Membranes for Biofuel Separation

Transcription

1 Membranes for Biofuel Separation Yan Wang and Tai-Shung Chung* Department of Chemical and Biomolecular Engineering National University of Singapore *Corresponding author Abstract: The growth of biofuel is not only dependent on the advances in genetic transformation of biomass into biofuel, but also on the breakthroughs in purification and separation techniques. Among many potential techniques, advances in membrane pervaporation-based hybrid technologies provide the most direct, effective and feasible separation approach to replace conventional distillation techniques. This article introduces the basics of membrane pervaporation, process design of pervaporation-distillation hybrid technologies, as well pervaporation membranes for biofuel dehydration and recovery. Keywords: biofuel, bioalcohol, pervaporation separation, pervaporation-distillation hybrid process, membrane. Introduction The fluctuation and high price of crude oil in recent years have raised worldwide concerns of energy security. Significant attention has been given to explore biofuel as an alternative energy source for its sustainability and lower emission of greenhouse gases. Technological breakthroughs have been made on both food-based first generation and non-food based second generation biofuels. To avoid food shortage for humanity, the non-food based biofuel research has received greater society support where non-food crops and agricultural residues (such as corn stover and plant trimmings) are the feedstock for the production of bioalcohol. Basically, bioalcohol is produced from the digestion of biomass by enzymes to release the stored sugars followed by yeast-based fermentation. For the second generation biofuel, several stages must be taken in order to produce biofuel from lignocellulose, i.e., (1) pretreatment to make the biomass amenable to hydrolysis; (2) hydrolysis to break down the lignocellulose into sugars; (3) purification of sugar solutions; (4) microbial fermentation; and (5) separation to produce 99.5% pure alcohol for energy uses. Depending on the process conditions, the fermentation broth typically contains water, acetone, butanol, ethanol and many others with various compositions, where alcohol content is in the range of 5-12% [1-4]. In order to produce alcohol of high purity, efficient concentration and separation technologies are in demand. At present, the separation process accounts for 60 to 80% of the overall production cost [5]. Conventional separation techniques for liquid mixtures include distillation, low-temperature crystallization, adsorption, extraction, etc. Among them, distillation is the dominant refinery process. However, due to the energy intensive nature, negative environmental impact, and complicated operation procedure, these techniques are generally not economical and practical to stand alone for the entire bioalcohol separation process. New biorefinery concepts and hybrid technologies must be developed. The integration of membrane-based pervaporation technology with the existing distillation infrastructure appears to be a promising solution in the bio-refinery field for being economical, energy efficient and environmentally benign. Using the distillation and membrane pervaporation hybrid technology, not only can one take advantage of plenty distillation facilities already in place, but also take advantage of the membranes unique features and superior performance to breakup azeotropic mixtures, therefore facilitating biofuel separation from biomass. This article is to provide a comprehensive summary of the pervaporation separation technology for biofuel applications, including the fundamentals of the pervaporation technology, hybrid process design, as well as conventional membrane materials and morphologies employed for bioalcohol dehydration and recovery. The future trends and challenges are also discussed briefly. 34

