Liquid nitrogen spray injection for direct-contact freeze concentration applications

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. This study investigated the feasibility of utilizing liquid nitrogen as the coolant to perform direct-contact freeze concentration. A proof-of-concept laboratory direct-contact freezer was developed, and experimental trials were conducted for various concentrations of the solute solution with 1 kg initial total solution weight. At the start of the process, cold nitrogen vapor, generated from liquid nitrogen due to the initially warm system, was introduced to pre-cool the cryogenic piping, contactor column, and liquid solution. Once the cryogenic injection system was sufficiently cooled down, liquid nitrogen was directly injected into the contactor column to cool the liquid solution and facilitate solidification. The experimental trials successfully demonstrated the formation of ice particles where the average size of ice particles decreased with an increase in the solute concentration of the solution. An ice growth rate of around 4 g/s was observed in the trials, along with an average partition coefficient of approximately 0.6. 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.