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Fuel 86 (2007) 1737–1748

Investigation of glass transition temperatures of Turkish asphaltenes

Muzaffer Yasar *, Solmaz Akmaz, M. Ali Gurkaynak

Department of Chemical Engineering, University of Istanbul, Avcilar 34850, Turkey

Received 13 October 2006; received in revised form 8 December 2006; accepted 18 December 2006Available online 30 January 2007

Abstract

Asphaltenes are a major concern in production operations, due to their role in conversion of vacuum residues by processes such ascoking, catalytic cracking and hydroconversion. Six Turkish asphaltenes were characterized by elemental content, proton NMR, carbonNMR, GPC, DSC. Differential scanning calorimetry (DSC) provides parameters for comparing the glass transition temperatures, endo-thermic behavior of asphaltene and a hypothetical melting temperature of the asphaltenes which is within the range of the pyrolysis tem-perature of asphaltenes. The glass transition temperatures points (Tg) of asphaltenes were determined using DSC. Hypothetical meltingpoint temperature (Tm) of the asphaltenes was also calculated. Among all the asphaltenes tested, Bati Raman showed much higher glasstransition temperatures than the others.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Asphaltenes; Glass transition; DSC

1. Introduction

Asphaltenes are the most complex fraction of crude oils.They consist of fused aromatic and naphtenic rings corewith short aliphatic side chains. Asphaltenes contain highamounts of sulfur oxygen, nitrogen and traces of metalssuch as nickel, vanadium and iron. Asphaltenes are themost polar of fraction of crude oil and defined asaromatic-soluble, and n-paraffinic-insoluble. Asphaltenes,unlike common chemicals, do not have any well-definedchemical or physical properties such as molecular weight,melting point, boiling point, vapor pressure, solubilityand chemical reactivity, by which they can be character-ized. These properties generally vary within certain limitsfor different asphaltenes, depending on the molecularweight, degree of substitution and on the method of prep-aration. Asphaltenes are thought to be responsible for themain cause of problems associated with catalytic process-ing of heavy oils and residues. Asphaltenes unfavorablyaffect the overall rate of hydrodesulfurization, take action

0016-2361/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2006.12.022

* Corresponding author. Tel.: +90 212 473 70 50.E-mail address: yasar@istanbul.edu.tr (M. Yasar).

as coke precursors and contribute to catalyst deactivationboth of which are serious problems encountered inattempts to make better and refine processes in the petro-leum industry [1–3]. Since these problems must be relatedto structure and composition, asphaltene have been com-prehensively characterized using various techniques includ-ing proton and carbon NMR, FTIR, GPC, elementalcontent. Ample information obtained with these methodshas provided important insights into the properties orstructures of crude oils and their fractions [4–12]. Grayet al. [13] investigated behavior of the melting of theasphaltene fraction in crude oils and bitumens. Grayreported that asphaltenes from Arab Heavy, Arab Light,Athabasca, Gudao, and Maya all showed evidence for for-mation of a liquid melt at temperatures between 214 and311 �C, which was significantly lower than initial reactiontemperatures. Ekulu et al. [14] used the differential scan-ning calorimetry (DSC) technique to detect changes ofthe oil microstructure as a function of temperature. Otherresearchers [15–17] have used DSC in asphalts studies.The molecular weight effect on the glass transition temper-atures (Tg) of asphalts has been studied by Huynh et al.[16]. Kumari [17] investigated the crystallization of asphaltsusing DSC and found that the exothermic heat content of

