Thin-layer modeling of microwave, microwave-convective, and microwave-vacuum drying of pharmaceutical powders

Common pharmaceutical excipients and active ingredients, wetted with specific solvents, were dried under selected microwave, microwave-convective, and microwave-vacuum conditions in an experimental drying system (2.45 GHz, 90 W). The generalized drying profile was independent of drying technique and material characteristics, with a constant rate stage and two falling rate periods being observed. Moisture removal rate was, however, dependent on the drying technique (pressure and presence/absence of external heating source), and powder and solvent properties (dielectric, physical and thermal). The experimental moisture loss data were fitted to selected semi-theoretical and empirical thin-layer drying equations. The correlations were compared according to two statistical tests, namely root mean square error and reduced chi-square. The microwave, microwave-convective, and microwave-vacuum drying behaviors were adequately represented by the Page, Logarithmic, Chavez-Mendez et al. and Midilli et al. equations, with the latter providing the best description of the experimental results.