Classification of Synthetic Membranes
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Classification of Synthetic Membranes:A membrane can be natural or synthetic, thick or thin, its structure can be homogeneous or heterogeneous, transport across membrane can be active or passive, passive transport can be driven by various means (e.g. pressure, concentration, electrical difference), neutral or charged. Membranes can be classified according to different viewpoints. The first classification is by nature:(1) Biological membranes. (2) Synthetic membranes. Synthetic membranes can be subdivided into 2 parts: organic (polymeric or liquid) Inorganic (e.g. ceramic, metal) membranes.
MEMBRANE FILTRATION PROCESSES :Membrane filtration is the separation of the components of a pressurized fluid, affected by polymeric or inorganic membranes. The pores (openings) in the membrane material are so small that a significant fluid pressure is required to drive the liquid through them; the pressure required varies inversely with the size of the pores. Membrane filtration processes have found a huge range of uses across a variety of industries. A selection of different applications from industrial sectors is given below: Beverage manufacturing - concentration of orange, tomato juices, etc Dairy processing - milk concentration and whey fractionation Electronics industry - supply of ultrapure water Medical - supply of sterile water Food processing - concentration and purification of sugars, enzymes Laboratory - supply of ultrapure water for high precision work Nuclear - concentration of radioactive contaminants Pharmaceutical - separation and purification of proteins, vitamins, vaccines Petroleum/Gas - cleaning of gas prior to usage Paper industry - treatment of waste streams containing pulp
Utility industry - desalination of seawater Metal industry - recovery of trace metals
There are now four commonly accepted categories or "classes" of membrane filtration, defined based on the size of the material they will remove from the carrier liquid. Moving from the smallest to largest pore size, these are :-
Membrane-based ProcessesThe following topics are discussed in greater details: Click on the desired image. (1) (2) (3) (4) (5) (6) (7) Reverse osmosis. Micro filtration. Ultra filtration . Nano filtration. Gas separation. Pervaporation. Electro dylasis.
(1) MICROFILTRATION:Microfiltration is a filtration process which removes contaminants from a fluid (liquid & gas) by passage through a micro porous membrane. A typical microfiltration membrane pore size range is 0.1 to 10 micrometres (m). Microfiltration is fundamentally different from reverse osmosis and nanofiltration because those systems use pressure as a means of forcing water to go from low pressure to high pressure. Microfiltration can use a pressurized system but it does not need to include pressure Microfiltration uses micro porous membrane to remove contaminants from a fluid. The function of microfiltration in principle is as same as that of reverse osmosis, ultra filtration and nanofiltration. The difference lies in terms of retention of the size of molecules. The pore size of microfiltration membrane range from 0.1 to 10 m. through microfiltration, suspended solids, bacteria or other impurities can be easily removed. The membrane used in microfiltration is porous enough to pass molecules of true solutions, even if they are large. Due to the small pores used in microfiltration, it can be used for sterilizing solutions.
Mechanism and Properties of Microfiltration :Adsorption and entrapment are the mechanism used for the conventional depth filtration whereas, microfiltration uses sieving mechanism. The filter with different pore sizes are used for retaining larger size particles than the pore diameter. This technology therefore can be used for various critical operations like sterile filtration of parental fluids, free-water for the electronics industry etc.
The materials used for making microfiltration membranes are natural or synthetic polymers like cellulose nitrate or acetate, polyvinylidene difluoride (PVDF), polyamides, polysulfide, polycarbonate, polypropylene etc. Apart from this some inorganic materials like alumina, glass, zirconia coated carbon etc. are also used for manufacturing the MF membranes. Microfiltration membrane should be selected keeping following aspects in mind: Mechanical strength of the membrane Their resistance to temperature Chemical compatibility of the membrane Hydrophobility, Hydrophilicity and Permeability of the membrane Cost and the manufacturing process of the membrane material.
