Coking Kinetics of Asphaltenes as a Function of

Coking Kinetics of Asphaltenes as a Function ofChemical StructureSamina Rahmani,†William C.McCaffrey,†Heather D.Dettman,‡andMurray R.Gray*,†Department of Chemical and Materials Engineering,University of Alberta,Edmonton, Alberta,T6G2G6,Canada,and National Centre for Upgrading Technology,1Oil Patch Drive,Devon,Alberta,Canada T9G1A8Received January9,2003.Revised Manuscript Received May15,2003The dependence of coking kinetics on the chemical structure of asphaltenes was examined by reacting five different asphaltenes in1-methylnaphthalene and tetralin at430°C and ca.9.8 MPa.The selected heptane-insoluble asphaltenes were Athabasca asphaltenes from Canada, Arabian Light and Arabian Heavy from Saudi Arabia,Maya from Mexico,and Gudao from China. The13C NMR aromaticity of the asphaltenes ranged from0.40to0.61,and the sulfur contents ranged from4.44wt%to7.47wt%.The cracking kinetics of the asphaltenes were consistent with a modified kinetic model for coke formation,incorporating phase separation and hydrogen transfer to the asphaltenes.The rate of cracking of asphaltenes in1-methylnaphthalene correlated with the content of aliphatic sulfur,and the yield coefficient for coke correlated with the aromaticity.These correlations allowed prediction of coking kinetics for Iranian Light and Khafji asphaltenes on the basis of average structural properties of asphaltenes.Hydrogen transfer to the different asphaltenes did not correlate with any single average structural property.IntroductionPrimary upgrading of bitumen and petroleum resi-dues to distillate products is achieved by either coking or catalytic hydroconversion.During coking processes, the objective is to maximize the yield of cracked products,while also producing a coke material of the desired quality.During hydroconversion processes,the process objective is maximum yield of cracked product without formation of coke deposits.In both types of processes,formation of coke limits the possible conver-sion to distillable liquid products.Improvements in the operability and efficiency of these units requires control or management of coke formation,which can only be achieved through a better understanding of the funda-mental mechanisms involved.Coke is defined operationally in petroleum refining as a carbonaceous material that is insoluble in an aromatic solvent such as benzene or toluene.The yield of coke in thermal and catalytic processes increases with the concentration of petroleum asphaltenes in the feed, defined as the pentane-or heptane-insoluble and toluene-soluble organic fraction of bitumen or vacuum residue.1,2 Heavy oil and residues can contain significant amounts of asphaltenes,in the range of10-30wt%;therefore, an understanding of asphaltene reaction pathways will help to develop an understanding of the mechanisms of coke formation.Asphaltene reaction pathways are af-fected by the chemistry of the feed material,the processing parameters of temperature and pressure, solvent properties,and the supply of reactive hydrogen.1 Prior treatments,such as distillation and blending of recycle material,will affect the feed composition in residue conversion processes.Any change in the feed composition may influence the rate and extent of coke formation,and also the product composition and yield. Despite the importance of feed composition,few studies have considered the relationship between thermal re-activity and chemical structure.3,4The a priori predic-tion of coke formation from the chemical structure of petroleum and bitumen residues during thermal con-version is not yet possible.The objective of this work was to examine the behavior of a range of heptane-insoluble asphaltenes in the liquid phase,to predict coking kinetics from feed properties.Feed asphaltenes were selected to give a range of average chemical properties.Based on the available data from the literature,5-7Athabasca as-phaltenes from Canada,Arabian Light and Arabian Heavy from Saudi Arabia,Maya from Mexico,and Gudao from China were selected.These asphaltenes*Author to whom correspondence should be addressed.Tel:(780) 492-7965.Fax:(780)492-2881.E-mail:murray.gray@ualberta.ca.