This document provides an overview of membrane separation processes. It discusses different types of membranes based on their structure like dense/non-porous, porous, asymmetric, composite and electrically charged membranes. It also describes various membrane separation techniques like microfiltration, ultrafiltration, reverse osmosis, dialysis, electrodialysis and pervaporation. Key applications and theoretical principles of each process are outlined. The document provides a comprehensive introduction to membrane separation technology.

1. Membrane Separation Process
Mass Transfer 2
B.Tech. 3rd Year
Instructor
U. K. Arun Kumar

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.

4. Classification - Membrane Separation Processes
Driving force for MSP
– Pressure difference
– Concentration difference
• Pressure Driven Processes
– Reverse Osmosis (RO)
– Nanofiltration (NF)
– Ultrafiltration (UF)
– Microfiltration (MF)

5. • Concentration Gradient Driven Membrane
process:
– Dialysis
– Membrane Extraction
– Electrodialysis
– Pervaporation

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

9. Membranes classification

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

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.

21. • Pressure driven membrane processes for
liquid separation
• Micro-Filtration
• Ultra-Filtration
• Nano-Filtration
• Reverse Osmosis

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

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

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

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.

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.

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

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

48. • The overall equation, after substitution
becomes,

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.

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

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