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.