†University of Alberta.‡National Centre for Upgrading Technology.(1)Speight,J.G.The Desulfurization of Heavy Oils and Residua; Marcel Dekker Inc.:New York,1981.(2)Speight,J.G.In Catalysis on the Energy Scene;Kaliaguine,S., Mahay,A.,Eds.;Elsevier:Amsterdam,1984.(3)Liu,C.;Zhu,C.;Jin,L.;Shen,R.;Liang,W.Fuel Process. Technol.1999,59,51-67.(4)Gray,M.R.;Jokuty,P.;Yeniova,L.;Nazarewycz,L.;Wanke,S.E.;Achia,U.;Krzywicki,A.;Sanford,E.C.;Sy,O.K.Y.Can.J.Chem. Eng.1991,69,833-843.(5)Zhou,J.;Mu,B.;Que,G.Prepr.Pap.s Am.Chem.Soc.,Div.Pet. Chem.1994,39(3),426-429.(6)Trauth,D.M.;Yasar,M.;Neurock,M.;Nigam,A.;Klein,M.; Kukes,S.G.Fuel Sci.Technol.Int.1992,10,1161-1179.(7)Ali,M.F.;Saleem,M.Arab.J.Sci.Eng.1994,19(2B),319-333.1048Energy&Fuels2003,17,1048-105610.1021/ef030007c CCC:$25.00©2003American Chemical SocietyPublished on Web06/17/2003span a range of sulfur and nitrogen contents,and a range of aromatic carbon content.The solvent 1-meth-ylnaphthalene was used in pressurized batch reactors in order to control the liquid environment during parison with tetralin solvent allowed explora-tion of the role of hydrogen transfer in a liquid medium of similar solubility parameter.8The phase separation model proposed by Wiehe 9and modified by Rahmani et al.8was used as a kinetic framework to analyze the coke yields.TheoryThe coke formation model proposed by Wiehe 9was extended by Rahmani et al.8to incorporate hydrogen transfer from solvents.During cracking,the asphaltene fraction was assumed to consist of A +,the unreacted asphaltenes with attached side groups,and two types of aromatic cores formed by cracking of the initial asphaltenes,each with different hydrogen-accepting capability.Cores that can accept enough hydrogen to change their solubility characteristics were designated A *A ,and cores that cannot accept hydrogen were designated A *NA .At any time,A *A +A *NA )A *,the total concentration of aromatic cores.The cores that accepted sufficient hydrogen from a donor such as tetralin (TN)were assumed to be converted to heptane-soluble material.The reaction model was as follows for asphaltenes in aromatic solvents:where H *is the fraction of product heptane solubles,k A is the first-order reaction rate constant for the thermal cracking of reactant asphaltene and d and c are stoichiometric coefficients.The apparent first-order rate constant for hydrogen transfer from solvent was the product of the rate constant (k ′)and the tetralin concentration,[TN].In the case of a closed reactor,the cracked distillate products (V )are retained and remain mainly in the liquid phase.The solubility limit (S L )in a closed reactor defines how much reacted asphaltene (A max/)can remain in solution in the liquid phase:where S )solvent (1-methylnaphthalene or tetralin).The asphaltenes in excess of the solubility limit (A ex /)then react to form toluene-insolubles,or coke.where TI )toluene-insoluble coke and k c is the first-order rate constant for formation of insolubles.Experimental MethodsMaterials.Asphaltenes from five different sources wereused to study the effect of asphaltene structure on the kinetics.Athabasca,Gudao,Arab Heavy and Arab Light asphalteneswere separated from the respective vacuum residues,and Maya asphaltenes were separated from the crude oil.The toluene-insoluble solids were removed from Athabasca by dissolving vacuum residue in 40parts of toluene for overnight and then filtering it on 0.22µm filter paper.Toluene was evaporated by rotary evaporation.The other feeds had insig-nificant concentrations of solids;therefore,this step was omitted.Heptane-insoluble asphaltene was separated by blending oil 1:1with toluene,then adding 40parts of heptane and mixing overnight.The asphaltene precipitate was recov-ered by filtration on a 0.22µm Millipore filter.The average molecular weights were obtained using vapor pressure osmometry (Westcan Corona model 232A)at the