Survey of Recent Methane Pyrolysis Literature A survey of methane pyrolysis data is presented and discussed M. S. Khan and Billy L. Crynes
heoretically, methane is important because of its T u n i q u e stability, and also because its decomposition is easiest to interpret. For kinetic and mechanistic investigations of hydrocarbon pyrolysis, the simple methane molecule lends itself to this type of study, more so than the more complex hydrocarbons. There are fewer product and intermediate species involved in methane pyrolysis compared with other hydrocarbons. I n the past ten years much work has been reported on high-temperature reactions of methane. Although its study has occupied many investigators, the reported data and results are so widely scattered i n the literature that it is difficult, if not impossible, to find a comprehensive summary. Therefore, the primary purpose of this review is to compile methane p)rolysis data for those interested in this area. The review is based on literature published in both technical journals and patents. Attention is given to literature published after 1960, since summaries of earlier work are available. Studies on shock-tube pyrolysis of methane are reviewed first, followed by a review of nonshock-tube investigations, which include both conventional and a variety of nonconventional reactors and techniques. Shock-lube Pyrolysis of Methane Skinner (74) presented shock-tube data on the pyrolysis of methane for a temperature range of 1200"-1400"K (1700"-2060°F) under homogeneous reaction conditions. Starting with methane, the pyrolysis is represented as occurring stepwise
CH4 + CzHe
+
CzH4 + CzH2
--t
C
At the highest temperature, the ethane and ethylene are short lived. Methane and ethane decompose by freeradical reactions of short and long chain lengths, respectively, while ethylene and acetylene decompose by molecular reactions not involving free radicals. [Benson and Haugen in J . Phys. Chem., 71, 1737 (1967)) disagree about the molecular decomposition of acetylene and ethylene. They suggest a free-radical scheme. ] Gas samples were analyzed by a vapor chromatograph before and after the reaction. The experimental overall 54
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
activation energy was found to be 101 kcal per g mol, which suggests that the initiating reaction is CH4 --.t CH3 H. The preferred scheme presented by Skinner is as follows:
+
CH, H
---t
+H CH3 + Hz
CH3
+ CHe+
2 CH3 4 CzHs
(1) (2)
(3)
The mechanism of conversion of methane to ethane is still unsettled, largely on the basis of observed activation energy. Investigators have reporred the overall activation energy varying from 86 to 103 kcal. The decomposition of methane in a single pulse shocktube reactor at 1656'-1965'K (2521 '-3077°F) was studied by Kevorkian et al. ( 4 ) . Argon was used as a diluent. The reaction followed first-order kinetics with exp (-93,000 ' an overall rate constant k = 1.32 X RT)/sec. The results indicate that a high-temperature homogeneous reaction of methane is not hydrogen inhibited as suggested by some workers. 4 reaction mechanism involving the decomposition of methane to CH2 (methylene) was proposed to explain the major observations.
CH:!
