VOL 5 9
NO. 1
JANUARY 1967
39
Though direc! synihesis of acetylene has been pursued for many years, undesirable product impurifies have prevented general adopfion of the synthesis. In this repor!, !he authors show tha! high yields of relaiively uncontaminafed mixfures of acetylene and hydrogen are possible
40
INDUSTRIAL AND ENGINEERING CHEMISTRY
Berthelot synthesized acetylene in the elecA lthough tric arc from carbon and hydrogen in 1862, acetylene is still made for the most part from calcium carbide rather than from hydrocarbon raw materials. The challenge of direct synthesis has stimulated the development of a number of commercial processes, and hundreds of patents have been issued on this subject during the past 30 years. A commercial process has generally proceeded from the development of a device for heating hydrocarbons at high temperatures to obtaining empirical information to establish its .characteristics rather than approaching the subject from the viewpoint of the kinetics involved. Techniques presently employed commercially include the use of a n electric arc as practiced by Huels (70) and more recently by Du Pont (7) ; regenerative stove techniques of which the Wulff (3) process is an example; flame techniques involving incomplete combustion with oxygen, such as the Sachsse process (BASF) (28), Societe Belge d 1'Azote (SBA) (23),and Tsutsumi (33); and admixture with hot combustion gases, as in the Hoechst (HTP)( 7 5 , Z U ) process. None of the above processes can make acetylene at a price competitive with that of ethylene, in spite of continuing improvements. The arc processes have the disadvantage of high energy consumption due to the unnecessarily high and largely uncontrolled temperatures employed. The regenerative stove technique is not able to operate at a sufficiently high temperature, together with a low contact time, to crack methane effectively. Techniques involving flame and combustion gas dilution operate adiabatically with a dropping temperature during the cracking process, a result undesirable for obtaining high conversions to acetylene. The pyrolysis products are also diluted with undesirable combustion products. Recent developments which have not as yet advanced to the commercial stage include the use of shock tubes (9) and plasma torches (29) for acetylene synthesis. Another interesting development is the feasibility of reactions of carbon with hydrogen and methane in high intensity arcs (7). For the present, in view of problems with high power consumption, materials of construction unable to withstand high temperatures, and design scaleup factors, it was felt that thorough exploration of conditions less severe would be profitable. Kinetic studies reported in this paper are aimed at obtaining suitable rate data for methane pyrolysis over a wide range of process conditions. This information can be used as the basis for process development employing optimum operating conditions under process conditions previously neglected owing to limitations of technique! employed. The results should also serve as the basis for further research to explain the reaction mechanism: involved. Our studies indicate that methane eithei alone (73) or diluted with hydrogen (72) can be pymlyzed to give high yields of relatively uncontaminated mixtures of acetylene and hydrogen. Early investigations of the kinetics of hydrocarbon
pyrolysis for acetylene production have been reviewed by Kramer and Happel (79). Most data have been obtained using some type of tubular reactor and at temperatures up to 1500' C. Methane decomposition is reasonably well correlated on the basis of first-order kinetics but conversion to acetylene and ethylene is more complicated and not well correlated. The effect of surface to volume of experimental apparatus had little influence either on conversion of methane or yield of products. Likewise, efforts to find catalysts for acetylene production from methane have not been successful so that all commercial processes in principle involve purely thermal pyrolysis. A recent comprehensive study in which mixtures of methane and hydrogen were cracked at atmospheric pressure has been reported by Kunugi et al. (27). This research was performed with tubular quartz reactors (10- to 22-mm. i.d.) packed with quartz fragments and heated externally by an electric furnace. Experiments were conducted at maximum reactor temperatures of 1100-1400" C. Reactor temperatures were taken as the maximum indicated by a Pt-Pt rhodium themnocouple inside a protection tube, and the true gas temperature was estimated to be about 100' C. lower. Data were correlated on the basis of reactor tempera-
tures as observed.
It was concluded that methane was
decomposed by three reversible simultaneous firstorder reactions producing ethylene, acetylene, and carbon and tar. A fourth irreversible first-order reaction was employed to correlate carbon and tar formation from acetylene. Results are in agreement with previous work concerning methane decomposition so that much more accurate acetylene yields are available than from previous studies. While hydrogen may reduce the rate of decomposition of methane even at higher temperatures, it also reduces the rate of carbon and tar formation even more. Kunugi et d.,however, reported a higher energy of activation of carbon and tar formation from methane than for acetylene formation from methane in the temperature range which they studied. This would indicate that the proportion of carbon and tar formation to acetylene formation would increase at temperatures above 1400' C. provided the same relationships applied. Our studies indicate that this is not the case. For those interested in further details of previously developed processes for acetylene production from hydrocarbons, a comprehensive survey has been published in the book by Miller (26).
