Pyrolysis of carbazole: Experimental results and kinetic study

Journal of Analytical and Applied Pyrolysis 113 (2015) 370–379

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Pyrolysis of carbazole: Experimental results and kinetic study Kangle Ding a,b,∗ , Yan Liu c , Qilin Xiao d , Yue Luo b , Huan Yang b a

Key Laboratory of Exploration Technologies for Oil and Gas Resources of Ministry of Education, Yangtze University, Jingzhou 434023, Hubei, China School of Chemistry and Environmental Engineering, Yangtze University, Jingzhou 434023, Hubei, China c School of Computer Science, Yangtze University, Jingzhou 434023, Hubei, China d School of Earth Environment and Water Resource, Yangtze University, Wuhan Campus, 430100 Hubei, China b

a r t i c l e

i n f o

Article history: Received 4 September 2014 Received in revised form 19 February 2015 Accepted 24 February 2015 Available online 14 April 2015 Keywords: Organic nitrogen compounds (ONCs) Carbazole Anhydrous thermal cracking Reaction mechanism Kinetics

a b s t r a c t Contents and types of organic nitrogen compounds (ONCs) in crude oil can critically affect the quality and the economic viability of oil products. In this article, anhydrous thermal cracking of carbazole was carried out using an autoclave at 425–525 ◦ C, and heating times ranging from 187.5 to 237.5 h. The pyrolysis products were qualitatively and quantitatively characterized by gas chromatograph- thermal conductivity detector-flame ionization detector, gas chromatography–mass spectrometry and elemental analyzer. Reaction mechanism and kinetics were investigated on the basis of the experimental data. The results show that primary reaction products are aromatic hydrocarbons, N-containing compounds as well as CH4 and H2 ; whereas NH3 , HCN and C2 H6 are minor products. At elevated temperatures degradation of carbazole undergoes simultaneously decomposition and condensation, involving cracking of C N bonds and C C bonds and forming of polycarbazole. Carbazole pyrolysis was demonstrated 0.8-order, and associated Arrhenius parameters of [E (KJ/mol), A (s−1 )] = [321.628, 1.832 × 1017 ] were determined. In comparison with dibenzothiophene, thermal stability of carbazole seems to be more unstable during thermal processing of residual oil. The present study may further improve our understanding of the evolution of complex ONCs in heavy oil refining. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Organic nitrogen compounds (ONCs) are generally undesirable components of crude oil. More than two thirds of the nitrogen content is in the form of the non-basic pyrrolic ring system such as carbazole and its derivatives [1–5]. Carbazoles mainly comprise a BP cyclic ring system with the incorporation of a nitrogen atom forming a third five-membered ring. In spite of low concentration in most oils, carbazole and its derivatives not only accelerate oil oxidation and reduce the useful lifetime of the finished products [6,7], but poison the catalysts used in petroleum refining. Additionally, ONCs in petroleum products forms NOx during combustion, which results in the environmental problems such as acid rain and photochemical smog. With current and future legislation demanding cleaner fuels, deeper removal of complex ONCs such as carbazoles and benzocarbazoles from fuels has become important. Although the activity of conventional hydrotreating catalysts for the removal of carbazoles and their alkyl derivatives has improved

∗ Corresponding author at: School of Chemistry and Environmental Engineering, Yangtze University, Jingzhou 434023, Hubei, China. Tel.: +86 716 8060442. E-mail address: [email protected] (K. Ding). http://dx.doi.org/10.1016/j.jaap.2015.02.029 0165-2370/© 2015 Elsevier B.V. All rights reserved.

considerably in recent years, a limit seems to have been reached. On the other hand, the thermal stability of carbazoles has been less reported in petroleum chemistry. Several pyrolysis experiments have been carried out on the thermal cracking of carbazoles in order to understand the reactions and mechanisms during pyrolysis of coal and heavy fuel oils [8–10]. These studies have drawn some useful information about the release of different volatilenitrogen. For example, it was found that both HCN and NH3 were determined as the major products of carbazole cracking [8–10]. By contrast, the pyrolysis of other pyrrolic nitrogen compounds such as pyrrole [11–13] and indole [14] showed that HCN was the major N-containing final product. However, it should be noted that most of previous pyrolysis experiments involving pyrrolic nitrogen compounds were conducted above 800 ◦ C, in a range of temperatures where cracking mechanisms and kinetics can be extremely different from those occurring under oil refining conditions. In recent years, Dartiguelongue et al. [15] elaborated a mechanistic model for dibenzothiophene (DBT) thermal cracking at 375–500 ◦ C in the gold bags. The chemical structure of dibenzothiophene is quite similar to that of carbazole; however, carbazole and dibenzothiophene would exhibit quite different evolutionary regularity at elevated temperatures due to the distinct chemical bonds of C N and C S. Moreover, it should be noted that the kinetic scheme for DBT

K. Ding et al. / Journal of Analytical and Applied Pyrolysis 113 (2015) 370–379

Fluid sampling/injection valve

Temp.

