5. What is a membrane?
A membrane is...
? ? ?
...a physical barrier (no necessarily solid)
that gives, or at least helps, the separation
of the components in a mixture.
Membrane Separations
7. - Membrane processes are not based in
thermodynamic equilibrium but based in the
different transport rate of each species through the
membrane.
- The membrane market is still growing. In the
1986-96 decade, the sales related to membrane
products and systems doubled.
- In 1998, these sales were over 5000 million €.
Membrane Separations
9. Advantages
Energy savings. The energy consumption is very low as
there is no phase change.
Low temperature operation. Almost all processes proceed
at room temperature, thus they can deal with compounds that
are not resistant at high temperatures.
Water reuse. When applied to recover water, they avoid the
transport of large water volumes and permit the reduction of
the Chemical Oxygen Demand (COD) loading in sewage
plants.
Recovery. Both the concentrate and the permeate could be
recovered to use.
Membrane Separations
10. Advantages
Compact operation. Which permits to save space .
Easy scale-up. Because usually they are designed in
modules, which can be easily connected.
Automatic operation. The most of the membrane plants are
managed by expert systems.
Tailored systems. In many cases, the membranes and
systems can be specifically designed according the problem.
Membrane Separations
11. Disadvantages
High cost. Membranes (and associated systems) are
costly, but for low selective separations.
Lack of selectivity. In many cases, the separation factors
are still insufficient.
Low fluxes. The permeat flowrate available are still too low
for some applications.
Sensitive to chemical attack. Many materials can be
damaged by acids, oxidants or organic solvents.
Lack of mechanical resistance. Many materials do not
withstand abrasion, vibrations, high temperatures or pressures.
Membrane Separations
12. - The membrane operations more widely used are
those based in applying a pressure difference
between both sides of the membrane.
• Microfiltration (MF).
• Ultrafiltration (UF).
• Nanofiltration (NF).
• Reverse osmosis (RO).
- Although similar in appearance, the involved
mechanisms in the separation can be very very
different.
Membrane Separations
Micro Filtration (MF)
(10-0.1m)
Bacteria, suspended particles
Ultrafiltration (UF)
(0.05-0.005m)
Colloids, macromolecules
Nanofiltration (NF)
5e-3-5.e-4 m
Sugars, dyes, divalent salts
Reverse Osmosis (RO)
(1.e-4-1e-5 m)
Monovalent salts, ionic metals
Water
Micro Filtration (MF)
(10-0.1m)
Bacteria, suspended particles
Ultrafiltration (UF)
(0.05-0.005m)
Colloids, macromolecules
Nanofiltration (NF)
5e-3-5.e-4 m
Sugars, dyes, divalent salts
Reverse Osmosis (RO)
(1.e-4-1e-5 m)
Monovalent salts, ionic metals
Water
13. Hemoglobin
(7 nm)
0.1 1 10 100 1000 10000
Pore diameter (nm)
Cells,
bacteria
and
polymers
Virus and
proteins
Vitamins
and sugars
Salts and
low molecular
weight
compounds
H2O
(0,2 nm)
Na+
(0,4 nm)
Glucose
(1 nm)
Influenza Virus
(100 nm)
Pseudomonas
Diminuta
(280 nm)
Staphylococcus
(1000 nm)
Starch
(10000 nm)
Microfiltration
Ultrafiltration
Nanofiltration
Reverse Osmosis
Emulsions
and colloids
Name of the membrane process in function
of the particle size.
Membrane Separations
16. - There are other separation operations where a
membrane is the responsible of the la selective
separation of the compounds:
• Dialysis.
• Electrodialysis (ED).
• Pervaporation.
• Gas permeation (GP).
• Liquid membranes.
- In others, the membrane is not directly
responsible for the separation but it actively
participates:
• Membrane extraction.
• Membrane distillation.
• Osmotic distillation.
Membrane Separations
19. - Synthetic membranes are solid barriers that allow
preferentially to pass specific compounds due to
some driving force.
(Very) Simple scheme for some mechanisms of
selective separation on a porous membrane.
+
+
+
+
+ +
+
Membrane Separations
20. - The separation ability of a synthetic material
depends on its physical, chemical properties.
