3D mathematical modelling to understand atypical heat transfer observed in vial freeze-drying

International audienceIn pharmaceutical freeze-drying, the position of the product container (vial) on the shelf of the equipmentconstitutes a major issue for the final product quality. Vials located at the shelf edges exhibit higherproduct temperature than vials located in the centre, which in turn often results in collapsed product. Aphysics-based model was developed to represent heat transfer phenomena and to study their variationwith the distance from the periphery of the shelf. Radiation, conduction between solids, and conductionthrough low-pressure water vapour were considered. The modelling software package COMSOLMultiphysics was employed in representing these phenomena for a set of five vials located at the borderof the shelf, close to the metallic guardrail. Model predictions of heat fluxes were validated against experimentalmeasurements conducted over a broad range of shelf temperatures and chamber pressures representativefor pharmaceutical freeze-drying. Conduction through low-pressure water vapour appearedas the dominant mechanism explaining the additional heat transfer to border vials compared to centralones. The developed model constitutes a powerful tool for studying heterogeneity in freeze-drying whilereducing experimental costs