Membranes technology

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. 

Further distinction has to be made between conventional filtration of fine particle less than 10 pm in diameter, and microfiltration. It would be unusual for the filtration of such particles on a conventional fiher cloth to be described as ndcrofiltratian. Thus microfihration is constituted by the filtration of small particles and by the medimn which is used for the filtration. Conventional fihration is undertaken on filter cloths with a very open structure, see Chapter 4, whereas membrane fihration is usua% concerned with fihration enq loying membrane media where the equivalent pore size is of the order of 10 pm, or less. These definitions are, however, becoming less distinct as it is now possible to obtain conventional fihration equ ment employing membrane-type fiher media, as discussed in Chapter 4, and crossflow microfilters enqploying conventional filter cloth. 

The use of membranes in cartridge microfihration has be discussed already in Chapter 6, Section 6.6. That section also contained a number of test procedures enq)loyed for membrane characterisation which will not be repeated here. This chapter provides details of membrane configuration other than cartridges, mathematical models to assist in the understanding and control of the processes. Industrial applications or investigations of microfiltration and to a lesser extent ultrafiltration are also discussed. 

Homogeneous uniform pore profile through filter 

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 

The efficiency of membrane separation increases with the permeability and the selectivity. Thin membranes are economic, since according to Equation (2.1) the gas flow is inverse proportional to the layer thickness. However thin polymeric films, which have favorable permeability and selectivity, are too weak to withstand the high pressure difference between permeate and retentate side. The economic breakthrough set in with the production of ultrathin compound polymeric membranes. These are designed as hollow fibres with a thick porous back-up layer for mechanical stability and a thin dense non porous membrane layer for gas separation. The porous layer only has a slight influence on gas separation. These hollow fibres are combined in a bundle, which is arranged in a cylindrical container [2.13]. Several of these bundles, also called modules, can be added to 

Carbon molecular sieve membranes, which are currently developed (2003) on different porous carriers in various geometries [2.15, 2.16], are an interesting alternative to the hollow fibre membranes. They promise higher permeability with higher selectivity at the same time. 

Pis are attractive membrane materials for gas separation because of their good gas separation and physical properties. Many attempts have been made to modify the chemical structure of Pis in order to construct both highly permeable and permselective membrane materials. Blends of Pis have been demonstrated to exhibit improved performance in gas separation applications.  

Crosslinking of the PI provides membranes with antiplasticization properties and good chemical resistance. Crosslinking can be effected by several methods, including  

UV light induced photochemical crosslinking reactions in benzo-phenone containing Pis, or 

The formation of interpenetrating networks using polymer blends, and subsequent thermal treatment at elevated temperatures. 

Dendrimers based on diaminobutane serve as crosslinking agents for fluo-rinated PI types and are active at room temperature.  

Reverse osmosis uses semipermeable membranes and high pressure to produce a clean permeate and a retentate solution containing salts and ions, including heavy metals. The technique is effective ifthe retentate solution can be reused in the process. The equipment tends to be expensive, and fouling of the membranes has been a common problem. Considerable research effort is being carried out on membrane processes, however, and they are likely to be more commonly applied in the future. Concentrations of dissolved components are usually about 34,000 ppm or less. 

Reverse osmosis employs a semipermeable membrane that allows passage of the solvent molecules, but not those of the dissolved organic and inorganic material. A pressure gradient is applied to cause separation of the solvent and solute. Any components that may damage or restrict the function of the membrane must be removed before the process is performed. Capital investment and operating costs depend on the waste stream composition. 

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Membrane separation Membrane s vitches Membrane technology... 

Improvements ia membrane technology, vahdation of membrane iategrity, and methods to extend filter usage should further improve the performance of membrane filters ia removal of viral particles. Methods to improve or extead filter life and iacrease flow rates by creating more complex flow patterns could possibly be the focus of the next generation of membrane filters designed to remove viral particles. 

Membrane Extraction. An extraction technique which uses a thin Hquid membrane or film has been introduced (80,81). The principal advantages of Hquid-membrane extraction are that the inventory of solvent and extractant is extremely small and the specific interfacial area can be increased without the problems which accompany fine drop dispersions (see Membrane technology). 

In order to maintain a definite contact area, soHd supports for the solvent membrane can be introduced (85). Those typically consist of hydrophobic polymeric films having pore sizes between 0.02 and 1 p.m. Figure 9c illustrates a hoUow fiber membrane where the feed solution flows around the fiber, the solvent—extractant phase is supported on the fiber wall, and the strip solution flows within the fiber. Supported membranes can also be used in conventional extraction where the supported phase is continuously fed and removed. This technique is known as dispersion-free solvent extraction (86,87). The level of research interest in membrane extraction is reflected by the fact that the 1990 International Solvent Extraction Conference (20) featured over 50 papers on this area, mainly as appHed to metals extraction. Pilot-scale studies of treatment of metal waste streams by Hquid membrane extraction have been reported (88). The developments in membrane technology have been reviewed (89). Despite the research interest and potential, membranes have yet to be appHed at an industrial production scale (90). 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

More recendy, two different types of nonglass pH electrodes have been described which have shown excellent pH-response behavior. In the neutral-carrier, ion-selective electrode type of potentiometric sensor, synthetic organic ionophores, selective for hydrogen ions, are immobilized in polymeric membranes (see Membrane technology) (9). These membranes are then used in more-or-less classical glass pH electrode configurations. 

PhenoHc-based resins have almost disappeared. A few other resin types are available commercially but have not made a significant impact. Inorganic materials retain importance in a number of areas where synthetic organic ion-exchange resins are not normally used. Only the latter are discussed here. This article places emphasis on the styrenic and acryHc resins that are made as small beads. Other forms of synthetic ion-exchange materials such as membranes, papers, fibers (qv), foams (qv), and Hquid extractants are not included (see Extraction, liquid-liquid Membrane technology Paper.). 

This article focuses on the commercial, ethylene-based ionomers and includes information on industrial uses and manufacture. The fluorinated polymers used as membranes are frequently included in ionomer reviews. Owing to the high concentration of polar groups, these polymers are generally not melt processible and are specially designed for specific membrane uses (see Fluorine compounds, organic—perfluoroalkane sulfonic acids Membrane technology). 

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

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