Pressure line

371

and was monitored using a thermocouple secured to the outer wall of the stainless steel autoclave. When the desired reaction temperature or time was reached, the stainless steel autoclave was withdrawn from the oven, air cooled for 30 min, and then rapidly cooled to room temperature by quenching in water. Afterwards, the tube reactor was recovered for detailed analysis of pyrolysis products. The pyrolysis experiments were repeated in quintuplicate, with essentially identical results for each replicate. 2.2. Gas analysis

Hastalloy closure

Hastalloy C-276 tube

Carbazole Stainless steel autoclave

Fig. 1. Schematic drawing of the thermal apparatus used in the laboratory experiments.

degradation was not rigorously demonstrated by Dartiguelongue et al. [15]. The present work is concerned with the thermal degradation of carbazole in closed confined system at elevated temperatures. Carbazole (CZ) is a good model compound that is representative of the nitrogen-containing compounds present in the greatest abundance in many petroleum samples [16–18]. The research on kinetics and mechanism of carbazole pyrolysis would be useful in predicting the variation and distribution of ONCs involved in deep denitrogenation units in oil refining. Our restricted knowledge of the effects of petroleum processing on stability of complex nitrogen compounds is another motivation.

After pyrolysis experiments, the sampling valve tube connected to the tube reactor was opened, allowing the headspace gaseous products to volatilize into the custom-built Teflon sampling bags (50 mL). Then, the composition of the collected gaseous products were determined using an Agilent 6890 gas chromatograph (Agilent Technologies, Palo Alto, CA, USA) equipped with a thermal conductivity detector, a flame ionization detector, an Agilent headspace G1888 autosampler (Wilmington, DE, USA), 3 J&W DBALC1capillary columns (30 m × 0.32 mm i.d., film thickness 1.8 ␮m; Wilmington, DE, USA) and 2 packed columns (stainless steel column packed with 20% SP-2100/0. 1% Carbowax 1500 on 100/120 Supelcoport; 30 m × 0.32 mm i.d., film thickness 0.25 mm). Gas chromatograph conditions were as follows: the initial flow rate of high purity nitrogen carrier gas is 3.8 mL/min at the initial pressure of about 1.86 × 104 Pa. The inlet temperature was 150 ◦ C with a split ratio of 20:1. The identification of peaks was achieved by comparing their retention times with those of standards purchased from Air Liquide America Specialty Gases LLC (Plumsteadville, PA, USA), Nu Chek Prep (Elysian, MN, USA) and Sigma–Aldrich (Sigma, St. Louis, MO, USA) under the same gas chromatography conditions. Peaks were integrated using Hewlett–Packard Chem Station software. The column oven temperature was initially set at 35 ◦ C for 15 min, and then it ramped to 200 ◦ C at 25 ◦ C/min and held at 200 ◦ C for 5.4 min. The detector temperature was set at 250 ◦ C, and the hydrogen and air flows were 30 mL/min and 400 mL/min, respectively.

2. Experimental 2.3. Analysis of acetone-soluble products 2.1. Anhydrous pyrolysis Carbazole is analytical pure, purchased from Sigma–Aldrich. Pyrolysis experiments were conducted in a 10.0-mL Hastalloy C276 tube reactor equipped with a Hastalloy fitting and a sampling valve connected to the tube with a capillary (Fig. 1). This valve allowed samples of the fluid to be obtained during the course of the experiments, and also allowed additional reactants to be periodically injected into the reaction cell. This reaction tube was confined within a stainless steel autoclave (Fig. 1). To remove any residual organic material derived from background sources, prior to loading the samples the open-ended Hastalloy C-276 tube was heated at 600 ◦ C for 3 h in the muffle furnace between the successive experiments. In each experiment, 50 mg of carbazole powder was loaded into the reactor and the tube was evacuated with a vacuum pump to remove air. The sealed tube reactor was subsequently placed in a separate stainless steel autoclave with an internal volume of 300 mL and then inserted into a pyrolysis oven. The reactor was first heated to 50 ◦ C directly and then to 425 ◦ C, 450 ◦ C, 475 ◦ C, 500 ◦ C and 525 ◦ C, following a program in different heating times of 187.5 h, 200 h, 212.5 h, 225 h, and 237.5 h, respectively. Because at the same reaction time higher temperature generally results in higher conversion of the reaction, shorter duration of experiment at higher temperatures or longer duration of experiment at lower temperatures is therefore utilized in the experiments in order to obtain the rational distribution of kinetic data. Heating rate was 2 ◦ C/h. Temperature was controlled to within 1 ◦ C of the set value,

