Mechanism of carbon formation during steamcracking of hydrocarbons

Carbon, 1977, Vol. IS, pp. 87-93.

Pergaman Press.

Printed in Great Britain

MECHANISM OF CARBON FORMATION DURING STEAMCRACKING OF HYDROCARBONS J. LAHAYE, P. BADIEand J. DUCRET Centre de Recherches sur la Physico-Chimie des Surfaces Solides, 24, Avenue de President Kennedy, 68200 Mulhouse, France (Received 15 June 1976) Abstract-The authors investigated carbon formation during steamcracking of cyclohexane, toluene and n-hexane. The kinetic study of deposition and the observation of deposits by electron microscopy show that carbonaceous materials may be formed as follows: (1) In the case of cyclohexane and toluene, the initial hydrocarbon reacts in the gas phase and forms hydrocarbon species which condense into droplets, striking substrate on which they solidify (dehydrogenation). (2) In the case of n hexane, the hydrocarbon yields gaseous intermediate species which react on the substrate and form carbonaceous products

1.INTRODUCTlON

steamcracking of cyclohexane, toluene and n hexane has been carried out in an apparatus described in the Appendix. This apparatus permits the measurement of the variation of mass of carbon deposits formed on the surface of a solid substrate (cylindrical or flat), made of quartz, refractory steel (Refractory Stainless SteelUgine Z 15 CNS 20.10) or electrode graphite. For the kinetic study, essentially quartz has been used because, with this type of material, the apparatus may be cleaned between two experiments by a simple air oxidation at 800°C. The flow systems used may be characterised by the three following parameters: -molal fraction of hydrocarbon [HI in the hydrocarbon-water mixture; -temperature T, measured with thermocouple (10) (cf Appendix); -residence time t inside of the reactor which depends on the flow-rate of gas mixture; it cannot be determined exactly because of the variation of the gas volume due to chemical reactions inside of the reactor; t has been arbitrarily defined from the composition of the initial mixture, presuming the temperature inside the whole reactor as the temperature measured with thermocouple (101. Carbon deposition is characterised by the deposition rate u of carbonaceous material (mg/cm*/mn). The results of the study of the morphology of carbon deposits will be given in a second part.

During thermal decomposition of hydrocarbons, carbonaceous materials are usually formed either on the wall of the reactor or in the gas phase (soot-carbon black). In many systems, a competition between both kinds of carbon formation occurs [ 1,2]. Palmer [3] assumes that the species responsible for carbon black formation could also produce a carbon deposit on the wall of the reactor. The morphology and internal structure of the carbon deposit depend upon the result of the competition between the diffusion of the different species towards the wall and the kinetics of the gas phase reactions producing carbon nuclei. This assumption is in agreement with most of the studies on pyrolytic carbon formation[4-61. It is also in agreement with the mechanism of carbon black formation established during the last years [2,7-91. Indeed, it has been shown that, in system producing thermal carbon black, the initial hydrocarbon reacts and yields polynuclear aromatics which condense into droplets which are pyrolysed into solid particles before reaching the reactor wall. The model developed by Lahaye and Prado agrees with previous investigations [ 10-131. Grisdale et al. [ 10,111, in studying the formation of carbon on quartz and ceramics, during the pyrolysis of different hydrocarbons, assumed the formation of aromatic hydrocarbons and their subsequent condensation. Parker and Wolfhard[ 121 as well as Sweitzer and Heller [13] in studying carbon black production in flames considered “oil droplets” as intermediates between initial hydrocarbon and final carbon particles. During the steamcracking of hydrocarbons to produce ethylene, carbon deposits are formed. In the present investigations, we determined the mechanisms responsible for their formation[14]. We have studied three hydrocarbons, representative of the mixtures usually treated in steamcracking: cyclohexane, toluene and n hexane.

