Supercritical gaseous extraction

Cumin oil is usually obtained by steam distillation of the milled spice hydrodiffusion gives a higher yield and, more recently, supercritical gaseous extraction is claimed to give oil closer to the aroma and taste of the spice (Eikani et al., 1999). The yields of cumin seed oil with steam distillation are 2.3-3.6%, with liquid carbon dioxide it is 4.5% and with ethanol it is 12%. The major components are cuminaldehyde, cuminyl alcohol, p-mentha and 1.3-dien-7-al, the minimum perceptible levels being at 0.2 ppm. Naik et al. (1989) reported that liquid C02 extraction was quicker than steam distillation for the quantitative extraction of cumin oil without loss of active flavour components, at 58 bar and 20°C. 

Fundamental studies on the adsorption of supercritical fluids at the gas-solid interface are rarely cited in the supercritical fluid extraction literature. This is most unfortunate since equilibrium shifts induced by gas phase non-ideality in multiphase systems can rarely be totally attributed to solute solubility in the supercritical fluid phase. The partitioning of an adsorbed specie between the interface and gaseous phase can be governed by a complex array of molecular interactions which depend on the relative intensity of the adsorbate-adsorbent interactions, adsorbate-adsorbate association, the sorption of the supercritical fluid at the solid interface, and the solubility of the sorbate in the critical fluid. As we shall demonstrate, competitive adsorption between the sorbate and the supercritical fluid at the gas-solid interface is a significant mechanism which should be considered in the proper design of adsorption/desorption methods which incorporate dense gases as one of the active phases. 

Supercritical fluid extraction has so far been applied mostly to solid samples and also, occasionally, to liquid ones (whether as such or following passage through a sorbent that was subsequently placed in the extraction chamber, a procedure also applicable to gaseous samples). 

Brinen, J., Depth Distribution of Light Stabilizers in Coatings Analyzed by Supercritical Fluid Extraction-Gas Chromatography and Time-of-Flight Secondary Ion Mass Spectrometry, Anal. Chem. 1998 70(18) 3762-3765. Baltussen, E., H.-G. Janssen, P. Sandra, and C.A. Cramers, A New Method for Sorptive Enrichment of Gaseous Samples Application in Air Analysis and Natural Gas Characterisation, HRC 1994 17(5) 312-321. 

Supercritical fluid solvents have been tested for reactive extractions of liquid and gaseous fuels from heavy oils, coal, oil shale, and biomass. In some cases the solvent participates in the reactions, as in the hydrolysis of coal and heavy oils with water. Related applications include conversion of cellulose to glucose in water, dehgnincation of wood with ammonia, and liquefaction of lignin in water. 

SEE is an instrumental approach not unlike PLE except that a supercritical fluid rather than a liquid is used as the extraction solvent. SFE and PLE employ the same procedures for preparing samples and loading extraction vessels, and the same concepts of static and dynamic extractions are also pertinent. SFE typically requires higher pressure than PLE to maintain supercritical conditions and, for this reason, SFE usually requires a restrictor to control better the flow and pressure of the extraction fluid. CO2 is by far the most common solvent used in SFE owing to its relatively low critical point (78 atm and 31 °C), extraction properties, availability, gaseous natural state, and safety. 

Isolation Methods. One of the isolation methods evaluated was a liquid-liquid extraction procedure using supercritical fluid (SCF) C02 as the extraction solvent (5). The effectiveness of SCF C02 as an extraction solvent compared to gaseous and liquid C02 is associated with the marked increase in the density of C02 at its critical temperature and pressure, resulting in increased solvating power. The extraction unit that was evaluated was an open system consisting of a SCF C02... 

To understand any extraction technique it is first necessary to discuss some underlying principles that govern all extraction procedures. The chemical properties of the analyte are important to an extraction, as are the properties of the liquid medium in which it is dissolved and the gaseous, liquid, supercritical fluid, or solid extractant used to effect a separation. Of all the relevant solute properties, five chemical properties are fundamental to understanding extraction theory vapor pressure, solubility, molecular weight, hydrophobicity, and acid dissociation. These essential properties determine the transport of chemicals in the human body, the transport of chemicals in the air water-soil environmental compartments, and the transport between immiscible phases during analytical extraction. 

The operating pressure is obtained from the vapor pressure and the partial pressure of the gaseous educts and products. In this process, the temperatures applied are between 150 and 500 °C. In recent times, supercritical fluids have attracted a great deal of attention as potential extraction agents and reaction media in chemical reactions. This has resulted from an unusual combination of thermodynamic properties and transport properties. As a rule supercritical reactions like hydrolysis or oxidation are carried out in water. Above the critical point of water, its properties are very different to those of normal liquid water or atmospheric steam. 

A substance is said to be in the gaseous state when heated to temperatures beyond its critical point. However, the physical properties of a substance near the critical point are intermediate between those of normal gases and liquids, and it is appropriate to consider such supercritical fluid as a fourth state of matter. For applications such as cleaning, extraction and chromatographic purposes, supercritical fluid often has more desirable transport properties than a liquid and orders of magnitude better solvent properties than a gas. Typical physical properties of a gas, a liquid, and a supercritical fluid are compared in Table 1. The data show the order of magnitude and one can note that the viscosity of a supercritical fluid is generally comparable to that of a gas while its diffusivity lies between that of a gas and a liquid. 

When the analytes are to be retained in a sorbent, the sample (which can be solid, semi-solid, liquid or gaseous) is inserted in the solid state into the extraction cell. Samples in the latter three forms are supported on an appropriate material in order to ensure effective attack by the supercritical fluid. Solid supports are not used for liquid, gaseous and semi-solid samples only, however. Some research work conducted so far on solid samples has involved not natural samples but synthetic ones prepared from a selected sorbent (a natural matrix where the presence of the analytes of interest was previously excluded or a synthetic support such as polyurethane foam or glass wool) with which a solution containing the analytes was homogenized. Quantitative evaporation of the analyte solvent is mandatory as any residual solvent may alter the polarity of the supercritical fluid and hence its action to an extent dependent on the particular fluid and solvent properties, and also on the amount of solvent retained. 

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