Chemical exergy partial pressure

Kotas [3] has drawn a distinction between the environmental state, called the dead state by Haywood [1], in which reactants and products (each at po. To) are in restricted thermal and mechanical equilibrium with the environment and the truly or completely dead state , in which they are also in chemical equilibrium, with partial pressures (/)j) the same as those of the atmosphere. Kotas defines the chemical exergy as the sum of the maximum work obtained from the reaction with components atpo. To, [—AGo], and work extraction and delivery terms. The delivery work term is Yk k kJo ln(fo/pt), where Pii is a partial pressure, and is positive. The extraction work is also Yk kRkTo n(po/Pk) but is negative. 

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 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 ... 

For pure components, the chemical exergy consists of the exergy that can be obtained by diffusing the components to their reference concentration cj0 with a partial pressure of Pi0. For an ideal gas, we obtain... 

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