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180 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 general agreement that phase behavior (thermodynam- ics) and transport play roles in the effects of process conditions on particle characteristics, it is not clear that a universal set of design guidelines currently exists. Hence, in summary, what appears to be needed in this CO2-based sub-field is research on building a true engineering model for such processes, where the input of fundamental thermophysical parameters allows for the design and operation of equipment that can deliver product with the desired characteristics. Indeed, the proliferation of acronyms associated with CO2 -based particle production (see Ref. [298]) lends the impres- sion that the various processes are in some way funda- mentally different from one another and thus, that one must experimentally evaluate each option (for a par- ticular solute) to determine the proper operating mode to produce a given particle size and distribution. The lack of a defined ‘unit operation’ with acknowledged theoretical underpinning makes it difficult to perform an engineering design and scale-up of such processes, hindering their wider use. Equipment for CO2-based particle production is rather treated as ‘custom’. Another avenue of research (in this area) that has received relatively scant attention in recent years is the use of CO2 to process/produce well-defined particles from pigments. It is known that pigment particle size (and extent of particle agglomeration) exhibits a strong effect on the ultimate color of the article receiving the pigment. Pigments are usually milled mechanically; the use of a CO2-based anti-solvent process could al- low for the production of pigments with good control over the size and size distribution. Texter [310] has reviewed a number of solution based methods (ho- mogeneous and multi-phase systems) for generating fine particles from pigments—most seem to rely upon controlled precipitation of pigment from a precursor solution (or emulsion) to form the particles. Here, nat- urally, CO2 presents some advantages as it can be read- ily separated from the organic solvent and it is itself benign. Whether such advantages allow CO2 -based processes to supplant traditional milling (which ob- viously uses no solvent) remains an open question, although preliminary results are promising [311]. 5.5. Industrial activity There has been an interesting spate of industrial activity on particle formation using carbon dioxide over the past 3 years, much of it not expressly tech- nology based. Bradford Particle Design (UK) helped pioneer the development of the ‘SEDS’ process (solution-enhanced dispersion by supercritical fluids), where ethanol is added to an aqueous solution while it is sprayed into CO2 to form particles. In early 2001, Inhale Therapeutics acquired Bradford Particle De- sign, demonstrating the interest by the pharmaceutical community in this technology. Interestingly, Bradford has previously announced that Bristol-Myers-Squibb had licensed their technology for use in pharmaceuti- cal manufacture; it is not clear as to the state of that al- liance at this time. At nearly the same time (late 2000) as the Bradford acquisition, Lavipharm (Greece) an- nounced the acquisition of Separex (France) and the purchase of a 30% stake in Phasex (US). Both Separex and Phasex are well known to the supercritical fluid community, having each worked on the fundamentals and design of numerous supercritical fluid processes. The review by Jung and Perrut lists many of the patents awarded on CO2-based processing for the gen- eration of fine particles. In addition to Bradford Parti- cle Design [301], a number of academics have patented aspects of the non-solvent route to particle produc- tion, including Randolph [312] and Sievers [302], at the University of Colorado and Subramaniam at the University of Kansas [313]. Regarding the PGSS type processes, many of the patents that have appeared are related to applications in the coatings industry, including the Unicarb Process (mentioned previously), and powder coatings applica- tions from Ferro (mentioned previously) and Morton [314]. 6. Milestones in green chemistry using CO2 Designating particular achievements as milestones is, of course, subjective. There are several types of milestones that one can consider with regards to green chemistry in carbon dioxide—purely scientific mile- stones, milestones in the dissemination of information on use of CO2 and milestones in commercialization. Perhaps the first true commercially successful ‘green’ applications of CO2 were the coffee decaffeination and CO2-based thermoplastic foaming processes scaled-up during the 1980s; these are milestones as they showed that one could successfully scale a CO2-based processPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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