The system hydrogen ‐ water up to 440°C and 2500 bar pressure

Abstract

Abstract The system H 2 — H 2 O has been studied isochorically from 0.5 to 90 mol‐% H 2 and up to 440°C and 2500 bar pressure using an autoclave containing two sapphire windows through which phase transitions could be observed at elevated temperatures and pressures. — The system was found to exhibit so‐called “gas‐gas” immiscibility with a critical curve proceeding to higher temperatures and pressures from the critical point of pure water. Within the range of these experiments, the critical temperature of H 2 —H 2 O mixtures does not change greatly from that of pure water (e.g. T c = 381.3°C at p c = 2520 bar for 38 mol‐% H 2 ). pVT measurements have been made in the homogeneous region and excess volumes have been calculated at 400°C and at different pressures. At 300 bar, the excess volumes are relatively large and positive (e.g. V E = 58.0 cm 3 mol −1 at 40 mol‐% H 2 ) whereas at 2500 bar, excess volumes of H 2 —H 2 O mixtures indicate only small positive or negative departures from ideality.