- Email: [email protected]
Design of novel direct contact membrane distillation membranes
Desalination 192 (2006) 105–111
Design of novel direct contact membrane distillation membranes M. Khayeta*, T. Matsuurab, J.I. Menguala, M. Qtaishatb a
Department of Applied Physics I, Faculty of Physics, University Complutense of Madrid, Avda. Complutense s/n, 28040, Madrid, Spain Tel. +34 (91) 3944454; Fax +34 (91) 3945191; email: [email protected] b Industrial Membrane Research Center, Department of Chemical Engineering, University of Ottawa, 161 Louis Pasteur, P.O. Box 450, Stn. A, Ottawa, Ontario K1N 6N5, Canada Tel. +1 (613) 562-5800, Ext. 6114; Fax +1 (613) 562-5172; email: [email protected]
Received 23 February 2005; accepted 24 June 2005
Abstract The desired characteristics needed for a membrane to be used in direct contact membrane distillation (DCMD) are outlined. A new approach for the design of novel membranes for desalination by DCMD is proposed. Porous hydrophobic/hydrophilic composite membranes have been prepared in one casting step, using the traditional phase inversion method of polymer solutions containing a hydrophilic host polymer and a fluorinated surface modifying macromolecule. The membranes comply with the conditions and properties required in DCMD process. Membranes of different mean pore sizes, pore size distributions, porosities, roughnesses, liquid entry pressures of water and thicknesses can be formed by varying the membrane preparation conditions. The structural characteristics and the DCMD performance of the first proposed porous composite membranes were compared with those of the commercial membranes most frequently used in this kind of process. The proposed membranes have potential to be used in DCMD for desalination because, compared to the commercial ones, these membranes possess a thinner hydrophobic porous top-layer responsible for the water vapour transport in DCMD and a thicker hydrophilic porous sub-layer filled with water to lower the conductive heat loss. The thickness of the hydrophobic layer was found to be an order of magnitude lower than that of the commercial membranes. Keywords: Direct contact membrane distillation; Membrane surface modification; Surface modifying macromolecules; Porous composite membranes
*Corresponding author. Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2005.06.047
106
M. Khayet et al. / Desalination 192 (2006) 105–111
1. Introduction Direct contact membrane distillation (DCMD) is one of the emerging non-isothermal membrane separation processes known for about forty years but still is in its development stage. The first patent of DCMD was obtained by Weyl in 1967 (filed in 1964) [1] and the first DCMD publication was made by Findley in 1968 [2]. Since then several applications in desalination, environmental/waste cleanup, water-reuse and food processing have been proposed. It is worth quoting that water desalination is the best-known DCMD application. Among the four membrane distillation (MD) configurations (1 direct contact membrane distillation, DCMD, 2 air gap membrane distillation, AGMD, 3 sweeping gas membrane distillation, SGMD, and 4 vacuum membrane distillation VMD), DCMD is the most widely used. This process refers to a thermally driven transport of water through microporous hydrophobic membranes. The membrane is maintained between a hot solution (feed side) and a cold pure water (permeate side). Due to the hydrophobic nature of the membrane, liquid water cannot penetrate into the pores of the membrane until the applied pressure exceeds the liquid entry pressure (LEP), which is a characteristic of each membrane. The transmembrane vapour pressure drop, which is the driving force, is created by the temperature difference established between the feed and the permeate sides. Under these conditions, evaporation takes place at the hot feed side and, after water vapour is transported through the membrane pores, condensation takes place at the cold permeate side, inside the membrane module. Therefore, both heat and mass transfers take place simultaneously through the membrane. It must be pointed out that most of the publications in MD are concerned with theoretical MD transport models and experimental studies on the effects of operating conditions. Capillary or flatsheet commercial microporous hydrophobic membranes, made of polypropylene (PP), polyvinyl-
idene fluoride (PVDF) and polytetrafluoroethylene (PTFE, Teflon), have been used in MD experiments despite the fact that these membranes were prepared primarily for microfiltration purposes. As well, many researchers are devoting their efforts towards identification of new applications for MD. Looking to the extended published papers in MD field, only few authors have considered the possibility of designing and manufacturing membranes for MD processes [3–15]. Far is to be known about DCMD membrane before DCMD process reaches the industrial scale. The main aim of this study is to propose a promising novel method for preparation of DCMD membranes. The authors of this study also profit this opportunity to call the attention of the MD researchers in particular, and the membrane engineers in general, to the design and preparation of membranes for MD, which may be as important as it is the MD modelling for the development of MD processes.
2. Desired characteristics for DCMD membranes As it is well known, a MD membrane must be porous and hydrophobic, with good thermal stability and excellent chemical resistance to feed solutions. The characteristics needed for DCMD membranes are the following: High liquid entry pressure (LEP): This is the minimum hydrostatic pressure that must be applied onto the liquid feed solution before it overcomes the hydrophobic forces of the membrane and penetrates into the membrane pores. LEP is characteristic of each membrane and permits to prevent wetting of the membrane pores. High LEP may be achieved using a membrane material with high hydrophobicity (i.e. large water contact angle) and a small maximum pore size. However, as the maximum pore size decreases, the mean pore size of the membrane decreases and the permeability of the membrane becomes low.
