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Environmental Science & Technology Article physical sorbents such as zeolites, an increase of the CO2 adsorption capacity was observed under humid conditions compared to dry conditions.7,11,25,33,35 However, substantial amounts of water are coadsorbed from moist gases.25,36 Formation of carbamates and carbamic acid was postulated as the main underlying CO2 adsorption mechanism on amine- modified silica under dry and humid conditions.37−40 Similar mechanisms were concluded for amine-modified cellulose.28 Additional adsorption of H2O presumably occurs through physical adsorption.35 As opposed to flue gases, the molar water content of air is typically 1−2 orders of magnitude higher than its CO2 content. Thus, water adsorption per gram of sorbent material can substantially exceed CO2 adsorption.25,36 This in turn implies significant heat requirements for water desorption during sorbent regeneration. The required heat of water desorption will typically be of the same order of magnitude as the heat of evaporation of the coadsorbed water.41 Corrosion and fouling due to condensed water vapor might also be of concern in an industrial large-scale implementation. Therefore, quantitative data on coadsorption of H2O are important for the design of a DAC process based on a solid sorbent material. Although several proposed DAC concepts are based on amine-functionalized materials, their coadsorption of water during CO2 capture has hardly been quantified.9,11,12 Data on H2O adsorption on an amine-based sorbent was shown for spacecraft air regeneration without CO2 concentration.41 Water adsorption isotherms on amine-grafted pore expanded mesoporous silica gel were measured36,42 but only for single component adsorption. Co-adsorption of CO2 and H2O on amine-functionalized silica was analyzed in column-break- relevant for this study. A detailed description and a schematic of the setup are contained in the Supporting Information. All experiments used dried, technical-grade pressurized air for adsorption obtained by compressing ambient air with a CO2 content varying between 400−510 ppm. By comparing several otherwise equal adsorption/desorption runs with different inlet CO2 concentration, it was verified that this variation has negligible effect on the results (see Figure S2, Supporting Information). This can be explained with the relatively flat shape − far beyond the Henry’s law regime − of the CO2 adsorption isotherms of amine-modified porous adsorbents even at CO2 concentrations of 400 ppm.7 To quantify the amount of H2O desorbed under vacuum conditions, the gas flow exiting the reactor was passed through one channel of a Nafion membrane gas dryer (Perma Pure MD-070-72S) before entering the vacuum pump. Dried pressurized air, controlled by an electronic mass flow controller (Bronkhorst EL-FLOW), was passed through the other channel as a “drying gas” in counter-current flow configuration. The piping between the reactor and the gas dryer was heated to avoid condensation. Nearly all water vapor contained in the desorption gas diffused through the Nafion membrane into the drying gas, as corroborated by blank experiments. This amount of H2O was quantified by measuring the RH of the drying gas at the exit of the channel with an electronic sensor (Vaisala HMP110). For comparison, the CO2 adsorption capacity for 10 h adsorption at 20 °C and 0/40% RH after N2 purge desorption was measured on an experimental setup described elsewhere.33 through experiments, but no concentrated CO was extracted.25 2 H2O2 In a previous paper, we analyzed the TVS process applied to an amine-functionalized solid sorbent material.33 The objective of the present paper is the detailed measurement of coadsorption of CO2 and H2O and the analysis of the effects of varying adsorption temperature and relative humidity (RH) ofair.Forthispurposewehavedesignedanexperimentalsetup that uses a membrane-based gas dryer to selectively extract the water vapor from the desorbed stream under vacuum conditions prior to its condensation, enabling an accurate mass balance. These measurements further enable a more reliable estimate of the energy requirements of the TVS process. ■ EXPERIMENTAL SECTION Synthesis and Characterization. The sorbent material (APDES-NFC-FD) was obtained through a one-pot reaction of an aqueous suspension of nanofibrillated cellulose (NFC) and 3-aminopropylmethyldiethoxysilane (APDES) similar to a procedure described elsewhere.28 Details of the synthesis and characterization are provided in the Supporting Information. The BET surface area of the APDES-NFC-FD sorbent material 2 was 12.2 m /g. Its amine content was 3.86 mmolN/g sorbent. Experimental Setup. The TVS adsorption/desorption cycles were performed on an experimental setup similar to a setup described elsewhere.33 A packed bed of 10 g of sorbent material that was contained in a 45 mm × 45 mm rectangular reactor with an oil-filled jacket for cooling and heating was used for cyclic adsorption and desorption. The bed length of 80 mm was chosen to obtain a low pressure drop because a sharp breakthrough, which would require a larger bed length, was not integrating the breakthrough profiles t=t ṅ·(c −c ) Data Processing. The CO2 and H2O adsorbed during a single TVS cycle Δq(ads) (mmol CO /g sorbent material) and CO2 2 Δq(ads) (mmol H O/g sorbent material) are determined by Δq(ads) =∫ ads air 0,CO2/H2O 1,CO2/H2O dt CO2/H2O t=0 ms (1) wheretadsistheadsorptiontime,nȧiristhemolarflowrateof the air stream, c0,CO2/H2O and c1,CO2/H2O are the CO2/H2O concentrations at the inlet and outlet of the reactor, and ms is the mass of the sorbent material contained in the reactor. The CO desorbed during a single TVS cycle Δq(des) (mmol CO /g) 2 CO22 is determined by integrating over the desorption process Δq(des) = CO2 ∫t=tdes ṅ CO2 dt t=0 m (2) s where tdes is the desorption time, and ṅCO2 is the measured molar flow rate of desorbed CO2. The H2O desorbed during a (des) single TVS cycle ΔqH2O (mmol H2O/g) is determined by integrating over the H2O concentration profile in the drying gas stream at the exit of the membrane gas dryer Δq(des) = H2O ∫t=tdes ṅ·c d d,H2O dt t=0 ms (3) 9192 dx.doi.org/10.1021/es301953k | Environ. Sci. Technol. 2012, 46, 9191−9198 where nḋ is the molar flow rate of the drying gas stream, and cd,H2O is the H2O concentration in the drying gas leaving the gas dryer calculated from its RH, temperature, and pressure. The baseline operational parameters of the adsorption/ desorption experiments are listed in Table 1. These were varied for the purpose of evaluating the effects of adsorptionPDF Image | Concurrent Separation of CO2 and H2O from Air PSA
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