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

Abstract

Due in part to the toxicity of the CO₂-SO₂ binary system, there are no density data available in the literature. Densities for this system were measured using a vibrating tube densitometer (VTD), Anton Paar DMA 512, and the forced path mechanical calibration (FPMC) method in the gas, liquid and supercritical phases at pressures up to 41.7 MPa. The mole fraction of the mixture was 0.9478 CO₂ + 0.0522 SO₂ at 298 K and 0.9503 CO₂ + 0.0497 SO₂ at 273, 283, 323 and 353 K. The compressibility factor, isobaric heat capacity and bubble points were also derived from the measured densities. The classical cubic equations of state, i.e., Soave-Redlich-Kwong (SRK), Peng-Robinson (PR) and Valderrama version of Patel-Teja (VPT) with the CO₂ volume correction term and Peneloux shift parameter in addition to a multi parameter EoS based on Helmholtz energy were evaluated using the measured density data. The most accurate EoSs for the investigated system were the multi parameter EoS and the PR-CO₂ with overall AAD of 0.6% and 1.0%, respectively.

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 (ppm_v). 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 ppm_v 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.