Ind. Eng. Chem. Res. 2005, 44, 7481-7489
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Pervaporation Separation of Water/2-Propanol Mixtures by Use of the Blend Membranes of Sodium Alginate and (Hydroxyethyl)cellulose: Roles of Permeate-Membrane Interactions, Zeolite Filling, and Membrane Swelling Boya Vijaya Kumar Naidu and Tejraj M. Aminabhavi* Membrane Separations Division, Center of Excellence in Polymer Science, Karnatak University, Dharwad-580003, India
In an effort to improve membrane performance of pristine sodium alginate (NaAlg) for 2-propanol dehydration, blend membranes of NaAlg with (hydroxyethyl)cellulose (HEC) (5, 10, and 20 mass %) were prepared. Membranes were prepared by solution casting and cross-linked by a twostage process as confirmed by Fourier transform infrared spectroscopy. The blend membrane of NaAlg with 10 mass % HEC gave the highest selectivity of 63 000 for 5 mass % water in feed mixture (highest selectivity achieved so far in the literature) by removing 99.97% of water, giving a flux of 0.04 kg/m2‚h. Incorporation of ZSM-5(40) zeolite in the blend membrane increased flux without affecting selectivity. Swelling results are used to study the membrane-solvent interactions. Sorption and diffusion selectivity values were computed from experimental data, which were comparable with the theoretically calculated values obtained from thermodynamic treatment based on Flory-Huggins theory. 1. Introduction Polymeric blends have been extensively studied in the literature for a variety of applications.1 In membranebased separation process, such as in pervaporation (PV) in separating water/organic mixtures, increased flux and selectivity are important. After the introduction of zeolites by Barrer,2 researchers have employed zeolitefilled3-6 membranes for PV separation of aqueousorganic mixtures. PV is a well-known membrane-based separation technique used to separate liquid mixtures on the basis of chemical potential difference, which acts as the driving force for molecular transport. Feed liquid diffuses through membrane and vaporizes on the other side, where pressure applied is lower than saturation vapor pressure. PV has the potential to separate azeotropic mixtures since the process has many inherent advantages over distillation, including reduced energy demands (only a fraction of the liquid is vaporized) and relatively inexpensive equipment (only a vacuum pump is required to create the driving force).7-9 Many PV studies have been reported for the separation of aqueous/organic mixtures10-15 using hydrophilic dense membranes; the process is not economical if organic concentrations are very low. The membrane with low hydrophilicity exhibits low water flux in dehydration, while the membrane made of natural polymers like sodium alginate (NaAlg), even though it exhibits low flux and selectivity, need modifications for improved stability and selectivity. In any case, polymeric materials, as successful dehydration membranes, should maintain a proper balance between hydrophilicity and hydrophobicity.16-18 Improvement in membrane performance has often been achieved by polymer modification including cross-linking, blending, grafting, * To whom correspondence should be addressed. Tel.: 91-836-2215372. Fax: 91-836-2771275. E-mail: aminabhavi@ yahoo.com.
incorporation of adsorbent materials, and so on.19-27 Membranes with high selectivity and flux are always preferred. As a consequence, the simultaneous increase of flux and selectivity has been a major challenge. The present paper addresses our continuing efforts to develop new membranes and/or to improve membrane performance of the existing membranes for PV separation of aqueous-organic mixtures. Sodium alginate (NaAlg) has been widely explored as a PV membrane for separation of a variety of aqueous/organic mixtures.17,23-26 Sodium alginate, derived from brown seaweeds, is a hydrophilic natural polymer that has also been used by other research groups19-22 for PV separation of water/organic mixtures, because the polymer can be easily cross-linked with aldehydes.20 One of the major industrial challenges in process engineering is to attempt the dehydration of 2-propanol, which is a widely used solvent in electronics and pharmaceutical industries, wherein its purity has great importance. It forms an azeotrope when mixed with 12.5 mass % water; its separation by conventional distillation requires high energy; additionally, the process poses environmental threats. Earlier, we have reported the preparation of blend membranes of NaAlg with PVA,23,24 and polyacrylamide-g-guar gum,25 and copolymeric membranes of polyacrylamide-g-NaAlg26 for PV separation of water/ 2-propanol mixtures. By doing so, we achieved the highest selectivity of 3600. In continuation of our ongoing efforts to develop new membranes based on NaAlg with higher selectivity, for the first time, we thought of blending NaAlg with (hydroxyethyl)cellulose (HEC), which is also a natural cellulose ether27 and provides sites for cross-linking reaction. HEC forms a compatible blend with NaAlg and both the polymers are derived from natural sources. To the best of our knowledge, there are no published reports in the earlier literature where such blend membranes were used in PV separation of water/2-
