Reaction calorimetric analysis of batch cooling crystallization processes: studies of urea in supersaturated water–methanol solutions

Abstract

A thermal energy balance model is developed for the monitoring of batch cooling crystallization processes and applied to the analysis of the crystallization of urea from an 80% methanol and 20% water solution using a 2-litre batch reaction calorimeter operating over a range of cooling rate from 0.1 °C min⁻¹ to 0.5 °C min⁻¹. Detailed calibrations and measurements to determine the parameters needed for the energy and material balance calculation of crystallization reveal the overall heat transfer coefficient, the solution heat capacity, the solubility, the metastable zone width, and the molar enthalpy of crystallization. The instantaneous enthalpy evolved as a result of nucleation and subsequent crystal growth are determined via applying the energy balance model. The rates of the nucleation and crystal growth and the solution supersaturation during the crystallization processes are subsequently calculated. For a typical crystallization process at 0.25 °C min⁻¹ cooling rate, the peak value of crystallization rate mainly due to the nucleation was approximately 30 g min⁻¹, and thereafter the growth rate slowly decreased from 7 g min⁻¹ and 3 g min⁻¹ following the change of solution supersaturation. The influence of seeding is also analyzed and the generic appliables of this approach is discussed.