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Membrane Separations

MicrofiltrationDan Libotean - Alessandro PattiPhD studentsUniversitat Rovira i Virgili, Tarragona, Catalunya

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Definition of a membrane

A membrane can be defined as a barrier (not necessarily solid) that separates two phases as a selective wall to the mass transfer, making the separation of the components in a mixture possible.

IDE

AL

ME

MB

RA

NE

Permeate Feed

Driving Force

RE

AL

ME

MB

RA

NE Phase 1Phase 2

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The growing use of MF

1. More attention paid to environmental problems linked to drinking and non-drinking water

2. Increased demand for water (using currently available sources more effectively)

3. Market power

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Membranes market in W. Europe

05

1015202530354045

MF Dialysis UF RO Other

% of total 1997 consumption in Western Europe

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Demand in U.S.A., 2001

MF has been used more and more to eliminate particles and microorganisms in untreated water, leading to a lower consumption of disinfectant and to a lower concentration of SPD (sub-products of disinfections).

MF has been used more and more to eliminate particles and microorganisms in untreated water, leading to a lower consumption of disinfectant and to a lower concentration of SPD (sub-products of disinfections).

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Cumulative capacity of MF

0

10

20

30

40

50

'86-'88 '89-'90 '91-'92 '93-'94 '95-'96

Number of plants

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Driving Forces

A driving force can make the mass transfer through the membrane possible; usually, the driving force can be a pressure difference (∆P), a concentration difference (∆c), an electrical potential difference (∆E).

Membranes can be classified according their driving forces:

A driving force can make the mass transfer through the membrane possible; usually, the driving force can be a pressure difference (∆P), a concentration difference (∆c), an electrical potential difference (∆E).

Membranes can be classified according their driving forces:

∆P ∆c ∆T ∆E

Microfiltration Pervaporation Thermo-osmosis Electrodialysis

Ultrafiltration Gas separation Membrane distillation Electro-osmosis

Nanofiltration Vapour permeation Membrane electrolysis

Reverse osmosis Dialysis

Piezodialysis Diffusion dialysis

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Pressure driven processes

MF10-300 kPa

RO0.5-1.5 MPa

NF0.5-1.5 MPa

UF50-500 kPa∆P=

Bacteria, parasites, particlesHigh molecular substances, virusesMid-size organic substances,multiple charged ions

Low molecular substances, single charged ions

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Pore size of MF membranes

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Pores and pore geometries

Porous MF membranes consist of polymeric matrix in which poresare present. The existence of different pore geometries implies that different mathematical models have been developed to describe transport phenomena.

Porous MF membranes consist of polymeric matrix in which poresare present. The existence of different pore geometries implies that different mathematical models have been developed to describe transport phenomena.

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Transport equations

The Hagen-Poiseuille and the Kozeny-Carman equations can be applied to demonstrate the flow of water through membranes. The use of these equations depends on the shapes and sizes of the pores.

1. Hagen-Poiseuille

x

PrJ

8

2

cylindrical pores

J – the solvent fluxP – pressure differencex – thickness of membranetortuosityviscosityr – the pore radiusε – surface porosity

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Transport equations

2. Kozeny-Carman

x

P

SKJ

2

3

S – surface area per unit volumeK – Kozeny-Carman constant (depends on the pore geometry)

closely packed spheres

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How to prepare MF membranes

1. Stretching

Semycristalline polymers (PTFE, PE, PP) if stretched perpendicular to the axis ofcrystallite orientation, may fracture in such a way as to make reproducible microchannels.The porosity of these membranes is very high,and values up to 90% can be obtained.

1. Stretching

Semycristalline polymers (PTFE, PE, PP) if stretched perpendicular to the axis ofcrystallite orientation, may fracture in such a way as to make reproducible microchannels.The porosity of these membranes is very high,and values up to 90% can be obtained.

Stretched PTFE membrane

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2. Track-etching

These membranes are now made by exposinga thin polymer film to a collimated bearn of

radiation strong enough to break chemical bonds in the polymer chains. The film is then etched in a bath which selectively attacks thedamaged polymer.

