Acetylene Formation in Thermal Decomposition of Hydrocarbons H. H. STORCH, Bureau of Mines Experiment Station, Pittsburgh, Pa.
T
Data on the pyrolysis of methane, ethane, same experiments described in ethylene, gasoline, and petroleum to yield acetymore detail in Tables IIA and cal c h a r a c t e r i s t i c s of IIB. Table I shows that neither methane should serve as lene are reviewed. All of the data on the thermal an antidote for overenthusiasm R1 nor Ra ever closer than about the chemical utilization decomposition of methane which do not involve a factor of 10 to the equilibrium of waste natural gas; and yet more than about 60 per cent total conversion are constants Kczrrzand KC*=(. In shown to be approximately predictable by the use most cases this factor is between this waste is sufficiently large, of two simple linear equations. The dehydrodespite t h e c o n s t r u c t i o n of 100 and 1000. Since the experilong-distance pipe lines for use ments of Table I were selected so genation of elhylene to acetylene appears to be a of the gas as f u e l , to j u s t i f y as to present those data of Tables research by both industrial and unimolecuzar Its lzinetics yet IIA and IIB which were closest government organizations. suficiently well known to write a rate equation. to equilibrium, it is apparent that The data o n gasoline and petroleum are of casual in discussing the data of Tables It was not u n t i l r e c e n t l y industrial interest but present nothing new of a n y 11.4 and I I B t h e r a t e s of the that any p r e c i s e information concerning the thermodynamics reverse reactions need not be theorefical interest. Some discussion of possible considered. a n d k i n e t i c s of the t h e r m a l decomposition of t h e m a j o r industrial processes utilizing the thermal deR e c e n t w o r k d o n e in t h e constituent of n a t u r a l gascomposition of methane or ethylene diluted with Bureau of Mines f u r n i s h e s a 75 to 90 per cent inert gas such as hydrogen or quantitative picture of the kin a m e l y , methane-was available. Kassel (7) Published a f e ~ carbon dioxide is presented. netics of the pyrolysis of methane months ago the r e s u l t s of his in static systems; and in systems calculations from spectroscopic ( c a r b o n f i l a m e n t lamps imdata of the free energies of methane, acetylene, ethylene, and mersed in liquid nitrogen) where the flow is sufficiently rapid ethane. From these figures the equilibrium constants to isolate primary products. Storch (16) showed that ethane plotted in Figure 1 were calculated for the three reactions: was the earliest product which could be isolated, 95 per cent conversion of the reacted methane to ethane being obtained in 2CH4 +s CzHz 3H2 experiments at low pressures of methane in a carbon filament 2H2 2CH4 s CzH, lamp immersed in liquid nitrogen. Similar experiments using CzH4 CzHz Hz liquid oxygen as the cooling medium yielded a mixture of Before much can be said about the kinetics of the thermal ethylene and acetylene. The mechanism suggested by Kassel decomposition of methane when conducted in flowing systems (6) for the thermal decomposition of methane involves the a t high temperatures, it is essential to ascertain the distance reactions, from equilibrium represented in the composition of the offCHc+CH2 Hz (1) gases, for, if this distance is too small, the kinetic picture b e C Hz CHI a CnH6 (2) comes complicated by the rates of the reverse reactions. CZH6 F-)c CZH4 HZ (3) Table I contains the two ratios, CaH4 G CzH1 Hz (4) CzHzd Cz Hz (5) (CzHc) (H2)' R1 = ( C 2 H d (H2)8 and R1 (CH4)' (CH4)' and yields the rate equation: calculated from the data of various experimenters using a(CHJ2 - b (Hz)' methane in flow systems a t temperatures from 1000" to - -d(CH4) = (6) dt c ( H P d ( H P e HZ f (CHd 1500" C. The experiment numbers of Table I refer to the HE relatively inert chemi-
++ +
*
+
+
+ + +
+
+
+
TABLEI. DISTANCE FROM EQUILIBRIUM IN PYROLYSIS OF METHANE TO PRODUCE HIGHERHYDROCARBONS EXPT. 4 5 49 47 54 53 52 13 14 55 56 57 17 18 58 59 30 61 62 63 64 65 66
TBMP.
CONTACT Tthfm
c.
