1. Membrane processes
Paul Ashall, 2009

2. Membrane processes Microfiltration (MF) Ultrafiltration (UF)
Nanofiltration (NF)
Reverse osmosis (RO)
Gas separation/permeation
Pervaporation (PV)
Dialysis
Electrodialysis
Liquid membranes
Etc
Paul Ashall, 2009

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
Baker p517, 518
Seader p715
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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 antibiotics;separation of mammalian cells from a liquid
Ultrafiltration – recovery of vaccines and antibiotics from fermentation broth
etc
Ref. Seader p715
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5. RETENTATE
FEED
PERMEATE
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6. RO (homogeneous dense solution – diffusion membranes)
‘pore’ diam. approx micron NF
UF (pore flow microporous membranes)
pore diam. approx micron
MF (pore flow microporous membranes)
pore diam.approx 1 micron
Membrane structure (dense, microporous, asymmetric, composite, membrane support)
Ref. Baker p4
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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
Baker fig. 1.1; table 1.1; fig. 1.2
Seader p719
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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
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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
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10. Membrane materials Polymers e.g. cellulose triacetate etc
Metal membranes
Ceramic membranes (metal oxide, carbon, glass)
Liquid membranes
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11. Membrane fabrication Isotropic Solution casting Melt extrusion
Track etch membranes (Baker fig. 3.4)
Expanded film membranes (Baker fig. 3.5)
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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.
Baker fig. 3.12
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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
Baker p134, 140, 141, 142, 143, 144, 145, 146, 147, 152, 153
Seader
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14. Spiral wound
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15. Spiral wound module
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16. Membrane filtration – Buss-SMS-Canzler
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17. Module designs RO – spiral wound UF – tubular, capillary, spiral wound
Gas separation – hollow fibres, spiral wound
PV – plate and frame
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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)
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19. Module selection criteria
Cost
Concentration polarisation
Resistance to fouling
Ease of fabrication of membrane material ΔP
Suitability for high pressure operation
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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
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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
>1000 NF
Size, charge, affinity
1nm RO
< 1nm
<200
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22. Typical operating pressure (bar) Feed recovery (%) Rejected species
Process technology
Typical operating pressure (bar)
Feed recovery (%)
Rejected species MF
0.5-2
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
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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
Baker p17, 18
transparency
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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
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25. c (for ideal mixtures) is the concentration in mol m-3
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
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26. Simple model (liquid flow through a pore using Poiseuilles law)
J = Δp ε d2
32 μ l
J = flux (flow per unit membrane area)
l = pore length
d = pore diam.
Δp = pressure difference across pore
μ = liquid viscosity
ε = porosity (π d2 N/4, where N is number of pores per cm2)
J/Δp – permeance
Typical pore diameter: MF – 1micron; UF – 0.01 micron
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27. Mechanisms for transport through membranes
Bulk flow
Diffusion
Solution-diffusion (dense membranes – RO, PV, gas permeation)
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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
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29. Separation of liquids Porous membranes
Asymmetric membranes/dense polymer membranes
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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.
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31. Microporous membranes
- are characterised by
Porosity (ε)
Tortuosity (τ) (measure of path length compared to pore diameter)
Average pore diameter (d)
Ref. Baker p 68 – Fig 2.30
Baker p68
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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
Baker p69, 73, 78, 279
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33. Filtration
Microfiltration (bacteria – potable water, 0.5 – 5 microns). Pore size specified.
Ultrafiltration (macromolecules, molecular mass – 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)
Baker p83
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34. Membrane types Dense High porosity Narrow pore size distribution
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35. Ultrafiltration(UF)
Uses a finely porous membrane to separate water and microsolutes from macromolecules and colloids.
Membrane pore diameter – 0.1 μm.
Nominal ‘cut off’ molecular weight rating assigned to membrane.
Membrane performance affected by:
Concentration polarisation
Membrane fouling
Membrane cleaning
Operating pressure
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36. Spiral wound UF module
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37. UF Membrane materials (Loeb- Sourirajan process)
Polyacrylonitrile (PAN)
PVC/PAN copolymers
Polysulphone (PS)
PVDF (polyvinylidene difluoride)
PES (polyethersulfone)
Cellulose acetate (CA)
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38. UF Modules Tubular Plate and frame Spiral wound Capillary hollow fibre
UF applications
Protein concentration
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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.
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40. MF Membrane materials Cellulose acetate/cellulose nitrate PAN – PVC
PVDF PS
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41. MF Modules Plate and frame
Cartridge filters (see Baker figs. 7.11/7.13, p288, 290)
Baker p 288, 290, 295, 297
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42. MF operation Fouling Backflushing Constant flux operation
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43. MF uses Sterile filtration of pharmaceuticals (0.22 μm rated filter)
Drinking water treatment
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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 > osmotic pressure is applied to more concentrated solution, solvent will diffuse from concentrated solution to dilute solution through membrane (reverse osmosis).
Paul Ashall, 2009

