A Study of Thermal-cracking Behavior of Asphaltenes (2003)

Chemical Engineering Science 58 (2003) 157–162www.elsevier.com/locate/ces

A study of thermal-cracking behavior of asphaltenes

Jinsheng Wang, Edward J. Anthony∗

CANMET Energy Technology Centre, Natural Resources Canada, 1 Hannel Drive, Ottawa, Ont., Canada K1A 1M1

Received 26 September 2001; received in revised form 17 April 2002; accepted 23 August 2002

Abstract

Asphaltenes are problematic substances for heavy-oil upgrading processes. Recently interesting .ndings on thermal-cracking kineticsof an asphaltenic residue were reported, but a proposed model which considered parallel reactions for oil + gas and coke formationcould not describe the behavior at higher temperatures. It was suggested that in such cases oils participated in secondary coke-formingreactions. Here we reexamine the data and give the expression for the oil + gas yield as a function of asphaltene conversion or residencetime, which describes the data well. Further, we show that an empirical relation for coke formation and asphaltene conversion gives areasonable description of the kinetics and prediction of the cracking behavior at high conversion level or long residence time, and thatthis method is much simpler. The maximum yield of oil + gas and the conversion level corresponding to the maximum yield can alsobe predicted easily. Further, our proposed approach is not dependent on assumed reaction orders of cracking.Crown Copyright ? 2002 Published by Elsevier Science Ltd. All rights reserved.

Keywords: Fuel; Kinetics; Mathematical modelling; Reaction engineering; Asphaltene; Thermal cracking

1. Introduction

In the petroleum industry the demand for light productsis increasing. To meet this demand, re.neries convert a por-tion of their residuals into light fractions. Such residues con-tain large amounts of asphaltenes, which are high molecularweight and non-volatile aromatic compounds.Asphaltenes cause many problems in oil production,

residual processing and heavy oil combustion (Callejas& Mart;<nez, 2000; Seki & Kumata, 2000; Rogel, 2000;Mart;<nez, Benito, & Callejas, 1997; Benito, Mart;<nez,Fern;andez, & Miranda, 1995; Wilt, Welch, & Rankin,1998; Artok et al., 1999; Victorov & Smirnova, 1998;Permsukarome, Chang, & Fogler, 1997; Speight & Long,1996), such as deactivation of catalysts and formation ofsludge or sediment, which clog fuel .lters, separators, andnozzles. They are precursors of coke formation in crackingreactions and cause undesirable properties in the products.Understanding cracking behavior of asphaltenes is of par-ticular importance to heavy-oil processing, as asphaltenescomprise the heaviest fraction.Mart;<nez et al. (1997) presented interesting experimental

results on the thermal cracking of asphaltene. They used a

∗ Corresponding author. Tel.: +1-613-996-2868;fax: +1-613-992-9335.

E-mail address: [email protected] (E. J. Anthony).

batch reactor to study the cracking of an asphaltenic residuefrom a synthetic crude obtained by coal liquefaction. Theamount of asphaltene and yields of two classes of products—oil+gas and coke were reported as a function of residencetime and a three-lump model was used to .t the data.

1k Oil + Gas

Asphaltene3k

2k Coke

where k1, k2 and k3 are the relevant reaction rate constants.It was reported that the data could be .tted well with the as-sumption of second-order reaction for asphaltene crackingat temperatures 425◦C, 435◦C and 450◦C, without consid-ering coke formation via secondary cracking of oils. How-ever, the data no longer .tted well at 475◦C. Therefore, itwas suggested that the secondary formation of coke due tocracking of oil became increasingly important with increas-ing temperature. Since the integration of the correspondingrate equations involving secondary cracking is not straight-forward, the concentration–time predictions for the productswere not developed for 475◦C.In this paper, we reexamine the asphaltene cracking data,

and derive the concentration and conversion or residencetime relation for the yields involving the secondary cracking

0009-2509/02/$ - see front matter Crown Copyright ? 2002 Published by Elsevier Science Ltd. All rights reserved.PII: S0009 -2509(02)00430 -X

mailto:[email protected]

158 J. Wang, E. J. Anthony / Chemical Engineering Science 58 (2003) 157–162

of oil, by direct integration of the rate equations. Further, wehave developed a relationship between the coke yield andconversion, which would make the predictions much easier.

