2. Introduction
• Membrane: is a thin barrier, placed between
two phases or mediums,
• It allows one or more species to selectively pass
from one medium to the other while retaining
the rest.
• It is done by a driving force.
• Membranes used for separation of mixtures are
called semi-permeable.
6. MSP – Advantages & Disadvantages
• Advantages
– Inherently simple
– Moderate operating temperature for high value
heat sensitive substances
– No change of phase occurs (except in
pervaporation)
– Same basic principles apply to most MSP
7. MSP - Disadvantages
• Membrane fouling and resultant flux decline
• Polymeric membranes have limited chemical
resistance to organic solvents
• A high degree of separation may not be
possible for many mixtures.
8. Desired properties of a membrane
• A good membrane should have
– Good permeability
– High selectivity
– Chemical stability and compatibility
– Mechanical strength
– Resistance to fouling and adsorption
– Amenability to casting of a thin film
– Suitability for fabrication of a module
10. • Polymeric membrane
– Dense or non porous membrane and Porous
membrane
• Dense or Non porous Membrane
– Isotropic or symmetric membrane
– Asymmetric membrane
– Composite membrane
• Porous Membrane
– Asymmetric membrane
– Microporous membrane
11. Dense Membranes
• A dense or non-porous membrane is a thin film
that allows selective passage of one or more
components of a mixture.
• No pores are present in the physical sense
• Permeating compound - dissolves at the
membrane surface
• Diffuses through the intermolecular space or free
volume within membrane
• Leaves at the opposite surface as permeate or
product
• Used – RO, gas separation process, pervoporation
12. • A dense membrane may have an asymmetric
or a composite structure.
• Asymmetric Membrane
– An asymmetric dense membrane has a thin dense
or non-porous permselective layer on a porous
substructure.
– The dense layer allows selective permeation (i.e.
perm-selective)
– The porous substructure offers necessary
mechanical strength to the membrane
• Allows high permeate flux
13. Symmetric or Isotropic Microporous
Membrane
• Symmetric microporous membranes are
functionally similar to conventional filters.
• In symmetric membrane the materials are
same
• But differ in their pore size and thickness
• Filters – separate relatively coarse particles in
suspension
14. • Microporous membranes: separate very fine
particles or colloids or even dissolved solutes
• Membranes are thinner than filters
• Microporous symmetric membranes
• have interconnected pores
• High porosity
• Used for microfiltration
• Pore size range from 0.1 – 10 μm.
• Particles >10 μm are rejected completely.
• Those smaller than <0.1 μm smallest pore size
freely pass through the pores.
15. Asymmetric Membranes
• Membrane should be as thin as possible for
high permeation flux
• Should have reasonable mechanical strength
and defect free.
• If membranes are too thin and mechanical
weak –
– Difficult to handle
– Develop pin-holes
• Practically impossible to have membrane <20
μm
16. • To overcome this problem
• Asymmetric membranes are made to contain
– A thin permselective layer (0.1 to 1.0 μm)
– Supported on a highly porous substructure
• The thin layer may be non-porous (RO
membrane) or with very fine pores (UF
membrane)
• Entire material is an integral part of same
material
17. Composite Membrane
• Composite membranes are functionally similar
to asymmetric membranes
• It contains
– A porous or dense, thin permselective upper layer
– Cast on a thick mechanically strong support.
• The thin layer and the support are made up
of different materials
18.
19. Electrically Charged Membranes
• They have ionic groups attached to the
membrane
• Forming fixed charged sites.
• In PEM fuel cell, the Nafion membrane is used
• Perfluoro ion exchange membrane has SO3
-
groups on a PTFE backbone
• This group give negatively charged fixed sites
• The cation can move freely within the
membrane matrix
20. • If the membrane contains a negative group
attached - conducts the cations freely
• The fixed charges repel the negative charges
and does not allow anions.
• Hence the name cation exchange membrane
• If the fixed charges are positive ,
• The membrane is called anion-exchange-
membrane.
22. Microfiltration
• Microfiltration refers to separation of fine
particles and colloids from a liquid or
particulates from a gas
• It uses porous membrane having pore sizes
range 100 to 104 nm or (0.1 to 10 μm) .
