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134 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 (photolithography, etching). These topics are included in Section 3.11 and Section 3.12. Fabric cleaning has recently been commercialized by two groups in the US (Micell, Inc., and Global Technologies/DryWash). Major issues confronting these groups in the future include design of inex- pensive surfactants that clean effectively in CO2, the design of high pressure cleaning equipment that ren- ders the process cost-competitive and competition from other ‘benign’ cleaning technologies (such as the use of high flash point alkanes, silicones and wa- ter). The use of silicones (Green Earth [49]) seems to present significant competition, as these materials are promoted as being more benign than PERC (they are, if TLV is any indication), they are used at one atmosphere (hence, equipment is relatively inexpen- sive) and their use is backed by some large, relatively wealthy corporations (GE for silicone production, Procter and Gamble for surfactant production [49]). Indeed, even the design of more efficient conventional dry cleaning equipment (i.e. that using perchloroethy- lene (PERC) as the solvent) represents a commercial challenge [50]; the volume of PERC used by dry cleaners in the US has dropped dramatically over the past decade primarily owing to the use of ‘tighter’ equipment (lower fugitive losses during cleaning). Indeed, significant consolidation occurred in the CO2-based dry cleaning industry during early 2002. Chart Industries, Inc., a member of the DryWash consortium, decided to exit the CO2-based dry clean- ing business [51] after several years of disappointing growth ($126,000 net sales in 2001); the connection to the consortium was maintained by some of their employees as a spin-out company (Cool Clean). Cool Clean recently purchased the Hangers franchising operation from Micell. Finally, intellectual property issues could complicate the use of carbon dioxide in fabric cleaning. Unilever, for example, has filed a number of patents (and continuations in part, etc.) on the use of surfactants in CO2 for the purpose of fabric cleaning [52], as well as on the general process where CO2 plus a surfactant is employed in fabric cleaning. In summary, this report will include several issues important to future cleaning applications for CO2, namely the design of effective, low-cost auxiliaries and the design of lower cost equipment for use in parts cleaning. 1.10. The effect of regulation on use of CO2 in green chemistry and chemical processing The extent to which conventional solvents are reg- ulated will have a profound effect on the extent to which CO2 is used as a solvent in the future. For example, we can examine the recent history of chlo- rofluorocarbons (vis-à-vis CO2 ). Chlorofluorocarbons (CFCs) were preferred as solvents for cleaning be- cause they are non-flammable, relatively non-toxic (TLV of chlorodifluoromethane is 1000 ppm [5]), and inexpensive. As a result of research performed during the 1970s and 1980s, it became apparent that CFCs contributed to the chemical erosion of the strato- spheric ozone layer, leading to the Montreal Protocols that outlined a timetable for the withdrawal of CFCs from use as solvents (and refrigerants, etc.). Carbon dioxide is often described as a potential substitute for CFCs in cleaning (and also refrigeration). Because CFCs exhibited a number of highly favorable proper- ties, without the regulation restricting their use, it is not likely that CO2 would have ever been considered as a viable competitor. Although CFCs represent a somewhat extreme case, regulation does exert more subtle effects on the use of CO2. This is most often seen when comparing the pluses and minuses of using conventional solvents to use of carbon dioxide. From an engineering perspec- tive, carbon dioxide is nearly always more difficult to employ as a solvent because one needs high-pressure equipment. Consequently, the extent to which a par- ticular solvent is regulated and hence, the obstacles to the use of such a solvent in a chemical process, can tip the scales either in favor or against use of CO2. For example, acetone is not currently on the list of com- pounds that require reporting under section 313 of the Emergency Planning and Community Right-to-Know Act (EPCRA, also known as the Toxics Release Inven- tory (TRI) [53]). Neither is it listed as a ‘Hazardous Air Pollutant’ [54] by the Office of Air Quality Plan- ning and Standards at the US EPA. Consequently, if a manufacturer was currently using carbon tetrachlo- ride, for example, in a process where some of the solvent was emitted to the atmosphere, a natural ap- proach to ‘greening’ the process might be to first deter- mine whether acetone could be substituted for carbon tetrachloride (the latter is included on both the TRI and classified as a hazardous air pollutant). Naturally,PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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