Some physico-chemical parameters of soot formation during pyrolysis of methane and methane-acetylene and methane-benzene mixtures

Twenty-FifthSymposium(International)on Combustion/TheCombustionInstitute,1994/pp. 653~i59

SOME PHYSICO-CHEMICAL PARAMETERS OF SOOT FORMATION D U R I N G PYROLYSIS OF METHANE A N D METHANE-ACETYLENE A N D M E T H A N E - B E N Z E N E MIXTURES P. A. TESNER AND S. V. SHURUPOV All-Russian Institute of Natural Gas 142717, Moscow obl., p/o Razvilka, VNIIGAS, Russia

Under isothermal conditions, pyrolysis of methane and its mixtures with acetylene and benzene was investigated at temperatures 1200 ~ ~ and methane concentrations 20-80%. Particle number density was shown to depend linearly on methane concentration, as was earlier demonstrated for acetylene. The equation of particle number density (N) was determined for methane as follows: N = 9.4 9 102~

exp(-39,400/T).

The activation energy of soot aerosol formation during methane pyrolysis was 328 kJ/mol. For both methane-acetylene and methane-benzene mixtures, inhibitionof the soot particles formation from methane was obtained. Soot particles were formed only from benzene or acetylene, whereas methane was consumed for particle growth. Promotion of heterogeneous methane decomposition on the surface of soot particles formed from acetylene or benzene was found.

Introduction Abundant literature is devoted to the soot formation process, and several detailed reviews are available [1-5]. However, the kinetics of soot formation under pyrolysis of hydrocarbons is not sufficiently investigated. The nonisothermal technique was employed in earlier work [6,7], so reliable kinetic parameters could not be obtained. We have developed an isothermal technique [8] to investigate the soot formation kinetics during pyrolysis of hydrocarbons, and simple kinetic parameters for acetylene pyrolysis have been obtained [9]. The present work deals with the investigation of the soot formation kinetics during pyrolysis of methane and of methane-acetylene and methane-benzene mixtures.

Experimental Technique The experimental technique used did not differ from that described in detail elsewhere [8]. The pyrolysis of hydrocarbons was carried out in a quartz tube 20 mm in diameter heated in an electric oven (Fig. 1). Isothermality of measurements was provided by heating a mixture of gases in channels 1 mm in diameter at gas flow velocity 90-100 m/s. The mixture of gases containing methane was fed along two channels of an alumina pipe 20-cm long, whose end was in the beginning of the constant-temperature zone. The flow rate of gas in each channel

was 800 cma/min. Additionally, 400 cm3/min of helium were fed to the inlet of the reactor to blow off the soot formed. Measurements made with wire thermocouples 0.2 mm in diameter have shown that the temperature of the gas in the channel 1 mm in diameter 5-7 mm from the end of the channel differs from that in the reaction space by no more than 10~ The soot carried out of the reactor by the gas stream was trapped in a bag filter. The soot deposited on the reactor walls at temperatures below 600 ~ did not differ in its surface area from the soot collected in the filter. Therefore, it was withdrawn and studied together with the soot from the filter. The soot deposited at higher temperatures was covered with pyrolytic carbon, and its surface area was smaller than that of the soot from the filter. This soot was withdrawn and investigated separately. Soot was weighed by ana/ytieal balance. The accuracy of soot yield measurements was _+5%. The yield of pyrolytic carbon deposited on the reactor walls was obtained by burning it in the air flow, followed by carbon dioxide reaction with alkali. The surface area of the soot was measured by nitrogen adsorption at the temperature of liquid nitrogen. For each soot sample, the measurements of the surface area were performed twice, and the average value of the surface area was taken. The accuracy of the surface area measurements was + 5%. Four sets of experiments have been performed under the following conditions.

653

1. At temperatures 1300 ~ and 1400 ~ and methane concentrations from 20 to 80%.

SOOT AND POLYCYCLIC AROMATIC HYDROCARBONS

654

T~F.NATUR~ H~LL~,41 X

yield (a) and surface area (A). Simple geometrical considerations were used for this purpose. The surface (F, cm 2) and the weight (G, g) of a soot particle of diameter d (cm) equal

~

F = nd ~

i~=]IL~ TWOCHANNEL ALUMINA TUBE

QUARTZ REACTOR

G = nd37/6 = nd~/3

where y is specific density of soot particles, ~ = 2.0 g/cm3. Therefore, the surface area (A, cm2/g) of the soot particle is

BF.NZENE

FIG. i. Diagramof the apparatusfor conducting pyrolysis under isothermalconditions. 35

N o = 1 / G = 3 / n d 3.

