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Membrane Separations

Membrane Separations

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Membrane Separations

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  1. Membrane Separations Microfiltration Dan Libotean - Alessandro Patti PhD students Universitat Rovira i Virgili, Tarragona, Catalunya

  2. Feed Permeate Definition of a membrane A membrane can be defined as a barrier (not necessarily solid) that separates two phases as a selective wall to the mass transfer, making the separation of the components in a mixture possible. IDEAL MEMBRANE REAL MEMBRANE Driving Force Phase 2 Phase 1 MF - UF - NF

  3. The growing useof MF 1. More attention paid to environmental problems linked to drinking and non-drinking water 2. Increased demand for water (using currently available sources more effectively) 3. Market power MF - UF - NF

  4. Membranes market in W. Europe MF - UF - NF

  5. Demand in U.S.A., 2001 MF has been used more and more to eliminate particles and micro organisms in untreated water, leading to a lower consumption of disinfectant and to a lower concentration of SPD (sub- products of disinfections).   MF - UF - NF

  6. Cumulative capacity of MF MF - UF - NF

  7. ∆P ∆c ∆T ∆E Microfiltration Pervaporation Thermo-osmosis Electrodialysis Ultrafiltration Gas separation Membrane distillation Electro-osmosis Nanofiltration Vapour permeation Membrane electrolysis Reverse osmosis Dialysis Piezodialysis Diffusion dialysis Driving Forces A driving force can make the mass transfer through the membrane possible; usually, the driving force can be a pressure difference (∆P), a concentration difference (∆c), an electrical potential difference (∆E). Membranes can be classified according their driving forces: MF - UF - NF

  8. Bacteria, parasites, particles High molecular substances, viruses Mid-size organic substances, multiple charged ions Low molecular substances, single charged ions Pressure driven processes MF 10-300 kPa UF 50-500 kPa NF 0.5-1.5 MPa RO 0.5-1.5 MPa ∆P= MF - UF - NF

  9. Pore size of MF membranes MF - UF - NF

  10. Pores and pore geometries Porous MF membranes consist of polymeric matrix in which pores are present. The existence of different pore geometries implies that different mathematical models have been developed to describe transport phenomena. MF - UF - NF

  11. J – the solvent flux • DP – pressure difference • Dx – thickness of membrane • - tortuosity h - viscosity • r – the pore radius • ε – surface porosity cylindrical pores Transport equations The Hagen-Poiseuille and the Kozeny-Carman equations can be applied to demonstrate the flow of water through membranes. The use of these equations depends on the shapes and sizes of the pores. 1. Hagen-Poiseuille MF - UF - NF

  12. closely packed spheres S – surface area per unit volume K – Kozeny-Carman constant (depends on the pore geometry) Transport equations 2. Kozeny-Carman MF - UF - NF

  13. How to prepare MF membranes • Stretching • Semycristalline polymers (PTFE, PE, PP) • if stretched perpendicular to the axis of • crystallite orientation, may fracture in such a • way as to make reproducible microchannels. • The porosity of these membranes is very high, • and values up to 90% can be obtained. Stretched PTFE membrane MF - UF - NF

  14. radiation source membrane polymer film etching bath How to prepare MF membranes 2. Track-etching These membranes are now made by exposing a thin polymer film to a collimated bearn of radiation strong enough to break chemical bonds in the polymer chains. The film is then etched in a bath which selectively attacks the damaged polymer. Track-etched 0.4 μm PC membrane MF - UF - NF

  15. Chemical phase inversion 0.45 μm PVDF membrane How to prepare MF membranes 3. Phase inversion (PI) Chemical PI involves preparing a concentrated solution of a polymer in a solvent. The solution is spread into a thin film, then precipitated through the slow addition of a nonsolvent, usually water, sometimes from the vapour phase. In thermal PI a solution of polymer in poor solvent is prepared at high temperatures. After being transformed into its final shape, a sudden drop in solution temperature causes the polymer to precipitate. The solvent is then washed out. MF - UF - NF

  16. HEAT pore How to prepare MF membranes 4. Sintering This method involves compressing a powder consisting of particles of a given size and sintering at high temperatures. The required temperature depends on the material used. MF - UF - NF

  17. Materials used PTFE, teflon PVDF PP PE Cellulose esters PC PSf/PES PI/PEI PA PEEK • Synthetic polymeric membranes: • Hydrophobic • Hydrophilic Ceramic membranes Alumina, Al2O3 Zirconia, ZrO2 Titania, TiO2 Silicium Carbide, SiC MF - UF - NF

  18. Stretching Phase inversion Track-etching Materials used 1. Polymeric MF membranes MF - UF - NF

  19. Anodec, anodic oxidation (surface) US Filter, sintering (cross section, upper part) Materials used 2. Ceramic MF membranes MF - UF - NF

  20. Modules A module is the simplest membrane element that can be used in practice. Module design must deal with the following issues: 1. Economy of manufacture 4. Minimum waste of energy 2. Membrane integrity against damage and leaks 5. Easy egress of permeate 3. Sufficient mass transfer to keep polarization in control 6. Permit the membrane to be cleaned MF - UF - NF

  21. Diameter tubular membrane assembly Modules: tubular • Membranes diameter: >0.5 mm • Active layer: inside the tube • Flux velocity: high (up to 5 m/s) • Tube: reinforced with fiberglass or stainless steel • Number of tubes: 4-18 • Flux: one or more channels • Cleaning: easy • Surface area/volume: low MF - UF - NF

