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streams of aqueous solution and CO2 (prior to exiting the high pressure environment at a nozzle), where the CO2 helps form (and dry!) an aerosol of the aqueous solution. These two processes are noted because they each accomplish the formation of small particles of valuable compound using entirely sustainable solvent systems—CO2 /water/ethanol by BPD and CO2 /water by Sievers et al. This mode of operation would seem to exhibit the highest green potential of the various CO2-based powder processes. 5.4. Comparisons with current processes The literature suggests that milling, crystalliza- tion and spray drying are currently the most com- mon means by which to generate powders (particles) from pharmaceuticals [299,303]. Milling [304] is a relatively energy-intensive process, but requires no solvent and is readily scalable. Milling (including jet pulverizing) has been demonstrated to be able to create particles in the 1–5 micron range. The design and performance characteristics of various types of mills are known and the process is readily scalable and can be rendered continuous [305]. However, tem- perature increases during milling can damage labile compounds and strict control over particle size and particle morphology may either be lacking or incon- sistent. Milling can create substantial waste if the distribution of particle sizes exhibits a substantial tail at the lower end of the scale. Replacement of milling with a CO2-based process would seem to owe more to product concerns than to ‘green’ concerns, if one of the various CO2 processes can generate product consistently with the correct characteristics (size, distribution, shape, morphology). Spray drying [305,306] involves the atomization of a solution (product in solvent), the mixing of the droplets with a hot gas (usually air) followed by the drying of the droplets to form the particles. Particles can be produced whose sizes range from 2 up to 500 microns; theory on design and operation of spray dry- ers has been well-studied. If one is employing water as the solvent, then the only significant ‘green’ com- plaint that one might have with spray drying is that water’s high heat of vaporization requires a significant energy input to the process. On the other hand, as in the case of milling, if the CO2-based process generates particles of higher quality (closer adherence to size and morphology constraints), at a competitive price, then the CO2 process could dominate despite poten- tially being less green. Obviously, if one is spray dry- ing from organic solution, then recycle of the solvent is an additional consideration. As for the cases of both milling and spray dry- ing, crystallization is an often-used industrial process where numerous variations are possible [305,307]. De- sign principles for crystallizers have been investigated in depth in the past and hence, procedures for the de- sign of crystallizers are readily available. If water is being used as the solvent, crystallization is already a relatively green process where perhaps high-energy in- put owing to the use of water as solvent (recall the need to dry the product) or the need to treat the wastewater from the process could be seen as negatives. Again, however, crystallization may not be able to produce the particle characteristics desired by the end-users. In summary, the use of carbon dioxide as a non- solvent for the production of particles (primarily phar- maceutical particles) is not substantially more ‘green’ than competing technologies (in some cases it could be less green). However, the use of CO2 could pro- vide better product, and hence its relatively green sta- tus provides no complications from a sustainability perspective. What seems to differentiate CO2 -based processes from their conventional competitors (crys- tallization, spray drying, milling) is a general lack of basic design equations that would allow ready creation of a design schematic given product specific inputs (the usual situation in computer-aided design of a unit operation or process). Research by DeBenedetti et al. during the 1990s [308] suggested that the process by which particles are created during spraying of a so- lution into CO2 could be modeled by considering the formation of fluid droplets and the transport of both CO2 and solvent between the continuous phase and the droplet phase. However, recent work by Randolph et al. [309] suggests that true droplets never form in the spray process and that particle formation can be de- scribed by gas phase nucleation and growth within the expanding plume. Whereas this may seem (to an out- sider) as merely an academic debate, accurate models of the particle formation process inevitably result in the identification of the correct dimensionless groups associated with the phenomena and the underlying mathematical relationships that will ultimately permit process design from first principles. While there is E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 179PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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