Abstract:

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 SO₄^2- and SO₄^2- 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 SO₄^2- 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.

Key words: supercritical water, sodium sulfate, potassium sulfate, solubility, molecular dynamics simulation

摘要：

超临界水氧化是一种能够高效无害化处理有毒和复杂有机废物的前沿技术。然而，无机盐溶解度在水临界点附近急剧降低而发生盐沉积，导致反应器堵塞，已成为了该技术大规模推广应用的基础性瓶颈问题。因此，本文对亚/超临界水中典型硫酸盐Na₂SO₄和K₂SO₄的溶解特性及机理进行了研究。获得了更宽温度和压力范围内Na₂SO₄和K₂SO₄在水中的溶解度数据。在84.13~540.46kg/m³水密度范围内Na₂SO₄和K₂SO₄溶解度分别增加了约8931倍和36211倍，相同条件下K₂SO₄溶解度高于Na₂SO₄。溶解度并非严格随水密度增加而增加，Na₂SO₄在25MPa和663.15K的高密度（215.18kg/m³）下溶解度（64.375mg/L）明显低于在21MPa和643.15K的低密度（195.73kg/m³）下溶解度（906.141mg/L），近似水密度下溶解度相差了约13倍，因为前者处于超临界态而后者处于亚临界态。溶解度在643.15~663.15K范围内下降速率远高于663.15~723.15K，与水密度和介电常数随温度的变化趋势一致。通过分子动力学揭示了Na₂SO₄和K₂SO₄二元盐水体系中的离子成核特性等微观机理。Cl^-的电荷/半径比低于SO₄^2-并且多原子离子结构的SO₄^2-的配位层比单原子离子结构的Cl^-多，所以氯盐溶解度高于硫酸盐。常温常压下会形成水合Na⁺、K⁺和SO₄^2-离子，水分子对盐离子的静电屏蔽作用较强。超临界条件下水分子的静电屏蔽效应减弱，离子碰撞聚集形成团簇，从而产生盐晶体。研究结果能够为超临界水技术的进一步发展提供指导。

关键词: 超临界水, 硫酸钠, 硫酸钾, 溶解度, 分子动力学模拟