Impact of hydrodynamics on the precipitation efficiency--application to HARDTAC reactor.

Precipitation of gypsum is studied in a HARDTAC (High-Aspect Ratio, Draft-Tube, Agitated Crystallizer) reactor, which is considered as the core crystallization unit of lots of wastewater treatment systems. Coupling Computational Fluid Dynamics (CFD) and population balance modelling to simulate precipitation can be a useful tool to come to a decision about upstream and downstream units. In the present study, we aim to validate such approach by investigating gypsum precipitation in a HARDTAC pilot unit and comparing experiments results with simulation. Measured nucleation and growth kinetics are used to feed the model. A comparison between experiments and simulations is presented in the case of gypsum precipitation with a given set of operating conditions. Good agreement is obtained for species concentrations, gypsum mass fraction and volumetric mean diameter but some discrepancies still remain between measured and simulated crystal size distribution.