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178 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 molecules in CO2, low pressures are sufficient to create solutions of CO2 in large molecules (or solu- tions of compounds in organic solvent), as suggested in Fig. 5. Hence, saturated solutions of CO2 (in ei- ther polymers or solute/solvent mixtures), sprayed through nozzles, have been used to generate fine particles. 5.2. Creating fine particles using CO2: non-solvent modes of operation and PGSS Jung and Perrut have written an excellent review of the use of supercritical fluids to generate fine particles [298]; other reviews have appeared recently as well [299]. These reviews describe the wide variety of ma- terials that have been micronized via CO2-based pro- cessing, and the various modes in which such particle processes operate. During the 1980s, Krukonis et al. [300] found that one could employ CO2 as a non-solvent to induce con- trolled precipitation of various solutes from organic solvent solution. The success of this approach derives from CO2’s generally feeble solvent power yet its mis- cibility with a variety of volatile organic solvents. The use of CO2 as a non-solvent to produce particles has expanded significantly since then, where the typical ‘process’ employs one of several nozzle designs in or- der to create an aerosol simultaneous with the induced phase separation. As shown in the review by Jung and Perrut [298], an extraordinary variety of materials (many bioactive compounds) have been processed via one of the many non-solvent routes, typically gener- ating micron-size particles and smaller. As noted in the section on polymer processing, the pressure required to create a concentrated mix- ture of polymer and CO2 is significantly lower than that required to create a dilute solution of polymer in CO2 (see Fig. 5). As such, a number of researchers have explored the use of gas-saturated solutions (of either CO2 in a polymer, or CO2 in an organic sol- vent/solute mixture) to produce fine particles. Here the CO2-saturated mixture is sprayed through a noz- zle and the rapid vaporization of CO2 creates an aerosol and removes any organic solvent. The work by Ferro Corporation on the generation of powder coating formulations using CO2 is an example of this type of processing, sometimes referred to as PGSS (particles from gas-saturated solutions). Although a variety of materials have been mi- cronized using carbon dioxide, it is clear that most of the industrial interest in such processes arises from pharmaceutical manufacturers. As such, we will focus on bioactive particle manufacture in discussing the green potential of these processes. 5.3. Production of fine pharmaceutical powders: is this green processing? To determine whether CO2-based particle forma- tion processes are ‘green’, one must first examine the ways in which particles are generated currently. First, it seems clear that the pharmaceutical industry is truly interested in the production of fine powders (particles) of controlled size and known purity. The design and testing of inhalable drugs is an ongoing area of sig- nificant research and business activity. The CO2-based particles processes described in the literature are green (and economical!) to varying de- grees. For example, while RESS employs CO2 as the only solvent, the need for high CO2 throughputs (ow- ing to low solubility of target compounds) means that the energy budget for such a process will be high (en- ergy needed for compression and purification of large volumes of CO2). On the other hand, processes such as PGSS or the various non-solvent modes of operation employ carbon dioxide at relatively low pressure and flow rates. Many of the anti-solvent processes employ organic solvents (DMSO most frequently), and hence care must be taken to ‘close the loop’ on these solvents to avoid lowering the sustainability of the process. Be- cause CO2-based particle production processes are, at most, at the pilot scale, it is not clear to what extent the organic solvent can actually be recycled. Further, if the particle process requires regulatory approval (for use in manufacture of pharmaceuticals), it is not clear to what extent solvent recycle will be permitted. Many pharmaceutical compounds are readily solu- ble in water, while being poorly soluble in even po- lar solvents such as DMSO. Researchers at Bradford Particle Design (BPD) dealt with this situation in a CO2-based non-solvent process by incorporating a co- solvent (an alcohol) that is miscible with both wa- ter and CO2 [301]. Use of a coaxial nozzle and this co-solvent allowed BPD to produce fine particles from a variety of water-soluble compounds. Sievers et al. [302] have dealt with this problem via use of collidingPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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