10.1021/ie050108t CCC: $30.25 © 2005 American Chemical Society Published on Web 08/09/2005
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propanol mixtures. Swelling and sorption selectivity experiments were also performed on these membranes, and experimental results are compared with the theoretically computed values based on Flory-Huggins theory.28 Effect of incorporation of a hydrophilic zeolite, viz., ZSM-5(40), on PV performance of the blend membrane was also investigated. Since Suzuki29 patented the first preparation of zeolite membranes in 1987, many types of zeolite membranes have been prepared and used in PV applications. Most previous studies have focused on silicalite-130 and ZSM-531 filled membranes, which have MFI structures, but MFI zeolites have a system of straight channels (pore size 0.53 × 0.56 nm) interconnected by zigzag channels (pore size 0.51 × 0.55 nm). Since hydrophilicity/hydrophobicity of zeolites can be controlled by the Si/Al ratio in the framework, high aluminum content zeolite, ZSM-5(40), with Si/Al = 40 and having a surface area of 400 m2/g, exhibited excellent water permselectivity, and hence its presence should improve flux in addition to maintaining dimensional stability of the blend membranes developed. Furthermore, results of this study have been analyzed by calculating interaction parameters between liquid components and membrane polymers. Membrane swelling, sorption, and diffusion anomalies have also been investigated. 2. Experimental Section 2.1. Materials. (Hydroxyethyl)cellulose (HEC) powder was purchased from Polysciences Inc.. Sodium alginate (NaAlg), glutaraldehyde (GA), 2-propanol, acetone, urea, formaldehyde, sulfuric acid, and hydrochloric acid (HCl) were purchased from s.d. Fine Chemicals, Mumbai, India. Double-distilled water was used; its purity was checked by conductivity and agreed with the literature value of 0.043 µS‚cm-1 within 2%. 2.2. Membrane Preparation. Blend membranes of NaAlg with HEC were prepared by solution casting. The required amount of NaAlg was dissolved in deionized water by stirring over a magnetic stirrer (Jenway, model 1103) for 24 h, to which different amounts of HEC (5, 10, and 20 mass %) were added. The membranes thus prepared were designated as NaAlg-HEC-5, NaAlgHEC-10, and NaAlg-HEC-20, respectively, while the pristine NaAlg was designated as NaAlg. The solution was mixed uniformly by stirring and filtered to remove any suspended particles. It was then poured onto a clean glass plate leveled perfectly on a tabletop kept in a dustfree atmosphere and dried at room temperature. Dried membranes were peeled off carefully from the glass plate. The zeolite-filled membrane was prepared in the same manner by dispersing 10 mass % ZSM-5(40) zeolite (sonicated for 30 min in water) before the membrane was cast. The membrane thus prepared was designated as NaAlg-HEC-10-ZSM-5(40). Preparation of blend membranes required a two-stage cross-linking. First, the membrane was cross-linked by immersion in a 75% aqueous/acetone mixture containing 2.5 mL of HCl and 2.5 mL of GA for about 24 h. After the membrane was removed from the cross-linking bath, it was washed with water repeatedly and dried in an oven at 40 °C. In the second stage, the first-stage crosslinked membrane was further cross-linked in a mixture of 2.5 mass % urea-2.2 mass % formaldehyde-2.5 mass % sulfuric acid containing 59 mass % aqueous ethanol at room temperature for 2 h. The membrane was washed repeatedly in deionized water and dried at room
temperature for about 24 h. Membrane thickness as measured by the micrometer screw gauge was in the range of 35-40 µm. 2.3. Fourier Transform Infrared Spectroscopic Studies. FTIR spectra of the cross-linked and un-crosslinked NaAlg-HEC-10 membranes in KBr pellets were recorded on a Nicolet model Impact 410 (Milwaukee, WI) in the wavelength range of 4000-400 cm-1. 2.4. Scanning Electron Microscopic Studies. Scanning electron micrographs of NaAlg-HEC-10 and NaAlg-HEC-10-ZSM-5(40) membranes were performed on a Leica Stereoscan-440 scanning electron microscope (SEM) equipped with Phoenix energy-dispersive analysis of X-rays (EDAX) available at National Chemical Laboratory, Pune (courtesy of Dr. S. B. Halligudi, Catalysis Division). 2.5. X-ray Diffraction Studies. The X-ray diffraction (x-RD) measurements of zeolite powder and NaAlgHEC-10 as well as NaAlg-HEC-10-ZSM-5(40) membranes were recorded on a Rigaku Geigerflex diffractometer equipped with Ni-filtered Cu K′R radiation (λ ) 1.5418 Å). The dried membranes of uniform thickness were mounted on a sample holder and the patterns were recorded in the range 0-50° at the speed of 5°/min. 2.6. Swelling Experiments. Dynamic and equilibrium swelling experiments on blend membranes as well as pristine NaAlg membranes were performed in water/ 2-propanol mixtures at 30 ( 0.5 °C by use of an electronically controlled incubator (WTB Binder, model BD-53, Tuttilgen, Germany) by following the procedures published earlier.32-34 Circularly cut (diameter ) 2.5 cm) disk-shaped membranes were stored in a desiccator over anhydrous calcium chloride maintained at 30 °C for at least 48 h before the