Control of an Optimal Extractive Distillation Process with Mixed-Solvents as Separating Agent

In this work, process control is studied in an optimal Extractive Distillation (ED) system to dehydrate ethanol using a glycerol-ethylene glycol mixture as separating agent. ED system simulation and optimization with respect to the total annual cost (TAC) was made using the software platform Aspen Plus. The control structure of the optimized system was evaluated in Aspen Dynamics. Two control strategies were studied: a conventional control and a modified control scheme. The conventional control structure uses the distillate and bottoms flow rates to control the top and bottom levels, respectively. The second strategy uses the entrainer makeup flow rate to control the sump level in the recovery column and the recovery column bottoms flow rate to control the entrainer feed flow rate to the extractive column. Dynamic simulations show the control strategy that allows obtaining a soft-regulating control while at the same time ethanol purity is above 0.995 mole fraction. In addition, the use of solvents mixture as entrainer reduces importantly the energy consumption and favors the process stability.