Selective surface flow

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. 

The energy barrier for surface migration, E, is then defined as 

The 2D-gas is characterized by a surface mean free path, A, inversely proportional to the surface concentration, Cg, and this value can be much larger than the spacing between adjacent surface sites. 

If the q/RT part of Equation 4.12 is increased, then Aj will no longer be controlled by collisions between adsorbed molecules. As q/RT increases, A decreases and is approaching the spacing between adjacent sites, and a hopping mechanism is 

If the Cg is low then a random walk diffusion of independent molecules can be expected, and would be given as 

Here Df is the diffusion coefficient for component i and dc,/dx,- is the driving force. The activated diffusion can be described by an Arrhenius type of equation  

the difference in transport activation energy and adsorption energy may be positive or negative. When AEs 0, transport due to selective surface flow will increase with decreasing temperature with AEs 0 it will decrease. Plainly stated, adsorption and hence selectivity increase with decreasing temperature. This is the opposite of temperature influence for molecular sieving separation (see below). 

Here Eg ms is the activation energy for diffusion in the molecular sieving process for CMS membranes. Nguyen et al. reported that the CMS membrane presents reasonable sieving effect for gas molecules with different kinetic diameters, which suggests that the CMS membrane is predominantly micro-porous with no major contribution from Knudsen diffusion or viscous flow in its overall mass transfer.  

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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 diffusion is governed by a selective adsorption of the larger (nonideal) components on the pore surface. The critical temperature, 7), of a gas will thus indicate which component in a mixmre is more easily condensable. The gas with the highest T will most likely be the fastest permeating component where a selective surface flow can take place. Eor a mixed gas an additional increase in selectivity may be achieved if the adsorbed layer now covering the internal pore walls restricts the free pore entrance so that the smaller nonadsorbed molecules cannot pass through. 

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.

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... 

The easily condensable CO2 molecule will follow a selective surface flow (SSF) mechanism and seriously hinder CH4 to permeate hence, high selectivities are obtained. 

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