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Detection of Trace Metals in Asphaltenes Using an Advanced Laser-Induced Breakdown Spectroscopy (LIBS) Technique

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Detection of Trace Metals in Asphaltenes Using an Advanced Laser-Induced Breakdown Spectroscopy (LIBS) Technique
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  1099 r 2009 American Chemical Society  pubs.acs.org/EF Energy Fuels  2010,  24,  1099  –  1105  :  DOI:10.1021/ef900973sPublished on Web 11/02/2009 Detection of Trace Metals in Asphaltenes Using an Advanced Laser-InducedBreakdown Spectroscopy  ( LIBS )  Technique Mohammed A. Gondal, † Mohammad N. Siddiqui,* ,‡ and Mohamed M. Nasr § † Laser Research Laboratory, Physics Department and   ‡ Department of Chemistry, King Fahd University of Petroleum and Minerals  ( KFUPM  ) , Dhahran 31261, Saudi Arabia, and   § Department of Natural Sciences, College of Dentistry and Pharmacy,Riyadh 11671, Saudi ArabiaReceived September 1, 2009. Revised Manuscript Received October 19, 2009 The significance of trace elements in the petroleum industry has increased because of the role of non-hydrocarbon components in the elucidation of the mechanisms of migration and srcin of crudes.Determining the amounts of trace elements is also very important in the petroleum industry for refiningand processing of crudes and even exploration. In the development of different instrumental analyticaltechniques for trace elemental analysis of crudes and its products, little attention has been devoted to thebroad variation in data because of poor sampling and the proper nature of the matrix. Accurate detectionof trace metals in asphaltene samples using conventional methods is not a trivial task. In this work, thedetermination of 13 trace elements in asphaltenes including Co, Fe, V, Ni, Pb, P, Mo, Ca, Si, Ti, Mn, Cd,andCuarereportedforthefirsttimeusinganadvancedlaser-basedanalyticaltechnique.Forthispurpose,laser-induced breakdown spectroscopy (LIBS) using a Nd:YAG laser at 1064 nm was employed todetermine the concentration of these trace elements present in four asphaltene samples derived from themarketableSaudiArabiancrudeoils.TheconcentrationdeterminedwithourLIBSsetupofdifferenttracemetals present in the same samples was also measured using a standard technique, such as inductivelycoupledplasma(ICP),andresultsachievedwithLIBSareincloseagreementwithICPresults.Thelimitof detection for these elements was also determined and reported. This is the first time, to the best of ourknowledge, the LIBS technique has been applied for the determination of trace metals in asphaltenes.The unique features of LIBS are no or little sample preparation requirement, fast and rapid analysis, and in situ  detection, which are lacking in the conventional analytical techniques. Introduction The consumption of crude oils is on the rise because of theincreasing demand for energy and fuels worldwide as a con-sequence of the growth in world population. The heavy petro-leumresidueconstitutesabout70%ofthedrilledcrudeoils,anda fairly low percent is being used without much processing. 1 Thus, the increasing demand for transportation fuel (gasolineand diesel fuel) has necessitated the need for processing theheavier residues containing higher contents of asphaltenes.Asphaltenes are considered to be the most troublesome frac-tions in petroleum cracking and refining processes. The ten-dencyofasphaltenestoprecipitateduringcrudeoilrecoverycancause severe consequences as a sharp decline in oil flow or evenblockage of pipelines and processing equipment. 2 Asphaltenesalso adversely affect the overall rate of hydrodesulfurizationduring catalytic hydroprocessing; they act as coke precursors,leading to catalytic deactivation and sludge formation, whichdiminishes the conversion efficiency achievable in the hydro-crackingprocess. 