Supercritical temperatures

The formation of acids from heteroatoms creates a corrosion problem. At the working temperatures, stainless steels are easily corroded by the acids. Even platinum and gold are not immune to corrosion. One solution is to add sodium hydroxide to the reactant mixture to neutralize the acids as they form. However, because the dielectric constant of water is low at the temperatures and pressure in use, the salts formed have low solubiHty at the supercritical temperatures and tend to precipitate and plug reaction tubes. Most hydrothermal processing is oxidation, and has been called supercritical water oxidation. 

For supercritical temperatures, it is satisfactory to ever-higher pressures as the temperature increases. For pressures above the range where Eq. (4-190) is useful, but below the critical pressure, the virial expansion in density truncated to three terms is usually suitable ... 

Adsorption and Desorption Adsorbents may be used to recover solutes from supercritical fluid extracts for example, activated carbon and polymeric sorbents may be used to recover caffeine from CO9. This approach may be used to improve the selectivity of a supercritical fluid extraction process. SCF extraction may be used to regenerate adsorbents such as activated carbon and to remove contaminants from soil. In many cases the chemisorption is sufficiently strong that regeneration with CO9 is limited, even if the pure solute is quite soluble in CO9. In some cases a cosolvent can be added to the SCF to displace the sorbate from the sorbent. Another approach is to use water at elevated or even supercritical temperatures to facilitate desorption. Many of the principles for desorption are also relevant to extraction of substances from other substrates such as natural products and polymers. 

McDonald, I. R. Singer, K., Machine calculation of thermodynamic properties of a simple fluid at supercritical temperatures, J. Chem. Phys. 1967, 47, 4766-4772... 

Hm for steam + n-heptane calculated by the above method is shown by the dashed lines in figure 6. Considering the simplicity of the model and the fact that no adjustable parameters have been used, agreement with experiment is remarkable. For mixtures of steam + n-hexane, benzene and cyclohexane agreement with experiment is much the same. At low densities the model reproduces the curvature of the lines through the results better than the virial equation of state. The method fails to fully reproduce the downward turn of the experimental curves at pressures near saturation, but does marginally better in this region than the P-R equation with k. = -0.3. At supercritical temperatures the model seems to... 

The supercritical fluid region, which encompasses all conditions of supercritical temperatures and pressures. 

Application of SFE necessitates a CO2 source, a pump to pressurize the fluid, an oven containing the extraction vessel, a restrictor to maintain a high pressure in the extraction line, an analyte collection vessel, and an overall system controller. CO2 is drawn from the bottom of the tank with a dip tube because the liquid is the more dense of the two phases. The substantial vapor pressure of the CO2 at ambient temperature helps to displace the liquid into the pump. CO2 remains a liquid throughout the pumping or compression zones and passes through small-diameter metal tubing as it approaches the extraction vessel. A preheating zone in front of the extraction vessel allows supercritical temperature, pressure, and density conditions to be applied immediately to the analyte matrix in the vessel. 

In SFC, the mobile phase is initially pumped as a liquid and is brought into the supercritical region by heating it above its supercritical temperature before it enters the analytical column. It passes through an injection valve where the sample is introduced into the supercritical stream and then into the analytical column. It is maintained supercritical as it passes through the column and into the detector by a pressure restrictor placed either after the detector or at the end of the column. The restrictor is a vital component it keeps the mobile phase supercritical throughout the separation and often must be heated to prevent clogging both variable- and fixed-restrictors are available. 

Comparison of the supercritical temperature and pressure conditions of some candidate fluids for industrial exploitation Q igure 3.1) may exclude those requiring extreme conditions, such as water, and others on environmental (SFg) or cost grounds (xenon). 

MODAR Inc. (Massachusetts. USA) developed the first reactor vessel [13]. It comprised an elongated, hollow cylindrical pressure-vessel, capped at both ends so as to define an interior reaction chamber. Defined within the reaction chamber are a supercritical temperature zone, in the upper region of the reactor vessel, and a subcritical temperature zone in the lower region of the reactor vessel. Oxidation of organics and oxidizable inorganics takes place in the supercritical temperature zone. Dense matter, such as inorganic material initially present and formed by reactions, if insoluble in the supercritical-temperature fluid, falls into the liquid phase provided in the lower-temperature, subcritical zone of the vessel. A perimeter curtain of downward-flowing subcritical-temperature fluid is established about a portion of the interior of the cylindrical wall of the vessel to avoid salt-deposits on the walls of the reactor vessel. 

Figure 6. Vibrational frequency shift for the CH stretch of ethane at two supercritical temperatures. The experimental and theoretical shifts are plotted relative to their zero density vapor phase values.

The fluorescence spectra show clear evidence of an enhancement in the solvent s density around the solute at slightly supercritical temperatures. To investigate this observation quantitatively, we define an average local density,... 

Figure 3 shows the relationship between the local and bulk density at two supercritical temperatures, and for three different values of R (R = R/si). 

Figure 3 Relationship between local and bulk densities at two supercritical temperatures (T7TC = 1.02, 1.145) for an infinitely dilute mixture of Lennard-Jones atoms with potential parameters chosen so as to simulate pyrene in carbon dioxide (see Table II). Molecular dynamics simulation.

The correlation length ( ) corresponding to simulations at the critical density and slightly supercritical temperature was calculated from the scaling equation = 0 (Tr -l)-o.63 where Tr is the reduced temperature, and a substance-specific amplitude (47). At Tr = 1.02, and using = 1.5 A for carbon dioxide (47), the... 

Carbon dioxide (CO2) has a low supercritical temperature (31°C) and pressure (73 atm). It is nontoxic and nonflammable and is available at high purity. Therefore, CO2 has become the solvent of choice for most SFE applications. Being nonpolar and without permanent dipole moment, supercritical CO2 is a good solvent for the extraction of nonpolar and moderately polar compounds. However, its solvating power for polar solutes is rather poor. Moreover, when the solutes bind strongly to the matrix, the solvent strength of CO2 is often inadequate to break the solute-matrix bond. 

Practically, the values of f c(r, p )r2dr are fitted in a wide range of densities by a polynomial of order 3 in density. Then, ppex is obtained by analytically integrating the resulting polynomial as a function of density. It should be stressed that this method involves only the direct correlation function c(r), but neither B (r) nor B r), which are known to be the keys of the IETs. It must be stressed that such a thermodynamic integration process is performed along an isotherm T. This method is only accurate for supercritical temperatures, but is not at all for lower temperatures. Furthermore, it is not adapted to a predictive scheme. 

Excess Chemical Potential (3pex of the Lennard-Jones Fluid at Supercritical Temperatures... 

Now, we turn to the entropy. In Fig. 15, the excess calculated entropy is plotted in comparison with the reference data drawn from MD simulation [34] owing to Eq. (86). For the three supercritical temperatures investigated, the curves follow the reference data extremely well from low densities up to p 0.7. Beyond this limit, the accordance becomes less and less satisfactory. This is not surprising since the excess chemical potential (see Fig. 13) is itself less accurately predicted for high densities. The bump observed on the curve at T 1.35 is also not surprising as it lies around the critical point located, with different theoretical and simulation approaches, at temperature between 1.30 and 1.35 and density between 0.30 and 0.35. 

We take, as a standard reference state, the state of the bulk liquid methane of the same temperature T in equilibrium with a vapor of fugacity fs. For supercritical temperatures, we assumed extrapolated effective values of fugacity... 

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