M. Khayet et al. / Desalination 192 (2006) 105–111
High permeability: The DCMD flux will “increase” with an increase in the membrane pore size and porosity, and with a decrease of the membrane thickness and pore tortuosity. In other words, to obtain a high DCMD permeability, the surface layer that governs the membrane transport must be as thin as possible and its surface porosity as well as pore size must be as large as possible. However, it must be mentioned here that it seems to exist a critical pore size equal to the mean free path of water vapor molecules for given experimental DCMD conditions. In DCMD process, air is always trapped within the membrane pores with pressure values close to the atmospheric pressure. Therefore, if the pore size is comparable to the mean free path of water vapor molecules, the molecules of water vapor collide with each other and diffuse among the air molecules. In this case, the vapor transport takes place via the combined Knudsen/molecular diffusion flow. On the other hand, if the pore size is smaller than the mean free path of water vapor molecules, the moleculepore wall collisions become dominant and Knudsen type of flow will be the responsible for mass transport in DCMD. It must be mentioned that for given experimental conditions, the calculated DCMD flux considering Knudsen mechanism is higher than that considering the combined Knudsen/molecular diffusion mechanism. For example, for the commercial polytetrafluoroethylene membrane TF200 (mean pore size 0.2 µm and porosity 80%) and a bulk temperature difference of 10 K, when considering Knudsen flux, the calculated DCMD flux is 2.3 (for feed temperature 45ºC) to 2.6 (for feed temperature 20ºC) times higher than the corresponding DCMD flux calculated from the combined Knudsen/ molecular diffusion flux. For the membrane TF450 having higher pore size (i.e. mean pore size 0.45 µm and porosity 80%), the difference between the calculated DCMD fluxes is even higher. The calculated flux using Knudsen type of flow is 3.7 (for feed temperature 45ºC) to 4.3 (for feed temperature 20ºC) times higher than that
107
using the combined Knudsen/molecular diffusion type of flow. This fact indicates that when the pore size of a given membrane is near the critical pore size (i.e. mean free path of water vapor molecules), the DCMD flux will not necessarily increase with the increase of the pore size. Under some operating conditions, it would be better to use membranes with lower pore size than the corresponding mean free path of water vapor molecules so that Knudsen type of flow will takes place leading to higher DCMD flux than the membranes having larger pore size where the combined Knudsen/ molecular diffusion flux is the responsible for mass transfer. Therefore, care must be taken to choose the appropriate membrane pore size taking into account the value of the mean free path of water vapor molecules and trying to lend the membrane to work under Knudsen type of flow. Low thermal conductivity: In DCMD heat loss by conduction occurred through both the pores and the matrix of the membrane. The conductive heat loss is greater for thinner membranes. Various possibilities may be applied to diminish the conductive heat loss by using: i) Membrane materials with low thermal conductivities. This does not necessarily guarantee the improvement of the DCMD process because most hydrophobic polymers have similar heat conductivities, at least the materials have thermal conductivities with the same order of magnitude. ii) Membranes with high porosity, since the conductive heat transfer coefficient of the gas entrapped within the membrane pores is an order of magnitude smaller than that of the membrane matrix. This possibility is parallel to the need of high DCMD permeability as the available surface area of evaporation is enhanced. iii) Thicker membranes. However, there is a conflict between the requirements of high mass transfer associated with thinner membranes and low conductive heat transfer through the membrane obtained by using thicker membranes.
108
M. Khayet et al. / Desalination 192 (2006) 105–111
iv) Composite porous hydrophobic/hydrophilic membranes, having a very thin hydrophobic layer responsible for the mass transfer and a thick hydrophilic layer, the pores of which are filled with water, to prevent heat loss through the overall membrane. This seems to be a relatively simple solution that fulfills all the above conditions for achieving high permeability and low thermal conductivity. 3. Composite hydrophobic/hydrophilic membranes: background As far as we know, the use of composite membranes in MD has been described first by Cheng and Wiersma [4] in a couple of patents. A cellulose acetate membrane was modified via radiation graft polymerization of styrene onto the surface, and a cellulose nitrate membrane was modified via plasma polymerization of both vinyltrimethylsilicon/carbon tetrafluoride and octafluorocyclobutane onto the surface. Wu et al. [5] used hydrophilic porous partitions such as cellulose acetate, the surface of which was treated via radiation graft polymerization of styrene to achieve the required hydrophobicity. In the same way, Kong et al. [6] employed a cellulose nitrate membrane modified via plasma polymerization of both vinyltrimethylsilicon/carbon tetrafluoride and octafluorocyclobutane. In the membrane literature, various physical and chemical techniques are carried out for membrane surface modification. There are several methods to make composite membranes, i.e. coating, grafting, plasma treatment, etc. Nevertheless, many of the used surface modification methods are complicated and require at least one additional step during the membrane formation process. The question to be answered is: How to maintain the pore structure in the surface coating process? A novel and simple method to prepare composite porous membrane for DCMD was proposed in a previous paper [16]. The membrane was prepared by the phase inversion method, in
