Carbon molecular sieve membranes separation performance

Kiyono M, Williams PJ, Koros WJ. Effect of pyrolysis atmosphere on separation performance of carbon molecular sieve membranes. 7 Afcmbr Sci 2010 359 2-10. 

Since the pioneering paper of Koresh and Soffer on carbon molecular sieve membranes in 1983 [300] much research has 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 publica-... 

A suitable polymer material for preparation of carbon membranes should not cause pore holes or any defects after the carbonization. Up to now, various precursor materials such as polyimide, polyacrylonitrile (PAN), poly(phthalazinone ether sulfone ketone) and poly(phenylene oxide) have been used for the fabrication of carbon molecular sieve membranes. Likewise, aromatic polyimide and its derivatives have been extensively used as precursor for carbon membranes due to their rigid structure and high carbon yields. The membrane morphology of polyimide could be well maintained during the high temperature carbonization process. A commercially available and cheap polymeric material is cellulose acetate (CA, MW 100 000, DS = 2.45) this was also used as the precursor material for preparation of carbon membranes by He et al They reported that cellulose acetate can be easily dissolved in many solvents to form the dope solution for spinning the hollow fibers, and the hollow fiber carbon membranes prepared showed good separation performances. 

Wang, L.-J., and Hong, F.C.-N. (2005b). Surface structure modification on the gas separation performance of carbon molecular sieve membranes. Vacuum 78, 1-12. 

In addition to the polymer and facilitated transport membranes, novel materials are being proposed and investigated to achieve membranes with economically attractive properties. Carbon molecular sieve (CMS) membranes prepared by pyrolysis of polyimides displayed much better performance for olefin/paraffin separation than the precursor membranes [39, 46, 47]. Results obtained with CMS membranes indicated properties well beyond the upper-bond trade-off curve, as shown in Figure 7.8. Nonetheless, this class of materials is very expensive to fabricate at the present time. An easy, reliable, and more economical way to form asymmetric CMS hollow fibers needs to be addressed from a practical viewpoint. 

Successful separation of alkanes and alkenes has been documented when microporous membranes have been used [79,138]. The physiochemical properties, size, and shape of the molecules will play an important role for the separation, hence critical temperatures and gas molecule configurations should be carefully evaluated for the gases in mixture. On the basis of gas properties and process conditions, the separation may be performed according to selective surface flow or molecular sieving (refer to Section 4.2 on transport). The transport may also be enhanced by having a Ag compound in the membrane. The Ag ion will form a reversible complex with the alkene, and facilitated transport results. Selectivities in the range of 200-300 have been reported for separation of ethene-ethane and propene-propane [138]. Successful separation of alkanes and alkenes will be important for the petrochemical industry. Today the surplus hydrocarbons in the purge gas are usually flared. Membranes which should be suitable for this application are the carbon molecular sieves (see Section 4.3.2) and nanostructured materials (Section 4.3.3). 

Ethylene has been separated from ethane by a silver nitrate solution passing countercurrent in a hollow fiber poly-sulfone.165 This separation has also been performed with the silver nitrate solution between two sheets of a polysilox-ane.166 A hydrated silver ion-exchanged Nafion film separated 1,5-hexadiene from 1-hexene with separation factors of 50-80.167 Polyethylene, graft-polymerized with acrylic acid, then converted to its silver salt, favored isobutylene over isobutane by a factor of 10. Olefins, such as ethylene, can be separated from paraffins by electroinduced facilitated transport using a Nafion membrane containing copper ions and platinum.168 A carbon molecular sieve made by pyrolysis of a polyimide, followed by enlargement of the pores with water at 400 C selected propylene over propane with an a-valve greater than 100 at 35°C.169... 

R. Nasir, H. Mukhtar, Z. Man, M.S. Shaharun, M.Z. Abu Bakar, Effect of fixed carbon molecular sieve (CMS) loading and various di-ethanolamine (DEA) concentrations on the performance of a mixed matrix membrane for CO2/CH4 separation, RSC Advances 5 (2015)60814-60822. 

Gas separation membranes combining the desirable gas transport properties of molecular sieving media and the attractive mechanical and low cost properties of polymers are considered. A fundamental analysis of predicted mixed matrix membrane performance based on intrinsic molecular sieve and polymer matrix gas transport properties is discussed. This assists in proper materials selection for the given gas separation. In addition, to explore the practical applications of this concept, this paper describes the experimental incorporation of 4A zeolites and carbon molecular sieves in a Matrimid matrix with subsequent characterization of the gas transport properties. There is a discrepancy between the predicted and the observed permeabilities of O2/N2 in the mixed matrix membranes. This discrepancy is analyzed. Some conclusions are drawn and directions for further investigations are given. 

