Thermodynamic properties of a CO2 – rich mixture (CO2 + CH3OH) in conditions of interest for carbon dioxide capture and storage technology and other applications
Graphical abstract
Introduction
Carbon dioxide capture and storage (CCS) is considered one of the most important technologies to reduce the world’s emissions of greenhouse gases. In the International Energy Agency’s two-degree scenario (2DS), CCS is expected to help reduce global emissions by storing approximately 7 gigatons per year by 2050 [1]. This amount is much greater than that used for enhanced oil and gas recovery purposes (approximately 50 megatons per year in the USA [2]). To optimize the process efficiency, the will have to be transported from the capture plants to reservoirs predominantly in high-pressure pipelines [3].
CCS technology comprises three main steps: anthropogenic capture, transport and storage. The design of each step is influenced by the thermodynamic procedure used to model the fluid behaviour [3]. Whether using an existing procedure or developing a new one, experimental data on the physicochemical properties of mixtures with the impurities typically present in anthropogenic are needed in wider composition, temperature and pressure ranges than those associated with CCS technology [4]. However, the paucity of experimental values precludes the development of a reference model for this technology, especially an equation of state (EoS), which is one of the most critical future challenges [5].
To develop an EoS, the essential data are the volumetric properties (pressure–density–temperature, ) and the vapor-liquid equilibrium, , although reliable values for the speed of sound, , and the isobaric specific heat capacity, are necessary as well. Using acoustic results to formulate EoSs is particularly attractive given that the speed of sound can be measured with outstanding precision over wide temperature and pressure ranges. Furthermore, all the thermodynamic properties of a fluid can be obtained from speed of sound measurements by integrating the partial differential equations that relate to other thermodynamic properties [6]. Currently, accurately measuring the speed of sound propagation in high-pressure fluids is one of the standard methods to precisely determine such fluids’ thermophysical properties [7].
In practice, reliable values, among others, are necessary to evaluate parameters related to transport, injection and storage [8]. Moreover, the speed of sound enables detection of the pressure drop along the pipeline and leaks, monitoring of changes in composition and the performance of seismic studies [9], [10], [11].
To estimate the temperature variations at various stages of the process, precise data from additional thermodynamic properties such as and the Joule–Thomson coefficient, μJT, are required [9], [12]. The solubility parameter, , provides information about the interactions between the injected fluid and other substances present in the storage reservoir.
All these properties are affected to a great extent by the nature and quantity of the impurities present in the anthropogenic , which, in turn, depend on the source and the capture and conditioning processes [5]. Although the main impurities are and water [4], [13], methanol can be present in transported and injected anthropogenic because of its use as a hydrate inhibitor and as a residue from pipeline drying. Thus, quantification of the effect of this impurity on the thermodynamic properties that influence CCS processes is necessary.
Furthermore, supercritical , , is the most widely used supercritical solvent in a broad range of applications, and methanol is one of the most common modifiers added to enhance the solvating power of to target polar species [14]. The solvent strength of a supercritical fluid solvent is related to its density, and it may be quantitatively represented by the solubility parameter [15]. In addition, and of the (CO2 + CH3OH) system acting as the mobile phase affect to resolution properties in supercritical fluid chromatography [16].
Density and have been widely studied in the literature [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37] for the (CO2 + CH3OH) system, however we note that little information is available on its volumetric behaviour at and at mole fractions of , , greater than 0.75 [19], [20], [21], [32], [34]. Values of have been obtained at [14], [38], [39], [40]; however, no numerical values for c, cp or μJT are available in the literature.
The aim of this work was to conduct an extensive thermodynamic study of a CO2 – rich mixture with under and conditions compatible with CCS and other applications. We therefore:
- (i)
Adapted and used an experimental apparatus to measure accurately the speed of sound in mixtures containing sufficiently dense compressed gases. We also determined the uncertainty of the experimental speed of sound measurements for the (CO2 + CH3OH) system.
- (ii)
Experimentally measured the following properties for the (CO2 + CH3OH) mixture with :
- –
The speed of sound, , between 263.16 K and 313.15 K and at pressures up to 194.49 MPa.
- –
The density, , from 263.15 K to 313.15 K and up to 20.00 MPa.
- –
- (iii)
Adapted and validated a calculation method to obtain and values and derived properties such as the volume-dependent solubility parameter, , and for systems containing compressed gases at pressures up to 195.0 MPa. All properties were obtained for (CO2 + CH3OH) with within the working temperature range. Several authors [41], [42], [43], [44], [45] have used the same fundamental approach for liquid compounds; however, this work represents its first application to compressed gases.
