The influence of phosphorus containing compounds on steam cracking of n-hexane

J. Anal. Appl. Pyrolysis 77 (2006) 133–148 www.elsevier.com/locate/jaap

The influence of phosphorus containing compounds on steam cracking of n-hexane Jidong Wang, Marie-Franc¸oise Reyniers, Guy B. Marin * Laboratorium voor Petrochemische Techniek, Universiteit Gent, Krijgslaan 281 S5, B-9000 Gent, Belgium Received 22 August 2005; accepted 26 February 2006 Available online 18 April 2006

Abstract In steam cracking of hydrocarbons P-containing compounds are used as additives to control coke formation and/or CO production. However, information concerning the influence of P-containing compounds on the decomposition of hydrocarbons during steam cracking is scarce. In this study, the influence of three P-containing compounds, hexamethyl phosphoric triamide (HMPA), tripiperidinophosphine oxide (TPyPO) and dioctyl phenylphosphonate (DOPP), on the conversion of n-hexane during steam cracking was investigated in a continuous flow micro-reactor with complete mixing. All these compounds accelerate the thermal decomposition of n-hexane. The effect of a P-containing additive on the conversion of n-hexane is quantitatively evaluated using the dimensionless group method proposed by LaMarca et al. [C. LaMarca, C. Libanati, M.T. Klein, Chem. Eng. Sci. 45 (1990) 2059]. To interpret the accelerating effect, the potential elementary radical reactions involved in the steam cracking of these P-containing compounds are proposed. Their rate coefficients are estimated based on thermochemical data. For all of these P-containing compounds the main coupling with n-hexane decomposition occurs through H-abstraction reactions. The accelerated effect can be traced back to the introduction of new initiation steps. The evaluated values are consistent with the experimentally measured values. # 2006 Elsevier B.V. All rights reserved. Keywords: n-Hexane; P-containing compounds; Steam cracking; Additives

1. Introduction The production of base chemicals such as ethylene, propylene and aromatics through steam cracking of hydrocarbons is an important field in industrial chemistry. Steam cracking of hydrocarbons is generally described as occurring through a radical chain mechanism. This comprises the initiation reactions via the dissociation of C–C bonds; propagation reactions including H-abstraction and b-scission; termination reactions via radical recombination and radical disproportionation. In steam cracking two important but undesirable reactions are the formation of CO and the coke deposition on the inner surface of the cracking coils and of the transfer line heat exchangers. In order to control the production of CO and coke deposition, chemicals such as sulfurcontaining compounds and phosphorus-containing compounds [2] are used as additives. Addition of these chemicals not only interferes with the formation of CO and coke

* Corresponding author. Tel.: +32 9 264 45 16; fax: +32 9 264 49 99. E-mail address: [email protected] (G.B. Marin). 0165-2370/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2006.02.008

deposition but can also, in principle, influence the conversion of hydrocarbons and the product distribution of the steam cracking process. Addition of an additive to hydrocarbons may introduce new initiation steps resulting in an increased conversion of hydrocarbons. The additive itself, or its decomposition products, may interfere with H-abstraction reactions. In this case, the effect of the additive on the conversion of hydrocarbons can be accelerating or suppressing, depending on the nature of the radicals derived from the additive. Finally, an additive may also interfere with the termination reactions via the addition of new termination steps, resulting in a decreased conversion of hydrocarbons. The influence of hydrogenated additives HY, such as H2S, HCl and HBr, on the thermal cracking of light hydrocarbons, e.g. ethane, propane, iso-butane and neo-pentane, at low conversion (T = 760–810 K) has been studied by Scacchi et al. [3,4], Niclause et al. [5] and Muller et al. [6]. The influence of these hydrogenated additives on the pyrolysis of these hydrocarbons was interpreted based on their decomposition mechanism. For the pyrolysis of ethane, b-scission of ethyl radicals is the rate-limiting step in the chain propagation process since its reaction rate is potentially slower than that of

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Nomenclature pre-exponential factor in the Arrhenius equation pre-exponential factor in the Arrhenius equation for H-abstraction (m3 mol
1 s
1 per hydrogen) Ai reactants subjected to cracking Ea activation energy (kJ mol
1) E1estimated the estimated effect of A2 on the decomposition rate of A1 in thermal cracking E1measured the measured effect of A2 on the decomposition rate of A1 in thermal cracking F flow rate of n-hexane in effluent (g h
1) FH 2 O flow rate of water feed (g h
1) F0 flow rate of n-hexane feed (mol h
1) DH enthalpies of reactions (kJ mol
1)  DHf standard enthalpies of formation (kJ mol
1) i.d. internal diameter of GC column (mm) k rate coefficients of reaction ki rate coefficient of b-scission reaction (s
1) kij rate coefficient of H-abstraction involving b radicals (m3 mol
1 s
1) 0 ki j rate coefficients of H-abstraction involving m radicals (m3 mol
1 s
1) nH the number of equivalent hydrogen atoms in a molecule Qi cracking products from Ai r reaction rate (mol m
3 s
1) r(A1) reaction rate of A1 in the cracking of pure A1 r(A1, A2) reaction rate of A1 in the cracking of the binary mixture of A1 and A2 S2 the molar ratio of component A2 to component A1 (mol mol
1) T temperature (K) V volume of reactor (cm3) X conversion of n-hexane (%) X(A1) the conversion of A1 in the cracking of pure A1 (%) X(A1, A2) the conversion of A1 in the cracking of the binary mixture of A1 and A2 (%)

v00ii

termination rate coefficients for reaction of mi + mi (m3 mol
1 s
1)

A AH

Greek symbols rate coefficient for initiation reactions (s
1) ai bi free radical formed by initiation of Ai and by bscission reaction of mi d steam dilution (kg steam/kg n-hexane) l1 the ratio of rate coefficients for H-abstraction from A2 by b1 to that from A1 by b2 mi free radical formed from H-abstraction reaction of Ai u1 the ratio of the rate coefficients for H-abstraction from A1 by b2 and b1 radicals r2 ratio of the rate coefficient of the initiation reaction of A2 to A1 vii termination rate coefficients for reaction of bi + bi (m3 mol
1 s
1) 0 vii termination rate coefficients for reaction of mi + bi (m3 mol
1 s
1)

the H-abstraction reactions. Therefore, the presence of HY type additives does not influence the propagation rate because HY type additives interfere only with H-abstraction reactions. However, the presence of HY can give rise to new termination reactions, resulting in a decrease in the conversion of ethane. For the pyrolysis of neo-pentane, H-abstraction is the ratelimiting step in the chain propagation process since the bscission rate of the neo-pentyl radical is potentially faster than that of the H-abstractions reactions. HY type additives act as catalysts and accelerate H-abstraction reaction. This results in an accelerating effect on the conversion of neo-pentane. Rebick [7] investigated the influence of H2S on the pyrolysis of nhexadecane at 770 K. H2S influences not only the conversion of n-hexadecane but also the product distribution. The overall cracking rate of n-hexadecane was found to increase with a 1/2order dependence on the H2S partial pressure. The yields of CH4, C2H6 and C2H4 were found to decrease markedly and significant amounts of C3–C14 normal paraffins appeared as products in the presence of H2S. The effect of H2S on the decomposition of n-hexadecane and the product distribution was attributed to its interference with H-abstraction steps in the chain propagation. During the last two decades P-containing compounds have been reported to be able to inhibit coke formation during steam cracking of hydrocarbons. The inhibiting effect of a Pcontaining additive and a P/S-containing additive on coke formation and CO production during steam cracking of hydrocarbons was reported by Boone [2]. However, the influence of these additives on the conversion of hydrocarbons was not mentioned. Ghosh and Kunzru [8] and Vaish and Kunzru [9] investigated the influence of triethyl phosphite and triphenyl phosphite on the coke formation during naphtha cracking at 1088–1108 K. Addition of these compounds (50–1000 ppm P) reduces the coke formation without affecting the product yields. Das et al. [10] and Chowdhury and Kunzru [11] investigated the influence of benzyl diethyl phosphite (100–500 ppm P) and triphenyl phosphine sulfide (50–100 ppm P) on the pyrolysis of naphtha in the temperature range of 1073–1103 K. Reduction in coke formation as well as CO production was observed. No influence on the yield of hydrocarbon products was noticed. Kisalus [12,13] reported that triphenyl phosphine (25– 100 ppm) and triphenyl phosphine oxide (25–100 ppm) could suppress the coke formation during n-hexane cracking without affecting the conversion of n-hexane at temperatures of 1053–1113 K. Tong [14,15] reported the influence of phosphoric triamide (200–500 ppm) on the steam cracking of n-hexane at 1043–1063 K and the influence of phosphonate/ thiophosphonate compounds on the steam cracking of heptane at 1053 K. Reduction of coke formation was observed. The influence of these compounds on the conversion of hydrocarbons was not reported.

