Industrial applications, supercritical carbon dioxide

Although numerous advantages are associated with the use of supercritical carbon dioxide (scC02) as an ecologically benign and user friendly reaction medium, systematic applications to metal-catalyzed processes are still rare. A notable exception is a recent report on the use of scC02 for the formation of industrially relevant polymers by ROMP and the eyelization of various dienes or enynes via RCM [7]. Both Schrock s molybdenum alkylidene complex 24 and the ruthe-... 

From an industrial point of view, homogeneous catalysis has significant advantages concerning selectivities and due to mild reaction conditions [47]. In fact, there is only a limited munber of processes established in industrial applications because of the disadvantageous separabihty of the catalyst from substrate and product. A possible and convenient solution for this limitation can be the application of supercritical carbon dioxide as part of a reaction system due to the following ... 

Plants and plant extracts have been used as medicine, culinary spice, dye and general cosmetic since ancient times. Plant extracts are seen as a way of meeting the demanding requirements of the modem industry. In the past two decades, much attention has been directed to the use of near critical and supercritical carbon dioxide solvent, particularly in the food pharmaceutical and perfume industries. CO2 is an ideal solvent because it is non-toxic, non-explosive, readily available and easily removed from the extracted products. At present the major industrial-scale applications of supercritical fluid extraction (SFE) are hop extraction, decaffeination of coffee and tea, and isolation of flavours, fragrances and other components from spices, herbs and medicinal plants [1-4]. 

The most common sc-fluid for industrial processing and benchtop research is supercritical carbon dioxide, chosen because of its moderate and easily attained critical temperature and pressure and its non-toxicity. Reactions in SC-CO2 produce similar results as reactions in nonpolar organic solvents. Its solvent polarity, empirically determined using solvatochromic dyes as polarity indicators (see Section 7.4), corresponds to that of hydrocarbons such as cyclohexane [221, 222]. Carbon dioxide has no dipole moment and only a small quadrupole moment, a small polarizabihty volume, and a low relative permittivity (er = 1.4-1.6 at 40 °C and 108-300 bar) [221, 223]. Thus, SC-CO2 is only suitable as a solvent for nonpolar substances, which unfortunately imposes considerable limitations on its practical applications. To overcome this limitation, more polar co-solvents (modifiers) such as methanol can be added to SC-CO2. 

Like supercritical carbon dioxide, supercritical water is a very interesting substance that has strikingly different properties from those of liquid water. For example, recent experiments have shown that supercritical (superfluid) water can behave simultaneously as both a polar and a nonpolar solvent. While the reasons for this unusual behavior remain unclear, the practical value of this behavior is very clear It makes superfluid water a very useful reaction medium for a wide variety of substances. One extremely important application of this idea involves the environmentally sound destruction of industrial wastes. Most hazardous organic (nonpolar) substances can be dissolved in supercritical water and oxidized by dissolved 02 in a matter of minutes. The products of these reactions are water, carbon dioxide, and possibly simple acids (which result when halogen-containing compounds are reacted). Therefore, the aqueous mixture that results from the reaction often can be disposed of with little further treatment. In contrast to the incinerators used to destroy organic waste products, a supercritical water reactor is a closed system (has no emissions). 

The attractiveness of supercritical carbon dioxide extraction is shown by the already existing industrial applications of hop extraction, decaffeination of tea and coffee, defatting of cocoa powder, and extraction of herbs and spices and is also demonstrated by the large number of patent applications and scientific publications in recent years. 

Supercritical carbon dioxide has been industrially used in a variety of processes, including coffee decaffeination, tea decaffeination, and extraction of fatty acids from spent barley, pyrethrum, hops, spices, flavors, fragrances, com oil, and color from red peppers. Other applications include polymerization, polymer fractionation, particle formation for pharmaceutical and military use, textile dyeing, and cleaning of machine and electronic parts. 

The other cleanup method utilizes the adsorptive properties of activated carbon to remove caffeine from the carbon dioxide before recycle (Zosel, 1981). An adsorption isotherm of the carbon dioxide-caffeine-activated carbon system, shown in figure 10.3, indicates that it is possible to adsorb the caffeine in the carbon dioxide-rich stream onto activated carbon (Krukonis, 1983a). In this industrial application an activated carbon bed is used to remove a component from a supercritical carbon dioxide-rich stream rather than having the activated carbon regenerated by the supercritical carbon dioxide. It is perhaps no surprise that spent activated carbon is difficult to regenerate with carbon dioxide, as discussed in chapter 8. 

As an extraction fluid, supercritical carbon dioxide is mostly used because its critical parameters can be rather easy obtained (Tc = 31°C, Pc = 74 bar) and is non-polluting. Other supercritical fluids are nitrogen protoxide (Tc = 36°C, Pc = 71 bar), ammonia (Tc = 132°C, Pc = 115 bar) and water (Tc = 374°C, Pc = 217 bar). The supercritical fluids can be removed at low temperatures, without any toxic wastes but the necessary high pressure can be dangerous in industrial applications. The extractors are tubular devices, pressure resistant where the (semi) solid sample is placed. 

Supercritical carbon dioxide extraction of natural products from solid plant matrices is currently an established application, with over one hundred industrial facilities of various sizes operating throughout the world. Several books have been published describing this process in detail - see, for example Brunner [1]. The main green credential of these processes is the replacement of volatile organic solvents. Their implementation resulted, however, from definite technological advantages. 

A recently developed industrial-scale application is a textbook example of the main advantages of supercritical carbon dioxide in extractions from natural products. The Diamond process, jointly developed by the Centre d Energie Atomique and the French company Sabate, [2] extracts the contaminant trichloroanisol (TCA) from cork stoppers for wine bottles. This contaminant is produced by fungi and is responsible for the infamous cork taint taste of wines, which has brought serious financial losses to the wine industry and damaged the image of cork as the ideal material for bottle stoppers. 

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