  22. Hollow fiber module (inside-out) Modules: hollow fiber • Fibers: 300 – 5000 per module • Fibers diameter: <0.5 mm • Flux velocity: low (up to 2.5 m/s) • Feed: inside-out or outside-in • Surface area/volume: high • Pressure drop: low (up to 1 bar) • Maintenance: hard • Cleaning: poor MF - UF - NF

  23. Symmetric membranes The cross section shows a uniform and regular structure surface cross section Symmetric ceramic membrane (Al2O3) MF - UF - NF

  24. 0.1/0.5 μm Porous irregular layer Porous with toplayer Same material! Asymmetric membranes 50/150 μm The active layer is supported over the porous layer. Cross-section of an asymmetric PSf membrane. MF - UF - NF

  25. Fouling and resistance Fouling depends on: concentration, temperature pH, molecular interactions Resistances-in-series model to describe the flux decline: J: flow ΔP: pressure drop η: viscosity Rm: membrane resistance Rc: cake resistance MF - UF - NF

  26. Fouling and resistance porous membrane gel layer The build-up layer and the clogging of the pores are referred to as a fouling layer. Rm= Rm(t=0)+Ra+Rp; Rc=Rg+Rcp Rtot=Rm+Rc MF - UF - NF

  27. Back-flushing • Heat treatment • pH adjustament • Addition of complexing agents • Chlorination • Adsorption onto active carbon • Chemical clarification • Hydraulic cleaning • Mechanical cleaning • Chemical cleaning • Electric cleaning • Reducing concentration polarisation • a1. Increasing flux velocity • a2. Using low flux membranes • b. Turbulence promoters • Narrow pore size distribution • Hydrophilic membranes Methods to reduce fouling 1. Pretreatment of the feed solution 2. Membrane properties 3. Module and process conditions 4. Cleaning MF - UF - NF

  28. permeate permeate Restorable pressure with back-flushing ΔP suspension suspension Irreversible fouling starting points Irreversible fouling J t permeate permeate Restorable flux with back-flushing starting points t Back-flushing MF - UF - NF

  29. Cake layer Dead end and cross-flow To reduce fouling two process modes exist: 1. Dead-end 2. Cross-flow Feed Feed Retentate Permeate Permeate MF - UF - NF

  30. Available MF membranes MF - UF - NF

  31. MF Pre Filter Disinfectants & Coagulants Residual disinfectant MF process applications • To replace four unit operations in the waste water • treatment process. Waste water MIX COAG/ FLOC SED FILT Water MF - UF - NF

  32. MF Pre Filter Coagulants MF process applications 2. To eliminate organic matter using MF after a pre-treatment with coagulants Waste water Water MF - UF - NF

  33. MF RO NF MF process applications 3. MF as pre-treatment for RO or NF Water Waste water Pre Filter Water MF - UF - NF

  34. Retentate: how will it be used? • Sent to a treatment plant • Discharged into a body of water • Sent to a storage facility • For ground applications • Recycled back to water source MF - UF - NF

  35. Some industrial applications • Waste-water treatment • Clarification of fruit juice, wine and beer • Ultrapure water in the semiconductor industry • Metal recovery as colloidal oxides or hydroxides • Cold sterilization of beverages and pharmaceuticals • Medical applications: transfusion filter set, purification of surgical water • Continuous fermentation • Purification of condensed water at nuclear plants • Separation of oil-water emulsions MF - UF - NF

  36. Membrane Separations Ultrafiltration & Nanofiltration

  37. Membrane separation MF - UF - NF

  38. Membrane separation MF - UF - NF

  39. Membrane separation MF - UF - NF

  40. Membrane characterization Membrane properties Membrane separation properties pore size pore size distribution free volume crystalinity rejection separation factor enrichment factor MF - UF - NF

  41. macropore f>50nm mesopore 2nm<f<50nm micropore f<2nm f = pore diameter Membrane characterization • Membranes • porous • nonporous MF - UF - NF

  42. The characterization of porous membranes 1. shape of the pore (pore geometry) MF - UF - NF

  43. 1. Pore geometries J – the solvent flux DP – pressure difference Dx – thickness of membrane t - tortuosity h - viscosity r – the pore radius e – the surface porosity Hagen-Poiseuille equation MF - UF - NF

  44. 1. Pore geometries S – the internal surface area K – Kozeny-Carman constant Kozeny-Carman relationship MF - UF - NF

  45. 1. Pore geometries top layer thickness 0.1-1mm sub layer thickness 50-150mm The flux is inversely proportional to the thickness. commercial interest MF - UF - NF

  46. The characterization of porous membranes 2. pore size distribution MF - UF - NF

  47. The characterization of porous membranes 3. surface porosity r – the pore radius np – number of pores Am – membrane area Microfiltration membranes: e 5-70% Ultrafiltration membranes: e 0.1-1% MF - UF - NF

  48. The characterization of porous membranes Characterization methods: • structure-related parameters (pore size, pore size distribution, top layer thickness, surface porosity) • permeation-related parameters (actual separation parameters using solutes that are more or less retained by the membranes - ‘cut-off’ measurements*) * ‘cut-off’ is defined as the molecular weight which is 90% rejected by the membrane MF - UF - NF

  49. The characterization of porous membranes MF - UF - NF

  50. Ultrafiltration ... separation of one component of a solution from another component by means of pressure and flow exerted on a semipermeable membrane, with membrane pore sizes ranging from 0.05 mm to 1nm. • is used begining with years ‘30 • the operating pressure 0.1-5 bar • typically used to retain macromolecules and colloids • the lower limit are solutes with molecular weights of a few thousands Daltons (1Dalton1.66.10-24g) • average flux around 50-200 GFD (~ 80-340 l/m2.h), at an operating pressure of 50 psig (~ 3,5bar) MF - UF - NF