Membrane Separation of Ethanol from Mixtures of Gasoline and

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Ind. Eng. Chem. Res. 2011, 50, 1023–1027

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Membrane Separation of Ethanol from Mixtures of Gasoline and Bioethanol with Heat-Treated PVA membranes Yohei Ueda,† Tomoyuki Tanaka,† Atsushi Iizuka,‡ Yuka Sakai,† Toshinori Kojima,† Shigeo Satokawa,† and Akihiro Yamasaki*,† Department of Materials and Life Sciences, Faculty of Science and Engineering, Seikei UniVersity, 3-3-1 Kichijoji-kitamachi, Musashino, Tokyo 180-8699, Japan, and Research Center for Sustainable Science and Engineering, Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, 2-1-1, Katahira, Sendai, Miyagi 980-8577, Japan

The pervaporation performance of heat-treated PVA membranes with varying degrees of saponification was examined for the separation of ethanol from mixtures with n-heptane, which simulates the on-board separation of ethanol from gasoline containing bioethanol. All of the PVA membranes showed high permselectivity for ethanol against n-heptane. The ethanol selectivity decreased slightly, but the permeation flux increased with decreasing degree of saponification. The ethanol weight fraction in the permeate was higher than 98% for all feed compositions studied for PVA-417 membrane with a degree of saponification of 78-81%. High ethanol selectivity was observed irrespective of the heat-treatment time, and a maximum flux was observed at 3 min heat treatment at 75 °C. The flux decreased with increasing heat-treatment temperature. The PVA membrane showed high ethanol selectivity (96.8% in the permeate) and flux (12.2 g · m-2 · h-1) for commercially available E10 (10 wt % ethanol) gasoline. Introduction Bioethanol-containing gasoline (BEG) has been introduced into the market in several regions including the United States and Europe, and the share of bioethanol is expected to increase because of environmental and energy-security concerns in many countries and areas. Combustion can be more easily controlled by adding ethanol to gasoline for the homogeneous-charge compression-ignition (HCCI) engine.1,2 This is mainly due to the high octane value of ethanol (about 113) compared to that of gasoline (about 88 for standard gasoline).3 In addition, a combustion engine with separate injection of ethanol-rich and ethanol-lean fuel has been proposed,4 where higher combustion efficiency can be achieved when ethanol-rich fuel is supplied for startup whereas gasoline-rich fuel is appropriate for highspeed operation. In this case, it is necessary to separate ethanol and gasoline components on a vehicle, and a compact separation unit should be loaded on the vehicle. Membrane separation is a potential option for such a separation process. However, no attempts have been made to develop membranes for the separation of the ethanol and gasoline components from BEG. In the present study, a membrane separation process for separating ethanol from BEG was developed. Separation of ethanol from mixtures with hydrocarbons has been studied by several authors. Okada and Matsuura5,6 investigated the pervaporation performance of hydrophilic membranes made from polymers such as cellulose triacetate and polyamide for ethanol/n-heptane mixtures and reported high selectivity for ethanol over n-heptane. Park et al.7,8 showed that membranes made from blends of the hydrophilic polymers poly(vinyl alcohol) and poly(acrylic acid) were selective to ethanol over toluene. Dutta and Sikdar9 showed that azeotropic mixtures of ethanol and benzene or ethanol and cyclohexane could be separated with perfluorosulfonic acid (PFSA; Nafion) * To whom correspondence should be addressed. E-mail: [email protected]. † Seikei University. ‡ Tohoku University.

