Abstract
The onset of nucleation of salicylamide in organic solvents has been measured for a total of 2911 nucleation experiments at a constant cooling rate. The experiments are divided into series of different experimental conditions and for each series, the mean values and distributions of the nucleation events have been recorded. Generally, a significant spread in the onset of nucleation among identical repeated experiments was observed. The solvent and the cooling rate were found to be the factors exerting the greatest influence on the onset of nucleation, whereas the expected effects of the solution volume and the saturation temperature were largely absent. A correlation is observed between the supersaturation ratio at the average onset of nucleation and a group of parameters extracted from the pre-exponential factor in the classical nucleation rate equation, which is in agreement with the concept that a higher attachment frequency facilitates nucleation. It is also found that there can be a profound difference in the metastable zone width between experiments in open and closed crystallizers.
Highlights
An undersaturated solution that is being cooled down will typically start to crystallize at a certain temperature below the saturation temperature of the solute
The metastable zone width for the nucleation of salicylamide at a constant cooling rate is shown to be fairly independent upon saturation temperature in the interval of 30–50 uC, while an increased cooling rate is shown to lead to a larger metastable zone in five common organic solvents
The average metastable zone width is found to decrease when the solvent is allowed to evaporate during the cooling crystallization compared to the case of a sealed crystallizer; in two out of five solvents this change is dramatic
Summary
An undersaturated solution that is being cooled down will typically start to crystallize at a certain temperature below the saturation temperature of the solute This gap between the point of saturation of a solution and the onset of crystallization is usually referred to as the metastable zone width, MZW. The MZW is not a thermodynamically defined property, but rather describes from a practical point of view the maximum level of supersaturation that can be operated in a process without having uncontrolled nucleation. It is commonly interpreted as the maximum level of supersaturation where primary nucleation will not dominate the process, even though the distinction between primary and secondary nucleation is not always clear-cut. From an industrial point of view, an overly narrow MZW may cause problems if a sudden temperature drop occurs in the process, e.g. as a result of transferring warm solutions between different reactors, or when performing polishing filtration
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