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The UniversiTy of Texas aT aUsTin • PeTroleUm exTension service
Plant Processingof Natural GasPlant Processingof Natural Gas
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Table ofContents
FIGURES viTABLES viiiFOREWORD ixACKNOWLEDGMENTS xiABOUT THE AUTHORS xiiiCHAPTER 1. Fundamentals 1
Fluid Properties 1Temperature 2Pressure3GravityandMiscibility 3Solubility 4
The Ideal Gas Law 4LiquidPhase 5VaporPressure 5BoilingPointandFreezingPoint 6
Hydrates 7Comparing Physical Properties 8Composition 10Heat Energy 10
HeatingValue 12Combustion 12
Flammability 13Applications 13
FlowDiagrams 14References 21
CHAPTER 2. Feed Gas Receiving and Condensate Stabilization 23Treating and Processing 23Design Basis and Specifications for Treatment Units 26
FeedGasBasis 26ProductSpecifications 27
Equipment Selection and Design 28PigReceivers 28SlugCatchers 30CondensateStabilizers 32CondensateStabilizerReboilers 32StabilizerOverheadCompressors 32GasandLiquidHeaters 32
References 33CHAPTER 3. Dew-Point Control and Refrigeration Systems 35
Process Descriptions 35Cost Estimate 35
SilicaGelProcess 36Glycol/PropaneSystem 37Glycol/J-TValveCoolingProcess 38ComparisonofDew-PointProcesses 40UnitSpecifications 40
The Refrigeration System 41Economizers 42Chillers 44PossibleProblems 44Multiple-StageRefrigeration 46
References 50
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CHAPTER 4. Hydrocarbon Treating 51Gas-Treating Processes 51Chemical Reaction 51
Amine-BasedSolvents 52Nonamine-BasedProcesses 57
Physical Absorption Processes 58Selexol® 59PropyleneCarbonateProcess 59Rectisol®Process 60
Mixed Chemical/Physical Absorption 60Sulfinol®Process 60
Adsorption on a Solid 61MolecularSieveProcess 61ActivatedCarbonProcess 62
Membrane Processes 62General Operating Considerations for Gas Treating 65
InletSeparation 65Foaming 65Filtration65Corrosion 65
References 66CHAPTER 5. Sulfur Recovery and Claus Off-Gas Treating 67
Sulfur Recovery 67ThermalProcess 67CatalyticRecovery 68
Claus Off-Gas Treating 70SCOTProcess 70
References 72CHAPTER 6. Dehydration and Mercury Removal 73
Dehydration 73InhibitorInjection 76
Dehydration Methods 80LiquidDesiccants 80SolidDesiccants 84DesignIssues 88
Mercury Removal Unit (MRU) 90DesignBasisandSpecifications 92DesignConsiderations 94EquipmentSelectionandDesign 95CaseStudy 95
References 96CHAPTER 7. NGL Recovery—Lean-Oil Absorption 97
Lean-Oil Absorption 98TheRecoverySystem 98Absorption 99WhyAbsorbersWork 99Presaturation 100PotentialProblems 102
The Rejection System 104HotRich-OilFlashTank 104Rich-OilDemethanizer 105PossibleProblems 107
The Separation System 108TheStill108OilPurification 109PossibleProblems 110
References 111
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CHAPTER 8. NGL Recovery—Cryogenic 113Typical Applications 115
TurboexpanderProcess 115Propane-RecoveryProcess 120Ethane-RecoveryProcess 121
Turboexpanders 122Cyrogenics 127References 130
CHAPTER 9. Fractionation and Liquid Treating 131Fractionation 131Packed Columns 134NGL Fractionation Plants 134
Deethanizer(DeC2)Column 136Depropanizer(DeC3)Column 137Debutanizer(DeC4)Column 137Deisobutanizer(DIB)orButaneSplitterColumn 137
Product Specifications 138Monitoring Fractionation Plant Operation 139Possible Operating Problems 140NGL Product Treating 141
Liquid—LiquidTreating 141Liquid—SolidTreating 143
References 143CHAPTER 10. Nitrogen Rejection Unit (NRU) 145
Nitrogen Rejection 145NRU Process Selection 145
PressureSwingAdsorption(PSA) 145CryogenicAbsorption 145Membranes 146CryogenicDistillation 146
Cryogenic NRU Processes 146Pretreatment 147Chilling 148CryogenicDistillation 148Recompression 148
NRU Processes 149References 152
APPENDIX. Figure and Table Credits 153GLOSSARY 159INDEX 183
Table of Contents
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About the Authors
Dr.DougElliot hasmore than 40yearsexperience in theoilandgasbusiness,devotedtothedesign,technologydevelopment, anddirectionof industrialresearch.DougiscurrentlyPresident,COOandcofounder(withBechtelCorporation)
ofIPSILLC,acompanyformedin1986todeveloptechnologyandpro-videconceptualdesignservicestooilandgasproducingandengineering,procurement,andconstructioncompanies.
PriortoIPSI,DougwasVicePresidentofOilandGaswithDavyMcKeeInternational.DougstartedhiscareerwithMcDermottHudsonEngineering in the early 1970s following apostdoctoral research as-signmentunderProfessorRikiKobayashiatRiceUniversity,wherehedevelopedaninterestinoilandgasthermophysicalpropertiesresearchanditsapplication.
Doughasauthoredor coauthoredover65 technicalpublicationsplus12patents.HeservedasamemberoftheGasProcessorsAssociationResearchSteeringCommitteefrom1972to2001andasChairmanoftheGPSA(GasProcessorsSuppliersAssociation)DataBookCommitteeonPhysicalProperties.DougservedasChairmanoftheSouthTexasSectionandDirectorof theFuelsandPetrochemicalDivisionof theAmericanInstituteofChemicalEngineers;andiscurrentlyamemberofthePETEXAdvisoryBoard.HeholdsaB.S.degreefromOregonStateUniversityandM.S.andPh.D.degreesfromtheUniversityofHouston,allinchemicalengineering.
DougisaBechtelFellowandaFellowoftheAmericanInstituteofChemicalEngineers.
