Comparative study of vapor-liquid phase equilibrium methods to measure partitioning coefficients of dissolved gases in hydrocarbon oils

Abstract

This work presents a comparative study of three categories of vapor-liquid phase equilibrium methods (VLE methods) for determining the partitioning coefficients (K) of gases (H2, O2, N2, CH4, CO, CO2, C2H2, C2H4, C2H6, C3H6 and C3H8) in gas-oil systems: i) a direct vapor-phase calibration method (VPC), ii) three indirect methods such as equilibrium partitioning in closed system (EPICS), phase ratio variation (PRV) and phase ratio calibration (PRC) and iii) two alternatives of the multiple headspace extraction method (MHE). The methods were primarily tested for their capability of delivering in a single application all the coefficients needed for the external calibration of a static headspace-gas chromatographic procedure used to assess the species in oil samples. The results showed that the VPC approach is the most qualified method in terms of overall precision and accuracy for measuring the coefficients. Excellent results are also noted when the individual species are considered: an RSD better than 2% is measured for all species on three independent trials, except for H2, O2, N2 and CO where a slightly higher level of uncertainty is observed. The results showed also for the PRC method a quality of results very close to what is achieved with the direct method. It has to be noted that these findings do not exclude the possibility that the other VLE methods could deliver better results than seen in this comparison if applied under the optimal species conditions. However, it is almost impossible to meet these conditions in this range of K-values for the gas-oil systems and even if they could have been met, multiple method applications would have to be envisaged. Finally, the gas-oil partitioning coefficients obtained with the VPC method were further validated by demonstrating their dependence with the temperature.