Crystallization 1 Batch Operation

The design and operation of industrial crystallizers is where developments in the laboratory are confirmed and their practical significance determined. In recent years, crystallization processes involving specialty chemicals and pharmaceuticals have increased. This has led increased interest in batch crystallization operation, optimization and desigrt At the same time, the advent of powerful computers and their routine avaUabilily has stimulated interest in the area of on-line control of crystallization process (both batch and continuous). Progress in batch crystallization is surrunarized in a number of recent papers and reviews 173-801. In this section I will discuss two areas which I think will have an impact in the next decade. 

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... 

Classification of batch crystallizers and batch crystallization operations according to the means by which supersaturation is created is still a widely accepted method. Therefore, the discussion of such operations may include cooling crystallization, evaporative crystallization, vacuum crystallization, antisolvent crystallization, reaction (reactive) crystallization, etc. The vacuum crystallization operation can be considered as a combination of the evaporative and cooling crystallization and thus will not be discussed separately. Reaction crystallization (precipitation) is discussed in detail in Chapter 6. 

Several features of secondary nucleation make it more important than primary nucleation in industrial crystallizers. First, continuous crystallizers and seeded batch crystallizers have crystals in the magma that can participate in secondary nucleation mechanisms. Second, the requirements for the mechanisms of secondary nucleation to be operative are fulfilled easily in most industrial crystallizers. Finally, low supersaturation can support secondary nucleation but not primary nucleation, and most crystallizers are operated in a low supersaturation regime that improves yield and enhances product purity and crystal morphology. 

Batch Crystallization. Crystal size distributions obtained from batch crystallizers are affected by the mode used to generate supersaturation and the rate at which supersaturation is generated. For example, in a cooling mode there are several avenues that can be followed in reducing the temperature of the batch system, and the same can be said for the generation of supersaturation by evaporation or by addition of a nonsolvent or precipitant. The complexity of a batch operation can be ihustrated by considering the summaries of seeded and unseeded operations shown in Figure 19. 

In all such laboratory studies, plant conditions and compositions should be employed as far as possible. Agglomeration rates tend to increase with the level of supersaturation, suspension density and particle size (each of which will, of course, be related but the effects may exhibit maxima). Thus, agglomeration may often be reduced by operation at low levels of supersaturation e.g. by controlled operation of a batch crystallization or precipitation, and the prudent use of seeding. Agglomeration is generally more predominant in precipitation in which supersaturation levels are often very high rather than in crystallization in which the supersaturation levels are comparatively low. 

Batch crystallizers are widely used in the chemical and allied industries, solar saltpans of ancient China being perhaps the earliest recorded examples. Nowadays, they still comprise relatively simple vessels, but are usually (though not always) provided with some means of agitation and often have artificial aids to heat exchange or evaporation. Batch crystallizers are generally quite labour intensive so are preferred for production rates of up to say 10 000 tonnes per year, above which continuous operation often becomes more favourable. Nevertheless, batch crystallizers are very commonly the vessel of choice or availability in such duties as the manufacture of fine chemicals, pharmaceutical components and speciality products. 

The concept of programmed operation can also be applied to other types of batch crystallization e.g. precipitation via drowning-out with miscible solvents (Jones and Teodossiev, 1988). 

Mathews and Rawlings (1998) successfully applied model-based control using solids hold-up and liquid density measurements to control the filtrability of a photochemical product. Togkalidou etal. (2001) report results of a factorial design approach to investigate relative effects of operating conditions on the filtration resistance of slurry produced in a semi-continuous batch crystallizer using various empirical chemometric methods. This method is proposed as an alternative approach to the development of first principle mathematical models of crystallization for application to non-ideal crystals shapes such as needles found in many pharmaceutical crystals. 

Heffels, S.K., de Jong, E.J. and Nienoord, M., 1994. Improved operation and control of batch crystallizers. In Particle design via crystallization, American Institute of Chemical Engineers Symposium Series, 87(284), 170-181. 

Roliani, S. and Bourne, J., 1990a. A simplified approach to the operation of a batch crystallizer. Canadian Journal of Chemical Engineering, 45, 3457-3466. 

Wayne Genck (7 8) has recently published several useful articles about batch crystallization. Often lab filtration after crystallization is done with a thin cake and no problem is observed. But when taken to the plant, this operation takes days to build and wash a cake. To avoid this problem it is best to operate a crystallizer that is properly seeded and cooled according to a profile that follows the equation in reference (7), slow at first and fastest at the end. The other reference (8) discusses the challenges without seeding. Experience by the author confirms that a large amount of seed crystal is required, about 1-2 % wt of the final crystal yield. 