J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

P-containing compounds, used as additives in thermal cracking of hydrocarbons, can in principle influence the decomposition of hydrocarbons due to their interference with the elementary steps in the free radical chain reactions. However, no report concerning the influence of P-containing compounds on the thermal decomposition of hydrocarbons has been published. This is probably due to the fact that most of the investigations with regard to the influence of P-containing compounds on the thermal cracking of hydrocarbons are carried out with complex mixtures of hydrocarbons such as naphtha and attention has been focused on the influence of P-containing compounds on coke formation. In this paper we report the effect of three P-containing compounds, hexamethyl phosphoric triamide (HMPA), tripiperidinophosphine oxide (TPyPO) and dioctyl phenylphosphonate (DOPP), on the steam cracking of a pure hydrocarbon compound, n-hexane at temperatures from 1123 to 1173 K. To interpret the influence of these P-containing compounds on the decomposition rate of n-hexane, a tentative predominant decomposition route of these P-containing compounds and the coupling reactions with the n-hexane decomposition are proposed based on the reaction rates of the possible elementary free radical reactions. Their effect on the decomposition rate of n-hexane is evaluated in terms of the dimensionless group method proposed by LaMarca et al. [1]. 2. Theoretical background LaMarca et al. [1] experimentally studied the pyrolysis of binary mixtures consisting of model compounds such as dibenzyl ether and phenethylphenyl ether and mathematically examined the corresponding rate expressions. The initiation, propagation and termination in the cracking of a pure hydrocarbon compound, Ai, to products biH and Qi can be represented by the elementary steps given in Fig. 1. For the cracking of a binary mixture consisting of A1 and A2, the chain coupling steps can be expressed by the elementary steps given in Fig. 2.

Fig. 1. Elementary steps in the thermal cracking of hydrocarbons.

135

Fig. 2. Elementary steps of the cross-coupling in a binary pyrolysis system.

The effect of the addition of a component A2 on the decomposition rate of component A1 in a binary mixture can be expressed in terms of dimensionless groups as E1 ¼

rðA1 ; A2 Þ ð1 þ r2 S2 Þ1=2 ð1 þ l1 u1 S2 Þ ¼ rðA1 Þ 1 þ l1 S2

(1)

If A1 represents n-hexane and A2 represents the P-containing additive, r(A1) is the reaction rate of n-hexane in the cracking of pure n-hexane; r(A1, A2) is the reaction rate of n-hexane in the presence of the additive; r2 is the ratio of the initiation rate coefficient of the additive to that of n-hexane, r2 = a2/a1; S2 is the molar ratio of the additive to n-hexane, S2 = A2/A1. The parameter u1 and l1 represent the cross-coupling between the propagation steps. u1 is the ratio of the rate coefficients for Habstraction from n-hexane by b2 radicals, i.e. radicals derived from the additive, and by b1 radicals, i.e. radicals derived from n-hexane, respectively, u = k21/k11; l1 is the ratio of rate coefficients for H-abstraction from the additive by b1 radicals to that of H-abstraction from n-hexane by b2-radicals , l = k12/ k21. Eq. (1) was derived based on two assumptions. The first one is that b-scission reactions are potentially fast relative to Habstractions. Consequently, the effect of the cross-coupling through the m-radicals as indicated in Fig. 2 can be neglected. The second assumption is the so called ideal termination, i.e. the radical recombination reactions have essentially zero activation energy and equal pre-exponential factors except for the statistical factor that renders the rate coefficient (collision number) for self-collisions one-half that of cross-collisions. The influence of the dimensionless groups on E1 has been discussed in detail in the paper of LaMarca et al. [1]. In summary, if the addition of the P-containing additive (A2) results in new initiation steps (r2 > 1), the decomposition of nhexane (A1) will be accelerated. The parameter u1 determines whether the cross-propagation steps will accelerate or inhibit the decomposition of n-hexane. If the radicals derived from the additive are more active in H-abstraction (u1 > 1), the decomposition of n-hexane will be accelerated. l1 and S2 amplify the accelerating or inhibiting effect that is directed by u1. Based on the study of the pyrolysis of a binary mixture, Savage [16] reached the similar conclusions. Eq. (1) provides an instantaneous measure of the effect of kinetic coupling during the pyrolysis of a binary mixture. If the values of the dimensionless group parameters in Eq. (1) are known, the effect of the additive on the decomposition of n-hexane can be estimated.

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J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

For a continuous flow stirred tank reactor (CSTR) with complete mixing, the conversion of n-hexane is given by Eq. (2): X¼

V r F0

(2)

where X is the conversion of n-hexane; V the volume of the reactor; F 0 is the feed flow rate of n-hexane [17]. In a CSTR reactor, E1, can thus be expressed as E1 ¼

rðA1 ; A2 Þ XðA1 ; A2 Þ ¼ rðA1 Þ XðA1 Þ

(3)

where X(A1) is the conversion of n-hexane in the cracking of pure n-hexane; X(A1, A2) is the conversion of n-hexane in the presence of the additive. To quantitatively evaluate the increase of the decomposition rate of n-hexane by means of Eq. (1), the cross-coupling steps occurring during the co-cracking of n-hexane and the Pcontaining compounds and the corresponding reaction rate coefficients need to be known. Therefore, the thermal cracking mechanism of n-hexane and the P-containing compounds are required. 3. Experimental The experiments were carried out in an electrobalance-microreactor unit. This unit has been described in detail previously by Reyniers and Froment [18]. The reactor is made of Incoloy 800 HT with a volume V = 5.23 cm3. Distilled water and nhexane are evaporated, mixed, and then preheated to about 873 K before entering the reactor. The preheated feed enters the reactor at high velocity through 24 narrow channels drilled at an angle of 158 with respect to the vertical, so that complete mixing can be achieved. The P-containing compounds were dissolved in the nhexane feed. Before the start of the cracking experiment, the nhexane feed and water were preheated, mixed and bypassed to a condenser, while nitrogen flowed through the reactor until conditions were stabilized. Then a sliding valve was set in such a position that the n-hexane-steam mixture was admitted to the reactor. After cracking the effluent was cooled, but not condensed, in a heat exchanger in which oil is circulated by an Ultratherm 330SCB, and led to a cyclone, in which tar was separated from the gas and in which nitrogen, the internal standard for the gas chromatographic analysis, was added. This gas stream was sent to on-line analysis. For the analysis of the cracking products, three GC’s are used. With a Packard 417 (GC1) equipped with a TCD-detector and a 2 m Carbosphere column (i.d. = 2 mm) H2, N2, CO and CH4 are analyzed. Hydrocarbons from C1 to C10 are analyzed using a Varian 3400 (GC2) equipped with a FID-detector and a 30 m DB-1-30 W capillary column (i.d. = 0.35 mm). Since propane and propylene cannot be properly separated using this column, another Packard 417 (GC3) equipped with a FID detector and a 6 m phenyliso-cyanate column (i.d. = 2 mm) is used to analyze C1 to C3 hydrocarbons.

