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regime (in the gas phase), more so than if either N2 or water vapor was added [9]. At this point it is not clear to what extent the non-explosive regime will expand further as one raises the density of the mixture (and hence the heat capacity). In a final intriguing note regarding safety advan- tages inherent to use of CO2 as a solvent, DuPont scientists [10] discovered that addition of CO2 to tetrafluoroethylene enhances the stability of that no- toriously difficult-to-handle monomer, although the exact mechanism for the enhanced stability has not been published. What has been revealed is that addi- tion of CO2 to TFE vapor inhibits runaway decom- position and explosion of the monomer. In addition, the CO2/TFE mixture behaves like an azeotrope, in that boiling of a mixture of the two does not signif- icantly change the concentration of either the liquid or the vapor. According to the DuPont patent [10], this ‘azeotrope-like’ behavior persists over a wide concentration range, behavior that is quite unlike that of typical azeotropic mixtures. The enhanced safety of CO2/TFE mixtures relative to pure TFE is one of the reasons that DuPont constructed a semi-works polymerization plant employing CO2 as solvent for the generation of fluoropolymers. 1.3. Environmental and safety disadvantages inherent to use of CO2 in a process Because CO2’s vapor pressure at room temperature is >60 bar, use of CO2 in a process clearly requires high-pressure equipment, creating a potential safety hazard relative to the same process operated at one at- mosphere operation. In addition, uncontrolled release of large quantities of carbon dioxide can asphyxiate bystanders owing to air displacement. These issues have not impeded the commercialization of CO2-based processes nor is it likely they will do so in the future. It should be remembered that the low density polyethy- lene polymerization process, first commercialized in the 1940s and still in operation today [11], runs con- tinuously at 2000–3000 bar and 520 K with a highly flammable component and hence, safe operation of a 100–200 bar CO2-based plant is readily achievable us- ing current technology. Operating an exothermic re- action in a high-pressure environment is accompanied by additional safety concerns versus the analogous re- action run at one atmosphere. Whether to use liquid or supercritical CO2 is a choice that actually involves safety as well as chem- istry considerations. While use of supercritical CO2 almost always involves use of higher pressure (to achieve the same solubility of a given substrate as in the liquid case), other factors should also be con- sidered. First, supercritical CO2 will exhibit a higher compressibility than liquid CO2, and hence the su- percritical fluid will be better able to absorb excess heat evolved from an exothermic reaction whose rate suddenly exceeds typical operating conditions. On the other hand, use of saturated liquid CO2 (in the presence of the vapor phase) would allow boiling to be used as a means to absorb excess heat. Use of supercritical CO2 (versus liquid) could avoid compli- cations owing to a phase separation occurring upon a departure from established temperature or pressure conditions within a given reactor. For example, if one is employing a mixture of oxygen, substrate, and liquid CO2 in a particular process, a sudden drop in pressure owing to a perturbation in the process could lead to formation of a flammable gaseous phase—use of a supercritical mixture could avoid this problem as no vapor–liquid separation will be encountered. Indeed, it should also be remembered that the Tc of a mixture of CO2 and other materials will differ from that of pure CO2 (see, e.g. Ref. [12] for useful correlations) and hence T-p conditions sufficient for supercritical operation with pure CO2 may create a liquid in the case of the mixture. 1.4. Chemical advantages to use of CO2 as a solvent Carbon dioxide can provide not only environmen- tal advantages, but also chemical advantages when ap- plied strategically, as described below. 1.4.1. CO2 cannot be oxidized In essence, carbon dioxide is the result of complete oxidation of organic compounds; it is therefore partic- ularly useful as a solvent in oxidation reactions. Use of almost any organic solvent in a reaction using air or O2 as the oxidant (the least expensive and most atom-efficient route) will lead to formation of byprod- ucts owing to reaction of O2 and the solvent. Indeed, the commercial anthraquinone process used to gener- ate H2O2 requires the removal and regeneration (or incineration) of substantial volumes of such solvent E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 123PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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