Industrial crystallization

N. S. Tavaie, Industrial Crystalli tion Plenum Piess, New Yoik, 1995. 

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. 

Many industrial crystallizers operate in a weU-mixed or nearly weU-mixed manner, and the equations derived above can be used to describe their performance. Furthermore, the simplicity of the equations describing an MSMPR crystallizer make experimental equipment configured to meet the assumptions lea ding to equation 44 useful in determining nucleation and growth kinetics in systems of interest. 

It is emphasized that the delta L law does not apply when similar crystals are given preferential treatment based on size. It fails also when surface defects or dislocations significantly alter the growth rate of a crystal face. Nevertheless, it is a reasonably accurate generahzation for a surprising number of industrial cases. When it is, it is important because it simphfies the mathematical treatment in modeling real crystallizers and is useful in predicting crystal-size distribution in many types of industrial crystallization equipment. 

In order to treat crystallization systems both dynamically and continuously, a mathematical model has been developed which can correlate the nucleation rate to the level of supersaturation and/or the growth rate. Because the growth rate is more easily determined and because nucleation is sharply nonlinear in the regions normally encountered in industrial crystallization, it has been common to... 

TABLE 18-6 Growth Rates and Kinetic Equations for Some Industrial Crystallized Products... 

A particularly interesting feature of industrial crystallization systems is the relatively wide range of particle sizes encountered. Particle sizes range over several orders of magnitude from the sub micron (nanometers) to several millimetres or more, i.e. from colloidal to coarse . Such particles comprise a large part of the world on a human scale and a great source of industrially generated wealth. 

Cate etal. (2001) propose a method for the calculation of crystal-crystal collisions in the turbulent flow field of an industrial crystallizer. It consists of simulating the internal flow of the crystallizer as a whole and of simulating the motion of individual particles suspended in the turbulent flow in a small subdomain (box) of the crystallizer. 

Secondary nucleation is an important particle formation process in industrial crystallizers. Secondary nucleation occurs because of the presence of existing crystals. In industrial crystallizers, existing crystals in suspension induce the formation of attrition-like smaller particles and effectively enhance the nucleation rate. This process has some similarity with attrition but differs in one important respect it occurs in the presence of a supersaturated solution. 

Tailoring of the particle size of the crystals from industrial crystallizers is of significant importance for both product quality and downstream processing performance. The scientific design and operation of industrial crystallizers depends on a combination of thermodynamics - which determines whether crystals will form, particle formation kinetics - which determines how fast particle size distributions develop, and residence time distribution, which determines the capacity of the equipment used. Each of these aspects has been presented in Chapters 2, 3, 5 and 6. This chapter will show how they can be combined for application to the design and performance prediction of both batch and continuous crystallization. 

Thus, methods are now becoming available such that process systems can be designed to manufacture crystal products of desired chemical and physical properties and characteristics under optimal conditions. In this chapter, the essential features of methods for the analysis of particulate crystal formation and subsequent solid-liquid separation operations discussed in Chapters 3 and 4 will be recapitulated. The interaction between crystallization and downstream processing will be illustrated by practical examples and problems highlighted. Procedures for industrial crystallization process analysis, synthesis and optimization will then be considered and aspects of process simulation, control and sustainable manufacture reviewed. 

Rossiter and Douglas (1986) state that the first step in process design is to generate a basic structure for the flowsheet i.e. the choice of unit operations and interconnections which can be analysed, refined and costed, and then compared to alternatives. Thus, the generation of an industrial crystallization flowsheet gives rise to a number of optimization problems for which a systematic hierarchical decision process for particulate systems was proposed ... 

As mentioned above, among many possible process variables, industrial crystallization frequently focuses on the optimization of particle size. In many cases this may be fixed by market demands, in others it may be a variable e.g. during the processing of intermediates. 

In practice, industrial crystallization processes are subject to a number of constraints, which tend to limit equipment selection. For example, since particle size and purity tend to be such important variables, equipment and operating conditions that induce minimum particle breakdown or achieve maximum crystal purity are normally desirable. 

Al-Rashed, M.H. and Jones, A.G., 1999a. Validation of a CFD model of a gas-liquid precipitation system. In Industrial Crystallization 99. (Rugby Institution of Chemical Engineers). Cambridge, 13-15 September 1999. Paper 157, p. 160. 

Bamforth, A.W., 1965. Industrial crystallization. London Leonard Hill. 

Botsaris, G.D., 1976. Secondary nucleation A review. In Industrial Crystallization, Ed. J.W. Mullin. Plenum Press New York, pp. 3-22. 

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. 

Bujac, P.B. and Mullin, J.W., 1969. A rapid method for the measurement of crystal growth rates in a fluidised bed crystallizer. Symposium on Industrial Crystallization. London, 1969. Rugby Institution of Chemical Engineers, pp. 121-129. 

Donnet, M., Jongen, N., Lemaitre, J., Bowen, P. and Hofmann, H., 1999. Better control of nucleation and phase purity using a new segmented flow tubular reactor Model system Precipitation of calcium oxalate. In 14th International Symposium on Industrial Crystallization. Cambridge, U.K., September 12-16, Institution of Chemical Engineers, CD ROM, pp. 1-13. 

Garside, J., Davey, R.J. and Jones, A.G. (eds.), 1991. Advances in Industrial Crystallization. Oxford Butterworth-Heinemann, ix + 244pp. 

Gutwald, T. and Mersmann, A., 1990. Determination of crystallization kinetics from batch experiments. In Industrial crystallization 90. Gamiiscli-Partenkirclien, September 1990. Ed. A. Mersmann, Dtisseldorf GVC-VDI, p. 331. 

Hostomsky, J. and Jones, A.G., 1993a. Modelling and analysis of agglomeration during precipitation from solution. In Industrial Crystallization 93. Ed. Z. Rojkowski, University of Warsaw, 1993, pp. 2037-2041. 

Jones, A.G., Chianese, A. and Mullin, J.W., 1984. Effect of fines destruction on batch cooling crystallization of potassium sulphate solutions. In Industrial Crystallization 84. Eds. S.J. Jancic and E.J. de Jong, Amsterdam Elsevier, pp. 191-194. 

Marcant, B., 1996. Prediction of mixing effects in precipitation from laser sheet visualisation. Industrial Crystallization 1996, Toulouse (Rugby Institution of Chemical Engineers), pp. 531-538. 

Myerson, A.S. (ed.), 2001. Handbook of Industrial Crystallization, 2nd edition. Oxford Butterworth-Heinemann. 

Nyvlt, J., 1982. Industrial Crystallization-The State of The Art, 2nd edition. Weinheim Verlag Chemie. 

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