Nucleation characteristics and dissolution mechanism of Na₂SO₄ salts under molten NaOH condition in supercritical water

AbstractSodium sulfate (Na₂SO₄) easily precipitates on the equipment surfaces, contaminating or clogging supercritical water (SCW) systems. Hydrothermal molten salt (HyMoS) technology shows promise for dissolving salt deposits and improving system safety. This work investigated the nucleation characteristics of Na₂SO₄ salt deposits in SCW and their removal upon addition of NaOH. Compared with a pure Na₂SO₄ solution, the binding probability of Na+ and SO₄2−, the diffusion coefficients of Na₂SO₄ clusters and water, and the total number of nucleated Na₂SO₄ particles were all reduced in the mixed solution. After autoclave experiments, Na₂SO₄ particles were observed to deposit only on the top of the autoclave, not on the walls or the bottom. This supports the conclusion that molten NaOH plays a positive role in dissolving Na₂SO₄ salt deposits. Na₂SO₄ dissolution was followed by the formation of NaOH-Na₂SO₄ HyMoS, which can be discharged as the effluent to fundamentally resolve the salt deposition problem.