Characterization, determination and elimination technologies for sulfur from petroleum: Toward cleaner fuel and a safe environment
Taw fi k A. Saleh*
,1ChemistryDepartment,KingFahdUniversityofPetroleum&Minerals,Dhahran31261,SaudiArabia
ARTICLE INFO Articlehistory:
Received23September2019
Receivedinrevisedform1January2020 Accepted4January2020
Keywords:
Analysis:environment Petroleum
Catalysis Sulfurremoval Analysis Fuel
ABSTRACT
Sulfur-containingcompoundsarethemostabundantcompoundsincrudeoil.Sulfurinliquidfueloil leadstotheemissionofsulfuroxidesandsulfateparticulatematterwhichnotonlyendangershealthand communitypropertybutalsoreducesthelifeofthecatalystsandenginesduetocorrosion.Various methods,withahighlevelofprecisionandsensitivity,havebeendevelopedtoanalyzesulfur-containing compounds.Ontheotherside,theremovalandcharacterizationofsulfur-containingcompoundsin crudeoilsandpetroleumproductsisofgreatimportance,notonlyforthedownstreamrefiningprocess, control/optimization,andenvironmentalcompliance,butalsoforupstreamgeochemicalstudiesfor explorationandproduction.Thisreviewsummarizestheanalyticalstrategiesandsomeofthemost importantandpromisingtechnologiesfortheremovalofsulfurfromoil.
©2020ElsevierB.V.Allrightsreserved.
1.Introduction
Petroleum can be defined as a complexmixture of hydro- carbons, non-hydrocarbons, and heteroatom-containing com- pounds.Itincludescrudeoil,condensate,naturalgasandsolids like bitumen, oil sand, andtar [1]. The petroleum consists of carbon,hydrogen,andheteroatomslikesulfur,nitrogen,oxygen, metals,etc.Amongheteroatom,sulfuristhemostabundantwith around0.03–6wt%innaturalgasandcrudeoils.Whenthetotal amountof sulfuris <0.42%,it is calledsweet crudeoil,while whentheamountofsulfurismorethanaround0.42%,itiscalled sourcrude oil.Sulfur-containingcompoundsare classifiedinto different types, as shown in Fig. 1. : (i) elemental sulfur, (ii) hydrogensulfide(H2S),mercaptans(thiols),sulfides(acyclicand cyclic),polysulfides(disulfides,trisulfides,etc.),thiophenesand others,Table1[2].Itisrecommendedthatsulfurcompoundsare removedintherefiningprocessastheycausethedeactivationof thecatalystsusedincrudeoilprocessingandcorrosionproblems inpipelines, alongwiththe pumping, andrefiningequipment.
Fromanenvironmentalpointofview,thesulfurleftinfuelsmay causetheemissionoftoxicgasesthatreactwithwaterandcause acidrain.Thus,thesegasesoracidproductscandamagebuildings and other materials. Therefore, there is a need for both the determinationoftheamountofsulfurinoilandformethodsto
removesulfur.Itshouldbenotedthatassessingtherefiningvalue of crudeoilrequires a full description of thecrude oilandits components, involving scores of properties. Nevertheless, two properties, i.e. API gravity (a measure of density) and sulfur content are important for quickly classifying and comparing crudeoils[3].
2.CharacterizationandAnalysisofsulfurcompounds
Sulfur,oxygen,andnitrogenareassociatedwithcarbonatoms invariousstructuralforms,andthesearethebuildingblocksofthe whole molecular structure of kerogen. Sulfur-containing com- pounds haveanadverseinfluenceontheoilsusedas transport fuels.Inaddition,thecombustionofsulfur-containingcompounds leadstotheemissionofSOxwhichisanimportantsourceofair pollution and acid rain. Several techniques for the qualitative characterizationandquantitativedeterminationofsulfurinfuels havebeendevelopedandapplied,Fig.2.
Sulphur is analyzed bythe bombmethod (ASTMD129)[4].
