Pervaporation separation processes: history, development and separation applications.
Pervaporation is one of the most popular areas of current membrane research, but the concept of pervaporation separation is not new. As early as 1917, it was recorded that water permeated through collodion films selectively |1~. However, an analysis of pervaporation literature and patents showed that most of the work in field was done in the last several years |2~.
It was the work by Binning and co-workers |4,5,6~ from 1958-1961 that established the principles and highlighted the potential of pervaporation separation. Although the research work was continued for many years and several patents were obtained, the permeation flux was too low to be economically useful. The solution to this problem was the invention of an integrally-skinned asymmetric membrane for reverse osmosis, but the commercial reverse osmosis membranes proved to be not very selective for pervaporation separation because of the defects in the selective skin layer.
The literature on pervaporation is extensive. A book published by Huang extensively covers the various aspects of pervaporation, such as transport principles, process thermodynamics, evaluation of membrane materials, and plant design as well as industrial applications |3~.
Membranes made of cellulose acetate, polyethylene, polyamide, polysulfone, and poly (vinyl alcohol), to name a few, have been tested for the separation of various mixtures including, for example, alcohols-water, acetone-water, benzene-methanol, toluene-heptane, benzene-cyclohexane, and isomeric xylenes.
During the last decade, many technical breakthroughs took place |11~.
In 1982-83 Gesellschaft fur Trenntechnik (GFT) Co., Germany, developed the first commercial pervaporation membrane for the dehydration of alcohol solutions. The membrane was prepared by depositing a thin layer of poly(vinyl alcohol) on an asymmetric polyacrylonitrile substrate using a proprietary technique |8,9~.
The first commercial pervaporation plant was built in 1988 in Bethenville in the Champagne region of France. Here, the fermentation of sugar beets produces a fermentation broth containing approximately 14% alcohol content. The feed stream is first concentrated using conventional distillation columns up to 85% and then the pervaporation separation units are used to bring the alcohol content to 99.9%. The capacity of this plant is 150,000 litres/day of alcohols of various qualities and could reach 400,000 litres/day.
This plant was designed and built by GFT of Germany which has since been absorbed into Carbon Lorraine Engineering.
Since 1987 Mitsui Zosen Co., Japan, has built many plants in Japan using the GFT membrane under license for various separation applications. This company has been instrumental in pioneering efforts to introduce pervaporation separation processes in the Japanese chemical industry and has already built some 10 pervaporation plants with applications in dehydration of ethanol, isopropanal and the separation of various types of organic aqueous systems |7~.
In 1988-89 Membrane Technology and Research (MTR) Inc., U.S.A. marketed a pervaporation system for the removal of small amounts of organic solvents from contaminated water.
Pervaporation membranes for the dehydration of ethanol have recently been developed by Texaco. The nature of the membranes used is proprietary and not disclosed by Zenon Inc. of Burlington, ON, which has been contracted under license to build a spiral wound module based on these membranes.
Kalsep, a subsidiary of British Petroleum, Great Britain, is developing a tubular, composite membrane which is claimed to be more efficient than the currently available membranes. It is believed that this membrane is made of polyacrylates supported on polysulfone substrate.
Pervaporation is a relatively new membrane separation process that has elements in common with reverse osmosis and membrane gas separation. In pervaporation, the liquid mixture to be separated (feed) is placed in contact with one side of a membrane and the permeated product (permeate) is removed as a low pressure vapor from the other side, Fig. 1. The permeate vapor can be condensed and collected or released as desired. The chemical potential gradient across the membrane is the driving force for the mass transport. The driving force can be created by applying either a vacuum pump or an inert purge (normally air or steam) on the permeate side to maintain the permeate vapor pressure lower than the partial vapor pressure of the feed liquid.
Among others, vacuum pervaporation is the most widely used mode of operation, while inert purge pervaporation is of interest if the permeate can be discharged without condensation.
Pervaporation can be described as a three-step process: solution-diffusion-evaporation. The separation is based on the selective solution and diffusion mechanism, i.e., the physical-chemical interactions between the membrane material and the permeating molecules, not the relative volatility as in distillation. Therefore, pervaporation is commonly considered to be a profitable complement to distillation for the separation of azeotropic and close-boiling mixtures, which requires at present the use of energy-intensive processes.
Unlike reverse osmosis, pervaporation is not limited by osmotic press and can be used for concentrating ethanol in an aqueous solution from 85 wt% to more than 99 wt%, while reverse osmosis is not applicable because an extremely high operating pressure is needed to overcome the osmotic pressure.
