Dynamics and control of a heat pump assisted azeotropic dividing-wall column for biobutanol purification

An efficient downstream process has been developed for the acetone–butanol–ethanol (ABE), allowing about 60% energy savings as compared to a conventional separation sequence. This is achievable in an azeotropic dividing-wall column (A-DWC) coupled with a heat pump and applying heat integration. The energy use is reduced to only 2.7 MJ/kg butanol (or 7.5% of the energy content of butanol), but to take full advantage of the energy savings offered by this highly integrated process, the process must be also controllable. This work presents the dynamics and control of this novel A-DWC process used for biobutanol purification. A rigorous, pressure-driven dynamic simulation has been developed in Aspen Plus Dynamics. After adding the required PID controllers (e.g. flow, pressure, liquid level and temperature) to the base-case process, the system can handle small and short disturbances (e.g. 5% change of feed flow rate for 10 min.) but if the disturbance persists for a long time the process shuts down. To solve this problem, the controllability of the system is improved by adding an additional reboiler and condenser to control the temperature on the feed-side stripping section. Moreover, adding concentration controllers ensures that the products purity is kept constant when feed flowrate or composition disturbances occur.