S-layers ultrafiltration membranes

FIG. 7 Schematic drawing illustrating the structure of (a) native or glutaraldehyde-treated S-layer containers as prepared for permeability studies according to the space technique and of (b) S-layer ultrafiltration membranes. 

B. Production and Rejection Characteristics of S-Layer Ultrafiltration Membranes (SUMs)... 

S-layer is crystalline bacterial cell surface layer of Bacillus coagulans E38-66/vl SUM is S-layer ultrafiltration membrane (Bacillus sphaericus CCM 2120). 

S-layer ultrafiltration membranes (SUMs) are isoporous structures with very sharp molecular exclusion limits (see Section III.B). SUMs were manufactured by depositing S-layer-carrying cell wall fragments of B. sphaericus CCM 2120 on commercial microfiltration membranes with a pore size up to 1 pm in a pressure-dependent process [73]. Mechanical and chemical resistance of these composite structures could be improved by introducing inter- and intramolecular covalent linkages between the individual S-layer subunits. The uni-... 

FIGURE 11.3 Gel-layer formation on surface of an ultrafiltration membrane made from (I) hydrophobic and (II) hydrophilic material. C, solute concentration Ci < C2 < C3, 1 adsorption layer, 2 gel-polarization layer, 3 membrane material. (From Cherkasov, A.N., Tsareva, S.V., and Polotsky, A.E., J. Membr. Sci., 104, 157, 1995.)... 

Brink, L.E.S. and Romijn, D.J. 1990. Reducing the protein fouling of polysulfone surfaces and polysulfone ultrafiltration membranes Optimization of the type of presorbed layer. 

U should be noted that this definition does not characterise the membrane nor the pores ot the membrane, but rather the size of the particles or molecules retained by it. "nw separation characteristics are determined by the large pores in the membrane.. Another factor of interest is the surface porosity. This is also a very important variable in determining the flux through the membrane, in combination with the thickness of the top layer or the length of the pore. Different miciofiltration membranes exhibit a wide range of surface porosity as discussed in chapter III. from about S to 70%. In contrast, the ultrafiltration membranes normally show very low surface porosities, ranging from 0.1-1. ... 

Nakao, S-I Nomura, T Kumura, S, Characteristics of Macromolecular Gel Layer Formed on Ultrafiltration Tubular Membrane, AIChE Journal 25, 615, 1979. 

Womans, J. G., Nakao, S. and Smolders, C. A. Journal of Membrane Science 20 (1984) 115. Flux limitation in ultrafiltration osmotic pressure model and gel layer model. 

Ceraver s entry into the microfiltration and ultrafiltration field followed a completely different approach. In 1980, it became apparent that the type of product made by Ceraver for uranium enrichment, which was a tubular support and an intermediate layer with a pore diameter in the microfiltration range, might be declassified. Ceraver therefore developed a range of a-AljOj microfiltration membranes on an a-AljOs support with two key features first, the multichannel support and second, the possibility to backflush the filtrate in order to slow down fouling. 

The layer of solution immediately adjacent to the membrane surface becomes depleted in the permeating solute on the feed side of the membrane and enriched in this component on the permeate side. Equivalent gradients also form for the other component. This concentration polarization reduces the permeating component s concentration difference across the membrane, thereby lowering its flux and the membrane selectivity. The importance of concentration polarization depends on the membrane separation process. Concentration polarization can significantly affect membrane performance in reverse osmosis, but it is usually well controlled in industrial systems. On the other hand, membrane performance in ultrafiltration, electrodialysis, and some pervaporation processes is seriously affected by concentration polarization. 

Example 30.5. Ultrafiltration tests with a 1.5-cm tubular membrane at — 25,000 gave a permeate flux of 40 L/m -h and 75 percent rejection for a 5 percent polymer solution. The polymer has an average molecular weight of 30,000, and the estimated diffusivity is 5 x 10 cm s. (fl) Neglecting the effect of molecular diffusion in the pores, predict the fraction rejected for a flux of 20 L/m -h, and predict the maximum rejection, (b) Estimate the fraction rejected for the low-molecular-weight fraction of the pol3mier with M 10,000. (c) If the selective layer thickness is 0.2 fan, does molecular diffusion have a significant effect on the rejection for case (a) ... 

Wijmans J.G., Nakao S., Van Den Berg J.W.A., Troelstra F.R. F.R., Smolders C.A. (1985), Hydrodynamic resistance of concentration polarization boundary layers in ultrafiltration. Journal of Membrane Science, 22, 117-135. 

Let us take polysulfone as an example. This is a polymer which is frequently used as a membrane material, both for microfiltration/ultrafiltration as well as a sublayer in composite membranes. These applications require an open porous structure, but in addition also asymmetric membranes with a dense nonporous top layer can also be obtained which are useful for pervaporation or gas separation applications. Some examples are given in table ni.S which clearly demonstrate the influence of various parameters on the membrane structure when the same system, DMAc/polysulfone(PSf), is employed in each case. How is it possible to obtain such different structures with one and the same system To understand this it is necessary to consider how each of the variables affects the phase inversion process. The ultimate structure arises through two mechanisms i) diffusion... 

Profiles in which this latter profile can be found are electrodialysis, per/aporation, gas separation, dialysis, diffusion dialysis, facilitated transport or carrier mediated transport and membrane contactors. The extent of the boundary layer resistance varies from process to process and even for a specific process it is quite a lot dependent on application. Table Vn.2 summarises the causes and consequences of concentration polarisation in various membrane processes. The effect of concentration polarisation is very severe in microfiltration and ultrafiltration both because the fluxes (J) are high and the mass transfer coefficients k (= EV8) are low as a result of the low diffusion coefficients of macromolecuiar solutes and of small particles, colloids and emulsions. Thus, the diffusion coefficients of macromolecules are of the order of lO ° to 10 m /s or less. The effect is less severe in reverse osmosis both because the flux is lower and the mass transfer coefficient is higher. The diffusion coefficients of low molecular weight solutes are roughly of the order of 10 m /s. In gas separation and pervaporation the effect of concentration polarisation is low or can be neglected. The flux is low and the mass transfer coefficient high in gas separation (the diffusion coefficients of gas molecules are of the... 

Cellulose acetate Loeb-Sourirajan reverse osmosis membranes were introduced commercially in the 1960s. Since then, many other polymers have been made into asymmetric membranes in attempts to improve membrane properties. In the reverse osmosis area, these attempts have had limited success, the only significant example being DuPont s polyamide membrane. For gas separation and ultrafiltration, a number of membranes with useful properties have been made. However, the early work on asymmetric membranes has spawned numerous other techniques in which a microporous membrane is used as a support to carry another thin, dense separating layer. 

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