(19b) Modeling Mass Transfer in Hydraulic Conveying of Salt Particles Using CFD-DEM Numerical Simulation

Hydraulic conveying of particulate solids is a common transportation method widely used in various fields and industries, such as the energy, pharmaceutical and agricultural. In most of hydraulic conveying systems there is neither mass transfer interaction nor dissolution of the particulate solids into the conveying fluid phase, mainly due to design considerations and the fact that the solids production processes are commonly separated from the transportation procedures. Nonetheless, there are processes which involve dissolution of the solid into the continuous phase, and result in a significant change in the two phase flow dynamics. One example which is being investigated in this research is the conveyance of salt particles via an unsaturated brine mixture.

The studied phenomenon in this research is the dissolution of the solid particles into the unsaturated brine which occurs during hydraulic conveying of salt particles. The observed multiphase system is constantly prone to create a thermodynamic balance, which derives a spontaneous dissolution of the solid salt into the mixture. The aim of the current study is to investigate this phenomenon via combined CFD-DEM numerical simulations. The mass transfer between the phases and its respective influence on the flow characteristics throughout these types of hydraulic conveying flows are the focus of our investigation.

Our multiphase flow system was modeled using an Euler-Lagrange model. The conveying brine takes place as a multi-species Euler continuum which its physical and chemical properties are described by various models taken from past works. The particulate NaCl matter is represented by a lagrangian discrete phase where each particle momentum, heat and mass transfer equations are considered. As for the computational implementation of the model, ANSYS Fluent was used for the computation of the Eulerian phase with an addition of User defined functions for all properties, such as viscosity, thermal conductivity and density. As for the discrete phase, our in-house BGU-DEM code [1], [2] integrated to Fluent computational scheme also by a set of user defined functions, and a full coupling via source terms of mass, momentum and energy are considered.

Three mass transfer models where investigated, and one model proposed by [3], was chosen for the hydraulic flow. The model was validated using experimental data collected by [4]; the simulations showed very good agreement with the experiments and approved the multiphase model as proper-based tool to describe dynamic flows which are characterized by dissolution of particles.

The main benefit from the Lagrangian representation comes with the attendance of a discrete approach which enable us to track each particle in the flow individually, and by that to extract additional dynamic and kinetic data about the physical problem we choose to simulate. A lot of research in the field of mass transfer in a solid-liquid interface was conducted and exhibited in past works. Also, many works have presented an investigation and characterization of particulate flows with an Euler-Lagrange approach. But very few works involved the dissolution of particles in a flow, and its study via a CFD-DEM simulation as done in our work. The conduction of this study unveils an innovative approach which presents a full coupling and the individual influence of each particle being dissolved over the flow characteristics and vice versa. Additionally, we obtained discrete dynamic data on the particles longitudinal and radial concentration, diameter distribution and variation, linear and angular velocity etc. which are all under the direct and indirect influence of the present mass transfer.

Furthermore, the numerical model was compared with experimental data collected in lab, of various static dissolution experiments of small NaCl particles (2-5 mm). The model shows a substantial over –estimation of the dissolution. One main explanation is the dynamical terms which the mass transfer models used in the simulations were derived from, that constrained our model to dynamical dissolution problems only, being investigated and presented in future research.
Reference

[1]	T. Brosh and A. Levy, "Modeling of Heat Transfer in Pneumatic Conveyer Using a Combined DEM-CFD Numerical Code," Drying Technology, vol. 28, no. 2, pp. 155-164, 2010.
[2]	T. Brosh, H. Kalman, A. Levy, I. Peyron and F. Ricard, "DEM–CFD simulation of particle comminution in jet-mill," Powder Technology, vol. 257, pp. 104-112, 2014.
[3]	G. A. Aksel'rud, A. E. Boiko and a. A. E. Kashcheev, "Kinetics of the solution of mineral salts suspended in a liquid flow," Journal of engineering physics, vol. 61, no. 1, p. 885–889, 1991.
[4]	L. Calcada, L. Martins, C. Scheid, S. Magalhaes and A.L.Martins, "Mathematical model of dissolution of particles of NaCl in well drilling: Determination of mass transfer convective coefficient," Journal of Petroleum Science and Engineering, vol. 126, pp. 97-104, 2015.