ProcessSafetyCalculations
RenatoBenintendi MScCEngFIChemE
Elsevier
Radarweg29,POBox211,1000AEAmsterdam,Netherlands
TheBoulevard,LangfordLane,Kidlington,OxfordOX51GB,UnitedKingdom
50HampshireStreet,5thFloor,Cambridge,MA02139,UnitedStates
# 2018ElsevierLtd.Allrightsreserved.
Nopartofthispublicationmaybereproducedortransmittedinanyformorbyanymeans,electronicormechanical, includingphotocopying,recording,oranyinformationstorageandretrievalsystem,withoutpermissionin writingfromthepublisher.Detailsonhowtoseekpermission,furtherinformationaboutthePublisher’spermissions policiesandourarrangementswithorganizationssuchastheCopyrightClearanceCenterandtheCopyright LicensingAgency,canbefoundatourwebsite: www.elsevier.com/permissions
ThisbookandtheindividualcontributionscontainedinitareprotectedundercopyrightbythePublisher(otherthan asmaybenotedherein).
Notices
Knowledgeandbestpracticeinthisfieldareconstantlychanging.Asnewresearchandexperiencebroaden ourunderstanding,changesinresearchmethods,professionalpractices,ormedicaltreatmentmaybecome necessary.
Practitionersandresearchersmustalwaysrelyontheirownexperienceandknowledgeinevaluatingand usinganyinformation,methods,compounds,orexperimentsdescribedherein.Inusingsuchinformationor methodstheyshouldbemindfuloftheirownsafetyandthesafetyofothers,includingpartiesforwhomtheyhavea professionalresponsibility.
Tothefullestextentofthelaw,neitherthePublishernortheauthors,contributors,oreditors,assumeany liabilityforanyinjuryand/ordamagetopersonsorpropertyasamatterofproductsliability,negligenceor otherwise,orfromanyuseoroperationofanymethods,products,instructions,orideascontainedinthe materialherein.
LibraryofCongressCataloging-in-PublicationData
AcatalogrecordforthisbookisavailablefromtheLibraryofCongress
BritishLibraryCataloguing-in-PublicationData
AcataloguerecordforthisbookisavailablefromtheBritishLibrary
ISBN:978-0-08-101228-4
ForinformationonallElsevierpublicationsvisitour websiteat https://www.elsevier.com/books-and-journals
PublisherDirector: JoeHayton
AcquisitionEditor: AnitaA.Koch
EditorialProjectManager: AnaClaudiaA.Garcia
ProductionProjectManager: SruthiSatheesh
CoverDesigner: VictoriaPearson
TypesetbySPiGlobal,India
Preface
Theaimofthisbookistoprovidethereaderwithsomeguidanceoncalculationsinprocess safety.Accordingly,theintentionoftheauthorwasnottoduplicateortoemulatethe manyexcellentliteratureworksproducedsincethemanyyearsofstudyonprocesssafety techniquesandmodels,butrathertobuild-upalogicalandfluidthreadtoovercomedoubts, uncertainties,anddifficultiesoftenmetincalculationexercises.Theavailableliterature sourcesoffereitherabroadrangeofdifferentmodelsandapproachesor,evenwhentheyare calculationsoriented,sometimesunavoidablyandfaultlesslyleavesomegapsinthecalculation criteria;thisisafeetobepaidtotherichnessandvarietyofdataandinformation.This bookhasadifferenttarget:toprovideaclearindicationonwheretogoinpracticalapplications whenacrossroadsismet,andwhenavailabledataaredifficulttobeconvertedintofiguresand findings.Nevertheless,thetheoreticalandconceptualbackgroundisdeemedtobeeffective inenablingtheusertoproperlyframethetopicsand,tosomeextent,someaspectsnotincluded intheexistingliteraturesourceshavealsobeendealtwith,fromprinciplestoapplications.
