LNG Cyrogenic Heat Exchanger Design Report

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ME492MATERIALSINENGINEERINGDESIGN

April5,2012

MaterialsandProcess

SelectionforaCryogenic

HeatExchanger

GROUP#4DawsonJamesJeffreyPowellJamesStevensonDerekVisvanathan

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ExecutiveSummary

Thereisanincreasingdemandfornaturalgasasitisthecleanestburningofallfossilfuels.

Whenpipelinescannotbeusedtotransportthenaturalgasitisliquefied(liquefaction

temperature163)asithasagreatlyreducedvolume,makingtruckandnavaltransportation

morefeasible.Inordertoachievethelowtemperaturesrequiredtoliquefythenaturalgasa

cryogenicheatexchangermustbeused.Thisreportoutlinestheselectionprocedureforthe

type,materialandprocessesrequiredtofabricateacryogenicheatexchanger.

Aspiralinshellheatexchangerutilizesthintubesfilledwithcoolantwoundontheinsideofa

cylindricalshell.Thisdesignmaximizestheamountofsurfaceareabetweenthetubingandthe

naturalgasallowingforahighheatflowfromthegastothecoolant.Thetubesrequireless

advancedprocessestofabricatethanfinsandthespiralshellheatexchangerrequires

significantlylessmaintenancethanaplateandfinexchanger.Heatexchangersusedforthis

purposearequitelargewithlengthsupto500manddiametersof5m.

Wroughtaluminum2026wasfoundtobethebestmaterialforthetubinginsidetheheat

exchanger.Aluminumalloyshaveahighthermalconductivityresultinginthedesiredheatflow

inthecoolant.Aluminumisalsolessexpensivethannickelalloysandwillnothaveadverse

reactionswiththenaturalgaswhichwasaproblemwithcopperandbrass.Alongwiththe

thermalconductivityandcost,aluminumwillbeabletowithstandthepressuresinsidetheheat

exchangerpassingallobjectives.

Tomaximizeheattransferthinwalledtubingwillbeused.Therearemanyprocessesthatare

abletocreatethethinwalledtubingbutwiredrawingwasfoundtobethebestoption.Wire

drawingcancreatesmallcrosssectionalareasrequiredforthethinwalltubingandis

compatiblewiththeselectedaluminumalloys.Wiredrawingcanalsoyieldtighttolerances.

Anotherprocessthatissuitableisrollforming.Withsimilarspecificationsaswiredrawingitwill

alsoperformtherequiredfunction.Wiredrawinghaslowertoolingandcapitalcoststhanroll

formingandisthereforethemostidealprocess.

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TableofContentsExecutiveSummary........................................................................................................................................i

ReasonsandProcessofLiquefyingNaturalGas...........................................................................................1

IntroductionandDesignStatement..............................................................................................................1

DesignConstraintsandSelectionCriteria.....................................................................................................2

Function:...............................................................................................................................................2

Constraints:..........................................................................................................................................2

Objectives:............................................................................................................................................2

FreeVariables:......................................................................................................................................2

MaterialIndices:...................................................................................................................................2

Conceptualdesigns.......................................................................................................................................4

MaterialSelectionforHeatExchangerTubes...............................................................................................5

Limits:.......................................................................................................................................................5

MaterialSelectionSummary....................................................................................................................6

ProcessSelectionforHeatExchangerTubes................................................................................................8

JustificationforMaterialsandProcesses.....................................................................................................9

CostEstimation...........................................................................................................................................11

FinalDesignandMaterials..........................................................................................................................11

SummaryandConclusion...........................................................................................................................12

Bibliography................................................................................................................................................13

Appendix.....................................................................................................................................................14

MaximizeHeatFlowPerUnitArea:........................................................................................................14

MaximizeHeatFlowPerUnitMass:.......................................................................................................15

MaximizeHeatFlowPerUnitCost:........................................................................................................16

Costestimationresults...........................................................................................................................18

1

ReasonsandProcessofLiquefyingNaturalGas

Naturalgasconsumptionisincreasingduetothedemandforcleanerenergyproduction.Inmanycases

pipelinesarenotavailabletosupplythenaturalgasdirectlyfromtheextractionplant.Initsnatural

gaseousstate,naturalgastakesupalargeamountofvolumewhichmakestransportationvery

expensive.Inordertodecreasetheshippingvolume,naturalgascanbeliquefiedwhichreducesits

volumetoapproximately1/600thofitssize.Themaindifficultyinliquefyingnaturalgasisits

classificationasacryogenicfluid,meaningitcondensesatbelow150C.