• Pore size and structure
• Design
• Chemical characteristics
• Electrical charge
Membrane Separations
21. - The membranes can be roughly divided in two
main 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
22. Main parameters.
- Rejection, R, if there is just one component (RO)
f
,
A
p
,
A
f
,
A
p
,
A
f
,
A
C
C
1
100
C
C
C
100
(%)
R
- Separation factor - Enrichment factor
B
A
B,f
A,f
B,p
A,p
A,B
/C
C
/C
C
α
A,f
A,p
A
C
C
Membrane Separations
for two or more component
23. Main parameters.
- In RO, often we use the Recovery (Y)
Qp: Permeate flowrate (m3/s)
Qf: Feed flowrate (m3/s)
100
Q
Q
(%)
Y
f
p
Membrane Separations
24. Main parameters.
- Passive transport in membranes. The permeate
flux is proportional to a given driving force (some
difference in a property).
(X)
Force
riving
D
)
A
(
onstant
C
(J)
Flux
Driving forces:
Pressure (total o partial)
Concentration
Electric Potential
Membrane Separations
25. Main parameters.
Membrane processes and driving force.
Process
Feed
phase
Permeate
phase
Driving
Force
Microfiltration L L ΔP
Ultrafiltration L L ΔP
Nanofiltration L L ΔP
Reverse Osmosis L L ΔP
Dialysis L L Δc
Electrodialysis L L ΔΕ
Pervaporation L G ΔP
Gas Permeation G G ΔP
Membrane Separations
26. Main parameters.
- Permeate flux.
d
P
8
r
A
Q
J
2
m
w
w
Jw: Solvent flux (m3/s·m2)
Qw: Solvent flowrate (m3/s)
Am: Membrane area (m2) r: Pore radius (m)
d: Membrane thickness (m)
: Viscosity (Pa ·s)
P: Hydraulic pressure difference (Pa)
: Tortuosity
In MF and UF, porous membrane model is assumed, where
the a stream freely flows through the pore. Then, the
transport law follows the Hagen-Poiseuille equation.
: Porosity
Membrane Separations
27. Main parameters.
- The above model is good for cylindrical pores.
However, if the membrane is rather formed by a
aggregated particles, then the Kozeny-Carman relation
works much better.
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)
: Viscosity (Pa ·s)
d
P
1
S
K
A
Q
J 2
2
3
m
w
w
Am: Membrane area (m2)
Membrane Separations
28. - In the operations governed by the pressure, a
phenomenon called concentration polarisation
appears, which must be carefully controlled. This
is due to the solute accumulation neighbouring the
membrane surface.
Feed
Polarisation layer
membrane
membrane
Permeate
Permeate
Formation of the polarisation layer.
Membrane Separations
30. - Fouling: Irreversible
reduction of the flux
throughout the time.
• Pore size reduction by
irreversible adsorption of
compounds.
• Pore plugging.
• Formation of a gel layer over
the membrane surface (cake).
Membrane Separations
31. - Membrane can be classified in several ways, but
always there are arbitrary classifications.
• Structure: symmetric, asymmetric
• Configuration: flat, tubular, hollow fiber
• Material: organic, inorganic
• Surface charge: positive, negative, neutral
• ...and even other divisions and subdivisions
Membrane Separations
32. - Structure:
• Symmetric. Also called homogeneous. A cross
section shows a uniform porous structure.
• Asymmetric. In a cross section, one can see two
different structures, a thin dense layer and below a
porous support layer.
- Integral: the layers are continuous.
- Composites: the active layer (thickness
0.1-0.5 μm) is supported over a highly
porous layer (50-150 μm), sometimes
both layers are of different materials.
Membrane Separations
33. Symmetric UF membrane of 0.45 m made of
cellulose acetate (Millipore).
Membrane Separations
34. Symmetric ceramic membrane of 0.2 m made of
alumina (Al2O3) (AnoporeTM).
Surface Cross section
Membrane Separations
38. - Configuration and modules
• Configuration: geometric form given to the
synthetic membranes.
• Module: name of the devices supporting one
or several membranes (housing).
The module seals and isolates the different streams. The
geometry and specific fluid movement through the confined
space characterises each module. The type of flux, the
transport mechanism and the membrane surface phenomena
depend on the module design.
Membrane Separations
39. - Configuration:
• Flat.
- The active layer is a flat.
- Synthesised as a continuous layer.
- Later, one can select a desired geometry (rectangle,
circle,...) to be placed in the module.
- Used in two kind of modules: plate-and-frame and
spiral wound.
- High surface area/volume ratio.