The opened tube reactor was transferred into another autoclave filled with 100 mL of acetone. The solution was then extracted for 2 h at 90 ◦ C. Because the solubility of carbazole in the acetone at 25 ◦ C is about 10 g/100 mL [19], the residual carbazole (at most 50 mg) can be completely dissolved in the 100 mL of acetone. Consequently, organic products in the acetone extract were analyzed by gas chromatography–mass spectrometry (GC–MS) with an Agilent 7890A gas chromatograph (Agilent Technologies, Palo Alto, CA, USA) interfaced to an Agilent 5975C mass spectrometer (electron ionization 70 eV) in scan mode from 40 to 450 m/z at 3.6 scans/s. Analyses were performed with a fused silica capillary column (HP5, 30 m × 0.25 mm × 0.25 ␮m, No. 19091S-433) with a temperature program of 40 ◦ C for 3 min, 20 ◦ C/min to 300 ◦ C, and final temperature of 300 ◦ C for 4 min. He gas was used as carrier gas. The ion source and quadrupole temperatures were maintained at 230 ◦ C and 150 ◦ C, respectively. Total ion current chromatograms were acquired and processed using G1701DA D.01.02 Standalone data analysis software (Agilent Technologies, Palo Alto, CA, USA) on a Pentium IV computer that was also used to control the whole system. Identification of organic compounds was facilitated by comparing the retention times and mass spectra of experimental products with the Wiley (G1035B; Rev D.02.00; Agilent Technologies, Santa Clara, CA, USA), Nist (Nist Mass Spectral Search Program; version 2.0f, Nist Data Center, Gaithersburg, MD, USA) libraries reference spectral bank and published data [20–22]. For quantification purposes, external calibration curves were generated to determine

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Table 1 Relative contents of gaseous products and the elemental analysis of the acetone-insoluble residue in the carbazole decomposition experiments. na means that the amount formed is too small to be accurately determined. nr means no acetone-insoluble residue. Final temperature

Heating time

Heating rate

Gas composition (%)

(◦ C)

(◦ C)

(h)

(◦ C/h)

H2

CH4

C2 H6

NH3

HCN

H/C

N/C

50 50 50 50 50

425 450 475 500 525

187.5 200 212.5 225 237.5

2 2 2 2 2

na na na 12.53 11.38

na na na 80.49 81.28

na na na 1.95 1.14

na na na 3.67 4.18

na na na 1.36 2.02

nr na na 0.623 0.61

nr na na 0.075 0.073

401.1

401.1

5.2 57

C2

C1

51 6.8

C3

8.6 39

H22

6.8 51

442.5

C4

56 9 .9

C5 H21

.3 520

57

C11 398

C8

5 5.5 20.4 43

N9

H17

5. 2

C7

491.9

C6

C10

.8

538.6

.8

C12 398

C13 399.2

398

391.6

H14

491.9

2.4. Residual solid analysis

H16

H15

5 5. 43

the amount of carbazole in the extract. Calibration curves were obtained from GC–MS analysis of standard solution. The concentrations of carbazole in the acetone were 10 mg/L, 25 mg/L, 50 mg/L, 100 mg/L, 250 mg/L, and 500 mg/L. The analysis was performed in triplicate for every concentration. The concentration of each standard (mg/L) was plotted against the average peak area of the replicates and the calibration curve and the correlation coefficient were found for carbazole (y = 1378.9x + 16696; r2 = 0.9999). The analysis of the extracted organic products was carried out at Beijing Risun Chemical Technology Institute Co., Ltd., China (courtesy of Shijuan Du).

Aceton-insoluble residue Atomic ratios

399.2

1 2 3 4 5

Initial temperature

538.6

No

H20

H19

56

9. 9

.5

H18

Fig. 2. The different bond dissociation energy of carbazole (KJ/mol).

Elemental compositions of C, H, and N of the acetone-insoluble residue were analyzed by using an elemental analyzer (Elementar, Vario E.L, Hanau, Germany). Accurately weighed 0.5 mg of sample was heated to 1150 ◦ C and the corresponding element was determined by using a thermal conductivity detector. The elemental analysis of the solid products was carried out at School of Chemistry and Environmental Engineering, Yangtze University (courtesy of Zhiyun Feng). 3. Results and discussion 3.1. Pyrolysis products After experiments, the relative contents of gaseous products and the elemental analysis of the acetone-insoluble residue are listed in Table 1. It was found that no gases were generated below 400 ◦ C. Traces of gases were observed between 425 and 475 ◦ C. However, because gaseous products at 425–475 ◦ C were always present at very low levels, their concentrations could not be quantified. The gaseous products at ≥500 ◦ C are mainly composed of CH4 and H2 as well as small amounts of NH3 , HCN and C2 H6 (Table 1). NH3 and HCN are derived from the primary cracking of carbazole and the secondary cracking of the N-containing products. The amounts of NH3 are greater than those of HCN at the same pyrolysis temperature (i.e., 500 ◦ C), which is thought to be due to the strength of the C N bond in comparison with the C C bond [23]. Based on the previous studies [10,24], the different bond dissociation energy of carbazole (KJ/mol) were calculated and listed in Fig. 2. It is evident that the chemical bond between the carbon and the nitrogen is weaker than the chemical bond connecting this carbon with the other carbon atoms within the aromatic ring system of carbazole. This may account for the different amounts of NH3

