T H E J O C R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
Dec., 1915
p a r t of the work, attention was given only t o t h e establishment of general principles a n d no a t t e m p t was made t o learn details. CONCLUSIOKS
REGARDING
THE
PREPARATION
OF
TRI-
&-ITR O T O L U E N E
1-Trinitrotoluene, of a satisfactory degree of purity, may be prepared from t h e mononitrotoluene obtained b y t h e nitration of t h e toluene-paraffinolefin mixtures produced b y the fractionation of “cracked” petroleum. a-From t h e laboratory point of view, at least, t h e poly-stage process seems best. T h e transformation from mono- t o di-product should involve a fair d e gree of heating a n d a moderate concentration of acid. T h e final stage of t h e nitration should be conducted with a considerable excess of strong acid, which acid need not be wasted since when drawn off i t m a y be used in a n earlier stage of t h e next cycle of nitration. GENERAL SGNMARY
I-The aromatic hydrocarbons produced b y t h e “cracking” of petroleum may be utilized for t h e preparation of nitro-products without being brought t o a high degree of purity. T h e foreign substances not easily removable are paraffins a n d these are not affected b y reagents of sufficient vigor t o transform t h e aromatics quantitatively into mono-nitro products. These compounds may be separated in a high degree of purity by a simple process of distillation. 11-The mononitrotoluene prepared b y t h e method indicated m a y be readily converted into trinitrotoluene which, when washed with h o t water, satisfies t h e ordinary commercial specification of a melting point of 7 j 0 C. C H E M I C A L S E C T I O N OF T H E P E T R O L E U M
u. s.BUREAUO F MINES
DIVISION
THERMAL REACTIONS OF AROMATIC HYDROCARBONS IN THE VAPOR PHASE’ By W. F. RITTMAN, 0. BYRONA N D G .
EGLOFS
Received September 8, 1915
I n connection with t h e phenomena of thermal decomposition or “cracking” of hydrocarbons a s t u d y has been outlined for t h e purpose of obtaining comprehensive d a t a regarding t h e whole field. A first series of experiments2 dealt with t h e production of gas a n d determined t h e influence of temperature, pressure a n d concentration on t h e yield a n d composition of t h e product. Another series of experiments3 dealt with liquid reaction products a n d indicated conditions favorable for t h e production of ( I ) open-chain hydrocarbons a n d ( 2 ) aromatic hydrocarbons; i t was shown t h a t t h e temperature range m a y be divided roughly as follows with regard t o t h e products of t h e cracking reaction: u p t o a b o u t 600’ C. t h e formation of low-gravity hydrocarbons of t h e open chain class is favored; from 600’ t o 800’ aromatic compounds are produced in large q u a n t i t y ; above 800’ t h e maximum products are carbon a n d gas. F r o m both a scientific a n d a commercial point of
’ Published
*
with t h e permission of t h e Director of t h e Bureau of Mines. R i t t m a n a n d Whitaker, THISJOURNAL, 6 (1914), 383 and 472. R i t t m a n , I b i d . , 7 (1915). 945.
1019
view t h e differences among various aromatic hydrocarbons are of almost, if not quite, as great importance as t h e differences among- classes. The present series of experiments has, therefore, been conducted for t h e purpose of establishing facts concerning t h e influence of conditions of temperature a n d pressure on interaromatic reactions. Four typical monocyclic hydrocarbons-cymene, xylene, toluene a n d b e n z e n e w e r e subjected t o cracking, each at three different temperatures a n d a t each temperature under four pressures. Some work has been done also with t h e solid hydrocarbons naphthalene a n d anthracene, b u t on account of experimental difficulties, t h e results obtained are only qualitative. F r o m t h e results of t h e s t u d y of reaction products, indications have been obtained which may be summarized in t h e following general way: T h e course of t h e reaction is in t h e direction I-(a) of decrease in size of molecule when t h e degree of saturation is unchanged. ( b ) Reactions may occur in t h e direction of de-hydrogenation either with increase or with decrease in size of molecule. a-Reverse reactions occur in practically negligible amount. HISTORICAL