+ Hz CH3 + CH3
CH4 -+ CH2
(1)
+ CH4 -+
(2)
+ CH3 CH2 + Hz CHI + H CH2 + Hz CH3
--.t
-.t
--t
CzH6
Products
+H CH3 + H B
4
CH3
CH4 (Termination)
(3) (4)
(5) (6)
The products of Step (3) are probably ethylene, acerylene, carbon, and hydrogen. Another kinetic analysis of methane from shock-tube studies, using a single pulse technique, has been worked out by Kozlov and Knorre (5). A methane-argon blend (1 : 9 ) ) at 4 atm, and 1700"-2200"R (2600°-35GG0F) was used as reactant gas, while helium was used as the accelerating gas. The reaction time was about 0.8 msec,
and the overall sequence was described as kl
CH4 + C2Hs
ka 4
CzH4
2 C2Hz -% C
This is essentially the same sequence suggested by Skinner (74). However, the initial step is believed to be H,, in agreement with Kevorkian et al. C H I -t CH2 (4) rather than with Skinner. Yano a n d Kuratani (78) of Tokyo University studied the thermal decomposition of equimolar mixtures ?f methane a n d tetradeuteromethane a t around 1560°K (2348"F), using a single pulse shock-tube. The initiation step of methane pyrolysis was concluded to be the splitting to a methyl radical a n d a hydrogen atom. Using adiabatic compression and expansion in the temperature range 1400°-25000K (2060"-2372°F) in argon a n d nitrogen, Volokhonovich et al. (76) reported another value for the first-order rate constant as jog kl = 13.04 22,1OO/T, T h e calculated activation energy was 100 kcal. The initial step was described as C H 4--t CzHeand the proposed scheme was as follows :
+
acetylene a n d higher hydrocarbons. They concluded that under the experimental conditions, the methane cracking was a heterogeneous, autocatalytic reaction, H, and with a n w'ith the initiation step of CH4 --t CH3 acceleration step of CH3 + C H Z H occurring o n the carbon surface. A study of the use of methane and hydrogen to form acetylene was carried out by Kunugi et al. (6). The methane and hydrogen mixture was cracked a t atmospheric pressure in a tubular quartz reactor (10-20 mm i.d.) packed with quartz. T h e reactor was heated externally by an electric furnace, a n d the reactor temperature varied &om 110O0-14O0"C (2012'-2552"F). At first the products were analyzed by chemical analysis, and later, by gas chromatography. Fop the proposed mechanism, the reaction rate constants and activation energies were given as
+
-
CH4 4 C Z H G --t C2H4 -.CzH2
kl
CH4
k4
ko
Nonshock-Tube Methane Pyrolysis T h e thermal decomposition of methane, over the range of 950"-1630°C (1742'-2966"F), a t different prkssures, a n d a t a contact time of less than one minute was studied by Schneider and Murgulescu (77, 72). The gaseous reaction products were analyzed chromatographically. T h e overall activation energy, assuming that the reaction is first-order, was determined to be 86.6 kcal. At atmospheric pressure, this method indicated that the thermal decomposition of methane follows two parallel reactions
C H 4 4 C + 2Hz
* CzHz -. C + T a r ha
CH4
-. Carbon black
+ 3 Hz
CzH4 ka
ka
This series scheme is also in agreement with those reported by Skinner (74) and by Kozlov and Knorre (5).
2 CH4 --t CzHz
+
(1) (2)
This is in contrast to the series reactions suggested by the shock-tube work. T h e maximum acetylene concentration in the reaction product increased with temperature ( 7 7). A decrease in the pressure favored the formation of acetylene a t a lower temperature (72). T o keep acetylene a n d ethylene concentration as high as possible, the reaction mixture was quickly quenched to inhibit polymerization reaction of the two compounds. Above 1035°C (1895°F)) higher gaseous hydrocarbons were formed, as well as aromatic hydrocarbons, especially benzene and naphthalene, together with tar and carbon (77). Shantorovich and Pavlov (73) studied thermal cracking of methane a t 0.2 mmHg, 1293"-1373"C (2359"2503'F)) and with 17 mmHg of helium in a porcelain tube of 1.8 cm diameter. T h e first-order rate constant was given by log k = 11.60 - 17,68O/T, a n d the approximate overall activation energy was 90 kcal. T h e products were identified by a mass spectrometer and were mainly ethylene, ethane and, a t long contact times,
CHI
C ko
log ko log ki log kz log(k2 log k4
+ Tar
(3)
4,358 - 5,922/T; Eo = 27.6 kcil - 12,158/T; E1 = 56.0 kc$l = 12,836 - 19,214/T; E2 = 88.5 kcal k3) = 13,453 - 20,30O/T; Ep+3) = 93.5 kcal = 4,251 - 5,515/T; E4 = 75.4kcal =
= 7,642
+
T h e results were explained by a methane decomposition model incorporating three reversible, simultaneous, firstorder reactions, as shown above, protlucing ethylene, acetylene, carbon, and tar. T o correlate carbon a n d tar forination from acetylene, a fourth reaction was proposed. Such models are, in fact, semiempirical and not truly mechanistic in nature. They only represent the overall reactions and do not show the free-radical scheme that occurs, a t least in part. T h e rate of carbon film formation during methane pyrolysis was studied by Palmer and Hirt (7). The electrical resistance of a carbon film formed on a glazed porcelain rod centrally located within a pyrolysis tube was used as a measure of film thickness. T h e gas to be pyrolyzed entered the annular reactor in a helium carrier. After a short induction period, carbon deposited within the reactor a t a steady rate. This suggests that the reaction is autocatalytic with respect to carbon. The effective decomposition rate constant for methane over the approximate temperature range 115O0-13OO0C (2102'-2372"F) was given by R = IO1*.' exp (-101 ktal/ R T ) sec-', i.e., E = 101 kcal. This value compares closely to the earlier study of Palmer and Knox ( 8 ) . While studying bond dissociation energies in small hydrocarbon molecules, they suggested that the best value for overall activation energy for the reaction CH4+ CH3
+H
was 101-102 kcal a t 0°K. VOL. 6 2
NO.
