Membrane separation processes microfiltration

Cross-flow-elec trofiltratiou (CF-EF) is the multifunctional separation process which combines the electrophoretic migration present in elec trofiltration with the particle diffusion and radial-migration forces present in cross-flow filtration (CFF) (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears later in this section) in order to reduce further the formation of filter cake. Cross-flow-electrofiltratiou can even eliminate the formation of filter cake entirely. This process should find application in the filtration of suspensions when there are charged particles as well as a relatively low conduc tivity in the continuous phase. Low conductivity in the continuous phase is necessary in order to minimize the amount of elec trical power necessaiy to sustain the elec tric field. Low-ionic-strength aqueous media and nonaqueous suspending media fulfill this requirement. 

Filtration Cross-flow filtration (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears earlier in this section) relies on the retention of particles by a membrane. The driving force for separation is pressure across a semipermeable membrane, while a tangential flow of the feed stream parallel to the membrane surface inhibits solids settling on and within the membrane matrix (Datar and Rosen, loc. cit.). 

Membranes used for the pressure driven separation processes, microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO), as well as those used for dialysis, are most commonly made of polymeric materials. Initially most such membranes were cellulosic in nature. These ate now being replaced by polyamide, polysulphone, polycarbonate and several other advanced polymers. These synthetic polymers have improved chemical stability and better resistance to microbial degradation. Membranes have most commonly been produced by a form of phase inversion known as immersion precipitation.11 This process has four main steps ... 

A limitation to the more widespread use of membrane separation processes is membrane fouling, as would be expected in the industrial application of such finely porous materials. Fouling results in a continuous decline in membrane penneation rate, an increased rejection of low molecular weight solutes and eventually blocking of flow channels. On start-up of a process, a reduction in membrane permeation rate to 30-10% of the pure water permeation rate after a few minutes of operation is common for ultrafiltration. Such a rapid decrease may be even more extreme for microfiltration. This is often followed by a more gradual... 

We can use the same filtration principle for the separation of small particles down to small size of the molecular level by using polymeric membranes. Depending upon the size range of the particles separated, membrane separation processes can be classified into three categories microfiltration, ultrafiltration, and reverse osmosis, the major differences of which are summarized in Table 10.2. 

The four developed industrial membrane separation processes are microfiltration, ultrafiltration, reverse osmosis, and electrodialysis. These processes are all well established, and the market is served by a number of experienced companies. 

In this chapter, we will introduce fundamental concepts of the membrane and membrane-separation processes, such as membrane definition, membrane classification, membrane formation, module configuration, transport mechanism, system design, and cost evaluation. Four widely used membrane separation processes in water and wastewater treatment, namely, microfiltration (MF), ultrafiltration (UF), nanofiltrafion (NF), and reverse osmosis (RO), will be discussed in detail. The issue of membrane foufing together with its solutions will be addressed. Several examples will be given to illustrate the processes. 

The investigation of Dean vortices and their application to membrane separation processes has been the subject of several experimental and theoretical studies concerning the improvement of microfiltration (ME), ultrafiltration (UF), and nanofiltration (NF),... 

As with reverse osmosis, ultrafiltration (UF) and microfiltration (MF) are pressure-driven membrane separation processes, with the membrane permselective for the solvent, usually water. MF and UF separate mainly by size exclusion of the solutes. MF retains particles of micrometer size UF retains particles of submicrometer size by ultramicroporous membranes. Typically, UF retains solutes in the 300 to 500,000 molecular weight range including biomolecules, polymers, sugars, and colloidal particles. 

Microfiltration (MF) is the most common pressure-driven membrane separation process and also the easiest to understand [1, 2]. It is simply conventional coarse filtration running at very low pressures (typically below 2 bar) owing to the open structure of the membranes. Darcy s law is apphcable showing a proportionality between the appHed pressure difference AP and the flux J through the membrane. 

Vapor permeation and pervaporation are membrane separation processes that employ dense, non-porous membranes for the selective separation of dilute solutes from a vapor or liquid bulk, respectively, into a solute-enriched vapor phase. The separation concept of vapor permeation and pervaporation is based on the molecular interaction between the feed components and the dense membrane, unlike some pressure-driven membrane processes such as microfiltration, whose general separation mechanism is primarily based on size-exclusion. Hence, the membrane serves as a selective transport barrier during the permeation of solutes from the feed (upstream) phase to the downstream phase and, in this way, possesses an additional selectivity (permselectivity) compared to evaporative techniques, such as distillation (see Chapter 3.1). This is an advantage when, for example, a feed stream consists of an azeotrope that, by definition, caimot be further separated by distillation. Introducing a permselective membrane barrier through which separation is controlled by solute-membrane interactions rather than those dominating the vapor-liquid equilibrium, such an evaporative separation problem can be overcome without the need for external aids such as entrainers. The most common example for such an application is the dehydration of ethanol. 

Commercial membrane separation processes include reverse osmosis, gas permeation, dialysis, electrodialysis, pervaporation, ultrafiltration, and microfiltration. Membranes are mainly synthetic or natural polymers in the form of sheets that are spiral wound or hollow fibers that are bundled together. Reverse osmosis, operating at a feed pressure of 1,000 psia, produces water of 99.95% purity from seawater (3.5 wt% dissolved salts) at a 45% recovery, or with a feed pressure of 250 psia from brackish water (less than 0.5 wt% dissolved salts). Bare-module costs of reverse osmosis plants based on purified water rate in gallons per day are included in Table 16.32. Other membrane separation costs in Table 16.32 are f.o.b. purchase costs. 

Finally, in Chap. 8, attempts are made to correlate the AFM parameters, such as nodule and pore sizes, to the membrane performance data. Membranes used for a variety of membrane processes, including reverse osmosis, nanofiltration, ultrafiltration, microfiltration, gas and vapor separation, pervaporation, and other membrane separation processes, are covered in this chapter. AFM parameters are also correlated to membrane biofouhng. This chapter also includes appUcations of AFM to characterize biomedical materials, including artificial organs cind drug release. 

Microfiltration (MF) Membrane separation process using microporous membranes with a pore size range of 0.1-10 pm. 

Membrane separations are techniques used industrially for removal of solutes and emulsified substances from solutions by application of pressure onto a very thin layer of a substance with microscopic pores, known as a membrane. Membrane separation processes include reverse osmosis (RO), ultrafiltration (UF), microfiltration (MF), dialysis, electrodialaysis, gas... 

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