Seconds
1000 1000
1.16 1.81 0.52 1.35 0.15 0.17 0.22 0.39 0.41 0.11 0.23 0.2 0.027 0,098 0.01 0.005 0.034 0.014 0.032 0.0032 0.0042 0.0032 0.0042
1100 1100 1150 1150 1150 1200 1200 1200 1200 1260 1300 1300 1450 1500 1500 1500 1500 1500 1500 1500 1500
RCZHI 1.97 x 8.84 x 2.89 X 7.41 x 2.56 X 8.55 x 3.12 X 6.25 X 2.67 X 2.17 x 1.07 X 2.3 X 2.5 X 5.1 x 1.56 2.27 1.30 3.1 X 2.5 X 2.1 x 7.5 x 2.1 x 1.0 x
10-3 10-4 10-6 10-4 10-j 10-5 10-8 10-j 10-3 10-4 10-2 10-1 10-3 10-8
lo-: 10-
lo-:
1010-3 lo-'
EQUILIBRIUM KCZHZ 5.0 x 10-3 5.0 X lo-* 7.95 x 10-2 7.95 x 10-2 2.82 X 10-1 2.82 x 10-1 2.82 X 10-1 8.92 x 10-1 8.92 x 10-1 8.92 x lo-' 8.92 X 10-1 3.39 7.42 118 251 251 251 251 251 251 251 251 251
56
RCPHI
4.8 x 8.16 X 2.13 X 2.66 X 2.11 x 4.39 x 5.0 X 1.8 x 1.0 x 9.0 x 1.9 x
10-4 10-8 10-4 10-8
lo-' 10-4 10-8 10-2 10-8 lo-' 10-2
... ...
... ... ... ... ... ... ... ... ... ...
EQUILIBRIUM KCZHI 5.76 X 5.76 X 2.24 X 2.24 X 4.47 x 4.47 x 4.47 x 7.77 x 7.77 x 7.77 x 7.77 x
10-2 10-1 10-1
10-1 lo-:
... ... ... ...
13.5
... ... ...
...
...
... ...
10-
10-1 10-1 10-1 10-1
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1931
DATAOF TABLE I TABLE IIA. DETAILED CONVERSION CHI
cos-
To CzHz
IN
TACT
EYPT. TIME,a
PRESSURE,
Mm. Hg
.'cr
+
OFF-
b Gas, c
%
%
ASD
%!?&
Total, e
%
RUDDER
ki =
CzH4,d
a
4.0 3.3
760 84.8 760 67.5
CONVERSION
OF
*
(ZE.8)4-dab (-) 760 X 2 f+c
TION
7.5 9.0 23.0 10.0 12.0 25.0 41.0 6.0 33.0 3 . 5 46.5 2.0 48.0
TEMPERATURE
C.
31 32 33 34 35 36
14.3 8.8
0.0012
....
760 760 760 760 760 760 760 760 760 760
4.0 4.0 7.5 5.0 3.5 6.0 5.0
13 0.39 14 0 . 4 1
105 58.5 105 63.0
15 0.0095 16 0.0149 17 0.0268 18 0 0975
151 64.4 11.0 20.0 4.0 8.0 85 83.2 118 l33.6 21.0 49.5 142 13.8 26.0 75.0
21.0 5.4 18.5 7.7
=
1400' C.
68.0 82.0 61.5 56.0 63.0
88.4 78.8 34.4 31.1 26.9
T E M P E R A T U R E = 1200'
TEMPERATURE
19 20 21 22 23
0.0077 134 17.8 0.0104 206 9.5 0.0180 82 21.7 0.0180 34 29.4 0.0234 34 21.9
20.0 27.0 44.0 36.0 38.0
= 1300'
T E M P E R A T U R E = 1500'
24 25
0,0012 60 52.7 26.0 74 21.5 52.5 0.0022 3.9 58.5 85 26 0.0063 99 4.75 65.5 27 0.0090 35 2.7 65.8 28 0.0152 29 0.0240 53 2.6 62.0 30 0.0339 73 2.2 58.0 cent methane used = f. ** Per Carbon from previous run left in
37 38 39 40 41
...
0.028
42 43 44 45 46
7.3
0.00085 0.00089 0.00100 0.00360
C.
...
... ...
