YOUSUF Ch. 4

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result discussion of master project .. A STUDY ON CORRELATION OF SAFETY, HEALTH AND ENVIRONMENTAL PROPERTIES AT INHERENT LEVEL: BENZENE SYNTHESIS ROUTES

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<p>CHAPTER 4RESULTS AND DISCUSSIONThissectiondiscussesalltheresultsofthestudy,whichwereobtainedbyapplyingthesixselectedindexmethodsonthethreeprocessroutesofbenzenesynthesiscasestudy. Afterunderstandingthecriteria andrequirementofalltheindexmethodsindetail, thefirststepwastocollectorestimatethedataneededforthe index calculation. The data for all the parameters is provided in Appendices D F. Then, the scoring of the hazard parameter was done based on the penalty systemofeachindexmethod,asdescribedinChapter2ofthisthesis. Itshouldalsobenotedthatthepenaltysystemforallindexesisconsistent,inthatahigher penaltyscore indicates a more unsafe or severe (more hazardous) situation.4.1 Benzene Synthesis Routes case studyThe six selected index methods were used to examine the inherent propertiesforthe threeprocessroutesofbenzeneproduction. Theprocessroutesare;toluenehydrodealkylation(TDA),pyrolysis gasolinehydrogenation(Pygas)andcatalyticreforming of naphtha764.1.1 Case study 1: Toluene hydrodealkylation process route (TDA)In the TDA process, toluene reacts with hydrogen to produce benzene and methane.Thereactiontakesplaceat630 C and23bar(Turtonet.al. 1998). TheTDAprocessroutecomprisesoftwonon-catalyticvapor-phasereactions. Thefirstreactionistheonlymainreactionwhichisaccompaniedbythesidereactionasshown below. The TDA process route is described in more details in Section 2.9.Main reaction: Toluene + Hydrogen Benzene + Methane (4.1)Side reaction: Benzene Diphenyl + Hydrogen (4.2)4.1.2 Case study 2: Pyrolysis gasoline hydrogenation process route (Pygas)Pyrolysis gasoline (Pygas) contains a mixture of about 100 chemicals so thatmanyhydrogenationreactionstakeplaceinthereactor. However,theC6cutrepresents 26.23% of the total chemicals fed to the reactor. This is the second largestfeedafterbenzene46.4%(materialbalancetable in Chapter2). Cyclohexeneisconsidered to be the key component of C6cut (Mostoufi et al., 2005). Based on that,the hydrogenation of cyclohexene is selected to be the main reaction chosen for thiscasestudy. Sincebenzenerepresents46%ofthetotalfeedintothereactor,itwasinvolvedintheassessmentofthiscasestudy. Methylcyclohexanewasselectedtorepresent the C7cut in the assessment. The selection of methylcyclohexane is basedon the availability of the data needed for the assessment.In pyrolysis gasoline hydrogenation, the C5cut is removed first by distillation(depentanizer) and sent via the overhead of the depentanizer column to the refinery.TheC9+cutisalsoremovedintheBTX(benzene,toluene&amp;xylene)distillationcolumnandsentviathebottomtotherefinery. The central C6 C8cutisthen77transportedasatopoutletfromBTXdistillationcolumntothereactorforhydrogenation. Thehydrogenationinthereactorisdoneinvaporphaseattemperatureof230 Candpressureof26bar. However,thebottomoutletis thentransportedthroughdifferentdistillationcolumnsformorepurificationandthentoproducebenzene(thePFDanddetaileddescriptionoftheprocess isprovided inChapter 2). The formula below shows the hydrogenation of cyclohexene.C6H10+ H2C6H12(4.3)4.1.3 Catalytic reforming of naphtha process routeNaphthafeednormallycontainsabout300chemicalcompounds.Componentsliken-heptane,n-octane,methylcyclohexane,toluene,ethylbenzene,andxylenesareusuallypresentinsignicantconcentrations. Thesecomponentsrepresent more than 63% of naphtha cut. The other components are present in muchsmaller amounts. All the compounds are present in the naphtha feed as paraffins (40 70 wt. %), naphthenes (20 50 wt. %), aromatics (2 20 wt. %) and olefins only(0 2 wt. %) (Antos and