2 Pervaporation Technology and Process Design for Biofuel Purification Pervaporation was first introduced in 1917 by Kober in his study on the selective permeation of water from aqueous solutions of albumin and toluene through collodion (cellulose nitrate) films [6]. The real breakthrough was achieved in 1980s, when GFT (Gesell-schaft für Trenntechnik, Hamburg, Germany) developed a poly(vinyl alcohol) (PVA) and polyacrylonitrile (PAN) composite membrane for the dehydration of alcohol/water azeotropic mixtures [7]. From then on to 1999, the pervaporation process has been progressively commercialized towards large scale processes with more than 90 industrial pervaporation units installed worldwide [8]. The separation characteristics of pervaporation are quite complex since a phase change (liquid to vapor) is involved in the process. In pervaporation, the membrane acts as a barrier layer between the feed liquid and the permeate vapor. The permeable components are sorbed into/onto the membrane, diffuse through the membrane and evaporate as permeates driven by vacuum or gas purge. The transport mechanism is schematically shown in Figure 1. The separation performance of a pervaporation membrane is not only dependent on the characteristics of the membrane and the liquid mixture to be separated, but also on the operation conditions such as feed composition, process temperature, and permeate pressure. In spite of numerous advantages of the pervaporation technology, using pervaporation alone for the entire process of biofuel purification is not economical because other conventional separation processes are already available, mature and reliable in industries. The pervaporation-based hybrid processes such as pervaporationdistillation, pervaporation-adsorption and pervaporation-reactor have been proposed. Among them, pervaporation-distillation is the most popular hybrid process for the production of bioalcohol. To separate ethanol-water mixtures, the pervaporation process could be employed to split its azeotrope before distillation (schematically shown in Figure 2A). It can be either used for biofuel dehydration using hydrophilic membranes, or biofuel enrichment via organophilic membranes. In most cases pervaporation is integrated as a final step for either the top or bottom product of the distillation column (Figure 2B) in order to achieve the required concentration of retentate and/or permeate. Depending on the layout of the pervaporation unit and end users requirements, the final ethanol concentration of the product may vary from 99.5 to wt%. Another alternative layout of the pervaporation-distillation hybrid process is to place pervaporation unit between two distillation columns (Figure 2C). By incorporating a pervaporation process into the distillation process, it could also reduce the number of trays by processing a side stream of the distillation column. A considerable cost can be saved because of the lower energy consumption, waste minimization and avoidance of using chemical entrainer. Pervaporation Membranes for Biofuel Dehydration and Recovery The pervaporation membrane is the heart of the pervaporation process to determine the separation efficiency for the biofuel dehydration and recovery. A desirable Figure 1. Transport mechanism in the pervaporation process. 35

3 A C B Figure 2. Pervaporation-distillation hybrid processes with various designs. pervaporation membrane should possess a high permeation flux and separation efficiency, as well as a long-term chemical and mechanical stability. To the present, existing commercial membranes for pervaporation separation are very limited, as summarized in Table 1 [9]. Compared with biofuel dehydration, the development of pervaporation membranes for bioalcohol recovery is still at the infancy stage. Generally, composite membranes consisting of a selective layer and a porous substrate layer are employed for industrial applications, where the thin selective layer provides the overall separation function, while the substrate offers major mechanical strength, lowers water sorption and minimizes material costs. A typical example is the GFT PVA composite membrane that comprises a PVA selective layer, a PAN supporting layer and a nonwoven support layer. On the other hand, compared to conventional flat-sheet composite membranes, the development of asymmetric hollow fiber membranes for biofuel separations has gained much attention in recent years. Hollow fibers have advantages of a larger membrane area, self-supporting structure, good flexibility, and ease of module fabrication and system operation. Recent studies by Chung s group [14-20] on duallayer hollow fiber membranes have shown promising pervaporation performance for biofuel separation without any intensive post thermal or chemical treatments. In terms of membrane materials, there are polymeric, inorganic and organic-inorganic hybrid membranes. Among the diversity of polymers, hydrophilic polymeric materials, PVA, chitosan and sodium alginate are common membrane materials for bioalcohol dehydration. While rubbery polymers are usually adopted as pervaporation recovery membranes, such as poly(dimethylsiloxane) (PDMS), poly [1-(trimethylsilyl)-1-propyne), 36

4 MEMBRANE COMPANY COMMENTS NaA ceramic membranes Jiangsu Jiuwu Hi-Tech, China Dehydration NaA zeolite membrane Misui, Japan Dehydration (feed 93% to product for % ethanol) NaA zeolite membrane Inocermic GmbH, Germany Dehydration (flux of 13 for 85 ethanol feed at 120 C) [10] Siftek TM polyimide hollow fiber Vaperma, Canada Dehydration ZeoSep A membrane I3 Nanotec LLC, USA Dehydration (flux of , separation factor of 20,000 40,000, for 90 ethanol feed at 75 C) [11] Aromatic polyimide membrane UBE America, USA Dehydration PERVAP cross-linked PVA membrane with PAN support Sulzer Chemtech, Switzerland Dehydration Cross-linked PVA membrane CM-Celfa AG, Switerland Dehydration Ceramic silica-based membrane Pervatech BV, Netherlands Dehydration (flux of 2 ; separation factor of 160 for 89 ethanol feed at 70 C) [12] HybSi organic-inorganic hybrid silica-based membrane ECN, Netherlands Dehydration with excellent hydrothermal stability at high temperature GKSS Simplex Complex polyelectrolytes/ PAN GKSS, Germany Dehydration Ceramic membranes IBMEM Industrial Biotech Membranes, Germany Ethanol dehydration Cross-linked PDMS membrane with support Membrane Technology and Research, USA Recovery PERVAP cross-linked PDMS membrane with PAN support Sulzer Chemtech, Switzerland Recovery PDMS-based membranes GKSS, Germany Recovery PDMS-based membranes Solsep BV, Apeldoorn, Netherlands recovery [13] Table 1. Commercial pervaporation membranes for biofuel dehydration and recovery. 37