1738 M. Yasar et al. / Fuel 86 (2007) 1737–1748

asphalts were mainly dependent on the prior cooling timerather than on the thermal history of the samples. Consid-erable research has been devoted to asphaltenes character-ization and their pyrolysis reactions [18–26], however, notmuch attention has been devoted to understand the possi-ble existence of crystalline structures and amorphous nat-ure in asphaltenes. Glass transition and melting points ofasphaltenes has been investigated by a few numbers ofresearchers [13,27–29]. Kopsch [28] was among the fewresearchers to study asphaltenes’ glass transition tempera-tures with DSC. The glass transition temperatures forasphaltenes from different sources such as Venezuela, Iran,Saudi Arabia, Russia and from a British offshore field wereinvestigated by Kopsch and reported to be 313 ± 6 �C,331 ± 9 �C, 353 ± 11 �C with three heating rates of 10, 20and 50 K/min, respectively [28]. Kopsch claimed thatasphaltenes which are precipitated with the same solventreact relatively uniformly regardless of the origin of thecrude oils. Zhang et al. [29] investigated the thermal behav-ior of four asphaltenes from different vacuum residuesusing DSC and TGA techniques. Zhang et al. found thatthe glass transition temperature of four asphaltene waslying near 120–130 �C. In order to gain more insights intothe catalytic processing of heavy oils and residues, it is nec-essary to understand the thermodynamic behavior ofasphaltenes under high temperatures. DSC has been widelyused to study the glass transition and enthalpy relaxa-tion of crude oils and their fractions. However, a smallamount of information has been reported on the thermalbehavior of asphaltene in the literature. Understandingthe thermal behavior of asphaltene would provide addi-tional insight into the processing problems of petroleumresidue during hydroprocessing.

2. Experimental methods

2.1. Samples

Petroleum feedstocks from the Yenikoy, Celikli, BatiRaman, Besikli, Garzan and Adiyaman fields in southeast-ern part of Turkey were provided by TPAO (TurkishPetroleum Corporation). Each crude were stored at atmo-spheric conditions for at least one week before use, to allowtime for degassing and enable free water and sediments tosettle out. Atmospheric petroleum residues were obtainedby removing the distillates boiling up to 350 �C using atmo-spheric distillation [30]. All solvents used in this study wereobtained from Merck and used as received. Cellulose thim-bles (28 · 80 mm) from Macherey-Nagel were used forSoxhlet extraction.

2.2. Asphaltene separation

Asphaltene fractions were prepared from the atmo-spheric residue in a four-step process. Residue was addedto n-heptane in a 1 to 40 weight ratio. The solution was stir-

red for 1 h at 60 �C, and then allowed to cool under contin-uous stirring for 4 h. The solution was then allowed tosettle overnight prior to filtering. The solution was filteredto remove the asphaltene precipitate. Maltenes were col-lected by evaporating the n-heptane from the filtrate. Theasphaltene was subject to Soxhlet extraction to verify theabsence of maltenes. Boiling heptane was used to verifythe absence of asphaltenes in the maltene fraction. Like-wise, boiling heptane was used to extract the asphaltenesto remove any residual maltenes. Solvent was removed byrotary evaporation. Also, extracting the asphaltenes withboiling toluene revealed the absence of coke in the six res-idues [18–20,30]. Asphaltene contents vary widely for thestudied crude oils as can be seen in Table 1a.

2.3. Analytical chemistry

The samples were characterized by API gravity, elemen-tal content, proton NMR, carbon NMR, GPC, and DSC[8,23,27–30]. All the analysis was carried out at Universityof Istanbul with exception of the GPC which was per-formed at Yildiz Technical University, Istanbul.

2.3.1. Elemental analysis

Elemental analysis was carried out on a Thermo Finni-gan Flash Model EA 1112 elemental analyzer was used forthe analysis of asphaltene samples for total carbon, hydro-gen, sulfur and nitrogen content. The oxygen content wascalculated from the balance. Appropriate standard refer-ence samples were used to prepare the calibration curves.

2.3.2. Molecular weight by gel permeation chromatography

(GPC)

Six asphaltenes obtained from the atmospheric residuewere subjected to GPC. The samples for GPC analysis wereprepared by dissolving the asphaltene in HPLC-grade tet-rahydrofuran (THF) to obtain a 1 gram per liter solutionand the solution was filtered through micro-pore filters(Millipore, 0.2 lm size). The GPC separations were per-formed on an Agilent Model GPC (1100 series) equippedwith a refractometer. Zorbax PSM columns (each of6.2 mm ID and 250 mm long) of pore size; 5 lm were used.HPLC-grade tetrahydrofuran as a mobile phase were used.A flow rate of 0.3 ml/minute was used. Injection volumewas 20 lL. A calibration curve for the GPC column wasconstructed from polystyrene in the range of 160 and20,000 Da. The instrument was operated at ambient tem-perature, while the refractometer was maintained at 35 �C[8]. Table 1 lists the number average molecular weight(Mn) and the weight average molecular weight (Mw) forthe six asphaltenes.

