The growth and dissolution of potassium sulphate crystals in a fluidized bed crystallizer

The nucleation, growth and dissolution characteristics of potassium sulphate have been studied in a laboratory scale fluidized bed crystallizer under carefully controlled conditions of temperature, supersaturation and solution velocity. Aqueous solutions of potassium sulphate can withstand high degrees of supercooling (∼ 13°C) even when agitated, but this is reduced to about 5°C when the crystalline phase is present. Nevertheless, this is still a high level of supercooling for an aqueous salt solution. The importance of measuring nucleation data for crystallizer design purposes under the appropriate conditions is clearly demonstrated. The ‘order’ of the nucleation process, m, defined by Rn = KnΔcm is about 9. The growth process, defined by Rg = KgΔcn is second‐order (n = 2) with an activation energy of 4.3 kcal/mole, and the dissolution process, Rd = KdΔcp, in first‐order (p = 1) with an activation energy of 3.4 kcal/mole. The growth step is therefore considered to be diffusion controlled, and this is confirmed by the observation that the growth rate of potassium sulphate in a fluidized bed is dependent on the size of the crystals.