Membrane Technology & Separation Processes (Electrodialysis & Concentration Polarization) Module- 23 Lec- 23 Dr. Shishir Sinha Dept. of Chemical Engineering IIT Roorkee
Membrane Technology & Separation
Processes
(Electrodialysis & Concentration
Polarization)
Module- 23
Lec- 23
Dr. Shishir Sinha
Dept. of Chemical Engineering
IIT Roorkee
Mixing vs Separation
Two substances a and b will mix or separate depending on free enthalpy of mixing(ΔGm
)
ΔGm
= ΔHm
– TΔSm
where ΔHm
is the enhalpy of mixing and ΔSm
is the entropy of mixing.
ΔGm
< 0:Spontaneous mixing ;
ΔGm
> 0:Spontaneous separation
In most cases and always when A and B are gases the mixing occurs spontaneously and
minimum amount of energy, Wmin
= ΔGm
The actual energy requirement for the separation will bemany times greater than Wmin
The actual energy requirement depends on the type of separation processes
Membrane Separations
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 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 €.
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.
Recovery. Both the concentrate and the permeate could be recovered to use.
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.
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.
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
- The membrane operations more widely used are those based in applying a pressure difference
between both sides of the membrane.
Micro Filtration (MF)(10-0.1m)Bacteria, suspended particles
Ultrafiltration (UF)(0.05-0.005m) Colloids, macromolecules
Nanofiltration (NF)5e-3-5.e-4mSugars, 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-4mSugars, dyes, divalent salts
Reverse Osmosis (RO)(1.e-4-1e-5 m)Monovalent salts, ionic metals
Water
• Microfiltration (MF).
• Ultrafil
• Nanofil
• Reverse
- Althoug
different.
Name of
tration (UF)
ltration (NF)
e osmosis (R
gh similar in
.
f the membr
).
).
RO).
n appearanc
rane proces
e, the involv
s as functio
ved mechan
n of the par
nisms in the
rticle size.
separation ccan be veryy very
- There a
separatio
• Dialysi
• Liquid
- In other
in:
• Membr
• Membr
• Osmoti
Membra
are other sep
on of the com
s.
membranes.
rs, the memb
rane extractio
rane distillati
c distillation
ane Separat
paration ope
mpounds:
• Gas perm
. • Pervapor
brane is not
on.
ion.
n.
ions
erations whe
meation (GP)
ration.
directly resp
Type
re a membra
). • Ele
ponsible for
of filtration
ane is the re
ectrodialysis
the separati
n.
esponsible o
s (ED).
on but it act
of the la sele
tively partici
ective
ipates
Membra
Membra
- Synthet
to some d
ane Separat
ane Separat
tic membran
driving force
ions
Simp
ions
nes are solid
e.
ple scheme o
barriers that
of a membr
t allow prefe
rane module
erentially to
e.
pass specificc compound
ds due
(Very) Si
Membra
- The sep
• Pore siz
• Design
• Chemic
• Electric
Membra
- The me
imple schem
ane Separat
paration abili
ze and struct
cal character
cal charge
ane Separat
embranes can
me for some
ions
ity of a synth
ture
ristics
ions
n be roughly
e mechanism
hetic materia
y divided in t
ms of selectiv
al depends o
two main gro
ve separatio
on its physica
oups: porous
on on a por
al, chemical
s and non po
ous membr
l properties.
orous.
rane.
- 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
/CCα
A,f
A,pA C
C
for two or more component
Membrane Separations
Main parameters.
- In RO, often we use the Recovery (Y)
100Q
Q(%)Y
f
p
Qp: Permeate flowrate (m
3
/s)
Qf: Feed flowrate (m
3
/s)
Membrane Separations
Main parameters.
- Passive transport in membranes. The permeate flux is proportional to a given driving force
(some difference in a property).
(X) ForcerivingD )A( onstantC (J)Flux
Driving forces:
Pressure (total o partial)
Concentration
Electric Potential
Membrane Separations
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
Main parameters.
- Permeate flux.
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.
dP
8r
AQJ
2
mw
w
Jw: Solvent flux (m
3
/s·m2
) Qw: Solvent flowrate (m
3
/s) Am
: Membrane area (m2
)
d: Membrane thickness (m) : Viscosity (Pa ·s) P: Hydraulic pressure difference (Pa)
r: Pore radius (m) : Porosity : Tortuosity
Membrane Separations
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.
dP
1SK
AQJ
22
3
mw
w
JW
: Solvent flux (m3
/s·m2
) QW
: Solvent flowrate (m3
/s)
S: Particle surface area (m2
/m3
) K: Kozeny-Carman constant
Am
: Membrane area (m2
) d: Membrane thickness (m) : Viscosity (Pa ·s)
Membrane Separations
- In the
appears,
the memb
Membra
- Concen
(It is not
Membra
operations g
which must
brane surfac
ane Separat
ntration polar
fouling!!!)
ane Separat
governed by
be carefully
ce.
