On the Potential for Homogeneous Nucleation of Salt From Aqueous Solution in a Natural Convection Boundary Layer

Recent studies have examined the rate of salt deposition by natural convection on a cylinder heated above the solubility temperature corresponding to the concentration of salt in the surrounding solution at conditions typical of the Supercritical Water Oxidation (SCWO) process (Hodes et al. [1,2], Hodes [3]). The total deposition rate of salt on the cylinder is the sum of the rate of deposition at the salt layer-solution interface (SLSI) formed on the cylinder and that within the porous salt layer. The rate of deposition at the SLSI cannot be computed without determining whether or not salt nucleates homogeneously in the adjacent (natural convection) boundary layer. A methodology to determine whether or not homogeneous nucleation in the boundary layer is possible is presented here. Temperature and concentration profiles in the boundary layer are computed under the assumption that homogeneous nucleation does not occur. If, under this assumption, supersaturation does not occur, homogeneous nucleation is impossible. If supersaturation is present, homogeneous nucleation may or may not occur depending on the amount of metastability the solution can tolerate. It is shown that the Lewis number is the critical solution property in determining whether or not homogeneous nucleation is possible and a simple formula is developed to predict the Lewis number below which homogeneous nucleation is impossible for a given solubility boundary and set of operating conditions. Finally, the theory is shown to be consistent with experimental observations for which homogeneous nucleation is absent or present.