32.2
47 48 49
0.00024 0.00051 0.00085 0.00033 0.00039
120
ki
a
(EQUAT I O N 8)
(I&)*
dab
e X 760 X 2
70
%
50 51 52 53 54
0.50 0.32 0.22 0.17 0.15
C.;
METHANE;
QO.O%
QUARTZ T E B E , 8/U
3.2 55.8 0.050 4.5 51.7 0.083 5 . 1 47.7 0.111 5 . 7 44.4 0.135 6 . 0 39.4 0.151 4.7 50.3 0.087
10MJo C.; 94.4%
760 29.9 760 37.1 760 41.6 760 49.5 760 55.5 = 1 0 6 0 ~c.; 760 40.7 76048.1 760 62.1 760 68.5 760 73.3
METHANE;
2.9 4.5 4.5 5.1 5.8 92.5%
51.2 43.8 39.9 32.2 27.6
1100' C.; 92.5%
760 760 760
60.0 68.4 78.2
METHANE;
C.; 92.5%
760 48.1 760 52.2 760 54.7 760 74.3 760 78.6
M E T H A N E ; 0.5
4.8 4.3 3.8
24.3 21.7 6.5
=
1.86
0.395 0.343 0.303 0.277 0.274 0.344
.... .... .... .. .. ....
0.0576
.. .... .. .. ......
QUARTZ TUBE,
0.0576
.... .... .... .. ..
1.6
0.317 0.300 0.249 0.285 0.315 a/n = 27 0.193 0.151 0.108 0.111 0.088
x 20 C M . Q U I R T 2 HOT ZONE
0.178 0.250 0.125
M E T H A N E ; 0.5
5.2 33.5 8.8 31.7 8.8 30.3 4 . 5 14.0 3.8 10.6
0.0576
CHROME-IRON T U B E , 8 / V
0.057 0.097 0.123 0.129 0.138
3 . 0 38.4 0.104 4.332.00.200 5 . 0 21.5 0.358 5 . 2 17.1 0.380 5 . 2 14.8 0.493 ST.4NLEY AND N l S H (14)
x
0.18
0.212 0.139 0.259
..
..
22 CM. QUARTZ H O T ZONE
0.657 0.991 1.38 0.824 0.707
0.513
0.0556 0.0647 0.0472 0.0456 0.0460
... ... ... ...
SMITE,GRANDONE. AND RALL(13) TEMPERATURE
55 56
tube.
0.11 0.23
=
1200° C.;
760 76.4 760 46.1
0.67
x
80.6 CM. S I L L I M A N I T E H O T ZONE
7 . 1 14.2 8 . 5 41.5
1.29 1.76
1.26
0.0476 0.0338
..
tube at pressures varying from 760 mm. a t 1000" C. t o 34 mm. a t 1500° C. It was necessary to correct the contact times listed by Rudder and Biedermann (11) for the expansion upon heating the gas to the reacting temperature. This correction was made assuming that their volume measurements were made at 27" C. The constants k L of Tables IIA and IIB derived from Rudder and Biedermann's data (11) are generally higher by a factor of about 2 than those calculated from Equation 8. The marked drop in the values of kl a t 1500" C. with increasing total conversion of methane is to be expected for Equation 8 applies only to the early stages of the pyrolysis. Rudder and Biedermann (11) used a granularcarbon resistance furnace to secure temperatures above 1000" C. Such furnaces, unless the carbon is specially prepared and carefully screened, are known to develop "hot spots." While this defect might possibly account for the deviation of Rudder and Biedermann's kl values (11) above 1000" C. from those given by Equation 8, there still remains the same discrepancy at 1000" C. A very common error which might account for this discrepancy is made in the use of platinum and platinumrhodium couples in a reducing atmosphere in the presence of
When the concentration of hydrogen is small enough the rate given by this mechanism is: (7)
The first rate equation accounts for the marked retardation of the rate by hydrogen leading to practically complete cessation of the reaction (in quartz bulbs) at temperatures as high as 1000° C. long before equilibrium is reached. The second rate equation accounts for the first order reaction observed by Kassel (6) in the early stages of the decomposition. The numerical value for the constant of Equation 7 is given by: log k = 11.864 - 17,352/T
1.35 0.87 0.52
1060'
760 26.9 760 31.1 760 34.5 760 37.3 760 42.7 760 31.0
T E M P E R A T U R E = 1160'
0,000061 0.000108 0,000230 0.000460 0.000252 0.000566 0.001015
...
3.7 1.6 0.6 0.45 0.30
TEMPERATURE
C.
30.5 254 63.0 286 92.0 146 90.0 100 93.8 62 94.0 39 95.0 28
5
9.0 4.5 3.25 2.5 2.0**
TEMPERATURE
0.024
1.26
=a
11.2 6.2 4.3 3.3 2.6 5.8**
TEMPERATURE
C.
