Ideal cascade theory applied to carbon monoxide isotope separation by pressure swing adsorption

摘要

A three-step design methodology was developed for applying ideal cascade theory to separate a dilute binary gas mixture by pressure swing adsorption (PSA). This methodology was tested via simulation and applied to the enrichment of (CO)-C-14 from 1 to 20 ppm in a mixture of its own isotopes. It was assumed that the presence and slight enrichment of (CO)-C-13 did not affect the performance of (CO)-C-14 throughout the cascade. An 8 stage cascade was analyzed, with 3 stages in the stripping section, 5 stages in the enriching section and the feed fed to stage 4. Each stage consisted of a 3-bed 3-step PSA process of varying size, all utilizing feed pressurization/feed, heavy reflux and countercurrent depressurization steps of equal duration, and the same total cycle time. The performance in terms of overall (CO)-C-14 recovery from an ideal cascade of PSA units was compared to that from a single PSA unit operating with the same overall (CO)-C-14 enrichment and same overall throughput as the cascade. The results showed that the cascade of PSA units compared to the single PSA unit exhibited increasingly higher overall recoveries as the overall enrichment decreased; the performance of both systems became identical at an overall enrichment of about 18; and at overall enrichments higher than 18 the single PSA unit exhibited only slightly higher overall recoveries than the cascade of PSA units. The better performing separation process in either case always required more power to operate. In general, this analysis showed how to design an ideal cascade of PSA units for binary isotope separation and that a cascade of PSA units may be more advantageous than a single PSA unit, depending on the desired performance and as long as the capital and operating costs are acceptable.