Model for Diffusion Limited Crystal Growth with and without Growth Rate Dispersion

Abstract

We show, in this report, how a population balance model differential equation describing batch crystal growth from solution can be solved in closed form for the case of diffusion limited growth with and without modeling the effects of growth rate dispersion. By letting the growth rate diffusivity be directly proportional to the supersaturation, a closed form solution can be found for the case of a separable distribution function of crystal sizes. This result requires that the ratio of the solute diffusion coefficient to the growth rate diffusivity coefficient, be restricted to odd integer values. This implies that when growth rate dispersion is present, varying the conditions of the solution can only lead to one or more equilibrium size distributions out of an infinite discrete set of possible size distribution functions. We deal with this restriction by letting the growth rate diffusivity coefficient be restricted to a spectrum of discrete values. Results are compared with experimental data for two cases of lactose crystal growth from solution, and one for sucrose, where it was suggested that there are two distinct types of kinetic behavior in the growing crystal ensemble; one of slow growing crystals and the other fast growing. Our model is able to describe the equilibrium distribution of these crystals as being composed of a combination of two size distribution functions, each with its own uniquely valued growth rate diffusivity coefficient.