1. Membrane Separations

2. Membrane separations used in DSP

3. Membrane separations: Introduction
Membranes are semi-permeable barrier used for
Particle-liquid separation
Particle-solute separation
Solute-solvent separation
Solute-solute separation
Applications
Product concentration
Product sterilization
Solute fractionation
Solute removal from solutions (desalination, demineralization)
Purification
Clarification

4. Factors utilized in membrane separations
Solute size
Electrostatic charge
Other non-covalent interactions
Diffusivity
Solute shape

5. Transport of material through a membrane
Pressure driven separation
(convective transport)
Retentate
Membrane module
Permeate
Feed
Membrane
Diffusion driven
separation
Retentate
Membrane module
Permeate
Feed
Sweep
Membrane

6. Dead-ended or conventional
filtration
Cross-flow filtration

7. Membrane materials Organic polymers Inorganic materials
Polysulfone (PS)
Polyethersulfone (PES)
Cellulose acetate (CA)
Regenerated cellulose
Polyamides (PA)
Polyvinylidedefluoride (PVDF)
Polyacrylonitrile (PAN)
Inorganic materials
Glass
Metals
Ceramics
Layers of chemicals
Pyrolyzed carbon

8. Membrane preparation by casting
Precipitation from vapour phase
This is achieved by penetration of the precipitant through a polymeric film from the vapour phase, which is saturated with the solvent used.
Precipitation by evaporation
The polymer is dissolved in a mixture of more volatile and less volatile solvents. As the more volatile component evaporates, the polymer precipitates to form the membrane.
Immersion precipitation
This involves immersion of the cast film in a bath of non-solvent for coagulation of the membrane material.
Thermal precipitation
The polymer is precipitated from solution by a cooling step.

9. Other membrane making procedures
Stretching:
This involves stretching a polymer film at normal or elevated temperature in order to produce pores of desired size.
Sintering:
Powdered material is sintered by compression with/without heating to give microporous membranes.
Slip casting:
Most inorganic membranes are prepared using this method.
The method involves coating repeated layers of uniform particles with decreasing sizes on porous support.

10. Other membrane making procedures
Leaching:
Some inorganic membranes are prepared by leaching technique.
Isotropic glass membranes are prepared by a combination of phase separation and acid leaching.
Track etching:
A homogeneous polymer film is exposed to laser beams or beams of collimated charged particles.
This breaks specific chemical bonds in the polymer matrix.
The film is then placed in an etching bath to remove the damaged sections thus giving rise to monodisperse pores

11. Isotropic
Membranes: Structural classification
Anisotropic

12. Porous membranes Isoporous membrane Microporous symmetric
Microporous asymmetric

13. Basic forms of membranes
Flat sheet membrane
Tubular membrane
Hollow fibre membrane

14. Factors influencing the performance of a membrane
Mechanical strength
Tensile strength, bursting pressure
Chemical resistance
pH range, solvent compatibility
Permeability to different species
Pure water permeability, sieving coefficient
Average porosity and Pore size distribution
Sieving properties
Nominal molecular weight cut-off
Electrical properties
Membrane zeta potential

15. Driving force in membrane separation
Transmembrane (hydrostatic) pressure (TMP)
Concentration or electrochemical gradient
Osmotic pressure
Electrical field
Partial pressure
pH gradient

16. Membrane processes that separate primarily based on size (pressure driven)
200 to 600 psi
1 to 50 psi
10 to 100 psi

17. Membrane processes that separate based on principles other than size
Pervaporation
Separates a volatile or low-boiling-point liquid from a non-volatile liquid.
The driving force is a vacuum on the gaseous side of the membrane.
It is a tool for separation of liquid mixtures, especially dehydration of liquid hydrocarbons

18. Membrane processes that separate based on principles other than size:
A stack of membranes is used, half of them passing positively charged particles and rejecting negatively charged ones; the other half doing the opposite.
An electrical potential is imposed across the membranes and a solution with charged particles is pumped through the system.
Positively charged particles migrate toward the negative electrode, but are stopped by a positive-particle-rejecting membrane.
Negatively charged particles migrate in the opposite direction with similar results.

