Applications of fluorocarbons for supercritical extraction in the petroleum industry.

Abstract

The majority of supercritical processes utilise carbon dioxide (CO2) as the principal solvent, because CO2 has many attributes that make it an ideal supercritical fluid (SCF) solvent. This study investigates the possibility of replacing CO2 with trifluoromethane or hexafluoroethane, because of the poor performance of CO2 in cases where more polar and heavier molecular weight solutes must be extracted. Several applications in the petroleum industry, such as oil sludge treatment and the treatment of contaminated soils, are discussed. Due to the large number hydrocarbons present in such applications, a selection of solutes that could be used to simulate a simplified stream were chosen for these investigations. These selected solutes were n-hexane, n-heptane, n-octane, n-nonane, n-decane, n-undecane, 1-hexene, 3-methylpentane, methylcyclohexane, toluene and water. High-pressure vapour-liquid equilibria and vapour-liquidliquid equilibria for binary systems containing either trifluoromethane or hexafluoroethane, with these solutes were measured using a static-analytic apparatus at temperatures of between (272.9 and 313.2) K. For several systems, the phase equilibria data were verified using bubble-point pressures measured with a static-synthetic, variable-volume cell. Parameters for thermodynamic models were obtained by regression of the experimental data for the binary systems. The models provide a good representation of the majority of the systems investigated, and were therefore also used to estimate portions of the critical locus curves. These critical locus curves were compared to the critical loci that were extrapolated from the sub-critical coexistence data as well as critical loci that were measured with a critical point determination apparatus. There is satisfactory agreement between the calculated, the extrapolated and the measured critical loci. The thermodynamic models were used to simulate the separation of several hydrocarbon-water emulsions using either CO2, trifluoromethane, hexafluoroethane or mixtures thereof. The simulations confirmed that trifluoromethane, hexafluoroethane as well as mixtures thereof, provide improved performances (recoveries and yields) when used as alternative solvents in the SCF extraction of these systems. An economic analysis of a SCF extraction process was performed to investigate the performance of the solvents, and if such SCF extraction processes, using a mixture of trifluoromethane and hexafluoroethane, would provide an economically competitive treatment process for hydrocarbon-water emulsions.