Gassing Crystallization at Different Scales: Potential to Control Nucleation and Product Properties

Gassing crystallization is an induced nucleation process during batch cooling crystallization with the aim to control the nucleation step and thus product crystal properties. All previous studies have been made at lab scale and show that the metastable zone or the supersaturation at which gassing is started is crucial for the success of gassing crystallization. Since the metastable zone width depends on many factors, the purpose of this paper was to verify the hypothesis that, especially for parameter combinations which result in broad metastable zone widths, the success of gassing crystallization is independent of crystallizer scale and geometry. The studies were made for the substance system succinic acid/water in a 1 L lab and a 30 L pilot scale crystallizer. The effect of gassing on the metastable zone width and the median diameter was evaluated for varying process parameters (saturation concentration, gassing supersaturation, cooling rate, and stirrer speed) and compared to normal cooling crystallization. After the application of gassing, metastable zone widths were narrower, median diameters were bigger, and reproducibility was enhanced. We found that for process parameters which resulted in broad metastable zone widths the effect of gassing on the median diameter was largest, independent of crystallizer scale and geometry. Gassing crystallization induces nucleation and affects product crystal properties, which works best for process conditions resulting in broad metastable zone widths.