Ceramic capillary membranes

Ceramic materials generally exhibit very good chemical, thermal and mechanical stability. For this reason, ceramic membranes are widely used for the filtration of liquids in the food, chemical and pharmaceutical industries and in bioprocess engineering. Ceramic membranes are available in various geometries, e.g. as plates, rotating discs, tubes or multichannel elements. However, these commercially available membranes are relatively heavy and their production is costly. The space density of the elements is in many cases far below 1000 m²/m³. The ceramic capillaries produced at Fraunhofer IGB are characterized by the fact that they combine the advantages of ceramic membranes with high packing densities and are both compact and lightweight.

Fraunhofer IGB has developed a cost-saving process for the continuous production of ceramic capillary membranes. Ceramic powders are dispersed in an organic system and the resulting spinning dope is formed by a phase inversion process. Thus, capillaries made of oxide, nitride and carbide ceramics or even metals become readily available. The membrane properties can be specifically adjusted by the spinning dope composition, the process parameters during spinning and the choice of temperature during subsequent sintering.

This process is used at Fraunhofer IGB to manufacture capillaries from α-Al₂O₃ with outer diameters of 0.5 to 2 mm and wall thicknesses of 0.05 to 0.2 µm. The pore size can be varied between 0.2 and 1 µm and the porosity between 25 and 70 percent. The capillaries show a narrow pore size distribution. Their mechanical stability is good, with flexural rupture stresses (three-point flexural rupture test) up to 125 MPa. These properties make the capillaries an excellent base material for composite membranes produced by depositing selective layers on the α-Al₂O₃ support.

Modules with α-Al₂O₃ capillary membranes without coating, i.e. microfiltration membranes, can be used directly for filtration of solutions, emulsions or heterogeneous liquids. The picture shows a 0.15 m² module for microfiltration applications. Such a module can be used up to 200°C and is pressure stable up to 8 bar (feed). The permeability for water is 1700 l/m² h bar. Such a module can be easily integrated into a process via stainless steel connectors.

Ceramic capillary membranes produced at Fraunhofer IGB combine the advantages of ceramic materials with high space densities while significantly reducing weight. The modules are suitable for microfiltration of aggressive media or for use at higher temperatures where polymer membranes fail.

The membranes can easily be adapted for other separation ranges (ultra-, nanofiltration) by surface modification. The areas of application range from life sciences to high-temperature applications in the chemical industry. Thus, the membranes also lend themselves as carriers of catalysts for a wide variety.