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Environmental Science & Technology Article Table 1. Operational Parameters of the CO2 and H2O Adsorption/Desorption Experiments parameter adsorption time (h) 5 desorption time (h) 1 adsorption flow rate (lN/min) 4 adsorption temperature (°C) 10/20/30 adsorption relative humidity (% at adsorption temperature) 20/40/60/80 adsorption CO2 concentration (ppm) 400−510 desorption temperature (°C) 95 desorption pressure (mbar) 50 temperature and RH. All other operational conditions, in particular those during desorption, were fixed. Each adsorption cycle started therefore from the same equilibrium baseline state of the sorbent material to facilitate comparison of the individual cycles with varying adsorption conditions. The adsorption time of 5 h was required to attain sufficient loading of the sorbent material but did not lead to full adsorption equilibrium. For each parameter combination, two adsorption/desorption cycles were performed to ensure reproducibility; the reported results represent the average of both cycles. All experiments use the same sorbent sample. Periodic repetitions of a standard cycle during the parametric study verified that no measurable ■degradation of the sample occurred. RESULTS AND DISCUSSION CO2 and H2O Adsorption/Desorption Capacities. The results of the TVS adsorption/desorption measurements are shown in Figure 1. The specific CO2 and H2O adsorption/ desorption capacities are plotted as a function of the air RH for adsorption temperatures of 10, 20, and 30 °C. The RH was varied between 20 and 80% at 10 and 20 °C, and between 20 and 60% at 30 °C, to prevent condensation in the piping in the latter case. The CO2 adsorption/desorption mass balance is well closed for all parameter combinations, with average deviation <2.5%. The corresponding H2O mass balance exhibits discrepancy for some data points, caused by inaccuracies in the RH measurement during adsorption. Due to the relatively high water content of air compared to its CO2 content, the H2O adsorption measurement is more sensitive to small sensor deviations than the CO2 measurement. Thus, the more accurate desorption values are used in the analysis that follows. At 10 °C adsorption temperature, the CO2 capacity increases from 0.36 mmol/g at 20% RH to 0.63 mmol/g at 80% RH, corresponding to amine efficiencies (moles CO2 adsorbed per moles of amine groups) of 0.09 and 0.16, respectively. At 20 °C adsorption temperature, the CO2 capacity varies from 0.39 mmol/g at 20% RH to 0.65 mmol/g at 80% RH. At 30 °C, it varies from 0.32 mmol/g at 20% RH to 0.50 mmol/g at 60% RH. The amine efficiencies are lower than reported elsewhere, because (i) the adsorption process was stopped after 5 h before attainment of the complete full capacity, and (ii) desorption under TVS conditions, while resulting in a stream of concentrated CO2, is known to yield lower capacities than desorption under inert purge gas, which results in a stream of diluted CO2.33 The purity of the desorbed CO2 for all data points was between 94.4% and 96.7%. The corresponding H2O capacity at 10 °C adsorption temperature increases from 0.87 mmol/g at 20% RH to 4.20 mmol/g at 80% RH. At 20 °C adsorption temperature, the H2O capacity increases from 0.94 Figure 1. Specific CO2 and H2O adsorption/desorption capacities of APDES-NFC-FD in the TVS cyclic process as a function of the air relative humidity for adsorption temperatures of 10 °C (a), 20 °C (b), and 30 °C (c). at 20% RH to 4.76 mmol/g at 80% RH. At 30 °C, it increases from 0.96 at 20% RH to 3.02 mmol/g at 60% RH. For the purpose of comparison with published data, Figure 1b contains in addition the values of the adsorption capacities of 0.82 and 0.94 mmol/g measured for 10 h adsorption at 0 and 40% RH, respectively, after N2 purge desorption. Evidently, the measurements reveal a strong promoting effect of the RH during adsorption on both CO2 and H2O capacities for all investigated adsorption temperatures. The influence is larger on H2O adsorption than on CO2 adsorption. For example, the H2O capacity increases by a factor of 5, while the CO2 capacity increases by a factor of 1.7 when the RH varies from 20% to 80% at 20 °C adsorption temperature. These results are in agreement with the generally known promoting effect of humidity on CO2 adsorption on amine-functionalized solid sorbent materials.31,34 The fact that a higher level of RH leads to a stronger increase of the CO2 capacity has been previously observed.25,36,43 H2O coadsorption of 4.7 and 7.29 mmol/g was observed for 27% and 64% RH, respectively, on amine functionalized mesoporous silica that was regenerated by purging inert gas.25 Our results quantify for the first time the promoting effect of the RH of air on the coadsorption/ 9193 dx.doi.org/10.1021/es301953k | Environ. Sci. Technol. 2012, 46, 9191−9198PDF Image | Concurrent Separation of CO2 and H2O from Air PSA
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