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J.C.Kuo (ChenChuan J.Kuo) is a34-yearveteranof thegasprocess-ing,gastreating,andliquefiednaturalgas(LNG) industry.As a senior advisor forChevron’sEnergyTechnologyCompany,he has served as the ProcessManager/
ProcessLeadformanyprojects,includingtheWheatstoneLNG,GorgonLNG,DeltaCaribeLNG,CasotteLanding,andSabinePassLNGterminalprojects.HehasalsoservedasthetechnicalprocessreviewerforAngola,Olokola,Algeria,andStockmanLNGprojects.BeforeworkingatChev-ron, J.C.was theTechnologyManager for IPSI, anaffiliateofBechtel,andservedastheProcessManager/ProcessLeadforthePemexCatarelloffshoreproject,theEgyptianLNG(Idku)trains1and2,ChinaShellNanHai,ChevronVenicegasplantde-bottleneck,TunisiaNRU,andAustralianSANTOSprojects.
J.C.isafrequentspeakerandpresenteratinternationalconferencessuchasfortheAmericanInstituteofChemicalEngineers,gasprocessingandtreatingconferences,andtheLNGSummit.HehascontributedtogasprocessingandLNG technology improvements throughapatent,abook,andmanypapers.Hehasalsoservedasco-chairoftheAIChELNGsessionsforthetopicalconferencesonnaturalgasutilization.HeisamemberofthesteeringcommitteefortheNorthAmericanLNGSummit.
HisdegreesincludeaB.S.fromChungYuanChristianUniversity,Taiwan,andanM.S.fromtheUniversityofHouston,bothinChemicalEngineering;andanM.S.inEnvironmentalEngineeringfromSouthernIllinoisUniversity.HeisaregisteredProfessionalEngineerinTexasandamemberofAIChE.Heisthepresidentofthe99PowerQiQongTexasdivisions.
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Dr.PervaizNasirhasmore than27years experience in theoil andgasbusiness.HeiscurrentlytheRegionalMan-agerGas/LiquidTreatingandSulfurPro-cesses,Americas,atShellGlobalSolutions.
PervaizstartedhiscareeratShellDe-velopmentCompanyin1981inresearchanddevelopmentandtechnicalsupport,mostlyrelatedtooilandgasprocessing.In1986,hemovedintolicensingandprocessdesignofShellGas/LiquidTreatingtechnologies.AsamemberofShellMidstreamfrom1991through1999,Pervaizwasresponsiblefortheoperationssupportandoptimizationofexistinggasplantsandthedevelopmentandstartupofnewgasprocessingfacilities.HethenjoinedEnterpriseProductsCompanyasDirectorofTechnology.AtEnterprise,Pervaizwasresponsiblefortheevaluationofnewbusinessventures/technologies ingasprocessing, liquefiednatrualgas (LNG),petrochemicals,etc.HereturnedtoShellGlobalSolutionsin2006.
PervaizholdsaB.S.fromMiddleEastTechnicalUniversity(Ankara),anM.S.fromUniversityofAlberta(Edmonton),andaPh.D.fromRiceUniversity(Houston),allinchemicalengineering.HeservedontheGasProcessorsAssociation(GPA)PhaseEquilibriaResearchSteeringCom-mittee from1983 through1990and iscurrentlyamemberof theGPALNGCommittee.Pervaizhasauthoredorcoauthoredover17externaltechnicalpublications.
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1 Fundamentals
Natural gasiscolorless,shapeless,andodorlessinitspureform.Itisafos-silfuelconsistingprimarilyofmethanewithquantitiesofethane,propane,butane,pentane, carbondioxide, nitrogen,helium, andhydrogen sulfide.Naturalgasiscombustible,givesoffagreatdealofenergy,iscleanburning,andemitslowlevelsofbyproductsintotheair.Itisanimportantsourceofconsumerenergyusedinhomestogenerateelectricity.
Thepetroleum industry classifiesnaturalgasby its relationship tocrudeoil in theunderground reservoir.Associated gas is the termusedfornaturalgasthatisincontactwithcrudeoilinthereservoir.Theassociatedgasmightbeagas capoverthecrudeoilinareservoirorasolutionofgasandoil.Nonassociated gasisfoundinagasphaseinreservoirswithoutcrudeoil.
Whetherassociatedornonassociated,gas production streams arehighlyvariableandcancontainawiderangeofhydrocarbonandnonhydrocarboncomponents.Thesestreamsmightincludevariousmixturesofliquidsandgasesaswellassolidmaterials.Thereareusuallysomenonhydrocarboncomponentsincludingnitrogen,helium,carbondioxide,hydrogensulfide,andwatervaporpresentinthestream.Traceamountsofothercomponents,suchasmercury,mightalsobepresent.
Naturalgasprocessingplantsusephysicalandchemicalprocessestoseparateandrecovervaluablehydrocarbonfluidsfromagasstream.Intheprocessingplant,allthepipes,containmentvessels,steamlines,tanks,pumps,compressors,towers,andinstrumentscontainagasorliquidundergoingsomekindoftreatmentprocess.
Duringtheprocessing,thenonhydrocarboncontaminantsmustbehan-dledproperlybecausetheyaffectgasbehaviorduringtreatment,impairtheefficiencyofprocessingoperations,orcandamagetheprocessingequipment.Forexample,thecontaminant,liquidmercury,weakensandbondswiththealuminumheat exchangersusedtoproducesupercooledfluids.Ifmercuryisnotremovedfromthegasearlyintheprocessingphase,itliquefiesandcol-lectsontheexchanger’ssurfaces,eventuallydestroyingtheheatexchangers.
FLUID PROPERTIES
Whenthereisapipe,asteamline,atank,apump,acompressor,atower,aninstrument,orevenafilledsamplecontainerinagasplant,italmostalwayscontainsafluid.
What isafluid?Afluidcanbeagas,a liquid,orasolid.Afluid isdefinedasanysubstancethatflowsfreelyunlessrestrictedorcontainedbyabarrier.Withouttheabilitytoassumeashapeofitsown,afluidassumestheshapeofthecontainerintowhichitisplaced.Bothgasesandliquidsareclassifiedasfluids.
Naturalgas treatment isbasedonthereactionsofreservoirfluids inphysicalandchemicalprocesses.Eachfluidhasauniquesetofpropertiesincludinggravity, solubility,andflammabilitycontrolling its response togivenstimuli.Agasprocessingplantoperatormustdeterminethespecificpropertiesandconditionsofitssourceofoilfieldfluids,orfeedstock,becauseeachoneisdifferent.Problemsthatoccurduringgasprocessingcomefromafundamentalmisunderstandingofthespecificfluidpropertiesorthephysicalandchemicallawsthatdeterminefluidbehavior.