Aitani, 1997). This is the composition of typical straight-run medium naphtha.As shown in Figure 2.8 for the reforming process, naphtha feed is heated upto400 540 Candthenfedunderpressureof10 20 bar. Naphthafeedpassesthroughseriesofcatalyst-equippedreactorsandfurnacesbetweenthereactorstokeep the reactions temperature at desired level. The reactions that take place in thisprocessare;hydrogenationofolefins,isomerizationofparaffinstoiso-paraffins,dehydrocyclizationofparaffinstonaphthenesandthentoaromaticsanddehydrogenationofnaphthenestoaromatics. Asthereactionstakeplaceineachreactor, there is gradually increase in the aromatics concentration from the first to the78final reactor. The formulas below show some of the reactions that take place in thereactors.2 6 6 14 64 ) ( H H C H C Hexane nlization Dehydrocyc+ (4.4)2 6 7 14 73 ) ( H H C H C ohexane Methylcyclation Dehydrogen+ (4.5)The product from the last reactor is called reformate which is transported intoahigh-pressureseparatortoremovethelightcutC1-C2 (AntosandAitani,1997).Reformate is then sent via the bottom outlet into a stabilizer for more purification byseparatingtheC3-C4cutsfromreformatecut(C5-C8+) (Yang etal., 2008).Reformate is then transported into a reformate process unit for further processing toproduce benzene as the desired product. More details on catalytic naphtha reformingprocess route are provided in Chapter 2.4.2 Calculation and discussion of inherent SHE indexesInthisstudy,sixindexbasedmethodswereselectedforconductingtheassessment on the three process routes of benzene.From the six methods, each twomethods represent one aspect of the three main aspects of inherent safety, health, orenvironment. Forinherentsafety,twobasedindexmethods;InherentSafetyIndex(ISI) (Heikkila et al., 1996) and iSafe (Palaniappan et al., 2004) were selected. Foroccupationalhealthassessment,ProcessRouteHealthinessIndex(PRHI)(HassimandEdwards,2006)and InherentOccupationalHealthIndex(IOHI) (HassimandHurme,2010) wereselected.Thesemethodsassesstheinherentoccupationalhealthinessofaprocess. Theinherentenvironmentalevaluationmethodsare; theenvironmentalhazardindex(EHI) (Cave and Edwards, 1997), and InherentEnvironmental Toxicity Hazard; IETH (Gunasekera and Edwards, 2006).794.2.1 Inherent safety index (ISI) calculationAs mentioned earlier in Chapter 2, the inherent safety index (ISI) consists oftwomainindexes. The chemicalinherentsafetyindex(ICI)andprocessinherentsafety index (IPI). For this case study, the calculation of the ICIindex is summarizedin Table 4.1.Table 4.1: Calculation of the sub-indexes of chemical inherent safety index ICIforthe TDA, Pygas and naphtha reforming case studiesTDA processroutesCalculations of the ISISub-indexes of the ICIChemicals IINTICORIRMIRSIFIEITIFETMainreactionToluene 3 04 03 1 2 5Hydrogen 4 0 NA 4 0 4Benzene 3 0 4 1 4 9Methane 4 0 4 1 0 4Penalty of worst chemical 4 0 4 0 4 1 4 9Total ICIfor the step; ICI= 4 + 0 + 4 + 0 + 9 = 17Pygas processroutes Sub-indexes of the ICIChemicals IINTICORIRMIFIEITIFETCyclohexene 4 034 1 2 7Hydrogen 4 0 NA 4 0 4Cyclohexane 3 0 4 1 2 7Benzene 4 0 4 1 4 9Methylcyclohexane 3 0 4 1 1 6Penalty of worst chemical 4 1 3 4 1 4 9Total ICIfor the step; ICI= 4 + 1 + 3 + (4 + 1 + 4) = 17Naphtha reformingroute Sub-indexes of the ICIChemicals IINTICORIRMIFIEITIFETn-Hexane 3 044 1 2 7Methyl cyclohexane 3 0 4 1 1 6Hydrogen 4 0 NA 4 0 4Benzene 4 0 4 1 4 9Toluene 3 0 3 1 2 6Penalty of worst chemical 4 1 3 4 1 4 9Total ICIfor the step; ICI= 4 + 1 + 3 + 9 =17Forthecorrosivenesssub-index (ICOR),based onthedataprovidedinAppendices D1, E1andF1 nochemical involvedinthethree processroutes has80corrosivenesspropertyonmetals;thereforethe ICORisassignedapenaltyof(0).This value is determined based on the penalty system of the ICORwhich is providedinChapter2 (seeTable2.4). IntheTDAcasestudy,thetwoheatreactionsub-indexes for main IRMand side reactions IRSwere calculated to be (- 4134.65 j/g) and(0 j/g) and hence are given penalties of 4 and 0, respectively. In Pygas and naphthareforming case studies no side reactions take place so that only the heat reaction forthemainreaction werecalculatedtobe - 1405j/g forPygasand3005.8j/gfornaphthareforming. Hence,the IRMwas given apenalty of3 foreach. Theheatreactionforthemainandsidereactionswascalculated usingEquation (4.3). Thecalculation in more details is provided in Appendix D1., , ts reac ts reacfproductdproductsfH H Htan tan_ _ = A (4.3)Where;H, is the heat of a reactionHf, is the heat formation of a chemicalThesub-indicesforhazardouspropertiesincludingflammability(IF),explosiveness(IE)andtoxicity(IT)areeachdescribedbytheflashpoint,explosionlimits and threshold limit value TLV-15 min for each chemical substance. First, thepenaltyfor each sub-index is assigned forall chemicals in the process route. Then,the largest penalty sum of these sub-indexes, which represents the worst chemical inthe process, is taken to calculate the ICIindex. The largest sum which is 9 was givenby benzene. Benzene was assigned a penalty of (4) as a highly flammable chemicalwithalowflashpointtemperature(-11.1 C). Ontheotherhand,benzenewasassignedapenaltyof(1)foritsexplosionlimits(1.2% - 7.8%). FortheIT,benzenewasassignedapenaltyof(4)ascarcinogenicchemicalwithaTLV-15minof 2.5ppm. This result is generalized to all three case studies since benzene presents in allthreeprocessroutes. Thesepenaltiesweregivenbasedonthepenaltysystemprovided in Table 2.4.In the same manner, Penalties are assigned to each sub-indexfor all chemicals as shown in Table 4.1 above. Data is provided in Append D1, E1and F1.81, 9 4 1 4 = + + = + +benzene T E FI I I (4.4)After determining the penalties for all chemical sub-indexes, the ICIvalue wasobtained for each process route by applying Equation (2.2) as shown in Table 4.1. Inthe TDA and naphtha reforming case studies the ICIwas given a value of 17 for each.On the other hand, ICIwas given a value of 16 in the Pygas case study. This can beinterpreted by that the reactions in the three process routes take place with differentheatreaction asmentionedearlierandhence theIRMsub-indexwasassignedapenalty of 4 in both TDA and naphtha reforming case studies, while it was assigned apenalty of 3 in Pygas case study.The process inherent safety index (IPI) was also calculated as shown in Table4-2 below. This index consists of five sub-indexes which are; inventory (II), processtemperature (IT), pressure (IT), equipment safety (IEQ) and process structure (IST).IntheTDAprocess,thereactiontakesplaceat 630 C and 23bar,whichcontributetothepenaltyof4and 1totheITandIP,respectively. InPygashydrogenationprocess,theoperatingtemperatureis230 Candtheoperatingpressure is 26 and hence ITand IPwere assigned a penalty of 2 for each. In naphthareforming process, the feed is heated up to 400 540 C and then fed under pressureof10 20bar. Basedonthat,penaltiesof3and1were assignedtoITandIPrespectively. Theequipmentsafetysub-index(IEQ)wasgivenapenaltyof 3theTDAprocess,whileitwasgivenapenaltyof4bothPygasandnaphthareformingprocess. Thisisbecause;theTDA,Pygasandnaphthareformingprocessinvolvehighhazardequipmentsuchasanexothermicreactor(TDAprocess),furnace,compressor,andseparationanddistillationsystems(seetheprocessflowdiagramFig.2-6,2-7 and2.8). Theprocessintheseroutesisconsideredunsafeforhavingsuchhazardousequipments. Hence,theISTwasassignedapenaltyof3fortheprocessroutesexceptforthe naphthareforming routewherethe processincludethreefurnacesinserieswhichmeans repeatedreheatisrequired sothattheISTwasgiven a penalty of 4 (data is provided in Appendices D2, E2 and F2).82Table 4.2: Calculation of the sub-indexes of process