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SIMILARITES AND DIFFRENCES• Both pressure driven processes.• Boundary between the two processes is nonprecise• The distinction is made because– slight difference in membrane permeability– the ability of nanofiltration to separate lowmolecular weight non-ionic molecules
SIMILARITIES AND DIFFERENCES• Both processes are used to separate low molecularweight solutes from a solvent.– Solvent is typically water– Solutes are typically salts or amino acids• Both have applications on an industrial and domesticscale• Nanofiltration more open polymer structure• Pure solvent flux much greater in Reverse Osmosis• Transport mechanisms are considered the same andgenerally thought of as primary solution diffusion
DEVELOPMENT OF REVERSE OSMOSIS• Originally developed by the US army to produce potablewater from brackish sources.• Research leading to commercial application began in theearly 1950’s at the University of Florida.• Advanced by Loeb and Sourirajan at University of California– first high flux asymmetric reverse osmosis membranes• Original membranes made of cellulose acetate highlysusceptible to chemical and biological degradation• Further research funded by US government resulted incellulose ester membranes– Higher flux and rejection of ionic species– Still susceptible to biological degradation
REVERSE OSMOSIS APPLICATIONS• Potable water from brackish sources– Recent advances have made the production of potable water from sea waterpossible• Used in conjunction with ultrafiltration and ion exchange to produce ultrapure water• Extensively used to clean waste waters with small solutes and high BOD– Starch recovery from potato processing• Concentration of fruit and vegetable juices– Superior flavour to those produced by heat concentration– Lower energy requirements than evaporation processes– Smaller plant cost and size– Very high dissolved salt concentration– High operating pressures– High fouling– Shortens membrane life
NANOFILTRATION APPLICATIONS• Advantages over reverse osmosis whenretention of mono – valent salts are notrequired– Much more permeable membrane– Higher solvent fluxes, often a factor of 10– Lower energy and capital costs
NANOFILTRATION APPLICATIONS• Potable Water– Remove colour from water drawn from peat lands– Remove humic and fulvic acids– Conventional treatments labour and chemical intensive– Suitable for small domestic supplies in isolated communities– Can operate unattended for long periods of time– Does not remove minerals• Sugar Concentration– Concentration of lactose from deproteinised whey stream– Concentrates lactose while allowing mono-valent ions to passthrough– Concentrated lactose stream used as feed for fermentation
NANOFILTRATION APPLICATIONS• Dye Production– Improved by using nanofiltration– 10% NaCl reduced to 0.2%– Concentrate product by 30%
MATERIALS• Originally derived from modified celluloseprecursor• Aromatic polyamides no used– Advantage – Significant lower tendency tobiological degradation– Disadvantage – Susceptible to damage from freechlorine• Drawn as hollow fibres by melt or dry spinning– Typical diameter – 100 µm– Wall thickness – 20 µm
MATERIALS• Significant advance in RO/NF technology due tocomposite membrane• Casting active layer on surface of UF membrane• First membrane of this type utilised the additionof polyethylenimine (PEI) to a polysulfone UFmembrane• Addition of toluene 2,4 diisocynate followed byheat drying improved salt rejection to > 95%
MATERIALS• FilmTec (Dow subsidiary) developed the firstcomposite membrane by interfacialpolymerisation• The membrane is wetted with polymer precursorand brought into contact with co-reactant• Polymerisation takes place at the interface• Formation of the polymer prevents furtherreaction taking place• m-phenylenediamine (wetting agent) & trimesoylchloride (non-aqueous co-reactant)
TRANSPORT METHOD• Both RO & NF separation achieved byapplication of pressure gradient• Small solutes therefore membranes areconsidered non-porous• Molecular volume of water and salts aresimilar therefore not a sieving process• Exact nature of transport is under debate– Preferential sorption-pore flow– Solution diffusion
PREFERENTIAL SORPTION-POREFLOW• Assumes thattransported speciesforms an adsorbedlayer at membraneinterface• layer occludes thepores preventing thepassage of certainmolecules
SOLUTION DIFFUSION• Assumes that moleculesare not convectedthrough pores butpreferentially dissolve inthe membrane• The transported speciesdiffuses throughpolymer down achemical potentialgradient
SOLUTION DIFFUSIONWhen 2 solutions of different concentration are separated bya semi-permeable the solvent will permeate through themembrane from the dilute to the concentrated phase (HWCto LWC)
SOLUTION DIFFUSIONIf a pressure is applied to the concentrated phase so that apressure difference is generated across the membrane thenet transport of the solvent will be reduced. The pressuredifference required to reduce the net transport of solvent tozero is known as the osmotic pressure difference.
