AST Sanjay Patel

AST Sanjay Patel

Classification of Membranes

According to Pore Size• Porous Membranes (0.1-10 µm)• Microporous Membranes (1-100 nm)• Non-Porous Membranes (0.5-5 nm)

According to Structure• Symmetric (Isotropic) Membranes• Asymmetric (Anisotropic) Membranes Single Material Membranes Composite Membranes

• Electrically Charged Membranes

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Classification of Membranes

According to Phases• Solid Membranes Organic (Polymer) Membranes Inorganic Membranes

Ceramic, Alumina, Zeolite, Silica• Metallic Membranes

Noble Metals, Pd, Ti• Liquid Membranes• Glass Membranes

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Symmetric (Isotropic) Membranes Same Chemical & Physical Structure throughout the Thickness of Membrane Similar to conventional Filters Porous or Microporous Rigid, Voided Structure Randomly Distributed and Interconnected Pores Used in Mircro-, Ultra- & Nano- Filtration

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Asymmetric (Anisotropic) Membranes Chemical &/or Physical Structure throughout the Thickness of Membrane is/are not Same Made of Same or Varying Chemical Composition Normally, Extremely thin surface layer followed by thick porous structure Non-porous or Microporous Used in RO, Dialysis, ED, Pervaporation, GP etc.

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Membrane Preparation

Preparation of Microporous Membranes Pressing & Sintering of Powder Stretching of Polymer Sheet Track-Etching Phase Inversion Expanded Film Tempelate Leaching

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Preparation of Asymmetric Membranes Integral Method Phase Inversion Interfacial Polymerization Thin film Composite Membranes Sol-Gel Process Chemical Vapour Deposition (CVD) Leached Hollow Fibers Anodic Oxidation Pyrolysis Plasma Deposition

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Membrane Material

Pore Size, µm

Mfg Process Applications

Ceramic & Metal Powders

0.1-20 Pressing & Sintering

MF

Homogeneous Polymer Sheets (PE, PTFE)

0.5-10 Stretching or

Expanded Films

MF, Artificial Blood Vessels, Burn Dressings

Homogeneous Polymer (PC)

0.02-10 Track-Etching MF

Polymer Solution (CN, CA)

0.01-5 &5 nm-5 µm

Phase Inversion

MF, UF, RO

Symmetric Microporous Membranes Preparation

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Steps:• Pressing fine powder into a Film or Plate• Film Thickness: 100-500 µm• SinteringProperties of Membrane:• Microporous structure (0.1-20 µm)• Irregular porous structure• Wide pore size distribution• Low porosity (10-30%)Membrane Material:• Glass, Graphite• Polymers• Metal Powders like SS, TungstenApplications:• Separation of Colloidal solutions

Pressing & Sintering

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SEM of Sintered PTFE

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Stretching or Expanded FilmsSteps:• Extruding polymer powder close to its melting point with rapid drawdown• Annealing (Temperature controlled Cooling)• 300% Stretching in perpendicular to extruding direction of this Homogeneous Polymer• Cooling to room temperatureProperties of Membrane:• Microporous structure (1-20 µm)• Fairly regular pore sizes• High porosity (up to 90%)• Flat sheet, Tubes, Capillary shapesMembrane Material:• Partially crystalline polymers like PP (trade name: Celgard)• PE • PTFEApplications:• MF: Acid & Caustic Solutions, Organic Solvents• Membrane Distillation, Artificial Blood Vessels, Burn Dressing etc.

