Theoretical study on microwave freeze-drying of an aqueous pharmaceutical excipient with the aid of dielectric material

A mathematic model of simultaneous heat and mass transfer for the dielectric material assisted microwave freeze-drying was derived and solved numerically using the finite-deference technique with two moving boundaries. Lactose, a typical pharmaceutical excipient, was used as the representative solid material in the aqueous solution to be freeze-dried. Silicon carbide (SiC) was selected as the dielectric material. Numerical results show that the dielectric material can significantly enhance the microwave freeze-drying process. Under typical operating conditions, the drying time is 43% shorter than that of ordinary microwave freeze-drying. Temperature variations at sublimation fronts were examined in order to determine the appropriate microwave power input. Profiles of temperature, ice saturation, vapor concentration, and pressure during freeze-drying are presented, and rate-controlling mechanisms are discussed.