Application of zeolite-filled pervaporation membrane Z. Gao, Y. Yue, and W. Li Departments of Chemist9 and Material Shanghai, People’s Republic of China
Composite hydrophilic membranes, consisting of KA, NaA, CaA, and NaX zeolites and polyvinylalcohol polymer were prepared. The pervaporation and separation characteristics of different alcohol-water systems through these membranes were investigated at temperatures ranging from 20 to 50°C. Distinct improvement on molecular transport and the molecular sieving effect of the zeolites have been observed. Pervaporation-aided catalytic esterification of acetic acid with ethanol and salicylic acid with methanol have been carried out in a membrane reactor, leading to a considerable increase in conversion and a reduction in reaction time as a result of continuous removal of water through the membrane. Acetone-water separation by pervaporation on KA- and CaA-filled polyvinylalcohol membrane has been studied as well, which gives even better separation results than that of the ethanol-water system. Application of the zeolite-filled pervaporation membrane to the methanol-acetone condensation reaction also promotes the reaction. Keywords: vapor&ion-aided
INTRODUCTION The separation of water-based mixtures through hydrophilic polymer membranes has been studied extensively. Huang and Jar&’ have investigated the pervaporation and separation characteristics of aqueous alcohol solutions through cellophane and polyvinylalcohol (PVA) membranes. Other types of polymer membranes, such as Ntion, sulfonated polyethylene, and sulfonated polypropylene membranes were studied by Gierke et al.,’ Cabasso and Liu,” and Xu et a1.4 Mulder et aL5 presented a membrane-controlled continuous fermentation process for the production of pure ethanol from sugar. Esterification of carboxylic acid with alcohols and phenol-acetone condensation reaction accompanied by membrane separation have been reported by Okamoto et a1.6 and Neel et al.’ Zeolite-filled hydrophobic membrane was introduced by Hennepe et a1.8 The addition of silicalite to silicone rubber membrane improved both the selectivity and flux of the membrane in separation alcohol fl-om dilute mixtures. In the present work various types of zeolite-filled hydrophilic membranes with PVA as the polymer matrix were prepared. The performance of these composite membranes for the separation of alcohol-water and acetone-water systems was investigated in comparison with the unfilled membrane. Application of some of the zeolite-filled pervaporation membranes Address reprint requests to Prof. Gao, Dept. of Chemistry, Fudan University, Shanghai 200433, People’s Republic of China. Received 17 September 1992; revised 20 July 1995; accepted 30 July 1995 Zeolites 16:70-74, 1996 0 Elsevier Science Inc. 1996 655 Avenue of the Americas,
to catalytic esterification and condensation reactions has been explored, expecting them to promote the reactions as a result of continuous removal of water produced in the reacting systems.
EXF’ERIMENTAL Zeolite preparation NaA and NaX zeolites are commercial pure zeolite powers. CaA and KA zeolites were prepared by ion exchanging NaA with relevant salt solutions until the exchange extended above 90%.
The PVA membrane was prepared by casting PVA solution containing 3% polymer, by weight, on a glass plate and allowing the solvent to evaporate under infrared radiation. The zeolite-filled composite membrane was prepared by adding a calculated amount of zeolite into the polymer solution and mixing thoroughly before casting. The dried membranes were heating at 160-200°C to ensure cross-linking. They were then soaked in water for more than 40 h. The thickness of the membranes prepared is around 70-80 Pm.
and reaction tests
The membrane was mounted in the pervaporation cell, which was then connected to the collection system and vacuum pump. Vacuum at the downstream side was maintained at a pressure of 0.1 mm Hg. Liquid nitro-
gen was used as a cooling agent for the cold traps. The temperature of the feed in cell was kept constant and could be varied between 20 and 70°C. The volume of the feed solution was 120 ml, and the effective membrane area was 22.9 cm’. The permeate collected in the cold trap for 2 h, was analyzed, and the flux and separation factor of the membranes were calculated from the equations:
where W = weight (g) of permeate with density (g/ml), t = permeation time (h) , A = membrane area (cm2) ; C, and 6: are the mole fractions of water in the permeate and feed, C, and G are the mole fractions of alcohols or acetone in the permeate and feed. The esterification and condensation reactions accompanied by pervaporation were carried out in the same type of membrane cell. Sulfonated cation exchange resin and sodium hydroxide were used as the acid and base catalysts for the reactions. Product samples were drawn periodically and analyzed to follow the reaction course.