More or less work seems t o have been done by previous investigators on t h e problem of thermal reactions of aromatic compounds b u t in no case has a systematic s t u d y been made. The information desired at present concerns t h e influence of conditions of temperature a n d pressure upon quantitative a n d qualitative relations a m o n g end products a n d this is not adequately supplied b y t h e literature. T h e classic researches of Bertholet’ have furnished a foundation for our knowledge in this general field. His series of thermal reactions is comprehensive a n d t h e results obtained are valuable, though not such as t o supply t h e needs of t h e present case. Ferko2 furnished some valuable information, particularly with regard t o t h e direction of reactions. He passed t h e vapors of several hydrocarbons, either pure, or mixed with ethylene gas, through a n iron t u b e heated over a 40 cm. length in a gas furnace. T h e products were condensed a n d their composition studied. The more important of t h e results obtained are summarized in Table I. Without attempting a TABLEI-SUMMARYOF RESULTSOBTAINEDB Y FERKO REACTIOX PRODUCTS PERCENTAGES OF ORIGINALMATERIAL
Benzene a n d ethylene 1320 6 . 0 0 . 0 1 . 3 Toluene . . . . . . . . . . . . . . 1300 1 1 . 5 1 3 . 8 0 . 5 Toluene and ethylene.. 1300 1 5 . 4 1 2 . 3 0 . 8 Naphthalene a n d ethyle n e . . . . . . . . . . . . . . . . 900 0 . 0 0 . 0 0 . 0 E t h y l b e n z e n e . . . . . . . . 500 1 5 . 0 1 . 0 2 . 0
0.0 0.0 0.0
0.0 22.7 0 . 0 2.1 0.0 0.0
0.0 13.3 4.0 0.0
0 . 0 0 . 8 1.1 3 . 0 1.1 1.0 0.0 0 . 0 1 . 5
0.0 44.5 0 . 1 0.0 0.6 2 . 2 2 . 6 0.4
detailed s t u d y of this table it m a y be seen t h a t no reaction products are obtained which are not in t h e order of decrease of saturation, or of molecular weight ‘ A n n . chim. p h y s . , [4] 9 (1866), 445; 12 (1867), 5; 16 (1869). 143. 2
Bn.,20 (1887).
660.
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
I020
T h u s benzene yields no toluene nor a n y other compound of equal saturation or of greater molecular weight. Naphthalene yields neither benzene nor toluene. From ethyl benzene, however, it is possible t o obtain toluene, benzene a n d compounds of lower saturation. T h e work of Ferko seems t o have been t h e most valuable of all hitherto done in this field b u t it gives little attention t o t h e influence of temperature a n d none t o t h a t of ,pressure. Haber' studied t h e thermal reactions of various substances, among t h e m benzene, a t temperatures ranging between 6 0 ~ - 8 0 0 C. ~ a n d g o o - ~ o o o C. ~ One of his observations is contrary t o t h e results of t h e present experiments. He states t h a t although benzene goes readily t o diphenyl it does not form naphthalene. McKee2 has, as a result of some qualitative experiments, concluded t h a t t h e order of hydrocarbon decomposition is: higher paraffins .--f lower paraffins + olefins + acetylenes .--f benzene a n d homologs + diphenyl + naphthalene, etc., tarry matter + carbon a n d gas. I n his experiments benzene was
-
TABLE 11-QUANTITATIVE No.
REACTIONS
Volume changes
-18000
4 fo 8
..,., .
-82000
7 t o 19
K
3
3 C i o H i 4 ~ S C ~ H e 6Hz .... , , , . . -68500 Cymene Benzene
3 t o 11
K
4
7 C s H i o Z 8 C 1 H s f 3Hz.. .. . Xylene Toluene
.. .. -90800
7 t o 11
5
3 C s H i o Z 4 C a H s f 3H2.. . . . . . Xylene Benzene
6
6 C 1 H s 2 7 C e H s f 3H2. Toluene Benzene
~ C I O H I ~ Z S C I If H I3Hz.. O Cymene Xylene
2
7CioH14,'lOC7He Cvmene
+
9Hn.. Toluene
+
,
. . -65600
.......
-40700
=
3 to 7
K
6 to 10
K =
~
a 4
Ber., 29 (1896), 2691. Jour. SOC.Chem. J n d , 27 (1904), 403. Jour. Chem. Soc., 101 (1912), 1453. Rittman, LOC.cil.
APPROXIMATE
6 . 8 X 1032
2.0
(CaHs)5 (Hz)S_ (CloHln)a
7 . 8 X 1040
(CeHs) 3 ~'(Hz) __
(CaHio) 3
(caHs)'(H~)~
G E N E R A L CommERATIoxs-In the present as well as in t h e earlier series of experiments cracking was produced in a system having a single (gaseous) phase. T h e characteristics a n d advantages of this method of procedure have been already discussed in detail.4 It is sufficient in t h e present connection t o recall t h e fact t h a t t h e basic reason for t h e employment of t h e one-phase system is t h a t it permits t h e independent regulation of t h e t w o factors of temperature a n d
-
K650
(C7Hs) 'O(H2) 9 (C10H14)7
(CaHio)'
THEORETICAL
1
EQUILIBRIA
K = (C?Hs)8(Hz)s
p a s s e d t h r o u g h a hot copper t u b e a n d t h e degree of decomposition determined b y change in specific gravity of the product. N o analysis or separation seems t o have been a t t e m p t e d upon t h e latter. Smith a n d Lewcock3 passed benzene through a n iron t u b e a n d at 800' C. obtained a 2 j per cent yield of diphenyl. T h e y were not, however, able t o reverse t h e reaction, which inability t h e y attributed t o experimental difficulties. They observed t h a t t h e quantity of diphenyl produced increased with t h e rate of feed, which indicates t h a t this is not an equilibrium product, b u t simply one which is intermediate.