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.
Paul Ashall, 2009

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.
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47. RO F P1 P2 P
Baker p25, p30, p34 R
P1 » P2
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48. Model Flux equations Salt rejection coefficient – R = [1- csl/cso]100
csl is salt concentration on permeate side
cso is salt concentration on feed side of membrane
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49. Water flux Jw = cwDwvw (ΔP – Δπ) RT z or Jw = A (ΔP – Δπ)
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
ΔP pressure difference across membrane
R gas constant
T temperature
Δπ osmotic pressure difference
z membrane thickness
A is water permeability constant
Note: (ΔP – Δπ) is approx. 50 atm.
transparency
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50. Salt flux Js = Ds Ss (Δcs) z or Js = B(cso – csl) = Bcso
Ds diffusivity (10-9 cm2/s)
Ss solubility coefficient of solute (= mol/cm3.atm for sodium chloride)
Δcs 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
Transparency R
Baker p195
Paul Ashall, 2009

51. where ρw is density of water (g cm-3)
Jw increases with ΔP and selectivity increases also since Js does not depend on ΔP.
csl = (Js/Jw) ρw
where ρw is density of water (g cm-3)
Paul Ashall, 2009

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
Baker p203
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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 )
Tubular membranes
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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)
> 99% salt rejection
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55. Example
See McCabe p893
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56. Applications UP water (spec. Baker pp 226, 227) Paul Ashall, 2009

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.
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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.
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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.
Baker p356
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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.
Paul Ashall, 2009

61. Pervaporation (PV)
Hydrophilic membranes (polyvinylalcohol - PVA) e.g. ethanol/water
Hydrophobic membranes (organophilic) e.g. poly dimethyl siloxane - PDMS
Baker p365, 366, 373
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62. PV Composite membrane (dense layer + porous supporting layer)
Ref. Baker p366
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63. Modules
Plate & frame (Sulzer/GFT)
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64. PV Solution –diffusion mechanism
Selectivity dependent on chemical structure of polymer and liquids
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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.
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66. Models Solution – diffusion model
Experimental evidence (ref. Baker pp 43 – 48)
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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
(= miρmγioG/ γiom ρisat)
where mi is molecular weight of i (g/mol), ρm is molar density of membrane (mol/cm3), γioG is activity of i in gas phase at feed side of membrane, γiom is activity of i in membrane at feed interface, ρisat 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
Transparencies(2)
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68. PV selectivity β = (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
Baker p359
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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
Baker p48
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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.
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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 absorbed by an ideal polymer
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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
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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.
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74. PV process design Vacuum driven process Condenser
Liquid feed has low conc. of more permeable species
Ref. Baker p 370
Baker p370
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75. Applications
Dehydration of solvents e.g. ethanol (see McCabe pp , fig /example 26.3)
Water purification/dissolved organics e.g. low conc. volatile organic compounds (VOC)/solvents in water with limited solubility
Organic/organic separations
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76. PV – hybrid processes using distillation
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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
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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.
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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.
Paul Ashall, 2009

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.
Paul Ashall, 2009

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 (λ). (pore size < 0.1μm; flux proportional to 1/(Mr)1/2 – Grahams law of diffusion)
Molecular sieving ( – μm membrane pore size)
Solution-diffusion (dense membranes)
(See Baker fig. 8.2, p 303)
Baker p303
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82. Knudsen diffusion
Knudsen diffusion occurs when the ratio of the pore radius to the gas mean free path (λ ~ 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.
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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/λ (pore radius/mean free path) > 1
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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
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85. Membrane selectivity α = 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 )
(Ref. McCabe ch. 26 pp859 – 860)
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86. Diffusion coefficients in polyethyleneterephthalate polymer (PET) (x 109 at 25oC, cm2 s-1)
CH4
PET
3.6
1.4
0.54
0.17
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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
Baker p332
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88. Modules
Spiral wound
Hollow fibre
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89. Flow patterns Counter-current Co-/counter Radial flow Crossflow
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90. System design Feed/permeate pressure (Δp = 1 – 20 atm.)
Degree of separation
Multistep operation
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91. Applications Oxygen/nitrogen separation from air (95 – 99% nitrogen)
Dehydration of air/air drying
Ref. Baker p350
Baker p350
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92. Other membrane processes
Ion exchange
Electrodialysis e.g. UP water
Liquid membranes/carrier facilitated transport e.g. metal recovery from aqueous solutions
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93. PV lab
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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
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95. continued
Perry’s 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 p 22
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