19. Electrodialysis (ED) Electrochemical process used to separate
charged particles from an
aqueous solution or from
other neutral solutes

20. Flux Throughput of material through a membrane Flux Depends on Fouling
Decline in flux due to fouling in a
constant driving force membrane
separation
Throughput of material through a membrane
Flux Depends on
Applied driving force
Resistance offered by membrane
Fouling
Increase in membrane resistance during a process
The decline in flux through a membrane with time in a constant force membrane process is due to fouling
Flux
Time
Pressure
Fouling in constant flux
membrane separation
Time

21. Fouling of membranes

22. Membrane element and module
Membrane element refers to the basic form of the membrane:
Flat sheet
Hollow fibre
Tubular
Membrane module refers to the device which houses the membrane element:
Stirred cell module
Flat sheet tangential flow (TF) module
Tubular membrane module
Spiral wound membrane module
Hollow fibre membrane module

23. Membrane modules Arrangement flat sheet TF module Stirred cell unit
Pilot plant scale flat sheet TF module
Small scale flat sheet TF module

24. Stirred cell Research and small-scale manufacturing
Used for microfiltration and ultrafiltration
Excellently suited for process development work

25. Flat sheet tangential flow module
Similar plate and frame filter press
Alternate layers of membranes, support screens and distribution chambers
Used for microfiltration and ultrafiltration

26. Spiral flow membrane module
Flat sheet membranes are fused to form an envelop
Membrane envelop is spirally wound along with a feed spacer
Filtrate is collected within the envelope and piped out

27. Spiral wound module

28. Millipore spiral wound module

29. Tubular membrane module
Cylindrical geometry; wall acts as the membrane
Tubes are generally greater than 3 mm in diameter
Shell and tube type arrangement is preferred
Flow behaviour is easy to characterise

30. Section of tubular module
Large scale tubular module

31. Hollow fibre membrane module
Similar to tubular membrane module
Tubes or fibres are mm in diameter
Fibres are prepared by spinning and are potted within the module
Straight through or U configuration possible
Typically several fibres per module

32. Hollow fibre module
Section of hollow fibre module

33. Comparison of different membrane modules
Type
Fluid flow regime
Membrane area/module volume
Mass transfer coefficient
Hold-up volume
Special remarks
TF flat sheet
Laminar- turbulent
Low
Low to moderate
Moderate
Can be dismantled and cleaned easily.
Spiral wound
Laminar
High pressures cannot be used.
Hollow fibre
Laminar-turbulent
High
Susceptible to fibre blocking.
Tubular
Turbulent
Moderate to high
Flow easy to characterize. Excellently suited for basic membrane studies.

34. Flow patterns
in membrane module

35. Membrane characterization
The performance of a membrane process depends on the properties of the membrane:
Mechanical strength
tensile strength, bursting pressure
Chemical resistance
pH range, compatibility with solvents
Permeability to different species
pure water permeability, gas permeability
Average porosity and pore size distribution
Sieving properties
Nominal molecular weight cut-off
Electrical properties
membrane zeta potential

36. Ultrafiltration Uses: Applications: Concentration of macromolecules
Purification of solvent by removal of solutes
Fractionation of macromolecules
Clarification
Retention of catalysts
Analysis of complex solutions for specific solutes
Applications:
Fractionation of biological macromolecules e.g. proteins, DNA
Concentration of polymer solutions
Removal of LMW solutes from protein solutions
Removal of cells and cell debris from fermentation broth
Virus removal from therapeutic products
Harvesting of biomass e.g., cells and sub-cellular products
Membrane bioreactors
Effluent treatment

37. Ultrafiltration membranes
Pores: 10 to 1000 Angstroms
Generally anisotropic (skin layer 0.2 to 10 micron thick)
Properties of an ideal ultrafiltration membrane:
High hydraulic permeability to solvent
Sharp “retention cut-off” properties: The membrane must be capable of retaining completely nearly all the solutes above some specified value, known as the molecular cut-off (MWCO).
Good mechanical durability
Good chemical and thermal stability
Excellent manufacturing reproducibility and ease of manufacture

38. Sharp and diffuse cut-off membranes

39. Ultrafiltration: Pore flow model
Hagen-Poiseuille’s law for permeate flux
of pure solvent
Jv = Permeate flux
m =Membrane porosity
dp = Average pore diameter
P =Transmembrane pressure
 =Viscosity
Lp =Average pore length
The pressure drop for a
Cross flow membrane module
Pi and Po inlet and outlet pressures on the feed side
Pf is the pressure on the filtrate side

40. Ultrafiltration: Flux equations
Pore flow model: UF of solvent
Rm = membrane
Hydraulic resistance
Resistance model: UF of solvent
Osmotic pressure model: UF of solution
Rcp = resistance due to conc. polarization
Rg = gel layer resistance
Rm = membrane hydraulic resistance

41. Concentration polarization
The composition at the feed-membrane interface differs from the composition in the bulk of the feed mixture.
This gradient in composition is generated by the separation performed by the membrane and, as such, cannot be avoided

