Separation of Ethanol−Water Mixtures by Pervaporation Using Sodium

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Separation of Ethanol-Water Mixtures by Pervaporation Using Sodium Alginate/ Poly(vinyl pyrrolidone) Blend Membrane Crosslinked with Phosphoric Acid Swayampakula Kalyani,† Biduru Smitha,‡ Sundergopal Sridhar,‡ and Abburi Krishnaiah*,† Biopolymers and Thermophysical Laboratory, Department of Chemistry, Sri Venkateswara UniVersity, Tirupatis517 502, India, and Membrane Separations Group, Chemical Engineering DiVision, Indian Institute of Chemical Technology, Hyderabads500 009, India

The blend membranes of sodium alginate (SA) and poly(vinyl pyrrolidone) (PVP) are prepared by physical mixing of SA and PVP. The membranes are crosslinked with phosphoric acid and used in the pervaporation separation of a water + ethanol mixture at 30 °C. Fourier transform infrared spectroscopy and ion exchange capacity confirmed the crosslinking reaction. Thermal stability and crystallinity are determined from TGA and XRD studies. The membrane performance is studied by calculating flux, selectivity, and pervaporation separation index. Sorption studies are carried out to evaluate the extent of interaction and degree of swelling of the membranes in pure liquids as well as in binary mixtures. The effect of experimental parameters such as feed composition, membrane thickness, and permeate pressure on separation performance of the crosslinked membranes is determined. The membrane appears to have good potential for breaking the aqueous azeotrope of 96 mass % ethanol with the selectivity of 364 and substantial water flux of 0.5 kg m-2 h-1 10 µm. 1. Introduction There has been increased interest in the use of the pervaporation (PV) membrane process for the separation of organic liquid mixtures in recent years. A major advantage of this process is the ability to separate azeotropic mixtures such as water-ethanol systems, since it can overcome the limit of liquid vapor equilibrium. With this particular characteristic, pervaporation has been proven as a highly efficient separation process for dehydration of organic solvents.1-4 Many hydrophilic polymer membranes have been investigated for the dehydration of a water/alcohol azeotropic mixture on the basis of solution diffusion theory.5,6 A good pervaporation membrane should exhibit high permeation flux and high separation selectivity. The simultaneous enhancement of both these parameters has been a challenge in pervaporation separation industries. For aqueous mixtures, hydrophilic polymers have good separation characteristics due to their strong affinity toward water.7-10 However, excess hydrophilicity of the membrane material is not necessarily suitable for the dehydration process, because it can give a low selectivity and poor membrane stability against the aqueous solution at a cost of high flux. High permeable polymer material, which is chemically modified or crosslinked, has an optimal combination of flux and membrane stability or selectivity. Different types of membranes made from natural polymers11-13 have been used in PV separation studies.14 However, due to the abundant availability, biocompatibility, and commercial viability, SA has been used to separate aqueous-organic mixtures.15-19 The performance of a pure SA membrane is not satisfactory because of a large volume between the chains.20 Its performance has been improved by modifying alginate with different methods such as blending, 21,22 grafting,23 and crosslinking.24 In continuation of our ongoing program of research in * Corresponding author. Tel.: +919393621986. [email protected]. † Sri Venkateswara University. ‡ Indian Institute of Chemical Technology.

E-mail:

