Joule-Thomson effect, 191 values

The book can be covered in one semester with the following schedule Chapter 1 4, class days Chapter 2 4 days Chapter 3 (omit kinetic theory) 3 days Chapter 5 2 days Chapter 6 (omit heating value and Joule-Thomson effect) 2 days Chapter 8 2 days Chapter 9 2 days Chapter 10 3 days Chapter 11 6 days Chapter 12 (omit differential vaporization) 3 days Chapter 13 3 days Chapter 14 (convergence pressure only) 1 day Chapter 16 2 days Chapter 17 2 days. This schedule leaves several class meetings for examinations or other topics desired by the instructor. 

Although equations (22.10) and (22.11) yield results that are not exact, they are, nevertheless, useful when direct measurements have not been made they provide an indication of the range within which cooling of a given gas by the Joule-Thomson effect is possible. Such,information would be of value in connectioc with the liquefaction of the particular gas. 

Several new p, V, T investigations of benzene have been reported. Thus Belousova and Zaalishvili measured the compressibility at various temperatures between 160 and 230 C, at 3.5 to 9 bar pressure. The compressibility at lower pressures was measured by Hajjar et al. (40 to 200 °C) and Knoebel and Edmister (40 to 100 C). Francis, McGlashan, and Wormald reported values of both B (from low-pressure compressibility measurements, 54 to 150 C) and (B - TdB/dr) (from Joule-Thomson effect measurements, 60 to 130 °C). In an analysis of published second virial coefficients, published values of T d B/dr (from measurements of the pressure coefficient of heat capacity), and their own values of (B - rdB/dT), Francis et al. concluded that at least some of the measurements must be wrong. Re-examination of the situation, with inclusion of recently published values of B, led us to the same conclusion. 

For any gas, the sign of the Joule-Thomson effect depends on tanperature and pressure. The positive effect for each gas is observed only in the limited interval of temperatures and pressures. For each gas there are values of temperature and pressure at which the Joule-Thomson effect is equal to zero (no temperature changes occur at gas expansion in vacuum). These points (T, p,) are called points of inversion. At these points, the influence of forces of attraction is completely compensated for by the influence of repulsion forces consequently the gas temperature does not change. The set of inversion points forms an inversion curve in a p-T diagram. 

Figure 3.27 presents the inversion curve for nitrogen. It can be seen that, to a given value p, two points of inversion can occur. The curve of inversion outlines two points of inversion for which a positive Joule-Thomson effect is observed. Values for the upper and lower inversion points for some gases at various pressures are given in Table 3.2. For the majority of gases, the upper point of inversion lies above room temperature. Hydrogen and helium are an exception. 

Figure 3.6 shows that pj.r. is negative at high temperatures and pressures. Therefore, a gas heats up as it expands under these conditions. At lower temperatures, the gas continues to increase in temperature if the expansion occurs at high pressures. However, at lower pressures, the slope, and hence, Hj.t., becomes positive, and the gas cools upon expansion. Intermediate between these two effects is a pressure and temperature condition where //j.t. = 0. This temperature is known as the Joule-Thomson inversion temperature Tt. Its value depends upon the starting pressure and temperature (and the nature of the gas). The dashed line in Figure 3.6 gives this inversion temperature as a function of the initial pressure. Note that when Joule-Thomson inversion temperatures occur, they occur in pairs at each pressured... 

Equations of state (EOS) offer many rich enhancements to the simple pV = nRT ideal gas law. Obviously, EOS were developed to better calculate p, V, and T, values for real gases. The point here is such equations are excellent vehicles with which to introduce the fact that gases cannot be really treated as point spheres without mutual interactions. Perhaps the best demonstration of the existence of intermolecular forces that can also be quantified is the Joule-Thomson experiment. Too often this experiment is not discussed in the physical chemistry course. It should be. The effect could not exist if intermolecular forces were not real. The practical realization of the effect is the liquefaction of gases, nitrogen and oxygen, especially. 

The Thomson effect is related to the release of extra heat in a conductor (in addition to the well known Joule heat released due to the finite value of the electric conductivity) owing to the combined action of heat and electro conduction. In physical chemistry, classical examples of conjugate processes in nonuniform systems are thermomechanical and mechanocaloric effects. 

The need for high effectiveness heat exchangers in cryocoolers is indicated by the low values of q, which gives the maximum ineffectiveness allowed for the heat exchanger, at which point net refrigeration is eliminated. With pure gases in a Joule-Thomson (JT) cryocooler starting from... 

The Joule-Thomson inversion curve as calculated from (1) is in some doubt. This is apparent at high temperatures where (1) predicts a maximum inversion temperature of 293 °K, whereas a more probable value would be 225 °K. Although the effect of such an uncertainty on the temperature-entropy diagram is expected to be small, further efforts are being made to reconcile this problem. 

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