Paul Ashall, 2007

Membrane processes

Paul Ashall, 2007

Membrane processes• Microfiltration

• Ultrafiltration

• Reverse osmosis

• Gas separation/permeation

• Pervaporation

• Dialysis

• Electrodialysis

• Liquid membranes

• Etc

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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

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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

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FEED

RETENTATE

PERMEATE

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• Membrane structure (dense, microporous, asymmetric, composite, membrane support)

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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

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Anisotropic (asymmetric)

• Thin active surface layer supported on thicker porous layer

• Composite – different polymers in layers

• Others – ceramic, metal, liquid

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Asymmetric membranes

Thin dense layer

Microporous support

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Membrane materials

• Polymers

• Metal membranes

• Ceramic membranes (metal oxide, carbon, glass)

• Liquid membranes

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Membrane fabrication

Isotropic

• Solution casting

• Melt extrusion

• Track etch membranes (Baker fig. 3.4)

• Expanded film membranes (Baker fig. 3.5)

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continued

Anisotropic

• Phase separation (Loeb – Sourirajan method) (see Baker fig. 3.12)

• Interfacial polymerisation

• Solution coated composite membranes

• Plasma deposition

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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

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Spiral wound module

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Membrane filtration – Buss-SMS-Canzler

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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)

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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

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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

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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

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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.

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Simple model (liquid flow through a pore using Poiseuilles

law)J = Δp ε d2

32 μ lJ = fluxl = pore lengthd = pore diam.Δp = pressure difference across pore μ = liquid viscosityε = 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

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Mechanisms for transport through membranes

• Bulk flow

• Diffusion

• Solution-diffusion (dense membranes – RO, PV, gas permeation)

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continued

• Dense membranes: transport by a solution-diffusion mechanism

• Microporous membranes: pores interconnected

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Separation of liquids

• Porous membranes

• Asymmetric membranes/dense polymer membranes

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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.

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Microporous membranes

• Porosity (ε)• Tortuosity (τ) (measure of path length compared

to pore diameter)• Pore diameter (d)

Ref. Baker p68 – Fig 2.30

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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

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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)

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continued

Crossflow operation (as opposed to ‘dead end’ filtration)

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Membrane types

• Dense

• High porosity

• Narrow pore size distribution

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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

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Spiral wound UF module

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UF

Membrane materials (Loeb- Sourirajan process)• Polyacrylonitrile (PAN)• PVC/PAN copolymers• Polysulphone• PVDF (polyvinylidene difluoride)• PES (polyethersulfone)• Cellulose acetate (CA)

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UF

Modules

• Tubular

• Plate and frame

• Spiral wound

• Capillary hollow fibre

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UF applications

• Protein concentration

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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.

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MF

Membrane materials

• Cellulose acetate/cellulose nitrate

• PAN – PVC

• PVDF

• PS

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MF

Modules

• Plate and frame

• Cartridge filters (see Baker figs. 7.11/7.13, p288, 290)

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MF operation

• Fouling

• Backflushing

• Constant flux operation

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MF uses

• Sterile filtration of pharmaceuticals (0.22 μm rated filter)

• Drinking water treatment

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Reverse osmosisMiscible 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).

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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.

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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

acetate20 – 50 atm. operating pressure.Concentration polarisation at membrane

surface.

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RO

F

R

PP1 P2

P1 » P2

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Model

• Flux equations

• Salt rejection coefficient

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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, cm3g-1

ΔP pressure differenceR gas constantT temperatureΔπ osmotic pressurez membrane thickness

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Salt flux

Js = Ds Ss (Δcs)

z

Ds diffusivity

Ss solubility coefficient

Δcs difference in solution concentration

Ref. Baker pp 34, 195

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Jw increases with ΔP and selectivity increases also since Js does not depend on ΔP.

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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

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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

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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

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Example

See McCabe p893

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Applications

• UP water (spec. Baker pp 226, 227)

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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.

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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.

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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.

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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.

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Pervaporation (PV)

• Hydrophilic membranes (PVA) e.g. ethanol/water

• Hydrophobic membranes (organophilic) e.g. PDMS

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PV

• Composite membrane (dense layer + porous supporting layer)

Ref. Baker p366

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Modules

• Plate & frame (Sulzer/GFT)

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PV

• Solution –diffusion mechanism

• Selectivity dependent on chemical structure of polymer and liquids

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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.

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Models

• Solution – diffusion model

• Experimental evidence (ref. Baker pp 43 – 48)

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continued

Ji = PiG (pio – pil)

l

Ji – flux, g/cm2s

PiG – 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

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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

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continued

Structure – permeability relationships• Sorption coefficient, K (relates

concentration in fluid phase and membrane polymer phase)

• Diffusion coefficient, D

Ref. Baker p48

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continued

Diffusion in polymers

• Glass transition temperature,Tg

• Molecular weight, Mr

• Polymer type and chemical structure,

• Membrane swelling,

• Free volume correlations

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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

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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

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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.

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PV process design

• Vacuum driven process• Condenser• Liquid feed has low conc. of more permeable

species

Ref. Baker p 370

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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

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PV – hybrid processes using distillation

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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

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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.

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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.

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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.

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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)

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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.

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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

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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

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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)

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Diffusion coefficients in PET (x 109 at 25oC, cm2 s-1)

Polymer O2 N2 CO2 CH4

PET 3.6 1.4 0.54 0.17

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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

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Modules

• Spiral wound

• Hollow fibre

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Flow patterns

• Counter-current

• Co-/counter

• Radial flow

• crossflow

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System design

• Feed/permeate pressure (Δp = 1 – 20 atm.)

• Degree of separation

• Multistep operation

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Applications

• Oxygen/nitrogen separation from air (95 – 99% nitrogen)

• Dehydration of air/air drying

Ref. Baker p350

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Other membrane processes

• Ion exchange

• Electrodialysis e.g. UP water

• Liquid membranes/carrier facilitated transport e.g. metal recovery from aqueous solutions

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PV demonstration

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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

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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