Peak identification and integration was performed using XChrom provided by Labsystem. A precisely known amount of N2 is added to the effluent and is used as an internal standard for the analysis on GC1. The amount of methane determined on GC2 and GC3 is used to correlate the analysis of the three GC’s. The conversion and product yields can thus be calculated based on the absolute flow rate of the effluent components. Conversion of n-hexane is calculated according to Eq. (4): Xð%Þ ¼

F0
F  100 F0

(4)

where F 0 is the flow rate of n-hexane feed and F is the flow rate of n-hexane in the effluent. 4. Results and discussion 4.1. Experimental results Prior to the investigation of the influence of the Pcontaining additive on the thermal cracking of n-hexane, blank runs without the P-containing compound were first carried out to establish a reference. Then cracking runs in the presence of a P-containing compound were performed. All data reported in this study were reproduced in at least two repeat runs and the values presented in Table 1 are the mean values over the repeat runs. At 1123 K and d = 0.35 kg steam/kg n-hexane, the conversion of n-hexane is 42.9%; at 1173 K and d = 0.5 kg steam/kg n-hexane, the conversion of n-hexane is 47.8%. Taking the values of the blank runs as references, the relative conversion of n-hexane in the presence of the P-containing compounds is given in Table 1. At 1173 K, addition of 578 ppm HMPA (100 ppm P) results in an increase in the conversion of n-hexane by 14%. At 1123 K, addition of 500 ppm TPyPO (52 ppm P) results in an increase in the conversion of n-hexane by 46%. At 1173 K, addition of 967 ppm TPyPO (100 ppm P) results in an increase in the conversion of n-hexane by 19%. At 1123 K, addition of 500 ppm DOPP (74 ppm P) results in an increase in the conversion of n-hexane by 35%. At 1173 K addition of 500 ppm DOPP (74 ppm P) results in an increase in the conversion of n-hexane by 28%. It is evident that in the presence of these P-containing compounds the decomposition of n-hexane during steam cracking is accelerated. Table 1 Relative conversion of n-hexane in the presence of P-containing compounds Additive Concentration of additive (ppm) Concentration of additive (10
4 mol m
3) Temperature (K) Steam dilution (kg kg
1) Relative conversion Mass balance (%)

HMPA 578 8.58 1173 0.5 1.14 99.6

TPyPO

DOPP

500, 967

500, 500

8.58, 5.70

6.13, 8.12

1123, 1173 0.35, 0.5 1.46, 1.19 101.6, 97.8

1123, 1173 0.35, 0.5 1.35, 1.28 101.3, 98.3

J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

137

Table 2 The simplified thermal cracking mechanism of n-hexane and the estimated kinetic parameters ([DH] (kJ mol
1); [Ea] (kJ mol
1); [k] (s
1) for initiation and b-scission (m3 mol
1 s
1) for H-abstraction) No.

Initiation R1 R2 R3 H-abstraction R4

DH

Reaction

k1

n-C6 H14
!CH3  þ 1-C5 H11  k2



n-C6 H14
!C2 H5 þ 1-C4 H9



k3

n-C6 H14
!1-C3 H7  þ 1-C3 H7  k4

CH3  þ n-C6 H14
!CH4 þ sec-C6 H13 

R5

C2 H5 þ n-C6 H14
!C2 H6 þ sec-C6 H13 

R6

1-C3 H7 þ n-C6 H14
!C3 H8 þ sec-C6 H13 

b-Scission R7 R8

k5

k

k7

1-C6 H13 
!C2 H4 þ 1-C4 H9  k8

1-C4 H9 
!C2 H4 þ C2 H5 

R9

2-C6 H13 
!C3 H6 þ 1-C3 H7 

R10

1-C3 H7
!C2 H4 þ CH3 

k9

k10

Ea 1123 K

Log A

k 1123 K

1173 K

370.5

16.3

361.2

0.32

1.72

364.1

16.3

354.8

0.63

3.31

367.0

16.3

357.7

0.46

2.46


26.9

6.1

41.4

1.50  10 4

1.81  104

4

1.17  104


10.0

6.1

45.6

9.53  10


10.1

6.1

45.6

9.55  10 3

1.18  104

93.6

13.5

122.9

6.10  10 7

1.07  108

93.4

13.5

122.7

6.23  10 7

1.09  108

7

1.04  108

93.9

13.5

123.2

5.90  10

98.2

13.5

127.5

3.73  10 7

6.67  107

7

6.60  107 1.21  108

k11

98.3

13.5

127.6

3.69  10

k12

92.4

13.5

121.7

6.93  10 7

R11

3-C6 H13 
!1-C5 H10 þ CH3 

R12

3-C6 H13 
!1-C4 H8 þ C2 H5 

4.2. Thermal decomposition of n-hexane A simplified free radical mechanism for the thermal cracking of n-hexane proposed by Moens and Froment [19] is shown in Table 2. The initiation reactions occur mainly via the dissociation of the C–C bonds since the bond dissociation enthalpy (BDE) of the C–C bonds is lower than that of the C–H bonds. CH3, C2H5 and 1–C3H7 radicals can be produced in the initiation reactions. When considering the effect of the Pcontaining additive on the decomposition rate of n-hexane, only the fastest H-abstractions need to be taken into account. Obviously, H-abstraction by a radical from the secondary carbons of n-hexane is faster than that from the primary carbons. Therefore, in Table 2 only the H-abstraction reactions involving the secondary carbons are presented. Since isomerization reactions among 1-C6H13, 2-C6H13 and 3C6H13 radicals do not produce new type of C-centered radicals, there is no need to consider these reactions. The increased conversion of n-hexane due to the presence of a Pcontaining additive implies that all three P-containing compounds do not interfere with the termination reactions. Therefore, both the isomerizations and the termination reactions are not considered in the simplified thermal cracking mechanism of n-hexane. The reaction rate coefficients can be obtained by estimating the values of pre-exponential factors (A) and activation energies (Ea) in the Arrhenius equation. For the thermal cracking of nhexane, the reaction enthalpies DH can be estimated from the standard enthalpy of formation of the compounds and radicals involved in the free radical reactions. The required values are given in Table 3. The estimated values of A, Ea as well as k’s at 1123 and 1173 K for the reactions involved in the simplified

mechanism of thermal cracking of n-hexane are given in Table 2. For the initiation reaction, A generally has a value of 10161 s
1 according to Benson [20]. The reported values of A via the dissociation of the C–C bond in C2 to C6 n-paraffins are in the range of 1016.1 to 1017.2 s
1, most of them being 1016.3 s
1 [21]. Therefore, a value of 1016.3 s
1 is selected for the initiation reactions. The values of Ea for the initiation reactions can be estimated according to the following equation: Eforward  DH þ Ereverse
ðDnÞRT

(5)

where DH is equal to the BDE of the breaking bond. Ereverse is the activation energy of the corresponding radical recombination, Ereverse = 0; Dn is the net change of the number of moles between reactants and products, Dn = 1 [22]. Based on Eq. (5), Table 3 Standard enthalpies of formation of gas phase compounds and radicals involved in the simplified free radical mechanism for n-hexane cracking ([BDE] (kJ mol
1), T = 298.15 K) Compound

DHf

Reference

Radical

DHf

Reference

H2 C2H4 C3H6 1-C4H8 n-C4H10 1-C5H10 1-C5H12 n-C6H14 C2H6 C3H8

0 52.5 20.4
0.1
126.1
20.9
146.4
167.2
84.7
103.8

[24] [24] [24] [24] [24] [24] [24] [24] [24] [24]

H CH3 C2H5 1-C3H7 1-C4H9 1-C5H11 1-C6H13 2-C6H13 3-C6H13 C6H5

218.0 145.7 119.0 100.0 78.1a 57.8a 37.0a 26.5a 26.5a 328.9

[24] [24] [25] [25] [22] [22] [22] [22] [22] [22]

a Values calculated based on BDE H–Cprimary = 422.2 kJ mol
1 and BDE H– Csecondary = 411.7 kJ mol
1.

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J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

the activation energy for the dissociation of n-hexane producing CH3 radicals (reaction R1) amounts to 361 kJ mol
1, while the formation of C2H5 radicals (reaction R2) and 1-C3H7 radicals (reaction R3) require an activation energy of 355 and 358 kJ mol
1, respectively. At temperatures between 650 and 840 K, the reported Ea is 357.0 kJ mol
1 for reaction R1 and 344.5 kJ mol
1 for reactions R2 and R3 [21]. At temperatures lower than 723 K, the reported Ea is 363.6 kJ mol
1 for R1, 357.3 kJ mol
1 for R2 and 366.8 kJ mol
1 for R3 [26]. The values of Ea calculated according to Eq. (5) are quite reasonable as compared to the reported values. From Table 2 it can be seen that in the temperature range from 1123 to 1173 K, the dissociation of n-hexane producing ethyl and n-butylradicals (reaction R2) has the highest rate coefficient. H-abstraction is a bimolecular process, requiring collision of two reaction partners, and is thus characterized by a negative DS#. Hence, the value of A is notably lower as compared to that of initiation reactions. However, the values of A for Habstraction reactions are not very sensitive to structure and are constrained to a rather narrow range, typically in the range of 105 to 106 m3 mol
1 s
1 per hydrogen. An exception is Habstraction by a H atom. The A-value is 100 times higher than that of H-abstraction by a polyatomic radical [22]. Here, a value of AH = 105.5 m3 mol
1 s
1 per hydrogen is selected for the H-abstraction reactions involving a polyatomic radical and a value of AH = 107.5 m3 mol
1 s
1 per hydrogen is selected for the H-abstractions involving a H atom. According to Poutsma [22] the values of A for H-abstractions can be calculated using Eq. (6): log A ¼ log AH þ logðnH Þ

