Semicontinuous, pressure-swing distillation

The merits of semicontinuous, pressure-swing, azeotropic distillation are e xamined. In continuous operation, two columns at different pressures are fe d streams having compositions on opposite sides of the azeotropic compositi ons. The distillates, which approach the azeotropic compositions at high an d low pressure, are cycled between the columns. In contrast, semicontinuous operation involves only a single distillation column, which has lower inve stment costs and, when the mixture to be separated is changed, shorter down times. An optimal-control algorithm is employed to determine desirable camp aigns, and to schedule pressure switch-over policies. Simulation results fo r the dehydration of tetrahydrofuran, involving a pressure-sensitive azeotr ope, indicate that switch-over between steady states occurs quickly with on -spec product removed during 93% of the campaign. The column achieves produ ction rates near 89% of the maximum throughput of a single column in the co ntinuous process and shows superior performance when compared to reverse-ba tch operation.