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

Transcript of 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

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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 Processes

The following topics are discussed in greater details: Click on the desired image.

(1) Reverse osmosis.(2) Micro filtration.(3) Ultra filtration .(4) Nano filtration.(5) Gas separation.(6) Pervaporation.(7) 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 (µm). 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 µm. 

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.

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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 producingo Drinking watero Irrigationo Industrial water reuse and makeup water

Dairy industry

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.

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.

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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

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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

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(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 90¡C 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 configuration is usually cross-flow. In UF, the membrane pore size is larger allowing some components to pass through the pores with the water. It is a separation/ fractionation process using a 10,000 MW cutoff, 40 psig, and temperatures of 50-60°C with polysulfone membranes. In UF milk, lactose and minerals pass in a 50% separation ratio; for example, in the retentate would be 100% of fat, 100% of protein, 50% of lactose, and 50% of free minerals.

Diafiltration is a specialized type of ultra filtration process in which the retentate is diluted with water and re-ultra filtered, to reduce the concentration of soluble permeate components and increase further the concentration of retained components.

Many of the Crystal Quest water coolers use the Ultra filtration water purification process. Ultra filtration (UF) is an important purification technology used for the production of high-purity water in the biochemical, food and beverage, and biopharmaceutical industries. When strategically combined with other purification technologies in a complete water system, UF is ideal for the removal of colloids, proteins, bacteria, pyrogens, and other organic molecules.

Basic Principles of Ultra filtration

Ultra filtration is a pressure-driven purification process in which water and low molecular weight substances permeate a membrane while particles, colloids, and macromolecules are retained. The primary removal mechanism is size exclusion, although the electrical charge and surface chemistry of the particles or membrane may affect the purification efficiency. Ultra filtration pore ratings range from approximately 1,000 to 500,000 Daltons, thereby making UF more permeable than nanofiltration (200 -- 1.000 Daltons).

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UF membranes are composed of a polymer, such as polysulfone or polyamide that is usually extruded into flat sheets or hollow fibers or cut into disks as required by the specific application. A small disk of UF membrane may be subject to rapid fouling and produce a low flow rate for many processes.

As a result, UF membranes are typically arranged in a configuration which maximizes surface area and reduces fouling by using a tangential flow design to reduce solute accumulation at the membrane surface. Tangential flow UF devices may be spiral-wound cartridges containing several square feet of membrane wrapped onto a central core tube or hollow-fiber cartridges containing dozens of thin UF membrane fibers.

Ultra filtration is process by which suspended materials and macromolecules are separated from wastewater by using membrane and pressure differential. The pressure differential in this method is lower than that of reverse   osmosis . Unlike reverse osmosis it does not rely on overcoming osmotic effects. For dilute solutions of large polymerized macromolecules, this process is fruitful. 

In microfiltration, membrane filter separates particles according to pore size. The membrane used in this ultra filtration system acts as a molecular sieve. The ultra filter used in the process is selectively permeable membrane which does not allow macromolecules above a certain size to pass through. It also retains colloids, microorganisms and pyrogens. However smaller molecules like solvents and ionized contaminants pass into the filtrate. 

The function that Ultra filtration processes perform are feed clarification, concentration of rejected solutes and fractionation of solutes. Ultra filtration (UF) however is not so effective against organic streams.

Pores of the surface layer of the membrane is relatively smaller than the pores in the support layer of the

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membrane. Material that passes through the fine pores can readily be transported through the sponge-like structure of the support layer. 

Whenever the mixture of solvent passes through the membrane some of the materials are retained that when get concentrated resist the flow. Thus when the solution is processed this localized concentration of solute usually lead to the precipitation of a solute gel over the membrane. Due to this, by controlling the rate of transport through the polarization layer the permeate rate can be effectively controlled.PROPERTIES OF MAMBRANE:-

Membrane characteristics include porosity, morphology, surface properties, mechanical strength and chemical resistance. Following have been used successfully as a Polymeric materials like polysulfone, polypropylene, nylon 6, PVC, acrylic copolymer etc. Some of the inorganic materials like ceramics, carbon based membranes, and zirconia etc. are also sometimes used as ultra filtration membrane. These membranes come in sheet, capillary and tubular forms. The liquid is filtered in two streams - dilute permeate passes perpendicularly through the membrane whereas, concentrate passes out the end of the media.

