Membrane processes

1
Membrane processes
2
Membrane processes

• Microfiltration (MF)
• Ultrafiltration (UF)
• Nanofiltration (NF)
• Reverse osmosis (RO)
• Gas separation/permeation
• Pervaporation (PV)
• Dialysis
• Electrodialysis
• Liquid membranes
• Etc

3
Membrane applications in the pharmaceutical
industry

• Ultra pure (UP) water (RO)
• Nitrogen from air
• Controlled drug delivery (Membrane Technology
  and Applications p13)
• Dehydration of solvents
• Waste water treatment
• Separation of isomers (e.g. naproxen) (Membrane
  Technology and Applications pp517, 518)
• Membrane extraction
• Sterile filtration

4
Specific industrial applications

• Dialysis hemodialysis (removal of waste
  metabolites, excess body water and restoration of
  electrolyte balance in blood)
• Microfiltration sterilization of
  pharmaceuticals purification of
  antibioticsseparation of mammalian cells from a
  liquid
• Ultrafiltration recovery of vaccines and
  antibiotics from fermentation broth
• etc
• Ref. Seader p715

5
RETENTATE
FEED
PERMEATE
6
RO (homogeneous dense solution diffusion membranes) pore diam. approx. 0.001 micron
NF pore diam. approx. 0.001 micron
UF (pore flow microporous membranes) pore diam. approx. 0.01 micron
MF (pore flow microporous membranes) pore diam.approx 1 micron

• Membrane structure (dense, microporous,
  asymmetric, composite, membrane support)
• Ref. Baker p4

7
Membrane types isotropic (physical properties
do not vary with direction)

• Microporous pores 0.01 to 10 microns diam.
  separation of solutes is a function of molecular
  size and pore size distribution
• Dense non-porous driving force is diffusion and
  solubility
• Electrically charged microporous

8
Anisotropic - physical properties that are
different in different directions (asymmetric)

• Thin dense active surface layer supported on
  thicker porous layer
• Composite different polymers in layers
• Others ceramic, metal, liquid

9
Asymmetric membranes

• Flux through a dense polymer film is inversely
  proportional to the thickness so it is necessary
  to make them as thin as possible. Typical
  asymmetric membranes are 50 to 200 microns thick
  with a 0.1 to 1 micron skin.

Thin dense layer
Microporous support
10
Membrane materials

• Polymers e.g. cellulose triacetate etc
• Metal membranes
• Ceramic membranes (metal oxide, carbon, glass)
• Liquid membranes

11
Membrane fabrication

• Isotropic
• Solution casting
• Melt extrusion
• Track etch membranes (Baker fig. 3.4)
• Expanded film membranes (Baker fig. 3.5)

12
continued

• Anisotropic
• Phase separation (Loeb Sourirajan method) (see
  Baker fig. 3.12)
• Interfacial polymerisation
• Solution coated composite membranes
• Plasma deposition of thin films from a gas state
  (vapor) to a solid state on substrate.

13
Membrane modules

• Plate and frame - flat sheets stacked into an
  element
• Tubular (tubes)
• Spiral wound designs using flat sheets
• Hollow fibre - down to 40 microns diam. and
  possibly several metres long active layer on
  outside and a bundle with thousands of closely
  packed fibres is sealed in a cylinder

14
Spiral wound
15
Spiral wound module
16
Membrane filtration Buss-SMS-Canzler
17
Module designs

• RO spiral wound
• UF tubular, capillary, spiral wound
• Gas separation hollow fibres, spiral wound
• PV plate and frame

18
Operating considerations

• Membrane fouling
• Concentration polarisation (the layer of solution
  immediately adjacent to the membrane surface
  becomes depleted in the permeating solute on the
  feed side of the membrane and enriched in this
  component on the permeate side, which reduces the
  permeating components concentration difference
  across the membrane, thereby lowering the flux
  and the membrane selectivity)
• Flow mode (cross flow, co-flow, counter flow)

19
Module selection criteria

• Cost
• Concentration polarisation
• Resistance to fouling
• Ease of fabrication of membrane material
• ?P
• Suitability for high pressure operation

