THE DECOMPOSITION OF T H E PARAFFIN HYDROCARBOKS* BY GCBTAV EGLOFF, R. E. SCHAAD, AND C. D. LOFVRY, JR.
I. Introduction Decomposition is one of the few general reactions of the paraffin hydrocarbons. It may be brought about by heat, by high temperature aided by pressureJ by electrical discharge, by alpha radiation, or by photolysis. Because it is a general reaction, and allows investigation of many characteristics of the hydrocarbons which their inertness makes difficult, if not impossible, to study in any other way, decomposition has been the subject of a vast amount of investigation. This field, made arduous by the inactivity of the original substances, the tendency of the primary reaction products to decompose and so obscure the mechanism of conversion, and the obstacles to be surmounted in obtaining pure starting materials and in separating reaction products, is slowly being solved. Much has been accomplished in the past, the field is an active one at present, and it promises rich rewards in the future. Why, one may ask, is study of the break-down of the paraffins important? To the worker in pure science it has given the opportunity to compare the stabilities of homologous compounds and to measure the effects of different groupings on the properties of molecules, particularly in isomeric substances. Consideration of this subject is related to some of the most interesting problems in chemistry-t he measurement of equilibrium, the hunting of reaction mechanisms, and the search for free radicals. Hydrocarbon decomposition is closely related to the origin of petroleum and other bitumens, and its study may enable us to understand the chemistry concerned in Xature’s piling-up of these vast stores of hydrocarbons. From an industrial point of view, no less than from that of abstract science, the decomposition of the paraffins is vital. At the present time the cracking process is converting each year vast volumes of paraffin and other hydrocarbons into over ~,ooo,ooo,ooogallons of gasoline. The decomposition of oils, largely paraffinic, for the production of gas, has been an industry for more than a hundred years. Thermal action is the basis of the conversion of natural gas, made up almost exclusively of paraffins, into millions of pounds of carbon black annually and now in part to motor fuel. Both gaseous and liquid paraffins are being transformed by high temperature and pressure into more reactive substance for use in chemical synthesis. To all industries based on these different phases of hydrocarbon pyrolysis, accurate knowledge of the decomposition reactions of the individual paraffins is of fundamental importance. *Paper from The Research Laboratory of Yniversal Oil Products Company, Chicago, Illinois
1618
GUSTAV EGLOFF, R. E. SCHAAD, A S D C. D. LOWRY, JR.
The significance of the work on the decomposition of the paraffin hydrocarbons is emphasized by consideration of the vast quantities available, particularly of the lower members of the series. In the table on page 1619 are given the estimated proportion and amounts of methane, ethane, propane, and butane present in commercial gases and in gasoline. By fractionation the individual hydrocarbons may readily be obtained in pure form from these sources. Their accessibility in such amounts and at low cost means that the investigator has an almost inexhaustible supply of raw material a t his command, and that any process which can be put to commerical utilization will have significant economic value. This study has been divided according to the method of decomposition employed, whether by thermal action, by electrical means, by alpha particles, or through photosensitization. Many of the products of the decomposition of a single hydrocarbon are alike by all methods of conversion. Some are different, and no doubt there are also differences in reaction mechanism.
11. Thermal Decomposition An individual paraffin hydrocarbon, subjected to the disruptive action of heat, may give rise to few or to many products. What these will be in any single experiment depends on the hydrocarbon decomposed, the temperature, the time of heating, and catalytic influences present. Carbon and hydrogen are almost universally produced, and they may be the sole products. Paraffin hydrocarbons lower than the substances undergoing reaction are often reported. Olefins and diolefins are frequently observed. The formation of naphthenes had been claimed in a few cases. Under carefully regulated conditions aromatics may be produced. Acetylene and perhaps its homologs are formed in small amounts at moderate temperatures, and acetylene may become the main product under conditions of very high temperature. 111defined products such as “pitch,” “unsaturated bodies,” and “resins” also frequently appear as reaction products. The Mechanzsms proposed.-The question of reaction mechanism in the decomposition of paraffin hydrocarbons has been discussed widely and vigorously. While the steps in certain simple breakdowns seem to be clear, one cannot yet claim to understand thoroughly the pyrolysis of even the simpler members of the series. First in chronological order among proposed mechanisms is the postulate of Berthelot ( I O ) that acetylene is at least an intermediate product in every hydrocarbon decomposition. When methane, for example, was decomposed, Berthelot believed that the following reactions occurred.
+ +
+ +
CHI % CIHP 3 H,, and z CHI % C?HB Ht C,Ha HP, and z C,H, zCHl C,H? H, C2H4 % C2HZ HP,and z C2H4ti C2H6 C&. This acetylene mechanism was given color by the fact that the paraffins were found to give aromatic hydrocarbons in pyrolysis and it was known that acetylene passed through a red hot tube similarly yielded benzene, naphthalene, etc. z
C2H6
+
+
+
DECOMPOSITION OF THE PARAFFIN HYDROCARBONS
1619
1620
GCSTAV EGLOFF, R. E. SCHAAD, AND C. D. LOWRY, J R .
Lewes (138), following Berthelot, lent his support to the acetylene theory. Later workers, taking the cue from Haber (101, 1 0 2 , 103) who could find acetylene only in traces among the decomposition products of hexane, looked askance a t the idea that acetylene was an essential intermediate. Of late, however, interest in this theory has been revived by Frolich and his co-workers (86), who found considerable amounts of acetylene among the products of methane decomposed at about IIZO'. Berthelot also believed that a hydrocarbon did not decompose directly into carbon and hydrogen, but that in general, the early changes in thermal reactions mere polymerizations, with elimination of hydrogen, producing complex hydrocarbons. U'here carbon appeared it was considered to be a decomposition product of a very complex molecule. Polymerization no doubt accounts for the formation of the pitchy and resinous substances that occur as products of hydrocarbon conrersion. But that it is a necessary preliminary to the appearance of carbon is unlikely. This assumption is made improbable in part by the fact that the carefully studied decomposition of methane to carbon and hydrogen has been found t o be a true equilibrium, approachable from both sides, so that it probably does not have a complex set of intermediate reactions. Second among the decomposition mechanisms is the conception that the paraffin hydrocarbon chain may break a t any point, with simultaneous shifting of hydrogen and formation of a saturated and an unsaturated molecule. If the break occurs bet'ween carbon atoms a paraffin and an olefin are the primary products; if b e t w e n a terminal carbon and hydrogen, the saturated molecule is hydrogen, and an olefin is formed having as many carbon atoms as the original paraffin. Several workers stated this theory in part before it 1%-asenunciated in entirety. Thorpe and Young ( 2 2 3 ) showed that the action of heat and pressure on paraffin wax yielded lower paraffins and olefins but no hydrogen. They concluded that the primary decomposition of a paraffin hydrocarbon produced an olefin and a lower paraffin by scission of the molecule near the center, without loss of hydrogen. I t was thought that butane, for example, would give ethylene and ethane. I n work on hexane, Haber adopted a slightly different point of view. He contended that the break alxays came a t the terniinal carbon, with separation of methane as the saturated hydrocarbon, and the remainder of the molecule as an olefin. Thus, with hexane he belieied the primary products to be methane and amylene. Hurd and Spence ( 1 2 0 ) and Hague and Wheeler (104)eventually showed that both Thorpe and Toung, and Haber were correct, for the reaction takes several, not a single path. As Hague and Kheeler stated, from experiments with the hydrocarbons from methane through hexnne "the primary decompositions can be represented by a series of equations indicating the rupture of the chain at any position, with the production of an olefin and the com-
DECOMPOSITION OF THE PARAFFIN HYDROCARBONS
162I
plementary lower paraffin, or at the limit, hydrogen”. With n-butane they observed as primary decompositions:
+
(i) CH, C H 2 C H 2 C H 3= CH2:CH2 CH, CH2 H, (ii) C H ~ C H Z C H Z C=H CH&H:CH2 ~ CH,.H, (iii) CH, C H 2 C H 2 C H 3= CH3CH2 CH:CH2 H.H.
+
+
“As the series is ascended the tendency for hydrogen to be eliminated, leaving an olefin with the same number of carbon atoms as the original paraffin, rapidly diminishes.” Confirming this statement] Hurd and Spence found in their work on butane that all three of the above reactions occurred, but that the first two accounted for the major portion of the chemical change. The substances they obtained were so nearly exclusively those called for by these reactions that there can be little doubt that they were the primary stable products. Butadiene was also identified, a fact which will later be shown to have considerable significance. A subject well suited for speculation which is presented by this reaction mechanism is the question of the intermediary formation of free radicals. Consider any one of the three reactions given. Does the shift of a hydrogen take place before or a t the time of rupture of the carbon chain, or is rupture the first step, to give as it necessarily must, a t least momentary existence of free alkyl radicals? That under certain conditions free radicals can be formed from hydrocarbons was shown by Thomson ( 2 2 2 ) who found evidence for the methyl radical, the bivalent methylene] and the trivalent i C H in the positive ray spectrum of methane. Hogness and Kvalnes (113) found positively charged methyl radicals as well as charged methane atoms in their mass spectrograph work. Some workers believe that radicals have far more significance in hydrocarbon decomposition than as transient intermediates. Bone and Coward (2 9 ) suppose that in the decomposition of ethane (and ethylene or acetylene as well) both breaking of the carbon chain and loss of hydrogen occur, resulting in the formation of .CH3, : CH2, or i C H groups. Hague and Wheeler (104) consider the first step in the decomposition of methane at relatively low temperatures to be the formation of hydrogen and methylene radicals. These radicals could then react in three ways: (a) Combine to form ethylene or by dehydrogenation, acetylene. (b) Split t o carbon and hydrogen. (c) Be hydrogenated to methane. Williams-Gardner ( 2 4 0 ) and Fischer (69, 70) have also made use of free radicals as decomposition intermediates. So far, in considering reaction mechanisms, only the primary changes have been considered. But if the decomposition continues, particularly a t high temperature] for more than a brief time interval] the products are not only those of primary decomposition, but others formed by secondary changes. To account for them, Berthelot depended on the known ability of acetylene to change into a variety of other substances. Many more recent workers
GUSTXV EGLOFF, R. E. SCHAAD, A N D C. D. LOTVRP, JR.
1622
stress ethylene, and consider this hydrocarbon the essential intermediate in the formation of higher products, especially aromatic hydrocarbons, from the paraffins. Davidson ( s i ) , not finding acetylene in the products of thermal decomposition of the ethane-propane fraction of natural gas, suggested that by elimination of hydrogen two molecules of ethylene unite to give butadiene (a frequent product of hydrocarbon decomposition) that the butadiene reacts with an additional molecule of ethylene to give cyclohexane (or cyclohexene), and that the cyclic compound undergoes further change to hydrogen and benzene. Hague and Wheeler employ much the same mechanism. Based on their study of the paraffins from methane to hexane they state: "Our experiments show : that ethylene readily polymerises to butylene which, by elimination of hydrogen, yields butadiene. From the presence of six-membered unsaturated hydrocarbons (liquids, b.p. 59" and 78") amongst the synthetic products from the paraffins we conclude that the butadiene reacts with ethylene, the suggested mechanism being: (a) CH2:CHz CHz:CH2-+C H 3 C H 2CH:CH2--t CH2:CH CH:CHz H Z (b) CHz:CHCH:CH2 CH2:CHz-+either CH2:CHCHzCH2 CH:CHz or CH2:CH.CH:CH CH:CHz HZ
+
/ \ CH2 I/ I CH CH \ / CH
-+-
+
+
+
-+-
/ \ CH It I CH CH \ /
CH
+ Hz
CH CH "Further, by analogy with the formation of diphenyl from benzene, it is reasonable to suppose that butadiene can condense with benzene, hydrogen being eliminated, to form naphthalene; and that anthracene and phenanthrene can be formed from the naphthalene in a similar manner." Favoring this mechanism, with its emphasis on olefins as intermediates, as against the acetylene theory, is the fact that on pyrolysis, ethane and propane, which yield olefins in abundance but less acetylene than does methane, give far higher yields of aromatics than does methane. The Relative Stability o j the Para$ns.-The differences in behavior of the individual paraffin hydrocarbons toward heat have been studied to some degree, but not intensively. Hague and Wheeler (104) found that among the normal members of the series from methane to hexane the stability decreased with increasing molecular weight. They reported decomposition temperatures in an electrically heated silica bulb as follows:
Methane Ethane
First evidence of change by pressure increase 683'
485"
Propane Butane
First evidence of change by pressure increase 460'
435O
DECOMPOSITION O F T H E PARAFFIN HYDROCARBONS
1623
Bone and Coward ( 2 7 , 28, 29) also found methane more stable than ethane. The temperatures of decomposition, and perhaps the relative stability also, are profoundly affected by catalytic influences. Some calculations of stability have been made based on thermodynamics. From the free energies of the hydrocarbons, Francis (79) concluded that “only the paraffins and the higher olefins have any thermodynamic stability whatever, and above 260°C. methane is the only stable hydrocarbon.” At very high temperatures, however, acetylene is formed rapidly from methane. On the basis of the Kernst heat theorem, von Wartenberg ( 2 3 2 ) calculated that methane should be the most stable member of the paraffin series. I n a study of thermal decomposition of mixtures of hydrocarbons obtained by fractionating Pennsylvania petroleum, made up largely of paraffins, Egloff and Moore ( 6 2 ) showed that stability was not always a direct function of the size of the paraffin molecule. “If stability is graphed against the boiling point of the paraffins, there results a curve having the maximum stability a t about 250°C. representing the compounds C12H16to CljH32, minima a t the lowest boiling points (CsH,2 to C9HzO)and the highest boiling points (CiJLo to CZ~H,).” I n regard to the stability of isomeric hydrocarbons, there is available only the observation of Hurd and Spence (120) that butane and isobutane are about alike in their resistance to heat. The study of relative stability has really been carried out just far enough to show its possibilities. The irregularities disclosed by Egloff and Moore, the lack of knowledge of the effects of branched chains, and the entire absence of comparative work on pure hydrocarbons above hexane make the subject plead for further investigation. A. Methane and Natural Gas Since methane is the simplest of the paraffin hydrocarbons, and the one which is available in largest amount, it is not surprising that the thermal decomposition of this hydrocarbon has been the subject of more thorough study than has the breakdown of any other member of the paraffin series. Research has been prompted not only by such essentially scientific questions as the mechanism of decomposition and the equilibria involved, but also by the commercial importance of the hydrocarbon and of the substances which can be produced from it. The temperature a t which methane starts to decompose on heating varies with the experimental conditions. I n contact with substances of little or no catalytic action methane has been reported to undergo change a t between 650’ and 700’. Hague and Wheeler (104) found in silica bulbs the first evidence of decomposition a t 683’. Catalysts markedly reduce this temperature. I n the presence of palladium there is decomposition at z50°, with nickel a t 320°, and with iron at 350”. The decomposition of methane into carbon and hydrogen is an equilibrium reaction, reaching half conversion of the hydrocarbon at about 5 2 5 ’ and ninety per cent at 750’. Formation of carbon and hydrogen is not the only
1624
GUSTAV EGLOFF, R. E. SCHAAD, A S D C. D. LOWRY, JR.
type of change which methane undergoes, however, and except in the presence of catalysts which favor this conversion (and most catalysts do favor it) the breakdown to the elements is accompanied by the formation of hydrocarbons containing more than one carbon atom in the molecule. These products include ethane, also olefins and diolefins, such as ethylene, propylene, and butadiene. Acetylene is frequently found and is favored by high temperature, as in the electric arc. By careful regulation of the conditions of pyrolysis-temperature, catalysts, time of heating, gas concentration-considerable amounts of aromatic hydrocarbons may be formed, among them, benzene, toluene, xylene, naphthalene, and anthracene. That the first steps in the decomposition of methane are the formation of free radicals, such as CH3, : CH,, and i CHI and their conversion to new compounds is assumed by a number of those who have worked in this field, including Bone and Coward ( 2 9 ) and Fischer (69, 70). The gymnastics of these radicals are summarized by Stanley and Kash ( 2 0 4 ) as follows: 2
CHI
1
+ Hz
CH,,CHs %
2
CHz:CH,
1 2 CHz: + Hz 1
S
C H i CH ts
2
CH3.
