Pore size limitations, ceramic membranes

Pore Size Limitations. Although there are many potential commercial applications for ultrafiltration using currently available ceramic membranes, the pore sizes in these membranes are seldom less than 40 A in diameter, thereby limiting their applications in gas separations and in ceramic catalytic reactors. 

The modelling of gas permeation has been applied by several authors in the qualitative characterisation of porous structures of ceramic membranes [132-138]. Concerning the difficult case of gas transport analysis in microporous membranes, we have to notice the extensive works of A.B. Shelekhin et al. on glass membranes [139,14] as well as those more recent of R.S.A. de Lange et al. on sol-gel derived molecular sieve membranes [137,138]. The influence of errors in measured variables on the reliability of membrane structural parameters have been discussed in [136]. The accuracy of experimental data and the mutual relation between the resistance to gas flow of the separation layer and of the support are the limitations for the application of the permeation method. The interpretation of flux data must be further considered in heterogeneous media due to the effects of pore size distribution and pore connectivity. This can be conveniently done in terms of structure factors [5]. Furthermore the adsorption of gas is often considered as negligible in simple kinetic theories. Application of flow methods should always be critically examined with this in mind. 

With ceramic membranes (typical pore size 0.2 pm) this legal limit of 10 ppm is easily obtained [18]. Average membrane flux is 150 1/m h values are reported between 125 and 6001/m h [19]. The interval between cleanings or the maintenance interval can amount to 1000 h. By combination of the membrane unit with a static separator the concentration factor can be as high as 180, the oil content in the final concentrate can amount to over 90%. The pay-back time for such an installation would be less than two years at a cost of NLG 94/m effluent, and related to the present costs of treatment [18]. 

Some limitations of inorganic ceramic membranes are its sensitivity to mechanical impact, vibration, temperatme, and pressure variations pore size, which is limited only to UF and MF applications need of high-capacity pump to obtain the recommended speed flow of 2-6 m s" and finally, high cost, which probably represents the main limitation to the application of inorganic membranes [6]. 

Ceramic UF and MF membranes are microporous sieves in which separation takes place on the basis of the size and speed of a particle through a tortuous path of the pore. Capillary action, adsorption phenomena and surface charge aU play roles in retention and separation. Although ceramic membranes can be manufactured with a sharp pore size distribution, they are limited in their size-exclusion capability. 

Polymeric membranes, even if largely used in different industrial sectors, can operate in limited conditions of pH and temperature. Ceramic membranes offer a greater chemical, mechanical and thermal stability on the other hand, the available pore size range is more limited. 

Permporometry has size limitations relating to the fact that micropores do not experience capillary condensation and cannot therefore be studied using this technique. However, the choice of condensable vapour influences the Kelvin radius and thus the size ranges of pores that can be analysed. Cao and co-workers used the permporometry technique to study the pore size distribution of y-alumina membranes with a pore radius ranging from about 2 to 10 nm (Cao et al., 1993). Their results indicated that the permporometry technique can effectively measure the active pores which show a sharp pore size distribution with an average Kelvin radius, of 9 nm. More recently other groups have utilised water vapour to push the limits of permporometry and investigate microporous ceramic membranes with pore size diameters between 0.5-30 nm (Tsuru et al, 2001). 

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