Liquid Nitrogen Enabled Direct Freeze Concentration

Freeze concentration processes, which remove water molecules as ice crystals to yield a product with increased solute concentration, typically avoid direct contact between the coolant and the solution. However, the non-toxic and inert nature of liquid nitrogen makes it suitable to act as a coolant in direct contact with the processed solution. Through innovative research combining theoretical and experimental approaches, the work covered micro-, meso-, and macro-scales on the boiling heat transfer mechanisms of liquid nitrogen in immiscible liquids and its application in direct-contact freeze concentration processes. At the micro-scale, for the first time, a two-dimensional quasi-steady-state heat conduction model is proposed, enabling accurate prediction of the evaporation rate of liquid nitrogen droplets during the spherical bubble stage. This model eliminates the reliance on empirical parameters inherent in existing simplified one-dimensional models and further explores the impact of convection during the later stages of evaporation. At the meso-scale, a novel dynamic solute distribution model is proposed and validated at the ice interface, increasing freeze concentration separation efficiency by approximately 40%. This work provides a solid theoretical foundation for understanding heat and mass transfer processes during liquid nitrogen cooling of multicomponent solutions and ice phase change. At the macro-scale, a multiphase flow reactor is designed and tested for direct-contact cooling multicomponent solutions with liquid nitrogen, demonstrating the feasibility of achieving freeze concentration through injection of liquid nitrogen for direct-contact cooling. This study demonstrated the potential of using liquid nitrogen as a direct-contact coolant in freeze concentration, identifying the challenges and providing valuable insights to improve the design and operation of the system.