start of the experiment. Mass measurements were made on a digital Mettler microbalance (model AE 240, Greifensee, Switzerland) sensitive to (0.01 mg. 2.7. Sorption Selectivity Experiments. Sorption selectivity was measured by adopting the procedure of Matsui and Paul.35 The cross-linked blend membrane was placed in a water/2-propanol mixture in an airtight test glass bottle kept in an electronically controlled incubator until sorption reached equilibrium. The membrane was removed at different intervals of time and pressed between smooth filter papers to remove any excess liquid droplets, and its weight was measured. The average of at least three independent measurements was taken to determine the maximum swelling. To determine the composition of liquid mixture, the membrane was sealed within 60 s into a glass test vessel connected to the vacuum line with a valve, which caused the evaporation of less than 2 mass % liquid out of the membrane. The bottom of the glass vessel was chilled in liquid nitrogen to avoid the evaporation of liquid from the membrane by pulling vacuum. The vessel and the sample tube were closed from the vacuum system, heated to evaporate the liquid from the membrane, and collected in the sample tube cooled under liquid nitrogen. The vapor was collected until further evaporation of liquid from the membrane was negligible. A standard curve of refractive index (measured on an Atago refractometer, Model 3T, Tokyo, Japan) vs composition of water in water/2-propanol mixture was used to determine the exact composition of the mixture in the sample tube and feed mixtures. Sorption selectivity RS was calculated from
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RS )
wim/wjm wif/wjf
(1)
where wim (wjm) is the weight fraction of component i (j) in the membrane and wif (wjf) is the weight fraction of component i (j)in the external liquid phase. 2.8. Pervaporation Experiments. The procedure used in PV experiments was described elsewhere.36,37 The effective membrane area was 32.43 cm2 and the weight of the feed mixture taken in the PV cell was 50 g. Temperature of the feed mixture was maintained constant by a thermostatic water jacket. Downstream pressure was maintained below 10 Torr with a vacuum pump (Toshniwal, Mumbai, India). Before the experiment, the test membrane was equilibrated for about 2 h with the feed mixture. After establishment of the steady state, permeate vapors were condensed in traps immersed in liquid nitrogen. PV experiments were performed with different feed mixtures of water/2-propanol. Adding the required amount of solvent mixture enriched the depleted solvent mixture in the feed compartment. The weight of permeate condensed in the trap was noted and permeate composition was determined by measuring refractive index and comparing it with the established graph of refractive index vs mixture composition. In the case of feed mixtures with lower amounts of water (5-12.5 wt %), the permeate was analyzed on a Nucon gas chromatograph (model 5765) provided with a thermal conductivity detector (TCD) equipped with a Tenax packed column of 1/8-in. i.d. and 2 m length. Oven temperature was maintained at 70 °C (isothermal), while injector and detector temperatures were maintained at 150 °C. The sample injection volume was 1 µL. Pure hydrogen was used as a carrier gas at a pressure of 1 kg/cm2. The GC was calibrated for the particular analytical conditions including column type, oven temperature, injector and detector temperatures, TCD current, and carrier H2 gas flow rates. Known compositions of water/2-propanol feed mixture were injected to obtain different response data from the GC, which was fed into the software for obtaining correct analysis values for unknown samples, that is, permeates. Pervaporation selectivity, RPV, was calculated as
RPV )
(
)(
)
PA 1 - FA 1 - PA FA
(2)
where FA is mass % water in the feed and PA is mass % water in the permeate. The flux, J (kilograms per square meter per hour), was calculated from the weight of liquids permeated, W (kilograms), effective membrane area, A (square meters), and measurement time, t (hours) as
J)
W At
(3)
At least three independent measurements of flux and RPV were taken under the same conditions of temperature and feed composition to confirm the steady-state pervaporation. Diffusion selectivity, RD, was calculated from the experimental RPV and RS values on the basis of the solution-diffusion theory:38
RPV ) RSRD
(4)
These data are compared in Table 1. It may be noted
Table 1. Pervaporation, Sorption, and Diffusion Selectivites of Different Membranes at 30 °C mass % of water in feed
pervaporation selectivity (RPV)
sorption selectivity (RS)
diffusion selectivity (RD)
NaAlg 10 20 30
650 230 130
100 30 15
6.5 7.6 8.6
10 20 30
NaAlg-HEC-5 900 150 300 50 160 25
6.0 6.0 6.4
10 20 30
NaAlg-HEC-10 30 000 1000 600 90 180 30
30 6.6 6.0
10 20 30
NaAlg-HEC-20 2200 500 230 40 90 10
4.4 5.7 9.0
