Figure 5: (a) In a schematic of the simulation approach (left), the colors denote water vapour content going from low (blue)
to high (dark red). In the time (t) evolution of the film on tablets with different shapes (right), thickness is shown in
shades of gray. (b) In a surface plot of intra-tablet coating uniformity for the bi-convex shape, smaller values mean better
The Euler-Lagrange CFD simulations show how different physical parameters of the coating spray affect the coating process
on a single tablet. The deposition behavior of the droplets on the tablet surface is analyzed in terms of film thickness and
its homogeneity on the tablet. Simulations estimate the effects of tablet shape, droplet diameter, air temperature, spray
rate of the coating, and water content of the coating solution on film formation. Simulation data allow analysis of important
process characteristics such as behavior of the undried film on a tablet or the inter-tablet uniformity of the tablet film,
measured as relative standard deviation (RSD), as a function of process parameters (see Figure 5). Small values of RSD translate to high uniformity. The surface plot in Figure 5 shows that lower atomizing pressure results in higher uniformity, but air temperature has little influence.
Spraying process in an industrial coater
CFD multiphase simulations were used to study the air and droplet flow inside a pan-coating device. This work simulated the
effects of spray-gun position and injection angle on spray losses and coating efficiency.
Figure 6: (a) Air flow and its effect on spray droplet movement. Flow direction is shown by gray arrows, the droplet diameter
is drawn to scale, and the color of the droplets denotes the water content (i.e., blue droplets are already dried particles);
(b) Effects of different spray gun positions and angles on the amount of spray loss (defined as the relative amount of mass
that has deserted the system without contact to tablets).
Figure 6a shows the spray inside the coater after 0.5 s. Droplet color and size is proportional to the water content. The big droplets
close to the spray nozzle had water content close to the initial spray-water fraction of 0.8. The small droplets at the right-hand
side were already partially or completely dried. A fraction of the spray moved towards the tablet bed but was then dragged
upwards, thus impacting the air inlet/outlet cylinder. The dry droplets followed the flow of drying air and left after a varying
residence time. Figure 6b shows that the amount of spray loss varies for different spray nozzle positions. Positions 1 and 3 performed significantly
better than the others, including the standard nozzle placement (B).
Johannes G. Khinast is a professor at the Graz University in Austria and Head of the Institute of Process and Particle Engineering
Scientific Director Research Center Pharmaceutical Engineering (RCPE).
Articles by Johannes G. Khinast
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