Simulation Studies of Divided Wall Distillation Column

Abstract

The divided wall column (DWC) system is a promising energy-saving alternative for separating multi-component mixtures. The innovation is in the structure of this equipment, namely there is a wall in it which divides the space in the tower so the feed and the side stream-product zones are separated. Divided wall column can effectively reduce energy requirement by up to 30% and also lowers engineering and hardware costs compared with conventional direct and indirect distillation sequences. Conventional ternary separations progressed from the (in-) direct sequences to thermally coupled columns such as Petlyuk configuration, and later to the DWC compact design that integrates the two distillation columns into one shell. However at the same time this integration also leads to changes in the control and operating mode. Despite its potential to make major savings in energy and capital costs in distillation, it has not been widely used in practice. One of the major fears in applying the technology is uncertainty regarding the control and operation of the arrangement. A variety of controllers are used for binary distillation columns, only a few control structures were studied for DWC. In most of the cases multi-loop PID controllers were used to steer the system to the desired steady state. Simulation of a DWC is a difficult task, as it involves a number of variables and a highly nonlinear problem. The steady state and dynamic simulation of DWC were done in ASPEN PLUS and ASPEN dynamics respectively. The system under study was ternary separation of the mixture benzene-toluene-xylene (BTX) in DWC. The simulations were carried out by varying the operational parameters assuming the structural variables as constant and the optimum values of vapor split and liquid split were obtained for the minimizing the reboiler duty. At steady state, the optimum liquid split and vapor split were found to be 0.603 and 0.45 respectively, for the separation of BTX ternary mixture. At optimum conditions the minimum reboiler duty was found to be 15770 kW. A conventional control structure based on PID control loops was used as a control basis. From the study of the control and dynamics of a DWC, it was found that the control structure enhanced by adding an extra loop controlling the heavy component composition in the top of the pre-fractionator, by using the liquid split as an additional manipulated variable, thus implicitly achieving minimization of energy requirements. The results of the dynamic simulations show that the concentration of the components in the product streams returned to the set point soon after the disturbance in form of change in feed flow rate or the feed composition was introduced.