Narrow CSD

Batch size based on cycle time fill, crystallize, dump, clean. Use for production capacity of <0.15 kg/s and where target is relatively uniform and narrow CSD although reproducibihty may be difficult between batches. Batch gives narrower CSD than continuous. 

Continuous CPR with classified lines and classified product removal gives increased crystal size and narrow CSD. Multistaging in series gives larger crystals and narrow CSD. 

Although most of the industrial crystallizers described in the literature (e.g., Bamforth 1965 Bennett 1984 Moyers and Rousseau 1987) are continuous crystallizers, they often can be operated in a batch mode. For capacity requirements less than 500kg/h, batch crystallization is usually more economically advantageous. Furthermore, if the product requires a relatively narrow CSD, a batch crystallizer clearly has the advantage. Several industrial batch crystallizers are deseribed here. 

CSD control in batch crystallization can provide a significant improvement in product quality and features and in downstream processing efficiency and economics. Two important technologies considered here include fines destruction and preparation of narrow CSD. 

Preparation of Narrow CSD. In batch crystallization, the withdrawal of crystal product is made only at the end of the batch... 

In general, in order to obtain a narrow crystal size distribution (CSD), undesired nucleation should be avoided. In batch crystallization, the use of an optimal quantity of seed crystals of an optimal size may be the way to obtain a narrow CSD. According to Kohl et al. [8], the metastable zone of organic compound systems can be quite wide. Therefore, primary crystallization occurs only at a very high level of supersaturation. 

Furthermore, Jagadesh et al. [18,19] have studied seeded batch-cooling crystallization without temperature control for aqueous potassium alum, and potassium sulfate solutions. In their approach, the solution was cooled according to a natural profile. They succeeded in attaining a monodispersed and relatively narrow CSD by controlling the level of supersaturation in such a way that the existing seed crystals grew and no nucleation occurred. 

The objectives in the operation of a crystallization process are to meet the product specification, i.e., a narrow CSD, maximum crystal purity, high yield, and an acceptable crystal morphology. Moreover, the manufacturer s requirements for economic and trouble-free operation should be met. The CSD and crystal morphology are important parameters for downstream operations of crystallization such as filtration and drying. 

Classified product removal (CPR) is frequently adopted in crystallizer practice, e.g. with an elutriator or hydrocyclone. As Randolph (1970) has pointed out, however, although this practice may produce a narrower CSD it also leads to a smaller product mean size Figure 9.9). The cases of mixed and classified product removal are compared on the population density plot in Figure 9.9a and the CSD implications are shown in Figure 9.9b. 

Usually, batch crystallization is used when a relatively low production capacity is required, e.g., below 50 t of product per day. When batch crystallization is equipped with the proper temperature control and seeding system, the crystallization conditions can be adjusted in such a way that the residence times of the crystals, of various sizes, can be kept about the same. Therefore, the CSD can be narrower in batch crystallization than in continuous crystallization, which is one of the significant differences between batch and continuous crystallization without fine removal or a classification method for the product. In practice, industrial continuous crystallization processes contain fines-removal or classification units, such as hydrocyclones, in order to produce crystals of a narrow CSD. 

In the FB crystallizer (Figure 51.9), the heated mother liquor first goes to the evaporator section where supersaturation takes place. The supersaturated solution then directly enters into the fluid bed of product crystals where the desupersaturation occurs and the crystals grow. This equipment is known for its capacity to produce large crystals of a reasonably narrow CSD. Nevertheless, FB crystallizers are not as commonly used as would be expected on the basis of the advantages of the large product size. The disadvantage of FB crystallization is, for some... 

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