Membrane Separations
40. Plate-and-Frame Membrane System.
Membrane Separations
Consists of layers of membranes separated by corrugated
structural sheets, alternating layers with feed material
flowing in and retentate flowing out in one direction,
while permeate flows out in the other direction.
43. - Configuration:
• Tubular.
- It is like a tube.
- Usually the active layer is inside.
- The permeate crosses the membrane layer to
the outside (this is, the feed flows inside).
- Low surface are/volume ratio.
- Several lengths and diameter (>10 mm).
- Modules grouping one or various membranes.
Membrane Separations
53. - Comparison between modular configurations.
Module
Parameter Tubular Spiral-wound Hollow fiber
Specific surface area (m2
/m3
) 300 1000 15000
Inside diameter or spread (mm) 20-50 4-20 0.5-2
Flux (L/m2
day) 300-1000 300-1000 30-100
Production (m3
/m3
per module & day) 100-1000 300-1000 450-1500
Space velocity (cm/s) 100-500 25-50 0.5
Pressure loss (bar) 2-3 1-2 0.3
Pretreatment Simple Medium High
Plugging Small Medium Elevated
Replacement Easy Difficult Impossible
Cleaning:
Mechanical
Chemical
Possible
Possible
Not possible
Possible
Not possible
Possible
Membrane Separations
54. - Comparison between modular configurations.
Modular configurations and processes.
Module
Operation Tubular Spiral-wound Hollow fiber
Reverse Osmosis A VA VA
Ultrafiltration VA A NA
Microfiltration VA NA NA
Pervaporation A VA VA
Gas Permeation NA VA VA
VA = Very appropriate; A = Appropriate; NA = Not appropriate
Membrane Separations
55. - Material:
• Organic.
- Made of polymers or polymer blends.
- Low cost.
- Problems with their mechanical, chemical
resistance.
Temperature
pH, Solvents
Pressure
Membrane Separations
65. • Electrodialysis (ED)
- Ionic Membranes (non porous).
- Based on polystyrene or polypropylene with
sulfonic and quaternary amine groups.
- ED with reverse polarization (EDR).
- ED at high temperature (60ºC).
- Thickness: 0.15-0.6 mm.
- ED with electrolysis.
Membrane Technology
66. • Electrodialysis (ED)
- Required membrane area
j+
VC
c+
in
VC
c+
out
0
dc
z
V
dA
j C
m
Mass balance (in equivalents)
Charge flow
m
dA
dI
i
F
j
combining
i
F
z
c
c
V
i
F
z
c
c
V
N
A
N
A out
in
out
in
C
m
T
η: global electrical efficiency (~0.5 commercial equipment)
F: Faraday constant (96500 C/eq) N: number cells in the equipment
i: electric current density (A/m2)
Am: membrane surface (m2)
j: cation flow (eq/m2 s)
z: cation charge (eq/mol)
Membrane Technology
67. • Electrodialysis (ED)
- Then the required energy, E (J), is
t
R
I
N
t
I
U
N
E C
2
C
as
then
F
z
c
c
V
A
i
I out
in
C
m
UC: potential gradient in a cell (V)
RC: total resistance in a cell ()
t
R
F
z
c
V
N
E C
2
C
C
2
C R
F
z
c
V
N
P
ó
P: required Power (J/s)
Membrane Technology
68. • Electrodialysis (ED)
Where, the required specific energy, (J/m3), is
C
2
C
C
R
F
z
c
V
t
V
N
E
Ê
La cell resistance can be estimated from a model
based on series of resistances where the resistances to
transport are considered through two membranes and
the compartments concentrate and diluted.
Membrane Technology
69. • Electrodialysis (ED)
- How to determine operational i?
t
F
i
c
c
z
D
t
F
i DM
D
M
Cation Transport
t
t
c
c
z
F
D
i
M
DM
D
If cDM
+ = 0
t
t
c
z
F
D
i
M
D
lim
Usually: i = 0.8ilim t: transport number
D: diffusion coefficient
Cationic Membrane
Limit boundary
cCM
+
cC
+
cDM
+
cD
+
- +
concentrate diluted
Membrane Technology
70. • Electrodialysis (ED)
- Intensity Evolution versus applied potential
i (A/m2)
ilim
Ohmic zone
U (V)
Resistance rise
Ionic water
splitting
Membrane Technology
71. • Electrodialysis (ED)
- Fields of application:
Water desalination.
- Competing to RO.
- Economically more interesting at very high or
very salt concentrations.
- Other fields of application:
Food Industry.
