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pubs.acs.org/est Concurrent Separation of CO2 and H2O from Air by a Temperature- Vacuum Swing Adsorption/Desorption Cycle Jan Andre Wurzbacher,†,‡ Christoph Gebald,†,‡,§ Nicolas Piatkowski,† and Aldo Steinfeld*,†,∥ †Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland ‡Climeworks Ltd., 8005 Zurich, Switzerland §EMPA Material Science and Technology, 8600 Dübendorf, Switzerland ∥Solar Technology Laboratory, Paul Scherrer Institute, 5232 Villigen, Switzerland *S Supporting Information ■ INTRODUCTION coextracted in a DAC process can thus become a valuable byproduct. Note that, if seawater is accessible, fresh water 2 extraction via reverse osmosis desalination23 is about 2 orders Among several strategies to mitigate anthropogenic CO emissions, capturing CO2 directly from ambient air − usually referred to as direct air capture (DAC) − has recently attracted increasing interest.1−12 The advantage of DAC is its ability to address present and past emissions from distributed and mobile sources, e.g., derived from the transportation sector. Fur- thermore, DAC systems need not be attached to the source of emission but can be logistically centralized and located next to the site of CO2 storage/processing or of vast renewable (e.g., solar) energy resources. In particular, CO2 extracted from the atmosphere can be processed to synthetic liquid hydrocarbon fuels using renewable energy in a closed material cycle.13−15 Thereby, DAC uniquely offers the possibility of a truly sustainable liquid fuel-based energy future.16,17 While some studies claim that DAC can potentially become competitive5,18 and others question its economic feasibility,19−21 it is evident that additional R&D on the fundamentals of DAC is required to reliably assess its ultimate industrial-scale applicability. If, additionally, H2O is coextracted from ambient air, major logistical benefits can be achieved in the production of synthetic liquid hydrocarbon fuels using concentrated solar en- ergy.13−15,22 Solar fuel production plants will be located in deserted regions of the earth’s sunbelt with vast direct solar irradiation but limited or no fresh water resources. Water of magnitude more energy efficient than water extraction from air via adsorption. Solid amine-functionalized materials have been identified as promising sorbents for DAC, as they offer relatively high specific CO2 capacities and uptake rates under extremely low CO2 partial pressures, such as in the case of ambient air.7−10,12,24−30 The vast majority of previous studies on these materials focused on maximizing their CO2 adsorption capacity, while sorbent regeneration was usually achieved by purging with an inert gas, yielding − again − highly diluted CO2. Desorption of concentrated, high-purity CO2 is evidently crucial for downstream applications, yet this issue remained mostly disregarded.31 A few studies applied steam stripping,32 moisture swing,6 or temperature-vacuum swing (TVS)11,33 processes to obtain concentrated CO2 from the air. Another intriguing advantage of amine-functionalized solid sorbents is their tolerance to air moisture.12,31,34 In contrast to Article ABSTRACT: A temperature-vacuum swing (TVS) cyclic process is applied to an amine-functionalized nanofibrilated cellulose sorbent to concurrently extract CO2 and water vapor from ambient air. The promoting effect of the relative humidity on the CO2 capture capacity and on the amount of coadsorbed water is quantified. The measured specific CO2 capacities range from 0.32 to 0.65 mmol/g, and the corresponding specific H2O capacities range from 0.87 to 4.76 mmol/g for adsorption temperatures varying between 10 and 30 °C and relative humidities varying between 20 and 80%. Desorption of CO2 is achieved at 95 °C and 50 mbarabs without dilution by a purge gas, yielding a purity exceeding 94.4%. Sorbent stability and a closed mass balance for both H2O and CO2 are demonstrated for ten consecutive adsorption−desorption cycles. The specific energy requirements of the TVS process based on the measured H2O and CO2 capacities are estimated to be 12.5 kJ/molCO2 of mechanical (pumping) work and between 493 and 640 kJ/molCO2 of heat at below 100 °C, depending on the air relative humidity. For a targeted CO2 capacity of 2 mmol/g, the heat requirement would be reduced to between 272 and 530 kJ/molCO2, depending strongly on the amount of coadsorbed water. © 2012 American Chemical Society 9191 dx.doi.org/10.1021/es301953k | Environ. Sci. Technol. 2012, 46, 9191−9198 May 21, 2012 July 22, 2012 July 23, 2012 Received: Revised: Accepted: Published: July 23, 2012PDF Image | Concurrent Separation of CO2 and H2O from Air PSA
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