Large-scale dynamic optimization of a low density polyethylene plant

This paper presents the optimal control policy of an industrial low-density polyethylene (LDPE) plant. Based on a dynamic model of the whole plant, op timal feed profiles are determined to minimize the transient states generat ed during the switching between different steady states. This industrial pr ocess produces LDPE by high-pressure polymerization of ethylene in a tubula r reactor. The plant produces different final products. The model consists of two parts, the first one corresponds to the reactor and the second to th e rest of the plant. The process has many time delays that are also incorpo rated into the optimization model. The resulting differential algebraic equ ation (DAE) plant model includes over 500 equations. The continuous state a nd control variables are discretized by applying orthogonal collocation on finite elements. The resulting NLP is solved with a reduced space interior point algorithm. The paper studies two cases of switching among different p olymer grades determining the optimal butane flow rates, in order to minimi ze the time to reach the steady state operation corresponding to the desire d new product quality. (C) 2000 Elsevier Science Ltd. All rights reserved.