presentation on theory of Membranes.ppt
- 1. Paul Ashall, 2007 Membrane processes
- 2. Paul Ashall, 2007 Membrane processes • Microfiltration • Ultrafiltration • Reverse osmosis • Gas separation/permeation • Pervaporation • Dialysis • Electrodialysis • Liquid membranes • Etc
- 3. Paul Ashall, 2007 Membrane applications in the pharmaceutical industry • UP water (RO) • Nitrogen from air • Controlled drug delivery • Dehydration of solvents • Waste water treatment • Separation of isomers (e.g. naproxen) (‘Membrane Technology and Applications’ pp517, 518) • Membrane extraction • Sterile filtration
- 4. Paul Ashall, 2007 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
- 6. Paul Ashall, 2007 • Membrane structure (dense, microporous, asymmetric, composite, membrane support)
- 7. Paul Ashall, 2007 Membrane types - isotropic • 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; diffusion; solubility • Electrically charged microporous
- 8. Paul Ashall, 2007 Anisotropic (asymmetric) • Thin active surface layer supported on thicker porous layer • Composite – different polymers in layers • Others – ceramic, metal, liquid
- 9. Paul Ashall, 2007 Asymmetric membranes Thin dense layer Microporous support
- 10. Paul Ashall, 2007 Membrane materials • Polymers • Metal membranes • Ceramic membranes (metal oxide, carbon, glass) • Liquid membranes
- 11. Paul Ashall, 2007 Membrane fabrication Isotropic • Solution casting • Melt extrusion • Track etch membranes (Baker fig. 3.4) • Expanded film membranes (Baker fig. 3.5)
- 12. Paul Ashall, 2007 continued Anisotropic • Phase separation (Loeb – Sourirajan method) (see Baker fig. 3.12) • Interfacial polymerisation • Solution coated composite membranes • Plasma deposition
- 13. Paul Ashall, 2007 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
- 15. Paul Ashall, 2007 Spiral wound module
- 16. Paul Ashall, 2007 Membrane filtration – Buss-SMS-Canzler
- 17. Paul Ashall, 2007 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)
- 18. Paul Ashall, 2007 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
- 19. Paul Ashall, 2007 Characteristics of filtration processes Process technology Separation principle Size range MWCO MF Size 0.1-1μm - UF Size,charge 1nm-100nm >1000 NF Size, charge, affinity 1nm 200-1000 RO Size, charge, affinity < 1nm <200
- 20. Paul Ashall, 2007 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
- 21. Paul Ashall, 2007 Models • Ficks law (solution-diffusion model) Free volume elements (pores) are spaces between polymer chains caused by thermal motion of polymer molecules. • Darcys law (pore flow model) Pores are large and fixed and connected.
- 22. Paul Ashall, 2007 Simple model (liquid flow through a pore using Poiseuilles law) J = Δp ε d2 32 μ l J = flux 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
- 23. Paul Ashall, 2007 Mechanisms for transport through membranes • Bulk flow • Diffusion • Solution-diffusion (dense membranes – RO, PV, gas permeation)
- 24. Paul Ashall, 2007 continued • Dense membranes: transport by a solution- diffusion mechanism • Microporous membranes: pores interconnected
- 25. Paul Ashall, 2007 Separation of liquids • Porous membranes • Asymmetric membranes/dense polymer membranes
- 26. Paul Ashall, 2007 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.
- 27. Paul Ashall, 2007 Microporous membranes • Porosity (ε) • Tortuosity (τ) (measure of path length compared to pore diameter) • Pore diameter (d) Ref. Baker p68 – Fig 2.30
- 28. Paul Ashall, 2007 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 pp69, 73
- 29. Paul Ashall, 2007 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)
- 30. Paul Ashall, 2007 continued Crossflow operation (as opposed to ‘dead end’ filtration)
- 31. Paul Ashall, 2007 Membrane types • Dense • High porosity • Narrow pore size distribution
- 32. Paul Ashall, 2007 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
- 33. Paul Ashall, 2007 Spiral wound UF module
- 34. Paul Ashall, 2007 UF Membrane materials (Loeb- Sourirajan process) • Polyacrylonitrile (PAN) • PVC/PAN copolymers • Polysulphone • PVDF (polyvinylidene difluoride) • PES (polyethersulfone) • Cellulose acetate (CA)
- 35. Paul Ashall, 2007 UF Modules • Tubular • Plate and frame • Spiral wound • Capillary hollow fibre
- 36. Paul Ashall, 2007 UF applications • Protein concentration
- 37. Paul Ashall, 2007 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.
