Carbon dioxide as an extractant

Carbon dioxide as an extraction solvent has the advantage of low critical temperature additionally, it is cheap, nontoxic, and nonexplosive. It is classified as a nonpolar solvent that can be modified to more polar solvent by the addition of organic solvents (modifiers) such as lower alcohols (e.g., methanol). 

The most important extraction technique nowadays is simple solvent extraction. The traditional solvent for extraction was benzene, but this has been superseded by other solvents because of concern over the possible toxic effects of benzene on those working with it. Petroleum ether, acetone, hexane and ethyl acetate, together with various combinations of these, are typical solvents used for extraction. Recently, there has been a great deal of interest in the use of carbon dioxide as an extraction solvent. The process is normally referred to as super-critical carbon dioxide extraction but, in fact, the pressures employed are usually below the critical pressure and the extraction medium is sub-critical, liquid carbon dioxide. The pressure required to liquefy carbon dioxide at ambient temperature is still considerable and thus the necessary equipment is expensive. This is reflected in the cost of the oils produced, but carbon dioxide has the advantage that it is easily removed and there are no concerns about residual solvent levels. 

The mechanics of increasing the pressure on the fluid can he done in a couple of ways. One would he to increase the temperature surrounding the hquid carbon dioxide [14]. As noted above, the pressme exerted on the hquid carbon dioxide in the cyhnder is a function of its ambient temperature. Only at higher ambient temperatures would the resultant system pressures be enough to reach the nearly hquid-hke densities of 0.1 to 1.0 g/mL that are required to use supercritical fluid carbon dioxide as an extraction solvent for extraction temperatures ranging from 40 C to 150°C. This approach is not widely used at this time. Liquid pumps (such as those used in liquid chromatographs) are more commonly used. 

Another very intere,sting and promising cleanup technique is supercritical fluid extraction (SFE). This method, which usually employs supercritical carbon dioxide as an extraction medium, shows a high selectivity toward nonpolar solutes. As a rule of thumb, if a solute is soluble in hydrophobic solvents such as heptane or sometimes even in more polar ones such as dichloromethane, it should also be soluble in supercritical carbon dioxide. Through addition of modifiers (e.g.. methanol, water, formic acid), even relatively polar molecules are extractable, although the in-... 

One limitation of carbon dioxide as an extractant is its polarity. In its supercritical state and at low densities, CO2 has a polarity close to that of hexane. Even at extremely high pressures the solubility parameter may not approach that which is required to solubilize and extract polar analytes. This limitation can be overcome by the use of another extraction fluid, which is more polar, or by adding a polar modifier to the CO2. The most commonly used modifier with CO2 has been methanol. Increased solubilities and recoveries of polar analytes have been reported when a polar modifier is added to a less polar supercritical fluid (66-68). The ability of the supercritical fluid to dissolve a particular analyte is not the only factor, which affects extraction efficiency. The degree to which the analyte partitions into the supercritical fluid fi om the solid-sample matrix depends greatly on the sorptive and active sites on the solid matrix and the polarity of the solute (64,69). The addition of a polar modifier or entrainer, such as methanol, to a supercritical fluid such as CO2, not only increases the solubility of polar analytes in the supercritical fluid, but also may help block sorptive sites on the surface of the sample matrix. 

After performing the bioconversion in an ionic liquid, the product needs to be recovered and the biocatalyst and the ionic liquid recycled. Relatively volatile products can be removed by evaporation. Alternatively, immiscible organic solvents can be used to extract the product, and the biocatalyst can be recycled as a suspension in the ionic liquid phase [58]. A more elegant, green method, which avoids the use of volatile organic solvents altogether, involves the use of supercritical carbon dioxide as the extractive phase [96, 147, 148]. 

Supercritical fluid extraction (SFE) is a method that circumvents some problems associated with conventional separation techniques. Carbon dioxide, as an inert, inexpensive, nonflammable, and environmentally acceptable gas is the solvent of choice because of its moderate critical temperature and pressure (76). SFE has been used effectively to refine marine oils and remove cholesterol, polychlorinated biphenyls (PCB), Vitamin E, and other components (77). The disadvantages of this process include the use of extremely high pressures and the high capital cost. 

