Supercritical fluid-liquid systems

We wish to emphasize that the design of experimental apparatus for pVTxy measurements of supercritical fluid-liquid equilibrium in fatty ester systems has to be undertaken with special care to avoid adsorption of the fatty acid on the sampling apparatus. Since the equilibrium fluid mixture passes through the sampling valve in our design, it is neither necessary nor advisable to use very small diameter lines. 

Lam, D.H., JangkamoUculchai, A. and Luks, K.D. (1990) Liquid-liquid-vapor phase equilibrium behavior of certain binary carbon dioxide + n-alkanol mixtures. Fluid Phase Equilibria, 60,131-141. Gurdial, G.S., Foster, N.R., Jimmy Yun, S.L. and Tilly, KJ3. (1993) Phase behavior of supercritical fluid-entrainer systems, in Siqtercritical Fluid Engineering Science, Fundamentals and Applications, E. Kiran and JJ. Brennecke (Eds.), ACS Symposium Series No. 514, pp. 34-45. 

A third approach (Figure 6.14.9c) uses organometaDic catalysts in sohd modifications, for example, covalently bonded (see above) or immobilized in supported liquid form (see below). Here the reaction remains a sohd/supercritical fluid biphasic system during reaction and again the SCCO2 is used as the mobile phase to contact the reactant intimately with the immobilized organometallic complex. 

The following is a partial list of two-phase systems encountered in separation processes involving interphase transport gas-liquid (alternatively vapor-liquid), liquid-liquid, solid-liquid, Uquid-ion exchange resin, solid-supercritical fluid, liquid-supercritical fluid, etc. The first four systems are used much more frequently. Note that the two phases in each system are immiscible. 

EXAMPLES OF REACTIONS IN IONIC LIQUIDS - SUPERCRITICAL FLUIDS BIPHASIC SYSTEMS... 

When biocatalysis is conducted in ionic liquid/supercritical fluids biphasic systems, the major function of ILs is to protect the enzyme, whereas the supercritical fluid dissolves the substrate and extracts the product Both enzyme stability and... 

This is currently the most important area of large-scale use of membrane contactors. Three general classes of two fluid phase systems are relevant gas-liquid (also vapor—liquid), liquid-liquid, and supercritical fluid-liquid. We will describe/touch upon the following aspects of each of such two-phase systems basis of contacting, membranes used for contacting, mass transfer issues, and applications. 

Membrane contactors provide a continuous process for contacting two different phases in which one of the phases must be a fluid. Whether using a flat-sheet, hollow-fiber, or spiral-wound type, the membrane acts as a separator for two interfaces as it has two sides compared to conventional separation processes, which involve only one interface in a two-phase system. Therefore, it allows the formation of an immobilized phase interface between the two phases participating in the separation process [9]. Generally, there are five different classes of contacting operations gas-liquid, liquid-liquid, supercritical fluid-liquid, liquid-solid, and contactors as reactors [10]. The most commonly used operation in industry are gas-liquid also known as vapor-liquid, liquid-liquid, and supercritical fluid-liquid. Each class of system has its own modes of operation but in this study, emphasis will be focused on the gas-Uquid contacting systans. Table 9.1 describes the membrane contactor in summary. 

Unfortunately, not much experimental work has been carried out on the combination of Supercritical fluid extraction and liquid chromatography systems (43, 44). One of the reasons for this arises from the difficulties in achieving compatibility between the extraction solvent and the FC mobile phase. Baseline perturbations have been... 

Another application of SFC-GC was for the isolation of chrysene, a poly aromatic hydrocarbon, from a complex liquid hydrocarbon industrial sample (24). A 5 p.m octadecyl column (200 cm X 4.6 mm i.d.) was used for the preseparation, followed by GC analysis on an SE-54 column (25 m X 0.2 mm i.d., 0.33 p.m film thickness). The direct analysis of whole samples transferred from the supercritical fluid chromatograph and selective and multi-heart-cutting of a particular region as it elutes from the SFC system was demonstrated. The heart-cutting technique allows the possibility of separating a trace component from a complex mixture (Figure 12.21). 

One example of normal-phase liquid chromatography coupled to gas chromatography is the determination of alkylated, oxygenated and nitrated polycyclic aromatic compounds (PACs) in urban air particulate extracts (97). Since such extracts are very complex, LC-GC is the best possible separation technique. A quartz microfibre filter retains the particulate material and supercritical fluid extraction (SPE) with CO2 and a toluene modifier extracts the organic components from the dust particles. The final extract is then dissolved in -hexane and analysed by NPLC. The transfer at 100 p.1 min of different fractions to the GC system by an on-column interface enabled many PACs to be detected by an ion-trap detector. A flame ionization detector (PID) and a 350 p.1 loop interface was used to quantify the identified compounds. The experimental conditions employed are shown in Table 13.2. 

A schematic diagram of a chromatograph for SFC is shown in Figure 6.10. In general, the instrument components are a hybrid of components developed for gas and liquid chromatography that have been subsequently modified for use with supercritical fluids. Thus, the. fluid delivery system is a pump modified for pressure control and the injection system a rotary valve similar to components used in liquid chromatography. The column oven and... 

Cortes, H. J., Campbell, R. M., Himes, R. P., and Pfeiffer, C. D., On-line coupled liquid chromatography and capillary supercritical fluid chromatography large-volume injection system for capillary SFC, ]. Microcol. Sep., 4, 239, 1992. 

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

The mass spectra of mixtures are often too complex to be interpreted unambiguously, thus favouring the separation of the components of mixtures before examination by mass spectrometry. Nevertheless, direct polymer/additive mixture analysis has been reported [22,23], which is greatly aided by tandem MS. Coupling of mass spectrometry and a flowing liquid stream involves vaporisation and solvent stripping before introduction of the solute into an ion source for gas-phase ionisation (Section 1.33.2). Widespread LC-MS interfaces are thermospray (TSP), continuous-flow fast atom bombardment (CF-FAB), electrospray (ESP), etc. Also, supercritical fluids have been linked to mass spectrometry (SFE-MS, SFC-MS). A mass spectrometer may have more than one inlet (total inlet systems). 

Kitchens, C.L. and Roberts, C.B. (2004) Copper nanoparticle synthesis in compressed liquid and supercritical fluid reverse micelle systems. Industrial and Engineering Chemistry Research, 43 (19), 6070-6081. 

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