CH
I
+ Hz
As has already been brought out, the acetylene which is thus formed vas considered by Berthelot ( I O ) and some other workers, to play a major r61e in subsequent changes, while Hague and Wheeler stress the formation of ethylene which they believe goes to butadiene and eventually is responsible for the production of aromatic hydrocarbons. This discussion of the thermal decomposition of methane is divided into three parts : studies of the methane-carbon-hydrogen equilibrium, investigations producing largely carbon and hydrogen, and work on the formation of higher products. To be sure, there is some overlapping where a single research has produced significant amounts of substances in both of the last two categories. KO doubt, also, some investigators hunting higher products, particularly in industrial researches, have failed to mention carbon and hydrogen, and those seeking carbon and hydrogen have paid little attention to small amounts of higher products. The classification, therefore, cannot claim absolute accuracy, but it is a convenient division. “fifethane,” as used in this paper, includes the synthetic hydrocatbon and also such samples of natural gas as have been shown by analysis to be free from other hydrocarbons. The gases which investigators described merely
1625
DECOMPOSITIOS OF T H E P A R A F F I S HYDROCARBOSS
as “natural gas” or those naturally occurring which have been shown to contain hydrocarbons in addition to methane are classed as “natural gas.” I. The Methane Equilibrium The decomposition of methane into its elements according to the equation
+
CHa e C zH? is a true equilibrium. Mayer and -4ltmayer (160) shows that a t high temperatures methane may be decomposed or reformed from its elements a t will. They approached the equilibrium from both sides (working under 750 mm.) at 470’ to 6 2 0 ’ ~ by passing methane and nitrogen over a catalytic mass of clay impregnated with nickel and cobalt in a Jena glass U-tube, or by passing mixtures of hydrogen and nitrogen over the carbon-covered catalyst. From the partial pressures of methane and hydrogen found experimentally they derived the following equation for the methane equilibrium: C T = -18,507
+ 5.9934
T log, T
+ 0.002936
T2
+ RT
log,
p2H% in which C is a characteristic constant and T the absolute temperature. The value - 18,507 is the molar heat of reaction a t o’C. The constant calculated from Mayer and Altmayer’s results varied from 18.8 to 2 2 . 0 . Starting with methane the average value of the constant C was 21.6 with nickel as the catalyst and 2 1 . 1 with cobalt; the constant was 20.8 with each catalyst when the initial gas was hydrogen. By using C = 21.1, Mayer and Altmayer calculated the equilibrium data given in Table I.
TABLE I Methane Equilibrium (hlayer and Altmayer) Methane per cent
Hydrogen per cent
Tzmp. C.
Methane per cent
Hydrogen per cent
98.79 96.90 93.12 86.16 76.80 69.86 62.53 60.71 51.16 46.69
1.21 3.10 6.88 13 84 23.20 30. I4 37.47 39.29 48.84 53.31
567 577 600 607 625 650
41.26 38.22 31.68 29.40 24.75 19.03 11 . 0 7 6.08 4.41 1.59
58.74 61.78 68.32 70.60 75.25 80.97 88.93 93.92 95.59 98.41
700
7 SO 800 850
Pring and Fairlie carried measurements of the equilibrium to higher temperatures. In their early work (177, 178) they were able to detect the formation of methane and ethylene (the latter amounting to but one per cent of the methane formed) from carbon and hydrogen, a t IZOO’ under pressures of I O to 60 cm. of mercury. No acetylene could be detected. They were unable to make equilibrium measurements. Later (179, 180) they were able
1626
GUST.4V EGLOFF, R. E. SCHAAD, AND C. D. LOWRP, JR.
to evaluate the methane-carbon-hydrogen equilibrium. As would be expected, the velocity of formation of methane from carbon and hydrogen was observed to increase at higher gas pressures. When carbon was used in the form of a compact rod or tube, either with or without a catalyst, the equilibrium conditions were reached in about two hours a t 1200' to 1300' under 30 to 50 atmospheres pressure. Above 1400' under the same pressures, fifteen minutes sufficed for attainment of the equilibrium, while at 2 0 0 atmospheres the reaction was still more rapid. Methane was the only saturated hydrocarbon reported between I 100' and 2 100' at pressures up to 2 0 0 atmospheres. The relative amount of methane formed increased with the pressure in close agreement with the law of mass action according to the equation:
C
+
2
H? S CHI.
All experiments at IO to 2 0 0 atmospheres pressure gave the same value for the equilibrium constant:
a t any temperature when the same modification of carbon was used. Under atmospheric pressure the amount of methane in equilibrium with hydrogen and graphite was 0.24 per cent a t 1200' and 0.07 per cent at I joo". For amorphous carbon "metastable equilibrium" values of 0.36 per cent of methane a t 1zoo0 and 0.21 per cent at I jooo were obtained. The equilibrium values for amorphous carbon were less definite thap those for graphite on account of the gradual change of amorphous carbon to graphite at the temperatures of the experiments. By repeatedly passing hydrogen and a mixture containing z per cent of methane and 98 per cent of hydrogen over amorphous carbon in a heated porcelain tube, Coward and Wilson (54) found that the percentage compositions of the equilibrium mixtures of methane and hydrogen at atmospheric pressure were as shown in the following table: 850'
Methane Hydrogen
1000"
2.5
1.1
97.5
98.9
IIOOi
0.6 99.4
At 650' they could not attain equilibrium within a reasonable time, even in the presence of nickel catalysts. Coward and Wilson suggested that the relatively high amounts of methane which Mayer and Altmayer (160)claimed as equilibrium values a t lower temperatures might be erroneous, due to the presence in the decomposition products of carbon monoxide, for which the earlier workers recorded no tests. Fonda and Van Aernem ( 7 7 ) found that Mayer and Altmayer's (160) equation for the decomposition of methane over nickel-impregnated asbestos in a quartz tube a t 440' and 550' under atmospheric pressure held fairly closely for mixtures (two per cent at the start) of methane with nitrogen and
1627
DECOMPOSITIOS O F T H E P A R A F F I S HYDROCARBONS
argon. They presented curves a t intervals of 100' from 300' to 800' showing the percentage of methane a t equilibrium as a function of the methane content of the original mixture. Cantelo's (41)thermodynamic treatment of the equilibrium:
+ 2H2 * CH,,
C
was essentially that used by Mayer and Altmayer, but somewhat simplified. His equation differed from theirs only in the first term, for whichheemployed the heat of reaction a t ordinary temperatures, - 18,900cal., while they used - 18,507 cal., the heat of reaction calculated for 0'. By utilizing the Sernst approximation formula: H log Kp = 2 B v 1.75 log T 2 VC 4.571T
+
+
in TI hich H, represents the heat of reaction developed at ordinary temperatures, T the absolute temperature, Zv the volume changes, and ZvC a summation of constants, Cantelo calculated the equilibrium constant K,, and the equilibrium concentrations of hydrogen and methane a t 50' intervals from 600" to 900' and a t 1000'. His values are given in Table 11.
TABLE I1 Methane Equilibrium Calculated Values of Equilibrium Constant and Equilibrium Concentrations (Cantelo) Temp. "C.
Methane per cent
600 650
0,077 0.039
6.9 3.5 .o
700
0.021
2
750
0.012
I .o
800
0.007
0.5
850 900
0.003
0.003
0.4 0.4
0.0015
0.2
1000
Hydrogen per cent
93 ' 1 96.5 98 . o 99 .o 99.5 99.6 99.6 99.8
Inexperimental work he did not reach these equilibrium concentrations, though he believed that they could be attained a t space velocities lower than those which he used. By applying the Nernst heat theorem, Saunders (187) developed the following equations for the methane equilibrium: log Kp
4,583 T
1.75
log T
+ 0.000630 T - 0.7
(1)
(for amorphous carbon)
log K, =
4 008
T
1.75 log T
+ 0.000630T - 0.7
(for graphite)
(11)
1628
GUSTAV EGLOFF, R. E. SCHAADL AND C. D. LOWRY, JR.
He showed that results (Table 111) calculated from these equations agreed with the experimental data of RIayer and Altmayer and of Pring and Fairlie.
TABLE I11 Comparison of Calculated and Observed Methane Equilibrium Constants (Saunders)
Temp. "C. "Abs.
log KD
Calcd.
327
600
2.4j
527
800
0.4j
Observer
Equation
Exptl. 2 .j
11.and A. >I. and A.
about
627
900
-0.21
0.4 -0.37
727
1000
-0.74
-1,oabout
11.and A. P. and F.
727
1000
-1.31
-1.99
-1.9
-2.45
- 2 . j
-2.79 -3.04 -3.22
-2.90 -3.16 -3.3 about
Itl. and A. P. and F. P. and F. P. and F. P. and F.
927 I 2 0 0 1127 1400 I327 1600 1 j 2 7 1800 I727
2000
>I. and A.
A little later, by means of the above equation (I) Cantelo (42) calculated values of the equilibrium constant K, between joo" and I 100' for thereaction:
+
+
C 2H2 % CH, 21,730 cal. These values of K,, together with the equilibrium concentrations of methane and hydrogen calculated from the constants are given in Table IV.
TABLE IV Methane Equilibrium Data calculated from Saunders' Equation Temp.
Kp
(Cantelo) Methane Hydrogen Temp. per cent per cent "C.
Calcd.
850 900
0.053 0.036
4.5 3.6
9j.j
1000
0.018
2.0
1100
0.010
0.8
98.0 99.2
"C.
Calcd.
500
600
4.6 0.93
63.9 37.9
700
0.23
750 800
0.17 0.086
16.3 13.0
7.5
36.1 62.1
83.7 87.0 92.5
KP
Methane Hydrogen per cent per cent
96.4
Scheffer, Dokkum, and A1 (189) studied the dissociation equilibrium of methane under atmospheric pressure a t temperatures between 480' and 680' by circulating first methane and in subsequent runs hydrogen through an electrically heated porcelain tube containing a layer of asbestos impregnated with nickel, and covered with active carbon by a preliminary decomposition of methane or ethylene. Several years earlier Scheffer (188) had indicated that gaseous equilibria can be represented by the equation: log K, = - A/T B,
+
DECOMPOSITION O F T H E P A R A F F I N HYDROCARBONS
1629
in which A represents energy of reaction, T absolute temperature, and B is a constant nearly equal to the entropy at unit concentration, or log K, =
- A'/T
+ B',
in which A' represents heat of reaction and B' is a constant nearly equal to the entropy a t unit partial pressure. The results of Scheffer, Dokkum, and A's (189)experiments on methane, when plotted, did not fall on a straight line, as would be expected if they conformed to the above general equation, but on two lines. This indicated not only the expected dissociation, but also a second equilibrium previously unknown. Scheffer and his associates postulated that the second equilibrium involved the formation of a nickel carbide capable of combining with hydrogen to produce methane. They thus assumed two simultaneous reactions: C zHg = CH4 18.8 Cal. (constant pressure) 2H2 = xNi CHI 11.4 Cal. (constant pressure). and Ni,C
+ +
+ + +
The experimental data supporting these reactions were summarized by the equations: ( I ) Methane equilibrium: 3742
4108
T
T
log KO = - - 2.648;log K, = - - 4.924 (2)
Carbide equilibrium:
The nickel carbide was stable up to about 420'~ its decomposition point being indicated by the point of intersection of the curves representing the two equilibria (Fig. I). Upon the basis of these equations, calculation of the degree of dissociation of methane a t atmospheric pressure gave the data listed in Table V. Even the extrapolation a t low and high temperatures gave little difference between the values calculated from the equations for:
Schenck, Krageloh, Eisenstecken, and Klas (191)studied the equilibrium relations in the systems represented by the following equations a t temperatures ranging from 300' to goo'.
+
CH4 = C 2H2 CH4 3Fe = Fe3C
+
+ 2H2
(1) (2)
To facilitate reaction, the iron was deposited on porous pieces of pumice. The iron carbide was prepared by the action of methane on iron a t 700'. The reactions between 300' and 500' were carried out in a Jena glass tube, containing the catalyst, heated to constant temperatures by contact with the vapors of boiling liquids. Porcelain tubes heated in an electric furnace were used for the reactions a t j o o o to 900'.
1630
GUSTAV EGLOFF, R. E. SCHAAD, A N D C. D. LOmRY, JR.
TABLE V Methane Equilibria in the Presence of Nickel (Scheffer, Dokkum, and AI) C 2H2 % CHn
+
TEmp. C
3 so1 400 450 500
550 600 6.50 700 800
900 1000 IIO0 I200
Concentrations calculated from equations for log Kc log K P H27O CHd% Hz7, CHi%
86.1 77.2
34.0 46.6 59.0 70. I 79.0 -85 ' 5 93.2 96.6 98.2 99 .o -99.4
Ni,C 3 50 4 00 450 500
55 0 600 650 1
17.9 24.1 30.9 37.9 44.8 51.5
57.7
66 .o 53.4 41 .o 29.9 21
.o
14.5 6.8 3.4 I .8 I
.o
0.6
13.6 22.6 34.0 46.6 59.2 70.2
79.0 85.5 93 . o 96 ' 5 98.1 98.9 99.3
77.4 66 . o 53.4 40.8 29.8 21
.o
14.5.
7 .o 3.5 1.9 1.1 0.7.
+ 2HZ%xXr\Ti + CH, 82.1 75.9 69.1 62.1 55.2 48.5 42.3
I7.9 24.2 30.9 37.9 44.8 51.3 57.4
82.1 75.8 69.1 62.1 55.2 48.7 42.6
Extrapolated values are enclosed in the brackets.
The progress of the reactions was followed by reading the pressures developed in the reaction tube a t the different temperatures. I n the presence of iron a t 302O, no methane was decomposed. At 350' about one per cent by volume of the methane was decomposed and iron carbide could be detected; a t 445' the decomposition amounted to about 4.4 per cent by volume. The fact that the same proportions of hydrogen and methane were obtained by passing pure hydrogen over iron carbide as were formed when methane was passed over iron proved the existence of a true equilibrium in the second reaction. The percentages of methane decomposed according to these two reactions were plotted against temperature as in Fig. I . As shown by this figure and the data in Table VI the curve for the second reaction had two branches above 695'; one along which hydrogen and methane were in equilibrium with iron and carbide-iron mixed crystals, and the other along which the gases were in equilibrium with iron carbide and the mixed crystals of carbide and iron.
