Helium Head Pressure Carbon Dioxide in Supercritical Fluid Extraction and Chromatography:  Thermodynamic Analysis of the Effects of Helium

Abstract

The Peng-Robinson equation of state (EOS) is employed to quantify the effects of entrained helium when helium head pressure carbon dioxide (HHPCO 2 ) is used in supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC). Naphthalene serves as a model analyte, and the unlike interaction energy parameters of the EOS are obtained from the respective second crossvirial coefficients. The Peng-Robinson EOS provides a reasonable description of the vapor-liquid equilibrium in HHPCO 2 storage tanks at room temperature. Variations in the content of helium in HHPCO 2 within the relevant range (between 0 and 5 mol %) are predicted to produce significant changes in the density and solubility parameter of HHPCO 2 , in the solubility of naphthalene in HHPCO 2 , and in the chromatographic retention of naphthalene if HHPCO 2 serves as the mobile-phase fluid. These results provide a computational indication that the use of HHPCO 2 in SFE and SFC should be avoided.Carbon dioxide is the most common solvent in supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC). To achieve a smooth operation, the working cylinder of a syringe pump should be filled completely with liquid CO 2 . When using a storage tank with pure CO 2 , it is necessary to cool the pump cylinder to secure that the CO 2 inside it will really be in the liquid state. Cooling-induced contraction of the piston seal may sometimes result in annoying leaks. Alternatively, storage tanks may be pressurized with helium; the liquid phase from the helium head pressure CO 2 (HHPCO 2 ) tanks can be transferred to the pump cylinder without cooling it.The practice of using HHPCO 2 in SFE and SFC initially involved a tacit assumption that helium is essentially insoluble in liquid CO 2 and that, therefore, the relevant thermodynamic properties of CO 2 are not altered appreciably by the presence of helium. Earlier papers 1-3 reported contradictory results with regard to the effects of entrained helium on reproducibility of retention times and peak areas in SFC. More recent work, however, presents mounting indications that the above assumption is questionable at best and that the entrained helium acts as a "negative modifier" in both SFE and SFC. Görner et al. 4 observed a considerable increase in SFC retention times when using HHPCO 2 as compared with pure CO 2 at the same temperature, pressure, and flow rate. They ascribed their observation to a decrease in the density of HHPCO 2 relative to that of pure CO 2 and used the Lee-Kesler equation of state 5 (EOS) to model the density decrease. Raynie and Delaney 6 reported a reduced efficiency of SFE with HHPCO 2 relative to that of SFE with pure CO 2 at the same temperature and pressure. King et al. 7 found that the presence of helium in the extraction fluid resulted in a substantial reduction in soybean oil solubility relative to that in pure CO 2 . Leichter et al. 8 used HHPCO 2 as a mobile-phase fluid in SFC of polycyclic aromatic hydrocarbons and observed a systematic decrease in rete...

Microbial Growth under Supercritical CO 2

“…Generally, culture vessels with initial headspace pressures of Ͼ100 atm lost between 5 and 25 atm of pressure, due to slow leakage through fittings, over the course of a multiple-week incubation, with greater losses associated with longer incubations. Unless specifically noted, all vessel incubation data reported here maintained scCO 2 headspace pressures of Ͼ72.9 atm, the critical pressure for CO 2 mixed with Յ3% inert helium at 37°C (35), or for incubations at lower pressures, final pressure (P final ) was Ն70% of P initial . All pressures were measured at room temperature (21°C).…”

Supercritical carbon dioxide extraction of lithium-ion battery electrolytes

“…The density is decreasing with increasing He-amount and the solvating power regarding aromatic analytes, e.g. naphthalene is worse [22]. Due to the non-aromatic nature of our samples and the intention of application oriented proof of principle experiments which do not include the optimization of recovery rates we chose sc HHPCO 2 as the reasonable system of choice for our investigations.…”

Experimental P−T−ρ Measurements of Supercritical Mixtures of Carbon Dioxide, Carbon Monoxide, and Hydrogen and Semiquantitative Estimation of Their Solvent Power Using the Solubility Parameter Concept

“…This effect has been clearly observed in the case of nitrogen and helium. 2,3 For these reasons, the need arises for characterization of the P-T-F behavior of CO 2 -syn-gas mixtures at supercritical conditions, collecting novel experimental data to validate a simple EOS that can become a precious tool to predict the properties of these mixtures under different operating conditions in the perspective of industrial utilization.…”