Phase Equilibrium and Thermodynamic Model for the Na+//Cl–-SO42–-HPO42–-H2O System

After the NaClO solution was used to purify acetylene gas, the solution contained significant amounts of NaCl, Na₂SO₄, and Na₂HPO₄. The phase equilibrium data is the most important basic information for recovering sodium salts. In this study, the solubilities and dry-salt phase diagram for the system (Na⁺//Cl^–-SO₄^2–-HPO₄^2–-H₂O) were systemically studied. The dry-salt phase diagram contained one invariable point, three univariate solubility curves, and three single salt crystallization regions corresponding to NaCl, Na₂SO₄, and Na₂HPO₄·12H₂O at 298.15 K and 0.1 MPa. Based on the ELEC-NRTL and Pitzer model embedded in the Aspen Plus, the phase equilibrium data for the ternary subsystem (Na⁺//Cl^–-SO₄^2–-H₂O, Na⁺//Cl^–-HPO₄^2–-H₂O, and Na⁺//SO₄^2–-HPO₄^2–-H₂O) at 298.15 K and 0.1 MPa taken from the literature were first predicted to verify the applicability of parameters from the default database and literature. Subsequently, the models were optimized by literature solubility data, and then, the obtained parameters can well predict the phase equilibrium data for the ternary subsystem at 298.15 K and 0.1 MPa. All of these will provide basis data for the separation of sodium salt from the Na⁺//Cl^–-SO₄^2–-HPO₄^2–-H₂O system.