3,4 Petroleumasphaltenesrepresentasolubilityclass of petroleum liquids and are considered an importantfactor that causes hindrance in petroleum operations. 5 The main problem in studying the composition of asphal-tenes is its inherited chemical complexity. It has also beenreported that asphaltenes consist of highly condensed poly-aromatic rings bearing long aliphatic and alicyclic substitu-ents along with metals and heteroatoms as part of a ringsystem. 6 - 9 Therefore, the petroleum industry has genuineconcern and interest in the determination of trace metalspresent in heavy crude oils and asphaltenes in particular.Trace metals are of great significance because of their vitalrole in the genesis of petroleum and its refining. The informa-tionconcerningthesrcin,migration,andmaturationofpetro-leum may be obtained from the true nature of these metalsand their abundance in petroleum. Trace metals occur in vary-ing quantities depending upon the nature of the crude. 10 - 12 *Towhomcorrespondenceshouldbeaddressed.Telephone: þ 966-3-8602529. Fax: þ 966-3-8604277. E-mail: mnahid@kfupm.edu.sa.(1) Speight, J. G.  Fuel Science and Technology Handbook ; MarcelDekkar: New York, 1990. (2) Cimino, R; Correra, S.; Del Bianco, A.; Lochhart, T. P. In Asphaltenes Fundamentals and Applications ; Sheu, E. Y., Mullins, O.,Eds.; Plenum Press: New York, 1995; p 97. (3) Bartholomew, C. H. In  Catalytic Hydroprocessing of Petroleumand Distillates ; Oballa, M. C., Shih, S. S., Eds.; Marcel Decker: New York,1994; p 42. (4) Miyauchi, Y.; de Wind, M. Hydroprocessing. Proceedings of theAkzoNobelCatalystsSymposium,Amsterdam,TheNetherlands,1994 ; pp 123 - 140. (5) Sheu, E. Y.  Energy Fuels  2002 ,  16 , 74. (6) Speight, J. G.  Fuel   1970 ,  49 , 134. (7) Hasan, M.; Siddiqui, M. N.; Arab, M.  Fuel   1988 ,  67   (8), 1131. (8) Shirokoff,J.W.;Siddiqui,M.N.;Ali,M.F. EnergyFuels 1997 , 11 ,561. (9) Siddiqui, M. N.  Pet. Sci. Technol.  2003 ,  21  (9 and 10), 1601. (10) Valkovic, V.  Trace Elements in Petroleum, Part 2 ; The PetroleumPublishing Company: Tulsa, OK, 1978; pp 62 - 83. (11) Yen, T. F.  Energy Resour.  1974 ,  1 , 447. (12) Yen, T. F. Chemical aspect of metals in native petroleum.In  TheRole of Trace Metals in Petroleum ; Yen, T. F., Ed.; Ann Arbor SciencePublishers: Ann Arbor, MI, 1975; p 31.  1100 Energy Fuels  2010,  24,  1099  –  1105  :  DOI:10.1021/ef900973s  Gondal et al. Thenatureofeachelementisalsoimportant,regardlessoftheirabundance, as being the potential source of environmentalpollution,thecauseofcorrosiontoequipment,andthepoison-ing of catalysts during refining. Metals are present in verycomplex form in crudes, and their removal is not an easytask. 13,14 Demetalationofpetroleumbecomesdifficultbecauseof the association of metals to heteroatoms present in asphal-tenefractions.Theheavymetals,suchasvanadiumandnickel,which are primarily associated with asphaltenes, occupy eitherheteroatom (N, S, and O) bounded sites or are stronglyassociated with the aromatic sheets of asphaltenes via  π  - π  bonding.Different analytical techniques have been applied in thedetermination and identification of trace metals in the crudeoilsandasphaltenes.AliteraturereviewcarriedoutbyDuycketal. 15 onthedeterminationof traceelementsincrudeoilandheavy molecular mass fractions, including saturates, aro-matics, resins, and asphaltenes (SARA), by inductivelycoupled plasma - mass spectrometry (ICP - MS), ICP - opti-calemissionspectroscopy(OES),atomicabsorptionspectros-copy (AAS), and spectrochemical analytical techniques wasdiscussed. Escobar et al. 16 used SARA, gas chromatography(GC) - MS, ICP - OES, and UV - vis techniques to study thediverse trace metals and biomarker-derived parameters fora suite of 30 crude oil samples. Dreyfus et al. 17 developeda method to analyze direct trace and ultra-trace metal ele-mentsincrudeoilanditsfractions(maltenes - asphaltenes)byICP - MS.Atechniqueforintroducingcrudeoilsdirectlyintoan ICP mass spectrometer based on the formation of oil-in-water microemulsions and greatly simplifying the determina-tion of trace metals in oil is reported. 