- (iv)
Compared either the experimental results or the calculated values of the aforementioned thermodynamic properties with two different formulation EoS: PC-SAFT [46], [47] and GERG [48], [49].
In summary, in this article we implement an experimental setup to measure c in mixtures containing compressed gases. We present the experimental results for the speed of sound (at pressures up to 194.49 MPa), and density (up to 20.00 MPa) for the (CO2 + CH3OH) mixture ; together, these results allow us to evaluate the predictive power of the PC-SAFT and GERG EoSs for these properties. From our c values and both our and the GERG EoS at a reference pressure, we calculate , , , and for pressures up to 195.0 MPa. This method of calculating thermodynamic properties up to high pressures, which is applied here to compressed gases for the first time, is validated by comparing the results with the values obtained from the PC-SAFT and GERG EoSs.
Section snippets
Chemicals
Methanol from Sigma–Aldrich (biotech. grade, mole fraction 0.9993) and carbon dioxide from Air Liquide (mole fraction >0.99998) were used without further purification. The details, including purities and sources of the materials used in this work are listed in Table 1.
Speed of sound data acquisition: experimental setup and procedure
To determine the speed of sound, we used a 5 MHz ultrasonic pulse device previously described for its application to pure fluids [50]. It was originally designed to work with liquids, and we demonstrated that it is also adequate
Calculation method of thermodynamic properties up to high pressures
Various methods have been proposed in the literature to obtain other thermodynamic properties from the speed of sound [51], thus exploiting the high precision of its experimental determination. These methods use experimental values of obtained at high pressures and several temperatures and and both at a reference pressure and as a function of temperature. From these data, calculated values of , the isobaric thermal expansivity, , the isothermal compressibility, , and are
Equations of state
In this work, we compared both our experimental values , and the calculated values explained previously with data obtained from the PC-SAFT and the GERG EoS using VLXE [58] and REFPROP 9.0 [54] software, respectively.
Experimental results obtained using the newly implemented apparatus to measure the speed of sound in mixtures
The system (CO2 + CH3OH) was studied to develop the experimental procedure for mixtures containing sufficiently dense compressed gases and to determine the uncertainty of the speed of sound measurements in those gases. The chosen compositions were at nominal temperatures of 263.15 K, 298.15 K and 323.15 K and pressures from 6.00 MPa to 190.04 MPa. The values used in this section are listed in Table S2.
The overall standard uncertainty of the experimental c
Discussion
The repeatability and overall standard uncertainty results obtained in this work, together with the agreement with the data from the literature, allow us to use our experimental and values to evaluate whether the PC-SAFT and GERG EoSs properly predict the studied thermodynamic behaviour for (CO2 + CH3OH). If the two EoSs are successful, we will use both to validate the method for calculating and up to 195.0 MPa.
Conclusions
An ultrasonic pulse apparatus and the experimental procedure were adapted to measure the speed of sound in mixtures containing compressed gases, and was measured for four (CO2 + CH3OH) mixtures ( at nominal temperatures and at pressures up to 190.04 MPa. The overall standard uncertainty of the experimental speed of sound was evaluated for the (CO2 + CH3OH) system, where the contributions of temperature, pressure, composition and
Acknowledgements
This research received funding from the Ministry of Economy and Competitiveness of Spain ENE2013-44336-R and from the Government of Aragon and the European Social Fund.
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Cited by (4)
High-pressure speed of sound in pure CO<inf>2</inf> and in CO<inf>2</inf> with SO<inf>2</inf> as an impurity using methanol as a doping agent
2016, International Journal of Greenhouse Gas ControlCitation Excerpt :For the studied compositions, c in the fluid increased with increasing pressure and decreasing temperature. We did not identify speed of sound data in the literature for this system, except data published by ourselves (Rivas et al., 2016), which are in agreement with this work. A detailed explanation of the application of the PC-SAFT EoS to CO2 + methanol is given in previous works (Gil et al., 2012; Rivas et al., 2016) in which we studied vapor-liquid equilibrium, critical locus, density and c over a wide range of temperature and pressure.
Thermodynamics of mixing methanol with supercritical CO<inf>2</inf> as seen from computer simulations and thermodynamic integration
2020, Physical Chemistry Chemical Physics