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Extrusion

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Stretching

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SEM of Stretched PTFE

SEM of Stretched PP

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Steps:• Exposure of Polymer Film to Charged Particles in Nuclear Reactor • Charged Particles produce Sensitized Tracks of broken bonds• Etching to control cylindrical pores

Properties of Membrane:• Microporous structure (0.02-10 µm) & 10-15 µm uniform pore sizes • Perfectly round cylindrical pores

Membrane Material:• Homogeneous 10-15 µm Polymers• Poly carbonate, Polyester

Applications:• Analytical Chemistry & Microbiological Laboratories• Ultra pure water production• Blood component deficiencies monitoring

Track-Etching

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SEM of Track-Etched Polycarbonate

SEM of Asbestos fibers accumulated on Polycarbonate

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Phase Inversion/Solution CastingSteps:• Preparation of Casting Solution i.e. Polymer + Solvent & Casting of it in 20-200 µm thick film• Exposure of solution to non-solvent i.e. water (should be in vaporized form)• Solvent + non-solvent miscible with each other or Miscibility Gap attained in three phase causes precipitation of polymer• During precipitation some solvent trapped inside solid phase which creates pores inside the membraneProperties of Membrane:• Microporous structure (0.1 to 20 µm range, uniform pore sizes)Membrane Material:• Cellulose Acetate, Cellulose Nitrate• Polymers like Nylon 66, Polysulfone, PolyvinylidenecdifluorideApplications:• Clarification of Turbid solutions• Removal of Bacteria, Enzymes

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Pore size controlling parameters:Polymer, Solvent, non-solvent, Polymer concentration, Precipitation temperature

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SEM of water vapor precipitation CN membrane

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Mechanisms of Phase Inversion

1. Thermogelation of Homogeneous Solution2. Evaporation of volatile solvent from Homogeneous Solution3. Addition of non-solvent to a Homogeneous solution

Resultant membrane may be Symmetric or Asymmetric Membrane

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Mechanism of Thermogelation

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Polypropylene structures. (a) Type I: open cell structure formed at lowcooling rates. (b) Type II: fine structure formed at high cooling rates

(a)(b)

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Mechanism of Volatile Solvent

Two compounds can also be used

of Polymer, Solvent & Non-solvent

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Mechanism of addition of non-solvent (in vapour or liquid form) addition

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Microporous Membrane preparation by Water Vapour Absorption

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Asymmetric Microporous Membrane Preparation

1. Phase Inversion2. Interfacial Composite Membranes3. Solution-coated Composite Membranes

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Phase Inversion

Principles are same as Symmetric Membrane PreparationAsymmetric structure is formed by varying the preparation parameters

Method investigated by Loeb-Sourirajan & still used to prepare some membranesIt is also known as Polymer Precipitation by Water

Steps:• Casting• Evaporation (Enhance surface quality)• Water Immersion Precipitation of Polymer (Results in Two major structure)• Annealing (Pore structure equilibrium)

Absorption of water and loss of solvent (OR Replacement of solvent by water)cause the film to rapidly precipitate from the top surface down

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Concentration profiles of Precipitant at various Times in Symmetric Membrane Preparation

Throughout the thickness of membrane same flat profile is observed

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Concentration profiles of Precipitant at various Times in Asymmetric Membrane Preparation

At the surface of polymer steep concentration profiles observedAt depth of polymer flat profile same as symmetric membranes is there

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Membrane matrixPolymer Rich

Water filled poresPolymer leanForms Pores

Determines Porosity

At C Precipitation startsPolymer looses solvent gains waterPolymer becomes Opaque

Solidification Point

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Precipitation starts instantaneously & continue as long as 30-60 S Initially precipitated polymer is semi-solid that can flow As precipitation proceeds, more solvent replaced by non-solvent which imbibed by the polymer rich phase (highly viscous) Once precipitated polymer solidifies, no bulk movement of polymers Final step of Precipitation (De-solvation of polymer) is solid gel where membrane structure is fixed Solid polymer phase forms the matrix of membrane & liquid solvent/non-solvent phase forms pores Line AB represents the average composition of whole membranes but in reality there are some/many lines for Asymmetric membranes

Surface layer of polymer precipitates rapidly, therefore two phases formed has no time to agglomerate resulting in finely microporous or dense layerThis precipitated dense layer becomes barrier that slows further loss of solvent and Imbibitions of non-solventThe increasingly slow precipitation from top to bottom of polymerBecause of slow precipitation more agglomeration & more time for separationof agglomerated clumps