18,19 Proton-inducedX-ray emission (PIXE) analysis was also used for the directdetermination of the distributions and abundances of tracemetals in crude oils. 20 Buenafama et al. 21 applied neutron-activationanalysisforthedeterminationsof17traceelementsin heavy crude oils. Ali et al. 22 used atomic absorption in astudy of the trace metals of crude oils from the different oil-producing fields of Saudi Arabia. It is worth mentioning thatall of these methods are cumbersome and highly expensiveand require sophisticated equipment and high-level technicalskills. In addition, these methods are highly time-consumingandrequirespecialchemicalsandsamplepreparationpriortoanalysis.To minimize coke formation and develop advanced andmore efficient technologies for heavy oil upgrading, a betterand comprehensive understanding of metal contents in as-phaltene is highly essential. To have a rapid and on-lineanalysis of asphaltene, advance techniques are required. Forthis purpose, laser-induced breakdown spectroscopy (LIBS)was developed locally and applied for the first time todetermine the maximum number of trace metals in asphal-tenes, which are not easily detectable with other conventionaltechniques, as mentioned above, because of different reasonson rapid-time scale. In LIBS, a plasma spark is created byfocusing the high-energy laser beam at the sample of asphal-tene and spectrally resolved emissions are recorded with aspectrometer having reasonable resolution (0.1 nm) to identi-fy the elements present in the asphaltene sample. This analy-tical technique enables the determination of elementalcompositions 23 - 25 of different trace elements present in theasphaltene samples. The unique features of LIBS are no orlittlesamplepreparationrequirement,fastandrapidanalysis,and  in situ  detection, which are lacking in the conventionalanalyticaltechnique,suchasatomicabsorptionandICP. 23 - 25 Yaroshchyketal.haveusedLIBSforthequantitativeanalysisof wear metals in engine oil. A limit of detection (LOD) wasdetermined for several trace elements. They compared theLIBS results to the ICP - AES analysis of the same samples,and a good relation was reported. 26,27 The work on LIBS forenvironmental and other analytical applications is a continu-ity of laser-based research activities being developed atthe Physics Department, KFUPM, including laser intensitydirection and ranging (LIDAR) and photoacoustic spectro-scopy. 28 - 31 Experimental Section SampleCollection. Thefollowingfourvarietiesofmarketablecrude oils produced by the Saudi Arabian Oil Company (SaudiAramco) were procured from the Ras Tanura refinery, SaudiArabia. The asphaltenes used in this study were isolated fromthefollowingfourSaudiArabiancrudeoils:(1)ArabBerri(AB)is a relatively high American Petroleum Institute (API) gravity(38.50   API), low-sulfur, and paraffinic-type crude oil and hasthe lowest asphaltene contents (2.93%). Arab Extra Light,which comes from the Berri, Saudi Arabia field, is producedfrom the upper Jurassic age Arab zone reservoirs, generallyoolitic and dolomitic limestones. (2) Arab Light (AL) is amoderatelyhigh-gravity(33.80  API),medium-sulfur,andmed-ium-paraffinic crude oil and has moderate asphaltene contents(6.83%). Arab Light is produced from the Ghawar field, whichis the largest onshore oil field in the world, and is also derivedfrom the upper Jurassic age Arab zone reservoirs. (3) ArabMedium(AM)isamedium-gravity(30.40  API)andparaffinic-wax-containing crude oil and has higher asphaltene contents(9.18%). Arab Medium is produced from the Jurassic age Arabzone reservoirs as multi-stage separated oil from a blend of the following fields: 65% Khursaniya, 25% Qatif, and 10% (13) Filby, R. H.; Branthaver, J. F.  