Design and Control of Extractive Dividing-Wall Column for Separating Methylal-Methanol Mixture

The design and control of extractive dividing-wall columns (EDWCs) have not yet been investigated, whereas those of dividing-wall columns (DWCs) and conventional extractive distillation columns (CEDCs) have been studied by many researchers. The control of EDWCs is more difficult than that of CEDCs and somewhat different from that of DWCs because of differences in the inner structure and interactions among control loops in EDWCs. In this work, the design and control of an EDWC for methylal/methanol separation are investigated using commercial chemical simulators (Aspen Plus and Aspen Dynamics). The optimum EDWC design based on the total annual cost (TAC) is screened first; then, the interaction between design and control is observed; and finally, two control structures for the EDWC are presented. The first proposed basic control structure featuring four composition controllers and an adjustable vapor split ratio alpha(v) can handle all disturbances well with small offsets in product purities, except for a 20% decrease in feed flow rate with a large deviation in methanol product purity. Then, an improved control structure with the Q(R)/F ratio is established. The large deviation is effectively suppressed, and all of the disturbances are well rejected with small offsets in product purities. It is also revealed that the vapor split ratio alpha(v) at the bottom edge of dividing wall must be adjusted at least for composition disturbances of key feed components.