Modeling The Multizone Circulating Reactor For Propylene Polymerization

The multizone circulating reactor technology developed by Besell is a brand new process technology for polypropylene production. This article will present a mathematical model for the reactor based on the investigation of the fluid hydrodynamics.First of all, the fluid hydrodynamics of the circulating fluidized bed was investigated. The effects of superficial gas velocity U_g and solid-circulation flux G_s on axial and radial distribution of the porosity were investigated. Results indicated that the axial distribution of Solid concentration through the bed is nonuniform with the higher concentration in the bottom section and lower concentration in the top section. As for the radial distribution, the solid concentration in the center part is much lower than it near the wall. The porosity increases with increasing gas velocity, whereas decreases with increasing solid-circulation flux. The U_g and G_s will have smaller influence on the solid concentration with the higher bed-height.Based on the investigation of the fluid hydrodynamics, the kinetic mechanism of propylene polymerization was combined to develop a model for the polypropylene production in the multizone circulating reactor. One and half dimension compartment model was used to simulate propylene polymerization in the reactor. Meanwhile a parameter sensibility analysis was also shown to present how each operating and design parameter, such as reactor size, the proportion of monomer to hydrogen, gas velocity, porosity and circulating-ratio, can change the reactor behavior and polymer characteristic. Results indicated that the operating and design parameters affect the product molecular weight through changing the active polymer concentration and the resident time of the solid in the reactor, and affect the molecular weight distribution through setting the proportion of the polymers with different properties which were produced in the different environments, called riser and downer. As a result, the most sensitive parameter to the molecular weight is the proportion of monomer to hydrogen, and the most sensitive parameter to the molecular weight distribution is the diameter and porosity of the riser. Bimodal curve was found when the amount of polymer produced in the riser equals to the downer.Based on the homopolymerization simulating result, a new concept of pilot-plant for copolymerization with propylene and ethylene was presented. The principle ofselecting the technological conditions was established.