Membrane Based Gas Separation past, presence and future

寄稿 (Contribution): 膜 (MEMBRANE),31(3),165 169(2006) Special Issue Trends in Gas Separation Membranes II Membrane Based Gas Separation past, presence and future Klaus- Viktor Peinemann GKSS-Research Center Geesthacht, Institute of Polymer Research, GKSS Forschungszentrum Geesthacht, Institut für Polymerforschung, Max-Planck-Str. 1, D-21502 Geesthacht, Germany Klaus-Viktor Peinemann is currently Senior Scientist at the Institute of Polymer Research at GKSS Research Center in Germany and he has a position as honorary professor at the University of Hannover. From 2002 to 2004 he served as President of the European Membrane Society and he is co-founder of the company GMT Membrantechnik in Germany. His main research interest is the development of multicomponent membrane materials, e.g. mixed matrix membranes. He has published some 80 papers in international journals, holds 15 membrane related patents and edited the book Membrane Technology in the Chemical Industry (jointly with S.P. Nunes), 2006. Besides his scientific activities Peinemann is ardent scholar and teacher of Tai Chi Chuan. The separation of gas mixtures with membranes has emerged from being a laboratory curiosity to becoming a rapidly growing, commercially viable alternative to traditional methods of gas separation within the last two to three decades. Membrane gas separation has become one of the most significant new unit operations to emerge in the chemical industry in the last 25 years 1). The gas separation membrane module business for 2004 is estimated at $ 170 million with an annual growth rate of 8%. Table 1 shows commercial applications and some of the major suppliers of membrane gas separation units. Organic polymers are currently the dominating material for gas separation membranes. During the last two decades dozens of new polymers have been described in the literature, which have been developed for gas separation. The largest group among these are probably polyimides 2). In spite of these efforts less than 10 polymers are used for industrial gas separations. Nearly all of these are technical polymers developed for totally different applications. Designer polymers were either too expensive or their advantage over commercial polymers were not sensational enough or they did not show the expected performance in real applications. The latter is especially true for modified (fluorinated) polyimides. Some of the 6FDA based polyimides showed tremendous separation abilities in the laboratory but failed in real life due to plastizisation or physical aging. Cellulose acetate, polysulfone and polyimides are by far the most important polymers for gas separation membranes. When we look at the volume streams treated, the old workhorse material cellulose acetate is probably still the dominat- ing polymer. The company Kvaerner Process Systems alone, which has acquired Grace Membrane Systems, has sold or operates membrane plants with CA-membranes for carbon dioxide separation for a total stream of more than 5 Mio m 3 (STP)/day. The world largest gas separation membrane plant runs in Quadirpur, Pakistan (built by UOP (Separex). It has a capacity of about 14 Mio m 3 (STP)/day of gas and removes carbon dioxide from natural gas. Again, the membrane material is cellulose acetate. Because of the failure of most newly developed polymeric membrane materials for industrial applications the research in the field of polymer development for gas separation membranes has lost some of it s dynamic and attractiveness. I do not expect revolutionary breakthroughs in this field in the coming years. I see gradually improvements and especially the market for oxygen enrichment for enhanced combustion will grow significantly due to these improvements. However, I predict breakthroughs in the field of heterogeneous membranes and nano-structured membrane materials and maybe in the field of ultra high-free volume polymers with intrinsic microporosity, which will be discussed below. When asked for the most intriguing invention in the area of polymeric gas separation membrane development my favourite is still the 25 year old Monsanto patent of Henis and Tripodi from 1980 Multicomponent membranes for gas separation (US Patent 4, 230, 463), which led to the first commercial hydrogen recovery plants for ammonia synthesis. Henis and Tripodi showed that integral asymmetric membranes with low selectivity due to imperfections can be transformed into highly selective membranes by a simple coating with a highly