However, due to some limitations, it is preferable to use the Microwave induced combustion (MIC) method for preparing samples.Additionally,hightemperaturescanbeutilizedtoprepare samples of oils as per ASTM D1552 and ASTM D4239. Then, pyrohydrolysis was used to overcome the limitations. Other methods, such as wavelength dispersive X-ray fluorescence spectrometry(WDXRF),andEnergy-dispersiveX-rayfluorescence spectrometry (EDXRF) were used as standard test methods to analyze a sample without the sample being treated prior to analysis,i.e.directanalysis.
* Correspondingauthor.
E-mailaddress:tawfi[email protected](T.A. Saleh).
1 http://faculty.kfupm.edu.sa/CHEM/tawfik/publications.html
https://doi.org/10.1016/j.teac.2020.e00080 2214-1588/©2020ElsevierB.V.Allrightsreserved.
ContentslistsavailableatScienceDirect
Trends in Environmental Analytical Chemistry
j o u r n a l h o m ep a g e: w w w . e l s e v i e r . c o m / l o c at e / t e a c
Other methods are alsoused including gas chromatography coupled with sulfur chemiluminescence detector (GC-SCD), inductively coupled plasma optical emission spectrometry
(ICP-OES), inductivelycoupled plasma-mass spectrometry (ICP- MS),laserablation-inductivelycoupledplasma-massspectrome- try(LA-ICP-MS),electrothermalvaporizationinductivelycoupled plasma spectrometry (ETV-ICP-MS), ion chromatography (IC), high-performanceliquidchromatography(HPLC),highresolution- continuum source-molecular absorption spectrometry Fig.1.Classificationofsulfurcompoundsincrudeoil.
Table1
Examplesofthesulfurcompoundspossiblyavailableinpetroleum.
Hydrogensulfide Pentanethiol C5mercaptan Carbonylsulfide dibenzothiophene C6mercaptan Methylmercaptan C2thiophene C7mercaptan 2-Ethylthiophene 2,5-Dimethylthiophene C8mercaptan 3-Ethylthiophene 2,4-Dimethylthiophene n-Heptylmercaptan Ethylmercaptan 2,3-Dimethylthiophene sulfides
Dimethylsulfide Diethylsulfide Benzothiophene Carbondisulfide n-Butylmercaptan 2-methylbenzothiophene Isopropylmercaptan n-Hexylmercaptane 5-methylbenzothiophene tert.-Butylmercaptan 3,4-Dimethylthiophene 7-methylbenzothiophene n-Propylmercaptan C3thiophene Methylbenzothiophene Ethylmethylsulfide tert.-Amylmercaptan isomers
Thiophene Tetrahydrothiophene n-Amylmercaptan sec.-Butylmercaptan Methyldisulfide Ethyldisulfide Isobutylmercaptan 3-Methylthiophene 2-Methylthiophene
Fig.2.MethodsofcharacterizationandAnalysisofsulfurcompoundsinpetroleumsamples.
(HR-CS-MAS),ion-selectiveelectrode(ISE),andx-rayfluorescence spectrometry(XRF)[6–10].
Generally, the analytical determination methods used dependonparametersincludinginstrumentalsensitivity,cost, analyte behavior, size of sample and suitability for routine analysis[5].Someofthemethodsrequiresamplepretreatment before analysis (chemical transformation or chromatographic separation)andthe typeof samplepretreatment usedshould notcontain reagentsthatinterferewith theanalysis[6].Such methods are considered as indirect analysis due to the requirement of pretreatment which is time consuming and involves possible contamination and the potential loss of volatile analytes. Analytical techniques that require samples toundergo preparations includethe ICP-OES, ICP-MS,and IC.
Thesemethodsrefertoanalyticaltechniqueswithnoneedfor preparationpriortotheanalytedeterminationwhicharecalled directmethods.
The most used techniques are GC-SCD, LC, and GC–MS. An importantdisadvantageofusingaGC–MSisthatthefragmenta- tionpatternofthecomplexhydrocarbonmatrixthatwillinterfere with the hetero-compounds of interest due to the fact that hydrocarboncompoundsareseveralordersofmagnitudelargerin concentration than the heteroatomic compounds. GC GC chromatogramsarereportedandcouldbecoupledwithaflame ionization detector (FID), sulfur chemiluminescence detector (SCD), nitrogen chemiluminescence detector(NCD) and a time offlightmassspectrometer(TOF-MS)[7].EvenforGCGC–TOF- MSthehetero-compoundscanstilloverlapwiththehydrocarbon matrix.Duetothis interference,onlya limitedamountof(non- overlapping)hetero-compoundscouldbetentativelyidentifiedin thisway.Tosolvethisproblem,aqualitativesolidphaseextraction (SPE)canbecarriedoutbeforeinjectionontheGCGC–TOF-MS.
