Analysis of Ice Propagation and Salt Diffusion in Freezing Desalination

Abstract

Freezing desalination (FD) is attractive and more sustainable option due to the low energy consumption and less maintenance. However, technical challenges such as low freezing rate and process complexity arising from the salt trapping in the produced ice hinder the commercialization of this method. In this work, freezing experiments of water-NaCl solution are carried out to investigate the ice propagation and its salinity distribution to understand the salt trapping phenomena. Results reveals subcooling followed by a homogenous temperature jump occurs in the solution at an early stage of freezing suggesting ice freezing happens to large solution bulk. The Ice-front in brine is also characterized with many sharp and disorderly ice tips and there are many channels filled with liquids inside the formed ice block. This heterogeneous growth of ice promotes salt trapping. The salt distribution in the formed ice reveals that the salinity gradient in that early freezing stage is quite small and it rises dramatically at the last several layers of ice. The salinity in the first 10% frozen ice is only half of initial salinity which suggests a suitable freezing fraction of initial solution in freezing desalination. Finally, the developed first principle model based on the obtained salinity gradient is proved to be reasonably accurate to predict the freezing time with acceptable discrepancies and less computation cost.