Recent Studies on Crystallization in Japan
Crystallization had been-generally considered as one of arts rather than chemical engineering, but now it becomes one of the most important fields of the advanced chemical engineering as a result of the progressive studies in this field. Application of crystallization theories to industrial problems is restricted only to a few excellent chemical engineers, because of the complicated phenomena of crystallization in comparison with other unit operations. That is: generation of seed crystals and growth of crystal should be considered on the basis of the supersaturated systems, in some cases independently, or in the other cases effecting each other. The mechanism of generation is generally considered to consist of tridimensional nucleation, secondary nucleation, and attrition. The mechanism of growth is studied by the two different steps: diffusion and surface reaction steps. When they are considered for industrial purposes, they are able to be applied to designs of crystallizers and crystallization processes in order to produce good crystals with high efficiency. The capacity of chemical equipment is usually expressed by the production rate, but size of crystals is added to the production rate for crystallizers. (For some cases, the product of production rate and crystal size in the same crystallizer is constant under optimum operational conditions.) Though crystallization is such a complicated unit operation as mentioned before, crystallization is one of the most important unit operations for separation processes. Since crystallization often becomes the key operation for a successfull process, the processes have been developed after thorough study and sometimes have been successfull only by excellent engineers. A t present, crystallization in the field of chemical engineering becomes more understood, and has been studied to such a level as to be available for industrial uses. I n the application of crystallization theories for industrial purposes, crystallization phenomena are simplified from the standpoint of chemical engineering differently from the scientific one, and phenomena in crystallizers are also simplified so as to be expressed by quantitative equations. Even though phenomena are simplified, large deviation between the set model and the actual phenomena ought not be permitted. I n this paper, chemical engineering concepts on crystallization are explained from the standpoint of design, and the theories for designs of crystallizers and the crystallization process are also discussed. At the final, the crystallizers and the crystallization processes originally developed recently in Japan, will be introduced as examples of advanced crystallization, and crystallization in the near future will also be mentioned.
1. Fundamental concepts of crystallization on chemical engineering
On scientific basis, crystallization is discussed in the three steps, supersaturation condition, crystal nucleation, and crystal growth. When crystallization is dis-