Density and virial coefficients of gaseous butane from 265 to 450 K at pressures to 3.3 MPa

The Burnett-isochoric (B-I) method has been used to measure gas densities and virial coefficients for butane from 265 to 450 K (IPTS, 1990; Preston-Thomas, 1990). Two independent B-I runs were performed but both with a base isotherm of 450 Il, which is well above the critical temperature of 425 K. Significant physical adsorption of butane molecules onto the highly-polished, stainless steel cell walls was found below 375 K in agreement with conclusions reached by Ewing and associates in comparing their sonic-velocity-based density virial coefficients with those from the P-V-T literature. Our data below 375 K were then corrected for adsorption errors by previously published procedures developed by our laboratory for highly polar gases. Using statistical weighting of our two B-I runs, recommended density second virial coefficients B(T) are reported from 265 to 450 K whereas third virial coefficients C(T) are reported only from 325 to 450 K as values below 325 Ii are too uncertain due to the vapor pressure dropping under 300 kPa. However, then the virial equation of state truncated after B(T) is sufficient to represent the gas densities. At the lower temperatures of this investigation, our B(T) values lie between those from sonic velocities and the more negative values from the P-T T-T literature, which are uncorrected for adsorption errors; we are closer to the values from sonic velocities and about 1/4 Of the way between the two sets. At the higher temperatures, we agree very closely with the better P-V-T measurements whereas the sonic-velocity-based values become increasingly more negative.