Phase supercritical

In terms of the solubilities of solutes in a supercritical phase, the following generalizations can be made. Solute solubiUties in supercritical fluids approach and sometimes exceed those of Hquid solvents as the SCF density increases. SolubiUties typically increase as the pressure is increased. Increasing the temperature can cause increases, decreases, or no change in solute solubiUties, depending on the temperature effect on solvent density and/or the solute vapor pressure. Also, at constant SCF density, a temperature increase increases the solute solubiUty (16). 

Although modeling of supercritical phase behavior can sometimes be done using relatively simple thermodynamics, this is not the norm. Especially in the region of the critical point, extreme nonideahties occur and high compressibilities must be addressed. Several review papers and books discuss modeling of systems comprised of supercritical fluids and soHd orHquid solutes (rl,i4—r7,r9,i49,r50). 

Baron Cagniard de la Tour [244] made the first reported observation of the occurrence of a supercritical phase in 1822. Tables 3.12 and 3.13 summarise some of the main useful features of SCFs. Several properties of SCFs make them ideal candidates as solvents for industrial extraction processes [245,246],... 

The concept of supercriticality is more complex if a two-component fluid is used. For most mixtures used, SFE must be carried out above a certain pressure to ensure that the fluid is in one phase. For MeOH-C02 mixtures at 50 °C the fluid is in one phase and can be described as supercritical above 95 bar, whatever the composition [284]. Compounds may also be added to the supercritical phase as a reactant rather than as a simple modifier. 

To avoid two phases, it has been suggested that the most efficient way to transport C02 is as its supercritical phase [8,9], which occurs at a pressure higher than 7.38 MPa and a temperature of more than 31.1 °C. To maintain these conditions, this type of transportation may require the use of booster stations in the pipeline layout to maintain the required pressure and temperature. 

Cationic polymerization represents another area in which C02 has proven to be a viable continuous phase. In this area, both the liquid and supercritical phases... 

The techniques discussed up to now use C02 as the mobile phase for substrates and products. Naturally, this restricts the applications to relatively non-polar and/or volatile components with sufficient solubility in the supercritical medium. An intriguing alternative for processing highly polar substrates are inverted aqueous systems. In this approach, a C02-philic catalyst resides in the non-polar C02 phase, while water-soluble substrates and products are contained in the aqueous layer [58, 59]. A very attractive and unique feature of the scC02/H20 system is that the stationary supercritical phase is never depressurized and hence the large energy input required for recompression is avoided. Furthermore, the aqueous solution is not contaminated with any organic solvent or catalyst residues, which is particularly important if the product is a fine chemical intended for direct further use in aqueous solution. 

Tsubaki, N., Yoshii, K., and Fujimoto, K. 2002. Anti-ASF distribution of Fischer-Tropsch hydrocarbons in supercritical-phase reactions. J. Catal. 207 371-75. 

Biphasic systems that contain the catalyst in the supercritical phase and the substrates/products in a second liquid phase can also be implemented. With water as the polar phase, these inverted systems are particularly attractive for the conversion of highly polar and/or low-volatile hydrophilic substrates with limited solubility in typical SCFs such as scC02. 

Figure 9. Reaction equipment used for C02 hydrogenation in the supercritical phase...

Figure 10. Syntheses of formic acid and its derivatives via the supercritical phase. TON = mol product mol catalyst,...

N. O. Elbashir, P. Dutta, A. Manivannan, M. S. Seehra and C. B. Roberts, Impact of cobalt-based catalyst characteristics on the performance of conventional gas-phase and supercritical-phase Fischer-Tropsch synthesis, Appl. Catal. A, 2005, 285, 169-180. 

W. Linghu, X. Li, K. Asami and K. Fujimoto, Supercritical phase Fischer-Tropsch synthesis over cobalt catalyst, Fuel Process. Technol., 2004, 85, 1121-1138. 

