Low-pressure supersonic separator with finishing adsorption: Higher exergy efficiency in air pre-purification for cryogenic fractionation

Air fractionation in cryogenic plants demands a Pre-Purification Unit for air compression and removal of water, carbon dioxide and trace-species. The conventional Pre-Purification Unit - FULL-TSA - adopts temperatures-wing adsorption (TSA) with an activated-alumina bed for dehumidification followed by a molecular-sieve bed removing carbon dioxide and trace-species. A novel scheme - SS-TSA - prescribes a Supersonic Separator abating similar to 98.5% of moisture followed by a small single-bed molecular-sieve removing remaining impurities. SS-TSA-HI is a SS-TSA variant allocating compression heat for molecular-sieve regeneration. This work compares thermodynamic performances of FULL-TSA, SS-TSA and SS-TSA-HI via exergy analyses and discloses better utilization of resources. Exergy analyses are supported by HYSYS simulator running with unit-operation extensions for calculating the phase-equilibrium sound speed and simulating the supersonic separator. Since exergy rates depend on Reference Environmental Reservoirs, two Reference formalisms are employed at 1 atm and 25 degrees C. Reference#1 prescribes air with 60% relative humidity (the raw air feed), while Reference#2 adopts water-saturated air. It is shown that Reference#1 is appropriate for overall system analysis, while Reference#2 is better for sub-systems analysis as it dramatically lowers cooling-water exergy flows. SS-TSA has 61% less exergy losses than FULL-TSA in the purification step as the supersonic separator accomplishes bulk dehumidification drastically reducing steam and nitrogen for bed regeneration. Reference#1 exergy efficiencies of FULL-TSA, SS-TSA and SS-TSA-HI attained 57.9%, 60.0%, and 60.3%, respectively.