.F. Cui, S. Chang, A.G. Fane, The use of gas bubbling to enhance membrane processes, Journal of Membrane Science 221 (2003) 1. [5] G. Ducom, C. Cabassud, Possible effects of air sparging for nanofiltration of salted solutions, Desalination 156 (2003) 267. [6] G. Ducom, H. Matamoros, C. Cabassud, Air sparging for flux enhancement in nanofiltration membranes: application to O/W stabilised and non-stabilised emulsions, Journal of Membrane Science 204 (2002) 221. [7] J. Balster, D.F. Stamatialis, M. Wessling, Towards spacer free electrodialysis, Journal of Membrane Science 341 (2009) 131–138. [8] F. Li, W. Meindersma, A.B. de Haan, T. Reith, Novel spacers for mass transfer enhancement in membrane separations, Journal of Membrane Science 253 (2005) 1–12. [9] F. Li, W. Meindersma, A.B. de Haan, T. Reith, Spacer for use in a membrane separation device and a membrane separation device comprising such a spacer, International Patent, WO 2004/112945 A1. [10] J. Balster, I. Pünt, D.F. Stamatialis, M. Wessling, Multi-layer spacer geometries with improved mass transport, Journal of Membrane Science 282 (2006) 351–361. [11] K.S. Spiegler, Polarization at ion exchange membrane-solution interfaces, Desalination 9 (1971) 367–385. [12] N.W. Rosberg, C.E. Tirell, Limiting currents in membrane cells, Industrial & Engineering Chemistry Research 49 (1957) 780–784. [13] V.K. Indusekhar, P. Meares, The effect of the diffusion layer on the ionic current from a solution into an ion-exchange membrane, in: D.B. Spalding (Ed.), Physicochemical Hydrodynamics II, Advance Publications, London, 1977, pp. 1031–1043. [14] C. Forgacs, N. Ishibashi, J. Leibovitz, Polarization at ion exchange membranes in electrodialysis, Desalination 10 (1972) 181–214. [15] I. Rubinstein, B. Zaltzman, O. Kedem, Electric field in and around ion exchange membranes, Journal of Membrane Science 125 (1997) 17–21. [16] I. Rubinstein, B. Zaltzman, Electro-osmotically induced convection at a permselective membrane, Physical Review E 62 (2000) 2238–2251. [17] J. Balster, M.H. Yildirim, R. Ibanez, R.G.H. Lammertink, D.F. Stamatialis, V. Jordan, M. Wessling, Morphology and micro-topology of cation exchange polymers and the origin of the overlimiting current, Journal of Physical Chemistry B 111 (2007) 2152–2165. [18] S.M. Rubinstein, G. Manukyan, A. Staicu, I. Rubinstein, B. Zaltzman, R.G.H. Lammertink, F. Mugele, M. Wessling, Direct observation of a nonequilibrium electro-osmotic instability, Physical Review Letters 101 (23) (2008) 1–4 (Article 236101). [19] E. Korngold, L. Aronov, O. Kedem, Novel ion-exchange spacer for improving electrodialysis I. Reacted spacer, Journal of Membrane Science 138 (1998) 165–170. [20] V.K. Shahi, S.K. Thampy, R. Rangarajan, The effect of conducting spacers on transport properties of ion-exchange membranes in electrodriven separation, Desalination 133 (2001) 245–258. [21] P. Dlugolecki, J. Dabrowska2, K. Nijmeijer, M. Wessling, Ion conductive spacers for increased power generation in reverse electrodialysis, Journal of Membrane Science 347 (2010) 101–107. [22] J.H. Balster, D. Stamatialis, M. Wessling, Ion-permeable membrane and its manufacture, Stichting Tech. Wetenschapp, EP Patent 1982757 (2008). [23] J. Balster, O. Krupenko, I. Pünt, D.F. Stamatialis, M. Wessling, Preparation and characterisation of monovalent ion selective cation exchange membranes based on sulphonated poly(ether ether ketone), Journal of Membrane Science 263 (2005) 137–145. [24] F.G. Wilhelm, I.G.M. Punt, N.F.A. van der Vegt, H. Strathmann, M. Wessling, Cation permeable membranes from blends of sulfonated poly(ether ether ketone) and poly(ether sulfone), Journal of Membrane Science 199 (2002) 167–176. [25] M. Mulder, Basic Principles of Membrane Technology, 2nd edition, Kluwer Academic Publishers, Dordrecht, 1996 (Chapter 3). [26] F.G. Wilhelm, I. Punt, N.F.A. van der Vegt, M. Wessling, H. Strathmann, Optimisation strategies for the preparation of bipolar membranes with reduced salt ion leakage in acid–base electrodialysis, Journal of Membrane Science 182 (2001) 13–28. [27] F.G. Wilhelm, I. Punt, N.F.A. van der Vegt, M. Wessling, H. Strathmann, Asymmetric bipolar membranes in acid–base electrodialysis, Industrial &. Engineering Chemistry Research 41 (2002) 579–586. [28] R.A. Robinson, R.H. Stokes, Electrolyte Solutions, 2nd edition, Butterworths, London, 1954.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Membrane with integrated spacer J. Balster a,1 , D.F. Stamatialis a,b,∗ , M. Wessling a a
Institute of Mechanics, Process and Control – Twente (IMPACT), The Netherlands MIRA Institute for Biomedical Technology and Technical Medicine, University of Twente, Membrane Technology Group, Faculty of Science and Technology, P.O. Box 217, 7500 AE Enschede, The Netherlands b
