Control of a Heat-Integrated Complex Distillation Configuration

Abstract

Several steadystate design studies of distillation systems that combine complex configurations (prefractionators and sidestream columns) and heat integration have appeared in the literature and have shown the potential for large reductions (50%) in energy consumption compared to conventional designs. However, the dynamics and control aspects of these highly interactive and multivariable systems have been virtually unexplored. This paper presents the results of computer simulation studies of the dynamic controllability of a complex heat-integrated distillation system for the ternary mixture of benzene, toluene and m-xylene. Both low and high-purity separations were explored.For the low-purity (95% product purities) separation, the complex heat-integrated system was found to be controllable and gave dynamic responses that were comparable to the simple conventional system. A diagonal multi-loop SISO controller structure was successful in controlling this 3x3 multivariable process. Controlled variables were the three product purities. Manipulated variables were the flow rates of reflux, sidestream and reboiler steam.For the high-purity (99.9%) separation, the complex heat-integrated system was found to be controllable but could only handle small (<10%) feed composition disturbances and was dynamically worse than the conventional system. The control structure used in the low-purity separation did not work in the high-purity separation because of severe nonlinearity. To decrease the nonlinearity, a control structure was developed that maintained the two impurities in the toluene sidestream product instead of the toluene purity. The additional manipulated variable needed in this modified process (now a 4x4 multivariable process) was achieved by changing the sidestream drawoff-tray location.