A new concept of air pre-purification unit for cryogenic separation: Low-pressure supersonic separator coupled to finishing adsorption

Abstract

In commercial cryogenic manufacturing of oxygen, air to the Cold-Box must pass through a Pre-Purification Unit (PPU) to remove water, CO2 and other impurities. The conventional PPU – FULL-TSA – comprises compression, cooling pre-dehydration and temperature-swing adsorption (TSA) for dehydration and CO2 removal, supplying treated air at 3.1 bar. This work discloses a new PPU concept – SS-TSA – prescribing a supersonic separator (SS) upstream to TSA handling 98.5% of dehydration, greatly lowering TSA costs. SS-TSA comprises compression, cooling pre-dehydration, SS dehydration and a smaller TSA for finishing dehydration and CO2 removal. A SS-TSA variant – TSA-HI – additionally recovers compression heat lowering heating costs. SS-TSA, FULL-TSA and SS-SS-TSA-HI were analyzed. Flowsheets were simulated in HYSYS with full thermodynamic SS modeling via a new HYSYS Unit Operation Extension – SS-UOE – rigorously calculating the multiphase sound speed. SS was designed for only 3.5% of head-loss, recovering 98.5% of water as super-cooled liquid, lowering make-up and chilled-water costs, while shrinking the TSA service to 10% of the FULL-TSA counterpart. For commercial-scale PPU considering 20 years of operation at 10% interest rate, the purified 3.1 bar air breakeven prices reached 5.28, 5.19, and 5.18 US$/kNm3, respectively for FULL-TSA, SS-TSA and SS-TSA-HI, establishing superiority of SS alternatives over the conventional FULL-TSA.