MEMBRANE TECHNOLOGYMembrane technology can be tracedback to 18th century scientists. However,during 19th and in the beginning of 20thcenturies membranes were being usedonly on laboratory scale to developphysical and chemical theories andwere not being used for industrial andcommercial purpose.
There were four main reasons whichprohibited the wide use of membraneseparation process, those obstacles were1. reliability, 2. efficiency, 3. cost and 4.limited choice, however over the last three decadesthese obstacles have been resolved andnow these days membrane separationprocesses are being widely used
DEFINITIONMembrane technology has been provenvery effective in separation andpurification process.A membrane can be defined as “A barrierwhich separates two phases and restrictstransport of various chemicals in aselective manner”
A membrane could be a homogeneous orheterogeneous, symmetric or asymmetric, solid orliquid in structure. Membranes can carry “a positiveor negative charge or be neutral or bipolar”.Membrane thicknesses vary from 100 micron toseveral mms. A membrane separates the feedstream into two streams permeate andconcentrate. Permeate is the portion of main feedstream which passes through the membrane whilethe concentrate contains the material which isrejected by the membrane
APPLICATIONS• Membrane technology has wide range of application in food and dairy industry• Waste streams treatment• Separation of milk fraction• Concentrating of protein• In cheese manufacturing to recover the protein from brine used in washed cheese manufacturing• In dairy industry for defatting of skimmed milk and whey streams• For the partial demineralization of whey• For the removal of bacteria from milk and whey• The choice of membrane depends on the application objective, however, the most commonly used membrane are,
Micro-porous membraneThese membranes are usually made up of materialslike ceramics, graphite, metal oxides and polymersetc. The pore size of these membranes varies from1 nm-20 microns. Membrane works like a fibre filterand separates by sieving mechanism (Srikanth2005). In structure and function microporousmembranes are similar to conventional filters,however, the pore size is very smaller as comparedto conventional filter. Microporous membranespores sizes range from 0.01 to 10 μm
Homogeneous membranesHomogeneous membranes are densemembranes through which moleculespass by pressure, concentration orelectrical potential gradient. Thesemembranes are used to separate thechemical species of similar size anddiffusivity when their concentrationdifference significant
Asymmetric membranesAsymmetric membranes consist of twoparts; thin skin layer (0.1-1.0 micron) layon highly porous (100-200 micron) thicksubstructure. The thin layer acts as aseparator and its separationcharacteristics depends on the membranematerial and its pore size. Porous sub layerhas a little impact on separation its mainpurpose is to give support to the thin layer
Liquid membranesThese membranes utilizethe carrier to transport thecomponents selectivelylike metal ions “atrelatively high rate acrossthe membrane interface”
There are, in fact, two basic types of liquidmembranes, an Emulsion Liquid Membrane (ELM),and an Immobilized Liquid Membrane (ILM), alsocalled a Supported Liquid Membrane. An ELM canbe thought of as a bubble inside a bubble inside abubble, and so on; the inner most bubble being theone recieving phase, all the others acting asseparation skins with carriers inside, and anythingoutside the bubble being the source phase. In anELM setup, there would be huge quantities of thesebubbles, of course, all doing the same thing.
An ILM is much simpler to visualize. Pretty muchwhat you have is some other kind of rigidmembrane, with lots of microscopic pores in it.Every one of these pores, then, is filled with thisliquid, and in that liquid, you have the organicliquid and the carrier liquid. What happens thenis that the ILM takes things from one side of therigid membrane and carries it to the other sidethrough this liquid phase. And that, my friends,is pretty a very brief model of what a LM is.
Membrane operationsAccording to driving force of the operationit is possible to distinguish:pressure driven operations microfiltration ultrafiltration nanofiltration reverse osmosis gas separation pervaporation
• concentration driven operations – dialysis – osmosis – forward osmosis• operations in electric potential gradient – electrodialysis – membrane electrolysis – electrophoresis• operations in temperature gradient – membrane distillation
Widely Used Membrane ProcessesThere are various types of membrane separation according to the specific industrial needs.The most widely used processes are,Reverse Osmosis (RO)Ultrafiltration (UF)Micro filtration (MF)Electro dialysis (ED)Gas SeparationPervaporation
Reverse Osmosis (RO)Reverse Osmosis is a high pressure membrane process whichoperates at a pressure between 30 -40 bars.This is a reverse ofnatural osmosis which works by putting the pressure on theconcentrated side of the membrane which overcome the naturalosmotic pressure.Reverse Osmosis membranes have the smallest pore size rangingfrom approximately 5-15 A° (0.5nm 1.5nm). Extremely small sizeof membrane pores only allow to pass through the smallestorganic molecules and unchanged solutes.More than 95-99%inorganic salts gets rejected by the membrane due to the chargerepulsion established at membrane surface.