H N

and HCN generated during the pyrolysis experiments. H2 , CH4 and C2 H6 would be produced through decomposition and/or condensation of carbazole as well as its secondary products. The formation of acetone-insoluble residue is presumably caused by the polymerization of carbazole at elevated temperatures [25]. Although these residuals most likely contained higher molecular weight heterocyclic nitrogen compounds even polycarbazoles, they were hardly dissolved in acetone or other general organic solvents, and therefore not GC-amenable. Elemental analysis was then carried out to clarify the composition of the residuals. According to Table 1, the H/C ratio of acetone-insoluble residue has decreased by 0.13–0.14 over the calculated H/C value for carbazole (0.75). Nevertheless, the N/C ratio for residues at 525 ◦ C is just around 0.01 lower than the calculated N/C value for carbazole (0.083), indicating that denitrification of carbazole by increasing temperature is unpractical for petroleum processing. Main products identified in the acetone extract are dominated by a variety of aromatic hydrocarbons along with N-containing compounds (Table 2 and Fig. 3), indicating a complex behavior exhibited by carbazole cracking. Relative contents of typical products identified in the acetone extract vs. temperatures were shown in Figs. 4–7, respectively. It was found that the contents of main pyrolysis products such as toluene, naphthalene, biphenyl, indene, fluorene, methylcarbazole and their derivatives increased with increasing temperatures (Figs. 4–7), in comparison with an inverse correlation between contents of carbazole (CZ) and reaction temperatures (Fig. 7). Throughout the pyrolysis experiments, anhydrous degradation of carbazole at elevated temperatures follows a free radical mechanism. The reaction extent of carbazole cracking is limited by the generation of radicals. The H radicals would initially originate from condensation of N-containing ring systems of carbazole, as shown in the following reaction:

H N + (2n-1) H .

n n

n≥ 2

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373

Table 2 GC/MS identification of acetone-soluble products formed during carbazole pyrolysis with heating rate of 2 ◦ C/h from 50 ◦ C to 500 ◦ C for 225 h. Peak no.

Retention time (min)

Compounds

Molecular formula

1

2.368

Methylcyclohexane

C7 H14

2

2.635

Toluene

C7 H8

3

3.311

Ethylbenzene

C8 H10

4

3.373

p-Xylene

C8 H10

5

3.563

o-Xylene

C8 H10

6

3.811

Cumene

C9 H12

7

4.049

Propylbenzene

C9 H12

8

4.106

1-Ethyl-3-methylbenzene

C9 H12

9

4.230

Aniline

C6 H7 N

10

4.263

1,2,4-Trimethyl-benzene

C9 H12

11

4.744

Indane

C9 H10

12

5.006

p-Aminotoluene

C7 H9 N

13

5.044

o-Toluidine

C7 H9 N

14

5.144

1-Methyl-indan

C10 H12

15

5.654

2,3-Dihydro-4-methyl-1H-indene

C10 H12

16

5.754

1,2,3,4-Tetrahydronaphthalene

C10 H12

17

5.939

Naphthalene

C10 H8

18

6.687

Indole

C8 H7N

19

6.744

2-Methylnaphthalene

C11 H10

Chemical structures

H2N

H2N

H2N

NH

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Table 2 (Continued). Peak no.

Retention time (min)

Compounds

Molecular formula

20

6.873

1-Methylnaphthalene

C11 H10

21

7.311

Biphenyl

C12 H10

22

7.430

2-Methyl-1,1 -biphenyl

C12 H12

23

8.658

Fluorene

C13 H10

Chemical structures

H N

24

10.078

Carbazole

C12 H9 N

25

10.477

2-Methylcarbazole

C13 H11 N

H N

H N

26

10.658

3-Methylcarbazole

C13 H11 N

27

10.735

9-Methyl-carbazole

C13 H11 N

N

2 11 17

24

9 4

21 13

3

7 5

1

2

8

4

18

12 10

15

6

3

14

5

19

27 25

16

20

6

7

23

22

8

26

9

10

11

12

Retention time (min) Fig. 3. Total ion chromatogram from GC–MS analysis of acetone-soluble products formed during carbazole pyrolysis with heating rate of 2 ◦ C/h from 50 ◦ C to 500 ◦ C for 225 h. For identification see Table 2.

K. Ding et al. / Journal of Analytical and Applied Pyrolysis 113 (2015) 370–379

15

10

10

Methylcyclohexane Toluene Ethylbenzene p-Xylene o-Xylene cumene Propylbenzene 1-ethyl-3-methylBenzene 1,2,4-trimethyl-Benzene

5

6

4

2

0

0 420

1, 2, 3, 4-tetrahydronaphthalene Naphthalene 2-methylnaphthalene 1-methylnaphthalene Biphenyl 2-Methyl-1,1'-biphenyl

8

Relative content (%)

Relative content (%)

20

375

440

460

480

500

520

540

420

440

460

o

520

540

Fig. 5. Contents of naphthalene, biphenyl and their derivatives vs. reaction temperatures.

Relative content (%)

8

6

Indene 1-methyl-Indan 2,3-dihydro-4-methyl-1H-Indene Fluorene

4

2

0

420

440

460

480

500

520

540

o

Temperature ( C) Fig. 6. Contents of indene, fluorene and their derivatives vs. reaction temperatures.

Specifically, methylation of carbazole generates 2-methylcarbazole (25), 3-methylcarbazole (26) and 9-methyl-carbazole (27). Methylation of benzene radicals generates toluene (2), ethylbenzene (3), p-Xylene (4), o-Xylene (5), cumene (6), propylbenzene (7), 1-ethyl-3-methylbenzene (8) and 1,2,4-trimethyl-benzene (10).

