Lecture 8 – Membrane (Part 1)

Dr. Hanee Farzana Hizaddin

KIL3007 Separation Processes 2

www.um.edu.my
Learning Outcomes
Principles of operation
Liquid permeation model

2
 Membranes and Membrane Processes

What is membrane?
The possible definition is (Hwang and
Kammermeyer, 1975):
A region of discontinuity interposed
between two phases
Permeate

A membrane is a selective barrier that

We will focus on the solid membranes. For
permits the separation of certain
gas and liquid membranes please refer:
species in a fluid by combination of
sieving and diffusion mechanisms N.N. Li, Separating of hydrocarbons with
liquid membranes, 1968, U.S. Patent
Based on the above definitions, the 3,410,794
membranes can be gas, liquid or solid, or
Q. Zhang and E.L. Cussler, Hollow fibre gas
combination of these phases. membranes AIChE J 31(1985)1548

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 What is Membrane?
• A region of discontinuity interposed between two phases or
• A layer of material which serves as a selective barrier between two phases

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 Membrane Processes
Advantages of membrane processes
Perform separation at lower operating
cost
Provide more valuable products Schematic Diagram of Pervaporation Process
Fewer undesirable side effects than
conventional separations method Membrane Processes
It passes some components much more • Reverse osmosis • Membrane reactor
rapidly than others
• Nanofiltration • Gas permeation
• Ultrafiltration • Membrane contactor
• Microfiltration • Membrane
• Pervaporation distillation
• Dialysis/ • Liquid membrane
electrodialysis processes
• Electrochemical
membrane processes
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 Classification of Membranes

a. Nature of membrane a) Natural

b) Synthetic

b. Structure of membrane a) Porous

b) Nonporous

a) Gas- gas
c. Application of membrane b) Gas- liquid
c) Liquid- liquid
d) Gas-solid
e) Liquid-solid

a) Adsorptive
d. Mechanism of membrane b) Diffusive
c) Osmotic
d) Ion-selective
e) Non-selective

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 Types of Membrane

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 Membrane Structure

From the morphological point of view, membranes can be categorised into;

Porous membrane
have tiny pores or pore networks within themselves
achieve separation mechanically by size exclusion (i.e. sieving) where
the rejected material may be either dissolved or suspended
depending on its size relative to that of the pore.
Dense membrane
do not have any pores and solute or solvent transport through these
take place by a solubilisation mechanism.
relies to some extent on physicochemical interactions between the
permeating component and the membrane materials, and to
separation process having the highest selectivity

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 Membrane Structure

From the structural point of view, membranes can be categorised into;

Symmetrical structure
has a similar structural morphology
at all positions within it

Asymmetrical structure
constituted of two or more structural
planes of non-identical morphologies

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 Types of Membrane

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 Types of Membrane

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 Filtration modes

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 Types of Membrane
Microfiltration (MF) Ultrafiltration (UF)
When pressure-driven flow through a Hydrostatic pressure forces a liquid
membrane or other filter medium is used to against a semi-permeable membrane
separate micron-sized particles from fluid, Behave like a physical sieve
the process is called microfiltration Highly porous membrane
Used to separate solutes with molar mass
within the range of 5 kDa to 500 kDa.

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 Types of Membrane
Nanofiltration (NF) Reverse osmosis
Nanofiltration is a liquid separation membrane • Reverse osmosis processes allow selective
technology positioned between reverse osmosis passage of a particular species (solvent),
while other species, i.e. solutes are
(RO) and ultrafiltration (UF) retained partially or completely.
NF rejects solutes approximately 1 nanometer in • It uses pressure to force a solvent through
size with molecular weights above 200 a membrane
NF has always been a difficult process to define • Highly efficient process for drinking water
purification.
and to describe. • The transport mechanism is solution-
Tight NF membranes are similar to RO diffusion
membranes, and loose NF membranes could be
classified as UF membranes

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 Gas Permeation

Knudsen flow separation is Molecular sieving Solution-diffusion

based on the inverse separation is based on separation is based on
square root ratio of the the diffusion and both solubility and
molecular weights A & B sorption characteristics. mobility factors in all
cases. Diffusivity favors
condensable molecule.

