Numerical study of the effect of process parameters on the crystal product quality of potassium chloride and benzoic acid grown in a tubular crystalliser

Abstract

High productivity and narrow size distribution are two of the most desired characteristics in a majority of crystallisation processes. From the industrial view, the need for rapid process development and scalable design is always of great consideration, making tubular crystalliser a potential candidate, particularly in continuous crystallisation process. Here we studied the effect of process parameters, namely flow residence time, solution inlet temperature and seed volume, crystalliser wall temperature, seed size distribution, and seed size on the quality of potassium chloride (KCl) and benzoic acid (BA) crystals from aqueous solution and from 1.5:1 (w/w) water/ethanol solutions, respectively. The cooling crystallisation for spontaneous crystallisation and seeded crystallisation was simulated using the commercial CFD software ANSYS Fluent 19.1. For spontaneous crystallisations, shorter flow residence time produced more uniform crystals, yet lower total number of crystals and smaller crystal size for both KCl and BA. With the same residence time, changing the velocity yielded little effect on CSD curves and the total crystal number. In seeded crystallisation, wall temperature reversely impacted the potassium chloride and benzoic acid product crystals. For potassium chloride, the crystal average size, the total number of crystals, and the product distribution’s standard deviation increased as the temperature decreased. However, for benzoic acid, higher wall temperature led to increases of those output parameters. The study of seeded crystallisation showed that larger seed dispersion or deviation caused almost no impacts on the product average size. However, larger seed deviation led to larger product deviation and, hence, industrially unfavourable CSD shape. In the study of seed size impact, the larger size ended up having larger product size and product standard dispersion.