An Unusual Control Structure for High Reflux Ratio Distillation Columns

Distillation control wisdom suggests that reflux flowrate should be used to control reflux-drum level when the reflux ratio is large (>3). In the typical case where a single-end temperature control structure is used, reboiler heat input is used to control a tray temperature at an appropriate location in the column. When the reflux-ratio is large and reflux is used to control reflux-drum level, a frequently used scheme controls the reflux ratio by measuring the reflux flowrate and ratioing the distillate flowrate to the reflux flowrate. However, in many single-end distillation control structures the inherent ability to handle feed composition disturbances is better for a fixed reflux-to-feed ratio than for a fixed reflux ratio. Using reflux to control reflux-drum level precludes the use of this structure. This dilemma can be resolved by using an unusual control structure in which the reflux-drum level is controlled by reboiler heat input and a tray temperature is controlled by manipulating the distillate flowrate. The effectiveness of this non-conventional structure is demonstrated in this paper for a highly-nonlinear azeotropic system of water, acetic acid and formic acid.