Critical temperatures of gases

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... 

The absolute values of the solubilities of gases are not at present calculable from any general law, although W. M. Tate (1906) finds in the case of aqueous solutions a relation with the viscosities of the solution (/x ), and water (/x0), the critical temperatures of the gas (T0), and of water (T. ), and the absorption coefficients ... 

Originally it was considered that to obtain a gas in the liquid state the sole necessity was pressure however, all gases possess a physical property known as critical temperature The critical temperature of a gas is that temperature above which the gas cannot be liquefied, however great the pressure to which it is subjected. 

Gases are almost always less dense than liquids because the molecules are so far apart. (Density is the mass in a given volume.) As the pressure on a gas increases, it gets denser because the molecules are squeezed closer together. After a certain point, the molecules are so close together that the gas turns into a liquid. But at a very high temperature, called the critical temperature of a gas, the gas won t turn into liquid no matter how high the pressure. At that point it s called a supercritical fluid. 

Recently, considerable attention has been paid to the use of compressed gases and liquids as solvents for extraction processes (Schneider et al., 1980 Dain-ton and Paul, 1981 Bright and McNally, 1992 Kiran and Brennecke, 1992), although the law of partial pressures indicates that when a gas is in contact with a material of low volatility, the concentration of solute in the gas phase should be minimal and decrease with increased pressure. Nevertheless, deviations from this law occur at temperatures near the critical temperature of the gas, and the concentration of solute in the gas may actually be enhanced as well as increased with pressure. 

The liquid-gas equilibrium line terminates at a point known as the critical point. The temperature and pressure that define the critical point are known as the critical temperature and the critical pressure. For example, nitrous oxide has a critical temperature of 36°C and a critical pressure of 72.45 bar (1051 psi). When the temperature and pressure exceed these critical values, the system becomes a supercritical fluid. Supercritical fluids have the flow properties of gases but densities similar to liquids, and supercritical fluids have no surface tension. Therefore, supercritical fluids are terrific solvents. For example, supercritical carbon dioxide is an excellent solvent for extracting caffeine from coffee without resorting to more toxic organic solvents like dichloromethane. 

The above relationship predicts a monotonic increase in p with increasing temperature as shown in Figure 3 for three different gases. Hence, the maximum amount of gas (by the Gibbs definition) adsorbed on the surface of the adsorbent can be attained at a lower pressure by operating close to the critical temperature of the adsorbed gas. Application of even higher pressures then p will result in a large increase in the... 

For separation of liquefied gases, the critical temperature of the distillate may be lower than the cooling water temperature, and refrigeration is nseded. The economic balance is still primarily between the first favorable and first unfavorable effects, but the refrigeration complicates the analysis. Optimization is required for selecting the best pressure, and can be lengthy and tedious if correctly performed. Shortcuts often lead to nonoptimum conclusions. Each case must be considered on its own merits. An example of such an optimization for an ethylene-ethane separation column, as well as of some optimization pitfalls, is described elsewhere (7). 

Oxygen was not obtained in the liquid state by Faraday m his classical investigations on the liquefaction of gases, because the refrigerating agents used by him did not suffice for the attainment of the critical temperature of the gas, above which it is impossible to effect liquefaction, no matter how great the pressure. 

The critical temperature of oxygen is -118 C and that of hydrogen is -240 C. These gases, therefore, cannot be liquefied at ordinary temperatures. Their critical pressures are 49.7 and 12.8 atm, respectively. 

The isotherm EFGH at 21.5 C shows a similar behaviour except that now the liquefication commences at a higher pressure and the horizontal portion FG, representing decrease in volume, becomes smaller. At still higher temperatures, the horizontal portion of the curve becomes shorter and shorter until at 31.1 C it reduces just to a point (represented by X). At this temperature, therefore, the gas passes into liquid state imperceptibly. Above 31.1 C, the isotherm is continuous. There is no evidence of liquefaction at all. Andrews concluded that if the temperature of carbon dioxide is above 31.1 C, it cannot be- liquefied, no matter how high the pressure may be. He called 31.1 C as the critical temperature of carbon dioxide. Since then, other gases have been... 

When carbon dioxide is heated beyond its critical point, with a critical temperature of tc = 31.0 °C, a critical pressure of pc = 7.38 MPa, and a critical density of Pc = 0.47 g cm , the gaseous and the liquid phase merge into a single supercritical phase (SC-CO2) with particular new physical properties very low surface tension, low viscosity, high diffusion rates, pressure-dependent adjustable density and solvation capability ( solvation power ), and miscibility with many reaction gases (H2, O2, etc.). It can dissolve solids and liquids. The relative permittivity of an sc-fluid varies linearly with density, e.g. for SC-CO2 at 40 °C, r = 1.4 1.6 on going from 108 to 300 bar. This... 

Values in parentheses are the critical temperatures of the gases in degrees Kelvin. 

Simple systems will be considered first. We shall start with (sub-)monolayer adsorption, that is adsorption in which all the adsorbate molecules are in contact with the adsorbent. It is noted that for gases (sub-) monolayer adsorption is met only at low relative pressures for relative pressures approaching unity, invariably condensation in more than one layer takes place. Physical adsorption at temperatures above the critical temperature of the gas is also restricted to a maximum of one complete monolayer. For adsorption from solution (chapter 2) monolayer adsorption is more usually the rule. 

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