Effects of Superheated Surface on the Deposition Behavior of Na2SO4 in Supercritical Water

Abstract

The reduced solubility of inorganic salts in supercritical water has a significant impact on the stable operation of desalination facilities as it may lead to surface fouling due to salt deposition. In this study, the solubility of Na2SO4 was experimentally determined to be 0.04–15.34 mmol/kg water at 23–25 MPa and 390–420 °C. To investigate the precipitation behavior of Na2SO4 in supercritical water, a reactor with a heating bar was designed and the deposition effect of salt on the superheated surface in an autoclave was tested at a temperature of 390 °C and a pressure of 23 MPa. Then, the deposition mechanism of salt in the autoclave was analyzed and the temperature field in the reactor was simulated using CFD commercial software. The experimental results showed that Na2SO4 was present on both the heating rod and the bottom of the autoclave with a loose salt layer. The simulation results indicated that the temperature near the heating rod was significantly higher than the bulk fluid temperature and it provided the temperature condition where the inorganic salt preferentially nucleated and precipitated. Nickel foam was chosen as the porous media carrier to investigate the selective precipitation of salt on different superheated surfaces. The results showed that nickel foam could collect a large amount of salt on the heating rod and change the state of the compact salt layer on the kettle bottom wall, providing a technical choice for salt recovery under supercritical conditions.