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of heat transfer is related both to the Prandtl number and Reynolds number [22]; Prandtl numbers for SCFs are typically lower than for liquids, while the Reynolds number for an SCF could be quite a bit higher (given that kinematic viscosity for SCFs is high) at constant velocity. Heat removal is important, in that inability to effectively remove heat could lead to loss of selec- tivity. Liquid CO2 could be useful in this regard, as boiling is often employed as a means by which to ab- sorb excess heat, although it must be remembered that CO2’s heat of vaporization is relatively low. 2.3. Heterogeneous hydrogenation in CO2 As mentioned above, the key ‘green’ driving force behind the use of a supercritical solvent rather than an organic solvent in a heterogeneous reaction is the elimination of transport resistance (owing to diffusion of the gas across the liquid–vapor boundary) and po- tentially a more efficient reaction. Ease of separation of products from reactants is also often mentioned, but not typically evaluated. Indeed, products and reactants may be more easily separated in the conventional ana- log via a simple distillation. Baiker [60] has reviewed progress in heterogeneous reactions in supercritical fluids up to 1999; we will focus on key discoveries prior to 1999 and significant strides made since then. Harrod et al. [61] have successfully performed the hydrogenation of fats and oils using supercritical propane; propane was employed to allow for solubil- ity of both the substrates (whose solubility in CO2 is poor) and hydrogen, which is completely misci- ble with any supercritical fluid. The homogeneous propane/H2/substrate mixture was fed into a packed bed containing a commercial Pd catalyst—extremely high reaction rates were indeed achieved (gas–liquid transport resistance being eliminated) and the concen- tration of trans fatty acids (an undesirable byproduct) was reduced. Hence, the green advantages to this reaction would include reduced waste content and smaller, more efficient reactors. However, the use of propane is problematic, and it is not clear whether the process advantages due to faster reaction rate balance the disadvantages deriving from use of a flammable solvent and the problems inherent to high-pressure process design/development. Further, the catalyst deactivated quickly, an important problem for both economic and sustainable reasons [57,59]. Tacke et al. [62] also investigated the hydrogena- tion of fats and oils (over a supported Pd catalyst), although they employed CO2 as the supercritical solvent. Again, rates were shown to be significantly higher in the supercritical case (6-fold increase in space-time yields) and selectivity and catalyst lifetime were also improved. Each of these features contributes to enhancing the green potential of the process, while the need for high pressure operation detracts both from the cost and the sustainability (energy, unit op- eration complexity). Macher and Holmquist [63] also examined the hydrogenation of an oil in supercritical propane; similar results to those found by Harrod were obtained. King et al. [64] examined the hydrogena- tion of vegetable oil and fatty acid esters over nickel catalysts using both CO2 and propane as supercritical solvents and under conditions where either one or two fluid phases existed in the reactor. This approach is interesting, as it ultimately could prove a useful engineering solution to the problem of solubilizing substrates in CO2 at moderate operating pressures. Indeed, Chouchi et al. [65] recently examined the hydrogenation of pinene (over Pd/C) in supercritical CO2. They found that the rate of the reaction was sig- nificantly faster in the two-phase regime (i.e. lower pressures) than when the pressure was raised to the point where only a single fluid phase existed. The rea- son for this seems clear; the Chouchi study was per- formed by charging a known amount of each of the ingredients to the reactor, then pressurizing with CO2. The partitioning of compounds between phases (in the two-phase system) must have been such that the con- centration of reactants in the lower phase was higher than under single-phase conditions. In other words, raising the pressure to create a single phase simply di- luted the reactants, lowering the rate. Note that the con- centration of CO2 in the lower phase (in the two phase system) was likely to be substantial, as CO2 should in- teract favorably with a volatile, low molecular weight compound, such as pinene. Further, the concentra- tion of hydrogen in the lower phase must also have been substantial to support the high rate observed, and hence we see that CO2 can swell an organic substrate significantly and carry substantial amounts of hydro- gen into a ‘swollen’ liquid phase. CO2 could therefore function as a ‘reversible diluent’, much in the same way that it is employed as a ‘reversible plasticizer’ in polymer science [66]. In this case, addition of CO2 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 137PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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