Ceramic hollow fiber membranes

In general, ceramic materials are chemically, thermally and mechanically stable. Thus ceramic membranes find frequent use in liquid filtration in food, chemical and pharmaceutical industries, as well as in the bioengineering processes.

Different geometries of ceramic membranes have been produced worldwide, e.g. plate, disk, tube, multichannel and honeycomb. However, the commercially available ceramic membranes are relatively heavy and their fabrication is rather expensive, with membrane areas rarely higher than 1,000 m²/m³.

Ceramic capillaries produced at the Fraunhofer IGB allow these advantages in combination with high packing densities, both compact and light-weighted.

Fraunhofer IGB has developed a cost-efficient process for the continuous production of ceramic capillaries via a phase inversion process. Ceramic powders are dispersed within organic binder systems. The resulting slurries are extruded through annular nozzles into aqueous baths. In this way, capillaries can be obtained from oxides, nitrides, carbides or even metals. The membrane characteristics can be tuned through the slurry composition, the spinning process parameters and the sintering procedures.

Typical α-Al2O3 have an outer diameter ranging between 0.5-2.0 mm, while the wall thickness is between 0.05-0.2 mm. So far, capillaries with a pore size of 0.2-1.0 mm have been produced with a porosity of 25-70 per cent. Another characteristic of these membranes is a well-defined pore size distribution. The bending strength of the capillaries reaches values up to 125 MPa indicating an excellent mechanical stability. Altogether the characteristics render these porous capillaries perfect basic materials for asymmetric membranes, which can be obtained by depositing further selective layers. Fig. 1 and Fig. 2 show SEM images for typical capillaries with different characteristics.

Modules with uncoated alpha-alumina capillary membranes, which means microfiltration membranes, can be directly used for the filtration of solutions, emulsions or heterogeneous liquids. A 0.15 m2 module for microfiltration is shown in Fig. 3. Such a module can be used up to 200°C and up to 8 bar. The permeability for water is in the range of 1700 l/m2 h bar. Such module can be integrated into technical processes by stainless steel connectors.

Ceramic capillary modules provide an optimum solution in the case of high temperature applications or aggressive media have to be handled. The automotive industry is expected to be a broad field of application for compact capillary membrane modules. The efficient removal of NOx from the air ventilated into the passenger compartment is still unresolved. Likewise, powerful catalytic burners are lacking for the reduction of soot emissions in the exhaust gas of diesel engines. Another obvious application is organic nanofiltration.