(456f) A CFD-DEM Based Model for Wet Granulation Process in a Rotary Drum

One of the common methods for production of particulate fertilizer products by wet-granulation is rotary drum or spherodizer. Wet-granulation is a complex thermo-chemical process that involves several simultaneously occurring mechanisms. In this process, recycled fine granules are sprayed with fresh fertilizer melt that layers and granulates in the drum. The output from the drum is screened to remove the oversize and undersize particles from the product. The oversized particles are crushed and remixed with the undersize to recycle. The rotating drum has hot gas blown in order to be able to dry the melt depositing on the particles. A fundamental industrial challenge for this operation is maintaining process system stability for the optimized production.

The system is needed to be maintained at a steady state where the production could be optimized. This needs a deeper understanding of the mechanism of granulation. This is possible only with a CFD-DEM based system where all aspects of the thermo-chemical process are considered. This work discusses such a model development.

This proposed model is essentially a Euler-Lagrange based method where all fluid (both gases and liquids) phases are modelled as a single continuous phase and granular particles with discrete phases. In this unique new model, the continuous phase is modelled as a mixture of fluid species with a continuity and momentum equation. The species are accounted for as scalars where the total fraction is balanced to 1. The continuous fluid phase consists of 4 species represented by fertilizer liquid (Fl), water liquid (Wl), water vapor (Wv) and air (a).

The discrete particle phase consists of a solidified core and when the particles are wetted by the fluid spray an outer melt layer exists. Thus, the Lagrangian particle phase consists of 3 species represented by fertilizer liquid (Fl), water liquid (Wl) and fertilizer solid (Fs). The core consists of fertilizer solid and outer layer is a mixture of fertilizer liquid and water liquid.

This model uses a two-way coupling for momentum, mass and heat transfer. The particle drag and lift forces provide the drift for the flowing particles in systems like the spherodiser. The mass exchange between phases exists in various forms. The melt when deposits on particles it exchanges mass with the particles. The melt also constantly releases water in the form of water vaporization. This exchange is a function of heat balance which depends on the prevailing temperature of the particle and water vapor pressure. Several fluid and particle properties account for the transfer coefficients between the phases.

The present model is developed or implemented in ANSYS Fluent 18.0 using user defined functions (UDF) (Fluent, 2017). All the source terms relating to mass, momentum, species and heat exchange between phases are managed by the UDF. This work presents a series of implementation tests and validation of model. After completion of the implementation test and validation a series of drum granulation processes were simulated. These first of its kind simulations are demonstrated as a model prototype.