Reducing Energy Usage in Extractive Distillation

Abstract

Butadiene 1:3 is separated from other C4-hydrocarbons by extractive distillation in a sieve plate tower. Prior to the development work to be described, the pressure in the extraction tower was controlled at a fixed value. The tower pressure-boilup control loop did not behave satisfactorily in the presence of non-condensables which entered with the feed. The capacity of the flooded reflux drum condenser for the tower was limiting production during summer months. The tower pressure control loop was put on manual. The pressure was allowed to drop to its lowest attainable value for the existing conditions of boilup and condenser cooling capability. This manner of operation is known as floating pressure control. By taking advantage of the higher relative volatility at the lower tower pressure, energy usage was reduced and there was an increase in production capacity. The tower operation at a lower temperature reduced tower and reboiler fouling. Substantial savings have resulted from these improvements. The annual energy consumption has been reduced by 25% and maximum productive capacity is higher by 15%. The rate of tower and reboiler fouling has not been fully quantified but is greatly reduced. A more stable tower operation has also contributed to higher productivity and reduced energy usage. Venting of non-condensables does not affect tower stability and the operators have adapted well to the new control strategy.