20
Aspects

• Crossflow (as opposed to dead end) cross flow
  velocity is an important operating parameter
• Sub-micron particles
• Thermodynamic driving force (P, T, c etc) for
  transport through membrane is activity gradient
  in membrane
• Flux (kg m-2 h-1)
• Selectivity
• Membrane area

21
Characteristics of filtration processes
Process technology Separation principle Size range Molecular weight cut off (MWCO)
MF Size 0.1-1µm -
UF Size,charge 1nm-100nm gt1000
NF Size, charge, affinity 1nm 200-1000
RO Size, charge, affinity lt 1nm lt200
22
Process technology Typical operating pressure (bar) Feed recovery () Rejected species
MF 0.5-2 90-99.99 Bacteria, cysts, spores
UF 1-5 80-98 Proteins, viruses, endotoxins, pyrogens
NF 3-15 50-95 Sugars, pesticides
RO 10-60 30-90 Salts, sugars
23
Models

• Ficks law (solution-diffusion model)
• Free volume elements (pores) are spaces between
  polymer chains caused by thermal motion of
  polymer molecules. Diffusivities in the membrane
  depend on size and shape of molecules and
  structure of polymer.
• e.g. RO, PV
• Darcys law (pore flow model)
• Pores are large and fixed and connected.
• e.g. UF, MF
• NF membranes are intermediate between UF and RO
  membranes

24
Darcys law

• Ji Di (ciom cilm)/l
• where l is membrane thickness, ciom is
  concentration of i on feed side of membrane, cilm
  is concentration of i on permeate side of
  membrane.
• J flux
• D diffusivity

25

• Fick's first law relates the diffusive flux to
  the concentration field, by postulating that the
  flux goes from regions of high concentration to
  regions of low concentration, with a magnitude
  that is proportional to the concentration
  gradient (spatial derivative). In one (spatial)
  dimension, this is
• where J -D(dc/dx)
• J is the diffusion flux in dimensions of mol
  m-2s-1(g cm-2 s-1) . J measures the amount of
  substance that will flow through a small area
  during a small time interval.
• D is the diffusion coefficient or diffusivity in
  dimensions of m2s-1(cm2s-1)
• c (for ideal mixtures) is the concentration in
  mol m-3
• x is the position, m
• dc/dx is concentration gradient

26
Simple model (liquid flow through a pore using
Poiseuilles law)

• J ?p e d2
• 32 µ l
• J flux (flow per unit membrane area)
• l pore length
• d pore diam.
• ?p pressure difference across pore
• µ liquid viscosity
• e porosity (p d2 N/4, where N is number of
  pores per cm2)
• J/?p permeance
• Typical pore diameter MF 1micron UF 0.01
  micron

27
Mechanisms for transport through membranes

• Bulk flow
• Diffusion
• Solution-diffusion (dense membranes RO, PV, gas
  permeation)

28
continued

• Dense membranes transport by a
  solution-diffusion mechanism. The driving force
  for transport is the activity (concentration)
  gradient in the membrane. For liquids, in
  contrast to gases, the driving force cant be
  changed over a wide range by increasing the
  upstream pressure since pressure has little
  effect on activity in the liquid phase.
• In PV one side of the membrane is exposed to feed
  liquid at atmospheric pressure and vacuum is used
  to form vapour on the permeate side. This lowers
  the partial pressure of the permeating species
  and provides an activity driving force for
  permeation.
• In RO the permeate is nearly pure water at 1 atm.
  and very high pressure is applied to the feed
  solution to make the activity of the water
  slightly greater than that in the permeate. This
  provides an activity gradient across the membrane
  even though the concentration of water in the
  product is higher than that in the feed.
• Microporous membranes pores interconnected

29
Separation of liquids

• Porous membranes
• Asymmetric membranes/dense polymer membranes

30
continued

• With porous membranes separation may depend just
  on differences in diffusivity.
• With dense membranes permeation of liquids occurs
  by a solution-diffusion mechanism. Selectivity
  depends on the solubility ratio as well as the
  diffusivity ratio and these ratios are dependent
  on the chemical structure of the polymer and the
  liquids. The driving force for transport is the
  activity gradient in the membrane, but in
  contrast to gas separation, the driving force
  cannot be changed over a wide range by increasing
  the upstream pressure, since pressure has little
  effect on activity in the liquid phase.