2
STUDY OF
(Formulas refer t o partial pressures) (C~HIO)'(HZ)~ K = (CioHin)
.. .. , .
1
12
pressure. I n a n y two-phase system each of these is a function of t h e other. There are numerous other advantages b u t these have received adequate discussion in earlier connections. E Q U I L I B R I U M RELATIONS-In each Of t h e t w o preceding communications applications have been given of t h e Nernst approximation formula for equilibrium constants. The use of this expression is particularly helpful in connection with t h e present series of experiments as t h e reactions are obtained upon pure chemical compounds instead of complex mixtures. It is possible, therefore, t o write equations which are entirely representative of what may occur instead of merely typical. T h e figures obtained are understood t o be representative only of general tendencies but are of much value when interpreted in this light. T h e figures in Table I1 represent equilibrium relations for a series of reactions, a n y or all of which may have occurred in t h e present experiments. A first point t o be considered is t h e influence of variations in pressure. All t h e reactions occur with a n increase in volume a n d therefore i t is t o be expected
if saturation remains unchanged.
Heats of reaction
Vol. 7 , No.
-7cTKjr
x
101'
Kaoo
K 7 2 5
x
1032
4.7
x
1032
1 . 4 X 1088
2.0
x
10'0
x
1.0
x
1044
4.7
2.8
1042
1.1
x
10
6 . 6 X 108
3.1
x
10'0
1.1
x
1013
2.1
x
1014
4.3
x
1015
2.9
x
1017
3 . 2 X 10'8
2.7
x
10'0
t h a t t h e effect of pressure will be inimical. This consideration is, however, complicated b y t h e necessity of taking account of another factor. T h e use of the Kernst formula assumes t h a t t h e reactions have proceeded until equilibrium has been reached a n d i t is doubtful if this is the case, especially for t h e reactions a t lower temperatures.l According t o t h e law of mass action, reaction velocity is proportional t o t h e product of t h e concentrations of t h e reacting substances a n d concentration is, in t h e present case, directly proportional t o pressure. I t appears, therefore, t h a t increase in pressure may be favorable u p t o a point where t h e reactions proceed with sufficient rapidity t o attain t o a state of equilibrium. Increase in pressure above this will undoubtedly t e n d t o shift t h e equilibrium unfavorably for t h e reactions indicated from left t o right in t h e equations. Consideration of t h e effect of temperature is also complicated. The constants invariably increase with temperature a n d this should indicate t h a t even greater degrees of heat would be favorable. It must be remembered, however, t h a t no one reaction can be considered as happening alone. E a c h major reaction is coincident with a number of by-reactions a n d t h e 1 Rittman, THISJOURKAL. 6 (1914). 1027; Jour. SOC.Chem. Ind., 33 (1914), 626; Whitaker and Alexander, THISJ O U R N A L . 7 (1915), 484.
