给水排水专业英语课件(李田)8

Unit8MixingandFlocculationNewwordsandexpressionscoagulatev.凝结Bloodcoagulateswhenitmeetsair.adj.凝结的settlingn.沉淀沉淀作用沉淀物dispersev.(使)分散,(使)散开,疏散Thepolicedispersedthecrowd.instantaneousadj.瞬间的即刻的即时的aluminiumn.铝hydroxiden.氢氧化物calciumn.钙carbonaten.碳酸盐vt.使变成碳酸盐,使充满二氧化碳Newwordsandexpressionsagglomeratevt.使成团使成块使凝聚gradientn.坡度梯度adj.倾斜的pilotplant试验工厂中试装置collisionn.碰撞冲突constraintn.约束,强制,局促geometryconstraintn.几何约束dynamicn.动力的动力学的动态的viscosityn.粘度粘滞性Newwordsandexpressionsshaftn.轴,杆状物theshaftofanarrowstaticmixern.静态混合器Parshallflumen.巴歇尔计量槽ruleofthumbn.经验法则经验方法empiricalrulecollidev.碰撞turbinen.涡轮impellern.推进器叶轮bafflen.挡板导流片v.阻碍vortexn.旋涡,旋风,涡流tapern.锥形,锥度v.逐渐变细,逐渐减少Clearly,ifthechemicalreactionsincoagulatingandsofteningawateraregoingtotakeplace,thechemicalmustbemixedwiththewater.Inthissectionwewillbegintolookatthephysicalmethodsnecessarytoaccomplishthechemicalprocessesofcoagulationandsoftening.Thebasisofwhatwillbepresentedappliestoanyoperationofmixing,flocculating,settling,orfiltering.1-1Mixing,orrapidmixingasitiscalled,istheprocesswherebythechemicalsarequicklyanduniformlydispersedinthewater.Ideally,thechemicalswouldbeinstantaneouslydispersedthroughoutthewater.Duringcoagulationandsofteningthechemicalreactionsthattakeplaceinrapidmixingformprecipitates.2-1Eitheraluminumhydroxideorironhydroxideformduringthecaseofcoagulation,whilecalciumcarbonateandmagnesiumhydroxideformduringthecaseofsoftening.Afterformingtheprecipitates,itisnecessarytobringthemintocontactwithoneanothersothattheycanagglomerateandformlargerparticles,calledflocs.Thiscontactingprocessiscalledflocculationandisaccomplishedbyslow,gentlemixing.2-2Inthetreatmentofwaterandwastewaterthedegreeofmixingismeasuredbythevelocitygradient,G.Thevelocitygradientisbestthoughtofastheamountofmixingtakingplace;thatis,thehighertheGvaluethemoreviolentthemixing.Thevelocitygradientisafunctionofthepowerinputintoaunitvolumeofwater.3-1whereG—velocitygradient(s-1);P—powerinput(W);V—volumeofwater(m3);μ—dynamicviscosity(Pa.s).3-2VPGItisgivenbyFromliterature,experience,laboratory,orpilotplantworkitispossibletoselectanoptimalGvalueforaparticularapplication.ThetotalnumberofparticlecollisionsisproportionaltoG•t0,wheret0isthedetentiontimeinthebasin.TypicaldesignvaluesforGandG•t0willbediscussedelsewhere.4-1Rapidmixingisprobablythemostimportantphysicaloperationaffectingcoagulantdoseefficiency.Thechemicalreactionincoagulationiscompletedinlessthan0.1s;therefore,itisimperativethatmixingbeasinstantaneousandcompleteaspossible.5-1RapidMixRapidmixingcanbeaccomplishedwithinatankutilizingaverticalshaftmixer,withinapipeusinganin-linemixer,Orinapipeusingastaticmixer.Lessefficientmethodsaresometimesused,SuchasParshallflumes,hydraulicjumps,baffledchannels,orairmixing.5-2verticalshaftmixerstaticmixerin-linemixerRapidmixtanksusuallyhavedetentiontimesof10to30swithvelocitygradientsof600to1000s-1.Thevolumeoftherapidmixtankseldomexceeds8m3becauseofmixingequipmentandgeometryconstraints.Themixingequipmentconsistsofanelectricmotor,gear-typespeedreducer,andeitheraturbineoraxialflowimpellerasshowninFig.2-4.6-16-2Theturbineimpellerprovidesmoreturbulenceandispreferredforrapidmixing.Thetanksshouldbehorizontallybaffledintoatleasttwoandpreferablythreecompartmentsinordertoprovidesufficientresidencetime.Theyarealsobaffledverticallytominimizevortexing.6-3Chemicalsshouldbeaddedbelowtheimpeller,thepointofthemostmixing.Somecommonrules-of-thumbarethatthedesignliquiddepthis0.5to1.1timesthebasindiameterorwidth;thattheimpellerdiameterbebetween0.30and0.50timesthetankdiameterorwidth;andthattheverticalbafflesextendintothetankabout10percentofthetankwidthordiameter.6-4Althoughtheymaybelarger,impellerdiametersnormallydonotexceed1mindiameter.Theliquiddepthmaybeincreasedtobetween1.1and1.6timesthetankdiameterifdualimpellersontheshaftareemployed.6-5Whendualimpellersareemployedongeardrivenmixers,theyarespacedapproximatelytwoimpellerdiametersapart.Twostraightbladeturbinesimpartabout1.9timesasmuchpowertothewaterasoneturbinealoneforthesamemotorpower.6-6FlocculationWhilerapidmixisthemostimportantphysicalfactoraffectingcoagulantefficiency,flocculationisthemostimportantfactoraffectingparticle-removalefficiency.Theobjectiveofflocculationistobringtheparticlesintocontactsothattheywillcollide,sticktogether,andgrowtoasizethatwillreadilysettle.7-1Enoughmixingmustbeprovidedtobringtheflocintocontactandtokeeptheflocfromsettlingintheflocculationbasin.Toomuchmixingwillsheartheflocparticlessothattheflocissmallandfinelydispersed.Therefore,thevelocitygradientmustbecontrolledwithinarelativelynarrowrange.7-2FlexibilityshouldalsobebuiltintotheflocculatorsothattheplantoperatorcanvarytheGvaluebyafactoroftwotothree.Theheaviertheflocandthehigherthesuspendedsolidsconcentration,themoremixingisrequiredtokeeptheflocinsuspension.7-3SofteningflocisheavierthancoagulationflocandthereforerequiresahigherGvalue.Anincreaseintheflocconcentration(asmeasuredbythesuspendedsolidsconcentration)alsoincreasestherequiredG.7-4Fl