ADSORPTIVE DISTILLATION

A traditional countercurrent approach to the fluid movement in a cascade or fixed-bed stages undergoing cyclic liquid phase adsorption-vapor phase desorption is used to uncover a simple-to-use design technique for the prediction of number of stages to effect a prescribed separation. Analogous to the McCabe-Thiele method, the derivation shows the q-parameter depends on reflux ratio, feed ratio and solids loading ratio. For uniform solids loading, the q-line always takes a slope of infinity. Stage-to-stage calculations proceed sequentially using simple graphical methods by defining a pseudo-equilibrium curve. For the usual case of non-linear liquid phase equilibria, it is shown that the pseudo-equilibrium curve depends on selectivity and the respective hold-ups associated with adsorption on the one hand, and drained solids on the other. The theoretical results are entirely amendable to straightforward digital computation for design.

An example calculation is given for the very difficult to separate binary isomer system ortho in meta xylene. Based on published data, graphical computations for separation of a 50/50 mixture of the isomers into products of 95% purity show that only seventeen stages are necessary using a reflux ratio twice the minimum.