developing newer membranes,25,26 we now extend our study on the pervaporation separation of water + ethanol mixtures using the blend membranes of SA and PVP. In the present investigation, attempts are made to test the applicability of SA/PVP blends, crosslinked with phosphoric acid, for dehydration of ethanol. The work also explores the effect of polymer swelling and sorption selectivity, pervaporation separation index on separation performance by varying water percentages in the feed mixture from 5 to 45 mass %. FTIR spectra, XRD, and thermal analysis have been used to characterize the membranes. The effect of other parameters such as permeate pressure and membrane thickness on separation performance has been evaluated. 2. Experimental Section 2.1. Materials. PVP, having average molecular weight of 360000, was purchased from PIDILITE Industries Ltd., Mumbai, India. SA, with an average molecular weight of 500000, was obtained from Aldrich Chemical Co. Ethanol, isopropanol, and phosphoric acid were purchased from Loba Chemicals, Mumbai, India. Dematerialized water (conductivity ) 0.02 S/cm), which is used for the preparation of feed solutions, was generated in the laboratory. 2.2. Preparation of Membranes. Membranes were prepared by solution casting and solvent evaporation technique. Seven different blend solutions of SA and PVP were prepared by mixing SA with PVP in the mass ratios of 1:1, 1:2, 1:3, 2:3, 3:1, 4:1, and 5:1. Of these blended ratios, 3:1 was found to be optimum. It is noted that an increase in any one of the polymers’ (PVP, SA) content in the blend renders the membrane brittle as evidenced by the membrane stability test. The stability of the membrane is assessed by bending the film. The membrane is considered stable if its mechanical strength is restored after bending it; i.e., the membrane does not break upon bending. The mechanical weakness, introduced by the addition of PVP as a component, has a profound effect as its content in the blend increases. Hence, among the various blending ratios of SA to

10.1021/ie060085y CCC: $33.50 © 2006 American Chemical Society Published on Web 11/17/2006

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Figure 1. Schematic of laboratory vacuum pervaporation setup.

PVP studied, the only one mechanically stable blend membrane of SA and PVP at 3:1 ratio is considered for PV studies. A 3 mass % solution of SA and 1 mass % of PVP in aqueous medium are prepared individually and mixed in the ratio of 3:1, respectively. The blend solution is then stirred for a period of 0.5 h for homogeneity and kept aside for 1 h to obtain a bubble free solution, which is cast on to a clean glass plate and dried at room temperature to obtain a dense nonporous membrane. The membrane formed is crosslinked with a solution of 3.5 vol % phosphoric acid in an isopropanol/water bath (90/10 vol %) at room temperature, and a crosslinking time of 3 h is found to be optimum. With an increase in concentration of phosphoric acid leading to digestion of membranes, low concentrations lead to the crosslinking of the membranes getting reduced and solubilized in water. Isopropanol acting as a nonsolvent for the polymers prevented dissolution of the membrane, but water present in the solution is responsible for membrane swelling, which further facilitated crosslinking in the presence of phosphoric acid. The extent of crosslinking gradient is controlled by adjusting the exposure time. After removing the membrane from the crosslinking bath, it is washed with water repeatedly and dried in an oven at 80 °C to eliminate the presence of residual acid, if any. The membranes are prepared by varying volumes of the blend solution of same composition, and the thickness of the each membrane is measured by a micrometer screw gauge at different locations. The average values of these is taken as the thickness of the particular membrane. 2.3. Membrane Characterization. 2.3.1. Fourier Transform Infrared (FTIR) Studies. The FTIR spectrum of uncrosslinked SA/PVP and phosphoric acid crosslinked SA, PVP, and SA/PVP (P-SA/PVP) membranes are scanned in the range

between 4000 and 400 cm -1 using Nicolet-740 and PerkinElmer-283B FTIR spectrometers by KBr pellet method. 2.3.2. X-ray Diffraction (XRD) Analysis. A Siemens D 5000 powder X-ray diffractometer is used to study the solid-state morphology of crosslinked and uncrosslinked SA/PVP membranes in powdered form. X-rays of 1.5406 Å wavelengths are generated by a Cu K source. The angle of diffraction is varied from 2° to 65° to identify the changes in the crystal structure and intermolecular distances between the intersegmental chains after crosslinking. 2.3.3. Thermal Gravimetric Analysis (TGA). The thermal stability of the polymer films was examined, using a Seiko 220TG/DTA analyzer, from 25 to 700 °C heated at 10 °C/min and flushed with nitrogen at 200 mL/min. The samples are subjected to TGA both before and after blending to determine the thermal stability and decomposition characteristics. 2.4. Sorption Characteristics. A preweighed sample is immersed in pure water and ethanol as well as binary mixtures of different compositions and is allowed to reach sorption equilibrium at room temperature (for at least 10 h). The swollen sample is removed from the solvent mixture, wiped with tissue paper to remove the surface liquid, and immediately weighed. The process is repeated until the films attained steady state as indicated by constant mass after a certain period of soaking time. The degree of swelling (DS), which characterizes the ability of the membrane to absorb the liquid mixture, is obtained as the ratio of mass of swollen polymer (Ms) and mass of dry polymer (Md).

