(161c) Distillation Control – Still an Issue

In distillation columns control there is no "one fits all "strategy. Thorough understanding of the column’s purpose and dynamic behaviour are critical in designing and commissioning a control strategy that can respond to the specific requirements. This is why it is still and will probably always be an issue.

Even if distillation control is no longer a hot research topic the problem is far from being over. The distillation columns are typically delivered or configured to control top product composition with reflux flow and the bottom composition with the heat input. The most probable outcome from this practice is that the overhead drum and tower bottom levels, being open loop unstable, stay in automatic with distillate and bottoms, respectively, as the manipulated variables; and the fractionation - energy balance - is left to the operator for keeping the products within specifications. The bigger the reflux ratio, Reflux over Distillate, the more difficult to maintain the level in the overhead accumulator and it will only take an enthalpy disturbance to have the controller challenged; a change in the reflux flow and the subsequent change in reboiler heat input will have to follow in order to keep the column in operation and the compositions in check. It is therefore common to have column products being over purified by using more energy than what is required, for this provides the operators with a slack to deal with the disturbances.

This is not supposed to be news, but since the pendulum has swung back again to remind the industry that it’s time to optimize the units operation, this paper’s objective is to show, by using actual cases, how material balance controls have been implemented to improve/stabilize several distillation columns that have invariably resulted in economic quantifiable results, with ROI measurable in days rather than months.

The specific cases to be presented are: The implementation and commissioning of new control strategies in the thermally coupled Benzene and Toluene tower at a petrochemical facility resulted in savings in excess of 1.0MM$/yr. The material balance control implemented in a xylene tower at a refinery where the savings in the order of 2 MM$/yr. are being accrued as a result of product recovery and energy optimization. A new debutanizer column whose controls were designed and commissioned for dual composition using energy balance configuration, and that after several weeks of fruitless attempts to stop the continuous oscillation in the compositions; a material balance control strategy was successfully implemented resulting in a reduction of butane composition variability reduced by almost 60%, making easier to maintain the required gasoline RVP.