Metal Complexes in Fossil Fuels ;American Chemical Society: Washington, D.C., 1987; ACS Symp. Ser. 344. (14) Filby, R. H.; Shah, K. R. Neutron activation methods for traceelements in petroleum. In  The Role of Trace Metals in Petroleum ; Yen,T. F., Ed.; Ann Arbor Science Publishers: Ann Arbor, MI, 1975. (15) Escobar, M.; Da Silva, A.; Azuaje, V.; Esteves, I.  Rev. Tec. Fac.Ing., Univ. Zulia  2007 ,  30 , 391  –  400. (16) Duyck,C.;Miekeley,N.;PortodaSilveira,C.L.;Aucelio,R.Q.;Campos, R. C.; Grinberg, P.; Brandao, G. P.  Spectrochim. Acta, Part B 2007 ,  62B  (9), 939  –  951. (17) Dreyfus, S.; Pecheyran, C.; Magnier, C.; Prinzhofer, A.; Liene-mann,C.P.;Donard,O.F.X.Elementalanalysisoffuelsandlubricants.American Society for Testing and Materials (ASTM) Special TechnicalPublication, 2005 ; STP 1468, pp 51 - 58. (18) Lord, C. J., III.  Anal. Chem.  1991 ,  63  (15), 1594  –  1599. (19) Zaki, N. S.; Barbooti, M. M.; Baha-Uddin, S. S.; Hassan, E. B. Appl. Spectrosc.  1989 ,  43  (7), 1257  –  1259. (20) Fischbeck,H.J.;Engel,M.H.;Ruffel,A.V.;Weaver,B.L. Nucl.Instrum. Methods Phys. Res., Sect. B  1987 ,  24 - 25  (part 2), 655  –  657. (21) Buenafama, H. D.; Lubkowitz, J. A.  J. Radioanal. Nucl. Chem. 1977 ,  39  (1), 293  –  300. (22) Ali, M. F.; Bukhari, A.; Saleem, M.  Ind. Eng. Chem. Res. Dev. 1983 ,  22 , 691. (23) Gondal, M. A; Hussain, T.; Yamani, Z. H.; Baig, M. A.  Talanta 2007 ,  72 , 642  –  649. (24) Gondal, M. A.; Hussain, T.  Talanta  2007 ,  71 , 73  –  80. (25) Gondal, M. A.; Hussain, T.; Ahmad, Z.; Bakry, A.  J. Environ.Sci. Health, Part A: Toxic/Hazard. Subst. Environ. Eng.  2007 ,  42 , 879  –  887. (26) Yaroshchyk, P.; Morrison, R. J. S.; Body, D.; Chadwick, B. L. Spectrochim. Acta, Part B  2005 ,  60 , 986  –  992. (27) Yaroshchyk, P.; Morrison, R. J. S.; Body, D.; Chadwick, B. L. Spectrochim. Acta, Part B  2005 ,  60 , 1482  –  1485. (28) Gondal, M. A.  Appl. Opt.  1997 ,  36 , 3195  –  3201. (29) Gondal, M. A.; Mastromarino, J.  Appl. Opt.  2001 ,  40 , 2010  –  2017. (30) Gondal, M. A.; Mastromarino, J.  Talanta  2000 ,  53 , 147  –  154. (31) Striganove, A.; Sventitski, N.  Table of Spectral Lines of Neutral and Ionized Atoms ; Plenum: New York, 1968.  1101 Energy Fuels  2010,  24,  1099  –  1105  :  DOI:10.1021/ef900973s  Gondal et al. Abu-Safah. (4) Arab Heavy (AH) is a relatively low-gravity(28.03   API), high-sulfur, and paraffinic-wax-containing crudeoil and has the highest asphaltene contents (13.5%). ArabHeavyiscomprisedofcrudeoilfromanoffshorefield,Safaniya,locatedabout125milesnorthwestoftheexportingterminalRasTanura and known to be the world’s largest offshore oil field.Safaniya oil is produced from the lower Cretaceous age Arabzone reservoirs.AllcrudeoilleavingtheSaudiAramcoproductionfields,withthe exception of those at Safaniya, Marjan, and Zuluf, are sourand contain toxic hydrogen sulfide. Sour crude is sweetened,stabilized, and pumped to the storage facilities before shipment.All crude oil samples studied in this work are stabilized crudefrom the storage facilities in the Ras Tanura refinery. Chemicals and Materials.  High-performance liquid chroma-tography (HPLC)-grade normal heptane, which has 99.99%purity, was procured from Fluka and used for the precipitationof asphaltenes. Separation of Asphaltenes.  First, 7.0 g of heavy residue and5 mL of HPLC-grade  n -heptane were transferred into the200 mL Pyrex beaker. The beaker was heated on the hot plateat around 70 - 80   C temperature for 15 min with constantswirlingtohomogenizethesolution.Thisresiduesolution,whenwell-mixed, was carefully transferred to a 2 L Pyrex flask, and700 mL of HPLC-grade  n -heptane was added to the same flask.The flask containing the residue solution was fitted with amechanical stirrer and placed on the water bath. The residuesolution was heated at 90   C on the steam bath with continuousstirring for about 2 h to maximize the solubility of the residue in n -heptane.After2hofmixing,theresiduesolutioncoveredwithaluminum foil was left on the working bench to cool at roomtemperature for about 24 h. The long