Process Optimization And Control Strategy Of Pressure-swing Distillation For Separating An Azeotropic Mixture Of N-heptane And Isobutanol

The mixture of n-heptane and isobutanol presents minimum-boiling behaviors under low pressures and presents maximum-boiling behaviors under high pressures. According to the azeotropic behaviors of the mixture in different pressures, the mixture with flow rate of 100kmol/h and with a composition of 20mol% n-heptane and 80mol% isobutanol was separated to study a conventional pressure-swing distillation with distillate flow recycle and a unconventional pressure-swing distillation with bottom flow recycle in this work.An economic model of total annual cost(TAC) was adopted to optimize the top stream recycling pressure-swing distillation(TPSD) and the bottom stream recycling pressure-swing distillation(BPSD) following the sequential iterative optimization procedure, which gives the minimal TAC values of 653,127.9$/y and 897,690.9$/y, respectively. On the basis of two optimal pressure-swing distillation(PSD) processes without heat-integration, two different PSD processes with partial and full heat-integration were investigated as well. The optimized TAC values of the TPSD with partial and full heat-integration were 575,622.8$/y and 553,701.0$/y, respectively. The optimized TAC values of the BPSD with partial and full heat-integration were 814,621.8$/y and 837,844.0$/y, respectively.Based on the optimal steady states of two PSD processes, the dynamic controls for the TPSD without and with full heat-integration and for the BPSD without and with partial heat-integration were investigated. The dynamic performances of two control structure were examined by introducing the flow rate and composition disturbances in feed stream. It was found that the QR1/F ratio control structure can improve the ability for handling the disturbances in a large extent in the TPSD without heat-integration. A pressure-compensated temperature control structure was good at handling the disturbances of feed flow rate and feed composition in the TPSD with full heat-integration. For the dynamic control of the BPSD process, the process with bottom recycle stream increases the control difficulty in base level and a cascade control structure with a level controller and a flow controller can tackle this problem. The studies showed that a pressure-compensated temperature controller applied in the sensitive stage in low pressure column can maintain the products' purity well in the dynamic control of the BPSD without heat-integration. A pressure-compensated temperature control structure and a composition controller performed well in dealing with feed disturbances for the control of the BPSD with partial heat-integration. For the dynamic control of two PSD processes, the heat-integrated process can result in more complicated control structure compared with the non-heat-integrated processes.