Evaluation of control configurations for a depropaniser

Abstract

Distillation columns are highly coupled and non-linear, and have major impact on the utilities consumption and product quality. By proper design of their controls, energy consumption, product variability, operator intervention and equipment downtime can be reduced. Thus, selection of proper controls for distillation columns is both challenging and critical. Although extensive literature have been published on various aspects of distillation control, viz., level controller tuning, ratioing manipulated variables and turndown operation, there is no comprehensive study on control evaluation considering all aspects and rigorous simulation. In particular, turndown operation has received little attention in control research. This work deals with the composition control of distillation columns considering depropaniser as an example and using rigorous simulation. Effect of level controller tuning, ratioing the manipulated variable and turndown operation on the performance of several control structures to reject step disturbances in feed flow rate and composition, and sinusoidal disturbance in feed composition, is studied. Both single- and dual-ended composition control of the depropaniser are considered. Results of this study show the need and importance of a comprehensive and rigorous analysis including column operation far away from the design conditions, for optimal design of column control. © 2008 The Institution of Chemical Engineers.