10

100

Aniline p-Aminotoluene o-Toluidine Indole 2-Methylcarbazole 3-Methylcarbazole 9-Methyl-carbazole Carbazole (CZ)

8

6

4

80

60

40

2

20

0

0 420

440

460

480

500

520

o

Temperature ( C) Fig. 7. Contents of ONCs vs. reaction temperatures.

540

Relative content of CZ (%)

After that, the generated H radicals attack the N-containing ring systems, resulting in the rupture of the ring [23]. With the depth of pyrolyses, the secondary cracking of products can also provide a source of active hydrogen and other radicals (e.g., CH3 • ), further accelerating the cracking of carbazole. Therefore, it is logically reasoned that the availability of radicals to a large extent affects the types and amounts of pyrolysis products. With increasing temperatures, carbazole undoubtedly goes through deep degradation; meanwhile, its N-containing ring systems would tend to combine with other ring systems to form increasingly larger and more stable heteroaromatic ring systems (i.e., polycarbazoles). Increased ring sizes mean that the rupture of the heteroaromatic ring systems becomes increasingly difficult and free radicals are more and more difficult to generate [23], ultimately leading a termination of the free radical reaction. A reaction scheme was tentatively proposed to explain how pyrolysis products were obtained (Fig. 8). Numbers reported here correspond to peak no. listed in Table 2. As discussed above, under pyrolysis conditions, carbazole undergoes simultaneously two kinds of reactions: condensation and decomposition. Condensation of carbazole produces polymers such as polycarbazole and H radicals. The latter is then involved in the following free radical reactions, forming H2 gas and other products. Decomposition of carbazole includes cracking of C N bonds and C C bonds of carbazole (24). Cleavage of two C N bonds of carbazole (24) leads to the formation of biphenyl (21) and ammonia NH3 . Cleavage of two C C bonds of carbazole (24) leads to the formation of indole (18) and butadiene. It should be pointed out that butadiene was not detected after experiments, which is similarly reported in Dartiguelongue et al. [15]. The disappearance of butadiene in the extract may be due to its unstable chemical properties under pyrolysis conditions, or being entirely consumed after the generation of aromatic hydrocarbons (i.e., naphthalene). Simultaneous cleavage of one C N bond and one C C bond of carbazole (24) generates aniline (9) and benzene radicals. Further cracking of indole (18) via cleavage of one C N bond and one C C bond would release benzal radical and HCN. Benzene radicals can combine with butadiene to form naphthalene (17). After depletion of butadiene, naphthalene (17) also would be generated via the combination of two benzene radicals, releasing methyl radicals. With the depth of pyrolyses, carbazole and the above generated simpler products including radicals would be involved in the continuous methylation. This process would produce a variety of secondary products with methyls. According to our experimental results, it seemed that most methyl radicals took part in the formation of other products (i.e., CH4 ), compared with the generation of trace of C2 H6 .

500

Temperature ( C)

Relative content of ONCs except CZ (%)

Fig. 4. Contents of toluene and its derivatives vs. reaction temperatures.

480 o

Temperature ( C)

376

K. Ding et al. / Journal of Analytical and Applied Pyrolysis 113 (2015) 370–379

Fig. 8. Reaction scheme illustrating the formation of identified products in Fig. 3. Numbers reported here correspond to peak no. listed in Table 2.

Similarly, methylation of benzal radical would produce indane (11), 1-methyl-indan (14) and 2,3-dihydro-4-methyl-1H-indene (15). P-aminotoluene (12), o-toluidine (13), 2-methylnaphthalene (19), 1-methylnaphthalene (20) and 2-Methyl-1,1 -biphenyl (22) may also be formed from methylation of aniline (9), naphthalene (17) and biphenyl (21), respectively. In contrast, methylcyclohexane (1) and 1,2,3,4-tetrahydronaphthalene (16) were observed as a result of the addition of H radicals on toluene (2) and naphthalene (17). The formation of fluorene (23) can be explained in two different ways: from dehydrocyclization of 2-methyl-1,1 -biphenyl (22), and/or from the addition of benzene radicals on benzal radicals followed by hydrogen transfer. It must be emphasized that not all possible steps are shown, and not necessarily in that order all the time, but those that are shown illustrate the possibilities of how a wide variety of products can be formed during thermal degradation of carbazole.

3.2. Kinetics The lack of experimental data and chemical model on pyrolysis of nitrogen aromatic compounds in the literature as well as the complex composition of reaction products introduce strong limitations in the predictability of detailed chemical kinetic models for carbazole cracking. Therefore, we tentatively evaluate carbazole thermal cracking by the determination of bulk kinetic parameters (activation energy E and frequency factor A), constrained by a complete experimental pyrolysis study. The reaction conversions of carbazole pyrolysis at different temperatures were calculated using the external standard method, and given in Fig. 9. The composition of pyrolysis products demonstrated that thermal cracking of carbazole is a complex chemical process, possibly comprising several parallel and consecutive reactions. These reactions are difficult to be analyzed separately and their quantitative contribution to the

K. Ding et al. / Journal of Analytical and Applied Pyrolysis 113 (2015) 370–379

377

Table 3 The linear regression coefficients for the thermal degradation of carbazole. Reaction order Regression coefficient