APPLICATIONS OF MICRO FILTERATION:Water Treatment and production Prior to other membrane treatment they are used for the pretreatment of surface water, seawater and municipal effluent. They are useful for producing o Drinking water o Irrigation o Industrial water reuse and makeup water
Dairy industry Others In chemical industry Microelectronics industry Fermentation producing sterile water for pharmaceutical industry Food & beverages industry where the process is used for concentrating fruit juices and alcoholic beverages. For biomass concentration and separations of soluble products Separates solvents from pigments in paints. Microfiltration is used for removing bacteria and spores from cheese milk, milk for powder production and market milk They also serve the same functions for whey They help in the fractionation of milk proteins For high protein WPC they serve the purpose of defatting the whey For sanitation of cheese brine, they remove bacteria, spores, yeast and mould.
F is a low pressure (10-100 psig) process for separating larger size solutes from aqueous solutions by means of a semi-permeable membrane. This process is carried out by having a process solution flow along a membrane surface under pressure. Retained solutes (such as particulate matter) leave with the flowing process stream and do not accumulate on the membrane surface. Retains large suspended solids Passes some suspended solids and all dissolved material Pore ranges from 0.1 micron to 3 micron.
CROSSFLOW MICROFILTRATION SYSTEMS
The PSI cross flow microfiltration process occurs in an array of permeable textile tubes. Manifolds are cast onto each end of the cloth filter to form modules which are connected to a pump for liquid inlet and to a back pressure valve at outlet. Upon introducing liquid flow into the tubes and regulating outlet pressure, suspended and colloidal matter in the liquid to be treated forms a thin film cake layer on the internal surface of each tube. This layer is called a "dynamic membrane" for its membrane-like characteristics. Other terms used are "filter layer" and "pre-coat layer". Should the quantity of suspended matter in the feed liquid be insufficient to form a filter layer, a small amount of filter aid compound is added to the initial feed. Filter layers or membranes of widely different characteristics can be produced by using different treatment chemicals. To become treated product liquid, or permeate, the feed water filters radially through the membrane layer and out of the textile tube walls for collection at the base of each filter module. The debris removed from the liquid becomes concentrated and is swept out of the tubes with the remaining liquid which is called reject concentrate. It is from the longitudinal or "cross flow" passage of the feed liquid along the filter cloth tubes that the process derives its name. PSI cross flow micro-filtration plants are of modular construction employing a number of manifolded filter modules. Modules are connected together either in parallel or in series with each other. Ease of cleaning is an important feature of the PSI cross flow microfiltration technology, distinguishing it from standard membrane microfiltration. In most cases, cleaning is simply a matter of momentarily stopping the feed resulting in tube collapse which causes the thin cake or membrane material to be dislodged and flushed out with the reject flow. In other applications, chemical cleaning in place is used. The core technology is based upon the highly specialized woven textile tubular array and its post weaving treatment as well as on the formation and maintenance of dynamic layers, or membranes, and cleaning techniques. A uniformly high quality permeate is achieved with the PSI cross flow microfiltration process. Removal of virtually all suspended solids down to about 0.1 micron has been demonstrated in countless laboratory and field trials. Other experimental work indicates that the system can be developed to produce a low pressure process to reject high molecular weight dissolved solids. The process has many advantages over conventional treatment process:
Most liquids are treatable without the addition of filter aid chemicals Specialized membranes can be formed to produce the desired quality of treated liquid by using standard chemistry. PSI cross flow microfiltration is a low pressure process (20 - 35 PSI) with a low cross flow velocity (3 - 6 fps) The single stage treatment produces a treated liquid having no suspended solids greater than approximately 0.10 micron. Cleaning of the filter cloth is easily accomplished Water recovery is very high: Usually greater than 97% Coagulation, settlement, clarification and filtration occur in the same process - usually without the need expensive of polymers or coagulant additives Liquids of wide pH range, from strongly acidic to strongly alkaline, can be processed at temperatures up to 90C Extensive civil works and structures are not required Operation is simple either manually or in the automatic mode
ULTRA FILTERATION:Ultra filtration (UF) designates a membrane separation process, driven by a pressure gradient, in which the membrane fractionates components of a liquid as a function of their solvated size and structure. The membrane configuratio