Diffusion mechanism of CO 2 in 13X zeolite beads
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Diffusion mechanism of CO2 in 13X zeolite beads Xiayi Hu • Enzo Mangano • Daniel Friedrich Hyungwoong Ahn • Stefano Brandani
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Received: 21 February 2013 / Accepted: 1 June 2013 Ó Springer Science+Business Media New York 2013
Abstract A systematic study of the diffusion mechanism of CO2 in commercial 13X zeolite beads is presented. In order to gain a complete understanding of the diffusion process of CO2, kinetic measurements with a zero length column (ZLC) system and a volumetric apparatus have been carried out. The ZLC experiments were carried out on a single bead of zeolite 13X at 38 °C at a partial pressure of CO2 of 0.1 bar, conditions representative of post-combustion capture. Experiments with different carrier gases clearly show that the diffusion process is controlled by the transport inside the macropores. Volumetric measurements using a Quantachrome Autosorb system were carried out at different concentrations. These experiments are without a carrier gas and the low pressure measurements show clearly Knudsen diffusion control in both the uptake cell and the bead macropores. At increasing CO2 concentrations the transport mechanism shifts from Knudsen diffusion in the macropores to a completely heat limited process. Both sets of experiments are consistent with independent measurements of bead void fraction and tortuosity and confirm that under the range of conditions that are typical of a carbon capture process the system is controlled by macropore diffusion mechanisms. Keywords Carbon dioxide Zeolite Mass transfer kinetics Adsorption Carbon capture
X. Hu E. Mangano D. Friedrich H. Ahn S. Brandani (&) Scottish Carbon Capture and Storage, School of Engineering, University of Edinburgh, King’s Buildings, Mayfield Rd, Edinburgh EH9 3JL, UK e-mail: [email protected]
1 Introduction CO2 is the most important anthropogenic greenhouse gas, with a value of 77 % of the total anthropogenic emissions. The combustion of fossil fuels, mainly used for the production of electrical energy (but also in the cement, refining, petrochemical, iron and steel industry and transport) is responsible for the greatest part of the CO2 emitted from anthropogenic sources (IEA 2009; Luis et al. 2012; Kuramochi et al. 2012). Carbon capture and storage applied to large point sources, represents at the moment the most mature mitigation technology available. Pressure or vacuum swing adsorption using nanoporous adsorbents represents a possible technology which can provide the solution that can meet the requirements for good separation efficiencies satisfying both environmental and energy targets (Chou and Chen 2004; Ebner and Ritter 2009; Gomes and Yee 2002; Ishibashi et al. 1996; Kikkinides et al. 1993; Xiao et al. 2008). Several studies have indicated zeolite 13X as one of the best adsorbents available commercially for post combustion applications. For this reason it is very often used as a benchmark material for the comparison with other candidates for CO2 separation processes (Chue et al. 1995; Harlick and Tezel 2004;
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