Transport mechanism, membranes molecular sieving

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.

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. 

The transport properties across an MIP membrane are controlled by both a sieving effect due to the membrane pore structure and a selective absorption effect due to the imprinted cavities [199, 200]. Therefore, different selective transport mechanisms across MIP membranes could be distinguished according to the porous structure of the polymeric material. Meso- and microporous imprinted membranes facilitate template transport through the membrane, in that preferential absorption of the template promotes its diffusion, whereas macroporous membranes act rather as membrane absorbers, in which selective template binding causes a diffusion delay. As a consequence, the separation performance depends not only on the efficiency of molecular recognition but also on the membrane morphology, especially on the barrier pore size and the thickness of the membrane. 

Figure 4.17 Transport mechanisms for gaseous mixtures through porous membranes (a) viscous How (b) Knudsen diffusion (c) surface diffusion (d) multi-layer diffusion (e) capillary condensation and (0 molecular sieving [Saracco and Specchia, 1994]...

Molecular sieving and the interactions of gas molecules with the membrane are possible alternatives. As discussed in Chapter 4, if surface diffusion is operative on a gas but not the other, it can enhance the separation factor. Although surface diffusion contribution decreases with increasing temperature, it becomes more important as the pore diameter becomes smaller. Therefore, it is possible that as inorganic membranes with smaUer pore sizes become available their separation performance may increase not only due to molecular sieving effects but also surface diffusion or other transport mechanisms. 

In many studies the separation factor, which is indicative of the membrane s ability to separate two gases in a mixture, is predominantly governed by Knudsen diffusion. Knudsen diffusion is useful in gas separation mostly when two gases are significantly different in their molecular weights. In other cases, more effective uansport mechanisms are required. The pore size of the membrane needs to be smaller so that molecular sieving effects become operative. Some new membrane materials such as zeolites and other molecular sieve materials and membrane modifications by the sol-gel and chemical vapor deposition techniques are all in the horizon. Alternatively, it is desirable to tailor the gas-membrane interaction for promoting such transport mechanisms as surface diffusion or capillary condensation. 

That is the reason why lower pore size membranes working by molecular-sieving mechanism have been tested with a view to perform continuous operations. The use of a zeolite membrane (pore diameter estimated from the Dubinin-Astakhov analysis [20] 1.1 nm) provided an interesting rejection of 0.98 [10] (see Table 7.1). In that case, caffeine adsorption was weak and zeolite membrane could not be easily fouled with the solute. Transport was mainly controlled by molecular sieving, as indicated by the good rejection rate also obtained with other molecules (e.g., lauric acid) having molecular weight close to caffeine. 

To surpass Robeson s upper bound, materials are emerging that rely on transport mechanisms other than solution-diffusion through glassy or rubbery polymeric materials. In particular, a number of materials have been developed that possess fixed microporosity (2 nm or less) in contrast to the activated, transient molecular gaps that give rise to diffusion in most polymers. These materials include amorphous and crystalline (zeolite) ceramics [68-69], molecular sieve carbons [70], polymers that possess intrinsic microporosity [71-72], and carbon nanotube membranes [73-76]. Transport in such materials is determined primarily by the average size and size distribution of the microporosity - the porosity can be tuned to allow discrimination between species that differ by less than one Angstrom in size. However, surface... 

Theory on Transport Mechanisms for Carhon Molecular Sieve Membranes... 

In membrane-based gas separation, the movement of penetrant gases is driven by the pressure gradient imposed between upstream and downstream. A membrane will separate gases oidy if some components pass through the membrane more rapidly than others, as shown in Fig. 3.3. There are three general transport mechanisms for membrane-based gas separation Knudsen diffusion, solution-diffusion, and molecular sieving [156,163]. A schematic representation of the mechanisms of membrane-based gas separations is shown in Fig. 3.4. 

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