Dependence of the membrane performance the zeolite content and size Table 1 gives the separation factor as a function
of the KA zeolite content in the membrane for ethanol-water system. The alcohol concentration at feed side is 80 vol%, and the temperature of the feed is 50°C. It can be seen that the flux rises with increasing zeolite content, whereas the separation factor is maintained upon addition of zeolite until 11.1% and then decreases. The explanation for this phenomenon may be that both water and ethanol transport are possible through the zeolite pores, and the selectivity of zeolite for ethanolwater binary mixture is similar to that of the polymer matrix. As the zeolite content goes above 14.9% the separation factor of the membrane decreases drastically, which is probably due to less selective “leaks” formed around the zeolite crystals during preparation. The influence of zeolite particle size on flux and selectivity of membrane for the same system is shown in Table 2. As expected, the flux increases with decreasing zeolite particle size, whereas the separation factor is unchanged at low zeolite content and decreases at high zeolite content because the more zeolite particles in the Table 1 Performance urn) content
of PVA membrane
0 4.8 11.1 14.9 20.0 27.3
(ml h-’ 140 145 164 188 216 288
as a function
of KA (5
Separation 40.0 40.0 40.0 25.9 11.3 5.74
membrane, the more chances of creating tive leak around the zeolite particles.
a less selec-
of feed concentration
Figure 1 illustrates the relationship between the membrane performance and the water concentration in thcra ethanol-water mixtures for PVA- and KA- (11.1%) 5 pm) filled PVA membranes at 50°C. The flux increases aimost exponentially with increasing water content in the feed, which is caused by a higher concentration of water in the membrane. At the same time, the separation factor is decreased, and the reduction is more pronounced at water content below 20%. This can be esplained by the plasticizing effect of water on the permeability of PV‘4.l As the water concentration is iiicreased, the amorphous regions of the polymer are swollen, and the polymer chain segments become moi e mobile thus decreasing the energy required for difftisive transport through the membrane and increasing the alcohol flux.
Pervaporation tests of ethanol (80 vol%)-water (20 ~01%) mixture were conducted at 20, 30, 40, and 5O’C for PVA- and KA- (11 .l wt%, 5 pm) filled PVA membranes. The effects of temperature on membrane flttx and the separation factor are shown in Figure 2. As tlie temperature is raised, the flux of both types of mernbranes is increased. Arrhenius-type plots were used to calculate pervaporation activation energies of pure ~ater. The activation energies of pure water permeatitig through PVA and PVA + KA membranes are approximately the same, which equals 4.5 Kcal/mol. The decrease in separation factor at high temperature is probably caused by thermal motion of polymer chains in the membrane. As the temperature is increased, the thermal agitation increases, and diffusive “holes” are produced, thus more alcohol molecules can diffuse through the membrane.’
Molecular sieving effect the membrane
of zeolites in
Composite membranes consisting of zeolites (11 wt%) with different pore sizes, such as KA, NaA, GA, and NaX, and PVA polymer were prepared, and pervaporation tests for various alcohol-water systems composed of alcohols (80 ~01%) with different molectile Table 2
as a function
of KA partrcle
size Ka content (w%) 4.8 4.8 4.8 4.8 11.1 11.1 11.1 11.1
KA size (pm) 5 1.5 5 1.5 5 1.5 5 1.5
Flux (ml h-’ m-*)
Separation factor -
30 30 50 50 30 30 50 50
61 66 144 168 70 82 164 205
54.7 54.7 40.0 40.0 54.7 40.0 40.0 30.0
Zeolite-filled Table 50°C
Binary mixture and membrane
Flux (ml h-’ m-‘)
CH,OH/H,O PVA PVA + KA PVA + NaA PVA + CaA PVA + NaX C,H,0H/H20 PVA PVA + KA PVA + NaA PVA + CaA PVA + NaX I-C,H,OH/H,O PVA PVA + KA PVA + NaA PVA + CaA PVA + NaX t-C,H,0H/H20 PVA PVA + KA PVA + NaA PVA + CaA PVA + NaX
183 235 258 323 376
15.5 15.5 13.8 10.4 8.5
140 164 172 194 214
40.0 40.0 36.6 22.3 19.4
146 179 183 190 216
233 410 328 233 233
118 135 140 157 170
516 1170 1170 1170 516
T/"C Figure water
sizes, such as methanol, ethanol, isopropyl alcohol, and tert-butanol, were performed at 50°C. The results are given in Table3. It can be seen clearly that the flux and separation factor of the membranes are related. to the molecular size of the alcohols and the pore size of the zeolites. Adding zeolite to the polymer improves the flux of the membrane; in general the larger the pore size of the zeolite, the higher the flux. When the alcohol molecules are compared, the flux of the membrane is observed to decrease as the diffusive cross-section and the molecular length of the alcohol are increased. For smaller alcohol molecules, such as methanol and ethanol, the separation factor of the zeolite-filled mem-
2 Effect of temperature on the mixture. 7, PVA; 2, PVA + KA.
branes for alcohol-water systems at a given water content is inferior to that of PVA membrane except for KA-filled membrane. However, for larger alcohol molecules, such as isopropyl alcohol and tert-butanol, the separation factors of the majority of A-type zeolite-filled membranes are improved markedly. The above experimental results indicate that adding A-type zeolites into the membrane facilitates the permeation of water and smaller alcohol molecules while it hinders the passage of larger alcohol molecules. Therefore, the superiority of the zeolite-filled membranes over unfilled membrane is more predominant for alcohols with larger molecular sizes because of the molecular sieving effect of the zeolites. The improvement on separation is not observed for NaX-filled membrane because of the large pore size of the zeolite.
