Analysis of control properties of thermally coupled distillation sequences for four-component mixtures

The control properties of thermally coupled distillation sequences for the separation of quaternary mixtures of hydrocarbons were compared to those of conventional distillation sequences. Two thermally coupled systems were investigated: a thermally coupled distillation sequence with side columns and a Petlyuk-type column. The preliminary steady-state design of the thermally coupled distillation sequences was obtained by starting from a conventional distillation sequence and then optimizing for minimum energy consumption. The control properties of the sequences considered were obtained by using the singular value decomposition technique and closed-loop dynamic simulations under feedback control. It was found that, for some frequencies, the thermally coupled distillation sequences present theoretical control properties similar to those of conventional distillation sequences, i.e., similar minimum singular values and condition numbers. Also, both types of distillation sequences show interactions when relative gain arrays were obtained. Then, when both types of distillation sequences were studied under the action of proportional integral controllers, the thermally coupled sequences also presented optimum dynamic responses similar to those obtained in the conventional distillation scheme. As a result, it can be concluded that thermally coupled distillation sequences can present theoretical control properties similar to those obtained in conventional distillation sequences. This result is significant because it lets one establish that the energy savings predicted for thermally coupled distillation sequences are achieved without introducing additional control problems.