A Practical Guide to Pan Coating and Scale-Up

An understanding of the pan-coating process based on first principles can support successful scale up.
Jun 19, 2012

There are various theories about how to scale up a solid dosage coating operation in a pan coater. This article provides a basic process understanding and scale-up theory based on first principles.

The coating process shown in Figure 1 has multiple, simultaneous, ongoing exchange processes. The first is a heat exchange between the inlet process air and the coater metal body, spray droplets, and tablets. The second is a mass exchange between the material sprayed and the inlet air stream, tablets, and coating pan.

There are certain losses (e.g., heat loss to the metal body and the environment), however, that cannot be characterized easily. An approximate mass and energy balance for the coating process in an equilibrium state can be calculated as shown in Equations 1–3, where H = enthalpy and m = mass.

The heat balance is shown in Equation 1:

The mass balance for the solvent is shown in Equation 2:

The mass balance for the solids sprayed is shown in Equation 3:

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.

This model can be used to calculate process efficiency and efficiency of the inlet air heat capacity. Efficiency of coating will generally improve as the process is scaled up because the bed depth will be increased, thus increasing the heat exchange between the process air and the tablets.