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2 Feed Gas Receiving and Condensate Stabilization
TREATING AND PROCESSING
Plantunitconfigurationsvarydependingonthetypeofcomponentsofthefeedgasandthefinalproductsdesiredforconsumeruse(fig.2.1).
Figure 2.1 Gas processing plant
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Feedgasfromvariousgasfieldsentersthegasplantthroughpipelinesandgoesthroughseveralunitsoftreatingandprocessing,asshowninfigure2.2.Themainunitsperformthefollowingfunctions:
• Removeoilandcondensates• Removewater• Separatethenaturalgasliquidsfromthenaturalgas• Removesulfurandcarbondioxide• Removeimpuritiessuchasmercury,oxygen,andBETX(benzene,
ethylbenzene,toluene,andxylenes)
Thefirsttreatingunitisthefeedgas-receivingsystemandtheconden-satestabilizationsystem.Condensateisalighthydrocarbonliquidobtainedbycondensationofhydrocarbonvapors.Itconsistsofvaryingproportionsofpropanebutanes,pentanes,andheaviercomponentswithlittleornomethaneorethane.Thefeedgasreceivingsystemseparatesthefeedgasintogases,aqueousliquid,andhydrocarbonliquidforfurtherprocessingatplantunitsdownstream(fig.2.3).
Thecondensatestabilizationsystemremovesthelightcomponentssuchasmethane,ethane,andpropane,dissolvedinthehydrocarbonliquidfromthefeedgasreceptionsystem(fig.2.4).Hydrocarbonliquidnormallycon-tainsalargeamountofdissolvedlightcomponentsbecauseofhighpipelinepressures.Theselightcomponentsneedtoberemovedtomeetcondensateproductandotherdownstream processingrequirements.
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3 Dew-PointControl andRefrigerationSystems
PROCESS DESCRIPTIONS
Rawgascomesfromproductionfieldsthroughthepipelinestothefeedgasreceivingunitandcondensatestabilizationunit.Therawgasthenflowstothegas-treatingunitandthentoadew-pointcontrolandrefrigerationunitoranatural gas liquid (NGL)recoveryunit.Anexportcompressionsystemissometimesusedafterthedew-pointcontrolunittopressurizethegastotherequirementsofthe pipeline grid.Finally,thegasissenttotheconsumersthroughapipelinegrid.
Adew-pointcontrolunithelpstopreventliquidcondensationinthepipelinegridundervariouspressuresandtemperatureconditions.Therearetwokindsofdew-pointcontrolrequired:awater dew-point controlandahydrocarbon dew-point control.Inwaterdew-pointcontrol,thereareseveraldehydration,orwaterremoval,methodsavailable,includingthesilica gel,glycol,andmolecular sieve.Hydrocarbondew-pointcontrolalsohasvariousmethodsavailable including refrigerated low-temperature separation (LTS),expander,Joule-Thomson (J-T), andsilicagel.Companiesmightuseglycolgasdehydrationforwaterdew-pointcontrolandarefrigerationcoolingsystemforhydrocarbondew-pointcontrol.MoreexplanationofgasdehydrationisgiveninChapter6.
Thepurposeofarefrigeration systemistoremoveheatfromthefeedstreaminaheatexchanger.Heatexchangersarereferredtoas evaporatorsorchillers andprovidetherequiredcoolinglevel forvariousgasprocess-ingapplications.Refrigerationsystemsuserefrigerant,calledworking fluid.Workingfluidisselectedbasedupontemperaturerequirements,availability,economics,andpreviousexperience.Theavailabilityofethaneandpropaneonhandatnaturalgasprocessingplantsmakesthesegasestheprimechoiceasworkingfluids.Ingasplants,propaneisnormallythepreferredrefrigerant.
COST ESTIMATE
Dew-pointdepressionisdefinedasthedifferenceindegreesbetweenthefeedgastemperatureandthedewpointofafluid.Thedew-pointdepressiondif-ferenceindegreesdeterminesthebestprocesstousefordew-pointcontrol.
Depending on the amount of dew-point depression required, aneconomicevaluationisdonetocompareinstallationcostsandoperatingcostsforthevariousprocesses.Usinganaverage80°F(45°C)dew-pointdepressionrequirement,thereareseveralprocessesavailableforthedew-pointcontrol.Threeofthemostwidelyusedprocessoptionsarethesilicagelprocess, theglycol/propane refrigerationprocess, and theglycol/J-Tvalvecoolingprocess.
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4 HydrocarbonTreating
GAS-TREATING PROCESSES
Hydrocarbonstreams,bothgaseousandliquid,mightcontaincontaminantssuchasH2SandCO2thatmustberemovedbeforefurtherprocessingandmarketing.RemovalofH2S,CO2,andothersulfurcompounds,commonlycalledacid gases,isnormallyreferredtoashydrocarbontreating orsweetening.
Treatedgasregulationsandspecificationsarestringentregardingre-sidualH2Sandothersulfurspecies.TypicalU.S.salesgascontractsrestrictthefollowing:
H2S <0.25grains/100scf(about4ppmv)Totalsulfurcompounds <5grains/100scf(about80ppmw)CO2 <2%mole
Acidgascomponentscanberemovedfromasourgasstreamby:• chemicalreactionusingliquidsorsolids;• physicalabsorptioninliquids;• adsorptiononsolids;• diffusionthroughmembranes.
Theacidgasremovalprocessescanbenonregenerativeorregenerative. Thenonregenerativeprocessesaresuitableonlywhentraceamountsofcon-taminantsmustberemovedand/orveryhighpurityoftreatedgasisdesired.NonregenerativeprocessesbecometoocostlywhentheH2Stoberemovedexceedsabout1tonperday.ExamplesofnonregenerabletreatingprocessesareSulfaTreat®andChemsweet®,bothmarketedbyC.E.Natco.
Regenerativeprocessesaremoreeconomicalforremovinglargerquan-titiesofcontaminants.AnexampleofaregenerativeprocessistheuseofanaqueousaminesolutiontoremovetheH2SandCO2fromasourgasstream.Theaminesolutionisthenregeneratedbyreducingitspressureandheatingittoabout250°F.Thesolutionisthencooledandrecycledforreuse.Regen-erativetreatingprocessescanbebroadlyclassifiedasthosethatdependon:
• chemicalreactionin – amine-basedsolvents, – nonaminebasedsolvents;• physicalabsorption;• mixedchemical/physicalabsorption;• adsorptiononasolid.
