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What is a membrane?
A membrane is...
? ? ?
...a physical barrier (no necessarily solid)that gives, or at least helps, the separationof the components in a mixture.
Membrane Separations
The sorting demon...
Membrane Separations
- Membrane processes are not based inthermodynamic equil ibrium but based in thedifferent transport rate of each species through themembrane.
- The membrane market is still growing. In the1986-96 decade, the sales related to membraneproducts and systems doubled.
- In 1998, these sales were over 5000 milli on €.
Membrane Separations
Membrane Separations
Advantages
• Energy savings. The energy consumption is very low asthere is no phase change.
• Low temperature operation. Almost all processes proceedat room temperature, thus they can deal with compounds thatare not resistant at high temperatures.
• Water reuse. When applied to recover water, they avoid thetransport of large water volumes and permit the reduction ofthe Chemical Oxygen Demand (COD) loading in sewageplants.
• Recovery. Both the concentrate and the permeate could berecovered to use.
Membrane Separations
Advantages
• Compact operation. Which permits to save space .
• Easy scale-up. Because usually they are designed inmodules, which can be easily connected.
• Automatic operation. The most of the membrane plants aremanaged by expert systems.
• Tailored systems. In many cases, the membranes andsystems can be specifically designed according the problem.
Membrane Separations
Disadvantages
• High cost. Membranes (and associated systems) arecostly, but for low selective separations.
• Lack of selectivity. In many cases, the separation factorsare still i nsuff icient.
• Low fluxes. The permeat flowrate available are still t oo lowfor some applications.
• Sensitive to chemical attack. Many materials can bedamaged by acids, oxidants or organic solvents.
• Lack of mechanical resistance. Many materials do notwithstand abrasion, vibrations, high temperatures or pressures.
Membrane Separations
Micro Filtration (MF)(10-0.1µm)Bacteria, suspended particles
Ultrafilt ration (UF)(0.05-0.005µm) Colloids, macromolecules
Nanofilt ration (NF)5e-3-5.e-4 µmSugars, dyes, divalent salts
Reverse Osmosis (RO)(1.e-4-1e-5 µm)Monovalent salts, ionic metals
Water
Micro Filtration (MF)(10-0.1µm)Bacteria, suspended particles
Ultrafilt ration (UF)(0.05-0.005µm) Colloids, macromolecules
Nanofilt ration (NF)5e-3-5.e-4 µmSugars, dyes, divalent salts
Reverse Osmosis (RO)(1.e-4-1e-5 µm)Monovalent salts, ionic metals
Water
- The membrane operations more widely used arethose based in applying a pressure differencebetween both sides of the membrane.
• Microfil tration (MF).
• Ultrafiltration (UF).
• Nanofilt ration (NF).
• Reverse osmosis (RO).
- Although similar in appearance, the mechanismsinvolved in the separation can be very verydifferent.
Membrane Separations
Hemoglobin(7 nm)
0.1 1 10 100 1000 10000
Pore diameter (nm)
Cells,bacteria
andpolymers
Virus andproteinsVitamins
and sugars
Salts andlow molecular
weightcompounds
H2O(0,2 nm)
Na+
(0,4 nm)
Glucose(1 nm)
Influenza Virus(100 nm)
PseudomonasDiminuta(280 nm)
Staphylococcus(1000 nm)
Starch(10000 nm)
Microfiltration
Ultrafiltration
Nanofiltration
Reverse Osmosis
Emulsionsand colloids
Name of the membrane process in functionof the particle size.
Membrane Separations
More examples.
Membrane Separations
... and still more.
Membrane Separations
- There are other separation operations where amembrane is the responsible of the la selectiveseparation of the compounds:
• Dialysis.• Electrodialysis (ED).• Pervaporation.
• Gas permeation (GP).
• Liquid membranes.
- In others, the membrane is not directlyresponsible for the separation but it activelyparticipates:
• Membrane extraction.• Membrane distill ation.• Osmotic distill ation.
Membrane Separations
Types of filtration operation.
Dead-end Cross-flow
Membrane Separations
Simple scheme of a membrane module.
MembraneFeed Retentate(Concentrate)
Permeate(Filtrate)
CA,r, CB,r
CA,p, CB,p
CA,f, CB,f
Membrane Separations
- Synthetic membranes are solid barriers that allowpreferentially to pass specific compounds due tosome driving force.
(Very) Simple scheme for some mechanisms ofselective separation on a porous membrane.
+
+
+
+
+ ++
Membrane Separations
- The separation ability of a synthetic materialdepends on its physical, chemical properties.