only one casting step, using polymer solution containing the host hydrophilic polymer and hydrophobic active additives, fluorinated surface modifying macromolecules (SMMs) [17–19]. Details on SMM preparation and characterization may be found in a recent review article [18]. The active additives can migrate to the air/polymer solution film interface during membrane casting and change its chemical and physical properties. The driving force for the migration of SMMs to the interface is the low surface energy of the fluorinated blocks. From X-ray photoelectron spectroscopy (XPS) and contact angle measurements it was found that the top surface of the formed membrane was more hydrophobic than the bulk membrane phase, while the properties of the bulk phase remained relatively unchanged as only a small amount of SMM was added. One of the various advantages of this method is the small amount of active additives required to modify the membrane surface, less than 5 wt % of the base polymer. It must be mentioned that, based on this principle, both dense and porous composite membranes can be prepared. In previous studies, various SMMs were synthesised and blended with the following base polymers: polyethersulfone (PES), polyvinylidene fluoride (PVDF), polyetherimide (PEI) and polyurethane (PU) [17– 19]. Compared to the unmodified membranes, the SMM blended membranes exhibit low surface energy (i.e. higher hydrophobicity of the top surface), higher mechanical strength, less fouling in ultrafiltration of aqueous solutions containing humic acid and cutting machine oil/water emulsion, higher pervaporation performance to remove chloroform from water, good potential to be used in biomedical applications, lower liquid and gas permeation fluxes due to their higher hydrophobicity and smaller pore size, and enhancement of the LEP of hydrophilic porous membranes, indicating that the SMM blended membranes can be used in MD process [17–20]. Moreover, it was observed that the pores of the SMM blended membranes decrease with the
M. Khayet et al. / Desalination 192 (2006) 105–111
increase of the host polymer concentration in the casting solution. Observing the tapping mode atomic force microscopy (AFM) images, the pore size of the top surface was found to be smaller than that of the bottom surface and the SMM blended membranes were smoother than the unmodified membranes [17]. Before DCMD experimental runs, various techniques may be used for characterization of the porous composite membranes, including the LEP of water measurement to prevent pore flooding during DCMD experiments, gas permeation test to determine the mean pore size and the effective porosity, atomic force microscopy (AFM) to determine the mean pore size, pore size distribution, porosity and roughness parameters of both the hydrophobic top and bottom surfaces, scanning electron microscopy (SEM) or transmission electron microscopy (TEM) to visualize the morphological structure of the membrane thickness, contact angle measurements of both the top and bottom surfaces of the composite membranes, X-ray photoelectron spectroscopy (XPS) to analyze the compositions near the membrane surface and mechanical tests. Finally, DCMD experiments may be carried out using the well known Lewis Cell employed in our previous study [16], in which both the feed and permeate liquids are stirred inside the cell by graduated magnetic stirrers, or using membrane modules working under tangential flows by means of circulating pumps. Preliminary DCMD experiments were carried out for pure water and different aqueous solutions of sodium chloride using SMM blended PEI membranes with different mean pore sizes, pore size distribution, effective porosity, roughness and LEP [16]. Distilled water and sodium chloride aqueous solutions were used as feed. The effect of the mean temperature, stirring rate and salt concentration were studied. Promising results were found. The proposed composite hydrophobic/hydrophilic membranes exhibited very high separation
109
factors (> 99.7 %) when sodium chloride aqueous solutions (up to 2 Molar) were used as the feed. Compared to the frequently used commercial poly-tetra-fluoro-ethylene (PTFE) membranes TF200 and TF450, the SMM blended PEI membranes having lower pore sizes and porosities than the above PTFE membranes showed values of DCMD flux at least of the same order of magnitude. Furthermore, the DCMD flux of one of the SMM blended PEI membrane was found to be higher than that of the PTFE membranes. This was attributed to two factors: 1) The very low path length between the liquid/vapor interfaces formed at both sides of the hydrophobic membrane pores; 2) The nature of the mass transport in DCMD for each membrane type. From the data of pore size and the mean free path of water vapor molecules, the mass transport through the pores of the SMM blended PEI membranes took place via the Knudsen flow, while the mass transport through the PTFE membranes occurs via the combined Knudsen/ordinary diffusion flow. This indicates that for the PTFE membranes, the water vapor molecules colloid with each other and diffuse among the air molecules present inside the membrane pores, which will hinder the transfer of the water vapor molecules. As stated earlier, the calculated DCMD flux for the same DCMD operating conditions is higher when Knudsen mechanism is considered. In addition, a theoretical model was developed to estimate the thickness of the hydrophobic toplayer of the membranes using the experimental parameters of the membranes (i.e. mean pore size, effective porosity, thickness) together with the DCMD flux and temperature polarization values reported elsewhere [16,21]. It was found that the thickness of the path length responsible for the mass transfer in DCMD of the SMM blended PEI membranes increased, from 4.4 ± 0.2 µm to 7.9 ± 0.3 µm, with the increase of the PEI concentration in the polymer casting solution and decreased with the increase of the pore size. For PTFE membranes, the thickness of the top layer responsible
110
M. Khayet et al. / Desalination 192 (2006) 105–111
of mass transport was found to be an order of magnitude higher than that of the SMM blended PEI membranes (49.4 ± 1.4 µm for TF200 and 51.5 ± 2.3 µm for TF450). These calculated values are slightly lower than the measured ones (54.8 ± 6.2 µm for TF200 and 60.0 ± 6.7 µm for TF450) using a Millitron micrometer (Mahr Feinpruf, type 1202 IC). This may be attributed to the membrane roughness effect or to the fact that the vapor/liquid interfaces, in each side of the membrane pores, are not located exactly on the membrane surface but inside the pores very near the membrane surface. It must be stated that the above reported values of the membrane thickness were determined using the experimental conditions: 500 rpm stirring rate, bulk temperature difference 10 K, feed temperature varying from 25ºC to 50ºC and distilled water used as feed [21]. 4. Future studies The composite porous hydrophobic/hydrophilic membranes seem to be promising for desalination by DCMD process. Membranes of larger pore size, up to 0.5 µm, higher porosity, up to 80%, thinner top-hydrophobic layer, less than 10 µm, thicker sub-hydrophilic layer, more than 90 µm, with larger pores than the top-hydrophobic layer are needed. It would be possible to prepare these membranes, of much higher productivity, by optimizing the membrane preparation conditions based on the proposed approach. The effect of SMM concentration in the polymer casting solution, hydrophilic polymer content, solvent evaporation time, total thickness (i.e. both hydrophobic and hydrophilic layers), etc. on final membrane structure and DCMD flux would be of great interest. On the other hand, theoretical models to study the SMM migration kinetics and thermodynamics, to estimate the thickness of the hydrophobic and the hydrophilic layers, to determine the conductive heat loss through the composite membranes, etc. would help to understand the mechanisms of transport in DCMD process.
