Thermodynamic analysis of a freeze-dryer utilizing hygroscopic solution

Freeze-drying is usually applied to dry bioproducts, pharmaceuticals, and high-valued foodstuffs. Due to high running cost and energy consumption of conventional freeze dryers, wider applications of freeze-drying technology are restricted. A freeze dryer based on a closed regeneration loop of hygroscopic solution is proposed here. Hygroscopic solutions such as LiCl-water solution are used as working fluid to absorb the vapor sublimated from the stuffs. The low vapor pressure of hygroscopic solution leads to a higher operating temperature of water vapor absorber compared with the operating temperature of traditional vapor condenser. Thermodynamic analyses are conducted to illustrate the advantages of this new freeze dryer. A case study performed with coffee shows a coefficient of performance improvement of 25.1% of the refrigeration system and a reduction of 14.6% of energy consumption, in which 8.0% cut of heat consumption is obtained because of the reuse of regeneration heat. Parametric analyses on the impacts of evaporation temperature and condensing temperature are also performed. It is demonstrated that the new freeze dryer has high potential of energy saving in wide operation ranges. The security issue of using hygroscopic solution in food freeze-drying has also been discussed. Considering other benefits altogether, it is believed that such a freeze dryer is promising in practical freeze-drying of foodstuffs.