Exergetic process efficiency

Hence, the exergetic process efficiency rj in percent, which is interpreted in the sense of a degree of quality, is calculated from the sum of the exergy flows ( in MW) passing the boundary of the system that are assigned to be an effort or benefit... 

Figure 8.4 Change in the exergetic process efficiency of the gasifier and of the gasifier inciuding raw gas cooling, comparing standard and advanced design for Pittsburgh No.

The cell temperature T c is again the temperature T of the process environment. The work wtcc produced by the Carnot cycle CC increases with higher Tpc and the work in/ i crev produced by FC decreases with lower 7jc as already expected. The work wtSyst of the system is independent of 7 if (or nearly independent in the case of the simplified process). The FC operates reversibly in both cases but the Carnot cycle CC does not operate completely reversible in the simplified process caused by the fact that a small part of the waste heat of FC is needed to heat air and fuel. The practical benefit of this combined fuel cell-heat reference cycle is the opportunity for using exergetic efficiencies to describe the operation of real cycles with this very simple model. The needed exergetic efficiency f is defined as... 

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... 

The analysis presented in this chapter is an example of how the principles of thermodynamics can be applied to establish efficiencies in separation units. We have shown how exergy analysis or, equivalently, lost work or availability analysis can be used to pinpoint inefficiencies in a distillation column, which in this case were the temperature-driving forces in the condenser and the reboiler. The data necessary for this analysis can easily be obtained from commonly used flow sheeters, and minimal extra effort is required to compute thermodynamic (exergetic) efficiencies of various process steps. The use of hybrid distillation has the potential to reduce column inefficiencies and reduce the number of trays. We note that for smaller propane-propene separation facilities (less than 5000bbl/day [10]), novel technologies such as adsorption and reactive distillation can be used. 

Exergetic efficiency r ex for hydrogen production by thermo water-splitting at a temperature Ta can be defined as the quotient of the recoverable work (here equal to the free enthalpy of water formation) divided by this quantity plus the sum of the exergy losses D = Ta AS in the process [Eq. (6)]. Thermal efficiency t T from a heat source at temperature T is linked to this efficiency by transformation of heat into work [Eq. (7)]. 

From these brief considerations an exergetic efficiency of the combustion process can be defined ... 

The exergetic efficiency of the total process can then be evaluated with equations (3a) and (7) as a function of the maximum process temperature. 

Figure 13 shows the exergetic efficiency vs. the maximum temperature for different combustion processes with and without intermediate reactions. As a comparison, the adiabatic, isobaric... 

Figure 13. Exergetic efficiency of methane combustion for different processes as a function of maximum process temperature.

The exergetic efficiency of combustion processes with intermediate reactions is theoretically higher than combustion in usual practice. 

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