10
OCTOBER 1970
55
T h e study of Tesner and Mar'yasin (75))which was reported in 1961, described specifically the formation of carbon during thermal decomposition of methane. Un. like Palmer and Hirt (7), they covered a temperature range from 1400"-1700°C (2552O-3092"F). A graphite rod (diameter 12 mm, length 500 mm) was located inside the reaction zone (18 m m diameter) of a high temperature furnace fitted with a molded graphite heater. A mixture of methane (2-10y0) and hydrogen was passed through the annular space between the rod and the walls of the reaction tube. The gas was passed through a t 4-10 l./min. Higher methane concentrations promoted soot formation, while lower concentrations resulted in gasified graphite. When nitrogen or helium was used instead of hydrogen for dilution of methane, different results were obtained. With methane concentrations in excess of 1%, soot formation could be observed a t a temperature of 1500°C (2732°F)) and it was impossible to study the thickness of the carbon deposit. The activation energy of the methane decomposition was 36 kcal. The dependency of the reaction rate on temperature was expressed by
r
=
2,042 C exp ( - 18,20O/T)
where C is the concentration of methane in g/cc. The rates were not affected by diffusion. Happel and Kramer ( 2 ) reported that methane, either alone or diluted with hydrogen, can be pyrolyzed to give high yields of relatively uncontaminated mixtures of acetylene and hydrogen over the temperature range of 1500"-2000°C (2732"-3632°F). XIetered h) drogen and methane feed gas were passed through an electrically heated reaction chamber brought to predetermined conditions, and then the reactor effluent gas stream was rapidly quenched. Maximum reaction temperature obtained was 1900°C (3452°F). Results wcre obtained for methane cracking under vacuum, and at atmospheric pressure using hydrogen dilution. The reactor design was a system of vertically mounted cylindrical tubes, which were progressively larger in diameter, and all were contained within the largest outermost gas-tight metal tube. The annulus between the tube was the reaction zone (alumina walls). These concentric cylindrical alumina tubes were placed inside a graphite cylinder, which served as the heating element. The model adopted for kinetic study involved the following reactions : (11
CH4+C+2Hz CH4
+
1/2 CzHz
C2Hz + 2 C
+ 3/2 HL
+ Hz
(2) (3)
Appropriate rate equations correlating the data were given as follow7s :
M . S. Khan zs an Assistant Edztor, Chemical Abstracts, Columbus, Ohio, and Bzlly L Crynes is an Associate Professor, School of Chemzcal Engzneering, Oklahoma State Unzcerszty, Stzllreater, Okla. 74074. AUTHORS
56
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
r3
=
k3
P2 CzHz 1 +kPHz
where Pz equals partial pressure of specie i, and k equals 1.535 atm-'. The rate constants were given by k , = A , exp ( - E , / R T ) and activation energy and frequency factor were given as follows: E1 = 16,200 cal/g mol A1 = 25 E z = 86,450 cal/g mol A2 = 3.219 X 10'0 E 3 = 17,700 cal/g mol A B = 2190
g mol/l. sec atm g mol/l. sec atm g mol/l. scc atm2
