hinkova membrane introduc web 11
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Separation and
purification techniques
I. Membrane processesII. Chromatographic separation
Andrea Hinkov,Department of Carbohydrate Chemistry and TechnologyB 45, tel.: 22044 3111E-mail: [email protected]
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I. Membrane separation processes
1.Classification of membrane processesFiltration set-up (dead-end, cross-flow), criterions of membraneprocesses classification (mechanism, driving force, membranetypes, properties of separated particles), basic terminologypermeate, retentate, filtrate, up-stream, down-stream process.
2. MembranesSeparation properties (permeability, porosity, pore size,selectivity, cut-off)Membrane classificationstructure, (porous, non-porous,symmetric, asymmetric, composite, homogenous, heterogeneous,mosaic, sandwich), material (organic, inorganic), shape (planar,
tubular, hollow fibre), function and production, bio-membranes,liquid membranes.
3. Membrane modulesCharacter of the flow along the membraneConstruction, types, set-up. Lab scale, industrial scale, special
modules (rotating, vibrating, membrane reactors)
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4. Filtration process kineticsFiltration theory, driving forces, character of separated particlesand molecules (charge, size, shape, affinity, isoelectric point)Transport mechanisms, filtration equation, concentration
polarization.
5. Permeate flux, membrane fouling and cleaningDescription of fouling
Factors affecting the membrane fouling (membrane properties,solution properties, character of the process), permeate fluxenhancement (back-flush, turbulent flow, pulsing flow, constantpressure), Cleaning and sanitation.
6. Individual membrane processesDiffusion, diffusion theory, diffusivity, solvationDialysis, osmosis, reverse osmosis, microfiltration, ultrafiltration,nanofiltration, pervaporation, das permeation, membranedistillation.
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I. Membrane Separation
Processes (MSP)
Filtration principle= separation of at least two fluid components
(gas or liquid) due to the different size.
MEMBRANE:
selective barrier
barrier between two phasesa phase that forms barrier hinders mass transport but enableslimited and controlled passage of certain components
liquid, solid, gas character (or combination of all)
1. Class if ications o f MSP .
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Membrane separates feed into:
Retentate (concentrate)
= enriched with substances retained by the membrane
Permeate (filtrate)
= a stream passing through the membrane, devoid ofsubstances retained by the membrane
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Basic mechanisms of substance separation
on the membrane:
Different particle size (sieving effect)Different charge of mixture components
Different diffusivity
Different solubility of components in the membrane
Aspects of membrane process's
classification:Driving force
Type of the membrane
Properties of separated particles
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Driving force of membrane processes
Pressure
Concentration
Chemical potential
Electric potential
Properties of separated particles:
SizeShape
Charge
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Membrane process's characteristics:
Driving force Retentate Permeate
OsmosisChemical
potential
Substances dissolved
in a solution, water
Water
DialysisConcentration
gradientBig molecules, water
Small molecules,
water
Microfiltration (MF) PressureSuspended solids
(e.g. yeasts), water
Substances dissolved
in a solution, water
Ultrafiltration (UF) PressureBig particles,
molecules (bacteria),
water
Small molecules,water
Nanofiltration (NF) PressureSmall molecules,
water, bivalent ions,
dissociated acids
Monovalent ions, non-
dissociated acids,
water
Reverse osmosis
(RO)Pressure Substances, water water
Elektrodialysis
(ED)Potential
Dissolved non-
ionogenic
substances, water
Ionised substances
dissolved in a solution,
water
Pervaporation (PV) Partial pressure Non-volatilemolecules, water
Volatile smallmolecules, water
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A general set-up of the filtration processCheryan M.: Ultrafiltration and Microfiltration Handbook, Technomic Pub. Co., 1998
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Filtration apparatus
membrna
Up-stream processes
Down-stream processes
membrane
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2. Membranes .