V O L 5 9 NO. 1
JANUARY 1967
41
Thermodynamics
A brief review of pertinent thermodynamic considerations is in order because of its important bearing on potential products. Primary products from hydrocarbon pyrolysis are not directly related to final equilibrium products, which in this case would be merely carbon and hydrogen together with small amounts of acetylene, ethylene, and methane. It is important therefore to select the species which are to be considered in equilibrium with each other. Duff and Bauer (8) have presented a set of calculations useful for our purpose involving the equilibrium composition of the C-H system at elevated temperatures. Two basic cases were treated by these authors. The first set of results was obtained under the assumption, at a particular temperature and pressure, that equilibrium was attained for various C-H species under the restriction that no solid carbon was allowed to precipitate. The second set of computations was performed under the assumption of complete equilibrium including the presence of solid carbon. The first case is significant kinetically. Not only is it thought to describe a situation which prevails in many shock-tube and detonation experiments ( 7 7) during intermediate stages of reaction, but any useful commercial process must operate under conditions in which carbon formation is minimized. Figure 1 is a plot of temperature ts. equilibrium partial pressure abstracted from Duff and Bauer's results. I t shows the most abundant species present at a C-H atomic ratio of 1/4 (corresponding to methane) and at a total pressure of '/lo atm. Actually a total of 58 species was included in the detailed calculations, although only 11 appear in this diagram. As would be expected from numerous pyrolysis studies, the formation of acetylene does not become important below 900" C. Not so generally appreciated is the fact that it attains a maximum at about 1700" C., falling off slowly at higher temperatures. It is interesting that benzene, ethylene, and C3H4(allene plus methylacetylene), often found in high temperature pyrolysis effluents, attain maximum concentrations several hundred degrees lower. Note, however, that the radicals CsH, C3H, and C4H begin to reach appreciable proportions ("0.01 atin.) at teinperatures over 2000" C. and probably account for the presence of higher acetylenes in the effluent from arc processes. Moderate changes in C-H ratio and pressure do not affect these conclusions greatly. Thus, we conclude that the temperature range between 1500" C. and 2000" C. should be an interesting one to explore kinetically in regard to hydrocarbon pyrolysis. In contrast, calculations allowing for equilibrium with solid carbon show that temperatures well o w r 2000" C. are required for any appreciable acetylene formation. Experimental Apparatus and Procedure
The experimental arrangement is shown in Figure 2. Metered hydrocarbon feed gas passes through an electrically heated reaction chamber brought to predeter42
INDUSTRIAL A N D ENGINEERING CHEMISTRY
mined conditions and then the effluent gas stream is rapidly quenched. Maximum reaction temperatures of 1900" C. may be attained. This represents an increase of several hundred degrees over previously published information on cracking in continuous flow systems. The h>-drocarbon feed is first passed from storage cylinders to Fisher and Porter float-type rotameters where flow rates are metered in the gas phase. Feed gas is then throttled through a needle valve before the reactor. The reactor design employed is a concentric system of vertically mounted cylindrical tubes, which are progressively larger in diameter, and all contained within the largest, outermost gas tight metal tube. The smallest diameter tube is an alumina thermocouple ell (type RA 1732, Norton Abrasives Co.), 0.2-inch 0.d. and 0.0623-inch i.d. A platinum-platinum 10% rhodium thermocouple wire is mounted within the thermocouple well and is therefore positioned along the vertical axis of the reactor unit. This thermocouple, which can be moved, is used to obtain a longitudinal temperature profile of the reactor (including preheat section) which can be up to 5 inches in length. The next larger cylinder is also constructed of alumina tubing with a 0.5-inch 0.d. and a 0.25-inch i.d. The annulus between the tubes is the reaction zone. These two concentric cylindrical alumina tubes are placed inside a graphite cylinder (type ATJ graphite, National Carbon Co.) which serves as the heating element. This resistance element is designed to use three-phase, high amperage, low voltage current up to 3 kva, which