Supercritical point

In the region of supercritical point, most properties of supercritical water vary widely. The most prominent of these is the heat capacity at constant pressure, which approaches infinity at the critical point. Even 25°C above Tc, at 80 bar away from Pc, the heat capacity of water is an order of magnitude greater than its value at higher or lower pressure. 

The phase diagram features four phase regions, three phase boundaries, and two points of particular interest the triple point (TP) and the supercritical point (CP). Values for TP and CP from The International Association for the Properties of Water and Steam6 (IAPWS) are 273.16 K and 611.657 Pa (IAPWS, 2002) and 647.096 K and 22.064 MPa (IAPWS, 2002), respectively. Three of the phases (solid, liquid, and gas) are bounded by equilibrium... 

A solvothermal process is one in which a material is either recrystallized or chemically synthesized from solution in a sealed container above ambient temperature and pressure. The recrystallization process was discussed in Section 1.5.1. In the present chapter we consider synthesis. The first solvothermal syntheses were carried out by Robert Wilhelm Bunsen (1811-1899) in 1839 at the University of Marburg. Bunsen grew barium carbonate and strontium carbonate at temperatures above 200°C and pressures above 100 bar (Laudise, 1987). In 1845, C. E. Shafhautl observed tiny quartz crystals upon transformation of freshly precipitated silicic acid in a Papin s digester or pressure cooker (Rabenau, 1985). Often, the name solvothermal is replaced with a term to more closely refer to the solvent used. For example, solvothermal becomes hydrothermal if an aqueous solution is used as the solvent, or ammothermal if ammonia is used. In extreme cases, solvothermal synthesis takes place at or over the supercritical point of the solvent. But in most cases, the pressures and temperatures are in the subcritical realm, where the physical properties of the solvent (e.g., density, viscosity, dielectric constant) can be controlled as a function of temperature and pressure. By far, most syntheses have taken place in the subcritical realm of water. Therefore, we focus our discussion of the materials synthesis on the hydrothermal process. 

Water possesses vastly different properties as a reaction medium in its supercritical state than in its standard state. The diagram in Fig. 14.7 is that of a pure substance and shows the regions of temperature and pressure where the substance exists as a solid, liquid, gas, and supercritical fluid. The supercritical point for water is met at a temperature of 400°C and above and at high pressure (about 25 MPa). At the supercritical point, water behaves as a nonpolar dense gas, and hydrocarbons exhibit generally high solubility. However, the solubility of inorganic salts is very low in such liquid. Note that the dielectric constant of water is 80 at the standard state reaches approximately 0 at the supercritical point the Aw of 10 14 at the standard state reaches approximately 10-24 at the supercritical point. 

The heat transfer to supercritical carbon dioxide was measured in horizontal, vertical and inclined tubes at constant wall temperature for turbulent flow at Re-numbers between 2300 and lxl 05. The influence of the variation of physical properties due to the vicinity of the critical point was examined, as well as the influence of the direction of flow. Therefore most of the measurements were conducted at pseudocritical points. At those supercritical points the behaviour of the physical properties is similar to the behaviour at the critical point, but to a lesser degree. At such points the heat capacity shows a maximum density, viscosity and heat conductivity are changing very fast. [1]... 

The temperature-dependent reaction rates of e with H+, NO3, and NO2 have been reported.As mentioned above, around the supercritical point, the dielectric constant of water is similar to nonpolar organic solvents, and the dissociation constants of inorganic salts are extremely small. It is thought that these properties would affect those ionic reactions which are Coulumbic-force related. As shown in Fig. 9, for the reaction with the rates strongly increase... 

In all the experiments, both the treatment time and decompression time were fixed as 1 hour, except one in which 5-hour decompression time was applied. The carbon dioxide treatments evaluated include 3 subcritical and 4 supercritical points as indicated in Table 1. 

As an alternative to SEE with carbon dioxide or other supercritical fluids, it was proposed that heating the organic solvents under pressure above their boiling point, but below their supercritical point, would enhance the speed of reaction and solvent strength. This technique, known as the pressurized solvent extraction (PSE), provides an easy method for extraction, reducing the... 

A computer program was developed for electrochemical equilibria calculations and graphical pH-potential diagram presentation of a one-metal/one-nonmetal/water system. The program can be used for temperatures and pressures exceeding the supercritical point of water. The calculations show that hematite (Fe203) is the oxidation product of iron in supercritical water, and the oxidation product of chromium in supercritical water is an ionic species, Cr04 . Passivation effect of chromium is lost in supercritical water. 

The exchange current density, open circuit potential and transfer coefficients were calculated for different alloys around the supercritical point of water. Passivation of 304 or 316... 

