Control properties of thermally coupled distillation sequences for different operating conditions

The understanding of the dynamic behavior of distillation columns has received considerable attention due to the fact that distillation is one of the most widely used unit operations in chemical process industries. Thermally coupled distillation sequences (TCDS) can provide significant energy savings with respect to the operation of sequences based on conventional distillation columns. TCDS exhibit a complex structure, with recycle streams, that appear to affect their controllability properties. One potential solution to this problem has been suggested through the operation of TCDS under conditions that do not provide minimum energy consumption. The basic idea is that if one changes the operation point, the control properties might change as well. In this work, we analyze the dynamic behavior of two TCDS structures under different operating points, including the one with minimum energy consumption. The control analysis properties are analyzed with the application of the singular value decomposition technique at zero frequency and closed-loop dynamic responses using standard PI controllers. The results show that the controllability properties of distillation sequences may change significantly depending on the selected operation point.