文稿分析课件module3v

1、42991: Advanced water and wastewater treatmentModule 3: Design of a Seawater Desalination PlantDr Hokyong ShonContentsIntroduction to NF/RODesign of NF/ROCase studies: - Sydney Desalination plantVideo: Sydney desalination plantSome slides are from Greg and Troy (2012)PollenRelative size of common ma

2、terialsm0.00010.0010.010.1110100ionicmolecular1000gelatinlatex / emulsionmilled flourPollendPmacromolecularmicroparticlemacroparticlesugarasbestospollentobacco smokeyeast cellsmetalionredbloodcellsendotoxinvirusesbacteriaalbumin proteincarbon blackaqueoussaltspaint pigmenthuman hairbeach sandactivat

3、edcarbonindigo blueatomicradiussyntheticdyeNF/RO membraneRange RORange NFNF/RO membraneNon-porousMembrane thickness increase = permeate flux decreaseActive (aka separation or skin) layer as thin as possibleMembrane structureAsymmetric (some NF membranes)Thin film composite (most NF and all RO membra

4、nes)Ultra thin dense skinPolysulfon-supportBacking layerSEM-PhotoTFC NF/RO membrane videoThe skin layer can be as thin as 20 nmTransport phenomenaMass transport across the membrane is diffusiveOsmosisConcentration polarisationPermeate fluxLimited by osmotic pressure and concentration polarisation (a

5、part from intrinsic membrane resistance & fouling/scaling) Osmotic pressureOsmotic pressureRO = Reverse OsmosisFO = Forward OsmosisCompoundConc. (mg/)Osmotic pressure (kPa)NaCl1000100MgSO4100025Sucrose10001.5Seawater350002700Osmotic pressurevant Hoff equation(in 1901Jacobus Henricus vant Hoff reciev

6、ed the 1st Nobel Prize in Chemistry for his work with solution) Osmotic pressure caused by component i:i=ciRTwhere is the osmotic pressurec is concentration (mol/L)R (the gas constant): 0.0821 (L.atm.mol-1.K-1)T is the solution temperature (in K)Be careful with dissociated solutes!Osmotic pressureSe

7、awater desalinationOsmotic pressure of seawater 30 barEnergy consumption 3 times the theoretical limit (3.5 kWh/m3)Recovery = (permeate flow)/(feed flow)FeedCF = 36,000 ppmBrineQB = 0.5 QFPermeateCP bulk conc. (a distance away from the membrane surface)Diffusion from the membrane surface back to the

8、 bulk solutionBack diffusion can be enhanced by a cross flow (high shear rate)All NF/RO process are in cross flow mode to limit the CP effectNF/RO membraneLower permeate flux (10 times less than MF/UF)Need high packing density module (either hollow fiber or spiral wound)Higher pressure (5 60 bar)Hol

9、low fiber is self supportedSpiral wound for all commercial NF/RO membranesGood or bad?NF/RO membraneStandardisation in module designBW4040: BW = brackish water; 40 = 4” in diameter; 40 = 40” in lengthSW2540: SW = seawater; 25 = 2.5” in diameter; 40 = 40” in lengthGoodCompatibility (pressure vessels,

10、 endcaps, etc)Simple designR&D benefits all end users/manufacturersBadLack of diversityNF/RO membranesPressure vesselsFiber glass or stainless steelUp to 8 spiral wound elements in a vesselVessels can be arranged into banks with desired number of stages or passesEasy just like a lego game!Operationa

11、l considerationsLearning objectivesUnderstand key design constraintsDifferentiate operational issues between different types of pressure driven membranesMembrane foulingHydraulic considerations in a membrane systemHollow fiber vs spiral woundIntroductionAn ideal operational regimeContinuous operatio

12、nZero or low maintenanceFully automatedFlexibility (can change flux/overall rejection)A real membrane filtration systemMembrane foulingDisruptive hydraulic conditionsRequire expert knowledgeNot flexible at all (but nevertheless modular)Module & Operating PressureTypical operating pressureHollow fibe

13、rUsed for low pressure membranesMF, UF (and possibly NF)Typically mechanically unsupportedMF & UF require backwash & chemical cleaningAs frequent as every 30 mins for MF (over the membrane lifetime of approx 5 yrs!) Mechanical stressWater hammerFiber breakageHollow fiberOther causes of fiber breakag

14、eDebris in feed water due to inadequate pretreatmentSharp metal foils introduced during constructionConsequence: feed water preferentially enter the broken lumenDecrease permeate qualityIncrease permeate flux Can be readily identifiedSpiral woundA pressure vessel typically houses several elementsFlu