Anotherreasonforliquefyingnaturalgasisthattheprocessremovesimpuritieswhichmeanonceit

reachesitsdestinationitonlyneedstoberegasifiedpriortodistribution.Liquidnaturalgasisalsonon

toxicandnoncorrosivehowever;itisexplosivewhenputincontactwithwater.Oncenaturalgasis

extracted,itfirstgoesthroughseveralcleaningprocesses.AnycondensatesareremovedalongwithCO2

mercury&H2S.Thenaturalgasalsogoesthroughadehydrationstagetoremoveanytraceamountsof

water.Thegasthengoesthroughseveralcoolingstageswithairfinheatexchangersandcompressors

untilitreachesthefinalcryogenicheatexchanger.Thecryogenicheatexchangerusesanotherliquefied

gas,typicallyliquidnitrogenoroxygenintheliquefactionprocess.Finally,theliquidnaturalgasisput

intocryogenicseacarriersorcryogenicroadtankersandshippedtothefinaldestination.

IntroductionandDesignStatement

Thematerialsandprocessselectionwillbedeterminedforacryogenicheatexchangerusedinthe

liquefactionofnaturalgas.Theprocessofliquefyingnaturalgasrequiresatemperaturebelow163

degreesCelsiusandinthisprocesstheothertraceelementspresentinthenaturalgasareseparatedout

leavingpureliquidmethane.Thisprocessoccursatambientpressureonthenaturalgassidebutthe

refrigerantthattravelsthroughtheheatexchangermaynotoperateatthispressuredependingonthe

typeofprocessutilized.Thereforethestressduetopressuredifferencemustbeconsideredalongwith

thethermalconductivityofthetubingmaterial.Therearetwotypesofheatexchangersusedinthis

application,oneisacoiledtubeheatexchangerandtheotherisfinandplateheatexchangertheyare

commonlymanufacturedusingaluminumandtitaniumalloys,thisreportwillinvestigatetheoptimum

materialforthistypeofheatexchanger.Theoptimumprocessformanufacturingwillalsobe

determined.

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DesignConstraintsandSelectionCriteria

Function: Cryogenic heat exchanger used to cool natural gas to its saturation temperature (-163C) at

which point it will liquefy and become liquefied natural gas (LNG).

Constraints:

Withstandthepressuredifferencebetweenworkingfluids, Operateattemperaturebelowto163 Moderateductilitysotubingcanbebent Doesnotcorrodeduetoworkingfluidsorbyproducts(suchasH2S) Haveexcellentlowtemperature(cryogenic)durability

Objectives:

Maximizetheheattransfer/flowperarea, Minimizecost,

FreeVariables:

Wallthicknessoftubing, Materialchoice

MaterialIndices:

Themethodofheattransferthroughthetubingwillbeconductionwhichisgivenby:

Where: istheheatflux( ),

isthethermalconductivity( ),

isthetemperaturedifferencebetweentheworkingfluids(,and

isthetubewallthickness().

Heatflow,,isgivenby:

Where: istheheatflow(),and

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isthesurfaceareaofthetubing() 2.

Substitutingtheheatfluxfromconductionyieldsthefollowing:

Wallthicknessisaremainingfreevariablewhichrelatesthepressuredifferencesbetweentheworking

fluids.

Where: isthematerialsyieldstrength(),

istheradiusofthetube(),

isthepressuredifferencebetweentheworkingfluids(),and

isthewallthickness.