2. Track-etching

These membranes are now made by exposinga thin polymer film to a collimated bearn of

radiation strong enough to break chemical bonds in the polymer chains. The film is then etched in a bath which selectively attacks thedamaged polymer.

Track-etched 0.4 μm PC membrane

radiation source

polymer film

etching bath

membrane

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3. Phase inversion (PI)

Chemical PI involves preparing a concentrated solution of a polymer in a

solvent. The solution is spread into a thin film, then precipitated through the slow addition of a nonsolvent, usually water,sometimes from the vapour phase.In thermal PI a solution of polymer in poor

solvent is prepared at high temperatures. After being transformed into its final shape, a sudden drop in solution temperature causesthe polymer to precipitate. The solvent is then washed out.

3. Phase inversion (PI)

Chemical PI involves preparing a concentrated solution of a polymer in a

solvent. The solution is spread into a thin film, then precipitated through the slow addition of a nonsolvent, usually water,sometimes from the vapour phase.In thermal PI a solution of polymer in poor

solvent is prepared at high temperatures. After being transformed into its final shape, a sudden drop in solution temperature causesthe polymer to precipitate. The solvent is then washed out.

Chemical phase inversion 0.45 μm PVDF membrane

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4. Sintering

This method involves compressing a powder consisting of particles ofa given size and sintering at high temperatures. The required temperature depends on the material used.

4. Sintering

This method involves compressing a powder consisting of particles ofa given size and sintering at high temperatures. The required temperature depends on the material used.

HEAT

pore

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Materials used

Synthetic polymeric membranes:

a) Hydrophobic b) Hydrophilic

Ceramic membranes

PTFE, teflonPVDFPPPE

Cellulose estersPCPSf/PESPI/PEIPAPEEK

PTFE, teflonPVDFPPPE

Cellulose estersPCPSf/PESPI/PEIPAPEEK

Alumina, Al2O3

Zirconia, ZrO2

Titania, TiO2

Silicium Carbide, SiC

Alumina, Al2O3

Zirconia, ZrO2

Titania, TiO2

Silicium Carbide, SiC

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Materials used

1. Polymeric MF membranes

Phase inversion

Stretching

Track-etching

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Materials used

2. Ceramic MF membranes

Anodec, anodic oxidation (surface) US Filter, sintering (cross section, upper part)

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Modules

A module is the simplest membrane element that can be used in practice.

Module design must deal with the following issues:

2. Membrane integrity against damage and leaks

2. Membrane integrity against damage and leaks

3. Sufficient mass transfer to keep polarization in control

3. Sufficient mass transfer to keep polarization in control

4. Minimum waste of energy4. Minimum waste of energy

5. Easy egress of permeate

5. Easy egress of permeate

6. Permit the membrane to be cleaned

6. Permit the membrane to be cleaned

1. Economy of manufacture1. Economy of manufacture

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Modules: tubular

Diameter tubular membrane assembly

• Membranes diameter: >0.5 mm

• Active layer: inside the tube

• Flux velocity: high (up to 5 m/s)

• Tube: reinforced with fiberglass or stainless steel

• Number of tubes: 4-18

• Flux: one or more channels

• Cleaning: easy

• Surface area/volume: low

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Modules: hollow fiber

Hollow fiber module (inside-out)

• Fibers: 300 – 5000 per module

• Fibers diameter: <0.5 mm

• Flux velocity: low (up to 2.5 m/s)

• Feed: inside-out or outside-in

• Surface area/volume: high

• Pressure drop: low (up to 1 bar)

• Maintenance: hard

• Cleaning: poor

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Symmetric membranes

Symmetric ceramic membrane (Al2O3)

surfacecross section

The cross section shows a uniform and regular structure

The cross section shows a uniform and regular structure

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Asymmetric membranes

Cross-section of an asymmetricPSf membrane.

Porous irregular layer

The active layer is supported over the porous layer.

The active layer is supported over the porous layer.

50/150 μm

Porous with toplayer

Same material!

