MEMBRANE PROCESSES

MEMBRANESEPARATION

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

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

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

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 :

PAJ 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 :

x

PrJ

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2Where r is the pore radius, Δx is the membrane thickness, η is he dynamic viscosity and is the tortuosity factor ehich is unity in the case of cylindrical pores.

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

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)

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 OPERATIONNEED

MUST BE CLEANED PERIODICALLY

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

INDUSTRIAL APPLICATIONS

1. Cold sterilization of beverages and pharmaceutical2. Cell harvesting

3. Clarification of fruit juice, wine and beer4. Ultrapure water in the semiconductor industry

5. Metal recovery as colloidal oxides or hydroxides6. Waste-water treatment

7. Continuous fermentation8. Separation of oil-water emulsions

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 mechanism

Membrane material : Polymeric, and ceramic

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

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.

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

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

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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 mechanism

Membrane 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

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.

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 RO NF

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

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.

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.

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2 Aromatic polyamides. These material also show high selectivities towards salts but their water flux is somewhat lower.

Polybenzimidazoles, polybenzimidazolones, polyamidedehydrazide, and polyimides

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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 coatingInterfacial polymerizationPlasma polymerization

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

Summary Of Nanofiltration

Membranes : Composite

Thickness : Sublayer : 150 um Toplayer : 1 um

Pore sizes : < 2 nm

Driving force : Pressure (10 – 25 bar)

Separation principles : Solution – diffusion

Membrane material : Polyamide (interfacial polymerisation)

Main application : Desalination of brackish and seawaterRemoval of micropollutentsWater softeningWastewater treatmentRetention of dyes (textile industry)

Summary Of Reverse Osmosis

Membranes : Asymmetric or composite

Thickness : Sublayer : 10 umToplayer : 1 um

Pore sizes : < 2 nm

Driving force : Pressure : brackish water (15 – 25 bar) seawater (40 – 80 bar)

Separation principles : Solution-diffusion

Membrane material : Cellulose triacetate, aromatic polyamide, polyamide and poly(ether urea) (interfacial polymerization)

Main application : Desalination of brackish and seawaterProduction of ultrapure water (electronic industry)Concentration of food juice and sugars (food industry) and the concentration of milk dairy industry)

Microfiltration Ultrafiltration Nanofiltration / reverse osmosis

Separation of particles Separation of macromolecules (bacteria, yeasts)

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

Osmotic pressure negligible Osmotic 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 structure Asymmetric structure

Thickness of separating layer:Symmetric structure : 10-150 umAsymmetric structure :1 um

Thickness of actual separating layer : 0,1-1,0 um

Thickness of actual separating layer : 0,1 – 1 um

Separation based on particle size

Separation based on particle size

Separation based on differences in solubility and diffusivity