Finallybysubstitutingthicknessamaterialindexbasedonbothyieldstrengthandthermalconductivity

canbeattained.

Inordertheminimizecosttheheatflowperunitmassisused(/).

2

/

2

/

2

4

/ 2

/

2

Oncethisindexisderivedmultiplyingmassbycostwillresultintheindexforcost.

Conceptualdesigns

Thereareseveraltypesofheat

exchangersthatareeffectivefor

theliquefactionofnaturalgas.

Eachexchangerwillusethe

coolantliquidmethaneinorder

tobringthenaturalgastoits

saturationtemperaturethrough

theuseofatypicalvapour

compressionrefrigerationcycle.

Oneofthesimplestheat

exchangerisashellandtubeheatexchangerwheremanysmalltubesareranthroughareservoirof

coolant.Thesmalltubesallowformaximumsurfaceareatobeincontactwiththecoolantallowingfor

thehighestheatflowfromnaturalgastothecoolant.Themajorityofheatexchangersarecounterflow

inwhichtheworkingfluidsarepumpedinoppositedirections.Sotheinletsideofthenaturalgaswillbe

theoutletsideofthecoolantandviceversa.Multipletubepassescanbeusedtoincreasetheamountof

surfaceareaandheattransferthatoccurs.Amodificationoftheshellandtubeheatexchangeristhe

Figure1 Shellandtubeheatexchanger

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spiralwoundheatexchanger.Inthiscasethenaturalgasisintheshellarea(AAshellstream)while

coolantrunsthroughthetubestreams.Thisconfigurationhasaveryhightubesurfaceareabutisfar

morecomplexthanatypicalshellandtubeheatexchanger.Plateandfinheatexchangerscanhave

highersurfaceareaforheattransfer.Theyarealsoabletowithstandhighpressures.Platesare

sandwichedtogetherwithsmallfinsinbetweentofurtherincreasethesurfacearea.Thistypeofheat

exchangerismoredifficulttomanufacturethanthosethatutilizetubing.Inadditiontomanufacturing

complicationstheincreasedsurfaceareamakesthefluidpathwaysverysmall.Thiscanleadtoan

increaseincloggingdependingontheworkingfluidsbeingused.

MaterialSelectionforHeatExchangerTubes

WhiledeterminingthematerialsneededforthetubesintheLNGcryogenticheatexchangerthreeobjectiveswereconsidered:

MaximizeHeatFlowperUnitArea MaximizeHeatFlowperUnitMass MaximizeHeatFlowperUnitCost

Limits:Elongation:>20%Strain

MaximumServiceTemperature:

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ThermalConductororInsulator:GoodConductor

TolerancetoCryogenicTemperatures:Excellent

MaterialSelectionSummaryUsinglevel2CESEduPackwiththelimitsstatedabove,thefollowingaretheninematerialsthat

passthelimitstage:

AgehardeningwroughtAlalloys Brass Bronze Commerciallypurelead Copper Leadalloys Nickel Nickelbasedsuperalloys NonagehardeningwroughtAlalloys

Thetopfivematerialsusinglevel2foreachobjectiveare:

MaximizeHeatFlowperUnitArea:

AgehardeningwroughtAlalloys Brass Copper Nickel NonagehardeningwroughtAlalloys

MaximizeHeatFlowperUnitMass:


MaximizeHeatFlowperUnitCost:

AgehardeningwroughtAlalloys NonagehardeningwroughtAlalloys Brass Bronze

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Copper Breakingdowneachalloyusinglevel3CESEduPackallowedforamoreaccuratematerial

selectionforeachofthestatedobjectives.Copper,brass,andbronzewererejectedfromthe

materialselectionprocessastheyhaveanadversereactionwithH2Swhichisabyproductof

theliquefactionprocess.Thefollowingarethetoptenmaterialsinorderofperformanceindex

forAluminumandNickelalloys:

MaximizeHeatFlowperUnitArea(seeFigure2):

