Membrane processes Paul Ashall, 2009

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

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 Paul Ashall, 2009

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, 2009

RETENTATE FEED PERMEATE Paul Ashall, 2009

RO (homogeneous dense solution – diffusion membranes) ‘pore’ diam. approx. 0.001 micron NF UF (pore flow microporous membranes) pore diam. approx. 0.01 micron MF (pore flow microporous membranes) pore diam.approx 1 micron Membrane structure (dense, microporous, asymmetric, composite, membrane support) Ref. Baker p4 Paul Ashall, 2009

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 Paul Ashall, 2009

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 Paul Ashall, 2009

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 Paul Ashall, 2009

Membrane materials Polymers e.g. cellulose triacetate etc Metal membranes Ceramic membranes (metal oxide, carbon, glass) Liquid membranes Paul Ashall, 2009

Membrane fabrication Isotropic Solution casting Melt extrusion Track etch membranes (Baker fig. 3.4) Expanded film membranes (Baker fig. 3.5) Paul Ashall, 2009

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 Paul Ashall, 2009

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 Paul Ashall, 2009

Spiral wound Paul Ashall, 2009

Spiral wound module Paul Ashall, 2009

Membrane filtration – Buss-SMS-Canzler Paul Ashall, 2009

Module designs RO – spiral wound UF – tubular, capillary, spiral wound Gas separation – hollow fibres, spiral wound PV – plate and frame Paul Ashall, 2009

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, 2009

Module selection criteria Cost Concentration polarisation Resistance to fouling Ease of fabrication of membrane material ΔP Suitability for high pressure operation Paul Ashall, 2009

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, 2009

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 200-1000 RO < 1nm <200 Paul Ashall, 2009

Typical operating pressure (bar) Feed recovery (%) Rejected species 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 Paul Ashall, 2009

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 Paul Ashall, 2009

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 Paul Ashall, 2009

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 Paul Ashall, 2009

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 Paul Ashall, 2009

Mechanisms for transport through membranes Bulk flow Diffusion Solution-diffusion (dense membranes – RO, PV, gas permeation) Paul Ashall, 2009

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 Paul Ashall, 2009

Separation of liquids Porous membranes Asymmetric membranes/dense polymer membranes Paul Ashall, 2009

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, 2009

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 Paul Ashall, 2009

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 Paul Ashall, 2009

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) Crossflow operation (as opposed to ‘dead end’ filtration) Baker p83 Paul Ashall, 2009

Membrane types Dense High porosity Narrow pore size distribution Paul Ashall, 2009

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, 2009

Spiral wound UF module Paul Ashall, 2009

UF Membrane materials (Loeb- Sourirajan process) Polyacrylonitrile (PAN) PVC/PAN copolymers Polysulphone (PS) PVDF (polyvinylidene difluoride) PES (polyethersulfone) Cellulose acetate (CA) Paul Ashall, 2009

UF Modules Tubular Plate and frame Spiral wound Capillary hollow fibre UF applications Protein concentration Paul Ashall, 2009

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, 2009

MF Membrane materials Cellulose acetate/cellulose nitrate PAN – PVC PVDF PS Paul Ashall, 2009

MF Modules Plate and frame Cartridge filters (see Baker figs. 7.11/7.13, p288, 290) Baker p 288, 290, 295, 297 Paul Ashall, 2009

MF operation Fouling Backflushing Constant flux operation Paul Ashall, 2009

MF uses Sterile filtration of pharmaceuticals (0.22 μm rated filter) Drinking water treatment Paul Ashall, 2009

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

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

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, 2009

RO F P1 P2 P Baker p25, p30, p34 R P1 » P2 Paul Ashall, 2009

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 Paul Ashall, 2009

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 Paul Ashall, 2009

Salt flux Js = Ds Ss (Δcs) z or Js = B(cso – csl) = Bcso Ds diffusivity (10-9 cm2/s) Ss solubility coefficient of solute (= 0.035 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

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

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 Paul Ashall, 2009

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, 2009

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, 2009

Example See McCabe p893 Paul Ashall, 2009

Applications UP water (spec. Baker pp 226, 227) Paul Ashall, 2009

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, 2009

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, 2009

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 Paul Ashall, 2009

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

Pervaporation (PV) Hydrophilic membranes (polyvinylalcohol - PVA) e.g. ethanol/water Hydrophobic membranes (organophilic) e.g. poly dimethyl siloxane - PDMS Baker p365, 366, 373 Paul Ashall, 2009

PV Composite membrane (dense layer + porous supporting layer) Ref. Baker p366 Paul Ashall, 2009

Modules Plate & frame (Sulzer/GFT) Paul Ashall, 2009

PV Solution –diffusion mechanism Selectivity dependent on chemical structure of polymer and liquids Paul Ashall, 2009

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, 2009

Models Solution – diffusion model Experimental evidence (ref. Baker pp 43 – 48) Paul Ashall, 2009

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) Paul Ashall, 2009

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 Paul Ashall, 2009

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 Paul Ashall, 2009

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. Paul Ashall, 2009

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 Paul Ashall, 2009

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 Paul Ashall, 2009

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. Paul Ashall, 2009

PV process design Vacuum driven process Condenser Liquid feed has low conc. of more permeable species Ref. Baker p 370 Baker p370 Paul Ashall, 2009

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. volatile organic compounds (VOC)/solvents in water with limited solubility Organic/organic separations Paul Ashall, 2009

PV – hybrid processes using distillation Paul Ashall, 2009

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, 2009

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, 2009

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

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

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 (0.0005 – 0.002 μm membrane pore size) Solution-diffusion (dense membranes) (See Baker fig. 8.2, p 303) Baker p303 Paul Ashall, 2009

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, 2009

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 Paul Ashall, 2009

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 Paul Ashall, 2009

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 859-860) (Ref. McCabe ch. 26 pp859 – 860) Paul Ashall, 2009

Diffusion coefficients in polyethyleneterephthalate polymer (PET) (x 109 at 25oC, cm2 s-1) CH4 PET 3.6 1.4 0.54 0.17 Paul Ashall, 2009

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 Paul Ashall, 2009

Modules Spiral wound Hollow fibre Paul Ashall, 2009

Flow patterns Counter-current Co-/counter Radial flow Crossflow Paul Ashall, 2009

System design Feed/permeate pressure (Δp = 1 – 20 atm.) Degree of separation Multistep operation Paul Ashall, 2009

Applications Oxygen/nitrogen separation from air (95 – 99% nitrogen) Dehydration of air/air drying Ref. Baker p350 Baker p350 Paul Ashall, 2009

Other membrane processes Ion exchange Electrodialysis e.g. UP water Liquid membranes/carrier facilitated transport e.g. metal recovery from aqueous solutions Paul Ashall, 2009

PV lab Paul Ashall, 2009

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 Paul Ashall, 2009

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. 2004 p 22 Paul Ashall, 2009