Analysis-Synthesis-and-Design-of-Chemical-ProcessesChapter-9利用经验准则确认过程设计的适用性

Chapter9

UtilizingExperience-basedPrinciplestoConfirmtheSuitabilityofaProcessDesign.

利用经验准那么确认过程设计的适用性

王卓9.1

TheroleofexperienceinthedesignprocessCheckingnewprocessdesignProvidingequipmentsizeandperformanceestimatesHelpingtotroubleshootproblemswithoperatingsystemsVerifying〔核实〕thereasonablenessofresultsofcomputercalculationsandSimulationsProvidingreasonableinitialvaluesfortheinputtoaprocesssimulatorrequiredtoachieveprogramconvergenceObtainingapproximatecostsforprocessunitsDevelopingpreliminaryprocesslayouts.Experiencedchemicalengineershaveformulatedanumberofexperienced-basedshort-cutmethodsandguidelinesthatareusedfor:1/19〔a〕正三角形排列〔b〕正方形排列换热管的排列：9.1.1IntroductiontoTechnicalHeuristicsandShot-CutMethods排列紧凑，管外流体湍动程度高，给热系数大。排列比较松散，给热效果较差，但管外清洗方便，对易结垢液体更为适用。2/19多组分精馏的FUG简捷算法：用芬斯克〔Fenske〕公式估算最少理论板数用恩特伍德〔Underwood〕公式估算最小回流比用吉利兰〔Gilliland〕图估算实际回流比下的理论板数用Kirkbride公式近似确定进料位置3/19化工常用金属材料特性：materialAdvantagesDisadvantagesCarbonSteel价格便宜，方便易得，耐磨损，制造标准，耐碱抗酸性比较差，容易变色和污染StainlessSteel耐大多数酸，不易变色，磨损小于低碳钢不耐氯化物腐蚀，更贵，制造更难达标，合金材料可能发生催化作用Monel-Nickel很少变色、污染，耐氯化物腐蚀不耐氧化环境，比较贵Hasteloy(哈氏合金)从Monel-Nickel上改良二来比Monel-Nickel还贵OtherExoticMetals具有特殊性能一般都非常贵4/199.1.2MaximizingthebenefitsobtainedfromexperienceAnengineermustbecapableoftransferringknowledgegainedfromoneormoreexperiencestoresolvefutureproblemssuccessfully.Experiencedengineersretainsabodyofinformation,madeuplargelyofheuristicsandshort-cutcalculationmethods,thatisavailabletohelpsolvenewproblemsTheprocessbywhichanengineerusesinformationandcreatesnewheuristicsconsistsofthreesteps:Predict、Authenticate(验证)、Re-evaluate，theyformthebasisofthePARProcess.PredictAuthenticateRe-evaluate5/19Example9.1估测一下温度为93℃，流速3.05m/s的水在直径为38mm管内流动的传热系数。根据以往经验，我们知道21℃流速1.83m/s的水在相同管内的传热系数为5250W/m2℃。下面用PAR过程分析：Step1—predict:假设流速和温度没有影响，预测传热系数为5250W/m2℃Step2—Authenticate/Analyze:利用下表物性数据计算ReynoldNumber(雷诺数):property21℃93℃Rateof(new/old)ρ(kg/m3)997.4963.20.966K(W/m℃)0.6040.6781.12Cp(kJ/kg℃)4.194.201.00μ（kg/m.s）9.8*10^-43.06*10^-40.3126/19利用Sieder-Tate方程验证假设: 两个条件下的传热系数之比：

显然，最初的假设，流速和温度没有影响是不对的。Step3—Re-evaluate/Rethink:对假设进行改进：1.温度对黏度的影响的影响必须考虑 2.温度对Cp, 和k的影响可以忽略 3.管径对h的影响不大 4.结果仅限于Sieder-Tate方程的适用范围内 ρ7/19根据上述假设，把21℃下的数值代入方程〔1〕，即得到估测水在38mm管内流动传热系数的经验式： foru’(m/s),(kg/ms)Theheuristicsorrulesarecontainedinanumberoftables(9.2-9.18)andapplytooperatingconditionsthataremostoftenencountered.TheinformationprovidedusedinthefollowingexampleandshouldbeusedtocheckanyinformationonanyPFD.9.2Technicalheuristicsintheapplicationof chemicalequipment8/19Example9.2参考第一章甲苯加氢工艺图1.3，表1.5和表1.7所给数据，估算下面设备尺寸和其它一些操作参数：V-101E-105C-101T-101下面分别对这几个设备进行估算：V-101—High-PressurePhaseSeparatorVaporflow=stream8=9200kg/h,P=23.9bar,T=38℃,Liquidflow=stream17+18=11570kg/h,P=2.8bar,T=38℃

ρv=8kg/m3andρl=8kg/m3(表1.7)Question:怎样估算V-101的尺寸〔直径和高度〕？9/19Weusethefollowingheuristics:Rule1