1) Aluminum,2026,wrought,T3511 2) Aluminum,2024,wrought,T4 3) Aluminum,6082,wrought,T4 4) Nickel,DuranickelAlloy301,annealed&aged 5) NickelCoCralloy,UDIMET700,bar 6) Nickel,PermanickelAlloy300,annealed7) NickelFeCralloy,UDIMET630,bar 8) NickelCoCralloy,AEREX350,coldworked,aged9) 45Ni3MoFesoftmagneticalloy10) Nickel,commercialpurity,grade200,soft(annealed)

MaximizeHeatFlowperUnitMass(seeFigure4):

1) Aluminum,2026,wrought,T3511 2) Aluminum,2024,wrought,T4 3) NickelFeCralloy,UDIMET630,bar 4) NickelCoCralloy,AEREX350,coldworked,aged 5) Nickel,DuranickelAlloy301,annealed&aged 6) NickelCoCralloy,UDIMET700,bar 7) NickelCoCralloy,EP741NP 8) Aluminum,6082,wrought,T4 9) NickelFeCralloy,D979,bar 10) 45Ni3MoFesoftmagneticalloy

MaximizeHeatFlowperUnitCost(seeFigure6):

1) Aluminum,2026,wrought,T3511 2) Aluminum,2024,wrought,T4 3) Aluminum,6082,wrought,T4 4) Aluminum,S520.0:LM10TB,cast 5) Aluminum,5154,wrought,O

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6) Aluminum,6060,wrought,T4 7) Aluminum,2024,wrought,T0 8) Aluminum,5251,wrought,O 9) Aluminum,commercialpurity,1080,wrought,O 10) 45Ni3MoFesoftmagneticalloy

Materialsthatmeeteachofthethreedesignobjectivesare:

1) Aluminum,2026,wrought,T3511 2) Aluminum,2024,wrought,T43) Aluminum,6082,wrought,T44) 45Ni3MoFesoftmagneticalloy

ProcessSelectionforHeatExchangerTubes

Thefieldofavailableprocesseswasfirstnarrowedusingthetreeselectiontouseonly

processescompatiblewithaluminum.Usinglimitselectionthelistwasnarrowedtoinclude

onlyprimaryshapingprocesses,circularprismaticobjectsandcontinuousmethods.Finallydue

tothethinwallthicknessrequired,processeswhichcouldntprovideathicknessoflessthan1

mmwereremoved.Thefinalfiveprocesseswerethenrankedaccordingtotherangeof

thicknessavailablebelow1mm.

Thetopfiveprocessesforshapingthetubesare:

1. Wiredrawing

2. Rollforming

3. Shapedrawing

4. Coldshaperolling

5. Powderextrusion

Wiredrawingconsistsofpullingasolidcylindricalblankthroughahardeneddieinorderto

reduceitscrosssectionalarea.Tubedrawingisasubsetofwiredrawingwherethecylindrical

blankhasahollowcenterandamandrelisplacedinsidetokeeptheinteriordiameteratthe

valuethatisrequired.Thisprocessallowsforawallthicknessdownto0.1mmandatolerance

of0.010.04mm.Rollformingconsistsoffeedingacontinuoussheetofmetalthroughrollers

inordertocreatetheshape.Fortheheatexchangertubes,anadditionalweldingprocess

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wouldhavetobedoneinordertosealtheseambetweenthesections.Shapedrawingissimilar

towireortubedrawingwhereacontinuousblankispulledthroughadietocreatethefinal

geometry.Thedifferenceisthatamandrelisnotusedtocreatethedesiredwallthickness

whichlimitstheminimumwallthickness.Coldshaperollingissimilartorollformingexceptthe

processisdoneatroomtemperatureandproducesbettersurfacefinish.Thedownsidetocold

shaperollingisthehigherstressesinvolvedwhichdontallowforasectionthicknesslessthan1

mm.finally,powderextrusionusesheatedloosemetalpowdereitherformedintoabilletor

placeddirectlyintoachamber.Thematerialisthenpressedthroughadiewhichformsitinto

therequiredshape.Powderextrusionhasthesameproblemascoldshaperollingwherethe

sectionthicknessmustbegreaterthan1mm.

