Experimental study on optimal operation of an actual-size batch crystallizer

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Batch crystallization can be used for the production of a variety of fine chemicals in small scale. It has flexibility in operation. But, the method for control of crystal size, which has been long desired has not be established yet. In this research project, the effect of seeding on the product crystal size distribution was investigated experimentally using both large and small scale batch cooling crystallizers. The aim of this research was to establish the method for crystal size control. Experimental work was made with the potassium alum system. As a cooling mode, quenching (natural cooling) method was mainly employed. For comparison, the other methods, linear and controlled cooling methods, were also tested.At first, a small scale crystallizer (12 L) was used to investigate the effect of seed amount and initial solution concentration on the crystal size distribution of product. The size distribution of product crystals has been found to become uni-modal under the condition of enoug … More h seeding over a critical seed concentration. Under this condition, the product crystals were all grown seeds. The same result was obtained for solutions for high initial concentration. Even for the large scale crystallizer (600L) used, the uni-modal crystal size distribution was confirmed similarly to be obtained under the condition of enough seeding.In addition to the above experiments, the generation rate of fine crystals was measured to clarify the mechanism for the seeding effect on the production of uni-modal size distribution. Agglomeration (or sticking) of fine crystals to seed crystals and the short period of high supersaturation were shown to play important roles. By comparing the quenching method with the other cooling modes, the cooling mode itself was shown not to be an important factor in producing crystals of uni-modal size distribution. 'It is the seed amount that plays an important role.The results obtained in this project is expected to be extended to a useful and practical design method of a batch crystallizer for the production of the crystals of controlled size. Part of the results obtained in this project have been published in international journals (5 papers) and presented in meetings and symposia (1 6 papers). Less