a r t i c l e
i n f o
Article history: Received 1 February 2010 Received in revised form 26 April 2010 Accepted 2 May 2010 Available online 10 May 2010 Keywords: Membrane with integrated spacer Concentration polarisation Limiting current Mass transport Electrodialysis
a b s t r a c t Many membrane processes are severely influenced by concentration polarisation. Turbulence promoting spacers placed in between the membranes can reduce the diffusional resistance of concentration polarisation by inducing additional mixing. Electrodialysis (ED) used for desalination suffers from concentration polarisation in particular. Using spacers there leads to higher cell resistance, and therefore to higher power consumption, because of the induction of the spacer shadow effect: ions do not migrate in areas where the spacer is located. The use of ion-conductive spacers can reduce this spacer shadow effect, however the spacers are still rather thick and the cell resistance stays high. This work tries to overcome the disadvantages of thick spacers while keeping the beneficial effect of turbulence promotion. We present the preparation and characterisation of a novel cation exchange membrane where the spacer is formed directly on the membrane surface and is hence integral part of the membrane. This membrane with integrated spacer is formed by surface patterning of a drying polymer solution in contact with a regular membrane spacer where capillary forces pull the solution towards the spacer strands where they solidify. Peeling of the original spacer leaves the membrane with the spacer topology integrated in its surface. Characterisation of this novel membrane in an ED process improves process hydrodynamics while avoiding the resistance increase corresponding to the shadow effect of a non-conductive spacer. Having a spacer directly on the membrane surface has also the advantage of a much simpler membrane module assembly since the spacer becomes superfluous. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Membrane-based separation applications have been rapidly gaining practical relevance of the past decades. However, many membrane processes are severely influenced by concentration polarisation. At the membrane surface, concentration gradients evolve mostly at the feed side due to an imbalance of membrane resistance and hydrodynamic boundary layer resistance. In membrane processes such as pervaporation, vapour permeation and ultrafiltration, concentration polarisation is observed by the fact that the transmembrane flux does not continue to increase with increasing driving force (transmembrane pressure), but it reaches a limiting flux (Jlim ). The boundary layer resistance approaches values equal to the membrane resistance or even higher and hence
∗ Corresponding author at: MIRA Institute for Biomedical Technology and Technical Medicine, University of Twente, Membrane Technology Group, Faculty of Science and Technology, P.O. Box 217, 7500 AE Enschede, The Netherlands. Tel.: +31 53 4894675; fax: +31 53 4894611. E-mail address: [email protected] (D.F. Stamatialis). 1 Parker Filtration & Separation B.V., P.O. Box 258, 4870 AG Etten-Leur, The Netherlands. 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.05.011
it contributes to the overall resistance [1]. Nowadays, in these processes, additional mixing is induced by turbulence promoting spacers placed in between the membranes or using air sparging [2–7]. Recently novel spacers having a multilayer structure (sandwich of spacers containing a bigger spacer in the middle with two thin outside spacers) were developed [8–10]. These spacers result in 30% higher Sherwood numbers at the same cross flow power consumption in comparison to optimal non-woven net spacers [8]. Electrodialysis (ED) used for desalination suffers from concentration polarisation in particular. There, the operating current density is limited by concentration polarisation. The limiting flux is now called limiting current density (ilim ) and the voltage drop across the membrane is a measure of the driving force [11–13]. Concentration polarisation in ED is caused by differences between ion transport number in the electrolyte solution and the ion exchange membrane. To get the maximum ion flux per unit membrane area it is desirable to operate at the highest possible current density. The consequences of concentration polarisation in ED are twofold. The difference in transport number leads to depletion of salt ions on the dilute side of the membrane, whereas at the same time the concentration near the membrane on the concentrate side increases (Fig. 1a) [11,14]. If in the dilute channel the salt concentration at the membrane surface is reduced to zero there are
186
