Ultrafiltration ceramic membranes

A major new application of ceramic membranes is in the area of ultrafiltration. Ceramic membranes can outperform organic polymer... 

Furthermore, ultrafiltration ceramic membranes have been found to be effective in removing some heavy metal pollutants. 

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. 

Cabero, M.L., Riera, F. A., and Alvarez, R., Rinsing of ultrafiltration ceramic membranes fouled with whey proteins Effects on cleaning procedures, J. Membr. Sci., 154, 239, 1999. 

Smaller-pore ultrafiltration ceramic membranes are also manufactured. Measured at 20°C for 0.2 pm pore membrane. 

Liithi and Luisi [44] have used a hoUow fiber membrane reactor for peptide synthesis catalyzed by a-chymotrypsin in microemulsion. Chang et al. [110] described the immobilization of lipase on liposomes, which, in turn, were solubilized in AOT/isooctane reversed micelles and used for the continuous glycerolysis of olive oil in an ultrafiltration cell. The half-Ufe of the Chromo. viscosum lipase was 7 weeks. The development of an ultrafiltration ceramic membrane bioreactor for the simultaneous lipolysis of olive oil and product separation in AOT/isooctane reversed micellar media has been also reported [106,107], Cutinase performance was also evaluated in a ceramic membrane reactor [9]. An attempt to minimize the surfactant contamination problem was based on the use of an electro-ultrafiltration method which can decrease the gel formation in the membrane surface, improving the filtration flux, achieving the separation of the AOT reverse micelles [187],... 

Large Plants The economics of microfiltration units costing about ilO " is treated under ultrafiltration. When ceramic membranes are used, the cost optimum may shift energy consumption upward to as much as 10 kWh/m. ... 

The discussion so far implies that membrane materials are organic polymers, and in fact most membranes used commercially are polymer-based. However, in recent years, interest in membranes made of less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafiltration and microfiltration applications for which solvent resistance and thermal stability are required. Dense, metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported liquid films are being developed for carrier-facilitated transport processes. 

Terpstra, R. A., B. C. Bonekamp and H. J. Veringa. 1988. Preparation, characterization and some properties of tubular alpha alumina ceramic membranes for microfiltration and as a support for ultrafiltration and gas separation membranes. Desalination 70 395-404. 

Uhlhom, R. J. R., K, Keizer and A. J. Burggraaf. 1989. Formation of and gas transport properties in ceramic membranes. In Advances in Reverse Osmosis and Ultrafiltration eds. T. Matsuura and S. Sourirajan, pp. 239-59. Nat. Res. Council Canada, Ottawa. 

Cabral and coworkers [253] have investigated the batch mode synthesis of a dipeptide acetyl phenylalanine leucinamide (AcPhe-Leu-NH2) catalyzed by a-chymotrypsin in a ceramic ultrafiltration membrane reactor using a TTAB/oc-tanol/heptane reverse micellar system. Separation of the dipeptide was achieved by selective precipitation. Later on the same group successfully synthesized the same dipeptide in the same reactor system in a continuous mode [254] with high yields (70-80%) and recovery (75-90%). The volumetric production was as high as 4.3 mmol peptide/l/day with a purity of 92%. The reactor was operated for seven days continuously without any loss of enzyme activity. Hakoda et al. [255] proposed an electro-ultrafiltration bioreactor for separation of RMs containing enzyme from the product stream. A ceramic membrane module was used to separate AOT-RMs containing lipase from isooctane. Application of an electric field enhanced the ultrafiltration efficiency (flux) and it further improved when the anode and cathode were placed in the permeate and the reten-tate side respectively. 

The ultrafiltration of the microemulsion is a very useful operation for separating water and oil in these mixtures [117-120]. Because of the limited availability of solvent stable membranes, most of the work pubHshed so far was performed using ceramic membranes, which show a high adsorption of surfactant at the membrane surface and comparably low rejection rates of reverse micelles. Using electro ultrafiltration, where the concentration polarisation phenomenon of the reverse micelles (using the ionic surfactant AOT) at the membrane surface is depressed by asymmetric high voltage electrical fields, the rejection rates can be increased,but not to economical values [121,122]. 

The casein micelles are retained by fine-pore filters. Filtration through large-pore ceramic membranes is used to purify and concentrate casein on a laboratory scale. Ultrafiltration (UF) membranes retain both the caseins... 

Ceramic membranes are quite important since microporous ceramics are the principal barrier in UFe separation. Similar devices are used for microfiltration membranes and to a lesser extent for ultrafiltration. Homogeneous films are transformed into microporous devices by irradiation followed by selective leaching of the radiation damaged tracks, by stretching (Cortex is one welldmown example), or by electrochemical attack on aluminum. A few membranes are made by selective leaching of one component from a solid, as in membranes derived from glass or by selective extraction of polymer blends. 

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. 

Ultrafiltration permselectivity of a ceramic membrane can be enhanced by applying an electric field to the surface of the electrically conductive membrane [Guizard et al., 1986a]. The electric field is created using two electrodes with the conductive membrane being the anode or cathode. Such conductive membranes can be prepared with the introduction of a proper amount of Ru02 to Ti(>2. 

The ultrafiltration process is operated in a batch mode at a temperature of about 50 C. Ceramic membranes with 0.1 or 0.2 pm pore diameter enable processing of this highly viscous and concentrated raw or pasteurized whole milk due to their inherent mechanical strength. The viscosity of the concentrate has been found to increase exponentially with the rise of protein content in the precheese. Polymeric membranes have also been considered not suitable for this process in view of their structural compaction under pressure and their difficulty of cleaning. 

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]. 

Concentration of lignosulfonates (LS) and reduction of sugars (S) in residual black liquors from chemical pulp production by ceramic membrane ultrafiltration... 

Ceramic membranes are the most often used asymmetric membranes. When the separative layer, which is usually in contact with the feed, is also photoactive, irradiation must be applied on this top layer. A second configuration can also be considered. It consists of a conventional asymmetric membrane without photoactive separative layers but with a photoactive coating deposited on the surface of the grains of the support. In this case, the irradiation is apphed on the opposite side of the membrane, in contact with the permeate. Such a configuration could be used for instance in the final treafment of wastewater with a low-ultrafiltration membrane which provides retention of colloids and macromolecules, whereas small unretained molecules like VOCs would be photo-oxidized on the other side of the membrane (Figure 25.14). 

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. 

---