SOLVENT EFFECTS IN CRYSTALLISATION PROCESSES

摘要

The classical nucleation theory and theories on crystal growth rate indicate good correlation between the solubility and nucleation and growth kinetics. The goal of this study was, for widely different solvents, to work out correlations; first between the solubility and crystallisation kinetic parameters as well as the mean crystal size, and second between the kinetic parameters and the mean crystal size. Two organic substances, benzil and 3-nitro-1,2,4-triazol-5-one (NTO), were chosen as model substances. As crystallisation kinetic parameters the primary and secondary (seeded) metastable zone widths and the growth rates were measured. The growth rates were measured both at a fixed supersaturation and at a fixed fraction of the measured secondary metastable zone width. The mean crystal sizes from standardised recrystallisations, both seeded and unseeded operations were also measured. The correlations were described by the correlation coefficients. The results showed generally poor correlation between the solubility and the kinetic parameters as well as the mean crystal sizes. Good correlation was only obtained for groups of similar solvents, i.e. a series of alcohols. Good correlation was observed between the crystal growth rates and the mean crystal sizes. The best correlation was observed for the growth rates measured at a fixed supersaturation. Measured at a fraction of the secondary metastable zone width showed less correlation. Poor correlation was observed between any metastable zone width and the mean crystal sizes. The secondary metastable zone width, measured by cooling a solution after seeding with a few crystals, only poorly described the secondary nucleation kinetics. Solvent effects, primary metastable zone width, secondary metastable zone width, crystal growth rate, crystal size.