Optimal Operation of an Industrial PVC Dryer

Abstract

This article describes the design of a control structure architecture for an industrial polyvinyl chloride (PVC) dryer, currently operating at Braskem Company (Marechal Deodoro, Alagoas, Brazil). The underlying motivation is to search for a control configuration that leads to optimal economic operation, while promptly rejecting disturbances at lower layers in the control hierarchy. We start by optimizing a nonlinear model of the process with respect to reducing energy consumption as a criterion of optimization for different disturbance scenarios. The results show that it is optimal to control the temperature level of the utilities serving the dryer and the outlet PVC moisture contents at their respective upper bounds. In addition, the flow of air to the dryer should be fixed at its optimum nominal setpoint despite disturbances. Application of this strategy results in a reduction of about 16% in energy consumption with respect to the current dryer operation policy and a 22% increase in throughput under nominal operation. In addition, almost perfect indirect control of the outlet PVC moisture was achieved by tightly controlling a temperature difference in the dryer. The proposed decentralized control configuration gives good dynamic performance for the outlet PVC moisture content with maximum settling time of about 1.8 h for the more difficult disturbance of increasing the inlet slurry moisture content by 40% and magnitude of overshoot of ca. 5% w.r.t. the optimum setpoint for an increase of 20% in PVC feed flow rate to the dryer.