Journal of Membrane Science 144 (1998) 161±171
Ethanol/water transport through silicalite membranes Mikihiro Nomura*, Takeo Yamaguchi, Sin-ichi Nakao Department of Chemical System Engineering, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan Received 6 November 1997; received in revised form 28 January 1998; accepted 29 January 1998
Abstract Transport mechanism of ethanol/water mixtures by pervaporation and vapor permeation through silicalite membrane was investigated. The activities of both ethanol and water in feed were controlled independently by vapor permeation to study the feed composition effects quantitatively. Also, adsorption experiments of single component and binary mixtures were carried out to measure the adsorbed mixture composition in the silicalite membrane. Ethanol permeance was independent of water concentration in feed. Water permeance, however, seriously decreased by the presence of ethanol in feed, and water permeation was restricted by ethanol. The adsorption±diffusion model was considered for the transport mechanism through the silicalite membrane. Ethanol selectively adsorbed to silicalite membrane from ethanol/water vapor. The diffusion coef®cients of water and ethanol were calculated based on the adsorption±diffusion model, and the results showed almost the same diffusivity between the single component and mixture feed case. Thus, high ethanol selective permeation through the silicalite membrane was explained by the ethanol selective adsorption to the silicalite membrane. # 1998 Elsevier Science B.V. Keywords: Silicalite membranes; Transport phenomena; Ethanol/water; Adsorption
1. Introduction Zeolites are hydrated aluminosilicates composed of crystalline structure with molecular sieving property and are inorganic materials with thermal, chemical and mechanical stability. Zeolite membranes are suitable for molecular sieving separation or pervaporation (PV) of organic solutions at high temperature. Recently, zeolite membranes have been prepared by several research groups. The reported zeolite membranes had polycrystalline structure and this structure might have intercrystalline region. *Corresponding author. Tel.: +81 3 8122111; fax: +81 3 56848402. 0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0376-7388(98)00043-X
The separation of ethanol/water mixture is of great importance for the production of ethanol from biomass. There are several reports showing good separation performance for ethanol/water mixture using zeolite membranes [1,2]. Kita et al. [1] made NaA type zeolite membrane by hydrothermal synthesis. NaA zeolite membrane is a water selective membrane, and the PV separation factor of water/ethanol system was over 10 000 at 348 K. For ethanol permselective membranes, Sano et al. [2] prepared polycrystalline silicalite membrane by the hydrothermal synthesis. The silicalite membrane showed high ethanol permselectivity, and a separation factor of 58 was realized at 333 K by PV. Silicalite membranes seem to have great potential for the ethanol recovery by PV. Hence,
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in this study, transport mechanism of water and ethanol through silicalite membrane was investigated. Transport mechanism of PV through polymeric membranes was studied by many research groups, and it was explained by the solution±diffusion model [3±8]. According to the solution±diffusion model, each component of the permeation molecules dissolves into the membrane and diffuses through the membrane due to the concentration gradient. Several research groups tried to investigate the transport mechanism through zeolite membranes [9±19]. Although, there were some reports concerning transport mechanism through zeolite membranes, the mechanisms were usually explained only from the permeation results. Vroon et al. [9] compared the single and binary permeation results, and suggested that the single ®le diffusion of molecules took place in the zeolite MFI membrane. The single ®le diffusion means that molecules cannot pass each other in the micropores by their molecular size. Baertsch et al. [10] also suggested the single ®le diffusion of aromatic hydrocarbons in the zeolite membrane. Funke et al. [11] pointed out that the adsorption effect might be the key factor of the separation of hydrocarbon isomers. In these reports, the adsorption to the membrane and the feed activity were not considered. The permeation through the zeolite membrane was usually explained by the adsorption±diffusion model which is based on the solution±diffusion model for polymeric membranes. Based on the adsorption±diffusion model, each component of the permeating molecules was adsorbed to the inlet of the zeolite micropores and diffused through the zeolite pores due to the concentration gradient. However, the detailed mechanism is not clear yet. Barrer [12] considered the surface adsorption effect, and the intracrystalline diffusion in zeolite ®lm was calculated using activated energy of desorption. Krishna and van den Broeke [13] employed Maxwell±Stefan description for the simulation through the zeolite membranes. Kapteijn et al. [14,15] and Bakker et al. [16] employed Maxwell± Stefan model for hydrocarbon permeation through silicalite membranes and they considered the effect of concentration gradient using Darken equation. Single component adsorption measurements were used for the calculation. The ethanol and water adsorption to silicalite was investigated by many research groups [20±23]. Mile-
