Inhibiting Effect of Tetralin on the Pyrolytic Decomposition of

Ind. Eng. Chem. Res. 2002, 41, 4689-4701

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APPLIED CHEMISTRY Inhibiting Effect of Tetralin on the Pyrolytic Decomposition of Hexadecane. Comparison with Toluene Roda Bounaceur, Ge´ rard Scacchi, and Paul-Marie Marquaire* De´ partement de Chimie-Physique des Re´ actions, UMR Nο. 7630, CNRS, INPL-ENSIC, 1 rue Grandville, BP 451, 54001 Nancy Cedex, France

Florent Domine´ Carbochem, 12 chemin Maupertuis, 38240 Meylan, France

Olivier Bre´ vart, Daniel Dessort, and Bernard Pradier TotalFinaElf, Avenue Larribau, CSTJF, 64018 Pau Cedex, France

A semidetailed kinetic model consisting of 225 free-radical reactions has been developed to describe the thermal cracking of n-hexadecane mixed with tetralin or toluene. The model was tested against available experimental data in the temperature range 360-440 °C. The observed inhibiting effect of tetralin and toluene is explained by the formation of radicals stabilized by resonance, via H-transfer processes, that lead to new termination reactions. A “factor of inhibition” FI is defined to measure the inhibiting effect of an additive on the decomposition of an alkane. This factor FI depends on the resident time, the concentration of the additive, the temperature, and the thermal stability of the inhibitor. For the mixture n-hexadecane-tetralin, the inhibiting effect increases when the temperature increases, whereas the inhibiting effect increases when the temperature decreases in the case of toluene. The comparison between tetralin and toluene shows that toluene may be considered as a pure inhibitor contrary to tetralin. Indeed, during the pyrolysis of tetralin, the formation of molecules such as 1-methylnaphthalene, naphthalene, or butylbenzene, which accelerate the chain mechanism by forming several new radicals by bimolecular initiation reactions, reduces the inhibiting effect of tetralin. A theoritical study of the mechanism of acceleration and inhibition of alkane pyrolysis was treated and led to a better understanding of interactions in a mixture. Introduction Understanding the thermal transformation of crude oils in sedimentary basins is a crucial aspect of petroleum exploration. Because crude oils are mostly made up of alkanes,1 a large number of studies have been devoted to the thermal cracking of these hydrocarbons.2-4 However, the effect that the numerous other chemical structures present in petroleum will have on the rate of alkane cracking remains poorly understood and cannot be quantified today, especially at the low temperatures characteristic of petroleum reservoirs, usually below 200 °C.1 These other chemical structures include, on the one hand, compounds with chemical bonds that are weaker than the C-C bonds of alkanes, such as β C-C bonds in alkyl aromatics, and nonaromatic C-S bonds [Benson5 estimates the bond dissociation energies (BDEs) to be 85, 72, and 77 kcal/mol for alkyl C-C, aromatic β C-C, and nonaromatic C-S bonds, respectively]. These bonds can homolyze more readily than C-C bonds in alkanes, thus generating reactive radicals than can initiate chain reactions and accelerate alkane thermolysis. On the other hand, petroleum contains molecules that are often thermally more stable than alkanes and that * Corresponding author. E-mail: paul-marie.marquaire@ ensic.inpl-nancy.fr.

can, moreover, generate resonance-stabilized radicals that strongly inhibit alkane decomposition. Tetralin (1,2,3,4-tetrahydronaphthalene) is such a molecule: its thermolysis causes the release of a H atom (“hydrogendonor” effect6), which produces the stable 1-tetralyl radical that inhibits alkane pyrolysis. This inhibiting effect has been demonstrated by Khorasheh and Gray,7 who observed that hexadecane diluted in tetralin thermolyzed 10 times slower than pure hexadecane, between 400 and 460 °C. The purpose of the present study is to investigate and quantify the impact of this inhibiting effect (or “H-donor effect”) on the kinetics of alkane thermolysis using a kinetic modeling approach. In a first step, we model the pyrolysis of a tetralin-hexadecane mixture to elucidate the main processes taking place. Tetralin is a constituent of petroleum and is the simplest hydroaromatic.1 It has long been recognized as an effective H donor.6 Normal hexadecane is commonly used as a model compound representative of petroleum alkanes, in both the refining and geochemical fields.8,9 The model used is an extension for the mixture of our recent model of tetralin pyrolysis.10 The validity of this extended model is tested against experimental data obtained in this study and available in the literature. In a second step, we attempt to generalize our understanding on inhibition by comparing the impact of tetralin on hexadecane pyrolysis with that of toluene,