0.72 0.56
16.0 28.0 14.0 23.0
TEMPERATURE
0.52 0.47 0.48 0.82 0.46 0.61 0.29 0.50 0.60 0.38
0.072 0.018 ... 0.078 ... 0.127 ... 0.056 0.068 ... ... 0.059 ... 0.124 ... 0,090 ... 0.040 ... 0.035
1.03 1.16 1.81 2 1.79 1.75 3.17 9 3.30 10 3.68 11 11.5 12 13.7
&
86.9 83.4 62.0 83.2 77.5 60.0 42.1 60.6 34.5 34.7
Mm. Hg
k1 =
W H E E L E R AND W O O D (18)
T E M P E R A T U R E = lO0Oo C.
3 4 5
kt =
+
e X
Sec.
7 . 0 0.0026 19.8 0.0031
CHd
CHI TO CON' IN CZHI TACT PRES-OFFTotal, EXPT.TIME,a SURE, b G.43,c C1H4, d e
BIEDERMANN (ti)
T E M P E R A T U R E = 900'
1 27.1 2 63.4
-
kz
OF C H 4
(8)
Equation 8 represents accurately data obtained by Kassel (6) in static experiments in quartz bulbs. Tables IIA and IIB give comparisons of the constants calculated from Equation 8 with values derived from the data of a number of experimenters. The data of Rudder and Biedermann (11) were obtained using pure methane in a quartz
TABLE IIB. DETAILED DATAO F TABLEI CHI IN
LXPT
CONTACT TIME,^
Seconds
PRESSURE.
b
iMm. H g
OFF-
GAS,^
Yo
CHI
C O A V E R S I O N OF
To CzHz CZHI, d
+
%
ki Total, e
-
0!!e
a
kz = ki (EQUATION
8)
dab (j*) e X 760 X 2
5% FISCHER A N D PICHLER (. 1.)
57 58 59 60
0.2 0.01 0.005 0.004
760
(15)
760 760
('()8 )
61 62 63 64 65 66
25.0 50.0
(1.8)
0.0135 0.0321
35 55
4.5 3.3
23.0 41.0
0.0032 0.0042 0.0032 0.0042
760 760 760 760
14.2 10.8 6.3 4.3
29.5 32.4 23.3 36.1
55.0
74.0
(31.3) (62.5) (68.8) (92.5)
1.57 62.5 137 231
3.7 65.0 120 398 RCDDER A N D BIEDERMAXN (11) 91.0 67.5 120.0 93.0 29.0 120.0 STORCH A N D GOLDEN (17) 36.7 83.5 120 47.4 113 ... 36.5 114 ... 61.0 145 ...
COMPN. OF R A W .\ 4I TERI +L
TEMP.
c.
0.031 0.00136 0.00065 0.00043
+
0.000037 0.000036
1500 1500
4 3 . 1 7 CHI 54 4 0 HI 65.9% CHI f 33:5$ H 1
0.000504 0.000792
1500 1500 1500 1500
&C;
O.OOOl56
0.000178
+ ;iq CHc iCHI + CH4
CHI
+
75% COz 7 5 7 cot 90% COz 90% C O i
58
.
INDUSTKlAL AND ENGINEERING CHEMISTRY
quartz. Under such conditions the couple is contaminated with silicon and the temperature readings are usually too low by 50" to 100" C. Wheeler and Wood (18) used 92.5 to 96 per cent methane (the remainder being nitrogen) a t 760 mm. in quartz and chrome-iron tubes. Their contact times are apparently correctly calculated so far as temperature correction is concerned. They do not, however, give the lengths of the tubes employed, nor do they state whether correction was made for temperature gradient in the tube in calculating their contact times. Their kl values for the single quartz and chrome-iron tubes are, like Rudder and Biedermann's (II), higher than those of Equation 8 by about a factor of 2. It is probable that the discrepancy is in both cases due to uncertainties in the time of contact and temperature. It may be well to state that a value of kl higher than that given by Equation 8 is improbable on theoretical grounds. Such high values can result only from erroneously estimated contact times or temperatures, if one assumes that the analysis of the product is correct, and that the heat of activation (that is, the slope of the curve obtained by plotting Equation 8) does not vary greatly with temperature. It may be thought that the very high values of k! found by Wheeler and Wood (18), when using the annulus between two concentric quartz tubes as the reaction chamber, may possibly be due to the enhanced surface exposure destroying the validity of the assumptions made when Equation 8 (which was based on the results of static experiments) is used on a flowing system. However, it should be noted that at contact times of 3.3 to 3.7 seconds the total methane conversions are little, if any, higher than those of the single quartz tube. It seems much more probable that the temperature gradient in the annulus was more sensitive to rates of flow higher than that corresponding to 3.7 seconds than was the single quartz tube. Stanley and Nash's data (14) were obtained using 92.5 per cent methane and 7.5 per cent nitrogen at 760 mm. pressure
,
are in fair agreement with those of Equation 8.