DECOMPOSITION OF THE PARAFFIN HYDROCARBONS
163 I
TABLE VI Methane Equilibria in the Presence of Iron (Schenck, Kriigeloh, Eisenstecken, and Klss)
CHd% C Temp.
"C
CHI per cent
Hz per cent I .67 23 .oo 36. IO 63.26
640 680 700
98.33 77.00 63.90 36.74 23.30 16.99 14.37
710 715
12.91 13.02
87.09 86.98
351 445 480 580 640 680 697 700 705 710
99.24 95.63 88.33 59.80 36.15 22.94 19.75
0.76 4.37 11.67 40.20 63.85 7 7 .ob 80.25
18.61
81.39
715
17.68
82.32
351 445 480
580
76.70 83 .OI 85.63
3Fe
720
725 7 40 7 50 760 880 800 8.50 880 900
-
-
16.22 13.30 12.15
.oo 11.09 8.49 I2
-
-
+
2Hg
Temp
CH4
HZ
"C
per cent
per cent
720
12.07 11.70 10.96 10.34 10.36 9.20 6.27 4.60 3.50
87.93 88.30 89.04 89.66 89.64 90.80 93.73 95.40 96.50
725
730
7 40 750 760 800
850 880
+ CH, % Fe3C + 2H1 -
17.32
82.68
-
84.67
-
.__
83.78 86.70 87.85 88 .oo 88.91 91 .SI
87.93
-
92.11
3.39 ' I .20
0.81
96.61 98.80 99.79
5.59?
Using the preceding method, Schenck, Kriigeloh, and Eisenstecken (190) quantitatively investigated the methane-hydrogen equilibria in the presence of cobalt over the temperature range 310' to 740'. At low temperature a secondary equilibrium involving cobalt carbide was found, but above 680' this reaction was no longer encountered. As shown in Table VI1 the mean
1632
GCSTAV EGLOFF, R . E. SCHAAD, A S D C. D. LOWRY, JR.
Fic. I Methane Equilibria in t h e presence of Cobalt, Iron, and Kickel.
.u
8 .-# p4
E
DECOMPOSITION O F T H E P A R t l F F I N HYDROCARBONS
I633
values of the results observed at different temperatures with cobalt show close agreement with the results previously obtained using iron. These data are plotted in Fig. I . T.4BLE 1’11 hlethane Equilibria in the Presence of Iron and Cobalt
(Schenck, Krageloh, and Eisensteckenl CHa+C + z H ~ CH,+~CO*CO,C+ z H ~ Over iron Over cobalt TEmp.
C.
CHI
per cent
-
Ha
per cent
CHa
99.20 94.41
0.80
-
5.59 23 .oo 35 96
97.90 85.93 82.16 74.50 j r .09 30 .oo 19.19
-
95.7
-
-
480
63.90
36. IO
64.04
710
36.74 23.27 16.99 14.37 12.91
720
12.07
740 750
10.34 10.36 6.27
63.26 76.73 83 .OI 8 j .63 8; .09 87.93 89.66 89.64 93 i 3
36.56 23.37 16.65 14.09 13.09 11 .83 10.85
-
640 680 700
800
CHa
per cent
310 360 445
508 580
Ha
per cent
4.30
7 7 .oo
-
-
-
per cent
-
63.44 76.63 83.35 8; .91 86.91 88.17
Hz
per cent
Traces 2.10
14.07 17.84 25.50
48.91 70.00
80.81
-
-
-
-
89.15
-
-
In the absence of catalysts some workers have found the methane equilibrium difficult if not impossible of attainment. Holliday and Exell (114) observed that the decomposition of methane in silica and porcelain bulbs at temperatures between 900’ and 1 2 0 0 ~was a t first rapid, “but later becomes so strongly retarded that for all practical purposes a condition of false equilibrium, far removed from the theoretical, is set up.” This is shown in the following table, which gives the quantities of methane undecomposed after 60 minutes’ heating in silica bulbs, and shows them to be much greater than the value calculated from Saunders’ (18j ) equation. T,emp.
C. -
816 928 1000
1118
Total pressure after 60 min. mm.
819.2 845.3 799.3 820.6
Partial pressure
of methane, mm.
Calcd. equil. value.
Actual
19 . o 9.3 j .6 3.2
534.8 326.7 166.7 66.4
value
Holliday and Exell suggested that the retardation of the decomposition of methane “is almost certainly due to the preferential adsorption of hydrogen
1634
GUSTAV EGLOFF, R. E. SCHAAD, AND C. D. LOWRY, JR.
by silica, which finally protects the surface so efficiently that further decomposition of methane is practically prevented.” At 900’ in the presence of a nickel catalyst, no retardation took place. Only three per cent of methane remained after 18 minutes’ heating, showing that under these conditions equilibrium was rapidly approached. The graphs in Fig. z summarize the values for the methane equilibrium obtained by the different investigators whose work has been reviewed. The agreement is seen to be good.
Production of Carbon and Hydrogen Many of the investigations of the thermal decomposition of methane have reported carbon and hydrogen as the sole or major products. Some of these studies have been undertaken for purely scientific purposes, others have had commercial motives. Spurred on by the small amounts of carbon black, usually not over a pound or two per thousand feet, obtained in the usual method of manufacture by the incomplete combustion of natural gas, investigators have studied the thermal decomposition of this gas and of pure methane in the hopes of obtaining increased yields of carbon. At the same time, the great increase in recent years in the demand for hydrogen, due to the success achieved in the synthesis of ammonia from hydrogen and nitrogen, and of methyl alcohol and other products from oxides of carbon and hydrogen, has been a further incentive to study the decomposition of methane into its elements. I t is reported that hydrogen containing less than one per cent of hydrocarbons can be prepared by the thermal decomposition of natural gas. Projects are under way for the employment of natural gas on a large scale in the manufacture of hydrogen for use in ammonia synthesis. Yields as high as 13.5 pounds of carbon black per 1000 cubic feet of natural gas have been reported in small-scale experimentation, while in large-scale work Yunker (244) obtained seven pounds per 1000 cubic feet. Yield is not the only consideration in this process, however, as carbon blacks vary greatly in quality. High temperatures, in general, cause the production of hard, graphitic carbon of low value, while that produced at low temperatures is far superior. To lower the temperature of decomposition as well as to direct the reaction toward carbon and hydrogen instead of toward the production of higher hydrocarbons, catalysts have been employed in most of the studies made. These have usually been metals or special types of carbon. I t should be kept in mind, in considering the work on the decomposition of methane to carbon and hydrogen, that the reaction concerned reaches equilibrium, and that the limiting yields are those given in Fig. 2 . Some investigators have practically attained equilibrium; others have fallen far short of this goal. X special type of decomposition is the “reforming” of natural gas, a thermal decomposition carried out to increase the volume and decrease the B.t.u. value of natural gas that is to be used to replace manufactured gas. 2.
DECOMPOSITION OF THE PARAFFIN H Y D R O C A R B O S S
163 j
The process is essentiallythe decompositionof theethanepresent,withthe production of hydrogen, carbon, methane, and some unsaturated hydrocarbons. The word “carbon” is apparently applied by investigators to cover a variety of black solids. To prove their being in reality carbon, extractions should have been made by solvents to show that they were not high molecular weight hydrocarbons, which are often present in pyrogenetic carbonaceous deposits. This precaution seems rarely, if ever, to have been taken.
a. Without Catalysts i. Xethane.-In 1905, Berthelot (20) decomposed methane a t high temperature in a silica tube which enclosed a concentric metal tube cooled by water circulating within it. The gas was passed through the annular space between the hot and cold tubes. Methane kept in the tube for one hour at 1300’ to 132 j o under an initial pressure of 370 mm. of mercury, yielded a shiny layer of carbon which adhered to the silica tube, and nearly pure hydrogen. Berthelot believed, as has been stated, that the carbon was the end product of a complex series of polymerizations with acetylene as an intermediate. Bone and Jerdan (31) observed that methane was resolved rapidly into its elements when admitted into an evacuated glazed porcelain tube previously heated to I I joo, so that only three per cent of the methane remained after three hours’ heating. They could not detect the formation of acetylene or ethylene. Bone and Coward ( 2 7 , 28, 29) decomposed methane at temperatures of 7 8 j 3 to 980°, in contact with porcelain, with production of only carbon and hydrogen when the methane was in a concentration of less than 60 per cent of the gas mixture, but there was some formation of unsaturated products above this concentration. From the character of the carbon produced by the decomposition of methane, which was deposited on the porcelain surfaces as a metallic-appearing coating, very different from the dull soft material obtained from ethylene, or acetylene, Bone and Coward (23, 24, 29) concluded that the breakdown at these temperatures was entirely a surface reaction, and they opposed the assumption of Berthelot (IO) and also of Lewes (138, 139) that the primary decomposition of methane involves the formation of acetylene and hydrogen. Recently Wheeler (237) has disagreed with this “suggestion that carbon produced during the thermal decomposition of methane is exclusively, or even mainly of a brilliant shining form. . . . Nor is the carbon produced during the thermal decomposition of ethylene necessarily a dull and soft variety. Both types of carbon, and a form intermediate in character between the two, can be produced from both hydrocarbons, the condition of the carbon depending upon the character of the heated surface over which the gas is passed, the rate of passage of the gas over that surface, and the temperature of decomposition.” Williams-Gardner (240) stated that the decomposition of methane into its elements began at temperatures above 800’ and that equilibrium could be attained at 850’ and above.
1636
GUSTAV EGLOFF, R. E. SCHAAD, A N D C. D. LOWRY, JR.
Between 600’ and I O O O ~ ,Yamaguti (243) measured the relative rates of decomposition of methane under atmospheric pressure in a silica tube by determining the hydrogen content of the gaseous products formed after heating a t each temperature for I O minutes. His data are given in Table T’III.
TABLE TI11 Decomposition of Methane during Ten Minutes’ Heating in a Silica Tube without Catalysts ( I S mm. X 400 mm.) (Yamaguti) T,emp.
C.
Hydrogen produced per cent
T,emp.
C.
Hydrogen produced per cent
600
0 . 2
3.4
650
0.6
14.7
700
0.9
22.1
750
I
.6
37.5
800
2.1
h few patents have been obtained on methods of producing carbon and hydrogen from methane and natural gas without catalysts. Hermann (109) produced carbon black from methane by passing the preheated gas under pressure through openings against a highly heated surface. I n order to minimize the formation of low-value graphitic carbon from the decomposition of methane, the Berlin Anhaltische Nachinenbau (8) heated the hydrocarbons in a series of three retorts maintained a t 700’ to goo’, 900’ to IOOO’, and 1200’ to 1400’~respectively. The carbon of greatest value was said to be that deposited in the retort a t the lowest temperature. Pictet ( I 7 5 ) obtained hydrogen and lampblack from methane by conducting it through a long, relatively narrow tube with the first part heated to IZOO’ to 1350’ and the latter portions to higher temperatures. Fernekes (67) claimed the conversion of methane into hydrogen and lampblack by passing the hydrocarbon a t atmospheric pressure through an alundum chamber heated electrically a t 1300’ to 2000’. h decomposition temperature of 1700’ and “upwards” was said to give the best results. At such a temperature the gas was “instantly decomposed and separated” into hydrogen and a light flocculent form of carbon free from “intermediate products, such as naphthalene.” Szarvasy (208, 209) claimed that “the formation of soot can be promoted and the fineness of the soot can be increased by diluting the methane that is to be subjected to the heat-splitting operation, with inert gas, that is to say, gases that do not produce carbon, such as for instance nitrogen or hydrogen, and thus diminishing the partial pressure of the methane.” Carbon dioxide was also recommended as a diluent. Spear ( 2 0 2 ) passed methane through a heated retort, obtaining hydrogen and carbon, of which a part deposited in the retort. Steam was then passed through the heated chamber to combine with the carbon and yield a mixture containing carbon monoxide and hydrogen.
DECOMPOSITION OF THE PARAFFIN HYDROCARBONS
I637
ii. Natural Gas.-In 1853, Magnus (156) stated that while marsh gas remained unchanged a t a temperature sufficient to soften the most refractory Bohemian glass, it decomposed a t a "white" heat into carbon and hydrogen. Although Gault and Trompier (94) did not disclose the details of their process, they mentioned six years ago that they were producing hydrogen by thermally decomposing natural gas of Vaux, France. The gas contained hydrocarbons as follows: Methane
Ethane
Propane
Butane
per cent: 88.38
4.70
1.43
0.73
They stated that they had determined dissociation curves of this gas a t temperature intervals of 50' from 600" to 900' for a constant rate of flow of about 2 0 0 cc. per hour. According to patents of Szarvasy ( 2 I I , 2 16) the decomposition of natural gas a t high temperatures yields a mixture of carbonaceous material and tarry products from which carbon electrodes may be made. Patents issued to the American Nitro Products Company (I) and to Brownlee and Uhlinger (35, 36) claim the production of carbon black and hydrogen by the decomposition of gaseous and liquid hydrocarbons. The process, which was said to be especially suitable for treating natural gas, is carried out by passing the hydrocarbon a t a pressure "materially" above atmospheric into a chamber filled with refractory material heated to 1400' or higher. Later, Uhlinger ( 2 2 7 ) stated that less frequent reheating of the reaction chamber was necessary if the methane or natural gas was preheated in a pipe coilwhich passed through a chamber heated to a temperature of 500' to 900'. The preheated gas was then passed into a furnace containing a refractory checkerwork that had been heated similarly to 1100' to 1300'. The carbon produced was collected in a precipitator, while the hydrogen passed from the reaction chamber. According to other patents of the same man ( 2 2 8 , 2 2 9 ) the refractory checker-work in the decomposition furnace was heated to 1200'-1400' before introduction of the hydrocarbon gas. The methane (221) might also be diluted with five times its volume of hydrogen or nitrogen before bringing it into contact with the hot checkerworlr. On the basis of work described by Mantel (I j 7 ) , the Chemical Research Institute of Warsaw (44) applied for a Polish patent on a process for producing carbon black and hydrogen from aliphatic and aromatic hydrocarbons, including natural gas. I n their process, an oven lined with firebrick and filled with granular material such as crushed firebrick or magnesia brick was heated to about 1400' by combustion of illuminating gas within it. The gas to be decomposed, illuminating gas or natural gas, was preheated to 400' and passed through it. The resulting carbon black was collected in dust-catching devices. Experiments showed that during the operation the temperature of the reaction chamber could be allowed to drop to 800" without greatly affecting the decomposition if the velocity of flow of the gas was decreased from an initial 1.1 meters per second to about 0.67 meter per second as the temperature fell.
1638
GUSTAV EGLOFF, R. E. SCHAAD, A N D C. D. LOWRY, JR.