Micro Analytical Laboratory at the University of Alberta.The samples were dissolved at a single concentration of ca.2mg/mL in o-dichlorobenzene and analyzed at a temperature of 130°C.Elemental analysis was performed on a Carlo Erba Elemental Analyzer 1108at the Micro Analytical Laboratory,University of Alberta.Thermal Cracking Experiments.Experiments were car-ried out batchwise in a 15mL microreactor made from Swagelock fittings and tubing.The reactor was loaded with 3g of reactant and then pressure tested with nitrogen at 4MPa.The gas was then vented,and the reactor was pressurized twice more with nitrogen and vented to purge residual oxygen.The reactor was then filled again with nitrogen to a pressure of 4MPa.The reactor was heated in a fluidized sand bath and agitated at ca.1Hz for the duration of the reaction interval,then quenched by plunging the reactor into cold water.The contents of the reactor reached the final temperature within 5min,and an initial nitrogen pressure of ca.9.8MPa.All of the reactions were carried out at 430°C.Approximately 85%of the solvents were estimated to be in the liquid phase at 430°C,using the Peng -Robinson equation of state (ASPEN Plus software,Aspen Technology,Cambridge,MA).Separation of Products.Liquid product and coke were washed out of the reactor with 40parts of toluene,then kept overnight at 70°C to ensure the extraction of liquid products from the solid coke.Coke was then removed from the toluene solution by filtration on a 0.22µm Millipore filter.Coke yield was determined by weighing the filter after drying in a vacuum at 70°C for 12h.Toluene was removed from the filtrate by rotary evaporation,the oil was then blended with 40parts of n -heptane to precipitate asphaltenes.Solid asphaltenes were recovered by filtration and vacuum-dried at 70°C for 12h to give the asphaltene yield.Heptane was removed from the heptane-soluble fraction by rotary evaporation and then by vacuum-drying at 70°C.GC Analysis of Solvents.The heptane-soluble fractions of the product were analyzed by GC in order to estimate the hydrogen transfer from tetralin to asphaltenes.Under thermal reaction conditions,tetralin can dehydrogenate to produce 1,2-dihydronaphthalene and naphthalene in succession,isomerize to form 1-methyl indan,or crack to form alkyl benzenes.The quantity of hydrogen transferred from tetralin was calculated from the amount of naphthalene formed,given that for every mole of naphthalene produced,4atoms of hydrogen were transferred from tetralin.Formation of 1,2-dihydronaphtha-lene was not detected.A Hewlett-Packard 5890GC using a HP-1cross-linked methyl silicone gum column (25m,0.32mm i.d.,0.17µm)equipped with a flame ionization detector (FID)and a com-puter for storing the chromatograms.The temperature pro-gram for the GC was as follows:initial oven temperature of 50°C,then heating to 300°C at a rate of 5°C/min and then held at constant temperature for 15min.Nuclear Magnetic Resonance (NMR)Spectroscopy.Samples were prepared for 1H NMR by mixing approximately 20mg of the sample with 700µL of deuteriochloroform (CDCl 3).Samples for 13C NMR samples were prepared by dissolving approximately 50mg of material in 700µL of CDCl 3.(8)Rahmani,S.;McCaffrey,W.;Gray,M.R.Energy Fuels 2002,16,148-154.(9)Wiehe,I.A.Ind.Eng.Chem.Res .1993,44,2447-2457.A +98k A(c -d )A *NA +dA*A +(1-c )(H *+V )(1)A *A 98k ′[TN]H *(2)A max /)S L (S +H *+V )(3)A ex /)A *NA+A *A -A max/(4)A ex /98k cTI (5)Coking Kinetics of Asphaltenes and Chemical Structure Energy &Fuels,Vol.17,No.4,20031049The NMR experiments were performed at room temperature (20(1°C)on a Varian XL-300NMR spectrometer,operating at299.943MHz for proton and75.429MHz for carbon.The proton spectra were collected with an acquisition time of2.1s,a sweepwidth of7000Hz,a pulse flip angle of30.8°(8.2µs),and a1-s recycle delay.These pulse recycle conditions permit the collection of quantitative spectra for all protonated molecular species in the petroleum samples where the maxi-mum spin lattice relaxation time(T1)is expected