Acknowledgements The authors of this work wish to gratefully acknowledge the financial support of the Spanish Ministry of Science and Technology (MCYT) through its project No. PPQ2003-03299 and the Middle East Desalination Research Center (MEDRC) for a grant that partially supported this study.
References [1] P.K. Weyl, Recovery of demineralised water from saline waters, US Patent 3,340,186 (1967). [2] M.E. Findley, Vaporization through porous membranes, Ind. Eng. Chem. Process Des. Dev., 6 (1967) 226–230. [3] Y. Fujii, S. Kigoshi, H. Iwatani and M. Aoyama, Selectivity and characteristics of direct contact membrane distillation type experiments: I and II, J. Membr. Sci., 72 (1992) 53–89. [4] D.Y. Cheng and S.J. Wiersma, Composite membranes for a membrane distillation system, US Patents 4,316,772 (1982); 4,419,242 (1983). [5] Y. Wu, Y. Kong, X. Lin, W. Liu and J. Xu, Surfacemodified hydrophilic membranes in membrane distillation, J. Membr. Sci., 72 (1992) 189–196. [6] Y. Kong, X. Lin, Y. Wu, J. Cheng and J. Xu, Plasma polymerization of octafluorocyclobutane and hydrophobic microporous composite membranes for membrane distillation, J. Appl. Polym. Sci., 46 (1992) 191–199. [7] M. Tomaszewska, Preparation and properties of flatsheet membranes from polyvinylidene fluoride for membrane distillation, Desalination, 104 (1996) 1– 11. [8] J.M. Ortiz de Zárate, L. Peña and J.I. Mengual, Characterization of membrane distillation membranes prepared by phase inversion, Desalination, 100 (1995) 139–148. [9] M. Khayet and T. Matsuura, Preparation and characterization of polyvinylidene fluoride membranes for membrane distillation, Ind. Eng. Chem. Res., 40 (2001) 5710–5718. [10] C. Feng, B Shi, G. Li and Y. Wu, Preliminary research on microporous membrane from F2.4 for membrane distillation, Sep. Purif. Tech., 39 (2004) 221–228.
M. Khayet et al. / Desalination 192 (2006) 105–111 [11] C. Feng, B Shi, G. Li and Y. Wu, Preparation and properties of microporous membrane from poly (vinylidene fluoride-co-tetrafluoroethyelene) (F2.4) for membrane distillation, J. Membr. Sci., 237 (2004) 15–24. [12] A. Larbot, L. Gazagnes, S. Krajewki, M. Bukowska and W. Kujawski, Water desalination using ceramic membrane distillation, Desalination, 168 (2004) 367–372. [13] L.Q. Shen, Z.K. Xu, Y.Y. Xu, Preparation and characterization of microporous polyethylene hollow fiber membranes, J. Appl. Polym. Sci., 84 (2002) 203–210. [14] Z.K. Xu, J.L. Wang, L.Q. Shen, D.F. Men and Y. Xu, Microporous polypropylene and polyethylene hollow fiber membranes. Part 1. Surface modificatin by the graft polymerization of acrylic acid, J. Membr. Sci., 196 (2002) 221–229. [15] J.M Li, Z.K. Xu, Z.M. Liu, W.F. Yuan, H. Xiang, S.Y. Wang and Y.Y. Xu, Microporous polypropylene and polyethylene hollow fiber membranes. Part 3. Experimental studies on membrane distillation for desalination, Desalination, 155 (2003) 153–156. [16] M. Khayet, J.I. Mengual and T. Matsuura, Porous hydrophobic/hydrophilic composite membranes: Application in desalination using direct contact
[17]
[18]
[19]
[20]
[21]
111
membrane distillation, J. Membr. Sci., 252 (2005) 101–113. M. Khayet, C.Y. Feng and T. Matsuura, Morphological study of fluorinated asymmetric polyetherimide ultrafiltration membranes by surface modifying macromolecules, J. Membr. Sci., 213 (2003) 159–180. M. Khayet, D.E. Suk, R.M. Narbaitz, J.P. Santerre and T. Matsuura, Study on surface modification by surface-modifying macromolecules and its applications in membrane-separation processes, J. Appl. Polym. Sci., 89 (2003) 2902–2916. M. Khayet and T. Matsuura, Progress in membrane surface modification by surface modifying macromolecules using polyethersulfone, polyetherimide and polyvinylidene fluoride base polymers: Applications in the separation processes ultrafiltration and pervaporation, Fluid/Particle Sep. J., 15(1) (2003) 9–21. M. Khayet and T. Matsuura, Application of surface modifying macromolecules for the preparation of membranes for membrane distillation, Desalination, 158 (2003) 51–56. M. Khayet, T. Matsuura and J.I. Mengual, Porous hydrophobic/hydrophilic composite membranes: Estimation of the hydrophobic-layer thickness, J. Membr. Sci., in press.
