MEMBRANE PROCESSES MEMBRANE - PowerPoint Presentation

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MEMBRANE PROCESSES

MEMBRANE

TECHNOLOGY

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PRESSURE DRIVEN MEMBRANE PROCESS

Various pressure-driven membrane process can be used to :

1. Concentrate

2. Purify a dilute (aqueous or non aqueous)

THE PARTICLE OR MOLECULAR SIZE AND CHEMICAL PROPERTIES

of solute DETERMINE

:

Structure of membrane

1.

Pore size

2.

Pore size distribution

PORE SIZE

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Various PROCESS CAN BE DISTINGUISH RELATED TO THE PARTICLE SIZE OF THE SOLUTE AND CONSEQUENTLY TO MEMBRANE STRUCTURE :

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Various PROCESS CAN BE DISTINGUISH RELATED TO THE PARTICLE SIZE OF THE SOLUTE AND CONSEQUENTLY TO MEMBRANE STRUCTURE :

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MICROFILTRATION

Is the membrane process which most closely resembles conventional coarse filtration.

Pore size:

10 – 0,05



m

Suitable for retaining suspensions and emulsions

The Darcy’s Law :

Where the permeability constant A contains structural factors such as the porosity and pore size (pore size distribution)

For laminar convective flows through a porous systems :

Where r is the pore radius,

Δ

x is the membrane thickness,

η

is he dynamic viscosity and is the tortuosity factor ehich is unity in the case of cylindrical pores.

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MEMBRANE FOR MICROFILTRATION

Can

be prepared from organic materials (polymers) and inorganic (ceramics, metals, glasses)

Various techniques can be employed :

1. Sintering

2.

Stretching

3. Track - etching

4. Phase inversion

Frequently, inorganic membranes are used instead of polymeric membranes because of their outstanding chemical and thermal resistances.

Process

Porosity

Pore size distribution

Sintering

Low / medium

Narrow / wide

Stretching

Medium / high

Narrow

/ wide

Track-etching

Low

Narrow

Phase inversion

High

Narrow / wide

POROSITIES AND PORE SIZE DISTRIBUTION

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These various techniques allow to prepare microfiltration membranes from virtually all kinds of materials of which polymers and ceramics are the most important.

HYDROPHOBIC POLYMERIC MEMBRANES

HYDROPHILIC POLYMERIC MEMBRANES

Polytetrafluoroethylene (PTFE)

Poly(vinylidene fluoride) (PVDF)

Polyproylene (PP)

Polyethylene (PE)

Cellulase

esters

Polycarbonate (PC)

Aliphatic

polyamide (PA)

Polyetheretherketone

(PEEK)

CERAMIC MEMBRANES

Alumina (Al2O3)

Zirconia (ZrO2)

Titania (TiO2)

Silicium carbide (SiC)

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MAIN PROBLEMS of MICROFILTRATION

Deposition of solute inside the pores of membrane or membrane surface

Concentration polarization and fouling

FLUX DECLINE

NEED

Careful control over the mode of process operation

NEED

Must be cleaned periodically

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TWO MODES OF FILTRATION:

DEAD END FILTRATION

The feed flow is perpendicular to the membrane surface, so that the retained particles accumulate and form a type of a cake layer at the membrane surface. The thickness of the cake increases with filtration times and consequently the permeation rate decreases with increasing cake layer thickness

CROSS FLOW FILTRATION

The feed flow is along the membrane surface, so that part of the retained solutes accumulate

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INDUSTRIAL APPLICATIONS

Cold

sterilization

of beverages and pharmaceutical

Cell harvesting

Clarification of fruit juice, wine and beer

Ultrapure water in the semiconductor industry

Metal recovery as colloidal oxides or hydroxidesWaste-water treatmentContinuous fermentation

Separation of oil-water emulsions

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Summary Of Microfiltration

Membranes :

(a)Symmetric porous

Thickness :

10 – 150 um

Pore sizes :

0,05 – 10 um

Driving force :

