Supercritical pressure range

As it has appeared in recent years that many hmdamental aspects of elementary chemical reactions in solution can be understood on the basis of the dependence of reaction rate coefficients on solvent density [2, 3, 4 and 5], increasing attention is paid to reaction kinetics in the gas-to-liquid transition range and supercritical fluids under varying pressure. In this way, the essential differences between the regime of binary collisions in the low-pressure gas phase and tliat of a dense enviromnent with typical many-body interactions become apparent. An extremely useful approach in this respect is the investigation of rate coefficients, reaction yields and concentration-time profiles of some typical model reactions over as wide a pressure range as possible, which pemiits the continuous and well controlled variation of the physical properties of the solvent. Among these the most important are density, polarity and viscosity in a contimiiim description or collision frequency. 

Optimizing solvents and solvent mixtures can be done empirically or through modeling. An example of the latter involves a single Sanchez-Lacombe lattice fluid equation of state, used to model both phases for a polymer-supercritical fluid-cosolvent system. This method works well over a wide pressure range both volumetric and phase equilibrium properties for a cross-linked poly(dimethyl siloxane) phase in contact with CO2 modified by a number of cosolvents (West et al., 1998). 

The effect of pressure on the measured bimolecular rate constant of the Diels-Alder reaction between maleic anhydride and isoprene was investigated in supercritical CO2 and subcritical propane. The reaction was carried out at 35°C in CO2 and 80°C in propane. The rate constants in supercritical CO2 agreed closely with the thermodynamic pressure effect predictions over the entire pressure range. The rate constants in the subcritical propane solvent significantly diverged from the thermodynamic pressure effect predictions and were found to deviate from this linear density dependence at the lower pressures studied. The results show solvent-solute and cosolvent-solute interaction (Reaves and Roberts, 1999). 

Fig. 35. Effect of phase behavior on palladium-catalyzed oxidation of benzyl alcohol to benzaldehyde in supercritical CO2 characterized by transmission- and ATR-IR spectroscopy combined with video monitoring of the reaction mixture (102). The figure at the top shows the pressure dependence of the reaction rate. Note the strong increase of the oxidation rate between 140 and 150 bar. The in situ ATR spectra (middle) taken at 145 and 150 bar, respectively, indicate that a change from a biphasic (region A) to a monophasic (B) reaction mixture occurred in the catalyst surface region in this pressure range. This change in the phase behavior was corroborated by the simultaneous video monitoring, as shown at the bottom of the figure.

Supercritical fluid separation processes operate at pressures ranging from 1000 to 4000 lb/in.2, pressures that might be considered high, especially in the foods and essential oils industries. However, because of the factors just listed, supercritical fluid extraction has become eco-... 

Table 10.3 provides some examples of organic compounds and their respective dielectric constants. Many organic compounds become miscible in supercritical water because they behave almost as a nonaqueous fluid. For example, at 25°C, benzene is barely soluble in water (solubility, 0.07 wt%) however, at 260°C, the solubility is about 7 to 8 wt% and is fairly independent of pressure. At 287°C, the solubility is somewhat pressure dependent, with a maximum of solubility of 18 wt% at 20 to 25 MPa. In this pressure range and at 295°C, the solubility rises to 35 wt%. At 300°C, the critical point of... 

The solubility of TB 1 in supercritic carbon dioxide over the pressure range from 8 to 19 MPa and from 308 to 328 K has been measured using a flow system. Models based on chemical association, which did not require the critical parameters of the solute, were used to correlate the experimental data (00JCED464). Addition of methanol dramatically enhances the solubility (00MI823). 

The NMR method we have developed gives a direct, in situ determination of the solubility and also allows us to obtain phase data on the system. In this study we have measured the solubilities of solid naphthalene in supercritical carbon dioxide along three isotherms (50.0, 55.0, and 58.5°C) near the UCEP temperature over a pressure range of 120-500 bar. We have also determined the pressure-temperature trace of the S-L-G phase line that terminates with the UCEP for the binary mixture. Finally, we have performed an analysis of our data using a quantitative theory of solubility in supercritical fluids to help establish the location of the UCEP. 

In this paper solubility measurements of synthetic and natural dyestuffs are presented using VIS-spectroscopy. The investigations concentrate on two different methods. I. P-carotene was measured as a function of temperature and pressure in near- and supercritical C02 (289 to 309 K, 10 to 160 MPa) and CC1F3 (297 to 326 K, 12 to 180 MPa), respectively, using a static method. II. Additionally, the solubilities of l,4-bis-(n-alkylamino)-9,10-anthraquinones (with n-alkyl = butyl, octyl) were determined with a dynamic method in temperature and pressure ranges from 310 to 340 K and 8 to 20 MPa, respectively this method permits a continuous purification from better soluble impurities as well as the measurement of solubilities at the same time. For both anthraquinone dyestuffs intersection points of the solubility isotherms were found in the plot of concentration versus pressure. This behavior can be explained by a density effect. 

The solubilities of furfural in supercritical CO2 were determined in the equilibrium cell at temperatures of 298, 313 and 333 K within a pressure range of 80... 

Adsorption and desorption measurements of nimesulide in supercritical carbon dioxide on activated carbon are determined at 313.1 K and in a pressure range of 160-220 bar. 

NMR in Supercritical/Pressurized Fluids. Any of the above systems can be easily modified to record NMR spectra of samples in supercritical/pressurized fluids (within the pressure range of the chosen device). ... 

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