Design of novel direct contact membrane distillation membranes M. Khayeta*, T. Matsuurab, J.I. Menguala, M. Qtaishatb a
Department of Applied Physics I, Faculty of Physics, University Complutense of Madrid, Avda. Complutense s/n, 28040, Madrid, Spain Tel. +34 (91) 3944454; Fax +34 (91) 3945191; email: [email protected] b Industrial Membrane Research Center, Department of Chemical Engineering, University of Ottawa, 161 Louis Pasteur, P.O. Box 450, Stn. A, Ottawa, Ontario K1N 6N5, Canada Tel. +1 (613) 562-5800, Ext. 6114; Fax +1 (613) 562-5172; email: [email protected]
Received 23 February 2005; accepted 24 June 2005
Abstract The desired characteristics needed for a membrane to be used in direct contact membrane distillation (DCMD) are outlined. A new approach for the design of novel membranes for desalination by DCMD is proposed. Porous hydrophobic/hydrophilic composite membranes have been prepared in one casting step, using the traditional phase inversion method of polymer solutions containing a hydrophilic host polymer and a fluorinated surface modifying macromolecule. The membranes comply with the conditions and properties required in DCMD process. Membranes of different mean pore sizes, pore size distributions, porosities, roughnesses, liquid entry pressures of water and thicknesses can be formed by varying the membrane preparation conditions. The structural characteristics and the DCMD performance of the first proposed porous composite membranes were compared with those of the commercial membranes most frequently used in this kind of process. The proposed membranes have potential to be used in DCMD for desalination because, compared to the commercial ones, these membranes possess a thinner hydrophobic porous top-layer responsible for the water vapour transport in DCMD and a thicker hydrophilic porous sub-layer filled with water to lower the conductive heat loss. The thickness of the hydrophobic layer was found to be an order of magnitude lower than that of the commercial membranes. Keywords: Direct contact membrane distillation; Membrane surface modification; Surface modifying macromolecules; Porous composite membranes
*Corresponding author. Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2005.06.047
106
M. Khayet et al. / Desalination 192 (2006) 105–111
1. Introduction Direct contact membrane distillation (DCMD) is one of the emerging non-isothermal membrane separation processes known for about forty years but still is in its development stage. The first patent of DCMD was obtained by Weyl in 1967 (filed in 1964) [1] and the first DCMD publication was made by Findley in 1968 [2]. Since then several applications in desalination, environmental/waste cleanup, water-reuse and food processing have been proposed. It is worth quoting that water desalination is the best-known DCMD application. Among the four membrane distillation (MD) configurations (1 direct contact membrane distillation, DCMD, 2 air gap membrane distillation, AGMD, 3 sweeping gas membrane distillation, SGMD, and 4 vacuum membrane distillation VMD), DCMD is the most widely used. This process refers to a thermally driven transport of water through microporous hydrophobic membranes. The membrane is maintained between a hot solution (feed side) and a cold pure water (permeate side). Due to the hydrophobic nature of the membrane, liquid water cannot penetrate into the pores of the membrane until the applied pressure exceeds the liquid entry pressure (LEP), which is a characteristic of each membrane. The transmembrane vapour pressure drop, which is the driving force, is created by the temperature difference established between the feed and the permeate sides. Under these conditions, evaporation takes place at the hot feed side and, after water vapour is transported through the membrane pores, condensation takes place at the cold permeate side, inside the membrane module. Therefore, both heat and mass transfers take place simultaneously through the membrane. It must be pointed out that most of the publications in MD are concerned with theoretical MD transport models and experimental studies on the effects of operating conditions. Capillary or flatsheet commercial microporous hydrophobic membranes, made of polypropylene (PP), polyvinyl-
idene fluoride (PVDF) and polytetrafluoroethylene (PTFE, Teflon), have been used in MD experiments despite the fact that these membranes were prepared primarily for microfiltration purposes. As well, many researchers are devoting their efforts towards identification of new applications for MD. Looking to the extended published papers in MD field, only few authors have considered the possibility of designing and manufacturing membranes for MD processes [3–15]. Far is to be known about DCMD membrane before DCMD process reaches the industrial scale. The main aim of this study is to propose a promising novel method for preparation of DCMD membranes. The authors of this study also profit this opportunity to call the attention of the MD researchers in particular, and the membrane engineers in general, to the design and preparation of membranes for MD, which may be as important as it is the MD modelling for the development of MD processes.
2. Desired characteristics for DCMD membranes As it is well known, a MD membrane must be porous and hydrophobic, with good thermal stability and excellent chemical resistance to feed solutions. The characteristics needed for DCMD membranes are the following: High liquid entry pressure (LEP): This is the minimum hydrostatic pressure that must be applied onto the liquid feed solution before it overcomes the hydrophobic forces of the membrane and penetrates into the membrane pores. LEP is characteristic of each membrane and permits to prevent wetting of the membrane pores. High LEP may be achieved using a membrane material with high hydrophobicity (i.e. large water contact angle) and a small maximum pore size. However, as the maximum pore size decreases, the mean pore size of the membrane decreases and the permeability of the membrane becomes low.
