Membranes and Membranes and Carbon Carbon

AdsorptionAdsorption

CE 547CE 547

Electrodialysis Electrodialysis MembranesMembranes

Pressure MembranesPressure MembranesRORONFNFUFUFMFMF

Electrodialysis Electrodialysis MembranesMembranes

Electrodialysis Units Electrodialysis Units (Cells) are composed (Cells) are composed mainly of:mainly of:

CathodeCathode AnodeAnode Concentrating Concentrating

compartmentscompartments Diluting compartmentsDiluting compartments Cation permeable Cation permeable

membranesmembranes Anion permeable Anion permeable

membranesmembranes

Filtering MembranesFiltering Membranes Sheet-like barriersSheet-like barriers Made of high capacity ion exchange Made of high capacity ion exchange

resinsresins Allow passage of ions but not waterAllow passage of ions but not water

Water in diluting compartments Water in diluting compartments is the product water, while water is the product water, while water in concentrating compartments in concentrating compartments is discharged to drain.is discharged to drain.

Power Requirement of Power Requirement of Electrodialysis UnitsElectrodialysis Units

In the Figure, there is:In the Figure, there is: Four membranes (C, A, C, A)Four membranes (C, A, C, A) Two diluting or deionizing compartmentsTwo diluting or deionizing compartments

Generally, If the number of membranes = mGenerally, If the number of membranes = m

Then, Then, The number of deionizing compartments = The number of deionizing compartments = m / 2m / 2

If QIf Q00 = flow to the Electrodialysis unit = flow to the Electrodialysis unit

Then Flow per deionizing compartment = Q0 / (m / Then Flow per deionizing compartment = Q0 / (m / 2)2)

If If CC00 = influent ion concentration = influent ion concentration (equivalents / unit volume)(equivalents / unit volume)

Then the total rate of inflow of ions = [CThen the total rate of inflow of ions = [C00]Q]Q00 / / (m / 2) (m / 2) (in equivalents / time / cell)(in equivalents / time / cell)

Since 1 equivalent = 1 Faraday = 96494 coulombsSince 1 equivalent = 1 Faraday = 96494 coulombs

Assume efficiency = Assume efficiency = Then the amount of electricity needed to remove Then the amount of electricity needed to remove the ions in one cell (in coulombs per unit time) the ions in one cell (in coulombs per unit time)

2

96494 00

m

QC

Since, amperes = coulombs / secondSince, amperes = coulombs / second

Therefore:Therefore:

(in amperes of current)(in amperes of current)

Since cells are connected in series, therefore, Since cells are connected in series, therefore, the same current will be used in whole unit.the same current will be used in whole unit.

2

96494 00

m

QC

In Electrodialysis, a new term called In Electrodialysis, a new term called Current Density (CD) is used.Current Density (CD) is used.

CD = current in mill amperes that flows CD = current in mill amperes that flows through a square centimeter of through a square centimeter of membrane perpendicular to the current membrane perpendicular to the current direction = mA / Acmdirection = mA / Acm

Where,Where,

mA = milliamperes of electricitymA = milliamperes of electricity

Acm = square centimeters of Acm = square centimeters of perpendicular areaperpendicular area

If,If,

= current efficiency= current efficiency

I = electric currentI = electric current

Then,Then,

2/

96494

2/

96494

00

00

m

QCI

or

m

QCI

Electromotive ForceElectromotive Force

R = resistance across the unit (ohms)R = resistance across the unit (ohms) I = current (amperes)I = current (amperes) E = emf (volts)E = emf (volts)

Then,Then,

Power is in wattsPower is in wattsAnother term is usedAnother term is used(CD / N) = current density to normality (CD / N) = current density to normality (N (N = normality)= normality)

IREemfForceiveElectromot ,,

R

m

QC2

00101072.3 EI P power, The

ExampleExample

Pressure MembranesPressure Membranes

They are membranes that are used to They are membranes that are used to separate solids from fluid by application of separate solids from fluid by application of pressure on the membranepressure on the membrane

