Membrane Separations
Microfiltration
Dan Libotean - Alessandro Patti
PhD students
Universitat Rovira i Virgili,
Tarragona, Catalunya
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.
IDEAL MEMBRANE
REAL MEMBRANE
Permeate Feed
Driving Force
Phase 2 Phase 1
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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
45
40
35
30
25
20
15
10
5
0
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 micro
organisms 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
50
40
30
20
10
0
'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:
∆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
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∆P= 10-300 kPa 50-500 kPa 0.5-1.5 MPa 0.5-1.5 MPa
Mid-size organic substances,
Bacteria, parasites, particles substances, single charged ions
molecular substances, viruses
HighLow molecular
multiple 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 pores
are 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
J – the solvent flux
r P 2
P – pressure difference
J
8 x
x – thickness of membrane cylindrical pores
- tortuosity
- viscosity
r – the pore radius
ε – surface porosity
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Transport equations
2. Kozeny-Carman
P 3
J
KS x
2
closely packed spheres
S – surface area per unit volume
K – Kozeny-Carman constant
(depends on the pore
geometry)
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How to prepare MF membranes
1. Stretching
Semycristalline polymers (PTFE, PE, PP)
if stretched perpendicular to the axis of
crystallite 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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How to prepare MF membranes
2. Track-etching
These membranes are now made by exposing
a 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 the
damaged polymer.
radiation source
membrane Track-etched 0.4 μm PC membrane
polymer film
etching bath
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How to prepare MF membranes
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, Chemical phase inversion
a sudden drop in solution temperature causes 0.45 μm PVDF membrane
the polymer to precipitate. The solvent is
then washed out.
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How to prepare MF membranes
4. Sintering
This method involves compressing a powder consisting of particles of
a given size and sintering at high temperatures.
The required temperature depends on the material used.
HEAT
pore
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Materials used
PTFE, teflon
Synthetic polymeric membranes: PVDF
PP
a) Hydrophobic PE
b) Hydrophilic
Cellulose esters
PC
Ceramic membranes PSf/PES
PI/PEI
Alumina, Al2O3 PA
Zirconia, ZrO2 PEEK
Titania, TiO2
Silicium Carbide, SiC
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Materials used
1. Polymeric MF membranes
Stretching
Phase inversion
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:
1. Economy of manufacture 4. Minimum waste of energy
2. Membrane integrity against 5. Easy egress of
damage and leaks permeate
3. Sufficient mass transfer to keep 6. Permit the membrane
polarization in control to be cleaned
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Modules: tubular
• 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 Diameter tubular membrane assembly
• Surface area/volume: low
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Modules: hollow fiber
• Fibers: 300 – 5000 per module
• Fibers diameter: 50nm
mesopore 2nm
micropore f<2nm
nonporous f = pore diameter
Process Driving force Membrane Pore Separation principle
Microfiltration pressure difference macropore filtration
(0.1 - 1 bar)
Ultrafiltration pressure difference mesopore filtration
(0.5 – 10 bar)
Nanofiltration pressure difference micropore filtration/
(5 – 20 bar) electrostatic interaction/
solution-diffusion
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The characterization of porous
membranes
1. shape of the pore (pore geometry)
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1. Pore geometries
J – the solvent flux
P – pressure difference
x – thickness of membrane
Hagen-Poiseuille equation
- tortuosity
- viscosity
ε r ΔP2 r – the pore radius
J – the surface porosity
8 η τ Δx
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1. Pore geometries
S – the internal surface area
K – Kozeny-Carman constant
Kozeny-Carman relationship
ε 3
ΔP
J
K η S 1 - ε Δx
2 2
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1. Pore geometries
top layer thickness
0.1-1mm
sub layer thickness
50-150mm
The flux is inversely proportional commercial
to the thickness. interest
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The characterization of porous
membranes
2. pore size distribution
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The characterization of porous
membranes
3. surface porosity
r – the pore radius
πr 2
ε np np – number of pores
Am Am – membrane area
Microfiltration membranes: 5-70%
Ultrafiltration membranes: 0.1-1%
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The characterization of porous
membranes
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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The characterization of porous
membranes
Characterization methods
Microfiltration Ultrafiltration
scanning electron microscopy gas adsorption-desorption
bubble-point method thermoporometry
mercury intrusion porometry permporometry
permeation measurements liquid displacement
rejection measurement
transmission 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 mm 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 bar
material directly influences the separation
nanofiltration membranes are considered intermediate between porous
and nonporous membranes
most of the nanofiltration membranes are charged
two models for the separation mechanism
1. permeation through a micropore
2. 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
The Donnan potential
RT a A RT a B
Ψ Don Ψ -Ψ
m
ln m ln m
z A F a A z BF a B
Y - the electrical potential z - the valence
R - the gas constant F - the Faraday constant
T - the temperature a - the activity of the solutes
“m” refers to the membrane phase, while “A” and “B” are the components in the
solution
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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 <1mm, sub layer ~50-150mm
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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