As in reverse osmosis, the liquid in contact with the membrane tends to dissolve into it and cause membrane swelling. Swelling tends to alter the membrane properties and generally leads to higher permeability and lower selectivity for the permeating species. On the other hand, the partial vapor pressure of a component on the permeate side of the membrane affects its permeation rate. Hence, the permeate vapor pressure must be kept low to maximize the driving force for the permeation.
It is important to note that phase change occurs in standard pervaporation processes. Further, the membrane temperature and the temperature gradient have significant effects on separation performance. Consequently, the energy needed must be at least equal to the heat of vaporization of the permeate. In principle, the heat of vaporization required can be supplied either in the feed liquid, or by a sweeping fluid on the permeate side, or directly to the membrane.
Pervaporation membrane units are modular in construction. There is no significant economy of scale so they can be used in either large or small processing.
Moreover it is easy to integrate pervaporation units with other suitable techniques so that the hybrid processes would be more effective than a scheme where the full reparation is affected by either technique alone.
Requirements for membranes
In developing pervaporation membranes, three issues must be addressed: membrane productivity, membrane selectivity, and membrane stability.
Membrane productivity is a measure of the quantity of a component that permeates through a specific area of membrane in a given unit of time. Permeation rate, or more commonly, permeation flux is frequently used to characterize membrane productivity. Permeation flux relates product rate to the membrane area required to make the separation. Note that permeation flux depends on both the intrinsic permeability and the effective thickness of a membrane. The commercialization of the pervaporation technique is, to a large extent, attributed to the engineering approach of making thin dense membranes, in asymmetric and composite forms.
When describing the selectivity of a membrane for the separation of a mixture composed of components A and B, the separation factor is defined in analogy to distillation
|alpha~ = (Y/1-Y) (1-X/X)
where X and Y are the molar fractions of the more permeable components A in the feed and permeate, respectively. When the separation factor is unity, no separation occurs. The membrane selectivity forms the basis for separating a mixture. Note that only when the concentration polarization in the feed phase is negligible, the selectivity indicated by the equation is an intrinsic property of the membrane. Otherwise the feed concentration on the membrane surface has to be substituted for X to obtain the intrinsic selectivity. Note also that the selectivity depends on the membrane material, the membrane morphology, the mixture to be separated, and the process operating conditions. In general, the separation factor must be determined experimentally.
Membrane stability is the ability of a membrane to maintain both the permeability and selectivity under specific system conditions for extended period of time. Membrane stability is affected by the chemical, mechanical and thermal properties of the membrane.
Compactness, flexibility, simplicity and versatility are some other strong points of pervaporation process. A comprehensive evaluation of the characteristics of pervaporation is available |3~.
The applications of pervaporation are generally classified into three categories: dehydration of solvents, organic compounds removal from aqueous solutions, and the separation of organic mixtures.
Currently, the dehydration of ethanol and ispropanol is the best developed application of pervaporation. The dehydration of other solvents such as glycols, acetone and methylene chloride are under development. According to Bruschke |9,10~, pervaporation can be used to dehydrate a wide variety of solvents and intermediates in the chemical processing industry.
Pervaporation can also be used in pollution control, solvent recovery and waste reduction. It has been shown that pervaporation is effective for the recovery of alcohols from fermentation broths and the removal of hazardous organic compounds from wastewater.
The separation of organic mixtures is the least developed application of pervaporation, but represents the largest opportunity for energy and cost savings. Pervaporation has shown to be promising for the separation of isomeric compounds and other organic mixtures.If the problems associated with membrane instability under relatively harsh conditions are solved, pervaporation would be used as a substitute or supplement to distillation for organic separation.
Research in Canada
Following the pioneering work of Binning et al. at the American Oil Co. in Texas in the late 1950s and the subsequent research carried out under Allan S. Michaels in the early 1960s, pervaporation research was started in Canada in 1967 in the laboratory of the author at the department of chemical engineering, University of Waterloo. His early work concentrated on the pervaporation separation of organic-organic mixtures and the development and theoretical principles which govern the transport processes, and diffusion through modified polyethylene membranes by his laboratory group.
This work eventually expanded to include the separation of aqueous organic mixtures (mainly ethanol-water and acetic acid-water systems using modified polyvinyl alcohol membranes by his graduate students).
More recently, pervaporation research has been started at the Institute of Environmental Chemistry of the National Research Council of Canada (NRC) under the direction of Takeshi Matsuura (now with the University of Ottawa) and there is also a project involving the pervaporation separation of dilute organic contaminants from waste waters at the department of chemical engineering at McMaster University under Jim Dickson. There is also a research group headed by B. Farnand involved in various applications of pervaporation in the petroleum industry at the EMR Canmet Laboratories in Ottawa. Activities on the industrial applications of pervaporations are still in an embryonic stage due to lack of funding both from the government and private sources, but there is some activity at Zenon Environmental Co. Inc. which is attempting to develop a spiral wound module for the pervaporation dehydration of ethanol-water systems.
