Critical temperature table

The supercritical fluid carbon dioxide, C02, is of particular interest This compound has a mild (31°C) critical temperature (Table 1) it is nonflammable, nontoxic, and, especially when used to replace freons and certain organic solvents, environmentally friendly. Moreover, it can be obtained from existing industrial processes without further contribution to the greenhouse effect (see Air pollution). Carbon dioxide is fairly miscible with a variety of organic solvents, and is readily recovered after processing owing to its high volatility. It is a small linear molecule and thus diffuses more quickly than... 

API, Technical Data Book, Method 4A1.1, Critical Temperature Table 4A1.1... 

Type I superconductors are usually pure metals with low critical temperatures, Table 6.9. The value depends on the crystal structure, pressure and impurities. In the superconducting stage, Type I super-... 

Tj. is the reduced temperature, T is the critical temperature, is the critical pressure, and is the modified Rackett parameter as given in the supplemental table for pure-component properties. 

Table A2.3.5 Critical temperatures predicted by mean-field theory (MFT) and the quasi-chemical (QC) approximation compared with the exact results.

The most common mobile phase for supercritical fluid chromatography is CO2. Its low critical temperature, 31 °C, and critical pressure, 72.9 atm, are relatively easy to achieve and maintain. Although supercritical CO2 is a good solvent for nonpolar organics, it is less useful for polar solutes. The addition of an organic modifier, such as methanol, improves the mobile phase s elution strength. Other common mobile phases and their critical temperatures and pressures are listed in Table 12.7. 

By combining Eqs. (8.42), (8.49), and (8.60), show that Vi°(52 - 5i) = (l/2)RTj., where T. is the critical temperature for phase separation. For polystyrene with M = 3 X 10, Shultz and Floryf observed T. values of 68 and 84°C, respectively, for cyclohexanone and cyclohexanol. Values of Vi° for these solvents are abut 108 and 106 cm mol", respectively, and 5i values are listed in Table 8.2. Use each of these T. values to form separate estimates of 62 for polystyrene and compare the calculated values with each other and with the value for 62 from Table 8.2. Briefly comment on the agreement or lack thereof for the calculated and accepted 5 s in terms of the assumptions inherent in this method. Criticize or defend the following proposition for systems where use of the above relationship is justified Polymer will be miscible in all proportions in low molecular weight solvents from which they differ in 5 value by about 3 or less. 

Properties. Tetrafluoroethylene (mol wt 100.02) is a colorless, tasteless, odorless, nontoxic gas (Table 1). It is stored as a Hquid vapor pressure at —20° C = 1 MPa (9.9 atm). It is usually polymerized above its critical temperature and below its critical pressure. The polymerization reaction is highly exothermic. 

Properties. VinyHdene fluoride is a colorless, flammable, and nearly odorless gas that boils at —82°C. Physical properties of VDF are shown in Table 1. It is usually polymerized above its critical temperature of 30.1°C and at pressures above 3 MPa (30 atm) the polymerization reaction is highly exothermic. 

Along the saturation line and the critical isobar (22.1 MPa (3205 psi)), the dielectric constant of water declines with temperature (see Fig. 10). In the last 24°C below the critical point, the dielectric constant drops precipitously from 14.49 to 4.77 in the next 5°C, it further declines to 2.53 and by 400°C it has declined to 1.86. In the region of the critical point, the dielectric constant of water becomes similar to the dielectric constants of typical organic solvents (Table 6). The solubiHty of organic materials increases markedly in the region near the critical point, and the solubiHty of salts tends to decline as the temperature increases toward the critical temperature. 

A study on the thermodynamic properties of the three SO phases is given in Reference 30. Table 1 presents a summary of the thermodynamic properties of pure sulfur trioxide. A signiftcandy lower value has been reported for the heat of fusion of y-SO, 24.05 kj /kg (5.75 kcal/kg) (41) than that in Table 1, as have slightly different critical temperature, pressure, and density values (32). 

