CO₂ Emission and Energy Assessments of a Novel Pre-Purification Unit for Cryogenic Air Separation Using Supersonic Separator

Abstract:

Thermal power plants with oxy-combustion CO₂ capture are featured by large scale oxygen demand, where cryogenic air separation is most suitable. In such context, a Pre-Purification Unit (PPU) is required, prior to air fractionation, to remove hazardous air contaminants – H₂O, CO₂ and several trace-species – preventing ingress into the Cold Box. The conventional PPU – named FULL-TSA – remove those contaminants by means of Temperature Swing Adsorption (TSA), ordinarily using double-layered bed with activated alumina for adsorbing H₂O and zeolitic molecular sieve for adsorbing CO₂ and further trace-species, which implicates in relatively high demand of low-pressure steam for impurities desorption. A novel pre-purification concept (SS-TSA) embraces a Supersonic Separator (SS) performing the bulk of separation service, abating nearly 98.5% of H₂O, followed by a finishing single-bed molecular sieve (MS) TSA step, which is featured by its relatively small size, for removing CO₂ and remaining impurities. This work presents the energy analysis, as well as the related indirect CO₂ emissions, of such a novel concept (SS-TSA) comprising air compression, cooling, SS dehydration and finishing MS-TSA against the conventional method fully based in TSA purification (FULL-TSA). Process simulation in HYSYS 8.8 assisted technical evaluation and comparison of alternatives, which included the use of two Hysys Unit Operation Extensions – SS-UOE and PEC-UOE – for rigorous thermodynamic SS modeling with phase equilibrium sound speed. SS was designed to impose only 1.4% of head loss, while shrinking TSA service to about 10% of FULL-TSA counterpart, also recovering super-cooled aqueous condensate that reduces water make-up and N₂ consumption for cooling. Changing from FULL-TSA to SS-TSA the average demand of low-pressure steam reduced from 1.37 to 0.16 MW. In terms of electricity demand the difference was quite small, referring to a tiny increase of 0.07 MW in SS-TSA comparatively to total power demand of 14.97 MW in FULL-TSA. Assuming a natural gas combined cycle cogeneration plant matching requirements to air compression and pre-purification process, equivalent reduction in CO₂ indirect emission was 20 kg/h for SS-TSA. These results point superiority of SS-TSA.

• Periodical: Materials Science Forum (Volume 965)
• Pages: 59-67
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.965.59
• Online since: July 2019
• Authors: George Victor Brigagão*, Lara de Oliveira Arinelli, José Luiz de Medeiros, Ofélia de Queiroz Fernandes Araújo
• Keywords: Air Dehydration, Air Pre-Purification, Cryogenic Air Separation, Supersonic Separator

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