31
Microporous membranes

• - are characterised by
• Porosity (e)
• Tortuosity (t) (measure of path length compared
  to pore diameter)
• Average pore diameter (d)
• Ref. Baker p 68 Fig 2.30

32
Microporous membranes

• Screen filters (see Baker fig. 2.31) separation
  of particles at membrane surface.
• Depth filters (see Baker fig. 2.34) separation
  of particles in interior of the membrane by a
  capture mechanism mechanisms are sieving and
  adsorption (inertial capture, Brownian diffusion,
  electrostatic adsorption)
• Ref. Baker pp 69, 73

33
Filtration

• Microfiltration (bacteria potable water, 0.5
  5 microns). Pore size specified.
• Ultrafiltration (macromolecules, molecular mass
  1000 106, 0.5 10-3 microns). Cut-off mol. wt.
  specified.
• Nanofiltration (low molecular weight,
  non-volatile organics from water e.g. sugars).
  Cut off mol. wt. specified.
• Reverse osmosis (salts)
• Crossflow operation (as opposed to dead end
  filtration)

34
Membrane types

• Dense
• High porosity
• Narrow pore size distribution

35
Ultrafiltration(UF)

• Uses a finely porous membrane to separate water
  and microsolutes from macromolecules and
  colloids.
• Membrane pore diameter 0.001 0.1 µm.
• Nominal cut off molecular weight rating
  assigned to membrane.
• Membrane performance affected by
• Concentration polarisation
• Membrane fouling
• Membrane cleaning
• Operating pressure

36
Spiral wound UF module
37
UF

• Membrane materials (Loeb- Sourirajan process)
• Polyacrylonitrile (PAN)
• PVC/PAN copolymers
• Polysulphone (PS)
• PVDF (polyvinylidene difluoride)
• PES (polyethersulfone)
• Cellulose acetate (CA)

38
UF

• Modules
• Tubular
• Plate and frame
• Spiral wound
• Capillary hollow fibre
• UF applications
• Protein concentration

39
Microfiltration (MF)

• Porous membrane particle diameter 0.1 10 µm
• Microfiltration lies between UF and conventional
  filtration.
• In-line or crossflow operation.
• Screen filters/depth filters (see Baker fig. 7.3,
  p 279)
• Challenge tests developed for pore diameter and
  pore size.

40
MF

• Membrane materials
• Cellulose acetate/cellulose nitrate
• PAN PVC
• PVDF
• PS

41
MF

• Modules
• Plate and frame
• Cartridge filters (see Baker figs. 7.11/7.13,
  p288, 290)

42
MF operation

• Fouling
• Backflushing
• Constant flux operation

43
MF uses

• Sterile filtration of pharmaceuticals (0.22 µm
  rated filter)
• Drinking water treatment

44
Reverse osmosis

• Miscible solutions of different concentration
  separated by a membrane that is permeable to
  solvent but impermeable to solute. Diffusion of
  solvent occurs from less concentrated to a more
  concentrated solution where solvent activity is
  lower (osmosis).
• Osmotic pressure is pressure required to equalise
  solvent activities.
• If P gt osmotic pressure is applied to more
  concentrated solution, solvent will diffuse from
  concentrated solution to dilute solution through
  membrane (reverse osmosis).

45
Reverse osmosis

• The permeate is nearly pure water at 1atm. and
  very high pressure is applied to the feed
  solution to make the activity of the water
  slightly greater than that in the permeate. This
  provides an activity gradient across the membrane
  even though the concentration of water in the
  product is higher than that in the feed.

46
Reverse osmosis

• Permeate is pure water at 1 atm. and room
  temperature and feed solution is at high P.
• No phase change.
• Polymeric membranes used e.g. cellulose acetate
• 20 50 atm. operating pressure.
• Concentration polarisation at membrane surface.