DS )

Ms Md

(1)

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Figure 2. FTIR spectra of (a) crosslinked SA, (b) crosslinked PVP, (c) uncrosslinked SA/PVP, and (d) crosslinked SA/PVP.

The sorption represents the fraction of the extracted liquid mixture by the membrane

sorption [%] )

Ms - M d × 100 Md

(2)

2.5. Determination of the Ion Exchange Capacity. The ion exchange capacity (IEC) indicates the number of mill equivalents of ions in 1 g of the dry polymer. To determine the degree of substitution by acid groups, the phosphorylated and unphosphorylated membranes of similar mass are soaked in 50 mL of 0.01 N sodium hydroxide solution for 12 h at room temperature. Then, 10 mL of the leftover solution is titrated with 0.01 N sulfuric acid. The membrane is regenerated with 1 M hydrochloric acid, washed with water until free from acid, and dried to a constant mass. The IEC (mequiv/g) is calculated according to

IEC )

B - P × 0.01 × 5 m

(3)

where B is the volume of sulfuric acid required to neutralize blank solution containing NaOH (mL), P is the volume of sulfuric acid required to neutralize NaOH solution in which the membrane is soaked, 0.01 is the normality of the sulfuric acid,

5 is the factor corresponding to the ratio of the volume of NaOH taken to dissolve the polymer and the volume of NaOH used for titration, and m is the sample mass (g). 2.6. Pervaporation Procedure. 2.6.1. Influence of Operating Conditions. Experiments are carried out with an indigenously constructed pervaporation manifold (Figure 1) operated at a low vacuum in the permeate side. The membrane area in the pervaporation cell assembly is ∼ 20 cm2. The experimental procedure is described in detail elsewhere.26 Permeate is collected for a duration of 8-10 h. Tests are carried out at room temperature (30 ( 2 °C) and repeated twice using fresh feed solution. The collected permeate is weighed after allowing it to attain room temperature in a Sartorius electronic balance (accuracy 10-4 g) to determine the flux and then analyzed by gas chromatography to evaluate membrane selectivity. 2.6.2. Flux and Selectivity Equations. In pervaporation, the flux J of a given species, say faster permeating component i of a binary liquid mixture comprising i (water) and j (ethanol) is given by

Ji )

Wi At

(4)

where Wi represents the mass of water in permeate (kg), A is the membrane area (m2), and t represents the evaluation time

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Figure 3. X-ray diffractograms of (a) uncrosslinked SA/PVP and (b) crosslinked SA/PVP.

Scheme 1. Structural Representation of SA/PVP Blend Membranes Crosslinked with Phosphoric Acid

(h). In the present study, though membranes of different thickness are utilized, the flux has been normalized and reported for thickness of 10 µm. The membrane selectivity (R) is the ratio of permeability coefficients of water and ethanol and can be calculated from their respective percentages in feed and permeate as given below.