42. Concentration polarization
Membrane
Back diffusion dC
= D ----- dx Cw Jv
Bulk feed Cb
Permeate Cp
Concentration polarization layer = b C x

43. Concentration polarization model UF of solution
Material balance in a control volume within the concentration polarization layer at steady state
Upon integration with boundary conditions
C= Cw at x=0 and C=Cb at x=δb we get concentration polarization equation for partially rejected solutes
For total solute rejection i.e Cp = 0
When Cw is equal to the gelation concentration, there will be no further increase in the value of Cw
Hence we write gel polarization equation as

44. Effect of transmembrane pressure on permeate flux
At constant TMP the permeate flux decreases as the feed concentration increases
When Cw = Cg
the permeate flux is independent of the TMP

45. Effect of feed concentration on permeate fluxk
Cs or Cg Cb Jv
Jlim
TMP
ln Cb
For a given feed concentration, the limiting flux increases with increases in mass transfer coefficient
Permeate flux decreases as the feed concentration is increased

46. Mass transfer coefficient
Affects back-diffusion of accumulated solute
Measure of the hydrodynamic conditions within the module
k = (D/b)
Mass transfer coefficient can be measured experimentally
Plot of limiting flux versus log of feed concentration
Plot of sieving parameter versus (Jv/k)
Mass transfer coefficient can be estimated using heat-mass transfer analogy
Dimensionless equations: Sherwood number as function of Reynolds number and Schmidt number

47. Mass transfer correlations
Sherwood number
Reynolds number
Schmidt number
Fully developed laminar flow (i.e. Re < 1000) in tubular membrane
Graetz-Leveque
Turbulent flow (i.e. Re > 2000) in tubular membrane
Dittus-Boelter
Fully developed laminar flow
Porter
 = 8 ul / d for tubes
 = 6 ul / b for rectangular channels (b = channel depth)

48. Determination of Si and k
ln(Sa/1-Sa)
Slope = 1/k
ln(Si/1-Si) Jv

49. Definition of dimensionless momentum and mass transfer numbers
Name
Description
Definition
Sherwood
Total mass transfer/Diffusive mass transfer
Sh = Kmtdh/D
Reynolds
Inertial forces/Viscous forces
Re = dhuρ/µ
Schmidt
Momentum transfer/mass transfer
Sc = µ/ρD
Peclet
Convective mass transfer/Diffusive mass transfer
Pe = dh/D
Grashof
Gravitational forces/Viscous forces
Gr = L3ρ2B’gΔt/µ2
Froude
Inertial forces/Gravitational forces
Fr = u2/gL

50. Problem
A protein solution (conc, 4.4 g/l) is being ultrafiltered using a spiral
wound membrane module which totally retains the protein.
At a certain transmembrane pressure the permeate flux is
1.3x10-5 m/s. the diffusivity of the protein is 9.5 x m2/s while
the wall concentration at this operating pressure is estimated to be
10 g/l.
Predict the thickness of the boundary layer.
If the permeate flux is increased to 2.6 x 10-5 m/s while maintaining the same hydrodynamic conditions within the membrane module, what is the new wall concentration?

51. Where there is total retention we can use the equation
This equation can be written as
The mass transfer coefficient is given by
Therefore
When Jv is increased to 2.6 x 10-5 m/s and k remains the same, the wall concentration can be obtained from the concentration polarization equation for totally retained solute

52. Effect of hydrodynamic parameters on permeate flux

53. Enhancement of permeate flux
By increasing the flow rate
By creating pulsatile flow
By pressure pulsing
By creating oscillatory flow
By flow obstruction using baffles
By generating Dean vortices
By generating Taylor vortex
By gas-sparging into the feed

54. Dean vortices
Dean vortices are secondary tangential flows that create a self-cleaning flow mechanism when induced within a cross-flow filtering system
The general principle of this technology is
to design, develop and use these tangential flows to sweep around a curve to “clean” the membrane, leading to enhanced filter performance and longer membrane life.

55. Taylor Vortex Specialised type of Couette flow
When the angular velocity of the inner cylinder is increased above a certain threshold
Couette flow becomes unstable and a secondary steady state characterized by axisymmetric toroidal vortices

56. Solute transmission through UF membranes
Amount of solute going through an UF membrane can be quantified in terms of the membrane intrinsic rejection coefficient (Ri) or intrinsic sieving coefficient (Si):
Cw is difficult to determine and hence it is more practical to use the apparent rejection coefficient (Ra) or the apparent sieving coefficient (Sa):

