Crystallization and deposition of mixed potassium and sodium salts in supercritical water

Abstract

The salt deposition problem seriously limits the industrialization application of hypersaline organic wastes in supercritical water gasification (SCWG). This study selected Na₂CO₃, K₂CO₃, Na₂SO₄, K₂SO₄, NaCl, and KCl to investigate the crystallization and deposition of mixed potassium and sodium salts via molecular dynamic simulations and experiments. The findings of molecular dynamics demonstrate that sodium salt will form a more compact cluster than potassium salt under the same conditions. The addition of potassium into sodium salt will increase the binding energy between salt and water molecules, indicating that adding potassium salt into sodium salt can hinder salt crystallization. Experiments confirm that the increase of K: Na can promote salt deposition into the reactor bottom under gravity instead of sticking to the reactor wall. As the K: Na increases from 0.0625 to 0.5, the proportions of Na₂CO₃, Na₂SO₄, and NaCl deposition at the reactor bottom increase from 38.16 %, 35.98 %, 55.05 %, to 50.39 %, 43.48 %, 68.59 %, respectively. In addition, the morphology analysis on salt deposition reveals that adding potassium salt into sodium salts can change the shape of mixed salts into that of potassium salts. The results provide fundamentals for salt management and separation in SCWG.