CHEMICAL REACTION
Intheseprocesses,H2Sand/orCO2arechemicallyboundtotheactiveingredi-entinthetreatingsolution.Therefore,theresiduegascanbetreatedtoretainonlyverylowlevelsofthesecontaminants.Thechemicalsolventprocessesincurrentcommercialprocessesuseweakbaseslikealkanolamines,alkali saltsolutions,potassium carbonate,orachelate solution.
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5 Sulfur Recovery and Claus Off-Gas Treating
SULFUR RECOVERY
Gastreatingplantsmuststrictlycomplywithlegal,government,andsafetystandardsandregulationsconcerningemissionsandpollution.Duringthetreatmentprocess,H2SandsomeormostoftheCO2areremovedfromthesourgasstream,asdiscussedinChapter4.Theseremovedsourgascompo-nentsmustbedealtwithcautiously.
Whiletheemissionsrequirementsvarywithgeography,mostcountriesdonotpermittheemissionofmorethanafewpoundsofsulfur(H2S,SO2,etc.)perdayintotheatmosphere.Tocontrolemissions,theacidgasstreamfromatreatingplantisfedtoasulfurrecoveryunit(SRU)whereH2Sandothersulfurcompoundsareconvertedandrecoveredasnontoxicelementalsulfur(S).ThetailgasfromtheSRUstillcontainssomesulfurcomponents.TheseareconvertedtoSO2inanincineratorbeforebeingdischargedintotheatmosphere.Ifhigh(99.8+%)sulfurrecoveryisdesired,theSRUtailgasisfedtoatail-gastreatingplantforfurtherreductionofsulfuremissions.
Thermal ProcessIn1883,anEnglishscientist,CarlFriedrichClaus,discoveredandpatentedaprocessinwhichH2Swasreducedtoelementalsulfurandwaterinthepresenceofacatalyst.
Claus’sformulaforthisprocessis:
H2S+½O2 = S+H2O
Thecontrolofthisexothermic,orheat-releasing,processwasdifficult,andconversiontoelementalSwaslow.ThemodifiedClausprocessusedtodayovercomesthecontrolandconversionproblemsbydividingtheClausprocessintothefollowingtwosteps:
Thermal StepInthisexothermicstep,theair-to-acidgasratioiscontrolledsothataboutNoftheH2SisoxidizedtoSO2.Forgasescontaininghydrocarbonsand/orammoniafromasourwaterstripper,enoughairisinjectedtoensurecom-pletecombustionofammoniaandhydrocarboncomponents.Theprocesstemperatureduringthisstepisfrom1,800°F–2,500°F.
Catalytic StepIn thismoderately exothermic catalytic reaction, the sulfurdioxide (SO2)formedinthethermalsectionreactswithunburnedH2Stoformgaseouselementalsulfur.Thecatalystusedinthisstepismadefromactivatedalu-minaortitaniumdioxide.
Otherthanthereactionsofhydrocarbonsandothercombustibles,thekeyreactionstakingplaceare:
Thermal Reaction:
H2S+1½O2 = SO2+H2O
Thermal and Catalytic Reaction:
2H2S+SO2 = 3S+2H2O
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6 Dehydration and Mercury Removal
DEHYDRATION
Allhydrocarbonfluids can retain somewater.Water is soluble in liquidhydrocarbonsandcanbeheld in thevaporphasebyhydrocarbongases.Whenaliquidorgasiscooled,itscapacityforcontainingwaterdecreases.Asaresult,itcanproduceliquidand/orsolidwatercalledhydrates.Hydratesareaseparateandproblematicpartofgasprocessing.
Figure6.1showshowthewatercontentofnaturalgasvarieswithtem-peratureandpressure.Forexample,at1,000psiaand100oF,water-saturatednaturalgascontainsabout62poundsofwaterpermillionstandardcubicfeet(MMscf)ofgas.At1,000psiaand0oF,thegascontainsonlyabout2poundsofwaterperMMscfofgas.
Thewaterdewpointofagasorliquidisthetemperatureatwhichfreewaterwillbegintoseparatefromthegasorliquid.Ifanaturalgasstreamat1,000psiacontains62poundsofwaterperMMscfofgas,itswaterdewpointis100oF.Ifitiscooledbelow100oF,freewaterwillbepresent.
Hydrateswillformifagasorliquidcontainingfreewateriscooledbelowitshydratetemperature.Thegraphshowninfigure6.2canbeusedtoestimatethehydratetemperatureofnaturalgas.Forinstance,a0.6specificgravitygashasahydrate temperatureofabout61oFat1,000psia. If thisgasmustbecooledbelowitshydratetemperature,eitherduetopipelinetransportation,pressurereductionforconsumption,orforprocessing,pre-cautionsmustbetakentopreventfree-waterdropoutthatcausesfreezingandformationofhydrates.
Hydratesaresolidcompoundsthatformascrystalsandresemblesnow.Theyarecreatedbyareactionofnaturalgaswithwater,andwhenformed,areabout10%hydrocarbonand90%water.Hydrateshaveaspecificgravityofabout0.98andwillfloatinwaterandsinkinhydrocarbonliquids.Freez-ingcanbeavoidedbyeitherremovingthewaterfromthegasorliquidpriortocoolingbelowthehydratetemperatureorbyusingahydrateinhibitortomixwiththewatercondensedduringcooling.
Dehydrationistheprocessofremovingwaterfromasubstance.Dehy-drationcanbeaccomplishedbyusingsolidsubstancessuchasthoseusedindry-beddehydrators.Itcanalsobedoneusingaliquid,suchastriethyleneglycol.A stripping gasinaTEGreboilercanalsobeused.Themorecommonhydrateinhibitorsaremethanolandethyleneglycol.Mostnaturalgasfacili-tiesuseoneormoreofthefollowingdehydrationprocesses:ethyleneglycolinjection,TEGdehydration,ordry-beddehydrators.
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7 NGL Recovery—Lean-OilAbsorption
Straightfromthewell,naturalgasisamixofhydrocarbons,includingmeth-ane,ethane,propane,andbutane.Italsocontainsmanynonhydrocarbons,suchasnitrogen,helium,carbondioxide,hydrogensulfide,andwater.Rawgasisprocessedtoseparatethesecomponents.Theseprocessescontinuetoimprovewithadvancedtechnology.