Allfractions wereinjected on theTOF-MS tohelp identify the species.
Methodscanalsobedividedintotwoclassesbasedonthetype ofsamples;nonvolatileandvolatilesamples.Forthenonvolatile samples,themolecularcharacterizationofsulfurcanbeperformed usingHPLCordirectliquidinfusionanditscombinationwithmass spectrometry (LC–MS), like Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and Orbitrap MS, associatedwithvariousatmosphericionizationtechniquesinclud- ingatmosphericpressurechemicalionization(APCI).Theuseof surface desorption ionization techniques, direct electrospray ionization(DSEI)anddesorptionatmosphericchemicalionization (DAPCI),forthemolecularcharacterizationofinsolublekerogens that have been characterized by x-ray near edge structure spectroscopy (XANES) and x-ray photoelectron spectroscopy (XPS)remaintobeexplored.
Gas chromatography (GC) is a common and efficient separation technique for sulfur compound mixture analysis.
Examples of GC based methods are mentioned here briefly.
Theseinclude(i)flamephotometricdetectors(FPD)whichmake use of the specific emission spectrum of excited sulfur moleculesto achieveselective detection.When coupled with GCforthe analysisof SCCs,AEDatomizescomponentseluted and separated by GC into excited states using microwave- inducedplasma.(ii)a sulfurchemiluminescentdetector(SCD) convertssulfur-containingcompoundsfromoxidative/reductive zoneswith ozone to formSO2 in an excitedstate(SO2*)and detectsitsspecificchemiluminescencespectrumwhilequench- ing to a ground state. It is a selective detector because it eliminates the signals of many other compounds that may interferewithsulfurdetection.(iii)Massspectrometry(MS)is another preferred detector for the characterization of sulfur compoundsbasedonfragmentationpatternsthatenhancethe selectivity.
2.1.Sulfurspeciation
Analysis of sulfur molecular species (speciation or detailed analysis) provides useful data for mitigating sulfur compound problemsinpetroleumproductsandfeedstock,providinggreater insightcomparedwithmeasuringtotalsulfurcontentalone.Sulfur speciation of petroleum samples can be performed using GC methods.First,thesulfurcompoundsareseparatedbyspecificgas chromatography columns. After that, sulfur compounds are detected by atomic emission detection, chemiluminescence detection, flamephotometric detection,mass spectroscopy and pulsedflamephotometricdetection.Theneachsulfurcompound canbeidentifiedbyretentiontimeanddeterminedbycomparing the peak area with the standard run calibration data. Sulfur speciation can be performed by element-selective detection methods suchas flame photometric detection (FPD), SCD, and atomicemissiondetection(AED)tosimplifythisproblemagreat deal[11].TheAmericanSocietyforTestingandMaterials(ASTM) method can be used for the analysis of more than 16 sulfur compounds[4].
Amethodforthedetailedanalysisandgroup-typeidentifica- tionofthesulfurcontainedinmulti-sulfurcompoundsingasoline rangepetroleumproductsbasedonseparationanddetectionwas reported by coupling capillary gas chromatography–atomic emission detection (GC–AED). In this method, the chemical treatmentofthesamplesisperformedbyselectivereactionsfor group-type classification of the peaks. Thus, various kinds of naphthascanbeanalyzed[3]
2.2.Hydrogensulfideanalysis
Hydrogen sulfide in petroleumsamples can be qualitatively detected by the doctor test, while quantitative analysiscan be performed by the lead acetate method, the cadmium sulfide method, and multiple headspace extraction. Thiol/mercaptan sulfurinpetroleumsamplesisdetected (qualitativetest)bythe doctormethod[12],andmeasured(quantitativetest)bythestain detector[13],andpotentiometricmethods.
