Burklé-Vitzthum Etal 2011

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Thermal evolution ofn- and iso-alkanes in oils. Part 1: Pyrolysis model for amixture of 78 alkanes (C1C32) including 13,206 free radical reactions

V. Burkl-Vitzthum a,, R. Bounaceur a, P.-M. Marquaire a, F. Montel b, L. Fusetti b

a Laboratory of Reactions and Process Engineering, LRGP CNRS-UPR 3349, Nancy University, ENSIC, BP 20451, 54001 Nancy, Franceb TOTAL Exploration and Production, 64018 Pau Cedex, France

a r t i c l e i n f o

Article history:

Received 3 January 2011

Received in revised form 11 March 2011

Accepted 21 March 2011

Available online 25 March 2011

a b s t r a c t

A mechanistic model consisting of 13,206 lumped free radical reactions has been developed to describe

the thermal evolution of a mixture of 78 alkanes: all n-alkanes from C1 to C32 and 46 branched alkane

model compounds from C4 to C32. The mixture was meant to represent the major part of the saturated

fraction of petroleum. The rate constants used are available from the literature. The lumping together

procedure is described and the model validated on the basis of several experimental results from the lit-

erature and relating to pure alkanes. The model is also compared to the saturated fraction obtained from

pyrolysis of Elgin oil at 372 C for up to 1000 h. The cracking global activation energy ofn-C15as well as

iso-C15is close to 69 kcal/mol in the range 200350 C. The implications of the model for geological res-

ervoirs will be discussed in a following paper.

2011 Elsevier Ltd. All rights reserved.

1. Introduction

In exploration, the composition of petroleum in deeply buried

reservoirs (T> 200 C) is of strategic interest. Questions regarding

the availability of exploitable petroleum reserves in future decades

can only be answered if the thermal stability of liquid reserves can

be predicted under geological conditions. The thermal stability of

petroleum is commonly modelled using kinetic parameters deter-

mined from laboratory pyrolysis of whole crude oils (e.g. Ungerer

and Pelet, 1987; Ungerer et al., 1988; Bhar et al., 1992, 1997a,b,

2008; Schenk et al., 1997; Dieckmann et al., 1998; Lewan and Ru-

ble, 2002; Lewan et al., 2006; Lehne and Dieckmann, 2007a,b) or of

pure hydrocarbons and simple mixtures. The experiments typically

use higher temperatures (300500 C) than those encountered in

reservoir rocks (80200 C), to compensate for geological time

(millions of years). Geological timetemperature conditions are

then applied to a kinetic model derived from experimental results.

Rate laws for the formation and destruction of hydrocarbons arenot easily obtained from experiments on the cracking of whole oils

because of the complexity of the chemical composition. This is eas-

ier when model compounds are used, i.e. pure hydrocarbons and

simple mixtures representing the reactivity families found in oil.

Alkanes are usually the most abundant class in non biodegraded

crude oils (Tissot and Welte, 1984). Pure n-alkanes have been

extensively studied to derive global kinetic parameters or detailed

free radical models (e.g. Ford, 1986; Weres et al., 1988; Domin,

1989; Domin et al., 1990; Song et al., 1994; Jackson et al., 1995;

Bhar and Vandenbroucke, 1996; Bounaceur et al., 2002b). Except

for the work ofDomin (1991)at very low conversion, to the best

of our knowledge, iso-alkane cracking has not been investigated

extensively. Bounaceur (2001) and Domin et al. (2002) first pro-

posed a free radical model taking into account a distribution of

n-alkanes from C1 to n-C30, similar to that found in the saturated

fraction of crude oils. Only ten individual branched alkanes were

included in their model as reactants, the branched alkanes being

formed via n-alkane cracking but not consumed afterwards. It

should be noted that their mechanism was based on a lumping to-

gether concept in order to keep the model to a reasonable size.