(6)

where nH is the number of equivalent hydrogen atoms. For the estimation of the activation energies of H-abstraction reactions, the Evans–Polanyi relation, Ea = E0 + aDH with 0 < a < 1, is usually used. Here, an empirical equation proposed by Laidler [23] for a wide variety of exothermic transfer reactions is adopted: Ea ¼ 48:1 þ 0:25DH ðkJ mol
1 Þ

(7)

where DH is the reaction enthalpy. According to Laidler [23], the activation energies for endothermic transfer reactions can best be calculated based on the reaction enthalpy of the reverse exothermic reaction leading to Eq. (8): Ea;endo ¼ 48:1 þ 0:75DH ðkJ mol
1 Þ

(8)

With these equations the only parameter required to obtain Ea for a H-abstraction reaction is the reaction enthalpy, DH. According to Eqs. (7) and (8), the estimated Ea for Habstraction from n-hexane by a methyl radical (reaction R4) is 41.4 kJ mol
1 and amounts to 45.6 kJ mol
1 for abstraction by an ethyl (reaction R5) or a n-propyl radical (reaction R6). These values agree very well with the values of 39.7 kJ mol
1 for R4, 43.5 kJ mol
1 for R5 and R6 as reported by Ebert et al. [21]. From Table 2 it can be seen that in the temperature range from 1123 to 1173 K, the H-abstraction by methyl radicals (reaction R4) has the highest rate coefficient indicating that methyl

radicals are potentially more reactive in H-abstraction reactions than ethyl and n-propyl radicals. For the b-scission reactions, A has a value of 1013 to 14
1 10 s . The activation energy is generally 25.1– 33.4 kJ mol
1 higher than the reaction enthalpy [20,22]. Here a value of 1013.5 s
1 is selected for A and 29.3 kJ mol
1 is selected for the estimation of the activation energy. Thus, the activation energy for a b-scission reaction can be estimated by Eq. (9): Ea ¼ DH þ 29:3 ðkJ mol
1 Þ

(9)

The rate coefficients listed in Table 2 indicate that the initiation reaction R2 in which an ethyl radical and a 1-butyl radical are formed is the fastest initiation reaction; abstraction of a H atom by CH3 from the secondary carbons of n-hexane is the fastest H-abstraction reaction. The concentration of nhexane in our experiments is less than 2 mol m
3. According to the rate coefficients in Table 2, the values of b-scission are greater than the values obtained by multiplying the Habstraction rate coefficients with the concentration of nhexane, implying that b-scission is potentially faster than Habstraction. As mentioned before, in these conditions the influence of the additive on the decomposition rate of n-hexane can be evaluated using Eq. (1). 4.3. The influence of P-containing additives on the decomposition of n-hexane In order to determine the influence of a P-containing additive on the decomposition rate of n-hexane, the mechanism and the kinetic parameters of the thermal cracking of the three P-containing compounds needs to be known. However, information concerning the thermal decomposition of the three P-containing additives used in this study is scarce. All three P-containing compounds accelerate the decomposition of n-hexane and thus lead to a higher conversion nhexane during steam cracking. In order to determine which factor is critical in determining the magnitude of the accelerating effect, a sensitivity analysis of the value of E1 to the parameters, u1, l1, r2, involved in Eq. (1) has been carried out. In doing the analysis, the value of S2, the molar ratio of the additives to n-hexane, was set to the value used in this study, i.e. 2.8  10
4. The value of r2 was varied from 1 to 5000 and the values of u1 and l1 were varied from 1.0  10
3 to 1.0  103. The analysis results indicate that when u1  1, variation of l1 from 1.0  10
3 to 1.0  102 does not result in a significant change of E1. The curves of E1 as a function of l1 for u1 = 100, 1000, 2500 and 5000 and r2 = 1000 and 5000 are shown in Fig. 3. It can be seen that at a given concentration of the additive, the magnitude of E1 depends mainly on r2 for values of l1 < 0.1 and 100 < u1 < 5000. Provided that the radicals derived from the additive are 100–5000 times more reactive in H-abstraction from nhexane (k21  100–5000k11) and at least 10 times more reactive than the methyl radical for H-abstraction from the

J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

139

difference of the BDE of the breaking bond and that of the forming bond. The values of the BDE’s are presented in Table 4. Also, the initiation reaction of a P-containing compound can produce a P-centered radical, P O. This radical may undergo isomerization, forming an O-centered radical PO [32,33]. This reaction is similar to the cis-trans isomerization of olefins since both of their transition state involves the breaking of a p-bond. For this kind of reactions, the pre-exponential factor A is 1013 s
1; the activation energy is approximately equal to the BDE of a p-bond [20]. 4.4. The influence of HMPA on the decomposition of nhexane Fig. 3. Variation of E1 as a function of l1. Solid lines and solid markers, r2 = 1000; dashed lines and hollow markers, r2 = 5000. Diamonds, u1 = 100; squares, u1 = 1000; triangles, u1 = 2500; circles, u1 = 5000.

additive (k12 < 0.1k21), the ratio of the initiation rate coefficients is the most critical factor. To quantitatively evaluate the accelerating effect of the Pcontaining additive on the decomposition of n-hexane a tentative mechanism for the thermal decomposition of the three P-containing compounds is constructed. As E1 is most sensitive to the rate coefficients of the initiation steps, the rate coefficients of the elementary steps involved in the thermal cracking of the P-compounds are estimated using the same method as that for n-hexane cracking. Since most of the standard enthalpies of formation of the P-containing compounds are not available in literature, the reaction enthalpies for the H-abstraction and b-scission are estimated from the

Table 4 Bond dissociation enthalpies (kJ mol
1) Bond

BDE

Reference

H–H CH3–H Cprim–H Csec–H NCH2–H (N, C, H)C–Ha (2C, H)C–Ha (2C)N–H (P)O–H P–H HO–H C6H5–H (C, 2H)C–C(C, 2H) (N, 2H)C–C(C, 2H) P–N P–C (C, H)N–C(C, 2H) C C (p bond) P O (p bond) P N (p bond) C N (p bond) P O (double bond)

435.9 438.9 422.2 411.7 388.7 385 (a) 399 (b, g) 382.5 460.0 321.9 497.4 464.0 367.8 342.8 288.4 286.3 342.8 271.7 221.5 183.9 263.3 535

[22] [22] [22] [22] [22] [27] [27] [22] [28] [29] [22] [22] [22] [22] [29] [30] [22] [31] [31] [31] [31] [29]

a

BDE’s in piperidine (cyclic C5H11N) [27].

The molecular structure of HMPA is shown in Fig. 4. The elementary free radical reactions potentially involved in the thermal cracking of HMPA are schematically presented in Fig. 4 and the corresponding kinetic parameters are given in Table 5. In HMPA the P–N bond is the weakest one. At elevated temperature the scission of the P–N bond should be easier than the other bonds. Therefore, the initiation of the decomposition of HMPA can be assumed to proceed via the scission of the P–N bond (R13), producing a (CH3)2N radical and a ((CH3)2N)2P(O) radical. The (CH3)2N may abstract a H atom from HMPA according to R14 or may undergo b-scission according to R19. The ((CH3)2N)2P(O) may abstract a H atom from HMPA according to R15, it may isomerize according to R18 or it can undergo b-scission according to R20. When the concentration of HMPA subjected to cracking is low, k19 > k14[HMPA] and k20 > k15[HMPA], implying that the b-scission rate of (CH3)2N and ((CH3)2N)2P(O) radicals is faster than that of H-abstraction. In addition, the b-scission rate coefficient of the ((CH3)2N)2P(O) radical is also 100 times higher than that of the competing isomerization reaction. Therefore, it can be concluded that the main route for the disappearance of (CH3)2N and ((CH3)2N)2P(O) radicals is b-scission. The b-scission rate coefficient of the (CH3)2N radical is approximately 30 times higher than that of the ((CH3)2N)2P(O) radical, indicating that the (CH3)2N radical is more reactive than the ((CH3)2N)2P(O) radical in the chain propagation process during the thermal cracking of HMPA. b-Scission of the (CH3)2N radical produces CH3N CH2 and a H atom. The latter is more reactive than the (CH3)2N radical in H-abstraction reactions. The ratio of the Habstraction rate from HMPA by a H atom (r17) to that by (CH3)2N radical (r14) can be expressed as r17 k17 ½H ¼ r14 k14 ½ðCH3 Þ2 N

(10)

Application of the steady-state approximation to determine the H atom concentration, leads to r17 k19 ¼ r14 k14 ½HMPA

(11)

140

J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

Fig. 4. Schematic overview of the elementary free radical reactions involved in the thermal cracking of HMPA.