Under the simulation conditions, the HMX was found to exist in a highly reactive dense fluid. Important differences exist between the dense fluid (supercritical) phase and the solid phase, which is stable at standard conditions. One difference is that the dense fluid phase cannot accommodate long-lived voids, bubbles, or other static defects, whereas voids, bubbles, and defects are known to be important in initiating the chemistry of solid explosives.107 On the contrary, numerous fluctuations in the local environment occur within a time scale of tens of femtoseconds (fs) in the dense fluid phase. The fast reactivity of the dense fluid phase and the short spatial coherence length make it well suited for molecular dynamics study with a finite system for a limited period of time chemical reactions occurred within 50 fs under the simulation conditions. Stable molecular species such as H20, N2, C02, and CO were formed in less than 1 ps. 

When transported in ships, C02 is usually stored in cold liquid phase. This state is preferred to the supercritical phase because the wall thickness required for maintaining a pressure of about 8 MPa would become unacceptably high. Depending on the size of the individual tanks, the necessary wall thickness could even exceed the material quality that could be manufactured with existing technologies. 

Supercritical fluids (e.g. supercritical carbon dioxide, scCCb) are regarded as benign alternatives to organic solvents and there are many examples of their use in chemical synthesis, but usually under homogeneous conditions without the need for other solvents. However, SCCO2 has been combined with ionic liquids for the hydroformylation of 1-octene [16]. Since ionic liquids have no vapour pressure and are essentially insoluble in SCCO2, the product can be extracted from the reaction using CO2 virtually uncontaminated by the rhodium catalyst. This process is not a true biphasic process, as the reaction is carried out in the ionic liquid and the supercritical phase is only added once reaction is complete. 

Whilst it is obviously valuable to measure the solubility of reagents in the SCF, it is important to be aware that the solubility in a multicomponent system can be very different from that in the fluid alone. It is also important to note that the addition of reagents and catalysts can have a profound effect on the critical parameters of the mixture. Indeed, at high concentrations of reactants, the mole fraction of C02 is necessarily lower and it may not be possible to achieve a supercritical phase at the temperature of interest. Increases in pressure (i.e. further additions of C02) could yield a single liquid phase (which would have a much lower compressibility than scC02). For example, the Diels-Alder reaction (see Chapter 7) between 2-methyl-1,3-butadiene and maleic anydride has been carried out a pressure of 74.5 bar and a temperature of 50 °C, assuming that this would be under supercritical conditions as it would if it were pure C02. However, the critical parameters calculated for this system are a pressure of 77.4bar and a temperature of 123.2 °C, far in excess of those used [41]. 

Pater, J.P.G., Jacobs, P.A., and Martens, J.A. (1998) 1-Hexene oligomerization in liquid, vapor and supercritical phases over beidellite and ultrastable Y zeolite catalysts. J. Catal., 179, 477. 

The mixture to be separated is fed, together with the entrainer, to the middle of the first column. Flere, the solvent, carbon dioxide and acetone, is supercritical to provide high solubility of the monoglycerides. The supercritical phase leaves the top of column I and goes to the lower part of column II. In column II, the binary solvent entrainer is subcritical and in the bottom of this column, the monoglyceride leaves, together with the entrainer. Part of it is returned as reflux to column I, whereas the rest goes to distillation for the separation of acetone. With a bottom temperature of... 

It could be expected, that combustion reactions and possibly flames can be produced in such dense supercritical mixtures. Technical aspects of hydrothermal oxydation at moderate pressures have already been tested and discussed [7,8]. The study of combustion and flames in supercritical phases offers several possibilities 1. The variation of pressure over wide ranges should influence reaction mechanisms and flame characteristics because the density can be changed from low, gas-like, to high, liquid-like, values. 2. The variable temperature of the dense, fluid environment can have an influence on reactions and flames. 3. The chemical and physical character of this environment can be varied considerably, for example by using supercritical water as the major component, as in the present experiments. Certainly, the knowledge of transport coefficients of gases involved is desirable. For water the viscosity has been determined to... 

