Zeolite membranes for the energy-efficient separation of CO₂ from biogas

Membrane technology is ideally suited for the energy-efficient removal of CO₂ from biogas. Inorganic membranes are suitable candidates for CO₂ separation because they have a high hydromechanochemical stability and good permeation properties. Zeolites, for example,have special properties, such as an intrinsic molecular sieving capacity, which means they can separate molecules of different sizes. Additionally, they are capable of preferential adsorption of certain gases. Chabasite (CHA) zeolites were chosen to fabricate membranesthat separate CO₂ from biogas (CH₄). Pore size in the CHA framework is similar to CH₄ molecules but larger than CO₂. In this research proj­ect, ultrahigh-flux CHA membranes with a high Si/Al ratio (SSZ-13) were produced for CO₂/CH₄ separation.

Asymmetric porous Al₂O₃ monotubular supports (10 mm outer diameter, 7 mm inner diameter, 200 nm average pore size in the separation layer, and a membrane surface area of ~17 cm²) were chosen for the zeolite mem­brane (top image).

To evaluate the separation properties of the membrane, the permeation of single gases (middle figure) and the more complex permeation of binary gases (bottom figure) were analyzed. The single gas permeation of SSZ-13 membranes was tested at room temperature (here the dominant separation mechanism is adsorption) and at 150 °C (diffusion). In both cases, CO₂ has the highest permeance of the tested gases. The ideal CO₂/CH₄ permselectivityreaches up to 112 with a CO₂ permeance of around 11 m³/(m²hbar).

During the mixed gas permeation measurements, a gas mixture of 50 vol % CO₂ and CH₄ each was evaluated at room temperature with two different feed pressures. At the feed pressure of 2.55 bar, the CO₂ permeance is high at ~5 m³/(m²hbar) and the permeance of CH₄ is around 0.033 m³/(m²hbar). Accordingly, a CO₂/CH₄ selectivity of > 150 was achieved. An increase of feed pressure to 6.05 bar led to a decrease of the CO₂ permeance down to ~2.5 m³/(m²hbar) and an increase of the per­meance of CH₄ to ~0.045 m³/(m²hbar). The CO₂/CH₄ selectivity of the SSZ-13 membranes decreased at higher pressure, but still reached a satisfactory value of ~57.

The results show that the novel zeolite membranes provide excellent separation performance – both in terms of CO₂/CH₄ selectivity and CO₂ permeance. These membranes thus offer, for the first time, the possibility of separating highly concentrated CO₂ from biogas in an energy-efficient manner in just a single membrane stage. The next step is now to upscale the membranes with partners from industry while maintaining the same separation performance.

We gratefully acknowledge the financial sup­port of the European Union through the proj­ect “INNOMEM” (grant no. 862330–INNOM­EM–H2O2–NMBP–TO–IND–2010–2020/H2O2–NMBP–HUBS–2019).