2.3.Totalsulfurdetermination
Incommonlyusedsulfurdeterminationmethods,thesampleis combustedwiththepresenceofoxygentoconvertthesulfurto sulfur dioxide, which is thencollectedand subsequentlydeter- mined by oxidative microcoulometry, ultraviolet fluorescence, non-dispersive infrared, titration methods and gravimetrically [14,15]. Other testmethods, namely, hydrogenolysis and rateo- metriccolorimetry[16],x-rayfluorescencespectroscopy[17]and gaschromatography[18],arealsoavailableandhaverecentlybeen usedforsulfurdeterminationinpetroleumsamples.
Sulfur in petroleum products and feedstock at very low concentrations (sub-ppm toppm) are measured by thehydro- genolysis and rateometriccolorimetry method.Sulfur in petro- leum and petroleum products can be detected using x-ray fluorescence (XRF). Commonly used methods are ASTM D2622 wavelength dispersive XRF (WDXRF), ASTM D4294 energy dispersive XRF (EDXRF) and ASTM D5453 (ultraviolet (UV)- fluorescence).Allthreemethodswerefoundtobeequivalentin the150–500mg/kg(ppm)sulfurrange.Withregardtolowsulfur fuels(gasoline,jetfuel,anddiesel)ASTMD5453isagoodmethod, witharounda1ppmsulfuraccuracy[1].
Forpetroleumandpetroleumproductsthataresufficientlylow involatilityandthatcanbeweighedaccuratelyinanopenboat,the ASTM D129 method (burning sample in a bomb with oxygen environment)canbeused[19].Forhigherboiling(>350For>175
C) fractions, ASTM D1552, a high-temperature combustion
techniquewithiodatetitrationorinfrareddetectioncanbeused [20].Totalsulfur,alongwithotherelements,inheavypetroleum fractionscanalsobedeterminedbyinductivelycoupledplasma/
atomicemissionspectroscopy(oropticalemissionspectroscopy) (ICP/AESorICP/OES)[21],flameatomicabsorptionspectrometry (AAS)andICP-MS[22,23].
3.Removalofsulfur
Removalofsulfur,ordesulfurization,isasignificantoperation forcrudeoilinapetroleumrefinery.Thisoperationisimportantto meetthe globaltrendstowards cleanerfuels. Rigorous environ- mentalconstraintshavelimitedthesulfurlevelsincrudeoilacross theglobe.Sulfurcontentincrudeoilandcrudeoilproductsposes anenormousthreattopeople,theenvironment,andthedesired conversionofthevaluableproductsthatcouldbeobtained.Inthe petroleum refining process, sulfur compounds are undesired because of their capability in catalysts deactivation and the consequentenvironmental pollution.Sulfur presence in oil and lubricants leads to the corrosion of pipelines and refinery equipment. Anotherharmful effectof sulfur is the emission of H2SandSO2duringthecombustionoffuel.
Stringentenvironmentalregulationslimitthesulfurcontentin crudeoiltobetween10–15ppm.Consequently,severalprocesses
havebeenrecommended forthesulfur removalfromfuels.For example, Babich and Moulijn, (2003) [24] wrote an extensive reviewoftheclassificationsofdifferentdesulfurizationprocesses.
Formorethanthreedecades,severaldesulfurizationtechnologies havebeenemployedforcleaneroilproducts.Atthepresenttime, commondesulfurizationtechniquesincludeOxidativedesulfuri- zation (ODS), adsorptive desulfurization, bio-desulfurization (BDS),andoxidation–extractiondesulfurizationhydrodesulfuriza- tion(HDS)(OEDS).Fig.3showsdesulfurizationmethodsbasedon theroleofhydrogenintheprocess.Thedesulfurizationmethod commonlyemployedinarefineryishydrodesulfurization.
3.1.Hydrodesulfurization(HDS)
Hydrodesulfurizationisthemostcommontechniquetoremove sulfurfromcrudeoilandcrudeoilintermediatesinthepetroleum industry[25].In thecrudeoil refinery,thehydrodesulfurization reaction is a majorprocess in upgrading heavy crude oil. The hydrogenfromcatalytic reformersgreatlyenhanceshydrogena- tion desulfurization [26]. During the hydrodesulfurization pro- cesses, sulfur atoms are removed from hydrocarbons [27].
