Lyophilization model of mannitol water solution in a laboratory scale lyophilizer
The paper reports on the development of a numerical model for the simulation of a lyophilization process in a vial. Experimental analysis is presented of lyophilization dynamics inside a single vial in a laboratory scale lyophilizer. The problems of lyophilization modelling of a mannitol water solution are covered in detail. The effects of the small scale of the laboratory device with respect to a correct definition of boundary conditions for the numerical simulations are described, especially the effect of the comparatively high temperatures of the chamber walls. In the numerical model, a 1D vial approximation of the governing equations of heat and mass transport with moving front between the frozen and porous part of the cake is used and solved in a time stepping nonlinear iteration procedure. A water vapour diffusion model, implemented in the mass conservation equations, based on the Knudsen model of diffusivities, is applied and linked to the typical pore size of the porous cake. A front tracking scheme with moving computational grid is applied, and a dedicated sub-model of surface layer ice sublimation is introduced, based on the one-sided vapour diffusion model. The comparison of the numerical and the experimental results show that the developed numerical model is able to capture the transition points from primary to secondary drying very accurately, with accompanying accurate capturing of the temperature levels inside of the drying material.
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