Nucleation and kinetics of mixed salts in scaling

Abstract

AbstractDuring membrane and thermal desalination, the saline solution becomes supersaturated. Once the solution becomes unstable, nucleation and crystallization of the salts occur resulting in a buildup of an unwanted deposit layer on transfer surfaces and increasing the mass‐ or heat‐transfer resistance of the system and the energy input for producing water. Due to the process complexity, research focused primarily on single salt fouling and applying the nucleation and kinetics of a single system in developing physical models for precipitation (crystallization) fouling. This article demonstrates that single salt nucleation and crystallization kinetics applied so far to crystallization fouling cannot be extended to real situations when a combination of various salts and solutes are present. Other noncrystallizing species, if present, may affect the surface energy of a crystal and alter the Gibbs free energy for homogeneous nucleation. Other crystallizing species may also act as a nucleus initiating nonhomogeneous nucleation with a lower Gibbs free energy and promoting nucleation. The presence and catalytic and inhibitory effects of dissolved and solid species on metastability of the solution should be considered in kinetic analysis of crystallization fouling. Kinetic relationships developed take into account the kinetics of coprecipitating salts with and without a common ion and in the presence of other soluble species. They can be easily extended to multiprecipitating systems, but further advances in experimental techniques are necessary to integrate the kinetic analysis and experimental data.