Binary Adsorption Equilibrium of Carbon Dioxide and Water Vapor on Zeolite HY

Introduction An understanding of how CO2 and H2O interact and affect each other’s behavior on adsorbent surfaces is essential for both theoretical study and industrial processes especially CO2 capture from wet flue gas streams. There are few reported studies on adsorption equilibrium for such a binary system. Rege and Yang [1] measured very low concentration of CO2/H2O vapor (<4.1% relative humidity) mixture adsorption on 13X zeolite and γ-alumina by means of FTIR spectroscopy and observed a likely enhancement of CO2 when the concentration of CO2 <300 ppm (parts per million). However, some of the adsorption processes deal with water vapor of medium or high relatively humidity . To the best of our knowledge, adsorption equilibria of CO2/H2O binary mixture with wide-range of H2O relative humidity are not available in the literature. In this work, single component and binary adsorption equilibria of CO2 and H2O on zeolite HY over a range of Si/Al ratios are measured and reported. The interaction of H2O with CO2 on the adsorbent surface and the effects of hydrophobicity and cation density were also studied. Experimental Zeolite HY CBV760 (Si/Al = 60) and HY HSZ-320 HOA (Si/Al = 5) were selected as the test adsorbents. CO2 adsorption isotherms on the above adsorbents were obtained by an ASAP 2010 Gas Adsorption Analyzer (Micromeritics, USA) at different temperatures over the pressure range of 0–118 kPa. Single H2O and H2O/CO2 binary isotherms were measured on a custom-built breakthrough apparatus named BIAU (Binary Isothermal Adsorption Unit) . The measurement of the binary isotherms was conducted by increasing the H2O partial pressure stepwise while keeping the total pressure constant. Results and Discussion The adsorption of CO2 on HY CBV760 can be easily and accurately described by the single-site Langmuir equation (Fig. 1a), and the isotherm is nearly linear under the experimental conditions, suggesting that the adsorption sites on the framework of CBV760 with very low charge density are quite homogeneous. For CO2 adsorption on HY HSZ-320 HOA (Fig. 1b) however, the dual-site Langmuir equation was used rather than the single-site one due to the increase of surface heterogeneity caused by higher density of surface cations (H) and cations which form strong bonds with CO2 molecules by ion-quadrupole interaction. The steep uptake of CO2 at low partial pressure indicates that there is a relatively strong interaction between CO2 molecules and the H on HY HSZ-320 HOA which is further supported by the association energy Q derived by Langmuir isotherms (Fig. 1a and 1b) of CO2 on HY increasing with the number of H in the HY framework. As shown in Fig. 2a, the adsorption of CO2 on HY CBV760 is slightly enhanced in the presence of low concentration of H2O, compared with pure CO2. The reason could possibly be the cooperative adsorption effect induced by forming (H2O)n-CO2 complexes where n = 1–3 [4, . It is known that for high Si/Al ratio HY with low charge density frameworks, there is no strong preference for water molecules to condense in the pores of this material. However, H2O has a larger dipole-moment than CO2 does and that makes the adsorption of H2O lower in competition with CO2 onto a hydrophobic CBV760 framework, and it follows the observation of depressed H2O loading in the presence of CO2 at low H2O humidity. At higher water humidity, CO2 loading is reduced, because the accessible pore volume for CO2 drastically decreases when the zeolite pores are filled up by H2O. In the case of HY HSZ-320 HOA (Fig. 2b), the adsorption of CO2 was reduced exponentially with the increase of H2O partial pressure, mainly because the adsorption of H2O shields the cations and the more hydrophilic nature of HY HSZ-320 HOA preferentially adsorbs H2O. 0 20 40 60 80 100 120 0.0 0.5 1.0 1.5 2.0 2.5 273.15 K 313.15 K 333.15 K Single-site Langmuir A ds or be d A m ou nt (m m ol /g ) CO2 Partial Pressure (kPa) (a) 0 20 40 60 80 100 120 0 1 2 3 4 5 A ds or be d A m ou nt (m m ol /g ) CO2 Partial Pressure 273.15 K 303.15 K 333.15 K Dual-site Langmuir (b) Figure 1. Adsorption isotherm of CO2 at three temperatures on (a) HY CBV760, where the parameters of the single-site Langmuir are: M = 9.67 mmol/g, b = 3.56E-7 kPa and Q = 20.524 kJ/mol; and on (b) HY HSZ-320 HOA, where the parameters of the dual-site Langmuir are: M1 = 0.615 mmol/g, b1 = 4.04E-9 kPa, Q1 = 44.222 kJ/mol, M2 = 6.733 mmol/g, b2 = 4.036E-7 kPa and Q2 = 23.588 kJ/mol. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 5 10 15 20 H2O in binary CO2 in binary H2O in pure CO2 in pure A ds or be d A m ou nt (m m ol /g ) H2O Partial Pressure (kPa) (a) 0.0 0.5 1.0 1.5 2.0 2.5 0 2 4 6 8 10 12 14 H2O in binary CO2 in binary CO2 in pure A m ou nt A ds or be d (m m ol /g ) H2O Partial Pressure (kPa) (b) Figure 2. Adsorption isotherm of H2O/CO2 binary mixture on (a) HY CBV760; and on (b) HY HSZ-320 HOA at 303.15 K with constant total pressure of 105.525 kPa where open symbols denote pure component adsorption equilibrium at corresponding partial pressure for comparison purpose. ConclusionBinary adsorption of H2O/CO2 on zeolite HY with different Si/Al ratios showed that CO2 adsorption onthe cations was largely depressed in the presence ofH2O whereas the CO2 adsorption on high Si/Alframework is enhanced by the cooperative coadsorption ofH2O. The interactions of H2O and CO2 onHY surface and the effect of hydrophobicity await more study by isotherm modeling andthermodynamic analysis and interpretation. References1. S.U. Rege and R.T. Yang, A noval FTIR method for studying mixed gas adsorption at low concentrations: H2Oand CO2 on NaX zeolite and γ-alumina. Chemical Engineering Science 56 (2001) 3781-3796.2. G. Li, P. Xiao, P.A. Webley, J. Zhang, R. Singh, M. Marshall, Capture of CO2 from high humidity flue gas byvacuum swing adsorption with zeolite 13X. Adsorption 14 (2008) 415-422.3. G. Li, P. Xiao, P.A. Webley, Binary adsorption equilibrium of carbon dioxide and water vapor on activatedalumina, in submission.4. M.T. Nguyen, G. Raspoet, L.G. 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