Exergy analysis with a flowsheeting simulator .2. Application; Synthesis gas production from natural gas

Several processes, producing synthesis gas from natural gas, have been analyzed by the exergy method, showing exergy analysis as a valuable diagnostic too!. In addition, a generally applicable and systematic way of performing exergy analyses and dealing with their results is illustrated. Exergy calculations have been carried out by user-defined subroutines, which are integrated with a flowsheeting simulator. The method of calculating exergies has been described in part 1 of this article. First, in order to systematically perform exergy analyses, the overall exergy loss for each process is determined. Absolute exergy losses based on final product yield, which is chosen to be methanol, are used for process comparison and diagnosis rather than exergetic efficiencies. Compared to the conventional steam reforming process, giving an overall exergy loss of approximately 8.5 GJ/t methanol, the exergy loss can be reduced to about 4.9 GJ/t methanol by application of the convective reforming option in combination with partial oxidation. Secondly, by considering progressively smaller subsystems within the overall process, locations of major exergy loss are revealed and their potential for improvement can be indicated. Finally, the minus value of the standard Gibbs energy of the overall reaction of each process, denoted as available reaction exergy, is compared to the exergy loss associated with this overall reaction. This comparison demonstrates that available reaction exergies should always be minimized to reduce exergy losses associated with chemical reactions. They cannot however, be eliminated completely when reactions are only thermally coupled. Further improvement can be attained by direct coupling of chemical reactions such that the overall Gibbs energy of reaction is still reasonably negative. This latter conclusion will hopefully result in a reconsideration of the chemical paths along which important chemicals are produced nowadays. Copyright (C) 1996 Elsevier Science Ltd