Near-critical density

The simulated and experimental variations of the end-of-run (i.e., 8 hr.) isomerization rates with density are compared in Figure 1. Details of the experiments are provided elsewhere [2, 3]. At subcritical densities, the extraction of coke precursors is insignificant. Hence, an increase in the concentration of the hexene and coke precursors (i.e., oligomers) leads to lower isomerization rates. At near-critical densities, the extraction of coke precursors becomes significant. Hence, the isomerization rate increases. Both the experimental and simulated rates show a decreasing trend when the density is increased from near-critical to supercritical values. This is attributed to pore-diffusion limitations as the fluid changes from gas-like to liquid-like. Above 2.0 pc, the isomerization rate increases with density as the ability of the reaction mixture to extract the coke precursors increases. 

In this paper, some recent experimental results regarding the density fluctuations in pure SCF are used to show that the local density enhancement in dilute SCR mixtures is mainly due to the near critical fluctuations in the solvent and an explanation is suggested for the negative partial molar volnme of the solute. This conclusion was also strengthened by a discussion, presented in the following section, based on the Kirkwood—Buff (KB) theory of solution. First, the problem will be examined in the framework of the Kirkwood—Buff theory of solution. Second, nsing experimental results about the near critical fluctuations in pure SCF, it will be shown that the density enhancement in dilnte SCR mixtures is mainly caused by the near critical density fluctuations in pure SCF. 

Figure 17 Cartoon representation of the empirical three-density-region solvation model epictmg mo ecu ar eve interactions for the three density regions (a) low-density region (b) near-critical density region (c) hquid-like region.

CE Bunker, Y-P Sun. Evidence for enhanced bimolecular reactions in supercritical CO2 at near-critical densities from a time-resolved study of fluorescence quenching of 9,10-bis(phenylethynyl)anthracene by carbon tetrabromide. J Am Chem Soc 117 10865, 1995. 

CE Bunker, Y-P Sun, JR Gord. Time-resolved studies of fluorescence quenching in supercritical carbon dioxide system dependence in the enhancement of bimolecular rates at near-critical densities. J Phys Chem A 101 9233, 1997. 

Knutson et al. (280) measured the kinetics of the Diels-Alder reaction of maleic anhydride (MA) and 2,3-dimethyl-1,3-butadiene (DMB) in SCF propane solutions at 100-140°C and 46-141 bar. Reaction to the product 4,5-dimethyl-CM-l,2,3,6-tetrahydrophthalic anhydride (DMTA) was evaluated with excess DMB as a reactive cosolvent and 2,2,2-trifluoroethanol (TFE) as an unreac-tive cosolvent (Scheme 21). Near-critical effects and cosolvent effects on reaction rates were analyzed from transition state theory. Rate constants increased with increasing pressure at 140 C, but were not significantly affected at 100°C and 120 C at near-critical densities. A similar lack of pressure dependence has been reported by Reaves and Roberts (281) for the Diels-Alder reaction of MA with isoprene in subcritical propane at 80°C. This minimal pressure effect is in contrast to those noted above for Diels-Alder reactions in SCCO2 where the reactants were at approximately equal and dilute concentrations. The influence of the unreactive cosolvent, TFT, on reaction rates was found to be minimal. These results suggest that the local reactant composition, as well as pressure, temperature, and cosolvent, can be used to control the reaction rate of such reactions in the near-critical region. 

The highly compressible nature of SCFs (especially in the near-critical density region) has made them uniquely applicable in materials processing—in particular, the production of particles, fibers, and Aims via rapidly expanding solutions of polymers and other materials in SCFs. 

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