Supercritical pure substance

Pure substance, phase behavior of, 24 663 Pure supercritical fluids, physical properties of, 24 4... 

The physical-chemical properties of a supercritical fluid are between those of liquids and gases supercritical fluids (SCFs) indicate the fluid state of a compound in pure substance or as the main component above its critical pressure (pc) and its critical temperature (Tc), but below the pressure for phase transition to the solid state, and in terms of SCF processing, a density close to or higher than its critical density. 

The supercritical cycle is shown on the T-s diagram of a pure substance (Fig. 2.35). The cycle is composed of the following four processes ... 

A supercritical fluid is a substance above its critical temperature and pressure. Figure 3.4 shows a phase diagram of a pure substance, where curve... 

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. 

Researchers usually use a diagram of pure component to discuss the definition of an SCF. This is reasonable because supercritical has clear meaning for a pure substance. A pure SCF has some unique features, especially near its critical point. The solvent properties (e g. density, dielectric constant, solubility parameter, difiusivity) are sensitive to pressure. Therefore, pressure is an effective variable factor to optimize the operating conditions in the applications. However, a SCF loses most of these features as it is far from the critical point. 

Let us now focus upon the critical temperature and consider a few of the definitions that can describe this invariant point. It is important to note that the critical point is defined by the temperature only the value of the critical pressure appears to have a lesser or secondary significance. The critical (or supercritical) fluid region exists at all pressures at or above the critical temperature for a pure substance. Above this critical temperature, there exists only one phase, completely independent of the pressure. That is, no matter how high (or how low) you cause the pressure to be, the one phase wiU not condense to a hquid. 

For every substance there is a temperature above which it can no longer exist as a liquid, no matter how much pressure is applied. Likewise, there is a pressure above which the substance can no longer exist as a gas no matter how high the temperature is raised. These points are called the supercritical temperature (T ) and supercritical pressure (Pe) respectively and are the defining boundaries on a phase diagram for a pure substance. Beyond these boundaries, the substance has properties that are intermediate between a liquid and a gas and is called a supercritical fluid. 

Figure 1 Generalized phase diagram for a pure substance showing the locations of the supercritical and near-critical regions.

On the other side, the physical data [9] of the pure substances as well as the structural formula are given in Figures 1 and 2 (see Annex) for Clark I and Clark II and for Lewisite and Adamsite in Fig. 3 and 4. Because of the low carbon content of Lewisite, the oxygen balance of this substance is much less negative (-7.72 g O2/ lOOg) than that of Adamsite (-34.01). This means, the demand of air for a complete oxidation is much less than in the case of Adamsite, see Table 2. The reaction products with water at 400°C and 220 bar, which correspond to an oxidative reaction of warfare agents under supercritical conditions, are also presented in this table. As we recognise, the main reaction products are CO2, H2, CH4 and HCl beside As and residual H2O. 

For application of supercritical CO2 as a medium in polymer processes, it is important to consider its interactions with polymers and monomers. In general, the thermodynamic properties of pure substances and mixtures of molecules are determined by intermolecular forces acting between the molecules or polymer segments. By examining these potentials between molecules in a mixture, insight into the solution behavior of the mixture can be obtained. The most commonly occurring interactions are dispersion, dipole-dipole, dipole-quadru-pole, and quadrupole-quadrupole (Fig. 1.8). 

Classically pure substances are solid, hquid, or gaseous. A supercritical fluid is a pure substance compressed and heated above its critical point see Rgure 14.13 (46). The major characteristics of a supercritical fluid are hquidlike density, gaslike diffusivity and viscosity, and zero surface tension. 

Figure 14.13 The pressure temperature phase diagram of a pure substance, emphasizing the supercritical fluid region. The critical point is the highest pressure and temperature at which a pure substance can exist in a vapor iquid equilibrium.

It is usual to classify a fluid as either a liquid or a gas. The distinction is important for a pure substance because the choice determines the treatment of the phase s standard state (see Sec. 7.7). To complicate matters, a fluid at high pressure may be a supercritical fluid. Sometimes a plasma (a highly ionized, electrically conducting medium) is considered a separate kind of fluid state it is the state found in the earth s ionosphere and in stars. 

At temperatures above the critical temperature and pressures above the critical pressure, the one existing fluid phase is called a supercritical fluid. Thus, a supercritical fluid of a pure substance is a fluid that does not undergo a phase transition to a different fluid phase when we change the pressure at constant temperature or change the temperature at constant pressure. ... 

In the early 1990s it appeared that supercritical-fluid extraction was going to be the future method of choice for extracting environmental soils and solid samples. SEE showed promising recoveries for many environment analytes and used very little solvent (64). As of 2001, it had not gained the widespread use that was predicted (7). SEE is very similar to the ASE technique described above, except that a supercritical fluid is used for the extraction rather than a solvent. Any pure substance that is above its critical temperature (Tc) and critical pressure (Pc) is defined as a supercritical fluid. The most frequently used extraction fluid is CO2. If CO2 is compressed to a pressure above 72.9 atm and heated to above 31.3°C, it becomes a supercritical fluid and exhibits physical properties between those of a gas and a liquid. Carbon dioxide is used most frequently in SEE as an extraction... 

Temperature-pressure diagram for a pure substance and the region of supercritical fluid extraction. 

Supercritical fluid extraction (SFE), a newly developed technique, is used for laboratorial and industrial purposes because it presents a series of advantages compared to the conventional extraction processes, especially for the extraction of thermolabile components [21,22]. It was first presented as a patent for decaffeination of coffee [23]. Since then, SFE has been used for many years as an alternative extraction method, which causes less pollution to the environment. The concept of the critical point was defined in 1822 as the highest pressure and temperature at which a pure substance could exist in vapor-liquid equilibrium. Above this point, supercritical fluid (SCF) is formed. These qualities make SCFs have higher diffiisivities and less degradation of solutes than ordinary solvents to extract active components. 

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. 

To design a supercritical fluid extraction process for the separation of bioactive substances from natural products, a quantitative knowledge of phase equilibria between target biosolutes and solvent is necessary. The solubility of bioactive coumarin and its various derivatives (i.e., hydroxy-, methyl-, and methoxy-derivatives) in SCCO2 were measured at 308.15-328.15 K and 10-30 MPa. Also, the pure physical properties such as normal boiling point, critical constants, acentric factor, molar volume, and standard vapor pressure for coumarin and its derivatives were estimated. By this estimated information, the measured solubilities were quantitatively correlated by an approximate lattice equation of state (Yoo et al., 1997). 

For a supercritical fluid (SCF) component, the pure component parameters were obtained by fitting P-v data on isotherms (300-380K). Preliminary data for these substances suggest that although the computed v is a weak function of temperature, exl is a constant within regression error. 

The first experiments reported here lead us to think that the impregnation of porous supports by drugs can be achieved by means of supercritical fluids. This one-step method yields a final product exempt from any residual trace of toxic solvent. The kinetics of the mass transfer is faster, besides the thermodynamics of the adsorption seems more favourable here. The main problem encountered up to now is the weak solubility of many active molecules in pure C02, which induces a limitation of the percentage of deposited product. However, this difficulty can be overcome by the use of few amount of an entrainer. In particular, ethanol which does not show any toxicity, would greatly extend the range of active substances which could be used. 

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