A cyclic adsorption process simulator was used to study novel high-temperature pressure swing adsorption (PSA) cycles. Based on the use of a K-promoted hydrotalcite-like (HTlc) adsorbent and six different (vacuum swing) stripping PSA cycles, these cycles were designed to process a typical stack gas effluent at 575 K containing 15 vol % CO 2 , 75 vol % N 2 , and 10 vol % H 2 O into CO 2 -depleted and CO 2 -enriched streams. More than a thousand (1260) simulations were conducted to the periodic state to study and interpret the effects of the light-product purge-to-feed ratio, the cycle step time, the high-to-low pressure ratio, the heavyproduct recycle ratio, and the feed throughput (θ) on the process performance. The cycle configuration was changed from 4-bed 4-step, 4-bed 5-step, and 5-bed 5-step designs that utilized combinations of light-reflux (LR) and/or heavy-reflux (HR) steps, and cocurrent depressurization (CoD) and/or countercurrent depressurization (CnD) steps. The process performance was judged in terms of the CO 2 purity in the heavy product (y CO 2 ,HP ), with the CO 2 recovery (R CO 2 ) and θ both being secondary process performance indicators. Any PSA process with a HR step outperformed any PSA process with only a LR step, regardless of whether a CoD step was added or not. The best performance was obtained from the 4-bed 4-step stripping PSA cycle with HR obtained from the CnD step, with y CO 2 ,HP ) 82.7 vol %, R CO 2 ) 17.4%, and θ ) 14.4 L STP h -1 kg -1 . The next best performance was obtained from the 5-bed 5-step stripping PSA cycle with LR and HR obtained from LR purge, with y CO 2 ,HP ) 75.5 vol %, R CO 2 ) 48.8%, and θ ) 23.1 L STP h -1 kg -1 . Overall, this study further substantiated the feasibility of a high-temperature stripping PSA cycle for CO 2 concentration from flue gas using an HTlc adsorbent. It also disclosed the importance of the PSA cycle configuration to the process performance, by gaining an understanding of and appreciation for the use of HR, and it exposed the rigor involved in determining the best PSA cycle sequence for a given application.

A nonequilibrium kinetic model was developed to describe the reversible adsorption and desorption behavior of CO 2 in a K-promoted hydrotalcite-like compound (HTlc). The model consisted of three reversible reactions. Two of the reactions were of the Langmuir-Hinshelwood type with slow and intermediate kinetics, and one was a mass-transfer-limited chemisorption process with very fast kinetics. To calibrate and test this model, a K-promoted HTlc was synthesized and studied to determine its dynamic behavior during CO 2 adsorption and desorption cycles carried out at 400 °C. A long cycle time adsorption (700 min) and desorption (700 min) experiment was carried out with a sample activated at 400 °C for 12 h in helium. With this experiment approaching equilibrium at the end of each step, it proved that the adsorption and desorption behavior of CO 2 in K-promoted HTlc was completely reversible. Then, the effect of the half-cycle time (15, 30, 45, 60, and 75 min) was studied with samples activated for 12 h in helium at 400 °C and cycled four times each, and the effect of the activation time (8, 12, 16, and 20 h) was studied with samples cycled twice with a 45-min half-cycle time. The former set of experiments proved that periodic behavior was achieved very quickly with cycling even when far removed from an equilibrium state; the latter set proved that the CO 2 working capacity was independent of the activation time. The model was fitted successfully to the long cycle time experiment. It then predicted successfully the dynamic and cyclic behavior of both the much shorter cycle time and different activation time experiments. This kinetic model accurately simulated the reversible adsorption and desorption behavior of the very fast, intermediate, and slow kinetic processes; the approach to periodic behavior during cycling; and the independence between the CO 2 working capacity and activation time. It also proved that the adsorption and desorption behavior was due to a combination of completely reversible adsorption, diffusion, and reaction phenomena.

A K-promoted hydrotalcite-like compound (HTlc) was synthesized and tested for its reversible CO 2 capacity between 250 °C and 500 °C. Nonequilibrium dynamic adsorption and desorption isotherms were measured between 65 Torr and 980 Torr, using steps of 20 or 50 Torr and a 45-min duration between steps. The absolute CO 2 capacity on K-promoted HTlc increased as the temperature decreased, with CO 2 loadings of 2.25 and 1.02 mol/kg, respectively, at 250 and 500 °C and 980 Torr. The CO 2 working capacity, which is defined as the change in CO 2 loading between 65 Torr and 980 Torr, exhibited a maximum at 450 °C, with a value of 0.55 mol/kg, compared to values of 0.11 and 0.46 mol/kg, which were observed at 250 and 500 °C, respectively. It was surmised that three temperature-dependent, highly coupled, completely reversible, equilibrium-driven but kinetically limited, reactions were taking place, with the first one being a rapid and reversible chemisorption of CO 2 that initiated and participated in the entire process.

This study evaluated nine stripping PSA cycle configurations, all with a heavy reflux (HR) step, some with a light reflux (LR) step, and some with a recovery (REC) or feed plus recycle (F+R) step, for concentrating CO 2 from stack and flue gas at high temperature (575 K) using a K-promoted HTlc. Under the process conditions studied, the addition of the LR step always resulted in a better process performance; and in all cases, the addition of a REC or F+R step surprisingly did not affect the process performance except at low feed throughputs, where either cycle step resulted in a similar diminished performance. The best cycle based on overall performance was a 5-bed 5-step stripping PSA cycle with LR and HR from countercurrent depressurization (CnD) (98.7% CO 2 purity, 98.7% CO 2 recovery and 5.8 L STP/hr/kg feed throughput). The next best cycle was a 5-bed 5-step stripping PSA cycle with LR and HR from LR purge (96.5% CO 2 purity, 71.1% CO 2 recovery and 57.6 L STP/hr/kg feed throughput). These improved performances were caused mainly by the use of a very small purge to feed ratio (γ = 0.02) for the former cycle and a larger one (γ = 0.50) for the latter cycle. The former cycle was good for producing CO 2 at high purities and recoveries but at lower feed throughputs, and the latter cycle was useful for obtaining CO 2 at high purities and feed throughputs but at lower recoveries. The best performance of a 4-bed 4-step stripping PSA cycle with HR from CnD was disappointing because of low CO 2 recoveries (99.2% CO 2 purity, 15.2% CO 2 recovery and 72.0 L STP/hr/kg feed throughput). This last result revealed that the recoveries of this cycle

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% CO 2 , 75% N 2 and 10% H 2 O. The effects of the purge-to-feed ratio (γ ), cycle step time (t s ) (with all four steps of equal time), and pressure ratio (π T ) on the process performance was studied in terms of the CO 2 recovery (R) and enrichment (E) at a constant throughput θ of 14.4 L STP/hr/ kg. R increased with increasing γ and π T and decreasing t s , while E increased with increasing t s and π T and decreasing γ . The highest E of 3.9 was obtained at R = 87% and π T = 12, whereas at R = 100% the highest E of 2.6 was obtained at π T = 12. These results are very encouraging and show the potential of a high temperature PSA cycle for CO 2 capture.