cooling time producesmore efficient precipitation of asphaltenes. The residue solutionwas filtered using a Millipore filtration apparatus with 0.8  μ m(37 mm) pore size filter paper. All insoluble material wasSoxhlet-extracted with 25 mL of toluene for 2 h at 110   Ctemperature and filtered again using the same filtering appara-tus. The insoluble material was removed as sludge (coke), andsoluble material (asphaltenes) was recovered after evaporatingtoluene completely. The asphaltenes were collected in a 250 mLbeaker and washed several times with small portions of   n -hep-tane, to remove any traces of maltenes, until washings becamecolorless. The recovered asphaltenes were dried in an inertatmosphere oven for about 2 h at 105   C to obtain a constantweight.Thefiltrate(maltenes)wasrecoveredbyevaporatingthe n -heptane on the steam bath using a rotavapor with continuousblowing of dry nitrogen until a constant weight of maltenes wasobtained. For LIBS measurement, the pellets of asphaltenesamples were prepared. A 4.0 g asphaltene sample was pouredin a stainless-steel dye having a cylindrical shape. The pellets of this sample were made in a hydraulic press machine by applyinga load of 12000 psi for a 0.5 h duration. These pellets have adiameter of 20 mm and thickness of 10 mm. To test thehomogeneity of our samples, several LIBS measurements wereperformed at different locations of the surface of the pelletsamples. Laser-Induced Breakdown Spectrometer Details.  A schematicdiagram of the laser-induced breakdown spectrometer appliedin this study is depicted in Figure 1 and is discussed in detail inearlier publications. 23 - 27 The LIBS system applied in this studyconsists of an Ocean Optics LIBS 2000 þ spectrometer, a specialhome-built chamber, OOILIBS software, and a Nd:YAG laser(Spectra Physics, model GCR100). The Nd:YAG laser candeliver a maximum pulse energy of 1 J with a pulse width of 8 ns and operate at a 10 Hz pulse repetition rate, operating inQ-switched mode. Here, 1064 nm radiations emitted at afundamental frequency from the Nd:YAG laser were appliedfor the production of the plasma spark at the asphaltene testsample. The laser energy was measured with a calibrated energymeter (Ophir model 300) for the study of the dependence of theLIBSsignaluponincidentlaserenergy.Thepulseenergyusedinthis experiment was in the range of 80 - 120 mJ. The light fromthe plasma spark is collected by a collimating lens using aUV-grade fused silica 1 m, multimode sampling fiber with aSMA connector and is transferred to the LIBS 2000 þ spectro-meter (Ocean Optics). Our LIBS 2000 þ has four spectrometermodules to provide high resolution [full width at half maximum(fwhm) of 0.1 nm] in the 200 - 620 nm wavelength region. Thedetector has a gated charge-coupled device (CCD) camerahaving 14336 pixels.This makes it possible to measure a LIBS spectrum over abroad spectral range (200 - 620 nm) simultaneously with spec-tral resolution (0.1 nm). The plasma emission was recorded at a90   angle to the laser pulse. Software built in the spectrometerreadthedatafromthechipandreconstructedthespectrum.Theconcentrations of different trace metals present in asphaltene Figure 1.  Schematic diagram of the LIBS system applied for the analysis of asphaltene samples.  1102 Energy Fuels  2010,  24,  1099  –  1105  :  DOI:10.1021/ef900973s  Gondal et al. samples were also measured with a calibrated ICP spectrometerto verify the results achieved with our calibrated LIBS method.For each LIBS analysis, a fresh asphaltene test sample was keptin the LIBS chamber. To test the homogeneity of our testsamples, several LIBS measurements were performed at thesurface of asphaltene test samples. For calibration purposes,iron, cobalt, and lead metals were used. All of these metals inpowder forms were of high purity (99.99%) and procured fromFisher Scientific of Pittsburgh, PA. For the construction of thecalibration curves, pure metals in powder form were mixed withthe Ras Tanura asphalt sample, which contains an asphaltenematrix. The metals were thoroughly mixed with a warm asphaltsample. ICP Analysis.  