Optimal Seeding Condition for Semi-Batch Type Evaporative Crystallization of a High Suspension Density Sodium Chloride Slurry in a Draft-Tube Stirred Vessel

概要

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For highly efficient evaporative crystallization in batch or semi-batch type operations, a high crystal growth rate, a high density of suspended crystalline particles in the slurry and a sharp crystal size distribution are all ideally required. On the other hand, the initial seed crystal conditions are also very important because they affect the crystal size and distribution during the entire period of operation. In the present study, the influence of seeding conditions (average diameter and mass) and heating rate on both the crystal growth rate of sodium chloride (NaCl) and the crystal size distribution were investigated in an optimal design draft-tube stirred vessel containing high suspension density slurries of sodium chloride particles up to 30 vol%. In order to determine the optimal seeding conditions to simultaneously achieve a high crystal growth rate and narrow size distribution, an optimal operation index Iop, was defined based on a comparison of the experimental data and a model calculation with the assumption of ideal crystal growth. The results show that Iop correlated with the particle number of seed crystals and had a maximum value. With Iop correlation, the appropriate seeding conditions for achieving a high crystal growth rate and sharp size distribution were determined.