Green extraction processes

Green extraction processes (viz., subcritical water and sub- and supercritical carbon dioxide extraction) are the most promising methods for industrial implementation. In fact, these methods are excellent for isolating anthocyanins and other polyphenols from grape processing wastes [196]. 

Several studies on the use of phosphonium ionic liquids for selective extraction of metal cations from aqueous solution were reported. Binnemans and co-workers described a green extraction process in which cobalt was separated as a tetrachlorocobaltate(ii) complex in a tri(he yl)tetradecylphosphonium chloride ionic liquid phase. This left behind nickel, magnesium and calcium in the aqueous phase. The overall process showed a vety low environmental impact since no additional organic solvents were required. Other processes to extract zinc(n) nickel(II) and cobalt(II), cadmium (11), palladium and... 

Ma.nufa.cture. Several nickel oxides are manufactured commercially. A sintered form of green nickel oxide is made by smelting a purified nickel matte at 1000°C (30) a powder form is made by the desulfurization of nickel matte. Black nickel oxide is made by the calcination of nickel carbonate at 600°C (31). The carbonate results from an extraction process whereby pure nickel metal powder is oxidized with air in the presence of ammonia (qv) and carbon dioxide (qv) to hexaamminenickel(TT) carbonate [67806-76-2], [Ni(NH3)3]C03 (32). Nickel oxides also ate made by the calcination of nickel carbonate or nickel nitrate that were made from a pure form of nickel. A high purity, green nickel oxide is made by firing a mixture of nickel powder and water in air (25). 

Tea oxidation is generally referred to as fermentation because of the erroneous early conception of black tea production as a microbial process.66 Not until 1901 was there recognition of the process as one dependent on an enzymically catalyzed oxidation.67 This step and further reactions result in the conversion of the colorless flavanols to a complex mixture of orange-yellow to red-brown substances and an increase in the amount and variety of volatile compounds. Extract of oxidized leaf is amber-colored and less astringent than the light yellow-green extract of fresh leaf and the flavor profile is considerably more complex. 

Lapkin AA, Peters M, Greiner L, Chemat S, Leonhard K, Liauw MA, Leitner W. (2010) Screening of new solvents for artemisinin extraction process using ab initio methodology. Green Chem 12 241-251. 

Market considerations may play an important role in the feasibleness of this new extraction process by selling this output as a green salable product. Analyses of our raffinates, according to the European Norm for oils, confirmed that the olive oil thus obtained has all the characteristics for human use. 

Enzyme-catalyzed reactions based on such biphasic systems have been shown to be promising alternatives for developing green chemical processes because of their physical and chemical characteristics [9]. By combining these media with enzymes, the possibilities of carrying out integral green biocatalytic processes has been already demonstrated [10-12]. Such biphasic systems can be used for both the biotransformation and extraction of products simultaneously, even under extremely harsh conditions, because of the different miscibilities of ILs and SC-CO2. 

Perry and Green (1984) give a useful summary of solubility data. Liquid-liquid equilibrium compositions can be predicted from vapour-liquid equilibrium data, but the predictions are seldom accurate enough for use in the design of liquid-liquid extraction processes. 

Since the first large-scale supercritical extraction process was commercialized for the decaffeination of green coffee with carbon dioxide a decade ago, scientists and engineers in the food industry have been paying considerable attention to this technique for similar separations, i.e., removal of cholesterol from butter, removal of cocoa butter from cocoa beans, and extraction of hops, spices, and... 

The supply problem was partially solved through improvements in the extraction process from Pacific yew, but a more radical solution was achieved by French groups under the leadership of Pierre Potier in Paris and Andrew Greene in Grenoble. They discovered that the needles of the European yew, Taxus baccata, contained a structural relative of taxol called 10-deacetylbaccatin III to the extent of around 1 g per 3 kg of needles. This natural product could be converted to taxol in four chemical steps in a process discovered by Robert Holton in the USA. Since the European yew grows relatively rapidly, the needles could be harvested and the supply of taxol was thus assured. 

The extraction techniques described in this book fulfill many of Anastas and Warner s principles. For example, the use of supercritical carbon dioxide (SC-CO2) as the sole extraction solvent results in a nonpolluting process (prevention of waste and safer solvents and auxiliaries). Other beneficial properties of supercritical CO2 include fast diffusivity and nearly zero surface tension, which lead to extremely efficient extractions. In Chapters 2-4, applications of SC-CO2 as an extraction solvent are described. Ethanol and water are also environmentally friendly solvents that can be used as extraction media in many applications (see Chapters 5-7). Pressurized hot water ( 100-200 °C) in particular is a safe and nonpolluting solvent that has a similar dielectric constant to polar organic solvents, such as ethanol or acetone. Hence, pressurized hot water is a viable green alternative to many current extraction processes that use toxic organic solvents. Similarly, pressurized hot ethanol is an excellent solvent for the extraction of most medium polar to nonpolar organic molecules. Some of the techniques, such as membrane-assisted solvent extraction, described in Chapter 10, use organic solvents but in much smaller amounts compared to classical extraction techniques. Other techniques, for instance solid-phase microextraction and stir-bar sorptive extraction, described in Chapter 11, use no solvents. 

Because SFE can provide much higher selectivity than conventional extraction methods, usually the SFE processes produce far less impurities coextracted with the active compounds. Pure active compounds can be easily isolated from the SFE produced herbal extracts using HPLC for medicinal studies. This green extraction technique provides an efficient way of obtaining active compounds from herbs with minimum waste production and appears attractive for manufacturing high quality herbal products and for isolation and identification of active natural products from herbs and plants in general. 

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