2.3.3. NMR analysis

The distribution of hydrogen and carbon according tostructural type in the crude oils and their fractions wereinvestigated by proton NMR and Carbon NMR, respec-

Table 1aChemical analysis of Garzan (G), Besikli (B), Adiyaman (A), Batman Yenikoy (BY), Batman Celikli (BC), Bati Raman (BR) crude oils

Analysis G B A BY BC BR

API 19.22 20.19 21.37 24.91 29.61 10.33% Asphaltene 8.63 5.23 6.99 2.65 1.48 28.11% Carbon 84.29 84.67 84.76 85.97 85.66 79.49% Hydrogen 11.55 11.48 12.23 13.30 14.10 9.26% Nitrogen + oxygen% Sulfur 1.58 1.48 1.34 3.74H/C 1.65 1.63 1.73 1.86 1.96 1.40S/C 0.007 0.0065 0.0059 0.0176

Table 1bChemical analysis of Garzan (GA), Besikli (BA), Adiyaman (AA), Batman Yenikoy (BYA), Batman Celikli (BCA), Bati Raman (BRA) asphaltenes

Analysis GA BA AA BYA BCA BRA

% Carbon 83.69 83.87 83.52 86.23 84.43 79.23% Hydrogen 7.96 7.90 7.51 8.46 8.41 7.23% Nitrogen + oxygen 4.35 4.36 5.17 3.18 4.75 4.71% Sulfur 4.00 3.87 3.80 2.13 2.41 8.83H/C 1.14 1.13 1.08 1.18 1.19 1.09S/C 0.018 0.017 0.017 0.009 0.011 0.042

Mn by GPC 579 865 593 610 849 1010Mw by GPC 812 990 859 857 943 1342

Proton NMR% HMe 11.93 15.56 22.63 23.42 23.95 11.26% HMy 38.74 41.90 39.31 32.91 39.36 11.26% HN 21.87 26.76 19.89 26.83 16.05 26.11% Ha 22.14 15.17 16.16 15.59 17.08 23.43% Har 5.32 0.69 2 1.25 3.56 3.07

Tg (�C) Heating rate 10 �C/min

Onset 142 181 227 234 nm 303Midpoint 216 186 216 230 nm 307Endpoint 219 192 246 249 nm 309

Tm (�C) 332 286 332 354 nm 472

Tg (�C) Heating rate 30 �C/min

Onset 208 nm 195 nm 250 284Midpoint 220 nm 203 nm 252 295Endpoint 240 nm 211 nm 255 292

Tm (�C) 338 nm 312 nm 388 454

Tg (�C) Heating rate 50 �C/min

Onset 208 196 86 212 230 285Midpoint 218 209 186 224 257 292Endpoint 230 247 195 230 261 298

Tm (�C) 335 322 286 345 395 449

M. Yasar et al. / Fuel 86 (2007) 1737–1748 1739

tively. Proton NMR and Carbon NMR spectra wererecorded in the pulse Fourier transform mode using a Var-ian Inova 500 spectrometer operating at 125 MHz for 13CNMR spectra and 500 MHz for 1H NMR spectra. Samplesfor 1H NMR measurements were prepared by adding0.5 ml of deuterated chloroform (CDCI3), solvent to 5–10 mg of the asphaltene in a 5 mm tube. 13C NMR mea-surements, 0.5 ml CDCI3, were added to 20–25 mg ofasphaltene. 1H NMR measurements were carried out witha spectral sweep width of 8 kHz, and a delay time of 1 s.Proton types were assigned based on shifts noted in Speight[2]. 13C NMR measurements were conducted with a spec-tral width of 31 kHz, a delay of 1 s. Results are reportedin Table 1a.