provides for sufficient heat input. The shell of the reactor is constructed of copper, as are the 0.5-inch thick plates used at the top and bottom. All the outer walls of the reactor are water cooled. .4 window in the outer wall enables a Leeds and Northrup optical pyrometer to be used for determination of the temperature of the outer wall of the larger diameter alumina tube. T o do this, a vertical channel is cut through the successive tubular sections which encircle the 0.5-inch 0.d. alumina tube. A longitudinal wall temperature of the 4-inch reaction section of the reactor wall is thus obtained. The limiting temperature Lvhich the optical pyrometer is capable of indicating is considerably higher than the maximum temperature reached by the furnace, thus providinq a means for
6RAPHlTE HEATING
A
1 OHIA IEFUUORV INSUUTIOW
SllWlESS SJEEL UDlATlOH S
Figwa 3. Ami1 maw
measuring (to within 5' C.) wall temperatures of the reactor in excess of 1700' C. Figure 3 provides a cross. sectional end view of the design described. On leaving the reaction section, the gas stream enters a stainless steel quench &ne where rapid quenching of the hot product gas mixture takes place. Although the metal stock of this quench zone is water cooled, the principal quenching action is achieved by mixing the hot effluent gas with cold recycled gas of the same composition. In this way, no dilution of the product gas mixture occurs, and overall contact times of less than 0.001 sec. can be obtained with quenching times of less than 0.0001 sec. This gas quenching system is well suited to high temperature pyrolysis study as it achieves the dual purpose of not affecting the accuracy of product gas analysis, while it simultaneously provides the rapid quenching necessary to prevent the further decomposition of primary products. Thus, the reactor satisfies the following basic requirements: provides high heat transfer rates into the gas phase; maintains low temperature gradients acmss reactor walls; permits rapid passage of reactant and products through the reactor system; quenches the
uhw WJI
reaction immediately a t the exit of the reactor; allows temperature and pressure drop measurements to be made in the reactor zone. After leaving the reaction and quench zones, the gas stream is passed through a cotton gauze filter to remove any entrained solid or liquid particles. The most common contaminants removed from the gas are small 'flakesof solid carhn which form as a product of pyrolysis in the reactor. There is negligible or no liquid product trapped during runs made above 1200" C., and runs below this temperature produce only small quantities of t a r l i e product. A pumping system of two rotary vane-type pumps (Leiman Pump Co., Newark, N. J.) is used a t this point. The first is a large pump which achieves the necessaq conditions for vacuum operation of the reactor. Thi AUTnORS John Happel is Chirman and Lconard Krame is m the s t d of the Defiartment of C h i c a l Engineering,
Nnu Yark University. They acknowledge thc assistance of Jack Famulmo and Paul Ast, and thc use of thc facilities of tlu Courant Institute of the M a t h a t i c a l Sciences, New York University, in making computn calculationsfor this study. VOL 5 9
NO. 1
JANUARY 1967
43
second, a smaller pump, req-cles a portion of the cooled effluent stream back to the quench zone. All connecting lines between the reactor and the pumping system are fitted with flexible hose connections to reduce vibration of the reactor as shown in Figure 2. The gas sampling system is located downstream of the large pump. At least two gas samples are collected during each experimental run and analpzed using either a Perkin-Elmer Model 154 vapor fractometer or a Consolidated Model Mass Spectrometer, Type 21 103C. A temperature profile of the reactor is taken while each run is in progress. The thermocouple is moved at short intervals inside the M-ell and serves to measure temperatures in runs where the maximum temperature does not exceed 1500" C. Simultaneously, the optical pyrometer is used to take wall temperatures over the visible section of the reactor. The junction of the thermocouple and the p)-rometer are in line and are mechanically coupled so that they move together when the profile is being taken. At that point in the reaction zone where the temperature is a maximum, the two temperature measurements, the wall temperature and the thermocouple well temperature, generally are within 5" C. of each other. TZ'here the maximum reactor temperature is higher than 1500' C., the thermocouple is removed and only the pyrometer profile is taken.
