Determination of Organic Partitioning Coefficients in Water-Supercritical CO₂ Systems by Simultaneous in Situ UV and Near-Infrared Spectroscopies

Abstract

CO2 injected into depleted oil or gas reservoirs for long-term storage has the potential to mobilize organic compounds and distribute them between sediments and reservoir brines. Understanding this process is important when considering health and environmental risks, but little quantitative data currently exists on the partitioning of organics between supercritical CO2 and water. In this work, a high-pressure, in situ measurement capability was developed to assess the distribution of organics between CO2 and water at conditions relevant to deep underground storage of CO2. The apparatus consists of a titanium reactor with quartz windows, near-infrared and UV spectroscopic detectors, and switching valves that facilitate quantitative injection of organic reagents into the pressurized reactor. To demonstrate the utility of the system, partitioning coefficients were determined for benzene in water/supercritical CO2 over the range 35-65 °C and approximately 25-150 bar. Density changes in the CO2 phase with increasing pressure were shown to have dramatic impacts on benzene's partitioning behavior. Our partitioning coefficients were approximately 5-15 times lower than values previously determined by ex situ techniques that are prone to sampling losses. The in situ methodology reported here could be applied to quantify the distribution behavior of a wide range of organic compounds that may be present in geologic CO2 storage scenarios.

Partition Coefficients of Organics between Water and Carbon Dioxide Revisited: Correlation with Solute Molecular Descriptors and Solvent Cohesive Properties

“…In the set of experimental data employed here, this concerns primarily the sources of K-factor data of benzene. 13,14,25 The present correlation indicates that the benzene data of Bryce et al 25 are more accurate than those of Yeo and Akgerman 13 and Ghonasgi et al 14 Compared with most previous K-factor correlations in CO 2 − water systems, 31,23,24 the present LSER correlations employ more explanatory variables. Consequently, larger effort is required here to assemble the set of explanatory variables.…”

“…Calculated (eq 9) versus experimental K-factors in the set containing the data of Sengupta et al, 16 Brudi et al, 18 Wagner et al, 20 Timko et al, 23 Burant et al, 24 and Bryce et al 25 (n = 282). Solutes: (○) acetophenone, 23 ( □ ) benzaldehyde, 23 (△) bromobenzene, 23 (▽) methyl vinyl ketone, 23 ( ◊ ) chlorobenzene, 23 (×) cyclohexane, 23 (+) cyclohexene, 23 (green ○) cyclopentene, 23 (green □ ) ethylbenzoate, 23 (green △) fluorobenzene, 23 (green ▽) hexane, 23 (green ◊ ) methylbenzoate, 23 (green ×) propylbenzoate, 23 (green +) tetrahydrofuran, 23 (red ○) toluene, 23 (brown ○) thiophene, 24 (brown □ ) pyrrole, 24 (brown △) anisole, 24 (brown ▽) benzene, 25 (brown ◊ ) 1,2dichloroethane, 16 (brown ×) 1,1,2-trichloroethane, 16 (brown +) 1,1,2,2-tetrachloroethane, 16 (orange □ ) phenol, 18 (orange △) benzyl alcohol, 18 (orange ▽) benzoic acid, 18 (orange ◊ ) 2-hexanone, 18 (orange ×) cyclohexanone, 18 (orange +) caffeine, 18 (blue ○) vanillin, 18 (blue □ ) aniline, 20 (blue △) benzaldehyde, GC-SPME detection, 20 (blue ▽) benzaldehyde, UV/vis detection. 20 Environmental Science & Technology inconclusive) guide as regards the relative accuracy of different sources of experimental K-factor data for a particular solute.…”

“…It should be noted, however, that the relatively satisfactory performance of the correlation in Figure may partly result from the fact that all experimental K -factor data by Timko et al refer to a single temperature and pressure setting (300 K and 8 MPa, respectively). Figure shows the correlation by eq of a more extensive K -factor data set ,,,− comprising 282 data points for 30 solutes. Performance of eq with this data set still appears to be relatively acceptable, and the statistical parameters of the correlation are listed in Table .…”

“…Partitioning of an organic solute i between both phases has often been characterized in terms of K -factor defined by where y i is the equilibrium molar fraction of the solute in the CO 2 -rich phase and x i is the equilibrium molar fraction of the solute in the water-rich phase. The experimental K -factor data were taken from the original sources. − However, some of the solutes, notably pentachlorophenol, 2,3,4,5-tetrachlorophenol, 3-methyl-4-chlorophenol, and 2-methyl-4,6-dinitrophenol and propiophenone were omitted because of the lack of vapor pressure data at the required temperatures (see below). Therefore, the most extensive set of experimental K -factor data comprised 455 data points for 44 solutes of diverse structures and falling within the temperature range of 297–363 K and the pressure range of 2.5–31.5 MPa.…”

“…Phase behavior of systems containing water, carbon dioxide and organics has been relevant to several environment- and energy-related fields including carbon capture and storage, − CO 2 sequestration and CO 2 -assisted enhanced oil recovery. , A minor but distinct area of application has also been supercritical fluid extraction of aqueous media as a sample treatment technique of analytical chemistry. − Consequently, partitioning of organics between supercritical CO 2 and water or brine is an important topic, and there have been a number of experimental studies aimed either at the determination of solute partition coefficients ( K -factors) in CO 2 –water systems at low concentrations of the organic solute − or at the study of phase equilibria at finite concentrations of all components. − In line with the experimental studies, several approaches to modeling the CO 2 –water–organic systems have been applied although the overall number of such models does not match the large attention received by the CO 2 –water binary system. Equation of state (EoS) models of the CO 2 –water–organic(s) systems such as those employing the Peng–Robinson EoS ,,, or the perturbed-chain statistical associating fluid theory provide good description of the phase equilibria, however, at the expense of introducing unlike interaction (binary) parameters.…”