Perovskite oxygen-conducting capillary membranes

The separation of oxygen from the air is of both economic and ecological importance for many large-scale commercial processes. To be able to use the methane contained in natural gas as a base material for the chemical industry, it needs to be partially oxidized to synthesis gas (syngas), a mix of carbon monoxide and hydrogen. So far it has chiefly been the provision of pure oxygen by means of cryogen air separation which has driven the costs for the industrial manufacture of syngas to a high level. During the past few years mixed conductive perovskites have increasingly come into focus as membrane materials for the selective separation of oxygen from air-gas mixtures.

To combine the special material properties of perovskites with an effective specific membrane surface, we have developed oxygen-conducting perovskite capillary membranes at the Fraunhofer IGB. Compared to conventional geometries (disks, pipes, multichannel elements) these membranes have the biggest packing density (separation area per volume) and an extremely low material consumption.

By means of a wet spinning process with subsequent sintering, the perovskite capillaries with an outer diameter from 750 µm to 3 mm and wall thicknesses from 80 to 300 µm are manufactured at pilot plant scale (Fig. 1). Gas-tight capillaries made of the perovskite material BaCoxFeyZrzO3-δ display an oxygen permeation of up to 10 ml min-1 cm-2 and an excellent selectivity (separation factor O2/N2 > 10,000) [1] at temperatures of 950 °C.

Together with partners from universities and industry the Fraunhofer IGB tested these membranes for various applications. The capillaries can be used for the production of extremely pure oxygen, for the partial oxidation of methane (POM). The splitting of water coupled with the POM utilizing these membranes facilitates the simultaneous production of pure hydrogen and syngas.

In future perovskite capillary membranes can also be used in the energy industry for the efficient utilization of primary energy sources by means of oxygenated air. The utilization of oxygen-separating membranes is also advantageous for “carbon dioxide-free power plants” which were proposed within the scope of climate protection. In these plants the carbon dioxide created during the incineration of fossil fuels is not discharged into the air, but captured and permanently disposed of (CO₂ sequestration): if the incineration is carried out with pure oxygen instead of air, it would considerably simplify the subsequent separation of CO₂.

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