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rial would lead to further improvements [19–21] since it overcomes the spacer shadow effect but still the cell resistance is rather high. In this work, we present the preparation and characterisation of a novel cation exchange membrane where the spacer is formed directly on the membrane surface [22]. We coin this new membrane architecture ‘membrane with integrated spacer’ indicating their integration into one entity making the regular spacer superfluous. Our results show that this novel membrane improves process hydrodynamics while avoiding the resistance increase corresponding to the shadow effect of a non-conductive spacer. 2. Experimental 2.1. Materials Sulfonated poly(ether ether ketone), S-PEEK, was prepared by sulphonation of PEEK 450PF from Victrex as described in Ref. [23]. Blends of S-PEEK with poly(ether sulfone) (PES) were prepared by adding the desired amount of polymers to N-methyl pyrrolidinone (NMP). 2.2. Ion exchange films
Fig. 1. (a) Schematic illustration of the concentration polarisation in electrodialysis. (b) Typical current density–voltage drop curve for an ion exchange membrane showing the Ohmic, the plateau and the overlimiting current region, taken from Ref. [1].
To prepare S-PEEK and S-PEEK/PES films, the polymers were added in the desired amount to NMP (20 wt% polymer in solution). After a minimum of 24 h of stirring the polymer solution was filtered through a 25-m metal filter. The films were prepared by the evaporation technique [24,25]. The solutions were cast on glass plates with a 0.5-mm casting knife. The films were dried for 1 week in N2 atmosphere at 40–80 ◦ C, then immersed in water and subsequently dried under vacuum at 30 ◦ C for 1 week and stored in 0.5 M NaCl. 2.3. Capillary force induced surface structuring
no more extra salt ions available to carry the electrical current and the limiting current density ilim is reached (Fig. 1b). Further increase of the driving force (V across the membrane and boundary layer) does not increase the ionic flux. The plateau region of the current–voltage curve has been reached. At some critical voltage drop, transport occurs again: the overlimiting current sets in. Its nature has been disputed for some time and the reader is referred to [15–18] for details. When, due to concentration polarisation, the salt concentration in the concentrate channel exceeds the solubility limits of the solution constituents, precipitation may occur, resulting in an additional electrical resistance and eventually in membrane damage. Generally, every ED unit installed in the world is designed following a simple rule: the maximum current flowing through a stack must be 80% of the limiting current density at the lowest concentration in the diluate, resulting in high membrane areas for the removal of salt in very dilute solutions [19,20]. The fluid flow in ED modules mainly takes place in flat channels with rectangular cross-sections, where the membranes form the walls and the channels are filled with spacer material separating the membranes. The spacer design itself has great effect on the process costs and has been investigated intensively in recent years [3,8–10]. The spacers act as mechanical stabilisers for the channel geometry and promote turbulence, which reduces the polarisation phenomena near the membranes, reducing the laminar boundary layer and increasing the mass transfer coefficient [2]. As mentioned before, another way of promoting turbulence in the channel is air sparging. Recent research showed the positive effect of the use of air sparging in an ED process [7]. However, the use of spacers or the use of air bubbles as turbulence promoters leads to an increase cell resistance and therefore to a higher power consumption of the separation process. Instead of using a non-conductive spacer the use of conductive spacer mate-
The “capillary force induced surface structuring” is a technique to replicate a spacer structure on a membrane surface. The polymeric membranes were prepared in two steps [22,26]. 1st step: the membrane itself was created. The S-PEEK/PES blend was cast on glass plate with a 0.5-mm casting knife. The film was dried for 1 week in N2 atmosphere at 40–80 ◦ C. 2nd step: the spacer structure was created on the membrane surface by casting a second polymer layer of S-PEEK/PES with a 0.3-mm casting knife onto the membrane layer created in step 1 and pressing the commercial spacer with the desired structure onto this layer (Fig. 2a). The capillary forces drag the polymer under the spacer fibres, leading to structure replication. After solvent evaporation (1 week in N2 atmosphere at 40–80 ◦ C) the spacer can be removed from the membrane by immersion into a water bath. The membrane was placed in a vacuum oven at 30 ◦ C until a constant weight was reached. The dry membrane was stored in 0.5 M NaCl solution. Here, for the preparation of the spacer structure, we used the same material as for the membrane fabrication. In principle any conductive polymer soluble in the same solvent as the membrane material could be used too. This would guarantee optimal adhesion of the spacer structure onto the membrane. Besides, using different casting thicknesses in the 2nd step, one would be able to vary the height of the replicated spacer structure. 