Transport in Carbon Membranes

Gas transport can occur in porous membranes by four different idealized mechanisms Knudsen diffusion, partial condensation/diffiision, selective adsorption/diffusion, and molecular sieving (Rao and Sirkar, 1993a,b). Knudsen diffusion occurs when the mean free path of the molecule is greater than the size of the pore therefore, the di sing gas molecule collides more often with the pore wall than with other molecules. Knudsen diffusion can be described using Eq. (23.1) (Hines and Maddox, 1985)  

For gas pairs of similar molecular weight, the selectivity based on Knudsen diffusion is very low. 

Selective condensation occurs when one or more of the components in the feed stream condenses in the pores of the membrane (Rao and Sircar, 1993b). Once the pores are filled, the condensed material diffuses across the membrane. The pore size required for selective condensation is generally larger than 30 A. Very high selectivity can be achieved if one of the components is noncondensible (Sakata and Yamamoto, 1986), but the separation is limited by the condensation partial pressure of the components, which is a function of pore size and shape. Therefore, this mechanism is difficult to control and is rarely used in carbon membranes. 

Singh (1997) and Singh and Koros (1996) have discussed the importance of entropic selectivity in molecular sieving materials such as CMS membranes. These authors analyzed the activation energy of diffusion for oxygen and nitrogen in an upper-bound 

For molecules that have different dimensions, pores can inhibit rotational freedom for one molecule, while allowing it for other molecules. For example, nitrogen can be approximated as spherocylindrical with a length of 4.07 A and diameter of 3.09 A (Singh, 1997). 

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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. 

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