At 1173 K, r14/r17 = 146/[HMPA], indicating that if the concentration of HMPA is in the moderate range, H-abstraction from HMPA by a H atom is potentially faster than that by a (CH3)2N radical. The ((CH3)2N)2P(O)N(CH3)CH2 radical formed by Habstraction may undergo b-scission according to R22 and R23.

Compared to R22, R23 can be ignored since its rate coefficient is 260 times lower than that of R22. According to the rate coefficients listed in Table 5, when the concentration of HMPA subjected to cracking is not high, the b-scission rate of the ((CH3)2N)2P(O)N(CH3)CH2 radical in R22 is potentially higher than the H-abstraction rate of R14 to R17, implying that

J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

141

Table 5 Elementary free radical reactions potentially involved in the thermal cracking of HMPA and the estimated kinetic parameters ([DH] (kJ mol
1); [Ea] (kJ mol
1); [k] (s
1) for initiation, b-scission and isomerization (m3 mol
1 s
1) for H-abstraction) No.

DH

Log A

Ea

k (1173 K)

295.3

16.3

285.5

3.84  10 3

6.3

6.8

52.8

2.81  10 4

66.9

6.8

98.3

2.66  10 2


50.16

6.8

35.56

1.65  10 5


46.8

6.8

36.4

1.51  10 7

221.5

13.0

221.5

1.40  10 3

125.4

13.5

154.7

4.08  10 6

158.8

13.5

188.1

1.32  10 5

66.9

13.5

96.2

1.65  10 9

k22

25.1

13.5

54.4

1.20  10 11

k23

79.4

13.5

108.7

4.55  10 8

Reaction

Initiation R13 H-abstraction R14

k13

ððCH3 Þ2 NÞ3 PðOÞ
!ððCH3 Þ2 NÞ2 PðOÞ þ ðCH3 Þ2 N k14

ðCH3 Þ2 N þ ððCH3 Þ2 NÞ3 PðOÞ
!ððCH3 Þ2 NÞ2 PðOÞNðCH3 ÞCH2  þ ðCH3 Þ2 NH

R15

ððCH3 Þ2 NÞ2 PðOÞ þ ððCH3 Þ2 NÞ3 PðOÞ
!ððCH3 Þ2 NÞ2 PðOÞNðCH3 ÞCH2  þ ððCH3 Þ2 NÞPðOÞ
H

R16

CH3  þ ððCH3 Þ2 NÞ3 PðOÞ
!ððCH3 Þ2 NÞ2 PðOÞNðCH3 ÞCH2  þ CH4

R17

H ððCH3 Þ2 NÞ3 PðOÞ
!ððCH3 Þ2 NÞ2 PðOÞNðCH3 ÞCH2  þ H2

Isomerization R18 b-Scission R19

k15

k16

k17

k18

ððCH3 Þ2 NÞ2 PðOÞ
!ððCH3 Þ2 NÞ2 PO k19

ðCH3 Þ2 N
!CH3 N¼CH2 þ H

R20

ððCH3 Þ2 NÞ2 PðOÞ
!ðCH3 Þ2 NPðOÞ¼NCH3 þ CH3 

R21

ððCH3 Þ2 NÞ2 PO
!ðCH3 Þ2 NPðOÞ þ ðCH3 Þ2 N

R22

ððCH3 Þ2 NÞ2 PðOÞNðCH3 ÞCH2 
!ððCH3 Þ2 NÞ2 PðOÞ þ CH3 N¼CH2

R23

ððCH3 Þ2 NÞ2 PðOÞNðCH3 ÞCH2 
!ððCH3 Þ2 NÞ2 PðOÞN¼CH2  þ CH3 

k20

k21

the assumption that b-scission is potentially faster than hydrogen abstraction is justified for the thermal decomposition of HMPA. Based on the above discussions, the predominant thermal decomposition route of HMPA can be depicted as shown in Fig. 4 (bold reaction arrows). At 1173 K, the initiation rate coefficient of HMPA is about 1000 times higher than the fastest initiation rate coefficient of nhexane, i.e. r2 = 1160. Therefore, when HMPA is subjected to thermal cracking together with n-hexane, it will play the role of initiator. Initiation of HMPA produces a (CH3)2N radical and a

((CH3)2N)2P(O) radical. As discussed above, the (CH3)2N radical is more reactive than the ((CH3)2N)2P(O) radical and undergoes b-scission reaction, producing a H atom. This H atom then abstracts a H atom from n-hexane initiating the chain propagation steps for the decomposition of n-hexane (R25). The H atom is about 100 times more active in H-abstraction from n-hexane than the CH3 radical, i.e. u1 = 94.4, leading to an increased rate of decomposition of n-hexane. The predominant coupling reactions between n-hexane and HMPA cracking are R24 and R25 as shown in Table 6. In R24 a CH3

Table 6 Predominant coupling reactions between the cracking of n-hexane and the P-containing compounds and the estimated kinetic parameters ([DH] (kJ mol
1); [Ea] (kJ mol
1); [k] (m3 mol
1 s
1)) No.

HMPA R24 R25 TPyPO R44 R45 DOPP R58 R59a a

DH

Reaction

k24

CH3  þ ððCH3 Þ2 NÞ3 PðOÞ
!CH4 þ  CH2 NðCH3 ÞððCH3 Þ2 NÞ2 PðOÞ k25

H þ n-C6 H14
!H2 þ sec-C6 H13  k44

CH3  þ ðC5 H10 NÞ3 PðOÞ
!CH4 þ  C5 H9 NPðOÞðC5 H10 ÞN2 k45

H þ n-C6 H14
!H2 þ sec-C6 H13  k58

CH3  þ C6 H5 PðOÞðOHÞ2
!CH4 þ  OPðOÞðOHÞC6 H5 k59

HO þ n-C6 H14
!H2 O þ sec-C6 H13 

Values of Ea and A are taken from Ref. [38].

Log A

Ea

k 1123 K

1173 K


50.2

6.8

35.6

1.40  10 5

1.65  105


24.3

8.1

42.0

1.40  10 6

1.69  106


50.2

6.6

35.6

8.83  10 4

1.04  105


24.3

8.1

42.0

1.40  10 6

1.69  106

21.1

5.8

64

6.6  10 2

8.9  102

7.9

15.8

1.78  10 7

1.92  107

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J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

Table 7 Estimated accelerating effect of the P-containing compounds on the thermal cracking of n-hexane HMPA (1173 K) S2 = HMPA/n-hexane (10
4) r2 = k13/k2 u1 = k25/k4 l1 = k24/k25 E1estimated E1measured

2.8 1160 94.4 0.094 1.15 1.14

TPyPO (1123 K) S2 = TPyPO/n-hexane (10
4) r2 = k26/k2 u1 = k45/k4 l1 = k44/k45 E1estimated E1measured

1.4 1581 93.3 0.063 1.11 1.46

TPyPO (1173 K) S2 = TPyPO/n-hexane (10
4) r2 = k26/k2 u1 = k45/k4 l1 = k44/k45 E1estimated E1measured

2.8 1160 94.4 0.059 1.15 1.19

DOPP (1123 K) S2 = DOPP/n-hexane (10
4) r2 = k47/k2 u1 = k59/k4 l1 = k58/k59 (10
5) E1estimated E1measured

2.0 4294 1187 3.7 1.36 1.35

DOPP (1173 K) S2 = DOPP/n-hexane (10
4) r2 = k47/k2 u1 = k59/k4 l1 = k58/k59 (10
5) E1estimated E1measured