Figure 15.21 shows a schematic representation of the SCCO2 treatment effect for promoting the internal diffusion of metal ions to prepare Rh and RhPt alloy nanoparticles in mesoporous FS-16 and HMM-1. The supercritical phase displays both liquid and gas properties at the same time. SCFs can also dissolve various metal precursors, which promotes their mobiUty and surface-mediated reaction to form nanoparticles by the hydrogen reduction in the mesoporous cavities of... 

A special area of HP NMR in catalysis involves supercritical fluids, which have drawn substantial attention in both industrial applications and basic research [249, 254, 255]. Reactions in supercritical fluids involve only one phase, thereby circumventing the usual liquid/gas mixing problems that can occur in conventional solvents. Further advantages of these media concern their higher diffusivities and lower viscosities [219]. The most commonly used supercritical phase for metal-catalyzed processes is supercritical CO2 (SCCO2), due to its favorable properties [256-260], i. e., nontoxicity, availability, cost, environmental benefits, low critical temperature and moderate critical pressure, as well as facile separation of reactants, catalysts and products after the reaction. 

As described earlier, high pressure cells have been developed for the use of noble gases as solvents for IR spectroscopic studies, either at low temperature, or at ambient temperature where the supercritical phase exists. A particular focus of this work was the study of reactive complexes containing coordinated noble gas atoms or molecular H2, the latter being particularly relevant to hydrogenation reactions. 

The most desirable choice, of course, would be water, since it has essentially no harmful effects on humans or the environment. The problem is that most organic substances do not dissolve in water. One of the most exciting alternatives with promise for use in organic syntheses, however, is another widely available and environmentally benign compound, carbon dioxide. The carbon dioxide used in organic reactions exists in a phase not generally familiar to most people, the supercritical phase. 

Carbon dioxide, as can most other substances, can exist in any one of three phases—solid, liquid, or gas—depending on temperature and pressure. At low temperatures, carbon dioxide exists as a solid ("dry ice") at almost any pressure. At temperatures greater than about -76°F (-60°C), however, carbon dioxide may exist as a gas or as a liquid, depending on the pressure. At some combination of temperature and pressure, however, carbon dioxide (and other substances) enters a fourth phase, known as the supercritical phase, whose properties are a combination of gas and liquid properties. For example, supercritical carbon dioxide (often represented as SCC02, SC-C02, SC-CO2, or a similar acronym) diffuses readily and has a low viscosity, properties associated with gases, but is also a good solvent, a property one often associates with liquids. The critical temperature and pressure at which carbon dioxide becomes a supercritical fluid are 31.1°C (88.0°F) and 73.8 atm (1,070 pounds per square inch). 

Fractionation. Kerr-McGee developed the ROSE process for separating the heavy components of cmde oil, eg, asphaltenes, resins, and oils, in the 1950s. This process was commercialized in the late 1970s, when cmde oil and utility costs were no longer inexpensive. In the ROSE process (Fig. 11), residuum and pentane are mixed and the soluble resins and oils recovered in the supercritical phase. By stepwise isobaric temperature increases, which decrease solvent density, the resin and oil fractions are precipitated sequentially. 

Density is a factor in the solvating power of a supercritical fluid the more dense the fluid, the more powerful its solvent strength. Since changing the temperature and pressure within the supercritical phase changes the density, a supercritical fluid can be made to possess a wide range of solvent power. This property together with its increased diffusion and lower viscosity makes supercritical fluid an attractive extraction medium. 

Wlien fluids and gases are heated above their critical temperatures and compressed above their critical pressures they enter a supercritical phase in which some properties, such as solvent power, can be altered dramatically. 

A commercial Pt-Sn on y-Al203 catalyst showed 2-3 times higher activity in the catalytic dehydrogenation of a mixture of Ci0—CJ2 alkanes to linear monoalkenes when applied in a supercritical phase.332 The strong shift of the equilibrium under supercritical conditions is believed to be due to the high solubility of the product in supercritical fluids or the rapid desorption of alkenes from the catalyst surface. 

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