Adsorption and Mass Transport in Zeolite Membranes

• 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.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Kemiteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Chemical Engineering (hsv//eng)

Nyckelord

Kemisk teknologi

Chemical Technology

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