2. Model and analyses

By using the three-lump scheme of Martinez et al. for abatch reactor the following relations can be obtained:

dCAdt

=−(k1 + k2)C2A; (1a)

dCOdt

= k1C2A − k3CO; (1b)

dCCdt

= k2C2A + k3CO; (1c)

where C denotes concentration and t is residence time. Thesubscripts A,O and C denote asphaltene, oil + gas and coke,respectively. In Eqs. (1b) and (1c) the secondary crackingof oil is assumed to be an apparent .rst-order reaction. Thesolution of Eq. (1a) is

CA =CA0

1 + CA0(k1 + k2)t; (2)

where CA0 is the initial concentration of asphaltene. For theoil + gas lump, the following relation is obtained from Eqs.(1a) and (1b)

dCOdCA

=−k1k1 + k2

+k3

k1 + k2

COC2A: (3)

With the initial condition CO =CO0 at CA =CA0, we obtainfrom Eq. (3)

CO = exp(−r3=CA)[(r1CA0 + CO0) exp(r3=CA0)− r1CA exp(r3=CA)− r1r3 Ein(r3=CA0)+ r1r3 Ein(r3=CA)]; (4)

where r1 = k1=(k1 + k2) and r3 = k3=(k1 + k2). Ein(x) =∫ x−∞ ex=x dx. Eq. (4) is similar to the one given by Week-man and Nace for gasoline yield in plug Mow reactors inMuid catalytic cracking (Weekman & Nace, 1970). The dif-ference of the present system is that CO0 �= 0, and the reactoris a batch one instead of a continuous-Mow one. When thesecondary cracking is negligible, r3 =0 and Eq. (4) reducesto

CO − CO0 = r1(CA0 − CA) (5a)

or

yO − yO0 = r1(yA0 − yA); (5b)

where y denotes weight fraction in the mixture. On the otherhand, with r3 = 0 we get a similar relation for coke yield

and asphaltene content from Eqs. (1a) and (1c)

CC − CC0 = r2(CA0 − CA) (6a)

or

yC − yC0 = r2(yA0 − yA); (6b)

where CC and CC0 denote the amount of coke and initialcoke, respectively. r2 = k2=(k1 + k2). Furthermore, by sub-stituting Eq. (2) for CA in Eqs. (5a) and (6a), the residencetime dependence of CO and CC (or yO and yC) can be ob-tained.

3. Results and discussion

From Eqs. (5b) and (6b) it is clear that a plot of (yO−yO0)or (yC − yC0) against (yA0 − yA) would yield a linear rela-tionship with zero intercept, if the model holds. The reporteddata are summarized in Table 1. The plots in terms of Eqs.(5b) and (6b) are shown in Fig. 1, where the lines repre-sent linear regression results. Indeed, for 425◦C and 435◦C,good linear relationships are shown. Moreover, the sums ofthe slopes for oil+gas and coke, as shown in the .gure, arevery close to unity. This con.rms the prediction from themodel, i.e., r1 + r2 = k1=(k1 + k2) + k2=(k1 + k2) = 1 whenthe secondary cracking is negligible. The plots for 450◦Cappear to deviate from Eqs. (5b) and (6b). Nevertheless, thesum of the slopes from linear regression is close to unity.However, the plots for 475◦C clearly show that Eqs. (5b)and (6b) no longer hold, and Eq. (4) which involves thesecondary cracking may be applicable.Eq. (4) contains two parameters r1 and r3, which reMect

the formation and cracking of oil + gas, respectively. Thevalues can be determined by .tting Eq. (4) to the data. On theother hand, for the sake of consistence to the model, the valueof r1 may be estimated from the temperature dependence ofr1. The value of r1 for 475◦C was estimated to be 0.52 usingthe Arrhenius equation

k = k0 exp(− ERT

): (7)

By presetting this value, we .tted Eq. (4) to the oil + gasdata, and as can be seen from Fig. 2, a reasonable .t isachieved. It can also be seen that the oil + gas fraction is pre-dicted to decrease as the conversion of asphaltene increasesfurther and reduce to zero at complete conversion of asphal-tene. This is not likely, however, since light gas would formalong with coke formation due to the secondary crackingof oil. Consequently, the .nal products would be coke andlight gas. For a better description of the system near com-plete conversion of asphaltene, the current three lumps maynot be adequate. Unfortunately, the gas yield was not sep-arately reported giving the required information to modifythe model. For the present discussion we have to keep thethree-lump framework and note that the relation may not bevalid in the region of very high conversion of asphaltenes.

J. Wang, E. J. Anthony / Chemical Engineering Science 58 (2003) 157–162 159

Table 1Reported product distribution (wt%) for thermal cracking of an asphaltene residue (Mart;<nez et al., 1997)

Residence time (min)

Before cracking 5 10 20 30 40Asphaltenes 65.5Coke 15.0Oil + gas 19.6

425◦CAsphaltenes 58.5 53.8 44.4 42.5 39.7Coke 17.0 19.3 23.6 24.1 25.1Oil + gas 24.5 26.9 32.0 33.4 35.2




conversion (yA0−yA), %

425oC

y = 0.392x

y = 0.6032x

0

10

20

30

oil+gas

coke

435oC y = 0.5607x

y = 0.4363x

0

10

20

30

oil+gas

coke

450oC

y = 0.4359x

y = 0.561x

0

10

20

30

oil+gas

coke

475oC

0

10

20

30

yiel

d, %

0 10 20 30 40

oil+gas

coke

Fig. 1. Plots in terms of Eqs. (5b) and (6b) for the dependence of oil+gasand coke yields on asphaltene conversion. The equations are results oflinear regression.