10.1021/ie0108853 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/21/2002

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Ind. Eng. Chem. Res., Vol. 41, No. 19, 2002

Scheme 1

which is an efficient inhibitor.11 Indeed, the removal of an aliphatic hydrogen atom forms the resonancestabilized benzyl radical. This leads us to make suggestions on the efficiency of various pyrolysis inhibitors. Mechanism Construction The pyrolysis mechanism of this mixture is made up of four parts: the pyrolysis mechanisms of pure coumpounds (tetralin, toluene, and hexadecane) and a series of cross-reactions. Tetralin Mechanism. The mechanism of tetralin pyrolysis has been written and validated in a previous paper.10 It includes 114 processes (see Appendix 1). A chemical analysis has been made from this model and leads to the following conclusions: (i) The chain length is low and equal to 5 (440 °C, 14 MPa, 70 min). (ii) Tetralin pyrolysis is very sensitive to impurities; the knowledge of the impurity level in the tetralin used for any experiment is thus very important in interpreting its kinetics. (iii) The production of 1-methylindane is due to ring contraction. (iv) The production of naphthalene is due to dehydrogenation reactions. (v) The production of n-butylbenzene is due to the ipso addition of a hydrogen atom to tetralin. (vi) The production of toluene is due to the ipso addition of a hydrogen atom to 1-methylindane, followed by a decomposition reaction. (vii) Alkanes and alkenes are mostly due to the cracking of the alkyl chain of n-butylbenzene. (viii) The formation of 1-methylindane decreases when the temperature increases, whereas those of n-butylbenzene and naphthalene increase with temperature. Toluene Mechanism. Because we are interested here in the comparison of the inhibiting effect of tetralin and toluene, thermal reactions for toluene pyrolysis have been included in the model. The mechanism of toluene pyrolysis is well-known; it proceeds via a free-radical mechanism,12 producing mainly benzene, methane, hydrogen, and dibenzyl molecules. Initiation proceeds by scission of the benzylic C-H bond (82 kcal/mol13). The formation of benzene results from an ipso addition reaction followed by the elimination of a methyl radical. The formation of methane or hydrogen is explained by H-transfer reac-

tions. Finally, the dibenzyl compound is formed by a termination reaction. The detailed mechanism is given in Appendix 1; it consists of 11 primary and secondary free-radical processes. There are no exploitable experimental data concerning the pyrolysis of toluene at low temperature. The mechanism will thus be written in order to explain the presence of the products observed but not validated quantitatively. A validation of the mechanism will be done in the case of the pyrolysis of the mixture toluenehexadecane. n-Hexadecane Mechanism. The pyrolysis of nhexadecane (n-C16H34), more generally alkanes, can be described by a free-radical mechanism.14 At low conversion, the elementary reactions include initiations, H transfers, radical decompositions by β scission, additions on double bonds, and terminations. It is clear that the detailed pyrolysis mechanism of a long-chain alkane will include hundreds of reactions: the number of processes in the detailed pyrolysis mechanism of n-hexane (nC6H14) is equal to 218 and to 1700 for n-tetradecane (nC14H30).14 To reduce the calculation time and the number of species and reactions, processes were lumped by types, which is an extension of a lumping by chemical species.15 The detailed explanation of how to write such a mechanism and how to calculate kinetics parameters will be the subject of a separate paper. At present, we will only give an example that illustrates our approach. Initiation reactions from n-C16H34 can generate several alkyl radicals by homolysis of C-C bonds as in Scheme 1, where kij is the kinetic parameter for the initiation process number j. Our objective is to reduce these eight processes into one, as follows:

C16H34 f r-alkyl + r-alkyl

ki

(R14)

where numbers of alkyl radicals

r-alkyl )