membranes. Although these studies mainly focused on the pervaporation mechanism and not on the development of the separation of ethanol from bioethanol-containing gasoline, they suggested that ethanol could be effectively separated by hydrophilic membranes. Ethanol is more hydrophilic than gasoline components, which are mainly aliphatic hydrocarbons. The solubility parameter of ethanol is 26.2, whereas that of heptane is 15.3, and the solubility parameters of other hydrocarbons in gasoline are in the range of 15-16. It can be expected, therefore, that ethanol is permselective when hydrophilic material is used as the membrane material. In this study, we have examined poly(vinyl alcohol) (PVA) as a candidate material for ethanol enrichment from BEG. PVA is one of the most hydrophilic polymers, with a solubility parameter of 27.46, and has excellent membrane formation properties. PVA membranes have been used for dehydration processes. For example, the company GFT established a process in the 1990s for the dehydration of bioethanol contaminated with trace amounts of water.10-12 Thus, the ethanol permeation rate through a PVA membrane should be low, but much higher than for hydrocarbons. To achieve both high permeation flux and high permselectivity, it is necessary to tune the hydrophilicity of PVA membranes. Two kinds of methods for tuning the hydrophilicity can be considered, namely, varying the degree of saponification or varying the heat treatment. PVA is produced by the saponification of poly(vinyl acetate) (PVAc), whereby acetyl groups in PVAc are transformed to hydroxyl groups by alkaline treatment. The degree of saponification is defined as the ratio of the number of hydroxyl groups to the sum of the number of hydroxyl and acetyl groups. With increasing degree of saponification, the hydrophilicity of PVA increases because hydroxyl groups are more hydrophilic than acetyl groups. Thus, the hydrophilicity of a PVA membrane can be controlled by changing the degree of saponification. Properties of PVA membranes can be controlled by the heat treatment. The heat treatment makes PVA membranes water-insoluble by either thermal cross-linking or an increase in the degree of crystallinity.13,14 The cross-linking

10.1021/ie1014662  2011 American Chemical Society Published on Web 12/13/2010

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Table 1. Properties of Membrane Materials and Membranes membrane or membrane material PVA membrane (Kuraray Co., Ltd.) PVA217 PVA417 PVA505

degree of polymerization

degree of saponification (%)

1500

98-99

1700 1700 500

87-89 78-81 72.5-74.5

occurs by heat treatment at temperatures higher than ∼150 °C, whereas the increase of the crystallinity could occur at temperatures above the glass transition temperature (85 °C). Thus, the separation performances of PVA membranes could be controlled by changing the heat-treatment conditions. Cross-linking by heat treatment was applied to PVA membranes with varying heattreatment times and temperatures. The membrane separation characteristics of PVA membranes with various degrees of saponification and after heat treatment under various conditions were studied experimentally for ethanol/heptane mixtures and for commercially available E10 gasoline.

Figure 2. Permeation flux (total) through various PVA membranes. The membranes had a thickness of about 50 µm and had been heat-treated for 3 min at 75 °C. Pervaporation conditions: temperature ) 30 °C, downstream pressure ) 1 kPa. Feed liquid: ethanol/n-heptane ) 75:25 (wt %).

Experimental Section Membrane Materials and Preparation. PVAs with different degrees of saponification were kindly supplied by Kuraray Co., Ltd., Tokyo, Japan, in the form of powders. The properties of the PVA used are summarized in Table 1. For comparison purposes, a commercially available PVA membrane (referred to hereafter as Kuraray PVA membrane) supplied by Kuraray Co., Ltd., was also used for the membrane separation experiments. PVA membranes were prepared by casting aqueous solutions of the PVA powder. The concentration was fixed at 7 wt %, and the amount of casting solution was adjusted so that the dry thickness of the resulting membrane was 40 µm. After casting, the membrane was dried in a desiccator at room temperature at least overnight and then heat-treated in a constant-temperature oven at a given temperature for a given time. Without heat treatment, the membranes easily dissolved in water, but after heat treatment, they became insoluble. Membrane Separation Experiments. Membrane separation performance was examined based on pervaporation, chosen because a negative pressure source is normally available on automobiles. The experimental setup is shown schematically in Figure 1. Mixtures of ethanol with n-heptane were used as a model feed liquid simulating BEG. The permeation temperature

