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Environmental Science & Technology Article per mole of CO2 separated are shown in Table 2 for 10, 20, and 30 °C adsorption temperatures and 20, 40, 60, and 80% RH. (4) (5) 1⎛p⎞ ·ln⎜ amb⎟ compη pump⎜p⎟ W = ·R·T pump ⎝ des ⎠ ⎛ sens ⎜ Δq(des) p,sorb Q=h +H2O·h des des,CO2 Δq(des) des,H2O Δq(des) ⎞ Q =⎜ 1 ·c +c + H2O·c ⎟· ⎝ CO2 (T −T ) p,CO2 Δq(des) CO2 p,H2O⎟ ⎠ des ads Δq (des) Figure 6. Requirement of heat at below 100 °C of the TVS process as a function of the specific CO2 adsorption/desorption capacity, for the measured H2O:CO2 capacity ratios at 20 °C adsorption temperature. ■ CO2 (6) For example, at 20 °C adsorption temperature and 20% RH, the measured amounts of desorbed CO2 and H2O are 0.39 mmol/g and 0.94 mmol/g, respectively, and the corresponding required total heat is 493 kJ/molCO2. On the other hand, increasing the RH to 80% while keeping all other parameters results in an increase in the amount of desorbed CO2 and H2O to 0.65 mmol/g and 4.76 mmol/g, respectively, while the corresponding required heat is increased to 639 kJ/molCO2. The required mechanical work for the vacuum pump is 12.5 kJ/ molCO2 for all cases. Thus, although a higher RH significantly promotes CO2 adsorption and, consequently, reduces the mass of the sorbent material per mole of adsorbed CO2 and the corresponding sensible heat input, the overall heat con- sumption may still increase with RH due to disproportionately higher coadsorption of H2O and the corresponding heat of desorption. Therefore, unless increased water adsorption is explicitly desired despite the associated energy penalty − e.g., when a source of fresh water such as that obtained via reverse osmosis desalination of seawater is unavailable − it will be more favorable to operate the DAC system in air with low RH. In contrast, the effect of the adsorption temperature on the energy consumption is less pronounced. For example, the total required heats for a TVS at 10 and 30 °C adsorption temperatures and 40% RH are 588 kJ/molCO2 and 575 kJ/ molCO2, respectively. These effects have evidently an impact on the economic viability of the DAC process, especially when evaluating the costs of the sorbent material and the energy requirements. Figure 6 shows the requirements of heat at below 100 °C of the TVS process as a function of the specific CO2 adsorption/ desorption capacity varying between 0.5 mmol/g (experimen- tally verified in this study) to 2 mmol/g (targeted for an optimized sorbent). The adsorption temperature is 20 °C. The specific H2O capacity is assumed to be 2.4, 3.9, 5.4, and 7.3 times that of CO2 as measured in this work for 20, 40, 60, and 80% RH, respectively. At a H2O:CO2 capacity ratio of 2.4, the heat requirement decreases from 429 to 272 kJ/molCO2 when the specific CO2 capacity increases by a factor of 4 from 0.5 to 2 mmol/g. At a H2O:CO2 capacity ratio of 7.3, it decreases from 687 to 530 kJ/molCO2. The required mechanical work remains constant at 12.5 kJ/molCO2 for all cases. A higher CO2 capacity beyond 2 mmol/g results in an ever decreasing heat requirement because the contribution of the sensible heat term becomes less significant. On the other hand, the influence of the specific H2O capacity on the heat requirement is much larger, which again justifies its accurate quantification as accomplished in this study. ENVIRONMENTAL IMPLICATIONS 9196 dx.doi.org/10.1021/es301953k | Environ. Sci. Technol. 2012, 46, 9191−9198 The sorbent material examined in this study is based on cellulose as solid support. This minimizes the environmental impact of large-scale manufacturing as it uses a natural, abundant, renewable feedstock. Evidently, the full advantage of such a renewable-based support is achieved provided its adsorption capacity is comparable to that of conventional sorbent materials based on nonrenewable supports. The quantification of adsorbed/desorbed CO2 and H2O allows for a more accurate estimation of the specific energy requirements of the TVS process and, consequently, its viability for future industrial implementation. The separation of CO2 from atmospheric air in combination with its processing to synthetic liquid hydrocarbon fuels with solar energy13−15,22 can significantly contribute toward mitigation of anthropogenic greenhouse gas emissions in the transportation sector. Especially for the case of CO2-neutral aviation fuels, this approach can overcome sustainability limitations of biofuels while avoiding the inherent restrictions associated with other alternative fuels, such as H2, that require major changes in aircraft design and infrastructure. Ultimately, combining DAC that concurrently separates CO2 and H2O with the solar conversion of CO2 and H2O into liquid fuels may reduce energy requirements and costs by avoiding long-range transportation of CO2 and eliminating the use of fresh water resources as well as driving the DAC process with waste heat ■recuperated from the solar process. ASSOCIATED CONTENT *S Supporting Information Material synthesis and characterization; description and schematic of experimental setup; figures; table with assump- tions. This material is available free of charge via the Internet at Corresponding Author *Phone: +41-44-6327929. Fax: +41-44-6321065. E-mail: aldo. steinfeld@ethz.ch. Notes T■he authors declare no competing financial interest. ACKNOWLEDGMENTS This work has been conducted in the framework of a joint project of ETH Zurich, the Swiss Federal Laboratories for h■ AUTHOR INFORMATION ttp://pubs.acs.org.PDF Image | Concurrent Separation of CO2 and H2O from Air PSA
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