Distillation column controllability analysis through heat pump integration

Abstract

Understanding the dynamic behavior of distillation column assisted heat pump has received considerable attention, since this integrated process is used to improve thermodynamic and energy efficiency. This work studied controllability of two processes of distillation column assisted bottom flashing heat pump and vapor recompression heat pump. Integration caused extra degrees of freedom, in which some manipulated-controlled variable pairings were promising. Controllability tools such as set-point tracking and disturbance rejection were applied to compare the input-output pairings and select pairings with the best controllability and disturbance rejection indices under decentralized control in a steady state condition and frequency domain. The results indicated that controllability of recompression of the top vapor stream was better than flashing of the bottom liquid stream of the column. In addition, results showed that reboiler heat duty and reflux rate in a distillation column could be replaced with other variables. Besides, when a heat value was used in a heat exchanger for keeping the temperature of the outlet stream at a specific value, heat inputs could be replaced with the corresponding temperatures for the manipulated variables. Therefore, using this method demonstrated improved controllability indices.