Experimental Study On Freeze Drying Of Porous Frozen Material With Initial Porosity From Aqueous Solution

Compared with traditional drying processes, freeze-drying is a relatively mild drying process, which has a unique advantage in drying of heat-sensitive materials, such as the pharmaceutical, special food and other organisms. Lyophilized products are of the highest quality, but the process tends to be highest energy consumption because of the phase change in the sublimation drying process. Shortening freeze-drying time, reducing energy consumption, and improving productivity have been technical problems in the field of freeze-drying.In order to save drying time and increase productivity, a novel idea has been proposed for the freeze-drying of liquid materials by creating an initially unsaturated frozen structure with liquid nitrogen ice-cream making method. However, the saturated samples were prepared by directly pouring the aqueous solution into the cylindrical containers and placed into the freezer. A laboratory-scaled multi-functional experimental apparatus of freeze-drying was designed and built. Preliminary experiments had been implemented, and results showed that freeze-drying can be significantly enhanced with the initially unsaturated frozen material.Experimental apparatus has been optimized on the basis of previously work. And mannitol also be selected as the primary solute in aqueous solution to be dried. Freeze-drying experiments were carried out using two kinds of frozen materials, the initially unsaturated one and the conventionally saturated one for comparison. Results show that freeze-drying can be indeed significantly enhanced with the initially unsaturated frozen material. SEM images of dried products display that such prepared initially unsaturated material has bigger void space and more tenuous solid matrix that are continuous and uniform than those of the conventional one. This would be beneficial to migration of sublimated vapor and reduction of mass transfer resistance. There is indeed an overall sublimation for the initially unsaturated frozen material, and a primary sublimation region still exists through examining temperature variations at different locations inside the material. Predominant drying rate-controlling factor is heat transfer for the initially unsaturated frozen material, and mass transfer for the conventional one. The sample physical dimension has a significantly effect on the freeze- drying process of the two kinds of materials. Freeze-drying can be enhanced with the initially unsaturated frozen material, until the mass of two samples with same diameter reaches a certain value. Combination of radiation heating with conduction heating is able to improve heat transfer in freeze-drying of initially unsaturated porous materials, further shorten drying time. It was found that operating pressure has less influence on the freeze-drying process under the experimental conditions.