Control Study to Enhance the Controllability of Heterogeneous Extractive Distillation: Cyclohexane/Cyclohexene Separation

Control strategy is crucial for the smooth operation of distillation systems, especially when the design flowsheet is complicated. In this work, a control strategy is proposed for a recently published heterogeneous extractive distillation system separating cyclohexane and cyclohexene. During the investigation of control strategy, a closed-loop sensitivity test is adopted to analyze the controllability of this distillation system, and the results show that some modifications for the steady-state design must be made to facilitate the implementation of suitable control structure. After redesigning the separation process, closed-loop sensitivity test is again performed for the purpose of successfully setting up a single-temperature control scheme for two distillation columns and also determine which variable/ratio is to be fixed. The results of open-loop and closed-loop sensitivity tests are taken into consideration together for the sake of selecting the appropriate stage for temperature control. In addition, a closed-loop sensitivity test is also used for the preinvestigation of throughput disturbance rejection to devise feedforward control action. According to the closed-loop dynamic responses, the proposed control strategy enables the heterogeneous extractive distillation system to properly reject ±20% feed composition disturbances and ±10% throughput changes.