Na2SO4–NaCl–H2O system with a binary homogeneous critical point: Phase equilibria at 475–520°C and to 130 MPa

Abstract

Phase equilibria were studied at temperatures 475–520°C and a pressure to 130 MPa in the ternary system Na₂SO₄–NaCl–H₂O with boundary binary subsystems of two types. In type 1 subsystems (the NaCl–H₂O subsystem in this work), there are no critical phenomena in saturated solutions. Type 2 subsystems (the Na₂SO₄–H₂O subsystem in this work) have terminal critical points p (G = L – \({S_{N{a_2}S{O_4}}}\)) and Q (L₁ = L₂ – \({S_{N{a_2}S{O_4}}}\)). It was shown that the ternary system contains two regions of three-phase equilibria ((G–L–S) and (L₁–L₂–S)), divided by a two-phase fluid region (F – \({S_{N{a_2}S{O_4}}}\)), and two types of monovariant critical curves ((G = L – \({S_{N{a_2}S{O_4}}}\)) and (L₁ = L₂ – \({S_{N{a_2}S{O_4}}}\))). With increasing temperature, these three-phase regions approach each other until the two-phase fluid equilibrium vanishes and the monovariant critical curves meet at a binary homogeneous critical point (G = L–S ⇔ L₁ = L₂ – \({S_{N{a_2}S{O_4}}}\)) at a maximal temperature of ~495°C and a pressure of ~75 MPa.