Supercritical fluid critical point

Compare the behavior of the ethylene-n-propanol system to a type I system, CO2-hexane, shown in Figure 13. Note that in both cases, solubility of the supercritical component in the liquid phase increases rapidly as the pressure increases. This phenomenon can lead to substantial swelling of the liquid phase. The solubility of the heavy component in the lighter phase does not increase rapidly with an increase in pressure until the mixture critical point is approached or an additional phase is formed. The conditions of these occurrences may be substantially removed from the pure supercritical fluid critical conditions. 

Figure A2.5.28. The coexistence curve and the heat capacity of the binary mixture 3-methylpentane + nitroethane. The circles are the experimental points, and the lines are calculated from the two-tenn crossover model. Reproduced from [28], 2000 Supercritical Fluids—Fundamentals and Applications ed E Kiran, P G Debenedetti and C J Peters (Dordrecht Kluwer) Anisimov M A and Sengers J V Critical and crossover phenomena in fluids and fluid mixtures, p 16, figure 3, by kind pemiission from Kluwer Academic Publishers.

Critical Point Properties for Selected Supercritical Fluids... 

Hydrothermal crystallisation processes occur widely in nature and are responsible for the formation of many crystalline minerals. The most widely used commercial appHcation of hydrothermal crystallization is for the production of synthetic quartz (see Silica, synthetic quartz crystals). Piezoelectric quartz crystals weighing up to several pounds can be produced for use in electronic equipment. Hydrothermal crystallization takes place in near- or supercritical water solutions (see Supercritical fluids). Near and above the critical point of water, the viscosity (300-1400 mPa s(=cP) at 374°C) decreases significantly, allowing for relatively rapid diffusion and growth processes to occur. 

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). 

Eig. 1. Schematic pressure—temperature diagram for a pure material showing the supercritical fluid region, where is the pure component critical point... 

Although modeling of supercritical phase behavior can sometimes be done using relatively simple thermodynamics, this is not the norm. Especially in the region of the critical point, extreme nonideahties occur and high compressibilities must be addressed. Several review papers and books discuss modeling of systems comprised of supercritical fluids and soHd orHquid solutes (rl,i4—r7,r9,i49,r50). 

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. 

Fe(CN)6]3-(aq) + 6 H20(1). substrate The chemical species on which an enzyme acts, superconductor An electronic conductor that conducts electricity with zero resistance. See also high-temperature superconductor. supercooled Refers to a liquid cooled to below its freezing point but not yet frozen, supercritical fluid A fluid phase of a substance above its critical temperature and critical pressure. supercritical Having a mass greater than the critical mass. 

Carbon dioxide and water are the most commonly used SCFs because they are cheap, nontoxic, nonflammable and environmentally benign. Carbon dioxide has a more accessible critical point (Table 6.13) than water and therefore requires less complex technical apparatus. Water is also a suitable solvent at temperatures below its critical temperature (superheated water). Other fluids used frequently under supercritical conditions are propane, ethane and ethylene. 

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]. 

Table 5.2 Critical points of some common supercritical fluids...

Above a sufficiently high temperature and pressure, called the critical point (shown by a red dot), the distinction between the gas phase and the liquid phase disappears. Instead, the substance is a supercritical fluid, with viscosity typical of a liquid but able to expand or contract like a gas. 

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. 

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). 

Figure 3.7) [241], Some consider the SCF state to be more extended and comprising the area of the phase diagram above Tc independent of p0 [242], Critical temperature and pressure are usually defined as the maximum temperature at which a gas can be converted to a liquid by an increase in pressure, and the maximum pressure at which a liquid can be converted to a gas by an increase in temperature, respectively. In a PT diagram the vaporisation curve ends at the critical point. At a temperature above the critical point, the vapour and liquid have the same density. The critical parameters for some common fluids in analytical studies are listed in Table 3.11, but others may be found elsewhere [243], in particular, rc = 31.3 °C and pc = 7.38MPa for the most common SCF (C02). Supercritical C02 (scC02) is widely used because of its convenient critical parameters, low cost, and safety aspects (low toxicity, nonexplosive). 

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). 

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