Supercritical Extraction. The use of a supercritical fluid such as carbon dioxide as extractant is growing in industrial importance, particularly in the food-related industries. The advantages of supercritical fluids (qv) as extractants include favorable solubiHty and transport properties, and the abiHty to complete an extraction rapidly at moderate temperature. Whereas most of the supercritical extraction processes are soHd—Hquid extractions, some Hquid—Hquid extractions are of commercial interest also. For example, the removal of ethanol from dilute aqueous solutions using Hquid carbon dioxide... 

Natural Products. Various methods have been and continue to be employed to obtain useful materials from various parts of plants. Essences from plants are obtained by distillation (often with steam), direct expression (pressing), collection of exudates, enfleurage (extraction with fats or oils), and solvent extraction. Solvents used include typical chemical solvents such as alcohols and hydrocarbons. Liquid (supercritical) carbon dioxide has come into commercial use in the 1990s as an extractant to produce perfume materials. The principal forms of natural perfume ingredients are defined as follows the methods used to prepare them are described in somewhat general terms because they vary for each product and suppHer. This is a part of the industry that is governed as much by art as by science. 

Supercritical fluid extraction (SFE) is a technique in which a supercritical fluid [formed when the critical temperature Tf) and critical pressure Pf) for the fluid are exceeded simultaneously] is used as an extraction solvent instead of an organic solvent. By far the most common choice of a supercritical fluid is carbon dioxide (CO2) because CO2 has a low critical temperature (re = 31.1 °C), is inexpensive, and is safe." SFE has the advantage of lower viscosity and improved diffusion coefficients relative to traditional organic solvents. Also, if supercritical CO2 is used as the extraction solvent, the solvent (CO2) can easily be removed by bringing the extract to atmospheric pressure. Supercritical CO2 itself is a very nonpolar solvent that may not have broad applicability as an extraction solvent. To overcome this problem, modifiers such as methanol can be used to increase the polarity of the SFE extraction solvent. Another problem associated with SFE using CO2 is the co-extraction of lipids and other nonpolar interferents. To overcome this problem, a combination of SFE with SPE can be used. Stolker et al." provided a review of several SFE/SPE methods described in the literature. 

This chapter reviews recent findings about the health benefits of phytochemicals present in fruits, vegetables, nuts, seeds, and herbs, including phenolics, carotenoids, sterols, and alkaloids. These phytochemicals are extracted using emerging technologies such as supercritical carbon dioxide (SC-CO2) extraction, PEF, MWE, HPP, UE, and OH. The impact of important parameters related to sample preparation (particle size and moisture content) and extraction process (temperature, pressure, solvent flow rate, extraction time, and the use of a cosolvent) on the efficiency of extraction and on the characteristics of the extracted products is evaluated based on an extensive review of recent literature. The future of extraction of phytochemicals is certainly bright with the... 

The solution is now poured and rinsed into a moderate-sized (500-750 c.c.) separating funnel containing 100 g. of ice, and the acid is buffered by slowly adding finely powdered sodium carbonate. The ester separates as an oil and is taken up in ether, but the funnel must not be stoppered and shaken until evolution of carbon dioxide has ceased. Extraction with ether is repeated and the combined 1 Annalen, 1882, 215, 1. 

The first use of supercritical fluid extraction (SFE) as an extraction technique was reported by Zosel [379]. Since then there have been many reports on the use of SFE to extract PCBs, phenols, PAHs, and other organic compounds from particulate matter, soils and sediments [362, 363, 380-389]. The attraction of SFE as an extraction technique is directly related to the unique properties of the supercritical fluid [390]. Supercritical fluids, which have been used, have low viscosities, high diffusion coefficients, and low flammabilities, which are all clearly superior to the organic solvents normally used. Carbon dioxide (C02, [362,363]) is the most common supercritical fluid used for SFE, since it is inexpensive and has a low critical temperature (31.3 °C) and pressure (72.2 bar). Other less commonly used fluids include nitrous oxide (N20), ammonia, fluoro-form, methane, pentane, methanol, ethanol, sulfur hexafluoride (SF6), and dichlorofluoromethane [362, 363, 391]. Most of these fluids are clearly less attractive as solvents in terms of toxicity or as environmentally benign chemicals. Commercial SFE systems are available, but some workers have also made inexpensive modular systems [390]. 