Amounts of components present in the effluent other than those appearing in the above equations were generally small. Ethylene was present in some of the low temperature runs. Since it appears to be largely converted to acetylene at higher temperatures, the appearance of acetylene plus ethylene might be taken to represent conversion by Reaction 2. Such an assumption did not change the value of the basic constants much nor did it improve the correlation for methane disappearance and acetylene yield. Consequently it was decided to represent net loss of methane to all side reactions as resulting in carbon formation for the purpose of material balance calculations employed in making the correlation. Actual carbon deposits were assumed to consist entirely of solid carbon, though the actual chemical composition of the carbon deposit will vary somewhat. Appropriate rate equations correlating the data are :
(4)
Kt
Correlation of Data
Several hundred runs were made in the above manner using a number of low molecular weight hydrocarbons, both pure and mixed with each other and with hydrogen. I n this paper, we confine our attention to results obtained for methane cracking under vacuum and at atmospheric pressure using hydrogen dilution. Axial temperature gradients of two or three hundred degrees existed in the reactor. Fortunately, this does not complicate the analysis since the one dimensional form of the equation of change is still applicable-axial transport of energy and mass by diffusion being small in comparison with axial transport of energy and mass by convection (6). The basic equations remain firstorder ordinary differential equations whose solution is obtainable by straightforward numerical integration. An analog computer study of various alternative models was used to establish a kinetic model involving the smallest number of arbitrary constants which would satisfactorily correlate the data. The model finally adopted involves the following reactions :
-
CH4 + C
CH4
I/z
+2
C2H2
C2H2-t 2 C
+
(1)
H2 3/2
HZ
+ Hz
(2)
(3)
We do not suppose that these equations involve all the reactions which occur or constitute a mechanistic study in any sense. They are reactions which have been separately studied by others; a higher degree of sophistication is not justified by the accuracy of the data obtained. 44
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
=
Ai
for i = 1, 2, 3
(7)
Let =
gram moles of CH4 converted gram mole feed
by
(4)
x2 =
gram moles of CH4 converted gram mole feed
by
(5)
=
gram moles of CzHz converted gram mole feed
by
(6)
x1
x3
Then,
rl d V B =
F dxl
r2
dV, = F dxz
73
dVB
=
(8)
F dxa
At any point in the reactor, where conversions are X I , x2, x3 the components present are: CH4, C(solid), Hz, C2H2. I n the final analysis of the data according to the above scheme the calculations were made on a high speed digital computer. For any given run the necessary information employed in the computer program is: -the recorded temperature profile through the reactor -the total pressure on the reactor (A@ was small in all cases) -the V R / Femployed for the run -the mole fractions of CH4 and Hz in the feed -the observed results: (a) (b)
yoconversion of CH? to all products yoconversion of CH4 to C2Hz
The program similar to the "gradient search technique" described by Zellnik (34) was designed to integrate Equations 8 through the reactor for each run. The object function to be optimized (minimized) war the following (i = no. of runs) :
=u i-1
c
-
calc. total conv. obs. total oonv. obs. total cow.
]'+ J)
calc. conv. to C Z H~ obs. conv. to C& obs. conv. to CzHZ
(9)
This form places equal weight in fitting both methane disappearance and acetylene appearance. For integration of the rate equations, the actual observed temperature profile for each run was incorporated into the routine. Seventy-one runs were correlated in this fashion by use of the data in Tables I and 11. The parameter values which correspond to the optimum ?e: Activation energy
El = 16,200 cal./gcam mole En = 86,450 cal./gram mole Ea = 17,700 cal./gram mole
'
//\--,-
,,,' ,'
Frequency factors
7
, _ , 7
AI = 25.00 gram moles/(sec.) (liter) (atm.) A Z = 3.219 X lO1O gram moles/(sec.) (liter) (atm.) Aa = 2190 gram moles/(sec.) (liter) k = 1.535 arm.-' For a ready appreciation of the results, it is convenient to plot them in the form shown in Figures 4 and 5 in which contact time or VR/F does not appear as an explicit variable. In these plots the 45' straight line from the origin represents 100% yields of acetylene based on methane content of the feed. The series of straight dotted lines at different angles from the origin show the yield in terms of the ratio of conversion to disappearance. These linea are based on the methane disappearance in a single pass; in effect, they represent what would ultimately be expected as overall results if all the unreacted methane in the product stream were to be continuously recycled through the reactor under the same conditions. It is of interest that at each temperature there is a limit on conversion to acetylene and that this limit is raised by increasing the average cracking temperature. Also, the highest disappearance of methane corresponds to the maximum conversion to acetylene. I n other words, at the highest temperatures it is possible to attain mixtures consisting substantially of hydrogen and acetylene. The advantages of operation in the temperature range of 1600" to 1800" C.thus seem to be dearly established from this kinetic study. Figures 6 and 7 complete the correlation picture by giving overall conversions as a function of VR/F for various temperatures. The exact space velocity for a given conversion is more subject to error than the relationships between conversion and selectivity plotted in Figures 4 and 5.
