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132 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 drops, is shown by Charpentier et al. [45] in the examination of the continuous polymerization of fluorinated monomers in carbon dioxide. Here, the monomers are soluble in CO2 (as are many vinyl monomers) while the polymers are insoluble (also a relatively general trend). Thus, monomer can be con- tinuously recycled through the continuously stirred tank reactor while the polymer precipitates and is collected. 1.7.1.5. Recover and reuse homogeneous catalysts and CO2 -philes. The discovery of CO2 -philes in the early 1990s allowed for the exploration of a number of processes in CO2 that had been hereto- fore untenable owing to CO2’s feeble solvent power. Highly CO2 -soluble surfactants and catalyst lig- ands became available, leading to a number of im- portant discoveries regarding chemistry in carbon dioxide. However, the new CO2-philes are signifi- cantly more expensive than their CO2-phobic coun- terparts and hence it is important to the economics of a CO2 -based process that any CO2 -philes used in the process be recycled as extensively as possi- ble. Note that recycle of CO2-philes not only makes good economic sense, but is also more sustainable than the case where the CO2-philes are simply dis- posed. Recovery and recycle of homogeneous catalysts is important whenever such catalysts are employed be- cause the metals employed in such catalysts are typi- cally expensive. In the case of a CO2-based process, the ligands are also likely to be expensive (they must be designed to exhibit high CO2 solubility) and hence the need for effective catalyst recycle is even more important. In summary, attention must always be paid to the economic viability of processes employing CO2 as reactant and/or solvent—while CO2 -based processes are generally thought to be ‘green’, their benefits will never be realized if the cost of such processes dwarfs conventional analogs. 1.7.2. Where would process improvements enhance opportunities for green chemistry in CO2? As in the previous section, examples described here are not directly related to green chemistry, but solution of such problems would greatly enhance the viability of CO2-based processes and are hence intimately tied to green chemistry in carbon dioxide. For example, there remains no truly efficient means by which to inject and remove granular solids from a high-pressure system (screw feeders have been tried with limited success). There are clearly a number of areas (food processing) where continuous injection and removal of solids would greatly enhance the economic via- bility of a CO2-based process, yet lack of the me- chanical means by which to accomplish this relegates the process to batch or semi-batch operation. Note that the chemical basis for continuous polyurethane foam production using liquid CO2 as the blowing agent (see Section 3.5.2) was established in the early 1960s, whereas commercialization only occurred af- ter development of the proper equipment in the early 1990s. Over the past decade, there has been significant academic and industrial interest in cleaning processes using CO2—cleaning of metal parts, electronics com- ponents, and fabrics. CO2 is ideally suited to such applications owing to its low viscosity and environ- mentally benign nature, yet mechanical issues com- plicate application of CO2 to these processes. For each of these applications, individual ‘pieces’ must be rapidly inserted into a high pressure chamber, the chamber sealed and pressurized, the ‘piece’ cleaned, the then chamber depressurized and emptied. At one atmosphere, such an operation is trivially simple to conduct and easy to scale (cost per part drops as chamber volume rises). The opposite is currently true for high-pressure operation; scale-up is non-trivial and the cost of the system rises rapidly as the size of the chamber rises. More efficient ‘piecework’ oper- ations at high pressure will not only render cleaning operations less expensive, but also coating and fabric dying operations. Finally, many proposed CO2 -based processes (including spin coating, lithography and developing, free meniscus coating) that are under ex- amination in academic/industrial laboratories would benefit greatly from breakthroughs in the design of equipment designed to efficiently transfer parts in and out of high-pressure environments. 1.8. Scope of this report This report will focus on CO2-based processes where chemical reactions are taking place (i.e. green chemistry) or materials are being processed to createPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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