Despite rapid advances in polymeric gas separation membrane performance in the 1980 s, recent efforts have yielded only small improvements. Six years ago, the upper bound tradeoff limit between O2 permeability and O2/N2 selectivity was constructed (i), and it still defines the effective performance bounds for conventional soluble polymers. Consequently, an alternate approach to gas separation membrane construction is suggested to exceed current technology performance. Molecular sieves, such as zeolites and carbon molecular sieves (CMS), offer attractive transport properties but are difficult and expensive to process. A hybrid process exploiting the processability of polymers and the superior gas transport properties of molecular sieves may potentially provide enhanced gas separation properties. 

Several reviews of carbon molecular sieve (CMS) membranes have been presented (Ismail and David, 2001 Saufi and Ismail, 2004). This chapter is a broad overview and considers the modes of transport in carbon membranes, the formation processes used for fabrication, and the separation performance of several membranes, in particular membranes produced in the last 10 years. In addition, emerging efforts to produce industrial-scale carbon membranes will also be reviewed. 

This cost differential can be tolerated only in applications in which polymeric membranes completely fail in the separation [78]. Demanding separation applications, where zeolite membranes could be justified, due to the high temperatures involved or the added value of the components, and have been tested at laboratory scale, are the following separation of isomers (i.e., butane isomers, xylene isomers), organic vapor separations, carbon dioxide from methane, LNG (liquefied natural gas) removal, olefines/paraffins and H2 from mixtures. In most cases, the separation is based on selective diffusion, selective adsorption, pore-blocking effects, molecular sieving, or combinations thereof. The performance or efficiency of a membrane in a mixture is determined by two parameters the separation selectivity and the permeation flux through the membrane. 

The nanoporous carbon membrane consists of a thin layer (<10pm) of a nanoporous (3-7 A) carbon film supported on a meso-macroporous solid such as alumina or a carbonized polymeric structure. They are produced by judicious pyrolysis of polymeric films. Two types of membranes can be produced. A molecular sieve carbon (MSC) membrane contains pores (3-5 A diameters), which permits the smaller molecules of a gas mixture to enter the pores at the high-pressure side. These molecules adsorb on the pore walls and then they diffuse to the low-pressure side of the membrane where they desorb to the gas phase. Thus, separation is primarily based on differences in the size of the feed gas molecules. Table 7 gives a few examples of separation performance of MSC membranes. ° Component 1 is the smaller component of the feed gas mixture. 

Figure 22.7 Descriptions of nanoporous carbon membranes (a) mechanism of transport through the molecular sieve carbon (MSC) membrane, (b) mechanism of transport through the selective surface flow (SSF) membrane, (c) separation performance of H2S—H2 mixtures by the SSF membrane, (d) schematic drawing of a two-stage SSF membrane operation for Fl2S—FI2/CH4 separation.

There is a growing interest in the development of gas separation membranes based on materials that provide better selectivity, thermal and chemical stability than those which already exist (i.e. polymeric membranes). Carbon membranes offer the best candidates for the development of new membrane technologies, because of their stability and molecular sieving capabilities. The most notable advantages of carbon membranes have been recently reviewed [8] in comparison to those of polymeric membranes, in order to highlight the factors that make carbon membranes very attractive and useful as separation tools. Table 11.2 inserted in the errd of the chapter summarizes the performance of carbon membranes for the separation of mixtures of permanent gases as reported in the literature. 

Sofer et al. ° is one of the earliest works in this topic. These membranes are important due to the improved trade-off upper limit between permeabiUty and selectivity compared with their polymer precursor membranes. Singh-Ghosal and Koros reported the Robeson s plot (Oj/Nj selectivity versus O2 permeability) for some carbon membranes and corresponding polymer membranes (Fig. 10.2). It is obvious that the performance of carbon membranes is much better than that of corresponding polymer membranes. Moreover, the permeance of the carbon membranes depends on the surface characteristics and the interactions between pores and gas molecules rather than on the bulk properties as for the polymer membranes. When carbon membranes separate molecules based on the molecular-sieving mechanism, the molecules have to overcome an energetic barrier created by the differences between pore dimension and gas molecules. 

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