Current research activity at Waterloo is focused on the synthesis and preparation of modified silicone based membranes and polyetherimide membranes for the pervaporation separation of aqueous systems containing dilute organic contaminants. Work on the pervaporation of ethanol-water and acetic acid-water systems is also continuing using various types of specially synthesized water selective crosslinked polyvinyl alcohol membranes.
Five international conferences on pervaporation separation processes have been held to date. This conference devotes itself exclusively to recent developments in the theory and practice and industrial applications of pervaporation separation processes. Attendance at this conference has ranged from 100 to 200 participants from around the world. It is an intimate conference of academics, industrial and governmental researchers and industrial membrane makers and practitioners of pervaporation where informal discussion is encouraged at a personal level leading to the exchange of ideas in new pervaporation concepts and innovations.
The sixth international the conference was held in Ottawa, September 1992, hosted by the NRC, Heidelberg Innovative Trennttechnik, GmbH, Germany, University of Ottawa and Zenon Environmental Inc.
This conference featured some 60 presentations from participants from Canada, France, Germany, Japan, the Netherlands, Sweden, South Korea, United States, China, Portugal and Russia. Robert Rautenbach, RWTH Aachen, gave a thoughtful lecture on the state of the art in pervaporation. The conference sessions dealt with pervaporation fundamentals, membranes, and module design and applications.
There was also a student paper competition presentation with participants from Canada, Germany, the Netherlands and China. The purpose of the student competition is to encourage participation in pervaporation among the younger generation of future scientists thus ensuring the lively future of pervaporation in the chemical industry. Takeshi Matsuura, University of Ottawa, who now occupies the newly established NSERC-British Gas Research Chair in Membrane Separation Processes, and Robert Bakish, Bakish Materials Corporation, served as co-chairmen of the conference.
The conference ended with a provocative lecture on the future of pervaporation by T. Asada, Mitsui Zosen, Japan, who was largely instrumental in the introduction of pervaporation membrane separation processes in the Japanese chemical industry. He believes that pervaporation alone cannot at present substitute for conventional separation processes such as distillation but must be used in conjunction with other processes as an adjunct in hybrid processes such as combining distillation with pervaporation.
The final session was a panel discussion on pervaporation including the importance of membrane and module design on the performance of pervaporation processes. The panel chairman was R.D. Behling, G.K.S.S. Research Centre, Geeshacht, Germany, who concluded that pervaporation stands at the threshold of becoming a major separation process in the next decade when used in combination with other conventional separation processes.
Robert Y.M. Huang, professor, department of chemical engineering, University of Waterloo; Xianshe Feng, graduate student, department of chemical engineering, University of Waterloo.
1. Kober, P.A., J. Amer. Chem. Soc., 39, 944 (1917).
2. Slater, C.S. and P.J. Hickey, in Proc. 4th Int. Conf. Pervaporation Processes in Chem. Ind., R. Bakish (Ed.), Bakish Materials Corp., Englewood, NJ, p. 476 (1989).
3. Huang, R.Y.M., Pervaporation Membrane Separation Processes, Elsevier Science Publishers B.V., Amsterdam, the Netherlands (1991).
4. Binning, R.C. and F.E. James, Petroleum Refining, 27, 214 (1958).
5. Binning, R.C., J.F. Jennings and E.C. Martin, U.S. Patent 3,035,060 (1962).
6. Binning, R.C., R.J. Lee, J.F. Jennings and E.C. Martin, Ind. Eng. Chem. 53, 45 (1961).
7. Asada, T. in Pervaporation Membrane Separation Processes, R.Y.M. Huang (Ed.), Elsevier, Amsterdam, the Netherlands, p. 491 (1991).
8. Neel, J. in Pervaporation Membrane Separation Processes, R.Y.M. Huang (Ed.), Elesvier, Amsterdam, the Netherlands, p. 1 (1991).
9. Bruschke, H.E.A., in Proc. 4th Int. Conf. Pervaporation Processes in Chem. Ind., R. Bakish (Ed.), Bakish Materials Corp., Englewood, NJ, p. 1 (1989).
10. Bruschke, H.E.A., in Proc. 5th Int. Conf. Pervaporation Processes in Chem. Ind., R. Bakish (Ed.), Bakish Materials Corp., Englewood, NJ, p. 1 (1991).
11. Baker, R.W. et al. Membrane Separation Systems: Recent Developments and Future Directions, Noyes Data Corp., Park Ridge, NJ (1991).
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|Author:||Huang, Robert Y.M.; Xianshe Feng|
|Publication:||Canadian Chemical News|
|Date:||Mar 1, 1993|
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