Membrane Pore Size Pressure

MF Larger than UF 70 kPa

UF 0.001 – 10 m 100 – 500 kPa

NF Between UF and RO 500 – 1400 kPa

RO 0.0001 – 0.001 1400 – 8300 kPa

Module DesignModule Design

TubularTubular Hollow fibersHollow fibers Spiral woundSpiral wound Plate and framePlate and frame

Tubular MembranesTubular Membranes

Tubular modules consist of a set of parallel tubes all penetrating a circular plate at either end of a tube bundle housed inside a larger shell, or shroud. Feed material is pumped through the tubes in a cross-flow manner. Permeate is collected in the shroud while the retentate passes out the other end of the tubes. Advantages include turbulent flow (providing good membrane/solution contact and removing retentate film buildup), relatively easy cleaning, easy handling of suspended solids and viscous fluids and ability to replace or plug a failed tube while the rest of the system runs. Disadvantages include high capital cost, low packing density, high pumping costs, and limited achievable concentrations.

Tubular MembranesTubular Membranes

Hollow Fiber MembranesHollow Fiber MembranesHollow-fiber membranes, another cross-flow arrangement, consist of hollow, hair-like fibers bundled together into either a U-shape or straight-through configuration. Tube bundles are inside a pressure vessel and feed material normally flows inside the tubes. Fibers in the straight-through design are somewhat larger and allow low levels of suspended solids. The finer strands in the U-tube cannot tolerate suspended solids. U-tubes tend to be used for reverse osmosis, and the straight-through design for ultrafiltration. Advantages include low pumping power, very high packing density, cleaning can be accomplished with backflushing, and ability to achieve high concentrations in the retentate. Disadvantages include the fragility of the fibers, inability to handle suspended solids well, difficult cleaning and, in the straight-through design, damage of one fiber requires replacement of the entire module.

Hollow Fibers Hollow Fibers MembranesMembranes

Spiral Wound Spiral Wound MembranesMembranes

Spiral membranes consist of two layers of membrane, placed Spiral membranes consist of two layers of membrane, placed onto a permeate collector fabric. This membrane envelope is onto a permeate collector fabric. This membrane envelope is wrapped around a centrally placed permeate drain (see wrapped around a centrally placed permeate drain (see picture below). This causes the packing density of the picture below). This causes the packing density of the membranes to be higher. The feed channel is placed at membranes to be higher. The feed channel is placed at moderate height, to prevent plugging of the membrane unit. moderate height, to prevent plugging of the membrane unit. Spiral membranes are only used for nano filtration and Spiral membranes are only used for nano filtration and Reverse Osmosis (RO) applications.Reverse Osmosis (RO) applications.

A spiral wound membrane module comprises a spiral wound A spiral wound membrane module comprises a spiral wound membrane element including a separation membrane having membrane element including a separation membrane having high back pressure strength. The flow rate of permeate in high back pressure strength. The flow rate of permeate in filtration of the spiral wound membrane element is preferably filtration of the spiral wound membrane element is preferably set to 0.5 to 2.0 mset to 0.5 to 2.0 m33 /m /m22 /day, and the filtration time is /day, and the filtration time is preferably set to 10 to 300 minutes. The flow rate of wash preferably set to 10 to 300 minutes. The flow rate of wash water in washing is preferably set to 1.0 to 4.0 mwater in washing is preferably set to 1.0 to 4.0 m33 /m /m22 /day, /day, and the washing time is preferably set to 10 to 300 seconds. In and the washing time is preferably set to 10 to 300 seconds. In such ranges, the ratio of the permeate volume in filtration to such ranges, the ratio of the permeate volume in filtration to the permeate volume in back wash reverse filtration is set to the permeate volume in back wash reverse filtration is set to be not more than 600. be not more than 600.

Spiral Wound MembranesSpiral Wound Membranes

Plate and Frame Plate and Frame MembraneMembrane

Plate and frame membranes were among the earliest configurations in the market. Plate and frame devices use flat membrane sheets with permeate collection between the sheets. The sheets are sealed around the edges but with a provision for permeate removal (usually by a tube). Several of these plates are stacked on top of each other and clamped together o form a module or cartridge. Many plate and frame systems are based on dead-end flow and are more subject to plugging. The most commercially significant application of the plate and frame design is in electrodialysis modules (or stacks), although some microfiltration units and one reverse osmosis module design are also based on this configuration.