Figure 3. Ethanol fraction in the permeate through various PVA membranes. The membranes had a thickness of about 50 µm and had been heat-treated for 3 min at 75 °C. Pervaporation conditions: temperature ) 30 °C, downstream pressure ) 1 kPa. Feed liquid: ethanol/n-heptane ) 75:25 (wt %).

was controlled at about 30 °C by immersing the permeation cell in a thermostat. The pressure on the downstream side of the membrane was kept at 1 kPa by a vacuum pressure controller, and the upstream pressure was atmospheric pressure. The permeate was collected in a cold trap immersed in a liquid nitrogen bath, and the permeation flux was determined by the weight change of the cold trap for a given sampling time, which was 1-2 h depending on the permeation flux. The composition of the permeate was determined by gas chromatography with thermal conductivity detection (GC-TCD, Shimadzu GC-14). Sampling and analysis were repeated at least three times for a given condition to confirm the steady-state permeation conditions. Results and Discussion

Figure 1. Experimental setup for pervaporation: (1) Membrane, (2) permeation cell, (3) thermostat, (4) cold trap, (5) liquid N2, (6) pressure control valve, (7) vacuum pump.

Effect of the Degree of Saponification. Figure 2 shows the permeation flux for a feed of 75 wt % ethanol and 25 wt % n-heptane. All of the membranes were heat-treated for 3 min at 75 °C. After the heat treatment, the PVA membranes became water-insoluble. The highest permeation flux was observed for the PVA-505 membrane, which had the lowest degree of saponification of the PVA membranes studied; that is, it was the most hydrophobic membrane. The Kuraray PVA membrane, which had the highest degree of saponification, showed the lowest permeation flux. The permeation flux thus increased with increasing degree of saponification. Figure 3 shows the ethanol weight fraction in the permeate. The ethanol concentration in the permeate was close to 100% for the membranes PVA-217 (99.96 wt %) and PVA-417 (99.70 wt %) and about 90% for the PVA-505 membrane. The

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Figure 4. Effect of heat-treatment time on the pervaporation performance of PVA-417 membrane with heat treatment at 75 °C. Pervaporation conditions: temperature ) 30 °C, downstream pressure ) 1 kPa. Feed liquid: ethanol/n-heptane )75:25 (wt %).

composition of the permeate for the Kuraray PVA membrane was not measurable because the amount of permeate obtained was too small for reliable analysis. Thus, the ethanol selectivity over n-heptane increased with increasing degree of saponification (i.e., increasing hydrophilicity). The influence of the degree of saponification on the ethanol selectivity was opposite to that on the permeation flux. The membranes with higher fluxes showed lower ethanol selectivities. This result is an example of the trade-off between the permeation flux and the selectivity. The PVA membranes and ethanol are more hydrophilic than n-heptane, so the affinity of ethanol for the PVA membrane is much stronger than that of n-heptane for the PVA membrane. The difference in affinity increases with increasing hydrophilicity of the membrane. This is the reason for the higher permselectivity of ethanol through more hydrophilic membranes. However, because the ethyl group in ethanol is hydrophobic, the higher hydrophilicity of the membrane would reduce the affinity of ethanol for the PVA membrane. This is the reason for the lower permeation flux through more hydrophilic membranes. Effects of Heat-Treatment Time and Temperature. Figure 4 shows the effect of heat-treatment time on the pervaporation performance of the PVA-417 membrane for a 75:25 (wt %) ethanol/n-heptane mixture. The heat-treatment temperature was fixed at 75 °C. The permeation flux was slightly decreased with increasing heat-treatment time, except at about 3 min, where the flux showed a maximum value. The ethanol weight fraction in the permeate was higher than 97% for all cases studied, irrespective of the heat-treatment time.

Figure 5. IR spectra of heat-treated PVA-417 membranes.

