Isochoric pVT and Phase Equilibrium Measurements for Carbon Dioxide + Nitrogen

Abstract

The authors have designed and constructed an isochoric apparatus for fast and accurate pressure-volume-temperature (pVT) and phase equilibrium measurements. The apparatus allows isochoric (p, T at constant V) measurements for noncorrosive fluids over a wide range of pressures (7--105 MPa) and temperatures (100--450 K). Densities can be derived from densities determined at one isotherm (p, V at constant T) with another apparatus, such as a pycnometer or a Burnett apparatus. Temperatures are accurate to {+-} 10 mK and precise to {+-}1 mK on ITS-90. Pressure measurements are accurate to {+-}10 kPa and precise to {+-}2 kPa. This apparatus requires about 20 min for temperature equilibration for measurements 2 K apart and about 50 min for measurements 10 K apart. The authors have measured 23 phase boundary points for three mole fractions, x{sub 1} = 0.3991, 0.4459, and 0.5037, for CO{sub 2} (1) + N{sub 2} (2) at temperatures from 207 to 268 K and pressures from 10.8 to 21.5 MPa. The estimated accuracy is {+-} 40 mK in temperature and {+-} 17 kPa in pressure. The isochoric measurements used to obtain these phase boundary points extend from 205 to 300 K and from 8 to 80 MPa. They have determined more » the saturation density for 20 of the phase boundary points. The saturation densities and the densities along the isochores were derived from density measurements at 300 and 285 K measured with a pycnometer estimated to be accurate to {+-}0.1%. The behavior of the solid + fluid phase boundary is discussed. « less