Study On Modeling Of Metallocene Polyethylene Fluidized Bed Reactor

Metallocene polyethylene is a new kind of polyethylene with excellent engineering performance which is produced in gas-phase ethylene polymerization process. It has become one of the hot focuses in polyolefin field to study metallocene polyethylene technology and its modeling problem. Chemical process simulation technique is a fast and effective study method which has a variety of advantages compared with experiment study such as avoiding a great resource consumption, reducing the amount of work and the research period. Although there has been many studies on modeling of ethylene polymerization process, but studies concentrated on mechanism modeling of metallocene polyethylene fluidized bed reactor are limited. Dynamic simulation of the metallocene polyethylene fluidized bed reactor start-up process is not yet found in the open literature. Study on steady-state and dynamic modeling of metallocene polyethylene fluidized bed reactor has been carried out in this paper which includes the following three aspects.Firstly, a reasonable simplification on the industrial metallocene polyethylene gas-phase process was proposed, and a single-site reaction kinetic model was developed. Polyethylene molecular weight distribution curve was obtained based on Flory distribution function. The accuracy and reliability of the model was validated by the data from industrial process.Secondly, some reasonable hypotheses were raised on the metallocene polyethylene fluidized bed reactor according to the technological process, and a dynamic model including mathematical equations of material balance, energy balance and reaction kinetic model was developed. The model was applied to the dynamic simulation of the start-up process of metallocene polyethylene fluidized bed reactor. The change of several variables over time during the start-up process were obtained and analyzed, which included polymer production rate, concentration of components in the reactor, conversion of monomer or comonomer, polymer average molecular weight, copolymer composition, temperature and pressure of the reactor.Thirdly, the method of computational fluid dynamics was taken advantage of to study the flow pattern of the gas and solid phases inside the metallocene polyethylene fluidized bed reactor. A CFD model based on Eulerian-Eulerian Two-Fluids Equation was developed by introducing the Kinetic Theory of Granular Flow(KTGF) and the drag force model to close the model equations. The rationality of the model was validated by comparing the simulation result with the calculation result from the classical formula regarding fluidized bed pressure drop and minimum fluidization velocity in the literature. The effect of solid phase boundary conditions, restitution coefficient, solid particle size and operational gas velocity on the fluidization was discussed and analyzed in the end.