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 Gas Permeation
Apart from the commercial process
for H2 recovery from synthesis gas,
gas separation is of interest in the
following:

• O2 enriched air for combustion

and medical applications
• N2 enrichment for Wafer-Fab,
metallurgy and inerting
• CO2 for enhanced oil recovery
• CH4 from biogas
• Helium recovery from natural
gas

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 Pervaporation

Pervaporation is a separation process where a liquid mixture is in direct

contact with one side of membrane and where the permeate stream is
removed in vapour state from the other side of the membrane.

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 Pervaporation

Because of the presence of the

membrane, the liquid- vapour
equilibrium is perturbed as
shown.
Separation of:
1. Azeotropic mixtures
2. Mixture of closed boiling point
component
3. Heat-sensitive products

Mole fraction water in liquid

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 Dialysis

Dialysis is basically a diffusion

process and it describes the
separation of substances in
solution by means of their
unequal diffusion rate through
porous membrane.
Therefore, the dialysis is
achieved by imposing a
concentration gradient across
the membrane.
Typical application for this
process is the artificial kidney.

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 Electrodialysis

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 Application of Electrodialysis in Caustic Soda Industry

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 Membrane-based Gas Absorption and Stripping
• Unlike the conventional membrane process that the
membrane is the selective layer towards the fluids to
be separated, the membranes used in absorption or
stripping processes act only as somewhat a "packing
material" .

The advantages and disadvantages:

• Provide larger surface area per unit volume compared
to the conventional absorption processes such as Absorbent in
packed columns.
• There is no flow constraint such as flooding and
loading point usually existed in the conventional
absorption processes.
• Membranes supply new mass transfer resistance, that
of membrane, which is not presented in packed
columns.

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 Membrane Reactors
• A membrane reactor is a device that combines a membrane separation or
distribution process with a chemical reactor in one unit
• The main feature of the reactor is to remove the reaction product out of
the reactor with the membrane so that equilibrium of the reversible
reaction is shifted and the reaction continues to proceed to the right
toward completion.

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 Video on reverse osmosis membrane

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 Transport in membranes
• Flow of solvent through the membrane:
1) Pore flow model
2) Solution diffusion model
3) Osmotic pressure model
• Solute rejection
• Fouling phenomena

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 Pore Flow Model
• Assume that ideal cylindrical pores normal to membrane surface
• For pure solvent, or for negligibly low solute concentrations, the permeate flux can be
calculated using the Hagen-Poiseuille’s law:
= membrane porosity
= average pore diameter
= transmembrane pressure
= = viscosity
= average pore length
= permeate flux (filtration rate per
unit membrane surface area)

• Permeate flux is very sensitive to the pore diameter, TMP and the membrane
porosity.
• Permeate flux decreases with increase in viscosity and membrane thickness.
• The pressure drop for a membrane module is given by:
= inlet pressures
= outlet pressures
= pressure on the filtrate side
=
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 Solution Diffusion Model
• Liquid permeation membrane processes or dialysis
• The basic principle of dialysis is illustrated in figure below.

Cit

Concentration profiles for liquid permeation

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 Solution Diffusion Model
The equilibrium distribution coefficient K’:
: concentration of solute A in bulk
= = = liquid phase (kg mol A/m3)
: concentration of A in the fluid
adjacent to the solid
: concentration of A in the solid at
The flux eq. through each phase are equals at steady state: the surface and is in equilibrium
with
= ( )= ( )= ( ) : concentration of A in the permeate
adjacent to the solid
: concentration of A in the solid at
= = the surface and is in equilibrium
Substitute and
with
: mass transfer coefficient
= ( )= ( )= ( ))= ( ) (m/s)
: permeance in the solid (m/s)
homework
L: thickness (m)
= : diffusivity of A in the solid (m/s)
permeance vs permeability
NA: flux of solute A (kgmolA/m2s)