, ,
. . .I.
. ..
..
..
DISAWFMUU OF YWYE (Ullrr PEI 1W YOUS FED)
Figwe 4. Methaw pyrolysis to w(vlma and hydrogen. Feed is pwe m a t h , pressure is 100 m. I@ at h comjmted isothmnal h#+?ahae
.&4
WES aC, DlumruCFR IN IldlBMEI"€ IN FED
F&re 5. PyrOlyEir of methaw-hydrogen mixtures. The feed is an ep.molar mixhaa of m a t h and hydrogen p r e s s ~ ~ise 1 a h . ot the cmputsd isothomnl h p m h m VOL 59
NO. I
JANUARY 1967
45
Components other than methane, acetylene, and hydrogen are present in much smaller proportions so that exact correlation has not been attempted. I n the vacuum runs listed in Table I, ethylene was the only component detected in appreciable proportions (over 0.1 vol. % in effluent). In runs at atmospheric pressure with hydrogen dilution (Table 11), diacetylene is also present. Note that a temperature profile is reported in Tables I and 11. Since those simultaneous reactions with different activation energies are involved it is in principle impossible to obtain a single equivalent isothermal temperature. However, from a practical standpoint it is possible to compute an average temperature based on the activation energy of the predominant Reaction 2. Values for average temperatures were computed using a scheme similar to that reported by Malloy and Seelig (24) with a subsequent addition of Ting (37). Calculations of conversion and selectivity using this simpliied procedure are in good agreement with the more accurate results reported here and plotted in Figures 4 and 5. In all cases the average temperature was within 100' C. of the maximum because of the high heat transfer rates possible in this reactor. For convenience the per cent difference of each data point from the correlation is also tabulated in Tables I and 11. The average deviation for all runs taken together is 15.1% for overall conversion and 19.4% for conversion to acetylene.
/
"
0 ~~
a-
40
Discussion
60
80
im
OIYTIIItAHQ M (14 (UOlES PEI IMMlB FED) Figure 6. Owroll convnsion in methatu pyolylis. T h c j e d is pure mlhonc,pressure is 100 mm. Hg at computed i s o t h a l kmperahue
m
Y ..
m
11
uoltraq Disumuwm 1* WUI cu, Imo
Figura 7. Owrollconwrrion in methonc-hydrogenmixhrras. ha feed ir on equzmolm mixture of msthntu hy&ogm, presswe & 7 &. at thc computed i s o t h o l kmpnahrrc
46
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
At the outset it is worth inquiring to what extent reaction kinetics is influenced by equilibrium considerations. For this purpose, we will use the extensive results of Duff and Bauer discussed earlier. In the case of the carbon-forming Reactions 1 and 3, conditions are far from equilibrium. In the case of Reaction 2, the presence of uncracked methane does not appear to be limited by equilibrium. Thus in our runs at high conversions (9573, a high temperature, ~ 1 7 0 0C.,~ is necessary for -g o d selectivity. At 2000" K. (1727' C.) the equilibrium concentration of methane corresponding to a mixture consisting of 25 vol. % CHI and 75 vol. % Hr at 1 atm. will be 0.35 vol. %. Conversion (95%) for such a mixture corresponds to 1.01 vol. % of residual CH,, assuming the entire decomposition proceeds via Reaction 2. With runs at high hydrogen dilutions at lower temperatures, methane equilibrium could be a factor. The presence of ethylene appears to be definitely influenced by kinetic considerations. At low conversions, ethylene concentrations are substantially higher than correspond to equilibrium, indicating that the sequence of steps leading to acetylene production goes through ethylene as an intermediate. At higher conversions, -80%, equilibrium proportions of ethylene are approached. In the case of vacuum operation at high temperatures, this corresponds to very small ethylene concentrations. Thus with pure methane at 0.1 a m . and a temperature of 2000" K., the equilibrium
