Alternative Control Structures for Distillation Columns with Partial Condensers

Abstract

Partial condensers are used in distillation columns when the distillate product is removed as a vapor stream. This approach is commonly employed when there are very light components in the feed to the column that would require a high column pressure or a low condenser temperature to completely condense these very volatile components. The use of a partial condenser can avoid the use of costly refrigeration in the condenser. Both the pressure and the level in the reflux drum need to be controlled, and several manipulated variables are available: reflux flow, distillate flow, and condenser heat removal. This paper explores three alternative control structures for this type of system, under two different design conditions: (1) a large vapor distillate flow rate (moderate reflux ratio) and (2) a very small vapor distillate flow rate (high reflux ratio). The preferred control structure depends on the control objectives of the entire process. For example, when the distillate goes to a downstream unit and large variability in its flow rate is undesirable, the control structure should control pressure with condenser heat removal, control level with reflux, and maintain a constant reflux ratio.