For
ions
risation.
ions
y the pressur
y controlled.
rmation of t
re, a phenom
This is due
the polarisa
menon calle
to the solute
ation layer.
ed concentra
e accumulat
ation polaris
tion neighbo
sation
ouring
- Fouling
• Pore siz
• Pore plu
• Formati
Membra
g: Irreversibl
ze reduction
ugging.
ion of a gel l
ane Separat
le reduction
by irreversi
layer over th
ions
of the flux th
ble adsorptio
he membrane
hroughout th
on of compo
e surface (ca
he time.
ounds.
ake).
- 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
- 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
Symmetr
Membra
Symmetr
Membra
ric UF memb
ane Separat
ric ceramic m
ane Separat
brane of 0.45
ions
Surfa
membrane of
ions
5 m made o
ce
f 0.2 m ma
of cellulose
ade of alumin
acetate (Mil
Cross sectio
na (Al2O
3) (A
llipore).
on
AnoporeTM
)
).
Asymme
Membra
Membra
etric ceramic
ane Separat
U
ane Separat
c membrane
ions
UF integral as
ions
made of -A
symmetric m
Al2O
3 (Memb
membrane m
bralox).
made of polyp
propylene.
Membra
- Configu
• Configu
• Module
The mod
through t
and the m
Membra
- Configu
• Flat.
ane Separat
uration and m
uration: geom
e: name of th
dule seals an
the confined
membrane su
ane Separat
uration:
ions
modules
metric form
he devices su
nd isolates th
d space chara
urface pheno
ions
RO comp
given to the
upporting on
he different
acterises eac
omena depen
posite membr
e synthetic m
ne or several
streams. The
h module. T
nd on the mo
ranes.
membranes.
membranes
e geometry
The type of f
odule design
s (housing).
and specific
flux, the tran
n.
c fluid move
nsport mecha
ement
anism
- The act
- Synthes
- Later, o
- Used in
- High su
Membra
Consists
with feed
out in the
Membra
tive layer is a
sised as a co
one can selec
n two kind of
urface area/v
ane Separat
of layers o
d material fl
e other direc
ane Separat
a flat.
ntinuous lay
ct a desired g
f modules: p
volume ratio.
ions
Pla
of membrane
lowing in an
tion.
ions
yer.
geometry (re
plate-and-fra
.
ate-and-Fram
es separated
nd retentate
ectangle, circ
ame and spira
me Membran
d by corruga
flowing out
cle,...) to be
al wound.
ne System.
ated structur
t in one dire
placed in th
ral sheets, a
ection, while
e module.
alternating l
e permeate f
layers
flows
Membra
Membra
ane Separat
ane Separat
ions
ions
Spiral-w
Spiral-w
wound modu
wound modu
ule.
ule.
- Configu
• Tubular
- It is like
- Usually
- The per
- Low su
- Several
- Module
Membra
Membra
uration:
r.
e a tube.
y the active l
rmeate cross
urface are/vo
l lengths and
es grouping o
ane Separat
ane Separat
layer is insid
ses the memb
lume ratio.
d diameter (>
one or variou
ions
D
ions
de.
brane layer t
>10 mm).
us membran
Different type
to the outside
nes.
es of tubular
e (this is, the
r modules.
e feed flows inside).
Membra
Cross sec
Membra
ane Separat
ction of hollo
ane Separat
ions
ow fiber (M
ions
Hollow
onsanto). Co
w fiber modu
omparison w
ule.
with a clip.
Hollow f
Membra
Hollow f
Membra
fiber cross se
ane Separat
fiber made o
ane Separat
ection of pol
ions
f polysulfon
ions
lyamide for
ne ( 1 mm
RO (DuPon
m) for UF (d
nt).
detail).
Hollow f
Membra
Membra
fiber cross se
ane Separat
ane Separat
ection of
ions
Hollow f
ions
1 mm (Mo
fiber surface
onsanto).
of polyproppylene (Celg
gard).
- 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
- 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
- Material:
• Organic.
- Made of polymers or polymer blends.
- Low cost.
- Problems with their mechanical, chemical resistance.
