Scaling up the coating process
Various articles on scale-up principles have been published (1–3). The following are basic scale-up principles that can be
applied easily in practice:
- 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 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. Using this principle, the following calculation will provide the pan RPM:
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- 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 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's 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.
Abhishek Kathuria is technical manager at Evonik Pharma Polymers.
References
1. R. Turton and X.X. Cheng, Powder Technology, 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, USA, 1994.
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