Abstract

In the present study, a new dynamic fluidized bed reactor (FBR) model is developed to account for the effect of bubble growth in the bed height on the dynamic behaviour of the reactor and the molecular properties of the polymer product. The model takes into account the existence of solid catalyst in both phases and consequently, the occurrence of polymerization reaction in both bubble and emulsion phases. A dynamic two-phase model is employed for predicting the key hydrodynamic parameters of the bed. A comprehensive kinetic model for ethylene polymerization in the presence of multiple-site Ziegler-Natta catalyst is considered to describe the number and molecular weight averages and molecular weight distribution of polymer in the FBR. The hydrodynamic model and the kinetic model have been coupled and solved simultaneously to simulate the performance of the fluidized bed reactor. The study incorporates the effects of the most important reactor parameters such as superficial gas velocity, mean particle size, inlet gas temperature, bubble size, recycle stream and chain transfer agent on the steady-state behaviour of the FBR. The proposed dynamic model is capable of predicting both the performance of the reactor and the polymer physiochemical properties.

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  • Rep rod uced with p erm ission of th e copyright ow ner

  • Further reproduction prohibited without p erm ission

  • 1 am thankful to professor Manuel Cuenca, Dr Stephen Wylie and Dr Philip Chan who served as members o f my thesis committee

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Summary

CHAPTER 1: IN TRODUCT IO N î

Polyethylene (PE) is one o f the largest synthetic commodity and engineering polymers and is widely used throughout the world for its versatile physical and chemical properties. M odern polyethylene plants are continuous processes using Ziegler-Natta catalyst in bulk, r, solution, slurry or gas phase. It is a dry process since it does not involve any liquid in the reaction zone It is free of mass and heat transfer limitations inherent to viscous slurry and bulk systems. The catalyst/polymer system in the gas phase fluidized bed reactor has a good heat transfer. This process can produce polymers with a wide range o f densities and molecular weight distributions (MWDs). Production o f high comonomer content polymers, such as high im pact ethylenepropylene copolymers is another major advantage of the gas phase process. Figure 1. 2: Schem atic m olecular structure o f low-density polyethylene (LDPE)

L iterature R eview
Poiym erization Fluidized Bed R eactor
Catalysts for Gas Phase Polym erization
Reaction M echanism of Ziegler-Natta Polymerization
The reaction mechanism is called
CHAPTER 3: HYDRODYNAMICS OF FLUIDIZED BEDS
M inim um Fluidization Velocity
Bubble Size
B u b b le V elo city
Bubble Fraction
Em ulsion Velocity
M ass Transfer Coefficients
HeatT ra n s fe r C o efficien ts
Reaction M echanism
K inetic M odel D evelopm ent
Reaction Rate for
Reaction Rate for Cocatalyst
Reaction Rate for Gaseous
M olar Balances on Living Polymer Chains a)
Molar Balances on Dead Polymer Chains
Reactor Model
Bubble Phase Molar
Emulsion Phase Molar Balance
Bubble Phase Energy Balance
Emulsion Phase
Dynamic Model
Reactor Operating at Steady-State
Number Average and Molecular Weight Average
Molecular Weight
Steady-State M odel
Dynam ic M odel
Steady-State and Isothermal
Steady-State and N on-Isothcrm al
Superficial Gas Velocibv' The superficial velocity is an important parameter
M ean Particle Size
Gas Feed Temperature
B ubble Size
Recycle Stream
Chain Transfer Agent
D ynam ic M odel
Findings
X10 icosc
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