Diffusivity supercritical mixtures

Supercritical Mixtures Dehenedetti-Reid showed that conven-tionaf correlations based on the Stokes-Einstein relation (for hquid phase) tend to overpredict diffusivities in the supercritical state. Nevertheless, they observed that the Stokes-Einstein group D g l/T was constant. Thus, although no general correlation ap es, only one data point is necessaiy to examine variations of fluid viscosity and/or temperature effects. They explored certain combinations of aromatic solids in SFg and COg. 

Organic chemists have been attracted for a variety of reasons to supercritical media as an environment for performing reactions. These reasons include, especially for C02 and H20, the environmental friendliness of the medium. The fact that supercritical fluids can be removed without a residue is an advantage. Other advantages include the solubility of gases within supercritical mixtures, the high diffusion rates, and the variable and adjustable density, solvent power, and dielectric constant of the medium. Ordinary gases, such as 02 and H2, are miscible with... 

Supercritical fluids have no surface tension and, like gases, rapidly diffuse to occupy the entire volume of the system. Like a gas, a SCF also mixes perfectly with other gases. As a result, the concentration of gases within a SCF may be much higher than in liquids. The concentration of hydrogen in a supercritical mixture of hydrogen (85 bar) and carbon dioxide (120 bar) at 50 °C is... 

In conventional crystallization processes, supersaturation (and hence nucleation) are caused by a thermal perturbation. Because of the inherently low thermal diffusivities of liquids, this approach is ahva3rs accompanied by the existence of temperature non-uniformities within the supersaturated liquid. This, in turn, gives rise to rather wide product size distributions. In the rapid expansion of a highly compressible supercritical mixture, on the other hand. 

SCFs, like gases, have no surface tension they diffuse rapidly to occupy the entire volume of a system. This also means that if other gases are introduced, they will also diffuse to fill the entire volume and mix perfectly. Unlike the solubility of gases in liquid solvents, which is relatively low and decreases as temperature increases, gases are totally miscible with SCFs and can be said to have perfect solubility. The concentration of hydrogen in a supercritical mixture of hydrogen... 

Enhanced-fluidity liquids (EELs) are mixtures that contain high proportions of liquefied gases, such as carbon dioxide [5]. Eluidity,/, is defined as the inverse of viscosity. EEL mixtures combine the positive attributes of commonly-used liquids, such as high solvent strength, with the positive attributes of supercritical fluids, such as low viscosity or high fluidity, low surface tension, high diffusivity. These attributes allow EELC to contribute to the quest for increased separation power. 

This definition cannot be applied directly to mixtures, as phase equilibria of mixtures can be very complex. Nevertheless, the term supercritical is widely accepted because of its practicable use in certain applications [6]. Some properties of SCFs can be simply tuned by changing the pressure and temperature. In particular, density and viscosity change drastically under conditions close to the critical point. It is well known that the density-dependent properties of an SCF (e.g., solubihty, diffusivity, viscosity, and heat capacity) can be manipulated by relatively small changes in temperature and pressure (Sect. 2.1). 

In the third variation a mixture of moisturized green coffee beans and activated carbon is filled into the extractor, and the activated carbon pellets used are just big enough to fit between the beans. For 3 kg of coffee beans, 1 kg of activated carbon is needed. At 220 bar and 90°C the caffeine in the supercritical CO2 diffuses directly out of the beans into the activated carbon. A CO2 circulation is not necessary. The required degree of decaffeination is reached after 6 to 8 hours. After extraction, the beans and activated carbon are separated by a vibrating sieve. 

In supercritical fluids, the possibility of local composition enhancements of cosolvent about a solute suggests that we should see enhancement of anion fluorescence if the water cosolvent clusters effectively about the 2-naphthol solute. Although in liquids the water concentration must be >30% to see anion emission, the higher diffusivity and density fluctuations in SCFs could allow stabilization of the anion at much lower water concentrations provided that the water molecules provide sufficient structure. Therefore the purpose of these experiments was to investigate 2-naphthol fluorescence in supercritical CO 2 with water cosolvent in the highly compressible region of the mixture to probe the local environment about the solute. 

Roberts, C. B. Zhang, J. Chateauneuf, J. E. Brennecke, J. F. Diffusion-Controlled Reactions in Supercritical CHF3 and C02/Acetonitrile Mixtures, J. Am. Chem. Soc. 1993b, 115, 9576-9582. 

Propane or propane/C02 mixtures as liquid, near-critical, or supercritical fluids enhance the solubility of fats and oils (Harrod et al., 2000 Weidner and Richter, 1999). The decrease in viscosity and increase in diffusivity results in a higher hydrogenation rate (Figure 14.4). Harrod et al. (2000) have also demonstrated activity increases by reducing mass-transfer limitations in supercritical propane. 

The combination of Deloxan-supported precious-metal fixed-bed catalysts and the use of liquid, near-critical, or supercritical C02 and/or propane mixtures creates new possibilities for continuous fixed-bed hydrogenations with significantly improved space-time yields and catalyst lifetimes. Short residence times and well-balanced diffusion and desorption of products and reactants results in a decrease in undesirable by-products and therefore higher selectivity. The characteristics of high-pressure hydrogenations in near-critical or supercritical fluids can be summarized as follows ... 

Some measurements have been made of self diffusion in pure ethylene and in ethylene-sulfur hexafluoride mixtures (22), but these measurements were made very close to the critical temperature and up to pressures of only about 100 bar. Proton spin-lattice relaxation times (T.) of ethylene have been measured at temperatures from 0°C to 50°C and pressures up to about 2300 bar (13). The relaxation time values were -M0—50 sec for much of the region studied. Several relaxation mechanisms contribute to this long relaxation time and make both the measurement and analysis of the relaxation times very difficult. For these reasons, we decided to limit our study to the measurement of the self-diffusion coefficient in supercritical ethylene (60. 

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. 

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