Another possibility is impurity occlusion, which is caused by either imperfect removal of mother liquor from the solids or adsorption of impurities on the crystal surface. When the crystals are dried, only the solvent is removed and the remaining impurities adhere to the surface the crystals, thus decreasing the purity of the final product. Proper washing and deliquoring is the best solution to minimize this problem for two reasons. First, the “dirty” mother liquor is removed and replaced with a clean solvent, so less impurity goes to the drying step. Second, the introduction of a fresh solvent leads to partial dissolution of the outermost layer of the crystals, thus removing adsorbed impurities on the crystal surface. Washing and deliquoring performance is affected by many factors, such as cake porosity and permeability, liquid viscosity, etc. . The choice of filter type often dictates the extent of possible deliquoring and washing. For example, a rotary vacuum filter has limited area for washing and deliquoring, making it unsuitable for reducing liquid content to very low level. A belt filter may be more appropriate, because it can be designed to have a larger area dedicated to washing.
If the impurity occlusion is due to adsorption, the problem can be minimized by producing larger crystals and, thus, lower surface area per unit mass. Larger particles can normally be obtained by reducing supersaturation, because nucleation rate would be slower, thus favoring growth of existing particles rather than formation of new small particles. An alternative solution is to decrease the impurity concentration in the mother liquor, such as by adjusting the purge ratio or the upstream reaction operating conditions.
Pinpointing the culprit
It is impossible to distinguish whether a problem is caused by SLE or kinetics just by observing the symptoms. However, it is often possible to single out the cause after a minimum of targeted testing. The first step is to study the SLE phase behavior. At this stage, obtaining a rough idea on the phase diagram frequently suffices. For example, it may be enough to calculate the diagram using the solubility equation  and melting point and heat of fusion data from Differential Scanning Calorimetry (DSC) analysis. Experimental data such as solubility and location of double saturation points are then gathered for verification, especially near the boundary, because it is the most important feature to identify.
Several techniques are available for determining impurity inclusion or occlusion. Crystals can be analyzed for impurity profile by partially dissolving a sample to different extents. If the impurities are found to penetrate deep inside the crystals, then inclusion is likely. A concentration profile that is roughly proportional to the surface area (diameter squared) may indicate adsorption during crystallization. On the other hand, if the impurities are mainly on the outermost layer, it is likely that occlusion is the key phenomenon. When it is difficult to distinguish between the different mechanisms, it is best to try different remedies based on the possible scenarios.
Build on a solid foundation
Understanding the root of the problem is essential in troubleshooting a crystallization process, because the success in finding the most appropriate solution depends on the ability to identify the underlying phenomenon that causes the symptoms. When product fails to meet purity requirements, crystallization, inclusion and occlusion of impurities are the three mechanisms to be considered. By studying the SLE behavior and testing the impurity profile, it should be possible to identify the responsible mechanism and take a suitable corrective action without much trial and error.
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Christianto Wibowo is a senior engineer at ClearWaterBay Technology, Inc., Walnut, Calif. E-mail him at firstname.lastname@example.org.