that after continuous use of PV membranes for more than 10 h, the membranes were quite sturdy without any degradation. The same membranes when used for the second run of experiments also retained their integrity. 2.9. Density of Polymer Membranes. Density of the polymer was measured by the benzene displacement method with a specific gravity bottle. Initially, benzenefilled bottle and empty bottle weights were taken. Then a weighed quantity of the polymer was introduced into the bottle. Excess benzene was wiped out with a tissue paper and the weight of the bottle along with benzene and polymer was taken. From these weights, the volume of the polymer was calculated, from which density of the polymer was calculated. 3. Results and Discussion 3.1. Cross-Linking of NaAlg-HEC Blend Membrane. In the present study, both the polymers belong to the polysaccharide family, which can be readily crosslinked with glutaraldehyde20,24,25 in acidic media. Initially, the blend membranes of NaAlg-HEC were crosslinked with GA. When these membranes were tested for PV experiments, no systematic variations of flux and selectivity were observed with increasing mass percentage of water in the feed, but the pristine NaAlg membrane cross-linked with GA exhibited systematic variations. This prompted us to further cross-link GAcross-linked blend membranes with a urea-formaldehyde-sulfuric acid (UFS) mixture, since urea-formaldehyde under acidic or basic conditions could be an effective cross-linking agent for cellulosic polymers.27,39 The blend membranes thus prepared by double crosslinking have shown a systematic variation of flux and selectivity with increasing amount of water in the feed. The same procedure was adopted for cross-linking pristine NaAlg membrane. The cross-linked NaAlgHEC blend membranes were mechanically strong and did not deteriorate during PV experiments. Cross-linking of blend membrane (see Figure 1) was confirmed by FTIR. Hydroxyl groups of NaAlg have reacted with the aldehyde group of GA to form crosslinks between NaAlg chains. The UFS system also effectively cross-linked the hydroxyl groups of HEC. Figure 2 displays the FTIR spectra of un-cross-linked, GA-cross-linked, and GA + UFS-cross-linked mem-
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Figure 3. Scanning electron micrographs of (a) NaAlg-HEC-10 and (b) NaAlg-HEC-10-ZSM-5(40).
Figure 4. x-RD tracings of zeolite (a) ZSM-5(40), (b) NaAlgHEC-10, and (c) NaAlg-HEC-10-ZSM-5(40) membranes.
Figure 1. Model structure of cross-linked NaAlg-HEC blend membrane.
Figure 5. Plots of swelling vs time at 10 mass % water in water/ 2-propanol feed mixtures for (O) plain NaAlg, (b) NaAlg-HEC-5, (4) NaAlg-HEC-10, and (2) NaAlg-HEC-20 membranes.
Figure 2. FTIR spectra of (a) un-cross-linked, (b) GA cross-linked, and (c) GA + UFS cross-linked NaAlg-HEC-10 membrane.
branes of NaAlg-HEC-10. The observed spectral shifts confirm the cross-linking reaction. The characteristic peak at ∼3444 cm-1 in all the membranes corresponds to O-H stretching vibrations of NaAlg and HEC polymers. A sharp peak at ∼1642 cm-1 corresponds to CdO stretching of the carboxylic group of NaAlg. Peaks that appeared in the range of 1116-1134 cm-1 in GA and GA + UFS-cross-linked membranes correspond to vibrations of C-O-C linkage, due to the formation of an ether linkage between the hydroxyl group and methylolurea/glutaraldehyde. The peak at ∼702 cm-1 is due to S-O-C linkage of the cross-linked chains, confirming the chemical reaction between HOSO3- and methylolurea.27 Such changes in FTIR spectra confirmed the successful cross-linking reaction. 3.2. SEM Analysis. Figure 3 compares the surface SEM images of ZSM-5(40) zeolite-filled membrane with
that of plain of NaAlg-HEC-10 blend membrane. The zeolite-filled membrane showed a molecular-level distribution of zeolite particles in the membrane. 3.3. X-RD Analysis. The X-ray diffractograms of pristine ZSM-5(40) zeolite, virgin NaAlg-HEC-10 blend membrane, and ZSM-5(40)-filled NaAlg-HEC-10 membrane are shown in Figure 4. Sharp peaks observed around 5-6° and 23-25° represent the crystalline nature of the zeolite. However, similar peaks with lesser intensity also appeared in the zeolite-filled membrane, confirming the incorporation of zeolite in NaAlg-HEC matrix, but peak intensities are somewhat lower due to the surrounding NaAlg-HEC matrix. 3.4. Swelling Studies. Molecular transport32-34 through polymeric membranes is a complex phenomenon because of the interactions between feed liquid components and the membrane, resulting in membrane swelling. The solution-diffusion model has been widely accepted for explaining the mass transport mechanism40 for pervaporation. Experiments reported here were designed and interpreted with special attention to these issues. Results of dynamic swelling of liquids at 30 °C through the blend membranes are displayed in Figure 5 for 10 mass % water-containing feed mixture. Membrane swelling is controlled by the diffusion of solvent molecules in relation to polymeric chain relaxation.41 Irregular trends in swelling curves could be the result of differences in the rate of molecular chain relaxation due to locally induced stresses in the polymer matrix and nature of the liquid molecules transporting across
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Figure 6. Equilibrium swelling vs HEC content of NaAlg-HEC blend membranes, in 10 mass % water containing water/2-propanol mixture at 30 °C for (b) total swelling, (2) water swelling, and (9) 2-propanol swelling.
