A Practical Guide to Pan Coating and Scale-Up

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.

Scaling up the coating process

Various articles on scale-up principles have been published (1–3). The following basic scale-up principles can be applied easily in practice.

Pan size. Batch size scale-up is largely dependent on the pan size. As a general rule, the bed depth in proportion to the pan depth should be kept constant. This proportion will help to achieve consistent tablet flow in the scale-up batch, assuming the pans have similar dimensions. However, the technique can also be useful even if the pans are not geometrically similar.

Pan RPM. RPM scale-up is based on the industry’s accepted principle of using the linear distance travelled by tablets in the pan per unit time as constant and can be applied to obtain similar dynamic conditions during scale-up. The linear distance is calculated as πD, where D is the pan diameter.

Pan RPM can be calculated using Equation 4.

Air flow scale-up. Air flow is frequently limited by the particular equipment and/or blowers installed on the system. It is important—particularly for functional coatings—to keep the drying kinetics constant as the process is scaled up. This action can be accomplished in accordance with the spray rate scale-up presented below.

Spray rate. Spray rate scale-up needs to be in step with air flow scale-up. The coordination should be based on more than the batch size increase because equipment that provides a five-fold batch size increase may only allow a two-fold increase in process air. As a general rule, product temperature should be constant across scales. It is often possible to increase the temperature to allow for higher spray rate during scale-up, but this increase depends on the sensitivity of the coating material, or polymer, and its film-forming properties.

The general principles described above underpin the tablet coating process and can support successful scale-up.

References

1. R. Turton and X.X. Cheng, Powder Tech.,150, 78–85 (2005).

2. S.C. Porter, "Scale-up of Film Coating," Pharmaceutical Process Scale-Up, M. Levin, Ed., (Marcel Dekker Inc, New York, USA, 2000), pp. 259–310.

3. G.B. Tatterson, Scaleup and Design of Industrial Mixing Processes (McGraw Hill, New York, 1994).

Abhishek Kathuria is manager of formulations at J&J Consumer Healthcare, Fort Washington, PA.