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[1] Choice of Polymer: High molecular wt polymers are preferable more than 40,000 daltons Polymer must be soluble in water-miscible solvent Typical polymers are CA, PS, polyvinylidine fluoride, polyetherimide, polyamides etc

[2] Choice of casting solution solvent Should dissolve polymer fairly High solubility (Good) Solvents: Dimethyl formamide, N-methyl pyrrolidone and dimethyl acetamideCharacteristics: Dissolve wide variety of polymers, casting solutions based on these precipitate rapidly when immersed in water to give microporous anisotropic membranes. Low solubility Solvents: acetone, tetrohydrofuran, dioxane, ethyl formate

Characteristics: Low solubility solvent based casting solution precipitates at slow rate which results in non-porous dense ineffective membrane OR Large irregular-sized pore diametersUsed as a modifier that helps in forming non-porous asymmetric membranes for RO, GP, Pervaporation etc.

Factors affecting Phase Inversion

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[3] Polymer Concentration Increasing polymer concentration reduces porosity and flux of membranes For a given membrane material high concentration results in sponge

structure & low concentration results in finger structure Typical concentrations:

UF:- 15-20 wt% RO & GP:- 25 wt% HF:- 35 wt%

[4] Precipitating medium or non-solventWATER is the BEST precipitating medium for phase inversion membranesOther mediums are: methanol, iso-propanol which has low precipitation rate.

[5] Modifiers Ranging from 2-20 wt% Change the membrane structure & performance significantly Low solubility solvents: useful for producing non-porous membranes ZnCl2, LiCl2, polyethylene glycol etc gives open membranes (high porosity)

and also make the membrane hydrophilic

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Typical compositions & conditions of Phase Inversion method

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AST Sanjay PatelPorosity of cellulose acetate membranes cast from 15-wt% solutionswith various solvents.

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Generally low-temperature precipitation produces lower flux, more retentive membranes.For this reason chilled water is frequently used to prepare cellulose acetate reverse osmosis membranes.

Addition of low solubility solvents such as acetone, tetrahydrofuran or dioxane will normally produce denser, more retentive membranes.

Increasing the polymer concentration of the casting solution will also make the membrane more dense.

Addition of salts such as zinc chloride and lithium chloride usually gives more open membranes.

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Selective Layer followed by Sponge Structure Formation

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Selective Layer followed by Finger Structure Formation

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Finger Structure Formation

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10 minAs Evaporation time increases the Pore size IncreasesAs non-solvent increases Porosity IncreasesAs Polymer concentration decreases Porosity Increases

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Casting machine

AST Sanjay PatelLarge scale UF & RO preparation by LS (PI) method

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Large scale Phase Inversion Membrane Preparation

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Large scale Microporous Asymmetric Membranes preparation by Thermalgelation

CA with solvent: Acetone and non-solvent: 2-methyl-2,4-pentanediol

CA + Solvent + non-solvent Rapid Cooling Rate :: AnisotropicSlow Cooling Rate :: Isotropic

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Schematic diagram of the high-pressure apparatus for the membrane formation:(1) Piston pump (2) Temperature controller (3) High-pressure cell (4) Pressuretransducer (5) Back pressure regulator.

Phase Inversion by super critical fluid CO2

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Phase Inversion by critical CO2

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Composite Membranes Preparation

1. Solution-coated composite membranes2. Dip-coating of Microporous support film in a polymer3. Interfacial polymerization of reactive monomers on Microporous support

4. Gas-phase deposition

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Casting of Barrier layer followed by lamination on Microporous support

Solution-coated composite membranes

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Industrial scale production of solution-coated composite membrane

Developed by General ElectricPolycarbonate-Silicon rubber membrane for Oxygen enriched Air

0.1-0.5 µm

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Direct casting of polymer solution on Microporous support