5 polyvinylidene fluoride, etc. Various chemical and physical modifications are generally employed to stabilize and improve their mechanical strength and long-term stability. Except polymeric membranes, inorganic membranes based on silica, alumina or zeolites have gained attention in recent years for biofuel separation. Since they are not subjected to any solvent-induced swelling and have a superior thermal, chemical and mechanical stability, they typically exhibit a greater flux and separation factor than most polymeric membranes, especially for some specific separations in harsh environments. However, the high cost and low processibility of inorganic materials still limit their applications in membrane separation. Therefore, the combination of polymeric membranes with inorganic membranes in various forms may open up new applications for membrane technology. A successful example is the Hybsi membrane developed by the Energy Research Centre at Netherlands. In the dehydration of n-butanol with 5% of water, the organic-inorganic hybrid membrane shows a high separation factor of over 4000 and an ultra-fast water transport rate of more than 20kg/m 2 h at 150 C, far superior to most polymeric membranes [21]. Conclusion and Future Trends It is well believed that the pervaporationbased hybrid technologies possess many benefits over conventional separation techniques for biofuel purifications. The major challenges against the industrialization of membrane pervaporation are the membrane reliability in harsh environments, the high cost of membrane production and module fabrication, as well as problems associated with the complex process design. In the industry, the pervaporationdistillation hybrid process is the most popular configuration for biofuel separation. However, future works should focus on the science and hybrid engineering so that one can wisely select the best hybrid system for specific fermentation types, production scales, product purity requirements, and available investment. Ongoing works on the development and exploration of pervaporation membranes with better separation and physicochemical properties for bioalcohol separation is still of paramount importance. Polymeric membranes with higher flux and separation efficiency in harsh operating environments are in demand. Inorganic membranes and organic-inorganic hybrid membranes may share the industrial market or take the dominant position of polymeric membranes gradually. Greater emphases should be placed on hybrid membranes and hollow fiber membranes with desirable membrane morphology and separation performance. Acknowledgements The authors would like to thank the Singapore National Research Foundation (NRF) for support through the Competitive Research Program on the project entitled New Biotechnology for Processing Metropolitan Organic Wastes into Value-Added Products (grant number: R ). References [1] S.E. Koonin, Getting serious about biofuels, Science, 311, (2006). [2] L.Y. Jiang, Y. Wang, T.S. Chung, X.Y. Qiao and J.Y. Lai, Polyimides membranes for pervaporation and biofuels separation, Prog. Polym. Sci., 34, (2009). [3] N. Qureshi, M.M. Meagher, J. Huang and R.W. Hutkins, Acetone butanol ethanol (ABE) recovery by pervaporation using silicalitesilicone composite membrane from fed-batch reactor of Clostridium acetobutylicum, J. Membr. Sci., 187, (2001). [4] T.C. Ezeji, N. Qureshi and H.P. Blaschek, Butanol fermentation research: Upstream and downstream manipulations, Chem. Rec., 4, (2004). [5] A.J. Ragauskas, C.K. Williams, B.H. Davison, G. Britovsek, J. Cairney, C.A. Eckert, W.J. Jr. Frederick, J.P. Hallett, D.J. Leak, C.L. Liotta, J.R. Mielenz, R. Murphy, R. Templer and T. Tschaplinski, The path forward for biofuels and biomaterials, Science, 311, (2006). [6] P.A. Kober, Pervaporation, perstillation and percrystallization, J. Amer. Chem. Soc., 39, (1917). [7] G.F. Tusel and H.E.A. Bruschke, Use of pervaporation system in the chemical industry, Desalination, 53, (1985). [8] A. Jonquieres, R. Clement, P. Lochon, J. Neel, M. Dresch and B. Chreticn, Industrial state-of-the-art of pervaporation and vapour permeation in the western countries, J. Membr. Sci., 206, (2002). [9] [10] S.L. Wee, C.T. Tye and S. Bhatia, Membrane separation process Pervaporation through zeolite membrane, Sep. Purif. Technol., 63, (2008). [11] i3 Nanotec, ZeoSep A Membrane, available from: 38

