membrane technology 10.8

8/8/2019 Membrane Technology 10.8

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Operating considerations

Membrane fouling Concentration polarisation (the layer of solution

immediately adjacent to the membrane surface becomesdepleted in the permeating solute on the feed side of themembrane and enriched in this component on the permeateside, which reduces the permeating componentsconcentration difference across the membrane, therebylowering the flux and the membrane selectivity)

Flow mode (cross flow, co-flow, counter flow)

Thermodynamic driving force (P, T, c etc) for transportthrough membrane is activity gradient in membrane

Flux (kg m-2 h-1)

Selectivity

Membrane area


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Membrane fabrication

Isotropic

Solution casting

Melt extrusion

Track etch membranes

Expanded film membranes

Anisotropic

Phase separation Interfacial polymerisation

Solution coated composite membranes


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Membrane preparation methods TIPS

CIPS PIPS

SIPS

Track etching Stretching


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The TIPS method of membrane

formation A homogeneous solution of a polymer and a high boiling, low

molecular weight diluents or latent solvent (which does notcause appreciable dissolution or swelling of the polymer at roomtemperature) is formed by blending at an elevated temperature

the solution is cast or extruded through a slit die (to form a flatsheet) or a spinneret (to form a hollow fiber or tube)

the extruded melt-blend or nascent membrane passes through anair gap and is then cooled either in a quench bath or on a chillroll. This removal of thermal energy results in the formation of

polymer rich and diluent rich phase, and , eventually, the

solidification of the polymer- rich phase. after , the solidification of the polymer- rich matrix phase, the

diluent is extracted.

the extractant is removed (typically by evaporation) to yield amicro porous membrane


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Post processing treatments, such as stretching to modifythe pore structure or chemical modification to create or

alter functional groups, are optional. Depending on the nature and strength of the polymer-

diluent interactions, the initial polymer concentration, andthe rate at which the thermal energy is removed, the melt-blend may undergo one of three phase separation

sequences: liquid-liquid TIPS, in which the melt-blend separates into

polymer-rich and polymer-lean liquid phases, followed bysolidification of the polymer-rich phase.

solid-liquid TIPS, in which the polymer crystallizes fromthe melt-blend.

liquid-solid TIPS , in which the diluent crystallizes priorto the crystallization of the polymerization.


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The majority of the membrane formation

projects focus on liquid- liquid TIPS

because it has proven to be a valuable

method of making commercial membranes


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Advantages

TIPS can be used to form micro porous membranes from high- performance polymers that are difficult to process and do notdissolve at room temperature, including semi-crystalline polymers

that are chemically resistant and thermally stable. For any particular polymer , the structure development kinetics (and

therefore the pore size, pore size distribution , and overall porosityor void fraction) can be controlled by the choice of diluent, polymerconcentration, and quench temperature or cooling rate.

Phase separation is induced by heat transfer, which is easier to

control than the multi component mass transfer involved intraditional phase inversion methods. As such, the TIPS processresults in membranes with fewer defects such as the macro voidsthat often weaken membranes.

Since heat transfer is far more rapid than diffusion in polymersolutions, TIPS permits membrane structure to be formed viaspinodal decomposition. Consequently, TIPS membranes can be

made to have a narrower pore size distribution than membranesprepared by phase inversion techniques

Membrane can be prepared with porosities as high as 90%.


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TIPS can produce a variety of micro- structures including non-porous, open cell, closed cell, isotropic (uniform pore structurethroughout), anisotropic ( a variation in pore size from one

surface to the other surface), or asymmetric( a relatively denseskin on the surface of an isotropic or anisotropic structure).

pore sizes ranging from 0.01 to 20 micrometer have beenreported.

Only one diluent is needed for casting solution, as compared to

other membrane production techniques in which at least onesolvent and at least one non-solvent are needed. Therefore, fewerparameters need to be controlled during the process.

A simple, one- component quench bath can be used, ascompared to DIPS techniques in which multi- component bathsare used to induce phase separation. Since no componentexchange takes place in the quench bath, the bath can be reusedwithout the costly purification needed in some phase inversion

processes.