15、x distribution?Pressure distribution?Spiral wound(Wilf, 2007)Permeate flux distribution & rejection vs element position in a pressure vesselSpiral wound - DiscussionEight perfectly identical elements butOperating condition?From what we know beforeFirst element: if permeate flux (j) is too high ?Last

16、 element: if cross flow velocity (v) is too low ?Design implicationRange ?Spiral woundPressure profile of a practical systemSpiral woundPressure profile of a practical system with booster pumpsColloidal/organic foulingScaling (silica or insoluble salts)Spiral woundExcessive differential pressure can

17、 cause structural damageMembrane telescoping!Spiral wound - innovationInternally stage design(Spiral wound elements have been standardised)Front elements with low permeability & high rejectionBenefitsBetter overall rejectionMore uniform flux distributionContentsIntroductionDesign objectives & constr

18、aintsSystem performanceMembrane/Operational constraintsTerminologyROSA designing exerciseReview of Assignment 3ASome slides from Iwes, 2012ObjectivesProduction volumePermeate qualitySystem flexibilityVariation in water demandDown-time for maintenanceVariation in feed water qualityMembrane utilizatio

19、nConstraintsMembrane constraintsElement recoveryFeed flow rate, retentate flow rate, and permeate flow rateDifferential pressureFeed water parametersFree chlorine, turbidity or SDI, hardness (i.e. Ca, Mg)Water temperatureTerminology used in RO systemMembrane element (spiral wound)Feed (Qf, Cf)Permea

20、te (Qf, Cf)Retentate/Reject/Concentrate/Concentrated brine(Qr, Cr)ROTerminology used in RO systemMembrane elementFeedPermeateRetentateRecirculationFeedPermeateRetentateTerminology used for RO systemTotal dissolved solids (TDS): Sum of concentrations of all dissolved ions in water (in ppm or mg/L)Uni

21、ts for TDS: parts per million (ppm) mg/L g/m3Osmotic pressure (): Thermodynamic property of the solution = cRTWhere : Activity coefficientc: concentrations (mol/L)R: Universal gas constant (0.0821 L.atm/mol/K)T: Absolute solution temperature in Kelvin (273+t)Transmembrane pressure: Pressure applied

22、to the membrane by the pump Net driving force: Net driving force that forces water through the membraneTerminology used for RO systemAverage permeate fluxCombined permeate flow divided by the total membrane area in the RO unit. It measures the volume of water that passes through the membrane per uni

23、t membrane area in a unit time (L/m2/h)Feed (Qf, Cf)Permeate (Qf, Cf)Retentate/Reject/Concentrate/Concentrated brine(Qr, Cr)ROTerminology used for RO systemFeed (Qf, Cf)Permeate (Qf, Cf)Retentate/Reject/Concentrate/Concentrated brine(Qr, Cr)ROSalt Passage Percentage of permeate salt concentration to

24、 feed salt concentration Terminology used for RO systemFeed (Qf, Cf)Permeate (Qf, Cf)Retentate/Reject/Concentrate/Concentrated brine(Qr, Cr)ROSalt Rejection Percentage of retentate salt concentration to feed salt concentrationSR (%) = (Cc/Cf)*100 = 100 - SPTerminology used for RO systemFeed (Qf, Cf)

25、Permeate (Qf, Cf)Retentate/Reject/Concentrate/Concentrated brine(Qr, Cr)RORecovery rate (R) Fraction or percentage of feed converted to permeateTerminology used for RO systemFeed (Qf, Cf)Permeate (Qf, Cf)Retentate/Reject/Concentrate/Concentrated brine(Qr, Cr)ROConcentration factorIncrease in salt co

26、ncentration in the RO system due to permeation of water and rejection of salts. Builds up salt concentration at the membrane surface.Terminology used for RO systemFeed (Qf, Cf)Permeate (Qf, Cf)Retentate/Reject/Concentrate/Concentrated brine(Qr, Cr)ROConcentration Polarization factor ()Increase in fe

27、ed salinity adjacent to the membrane surface due to water transport across the membrane while leaving behind salts. TerminologyMembrane elementFeedPermeateRetentatePressure/Housing vessel: (several elements in one vessel)Each vessel has permeate and concentrate portFlow dynamics in RO elementFlow dy

28、namics for a single elementFeed 8 m3/hPermeate 1 m3/hRetentate 7 m3/hR = 12.5 %Two elements in seriesR1 = 12.5 % R2 = 10.9%Rejection of second elementR2 0.125 x (1-0.125) = 0.109 = 10.9%Normally each RO vessel/housing contains about 6 to 7 elements in seriesTerminology used in RO systemRO system tra