57. Rejection coefficient: Older theory  new theory
for  < 1
In other words, Ra is constant for a solute-membrane system.
for   1
 = (di / dp)=solute-pore diameter ratio
It is now recognised that rejection coefficients depend on operating and environmental parameters such as pH
Ionic strength
System hydrodynamics
Permeate flux

58. Sieving coefficients Intrinsic sieving coefficient
Depends on solute-membrane system
Depends on physicochemical parameters such as pH and ionic strength
Depends on permeate flux
Apparent sieving coefficient
Depends on solute-membrane system
Depends on physicochemical parameters such as pH and ionic strength
Depends on permeate flux
Depends on system hydrodynamics

59. Effect of permeate flux on intrinsic sieving coefficient

60. Effect of permeate flux and mass transfer coefficient on apparent sieving coefficient

61. Effect of permeate flux on apparent sieving coefficient

62. Effect of cross flow velocity on apparent sieving coefficient

63. Determination of intrinsic sieving coefficient and mass transfer coefficient

64. Solute fractionation Enhancement of fractionation
For fractionation of a binary mixture of solutes, it is desirable to achieve maximum transmission of the solute desirable in the permeate and minimum transmission of the solute desirable in the retentate.
Enhancement of fractionation
pH optimization
Feed concentration optimization
Salt concentration optimization
Membrane surface pre-treatment
Optimization of permeate flux and system hydrodynamics

65. Microfiltration
Microfiltration separates micron-sized particles from fluids.
The modules used for microfiltration are similar to those used in ultrafiltration.
Microfiltration membranes are microporous and retain particles by a purely sieving mechanism.
Typical permeate flux values are higher than ultrafiltration processes even though the processes are operated at much lower TMP.
Microfiltration can be operated either in dead-ended (normal flow) mode or cross-flow mode

66. Tortuous path of micro- and ultrafiltration

67. Applications of microfiltration
Cell harvesting from bioreactors
Virus removal for solutions
Clarification of fruit juice and beverages
Removal of cells from fermentation media
Water purification
Air filtration
Sterilization

68. The permeate flux in microfiltration
Jv = Permeate flux
P = Pressure difference across the membrane
RM = Membrane resistance
RC = Cake resistance
 = Liquid medium viscosity
The cake resistance
r = Specific cake resistance
VS = Volume of cake
AM = Area of membrane
For micron sized particles, r is given by
 = Porosity of cake
ds = Mean particle diameter

69. Dialysis The mode of transport in dialysis is diffusion
Separation occurs because
small molecules diffuse more rapidly than larger ones
Also due to the degree to which the membrane restricts transport of molecules usually increases with solute size
Concentration profile in dialysis
Bulk concentration on upstream side (C1)
Membrane
Bulk concentration
on downstream side (C2)
Boundary layers

70. The rate of mass transport or solute flux (N) is directly proportional to the difference in concentration (C) at the membrane surfaces
S is a dimensionless solute partition coefficient
Deff is the effective diffusivity of the solute within the membrane
d is the membrane thickness
Deff and d can be combined and termed the membrane mass transfer coefficient (KM) for a given membrane-solute system

71. In terms of mass transfer coefficient
The membrane resistance alone seldom governs the overall mass transport.
The liquid boundary layers on either side of the membrane also contribute to resistance to transport
RO is the overall resistance,
R1 is the resistance on the upstream surface
R2 is the resistance on the downstream surface
In terms of mass transfer coefficient
KO is the overall mass transfer coefficient
K1 and K2 are the mass transfer coefficients on the upstream and downstream sides
C1 and C2 are the upstream (feed) and downstream (dialysate) concentrations

72. Dialysis
Counter-current flow
Co-current flow C1 C1 C3 C3 C4 C4

73. Applications of dialysis
Removal of acid or alkali from products
Removal of alcohol from beer (to make alcohol free beer)
Removal of salts and low molecular weight compounds from solutions of macromolecules
Concentration of macromolecules
Dialysis provides a tool for controlling the chemical species within a reactor
Purification of biotechnological products
Haemodialysis

74. Liquid membrane technology
Liquid membrane extraction involves
the transport of solutes across thin layers of liquid interposed between two otherwise miscible liquids.
There are two types of liquid membrane processes:
Emulsion liquid membrane (ELM) processes
Supported liquid membrane (SLM) processes

75. Liquid surfactant membrane process

76. Supported liquid membrane

77. Advantages of membrane adsorbers
Low process time
Low recovery liquid volume
Possibility of using higher flow rates
Lower pressure drop
Less column blinding
Ease of scale-up
Fewer problems associated with validation (if a disposable membrane is used)

78. Different types separation chemistries are used in membrane adsorption
Affinity binding
Ion-exchange interaction
Reverse phase and hydrophobic interaction

