Supercritical fluid states

The principal solvents that have been used are alcohols such as ethanol, methanol, and propanol, and organic acids such as formic or acetic acid, but other solvents iaclude esters, ethers, phenols, cresols, and some amines. Even solvents such as CO2 and NH in the supercritical fluid state have been tried as solvents. 

To access the supercritical fluid state, we must have conditions in excess of the critical temperature and pressure. Given the rating of the autoclave, ammonia would not be suitable because one could not access the supercritical state as a result of the pressure limitation. Methylamine would not be suitable for a room temperature extraction because its Tc is too high. Either methane or tetrafluoromethane would be suitable for this application. 

A supercritical fluid exists when a substance is heated above its critical temperature and pressure and is unable to be condensed to a liquid by pressure alone. A typical supercritical fluid is carbon dioxide, which, at temperatures above 31°C and pressures above 73 atm, exists in a supercritical fluid state where individual molecules of the compound are held by less restrictive intermolecular forces and molecular movement resembles that of a gas (1 atm =101 325 Pa). 

Due to its compressibility in the liquid (near the critical point) and in the supercritical fluid state, the dielectric constant and density, and thus the solvent quality of C02, are tunable with pressure and temperature (Keyes and Kirkwood, 1930). As illustrated in Figure 1.2, this compressibility provides for control of the density and therefore solvent-dependent properties such as dielectric constant and overall solvent strength (Giddings et al., 1968). While supercritical C02 can have high liquidlike densities, it shares many of the... 

The extraction medium in SFE is supplied from a cylinder to a pump where it is compressed to the desired pressure in the critical range. Next, the fluid in this form reaches the vessel containing a sample situated in a chamber heated to the critical temperature. Here, the substance, already in the supercritical fluid state, extracts the analytes, and the extract is collected in a special receiver. Figure 19.6 shows a diagram of the instruments used for SFE. 

An increasingly popular alternative to liquid chromatography and gas chromatography is supercritical fluid chromatography (SFC). When a substance is heated above its critical pressure and temperature, it may exist in a supercritical fluid state. Carbon dioxide is an example of a supercritical fluid. At temperatures above 31°C and pressures exceeding 73 atmospheres, the individual molecules of the compound are able to overcome restrictive intermolecular forces and move in a manner more similar to that of a gas. 

Supercriticial Solvents. Although it was known in 1879 that supercritical fluids had solvent properties (180), supercritical extraction was not extensively developed until the early 1980s. This method uses organic or inorganic compounds as solvent, at or usually above their critical temperature and pressure where they are known as supercritical fluids. In a supercritical fluid state, common gases such as carbon dioxide have the properties and extractive capacity of a liquid. The compound most used in supercritical extractions is carbon dioxide. Carbon dioxide can exist as a gas, liquid, or solid, depending on pressure and temperature conditions. However, at or above its critical point. CO2 can only exist as a supercritical fluid. 

We conducted a quantiun-based molecular dynamics simulation of HMX at a density of 1.9 g/cm and temperature of 3500 K for up to 55 picoseconds has been conducted. These are conditions similar to those encountered at the Chapman-Jouget detonation state. Thus, although we do not model the entire shock process, we can provide some insight into the nature of chemical reactivity under similar conditions. Under the simulation conditions HMX was found to be in a highly reactive dense supercritical fluid state. We estimated effective reaction rates for the production of H2O, N2, CO2, and CO to be 0.48,... 

Nonvariant regions are points in the phase diagram (triple points r, i = 3) indicating equilibrium of three phases (g, 1, s or s, s, 1 or s, s, g, etc.). The vapor pressure curve (equilibrium 1 g) terminates in a critical point C. Above the critical temperature, liquid and gaseous states are indistinguishable (supercritical fluid state). 

Supercritical Fluid Chromatography (SFQ and Supercritical Fluid Extraction (SFE) A separation technology similar to other extraction and chromatographic methods, but in which the mobile phase is actually a fluid in its supercritical fluid state. A supercritical fluid is a fluid that is held above its critical temperature and pressure, and for which no application of additioncJ pressure can result in the development of a liquid phase. Supercritical fluids are unique in that while they possess liquid-like densities, the mass transfer behavior is superior to that of liquids. Supercritical fluid chromatography remains a niche method that is applicable to pharmaceuticals and other high relative molecular mass solutes. Supercritical extraction, on the other hand, is more widely used as a sample preparation method, especially in pharmaceutical analyses, polymers, and environmental analyses. 

Supercritical fluid states and natural-circulation loops... 

The historical investigations in the 1960s into supercritical fluids for NCLs and nuclear-powered machines were mentioned earlier. Chatoorgoon (1986, 2001) and Chatoorgoon et al. (2005a,b) presented additional early sources in addition to making new contributions. The renewed interest in supercritical fluid states for nuclear power applications has resulted in many new experimental, analytical, and niunerical investigations. 

Considerations of distributions across the flow channel, transverse to the primary flow direction, were first included in basically one-dimensional models by approximating the temperature distribution in the fluid parallel to the flow direction. Recently there is an increasing application of CFD to various single- and two-phase thermal-hydraulic analyses, including NCLs and supercritical fluid states, in nuclear power systems. These approaches also allow for resolution of the thermal stratification in horizontal and vertical sections of the loop as well as resolution of gradients normal to the primary flow direction and the consequent effects on calculated stability. Fully three-dimensional analyses are becoming the norm, but only for simple idealized single-phase cases. 

Detailed descriptions of all of the flow and heat transfer mechanisms is beyond the scope of the present text. Furthermore, it is difficult to provide a definitive list of suggested data sets and correlations for all possible combinations of flow and heat transfer mechanisms, especially when flow-channel geometry effects and supercritical fluid states are taken into consideration. See the literatore review in this chapter for additional information. 

The supercritical state of water is the most extreme one of common solvents. Its critical temperature Tq is 374 °C and the critical pressure is 22,059 MPa. The properties of liquid and supercritical fluid states are quite different. Supercritical water, e.g., is less polar than liquid water and a solvent for hydrophobic substances. It can be expected that electrochemistry in supercritical water would provide useful information about natural hydrothermal processes, e.g. oxide formation [218]. The majority of research papers, however, were focused on analytical investigations, preferably potentiometric pH measurement [210-212, 216, 219, 220, 253]. The chemistry of different redox couples has been studied as well [223, 224, 226,... 

---