New experimental density data and derived thermophysical properties of carbon dioxide – Sulphur dioxide binary mixture (CO2 - SO2) in gas, liquid and supercritical phases from 273 K to 353 K and at pressures up to 42 MPa
Introduction
High demand for energy due to the rapid economic growth resulted in an ever increasing use of fossil fuels, i.e. coal, oil and natural gas following the industrial revolution. The effect of this has been to increase the amount of greenhouse gases such as CO2 in the atmosphere. Carbon capture and storage (CCS) is the name given to technology based solutions aimed at reducing CO2 emissions to the atmosphere [1]. This technology comprises capturing CO2 during burning of fossil fuels, compression and transport mainly via pipelines and injection into geological storage basins. The captured CO2 will contain ranges of impurities depend on the source of fossil fuel as well as the capturing technology [2] [3].
Sulphur dioxide can be present in heavy oil production or in flue gas in the post-combustion or oxyfuel processes in coal-fired power plants. In the MEA-based absorption processes, impurities such as O2, NO2 and SO2 can lead to severe operational problems such as foaming, viscosity increase and formation of heat-stable salts. The typical range of SO2 in the MEA-based post-combustion processes is 500–3000 part per million by volume (ppmv). Flue gas desulphurisation (FGD) units with wet SO2 scrubbers can absorb 80-95% of the SO2 in the flue gas before entering the CO2 absorber. However, in case of failure in the FGD unit, 75% of the SO2 could be absorbed by MEA as it is not selective a solvent to acid gases [2]. The presence of SO2 can make problems such as corrosion in the presence of water [4] in the transportation of captured CO2 with impurities from both MEA-based post-combustion or oxyfuel processes [5] [6].
Due to the toxicity of SO2, the Immediately Dangerous to Life and Health (IDLH) concentration of this component is set to 100 ppmv by the National Institute for Occupational Safety and Health (NIOSH) [7]. Shell Cansolv also commissioned an integrated system to capture CO2 and SO2 simultaneously in a commercial scale post-combustion coal fired power plant in Saskatchewan. The captured CO2 is transported for the CO2 Enhanced Oil Recovery (CO2-EOR) in Weyburn oil field and SO2 is used to produce sulphuric acid as a valuable by-product [8].
Similar to injection of acid gases, i.e., injection of CO2 - H2S [9] [10], technically, CO2 and SO2 can be co-stored in deep saline aquifers and this effectively reduces the capture cost by avoiding SO2 removal costs [11] [12] [13]. However, the reactivity of SO2 with rock in the presence of water and acids that form is an issue of concern [14] [15] [16]. The geochemical effects can reduce the pH of the formation water, change the porosity of reservoir rock and cause mineral dissolution and sulphate precipitation [11] [17] [18] [19] [20].
A proper understanding of the thermodynamic and transport properties of CO2-SO2 systems is required as an input to feasibility studies and equipment sizing in the above processes [21]. The presence of impurities in the captured streams of CO2 would affect the thermophysical properties, in particular density and viscosity, of high CO2 content mixtures. Equations of state (EoSs) should be evaluated using the experimental density and vapour-liquid equilibrium (VLE) data. The lack of experimental data for CO2-SO2 systems is certainly due to toxicity of system [22]. The only available data for this system is reported by Caubet [23] in 1904. A comprehensive thermodynamic behaviour of CO2-SO2 mixture was studied experimentally by Coquelet and co-workers [24] [25], for transport purposes of CO2 mixtures in a CCS context. Only VLE data are available at 263.15 K and 333.21 K and at pressures ranging from 0.1 to 8.8 MPa.
In this work, the densities of approximately 95 mol% CO2 with 5 mol% SO2 were measured using a Vibrating Tube Densitometer (VTD) in the gas, liquid and supercritical phases. The measurements were carried out at five isotherms 273, 283, 298, 323 and 353 K at pressures up to 42 MPa. Thermodynamic properties such as compressibility factor, isobaric specific heat capacity and bubble points were obtained from the measured densities. The measured densities also were employed to evaluate the classical cubic EoSs, i.e., Soave-Redlich-Kwong (SRK-EoS) [26], Peng-Robinson (PR-ES) [27], and Valderrama modification of the [28] Patel-Teja (VPT-EoS) [29] EoSs. Then, to improve the density prediction, the CO2 volume correction term [30] and Peneloux shift parameter [31] were introduced to those EoSs. Also, a multi parameter EoS based on Helmholtz energy [32] [33] with the short industrial equation from Lemmon & Span for pure SO2 [34] (see Appendix A) and Span &Wagner EoS for pure CO2 [35] was evaluated using the measured density data and derived thermodynamic properties.
Section snippets
Experimental part
Experimental work was carried out in the high safety laboratory at CTP - Centre of Thermodynamics of Processes research group at Mines ParisTech in France.
Specific heat capacity calculations
The residual specific heat capacity () can be calculated from the measured density data through the following equation [44]:
In this equation, the gradient of molar volume with temperature at each constant pressure were plotted using the measured densities at the five measured isotherms. The procedure to calculate the specific heat capacity from Equation (3) has also been described in a previous publication for the CO2-H2S system [36]. A similar procedure was
Results and discussions
Densities of CO2-SO2 binary systems were measured using vibrating tube densitometer (VTD), Anton Paar DMA 512 in the gas, liquid and supercritical phases at pressures up to 41.7 MPa. The composition of the binary system was 0.9478 CO2 + 0.0522 SO2 at 298 K and 0.9503 CO2 + 0.0497 SO2 at 273, 283, 323 and 353 K. At each isotherm, the densitometer was firstly calibrated using pure CO2 with a forced path mechanical calibration (FPMC) technique [38]. The measured densities along with their
Conclusion
The densities of CO2-SO2 binary mixture were measured using VTD densitometer, Anton Paar DMA 512, in the gas, liquid and supercritical phases. The densitometer was first calibrated using pure CO2 and the FPMC calibration technique. Then, the densities of a 0.9503 CO2 – 0.0497 SO2 mixture were measured at temperatures of 273, 283, 323 and 353 K. In addition the densities of 0.9478 CO2 – 0.0522 SO2 were measured at 298 K. The overall average uncertainty of the measured densities with a confidence
Acknowledgements
This work was a part of the JIP project” Impact of Common Impurities on Carbon Dioxide Capture, Transport and Storage” [30] which the phase-I was conducted jointly at Heriot-Watt University in Edinburgh, UK and MINES ParisTech in France in 2011–2014. The authors would like to gratefully acknowledge the sponsors of the project: Chevron, GALP Energia, Linde AG Engineering Division, OMV, Petroleum Expert, Statoil, TOTAL and National Grid Carbon Ltd. The thermophysical properties of CO2-rich fluids
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