Single-Column PSA Process

Displacement desorption requires a somewhat more complex process scheme than either the pressure swing or thermal swing system and it is therefore generally used only in situations where the simpler methods fail. As an example of this type of process we consider the widely used adsorption separation of medium molecular weight (C,o-C,g) linear paraffins from mixtures with branched chain and cyclic isomers. 

There is considerable economic incentive for such a separation since the medium-chain paraffins are required for the manufacture of synthetic detergents while their removal reduces the freezing point of the raffinate stream, enhancing its value as jet fuel for high-flying aircraft. The difference in relative volatility between linear, branched, and cyclic isomers is small so that such a separation cannot be achieved simply by distillation. 

Several different adsorption processes for the separation of linear paraffins have been developed including Ensorb (Exxon), IsoSiv (Union Carbide), T. S. F. (Texaco), the Shell process, and the Leuna Werke process. The latter has been called Parex (paraffin extraction) but the choice of name is unfortunate because of possible confusion with the UOP Parex process for separation of xylene isomers. All these processes use a 5A molecular sieve, generally in binderless form to minimize nonselective adsorption. The C,o-C,g linear paraffins are strongly adsorbed even at temperatures as high as 350°C. Thermal swing desorption is not feasible since the temperature required for desorption is so high that coking would occur. The alternatives are therefore vacuum desorption, which is used in some versions of the IsoSiv process, or displacement desorption which is used in most if not all of the other processes. 

Ammonia is a good choice as a desorbent since, because of its high dipole moment, it is much more strongly adsorbed than would be expected simply on 

FIGURE llJl. Photograph of sieve bed and associated ductwork for the Ensorb unit at Baytown refinery. (Courtesy of Aromatics Technology Division, Exxon Chemical Company.) 

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Figure k. Light gas enrichment and recovery in product as a function of product cut in single-column PSA process. 

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