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Date of Award

4-13-1993

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Supervisor

Archie E. Hamielec

Co-Supervisor

John F. MacGregor

Abstract

The work described in this thesis can be broken down into two separate parts. The first deals with the mathematical and computer modelling of industrially important methods for the production of polyolefins, specifically: (a) the high pressure tubular reactor process, (b) the high pressure autoclave type reactor process, and (c) the low pressure Ziegler Natta reactor process using heterogeneous catalysts. The second part deals with the chemical modification of these polymers by free radical methods during the extrusion process. Free radical copolymerization of ethylene in high pressure reactors is considered. Kinetic mechanisms to describe the polymerization rate and polymer properties, including copolymer composition, molecular weight, branching frequencies, melt flow index and polymer density, have been proposed. The method of moments is used, in conjunction with pseudo kinetic rate constants to allow for copolymerization, to calculate the molecular weight averages. Based upon this kinetic scheme, a mathematical model has been derived and implemented as computer programs to simulate commercial tubular and autoclave type reactors. The model parameters were fit to steady state data from industrial reactors. The dynamic autoclave reactor model includes the two phase kinetics due to polymer-monomer solubilities and phase separation in the reaction mixture. Gel formation from crosslinking reactions is also analyzed. A mixing model is developed to represent the flow inside the reactor. In the simulation PID control equations are used to maintain operation at the unstable steady state. A sensitivity study is performed on the mixing model parameters and on some of the kinetic parameters. The model is compared to steady state temperature, initiator flow and conversion data from commercial reactors. Many polyolefins are produced using heterogeneous Ziegler Natta catalysts either in gas phase, bulk or slurry type reactors. These properties include broad molecular weight and copolymer composition distributions. A kinetic model has been derived, based upon multiple catalyst sites of differing reactivities, for the Ziegler-Natta copolymerization of olefins. The model predicts the rate of polymerization, the copolymer composition and the molecular weight distribution of the polymer produced as well as accounting for the observed broad copolymer composition and molecular weight distributions. This dissertation deals with the development of mathematical models to relate the molecular modifications, scission, branching, crosslinking and grafting. The models, based upon generally accepted kinetic mechanisms and certain assumptions about the nature of simultaneous scission and crosslinking, can predict the molecular weight averages, degrees of crosslinking, scission and grafting, and the sol/gel ratio. A mathematical formulation to describe simultaneous random scission and crosslinking has been presented in the literature but only solved by assuming that scission and crosslinking occur serially. For this work, an algorithm to numerically solve this equation was developed. The results of these calculations were compared to the classical solutions to this problem and to the experimental data gathered in this project. This model, although applied to polyolefin modification, has applications for other systems where simultaneous random scission and crosslinking of polymers is encountered. (Abstract shortened by UMI.)

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