Treatment of heavy metal polluted water.
Membrane Technology
72. • Electrodialysis (ED)
- Examples:
Production of drinking water from salty water.
Water softening.
Nitrate removal.
Lactose demineralization.
Acid removal in fruit juice.
Tartrate removal from wines.
Heavy metal recovery.
Production of chlorine and sodium hydroxide.
Membrane Technology
78. • Pervaporation
- Industrial applications.
- Alternative to distillation when thermodynamic
limitations.
Low energy costs.
Low investment costs.
Better selectivity, without
thermodynamic limitations.
Clean and closed operation.
No process wastes.
Compact and scalable units.
Membrane Technology
80. • Pervaporation.
- Do not mistake with a distillation where a membrane
is just separating phases.
- Three steps mechanisms:
Selective absorption on the membrane.
Dissolution at the membrane.
Diffusion through the membrane.
Membrane Technology
81. • Pervaporation
- The membrane is active in this process.
- The permeability coefficient (P) of a compound
depends on the solubility (S) and the diffusivity (D),
in the polymeric phase, of the crossing compound
Pi = Si (ci, cj)· Di (ci, cj)
- Simplificated transport equation:
p
i
o
i
i
i
i
i p
y
p
x
d
P
J
Ji: flux of component i d: membrane thickness xi: molarfraction in liquid i: activity coefficient
pi
o: vapour pressure yi: molar fraction at permeate pp: pressure at permeate side
Membrane Technology
82. • Pervaporation
- Main membrane parameters:
- Separation factors - Enrichment factors
B
A
B,f
A,f
B,p
A,p
A,B
/C
C
/C
C
α
A,f
A,p
A
C
C
Membrane Technology
83. • Pervaporation
Pervaporation process of an ethanol/water mixture with a PVA
membrane.
0.0
0.2 0.4 0.6 0.8 1.0
0.2
0.4
0.6
0.8
1.0
azeotrope
pseudoazeotrope
Ehtanol
at
permeate
(vapour)
Ethanol at feed (liquid)
Phase equilibria
pervaporation
0.0
Membrane Technology
84. Intermediat
tank
Condenser
Pervap. unit
Feed
Ethanol >90% w/w
Permeate
Boiler
Water
Distillation
column
Ethanol >99.95% w/w
Ethanol 20-80%
w/w
Ethanol 15% w/w
• Pervaporation
Combination of distillation and pervaporation for
the production of pure ethanol.
Plant for production
of ethanol from
sugar (Bethéniville,
France).
Membrane Technology
86. • Pervaporation
- Organic compounds recovery.
For volatile compounds.
Economically competitive.
Hydrophobic membranes: PDMS and
derivatives.
- Azeotrope breaking of organic compounds.
Studied at lab scale.
Low selectivity.
Membrane Technology
87. • Pervaporation
Lab scale separations reported.
Mixture Membrane Selectivity
Ethylbenzene/xylene Polyethylene Not available
p-xilene/o-xilene Polyethylene Not available
m-xilene/p-xilene Polypropilene m-Xylene
Dichlor ethane/trichlor
ethane
Poliamide/polyeth Dichlorethane
Benzene/cyclohexane Polyimide Benzene
Acetone/cyclohexane Polyimide Acetone
Membrane Technology
88. • Pervaporation
Hybrid process: extractive distillation and pervaporation
for the production of pure benzene and cyclohexane .
Pure cyclohexane
Pervaporation
unit
Pure benzene
Feed
Solvent
C
o
l
u
m
n
1
C
o
l
u
m
n
2
Membrane Technology
89. • Gas permeation
- Since 50’s.
- Membranes: porous and no porous.
- Several possible mechanisms for gas transport:
Knudsen Flow.
- The last two are selective.
Solution-diffusion.
X Viscous Flow.
Membrane Technology
90. • Gas Permeation
- Knudsen Flow (porous membranes). When the
porous diameter is on the range of the average free
space of the molecule (kinetic theory for gases).
i
k
i P
T
R
D
d
J
M
T
R
8
r
3
2
Dk
i
j
j
i
M
M
J
J
Transport equation
Knudsen diffusivity
: porosity d: membrane thickness
: tortuosity R: gas constant
T: temperature P: transmembrane P
r: porus radi
M: MW
Enrichment
Membrane Technology
91. • Gas permeation
-Solution-diffusion (non-porous membranes).
j
i
j
i
j
i
ij
S
S
D
D
P
P
The selectivity is referred to the separation factors of the
compounds to be separated
Pi = Si· Di
There are “slow” and “fast gases” for a determined membrane.
