Separation microfiltration

FIGURE 4.1 Block diagram of combined enzymatic hydrolysis and two-stage membrane separation (microfiltration (MF) and ultrafiltration) to remove sngars while retaining undigested biomass and enzymes. Reprinted from Andric et al. (2010b) with permission from Elsevier. 

A. E. Ostermann and E. Pfleiderer, "AppHcation of the Principle of Cross-Flow in SoHd/Liquid Microfiltration," in the Proceedings of the Symposium on Economic Optimi tion Strategy in SolidjFiquid Separation Processes, SocifitH Beige de Filtration, Louvaine-la-Neuve, Belgium, Nov. 1981, pp. 123-138. 

Phase Separation. Microporous polymer systems consisting of essentially spherical, intercoimected voids, with a narrow range of pore and ceU-size distribution have been produced from a variety of thermoplastic resins by the phase-separation technique (127). If a polyolefin or polystyrene is insoluble in a solvent at low temperature but soluble at high temperatures, the solvent can be used to prepare a microporous polymer. When the solutions, containing 10—70% polymer, are cooled to ambient temperatures, the polymer separates as a second phase. The remaining nonsolvent can then be extracted from the solid material with common organic solvents. These microporous polymers may be useful in microfiltrations or as controlled-release carriers for a variety of chemicals. 

The seminal discovery that transformed membrane separation from a laboratory to an industrial process was the development, in the early 1960s, of the Loeb-Sourirajan process for making defect-free, high flux, asymmetric reverse osmosis membranes (5). These membranes consist of an ultrathin, selective surface film on a microporous support, which provides the mechanical strength. The flux of the first Loeb-Sourirajan reverse osmosis membrane was 10 times higher than that of any membrane then avaUable and made reverse osmosis practical. The work of Loeb and Sourirajan, and the timely infusion of large sums of research doUars from the U.S. Department of Interior, Office of Saline Water (OSW), resulted in the commercialization of reverse osmosis (qv) and was a primary factor in the development of ultrafiltration (qv) and microfiltration. The development of electro dialysis was also aided by OSW funding. 

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. 

The original expanded film membranes were sold ia roUs as flat sheets. These membranes had relatively poor tear strength along the original direction of orientation and were not widely used as microfiltration membranes. They did, however, find use as porous inert separating barriers ia batteries and some medical devices. More recentiy, the technology has been developed to produce these membranes as hoUow fibers, which are used as membrane contactors (12,13). 

In reverse osmosis membranes, the pores are so smaH, in the range 0.5— 2 nm in diameter, that they ate within the range of the thermal motion of the polymer chains. The most widely accepted theory of reverse osmosis transport considers the membrane to have no permanent pores at aH. Reverse osmosis membranes are used to separate dissolved microsolutes, such as salt, from water. The principal appHcation of reverse osmosis is the production of drinking water from brackish groundwater or seawater. Figure 25 shows the range of appHcabHity of reverse osmosis, ultrafiltration, microfiltration, and conventional filtration. 

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. 

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

Polymer Membranes These are used in filtration applications for fine-particle separations such as microfiltration and ultrafiltration (clarification involving the removal of l- Im and smaller particles). The membranes are made from a variety of materials, the commonest being cellulose acetates and polyamides. Membrane filtration, discussed in Sec. 22, has been well covered by Porter (in Schweitzer, op. cit., sec. 2.1). 

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. 

Membrane Porosity Separation membranes run a gamut of porosity (see Fig. 22-48). Polymeric and metallic gas separation membranes, electrodialysis membranes, pervaporation membranes, and reverse osmosis membranes are nonporous, although there is hnger-ing controversy over the nonporosity of the latter. Porous membranes are used for microfiltration and ultrafiltratiou. Nanofiltration membranes are probably charged porous structures. 

Pores Even porous membranes can give very high selectivity. Molecular sieve membranes exist that give excellent separation factors for gases. Their commercial scale preparation is a formidable obstacle. At the other extreme, UF,3 separations use Knudsen flow barriers, with aveiy low separation factor. Microfiltration (MF) and iiltrafiltra-tion (UF) membranes are clearly porous, their pores ranging in size from 3 nm to 3 [Lm. Nanofiltration (NF) meiTibranes have smaller pores. 

Cross Flow Most membrane processes are operated in cross flow, and only a few have the option to operate in the more conventional dead-end flow. In cross flow, the retentate passes parallel to the separating membrane, often at a velocity an order of magnitude higher than the velocity of the stream passing through the membrane. Microfiltration is the major membrane process in which a significant number if apphcations may be run with dead-end flow. 

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. 

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.). 

Tangential crossflow filtration Process where the feed stream sweeps the membrane surface and the particulate debris is expelled, thus extending filter life. The filtrate flows through the membrane. Most commonly used in the separation of high-and-low-molecular weight matter such as in ultrapure reverse osmosis, ultrafiltration, and submicron microfiltration processes. 

However in the case that SWCNTs were purified by the centrifugal separation using an aqueous solution of cationic surfactants or by the microfiltration... 

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 ... 

The solid-liquid separation of shinies containing particles below 10 pm is difficult by conventional filtration techniques. A conventional approach would be to use a slurry thickener in which the formation of a filter cake is restricted and the product is discharged continuously as concentrated slurry. Such filters use filter cloths as the filtration medium and are limited to concentrating particles above 5 xm in size. Dead end membrane microfiltration, in which the particle-containing fluid is pumped directly through a polymeric membrane, is used for the industrial clarification and sterilisation of liquids. Such process allows the removal of particles down to 0.1 xm or less, but is only suitable for feeds containing very low concentrations of particles as otherwise the membrane becomes too rapidly clogged.2,4,8... 

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... 

The sample preparation in LC analysis is as important as the chromatographic separation itself. The procedure will often require considerable skill copied with a basic understanding of chromatographic methodology. The analyst will need to have some familiarity with micro techniques including general micro-manipulation, microfiltration, centrifugation and derivatization. 

The microalgae are cultured in bioreactors under solar or artiflcial light in the presence of carbon dioxide and salts. The bioreactors may be closed systems made of polyethylene sleeves rather than open pools. Optimal conditions for pigment production are low to medium light intensity and medium temperatures (20 to 30°C). Pigment extraction is achieved by cell breakage, extraction into water or buffered solution, and centrifugation to separate out the filtrate. The filtrate may then be partly purified and sterilized by microfiltration and spray dried or lyophilized. 

Production of the color involves centrifugal separation of the biomass, cell breakage, and extraction. Use of a salt solution rather than water as an extraction medium increases stability of the color during extraction. Methods for partial exclusion of the polysaccharide from the color extract in order to enhance resolubilization of the dried color were developed. These processes include either microfiltration or co-precipitation of the polysaccharide with an added positively charged polysaccha-... 

Brief Examples Microfiltration is the oldest and largest membrane field. It was important economically when other disciphnes were struggling for acceptance, yet because of its incredible diversity and lack oT large apphcations, it is the most difficult to categorize. Nonetheless, it has had greater membrane sales than all other membrane apphcations combined throughout most of its history. The early success of microfiltration was hnked to an ability to separate microorganisms from water, both as a way to detect their presence, and as a means to remove them. Both of these apphcations remain important. 

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