The purpose of the present work was to construct a model that

would be able to take into account the cracking of a complete dis-

tribution ofn-alkanes, as well as of a distribution of branched al-

kanes from C1 to C32. A lumping together procedure similar to

that ofDomin et al. (2002) was applied. Formation, as well as con-

sumption, radical reactions for every alkane (linear or branched)

from C1 to C32 were included in the model. After justification ofthe choice of model compounds for branched alkanes, the elabora-

tion of the lumped free-radical mechanism is detailed and the

model validated on the basis of literature experimental data. Final-

ly, the cracking of the distributions ofn- and iso-alkanes character-

istic of petroleum (Elgin oil, North Sea) is simulated and compared

with the experimental results. The implications of the model for

geological reservoirs will be discussed in a following paper.

2. Branched alkane model compounds

The structures of the branched alkanes in crude oils are so

numerous that most are included in the so called unresolved com-

0146-6380/$ - see front matter 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.orggeochem.2011.03.017

Corresponding author. Tel.: +33 383175093; fax: +33 383378120.

E-mail address: [email protected](V. Burkl-Vitzthum).

Organic Geochemistry 42 (2011) 439450

Contents lists available at ScienceDirect

Organic Geochemistry

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / o r g g e o c h e m
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plex mixture (UCM). Most of the identified structures are mono-

methyl alkanes, particularly mid-chain isomers (Jackson et al.,

1986; Klomp, 1986; Fowler and Douglas, 1987; Hoffmann et al.,

1987; Kissin, 1987; Summons, 1987; Summons et al., 1988a,b),

butKissin (1987) suggested that all monomethyl isomers can be

found in approximately the same amount. In the medium molecu-

lar weight (MW) range (C10C25), the most remarkable compounds

belong to the isoprenoid series and they frequently amount to ca.

1% of a crude oil, with pristane and phytane the most abundant

(Tissot and Welte, 1984). Components withn-alkyl branches long-

er than methyl also have been identified by several authors

(Kurashova et al., 1989; Gough and Rowland, 1990; Killops and

Al-Juboori, 1990; Gough et al., 1992; Warton et al., 1997).

Furthermore, it should be noted that in numerous biological

systems and sedimentary organic matter, some positions for the

methyl in monomethyl alkanes and their functionalised counter-

parts are favoured, particularly at C-2 and C-3 (Eglinton and Ham-

ilton, 1967; Tornabene et al., 1970; Downing, 1976; Kolattukudy,

1976; Kaneda, 1977; Dowling et al., 1986; Shiea et al., 1991; Gun-

stone, 1996).

The goal of this study was to construct an alkane cracking

mechanism including all the n-alkanes from C1 to n-C32, as well

as branched alkanes from C4to C32. Obviously, it was not possible

to take into account all the existing branched alkanes. That is why

we had to define model compounds, at least one for each chain

length, on the basis of the previous observations.

All branched alkanes from C4 to C6 (Fig. 1) are included in the

mechanism in order to predict the composition of the generated

gas in detail.

For odd carbon numbers between C7 and C31, one model com-

pound per chain length was chosen: the 2-monomethyl alkane.

This choice enabled us to represent one of the biologically favoured

methyl positions.

For even carbon numbers between C8and C32, two model com-

pounds per chain length were defined. The first represents another

favoured methyl position, i.e. mid-chain. In addition to a biological

origin, branched alkanes are formed during alkane cracking viaaddition of alkyl radicals to alkenes. Most are dialkyl alkanes with

the alkyl groups on neighbouring carbons, which are probably dif-

ficult to identify in crude oils because of analytical limitations. That

is why we chose dimethyl alkanes as the third type of model com-

pound. In order to limit the number of elementary reactions, the

structures of the dimethyl alkanes, as well as the mid-chain mono-

methyl alkanes, were chosen to be symmetrical. The three types of

branched model compounds are summarized inFig. 1.

Overall, the model comprises 32 n-alkanes and 46 iso-alkanes as

reactants.

