Supercritical phenomena

There were also several experiments involving current topics that did not fit neatly into a category. These included studies of supercritical phenomena (140, 141), cometary spectroscopy (142), experimental studies on the thermodynamics of heat pumps (143), a study of industrially relevant phase transfer catalysts (144), and finally an electrochemical study of a commercial soap (145). Both the environmental and the miscellaneous experiments can be found listed in Table X. 

In view of the interest in this field, an experimental investigation was undertaken to determine the applicability of supercritical phenomena to the separation of butadiene from C4 mixtures. In particular, the separation of 1-butene from 1,3-butadiene is a key factor in the separation process. Results of these studies are considered in light of predictions obtained from a representative equation of state in the retrograde region of the SC solvent-solute VLE envelope. 

In general, and provided the pressure is not high enough for supercritical phenomena to exist—usually true of pressures below 25 bar except in the case of He or H2—we can make the distinction between liquid and gas simply on the basis of density. A liquid has a relatively high density that is insensitive to changes in temperature and pressure. A gas, on the other hand, has a relatively low density that is sensitive to temperature and pressure and that approaches zero as pressure is reduced at constant temperature. 

There are two general types of fugacity models equations of state and liquid-state activity-coefficient models. An equation of state is an algebraic equation for the pressure of a mixture as a function of the composition, volume, and temperature. Through standard thermodynamic relationships, the fugacity, enthalpy, and so on for the mixture can be determined. These properties can be calculated for any density therefore, both liquid and vapor properties, as well as supercritical phenomena, can be determined. 

Watei has an unusually high (374°C) ctitical tempeiatuie owing to its polarity. At supercritical conditions water can dissolve gases such as O2 and nonpolar organic compounds as well as salts. This phenomenon is of interest for oxidation of toxic wastewater (see Waste treatments, hazardous waste). Many of the other more commonly used supercritical fluids are Hsted in Table 1, which is useful as an initial screening for a potential supercritical solvent. The ultimate choice for a specific appHcation, however, is likely to depend on additional factors such as safety, flammabiUty, phase behavior, solubiUty, and expense. 

Examples are the benefits in the area of extraction of vegetable cuticular waxes being separated from the more valuable essential oils, using supercritical CO2 (Stassi and Schiraldi, 1994). A molecular understanding of how a phenomenon like supercritical behavior affects solvent properties is important (Kazarian and Poliakoff, 1995). 

Since in the critical point the bubble point curve (l+g—tf) and the dew-point curve (l+g-+g) merge at temperatures between 7C and 7 , an isotherm will intersect the dew-point curve twice. If we lower the pressure on this isotherm we will pass the first dew-point and with decreasing pressure the amount of liquid will increase. Then the amount of liquid will reach a maximum and upon a further decrease of the pressure the amount of liquid will decrease until is becomes zero at the second dew-point. The phenomenon is called retrograde condensation and is of importance for natural gas pipe lines. In supercritical extraction use is made of the opposite effect. With increasing pressure a non-volatile liquid will dissolve in a dense supercritical gas phase at the first dew point. 

Studiengesselschaft Kohle m.b.H. (2) reported the effect of temperature on solubility level in supercritical gas. The solubility is highest within 20 K of the critical temperature and decreases as temperature is raised to 100 K above the critical temperature. At temperatures near the critical temperature, a sharp rise in solubility occurs as the pressure is increased to the vicinity of the critical pressure and increases further as the pressure is further increased. Less volatile materials are taken up to a lesser extent than more volatile materials, so the vapor phase has a different solute composition than the residual material. There does not seem to be substantial heating or cooling effects upon loading of the supercritical gas. It is claimed that the chemical nature of the supercritical gas is of minor importance to the phenomenon of volatility amplification. Ethylene, ethane, carbon dioxide, nitrous oxide, propylene, propane, and ammonia were used to volatilize hydrocarbons found in heavy petroleum fractions. 

The picture of a solute molecule stabilized in solution by a local environment where the solvent s concentration differs considerably from the bulk value is consistent with experiments and simulation. The encouraging agreement between the basic trends found in experiments and simulations should not obscure the fact that Lennard-Jones atoms are a pedestrian representation of the actual molecules studied in the fluorescence experiments. Caution must therefore be exercised when comparing simulations and experiments. At the same time, the very fact that such a crude model is able to capture the essential physics of the phenomenon under investigation lends further support to the notion that local density augmentations are common to all attractive supercritical systems. 

There is one more unique feature of supercritical fluid solvents that will be a recurring theme in this chapter. Several studies have demonstrated that near the critical point, the density of the solvent about a solute is enhanced relative to the bulk density (solvent/solute clustering). As such, the mobility of the solute may be impeded to an extent greater than expected on the basis of the bulk viscosity. This phenomenon may also affect reactivity for reactions that are diffusion-controlled or for which cage effects are important, particularly near the critical point (vide infra). 

High shear stresses are known to favour the most stable forms of the cocoa butter. The same phenomenon is probably also appearing in the supercritical generation of cocoa butter particles when the particles are generated in the expansion vessel with a huge drop in pressure through a small nozzle, they are exclusively made of form V—when generated in the first vessel with a slow drop in pressure form IV is present and even predominant. This is a track for further studies. 

In 1958, Pitzer (141), in a remarkable contribution that appears to have been the first theoretical consideration of this phenomenon, likened the liquid-liquid phase separation in metal-ammonia solutions to the vapor-liquid condensation that accompanies the cooling of a nonideal alkali metal vapor in the gas phase. Thus, in sodium-ammonia solutions below 231 K we would have a phase separation into an insulating vapor (corresponding to matrix-bound, localized excess electrons) and a metallic (matrix-bound) liquid metal. This suggestion of a "matrix-bound analog of the critical liquid-vapor separation in pure metals preceeded almost all of the experimental investigations (41, 77, 91,92) into dense, metallic vapors formed by an expansion of the metallic liquid up to supercritical conditions. It was also in advance of the possible fundamental connection between this type of critical phenomenon and the NM-M transition, as pointed out by Mott (125) and Krumhansl (112) in the early 1960s. 

In the case of a real liquid in an open channel, it is necessary to differentiate between the behavior at subcritical and supercritical velocities. Subcritical flow in a rectangular channel has been investigated experimentally and has been found to conform fairly well to ideal conditions, especially within the first part of the bend [44], As the flow continues around the bend, the velocity distribution becomes complicated by the phenomenon of spiral flow, which for open channels is analogous to the secondary counterrotating currents found at bends in closed pipes. 

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

In order to study the catalyst deactivation phenomenon under supercritical conditions and the difference between the liquid phase (LP) and supercritical fluid phase (SCFP) reactions, experiments were carried out in an isothermal tubular reactor (D=I2 mm, L=600 mm) packed with grounded Y-type zeolite pellets of 60 mesh. The experimental equipment for the LP and SCF reaction processes is illustrated in Figure 1. 

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