The Thermodynamic Properties of CO2 up to 200 MPa Including the Critical Region, Calculated by a New Combined Equation of State with Few Parameters

Abstract

Heat capacities Cv, Cp, and sound velocity W of carbon dioxide are calculated on the basis of a new thermal combined equation of state (EoS) with a low number of regulated constants. This equation includes a new regular EoS with 11 coefficients, a scaling EoS with six coefficients, and a regular transition function containing two adjustable parameters. Calculating results for thermodynamic properties of CO2 in the regular behavior range of up to 200 MPa correspond to the accuracy level of modern practical equations of state with large numbers of determined constants. This makes it possible to calculate the properties of CO2 in the critical region, taking into account the achievements of scaling theory. To determine the constants of the calculated equations, only p,ρ,T-data for CO2 were used. The experimental and tabular data on Cv, Cp, S, H, and W were not involved except isochoric heat capacity Cv in the ideal gas state. These data were used to compare the calculated results with the experimental and tabulated values. The universal critical exponents were taken in accordance with the three-dimensional Ising model. The average errors in describing the thermal properties of CO2 do not exceed errors: rms error is 0.93 % in pressure and max deviation in Cv is 10 %. Divergences in the critical region relate to application of the scaling equation of state, in contrast to the regular equations. The comparison was made with calculated Cv, Cp using the current crossover EoSs for CO2.