Hydrodesulfurization involves a reaction with hydrogen. This reactionoccursinthepresenceofanappropriatecatalystforthe conversionoforganosulfurcompoundstosulfur-freecompounds Fig.3.ClassificationofDesulfurizationtechniques.
andH2S.Consequently, inacrude oil refinery,theconventional ClausprocessconvertstheH2Sintoelementalsulfur[28].
Generally,hydrodesulfurizationoperatesathightemperatures and pressurestoachieve desulfurization [29]. The temperature rangesfrom300Cto400Candthepressuretowithin50barand 100 bar. In addition to these temperatures and pressures, hydrodesulfurizationalso requiressuitable catalysts to produce sulfur-freecompounds[30,31].
Thesupportmaterialusedinthehydrodesulfurizationcatalyst isalsoimportant.Currently,Alumina(Al2O3)isusedasasupport material.Thisisduetoitsexcellentstrengthsandtexture[30–35].
Nonetheless,aluminaasasupportmaterialcouldleadtolowHDS activity[36].Studies[37]showedthattheinteractionofAl2O3and metal is much stronger because this could migrate to the octahedralortetrahedralsites, orboth, ontheoutersurfaceof themetaloralloy.However,previousprocessesusedco-containing smectite clay. Co-containing smectite clay showed excellent hydrodesulfurization of thiophene activity [38]. Thus, co-clay catalysts modified with noble metals are effective
hydrodesulfurization catalysts [39]. Recentadvances have been madeinsynthesizingnewsupportsusingvariousmetal(oralloy) supplements to maintain the metal support link toachieve greaterHDSactivity[40].Severalstudieshaverevealedthatthe use of phosphorus reduces coke formation and ultimately increasesHDSactivity[41,42].Thehydrodesulfurizationreaction ofthiopheneispresentedinScheme1[43]
The reactions occur primarily through two parallel routes namely; hydrogenolysis(HYD) and direct desulfurization(DDS) pathways.Thiophenehydrodesulfurization(ThHDS)wasreported withaMoS2catalystusingtheMoedge,theSedge,andtheMoS2
link.ThHDSreactiontakesplaceviatheHYDrouteontheMoedge, S edge, and MoS2 edge depending on the temperature. The selectivity of the products changes according to the dominant reactionroutes[44].
Likewise,Raoetal.[40]usedgraphene-liketiny-layersofMoS2
andonCo,Ni-nanoparticlescoveredwithafew-layersofMoS2in the formation of n-butane from thiophene. They achieved a percentconversionof64%at480CusingbulkMoS2,whileusing MoS2 coveredbyNiorConanoparticles,theconversionroseto above98%at375 C.Bejetal.[45] synthesizedACF-supported NiMo catalyst for thiophene hydrodesulfurization at operating conditionsof1barand350C.TheresultshowedthatatW/FAoof 115(kgcatalyst-h)/kmol,thecatalystsgave90%conversion.Also, the catalytic activity was maintained for 30 h during the experiment.Theauthorscomparedtheactivationenergyandrate constantwiththevaluesreportedinearlierworkforthesupported catalysts. Forthecatalyst,theyfoundthatafaster reactionrate withthelowestactivationenergywhencomparedtothevalues analyzedinpreviouswork[46].
Table2
Examplesofthecatalystusedforhydrodesulfurizationofthiophenes.
