MEMBRANE TECHNOLOGY

Membrane technology can be traced back to 18th century scientists. However, during 19th and in the beginning of 20th centuries membranes were being used only on laboratory scale to develop physical and chemical theories and were not being used for industrial and commercial purpose.

There were four main reasons which prohibited the wide use of membrane separation process, those obstacles were 1. reliability, 2. efficiency, 3. cost and 4. limited choice, however over the last three decades these obstacles have been resolved and now these days membrane separation processes are being widely used

DEFINITION

Membrane technology has been proven very effective in separation and purification process.

A membrane can be defined as “A barrier which separates two phases and restricts transport of various chemicals in a selective manner”

A membrane could be a homogeneous or heterogeneous, symmetric or asymmetric, solid or liquid in structure. Membranes can carry “a positive or negative charge or be neutral or bipolar”. Membrane thicknesses vary from 100 micron to several mms. A membrane separates the feed stream into two streams permeate and concentrate. Permeate is the portion of main feed stream which passes through the membrane while the concentrate contains the material which is rejected 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 membrane

These membranes are usually made up of materials like ceramics, graphite, metal oxides and polymers etc. The pore size of these membranes varies from 1 nm-20 microns. Membrane works like a fibre filter and separates by sieving mechanism (Srikanth 2005). In structure and function microporous membranes are similar to conventional filters, however, the pore size is very smaller as compared to conventional filter. Microporous membranes pores sizes range from 0.01 to 10 μm

Homogeneous membranes

Homogeneous membranes are dense membranes through which molecules pass by pressure, concentration or electrical potential gradient. These membranes are used to separate the chemical species of similar size and diffusivity when their concentration difference significant

Electrically charged membrane

These membranes consist of highly swollen gels which carry fixed positive or negative charged. Their main potential is in electrodialysis.

Asymmetric membranes

Asymmetric membranes consist of two parts; thin skin layer (0.1-1.0 micron) lay on highly porous (100-200 micron) thick substructure. The thin layer acts as a separator and its separation characteristics depends on the membrane material and its pore size. Porous sub layer has a little impact on separation its main purpose is to give support to the thin layer

Liquid membranes

These membranes utilize the carrier to transport the components selectively like metal ions “at relatively high rate across the membrane interface”

There are, in fact, two basic types of liquid membranes, an Emulsion Liquid Membrane (ELM), and an Immobilized Liquid Membrane (ILM), also called a Supported Liquid Membrane. An ELM can be thought of as a bubble inside a bubble inside a bubble, and so on; the inner most bubble being the one recieving phase, all the others acting as separation skins with carriers inside, and anything outside the bubble being the source phase. In an ELM setup, there would be huge quantities of these bubbles, of course, all doing the same thing.

An ILM is much simpler to visualize. Pretty much what you have is some other kind of rigid membrane, with lots of microscopic pores in it. Every one of these pores, then, is filled with this liquid, and in that liquid, you have the organic liquid and the carrier liquid. What happens then is that the ILM takes things from one side of the rigid membrane and carries it to the other side through 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 operation it is possible to distinguish:

pressure driven operations microfiltrationultrafiltrationnanofiltrationreverse osmosisgas separationpervaporation

• 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 Processes

There 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 Separation

Pervaporation

Membrane ProcessApplied pressure psi (kPa) Minimum particle

size removedApplication (type, average removal efficiency %)

Microfiltration 4-70 (30-500) 0.1-3 μmParticle/turbidity removal (>99%)Bacteria/protozoa removal (>99.99 %)

Ultrafiltration 4-70 (30-500) 0.01-0.1 μmParticle/turbidity removal (>99 %)Bacteria/protozoa removal (>99.999 %) TOC removal (<20%)Virus removal/(partial credit only)

Nanofiltration 70-140 (500-1000) 200-400 daltonsTurbidity removal (>99%)Color removal (>98%)TOC removal (DBP control) (>95%) Hardness removal (softening) (>90%)Synthetic organic contaminant (SOC)removal (500 daltons and up) (0-100%) Sulfate removal (>97%)

Virus removal (>95%)

Hyperfiltration (Reverse

Osmosis)

140-700 (1000-5000) 50-200 daltonsSalinity removal (desalination) (>99%)Colour and DOC removalRadionuclide removal (not including radon) (>97%)Nitrate removal (85 -95%)Pesticide/SOC removal (0-100%)Virus removal (> 95%)As, Cd, Cr, Pb, F removal (40 to >98%)

Reverse Osmosis (RO)Reverse Osmosis is a high pressure membrane process which operates at a pressure between 30 -40 bars.This is a reverse of natural osmosis which works by putting the pressure on the concentrated side of the membrane which overcome the natural osmotic pressure.

