Carbon molecular sieve selective surface flow

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). 

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.

CMS manhranes are composed of microporous carbon (as libers, on tubes or flat sheets) prepared from carbonization of polymeric precursors under controlled conditions. Precursors that are mostly used are cellulose or Pis. Depending on the manbrane pore size and the process conditions, separation may take place according to (1) molecular sieving < 5 A), (2) selective surface flow (5 A < < 12 A), (3) Knudsen diffusion... 

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]. 

Surface-selective flow membranes made of nanoporous carbon, which is a variation of molecular sieving membranes, were developed by Rao et al. (1992) and Rao and Sircar (1993). The membrane can be produced by coating poly(vinylidene chloride) on the inside of a macroporous alumina tube followed by carbonization to form a thin membrane layer. The mechanism of separation is by adsorption-surface-diffusion-desorption. Certain gas components in the feed are selectively adsorbed, permeated through the membrane by surface diffusion, and desorbed at the low-pressure side of the membrane. This type of membrane was used to separate H2 from a mixture of H2 and CO2 (Sircar and Rao, 2000), and their main advantage is that the product hydrogen is at the high-pressure side eliminating the need for recompression. The membrane, however, is not industrially viable because of its low overall separation selectivity. In addition, since the separation mechanism involves physical adsorption, operation at low temperatures is required. 

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