47
RO
F
P1
P2
P
R
P1 P2
48
Model

• Flux equations
• Salt rejection coefficient
• R 1- csl/cso100
• csl is salt concentration on permeate side
• cso is salt concentration on feed side of
  membrane

49
Water flux

• Jw cwDwvw (?P ?p)
• RT z
• or Jw A (?P ?p)
• Dw is diffusivity in membrane, cm2 s-1 ( 10-6)
• cw is average water conc. in membrane, g cm-3 (
  0.2)
• vw is partial molar volume of water, cm3g-1
• ?P pressure difference across membrane
• R gas constant
• T temperature
• ?p osmotic pressure difference
• z membrane thickness
• A is water permeability constant
• Note (?P ?p) is approx. 50 atm.

50
Salt flux

• Js Ds Ss (?cs)
• z
• or Js B(cso csl) Bcso
• Ds diffusivity (10-9 cm2/s)
• Ss solubility coefficient of solute ( 0.035
  mol/cm3.atm for sodium chloride)
• ?cs difference in solution concentration on feed
  side and permeate side of membrane - (cso csl)
• B salt permeability constant
• Note selectivity increases as P increases
• Ref. Baker pp 34, 195

51

• Jw increases with ?P and selectivity increases
  also since Js does not depend on ?P.
• csl (Js/Jw) ?w
• where ?w is density of water (g cm-3)

52
Membrane materials

• Asymmetric cellulose acetate
• Polyamides
• Sulphonated polysulphones
• Substituted PVA
• Interfacial composite membranes
• Composite membranes
• Nanofiltration membranes (lower pressure, lower
  rejection used for lower feed solution
  concentrations)
• Ref. Baker p203

53
RO modules

• Hollow fibre modules (skin on outside, bundle in
  sealed metal cylinder and water collected from
  fibre lumens individual fibres characterised by
  outside and inside diameters)
• Spiral wound modules (flat sheets with porous
  spacer sheets, through which product drains, and
  sealed edges a plastic screen is placed on top
  as a feed distributor and sandwich is rolled in
  a spiral around a small perforated drain pipe)
  (see McCabe fig. 26.19)
• Tubular membranes

54
Operational issues

• Membrane fouling
• Pre-treatment of feed solutions
• Membrane cleaning
• Concentration polarisation (higher conc. of
  solute at membrane surface than in bulk solution
  reduces water flux because the increase in
  osmotic pressure reduces driving force for water
  transport and solute rejection decreases because
  of lower water flux and greater salt conc. at
  membrane surface increases solute flux) (Baker
  ch. 4)
• gt 99 salt rejection

55
Example

• See McCabe p893

56
Applications

• UP water (spec. Baker pp 226, 227)

57
Dialysis

• A process for selectively removing low mol. wt.
  solutes from solution by allowing them to diffuse
  into a region of lower concentration through thin
  porous membranes. There is little or no pressure
  difference across the membrane and the flux of
  each solute is proportional to the concentration
  difference. Solutes of high mol. wt. are mostly
  retained in the feed solution, because their
  diffusivity is low and because diffusion in small
  pores is greatly hindered when the molecules are
  almost as large as the pores.
• Uses thin porous membranes.

58
Electrodialysis

• Ions removed using ion selective membranes
  across which an electric field is applied.
• Used to produce potable water from brackish
  water. Uses an array of alternate cation and
  anion permeable membranes.

59
Pervaporation (PV)

• In pervaporation, one side of the dense membrane
  is exposed to the feed liquid at atmospheric
  pressure and vacuum is used to form a vapour
  phase on the permeate side. This lowers the
  partial pressure of the permeating species and
  provides an activity driving force for permeation.

60
PV

• The phase change occurs in the membrane and the
  heat of vapourisation is supplied by the sensible
  heat of the liquid conducted through the thin
  dense layer. The decrease in temperature of the
  liquid as it passes through the separator lowers
  the rate of permeation and this usually limits
  the application of PV to removal of small amounts
  of feed, typically 2 to 5 for 1-stage
  separation. If a greater removal is needed,
  several stages are used in series with
  intermediate heaters.

61
Pervaporation (PV)

• Hydrophilic membranes (polyvinylalcohol - PVA)
  e.g. ethanol/water
• Hydrophobic membranes (organophilic) e.g. poly
  dimethyl siloxane - PDMS

62
PV

• Composite membrane (dense layer porous
  supporting layer)
• Ref. Baker p366

63
Modules

• Plate frame (Sulzer/GFT)

64
PV

• Solution diffusion mechanism
• Selectivity dependent on chemical structure of
  polymer and liquids

65
PV

• Activity driving force is provided by difference
  in pressure between feed and permeate side of
  membrane.
• Component flux is proportional to concentration
  and diffusivity in dense membrane layer.
• Flux is inversely proportional to membrane
  thickness.

