Title: Membrane processes
1Membrane processes
2Membrane processes
- Microfiltration
- Ultrafiltration
- Reverse osmosis
- Gas separation/permeation
- Pervaporation
- Dialysis
- Electrodialysis
- Liquid membranes
- Etc
3Membrane 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
4Specific industrial applications
- Dialysis hemodialysis (removal of waste
metabolites, excess body water and restoration of
electrolyte balance in blood) - Microfiltration sterilization of
pharmaceuticals purification of
antibioticsseparation of mammalian cells from a
liquid - Ultrafiltration recovery of vaccines and
antibiotics from fermentation broth - etc
- Ref. Seader p715
5RETENTATE
FEED
PERMEATE
6- Membrane structure (dense, microporous,
asymmetric, composite, membrane support)
7Membrane 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
8Anisotropic (asymmetric)
- Thin active surface layer supported on thicker
porous layer - Composite different polymers in layers
- Others ceramic, metal, liquid
9Asymmetric membranes
Thin dense layer
Microporous support
10Membrane materials
- Polymers
- Metal membranes
- Ceramic membranes (metal oxide, carbon, glass)
- Liquid membranes
11Membrane fabrication
- Isotropic
- Solution casting
- Melt extrusion
- Track etch membranes (Baker fig. 3.4)
- Expanded film membranes (Baker fig. 3.5)
12continued
- Anisotropic
- Phase separation (Loeb Sourirajan method) (see
Baker fig. 3.12) - Interfacial polymerisation
- Solution coated composite membranes
- Plasma deposition
13Membrane 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
14(No Transcript)
15 Spiral wound module
16Membrane filtration Buss-SMS-Canzler
17Operating 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)
18Aspects
- 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
19Characteristics of filtration processes
Process technology Separation principle Size range MWCO
MF Size 0.1-1µm -
UF Size,charge 1nm-100nm gt1000
NF Size, charge, affinity 1nm 200-1000
RO Size, charge, affinity lt 1nm lt200
20Process 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
21Models
- 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.
22Simple model (liquid flow through a pore using
Poiseuilles law)
- J ?p e d2
- 32 µ l
- J flux
- l pore length
- d pore diam.
- ?p pressure difference across pore
- µ liquid viscosity
- e porosity (p d2 N/4, where N is number of
pores per cm2) - J/?p permeance
- Typical pore diameter MF 1micron UF 0.01
micron
23Mechanisms for transport through membranes
- Bulk flow
- Diffusion
- Solution-diffusion (dense membranes RO, PV, gas
permeation)
24continued
- Dense membranes transport by a
solution-diffusion mechanism - Microporous membranes pores interconnected
25Separation of liquids
- Porous membranes
- Asymmetric membranes/dense polymer membranes
26continued
- 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.
27Microporous membranes
- Porosity (e)
- Tortuosity (t) (measure of path length compared
to pore diameter) - Pore diameter (d)
- Ref. Baker p68 Fig 2.30
28Microporous 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
29Filtration
- 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)
30continued
- Crossflow operation (as opposed to dead end
filtration)
31Membrane types
- Dense
- High porosity
- Narrow pore size distribution
32Ultrafiltration(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
33Spiral wound UF module
34UF
- Membrane materials (Loeb- Sourirajan process)
- Polyacrylonitrile (PAN)
- PVC/PAN copolymers
- Polysulphone
- PVDF (polyvinylidene difluoride)
- PES (polyethersulfone)
- Cellulose acetate (CA)
35UF
- Modules
- Tubular
- Plate and frame
- Spiral wound
- Capillary hollow fibre
36UF applications
37Microfiltration (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.