0.1/0.5 μm

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Fouling and resistance

Fouling depends on: concentration, temperature pH, molecular interactions

Fouling depends on: concentration, temperature pH, molecular interactions

cm RR

PJ

Resistances-in-series model to describe the flux decline:Resistances-in-series model to describe the flux decline:

J: flowΔP: pressure dropη: viscosityRm: membrane resistanceRc: cake resistance

time, t

flu

x,

J

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Fouling and resistance

m

a

p

R

R

R

gcp RR

porousmembrane

gel layerThe build-up layer and the cloggingof the pores are referred to as a fouling layer.

onpolarizatiionconcentratR

formationlayergelR

membraneR

adsorptionR

blockingporeR

cp

g

m

a

p

:

:

:

:

:

Rm= Rm(t=0)+Ra+Rp; Rc=Rg+Rcp

Rtot=Rm+Rc

Rm= Rm(t=0)+Ra+Rp; Rc=Rg+Rcp

Rtot=Rm+Rc

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Methods to reduce fouling

1. Pretreatment of the feed solution1. Pretreatment of the feed solution

2. Membrane properties2. Membrane properties

3. Module and process conditions3. Module and process conditions

4. Cleaning4. Cleaning

a. Reducing concentration polarisationa1. Increasing flux velocitya2. Using low flux membranes

b. Turbulence promoters

a. Reducing concentration polarisationa1. Increasing flux velocitya2. Using low flux membranes

b. Turbulence promoters

a. Narrow pore size distributionb. Hydrophilic membranes

a. Narrow pore size distributionb. Hydrophilic membranesa. Heat treatmentb. pH adjustamentc. Addition of complexing agentsd. Chlorinatione. Adsorption onto active carbonf. Chemical clarification

a. Heat treatmentb. pH adjustamentc. Addition of complexing agentsd. Chlorinatione. Adsorption onto active carbonf. Chemical clarification

a. Hydraulic cleaningb. Mechanical cleaningc. Chemical cleaning d. Electric cleaning

a. Hydraulic cleaningb. Mechanical cleaningc. Chemical cleaning d. Electric cleaning

Back-flushing

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Back-flushing

suspension

permeate

permeate

suspension

permeate

permeate

J

t

Restorable fluxwith back-flushing

Irreversible fouling

starting points

ΔP

t

Restorable pressurewith back-flushing

Irreversible fouling

starting points

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Dead end and cross-flow

To reduce fouling two process modes exist:

Feed

PermeatePermeate

Feed Retentate

1. Dead-end 2. Cross-flow

Cake layer

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Available MF membranes

Pore size, μm Module Material Membrane area per module, m2 Producer

2, 3, 5 T C 0.02 – 7.1 US Filters

1.4 T C 0.005 – 7.4 US Filters

1 T C 0.09 – 10.0 CTI TechSep

0.45 T C 0.13 – 11.5 Ceramen

0.45 FH PSf 0.01 – 3.7 AG Technology

0.2 T C 0.02 – 7.1 US Filters

0.2 FH PP 2.0 Akzo

0.2 FH PP/PF 10.8 – 15 Memtec

0.1 T C 0.02 – 7.1 US Filters

0.1 FH PSf 0.01 – 3.7 AG Technology

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MF process applications

1. To replace four unit operations in the waste water treatment process.

COAG/FLOC

SEDMIX FILT

Disinfectants &Coagulants

Wastewater

Water

Residualdisinfectant

MFPre Filter

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2. To eliminate organic matter using MF after a pre-treatment with coagulants

Water

Wastewater

Coagulants

MFPreFilter

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Wastewater

3. MF as pre-treatment for RO or NF

Water

MFPre

Filter

RO

NF Water

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Retentate: how will it be used?

