Transcript Membranes - Dublin Institute of Technology

Membrane processes
Paul Ashall, 2009
Membrane processes
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Microfiltration (MF)
Ultrafiltration (UF)
Nanofiltration (NF)
Reverse osmosis (RO)
Gas separation/permeation
Pervaporation (PV)
Dialysis
Electrodialysis
Liquid membranes
Paul Ashall, 2009
Etc
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
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
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RETENTATE
FEED
PERMEATE
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• Membrane structure
(dense, microporous,
asymmetric, composite,
membrane support)
Ref. Baker p4
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RO
(homogeneous
dense solution
– diffusion
membranes)
‘pore’ diam.
approx. 0.001
micron
NF
‘pore’ diam.
approx. 0.001
micron
UF (pore flow
microporous
membranes)
pore diam.
approx. 0.01
micron
MF (pore flow
microporous
membranes)
pore
diam.approx 1
micron
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
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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
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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
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Membrane materials
• Polymers e.g. cellulose triacetate etc
• 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 of thin films from a gas
state (vapor) to a solid state on substrate.
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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
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in a cylinder
Spiral wound
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Spiral wound module
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Membrane filtration – Buss-SMS-Canzler
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Module designs
• RO – spiral wound
• UF – tubular, capillary, spiral wound
• Gas separation – hollow fibres, spiral
wound
• PV – plate and frame
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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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Module selection criteria
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Cost
Concentration polarisation
Resistance to fouling
Ease of fabrication of membrane material
ΔP
Suitability for high pressure operation
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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
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• Membrane area
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, 1nm
affinity
RO
Size, charge, < 1nm
<200
affinity
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200-1000
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.
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
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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
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• 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 m2s1(cm2s-1)
• c (for ideal mixtures) is the concentration in mol m-3
• x is the position, m
• dc/dx is concentration gradient
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Simple model (liquid flow
through a pore using Poiseuilles
law)
2
J = Δp ε d
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
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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. 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
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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
- 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
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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 pp 69, 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)
• 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 Paul Ashall, 2009
Spiral wound UF module
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UF
Membrane materials (Loeb- Sourirajan process)
• Polyacrylonitrile (PAN)
• PVC/PAN copolymers
• Polysulphone (PS)
• PVDF (polyvinylidene difluoride)
• PES (polyethersulfone)
• Cellulose acetate (CA)
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UF
Modules
• Tubular
• Plate and frame
• Spiral wound
• Capillary hollow fibre
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 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).
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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
acetate
20 – 50 atm. operating pressure.
Concentration polarisation at membrane
surface.
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RO
F
P1
P
R
P1 » P2
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P2
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
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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.
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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
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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)
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Membrane materials
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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)
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Ref. Baker p203
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
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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.
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.
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Pervaporation (PV)
• Hydrophilic membranes (polyvinylalcohol PVA) e.g. ethanol/water
• Hydrophobic membranes (organophilic) e.g.
poly dimethyl siloxane - 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.
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
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
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
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
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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 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.
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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. volatile organic compounds
(VOC)/solvents 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.
Paul Ashall, 2009
Gas separation
The transport of gases through dense (non-porous)
polymer membranes occurs by a solutiondiffusion 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)
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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 (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)
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Diffusion coefficients in
polyethyleneterephthalate
9
o
polymer (PET) (x 10 at 25 C,
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 lab
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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, McGrawHill, 6th edition, 2001
• Transport Processes and Unit Operations, C. J.
Geankoplis, Prentice-Hall, 3rd edition, 1993
• Membrane Processes: A Technology Guide, P. T.
Paul Ashall,
20091998
Cardew and M. S. Le,
RSC,
continued
• Perry’s Chemical Engineers’ Handbook, 7th
edition, R. H. Perry and D. W. Green, McGrawHill, 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.), WileyVCH, 2001
• Chem. Eng. Progress, vol. 100 no. 12, Dec. 2004 p
22
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