Separation properties of the membrane are given by:
permeability, selectivity and cut-off
A) Permeability
Permeation = a transport of atoms, molecules and ions in apermeable media due to the gradient (of concentration,temperature, pressure, electric potential)
Porosity
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A flow through ideal semipermeable
membrane can be expressed as:
J = A (PT- F)
J = permeate flow, expresses a velocity of component
passage through the membraneA = permeation coefficient (reciprocal resistance)
PT= trans-membrane pressureF= osmotic pressure of solvent
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B) Selectivity
Pore sizeFrequ
ency(numberofpores)
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C) Cut-off
100 %
90 %
MWCO
Velikost pr (MW)
R
ejekce(%)
ideln membrna
R
nzk selektivita
reln membrna
M1 M2 M3 M4
Deffinition: 90 % of molecules with a molecular weightcorresponding to cutt-off do not pass through the membrane
Low selectivity
Real membrane
Ideal membrane
Pore size
Re
jection
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Membrane classification:
Membrane origin (natural, synthetic)
Membrane structure (porous, non-porous, surface
morphology)Membrane application (gas - gas, gasliquid, liquid
liquid separations)
A mechanism of separation (adsorption, diffusion,
ion exchange, osmotic pressure, inert membrane)
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Membrane structure:
1. Micropo rous membrane
isotropic
anisotropic
isotropic membrane
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2. Asymmetr ic membrane (w i th act ivelayer)
A thin active layer is deposed on a support
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Membranes material
1. Organiccellulose acetate (low costs, low price, wide range of poresizes, hydrophilic characterreduced fouling)
polyamide (low resistance to Cl2, biofouling)
polysulphone (high temperature, pH and chemical Cl2resistance)
other polymers: nylon, PVDF, PTFE, PP, polycarbonate)
2. Inorganic (= mineral, ceramic)also metallic (stainless steel)
support layer: ceramic, Al2O3, TiO2
separation layer: TiO2, zirconium, carbon-titanium, carbon-zirconium
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posi t ives:
+inert toward most of chemical solutions (exceptions: HF, H3PO4Al membranes)
+high temperature resistance (350 C)steam sterilization
+wide pH range(113)+high pressure resistance (1 MPa)
+high lifetime
+back-flushing possible
drawbacks:
-high pore size (UF, MF, opened NF membranes)
-high pump discharge needed (26 m/s tangential velocity)
-high producing costshigh prices (high investment costs)
Ceramic membranes:
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Plate
Tubular (channel inside > 4 mm)
Capillary (low diameter)
Hollow fibres (inner diameter 0.23 mm)
Spiral-wound
Pleated sheet cartridges (dead-end filtration)
Membranes shapes
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Tubular membrane
Cheryan M.: Ultrafiltration and
Microfiltration Handbook, Technomic Pub.
Co., 1998
Membralox
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Pleated-sheet membrane
Cheryan M.: Ultrafiltration and Microfiltration Handbook, Technomic Pub. Co., 1998
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Spiral wound membrane
www.mtrinc.com
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Biomembranes
Enzyme immobilization on the membranes surface
- adsorption, chemical bonds
Liquid membranes
a) ELM = Emulsion Liquid Membraneimmiscible liquids
b) ILM = Immobilized Liquid Membrane
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3. Membrane modu les .
Rotary membranes: tubular and diskVibrating membranes: disk
Rotary tubular module Rotary disk module
Cheryan M.: Ultrafiltration and Microfiltration Handbook, Technomic Pub. Co., 1998
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Connections of several modules
Cheryan M.: Ultrafiltration and Microfiltration Handbook, Technomic Pub. Co., 1998
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www.airproducts.co.za
Connections of several modules
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Filtration area enlargement
Plate modules
www.esemag.com
www.dayton-knight.com
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Membrane reactors
Membrane used:
Hollow fibre, flat, rotating cylinder,
Advantages:
Easier cleaning
One-step operation (chemical reaction and separation in one stage)
Placement:
Inside the bioreactor
or in outside recycling pipe
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4. Fi l t rat ion k inet ics .
