The influence of the freezing process on vapour transport during sublimation in vacuum-freeze-drying of macroscopic samples

Abstract The influence of freezing on mass transfer during subsequent sublimation was examined in macroscopic samples (cm scale). A matrix with regions of pure ice and regions of concentrated solution forms during freezing. In macroscopic samples, resulting from local differences in solidification conditions, this matrix is irregular, leading to local differences in mass transfer properties in the drying sample. A characteristic local distribution of diffusion coefficients in frozen, drying macroscopic samples could be analysed with differences amounting up to 425%. Sample segments at positions where solidification started yield small diffusion coefficients, the value increasing with the distance from this position, reaching a maximum in a layer right below the sample surface. A covering layer forms a limiting barrier against vapour transport. In macroscopic samples, a decrease of the applied cooling rate leads to a significant shift of the profile of space dependent diffusion coefficients to higher values and therefore to reduced drying times.