∑ i)1

alkyl radicals (E1)

If we want reaction (R14) to represent all of the eight

Ind. Eng. Chem. Res., Vol. 41, No. 19, 2002 4691

processes written above [(R1)-(R8)], we have to respect the relation concerning the rate of initiation:

Table 1. Experimental Conditions Used for the Pyrolysis of the Mixture Tetralin-Hexadecane27 exptl conditions

8

rate of reaction (R14) )

rate of reaction (Rj) ∑ j)1

(E2)

Then

ki[C16H34] ) ki1[C16H34] + ki2[C16H34] + ki3[C16H34] + ki4[C16H34] + ki5[C16H34] + ki6[C16H34] + ki7[C16H34] + ki8[C16H34] Therefore 8

ki )

kij ∑ j)1

(E3)

We deal with all types of processes in the same way, and we obtain a lumped mechanism for the pyrolysis of n-hexadecane, as shown in Appendix 1, containing only 10 reactions. Cross-reactions between n-Hexadecane, Tetralin, and Toluene. We have to include the major crossreactions between n-hexadecane, tetralin, and toluene. Included are initiation, H transfer, addition, and termination reactions. Initiation reactions are reverse radical disproportionation (RRD) reactions such as (R9) and (R9′):

Several H-transfer reactions were written such as (R10) and (R10′):

Additions leading to alkylbenzenes were included such as (R11) and (R11′):

Finally, terminations between alkyl and aryl radicals were added as reactions (R12), (R13), and (R13′):

The mechanism written is able to model the pyrolysis of the mixture n-hexadecane, tetralin, and toluene. It consists of 225 free-radical mechanisms with kinetic data in Arrhenius form, and it is given in Appendix 1.

mixture (mol %) 94% tetralin + 6% hexadecane 70% tetralin + 30% hexadecane

T ) 380 °C and P ) 500 bar 168 h

288 h

T ) 360 °C and P ) 500 bar 500 h

500 h

In Appendix 2, the correspondence between the names used in the mechanism and the chemical formula is given. Estimation of Rate Constants and Thermochemical Properties. Activation energies E and preexponential factors A were estimated by methods detailed by Bounaceur et al.10,14 For some reactions, the compilation of Allara and Shaw16 or the NIST database17 was used. For other reactions, the parameters were estimated from analogous reactions, with corrections taking into account thermochemistry. We also used the software KINGAS,18 which is a computer program for the estimation of rate parameters by the methods of Benson.5 The reaction-family concept was also used to estimate the rate constant for several processes. This is based on the use of a linear free energy relationship19,20 (LFER). The method used, validated by Malhotra and McMillen21 and Walter and Klein,22 is based on the Evans-Polanyi23 relationship, which correlates rate constants to heats of reaction for free-radical reaction families. The calculation of the heats of formation ∆fH° and entropies S° of the species used (both molecules and radicals) was estimated as detailed by Bounaceur et al.10,14 Different methods were used such as reviews, extrapolation from analogous compounds, BDEs, group additivities, or the software THERGAS,24 which is a computer program for the evaluation of thermochemical data for molecules and free radicals using the methods of Benson5 and Yoneda.25 Experimental Validations Several validations, using the software CHEMKIN II,26 have been performed to test the mechanism written in Appendix 1. Mixture Tetralin-Hexadecane. The experimental results come from two different origins: a study carried out by Dessort27 (from Elf-Exploration-Production, confidential report, 1996) and another coming from Khorasheh and Gray7 (1993). (a) First Study (Dessort,27 1996). Two different mixtures and several experimental conditions were used. Table 1 gathers these various conditions. The tetralin used for these experiments is not pure, but we know exactly the impurity levels. The knowledge of these impurity levels is very important because tetralin pyrolysis is very sensitive.10 After minor rate coefficient adjustments to improve the fit between model and experimental results (the rate coefficient adjustments are listed in Appendix 1), we obtain the values of Table 2. We can conclude that a good agreement is reached between the experimental and simulated values for all compounds except for tetralin and alkanes at high conversion. There are different possible reasons to explain these discrepancies. The first one is a possible error in the quantification of light and heavy alkanes (note that more than 80% of alkanes have a number of carbons of less than 4). The second possible reason is a