Membrane Technology
92. • Gas permeation
- Driving force: partial pressure gradient.
- Working pressure: up to 100 bar.
- Non-porous polymeric membranes:
PDMS, CA, PS, PES i PI
- Ceramic Membranes (small pores for Knudsen).
- Metallic membranes (Pd and Ag alloys).
Membrane Technology
93. • Gas permeation
- Asymmetric membranes.
- Thin polymer on a structural porous material.
- Preferred configuration Hollow Fiber or Spiral,
others like flat or tubular also possible.
- Applied in petrochemistry.
Purification of H2, CO2, CH4 and gaseous
hydrocarbons of difficult distillation.
Nitrogen purification.
Membrane Technology
94. • Gas permeation
- Some examples:
Enrichment, recovery and dehydration of N2.
H2 recovery in residual flows of proceses, purge o
natural gas.
Adjust of the ratio H2/CO synthesis gas.
Acid gas removal (CO2, H2S) from natural gas.
Helium recovery from natural gas and other
sources.
VOC removal from process flow.
Membrane Technology
95. Residuals gases
Gas
permeation
to fuel-gas
Hydrogen
Recycle of n-C4
Unitat de
isomerització
n-Butane
Isobutane
Recycle
H2 (96%)
• Gas permeation
Hydrogen recovery in a butane isomeration
plant.
A typical PRISM®
Separator (Airproducts)
Membrane Technology
96. • Liquid Membranes
- A liquid barrier between to phases.
- Not yet industrial uses.
- Driving force: chemical potential, concentration.
Emulsion (ELM).
- Two configurations:
Supported Liquid Membranes (SLM).
Membrane Technology
97. • Liquid Membranes
Possible
configuration
for LM.
Emulsion liquid Mem.
Organic liquid + surfactant (membrane)
Receiving phase
Aqueous phase
Porous
Support
Organic liquid impregnated
into the pores
SLM
Membrane Technology
98. • Liquid Membranes
- Advantages:
High flows due to the transport velocity in liquids.
- Drawbacks:
Selective separations due to the presence of specific reagents.
Pumping effect (against the gradient) due to the carrier
equilibrium.
Small quantities of solvent lets to the application of expensive
solvents.
Low stability of emulsions in ELM.
Leaching out of organic phase from the pores of a SLM .
Membrane Technology
99. • Liquid Membranes
Facilitated Transport in Liquid Membrane.
Liquid Membrane
M + B MB
B: selective carrier
M: selectively separated
M M
B
MB
N
P
O
Ag+
O
O
O
O
O
O diphenyl-18-crown-6
O
O
O
O
O
O
Ag+
Membrane Technology
100. • Liquid Membranes
- ELM: low practical interest
- SLM: lab scale and few applications restricted
high added value compounds.
- Hydrophobic Membranes (PE, PP ...).
- Hollow fibers.
- Potential applications:
Selective removal and concentration of cations in solution.
Selective separation of gases.
Recovery of acid or basic compounds.
Organic compound separation in complex mixtures.
Membrane Technology
101. • Other Techniques
- Membrane distillation.
A hydrophobic membrane separates two aqueous
phases.
The volatile compounds cross the membrane and
condensate.
The hydrophobic membrane avoids the aqueous
phases to get into the membrane.
The driving force in the temperature gradient.
Membrane Technology
102. • Other techniques
- Membrane distillation.
Driven by the phase equilibrium in both sides of
the membrane.
The membrane acts just like a physical barrier.
Some applications:
Water demineralization.
Inorganic acid or salt concentration.
Ethanol extraction at the fermentation.
Membrane Technology
103. • Other techniques
- Osmotic distillation.
Similar to membrane distillation.
Both phases at the same temperature.
The osmotic pressure is risen by adding
appropriate compounds to the receiving phase.
The partial pressure gradient due to the osmotic
pressure is the driving force.
Attractive to the food industry provided it
maintains the temperature.
Alcohol removal from wine and beer.
Fruit juice enrichment.
Membrane Technology
104. • Other techniques
- Membrane extraction.
The membrane acts as a barrier to separate
immiscible phases.
It has to assure immiscibility between phases.
Hollow Fiber membranes have high area.
It makes possible to avoid the separation at
decanting of the phases at the end.
Lab scale research.
Membrane Technology