The finer the granules filling the oven the more complete was the decomposition of the hydrocarbon. The decrease in size of these granules was limited, however, as w r y small granules restricted the flow of gas. Use of different materials for the granular filler had no influence upon the degree of decomposition of the hydrocarbon. The composition of the gaseous product was practically independent of that of the original gas. The hydrogen produced contained, in some cases, as little as 0 . 7 per cent of methane. Frank (80) claimed to produce hydrogen by passing natural gas into a blast furnace, a generator filled with firebrick, or a refractory “stove” heated to a t least 1 2 0 0 . ~The gas began to decompose a t 800°, and at temperatures above 1200’ practically complete dissociation to carbon and hydrogen was obtained. iii. “Reforming” Natural Gas.-Natural gas has a higher calorific value than the manufactured gases usually furnished for domestic use. Where a city uses both natural and manufactured gas it is sometimes desirable to reduce the B.t.u. content of natural gas before passing it into the city mains. If the gas is subjected to a partial thermal decomposition it will be increased in volume, due to the formation of hydrogen, and at the same time reduced in calorific value. This process is known as “reforming.” Masser (159) carried out the reforming process by injecting natural gas against hot brickwork in a standard oil-gas generator. The cycle of operation consisted of a Io-minute heating period and a zo-minute gas-decomposing period. Typical analyses of Signal Hill gas and of reformed gas made from it are given in Table IX.
TABLE IX Analyses of Signal Hill Natural Gas and of Reformed Gas B.t.u. Signal Hill Gas per cent 1208
0.0
66.8
(Masserl
j50 B.t.u. Reformed Gas Per cent
48.3 33 . o
700 B.t.u. Reformed Gas per cent 40.0
46.8
.o
0.0
0.7
I .2
4.3
0.0
13 . 6
I .9
2 . 1
5.4 1.4
0.1
0.3
0.1
I .o
1.5
2
29.5
0
.o
The absence of ethane from the reformed gas caused RIasser to regard the primary reaction taking place as the decomposition of ethane. The analyses in Table X afford a more detailed picture of the reforming process.
DECOhlPOSITION OF THE PARAFFIN HYDROCARBONS
I639
TABLE X Quantiative Chemical Comparison of Natural Gas and Reformed Gas (Masser) Xatural Gas Reformed Gas I O 0 0 CU. ft. - 750 B.t.U. 655 cu. ft. - 1 1 7 5 B.t.u. Per cent Total Total Per cent Total Total by vol. cu. ft. pounds by vol. cu. ft. pounds H2
CHa C2H6 CnHBn
co
0.o
0.000
70.1
459.0
26.8
1jj.0
19.435 13.900
0.9 0 .o
5.9
0.437
5.1
51 . o
0.0
0.000
4.4
44 . O 11.0
0.0
36.7 50.3 0.0
367 . o 503.0 0.0
c02
1.3
I .I
0.1
8.5 0.65
0,989
0 2
0.0;;
0.1
I .o
S2
0.8
j.2
0.386
2.3
23 . o
Ultimate Analysis
C
H? 0 2
K?
1.945 21.300 0.000
3.780 3.260 I . 163 0.086 I ,708
35.202
33.242
Pounds
Pounds
26,237 7 , io7 0.874 0.386
20.929 7.810 2.795 1.708
35.202
33,242
Hough and RlcLaughlin (116) reformed natural gas mixed with air. Ode11 (16j)reformed in a water gas generator in the presence of steam, obtaining both thermal decomposition and reaction with the steam. b. With Catalysts In considering the effect of various contact substances on methane decomposition, it is not easy to distinguish between the influence of surface alone, and specific catalytic action. Firebrick and porcelain appear to exert only a surface action, and cases where they were used have therefore been considered under “noncatalytic” decomposition. Silica apparently exerts at times an accelerating and at other times a retarding influence, A few other substances have been reported t o check decomposition, while to a large number of carbonaceous materials, metals, and metal compounds, definite catalytic properties have been attributed. I n Table XIV on page 1648 is given a survey of all the catalysts that have been used for decomposing methane to carbon and hydrogen. i. Refractory RIaterial That silica sometimes increases the rate of methane decomposition is evident from the observation of Campbell and Slater (40)that when methane was exposed at 900’ to 1000’ to silica tubing, and t o glazed porcelain tubing
1640
GUSTAV EGLOFF, R . E. SCHAAD, A N D C. D. LOWRY, JR.
of the same size and shape, the rate of decomposition was greater in the presence of the silica. Fischer and Bahr ( 7 1 ) formed more tar in a quartz tube than in one of porcelain. The effect of silica is not easy to estimate, however, as Slater (196) later found that when methane was heated at 910' for periods of five and ten minutes in a porcelain tube packed with finely divided silica, less hydrogen was formed than when the tube was empty. He observed that finely divided baryta, alumina, and magnesia also retarded the decomposition of methane more than did silica.
ii. Carbonaceous Material Methane.-Carbon appears to have a considerable effect on methane decomposition, the effect varying with the condition of the carbon employed. At temperatures around 1000' most carbonaceous materials, notably graphitic carbon, appear to catalyze the decomposition of methane to its elements, while, at slightly lower temperatures, certain forms of carbon favor the formation of aromatic hydrocarbons (43). In many of the decompositions recorded as non-catalytic, carbon was present, and no doubt it exerted some action. In this section mill be considered, however, only those cases in which c:irbon \vas intentionally employed. Slater (196) observed that the decomposiiion of methane in a porcelain tube at 910' \xis acce1er:ited by the presence of charcoal, graphite, and carborundum. Hydrogen was ihe only gaseous product. Smolcnski ( r g 8 ) partially clecomposetl methane in the presence of carbon black at iooo0. Constable [ , 5 I j used a graphite film supported on china clay rods and heated electrically from 800" to 1200'. Pimmersbach ( 1 9 j ) reportcd that the methane in a coke oven gas was partially decomposed i v h i pissed over carbon-covered chamotte at 800' or higher, and that the decomposition wvas p:irticularly rapid aboi-e goo". Khen Ilollings and Cobb (,I I j ' i passed a giis mixture containing about equal volumes of methane and hydrogen through a layer of coke in a horizontal porcelain tube heated electrically to goo0, z per cent of the methane was decomposed with a heating period of j j seconds. .It I 100' with gas mixtures of the same composition, 6; per cent of the methane mas decomposed during a heating period of 46 seconds. X process patented by Hose (182) "comprises conducting methane, together with oxygen mingled therewith in insufficient quantity to support combustion, through a discontinuous mass of resistance in an electric circuit and heated thereby, whereby the methane will be dissociated into carbon and a gas containing a preponderating amount of hydrogen." The combustion and dissociation were carried out in a carbon brick checkerwork which was heated before admitting the methane, preferably to a temperature a little higher than the ignition point of the hydrocarbon. Szarvasy ( 2 I O , z I I ) proposed graphitizing carbon electrodes by heating them electrically in an atmosphere of methane t o a temperature a t which the gas was decomposed. At the same time carbon was precipitated from which
DECOMPOSITION OF THE PARAFFIN HYDROCARBONS
1641
more electrodes could be made. He ( 2 12) also claimed that yields of carbon larger than any previously obtained from methane resulted when the decomposition of this hydrocarbon was carried out in internally heated chambers containing incandescent carbon fragments. Under these condit,ions most of the carbon deposited on the incandescent carbon surfaces and little on the walls of the retorts. The same worker (213) also proposed to produce pure carbon from methane by carrying out the deconiposition in externally heated tubes of retort carbon, previously produced from methane, or ( 2 14) by passing methane through a rotating cylinder containing glowing fragments of carbon. To produce a mixture of hydrogen and nitrogen suitable for ammonia synthesis, Szarvasy ( 2 1 j) passed methane and nitrogen through a chamber containing incandescent carbon, and claimed that the decomposition \?as facilitated by the nitrogen. Natural Gas.-A method for completely cracking natural gas to hydrogen and carbon at economical space velocities was described recently by Yunker (244). X bed of coke in a water-gas generator mas raised by passage of a current of air, in accordance with water-gas practice, to a temperature higher than the decomposition temperature of methane. The air was then shut off and natural gas turned into the generator. The decomposed gas passed through cooling coils to carbon traps and thence to a gas holder. Seven pounds of carbon black, suitable for the manufact'ure of tires, was recovered per 1000 cubic feet of natural gas. Each cubic foot of natural gas yielded 2 . 2 j cubic feet of cracked gas, which was ninety per cent of hydrogen. The capacity of the generator for the production of cracked gas was approximately equal to its capacity for the generation of water gas. Since several million cubic feet of gas and almost a ton of carbon black were made in the experimental work, Yunker believed the process suitable for commercial use. According to a patent of Lessing (137) coal gas, considered as largely methane, yielded hydrogen when passed under pressure through externally heated retorts packed with coke, charcoal, or carbon maintained at a ternperature of 1000' t o 1300'. Frank (80) produced hydrogen by subjecting a gas rich in methane, as natural gas, to decomposition in a blast furnace or generator filled with incandescent coke or with refractory material such as firebrick or refractory stone. The gas began to decompose a t about' 800' and was dissociated "completely" into carbon and hydrogen when temperatures exceeding I 200' were attained. I n the process of Lowe ( I j4), a shallow layer of coke, coal, or other suitable carbonaceous material is brought to a high temperature by an air blast. A brick checkerwork above it is heated from 870" to 1245' by the combustion of oil or gas. Satural gas or oil is then passed into the space above the checkerwork and on passing downward is decomposed. Excess coke is removed from time to time and the brick checkerwork reheated. The hydrocarbons may be atomized into the apparatus by air or steam. Battig (6, 7 ) claimed that methane was decomposed into carbon and hydrogen by passage through a generator containing coke heated to about
1642
GUSTAV EGLOFF, R. E. SCHAAD, .4ND C. D. LOWRY, J R .
IOOO', or through ovens. The heat absorbed in the decomposition of methane cooled the coke, and the carbon produced deposited on the coke.
iii. Metallic Catalysts A number of metals have a pronounced effect on methane decomposition, turning the reaction in the direction of carbon and hydrogen formation, and markedly decreasing the initial temperature of thermal breakdown. Thus in the presence of palladium, decomposition is reported at 2 joO, with nickel a t 320°, and with iron at 350°, whereas in the absence of catalysts the hydrocarbon is stable to 650" or higher. Cobalt is nearly as active as iron, and aluminum, magnesium, manganese, molybdenum, and tungsten have also been used. With a number of these metals the carbon of the methane combines during decomposition to form carbides. Oxides of the alkali metals coated on copper or on platinum have also been employed to assist the decomposition of methane. The metals are taken up in alphabetical order. Aluminum-Kusnetsov (13 3 ) found that almost pure hydrogen was obtained when methane was decomposed in contact with aluminum at its temperature of fusion (6 j 9 O ) . All the carbon dropped out inside the heating tube, part combining with the metal to form a carbide and the remainder being deposited upon it. Cobalt.-The studies of Mayer and illtmager (160) and of Schenck, Krageloh, and Eisenstecken (190) over the temperature ranges 470' to 620' and 310' to 740°, respectively, which have been fully discussed in considering the methane equilibrium, showed that decomposition h a y be accelerated by a cobalt catalyst. I n order to obtain a gas suitable for filling balloons, Xauss (162) passed coal gas over a cobalt catalyst at 250" to 3 o o 0 s o as to convert its carbon monoxide content into methane, and heated the gaseous product at rooo0 to IZOO', with clay or coke, impregnated with cobalt, to convert the methane int o carbon and hydrogen. Copper.-Slater (196) found that the presence of copper surfaces increased the rate of decomposition of methane in a glazed porcelain tube a t 910". That copper is not as active as nickel, iron, or steel in promoting the decomposition of methane to its elements was shown by Chamberlin and Bloom ( 4 3 ) . In the pyrolysis of natural gas the highest yields of benzene were obtained in copper tubes. Iron.-In 1860, Buff and Hofmann (3;) observed that methane was decomposed very slowly by an iron wire heated electrically to incandescence. The wire became brittle through combination of the metal with carbon. Over finely divided iron, Slater (196) found that the rate of decomposition of methane at 910' was greater than that observed in the presence of quicklime, copper, carborundum, graphite, or charcoal. After the hydrocarbon was heated with the powdered iron for ten minutes, the gaseous products contained 73.1 per cent of hydrogen.
DECOMPOSITION OF T H E PARAFFIN H Y D R O C A R B O N S
1643
Smolenski (198) partially decomposed methane a t 1000' with iron as a, catalyst. I n their investigation of the methane equilibrium, Schenck, Kriigeloh, Eisenstecken, and Klas ( r g ~ found ) that methane began to decompose in the presence of iron at about 350'. Stanley and Nash (204), working at higher temperatures, also found that an iron catalyst increased the conversion of methane into its elements. \\-heeler and Wood (238) stated that methane was decomposed "exclusively into its elements" and no liquid products were obtained when i t was heated in iron tubes at 900' to 1000'. Chamberlin and Bloom (43) obtained similar results. They reported that steel also favored the dissociation of methane to its elements. Patents of Kauss (162) claimed the decomposition of methane to carbon and hydrogen a t Iooo'to Inoo'in the presence of iron mixed with clayor coke. Magnesium.-When in contact with magnesium, methane was reported by Xovak (164) to decompose at about 600°, giving besides carbon and hydrogen, a magnesium carbide, Mg,Cs. The reaction was slow below 720°, but increased rapidly above this temperature. The amount of the carbide Mg,C3 produced was greater than the formation of free carbon except a t 780°, where the latter predominated. The maximum production of Mg,C3 occurred at, 760', where it amounted to jo per cent of the reaction products. Another carbide of magnesium, MgCz, was detected only a t 733'. Earlier Parkinson ( I 65) had observed that magnesium filings tarnished when heated t,o redness in methane for five minutes, but there was only slight separation of carbon. Lidov and Kusnetsov (141) secured about half decomposition of methane into carbon and hydrogen when the gas was passed over magnesium. As the same proportion of ethane, ethylene, and acetylene was decomposed, these investigators suggested that t'his decomposition might be used as a method for quantitative determination of the four hydrocarbons. Manganese.-Decomposition of methane in contact with manganese at 600' to 900' by Hilpert and Paunescu (112) was characterized by the formation of carbides. Molybdenum.-Methane was decomposed and a "fairly pure'' carbide containing 7 to 8 per cent of carbon was formed when Hilpert and Ornstein ( I I I ) heated molybdenum in the gas between j o o o and 800'. Nickel.-Xickel has a powerful catalytic action accelerating the decomposition of methane to its elements. At as low a temperature as 320' Sabatier and Senderens (185) found slow decomposition of the hydrocarbons in the presence of nickel. The breakdown became pronounced at about 390°, with deposition of carbon on the metal. The highest yield of carbon black that has been reported-13.5 pounds per 1000 cu. ft.-was obtained by Chamberlin and Bloom (43) when using a clean nickel tube. Holliday and Exell ( I 13) found nickel a particularly satisfactory decomposition catalyst, because the reaction continued at a good rate when it was used, whereas in silica a retardation t o far below the initial speed quickly set in.
GUSTAV EGLOFF, R. E. SCHAAD, A N D C. D. LOWRY, JR.
1644
In methane decomposition, as in many other catalytic processes, the conditions under which a nickel catalyst is prepared determine its efficiency. Yumagati (243) studied the effectiveness of nickel catalysts prepared in a number of ways in an apparatus practically identical with that used by Slater (196). The catalysts were prepared by the method of Armstrong and Hilditch (4) and reduced by hydrogen, a t temperatures one hundred degrees apart between 300' and 800'. I n the decomposition tests an evacuated silica tube containing a catalyst was heated t o about 850'~ and methane introduced until the pressure reached atmospheric. The gas was permitted to remain in the heated tube for I O minutes, and its hydrogen content then determined. Among catalysts which were reduced for 12 hours, maximum decomposition (j1.13 per cent of the methane treated) was observed in the presence of the catalysts which had been reduced a t 600'. A summary of the results is shown in Table XI.