- 38. Paul Ashall, 2007 MF Membrane materials • Cellulose acetate/cellulose nitrate • PAN – PVC • PVDF • PS
- 39. Paul Ashall, 2007 MF Modules • Plate and frame • Cartridge filters (see Baker figs. 7.11/7.13, p288, 290)
- 40. Paul Ashall, 2007 MF operation • Fouling • Backflushing • Constant flux operation
- 41. Paul Ashall, 2007 MF uses • Sterile filtration of pharmaceuticals (0.22 μm rated filter) • Drinking water treatment
- 42. Paul Ashall, 2007 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).
- 43. Paul Ashall, 2007 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.
- 44. Paul Ashall, 2007 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.
- 45. Paul Ashall, 2007 RO F R P P1 P2 P1 » P2
- 46. Paul Ashall, 2007 Model • Flux equations • Salt rejection coefficient
- 47. Paul Ashall, 2007 Water flux Jw = cwDwvw (ΔP – Δπ) RT z Dw is diffusivity in membrane, cm2 s-1 cw is average water conc. in membrane, g cm-3 (~ 0.2) vw is partial molar volume of water, cm3 g-1 ΔP pressure difference R gas constant T temperature Δπ osmotic pressure z membrane thickness
- 48. Paul Ashall, 2007 Salt flux Js = Ds Ss (Δcs) z Ds diffusivity Ss solubility coefficient Δcs difference in solution concentration Ref. Baker pp 34, 195
- 49. Paul Ashall, 2007 Jw increases with ΔP and selectivity increases also since Js does not depend on ΔP.
- 50. Paul Ashall, 2007 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
- 51. Paul Ashall, 2007 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
- 52. Paul Ashall, 2007 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
- 53. Paul Ashall, 2007 Example See McCabe p893
- 54. Paul Ashall, 2007 Applications • UP water (spec. Baker pp 226, 227)
- 55. Paul Ashall, 2007 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.
- 56. Paul Ashall, 2007 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.
- 57. Paul Ashall, 2007 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.
- 58. Paul Ashall, 2007 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.
- 59. Paul Ashall, 2007 Pervaporation (PV) • Hydrophilic membranes (PVA) e.g. ethanol/water • Hydrophobic membranes (organophilic) e.g. PDMS
- 60. Paul Ashall, 2007 PV • Composite membrane (dense layer + porous supporting layer) Ref. Baker p366
- 61. Paul Ashall, 2007 Modules • Plate & frame (Sulzer/GFT)
- 62. Paul Ashall, 2007 PV • Solution –diffusion mechanism • Selectivity dependent on chemical structure of polymer and liquids
- 63. Paul Ashall, 2007 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.
- 64. Paul Ashall, 2007 Models • Solution – diffusion model • Experimental evidence (ref. Baker pp 43 – 48)
- 65. Paul Ashall, 2007 continued Ji = Pi G (pio – pil) l Ji – flux, g/cm2 s Pi G – gas separation permeability coefficient, gcm. cm-2 s-1 . cmHg-1 l – membrane thickness pio – partial v.p. i on feed side of membrane pil – partial vp i on permeate side
- 66. Paul Ashall, 2007 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
- 67. Paul Ashall, 2007 continued Structure – permeability relationships • Sorption coefficient, K (relates concentration in fluid phase and membrane polymer phase) • Diffusion coefficient, D Ref. Baker p48
- 68. Paul Ashall, 2007 continued Diffusion in polymers • Glass transition temperature,Tg • Molecular weight, Mr • Polymer type and chemical structure, • Membrane swelling, • Free volume correlations
- 69. Paul Ashall, 2007 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
- 70. Paul Ashall, 2007 Dual sorption model Gas sorption in a polymer occurs in two types of site (equilibrium free volume and excess free volume (glassy polymers only)). Baker pp56-58
- 71. Paul Ashall, 2007 continued Flux through a dense polymer is inversely proportional to membrane thickness. Flux generally increases with temperature (J = Jo exp (-E/RT). An increase in temperature generally decreases membrane selectivity.