Expressed as a volumetric permeate flow through 1 m2of themembrane under given conditions (temperature and pressure)
lh-1m-2
Affected by the permeability and pore density (porosity)
Low permeability can be compensate by higher membranearea
In general:
resistance
forcedrivingareaflowmass _
_
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Filtration theory:
Pressure-driven processes: driving force = pressure gradient
PFT PPP
PF
TPReal driving force:
Pressure drop :
Osmotic pressure within membrane:
ez membrnou
solvatace
Driving force of the process: TP
Cross section of the membrane
Active
layer
Support
layer
Feed
Permeate
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Concentration polarization:
Higher concentration of components in boundary layer close to themembrane comparing to bulk
Forming of concentration profileunder extreme conditionsgelor precipitate formation, secondary membrane
Increased membrane resistance
University of Minnesota Duluth; www. d.umn.edu
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www.yale.edu
Concentration polarization:
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Pressure-driven membrane processes
dx
dcDJs
012 cGdx
dcDcG
Driving force = pressure gradient
Real driving force: reduced by pressure drop in polarizationlayer, secondary membrane or gel layer
1cJJS Convective flow Js
Diffusion velocity
(neglected concentration
gradient c)
Filtration equation:G= specific membrane
permeability
Permeate flow Diffusion Convection
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Factors influencing fouling effect
a) Membrane properties
Pore size (permeability)Active layer thickness
An affinity of dissolved components for the membrane
b) Character of filtered medium
Viscosity
Ionic strength
pH
Density
Concentration
Reactivity of molecules with the membraneShape and size of separated molecules
Sample pre-treatmentprecipitation, addition of ballast material,pre-filtration (to improve the size ratio between molecules andpores)
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Membrane purification
Back-flushingCIP (Cleaning in place) chemicals, detergents, high temperature
Steam sterilization (ceramic)
Chemically: ozone, Cl2corrosive action, NaClO, HNO3, NaOHceramic membrane)
Mechanically (after removal, sheet membranes)
Enzymatically
c) Conditions of process operation
Pressure (increasing during filtration, consider irreversiblefouling!limit pressure)
TemperatureHydrodynamics (character of a flow along membrane)maintain turbulent flow:
Pulsation, ultrasound, air bubbling
Back-flushing (asymmetric and composite
membranesa risk of active layerdetaching)
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Pure water flux Jv
Importance:
To estimate an efficiency of membrane cleaningA ratio of pure water flux before filtration and after membrane
cleaning should be less than 20%
Definition:Permeate flow during filtration of pure water at 20 C and given
pressure calculated on 1 m2of membrane area:
JVpure water flux (l.h-1
.m-2
)JPpermeate flow
kt temperature coefficient (20 C)
S membrane area (m2)S
kJJ
tPV
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Filtration efficiency
Permeate and retentate properties
Analytical methods
Concentration of feed
Concentration factor / ratioVCR (VCF) volumetric
MCR (MCF) mass
Definition:
VFfeed volume
VRretentate volumeR
F
V
VVCR
Separation efficiency Rejection retention
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Separation efficiencyRejection, retentionProperties of permeate and rententate
Analytical methods
Rejection factor R
Expresses a relationship betweenconcentration above and under themembrane
cidownst ream, ciupstreamconcentration ofcomponent (i )under, respectively above,the membrane
Apparent rejectionif cupstreamequals tothe concentration in the bulk
Intrinsic rejectionif cupstreamequals to theconcentration at the membrane surface
Retention factor r
Refers to feed and permeate
upstream
downstream
ci
ciR 1
iF
iP
c
cR 1
iR
iP
c
cr 1
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A choice of separation processPermeate flux Separation efficiency
Determine demanded properties of the product
Define required purity90 % industrial enzymes, organic acids,
99 % sugar solutions
99.9 %
99.99 % vaccines
Physical and chemical properties (stability !)
Define initial properties of mixture
Composition, chemical and physical properties, raw materialproperties, etc.
Choose the processDifferent properties of products and contaminants
Use different physico-chemical properties of both
Remove the highest concentration of pollutants at the beginning
The most efficient process should be among first
The most expansive and demanding process should be among latest
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6. Membrane techn iques .