I
Vol. 26, No. 1
'
~
I 2
\
the kl valies of Tables IIA a n d I I B w i t h
'
o Equation 8 is, on the whole, w i t h i n A50 per c e n t . T h i s i s fairly good evidence -' that the initial rate-2 determining steps are the s a m e o v e r t h e whole temperature -8 range up to 1500"C., and that Eauation 8 will predict *approxi9 8 mately the total per10.000 7 c e n t a g e of methane T reacted a t any pres- FIGURE2. SECONDRATECONSTANTS sure in flow systems when the temperature and time of contact are given. Such prediction is, however, limited to not more than about 60 per cent total conversion of methane per pass, It is probable that most of the deviations from the predictions of Equation 8, noted in connection with Tables IIA and IIB, are due to unavoidable temperature gradients ip flowing systems. It now remains to discover, if possible, Some way of predicting the distribution of the products; that is, what fraction of the reacted methane will appear as unsaturated 2carbon gas (acetylene ethylene) and what fraction as polymers and carbon? Those familiar with the literature on the pyrolysis of methane will recall that, in general, larger percentages of acetylene are obtained in the product if lower pressures are employed, but that the partial pressure of acetylene ethylene is approximately constant for a given tem-
!
+
+
INDUSTRIAL AND ENGINEERING
January, 1934
TABLE111.
PYROLYSIS OF
59
CHEMISTRY
ETHYLENE DATA
(Time of contact, 0.004 t o 0.005 second) F I 5 C H E R AND PICHLER'B
T4Br.E EXPT. T E M P .
:a)
(0)
1
1
19
1000
1 4 1 1
9 3b 12 20
1100 1100
1000
1100 1100
-FISCHER A X D PICHLER'S DATAF R O Y TABLE4 @)-PRODUCT8 A 0 PERCENTAGE OF PRODCCTS AS P E R C E N T A Q ~ REACTED C ~ H A ~ OF REACTED CnHP CZHI PolyCiHz I N PRESSURE CzHc PolyCzHz I N N? REACTEDCzHa mers CHI O F F - G A B EXPT.TEMP. C2H4 Hn REACTEDC2H2 mers CHI OFF-GAJ
AtmosDhere
c'.
; 1000 11 1000
1
4
P R E s sE R E C?Hc HZ 1.0 0.1 0.5 0.5 1.0 0.1 0.5 0.5
...
...
...
... 0.5 ...
0.5
...
.
...
... ...
0.5
. .. . ..
DATA(9)
. ..
0.5
%
% 8.5 13.0 6.4 9.9
16.5 32.3 30.3
20.9 26.0 20.0 27.0
23.9 95.0 43.0 43.4
12.5
83.5 58.1 45.3 51.6
64.2 O?
38.0 49.6
0 8.5 42.2 19.2
1.3 3.6 0.4 1.4
9.6 2.3 19.0 7.0
4.3 18.8 3.9 5.2
... ... . ..
e
c.
5 7 10 12 14 18
1300 1300 1320 1300 1280 1300
9 11 13 16 19 20
1400 1380 1400 1400 1400 1410
Btmc)sphere 0.1 .... 0.017 0.05 0.05 0.0085 0.0085 0.075 0.025 0.0143 0.0857 0.017 0.05
0.0085
9.5 10.6 0.025 46.2 29.4 60. 3 1 IO 1200 1.0 7.3 23.6 0.0143 25.2 54.4 67.4 4 4 1200 0.1 ... 9.4 15.8 0.0143 36.7 45.1 47.5 13 1200 1 0.5 0.5 1 21b 1200 0.5 . . . 0 . 5 54.4 42.0 48.7 9 . 4 9.3 a The sum of these three figures subtracted from 100 gives the percentage conversion to ethylene. b Some carbon formed.
