Semicontinuous, Pressure-Swing Distillation

The merits of semicontinuous, pressure-swing, azeotropic distillation are examined. In continuous operation, two columns at different pressures are fed streams having compositions on opposite sides of the azeotropic compositions. The distillates, which approach the azeotropic compositions at high and low pressure, are cycled between the columns. In contrast, semicontinuous operation involves only a single distillation column, which has lower investment costs and, when the mixture to be separated is changed, shorter downtimes. An optimal-control algorithm is employed to determine desirable campaigns, and to schedule pressure switch-over policies. Simulation results for the dehydration of tetrahydrofuran, involving a pressure-sensitive azeotrope, indicate that switch-over between steady states occurs quickly with on-spec product removed during 93% of the campaign. The column achieves production rates near 89% of the maximum throughput of a single column in the continuous process and shows superior performance when compared to reverse-batch operation.