CFD Modeling of a Horizontal Plug Flow Reactor for HIPS Production

I.L. GAMBA

Houston

Encuentro; TOTAL VI Meeting Houston April 2011; 2011

TOTAL Petrochemiclas USA

High-Impact Polystyrene (HIPS) is a composite material produced by polymerizing styrene in the presence of a rubber. Final product is composed by a matrix of free polystyrene with dispersed rubber particles. In turn, these particles are themselves heterogeneous and contain occluded free PS. Improvement of the HIPS production process is sought to better meet quality product requirements as well as to reduce cost. As far as complex and intricate physical phenomena are concerned, how and what precisely happens in the polymerization reactors remains uncertain. By integrating each chemical and physical phenomenon, Computational Fluid Dynamics (CFD) analyses can help in improving the understanding of the process that leads to rubber particle formation, which is critical in controlling HIPS properties. An extensive literature search shows that CFD has been used only to study the hydrodynamics of the reactors in isothermal condition and without polymerization. Nevertheless, a CFD model can integrate fluid dynamics together with kinetics of polymerization, energy and mass balances, exothermicity, reactor cooling, etc into the simulation of an industrial reactor. Therefore it could bring insight in the influence of hydrodynamics on the product quality. The main objective of this project is to develop a model for the continuous prepolymerization reactor that considers the hydrodynamics and kinetic phenomena that could help to better understand what happens inside the reactor. This tool could be used to determine the appropriate operating conditions to obtain an a priori-specified-characteristics product. The first phase of the project included two major steps. As a first step, the hydrodynamics of a homogeneous Continuous Stirred Tank Reactor (CSTR) was studied using CFD. A three dimensional model of the stirred tank was built and simulated with the aid of the CFD commercial package Fluent. From the complete velocity distribution of the reactor obtained, an advection-diffusion equation for a tracer was solved to estimate the RTD. CFD results were validated with experimental data. The second step was the development of a 0-dimensional polymerization model that is used to predict global variables, molecular structure and physicochemical properties. Although this mathematical model was not coupled with the CFD model, it was used to calculate the evolution of the variables (i.e. conversion, viscosity, molecular weight) along each residence time, obtaining variables time distributions. The second phase of the project consists of developing a model that simulates a Horizontal Plug Flow Reactor (HPFR) under polymerization conditions using CFD. That is, incorporate the kinetics into the CFD model. The HPFR consists of several stages. However, as the geometry and the expected pattern of the flow solution have a periodically repeating nature, only a single representative module of the tubular reactor has been built using periodic boundary conditions. In this way, the overall mesh size is reduced together with the computational cost.