166 Peinemann: Membrane Based Gas Separation past, presence and future Table 1 Commercial application and major suppliers of membrane gas separation units permeable polymer or liquid. A special class of polymers are polymers with very high free volumes, like some functionalised polyacetylenes, which are bridging the gap between microporous and dense polymeric materials. The best known examples for these polymers are poly (1-trimethylsilyl-1-propyne (PTMSP) and poly (4-methyl-2-pentyne (PMP). Although the PMP was already synthesized in 1982 3), its high gas permeabilities were first published 1996 by Pinnau et al. 4) at MTR, Menlo Park. MTR is currently evaluating the performance of PMP membranes for hydrocarbon separation. An attractive application of membranes in this field is natural gas hydrocarbon dewpointing. This means the separation of higher hydrocarbons like butane present in natural gas from methane. The performance of PTMSP and PMP for hydrocarbon separation is superior to all other known polymers. Main reason, which have prevented large scale application of these materials as membranes, is their property to strongly absorb low vapour pressure components leading to a drastically reduced permeability. It has been reported recently, that flux and even selectivity of PMP and PTMSP can be enhanced by the addition of nano-particles 5, 6). Merkel et al. added fumed silica to PMP and observed a simultaneous increase of butane flux and butane/methane selectivity. This unusual behaviour was explained by fumed-silica induced disruption of polymer chain packing and an accompanying increase in the size of free volume elements through which molecular transport occurs. Gomes et al. incorporated nano-sized silica particles by sol-gel technique into PTMSP and found also for this polymer a simultaneous increase in flux and selectivity. It has to be studied, if physical aging of the polyacetylenes is reduced by the addition of nano-particles. A new class of polymers with high free volumes has been introduced recently by Budd et al. 7). The molecular structure of these polymers contains sites of contortion (e.g. spiro-centers) within a rigid backbone (e.g. ladder polymer). The inventors call this polymer class polymers of intrinsic microporosity (PIMs), because their porosity arises as consequence of the molecular structure and is not generated solely through processing. The gas permeation properties of membranes formed from PIM-1 were investigated at the GKSS Research Center 8). With an oxygen permeability of 370 Barrer and an O 2/N 2-selectivity of 4.0 the PIM-1 shows an extraordinary behaviour as gas separation polymer. However, long-term measurements revealed a physical aging of PIM-1 analogous to PTMSP, which resulted in reduced permeabilities. When the aging problems can be solved, the PIMs will be a highly interesting polymer class for fabrication of gas separation membranes. The research in high-free volume polymers with intrinsic microporosity will remain very attractive. Large scale membrane applications for gas and vapour separation will emerge. Transport properties through some high free volume polymers are similar to transport characteristics through microporous inorganic materials. One attractive inorganic material for gas separation is carbon. Since the pioneering paper of Koresh and Soffer on carbon molecular sieve membranes in 1983 9) much research has

膜 (MEMBRANE),Vol. 31 No. 3(2006)167 been carried out in the field of carbon-based gas separation membranes. Selectivities and permeabilities far above the performance of the best polymers have been obtained for carbon molecular sieve membranes by many researchers. One example is a recent publication of Koros et al. 10), in which striking selectivities for gas pairs like oxygen/nitrogen and carbon dioxide/methane are described. In spite of these findings carbon molecular sieve membranes have not found their way into industrial separation processes. One reason for this might be the inherent brittleness of carbon materials, high price and aging of the carbon surface by chemical surface reactions are other difficulties. Very close to commercialisation were the nanoporous carbon membranes developed mainly by Air Products in the early 90ies. They were one of the highlights in membrane development for gas separation from 1990 to 2000. They can be produced by different methods. The most advanced membranes of this kind have been produced by Air Products first published 1993 11). Air Products called this membrane Selective Surface Flow (SSF TM ) membrane. Due to its unmatched selectivity the nanoporous carbon membrane looks very attractive for hydrogen enrichment of refinery off-gases with low hydrogen content, e.g. FCC (fluidized catalytic cracker) off-gases. It is much more attractive than hydrogen selective membranes, because the hydrogen remains on the high-pressure side of the membrane and can be fed into a pressure swing unit for further purification. The drawback of