2.4. Thermal treatment of asphaltenes

Thermal properties of the asphaltenes were investigatedby differential scanning calorimeter measurements to inferthe possible changes in the asphaltene molecules and theirbehavior during refinery.

2.4.1. Differential scanning calorimeter (DSC)

measurements

The DSC thermograms of the asphaltenes were mea-sured with a Mettler-Toledo STAR system and DSC822measuring module equipped with an intracooler. TheDSC module was calibrated with indium and zinc stan-dards. For each measurement, about 5 mg of asphaltene

1740 M. Yasar et al. / Fuel 86 (2007) 1737–1748

were weighed into standard ceramic crucibles with piercedlids and nitrogen purging were used for sampling. Lightmolecular weight components adsorbed on asphaltenemay contribute to the DSC scan over the temperaturerange of 25–300 �C. This makes it difficult to get accurateinformation about the glass transition temperatures ofthe asphaltenes [29]. In order to eliminate the effect of evap-oration of light component, the asphaltene samples weresubjected to repeated heating and cooling DSC scanswithin temperature range of 25–300 �C. The evaporationof light molecular weight components were performedtwice for each asphaltene. It is also well known that thethermal history of the sample affects. After repeated andcooling DSC scans, the curves recorded with three differentheating rates of 10, 30 and 50 �C/min over the temperaturerange of 25–600 �C with nitrogen flow rate of 50 mL/min.Liquid nitrogen was used for cooling. The curves measuredwere imported into a PC with Star V6.0 software. The glasstransition temperature was determined according to ASTME1356 [31]. This gives the midpoint temperature betweenthe extrapolated onset and endset. Differences in the glasstransition temperature so determined can arise becausethe value obtained depends on how the tangents are drawn.

3. Results and discussion

3.1. Structural proporties of asphaltenes

Proton NMR, carbon NMR, GPC and elemental anal-ysis used to study the molecular origin of the endothermicevents that occur in the asphaltene molecules during ther-mal processing. The total asphaltene content of the six par-ent crude oils were totally different. However, the protonNMR studies indicated that the chemical structures ofthe asphaltene molecules were very close for six asphalt-enes. The asphaltene samples were characterized for ProtonNMR, carbon NMR, elemental analysis and molecularweight by GPC. The results are given in Table 1. Severalinteresting trends are evident. H/C ratios were similar forGarzan and Besikli crude oils (1.65, 1.63) and their isolatedasphaltenes (1.14, 1.13), respectively. While Adiyamancrude oil (1.70) had higher H/C ratio than Bati Ramancrude oil (1.40), H/C ratio were similar for the Adiyaman(1.08) and Bati Raman asphaltene (1.09). Batman Yenikoyand Batman Celikli crude oils followed the same trend.Batman Celikli crude oil (1.96) had higher H/C ratio thanBatman Yenikoy crude oil (1.86), H/C ratio were similarfor Batman Celikli Asphaltene (1.19) and Batman YenikoyAsphaltene (1.18). The elemental analysis indicated thatboth Bati Raman and Adiyaman asphaltenes have the low-est hydrogen content, while both Batman Yenikoy andBatman Celikli have the highest. The elemental analysisof these asphaltene is given in Table 1 and indicates sulfurcontents decrease from Bati Raman asphaltene (8.83) toBatman Yenikoy asphaltenes (2.13). However, the oxy-gen + nitrogen contents increase from Batman Yenikoyasphaltenes (2.13) to Adiyaman asphaltenes (5.17). The

