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Binary Adsorption Equilibrium of Carbon Dioxide and Water Vapor on Activated Alumina Gang Li, Penny Xiao, and Paul Webley* Cooperative Research Centre for Greenhouse Gas Technologies, Department of Chemical Engineering, Monash University, Wellington Road, Clayton, Victoria 3168, Australia Received March 30, 2009. Revised Manuscript Received June 6, 2009 Adsorption equilibria of a CO2/H2O binary mixture on activated alumina F-200 were measured at several temperatures and over a wide range of concentrations from 4% to around 90% of the saturated water vapor pressure. In comparison with the single-component data, the loading of CO2 was not reduced in the presence of H2O, whereas at low relative humidity the adsorption of H2O was depressed. The binary system was described by a competitive/ cooperative adsorption model where the readily adsorbed water layers acted as secondary sites for further CO2 adsorption via hydrogen bonding or hydration reaction. The combination of kinetic models, namely, a Langmuir isotherm for characterizing pure CO2 adsorption and a BET isotherm for H2O, was extended to derive a binary adsorption equilibrium model for the CO2/H2O mixture. Models based on the ideal adsorbed solution theory of Myers and Prausnitz failed to characterize the data over the whole composition range, and a large deviation of binary CO2/H2O equilibrium from ideal solution behavior was observed. The extended Langmuir-BET (LBET) isotherm, analogous to the extended Langmuir equation, drastically underestimated the CO2 loading. By incorporating the interactions between CO2 and H2O molecules on the adsorbent surface and taking into account the effect of nonideality, the realistic interactive LBET (R-LBET) model was found to be in very good agreement with the experimental data. The derived binary isosteric heat of adsorption showed that the heat was reduced by competitive adsorption but promoted by cooperative adsorption.
1. Introduction The adsorption of CO2 on solid adsorbents is receiving increasing attention in both experimental and theoretical studies due to the interest in CO2 capture technologies. Water vapor is commonly found coexisting with CO2 such as in air prepurification1 and postcombustion CO2 capture applications.2 It is known to substantially affect the CO2 adsorption processes. An understanding of how CO2 and H2O interact and affect each other’s behavior on adsorbent surfaces is therefore essential. In spite of the importance of this system, there are relatively few adsorption equilibrium data sets available in the literature for this binary system. Rege and Yang3 measured very low concentration of CO2/H2O vapor (10 mol/kg), corresponding to RH above 55%, the isosteric heat levels off at 43.3 kJ/mol as multilayer adsorption of water in the form of clustering and polymerization become dominant. This value is compatible with the heat of water liquefaction of 44.2 kJ/mol,50,51
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Langmuir 2009, 25(18), 10666–10675
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Figure 9. Estimated isosteric heat of adsorption of component (a) H2O and (b) CO2 of the binary gas mixture at constant partial pressure of the other component on activated alumina F-200. Symbols denote data points calculated from binary R-LBET model.
indicating that the assumption of the multilayer mechanism is still adequate to characterize the adsorption of H2O, even though multilayer adsorption may possibly merge into capillary condensation at higher relative humidity region. In the presence of CO2, the H2O isosteric heat significantly decreases within the region of lower water loading (