inherent safety index IPIfor theTDA, Pygas and naphtha reforming case studiesTDA processroutesCalculations of the ISISub-indexes of the IPIMainreactionChemicals IIITIPIEQISTToluene3 4 1 3 3HydrogenBenzeneMethaneTotal IPIfor the route = 3 + 4 + 1 + 3 + 3 = 14Pygas process route Sub-indexes of the IPIChemicals IIITIPIEQISTCyclohexene4 2 2 4 3HydrogenCyclohexaneBenzeneMethylcyclohexaneTotal IPIfor the route = 4 + 2 + 2 + 4 + 3 = 15Naphtha reforming route Sub-indexes of the IPIChemicals IIITIPIEQISTn-Hexane3 3 1 4 4Methyl cyclohexaneHydrogenBenzeneTolueneTotal IPIfor the route = 3 + 3 + 1 + 4 + 4 = 15The inventory (Q) of chemicals was calculated to be 73 ton for TDA process,318.5tonforPygasprocessand127.6tonfornaphthareformingprocess. Thegreater is the inventory the higher is the penalty. Based on that, the IIwas assigned apenalty of 3 for the TDA and naphtha reforming routes, while it is assigned a penaltyof4forthePygasprocessroute. Thecalculationoftheinventorywasdone bymultiplyingthethroughputintothemajorvessels(F)withtheresidencetime()ofthese vessels(seeEquation4.1). Majorvesselsforeachprocessrouteareasillustrated in Figures 2.6, 2.7 and 2.8. The calculation of the inventory for all routesis shown in Table 4.3. = F Q (4-1)83Table 4.3: Calculation of the inventory in the TDA, Pygas and naphtha reformingprocess routesToluene hydrodealkylation process routUnit Feed (t/h) (h) Inventory, Q (ton)Mixer (V-101) 13.27 1 13.27Reactor (R-101) 20.86 0.083 1.73Flash drum(V-102) 20.9 1 20.9Flash drum(V-103) 11.6 1 11.6Distillation column (T-101) 25.6 1 25.6Total inventory 73.1Pygas hydrogenation process routeUnit Feed (t/h) (h) Inventory, Q (ton)Depentanizer(DA-17002) 24124BTX-Tower (DA-17003) 18.428118.428DPG-2 reactor (DC-17101) 16.410.0831.362Flash (FA-17201) 16.412116.412Stabilizer(DA-17202) 11.864111.864Splitter (DA-17001) 11.839111.839Ex. distillation (DA-17901) 31.84131.84Stripper (DA-17902) 148.5971148.597Distillation C. (DA-17203) 18.053118.053Clay Tower(FA-17203) 18.053118.053Clay Tower(FA-17204) 18.053118.053Total inventory Q 318.5Naphtha reformingUnit Feed (t/h) (h) Inventory, Q (ton)Reactor(R-100) 27.3211 0.083 2.267Reactor(R-100) 27.3211 0.083 2.267Reactor(R-100) 27.3211 0.083 2.267Flash drum(V-100) 27.3211 1 27.3211Stabilizer (V-101) 25.9023 1 25.9023Splitter(V-102) 25.901 1 25.901Ex. Distillation tower (T-100) 25.901 1 25.901Benzene tower(T-101) 17.358 1 17.358Total inventory Q 129.1844Afterdeterminingthepenaltyofeachsub-index,theIPIvalueswerecalculatedtobe15forbothTDAandPygasprocessroutesand14fornaphthareforming route as shown in Table 4.2 above. The total inherent safety index value(IISI) was obtained for all process routes by using (Eq. 2-1) as follows;84, 31 14 17 = + = + =PI CI TDA ISII I I, 32 15 17 = + = + =PI CI Pygas ISII I I, 32 15 17 = + = + =PI CI TDA ISII I I4.2.2 i-Safe index calculationTheiSafeindexmethodconsistsoftwoindexeswhicharetheindividualchemicalindex(ICI)andtheindividualreactionindex(IRI). TheICIconsistsofreactivity(Nr),flammability(Nf),explosiveness(Ne)andtoxicity(Nt)sub-indexes.Meanwhile,theIRI consistsof processyield(Ry),temperature(Rt),pressure(Rp),and the heat reaction (Rh) sub-indexes (Palaniappan et al., 2004). The iSafe methodisalmostsimilartotheISImethodexceptfortheprocessyieldsub-indexwhichisfromthe PIIS (EdwardsandLawrence,1993). TheiSafesub-indexesarescoredbased on the same penalty system developed for the ISI.As shown in Table 4.4 for the ICI index, a penalty was assigned to each sub-indexbasedonthedatacollectedforthe relevant parameter. Thepenaltyofthereactivitysub-index (Nr) isbasedontheNFPAreactivityratingdataforeachchemicalsubstance,whereastheotherthreesub-indexes(Nf, Ne, Nt)areevaluatedbasedonthedataofflashpoint,explosionlimitsandthresholdlimitvalueTLV-15min, respectively foreach chemical. The ICI is...</p>