Theoretical analysis for freeze-drying of initially unsaturated porous material

To improve the process economy, freeze-drying of initially unsaturated frozen material from aqueous solution is put forward in the present investigation. A novel heat and mass transfer model of freeze-drying with hygroscopic effect was derived on the basis of a new adsorption-desorption relationship. The finite-volume method was adopted in solving the governing equations. Mannitol, a typical pharmaceutical excipient, was selected as the solute in aqueous solution to be dried. Experimental results show that the freeze-drying process can be enhanced indeed using the initially unsaturated frozen material, and drying time decreases with increase in the initial porosity, ε₀(1-S₀). There is the shortest drying time in correspondence with about 0.30-0.35 of initial saturation, S₀. According to the profiles of temperature and saturation, heat and mass transfer mechanism was discussed and the drying rate-controlling factor was analyzed. Inspection on the effective mass diffusivity, K_S and the effective heat conductivity, λ+K_TΔH displays that the rate-controlling factor would change from mass transfer to heat transfer with increase in instantaneous porosity, ε₀(1-S).