66
Models

• Solution diffusion model
• Experimental evidence (ref. Baker pp 43 48)

67
continued

• Ji PiG (pio pil)
• l
• Ji flux, g/cm2s
• PiG gas separation permeability coefficient, g
  cm. cm-2 s-1. cmHg-1 ( DiKiG)
• KiG is gas phase sorption coefficient
• ( mi?m?ioG/ ?iom ?isat)
• where mi is molecular weight of i (g/mol), ?m is
  molar density of membrane (mol/cm3), ?ioG is
  activity of i in gas phase at feed side of
  membrane, ?iom is activity of i in membrane at
  feed interface, ?isat is saturation vapour
  pressure of i.
• l membrane thickness
• pio partial v.p. i on feed side of membrane
• pil partial v.p. i on permeate side

68
PV selectivity

• ß (cil/cjl)
• (cio/cjo)
• cio conc. i on feed side of membrane
• cil conc. i on permeate side of membrane
• cjo conc. j on feed side
• cjl conc. j on permeate side

69
continued

• Structure permeability relationships.
• Membrane permeability is dependent on solute
  diffusion coefficient and absorption in membrane.
• Sorption coefficient, K (relates concentration in
  fluid phase and membrane polymer phase)
• Diffusion coefficient, D m2/s
• Ref. Baker p48

70
continued

• Diffusion in polymers
• Glass transition temperature,Tg
• Molecular weight, Mr
• Polymer type and chemical structure,
• Membrane swelling,
• Free volume correlations pores and spaces
  produced between polymer chains as a result of
  thermal motion of polymer molecules.

71
continued

• Sorption coefficients in polymers vary much less
  than diffusion coefficients, D.
• nim pi/pisat , where nim is mole fraction i
  absorbed, pi is partial pressure of gas and pisat
  is saturation vapour pressure at pressure and
  temperature of liquid.
• Vi pi/pisat , where Vi is volume fraction of
  gas 2.72 absorbed by an ideal polymer

72
Dual sorption model

• Gas sorption in a polymer occurs in two types of
  site - (equilibrium free volume and excess free
  volume (glassy polymers only where additional
  free volume is frozen in during synthesis )).
• Baker pp 56-58

73
continued

• Flux through a dense polymer is inversely
  proportional to membrane thickness.
• Flux generally increases with temperature (J Jo
  exp (-E/RT) i.e. a Arrhenius relationship an
  exponential relationship with temperature.
• An increase in temperature generally decreases
  membrane selectivity.

74
PV process design

• Vacuum driven process
• Condenser
• Liquid feed has low conc. of more permeable
  species
• Ref. Baker p 370

75
Applications

• Dehydration of solvents e.g. ethanol (see McCabe
  pp886-889, fig. 26.16/example 26.3)
• Water purification/dissolved organics e.g. low
  conc. volatile organic compounds (VOC)/solvents
  in water with limited solubility
• Organic/organic separations

76
PV hybrid processes using distillation
77
continued

• Measures of selectivity
• Rate (flux, membrane area)
• Solution diffusion model in polymeric membranes
  (RO, PV etc)
• Concentration polarisation at membrane surface
• Membrane fouling
• Batch or continuous operation

78
Gas separation

• When a gas mixture diffuses through a porous
  membrane to a region of lower pressure, the gas
  permeating the membrane is enriched in the lower
  mol. wt. component(s), since they diffuse more
  rapidly.

79
Gas separation

• The transport of gases through dense (non-porous)
  polymer membranes occurs by a solution-diffusion
  mechanism.The gas is absorbed in the polymer at
  the high pressure side of the membrane, diffuses
  through the polymer phase and desorbs at the low
  pressure side. The diffusivities in the membrane
  depend more strongly on the size and shape of the
  molecules than do gas phase diffusivities.