2.0 3006 1061 5 1.27 1.28

radical abstracts a H atom from HMPA and in R25 a H atom abstracts a H from n-hexane. The estimated reaction rate coefficients for these reactions are presented in Table 6. The rate coefficient for H-abstraction from n-hexane by a H atom is about 10 times higher than the rate coefficient for H-abstraction from HMPA by the CH3 radical, i.e. l1 = 0.094 and the accelerated decomposition of n-hexane directed by u1 is thus somewhat attenuated. Based on this tentative reaction scheme, it can be concluded that the parameter controlling the increased decomposition of n-hexane is the ratio of the initiation rate coefficients r2. The values of S2, r2, u1, l1, and E1 are given in Table 7. For HMPA the estimated E1 value by means of Eq. (1) is 1.15 which is consistent with the experimentally measured value, E1 = 1.14. 4.5. The influence of TPyPO on the decomposition of nhexane The molecular structure of TPyPO is shown in Fig. 5. To the best of our knowledge, the thermal elimination of phosphoric amides derived from secondary amines containing b H-atoms

has not been reported in literature. Therefore, non-radical elimination of TPyPO was not considered. There are two types of H atoms in TPyPO, those connected with carbon atoms which are in a-position to the nitrogen atom and those connected with the carbon atoms which are in b- and gpositions to the nitrogen atom. The BDE of a C–H bond in aposition is 385 kJ mol
1 which is 14 kJ mol
1 lower than that of the C–H bond in b- and g-positions [27]. Therefore, H atoms connected with the a-carbons are more reactive than the others in H-abstraction reactions. In considering the H-abstraction reactions involving TPyPO, only those hydrogen atoms connected with a-carbons are taken into account. The possible elementary reactions involved in the thermal decomposition of TPyPO are schematically presented in Fig. 5 and the estimated reaction rate coefficients at 1123 and 1173 K are given in Table 8. As in HMPA, the P–N bond is also the weakest one in TPyPO. The initiation reaction of TPyPO can be assumed to proceed according to reaction R26, producing a C5H10N radical and a (C5H10N)2P(O) radical. The C5H10N radical may abstract a H from TPyPO according to R27 or may undergo b-scission according to R35 and R36. The (C5H10N)2P(O) radical may abstract a H from TPyPO according to R32, may isomerize according to R34 or may undergo b-scission reaction according to R40. The rate coefficients in Table 8 indicate that when the concentration of TPyPO subjected to cracking is low, the b-scission rate of the C5H10N radical and the (C5H10N)2P(O) radical is potentially faster than that of the H-abstraction. For the C5H10N radical, the b-scission rate coefficient k36 is 100 times higher than k35. For the (C5H10N)2P(O) radical, the b-scission rate coefficient k40 is about 100 times higher than the isomerization rate coefficient k34. Therefore, it can be concluded that the predominant reaction of C5H10N radical is b-scission R36; the predominant reaction of (C5H10N)2P(O) is b-scission R40. The b-scission rate coefficient of the C5H10N radical according to R36 is approximately 3.5  103 times higher than that of the (C5H10N)2P(O) radical according to R40, indicating that the C5H10N radical is more reactive than the (C5H10N)2P(O) radical in the chain propagation process. The b-scission of a C5H10N radical produces a CH2 N(CH2)3CH2 radical. This radical is able to undergo successive b-scissions according to reaction R37–R39. When the concentration of TPyPO subjected to cracking is not high, the b-scission rate of R37–R39 is potentially faster that the H-abstraction rate of R28– R30, respectively. It is reasonable to assume that the CH2 N(CH2)3CH2 radical undergoes successive b-scissions R37–R39 until a H atom and hydrogen cyanide are formed. Using the steady-state approximation to CH2 N(CH2)3CH2, CH2 NCH2CH2, CH2 N, and H, the ratio of H-abstraction rate by a H atom to that by a C5H10N radical from TPyPO can be expressed as r31 k35 þ k36 k36   r27 k27 ½TPyPO k27 ½TPyPO

(12)

At 1123 K, r31/r27 = 1.99  104/[TPyPO]; at 1173 K, r31/ r27 = 2.57  104/[TPyPO]. This means that when the concen-

J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

143

Fig. 5. Schematic overview of the elementary free radical reactions involved in the thermal cracking of TPyPO.

tration of TPyPO subjected to cracking is low, H-abstraction by a H atom from TPyPO is faster than that by the C5H10N radical in the chain propagation process. The (C5H10N)2P(O)NC5H9 radical formed from TPyPO through H-abstraction may undergo b-scissions according to R41–R43. Comparing the values of the rate coefficients R41– R43, it can be seen that R41 is the predominant route for the decomposition of the (C5H10N)2P(O)NC5H9 radical . The bscission rate of the (C5H10N)2P(O)NC5H9 radical, R41, is potentially faster than the H-abstraction rate of R27 to R33 when the concentration of TPyPO subjected to cracking is not high. The assumption that b-scission occurs potentially faster than H-abstraction is justified for the thermal cracking of TPyPO. Based on the above discussions, the predominant route for the thermal cracking of TPyPO can be depicted as shown in Fig. 5 (bold reaction arrows). At 1123–1173 K, the initiation rate coefficient of TPyPO is more than 1000 times higher than that of n-hexane, i.e. r2 = 1581 at 1123 K and 1160 at 1173 K. Therefore, when TPyPO is subjected to thermal cracking with n-hexane, it will

act as an initiator. Initiation reaction of TPyPO produces a C5H10N radical and a (C5H10N)2P(O) radical. As discussed above, the C5H10N radical is more reactive than the (C5H10N)2P(O) radical and thus plays a predominant role in the chain propagation process. Once produced, the C5H10N radical undergoes b-scission first, producing a H atom. In turn, this H atom abstracts a H atom from n-hexane, initiating the chain propagation steps for the decomposition of n-hexane (R45). The H atom is about 100 times more active in Habstraction from n-hexane than the CH3 radical, i.e. u1 = 93.3 at 1123 K and 94.4 at 1173 K, leading to an increased rate of decomposition of n-hexane. The predominant coupling reactions between n-hexane and TPyPO are R44 and R45 as shown in Table 6. In R44 a CH3 radical formed by n-hexane abstracts a H atom from TPyPO and in R45 a H atom formed from TPyPO abstracts a H atom from n-hexane. The rate coefficient for H-abstraction from n-hexane by a H atom is about 15 times higher than the rate coefficient for H-abstraction from TPyPO by the CH3 radical, i.e. l1 = 0.063 at 1123 K and 0.059 at 1173 K and the accelerated decomposition of n-hexane directed

144

J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

Table 8 Elementary free radical reactions potentially involved in the thermal cracking of TPyPO and the estimated kinetic parameters ([DH] (kJ mol
1); [Ea] (kJ mol
1); [k] (s
1) for initiation, b-scission and isomerization (m3 mol
1 s
1) for H-abstraction) No.

