Internal exergy loss

Equation 13.17 explicitly mentions the useful exergy flows coming out of the process because exergy can be lost in two different ways. First, exergy is lost in any real process as a result of irreversibility in the process itself, and such losses are called internal exergy losses. Second, exergy can be lost via waste streams that are not yet at equilibrium with the natural environment. 

Erejea = H2 - H10 - T0 S2 -, S 2 0). We may call Erejea the external exergy loss in contrast to which is called the internal exergy loss. 

The exergy balance consists of internal exergy losses. Irreversible processes may cause the distribution of exergy losses within the volume, and the partition of exergy losses may help in understanding the thermodynamic performance of the system. 

Calculation of Internal Exergy Loss in a Heat Exchanger... 

Thus, we lose 783 kj of useful work for every minute the heat exchanger operates This value represents the internal exergy loss in this process. Said another way, 783 kJ of work is lost because heat is transferred nonreversibly due to the temperature difference. If we could place a Carnot engine between the steam and air, we could generate 783 kj/min of power. 

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. 

In technical combustion processes, this loss of exergy can be as high as 50%. If the combustion process is adiabatic, but at constant volume, then the internal energy stays constant. Usually the exergy losses are somewhat smaller than in the previous case (see Figure 3). 

Local gross exergy loss 8external exergy losses in the used technology for the major product and byproduct, and it can be calculated from the following steady-state exergy balance ... 

Figure 15.3 Exergy analysis results for the process units in the FP-FC system, (a) Internal and external exergy losses (b) rational efficiencies.

In the air separation process (see Fig. 2.3A) liquid high pressure air (c), resulting from the internal compression of oxygen, is expanded via a throttle valve (22) into the pressure column (12). Alternatively this expansion can be performed in a so called dense fluid expander . This is a turbine for the expansion of a liquid or very dense supercritical cold fluid. A turbine expansion produces less exergy loss than a throttle expansion. Owing to this the use of a dense fluid expander reduces the work for gas separation or liquefaction. 

Let us comment on features of heat exchanger HX-207 in the CEA flow sheet. The temperature profile is given in Figure 7. The hot flow is cooled from 850 to 375°C, whereas the cold flow is heated from 305.4 to 403.3°C. The entrance pinch is ca. 450°C, which is thermodynamically not favourable (high loss of exergy). A better use of the available heat could be found inside the section, for example by optimising the internal heat exchanges with pinches of ca. 50°C. 

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