JustificationforMaterialsandProcesses

Inordertodetermineappropriatematerialsforthecryogenicheatexchangertubes,objectives

andconstraintshadtobedefined.Themainconstraintsassociatedwiththeheatexchanger

tubematerialare:

MinimumElongationof20%Strain MaximumServiceTemperatureLessThan163C GoodThermalConductor ExcellentTolerancetoCryogenicTemperatures

Additionally,objectivesweredeterminedtoincorporateperformanceindiceswithourmaterial

selections.Sincetheheatexchangertubesmainpurposeistotransferheatfromonefluidto

another,thermalconductivityisamajoraspectwiththismaterialselection.Therefore

consideringmass,area,andcostwiththermalconductivity;threeobjectiveswereconsidered:

MaximizeHeatFlowperUnitArea MaximizeHeatFlowperUnitMass MaximizeHeatFlowperUnitCost

Usinglevel2CESEduPack2011withthelimitsandobjectivesstatedabove,thetopfive

materialsdeterminedwerefoundtobe:

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Duetohighcorrosionrateswithhydrogensulphide,whichisabyproductofnaturalgas,

copperandbrasswereeliminatedfromourselection.Withcopperandcopperalloysremoved

fromourselectionlist,alevel3CESEduPackanalysisofthetopmaterialswasconducted

incorporatingthethreeperformanceindicesstatedearlier.Thisproducedthetopthree

materialswhichallperformedwellineachobjective.Therefore,thetopthreeoverallmaterials

fortheheatexchangertubes,inorderofperformanceindex,are:

5) Aluminum,2026,wrought,T3511



Wiredrawingwaschosenforitswiderrangeofavailablecrosssectionalareas.Thisprocessisalso

simplerthanrollforming,whichmakesitlessexpensive.Wiredrawingalsoconsistsofoneprocess

doneinstepsunlikerollformingwhichwouldrequireanextraweldingstep.Wiredrawingallowsa

rangeofcrosssectionalareasof.01to10mmwithanexcellenttolerancerangeof0.01to0.04mm.

Theprocessisalsocontinuouswhichallowsfortheproductionofverylongtubes,thisisidealdue

toatypicalheatexchangerbeing500minlength.

Figure3Wiredrawingprocess

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CostEstimation

Thetotalamountoftubingwasbasedonanaverageweightof91000kg.Thecomparisonof

potentialmaterialscanbeseeninTable1oftheappendix.Fromthetableitcanbeseenthat

Aluminum6082isthecheapest,withacostperunitweightof$1.258/kgwhichbringsthetotal

costofthematerialto$243,780.Thiscostdoesnotincludethecontainmentvesselandthe

weldedmountingstructure.

Forthechosenmaterialmorefactorswereconsideredsuchasthermalconductivityand

aluminum,2026,wrought,T3511waschosen.Outofthetwoavailableprocesses,onlythe

capitalcostandthetoolingcostcouldbeconsidered.Thecheapestprocessturnedouttobe

wiredrawingasseeninTable2withacombinedcostof$104,720.Thetotalcostoftheheat

exchangertubingincludingmaterialandprocesscameto$360,430.

FinalDesignandMaterials

Astherearesignificantbyproductscreatedintheliquefactionofnaturalgastheplateandfin

heatexchangerwasrejectedbecausethesmallpathwaysmaybecomecloggedduring

operation.Itwasdecidedthataspiralwoundheatexchangerwouldbettersuitedforthis

application.Thisheatexchangerhasmuchhighersurfaceareaoftubingresultinginhigherheat

flowfromthenaturalgastothecoolantwhencomparedwithamoreconventionaltubeinshell

exchanger.Also,thermalexpansionandcontractioncanoccurinaspiraltubeheatexchanger

withoutbreaking.Thetubingforthisheatexchangerwillbemadeoutofthefollowing

aluminumalloys(inorderfrommosttoleastdesirable):




Aluminumhasahighthermalconductivityallowingformaximumheattransferthroughoutthe

heatexchanger.Aluminumalloyswillnotreactwithanyofthecoolants,naturalgasorby

productsfromtheliquefactionsothereisnoriskofcorrosion.Aluminumisalsoreadily

availableandinexpensivematerialthatiswidelyusedinheatexchangersandother

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mechanism.Thereisanaddedadvantagebecausealuminumcanbebenttocreatecomplex

pipingnetworkswithoutbreakingandiscompatiblewiththeprocessselected,wiredrawing.