Previously,whenkerosenewasahighlyvaluedproductandnaturalgaswassimplyanunwantedbyproduct,mostnaturalgaswaswasted.Gaswascommonlyflaredoff—aprocessthatoftencontinueddayandnight.Littlewasdonetocaptureanygasproducts.Afewoperatorsputtrapsintheirlinestocatchdrip gas.Aformofgasoline,dripgasnaturallycondensesasitcomesfromthenaturalgaswellsandcoolsinfield-gatheringlines.
Gasolinebegantosurpasskerosenesalesaround1912.Thefirstgasprocessorslearnedtoincreasetheyieldofdripgasbycompressingthegasandallowingittocool.Withthedemandforfuelgasolinegrowingquickly,producersbegantryingtogetmoreusableproductsfromtheoilandgas.Anincreased-yieldprocess,calledlean oil absorption,wasdevelopedinthe1920s.Leanoilisahydrocarbonliquid,usuallylighterthankerosene.Incontactwithnaturalgas,leanoilabsorbssomeoftheheavierhydrocarbonsfromthegas,whichcanbeseparatedfromtheoillater.Usingthisprocess,operatorsrecoveredmoregascondensateaswellasbutane,agasthatliquefieseasilyunderpressure.ThiswasthebeginningoftheNGLmarket.
Inthe1950s,processorsimprovedtheyieldfromlean-oilabsorptionbyaddingrefrigerationtotheprocess.Advancingtechnologyhasaddedthedevelopmentofbetterrefrigerationequipment,lowerprocessingtempera-tures,andnewwaystomarketnaturalgas.
Lean-oilabsorptionisonlyoneofmanywaystoseparatethevariousproductsinnaturalgas.Insteadofusinglean-oilabsorption,plantsmightusealessexpensiveprocessbyrefrigeratingthegastoremovethepropaneandheavierhydrocarbons.Manyofthenewestgasprocessingplantsonlyproducea singleNGLproduct calledY-grade,which is then shipped toanotherplantforfurtherseparation.
Chapter 7presents theolder, but still commonlyused, lean-oil ab-sorptionprocess.Chapter8coversthenewerprocessesofrefrigerationandturboexpansion.However,thecompleterangeofprocesspossibilitiescannotbefullyexploredinthesechaptersbecausegasplantsoftenusecombina-tionsandmodifiedversionsoftheseprocesses,dependingontherangeofproductsproduced.
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8 NGL Recovery-Cryogenic
Theheavierhydrocarbons,generallyreferredtoasNGLs,mightneedtoberemovedtocontrolthehydrocarbondewpointand/orthegasheatingvalue.However,heavierhydrocarbonscanalsobeasourceofincomeforthegasproducer.Thehistoryoftheevolutionofliquidsrecoveryfacilitiesfrom simple oil absorption to cryogenic expanderprocesses is complex(Elliot,1996).
Lean-oilabsorptionprocesses,suchastheambientandrefrigeratedprocesses,werecommonlyuseduntiltheearly1970s,asshowninfigure8.1.
Figure 8.1 Lean-oil absorption process and cryogenic process
The refrigeratedoil absorptionprocesswas introduced in 1957bymodifyingtheambientoilabsorptionprocesstooperateatlowertempera-tures.ThisallowedtheuseoflowermolecularweightoilswhichrecovermoreNGLsthanthehighermolecularweightoils,usedintheambientprocess.Atatemperatureof–40oF,therefrigeratedoilabsorptionprocesscouldbeusedtorecoverupto40+%ethaneand90+%propaneinthefeedgas.
Amajorshiftinthegasprocessingindustrybeganwhenthefirstlow-temperatureexpanderplantwasbuiltandbroughtintooperationin1964.Thebasicdesignofthisplantremainstoday.Duringthe1970s,theexpanderplantbecamethedominantprocessschemeusedtorecoverNGLsfromfeedgas.Itisanefficientprocessforhighethaneandpropanerecovery.
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9 Fractionation and LiquidTreating
FRACTIONATION
Thebasisformosthydrocarbonphaseseparationsistheequilibriumflashprocess.Aflashprocessgivesa“sloppy”or impreciseseparationof thecomponentsinamixture.Theliquidandvaporphasesoftheflashprocessstillcontainallthecomponentsthatwerepresentinthefeedgas.Tobetterseparate the feedcomponents,aprocesscalledfractionation,ordistilla-tion,wasdeveloped.Fractionationispossiblewhenthecomponentstobeseparatedhavedifferentboilingpoints.Thehigher thedifference in theboilingpoints,theeasieritistoseparatethecomponents.
Duringfractionation,amixtureisseparatedintoindividualcompo-nentsorgroupsofcomponents.Fractionationisacountercurrent operationinwhichvapormixtures are repeatedly brought in contactwith liquidmixtureshavingsimilarcompositionasthevapors.Theliquidsareattheirbubble pointsandthevaporsareattheirdewpoints.Bubblepointisthetem-peratureatwhichthefirstbubbleofgasformsinliquid.Partofthevaporcondenses,andpartoftheliquidvaporizesduringeachcontact.Thevaporbecomesricherinthelighterorlowerboilingcomponents,andtheliquidbecomesricherintheheavierorhigherboilingcomponents.
Afractionatingcolumncanbeviewedasacombinationofabsorptionandstrippingcolumns.Figure9.1isaschematicdiagramofafractionationcolumnwiththeassociatedandperipheralequipment.
Thecoolinginacondenserisdoneeitherbyair,coolingwater,orarefrigerantsuchaspropane.Thecolumnpressurenormallydeterminesthecoolingmediumthatisused.Reboilerheatisprovidedbysteam,hotoil,hotmediumfluids,hotcompressordischargegas,orahotprocessstream.
Thenumberoftraysortheheightofthepackedsectioninafraction-ationcolumndependsuponthenumberofvapor/liquidcontactsrequiredtomake the desired separation. Fractionation columns usevalve trayswithdowncomers orpipesthatmovethe liquidfromonetraytotheonebelow.Thevalvesopeneitherpartiallyorfullybyvaporflowingthroughthetray.Aweir maintainsliquidlevelonthetray.Liquidflowsacrossthetray,overtheweir,andthroughadowncomerordownpipetothetraybelow.Large-diametercolumnsmighthavetwoorfourliquidflow-passesoneachtray.Figure9.2showsflowthroughvaporpassagesonatrayinafractionationcolumn.
