| Compared with spray drying or other contact drying processes, freeze-drying is a mild drying process which plays a irreplaceable role in drying of heat-sensitive materials such as foods, Pharmaceuticals and biological products. It is well known that among all current drying techniques, freeze-drying products are of high quality. However, due to the phase change of moisture and the low energy efficiency of the process, freeze-drying has become one of the most energy consumption operation units in chemical engineering. How to reduce the energy cost through decreasing the drying time has been the hot spot of research in the past decades.To overcome disadvantages of freeze-drying that is usually lengthy and expensive, a novel idea, freeze-drying of initially unsaturated porous frozen material, was proposed here. An experimental investigation was performed aiming at verifying the effect of unsaturated frozen materials with a certain initial porosity on freeze-drying of liquid materials. A multifunctional freeze-drying apparatus on the lab scale was designed, made and assembled. Mannitol was selected as the primary solute in solution. Liquid nitrogen ice-cream making method was used to prepare frozen materials with different initial saturations. The initial unsaturated frozen material was prepared, which has the same material quantity and initial moisture content with the conventional frozen material. The contrast experiments of the two kinds of frozen material were performed under the same operating conditions.Results show that compared with conventional freeze-drying, the unsaturated frozen material can significantly shorten the drying time and enhance the freeze-drying process under the same material quantity,initial moisture content and operating conditions. The drying time of the unsaturated material (So=0.28) was at best36.4%shorter than the convention’s (So=1.00). The smaller the initial saturation is, the shorter the drying time is and also the lower the moisture content of dried products. The drying time of the two kinds of frozen materials decreases with the increasing ambient temperature. The initial unsaturated frozen material shows a little sensitive to operating pressure, and the drying time decreases with the increase of operating pressure. However, the initial saturated sample shows little sensitive to operating pressure, and we cannot find any significant change of the drying time with different operating pressures. Freeze-drying mechanism shows some differences between the two kinds of frozen materials. There is no clear sublimation interface in the initial unsaturated frozen material when ice crystal begin to sublimate. Consequently, the sublimation drying time can be significantly decreased. |