SOLUTION DIFFUSIONTo relate the flux of a species to the chemicalpotential we need to establish the differences inchemical potential for the species across amembraneFor isothermal conditions the chemical potentialof the solvent in the concentrated phase isexpressed as
SOLUTION DIFFUSIONThe chemical potential of the solvent in thedilute phase is expressed similarly asThe difference in chemical potential can thusbe expressed as
SOLUTION DIFFUSIONFor dilute solutions the activity of the solventwill be approximately equal on both sides.Therefore the potential is highly dependant onthe pressure difference
SOLUTION DIFFUSIONThere will be a net flow of solvent through themembrane until the chemical potentials inboth phases are equalWhich gives:
If the dilute phase consists of pure solvent then the osmotic pressure ofthe concentrated phase can be expressed asFor dilute solutionsIf the solute is denoted as component jThereforeSOLUTION DIFFUSION
SOLUTION DIFFUSIONAlso for dilute solutionsThereforeIf the salt dissociates then the above relationship has to be modified, asthe number of moles of solute will be increased. Therefore:
Osmotic Pressure of Real Solutions• Previous expressions assume that solutionbehaviour is ideal• π of solutions departs significantly from idealbehaviour at high concentrations• It is possible to estimate π using a virial expansion• Unfortunately coefficients are not always readilyavailable over the range of concentrationsrequired
Osmotic Pressure of Real Solutions• The vapour pressure of the solution can alsobe used to estimate the osmotic pressure withreasonable accuracy
Trans-membrane flux of solventFrom Fick’s lawFor isothermal operation the expression can bereduced toi.e. for constant retentate conditions the flux willbe linear w.r.t. the trans-membrane pressure
TRANS-MEMBRANE FLUX OF SOLUTEFrom Fick’s law:As before this reduces toIndependent of pressure as is the rejectioncoefficient
TRANS-MEMBRANE FLUX OF SOLUTEThe concentration of the solute in the permeate canbe written asthe rejection coefficient becomeswe can see that as the pressure drop across themembrane increases so does the rejectioncoefficient
MEMBRANE FOULING• Membrane fouling is the main cause ofpermanent flux decline and loss of productquality in Reverse Osmosis systems, so foulingcontrol determines system design andoperation.
SOURCES OF FOULING• There are four Categories1. Scale2. Silt3. Bio Fouling4. Organic Fouling
SCALE• Cause– Precipitation of dissolved metal salts in the feedwater on the membrane surface.• Salts that form Scale– Calcium Carbonate– Calcium Sulphate– Silica Complexes– Barium Sulphate
METHOD TO DETERMINE• Scaling is determined by calculating ExpectedConcentration Factor in the feed Brinesolution.Where
• Scaling is not a problem if and only if–Concentration Factor < 2 or–Recovery Rate = 50%• Most systems are operated at Recovery Rate of80 – 90%, the Concentration Factor Exceeds 2
CONTROL OF SCALE• Calcium Carbonate Scale is a commonproblem and is controlled by the acidifying thefeed.• Others can be reduced by adding AntiscalantChemicals such as SodiumHexametaphosphate.
SILT• Cause– It is caused by the suspended particulates of alltypes that accumulate on the membrane surface.• Typical Sources– Particulates– Iron Corrosion Products– Precipitated Iron Hydroxide– Algae– Fine Particulate Matter
METHOD TO DETERMINE SILT• SILT DENSITY INDEX (SDI) of the Feed Water– The time required to filter a fixed volume of waterthrough a standard 0.45µm pore microfiltrationmembrane.Ti = Total Elapsed Test TimeTf = Time required to collect 500ml WaterTt = Total Elapsed Test Time
If:-• SDI <1 ---- RO can run several years withoutcolloidal fouling• SDI< 3 ---- RO can run several months withoutcleaning• SDI 3-5 ---- RO need Cleaning on regular basis• SDI > 5 ---- Unacceptable and additional pre-treatment is required.
BIOFOULING• Cause– Growth of Bacteria on Membrane Surface.– Membrane itself becomes nutrient for bacteria• Industrial examples are– Cellulose Acetate Membrane– Ployamide Fibres• Solution is sterilization by heat, chlorine orchemicals.
ORGANIC FOULING• Causes– Attachment of materials such as oil or grease ontothe membrane surface.• Such fouling may occur accidentally inmunicipal drinking water system, but is morecommon in industrial applications in which ROis used to treat a process or effluent stream.
CONTROLLING ORGANIC FOULING• Organic Materials can be removed by– Filtration– Carbon Adsorption