6 [12] B.V. Pervatech, Applications and preferred combinations, available from: [13] P. Peng, B. Shi and Y. Lan, A review of membrane materials for ethanol recovery by pervaporation, Sep. Sci. Technol., 46, (2011). [14] Y. Wang, S.H. Goh, T.S. Chung and N. Peng, Polyamide-imide/polyetherimide dual-layer hollow fiber membranes for pervaporation dehydration of C1-C4 alcohols, J. Membr. Sci., 326, (2009). [15] L.Y. Jiang, H. Chen, Y.C. Jean and T.S. Chung, Ultra-thin polymeric interpenetration network with separation performance approaching ceramic membranes for biofuel, AIChE J., 55, (2009). [16] Y. Wang, T.S. Chung, B. Neo and M. Gruender, Processing and engineering of pervaporation dehydration of ethylene glycol via duallayer polybenzimidazole (PBI) /polyetherimide (PEI) membranes, J. Membr. Sci., 378, (2011). [17] G.M. Shi, Y. Wang, and T.S. Chung, Dual-layer PBI/P84 hollow fibers for pervaporation dehydration of acetone, AICHE J., 58, (2012). [18] N. Widjojo and T.S. Chung, Pervaporation dehydration of C2-C4 alcohols by 6FDA-ODA-NDA/Ultem dual-layer hollow fiber membranes with enhanced separation performance and swelling resistance, Chem. Eng. J., 155, (2009). [19] R.X. Liu, X.Y. Qiao and T.S. Chung, Dual-layer P84/polyethersulfone hollow fiber for pervaporation dehydration of isopropanol, J. Membr. Sci., 294, (2007). [20] Y. Wang, M. Gruender and T.S. Chung, Pervaporation dehydration of ethylene glycol through polybenzimidazole (PBI)-based membranes. 1. Membrane fabrication, J. Membr. Sci., 363, (2010). [21] H.L. Castricum, R. Kreiter, H.M. van Veen, D.H.A. Blank, J.F. Vente and J.E. ten Elshof, High-performance hybrid pervaporation membranes with superior hydrothermal and acid-stability, J. Membr. Sci., 324, (2008). About the Authors Dr. Wang Yan is a research fellow and project leader working in Prof. Chung s membrane research group. She received her bachelor degree in 1996 from Hefei University of Technology (Anhui Province, China), M.Sc. degree in 2003 and PhD degree in 2009 from Department of Chemistry, National University of Singapore (NUS). Her research interests mainly include: (1) development and applications of miscible polymer blends; (2) polymeric-based membranes for pervaporation separation of liquid mixtures, as well as (3) the purification and separation of biofuels from the fermentation broth. Her PhD thesis won the World Future Foundation PhD Prize of $10,000 in Environmental and Sustainability conferred by the Government of Singapore in So far she has published 17 journal papers, 20 conference papers, two book chapters and four patents. Professor (Neal) Tai-Shung Chung is a Professor and Provost s Chair of the Chemical and Biomolecular Engineering Department at the National University of Singapore (NUS). Before joining NUS in 1995, he worked in Hoechst Celanese for 13 years and Aeroquip for two years in USA. His research strengths are in polymeric membranes for water, energy, biofuel and CO 2 capture. He has won research grants of $48 millions and has trained 31 PhD, 14 Master students and 50 post-doctors. Currently, he has one book, 16 book chapters, 50 patents, 430 journal papers. He has received numerous honors and awards. As a Senior Consultant in Hyflux (Singapore) in , he led and built the Hyflux membrane research team and was a co-inventor of Hyflux Kristal 600 ultrafiltration hollow fiber membranes. 39