Thebookisthefinalstepofalongtriptheauthorstartedin1988,when,morethan10yearsafter theincidentsofFlixboroughandSeveso,andsomeyearsaftertheunresolvedtragedyof Bhopal,theSevesoIdirectivewasactuatedinItaly.ItisdoubtlessthatthisEuropeanlegislative acthasgivenatremendousimpulsetothedevelopmentofsystematicmethodsandtechniques inprocesssafetyengineering.Intheninetiestheauthorwasinvolvedasateacherinthefinal partoftheChemicalPlantscourseheldattheChemicalEngineeringfacultyattheUniversityof Salerno(Italy),providingsomeguidanceaboutprocessriskassessmentmethodologies.Inthe sameyears,alongexperienceacquiredasaninstructorwithinthecourseforRiskAnalysis, managedbytheItalianInspectoratesengagedintheSevesoDirectivesafetyreportsassessment, clearlyshowedhowdifficultandchallengingitwastorelatetheorytorealcases.Specifically, evenifchemicalengineers,andengineersingeneral,shouldhaveathoroughknowledgeof backgroundconceptsunderpinningprocesssafetystudies,theexperiencehasshownthat culturaltransitionfromprocesstoprocesssafetyengineeringisneitherautomaticnoreasy.The authorhasanalysedthisaspectinarecentarticle(Benintendi,2016),wherehehaspointedout thattheeffectivenessof adding-on basicprocesssafetyconceptstotheuniversitybackgroundis notalwayshigh.Thecombinationoftheexperienceacquiredfromprocesssafetyteaching, tutoring,andlecturingatseveraluniversitiesintheUK,Italy,Asia,andUSA,withthe
professionalexpertisedevelopedinalmost30years’work,hassuggestedthattheprovisionof basicconceptsalreadycalibratedonprocesssafetyismuchmoreeffective.
Inthisrespect,thisbookincludesafirstpartwherebasicconceptsofchemistry, thermodynamics,reactorengineering,hydraulics,andfluid-dynamicsarereviewedwitha specificfocusonprocesssafetyscenarios.Dozensoffullyresolvedexamplesfocusingon processsafetyapplicationshavebeenincluded.This Fundamentals sectionendswithone chapterdealingwithstructuralanalysisforprocesssafetyandanotheroneincludingastatistics andreliabilityoverview,aimingtoprovidethebasicconceptstoproperlymanagethe probabilisticaspectofriskassessmentstudies.Allthesefirstchaptersincludemanyliterature data,withtheintentiontoprovidetheuserswithacompletetoolfortheircalculations.
The ConsequenceAssessment sectionisorganisedaccordingtothetypicalsequentialoutcomes followingareleaseafterlossofcontainment.Someeffortshavebeenmadetoensurethatall potentialgapsanduncertaintiesinthecalculationswerecoveredandovercome,basedonthe professionalinvolvementoftheauthorinmanyprojectsdealingwithoilandgas, petrochemical,pharmaceutical,finechemistry,food,andenvironmentalsubjects.Inthis respect,theuserswillbedrivenacrossarelativelysimpleanddirectroute,unlikewhathappens whentheygototheliterature,whereobviouslyamuchwiderspreadofmethodsisprovided. Chapter7 focusesonreleasesfromcontainmentsandfrompools:onthebasisofthetheoretical backgroundprovidedinthe Fundamentals section,asystematicanalysisofpossiblescenarios hasbeencarriedout,withthesupportofmanyfullyresolvedexamples.Releaseofcarbon dioxidehasbeendealtwithindetail,duetotherelativelynewhazardousscenariospresented aftertheintroductionofCarbonCaptureandStorage(CCS)process,andtothespecificnature ofthissubstance,whichshowsasolid-liquidequilibriumbelowthetriplepointanddoesnot fullybehaveaccordingtoequilibriumthermodynamics. Chapter8 presentsdispersionmodels; intheauthor’sintention,theefforthasbeenmadetoresolvethevariousuncertaintiesmetby processsafetyengineersonwhichmodeltoadopt,whichregimetoselect,whichphaseofthe dispersionroutetoidentify,andwheretolocaliseit.Keyparametershavebeenidentifiedto drivethisapproachwiththesupportofmanyexamples.Eisenberg’smodelforflashfireand Kalghatgisolid’sflamemodelforjetfirehavebeenselectedfortheirsimplicity,completeness, androbustnessin Chapter9,whichcoversfire.Aspecificfocushasbeenmadeonignition sources,accordingtothesystematicBSEN1127-1standard,withtheaimtoreducethe incompletenessoftheapproachoftenfollowed. Chapter10 dealswithgasandvapour explosions,consistingofallofscenariospotentiallyresultinginsignificantoverpressures, includingBLEVE,RapidPhaseTransition,andthermalrunaway.TheMultiEnergyMethod (MEM)hasbeenfittedwiththefindingsoftheGAMEprojects,andthishasbeenveryeffective inremovingthetraditionallargelevelofsubjectivityanduncertaintyinblastcurveselection.A MEMdetailed,andfullyresolved,examplehasshownaverygoodconsistencywiththe findingsoftheBaker,Strehlow,andTang(BST)method. Chapter11 hasbeenincludedto coverdustexplosions.Inadditiontothemodelsdescribingtheprimaryandthesecondary
explosions,someHAZIDcasesrelatingtodustprocessingequipmenthavebeenincluded, accordingtothegreatemphasisthemachineryandtheATEXdirectiveshaveputonthis specificaspect.AcasestudydealingwiththeImperialSugarCompanyhasbeenanalysedand verifiedagainstsomecalculationfindings.