Hence,

9

"'

3

,,t

3O

The number of particles in 1 g of soot (No, g - l ) is

4o

4 ,A,~,

A = F / G = 3/d.

,',

~.

35

No = 3A3/33n = A3/9n.

(1)

N

If the surface area (A) is in m2/g, then i

G~

I,

O

~

3o

O

25

o

Q = N O 9 a.c

15 i

i

20

40 ~TIOM

i

,

(2)

The flux of soot particles (Q, rain- 1) was calculated according to the following equation:

1

Co

2O

N O = 1012A3/9n.

J

60

i

80

OF t~'T~qE, 2:

FIG. 2. Surface area and soot yield vs methane concentration. 1, 2--temperature 1300 ~ 3, 4--temperature 1400 ~ 2. At constant methane concentration 20% and temperatures from 1200 ~ to 1400 ~ 3. Pyrolysis of a methane-acetylene-helium mixture at constant concentration of helium 20% and at temperature 1300 ~ 4. Pyrolysis of a methane-benzene-helium mixture at constant concentration of methane 20% and at temperature 1300 ~ Results

The results of the first set of experiments, namely the values of the surface areas and the yield of the soot trapped by the bag filter and deposited in the cold part of the reactor tube are given in Fig. 2. The results obtained reveal that the surface area is independent of the methane concentration. It can be mentioned that, at methane concentrations less than 20%, the soot yield increases, whereas the surface area decreases. The results obtained (Fig. 2) were processed as follows. For each experiment, the soot particle number density (N) was calculated on the basis of soot

(3)

where c is the flux of the hydrocarbon's carbon (g/rain) and a is the yield of soot trapped by the filter and deposited in the cold part of the reactor. Soot particle number density was calculated according to the next equation, supposing that coagulation between soot particles does not change the surface area (see Discussion). N = K.

Q/P

(4)

where N is soot particle number density (cm-3), K is a fitting coefficient taking into account the soot deposited in the hot zone of the reactor and equal to 1.5, and P is the gas flow rate at the temperature of the experiment (cm3/min). A peculiarity of the experimental technique for investigation of soot formation under pyrolysis of hydrocarbons in the reactor tubes consists of the formation of diverse carbon forms. Their approximate balance at maximum soot yield was as follows (wt.%). 1. The soot carried out into the bag filter and deposited in the cold part of the reactor at temperatures below 600 ~ representing 40%. 2. The soot deposited on the walls of the reactor hot zone, representing 23%. 3. Pyrolytic carbon film on the reactor walls, representing 18%. Thus, at the highest degree of methane decomposition, nearly 80% of the carbon from methane is converted to soot and pyrolytic carbon, and 20% remains in the pyrolysis gas. The ratio of the surface area of soot trapped by the filter and deposited in the cold part of the reactor

PHYSICO-CHEMICAL PARAMETERS OF SOOT FORMATION DURING PYROLYSIS N/lo 9 14

N/109

655

TABLE 1 Calculation of the particle number density at different temperatures

45 0

40

12

,~

" 2

o

10

.,i~ 30

o

Surface Soot Particle number density Temperature area, yield, [calculated by Eq. (4)], (K) A (mVg) a (wt.%) N (cm -a)

1

1473 1523 1573 1623 1673

25 6

,I 2

5 i

i

I

20

I

40

i

i

i

60

IgN,

14.0 16.5 20.5 25.0 31.0

15.0 24.0 30.0 34.5

0.45" 1.20" 2.70' 5.70" 1.20"

38.0

109 l0 s l0 g 109 I0 l~

(era-3)

10.5

i

80

I0.0

fI~I~'I'RATIO8 OF ~l'ltkllg, Z

FIG. 3. Particle number density vs methane concentration. 1--temperature 1300 ~ 2--temperature 1400 ~

to the surface area of soot deposited in the hot zone of the reactor is 1.05. So the ratio of soot coated with pyrolytic carbon to that deposited in the hot zone is (1.05) a = 1.16. Therefore, the real yield of soot deposited in the hot zone amounts to 23/1.16 = 20 wt.%. Thus, the ftting coefficient taking into account the fraction of soot particles deposited on the hot walls of the reactor can be estimated for the first approximation as follows: K = (40 + 20)/40 = 1.5. The results of the calculation are given in Fig. 3. The results obtained clearly certify that the particle number density N is of first order in methane coneentration. A similar dependency was also observed for acetylene [9] and lately demonstrated for benzene. It should be mentioned that the linear dependence obtained for particle number density on hydrocarbon concentration is not influenced by inaccurate estimation of the fitting coefficient K. The results obtained for the second set of experiments are summarized in Table 1 and in Fig. 4. Dependence of the soot particle number density on temperature and methane concentration is approximated by the following equation: N = 9.4" 102~

exp(-a9,400/T)