50-100 µm

0.5-2 µm

Dip-coating composite membranes

Developed by UOP

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Interfacial composite membranes

Microporous membrane as a supportDense non-porous skin a barrier (<0.1 µm)

Microporous UF polysulfonesupport membrane

Non-solvent + reactant diacid chloride in HexaneReactive

Polyamine polymer deposition

Cross-linking improves selectivity70-80% RO membranes are ICM

Polyamide Membrane

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Polyamide Membrane

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Interfacial composite membranes

Polyethyleneimine Membrane (NS100)

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Polyethylenimine in waterTolune-2,4-diisocynate or isophthaioyl chloride in Hexane

NS100

CH2CH2 group

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NS100

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Industrial scale production of ICM membranes

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Hollow Fiber Membranes Preparation

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Microporous Hollow fiber membranes making

Bore forming fluid

Casting solution

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Asymmetric Hollow fiber membranes making

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Out side & InsideOut sideInsidePrecipitation Patterns

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Preparation Parameters of Hollow fiber membranes

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Inorganic Metal Oxide MembranesAl, Ti, Si - Oxides

Chemically InertStable at high temperature0.01 – 10 µmMF, UFPreparation Methods: Slipcoating-Sintering Solgel

Ceramic Membranes Preparation

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Ceramic Membranes Preparation

Cellulosic polymer or PVA acts a binder & viscosity enhancer to form Suspension Slip Coating Method

Making Porous Tube moldingFine grain particles + BinderMolding followed by SinteringResults in Large pore dia tubes

Finer grain particles + Binder (Slip Suspension)

Coating of one surface of Porous Tube with Slip Suspension

Drying

Sintering 500-800 oC Ceramic Membrane

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ssCeramic Membranes

Sol-Gel Methods

Hydrolysis by excess water Catalyst: acidFormation of Colloidal Solution

Cooling & Coating on Microporous support

Ageing or Peptization

Minimum water

Peptization: Formation of stable dispersion of colloidal particles in water

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Particulate Sols

Polymeric Sols

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Ion-exchange Membranes Preparation

Cation-exchange Membrane

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Moieties used as fixed charges in Cation-Exchange Membranes:-SO3- , -COO- , -PO3

2- , -AsO32-

Moieties used as fixed charges in Anion-Exchange Membranes:-NH3

+ , =NH2+ , =S+

Desired Properties of Membranes: High Perselectivity Low electrical Resistance Mechanically Stable Chemically Stable for pH 1-14

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Cation-exchange Membrane

Phenol Polycondensation

P-phenosulfonic acidBrown colored crystalline

P-phenosulfonic acid +Formaldhyde 8-10 hrs at 90oC

Phenol Sulfonation

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Cation-exchange Membrane

Polymerization of Styrene & Divinylbenzene

Sulfonation

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Anion-exchange Membrane

Polymerization of Styrene & divinylbenzene Amination

Trimethylamine

Chloromethyl ether

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Microporous Carbon Membranes

Pressing at high pressure of Highly porous Carbon Powder

Pyrolysis of Polyamide or Plyacrylonitrile membraneUnder Vaccum & 500-800 oC to produce highly porous carbonPore Size: 10-20 ÅProduced by: Koresh & Soffer, Air Products

Solution coating on ceramic support of Highly porous Carbon Powder

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Metal Membranes

Pd, Pt, AgPalladium for H2 Separation99.9% in once through processSymmetric: 25 µmAsymmetric: 0.05 µm

Microporous Glass Membranes

Prepared by Corning Inc only Trade name: Vycor

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Factors to be considered in preparation of Membranes

Selection of Membrane material, Solvent, non-solvent, modifiers Preparation method Type of membrane to be prepared (Symmetric or Asymmetric) Chemical and Mechanical strength Type of module to be used Ability to produce high flux with desired purity Defect free production Lowest thickness and high surface area Large scale production Economic and efficient manufacturing

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& Configurations