Macro-characteristics and micro-mechanisms of Na₂SO₄ precipitation dissolved by KNO₃ molten salt in a continuous supercritical water system

This work aims to explore the ability of molten salt to solve salt deposition in supercritical water (SCW) related technologies including supercritical water oxidation and supercritical water gasification, with KNO₃ and Na₂SO₄ as examples. In the pure KNO₃ solution, the two-phase layering of high-density KNO₃ molten salt (settling at the reactor bottom) and low-density saturated KNO₃-SCW salt solution (flowing out at the top outlet of the reactor) was formed in a kettle-reactor with about 6.5 ratio of depth to inner diameter, thereby improving the accuracy of measured solubilities. The precipitation macro-characteristics of mixed KNO₃ and Na₂SO₄ in SCW were investigated under different feed concentration conditions. The results showed that Na₂SO₄ deposition on the reactor sidewall could be reduced by more than 90 % when the mass ratio of KNO₃ to Na₂SO₄ in the feed was only 0.167. No visible salt deposition was observed on the sidewall when the ratio was 0.374. All solid deposited salts were converted into the liquid molten salt as the ratio reached 3.341, and thus could easily flow out of the reactor, without plugging. ‘Molten salt dissolution’ mechanism may provide a more plausible explanation for mixed KNO₃ and Na₂SO₄ in SCW. In addition, the precipitation micro-mechanisms of mixed KNO₃ and Na₂SO₄, and the critical conditions of avoiding sidewall deposition and reactor plugging were proposed. This work is valuable for overcoming the salt deposition problem in SCW-related technologies.