THE INFLUENCE OF THE FREEZING PROCESS ON VAPOR TRANSPORT DURING SUBLIMATION IN VACUUM-DRYING FREEZE-DRYING OF MACROSCOPIC SAMPLES

The influence of freezing on mass transfer during subsequent sublimati on was examined in macroscopic samples (cm scale). A matrix with regio ns of pure ice and regions of concentrated solution forms during freez ing. In macroscopic samples, resulting from local differences in solid ification conditions, this matrix is irregular, leading to local diffe rences in mass transfer properties in the drying sample. A characteris tic local distribution of diffusion coefficients in frozen, drying mac roscopic samples could be analysed with differences amounting up to 42 5%. Sample segments at positions where solidification started yield sm all diffusion coefficients, the value increasing with the distance fro m this position, reaching a maximum in a layer right below the sample surface. A covering layer forms a limiting barrier against vapour tran sport. In macroscopic samples, a decrease of the applied cooling rate leads to a significant shift of the profile of space dependent diffusi on coefficients to higher values and therefore to reduced drying times .