SIMULATION OF TUBULAR HIGH PRESSURE POLYETHYLENE REACTOR

Abstract

A computer model has been developed for tubular high pressure polyethylene reactor.Simplifications (plug flow, absence of axial mixing) have been made which are shown to be justifiable in high throughput reactors.However,the steady state free radical approximation is not made, and variation in the physical properties of the fluid along the reaction coordinate are taken into account explicitly.Emphasis is made on realistic kinetics including long chain branching,use of best rate constants available and realistic simulation of all physical properties along the reaction coordinate. The governing equations which are found to be highly nonlinear and stiff ' differential equations,are solved using Semi-implicit Runge-Kutta method (Rosenbrock's Algorithm).Experimental data used by Brandolin et al. has been used to verify the model. This model permits a calculation, for any point along the reaction coordinates, of transport properties,the first several moments of the radical and polymer size distribution, and of long chain branching.Because the reaction is exothermic, the polymerization reaction accelerates rapidly after a certain rate of initiation has been achieved. The radicals concentration, that increases until the temperature peak is reached, begins to decrease, still allowing a further significant polymerization in the last part of reactor. To a first approximatiOn,conversion and polymer properties are determined by the size and loCation of the temperature interval available for polymerization.Changes in conversion which can be brought about by changes in reactor design and operating variables, such as jacket temperature, Wall heat transfer coefficient etc. are shown to be of a magnitude which is commercially significant. Temperature profiles, monomer and initiator conversion,the number average degree of polymerization (X n) ,and the molecular weight distribution(MWD)are calculated for a typical set of • operating conditions as they might exist in a simple high throughput reactor.The response of the model to changes in operating variables is then examined. The model can be extended readily to simulate reactors with multiple initiator or monomer injection and to reactors in which laminar conditions prevail in the high conversion zone