1. Sent to a treatment plant2. Discharged into a body of water3. Sent to a storage facility4. For ground applications 5. Recycled back to water source

1. Sent to a treatment plant2. Discharged into a body of water3. Sent to a storage facility4. For ground applications 5. Recycled back to water source

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Some industrial applications

1. Waste-water treatment2. Clarification of fruit juice, wine and beer3. Ultrapure water in the semiconductor industry4. Metal recovery as colloidal oxides or hydroxides5. Cold sterilization of beverages and pharmaceuticals6. Medical applications: transfusion filter set, purification of

surgical water 7. Continuous fermentation8. Purification of condensed water at nuclear plants 9. Separation of oil-water emulsions

1. Waste-water treatment2. Clarification of fruit juice, wine and beer3. Ultrapure water in the semiconductor industry4. Metal recovery as colloidal oxides or hydroxides5. Cold sterilization of beverages and pharmaceuticals6. Medical applications: transfusion filter set, purification of

surgical water 7. Continuous fermentation8. Purification of condensed water at nuclear plants 9. Separation of oil-water emulsions

Membrane Separations

Ultrafiltration & Nanofiltration

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Membrane separation

SPECIES RANGE OF DIMENSIONS (NM)Yeasts and fungi 1000-10000Bacteria 300-10000Oil emulsions 100-10000Colloidal solids 100-1000Viruses 30-300Proteins, polysaccharides 2-10Enzymes 2-5Common antibiotics 0.6-1.2Organic molecules 0.3-0.8Inorganic ions 0.2-0.4Water 0.2

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Membrane separation

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Membrane separation

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Membrane characterization

pore size

pore size distribution

free volume

crystalinity

pore size

pore size distribution

free volume

crystalinity

Membrane properties Membrane separation properties

rejection

separation factor

enrichment factor

rejection

separation factor

enrichment factor

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Membrane characterization

Membranes porous

nonporous

Process Driving force Membrane Pore Separation principleMicrofiltration pressure difference

(0.1 - 1 bar)macropore filtration

Ultrafiltration pressure difference(0.5 – 10 bar)

mesopore filtration

Nanofiltration pressure difference(5 – 20 bar)

micropore filtration/electrostatic interaction/solution-diffusion

macropore >50nmmesopore 2nm<<50nmmicropore <2nm = pore diameter

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The characterization of porous membranes

1. shape of the pore (pore geometry)

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1. Pore geometries

ΔxΔP

τη8rε

J2

Hagen-Poiseuille equation

J – the solvent fluxP – pressure differencex – thickness of membranetortuosityviscosityr – the pore radius – the surface porosity

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1. Pore geometries

ΔxΔP

ε1SηK

εJ 22

3

Kozeny-Carman relationship

S – the internal surface areaK – Kozeny-Carman constant

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1. Pore geometries

top layer thickness0.1-1m

sub layer thickness50-150m

The flux is inversely proportional to the thickness.

commercial interest

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2. pore size distribution

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3. surface porosity

m

2

p Arπ

nε

r – the pore radius

np – number of pores

Am – membrane area

Microfiltration membranes: 5-70%

Ultrafiltration membranes: 0.1-1%

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Characterization methods: structure-related parameters

(pore size, pore size distribution, top layer thickness,

surface porosity) permeation-related parameters

(actual separation parameters using solutes that are more or

less retained by the membranes - ‘cut-off’ measurements*)

* ‘cut-off’ is defined as the molecular weight which is 90% rejected by the membrane

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Characterization methodsMicrofiltration Ultrafiltrationscanning electron microscopy gas adsorption-desorptionbubble-point method thermoporometrymercury intrusion porometry permporometrypermeation measurements liquid displacement

rejection measurementtransmission electron microscopy

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Ultrafiltration

... separation of one component of a solution from another component by

means of pressure and flow exerted on a semipermeable membrane, with

membrane pore sizes ranging from 0.05 m to 1nm.

is used begining with years ‘30

the operating pressure 0.1-5 bar

typically used to retain macromolecules and colloids

the lower limit are solutes with molecular weights of a few thousands Daltons (1Dalton1.66.10-24g)

average flux around 50-200 GFD (~ 80-340 l/m2.h), at an operating pressure of 50 psig (~ 3,5bar)

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Ultrafiltration

Membranes used:polymeric

- polysulfone/poly(ether sulfone)/sulfonated polysulfone

- poly(vinylidene fluoride)

- polyacrilonitrile

- cellulosics

- polyimide/poly(ether imide)

- aliphatic polyamides

- polyetheretherketone

ceramic

- alumina (Al2O3)

- zirconia (ZrO2)

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Ultrafiltration

Process performance do not depend only to the intrinsic

membrane properties, but also to the occurence of

different phenomena:

concentration polarization

fouling

adsorption

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Concentration polarization

The concentration of removed species is higher near the

membrane surface than it is in the bulk of the stream.