to be less than20s.The spectra,resulting from128scans and using 0.3-Hz line broadening,were referenced to the residual chloroform resonance at7.24ppm.The quantitative carbon spectra were acquired using an acquisition time of0.9s and a sweepwidth of16500Hz.A flip angle of31.9°(5.7µs)and a4-s delay were used.These parameters are quantitative for carbons with spin lattice relaxation times(T1)of the order of30s.Reverse-gated waltz proton decoupling was used to avoid nuclear Overhauser effect enhancements of the carbon signals.The spectra were the result of15000scans.Line broadening at10Hz was used to improve the signal-to-noise ratio of the spectra.All spectra were referenced to the CDCl3resonance at77.0ppm.The chemical shift assignments for the1H and13C NMR are shown in Table1and Table2,respectively.These assign-ments were based on model compound assignments,10,112-D NMR spectroscopic techniques such as HETCOR(hetero-nuclear chemical shift correlation),and COSY(homonuclear correlation spectroscopy)12as well as1-dimensional techniques such as DEPT(distortionless enhanced polarization trans-fer).13,14The final calculations performed to obtain the quanti-ties of the different carbon types are shown in Table3.15The aromatic CH’s,terminal methyls,and the methyls of aromatic ethyls were determined directly from the carbon spectra.These groups were based on minimal assumptions and may be considered to be the most accurate.The other groups listed below were calculated using data from both proton and carbon spectra or had to be estimated due to spectral overlap and are therefore subject to larger margins of error.The substituted quaternary carbon(Qar-S) content was determined from the proton spectra by converting the R-to-aromatic CH2and CH3protons to their respective amounts of carbon.The remaining amount of quaternary carbon was considered to be bridgehead carbon(Qar-P).The methyl carbons attached to aromatic rings(Ar-CH3)were determined using the proton spectral region HP2.The remain-ing R-to-ring CH3’s were assumed to be attached to cyclopar-affinic rings(Cy-CH3).The content of olefinic carbon would have been obtained from the proton spectra;however,olefinic resonances were not detected in the asphaltene samples.Half of region CP2was estimated to be aliphatic CH’s,and the total aliphatic CH’s were calculated as the sum of region CP1and one-half of region CP2.The CH2carbons in paraffinic chains longer than5carbons were calculated from region CP4after the contributions of CH2carbon from aromatic-attached ethyls and R-to-aromatic cycloparaffinic CH2were removed.The amount of cycloparaffinic CH2’s R to aromatic rings were estimated as half of the cycloparaffinic region CP5.Finally, the area of region CP5was used as an indicator of cyclopar-affinic CH2content.Sulfide Analysis.Aliphatic sulfides were measured by selectively oxidizing them to sulfoxides followed by infrared spectroscopy(IR)determination,as described by Green et al.16 About0.2g of sample was dissolved in25mL of toluene plus 5mL of methanol,and then about0.2g of tetrabutylammo-nium periodate was added as oxidant.Tetrabutylammonium periodate was obtained from Aldrich Chemical Co.,Inc.The resulting mixture was magnetically stirred and refluxed for 30min in a100mL round-bottom flask connected with a short condenser.The oxidized mixture was then cooled and extracted in a separatory funnel three times,with about100mL of high purity water per extraction.The organic phase was dried in a rotary evaporator,followed by an additional2h drying in a vacuum oven at about70°C.A measured quantity of the dried product,at about0.2g,was dissolved in dicloromethane to make a solution of4mL,and its sulfoxide content was measured by IR absorbance.All IR measurements were performed with a FTS6000infrared spectrometer with a removable cell containing KBr windows and a0.015mm Teflon spacer.Background was subtracted from the spectrum of the sample solution.Sulfoxides were measured via their maximum absorbance near1025cm-1.The molar absorptivity of245L mol-1cm-1was used to calculate sulfoxide concentration in all the asphaltenes.16(10)Snape,C.E.;Ladner,W.R.Anal.Chem.1979,51,2189-2198.