Ref: Transport Processes and Separation Process Principles (Geankoplis)

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 Solution Diffusion Model
Solving the eq. for concentration difference:

= = =

Final equation:

=
1 1 1
+ +

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Example 1
A liquid containing dilute solute A at a concentration =3×
10 kmol/m3 is flowing rapidly by a membrane of thickness =
3.0 × 10 m. The distribution coefficient = 1.5 and =
1.0 × 10 m2 s in the membrane. The solute diffuses through the
membrane and its concentration on the other side is = 0.50 ×
10 . The mass-transfer coefficient is large and can
be considered as infinite and = 2.02 × 10 .

Calculate the flux and the concentrations at the membrane

interfaces.
Example 1 – Solution
=
1 +1 +1

L
Example 2 - Dialysis to Remove Urea from Blood
Calculate the flux and the rate of removal of urea at steady
state in g/h from blood in a cuprophane (cellophane)
membrane dialyzer at 37°C The membrane is 0.025 mm thick
and has an area of 2.0 m2. The mass-transfer coefficient on the
blood side is estimated as = 1.25 × 10 and that on
the aqueous side is 3.33 x 10-5 m/s. The permeability of the
membrane is 8.73 x 10-6 m/s. The concentration of urea in the
blood is 0.02 g urea/100 ml and that in the dialyzing fluid will
be assumed as 0.
Example 2 – Solution

91x109 0mR 360

8
convert to per hour
.

x 2 .
 Osmotic pressure of solutions
Osmotic pressure, of a solution is proportional to the
concentration of the solute and temperature T:

=
: osmotic pressure (Pa, N/m2)
: number of kg mol of solute (total
number of ions of a solute)
: volume of pure solvent water
associated with n kg mol of
solute(m3)
: gas law constant 0.082057 m3·atm/
kg mol·K
: temperature (K)

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Example 3 – Osmotic pressure
Calculate the osmotic pressure of a solution containing 0.10 g
mol NaCl/1000 g H20 at 25°C.
Example 3 – Solution
2 ions
(Nat (i) ,

↑ basis of solvent
Ikg

0082057 m2 atm
0 .
0002kmol x 0 .
.

x (298 15)
.

Kmol 1
T
.

= 4 88 atm
.

119
997 .

09/m3
 Osmotic Pressure Model
for solvent and
permeability constant -

solute

Diffusion of solvent through membrane:

w = solvent ,
m = membrane = ( ) = : solvent flux (kg/m2·s)
constant (solvent)
: solvent membrane permeability
-/ (kg solvent/m·s·atm)
= ( ) : membrane thickness (m)
: solvent permeability constant (kg
Diffusion of solute through membrane: solvent/ m2·s·atm)
x
coefficient : hydrostatic pressure difference
= ( ) (atm)
=
: osmotic pressure difference (atm)
/constant (solute) : solute flux (kg/m2·s)
: diffusivity of the membrane (m2/s)
= ( )
: distribution coefficient
: solute permeability constant (m/s)
At a steady state, solute diffusing through the membrane : solute concentration in feed (kg
must be equal to the amount of solute leaving the solute/m3)
downstream: : solute concentration in permeate
(kg solute/m3)
If permeate stream is dilute, : concentration of solvent in
= is approximately the density of permeate (kg solvent/m3)
the solvent

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KKEK3159-NAH 38
 Solute rejection in RO
Solute rejection in RO: lower C2 is better

high R value the better it is

,

= =1

Solute rejection in RO:

