dc.description.abstract | Crystallization is a widely used process for liquid solid separation. The products from this process are crystals, which can be described by a distribution function called “crystal size distribution function (CSD)”. The properties of the crystals affect the efficiency of downstream process, such as filtration or drying. During the process both crystal nucleation and crystal growth happens, and most of the time, the crystal nucleation is undesirable. A well-controlled crystallizer can produce crystals with less crystal nucleation.
Researchers have optimize a crystallization processes by improving the seed properties or the cooling policy. In both cases an objective function is required. In this work we compare the objective function that researchers have used, to see which objective function is best when optimizing the cooling policy for a batch crystallizer.
The result shows that some of the objective functions are minimized by producing a large amount of nuclei. However, for a seeded batch crystallizer the idea is to grow the seed crystals while suppressing the nucleation. Moreover, in industrial practice, the product crystals would probably be filtered so that fines (nucleated crystals) would be removed. Therefore, for each objective function we also determine the objective value after the nucleated crystals are removed, to see whether the result from each objective function is still the best. After the analysis we conclud that the objective function “minimizing the nucleated crystal mass” is better than others.
In this work we also discuss the utility of changing seed properties when using “minimizing weight coefficient of variation” as objective function. We found that if the seed distribution is too wide, the system would be more likely to generate a narrow distribution crystal by excess nucleation. To prevent excess nucleation and achieve a narrow product CSD, a large seed loading and a narrow seed distribution helps.
Finally, we also considered the effect of using different nucleation parameters. We changed the exponent on third moment term in nucleation rate equation. The result shows that for higher value of the exponent, the nucleation rate is suppressed, and the performance is better when the growth rate trajectory is optimized using the objective function “minimizing nucleated crystal mass.” The optimized result also mention that for higher value of the exponent on third moment term in nucleation rate equation, higher growth at the beginning of the batch is desirable. | en |