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10 Nitrogen Rejection Unit (NRU)
NITROGEN REJECTION
Nitrogen(N2)isaninertgasfoundinvaryingamountsinnaturalgasres-ervoirs.Nitrogen rejection is anecessaryprocess inmaintainingadesiredBtuvalueforsalesgasandpipelinespecifications.NitrogenisaddedtoorremovedfromthesalesgastoadjustitsheatingorBtuvalue.AddingnitrogenlowerstheBtuvalue,whileremovingnitrogenraisestheBtuvalue.How-ever,thereisalimittothemaximumamountofN2orinertgasesallowed.Nitrogenhasanadditionaluseintheenhanced oil recovery(EOR)processesandforincreasingoilproductionthroughreservoirinjection.
Nitrogenisremovedfromthefeedgasatlowtemperaturesinanitrogen rejection unit (NRU)designedaccordingto:
• Inletgascomposition• Inletgaspressure• Productspecifications• Ventnitrogen• Residuegasheatingvalue• Hydrocarbonrecoveryrequired
NRUsoperatebestunderstablecompositions,inletrates,temperatures,andpressures,andmustbedesignedtoefficientlyoperateoverabroadrangeofnitrogengascompositions(5%–80%).Thequantityofnitrogeninthefeedgasisgenerallythemainfactorinselectinganitrogenremovalprocess.
NRU PROCESS SELECTION
Thereare four categoriesofprocesses currently available for removalofnitrogenfromnaturalgas.
Pressure Swing Adsorption (PSA) Pressure swing adsorption isatechnologyusedtoseparatenitrogenunderpres-sureaccordingtoitsmolecularcharacteristicsandattractiontoanadsorbentmaterialatnear-ambienttemperatures.Specialadsorptivematerialsareusedasamolecularsieve,adsorbingthehydrocarboncomponentsathighpressure.Theprocessthenswingstolowpressuretodesorbtheadsorbentmaterial.Methaneisproducedduringthedesorptionstepatrelativelylowpressurenearambientorundervacuuminsomecases.Itoftenrequirespretreatmentandhashighcapitalandcompressioncosts.Therecoveryofmethaneisgenerallymoderate.
Cryogenic Absorption Thecryogenic absorption process useschilledhydrocarbonoiltoabsorbthebulkofthemethaneandachieveaseparationofnitrogenfromnaturalgas.Theabsorbedmethaneisstrippedofftheoilinaregeneratorandsubsequentlycompressedbacktothepipelinepressure.Theneedtoabsorbthebulkofmethanerequiresalargeoilcirculationflowandequipmentsize.Therefore,itismostsuitableforhighnitrogencontentstreams.Ithasnotbeenwidelyusedcommercially.
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Indexabsolutepressure,3absolutezero,2absorbers,98absorberworkings,99–100.absorption,36,51absorptionoil,99activatedcarbonprocess,62adiabaticexpansion(JT),148adsorbentlife,89adsorptiononasolid,61–62air-to-fuelratio,13alkalisaltsolutions,51alkanolamines,51,141amalgamation,92amalgamcorrosion,92ambienttemperature,58AmericanPetroleumInstitute(API),28AmericanSocietyforTestingandMaterials(ASTMInternational),109AmericanSocietyofMechanicalEngineers(ASME),28antiagglomerants(AAs),76APIgravity,3applications,gasprocessinggeneral,13–21applications,NGLrecovery-cryogenic ethane-recoveryprocess,121–122 propane-recoveryprocess,120 turboexpanderprocess,115–119aromatics,26associatedgas,1
biologicaloxidationprocess,57boilingpoint,6bottomstemperature,106Britishthermalunits(BTUs),10bubblecap,18bubble-pointline,116bubble-points,131butanesplittercolumn,137
carbondisulfide(CS2),53casestudy,95catalyticrecovery,68,70catalyticstep,67caustictreating,141centrifugalcompressors,32chelatesolution,51,57
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chemicalreactions amine-basedsolvents,52–53 biochemicalprocesses,57–58 diethanolamine(DEA),53 diglycolamine(DGA),53 formulatedamines,54 hinderedamines,54 hotpotassiumcarbonateprocess,57 methyldiethanolamine(MDEA),54 monoethanolamine(MEA),53 nonamine-basedprocesses,57–58 redoxprocess,57chillers,35,44chilling,148Claus,CarlFriedrich,67Clausoff-gastreating,70Clausprocess,54closedlooppropanerefrigerationsystem,38coalescentliquidlevelproblem,95coalescers,141cobalt(II)carbonylsulfide,53coldbox,152combustion,12–13comparisonofdew-pointprocesses,40composition,10compressors,1,32condensateproductstoragetanks,28condensatestabilizerreboilers,32condensatestabilizers,32condensers,16condensing,5contactorvessel,52contaminants,1controllers,44coolers,38coreexchangers,127corrosion,25,65corrosivity,53costestimate,35countercurrentoperation,131criticalpressures,3criticaltemperatures,3cryogenicabsorption,145cryogenicdistillation,146,148cryogenicNRUprocesses chilling,148 cryogenicdistillation,148 nitrogenrejectionunits(NRUs),146–148 pretreatment,147–148 recompression,148cryogenics,127–129
debutanizer,28debutanizer(DeC4)column,137deethanizer,120deethanizer(DeC2)column,137degradationproducts,53dehydration,4,73–80dehydrationandmercuryremoval,73–95dehydrationmethods designbasisandspecifications,92–93 designissues,88 liquiddesiccants,80,82–84 mercuryremovalunit(MRU),90 molecularsieveprocess,85–86 silicagelsandactivatedammonia,85 soliddesiccants,84–85deisobutanizer(DIB)column,137demethanizing,104depentanizer,28depropanizer(DeC3)column,137desiccants,147designbasisandspecificationsfordehydrationandmercuryremoval aboutgenerally,92 casestudy,95 designconsiderations,94–95 equipmentselectionanddesigns,95 gasdehydrationunits,92 generalconsiderations,92 technologyselection,92designbasisandspecificationsfortreatmentunits condensateproductstoragetanks,28 feedcasebasis,26 feedgas,26–28 productspecifications,27designcases,26designconsiderations designbasisandspecificationsfordehydration andmercuryremoval,94–95 free-liquidremoval,94 mercuryremovalcosts,94 MRUlocation,94 pressuredrop,94 superficialvelocityandresidencetime,95designissues adsorbentlife,89 dehydrationmethods,88 pooroutletwaterdewpoint,88–89 pressuredrop,89 switchingvalvesforadsorptionandregeneration operation,90desorption,37dew-pointcontrol,25
