Membrane process & its different aspect
- 2. Types of membrane separation process • Microfiltration • Ultrafiltration • Reverse osmosis • Gas separation/permeation • Pervaporation • Dialysis • Electrodialysis • Liquid membranes • Etc
- 3. Membrane applications • UP water (RO) • Nitrogen from air • Controlled drug delivery • Dehydration of solvents • Waste water treatment • Separation of isomers • Membrane extraction • Sterile filtration
- 4. Specific industrial applications • Dialysis – hemodialysis • Microfiltration – sterilization of pharmaceuticals; purification of antibiotics. • Ultrafiltration – recovery of vaccines and antibiotics from fermentation broth
- 5. Membrane classification • Membrane structure (dense, microporous, asymmetric, composite, membrane support)
- 6. 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 Membrane classification
- 7. Anisotropic (asymmetric): • Thin active surface layer supported on thicker porous layer • Composite – different polymers in layers • Others – ceramic, metal, liquid Membrane classification
- 9. • Polymers • Metal membranes • Ceramic membranes (metal oxide, carbon, glass) • Liquid membranes Membrane classification Different material of construction are used for different separation process
- 11. • 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. Principle
- 12. Principle • 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
- 13. • 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 Membrane modules
- 14. Membrane modules Illustration of Spiral wound :
- 15. Membrane modules Illustration of hollow fiber :
- 16. 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)
- 17. 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
- 18. 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 Characteristics of filtration processes
- 19. 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
- 20. UF Membrane materials (Loeb- Sourirajan process) • Polyacrylonitrile (PAN) • PVC/PAN copolymers • Polysulphone • PVDF (polyvinylidene difluoride) • PES (polyethersulfone) • Cellulose acetate (CA)
- 21. UF membrane module Modules • Tubular • Plate and frame • Spiral wound • Capillary hollow fibre
- 22. UF applications • Protein concentration
- 23. 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.
- 24. MF Membrane materials • Cellulose acetate/cellulose nitrate • PAN – PVC • PVDF • PS
- 25. MF Modules • Plate and frame • Cartridge filters
- 26. MF operation • Fouling • Back flushing • Constant flux operation
- 27. MF uses • Sterile filtration of pharmaceuticals (0.22 μm rated filter) • Drinking water treatment
- 28. • 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). Reverse osmosis (RO)
- 29. • 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. Reverse osmosis (RO)
- 30. Reverse osmosis (RO) 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.
- 31. RO 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)
- 32. 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
- 33. 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
- 34. 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.
- 35. Electro dialysis • 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.
- 36. 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.
- 37. Reference • 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 • MembraneTechnology in the Chemical Industry, S. P. Nunes and K.V. Peinemann (Eds.),Wiley-VCH, 2001