Distillation pressure control troubleshooting—The hidden pittfalls of overdesign

The operating pressure of a distillation column is one of the main handles with which to control and optimise separation as it affects most other parameters as well as the overall stability of the column. It is therefore one of the most important parameters to control.Controlling a distillation column, designed to be operated under a vacuum, at the intended operating pressure can prove to be much more difficult than for positive pressure columns especially during the initial start-up of the column. Determining the cause of pressure instability can be a daunting task as the column will most likely be under turndown conditions, the inventory might differ from the intended composition, the control loops will be in manual or not tuned yet or the cause might even be from an equipment design perspective.During the initial start-up of a vacuum operated distillation column several problems were encountered that prohibited the column from being operated at its intended pressure. This caused severe instabilities in the column and also affected the downstream sections of the plant. This paper focuses on these problems and the troubleshooting to determine the causes of the pressure instability.During troubleshooting a number of key factors that contributed to the pressure instability were identified, with the main cause being the design and expected air ingress into the column. This influenced the design of several pieces of equipment in and around the column. A deviation from the design air ingress meant that several design parameters had to be re-evaluated in order to eventually reach the intended operating pressure.A conventional pressure control philosophy was implemented, but changes to this were required in order to achieve the necessary stability in the column. The improved control philosophy has several benefits, such as a reduced number of variables to be manipulated by the operator, smoother change over between the primary pressure controller and the setpoint high controller, as well as optimising the product losses to the vacuum system.

► Pressure control problems were encountered during the initial start-up of vacuum operated distillation column.
► A Lack of air ingress caused an over design in condenser area.
► Nitrogen were added to substitute for the lack of air ingress.
► Control philosophy changes together with trip setting changes allowed for tight pressure control.