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Index 185
dew-pointcontrolandrefrigerationsystems,35–49 chillers,44 costestimateof,35 economizers,42 multiplestagerefrigeration,46 problems,44–46 processdescriptions,35–46 refrigerationsystems,41dew-pointdepression,35diethanolamine(DEA),52diethyleneglycol(DEG),37differentialpressureindicator(DPI),102diisopropanolamine(DIPA),60distillation,108downcomers,131downstreamprocessing,23dripgas,97driveassembly,45drygasloss,12drypointtemperature,109drystills,108
economizers,42elementalsulfur,57entrainedwatervapor,12equilibrium,5equipmentselectionanddesign condensatestabilizerreboilers,32 condensatestabilizers,32 feedgas,28 gasandliquidheaters,32–33 mercuryadsorber,95 mercuryremovalafter-filter,95 pigreceivers,28slugcatchers,30stabilizeroverheadcompressors,32ethane-recoveryprocess,121–122ethyleneglycol(EG),37,77eutecticfreezingpoint,77evaporation,41evaporators,35exchangers,127exothermicprocess,67expanders,35
Fahrenheitscale,2feedgasbasis,26feedgasreceivingandcondensatestabilization,23–33 designbasisandspecificationsfortreatment units,26–28
equipmentselectionanddesign,28 treatingandprocessing,23–26feedstock,1filtration,65firepoint,13flammability,13flashing,32flashpoint,13flashtank,77flooding,140flow-controlvalve,25flowdiagrams,14,16,18,19-20flowrate,28fluegas,12fluid,1fluidproperties,1–4 gravity,3 miscibility,3 pressure,3 solubility,4 temperature,2–3foaming,65fractionation,131–133fractionationandliquidtreating,131–143fractionation,131–133 monitoringoffractionalizationplants,139–140 NGLfractionationplants,134–136 NGLproducttreating,141–143 operatingproblems,140 packedcolumns,134 productspecification,138fractionationcolumn,26free-liquidremoval,94freezing,103freezingpoint,6Fuller’searth,143fundamentals combustion,13 composition,10 fluidproperties,1–4 heatenergy,10–12 hydrates,7–9 idealgaslaw,4–6
gallonsperminute(gpm),100gasandliquidheaters,32–33gascap,1gaschiller,41gasdehydrationunits,92gas/gasexchanger,38
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186 Plant Processing of Natural Gas
gaspermeation,62GasProcessorsSuppliersAssociation(GPSA),6gasproductionstreams,1gassub-cooledprocess(GSP),119gassurgetank,41gastreatment,1gastreatmentunit,26gaugepressure,3generalconsiderationsfordehydrationandmercuryremoval,92generaloperatingconsiderationsforgastreating,65glycol,4glycolcryogenicprocess,38glycolinjectionproblems,78–80glycol/J-Tvalvecoolingprocess,38–40glycol/propanesystem,37–38GPSA Engineering Data Book,28,84gravity,3grossheatingvalue,12
heatenergy,10–12heatingvalue,12heavy-liquidproduct,20high-integritypressureprotectionsystem(HIPPS),28high-liquidlevelalarm(HLA),102horsepower,10hot-richoilflashtank,104hydrateplug,7hydrates,7,8–9,73hydrocarbondew-pointcontrol,35hydrocarbonfluids,1hydrocarbontreating,51–65 adsorptiononasolid,61–62 chemicalreactions,51–58 gastreatingprocesses,51 generaloperatingconsiderationsforgastreating,65 membraneprocesses,62–64 mixedchemical/physicalabsorptionprocesses,60 physicalabsorptionprocesses,58–60hydrocarbontreatingunits,27hydrogensulfide,1
idealgaslaw aboutgenerally,4 boilingpoint,6 freezingpoint,6 liquidphase,5 vaporpressure,5–6immiscibleliquids,3incorrectcoalescerliquidlevel,95inhibitorinjection,76–80initialboilingpoint,13
inletseparation,65interstagecoolers,32
Joule-Thomson(J-T),35Joule-Thomson(J-T)expansion,117Joule-Thomson(J-T)valvesystem,38
Kelvinscale,2kinetichydrateinhibitors(KHIs),76KOdrum(knockoutdrum),36
latentheat,11leanglycol,77leanoil,44leanoilabsorption aboutgenerally,97 absorberworkings,99–100. potentialproblemswith,102–103 presaturation,100 recoverysystem,98–103levelcontrollers,44levelcontrolvalve,38liquiddesiccants,80,82–84liquidexpansion,6liquid-liquidtreating,141–142liquidphase,5liquidseparator,38liquid-solidtreating,143lowdosagehydrateinhibitor(LDHI),76lowtemperatureseparation(LTS),35
magneticbearings,126matter,10membraneprocesses,62–64membranes,146mercaptans,26mercuryadsorber,95mercuryembrittlement,92mercuryremovalafter-filter,95mercuryremovalcosts,94mercuryremovalunit(MRU),90Meroxprocess(commercialprocess),142metalsulfideoncarbonoralumina,93metalsulfidesystems,93methane,8methanol,7methyldiethanolamine(MDEA),52methylethyleneglycol(MEG),37millionstandardcubicfoot(MMscf),100miscibility,3
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miscibleliquids,3mixedchemical/physicalabsorptionprocesses,60mole,10molecularpercent,10molecularsieve,35,93molecularsieveprocess,61,85–86molecularweight,10molecule,2monitoringoffractionalizationplants,139–140MRUlocation,94multiplestagerefrigeration,46mutuallysolubleliquids,4
naturalgas,1naturalgasliquid(NGL)fractionationplants,134-136 aboutgenerally,133–136 butanesplittercolumn,137 debutanizer(DeC4)column,136 deethanizer(DeC2)column,137 deisobutanizer(DIB)column,137 depropanizer(DeC3)column,137naturalgasliquid(NGL)producttreating,141–143 liquid-liquidtreating,141–142 liquid-solidtreating,143naturalgasliquid(NGL)recovery-cryogenic,113–129 aboutgenerally,113–114 applications,115–122 cryogenics,127–129 truboexpanders,122–126naturalgasliquid(NGL)recovery-leanoilabsorption,97–111 leanoilabsorption,98–103 rejectionsystem,104–107 separationsystem,108–111naturalgasliquid(NGL)recoveryoperations,77naturalgasliquids(NGLs),35“NaturalGasolineSpecificationsandTestMethods”(GPAPublication3132),138netheatingvalue,12nitrogenrejectionunits(NRUs) cryogenicNRUprocesses,146–148 NRUprocesses,149–152 processselection,145–146nonassociatedgas,1nonregenerativeacidgasremoval,51