The residence time dependence of the oil + gas fractioncan be obtained by combining Eq. (4) with Eq. (2). How-ever, this would make the expression for CO quite compli-cated.The expression for CC , the coke yield would also be com-

plicated, since it should be obtained from an overall materialbalance

yO − yO0 + yC − yC0 = yA0 − yA: (8)

What is desirable is a simple relation for oil+gas and as-phaltene contents.A simple relation between coke yield and conversion was

given for catalytic cracking of gas oils (Wilson, 1997)

yC = k�

1− � + C0; (9)

where yC is the weight fraction of coke. k is a coePcient.�= 1− y1 and y1 is the weight fraction of unconverted gasoil. C0 is related to the content of carbon residue in gas oil.By analogy with catalytic cracking of gas oils, we assumethe following relation for the asphaltene system:

yC = kyA0 − yAyA

+ yC0: (10)

The diQerence between the two systems is that the weightfraction of gas oil in the feed is unity, whereas the fractionof asphaltene in the feed is yA0. If this relation of Eq. (10)holds, i.e., there is a linear relationship between yC and(yA0 − yA)=yA, the oil+gas content as a function of theasphaltene content can be determined from material balance.As shown in Fig. 3, the plots in terms of Eq. (10) can be


0

10

20

30

40

50

60

0 10 20 30 40 50 60 70

measured

predicted by eq.(11)

predicted by eq.(4)

yo

il+

ga

s, %

yA0−yA, %

Fig. 2. Oil+gas yield as a function of asphaltene conversion at 475◦C. The symbols represent measured yield and the curves represent predicted yield.

y = 17.371x + 15

R2

= 0.9522

0

5

10

15

20

25

30

35

40

45

0 0.2 0.4 0.6 0.8 1 1.2 1.4

(yA0−yA)/yA

coke y

ield

, %

425°C

435°C

450°C

475°C

Fig. 3. Correlation of coke yield with asphaltene conversion in terms of Eq. (10). The equation is the result of linear regression for the data of 475◦Cand R2 is the square of correlation coePcient.

taken as linear, and converging to yC=yC0=0:15 at yA=yA0.A linear regression of the plotted data for 475◦C yieldedk = 0:17 for the relation given by Eq. (10). To obtain theoil+gas fraction as a function of the asphaltene fraction, weuse the relation of the material balance Eq. (8). SubstitutingEq. (8) into Eq. (10), we obtain

yO = yO0 + (yA0 − yA)(1− k

yA

): (11)

Predicted yO based on Eq. (11) is shown in Fig. 2, in com-parison with the .tting by Eq. (4). It can be seen that Eq.(11) also gives a reasonable prediction, although it does notcontain any term related to the secondary cracking of oil.Near complete conversion of asphaltene the two equationspredict diQerent rates for the decrease of oil + gas fraction.It should be noted here that Eq. (11) is not supposed to pre-dict the conversion level at which the oil + gas fraction re-duces to zero, since it is based on the expression given by

Eq. (10), which becomes in.nite at the complete conversion(yA = 0). However, as has been discussed earlier, Eq. (4)may not be valid at this conversion level either. At least inthe medium range of conversion, Eq. (11) would be useful.In addition to this simplicity, the prediction for the oil+gasfraction with Eq. (11) requires only the data of asphalteneand coke fractions. Moreover, this prediction involves noassumptions of reaction orders for the cracking of asphalteneand the cracking of oil + gas.Eq. (11) also predicts a conversion level at which maxi-

mum oil+gas yield is obtained. With dyO=dyA=0 Eq. (11)yields

y∗A =√kyA0: (12)

Using the values k = 0:17 from the coke data in Fig. 3 andyA0=0:655 from Table 1, we obtain y∗A=0:33. Accordingly,at conversion level yA0−y∗A=0:325 the maximum oil+gasyield is expected. This yield is calculated to be 0.354 by

J. Wang, E. J. Anthony / Chemical Engineering Science 58 (2003) 157–162 161

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100 120

yo

il+g

as

t, min

Fig. 4. Oil+gas yield as a function of residence time. The symbols represent measured yield for 475◦C and the curve represents predicted yield.

