Improving dual composition control in continuous distillation by a novel column design

How a novel design of continuous distillation columns may impact the performance of conventional controllers when it is required to accurately control the purity of both products in a binary separation is analyzed. The proposed column layout is characterized by the presence of a middle vessel, to which two streams are fed: the liquid flow coming from the rectifying section and the feed to be separated. The actual liquid flow to the column is determined by the middle-vessel level controller. For different control configurations and conventional proportional-integral control, it is shown that using a middle-vessel column provides a way to reduce the interaction between the composition loops, in such a way that the control performance of middle-vessel columns may be made remarkably superior to that of the conventional columns. The results obtained are supported by a theoretical analysis based on the frequency-dependent relative gain array tool.