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Table 2. Comparison between Experimental27 and Simulated Values 94% tetralin + 6% n-C16H34 500 h and 360 °C

0h expa indan methylindan tetralin C1-C16 alkyltetralin alkanes (C1-C15, C17+) n-C16H34 methylpropylbenzene butylbenzene methylpropenylbenzene toluene ethylbenzene o-xylene naphthalene dihydronaphthalene

simb

93.76

93.76

5.60

5.60

0.02 7.96 85.15 0.06 0.05 4.94 0.58

0.64

0.64

tetralin n-C16H34 a

expa

1.23

168 h and 380 °C

simb

expa

70% tetralin + 30% n-C16H34 288 h and 380 °C

simb

expa

simb

Mole Fraction (%) 0.01 0.04 0.03 7.42 9.38 9.29 83.90 81.44 80.30 0.50 0.58 0.79 0.90 0.50 1.24 4.62 4.75 4.31 0.05 0.20 0.65 0.93 1.15 0.07 0.32 0.09 0.20 0.08 0.04 0.01 0.03 1.10 2.00 1.48 0.02 0.02

0.35 19.58 44.36 1.51 23.60 2.50 0.34 1.86 0.19 0.91 0.31 0.04 4.15 0.07

0.10 16.00 69.80 1.37 2.23 3.46 0.45 2.06 0.40 0.53 0.10 0.08 2.40 0.04

Conversion (wt %) 14 16

8 10

40 44

500 h and 380 °C

0h expa

simb

69.59

69.59

30.00

30.00

0.15

0.15

0.26

0.26

expa

simb

0.17 8.59 28.89 3.48 37.60 14.52 0.12 1.08 0.09 0.32 0.12 0.03 3.53 0.04

0.13 12.00 42.60 5.86 12.20 13.90 0.40 2.20 0.27 0.90 0.15 0.06 2.60 0.02

40 29

exp ) experimental. b sim ) simulated.

Table 3. Comparison between Experimental7 and Simulated Values tetralin-hexadecane (5 mol %) compound

Xi/Xbut experimental

Xi/Xbut simulated

1-methylindan naphthalene n-butylbenzene indan ethylbenzene toluene styrene propenylbenzene C20

22.00 4.20 1.00 0.44 0.48a 0.13a 0.04a 0.03a 0.85a

19.00 3.30 1.00 0.40 0.41 0.16 0.07 0.04 1.85

a In the absence of precision from the authors, we adopted the values in the case of the pyrolysis of pure tetralin.7,10

possible hydrogenation of tetralin followed by decomposition into alkanes. It is important to notice that both Khorasheh and Gray7 (1993) and Chaverot28 (1985) had noticed the same phenomena by pyrolyzing neat tetralin at 400 and 800 °C, respectively. Finally, it is clear that the mechanism written has to be completed by several secondary reactions to fit the model at high conversion.

Nevertheless, the same mechanism can represent in a satisfactory way the experimental results under various experimental conditions. (b) Second Study (Khorasheh and Gray,7 1993). The second test of the model was performed by comparing the model output with the experimental data of Khorasheh and Gray7 about the pyrolysis of the mixture tetralin-hexadecane (5 mol %) at 440 °C, 139 bar, and a residence time of around 1 h. The authors give only the molar ratio of product i (Xi) relative to that on n-butylbenzene (Xbut). They found that these values were quite similar to those of the pyrolysis of pure tetralin except for 1-methylindan, naphthalene, and indan. Starting from the same composition of Khorasheh and Gray,7 we have simulated (with the mechanism written in Appendix 1) the pyrolysis of the mixture tetralin-hexadecane (5 mol %) at 440 °C, 139 bar, and a residence time of 1 h. The result of the simulation is shown in Table 3. In Figure 1, the variation of the molar selectivities (mol/100 mol of hexadecane decomposed) of the total of alkanes, alkenes, and alkyltetralins is shown.