2.2 Experimental results The influence on the deposition rate of carbon u, of the residence time t, of the initial molal fraction [HI, of the hydrocarbon in the hydrocarbon-water mixture and of the temperature T are represented by the curves of Figs. 1, 2 and 3, respectively. The curves of Fig. 3 permit the computation of the apparent activation energies of the reactions of steamcracking of the three hydrocarbons. The values obtained are shown in Table 1, together with the activation energy of thermal decomposition of these hydrocarbons during homogeneous processes, in the absence of water vapor[l5,16].

2. KINETIC STUDY 2.1 Experimental The kinetic study of carbon formation during the 87

J. LAHAYE et al.

88

.4

.’

-3

-5

-4

/ I

-6

X

0

/

-6

/

/,=’ -7

/x /

X -7

-8

t 0.5

I

I. 5

2

t, set

Fig. 1. Influence of residence time t on the rate of carbon formation u. x , n hexane; 0, cyclohexane (T = 860°C; [If],, = 0.47); 0, toluene (T = 860°C; [HI,, = 0.42).

85

9

9.5

I04/T

Fig. 3. Influence of temperature T (“K) on the rate of carbon

formation u. x, n hexane; 0, cyclohexane ([&= 0.47; t = 0.7 set); 0, toluene ([HI,, = 0.42; t = 0.7 set). 3-

[Hlo

Fig. 2. Intluence of molar fraction [HI0 on the rate of carbon formation u. x, n hexane; 0, cyclohexane; 0, toluene (T = 860°C; t = 0.7 set).

These two different modes of behavior will be studied separately. Cyclohexane and foluene. The kinetics of steamcracking of both hydrocarbons corresponds to a firstorder reaction. The activation energies of carbon formation by steamcracking and of homogeneous decomposition of the corresponding hydrocarbons are equal within experimental error. It shows that the controlling kinetic step is a gas phase reaction. Let us assume an intermediate species M between the initial hydrocarbon H and the carbon deposit to be formed according to the equation:

3. INTFJtPmATION

The carbon deposition rate on quartz increases with temperature and residence time during the steamcracking of the three hydrocarbons. It increases also with the molal fraction for cyclohexane and toluene, while it remains constant for n hexane.

qHAM.

The deposition rate of carbon v will be proportional to the rate of formation of the intermediate species and to

Table 1. Activation

Homogeneous

Energy

(Kcal/mole)

ther-

mal decomposition in absence of water vapor

52.7 64.5

187

89

Mechanism of carbon formation during steamcracking of hydrocarbons

the surface of the substrate. v can be written u = Zk [HI, e+

(1)

where Z is a constant. If the rate constant k is expressed as a function of the activation energy of the reaction (E) and of the temperature (T) (as an Arrhenius relation with a preexponential factor a), then Log u = Log (aZ[E&) - +-

qta e-(E’Rr).

(2)

The logarithm of the carbon deposition rate 21will then be a linear function of T-’ if the term qta exp (- (EIRP)) is negligible compared with EIRT. This result has been verified in the whole range of temperatures studied for toiuene. The expression exp (- (EIRT)) is an increasing function of temperature, and, effectively, it is in the lower temperature range that expression (2) is verified in the case of cyclohexane. n hexane. During the steamcracking of n hexane: -the carbon deposition rate is independent of the initial molar fraction [Z& of the hydrocarbon; -in the interval OS-l.5 set the deposition rate is a linear function of the residence time; -in the interval 800-890°C the deposition rate follows an Arrhenius plot; -the apparent activation energy of carbon formation is different from the value given in the literature for the homogeneous decomposition of n hexane diluted with an inert gas. A reaction rate independent of the initial molar fraction of the reagent is, by definition, a zero-order reaction. This type of kinetics is generally attributed to heterogeneous reactions. Considering the temperature and the pressure of the system, the initial hy~ocarbon reacts in the gas phase and yields intermediate species (I) which react on the reactor wall and on the substrate. As the concentration of intermediate species in the gas phase [Zg] increases, their surface concentration [Z5] increases and reaches a limit. Thereafter the surface concentration becomes independent of the gas phase concentration and the order of the reaction becomes zero. This explanation implies that the surface reaction rate is lower than the diffusion rate of intermediate species towards the surface. The reaction of carbon formation can be written

