In the manufacture of propellants, drying is a crucial step in which liquid solvents are extracted from the propellants' microstructure in order to impart the desired physical and chemical qualities. Therefore, the prediction of the effective diffusivity, a fundamental characteristic defining the rate of solvent transfer during vacuum drying of propellants, is particularly valuable for determining optimal drying conditions in the operation of drying systems and optimizing the drying process. This paper presents an elegant way to estimate the lumped value of the effective diffusivity for the mass transfer process in the course of drying propellants. Analytical solutions for the effective diffusivity at planar, cylindrical, and spherical geometries are obtained. These solutions are used to investigate effects of the propellant geometry on the drying process. Using experimental data for the transient moisture content, it becomes possible to determine geometries, which should be preferred for a given function of the mass flux.