the nanoporous carbon membrane is that water vapour and higher hydrocarbons should be removed before the membranes separation because they adsorb very strongly in the membrane pores. Air Products SSF-membrane was field tested at different refinery sites 12). Surprisingly, Air Products discontinued the work in this area in 2003. One reason could have been the aging/deactivation of the membranes in the presence of water vapour. In spite of this drawback microporous carbon membranes will find their way into future industrial applications. Another type of inorganic membranes for gas separation, which has to be mentioned here are perovskite membranes for air separation. It is known for a long time that certain dense ceramic materials are good conductors of oxygen at elevated temperatures. Oxygen transport through an ionic conductor is a result of oxygen ionic conduction mechanisms that involve oxygen defects such as lattice vacancies. One of the best known ceramic oxygen conductors is yttria-doped zirconia, which is widely used in high temperature oxygen sensors. Oxygen is transported through these materials as O 2- ion. Hence, when oxygen permeates through these materials there must be a flow of electrons in the opposite direction. Oxygen conducting ceramics like doped zirconia are good oxygen conductors but poor electronic conductors. The electronic conductivity of yttria stabilized zirconia is three to four orders of magnitude lower than its ionic conductivity. The oxygen can be pumped through the material by an external electrical field. However, a simple calculation reveals that this is not economic for oxygen separation due to the high electricity consumption. The situation changes when the ceramic material is a good conductor for both -oxygen ions and electrons. These materials are referred to as ionic-electronic mixed conductors. With these a high oxygen flux can be obtained without an external electrical field. One of the first papers which stimulated large interest in this research was published in 1985 by Teraoka et al 13). Teraoka reported high oxygen fluxes of up to 2.4 cm 3 /min cm 2. With today s optimised perovskite compositions even much higher fluxes in the range 6 to 7 cm 3 /cm 2 min can be obtained 14). The promising prospect of these membranes is not in the first place the production of oxygen, but their application in membrane reactors for the partial oxidation of natural gas. The mixed conducting membrane eliminates the cryogenic air separation plant and it forms a safety barrier between the natural gas and air. The membrane becomes more productive in a configuration like this, because the slow oxygen desorption at the permeate side is enhanced by the chemical reaction. The perovskite type ceramic membranes have attracted much attention from major chemical and petrochemical companies in the USA. Companies currently involved in the development of the mixed conducting ceramic membranes include Air Products, Praxair, BP and Amoco. The largest currently existing consortium developing this technology is headed by Air Products and sponsored by the US Department of Energy. The team includes Ceramatec, Eltron Research, McDermott Technologies, Pennsylvania State University, Siemens Westinghouse and Texaco Gasification. A pilot scale prototype unit with an oxygen production rate of 5 tons per day is scheduled for the first half of 2006. It looks like that the mixed conducting membrane technology will represent a major breakthrough in industrial application of inorganic membranes. The combination of the superior gas selectivities of molecular sieves or other selective absorbents with the processibility of polymeric membranes have attracted many researchers. The hybrid membranes consisting of inorganic molecular sieves and polymers are often referred to as mixed matrix membranes. The term mixed matrix membrane has been introduced by Kulprathipanja 15), who performed pioneering work in the field of polymer/zeolite hybrid membranes. Kulprathipanja showed that the CO 2/H 2 selectivity of cellulose acetate could be reversed by addition of silicalite. The silicalite-ca membrane had a CO 2/H 2 selectivity of 5.1,

