Ultrafiltration compaction

In 1966, Cadotte developed a method for casting mlcroporous support film from polysulfone, polycarbonate, and polyphenylene oxide plastics ( ). Of these, polysulfone (Union Carbide Corporation, Udel P-3500) proved to have the best combination of compaction resistance and surface microporosity. Use of the mlcroporous sheet as a support for ultrathin cellulose acetate membranes produced fluxes of 10 to 15 gfd, an increase of about five-fold over that of the original mlcroporous asymmetric cellulose acetate support. Since that time, mlcroporous polysulfone has been widely adopted as the material of choice for the support film in composite membranes, while finding use itself in many ultrafiltration processes. 

The CHEMIC process does not effectively remove arsenic from solution. Removal efficiency for arsenic is only about 10 to 35%. Also, the ultrafiltration membranes used in the process can become fouled due to compaction of the membrane, physical aging by extended operation, and plugging of the membrane pores by particles. 

HANSHAW (B.B.) and COPLEN (T.B.), 1973. Ultrafiltration by a compacted clay membrane II - sodium ion exclusion at various ionic strengths. Ceochim. Acta. 37, 3211-27. 

Experience has shown that the best long-term performance of an ultrafiltration membrane is obtained when the applied pressure is maintained at or just below the plateau pressure ps shown in Figure 6.7. Operating at higher pressures does not increase the membrane flux but does increase the thickness and density of retained material at the membrane surface layer. Over time, material on the membrane surface can become compacted or precipitate, forming a layer of deposited material that has a lower permeability the flux then falls from the initial value. 

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. 

Aerts, R, Greenberg, A.R., Leysen, R., Krantz, W.B., Reinsch, V.E. and Jacobs, PA. 2001. The influence of filler concentration on the compaction and filtration properties of Zirfon -composite ultrafiltration membranes. Sep. 663-669. 

Nanohybrid materials have been furthermore used for ultra-/nanofiltration applications. Nanofiltration is a pressure-driven membrane separation process and can be used for the production of drinking water as well as for the treatment of process and waste waters. Some apphcations are desalination of brackish water, water softening, removal of micropollutants, and retention of dyes. Ultrafiltration membranes based on polysulfones filled with zirconia nanoparticles are usually prepared via a phase-inversion technique and have been used since 1990 [328]. Various studies were done in order to assess the effect of the addition of Zr02 to polysulfone-based ultrafiltration membranes [329] and the influence of filler loading on the compaction and filtration properties of membranes. The results indicate that the elastic strain of the nanohybrid membranes decreases and the time-dependent strain... 

Figure 3 is a scanning electron micrograph of polyacrylonitrile hollow fiber used for ultrafiltration. A large number of voids can be seen in the wall and there are thin and compact layers on both the outer and inner surfaces of the wall. The balance between these two kinds of structures affect the properties of the membrane. The electron micrograph of a cross section of the outside surface of the polyacrylonitrile hollow fiber shows that the pore size of the network close to the outer surface is smaller than that of the inner part. 

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