Pressure exergy

Under constant pressure, exergy change (AE) in case of raising temperatures from T, to T2 is given as follows ... 

Pressure exergy of cooling water Since cooling water used in the multi-jet condenser is compressed to Pc hy a pump, its pressure exergy must be evaluated. 

TABLE IV Mass Flow Rate, Temperature, Pressure, Exergy Flow Rates, Cost Per Exergy Unit, and Cost Flow Rate for Each Stream in the Base-Case Design of the Cogeneration System... 

Point Temperature (°F) Pressure (psia) Specific Entropy (Mass) Btu/lbm/R Specific Enthalpy (Mass) Btu/lbm Reduced Pressure Exergy (Btu/lbm)... 

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. 

Fig. 2.9 illustrates this approach of tracing exergy through a plant. The various terms in Eq. (2.49) are shown for an irreversible open gas turbine plant based on the JB cycle. The compressor pressure ratio is 12 1, the ratio of maximum to inlet temperature is 5 1 (T,nax = 1450 K with To = 290 K), the compressor and turbine polytropic efficiencies are... 

Fig. 5.10 shows the exergy losses as a fraction of the fuel exergy (including the partial pressure terms referred to in Section 2.4) for the General Electric LM 2500 [CBT]ic plant. 

The first term on the right-hand side of this equation expresses the amount of work available due to differences in pressure and temperature with the environment. The second term, the chemical exergy, expresses the amount of work available due to the differences in composition with respect to the environment. The superscript in Ex, expresses that the chemical exergy is considered at ambient conditions. 

Table 6.2 lists exergy values for methane. It is clear from this table that methane carries an impressive amount of exergy as chemical exergy. Further, the table shows (1) the influence of increased pressure and temperature on the physical exergy and (2) that this latter contribution of exergy is nearly two orders smaller than the chemical contribution. Chemical exergy is the exclusive subject of Chapter 7. 

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... 

Recall that exergy values reflect the extent to which a compound or mixture is out of equilibrium with our environment. Examples are differences in pressure and temperature with the environment. Differences in temperature lead to heat transfer, while differences in pressure lead to mass flow. Chapter 6 shows that the physical exergy represents the maximum amount of work that can be obtained from a system by converting a system s pressure and temperature to those of our environment. 

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 ... 

The conditions of a natural gas reservoir are 30 MPa and 100°C. The gas, assumed to be pure methane, is spontaneously expanded to a pressure of 7MPa (Figure 8.1). Assuming that this expansion is adiabatic, calculate the amount of work that is lost in the process, and express it as a fraction of the originally available amount of work per mole of gas in the reservoir. Carry out this calculation while making a distinction between the physical and chemical exergy of the gas. 

As the table shows, the exergy is not a strong function of temperature or pressure. For the illustrative purposes of our analysis, we use the value of 831.6 kj/mol. 

The extruder is substituted by a deep flash vessel (which operates at 150 mbar), a gear pump, and a static mixer. The only exergy input is the energy requirement of the pump and the compressor, which removes the gas. The pump only needs to increase the pressure of the polymer such that it can... 

By computing the pressure drop in the static mixer and the pressure drop in the granulating head and adding this to the exergy requirements of the compressor, it is possible to compute the exergy input of the alternative scheme, and compare this with the regular extruder (Figure 11.7). 

Examples of such external exergy losses are the release of hot flue gases or high-pressure gas to the atmosphere. Both the internal and the external exergy losses are in principle inefficiencies, and the exergy used efficiently in the process is therefore only the exergy of products and the exergy of waste products, provided they are made useful in other processes. 

The exergy of an ideal gas below and above T0 is positive for pressures above and equal to ambient pressure. But its exergy below ambient pressure is negative at ambient temperature. How should we interpret this ... 

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