Microfiltration membrane materials

Most ceramic membranes are made from oxide powders. Z1O2 and AI2O3 are the most common compounds for microfiltration membrane materials. Meso-porous UF membranes usually also consist of oxides. AI2O3, Zr02, Ti02, Ce02 are most commonly used. Membranes from non-oxide compounds such as C, are also noted in literature (see Ref. [2] for an example). 

Meyer E. and Lim H.S., 1989. New nonwoven microfiltration membrane material, Fluid/Particle Separ. 7., 2, 17-21. 

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

Ultrafiltration separations range from ca 1 to 100 nm. Above ca 50 nm, the process is often known as microfiltration. Transport through ultrafiltration and microfiltration membranes is described by pore-flow models. Below ca 2 nm, interactions between the membrane material and the solute and solvent become significant. That process, called reverse osmosis or hyperfiltration, is best described by solution—diffusion mechanisms. 

Process Description Microfiltration (MF) separates particles from true solutions, be they liquid or gas phase. Alone among the membrane processes, microfiltration may be accomphshed without the use of a membrane. The usual materials retained by a microfiltration membrane range in size from several pm down to 0.2 pm. At the low end of this spectrum, very large soluble macromolecules are retained by a microfilter. Bacteria and other microorganisms are a particularly important class of particles retained by MF membranes. Among membrane processes, dead-end filtration is uniquely common to MF, but cross-flow configurations are often used. 

Porous metals have long been commercially available for particulate filtration. They have been used in some cases as microfiltration membranes that can withstand harsh environments, or as porous supports for dynamic membranes. Stainless steel is by far the most widely used porous metal membrane. Other materials include silver, nickel. Monel, Hastelloy and Inconel. Their recommended maximum operating temperatures range from 200 to 650°C. Elepending on the pore diameter which varies from 0.2 to 5 microns, the water permeability of these symmetric membranes can exceed 3000 L/h-m -bar and is similar to that obtained with asymmetric ceramic microfiltration membranes. Due to the relatively high costs of these membranes, their use for microfiltration has not been widespread. 

Figure Three (3) illustrates the mechanism of cross flow microfiltration. Microfiltration involves the removal of insoluble particulate materials ranging in size from 0.1 to 10.0 microns (1000 to 100,000 angstroms). Microfiltration membrane polymers include ...

The first major application of microfiltration membranes was for biological testing of water. This remains an important laboratory application in microbiology and biotechnology. For these applications the early cellulose acetate/cellulose nitrate phase separation membranes made by vapor-phase precipitation with water are still widely used. In the early 1960s and 1970s, a number of other membrane materials with improved mechanical properties and chemical stability were developed. These include polyacrylonitrile-poly(vinyl chloride) copolymers, poly(vinylidene fluoride), polysulfone, cellulose triacetate, and various nylons. Most cartridge filters use these membranes. More recently poly(tetrafluo-roethylene) membranes have come into use. 

The key innovation that has led to the increased use of cross-flow microfiltration membrane modules in the last few years has been the development of back-pulsing or backflushing to control membrane fouling [9-11]. In this procedure, the water flux through the membrane is reversed to remove any particulate and fouling material that may have formed on the membrane surface. In microfiltration several types of backflushing can be used. Short, relatively frequent flow reversal lasting a few seconds and applied once every few minutes is called... 

Figure 7.16 An illustration of the efficiency of back-pulsing in removing fouling materials from the surface of microfiltration membranes. Direct microscopic observations of Mores and Davis [9] of cellulose acetate membranes fouled with a 0.1 wt% yeast suspension. The membrane was backflushed with permeate solution at 3 psi for various times. Reprinted from J. Membr. Sci. 189, W.D. Mores and R.H. Davis, Direct Visual Observation of Yeast Deposition and Removal During Microfiltration, p. 217, Copyright 2001, with permission from Elsevier...