Chapter12 dealswithQRAtechniques,includingtheexceedancefrequencycurvebuild-up,the ALARPmodeldemonstration,theFNcurves,andthepartscount.Someapplicationshavealso beengiveninthischapter.
Inthisbook,unlessotherwisespecified,allunitsareexpressedaccordingtotheInternational System(SI)ormkssystem.Thisbookaimstosupportscientistsandengineersworkingin processsafetyengineering.Itisworthrepeatingthatitisabookofcalculationsofferingalarge numberofdatausefulforthispurpose.Theauthorguessesthatitisnotfreefrommistakesand defects,andtheauthorapologiesinadvanceforthat.Hewillbegratefulforanycontributions readerswillwishtogivehim,toensurethattheobjectivesthewriterhadinhismindcanbefully achieved.
Reading(Berkshire),30April2017
Reference
Benintendi,R.,April2016.Thebridgelinkbetweenuniversityandindustry:akeyfactorforachievinghigh performanceinprocesssafety.Educ.Chem.Eng.15,23–32. IChemE,Elsevier.
Massbalanceonagenericspacedomain.
where:
- Win isthemassenteringthespacedomain.
- Wout isthemassleavingthespacedomain.
- Wgen isthemassgeneratedorconverted.
- Wacc isthemassinventoryvariation.
AccordingtoLavoiser’sprinciple, Wgen existsonlyforcomponentswhicharetransformedinto others.
Processsafetyengineeringentailsabroadrangeofcomplexandvariablescenarioswherefull understandingofstoichiometryandmassbalancesisnecessarytoproperlyanalyseandassess therelatedprocessandplantconfigurations.Somecasesarediscussedhere,andspecific scenarioshavebeenanalysedinthenextparagraphs.
1.1.2ChemicalReactions
Alowpressurevesselcontainsastoichiometricmixtureofcarbonmonoxideandpureoxygenat ambienttemperature To (Fig.1.2).
Thesystemundergoesachemicalreactionthatconvertsallcarbonmonoxideintocarbon dioxideandisassumedtobeatthermalequilibriumsothatinitialambienttemperatureis attained.Applicationofidealgaseslaw,with V and To constant,gives:
where N1 and No arethefinalandinitialnumberofkmolesofproductandreactantsrespectively, whichinthisspecificcasecoincideswiththereactionstoichiometriccoefficient srp:
Fig.1.1
Itcanbeconcludedthereaction,assuminganoverallisothermalandisochoriccondition, causesa33%pressuredrop,whichcouldresultinacatastrophicoutcomeforthevessel.
1.1.3JetFlowsFromPressurisedSystems
Jetflowsfrompressurisedcontainmentsarefrequentinprocesssafety.Theconsequencesof toxicorflammablecompounddispersionstrictlydependonthejetdynamics.Thescenario shownin Fig.1.3 illustratesthereleaseofhydrogensulphidefromapipeline.
Thetoxicgasisreleasedwithamassflowrateof WH2 S .Airisentrainedintothejetaslongas thisisdeveloped,resultinginaprogressivedilutionofH2S.Dependingontheeffectofthe entrainment,toxicconcentrationsareproportionallyreduced,whileflammabilitywillbe promotedbyairmixingwithinaspecificregionofthejet.Assumingasteadystatevalueof
Fig.1.3
Fig.1.2
Oxidationofcarbonmonoxideleadingtovesseldepressurisation.
WH2 S ,andindicatingwith WAIR(z)theairentrainmentmassflowrateperlengthunitalong z,the massbalanceat z ¼ h canbewrittenas:
Ithasbeenshownhowimportantthecorrectmanagementofthisbalanceisinjetflow consequenceassessmentstudies.
1.1.4FlashFlow
Aflashflowisthereleaseofaliquidfromacontainmentwheretheoperatingtemperatureis significantlygreaterthanitsdownstreamboilingtemperature,typicallythenormalboiling temperature.Theliquidisforcedtovaporiseafractionofittoreachthedownstream equilibriumcondition.ThisisthecasewithLPGstoredatambienttemperature(Fig.1.4).