Properties of Light and Heavy Water. Selected physical properties of light and heavy water are Hsted ia Table 3 (17). Thermodynamic properties are given ia Table 4. The Hquid plus vapor critical-temperature curve for xT) (1 )H2 ) mixtures over the entire concentration range has been reported (28). 

Unpublished data of General Chemicals Division, Allied Chemical Company. Used by permission, c = critical temperature. No material in SI units appears in the 1993 ASHRAE Handbook—Fundamentals (SI ed.). Tables and a chart to 50 ata, 200 C are given by Mathias, H. and H. J. Loffler, Techn. Univ. Berhn rept., 1966 (42 pp.). A chart to 1500 psia, 500 F was given by Mears, W. H., E. Rosenthal, et al.,y, Chem. Eng. Data, 11, 3 (1966) 338-. l43. 

Be is the critical pressure, MPa. Values of Ap from Table 2-383 are summed for each part of the molecule to yield X Ap. Calculation of the Platt number is discussed under Critical Temperature. Errors in average 0.07 MPa and are less reliable for compounds with 12 or more carbon atoms. 

Various methods are available for estimation of the normal boiling point of organic compounds. Lyman et al. review and give calcula-tional procedures for the methods of Meissner, Miller, and Lydersen/ Forman-Thodos. A more recent method that has been determined to be more accurate is the method of Pailhes, which reqmres one experimental vapor pressure point and Lydersen group contributions for critical temperature and critical pressure (Table 2-385). 

As can be seen in the table above, the upper two results for heat transfer coefficients hp between particle and gas are about 10% apart. The lower three results for wall heat transfer coefficients, h in packed beds have a somewhat wider range among themselves. The two groups are not very different if errors internal to the groups are considered. Since the heat transfer area of the particles is many times larger than that at the wall, the critical temperature difference will be at the wall. The significance of this will be shown later in the discussion of thermal sensitivity and stability. 

Table 4.5 Critical temperature and pressure data for common gases ...

TABLE 6.1. Boiling Point, Critical Temperature and Pressure, and Measured... 

The second method can be applied to mixtures as well as pure components. In this method the procedure is to find the final temperature by trial, assuming a final temperature and checking by entropy balance (correct when ASp t, = 0). As reduced conditions are required for reading the tables or charts of generalized thermodynamic properties, the pseudo critical temperature and pressure are used for the mixture. Entropy is computed by the relation. See reference 61 for details. ... 

Table 9.1 lists the critical temperatures of several common substances. The species in the column at the left all have critical temperatures below 25°C. They are often referred to as permanent gases. Applying pressure at room temperature will not condense a permanent... 

Chueh s method for calculating partial molar volumes is readily generalized to liquid mixtures containing more than two components. Required parameters are and flb (see Table II), the acentric factor, the critical temperature and critical pressure for each component, and a characteristic binary constant ktj (see Table I) for each possible unlike pair in the mixture. At present, this method is restricted to saturated liquid solutions for very precise work in high-pressure thermodynamics, it is also necessary to know how partial molar volumes vary with pressure at constant temperature and composition. An extension of Chueh s treatment may eventually provide estimates of partial compressibilities, but in view of the many uncertainties in our present knowledge of high-pressure phase equilibria, such an extension is not likely to be of major importance for some time. 

A gas can be liquefied by applying pressure only if it is below its critical temperature (Fig. 8.14). For example, carbon dioxide can be liquefied by applying pressure only if its temperature is lower than 31°C. According to Table 8.4, the critical temperature of oxygen is —118°C, and so we know that it cannot exist as a liquid at room temperature whatever the pressure. 

Self-Test 8.6A Identify trends in the data in Table 8.4 that indicate the relationship of the strength of London forces to the critical temperature. 

TABLE 8.4 Critical Temperatures and Pressures of Selected Substances... 

Table 6.13 Critical temperature (T ), pressure (Pc), density (Z>c) and molar volume (Vc) for selected substances...

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. 

---