ethylene concentration is 0.01 vol. yo and indeed no ethylene can be detected in the effluent. However, with hydrogen dilution appreciable ethylene should be present even at high temperatures. Thus with a mixture consisting of 25 vol. % methane and 75 vol. % hydrogen at 1 atm., the equilibrium concentration of ethylene at 2000" K. is 0.06 vol. %. Several tenths of a per cent of ethylene are present in many cases. In the case of diacetylene, none is detected in vacuum operations though appreciable proportions should be present at equilibrium. Thus, with pure methane feed at 0.1 atm., equilibrium at 1500" K. (1227" C.) corresponds to 0.33 vol. % of diacetylene and at 2000" K. (1727" C.) corresponds to 0.73 vol. % of diacetylene. O n the other hand, with hydrogen dilution at atmospheric pressure appreciable diacetylene is found experimentally. For a mixture of 25 vol. yo CH4 and 75 vol. % Hz at atmospheric pressure, the equilibrium concentration of diacetylene is 0.01 vol. yo at 1500" K. and 0.10 vol. yoat 2000" K. Actually close to 0.2 vol. yo is present over a range of temperatures. Perhaps it is formed by hydrogenation of carbonaceous intermediates or from ethylene with intermediate formation of butadiene. In correlation of the rate data obtained, we have followed the formulation of Hougen and Watson (74) in expressing the reaction rates in terms of partial pressures of the gaseous components involved instead of concentrations. Thus, if concentration units are employed, the reaction velocity constant for a first-order reaction, k,, will be sec.-l whereas those for a first-order reaction Ki, expressed in terms of partial pressures have been given here as gram moles/(sec.) (liter)(atm.) The relationship between these "constants" is ki = R T K i
(10)
If transition state theory is accepted, the use of units based on partial pressures will be more correct though, as Benson (2) has pointed out, the theory is not sufficiently advanced to justify this choice rigorously. Obviously the choice can make a significant difference when comparing results over a wide range of temperatures. All of our data have been correlated on the basis of reaction rate per unit volume of reactor. The reactions with low activation energies are probably heterogeneous, but the heterogeneity may be associated not only with gross effects like surface-to-volume geometry of the reactor but also with the presence of fine particles of carbonaceous materials. In addition, of course, theoretically there is no way to justify the simple addition of the kinetic equations for correlation purposes because they are not simple mechanistic steps. It seems unlikely, however, that the observations obtained can be correlated by fewer equations, involving a smaller number of empirical constants. Reaction 1, the decomposition of methane to form hydrogen and carbon, has not been widely studied. The most interesting quantitative study is that of Mar'yasin and Tesner (25) on the kinetics of carbon
surface growth in the thermal decomposition ol methane in the 1420-1700" C. temperature range. Experiments were carried out at methane concentrations or 2 to 10 vol. % using hydrogen as a diluent. Their results are correlated in terms of area exposed to the reacting gaseous mixture with a rate constant reported as
r
=
2042 C e-361000/RT gram/(sq. cm.)(sec.)
(11)
If Reaction 1 is expressed in this form, we have =
17.7 c e--16,200/RT
gram/(sq. cm.)(sec.)
(12)
Agreement is better than appears by comparing these equations. Thus at 1600" C., the reaction rate constant (corresponding to rate of carbon deposition at unit methane concentration) from Equation 11 is 0.12. cm./ sec., whereas that from Equation 12 is 0.23 cm./sec. Our apparatus surface-to-volume ratio is 31.8 sq. cm./ cc. whereas that of Tesner is 6.7 sq. cm./cc. We hesitate to adopt this method of correlation without further study, however, because work of other investigators (5) indicates much higher rates of decomposition of pure methane under vacuum than obtained by Mar'yasin and Tesner. Reaction 2, the main reaction involved and the one which produces acetylene from methane, when expressed in concentration units (using 1600" C. to match constants) becomes : k , = 4.94
x
1012
sec.-l
e--863450/RT
(13 )
which is in reasonable agreement with both high temperature shock-tube studies ( ? I , 26, 30) and earlier low temperature work (79). A survey by Kozlov and Knorre (18) which presents some new additional data gives 4.5 X 1013 e - g l , O O O / R T kc sec. -l (14) At 1600" C., our Equation 13 gives k , = 0.41 x 10d whereas Equation 14 predicts k , = 1.05 x IO3. It seems to be generally accepted that methane pyrolysis proceeds through intermediate formation of ethane. The mechanism of conversion of CH4 to C2Hsis still not entirely established. Lower activation energies are in accordance with the sequence CH4-+ CH2 CH2
+ Hz ( E
+ CH4
-+
'v
85 kcal.)
