亚/超临界水中典型硫酸盐 Na₂SO₄和 K₂SO₄的溶解特性及机理

Supercritical water oxidation is an effective technology capable of efficiently and harmlessly treating toxic and complex organic wastes. However, inorganic salt solubility decreases dramatically near the critical point of water, and deposited salt can lead to reactor clogging, which has become a fundamental bottleneck for the large-scale application of this technology. Therefore, the dissolution characteristics and mechanisms of typical sulphates Na₂SO₄ and K₂SO₄ in sub-/supercritical water were investigated in this study. Na₂SO₄ and K₂SO₄ solubilities in water were obtained over a wider temperature and pressure range. It was found that the solubilities of Na₂SO₄ and K₂SO₄ increased about 8931 times and 36211 times in the water density range of 84.13—540.46kg/m³, respectively. The solubility of K₂SO₄ was higher than that of Na₂SO₄ under the same conditions. Solubility did not strictly increase with increasing water density. Na₂SO₄ solubility (64.375mg/L) at high density (215.18kg/m³ at 25MPa and 663.15K) was significantly 13 times lower than that (906.141mg/L) at low density (195.73kg/m³ at 21MPa and 643.15K). Because the former was in the supercritical state while the latter was in the subcritical state. The decreasing rate of solubility at 643.15—663.15K was much higher than that at 663.15—723.15K, which was consistent with the trend of water density and dielectric constant with temperature. Micro-mechanisms such as ion nucleation properties in the binary brine system of Na₂SO₄ and K₂SO₄ were revealed by molecular dynamics. Cl⁻ had a lower charge/radius ratio than SO₄²⁻ and SO₄²⁻ with polyatomic ionic structure had more coordination layers than Cl⁻ with monatomic, thus the solubility of chloride salts was higher than that of sulphate salts. Hydrated Na⁺, K⁺ and SO₄²⁻ ions could be formed at room temperature and pressure, and water molecules had a strong electrostatic shielding effect on salt ions. The electrostatic shielding effect was weakened under supercritical conditions, and ions collided and aggregated to form clusters, resulting in salt crystals. The results could provide a guidance for the further development of supercritical water technology.