Reverse Osmosis membranes have the smallest pore size ranging from approximately 5-15 A° (0.5nm 1.5nm). Extremely small size of membrane pores only allow to pass through the smallest organic molecules and unchanged solutes.More than 95-99% inorganic salts gets rejected by the membrane due to the charge repulsion established at membrane surface.

As compare to basic membrane methods like microfiltration(MF), ultrafiltrtation(UF)and Nanofiltratiion(NF) recerse Osmosis can remove the smallest particles retaining particles smaller than 0.001 microns. Reverse Osmosis can remove the particles 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 most of the aqueous salts and metal/non-metal ions including NO3-1, iron and manganese.

applicationReverse Osmosis (RO) technique is extensively applied in the following fields

Conversion of sea or brackish water into potable water

To get the ultrapure water for food processing and electronic industries

To get the pharmaceutical grade water

For chemical, pulp and paper industry usable water

Usage in waste treatment

Future applicationsReverse Osmosis technique could have a good potential to use in the future in the following sectors

Municipal and industrial waste treatment applicayions

To process the water for boilers

To de-water feed streams

To process high temperature feed streams etc

Micro filtration (MF)

Microfiltration is a low pressure membrane system which operates at between 0.1 to 0.5 bars. Cross flow membranes are used and the suspended particles in the range of 0.05 to 10 microns can be removed. At present MF membrane technology is the most widely used membrane technology its application and sale is more as compare to the rest of all membrane technologies. MF has too many small applications, essentially it is a sterile filtration with pore size 0.1-10.0 microns, this range of pore size can not let micro-organisms to pass through.

applicationsMicrofiltration technology is widely used

To prepare parenterals and sterile water for pharmaceutical industry

Concentration of fruit juices for food and beverages industry

In chemical industry

In microelectronics industry

For fermentation

Usage in laboratory/analysis

Future applicationsIn future Microfiltration technology has the potential to use in the following sectors

Biotechnology sector for the concentration of biomass and separation of soluble products

Diatomaceous earth displacement

During the treatment of non-sewage water to remove intractable particles from oily fluids and aqueous wastes which contain toxic s and stack gas

To separate solvents from pigments in paints industry

Ultrafiltration (UF)

Ultrafiltration is mainly used to separate a mixture which consists of desirable and undesirable components.

Ultrafiltration process operates between 2-10 bars but in some cases it goes up to 25-30 bar. Ultrafiltration (UF) can retain particles from 1000 – 1000 000 molecular weight. Ultrafiltration system can be based on hollow fibre, spiral wound or plate and frame membranes.

Ultrafiltration can be used to produce many new products by fractionation of the components like fat, protein etc and can also be used to improve the functional properties of the product. One of the very important applications of ultrafiltration is its application in recovering and concentrating of valuable small components like enzymes from cow milk. Ultrafiltratin technique is also being used successfully for the isolation of important components from food processing waste

Ultrafltration by using membranes of polyether sulfone and plyvinylpyrlidone can remove the polyphenols which are responsible for browning colour and haze forming in apple juice.

Electrodialysis (ED)

Like Reverse Osmosis, ED can remove the particles smaller than 0.001microns but the condition is that the particles must be charged ions. It can not remove non ionic dissolved species or microbes. Electrodialysis is an electrochemical process in which ions pass through an ion selective semipermeable membrane because of their attraction to the electrically charged membrane surface.

Ions get transported through membrane from one solution to another under the influence of electrical potential. ED system consists of anion and cation membranes which place in electric field. The cation selective membrane only let pass through the cation ions, while the anion selective membrane will let only cation ions.

applicationsED technique can be applied to for several types of separations like,

To separate and concentrate salts, acids and bases from aqueous solutions

To separate and concentrate monovalent ions from multiple charged componenets

To separate ionic compounds from uncharged molecules

At present ED technique is being widely used

In the production of potable water from sea or brackish water

In electroplating rinse recovery

In desalting of cheese whey

In the production of ultrapurewater etc

Future applicationsfuture applications for ED are

To de-ionize water from conductive spacers

To treat radioactive wastewater by using radiation resistant membranes

For the de-acidification of fruit juices

To recover heavy metal

To recover organic acids from salts

To control pH without adding acid or base

To regenerate ion-exchange resins with improved process design

To 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

applications

Gas separation technology is being used in the separation and recovery of hydrogen , natural gas processing, upgrading of landfill gas, separation of air, production of nitrogen, dehydration of air and recovery of helium etc.

Future applications

In future Gas Separation technology has the following potential applications

Air enrichment by N2

Enrichment of air by low level O2

H2 and acid gas separation from hydrocarbons

Recovery of helium

Dehydration of natural gas

Pervaporation

Pervaporation is a membrane based process to separate miscible liquids. Pervaporation process is very effective as compare to conventional techniques to separate the mixtures of close boiling point or azeotropic mixtures.

• 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