3. Elaboration of reaction scheme

It is now widely accepted that elementary free radical reactions

can adequately describe the thermal transformation of most spe-

cies found in crude oils (e.g. Ford, 1986; Savage and Klein, 1988;

Weres et al., 1988; Domin, 1989, 1991; Domin et al., 1990,

2002; Jackson et al., 1995; Bounaceur et al., 2002a,b; Bhar et al.,

2002; Burkl-Vitzthum et al., 2004, 2005; Fusetti et al., 2010).The proposed model includes 78 alkanes as reactants, each of

which can potentially undergo hundreds or thousands of free rad-

ical reactions. For example, the mechanism of n-C6 pyrolysis, at

only low conversion, comprises 156 reactions (Domin et al.,

1990) and includes 53 species (molecules and radicals). A detailed

model of our mixture at high conversion would thus require mil-

lions of reactions (the detailed mechanism of each pure alkane

and the related cross reactions) and the resulting computing time

would be unmanageable. Some simplification is therefore required,

which means that reactions and some species must be lumped to-

gether. The work is inspired by the previous models ofBounaceur

(2001) and Domin et al. (2002), but the method applied here is

slightly different. Indeed, the part of the mechanism concerning

n-alkanes in the model of Domin et al. (2002) was first written

using an automated procedure and the number of reactions was re-

duced afterwards by lumping together radicals with the same

chain length and by lumping together some products as well. In

contrast, a lumping together procedure was applied immediately

in this study. All rate constants were derived from Allara and Shaw

(1980)or from the NIST Kinetics Database.

3.1. Unimolecular initiation

At high pressure (several hundred bar) and low or intermediate

temperature (200400 C), initiation rates are negligible vs. propa-

gation rates (Bounaceur, 2001). Therefore all radicals formed by

initiation steps, whatever their structure, can be lumped together

in a single one and so, for each alkane, there is only one initiation

step in the model (Fig. 2). Simulations have enabled validation of

such an assumption.

For each alkane from C2to C6, an average value of activation en-

ergy was estimated, and for C7+, the same value was taken, what-

ever the number of carbons in the molecule. Indeed for long

chain alkanes, the chain end effect can be neglected. Frequency fac-

tors are approximately proportional to the number of CC bonds. A

reference value was set according to the literature and all fre-

quency factors were then calculated.

Fig. 1. Model compounds for branched alkanes. Fig. 2. Examples of each type of lumped free radical reaction.

440 V. Burkl-Vitzthum et al. / Organic Geochemistry 42 (2011) 439450


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3.2. Decomposition via b-scission

In the mechanism, radicals are lumped together, i.e. the position

of the single electron is not precisely defined. For example, C3H7represents the primary propyl radical as well as the secondary one.

However, since it is highly important to determine precisely the

products formed from each decomposition as well as the corre-

sponding rate constant, each radical was un-lumped and its

decomposition precisely analyzed. For example the lumping to-

gether of the 2-methylbutyl radical (iso-C5) represents four differ-

ent radicals depending on the position of the single electron. These

radicals undergo five decompositions (Fig. 3) which are repre-

sented by three decompositions lumped together in the mecha-

nism (Fig. 2) because some of them lead to the same products. It

should be noted that all alkenes with the same number of carbons

are lumped together in a single class. Moreover, the branched rad-

icals included in the mechanismhave the same structure as the iso-

alkane model compounds. Therefore, when a branched radical is

formed, it is replaced in the mechanism by the radical that has

the closest structure (example inFig. 4).

The determination of the rate constants is a critical step: the

rate of a lumped reaction must be equal to the sum of the rates

of the reactions for which it stands; this means that, for the previ-

ous example, where the first, third and fourth decompositions in

Fig. 3are lumped together in the first decomposition in Fig. 2:

kaisoC5 radical k1 1ary

radical k3 3ary

radical 2 k4

2ary radical;

where 1ary stands for primary, 2ary for secondary and 3ary for

tertiary.

By implementing the intrinsic distribution of each type of

radical:

kaisoC5 radical k1 %1ary isoC5 radical k3 %3

ary

isoC5 radical 2 k4 %2ary

isoC5 radical

This leads to:ka= k1 %1ary + k3 %3

ary + 2 k4 %2ary.

