Ceramic membranes extrusion

Fig. 4. Configuration of a ceramic membrane reactor for partial oxidation of methane. The membrane tube, with an outside diameter of about 6.5 mm and a length of up to about 30 cm and a wall thickness of 0.25-1.20 mm, was prepared from an electronic/ionic conductor powder (Sr-Fe-Co-O) by a plastic extrusion technique. The quartz reactor supports the ceramic membrane tube through hot Pyrex seals. A Rh-containing reforming catalyst was located adjacent to the tube (57).

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... 

The porous structure of ceramic supports and membranes can be first described using the lUPAC classification on porous materials. Thus, macroporous ceramic membranes (pore diameter >50 nm) deposited on ceramic, carbon, or metallic porous supports are used for cross-flow microfiltration. These membranes are obtained by two successive ceramic processing techniques extrusion of ceramic pastes to produce cylindrical-shaped macroporous supports and slip-casting of ceramic powder slurries to obtain the supported microfiltration layer [2]. For ultrafiltration membranes, an additional mesoporous ceramic layer (2 nm<pore diameter <50 nm) is deposited, most often by the solgel process [11]. Ceramic nanofilters are produced in the same way by depositing a very thin microporous membrane (pore diameter <2 nm) on the ultrafiltration layer [4]. Two categories of micropores are distinguished the supermicropores >0.7 nm and the ultramicropores <0.7 nm. 

The extrusion of porous ceramic tubes, which are the starting point of ceramic membranes, is treated in Chapter 5. 

Besides the abovementioned advantages, ceramic hollow-fiber membranes unveil further superiorities over other ceramic membranes. In addition to a substantially higher surface area/volume ratio due to the smaller radius, the unique microchannels inside the membrane wall play an important role in lowering mass transfer resistance and elevate the accessible geometric and specific surface areas. Such a structure is formed due to the concurrence of several process phenomena during the membrane fabrication, which is different from the conventional ram-extrusion-based process where only a simple symmetric structure can be delivered. 

The mechanical properties of the membrane are essential in operation and module design. For instance, hollow carbon fibers fabricated by pyrolysis of polymers are seemingly too brittle for practical applications [111]. Ceramic capillaries prepared by extrusion are much stronger, but appear limited in maximum length due to... 

Extrusion can be defined as paste flow and extrudate formation from ceramic paste [1]. A membrane support must provide a high mechanical resistance and permeability. Tubular configurations correspond to this criterion and are adapted to the tangential filtration. 

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. 

Because the ceramic porous structure depends on the shape of individual grains and the way they are packed, different factors can affect the two major characteristics of membrane supports, mechanical strength and porosity. The pseudoplastic behavior of the paste during extrusion is responsible for an exponential dependence of extrusion velocity versus applied pressure. High pressures to increase support permeability and strength have been emphasized in the literature [14]. These can be linked to a better (more dense) particle pack-... 

Novel polymer-ceramic nanocomposite membranes were fabricated, characterized, and tested for their barrier performance. Atomic layer deposition (ALD) was used to deposit alumina films on primary, micron-sized (16 and 60 pm) high-density polyethylene (HOPE) particles at a rate of 0.5 nm/cyde at 77 °C. Well-dispersed polymer-ceramic nanocomposites were obtained by extruding aluminapolymer particle size. The diffusion coefficient of fabricated nanocomposite membranes can be reduced to half with the inclusion of 7.29vol.% alumina flakes. However, a corresponding increase in permeability was also observed due to the voids formed at or near the interface of the polymer and alumina flakes during the extrusion process, as evidenced by electron microscopy [1]. 

Tubular membranes are usually prepared by a plastic extrusion method [28]. Calcined and milled ceramic powder is mixed with several additives to make a slip with enough plasticity to easily be formed into tubes while... 

To deliver various microstructured ceramic hollow-fiber manbranes, a number of technical parameters need to be considered, such as suspension rheology, suspension composition and viscosity, air-gap, extrusion rate of suspension, and flow rate of internal coagulant, which have been systematically investigated previously [36-38]. This section addresses how to use these technical parameters to form the membrane morphologies listed in Table 10.1. 

Ceramic hollow-fiber support can be fabricated by a phase inversion-based extrusion/sintering technique [3], which allows a more flexible control over the membrane macro/microstructures by adjusting fabricating parameters such as air-gap, extrusion rate, internal coagulant composition, and the amount of nonsolvent additive in the spinning suspensions [4]. Such unique structural diversity delivers... 

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