M. Khayet et al. / Desalination 192 (2006) 105–111
High permeability: The DCMD flux will “increase” with an increase in the membrane pore size and porosity, and with a decrease of the membrane thickness and pore tortuosity. In other words, to obtain a high DCMD permeability, the surface layer that governs the membrane transport must be as thin as possible and its surface porosity as well as pore size must be as large as possible. However, it must be mentioned here that it seems to exist a critical pore size equal to the mean free path of water vapor molecules for given experimental DCMD conditions. In DCMD process, air is always trapped within the membrane pores with pressure values close to the atmospheric pressure. Therefore, if the pore size is comparable to the mean free path of water vapor molecules, the molecules of water vapor collide with each other and diffuse among the air molecules. In this case, the vapor transport takes place via the combined Knudsen/molecular diffusion flow. On the other hand, if the pore size is smaller than the mean free path of water vapor molecules, the moleculepore wall collisions become dominant and Knudsen type of flow will be the responsible for mass transport in DCMD. It must be mentioned that for given experimental conditions, the calculated DCMD flux considering Knudsen mechanism is higher than that considering the combined Knudsen/molecular diffusion mechanism. For example, for the commercial polytetrafluoroethylene membrane TF200 (mean pore size 0.2 µm and porosity 80%) and a bulk temperature difference of 10 K, when considering Knudsen flux, the calculated DCMD flux is 2.3 (for feed temperature 45ºC) to 2.6 (for feed temperature 20ºC) times higher than the corresponding DCMD flux calculated from the combined Knudsen/ molecular diffusion flux. For the membrane TF450 having higher pore size (i.e. mean pore size 0.45 µm and porosity 80%), the difference between the calculated DCMD fluxes is even higher. The calculated flux using Knudsen type of flow is 3.7 (for feed temperature 45ºC) to 4.3 (for feed temperature 20ºC) times higher than that
107
using the combined Knudsen/molecular diffusion type of flow. This fact indicates that when the pore size of a given membrane is near the critical pore size (i.e. mean free path of water vapor molecules), the DCMD flux will not necessarily increase with the increase of the pore size. Under some operating conditions, it would be better to use membranes with lower pore size than the corresponding mean free path of water vapor molecules so that Knudsen type of flow will takes place leading to higher DCMD flux than the membranes having larger pore size where the combined Knudsen/ molecular diffusion flux is the responsible for mass transfer. Therefore, care must be taken to choose the appropriate membrane pore size taking into account the value of the mean free path of water vapor molecules and trying to lend the membrane to work under Knudsen type of flow. Low thermal conductivity: In DCMD heat loss by conduction occurred through both the pores and the matrix of the membrane. The conductive heat loss is greater for thinner membranes. Various possibilities may be applied to diminish the conductive heat loss by using: i) Membrane materials with low thermal conductivities. This does not necessarily guarantee the improvement of the DCMD process because most hydrophobic polymers have similar heat conductivities, at least the materials have thermal conductivities with the same order of magnitude. ii) Membranes with high porosity, since the conductive heat transfer coefficient of the gas entrapped within the membrane pores is an order of magnitude smaller than that of the membrane matrix. This possibility is parallel to the need of high DCMD permeability as the available surface area of evaporation is enhanced. iii) Thicker membranes. However, there is a conflict between the requirements of high mass transfer associated with thinner membranes and low conductive heat transfer through the membrane obtained by using thicker membranes.
108
M. Khayet et al. / Desalination 192 (2006) 105–111
iv) Composite porous hydrophobic/hydrophilic membranes, having a very thin hydrophobic layer responsible for the mass transfer and a thick hydrophilic layer, the pores of which are filled with water, to prevent heat loss through the overall membrane. This seems to be a relatively simple solution that fulfills all the above conditions for achieving high permeability and low thermal conductivity. 3. Composite hydrophobic/hydrophilic membranes: background As far as we know, the use of composite membranes in MD has been described first by Cheng and Wiersma [4] in a couple of patents. A cellulose acetate membrane was modified via radiation graft polymerization of styrene onto the surface, and a cellulose nitrate membrane was modified via plasma polymerization of both vinyltrimethylsilicon/carbon tetrafluoride and octafluorocyclobutane onto the surface. Wu et al. [5] used hydrophilic porous partitions such as cellulose acetate, the surface of which was treated via radiation graft polymerization of styrene to achieve the required hydrophobicity. In the same way, Kong et al. [6] employed a cellulose nitrate membrane modified via plasma polymerization of both vinyltrimethylsilicon/carbon tetrafluoride and octafluorocyclobutane. In the membrane literature, various physical and chemical techniques are carried out for membrane surface modification. There are several methods to make composite membranes, i.e. coating, grafting, plasma treatment, etc. Nevertheless, many of the used surface modification methods are complicated and require at least one additional step during the membrane formation process. The question to be answered is: How to maintain the pore structure in the surface coating process? A novel and simple method to prepare composite porous membrane for DCMD was proposed in a previous paper [16]. The membrane was prepared by the phase inversion method, in
