Porous ceramic membranes sintering

Inorganic membranes (29,36) are generaUy more stable than their polymeric counterparts. Mechanical property data have not been definitive for good comparisons. IndustriaUy, tube bundle and honeycomb constmctions predominate with surface areas 20 to 200 m. Cross-flow is generaUy the preferred mode of operation. Packing densities are greater than 1000 /m. Porous ceramics, sintered metal, and metal oxides on porous carbon support... 

Experiments were conducted at the University of Magdeburg to examine the partial oxidation of ethane to ethylene by dosing oxygen into the fluidized bed of porous catalysts using immersed sintered metal and ceramic membranes. These studies were related to a DFG (German Research Association) research group (DFG-Nr. FOR 447/1-1) Membrane supported reaction engineering in the subproject Fluidized-bed membrane reactor . 

Some of those developments at Oak Ridge were believed to spin off in some form at Union Carbide and some aspects of the efforts led to the commercialization of dyiuunically formed membranes primarily for ultrafiltration and hypeiTiltration (reverse osmosis) applications. In these dynamic membranes, a mixture of zirconium hydroxide and polyacrylic acid deposited on a porous support which provides the necessary mechanical strength. The support is mostly made of porous carbon although porous ceramic and stainless steel are also used. These non-sintered membranes, in great contrast to most of the membranes discussed in this book, are formed in situ and require periodic regeneration with new zirconium hydroxide and polyacrylic acid. 

Many commercial ceramic membranes nowadays come in the form of a monolith consisting of multiple, straight channels parallel to the axis of the cylindrical structure (Figure 3.6). The surfaces of the open channels are deposited with permselective membranes and possibly one or more intermediate support layers. The porous suppon of these multi-channel structures are produced by extrusion of ceramic pastes described above with a channel diameter of a few millimeters. Their lengths are somewhat limited by the size of the furnaces used to dry, calcine and sinter them and also by such practical considerations as the total compact weights to be supported during heat ueatment and the risk of distortion in the middle section. It should be noted that this type of honeycomb... 

Microfiltration and ultrafiltration are the two main filtration techniques for which ceramic membranes have been widely used to date. As described in Section 6.2.1.2, MF and UF ceramic membranes exhibit macro- and mesoporous structure, respectively, which result from packing and sintering of ceramic particles. Liquid flow in such porous media is convective in nature and the simplest description of permeation flux, J, is given by the Darcy s equation [20] ... 

The MF membranes are usually made from natural or synthetic polymers such as cellulose acetate (CA), polyvinylidene difiuoride, polyamides, polysulfone, polycarbonate, polypropylene, and polytetrafiuoroethylene (FIFE) (13). Some of the newer MF membranes are ceramic membranes based on alumina, membranes formed during the anodizing of aluminium, and carbon membrane. Glass is being used as a membrane material. Zirconium oxide can also be deposited onto a porous carbon tube. Sintered metal membranes are fabricated from stainless steel, silver, gold, platinum, and nickel, in disks and tubes. The properties of membrane materials are directly reflected in their end applications. Some criteria for their selection are mechanical strength, temperature resistance, chemical compatibility, hydrophobility, hydrophilicity, permeability, permselectivity and the cost of membrane material as well as manufacturing process. 

As a general conclusion to this part dedicated to nanofiltration with ceramic membranes one can assume that the general behaviour of these membranes can be assimilated to the behaviour of electrically charged organic nanofiltration membranes. However some specificities exist with ceramic nanofilters due to a sintered metal oxide grains derived porous structure and an amphoteric character... 

The whole study of this research has been divided into two parts preparation of porous substrate and deposition of thin palladium membrane. This paper reveals only the first one, i.e. the fabrication of porous ceramic tubes by extrusion method. Early ceramic supports for palladium membrane were made of AI2O3 In recent work, we attempted to examine the properties of two kinds of ceramic materials, AI2O3 and YSZ (yittria stabilized zirconia). The extrusion of those ceramic materials was carried out by mixing with additives in various portions. After that, they were sintered at temperature between 1200 - 1450 °C in order to investigate the effect of sintering temperature on pore size and porosity of porous support. The mechanical strength was also inspected to clarify the most appropriate sintering temperature for each ceramics support. 

Composite membranes also employ dense cermets fabricated by sintering together mixed powders of metal and ceramic [10-12], Examples include powders of Pd and its alloys sintered with powders of perovskites [11,12], niobium sintered together with AI2O3 [12], and nickel sintered with proton-conducting perovskites. Layers of dense cermets, 25-100 xm thick, are supported by porous ceramic tubes. Cermets employing chemically reactive metals, Nb, Ta, U, V, Zr, and their alloys, are typically coated with Pd and alloys thereof [11,12],... 

Palladium cermets have a number of advantages over thin films of Pd supported by porous ceramics. For systems which wet, sintering cermets at very high temperatures (well above membrane operating temperatures) produces dense, pinhole-free composites (see Fig. 8.5). Because Pd is closely confined within a matrix of ceramic and because small, individual, micron-size Pd crystallites already possess a small surface-to-volume ratio of low surface energy, the Pd has relatively low driving... 

It is often found that the sintered metal and porous ceramic supports that have been used for many academic membrane studies are very expensive, sometimes even exceeding the cost of the palladium alloy membrane by several fold. This situation cannot be accepted for commercial membrane modules, especially when a large membrane area of up to several hundred to several thousand square meters is required. Some of the least expensive membrane supports include tension springs for tubular membranes and woven wire mesh for planar membranes, but even these supports can be costly, and further development of exceptionally low cost membrane supports is needed. 

The most common methods for manufacturing thin metal membranes include rolled foil, drawn tubes, and films deposited onto porous substrates (ceramic or sintered metal). Usually, electroless plating or electrolytic plating are the methods used to deposit the permselective metal onto the porous substrates although vapor deposition methods have been the subject of much research effort However, to date, vapor deposition methods have not proven to be a superior membrane fabrication method. There are pros and cons to each of these methods, but commercial membrane modules have only succeeded using rolled foil and drawn tubular membranes. 

This is difficult to achieve in practice, but it is always a good practice to comparatively evaluate candidate supports for thickness, porosity, and tortuosity. Support materials that have been used include porous ceramics, porous metal such as sintered metal, metal screens, metal mesh, and slotted metal plates. Metal mesh has been demonstrated to offer favorable performance and attractive cost The thin Pd-40Cu membranes shown in Fig. 5.4 have been supported on a 70 x 70 mesh the impressions on the membrane surface are caused by the underlying support When evaluating the acceptable pore diameter for the membrane support, a general rule is that the pores or openings in the support should not be greater than about four times the membrane thickness. If the membrane is very... 

---