• Traditionally used for separation of
microorganisms.
• Separation of yeast from fermentation broth
23.
24. • 1G – MF membranes were made from
nitrocellulose.
• Separation occurs by sieving mechanism
• low pressure difference about 2 bar.
• Two types of flow and filtration arrangements
are –
– Cross flow filtration
– Dead-end filtration.
25. • Cross-flow filtration.
– Feed flows parallel to membrane surface
– Most of particles or solute are swept away with
the flowing feed liquid
– A part of liquid is re-circulated if required.
• Dead-end filtration
– Feed flow is normal to membrane surface
– Retained particles or solute form a cake on
membrane surface.
– Cake growth offers filtration resistance
26.
27.
28. Applications
• 1. Making small-scale lab separations for
microbiological analysis – of soft drinks,
wines, pharmaceutical products.
• Sterilization and clarification in food and
beverage, clarification of cheese etc.
• Harvesting of cells from a fermentation broth
• Detection and analysis of particulate
contaminants in air.
29. Theoretical principles
• In MF, the objective is to concentrate a
suspension - by forcing the liquid through
the pores
• Calculating solvent flux in membrane module
is essential for design of MF device
• The flux may be expressed by Darcys law
• K’ – is a constant
30. • If the flow of the fluid through the pore is
laminar,
• Hagen-Poiseulle equation can be used
31. • Sometime the membrane structure
resembles like a very thin packed bed (with
void spaces)
• The pressure drop relation for such a
membrane is give by Kozeny-Karman
equation
32. Ultra Filtration (UF)
• Separation or concentration of a large molecular
weight solute or a colloidal suspension
• Approximate mol. wt. range 1000 to 80,000
Dalton (unit of mass equivalent to H atom)
• Membrane pores size - range 1 to 100 nm.
• Driving force for UF is - ΔP
• Separation occurs by sieving mechanism.
• Separates larger molecules from smaller ones
which pass through the membrane
33.
34. • Application of Ultra Filtration
Application of UF
Paint
Recovery
Latex
Recovery
Water
treatment
Separation of
oil – water
Paper
industry
35. Configuration of a UF unit
• Since a single membrane module does not
provide large area,
• A number of modules are used in parallels
depending on feed rate.
• Two configurations are used
– Recycle configuration
– Tapered configuration
36. Recycle configuration
• A part of the retentate is reclycled back to the inlet
of the module
• To achieve higher concentration of rententate
• Increases cross flow velocity
• Reduces membrane
fouling
37. Tapered Configuration
• Modules are arranged in a parallel-series
pattern
• Retentate volume decreases after liquid
passes through a module
• Therefore, a lesser number of modules are
provided in successive stages.
38.
39. Reverse Osmosis
• When aqueous solution is kept separated
from water by a semi-permeable membrane
in two compartment cell,
• Water diffuses through the membrane into
higher concentration compartment.
• This is due to the difference in chemical
potential of water in two compartments.
• Chemical potential of pure water is larger
than water containing a dissolved solute.
40.
41. • When the level of solution is maintained at a
certain elevated position, the flow of solute to
higher concentration side stops.
• This condition is called osmotic equilibrium.
• The extra pressure due to the elevated level of
solution is –osmotic pressure (π)
• The chemical potential of water in a solution
increases if it is maintained at an elevated
pressure.
42. • When an extra pressure higher than the osmotic
pressure is applied, the chemical potential of
water in solution becomes larger than pure
water.
• So that , water flows from solution to the pure
water side.
• This is reverse osmosis.
• Driving force: difference in chemical potential of
water on both sides.
• RO is most important technique for desalination
of brackish (1000 – 5000 ppm salt)
43. • Or sea water (35,000 ppm or 3.5 % salt)
• Commercial exploitation was not possible
until the 1960s
• The development of high flux, asymmetric
cellulose acetate membrane by (Loeb and
Sourirajan, 1963) made it commercially
possible.
• Over 12,500 industrial scale desalination
plants are operating worldwide.
• With an average production of 23 milliion m3
per day of drinking water
44.