comparison of the sulfur content of six Turkish crude oilsand their asphaltene fraction showed that sulfur atomsare concentrated in the asphaltene fractions of the crudeoils. A high H/C ratio indicates the presence of long ali-phatic straight chain compounds, whereas a low H/C ratiois a good indication of the presence of polynuclear aro-matic system [32]. The elemental analysis results led to con-clusion that Adiyaman and Bati Raman asphaltenes are themost hydrogen deficient followed by Besikli, Garzan, Bat-man Yenikoy and Batman Celikli asphaltenes. The H/Cratios shown in Table 1 suggest that Adiyaman and BatiRaman asphaltenes contain more fused polynuclear aro-matic rings, whereas Batman Celikli and Batman Yenikoyasphaltenes possess a less condensed aromatic rings withlonger paraffinic side chains in their molecular structure.The differences in the elemental content of asphaltenesand the NMR results emphasize the fact that there is a dif-ference in the size and the morphology of the asphaltene.To understand the true nature of this difference, the asphal-tene the molecular weight were determined by GPC. Thenumber average molecular weight (Mn) and the weightaverage molecular weight (Mw) measurement by GPC forsix asphaltene ranged from 580 to 1010 and 812 to 1342,respectively. Molecular weight measurements indicatedthat Bati Raman asphaltene were heavier than those ofGarzan, Besikli, Adiyaman, Batman Yenikoy, BatmanCelikli asphaltenes.

3.2. Glass transition temperatures of asphaltenes

The inspection of literature showed that there is a fewstudy concerning the glass transition of petroleum asphalt-enes. In this study thermal properties of six asphaltenesobtained from six different Turkish crude oils were investi-gated by differential scanning calorimeter (DSC) to infertheir behavior during refinery purposes. Glass transitionphenomena are generally accompanied by a sudden changein the specific heat capacity. As a result a baseline shiftshould be observed on DSC thermograms. RepeatedDSC scans with three heating rate of Garzan, Besikli,Adiyaman, Batman Yenikoy, Batman Celikli and BatiRaman asphaltenes demonstrated an obvious baseline shiftin 186–307 �C temperature range, which can be accepted asglass transition range for the asphaltenes. Heating rate isone of the parameters that can affect DSC results signifi-cantly. Three heating rates have been tested. The glass tran-sition temperature of asphaltenes measured at differentheating rates is listed in Table 1b. It can be seen that thefirst endothermal peak area increases significantly as theheating rate increases from 10 �C/min to 50 �C/min, butthe total enthalpy did not show a big increase. The com-bined area of both peaks seems to increase a little. Tg

temperatures for Garzan, Besikli and Batman Celikliasphaltenes exhibit a small increase as heating rateincreases. Tg temperatures for Adiyaman and BatmanYenikoy asphaltenes exhibit a small decrease as Tg heatingrate increases. There are three main reasons for the temper-

M. Yasar et al. / Fuel 86 (2007) 1737–1748 1741

ature differences. First of all, the glass temperature deter-mined according ASTM E1356 is not thermodynamicallydefined but is influenced by the actual shape of the curve.Second, the DSC thermogram is deformed at increasingheating rates due to temperature gradients in the sampleand sensor. Third, the thermal glass transition is a kineticprocess controlled by cooperative molecular rearrange-ments in which time is an important factor. The glass tran-sition measurement result of six Turkish asphaltene showedthat there is not one correct glass temperature of asphalt-enes. Any glass transition temperature quoted must there-

Fig. 1a. DSC Thermogram for Adiyaman

Fig. 1b. DSC Thermogram for Adiyaman

Fig. 1c. DSC Thermogram for Adiyaman

fore be accompanied by the details of measuringtechnique, the heating rate, the thermal and physical sam-ple pretreatment [33]. DSC thermograms of the asphaltenesare given in Figs. 1a–1c, 2a–2c, 3a–3c, 4a–4c, 5a–5c, 6a–6c.Figs. 1–6 display the DSC thermograms used for determi-nation of the glass transition temperatures. All six asphalt-enes showed similar calorimetric signals in the temperaturerange studied. The purposes of using three heating rate isto show the effect of this parameter on the glass transitiontemperature. As shown in Figs. 1–6, an obvious baselineshift is present at temperatures between 186–307 �C and

asphaltene at 10 �C/min heating rate.

asphaltene at 30 �C/min heating rate.

asphaltene at 50 �C/min heating rate.

Fig. 2a. DSC Thermogram for Bati Raman asphaltene at 10 �C/min heating rate.

Fig. 2b. DSC Thermogram for Bati Raman asphaltene at 30 �C/min heating rate.

