Broadening the Polyethylene Molecular Weight Distribution by Periodic Variation of the Hydrogen Feed Rate

Abstract

The ability to control the molecular weight distribution of the created polyethylene in gas-phase fluidized-bed reactors is discussed. This objective is achieved via nonlinear model predictive controller utilizing the hydrogen feed rate as the only manipulated variable. The idea for limited usage of manipulated variables is to avoid economically unfavorable operations such as excessive purge and/or reduced production rate. The simulation results indicated successful implementation of the control algorithm to attain the desired molecular weight distribution. The success depends on the improved hydrogen activities inside the reactor by employing a modified catalyst that is responsive to hydrogen variation and allowing a wider range for hydrogen feed rates. Polyethylene with a pre-specified molecular weight distribution is produced in a single fluidized-bed reactor using a multi-site Ziegler-Natta catalyst. The control objective is successfully achieved, which requires forcing reactor input into cyclic operation using a nonlinear model predictive algorithm. The hydrogen-to-monomer molar ratio is successfully altered by regulating the hydrogen feed while keeping monomer feed rate constant, which makes reactor operation easier. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.