Micro-porous products can be readily prepared in the form ofsheet, fibres, hollow fibres, spheres, and blocks of various sizes

and shapes


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Applications

Membranes and membrane support materials: batteryseparators: closed- cell and open cell foams for thermaland sound insulators: breathable/disposable surgical

garments, diapers, tents, and packaging materials:medicated wound and burn dressings: controlled releasedevices of various geometries: and high surface areasfibres for improved dyeabiltiy and reduced density.

Most of the TIPS membranes produced on a commercialscale are used for micro filtration, plasmapheresis,controlled drug release, membrane distillation, and lithiumion battery separators.


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DialysisA process for selectively removing low mol. wt. solutes from

solution by allowing them to diffuse into a region of lowerconcentration through thin porous membranes. There is

little or no pressure difference across the membraneand the flux of each solute is proportional to theconcentration difference. Solutes of high mol. wt. aremostly retained in the feed solution, because theirdiffusivity is low and because diffusion in small pores is

greatly hindered when the molecules are almost as large asthe pores.

Uses thin porous membranes.


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PV Membranes

Composite membrane (dense layer + poroussupporting layer)

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

Hydrophobic membranes (organophilic)

Modules Plate & frame


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Mechanism Solution diffusion

Selectivity dependent on chemical structureof polymer and liquids

Activity driving force is provided bydifference in pressure between feed andpermeate side of membrane.

Component flux is proportional toconcentration and diffusivity in densemembrane layer.

Flux is inversely proportional to membranethickness.


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Gas permeations Mechanisms Convective flow (large pore size 0.1 10

m)

Knudsen diffusion (pore size < 0.1m)

Molecular sieving (0.0005 0.002 m)

Solution-diffusion (dense membranes)


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Knudsen diffusionKnudsen 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 permeatepreferentially.


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Poiseuille flowIf the pores of a microporous membrane are

0.1 micron or larger, gas flow takes place

by normal convective flow.i.e. r/ > 1


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Transport of gases through dense

membranesJA = QA (pA1 pA2)

QAis permeability (L (stp) m-2 h-1 atm-1)

pA1 partial pressure A feed

pA2 partial pressure A permeate


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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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Applications

Oxygen/nitrogen separation from air (95

99% nitrogen)

Dehydration of air/air drying


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STRETCHED SEMI-CRYSTALLINE

POLYMER This method is used for only membrane which uses

crystalline materials for its construction. PROCESS

The crystalline membrane is made by rolling.

It is stretched perpendicular to the axis of crystallineorientation.

If stretched perpendicular to the axis of crystallineorientation may fracture in such a way as to make areproducible microchannels.

This microchannels act as pores for the membrane.

PROPERTIES

Stretched polymers have unusually large fractions of openspace, giving the fluxes. Hence they are used formicrofiltration

Most of the material are hydrophobic.

EXAMPLES

tefflon & polyolefin.


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NUCLEATION TRACK ETCHING

Process of forming well defined pores by exposing a dense film to ionbombardment followed by etching of the damaged region

PROCESS

These membranes are made by exposing a thin polymer film to acollimated beam of radiation strong enough to break chemical bonds in thepolymer chain.

This is achieved with a collimated beam of particle by separating auranium or californium source from the film evacuating the interveningspace, and exposing the assembly to thermal neutans.

Tracks can be developed in inorganic material when a massive chargedparticle (positive ion) passes through inorganic material,it propels electronout of the atom in crystal lattice,creating an ion-explosion spike.

In organic polymer, broken repulsion, thereby disrupting the regular lattice.

The larger the separation and the smaller the source area, the morecomplete the resulting hole alignment will be.

The film is then etched in a bath which selectively attacks the damagedpolymer.

Reacks can be enlarged by leaching with suitable reagents.

Nuclepore membranes are characterized by cylindrical pores with a narrowpore size distribution.

The technique produces photogenic pores.


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Tracks can be developed in non conducting substancesboth organic or inorganic films.

Flux obtained in this type of membranes is low.

High pore density cannot be achieved

The choice of the etchants varies with the chemical natureof the polymeric film, concentration, temperature, and theorientation of the attacked surface.

Length of the fission track varies with the nature of thesource.

Normal sodium hydroxide is widely used.