stone and Bibby [20], Bul et al. [22] and Lin and Ma [23] carried out batch method to evaluate the ethanol adsorption from ethanol aqueous liquid. Ethanol adsorption isotherm showed Langmuir type adsorption and saturation amounts were about 1.5± 1.8 mol kgÿ1. Lin and Ma [23] also carried out a HPLC technique for the measurement of adsorption amounts and diffusion coef®cients in the silicalite. Klein and Abraham [21] measured the total adsorption amount on silicalite gravimetrically using water and ethanol vapor. They employed the Langmuir mixture model in prediction of ethanol adsorption from ethanol aqueous solution. There were, however, no reports investigating the binary adsorption to the silicalite membrane and the effect of the feed activity change for the transport mechanism. Although, we should consider the feed activity of each component instead of concentration, liquid feed activity cannot be controlled independently. Thus, in this paper, we employed vapor permeation (VP) experiments to control each feed activities independently. The permeation results of PV and VP were estimated by the feed activity. Single and binary adsorption measurements were carried out. The transport mechanism of ethanol/water mixtures through silicalite membrane will be explained by the adsorption diffusion model using PV, VP and adsorption measurements. 2. Experimental 2.1. Synthesis of the silicalite membrane Synthesis method of silicalite membrane reported by Sano et al. [2] was followed, and the detailed procedure was described elsewhere [24]. A hydrogel was made with colloidal silica, tetrapropylammonium bromide (TPABr), sodium hydroxide and pure water (0.1 TPABr±0.05 Na2O±1 SiO2±80 H2O). The pore diameter of the porous stainless steel substrate was 10 mm. Hydrothermal synthesis was made at 443 K for 48 h. In order to remove the TPA template, the membrane was calcinated at 773 K for 20 h. Polycrystalline silicalite membranes were formed on the surface of the stainless steel support, so the effect of support characteristics on the transport behavior is negligible. The amount of the silicalite was
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Fig. 1. SEM image of the silicalite membrane.
about 1 g, and the thickness of the silicalite layer was about 400 mm. Fig. 1 shows the SEM microphotographs of the surface side and cross-section of the silicalite membrane. The silicalite crystals are arranged randomly, and they are inter-grown with each other. From the cross-sectional view, the surface polycrystalline structure looks denser, and the support side is looser. The surface dense layer looks a few micron thick, and may represent the effective membrane thickness. 2.2. Pervaporation and vapor permeation Permeation of ethanol/water mixtures through the silicalite membrane was carried out by PV and VP at 303 K. Before each separation experiment, the silicalite membrane was calcinated at 473 K in the air to remove any adsorbates, and N2 permeance was used to check the initial membrane properties. Each silicalite membrane had some difference in the permeance properties. Thus, one silicalite membrane was employed for all the PV and VP measurements. PV performance is usually evaluated by total ¯ux (kg mÿ2 hÿ1) and separation factor (dimensionless). Separation factor of ethanol aqueous solution was calculated from the following equation:
Separation factor
XEtOH =XH2 O perm =XEtOH =XH2 O feed where XEtOH and XH2 O are molar fractions of ethanol and water, respectively. In this study, permeance (mol mÿ2 sÿ1 Paÿ1) and activity in feed (dimensionless) were employed to compare with PV and VP results. We assumed that PV results must be the same as VP results with a saturated vapor of the same feed liquid. For permeance calculation, the PV ¯ux of each component was divided by the vapor pressure of the same in feed. The activity in feed was de®ned as the ratio of the actual vapor pressure to the saturated vapor pressure. The vapor pressure of each component was calculated using Antoine's equation and Wilson's equation. PV was performed using the apparatus shown in Fig. 2. The feed solution was placed on one side of the membrane, and the solution was well stirred to keep the concentration and temperature constant in the feed vessel. The other side of the membrane was outgassed by the vacuum pump, and the permeate vapor was trapped by liquid nitrogen. PV was carried out at 303 K. Fig. 3 shows the VP apparatus. Water and ethanol were vaporized at the liquid bubbler using helium as
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Fig. 2. Schematic figure of the pervaporation apparatus: (1) heater; (2) stirrer; (3) thermometer; (4) silicalite membrane; (5) Pirani gauge; (6) cold trap (liquid N2); (7) vacuum pump.
Fig. 3. Schematic diagram of the vapor permeation apparatus.
carrier gas. The bubbler temperature was controlled between 268 and 303 K. The feed pressure of water and ethanol vapor was calculated by Antoine's equation and Wilson's equation. Feed vapor was introduced into the membrane module, and the permeate was carried by helium sweep gas. The feed gas and the permeate were directly supplied into gas chromatography (Shimadzu, GC-8A) to measure each component concentration in feed and permeate vapor. Carrier gas rate was 5.010ÿ6 m3 sÿ1, and sweep gas rate was between 0.210ÿ6 and 4.510ÿ6 m3 sÿ1. The membrane module was kept at 303 K in the air oven. Effective membrane area for PV and VP cell were 1.2610ÿ3 and 5.010ÿ4 m2, respectively.
