Dynamic controllability comparison of different side-stream extractive distillation processes with intermediate boiling entrainer

Abstract

It is of great significance to simultaneously consider the economy and controllability for promoting the sustainability of the process industry. In the paper, the controllability of different energy-efficient side-stream extractive distillation (SSED) processes using intermediate boiling entrainer including two double column side-stream extractive distillation (DC-SSED) processes and one single column side-stream extractive distillation (SC-SSED) process are explored for separating the methanol/toluene azeotrope with triethylamine as entrainer. The side stream withdrawal locations of the DC-SSED processes are located in the rectifying section (DC-SSED (a)) and extraction section (DC-SSED (b)), respectively. The recycled intermediate boiling entrainer concentration is 45 mol% for the SC-SSED process. The two robust control strategies constructed for the DC-SSED (b) process are also suitable for the DC-SSED (a) process in resisting the same feed disturbances, which do not adopt the construction concept of the feasible control strategy for the SSED separation process with heavy entrainer. The difference between the two novel control strategies exists in how to control the methanol product purities via reflux ratios. Simultaneously, the two robust control strategies are constructed to resist the same feed disturbances for the SC-SSED process and compared to the two DC-SSED processes in terms of dynamic performances. The dynamic performances demonstrates that the DC-SSED (a) process with worst energy and economic performances is superior to the SC-SSED process and the DC-SSED (b) process from the perspective of the total integral squared error. However, the number of control loops used in the control strategies constructed for the SC-SSED process with optimal energy and economic performances is much smaller than that used in the control strategies constructed for the DC-SSED process, especially the number of composition control loops. Overall, the dynamic performances and economic performances of different processes may not be consistent, and the paper provides some advice for the construction of the robust control strategies for the SSED separation processes with intermediate boiling entrainer.