Limitations in the Operation and Control of Continuous Middle-Vessel Distillation Columns with a Draw Stream

Abstract

This paper studies the limitations in the operation and control of a continuous middle-vessel distillation column with a draw stream in the middle vessel. This separation layout can be of interest to cases where the most and less volatile components of fractions of a mixture are desired in high concentrations, as in the processing of some oil streams. It was found that to maintain distillate and bottoms products within specifications, large recycling rates can be required to compensate for the storage effects of the middle vessel. This is because, contrary to the case where a draw stream is not present, in the studied distillation column layout the middle vessel alters material balances. In this form, the middle vessel acts as both a dynamical damper to reduce bottoms and distillate control loops and an accumulator for intermediate fractions. That includes some residuals of nonseparated most and less volatile components. The control objective is to track prescribed product compositions of the most and less volatile components by means of manipulations of the reflux and vapor boilup rates. Given that the middle vessel introduces a sluggish controlled response, the recycling stream from the middle vessel to the column is taken as an additional control input to improve the operation of the column and the control performance. In this way, a rectangular control configuration is obtained, which is regularized (i.e., squared) with a habituating control parameter. It is shown that this habituating control configuration can satisfy the control objectives more easily because it provides a balance between the separation and mixing tasks provided by the distillation column and the middle vessel, respectively.