R)

y(1 - x) x(1 - y)

(5)

Here, y is the permeate mass content of water (%), and x is its feed mass content. Pervaporation separation index (PSI) is a measure of the separation capability of a membrane and is expressed as a product of selectivity and flux.27

PSI ) JR

(6)

2.7. Analytical Procedure. The feed and permeate samples were analyzed using a Nucon gas chromatograph (GC model 5765) installed with a thermal conductivity detector (TCD) and

packed column of 10% DEGS on 80/100 Supelcoport of 1/8 in. i.d. and 2 m length. The oven temperature is maintained at 70 °C (isothermal) while the injector and detector temperatures are maintained at 150 °C each. The sample injection size is 1 µL, and pure hydrogen is used as the carrier gas at a pressure of 1 kg/cm2. The GC response is calibrated for this particular column and conditions with known compositions of ethanol-water mixtures, and the calibration factors are fed into the software to obtain correct analysis for unknown samples. 3. Results and Discussion Scheme 1 represents the structures of the polymers used in the study and also represents the structure of SA/PVP blend crosslinked with phosphoric acid. The hydroxyl groups present in H3PO4 react with the acetate group of SA and alkyl group of PVP resulting in the formation of a covalent bond. This is confirmed by FTIR and IEC. It is noticed that both the homopolymers and the SA/PVP blends are optically clear to the naked eye. No separation into two layers or any precipitation

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Figure 4. TGA curves for (a) uncrosslinked SA/PVP and (b) crosslinked SA/PVP.

Figure 5. Effect of mass % of water in feed on percentage of sorption studies of (a) crosslinked SA, (b) crosslinked PVP, and (c) crosslinked SA/ PVP.

Figure 6. Effect of feedwater composition on PV performance of SA/ PVP (membrane thickness 60 µm and permeate pressure at 0.5 mm Hg).

is noticed when allowed to stand for 1 month at room temperature. An estimation of the number of groups present before and after crosslinking gives an idea of the extent of crosslinking. 3.1. Membrane Characterization. 3.1.1. Ion Exchange Capacity (IEC). The amount of residual hydroxyl and acetate groups in blend membranes after crosslinking is estimated from IEC studies. It is found that the uncrosslinked SA/PVP blend had an IEC of 0.53 mequiv/g, whereas phosphoric acid crosslinked polymer exhibited an IEC of 0.32 mequiv/g. The IEC, which is equivalent to the total number of acetate and hydroxyl groups present in the membrane, decreased upon crosslinking because some hydroxyl groups and acetate groups are consumed during the reaction.28 A model of the possible interaction shown in Scheme 1 represents the crosslinking reaction occurring between SA/PVP blend and phosphoric acid. To the best of our knowledge, it is the first kind of study wherein phosphoric acid is employed as a crosslinking agent and the

membrane could withstand the solvent environment and pervaporation conditions employed in this study. 3.1.2. Fourier Transform Infrared Spectroscopy (FTIR) Studies. Figure 2 shows the FTIR spectra of phosphoric acid crosslinked SA (Figure 2a) and PVP (Figure 2b), uncrosslinked SA/PVP (Figure 2c) and phosphoric acid crosslinked SA/PVP (Figure 2d) blends. The FTIR spectrum of uncrosslinked SA/ PVP blend membrane shows the prominent peaks of CdO stretching in both SA and PVP at 1689 and 1721 cm-1, respectively. Whereas in the case of P-SA, P-PVP, and P-SA/ PVP blend, the CdO stretching is observed at 1759, 1738, and 1750 cm-1, respectively. The CdO peaks of SA in P-SA and P-SA/PVP blended membrane showed a shift from 1689 cm-1 (uncrosslinked SA/PVP) to 1759 cm-1 and 1750 cm-1, respectively, but there is no considerable change in the CdO peak for PVP in their crosslinked forms. This shift may be attributed to the formation of the -OC-O-P- bond due to the interaction of the hydroxyl group of phosphoric acid with the acetate group

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Figure 7. Effect of membrane thickness on PV performance (azeotropic feed composition 4 mass % of water and permeate pressure at 0.5 mm Hg).

