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Environmental Science & Technology Article desorption capacities of CO2 and H2O for the TVS process extracting pure CO2 from air. While the detailed mechanisms of binary adsorption of CO2 and H2O are not yet fully understood, hydrogen bonding with the surface functional groups and multilayer adsorption have been proposed as general mechanisms for the coadsorption of H2O and CO2 on porous sorbent materials.44 On cellulose fibrils, H2O molecules are assumed to mainly adsorb in multilayers on the cellulose hydroxyl surface groups.45 Besides the chemical interaction of H2O and CO2 with the surface functionalities, the smaller van der Waals diameter of H2O molecules compared to that of CO2 molecules44 can be one reason for the observed H2O capacities largely exceeding the corresponding CO2 capacities. Moreover, multilayer adsorption of H2O is presumably responsible for the stronger increase of theH2OcapacitywithRHvis-a-̀visthatoftheCO2capacity. The TVS adsorption/desorption CO2 capacities achieved in this study for an amine-functionalized nanofibrilated cellulose sorbent are about twice higher than those obtained previously for an amine-functionalized silica gel sorbent,33 while adsorption time was only 5 h instead of 24 h. This is associated with the higher amine content of the APDES-NFC-FD sorbent used in this study and its more favorable adsorption kinetics.28 In another TVS study, a CO2 capacity of 0.13 mmol/g under moist conditions was reported for an amine-grafted silica sorbent.11 From Figure 1 and Figure S3 in the Supporting Information, which shows the capacities as a function of the adsorption temperature, it can be seen that the influence of the adsorption temperature on the CO2/H2O capacities at fixed RH is minor. For example, the CO2 capacities for adsorption temperatures of 10, 20, and 30 °C at a fixed 40% RH are 0.42, 0.44, and 0.38 mmol/g, respectively. The corresponding H2O capacities are 1.67, 1.69, and 1.74 mmol/g, respectively. Since the RH at different adsorption temperatures does not reflect the absolute water content of the air, the measured capacities are also plotted as a function of the partial pressure of H2O in Figure 2. For clarity, only the desorption values are shown. It becomes evident that, at constant H2O partial pressure, both CO2 and H2O capacities strongly decrease with increasing adsorption temperature. For example, at a partial H2O pressure of about 9 mbar, the H2O capacities at 10, 20, and 30 °C are 4.20, 1.69, and 0.96 mmol/g, respectively. The corresponding CO2 capacities are 0.63, 0.44, and 0.32 mmol/g, respectively. This coherence is a direct consequence of the relation between the three variables H2O partial pressure, temperature, and RH, of which only two are independent and the respective third one can be calculated from the corresponding vapor pressure of H2O. Therefore, describing the H2O/CO2 capacities as a function of the relative humidity turns out to be most convenient, since this representation is nearly independent of the adsorption temperature in the temperature range considered in this study. This is in agreement with other studies in which water adsorption was observed to be only marginally influenced by temperature at constant RH.46,47 In fact, both the strong temperature dependence of the adsorption capacities at constant H2O partial pressure (Figure 2) and the negligible dependence at constant RH are predicted from thermodynamic principles. According to the Clausius−Clapeyron equation, the temperature dependence of the adsorption capacity scales with the isosteric heat of adsorption. As shown elsewhere,48 when the H2O capacity is expressed as a function of H2O partial Figure 2. Specific CO2 (a) and H2O (b) desorption capacities in a TVS cyclic process as a function of the H2O partial pressure for adsorption temperatures of 10, 20, and 30 °C. pressure, the corresponding isosteric heat of adsorption to be used for the evaluation of the Clausius−Clapeyron equation is the total heat of adsorption, i.e. the sum of the “net” heat of adsorption and the latent heat of evaporation. On the other hand, when the H2O capacity is expressed as a function of RH, the Clausius−Clapeyron equation needs to be evaluated by using the “net” heat of adsorption only. The latter is relatively small, since the heat of adsorption for amine modified sorbent materials is known to approach the value of the heat of H2O evaporation.41 While thermodynamics predict a marginal decrease of the CO2/H2O capacities with increasing temperature, the observed minimal increase for several data points (Figure S3, Supporting Information) is attributed to − besides measurement uncertainty − the nonideal behavior of the binary CO2/H2O system.44 For the CO2 capacities, further overlapping of equilibrium and kinetic effects is likely. To compare the rates of CO2 and H2O adsorption, the breakthrough curves at 20 °C adsorption temperature and 40% RH are shown in Figure 3. Breakthrough of water occurs much faster than that of CO2, which is consistent with previous results obtained with diluted CO2 streams.25 After approx- Figure 3. CO2 and H2O adsorption breakthrough curves at 20 °C adsorption temperature and 40% RH. 9194 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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