79. Membrane adsorption
Membrane adsorption processes are carried out in two different modes, pulse and step.
Based on the membrane geometry, three types of membrane adsorbers are used:
 Flat sheet
Radial flow
Hollow fibre

80. Limitations of Packed bed adsorption
High pressure drop
Increase in pressure drop during operation
Column blinding by proteins
Dependence on intraparticle diffusion for the transport of proteins to their binding sites
High process time (due to iv)
High flow rates cannot be used
High recovery liquid volume
Radial and axial dispersion resulting from the use of polydisperse media
Problems associated with scale-up

81. Schematic diagram of the membrane emulsification process

82. Limitations
1. Film diffusion
3. Binding kinetics
Limitations
1. Film diffusion
2. Pore diffusion
3. Binding kinetics

83. Problem
The intrinsic and apparent rejection coefficients for a solute in an ultrafiltration process were found to be 0.95 and 0.63 respectively at a permeate flux value of 6 x 10-3 cm/s.
What is the solute mass transfer coefficient?

84. Solution:
Sa = 1-Ra = = 0.37
Si = 1-Ri = = 0.05

85. Problem
A solution of dextran ) MW=505 kDa is ultrafiltered through a 25 kDa MWCO
Membrane. The pure water flux values and the dextran UF permeate flux values
at different TMP are given below
The osmotic pressure is given by the following correlation
where Δπ is in dynes/cm2 and Cw is in %w/v.
Calculate the membrane resistance and the mass transfer coefficient for
dextran assuming that Rg and Rcp are negligible
ΔP (kPa)
Pure water flux (m/s)
Jv (m/s) 30
9.71x10-6
6.24x10-6 40
1.23 x 10-5
7.08x10-6 50
1.57x10-5
7.63x10-6 60
1.87x10-5
8.02x10-6

86. Solution
The MWCO of the membrane is 25 kDa while the MW of dextran is 505 kDa.
Hence we can safely assume that dextran is totally retained.
Pure water ultrafiltration is governed by
Rm = 3.33 x 109 Pa.s/m

87. Since Rcp and Rg are negligible the equation becomes
We also know that
Since Rcp and Rg are negligible the equation
becomes
Using the above equation we can calculate the osmotic pressure for every value of transmembrane pressure
ΔP (kPa)
Pure water flux (m/s)
Jv (m/s)
Δπ (kPa)
Δπ (dynes/cm2) Cw
(%w/v)
(g/l) K
(m/s) 30
9.71x10-6
6.24x10-6
9.22
92208
7.63
76.31
3.07x10-6 40
1.23 x 10-5
7.08x10-6
16.42
164236
10.04
100.43
3.7x10-6 50
1.57x10-5
7.63x10-6
24.59
245921
11.99
119.94 60
1.87x10-5
8.02x10-6
33.29
332934
13.61
136.08

88. The correlation for osmotic pressure given in the problem can be rearranged to
Using the above equation the wall concentration at different TMP can be calculated.
The mass transfer coefficient in an UF process with total solute retention is given by
Rearranging the equation we get

89. Problem:
Obtain expressions for the optimum concentration for minimum process time in the diafiltration of a solution of protein content S in an initial volume V0.
(a) If the gel-polarisation model applies.
(b) If the osmotic pressure model applies.
It may be assumed that the extent of diafiltration is given by:

90. Solution Gel polarization model
In this case the bulk concentration is Cb = S/V0
The volume Vd liquid permeated, Vp = VdS/Cb
The process time per unit area, t = Vp/J

91. Assuming Cb and K as constant, then
If, at the optimum concentration C b* and dt/dCb = 0, then:
and

92. Assuming that the osmotic pressure model applies
Substituting for Δπ At C = Cw
If Δπ is much greater than JRmµ, then
As before V = Vp/Vo and Cf = S/Vo

93. The process time t is a minimum when dt/dCb = 0 that is when

95. Bubble Point Bubble point is a function of Method
Pore size
Filter medium
Wettability
surface tension
angle of contact.
Method
Fillter membrane is wetted and a gradual increasing gas pressure is applied.
The bubble starts forming from the largest pore first.
The gas pressure at this time is the bubble point for the membrane.
This is an indirect measurement of the size of the largest pore on the filter.
It does not indicate the variability of pore sizes or irregularity of the membrane

97. Bubble Point Test
Bubble point is the minimum pressure required to force air through a filter that has been wetted with water or alcohol
The bubble point method is the most widely used for pore size determination.
It is based on the fact that, for a given fluid and pore size with a constant wetting, the pressure required to force an air bubble through the pore is inverse proportion to the size of the hole.