(74b) CFD-DEM Simulation of the Heat Transport in a Wurster Coater

The Wurster process is widely applied to coat particles in the pharmaceutical industry. Beads are often coated with a functional film that controls the release of drug substances or masks a specific taste. Multiple films can be coated onto the pellets. The coating process generally takes place in the Wurster tube, in which the particles are also transported centrally upwards. Outside the Wurster tube the particles are moving in a downward direction. Thus, the particle flow resembles a torus shaped recirculation pattern.

The spray applied through the spray nozzle onto the particles and the solvent temperature is usually lower than the fluidization air temperature. The inlet temperature of the fluidization air is set in such a way, that the drying process of the sprayed particle is accelerated and to facilitate a good film forming temperature. Particles are sprayed in the Wurster tube. Drying starts immediately and the biggest part of the energy is absorbed in the Wurster tube. After a short while an equilibrium of the temperature inside the Wurster coater is reached. This equilibrium depends on the spray rate, liquid temperature and used solvent.

In this work a model to simulate the heat transport in a CFD-DEM simulation is presented. The eXtended Particle System XPS is used for the DEM simulation and is coupled to AVL Fire® for the CFD simulation¹. Drag as well as the energy information are exchanged between the fluid and solid phase at every time step.

First the validity of the model is shown. A lab scale Wurster coater² is used to show the influence the spray rate and fluidization air flow rate has on the heat transfer. To model the influence of the different spray rate the particle temperature is set constant. The influence the fluidization air flow rate has on the heat transfer rate is shown. A correlation between fluidization air flow rate and temperature difference was discovered. At last the heat transfer in the Wurster coater at various scales going from the lab up to production scale sized processes will be shown.

References

1. Jajcevic D, Siegmann E, Radeke C, Khinast JG. Large-scale CFD–DEM simulations of fluidized granular systems. Chem Eng Sci. 2013;98:298-310.

2. Liang L, Remmelgas J, van Wachem BGM, et al. Residence time distributions of different size particles in the spray zone of a Wurster fluid bed studied using DEM-CFD. Powder Technol. 2015;280:124-134.