Figure 1. Variation of the molar selectivities of the sum of alkanes, alkenes, and alkyltetralins versus conversion of hexadecane in the pyrolysis of a mixture hexadecane (5%)-tetralin. Comparison with experimental data7 at 440 °C and 139 bar. Points: experimental values. Lines: simulated values.


Figure 2. Variation of the molar selectivities of the sum of alkanes, alkenes, and alkylbenzene versus conversion of hexadecane in the pyrolysis of a mixture hexadecane (5%)-toluene. Comparison with experimental data11 at 420 °C and 139 bar. Points: experimental values. Lines: simulated values.

butylbenzene (Figure 4). Indeed, for the mixture of n-hexadecane + 5 mol % of tetralin, the decomposition of the alkane decreases by 80% after 1 day [(16.9 - 3.4)/ 16.9 ) 80%], 75% after 2 days, and 71% after 4 days. Adding 5 or 20 mol % of tetralin reduces the conversion of n-hexadecane. After 4 days at 380 °C, tetralin decreases the decomposition by about 71% for 5 mol % and 76% for 20 mol %. Nevertheless, if the percentage of tetralin in n-hexadecane is increased, the inhibiting effect is reduced and, in this case, tetralin reduces the decomposition by about 55% for 4 days. This phenomenon could be explained by the formation of butylbenzene during the pyrolysis. Via homolysis reactions, butylbenzene formed several radicals. Figure 3. Inhibiting effect of tetralin on the decomposition of n-hexadecane at 380 °C and 500 bar.

We can conclude that we obtain a rather good agreement between the experimental and simulated values and this with the same mechanism in Appendix 1. Mixture Toluene-Hexadecane. Khorasheh and Gray11 studied the pyrolysis of the mixture toluenehexadecane (5 mol %) at 420 °C and 139 bar. Using the same conditions, we have simulated the experimental results of the authors. Figure 2 shows the variation of the molar selectivities (mol/100 mol of hexadecane decomposed) of the total of alkanes, alkenes, and alkylbenzenes. The agreement between the experimental data and the simulated values is rather good. After the validation of the mechanism on different mixtures and experimental conditions, we will study, by only simulation, the inhibiting effects of tetralin and toluene on the pyrolysis of hexadecane. Modeling Results: Inhibiting Effects of Tetralin and Toluene on the Pyrolysis of Hexadecane Inhibiting Effect of Tetralin. Simulations of pure n-hexadecane and n-hexadecane diluted in various amounts of tetralin were performed at 380 °C and 500 bar and for a duration of up to 4 days. The conversion of n-hexadecane is reported in Figure 3, and it is clear that adding tetralin reduces the conversion of n-hexadecane. With time, the effect of tetralin is slightly reduced because of the decomposition of the latter into several products such as 1-methylindane, naphthalene, and

Via H transfer with n-hexadecane, these new radicals will form n-C16H33 radicals and thus participate in the consumption of n-hexadecane. Another possible explanation could be the increasing presence of a compound (such as 1-methylindane or naphthalene) which reacts by bimolecular homolysis (Figure 5). This would have as a consequence an increase of the rate of pyrolysis by creation of new radicals and thus a reduction of the inhibiting effect of the tetralin. In conclusion, tetralin is not a pure inhibitor because of the formation of compounds (such as 1-methylindane, naphthalene, or n-butylbenzene) which accelerate the consumption of alkanes. Inhibiting Effect of Toluene. Some similar simulations were performed at 380 °C, 500 bar, and a residence time of 4 days with a mixture of n-hexadecane and 5, 20, and 70 mol % of toluene. Figure 6 shows the conversion of n-C16H34 in different mixtures. It can be seen that toluene considerably reduces the conversion of n-hexadecane. This effect is accentuated with an increasing concentration of toluene. We can conclude that toluene is a pure inhibitor.

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Figure 6. Inhibiting effect of toluene on the decomposition of n-hexadecane during its pyrolysis at 380 °C and 500 bar.

important the inhibitor effect is. In this case we can conclude that toluene is a pure inhibitor.

Figure 4. Kinetic curves for the decomposition of tetralin and the formation of 1-methylindane, n-butylbenzene, and naphthalene during the pyrolysis of n-hexadecane + 20 mol % of tetralin (380 °C and 500 bar).