As compare to basic membrane methods likemicrofiltration(MF), ultrafiltrtation(UF)andNanofiltratiion(NF) recerse Osmosis can remove thesmallest particles retaining particles smaller than0.001 microns. Reverse Osmosis can remove theparticles down to the molecular weight of 100.Rverse Osmosis(RO) can effectively remove sand,silt, clay, algae, protozoa(5-10 microns)bacteria(0.4-30 microns), viruses (0.004 -6 microns)humic acids, organic/inorganic chemicals and mostof the aqueous salts and metal/non-metal ionsincluding NO3-1, iron and manganese.
applicationReverse Osmosis (RO) technique is extensively applied in the following fieldsConversion of sea or brackish water into potable waterTo get the ultrapure water for food processing and electronic industriesTo get the pharmaceutical grade waterFor chemical, pulp and paper industry usable waterUsage in waste treatment
Future applicationsReverse Osmosis technique could have a good potential to use in thefuture in the following sectorsMunicipal and industrial waste treatment applicayionsTo process the water for boilersTo de-water feed streamsTo process high temperature feed streams etc
Micro filtration (MF)Microfiltration is a low pressure membrane systemwhich operates at between 0.1 to 0.5 bars. Crossflow membranes are used and the suspendedparticles in the range of 0.05 to 10 microns can beremoved. At present MF membrane technology isthe most widely used membrane technology itsapplication and sale is more as compare to the restof all membrane technologies. MF has too manysmall applications, essentially it is a sterile filtrationwith pore size 0.1-10.0 microns, this range of poresize can not let micro-organisms to pass through.
applicationsMicrofiltration technology is widely usedTo prepare parenterals and sterile water for pharmaceutical industryConcentration of fruit juices for food and beverages industryIn chemical industryIn microelectronics industryFor fermentationUsage in laboratory/analysis
Future applicationsIn future Microfiltration technology has the potential to use in the followingsectorsBiotechnology sector for the concentration of biomass and separation ofsoluble productsDiatomaceous earth displacementDuring the treatment of non-sewage water to remove intractable particlesfrom oily fluids and aqueous wastes which contain toxic s and stack gasTo separate solvents from pigments in paints industry
Ultrafiltration (UF)Ultrafiltration is mainly used to separate a mixture whichconsists of desirable and undesirable components.Ultrafiltration process operates between 2-10 bars but insome cases it goes up to 25-30 bar. Ultrafiltration (UF) canretain particles from 1000 – 1000 000 molecular weight.Ultrafiltration system can be based on hollow fibre, spiralwound or plate and frame membranes.
Ultrafiltration can be used to produce many newproducts by fractionation of the components likefat, protein etc and can also be used to improvethe functional properties of the product. One ofthe very important applications of ultrafiltration isits application in recovering and concentrating ofvaluable small components like enzymes fromcow milk. Ultrafiltratin technique is also beingused successfully for the isolation of importantcomponents from food processing waste
Ultrafltration by usingmembranes of polyethersulfone and plyvinylpyrlidonecan remove the polyphenolswhich are responsible forbrowning colour and hazeforming in apple juice.
Electrodialysis (ED)Like Reverse Osmosis, ED can remove theparticles smaller than 0.001microns but thecondition is that the particles must becharged ions. It can not remove non ionicdissolved species or microbes. Electrodialysisis an electrochemical process in which ionspass through an ion selective semipermeablemembrane because of their attraction to theelectrically charged membrane surface.
Ions get transported through membranefrom one solution to another under theinfluence of electrical potential. EDsystem consists of anion and cationmembranes which place in electric field.The cation selective membrane only letpass through the cation ions, while theanion selective membrane will let onlycation ions.
applicationsED technique can be applied to for several types of separations like,To separate and concentrate salts, acids and bases from aqueous solutionsTo separate and concentrate monovalent ions from multiple charged componenetsTo separate ionic compounds from uncharged moleculesAt present ED technique is being widely usedIn the production of potable water from sea or brackish waterIn electroplating rinse recoveryIn desalting of cheese wheyIn the production of ultrapurewater etc
Future applicationsfuture applications for ED areTo de-ionize water from conductive spacersTo treat radioactive wastewater by using radiation resistant membranesFor the de-acidification of fruit juicesTo recover heavy metalTo recover organic acids from saltsTo control pH without adding acid or baseTo regenerate ion-exchange resins with improved process designTo recover acid from etching baths etc
Gas Separation• Gas separation technology is nearly eleven years old but has been proven one of the most important technology. Membranes made up of polymers and copolymers in the form of flat film or hollow fibre are being used in gas separation. Gas separation technology has the advantages of• Light in weight• Low labour• Easy expansion• Operatable at partial capacity• Involves low maintenance• Needs less energy• Economical so for small sizes
applicationsGas separation technology isbeing used in the separation andrecovery of hydrogen , natural gasprocessing, upgrading of landfillgas, separation of air, productionof nitrogen, dehydration of airand recovery of helium etc.
Future applicationsIn future Gas Separation technology has the following potential applicationsAir enrichment by N2Enrichment of air by low level O2H2 and acid gas separation from hydrocarbonsRecovery of heliumDehydration of natural gas
PervaporationPervaporation is a membrane basedprocess to separate miscible liquids.Pervaporation process is veryeffective as compare to conventionaltechniques to separate the mixturesof close boiling point or azeotropicmixtures.
• Pervaporation technique works by absorbing one of the components of the mixture by the membrane, its diffusion across the membrane and then evaporation, partial vacuum applied to the underside of the membrane makes permeate vapour.
• Based on this, hydrophilic membranes are used for dehydration of alcohols containing small amounts of water and hydrophobic membranes are used for removal/recovery of trace amounts of organics from aqueous solutions.• Pervaporation is a very mild process and hence very effective for separation of those mixtures which can not survive the harsh conditions of distillation.
applications• Pervaporation has been used to• To separate ethanol water mixture• To recover solvent• To separate heat sensitive products• To enrich organic pollutants
adavantages• Pervaporation has certain advantages over other separation techniques which are• Its modular membrane design• It is economical and effective to separate mixtures of substances with small difference in boiling points.• Reduced capital cost as compared to conventional techniques