Salt Separation During Supercritical Water Oxidation of Human Metabolic Waste: Fundamental Studies of Salt Nucleation and Growth

Supercritical water oxidation (SCWO) is currently being considered as a means of waste/water recycle for long-term space missions. Solid salts are produced in the process of oxidizing organic material in supercritical aqueous ionic solutions. Knowledge of nucleation and crystal growth rates of salts in supercritical water is needed for efficient salt separator design in the waste treatment process. This paper describes an experimental and theoretical program which is currently underway at MIT under sponsorship from NASA-JSC (Grant # NAG9-252) to obtain a fundamental understanding of these phenomena. The extension and application of conventional thermodynamic models for the prediction of salt solubility in supercritical water is being explored. An apparatus has also been constructed for high temperature and pressure aqueous salt solution studies. An optical cell with sapphire (α-Al₂O₃) windows is being used to perform experiments examining phase behavior and precipitation in salt-water systems above and near the critical point of water (374 °C and 221 bar).