(13c) Impurity Management in Integrated Crystallization Systems

Crystallization is a widely used technique for separation and purification, partly because it is thermodynamically capable of producing pure products. However, impurities may be trapped inside the crystals due to fast growth, adsorbed on crystal surface, or precipitated out from the mother liquor retained in the voids among the crystals after solid/liquid separations. To achieve the target product purity, post-crystallization steps such as filtration, washing, deliquoring, and recrystallization, and so on must be designed properly together with the crystallization process itself. This presentation shows how the problem of managing the impurity content in the product of a crystallization process should be approached in consideration of the entire train of crystallization and post-crystallization steps. A workflow is proposed, starting from the investigation of the impurity level in variation of several factors, such as growth rate, solvent concentration, particle size distribution (PSD), and the yield of crystallization. The impurity inclusion profile within the crystals is also determined, as it provides important information whether the impurity level could be minimized by controlling the size of the crystals or by partial dissolution. Next, the ways of reducing the surface impurity level by properly designing the downstream processing systems are evaluated. Empirical models are used to optimize the performance of washing and deliquoring steps so as to achieve the target product purity. In practice, washing and deliquoring are performed together with solid/liquid separation in the same unit, such as a rotary filter, a belt filter, or a centrifuge. The models can also be used for targeting equipment specifications. A case study concerning the production of Vitamin C (ascorbic acid) is discussed to illustrate the workflow.