It should be noted that the second and fifth decompositions in

Fig. 3lead to different products and so cannot be lumped together

and remain un-lumped in the mechanism (Fig. 2, second and third

decompositions).

On the basis of the work ofBounaceur (2001), the intrinsic dis-

tribution of each radical was estimated (Table 1). Then, depending

on the type of decomposition, three reference rate constants were

set (Table 2) and the rate constant of every lumped decomposition

was calculated independently.

3.3. H transfer

H transfers between each alkane and lumped radical are in-

cluded in the mechanism (Fig. 2). Activation energy depends on

the class (primary, secondary or tertiary) of the acceptor radical

and on the class of the carbon bearing the transferred hydrogen.

Reference values were estimated according to the literature (Table

3). An average value (1011 cm3 mol1 s1) was taken for the fre-

quency factor of a single H transfer. In the mechanism, the H trans-

fers are lumped and therefore: Alumped =P

Asingle, which implies

that Alumped is proportional to the number of H transfers the

lumped step stands for.

3.4. Addition

Average values for the activation energy and frequency factors

were estimated knowing that they are inversely proportional tothe number of carbons in the radical. It should be noted that every

C33+ radical formed by addition is lumped into a single class, what-

ever its number of carbons, which is considered as stable in the

model (Fig. 2).

3.5. Isomerization

Obviously, because of the lumpingtogether of radicals, no isom-

erization reaction is explicitly written into the mechanism, but the

intrinsic distribution of each radical is implicitly taken into account

by way of decomposition rate constants.

3.6. Termination

Termination steps between all radicals were lumped together

(Fig. 2), because, like initiation steps, their rates are negligible vs.

Fig. 3. Detailed decompositions viab-scissionon the example ofiso-C5. For the first

radical, two different bonds break, leading to different products. The arrow colourcorresponds to the breakage colour. (Forinterpretation of the references to colour in

this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Replacement of a formed radical by the closest structure.

V. Burkl-Vitzthum et al. / Organic Geochemistry 42 (2011) 439450 441


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propagation rates under our conditions. As a consequence, all ter-

mination steps lead to the same alkane and a reference rate con-stant was used. Our simulations confirmed that the

concentration of this end member alkane was negligible.

Neither cyclization nor aromatization reactions were included

in the mechanism because aromatics are negligible up to 50% con-

version and the amounts remain low for higher conversions (Song

et al., 1994; Bhar and Vandenbroucke, 1996) even at high temper-

ature (450 C). In previous studies (Bounaceur, 2001; Bounaceur

et al., unpublished data), it was shown that the importance of these

reactions with regard to alkane cracking decreases when the tem-

perature decreases, and becomes minor below 300 C, particularly

at geological temperatures. Indeed, cyclization reactions require

the formation of alkenyl radicals by H transfer to an alkene, which

react further by way of intramolecular addition to form a ring.

There is a competition between these H transfers and the additionof alkyl radical to alkene. The addition activation energy is from 3

to 5 kcal/mol lower than H transfer activation energy and so, when

the temperature decreases, the consumption of alkenes by addition

is favoured to the detriment of the cyclization route. So, cyclization

and aromatization reactions probably do not affect the n- andiso-

alkanes distribution to a significant extent. Nevertheless, this

choice constitutes a limitation of the model.

Overall, the model comprises 13,206 free radical reactions and

193 species.

4. Stoichiometric equations corresponding to n- oriso-alkane

cracking

A free-radical mechanism such as the present one is based ontwo generic stoichiometric equations (in moles):

Cracking equation:

Alkane Cn ! Alkane-minusCn1 to n2 1-alkene Cn2 to n1

Alkylation equation:

Alkane Cn 1-alkene Cn2 to n1 ! Alkane-plusCn2 t o 2n1

where alkane-minus corresponds to an alkane whose molecularweight (MW) is lower than the reactant and alkane-plus to an al-

kane whose MW is higher than the reactant.

