EXERGY ANALYSIS WITH A FLOWSHEETING SIMULATOR .1. THEORY - CALCULATING EXERGIES OF MATERIAL STREAMS

This paper presents a method for calculating the absolute exergy of mu lticomponent liquid, vapour or two-phase flows. The method is differen t from those described by most authors in that it enables a clear divi sion of the total exergy of a material stream into three terms such th at the exergy change of mixing is calculated separately from the chemi cal and the physical exergy. The method has been implemented in a set of external subroutines which have been integrated with a flowsheeting simulator, in order to calculate exergies of material streams along w ith the traditional energy- and mass-balance calculations. Except for an external database containing values of standard chemical exergies, the exergy calculations require no additional input data. Exergies are calculated as an extensive stream property and are therefore accessib le to various procedures supported by the flowsheeting simulator, e.g. optimization. Furthermore, this also enables the determination of exe rgy losses by simple accounting. A complete exergy analysis, of course , must also incorporate the interpretation of these exergy losses and a suggestion or indication of ways to reduce them. These will be discu ssed in part 2 of this article with the case of synthesis gas producti on from natural gas, showing the exergy method as a useful diagnostic tool for analyzing chemical processes. Copyright (C) 1996 Elsevier Sci ence Ltd