Modeling, Simulation and Optimal Control of Ethylene Polymerization in Non-Isothermal, High-Pressure Tubular Reactors

Low-density polyethylene (LDPE) is a very important polymer, which is usually produced through a free radical polymerization process in high-pressure tubular reactors. In this study, the mathematical model of this process is developed on the basis of a comprehensive reaction mechanism to accurately determine polymerization rate, and polymer properties under extreme temperature conditions. The model comprises of (i) the mass balances of monomer, initiator, solvent, live radicals and dead polymer chains, and (ii) the energy balances of a tubular LDPE reactor, and its jacket under steady state condition. The model considers the variations in the density and viscosity of reactants along reactor length. Computer simulation of the model is successfully carried out to accurately describe the performance of a non-isothermal, high-pressure, industrial LDPE reactor with multiple initiator injections. Based on the developed non-linear model, the optimal control of the industrial reactor is carried out using genetic algorithms to maximize monomer conversion using jacket temperature as a control function of reactor length. The application of optimal control leads to significant improvements in the final monomer conversion, and capacity utilization of the reactor.