Surface-selective flow membranes

S. Sircar, M.B. Rao, M.A. Thaeron, Selective surface flow membrane for gas separation. Separation Science i Technology 34 (1999) 2081. 

Rao and Sircar [5-7] introduced nanoporous supported carbon membranes which were prepared by pyrolysis of PVDC layer coated on a macroporous graphite disk support. The diameter of the macropores of the dried polymer film was reduced to the order of nanometer as a result of a heat treatment at 1,000°C for 3 h. These membranes with mesopores could be used to separate hydrogen-hydrocarbon mixtures by the surface diffusion mechanism, in which gas molecules were selectively adsorbed on the pore wall. This transport mechanism is different from the molecular sieving mechanism. Therefore, these membranes were named as selective sitrface flow (SSF ) membranes. It consists of a thin (2-5 pm) layer of nanoporous carbon (effective pore diameter in the range of 5-6 A) supported on a mesoporous inert support such as graphite or alumina (effective pore diameter in the range of 0.3-1.0 pm). The procedures for making the selective surface flow membranes were described in [5, 7]. In particular, the requirements to produce a surface diffusion membrane were shown clearly in [7]. 

Paranjape, M., Clarke, P. F., Pruden, B. B., Parrillo, D. J., Thaeron, C., and Sircar, S. (1998). Separation of bulk carbon dioxide-hydrogen mixtures by selective surface flow membranes. Adsorption 4, 355-360. 

Rao, M.B. and S. Sircar, Nanoporous carbon membranes for separation of gas mixtures by selective surface flow, /. Membr. Sci., 85(3), 253-264, 1993a. 

In addition to the particulate adsorbents listed in Table 16-5, some adsorbents are available in structured form for specific applications. Monoliths, papers, and paint formulations have been developed for zeolites, with these driven by the development of wheels (Fig. 16-60), adsorptive refrigeration, etc. Carbon monoliths are also available as are activated carbon fibers, created from polymeric materials, and sold in the forms of fabrics, mats, felts, and papers for use in various applications including in pleated form in filters. Zeolitic and carbon membranes are also available, with the latter developed for separation by selective surface flow [Rao and Sircar, J. Membrane Sci., 85, 253 (1993)]. 

Selective Surface Flow (SSF) membrane technology, 13 795 Selectivity... 

Selective surface flow is, as Knudsen diffusion, associated with transport through microporous membranes, usually inorganic materials. The mechanism of surface diffusion is disputed and several different approaches have been proposed in the literature. 

Figure 4.11 illustrates a carbon membrane with pores in the range suitable for molecular sieving [78]. As expected, there is a clear and indisputable correlation between flux and molecular size. In Figure 4.12, the carbon membrane is more open (pore size in the range 6-10 A). The gas pair reported is CO2 and CH4, and as can be seen, the selectivity is clearly in favor of CO2 indicating selective surface flow. The critical temperatures, 7)., and Lennard-Jones diameters, for the two gases are... 

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). 

Two types of nanoporous activated carbon membranes have been developed for continuous gas separation applications [25]. They are (i) the MSC membrane produced by the Carbon Membranes Limited, Israel [26], and others [25, 27, 28], and (ii) the selective surface flow (SSF) membrane produced by the Air Products and Chemicals, Inc., USA [29]. Both membranes consist... 

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.

As can be seen from the table the pure gas selectivities of the nanoporous carbon membrane are quite low, e.g. 1.19 for butane/hydrogen. However, for the mixture given in Tab. 7.6 the butane/hydrogen selectivity increases to 94. The reason for this is that the butane is selectively absorbed over hydrogen at the carbon pore wall and because the pores are so small the pathway for hydrogen is blocked. This effect of selective surface flow and pore blocking was first observed by Barrer et al. [303]. Due to its unmatched selectivity the nanoporous... 

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