Aluminumhasloweryieldstrengththannickelalloysbutthe2000seriesaluminumalloyswill

besufficientforthisfunction.Typicallytheheatexchangersusedtoliquefynaturalgasare5m

indiameterandrangebetween200mand500mlong.Usingaluminumwillkeepweightand

costdownwhilethespiralheatexchangerwillreducethemaintenancerequiredwhich,

consideringitssize,isvital.

SummaryandConclusion

Themostdesirablematerialswereallagehardenedaluminumalloyswhicharelowincost,

havehighyieldstrengthandhighthermalconductivity.Thesematerialsweresimilartothose

currentlyusedinthesetypesofnaturalgasheatexchangerswhichtypicallyusealuminum.

Othermaterials,suchascopper,wouldbeattractivehadtheynothavehadadversereactions

withbyproductsofthereaction.Aluminumisalsoductilesoitcanbebentintothespiral

shapesonceithasbeenmanufacturedintothinwalledtubes.Perhapsthemostimportant

constraintthataluminummetwasitslowminimumoperatingtemperature(below200C).

Afterpassingallconstraintsaluminumscoredhighinallthreeofthematerialindices:heatflow

perunitarea,heatflowperunitmassandheatflowperunitcost.

Wiredrawingistheprocessthatbestfitstherequirementsformakingsmallthinwalledtubes

whilestillhavinglowcostswhencomparedtoothermethods.Asanaddedbenefittherewillbe

noadditionalprocessesrequiredotherthanthefinalassembly.Afterresearchingcurrent

materialsandprocesswefoundthatthemethodsandmaterialsselectedforthecryogenicheat

exchangeraresimilartowhatiscurrentlyused.

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Bibliography

AirProducts.(2008,February).RetrievedMarch23,2012,from

http://www.airproducts.com/~/media/Files/PDF/industries/energylngbrochure0408.ashx

Ashby,M.F.(2011).MaterialsSelectioninMechanicalDesign4thEd.Burlington:Butterworth

Heinemann.

TheLindeGroup.(2008,December09).GryogenicHeatExchangersforLNGPlants.Retrieved

March23,2012,from

http://www.hts.org.uk/downloads/Linde_LNG_HEX_09Dec2008_Extract.pdf

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Appendix

MaximizeHeatFlowPerUnitArea:MaterialIndices: Slope=1

Figure4Level2CESEduPack2011Materials

Figure5Level3CESEduPack2011Al,NiAlloys

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MaximizeHeatFlowPerUnitMass:

MaterialIndices:

Slope=1

Figure6Level2CESEduPackMaximizeHeatFlowperUnitMass


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MaximizeHeatFlowPerUnitCost:

MaterialIndices:

Slope=1

Figure8Level2CESEduPackMaximizeHeatFlowperUnitCost


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Figure10Level3CESEduPackProcessrankofsectionthicknes

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Costestimationresults

Table1Materialcost

Material UnitCost($/kg) Avg.TubingWeight

(kg)

TotalCost

($)

Aluminum,2026,wrought,

T3511

2.81 91,000 255,710

Aluminum,2024,wrought,T4 2.85 91,000 259,350

Aluminum,6082,wrought,T4 2.58 91,000 234,780

Table2Processcost

Process CapitalCost

($)

ToolingCost

($)

TotalCost($)

WireDrawing 95,200 9,520 104,720

RollForming 667,000 19,000 686,000

LNG Cyrogenic Heat Exchanger Design Report

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