Pressure (< 2 bar)Separation principles :

Sieving mechanismMembrane material :

Polymeric, and ceramicMain application :

Analytical applicationsSterilization (food, pharmaceutical)Ultrapure water (semiconductor)Clarification (beverages)Cell harvesting and membrane bioreactor (biotechnology)Water treatment

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ULTRAFILTRATION

Is a membrane process whose nature lies between nanofiltration and microfiltration

Pore size:

0,05



m

– 1 NM

UF is typically used to retain macromolecules and colloids from a solution. UF and MF can

both

be consider as porous membrane where rejection is determined by the size and shape of the solutes relatives to the pore size in

the

membrane and where the transport of solvent is directly proportional to the applied pressure.

In fact both UF and MF involve similar membrane processes based on the same separation principle. However, an important difference is that UF membrane have an asymmetric structure with a much denser top layer (small pore size and lower surface porosity) and consequently a much higher hydrodynamic resistance.

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MEMBRANE FOR ULTRAFILTRATION

Most of UF membrane used commercially these day are prepared from :

POLYMERIC MEMBRANES

Polytetrafluoroethylene (PTFE)

Poly(vinylidene fluoride) (PVDF)

Polyacrylonitrile

Polyimide

Polyetheretherketone

Aliphatic polyamides

Cellulosics

INORGANIC (CERAMIC)

Alumina (Al2O3)

Zirconia (ZrO2)

BY PHASE INVERSION PROCESS

SOL GEL TECHNIQUE

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APPLICATIONS

FOOD AND DAIRY INDUSTRY

PHARMACEUTICAL INDUSTRY

TEXTILE INDUSTRY

CHEMICAL INDUSTRY

METALLURGY

PAPER INDUSTRY

LEATHER INDUSTRY

Recovery of whey proteins

Recovery of potato starch and proteins

Concentration of egg product

Clarification of fruit juices and alcoholic beverages

1

2

3

4

5

6

7

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Summary Of Ultrafiltration

Membranes :

Asymmetric porous

Thickness :

150 um (or monolithic for some ceramics)

Pore sizes :

1– 100 nm

Driving force :

Pressure (1-10 bar)Separation principles :

Sieving mechanismMembrane material :

Polymeric (PS, Polyacrylonitrile)Ceramic (zirconium oxide, aluminum oxide)

Main application :Dairy (milk, whey, cheese making)Food (potato starch and protein)Metallurgy (oil-water emulsion, electropaint recovery)

Textile (indigo)Pharmaceutical (enzymes, antibiotics, pyrogens)Automotive (electropaint

)Water treatment

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REVERSE OSMOSIS AND NANOFILTRATION

FORCE

Salt solution

Pure water

Membrane

“Complete barrier to dissolved salt”

RO and NF are used when low molecular weight solutes such as inorganic salts or small organic mocules such as glucose, and sucrose have to be separated from solvent.

PRINCIPE OF REVERSE OSMOSIS

The membrane is permeable to the solvent (water) but not to the solute (salt). In order to allow water to pass through the membrane, the applied pressure must be higher than the osmotic pressure.

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Both process are considered as one process since the basic principles are the same.

NF membranes are the same as RO membranes only the network structure is more open.

Comparison of retention characteristic between

nanofiltration

(NF) and

reverse

osmosis (RO) are listened :

Solute

RONF

Monovalent ions (Na, K, Cl, NO3)>98%

<50%Bivalent ions (Ca, Mg, SO4, CO3)

>99%>90%Bacteria and viruses>99%<99%

Micro solutes (Mw >100)>90%

>50%Micro solutes (Mw < 100)0-99%

0-50%

The pressure used in reverse osmosis range from 20 – 100 bar and in

nanofiltration from about 10 – 20 bar, which much higher than those used in ultrafiltration

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MEMBRANES FOR RO AND NF

The flux is approximately inversely proportional to the membrane thickness and for this reason most reverse osmosis membranes have an asymmetric structure with a thin dense

toplayer

(thickness ≤ 1um) supported by a porous

sublayer

(thickness 50 – 150

um)

The

resistance towards transport being determined mainly by

the dense

toplayer

. An asymmetric membrane structure can be distinguished: (i) integral asymmetric membranes, and (ii) composite membranes.

