Standard chemical exergy

Exergy is a convenient concept if one wishes to assign a quantitative quality mark to a stream or a product. This quality mark expresses the maximum available work or potential to perform work because of its possible differences in pressure, temperature, and composition with the prevailing environment. The physical exergy, Exphys, only accounts for the differences in pressure and temperature the standard chemical exergy, Ex/hf.rn, accounts for the difference in composition with the environment at the environment s pressure and temperature. Thus... 

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. 

Exergy values for the elements in their stable modification at T0 = 298.15 K and P0 = 101.325 kPa are called standard chemical exergy values Ex. For the calculation of the chemical exergy value of all kinds of substances, the standard chemical exergy values of all elements are required. 

Standard Chemical Exergy Values at P0/ T0 of Various Components Present in Air... 

For the remaining elements, reference compounds have been chosen, as they occur in seawater or in the lithosphere, the earth s crust. An important aspect of this choice has been that the calculated exergy values of most compounds should be positive. Table 7.3 lists the standard chemical exergy values of the elements as presented in Szargut s well-known standard work [1]. Chapter 8 gives an example, the adiabatic combustion of H2, to illustrate the use of these exergy values in an interesting application. 

Table 7.3 is useful for the calculation of the standard chemical exergy values of compounds. We illustrate this for methane and start from its hypothetical formation reaction at standard conditions ... 

In general, we can calculate the standard chemical exergy of a component from the standard chemical exergy of its elements with the equation... 

Standard Chemical Exergy Values of Selected Compounds... 

Suppose we deal with a process in which iron, Fe, has to be used as a reactant, for example, in a reduction reaction. The standard chemical exergy of Fe is 376.4 kj/mol. If we wish to carry out a thermodynamic or exergy analysis of this process, this value is not appropriate. After all, to put the exergy cost of the product, for which Fe was needed as a reactant, in proper perspective, we need to consider all the exergetic costs incurred in order to produce this product all the way from the original natural resources— iron ore and fossil fuel in this example. The production of iron from, for example, the iron ore hematite and coal has a thermodynamic efficiency of about 30% [1], and therefore it is not 376.4 kj/mol Fe that we need to consider... 

We can now compute the exergy of the gaseous Standard Chemical Exergy ... 

Table 7.2 gives the standard chemical exergy of H20 and 02, respectively. Calculate with the help of Equation 7.13 the standard chemical exergy of H2. The standard Gibbs energy of formation for H20 can be found in [1],... 

From the foregoing discussion, it follows that the standard exergy of one of the reactants can be estimated by use of the standard affinity of the reaction, provided that we know the values of the standard exergy of the other reactants and products. The numerical values of the molar exergy thus obtained of various chemical substances in the standard state (temperature T° = 298 K, pressure p° = 101.3 kPa, activity a° = 1) are tabulated as the standard chemical exergy of chemical substances in the literature on engineering thermodynamics [Ref. 9.]. 

In chemical thermodynamics the standard chemical potential ut of a compound i is defined as the molar free enthalpy Ag° for the formation of the compound from its constituent elements j in their stable molecular form in the standard state, and their chemical potential values are set zero in the standard state fit-Ag°f. In exergy engineering the standard molar exergy e° of a compound i is defined as consisting of the molar free enthalpy Ag°f for the formation of the compound in the standard state from its constituent elements and the stoichiometrical sum of the standard chemical exergy values e° of the constituent elements j in their stable state at the standard temperature T° and pressure p° ef- Ag°f + 2 vy e°. 

For substances which are not present in the atmosphere but in the ocean, we can take the reference species of zero exergy level at the most stable state of their existence in seawater. For example, metallic sodium takes its reference level at the state of sodium ions in seawater and the standard chemical exergy e a of metallic sodium is equivalent to the free enthalpy... 

In the case of solid substances the reference species is often set at the most stable solid compounds in lithospheric rocks. For example, metallic iron is most stable in the form of its oxides. The standard chemical exergy of metallic iron can then be obtained from the standard affinity Aaf of the formation of iron oxide, Fe +0.75O2 = 0.5Fe2O3 A° = e e + 0.75s 2 - 0.5 pe2Oj and = 0 hence e°c = A° -0.75e° . Table 10.3 shows the standard molar chemical exergy of a few substances relative to the solid reference species in the lithosphere at the standard temperature and pressure. 

Standard chemical exergy values, in units of kJ/kmol, are based on reference conditions T0 and P(h such as 298.15 K (536.67 R) and 1 atm, respectively, and consist of a set of reference substances with standard concentrations of gaseous, liquid, and solid components. The standard chemical exeigy tables often simplify the application of exeigy principles. 

X ch,i, where Abf is the standard chemical exergy of formation of the resoiuce from elements, and (>ch,i is the standard chemical exergy of the /th element relative to its reference species. Tabulated values of free energy can be used for this purpose. 

The standard chemical exergy of a substance not present in the environment can be calculated by considering a reversible reaction of the substance with other substances for which the standard chemical exergies are known. For energy-conversion processes, calculation of the exergy of fossil fuels is particularly important. 

The physical exergy Eph is equal to the maximum amount of work obtainable when a compound or mixture is brought from its temperature T and pressure P to environmental conditions, characterized by environmental temperature T and pressure Pq. The standard chemical exergy of a pure chemical compound Ech is equal to the maximum amount of work obtainable when a compound is brought from the environmental state, characterized by the environmental temperature To (298.15 K) and environmental pressure Po (1 atm), to the dead state, characterized by the same environmental conditions of temperature and pressure, but also by the concentration of reference substances in a standard environment. 

The chemical exergy is associated with the departure of the chemical composition of a system from that of the environment. For the sake of simplicity, the chemical exergy considered in the analysis is rather a standard chemical exergy that is based on the standard values of the environmental temperature of T, = 298 K and pressure... 

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