Plate and Frame Plate and Frame MembranesMembranes

Dead-End vs Crossflow Dead-End vs Crossflow FiltrationFiltration

CrossflowCrossflow

MicrofiltrationMicrofiltration

Membranes with a pore size of 0.1 – 10 µm perform Membranes with a pore size of 0.1 – 10 µm perform microfiltration. Microfiltration membranes remove all microfiltration. Microfiltration membranes remove all bacteria. Only part of the viral contamination is caught up bacteria. Only part of the viral contamination is caught up in the process, even though viruses are smaller than the in the process, even though viruses are smaller than the pores of a microfiltration membrane. This is because pores of a microfiltration membrane. This is because viruses can attach themselves to bacterial biofilm. viruses can attach themselves to bacterial biofilm. Microfiltration can be implemented in many different Microfiltration can be implemented in many different water treatment processes when particles with a diameter water treatment processes when particles with a diameter greater than 0.1 mm need to be removed from a liquid.greater than 0.1 mm need to be removed from a liquid.

Examples of micro filtration applications are:Examples of micro filtration applications are: Cold sterilisation of beverages and pharmaceuticalsCold sterilisation of beverages and pharmaceuticals Clearing of fruit juiceClearing of fruit juice Separation of bacteria from water (biological wastewater Separation of bacteria from water (biological wastewater

treatment)treatment) Effluent treatmentEffluent treatment Separation of oil/ water emulsionsSeparation of oil/ water emulsions Pre-treatment of water for nano filtration or Reverse Pre-treatment of water for nano filtration or Reverse

OsmosisOsmosis Solid-liquid separation for pharmacies or food industriesSolid-liquid separation for pharmacies or food industries

UltrafiltrationUltrafiltration

For complete removal of viruses, ultra For complete removal of viruses, ultra filtration is required. The pores of ultra filtration is required. The pores of ultra filtration membranes can remove particles filtration membranes can remove particles of 0.001 – 0.1 µm from fluids.of 0.001 – 0.1 µm from fluids.

Examples of fields where ultra filtration Examples of fields where ultra filtration is applied are:is applied are:

The dairy industry (milk, cheese)The dairy industry (milk, cheese) The food industry (proteins)The food industry (proteins) The metal industry (oil/ water emulsions The metal industry (oil/ water emulsions

separation, paint treatment)separation, paint treatment) The textile industry The textile industry

NanofiltrationNanofiltrationNanofiltration is a technique that has prospered over the past Nanofiltration is a technique that has prospered over the past few years. Today, nanofiltration is mainly applied in drinking few years. Today, nanofiltration is mainly applied in drinking water purification process steps, such as water softening, water purification process steps, such as water softening, decoloring and micro pollutant removal. During industrial decoloring and micro pollutant removal. During industrial processes nanofiltration is applied for the removal of specific processes nanofiltration is applied for the removal of specific components, such as coloring agents. Nanofiltration is a components, such as coloring agents. Nanofiltration is a pressure related process, during which separation takes place, pressure related process, during which separation takes place, based on molecule size. Membranes bring about the separation. based on molecule size. Membranes bring about the separation. The technique is mainly applied for the removal of organic The technique is mainly applied for the removal of organic substances, such as micro pollutants and multivalent ions. substances, such as micro pollutants and multivalent ions. Nanofiltration membranes have a moderate retention for Nanofiltration membranes have a moderate retention for univalent salts.univalent salts.

Other applications of nano filtration are:Other applications of nano filtration are: The removal of pesticides from groundwaterThe removal of pesticides from groundwater The removal of heavy metals from wastewaterThe removal of heavy metals from wastewater Wastewater recycling in laundriesWastewater recycling in laundries Water softeningWater softening Nitrates removalNitrates removal