[Z], increases with residence time of hydrocarbon in the reactor but in view of the zero-order reaction, [I], remains constant. Then, the carbon deposition rate should be independent of the residence time, in contradiction with experimental results. In fact, in the gas phase many chemical species are present. Those leading to carbon are represented by Z. f represents the species reaching the substrate without producing carbon. At the steady state, it may be assumed that the surface ratios of the substrate S1 and ,S, occupied by the adsorbed species (I) and (J) are proportional to their concentrations [I], and [ZJ, in the gas phase:

A sufficient prerequisite for the logarithm of the carbon deposition rate to be a linear function of the residence time t is Log @& = c, t C*f VI,

C, and C, being two constants. Species (J) and (Z) can be formed either in com~titive reactions (case 1) or in consecutive reactions (case 2) H-

1

J,

“i

By analogy with the results obtained with toluene and cyclohexane, all gas phase reactions are assumed to be first-order reactions. Let us consider both possibilities for (Z) and (J) formation Case 1

y

= Z&Y]

(6)

$4Zl = &[H].

(7)

After integration, eqns (6) and (7) yield

VI,

(3)

with C: carbon deposit; R: gaseous reactants (essentially hydrogen). In assuming reaction (3) to be first order with respect to L, the Arrhenius relation between Logv and l/T becomes evident. The in~uence of residence time is somewhat most dif8cult to understand. Indeed, for residence times between 0.5 and 1.5 see, the logarithm of the deposition rate is a linear function of t. As shown above, the deposition rate is determined by the decomposition rate of intermediate species on the substrate,

k2

H-Z,

IJI s=-L? Z,+ wall~wall~C+~

(5)

k k;’

(8)

In that case, the ratio of gas phase concentration of species (I) and (J) is independent of t and it does not explain the experimental results. Case 2: In agreement with Benson[l7],

d -=WI dt

F

- k?fH]

= k,[H] - k:[J],

(9)

(10)

J. LAHAYEet al.

90

4.1 Carbonaceous

(11) toluene By integration of eqns (9)-(ll), it can be shown that

m, _ k: - k, -_VI,

1 - exp ( - k,t)

k3 exp(-kgt)-exp(-kit)’

(12)

Equation (12) is independent of [Hlo. If the activation energy of the gas phase reactions are of the order of magnitude of 50 kcal/mole, and the preexponential factors of the Arrhenius expression of the rate constants close to 10’*[16],eqn (12) becomes 1 I&J-k, k3 exp(-kgt)-exp(-kit)’ VI,

(13)

For case 2 to be different from the reaction H-I,

it is necessary that k, > k:. If k, B k; eqn (13) becomes

VI, Logm=Logkl

kk:+k,T 3 I

an expression identical to (5). Thus, the kinetic interpretation is in agreement with all experimental results. It also shows the presence, in the gas phase, of species (J) intermediates between the initial hydrocarbon and the precursor (I) of carbon. Contrary to the case of cyclohexane and toluene, carbon deposits formed by steamcracking of n hexane are formed by decomposition of gaseous species at the surface of the substrate. The kinetics of carbon formation is controlled by the heterogeneous decomposition on the substrate of species produced in the gas phase. The kinetic study of steamcracking of n hexane, cyclohexane and toluene points to a marked difference in behavior between the different hydrocarbons. The kinetics of carbon formation from n hexane is controlled by an heterogeneous gas-substrate reaction. The controlling step of steamcracking of cyclohexane and toluene is a gas phase reaction. In the latter case, the kinetic study gives no information on the physical state (gas, liquid or solid) of the carbonaceous species which strike the substrate. Observation of deposits by electron microscopy completes and prefects the kinetic study.