Supercritical carbon dioxide extraction (SCDE) is an ex sitn process for the treatment of low-level solid mixed and land disposal restricted (LDR) wastes. SCDE can extract hazardons solvents from waste snbstrates to prodnce land-disposable, low-level wastes. The process employs the snpercritical finid carbon dioxide as a solvent. This finid is noncombustible, nontoxic, and environmentally safe. In its supercritical state, carbon dioxide can dissolve organic contaminants allowing the fluid to quickly penetrate and facilitate transfer out of a contaminated matrix. 

Some more exotic procedures can be suggested as well The combination of ILs wifh fhe use of supercritical carbon dioxide (scCOj) as an extractant represents a potential combination for the reaction of synthesis and downstream separation, as well as for purification. 

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 current trend of consumer preference towards natural products requires new processing methods for spice-oils and extracts, without the addition of external material. In recent years there has been an increased interest in supercritical and subcritical extraction [26,27], which use carbon dioxide as a solvent [34,35,36]. Carbon dioxide (CO2) is an ideal solvent for the extraction of natural products because it is non-toxic, non-explosive, readily... 

It has been shown [73] that carbon dioxide is less efficient as an extractant for the heavier polycyclic aromatic hydrocarbons than nitrous oxide and Freon-22. This deficiency can be remedied by using a mixture of water, methanol and methylene dichloride [73]. 

SCF). This fluid does not any more have a free surface, that characterizes a liquid as opposed to a vapour, but may serve as a useful solvent just the same. Some substances that are gases at ambient conditions can be compressed by high pressures to become supercritical fluids and solvents, a well-known example being carbon dioxide, used extensively as an extractant for foodstuffs and pharmaceuticals. Some physical properties—the critical temperature Tc, pressure Pc, and density dc—of supercritical solvents are shown in Table 3.3. 

The liquid-gas equilibrium line terminates at a point known as the critical point. The temperature and pressure that define the critical point are known as the critical temperature and the critical pressure. For example, nitrous oxide has a critical temperature of 36°C and a critical pressure of 72.45 bar (1051 psi). When the temperature and pressure exceed these critical values, the system becomes a supercritical fluid. Supercritical fluids have the flow properties of gases but densities similar to liquids, and supercritical fluids have no surface tension. Therefore, supercritical fluids are terrific solvents. For example, supercritical carbon dioxide is an excellent solvent for extracting caffeine from coffee without resorting to more toxic organic solvents like dichloromethane. 

In contrast to alkamides, alternative extraction solvents such as SF carbon dioxide appear to be ineffective as an extraction solvent for CAP removal (Catchpole et al., 2002 Sun et al., 2002). Conditions evaluated by these researchers include pressures of 31 - 55 MPa and temperatures between 41 and 60°C. In both studies, ethanol was used as a solvent modifier, but the supercritical carbon dioxide was not modified sufficiently to promote the extraction of CAP. The addition of 10% methanol to the supercritical carbon dioxide at 25 MPa and 60°C was sufficient to promote the extraction of rosmarinic acid, a compound with similar structure features as cichoric acid (Bicchi et al., 2000). Thus, additional work is needed to determine if SFE can be used as a method to remove CAP. 

In order to obtain a commercial loading of the near-critical extractant, the extraction is sometimes carried out at enhanced pressures in the droplet regime. In such cases the liquid phase does not flow downwards as a film adhering to the packings of a column as is usually assumed, rather it falls down as a swarm of droplets. On the basis of the separation of a mixture of partial glycerides the behavior of packed columns in the droplet regime (instable flowing films) the efficiency of different column installations are compared. A mixture of 55 wt.% propane and 45 wt.% carbon dioxide is used as an extractant. 

Energetic materials like TNT or other nitroaromatic compounds are readily soluble in liquid and supercritical carbon dioxide (SC-CO2). Extraction processes using SC-CO2 as an extracting solvent (supercritical fluid extraction SFE) permit the discharging of TNT and its breakdown products out of contaminated soils or other matrices [1],... 

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