Under our conditions [low temperature (200400 C) and high

pressure (1001000 bar)], addition to alkenes is fast, so both equa-

tions lead to (in moles):

2 AlkanesCn ! Alkane-minusCn1 to n2

Alkane-plusCn2 t o 2n1

Indeed, the amounts of alkenes are low and become totally neg-

ligible at low temperature experimentally as well as in simulations,

as in petroleum (Tissot and Welte, 1984).

Moreover, in the mechanism, the formation of alkanes-plus is

due to the addition of alkyl radicals to alkenes. Because primary

radicals are negligible, alkanes-plus are mainly iso-alkanes.

So the above analysis leads to:

In the case ofn-alkanes (in moles):

2 n-Alkanesn-Cn ! n-Alkane-minusn-Cn1 to n2

iso-Alkane-plusCn2 t o 2n1

In the case of iso-alkanes (in moles):

2 iso-Alkanes n-Cn ! n-or iso Alkane-minusCn1 to n2

iso-Alkane-plusCn2 t o 2n1

Then, the produced alkanes react according to the same stoichi-

ometric equations.

5. Pyrolysis of a pure n-alkane

The mechanism was tested on pure n-C15 as an example. The

model was simulated using the software CHEMKIN II (Kee et al.,

1989).Fig. 5plots logxi (molar fraction ofn- oriso-alkane Ci) vs.

the number i of carbons, after 1 week (low conversion) and after

15 yr (high conversion).

At low conversion, n-C15 obviously remains predominant. Al-

kanes-minus are mainly n-alkanes from C2 to n-C13 in almost the

same proportions. The formation of methyl radicals via decompo-

sition by b-scission is slightly more difficult than for the other alkyl

radicals (Table 2). This is why the amount of CH4is lower than the

amounts of C2C13. Furthermore, the decomposition via b-scission

of the C15radicals cannot lead to the formation of a C14radical, son-C14cannot be formed primary: its amount is negligible. Alkanes-

plus are mainly iso-alkanes from C17 to C29. Indeed, alkanes-plus

are formed by addition of the predominant radicals that are the

C15radicals, on alkenes comprising between C2and C14(decompo-

sition via b-scission of C15 radicals). Moreover it should be noted

thatn-alkanes-minus andiso-alkanes-plus are formed in the same

proportion, as illustrated by the above stoichiometric equation.

At high conversion, n-C15does not predominate anymore. Dis-

tributions ofn-alkanes and ofiso-alkanes are obtained and the evo-

lution of log(xi) vs.i is almost linear for iP 10. It should be noted

that methane is much less reactive than the other alkanes and

therefore it accumulates. The C33+ compounds accumulate but, as

mentioned above, this is an artifact due to the fact that their con-

sumption is not taken into account in the mechanism. Further-more, the amount of C2H6 is less than the amount of C3C10

Table 1

Estimation of internal distribution of radicals (x is the number of secondary (2ary)

carbons and y, the number of tertiary (3ary) carbons).

Iso-CnH2n+2 Primary

radicals (%)

Secondary radicals

(equidistribution)

Tertiary radicals

5 6n 67 7.5 2ary% = (100-1ary%)/

(x+ 2y)

3ary% = 2 2ary%

n= 8 7.5 2ary% = (100-1ary%)/

(x+ 1.5y)

3ary%=1.5 2ary%

9 6n 615 5 Ditto 3ary%=1.5 2ary%

16 6n 622 4 Ditto 3ary%=1.5 2ary%

23 6n 629 3 Ditto 3ary%=1.5 2ary%

n> 29 2 Ditto 3ary%=1.5 2ary%

Table 2

Decomposition via b-scission: reference rate constants depending on type of CC

bond.

Type of CC bond Frequency factor

(1013 s1)

Activation energy (cal/

mol)

1ary C2ary C 1.5 30,000

2ary C2ary C 2.5 27,700

2ary (or 1ary) C3ary

C

2.5 27,200

Table 3

H transfer: reference activation energy depending on type of radical and type of

transferred hydrogen.