perature. Upon esamination of the available data it was found that for constant temperature:
+
% conversion of CH4 to (C2H2 C2H4) X av. partial pressure of CH4 X contact time % conversion of CH4to total products = constant = Icz Since the denominator = k , x contact time, then kz = percentage conversion to (C2Hz CzH4) X average partial pressure of CH4. It was thought desirable, however, to calculate k z directly from the data without employing any specific value of kl. The values of k z given in Tables IIA and IIB are subject more or less to the same uncertainties of temperature and time of contact as are those of kl. I n general the agreement at 1000", 1050°, 1100", llFiO", and 1200" C. is good. The 1500" C. kz values from Rudder and Biedermann's data ( 1 1 ) are lower than those of other experimenters (Fischer and Pichler, 1, and Storch and Golden, 17) by a factor of about 10. Rudder and Biedermann's 1300" and 1400' C. k z values (11) are also lower than would be expected from Fischer and Pichler's ( 1 ) and Storch and Golden's (17) data. The average values of log k ) for each experimenter and each temperature are plotted against the reciprocal of the absolute temperature in Figure 2. The straight line drawn through the points is obviously fixed in slope within about 10 per cent. The heat of activation thus obtained is about 64,000 calories. This exothermic heat value is probably a complex quantity made up of differences between the separate heats of activation of several reactions. I n discussions with L. S. Kassel concerning the probable significance of the equation
+
%
%
%
%
%
82.2 93.4 80.6 85.8 62.8 61.6
72.0 66.3 75.4 76.3 84.8 81.2
22.6 30.2 19.4 17.1 2.9 13.2
4.6 3.5 4.3 5.6 8.9 5.6
31.8 32.0 24.4 23.5 10.9 6.8
. . . . 98.5 0.05 95.2 0,0085 9 8 . 7 0.075 97.0 91.1 0.0857 0.0867 100.0
59.0 74.0 73.5 87.4 92.3 90.0
39.0 22.3 23.5
2.0 3.7 2.7 1.5 4.6 8.0
28.8 27.0 24.6 16.4
....
11.1
3.1 2.0
11.1
11.2
Thus it is now possible to represent the entire mass of data on methane pyrolysis by two straight lines-that is, Figure 2 and a similar plot for Equation 8. Since the values of k , and kz remain sensibly constant for large variations of tube diameter, composition of tube material, and composition of reacting gas (excluding more than about one per cent of higher hydrocarbons), the results of a good portion of the testing preliminary to pilot-plant operation can be predicted.
THERMAL DECOMPOSITION OF HIGHERHYDROCARBONS To YIELDACETYLENE
Fischer and Pichler (2) have recently published some data on the dehydrogenation of ethylene a t temperatures from 1000" to 1400" C. Their results were recalculated so as to show clearly the percentage of ethylene reacted, and the distribution of the products between ethylene and pollmers. These data are presented in Table 111. The polymers coiisisted of about 50 per cent benzene, the remainder being oils of higher molecular weight. Little or no carbon was obtained under the conditions chosen for the experiments. The ratios of (acetylene hydrogen) to ethylene which may be calculated from the original data of Fischer and Pichler (6) are all ten to twenty-five times smaller than the equilibrium values which may be obtained from Figure 1. The percentage of ethylene reacted is constant within about 25 per cent of its value for any given temperature, being almost independent of the partial pressure of ethylene. This is characteristic of a unimolecular reaction. There is, however, a definite trend toward somewhat higher conversions a t the lower pressures. Evidence of retardation of the dehydrogenation rate by hydrogen is observed especially in the 1300" kz = yo conversion to CZ X av. partial pressure of CH4 C. tests (compare experiments 14 and 18 with 5,7,10,and 12). he suggested that the polymerization reactions were depend- Pressure seems to affect the distribution of the products ent upon some power of the acetylene pressure sufficiently markedly, a lower partial pressure of ethylene resulting in a greater than unity to keep the percentage conversion of higher conversion to acetylene. There is, however, no simple methane to acetylene apparently constant for a given methane inverse proportionality here as is the case in methane pyrolypressure. Pease (10) and later Schlapfer and Brunner (16) sis. This pressure effect is easily noted in the lOOO", 1100". studied the kinetics of acetylene polymerization. Both ex- and 1200" C. experiments. Thus a tenfold decrease in perimenters found the reaction to be bimolecular and homo- ethylene pressure from 1.0 to 0.1 atmosphere results in an geneous a t 400" to GOO" C. and pressure close to atmospheric. increase of about fourfold in the ratio of percentage converPease (10) reported about a 50 per cent decrease in rate upon sion to acetylene/(percentage conversion to polymers). The packing the tube, indicating some chain reaction. It is prob- effect of dilution with hydrogen is also to increase this ratio. able that a t lower pressures (that is, 0.5 to about G cm. of This beneficial effect of hydrogen dilution is quite marked a t mercury) and temperatures of 1000" to 1500" C. the reaction 1300" and 1400" C. mould be largely homogeneous and bimolecular, although the From a practical point of view the experiments a t 1300" possibility of some chain reaction still remains. and 1400" C. and low pressures with hydrogen dilution are of Whatever the explanation of the apparent constancy of kz most interest. Thus experiment 20 shows 100 per cent conits usefulness is obvious in predicting with the help of Equa- version with only 2 per cent going to polymers and 8.0 per cent tion 8 the conversion to acetylene ethylene and the total t o methane. It is, however, doubtful whether an air-tight conversion for any given contact time and methane pressure. system can be operated a t 1300" to 1400" C. on a large scale.