(11)Thiel,J.;Gray,M.R.AOSTRA J.1988,4,63-73.(12)Sarpal,A.S.;Kapur,G.S.;Chopra,A.Fuel1996,75,483-490.(13)Netzel,D.A.Anal.Chem.1987,59,1775-1779.(14)Kotlyar,L.S.;Morat,C.;Ripmeester,J.A.Fuel1991,70,90-94.(15)Japanwala,S.;Chung,K.H.;Dettman,H.D.;Gray,M.R. Energy Fuels2002,16,477-484.(16)Green,J.B.;Yu,S.K.T.;Pearson,C.D.;Reynolds J.W.Energy Fuels1993,7,119-126.Table1.Chemical Shifts of Proton Spectral Regionsregion chemical shifts(ppm)structural typeHA110.7to7.4polyaromaticHA27.4to6.2monoaromaticHO1 6.2to5.1olefinic CHHO2 5.1to4.8olefinic CH2HO3 4.8to4.3olefinic CH2HP1 4.3to2.4R-to-aromatic CH2 HP2 2.4to2.0R-to-aromatic CH3 HP3 2.0to1.09paraffinic CH2HP4 1.09to-0.5paraffinic CH3 Table2.Chemical Shifts of Carbon Spectral Regionsregion chemical shifts(ppm)structural typeCA1190to170oxygenatedCA2170to129quaternary aromaticCA3129to115.5aromatic CHCA4115.5to113.5olefinic CH2CA5113.5to100olefinic CH2CP170to45paraffinic CHCP245to32.7paraffinic CH&CH2CP332.7to30.8chainγ-CH2 -to-aromatic CH2CP430.8to28.5chainδ-CH2R-to-aromatic naphthenes,aromatic-attached ethyl CH2CP528.5to25cycloparaffin CH2CP625to21.9chain -CH2,R to aromatic or isobutylCH3CP721.9to17.6R-to-ring CH3CP817.6to14.7aromatic-attached ethyl CH3CP914.7to12.3chain R-CH3CP1012.3to0branched-chain CH3Table3.Group Classifications of Chemical SpeciesReported for Asphaltene Samplescarbon chemical species spectral region aromatic CH(CHar)CA3methyl of an aromatic ethyl(E-CH3)CP8terminal methyl of a paraffinic chain CP9paraffin-substituted quaternary(Qar-S)HP1+HP2convertedto C wt% polyaromatic quaternary(Qar-P)CA2-Qar-SR-to-aromatic ring methyl(Ar-CH3)HP2converted toC wt%paraffinic CH(CH)CP1+CP2/2paraffin CH2in>C5chains(CHAIN)CP4-CP8-CP5/2 cycloparaffin CH2(NAPH)CP51050Energy&Fuels,Vol.17,No.4,2003Rahmani et al.Results and DiscussionProperties of Feed Asphaltenes.Molecular weights of the feed asphaltenes are presented in Table 4.The data from VPO may suffer from the self-association of asphaltenes even after using a strong solvent,17but on a relative basis Arab Light had the lowest molecular weight of the five samples.The other molecular weights were not significantly different from each other consid-ering the error involved in the measured molecular weight,which is typically (10%of the molecular weight.Table 4also lists the elemental analyses of the feed asphaltenes.Arab Light and Arab Heavy asphaltenes were low in nitrogen (0.83and 0.91wt %),while the other asphaltenes had similar nitrogen content (1.17∼1.27wt %).The asphaltene from Gudao vacuum residue was highest in H/C atomic ratio (1.41)and Arab Light asphaltene was the lowest (1.07),while the others were similar (1.16∼1.18).Athabasca and Arab Heavy as-phaltenes had the highest sulfur content (7.47and 7.15wt %),whereas Gudao and Arab Light had the lowest sulfur contents (4.44and 5.67wt %).Sulfide contents ranged from 32to 58%of the total sulfur.NMR Analysis.The data of Table 5list the concentra-tions of carbon groups in the asphaltenes.Arab Light had the highest percentage of quaternary aromatic carbon (Qar)and Gudao had the lowest.All other asphaltenes had comparable quantities of quaternary carbon.A similar trend was observed for protonated aromatic carbon.Arab Light had the highest total aromaticity and Gudao the lowest.NMR data were also used to calculate aromatic cluster size,i.e.,the number of carbons in an aromatic cluster.For example,naph-thalene has an aromatic cluster size of 10.This param-eter was calculated using the aromatic functional group contents and the method described by Solumn et al.18The Arab Light asphaltenes had the largest cluster size (21),followed by Athabasca and Maya (19),Arab Heavy (17),and lowest in Gudao asphaltenes (15).The asphaltenes also differed in the concentrations of aliphatic carbon groups (Table 5).Paraffin CH,indicating branched chains or substituted cycloparaffins,were highest in Arab Heavy and Gudao asphaltenes