Transferred hydrogen radical CH4 CH3 CH2 CH

CH3 8200 9600 9600

C2H

5 14,500 11,600 11,600

Linear 15,500 12,900 12,200 12,200

Branched 16,000 13,500 12,200 12,200



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because it cannot be formed by addition, only by decomposition

viab-scission.

Finally it is important to note that, for i > 29, the aforemen-

tioned trend is not observed anymore. In the following paragraph

these break points are analyzed in order to understand whether

they have a chemical meaning or whether they are due to the

mechanism, which does not take into account the cracking of the

C33+ fraction.

6. Effect of number of carbons

In order to answer the previous question, a reduced mechanism

was written which comprisesn- andiso-alkanes up to C27. Pyroly-sis ofn-C12is simulated up to the same conversion extent asn-C15previously. The product distributions show the same trends as for

the full mechanism, but the break points are moved to C24(Fig. 6).

Therefore, these break points are linked to the maximum number

of carbons accounted for by the mechanism and have no chemical

meaning: this is an artifact of the mechanism, more precisely for

the C33+ compounds which are formed, but do not further react.

Hence, every conclusion relating to the longest alkanes has to be

taken with caution.

7. Experimental validation for pure alkanes as reactants

7.1. Conversion

The mechanism was tested on several pure n-alkanes with

chainlength varying from 6 to 25, submitted to temperatures rang-

ing from 300 to 450 C, pressures from 120 to 700 bar and conver-

sions up to 94%. All the data were taken from the literature

(Domin, 1989; Song et al., 1994; Jackson et al., 1995; Bhar and

Vandenbroucke, 1996; Burkl-Vitzthum et al., 2004; Lannuzel,

2006; Lannuzel et al., 2010). The CHEMKIN II software uses the

perfect gas law to calculate the concentrations; consequently, for

each condition, the initial concentration was first calculated sepa-

rately by using the PengRobinson equation of state, in order to

evaluate the perfect gas pressure that leads to the same initial con-

centration. This pressure corresponds to the input value.Figs. 79

plot the conversion vs. time for all data. Agreement between the

experimental and the simulated conversions is observed for everyn-alkane and under each pressuretemperature-time condition.

Concerning iso-alkanes, to the best of our knowledge, few

experimental data are available at high pressure and intermediate

temperature (300400 C). We only found the results ofDomin

(1991) relating to 2,4-dimethylpentane pyrolysis at 357 C,

210 bar and low conversion (up to 5%). Our model does not de-

scribe this alkane, but the corresponding lumped species is 2-

methylhexane. By comparing experimental 2,4-dimethylpentane

conversion to the simulated 2-methylhexane conversion (Fig. 10),

agreement was obtained.

7.2. Product distribution

Conversion is not the only parameter to be checked in order tovalidate the model; product distribution also needs to be assessed.

Jackson et al. (1995)defined four fractions to describe the product

distribution (wt%) aftern-C16pyrolysis: C1C4, C5C9, C10C15and

C16+. For each fraction, the experimental and simulated mass bal-

ances were represented vs. conversion. Data, at all temperatures

and pressures are gathered (Fig. 11). The composition of each frac-

tion is well predicted by the model up to 30% conversion. Agree-

ment between the experimental and simulated results is

satisfactory up to 50% conversion. Above 50% discrepancies appear;

in particular the C16+fraction is overestimated and the C1C9frac-

tions are underestimated, whereas the C10C15fraction is correctly

described, as well as conversion (Figs. 79). This can be easily ex-

plained as the C33+species are lumped together in the mechanism

and, once formed, do not react further and accumulate at the ex-pense of low MW compounds. This phenomenon only becomes

important at high conversion, which explains the observed

discrepancies.

Table 4compares the simulated results with some data taken

from Bhar and Vandenbroucke (1996) who defined three fractions

after n-C25 pyrolysis: C1C4, C7C14 and C14+. As previously, at

intermediate conversion (37%) the agreement is totally satisfac-

tory, whereas at high conversion (90%), the light alkanes are under-

estimated and the heavy ones overestimated.

