New Pressure Swing Adsorption Cycles for Carbon Dioxide Sequestration
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New Pressure Swing Adsorption Cycles for Carbon Dioxide Sequestration STEVEN P. REYNOLDS, ARMIN D. EBNER AND JAMES A. RITTER∗ Department of Chemical Engineering, Swearingen Engineering Center, University of South Carolina, Columbia, SC 29208, USA [email protected]
Abstract. A rigorous pressure swing adsorption (PSA) process simulator was used to study a new, high temperature PSA cycle, based on the use of a K -promoted HTlc adsorbent and a simple, 4-step, Skarstrom-type, vacuum swing cycle designed to process a typical stack gas effluent at 575 K containing (in vol%) 15% CO2 , 75% N2 and 10% H2 O. The effects of the purge-to-feed ratio (γ ), cycle step time (ts ) (with all four steps of equal time), and pressure ratio (πT ) on the process performance was studied in terms of the CO2 recovery (R) and enrichment (E) at a constant throughput θ of 14.4 L STP/hr/ kg. R increased with increasing γ and πT and decreasing ts , while E increased with increasing ts and πT and decreasing γ . The highest E of 3.9 was obtained at R = 87% and πT = 12, whereas at R = 100% the highest E of 2.6 was obtained at πT = 12. These results are very encouraging and show the potential of a high temperature PSA cycle for CO2 capture. Keywords: pressure swing adsorption, hydrotalcite, carbon dioxide, sequestration, mathematical modeling Introduction It is now generally accepted by most climate scientists that increasing global temperatures over the last 50 years are the result of increased atmospheric concentrations of greenhouse gases such as methane (CH4 ), nitrous oxide (N2 O) and, most especially, carbon dioxide (CO2 ). Since the beginning of the industrial revolution, atmospheric concentrations of CO2 have increased nearly 30%, CH4 concentrations have more than doubled, and N2 O concentrations have risen by about 15%. These increases have enhanced the heattrapping capability of the earth’s atmosphere via the greenhouse effect. Predictions of global energy use in the next century suggest a continued increase in carbon emissions and rising concentrations of CO2 in the atmosphere unless major changes are made in the way humans produce and use energy, in particular how humans manage carbon (Reichle et al., 1999). There are three courses of action that can be taken to stabilize the CO2 concentration in the atmosphere. The first approach is increased efficiency of primary ∗ To
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energy conversion. This will decrease the amount of fossil fuels needed to provide the same energy service. The second approach is to use lower-carbon or carbonfree energy sources, with the obvious outcomes of less or no CO2 production. The final approach is carbon sequestration, which involves the capture and storage of carbon. This last approach is probably the newest means being studied to manage CO2 in the environment (White et al., 2003). The most likely options for CO2 separation and capture include (1) chemical and physical absorption, (2) physical and chemical adsorption, (3) low-temperature distillation, and (4) gas separ
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