Modelling and Optimization of the Tube Reactors in an Ethylene Dichloride Cracker Using Pontryagin Optimal Control Theory

Abstract

Ethylene dichloride (EDC) cracker is the heart of vinyl chloride monomer (VCM) unit, so predicting the behavior of this cracker is mostly considered. In this work, mass, energy and momentum balances have been applied to the tube reactors in ethylene dichloride cracker and the obtained differential equations are solved simultaneously using the characterizations of an existing unit and the suitable numerical method to predict the reactor behavior. Profiles of concentration, temperature and
pressure along the reactor have been obtained and compared
with the operating data. Optimization of ethylene dichloride thermal cracking reactor for vinyl chloride monomer production has also been done in this work. The objective function for this problem depends on length of reactor which finds optimum external wall temperature profile along the reactor and can optimize the profile of heat transfer rate to the process fluid along the reactor in order to maximize the VCM product at the outlet of the reactor. Mass, energy and momentum differential
equations derived from the modeling of reactor are the constraints of optimization problem. The present problem has been converted to an optimal control problem format by using the Pontryagin theory to solve it. Visual Fortran has been used for programming and optimum profiles of state and control variables have been analyzed as results of program. Finally, the results of both the usual method and the method using Pontryagin optimal theory are compared with operating data. This work shows Pontryagin optimal control theory can be applied to other cases and in general it has computational advantages, accuracy and also robustness.