0 0.60203

0.2 0.82134

0.4 0.94019

Reaction conversion (%)

80

60

40

20

0 440

460

480

500

520

540

Fig. 9. Plot of reaction conversions under different temperatures.

global reaction process is virtually impossible to assess. Under such conditions, the mathematical model is useful to understand and verify the validity of the assumptions implied by decomposition of carbazole. In the present study, anhydrous degradation of carbazole is assumed as n-order reaction, and its kinetic equation is written as follows, dx = Ae−E/RT (1 − x)n dt

(1)

where x is reaction conversion, t is reaction time, n is reaction order, and dx/dt is reaction rate. A is the apparent frequency factor, E is apparent activation energy, R is the gas constant, and T is absolute temperature. Considering constant rate heating condition ˇ = dT/dt, Eq. (2) is obtained, dx ˇ ln dT (1 − x)n

 = ln A −

E RT

(2)

-7

1 n= 0.8 n= .6 n=0 .4 n=0 .2 0 n= n=0

-8

ln[βdx/dT/(1-x) ]

-9 -10 -11 -12 -13 -14 -15 -16 -1.7x10

1 0.98261

-1.7x10

-1.6x10

-1.6x10

-1.5x10

-1/RT (J/mol) Fig. 10. Plots of −1/RT vs. ln

 dx

Temperature (◦ C)

k (s-1 )

t0.5 (carbazole) (s)

480 500 550 600 700 800 900 1000

9.031 × 10−6 3.410 × 10−5 7.124 × 10−4 1.051 × 10−2 9.972 × 10−1 4.051 × 101 8.750 × 102 1.166 × 104

7.167 × 104 1.898 × 104 9.085 × 102 6.160 × 101 6.491 × 10−1 1.598 × 10−2 7.397 × 10−4 5.549 × 10−5

Based on the conversions at different temperatures, and taking different  n values  (0–1), the fitted linear regression lines for −1/RT vs. ln

o

Temperature ( C)



0.8 0.99544

Table 4 The reaction rate constants and half-lives for thermal degradation of carbazole at different temperatures, assuming that cracking reactions follow a 0.8-order kinetic law.

100

420

0.6 0.988

ˇ dT (1−x)n



.

-1.5x10

ˇ dx dT (1−x)n

can be drawn and shown in Fig. 10. When the

linear regression coefficient is closest to 1, the corresponding value of n is the reaction order. The linear regression coefficients for the thermal degradation of carbazole were listed in Table 3, and it can be evidently concluded that the reaction order is 0.8. For n = 0.8, Eq. (2) becomes,



ln

ˇ dx dT (1 − x)0.8



= ln A −

E RT

By using the linear regression of −1/RT vs. ln

(3)



ˇ dx dT (1−x)0.8

 in Eq.

(3), the apparent activation energy E and apparent frequency factor A can be determined as 321.628 KJ/mol and 1.832 × 1017 s−1 by the slope and intercept of the regressed line, respectively (Fig. 10). The reaction rate constants and half-lives for carbazole cracking are determined using Eq. (3). The results are listed in Table 4. Rate constants determined experimentally using 0.8-order kinetics extrapolate to carbazole half lives of 7.167 × 104 –9.085 × 102 s in petroleum coking (480–550 ◦ C). Since the general petroleum coking time is just several seconds [26], complete removal of carbazole by coking is not an optimal choice for deep denitrogenation units in oil refining. Nevertheless, in medium and high temperature carbonization of coal (700–1000 ◦ C) the reaction rates of carbazole cracking increase with increasing temperatures (Table 4), indicating the fate of carbazole in coal processing is most likely influenced by elevated temperatures. According to the Arrhenius equation k = Ae−E/RT , Ln (k) of pyrolysis of carbazole (CZ) and dibenzothiophene (DBT) [15] at different temperatures are illustrated in Fig. 11. For the sake of discussion, the rate constant of carbazole pyrolysis and that of dibenzothiophene pyrolysis are marked as kCZ and kDBT , respectively. It is evident that both kCZ and kDBT increase with the increasing temperature. Especially, present kinetic calculations confirmed that kCZ equals kDBT at 378.58 ◦ C. When temperature is lower than 378.58 ◦ C kDBT is consistently larger than kCZ , implying that the thermal degradation rate of dibenzothiophene is faster than that of carbazole in typical oil & gas reservoirs (100–200 ◦ C) [27] and during oil shale in-situ conversion processes (340–370 ◦ C) [28]. On the other hand, kCZ gradually exceeds kDBT above 378.58 ◦ C. This indicates that during thermal processing of heavy oil and residual oil, thermal stability of carbazole seems to be more susceptible to visbreaking (400–450 ◦ C) [26] and coking (480–550 ◦ C) [26] than dibenzothiophene. Because the goal of this study was to

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0 o

378.58 C

-10 -20

ln (k)

-30

CZ DBT

-40 -50 -60 -70 -80 -90 0

50

100 150 200 250 300 350 400 450 500 550 600 650 o

Temperature ( C) Fig. 11. Ln (k) of carbazole pyrolysis vs. temperature and comparison with data obtained for dibenzothiophene [15]. k = the rate constant, carbazole = CZ, dibenzothiophene = DBT.

