1. Membrane Separations Microfiltration Dan Libotean - Alessandro Patti
PhD students
Universitat Rovira i Virgili,
Tarragona, Catalunya

2. 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.
REAL MEMBRANE
IDEAL MEMBRANE
Feed
Permeate
Driving Force
Phase 2
Phase 1
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3. 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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4. Membranes market in W. Europe
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5. 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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6. Cumulative capacity of MF
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7. 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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8. Pressure driven processesMF
kPa UF
kPa NF
MPa RO
MPa
∆P=
Mid-size organic substances,
multiple charged ions
Bacteria, parasites, particles
High molecular substances, viruses
Low molecular substances, single charged ions
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9. Pore size of MF membranes
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10. 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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11. 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
cylindrical pores
J – the solvent flux
DP – pressure difference
Dx – thickness of membrane
- tortuosity
h - viscosity
r – the pore radius
ε – surface porosity
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12. Transport equations 2. Kozeny-Carman closely packed spheres
S – surface area per unit volume
K – Kozeny-Carman constant
(depends on the pore
geometry)
closely packed spheres
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13. How to prepare MF membranes
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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14. 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
polymer film
etching bath
membrane
Track-etched 0.4 μm PC membrane
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15. 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,
a sudden drop in solution temperature causes
the polymer to precipitate. The solvent is
then washed out.
Chemical phase inversion
0.45 μm PVDF membrane
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16. 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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17. Materials used PTFE, teflon PVDF Synthetic polymeric membranes: PP PE
Cellulose esters PC
PSf/PES
PI/PEI PA
PEEK
Synthetic polymeric membranes:
Hydrophobic
Hydrophilic
Ceramic membranes
Alumina, Al2O3
Zirconia, ZrO2
Titania, TiO2
Silicium Carbide, SiC
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18. Materials used 1. Polymeric MF membranes Stretching Phase inversion
Track-etching
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19. Materials used 2. Ceramic MF membranes
Anodec, anodic oxidation (surface)
US Filter, sintering (cross section, upper part)
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20. 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
damage and leaks
5. Easy egress of
permeate
3. Sufficient mass transfer to keep
polarization in control
6. Permit the membrane
to be cleaned
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21. Modules: tubular • Membranes diameter: >0.5 mm
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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22. Modules: hollow fiber • Fibers: 300 – 5000 per module
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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23. Symmetric membranes The cross section shows a uniform
and regular structure
cross section
surface
Symmetric ceramic membrane (Al2O3)
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24. Asymmetric membranes Same material! 0.1/0.5 μm Porous with toplayer
Porous irregular layer
50/150 μm
The active layer is supported
over the porous layer.
Cross-section of
an asymmetric
PSf membrane.
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25. Fouling and resistance
Fouling depends on: concentration, temperature
pH, molecular interactions
Resistances-in-series model to describe the flux decline:
J: flow
ΔP: pressure drop
η: viscosity
Rm: membrane
resistance
Rc: cake resistance
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26. Fouling and resistance
porous
membrane
gel layer
The build-up layer and the clogging
of the pores are referred to as a
fouling layer.
Rm= Rm(t=0)+Ra+Rp; Rc=Rg+Rcp
Rtot=Rm+Rc
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27. Methods to reduce fouling
Back-flushing
1. Pretreatment of the feed solution
Heat treatment
pH adjustament
Addition of complexing agents
Chlorination
Adsorption onto active carbon
Chemical clarification
2. Membrane properties
Hydraulic cleaning
Mechanical cleaning
Chemical cleaning
Electric cleaning
Narrow pore size distribution
Hydrophilic membranes
Reducing concentration polarisation
a1. Increasing flux velocity
a2. Using low flux membranes
b. Turbulence promoters
3. Module and process conditions
4. Cleaning
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28. Back-flushing ΔP t J t suspension permeate suspension permeate
Restorable pressure
with back-flushing
Irreversible fouling
starting points J t
Restorable flux
with back-flushing
Irreversible fouling
starting points
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29. Dead end and cross-flow
To reduce fouling two process modes exist:
1. Dead-end
2. Cross-flow
Feed
Feed
Retentate
Cake layer
Permeate
Permeate
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30. 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
0.005 – 7.4 1
0.09 – 10.0
CTI TechSep
0.45
0.13 – 11.5
Ceramen FH
PSf
0.01 – 3.7
AG Technology
0.2 PP
2.0
Akzo
PP/PF
10.8 – 15
Memtec
0.1
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31. MF process applications
To replace four unit operations in the waste water
treatment process.
