Heat and mass transfer analysis and optimization of freeze desalination utilizing cold energy of LNG leaving a power generation cycle

Abstract

Freeze desalination (FD) works upon the separation of impurities from pure water during ice crystals formation. The required cold source could be supplied by the cold energy of liquefied natural gas (LNG). In the current study, freeze desalination of seawater is explored by directly exploiting the cold energy of LNG within an appropriate range of temperature after producing work in a power generation cycle. A detailed discussion has been given on the inlet temperature of LNG to the FD unit for the first time. The direct utilization has the privilege of eliminating the addition of a secondary refrigerant and its refrigeration cycle to the FD process. A multi-objective optimization is conducted to specify the optimal conditions to acquire the highest ice mass with lowest salinity. Results show that decreasing LNG inlet temperature from −10 to −60 °C improves the ice mass production from 60.9 to 977.6 g, whereas diminishing its quality by 90%. Also, increasing Reynolds number of LNG from 4000 to 32,000 leads to the production of 492 g more ice and increment of ice salinity from 1.22 to 1.56%. Finally, conducting multi-stage freezing reveals that for Reynolds numbers below 16,000, potable water is achievable after three-stage desalination.