Catalyst ReactionCondition Reactortype Activity/Conversion(%)) Rf
ACF-supportedNiMo 1barand350C. SStubularreactor 90.0 [37]
graphene-likefew-layerMoS2, 450C stainless-steelmicroreactor 64.0 [40]
Co/Ni-nanoparticle-coveredfew-layerMoS2 375C stainless-steelmicroreactor 98.0 [40]
MoCozeolite/activecarbon at30050bar. fix-bedreactor 98%conversion [35]
CoMosupportedAlMC41 350C fixedbedcontinuousflowmicroreactor 53 [47]
CoMoonMCM41andMCM48 400C,20bar stainless-steelmicroflowreactor 50 [48]
Molybdenumnitridesupportedonalumina 360C atmosphericpressureflowreactor 79.0 [49][45],
MolybdenumCarbidesupportedonalumina 420C atmosphericpressureflowreactor 23.0 [49][50],
g-Alumina-SupportedMolybdenum OxideMoO-gAl3O2
400C,1bar quartztube(innerdiameter8mm), micro-flowreactor
1.5m3mol 1s-l) [51]
Pt/FSM16 350C,1bar fixedbedflowreactor 95.0 [52]
Pt-Pd/FSM16 400C fixedbedflowreactor 73.0 [53]
Pt-Rh/FSM16 400C fixedbedflowreactor 55.0 [53]
Pt-Rhnanoparticle/FSM16 350C fixedbedflowreactor 50.0 [53]
MoS2 350C continuous-flowmicroreactor 11.0 [54]
CMOS/Al2O3 1bar asingle-passmicroreactor 60gmol1s 1 [55]
NiMoS/Al2O3 1bar asingle-passmicroreactor 80gmol1s 1 [55]
CoNiMoS/Al2O3 350C,1bar asingle-passmicroreactor 100gmol 1s 1 [55]
MoS2impregnatedoncobalt 400C,1bar microreactor 28.8 [56]
WS2nanotubescoveredwithCo 600C 12.0 [57]
MoS2nanotubesdecoratedwithNi. 250–350,1bar single-passstainless-steelmicroreactor 90 [58]
Como(CoO+MoO)onAMC 400C 54.0 [59]
ComoonTi 350C 90.0 [59]
Tungstenphosphide 400C 67.0 [59]
Tungstennitrideong-alumina 300C,atmosphericpressure differentialmicroreactor 4.8rate/g [60]
AupromotedNi/SiO2 340C,1bar continuousflowfix-bedmicroreactor 35.0 [61]
Au/SAPO-11 300C,1bar fix-bedmicroreactor 35.0 [62]
Au2/SAPO-11 300C,1bar fix-bedmicroreactor 35.0 [62]
(Rh2P)on(Al2O3) 350C,10bar fixed-bedflowreactor 1.8timeshigherthanthat ofRhP(A)/Al2O3catalyst
[63]
Molybdenaontitania 350Cand1bar all-Pyrexdifferentialmicroreactor 1.32gmol1s 1 [64]
CoMo/Al2O3 300C-4000C,1-30bar asingle-passmicroreactor 28gmol1s 1 [65]
CoMo/Nb2O5 400C,1bar fixed-bedflowreactor 3.25gmol1s 1 [65]
Mo/Al2O3 260C,10bar fixbedmicro-reactor 0.26gmol1s 1 [66]
CoMo/USY 200-300C,1bar theyieldof2,88% [67]
Scheme1.Hydrodesulfurizationreactionofthiophene.
3.2.CatalystsusedinHDS
The Thiophene hydrodesulfurization reaction requires the presenceofa catalyst for theformationof theend product- n- butane.Thechoiceofthecatalystnotonlyrelatestothecatalyst activitybutalsotomanyothercriteriasuchasstability,selectivity, regeneration,easeofactivation,pressuredropbuild-upandcost [27].Inaddition,theoperatingconditionsarealsopertinenttothe typeofcatalystthatgivestheoptimumconversion.Hence,catalyst selection involves a holistic investigation ofthe situation.Table2lists somecatalystsusedinthehydrodesulfurizationofthiophene.
3.3.Oxidativedesulfurization(ODS)
ODSisapromisingtechnologyforthereductionofsulfuratlow temperatures(50 C) and atmospheric pressure [1,68]. Heavy sulfides are oxidized by adding oxygen to the sulfur using appropriateoxidants(suchasorganicandinorganicperoxyacids, catalyzed hydro-peroxides, peroxy salts, NO2, tert-butyl-hydro- peroxide,O3)withoutbreakinganycarbon–sulfurbonds,yielding thesulfoxideandsulfone,respectively.Theseoxidizedcompounds are then extracted or adsorbedfrom the light oil due to their increasedrelativepolarity.Thus,theODSisbasicallyatwo-stage process;oxidation,followedbyliquidextraction.Oxidationofthe DBT derivatives to the corresponding sulfones increases their polarityand molecularweight, Scheme 2. This facilitates their separationbyextraction,distillation,or adsorption.ODScanbe assisted by plasma, radiation, ultrasound, photo-oxidation and electrochemicalcatalyticoxidation.TheextractioninODScanbe donebyionicliquids.