Fig. 2c. DSC Thermogram for Bati Raman asphaltene at 50 �C/min heating rate.

1742 M. Yasar et al. / Fuel 86 (2007) 1737–1748

this present provides clear evidence that each of six asphalt-enes undergoes a glass transition. The glass temperatures ofGarzan, Besikli, Adiyaman, Batman Yenikoy, Batman Cel-ikli and Bati Raman asphaltenes were determined as lyingnear (216–220) �C, (186–209) �C, (186–216) �C, (224–230)�C, (252–257) �C and (292–307) �C. The Tg values ofasphaltenes observed in this study were between thosereported by Kopsch [28] and Zhang et al. [29]. Kopschand Zhang reported the DSC thermograms of asphaltenesfrom 200 to 550 �C and from 100–230 �C, respectively. TheDSC traces in Figs. 1-6 show close resemblance to thosepreviously reported by Karacan and co-workers [27]. It

was not possible to make a direct comparison withKopsch’s and Zhang’s results because of the differencesin both molecular structure and chemical compositions ofthe asphaltenes used.

3.3. Melting points of asphaltenes

DSC is the most suitable technique enabling identifica-tion of polymers on the basis of their melting behavioror, in the case of amorphous polymers, of the glass transi-tion. The literature does not supply much informationabout melting point or melting range temperatures because

Fig. 3a. DSC Thermogram for Besikli asphaltene at 10 �C/min heating rate.

Fig. 3b. DSC Thermogram for Besikli asphaltene at 30 �C/min heating rate.

Fig. 3c. DSC Thermogram for Besikli asphaltene at 50 �C/min heating rate.

M. Yasar et al. / Fuel 86 (2007) 1737–1748 1743

there is a widespread conception that asphaltenes are solidsthat pyrolyze before melting. The difficulty in observingasphaltene behavior at refinery process temperatures is thatthe functional groups, such as carboxylic acids, tend todecompose at temperatures below 350 �C. This propertyhas hindered the application of conventional methods,such as the capillary melting point method. As a complexmixture of components, asphaltenes may not display a pre-cise melting point. Gray et al. claimed that the majority of

the asphaltenes must exist in either a molten phase or as asolution at process temperatures typical of distillation orreaction. Their results are qualitatively consistent withmelting behavior under slow heating conditions reportedby Zhang et al. [29] confirm melt formation at temperaturesabove the glass transitions observed at 120–130 �C. Tanakaet al. [34] also observed a persistent fractal structure inMaya asphaltenes that was fundamentally differentfrom asphaltenes from Middle Eastern crude oils. More

Fig. 4a. DSC Thermogram for Celikli asphaltene at 10 �C/min heating rate.

Fig. 4b. DSC Thermogram for Celikli asphaltene at 30 �C/min heating rate.

Fig. 4c. DSC Thermogram for Celikli asphaltene at 50 �C/min. heating rate.

1744 M. Yasar et al. / Fuel 86 (2007) 1737–1748

persistent structure in the liquid phase would be expectedto increase the melting point of the material, as observedfor Maya in their study. Kopsch [28] reported a glass tran-sition temperature for Kirkuk asphaltenes of 314–353 �Cfrom DSC for heating rates of 10–50 K/min In order toestimate the hypothetical melting points of the asphaltenes,Kopsch used a relationship of the glass transition temper-ature and the melting temperature, Tg = 0.65 Tm. In thisstudy we used the same approximation to calculate thehypothetical melting points of Turkish asphaltenes. The

Tg midpoint values of Garzan asphaltenes obtained withthree different heating rates of 10, 30 and 50 K/min equalto 216, 218 and 220 �C resulted in hypothetical Tm valuesof 332, 330 and 338 �C, respectively. The hypothetical melt-ing points of Garzan asphaltenes were at the process tem-peratures typical of atmospheric distillation. Thehypothetical melting points of Besikli, Adiyaman, BatmanYenikoy, Batman Celikli, Bati Raman asphaltenes werecalculated by using the same approximation and given inTable 1. The hypothetical melting points of Batman Celikli

Fig. 5a. DSC Thermogram for Garzan asphaltene at 10 �C/min heating rate.