2.3. Adsorption measurement Ethanol/water adsorption to the silicalite membrane was employed using the gravimetric and the adsorption±desorption method as explained below. For the measurement, silicalite membrane sample was peeled off from the porous stainless steel substrate, and calcined at 773 K in the air to remove any previously adsorbed molecules. The overall concentration both in the silicalite pores and in the intercrystalline region may be measured using the silicalite membrane as a sample. Fig. 4 shows the gravimetric method apparatus for a single component adsorption measurement. The sili-
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Fig. 4. Schematic diagram of the gravimetric adsorption apparatus.
calite membrane was placed on the quartz basket. After the sample chamber was well evacuated, the feed vapor was introduced. The weight change by the adsorption was measured by the electric balance. Silicalite sample was kept at 303 K in the air bath. The feed vessel for each adsorbate was controlled between 268 and 303 K in the water bath. Feed solutions were degassed by freezing and thawing four times before the measurements. Single and binary adsorption were measured by the adsorption±desorption method using the apparatus
shown in Fig. 5. The silicalite membrane was placed in the sample chamber. Initially, the sample chamber was evacuated at 473 K for more than 2 h, following which the sample temperature was maintained at 303 K. The feed vapor was introduced into the sample chamber. Adsorption was carried out until the equilibrium was reached. The equilibrium time was found by trial and error method. The adsorption period was at least 870 min. After reaching the equilibrium, the sample room was heated at 473 K to remove all the adsorbates, and the adsorbates were collected in the
Fig. 5. Schematic diagram of the binary adsorption apparatus.
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U-shaped cold trap kept in liquid nitrogen. The sample was heated more than 1 h to ensure that all the adsorbates were removed. The collected adsorbates were vaporized at 373 K, and the vapor was supplied into the gas chromatography (Shimadzu, GC-14A) directly. The concentration and amount of the adsorbates were measured by gas chromatography. The blank vapor concentration in the sample chamber was measured by the same procedure without the silicalite membrane sample. The volume of the sample room and the sample were 2.810ÿ6 m3 and about 510ÿ8 m3, respectively. The ethanol adsorption between ethanol activities 0.2±0.8 were measured by gravimetric method. Other adsorption values were measured by the adsorption± desorption method. 3. Permeation model The permeation through a zeolite membrane was usually explained by the adsorption±diffusion model. In such a model, the permeation was divided into two steps as the adsorption part and the diffusion part. One purpose of this paper is, hence, to make clear the method of using the adsorption data for analyzing the permeation ¯ux. Thus, we assumed the following assumptions for calculating the diffusion coef®cients: 1. Diffusion coef®cient (Di) is assumed to be constant across the membrane. This means that the diffusion coef®cient in this model represents an overall diffusion coef®cient in a membrane. The effective membrane thickness (x) was not clear and all the permeation experiment was carried out using the same silicalite membrane. Thus, diffusion coef®cients divided by the membrane thickness (Di/x) was estimated instead of diffusion coef®cients (Di). 2. Equilibrium was assumed to be reached at the interface between feed and silicalite membrane, and the solution concentration in the membrane at the feed interface (Ciÿfeed) has been considered equivalent to the adsorbate concentration in the adsorption experiments. 3. The concentration at the permeate interface (Ciÿperm) and the vapor pressure at the permeate side (piÿperm) have been considered as zero because
the activities of permeate side vapor of both PV and VP are below 0.01. The error caused by this assumption could be less than 8% in our experimental conditions. 4. The diffusion in the membrane follows Fick's law and so the diffusion coefficients were calculated using ÿ Di ÿ Ji Pi piÿfeed ÿpiÿperm Ciÿfeed ÿ Ciÿperm x (1) 4. Results and discussion 4.1. Pervaporation results Fig. 6 shows the PV results through silicalite membrane at 303 K. The ethanol concentration in permeate was higher than vapor±liquid equilibrium in all feed composition. The maximum separation factor was 64 at 4.65 wt% feed ethanol concentration. Total ¯ux gradually increased with increasing feed ethanol concentration.
Fig. 6. Pervaporation of water/ethanol systems through silicalite membrane.
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Fig. 7. Ethanol permeance and water activity plotted against ethanol activity in feed. PV ethanol/water liquid mixture (*) and VP pure ethanol vapor (*).