Reverse OsmosisReverse OsmosisReverse Osmosis is based upon the fundamental pursuit Reverse Osmosis is based upon the fundamental pursuit for balance. Two fluids containing different for balance. Two fluids containing different concentrations of dissolved solids that come in contact concentrations of dissolved solids that come in contact with each other will mix until the concentration is with each other will mix until the concentration is uniform. When these two fluids are separated by a semi uniform. When these two fluids are separated by a semi permeable membrane (which lets the fluid flow through, permeable membrane (which lets the fluid flow through, while dissolved solids stay behind), a fluid containing a while dissolved solids stay behind), a fluid containing a lower concentration will move through the membrane lower concentration will move through the membrane into the fluids containing a higher concentration of into the fluids containing a higher concentration of dissolved solids. After a while the water level will be dissolved solids. After a while the water level will be higher on one side of the membrane. The difference in higher on one side of the membrane. The difference in height is called the osmotic pressure. By pursuing height is called the osmotic pressure. By pursuing pressure upon the fluid column, which exceeds the pressure upon the fluid column, which exceeds the osmotic pressure, one will get a reversed effect. Fluids osmotic pressure, one will get a reversed effect. Fluids are pressed back through the membrane, while dissolved are pressed back through the membrane, while dissolved solids stay behind in the column. Using this technique, a solids stay behind in the column. Using this technique, a larger part the salt content of the water can be removed.larger part the salt content of the water can be removed.

Reverse OsmosisReverse Osmosis

The applications of Reverse Osmosis The applications of Reverse Osmosis application are:application are:

Water softeningWater softening Drinking water productionDrinking water production Process water productionProcess water production Ultra pure water production (electronic Ultra pure water production (electronic

industries)industries) Concentration of molecular solvents for Concentration of molecular solvents for

food and dairy industriesfood and dairy industries

Feed Pre-TreatmentFeed Pre-TreatmentThe pre-treatment of feed water for nanofiltration or The pre-treatment of feed water for nanofiltration or Reverse Osmosis installations greatly influences the Reverse Osmosis installations greatly influences the performance of the installation. The required form of pre-performance of the installation. The required form of pre-treatment depends on the feed water quality. The purpose treatment depends on the feed water quality. The purpose of pre-treatment is reducing the organic matter content of pre-treatment is reducing the organic matter content and the amount of bacteria, as well as lowering the MFI.and the amount of bacteria, as well as lowering the MFI.

The organic matter content and the amounts of bacteria The organic matter content and the amounts of bacteria should be as low as possible to prevent the so-called should be as low as possible to prevent the so-called biofouling of membranes. The application of a pre-biofouling of membranes. The application of a pre-treatment has several benefits:treatment has several benefits:

Membranes have a longer life-span when pre-treatment is Membranes have a longer life-span when pre-treatment is performedperformed

The production time of the installation is extendedThe production time of the installation is extended The management tasks become simplerThe management tasks become simpler The employment costs are lowerThe employment costs are lower

This is an overview of the main pre-treatment techniques and the substances, which are reduced during these processes.

Pre-treatment CaCO3 SO4 SiO2 MFI Fe Al Bacteria

Organic matter

Acid dosage X       O      

Anti-scalant O X            

Softening and ion exchange X X            

Preventive cleansing O   O O O O O X

Adjusting of process parametres   O X          

Quick filtration     O O O O    

Flocculation     O X O O    

Micro and ultra filtration     X X O O O X

Candle filtres     O O O O O  

X = highly effectiveO = effective pre-treatment

OsmosisOsmosis

Factors affecting solute Factors affecting solute rejection in ROrejection in RO

functional groups present in functional groups present in the membranethe membrane

nature of membrane surfacenature of membrane surfacesize of solute moleculesize of solute moleculepHpHpressure of other solutespressure of other solutes

Types of MembranesTypes of MembranesCellulose AcetateCellulose Acetatepolyethylene Amine (PA-100)polyethylene Amine (PA-100)Polyether Amine (PA – 300)Polyether Amine (PA – 300)Film Tec (FT – 30) [meta-Film Tec (FT – 30) [meta-

phenylene diamine + trimesoyl phenylene diamine + trimesoyl chloride)chloride)

NS – 200 (2-hydroxyl-methyl furan NS – 200 (2-hydroxyl-methyl furan + H2SO4)+ H2SO4)

PEC – 1000 (2-hydroxyl-methyl PEC – 1000 (2-hydroxyl-methyl furan)furan)