This scheme is similar to that proposed by Kunugi et al. ( 6 ) ) excluding reversible reactions and ethylene formation. There is good agreement on the activation energy of the acetylene formation step, 86.4 and 88.5 kcal, respectively. Another recent kinetic study of methane pyrolysis was undertaken by Eisenberg and Bliss ( 7 ) . A tubular reactor was employed in the study that provided for the sudden heating of methane to reaction conditions by using a hot stream of nitrogen, followed with a cold nitrogen quench. The method was fully tested and exhibited flat temperature profiles (1"-2°C) so that varying contact times were possible. Runs made at 1100"1200°C (2012"-2192°F) showed conclusively that methane pyrolysis was not a first-order reaction in this temperature range, as most often reported. hIethane rate of pyrolysis was described roughly with the normal growth curve-an accelerating portion, a straight line, and a decelerating region. The rate was accelerated by ethane in the feed in agreement with Schneider (70) and inhibited by hydrogen, which is in disagreement with Kevorkian et al. ( 4 ) . Surface effects were minor. A ne" kinetic model was proposed as follows : (11
C H ~ ~ C H ~ + H CH3
+ C H 42 C2H6 + H
(2)
C2HB-% 2 C H a
(3)
2 Hz + CzH4 2 H:! + CzHs 2 2 C 4-
C2HB
H2
+ CH3 4 CH, + H
Ha
(4)
(5)
The rate constants were given by kl k:! k3 kq k5
= = = = =
4.50 3.60 3.05 3.01 4.00
X X X X X
sec-' l o 6 l./mol sec l o 2 sec-I l o 2 sec-' l o 4 l./mol sec
Noteworthy about this particular study was the use of the free-radical scheme to mechanistically model methane pyrolysis using a digital computer. The flow system studies of Happel and Kramer (2) predicted an overall activation energy of 85 kcal, while
hydrogenation to acetylene was the only reaction of ethylene during the pyrolysis of methane.
shock-tube studies--i.e., Skinner (74)-predicted an overall activation energy of 103 kcal. I n the hope of understanding the discrepancy between ihese two studies, a new study of thermal decomposition of methane was carried out by Palmer et al. ( 9 ) . A flow system similar to that used by Eisenberg and Bliss (7) was employed. T h e reactor was basically a hot porcelain tube of 5 m m i.d. Methane a t a concentration of about 1-20 vol yo was carried into the reactor in a stream of helium at a total pressure of 740 mmHg. Products were analyzed chromatographically. Residence times ranged from 0.1 to 0.9 seconds, with a 1323"-1523 O K (1922°-22820F) temperature range. From the study, Palmer et al. concluded that nucleation of carbon in the gas phase causes the decomposition to accelerate because of heterogeneous decomposition of methane on the nuclei. T h e rate was approximately first-order. This study supported the results obtained by Skinner (74), and suggested that the shock-tube data was the best available representation of the rate constant for the homogeneous first step in the decomposition mechanism. Yampal'Skie et al. (77) studied the methahe pyrolysis in a graphite reactor in an atmosphere of argon a t 1300220OOC (2373"-3992"F) for 0.0001-0.001 6ec contact times. They showed that the conversion of methane via dehydrogenation to ethylene, and then to acetylene was accompanied by direct decomposition, and was catalyzed by the graphite surface. This model incorporated the features of both the series and parallel schemes predicted by others. T h e elemental decomposition and condensation reactions were significantly reduced by using hydrogen as diltlent, and conducting the reaction under laminar flow conditions to obtain a selectivity of over 80% for the formation of ethylene and acetylene. Besides its effect in suppressing both the elemental decomposition and the condensation reaction, hydrogen also decreased the overall conversion of methane, which was a first-order reaction. I t was also reported that de-