+
+
60
I N D U S T R I A L AIVD E N G I N E E R I N G C H E M I S T R Y
Vol. 26, No. 1
TABLEIV. FISCHER AXD PICHLER'S RESULTS ON PYROLYSIS OF GASOLINE AND PETROLEUM -REACTION
TEMP. O
C.
RAWMATERIAL
PRESSURB Character
Mm. Hg
Material
Ce./hour
TUBEDiam. Cm.
Gaaoline 100 Porcelain 1.6 Gasoline 100 Porcelain 1.6 Gasoline 100 Porcelain 1.6 Gasoline 12005 76 Hydrogen 60,0!! 900 760 Petroleum 106 Porcelain 1.6 900 760 Petroleum 95 Steel 1.5 900 760 Petroleum 60 Copper 1.2 Petroleum 60 1260b 76 Hydrogen 60,000 Porcelsin O. 19.4 per cent to. light,oil and 6.3 per cent to tar and carbon in this experiment. b 24 per cent to light oil and 12 per cent to tar and carbon in this experiment.
700 800 900
760 760 760
{
1
Fischer and Pichler (2) also give some results on the conversion of ethane to acetylene. These data are practically identical with those obtained for ethylene, under the same conditions. It appears therefore that the dehydrogenation of ethane to ethylene is much more rapid a t 1000" to 1400' C. than is the dehydrogenation of ethylene to acetylene. At 1200" C. the rate equation given by Marek and McCluer (8) for the dehydrogenation of ethane to ethylene indicates that equilibrium would be reached in about 10-5 seconds. It is also probable that propane and butane would behave very much like ethane, except for the production of considerably more methane. I n another recent article Fischer and Pichler (3) describe some experiments on the use of gasoline and petroleum as raw materials for acetylene production. Some of their results are given in Table IV. There is little of theoretical importance connected with the data of Table IV, but the results with petroleum as the raw material are potentially of industrial interest. The higher results obtained with a copper tube as compared with steel or porcelain are probably due to more efficient heat transfer rather than to any specific catalysis.
Length Cm.
10 10 10 60 10 10
PRODUCTS AS PERCENTAQE~ OF RAW M A T E R I A L CRACKED
CzHa
0.4
i:e 50.4 0.5 0.0
ClHd
21 45 40
5.4 22.2
10
1.2
23.3 31.0
60
28.5
18.4
CaH6
... ...
...
