467270 Comparison of the Static and Dynamic Freeze-Drying By Means of Modelling
The freeze-drying is slow due to a low partial pressure of the vapor given by the low temperature as well as to increasing heat and mass transfer resistances. Specifically, the drying velocity is reduced with the growing dried layer. Consequently, of the conductive heat transfer to the ice front as well as of the diffusive vapor transfer from the ice front are slowed down.
To produce starter cultures, the biomass is increased, harvested and frozen by dropping into liquid nitrogen. Particles of approx. 3 to 4 mm are formed. This standard process is followed by dynamically freeze drying the pellets. Dynamical freeze drying describes the process of freeze drying in a vacuum contact drying, i.e. drying during continuous movement of the particles in a solid mixer. The formed dry layer on the surface is scraped off, so that at every time ice-crystals are present on the surface and sublimation without diffusion hindrance can proceed. It was shown, that such a freeze-drying can be carried out steadily in the constant drying rate regime. In addition a comparison of required drying times of this “dynamic freeze-drying process” with static freeze-drying at optimum drying conditions will be given. Actual heat transfer coefficient were determined in dependency of the dryer loading, drying-temperature and rotary frequency of the mixing element starting at a mean particle size of about 4 mm. A calculation of the whole drying process gives the registered energy which leaves the drying arrangement as steam and accordingly also the required time to remove all 'frozen' water. The obtained drying time is compared with the drying time of the conventional drying.