Acoustic Determination of the Helium Content of Carbon Dioxide from He Head Pressure Cylinders and FT-IR Studies of the Density of the Resulting Supercritical CO₂:  Implications for Reproducibility in Supercritical Experiments

Abstract

The speed of sound reaches a minimum value at the critical point of a fluid. An acoustic technique is used to measure the critical pressures of mixtures of He + CO 2 of known composition (0.1-3 mol % He). These data are then used to establish the composition of CO 2 samples from commercial He head pressure cylinders. The He content of apparently similar samples of HHPCO 2 is widely different. Only one sample (out of 5) showed the 3 mol % He predicted for such cylinders. These findings can only be explained if the He and CO 2 within our cylinders were not fully equilibrated. If such lack of equilibrium is common, the He content of HHPCO 2 from a given cylinder will be unpredictable and largely dependent on the particular history of that cylinder (i.e., its treatment during storage and handling). FT-IR measurements of the Fermi triad absorption bands of CO 2 (∼5000 cm -1 ) were then used to quantify the difference in density between pure CO 2 and HHPCO 2 containing 2.2 mol % He at the same temperature and pressure. Striking differences in density were observed close to the critical point. The implications of these results for reproducibility in supercritical experiments are discussed. It is recommended that, if possible, HHPCO 2 should be avoided for all studies involving CO 2 close to its critical point.

Determination of thermodynamic properties by supercritical fluid chromatography

“…Other unexpected deleterious effects that might occur with HHPCO 2 were discussed by Parcher and Xiong [132] and by Wells et al [133]. Although thermodynamic implications of entrained helium may simply and fairly accurately be treated by a cubic EOS [134], the principal cause of the unexpected effects of HHPCO 2 is probably the lack of thermodynamic equilibrium in HHPCO 2 tanks [135]. The estimated low diffusivity of helium in liquid CO 2 suggests that, after a rapid pressurisation of a CO 2 tank with helium, equilibration may take months, if not years [135].…”

“…Although thermodynamic implications of entrained helium may simply and fairly accurately be treated by a cubic EOS [134], the principal cause of the unexpected effects of HHPCO 2 is probably the lack of thermodynamic equilibrium in HHPCO 2 tanks [135]. The estimated low diffusivity of helium in liquid CO 2 suggests that, after a rapid pressurisation of a CO 2 tank with helium, equilibration may take months, if not years [135]. Therefore, the composition of the liquid drawn from the tank through a dip tube depends not only on temperature and pressure within the tank but also on storage history and liquid level in the tank.…”

“…To illustrate the important effects of entrained helium, Fig. 5 shows the density differences between pure CO 2 and HHPCO 2 as obtained from acoustic measurements [135]. Consequently, a growing number of authors have concluded that the use of HHPCO 2 tanks should be discouraged [128,131,132,[134][135][136].…”

“…5 shows the density differences between pure CO 2 and HHPCO 2 as obtained from acoustic measurements [135]. Consequently, a growing number of authors have concluded that the use of HHPCO 2 tanks should be discouraged [128,131,132,[134][135][136]. Wells et al [136] utilised the high-pressure behaviour of He-CO 2 system (type III phase behaviour) in a remarkable demonstration of gas-liquid chromatography with a dynamic stationary phase formed by the CO 2 -rich liquid at temperatures below the critical point of the He-CO 2 mixtures and pressures above the vapour pressure of pure CO 2 .…”

Peer Reviewed: Unified Chromatography with CO2-Based Binary Mobile Phases

“…In this example, the isotherms were generated from experimentally measured data (23,24). The critical points were extrapolated from experimental data reported by Kordikowski et al (25). The phase diagram in Figure 4 is discontinuous in composition.…”