Highly Accurate PvTx Measurements On Supercritical CO₂ Mixtures And The Analysis At The Vicinity Of The Critical Point

Carbon capture and storage(CCS)is considered to be an important strategy in reducing global carbon emissions.The density of the carbon dioxide mixtures is a fundamental parameter in the process design for CCS.However,to the best of our knowledge,the density data situation is poor and the available data are mostly of lacking accuracy.Besides,fluids in the transportation and storage sections in the CCS chain are generally in supercritical state.At the vicinity of the critical point in the supercritical state,the density of a fluid changes significantly with a tiny variation in the temperature or pressure,and the second order partial derivatives of Gibbs energy,e.g.isobaric heat capacity,and isothermal compressibility,have a maximum locus.Therefore,it is significant to provide highly accurate experimental data for the fluid mixtures relevant for CCS and to investigate the characteristic of fluids at the vicinity of the critical point in the supercritical state.Comprehensive density measurements were performed on(CO2+N2),(CO2+Ar),and(CO2+N2+Ar)mixtures with a single-sinker magnetic suspension densimeter and a two-sinker magnetic suspension densimeter at temperatures from(273.15 to 423.15)K with pressures up to 33 MPa.The single-sinker magnetic suspension densimeter system was built in our laboratory.With all the measurement and composition effects considered,the expanded combined uncertainty(k = 2)in the density determination was generally from 0.04% to0.3%.Comparisons between the experimental data and the EOSs showed that even the reference multi-parameter EOS yielded relatively high deviation in estimating density at the vicinity of the critical point in the supercritical state near Widom line.Our new experimental data could be used to further improve the performance of multi-parameter equations of state(EOS)for mixtures.Besides,the fluid characteristic at the vicinity of the critical point in the supercritical region was investigated by the single-sinker magnetic suspension densimeter and a self-developed EOS method.Special attention was drawn on the topology relation between the critical point and the maximum loci of thesecond order partial derivatives of Gibbs energy.The mixture(CO2+CH4)was measured and the maximum loci of(??/?T)p?(??/?p)Twere calculated.The intersection point of the two maximum loci was very close to the critical point.An EOS method was developed to calculate the maximum loci of the second order partial derivatives of Gibbs energy for industrial application.With this method,(CH4+C2H6)and(CH4+C2H6)were investigated.The results showed that for pure materials,the critical point is on the extension lines of the second order partial derivatives of Gibbs energy,while it is not true for binary mixtures.