Advantages Following are the advantages of Ultra filtration:

Removes particles, pyrogens, microorganisms, and colloids above their rated size effectively. Highest quality of water is produced using least amount of energy. They are easy to install. Maintenance is cheaper and easier.

APPLICATION OF ULTRAFILTRATION:-

For biological molecule concentration. For recovering electro paint. Waste treatment of oil emulsion. Whey treatment in dairy industries. Waste treatment of pulp mill. Producing pure water for electronics industry. For concentrating textile sizing. Heat sensitive proteins concentration for food additives. Gelatin concentration. Preparing Enzyme and pharmaceutical products.

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GAS SEPARATION:-

Gas separation, as the name implies, is the separation of gaseous components, and this is achieved using the selective properties of membranes. Both porous and dense membranes can be used as selective gas separation barriers. The mechanism of gas permeation in porous membranes is shown in the Figure.

If the pores are relatively large, i.e. from 0.1 - 10 micrometer, gases permeate the membrane by convective flow and no separation occurs. If the pores are smaller than 0.1 micrometer, then the pore diameter is the same size as or smaller than the mean free path of the gas molecules. Movement through such pores is governed by Knudsen Diffusion. Finally, if the membrane pores are extremely small, on the order of 5 - 20 Angstrom, then gases are separated by molecular sieving. Transport of this type is complex and includes both diffusion in the gas phase and diffusion of adsorbed species along the surface of the pores (surface diffusion).

The mechanism of gas permeation in dense membranes is known as solution diffusion and is shown in the Figure.

Applications of Gas Separation Operations :-

The following processes are briefly discussed:

(1) Air Separation

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(2) Hydrogen Recovery (3) Natural Gas Liquids Removal (4) Natural Gas Dehydration (5) Vapor-Gas Separation

PERVAPORATION:-

Pervaporation separation technique is a fractionation process that uses dense or composite membranes as the separation barrier between the feed liquid and permeated vapor. Pervaporation is attractive when it is difficult to apply distillation such as in fractionation of azeotropic mixtures, close boiling point components and isomeric mixtures.1 Transport through a pervaporation membrane takes place by a solution-diffusion mechanism; i.e., the permeation rate is a function of solubility and diffusivity of the components to be separated. Total selectivity is the product of sorption selectivity and diffusion selectivity. The component that is sorbed preferentially permeates preferentially too.

Pervaporation: is a process in which a liquid stream containing two or more components is placed in contact with one side of a non-porous polymeric membrane while a vacuum or gas purge is applied to the other side. The components in the liquid stream sorbs into the membrane, permeate through the membrane, and evaporate into the vapor phase (hence the word pervaporate). The vapor, referred to as "the permeate", is then condensed. Due to different species in the feed mixture having different affinities for the membrane and different diffusion rates through the membrane, a component at low concentration in the feed can be highly enriched in the permeate. Further, the permeate composition may widely differ from that of the vapor evolved after a free vapor-liquid equilibrium process. Concentration factors range from the single digits to over 1,000, depending on the compounds, the membrane, and process conditions.

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Pervaporation, in its simplest form, is an energy efficient combination of membrane permeation and evaporation. It's considered an attractive alternative to other separation methods for a variety of processes. For example, with the low temperatures and pressures involved in pervaporation, it often has cost and performance advantages for the separation of constant-boiling azeotropes. Pervaporation is also used for the dehydration of organic solvents and the removal of organics from aqueous streams. Additionally, pervaporation has emerged as a good choice for separation heat sensitive products. Pervaporation involves the separation of two

Or more components across a membrane by differing rates of diffusion through a thin polymer and an evaporative phase change comparable to a simple flash step. A concentrate and vapor pressure gradient is used to allow one component to preferentially permeate across the membrane. A vacuum applied to the permeate side is coupled with the immediate condensation of the permeated vapors. Pervaporation is typically suited to separating a minor component of a liquid mixture, thus high selectivity through the membrane is essential. Figure 1 shows an overview of the pervaporation process. Pervaporation can used for breaking azeotropes, dehydration of solvents and other volatile organics, organic/organic separations such as ethanol or methanol removal, and wastewater purification.Characteristics of the pervaporation process include:-1. Low energy consumption2. No entrained required, no contamination3. Permeate must be volatile at operating conditions4. Functions independent of vapor/liquid equilibrium

Batch pervaporation is a simple system with great flexibility, however a buffer tank is required for batch operation. Continuous pervaporation consumes very little energy, operates best with low impurities in the feed, and is best for larger capacities. Vapor phase permeation is preferred for direct feeds from distillation columns or for streams with dissolved solids.

