Ceramic membranes zirconia

Milk protein standardization for continuous cheese making can also be done by ultrafiltration using ceramic membranes. Zirconia membranes with an average molecular weight cut-off (MWCO) of 70,000 daltons on carbon supports have been used for this purpose. The objective for this application is to concentrate either the whole volume of the milk to a volume concentration factor of 1.3 to 1.6 or just a fraction of the feed volume to a volume concentration factor of 3 to 4 followed by mixing the concentrate with raw milk to reduce the requirement of milk storage space [Merin and Daufin, 1989]. 

In ceramic membranes (zirconia), however, the pretreatment was ineffective. In these membranes, the permeate flux (hexane) decreased over time and between successive tests, indicating a possible interaction between the solvent and the membrane material, or adsorption of solvent on the membrane surface. 

Fig. 16.25. Mixture of zirconia and alumina coated on the ceramic membrane.

The vast increase in the application of membranes has expanded our knowledge of fabrication of various types of membrane, such as organic and inorganic membranes. The inorganic membrane is frequently called a ceramic membrane. To fulfil the need of the market, ceramic membranes represent a distinct class of inorganic membrane. There are a few important parameters involved in ceramic membrane materials, in terms of porous structure, chemical composition and shape of the filter in use. In this research, zirconia-coated y-alumina membranes have been developed using the sol-gel technique. 

Figure 16.23 presents the alumina-coated ceramic membrane. There were opportunities to fabricate a crack-free ceramic membrane coated with y-alumina. The supported zirconia-alumina membrane on the ceramic support shows an irregular surface. The non-uniform surface of ceramic support causes the irregular surface on the top layer of the membrane. Some of the membrane sol was trapped in the porous ceramic support during coating, and caused the irregularity of the membrane surface. 

The zirconia membrane was obtained in a unique manner. Figure 16.24 shows light micrographs of the zirconia-alumina membrane coated on the ceramic support. The non-unifomity and crater-filled surface of the ceramic support was covered by the zirconia-alumina membrane layer. Zirconia was mounted by very thin or nano-layers on the ceramic membrane. 

A combination of alumina and zirconia was used as a strong nano-film on the ceramic membrane. SEM micrographs are shown in Figure 16.25. Observation by SEM shows that the zirconia-alumina membrane layer was properly adhered and could stand on the top of the porous ceramic support. 

We have successfully developed a new inorganic ceramic membrane coated with zirconium and alumina. A thin film of alumina and zirconia unsupported membrane was also fabricated. The successful method developed was the sol-gel technique. 

Ceramic membranes were first developed in the 1940s for uranium isotope enrichment processes. Important progress has been made since that time, mainly due to the improved knowledge of the physicochemical properties of the membrane precursors. Most CMR studies concern alumina membranes other oxides such as silica, titania, or zirconia are much less frequently mentioned. 

Mixed ion and electronic conducting ceramic membranes (e.g. yttria-stabilized zirconia doped with titania or ceria) can be slip cast into a tubular form from the pastes containing the constituent oxides in an appropriate proportion and other ingredients and the cast tubes are then subject to sintering at 1,200 to 1,500X to render them gas impervious [Hazbun, 1988]. 

One way to improve chemical stability of a ceramic membrane is to introduce another oxide to the system as mentioned previously. On the other hand, even a small quantity of an ingredient present in the membrane composition may appreciably change the resulting chemical resistance in an undesirable direction. An example is cakia-stabilized zirconia which is likely to offer less resistance to acids than pure zirconia. 

The year 1980 marked the entry of a new type of commercial ceramic membrane into the separation market. SPEC in France introduced a zirconia membrane on a porous carbon support called Carbosep. This was followed in 1984 by the introduction of alumina membranes on alumina supports, Membralox by Ceraver in France and Ceraflo by Norton in the U.S. With the advent of commercialization of these ceramic membranes in the eighties, the general interest level in inorganic membranes has been aroused to a historical high. Several companies involved in the gas diffusion processes were responsible for this upsurge of interest and applications. 

Although some inorganic membranes such as porous glass and dense palladium membranes have been commercially available for some time, the recent escalated commercial activities of inorganic membranes can be attributed to the availability of large-scale ceramic membranes of consistent quality. As indicated in Chapter 2, commercialization of alumina and zirconia membranes mostly has been the technical and marketing extensions of the development activities in gas diffusion membranes for the nuclear industry. 

The above process for recycling spent aqueous alkaline cleaners for metal manufacturing plants can utilize other ultrafiltration ceramic membranes with a mean pore diameter of 5 to 100 nm, although zirconia membranes are preferred [Bhave et al., 1993]. A crossflow velocity of 3 m/s and a TMP of less than 5 bars are recommended. 

Thermal stability. Thermal stability of several common ceramic and metallic membrane materials has been briefly reviewed in Chapter 4. The materials include alumina, glass, silica, zirconia, titania and palladium. As the reactor temperature increases, phase transition of the membrane material may occur. Even if the temperature has not reached but is approaching the phase transition temperature, the membrane may still undergo some structural change which could result in corresponding permeability and permselectivity changes. These issues for the more common ceramic membranes will be further discussed here. 

As to ceramic membranes [3,4] the focus has been so frir in particular on amorphous porous aluminas and silicas. Other inorganics studied include titania, zirconia, non-oxide ceramics (carbides), and microporous carbons. 

Compared to modules based on cylindrical elements, flat ceramic membrane modules are not developed in a large extent and are limited to date to small liquid volume treatment [27]. Flat ceramic membranes are generally implemented as disks in laboratory scaled cells, offering a limited filtration surface area. Indeed a diameter of 90 mm that is one of the largest available dimensions for these membrane disks results in a filtration surface of -56 cm. Anopore alumina membranes supplied by Whatman or ATZ ceramic membrane disks with zirconia or titania top-layers from Sterlitech are typical examples of these commercially available flat ceramic membranes. Sterlitech ATZ ceramic membrane disks and the corresponding membrane holder are shown in Figure 6.16. 

Supported, multilayered (as5onmetric) - dense oxide or metal - porous ceramic membranes alumina, zirconia, titania, carbon - composite ceramic-metal, ceramic-ceramic layers on porous support tube, disk multilayers on porous support plate, disk, tube, monolith... 

G.Z. Cao, H.W. Brinkman, J. Meijerink, K.J. de Vries and A.J. Burggraaf, Pore narrowing and formation of ultra thin yttria-stabilised zirconia layers in ceramic membranes by chemical vapor deposition. /. Am. Cer. Soc., 76 (1993) 2201-2208. 

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