Heat Flux Heterogeneity in the Sublimation Chamber

Because the heat flux is not evenly distributed through the sublimation chamber, resulting principally from the relative importance of the radiative flux component coming from the shelves and from the side-walls of the freeze-dryer, each vial of the whole batch does not receive the same overall heat flux. In fact, it is well knovm that the vials located at the edges of each shelf present shorter sublimation and desorption times and higher temperature profiles. This phenomenon can be investigated by 

Consequently, for the frequent situation of sublimation times controlled by the heat transfer, the reduction of the operating costs related to the drying time relies on the optimization of the thermal resistance by searching for an optimum of the total gas pressure or, if possible, by adapting the shape of the vial bottom. The latter can be done by reducing the space between the shelf and vial bottom as far as possible or by 

Experimental-5 ml molded vials Experimental- ml tubing vials Experimental-4 ml tubing vials Identified model-5 ml-molded vials Identified model-7 mi-tubing vials Identified model-4 mi-tubing vials 

Furthermore, it is worth noting that the discussed heat transfer coefficient values are largely dependent on the vial type (molded or tubing), on the vial size, and also on the total gas pressure in the pressure range - between 10 and 70 Pa - usually selected for freeze-drying of sensible drugs (proteins, vaccines, etc.) (Pikal et al., 1984). 

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