Figure 8. Effect of permeate pressure on PV performance (azeotropic feed composition 4 mass % of water and membrane thickness 60 µm).

of SA. All the membranes synthesized (Figure 2a, c, and d) showed characteristic peaks appearing in the range 3000-3600 cm-1 corresponding to O-H stretching vibrations. It can also be noted that the PdO remains intact and does not participate in the reaction, which can be confirmed by the presence of a peak in the range 936-1021 cm-1 in all the crosslinked membranes. The characteristic deformation peak in PVP (uncrosslinked SA/PVP blend) noted at 1150 cm-1 represents the CsN (tertiary amine) stretching. The formation of new peaks (Figure 2b,d) at 1484 and 1415 cm-1 corresponds to the interaction of the hydroxyl group of phosphoric acid with the -CH- group present in the linear chain of PVP resulting in the formation of a -P-C-N (tertiary amine) bond. From the above, it can be concluded that phosphoric acid acts as a crosslinking agent to both SA and PVP. The model structure proposed in Scheme 1 is well in accordance with the FTIR spectra. 3.1.3. X-ray Diffraction (XRD) Studies. X-ray diffraction studies not only indicate the nature of the compounds but also enable the identification of the space between the clusters of the polymer chains. It is a well-known fact that on crosslinking the polymer chains are close to one another, and thus, a reduction in the cluster space may be encountered. The d-spacing (d) value gives an indication of cluster space existing in the polymer before and after crosslinking. The X-ray diffractograms of crosslinked and uncrosslinked SA/PVP blend membranes are shown in Figure 3(a,b). The XRD spectrum of the blend shows the amorphous nature with broad peaks at 2θ ) 10° and 20°. These two peaks are related to two types of crystals: crystal 1 and crystal 2. Crystal 1, which corresponds to the peak at 12.2, is generally responsible for the separation of compounds. This indicates the presence of higher free volumes in the blend, which might result in a reduction in selectivity but enhanced fluxes. The diffraction peaks are observed at 2θ ) 7°-9° (crystal 1) and 2θ ) 20° (crystal 2) for PVP and at 2θ ) 13° for alginate.29 Of the two peaks observed for PVP, crystal 1 is responsible for the separation, since it corresponds to the alkyl functional group. It would thus be interesting to know if these groups might have further undergone significant changes after crosslinking. The reduction in effective d-spacing from 12.5 Å (2θ ) 20°) for the uncrosslinked blend to 12.2 Å for the crosslinked one, suggests the occurrence of reaction between the carboxylic groups of SA and hydroxyl groups of phosphoric acid. On the other hand, the reduction in effective d-spacing from 9.5 Å for the uncrosslinked blend (Figure 3a) to 9.3 Å in

the crosslinked blend membrane (Figure 3b) indicates the shrinkage in cell size or intersegmental spacing occurring due to crosslinking between the CH group of PVP and phosphoric acid, which would improve the selective permeation of the membrane. 3.1.4. Thermogravimetric Analysis (TGA) Studies. The TGA curves of the crosslinked and unmodified blend are shown in Figure 4. The TGA curve of the SA/PVP blend (Figure 4a) shows its first weight loss stage occurring at 216-280 °C followed by a final decomposition at 280 °C. The weight loss stage can be attributed to the decomposition (Td) of the uncrosslinked blend at 250 °C. The TGA curve of the P-SA/PVP blend (Figure 4b) also exhibits its first weight loss stage at 200-260 °C followed by a final decomposition at 260 °C. The Td of the crosslinked blend reduced to 230 °C upon crosslinking. It is worth mentioning that there is a negligible change in the Td before and after crosslinking, which is indicative of the thermal stability of the crosslinked blend. 3.2. Pervaporation Results. 3.2.1. Effect of Feed Composition. The effect of liquid feed composition on the extent of pervaporation is investigated over a wide range of feed composition at 30 °C. For this study, the membrane thickness and permeate pressure are kept constant at 60 µm and 0.5 mm Hg, respectively. The effect of percentage of water in the flux on equilibrium sorption percentage and degree of swelling of the membrane in aqueous ethanol is shown in Table 1. Figure 5 shows the comparison of the sorption percentage of the SA/ PVP blend membrane with their individual crosslinked membranes. Swelling behavior is less in the crosslinked SA/PVP blend membrane when compared with their individual crosslinked membranes. The percentage of water uptake from the binary feed mixtures increases with increase in water percentage in the feed mixture, signifying that the SA/PVP blend membrane shows the possibility of attaining enhanced flux. However, absorption of large amounts of water at higher water percentage in the feed could cause increased swelling and subsequent fall in membrane selectivity due to plasticization of the polymer chains. Expectedly, a rise in the feedwater percentage produced an increase in the water-normalized flux from 0.514 to 0.81 kg m-2 h-1 (Figure 6). At higher feedwater percentages, the membrane swells appreciably because of the availability of more water molecules for sorption and diffusion. The preferential interaction with water molecules causes the membrane to swell, leading to plasticization and unrestricted and quicker transport