Discussion: Comprehension of the Mechanism of Inhibition Principle of the Action of an Inhibitor: Mechanism µH-YH. The theoretical study of the mechanisms of acceleration and inhibition of alkane pyrolysis was treated in an exhaustive way by Niclause et al.29 For this paper, we will limit ourselves to the phenomena of inhibition. At low conversion and low temperature, Bounaceur et al.14 have shown that the pyrolysis of an alkane (“µH”) can be schematized by the following free-radical cracking mechanism where “µ•” and “β•” are free radicals:

Initiation Propagation I Termination

Figure 5. Molar fraction of 1-methylindane, naphthalene, and butylbenzene at 380 °C, 500 bar, and a residence time of 4 days and for different mixtures.

“Factor of Inhibition” (FI). To measure the inhibiting effect from an additive, we can define FI as follows (eq E4):

FI )

conversion of reactant without additive (E4) conversion of reactant with additive

An FI above 1 indicates an inhibiting effect, whereas an FI below 1 indicates an accelerating effect on the pyrolysis of an alkane. Figure 7 shows the variation of FI for pyrolyses at 380 °C and 500 bar of mixtures n-hexadecane-tetralin at different concentrations (in mol %) and different residence times. This figure clearly shows that the inhibiting effect diminishes with time for a fixed concentration of tetralin. For a fixed residence time, between 0 and approximately 20 mol % of tetralin, the more the molar fraction of tetralin grows, the more the inhibiting effect on the pyrolysis of n-hexadecane is important. With a concentration of above 20 mol % of tetralin, the effect is reversed. If we calculate FI versus time and concentration in toluene, we obtain Figure 8. Contrary to tetralin, the more important the concentration of toluene is, the more

µH f free radical

{

•

(i) •

µ f alkene + β β• + µH f alkane + µ• µ• + µ• f µ-µ

(2) (3) (µµ)

In the presence of an H-donor molecule (“YH”), µ• and can react by H-transfer reactions with YH to yield new radicals “Y•”: β•

β• + YH f alkane + Y• •

•

µ + YH f µH + Y

(4β) (4µ)

In the same way, Y• could react with µH by a H-transfer reaction:

Y• + µH f YH + µ•

(5)

Note that reaction (5) is the reverse of reaction (4µ). Finally, in the presence of YH, two new termination reactions can appear:

Y• + Y• f Y-Y •

•

Y + µ f Y-µ

(YY) (Yµ)

Then, in the presence of an additive YH, the freeradical mechanism of the pyrolysis of an alkane µH can be schematized by the following reactions:

Initiation Propagation I

µH and YH f free radical

{

µ• f alkene + β• β• + µH f alkane + µ•

(i) (2) (3)


Figure 7. Variation of FI vs time and mole % of tetralin in the pyrolysis of mixture n-hexadecane-tetralin (380 °C and 500 bar).

Figure 8. Variation of FI vs time and mole % of toluene in the pyrolysis of mixture n-hexadecane-toluene (380 °C and 500 bar).

Propagation I′

{

µ• f alkene + β• β• + YH f alkane + Y• Y• + µH T µ• + YH

Termination

{

µ• + µ• f µ-µ Y• + Y• f Y-Y Y• + µ• f Y-µ

(2) (4β) (5) and (4β)

(µµ) (YY) (Yµ)

Without YH there is only one propagation: propagation I [reactions (2) and (3)]. In the presence of YH, a new chain reaction appears: propagation I′ [reactions (2), (4β), and (5)], sharing the same process (2) with propagation I and leading to the same stoichiometric equation, µH ) alkane + alkene. We can schematize the propagation reactions as follows: Figure 9. Inhibition of the decomposition of n-hexadecane by H-donor molecules (in the case of toluene).

