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Mass transport of CO2 over CH4 controlled by the selective surface barrier in ultra-thin CHA membranes
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.ORCID iD: 0000-0002-7792-1348
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.ORCID iD: 0000-0003-2656-857x
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.ORCID iD: 0000-0003-1053-4623
2022 (English)In: Microporous and Mesoporous Materials, ISSN 1387-1811, E-ISSN 1873-3093, Vol. 332, article id 111716Article in journal (Refereed) Published
Abstract [en]

The adsorption and mass transport of CO2 and CH4 in CHA zeolite were studied experimentally. First, large and well-defined CHA crystals with varying Si/Al ratios and morphologies ideal for adsorption studies were prepared. Then, adsorption isotherms were measured, and adsorption parameters were estimated from the data. In the next step, permeation experiments for pure components and mixtures were conducted for a defect-free CHA membrane with a Si/Al ratio of 80 and a thickness of 600 nm over a wide temperature range. A maximum selectivity of 243 in combination with a CO2 permeance of 70 × 10−7 mol/(m2 s Pa) was observed for a feed of an equimolar CO2/CH4 mixture at 273 K and 5.5 bar. Finally, a simple model accounting for adsorption and diffusion through the surface barriers and the interior of the pores of the membrane was fitted to the permeation data. The fitted model indicated that the surface barrier was a surface diffusion process at the pore mouth with higher activation energy than the diffusion process within the pores. The model also showed that the highly selective mass transport in the membrane was mostly a result of a selective surface barrier and, to a lesser extent, a result of adsorption selectivity.

Place, publisher, year, edition, pages
Elsevier, 2022. Vol. 332, article id 111716
Keywords [en]
Activation energy, Adsorption, Mass transport, Surface barrier, Surface diffusion
National Category
Chemical Process Engineering
Research subject
Chemical Technology
Identifiers
URN: urn:nbn:se:ltu:diva-89075DOI: 10.1016/j.micromeso.2022.111716ISI: 000761764300006Scopus ID: 2-s2.0-85123872231OAI: oai:DiVA.org:ltu-89075DiVA, id: diva2:1634147
Funder
Swedish Research Council FormasSwedish Research CouncilBio4Energy
Note

Validerad;2022;Nivå 2;2022-02-21 (hanlid)

Available from: 2022-02-01 Created: 2022-02-01 Last updated: 2022-07-04Bibliographically approved
In thesis
1. Adsorption and Mass Transport in Zeolite Membranes
Open this publication in new window or tab >>Adsorption and Mass Transport in Zeolite Membranes
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Zeolites are commonly used as adsorbents and catalysts in the industry due to their well-defined pores of molecular dimensions. Zeolites offer porous structure, which consists of interlinked alumina and silica tetrahedra with shared oxygen atoms. Zeolites can also be prepared as intergrown films on porous supports, which results in zeolite membranes. CHA and MFI are two promising zeolites that can be used as membranes for biogas and syngas separation and upgrading since their pore size is suitable.

Membrane technology is considered an energy-lean gas separation method that offers a straightforward process with compact equipment and high efficiency. Compared with polymeric membranes, zeolite membranes offer higher permeance and stability due to their porous structure and ceramic nature. Since zeolite membranes are expensive and a higher flux would reduce the needed membrane area, thin membranes with high flux are of great interest. However, to enable the design of zeolite membrane processes, it is vital to enhance the fundamental understanding of the mass transport in the materials.

In this study, zeolite membranes of different types, i.e., CHA and MFI, were evaluated for separation of various gas mixtures. MFI disc membranes were evaluated for the separation of equimolar CO2/H2 mixtures under both dry and humid conditions, as well as for the separation of ternary CH4/N2/He mixtures. High selectivity and high CO2 fluxes were observed during CO2/H2 separation under both dry and humid conditions. The MFI disc membrane also displayed a high performance for separation of ternary CH4/N2/He mixtures. The results indicated that MFI membranes are promising candidates for separation of CO2 from the gas mixtures and for helium recovery from natural gas. Tubular CHA membranes, with lengths of 10 and 50 cm, were also investigated for CO2/CH4 separation under industrially relevant conditions. A maximum CO2/CH4 separation selectivity of 198 combined with a CO2 permeance of 14×10-7 mol/(m2·s·Pa) was observed for humid gas. The results verified the feasibility of these membranes for industrial gas separations.

After verifying the high performance of CHA zeolite membranes for gas separation under industrial conditions, CHA zeolite crystals with various Si/Al ratios were synthesized and the adsorption of CO2 and CH4 in the materials were studied. Subsequently, the mass transport through ultra-thin MFI and CHA zeolite disc membranes was measured and a model accounting for the adsorption and diffusion through the surface barriers and in the pores was developed. The model was successfully fitted to both single component and mixture permeation data. The fitted model indicates that the mass transport through ultra-thin membranes is controlled by the surface barriers. It revealed that the surface barrier is a surface diffusion process at the pore mouth with an activation energy that is higher than for the surface diffusion in the pores. Furthermore, the fitted model indicated that the high selectivity of CHA membranes is mostly due to a highly selective surface barrier and, to a lesser extent, is a result of adsorption selectivity.

In the last part of the work, a process for upgrading biogas was designed by using the developed model. The process was compared with a process based on hollow fiber polymeric membranes. It was concluded that the zeolite membrane processes were much more compact and had a much lower demand for electricity than the polymeric membrane process.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2022
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Chemical Engineering
Research subject
Chemical Technology
Identifiers
urn:nbn:se:ltu:diva-89354 (URN)978-91-8048-032-1 (ISBN)978-91-8048-033-8 (ISBN)
Public defence
2022-04-22, E632, Luleå tekniska universitet (LTU), E-huset, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2022-02-21 Created: 2022-02-20 Last updated: 2022-04-01Bibliographically approved

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Nobandegani, MojtabaYu, LiangHedlund, Jonas

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