substituting y∗A into Eq. (11), and this is indeed close tothe observed maximum yield 0.373. With the three-lumpmodel the maximum yield can also be predicted. By lettingdyO=dyA = 0 in Eq. (3) we obtain

yOmax =k1CFk3

y∗2A ; (13)

where CF is the concentration of the feed. The value of y∗Acan be obtained by combining Eq. (13) with Eq. (4). How-ever, numerical computation is required since an analyticalexpression for y∗A can not be derived.The oil+gas fraction can also be predicted as a function

of residence time. For this purpose a relation between as-phaltene content and residence time is needed. Eq. (2) givesthis relation, but it was obtained with the assumption ofsecond-order reaction for asphaltene conversion. In reality,the data at 475◦C deviate signi.cantly from the assumption,as was observed from a plot in terms of Eq. (2) (not shown).Although a higher reaction order might give a better .t ofthe asphaltene conversion data, for convenience of treatmentwe just take the second-order approximation. With the valueof the rate constant in terms of Eq. (2) determined from alinear regression, the predicted oil+gas yield as a functionof residence time is shown in Fig. 4. In comparison with thereported data, the prediction appears to be reasonable. Thetime at which the maximum oil+gas yield is reached canalso be predicted, from Eqs. (2) and (12), as

t∗ =

√yA0=k − 1

CA0(k1 + k2): (14)

As has been seen earlier, Eq. (10) is an analogy of Eq. (9),which is an empirical relation in catalytic cracking. The pre-diction for the oil+gas selectivity based on the resultant Eq.(11) is not related to the three-lump model, except for theprediction of the residence time dependence where the as-sumption of the second-order reaction for asphaltene crack-ing is used. It should be noted that for other reaction orders,

the approach is the same. Of course, the prediction of thetime dependence can be improved if a good correlation ofasphaltene cracking with the residence time is established.Although we are not able to give a theoretical foundationto Eq. (10), the equation enables a reasonable descriptionand prediction for the thermal cracking of asphaltene, andthe expression for the yields and the manipulation of dataare much simpler than those obtained for the three-lumpmodel. Since the relationship given by Eq. (10) is linear, itcan be determined with a microreactor, and then used forextrapolation and prediction of the yield in large plants.An implication from the above results is that, the oil+gas

yield at low temperatures may also decrease with increas-ing conversion, but at higher conversion levels than thosestudied by Mart;<nez et al. So long as the relation in Eq.(10) holds, the decrease is expected according to Eq. (11).This is also consistent with the three-lump model. As thefraction of the oil+gas lump increases to higher level, thesecondary cracking, whose rate depends on the concentra-tion of oil+gas, would become signi.cant and result inthe decrease. However, this prediction should be veri.edexperimentally.

4. Conclusions

Thermal cracking of asphaltenes occurs in importantheavy-oil upgrading processes such as coking and visbreak-ing. Our analyses con.rmed that at lower temperaturesthe three-lump model which considered parallel reactionsof oil+gas and coke formation described the cracking be-havior well, whereas at higher temperature the secondarycracking of oil may be considered. Unfortunately, theresulting expression for the oil+gas yield is quite compli-cated. With an empirical relation for the coke yield and theasphaltene conversion, a much simpler expression for theoil+gas yield can be obtained and a reasonable descriptionhas been achieved. Prediction of the maximum yield is also


much simpler. This approach is not dependent on the re-action orders of cracking of asphaltene and oil. Moreover,since the empirical relation is linear, it can be determinedwith a microreactor, and then used for extrapolation andprediction of the yield in large reactors. The relations wedeveloped here have the potential to be useful in describingthermal-cracking processes for heavy oils.

Notation

C concentration, kg=m3

C0 a constant in Eq. (9) related to the content of carbonresidue in gas oil

Cj0 initial concentration of component j, kg=m3

E activation energy in Arrhenius equationEin(x) =

∫ x−∞ exx dx

k a coePcient in Eq. (9) relating coke fraction withgas oil fraction

k1 rate constant oil + gas formationk2 rate constant for coke formation from asphaltene

crackingk3 rate constant for coke formation from oil + gas

crackingR gas constant in Arrhenius equationri =ki=(k1 + k2) (i = 1; 2; 3)T temperature, Kt residence time, st∗ time corresponding to maximum oil + gas yieldy weight fractiony∗A weight fraction of asphaltene corresponding to

maximum weight fraction of oil + gasyj0 initial weight fraction of component j� (=1 − y1), where y1 is the weight fraction of un-

converted gas oil

Subscripts

A asphalteneC cokeF feedO oil + gas

References

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Callejas, M. A., & Mart;<nez, M. T. (2000). Hydroprocessing of a Mayaresidue. 1. Intrinsic kinetics of asphaltene removal reactions. Energyand Fuels, 14, 1304–1308.

Mart;inez, M. T., Benito, A. M., & Callejas, M. A. (1997). Thermalcracking of coal residues: Kinetics of asphaltene decomposition. Fuel,76, 871–877.

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Documents

A Study of Thermal-cracking Behavior of Asphaltenes (2003)