Joule-Thompson inversion

Fig. 8.5. Enthalpy of water as a function of temperature and pressure. Isenthalps are labelled in kcalmop. The Joule-Thompson inversion curve is the locus of conditions where the isenthalps change slope from positive to negative. After Helgeson and Kirkham (1974a). The effect of adding NaCl is from Wood and Spera (1984).

Where they have a positive slope, water cools on adiabatic expansion and warms if adiabatically compressed, and the two regions are separated by the Joule-Thompson inversion curve. Much the same information is contained in the enthalpy-pressure diagram (Figure 8.6), where it can be seen that constant enthalpy changes in pressure lead to increases in temperature in one region and decreases in another. The effect of dissolved NaCl on the Joule-Thompson coefficient has been calculated by Wood and Spera (1984), and the effect will be similar for other electrolytes. Because the addition of most electrolytes to water results in a decrease in V and in a, fijT is smaller, and the net effect is to move the inversion curve to higher temperatures, as shown in Figure 8.5. 

Figure 6.4 Isotherms on logP versus enthalpy in kJ moh for water. Dashed contours are the Joule-Thompson coefficient, The contour for /ajj = 0 is the Joule-Thompson inversion curve. Data from the program steam of Harvey et al. (2000).

The effect of dissolved NaCl on the Joule-Thompson coefficient has been calculated by Wood and Spera (1984), and the effect will be similar for other electrolytes. Because the addition of most electrolytes to water results in a decrease in V and in a, is smaller, and the net effect is to move the inversion curve to higher temperatures and lower pressures. 

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