H,O vol. % Figure 1 centration.
Membrane performance 7, PVA; 2, PVA + KA.
as a function
N ‘E 2 2 3 E
T/“C of water
Figure 3 Membrane performance ture. 1, PVA; 2, PVA + KA (I 1 wt%);
for an acetone-water mix3, PVA + CaA (I 1 wt%).
Separation of acetone-water
of acetic acid with ethaKA (11 wt%); 4, PVA + CaA
Pervaporation tests of acetone-water solution for zeolite-filled and unfilled membranes were conducted at 20-50°C. The acetone concentration at the feed side is 80 ~01%. The results are illustrated in Figure 3, from which it can be seen that a better separation of the acetone-water system was realized for all membranes as compared with the ethanol-water system. In particular, the KA-filled membrane possesses a higher flux than the unfilled PVA membrane without the expense of good selectivity.
Esterification of acetic acid (40 ml) with ethanol (80 ml) was carried out in the membrane cell under 70°C in the presence of 6 g of sulfonated cation exchange resin. [email protected]
4 shows the time course of the esterifrcation reaction accompanied by pervaporation on various types of membranes in comparison with the blank reaction. Pervaporation exerts a significant effect on the conversion of the reactants. Continuous removal of water from the system makes it possible to pass over the equilibrium limit (79%)) which is approached slowly in the blank reaction. The times required for the reacnon to attained 95% conversion for PVA, PVA + KA, and PVA + CaA membranes are 20.0, 11.3, and 10.0 h, respectively. The esterification of salicylic acid with methanol is less reactive. In the presence of 10 g of sulfonated cation exchange resin, it takes 150 h for salicylic acid (20 g) and methanol (100 ml) to reach chemical equilibrium (conversion 65%) at 60°C. Figure 5 shows that the investigated reaction is accelerated considerably as a result of removal of water by pervaporation. The two zeolite-filled membranes perform better than the unfilled membrane. After reacting for 30 h, the conversion of the reaction on PVA, PVA + KA, and PVA + CaA
2. Gas et ai.
t/h Figure 4 Time course of esterification nol. 7, blank reaction; 2, PVA; 3, PVA+ (11 wt%l.
Figure 5 Time course of esterification of salicylic acid with methanol. 1, blank reaction; 2, PVA; 3, PVA+ KA (I 1 wt%); 4, PVA + CaA (11 wt%).
membranes is 1.35, 1.0, and 1.70 times greater than that of the blank reaction, respectively. The above results indicate that the preferential pervaporation through a hydrophilic membrane could be used to assist the esterification reaction by shifting the chemical equilibrium. The promotion effect of zeolitcs can be explained by the increase in water transport through the zeolite crystals, so a membrane with larger flux, such as CaA + PVA, is more favorable for the r-c’action.
Pervaporation-aided The reaction dimethoxypropane
of methanol with acetone to form 2,?was used as an example for the al>-
t/h Figure 6 Time course of the methanol-acetone condensation reaction. 7, blank reaction: 2, PVA; 3, PVA + KA (I 1 wt%); 4, PVA + CaA (11 wt%).
plication of pervaporation membrane to condensation reactions. 30 ml of acetone, 100 ml of methanol, and 0.5 g of NaOH were added into the membrane cell, and the reaction was run at 50°C. Figure 6illustrates the time course of the condensation reaction on various types of membranes as compared with the blank reaction, The results obtained are similar to that of the esterification reaction. After reacting for 12 h, the conversion of the reaction on WA, WA + KA, and PVA + CaA membranes is 1.20, 1.30, and 1.34 times greater than that of the blank reaction, respectively.
CONCLUSIONS The performance of hydrophilic pervaporation membrane can be enhanced by adding a zeolite filler to the membrane. Zeolite facilitates the permeating of smaller molecules but hinders that of larger molecules. In this manner it improves the flux and separation factor of the membrane. Chemical reactions generating water as a product can be assisted by pervaporation through membranes. The continuous removal of water from the
reacting system will lead to an increase in conversion and a reduction in reaction time. Zeolite-filled membranes perform better in pervaporation-aided reactions because of enhancement on the water transport of the membranes.
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