The development of the Borstar PE process, by Borealis, is a relatively recent development in multi-reactor processes. The foundation of this process is the utilization of supercritical propane as diluent in the slurry loop reactor.438 Operating the slurry loop in a supercritical condition provides several advantages over the tradition diluent (isobutane). The solubility of PE drops markedly at the supercritical point of propane, allowing the process to operate... 

Some of the particles were studied after supercritical point drying. The particles were dispersed in acetone by several centrifugation steps (45,000 X g). In a critical point dryer (Balzers Union, CPD 020), the carbon-coated grids and the dispersion in acetone were placed. Under pressure, the acetone was exchanged against liquid CO2 after increasing the temperature and passing the critical point, the particles adsorbed on the film were supercritically dried. 

When water is used as the solvent, PLE is referred to as superheated water extraction, subcritical water extraction (SWE), or pressurized (hot) water extraction (PWE). Hot water is very effective as an extraction solvent for PAHs from soil and sediment. Superheated water is water above the boiling point but below the supercritical point, and under sufficient pressure to maintain... 

Chapter 8 briefly introduced the concept of supercritical fluids in the context of undersea thermal vents. The supercritical point for water occurs at a temperature of 705°F (374°C) and a pressure of 222.3 bar (atmosphere). Above this temperature, no pressure can condense water to its liquid state. For carbon dioxide (CO2), the critical temperature (88.0°F or 31.1°C) and critical pressure (73.8 bar) are much lower. Above the supercritical point, CO2 behaves as a liquidlike gas liquidlike densities, gaslike viscosities. The solubility properties of supercritical CO2 are mnable by varying temperature and/or pressure. Density and dielectric constant increase with increasing pressure and decreasing temperature. Water and ionic substances are insoluble in supercritical CO2. The ability of supercritical CO2 to dissolve and extract relatively non-polar substances has been known for decades. The range may be extended by adding polar solvents such as methanol or acetone. The addition of surfactants helps to disperse microscopic particles to form colloidal suspensions. Carbon dioxide is nonflammable, nontoxic, and inexpensive. 

I. A fluid that is above the supercritical point is injected into an environment at conditions above the critical point of the injectant (Tr,j j and Tr.env... 

When residual solvents and other impurities, such as monomers and polymeric precursors, remain in the material layer, a deterioration of the device could occur. Between the individual processing steps, these impurities can be removed by extraction with supercritical fluids. When the process can be performed under comparatively low-temperature and low-pressure conditions, the impurities can be removed, suppressing chemical physical changes of the layer [102]. For example, while the supercritical point of ethanol is 241 °C at 6.1 MPa, the cleaning process can be performed at 260 °C and 8.0 MPa. Using carbon dioxide, a temperature of 80 °C and a pressure of 15 MPa can be used. 

The supercritical extraction methods avoid the liquid/vapor boundary line by bringing the solvent above its supercritical point and then removing it from the sol-gel matrix as a supercritical fluid. In this state there is no liquid-vapor interface and therefore no capillary stresses due to the receding meniscus. The supercritical extraction technique was first developed by Kistler [22] in the 1930s. There are several types of supercritical extraction methods in use. They include high temperature, low temperature, and rapid supercritical techniques, each of which is described in more detail below. 

Fig. 2 The phase transitions involved in water-air, ethanol-air, or pentane-air-drying green arrowy, freeze-drying blue arrow) -, and supereritieal drying red arrow) of wet porous silicon. For air-drying, the liquid L in the pores immediately becomes a gas G via evaporation. For freeze-drying, it is first solidified (5) and then becomes a gas via sublimation around the triple point. For supercritical drying, it is taken around the supercritical point to become a gas...

As the temperature of water increases to the critical point, its electrolytic conductance rises sharply independent of the pressure. This is attributed to decrease in its viscosity over this range. However, near the supercritical point, the conductance begin to drop off... 

Most often the fluid is carbon dioxide, and it is used purely as a solvent. Carbon dioxide s great advantage is that its critical point is relatively attainable (3 PC and 74 bar), and it is readily available and inexpensive. Propane is another candidate, as its supercritical point is 96°C and 43 bar. Water is often considered as a supercritical solvent. However, water s critical point occurs at 374°C and >200 bar, so its use is much less convenient and use of CO2 is dominant. The key advantages of supercritical operation are ... 

Martin and Beller [77] conducted the reaction either in supercritical NHj or close to the supercritical point of NHj (,T = 132.35 °C,p = 133.5 bar) by employing a dual catalytic system consisting of [lrCl(COD)]2/Rh(acac)(CO)2 in the ratio 5 1 (CO/H2 =1 3,190-240 bar 140 C). Noteworthy, the position of the critical point is dependent on the ratio NHj/olefin and water, the latter being a product of the imine/enamine formation in the reaction system [78]. 

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