Inorganic porous hollow fiber membranes : with tunable small radial dimensions

Abstract

The objectives of this thesis are twofold. The first aim is to develop of robust coating procedures for thin supported films onto porous ceramic supports. The second aim is the development of a preparation methodology for high quality porous inorganic membranes, with large membrane surface area. A robust method is presented for applying a high quality micro-porous silica top layer on a commercial tubular support. The coating method is straightforward and allows for a reproducible coating of large surface area high performance silica membranes. To improve the surface-to-volume ratio and enhance the mechanical stability, hollow fibers of stainless steel have been developed via a dry wet-spinning process. The morphology of the fibers is predominantly determined during the dry-wet spinning process and can be tuned by changing the spinning conditions and the composition of the spinning mixture. In analogy to their ceramic counterpart the morphology is preserved upon sintering, apart from shrinkage due to densification. The fibers combine a high strength, with a large nitrogen permeance. The surface-to-volume ratio is further increased by the development of hollow fibers with exceptionally small radial dimensions. Viscous deformation of the green fibers allows for regulated reduction of the macro void volume resulting in a substantial reduction of the outer diameter. Depending on the particle loading different outer diameters (250 – 750 μm) can be achieved, using only a single spinneret with fixed dimensions. Fibers of other materials were prepared, demonstrating that the method presented in chapter 5 is generic and versatile. It allows for the preparation of a variety of inorganic membranes with small radial dimensions. At low particle concentration, extensive shrinkage of hollow fibers can be achieved above the Tg of the polymer, irrespective of the type and nature of the particles added. Increasing the particle loading results in differences in shrinkage behavior for different powders. A particle specific concentration range can be identified in which extensive shrinkage is observed and fibers with adequate structural integrity are obtained.