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Table 1. Effect of Feed Composition on Degree of Swelling, Permeate Water Percentage, and Water Flux sample number

mass % of water in feed

mass % of water in permeate

degree of swelling

water flux (kg m-2 h-1)

PSI

1 2 3 4 5 6

4.537 10.139 15.673 21.396 30.374 42.044

94.531 91.474 89.645 85.810 79.296 75.348

1.20 1.27 1.35 1.42 1.65 1.81

0.0857 0.0912 0.105 0.116 0.121 0.135

187.009 52.032 29.344 15.462 6.373 3.412

Table 2. Comparison of Flux and Selectivity of SA/PVP Blend Membranes with Values Reported in Literature Feed Composition sample number 1 2 3 4 5 6 7 8 9 10 11

membrane

water (%)

ethanol (%)

selectivity

novel two ply composite CS/SAa PVA/PAAM CS/SA zeolite PVA SA PVA/PVS silicone rubbler KE 108 CTSN-BCM CTSN-PVA P-SA/PVP

5.0 5.0 13.5 30.0 30.0 10.0 6.2 5.0 5.0 5.0 4.6

95.0 95.0 86.4 70.0 70.0 90.0 93.8 95.0 95.0 95.0 95.4

1110 1300 436 2140 6.4 120 700 125 10 22.0 364

flux (kg m-2 h-1) 0.07 0.80 0.22 2.10 1.20 0.29 0.50 0.14 42.8 1.7 0.09

ref 31 7 32 10 10 33 34 35 36 36 present work

a CS/SA ) chitosan/sodium alginate; PVA/PAAM ) poly(vinyl alcohol)/poly(acryl amide); SA ) sodium alginate; poly(vinyl alcohol)/poly(styrene sulfuric acid); P-SA/PVP ) phosphorylated sodium alginate/poly(vinyl pyrrolidone); CTSN-BCM ) chitosan impregnated bacterial cellulose; CTSNPVA ) chitosan-poly(vinyl alcohol).

of both volatile components through the barrier. On swelling, the polymer chains become more flexible, and hence, the transport through the membrane becomes easier for both the feed components resulting in high flux.30 Hence, the selectivity decreased from 364 at 4.53 mass % when feedwater percentage is increased from 4.53 to 42.04 mass %. Higher water percentages render greater swelling of the membranes that enables permeation of both components into the permeate stream yielding higher flux but a drop in selectivity. Thus, the swelling increases with increasing percentage of water in the feed leading to reduction in membrane selectivity. However, it is worth mentioning that the membrane showed promising results for dehydrating feeds having 5%-25% water. Moreover, the azeotropic composition of 96 mass % ethanol is easily broken by pervaporation. 3.2.2. Effect of Membrane Thickness. The effect of varying membrane thickness on separation performance is studied at constant feed composition (azeotropic) and permeate pressure (0.5 mm Hg) by synthesizing membranes of thickness ranging from 30 to 150 µm. With an increase in the membrane thickness, a gradual reduction in the flux from 0.201 to 0.0318 kg m-2 h-1 is observed as can be clearly observed from Figure 7. Though the availability of polar groups enhances with an increase in the thickness, flux decreases since diffusion become increasingly retarded as the feed molecules have to travel a greater distance to reach the permeate side. The percentage of water in the permeate varied from 88.60 to 97.25 mass % which suggests that the selectivity increased from 163.61 to 745.20. The interaction between the permeating component and membrane on the feed side produces a swelling phenomenon. The membrane thickness increases due to the sorption of water from the feed mixture at the upstream phase, and there will be a formation of a complex gradient in swelling between the upstream and downstream phases, which results in improving the selectivity as observed in the present case. Thus, in the pervaporation process, the upstream layer of the membrane is swollen and plasticized due to absorption of feed liquid. In contrast, the downstream layer is virtually dry due to continuous evacuation in the permeate side, and therefore, this layer forms