Pervaporation for SeparationLiquid transport in pervaporation is described by various solution-diffusion models1. The steps included are the sorption of the permeate at the interface of the solution feed and the membrane, diffusion across the membrane

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due to concentration gradients (rate determining steps), and finally desorption into a vapor phase at the permeate side of the membrane. The first two steps are primarily responsible for the permselectivity1. As material passes through the membrane a "swelling" effect makes the membrane more permeable, but less selective, until a point of unacceptable selectivity is reached and the membrane must be regenerated.The other driving force for separation is the difference in partial pressures across the membrane. By reducing the pressure on the permeate side of the membrane, a driving force is created. Another method of inducing a partial pressure gradient is to sweep an inert gas over the permeate side of the membrane. These methods are described asVacuum and sweep gas pervaporation respectively.

Of the membrane and a fraction of the feed (permeate) passes through the membrane and leaves in the vapor phase on the opposite side of the membrane. The "vapor phase" side of the membrane is either kept under a vacuum or it is purged with a stream of inert carrier gas. The permeate is finally collected in the liquid state after condensation. The liquid product is rich in the more rapidly permeating component of feed mixture. The retentate is made up of the feed materials that cannot pass through the membrane.

Industrial Applications:- Established industrial applications of pervaporation include:-

The treatment of wastewater contaminated with organics Pollution control applications Recovery of valuable organic compounds from process side streams Separation of 99.5% pure ethanol-water solutions Harvesting of organic substances from fermented broth

Other products separated or purified by pervaporation include:Alcohols Ketones

Methanol Acetone Ethanol Butanone Propanol (both isomers) Methyl isobutyl ketone (MIBK) Butanol (all isomers) Amines Pentanol (all isomers) Triethylamine Cyclohexanol Pyridine Benzyl alcohol Aniline

Aromatics Aliphatic Benzene Chlorinated hydrocarbons (various)

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Toluene Dichloro methane Phenol Perchloroethylene

Ester Ethers Methyl acetate Methyl tert-butyl ether (MTBE) Ethyl acetate Ethyl tert-butyl ether (ETBE) Butyl acetate Di-isopropyl ether (DIPE)

Organic Acid Tetra hydro furan (THF) Acetic acid Dioxin

ELECTRODIALISIS:-

Electro dialysis (ED) is a membrane separation process that utilizes an electrical potential difference as a driving force for moving salt ions in solutions. The membrane is selective in that it'll only permit the passage of either anions or cations but not both. Thus separation is due to charge rather than size differences. This process can be used to separate differently charged molecules of similar sizes.

The process is based on the principle that most salts that dissolved in water are ionic, being either positively charged (cation, such as Na+, Ca2+, Mg2+) or negatively charged (anion, such as Cl-, CO3

2-). These ions are attracted to electrodes with an opposite electric charge (i.e. anion to cathode or positively-charged electrode, and cation to anode or negatively-charged electrode).

In the ion-exchange membranes, charged groups are attached to the polymer backbone of the membrane material. The fixed charged groups partially or completely exclude ions of the same charge from the membrane.

This means that an anionic membrane with fixed positive groups excludes positive ions but is freely permeable to negatively charged ions. Similarly, a cationic  membrane with fixed negative groups exclude negative ions but is freely permeable to positively charged ions. An example for the latter is shown in the Figure.

A typical ED system has up to 100 or more unit cells (cell pairs) in a plate-and-frame arrangement where the cation and anion exchange membranes are placed in an alternating pattern between the anode and cathode electrodes of a direct current source. As shown in the Figure, the cation-selective and anion-selective membranes are arranged in alternative fashion. The space (channel) bounded by the 2 different membranes is called a cell. A unit cell or cell pair is made up of 2 cells: - the ion-depleted cell for the passage of fresh water (the fresh water channel or desalted water channel), and the ion-concentrated cell for the passage of brine (the brine channel). The cells alternate between the fresh water channel and the brine channel. Each cell is connected by its own feed/product stream and brine stream. The unit cells and electrodes are arranged horizontally or vertically. The entire unit is called a stack. The entire stack is commercially available from manufacturers as such.

