Exergy analysis of industrial processes

Abstract

The usefulness of the exergy concept for the analysis of industrial processes is illustrated in literature in detail. Still the use of exergy analysis meets much resistance in industrial engineering departments. Reasons for this resistance might be the fact that exergy analysis is often presented as the consequence of the second law in thermodynamics and that exergy loss in a process is related to entropy differences between in- and outputs of the process. Both second law and entropy are often not used explicitly in the design of processes. A theoretical approach is outlined leading directly from the basic equations of thermodynamics to the analysis of complicated irreversible processes. The use of steady state processes is fundamental for the analysis. Complicated processes are aggregated into a limited number of irreversible unit operations connected by thermodynamically defined flows. Flows are described as originating from or going to reservoirs. Thermodynamic analysis is performed by just using changes in the reservoirs. The essential aspects are obtained directly in this approach, for example, exergy losses in the unit operations. Exergy efficiencies can be determined only if the proper environmental reference system is used. The major difference between exergy analysis and the usual engineering approach is not with respect to the use of the second law, but with the use of different reference states for calculating values of thermodynamic functions. It is suggested to drop the term second law analysis and to use exergy analysis in stead.