29、inSingle stage/pass system2 Stage RO system2 pass RO systemFeed RetentatePermeateRO VesselRO VesselRO VesselSingle stage/pass systemTerminology used in RO systemTwo-stage RO systemRetentate of first vessel goes to second vesselHigher recovery rates achievedSecond vessel receives feed with higher TDS

30、 or Feed RetentatePermeateRO VesselStage 1Stage 2RO VesselRO VesselRO VesselRO VesselRO VesselTerminology: Schematic of a three-stage RO trainTerminology used in RO systemTwo pass RO systemPermeate of first vessel goes to second vessel (2 pass)Higher permeate quality is achievedApplied when RO membr

31、ane rejection is low & permeate does not meet the target qualityFeed RetentatePermeateRO VesselPass 1Pass 2RO VesselRO VesselRO VesselRO VesselRO VesselTerminology used in RO systemPressure exchange Parameters for design of RO plants (manual)Plant capacity (m3/day)Feed water qualityTDS or Membrane p

32、roperties Average permeate flux (APF)Salt rejection rate (SR)Operating parametersFeed flow rate (Qf)Recovery rate (R)Basic RO design by manual calculations: ExampleExample of Wastewater Reuse RO Plant Assume each train has a capacity of 300 m3/day. Use ROSA to size the RO feed pump for one train. Wh

33、at conditions do you use? Calculate the number of pressure vessels, elements per pressure vessel, total number of elements, etc. ROSA ExerciseBW30-400 membraneFeed waterSecondary sewage effluentTDS 1800 ppmWater temperature 25oCProduction volumeApproximately 300 m3/dReverse Osmosis System Analysis (

34、ROSA)ROSA ExerciseStep 1: Consider Feed Source, Feed Quality, Feed / Product Flow, and Required Product QualityStep 2: Select the Flow Configuration and Number of PassesStep 3: Select Membrane and Element TypeStep 4: Select Average Membrane Flux (Design Flux)Step 5: Calculate the Number of Elements

35、NeededStep 6: Calculate the Number of Pressure Vessels NeededStep 7: Select the Number of StagesStep 8: Select the Staging Ratio (Array Ratio)Step 9: Balance the Permeate Flow RateStep 10: Example Step 1Step 1: Consider Feed Source, Feed Quality, Feed / Product Flow, and Required Product QualityThe

36、membrane system design depends on the available feed water and the application. The system design information and the feed water analysis should, therefore, be collected first.A) Choose feed water source (e.g. well water with SDI 3, surface water with SDI 5, etc.)B) Choose overall feed water concent

37、ration in TDS (ppm) or individual (specific) ions.C) Individual ion concentration from water analysis is always preferred.Step 2Step 2: Select the Flow Configuration and Number of PassesThe standard flow configuration for a membrane system is plug flow, where the feed volume is passed once through t

38、he system.An RO / NF system is usually designed for continuous operation and the operating conditions of every membrane element in the plant are constant with timeA permeate staged (double pass) system is the combination of two conventional RO systems where permeate of the first system (first pass)

39、es the feed for the second system (second pass). Step 3Step 3: Select Membrane and Element TypeElements are selected according to feed water salinity, feed water fouling tendency, required rejection and energy requirements. The standard element size for systems greater than 2.3 m3/hr (10 gpm) is 8-i

40、nch in diameter and 40-inch long. Smaller elements are available for smaller systems.For high quality water applications where very low product salinity is required, ion exchange resins are frequently used to polish RO permeate.Step 4Step 4: Select Average Membrane Flux (Design Flux)RO / NF systems

41、are usually designed for a specific permeate flow rate (GPD or l/h) and a specific system recovery. These numbers, and the specific feed water source, are the information required to estimate the number of membrane elements, pressure vessels and stages as flows:Select the design flux (GFD or l/m2h)

42、based on pilot data, customer experience or the typical design fluxes according to the feed source from the Membrane System Design Guidelines.Step 5Step 5: Calculate the Number of Elements Needed Total number of elements needed = (design permeate flow rate) / (design flux) / (active membrane surface

43、 area of selected element)Tip: For 8-inch elements, model number indicates active membrane surface area. (e.g. DOW DOW FILMTEC BW30-400 element has 400 ft2 (37.2 m2) of active membrane surface area.Step 6Step 6: Calculate the Number of Pressure Vessels NeededA) Total number of pressure vessels neede

44、d = (total number of elements) / (number of elements in pressure vessel)B) Round up to the nearest integer.C) For large systems, 6-element vessels are standard, but vessels with up to 8 elements are available. For smaller and / or compact systems, shorter vessels may be selected.D) Although the appr