Terminology:
Diffusion
= a movement of molecules in liquid phase in concentration
gradient (from higher concentrated areas to the lower
concentrated ones)
1st Fick law:
Describes a flow of liquid through the membranes (J), which is
reciprocal to the concentration gradient (dC) along the
membrane (dx), where D= diffusion coefficient (diffusivity):
dx
dCDJ
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Dialysis
A transport of small molecules
(dissolved in liquid) acrossthe membrane (from
hypertonic to hypotonic
media); kidneys
driving force: concentration
gradientOsmosis
A transport of solvent across
the membrane, which retains
dissolved molecules (fromhypotonic to hypertonic media)
driving force: chemical potential
gradient
www.visionengineer.com/ env/reverse_osmosis.shtml
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Reverse osmosis (RO)
A process reversal toosmosis.
Driving force: pressuregradient through the
membrane, needs to behigher than osmoticpressure high workingpressure (310 MPa).
peffective>posmotic
Separation of particles of 10-4 m = only solventmolecules permeate throughthe membrane
www.visionengineer.com/ env/reverse_osmosis.shtml
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Pressure-driven membrane processes
(MF, UF, NF a RO)
Conventional filtration - separation of particlesbigger than 10 m.
BUT: comparing to the conventional filtration,where only hydrostatic pressure is needed as adriving force (or maximum pressure of 0.10.5MPa) - membrane filtration (UF, NF and RO,especially requires higher pressure gradients).
The smaller pore size, the higher pressure gradientis needed.
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Separation
aspects of
membraneprocesses
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Separates particles within the range of 10 10-1 m (i.e.bacteria, yeasts, suspended solids, high molecular substances;
M>106)Particles bigger than 5 - 10 m are better to separate byconventional filtration (dead-end filtration).
Pressure difference needed: 0.02 do 0.5 MPa
Refining technique, suspended solids are separated fromdissolved substances.
Microfiltration (MF)
Ultrafiltration (UF)Separates particles within the range of10-210-3 m (i.e. bacteria,
viruses, colloids, macromolecular substances, MWCO 5000 500
000)Pressure difference needed: 0.1 do 1 MPa
a technique for mutual concentration and fractionating of
molecules or fine colloid suspensions
A sieving effectis a separation mechanism of both processes (MF
and UF).
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Nanofiltration (NF)
Relatively new medium pressure membrane process forseparation of compounds within a size of 10-310- 4 m.
Separation mechanism:
1. sieving effect(large molecules, e.g. sucrose)
2. electrostatic forces between membrane and particlespresent in solution (ion separation).
Most of commercially produced NF membranes are negativelycharged.
NF membranes separate dissociated compounds from nondissociated (e.g. organic acids pass the membrane more easy atlow pH, but are retained at high pH in a form of their salts,MWCO < 500).
Membrane cut-off expressed as molecular weight or in Daltons
Operation pressure 2 - 4 MPa
NF phenomena
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NF phenomena The separation mechanism of NF is not totally explained
Single salt rejection: the higher is the salt concentration thelower is the salt rejection: NF membranes negatively charged
polarization layer is formed (and the effective charge of themembrane is hidden and co-ions can easily pass through themembrane)
Ion hydration (solvation)affects the separation: e.g. NaNO3retention is lower than the one of NaCl because of higherhydration of NaNO3molecule in water solution.
High viscositycaused by salt or organic molecules preventsfrom the back diffusion in the concentration polarisation layerand ions cumulate in permeate.
Donnan effectobserved at cheese whey NF (at high VCR)membrane showed a negative rejection of Cl-which preferably
gathered in permeate: Proteins and other oraganic compoundsare concentrated above the membrane (at pH 6.2 occures gelformation and negatively charged layer). This enhance thecation transport through the membrane. Due to theelectroneutrality principle, some anions must pass themembrain as well to keep the electro-neutralityonly the
smallest one can pass (even against the concentration gradient;(Cuartas-Uribe, 2006).
