Experimental investigation on freeze drying with dielectric material assisted microwave heating

Aiming at experimentally verifying the beneficial effect of the dielectric material on freeze drying with microwave heating, a laboratory scaled microwave freeze drying apparatus was designed, fabricated and assembled. Sintered silicon carbide (SiC) was used as the dielectric material as well as quartz as a reference substance. Mannitol, a typical pharmaceutical excipient, was selected as the solute in the being freeze dried aqueous solution. The results show that use of dielectric material in microwave freeze drying can effectively enhance the freeze drying rate, and 20% of drying time can be saved compared with conventional freeze drying under the operating conditions examined. Microwave heating was found to take effect gradually, and the importance of microwave heating becomes more pronounced at the latter half period of drying. It was also found that, when the solid content in the solution being freeze dried is very low or the solid product has a very small dielectric loss factor, the effect of the microwave heating is not evident.