DH

Reaction

Initiation R26

1123 K

1173 K

286.0

9.96  102

3.84  103

6.3

6.6

52.8

1.39  104

1.77  104


33.4

6.6

39.7

5.64  104

6.77  104


33.4

6.6

39.7

5.64  104

6.77  104

6.3

6.6

52.8

1.39  104

1.77  104


46.8

8.6

36.4

8.07  106

9.53  106

66.9

6.6

98.3

1.07  102

1.68  102


58.5

6.6

33.5

1.10  105

1.29  105

221.5

13.0

221.5

4.98  102

1.37  103

k35

125.4

13.5

154.7

2.01  106

4.08  106

k36

79.4

13.5

108.7

2.77  108

4.55  108

7

8.20  107

k27

C5 H10 N þ ðC5 H10 NÞ3 PO
!ðC5 H10 NÞ2 PðOÞNC5 H9  þ HNC5 H10

R28

CH2 ¼NðCH2 Þ3 CH2  þ ðC5 H10 NÞ3 PO
!ðC5 H10 NÞ2 PðOÞNC5 H9  þ CH2 ¼NðCH2 Þ3 CH3

R29

CH2 ¼NCH2 CH2  þ ðC5 H10 NÞ3 PðOÞ
!CH2 ¼N
CH2 CH3 þ ðC5 H10 NÞ2 PðOÞNC5 H9

R30

CH2 ¼N þ ðC5 H10 NÞ3 PO
!ðC5 H10 NÞ2 PðOÞNC5 H9  þ CH2 ¼NH

R31

H þ ðC5 H10 NÞ3 PO
!ðC5 H10 NÞ2 PðOÞNC5 H9  þ H2

R32

ðC5 H10 NÞ2 PðOÞ þ ðC5 H10 NÞ3 PO
!ðC5 H10 NÞ2 PðOÞNC5 H9  þ ðC5 H10 NÞ2 PðOÞH

R33

ðC5 H10 NÞ2 PO þ ðC5 H10 NÞ3 PO
!ðC5 H10 NÞ2 PðOÞNC5 H9  þ ðC5 H10 NÞ2 POH

R36

k

16.3

H-abstraction R27

b-Scission R35

Ea

295.3

k26

ðC5 H10 NÞ3 PO
!ðC5 H10 NÞ2 PðOÞ þ C5 H10 N

k28

k29

k30

k31

k32

Isomerization R34

Log A

k33

k34

ðC5 H10 NÞ2 PðOÞ
!ðC5 H10 NÞ2 PO C5 H10 N
!C5 H9 N þ H C5 H10 N
!CH2 ¼NðCH2 Þ3 CH2  k37

96.1

13.5

125.4

4.63  10

k38

71.1

13.5

100.4

6.79  108

1.07  109

6

4.08  106

R37

CH2 ¼NðCH2 Þ3 CH2 
!CH2 ¼N
CH2 CH2  þ C2 H4

R38

CH2 ¼N
CH2 CH2 
!CH2 ¼N þ C2 H4

R39

CH2 ¼N
!HCN þ H

R40

ðC5 H10 NÞ2 PðOÞ
!C5 H10 NPðOÞ¼NðCH2 Þ4 CH2 

R41

ðC5 H10 NÞ2 PðOÞNC5 H9 
!ðC5 H10 NÞ2 PðOÞ þ C5 H9 N

k39

k40

k41

125.4

13.5

154.7

2.01  10

158.8

13.5

188.1

5.61  104

1.32  105

25.1

13.5

54.4

9.34  1010

1.20  1011

8

1.07  109 8.20  107

k42

71.1

13.5

100.4

6.79  10

k43

96.1

13.5

125.4

4.63  107

R42

ðC5 H10 NÞ2 PðOÞNC5 H9 
!ðC5 H10 NÞ2 PðOÞN¼CHðCH2 Þ3 CH2 

R43

ðC5 H10 NÞ2 PðOÞNC5 H9 
!ðC5 H10 NÞ2 PðOÞNðCH¼CH2 ÞðCH2 Þ2 CH2 

by u1 is thus somewhat attenuated. The estimated reaction rate coefficients for these coupling reactions are presented in Table 6. Based on this tentative reaction scheme, it can be concluded that the parameter controlling the increased decomposition of n-hexane is the ratio of the initiation rate coefficients r2. The values of S2, r2, u1, l1, and E1 are given in Table 7. The estimated E1 value at 1123 K for n-hexane cracking with addition of 500 ppm TPyPO amounts to 1.11, the measured value of E1 is 1.46. At 1173 K with 967 ppm TPyPO the estimated E1 value is 1.15, which is consistent with the experimentally measured value, E1 = 1.19. From Eq. (1), it can be seen that increasing the concentration of P-containing compounds, i.e. increasing S2, will increase E1. On the other hand, increasing temperature results in a reduction of r2 and consequently E1 is decreased. For TPyPO, increasing temperature from 1123 to 1173 K leads to a reduction of r2 from 1581 to 1160. In contrast, u1 and l1 are not very sensitive to the change in temperature as indicated by the values in Table 7. This

can be attributed to the small difference in activation energies between the H-abstractions involved in the main coupling reactions between n-hexane and TPyPO (see Table 6). 4.6. The influence of DOPP on the decomposition of nhexane The pyrolysis of diethyl methylphosphonate (DEMP) and diisopropyl methylphosphonate (DIMP) has been investigated by Zegers and Fisher [34,35]. At 802–907 K the decomposition products of DEMP include ethylene, ethanol, ethyl methylphosphonate, and methylphosphonic acid (MPA). At 700–800 K the decomposition products of DIMP include propylene, isopropanol, isopropyl methyl phosphonate, and MPA. Thermal decomposition of DEMP and DIMP was assumed to proceed by two stages [34,35]. The first stage corresponds to the unimolecular decomposition of the parent compound into an olefin and a mono-alkyl methylpho-

J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

145

Table 9 Elementary free radical reactions involved in the thermal cracking of PPOA and the estimated kinetic parameters ([DH] (kJ mol
1); [Ea] (kJ mol
1); [k] (s
1) for initiation, b-scission and isomerization (m3 mol
1 s
1) for H-abstraction) No.

Decomposition R46 Initiation R47 H-abstraction R48

DH

Reaction

k47

ðHOÞ2 PðOÞC6 H5
!ðHOÞ2 PðOÞ þ C6 H5  k48

C6 H5  þ ðHOÞ2 PðOÞC6 H5
!C6 H5 PðOÞðOHÞO þ C6 H6

R50

H þ ðHOÞ2 PðOÞC6 H5
!C6 H5 PðOÞðOHÞO þ H2

R51

HO þ ðHOÞ2 PðOÞC6 H5
!C6 H5 PðOÞðOHÞO þ H2 O

b-Scission R54

k49

k50

k51

1123 K

1173 K

286.0

16.3

276.7

2.70  103

9.95  10 3


4.0

5.8

47.1

4.06  103

5.04  10 3

125.4

5.8

142.2

1.54  10
1

2.95  10
1

24.1

7.8

66.2

5.26  103

7.11 10 3


33.4

5.8

39.8

8.9  103

1.07  10 4 3

4.55  10 3

0

5.8

48.1

3.65  10

k53

221.5

13.0

221.5

4.95  102

1.36  10 3

k54

225.7

13.5

255.0

4.34  101

1.39  10 2

121.2

13.5

150.5

3.15  106

6.27  10 6

64.8

13.5

94.1

1.33  109

2.04  10 9

k52

ðHOÞ2 PO þ ðHOÞ2 PðOÞC6 H5
!PðOHÞ3 þ C6 H5 PðOÞðOHÞO ðHOÞ2 PðOÞ
!ðHOÞ2 PO ðHOÞ2 PðOÞ
!HOPðOÞ2 þ H

R55

ðHOÞ2 PO
!HOPðOÞ þ HO

R56

C6 H5 PðOÞðOHÞO
!HOPðO2 Þ þ C6 H5 

R57

k

k46

ðHOÞ2 PðOÞ þ ðHOÞ2 PðOÞC6 H5
!C6 H5 PðOÞðOHÞO þ ðHOÞ2 PðOÞH

Isomerization R53

Ea

ðC8 H17 OÞ2 PðOÞC5 H5
!2C8 H16 þ ðHOÞ2 PðOÞC6 H5

R49

R52

Log A

k55

k56 k57

C6 H5 PðOÞðOHÞO
!C6 H5 PðOÞ2 þ HO

sphonate via a six-member ring transition state. The second stage involves two competing pathways for the unimolecular decomposition of mono-alkyl methylphosphonate. The first pathway leads to an alcohol and methyl dioxophosphorane; the second pathway leads to an olefin and MPA. Kim et al. [36] used phosphonic acid esters as thermally latent initiators for the polymerization of glycidyl phenyl ether. They found that di-1-phenylethyl phenylphosphonate di-tertiary butyl phenylphosphonate and di-cyclohexyl phenylphosphonate decompose into an olefin and phenylphosphonic acid (PPOA) upon heating to 423–443 K. Since DOPP has the same molecular structure as the above mentioned phenyl phosphonate esters, it seems reasonable to assume that DOPP first decomposes to PPOA and octene according to reaction R46 as shown in Table 9. This reaction may already occur before DOPP enters the reactor since the temperature in the preheating section is about 880–823 K if the cracking temperature in the reactor is 1123–1173 K. Therefore, the species that actually influences the cracking of n-hexane is most likely PPOA. The molecular structure of PPOA is shown in Fig. 6. The values of the BDE’s in PPOA are given in Table 4. The weakest linkage in PPOA is the C–P bond for which the BDE amounts to 286 kJ mol
1 as reported by Smith and Patrick [30]. De Lijser et al. [37] investigated the pyrolysis of methyl dimethyl phosphonate in the presence of hydrogen. It was found that 40% of the phosphorus fed to the reactor was in the form of

160.0

13.5

192.3

3.6  10

4

8.6  10 4

phosphorus acid. This also supports the assumption that the C– P bond in PPOA is the weakest one. In PPOA there are two types of hydrogen atoms, those connected with the carbon atoms in the phenyl group and those connected with oxygen atoms. The BDE of C–H in benzene, 464 kJ mol
1 [22], can be taken as a reference for the C–H bonds in the phenyl group. H-abstraction from the phenyl group results in the formation of a relative unstable vinyl type of radical. H-abstraction from the PO–H bond would result in a resonance stabilized O-centered radical. Therefore, it seems reasonable to assume that the H atoms connected with oxygen are more active in H-abstraction reactions than those connected to the carbon atoms in the phenyl group. Based on theoretical calculations, Leroy et al. [28] proposed a BDE of 460 kJ mol
1 for the PO–H bond. The free radical reactions potentially involved in the thermal decomposition of PPOA are schematically presented in Fig. 6 and the estimated reaction rate coefficients at 1123 and 1173 K are given in Table 9. The initiation reaction of PPOA can be assumed to proceed according to reaction R47, producing a phenyl radical and a (HO)2P(O) radical. The phenyl radical can abstract a H atom from PPOA according to R48. The (HO)2P(O) radical may abstract a H atom from PPOA according to R49, may isomerize according to R53 or undergo b-scission according to R54. When the concentration of DOPP subjected to cracking is low, the isomerization rate of R53 and the b-scission rate of R54 of the

146

J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

Fig. 6. Schematic overview of the elementary free radical reactions involved in the thermal cracking of DOPP.