Figure 8. Selectivity vs mass % water in feed at 30 °C: (O) plain NaAlg; (b) NaAlg-HEC-5; (4) NaAlg-HEC-10; (2) NaAlgHEC-20.
Figure 9. Water flux vs mass % water in feed at 30 °C. Symbols have the same meanings as in Figure 8.
Figure 7. Sorption selectivity vs HEC content in NaAlg-HEC blend at 10 mass % water.
the membrane. On the other hand, a regularly increasing mass uptake was observed due to ingression of higher amount of water through the voids of the membranes. However, the time required to attain equilibrium swelling varied depending upon the morphology of the membrane. For instance, as shown in Figure 5, with the pristine NaAlg membrane, swelling reached equilibrium within 25 min, whereas for NaAlg-HEC blend membranes, swelling was reached before 25 min. Swelling experiments were continued for longer times to ensure complete equilibrium, which followed the sequence NaAlg-HEC-20 > NaAlg-HEC-10 > NaAlg-HEC-5 > NaAlg. Thus, compared to swelling of blend membranes with the pristine NaAlg membrane, after incorporation of a higher amount of HEC, swelling was also higher. Equilibrium swelling results of the feed mixture containing 10 mass % water are compared in Figure 6 with those of the individual components (water or 2-propanol) of the feed mixture for all the membranes. The total mixture and water equilibrium swelling data increased with increasing HEC content of the blend, but a reverse trend was observed for 2-propanol. This may be due to an increase in the free volume of the blend membrane with increasing HEC content, since the hydroxyethyl groups of HEC are bigger than the carboxylic group of NaAlg. Also, it may be noted that the equilibrium swelling curves of the feed mixtures and that of water follow the same pattern, which is quite different than that of the swelling curve of 2-propanol. Sorption selectivity as a function of HEC content in the blend membrane is shown in Figure 7. NaAlg-HEC-10 membrane exhibited higher sorption selectivity than all other membranes. Hence, it was selected for further detailed investigation.
Figure 10. Composition of water in permeate vs mass % water in feed at 30 °C. Symbols have the same meanings as in Figure 8.
3.5. Pervaporation Results. The PV results expressed in terms of selectivity and flux of the blend membranes are compared, respectively, in Figures 8 and 9, with the pristine NaAlg membrane. For NaAlg-HEC10 blend membrane with 10 mass % water-containing feed mixture, selectivity was 30 000, the highest so far achieved in the literature with 99.97 wt % water in the permeate (see Figure 10). For 5 mass % HEC-containing blend membrane, selectivity for 10 mass % watercontaining feed was comparatively very small (i.e., RPV ) 900), whereas for 20 mass % HEC-containing blend membrane, selectivity was 2200. Thus, the optimum selectivity for water could be achieved only for the blend membrane containing 10 mass % HEC. However, the selectivity of pristine NaAlg at 10 mass % water in the feed was lowest (i.e., RPV ) 650). With increasing amount of water in the feed, selectivity of the pristine NaAlg membrane decreased considerably for 20-50 mass % water in the feed. Water flux values (see Figure 9) of the blend membranes were higher than those observed for the pristine NaAlg membrane, but no drastic improvement in water flux was observed even after the addition of HEC. In the present study, NaAlg-HEC-10 blend membrane exhibited the highest selectivity to water at the lower water composition of the feed mixture (i.e., 10 mass % of water). This prompted us to perform PV
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Table 2. Pervaporation Data for Water/2-propanol Mixtures for NaAlg-HEC-10 Blend Membrane near the Azeotropic Composition at 30 °C mass % of water in feed
mass % of water in permeate
water flux, kg/m2‚h
selectivity
5 7.5 10 11.5 12.5 15
99.97 99.97 99.97 99.97 99.95 99.63
0.04 0.07 0.09 0.09 0.09 0.09
63 000 41 000 30 000 25 000 14 000 1 500
Table 3. Pervaporation Results of ZSM-5(40)-Filled NaAlg-HEC-10 Blend Membrane at 30 °C mass % of water in feed
mass % of water in permeate
water flux, × 102 kg/m2‚h
selectivity
10 12.5 15
99.97 99.95 99.63
0.20 0.22 0.25
30 000 14 000 1 500
