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

Supercritical phase equilibria in the ternary system K 2 SO 4 –KOH–H 2 O 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–H 2 O); 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 S O 4 ) and Q ( l 1 = l 2 − S K 2 S O 4 ) in saturated aqueous solution (K 2 SO 4 –H 2 O). The ternary system has two three-phase equilibria (g–l–s) and (l1–l2–s), separated by a two-phase supercritical fluid region ( f l − S K 2 S O 4 ) , and two types of monovariant critical curves ( g = l − S K 2 S O 4 ) and ( l 1 = l 2 − S K 2 S O 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 S O 4 ↔ l 1 = l 2 − S K 2 S O 4 ) at maximum temperature ~445°C and 51–52 MPa.