- 72. Paul Ashall, 2007 PV process design • Vacuum driven process • Condenser • Liquid feed has low conc. of more permeable species Ref. Baker p 370
- 73. Paul Ashall, 2007 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. VOC in water with limited solubility • Organic/organic separations
- 74. Paul Ashall, 2007 PV – hybrid processes using distillation
- 75. Paul Ashall, 2007 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
- 76. Paul Ashall, 2007 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.
- 77. Paul Ashall, 2007 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.
- 78. Paul Ashall, 2007 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.
- 79. Paul Ashall, 2007 Mechanisms • Convective flow (large pore size 0.1 – 10 μm; no separation) • Knudsen diffusion (pore size < 0.1μm; flux α 1/(Mr)1/2 ) • Molecular sieving (0.0005 – 0.002 μm) • Solution-diffusion (dense membranes) (See Baker fig. 8.2, p303)
- 80. Paul Ashall, 2007 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.
- 81. Paul Ashall, 2007 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 > 1
- 82. Paul Ashall, 2007 Transport of gases through dense membranes JA = QA (pA1 – pA2) QA is permeability (L (stp) m-2 h-1 atm-1 ) pA1 partial pressure A feed pA2 partial pressure A permeate
- 83. Paul Ashall, 2007 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 (Ref. McCabe ch. 26 pp859 – 860)
- 84. Paul Ashall, 2007 Diffusion coefficients in PET (x 109 at 25o C, cm2 s-1 ) Polymer O2 N2 CO2 CH4 PET 3.6 1.4 0.54 0.17
- 85. Paul Ashall, 2007 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
- 86. Paul Ashall, 2007 Modules • Spiral wound • Hollow fibre
- 87. Paul Ashall, 2007 Flow patterns • Counter-current • Co-/counter • Radial flow • crossflow
- 88. Paul Ashall, 2007 System design • Feed/permeate pressure (Δp = 1 – 20 atm.) • Degree of separation • Multistep operation
- 89. Paul Ashall, 2007 Applications • Oxygen/nitrogen separation from air (95 – 99% nitrogen) • Dehydration of air/air drying Ref. Baker p350
- 90. Paul Ashall, 2007 Other membrane processes • Ion exchange • Electrodialysis e.g. UP water • Liquid membranes/carrier facilitated transport e.g. metal recovery from aqueous solutions
- 91. Paul Ashall, 2007 PV demonstration
- 92. Paul Ashall, 2007 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
- 93. Paul Ashall, 2007 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
Editor's Notes
- #3: Baker p517, 518 Seader p715
- #7: Baker fig. 1.1; table 1.1; fig. 1.2 Seader p719
- #12: Baker fig. 3.12
- #13: Baker p134, 140, 141, 142, 143, 144, 145, 146, 147, 152, 153 Seader
- #21: Baker p17, 18 transparency
- #27: Baker p68
- #28: Baker p69, 73, 78, 279
- #29: Baker p83
- #39: Baker p 288, 290, 295, 297
- #45: Baker p25, p30, p34
- #47: transparency
- #48: Transparency R Baker p195
- #50: Baker p203
- #54: Baker p226, 227
- #57: Baker p356
- #59: Baker p365, 366, 373
- #65: Transparencies(2)
- #66: Baker p359
- #67: Baker p48
- #72: Baker p370
- #79: Baker p303
- #85: Baker p332
- #89: Baker p350