Reducing the energy demand of continuous distillation processes by optimal controlled forced periodic operation

Abstract

In a continuous distillation process, it is generally believed that steady-state conditions represent the optimal operation mode from the viewpoint of required energy supply and that any disturbance of steady-state conditions (e.g. an oscillating profile of reflux ratio or reboiler heat rate) will cause a higher energy demand if the product specifications (expressed as average values) have to be fulfilled. However, this assumption has been disproven by some theoretical and experimental studies in the past, where the distillation process was studied under unsteady-state conditions, to reduce the energy demand. In these studies, both the reflux and the vapor flow rates have been controlled periodically. In the present paper, dynamic optimization methods are applied to find the optimal profiles for all possible control variables (flow rates of feed, products, reflux and vapor). Two examples are discussed. In the first example, an ideal ternary mixture is separated into two fractions, and in the second example, the same mixture is separated into three fractions using a column with a sidestream. For both examples, it is demonstrated that appropriate control profiles for the main control variables can reduce the required energy supply compared with steady-state conditions. The results were obtained on the basis of a simple mathematical model of a distillation column with the assumption of constant relative volatilities and equimolar evaporation and condensation. To generate the control profiles, a new software package for the solution of large-scale optimal control problems was used. The optimal control profiles represent a decoupling of feed supply, removal of distillate product, removal of bottom product, and, in the second example, withdrawing of the sidestream.