Fig. 5b. DSC Thermogram for Garzan asphaltene at 30 �C/min heating rate.

Fig. 5c. DSC Thermogram for Garzan asphaltene at 50 �C/min heating rate.

M. Yasar et al. / Fuel 86 (2007) 1737–1748 1745

and Bati Raman asphaltenes are higher than those of Gar-zan, Besikli, Adiyaman, Batman, Yenikoy asphaltenesindicating the process temperatures typical of pyrolysis.

3.4. Pyrolysis of Garzan asphaltenes

In order to determine the thermal behavior of asphalt-enes at high temperature, Garzan asphaltene was also sub-jected to pyrolysis at temperature of 375 �C for holdingtimes ranging from 10 to 120 min in microbatch reactors.

Pyrolysis reactions were performed as described elsewhere[30]. To summarize, reaction products were recovered asgas, maltene (heptane-soluble material) and asphalteneand coke fractions. Maltene, asphaltene, and coke productfractions were isolated by a solvent extraction sequence;gas yield was determined gravimetrically. Asphaltene con-version was slow and reacted predominately to malteneand coke at 375 �C. The time dependence of the productyields of Garzan asphaltene at 375 �C for 120 min are illus-trated in Fig. 7. Asphaltene yield decreased with reaction

Fig. 6a. DSC Thermogram for Yenikoy asphaltene at 10 �C/min heating rate.

Fig. 6b. DSC Thermogram for Yenikoy asphaltene at 30 �C/min heating rate.

Fig. 6c. DSC Thermogram for Yenikoy asphaltene at 50 �C/min heating rate.

1746 M. Yasar et al. / Fuel 86 (2007) 1737–1748

time. The maxima exhibited by Garzan asphaltene at reac-tion temperature of 375 �C is indicative of secondary crack-ing and/or condensation reactions. The ultimate gas yieldfor Garzan asphaltene was about 3% at 375 �C. Severalreaction pathways have been proposed to describe asphal-tene pyrolysis in the literature [20–26]. The pathwaysproposed by Schucker and Keweshan [22] allows for theformation of volatile products from asphaltenes. The

model proposed by Savage et al. [25,26] allows for the for-mation of maltene, gas and coke from asphaltene. Themodel also includes secondary cracking steps for formationof gas from coke and maltene. Neurock et al. [23] proposeda model based primarily on three parallel steps for asphal-tene decomposition to gas, maltene and coke. Our previousstudies have provided information on asphaltene pyrolysispathways of Hondo, Maya, Arabian Light and Arabian

0

10

20

30

40

50

60

70

80

90

100

0 10 20 40 60 90 120

Time (min.)

yie

ld %

Gas

Maltene

Asphaltene

Coke

Fig. 7. Temporal variations in the four product classes from pyrolysis ofGarzan asphaltene pyrolysis at 375 �C.

M. Yasar et al. / Fuel 86 (2007) 1737–1748 1747

Heavy and Yenikoy asphaltenes [18–20]. Our reactionmodel allows for the formation of maltene, gas and cokefrom asphaltene.

4. Conclusions

In conclusion, the glass temperatures of Garzan, Besikli,Adiyaman, Batman Yenikoy, Batman Celikli and BatiRaman asphaltenes were determined as lying near 186–307 �C. The results show that the glass transition can bemeasured at different heating rates. Each rate, however,yields different values for the glass transition temperature.There is not just one correct glass transition temperaturefor asphaltenes. Any glass transition temperature excerptedmust be accompanied by details of the measuring tech-nique, the heating rate, the sample pretreatment. The heatflow effect between 350 and 550 �C can be attributedmainly to pyrolysis of the asphaltene molecule. The presentstudy has provided further insight into the thermal behav-ior of asphaltenes. Among all the asphaltenes tested,Batman Celikli and Bati Raman showed much higher Tg

glass transition temperature than the others.

Acknowledgement

This work was supported by the T.R. Primary MinistryState Planning Organization, Project Number 98K122030.

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