and lowest in Arab Light.Chain paraffin CH 2groups that are at least six carbons in length were highest in Gudao asphaltenes.All other asphaltenes had similar values.Cycloparaffinic (i.e.,naphthenic)CH 2,indicating car-bons that are at least to an aromatic ring or not attached to an aromatic ring,were highest in Gudao asphaltenes followed by Athabasca.CH 2groups that are R or either to alkyl attachment sites in rings or chains,or to terminal methyls in chains,were lowest in ArabLight asphaltenes.The sum of cycloparaffinic CH 2and these R -and -CH 2groups,roughly representing the total cylcoparaffinic carbon,was higher in Gudao and Athabasca compared to the other asphaltenes.Overall,the five samples were very different in elemental compositions and structural parameters.This result suggests that the five samples used in this study were representative of a range of variations in chemical and structural compositions in petroleum asphaltenes.Thermal Cracking of Asphaltenes in 1-Methyl Naphthalene.All the asphaltenes were reacted in 1-methyl naphthalene (MN)for a series of reaction times.A concentration of 25wt %initial asphaltene was used for all of the reactions.Figure 1shows the product asphaltene yield from the thermal cracking of five different asphaltenes in 1-methyl naphthalene.Gudao and Arab Light cracked more slowly than the other three asphaltenes.These two asphaltenes had the lowest amount of sulfur among the five.Yields of residual asphaltenes after long reaction times were not significantly different between the samples.After long reaction times,the unreactive asphaltene cores should remain in the solution and contribute to the product asphaltenes.The similar yields of residual asphaltenes from different feeds suggests that their solubilities in 1-methyl naphthalene were similar,or in other words,structurally the asphaltene cores from different asphaltenes were not very different from each other after reaction.To test this hypothesis,NMR spectra of residual asphaltenes from highly paraffinic Gudao and moderately aromatic Athabasca asphaltenes were collected.The difference in aromaticity between the two asphaltenes decreased from 15%in the feed to only 4%in the product.The aromaticity (without olefin)of the product asphaltenes became 73%for Athabasca and 69%for Gudao.Aliphatic carbon decreased signifi-cantly in both cases.Although the starting materials were very different in chemical structure,after they went through the thermal cracking they became more similar in structure.This similarity was consistent with the yields of residual asphaltenes after 60min in Fig-ure 1.(17)Altgelt,K.H.;Boduszynski,position and Analysis of Heavy Petroleum Fractions ;Marcel Dekker:New York,1994;Chapter 4.(18)Solum,M.S.;Pugmire,R.J.;Grant,D.M.Energy Fuels 1989,3,187-193.Table 4.Chemical Properties of the Feed Asphaltenes feed asphaltene H/C ratio sulfur (wt %)sulfide S (wt %)nitrogen (wt %)molecular weight Arab Heavy 1.167.15 3.270.922600Arab Light 1.07 5.67 1.790.831800Athabasca 1.197.47 3.78 1.282900Gudao 1.41 4.44 2.28 1.182800Maya1.176.613.861.192500Figure 1.Residual asphaltene yields from the thermal cracking of asphaltenes in 1-methylnaphthalene.Curves are drawn from the model eqs 1and 3-5using the parameters of Table 6.Initial concentration was 25wt %asphaltene in solvent.Data for Arab Heavy were almost identical to Atha-basca at 20and 40min.Coking Kinetics of Asphaltenes and Chemical StructureEnergy &Fuels,Vol.17,No.4,20031051The data of Figure 2show the coke yield from the same sets of experiments.Gudao had the lowest coke yield among all asphaltenes.The other four samples clustered together.The data showed that Arab Light reacted more slowly,but ultimately produced the high-est amount of coke.Data from the reactions in 1-meth-ylnaphthalene were used in the modified phase sepa-ration kinetic model.8The solubility limit,S L ,and first-order rate constant for coke formation,k C ,were taken to be the same for all the asphaltenes as for Athabasca asphaltenes,assuming that the asphaltene