C t ff f d i b
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Cut-off of pressure-driven membrane processes
Pervaporation (PV)
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Pervaporation (PV)
Mixture separation due to the evaporation through the porous membrane-
selective barrier between two phases:
Liquid feedwet side of the membraneswelling (atmospheric pressure) Gas permeatedry side of the membranealmost dry (low pressure of
vapours)
Principlephase change
1. sorption
2. diffusion3. desorption and evaporation http://chemelab.ucsd.edu/pervap/
ROZTOK
Vlhk strana Such strana
MEMBR NAPERMET
Sorpce
Desorpce
Transp
ort-d
ifze
P
ciF
iF
P
C
iP
iP
Hnac sla: vyjden jako i i p
Solution Permeate
Wet side Dry side
Driving force: iexpressed as pi
MEMBRANE
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Pervaporation (PV)
Driving force: chemical potential difference (expressedas a partial pressure), the pressure behind themembrane is lower, which causes evaporation followedby condensation of vapours.
Separation mechanisms:different diffusivity
Separation of volatile compounds (hexane, toluene, ethanol)from liquid mixtures (dehydration of organic solvents),separation of azeotropic mixtures, pollutants and impuritiesremoval, solution concentration
PV membrane:
Composite membranes (active layer)Hydrophobic membranesseparation of organic solvents(polysulfone, polydimethylsiloxane, polyamide)
Hydrophilic membranepolar solutions (water, watervapours)glass, crystalline polymers of hydrophilic nature)
P ti t
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Pervaporation set-upModules - membranes:Capillary, hollow fibre, planar, spiral wound, tubular
Minimal piP:
1. Permeate sidemaintain a vacuum (20
30 mbar)2. Permeate side purified by sweeping gas removal of desorbed compounds)
3. Temperature reduction on permeate side lower piof component i (-20 C,opt. T = 50 C)
Feed, l
Permeate, g
membrane The effect of temperature
Permeate side - latent heat of vaporization causes
cooling
Isothermic separation(tF= tP) the flow profile
through the membrane is shorterfaster process.
F F F
R RR
P
P
P
ad 1. ad 2. ad 3.
K K
condenser condenser
Vacuum
pumppump
G ti
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Gas permeation
An analogue of pervaporation,but uses non-porousmembranes
The same phaseon both sizeof the membrane (gas)
Concentration gradient isprovided - sweeping gas
Separation mechanism:
different velocity of gaspermeation (sorption,
diffusion, sieving effect,desorption)
Feed (higher pressure)
Permeate (lower pressure)
Polymeric
membrane
M b f ti
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Membranes for gas permeation
Composite structure
Porous PS(polysulphone) coated by thin layer of rubber (PDMS;
polydimethylsulphoxan) placed on macroporous supportPDMSlow selectivity, high permeability
PSreversal
PDMS
Macroporous
supportPS
Applications:
Separation of CO2, CH4from natural gas and biogas)H2S removing from natural gases
N2O2 separation
Gas drying (water removing)
Separation of organic pollutants from air
Membrane distillation (MD)
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Membrane distillation (MD)Uses both distillation and membrane separationporous hydrophobic
membrane (permeable for water vapours but not for liquid water
Driving force: temperature and pressure gradient
Principle:
One side of the
membrane: liquid is heated
evaporation, vapors pass
through
Other side: cooling
vapor condensation
water removing
The separation is influenced only by the equilibrium liquid-vapours (themembrane has no effect):
Limitunsuitable for azeotropic mixture separation (azeotropic point =
identical composition of gas and liquid phase)
Necessary requirementthe membrane must not be soddenpore
blocking, application for hydrophobic solutions
Velocity of permeation is given by thigh t ensuresspeed and selectivity
M b f th b di till ti
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Membranes for the membrane distillation
Hydrophobic (non-polar) Microporous Material: PTFE (polytetrafluorethylene = teflon)
PP (polypropylene)
SeparationsMixtures EtOHwater (up to 30 - 40 vol. %)Water solutions of saltsdesalination (e.g. water for
heating systems)Sea water desalination
DrawbacksLow selectivityLimited application
Applications