Ceramic membranes nanofilters

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. 

Lefebvre X, Palmeri J, Sandeaux J, Sandeaux R, David P, Maleyre B, Guizard C, Amblard P, Diaz J-F, and Lamaze B. Nanofiltration modelling A comparative study of the salt filtration performance of a charged ceramic membrane and an organic nanofilter using the computer simulation program NANOFLUX. Sep. Purif. Technol. 2003 32(1-3) 117-126. 

Major developments of membrane processes using ceramic membranes have been aimed at microfiltration or ultrafiltration applications. Up to now the most important applications for these membranes are found in aqueous media for the separation of particles, bacteria, colloids, macromolecules. Recently, ceramic nanofilters based on sol-gel derived microporous materials have been described [20]. They extend separation capability of ceramic membranes to ions and organics. 

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... 

Since the first industrial developments in cross-flow MF in early 1980s, significant progress was achieved with ceramic membranes presently, affording new products with improved permeate flux, separation selectivity, and system compactness. Effectively, monolithic and hollow fiber elements have resulted in a significant increase in membrane surface to volume ratio, closer to the compactness of polymeric membrane systems. Ceramic nanofilters are able... 

As the newest development of the liquid filtration family, nanofiltration (NF) is capable of retaining small molecules from 200 to 1000 Da, and multivalent ions. The main current applications of NF polymeric membranes are dealing with the production of drinking and process water, the sulphate removal of seawater or the desalination of cheese whey. Ceramic nanofilters were... 

The main characteristics of nanofiltration membranes made of oxide ceramics is that they exhibit a microporous structure with charged pore walls depending on pH and ionic strength of feed solutions. Three main cases are distinguished in the discussion of mechanisms involved in permeation and separation processes using microporous ceramic nanofilters ... 

In the presence of solutes with small molecular weights, concentration polarization is likely to occur but with much less effect than in the case of ultrafiltration as explained in Section 12.2.1. A theoretical model concerning separation of sucrose and raffinose by ultrafiltration membranes has been proposed by Baker et al. [53] which assumes transport of solvent and solute exclusively through pores. This model can apply to ceramic nanofilters as they exhibit a porous structure with a pore size distribution. The retention characteristics of a given membrane for a given solute is basically determined by its pore-size distribution. The partial volume flux jy through the pores which show no rejection to the solute can be expressed as a fraction of the total volume flux fy. 

Salt rejection of a single electrolyte by a nanofiltration membrane in the absence of Donnan contribution can be described by Eqs. (12.9) and (12.10) according to the work of Spiegler and Kedem [57]. With ceramic nanofilters the Donnan contribution has to be taken into account due to the amphoteric... 

Both calculations by Perry and Schirg have been performed to describe and to predict the rejection characteristics of organic nanofiltration membranes when ionic and charged molecular solute mixtures are used in the feed solution. Recently experiments were carried out with ceramic nanofilters [67] which showed that similar properties can be obtained. As an example, results concerning the rejection of a dye/electrolyte mixture at pH = 9 through a zirconia nanofilter are reported in Table 12.5. 

These membranes are achievable using the concept of nanophase ceramics. According to literature, this new class of materials can result from the emphasis of some new ceramic processes, such as the condensation of gaseous atomic clusters [30] or the sol-gel process [31]. This last method, which has been successfully applied to ultrafiltration membranes, was used recently to prepare ceramic nanofilters. Nanophase materials deal both with the nanometer-sized particle and with the nanometer pore size aspects. The nanopore aspect is central to membrane technologies because of the need for selective separation processes at the molecular level. 

Other relevant examples of application for ceramic nanofilters have been reported [124], The first separation problem in which ceramic NF membranes are likely to compete with polymeric membranes is the concentration of pharmaceutical components in their reaction solvents. Ceramic nanofllters are also of interest in the agrofood indnstry for separation processes working at relatively high temperatnre and/or in the presence of organic solvents. Another potential application is in the production of chemically modified sugars in which the N-methyl-pyrrolidone (NMP), nsed as the solvent for the reaction, needs to be removed from the product (minimum molecular weight about 1000 Da) down to 0.1%. In this case, diaflltration with a ceramic NF membrane has shown to be an efficient way of decreasing the NMP content down to the intended low NMP residual... 

Benfer, S., Popp, U., Richter, H., Siewert, C., and Tomandl, G. (2001). Development and characterization of ceramic nanofiltration membranes. Sep. Purif. Technol 22-23 231-237. Vacassy, V., Guizard, C., Thoraval, V., and Cot, L. (1997). Synthesis and characterization of microporous zirconia powders application in nanofilters and nanofiltration characteristics. J Membr. Sci. 132 109-118. 

Liquid Separation. Ceramic ultrafilters and nanofilters for liquid mass separation are now produced at an industrial scale and most often are synthesized by the sol-gel process. These membranes operate according to a cross-flow filtration process in which the feeding fluid flows over the surface of the membrane and the permeate flow is recovered across the membrane in a perpendicular direction to the feed flow. The driving force for liquid flow... 

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