investigate thermal degradation of carbazole in closed confined system at elevated temperatures, the design of the experiments was not optimized to evaluate the effects of other compounds in crude oil on DBT or CZ cracking. However, it should be noted that some chemical factors such as water and mineral catalysis could modify the above results in a yet unknown manner. Also, physical factors such as the opening of the system and the short reaction time resulting in small extents of conversion could render the effects evidenced in this study difficult to observe in oil refining. Future studies would be required to investigate the contribution of compounds in crude oil, water and minerals to the thermal degradation of carbazole, and to further test the observed trends in the present experiments. When available, these information will constitute a valuable input to modeling the evolution of carbazole and dibenzothiophene in oil refining, which is beyond the scope of the this article and will be investigated in the next work. The present results of the simulation experiments could not yet be directly applied to oil refining. Nevertheless, the results of this study are theoretically useful for further understanding of complex ONCs and OSCs distributions and variation in deep processing of heavy oil. 4. Conclusions Thermal cracking of carbazole was performed using an autoclave in the temperature range 425–525 ◦ C to investigate degradation of complex ONCs under elevated temperatures. Based on the results of this study, the following conclusions are obtained: (1) The major products from anhydrous pyrolysis of carbazole at ≥500 ◦ C are a variety of aromatic hydrocarbons, N-containing compounds as well as CH4 and H2 , whereas NH3 , HCN and C2 H6 are minor products. After experiments, some acetone-insoluble residue was found and presumably attributed to the polymerization of carbazole at elevated temperatures. (2) Carbazole thermolysis proceeds through a series of chemical reaction steps involving cracking of C N bonds and C C bonds, hydrogen transfer, as well as condensation of carbazoles. Nevertheless, a free radical mechanism is believed to be dominant during the anhydrous pyrolysis of carbazole. (3) The conversion of carbazole increased with an increase in temperature, which agrees with previous studies. An overall reaction kinetic model based on experimental results was developed to account for the observed carbazole conver-

sion under different temperatures. Based on the experimental data, the reaction order for carbazole degradation is determined as 0.8. The calculated apparent activation energy E is 321.628 KJ/mol and the apparent frequency factor A is 1.832 × 1017 s−1 . (4) The rate constant of carbazole kCZ is consistently smaller than that of dibenzothiophene kDBT below 378.58 ◦ C, whereas an inverse relationship between kCZ and kDBT takes hold above 378. 58 ◦ C. Especially, kCZ equals kDBT at 378.58 ◦ C. These calculated trends indicate that the thermal degradation rate of dibenzothiophene is faster than that of carbazole in typical oil & gas reservoirs and during oil shale in-situ conversion processes. However, thermal stability of carbazole seems to be more susceptible to thermal processing of heavy oil and residual oil than dibenzothiophene. Other compounds, water, metallic ions and or oxides in residual oil may also affect the stability of carbazole and dibenzothiophene, which will be investigated in the next work. Acknowledgments This research was financially supported by the National Natural Science Foundations of China (Ratification No.: 41472095, 40902034) and PetroChina Innovation Foundation (No.: 2012D5006-0104). Two anonymous reviewers are thanked for their critical and constructive reviews of our paper. Editor D. Fabbri is gratefully acknowledged for his helpful review and instructions. References [1] R.V. Helm, D.R. Latham, C.R. Ferrin, J.S. Ball, Identification of carbazole in Wilmington petroleum through use of gas–liquid chromatography and spectroscopy, Anal. Chem. 32 (1960) 1765–1767. [2] H. Clegg, B. Horsfield, H. Wilkes, J. Sinninghe-Damsté, M.P. Koopmans, Effect of artificial maturation on carbazole distributions, as revealed by the hydrous pyrolysis of an organic-sulphur-rich source rock (Ghareb formation, Jordan), Org. Geochem. 29 (1998) 1953–1960. [3] M. Li, S.R. Larter, D. Stoddart, M. Bjorùy, Fractionation of pyrrolic nitrogen compounds in petroleum during migration: derivation of migration-related geochemical parameters, in: J.M. Cubitt, W.A. England (Eds.), The Geochemistry of Reservoirs, 86, Geological Society Special Publication, 1995, pp. 103–123. [4] D.P. Stoddart, P.B. Hall, S.R. Larter, J. Brasher, M. Li, M. Bjorùy, The reservoir geochemistry of the Eldfisk Field, Norwegian North Sea, in: J.M. Cubitt, W.A. England (Eds.), The Geochemistry of Reservoirs, 86, Geological Society Special Publication, 1995, pp. 257–279. [5] S.R. Larter, B.F.J. Bowler, M. Li, M. Chen, D. Brincat, B. Bennett, K. Noke, P. Donohoe, D. Simmons, M. Kohnen, J. Allan, N. Telnaes, I. Horstad, Molecular indicators of secondary oil migration distances, Nature 383 (1996) 593–597. [6] J.W. Frankenfeld, W.F. Taylor, Effects of nitrogen compounds on deposit formation during synfuel storage, preprint papers – American chemical society, Div. Fuel Chem. 23 (1978) 205–214. [7] C.D. Ford, S.A. Holmes, L.F. Thompson, D.R. Latham, Separation of nitrogen compound types from hydrotreated shale oil products by adsorption chromatography on basic neutral alumina, Anal. Chem. 53 (1981) 831–836. [8] R. Guan, W. Li, H. Chen, B. Li, The release of nitrogen species during pyrolysis of model chars loaded with different additives, Fuel Process. Technol. 85 (2004) 1025–1037. [9] J.P. Hämäläinen, M.J. Aho, J.L. Tummavuori, Formation of nitrogen oxides from fuel-N through HCN and NH3 : a model-compound study, Fuel 73 (1994) 1894–1898. [10] S. Yuan, J. Li, X. Chen, Z. Dai, Z. Zhou, F. Wang, Study on NH3 and HCN formation mechanisms during rapid pyrolysis of pyrrolic nitrogen, J. Fuel Chem. Technol. 39 (2011) 801–805 (in Chinese with english abstract). [11] J.C. Mackie, M.B. Colket III, P.F. Nelson, M. Esler, Shock tube pyrolysis of pyrrole and kinetic modeling, Int. J. Chem. Kinet. 23 (1991) 733–760. [12] A.E. Axworthy, V.H. Dayan, G.B. Martin, Reactions of fuel-nitrogen compounds under conditions of inert pyrolysis, Fuel 57 (1978) 29–35. [13] A. Lifshitz, C. Tamburu, A. Suslensky, Isomerization and decomposition of pyrrole at elevated temperatures: studies with a single-pulse shock tube, J. Phys. Chem. 93 (1989) 5802–5808. [14] A. Laskin, A. Lifshitz, Isomerization and decomposition of indole. Experimental results and kinetic modeling, J. Phys. Chem. A 101 (1997) 7787–7801.