Inhibiting Effect of Toluene and Tetralin. The previous general mechanism can be illustrated in the case of mixtures of hexadecane and toluene (or tetralin) in Figure 9. Without toluene (respectively tetralin), n-hexadecane reacts by H transfer with a radical to form a molecule and a radical n-C16H33• [reaction (R21)]. This radical

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Figure 10. Stabilization by resonance of benzyl (top) and 1tetralyl (bottom) radicals.

decomposes by β scission to form another radical and an alkene [reaction (R20)]. These two processes form the chain propagation I, the rate of which being limited by the β scission [reaction (R20)]. In a mixture with toluene (respectively tetralin), the radicals formed in reaction (R20) can react with toluene (respectively tetralin), leading to the benzyl radical [respectively the 1-tetralyl radical; reaction (R24)]. The radical n-C16H33• can also react by H transfer [reactions (R22) and (-R22)] with toluene (respectively tetralin) to form the benzyl radical (respectively the 1-tetralyl radical), which is stabilized by resonance (Figure 10), and therefore less reactive, and will react essentially in termination reactions [reaction (R23)].

Thus, the action of toluene (respectively tetralin) consists of the consumption of alkyl radicals, coming from the decomposition of n-hexadecane, to form more stable radicals, which can react by termination processes. The consequence is a decrease of the concentration of radicals n-C16H33• and then a diminution of the rate of pyrolysis of n-hexadecane because this rate is always given by r ) kR20[n-C16H33.]. It is important to notice that, in this case, the hypothesis that toluene (respectively tetralin) did not bring new important initiation processes was made. In the contrary case, there would be competition between the accelerator effect, because of the generation of new radicals from initiation processes, and the inhibition effect, because of the stabilized radical. Effect of Temperature. We have seen (Figure 9) that principal reactions involved in inhibition are H transfer [reactions (R22) and (-R22)] and termination [reaction (R23)]. Terminations by recombination have (generally) activation energies close to zero, whereas

Figure 11. Variation of FI as a function of temperature for the mixture n-hexadecane + 20 mol % of toluene (500 bar).

Figure 12. Variation of FI as a function of temperature for the mixture n-hexadecane + 20 mol % of tetralin (500 bar).

Ind. Eng. Chem. Res., Vol. 41, No. 19, 2002 4697 Table 4. Total Rate of Initiation for Different Mixtures and Different Temperatures at 10% of Conversion of n-Hexadecane rate of initiation (R) (mol‚cm-3‚s-1) pure n-hexadecane n-hexadecane + 20 mol % tetralin n-hexadecane + 20 mol % toluene

350 °C

400 °C

450 °C

3.3 × 10-14 5.0 × 10-12 (152) 8.0 × 10-14 (2.4)

4.0 × 10-12 6.0 × 10-11 (15) 5.0 × 10-12 (1.25)

2.0 × 10-10 5.0 × 10-10 (2.5) 2.0 × 10-10 (1)

H-transfer reactions have activation energies in the range 10-20 kcal/mol. The importance of the inhibiting effect with temperature is closely related to the relative importance of processes (R22) and (-R22). The enthalpy of reaction (R22) is related to activation energies by the following relationship:

∆rH°R22 ) ER22 - E-R22

(E5)

where ∆rH°R22 ) enthalpy of reaction (R22), ER22 ) activation energy of process (R22), and E-R22 ) activation energy of process (-R22). In both cases (n-hexadecane + tetralin or toluene), the average values of ER22 and E-R22 are 12 and 22 kcal/ mol, respectively (see Appendix 1). Then, using eq E5, we find a value of ∆rH°R22 equal to -10 kcal/mol. This negative value implies that, as the temperature decreases, the equilibrium shifts on the side of reaction (R22), and then an increasing of FI is expected. Variations of FI as a function of temperature are presented in Figures 11 and 12. In the case of a mixture of n-hexadecane + 20 mol % of toluene (Figure 11), the inhibiting effect is more pronounced at low temperature as expected, but this trend is inverse in the case of a mixture of n-hexadecane + 20 mol % of tetralin (Figure 12). It is not very easy to explain the behavior of tetralin, but similar results have been obtained in different investigations. Yoon et al.30 found a conversion of pure dodecane at 450 °C of 65% and 36%, with a mixture of dodecane + 10 mol % of tetralin corresponding to a value of FI of 1.8; at 425 °C the value of FI is only 1.2 (conversion of pure dodecane equals 65% and 55% in the mixture). Considering that the chemical behaviors of dodecane and n-hexadecane are similar, we can conclude that the effect of inhibition of tetralin is more pronounced at high temperature. If we try to calculate, via simulation, only the rate of initiation, we can attempt to explain such a comportment. Table 4 presents values of the total rate of initiation for different mixtures at several temperatures. At each case the conversion of pure n-hexadecane is around 10%. We define R as follows:

