Batch crystallizer

Bastnasite [12172-82-6] Bastnasite [68909-13-7] Batch crystallization Batch dyeing paper Batch esterification BATCHFRAC Batch furnaces Batch injection analy ... 

Crystallization batches range from 30,000 to 60,000 Hters for each pan. Continuous centrifugals are typically used for second, third, and affination steps continuous vacuum pans are less common but are used in the U.S. for intermediate strikes. Most horizontal batch crystallizers have been replaced by continuous units, and all are designed for controlled cooling of the massecuite to maintain supersaturation. 

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. 

Fig. 20. CSD characteristics from batch crystallization without seeding.

The advantages of selective removal of fines from a batch crystallizer have been demonstrated (66,67). These experimental programs showed narrowiag of crystal size distributions and suggest significant reductions ia the fraction of a product that would consist of fines or undersize material. 

Cooling Ciystalli rs use a heat sink to remove both sensible heat from the feed stream and the heat of crystallization released as crystals are formed. The heat sink may be no more than the ambient surroundings of a batch crystallizer, or it may be cooling water or another process stream. 

Batch Crystallization Batch crystalhzation has been practiced longer than any other form of ciystaUization in both atmospheric tanks, which are either static or agitated, as well as in vacuum or pressure vessels. It is still widely practiced in the pharmaceutical and fine chemical industry or in those applications where the capacity is veiy small. The integrity of the batch with respect to composition and history can be maintained easily and the inventoiy management is more precise than with continuous processes. Batch ciystalhzers can be left unattended (overnight) if necessary and this is an important advantage for many small producers. 

Several authors have presented methods for the simultaneous estimation of crystal growth and nucleation kinetics from batch crystallizations. In an early study, Bransom and Dunning (1949) derived a crystal population balance to analyse batch CSD for growth and nucleation kinetics. Misra and White (1971), Ness and White (1976) and McNeil etal. (1978) applied the population balance to obtain both nucleation and crystal growth rates from the measurement of crystal size distributions during a batch experiment. In a refinement, Tavare and... 

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. 

It was shown in Chapter 3 that supersaturation, or concentration driving force, is essential for any crystallization. In a batch crystallizer supersaturation can be generated in several ways, either solely or in combination ... 

As mentioned above batch crystallizers are usually simple vessels provided with some means of mechanical agitation or particulate fluidization. These have the effect of reducing temperature and concentration gradients, and maintain crystals in suspension. Baffles may be added to improve mixing and heat exchange or vacuum systems may be added, as appropriate. Various design combinations are available and some are illustrated in Figure 7.1. 

Thus for batch crystallizations the mass yields and heat loads are given by ... 

Therefore, if the solubility data for a substance are known, it is a simple matter to calculate the potential yield of pure crystals that could be obtained from batch crystallization (equations 7.4 and 7.6). Conversely, the degree of evaporation to produce a specified yield may be estimated (equation 7.8). 

A theoretical analysis of an idealized seeded batch crystallization by McCabe (1929a) lead to what is now known as the AL law . The analysis was based on the following assumptions (a) all crystals have the same shape (b) they grown invariantly, i.e. the growth rate is independent of crystal size (c) supersaturation is constant throughout the crystallizer (d) no nucleation occurs (e) no size classification occurs and (f) the relative velocity between crystals and liquor remains constant. 

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. 

Batch crystallizers are widely used for relatively small tonnage (say <10 kTe/yr) dyestuffs, fine chemicals, pharmaceuticals and other speciality chemicals. The... 

Bohlin, M. and Rasmuson, A.C., 1992. Application of controlled cooling and seeding in batch crystallization. Canadian Journal of Chemical Engineering, 70, 120-126. 

Brown, D.J. and Boyson F., 1987. Modelling of fluid flow in a batch crystallizer. In Industrial Crystallization 87. Eds. J. Nyvet, S. Zacek, Bechyne, Czechoslovakia, September 1987. Academia Prague and Elsevier, 1989, pp. 547-550. 

Chang, C.-T. and Epstein, M.A.F., 1982. Identification of batch crystallization control strategies using characteristic curves. American Institute of Chemical Engineers Symposium Series, 78(215), 68-75. 

Chianese, A., Di Berardino, F. and Jones, A.G., 1993. On the effect of crystal breakage on the fine crystal distribution from a seeded batch crystallizer. Chemical Engineering Science, 48, 551-560. 

Farrell, R.J. and Yen-Cheng Tsai, 1994. Nonlinear controller for batch crystallization Development and experimental demonstration. In American Institute of Chemical Engineers National meeting. Atlanta, Paper 89e. 

Girolami, M.W. and Rousseau, R.W., 1985. Initial breeding in seeded batch crystallizers. Industrial and Engineering Chemistry Research, 25, 66-70. 

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. 

Jones, A.G., 1984. The Design of Well-Mixed Batch Crystallizers. SPS DR17. (Harwell/ Warren Spring Separation Processes Service), 40pp. 

Jones, A.G., Budz, I. and Mullin, I.W., 1987. Batch crystallization and solid-liquid separation of potassium sulphate. Chemical Engineering Science, 42, 619-629. 

Jones, A.G., Chianese, A., 1988. Fines destruction during batch crystallization. Chemical Engineering Communications, 62, 5-16. 

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