off-gases,12oil-filmresonance,125oilpurification,109–110oil-to-gasratio,100oilwhip,125oilwhirl,125
operatingproblems,140.Seealsoproblemsoverheadproduct,134oxidation,57
packedcolumns,134paraffins,88physicalabsorptionprocesses propylenecarbonateprocess,59–60 Rectisol®process(commercialproduct),60 Selexol®(commercialproduct),59physicalpropertiescomparison,8–9pigging,30piggingfrequency,28pigreceivers,25,28pipelinegrid,35platefinexchangers,127pooroutletwaterdewpoint,88–89potassiumcarbonate,51poundspersquareinch(psi),3poundspersquareinchabsolute(psia),3poundspersquareinchgauge(psig),3prefractionator,150presaturation,100presaturationsystems,100pressure,3pressuredrop,89,94pressureindicators(PIs),44pressureswingadsorption,145pretreatment,147–148problems withdew-pointcontrolandrefrigerationsystems,44–46 withethane/butaneinpropane,44 withfractionators,140 inglycolinjectionsystems,78 withinadequateinletseparation,65 withleanoilabsorption,102–103 withmethanol,77 intherecoverysystem,102 withrejectionsystem,107 withretrogradecondensation,25 withseparationsystem,110–111 solvedbythegasfeedreceptionandcondensate stabilizationunitequipment,33 withsulfur-impregnatedcarbonsystems,92processdescriptions comparisonofdew-pointprocesses,40 dew-pointcontrolandrefrigerationsystems,35–46 glycol/J-Tvalvecoolingprocess,38–40 glycol/propanesystem,37–38 silicagelprocess,36–37 unitspecifications,40
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processengineers,2processselection cryogenicabsorption,145 cryogenicdistillation,146 membranes,146 nitrogenrejectionunits(NRUs),145–146 pressureswingadsorption,145productspecifications,27,138propane,3propane-recoveryprocess,120propylenecarbonateprocess,59–60
quenchcolumn,70
Rankinescale,2reboilers,16reciprocatingcompressors,32reclaimers,53recompression,148reconcentrators,38recoverysystems,98–103rectificationsection,21rectifying,21Rectisol®process(commercialproduct),58–60reflux,20,108refrigerationsystems,35,41regeneration,26regenerationgas,36regenerativeacidgasremoval,51regenerativemolecularsieve,93ReidVaporPressure(RVP)requirement,28,133rejectionsystem,98 hotrich-oilflashtank,104 possibleproblemswith,107 rich-oilflashdemethanizer,105–106retrogradecondensation,25richglycol,77richoil,99rich-oildemethanizer(ROD),105rich-oildemethanizerbottoms,105rich-oilflashdemethanizer,105–106rich-oilfractionator,107royaltycharges,59
safetyinstrumentalsystems(SIS),28SCOTprocess(commercialproduct),67,70Selexol(commercialproduct),59sensibleheat,11separationsystem,98
oilpurification,109–110 possibleproblemswith,110–111 still,108–109separationtower,18shell-and-tubeexchangers,16ShellClausOff-GasTreating(SCOT)process,67,70silicagelprocess,35,36–37silicagelsandactivatedalumina,85slipping(rejection),54slugcatchers,25,30soliddesiccants,84–85solubility,4“SpecificationandTestMethodsforLPGas”(GPAPublication2140),138specificgravity,3split-vaporprocess,119stabilizationcolumn,32stabilizeroverheadcompressors,32stackloss,12stagedseparation,42standardcubicfoot(scf),12stericallyhinderedamine-basedsolvents,54stills,107,108–109strippingsection,20subambienttemperature,58Sulfinol®process(commercialproduct),60sulfur-impregnatedactivatedcarbon,92–93sulfurrecovery,67–70 catalyticrecovery,68,70 catalyticstep,67 thermalandcatalyticreaction,67 thermalprocess,67 thermalreaction,67 thermalstep,67sulfurrecoveryandClausoff-gastreating Clausoff-gastreating,70 sulfurrecovery,67–70sulfurrecoveryunits(SRUs),54,67superficialvelocityandresidencetime,95sweeteningacidgases,51switchingvalvesforadsorptionandregenerationoperation,90
tailgas,54technologyselection designbasisandspecificationsfordehydration andmercuryremoval,92 metalsulfideoncarbonoralumina,93 regenerativemolecularsieve,93sulfur-impregnatedactivatedcarbon,92–93temperature,2–3
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temperaturedifferential,12temperatureindicators(TIs),44thermalandcatalyticreaction,67thermalprocess,67thermalreaction,67thermalstep,67thermocouples,88thermodynamicinhibitors,76thiobacillusbacteria,57–58totalinstallationcost(TIC),93towers,67towerwithtrays,18trays,18treatingandprocessing,23–26triethyleneglycol(TEG),37triple-columncycle,149truboexpanders,122–126turboexpanderprocess,115–119turboexpansion,97
turndownratio,26Twister™Supersonicseparator(commercialproduct),40
unitspecifications,40upstream,25
vacuum,59valvetrays,131vaporization,53vaporizing,5vaporpressure,5–6vapors,4volatility,14volumeexpansion,6
waterdew-pointcontrol,35weirs,131wetstills,108workingfluid,35
Y-grade,97
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To obtain additional training materials, contact:
PETEXThe University of Texas at Austin
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Catalog No. 3.11020ISBN 0-88698-223-5
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Cover PageTitle PageTable of ContentsFiguresTablesForewordAcknowlegmentsAbout the AuthorsChapter 1: FundamentalsChapter 2: Feed Gas Receiving and Condensate StabilizationChapter 3: Dew-Point Control and Refrigeration SystemsChapter 4: Hydrocarbon TestingChapter 5: Sulfur Recovery and Claus Off-Gas TreatingChapter 6: Dehydration and Mercury RemovalChapter 7: NGL Recovery -- Lean-Oil AbsorptionChapter 8: NGL Recovery -- CryogenicChapter 9: Fractionation and Liquid TreatingChapter 10: Nitrogen Rejection Unit (NRU)AppendixGlossaryIndex