Computational simulation method
DEM simulations of non-spherical particles in different coater geometries were used to study the particle flow inside the
coater and the residence time of the tablets in the spray zone (EDEM software v2.3, DEM Solutions). Material attributes of
a sample placebo tablet were experimentally quantified as a control. DEM simulations consider the body forces and tangential
contact forces on the tablets. Newton's equations are solved to calculate the velocity and rotational velocity of the tablet.
Appropriate contact models, which are applied at the contact points of the tablets, account for tablet–tablet and tablet–wall
collisions. A contact model based on the Hertz–Mindlin theory was used in this study.
Analyses of the local coating process and air- and spray-flow in the coating chamber were made using CFD multiphase simulations
(FIRE v2009, AVL List). The software was used to study the interaction between the coating spray and the tablets, both locally
(i.e., for a single tablet) and globally (i.e., by calculating the air flow in the whole coater). To this end, spray droplets
were simulated with a discrete droplets method (DDM) Euler-Lagrange approach. Solvent evaporation is taken into account in
order to estimate effects of drying-air flow. A two-dimensional model that incorporates sub-models for interfacial shear force,
film evaporation, and heat transfer between the film, solid wall, and air describes the undried coating film.
DEM investigation of tablet movement
shows a DEM simulation for a continuous coater (Driaconti, DRIAM Anlagenbau). The tablets are colored according to their velocity,
with the fastest near the top, at the upper part of the coater (red region). The spray nozzle is typically positioned in this
region, because the tablets should move through the spray zone as fast as possible to eliminate over-wetting.
Figure 3: DEM simulations in a continuous-cycled coater (DriaConti, Driam). Tablet color indicates velocity from slow (blue)
to fast (red).
Residence time of tablets in the spray zone is an important parameter. While residence time is difficult to determine from
experiments, it is readily available from DEM simulation data. Figure 4 shows the fractional residence time (i.e., the residence time relative to the total time) for a simulation time of 60 s (9).
Both tablet shape (quantified as length-to-height ratio) and fill level (ratio of tablet volume to total drum volume) influence
the time that a single tablet spends exposed to the spray. To minimize coating variability or required coating process time,
the fractional residence time should be as high as possible.
Figure 4: Average fractional residence time in the spray zone as a function of length-to-height ratio of tablet shape (round,
bi-convex, and oval in increasing order) and coater fill levels.
In Figure 4, lower fill levels generally perform better than high levels, as is expected. The dependence of residence time on fill level,
however, is not linear. Spherical and oval tablets show similar dependence on fill level. Bi-convex tablets, on the other
hand, show a qualitatively different behavior; they are less affected by fill level and, therefore, outperform the other shapes
at low fill levels.