(359f) Design and Control of Pressure-Swing Distillation with Partial Heat-Integration

Yingzhe Yu, Yamei Sun, Guiming Li, Xiuqin Dong, Minhua Zhang^*

Key Laboratory for Green Chemical Technology of Ministry of Education, R&D Center for Petrochemical Technology, Tianjin University, Tianjin 300072, P. R. China

*Corresponding author   Tel. & fax: +86-22-27406119,  E-mail address: mhzhang@tju.edu.cn

Ethanol and ethyl acetate are separated using partially heat-integrated pressure-swing distillation in this paper. The heat removed from high-pressure column condenser is transferred into the low-pressure column reboiler. An auxiliary reboiler using steam as a heating agent provides the remaining energy. The steady-state simulation and dynamic control is implemented based on Aspen Plus and Aspen Dynamics. Aspen Sensitivity analysis is performed to obtain the vital operating parameters and geometric parameters according to the steady-state simulation results and to choose the suitable temperature control point of distillation column. After the process is designed, three different kinds of control structures are presented and explored in this paper: constant reflux ratio control structure, constant R/F ratio control scheme and improved control structure with QR/F fixed.

For dynamic simulation, feed flow rate disturbance and feed composition disturbance are used to evaluate dynamic performance of each control scheme. Control structure with reflux ratio fixed is quite common industrially and the constant R/F control scheme is proposed by Grassi. However, the dynamic simulation results suggest that, the proposed constant R/F control structure with sensitive plate temperature fixed is unable to maintain the two bottom product at the respect quality. The bottom product concentration deviate their quality specification in the new steady state and more ethyl acetate escape from the low-pressure column. In contrast, the control structure with fixed reflux ratio is more effective. It can handle the feed composition disturbance well. The system spends less time to reach a new steady state, and the bottom product concentration is close to the expected quality. But, the constant reflux ratio control scheme cannot maintain the quality specification when feed flow rate disturbance is added. The problem can be greatly improved by using a feed forward control structure in which the distillation column reboiler heat duty and the feed flow rate are proportional. The QR/F ratio depends on the temperature controller. It can provide good dynamic control for both large feed flow rate and feed composition disturbance for this partial heat-integration pressure-swing distillation. This paper provides guidance to choose appropriate control scheme industrially.