38MF
- Membrane materials
- Cellulose acetate/cellulose nitrate
- PAN PVC
- PVDF
- PS
39MF
- Modules
- Plate and frame
- Cartridge filters (see Baker figs. 7.11/7.13,
p288, 290)
40MF operation
- Fouling
- Backflushing
- Constant flux operation
41MF uses
- Sterile filtration of pharmaceuticals (0.22 µm
rated filter) - Drinking water treatment
42Reverse 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 gt osmotic pressure is applied to more
concentrated solution, solvent will diffuse from
concentrated solution to dilute solution through
membrane (reverse osmosis).
43Reverse 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.
44Reverse 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.
45RO
F
P1
P2
P
R
P1 P2
46Model
- Flux equations
- Salt rejection coefficient
47Water flux
- Jw cwDwvw (?P ?p)
- 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
- ?P pressure difference
- R gas constant
- T temperature
- ?p osmotic pressure
- z membrane thickness
48Salt flux
- Js Ds Ss (?cs)
- z
- Ds diffusivity
- Ss solubility coefficient
- ?cs difference in solution concentration
- Ref. Baker pp 34, 195
49- Jw increases with ?P and selectivity increases
also since Js does not depend on ?P.
50Membrane 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
51RO 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
52Operational 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) - gt 99 salt rejection
53Example
54Applications
- UP water (spec. Baker pp 226, 227)
55Dialysis
- 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.
56Electrodialysis
- 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.
57Pervaporation (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.
58PV
- 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.
59Pervaporation (PV)
- Hydrophilic membranes (PVA) e.g. ethanol/water
- Hydrophobic membranes (organophilic) e.g. PDMS
60PV
- Composite membrane (dense layer porous
supporting layer) - Ref. Baker p366
61Modules
62PV
- Solution diffusion mechanism
- Selectivity dependent on chemical structure of
polymer and liquids
63PV
- 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.
64Models
- Solution diffusion model
- Experimental evidence (ref. Baker pp 43 48)
65continued
- 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
66PV selectivity
- ß (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
-
67continued
- Structure permeability relationships
- Sorption coefficient, K (relates concentration in
fluid phase and membrane polymer phase) - Diffusion coefficient, D
- Ref. Baker p48
68continued
- Diffusion in polymers
- Glass transition temperature,Tg
- Molecular weight, Mr
- Polymer type and chemical structure,
- Membrane swelling,
- Free volume correlations
69continued
- 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 -
70Dual 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
71continued
- 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.
72PV process design
- Vacuum driven process
- Condenser
- Liquid feed has low conc. of more permeable
species - Ref. Baker p 370
73Applications
- 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
74PV hybrid processes using distillation
75continued
- 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
76Gas 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.
77Gas 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.
78continued
- 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.
79Mechanisms
- Convective flow (large pore size 0.1 10 µm no
separation) - Knudsen diffusion (pore size lt 0.1µm flux a
1/(Mr)1/2) - Molecular sieving (0.0005 0.002 µm)
- Solution-diffusion (dense membranes)
- (See Baker fig. 8.2, p303)
80Knudsen diffusion
- Knudsen diffusion occurs when the ratio of the
pore radius to the gas mean free path (? 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.
81Poiseuille 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/? gt 1
82Transport 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
83Membrane selectivity
- a 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)
84Diffusion coefficients in PET (x 109 at 25oC, cm2
s-1)
Polymer O2 N2 CO2 CH4
PET 3.6 1.4 0.54 0.17
85Membrane 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
86Modules
- Spiral wound
- Hollow fibre
87Flow patterns
- Counter-current
- Co-/counter
- Radial flow
- crossflow
88System design
- Feed/permeate pressure (?p 1 20 atm.)
- Degree of separation
- Multistep operation
89Applications
- Oxygen/nitrogen separation from air (95 99
nitrogen) - Dehydration of air/air drying
- Ref. Baker p350
90Other membrane processes
- Ion exchange
- Electrodialysis e.g. UP water
- Liquid membranes/carrier facilitated transport
e.g. metal recovery from aqueous solutions
91PV demonstration
92Reference 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
93continued
- Perrys 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