Temperature
pH, Solvents
Pressure
Membra
Membra
Membra
ane Separat
ane Separat
ane Technol
ions
Polypr
ions
Polyt
ogy
ropylene wit
tetrafluoroet
th 0.2 m po
tylene with 0
ores (Accure
0.2 m pores
el).
s.
• Dialysi
- Applied
- Low ind
- Ions &
- Ionic M
- Driving
- S
Membra
• Dialysi
- Artificia
- NaOH r
industry)
s
d since the 7
dustrial inter
species of lo
Membranes (j
g Force: conc
low and low
ane Technol
s
al kidney.
recovery in t
).
0’s.
rest.
ow MW (<
just like ED)
centration gr
w selective.
ogy
textile efflue
100 Da).
).
radient.
ents, alcoholl removal froom beer, saltts removal (p
pharmaceutiical
Membra
• Dialysi
Looks no
Membran
Membra
• Electro
- First ap
- Ion Sep
- Ionic M
ane Technol
s
ot very impo
ne and modu
ane Technol
dialysis (ED
pplications ba
parations.
Membranes (n
ogy
ortant...?.
ule markets
ogy
D)
ack at 30’s.
non porous)
HD
.
GS PV
RO
V ED
MF
UFF
- Driving
- Potentia
- Flat con
- Hundre
- Orthogo
Membra
• Electro
Membra
• Electro
g Force: grad
al: 1-2 V.
nfiguration.
eds of anioni
onal electric
ane Technol
dialysis (ED
ane Techno
dialysis (ED
dient in elect
c and cation
al field.
ogy
D)
logy
D)
trical potenti
nic membran
ial.
nes placed altternatively.
Membra
• Electro
- Ionic M
- Based o
- Thickne
- ED with
- ED at h
- ED with
Membra
ane Technol
dialysis (ED
Membranes (n
on polystyren
ess: 0.15-0.6
h reverse po
high tempera
h electrolysi
ane Technol
ogy
D)
non porous)
ne or polypr
6 mm.
larization (E
ature (60ºC).
is.
ogy
.
ropylene with
EDR).
h sulfonic annd quaternarry amine grooups.
• Electrodialysis (ED)
- Required membrane area
Mass balance (in equivalents)
0m Cj dA V z dc
Vc
C out
j
Vc
c in
Charge flow i: electric current density (A/m2)
Am
: membrane surface (m2)
m
j F dIi
dA
combining
C in out in out
T m
N V c c z F V c c z FA N A
i i
η: global electrical efficiency (~0.5 commercial equipment) j: cation flow (eq/m2 s)
F: Faraday constant (96500 C/eq)
N: number cells in the equipment
z: cation charge (eq/mol)
Membrane Technology
• Electrodialysis (ED)
- Then the required energy, E (J), is
2C CE N U I t N I R t U
C: potential gradient in a cell (V)
RC: total resistance in a cell ()
as
C in out
m
V c c z FI i A
then
2
CC
VE N R t
c z F
2
CC
Vó P N R
c z F
P: required Power (J/s)
Membrane Technology
• Electrodialysis (ED)
Where, the required specific energy, (J/m3
), is
2
C CC
EE V R
N V
c z F
t
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.
Membra
• Electro
- How to
Cation
iMtF
Di
F
If cDM
+
=
lim
Di
ane Technol
dialysis (ED
determine o
Transport
zD
Dc c
z D DM
M
c c
t t
= 0
z D
M
F c
t t
ogy
D)
operational i
iDMct
F
M
i?
Usually:
t: transpo
D: diffus
Membra
• Electro
- Intensit
Membra
• Electro
- Fields o
W
- Compet
- Econom
- Other fi
F
T
i = 0.8ilim
ort number
ion coefficie
ane Technol
dialysis (ED
ty Evolution
ane Technol
dialysis (ED
of applicatio
Water desalin
ting to RO.
mically more
fields of appl
ood Industry
Treatment of
ent
ogy
D)
versus appl
ogy
D)
n:
nation.
e interesting
lication:
y.
heavy metal
ied potential
at very high
l polluted wa
l
h or very salt
ater.
t concentrati
ions.
Membrane Technology
• 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
• Electrodialysis (ED)
elect
Membra
• Electro
trolytic Cell
ane Technol
dialysis (ED
l for the pro
ogy
D)
oduction of
m
chlorine an
membrane.
nd sodium hhydroxide wwith cationicc
Electr
Membra
• Electro
rolytic cell f
ane Technol
dialysis (ED
for the prod
ogy
D)
Hydroge
duction of su
m
en fuel cell
ulfuric acid
membrane.
with a catio
and sodium
onic membr
m hydroxide
rane.
e with bipollar