TABLE SI Influence of the Temperature of Reduction of a Sickel Catalyst on the Decomposition of Methane at 8j o o (Yamaguti) Temp. of redpion
C.
300 400 500
Temp. of re@tion
Per cent of methane decomposed in I O mms.
22.1
600
32.54 41 7 5
TOO
51.13 48. j o 46.51
Per cent of methane decomposed in I O mins.
C.
800
The amount of methane decomposed during IO minutes depended also upon the time of reduction of the catalyst. As indicated by Table XII, the catalyst which had been reduced a t 600' for 18 hours was the most active in promoting the dissociation of the hydrocarbon. This catalyst contained 79.5 per cent of free nickel. A deposit of 0.6 gram of carbon on one gram of the catalyst had practially no effect on its activity.
TABLE XI1 Influence of Time of Reduction of a Xickel Catalyst on the Amount of Methane decomposed Catalyst reduced at 600". Time of decomposition, ten minutes a t 850'. (Ysmaguti) Time of reduction, hours
Per cent of methane decomposed
8
45.99 48.23 51 .I3 53.09
IO I2
I4 16 18
j7.20
63 . s o
Time of reduction, hours
19 20 22
24 26 30
Per cent of methane decomposed
53.55 31 .oo 30.11 31.14 30.08 30.20
DE COMPOSITION O F THE P A R S F F I N HYDROCARBONS
I645
Cantelo (41) found that, of a number of catalysts tried, asbestos impregnated with nickel-nickel oxide was the most effective for hastening the formation of carbon b!ack from methane a t temperatures of joo" and higher. -4 summary of results obtained with this catalyst is given in Table SIII. Reference has previously been made to llayer and Xltmayer's (160) study of the methane equilibrium at temperatures between 470' and 620' in the presence of clay fragments impregnated with nickel. Scheffer, Dokkum, and -41 (189) prepared a nickel catalyst, also for use in an equilibrium investigation, by reducing nickel hydroxide with hydrogen and then depositing carbon upon it by cracking methane or ethylene. Stanley and S a s h (204) observed that the decomposition of methane in porcelain tubes at 600' to 900' was increased by the presence of nickel. By conducting methane under atmospheric pressure over reduced nickel in a long glass tube a t 500Oto 600°, Ipatiev (127) brought about a slight decomposition. .4t 465' to j18" under pressure, nickel caused some change, but a zinc dust catalyst had no action. Fonda and Van Aernem ( 7 7 ) observed that methane, contaminating inert gas to be used for filling incandescent lamps, could be removed by passing the gas first through finely divided nickel and then through copper oxide, both a t 500'. The nickel decomposed the methane into carbon and hydrogen, and the latter was then oxidized by the heated copper oxide. With a view of utilizing the natural gas of Echigo Province, Japan, Kusama and Uno (133) thermally decomposed methane using pure nickel and a mixture of nickel and kieselguhr as catalysts. The mixture of nickel and kieselguhr was equally as suitable after being deactivated by use in the reduction of naphthalene as when newly prepared. The carbon resulting from the decomposition was graphitic and contained nickel. The hydrogen produced always contained about 2 0 per cent of methane, but this gaseous product was used directly, with satisfactory results, for reducing naphthalene. Chamberlin and Bloom (43) decomposed natural gas into carbon and hydrogen by passage through a nickel tube heated a t temperatures up to 880'. I n small scale runs they obtained 13.j pounds of carbon black per 1000 feet of gas treated. iifter the tube had been used for some time, however, the carbon production was much less. Nauss (162) covered in patents the decomposition of methane into its elements a t 1000' to 1200' in the presence of nickel. Palladium. Wieland (239) found that methane was split into carbon and hydrogen by dry palladium black at about 250'. Coquillion (53) observed that the dissociation of methane into its elements took place when the gas was passed over a "red-hot" spiral of palladium wire. Platinum. The action of platinum on methane has been the subject of two researches by Schwab and Pietsch. In the first (193) they brought the gas at an initial pressure of about 0.04 to 0.06 mm. of mercury into contact with a platinum band 0.01 mm. thick, alone or coated with a mixture of calcium oxide and barium fluoride and heated to 1100' to 1278' by a low-voltage current. The reaction seemed to be monomolecular, and apparently only
1646
GCSTAV EGLOFF, R. E. SCHAAD, AND C. D. LOTVRY, JR. TABLE
XI11
Decomposition of Methane in the Presence of Nickel-Nickel Oxide Tgmp. C.
Catalyst
600
None
600
(Cantelo) Rate, cc. Vol.CH4 used, cc. per min.
Space velocity'
2;oo
2650
Ni-Xi0
3 100
3 400
41
600
Xi-Xi0
3500
3 800
46
7 00
Sone
4050
4000
700
Xi-Xi0
3300 3500
4150 4000
7 60
Sone
2 700
2 900
780
Si-NO
2 700
42 5 0
20
30
30
Fa 0
88 . o
IO
F
8I,2
17.2
0
600
F
81.3
17.7
j00
0
95.6 87.6
600
600
0 2
F 700
0
7 80
52
6.9 8.7
.3
0.6
0 2
0
.o
I .2
1.3 0.9
0.0
0.3
0.0
1.7
0.0
0.0
0 .o
.o
0.0
I
0 .o
0.5
0.5
0.0
1.1
0.0
0.0
I .o
0.3
0.0
0.4
0 . 2
0.0
0
.o
0.0
0.8
0.0
0
.o
0.0
0.0
0.0
0.0
0.0
.6
0.0
62.1
8.4 43.2 37.9
92.9 85.7
6.5
0.0
0.0
F
12.2
0.6
0.3
0.6 0.8
0 F
83.5
7.9 71.9
4.0
2.4
0.9
1.3
0
1.9
2.5
0.0
F F 7 60
91 . o 56 . o
43 43
Analysis of gas, per rent (N2 free basis) Hz CzH1 co coz
CH4 91 .2 88.9
0
23.6
.o
Liters of gas per liter of catalyst space per hour. Original gas. 9 Final gas. 1
Vol. of resulting gas, cc.
0.4
DECOMPOSITION O F T H E P A R A F F I N HYDROCARBOXS
1647
those methane molecules colliding with the wire were decomposed. The catalytic action of the oxide-coated platinum was less than that of the pure platinum. Later (194) they stated that the decomposition was accelerated when a potential difference of between I j and 16 volts was applied between the band and a platinum-gauze electrode. Tungsten.-Decomposition of methane, indicated by the formation of tungsten carbide, was obtained by Hilpert and Ornstein ( I I I) by heating the finely divided metal or its trioxide a t 800' in a Jena glass tube in a I :I mixture of methane and hydrogen. At 800" and 900' most of the carbon was converted into the carbide, but at 1000' most of it separated in the free form. When a tungsten filament was heated in methane a t low pressure (in the order of 0.001to 0.01mm.), Langmuir (135) found that carbon was "taken up" by the wire, as proved by its increase in electrical resistance, and decrease in the temperature coefficient of the resistance. In some cases the pressure increased to almost twice its original value, indicating nearly complete decomposition of the hydrocarbon. Ilranium.-That uranium facilitated the decomposition of methane t o its elements was noted by Stanley and Yash (204). iv. Compounds of Metals Methane.-Bone and Coward (23, 2 4 , 29) observed that at 1000' the decomposition of methane was extremely rapid in hot tubes packed with fragments of quicklime. After twenty-five minutes' heating, the product contained only 1.9 per cent of methane; and at the end of an hour at 1030°,when equilibrium was practically established, only 0.7 per cent of methane remained. Slater (196) also found that at 910' the presence of quicklime accelerated methane decomposition. A nine per cent increase in volume was obtained when Fischer and Bahr ( ; I ) passed methane over zinc oxide at 850'. The addition of I per cent of potassium oxide (in the form of potassium carbonate) to the zinc oxide permitted the temperature to be lowered to 700°, with but slight lessening of decomposition when the gas was passed at the same rate. At 850' using alkalized zinc oxide the increase in volume was 50.9 per cent. Increase in the quantity of alkali carbonate added up to j per cent did not further change the results. Fischer and Bangert (72) decomposed methane with formation of barium carbide by passing the gas for 30 minutes under j to 18 mm. pressure over a I O : ~ : Iby weight mixture of barium, calcium, and ferric oxides heated to between 8.50~and I I jo'. Similarly, decomposition of the hydrocarbon and formation of a manganese carbide ( M n j C 2 ) xpreviously unknown, took place when methane (73) was conducted over manganic oxide or manganese dioxide at 105o'to 1200'. v. Summary In Table XIT' are listed the catalysts which have been found to accelerate the decomposition of methane or natural gas to carbon and hydrogen.
1648
GUSTAV EGLOFF, R. E. SCHABD, i \ S D C. D. LOWRT, JR.
TABLE XII’ Catalysts promoting the Decomposition of Methane and Satural Gas t o Carbon and Hydrogen Methane
Catalysts I.
2.
Catalysts
References
Carbonaceous Materials Carbon 198 Carbon brick I82 Carbon-chamot te I95 Carbon fragments 210-215 Charcoal 196 Coke 115 Graphite 51, 196 Carborundum 196
I,
Metallic Catalysts 160, 162, 190 Cobalt Nickel I * i , 133,
2.
c-
160, 162, 185, 189, 204, 243
Nickel-nickel oxide Nickelkieselguhr Iron
Copper Aluminum Magnesium Manganese Molybdenum Palladium Platinum Tungsten Uranium
Katural Gas References
Carbonaceous Materials Carbon I3 7 Charcoal I37 Coke 6, 7, 80, 137, Coal “Carbonaceous material”
154, 241 154
154
bletallic Catalysts Cobalt 162 Kickel 43, 162 Iron 43 Methane
3. Metallic Compounds Barium oxide iz Calcium oxide 23, 24, 29, 196
Ferric oxide jz Manganese oxide 73 Zinc oxide 71 Zinc oxidepotassium oxide 7 1
Production of Higher Hydrocarbons As has been pointed out, the thermal decomposition of methane may go almost exclusively t o carbon and hydrogen, or may produce a considerable amount of other hydrocarbons. These include ethane, olefins and diolefins, acetylene, and aromatic hydrocarbons. Sapthenes have not been reported. The formation of these higher products is favored by the absence of most substances having catalytic action, although it is assisted by special catalysts. Olefins are predominantly low-temperature products, being produced from the lowest temperature of methane decomposition to somewhat over 3.
DECOMPOSITION O F T H E P A R A F F I N HYDROCARBONS
I649
1000'. Ethylene up to 2 . 8 per cent of the gaseous products has been reported at IOOO' while in patents conversion into ethylene or other olefins of 80 per cent of the methane passed is claimed through the use of diminished pressure. Butadiene is frequently observed. Often the production of individual olefins is not reported, but a figure for "unsaturated hydrocarbons" is given, which has been as high as 8.8 per cent of the methane decomposed. Acetylene, while produced in small amount at almost all temperatures, is primarily a high temperature product. Having a negative heat of formation, it is necessarily most stable a t elevated temperatures and is formed from methane in considerable amounts a t 2 joo" to 3 0 0 0 ~ . The best yields are those of Peters and Meyer (173), who obtained acetylene equivalent to from 3 5 to 67 per cent of the methane used when the gas was passed very quickly, a t 50 mm. pressure through a spiral of tungsten wire at 2200" to 3000". Nearly as good results have been obtained a t the high temperatures of the electric arc.
TABLE
xv
Production of Benzene and of Total Liquid Products by Decomposition of Methane Maximiim yield, gallons per 1000 cu. ft. of methane Total oils and tars
Benzene fraction
0,537 0.46
Temperature a t which maximum yield was produced.
Time of contact sec.
Reference
"C. I
130
0.2
IIjO
0.146
I075
0.31
0.2
1050
0,Ijj
0.133 0.314**
1000 82 j
0.044* 0.6 31
to 6 8
7 .4* 52
*
21*
Fischer (70) Stanley and Nash (204) T'ysoky (233) Wheeler and Wood (238) Hague and Wheeler (104) Chamberlin and Bloom (43)
*Calculated from data reported in the original articles. **Gas treated contained 71.2 per cent methane, 23.3 per cent ethane, and 5.5 per cent carbon dioxide and nitrogen.
Some samples of natural gas have given higher yields than those cited, but this has been due to the presence of hydrocarbons above methane. Aromatic hydrocarbons, which are much sought products of the decomposition of natural gas, are apparently formed under rather special conditions. These include a temperature higher than that found best for olefin production, usually 8 j o " to IZOO', a carefully regulated time of heating, and selected catalysts, notably a particular form of carbon. The best jields of benzene and of total liquid products (largely aromatic) that have been obtained by different investigators are summarized in Table XT'. Besides benzene, the aromatic compounds identified in the products of methane decomposition are toluene, xylene, naphthalene, and anthracene. I n the pages following, the investigations in which production of higher products from methane has been reported, are outlined in roughly chronological order. On pages 1667-1672 is given a tabular summary of this work.
16jo
GUSTAV EGLOFF, R. E. SCHAAD, A T D C. D. LOWRY, JR.
a. Early Work I n 1840, Bischoff ( 2 1 ) found that mine gas decomposed slowly when it was passed through a “white-hot” porcelain tube. Because of an odor similar to that of petroleum or turpentine which remained in the tube, he believed that the hydrocarbon was not merely broken down into its elements, but that liquid or solid hydrocarbons were also formed. I n a number of publications which appeared during the period 1862 to 1869, Berthelot reported that decomposition of methane, confined in sealed tubes of difficulty fusible glass (9, I O ) , yielded propylene ( I S ) , benzene (11, I S ) , and naphthalene (9, 14). Much of the methane remained undecomposed. The same results were obtained by passing methane through red hot porcelain tubes. About 1.8 per cent of acetylene, 1.4 per cent of other unsaturated hydrocarbons, and some benzene were present in the products of decomposition of methane obtained by Levies (138) using a platinum tube heated to 1 0 0 0 ~ . Methane required a higher tcmperature for conversion into acetylene than did ethane and ethylene. Lewes believed, as did Berthelot (91,that the benzene was formed by polymerization of acetylene. I n later work he studied the decomposition of methane at a number of temperatures and obtained much hydrogen, but little acetylene, as shown in Table SI I.
TABLE STT Thermal Decomposition of Methane in Platinum Tubes Temgeratuie C.