the restrictive barrier which allows only interacting and smaller sized molecules such as water to pass through it. 3.2.3. Effect of Permeate Pressure. The separation characteristics of the SA/PVP blend membrane are studied for varying permeate pressures from 0.5 to 10 mm Hg, at a constant membrane thickness (60 µm) and 95.46% ethanol feed. Figure 8 shows that the membrane exhibits a considerable lowering of normalized flux from 0.51 to 0.11 kg m-2 h-1 10 µm with increasing permeate pressures (decreasing vacuum) for the same degree of swelling (1.2) in azeotropic composition. Diffusion through the membrane is the rate-determining step in the pervaporation process, and the diffusing water molecules experience a larger driving force under high vacuum, which enhances the desorption rate at the downstream side. Lower vacuum reduces the driving force, thus slowing desorption of molecules. In such cases, the relative volatilities of the two components of the mixture govern the separation factor of the membranes. Ethanol, having higher vapor pressure than water, permeates competitively with the latter, thus lowering the water percentage in permeate. Therefore, selectivity decreased from 364 to 97.908 as the pressure increased from 0.5 to 10 mm Hg. Pervaporation performance of the P-SA/PVP membranes is compared with the literature data and is reported in Table 2. From the table, it is observed that a marginally good selectivity and fluxes render the covalently crosslinked blend suitable for dehydration of ethanol/water mixtures. Furthermore, the ease in fabrication of these membranes associated with low cost render them more attractive for pervaporation separation of aqueous alcohol mixtures. 4. Conclusions In this study, the blend membranes of SA and PVP are prepared and crosslinked with phosphoric acid for pervaporationbased dehydration of the ethanol/water mixture. The number of groups crosslinked in the SA/PVP blend polymer is identified from the IEC studies. Characterization of the crosslinked membranes by FTIR and XRD confirms the crosslinking reaction. The membrane shows adequate thermal stability to withstand the PV experimental conditions. FTIR spectroscopy