2.4. Characterisation of the ion exchange membrane layers The tailor made ion exchange membrane layers were characterised by measurements of the ion exchange capacity (IEC), water uptake (w), permselectivity (P) and electrical resistance (R). These properties were used to calculate the membrane sulphonation degree (SD), the specific membrane conductivity (Cond), and
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Fig. 2. (a) Illustration of the capillary force induced spacer replication. (b) Representation of the spacer structure (the spacer height is 1.1 mm) used for the preparation of a membrane with integrated spacer. (c) Photos of a membrane with integrated spacer, showing the surface structure including zoom into two different membrane parts.
the charge density (cchar ) of the membranes (more details in Refs. [23,24]). The membrane was visualised by Optical (Zeiss Axiovert 40 MAT) Microscope. 2.5. Measurement of the limiting current density The influence of the spacer structured membrane surface on the mass transfer was evaluated by limiting current density measure-
ments at different flow velocities in a simplified bipolar membrane ED system, consisting of a single dilution and concentration channel pair. The ED system is a semi-automated lab scale system used in batch recycle mode. To be comparable to industrial ED application, a commercial plate and frame membrane module for small pilot applications was chosen for this research. The membrane module (FuMATech, Germany) has a membrane area of 100 cm2 (10 cm × 10 cm) for each membrane (see more details in Ref. [27]).
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Table 1 Properties of the S-PEEK/PES cation exchange film. Membrane
SD S-PEEK
dwet [m]
Cond [mS/cm]
IEC [mol/kgdry ]
w [kgwater /kgdry ]
cchar [mol/L]
P [%]
60% S-PEEK 40% PES
80%
80
1.9
1.3
0.34
3.8
96
For the tests, the commercial anion exchange membrane, AMX, and cation exchange membrane, CMX (Astom, Japan) were used. Current–voltage curves were obtained under direct current with the stack configuration shown in Fig. 3. The determination of limiting current density of the central membrane is described in detail in Ref. [10]. In the compartments 1 and 6, a 0.5 M Na2 SO4 solution was applied as electrolyte. In the compartments 2 and 5, 0.5 M NaCl was applied as shielding solution, and in the compartments 3 and 4, 0.1 M NaCl was used as measurement solution. The physical properties of a 0.1 M NaCl solution are: = 1088 kg/m3 ; = 1 mPa s; bl D = 1.48 × 10−9 m2 /s; tNa + = 0.39 [28]. 3. Results and discussion Table 1 presents the properties of the S-PEEK/PES membranes (without spacer structure, prepared as described in Section 2.2). The characteristics are comparable with commercially available cation exchange membranes [23]. Fig. 2b shows the spacer and its geometrical properties used in Section 2.3 to prepare the membrane with integrated spacer from the S-PEEK/PES blend. The spacer has a rectangular symmetrical structure with a flow attack angle of 45◦ . The use of this spacer for the spacer replication technique [22] resulted in a membrane shown in Fig. 2c. The spacer structure formed is a direct copy of the used spacer. The channels present in the structure itself show that the polymeric solution, in which the spacer was pressed for replication, was dragged under the filaments of the spacer due to capillary forces. Therefore a membrane with conductive spacer on the surface made of the same material was produced in an easy way. In this research it is shown, that the evaporation technique results in a direct copy of the spacer filaments. However, we have also used this method for the preparation of porous membranes through a phase inversion process. A negative copy of the spacer structure could be achieved by direct phase inversion in a nonsolvent bath after pressing the spacer into the liquid polymeric film. By applying longer times after pressing the spacer structure into the liquid polymeric film before phase inversion, also a direct copy of the spacer structure on the surface of the produced membranes can be achieved. Therefore the spacer replication technique can be used for all flat sheet membranes prepared by solvent evaporation or phase inversion [22].
