Thermophysical properties of hydrocarbons with dissolved carbon dioxide
Creators: name: Tay, Weparn Julian
Publisher: name: Imperial College London
Year: 2017

description: Although the thermophysical properties of hydrocarbons have been studied extensively, there remain important gaps in the literature for high-temperature and high-pressure (HTHP) conditions and for mixtures of hydrocarbons with CO2 and other inorganic gases. In this work, thermophysical properties of hydrocarbons with the addition of carbon dioxide (CO2) were studied under conditions of elevated temperatures and pressures that are representative of CO2 injection into hydrocarbon fields for purposes of carbon storage, enhanced oil recovery (EOR) or both. The main focus of this research is to investigate the thermophysical properties experimentally mainly density and sound-speed, in relation to (hydrocarbon + CO2) systems. The representative hydrocarbons chosen were: n-pentane, n-heptane, n-nonane and methylbenzene. Speed-of-sound measurements for (n-pentane, n-heptane, n-nonane, methylbenzene, CO2 + n-nonane and CO2 + methylbenzene) were performed using the double path pulse-echo technique in a state-of-the-art apparatus. It is based on an ultrasonic cell with a single 5 MHz ultrasound transducer placed between two unequally spaced parallel reflectors. The speed-of-sound measurements were carried out at temperatures ranging from (263.15 to 473.15) K and at pressures up to 390 MPa. The path-length of the ultrasonic cell was pre-calibrated with deionised water at T = 298.15 K and p = 1 MPa against the speed-of-sound given by the 1995 equation-of-state formulation of the International Association for the Properties of Water and Steam (IAPWS-95) which for that state point, has an uncertainty of 0.005%. The expanded uncertainty of the sound-speed measurements presented in this work, in general, is less than 0.1 %. The experiment sound-speed data collected were fitted with an empirical polynomial correlation with average absolute relative deviations (∆AARD) of approximately 0.06 % which is consistent with the specified uncertainty of the measurements. The addition of CO2 decreases the overall speed-of-sound in the CO2 + hydrocarbons mixtures. Density measurements were carried out for (n-heptane, n-nonane, methylbenzene, CO2, CO2 + n-heptane, CO2 + n-nonane and CO2 + methylbenzene) with a vibrating-tube densimeter (VTD) at a temperature range of (283.15 to 473.15) K and pressures up to 68 MPa. The measurements were associated with an absolute uncertainty of less than 1 kg∙m-3. Different calibration methods of the VTD were reviewed and a new calibration method based on the physical parameters of the densimeter was proposed. The VTD used in the present work was calibrated under vacuum, helium and water over the full temperature and pressure range of interest. The experimental densities show a typical density trend for liquids: increase with pressure but decrease with temperature. The experimental density for pure components was fitted by the modified Tait equation, permitted the interpolation/extrapolation of the data as a function of temperature and pressure to compare with the available literature. The average absolute relative deviations between the experimental data and the predictions were approximately 0.04 % across the pressure and temperature range investigated. For the density of the binary mixtures, a density crossover was observed with the addition of CO2. A non-linear empirical correlation was developed which fitted the experimental data with an ∆AARD of approximately 0.1 %. The excess molar volumes were calculated from the binary mixtures densities, showing a highly non-ideal mixing behaviour of the binary systems studied. The results were correlated with the Redlich-Kister polynomial equation. Bubble pressures for (CO2 + n-heptane and CO2 + methylbenzene) were obtained by combining the binary mixtures densities with the saturated-phase densities. In the cases where the speed-of-sound and density for pure hydrocarbons were measured (n-heptane, n-nonane and methylbenzene), the experimental data were combined with knowledge of isobaric heat capacity to derive a complete spectrum of thermodynamic properties over the range of temperatures and pressures covered by the sound-speed measurements by means of thermodynamic integration. In addition, a study on the thermophysical properties of two types of hydrocarbon oils has been carried out. Viscosity and isobaric heat capacity measurements have been included on top of the properties mentioned above; empirical correlations were developed for the measured properties. Lastly, the experimental data for the hydrocarbon systems have been compared with predictions calculated from various types of equations of state (EoSs): PPR78, SAFT-γ Mie, and the multi-parameter Helmholtz energy explicit equations of state. It can be concluded that the agreement between the experimental data and predictions from these models is satisfactory. However, there is scope for further improvement in the performance of the equations of state in relation to the binary mixtures considered in this work. The present work greatly extends the available literature by providing an accurate set of both experimental density and speed-of-sound data over a large temperature range of (283.15 to 473.15) K with pressures up to 390 MPa. The results obtained in this study will permit equations of state and other thermodynamic models to be tested in a more rigorous manner. Further experimental studies and research areas were identified at the end of the thesis.