(5)

where [CH4] is concentration of methane, mole fraction, and T is temperature, K. The methane-acetylene and methane-benzene mixtures (the third and fourth sets of experiments) were investigated using the technique described in

9,5

9,0

8,5 i

i

5,9

6.1

I

6.3

I

6.5

I

6.7

6.9

104/T, K"~

FIC. 4. Particle number density. Concentration of methane is 20%. Ref. 8. All the experiments were performed at temperature 1300 ~ The results were processed in the same way as in the work of Ref. 8. Fluxes of soot particles (Q, s -1) were obtained for both the individual hydrocarbons and their mixtures. The ratios of additive soot fluxes to experimental ones as well as to the soot fluxes of benzene or acetylene were calculated (Tables 9, and 3). In addition, the surface areas were calculated supposing that only acetylene or benzene can form soot particles, whereas methane is consumed only for the particle growth accorddr~g to Eq, (3) of fief. 8: A,~ = An~(1 + H) 1/3

(6)

where Am is the surface area of soot formed during

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SOOT AND POLYCYCLIC AROMATIC HYDROCARBONS TABLE 2 Fluxes of soot particles (Q/1012, s-1) for benzene-helium-methane(20%) mixture

Experiment (E) Additive calculation (A)

Benzene concentration (%) 0.4

1.0

1.7

3.8

4.5

8.0

E (mixture) E (benzene) A A/E (mixture) E/E (benzene)

0.19 0.25 0.46 2.42 0.76

0.60 0.62 0,83 1,38 0,97

1.02 1.06 1.26 1.23 0.96

2.10 3.36 2.56 1.22 0.90

2.65 2.80 3.00 1.13 0.95

4.80 5.00 5.16 1.07 0.96

TABLE 3 Fluxes of soot particles (Q/10a2, s -1) for methane-acetylene-helium(20%) mixture Experiment (E) Additive calculation (A)

Acetylene concentration, % 2.0

4.0

7.0

10.0

13.0

20.0

E (mixture) E (acetylene) A A/E (mixture) E/E (acetylene)

0.50 0.22 1.49 2.98 2.27

0.89 0.54 1.78 2.00 1.64

1.45 1.06 1.95 1.34 1.37

2.30 1.24 2.38 1.03 1.85

2.93 1.61 2.70 0.92 1.81

3.55 2.49 3.47 0.98 1.42

pyrolysis of the hydrocarbon mixture (m2/g), A. is the surface area of soot formed during pyrolysis of acetylene or benzene (m2/g), and H is the increase in the mass of soot particles from acetylene or benzene due to decomposition of methane molecules. When calculating the weight of the soot particles formed from methane molecules of methane-acetylene and methane-benzene mixtures, it was observed that degree of methane decomposition in the presence of hydrocarbons exceeded that from the methane-helium mixture. Taking this fact into consideration, the soot fluxes (q, g/s) for hydrocarbon mixtures and individual hydrocarbons, the additive soot fluxes, and the experimental values of the surface areas of hydrocarbon mixtures are given in Figs. 5 and 6. The curves obtained by Eq. (6) are also presented in these figures. The difference between the ordinates of the soot flux curves for the hydrocarbon mixtures and the additive calculation (the shaded area in Figs. 5 and 6) reveals the observed soot yield increase. For the methane-benzene mixture, a soot flux increase equal to 100 and 35% was observed at benzene concentrations 1 and 8%, respectively. For the methane-acetylene mixture, the soot flux increase was 6 and 13% for acetylene concentrations 4 and 16%, respectively. The higher promotion effect observed for the methane-benzene mixture compared with methaneacetylene and acetylene-benzene mixtures [8] can be explained as follows. First, the surface area of the soot formed from acetylene and benzene (34.0 and

30

~.i03 7,0

25

Z~~

20~

6,0

/~~~.~~ 5,0

~,,~ ~"

r 3,0 10

~

0 2,0

5

1.0

2,0

4,0

CO~ff~T'RATIOI'4OF

6,0

8,0

BE4 IZH~ I,Z

FIG. 5. Surfaceareaand sootfluxvs benzeneconcentration. 1, 2, 3--benzene-helium-methane (20%) mixture; 4--benzene-heliummixture; 2, 4---experimentalsoot flux; 3--additive soot flux.