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Figure 6. Effect of the heat-treatment temperature on the pervaporation performance of PVA-417 membrane with heat treatment for 3 min. Pervaporation conditions: temperature ) 30 °C, downstream pressure ) 1 kPa. Feed liquid: ethanol/n-heptane )75:25 (wt %).

Figure 7. Pervaporation performances for ethanol/n-heptane mixtures with various feed compositions. Membrane: PVA-417 heat-treated for 3 min at 75 °C. Pervaporation conditions: temperature ) 30 °C, downstream pressure ) 1 kPa.

The heat treatment could change the properties of the physical or chemical structure of the PVA membrane. Figure 5 shows IR spectra of the PVA membranes after heat treatment at 75 °C. From the IR spectra, no marked changes in absorption were observed upon changing the heat-treatment time. It is known that changes in the crystallinity or cross-linking occur at temperatures above 75 °C.14 Thus, the cross-linking or crystallinity of the PVA-417 membrane was little affected by the heat treatment at 75 °C. Morphological changes in the PVA at the very surface of the PVA membrane caused by the heat treatment

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Figure 8. Gas chromatograms of the feed (E10 gasoline containing bioethanol) and the permeate of the E10 gasoline through the PVA-417 membrane heat-treated for 3 min at 75 °C. Pervaporation conditions: temperature ) 30 °C, downstream pressure ) 1 kPa.

would result in differences in the permeation flux and ethanol permselectivity. The morphological changes in the PVA membrane can be confirmed by the fact that the PVA membranes became insoluble in water after the heat treatment; without heat treatment, the PVA membranes were soluble in water. The maximum flux observed for the PVA membrane treated for 3 min is unclear at the present stage. However, the effect of the heat-treatment temperature was examined at a fixed treatment time of 3 min. Figure 6 shows the effect of the heat-treatment temperature on the permeation flux and ethanol permselectivity of the PVA417 membrane, for a 75:25 (wt %) ethanol/n-heptane mixture. The permeation flux gradually decreased with increasing heattreatment temperature. The ethanol permselectivity was slightly higher for the membrane treated at 75 °C than at 65 or 100 °C, but the difference was almost negligible. The decrease in the flux for the PVA-417 membrane treated at 100 °C can be attributed to the increase in the degree of crystallinity of PVA, because it has been reported that the crystallinity increases upon heat treatment above the glass transition temperature of 85 °C. Effect of Feed Composition. Figure 7 shows that the pervaporation performances of the PVA-417 membrane heattreated for 3 min at 75 °C increased with increasing ethanol fraction in the feed. The ethanol content of the feed was higher than 98% for all feed compositions, and the total permeation flux was almost equal to that of ethanol. These results demonstrate that high-purity ethanol would be obtained by membrane separation with cross-linked PVA membranes for all compositions of ethanol and gasoline. The decreased permeation flux with decreasing ethanol fraction in the feed can be explained in terms of the activity of ethanol in the membrane. The ethanol activity decreases with increasing n-heptane concentration in the feed. The permeation flux is proportional to the chemical potential difference, which is a function of the activity difference across the membrane. In addition, it was observed that the membrane was hardened by contact with n-heptane, which, in turn, reduces the ethanol permeation.

