A modified fully thermally coupled distillation column is proposed, with utilization of the existing distillation columns of the conventional system, and its control scheme is suggested here. The proposed distillation system is applied to a benzene-toluene-xylene (BTX) separation process, of which the system design and control performance evaluation are conducted using the HYSYS software. The performance of the suggested 3 × 3 control is examined in the set-point tracking of product specification and the regulation for the changes of feed flow and composition. The pairings of three proportional-integral control loops are the reflux flow and the specification of overhead product, the prefractionator vapor flow and that of the side product, and the vapor boil-up rate and that of the bottom product. The multi-variable controllability using various indices is investigated for the proposed control scheme, and the controllability is compared with that of the cross-pairing between the control loops of the side and bottom products. IntroductionWhen the divided wall column (DWC) -a kind of the fully thermally coupled distillation column (FTCDC) -replaces a conventional distillation system, the existing columns are not utilized and a totally new column is built. Although the energy efficiency and operability of the DWC have been proved from the practical applications in many processes for a long time [1,2], the replacement of the currently operating process with the DWC is not preferred by field engineers due to the cost problem and the difficulty of the new column construction.Another difficulty in the FTCDC implementation is that the specification control of the products in the FTCDC is more complex than that of conventional binary distillation columns, because the FTCDC, composed of a prefractionator and a main column interlinked with two-way transfers, produces three products in a column. While the specification control of overhead and bottom products of the FTCDC is similar to the binary column, the control of the side draw is not simple. It has two impurities of the lightest and heaviest components, which means that refining with increased reflux flow or vapor boilup does not always help to improve the specification of the side draw. Generally, the raised reflux flow and vapor boilup elevate the specification, but the effect of the increase is limited in the case of the FTCDC control [3].There have been two groups of studies to improve the operability of the FTCDC. One deals with devising a new column structure for better controllability of the column. Agrawal and Fidkowski [4] proposed more operable arrangements of distillation sections in the FTCDC for easy vapor transfer, and later they introduced modified structures of the FTCDC by eliminating one of the two-way transfers between the prefractionator and the main column for easy operation of the system [5]. Wolff and Skogestad [3] examined various control structures of the FTCDC to find an optimum control scheme. A configuration of partially separated main col...