Membrane Separations

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Membrane Separations

• Microfiltration

Dan Libotean - Alessandro Patti PhD
students Universitat 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.
IDEAL MEMBRANE
REAL MEMBRANE
Driving Force
Phase 1
Phase 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
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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
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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
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Pressure driven processes
MF 10-300 kPa
RO 0.5-1.5 MPa
NF 0.5-1.5 MPa
UF 50-500 kPa
?P
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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
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Transport equations
2. Kozeny-Carman
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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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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.
Track-etched 0.4 µm PC membrane
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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,
a sudden drop in solution temperature
causes 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.
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Materials used
PTFE, teflon PVDF 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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Materials used
1. Polymeric MF membranes
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Materials used
2. Ceramic MF membranes
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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
4. Minimum waste of energy
1. Economy of manufacture
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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Modules tubular
Membranes diameter gt0.5 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
Fibers 300 5000 per module Fibers
diameter lt0.5 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
The cross section shows a uniform and regular
structure
surface
cross section
Symmetric ceramic membrane (Al2O3)
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Asymmetric membranes
50/150 µm
The active layer is supported over the porous
layer.
Cross-section of an asymmetric PSf membrane.
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Fouling and resistance
Fouling depends on concentration,
temperature pH, molecular interactions
Resistances-in-series model to describe the flux
decline
J flow ?P pressure drop ? viscosity Rm
membrane resistance Rc
cake resistance
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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(t0)RaRp RcRgRcp RtotRmRc
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Methods to reduce fouling
1. Pretreatment of the feed solution
2. Membrane properties
3. Module and process conditions
4. Cleaning
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Back-flushing
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Dead end and cross-flow
To reduce fouling two process modes exist
1. Dead-end
2. Cross-flow
Feed
Feed
Retentate
Permeate
Permeate
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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

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

Waste water
COAG/ FLOC
SED
MIX
FILT
Water
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MF process applications
2. To eliminate organic matter using MF after a
pre-treatment with coagulants
Waste water
Water
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MF process applications
3. MF as pre-treatment for RO or NF
Water
Waste water
Pre Filter
Water
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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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Some industrial applications

1. Waste-water treatment
2. Clarification of fruit juice, wine and beer
3. Ultrapure water in the semiconductor industry
4. Metal recovery as colloidal oxides or hydroxides
5. Cold sterilization of beverages and
   pharmaceuticals
6. Medical applications transfusion filter set,
   purification of surgical water
7. Continuous fermentation
8. Purification of condensed water at nuclear plants
   
9. Separation of oil-water emulsions

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Membrane Separations

• Ultrafiltration Nanofiltration

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Membrane separation
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Membrane separation
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Membrane separation
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Membrane characterization
Membrane properties
Membrane separation properties
pore size pore size distribution free
volume crystalinity
rejection separation factor enrichment factor
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Membrane characterization

• Membranes
• porous
• nonporous

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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 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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1. Pore geometries
S the internal surface area K Kozeny-Carman
constant
Kozeny-Carman relationship
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1. Pore geometries
top layer thickness 0.1-1mm
sub layer thickness 50-150mm
The flux is inversely proportional to the
thickness.
commercial 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 np number of pores Am
membrane area
Microfiltration membranes e ? 5-70 Ultrafiltrati
on membranes e ? 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
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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
  (1Dalton?1.66.10-24g)
• 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
  psig0.068948 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 lt 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
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 lt1mm, sub layer
  50-150mm
• asymmetric membranes (prepared by phase inversion
  techniques)
• - cellulose esters
• pH range 5-7, temperature lt 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