Particle Size Improvement by a Countercurrent Tower Crystallizer
A continuous, staged-feed column Crystallizer was investigated to obtain enlarged product size distribution in precipitation from solution. Stage population balances, in moment form, were solved analytically for several single-feed configurations. It was found that internal staging permits control of the solids concentration profile, and if collision nucleation is unimportant then countercurrent operation yields the greatest product size enlargement. Nucleation kinetic parameters for a given salt indicate the potential product size improvement, compared to product from an MSMPR crystallizer of equal volume.
SCOPE
In crystallization processes the concept of staging typically appears in the form of cascades of separate stirred-tank crystallizers. Generally, such arrangements permit closer control of product crystal size distribution (CSD) than does a large single crystallizer of identical total volume (Larson and Wolff, 197 1). A more economical approach may be to incorporate these individual crystallizers into a single unit while retaining the benefits of staging. This is the motivation for the current investigation of a staged-feed column crystallizer for enlargement of product CSD.
Previous studies involving column crystallizers have dealt exclusively with improving the purity of crystals produced from melts Mullin, 1980; Takegami et al., 1984). Mathematical development of simulation models for countercurrent column melt crystallizers has been presented by , and more recently by . However, these examples do not deal directly with product CSD. For crystallization from solution, product CSD is recognized as the primary indicator of performance.
The configuration of the multistage crystallizer is essentially a mixed column, which may be divided by internal baffles into a number of cylindrical stages.