Summatibn and lnterprefation
Although there has been considerable interest and research on methane pyrolysis in recent years, the kinetics and mechanism are not yet fully understood. Different techniques used in the study of methane pyrolysis included the shock tube, flow and static reactor, plasma jet, steam cracking, etc. T h e literature indicates that more emphasis has been given to shock-tube studies of the decomposition of methane than to studies using flow or static reactor systems. I n shock-tube studies, it is possible to investigate reaction behavior a t considerably higher temperatures, a n d with extremely short residence times. The studies investigated in this review have been condensed and are presented in Tables I a n d 11. Shown are operating conditions, results, reactions, a n d other informati on. Operating conditions. Data reported here covered a temperature range of 927 '-2200 "C (1700 "-3992 O F ) . Much of these data were taken a t essentially atmospheric pressure, but some wark was reported as high as 4 atm (5). Various diluents, such as argon, hydrogen, tetradeuteromethane, and helium were used. Order of reaction. T h e overall order of reaction is generally accepted as being first-order. Contrary to the other workers, Eisenberg and Bliss ( 7 ) reported that the reaction was not first-order, while Palmer et al. (9) reported that the reaction was approximately first-order. As with most pyrolysis reactions, the overall decomposition of methane approximately follows a firskorder reaction. However, the mechanism of pyrolysis involves free radicals and probably some molecular steps. With increasing conversions and temperatures, various secondary reaction steps can become important. Also, evidence now exists to illustrate the importance and influence of surfaces and solid carbon that may form during ~~
TABLE I. S U M M A R Y Operating Condztions 927°-11270C (1700-206O'F)
1383O-1 G 9 2 O C (2521°-3077'F)
Rcactor or Method Used Shock-tube reactor
OF SHOCK-TUBE STUDIES OF METHANE PYROLYSIS
Overall activatton energy E = 101 kcal
First-order reaction, Initiating step; CH4 -+ CHa f Ha Not hydrogen inhibited
Overall activation energy E = 93 kcal (accurate within
+
Single pulse shocktube reactor
*IO%) Frequency factor A = 1 32 X 10'4 sec-1
1427°-11270C (2600-3100°F) Pressure: 4 atm Reaction time: 0.8 msec CHa-Ar blend (1 : 9 )
Single pulse shocktube reactor
Initiating step: CHI -+ CHz 4- Ha
1287'C (2348'F) CHg-Tetradeuteromethane mixture
Single pulse shocktube reactor
Initiating step: CH4- CHa f H
112?0-13000C
Thermal cracking under adiabatic compression and expansion in argon and nitrogen
First-order reaction
(2060°-2372'F)
-
Kinetzc Constonts
Results FirBt-order teaction, Initiating step' CHc- CHs H
Reactions
- -+
CHI -+ Czks CzHe -+ CaHz OH4 -+ CzHo goes by CHI CHI H H f CH4 CHa Hz 2 CHa --t CzH6
-
Reference
C
+
+
Kevorkian et al ( 4 )
CH4 -+ CHz Hz CH4 -+ CHs f CH3 C H I -+ CzHs -+ Products CHa Hz -+ CHa H CHa f Hz CHI f H CHs Hz CHa(Termination)
CHz CHfa
+ + +
CH4-
--
CzH6
Skinner (74)
+
CzH4 -+ C2Hz
-
C
Kozlob and Knorre ( 5 )
Yano and Kuratani (78)
Overall activation energy: E = 100 kcal Rate constant: log k i = 13.04 - 22,i oo/ 7-
CHa -+
- -
CzH6 CzH4 -+ CzHz Carbon black
VOL. 6 2 NO.
