Operation of Dividing-Wall Distillation Columns. 3. A Simplified Double Temperature Difference Control Scheme

Although the double temperature difference control (DTDC) scheme outperforms the temperature difference control (TDC) scheme in controlling dividing-wall distillation columns (DWDCs), four additional temperature measurements are required and result in a more expensive and complicated control structure. In order to overcome its drawbacks and still keep its advantages, we proposed a simplified DTDC (SDTDC) scheme, which includes two temperature control (TC) and two DTDC loops. While the two TC loops are designed, respectively, to regulate the top and bottom products, the two DTDC loops are designed to control tightly the prefractionator and intermediate product. The design strategy makes deliberately use of the operating characteristics of the DWDC and can consequently facilitate its product control and robustness to the operating condition changes. The control of a DWDC, fractionating a benzene/toluene/o-xylene mixture, is studied to compare the SDTDC scheme against the TDC and DTDC schemes. Although sharing an equal number of temperature measurements, the SDTDC scheme is superior to the TDC scheme with comparable or even reduced static offsets in the three products and enhanced capability of handling feed composition disturbances. The SDTDC scheme leaves slightly greater static offsets than the DTDC scheme, but its reduced cost and alleviated complication justify it to be a competitive alternative for the operation of the DWDC. The SDTDC scheme can be further reinforced through the formation of TDC loops in the rectifying and/or stripping sections with the available temperature measurements in the two sections along the dividing wall, and this increases the diversity of the proposed control strategy. A systematic approach is given to search for the tight control scheme for the DWDC with as a small number of temperature measurements as possible.