Result:

a boundary layer of substantially high concentration

permeate of inferior quality

Resolution:

high fluid velocities are maintaned along the membrane

surface

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Fouling

Build-up of impurities in the membrane that can keep it

from functioning properly.

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Ultrafiltration

Crossflow Mode

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Ultrafiltration

Dead End Mode

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Cleaning

Cleaning in Backwash mode

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Cleaning

Cleaning in Forward Flush mode

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Adsorption

The main factor enhancing this phenomenon is hydrophobic

interaction between the surface of the membrane and substance

molecules.

Hydrophobic groups are more prone to adsorbtion than

hydrophilic groups

Hydrophobic Hydrophilic

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Adsorption

The number of molecules adsorbed on the surface, can be

reduced by modifying hydrophobic membrane surface to

hydrophylic membrane surface.

It is also easy to clean a hydrophilic membrane.

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Ultrafiltration

Applications:food and dairy industry (the concentration of milk and cheese making, the recovery of whey proteins, the recovery of potato starch and proteins, the concentration of egg products, the clarification of fruit juices and alcoholic beverages)

pharmaceutical industry (enzymes, antibiotics, pyrogens)

textile industry

chemical industry

metallurgy (oil-water emulsions, electropaint recovery)

paper industry

leather industry

sub layers in composite mebranes for nanofiltration, reverse osmosis, gas separation or prevaporation

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Ultrafiltration

Factors affecting the performance:

flow across the membrane surface

high flow velocity high permeate rate

operating pressure

due to increased fouling and compaction, pressures rarely exceed 100 psig (1 psig=0.068948 bar)

operating temperature

high temperature high permeate rate

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Nanofiltration

...used when low molecular weight solutes as inorganic salts or small organic molecules (glucose, sucrose) have to be separated.

pore size < 2 nm the operating pressure 10-20 barmaterial directly influences the separationnanofiltration membranes are considered intermediate between porous and nonporous membranesmost of the nanofiltration membranes are chargedtwo models for the separation mechanism

1. permeation through a micropore2. the solution-diffusion into the membrane matrix

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1. The permeation mechanism

...is explained in terms of charge and/or size effects.

uncharged solutes sieving

charged components Donnan exclusion mechanism

mB

B

BmA

A

A

mDon a

aln

FzRT

aa

lnFz

RTΨΨΨ

- the electrical potential z - the valenceR - the gas constant F - the Faraday constantT - the temperature a - the activity of the solutes“m” refers to the membrane phase, while “A” and “B” are the components in the solution

The Donnan potential

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2. The solution-diffusion mechanism

membrane behaves as a nonporous diffusion barrier

each component dissolves in the membrane in accordance with an equilibrium distribution law

each component diffuses through the membrane by a diffusion mechanism in response to the concentration and pressure differences

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Nanofiltration

Membranes for which the Donnan exclusion seems to play an important role

negatively charged membrane pozitively charged membrane

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Nanofiltration

Membranes for which the diffusion seems to play an important role

nonporous membrane

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Nanofiltration

Membranes used:asymmetric structure: top layer <1m, sub layer ~50-150m

asymmetric membranes (prepared by phase inversion techniques)

- cellulose esters

pH range 5-7, temperature < 30oC (for avoiding the hydrolysis

of the polymer)

- polyamides

- polybenzimidazoles, polybenzimidazolones, polyamidehydrazide, polyimides

composite membranes

- first stage is preparing the porous sub layer

- placing a thin dense layer on the top of the sub layer: dip coating, in-situ polymerization, interfacial polymerization, plasma polymerization

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Nanofiltration

Applications:

desalination of brackish and seawater to produce potable water

producing ultrapure water for the semiconductor industry

retention of bivalent ions such as Ca2+, CO32-

retention of micropollutants and microsolutes such as: herbicides, insecticides, pesticides, dyes, sugar

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