Theliquidmass W splitsintotheflashedvapourfraction XV andtheliquidfraction XL.Itis:
1.1.5AbsorptionandAdsorption
Removaloftoxicordangerouscompoundscanbeaccomplishedviamasstransferunits,suchas absorberoradsorptiontowers.Atypicalexampleistheaminetreatmentofsourgas(Fig.1.5), orabsorptionofcarbondioxidewithsodiumhydroxide.Forsourgastreatment,neglecting changesofflowrates Q and q,themassbalanceofH2Scanbesimplifiedconsideringthe concentrationofsulphur S
Emptyingofnitrogenblanketedtank.
Solution
Thetankheadspaceisassumedtobeoccupiedbynitrogenonly.Applicationofidealgaslaws:
ImposingthatpressureismaintainedconstantbythePCV.Itis:
Simplifyingandrearranging:
Consideringthat dV dt ¼ Q andthat V ¼ Vo + Q t ,Eq. (1.20) maybewrittenas:
Separating:
Solving:
where nN2o isthemolarnitrogenamountof Vo at t ¼ 0.Thisresultisintuitivebuthasbeen rigorouslyobtainedhereviatheapplicationofmassbalances.
1.2StatesofSubstancesinProcessSafety
Substancesinprocesssafetycanbepresentinthefollowingforms:
1.2.1GasesandVapours
Gasisafundamentalstateofsubstancesatatemperaturehigherthantheircritical temperature.Hydrogenandmethanehavetoberegardedasgasesatambient temperature,whereaspropaneandsulphurdioxidearevapoursandcanbecondensed bycompression.
1.2.2Liquid
Liquidsarethecondensedphaseofvapours.Theycanbeinequilibriumwiththeirvapoursat anytemperature,andvapourpressureistheequilibriumpressureexertedbyvapourabove theliquids.Liquidscanbemiscibleorimmiscible,polarornon-polar,andthisbehaviour stronglyaffectsthereleaseanddispersionscenarios.
1.2.3Dusts
Inadditiontobeingcombustible,dustswhicharefinelydividedsolidparticlescanbe explosive.Accordingto BS-EN60079-10-2:2015,combustibledusts,500 μmorlessin nominalsize,mayformanexplosivemixturewithairatatmosphericpressureand normaltemperatures.Particulartypesofsolidparticles,includingfibres,arecombustible flyings,greaterthan500 μminnominalsize,whichmayformanexplosivemixturewith airatatmosphericpressureandnormaltemperatures(BS-EN60079-10-2:2015).
Asforgasesandvapour,themechanismofdustexplosionconsistsoftherapidreleaseofheat duetothechemicalreaction:
Fuel+oxygen ! oxides+heat
Metaldustscanalsoexothermicallyreactwithnitrogenandcarbondioxideaccordingto Eckhoff(2003),whichclassifiesexplosivedustsasfollows:
-Naturalorganicmatters
-Syntheticorganicmaterials
-Coalandpeat -Metalssuchasaluminium,magnesium,zinc,andiron.
Bothstates,normalandstandard,areassumedinthisbooktobeat273.15Kand1atm,according to Hougenetal.(1954).Undertheseconditionsthenormalmolarvolumesareasfollows:
Volumeof1mol ¼ 22:414L
Volumeof1kmol ¼ 22:414m3
1.3.4PartsperMillion(GasandLiquidPhase)
ppmw(weight)—typicalinliquids
Ifliquidiswater,assumingwaterdensityas1000kg/m3 or1000g/L:
1.3.5PartsperMillion(GasPhase)
whereallsymbolsareknown.
1.3.6MolarConcentration(AqueousSolutions)
Itisdenotedas[X]andindicatesmoles/litre.
1.3.7ConcentrationUnitsConversionSummary
See Table1.1.
Example1.4
TheIDLH(immediatelydangeroustolifeandhealth)ofsulphursulphideis100ppmv.Calculate itasmg/Nm3 andasmolarfraction.(MW 34)
Solution
Example1.6
100kgofsolidsulphurareburntinacombustoratatmosphericpressure.Knowingthat10% ofairexcessisused,findthepartialpressureofnitrogenintheoffgas.(Sulphurmolecular weight:32,nitrogen:28,oxygen:32).
Solution
Thecombustionreactionis:
100kgofsulphurareequivalentto3.125kmol.Fromthereactionstoichiometryandconsidering 10%ofairexcess:
Thisresultisintuitive,duetheequimolarS/O2 ratio.
