ConclusionA laboratory scale microwave freeze drying apparat

Conclusion
A laboratory scale microwave freeze drying apparatus was
developed aimed at experimentally verifying the beneficial
effect of a dielectric material on freeze drying with microwave
heating.Sintered SiC was used as the dielectric material.
Mannitol,a typical pharmaceutical excipient,was
selected as the solute in aqueous solution.The experimental
results show that the use of the dielectric material in microwave
freeze drying can effectively enhance the freeze drying
rate.More than 20% of drying time was saved compared
with conventional freeze drying under the operating conditions
tested.A porous media mathematical model of freeze
drying with hygroscopic effect was derived and solved
numerically using the finite difference method with a moving
boundary.The hygroscopic effect was quantified based on a
modified adsorption–desorption relationship.For mannitol,
the ratio of vapor pressures between the adsorption–desorption
equilibrium and the thermodynamic equilibrium follows
the square root of the moisture saturation.Comparisons of
drying curves between experimental data and theoretical predictions
displayed good agreements.Simulation results also
show that when the solid content of the solution to be freeze
dried is very low or the solid product has a very small loss
factor,microwave heating is less effective unless dielectric
materials are employed.Drying rate controlling factor was
analyzed to be mass transfer in the primary drying stage and
heat transfer in the secondary drying stage.
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