PHYSICO-CHEMICAL PARAMETERSOF SOOT FORMATION DURING PYROLYSIS 30

A

25

20

i

5

2,0 ~

5........,..

1,0 4

8 12 C O N ( ~ r R A T I O N OF ~

16

20 , Z

FIG. 6. Surface area and soot flux vs acetylene concentration. 1, 2, 3--acetylene-methane-helium (20%) mixture; 4--methane-helium mixture; 5~acetylene-helium mixture; 2, 4, ~xperimental soot flux;3--additive soot flux.

32.0 me/g, respectively, at 1300 ~ is higher than that of soot formed from methane (20.5 me/g). Second, during the pyrolysis of the acetylene-benzene mixture [8], the mass of the acetylene molecules from which soot particles were formed was three times less than the mass of the benzene molecules consumed for only the growth of soot particles. In the experiments with methane mixtures, the ratio of the masses of soot particles formed from acetylene or benzene was two or six times higher than the mass of methane consumed in soot particle growth. Therefore, the total soot particle surface on which particles grow was higher, thus permitting the observation of the promotion effect. For these reasons, the promotion effect for the methane-benzene mixture was higher than for the methane-acetylene mixture. We have not observed the promotion effect in the work of Ref. 8 for this reason. So the soot yield was taken as additive. Because of the observed weight increase in the carbon formed from methane, the particle weight gain [H in Eq. (6)] caused by particle growth from methane was calculated from the experimental data obtained. Coincidence of the curves obtained by calculation according to Eq. (6) with the experimental curves for both mixtures can be considered satisfactory. But careful analysis reveals an essential difference. The results presented in Table 2 for the methanebenzene mixture show the same tendency as for the

657

acetylene-benzene mixture [8]. The ratio of the additive soot flux to the experimental mixture flux exceeds 1. The fact that this ratio approaches 1 with increasing benzene concentration is attributed to decreasing soot flux from methane to additive soot flux. The ratio of the experimental soot flux to that from benzene approaches i for all benzene concentrations (except benzene concentration 0.4%). This fact permits us to draw the conclusion that benzene fully inhibits soot particle inception from methane, and the latter is consumed for only soot panicle growth. Another result was observed for the methane-acetylene mixture (TabLe 3). The calculation by Eq. (6) satisfactorily coincides with the experimental data, and the ratio of the additive soot flux to the experimental mixture flux exceeds 1 at a low acetylene concentration. It should be mentioned that the ratio of the experimental soot flux to that for acetylene exceeds i for all experiments. The experimental results obtained reveal that, in spite of the acetylene inhibition of the soot particle inception from methane, the inhibition does not result in complete cessation of the particle inception from methane. This point is considered in the discussion. The observed inhibition of the soot particle formation from methane in the presence of acetylene and benzene can be attributed to a longer induction period for methane compared with both acetylene and benzene as was explained in Ref. 8.

Discussion The linear dependence of particle number density upon methane concentration obtained during pyrolysis of methane is also confirmed for acetylene [9] and lately for benzene. The soot particle number densities from both methane and acetylene are expressed by similar equations: methane N = 9.4 1020 [CH4] exp(-39,400/T) acetylene N = 3.0 1022 [C2H2] exp(-41,000/T) Some remarks were presented in works 10 and 11 that throw a shadow of doubt upon the results obtained from the investigation of soot formation during hydrocarbon pyrolysis by the calculation used in the present work, as the coagulation of soot particles was not taken into consideration. Coagulation certainly occurs during the process of soot particle formation. The optical measurements made during hydrocarbon pyrolysis in shock tubes and at fiat-flame combustion show the maximmn soot particle number density. The descending part of the