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Salt solution (such as seawater) enters in parallel path through all the cells while an electric potential is maintained across the electrodes. The positively charged cations in the solution migrate toward the cathode, and the negatively charged anions migrate toward the anode.

Cations easily pass through the negative charged cation exchange membrane but are retained by the positively charged anion exchange membrane. Similarly anions pass through the positively charged anion exchange membrane but are retained by the negatively charged cation exchange membrane. 

For example, in the fresh water channel, the anions are attracted and diverted towards the cathode, while at the same time, the cations are attracted and diverted towards the anode, in an opposite direction to the movement of anions. The anions pass through the anion-selective membrane into the brine channel. The anions cannot pass through the next membrane because it is a cation-selective membrane. The anions are therefore trapped in the brine channel.

Likewise, the cations pass through the cation-selective membrane into the brine channel. The cations are also trapped in the brine channel as the next membrane is anion-selective.

Thus, the freshwater channel is depleted of both cations and anions through migration across the membranes. At the same time, the brine channel next to the freshwater channel becomes enriched in cations and anions. 

Advantages and Disadvantages of ED (and EDR) System  :-

Advantages:

ED system separates without phase change, which results in relatively low energy consumption. When brackish water is desalted by ED system, the product water needs only limited pre-treatment.

Typically only chlorination for disinfection is required. Because ED system removes only ionized species, it is particularly suitable for separating non-ionized

fro ionized components. Osmotic pressure is not a factor in ED system, so the pressure can be used for concentrating salt

solutions to 20% or higher.

 Disadvantages:

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Organic matter, colloids and SiO2 are not removed by ED system. Feed water pre-treatment is necessary to prevent ED stacks fouling. Elaborate controls are required, and keeping them at optimum condition can be difficult. Selection of materials of construction for membranes and stack is important to ensure compatibility

with the feed stream.

NANO FILTERATION:-

A nanofiltration filter has a pore size around 0.001 micron. Nanofiltration removes most organic molecules, nearly all viruses, most of the natural organic matter and a range of salts. Nanofiltration removes divalent ions, which make water hard, so nanofiltration is often used to soften hard water.

The Nanofiltration process is a Reverse Osmosis process using a relatively open RO membrane, allowing water and small univalent ions (Na+, K+, and Cl-) to pass.

Typical APV NF applications are:

Dairy

Whey partial demineralization and concentration o For WPC production dedicated to special applications such as baby foods o As a pre-treatment for Ion Exchange and Electro dialysis o For reduction of the salt content of cheddar whey

Milk partial demineralization o Special market milks o Special milk powders

UF permeate partial demineralization o Fermentation o Special applications such as baby foods o Special lactose powders

The “NFX” Nanofiltration membrane provides high quality liquid separation with tremendous efficiency.

NFX membranes have an approximate molecular weight cut-off of 150-300 Daltons, with an average

MgSO4 rejection of 99% or higher. This rejection is a significant improvement over conventional NF

membranes that only promise 98% rejection.

The NFX membrane is ideal for rejecting divalent and multivalent ions, while monovalent ion rejection is

dependent on the concentration in the feed stream. Nanofiltration membranes operate at lower pressures

than reverse osmosis membranes, offering lower energy and equipment costs for applications that do not

require high salt rejection rates.

Synder NFX membranes greatly reduce levels of heavy metals, hardness, nitrates, sulfates, tannins, color

and TDS including moderate levels of salt from feed water streams. In addition, pH is virtually unchanged

from the incoming source so the permeate is not aggressive and will not cause increased corrosion.

NFX membranes can be retrofitted with all major brands and models.

