Liquid solutions supercritical extraction

Supercritical Fluid Chromatography (SFQ and Supercritical Fluid Extraction (SFE) A separation technology similar to other extraction and chromatographic methods, but in which the mobile phase is actually a fluid in its supercritical fluid state. A supercritical fluid is a fluid that is held above its critical temperature and pressure, and for which no application of additioncJ pressure can result in the development of a liquid phase. Supercritical fluids are unique in that while they possess liquid-like densities, the mass transfer behavior is superior to that of liquids. Supercritical fluid chromatography remains a niche method that is applicable to pharmaceuticals and other high relative molecular mass solutes. Supercritical extraction, on the other hand, is more widely used as a sample preparation method, especially in pharmaceutical analyses, polymers, and environmental analyses. 

As an alternative to distillation, extraetion with a eo-solvent that is poorly mis-eible with the ionie liquid has often been used. There are many solvents that can be used to extract product from the ionic liquid phase, whether from a monophase reaction or from a partially miscible system. Typical solvents are alkanes and ethers (15). Supercritical CO2 (SCCO2) was recently shown to be a potential alternative solvent for extraction of organics from ionic liquids (22). CO2 has a remarkably high solubility in ionic liquids. The SCCO2 dissolves quite well in ionic liquids to facilitate extraction, but there is no appreciable ionic liquid solubilization in the CO2 phase in the supercritical state. As a result, pure products can be recovered. For example, about 0.5 mol fraction of CO2 was dissolved at 40°C and 50 bar pressure in [BMIMJPFe, but the total volume was only swelled by 10%. Therefore, supercritical CO2 may be applied to extract a wide variety of solutes from ionic liquids, without product contamination by the ionic liquid (29). 

In a supercritical extraction process a solvent is contacted with a solute at conditions near a critical point of the solvent plus solute mixture. The mixture may exhibit multiphase behavior invoving vapor, liquid, and solid phases, depending on the mixture composition and temperature and pressure conditions. 

Extraction or separation of dissolved chemical component X from liquid phase A is accomplished by bringing the liquid solution of X into contact with a second phase, B, given that phases A and B are immiscible. Phase B may be a solid, liquid, gas, or supercritical fluid. A distribution of the com-... 

Although C02 is the most common solvent for supercritical extraction processes because of it s abundance, non-toxicity and non-flammability, other compounds may prove to be better solvents in certain instances. In choosing a solvent, a balance between solubility and selectivity has to be struck. In the case of solutes with a melting point well below the decomposition temperature, it is usually desirable to perform a liquid-supercritical fluid extraction to circumvent the problems associated with handling solids at high pressures. In... 

A recent development in liquid-liquid extraction has been the use of supercritical fluids as the extraction-solvent. Carbon dioxide at high pressure is the most commonly used fluid. It is used in processes for the decaffeination of coffee and tea. The solvent can be recovered from the extract solution as a gas, by reducing the pressure. Supercritical extraction processes are discussed by Humphrey and Keller (1997). 

Type III behavior indicates the most extreme asymmetry between the components of a binary mixture. Nearly all H2 systems supply striking examples of type III behavior. CO2 mixtures with 2,5-hexanediol and 1-dodecanol are also classified as type III. The system CO2 -I- n-tridecane is peculiar because it was classified by van Konynenburg and Scott as type III, whereas Enick et al. have classified it as type IV, owing to experimental identification of a three-phase region. The system CO2 -I- n-tetradecane is a variation on type III, where the solute-rich locus terminates in a solid(wax)-liquid-liquid boundary. Several important systems fall into a similar category. For example, CO2 + naphthalene is commonly used as a model system for supercritical extraction. The naphthalene system differs from the n-tetradecane system in that the solute-rich locus terminates at a higher temperature... 

In supercritical extraction, the components are transferred from a liquid phase to a supercritical dense phase at equilibrium with the liquid. A system would be suitable for supercritical extraction if the extracted component, or solute, is adequately soluble in the supercritical solvent, and its solubility is a strong function of the solvent density. This phenomenon facilitates the recovery of the solute from the solvent since lowering its pressure or raising its temperature will lower its density. The solubility of the solute drops at the lower solvent density allowing the solute to be recovered, possibly with no need for a distillation column following the extractor. 

In absorbers or strippers, components are transferred from the vapor phase to the liquid phase or vice versa. In liquid-liquid extraction, components are transferred from one liquid phase to the other. In supercritical extraction, components (the solute) are transferred from the liquid phase to the supercritical phase (the solvent) at equilibrium with the liquid. 

One of the benefits of using supercritical fluids as the solvent is the strong dependence of the solubility of the solute on the solvent density. This is a property that could be exploited for facilitating the separation of the solute from the solvent as it leaves the column, by dropping its pressure or raising its temperature, thereby lowering its density and the solubility of the solute. As a result, the extract separates into a liquid solute and a vapor solvent. Another favorable property of supercritical fluids as solvents is the high diffusivity of the solute in these fluids compared to that in liquids. Supercritical fluids also have a substantially lower viscosity than liquids. Because of these properties the mass transfer rate of the solute... 

