2 PRESSURE DRIVEN MEMBRANE PROCESS Various pressure-driven membrane process can be used to :1. Concentrate2. Purify a dilute (aqueous or non aqueous)THE PARTICLE OR MOLECULAR SIZE AND CHEMICAL PROPERTIES of solute DETERMINE :Structure of membrane1. Pore size2. Pore size distributionPORE SIZE
3 Various PROCESS CAN BE DISTINGUISH RELATED TO THE PARTICLE SIZE OF THE SOLUTE AND CONSEQUENTLY TO MEMBRANE STRUCTURE :
4 Various PROCESS CAN BE DISTINGUISH RELATED TO THE PARTICLE SIZE OF THE SOLUTE AND CONSEQUENTLY TO MEMBRANE STRUCTURE :
5 MICROFILTRATIONIs the membrane process which most closely resembles conventional coarse filtration.10 – 0,05 mPore size:Suitable for retaining suspensions and emulsionsThe Darcy’s Law :Where the permeability constant A contains structural factors such as the porosity and pore size (pore size distribution)For laminar convective flows through a porous systems :Where r is the pore radius, Δx is the membrane thickness, η is he dynamic viscosity and is the tortuosity factor ehich is unity in the case of cylindrical pores.
6 MEMBRANE FOR MICROFILTRATION Can be prepared from organic materials (polymers) and inorganic (ceramics, metals, glasses)Various techniques can be employed :1. Sintering2. Stretching3. Track - etching4. Phase inversionFrequently, inorganic membranes are used instead of polymeric membranes because of their outstanding chemical and thermal resistances.POROSITIES AND PORE SIZE DISTRIBUTIONProcessPorosityPore size distributionSinteringLow / mediumNarrow / wideStretchingMedium / highTrack-etchingLowNarrowPhase inversionHigh
7 Polytetrafluoroethylene (PTFE) Poly(vinylidene fluoride) (PVDF) These various techniques allow to prepare microfiltration membranes from virtually all kinds of materials of which polymers and ceramics are the most important.Polytetrafluoroethylene (PTFE)Poly(vinylidene fluoride) (PVDF)Polyproylene (PP)Polyethylene (PE)HYDROPHOBIC POLYMERIC MEMBRANESCellulase estersPolycarbonate (PC)Aliphatic polyamide (PA)Polyetheretherketone (PEEK)HYDROPHILIC POLYMERIC MEMBRANESAlumina (Al2O3)Zirconia (ZrO2)Titania (TiO2)Silicium carbide (SiC)CERAMIC MEMBRANES
8 MAIN PROBLEMS of MICROFILTRATION Deposition of solute inside the pores of membrane or membrane surfaceConcentration polarization and foulingFLUX DECLINENEEDCareful control over the mode of process operationNEEDMust be cleaned periodically
9 TWO MODES OF FILTRATION: DEAD END FILTRATIONThe feed flow is perpendicular to the membrane surface, so that the retained particles accumulate and form a type of a cake layer at the membrane surface. The thickness of the cake increases with filtration times and consequently the permeation rate decreases with increasing cake layer thicknessCROSS FLOW FILTRATIONThe feed flow is along the membrane surface, so that part of the retained solutes accumulate
10 INDUSTRIAL APPLICATIONS Cold sterilization of beverages and pharmaceuticalCell harvestingClarification of fruit juice, wine and beerUltrapure water in the semiconductor industryMetal recovery as colloidal oxides or hydroxidesWaste-water treatmentContinuous fermentationSeparation of oil-water emulsions
11 Summary Of Microfiltration Membranes :(a)Symmetric porousThickness :10 – 150 umPore sizes :0,05 – 10 umDriving force :Pressure (< 2 bar)Separation principles :Sieving mechanismMembrane material :Polymeric, and ceramicMain application :Analytical applicationsSterilization (food, pharmaceutical)Ultrapure water (semiconductor)Clarification (beverages)Cell harvesting and membrane bioreactor (biotechnology)Water treatment
12 0,05 m – 1 NM ULTRAFILTRATION Is a membrane process whose nature lies between nanofiltration and microfiltration0,05 m – 1 NMPore size:UF is typically used to retain macromolecules and colloids from a solution. UF and MF can both be consider as porous membrane where rejection is determined by the size and shape of the solutes relatives to the pore size in the membrane and where the transport of solvent is directly proportional to the applied pressure.In fact both UF and MF involve similar membrane processes based on the same separation principle. However, an important difference is that UF membrane have an asymmetric structure with a much denser top layer (small pore size and lower surface porosity) and consequently a much higher hydrodynamic resistance.