K. Ding et al. / Journal of Analytical and Applied Pyrolysis 113 (2015) 370–379 [15] C. Dartiguelongue, F. Behar, H. Budzinski, G. Scacchi, P.M. Marquaire, Thermal stability of dibenzothiophene in closed system pyrolysis: experimental study and kinetic modeling, Org. Geochem. 37 (2006) 98–116. [16] C.S. Creaser, F. Krokos, K.E. O’neill, M.J.C. Smith, P.G. McDowell, Selective chemical ionization of nitrogen and sulfur heterocycles in petroleum fractions by ion trap mass spectrometry, J. Am. Soc. Mass Spectrom. 4 (1993) 322–326. [17] C.S. Hsu, K. Qian, W.K. Robbins, Nitrogen speciation of polar petroleum compounds by compound class separation and on-line liquid chromatography–mass spectometry (LC–MS), J. High Resolut. Chromatogr. 17 (1994) 271–276. [18] G.W. Mushrush, E.J. Beal, D.R. Hardy, J.M. Hughes, Nitrogen compound distribution in middle distillate fuels derived from petroleum, oil shale, and tar sand sources, Fuel Process. Technol. 61 (1999) 197–201. [19] Z.B. Zhao, The solvent choice of the refined process of separating anthracene, in: M. Sc. Thesis, Taiyuan University of Technology, 2003, 2015. [20] H. Clegg, H. Wilkes, B. Horsfield, Carbazole distributions in carbonate and clastic source rocks, Geochim. Cosmochim. Acta 61 (1997) 5335–5345. [21] B. Bennett, A. Lager, C.A. Russell, G.D. Love, S.R. Larter, Hydropyrolysis of algae bacteria, archaea and lake sediments: insights into the origin of nitrogen compounds in petroleum, Org. Geochem. 35 (2004) 1427–1439.

379

[22] L. Berwick, P. Greenwood, R. Kagi, C. Jean-Philippe, Thermal release of nitrogen organics from natural organic matter using micro scale sealed vessel pyrolysis, Org. Geochem. 38 (2007) 1073–1090. [23] C. Li, L. Tan, Formation of NOx and SOx precursors during the pyrolysis of coal and biomass. Part III. Further discussion on the formation of HCN and NH3 during pyrolysis, Fuel 79 (2000) 1899–1906. [24] Y. Luo, Hand Book of Bond Dissociation Energies in Organic Compounds, CRC Press, Boca Raton, Florida, 2002. [25] Z. Zhao, W. Li, B. Li, H. Chen, Effect of Na, Ca and Fe on evolution of fuel–nitrogen during pyrolysis and combustion of model compound, J. Fuel Chem. Technol. 30 (2002) 294–299 (in Chinese with english abstract). [26] C.M. Xu, Z.H. Yang, Petroleum Refinery Engineering, 4th ed., Petroleum Industry Press, Beijing, 2009 (in Chinese). [27] M.M. Cross, D.A.C. Manning, S.H. Bottrell, R.H. Worden, Thermochemical sulphate reduction (TSR): experimental determination of reaction kinetics and implications of the observed reaction rates for petroleum reservoirs, Org. Geochem. 35 (2004) 393–404. [28] S. Lee, J.G. Speight, S.K. Loyalka, Handbook of Alternative Fuel Technologies, CRC Press, Boca Raton, Florida, 2007.