Microfiltration description

Process Description Microfiltration (MF) separates particles from true solutions, be they liquid or gas phase. Alone among the membrane processes, microfiltration may be accomplished without the use of a membrane. The usual materi s retained by a microfiltra-tion membrane range in size from several [Lm down to 0.2 [Lm. 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. 

Process Description Gas-separation membranes separate gases from other gases. Some gas filters, which remove hquids or sohds from gases, are microfiltration membranes. Gas membranes generally work because individual gases differ in their solubility and diffusivity through nonporous polymers. A few membranes operate by sieving, Knudsen flow, or chemical complexation. 

Cross-flow is the usual case where cake compressibility is a problem. Cross-flow microfiltration is much the same as cross-flow ultrafiltration in principle. In practice, the devices are often different. As with UF, spiral-wound membranes provide the most economical configuration for many large-scale installations. However, capillary devices and cassettes are widely employed, especially at smaller scale. A detailed description of cross-flow microfiltration had been given by Murkes and Carlsson [Crossflow Filtration, Wiley, New York (1988)]. 

Ideally, cross-flow microfiltration would be the pressure-driven removal of the process liquid through a porous medium without the deposition of particulate material. The flux decrease occurring during cross-flow microfiltration shows that this is not the case. If the decrease is due to particle deposition resulting from incomplete removal by the cross-flow liquid, then a description analogous to that of generalised cake filtration theory, discussed in Chapter 7, should apply. Equation 8.2 may then be written as ... 

In the following section, film and gel-polarisation models are developed for ultrafiltration. These models are also widely applied to cross-flow microfiltration, although even these cannot be simply applied, and there is at present no generally accepted mathematical description of the process. 

Microfiltration and ultrafiltration are the two main filtration techniques for which ceramic membranes have been widely used to date. As described in Section 6.2.1.2, MF and UF ceramic membranes exhibit macro- and mesoporous structure, respectively, which result from packing and sintering of ceramic particles. Liquid flow in such porous media is convective in nature and the simplest description of permeation flux, J, is given by the Darcy s equation [20] ... 

In this thesis, the initial chapters provide an introduction with a description of the organics (Chapter 2), the membrane processes (Chapter 3), and a general materials and methods section (Chapter 4). This is followed by the results of the different processes, microfiltration (Chapter 5), ultrafiltration (Chapter 6),... 

Membrane filtration has many similarities to conventional filtration, and the mathematical description of the process uses many ccmcepts already introduced in Chapter 2. However, there are rignificant differences in the terminology enqiloyed the filtrate is referred to as the permeate , the residual slurry or suspension from the filtration is called the retentate and the permeate filtration rate is the flux rate , which in microfiltration is conventionally reported in the emits of litres per square metre of membrane area per hour (1 m h ). This rate is equivalent to the superficial liquid velocity through the menibrane. In nearly aU the instances of constant-pressure... 

Description Reverse osmosis Nanofiltration Ultrafiltration Microfiltration Macrofiltration... 

The contribution of convective flow is the main term in any description of transport through porous membranes. In nonporous membranes, however, the convective flow term can be neglected and only diffusional flow contributes to transport.It can be shown by simple calculations that only convective flow contributes to transport in the case of porous membranes (microfiltration). Thus, for a membrane with a thickness of 100 pm, an average pore diameter of 0.1 pm, a tortuosity C of 1 (capillar) membrane) and a porosity e of 0.6, water flow at 1 bar pressure difference can be calculated from the Poisseuille equation (convective flow), i.e. 

The description given here can be applied in general. However, a distinction must be made for pressure-driven processes such as microfiltration, ultrafiliration and reverse osmosis. Here the feed consists of a solvent (usually water) and one or more solutes. In general, the concentration of the solute(s) is low and the separation characteristics of the membrane are always related to the solute(s). On the other hand, in liquid separation (pervaporation) and gas separation the terms solvent and solute are best avoided. 

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