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for example. Further, addition of any third component (here, a gas such as H2 or CO) to a mixture of CO2, substrate (and catalyst, perhaps) will alter the phase behavior of the mixture. Because commonly used re- actant gases, such as H2, O2 and CO, exhibit low criti- cal temperatures [12], at typical reaction temperatures (273–373 K), the density of these gases, even under relatively high pressures used to compress CO2, will be quite low (more gas-like than liquid-like). As such, we expect these gases to behave as non-solvents to- wards the substrate and/or catalyst [20]. Thus, addition of large amounts of reactant gas to the mixture may solve one problem (diffusion limitations) and create another (phase separation). 1.4.6. CO2 exhibits solvent properties that allow miscibility with both fluorous and organic materials Carbon dioxide is miscible with a variety of low molecular weight organic liquids, as well as with many common fluorous (perfluorinated) solvents. The literature has shown previously that one can create a homogeneous mixture of certain fluorous and organic liquids at one temperature, where phase separation occurs upon a temperature increase or decrease. Re- cently, Eckert et al. has shown that one can employ CO2 as a phase separation ‘trigger’ in much the same way—the addition of CO2 (at pressures as low as 20–30 bar) to a mixture of organic and fluorous liquids creates a homogeneous single phase, while removal (through depressurization) returns the system to a two-component, two-phase system [21]. 1.4.7. CO2 exhibits a liquid viscosity only 1/10 that of water At liquid-like densities, CO2’s viscosity is only 1/10 that of water and hence Reynolds numbers (ρVD/μ, where V is fluid velocity, ρ is density and μ is the viscosity) for flowing CO2 will be approximately ten times those for conventional fluids at comparable fluid velocity. Because convective heat transfer is usually a strong function of Reynolds number, heat transfer in a CO2 mixture can be expected to be excellent. On the other hand, CO2’s physical properties also lead to significant natural convection causing problems in some coatings processes. The extent to which natural convection is an issue is directly related to the magni- tude of the Grashof number [22], which itself scales as a gas-like viscosity, Grashof numbers for CO2-based processes can be significantly higher than for analo- gous liquid processes. The surface tension in carbon dioxide is much lower than that for conventional organic solvents and the diffusivity of solutes is expected to be considerably higher, owing to CO2’s low viscosity. Consequently, CO2 would be expected to wet and penetrate com- plex geometries better than simple liquids. Further, so- lutes would be expected to diffuse faster within cata- lyst pores where CO2 is the solvent than in analogous systems using conventional liquids. 1.5. Chemical disadvantages to use of CO2 as solvent Carbon dioxide exhibits some inherent disadvan- tages where chemistry is concerned; some of these are unique to CO2 while others are common to any num- ber of solvents. 1.5.1. CO2 exhibits a relatively high critical pressure and vapor pressure As mentioned above, CO2 exhibits high critical and vapor pressures; these characteristics guarantee higher capital costs for a CO2-based process relative to one using a conventional solvent, as well as the need for specialized equipment for laboratory work. Exother- mic reactions pose special problems for operation in CO2, given that high pressure is the baseline situation. 1.5.2. CO2 exhibits a low dielectric constant Carbon dioxide exhibits a dielectric constant of ≈1.5 in the liquid state; supercritical CO2 will exhibit values generally between 1.1 and 1.5, depending upon density. This low dielectric can be both a process disadvantage and a chemistry disadvantage. Some reactions, for example, require polar solvents for best results. Further, low dielectric constant also suggests poor solvent power, and hence solubility in CO2 can require much higher pressures for certain classes of solute than more polar compressible fluids (fluoro- form, for example, which exhibits a liquid dielectric of ≈10). On the other hand, the thermodynamic inter- action between CO2 and non-polar methylene groups is not particularly favorable and hence, ethane is often E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 125 ρ2/μ2. Because CO exhibits a liquid-like density and 22 a better solvent for hydrocarbons than CO .PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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