2 CH3
2 CH3 -+ CzHs
(1 5) (16 )
(17)
Reaction 3, involving the decomposition of acetylene when expressed in terms of concentration units, becomes k , = 5.05
x
10ioe-*7~700/RT cc./(gram mole) (sec.)
(1 8)
A recent survey by Palmer and Dormish (27) which includes some new data (using helium dilution) proposes for the range from 1500-2500" K., k c -- 3.2 X 1014 e-50,000/RT
cc./(gram mole)(sec.)
(19)
At 1600" C., Equation 16 gives a value for the reaction velocity constant of 4.4 X lo8, whereas Equation 17 gives 9.5 X IO8. Thus our data show a carbon formation rate in agreement with data obtained by direct pyrolysis VOL. 5 9
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JANUARY 1967
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9 1 1
I
I
VOL 5 9
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of acetylene. We also obtained data some time ago on direct pyrolysis of hydrogen-acetylene mixtures and found a rate equation which is in substantial agreement with Palmer and Dormish in the temperature range of 125G-1350° C., namely
k, = 7.94 X 1014G-52’800’RT cc./(grammole)(sec.)
(20) Absolute values of the rate at about 1600” C. obtained in the present work are lower than the shock-tube data of Bradley and Kistiakowsky ( 4 ) although the activation energy reported by these authors and others is much higher ( E 1: 50 kcal.). Probably the data reported here are substantially influenced by heterogeneous decomposition on carbon particles formed in the reactant stream. The presence of hydrogen seems to be reflected mostly in its tendency to suppress formation of carbon from acetylene by- Reaction 3. This is probably a heterogeneous process related to the adsorption of hydrogen on the soot deposits present in the reaction system. We have not been able to establish any effect of hydrogen, other than dilution, in the other two reactions. I n making the calculations for rate constants, radial temperature and diffusional gradients were neglected as has been customary in calculations of this type. Itre thought it desirable to establish the possible magnitude of such effects because at sufficiently high temperatures and heating rates in flow systems corrections might be necessary. It appears that for methane pyrolysis up to temperatures of 1700”C., the actual rate constant might be up to 10% higher than reported (32), provided the gas is completely transparent and picks up heat entirely by convection to a laminar flow. Radiant transfer does not appear to contribute appreciably to overall heat pickup (231, but the reactor gases may be quite opaque even in thin layers because of the substantial proportions of carbon soot formed during the reaction. Therefore, it was not thought advisable to make any correction to the observed rates. The reaction velocity constants reported appear to be in reasonable agreement with more exact kinetic studies reported in the literature insofar as these studies are applicable. There is considerable evidence that the presence of soot deposits make heterogeneous effects more predominant than in high temperature shock-tube studies. Applications
The basic kinetic data reported above should be useful in the design of equipment for maximizing acetylene production by cracking of natural gas. For this purpose there are a number of different devices which might possibly be employed. I n our own program, we have chosen to conduct pilot plant studies on a n electrically heated system in many respects similar to the laboratory device described above but having throughputs approximately one hundred times greater. Lower surface-to-volume ratios were employed, and therefore temperature measurement and interpretation of data are not so quantitative as in the case of laboratory data. Kinetic results appear to be 50
INDUSTRIAL A N D ENGINEERING CHEMISTRY
roughly in agreement with the smaller scale experiments so that the scaleup problem is one of obtaining runs of
sufficient length to minimize maintenance costs. Data are currently being obtained along these lines. Preliminary economic studies have also been made for commercial production of acetylene based on pyrolysis in the range of 1600-1800” C. at a level of 50,000,000 lb./)-ear. Several processing variations show costs of acetylene of about 5 0 per pound. Operation at high conversions of methane will enable an effluent consisting of almost pure hydrogen to be readily produced, correspondingly reducing acetylene cost to 3.5 # per pound if by-product hydrogen is priced at 30$ per 1000 s.c.f. Acetylene at such a price will compete successfully with ethylene so that it will again be firmly established as a petrochemical building block. Demand for hydrogen in various petroleum refinery processes also appears to be increasing. Production of acetylene, instead of carbon monoxide, as a coproduct has obvious advantages. NOMENCLATURE
A
= frequency factor, gram moles/( sec.)(liter-atm.)
c = methane concentration,
gram/cc.
E = activation energy, cal./gram mole F = feed rate, gram moles/sec. k, ni
=
$3
=
R = 7i
=
T = t
v,
rate constant in concentration units, cc./(gram mole)(sec.)
