Group-type separation and simulated distillation: a niche for SFC

Abstract

IntroductionApplication of supercritical-fluid chromatography (SFC) for the separation of petroleum compounds really started in the '80s. SFC combines many advantages of gas chromatography (GC) and liquid chromatography (LC): its efficiency is close to GC, it works with GC detectors and involves mobile phases whose solvating power can be tuned as in LC; thus, SFC fills the gap between GC and LC. Petroleum applications are a niche for SFC because it combines GC detectors to LC-like mobile phase: high molecular-weight compounds that could not elute from a GC column elute in SFC, while universal and sensitive GC detection, such as with the flameionization detector (FID), can still be used. As a consequence, typical SFC applications [1][2][3][4] are GC-like separations, e.g. simulated distillation (simdis), and LC-like separations, e.g. hydrocarbon group-separations. However, a GC-like separation does not mean that an open-tubular column must be used. In fact, both packed and capillary columns can be used in SFC, both having their advantages and drawbacks.Simdis is a routine GC application; however, SFC is very attractive for eluting hydrocarbons having more than 80 carbons, which are difficult to elute in GC or high-temperature GC (HTGC) without cracking, because their elution can be obtained on packed or open-tubular columns at much lower temperature compared to HTGC. As SFC can also elute hydrocarbons starting from C 20 to more than C 130 , it is more a competitor than an alternative to GC.The most studied type of LC applications in the petroleum industry is hydrocarbon group-type analysis. SFC's best features for this application are the detection capabilities of FID and the properties of carbon dioxide as a mobile phase. Moreover, FID provides easy quantitation of hydroRecent advances in supercritical-fluid chromatography (SFC) of petroleum fractions are reviewed. Simulated distillation (simdis) still requires some improvements of the hardware so that routine analysis of heavy fractions can be performed. Compared to gas-chromatography simdis, SFC simdis extends the range of this application up to C 140 hydrocarbons, while multi-detection would allow the differentiation of aromatic and non-aromatic hydrocarbons. SFC has been registered by ASTM for hydrocarbon group-type analysis using carbon dioxide. It enables fast separation of aromatic and non-aromatic fractions, subfractionation of aromatics and easier quantitation than in liquid chromatography. Resins and asphaltenes of heavier samples can be backflushed from the column. The method could be further improved for better resolution and quantitation via multiple detection and more selective stationary and mobile phases (CO 2 -SF 6 mixtures for example).Keywords: SFC, simulated distillation, group-type separation, petroleum compounds Article available at http://analusis.edpsciences.org or http://dx.doi.org/10.1051/analusis:1999270681 carbon groups because of the similarity of response factors of most hydrocarbons. Replacing a LC by a SFC method is also worth con...

Supercritical Fluid and Unified Chromatography

“…The acceptance of these methods by workers in the petroleum industry has grown in recent years. Illustrating this point, Thiebaut and Robert provided a description of the use of SFC methods for simulated distillation and for an ASTM-registered procedure for group-type separations (106). They demonstrated improved performance by using multiple detectors and better selectivities using novel stationary and mobile phases.…”

High Temperature Simulated Distillation of Bitumen Fractions with Open Tubular Capillary Columns Depleted in Silicone/Siloxane Stationary Phases

“…Sample decomposition above 370 °C was evidenced from these experiments, and supercritical fluid chromatography (SFC) was proposed as a feasible low temperature alternative for SimDist determination . SFC has been further developed for SimDist analysis as a mild technique that avoids TC, − providing resolution of molecules up to 140 carbon atoms and analyses that reported up to C162 (about 800–820 °C) . However, solvent overlapping at the chromatogram’s initial portions is at present an unresolved issue for SFC, demanding ancillary HTSD for quantitative analysis correction .…”

Determination of Hydrocarbon Group-Type of Diesel Fuels by Gas Chromatography with Vacuum Ultraviolet Detection

“…Group-type separations of diesel fuels have been performed with gas chromatography (GC), − high-performance liquid chromatography (HPLC), − LC-GC, , SFC-GC, SFC-MS, and recently a number of hyphenated techniques have been reviewed. − Most commonly used HPLC detectors yield a nonuniform response for saturates and aromatics, ,− mandating the use of complex calibrations. However, the use of an FID provides reliable mass quantification of the different hydrocarbon group-types. ,− When more complex hyphenated techniques are used, reliable quantification becomes more difficult, especially when mass spectrometric detectors are used, as they too give nonuniform responses in group-type separations . Finally, it should be noted that for group-type separations, the ultimate goal is not the separation and quantification of each individual compound but the quantification of families of compounds eluting in distinct regions so that the relative proportion of each family of compounds can be determined.…”