Optimal operation of rapid pressure swing adsorption with slop recycling

Rapid pressure swing adsorption (RPSA) is a cyclic process operating, basically, in three phases: a pressurization, a delay, and a depressurization phase. A new, modified operation is suggested by the addition of either a raffinate recycle phase or an extract recycle phase, during which raffinate respectively extract is returned to the feed tank. This feasibility study focuses on design based on the optimal process operation. The degree of difficulty of separation is used as an independent variable to indicate under what condition slop recycle is useful. Slop recycle proves to increase the recovery and in some case the productivity. The addition of a raffinate recycle phase is only profitable for difficult separations, when the composition profile is inclined towards a concave shape. The addition of an extract recycle is nearly always profitable as during the depressurization phase over some period the product fraction in the exhaust exceeds the fraction in the feed. For high degrees of difficulty the improvement in the recovery is over 100%, and for easy separations this is still about 40%. All simulations are carried out for a hypothetical binary mixture, a product purity of 90%, a cyclic stationary state, and optimized switching times.