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SINTERING

Sintering is a method for making objects from powder, by heating the

material in a sintering furnace below its melting point (solid statesintering) until its particles adhere to each other. Sintering istraditionally used for manufacturing ceramic objects

ADVANTAGES OF SINTERING

The possibility of very high purity for the starting materials and theirgreat uniformity

Preservation of purity due to the restricted nature of subsequentfabrication steps

Stabilization of the details of repetitive operations by control of grainsize in the input stages

Absence of binding contact between segregated powder particles orinclusions (called stringering), as often occurs in melt processes

No requirement fordeformation to produce directional elongation ofgrains

The possibility of creating materials of uniform controlled porosity.


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PROCESS

Sintering is part of the firing process used in themanufacture of pottery and other ceramic objects. SomePOLYMER have a lower affinity for water and a lower

plasticity index than clay, requiring organic additives in thestages before sintering. The general procedure of creatingMEMBRANES via sintering of powders includes:

Mixing water, binder, deflocculant, and unfiredPOLYMER to form a slurry

Spray-drying the slurry Putting the spray dried powder into a mold and pressing it

to form a green body (an unsintered item)

Heating the green body at low temperature to burn off thebinder

Sintering at a high temperature to fuse the POLYMERtogether

There are two types of sintering: with pressure (alsoknown as hot pressing), and without pressure. Pressurelesssintering is possible with a nanoparticle sintering aid and

bulk molding technology. A variant used for 3D shapes iscalled hot isostatic pressing.


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PIPS-PHASE INVERSION

PHASE SEPERATION Phase inversion refers to the process b which a polymer solution (in which the solvent system iscontinuous phase) inverts into swollen threedimensional macromolecular networks or gel (where

the polymer is the continuous phase). In thin film formsuch a gel constitute a phase inversion membrane.

There are four types of phase inversion processes:

the dry process

the wet process thermal process

the polymer assisted phase inversion process (PAPI)


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Chemical Phase inversion:

This method can be used to produce very largequantities of symmetric membranes. The process

produces tortuous flow membranes. It involves

preparing a concentrated solution of a polymer in

a solvent.

The solution is then spread into a thin film and

then precipitated through the addition of a non

solvent usually water. This technique is capable of

producing fairly uniform membranes versatile.


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The dry Process:

The dry or complete evaporation process is the oldest of the phaseinversion processes. A dissolved polymer is precipitated by

evaporation of a sufficient amount of solvent to form a membranestructure.

Appropriate mixtures of additives are present in solutions with thepolymer to alter its precipitation tendency during solvent evaporation.

The solvent that occupies the void volume of the porous membrane iscompletely evaporated after the structure has stabilized. The originalsolvent may also be displaced by the solvent of the bathins solution.

Stages in the membrane formation:

Loss of solvents and the inversion of a clear one phase solution into aturbid two phase (Sol 2) solution.

Gelation. This is accompanied by a diminution in the reflectivity of thecast solutions.

concentrations of the gel with or with out synergetic

Capillary depletion. Here the non solvent liquid encompasses by thegel departs leaving behind empty capillaries

loss of residual solvents (final drying)


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Wet- Phase separation membranes formation:

Process in which a dissolved polymer isprecipitated by immersion in a non-solvent bath toform a membrane structure

The viscous polymer solution is allowed to

partially evaporate after which it is immersed intoa nonsolvent gelation bath wherever whatever leftof the solvent- pore former system is exchangedfor the nonsolvent. Or the viscous polymersolution may be immersed into the non- solventgelation bath for the exchange of the solventsystem for non solvent. The end products of thewet processes are water swollen membranes


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Polymer assisted phase inversion process (PAPI):

The 4 polymer assisted phase inversion process utilizes asolution consisting of a solvent and two physicallycompatible polymers to cast a dense film with amorphology known as interpenetrating polymer network(IPN).

after complete (dry PAPI) or partial (wet PAPI) solvent

evaporation, the IPN film is immersed in a liquid, usuallywater, which is a solvent for one of the polymers and anonsolvent for the other.

The insoluble network which remains after leaching is askinless micro porous PAPI process membrane. The

polymer which is leached is acting the role of a poleformer. A potential application for PAPI processmembrane is to serve as micro porous supports for thinfilm composites.


