Thiourea Nucleation\nBehavior Investigation Based on\nMetastable Zone Width and Applied for Continuous Crystallization Designing

Batch crystallization of thiourea has been industrially\nemployed\nfor decades. However, studies of the continuous crystallization and\nnucleation mechanism of industrial thiourea, especially in the presence\nof synthesis-related impurities, remain scarce. Herein, the nucleation\nmechanism of industrial thiourea was investigated under varying conditions.\nResults revealed that the nucleation constant decreased with increasing\nsaturation temperature, signifying a progressive acceleration in the\nnucleation rate. Notably, when the saturation temperature surpassed\n301 K, the nucleation mechanism shifted from progressive to instantaneous.\nFurthermore, both the critical nucleation size and critical Gibbs\nfree energy decreased as the saturation temperature rose. Leveraging\nthese insights, a continuous crystallization apparatus was designed,\nwhich was experimentally validated to maintain operational stability\nand achieved a target product purity of 79 wt %, compared to 40 wt\n% from batch crystallization. These findings offer significant implications\nfor optimizing the crystallization process of industrial thiourea.