• Pore size and structure
• Design
• Chemical characteristics
• Electrical charge
Membrane Separations
- The membranes can be roughly divided in twomain groups: porous and non porous.
- Porous membranes give separation due to...
• size• shape• charge
...of the species.- Non porous membranes give separation due to...
• selective adsorption• diffusion
...of the species.
Membrane Separations
Main parameters.
- Rejection, R, if there is just one component (RO)
−⋅=
−⋅=
f,A
p,A
f,A
p,Af,A
C
C1100
C
CC100 (%)R
- Separation factor - Enrichment factor
B
A
B,fA,f
B,pA,pA,B /CC
/CC.
ββ==
A,f
A,pA C
C =β
Membrane Separations
for two or more component
Main parameters.
- In RO, often we use the Recovery (Y)
Qp: Permeate flowrate (m3/s)
Qf: Feed flowrate (m3/s)
100Q
Q(%)Y
f
p ⋅=
Membrane Separations
Main parameters.
- Passive transport in membranes. The permeateflux is proportional to a given driving force (somedifference in a property).
(X) ForcerivingD )A( onstantC (J)Flux
⋅⋅=
Driving forces:
• Pressure (total or partial)• Concentration• Electric Potential
Membrane Separations
Main parameters.Membrane processes and driving force.
ProcessFeedphase
Permeatephase
DrivingForce
Microfilt ration L L û3Ultrafiltration L L û3Nanofil tration L L û3
Reverse Osmosis L L û3Dialysis L L ûF
Electrodialysis L L ûüPervaporation L G û3
Gas Permeation G G û3
Membrane Separations
Main parameters.
- Permeate flux.
dP
8r
AQJ
2
mw
w∆⋅τ⋅µ⋅
⋅ε==
Jw: Solvent flux (m3/s·m2)
Qw: Solvent flowrate (m3/s)
Am: Membrane area (m2) r: Pore radius (m)
d: Membrane thickness (m)
µ: Viscosity (Pa ·s)
∆P: Hydraulic pressure difference (Pa)
τ: Tortuosity
In MF and UF, porous membrane model is assumed,where the stream freely flows through the pores. Then,the transport law follows the Hagen-Poiseuill e equation.
ε: Porosity
Membrane Separations
Main parameters.
- The above model is good for cylindrical pores. However,if the membrane is rather formed by aggregated particles,then the Kozeny-Carman relation is preferred.
JW: Solvent flux (m3/s·m2)QW: Solvent flowrate (m3/s)S: Particle surface area (m2/m3)
K: Kozeny-Carman constant d: Membrane thickness (m)µ: Viscosity (Pa ·s)
( ) dP
1SK
AQJ
22
3
mw
w∆⋅
ε−⋅⋅µ⋅ε==
Am: Membrane area (m2)
Membrane Separations
- In the operations governed by the pressure, aphenomenon called concentration polarisation appears,which must be carefully controlled. This is due to thesolute accumulation neighbouring the membrane surface.
FeedPolarisation layer
membrane
membrane
Permeate
Permeate
Formation of the polarisation layer.
Membrane Separations
- Concentration polarisation.
(It is not fouling!!!)
Membrane Separations
- Fouling: Irreversiblereduction of the fluxthroughout the time.
• Pore size reduction byirreversible adsorption ofcompounds.
• Pore plugging.
• Formation of a gel layer overthe membrane surface (cake).
Membrane Separations
- Fouling: Cake.
• The presence of a cake is included in the transportmechanism by the addition of a new resistance due tothe cake layer.
Flux: Resistance (Rc��( )cmv RR
PJ
+∆= ccC rlR ⋅=
rc: specific resistance of the cake lc: cake thickness
Membrane Separations
])d[(
)1(180r
32s
2
c εε−= ]A)1([
ml
s
sc ε−ρ
=
Kozany – Carman relationship:
εε: cake layer porosityms: cake massρρs – solute densityA – membrane areads: solute particle diameter
Membrane Separations
cb: bulk concentrationcc: cake concentration
If the rejection is 100% in dead-end filtration,
Ac
Vcl
c
bc =
V: filtrate volume
Then the flux is
+
∆==
Ac
VcrR
PdtdV
A1
J
c
bcm
AV
cP
cr
J1
J1
c
bc
w
⋅∆
⋅+=
Jw: water flux
- Fouling: Cake.
- Fouling: back pressure.
Membrane Separations
Flux versus time behaviour in a given microfiltration process with and without back-flushing After a given period of time, the
feed pressure is released and thedirection of the permeate reversedfrom the permeate side to the feedside in order to remove the foulinglayer within the membrane or at themembrane surface.
Alternate pressuring anddepressuring and by changingthe flow direction at a givenfrequency.