deposits

from

cyclohexane

and

Deposition on graphite and quartz substrates. The carbon deposited on graphite substrates during steamcracking of cyclohexane (Fig. 4) consists of tridimensional aggregates of spherical particles about 8 pm dia. This observation indicates that carbon deposits result from impacts with the substrate of liquid or solid spherical particles formed in the gas phase. With a quartz substrate, the deposit appears as a smooth film in spite of the presence of some globules (Fig. 5). Therefore, liquid droplets are responsible for carbon film formation. In analogy with the mechanism of carbon black formation, described elsewhere[7,8,10-131 it may be assumed that the liquid droplets consist of polynuclear aromatic hydrocarbons formed by chemical reaction of the initial hydrocarbon in the gas phase followed by nucleation in the gas phase, of the molecules formed. The droplets spread on the quartz; they do not wet the porous graphite substrate and therefore, keep their spherical shape. The difference in behavior between graphite and quartz substrates may be compared to the results we obtained in the study of wettability of cokes by tars, carried out in order to have a better understanding of the preparation of artificial graphite [ 181.The

Fig. 4. Steamcracking of cyclohexane. Carbon deposit on graphite(X 860).

4. MORPEOLOGY OF CARBON DEFOSlTS STUDIED BY JXJXTRON MICROSCOPE

The carbonaceous materials formed in the apparatus previously described have been observed with a JEOL 100 B electron microscope equipped with a scanning attachment. Three different substrates have been used: quartz, electrode graphite and refractory steel.

Fig. 5. Steamcrackingof cyclohexane.Carbon depositon quartz (Xlam.

Mechanism of carbon formation during steamcracking of hydrocarbons materials decreases with its porosity. The viscous microdroplets are converted into solid particles essentially by dehydrogenation. The carbon deposited on graphitic substrates during steamcracking of cyclohexane has an elemental composition of 99.21% carbon and 0.71% hydrogen.

wettability of carbonaceous

Fig. 6. Steamcracking of cyclohexane. Carbon deposit on refractory steel ( X 1000).

Fig. 7. Steamcracking of heptane at 830°C.Carbon filament formed on refractory steel (X 2.106).

91

Deposition on refractory steel substrate. Carbon deposited on refractory steel presents an unusual morphology (“cauliflower” morphology). It appears that droplets are caught at the substrates surface on preferential sites, each site giving rise to the formation of a particle magma approximately semispherical (micrograph 6). It is well known[l9] that filaments with a coaxial hollow channel are formed when a hydrocarbon decomposes on a heated transition metal. As an example, micrograph 7 represents such a filament and micrograph 8 its extremity (presence of a metal particle). The filaments are not randomly distributed on the surface; they are formed on preferential sites. During the steamcracking of cyclohexane and toluene, before the substrate is completely covered with carbon, filaments are formed simultaneously with liquid microdroplets produced in the gas phase. In agreement with micrograph 9, it may be assumed that filaments or groups of filaments are able to trap the droplets before they reach the solid substrate leading to a build-up of particle magma. It is noticed that a “cauliflower” shaped particle magma is not specific to carbon; it can be formed in the production of silica from gaseous silicon compounds. The morphology of such magmas has not been explained.

4.2 Carbonaceous deposits from n hexane Contrary to cyclohexane and toluene, the kinetics of steamcracking of n hexane is rate controlled by the heterogeneous decomposition of gaseous species of the

Fig. 8. Steamcracking of heptane at 830°C. Extremity of carbon filament on refractory steel (X 3.106).

J. LAHAYE et al.

92

Hydrocarbon

Fig. 11. Experimental device. Fig. 9. Steamcracking of cyclohexane. Carbon deposit on refractory steel ( X 30,000).

cracking of hydrocarbons. The mechanism, in the case of cyclohexane and toluene may be compared with the mechanism generally assumed to describe the formation of carbon blacks. Indeed, in both cases, nucleation of droplets occurs. At the temperature of steamcracking (800-103O”C),the chemical evolution is too slow for the liquid droplets to solidify before reaching the substrate. At the temperature of carbon black formation (above 1KKPC)the solidification of the droplets occurs in the gas phase. Acknowledgements-This work was supported by C. D. F.Chimie. The authors gratefully acknowledge Drs. G. Henrich and P. Gillet for their comments on the results.