4.2. Single component vapor permeation and pervaporation of mixture The interaction between water and ethanol was investigated by single component VP and mixture PV. Fig. 7 shows ethanol permeance under mixture (PV) and single component (VP) at 303 K as a function of ethanol activity in feed. The closed keys show PV results for ethanol/water mixture, and open keys show single component ethanol permeance by VP. There is almost no difference between binary PV and single component VP for the ethanol permeance at the same ethanol activities in feed. This ®gure indicates that water molecules have almost no effect on ethanol transport. Fig. 8 shows the comparison between water permeances by PV and VP at 303 K as a function of water activities in feed. The single component water permeance remained constant with the water activity change in feed. On the other hand, the PV mixture
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Fig. 8. Water permeance and ethanol activity plotted against water activity in feed. PV ethanol/water liquid mixture (&) and VP pure ethanol vapor (&).
results showed lower water permeance than the single component VP results. Water permeance might be reduced by the presence of ethanol in feed. 4.3. Binary components vapor permeation Water permeance might be reduced by the presence of ethanol in feed as shown in Fig. 8. Activities of the coexisting ethanol in feed, however, changed from 0.11 to 0.82 in this experiment. For a clearer understanding, the activity of one component was kept constant and the other was changed in the VP experiments detailed in this section. Fig. 9 shows the water and ethanol permeance by VP as a function of the water activities. In this case, ethanol activity in feed was kept constant at about 0.5, and only water activity was changed. Both ethanol and water permeance were almost constant against water activities in feed. Ethanol permeance was almost independent of water activity.
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Fig. 9. Water and ethanol permeance when the ethanol activity in feed was constant. Water permeance (&, &) and ethanol permeance (*, *). Open keys represent the pure permeance and closed keys represent the mixture permeance.
Fig. 10 shows the constant water activity case. Water activity was kept constant, and only ethanol activity was changed. The water permeance decreased with increasing ethanol activity, although the water activity in feed was constant. This means that the water permeation was restricted by coexisting ethanol. Ethanol permeance was almost same as the single component VP and PV with the same ethanol activities (see Fig. 7). Hence, it is clear that the permeance of ethanol through the silicalite membrane depend only on the feed ethanol activities, whereas water permeance was seriously affected by the ethanol activities. 4.4. Adsorption measurements and diffusion coefficients From the results mentioned above, extremely high ethanol selectivity of the silicalite membrane may be caused due to the interference on water transport by
Fig. 10. Water and ethanol permeance when the water activity in feed was constant. Water permeance (&, &) and ethanol permeance (*, *). Open keys represent the pure permeance and closed keys represent the mixture permeance.
ethanol. It is not clear whether adsorption or diffusion effect is the dominating factor on overall transport. Single and binary adsorption measurements and diffusivity calculation were carried out to realize the relative importance. Fig. 11 shows single component adsorption to the silicalite membranes at 303 K. The amounts of ethanol adsorbed were almost constant, while the amount of water adsorbed increased linearly as the water activity increased. The ®tting curve for ethanol adsorption was calculated using Langmuir isotherm (K1.2 10ÿ3 Paÿ1). Single component ethanol adsorption results were similar to silicalite powder results [2,25]. Fig. 12 shows amount of adsorption with ethanol/ water mixture as a function of ethanol activity in feed. Water activity in feed was kept at about 0.54 and only ethanol activity was changed. Although water activity in feed was kept constant, the amount of water adsorption obviously decreased because of the coexistence of ethanol in feed. On the other hand, ethanol adsorption from ethanol/water mixture agreed with the Langmuir
M. Nomura et al. / Journal of Membrane Science 144 (1998) 161±171
Fig. 11. Adsorption measurements of single component. Water adsorption (&) and ethanol adsorption (*).
Fig. 12. Adsorption measurements of binary components. Water adsorption (&, &) and ethanol adsorption (*, *). Open keys represent the pure permeance and closed keys represent the mixture adsorption. Water activities were kept at about 0.54.
curve calculated by single component adsorption results, even though there were some experimental errors. Water adsorption from water ethanol mixtures were so small that a detailed quantitative explanation is dif®cult to make. Silicalite membrane showed ethanol selective adsorption, and the amount of ethanol adsorbed was about 10 times larger than that of water when water activity in feed was 0.54 and ethanol activity was 0.059. This water and ethanol adsorption amounts agreed well with the prediction by Langmuir mixture model [21].
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Fig. 13. Diffusion coefficients of single component. (&) represents water diffusion coefficients and (*) represents ethanol.
