Heuristics for synthesis and design of pressure-swing adsorption processes

Simulation based synthesis and design of adsorptive enrichment of CO from tail gas having 51 % CO are presented. The adsorption breakthrough curve simulation using this feed gas composition, helped to provide a starting guess of the adsorption step duration in a pressure-swing adsorption cycle for meeting the purity and recovery targets. Use of smaller bed dimensions facilitated the simulation of many cycles. These simulations helped to decide the operating pressure range, operating temperature, constituent steps of the cycle, their sequence, direction of pressurization of the bed, number of beds in the cycle and the composition of the streams to be used for pressurization and/or rinse and/or purge steps. Only an optimally designed pressure-vacuum-swing adsorption cycle achieves the stiff separation targets of getting an extract having 80 % pure CO at 80 % recovery in a single-stage with an adsorbent that uses physical adsorption and offers CO/CH4 sorption selectivity of just 2.44. Additional simulations are done wherein the bed sizes and velocities are increased to predict the performance of a large-scale unit. These require deciding only the durations of the steps that are finalized from the small-scale unit simulations. These durations were kept fixed and the bed dimensions were varied till the separation targets are obtained for the particular feed rate. The scale-up criteria was matching residence times in the bed. A strategy for treating a feed gas having only 30 % CO is also discussed and a novel concept of cascaded PSA is evaluated using simulations. Some heuristics are evolved from the studies.