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confirms the predicted interaction between the SA/PVP blend and the cross-linker. With increasing feedwater composition, the membrane exhibits a reduction in selectivity and an improvement in flux due to increased swelling. As expectedly, with increasing membrane thickness, selectivity improves, but flux decreases. Higher permeate pressure causes a reduction in both flux and selectivity. Pervaporation could be combined with distillation in an integrated process wherein the former overcomes the azeotropic barrier of ethanol (95.463%) after which the latter is applied to achieve final purity. Literature Cited (1) Lee, K. R.; Liu, M. J.; Lai, J. Y. Pervaporation Separation of Aqueous Alcohol Solution through Asymmetric Polycarbonate Membrane. Sep. Sci. Technol. 1994, 29, 119. (2) Takegami, S.; Yamada, H.; Tsujii, S. Dehydration of Water/Ethanol Mixtures by Pervaporation using Modified Poly (Vinyl Alcohol) Membrane. Polym. J. 1992, 24, 1239. (3) Song, K. M.; Hong, W. H. Dehydration Of Ethanol and Isopropanol using Tubular Type Cellulose Acetate Membrane with Ceramic Support in Pervaporation Process. J. Membr. Sci. 1997, 123, 27. (4) Huang, R. Y. M. PerVaporation Membrane Separation Process; Elsevier Science: New York, 1991. (5) Mochizuki, A.; Sato, Y.; Ogawara, H.; Yamashita, S. Pervaporation Separation of Water/Ethanol Mixtures through Polysaccharide Membranes. I. The Effects of Salts on the Perm Selectivity of Cellulose Membrane in Pervaporation. J. Appl. Polym. Sci. 1989, 37, 3357. (6) Bruschke, H. E. A. Japan Patent Kokai, 59-109204, 1984. (7) Ruckenstein, E.; Liang, L. Pervaporation of Ethanol-Water Mixtures through Polyvinyl Alcohol-Polyacrylamide Interpenetrating Polymer Network Membranes Unsupported and Supported on Polyethersulfone Ultrafiltration Membranes: A Comparison. J. Membr. Sci. 1996, 110, 99. (8) Kim, H. J.; Jo, W. H.; Kang, Y. S. Modified Free-Volume Model for Pervaporation of Water/Ethanol Mixtures through Membranes Containing Hydrophilic Groups or Ions. J. Appl. Polym. Sci. 1995, 57, 63. (9) Nam, S. Y.; Chun, H. J.; Lee, Y. M. Pervaporation Separation of Water-Isopropanol Mixture using Carboxymethylated Poly (Vinyl Alcohol) Composite Membranes. J. Appl. Polym. Sci. 1999, 72, 241. (10) Shah, D.; Kissick, K.; Ghorpade, A.; Hannah, R.; Bhattacharyya. Pervaporation of Alcohol-Water and Dimethylformamide-Water Mixtures using Hydrophilic Zeolite Naa Membranes: Mechanisms and Experimental Results. J. Membr. Sci. 2000, 179, 185. (11) Bhat, N. V.; Wavhal, D. S. Preparation of Cellulose Triacetate Pervaporation Membrane by Ammonia Plasma Treatment. J. Appl. Polym. Sci. 2000, 76, 258. (12) Huang, R. Y. M.; Moon, G. Y.; Pal, R. Chitosan/Anionic Surfactant Complex Membranes for the Pervaporation Separation of Methanol/MTBE and Characterization of the Polymer/Surfactant System. J. Membr. Sci. 2001, 184, 1. (13) Jiraratananon, R.; Chanachai, A.; Huang, R. Y. M.; Uttappap, D. Pervaporation Dehydration of Ethanol-Water Mixtures with Chitosan/ Hydroxyethylcellulose (CS/HEC) Composite Membranes. I. Effect of Operating Conditions. J. Membr. Sci. 2002, 195, 143. (14) Feng, X.; Huang, R. Y. M. Liquid Separation by Membrane Pervaporation: A Review. Ind. Eng. Chem. Res. 1997, 36, 1048. (15) Aminabhavi, T. M.; Vijayakumar N. B.; Sridhar, S. Computer Simulation and Comparative Study on the Pervaporation Separation Characteristics of SA and its Blend Membranes with Poly (Vinyl Alcohol) to Separate Aqueous Mixtures of 1,4-Dioxane Or Tetrahydrofuran. J. Appl. Polym. Sci. 2004, 94, 1827. (16) Toti, U. S.; Aminabhavi, T. M. Synthesis and Characterization of Polyacrylamide Grafted SA Membranes for Pervaporation Separation of Water + Isopropanol Mixtures. J. Appl. Polym. Sci. 2004, 92, 2030. (17) Toti, U. S.; Aminabhavi, T. M. Different Viscosity Grade SA and Modified SA Membranes in Pervaporation Separation of Water + Acetic

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ReceiVed for reView January 19, 2006 ReVised manuscript receiVed September 30, 2006 Accepted October 11, 2006 IE060085Y