( )
=

Scar
1+ ( )
eat
use
= = : atm -1

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 Example 4
Experiments at 25°C were performed to determine the permeabilities of a cellulose acetate
membrane. The laboratory test section shown in the figure has membrane area A = 2.00 x 10-3 m2 .
The inlet feed solution concentration of NaCl is =10.0 kg NaCl/m3 solution (10.0g NaCI/L,
=1004 kg solution/m3). The water recovery is assumed low so that the concentration c1 in the
entering feed solution flowing past the membrane and the concentration of the exit feed solution are
essentially equal. The product solution contains 2= 0.39 kg NaCl/m3 solution ( = 997 kg
solution/m3) and its measured flow rate is 1.92 x 10-8 m3 solution/s. A pressure differential of 5514
AsAn ,

kPa (54.42 atm) is used. Calculate the permeability constants of the membrane and the solute
rejection R.
Example 4 – Solution

density A
 -
flow
by solvent/s (linear
Given to m From Table 13 9-1 7 80 atm
rate ,
we want
change to . .

,
X ,
= .

interpolation
8 39
substitute into 13 9-1 the
predicted 71 = atm
.
.
,

NW = (1 . 92x10
·
8
m3/s)(997kg solvent (m3) (2 .
00x10-3m2) which is higher than experimental value e.

by solvent/s
NW =
9 57
.
x 10 . m

997 996 6 #20 .

C2 997 34 ky
=
=
0
-
- .
.

(9 57 3)(0 39)
·

. x 10 .

997 (0 39x1000)
gmol H20 .
.

= 0 00670 .
NaCl
kg
3 744x10-6 solute Nucl/s m 996 6x58 45
kg
= .
.
.
.

From Table 13 9-1

.
,
Me = 0 32
.
atm

To determine the osmotic pressures from Table 13 9-1

. ,

1
10 kg NaCl 1004 (m3
C, in
kg solution

(p ,
=
1004) So 1004-10 =

994kg H20 in 1m3 solution .

Molecular NaCl 58 15
weight
= .

110(
1000
=> 0 .

1721g mol NaCI/kg H2O

994X58 45 .
 long NaC1/m3 (in feed solution)

0 = 1004 kg solution /m

for NaCl in feed

only
0
= 1004-10
=
994bg He0/m3

C =
10gNall , emol Nac

994kg He0 58 . NaC

45
1000 mol NaCl
x
Nach
Amol
0 1721 mol Nach =
.

ng He
interpolate to find it,
0 1721
.
-
0 . 1 x -

4 56 .

0 5 -
0 1 22 55 4 56
.
.
-
.
.

x = π =
7 8 atm .

0 .

39kgNaCI/m3 (product)
P2 =
997kg solution/m3
in
only HeO product

#
% = 997 -
0 39.

=
996 61 .

kg #20 /m

% :
0 .

39k
NaCl Itemol
Nac
,
996 61 kgH20 58
454gNaC
.

1000 mol Nach

interpolate to find its
x
0 0067

&
0 2 0
#mol
-

-
-
.

Nach
I

0 01 -

0 0 47 0
% 0 0067KgNacI
. -

= .

kg +20
i = 0 32 atm
.

Di = T1 , T12
=
7 8 .
-

0 32 . = 7 .
48atm
Gr
 Fouling
• The decline in permeate flux with time in ultrafiltration processes is generally due to fouling.
• Fouling is an undesirable effect and a lot of attention has been devoted to prevention of fouling or to
put it more pragmatically, minimisation of fouling.
• One or both of two mechanisms generally cause fouling.
• Control of concentration polarization can help in reducing fouling since extensive build-up of solute
molecules is found to promote rapid protein adsorption and fouling.
• If the wall concentration ( ) reaches the point where solute precipitates, this gel layer can provide
an additional resistance in series with the membrane itself.
• Environmental conditions such as pH and salt concentration have also been found to have a
profound influence on the rate and extent of membrane fouling for a given solute – solvent system.
• The fouling layer may be removed to a certain extent by membrane cleaning.
• However, some irreversible fouling may also occur, which over time may necessitate membrane
replacement.
• It must be noted that fouling is not the only reason for decrease in permeate flux with time.
• With certain types of membranes, membrane compaction over a certain time period may also result
in flux decline.

KIL 3007-NAH 44
Reflection
Membrane for separation of mixtures
Liquid permeation