Membrane Performance is Membrane Performance is measured by:measured by:

FluxFlux Quality of productQuality of product

Flux is a function of:Flux is a function of: Membrane thicknessMembrane thickness Chemical composition of feedChemical composition of feed Membrane porosityMembrane porosity Time of operationTime of operation Pressure across the membranePressure across the membrane Water temperatureWater temperature

Quality of product is a function of:Quality of product is a function of: rejection ability of the membranerejection ability of the membrane

Flux DeclineFlux Decline

The general trend of flux curve can be The general trend of flux curve can be represented by:represented by:

WhereWhereF = fluxF = fluxt = timet = timeK = constantK = constant

This is a straight-line equation, so, at least two This is a straight-line equation, so, at least two data points are necessary to calculate m and K.data points are necessary to calculate m and K.

KtmF

KtF m

ln)ln()ln(

Flux as a Function of Flux as a Function of PressurePressure

Where,Where,RRmm = resistance due to membrane = resistance due to membrane = viscosity= viscosityP = transmembrane pressureP = transmembrane pressure

PR

Fm

1

Flux as a Function of Flux as a Function of TemperatureTemperature

Since F is a function of Since F is a function of and and is is a function of temperature. a function of temperature. Therefore, F is a function of Therefore, F is a function of temperature.temperature.

Percent Solute Rejection or Percent Solute Rejection or RemovalRemoval

This is a very important parameter in designing an RO processThis is a very important parameter in designing an RO process

Where,Where,R = percent rejectionR = percent rejectionQQ00 = feed flow rate = feed flow rate[C[C00] = feed concentration of solutes] = feed concentration of solutesQQpp = permeate flow rate = permeate flow rate[C[Cpp] = permeate concentration of solutes] = permeate concentration of solutesQQcc = concentrate flow rate = concentrate flow rate[C[Ccc] = concentrate concentration of solutes] = concentrate concentration of solutesi refers to the ii refers to the ithth solute species solute species

100

100

00

0

0

i

cic

oi

pipoi

CQ

CQR

CQ

CQCQR

Carbon AdsorptionCarbon Adsorption

Adsorption: the process of concentrating solute at Adsorption: the process of concentrating solute at the surface of a solid by the virtue of attraction.the surface of a solid by the virtue of attraction.

AdsorptionAdsorption ChemicalChemical chemisorption (weak) chemisorption (weak) PhysicalPhysical van der Waals adsorption (stronger due to van der Waals adsorption (stronger due to

chemical bonding)chemical bonding)

AdsorbateAdsorbate: the solute: the soluteAdsorbentAdsorbent: solid that adsorbs the solute: solid that adsorbs the soluteActivationActivation: enhances the capacity of adsorption and : enhances the capacity of adsorption and is accomplished by subjecting the char of carbon is accomplished by subjecting the char of carbon (coal) to an oxidizing steam at high temperature. (coal) to an oxidizing steam at high temperature. The char becomes a very porous structure (increase The char becomes a very porous structure (increase area for adsorption; 1 gram of char coal has an area for adsorption; 1 gram of char coal has an adsorption area of 1000 madsorption area of 1000 m33)) PACPAC = powdered activated carbon = powdered activated carbonGAC GAC = granular activated carbon= granular activated carbonACAC = activated carbon = activated carbon

Activation TechniqueActivation TechniqueIt is the process of enhancing a It is the process of enhancing a characteristic. Activation techniques used in characteristic. Activation techniques used in manufacturing of AC depends on the nature manufacturing of AC depends on the nature and type of raw materials.and type of raw materials.

Types of ActivationTypes of Activation Chemical activationChemical activation Steam activationSteam activation

Chemical Activation: Chemical Activation: Used with wood-Used with wood-based raw materialsbased raw materials

Phosphoric pentoxide (PPhosphoric pentoxide (P22OO55)) Zinc chloride (ZnClZinc chloride (ZnCl22))

Activation ProcessActivation Process raw material is mixed with the chemicalraw material is mixed with the chemical heated at 500 – 800heated at 500 – 800 C C washedwashed drieddried ground to desired sizeground to desired size

Characteristics of ACCharacteristics of AC very open pore structurevery open pore structure good for large moleculesgood for large molecules

Steam ActivationSteam Activation

It is used for coal and coconut shell raw It is used for coal and coconut shell raw materials at a temperature of 800 – 1100materials at a temperature of 800 – 1100 C C in the presence of superheated steam.in the presence of superheated steam.