only one casting step, using polymer solution containing the host hydrophilic polymer and hydrophobic active additives, fluorinated surface modifying macromolecules (SMMs) [17–19]. Details on SMM preparation and characterization may be found in a recent review article [18]. The active additives can migrate to the air/polymer solution film interface during membrane casting and change its chemical and physical properties. The driving force for the migration of SMMs to the interface is the low surface energy of the fluorinated blocks. From X-ray photoelectron spectroscopy (XPS) and contact angle measurements it was found that the top surface of the formed membrane was more hydrophobic than the bulk membrane phase, while the properties of the bulk phase remained relatively unchanged as only a small amount of SMM was added. One of the various advantages of this method is the small amount of active additives required to modify the membrane surface, less than 5 wt % of the base polymer. It must be mentioned that, based on this principle, both dense and porous composite membranes can be prepared. In previous studies, various SMMs were synthesised and blended with the following base polymers: polyethersulfone (PES), polyvinylidene fluoride (PVDF), polyetherimide (PEI) and polyurethane (PU) [17– 19]. Compared to the unmodified membranes, the SMM blended membranes exhibit low surface energy (i.e. higher hydrophobicity of the top surface), higher mechanical strength, less fouling in ultrafiltration of aqueous solutions containing humic acid and cutting machine oil/water emulsion, higher pervaporation performance to remove chloroform from water, good potential to be used in biomedical applications, lower liquid and gas permeation fluxes due to their higher hydrophobicity and smaller pore size, and enhancement of the LEP of hydrophilic porous membranes, indicating that the SMM blended membranes can be used in MD process [17–20]. Moreover, it was observed that the pores of the SMM blended membranes decrease with the
M. Khayet et al. / Desalination 192 (2006) 105–111
increase of the host polymer concentration in the casting solution. Observing the tapping mode atomic force microscopy (AFM) images, the pore size of the top surface was found to be smaller than that of the bottom surface and the SMM blended membranes were smoother than the unmodified membranes [17]. Before DCMD experimental runs, various techniques may be used for characterization of the porous composite membranes, including the LEP of water measurement to prevent pore flooding during DCMD experiments, gas permeation test to determine the mean pore size and the effective porosity, atomic force microscopy (AFM) to determine the mean pore size, pore size distribution, porosity and roughness parameters of both the hydrophobic top and bottom surfaces, scanning electron microscopy (SEM) or transmission electron microscopy (TEM) to visualize the morphological structure of the membrane thickness, contact angle measurements of both the top and bottom surfaces of the composite membranes, X-ray photoelectron spectroscopy (XPS) to analyze the compositions near the membrane surface and mechanical tests. Finally, DCMD experiments may be carried out using the well known Lewis Cell employed in our previous study [16], in which both the feed and permeate liquids are stirred inside the cell by graduated magnetic stirrers, or using membrane modules working under tangential flows by means of circulating pumps. Preliminary DCMD experiments were carried out for pure water and different aqueous solutions of sodium chloride using SMM blended PEI membranes with different mean pore sizes, pore size distribution, effective porosity, roughness and LEP [16]. Distilled water and sodium chloride aqueous solutions were used as feed. The effect of the mean temperature, stirring rate and salt concentration were studied. Promising results were found. The proposed composite hydrophobic/hydrophilic membranes exhibited very high separation
109
factors (> 99.7 %) when sodium chloride aqueous solutions (up to 2 Molar) were used as the feed. Compared to the frequently used commercial poly-tetra-fluoro-ethylene (PTFE) membranes TF200 and TF450, the SMM blended PEI membranes having lower pore sizes and porosities than the above PTFE membranes showed values of DCMD flux at least of the same order of magnitude. Furthermore, the DCMD flux of one of the SMM blended PEI membrane was found to be higher than that of the PTFE membranes. This was attributed to two factors: 1) The very low path length between the liquid/vapor interfaces formed at both sides of the hydrophobic membrane pores; 2) The nature of the mass transport in DCMD for each membrane type. From the data of pore size and the mean free path of water vapor molecules, the mass transport through the pores of the SMM blended PEI membranes took place via the Knudsen flow, while the mass transport through the PTFE membranes occurs via the combined Knudsen/ordinary diffusion flow. This indicates that for the PTFE membranes, the water vapor molecules colloid with each other and diffuse among the air molecules present inside the membrane pores, which will hinder the transfer of the water vapor molecules. As stated earlier, the calculated DCMD flux for the same DCMD operating conditions is higher when Knudsen mechanism is considered. In addition, a theoretical model was developed to estimate the thickness of the hydrophobic toplayer of the membranes using the experimental parameters of the membranes (i.e. mean pore size, effective porosity, thickness) together with the DCMD flux and temperature polarization values reported elsewhere [16,21]. It was found that the thickness of the path length responsible for the mass transfer in DCMD of the SMM blended PEI membranes increased, from 4.4 ± 0.2 µm to 7.9 ± 0.3 µm, with the increase of the PEI concentration in the polymer casting solution and decreased with the increase of the pore size. For PTFE membranes, the thickness of the top layer responsible
110
M. Khayet et al. / Desalination 192 (2006) 105–111
of mass transport was found to be an order of magnitude higher than that of the SMM blended PEI membranes (49.4 ± 1.4 µm for TF200 and 51.5 ± 2.3 µm for TF450). These calculated values are slightly lower than the measured ones (54.8 ± 6.2 µm for TF200 and 60.0 ± 6.7 µm for TF450) using a Millitron micrometer (Mahr Feinpruf, type 1202 IC). This may be attributed to the membrane roughness effect or to the fact that the vapor/liquid interfaces, in each side of the membrane pores, are not located exactly on the membrane surface but inside the pores very near the membrane surface. It must be stated that the above reported values of the membrane thickness were determined using the experimental conditions: 500 rpm stirring rate, bulk temperature difference 10 K, feed temperature varying from 25ºC to 50ºC and distilled water used as feed [21]. 4. Future studies The composite porous hydrophobic/hydrophilic membranes seem to be promising for desalination by DCMD process. Membranes of larger pore size, up to 0.5 µm, higher porosity, up to 80%, thinner top-hydrophobic layer, less than 10 µm, thicker sub-hydrophilic layer, more than 90 µm, with larger pores than the top-hydrophobic layer are needed. It would be possible to prepare these membranes, of much higher productivity, by optimizing the membrane preparation conditions based on the proposed approach. The effect of SMM concentration in the polymer casting solution, hydrophilic polymer content, solvent evaporation time, total thickness (i.e. both hydrophobic and hydrophilic layers), etc. on final membrane structure and DCMD flux would be of great interest. On the other hand, theoretical models to study the SMM migration kinetics and thermodynamics, to estimate the thickness of the hydrophobic and the hydrophilic layers, to determine the conductive heat loss through the composite membranes, etc. would help to understand the mechanisms of transport in DCMD process.
