Prediction of particle size distribution in suspension polymerization reactors: effect of turbulence nonhomogeneity

The quantitative description of particle size distribution development in suspension polymerization reactors is very complex. The exact mechanisms of breakage and coalescence/aggregation of the polymerizing drops are generally not very well understood, and are closely related and controlled by the spectrum of turbulent energy dissipation rate in the reactor. In the present investigation, a two-compartment population balance model was developed for taking into account the large spatial variations of the local turbulent kinetic energy, in order to predict the evolution of droplet sizes in a high holdup (i.e., 47-50 vol%) suspension polymerization system as a function of the most important process conditions, such as type of suspending agent, monomer/water-phase ratio, polymerization temperature, quality of agitation, and evolution of the dispersed-phase density, interfacial tension and viscoelasticity during the polymerization. Phenomenological expressions of the literature were modified for drops in the viscous dissipation range and were applied for describing the breakage and coalescence rates of the polymerizing dispersed phase as a function of the basic hydrodynamics and evolving physical properties of the system. Computational fluid dynamics simulations were used for estimating the volume ratio of the impeller and circulation regions, the ratio of turbulent dissipation rates and the exchange flow rate of the two compartments at different agitation rates and continuous-phase viscosities. The theoretical model can predict reasonably well the experimentally observed inhomogeneities of the drop size distribution as well as the evolution of particle size distribution in VCM suspension polymerization, especially considering the various assumptions in formulating the drop breakage and coalescence rates and in the two-compartment approximation of the inhomogeneities of the turbulent flow field in the suspension polymerization reactor. (C) 2000 Elsevier Science Ltd. All rights reserved.