13 MEMBRANE FOR ULTRAFILTRATION Most of UF membrane used commercially these day are prepared from :POLYMERIC MEMBRANESPolytetrafluoroethylene (PTFE)Poly(vinylidene fluoride) (PVDF)PolyacrylonitrilePolyimidePolyetheretherketoneAliphatic polyamidesCellulosicsBY PHASE INVERSION PROCESSINORGANIC (CERAMIC)Alumina (Al2O3)Zirconia (ZrO2)SOL GEL TECHNIQUE
14 APPLICATIONS Recovery of whey proteins Recovery of potato starch and proteinsConcentration of egg productClarification of fruit juices and alcoholic beverages1FOOD AND DAIRY INDUSTRY2PHARMACEUTICAL INDUSTRY3METALLURGYCHEMICAL INDUSTRY4TEXTILE INDUSTRY56LEATHER INDUSTRY7PAPER INDUSTRY
15 Summary Of Ultrafiltration Membranes :Asymmetric porousThickness :150 um (or monolithic for some ceramics)Pore sizes :1– 100 nmDriving force :Pressure (1-10 bar)Separation principles :Sieving mechanismMembrane material :Polymeric (PS, Polyacrylonitrile)Ceramic (zirconium oxide, aluminum oxide)Main application :Dairy (milk, whey, cheese making)Food (potato starch and protein)Metallurgy (oil-water emulsion, electropaint recovery)Textile (indigo)Pharmaceutical (enzymes, antibiotics, pyrogens)Automotive (electropaint)Water treatment
16 REVERSE OSMOSIS AND NANOFILTRATION RO and NF are used when low molecular weight solutes such as inorganic salts or small organic mocules such as glucose, and sucrose have to be separated from solvent.PRINCIPE OF REVERSE OSMOSISFORCEThe membrane is permeable to the solvent (water) but not to the solute (salt). In order to allow water to pass through the membrane, the applied pressure must be higher than the osmotic pressure.Salt solutionMembrane“Complete barrier to dissolved salt”Pure water
17 The pressure used in reverse osmosis range from 20 – 100 bar and in nanofiltration from about 10 – 20 bar, which much higher than those used in ultrafiltrationBoth process are considered as one process since the basic principles are the same.NF membranes are the same as RO membranes only the network structure is more open.Comparison of retention characteristic between nanofiltration (NF) and reverse osmosis (RO) are listened :SoluteRONFMonovalent ions (Na, K, Cl, NO3)>98%<50%Bivalent ions (Ca, Mg, SO4, CO3)>99%>90%Bacteria and viruses<99%Micro solutes (Mw >100)>50%Micro solutes (Mw < 100)0-99%0-50%
18 MEMBRANES FOR RO AND NFThe flux is approximately inversely proportional to the membrane thickness and for this reason most reverse osmosis membranes have an asymmetric structure with a thin dense toplayer (thickness ≤ 1um) supported by a porous sublayer (thickness 50 – 150 um)The resistance towards transport being determined mainly by the dense toplayer.An asymmetric membrane structure can be distinguished: (i) integral asymmetric membranes, and (ii) composite membranes.
19 Integral Asymmetric Membranes Both toplayer and sublayer consists of the same material. These membrane are prepare by phase inversion technique. The polymeric material from which the membrane it to be prepared is soluble in a solvent or a solvent mixture.1cellulose esters. This materials are very suitable for desalination because of their high permeability towards water in combination with a (very) low solubility towards salt.An important class of asymmetric membranes are :2Aromatic polyamides. These material also show high selectivities towards salts but their water flux is somewhat lower.3Polybenzimidazoles, polybenzimidazolones, polyamidedehydrazide, and polyimides
20 Composite MembranesThe second type of structure frequently used in RO while most of the NF membrane are in fact composite membrane.In such membranes the top layer and sublayer are composed of different polymeric materials so that each layer can be optimized separately.The first stage is the are preparation of the porous sublayer. Important criteria for this sublayer are surface porosity and pore size distribution and asymmetric ultrafiltration membranes are often used. Different methods have been employed for placing a thin dense layer on top of this sublayer :dip coatingInterfacial polymerizationPlasma polymerization
21 APPLICATIONROpurification water, desalination of brackish and seawater to produce potable waterProduction of ultrapure water for the semiconductor industryConcentration step particularly in the food industry ( concentration of fruit juice)Galvanic industry (concentration of waste stream)Dairy industry (concentration of milk to prior cheese manufacture)NFWhen a high retention is required for NaCl with high feed concentrations reverse osmosis reverse osmosis is the preferred process. In other cases with much lower concentrations, divalent ions and micro solutes with molecular weight nanofiltration is the preferred process. Since the water permeability is (much) higher in nanofiltration the capital cost for a certain application will be lower.
22 Summary Of Nanofiltration Membranes :CompositeThickness :Sublayer : 150 umToplayer : 1 umPore sizes :< 2 nmDriving force :Pressure (10 – 25 bar)Separation principles :Solution – diffusionMembrane material :Polyamide (interfacial polymerisation)Main application :Desalination of brackish and seawaterRemoval of micropollutentsWater softeningWastewater treatmentRetention of dyes (textile industry)
23 Summary Of Reverse Osmosis Membranes :Asymmetric or compositeThickness :Sublayer : 10 umToplayer : 1 umPore sizes :< 2 nmDriving force :Pressure : brackish water (15 – 25 bar)seawater (40 – 80 bar)Separation principles :Solution-diffusionMembrane material :Cellulose triacetate, aromatic polyamide, polyamide and poly(ether urea) (interfacial polymerization)Main application :Desalination of brackish and seawaterProduction of ultrapure water (electronic industry)Concentration of food juice and sugars (food industry) and the concentration of milk dairy industry)
24 MicrofiltrationUltrafiltrationNanofiltration / reverse osmosisSeparation of particlesSeparation of macromolecules (bacteria, yeasts)Separation of low MW solute (salt, glucose, lactose, micropollutant)Osmotic pressure negligibleOsmotic pressure high(1-25 bar)Applied pressure low (< 2bar)Applied pressure low (1-10 bar)Applied pressure high (10-60 bar)Symmetric structureAsymmetric structureThickness of separating layer:Symmetric structure :umAsymmetric structure :1 umThickness of actual separating layer : 0,1-1,0 umThickness of actual separating layer : 0,1 – 1 umSeparation based on particle sizeSeparation based on differences in solubility and diffusivity