Supercritical fluids critical density

Thermal expansion causes liquid water to become less dense as the temperature increases. At the same time, the liquid vapor becomes more dense as the pressure rises. For example, the density of water varies from 1.0 g/cm3 at room temperature to 0.7 g/cm3 at 306°C. At the critical point, the densities of the two phases become identical and they become a single fluid called supercritical fluid. Its density at this point is only about 0.3 g/cm3 (Figures 1.4 and 1.5). 

Below the critical temperature, a phase transition occurs when compressing a gas. The formation of a liquid phase is usually first noted by the formation of droplets on the walls of the container. At temperatures above the critical temperature, a substance can be continuously compressed without a separate liquid phase forming. Under such conditions, the substance is a gas, because it continues to fill its container. However, because densities comparable to those of the liquid can be reached by such compression, it is customary to call a substance above its critical temperature a supercritical fluid, where the term fluid (from flow) refers to either liquid or gas. Supercritical fluids, with densities comparable to liquid and high thermal energy, can be exceedingly good solvents and have found use recently in processes such as decaffeination of coffee. 

A supercritical fluid is formed whenever a substance is heated above its critical temperature. The critical temperature of a substance is the temperature above which a distinct liquid phase cannot exist, regardless of pressure. The vapor pressure of a substance at its critical temperature is itscriticalpressure. At temperatures and pressures above its critical teniperaiure and pressure (its critical point), a substance is called a supercritical fluid. Supercritical fluids have densities, viscosities, and other properties that are intermediate between those of the substance in its gaseous and liquid states. Table 29-1 compares some properties of supercritical fluids to those of typical gases and liquids. I hesc properties are important in gas. liquid, and supercritical fluid chromatography and extractions. 

The properties of fluids under supercritical conditions are considered ideal for extracting substances from exhausted activated carbons. Two supercritical fluids are of particular interest, carbon dioxide and water. Carbon dioxide has a low critical temperature of 304 K and a moderate critical pressure of 73 bar, while water has a critical temperature of 647 K and a critical pressure of 220 bar. The character of water at supercritical conditions changes from one that supports only ionic species at ambient conditions to one that dissolves paraffins, aromatics, gases and salts [65]. These supercritical fluids exhibit densities similar to those of liquids (high solvent strengths) and diffusion coefficients similar to those of gases (excellent transport characteristics), enabling them to effectively dissolve and/or desorb contaminants from the carbon surface and to easily enter/exit even the smallest pores and carry away any... 

Water becomes less dense due to thermal expansion with increase in temperature. The density of water is 1.0 g/cm at room temperature, which changes to 0.7 g/cm at 306 C. At critical point, the densities of the two phases become identical and they become a single fluid, which is called supercritical fluid. The density of water at this point is 0.3 g/cm In the supercritical region, most of the properties of water vary widely. The most important of these is the heat capacity at constant pressure, which approach infinity at the critical point. Also, the dielectric constant of dense, supercritical water ranges from 5 to 20 on variation of applied pressure. 

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]. 

On the right t is plotted against the reduced density (density/critical point density) at a reduced tesperature of 1.03 (temperature/critical point teiqperature) for several cosnon supercritical fluids. (Reproduced with permission from ref. 9. Copyright American Chemical Society). 

In general, the properties of supercritical fluids make them interesting media in which to conduct chemical reactions. A supercritical fluid can be defined as a substance or mixture at conditions which exceed the critical temperature (Tc) and critical pressure (Pc). One of the primary advantages of employing a supercritical fluid as the continuous phase lies in the ability to manipulate the solvent strength (dielectric constant) simply by varying the temperature and pressure of the system. Additionally, supercritical fluids have properties which are intermediate between those of a liquid and those of a gas. As an illustration, a supercritical fluid can have liquid-like density and simultaneously possess gas-like viscosity. For more information, the reader is referred to several books which have been published on supercritical fluid science and technology [1-4],... 

Most supercritical fluid chromatographs use carbon dioxide as the supercritical eluent, as it has a convenient critical point of 31.3°C and 72.5 atmospheres. Nitrous oxide, ammonia and n-pentane have also been used. This allows easy control of density between 0.2g ml-1 and 0.8g ml-1 and the utilization of almost any detector from liquid chromatography or gas chromatography. 

Supercritical fluids represent a different type of alternative solvent to the others discussed in this book since they are not in the liquid state. A SCF is defined as a substance above its critical temperature (Tc) and pressure (Pc)1, but below the pressure required for condensation to a solid, see Figure 6.1 [1], The last requirement is often omitted since the pressure needed for condensation to occur is usually unpractically high. The critical point represents the highest temperature and pressure at which the substance can exist as a vapour and liquid in equilibrium. Hence, in a closed system, as the boiling point curve is ascended, increasing both temperature and pressure, the liquid becomes less dense due to thermal expansion and the gas becomes denser as the pressure rises. The densities of both phases thus converge until they become identical at the critical point. At this point, the two phases become indistinguishable and a SCF is obtained. 

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. 

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