Fig. 10. Steamcracking of n hexane. Carbon deposit on quartz ( x 3000).

solid substrate. Such a decomposition is expected to produce a film of carbon. As predicted (Fig. lo), the tilm is smooth, presenting, however, some holes but no rugosity or particle shaped areas. 5. CONCLUSION The comparison of kinetic data and observations by electron microscopy permitted the determination of the mechanism

of carbon

deposition

during

the

steam-

1. J. D.Brooks, W. R. Hesp and D. Rigby, Carbon 7,261 (1969). 2. J. Lahaye and G. Prado, In Chemisfry and Physics of Carbon (Edited by P. L. Walker, Jr. and Peter A. Thrower), Vol. 14. Marcel Dekker, New York (1976). 3. H. B. Palmer, J. Chim. Phys. Special Issue, 87 (1%9). 4. H. B. Palmer and C. F. Cullis, In Chemisrry and Physics of Carbon (Edited by P. L. Walker, Jr.), Vol. 1, p. 265. Marcel Dekker, New York (1965). 5. F. Tombrel et J. Rappeneau, Les Carbones, Tome II, p. 783. Masson et Cie, Paris (1%5). 6. J. C. Bokros, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.), Vol. 5, p. 3. Marcel Dekker, New york (1%9). 7. G. Prado and J. Lahaye, J. Chim. Phys. ll-12,1678 (1973);4, 483 (1975). 8. J. Lahaye, G. Prado and J. B. Donnet, Carbon 12, 27 (1974). 9. J. Lahaye and G. Prado, Petroleumlkived Cargons (Edited by M. L. Deviney and T. M. O’Grady)A.C.S. Symposium Series 21, 335 (1976). 10. R. 0. Grisdale, A. C. Pfister and W. van Roosbroeck, Bell. System Tech. J. 30, 271 (1951). 11. R. 0. Grisdale, J. Awl. Phys. 24, 1082(1953). 12. W. G. Parker and H: G. Wolfhard, J. Chem. Sot. 2038(1950). 13. C. W. Sweitzer and G. L. Heller, Rubber World 134, 855 (1956). 14. P. Badie, Ph.D. Dissertation Thesis, Universite Louis

Pasteur de Strasboura et Centre Universitaire du Haut-Rhin (1975). 15. G. M. Badger, Prog. Physical Organic Chem. 3, 1 (1975).

Mechanism of carbon formation during ste~~racking of hydrocarbons 16. L. Szepesy, V. Illes, K. Welther and J. Simon, Actu Chimica Scientiarum Hungaricae 78(4), 341 (193). 17. S. W. Benson The Foundations of Chemical Kinetics.

McGraw-Hill, New York (l%O). 18. J. Lahaye, J. P. Aubert and A. Buscailhon, 4th London Int. Carbon and Graphite Conf., Extended Abstracts, p. 30 (1974). 19. R. T. K. Baker, P. S. Harris and R. B. Thomas, Carbon 72 (Baden-Baden), Preprints p. 291. APPENDE Description of the apparatlis

The apparatus (Fig. 11) includes three main parts: (1) A Mac Bain type of balance (I) (coiled quartz spring), thermostated with water at 85°C. The substrate (2) is suspended from the spring with a quartz thread. A stream of nitrogen (9I/hr)

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is passed through the thermostat jacket producing a slight overpressure at the 2 mm diameter neck (4). (2) Injection device-the liquid hydrocarbons and water are injected with a double head metering pump (Lewa Herbert Ott KG, Type F1.2). The flow delivered is from 0 to 400cm’/hr for one head (hydrocarbon) and from 0 to iSOcm’/hr for the other (water). Water is vaporised at 500°Cin furnace (5). Water vapor and hydrocarbon are mixed in (6). The gas stream is preheated in (7). (3) The reactor is a quartz cylindrical tube of 2cm i.d. and 85 cm long. It is heated with a furnace with three independent adjustable resistances in order to obtain a flat thermal profile (ADAMEL RT 5 HT). The temperature T is measured with a PtiPt - Rh 10%thermocoupIe, located at the external wall of the reactor, between wall and furnace, at the level of the suspended substrate.