Acknowledgements The authors of this work wish to gratefully acknowledge the financial support of the Spanish Ministry of Science and Technology (MCYT) through its project No. PPQ2003-03299 and the Middle East Desalination Research Center (MEDRC) for a grant that partially supported this study.
References [1] P.K. Weyl, Recovery of demineralised water from saline waters, US Patent 3,340,186 (1967). [2] M.E. Findley, Vaporization through porous membranes, Ind. Eng. Chem. Process Des. Dev., 6 (1967) 226–230. [3] Y. Fujii, S. Kigoshi, H. Iwatani and M. Aoyama, Selectivity and characteristics of direct contact membrane distillation type experiments: I and II, J. Membr. Sci., 72 (1992) 53–89. [4] D.Y. Cheng and S.J. Wiersma, Composite membranes for a membrane distillation system, US Patents 4,316,772 (1982); 4,419,242 (1983). [5] Y. Wu, Y. Kong, X. Lin, W. Liu and J. Xu, Surfacemodified hydrophilic membranes in membrane distillation, J. Membr. Sci., 72 (1992) 189–196. [6] Y. Kong, X. Lin, Y. Wu, J. Cheng and J. Xu, Plasma polymerization of octafluorocyclobutane and hydrophobic microporous composite membranes for membrane distillation, J. Appl. Polym. Sci., 46 (1992) 191–199. [7] M. Tomaszewska, Preparation and properties of flatsheet membranes from polyvinylidene fluoride for membrane distillation, Desalination, 104 (1996) 1– 11. [8] J.M. Ortiz de Zárate, L. Peña and J.I. Mengual, Characterization of membrane distillation membranes prepared by phase inversion, Desalination, 100 (1995) 139–148. [9] M. Khayet and T. Matsuura, Preparation and characterization of polyvinylidene fluoride membranes for membrane distillation, Ind. Eng. Chem. Res., 40 (2001) 5710–5718. [10] C. Feng, B Shi, G. Li and Y. Wu, Preliminary research on microporous membrane from F2.4 for membrane distillation, Sep. Purif. Tech., 39 (2004) 221–228.
M. Khayet et al. / Desalination 192 (2006) 105–111 [11] C. Feng, B Shi, G. Li and Y. Wu, Preparation and properties of microporous membrane from poly (vinylidene fluoride-co-tetrafluoroethyelene) (F2.4) for membrane distillation, J. Membr. Sci., 237 (2004) 15–24. [12] A. Larbot, L. Gazagnes, S. Krajewki, M. Bukowska and W. Kujawski, Water desalination using ceramic membrane distillation, Desalination, 168 (2004) 367–372. [13] L.Q. Shen, Z.K. Xu, Y.Y. Xu, Preparation and characterization of microporous polyethylene hollow fiber membranes, J. Appl. Polym. Sci., 84 (2002) 203–210. [14] Z.K. Xu, J.L. Wang, L.Q. Shen, D.F. Men and Y. Xu, Microporous polypropylene and polyethylene hollow fiber membranes. Part 1. Surface modificatin by the graft polymerization of acrylic acid, J. Membr. Sci., 196 (2002) 221–229. [15] J.M Li, Z.K. Xu, Z.M. Liu, W.F. Yuan, H. Xiang, S.Y. Wang and Y.Y. Xu, Microporous polypropylene and polyethylene hollow fiber membranes. Part 3. Experimental studies on membrane distillation for desalination, Desalination, 155 (2003) 153–156. [16] M. Khayet, J.I. Mengual and T. Matsuura, Porous hydrophobic/hydrophilic composite membranes: Application in desalination using direct contact
[17]
[18]
[19]
[20]
[21]
111
membrane distillation, J. Membr. Sci., 252 (2005) 101–113. M. Khayet, C.Y. Feng and T. Matsuura, Morphological study of fluorinated asymmetric polyetherimide ultrafiltration membranes by surface modifying macromolecules, J. Membr. Sci., 213 (2003) 159–180. M. Khayet, D.E. Suk, R.M. Narbaitz, J.P. Santerre and T. Matsuura, Study on surface modification by surface-modifying macromolecules and its applications in membrane-separation processes, J. Appl. Polym. Sci., 89 (2003) 2902–2916. M. Khayet and T. Matsuura, Progress in membrane surface modification by surface modifying macromolecules using polyethersulfone, polyetherimide and polyvinylidene fluoride base polymers: Applications in the separation processes ultrafiltration and pervaporation, Fluid/Particle Sep. J., 15(1) (2003) 9–21. M. Khayet and T. Matsuura, Application of surface modifying macromolecules for the preparation of membranes for membrane distillation, Desalination, 158 (2003) 51–56. M. Khayet, T. Matsuura and J.I. Mengual, Porous hydrophobic/hydrophilic composite membranes: Estimation of the hydrophobic-layer thickness, J. Membr. Sci., in press.