10
Volokhonovich e t o l . (lis)
OCTOBER 1970
57
TABLE I f . Operuting Condilions
S U M M A R Y OF NONSHOCK-TUBE S T U D I E S OF METHANE PYROLYSIS
Reactor o r Method Used
Kinefic Constants
Rerulis
95Oo-163O0C (1 742°-29660F)
First-order reaction, Maximum CZHI
Atmospheric and other pressures, contact time less than 1 min
Concentration increases with temperature and with decrease in pressure
1293°-13730C (2359'-2503'F) 0.2 mmHg pressure
Overall activation energy: E = 86.6 kcal
Thermal cracking First-order reaction, in porcelain tube Initiating step: of 1.8 cm CHI- Cya f H diameter Accelerating step: CHs+ CHz H Heterogeneous autocatalytic reaction
Overall activation energy: E = 90 kcal Rate constant: logk = 11.60 - 1 7 , 6 8 0 / 3
Tubular quartz reactor packed with quartz
Activation energies: Eo = 27.6 kcal E1 = 56.0 kcal Ez = 88.5 kcal E z + 3 = 93.5 kcal E d = 75.4 kcal Rate constants: 5,992/T log eo = 4,358 logki 7,642 12,158/T 19,214/3' log ka = 12,836 log (kz ka) = 13,453 -20,300/ T lOgk4 = 4,251 - 5,515/T
+
Reactions
Reference
2 CHI -c CzHa f 3 Hz CH4 -+ C 2 Hz
+
Schneider and Murgulescu ( 1 7 , 12)
Shantorovich and ( 13)
Pavlov
kl 1100~-1~00"c (2012°-22520F) Atmospheric pressure CH, - Hz used as mixture
First-order reversible reaction
+
---
130O0-22OO0C (2372'-3992"F) Contact time: 0.0001-0.001 sec
First-order reaction. Graphite reactor Hydrogen inhibits and in an atmosphere decreases the overall of argon conversion of methane
1150°-1 300°C (21 02"-2372'F)
Annular reactor with porcelain rod
First-order reaction, Rate of carbon film formation was studied Initiating step: CHI -+ CH3 f H
1400°-17000C (2552°-30920F) Mixture of methane @-lo%) and hydrogen was used
Reactor with a graphite rod (diameter 12 mm, length 500 mm)
Rate of formation of car- Activation energy: E = 36 kcal bon during thermal decomposition was dis- Reaction rate: 7 = 2,042 Cexp cussed. Higher methane concentra(-18,20O/T) tion promoted soot formation while lower concentrations resulted in gasified graphite. No diffusionaleffects
1500°-20000C (2732'-3632'F) Pressure: 70-800 mm Hg Mixture of hydrogen and methane was used
Annular reactor with alumina walls
First-order reaction in methane
11000-120p~c (2012O-21 92OF)
Not a first-order reacTubular reactor, heated by hot tion, nitrogen followed Initiating step: CH, -+ CHa f H with a cold nitrogen quench Ethane accelerate the rate. Surface effects are minor. Hydrogen ' inhibits
ko k? CH4
58
l h e r m a l decomposition of methane by addition of ethane
First-order reaction, ethane exhibit strong accelerating effects that decrease with increasing temperature. Not observed above 15OO0C. T h e composition of product is not influenced by addition of methane
105Oo-125O0C (1 922°-22820F) Pressure: 740 mmHg 120 vol % CH4 in helium Residence time: 0.1-0.9 sec
T h e rate is approximateTubular reactor basically a hot ly first-order. Claimed porcelain tube. that carbon particle accelerate decomposiMethane entered the reactor in a tion. Heterogeneous stream of helium effects were noted
INDUSTRIAL A N D ENGINEERING CHEMISTRY
ka
CHI
C f Tar
ko
CH4-+ CzH4 -+CtHn -+ 2 C HZ CHI -+ 2 Ha C
Yampdl'Skle el (I!. (77)
CHI -+
Palmer and Hirt (7)
-+ CHs -+ H
Tesner and Mar'yasin (75)
Activation energies: CHa -+ C 4- 2 Ht CH4 -+ 1/2 C?Ha 3/2 H2 E1 = 16.2 kcal Et = 86.45 kcil CzHz-2C f Hz Ea = 17.70 kcal Frequency factors: A I = 25 g mol/l. sec atm Az = 3.219 X 10'0 g mol/l. sec atm A3 = 2190 g mol/l. sec atmz
+