Microfiltration membranes usually have a nominal pore diameter in the range of 0.1-10 pm. However, the membrane specification is not an absolute parameter. The membranes usually present a pore size distribution around the nominal value and the shape of the bioparticles can determine whether they are retained or pass through the membrane. The membranes are manufactured from polymers, such as Teflon, polyester, PVC (polyvinyl chloride), Nylon, polypropylene, polyethersulfone, and cellulose, or from inorganic materials, such as ceramic and sinterized stainless steel. 

Inorganic membranes have also been studied. Thus, AFM has been used to probe the surface morphology and pore structure of micro- and ultrafiltration membranes, both in contact and noncontact mode, the latter being very suitable for soft and delicate materials. One of the first reports concerned alumina microfiltration membranes (Anapore) [45] and the authors performed statistical analysis to obtain the pore size distribution from the AFM... 

Both microfiltration (02 m) and ultrafiltration (4 nm) alumina membranes are very effective in removing bacterias. For example, the bacteria level of a lagoon water is reduced from 1,000-5,000/cm to 0.03-0.4/cm and 0.03-0.1/cm with the microHltration and ultrafiltration membrane, respectively [Castelas et al., 1984]. The total coliform level drops from 50-500/cm to zero for both types of membranes. The accompanying permeate flux is 600-1,200 L/hr-m for 70 hours for the microfiltration membrane when the water contains a low level of colloids and only 200 L/hr-m for 20 hours when the concentrations of colloids and organic materials are high. The ultrafiltration flux varies between 100 and 250 L/hr-m for 1,000 hours of operation. 

Dextran is mainly used as a coating material on available microfiltration membranes. Breifs and Kula [36] reported the use of dextran-coated nylon membranes as affinity media. The ligand was first coupled with dextran (both by adsorption and... 

Microfiltration membranes can be used as pretreatment for other membrane technologies and to remove microbes and total suspended sohds (TSS) including fibers and particles. Retention of salts and dissolved organics is negligible, if they are not bound to the suspended sohds. MF can be used for the recovery of coating color pigments. MBRs generally use UF or MF membranes. The materials used in microfiltration are polyvinylidenefluoride (PVDF), polypropylene, polyethylene, polysulfone, polyether suUbne, Teflon, and ceramic materials. 

Suppose further that at first we have suspension dip-coating in mind for the preparation of the layer to be obtained. The coating should be suitable as a substrate for a microfiltration membrane (layer 3) with a pore diameter of 200 nm. Which coating compoimd material is most appropriate This depends on the application and on the substrate material. When there is no reason not to use alumina, this is the best choice because thermal shock cracks can then be avoided during heat treatment (sintering) of the coating. 

Solvent and solute interactions with membrane material during microfiltration and ultrafiltration are then described and related to process performance, i.e. permeability and rejection. It is worth recalling that molecules in the colloidal range, roughly between one and one hundred nanometers, are separated by ultrafiltration, while microfiltration retains larger pcuticles. 

Yeast rests in fermenting cellars in beer breweries typically have a composition of 90% beer and 10% solids, mainly yeast. The amount of this waste material is 2-3% of the annual output. It can be sold as cattle feed or discharged. In a system with 4 m 0.4 pm ceramic microfiltration membranes, beer recovery amoimts to 42-62% the concentrate contains 23% solid matter [51]. Fluxes in... 

Microfiltration membranes are similar to UF membranes but have larger pores. Microfiltration membranes are used to separate particles in the range of 0.02-10 pm from liquid or gas streams. Commercial MF membranes are made from a wide variety of materials including polymers, metals, and ceramics. A wide variety of membrane module designs are available including tubular, spiral wound, pleated sheet, hollow fiber, and flat sheet designs. Some modules are best suited for crossflow filtration, and others are designed for dead-end filtration. In dead-end filtration, the feed liquid flows normal to the surface of the membrane, and retained particles build up with time as a cake layer on the membrane surface or within the pores of the membrane. 

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