Phenomena, methods of operation, etc. have been studied extensively for the use of crystallization in separation processes. Although much remains to be learned about such processes, relatively little attention has been given to the other functions and the purpose of this work was to examine the role of various process variables in determining the purity of crystals recovered from a batch crystallizer. The system studied experimentally was a model system of amino acids, and the key variables were the composition of the liquor from which a key amino acid was crystallized, the rate at which supersaturation was generated by addition of an acid solution to reduce solubility, and the degree of mixing within the batch unit. 

In this section, a brief description of the necessary experiments to identify the kinetic parameters of a seeded naphthalene-toluene batch crystallization system is presented. Details about the experimental apparatus and procedure are given by Witkowski (12). Operating conditions are selected so that the supersaturation level is kept within the metastable region to prevent homogeneous nucleation. To enhance the probability of secondary nucleation, sieved naphthalene seed particles are introduced into the system at time zero. 

Example 16.5. Teflon heat transfer tubes that are thin enough to flex under the influence of circulating liquid cause a continual descaling that maintains good heat transfer consistently, 20-65 Btu/(hr)(sqft)(°F). Circulating types such as Figures (d) and (e) of ten are operated in batch mode, the former under vacuum if needed. High labor costs keep application of batch crystallizers to small or specialty production. 

Crystallization equipment can vary in sophistication from a simple stirred tank to a complicated multiphase column, and the protocol can range in complexity from simply allowing a vat of liquor to cool to the careful manipulation required of batch cyclic operations. In principle, the objectives of these systems are the same to produce a product meeting specifications on quality at an economical yield. This section will examine some of the considerations that go into the selection of a crystallizer so as to meet these objectives. 

One of the first decisions that must be made is whether the crystallizer operation is to be batch or continuous. In general, the advantages of each type of operation should be weighed in choosing one over the other, but more often the decision rests on whether the other parts of the process are batch or continuous. If they are batch, then it is likely that the crystallizer also should be batch. 

Batch crystallizers can be used in a campaign to produce a particular product and in a second campaign to produce another product. Generally, it is not possible to operate continuous processes in this way. Batch crystallizers can handle viscous or toxic systems more easily than can continuous systems, and interruption of batch operations for periodic maintenence is less difficult than dealing with interruptions in continuous processes. The latter factor may be especially important in biological processes that require frequent sterilization of equipment. Batch crystallizers can produce a narrow crystal size distribution, whereas special processing features are required to narrow the distribu-... 

It may be easier to operate a continuous system so that it reproduces a particular crystal size distribution than it is do reproduce crystal characteristics from a batch unit. Moreover, the coupling of several transient variables and nucleation make it difficult to model and control the operation of a batch crystallizer. 

The above equations can be applied to any batch crystallization process, regardless of the mode by which supersaturation is generated. For example, suppose a model is needed to guide the operation of a seeded batch crystallizer so that solvent is evaporated at a rate that gives... 

It is clear that stringent control of batch crystallizers is critical to obtaining a desired crystal size distribution. It is also obvious that the development of a strategy for generating supersaturation can be aided by the types of modeling illustrated above. However, the initial conditions in the models were based on properties of seed crystals added to the crystallizer. In operations without seeding, initial conditions are determined from a model of primary nucleation. 

Suppose that the batch crystallizer is seeded with a mass of crystals with a uniform size of Lseed. The number of seed crystals is Nseed, and, as the operation is to be free from nucleation, the number of crystals in the system remains the same as the number of seed crystals. The initial values of total crystal length, total crystal surface area, total... 

The flow scheme of the process(17) is represented in Fig. 2. The required throughput rate of 5 kg Pu/day is obtained in a batch-type operation, where a 5 to 10% substoichometric oxalate precipitation is performed by adding solid oxalic acid to a 3 M HNO - 100 g/L Pu(NO3) 4 solution at 80°C in about 2 hours. Up to 95% of the Pu is precipitated as uniform crystals of 20 yum average size and filtered. After washing and calcination, the average analysis of this product shows less than 1000 ppm total metallic impurities. When evaporating the filtrate to about 5% of its original volume, nitric acid is recovered, and most of the oxalic acid is destroyed. This results from sump temperatures of up to 123°C and the presence of Pu(VI). 

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