To verify our results for the determination of trace metals present in asphaltene samples, ICP analysis wasalso performed on an ARL model 3580 OES ICP atomicemission spectrometer. For the ICP analysis, each individualsample of asphaltene was digested with nitric acid (99.99%,Fisher Scientific, Pittsburgh, PA) and left overnight. The result-ing residue was then ashed at 500   C. The ashed sample wasfurther diluted with nitric acid (99.99%, Fisher Scientific,Pittsburgh, PA), and the resulting solution was analyzed fortrace metals using an ICP spectrometer calibrated using refer-ence standards of three levels of accuracy. Results and Discussion InLIBS,therearemanyprocessesthatoccurwhenapulsedlaser beam interacts with any solid material, resulting in theproduction of intense plasma, thermionic emission, sampleheating,melting,atomization,excitation,andionization. ThetraceelementspresentinthesamplescanbeidentifiedbyLIBSspectral analysis. There are many variables that influence theLIBS signal intensity for the investigation of solid samples. Figure 2. TypicalLIBSspectraoftheArabLightasphaltenesamplerecordedintheregionof200 - 550nm.ThechangeinLIBSsignalintensitydepending upon the concentration of trace metals present in the sample can be noticed from the  y -axis scale. Figure 3.  Typical LIBS spectra of the Arab Medium asphaltene sample recorded in the region of 200 - 570 nm.  1103 Energy Fuels  2010,  24,  1099  –  1105  :  DOI:10.1021/ef900973s  Gondal et al. These are the laser pulse width, shape, spatial and temporalfluctuation of the pulse and power fluctuation, laser wave-length, laser energy, and the physical and chemical propertiesof the target material.To enhance the sensitivity of the LIBS system, for analysisof asphaltene samples, the optimal experimental conditionsmentioned above, which can affect the LOD in LIBS, wereexplored.Hence,priortotheanalysisoftestsamples,differentparameters, such as laser energy, delay time, focusing lens forincidentlaserradiation,andcollectinglensforlaser-producedplasmaemission,wereoptimized.Thereproducibilityforeachdata point was calculated with a confidence level of 95%. Spectral Analysis and Trace Element Identification Usingthe LIBS Technique.  Figures 2 - 5 depict the emission spectraof the asphaltene samples for the spectral region of 200 - 592 nm. The laser pulse energy was 100 mJ. The distancebetweentheopticalfiberandtheplasmawas10mm.TheLIBSspectra of the element under investigationwere recorded in theabove-mentionedspectralregiontofindthemostsensitivelinesforeachelement.ThemajorelementsdetectedinthesampleareCo,Fe,V,Ni,Pb,P,Mo,Ca,Si,Ti,Mn,Cd,andCu.Thetraceelements present in different asphaltene samples detected byourLIBSsystemandcounterverifiedbyICParelistedinTable1. The fingerprint wavelength of each element is also listed inTable 1. Table 2 shows the trace metals and their spectralassignments determined in different asphaltene samples usingLIBS. These emission lines have minimal interference fromother emission lines, do not involve the ground state, so thatself-absorption is almost absent, and are intense enough.Because of these reasons, these lines are useful for quantitativeanalysis. All of the spectral lines for the above-mentionedelements recorded with our LIBS setup were identified usingthe work performed by Striganove et al. 31 and also usingthe National Institute of Standards and Technology (NIST)atomic spectral database. 32 Figure 4.  Typical LIBS spectra of the Arab Heavy asphaltene sample recorded in the region of 200 - 570 nm. Figure 5.  Typical LIBS spectra of the Arab Berri asphaltene sample recorded in the region of 200 - 570 nm. (32) http://physics.nist.gov/PhysRefData/ASD/index.html?nist_ato-mic_spectra.html.
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