Reference Equations of State for the Thermodynamic Properties of Fluid Phase $n$-Butane and Isobutane Available to Purchase

New formulations for the thermodynamic properties of fluid phase $n$-butane and isobutane in the form of fundamental equations explicit in the Helmholtz energy are presented. The functional form of the correlation equations for the residual parts was developed simultaneously for both substances considering data for the thermodynamic properties of ethane, propane, $n$-butane, and isobutane. Each contains 25 coefficients which were fitted to selected data for the thermal and caloric properties of the respective fluid both in the single-phase region and on the vapor–liquid phase boundary. This work provides information on the available experimental data for the thermodynamic properties of $n-$ and isobutane, and presents all details of the new formulations. The new equations of state describe the $pρT$ surfaces with uncertainties in density of 0.02% (coverage factor $k=2$ corresponding to a confidence level of about 95%) from the melting line up to temperatures of 340 K and pressures of 12 MPa. The available reliable data sets in other regions are represented within their experimental uncertainties. The primary data, to which the equation for $n$-butane was fitted, cover the fluid region from the melting line to temperatures of 575 K and pressures of 69 MPa. The equation for isobutane was fitted to primary data that cover the fluid region from the melting line to temperatures of 575 K and pressures of 35 MPa. Beyond the range described by experimental data, the equations yield reasonable extrapolation behavior up to very high temperatures and pressures. In addition to the equations of state, independent equations for the vapor pressures, the saturated-liquid and saturated-vapor densities, and the melting pressures are given. Tables of thermodynamic properties calculated from the new formulations are listed in Appendix 2. Additionally, a preliminary equation of state for propane is presented that was developed in the course of the simultaneous optimization. This equation has the same functional form as the equations of state for $n-$ and isobutane.