The role of diffusional resistance on crystal growth: Interpretation of dissolution and growth rate data

Experimental results on crystal growth and dissolutions kinetics were taken from literature to test two theoretical models describing the crystal growth from solutions. One of the models, designated by two-step model (TSM), is being used for many years to describe the relation between crystal growth rates with supersaturation, temperature and hydrodynamic conditions. The other model was presented in a previous work and will be called concurrent-step model (CSM). The chosen literature data allowed calculating the mass transfer coefficients during crystal growth, for different hydrodynamic conditions. The obtained results are interpreted taking into consideration well-established mass transfer theories. According to the TSM, the measured crystal growth kinetics can only be explained by means of an unrealistic variation of the mass transfer coefficient with the relative crystal-solution velocity. Conversely, mass transfer coefficients obtained by the CSM were confirmed by appropriate semi-theoretical correlations, both in their order of magnitude and in their behaviour. In addition, crystal growth and dissolution experiments of sucrose were carried out at in a batch crystallizer for different agitation speeds. The resulting kinetics are used to test the CSM in a system that is significantly different from the inorganic salts used in the analysed literature works. As predicted by this model, the existence of an adsorbed layer in the crystal surrounding is likely to have affected the solute molecular diffusivity in the medium. Based on this premise, the results obtained with sucrose are well described by the CSM.