8. Apparent activation energy

Values of apparent cracking activation energy in the case of

puren-C15andiso-C15 were calculated at several temperatures ofT0 between 200 and 350 C. In fact, the simulations were done at

Fig. 5. Simulated product distribution obtained from n-C15cracking at 350 C.



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very short and constant conversion. The initial consumption rate r0was calculated by using CHEMKIN, and by plotting ln r0 vs. 1/RT

(where R is the perfect gas constant and Tthe temperature in K)

with T= T0, T0 2, T0 + 2, the slope corresponds to the apparent

Ea. For both alkanes and whatever the temperature between 200

and 350 C, the apparent Ea was found to be constant and close

to 69 kcal/mol.

This value is consistent forn-alkanes with experimentalEaofn-

C25 (68.2 kcal/mol) determined by Bhar and Vandenbroucke

(1996) and used for the n-C14+ fraction byVandenbroucke et al.(1999) in their kinetic modelling of petroleum cracking. But in

the same model, for the other saturated C14+ fraction, which is

composed of cyclo- and branched alkanes, Ea is set to 59 kcal/

mol. Nevertheless, the authors concluded that it may be better to

group the n- and iso-alkanes on the one hand, and the cycloalkanes

on the other hand. The conclusion ofVandenbroucke et al. (1999)

appears to be justified on the basis of this study and of another

study of cycloalkane cracking (Bounaceur et al., unpublished data)

that showedEa close to 55 kcal/mol at 200 C.

9. Saturated fraction of Elgin oil: comparison between

experimental and simulated results at 372 C

The mechanism was tested on thermal cracking experimentsinvolving a whole oil sample from the Elgin Field (North Sea,

UK). This sample has abundant saturated hydrocarbons and negli-

gible polar compounds (Domin et al., 2002; Vandenbroucke et al.,

1999), making it well adapted to our model. Pyrolysis experiments

with the sample were conducted by ELF (TOTAL) in 19921993 but

the results were not published until now. The temperature was set

at 372 C, the initial pressure was ca. 400 bar, for several time peri-

ods up to 1000 h; the experimental conditions are summarized in

Table 6. Closed cells (20 cm3) were loaded with ca. 5 g Elgin oil and

added to the same oven at the same time. The C1C10fraction was

analyzed using GCMS (gas chromatographymass spectrometry)and the C11+ fraction was analyzed using GPC (gel permeation

chromatography) by applying the method ofSynovec and Yeung

(1984), which uses toluene and tetrahydrofuran as solvents. The

product distribution (Fig. 12), e.g. logxi (xi molar fraction of all

hydrocarbons,n-,iso- and cycloalkanes as well as alkenes and aro-

matics) vs. the number i of carbons, was determined for several

time periods up to 1000 h. Nevertheless, it should be noted that

then- andiso-alkanes represent more than 90% molar of the oil.

In order to test our mechanism, the molar composition of the

saturated fraction (Table 5) was deduced from the analysis. The

n-Ci/iso-Ci ratio is not known fori > 10 because GPC does not sep-

arate n-alkanes fromiso-alkanes, so it was set to 0.5 according to

Vandenbroucke et al. (1999). This is an approximation as these

authors analyzed a different sample from Elgin Field, the composi-tion of which is somewhat different due to inhomogeneity within

Fig. 6. Simulated product distribution obtained by n-C12 cracking (reduced mechanism) and by n-C15cracking (full mechanism) at 350 C: (a)linear alkanes and (b) branched

alkanes.



