Dual Product Control of a Binary Distillation Column
Consider an ideal equimolar binary mixture of benzene and toluene at . This mixture, with a thermal quality equal to 0.5, is fed to a 10-stage column with a total condenser and partial reboiler. The feed stage is stage 7. The feed flow rate is taken to be .
This Demonstration shows how one can perform dual product control (i.e. produce distillate and bottom compositions determined by the two setpoints).
Because of the interaction between the top and bottom control loops, the conventional two-point feedback control does not give good results. For example, if distillate purity needs to be adjusted, the reflux ratio is increased, which negatively affects the bottom purity. Thus, the reboil ratio is manipulated by the bottom control loop, which increases the overhead vapor flow rate and affects the top control loop.
The results show that the ratio control scheme suggested by Rijnsdorp [1, 2] and Rijnsdorp and Van Kampen  gives very good results.