5 1748. J.A. Nollet discovers osmosis phenomena. HISTORY1748. J.A. Nollet discovers osmosis phenomena.1855. A. Fick enunciates a law to describe membrane diffusion. 1887, J.A. Van’t Hoff expose first theorical explanations.Gibbs adduce a scientific base these theories are completed. However, osmosis is banished from one’s mind.
6 HISTORY: Desalination First attempt, J.E. Breton and C.E. Reid who is recognized as inventor of reverse osmosis. Nevertheless, they obtain low volume of drinking water.1960. S. Loeb and S. Sourirajan prepare cellulose acetate membranes.1962. Pilot plant in California. 1965, 4th june: Incorporation to water supply.
7 1963. U. Merten propound mathematical equations which describe solute and solvent flow across membrane.1968. J. Westmoreland and D.T. Bray design and patent spiral configuration.1917. Patent of aromatic polyamide.First industrial plants to produce drinking water appear in the second half of 70’s.
8 There are some process which both solvent and solute are recovered. WHY IS USEFUL?There are many applications for reverse osmosis. These can be included in two general groups:Recovery of solventRecovery of soluteThere are some process which both solvent and solute are recovered.
9 Production of drinking water Treatment of urban waste water SOME USESProduction of drinking waterTreatment of urban waste waterProduction of water for industrial usesTreatment of different wastesConcentration of fruit juices, white of an egg, whey...Fermentation
10 PRESSURE RETARDED OSMOSIS This process enables to generate energy from a concentration difference.The water flow at a pressure water P < Power per unit membrane area:Practical problems:Salt fluxConcentration polarisationfresh waterturbinesaline water
11 PRESENT AND FUTUREReverse osmosis has have a high increase since 60’s. This increase has been promoted by its application to desalation and waste treatmentNowadays water market hold majority economic resources for investigation in osmosis field.
12 PRESENT AND FUTURERemoval and selective concentration of certain substances are less developed than production of clean waterHowever, their low development may involve espectacular innovations and new applications during next years.
13 PARAMETERS Qa Ca Qp Cp Pa pa Pp pp Qr Cr Pr pr Q - Flux C - ConcentrationP - Pressurep - Osmotic pressure
14 A, permeability coeficient (for T and salinity fixed) [m3/d·m2·bar] PARAMETERSA, permeability coeficient (for T and salinity fixed) [m3/d·m2·bar]Y, recoveryR, rejection
15 Fc, concentration factor PARAMETERSPs, pass of saltsFc, concentration factor
16 There are two forces which rule solvent and solute fluxes: EQUATIONS OF PROCESSThere are two forces which rule solvent and solute fluxes:Solvent: Pressure gradientJa = A(DP - Dp)Solute: Concentration gradientJs = B·DC + M·JaCm
21 SOLVENT FLUX AND REJECTION Solvent flux and rejection are two important parametersStudy of these and their relation with other parameters are useful to optimize the process. Some of these parameters are pressure, Ca, feed temperature, recovery, feed pH.