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the field. The model was simulated using the mixture inTable 5as

input composition, at 372 C and 400 bar; the product distributions

are represented inFig. 13. A detailed comparison betweenFigs. 12

and 13 is complex because Fig. 12 includes all hydrocarbons

whereasFig. 13only represents n- and iso-alkanes. Nevertheless

the evolution of the distribution up to 1000 h appears satisfactory:

experimentally as well as via simulation, the evolution of the light

compounds is negligible, particularly up to 500 h and for iP 10 the

distribution follows a straight line whose slope increases with

time. To better characterize the evolution of the C10+fraction, the

experimental and simulated average ratios of xi+1/xi with

15 < i< 25 were calculated for all time periods (Table 7). This ratio

is perfectly simulated up to about 200 h. However it stabilizes

experimentally from 200 h, whereas its decrease continues in the

simulation. The explanation of this apparent experimental stabil-

ization remains unclear, but it may be because of cycloalkanes that

crack to alkanes and are not taken into account in the model. Com-

parisons of the evolution of the experimental and the simulated

molar fractions for individual alkanes show good agreement

(Figs. 14 and 15).

Fig. 7. Comparison between experimental and simulated n-alkanes conversion from 299 to 357 C.

Fig. 8. Comparison between experimental and simulated n-alkanes conversion from 369 to 400 C.



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To conclude, all trends are well represented by the model and so

it is also validated in the case of a mixture as complex as an oil.

10. Conclusions

We have proposed the first cracking model ofn- and iso-alkanes

mixtures that represents their formation as well as their consump-

tion. It includes all types of free radical reactions that can be rea-

sonably expected to take place in the pyrolysis of alkanes below

450 C, except cyclization and aromatization reactions, but these

reactions are believed to be minor, especially at low temperature.

The rate constants for elementary processes have been estimatedon the basis of the literature and were not adjusted. The approach

is not empirical and uses reactions representing what happens at

the molecular scale.

Lumping together reaction types and species with similar struc-

tures and defining model branched alkanes allow limiting the

model to a reasonable size. The pyrolysis of a mixture containing

32 n-alkanes and 46iso-alkanes is represented by 13,206 reactions

involving 193 species. Software like CHEMKIN II is able to solve

this system easily. The model was compared with several literature

experimental results with pure alkanes. With respect to conver-

sion, the agreement is totally satisfactory but product distributions

are well simulated only up to 50% conversion. Above 50%, gas is

underestimated and heavy alkanes overestimated because of C33+which, once formed, does not react further in our model. Moreover,

Fig. 9. Comparison between experimental and simulated n-alkanes conversion at 425 and 450 C.

Fig. 10. Comparison between experimental and simulated iso-C7

conversion at 357 C.



9/12

the model was compared with the pyrolysis results for Elgin oil. In

this case, the global evolution of the saturated fraction is well rep-

resented by the model but a quantitative comparison is difficult.

Indeed, Elgin oil contains cycloalkanes and aromatics (


10/12

Finally, one majorlimitation of themodel is probably theaccumu-

lationof the C33+ fraction at high conversion. One solution would be,

at eachcalculation steptime,to extrapolate thealkanedistribution on

the basis of the middle MWcompound distribution and to compare

the extrapolation to the simulated results. The difference between

the extrapolation and the simulation corresponds to the accumula-

tion which could be equally shared among the C1C32 alkanes for

the following step time. This method would avoid the accumulation

of C33+ as well as significant deviation of the C1C32 distribution,but its computational implementation remains unsolved.

Fig. 12. Experimental product distribution of Elgin saturated fraction after whole oil pyrolysis at 372 C and 400 bar as initial pressure, up to 1000 h.

Fig. 13. Simulated cracking (372 C, 400 bar) of a linear-branched alkanes mixture in the same proportions as in Elgin oil.

Table 7

Comparison between experimental and simulated xCi+1/xCi molar fraction ratio

(average value from C15to C25).

Duration (h) Experimental xCi+1/xCi SimulatedxCi+1/xCi

0 0.85 0.85

30 0.84 0.84

100 0.82 0.82

200 0.78 0.80

500 0.78 0.74

1000 0.79 0.65



11/12

Acknowledgements

We are grateful for valuable comments and suggestions fromC.C. Walters and an anonymous reviewer.

Associate EditorR. di Primio

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