Rate constants. k l = 4.50 x 10-3 sec-1 kz = 3.60 X 108 I./mol sec ka = 3.05 x 102 s e c - ~ k4 = 3.01 X 10%sec-1 ks = 4.00 X 104 ]./mol sec
Happel and Kramer ( 2 )
k1
CHI -+ CHa f H kz
CHI $. CH4
-+
Eisenberg and Bliss ( 7 )
CZHSf H
k3 -+
ki CZHS-+ H Z Hz Ha
105Oo-155O0C (1922°-28220F) Pressure: 1 atm
CzHa -+ C 4- T a r
d
ko
+
Overall activation energy: E = 101 kcal Rate constant: k = 10lr.lexp (-101 kcal/RI') sec-1
Kunugi el ai. (6)
CH4 F? C2H4
+ CZH4 ks
-*
kl ++ C?Hz-+ 2 C f Hz C H s +ki CHI 4-H Schneider (10)
Palmer et 01. (9)
reaction. All these phenomena tend to influence the reaction scheme to make a true order of reaction somewhat meaningless for these high-temperature pyrolysis reactions. The observed order of reaction and overall activation energy varies with operating or experimental conditions. Nevertheless, the degree of refinement of experiments and sophistication of pyrolysis models probably do not justify design equations using other than a first-order reaction. Initiation step. Most of the workers agreed that the reaction mechanism involves a free-radical scheme, or a t least partly free-radical. The disagreement about mechanism is especially concentrated over the initiation step between
+H CHz + Hz
C H 4 4 CH8
(11
CH4 +
(2)
Most of those who investigated methane pyrolysis in a temperature range of about 900"-1400°C (1652'2552'F) suggested that decomposition followed the first reaction. Those reporting decomposition data in the range 1383O-1927OC (2521 '-3100°F) favored the second reaction. Probably in the high-temperature decomposition of methane, both reactions are significant. T h e question of initiation is not yet resolved. Activation energy. Those reporting first-order decomposition of methane indicated the value of overall activation energy ranged from 83 kcal to 103 kcal. Shock-tube studies generally predicted a higher value of overall activation energy than nonshock-tube studies. I n shock-tube studies, the overall activation energy was around 103 kcal, as reported by Skinner (74) and Kevorkian et al. (4),while in flow systems it was about 85 kcal. This suggests that heterogeneous effects were entering into the kinetics to yield lower E values in conventional reactors. Some workers did refer to these surface effects (7, 7, 9, 13, 75). I n shock tubes, for all practical purposes, a "wall-less" reactor exists. This is true because gas-gas collisions are many times greater than gas-wall collisions. Kevorkian ( 3 ) indicated there are about 106 gas collisions for each wall collision. Although most shock-tube data predicted a n E value of around 100-103 kcal, the data of more conventional reactors did not agree. Table I11 shows that nonshocktube studies vary widely in predicting activation energy. However, these nonshock-tube studies generally favor lower values for activation energy. Effects of ethane. At lower temperatures, ethane seemed to accelerate the reaction; however, this effect disappeared with increase of temperature, and was not observed above 1500°C (2732'F), as shown by Schneider (70). This suggested that ethane probably was a ready and relatively easy source of free radicals, which in turn accelerated methane disappearance. At high temperature, sufficient energy is available to produce ample free radicals from methane and hence, the ethane contribution was not as significant. Hydrogen effects. Eisenberg and Bliss ( 7 ) and Yampal'Skie et al. (77) noted the effects of hydrogen,
TABLE 111. ACTIVATION ENERGY VALUES OF NONSHOCK-TUBE STUDIES PREDICTED BY VARIOUS INVESTIGATORS
