(429e) Thermal Stability of Ethylene Polymerization in Fluidized-Bed Reactor

Ethylene polymerization in fluidized bed reactor is a simple process and is easier for production separation. Thus fluidized bed reactor is widely used in ethylene and propene polymerization. In order to improve the production rate of polyethylene in fluidized bed reactor, condensed mode operation and super-condensed mode operation are developed. With inert condensed compound such as i-pentane or n-hexane or comonomer such as n-hexene, the recycle stream can become a gas-liquid mixture under the reasonable temperature, thus more heat of reaction can be removed by the recycle stream and the production rate will be dramatically increased.

Ethylene polymerization is a rapid and highly exothermic process. As the melting temperature of polyethylene near the reaction temperature, thermal instability of fluidized bed may cause the oscillation of reactor temperature, and even induce polyethylene melting and plant stop. Therefore, thermal stability and dynamical behavior of fluidized bed not only concern design and scale up of fluidized bed reactor, but also concerns the real time monitoring, handling of abnormal conditions and safety production.

Several methods have been employed to analyze the stability and dynamic behaviors, such as mathematics deduction method, nonlinear analysis method, process simulation method, etc. The mathematics deduction method is based on strict mathematical derivation, however, it cannot be used to solve relatively high dimensional systems. Nonlinear analysis method is used to solve relatively high dimensional systems. Meanwhile, the process simulation method is applied to solve large complex chemical process but this method still exist some disadvantages in stability analysis. In this work, a simple model of fluidized bed reactor is proposed to get the thermal stability criterion. Then effects of structural parameters of fluidized bed and operation parameters on thermal stability of ethylene polymerization process are investigated based on a detailed mathematical model of fluidized bed.

In the simple model of fluidized bed reactor, concentration of reactants and reactor pressure are assumed to be constant. Therefore, the simple model is a first order ordinary differential equation group with respect to catalyst concentration and reactor temperature. With mathematics deduction, the coefficient matrix of and thermal stability criterion are obtained. The calculation results show that space-time yield of polyethylene on the Hopf bifurcation points is proportion to catalyst activation energy, reactor height and square of reaction, and is in inverse proportion to superficial velocity and density of recycle gas.

Based on the two phase model, a multi zone model is proposed in order to describe ethylene polymerization in fluidized bed in condensed mode and super-condensed mode. In the multi zone model, fluidized bed is composed of emulsion phase and bubble phase, and the emulsion phase is divided into gas-liquid-solid zone and gas- solid zone. The evaporation of liquid occurs in the gas-liquid-solid zone with a certain thickness liquid film on the surface of solid particles. Furthermore, the liquid evaporation rate obeys the equilibrium evaporation principles. Then the structural parameters and operation parameters of fluidized bed reactor are chosen to trace solution curves and bifurcation points referred to the catalyst feed rate.

The calculation results of multi zone model show that there are Hopf bifurcation points and limit points in the solution curves, which divide the solution curve into three parts-stable part with low temperature, unstable part and stable part with high temperature. Moreover, the steady state multiplicity of fluidized bed reactor occurs and the temperature range of stable part with low temperature shrinks with increase of activation energy of catalyst and condensed compounds in the recycle stream. In addition, thermal stability criterion based on a simple model fit well with the prediction of multi zone model.