(HO)2P(O) radical is potentially faster than the Habstraction rate of R49. In addition, the rate coefficient of isomerization R53 is 10 times higher than that of the bscission R54. This means that the predominant reaction for the disappearance of the (HO)2P(O) radical is the isomerization reaction R53. Isomerization of (HO)2P(O) radical according to R53 produces an oxygen-centered radical (HO)2PO. This radical

can abstract hydrogen from PPOA according to R52 or undergo b-scission according to R55. When the concentration of PPOA subjected to cracking is low, the b-scission rate of (HO)2PO radical is potentially faster than the H-abstraction rate of reaction R52. Therefore, the main route for the disappearance of (HO)2PO radical is the b-scission R55. The isomerization rate of the (HO)2P(O) radical is also potentially faster that the H-abstraction rate of R48 in which a

J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

phenyl radical abstracts a hydrogen atom from PPOA if the concentration of DOPP subjected to cracking is low. The (HO)2PO radical formed from the isomerization of the (HO)2P(O) radical undergoes a fast b-scission, producing HOP(O) and HO. The HO radical is more reactive in the Habstraction R51 than the phenyl radical in the H-abstraction R48. Therefore, it is reasonable to assume that the HO radical plays a predominant role in the chain propagation process in the thermal cracking of PPOA. The C6H5P(O)(OH)O radical formed from PPOA through H-abstraction may undergo b-scission according to R56 and R57. The rate coefficients of R56 and R57 indicate that R56 is the main route for the decomposition of the C6H5P(O)(OH)O radical. When the concentration of PPOA subjected to cracking is low, the b-scission rate of the C6H5P(O)(OH)O radical is potentially faster than the Habstraction rate of R48 to R52. Therefore, the assumption that the b-scission rate is faster than that of H-abstraction reactions is justified for the thermal cracking of DOPP. Based on the above discussion, the predominant route for the thermal cracking of DOPP can be depicted as shown in Fig. 6 (bold reaction arrows). At 1123 and 1173 K, the initiation rate coefficient of PPOA is 4294 and 3006 times higher than that of n-hexane. Therefore, when DOPP is subjected to thermal cracking with n-hexane, it will act as an initiator. Initiation reaction of PPOA produces a C6H5 and a (HO)2P(O) radical. The (HO)2P(O) radical undergoes fast isomerization, producing a (HO)2PO radical which then undergoes fast b-scission producing HOP(O) and HO radicals. The HO radical abstracts a H atom from n-hexane, initiating the chain propagation steps for the decomposition of n-hexane. The predominant coupling reactions between n-hexane and PPOA are R58 and R59 as shown in Table 6. In R58 a CH3 radical formed by n-hexane abstracts a H atom from PPOA and in R59 a HO radical formed from PPOA abstracts a H atom from n-hexane. In the temperature range of 1000–1200 K, the kinetic parameters for H-abstraction from alkanes by the HO radical can be determined as Ea = 15.8 kJ mol
1 and log A = 7.9 [37]. The values of S2, r2, u1, l1, and E1 are given in Table 7. At 1123 and 1173 K the HO radical is some 1000 times more active in H-abstraction from n-hexane than the CH3 radical, leading to an increased rate of decomposition of nhexane leading to values of u1 of 1187 and 1061, respectively. The rate coefficient for H-abstraction from n-hexane by the HO radical is some hunderd thousand times higher than the rate coefficient for H-abstraction from PPOA by the CH3 radical, i.e. l1 = 3.7  10
5 at 1123 K and 5  10
5 at 1173 K and the accelerated decomposition of n-hexane directed by u1 is thus somewhat attenuated. Based on this tentative reaction scheme, it can be concluded that the parameter controlling the increased decomposition of n-hexane is the ratio of the initiation rate coefficients r2. The estimated E1 value at 1123 K for n-hexane cracking with addition of 500 ppm PPOA amounts to 1.36, the measured value of E1 is 1.35. At 1173 K with 500 ppm PPOA the estimated E1 value is 1.27, the measured

147

value of E1 is 1.28. Both are thus consistent with the experimentally measured values. As can be seen form Table 7, an increase in the cracking temperature from 1123 to 1173 K results in a reduction of r2 from 4294 to 3006. As a consequence, the accelerating effect of E1 decreases from 1.35 to 1.28. Here, again no significant changes in the value of u1 and l1 are observed with increasing cracking temperature. 4.7. The influence of addition of P-compounds on the conversion of n-hexane during steam cracking All three P-compounds accelerate the decomposition of nhexane and thus lead to a higher conversion of n-hexane during steam cracking. As discussed above, the accelerating effect of the P-containing compounds on the thermal decomposition of n-hexane is mainly due to the introduction of new initiation steps. At 1173 K, the value of r2 for HMPA and TPyPO is the same, being 1160. For DOPP, the value of r2 at 1173 is about 3000. Thus, the r2 value of DOPP is about 2.6 times higher than that of HMPA and TPyPO. This difference in the values of r2 for the three compounds can be traced back to their molecular structure. DOPP readily decomposes to PPOA before entering into the reactor. Therefore, the actual compound interfering with n-hexane decomposition is PPOA. In PPOA the P–C bond is weaker as compared to the P–N bond in HMPA and TPyPO, while in HMPA and TPyPO the P–N bond has the same strength. As compared to HMPA and TPyPO, a higher promotional effect of DOPP on the decomposition of n-hexane is expected. No significant difference in the promotional effect for HMPA and TPyPO is expected since the values of r2 are the same for these two compounds. This is consistent with the experimental results as shown in Table 7. Previously Kunzru and co-workers [8,9] used 50– 1000 ppm P as triethyl phosphite (TEP) and triphenyl phosphite (TPP) as additives in the steam cracking of naphtha at 1088 K. These authors did not observe any significant influence of these additives on the conversion of naphtha. According to the authors, decomposition of TEP and TPP proceeds via the scission of either a P–O bond or a C–O bond. Since the BDE of the P–O bond (384.6 kJ mol
1) is higher than that of the C–C bond in n-hexane (364.1 kJ mol
1), TEP and TPP cannot act as initiators via the scission of a P–O bond. Although the C–O bond (359.5 kJ mol
1) is somewhat weaker than the C–C bond in n-hexane, at 1088 K the value of r2 would merely amount to 1.7. Therefore, it can be expected that the use of TEP and TPP as additives during steam cracking of hydrocarbons will have hardly any influence on the conversion. 5. Conclusions The influence of three P-containing compounds, HMPA, TPyPO and PPOA on the conversion of n-hexane during thermal cracking in a continuous flow micro-reactor with complete mixing was experimentally investigated. Addition of the P-containing compound results in an increase of the conversion of n-hexane.

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J. Wang et al. / J. Anal. Appl. Pyrolysis 77 (2006) 133–148

To interpret the accelerating effect of the P-containing compounds on the decomposition of n-hexane, a tentative predominant route of the thermal cracking of the P-containing compounds is deduced based on the reaction rate of the elementary free radical reactions which are potentially involved in the thermal cracking process. According to the proposed predominant decomposition route, the main coupling reactions between the P-containing compounds and n-hexane in steam cracking are proposed to occur through H-abstraction reactions. For all three P-containing compounds the increase of the conversion of n-hexane during steam cracking can be traced back to the introduction of new initiation steps. The estimated increase in conversion of n-hexane is consistent with the experimentally measured values.

[11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

[21] [22] [23] [24]

Acknowledgement Jidong Wang is grateful to BASF Antwerp N.V. for financial support.

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