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Applications

Brackish water treatment (desalination / drinking water)

Eliminate odor, color, THM precursors from ground water

Reverse Osmosis pre-treatment (desalination / drinking water)

Removal of hardness from seawater, partial NaCl removal

Lactose concentration, whey protein fractionation (dairy)

Dye effluent & PVA recovery / removal of salt & sulfite (textile)

Mining waste purification / metals recovery

Antibiotics production (Pharma), Dialysis, blood serum processing

Food & Beverage, Maple Syrup, Corn Wet Processing, Dealcoholization, Juice

processing, fragrance recovery

Hydrogen Production, Power Gen, Boiler, Cooling Tower Water treatment

Pulp & Paper processing / sulfite liquor clean up

Pollution Control: nuclear coolant clean up, nitrate reduction (surface/well water), surfactant recovery,

landfill leach ate, COD/BOD, color, colloidal silica, and particulate removal.

REVERSE OSMOSIS :-

 Reverse osmosis affects separation of very small solutes, including salts with ionic radii in the angstroms range. The solute moves through the membrane mainly under concentration gradient forces, while the solvent transport is dependent on the hydraulic pressure gradient. Pores in reverse osmosis membranes are so small they have not

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yet been resolved, even by the most advanced microscopic techniques. They are generally regarded to be in the 4 to 8 angstroms range, four orders of magnitude smaller than the finest of the normal-flow particle filters.

 

Basic Concepts

Consider the 2 tanks connected as shown in the Figure. The 2 tanks are joined by a pipe which contains a semi-permeable membrane. A pure solvent, such as water, is contained in one tank and a saline solution (ordinary salt dissolved in water) in the other tank. The semi-permeable membrane will allow the passage of the water molecules, but not the salt ions.

Because the water molecules are less concentrated in the saline solution (due to the presence of the salt ions), a water concentration gradient will exist across the membrane between the pure water and the saline solution. This water concentration gradient will cause the flow of water molecules from the pure water side across the membrane to the saline solution side.

A similar salt concentration gradient will exist in the opposite direction that will cause the flow of salt ions across the membrane from the saline solution side to the pure water side. Although the concentration gradient exists, the membrane will prevent the flow of the salt ions across it.

This process of osmosis will stop when the pressure created by the elevated level of saline solution is equal to the driving pressure of the water concentration gradient, known as the osmotic pressure. The condition is known as osmotic equilibrium. This is shown in the Figure.

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To reverse the flow of water molecules, an external pressure greater than the osmotic pressure must be applied to the saline solution side. This results in the reverse flow of water molecules across the membrane back to the pure water side.. This flow is termed the reverse osmosis, and forms the basis of separation whereby a solution can be concentrated by forcing out the solvent.

RO Membranes and Materials

Spiral wound membranes 8-in in diameter and 40-in in length are the type most commonly used for RO. RO membranes can be grouped into 3 main categories:

Seawater membranes operated with 3 - 5 wt% salt solutions at pressures of 800 - 1000 psi. Brackish water membranes operated with 2,000 - 10,000 ppm salt solutions at pressures 200 - 400 psi. Ultrapure water low-pressure nanofiltration membranes operated with 200 - 500 ppm salt solutions at

pressures 100 - 150 psi.

Operation of Reverse Osmosis (RO) Systems

A simple schematic of an RO system is shown in the Figure. The feed water is pumped into a pressure vessel containing the semi-permeable RO membranes. The RO device is also known as a permeator. The pressurized concentrated liquid, called brine or reject, is let down to atmospheric pressure through a flow-regulating valve. The purified water, called permeates or product water, is recovered at low or atmospheric pressure.

In its simplest design form, an RO system consists of a pump to pressurized the feed water, an RO device, and a throttling (flow control) valve on the brine outlet to control the conversion. A typical RO plant uses this basic design in modular form to achieve the desired product flow and water quality.

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There are 2 basic types of pumps used in RO systems - centrifugal pumps and positive-displacement pumps. The piping configuration for both pump systems is shown in the Figure.

For the centrifugal pump, a throttling valve on the discharge line controls the feed pressure to the RO system. A back-pressure valve for re-circulation of about 10% of the flow can be used when the pump capacity is greater than needed to supply the RO system.

For the positive-displacement pump, a relief valve is required for protection again high pressure. Flow restrictions, such as a shutoff valve, must never be used between the pump and the relief valve.

The back-pressure valve controls the system pressure and allows for re-circulation of about 10% of the flow when the pump capacity is greater than needed to supply the RO system. Pulsation dampeners are required on the pump inlet and outlet lines to minimize pressure fluctuations.

