Yan Wang and Tai-Shung Chung (Department of Chemical and Biomolecular Engineering
National University of Singapore)
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.
biofuel, bioalcohol, pervaporation separation, pervaporation-distillation hybrid process, membrane.
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 . 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 bio-refinery 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.
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 . 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 . 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.
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 pervaporation-distillation, 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 99.95 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 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 . 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 dual-layer 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), 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/m2h at 150°C, far superior to most polymeric membranes.
Conclusion and Future Trends
It is well believed that the pervaporation-based 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 pervaporation-distillation 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.
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-279-000-311-281).
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