1.4.2KineticsandEquilibriuminGasReactiveMixtures
ThegasphasereactionrateofthegeneralchemicalreactionpresentedinEq. (1.2) maybe writtenas:
where rf istheforwardreactionrate, kf isthekineticconstant,and pRi arethereactant’s partialpressures.Somereactionsmaybereversible,thereforeasimilarequationmaybewritten forthebackwardreaction:
Atequilibriumthetworeactionratesarethesame:
where KP istheequilibriumconstant.
Manyimportantreactivemixturesinprocesssafetyreachtheequilibrium.
1.4.3LiquidSolutions
Liquidsolutionsareobtainedbydissolvinggaseous,liquid,orsolidsubstancesinliquids. Dependingonthenatureandthebehaviourofthedissolvedsubstances(solute),andofthe liquid(solvent),awiderangeofphysical–chemicalscenariosmaybeobtained,whichhaveto bewellunderstoodinorderforthemtobeproperlyanalysedprocesssafetywise.
Liquid–liquidsolutions
Miscibleliquidsformhomogeneoussolutions,whereasimmiscibleliquidsformtwophase dispersedemulsions.Ageneralcriterionusedtoestablishwhetherornottwoormoreliquids aremiscibleiscomparingtheirpolarfeatures.Theoldsaying likedissolveslike isaveryuseful ruleofthumb.Therefore,polarspecies,suchaswater,havetheabilitytoengageinhydrogen bonding.Alcoholsarelesspolar,butcanformhydrogenbondingaswell.Duetoitsstrong polarity,waterisanexcellentsolventformanyionicspecies.Non-polarspeciesdonothavea permanentdipole,andthereforecannotformhydrogenbonding.Organiccovalentliquids,such asmanyhydrocarbons,fallwithinthiscategory.
Thefollowinggeneralcriteriacanbeadoptedtopredictsolubilityofchemicals:
-Symmetricstructuremoleculeshaveaverylowdipolemomentandarenotdissolved bywater
-MoleculescontainingO HandN Hcanformhydrogenbonds
-Moleculescontainingfluorineandoxygenareexpectedtohaveahighdipolemoment -Purehydrocarbons,oilandgasoline,arenon-polarorweakmolecules
Dipolemomentgivesjustaverygeneralindicationofsolubilityofmolecules. Table1.2 includesthedipolemomentforsomeorganicandinorganicsubstances.
Acommonpracticeistoassumethefollowingscaleofpolaritywithrespecttothedipole moment:
-Dipolemoment < 0.4:Nonpolarmolecule.Behaviourequivalenttohomopolar covalentbond.
-0.4 < Dipolemoment < 1.7:Polarmolecule.Behaviourequivalenttoheteropolar covalentbond.
-Dipolemoment > 1.7.Verypolar(ionic)molecule.
Table1.3ParametersofHoffman’sequation
and b and TB (normalboilingtemperature)areincludedin Table1.3.
ForC7+ fractionsthefollowingequationscanbeused:
Pressureisgiveninbar.
1.4.4AzeotropicMixtures
ApplicationofRaoult’lawshowsthatmixturesofvapourcompositionaregenerallydifferent fromliquidcomposition,duetothedifferentvolatility(vapourpressures)ofthecomponents. Thisisnotalwaystrue,becausesomemixturesbehaveasasinglepurecompoundin correspondencetoaspecificcompositionandtemperatureatgivenpressures.Azeotropic compositionisfoundatconcentrationswherevolatilityisreversed,asshownin Fig.1.8.that representsthemixturesoftwopuresubstances, A and B.Intheleft-handzone,component A is morevolatilethancomponent B,whereasintheright-handzoneitistheopposite.Therefore
Fig.1.8
Liquidboilingpointsandvapourcondensationtemperaturesforminimum-boilingazeotrope mixturesofcomponents A and B
point Q intheliquid–vapourequilibriumzonecorrespondstoaliquidthatismorerichin B and toavapourthatismorerichin A thantheoriginalcomposition.Forpoint P itistheopposite.In correspondencetotheazeotropiccomposition,andtotheazeotropictemperature,vapourwill havethesamecompositionasliquid.
Table1.4 includessomeazeotropicmixtureswiththeindicationoftheazeotropiccomposition (firstcomponent)oftheazeotropictemperatureat1atm.
Table1.4Azeotropicmixturesat1atm(Dean,1999)