4.7 5.3 5.5 0.8
CZH6
CHI
4
5 17
7 3 4.6 0.0 2.3
19
14 24.7
0.0
22.5 27.1
1.1
15.2
The necessity of employing temperatures of 1500" to 1600" C. practically limits the choice of refractory materials to alundum or silicon carbide. Either of these materials in the shape of a checkerwork could be blasted up to reacting temperature with preheated natural gas and compressed air, and the diluted methane then admitted, thus making a cyclic process with direct heat transfer. It seems unwise to attempt to secure the necessary energy by heat transfer through the walls of closed chambers. The conversion of ethylene, ethane, propane, or butane to acetylene by thermal decomposition is a much simpler engineering problem, for temperatures of 1200" C. may be readily maintained by heat transfer through chrome alloy tubes. It would be of industrial interest to extend Fischer and Pichler's experiments ( 2 ) with ethylene decomposition a t 1200" C. t o mixtures containing 75 to 90 per cent of diluents such as hydrogen and carbon dioxide, and to make similar tests with propane and butane. Considerable data have been publishnd on the synthesis of acetylene from methane by means of an electric discharge through the gas (15) or by passing methane through an electric arc ( 5 ) . Cost estimates made by the writer (based on 0.25 cent per kilowatt-hour for power) indicate that such INDUSTRIAL POSSIBILITIES OF ACETYLENE PRODUCTION BY processes cannot compete with calcium carbide a t its present PYROLYSIS OF HYDROCARBONS price. There are also several patented processes for the The results obtained by the thermal decomposition of production of acetylene from methane by partial oxidation of methane a t temperatures of about 1500" C. indicate that for the latter (4,9). The writer's judgment is that such processes efficient conversion of methane to acetylene i t is essential to would be economically feasible in the United States only when air was employed, and the resulting by-product mixture of operate a t low partial pressures of methane. From a praccarbon oxides, nitrogen, and hydrogen purified for use in the tical point of view the use of total pressures below atmospheric synthesis of ammonia, The very large capacity of present is undesirable because of the severe engineering difficulties synthetic ammonia plants argues strongly against any further involved in constructing a gas-tight system containing re- installations in areaa where natural gas is available a t very fractory chambers a t 1500" C. Dilution with some cheap and low cost. relatively inert gas seems to be the alternative to operation a t subatmospheric pressure. The results of Fischer and PichLITERATURE CITED ler ( 1 ) using coke-oven gas indicate that 50 to 75 per cent con(1) Fischer, F.,a n d Pichler, H., Brennstof-Chem., 13, 381 (1932). versions to acetylene may be obtained, with but little carbon (2) Ibid., 13,406 (1932). (3) Zbid., 13,441 (1932). deposition, a t 1500" to 1600" C. and 0.005 to 0.004 second (4) Fisoher, F., a n d Pichler, H., F r e n c h P a t e n t 719.035 ( J u n e 23, contact time, if the methane is diluted sufficiently with hydro1931). gen. It is probable that if hydrogen is the only diluent (5) Frolich, P. K., e t al., IND.ENQ.CHEM., 22, 20 (1930). [Fischer and Pichler ( 1 ) also had 25 per cent nitrogen 4(6) Kassel, L. S., J. Am. Chem. SOC.,54, 3949 (1932). (7) Ibid., 55, 1351 (1933). carbon monoxide carbon dioxide in their gas] that the (8) M a r e k , L. F.,a n d McCluer, W. B., IND.ENG.CHEM.,23, 878 total conversions will be somewhat smaller. (1931). The results of Storch and Golden (16) using carbon dioxide (9) Mittasch. A.,e t al., U. S. P a t e n t 1,823,503(Sept. 15, 1931). as a diluent are of interest because the carbon dioxide keeps (10) Pease, R. N.,J. Am. Chem. SOC.,51,3470 (1929). the refractory surfaces free of deposit,ed carbon, and because (11) R u d d e r , D.,a n d Biedermann, H., Bull. SOC. chim., 47,710 (1930). the process is flexible in that very little or almost complete (12) Schliipfer, P.,a n d B r u n n e r , M., Helu. Chim. Acta, 13, 1125 conversion of the excess methane (which does not react to (13) S m(1930). i t h , H.M., Grandone, P., a n d Rall, H. T., Bur. Mines, Rept. form acetylene) to water gas may be obtained. The conZnuestigations 3143 (1931). versions per pass are 30 to 36 per cent to acetylene ethylene, (14) Stanley, H. M., and Nash, A. W., J . SOC.Chem. Zd., 48, 1T (1929). and 37 to 61 per cent total when the time of contact is about (15) Storch, H.H., Bur. Mines, Circ. 6549 (1932). 0.0035 second and the carbon dioxide content 75 to 90 per (16) Storch, H.H., J . Am. Chem. SOC., 54,4185 (1932). cent. The percentage of acetylene in the product after (17) Storch, H.H., a n d Golden, P., IND. ENQ.CHEM., 25, 768 (1933). scrubbing out the carbon dioxide is approximately the s a m e (18) Wheeler, R.V.,and Wood, W. L., Fuel, 9,567 (1930). namely, about 9 per cent-as that of Fischer and Pichler's RBCBIYEDSeptember 7, 1933. Publiahed by permiasion of the Director, product using coke-oven gas. U. 8. Bureau of Mines. (Not aubject to copyright.)
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