High pressure phase equilibria and critical phenomena of water + iso-butane and water + n-butane systems to 695 K and 306 MPa

The equilibrium surface (x, p, T, Vm) for water+iso-butane was measured and additional equilibrium data for water+n-butane are reported. Mole fractions of water for water+iso-butane from 0.5 to 0.98, pressures up to 306 MPa and temperatures up to 695 K were studied. Critical curves, represented as the envelope of the isopleths starting at the critical point of water (647 K), went through a temperature minimum and then increased at higher pressures and higher temperatures, giving evidence for (gas+gas) equilibria of the third type. The results are compared with the phase equilibrium of previously measured (water+n-alkane) mixtures. The excess molar volumes, VE, and excess molar Gibbs energies, GE, for water+n-butane on the phase equilibrium surfaces have been calculated for 600 and 620 K from 40 to 200 MPa. All the VE and GE values were found to be positive.