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Integral Asymmetric Membranes

Both

toplayer

and

sublayer

consists of the same material. These membrane are prepare by phase inversion technique. The polymeric material from which the membrane it to be prepared is soluble in a solvent or a solvent mixture.

An important class of asymmetric membranes are :

cellulose esters.

This materials are very suitable for desalination because of their high permeability towards water in combination with a (very) low solubility towards salt.

1

2

Aromatic

polyamides.

These material also show high

selectivities

towards salts but their water flux is somewhat lower.

Polybenzimidazoles, polybenzimidazolones, polyamidedehydrazide, and polyimides

3

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Composite Membranes

The second type of structure frequently used in RO while most of the NF membrane are in fact composite membrane.

In such membranes the top layer and

sublayer

are composed of different polymeric materials so that each layer can be

optimized

separately.

The first stage is the are preparation of the porous

sublayer. Important criteria for this sublayer

are surface porosity and pore size distribution and asymmetric ultrafiltration membranes are often used. Different methods have been employed for placing a thin dense layer on top of this sublayer :

dip coating

Interfacial polymerizationPlasma polymerization

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APPLICATION

purification water, desalination of brackish and seawater to produce potable water

Production of ultrapure water for the semiconductor industry

Concentration step particularly in the food industry ( concentration of fruit juice)

Galvanic

industry (concentration of waste stream)

Dairy industry (concentration of milk to prior cheese manufacture)

RO

When a high retention is required for

NaCl with high feed concentrations reverse osmosis reverse osmosis is the preferred process. In other cases with much lower concentrations, divalent ions and micro solutes with molecular weight

nanofiltration is the preferred process. Since the water permeability is (much) higher in nanofiltration the capital cost for a certain application will be lower.

NF

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Summary Of

Nanofiltration

Membranes :

Composite

Thickness :

Sublayer

: 150 um

Toplayer

: 1 umPore sizes :

< 2 nmDriving force :

Pressure (10 – 25 bar)

Separation principles :Solution – diffusionMembrane material :Polyamide (interfacial polymerisation

)Main application :Desalination of brackish

and seawaterRemoval of micropollutentsWater softeningWastewater treatment

Retention of dyes (textile industry)

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Summary Of Reverse Osmosis

Membranes :

Asymmetric

or composite

Thickness :

Sublayer

: 10 um

Toplayer : 1 umPore sizes :

< 2 nmDriving force :

Pressure : brackish water (15 – 25 bar) seawater (40 – 80 bar)Separation principles :

Solution-diffusionMembrane material

:Cellulose triacetate, aromatic polyamide, polyamide and poly(ether urea) (interfacial polymerization)Main application :Desalination of brackish and seawater

Production of ultrapure water (electronic industry)Concentration of food juice and sugars (food industry) and the concentration of milk dairy industry)

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Microfiltration

Ultrafiltration

Nanofiltration

/ reverse osmosis

Separation

of particles

Separation of macromolecules (bacteria, yeasts)

Separation of low MW solute (salt, glucose, lactose, micropollutant)

Osmotic pressure negligibleOsmotic pressure negligible

Osmotic pressure high (1-25 bar)Applied pressure low (< 2bar)

Applied pressure low (1-10 bar)Applied pressure high (10-60 bar)

Symmetric structureAsymmetric structure Asymmetric structureAsymmetric structureThickness of separating layer:

Symmetric structure : 10-150 umAsymmetric structure :1 umThickness of actual separating layer : 0,1-1,0 um

Thickness of actual separating layer : 0,1 – 1 umSeparation based on particle size

Separation based on particle sizeSeparation based on differences in solubility and diffusivity