Accurate experimental (p, ρ, T) data of the (CO2 + O2) binary system for the development of models for CCS processes
Introduction
Design and operation of Carbon Capture and Storage (CCS) processes need reliable thermodynamic models able to accurately describe the behaviour of the fluid mixtures of CO2 with other gases [1], [2]. Besides CO2, the main components involved in those processes related to CCS technologies are N2, O2, Ar, H2O, H2, CO, H2S, and SO2. An approved thermodynamic model able to describe fluid mixtures of these components over an extended p,T-region is the GERG-2008 equation of state [3], which is based on a multi-fluid mixture model and explicit in the reduced Helmholtz energy. However, this EoS was originally developed for natural gas mixtures and thus does not a priori guarantee a high accuracy for mixtures with a composition far from that of typical natural gas mixtures, such as CCS mixtures with perhaps no methane present in them at all, but with CO2 as the main compound.
Some specific instances of EoS have recently been developed for mixtures involved in CCS processes. Demetriades and Graham [4] proposed a pressure-explicit EoS for mixtures of CO2 with small quantities (impurities) of N2, O2, and H2. The range of validity of this model is for pressures up to 16 MPa and temperatures between 273 K and the critical temperature of CO2. A recent research work [5] has proposed a specific model for the binary mixture (CO2 + CO). Gernert and Span proposed an equation of state for the calculation of thermodynamic properties of humid gases, combustion gases, and CO2-rich mixtures typical in CCS processes, the so-called EOS-CG (Equation of State for Combustion Gases and Combustion Gas-like Mixtures) [6]. This equation of state, with a functional structure similar to the GERG-2008, based on a multi-fluid mixture model explicit in the reduced Helmholtz energy, has a wider range of validity in temperature and pressure, and more components are considered. The EOS-CG has been developed for 6 constituting pure components: CO2, N2, O2, Ar, H2O, and CO. Unfortunately, binary specific departure functions were developed (or, in some case, taken from the GERG-2008 EoS) for only some of the 15 resulting binary mixtures, those for which sufficiently accurate experimental data were available. As for the binary mixture (carbon dioxide + oxygen), no departure function has yet been developed due to limited experimental data.
High-accuracy density data are of great relevance for the development of reliable equations of state for CCS processes [7]. In this work, accurate density measurements for three binary mixtures of carbon dioxide with oxygen (nominal amount-of-substance fraction x(O2) = 0.05, 0.10, 0.20) are presented. Measurements were performed at temperatures between (250 and 375) K and pressures up to 13 MPa, using a single-sinker densimeter with magnetic suspension coupling, which is one of the state-of-the-art methods for density determination over wide ranges of temperature and pressure. In order to achieve the highest accuracy in composition, the binary mixtures for this investigation were prepared gravimetrically according to the ISO 6142–1 [8], a method that qualifies for the production of reference materials. The experimental results are compared with the calculations by both the GERG-2008 equation of state and the EOS-CG, and also with the limited experimental data available in the literature.
Section snippets
Mixture preparation
Three (CO2 + O2) binary mixtures were prepared at the Federal Institute for Materials Research and Testing (Bundesanstalt für Materialforschung und -prüfung, BAM) in Berlin, Germany, following the recommendations given in the standard ISO 6142–1 [8].
Purity, supplier, molar mass, and critical parameters of the pure compounds (obtained from the reference equations of state for carbon dioxide [9] and oxygen [10]) are given in Table 1. Table 2 shows the gravimetric composition and its corresponding
Experimental results
Table 5, Table 6, Table 7 show the 162 experimental (p, ρ, T) data measured for the three (CO2 + O2) binary mixtures. The temperature, pressure, and density of each measured point were calculated as the arithmetic mean of the last ten consecutive measurements of a series of thirty. Table 5, Table 6, Table 7 also show the expanded uncertainty in density U(ρexp) (k = 2), calculated by Eq. (10) and expressed in absolute density units and as a percentage of the measured density.
The experimental
Discussion of the results
Fig. 2 shows the relative deviations of the experimentally determined density data of the (0.95 CO2 + 0.05 O2) mixture from the corresponding density data calculated by the GERG-2008 (a) and the EOS-CG (b) models. In the same way, Fig. 3, Fig. 4 show the deviations for the (0.90 CO2 + 0.10 O2) mixture and the (0.80 CO2 + 0.20 O2) mixture, respectively.
Both equations of state claim an uncertainty in density of 1.0% for mixtures of CO2 and O2, over the temperature range from (250 to 450) K and at
Conclusions
New (p, ρ, T) high-precision experimental data for three binary mixtures of carbon dioxide and oxygen, with nominal compositions of (0.95 CO2 + 0.05 O2), (0.90 CO2 + 0.10 O2), and (0.80 CO2 + 0.20 O2) at temperatures between (250 and 375) K and pressures up to 13 MPa, are reported. The experimental device used was a single-sinker densimeter with magnetic suspension coupling. The mixtures were prepared gravimetrically, which qualifies them as metrologically traceable reference mixtures.
The new
CRediT authorship contribution statement
Daniel Lozano-Martín: Investigation, Formal analysis, Visualization. Gerald U. Akubue: Investigation. Alejandro Moreau: Investigation. Dirk Tuma: Resources, Writing - review & editing. César R. Chamorro: Conceptualization, Supervision, Writing - original draft.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors wish to thank the Ministerio de Economía, Industria y Competitividad for their support through the project ENE2017-88474-R Junta de Castilla y León project VA280P18. The second author is grateful to the members of his affiliation group and program Erasmus + KA107-36589 ICM-UVa.
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