Liquid separation membranes ultrafiltration

A biological step is always necessary to remove the carbonaceous fraction from the influent wastewater suspended biomass treatments are the most common. These entail long SRTs (>25-30 d), and compartmentalization of the biological reactor is necessary for the removal of recalcitrant compounds. Furthermore, as many micro-pollutants tend to adsorb/absorb to the biomass flocks, efficient solid/ liquid separation can greatly improve their removal from wastewater and, at the same time, guarantee consistently good effluent quality. MBRs have been suggested for this purpose by many authors [9, 58, 80, 93], some of whom found that ultrafiltration (UF) membranes are more efficient than MF membranes [9, 93]. 

Two other major factors determining module selection are concentration polarization control and resistance to fouling. Concentration polarization control is a particularly important issue in liquid separations such as reverse osmosis and ultrafiltration. In gas separation applications, concentration polarization is more easily controlled but is still a problem with high-flux, highly selective membranes. Hollow fine fiber modules are notoriously prone to fouling and concentration polarization and can be used in reverse osmosis applications only when extensive, costly feed solution pretreatment removes all particulates. These fibers cannot be used in ultrafiltration applications at all. 

In the discussion of concentration polarization to this point, the assumption is made that the volume flux through the membrane is large, so the concentration on the permeate side of the membrane is determined by the ratio of the component fluxes. This assumption is almost always true for liquid separation processes, such as ultrafiltration or reverse osmosis, but must be modified in a few gas separation and pervaporation processes. In these processes, a lateral flow of gas is sometimes used to change the composition of the gas on the permeate side of the membrane. Figure 4.14 illustrates a laboratory gas permeation experiment using this effect. As the pressurized feed gas mixture is passed over the membrane surface, certain components permeate the membrane. On the permeate side of the membrane, a lateral flow of helium or other inert gas sweeps the permeate from the membrane surface. In the absence of the sweep gas, the composition of the gas mixture on the permeate side of the membrane is determined by the flow of components from the feed. If a large flow of sweep gas is used, the partial... 

Ultrafiltration Liquid Microporous membrane with pressure gradient Separation of whey from cheese... 

Liquid separation. Separation can take place between solvents and solutes, macromolecules or particles or between species in liquid media by the effect of size exclusion. That is, those molecules or colloids larger than the size of the membrane pores will be retained or rejected while those smaller ones can pass through the membrane. The size exclusion mechanism predominates in pressure driven membrane processes such as microfiltration, ultrafiltration and even nanofiltration which has a molecular selectivity on the order of one nanometer. 

The use of two types of liquid membranes is described in [302] liquid emulsion membranes (LEMs), and supported liquid membranes (SLMs), where isoparaffin or kerosene and their mixtures were used as organic phases. A surfactant of the type of Span 80 served as emulsifier. LEMs are used, for example, for selective separation of L-phenylalanine from a racemic mixture of L-leucine biosynthesis as well as conversion of penicillins to 6-APA (6-aminopenicillanic acid). SLMs have a higher stability. A number of their commercial applications have been studied, e.g. in separation of penicillin from fermentation broth, as well as in the recovery of citric acid, lactic acid and some aminoacids. Compared with other separation methods (ultrafiltration, ultracentrifugation and ion exchange), LEMs and SLMs are advantageous in the separation of stereospecific isomers in racemic mixtures. 

The first field for which concentration polarization was deeply investigated (since the 1960s ) was liquid separation by membrane processes, such as ultrafiltration and reverse osmosis. On the contrary, for a long time it had been generally accepted that concentration polarization had only a negligible effect on membrane performance in gas separation. This was justified by the fact that membranes were quite thick and permeating flux very low, and, moreover, that... 

Appropriate membrane technologies in the context of solid-liquid separation are microfiltration and the use of the more open membranes in ultrafiltration. Other membrane processes, including the pressure-driven processes of hyperfihration and reverse osmosis, are concerned primarily with the removal of dissolved species fi om a solvent and shall not be considered. The boundary between the finer end of microfiltration and the coarser end of ultrafiltration is not sharp, and ultrafiltration is used for fine colloid-liquid separation. The start of the regions of ndcrofiltratian, ultrafihration and hyperfiltration occurs, approximately, with the fihration of particles of diameter 10, 0.1 and 0.005 rm, respectively. 

Surfactant enhanced chemical separations are obtained through coacervation, liquid membranes, ultrafiltration, foams and/or other interactions with phospholipids, proteins and biomolecules. All these topics were deliberately excluded. They are well exposed in the literature [1-3]. 

Fermentation-derived lactic acid can be separated by several recoveiy processes, which include calcium precipitation, solvent extraction and electrodialysis.Other recovery techniques have also been reported such as direct distillation, adsorption, liquid surfactant membrane extraction, chromatographic approaches, ultrafiltration, reverse osmosis, drying, conventional electrodialysis as well as bipolar membrane electrodialysis. ... 

This part, on applications, covers the following unit operations 8. Evaporation 9. Drying of Process Materials 10. Stage and Continuous Gas-Liquid Separation Processes (humidification, absorption) 11. Vapor-Liquid Separation Processes (distillation) 12. Liquid—Liquid and Fluid-Solid Separation Processes (adsorption, ion exchange, extraction, leaching, crystallization) 13. Membrane Separation Processes (dialysis, gas separation, reverse osmosis, ultrafiltration) 14. Mechanical-Physical Separation Processes (filtration, settling, centrifugal separation, mechanical size reduction). 

In its most general sense, reverse osmosis describes any pressure-driven process that depends on preferential diffusion of a liquid through a membrane that is selectively permeable to the liquid. Illustrated in Figures 5.5 and 5.6, reverse osmosis is a very useful and well-developed technique for the purification of water. Basically, it consists of forcing pure water through a semipermeable membrane that allows the passage of water but not of other material. This process, which is not simply sieve separation or ultrafiltration, depends on the preferential sorption of water on the surface of a porous cellulose acetate or polyamide membrane. Pure water from the sorbed layer is forced through pores in the membrane under pressure. If the thickness of the sorbed water layer is d, the pore diameter for optimum separation should be 2d. The optimum pore diameter depends on the thickness of the sorbed pure water layer and may be several times the diameters of the solute and solvent molecules. 

Ultrafiltration is a pressure-driven filtration separation occurring on a molecular scale (see Dialysis Filtration Hollow-fibermembranes Membrane TECHNOLOGY REVERSE osMOSis). Typically, a liquid including small dissolved molecules is forced through a porous membrane. Large dissolved molecules, coUoids, and suspended soHds that caimot pass through the pores are retained. 

Nonselective membranes can assist enantioselective processes, providing essential nonchiral separation characteristics and thus making a chiral separation based on enantioselectivity outside the membrane technically and economically feasible. For this purpose several configurations can be applied (i) liquid-liquid extraction based on hollow-fiber membrane fractionation (ii) liquid- membrane fractionation and (iii) micellar-enhanced ultrafiltration (MEUF). 

The discussion so far implies that membrane materials are organic polymers, and in fact most membranes used commercially are polymer-based. However, in recent years, interest in membranes made of less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafiltration and microfiltration applications for which solvent resistance and thermal stability are required. Dense, metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported liquid films are being developed for carrier-facilitated transport processes. 

---