3.4.Adsorptivedesulfurization
Adsorptionisappliedfortheremovalofsulfurcompoundsfrom liquidhydrocarbonfuels.Removalofsulfurcompoundshasbeen reported over zeolites, aluminosilicates, activated carbon (AC), alumina,zincoxide,etc.However,only afewadsorbentshaveshown highselectivityfordifficulttohydrotreatsulfurcompoundssuchas 4,6-dimethylDBT.Variousadsorbents,suchasactivatedcarbonand its modifications, silica, and zeolites, have been reported and researchgapshave beenidentified [69–77].Reactive adsorption desulfurizationcombinestheadvantagesofboththecatalyticHDS andadsorptiondesulfurization.Thus,sulfur-containingmolecules reactwithasolidadsorbentinthepresenceofhydrogen.
3.5.Biodesulfurization
Widespread attentionhasbeendirectedtowardbiodesulfu- rization(BDS)becauseofitsgreenprocessingoffossilfuelsforthe removalofsulfurcompounds.Manybacterialspeciesthathadthe abilitytoconsumeDBTsastheirenergysourcewereisolatedfrom theirnatural habitatsalthough theseisolated microbial species could not specifically remove sulfur from DBTs. Some of the isolated microorganisms used thiophens as carbon and sulfur sources.OthersusedmetabolizedDBTsasacarbonsourceand,ina series of oxidizing steps, converted them into several water- solublematerials[1].Theaccumulationofthesewater-solubleend products considerably inhibited microbial growth and DBT
oxidation.Somebacterialspecies,suchasArthrobacter,Brevibac- terium,Pseudomonas,Gordona,andRhodococcusspp,havenow been identified as being capable of bio-transforming DBT and growingonitasasolesulfursource.
4.Conclusionandprospects
Characterizationandanalysisofsulfurcompoundsisacritical elementinpetroleumsciencesinceitallowsforimprovedevalua- tion of the refinery process. With regards to sulfur detection, severalmeritsshouldbeconsideredwhenselectingamethodfor sulfur-containingcompoundsanalysisincludingprecision,accu- racy,sensitivitylinearity,andstabilitywithlong-termreproduc- ibility.In addition,thedata processing,samplepreparationand pretreatment, simplicity of the method, interference, cost of analysis, and suitability for routine analysis should also be considered.Sulfurremovalfromoilsisanessentialsteptoprotect refineryequipmentandcatalysts,inadditiontoproducingclean low-sulfur fuels and products. Hydrodesulfurization remains a powerful sulfur removal technology in a refinery, however, additionaldesulfurizationtechnologiessuchasoxidation,absorp- tionandbiodesulfurizationareemerging.Acombinationofthese toolsmaycontributetothecurrentpracticeofhydrodesulfuriza- tion.Thestudyofthiophenehydrodesulfurizationremainsanarea ofinterestforresearcherstoprovidethebestsuitablecatalystand kinetic design. Anotherchallenging areaof researchfor hydro- desulfurizationprocesses is operation at mild conditions in an optimalreactor.Thedevelopmentoftechnology,theupgradingof equipment, and the innovation of detection and molecular characterization methods have deepened our understandingof sulfurcompoundsin oils,which can,inturn,monitoreffective upstream exploration/production and downstream processing, along with lowering environmental risk. Consequently, these approaches stillneed furtherresearch,especially in theareaof designingappropriateselectivemethods.
DeclarationofCompetingInterest Noconflictofinterest
Acknowledgements
The authors would like to acknowledge thesupport by the Research and Development Office (RDO) in the Ministry of Education(MoE)underHighQuality/ImpactResearchPublication Initiative,SaudiArabia.
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