For large-scale RO plants, continuous monitoring is used for such factors as temperature, feed pH, feed conductivity, brine and product conductivities, feed chlorine content, energy consumption, etc.

Advantages and Disadvantages of RO Systems :-

Advantages:

RO performs a separation without a phase change. Thus, the energy requirements are low.  RO systems are compact, and space requirements are less than with other desalting systems, e.g.

distillation. RO equipment is standardized - pumps, motors, valves, flow meters, pressure gages, etc. Thus, the

learning curve for unskilled labour is short. Many RO systems are fully automated and designed to start-up and shutdown automatically through

interlocks. Thus, RO plants usually require little labour. Due to their modular design, maintenance is easy. Scheduled maintenance can be performed without

shutting down the entire plant. The modular design also makes expansion an easy option.

Disadvantages:

The applied pressure must exceed the osmotic pressure to obtain product flow and to separate the solute from the solvent. The maximum feed pressure for seawater devices varies from 800 - 1000 psig,

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while the limit for brackish water varies from 400 - 600 psig. Due to the high pressure requirement (about 200 psig or more above the osmotic pressure) RO is usually not applicable for concentrated solutions.

Because all RO membranes and devices are susceptible to fouling, the RO process usually cannot be applied without pretreatment.

RO feed streams must be compatible with the membrane and other materials of construction used in the devices. If the feed stream contains incompatible compounds, these must be removed in pretreatment, or another compatible device and/or membrane must be considered.

APPLICATIONS OF REVERSE OSMOSIS:-

Drinking water :-All over the world, the technology of reverse osmosis in providing safe drinking water in households is now common both for homes and big establishments. In the U.S. military where it was first developed, R.O.W.P.U.s (Reverse Osmosis Purification Units) produces 12,000 to 60,000 gallons of water for 1,000 to 6,000 soldiers.The purified water is also safe from NBC (nuclear/biological/chemical agents) after the reverse osmosis process.

Wastewater:- In big cities and progressive areas, collected rainwater is purified with reverse osmosis and used for irrigation of landscapes and industrial cooling. For big power plants, reverse osmosis is used to remove the minerals from water used in the boilers. The water has to be pure and free from minerals that leave deposits on the machinery and cause corrosion and other damages. In Singapore, the authorities had announced their intention to use reverse osmosis to treat their wastewater before discharging them back into the reservoirs.

Food industry   :-Reverse osmosis is used in the concentration (thickening) of food liquids (orange juice, tomato juice) that lose their nutritional values if processed with heat.It is also used in the dairy industry in the production of whey protein powder and the concentration of milk to reduce shipping and storage costs. It is already used by the wine industry, although it had been frowned before. Now, reverse osmosis machines are used by many wineries in France, even by well-known companies. The machines were used to concentrate the grape juices, and removing taints as acetic acid, smoke, and the removal of alcohol in some..

Car wash :-In places where there is ‘hard water’, enterprising car wash entrepreneurs employ the use of reverse osmosis machines.Hard water causes water spotting on vehicles. Reverse osmosis removes the heavy minerals in their original water. Reverse osmosis also reduces demands from customers for drying their vehicles which adds costs. 

Reef aquariums :-Many reef aquarium keepers are now using reverse osmosis systems to produce water for their artificial mix of seawater. They found that ordinary tap water often contains excessive amounts of chlorine, chloramines, heavy metals, and many other chemicals that are bad for the reef environment in their aquariums. Other contaminants such as nitrogen compounds and phosphates lead to excessive algae growth.Today, reef aquarium owners use the combination of reverse osmosis machines and deionization because of low ownership costs and minimal running costs.

Desalination :-In places where there is limited water, authorities use reverse osmosis technology to desalinate the sea for their drinking water. In the Middle East especially in Saudi Arabia, large reverse osmosis and multistage flash desalination plants are in harness. The energy requirements are large, but they are offset by the fact that these countries are oil-producers.So far, these are just some of the many applications of reverse osmosis, although there had been other small uses as well using the principle of the process (in hydrogen production, organics removal, etc). It will not be a surprise if more uses will be added to this list.

Page 21: Classification of Synthetic Membranes