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136 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 rate significantly. With some exceptions (described below), it is not likely that use of a supercritical sol- vent will enhance either the economic viability or the sustainability of a gas-phase hydrogenation. Two areas where addition of CO2 might benefit a gas-phase hydrogenation are flammability and catalyst defouling; addition of CO2 to a mixture of hydrogen and a substrate will enlarge the non-flammable region, while CO2 could help to prevent catalyst fouling by dissolving compounds that contribute to coke forma- tion [57]. 2.2. Liquid-phase hydrogenations: advantages to use of supercritical solvents A number of hydrogenations (synthesis of unsatu- rated fatty acids, reduction of fatty esters to alcohols) are conducted commercially in organic solvent and re- placement of these solvents with benign carbon diox- ide will reduce both liability (reduced flammability, potential toxicity issues) and the potential for VOC emissions owing to fugitive losses. In addition, use of any supercritical fluid in a liquid-phase hydrogenation process can significantly alter the relative importance of fundamental processes governing the rate expres- sion. In a three-phase hydrogenation, the rate can be governed purely by the kinetics of the reaction, but more likely will depend on the rate at which hydro- gen diffuses from the gas phase to the active sites on the catalyst. The overall rate of transport is itself gov- erned by three resistances in series: (1) the resistance to transport of H2 across the gas–liquid interface; (2) the resistance to transport of H2 through the liquid to the surface of the catalyst; and finally (3) resistance to transport of H2 within the pores of the catalyst. Given that the overall rate is related to the sum of the resis- tances in series [58], one term can easily dominate the expression for the overall rate. Use of a supercritical fluid solvent (as opposed to a traditional liquid) elim- inates the gas–liquid interface, as low Tc gases such as H2, O2 and CO are completely miscible with flu- ids above their critical point. However, this does not necessarily mean that the reaction will be kinetically controlled, as one must deal with the remaining two resistances to transport (bulk liquid to solid surface, interpore diffusion). Because the diffusion constant is embedded in each of these resistances, the use of a supercritical fluid can also aid in their elimination, although simply switching from a conventional liquid to a supercritical fluid solvent for hydrogenation by no means guarantees that the reaction rate will depend solely on the underlying kinetics. It should be noted that significant effort is expended in hydrogenation reactor design to ensure that H2 is well dispersed in the liquid phase—effective sparg- ing greatly increases the contact surface area between the phases and hence the rate at which H2 diffuses into the liquid. If use of a supercritical fluid allows for a reactor redesign (for example, plug-flow versus continuous-stirred tank given that gas sparging is un- necessary), then it may be possible to enhance the selectivity of the reaction through reactor design im- provement, reducing waste. Indeed, selectivity is a major concern in any chemi- cal process—hydrogenation is no exception. It is well known that solvents affect the yield and selectivity of various hydrogenation reactions where ‘one very use- ful, although fallible, generality is that in a series of solvents, the extremes in selectivity will be found at the extremes of the dielectric constant. . . ’ [56]. The supercritical fluids most often employed as hydro- genation solvents, propane and CO2, exhibit dielectric constants at the lower end of the scale (1.5–1.7) and we might expect to see an effect on selectivity if a polar solvent is replaced by CO2. In addition, the physical properties of supercritical fluids are readily varied over a significant range through changes to pressure and temperature and it may be possible to af- fect selectivity by altering these variables. Finally, the addition of CO2 or operation above the critical point of the reactant mixture could aid in coke removal from the catalyst, prolonging its life or maintaining favorable selectivity [57]. Clearly, enhancing selec- tivity of a reaction will ultimately reduce the volume of byproducts generated and potentially the volume of waste emanating from a particular process. Hydrogenation is generally exothermic and remov- ing heat from the process is thus more of a problem than injecting heat [59]. In this case, the use of a super- critical fluid may or may not be advantageous. Liquids are useful as heat transfer fluids in that one can employ the heat of vaporization to absorb excess heat. Convec- tive heat transfer, which will depend upon both fluid velocity and fluid physical properties, may or may not be more successful in a supercritical fluid, depending upon the exact conditions. For example, the magnitudePDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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