cores were not significantly different from each other.The asphal-tene cracking rate constants,k A ,and the stoichiometric coefficient,c ,were estimated by minimizing the sum of squares of the error following the same procedure as described in the work by Rahmani et al.8The parameter d ,giving the split between cores that could and could not accept hydrogen,and the rate constant for hydrogen transfer (k ′)were not needed in the absence of tetralin.The data of Figures 1and 2show the fit of the experimental data to the model.Estimated parameters,k A and c ,are presented in Table 6.The error estimates represent the 95%confidence interval in the parameter estimation.The standard deviation for each estimated parameter was calculated by using the principal diago-nal terms in the variance -covariance matrix.19ArabLight had a significantly lower cracking rate constant compared to Athabasca and Arab Heavy,while the stoichiometric coefficient,c ,for Gudao was significantly lower than the other asphaltenes.Thermal Cracking of Asphaltenes in Tetralin.Product Yield.All the asphaltenes were reacted in the presence of tetralin for 40min.From the study with Athabasca asphaltene 8it was found that coke formation was almost complete after 40min;therefore,this reaction time was used to minimize the error involved due to the initial heating time versus overcracking of the products at long reaction times.A concentration of 25wt %initial asphaltene was used for all of the reactions.Coke and product asphaltene yields were measured for each experiment.The data of Figure 3show that the coke yield was suppressed by the addition of tetralin for all asphal-tenes,in comparison to reaction in a solvent of equiva-lent solubility parameter (1-methylnaphthalene).As-phaltene from Gudao vacuum residue produced the lowest amount of coke,whereas coke yields from all other asphaltenes were not significantly different from each other.The residual asphaltene yield varied sig-nificantly between the different feeds,which suggested that hydrogen transfer to give asphaltene conversion to heptane-solubles did not occur to the same extent for all asphaltenes.Data from the reactions in tetralin can be interpreted by the modified phase separation kinetic model.8For each asphaltene,the rate constants,k A ,and stoichio-metric coefficients,c ,from reaction in 1-methylnaph-thalene (Table 6)were assumed to apply to reaction in tetralin.The stoichiometric coefficient,d ,was estimated from the difference in heptane-solubles plus volatiles (gas and light naphtha)formed when each asphaltene was reacted in tetralin and 1-methylnaphthalene.8The rate constant for hydrogen transfer,k ′,was assumed(19)Himmelblau,D.M.Process Analysis by Statistical Methods ;John Wiley &Sons:New York,1970.Table 5.NMR Data for Concentration of Carbon Typescarbon typesArab Heavy (%C tot )Arab Light (%C tot )Athabasca (%C tot )Gudao (%C tot )Maya (%C tot )total aromatic (f a )5261554053Quaternary (Qar)26.634.42921.428.2polyaromatic Qar 17.126.219.812.219.6alkyl-substituted Qar 9.58.29.29.28.6aromatic CH 24.626.524.618.424.6paraffin CH11.47.18.610.68.3paraffin CH 2,>5C8.98.48.216.78.1methyl of aromatic ethyl 1.1 1.2 1.2 1.2 1.5terminal paraffinic methyl 2.3 2.1 2.3 2.9 2.8cycloparaffinic CH 23.7 3.64.75.1 3.8CH 2R and to chain ends,attachment sites 15.712.615.517.116.1aromatic CH 32.3 2.1 2.4 2.2 2.2cylcoparaffinic CH 32.93.43.53.83.6Figure 2.Coke yields from the thermal cracking of asphal-tenes in 1-methylnaphthalene.Curves are drawn from the model eqs 1and 3-5using the parameters of Table 6.Table 6.Estimated Kinetic Parameters for DifferentAsphaltenes in 1-Methylnaphthalene a ,b feed asphaltene cracking rate constant,k A(min -1)yield coefficient,c Arab Heavy 0.17(0.040.52(0.05Arab Light 0.11(0.010.63(0.03Athabasca 0.15(0.030.54(0.03Gudao 0.10(0.030.37(0.02Maya0.20(0.090.60(0.05aError estimates are for 95%confidence intervals.b The other kinetic constants in eqs 1and 3-5were S L )0.0295g/g and k C )0.17min -1.1052Energy &Fuels,Vol.17,No.4,2003Rahmani etal.。