Ultrafiltration anisotropic membranes

The transition between reverse osmosis membranes with a salt rejection of more than 95 % and molecular weight cutoffs below 50 and ultrafiltration membranes with a salt rejection of less than 10% and a molecular weight cutoff of more than 1000 is shown in Figure 2.42 [74], The very large change in the pressure-normalized flux of water that occurs as the membranes become more retentive is noteworthy. Because these are anisotropic membranes, the thickness of the separating layer is difficult to measure, but clearly the permeability of... 

The production by Loeb and Sourirajan of the first successful anisotropic membranes spawned numerous other techniques in which a microporous membrane is used as a support for a thin, dense separating layer. One of the most important of these was interfacial polymerization, an entirely new method of making anisotropic membranes developed by John Cadotte, then at North Star Research. Reverse osmosis membranes produced by this technique had dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Souri-rajan process. Almost all reverse osmosis membranes are now made by the interfacial polymerization process, illustrated in Figure 3.20. In this method, an aqueous solution of a reactive prepolymer, such as a polyamine, is first deposited in the pores of a microporous support membrane, typically a polysul-fone ultrafiltration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, such as a diacid chloride in hexane. The amine and acid chloride react at the interface of the two immiscible... 

Let us examine the TFF technique and a typical process regime. Tangential Flow Filtration is the general term used to describe filtration where cross flow parallel to the filter surface is used to enhance filtration rate. This is in contrast to dead ended filtration where the fluid path is solely through the filter. If the membrane used to make the separation is microporous (0.2-0.1+5 urn pore size), the technique is more specifically called mdcroporous tangential flow filtration. Ultrafiltration, a subset of Tangential Flow Filtration, employs a finer, anisotropic membrane able to retain macromolecules, albumin for instance. We will emphasize the use of ultrafiltration membranes in this discussion. 

Low Pressure Ultrafiltration of Sucrose and Raffinose Solutions with Anisotropic Membranes," J. Pky. Ckm., 238 (1972) coauthors R. W. Baker and H. Strathmann. 

The theory of permeation through microporous membranes in ultrafiltration and microfiltration is much less developed and it is difficult to see a clear path forward. Permeation through these membranes is affected by a variety of hard-to-compute effects and is also very much a function of membrane structure and composition. Measurements of permeation through ideal uniform-pore-diameter membranes made by the nucleation track method are in good agreement with theory. Unfortunately, industrially useful membranes have nonuniform tortuous pores and are often anisotropic as well. Current theories cannot predict the permeation properties of these membranes. 

Mesoporous dp = 2. .. 50 nm Carrier in liquid Affinity ligand in solid matrix anisotropic, isotropic track-etched immobilized in isotropic porous membrane isotropic, anisotropic Dialysis Carrier-mediated separation Ultrafiltration Electro- ultrafiltration... 

Membranes for Reverse Osmosis. The first commercially successful membrane was the anisotropic or asymmetric structure invented by Loeb and Sourirajan (1960 cited by Sourirajan, 1970). It is made of cellulose acetate and consists of a dense layer 0.2-0.5 jjim diameter. The thin film has the desired solute retention property while offering little resistance to flow, and the porous substructure offers little resistance to flow but provides support for the skin. The characteristics of available membranes for reverse osmosis and ultrafiltration are listed in Tables 19.2 through 19.4. 

In the equilibrium dialysis/ultrafiltration method for determining free testosterone in blood, a sample is first equilibrated with radioactive testosterone. Free steroid is then separated from bound steroid by filtration through an anisotropic, hydrophilic ultrafiitration membrane. The driving force for ultrafiltration is provided by centrifugation at 1000 to 2000 xg. Filtrate containing free steroid collects in the filtrate cup, whereas protein-bound steroid remains above the filter. Radioactivity in the filtrate is a measure of... 

Figure 1 Contact angle on four serum proteins deposited by ultrafiltration on anisotropic cellulose acetate membranes. Errors indicated are 90% confidence limits. For graphical reasons errors are shown only in some cases errors are similar...

The amount of colloidal silica present in an aqueous solution of high ratio alkali metal silicate can be determined for example by ultrafiltration. Ultrafiltration refers to the efficient selective retention of solutes by solvent flow through an anisotropic skinned membrane such as the Amicon Diaflo ultrafiltration membranes made by the Amicon Corporation of Lexington, Mass. In ultrafiltration solutes, colloids or particles of dimensions larger than the specified membrane cut-ofT are quantitatively retained in solution, while solutes smaller than the uniform minute skin pores pass unhindered with solvent through the supportive membrane substructure. 

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