Chemical process systems analysis using thermodynamic balance equations with entropy generation. Revaluation and extension

摘要

Modeling inconsistencies exhibited in previous work (O'Connell, 2017) associated with post-combustion methanolamine (MEA) and ammonia (NH3) absorption processes have been revealed. The origin of the problem was that entropies for ionic reactions were not evaluated from input model equilibrium constants regressed from data. Revised calculations have been made using only properties of formation. Positive and consistent entropy generation rates (S˙gen) are now found for all units and sections, and process convergence was achieved for multiple sections rather than for only single sections as before. Only minor changes in material, energy and overallS˙genappeared in the new simulations. Results forS˙genvalues show that the greatest irreversibilities for the MEA process are in the stripping section, though significant effects appear in the chiller, heat exchanger, and stripper flash. For the NH3process, roughly equal and large contributions toS˙genare in the absorption, heat exchange, and stripping sections. Process improvements should focus on these sections. Applying the methodology to proposed changes would quantitatively reveal the amounts of increased efficiency.