Fig. 3. Schematic of the membrane arrangement in the electrodialysis stack taken from Ref. [10].
In order to test the influence of the membrane with integrated spacer in ED process, the limiting current densities of the flat side and the structured side of the same membrane facing the anode were investigated (Fig. 4) at different flow rates of 40–80 L/h (corresponding to 3.7–7.4 cm/s). The measured limiting current densities and therefore the corresponding mass transfer coefficient of the membrane with integrated spacer side facing the anode are significantly higher in comparison to the flat side facing the anode. Already at a flow rate of 40 L/h, the limiting current density is more than 30% increased compared to the flat membrane surface. At higher flow rate, the difference in limiting current density compared to the flat surface seems to increase further. In earlier work the influence of novel spacer structures on the limiting current density in a comparable system was evaluated [10]. At low flow velocities, the limiting current density of the single spacer system reported there is comparable with that of a membrane with integrated spacer reported here in a spacer free cell. Only at higher flow velocities, the spacer system caused increase in mass transfer. In [10], we also showed the importance of having a multilayer spacer system containing a big middle spacer to divert the flow from the bulk to the channel walls, and two thin net spacers close to the membrane surface. Using two “membranes with integrated spacer” (as developed in this work) we in fact need only one middle spacer to achieve similar mass transfer enhancement. A comparison of the resistance of the flat side of the membrane with the side containing the spacer structure (measured in a 0.1 M NaCl solution with a six-compartment cell as described in Refs. [23,24]) shows a resistance decrease of 3 cm2 when the side with the spacer faces the anode. Our results suggest that the membrane with integrated spacer combines the enhanced hydrodynamics and the corresponding increase in mass transfer due to the use of a spacer system, while avoiding the resistance increase corresponding to the shadow effect of a non-conductive spacer. The increased surface area and height of a spacer containing membrane leads even to a decrease in total cell resistance.
Fig. 4. Dependence of the limiting current density on the liquid flow rate. Comparison of flat membrane with a membrane with integrated spacer.
J. Balster et al. / Journal of Membrane Science 360 (2010) 185–189
4. Conclusions and outlook This work presented the preparation of a membrane with integrated spacer and its application in an ED process. This novel membrane improves the hydrodynamics of the process and leads even to a decrease in process resistance. In electro-deionisation processes, these membranes are expected to lead to a tremendous decrease of resistance. The space between the membranes in the dilution compartment, which gives the main resistance, would be filled with conducting material. Besides, the membrane module can be made in such way that the next membrane with integrated spacer with the opposite charge would be in direct contact with the other leading to water splitting inside this compartment. Currently this is achieved by applying ion exchange particles. In recent articles [8–10] the advantages of using threedimensional spacer system (sandwich of two small outside spacers close to the membrane surface and a bigger spacer in the middle) was discussed. Using the membrane with integrated spacer (producing the small spacers on the membrane surface) the same positive effect can probably be achieved using only one, the middle spacer. This is also expected to have a significant improvement on concentration polarisation resulting in decrease of resistance compared to the existing systems. Generally the advantage of applying the spacer directly on the membrane surface will lead to a much simpler membrane module production like for example the preparation of spiral wound modules. Acknowledgement This CW/STW project is financially supported by the Netherlands Organisation for Scientific Research (NWO). List of symbols cb concentration in the bulk [mol/L] charge density [mol/L] cchar cm concentration at the membrane surface [mol/L] Cond conductivity [mS/cm] dF diameter of the spacer filament D diffusion coefficient [m2 /s] h height of the spacer [m] i current density [A/m2 ] ilim limiting current density [A/m2 ] IEC ion exchange capacity [mol/kgdry ] Jlim limiting flux [mol/(m2 s)] l distance between the spacer filaments [m] P permselectivity [%] R area resistance [ cm2 ] SD sulphonation degree [%] U voltage drop [V] w water uptake [kgwater /kgdry ] CE cation exchange ED electrodialysis NMP N-methyl-2-pyrrolidinone PEEK poly(ether ether ketone) PES poly(ether sulphone) S-PEEK sulphonated poly(ether ether ketone) Greek letters ␣ flow attack angle [◦ ] ˇ angle between crossing filaments [◦ ]
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