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Dense membranes:

Dense membranes may be formed by a variety of techniques:

solution methods melt processing

direct polymerization

Membranes can be prepared by dissolving a membrane forming polymer insome solvent. The solution is then spread on a support and the solvent isevaporated to leave the membrane. The detailed characteristics of the finalmembrane depend on the nature of the solution and the mode of preparation of

the final membrane. There are several techniques for producing dense membranes. The technique

of dissolving the molecules insolvent, spreading the solution on some supportand allowing the solvent to evaporate is quite commonly used. The nature ofthe final membrane is dictated by the polymer concentration in solution, thechoice of the solvent and aggregation of molecules while they are still insolution. A solvent in which solvent polymer interactions is small relative to

the polymer-polymer interactions produce a solution with a high percentage ofaggregate molecules. The properties of such a polymer are influenced by theaggregated molecules. If the same polymer is dissolved in a solvent withstrong solvent-molecules interactions. Thus a wide range of membranestructures may be produced from a single polymer.


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Dense membranes from polymer solutions (cast films) are prepared bydissolution of a polymer substrate in a solvent medium, followed by theapplications of a liquid film onto a suitable substrate and complete evaporation

of the solvent to form a dense film. The nature of both the polymer and thesolvent are important in determining the morphologies of amorphous and semicrystalline films.

The polymers to be used are dissolved, filtered, re-precipitated, filtered anddried prior to dissolution in the solution from which membrane are to be cast.The solvents to be used for membrane formation are pure solvents.

Dense membranes can be subjected to post-formation treatments which serve

to modify their structural and performance characteristics. Thermal annealing,particularly at temperatures in excess of the glass transition temperature can beutilized to increase both crystallite size and the extent of crystallinity.

Annealing of amorphous films has the effect of diminishing the average inter-chain displacement. Increasing the chain mobility by the inclusion ofplasticizers or by subjecting dense membranes to an atmosphere of solventvapors can also promote crystallization even at room temperature.

The crystallinity of polycarbonate films, for example, is increased by exposureto acetone vapor. The application of stress, particularly in the presence oflasticizers, has been utilized to increase crystallinity. Thus the crystallinity ofpolyethylene terephthalate, polycarbonate and cellulose membranes increasewhen subjected to stress under water amounting to 15% of ultimate strength.


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There are a number of factors that influence the finalstructure of the membrane.

Membranes can be formed from a specific solution, but

their final structure can still be influenced by theevaporating atmosphere, temperature and rate ofevaporation. The nature of the finished membrane can bealtered by applying stress, by spontaneous crystallization,and by thermal annealing.

The neat (pure material in a liquid or molten state) can beextruded to produce membranes in the absence of solvent.The structure of the membrane is dictated by the coolingconditions.


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Porous membranes

The porous membranes can be made from densemembranes by introducing solvent which inducesswelling. The solvent is then removed from swollenmembrane to produce the final porous membrane.

Porous membranes can also be generated directlyfrom a molecular solution. The phase inversions

membranes are formed from polymer- solventsystems in which the final structures of themembranes is produced while the solvent is stillpresent in the membrane.


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CIPS

As a new kind of preparation technique for porous polymermaterials, cryogenic induced phase separation (CIPS) process isperformed as follows:

(1) a homogeneous solution is formed at an elevated temperature byblending the polymer with a high boiling point, low molecular weightdiluents;

(2) the solution is cast into the desired shape and cooled at acontrolled rate to induce phase separation:

(3) the diluents is removed (usually by extraction) to produce thechitosen fluid. However, chitosen can easily dissolve in a dilute acidsolution. When the chitosen in the solution is crosslinked and cooled,the phase separation will occur because of the water (solvent)

crystallization (ice crystals). (4) Once the ice crystal is removed by the organic non solvent

extraction, chitosan porous membrane or another desired shape of thematerial (e.g. chitosan sponge) can be produced


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Cryogenic induced phase separation is process in which adissolved polymer is precipitated or coagulated by

controlled cooling to form a membrane structure. The solution of polymer in a poor solvent is prepared at an

elevated temperature. After being formed into its finalshape, a sudden drop in solution temperature causes the

polymer to precipitate.

The solvent is then washed out. Membrane may be spun athigh rates using this technique. This is latest technologywhere very low temperatures are used. The membranesdeveloped using this technique is used in:

Recovery of hydrogen from gases.

In oil refineries

Enrichment of oxygen

Removal of H2S from natural gas

Air separation.

membrane technology 10.8

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