Paul Ashall, 2007 Membrane processes Microfiltration Ultrafiltration Reverse osmosis Gas separation/permeation Pervaporation Dialysis Electrodialysis Liquid membranes Etc
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
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
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
Paul Ashall, 2007 Anisotropic (asymmetric) Thin active surface layer supported on thicker porous layer Composite – different polymers in layers Others – ceramic, metal, liquid
Paul Ashall, 2007 Asymmetric membranes Thin dense layer Microporous support
Paul Ashall, 2007 Membrane materials Polymers Metal membranes Ceramic membranes (metal oxide, carbon, glass) Liquid membranes
Paul Ashall, 2007 continued Anisotropic Phase separation (Loeb – Sourirajan method) (see Baker fig. 3.12) Interfacial polymerisation Solution coated composite membranes Plasma deposition
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
Paul Ashall, 2007 Membrane filtration – Buss-SMS-Canzler
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)
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
Paul Ashall, 2007 Characteristics of filtration processes Process technology Separation principle Size rangeMWCO MF Size0.1-1μm- UF Size,charge1nm-100nm>1000 NF Size, charge, affinity 1nm200-1000 RO Size, charge, affinity < 1nm<200
Paul Ashall, 2007 Process technology Typical operating pressure (bar) Feed recovery (%) Rejected species MF0.5-290-99.99Bacteria, cysts, spores UF1-580-98Proteins, viruses, endotoxins, pyrogens NF3-1550-95Sugars, pesticides RO10-6030-90Salts, sugars
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.
Paul Ashall, 2007 Simple model (liquid flow through a pore using Poiseuilles law) J = Δp ε d 2 32 μ l J = flux l = pore length d = pore diam. Δp = pressure difference across pore μ = liquid viscosity ε = porosity (π d 2 N/4, where N is number of pores per cm 2 ) J/Δp – permeance Typical pore diameter: MF – 1micron; UF – 0.01 micron
Paul Ashall, 2007 Mechanisms for transport through membranes Bulk flow Diffusion Solution-diffusion (dense membranes – RO, PV, gas permeation)
Paul Ashall, 2007 continued Dense membranes: transport by a solution- diffusion mechanism Microporous membranes: pores interconnected
Paul Ashall, 2007 Separation of liquids Porous membranes Asymmetric membranes/dense polymer membranes
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.
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
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
Paul Ashall, 2007 Filtration Microfiltration (bacteria – potable water, 0.5 – 5 microns). Pore size specified. Ultrafiltration (macromolecules, molecular mass 1000 – 10 6, 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)
Paul Ashall, 2007 continued Crossflow operation (as opposed to ‘dead end’ filtration)
Paul Ashall, 2007 Membrane types Dense High porosity Narrow pore size distribution
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
Paul Ashall, 2007 UF Modules Tubular Plate and frame Spiral wound Capillary hollow fibre
Paul Ashall, 2007 UF applications Protein concentration
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.
Paul Ashall, 2007 MF Membrane materials Cellulose acetate/cellulose nitrate PAN – PVC PVDF PS
Paul Ashall, 2007 MF Modules Plate and frame Cartridge filters (see Baker figs. 7.11/7.13, p288, 290)
Paul Ashall, 2007 MF uses Sterile filtration of pharmaceuticals (0.22 μm rated filter) Drinking water treatment
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).
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.
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.
Paul Ashall, 2007 Model Flux equations Salt rejection coefficient
Paul Ashall, 2007 Water flux Jw = c w D w v w (ΔP – Δπ) RT z D w is diffusivity in membrane, cm 2 s -1 c w is average water conc. in membrane, g cm -3 (~ 0.2) v w is partial molar volume of water, cm 3 g -1 ΔP pressure difference R gas constant T temperature Δπ osmotic pressure z membrane thickness
Paul Ashall, 2007 Salt flux Js = Ds Ss (Δc s ) z Ds diffusivity Ss solubility coefficient Δc s difference in solution concentration Ref. Baker pp 34, 195
Paul Ashall, 2007 Jw increases with ΔP and selectivity increases also since Js does not depend on ΔP.
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
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
Paul Ashall, 2007 Applications UP water (spec. Baker pp 226, 227)
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.
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.
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.
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.
Paul Ashall, 2007 Pervaporation (PV) Hydrophilic membranes (PVA) e.g. ethanol/water Hydrophobic membranes (organophilic) e.g. PDMS
Paul Ashall, 2007 Modules Plate & frame (Sulzer/GFT)
Paul Ashall, 2007 PV Solution –diffusion mechanism Selectivity dependent on chemical structure of polymer and liquids
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.
Paul Ashall, 2007 Models Solution – diffusion model Experimental evidence (ref. Baker pp 43 – 48)
Paul Ashall, 2007 continued Ji = P i G (p io – p il ) l Ji – flux, g/cm 2 s P i G – gas separation permeability coefficient, gcm. cm -2 s -1. cmHg -1 l – membrane thickness p io – partial v.p. i on feed side of membrane p il – partial vp i on permeate side
Paul Ashall, 2007 PV selectivity β = (c il /c jl ) (c io /c jo ) c io conc. i on feed side of membrane c il conc. i on permeate side of membrane c jo conc. j on feed side c jl conc. j on permeate side
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
Paul Ashall, 2007 continued Diffusion in polymers Glass transition temperature,Tg Molecular weight, Mr Polymer type and chemical structure, Membrane swelling, Free volume correlations
Paul Ashall, 2007 continued Sorption coefficients in polymers vary much less than diffusion coefficients, D. n im = p i /p isat, where n im is mole fraction i absorbed, pi is partial pressure of gas and pisat is saturation vapour pressure at pressure and temperature of liquid. Vi = p i /p isat, where Vi is volume fraction of gas 2.72absorbed by an ideal polymer
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
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.
Paul Ashall, 2007 PV process design Vacuum driven process Condenser Liquid feed has low conc. of more permeable species Ref. Baker p 370
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
Paul Ashall, 2007 PV – hybrid processes using distillation
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
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.
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.
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.
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.
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
Paul Ashall, 2007 Transport of gases through dense membranes J A = Q A (p A1 – p A2 ) Q A is permeability (L (stp) m -2 h -1 atm -1 ) p A1 partial pressure A feed p A2 partial pressure A permeate
Paul Ashall, 2007 Membrane selectivity α = Q A /Q B = D A S A /D B S B D is diffusion coefficient S is solubility coefficient (mol cm -3 atm -1 ) i.e. c A = p A S A, c B = p B S B (Ref. McCabe ch. 26 pp859 – 860)
Paul Ashall, 2007 Diffusion coefficients in PET (x 10 9 at 25 o C, cm 2 s -1 ) Polymer O2O2 N2N2 CO 2 CH 4 PET22.214.171.1240.17
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
Paul Ashall, 2007 Modules Spiral wound Hollow fibre
Paul Ashall, 2007 Reference texts Membrane Technology and Applications, R. W. Baker, 2 nd 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, 6 th edition, 2001 Transport Processes and Unit Operations, C. J. Geankoplis, Prentice-Hall, 3 rd edition, 1993 Membrane Processes: A Technology Guide, P. T. Cardew and M. S. Le, RSC, 1998
Paul Ashall, 2007 continued Perry’s Chemical Engineers’ Handbook, 7 th 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