45. Models for water/solute transport in RO –
Solution-Diffusion Model
• This model assumes that sorption of both the
solute and the solvent occurs at the upstream
section of the membrane
• Followed by diffusion through the non-porous
and homogeneous permselective layer
• And exits the membrane at the other side of
membrane
46. • Diffusion flux of solvent is given by
• The change in chemical potential of the
solvent is given by
• Substituting equation 2, in equation 1. gives
47. • Δμm - is the difference in chemical potential
across the homogeneous membrane layer of
thickness, lm
• Expressing the above equation in terms of
measurable quantities
49. • For the salt (flux), - i.e., solute flux
• Contribution of pressure towards chemical
potential difference is negligible,
• So the salt flux is viewed as diffusive and is
expressed as
50. • Maximum rejection of NaCl is about 70 %
• Permeation flux: 0.1 to 1 m3/m2.day
• Application:
• Water reclamation
• Food industries – concentration of dextrose
syrup
• Chemical: metal recovery, recovery of textile
dyes, cleaning paper mill efflueents
51. Concentration Driven Process
• Dialysis - is diffusional transport of one or
more dissolved species – through a thin
permselective barrier.
• The membrane is place between two aqueous
solutions at different concentrations
• Separation is by concentration driving force
• No bulk flow of solvent or solution through
membrane
52. • Since smaller molecules diffuse faster than
larger ones,
• Dialysis separate such molecules from
solution.
• NaOH (17-20 %) in the viscose liquor can be
recovered as 9-10 % solutions by dialysis
• A dialysis unit is called dialyser.
• The feed-side liquid leaving the unit is called
dialysate (rententate)
53. • And that leaving the permeate side is
diffusate (permeate)
• A membrane that swells substantially in
contact with the solvent is selected,
• Because the process involves diffusion
through dense membranes
• Swelling of a polymer increases the spaces
between the polymer chains and facilitates
diffusional motion of molecules through it.
54. • Applications
• In medical purposes-haemodialysis
• Dialysis offers a passive envirionment for
transport solutions are not in direct contact
• No pressure is applied as in RO
• Many breweries use dialysis to reduce alchol
content of beer
55. Electrodialysis
• Electrodialysis (ED) is a process of removal of
salts from an aqueous solution
• By transport through an electrically charged
membrane.
• Cell is divided into a number of
compartments by placing pieces of a cation
exchange membrane and an anion exchange
membrane alternatively
56. • At the two ends are placed a cathode and an
anode connected to a DC power supply
• Example NaCl,
• Na+ ions pass through the cation-exchange
membrane that has a fixed negative charges
• But anions are rejected
• Anion exchange membrane allows Cl- ions to
pass through.
• Thus the solution passing through the
compartment are depleted of the salt.
57. • The adjacent compartments get enriched in
the salt.
• Desalinated water and concentrated brine
leave from adjacent compartments.
• Applied electrical potential is the driving force
of the process.
• It is widely used for desalination of brackish
water as an alternative to RO.
58.
59. Pervaporation
• Pervaporation combines - permeation of one or
more species and its subsequent vaporization.
• Pervaporation: permeation + vaporization
• A component in the feed, that has large affinity
for the membrane gets sorbed (dissolved) on
the feed-side membrane
• The solute then – diffused through the
membrane
• Vaporizes at the product side of the membrane
60. • Vaporization occurs at the product side under
vacuum
• The vapour product – having a target
compound at a much larger conc. than in the
feed is condensed and recovered as liquid.
• Pervaporation cannot be an alternative for
the conventional processes
• Like distillation and liquid extraction
• The diffusional flux through a membrane is
generally low
61. • Also right membrane that would provide high
flux and satisfactory separation factor is not
always available.
• Industrial application:
– Preparation of absolute alcohol
– Dehydration of solvents
62.
63.
64. Salt and Light
• “You are the salt of the earth.
• But if the salt loses its saltiness, how can it be
made salty again? It is no longer good for
anything, except to be thrown out and trampled
underfoot.
• You are the light of the world.
• A town built on a hill cannot be hidden. Neither
do people light a lamp and put it under a bowl.
Instead they put it on its stand, and it gives light
to everyone in the house