45、oach described in the following sections apply for all systems, it is especially applicable for 8-inch systems with a larger number of elements and pressure vessels, which then can be arranged in a certain way. Small systems with only one or a few elements are mostly designed with the element in ser

46、ies and a concentrate re-circulation for maintaining the appropriate flow rate through the feed / brine channels.Step 7Step 7: Select the Number of StagesTable 1 Number of Stages of a Brackish Water SystemTable 2 Number of Stages of a Seawater SystemStep 8Step 8: Select the Staging Ratio (Array Rati

47、o)For a system with four vessels in the first stage and two vessels in the second stage the staging ratio is 2:1. A three-stage system with four, three, and two vessels in the first, second, and third stage respectively has a staging ratio of 4:3:2. In brackish water systems, staging ratios between

48、two subsequent stages are usually close to 2:1 for 6-element vessels and less than that for shorter vessels. In two-stage seawater systems with 6-element vessels, the typical staging ratio is 3:2.Step 9Step 9: Balance the Permeate Flow RateThe permeate flow rate of the tail elements of a system (the

49、 elements located at the concentrate end) is normally lower than the flow rate of the lead elements. This is a result of the pressure drop in the feed / brine channel and the increase of the osmotic pressure from the feed to the concentrate. The ratio of the permeate flow rate of the lead element an

50、d the tail element can e very high under certain conditions:- High system recovery- High feed salinity- Low pressure membranes- High water temperature- New membranesThe goal of a good design is to balance the flow rate of elements in the different positions. This can be achieved by the following mea

51、ns:- Boosting the feed pressure between stages: preferred for efficient energy use- Apply a permeate backpressure only to the first stage of a two-stage system: low system cost alternative- Hybrid system: use membranes with lower water permeability in the first positions and membranes with higher wa

52、ter permeabilities in the last positions: e.g. high rejection seawater membranes in the first and high productivity seawater membranes in the second stage of a seawater RO systemThe need for flow balancing and the method used to balance the flow can be determined after the system has been analyzed w

53、ith ROSA (Reverse Osmosis System Analysis).Given Conditions: Feed source: brackish surface supply water, SDI 5 Required permeate flow = 720 m3/d 6-element pressure vessels to be used 75% recoverySteps: Brackish surface supply water with SDI 5; total permeate flow = 720 m3/d Select plug flow FILMTECB

54、W30-365 element (BW element with active membrane area of 33.9 m2 mended average flux for surface supply water feed with SDI 95%Membrane area of each element (8040)= 37 m2 Operating parametersApplied pressure = 10 barFeed flow rate Qf = 300 m3/dayRecovery rate (R) = 80%Applied pressure = 10 barCalcul

55、ate the following No. of RO elements required TDS or the permeateVolume of retentate or concentrate produced per dayRO System Design SoftwareRO System Design SoftwareWater quality inputsPhysical propertiesTemperature: Influences viscosity and determines pumping requirementspH and alkalinity: Propert

56、ies are linked. pH control is critical to control pretreatment (coagulation) and scale formation. Sufficient alkalinity is a requirement to control pHTotal Dissolved Solids (Default input): Influence RO operating pressureInorganic inputsCalcium (Ca2+) and Magnesium (Mg2+): influences scale formation

57、Sodium (Na+) and Potassium (K+): Influences osmotic pressure, corrosion and sodium adsorption ratio (irrigation applications)Barium (Ba2+) & Strontium (Sr2+): slightly soluble metals, special problem for RO due to sulfate salt solubility limits.Silica: present in water as silicate SiO2 slightly solu

58、ble. Fouls RO at concentration (150-200 ppm)Bicarbonate (HCO3-): alkalinityCarbonate (CO3-): alkalinityChloride (Cl-): monovalent halideHydrogen Sulfide (H2S) Sulfate (SO42-): divalent ion of sulfur and oxygenSilt Density Index (SDI)SDI is calculated number based on filtration of a sample through a

59、0.45 micron filter pad. SDI readings are used to indicate the “fouling” tendency of a water for use as RO feedwater.SDI is calculated by following formula:Where T1 is time in seconds to filter initial 500 ml of sample andT2 is time in seconds to filter final 500 ml of sampleand 15 minutes is allowed

60、 to pass between timed sample intervals.Important for RO membrane warranty.RO System Design SoftwareCarbonate scaling potential may be assessed by LSILangelier Saturation index (LSI) Scale Potentialnegative less than zero: No scale potential. Water will dissolve CaCO3.positive greater than zero: Sca