27 Is the most important application of reverse osmosis DESALINATIONIs the most important application of reverse osmosisThere are many osmosis desalination plants constructed or in process of constructionIn 1987, reverse osmosis represented 25% of total worldwide desalination capacity by all methods
28 FEED CONSIDERATIONS (DESALINATION) Depending water source:SuperficialWellSalinity of water used. Feed of process is usually seawater or brine:Seawater: g/l Brine: g/l
29 PRINCIPAL SECTIONS: DESALINATION Water feedPretreatmentReverse osmosisComplementary treatment
32 TECHNOLOGY COMPARISON There are another technologies as destilation and electrodialysis to desalinate waterComercialy, destilation is used for seawater, electrodialysis for saltwater and reverse osmosis for both kind of water
36 Membrane SelectionMembrane accounts for 15 to 40 percent of the price in reverse osmosis.Membranes must be replaced periodicallyCAREFUL MEMBRANESELECTION IS ESSENTIALSELECTION CRITERIA:Chemical toleranceMechanical suitabilityPriceCleanibilitySeparation performanceGOOD DESIGN:Consistent performanceNeeds less frequent membrane cleaningReasonable consum of powerLittle operational attention
37 Pore size in reverse osmosis Salts and low molecular weight compounds
38 Pressure and flux range Membrane processPressure range (bar)Flux range (l/m2·h·bar)Microfiltration0,1 - 2,0>50Ultrafiltration1,0 - 5,010 – 50Nanofiltration5,0 – 201,4 - 12Reverse Osmosis0,05 - 1,4The pressures used in reverse osmosis range from 20 to 100 bar and the flux from 0,05 to 1,4 l / m2·h
39 Features of membranes in RO In contrast to MF and UF the choice of material directly influences the separation efficiency through the constants A and B.A hydrodynamic permeability coefficientB solute permeability coefficientThe types of membranes used are porous, and it can be asymmetric or composite. Usually they are formed by a toplayer (about 1 mm) and a sublayer (150 mm)The flux through the membrane is as important as its selectivity towards various kinds of solute. The flux is approximately inversely proportional to the membrane thickness, and for this reason membranes have an asymmetric structure.The resistance towards transport is determined mainly by the dense toplayer.The function of the sublayer is mainly support.
40 Types of membranes (I) INTEGRAL MEMBRANES Both toplayer and sublayer consist of the same material.Materials used: cellulose esters (especially diacetate & triacetate), polybenzimidazoles.Prepared by phase inversion techniques.COMPOSITE MEMBRANESToplayer and sublayer composed of different polymeric materials such as aromatic polyamidesThe support material is commonly polysulfones while the thin film is made from various types of polyamides, polyureas, etcPrepared by dip coating, in-situ and interfacial polymerization.
41 CTA membranes (Cellulose TriAcetate) Main Membrane TypesCTA membranes (Cellulose TriAcetate)Low stability against chemicals, temperature and bacteria.Typically needs a sediment prefilterLow duration (18-24 months)High sensibility to hydrolysisHigh work pressuresHigh permeabilityAllow contact with chlorine in water.Low costHigh removal percentage of sales %
42 TFC membranes (Thin Film Composite) Main Membrane TypesTFC membranes (Thin Film Composite)Synthetic membranesHigher rejection to many chemicals than CTA membranesHigh duration (3-5 years)High removal percentage 94-99%Low work pressures.High chemical stabilityHigh sensibility to the oxidants and chlorine.Typically needs a carbon prefilter.Easy foulingHigh cost
44 Types of membranes (II) Very low pressure membranes:Work pressures between 5-10 bars used to desale water with a salt content in a range of mg/lDesigned to compete with the ion exchange resinsLow pressure membranesRange pressure: bars Salt content ( mg/l)Desalation of water or removal of compounds such as nitrates, organic substances ..Medium pressure membranesRange pressure: bars Salt content ( mg/l)At the beginning used in desalation of waters with high salt content, but now used in multiple processes of separation and concentration.High pressure membranesConceived to obtain potable water from the sea water in a single pass.Osmotic pressure of sea water (till 35 bar in the Red Sea) Range pressure (50-80 bar)Recommendations of O.M.S (salt < 500 mg/l) salt reject 99%
45 Prices of membranes CTA Membranes TFC Membranes Model l/m2 · h Size Mem-S-253,941.8" x 12"69$Mem-S-406,302.0" x 12"79$Mem-S-609,5089$Mem-S-9014,299$TFC MembranesModell/m2 · hSizePriceXTFCH-15023,72.5" x 14"112$XTFCH-35055,12.5" x 21"174$XTFCH-750118,32.5" x 40"247$XTFCH-1000157,74.0" x 21"361$XTFCH-2000315,44.0" x 40"489$XTFCH-2600410540$
46 How to prepare RO membranes Phase inversion techniquesChemical PI is a process whereby a polymer is transformed in a controlled manner from a liquid to solid state. The process is very often initiated from the transition from one liquid state in two liquids. One of the liquid phases (the high polymer one) will solidify forming a matrix.Immersion Precipitation is the most used PI technique currently specially for flat or tubular membranes. The only requirement is that the polymer must be soluble in a solvent or a solvent mixture.
