Figure 1: Pan-coating process.
|
The coating process indicated in Figure 1 is an operation that comprises multiple, simultaneous ongoing exchange processes. These processes include:
- Heat exchange between inlet process air and the coater metal body, spray droplets and tablets.
- Mass exchange between material sprayed, and the inlet air stream, tablets and coating pan.
However, there are certain losses, such as heat loss to the metal body and the environment, that cannot be characterised easily.
An approximate mass and energy balance for the operation can be calculated as below.
Equilibrium state at coating process
Heat balance:
As with any theoretical analysis, there are assumptions built into the model described above. For example:
- Leftover material in lines are not considered.
- Heat loss to equipment body and environment are not considered.
- Any unrecoverable mass loss in the process is not accounted for.
These steps can be used to measure process efficiency and efficiency of the inlet air heat capacity. These actions are beneficial
because the process scientist may realise that only a small fraction of heat–mass uptake capacity is being utilised. It may
or may not be possible to change this. As long as the total process reconciliation is acceptable (e.g., within 2%), the process
efficiency can be calculated as a ratio of tablet weight gain to the weight of solids sprayed. As the process is scaled up,
the increased bed depth will generally improve coating efficiency, thus providing better opportunity for process air to exchange
heat with the tablets.
Similarly, spray gun positioning, relative to the bed and to the inlet air plenum, will affect the drying process of spray
droplets. In general, the greatest impact depends upon the distance of the spray gun from the bed, atomisation/fan air pressures
and nozzle type/diameter.
