Modeling of a CSTR for HIPS Production: Prediction of RTD with CFD

I.L. GAMBA

Houston

Encuentro; TOTAL VI Meeting Houston April 2010; 2010

TOTAL American Services

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. The industrial HIPS process involves 4 stages: dissolution, prepolymerization, finishing, and devolatilization. The prepolymerization is carried out in a series of CSTRs and during this stage the morphology is developed. In this step the fluid-dynamic phenomena is vital since it affects the particle morphology and therefore the material final properties. The main objective of this work is to develop a model for the continuous prepolymerization reactor that considers the fluid-dynamics and kinetic phenomena. This tool that could be used to optimize and select reactors, impellers and reaction conditions for the production of a priori specified grades of HIPS. As a first step in this project, the fluid-dynamics of a homogeneous CSTR was studied using CFD. A model of the stirred tank was constructed in Fluent’s geometry and grid generation tool using a rotating mesh in the region of the impeller and a fixed mesh elsewhere. From the complete velocity distribution of the reactor, an advection-diffusion equation for a tracer was solved to estimate the RTD of the reactor. Particle tracking method was used too but with not good results. The second step was a 0-dimensional polymerization model that is used to predict global variables, molecular structure and physicochemical properties. This mathematical model was used to calculate the evolution of the variables (i.e. conversion, viscosity, molecular weight) along each residence time, obtaining variables time distributions. In order to validate CFD results, we are developing an experiment to measure the RTD of a laboratory CSTR with a stimulus-response tracer method, operating at different feed flow rates and impeller rpms. The objective is to compare these measurements with the predictions for the RTD using CFD coupled with a dynamic two-species, fluid and tracer, mass balance operating with a step change of the concentration of tracer in the feed.