Hydrogen per cent
1000
1.j;
1200
1300 I joo
(Len es I Paraffins, largelj unchanged methane. per cent
Acetjlene per cent ~
8.53 10.37
90
0 0;
88. j 3
78.66
19.22
0.39 0 96
Working at 9 8 5 O , Bone and Coward ( 2 7 , 28, 29) found olefins, acetylene, and aromatic hydrocarbons among the decomposition products at high methane concentrations, such as above 60 per cent. The presence of aromatics was indicated by the appearance of a slight mist in the condensing system. At 1150’t o 1160’ no acetylene could be detected and aromatic hydrocarbons were formed only during the first minute of each run. De Boistesselin and Dubosc (j9) are reported to have secured unsarurated hydrocarbons by passing methane over carbon impregnated with copper oxide, at 400° to 450’. They claimed that isoprene, butadiene, and higher olefins were obtained, the proportions being dependent upon the temperature. The composition of the olefin fraction was approximately as follows: Per cent:
Ethylene 36
Propylene 42
Higher olefins 21
DECOMPOSITION O F T H E PARAFFIN HYDROCARBONS
16j1
Elworthy (64) pointed out that De Boistesselin and Dubosc did not disclose the yield of olefins on the basis of the methane treated. In an attempt to duplicate and supplement their results, he passed natural gas containing 80.3 per cent of methane and 7.6 per cent of ethane through a heated quartz tube, using as catalyst a mixture of pumice, carbon black, and copper oxide. KO acetylene was found in the gaseous products, although the analytical method (234) used was capable of detecting as little as 0.03 per cent of this substance. Elworthy concluded that: (I) The amount of unsaturated hydrocarbons formed was always small. (2) The optimum temperature was about 880'. (3) The ethylene formed probably resulted from the decomposition of ethane and did not arise from methane. Garner (87, 88) stated that he had decomposed natural gas in electrically heated quartz or steel tubes at 400' to 1400' in the presence of pumice-nickel, pumice-copper, charcoal, finely divided copper, finely divided iron, aluminum, aluminum chloride, and silica. He reported no data but claimed that the decomposition varied with the catalyst used, one catalyst under a definite set of conditions giving a high percentage of acetylene while another produced none. -1process of carbon black manufacture based on his results gave good yields of carbon and a discharge gas containing 6. j j per cent of unsaturated hydrocarbons, mainly ethylene. b. Recent Investigations and Developments W o r k directed tou ard Production of Aromatzc Hydrocarbor~s.-During the last few years, the investigation of the conversion of methane, or more often natural gas, into higher hydrocarbons, especially aromatic hydrocarbons, has been particularly active. Studies have been made independently in Germany, Great Britain, and the United States. In 192;, Williamson (241) stated that "valuable liquid products," particularly benzene, were being produced in a semi-commercial scale plant of the Anglo-Persian Oil Company by subjecting stripped low-pressure gas from the Persian oil fields to high temperatures under pressure. Recent large scale tests made by that company, have yielded 2 2 0 gallons of benzene per I,OOO,OOO cubic feet of gas containing 80 per cent methane. Gas rich in higher homologs of methane yielded 770 gallons of benzene per ~,ooo,ooocubic feet. During the same year (192;), Dunstan and Wheeler (60) applied for a patent on the production of aromatic hydrocarbons by subjecting gaseous paraffins to high temperature. I n applying their process to methane, the hydrocarbon was first preheated and then passed through reaction tubes at 850' t o 975'. The gaseous products were allowed to expand, cooled, and the products, carbon and aromatic hydrocarbons, separated. The yield of aromatics was increased by diluting the gas with steam before cracking. Fischer (69, 70) at the same time was conducting extensive investigations of methane decomposition in an endeavor to increase the small yields of aromatic and unsaturated hydrocarbons reported by previous workers. He predicted that the time of heating would be of as great importance as tempera-
1652
GUSTAV EGLOFF, R. E. SCHAAD, AND C . D. LOWRY,
JR.
ture in the formation of higher hydrocarbons by the decomposition of methane. His first experiments were conducted in a porcelain tube, 16 mm. in diameter, at temperatures between goo0 and I I ~ O ’ , on a gas initially containing 93.0 per cent of methane, 1.8 per cent of heavy hydrocarbons, and no hydrogen or ethane. The results are given in Table XVII.
TABLE XVII Decomposition of Methane T,emp. C.
Time hours
900 975
__
1000
-
Liters per hour
(Fischer) Expansion per cent
4 4
3.85 70
Analysis of gas’ after the experiment. in per cent Heavy H1 CHP H.C.
-
-
__
I2
2.8
23.0
66.5
I9 6
2’7
39.3
2.6
12.6
55.0 79.7 7 7 4 50.4
3
11002
4 4
70
IO
3.4
14’7
1 1 jo3
1213
70
21
4.2
41.8
1050‘
* No ethane.
16.5 g oil, 1.6 g tar, very little carbon. 9 g oil, 4.5 g tar, I g carbon.
2
3
4.2 g oil, 3.5 g tar, 3.1 g carbon.
These experiments showed that a relatively high temperature was required to produce a “satisfactory” yield; that the appearance of free carbon increased rapidly with rising temperature; and that the undesirable decomposition to carbon and hydrogen could be prevented by passing the gas at a suitable velocity. Fischer also used quartz capillary heating tubes of 3 mm. inside diameter, and he proved that a t 1130’ quartz does not accelerate the decomposition of methane into carbon and hydrogen. With these tubes the yield of oil and tar reached 12.9 per cent by weight of the methane treated. With the intention of keeping the reaction temperature as low as possible, the influence of different catalysts was investigated. Thin iron or molybdenum wires within small porcelain tubes had no influence. Caustic potash in an iron boat in the reaction tube at 1 0 0 0 ~produced a gas containing 56 per cent of hydrogen, and large quantities of carbon were formed, but no oil or tar. If the methane was diluted with carbon monoxide, carbon dioxide, or nitrogen, the temperature had t o be higher to obtain the same degree of decomposition than that used when the hydrocarbon was alone. T o manufacture a larger quantity of light oils and tar, long runs were conducted at 1100’with a gas throughput of 60 to j o liters per hour. After 48 hours the tubes were still sufficiently clear of carbon to be operative. X run of this duration yielded a rather watery tar and 140 to I jo cc. of light oil. I n the case of low gas velocities the carbon formed was chiefly soot-like, voluminous, and light; while with the high velocity which was used in the longer runs, the carbon stuck tightly to the walls of the tubes.
DECOMPOSITION OF THE PARAFFIN HYDROCARBONS
1653
The tar formed began to boil at about zoo’. “The distillate which passed over up to 250’ (14 per cent) solidified completely. I t consisted chiefly of naphthalene. Of the fraction 250’ to 360” (29 per cent), 90 per cent consisted of a yellowish brown, green fluorescent oil; the rest consisted of solid, cyclic hydrocarbons from which anthracene was isolated.” The light oil recovered by adsorption on activated carbon was driven from the latter by superheated steam. The first fraction (7.8 per cent by weight of the light oil) boiled up to 55’ and consisted mainly of unsaturated hydrocarbons. The second fraction, boiling from 55’ to 85’, was chiefly benzene. A toluene fraction constituting approximately 8 per cent, a xylene fraction amounting to 9 per cent, and naphthalene equal to I O per cent of the total light oil were also obtained. The uncondensed gases contained acetylene besides hydrogen and methane. Fischer’s experiments “confirmed the supposition that for the production of higher hydrocarbons by way of the thermal decomposition of ethane, the duration of the heating is of just as great importance as the temperature. If the duration of heating exceeds sixty seconds, there is usually time enough to separate all the hydrogen from the methane molecule.” Fischer observed that there was no “remarkable” decomposition of methane below 900’ in porcelain tubes without catalysts even a t low gas velocity. A temperature of IZOO’, with all the velocities (up to 7 0 liters per hour) investigated by Fischer, was too high for the purpose in mind. “There is, certainly, an optimum temperature for the splitting up of the methane and another for the combination of the fragments.” According to Fischer’s results, catalysts were not desirable during the methane cleavage if the separation of carbon was to be prevented. “But during the cooling of the gas they can play a role by influencing the polymerization of the radicals of low molecular compounds resulting from the methane.” Fischer stated also that increased pressure was disadvantageous for the methane cleavage itself, but that it would aid polymerization of the resulting products. He suggested that the decomposition reaction be carried out under diminished pressure, and that further treatment of the gas be effected under pressure. I n resume, Fischer (70) stated that his experiments showed “that methane can be converted into benzene and other benzene hydrocarbons, if it is heated to temperatures of 1000’to IZOO’, when keeping the period of heating, however, shorter than I O seconds.” The process can be carried out under ordinary pressure without catalysts. “Conditions can be so adjusted that the chief reaction product consists of benzene hydrocarbons, more than half of which is benzene. The unsaturated hydrocarbons in the residual gas can be reduced to 4 per cent, and the separation of carbon can be almost completely prevented.” Fischer and Bahr (71) found that when Hamburg natural gas (containing 9 2 . 5 per cent of methane, 6.6 per cent of nitrogen, 0.5 per cent of oxygen, and 0.1 per cent each of carbon dioxide and heavy hydrocarbons) was passed through a heated porcelain tube, tar formation began sharply a t 8 5 0 ~ . The
I654
GUSTAV EGLOFF, R. E. S C U D , AND C. D. LOWRY, JR.
inner walls of the heating tube became covered with a hard carbon film, and naphthalene deposited in the receiver. Tar formed a t a lower temperature in only a few cases. At 850" the formation of tar and naphthalene was greater in a quartz tube than in one of porcelain. The decomposition of methane was essentially the same in a porcelain tube filled with porcelain fragments as in the empty tube. The formation of tar was greater when the natural gas was laden with water vapor than when the gas was dry. At 850°, 42.6 per cent of the moist gas disappeared, while under similar treatment the dry gas decreased in volume but 2 2 . 7 per cent, Tar was not formed and the production of naphthalene was small when finely divided graphite replaced the porcelain fragments as contact material. Drops of a heavy unsaturated oil resulted when dry methane was passed over barium sulfate or barium oxide in a porcelain tube at 630' to 650", but its quantity was too small to permit identification of the hydrocarbons. Benzene and naphthalene were detected in the products obtained by passing a mixture of 15 per cent of methane and 85 per cent of nitrogen through a glazed porcelain t,ube at 850'. Wheeler (236) claimed priority over Fischer in the production of higher hydrocarbons, especially aromatics, by the thermal treatment of methane. His experimental work was reported with Wood (238). Static experiments in which methane initially at 400 mm. pressure was heated at various temperatures in a quartz bulb of about 50 cc. capacity, showed that the decomposition point of the gas was between 650" and 685". Pressure changes in the reaction bulb at 'joo', goo0, and 900' indicated an abnormally rapid rate of decomposition during the first ten minutes, followed by a period of nearly constant speed. Experiments in which methane was circulated in a closed system, through a quartz bulb-tube of 50 cc. capacity heated a t 900' and 9 jo' and the course of the decomposition followed from the change in pressure, gave time-pre sure curves similar to those obtained during the static experiments. At 950°, liquid products were formed during the first few minutes of heating, and later, crystals of naphthalene were deposited in the cooled condensers. Liquid hydrocarbons were formed during the passage of methane, in "stream" experiments, a t rates of 2 t o 60 liters per hour through a quartz tube 130 cm. long and of 3 cm. internal diameter kept a t temperatures higher than 875". Benzene was an important product of the pyrolysis of methane between 875' and 1100'. By bromination the gaseous products of decomposition a t 950' were shown t o contain ethylene and butadiene. The liquids of high boiling point contained naphthalene and anthracene. Carbon smoke appeared above 1000' and seemed due to decomposition throughout the mass of the gas, as distinct from the lustrous surface deposit of carbon which formed on the walls of the reaction tube a t lower temperatures. The optimum temperature for the production of benzene under the conditions of experiment appeared to be about iojo'. The best yield of benzene was 0.2 gallon per 1000cu. ft. of methane. This was in a horizontal
DE COMPOSITIOX O F T H E PAR.4FFIS HYDROCARBONS
TABLE XT'III Decomposition of JIethone in Porcelain and Quartz Tubes (Wheeler and Wood)
Yields in grams per 2 2 . 4 liters of methane passed Comparison of porcelain and quartz reaction tubes Temp.
"C.
950
Porrelain tube Kate of Crude flow, oil liters per hr. 1.9 I .j4
--
3 ' 1
10.0
.o
1:
0 .
j6
0.4.; 0.34
Benzene fraction
37 0.33 0.26 0 . 2 0
0
Rate of flow. liters per hr.
0.60 0.66
0.45
10.0
0.50
Ij.0
0.39
0.31 0.23
2
.o
j .O
19.o IO00
7.i
0.47
0.2j
0.1j
0.96
1.11
0.55 o 61 0.61
0.96
0.54
14.2
0.98
20
28.3
0.77
3.; I
0.62 0.40
0.55 0 43 0.34
i)
42 .o .6
0
20.0
I
SI
91 06
0.42
;.8
20.0
I?
10.50
0.8j
Quartz tuhc Crude Benzene oil fraction
4
I .04
31 6 43 4
0.77 0.46
0.42
10.9 19.9
0.82
0
0.58
I .20
'
0.25
0 20 0 . j 2
48
32.2
I.oj
0.5j
30.2
1.33
0.62 0.64
42 . 3
0
9'
44.8
I .20
0.57
3 . 5
0.78
0.54 0.45
60.0
I
.03
0.49
('omparison of pocked and unpacked auartz tubes Temp
'C.
950
1000
IOjO
t-nl)acktd tiihe Rate ot Crude AOW, oil liters p w hr.
6.5
0
65
Benzene fraction
Kate of flO\V,
Packed tub? Crude Benzene oil fraction
liters per hr.
0.28
j .8 12.3 1 7 .o
0.7;
0.64
0.34 0.33
0.50
0.1;
i .4
0.78
0.28
10.8
0.78
0.37
73
0.33
I0.I
0.66
13.9
0.62
0.30
17.7
0.53
0.18
14.8
0.53
0.14
19.0
0.62
0.32
22.8
0.61
0.29
14.5 23.4
0
0.67
0.28
30.8
0.s9
0.18
9.8;
0.31 0.34
0.31
'
0.75
0.42
24.5
0.56 0.66 0 . j6
0.5;
30.0
0.77
40.0
0.96 0.74
0.44
40 . o
0.66
0.35 0.30
46 .o
0.90
0.45
43 . o
0.44
0.1;
20.4
0.82
0
27.7
0.79
0.44
33 ' 1 38.3
'
43
1;.2
0.28
1656
GUSTAV EGLOFF, R. E. SCHAAD, AND C. D. LOWRY, JR.
TABLE XIX The Effect of Dilution on the Decomposition of Methane (Wheeler and Wood)
Yields in grams per
22.4
liters of methane passed, a t
Dilution with Nitrogen nitrogen Crude Benzene oil fraction per cent by vol. 9
1.05
25
0.58 0.49
I
0.0
27.7
0.88
41.1
0.8;
53.5
0.84
0.48 0.46
70.3
0.82
0.45
IOOO'.
Dilution with Hydrogen Hydrogen Crude Benzene per cent oil fraction by vol.
8 .O 13.6 22.6 31 . o 43.7
.04
0.82
0.59
0.36
0.60 0.45 0.36 0.24
0.21
0.13
0.04
0.03
TABLE SX Eecomposition of Methane in a Silica Tube (Stanley and Nash) Temp.
"C.
