Extraction with supercritical solvent

It may be noted that if the solvent is liquid under ambient conditions, the mechanical energy requirements of the pressure reduction method of solute recovery may be considerably reduced since repressurisation of the solvent in liquid form may be used rather than repressurisation of gaseous solvent. The pilot plant used by the UK National Coal Board (now British Coal) for the supercritical extraction of coal was of this type [17]. However, the decrease in mechanical energy requirements is balanced by increased thermal requirements. 

An alternative separation technique for the selective removal of components from supercritical solution is adsorption [18-21]. Although not discussed in detail here, this is potentially a very powerful method, particularly for the removal of trace components. 

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As reported in a lot of reviews, extractions with supercritical solvents have a very promising commercial potential. Until now the commercialization is mainly restricted to batchwise extraction of solids with carbon dioxide (e g. decaffeination of coffee and tea, extraction of hop). Laboratory experiments and operation of small-scale pilot plants gave favourable economic values for continuous extraction of liquids with C02 and other gases. Only a few extractions with C02 or C HS are performed already on a small industrial scale. For research purposes and product development a new high pressure counter-current extraction plant was erected. To get greater amounts of product the explosionproof plant was constructed in pilot scale using a special modular concept and an effective visual control system. 

For reasons discussed in section 1.3, extractions with supercritical solvents (i.e. solvents whose reduced temperatures and pressures exceed unity under the extraction conditions) are not normally carried out at very high reduced temperatures and it is convenient to classify extractions of this type together with extractions with marginally subcritical solvents as near-critical extraction operations. The two types of operation are closely related and many of the convenient properties of supercritical solvents are shared by marginally subcritical ones also. 

Several standard methods for the quantitative analysis of food samples are based on measuring the sample s mass following a selective solvent extraction. For example, the crude fat content in chocolate can be determined by extracting with ether for 16 h in a Soxhlet extractor. After the extraction is complete, the ether is allowed to evaporate, and the residue is weighed after drying at 100 °C. This analysis has also been accomplished indirectly by weighing a sample before and after extracting with supercritical GO2. 

When ionic liquids are used as replacements for organic solvents in processes with nonvolatile products, downstream processing may become complicated. This may apply to many biotransformations in which the better selectivity of the biocatalyst is used to transform more complex molecules. In such cases, product isolation can be achieved by, for example, extraction with supercritical CO2 [50]. Recently, membrane processes such as pervaporation and nanofiltration have been used. The use of pervaporation for less volatile compounds such as phenylethanol has been reported by Crespo and co-workers [51]. We have developed a separation process based on nanofiltration [52, 53] which is especially well suited for isolation of nonvolatile compounds such as carbohydrates or charged compounds. It may also be used for easy recovery and/or purification of ionic liquids. 

For the analysis of organic additives in polymeric materials, in most cases, prior extraction will be necessary. Depending on the nature of the additive, many different approaches are employed. These include soxhlet extraction with organic solvent or aqueous media, total sample dissolution followed by selective precipitation of the polymer leaving the additive in solution, assisted extraction using pressurised systems, ultrasonic agitation and the use of supercritical fluids. In trace analysis, solid phase extraction (SPME) from solution or solvent partition may be required to increase the analyte concentration. 

In some cases, components extracted with supercritical carbon dioxide are immediately dissolved in a solvent after extraction. Under what conditions might this be advisable ... 

Agricultural processing will still incorporate solvents. As an example, soybean flakes were extracted with supercritical carbon dioxide to produce a solvent-free, good-quality soybean oil. During the SFE process, volatile compounds were trapped on a porous polymer trap attached at the exhaust port of the SFE apparatus. The volatile profile obtained from the sorbent trap was found to be similar to the headspace profile from the SFE/soybean oil removed during the same extraction. In addition, crude soybean oil was heated in a stirred reactor and the volatiles, which were stripped by supercritical carbon dioxide in an attempt to improve oil properties, were collected on sorbent traps and analyzed by the above method for comparison. The described methodology permits the characterization of volatiles and semivolatUes in SEE soybean oil and can be used to monitor the extraction and quality of the resultant oil (Snyder and King, 1994). 

The resinoids described above should be distinguished from prepared oleoresins (e.g., pepper, ginger, and vanilla oleoresins), which are concentrates prepared from spices by solvent extraction. The solvent that is used depends on the spice currently, these products are often obtained by extraction with supercritical carbon dioxide [223a]. Pepper and ginger oleoresins contain not only volatile aroma compounds, but also substances responsible for pungency. 

Supercritical fluid extraction uses a supercritical fluid (Box 25-2) as the extraction solvent.20 C02 is the most common supercritical fluid because it is inexpensive and it eliminates the need for costly disposal of waste organic solvents. Addition of a second solvent such as methanol increases the solubility of polar analytes. Nonpolar substances, such as petroleum hydrocarbons, can be extracted with supercritical argon.21 The extraction process can be monitored by infrared spectroscopy because Ar has no infrared absorption. 

Free convection is fluid flow, induced by density gradients owing, for example, to temperature gradients. In gas extraction the supercritical solvent is subject to density variation with only slight changes in pressure and temperature. Furthermore, flow velocities within the processing equipment are low, so that flow owing to free convection may be important. Therefore, conditions for free convective flow must be considered in such types of systems. For isothermal vertical plates ... 

