Comparison of Knudsen Diffusion and the Dusty Gas Approach for the Modeling of the Freeze-Drying Process of Bulk Food Products
Patrick Levin; Moritz Buchholz; Vincent Meunier; Ulrich Kessler; Stefan Palzer; Stefan Heinrich
Freeze-drying is generally used to achieve high quality products and preserve thermal sensitive components: however, it is also considered as a high energy and costly process. Modeling of the process can help to optimize the process to reduce these drawbacks. In this work, a mathematical model is presented to predict the heat and mass transfer behavior for freeze-drying of porous frozen food particles during freeze-drying to optimize the process. For the mass transfer, a comparison between Knudsen diffusion and the more complex dusty-gas approach is performed. Simulation results of a single particle are validated by experiments of single-layer drying to extend the usage of this model from a single particle to a particle bed. For the moisture transfer, adaption parameters are introduced and evaluated. A comparison shows a good agreement of the model with experimental results. The results furthermore suggest a strong correlation of the drying kinetics with pore size and particle porosity. An increase in the pore diameter strongly improves the overall mass transfer rates and hence is a suitable parameter for an effective increase of the drying rates in freeze-drying.
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