Integrated Design and Control of Semicontinuous Processes Using Mixed Integer Nonlinear Dynamic Optimization

Semicontinuous separation is a distillation based process intensification technique devised for the purification of ternary mixtures to desired purities. It is particularly promising for small scale production processes where the semicontinuous system has a lower total annualized cost relative to the conventional continuous process, thus it can be a suitable option for the purification of biofuels. Semicontinuous systems are dynamic, control driven processes that work in a stable limit cycle with several modes of operations. Therefore, designing the system and its controllers is a challenging task which has not been thoroughly considered in the literature. In this work, for the first time, a methodology is presented to simultaneously obtain the optimum design parameters of the semicontinuous system such as the number of column stages, feed and side stream locations, as well as the optimum tuning parameters for the PI control structure. To design the semicontinuous process, it is formulated as a mixed integer nonlinear dynamic optimization problem using the equation oriented gPROMS software which has built-in deterministic optimization packages. The benzene, toluene and o-xylene mixture is chosen as a case study to demonstrate the methodology. The optimization took 1.03 CPU hours and the optimum design parameters are obtained. The results show that the total annualized cost of the designed semicontinuous system is lower than the respective conventional continuous process.