Modeling the Attrition Process in a 22 Liter Draft Tube Crystallizer

Abstract

In Draft Tube (DT) crystallizers secondary nuclei are predominantly produced due to crystals colliding with the impeller. The formation step of these nuclei is a complex process depending on the local two-phase flow pattern in the vicinity of the impeller and on the attrition behavior of the crystalline material. In order to develop crystallizer models with a predictive value for different operational conditions or for different scales of operation a mechanistic description of the secondary nucleation rate is a prerequisite.

In this paper two model frameworks (Gahn et al., 1997 and Ó Meadhra et al., 1996) have been compared using experimental results obtained from a 22 liter DT crystallizer. First, both model frameworks were compared on the basis of the volumetric attrition rate. A discrepancy (factor 20) between both model frameworks was observed. In a second approach both models were tested by comparison of the trend of the predicted median crystal size, L₅₀, with the experimental one in a dynamic experiment. Both models are in good agreement with the experimental data regarding the predicted steady-state L₅₀. The dynamic behavior predicted by the models, however, differs from the measured behavior regarding the oscillation frequency, phase and amplitude.