Factors Affecting Batch Crystallization

Mixing is an important factor affecting batch crystallization. On one hand, sufficient mixing is required to maintain crystals in suspension, to assure an adequate rate of energy transfer, and to achieve uniformity of suspension properties throughout the crystallizer. On the other hand, the effect of mixing on batch crystallization is largely system-dependent. 

Factors Affecting the Purity of L-Isoleucine Recovered by Batch Crystallization... 

Batch crystallization factors affecting purity of recovered crystals, 85-99... 

The quality, productivity, and batch-to-batch consistency of the final crystal product can be affected by the conditions of the batch crystallizer. Several factors considered here include batch cycle time, supersaturation profile, external seeding, fouling control, CSD control, growth rate dispersions, and mixing. 

The supersaturation profile in a batch crystallizer has a profound effect on the nucleation and growth processes and the resulting CSD. It can also affect other factors (e.g., batch cycle time) related to the batch crystallization operation. Figure 10.13 shows schematically a supersaturation profile in a batch crystallization experiment (Nyvlt et al. 1985). At / = 0, the batch crystallizer is filled with a just-saturated solution that contains crystals with a negligible surface area. The solution begins to be supersaturated at a constant rate, and the supersaturation increases until it reaches the limit of the metastable zone (Acmoi)- At this point, nucleation... 

Another key variable in batch cooling is seeding. The difficulty is in determining the seed point, which is ideally when the batch temperature first crosses the saturation curve. However, this temperature can be affected by batch-to-batch variations in several factors, including the actual concentration of the material to be crystallized, as well as by impurities that can affect the solubility. If the seed is added at a temperature above the solubility temperature, some or all of it can dissolve, resulting in uncontrolled nucleation. If the seed is added at a temperature too far below saturation, the product may have already nucleated. In either case, the increase in nucleation could result in a decrease in impurity rejection and/or a change in particle size distribution and other physical attributes. 

The critical mixing factors in a stirred tank at e impeller speed and type, as well as their influence on local turbulence and overall circulation. Since all aspects of these factors cannot be maintained constant on scale-up either locally or globally, the extent to which changes in the crystallizing environment will affect nucleation is extremely difficult to predict. To the mixing issue must be added the uncertainties caused by soluble and insoluble impurities that may be present in sufficiently different concentrations from batch to batch to cause variation in induction time, nucleation rate, and particle size. 

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