Numerical investigation of the influence of the activity coefficient on barium sulphate crystallization

We investigate in this paper the influence of the value and computational method for the activity coefficient on the crystallization process of BaSO₄. We consider for this a mixing-controlled configuration based on a coaxial pipe mixer. The corresponding flow is turbulent and this problem is solved using numerical simulations until reaching steady-state conditions. The precipitation model has been coupled with the method of moments to describe the crystal population, taking the first four moments into account. All supplementary physical models and transport equations have been defined as User-Defined Functions and Scalars and compiled into a commercial Computational Fluid Dynamics code (here Fluent 6.1). The simulations have been performed for various inlet concentrations with different values and models for computing the activity coefficient in order to analyze its influence on the product quantity and on the crystal size distribution at the pipe outlet. The supersaturation ratio, containing the activity coefficient in its definition, is the driving force for the crystallization process. This is the reason why, as concluded from the present results, it is important to model correctly and accurately the activity coefficient in order to obtain reliable data, a point which has been often neglected in the literature. Even relatively small variations of this coefficient can considerably affect the results, in particular for fast reactions.