Separation of E10 Gasoline with the PVA Membrane. Figure 8 shows chromatograms of the feed and permeate of commercially available E10 gasoline with PVA-417 membrane heat-treated at 75 °C for 3 min. The peaks at about 2-min retention time indicate ethanol. The ethanol concentration in the permeate was 96.8 wt %, and the permeation flux was 12.2 g · m-2 · h-1. The permeation performance was unaffected by contact with E10 gasoline during the experiment (about 10 h). These results demonstrate that PVA membranes can be applied for the separation of ethanol from gasoline components in E10 gasoline. Conclusions Excellent selectivity of ethanol for ethanol/n-heptane mixtures (as a model for bioethanol-containing gasoline) and commercially available E10 gasoline was observed for heat-treated PVA membranes. The permeation performance suggested that pervaporation was significantly affected by the hydrophilicity of the membrane. Heat-treated PVA membranes showed very high selectivity (>90%) for ethanol against n-heptane or gasoline components. A higher degree of saponification of the PVA resulted in a higher ethanol selectivity and lower flux. A higher heat-treatment temperature gave a lower flux but affected the ethanol selectivity only slightly. Finally, an increased heattreatment time had little effect on the ethanol selectivity, and the flux showed a maximum at a 3-min heat-treatment time at 75 °C. Acknowledgment The authors are thankful for financial support by Ministry of Education, Culture, Sports, Science and Technology, MEXT, Japan under the program of Grant-in-Aid for Building Strategic Research Infrastructures. The authors thank Kuraray for the supply of PVAs and membranes. Literature Cited (1) Hashimoto, K. Effect of Ethanol on the HCCI Combustion; SAE Paper 2007-1-2038; SAE International: Warrendale, PA, 2007.

Ind. Eng. Chem. Res., Vol. 50, No. 2, 2011 (2) Kamio, J. Internal combustion engine system. U.S. Patent 7,370,609 B2, 2008. (3) Wyman, C., Ed.; Handbook on Bioethanol: Production and Utilization; CRC Press: Boca Raton, FL, 1996. (4) Kamio, J.; Kurotani, T.; Kuzuoka, K.; Kubo, Y.; Taniguchi, H.; Hashimoto, K. Study on HCCI-SI Combustion Using Fuels Containing Ethanol; SAE Paper 2007-01-4051; SAE International: Warrendale, PA, 2007. (5) Okada, T.; Matsuura, T. A new transport model for pervaporation. J. Membr. Sci. 1991, 59, 133. (6) Okada, T.; Matsuura, T. Predictability of transport equations for pervaporation on the basis of pore-flow mechanism. J. Membr. Sci. 1992, 70, 163. (7) Park, H. C.; Meertens, R. M.; Mulder, M. H. V.; Smolders, C. A. Pervaporation of alcohol-toluene mixtures through polymer blend membranes of poly(acrylic acid) and poly(vinyl alcohol). J. Membr. Sci. 1994, 90, 265. (8) Park, H. C.; Meertens, R. M.; Mulder, M. H. V. Sorption of alcoholtoluene mixtures in poly(acrylic acid)-poly(vinyl alcohol) blend membranes and its role on pervaporation. Ind. Eng. Chem. Res. 1998, 37, 4408. (9) Dutta, B. K.; Sikdar, S. K. Separation of azeotropic organic liquid mixtures by pervaporation. AIChE J. 1991, 37, 581.

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(10) Wesslein, M.; Heintz, A.; Lichtenthaler, R. N. Pervaporation of liquid mixtures through poly(vinyl alcohol) (PVA) membranes. I. Study of water containing binary systems with complete and partial miscibility. J. Membr. Sci. 1990a, 51, 169. (11) Wesslein, M.; Heintz, A.; Lichtenthaler, R. N. Pervaporation of liquid mixtures through poly(vinyl alcohol) (PVA) membranes. II. The binary systems methanol/dioxane and the ternary system water/methanol/ 1-propanol. J. Membr. Sci. 1990b, 51, 181. (12) Frank, L. Membrane process opportunities and challenges in the bioethanol industry. Desalination 2010, 250, 1067. (13) Gohil, J. M.; Bhattacharya, A.; Ray, P. Studies on the Cross-linking of Poly(vinyl alcohol). J. Polym. Res. 2006, 13, 161–169. (14) Hasimi, A.; Stavropoulou, A.; Papadokostaki, K. G.; Sanopoulou, M. Transport of water in polyvinyl alcohol films: Effect of thermal treatment and chemical crosslinking. Eur. Polym. J. 2008, 44, 4098–4107.

ReceiVed for reView July 9, 2010 ReVised manuscript receiVed November 17, 2010 Accepted November 22, 2010 IE1014662