Activation ProcessActivation Process the product is gradedthe product is graded screenedscreened de-dustedde-dusted

Characteristics of ACCharacteristics of AC exhibits a fine pore structureexhibits a fine pore structure good for small moleculesgood for small molecules

Adsorption CapacityAdsorption Capacity

This is determined by an isothermThis is determined by an isotherm

What is an ISOTHERM?What is an ISOTHERM?

Isotherm is an equation relating the amount Isotherm is an equation relating the amount of solute adsorbed onto the solid and the of solute adsorbed onto the solid and the equilibrium concentration of solute in equilibrium concentration of solute in solution at a given temperature.solution at a given temperature.

Most common isotherms in Environmental Most common isotherms in Environmental Engineering Applications are:Engineering Applications are:

FreundlichFreundlich LangmuirLangmuir

Freundlich IsothermFreundlich Isotherm

See FigureSee Figure

X = mass of adsorbateX = mass of adsorbateM = mass of adsorbentM = mass of adsorbent[C] = concentration of adsorbate in solution in [C] = concentration of adsorbate in solution in equilibriumequilibriumk and n = constantsk and n = constants

nCkM

X 1

Langmuir IsothermLangmuir Isotherm

See FigureSee Figure

a and b = constantsa and b = constants

][1

][

Cb

Cab

M

X

The equilibrium takes place when the The equilibrium takes place when the rate of adsorption is equal to the rate rate of adsorption is equal to the rate of desorptionof desorption

kkss = proportionality constant = proportionality constant

(X / M)(X / M)ultult = maximum (X / M) = maximum (X / M)

(X / M) = (X / M) at any time(X / M) = (X / M) at any time

rsM

X

M

XCkadsorptionofrate

ults

Then at equilibriumThen at equilibrium

M

Xkrdesorptionofrate dd

M

Xk

M

X

M

XCk d

ults

Solving for (X / M) will yield Langmuir Solving for (X / M) will yield Langmuir Isotherm with:Isotherm with:

d

s

ult

k

kb

M

Xa

Freundlich and Langmuir isotherms Freundlich and Langmuir isotherms can be expressed as straight-line can be expressed as straight-line equationsequations

)(][11][

)(]ln[1

lnln

LangmuirCaabMX

C

FreundlichCn

kM

X

Two data points are required to solve Two data points are required to solve the equationsthe equations

[C[C00] = solute concentration before ] = solute concentration before adsorption onto mass of adsorbent adsorption onto mass of adsorbent (M)(M)

V = sample volumeV = sample volume

VCCX

M

XaCapacityAdsorption

ult

0

Determination of the Freundlich Determination of the Freundlich ConstantsConstants

Two data points are required, but, in case where more Two data points are required, but, in case where more than two points are available, the data points must be than two points are available, the data points must be reduced to two pairs.reduced to two pairs.

l

CnM

X

k

M

Xlm

M

Xl

ClmCln

then

pairCn

klmCn

kM

X

pairCn

klCn

kM

X

l l

m

l

l

m

l

l

m

l

m

l

m

l

ndm

l

l l lst

l

1 1

1 1

1 1

1 1 1 1

1 1 1 1

]ln[1

exp

lnln

]ln[]ln[

)2(]ln[1

ln)(]ln[1

lnln

)1(]ln[1

ln]ln[1

lnln

Determination of the Langmuir Determination of the Langmuir ConstantsConstants

l l

m

l

l

lm

l

ndm

l

m

l

stl l

CMX

Ca

lb

MX

Clm

MX

Cl

ClmCla

then

pairCaab

lmMX

C

pairCaab

lMX

C

1 1

1 1

11

1 1

1 1

)(

)(

][)(

)(

][

][][

)2(11

)()(

)1(11

)(

Study Examples 8.5 and Study Examples 8.5 and 8.68.6