experiments for lower feed compositions of water, that is, below 10 mass %. However, a few additional experiments were performed at 12.5 and 15 mass % water in the feed, due to the fact that water/2-propanol forms an azeotrope at 12.5 mass % water. These data are presented in Table 2. At 5 mass % water, the observed selectivity was as high as 63 000, almost reaching the infinity value, thereby pushing the limit of membrane selectivity to water compared to the previously investigated NaAlg-based membranes. However, by increasing the amount of water in the feed, selectivity decreased considerably, while flux did not change much. Realizing the moderate increase in flux of the blend membranes compared to the pristine NaAlg membrane, we have incorporated 10 mass % ZSM-5(40) zeolite into NaAlg-HEC-10 blend membrane. These results are presented in Table 3. For this filled membrane, flux increased considerably for 10 mass % water-containing feed from its initial value (for virgin membrane) of 0.09 to a highest value of 0.2 kg/m2‚h without affecting the selectivity. This increase in flux can be assigned to high water uptake capacity of the zeolite particles due to the pores created in the membrane, thereby enhancing the permeation rate of water. However, the other blend membranes showed reduced fluxes compared to zeolitefilled membrane. At this stage, a great deal still remains to be understood about the transport mechanism of various species through the zeolite-filled membranes. The transport mechanism of aqueous species through the zeolites in the presence of a nonzeolitic matrix is more complex than in the pristine NaAlg membrane. The transport of aqueous species could take place through both molecularly selective zeolite microcrystals as well as the less selective interstitial regions between the zeolite particles and the polymeric membrane matrix. The present data suggest that, during PV, water permeates through both zeolitic and nonzeolitic pores because of its smaller diameter as evidenced by an increase in flux. Note that the PV experiments of this study were performed only at room temperature. However, it has been our experience that an increase of temperature could increase the flux, but with a decrease in selectivity. 3.6. Comparison between PV and Vapor Liquid Equilibrium Results. The major difference between PV separation via membranes and that of simple distillation is that the latter is not environmentally benign for separation of azeotropic mixtures. If one
Figure 11. Comparison of vapor liquid equilibrium curve (2), with PV data (b) for water (1) + 2-propanol (2) mixtures at 30 °C for NaAlg-HEC-10 membrane.
attempts to separate a water/2-propanol mixture by simple distillation, benzene (a deadly carcinogen) has to be added as an entrainer. However, the advantage of PV is that no such entrainer is required to be added since the membrane acts as a third phase to achieve an effective separation. To see this effect, we have constructed a vapor-liquid equilibrium (VLE) curve from the literature42 for comparison with the PV curve in Figure 11. It is observed that the PV curve lies well above the VLE curve, indicating successful dehydration of 2-propanol near the azeotropic composition. 3.7. Comparison with Literature. Superiority of the present membranes in separating water/2-propanol mixtures can be judged by comparing our results (see Table 4) with those of the published literature data.17,24-26,43,44 It is remarkable to note that NaAlgHEC-10 membrane gave the highest selectivity in the range of 63 000-14 000 for feed mixture compositions varying from 5 to 12.5 mass % (the latter being azeotropic composition) of water. Flux values of the membranes for the same composition range of water in the feed mixture varied between 0.04 and 0.09 kg/m2‚h. Upon adding 10 mass % hydrophilic ZSM-5(40) zeolite into the membrane, we observed an increase in flux from 0.09 to 0.2 kg/m2‚h for 10 mass % water in the feed, which is also the highest value reported so far in the literature. Compared to the performance of the present membranes, NaAlg membranes loaded with 30% NaY zeolite gave very poor selectivity43 of only 600 with a flux of 0.15 kg/m2‚h for 5 mass % water in the feed containing water/2-propanol mixture. Thus, the choice of the proper zeolite is the key to success to increase both selectivity and flux. On the other hand, our earlier membranes17 prepared from NaAlg with 5 mass % poly(vinyl alcohol) and 10 mass % poly(ethylene glycol) have given a selectivity of 3600 (at the expense of moderate flux of only 0.07 kg/ m2‚h) for 10 mass % water-containing feed mixture. Even the composite membrane of NaAlg and chitosan44 gave a selectivity of 2000 with a flux of 0.06 kg/m2‚h for 10 mass % water-containing feed mixture. NaAlg blended with acrylamide-g-guar gum membrane25 gave a selectivity of 900, a much higher value than those of NaAlg + NaY membranes.43 But selectivity and flux of NaAlg and PVA membranes24 were in the range of 0.03-0.039 kg/m2‚h and 90-200, respectively, which are much lower than those observed for pristine NaAlg membrane for which selectivity was 350, while flux was 0.06 kg/m2‚h. However, the present NaAlg-HEC-10 blend membrane showed much better flux and selectivity than all other NaAlg-based membranes studied hitherto in the literature.
Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005 7487 Table 4. Comparison of PV Performance of the Present Membranes with Literature mass % of water in feed
flux (kg/m2‚h)
selectivity
reference
NaAlg-HEC-10 (GA + UFS cross-linked) NaAlg-HEC-10 (GA+UFS cross-linked) NaAlg-HEC-10 (GA + UFS cross-linked) NaAlg-HEC-10 (GA + UFS cross-linked) NaAlg-HEC-10 (GA + UFS cross-linked)
5 7.5 10 12.5 15
0.04 0.07 0.09 0.09 0.09
63 000 41 000 30 000 14 000 1 500
this work this work this work this work this work
NaAlg-HEC-10-ZSM-5(40) (zeolite-loaded, GA + UFS cross-linked) NaAlg-HEC-10-ZSM-5(40) (zeolite-loaded, GA + UFS cross-linked) NaAlg-HEC-10-ZSM-5(40) (zeolite-loaded, GA + UFS cross-linked)
10 12.5 15
0.2 0.22 0.25
30 000 14 000 1 500
this work this work this work
NaAlg (30% Na-Y zeolite-loaded)
5
0.15
600
NaAlg + 5% PVA + 10% PEG
10
0.07
3600
NaAlg NaAlg-GG-g-PAAm NaAlg-GG-g-PAAm
10 10 10
0.05 0.06 0.04
400 800 900
25 25 25
NaAlg /PVA (75:25) NaAlg-PVA (50:50) NaAlg-PVA (25:75)
0.030 120 90
200 24 24
24
0.034 0.039
composite membrane
10
0.06
2000
44
membranea
43 17, 26
a
NaAlg, sodium alginate; HEC, (hydroxyethyl)cellulose; PAAm, poly(acrylamide); PVA, poly(vinyl alcohol); PEG, poly(ethylene glycol); composite membrane, sodium alginate and chitosan.
4. Thermodynamic Treatment: Preferential Sorption PV separation is governed by the chemical nature of the macromolecular network. To understand the PV separation at the molecular level is complicated, but phenomenologically, one can study the preferential interaction parameter of a liquid with the membrane. Several approaches for selecting membranes have been proposed, but the ideal situation to describe mass transfer can be studied by a solution-diffusion model.38 However, for choosing an efficient membrane for a specific separation problem, it is desirable to achieve good membrane permeability and selectivity, but the thermodynamic conditions of feed flow and permeate cannot be overlooked. To consider these effects, it is important to partition the pervaporation selectivity, RPV, into solubility selectivity, RS, and diffusion selectivity, RD. Here, the thermodynamic part dominates RPV since RS increased with increasing amount of HEC in the blend membrane up to 10 mass % and then declined, while RD did not show any systematic variation (see Table 1). The observed differences in selectivity were explained by the thermodynamic theory of Aminabhavi and Munk45 developed for polymers in a mixed solvent (a threecomponent system) to compute preferential interaction of polymers in binary mixtures using Flory-Huggins free energy mixing term (∆Gmix). Later, Mulder and Smolders40 extended this concept to explain the PV results. According to the well-known Flory-Huggins theory, we have
( ) () (
) ( ) ( )
Φ1 v1 V1 Φ2 - ln ) - 1 ln Φ2 v2 V2 v2 V1 χ12(Φ2 - Φ1) - χ12(v1 - v2) - ΦP χ1P - χ2P (5) V2
ln RS ) ln
Here, Φi is the volume fraction of the ith component in the swollen polymer membrane, vi is the volume fraction of the ith component in the external liquid phase, and Vi is the respective molar volume. Subscripts 1, 2, and P refer to water, 2-propanol, and the membrane polymer, respectively. The values of Vi at 30 °C for water
and 2-propanol were taken from the literature.46 The volume fraction, φP, of the polymer in the swollen state was calculated from47
[ ( ) ( )]
ΦP ) 1 +
FP Ma FP FS Mb FS
-1
(6)
where FP and FS are the densities of polymer and solvent, respectively, and Mb and Ma are the mass of the membrane before and after swelling. Molar volume of the binary mixtures of water/2-propanol was calculated from48
V)
(x1M1 + x2M2) Fm
(7)
where x1 and x2 are, respectively, mole fractions of components 1 and 2 of the mixture; M1 and M2 are the corresponding molecular weights; and Fm is the density of the solvent mixture (measured by Anton Paar digital densitometer). The interaction parameter, χ12, between water and 2-propanol was then calculated from the equation40,49
χ12 )
[x1 ln (x1/v1) + x2 ln (x2/v2) + (∆GE/RT)] (8) x1v2
