High-pressure experimental vapour-liquid-liquid equilibrium measurements and modelling for natural gas processing: Equipment validation, and the system CH₄+nC₆H₁₄+H₂O

Despite its relevance for oil and gas separation processes, surprisingly few experimental three-phase data are available in the open literature for gas-condensate-water systems. In support of proposed subsea processing facilities, a three-phase equilibrium measurement apparatus has been recommissioned for the measurement of C1+nC6+H2O systems between 303 and 323 K.

The apparatus has been validated by comparison with literature VLE data for C1+H2O and C1+nC6, with the experimental uncertainty estimated at ± 9% (95% confidence interval). 81 new three-phase C1+nC6+H2O data points have been generated for three different feed mixtures. The relative compositions in each phase are strongly correlated to the experimental feed composition, while the solubility-temperature trends indicate complex interactions in the system.

The repeatability of the measurements ranges from below 1% for major components up to 10–20% for trace components in the respective phases. Experimental standard deviations are comparable with those reported for literature data. The data have been modelled using the CPA and sPC-SAFT equations of state.

Satisfactory results were achieved for major components (accuracy between 1 and 6%), while order of magnitude errors were observed for certain trace components (e.g. water in the HC-rich phase). Overall, sPC-SAFT provided slightly better predictions compared to the CPA, but the differences are rather modest.