1060
Gas rate liter/ hour
Per cent expansion of gases
3.42 4.26 6.52
I2
I .07
29 16
O.OI2j
2.2
0.0112
2.0
4.2
2.33
0.0183
3.1
0.0282
8.0
3.52
12
o.ozj4
4.3
0.0285
4.9 4.9
5.98 7.50 8.90
IO
0.012.;
2.2
0
019;
3.4
6.5
8
0.011:
2.0
0.0064
1.1
3.1
6
O.OIOO
r . j
0.0046
2.3
o.orjr
0.8 3.0
2.j
O.OI3j
I IO0
8 7
4.08~ 1150
0 . 0 2 5 0]TaLi,nd
44.3 '3
0.1580
2.7
9 2
5.3
7.5
28
0.0188
3.2
0.02jo
9.47
21
0.02j2
4.j
0.0308
4.3 5.4
10.1
18
I1
13.8 .6
20.8
j
1
Tar ohtained Per cent g.,'l. of Per cent conversion methane on ofmethane methane into oil and tar
6.02
1:
I200
Light oil obtained g.;l. ot Per cent methane on methane
.62?
16.l 3
o.0281
4.8
0,0352
6.2
j
0.0129
2.2
0.0035
0.6
2.8
small
.o
5
0.0010
1.7
15
0.02j3
4.3
0.0179
3.1
7.4
11
0.029;
j.1
o.016j
2.9
8 .o
Silica tube coated internally with graphitic carbon.
1 . j
* Methane mixed with equal volume of steam. Gas rate given here is of the steam-free gas. 3
Gas rate found to give maximum production of tar fog.
DECORlPOSITION O F T H E PARAFFIN HYDROCARBONS
1657
TABLE XX (continued) Analysis of reaction gas by method of Bone and Wheeler in percent Temp. "C. Gasrate l./hr.
1060
1100
1150
1200
0%
Nz
+ CH,
C2H2
CnHzn
3.42 4.26 6.52
0.1
1.1
1.1
0.8
1.5
15.3 14.3
82.2
0.0
0.9 0.8 0.7
0.9
1.1
IO,^
86.0
2.33 3.52 5.98
0.3 0.3
1.0
1.2
1.1
0.8
1.4 1.4
29.7 21.6
0.0
1.0
1.0
0.4 0.4 0.3
1.0
12.5
66.0 74.4 84.2
1.3 1.3 1.6 1.6 1.7
41.8 37.0 32.8 14.9
0.2
6.02 9.47 13.8 17.6 20.8 7.62
81.5
1.9
1.3
2 . 0
1.5
1.4 1.0
0.2
0.2
2.3 1.3 0.9
0.4 0.5 0.4
0.9
0.9
0.1
2.1
1.1
2.2
31 o
53,2 57.7 61.3 80.5 84.9 63.j
1.9
1.1
1.6
25.2
70.1
0.1
0.2
0.5
16.1
CO
H2
COz
0.1
10.5
Analysis of reaction gas by low temperature condensation and distillation Temp. "C. I IO0
I150
Gas rate l./hr.
Ratio
CgH,
C?H? C?H4
Per
0.62
0.65 0.91 I .28
-
.07
2.33 3.52 5.98 7.50 8.90 4.08
0.92 I .08 0.82 0.65 0.58 1.13
0.84 0.78 0.66 0.71
.67
I .20
I
2.10
I .25
I
2.21
I .25
7.77
.06 0.83 2.48
0.89 0.85 I .03
1.19 0.96
2.02
I .I2
I
6.02 9.47 13.8 17.6 20.8
7.62 1200
C?Hi Per cent
C& Per cent 0.40
16.1
I
I
I .OI
I
,os
0.99 0.83
cent
0.08
0.08 0.09 0.08 0.08 0.10
1.15
.38 .68
0.08 0.06 0.08 0 .IO
0.10
0.06 0.05
quartz reaction tube, which gave better results than a vertical tube. As shown by the data in Table XVIII, below 1050' somewhat better yields of benzene and above 1 0 5 0 ~ markedly greater yields of oil were obtained with quartz than with porcelain tubes. Experiments in a quartz tube packed with
1658
GUSTAV EGLOFF, R. E. SCHAAD, A S D C. D LOWRY, J R
quarter to half inch pieces of quartz showed that this added surface had little, if any, effect. Similarly copper tubes had but slight catalytic action. LIethane was decomposed into its elements and no condensible products obtained when an iron tube was used. As shown by Table XIX, dilution of the methane with nitrogen had only a slight effect on the yield of aromatic hydrocarbons at IOOO’, but the yields were decreased by the presence of hydrogen in the gas entering the reaction tube. Stanley and S a s h (203) have also produced higher hydrocarbons from methane. At their Birmingham laboratory, methane was “found to yield, on subjection to passage through a silica tube at 1100’(heating time of I Z seconds), approximately I O per cent of liquid and solid hydrocarbons (free from carbon black) calculated on the original ~iethane,while, in other experiments, the gaseous product of the reaction contained 0.9 per cent of acetylene as well as olefins. Analysis of the acetylene, regenerated from its metallic derivatives, indicated that there was not an appreciable quantity of its higher homologs present.” I n a more complete report Stanley and Kash ( 2 0 4 ) stated that gaseous, liquid, and solid hydrocarbons were produced when a stream of gas containing methane 92.5 per cent, nitrogen 6.6 per cent, carbon monoxide 0.5 per cent, and water vapor 0.3 per cent was passed at rates of 1.07 to 2 0 . 8 liters per hour through transparent silica tubes 5 mm. in internal diameter heated by an electric furnace to temperatures between 1000’ and 1200’. The liquid and solid products (Table XX) were chiefly aromatic, mainly benzene and naphthalene. The gaseous products, analyzed by a low-temperature condensation and distillation method, were shown to contain hydrogen, acetylene, ethylene, and small quantities of ethane, higher olefins, and aromatic hydrocarbons. At any given temperature the proportion of methane converted into hydrocarbons of higher molecular weight, increased rapidly to a maximum with increasing gas rate. As the gas rate was further increased, there was a decrease in the production of higher hydrocarbons, at first rapid and later more gradual. An increase in the temperature of the reaction caused an increase in the gas velocity at which the maximum production of hydrocarbons occurred. The ratio of the acetylene to the ethylene content in the reaction gas increased with increase in gas velocity for a given reaction temperature, reached a sharp maximum (which appeared to coincide with the conditions for the formation of maximum light oil and tar), and then decreased rapidly. A4ccumulationof carbon in the reaction tube caused a decrease in the yield of higher hydrocarbons. The addition of an equal volume of steam to the methane lowered the gas rate at which maximum production of tar and 011 took place. The liquid and solid products, when steam was used, apparently did not contain oxygenated compounds. Stanley and Kash believed that all diluting gases would exert the same effect as steam. The best yields of higher hydrocarbons were obtained at 1150’ with a heating period of 0.6 second. “Under these conditions the yield of light 011
DECOMPOSITION O F T H E PARAFFIN HYDROCARBONS
I659
and tar was I I . O ? calculated on the methane used, whilst the quantities of acetylene and ethylene in the reaction gas correspond with a total conversion of methane into these gases of 8.8C;. Thus the total conversion of methane into higher hydrocarbons was nearly zoc/'c of the theoretical," while only about 6 per cent of methane was decomposed into its elements. The yield of light oil was 4.8 per cent or 0 . 2 gallon per 1000 cubic feet of methane treated. Hague and Kheeler (104) obtained liquid products and ethylene in small yields by decomposition of methane in a vertical, electrically heated quartz tube 70 cm. long and of 2 . 2 cm. internal diameter. I n each experiment, 1 2 to 16 liters of the hydrocarbon was passed downward through the tube at the rate of 4 liters per hour (67 cc. per minute). The resultant gases were conducted through three condensers cooled by carbon dioxide snow. Before and after each experiment, the reaction tube was swept out with nitrogen, and between runs carbon which had deposited in the tube was burned out. Table S S I summarizes the results.
TABLE SSI The Decomposition of Methane T$mp. C.
900
950
Per cent increase in volume ~
3.i
1000
16.6
IOjO
2i.j
(Hague and Wheeler) Yield. Der cent bv n.eiphtAof methane Total Benzene Carbon liquids fraction
Trace
0.6
3.2
1.4
1.7
4.5
0.4 0.3
1.2
-
.o 3.7 2.6
I ,I
2
Analvsis of eas in per c e h by Glume Higher C2HI H2 olefins 0.9
CHa
82.6 63.9
2.8 3.i 2.8
42.6
j4,2
2.1
54.3
43.3
14.0 31.5
Benzene and other aromatic hydrocarbons began to appear between gooo and 9 j 0 ° . About 9 j per cent of the liquid product boiling below 1iOOwas benzene. The liquids of higher boiling point contained naphthalene and anthracene. The gases contained butadiene as well as ethylene. According to experiments of Vysoky (233) benzene began to appear as a product of methane pyrolysis in quartz tubes between 800' and 850'. Yields of aromatic hydrocarbons (determined by conversion to mixtures of hexabrombenzene and pentabromtoluene, and computed as consisting of 80 per cent benzene and 2 0 per cent toluene), increased with the temperature of the tube, and at 1075' amounted to 2 4 . 7 mgm. per liter of original methane. At 1000' an increase in reaction time from 31 to 68 seconds was accompanied by an increase in yield of aromatic hydrocarbons from 13.4 to 18.77mgm. per liter of methane treated. For comparison it should be noted that Wheeler and Wood (238) found that the production of aromatics a t 1000' decreased from 27.2 t o 10.7 mgm. per liter of methane when the reaction time was increased from 30 to 1 2 3 seconds. In other experiments a mixture of 87.1 per cent methane, j.4 per cent oxygen, and 5.5 per cent nitrogen when thermally treated at gjo", I O O O ~ and ,
1660
GUSTAV EGLOFF, R E. SCHAAD, A S D C . D. LOWRY, JR.
1050' gave smaller yields of benzene than methane alone under parallel conditions. Silver did not increase the formation of benzene from the methaneoxygen mixture, while copper and copper oxide turned the decomposition away from aromatic formation.
TABLE XXII Thermal Treatment of Natural Gas Tube
(Chamberlin and Bloom) Internal Heated Natural Rate cu.m. diam. length per hour. gas em. cm.
Oil or tar fog first visible at "C.
Fused silica
1.9
30
W.Va.'
o 085
Fused silica
3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6
45 45 45 45 46 46 46 46 46 46 46 92
La2 La. La. La. W.Va. W .T'a . W'.Va. W.Va. W,Va. 7.7' .Va. W.Va. W.Va.
0.091
3.5 3.5 3.5
45 45 45
La. La. La.
0.0~~-0.19 0,198
Nickel
3.8
45
W.Va.
0 . IO
Silica (containing nickel gauze)
3.5 3.5
illonel metal
3.5
45
La.
Copper
3.5 3.5
45 45
W.Va. La.
91.5 91.5
La. La.
Steel
Fused silica
10.2
10.2
6j o
0.10
0.093 0.10
0.0'iI
0.08 0.091 0.08 0.082
0.085 0.082 0.042
630
0.I02
R.Va. La. 0.156
670
0.10
565 450
0.085 0.142 0.136
Steel
15.2
549
2.265 to 2.832
Clay
15.2
549
2.265 to 2.832
830
800
DECOMPOSITION O F THE P A R A F F I N HYDROCARBOKS
1661
TABLE XXII (continued Yields per 1000 cu. m. of natural gas treated .It ‘C. Benzene, CloHs-tars, Other products. liters. grams.
Tube
Fused silica
800-8jo
Fused silica
585 620
750 846 6503 7j03
8503 8604 I 040‘
860~ 7j04
Steel
43,73
Trace
8% C. 5.4%’ oils
I .20
Trace
7.23 15.23 24.05 6.95 4.01 4.01
282.70 2 8 2 70 1060 I O
s o fog s o fog
2 8 . IO
38.90 20.20
2.67
82 j 800
41.95
7 50
27 .o
Light fog Considerable heavy fog Xhite vapors None 2401 I O Brown fumes 2 4 0 1 . I O Anthracene in fumes 9604 30 Heavy pitch, brown fumes By Cottrell precipitator 187 g. 1,0801. I O condensate from fog. Heavy pitch with carbon Yellow fumes, red oil Heavy dark fumes, carbon
24.6 714.00
Xckel
880
Silica (containing nickel gauze)
700
66.60 kg. of carbon
700
I j.64
llonel metal
700-7
jo
None
24-275
Copper
600
13.3~
Fused silica
880
6.67
Steel
760
13.73’
Clay
“Little”
2 I;
kg. of carbon kg. of carbon
Some anthracene
Small amount
* T h e W. Va. natural gas gave the analysis: COS 0.80, CHI 71.20, CaH, 23.30, and Na 4.70 per cent. 2 The La. natural gas gave the analysis: COa o.jo, CH; 94.12, CIH, 3.44, and N1 1.94 per cent. 3 The carbon was burned from the tube after each test. 4 The carbon was permitted to accumulate during these four tests. 5 On continuing these tests the yield of oil dropped off finally to zero. ‘Gas flow of 0.085 cu. m. per hr. ’The oil yield decreased as the tests progressed due t o separation of carbon in the tubes
Chamberlin and Bloom (43) obtained aromatic hydrocarbons, principally benzene, naphthalene, and anthracene, along with acetylene, ethylene, and other olefins, by thermally treating natural gas at temperatures under 900’ in tubes of silica, steel, nickel, monel metal, copper, and clay. Their results are given in Table XXII.
1662
GCSTSV EGLOFF, R. E. SCHAAD, A N D C. D. LOWRY, JR.
The apparatus used ranged from laboratory equipment to a semi-commercia1 installation operated in Philadelphia during 1926 and 1927. At the lowest temperature used (600°), copper tubes gave the highest yields of benzene, but the metal was consumed after a short period of use. At intermediate temperatures (800’ to 8 j o o ) silica produced the highest yields of benzene. I t was the most satisfactory material employed as it was not changed by heat, or by the products of thermal treatment. Iron, nickel, and various forms of carbon tended to catalyze the decomposition of methane into its elements. It was noticed that carbon resulting from the decomposition of natural gas a t temperatures from 4 j o o to 900’ had a selective action aiding the conversion of paraffins into aromatics. This activity was easily destroyed by socalled partial oxidation, by the formation of carbon-metal compounds, or by the conversion of the carbon into the graphitic form. The optimum temperature for the formation of benzene varied with the tube material and also depended on the nature of the carbon within the tubes. I n order to obtain maximum yield, it was found best to keep the temperature as low as possible. The oil yield became negligible when the concentration of hydrogen reached j o to 60 per cent of the volume of the gas being treated. From a comparison of their results with analysis of gas from the by-product coke oven, making the assumption that the gases in the coke oven reached equilibrium, these investigators concluded that in their work they had attained the maximum practical yield of benzol from natural gas, about z i to 40 liters per 1000cubic meters. A study (197)now in progress in the petroleum laboratory of the Bureau of Mines at Bartlesville, Oklahoma, has yielded benzene, toluene, naphthalene, and anthracene, as products of natural gas decomposition in electrically-heated tubes made of a mixture of alumina and silica. Some carbon, hydrogen, acetylene, and ethylene formed a t the same time. I t was noted that when the methane content of the exit gas became less than 60 per cent the yields of unsaturated aromatic hydrocarbons were low and carbon clogged the apparatus. This work on the conversion of natural gas to benzene has progressed far enough, it is stated, to “indicate that the process is already almost as efficient as the present carbon black production methods.” This might be called dubious praise. Production mainly of 0kfins.- Jones (130) obtained small amounts of olefinic hydrocarbons when natural gas containing 92.j per cent of methane,
TABLEX X I I I Olefins produced by the Decomposition of Methane in a Quartz Capillary Tube at 1000’ (Jones)
Gas pressure, cm. Olefins, per cent a t 100em. per sec. Olefins, per cent at zoo cm. per see.
