A new criterion for vanishing points of Widom lines of CO2/natural gas mixture: Application to pipeline and compressor units in CCUS

Abstract

The Widom lines of mixture indicate the abnormal response behavior of properties of mixed supercritical fluid near the critical point, determining operating conditions in supercritical region of ground equipment in CCUS. Peng-Robinson equation (PR-EOS) is used to calculate Widom lines of isothermal compressibility coefficient, isobaric expansion coefficient, isobaric specific heat, and density of CO2/natural gas quinary mixture. A new criterion is proposed to calculate the vanishing points of Widom lines using the coefficient of variation (CV) of response function distribution, with a clear statistical meaning and an appropriate range, compared with existing criteria. Based on the new criterion, the effect of CO2 concentration on the thermodynamic fluctuations in the near-critical region of CO2/natural gas mixture is analyzed. It is found that thermodynamic fluctuations of mixture are controlled by CO2 concentration and fluctuations can be considered relatively weak when CO2 concentration is below 60 %. The selection and control of operating conditions of pipeline and compressor unit in CCUS application are discussed to cope with the Widom region movement caused by changes in CO2 concentration.