Crystallization equipment fluidized beds

We have proposed a fluidized bed type process, which can be applied to phosphate removal from wastewater containing phosphate 2-23 mg/jg as P.By the results of experiments using equipment of capacity l-4m3 /day, factors such as supersaturation, recirculation ratio and space velocity were recognized to affect crystallization rate or phosphate removal efficiency. By mathematical analysis, we could obtain the characteristic equation for fluidized bed process, to agree well with experimental results. 

The structure of a experimental fluidized bed crystallizer (FBC) is shown in Fig. 12.4, where the crystallizer is actually a universal equipment for the measurement of crystal-growth rate. The solution enters the FBC at its bottom, and leaves the FBC by overflow. All the other parts of the experimental system are the same as shown in Fig. 12.3, and so are not shown in Fig. 12.4. The operation procedure for the FBC is the same as for the ISC. For convenience of comparison, the corresponding conditions, temperature and concentration of the solution, operated in the ISC and the FBC are rigorously controlled to be the same, with the deviation of the operating temperature no greater than 0.05 °C. 

Fluidized bed crystallizers to avoid the need for heavy-magma, high-flux filtration equipment. 

Fines Destruction. In the operation of industrial crystallizers, one would usually want to avoid the fines (i.e., small crystals) since they may cause difficulties in downstream processing equipment (e.g., filtration) and affect both product quality and process economics. Excessive fines may also require a relatively long batch run time to achieve the desired final size of the product crystals. Karpinski (1981) proposed a controlled dissolution of secondary nuclei in order to improve CSD from fluidized bed crystallizers. Jones et al. (1984) first described the application of fines destruction in batch crystallization of potassium sulfate solutions. Their study demonstrated the experimental feasibility of this technology to dramatically reduce the amount of fines in the final product CSD. Their theoretical predictions, obtained from population balance models, agreed with the experimental results. 

As described above, the Oslo-Krystal unit is a fluidized-bed agitated crystallizer in which the gentle action minimizes secondary nucleation and allows large crystals to grow. Oslo-Krystal vacuum crystallizers can be of the open Figure 8.51) or closed Figure 8.45) types. In the former the crystallization zone is at atmospheric pressure. In the latter all parts of the equipment are under reduced pressure. 

Among engineers, population balance concepts are of importance to aeronautical, chemical, civil (environmental), mechanical, and materials engineers. Chemical engineers have put population balances to the most diverse use. Applications have covered a wide range of dispersed phase systems, such as solid-liquid dispersions (although with incidental emphasis on crystallization systems), and gas-liquid, gas-solid, and liquid-liquid dispersions. Analyses of separation equipment such as for liquid-liquid extraction, or solid-liquid leaching and reactor equipment, such as bioreactors (microbial processes) fluidized bed reactors (catalytic reactions), and dispersed phase reactors (transfer across interface and reaction) all involve population balances. 

Mixing can be provided by application of an agitator in a stirred tank reactor or a fluidized bed or forced circulation-type crystallizer. A variety of impellers of different geometry, for larger vessels also equipped with several turbines, are available. An extended discussion of mixing in crystallization is found in Chapter 13. 

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