455518 Phase Behavior and Density Measurements of (CO2 + Toluene) at Temperatures from (298 to 448) K and Pressures up to 65 Mpa
Carbon Capture and Storage (CCS) is a crucial technology in the drive to reduce anthropogenic CO2 emissions with the aim of keeping the global temperature rise below 2 oC above pre-industrial levels.1 According to an IEA report in 2015, in all sectors the amount of CO2 captured and stored needs to be about 6 billion tonne per year until 2050 to achieve the 2 oC target.2 CCS technologies can be applied in many industrial processes such as natural gas, hydrogen, steel and cement production, power generation and so on. Early commercial projects have been achieved by combining CCS with enhanced oil recovery (EOR) technology.
The understanding of the thermophysical properties for CO2 + hydrocarbons systems at reservoir conditions is very important for the correct design and optimization of carbon storage and EOR. The available saturated-phase densities, high-pressure liquid densities and phase behaviour data for these mixtures are limited, especially at reservoir conditions.3 These thermophysical properties are essential to determine the amount of CO2 that can be stored in depleted oil reservoirs as well as to understand the convective transport through the reservoir. Such experimental data are also important in a wide range of industrial applications. Furthermore, the thermodynamic models used for reservoir and/or process simulation are optimized and/or validated with these experimental data. In this work, new density and phase behaviour measurements of (CO2 + toluene) systems have been made at temperatures from (298 to 450) K and pressures up to 70 MPa. The measurements have been carried out in specially-developed apparatus, previously reported by our group.4,5,6 New empirical models have also been developed to represent the densities for this particular mixture. Finally, the new density and vapour-liquid-equilibrium data are compared with the predictions of the SAFT-γ Mie7 and Peng-Robison equations of state. A key feature of the present study is that it incorporates both phase-equilibrium and phase-density measurements over extremely wide ranges of temperature and pressure, permitting the thermodynamic models to be tested in a rigorous way.
Results and Modelling
Dew- and bubble-point measurements for (CO2 + toluene) were carried out at T/K = 298.15, 323.15, 373.15 and 423.15 and at pressures up to 20 MPa by means of a synthetic method described previosuly.4 Critical points were also determined by direct visual observation. These vapour-liquid-equilibria results were found to be in good agreement with data existing in the literature.
Saturated-phase density measurements for (CO2 + toluene) were performed along six isotherms from 298 K to 450 K and at pressures up to the critical. On most of the isotherms investigated, the bubble-point densities decreased, and the dew-point densities increased, monotonically with increasing pressure. However, at T = 298 K and T = 323 K, the bubble-point densities initially increased with increasing pressure until a maximum was reached after which they deceased monotonically to the critical point. The experimental dew- and bubble-point data were fitted on each isotherm, with absolute average deviations of about 2 kg·m-3, using an empirical function incorporating an appropriate non-analytic term. This permitted the critical pressure to be estimated precisely and the results were found to be in close agreement with previous data.8
The high-pressure compressed-liquid densities were measured with a vibrating U-tube densimeter as a function of pressure along nine isotherms at T/K = 283, 298, 323, 348, 373, 398, 423, 448 and 473 at pressures up to 70 MPa. The carbon dioxide mole fractions in the binary mixtures studied were 0.0, 0.2, 0.4, 0.6 and 0.8 and the overall uncertainty of the density measurements was ≤ 1 kg·m-1 at 95 % confidence. In order to maintain the homogeneity of the mixture, only pressures above the bubble curve were considered. An empirical model was used to correlate the high-pressure liquid densities along isotherms. To verify the consistency of the data, we estimated the bubble pressures at each composition by combining the high-pressure liquid-phase and the saturated-phase density data. Bubble points obtained from these densities measurements matched the ones determined directly from the VLE measurements.
The new density and vapour liquid equilibrium data were compared with the predictions of the SAFT-γ Mie equation of state, which implements a group-contribution approach and the Mie potential to represent segment-segment interactions. The results showed that the SAFT-g Mie model predicted the phase behaviour, saturated-phase densities and high-pressure compressed-liquid density for this mixture very well. For comparison, we also tested the using Peng-Robison equation of state with the classical mixing rules and one binary parameter. As expected, this simple model represented the vapour liquid equilibrium quite well but gave inferior results for the saturated-phase and compressed-liquid densities.
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We gratefully acknowledge the funding of QCCSRC provided jointly by Qatar Petroleum, Shell, and the Qatar Science and Technology Park, and their permission to publish this research.