Solubility and Critical Phenomena in Ternary Aqueous Solutions Containing Type-2 Salt and Alkali

Supercritical phase equilibria in the ternary system K₂SO₄–KOH–H₂O at 420–500°C and up to 130 MPa pressure with binary boundary subsystems of different types are studied. The binary subsystem of type 1 features no critical phenomena in saturated (l = g) aqueous solution and no phase separation (l1–l2) (KOH–H₂O); the binary subsystem of type 2 is characterized by immiscibility of the liquid phase and has two critical end-points \(p(g = l-_{S_{K_{2}SO_{4}}})\) and \(Q(l_{1} = l_{2}-_{S_{K_{2}SO_{4}}})\) in saturated aqueous solution (K₂SO₄–H₂O). The ternary system has two three-phase equilibria (g–l–s) and (l1–l2–s), separated by a two-phase supercritical fluid region \((fl-_{S_{K_{2}SO_{4}}})\), and two types of monovariant critical curves \((g=l-_{S_{K_{2}SO_{4}}})\) and \((l_{1}=l_{2}-_{S_{K_{2}SO_{4}}})\). The three-phase regions approach each other upon temperature increase up to the point where the two-phase supercritical equilibrium disappears, and the two mentioned monovariant critical curves are joined into a double homogeneous critical point \((g=l-_{S_{K_{2}SO_{4}}} \leftrightarrow l_{1} = l_{2}-_{S_{K_{2}SO_{4}}})\) at maximum temperature ~445°C and 51–52 MPa.