Decontamination of soils using supercritical fluids is an attractive process compared to extraction with liquid solvents because no toxic residue is left in the remediated soil and, in contrast to thermal desorption, the soils are not burned. In particular, typical industrial wastes such as PAHs, PCBs, and fuels can be removed easily [7 to 21]. The main applications are in preparation for analytical purposes, where supercritical fluid extraction acts as a concentration step which is much faster and cheaper than solvent-extraction. The main parameters for successful extraction are the water content of the soil, the type of soil, and the contaminating substances, the available particle-size distribution, and the content of plant material, which can act as adsorbent material and therefore prolong the extraction time. For industrial regeneration, further the amount of soil to be treated has to taken into account, because there exists, so far, no possibility of continuous input and output of solid material for high pressure extraction plants, so that the process has to be run discontinuously. 

Solvent removal from the gels has a strong influence on the structure of the materials. Simple evaporation of solvent gives the xerogel, whereas extraction with supercritical C02 gives the aerogel with much larger pore volume and pore diameter (Miller et al., 1994 Hutter et al., 1995). 

Samples of sand spiked with 36 nitroaromatic compounds, 19 haloethers, and 42 organochlorine pesticides, and a standard reference soil (certified for 13 polynuclear aromatic hydrocarbons, dibenzofuran, and pentachlorophenol) were extracted with supercritical carbon dioxide in a two- or four-vessel supercritical fluid extractor to establish the efficiency of the extraction and the degree of agreement of the parallel extraction recoveries. Furthermore, the many variables that influence the extraction process (e.g., flowrate, pressure, temperature, moisture content, cell volume, sample size, extraction time, modifier type, modifier volume, static versus dynamic extraction, volume of solvent in the collection vessel, and the use of glass beads to fill the void volume) were investigated. 

In addition, a supporting material (such as Perfil 100 ) was also added into the reaction mixture then the solvent was evaporated and the solid residue was extracted with supercritical carbon dioxide at 150 bar pressure. As an example, resolution of 37 by supercritical extraction is outlined in Scheme 17 and the results are sumarised in Table 10. 

Processes for supercritical extraction of oils have been described in numerous literature references, including Paulaitis et al. ( ), Ely and Baker (2), Gerard (3.), Stahl et al. (4K and Robey and Sunder (5). The literature lacks detailed phase equilibrium data on multicomponent essential oils with supercritical solvents in the proximity of the solvent critical temperature. 

Extractions or extractive distillations with supercritical solvent need to be performed at as high as possible a solubility of oil in the extract or vapor phase in order to reduce the solvent or carrier gas requirement. From our lemon oil-carbon dioxide phase diagrams, it appears that the highest practical solubility level is 0.9 mole % (2.8 wt ) essential oil. This is obtainable at 313 K. At lower temperature, sensitivity of solubility to pressure requires that solubility be lower (e.g., 0.3 mole % at 308 K). 

This study indicates that extraction with supercritical water could be an attractive route for liquefaction of Victorian brown coals (but probably not black Australian coals). The low cost and ready availability of the solvent (water), the relatively high H/C atomic ratios of the extracts, and also as no hydrogen or coal-drying are required, are positive factors. Higher yields can be obtained when a strong base or a hydrogen-donor is added to the water. 

Other reagents and solvents were obtained from commercial sources. The samples were prepared by mixing various chiral bases with racemic acids in 0.5 1 molar ratio. A porous supporting material (Perfilt), impregnated with these mixtures, was put into the extractor vessel and extracted with supercritical carbon dioxide. 

Supercritical fluid extraction (SEE) has become a method of choice for the extraction of plant material [14]. It represents an interesting alternative technique compared to conventional liquid-solid extraction, with lower solvent consumption and working temperature. The free bases of hyoscyamine and scopolamine are extractable with... 

Extraction may also be performed with supercritical solvents. Ionic liquids have been described as designer solvents that is, their properties can be adjusted to suit the requirements of a particular process. Properties such as melting point, viscosity, density, and hydrophobicity can be modified by simple changes in the structure of the ions (Table 12.4). For example, the melting points of... 

In addition to common organic solvents, supercritical fluids (scf s) can be used for a great variety of extraction processes [158 165], Supercritical fluid extraction (SFE), mostly carried out with SC-CO2 as eluant, has many advantages compared to extractions with conventional solvents. The solvent strength of a supercritical fluid can easily be controlled by the pressure and temperature used for the extraction at a constant temperature, extraction at lower pressures will favour less polar analytes, while extraction at higher pressures will favour more polar and higher molar mass analytes. As supercritical fluids such as CO2 and N2O have low critical temperatures (tc = 31 °C and 36 °C, respectively), SFE can be performed at moderate temperatures to extract thermolabile compounds. Typical industrial applications using SC-CO2 include caffeine extraction from coffee beans [158] as well as fat and oil extraction from plant and animal tissues [165]. For some physical properties of supercritical solvents, see Section 3.2. 

It is possible to obtain lipids from microalgae by conventional solvent extraction. However, extraction with supercritical (SC) CO, is emerging as a potential alternative to obtain lipids from natural sources (13-19). 

With CO2 as a solvent, an investigation by Eggers (6) indicates that high-pressure extraction with supercritical separation is energetically superior over subcritical separation in a wide range of extraction pressures and temperatures. In the CO2 solvent ex-... 

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