IO
20
0.13
0.78
Trace o .z;
30
40
jo
.zj
I . jj
1.75
I
0.49
0.38
0.16
I
60
jo
.61
0.84
0 .Ij
Trace
1663
DECOJIPOSITIOS O F T H E PARAFFIS HYDROCARBONS
zz$ &.zz
m
bp
.-
k
N
O
0 O
o o o
W 0
-o
10w.100
"
"
0
C0 )
" 0
;
;
0
0
0
0
0
0
0
0
o o o o o o o o
1664
GUSTAV EGLOFF, R . E . SCHAAD, AND C. D. LO'R'RY, JR.
3.5 per cent of hydrogen, and 3.9 per cent of nitrogen was passed under pressures of I O to 7 0 cm. of mercury through a quartz capillary tube heated a t 700' to 1080". The gas was passed through the quartz tube ten or twelve times at constant pressure, and the increase in pressure at constant volume noted after each passage. "At 700' the increase in pressure was very small, at 800' and a rate of flow of zoo cm. per second the increase after ten passages through the tube was 2.j per cent, whilst a t 1080' and a rate of I O O cm. per second it was 32.0 per cent for the same number of passages." "Traces" of olefins were formed a t 800" and 900" and "appreciable" amounts a t IOOO', as shown in Table SSIII. "Small amounts of acetylene were formed at all temperatures and pressures, the maximum yield occurring at a pressure of 30-40 cm. Easily condensable hydrocarbons were not formed." The main course of the methane decomposition in this work was to carbon and hydrogen. Production mainly o j Acetylene.-For the purpose of producing acetylene, Peters and Meyer (173) passed methane through porcelain tubes under pressures of 2 0 to 760 mm. at temperatures from 11j o " to I~OO', and in other experiments through a spiral of tungsten wire heated electrically to about 2200' to 3000'. As shown by their results in Table XXIV, the yield of acetylene was greatest a t the highest temperature employed, reaching as a maximum a quantity equivalent to 66 per cent of the methane passed through the tube. I n the higher range of temperature there was no formation of benzene or light oils. This yield of acetylene is about equal to the best that has been obtained by use of the electric arc (see Electrical Decomposition page (1722). Hydrogen cyanide, which results from the actionof electrical discharges (74) on mixtures of methane and nitrogen, was not produced in detectable quantities by the thermal decomposition of such a mixture. Peters and Meyer gave curves showing the ranges of reaction temperature and heating time necessary for the maximum production of carbon, acetylene, and benzene from methane or coke-oven gas. For example, acetylene was shown as the main product of the exposure of methane for less than 0.001 second t o a temperature of 2000°, but the separation of carbon predominated with a longer time of heating. In a quartz tube at IIZO', with a time of contact of 0.06 second, Frolich, White, and Dayton (8 j ) were able to convert about I I per cent of the entering methane into acetylene with the simultaneous formation of only small amounts of other hydrocarbons and carbon. If the exposure was prolonged to 0.6 second, however, there was considerable deposition of carbon and polymerization, with an acetylene yield of less than j per cent on a basis of entering methane. Further increase in the contact time to more than one second had little effect upon the yield of acetylene, but markedly increased the formation of carbon and polymers. These investigators state that their results "support the conclusion that acetylene is an intermediate in the production of benzene, and other polymers
DECOiMPOSITION OF THE PARAFFIN HYDROCARBONS
166 j
by high temperature cracking of methane.” They found no noticeable difference in acetylene yield as the gas pressure was varied from 2 j to 760 mm. Jones (130) had claimed a maximum formation of acetylene a t 30 to 40 mm. His yields, however, were low and the time of contact was not stated. Patents.-The conviction that the thermal treatment of methane and natural gas will acquire commercial importance is indicated by the number of patents which have been taken out on this process. According to patents of the Compagnie de Bethune (45), methane or gas containing it was treated under pressure at zooo to b o 0 , using as catalysts ferric oxide, a metallic oxide mixture, or the corresponding metals obtained by reduction. The formation of ethane and butane was claimed (No other report has been made of butane formation.) and also of liquid hydrocarbons which in large measure distilled from 38O to 100’. The presence of hydrogen or oxygen in the mixture under treatment seemed to facilitate the reaction. According to another patent (46) the hydrocarbon was to be decomposed under pressure a t 2 5 0 ” to 300’ in the presence of a metal oxide catalyst. A patent of Goudet (95) claimed that “a hydrocarbon mixture resembling crude petroleum is obtained by subjecting methane, either alone or with the addition of hydrogen-combining gases, such as acetylene and carbon monoxide, to a decomposition temperature of 500’ to 950°, cooling the dehydrogenated products, and then passing them over a catalytic material to furnish higher hydrocarbons; the three stages are each conducted under increased pressure.” According to another Goudet patent (96) “methane is treated with nickel, iron, cobalt, or their compounds a t a pressure up to 1000 kg. per square centimeter, the migration of the hydrogen being facilitated by the addition of alkali or alkaline earth metals or of chlorine, bromine, iodine, sulfur, selenium, and tellurium.” The Compagnie GBn6rale des Produits de Synthkse (48) reported that methane and ethylene hydrocarbons were polymerized by heating at 80” in the presence of a cerium sulfate catalyst. The polymerized product was said to be suitable for use as motor fuel or as a solvent. S o manipulative details of the process were given. Starke and Starke ( 2 0 6 ) claimed to obtain benzene and naphthalene as by-products in the purification of natural gas. The process consisted in passing natural gas under atmospheric pressure at a temperature of 5 2 5 ’ to 8 7 5 O over a catalytic mass formed by calcining a mixture of 2 0 per cent of iron or magnetic iron oxide, 40 per cent of magnesium oxide, and 40 per cent of sodium or potassium carbonate. Ellis (63) claimed to obtain “ethylene, propylene, and the like” by passing casinghead gas over nickel at a temperature between j o o o and ioo’ and under a pressure of about half an atmosphere. Knapp (131) described a process for transforming the methane content of natural gas into acetylene. The natural gas, which might be preheated, was passed through a bed of coke, from which air was excluded, heated electrically
1666
GUSTAV EGLOFF,
R. E.
SCHAAT), A S D C . D. LOWRY, JR.
from 2 500' to 3500'. He claimed that the methane combined with the carbon in the layer of coke and formed acetylene. Several patents of I. G. Farbenindustrie -4.-G. cover the production of benzenoid hydrocarbons by the thermal treatment of natural gas or distillation gases containing methane. At temperatures (121) of 600' to goo0 and under pressures from so to 1000atmospheres, the gases are passed over active silica, active charcoal, a carbonate, oxide, or hydroxide of an alkaline earth metal, or of selenium, tellurium, or thallium. According to an example given, a liquid rich in benzene hydrocarbons is obtained by passing natural gas, consisting essentially of methane but also containing ethane and propane under 150 atmospheres pressure through a vessel filled with active silica maintained at 600'. Under ordinary pressure (I 2 5 ) the passage of gas mixtures containing methane over the above catalysts a t joo" to goo0 was claimed to produce "high yields" of benzene hydrocarbons. Acetylene ( I z 2 ) and ethylene were claimed as products of the rapid passage of methane under pressure over the catalysts mentioned above at temperatures of 700' to IOOO' or higher. Acetylene was the main product at very high temperatures and ethylene at somewhat lower temperaturs. As an example, it was claimed that methane was converted into acetylene by passing it rapidly through a rock crystal tube containing fragments of magnesia heated to 950'. If the speed of gas flow was reduced, naphthalene and benzene were formed also. A further patent (123) covers a similar process. The SociBtB Anonyme le PBtrole (166, I 9 9 ) , to convert methane into substances of "higher carbon content,'' heats the hydrocarbon gradually and evenly under a pressure of 20 mm. to 750'. From natural gas of Vaux, France, containing 80 per cent of methane, 6 per cent of hydrogen, 2.5 per cent of carbon dioxide, and 2.6 per cent of propane and higher hydrocarbons, was produced mainly ethylene and its homologs, with some hydrogen and saturated hydrocarbons. The formation of naphthalene, tar, and aromatics was avoided. I n later patents ( 2 0 0 ) it was stated that the temperature might reach 9joO. By the use of this process Olivier (167) claimed that it was possible, by a single operation, to convert 80 per cent of the methane content of Vaux natural gas into ethylene or other olefinic hydrocarbons. Aromatic hydrocarbons are prepared by Fischer and Pichler ( j j ) by heating methane at temperatures of 1000' to 1100' for 0.2 to 0.05 second. Lewis (140) claimed that benzene, toluene, anthracene, and naphthalene as well as free carbon were formed when purified natural gas containing only methane and ethane was passed at a pressure of 5 to I O inches of water through a tube heated from 600' to 900'. c. Summary The products which have been obtained by thermal treatment of methane and natural gas are summarized in Tables XXT' to XXIS, inclusive. These outlines do not include the substances claimed in patents.
1667
DECOMPOSITIOPI‘ O F THE PARAFFIN HYDROCARBONS
TABLE XXV Thermal Decomposition of Methane Products obtained at Different Temperatures in Silica Tubes TEmp.
C.
800
Tube diam., internal I
mm.
Pressure, rate of flow, etc.
Products
to 7 0 cm. “Olefins” Hg. Acetylene
IO
Yields
References
“Traces” -
130
850
dry CHI
8.50
C H 4 + H 2 0 Tar vapor
Better than from dry CH4
“Stream” expts.
Not stated
238
“Trace”
104
875I IO0
900
I
mm.
2.2
Benzene
71
71
to 7 0 cm. “Olefins” Hg. Acetylene
2 to 60 l./hr.
950
9jo
IO
Carbon, Greater than naphthalene, in porcelain and tar
cm. 4 l./hr.
Ethylene Butadiene Naphthalene Anthracene Carbon Hydrogen
3 I .s% of gaseous
Ethylene
3.7 7cof gaseous
“Higher olefins” Butadiene Benzene fraction
0 . 9 7 ~of gaseous
Carbon
Appeared a t
product
product (max.)
“Stream” expts.
1000
1000
1000
I
mm.
2.2
cm./sec. “Olefins” at jomm. Hg.
IOO
cm. 4 l./hr.
Carbon Hydrogen Ethylene “Higher olefins” Benzene fraction Naphthalene
product (max.) 1.17~ by wt.
of the CHI treated IOOOO
238
1 . 7 5 % of CH4
130
1 . 4 7 by ~ wt. of CH,
104
treated (max. between I O and 7 0 cm. pressure)
42.6Yc of gaseous
product of gaseous product 0.4% of gaseous product 3 . 2 7 0 by wt . of CH, (Max. between 900’ and 2.8%
I05O0)
-
1668
GUSTAY EGLOFF, R . E . SCHAAD, AND C. D. LOWRY, J R .
TABLE XXV (Continued) Thermal Decomposition of Methane 5 m p . Tube
C.
diam., internal
1000
8 mm.
1050
2.2
Pressure, rate of flow, etc.
Products
31 see. con- Benzene tact time 68 sec. con- Benzene tact time
cm. 4 l./hr.
Yields
References
1.9% by wt. of CHI 2.7%
233
by wt. of CHI
Carbon
4.5% by wt. of CHa (greatest formation up to
Hydrogen
54.3% of gaseous
104
10go”)
product (greatest formation UD to 1050”)
Ethylene “Higher olefins” Benzene fraction “Stream” expts.
1050
Benzene
yc of gaseous product
2.I
0.3y0of gaseous product 1.7% by wt. of CH,
0.2
gal./Iooo cu.
238
ft. CH4 (best yield; 1000’is optimum temp.)
1075
8 mm.
68 see. con- Benzene tact time
1100
3 mm.
60 to 70
l./hr.
II00
1
Heating time, 1 2 secs.
Carbon Hydrogen ‘Tnsatd.” H.C.’ Acetylene Benzene To1u en e Xylene Kaphthalene Anthracene “Tar” “Light” oil
3 . 5 7 G by wt. of CHb
7.8Yc of the light oil 49Yc of the light oil
8%) of the light oil 9p6 of the light oil 10%
of the light oil
About 1 0 7 p of CHI About 3 cc. per hr.
“Liquid and solid” H.C. Olefins Not stated Acetylene 0.97~of gaseous product
Hydrocarbons are referred to here by “H.C.”
233
1669
DECOMPOSITION OF THE PARAFFIN HYDROCARBONS
TABLE XXV (Continued) Thermal Decomposition of Methane Temp. Tube Pressure, rate “C. diam , of flow, etc. internal I120
1150 j
I 300I325O
mm.
0.6 sec.
Acetylene
0.06 sec.
Acetylene
13.8 l./hr.; heating time. 0.6 sec.
Carbon Hydrogen
“Hot-cold” tube
Yields
Products
“Less than 5%” CH, 11% of CHI
1 Ethylene Acetylene 1 Light oil
References
of
6% of CHd decomposed
85 2 04
8.8% of CH4
decomposed 11% of CH, decomposed 4.8% of CHI =0.2 gal./Iooo cu. ft.
and tar Light oil (mainly benzene) Carbon Hydrogen
-
20
TABLEXXVI Thermal Decomposition of Methane Products obtained a t Different Temperatures in Porcelain Tubes Teomp.
Tube Initial gas, diam , rate of flow, internal and catalyst
6306 50
BaS04 or BaO
850
“Stream”
C.
850
flow
Products
“Unsatd. oil” Too small for identification of individual H.C.’ Carbon, naphthalene, and
Mixt.: I j% Benzene CH4+85% and naphx* thalene
Hydrogen and “heavy H.C.” 985 1 2 mm. CH4 at rest “Unsatd.” H.C. Acetylene Iojo 16 mm. 4 l./hr. Carbon Hydrogen goo
Yields
1
71
-
71
Formed only when CH4 conc. was greater than 60% “very little” 12.6% of gaseous product “Heavy, 2.6y0 of gaseous H.C. product “oil” 1.6 g./hr. 1 In this table hydrocarbons are referred to aa “H.C.” *Tarformation started a t 850’.
71
-
Not stated
16mm. 4 l./hr.
References
69, 7 0
27 2 8 , 29
69, 7 0
GUSTAV EGLOFF, R.
1670
E.
SCHAAD, AND C. D. LOWRY, JR.
TABLE XXVI (Continued) T,emp. C.
Tube Initial gas, diam., rate of flow, internal and catalyst
1100
16mm. 4 l.,’hr.
Products
Yields
References
/ hr. / hr.
“Tar” Carbon Hydrogen
0.4 g.
“Heavy H.C.” “Oil” ‘lTar”
3.470 of gaseous
Carbon Hydrogen
1.8 g./hr. 4 1 . 8 7 ~of gaseous product 4.2 yc of gaseous product 2 . 5 g./hr. 2.1 g./hr. Formed only during first minute of heating
69, 7 0
36y0 of CHI treated
173
0.25
g.
69,
14.770 of gaseous
7O
product
1150
16mm. 4 l./hr.
product g./hr. 1.1 g./hr.
2.25
“Heavy H.C.”