Residual
disinfectant
Waste
water MF
Pre
Filter
MIX
COAG/
FLOC
SED
FILT
Disinfectants &
Coagulants
Water
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32. MF process applications
2. To eliminate organic matter using MF after a pre-treatment
with coagulants
Waste
water MF
Pre
Filter
Water
Coagulants
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33. MF process applications
3. MF as pre-treatment for RO or NF
Water RO
Waste
water MF
Pre
Filter NF
Water
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34. Retentate: how will it be used?
Sent to a treatment plant
Discharged into a body of water
Sent to a storage facility
For ground applications
Recycled back to water source
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35. Some industrial applications
Waste-water treatment
Clarification of fruit juice, wine and beer
Ultrapure water in the semiconductor industry
Metal recovery as colloidal oxides or hydroxides
Cold sterilization of beverages and pharmaceuticals
Medical applications: transfusion filter set, purification of surgical water
Continuous fermentation
Purification of condensed water at nuclear plants
Separation of oil-water emulsions
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36. Ultrafiltration & Nanofiltration
Membrane Separations
Ultrafiltration & Nanofiltration

37. Membrane separation
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38. Membrane separation
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39. Membrane separation
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40. Membrane characterization
Membrane properties
Membrane separation
properties
pore size
pore size distribution
free volume
crystalinity
rejection
separation factor
enrichment factor
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41. Membrane characterization
Membranes
porous
nonporous
macropore f>50nm
mesopore 2nm<f<50nm
micropore f<2nm
f = pore diameter
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42. The characterization of porous membranes
1. shape of the pore (pore geometry)
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43. 1. Pore geometries J – the solvent flux DP – pressure difference
Dx – thickness of membrane
t - tortuosity
h - viscosity
r – the pore radius
e – the surface porosity
Hagen-Poiseuille equation
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44. 1. Pore geometries S – the internal surface area
K – Kozeny-Carman constant
Kozeny-Carman relationship
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45. 1. Pore geometries top layer thickness 0.1-1mm sub layer thickness
The flux is inversely proportional
to the thickness.
commercial
interest
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46. The characterization of porous membranes
2. pore size distribution
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47. The characterization of porous membranes
3. surface porosity
r – the pore radius
np – number of pores
Am – membrane area
Microfiltration membranes: e  5-70%
Ultrafiltration membranes: e  0.1-1%
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48. 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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49. The characterization of porous membranes
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50. 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 bar
typically used to retain macromolecules and colloids
the lower limit are solutes with molecular weights of a few thousands Daltons (1Dalton g)
average flux around GFD (~ l/m2.h), at an operating pressure of 50 psig (~ 3,5bar)
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51. Ultrafiltration - alumina (Al2O3) 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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52. 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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53. 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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54. Fouling Build-up of impurities in the membrane that can keep it
from functioning properly.
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55. Ultrafiltration
Crossflow Mode
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56. Ultrafiltration
Dead End Mode
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57. Cleaning
Cleaning in Backwash mode
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58. Cleaning
Cleaning in Forward Flush mode
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59. 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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60. 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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61. 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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62. 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= bar)
operating temperature
high temperature high permeate rate
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63. 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 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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64. 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
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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65. 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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66. negatively charged membrane pozitively charged membrane
Nanofiltration
Membranes for which the Donnan exclusion seems to play an important role
negatively charged membrane pozitively charged membrane
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67. Membranes for which the diffusion seems to play an important role
Nanofiltration
Membranes for which the diffusion seems to play an important role
nonporous membrane
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68. Nanofiltration - first stage is preparing the porous sub layer
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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69. 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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