where ∆GE (joules per mole) is the excess Gibbs free energy of mixing, R is the molar gas constant (joules per mole per kelvin), and T is the temperature (kelvins). Values of ∆GE were calculated from the thermodynamic data on activity coefficients, γ, of the mixtures as
∆GE ) RT(x1 ln γ1 + x2 ln γ2)
(9)
In the absence of direct experimental data on γ1 and γ2, we have used the van Laar equation at 30 °C to compute the activity coefficient, γi, of component i in the mixture:
(
ln γi ) Aij
Ajixj Aijxi + Ajixj
)
2
(10)
7488
Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005
Table 5. Interaction Parameters and Computed Sorption Selectivity for Different Membranes with Water/2-Propanol Mixtures at 30 °C interaction parameter
sorption selectivity
mass % of water in feed χ12
water/2-propanol [eq 8]
water/polymer (χ1P) [eq 11]
2-propanol/polymer (χ2P) [eq 11]
exptl [eq 1]
calcd [eq 5]
NaAlg NaAlg NaAlg
10 20 30
1.88 1.26 1.08
0.34
1.21
100 30 15
90 25 15
NaAlg-HEC-5 NaAlg-HEC-5 NaAlg-HEC-5
10 20 30
1.88 1.26 1.08
0.31
1.29
150 50 25
140 40 15
NaAlg-HEC-10 NaAlg-HEC-10 NaAlg-HEC-10
10 20 30
1.88 1.26 1.08
0.28
1.31
1000 90 30
950 70 20
NaAlg-HEC-20 NaAlg-HEC-20 NaAlg-HEC-20
10 20 30
1.88 1.26 1.08
0.29
1.22
500 40 10
400 30 9
membrane
The van Laar parameters, Aij for water and Aji for 2-propanol, were taken from the literature.50 Polymer-solvent interaction parameter χiP was calculated from Flory-Higgins theory:28,31
χiP )
Vi(δP - δi)2 RT
(11)
branes. Overall, the results are very promising, superseding much of the previously published data in the literature on sodium alginate-based membranes, used in PV separation of a water/2-propanol mixture. However, a tradeoff between flux and selectivity is to be determined for future industrial applications of these membranes.
where δi is the solubility parameter (joules1/2‚centimeter-3/2) of the ith component. Solubility parameters δP of NaAlg and HEC polymers were estimated to be 61.48 and 42.8 J1/2‚cm-3/2, respectively, from the atomic group contribution method.51,52 Solubility parameters for water and 2-propanol were taken from the literature.46 Solubility parameters of the blend polymers NaAlg-HEC-5, NaAlg-HEC-10, and NaAlg-HEC-20 were calculated from the additive relation:
Acknowledgment
δ ) w1δ1 + w2δ2
Literature Cited
(12)
Here, w1 and w2 are weight fractions and δ1 and δ2 are solubility parameters of NaAlg and HEC, respectively. Interaction parameters calculated from eqs 8 and 11 along with the experimental and calculated sorption selectivity data are presented in Table 5. Calculated values of RS are similar in magnitude but showed trends analogous to those of the experimental values (see Table 1). This suggests that the procedure used is appropriate. 5. Conclusions In this study, novel blend membranes of sodium alginate were prepared with varying amounts of (hydroxyethyl)cellulose. The blend membrane prepared with 10 mass % (hydroxyethyl)cellulose was chosen for the detailed study, since it exhibited the optimum membrane performance. Pervaporation data were compared with the pristine sodium alginate membrane. Results are quite promising since we could push the limit of highest selectivity to water by fabricating the blend membrane comprising 10 mass % containing (hydroxyethyl)cellulose. Pervaporation was further affected by the addition of 10 mass % zeolite [ZSM-5(40)]. Flux increased greatly when the NaAlg-HEC-10 blend membrane was incorporated with ZSM-5(40) since ZSM5(40), being hydrophilic, could enhance water flux to almost double the original value compared to virgin blend membrane. The roles of permeate-membrane interactions, zeolite-filling, and membrane swelling all supported the pervaporation performance of the mem-
This research was funded by the Council of Scientific and Industrial Research (CSIR), New Delhi, Grant 80(0042)/02/EMR-II. We thank the University Grants Commission (UGC), New Delhi, India, for major funding (F1-41/2001/CPP-II) to establish the Center of Excellence in Polymer Science at Karnatak University, Dharwad, India. This paper is CEPS Communication No. 58.
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Received for review January 27, 2005 Revised manuscript received July 6, 2005 Accepted July 12, 2005 IE050108T
