DYNAMIC MODELING OF SUSPENSION CRYSTALLIZERS, USING EXPERIMENTAL-DATA

A model for the dynamics of a single-stage suspension crystallizer is developed which serves as a basis for process analysis and the design of controllers. A population balance for the dynamics of the crystal size distribution (CSD) with mass and heat balances is described as well as empirical relations for the separation efficiency of classified particle removal systems, the initial CSD, and the crystallization kinetics. A continuous pilot crystallizer is used that is equipped with a separator and dissolver for fine crystals and a CSD sensor based on forward light scattering. The process and sensor are modeled separately. The sensor model is based on Fraunhofer light scattering theory assuming rectangular-shaped particles. CSD dynamics data are obtained from startup experiments with the pilot plant at different process conditions. Experimental process data show a strong effect of fines and the slurry retention time on the CSD dynamics, A nonlinear parameter estimation procedure determines the empirical parameters directly from raw sensor data, The model fits accurately to the measured data. Evidence is found for the existence of a population of slow growing crystals with a growth rate approximately ten times lower than the fast growing crystals. A strong correlation is found between the total surface area of crystals with a size larger than 600 mu m and the nucleation rate.