Reference Components from Air

In our environment, there are many substances that, like oxygen in our atmosphere, cannot further diffuse and/or react toward more stable configurations and may be considered to be in equilibrium with the environment. Neither chemical nor nuclear reactions can transform these components into even more stable compounds. From these components, we cannot extract any useful work, and therefore an exergy value of OkJ/mol has been assigned to them. This has been done for the usual constituents of air N2,02/ C02/ H20, DzO, Ar, He, Ne, Kr, and Xe at T0 = 298.15 K and P0 = 99.31 kPa, the average atmospheric pressure [1]. Their partial pressures P in air are given in Table 7.1. 

From these data, we can calculate the chemical, exergy values of these components in the pure state at P0 and T0. Air at these conditions can, to a good approximation, be considered as an ideal gas therefore, separation 

As discussed in Chapter 2, the change in entropy associated with taking 1 mol of an ideal gas isothermally from pressure Pt to a pressure P2 is given by 

From this equation, we can show [2] that the standard chemical exergy at P0 and T0 of a pure component can be calculated from its partial pressure Pt in air with Equation 7.6  

The standard chemical exergy values for the main constituents of air as listed in Table 7.1 are given in Table 7.2. 

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