Thermal Stability Of Liquid-containing Fluidized Bed Reactor And Control Of Polyethylene Properties

Liquid-containing gas-solid fluidized bed ethylene polymerization reactor is one of the most important devices of the ethylene polymerization process under the condensed mode operation.Operation safety,stability,and efficiency are the most issues for a liquid-containing fluidized bed reactor.The industrial data shows that the liquid-containing fluidized bed reactor usually encounters with large fluctuations of the reactor temperature,the pressure and the concentrations of reactants during the production rate change and grade transition with a high condensed medium.The production rate of ethylene of the liquid-containing fluidized bed reactor is usually twice as large as that of the conventional gas phase fluidized bed reactor.And the mass transfer and heat transfer behaviors are very complex and very sensitive to the reactor temperature with the fluidized bed reactor operated under the condensed mode operation.Then the stable operation region becomes smaller when the gas phase operation is switched to the condensed mode operation.Therefore,without a deep understanding of the thermal stability of the liquid-containing fluidized bed reactor and efficient control strategy,the bad fluidized states may happen.Besides,the understanding of thermal stability of the liquid-containing fluidized bed and the relationship between the control strategies and the micro properties of polyethylene are important for the operation safety of the liquid-containing fluidized bed reactor and developing high-performance polyethylene productsIn this paper,the liquid-containing fluidized bed olefin polymerization reactor is investigated.The main work of this paper consists of the thermal stability of the liquid-containing fluidized bed and control strategies of the micro properties of polyethylene More specially,this work includes three parts as listed:1.According to the characteristics that the continuous gas phase,the dispersed liquid and solid phase coexist in liquid-containing fluidized bed reactors,an equilibrium multicomponent evaporation model and a two-phase model are coupled in order to describe liquid-containing fluidized bed ethylene polymerization reactors.Effects of operation parameters on the thermal stability of the liquid-containing fluidized bed reactor are investigated based on the bifurcation theory.Simulation results show that increasing the catalyst activiation energy and condensed medium content will reduce the stable operation regions of a liquid-containing fluidized bed reactor and multiple steady states appear when the reactor temperature is open-loop controlled.Further,a population balance model is combined with the proposed two-phase model in order to investigate the influences of particle growth and aggregation on the thermal stability of FBRs.It is found that both the catalyst size and the superficial gas velocity had significant effects on the thermal stability of FBRs.The stable operation region shrinks with the increasing of catalyst size and decreasing of the superficial gas velocity.2.A dynamical model consists of a liquid-containing fluidized bed reactor model and a heat exchanger model is proposed to investigate impacts of operation parameters on the thermal stability of the ethylene polymerization process.The BWRS Eos is applied to predict the gas-liquid equilibrium of the recycle materials in the heat exchanger.The stability of the steady states is determined by a small perturbation method.The parameter space with the catalyst feed rate and the liquid feed rate as coordinates is divided into a stable region with low temperature,an unstable region,an oscillation region,and a stable region with high temperature.The stable operating region with the reactor temperature under the closed loop controller is much larger than that for the temperature under the open loop controller.In addition,the oscillation of reaction temperature is determined by the reactor temperature control loop and the reactor pressure control loop3.In order to control the molecular chain structure of polyethylene,influences of the condensed medium content and catalyst feed rate on the axial temperature profile,the axial concentration profile,the molecular weight distribution,and the comonomer content distribution are studied.A multi-zone model for the liquid-containing fluidized bed reactor is proposed according to the solid flow pattern in the gas-solid fluidized bed.The deconvolution of the measured molecular weight distribution(MWD)and comonomer content distribution is applied for estimating kinetic parameters of multi-site type catalyst.When the liquid feed rate is much higher,the temperature of the reactor bottom,the concentration of 1-hexene and the ratio of 1-hexene to ethylene increase,while the ratio of hydrogen to ethylene decrease,which will result in the increase of comonomer content in the whole polyethylene and its high molecular chains.