Supercritical fluids critical point properties

Critical Point Properties for Selected Supercritical Fluids... 

Supercritical Fluid Extraction. Conditions can be generated that allow materials to behave differently from their native state. For example, boiling points are defined as that temperature at which a liquid changes to a gas. If the liquid is contained and pressure exerted, the boiling point changes. For a particular liquid, a combination of pressure and temperature will be reached, called the critical point, at which the material is neither a liquid nor a gas. Above this point exists a region, called the supercritical region, at which increases in both pressure and temperature will have no effect on the material (i.e., it will neither condense nor boil). This so-called supercritical fluid will exhibit properties of both a liquid and a gas. The supercritical fluid penetrates materials as if it were a gas and has solvent properties like a liquid. 

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. 

Above a certain temperature and pressure, marked by the critical point, a single supercritical fluid exists its properties are markedly different from normal gases or liquids. 

A supercritical fluid is defined as one that is above its thermodynamic critical point, as identified by the critical pressure (p ) and critical temperature (Tc). Supercritical fluid behavior can be peculiar because of the variation of theimophysical properties such as density and specific heat near and at the critical point. Supercritical fluids have some properties similar to liquids (e.g., density), and some properties that are comparable to those of gases (e.g., viscosity). Thus, they cannot be considered either a liquid or a gas. 

Supercritical Fluid Extraction. Supercritical fluid (SCF) extraction is a process in which elevated pressure and temperature conditions are used to make a substance exceed a critical point. Once above this critical point, the gas (CO2 is commonly used) exhibits unique solvating properties. The advantages of SCF extraction in foods are that there is no solvent residue in the extracted products, the process can be performed at low temperature, oxygen is excluded, and there is minimal protein degradation (49). One area in which SCF extraction of Hpids from meats maybe appHed is in the production of low fat dried meat ingredients for further processed items. Its apphcation in fresh meat is less successful because the fresh meat contains relatively high levels of moisture (50). 

Supercritical fluid chromatography (SFC) refers to the use of mobile phases at temperatures and pressures above the critical point (supercritical) or just below (sub-critical). SFC shows several features that can be advantageous for its application to large-scale separations [132-135]. One of the most interesting properties of this technique is the low viscosity of the solvents used that, combined with high diffusion coefficients for solutes, leads to a higher efficiency and a shorter analysis time than in HPLC. 

Supercritical fluids (SCFs) are compounds that exist at a temperature and pressure that are above their corresponding critical values [70,71]. They exhibit the properties of both gases and Hquids. With gases, they share the properties of low surface tension, low viscosity, and high diffusivity. Their main Hquid-like feature is the density, which results in enhanced solubility of solutes compared with the solubility of gases. Furthermore, the solubility of solutes can be manipulated by changes in pressure and temperature near the critical point [72]. 

A supercritical fluid exhibits physical-chemical properties intermediate between those of liquids and gases. Mass transfer is rapid with supercritical fluids. Their dynamic viscosities are nearer to those in normal gaseous states. In the vicinity of the critical point the diffusion coefficient is more than 10 times that of a liquid. Carbon dioxide can be compressed readily to form a liquid. Under typical borehole conditions, carbon dioxide is a supercritical fluid. 

SEE is an instrumental approach not unlike PLE except that a supercritical fluid rather than a liquid is used as the extraction solvent. SFE and PLE employ the same procedures for preparing samples and loading extraction vessels, and the same concepts of static and dynamic extractions are also pertinent. SFE typically requires higher pressure than PLE to maintain supercritical conditions and, for this reason, SFE usually requires a restrictor to control better the flow and pressure of the extraction fluid. CO2 is by far the most common solvent used in SFE owing to its relatively low critical point (78 atm and 31 °C), extraction properties, availability, gaseous natural state, and safety. 

Supercritical fluid chromatography (SFC) is an intermediate chromatographic technique between GC and HPLC. It depends upon the fact that when a fluid becomes supercritical (both the temperature and pressure are at or above its critical point) it develops some of the solvating properties of a liquid whilst retaining the low viscosity of a gas. Hence, mass transfer (essential to efficient chromatography) is more akin to that of GC than HPLC, but many compounds can be chromatographed at temperatures much lower than what would be required by GC, so some thermally labile compounds are amenable to SFC where they would degrade under GC conditions [28]. 

The liquid state exists only below the critical point pressure and above the triple point pressure. When a vapor below the triple point pressure is cooled down, we encounter a discontinuous and abrupt phase change to solid but, above the critical point pressure, a cooled vapor turns into the supercritical state where the properties of the fluid... 

The process employs the supercritical fluid carbon dioxide as a solvent. When a compound (in this case carbon dioxide) is subjected to temperatures and pressures above its critical point (31°C, 7.4 MPa, respectively), it exhibits properties that differ from both the liquid and vapor phases. Polar bonding between molecules essentially stops. Some organic compounds that are normally insoluble become completely soluble (miscible in all proportions) in supercritical fluids. Supercritical carbon dioxide sustains combustion and oxidation reactions because it mixes well with oxygen and with nonpolar organic compounds. 

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). 

The process involves the use of supercritical fluids rather than liquids as solvents. A fluid is in the supercritical state when its pressure and temperature exceed the pl ical properties which defines its critical point. Carbon dioxide is by far the most widely used supercritical solvent. Many other selected fluids have potential use for SFE technologies. 

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