Numerical modelling of a counter-current spray drying tower

Spray drying of atomised solutions or slurries is one of the most common methods for the production of a wide variety of particulate products in the chemical, food, personal care products and pharmaceutical industries. The modelling of a spray drying process for the manufacture of detergent powder in a counter-current tower is carried out using two different approaches: a simple one-dimensional multiphase plug-flow modelling and a more rigorous CFD modelling approach. Both approaches are coupled with an existing semi-empirical slurry droplet drying model. The plug-flow model considers heat, mass and momentum transfer between the polydispersed droplets/particles and gas phase, along the tower height. In the CFD model, based on the Eulerian-Lagargian method, the three-dimensional turbulent swirling gas flow is fully coupled with the droplets/particles motion along with particle-wall interactions via the heat, mass and momentum exchanges. The simulation results are compared with the experimental data collected from a large scale pilot-plant spray drying tower and a reasonable agreement with the measured powder outlet temperature, moisture content, and exhaust gas temperature is obtained, considering the complexity of the process and the accuracy of the measured data. The plug-flow model gives similar qualitative trends compared to the CFD model and can be a useful supplement for quick determination of operating conditions for pilot-plant trials that would enable more extensive and accurate optimisation of the process. The more computationally expensive CFD model can be used for tackling operational and product quality issues including wall deposition and thermal degradation. The gas flow and temperature profiles, and droplet/particle trajectories obtained from the CFD modelling results are used to propose a zonal modelling approach to model spray tower in a computationally efficient manner. This approach can be used to develop models for process optimisation of counter-current spray drying towers, as reliably as the CFD model.