658

SOOT AND POLYCYCLICAROMATICHYDROCARBONS

curve on this maximum is interpreted as a coagulation. Unfortunately, the optical measurements do not make it possible to answer the question as to what particles are responsible for the decrease in particle number density at the initial stage of soot formation. Assuming that there occurs a coagulation of the spherical soot particles fonaaed, one has to use the term "coalescence" [12], i.e., a concept of liquid drops. It is very difficult to agree upon this concept. There are no experimental data available confirming that soot particles made of carbon atoms might be liquid at 2000 K. It may be supposed that a decrease of soot particle number density is connected with a molecular process of heterogeneous decomposition of the longchain polyyne molecules, as well as of soot particle nuclei on the surface. Krestinin [13] supposes and proves by calculation that the soot particle nuclei are made up of long-chain polyyne molecules. These particles contain up to 25-30 carbon atoms, so the optical systems can register them along with soot particles and tar. The chain structure of soot is an outcome of the particle coagulation. But this coagulation being accompanied by a decrease in soot particle number does not result in the decrease of the soot surface area. This is a well-known result, and it was proved by coincidence of the data obtained by measuring the soot surface area by the use of both adsorption and electron microscopy [1, p. 113]. The particle number density being measured in our work is the soot particles leaving the zone of soot formation. Certainly, we know little about the processes occurring in this zone, but the spherical soot particles leaving this zone have some concrete physical meaning. They are the elementary particles that determine the soot dispersion properties. The experimental results for the pyrolysis of methane-acetylene and methane-benzene mixtures allow us to conclude that acetylene is the main intermediate product on the pathway methane -* soot. Actually, it is a well-known fact that acetylene is the first compound formed at methane pyrolysis. Benzene is also formed at methane pyrolysis, but later. But in our work [8], it was shown that benzene cannot form soot particles in the presence of acetylene. Soot formation from methane via a stage of acetylene generation explains the observed difference in soot formation from methane-benzene and methaneacetylene mixtures. Obviously, the soot particles formed from acetylene inhibit the soot inception from methane as well as during pyrolysis of a methane-benzene mixture. But acetylene does not influence additional generation of acetylene molecules from methane. Acetylene formed additionally from methane molecules increases the concentration of acetylene and results in a surplus of soot particles,

thus giving an impression of formation of the soot particles from methane. Soot formation during pyrolysis of a methane-benzene mixture has been investigated in other work [7] at the same concentrations of the compounds and the same temperatures as in this work. The authors also observed the absence of additivity in particle number density. But they did not reveal the likeness of the soot fluxes from mixtures with the benzene ones. This can be attributed to nonisothermality of the experimental technique used.

Conclusions

1. During methane pyrolysis under isothermal conditions, the particle number density of soot depends linearly on methane concentration. The particle number density is described by the following equation: N = 9.4- 102~

exp(-39,400/T).

2. During pyrolysis of methane-benzene mixtures, soot particles are formed only from benzene, and methane is consumed only for particle growth. 3. Promotion of soot formation from methane is observed at pyrolysis of both methane-acetylene mad methane-benzene mixtures. Acknowledgment

The research described in this publication was made possible in part by Grant N FiNA000from the International Science Foundation.

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5. 6. 7. 8.

(D. C. Siegla and G. W. Smith, Eds.), New York, Plenum Press, 1981. Lahaye, J., and Prado0 G., in Chemistry and Physics of Carbon (P. L. Walker and P. A. Thrower, Eds.), Marcel Dekker, New York, 1978, Vol. 14, p. 167. Tesner, P. A., Formation of Solid Carbon fi'om Hydrocarbons in Gas Phase, Moscow,Khimiya, 1972. Prado, G., and Lahaye, J., in Particulate Carbon (D. C. Siegla and W. G. Smith, Eds.), New York, Plenum Press, 1981, p. 143. Tesner, P. A., and Shurupov,S. V., Combust. Sci. Technol. 92:61 (1993).

PHYSICO-CHEMICAL PARAMETERS OF SOOT FORMATION DURING PYROLYSIS 9. Krestinin, A. V., Tesner, P. A., and Shurupov, S. V., in Extended Abstracts of the Second International Conference on Carbon Black, Centre National Recherche Scientifique, Mulhouse, France, 1993, p. 23. 10. Wersborg, B. L., Howard, J. B., and Williams, G. C., Fourteenth Symposium (International) on Combus-

659

tion. The Combustion Institute, Pittsburgh, 1973, p. 929. 11. Harris, S. J., Weiner, A. M., and Ashcraft, C. C., Combust. Flame 64:65 (1986). 12. Harris, S. J., and Weiner, A. M., Annu. Rev. Phys. Chem. 36:31 (1985). 13. Krestinin, A. V., Khim. Fiz. 13:121 (1994).