hinkova membrane introduc web 11

8/13/2019 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



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Connections of several modules



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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

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