Fig. 13 shows the diffusion coef®cients in the membrane for single component adsorption which was calculated by adsorption±diffusion model mentioned above. The permeance data and the adsorption measurements are taken from Figs. 7 and 11, respectively. While the diffusion coef®cients of water were almost constant, the ethanol diffusion coef®cients gradually increased with increasing ethanol activity. This might be the effect of the concentration gradients in the membrane or intercrystalline region. In this study, the diffusion coef®cients for single component and binary components were compared with at the same activity in feed. Thus, we neglected this diffusion coef®cients gradient. The diffusion coef®cients for binary components adsorption case are shown in Table 1. The table shows the diffusion coef®cients when the water activity in feed was about 0.54. The permeance data and the adsorption data used for the calculation are from Figs. 10 and 12, respectively. The diffusion coef®cients of ethanol were almost same as the extrapolated single component results in Fig. 13. Water diffusion seems to be increased by the presence of ethanol in feed. However, it might not be different considering the adsorption error in estimation of adsorbed volume of water. Table 1 shows the water permeance, the amount of adsorption and diffusion coef®cients of water for binary mixture. The water activity in feed was kept constant, and only ethanol activity was varied. The permeance and the amounts of water adsorption
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Table 1 Diffusion coefficients of water and ethanol with water permeance and water adsorption (temperature 303 K) Feed activity (dimensionless) Water Ethanol Water permeance (10ÿ7 mol mÿ2 sÿ1 Paÿ1) Water adsorbed (mol kgÿ1) Di/x (10ÿ7 m sÿ1) Water Ethanol
0.55 0 15.8 1
0.53 0.059 8.4 0.2
0.53 0.14 5.1 0.1
0 0.22 Ð Ð
3 Ð
7 1
8 3
Ð 2
seriously decreased with increase in the ethanol activity, while the water diffusion coef®cients were not changed. Thus, the high ethanol permselectivity through the silicalite membrane was due to its competitive adsorption to the silicalite membrane. 5. Conclusion The transport mechanism of water/ethanol mixture through silicalite membrane was investigated by the combination of PV, VP and adsorption measurements using the adsorption±diffusion model. Only water permeation was greatly restricted by the ethanol in feed. On the other hand, ethanol permeance was slightly affected by water. Ethanol selective adsorption, hence, is the main reason for the high ethanol permselectivity of the silicalite membrane. 6. Symbols Ci Di Ji K pi Pi x X
concentration in the membrane (mol mÿ3) diffusion coefficient (m2 sÿ1) flux (mol mÿ2 sÿ1) Langmuir constant (Paÿ1) vapor pressure (Pa) permeance (mol mÿ2 sÿ1 Paÿ1) membrane thickness (m) molar fraction (dimensionless)
6.1. Subscripts EtOH H 2O feed
ethanol water feed side
perm i
permeate side i component
Acknowledgements This research was supported by NEDO International Joint Research Grant. The authors are grateful to Mr. Hideo Suematsu and Miss Momoyo Aizawa of Japan High Polymer Center for the gravimetrical adsorption measurements, and wish to acknowledge helpful advises and discussions with Mr. Takashi Sugawara and Mr. Balagopal N. Nair. References [1] H. Kita, K. Horii, Y. Ohtoshi, K. Tanaka, K. Okamoto, Synthesis of a zeolite NaA membrane for pervaporation of water/organic liquid mixtures, J. Mater. Sci. Lett. 14 (1995) 206. [2] T. Sano, H. Yanagishita, Y. Kiyozumi, F. Mizukami, K. Haraya, Separation of ethanol/water mixture by silicalite membrane on pervaporation, J. Membr. Sci. 95 (1994) 221. [3] R.C. Binning, R.J. Lee, J.F. Jennings, E.C. Martin, Separation of liquid mixtures by permeation, Ind. Eng. Chem. 53 (1961) 45. [4] D.R. Paul, J.D. Paciotti, Driving force for hydraulic and pervaporative transport in homogeneous membranes, J. Poly. Sci. 13 (1975) 1201. [5] C.H. Lee, Theory of reverse osmosis and some other membrane permeation operations, J. App. Poly. Sci. 19 (1975) 83. [6] M.H.V. Mulder, C.A. Smolders, On the mechanism of separation of ethanol/water mixtures by pervaporation I. Calculations of concentration profiles, J. Membr. Sci. 17 (1984) 289. [7] T. Kataoka, T. Tsuru, S. Nakao, S. Kimura, Permeation equations developed for prediction of membrane performance in pervaporation, vapor permeation and reverse osmosis based on the solution±diffusion model, J. Chem. Eng. Japan 24 (1991) 326.
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