Temp., "C
E, (kcal)
Rqerence
1150-1300 1127-1300 1293-1373 950-1636
101 100 90 86.6
Palmer and Hirt (7) Volokhonovich et al. (76) Shantorovich and Pavlov (73) Schneider and Murgulescu (77,72)
and both groups claimed that hydrogen inhibited the reaction in the range 1100"-2200°C (2012"-3992"F). They pointed out that hydrogen decreased the overall conversion of methane and condensation reactions. Contrary to this, Kevorkian et al. (4)observed that hydrogen did not inhibit the reaction in the range 1383O-1692OC (252lo-3077'F). Sufficient data are not yet available to resolve this question of hydrogen dilution effect. Surface effects. Surface effects are known to be important in kinetic studies of some reactors. Nevertheless, these effects are not always incorporated in the discussion of the overall kinetics. Some works did mention the importance of these effects. All the data of Happel and Kramer (2) have been correlated on the basis of reaction per unit volume of reactor. This study indicates that reactions with low activation energies are probably heterogeneous, and this heterogeneity may be related to (1) the effects of surface to volume geometry of reactors, and (2) the presence of fine particles of carbonaceous material. Tesner and Mar'yasin's quantitative study (75) covers the kinetics of carbon surface growth in the thermal decomposition of methane. Eisenberg and Bliss (7) made special runs to evaluate surface effects and indicated that these effects are of minor importance. Palmer et al. ( 9 ) provide confirmation of this conclusion. The above comparison of experimental activation energies obtained from shock-tube and conventional reactor data indicate possible heterogeneous effects. REFERENCES" (1) Eisenberg, B. and Bliss, H., Chem. Eng. Progr., Symp. Ser., 72, 3 (1967). 59, 39 (1967). (2) Happel, J. and Kramer, L., INn. ENG.CHEM., (3) Kevorkian, V., "Advances in Petroleum Chemistry and Refinery," Vol. V, J. J. McKett, Ed., Interscience Publ., New York, 1962, p 369. (4) Kevorkian, V., Heath, C. E., and Boudart, M., J . Phys. Chem., 64, 964 (1960). (5) Kozlov, G. I. and Knorre, V. G.,Inrh.-Fir. Zh., 4 (7), 11 (1961). (6) Kiinugi, T., Tamura, T., and Naito, T., Chem. Eng. Progr., 57, 43 (1961). (7) Palmer, H. B. and Hirt, T. J., J. Phys. Chem., 67, 709 (1963). (8) Palmer, H. B. and Knox, B. E., Chem. Rev., 61, 247 (1961). (9) Palmer, H. B., Lahaye, J., and Hou, K . C., J. Phys. Chem., 72, 348 (1968). (10) Schneider, I. A., Z. Phys. Chem. (Lei,bti,c),223, 234 (1963). (11) Schneider, I. A. and Murgulescu, I. G., h a d . Repub. Pop. Ram., Stud. Cercet. Chem., 367 (August, 19GO). (12) Schneider, I. A. and Murgulescu, I. G., Z. Phys. Chem. (Leipzix), 218, 338 (1961). (13) Shantorovich, P. S. and Pavlov, B. V., Znt. Chem. Eng., 2, 415 (1962). (14) Skinner, G. B., PTOC.Diu. Fuel Chem., American Chemical Society Meeting, St. Louis, Missouri, March 21-30, 1961. (15) Tesner, P. A. and Mar'yasin, I . I.., Int. Chem. Eng., 2, 303 ( 1 9 6 2 ) . (16) Volokhonovich, I . E., et d.,Dokl. Akod. Nauk.SSSR, 146, 287 (1962). (17) Yampal'Skie, Yu. P., Gordon, M . D., and Lavrovskie, K. P., Neftekhimiya, 68 (21, 198 (1968). (18) Yano, Y.and Kuratani, K., Bull. Chem. Soc. Jop., 41, 4, 299 (1968). a One reviewer has suggested another shock-tube study that has not been included in this report: Rao, V. V., Mackay, D., and Trass, O., Can. J.Chem. Eng., 43, 183 (1965).
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