168 Peinemann: Membrane Based Gas Separation past, presence and future whereas the pure CA membrane exhibited a selectivity of 0.77. Hennepe, from the university of Twente, proved for the first time that the incorporation of silicalite in PDMS increased the ethanol/water selectivity significantly under steady state conditions 16) in pervaporation experiments. Later it was shown by Jia et al. 17) that using the same approach (silicalite in PDMS) the gas selectivity could be also changed due to a molecular sieving effect. However, the effects were too small to be of any interest for practical applications. One problem of these membranes was that the permeability ratio P PDMS/P zeolite was too high. Another practical challenge is to improve the compatibility between inorganic molecular sieves and glassy polymers in order to eliminate gas diffusion pathways at the interface between polymer and zeolite. Additional to voids one might also find regions of reduced polymer permeability close to the filler surface. A number of non-ideal effects in mixed matrix membranes are discussed in a paper of Moore et al. 18). There are still manufacturing problems to be solved, before mixed matrix membranes will be introduced in commercial gas separation. We see an increasing number of patents filed by big companies active in gas separation 19 ~ 22), and it can be concluded, that mixed matrix membranes are on the brink of practical application. The development of new mixed matrix materials for gas separation will remain an attractive research field. Besides classical zeolites and carbon molecular sieves new selective adsorbants have to be considered. One candidate e.g. might be metal-organic frameworks (MOF s). Recent findings suggest that MOF s can revolutionise gas separation and storage 23). MOF s are nanoporous metalorganic soft materials analogous of zeolites, but with all the chemical diversity of polymeric compounds. MOF s exhibit a very high porosity with exactly tailorable pore sizes. One example is a manganese formate MOF, which has be reported recently 24). This material, which features permanent porosity with pore aperture diameters of about 0.45 nm, sorbs carbon dioxide but essentially completely excludes methane and nitrogen. Hence, Mixed matrix membranes containing this MOF might be very attractive for natural gas and biogas purification. This is just one example for potential future developments of mixed matrix membranes. A special case of mixed matrix membranes are facilitated transport membranes, which have been developed for oxygen, carbon dioxide and olefin separation. We worked for quite a while on the development of fixed carrier membranes for olefin separation and gave our first presentation 16 years ago 25). The idea was to mix silver salts with polymers using a high salt/polymer ratio. High mixed gas selectivities and fluxes could be obtained. However, it turned out, that in spite of significant efforts these membranes could not be modified to be stable more than about one month. Many other research groups followed a similar approach with no better results. My prognosis for silver salt containing membranes for olefin separation: they will never make it into technical applications. The situation does not look much better for fixed carrier membranes for oxygen separation containing organo-transition metal compounds. Instability will deter commercial development. The third potentially large application for carrier facilitated transport is carbon dioxide separation. An interesting approach are fixed carrier membranes using aminated polymers. Recently high ideal selectivities have been published by Hägg and coworkers using polyvinylamine membranes 26). Long-term stability and real gas conditions will be a big challenge for these membranes. Basically, I agree with Richard Baker, who sees carrier facilitated membrane still in the laboratory in 2020 27). References 1) Prasad R, Shaner RL, Doshi KJ : Comparison of membranes with other gas separation technologies. in : Polymeric gas separation membranes, Paul DR, Yampolskii YP (Eds.), CRC Press, Boca Raton (1994) 2) Langsam M : Polyimides for gas separation. in: Polyimides fundamentals and applications, Gosh MK, Mittal KL (Eds.), Marcel Dekker, New York (1996) 3) Masuda T, Kawasaki M, Okano Y, Higashimura T : Polym. J. (Tokyo),, 371 (1982) 4) Morisato A, Pinnau I : Synthesis and gas permeation properties of poly(4-methyl-2-pentyne), Journal of Membrane Science,, 243 (1996) 5) Merkel TC, Freeman BD, Spontak RJ, He Z, Pinnau I, Meakin P, Hill AJ : Ultrapermeable, reverse-selective nanocomposite membranes, Science,, 519-522 (2002) 6) Gomes D, Nunes SP, Peinemann KV : Membranes for gas separation based on poly(1-trimethylsilyl-1-propyne)-silica nanocomposites, Journal of Membrane Science, (1), 13-25 (2005) 7) Budd PM, Ghanem BS, Makhseed S, McKeown NB, Msayib KJ, Tattershall CE : Polymers of intrinsic microporosity (PIMs) : robust, solution-processable, organic nanoporous materials, Chemical Communications, (2), 230-231 (2004) 8) Budd PM, Msayib KJ, Tattershall CE, Ghanem BS, Reynolds KJ, McKeown NB, Fritsch D : Gas separation membranes from polymers of intrinsic microporosity, Journal of Membrane Science, (1-2), 263-269 (2005) 9) Koresh JE, Sofer A : Molecular-Sieve Carbon Permselective Membrane.1. 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