Gas/liquidphaseseparatorsareusuallyvertical.Rule2Optimumlength/diameter=3,buttherange2.5to5is common.Rule3Holdtimeis5minforhalf-fullgas/liquidseparators,5-10 minforaproductfeedinganothertower.Rule4Gasvelocityingas/liquidseparators,u=k(ρl/ρv-1)0.5

m/s K=0.0305forvesselswithoutmeshentrainers.Rule5Goodperformancecanbeexpectedatvelocitiesof30- 100%ofthosecalculatedwiththegivenk;75%ispopular.10/19FromRule4，u=0.0305(850/8-1)0.5=0.313m/sFromRule5,uact=0.75u=0.75*0.313=0.23m/sMassflowrateofvapor=9200/3600=2.56kg/s=uρvπD2/4 D=1.33mFromRule3,volumeofliquid=0.5LπD2/4=0.726Lm35minutesofliquidflow=5*60*11570/850/3600=1.13m30.726L=1.13 L=1.56mFromRule2,L/Dshouldbeinrange2.5to5.ForourcaseL/D=1.56/1.33=1.17,isoutofrange.WeshouldchangetoL=2.5D=3.3m11/19Conclusion:HeuristicsabovesuggestthatV-101shouldbeaverticalvesselwithD=1.33m,L=3.3mFromTable1.7,weseethattheactualV-101isverticalvesselwithD=1.1m,L=3.5mWeshouldconcludethatthedesignofV-101inChapter1isconsistentwiththeheuristics.b.E-105Productcooler热物流：105℃38℃Q=1085MJ/h=301kw(表1.7)

Question:怎样估算换热器E-105的传热面积？12/19Weusethefollowingheuristics:Q=KAFΔTmRule1ForconservativeestimatesetF=0.9forshellandtube exchangerswithnophaseChanges.Rule2Coolingwaterinletis30℃,maximumoutlet45℃。Rule3Heattransfercoefficientsforestimatingpurposes,W/m2℃： Watertoliquid,850;condensers,850;liquidtoliquid,280; liquidtogas,60;gastogas,30;reboiler1140.Rule4Double-pipeexchangeriscompetitiveatdutiesrequiring2FromtheRulewecanset:F=0.9，K=850W/m2℃Waterentersat30℃andleavesat40℃13/19ΔTm=[(105-40)-(38-30)]/ln[(105-40)/(38-30)]=27.2℃A=Q/KΔtmF=301000/850/27.2/0.9=14.46m2FromRule4,thisexchangershouldbeadouble-pipeormultiple-pipedesign.Comparingtheresults:Heuristic:Double-pipedesign,Area=14.46m2Table1.7:Multiple-pipedesign,Area=12m2Theheuristicanalysisisclosetotheactualdesign.c.C-101Question:怎样估算压缩机的功率？From

Table1.7,flow=6770kg/h,T1=38℃=311K,P1=23.9bar,P2=25.5bar14/19Weusethefollowingheuristics:Rule:Theoreticalreversibleadiabaticpower(理论绝热可逆功率) Wrev,adiab=mz1RT1[(P2/P1)a-1]/a.whereT1=inlettemperature, R=GasConstant=8.314J/molK,z1=compressibility(压缩系数), m=molarflowrate,a=(k-1)/kandk=Cp/Cvm=molarflowrate(Sream5+Stream7)=(758.8+42.6)/3600=0.223kmol/sk=1.41〔assume〕anda=(k-1)/k=0.2908Wrev,adiab=mz1RT1[(P2/P1)a-1]/a=0.223*1.0*8.314*311*[(25.5/23.9)0.2908-1]=37.7kWUsingacompressorefficiencyof75%Wactual=37.7/0.75=50.3KWFromTable1.7,W=49.1KW,theresultsareveryclose.15/19d.T-101xD=0.9962,xW=0.0308,αD=2.44,αW=2.13,F=142.2kmol/h,D=105.6kmol/h，α=〔αDαW〕0.5=2.28Question:怎样估算塔径、回流比、板数、全塔压降？Weusethefollowingheuristics:Rule1：minRule2:Optimumnumberofstagesapproximately2NminRule3:Nmin=ln{[xD/(1-xD)]/[xw/(1-xw)]}/lnα(Fenske方程)Rule4:Rmin=(F/D)/(α-1),whenfeedisatthebubblepoint.Rule5:Useasafetyfactorof10%onnumberoftrays16/19Rule6:Lmax=53m(windloadandfoundationconsiderations)andL/D<30Rule7:Peakefficiencyoftraysisatvaluesofthevaporfactor Fs=uρv0.5=1.2→1.5m/s(kg/m3)0.5Rule8:PressuredroppertrayΔPtray=0.007barRule9:Trayefficienciesfordistillationoflighthydrocarbonand aqueoussolutions(水溶液)are60-90%.UsetheRulesabove,weget:Nmin=ln{[0.9962/(1-0.9962)]/[0.0308/(1-0.0308)]}/ln2.28=10.9Rmin=F/D/(α-1)=(142.2/105.6)/(2.28-1)=1.05RangeofR=(1.2→1.5)Rmin=1.26→1.58Ntheoretical≈2*10.9=21.8εtray=0.6Nactual≈(21.6/0.6)*1.1=40traysρv=6.1kg/m3u=(1.2→1.5)/6.10.5=0.49→0.60m/s17/19Vaporflowrate(stream13)=22700kg/h体积流率v=1.03m3/sDtower=[4v/πu]0