Ceramics, macroporous

The most important technique for perfusion culture methods is to separate the concentrated cells and conditioned medium from the suspended culture broth. As noted above, the separation methods chiefly used are filtration with tubular and flat membranes as well as ceramic macroporous filters. These membrane reactors can be employed for both anchorage-dependent and suspension growing cells. Static maintenance type systems are commercially available for disposable reactors, and small size unit reactors from 80 ml to 1 liter are used for continuous production of monoclonal antibodies with hybridoma cells. The maintainable cell densities are about 10 -10 cells/ ml which is essentially mouse ascites level. However, in these systems, the cell numbers cannot be counted directly because the cells adhere to membranes or hollow fibers. Therefore, the measurement of cell density must use indirect methods. Such indirect methods include the assaying of the quantities of glucose consumption and the accumulation of lactate. The parameters of scale-up have not yet been established for these static methods. 

Figure 7-15, Multichannel geometry of a, ceramic macroporous membrane coated at the surface of an alumina support by the slip-casting method.

Porous ceramic membrane layers are formed on top of macroporous supports, for enhanced mechanical resistance. The flow through the support may consist of contributions due to both Knudsen-diffusion and convective nonseparative flow. Supports with large pores are preferred due to their low resistance to the flow. Supports with high resistance to the flow decrease the effective pressure drop over the membrane separation layer, thus diminishing the separation efficiency of the membrane (van Vuren et al. 1987). For this reason in a membrane reactor it is more effective to place the reaction (catalytic) zone at the top layer side of the membrane while purging at the support side of the membrane. 

Passuti, N., Daculsi, G, Rogez, J. M., Martin, S., and Bainvel, J. V., Macroporous calcium phosphate ceramic performance in human spine fusion. Clinical Orthop. 248, 169-176 (1989). 

Such ceramic hollow hbers can be assembled in fully ceramic modules of a thousand hbers exhibiting surfaceivolume rahos of about 2 m L-1. Different types of mesoporous, microporous, or dense separative layers have been deposited usually on the outer surface of macroporous hoUow hbers using y-alumina, ... 

Those monoliths can be produced from a piece of structured foam polymer with macropores. The piece of polymer is soaked in a sol that will form a ceramic of the desired material after heat treatment. The sol-soaked structure is dried, and it is burned at a suitable temperature to remove the polymer. The remaining structure will be a ceramic one with macropores, permitting the wall-flow of gases. This technique is also used to produce heat plates and pipes with macroporous walls for gas separation purposes. The polymers used are often derived from polyurethanes [45-46]. 

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. 

Basic mechanisms involved in gas and vapor separation using ceramic membranes are schematized in Figure 6.14. In general, single gas permeation mechanisms in a porous ceramic membrane of thickness depend on the ratio of the number of molecule-molecule collisions to that of the molecule-wall collisions. In membranes with large mesopores and macropores the separation selectivity is weak. The number of intermolecular collisions is strongly dominant and gas transport in the porosity is described as a viscous flow that can be quantified by a Hagen-Poiseuille type law ... 

So far, essentially three different approaches have been reported for the preparation of zeolitic membranes [119]. Tsikoyiannis and Haag [120] reported the coating of a Teflon slab during a "regular" synthesis of ZSM-5 by a continuous uniform zeolite film. Permeability tests and catals ic experiments were carried out with such membranes after the mechanical separation of the coating from the Teflon surface [121]. Geus et al. [122] used porous, sintered stainless steel discs covered with a thin top layer of metal wool to crystallize continuous polycrystalline layers of ZSM-5. Macroporous ceramic clay-type supports were also applied [123]. 

This novel route involves the retention of colloidal dispersions of zeolites (silicalite-1 described here) onto the surface of macroporous ceramic substrates. Silicalite-1 colloids were synthesised as described by Schoemann [14]. These were characterised by SEM and filtered through ceramic alumina discs. 

Synthesis conditions were established which either favoured the growth of a well-erystallised zeolite layer on the surface of the ceramic support or the preferential formation of a zeolite phase within the macropores of the alumina sub-layer. To obtain defect-fi ee and stable zeolite membranes, growth within the sub-layer is preferred. We will briefly illustrate the formation of these two distinct membrane structures here. 

The amorphous (nonordered) mesoporous materials such as ordinary SiC>2 aerogel and porous glass possess mesopores, but the channels or pores are irregular and the pore sizes distribute over a wide range. Most macroporous materials such as ceramics and cement have the same characteristics irregular pores and wide pore-size distribution. 

An inorganic membrane can be described as an asymmetric porous ceramic formed by a macroporous support with successive thin layers deposited on it. The support provides mechanical resistance to the medium. The successive layers are active in microfiltration (MF), ultrafiltration (UF) or nanofiltration (NF), depending on their pore diameters. 

Fig. 12.12. Influence of zeta-potential (Stem-layer thickness 1) and Streaming-potential (electrokinematic flow) on ion rejection and volume flux for porous ceramic membranes exhibiting negatively charged pore walls. Cases of micropores (nanofiltration), mesopores (ultrafiltration) and macropores...

Jia, Y. et al., Macroporous ZrO, ceramics prepared from colloidally stable nanoparticles building blocks and organic templates, J. Colloid Interf. Sci., 291, 292. 2005. 

Chen, Y., Xiangli, F., Jin, W., Xu, N. (2007). Organic-inorganic composite membranes prepared by self-assembly of polyelectrolyte multilayers on macroporous ceramic supports. J. Membr. Sci., 302, 78-86. 

Three-dimensionally ordered macroporous ceramic with high LR ion conductivity was prepared by colloidal crystal templating method using monodispersed polystyrene beads [12]. Monodispersed polystyrene beads with 3 pm diameter were dispersed in water and then filtrated by using a membrane filter under a small pressure difference. After this treatment, polystyrene beads were accumulated on the membrane filter with closed pack structure, as shown in Fig. 4.2. Then, the membrane consisting of accumulated polystyrene beads was removed from the membrane filter and put on a glass substrate. After drying at room temperature, the... 

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