80
continued

• Gas separation processes operate with pressure
  differences of 1 20 atm., so the thin membrane
  must be supported by a porous structure capable
  of withstanding such pressures but offering
  little resistance to the flow of gas. Special
  methods of casting are used to prepare asymmetric
  membranes, which have a thin, dense layer or
  skin on one side and a highly porous
  substructure over the rest of the membrane.
  Typical asymmetric membranes are 50 to 200
  microns thick with a 0.1 to 1 micron dense layer.

81
Mechanisms

• Convective flow (large pore size 0.1 10 µm no
  separation)
• Knudsen diffusion pore diameter same size or
  smaller than the mean free path of gas molecules
  (?). (pore size lt 0.1µm flux proportional to
  1/(Mr)1/2 Grahams law of diffusion)
• Molecular sieving (0.0005 0.002 µm membrane
  pore size)
• Solution-diffusion (dense membranes)
• (See Baker fig. 8.2, p 303)

82
Knudsen diffusion

• Knudsen diffusion occurs when the ratio of the
  pore radius to the gas mean free path (? 0.1
  micron) is less than 1. Diffusing gas molecules
  then have more collisions with the pore walls
  than with other gas molecules. Gases with high D
  permeate preferentially.

83
Poiseuille flow

• If the pores of a microporous membrane are 0.1
  micron or larger, gas flow takes place by normal
  convective flow.i.e. r/? (pore radius/mean free
  path) gt 1

84
Transport of gases through dense membranes

• JA QA (pA1 pA2)
• QA is permeability (L (stp) m-2 h-1 atm-1) flux
  per unit pressure difference
• pA1 partial pressure A feed
• pA2 partial pressure A permeate
• JA flux

85
Membrane selectivity

• a QA/QB DASA/DBSB
• D is diffusion coefficient
• S is solubility coefficient (mol cm-3 atm-1) i.e.
  cA pASA, cB pBSB
• A high selectivity can be obtained from either a
  favourable diffusivity ratio or a large
  difference in solubilities.
• (Ref. McCabe ch. 26 pp 859-860)
• (Ref. McCabe ch. 26 pp859 860)

86
Diffusion coefficients in polyethyleneterephthalat
e polymer (PET) (x 109 at 25oC, cm2 s-1)
Polymer O2 N2 CO2 CH4
PET 3.6 1.4 0.54 0.17
87
Membrane materials

• Metal (Pd Ag alloys/Johnson Matthey for UP
  hydrogen)
• Polymers (typical asymmetric membranes are 50 to
  200 microns thick with a 0.1 to 1 micron skin)
• Ceramic/zeolite

88
Modules

• Spiral wound
• Hollow fibre

89
Flow patterns

• Counter-current
• Co-/counter
• Radial flow
• Crossflow

90
System design

• Feed/permeate pressure (?p 1 20 atm.)
• Degree of separation
• Multistep operation

91
Applications

• Oxygen/nitrogen separation from air (95 99
  nitrogen)
• Dehydration of air/air drying
• Ref. Baker p350

92
Other membrane processes

• Ion exchange
• Electrodialysis e.g. UP water
• Liquid membranes/carrier facilitated transport
  e.g. metal recovery from aqueous solutions

93
PV lab
94
Reference texts

• Membrane Technology and Applications, R. W.
  Baker, 2nd edition, John Wiley, 2004
• Handbook of Industrial Membranes, Elsevier, 1995
• Unit Operations in Chemical Engineering ch. 26,
  W. McCabe, J. Smith and P. Harriot, McGraw-Hill,
  6th edition, 2001
• Transport Processes and Unit Operations, C. J.
  Geankoplis, Prentice-Hall, 3rd edition, 1993
• Membrane Processes A Technology Guide, P. T.
  Cardew and M. S. Le, RSC, 1998

95
continued

• Perrys Chemical Engineers Handbook, 7th
  edition, R. H. Perry and D. W. Green,
  McGraw-Hill, 1998
• Separation Process Principles, J. D. Seader and
  E. J. Henley, John Wiley, 1998
• Membrane Technology in the Chemical Industry, S.
  P. Nunes and K. V. Peinemann (Eds.), Wiley-VCH,
  2001
• Chem. Eng. Progress, vol. 100 no. 12, Dec. 2004 p
  22