Study on the effect of the operating conditions on the crystal size distribution of potassium chloride crystal by numerical simulation

Abstract

Narrow crystal size distribution and high crystal production rate is the final goal for many industrial applications such as pharmaceutical and agro-chemical industries. Operating conditions in a plug flow crystallizer can affect crystal size distribution and crystal production rate. The effect of operating conditions of inlet velocity and wall temperature on the crystal size distribution (CSD) and crystallization rate of potassium chloride in cooling crystallization was studied by numerical simulation using the commercial CFD software ANSYS Fluent 18. Inlet velocity determined the shape of crystal size distribution and the presence of hot zones close to the center of crystallizer at which crystallization cannot happen. Lower velocity gave increased residence time, which resulted in more number of crystals and a broader CSD. With lower inlet velocity, increased residence time gave longer cooling time and smaller hot zones. With low wall temperature, total number of particles increased but still 32% of the reactor was a hot zone. To quantify the fraction of hot zones, the aspect ratio of crystallizer was modified. The aspect ratio with a fraction of 4% of hot zone in the crystallizer was $10^{-3}$ or 1mm radius for the 1m length of reactor used in this study. To validate the results obtained by simulation, the results of acetyl salicylic acid cooling crystallization in ethanol obtained by Eder et al.[1] were replicated using numerical simulation.