As noted above, even for supercritical fluids at hquid-Hke densities, there is no heat of vaporisation to move the fluid to gas-hke densities -- just lower the pressm-e at a given temperature. With typical hquids, heat must be supplied to evaporate them. That means that when a chemist wants to remove the solvent from a solution of liquid solvent-plus-extract, heat is usually appHed and the temperature of the solution is elevated to the boiling point of the liquid solvent. If the raffinate (what is left after extraction) is the desired product, usually there is some liquid solvent left in it as well, so heat is often used again to elevate the temperature. This can he a problem when working with thermally labile components and matrices. With a supercritical flvud, when pressure is lowered sufficiently, the supercritical solvent is effectively removed from both the extract and the raffinate. Usually heat is only input to offset the cooling due to the expansion process so very mild temperatures can be used during supercritical solvent removal. 

The invention is a method to facilitate recovery of the solvent from the asphalt or solvent refined coal. The heavy phase from a supercritical extraction is heated to lower the solution viscosity and it is sent to a high shear mixer where it is admixed with steam. The intimately-contacted mixture is then conveyed to a solvent separation vessel. The solvent-steam mixture is withdrawn from the separator and sent to a condenser and solvent-water separator, llie heavy liquid phase from the solvent separation vessel is withdrawn and sent to storage. 

We now want to consider the extent to which a solid is soluble in a liquid, a gas. or a supercritical fluid. (This last case is of interest for supercritical extraction, a new separation method.) To analyze these phenomena we again start with the equality of the species fugacities in each phase. However, since the fluid (either liquid, gas, or supercritical fluid) is not present in the solid, two simplifications arise. First, the equilibrium criterion applies only to the solid solute, which we denote by the subscript 1 and second, the solid phase fugacity of the solute is that of the pure solid. Thus we have the single equilibrium relation... 

A solute that is a liquid at the temperature of a supercritical extraction has critical properties much closer to the supercritical fluid s critical properties than the solids discussed above. Since the mixture is somewhat less molecularly as3nranetric, the critical temperature of the supercritical fluid lies above the melting temperature of the solute. As a result, the vaporization (L2V) curve is generally the only pure solute property to be of concern on mixture P-T traces. Many literature references are available for mixtures that fall into this category, because these systems comprise the bulk of high pressure vapor-liquid equilibria research of the last century, however, most data are for hydrocarbon related systems and the current interest extends beyond these systems. A few cases of special interest will be mentioned and further information may be found in other references (4-10). 

Wang, J. S., Sheaff, C. N., Yoon, B., Addleman, R. S., Wai, C. M. (2009). Extraction of uranium from aqueous solutions by using ionic liquid and supercritical carbon dioxide in conjunction. Chemistry-A European Journal, 15, 4458-4463. 

In Section 11.4, it was shown how suitable solvents can be selected with the help of powerful predictive thermodynamic models or direct access to the DDB using a sophisticated software package. A similar procedure for the selection of suitable solvents was also realized for other separation processes, such as physical absorption, extraction, solution crystallization, supercritical extraction, and so on. In the case of absorption processes or supercritical extraction instead of a g -model, for example, modified UNIFAC, of course an equation of state such as PSRK or VTPR has to be used. For the separation processes mentioned above instead of azeotropic data or activity coefficients at infinite dilution, now gas solubility data, liquid-liquid equilibrium data, distribution coefficients, solid-liquid equilibrium data or VLE data with supercritical compounds are required and can be accessed from the DDB. 

In the second method the solution is sprayed through a nozzle into compressed carbon dioxide the process is termed as precipitation with compressed antisolvent (PCA) [33] and liquid or supercritical antisolvents can be employed. In the case of continuous flow of the solution and of the antisolvent the process is termed also as aerosol solvent extraction system (ASES) [34], in the case of countercurrent flow and supercritical antisolvent precipitation (SAS) in the case of co-current flow [35]. 

By changing the density of the supercritical fluid, different fractions may be selectively extracted from the complex mixture or sample matrix. On decompression, the extracted solutes are precipitated and may be collected for injection into a GC or SFC for analysis. Figure 8.4 shows a simple apparatus for on-line SFE/SFC solutes extracted from the sample matrix are deposited from the end of a restrictor into the internal loop of the microinjection valve of the capillary SFC. The valve loop contents are subsequently switched into the SFC column by means of liquid or supercritical carbon dioxide. 

A fluid in the supercritical region can have a density comparable to that of tbe liquid, and can be more compressible than the liquid. Under supercritical conditions, a substance is often an excellent solvent for solids and liquids. By varying the pressure or temperature, the solvating power can be changed by reducing the pressure isothermally, the substance can be easily removed as a gas from dissolved solutes. These properties make supercritical fluids useful for chromatography and solvent extraction. 

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