R) global rate of initiation of mixtures (E6) global rate of initiation of pure n-hexadecane Concerning the mixture n-hexadecane-tetralin, the ratio R is equal to 2.5 at 450 °C, 15 at 400 °C, and 152 at 350 °C. Then, when the temperature decreases, the relative importance of the total rate of initiation increases drastically compared to pure n-hexadecane (principally by RRD with alkene and dihydronaphthalene). In fact, in this case, the more the temperature decreases, the more the importance of new initiations

coming from tetralin is, with a consequence of a reduction of the inhibiting effect. For the mixture n-hexadecane-toluene, the ratio R is equal to 1.0 at 450 °C, 1.25 at 400 °C, and 2.4 at 350 °C. In this case, the accelerating effect due to RRD reactions is not enough to compensate the inhibiting effect. Conclusion A kinetic model for the pyrolysis of n-hexadecane in a mixture with tetralin or toluene has been written. This model includes 225 processes with kinetic parameters in an Arrhenius form and is suitable for several simulations. The general conclusion is that the preferential H transfer from tetralin or toluene rather than n-hexadecane inhibits the chain mechanism for conversion of the alkane. For the mixture of n-hexadecane-tetralin this inhibiting effect increases when the temperature increases, whereas the inhibiting effect increases when the temperature decreases in the case of toluene. The comparison between tetralin and toluene shows that toluene may be considered as a pure inhibitor in contrast to tetralin. Indeed, during the pyrolysis of tetralin, the formation of molecules such as 1-methylnaphthalene, naphthalene, or butylbenzene, which accelerates the chain mechanism by forming several new radicals, reduces the inhibiting effect of tetralin. The studies show that for mixtures between 0 and approximately 20 mol % in tetralin, the higher the concentration is, the greater the inhibiting effect is, and for mixtures with more than 20 mol % in tetralin, the observed effect is reversed. Regarding mixtures with toluene, the inhibiting effect increases when the concentration increases. It appears that the inhibiting effect of a molecule YH on the pyrolysis of an alkane µH depends on the concentration of YH, the temperature, the residence time, the facility of YH to decompose, and the nature of the product of decomposition. In a general way, there is always a competition between the inhibiting and the accelerating effects of an additive YH on the pyrolysis of an alkane µH, and the more pronounced effect will carry it. Last, we could define FI to measure the inhibiting effect of an additive on the pyrolysis of an alkane. In conclusion, this paper leads to a better understanding of the mechanism of inhibition of alkane pyrolysis and the influence of temperature, concentration, and residence time on this inhibition. It appears from the above results and discussion that the applications of the model are promising. This study may be helpful for petroleum exploration to understand the role and kinetic influence of “H-donor” molecules and therefore explain the thermal stability of alkanes in petroleum.4,31 The application to the geological temperature (100-200 °C) and the implication for the thermal stability of crudes oils in reservoirs will be the subjecta of a future paper. Another application could be the improvement of jet fuel thermal stability. Indeed, the advanced aircraft of the future will depend on the jet fuel as the primary coolant for dissipating the heat generated from various aircraft components. This will expose the fuel to severe conditions, and the thermal decomposition of the fuel (composed of a majority of alkanes) at high temperatures may lead to the formation of detrimental solid deposits. A possible solution to avoid this decomposition is to use “H-donor” molecules30,32 to reduce the decomposition of alkane molecules.

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Appendix 1: Mechanism for the pyrolysis of n-hexadecane-tetralin or n-hexadecane-toluene mixtures


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Appendix 2: Name of compounds used in the mechanism (Appendix 1)

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Received for review October 28, 2001 Revised manuscript received May 10, 2002 Accepted May 31, 2002 IE0108853

Inhibiting Effect of Tetralin on the Pyrolytic Decomposition of

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