47 How to prepare RO membranes Dip coatingVery simple and useful technique for preparing composite membranes with a very thin but dense toplayer.An asymmetric membrane (often of the type used in UF) is immersed in the coating solution where a thin layer adheres to itThe film is put in a oven where the solvent evaporates and crosslinking occurs.
48 How to prepare RO membranes Interfacial polimerisationA polymerisation reaction occurs between two very reactive monomers at the interface of two immiscible solvents.Support layer (usually a UF membrane) immersed in an aqueous solution. Film immersed in a water immiscible solvent.Reaction of the two monomers forms a dense polymeric toplayer.The process takes advantage of the self-inhibiting character of the reaction.
49 Polarisation & Membrane fouling Performance in RO is diminished by polarization and fouling phenomena.Polarisation: reversible processes related with the increase of concentration over the bulk when we are closed to the membrane.Fouling: deposition of retained particles, colloids, macromolecules, salts, etc … on or in the membrane. Typically more important in micro and ultrafiltration, but it becomes important also when we use hollow fiber or spiral wound configurations.
50 Fouling description and tests The cake layer resistance can be written from a mass balance as:Now the flux may be written as:or
51 MFI fouling test: Integrating the before expression: The use of MFI values has the next advantages:By comparing various solution, different fouling behavior can be observedA maximum allowable MFI value can be given for a specific plantFlux decline can be predicted to some extentOther tests used: silting index (SI), plugging index (PI), silt density index (SDI)
52 Reducing foulingPretreatment of the feed solution: ph-adjustement, pre-microfiltration, pre-ultrafiltration, adsorption onto active carbon …Membrane properties: use of hydrophilic membranes and dense membranes reduces the fouling.Module and process conditions: fouling decreases as concentration polarization decreases. In this order is convenient to work with high flow velocities.Cleaning:HydraulicMechanicalChemicalElectric
53 Cleaning needsAs a general rule is necessary to realize a cleaning of the membranes when one of the next situations occurs:Salts filtration increases in more than 15% over the last value.Production changes (increase or decrease) in more than 10%Reject flow changes in more than 10%Drop pressure in the modules increases in more than 20%Feed pressure increases in more than 10%Long plant stops (> 1week)Before applying any reactant to regenerate membranes.
54 Chemical cleaningProblem identification: the chemical product to be used will depend on the nature of the deposit, so the first thing to do is identify the type of fouling.Types of foulingPrecipitations: mineral salts and metallic oxidsDeposits: high size particles, colloids, biological developmentsOthers: colloidal sulfur, oils, organic compounds ..Fouling effectsWe can also identify the effect produced by the effects produced in the pressure drop of the equipment, the flow permeate and the filtrate of salts.
59 Membrane configurations in RO (I) Spiral-wound configurationNext logical step from a flat membrane but with higher packing density300 – 1000 m2/m3Permeate is collected in the central tube
60 Membrane configurations in RO (III) Tubular configurationNot self supporting in contrast to hollow fiber –modulesPermeate crosses the membrane layer to the outsideLow surface-volume ratioUsually the active layer is inside
61 Membrane configurations in RO (II) Capillary/hollow fiber configurationFibers diameter: <0.5 mm• Flux velocity: low (up to 2.5 m/s)• Feed: inside-out or outside-in• Surface area/volume: high• Pressure drop: low (up to 1 bar)• Maintenance: hard• Cleaning: poor
62 Comparison of module configurations TubularPlate and frameSpiral-woundCapillaryHollow-fiberPacking densityLOWVERY HIGHInvestmentHIGHFouling tendencyCleaningGOODPOORMembrane replacementYESNO