Penny Xiao

CO2 capture by adsorption: Materials and process development

Abstract Vacuum swing adsorptive (VSA) capture of CO 2 from flue gas and related process streams is a promising technology for greenhouse gas mitigation. Although early reports suggested that VSA was problematic and expensive, through the application of more logical process configurations that are appropriately coupled to the composition of the feed and product gas streams, we can now refute this early assertion. Improved cycle designs coupled with tighter temperature control are also helping to optimise performance for CO 2 separation. Simultaneously, new adsorbent materials are being developed. These separate CO 2 by selective (acid-base) reaction with surface bound amine groups (chemisorption), rather than on the basis of non-bonding interactions (physisorption). This report describes some of these recent developments from our own laboratories and points to synergies that are anticipated as a result of combining these improvements in adsorbent properties and VSA process cycles.

Binary Adsorption Equilibrium of Carbon Dioxide and Water Vapor on Activated Alumina

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.

One-step fabrication of ZIF-8/polymer composite spheres by a phase inversion method for gas adsorption

This paper reports a phase inversion method for the preparation of macroporous polysulfone (PS) composite spheres through a single orifice spinneret. Surfactant F127 was pre-added in the polymer solution as a surface pore-forming agent, and different amount of zeolitic imidazolate framework-8 (ZIF-8) particles were incorporated to form the ZIF-8/polysulfone (ZIF-8/PS)-composite spheres. ZIF-8 and polymer acted as the adsorbent and binder in the final composite spheres, respectively. The fabrication conditions, such as the types of the surfactant, the amount of the surfactant, and ZIF-8 added in the polymer solution, were investigated. Nitrogen and carbon dioxide sorption analysis indicated the ZIF-8/PS composite spheres had similar properties as the pure ZIF-8 particles, and the active sites of ZIF-8 in the polymer composites were well exposed. The composite spheres exhibited advantages of easy handling and recycling over ZIF-8 particles, and this phase inversion method can be extended to prepare other polymer composite spheres.

Effect of water vapor from power station flue gas on CO2 capture by vacuum swing adsorption with activated carbon

Abstract Due to the high absolute humidity of real flue gas, activated carbon, a hydrophobic adsorbent, was used to selectively adsorb CO2 by vacuum swing adsorption in this study. The objective of this work is to study the feasibility and advantage of CO2 capture along with simultaneous moisture removal by activated carbon and the effect of H2O on CO2 capture from wet flue gas streams. Through experiment and analysis, the “S” shape isotherms of water indicated water was easier to be desorbed from activated carbon. Then a cone shape model was proposed to depict water distribution inside the adsorption bed. As a consequence, water vapor hardly influenced the CO2 capture performance. Moreover, the process can be operated under a relatively high vacuum pressure and short evacuation time. The preliminary results showed that our one-bed VSA process could yield a good CO2 recovery of over 80% and a reasonable purity of 43%.

Converting 3D rigid metal–organic frameworks (MOFs) to 2D flexible networks via ligand exchange for enhanced CO2/N2 and CH4/N2 separation

We report a synthetic strategy for constructing a novel flexible MOF from a rigid parent structure by ligand exchange. This is the first reported study on introducing flexible heterogeneity into a rigid structure via substantial structural rearrangement. The daughter material exhibits enhanced gas separation selectivity compared with the parent.

Adsorption of CO2, N2, and CH4 in Cs-exchanged chabazite: A combination of van der Waals density functional theory calculations and experiment study

The crucial role of dispersion force in correctly describing the adsorption of some typical small-size gas molecules (e.g., CO2, N2, and CH4) in ion-exchanged chabazites has been investigated at different levels of theory, including the standard density functional theory calculation using the Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional and van der Waals density functional theory (vdWDFT) calculations using different exchange-correlation models - vdW_DF2, optB86b, optB88, and optPBE. Our results show that the usage of different vdWDFT functionals does not significantly change the adsorption configuration or the profile of static charge rearrangement of the gas-chabazite complexes, in comparison with the results obtained using the PBE. The calculated values of adsorption enthalpy using different functionals are compared with our experimental results. We conclude that the incorporation of dispersion interaction is imperative to correctly predict the trend of adsorption enthalpy values, in terms of different gas molecules and Cs(+) cation densities in the adsorbents, even though the absolute values of adsorption enthalpy are overestimated by approximate 10 kJ/mol compared with experiments.

CO2 capture at elevated temperatures by cyclic adsorption processes

Capture of CO2 from high temperature process streams, such as those arising in the Integrated Gasification Combined Cycle (IGCC) process, is receiving increasing attention. In this paper, we report the performance of zeolite 13X for CO2 capture from IGCC streams at elevated temperatures. Isotherms of CO2 adsorption on zeolite 13X at 90 °C, 120 °C, and 200 °C were measured and the fitted parameters were used for cyclic adsorption process design. Four different cycles were designed and tested with gas mixtures representing syngas compositions from a coal gasification pilot plant. These four cycles were then applied to CO2 separation from real coal gasification syngas. The experiments carried out with real syngas showed that zeolite 13X can be used for CO2 capture with acceptable performance even with impurities present in the syngas.

Biogas upgrading through kinetic separation of carbon dioxide and methane over Rb- and Cs-ZK-5 zeolites

Eight-membered ring (8 MR) zeolites hold large potential for industrial CO2 separations such as biogas separation. They offer large selectivity due to the constrained environment for adsorption, especially when large cations are present in the interconnecting windows. The Rb- and Cs-exchanged ZK-5 zeolites (8 MR KFI type zeolites) were studied for kinetic CO2/CH4 separation. First, Rb-ZK-5 and Cs-ZK-5 were thoroughly characterized via chemical analysis, argon porosimetry, X-ray diffraction and Rietveld refinements. Afterwards, the CO2/CH4 separation potential of both adsorbents was assessed via the measurement of kinetic and equilibrium data (T = 261.15 - 323 K), breakthrough measurements at 303 K (P = 1 - 8 bar), and simulations of their performance. The high occupation of the central 8 MR sites with large cations causes strong diffusional limitations for CH4 on Rb-ZK-5 and Cs-ZK-5. As a result, both zeolites effectively separate CH4 from CO2 with very high selectivities (α = 17 at 1 bar and 303 K). Despite their very large CO2 selectivities, the performance of Rb-ZK-5 and Cs-ZK-5 was still lower than for the benchmark 13X zeolite on a larger scale. Future research needs to further unravel the adsorption mechanism on low-silica 8 MR zeolites and their corresponding potential in separation processes such as biogas purification.

Temperature controlled invertible selectivity for adsorption of N 2 and CH4 by molecular trapdoor chabazites

We report an unusual operating regime for a chabazite zeolite in which the adsorption selectivity for N2 over CH4 inverts from being more selective for N2 at 253 K, to becoming less selective with increasing temperature and eventually becoming selective for CH4 over N2 above 293 K.

CO2 Capture by Vacuum Swing Adsorption Using F200 and Sorbead WS as Protective Pre-layers

Abstract In order to solve the water issues when 13X zeolite was applied to capture CO 2 from wet flue gas by vacuum swing adsorption process, multi-layered adsorption system was considered regarding activated alumina F200 and silica gel based Sorbead WS as pre-layer materials. LBET (extended Largmuir-BET) model and extended CMMS (cooperative multimolecular sorption) equation were simulated respectively to describe water loading on F200 and Sorbead WS. The two equations can be well added into our in-house simulator to simulate double-layered CO 2 -VSA (vacuum swing adsorption) process. Results indicated that water can be successfully stopped in pre-layers with a good CO 2 capture performance.