Helium critical temperature Table

Supercritical Fluid. To be useful as a mobile phase in chromatography, a supercritical fluid must have a relatively low critical temperature and pressure, and a relatively high density/solvating power at experimentally accessible pressures and temperatures. The former criterion excludes water and most common organic solvents, whereas the latter excludes such low-boiling substances as helium, hydrogen, and methane. Commonly used fluids are listed in Table I. 

There are a vast number of different sorts of superconducting compound. Examples of these are summarized in Table 7.2. They include organic polymers and intercalation compounds (such as RbjCgjj), as well as ceramic oxides and sulfides. In all these compounds the critical temperature is still below the boiling point of liquid nitrogen. Hence, these materials would be very expensive to use in everyday life (as a litre of liquid helium costs 20 times as much as liquid nitrogen). 

In addition, the critical temperature Tc, the critical pressure pc, the critical density Qc, the triple-point temperature Ttr, and the triple-point pressure ptr are given for some elements. For the element helium, the table also contains data for the A, point, at which liquid helium passes from the normal-fluid phase helium I (above the A point) to the superfluid phase helium II (below the A point), for " He and He. 

The SSTAR reactor is coupled to a supercritical carbon dioxide (S-CO2) Brayton cycle power converter. It provides higher cycle efficiency than a helium ideal gas Brayton cycle or a Rankine saturated steam cycle operating at the same core outlet temperature. A key contributor to the high efficiency is the low amount of work (PdV work) to compress S-CO2 immediately above its critical temperature - due to the high S-CO2 density. Table XXII-6 compares the densities of S-CO2 at cycle conditions versus those for helium in the Eskom Pebble Bed Modular Reactor (PBMR) as well as typical liquid coolants the S-CO2 density is more like that of an ordinary liquid. Thus, the S-CO2 temperature and pressure at the low end of the cycle are designed close to but slightly greater than the critical temperature (30.98°C) and pressure (7.373 MPa) to exploit the small PdV work of compression. 

Table 13.1). In the solid P(CH4) > P(CD4) but the curves cross below the melting point and the vapor pressure IE for the liquids is inverse (Pd > Ph). For water and methane Tc > Tc, but for water Pc > Pc and for methane Pc < Pc- As always, the primes designate the lighter isotopomer. At LV coexistence pliq(D20) < Pliq(H20) at all temperatures (remember the p s are molar, not mass, densities). For methane pliq(CD4) < pLiq(CH4) only at high temperature. At lower temperatures Pliq(CH4) < pliq(CD4). The critical density of H20 is greater than D20, but for methane pc(CH4) < pc(CD4). Isotope effects are large in the hydrogen and helium systems and pLIQ/ < pLiQ and P > P across the liquid range. Pc < Pc and pc < pc for both pairs. Vapor pressure and molar volume IE s are discussed in the context of the statistical theory of isotope effects in condensed phases in Chapters 5 and 12, respectively. The CS treatment in this chapter offers an alternative description.

When we look at the critical states and triple points of other gases, we find the situation shown in table 4.34. The liquid phase exists only when the pressure is between the critical and the triple-point pressures. If we cool down hydrogen, helium or water at room temperature and pressure, we will get liquids before we get solids. But if we cool down CO2 from room temperature and pressure, we get dry ice rather than liquid carbonic to obtain liquid carbon dioxide we have to raise the pressure to at least 5.1 atm to exceed the triple-point pressure. The melting point is not as sensitive to the pressure as the boiling point, which is stated usually for a room pressure of 1 atm, which prevails at sea level on Earth and not in Colorado or the Himalayas. 

The experimental kinetics of accumulation of the Frenkel defects - F centres in alkali-halide crystals at liquid-helium temperatures - was studied in [17] and [40] within the framework of a model that yields a logarithmic dependence of the concentration of defects on the irradiation dose - equation (6) of Table 7.6). Although we criticized this relationship above, at low radiation doses it can be represented as a polynomial in powers of uqVq resembling equations (3) to (5). At the same time cogent arguments exist favoring the... 

At 17.4°, 20.4° and 21.8°K, there appear to be no critical points for the helium-hydrogen mixtures this is indicated by phase equilibrium data calculated at these temperatures at pressures up to 7000 psia. If a critical point were reached, thepylx curves for helium and hydrogen could converge to the same value at the critical point, but there appears to be no tendency toward convergence even at pressures of 7000 psia. Unfortunately, no experimental data exist to verify this calculation. Virial coefficients calculated from the correlation are in good accord with the experimental data of Varekamp and Beenakker [ ], as shown in Table V. 

Several fluids have been used as supercritical solvents Table 11.4 shows the physical properties of several common SFs (95). The most common solvent is carbon dioxide because it has critical values that are easy to obtain (96), is nontoxic, becomes a gas at ambient temperatures and pressures, is inexpensive, and is mutually soluble with many organic compounds. Carbon dioxide can be obtained as a liquid from cylinders equipped with a dip tube. The cylinder headspace is often pressurized with 1500 psi of helium, which conveniently allows the liquid to be transferred to a delivery system. 

The specific heat of water (see Fig. A3.9(b)) (as well as of other fluids, for example, for carbon dioxide, see Fig. A3.18 and Fig. A3.26 for helium) has a maximum value at the critical point. The exact temperature that corresponds to the specific heat peak above the critical pressure is known as the pseudocritical temperature (see also Figs. A3.23 and A3.24, and Table A3.2 for water and carbon dioxide). For water at pressures approximately above 300 MPa and for carbon dioxide at pressures above 30 MPa (see Fig. A3.24), a peak (here, it is better to say a hump ) in specific heat almost disappears therefore, the term such as a pseudocritical point no longer exists. The same applies to the pseudocritical line. 

Helium-4 is by far the more common of the two isotopes. Ordinary helium gas contains about 1.3 x 10 " percent helium-3, so that when we speak of helium or liquid helium, we normally are referring to helium-4 (molecular weight 4.0026). Liquid helium-4 has a normal boiling point of 4.224 K and a density at the normal boiling point of 124.96 kg/m, or about one-eighth that of water. Liquid helium has no solidification point at normal atmospheric pressure. In fact, liquid helium does not solidify under its own vapor pressure even if the temperature is reduced to absolute zero. Saturated liquid helium must be compressed to a pressure of 2.53 MPa before it will solidify. Liquid helium-4 is odorless and colorless and somewhat difficult to see in a container, since its index of refraction is so near that of the gas ( = 1.02 for liquid He). The heat of vaporization of liquid He at the normal boiling point is 20.73 kJ/kg, which is only 1/110 that of water. Table 2.7, prepared by McCarty, presents densities for helium-4 at the critical point, normal boiling point, lower lambda point, and upper lambda point. 

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