= gram moles of species
= =
i
partial pressure of species i, atm. abs. gas constant 0.082 liter-atm./(gram mole)( K . ) reaction rate, gram moles/( liter)(sec.) temperature, K. time, sec. reactor volume, liters
REFER ENCES (1) Baddour, R . F., Blanchet, J. L., 1x0. END. C H E WPROCESS DESIONDEVELOP. 3, 258 (1964). (2) Benson, S . W., “ T h e Foundations of Chemical Kinetics,” McGraw-Hill, 1960. (3) Bogart, M . J . P . , Shiller, G. R., Coberly, C. J.. Petrol. Process. 8 , 377 (1953). (4) Bradley, J. N.: Kisriakowsky, G . B., J.Chem. Phys. 35, 264 (1961). (5) Brown, A. R., Wati, I V . , “Industrial Carbon and Graphite,” p. 86, Wiley, 1958. (6) Chambre, P . L., Appl. Sci. Res. A9, 157 (1959). (7) Chem. Week, p. 64 ( J a n . 18, 1964). (8) Duff, R . E., Bauer, S. H . , “ T h e Equilibrium Composition of the C / H System at Elevated Temperatures,” Los Alamos Scientific Laboratory Report LA-2556 Chemistry, T.1.D.-4500 16th E d . , OTS, U. S . n e p t . of Commerce, Washingto; 25, D. C. (1961); seealso .I. Chem. Phys. 36, 1754 (1962). (9) Esso Research and Engineering Co., Brit. Patent 900,844 (1962). (10) Gladisch, H.,Pelroi. Rejner41 (6), 159 (1962). (11) Glick, H . S., “Seventh International Symposium on Combustion,” p. 98 (1958). (12) Happel, J., 5 . S. Patent 3,156,734 (1964). (13) Happel, J., Kramer, L . , Ibid., 3,156,733 (1964). (14) Hougen, 0. A., Watson, K. M., “Chemical Process Principles,” Part Three, Wiley, 1947. (15) Kamptner, H. K . , Erdoel Kohla lfi, 547 (1963). (16) Kevorkian, V., Heath, C . E., Boudart, h l . , J. Phys. Chem. 64, 964 (1960). (17) Kistiakowsky, G . B., Zinrnan, 1%’. G., J . Ciiem. P.hys. 23, 1889 (1955). (18) Kozlov, G . I., Knorre, V. G . , Combust. Flame 6 , 253 (1962). (19) Kramer, L . , Happel. J.. in “ T h e Chemistrv of Petroleum Hydrocarhons,” B. T. Brooks et ai., eds., Vo!. 11, Reinhold, 1955,’ (20) Krekeler, H., Wirtz, R . : Pechtold, pi., Erdoel Kohle 12, 353 (1959). (21) Kunugi, T., Tamura, T., S a i t o ? T., Chem. Eng. Pmg. 57, 43 (1961). (22) h e : Richard H . C., Happel, J., I K D . END.C H E M . FUNDAMENTALS 3 (2), 167 (1964). (23) Leroux, P. J., Mathieu, P. hl.,Chern. E n g . Prog. 57, 54 (1961). (24) Malloy, J. B., Seelig, H. A . ?A.I.Ch.h’. J . 1, 528 (19S5). (25) Mar’yasin, I. L . , Tesner, P. A . , Zntern. Chem. E n g . 2, 303 (1962). (26) Miller, S . A , , ”Acetylene. I t s Propcrties Manufacture and Uses,” Vol. I, Ernest Benn, London, 1965. (27) Palmer, H . B., Dormish, F. L., J . Phjs. Chem. 68, 1553 (1964). (28) Sachsse, H . , Chern.-Eng. Technik 26, 245 (1934). (29) Sennewald, K., Schallos, E., Pohl. F., Ibid.,35, 1 (1963). (20) Skinner, G. R . , Ruehrwein. R. A.: J. Phyr. Chem. 63, 1736 (1959). (31) Ting, A . P., A.1.Ch.E. J. 4, 246 (1958). (32) Tromhetta, 41. L . , Happel, J., Zbid., 11, 1041 (1965). (33) Tsutsumi, S., Peiroi. Rejner 37 ( 3 ) , 169 (1058). (34) Zellnik, H. E., Sondak, N. E., Davis, R . S., Chem. E n g . Prog. 5 8 , 35 (1962).
