Download Membrane Separation

Membrane
Separation
By
Nitish Gupta
Chemical Engineering 3rd year
Meerut Institute of Engineering and Technology
1
1. Overview
• Membrane separation process is a feed
consisting of a mixture of two or more
components are partially separated by
means of semi permeable barrier (the
membrane) through which one or more
species move faster than another or
other species.
2
• Part of the feed that does not pass
through membrane, i.e. retained is
called Retentate.
• Part of the feed that passes through
the membrane is called permeate.
3
2. Why Membrane Separation
•
•
•
•
•
Saves large amount of energy.
Long life membrane.
Defect free.
More compact
More easily operated, controlled and
maintained.
4
3.Desirable attributes
• Good permeability.
• High selectivity.
• Chemical and Mechanical compatibility with
the processing environment.
• Ability to withstand large pressure
differences across the membrane thickness.
5
4. Membrane materials
• All the membrane materials are made
from synthetic and natural polymer.
• These polymers can be formed by
addition, condensation or by the
copolymerization processes.
• Thermoplastic polymer membranes can
be used for the temperature upto glass
transition temperature.
• Thermosetting polymers never melts.
6
• Membranes made of glassy polymers can
operate below or above the glass
transition temperature.
• Aromatic
polyamides
(also
called
aramids) are high melting crystalline
polymers that have better long term
thermal stability and higher resistance
to solvents than do aliphatic polyamide,
such as nylon.
• Polycarbonate differs from most other
polymers because they can be extruded
into various shape,films and sheets.
8
4. Permeance factor
To be effective for separating a mixture
of chemical component a polymer
membrane should have high permeance
and a high permeance ratio.
The permeance of the given species
diffusing through membrane of given
thickness is analogous to mass transfer
coefficient. i.e. flow rate of that
species per unit cross sectional area of
membrane per unit driving force
( pressure, concentration etc.)
9
The membrane which consist of thin
dense skin about .1 to 1 micrometer in
thick is called perm selective layer.
Ni = Pm / lm (driving force)
where, Pm is Permeability
lm is Membrane thickness.
10
5. Transport in membranes
It is important to understand the nature
of transport phenomenon in membranes
because both the permeance and driving
force depends on the mechanism of
transport.
Membranes can be macro porous, micro
porous and dense ( non-porous).
11
5 (a) Porous Membranes
• If the pore diameter is larger as
compared to the molecular diameter and
a pressure differences exists across
the membrane, bulk or convective flow
through the pores occurs. Such a flow is
generally undesirable because it is not
perm selective and therefore no
separation between component and feed
occurs.
12
Contd.
• If fugacity, activity, concentration
differences
exists
across
the
membrane but the pressure is same on
the both side, perm selective diffusion
of the component through the pores will
takes place.
Contd.
13
14
5 (b) Non porous Membranes
In this gas or liquid components absorbs
into the membrane at the upstream
face diffusion through the solid
membrane and desorbs at the face.
Diffusivities of solid is less than gas as
well as liquid.
water vapor in air is .25
water in ethanol liquid is 1.2*10-5
water in cellulose acetate is 1.0*10-8
15
6. Some Industrial Application
of Membrane Separation
• Dialysis
• Gas permeation
• Reverse osmosis
• Pervaporation
16
DIALYSIS
• Dialysis is membrane processes that
use pressure difference as the driving
force.
• It is of great importance in
hemodialysis, in which urea, creatine,
uric acid, phosphate and chlorides are
removed from blood without removing
essential higher molecular weight
compounds and blood cells. This dialysis
is called as artificial kidney.
17
18
REVERSE OSMOSIS
• It refers to the passage of solvent such
as water, through a dense membrane
i.e. permeable to the solvent, but not
the solute.
• It was first experimented by Nollet
with water, an alcohol and an animal
bladder.
• Reverse osmosis is applied to the
desalination and purification of sea
water, brackish water and waste water. 19
20
PERVAPORATION
• pervaporation is the combination of
two
words
perm
selective
and
evaporation.
• Here vaporization may occur near the
downstream face of the membrane, such
that the membrane can considered to
operate with two zones, a liquid phase
zone and a vapor phase zone as shown in
the figure.
21
22
Contd.
• It is used for the removal of volatile
organic compounds ( VOCs).
• Dehydration of ethanol.
• Removal of organics from water.
It is best applied when the heat solution
is dilute in the permeant because
sensible heat of feed mixture provides
the
enthalpy
permeate.
of
vaporization
to
23
Thank you
24