Residence Time Distribution of Liquid and Solid Phases in a Novel Staged Crystallizer

Abstract

A novel vertically staged continuous crystallizer is proposed in this work. It consists of a series of compartments stacked one above another with partition plates in between. A stirrer shaft with multiple impellers provides agitation in each compartment. Solution to be crystallized enters the crystallizer from the top and flows downward through all stages. Crystal product and mother liquor are collected from the bottom. Openings in partition plates with accessories attached serve as passages for slurry flow, allowing for control over interstage mixing of liquid phase and residence time distribution (RTD) of crystals. The hydrodynamics of such a crystallizer is investigated in a two-stage experimental setup. Water, PVC particles, and glass beads are used to investigate the influences of design and operating factors on interstage mixing of liquid phase and RTDs of both liquid and solid phases. Results show that interstage mixing of liquid phase can be repressed successfully to ensure the establishment of supersaturation gradient across stages. RTD of solid phase can be manipulated by adjusting the feeding rate of liquid phase and opening size in the partition plate. Average residence time of solids deceases with increasing particle size, leading to classification of particles. A model is proposed for the RTD of solid phase which agrees with experimental data very well.