An examination of the crystallization of urea from supersaturated aqueous and aqueous-methanol solutions as monitored in-process using ATR FTIR spectroscopy

In-process ATR FTIR spectroscopy combined with optical turbidometry is applied to the measurement of reactant supersaturation during temperature-programmed studies of the batch crystallization of urea from aqueous and mixed aqueous/methanolic solvent systems at the 400 mL scale size (pitched blade impeller) for solute compositions in the 35-65 w/w% range. The metastable zone widths were found to be in the range 3-12°C and critical supersaturations in the 1.05-1.18 range, respectively, inversely dependent upon solute concentration. Only a slight dependency of the metastable zone width with reactor heating/cooling rates, indicative of fairly matched rates of nucleation and solubility change, is found. Cross-correlation between FTIR and turbidometic measurements reveal, as expected, the former technique to be more sensitive to the detection of the crystallization onset. No evidence was found for phase separation (oiling-out) prior to nucleation, in contrast to previous studies of citric acid (Groen, H.; Roberts, K. J. J. Phys. Chem. B 2001, 105, 10723-10730). Crystallization enthalpic release at the onset point is found to affect transient crystal dissolution and regrowth following nucleation. Good FTIR calibration, with excellent correlation to known solubility data, using a univariant IR transmittance (peak intensity) model is achieved for both single and mixed solvent systems, this despite slight variation in the IR peak positions with temperature and concentration. Attempts to use the calibration data at higher solute concentrations beyond the calibration range were not successful, this being associated with a significant underestimation of the solution supersaturation. A forward look to improvements in FTIR calibration using multivariant chemometric approaches is highlighted.