Improved Control Strategies for Semicontinuous Distillation

Improved Control Strategies for Semicontinuous Distillation

Pranav Bhaswanth Madabhushi, Thomas A. Adams II

Department of Chemical Engineering McMaster University, 1280 Main St. West, Hamilton, Ontario, Canada, L8S 4L8.

Semicontinuous distillation processes have undergone certain design and control configuration changes in the past decade that significantly improved their controllability and lowered the total annualized cost. Adams and Seider, 2008, modified the traditional Distillate-Bottom (DB) control configuration to include the control of the side stream flow rate () using a model-based heuristic called the “ideal side draw recovery arrangement (ISR)” for reducing cycle time (T). Although this heuristic soon became the standard in the studies that followed for almost a decade, its effect on the cycle time and on the system dynamics were not well understood.

In the current study, we compare the performance and dynamic characteristics of two new heuristics that were introduced to reduce cycle time. The effect of the heuristics was demonstrated on a case study involving the separation of benzene, toluene and, o-xylene. Results show that they performed better in comparison to the status quo, with a remarkable 25% reduction in total annualized cost per total annual production of the toluene (TAC) while maintaining product purities. This shows that the ideal side draw recovery heuristic is inherently sub-optimal even though it is effective. To find the optimal side stream flow rate function, dynamic optimization was used with the objective of minimizing TAC without compromising product purities and cycle periodicity. It was however found that dynamic optimization did not return better results when compared to the two newly introduced heuristics, which were used as initial guesses.

References

·         Adams, T.A., Seider, W.D., 2008. Semicontinuous distillation for ethyl lactate production. AIChE Journal 54, 2539–2552. doi:10.1002/aic.11585

·         Pascall, A., Adams II, T.A., 2013. Semicontinuous separation of dimethyl ether (DME) produced from biomass. The Canadian Journal of Chemical Engineering 91, 1001–1021. doi:10.1002/cjce.21813

Figure 1 Semicontinuous distillation system with the control configuration as suggested by Pascall and Adams, 2013. CC represents concentration control, LC represents level control and PC represents pressure control.

Figure 2 Phase plane plot when ideal side draw recovery heuristic is used to control the side stream flowrate. PR, RQ and QP representing the Charging, Separating and Discharging modes.