Dec., 1915
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
establishment of conditions which apparently are favorable for t h e main reaction m a y in actual fact hinder i t on account of greater favoring of minor reactions; for example, benzene formation would seem t o be favored b y increasing temperature even beyond t h e 800' limit. It appears, however, t h a t even a t this mark t h e reaction has suffered on account of greater favoring of other products a n d t h e maximum yields of benzene occur in t h e 650' a n d 7 2 5 ' runs. T h e chief value of t h e constants lies in t h e indication t h e y give of t h e general tendencies of reactions. An examination of t h e values in Table I1 makes evident t h e fact t h a t all t h e reactions should occur principally in t h e direction indicated from left t o right in t h e equations. T h e magnitude of t h e reverse reactions would be proportional t o t h e reciprocals of t h e constants a n d these in most cases are negligibly small. E X P E R I M E N TA L
T h c scheme of procedure followed in t h e present experiments was, with slight exceptions, identical with t h a t described i n t h e earlier communications.l T h e a p p a r a t u s consisted of a n electrically heated cracking furnace equipped a t one end with a n oil-feed cup a n d a vaporizing device, a n d at t h e other with a condenser, receivers for liquid reaction products, a n d a pressureregulating device. One modification in t h e making of t h e "run" is worthy of mention. I n t h e earlier series of experiments pressures were built u p in t h e cracking furnace b y t h e decomposition of some of t h e oil used in t h e run. This necessitated t h e introduction of a source of error, which in t h e more recent experiments, i t has been found possible t o eliminate. Before t h e beginning of each pressure r u n t h e system was connected with a compressor a n d natural gas2 pumped in until t h e gauge gave t h e proper reading. T h e compressor mas t h e n s h u t off b y closing a valve a n d t h e r u n started. For all t h e liquid hydrocarbons t h e original material was allowed t o flow into t h e furnace at t h e r a t e of about 6 cc. per minute. I n t h e cases when t h e solid hydrocarbons, naphthalene a n d anthracene, were used a s starting-out material i t was, of course, necessary t o keep t h e m in a molten condition during t h e r u n b y applying heat t o t h e feed cup. E v e n t h e n considerable experimental difficulty was involved a n d no a t t e m p t was made t o obtain results of more t h a n qualitative accuracy. T h e supplementary determinations were made upon quantities of recovered oil too small t o permit separation of pure products. A single analytical distillation was made through a n 8-in. Hempel column a n d cuts made at IOO', I Z S ' , Ijoo, a n d zoo'. T h e first cut was t a k e n as representative of benzene, t h e second of toluene, t h e t h i r d of xylene, a n d t h e fourth of cymene. t h e distillation was continued with a Above 200' shorter fractionating column a n d cuts made at intervals of 50'. These distillates were chilled a n d ex-
' Whitaker
and R i t t m a n , LOC.a t . ; Rittman, LOC.c i l . N a t u r a l gas was used because i t is largely methane, which is one of t h e end products of all cracking reactions.
I021
amined for t h e separation of solid naphthalene a n d anthracene. On account of t h e smallness of t h e quantities obtained no a t t e m p t was made t o obtain quantitative results for t h e solid products. T h e total scheme of procedure m a y be summarized as fol ows: "RUN"
OR MAJOR EXPERIMENT
I-Cracking of a quantity (generally 500-600 cc.) of hydrocarbon under regulated conditions of temperature and pressure. 2-Determination of quantities of liquid and solid reaction products. SUPPLEMENTARY DETERMIFATIONS I-Distillation of liquid reaction products through Hempel fractionating columns. n-Determinations of spec;fic gravities of total reaction products and of distillation cuts. E X P E R I M E N T A L MAT E R I A L B E N Z E N E was t h e purest material commercially obtainable. T h e sample used was particularly good as is shown b y t h e d a t a in Table 111. T O L U E N E a n d X Y L E N E were t h e "chemically pure" market products. C Y M E N E also was a commercial product.
TABLE111-ORIGINAL PRODUCTS USED I N EXPERIMENTS PRODUCT
... , . Toluene . . . . . , , Benzene..
' '' '
' '
'
Cymene.. . . . . .
SP. gr.o l.50/15 FRACTIONAL DISTILLATION 0.879 98 per cent between 80 a n d X I o C. Temp. 100-110 110-115a C. 0.869 % lst98drop 98-100 o,2 92,0 4.0 88-120 1 2 0 - 1 3 i 0 C. 0 . 862 lst88drop 0.7 9 9 , 3 (flask dry) Temp. 16.5' 17O-li9' C. 0.863 % Ist drop 90.0
{
t "Ep' {
GEh-ERAL R E S U L T S O F E X P E R I M E N T S
T h e entire collection of d a t a obtained in t h e present series of experiments appears in Table IV. This table is, on account of its size a n d necessity arrangement, not particularly readable a n d special tables have been prepared t o set forth t h e various indications obtained: it does, however, show t h e scope a n d magnit u d e of t h e work a n d indicates t h e nature of t h e measurements which have been made. Results are recorded in terms of percentage b y volume except in cases where either starting-out material or reaction product was a solid. I n such instances mass relations are given. It will be noted t h a t for t h e four liquid hydrocarbons-cymene, xylene, toluene a n d benzene-the series of experiments are complete. Quantitative results for t h e distillations are given for cuts u p t o zooo a n d above t h a t temperature t h e indications for naphthalene a n d anthracene are given b y plus (+) a n d minus (-) signs. For t h e experiments where naphthalene a n d anthracene were starting-out material no distillates were obtained below zooo C. a n d all t h e results are therefore qualitative. A N A L Y S E S O F QUANTITATIVE
(a)
P E R C E N T A G E YIELD
RESULTS (TABLE
OF LIQUID
REACTION
v) PROD-
UCTS decreases with increase in temperature a n d in general with increase in pressure. This percentage represents t h e total of liquid products a n d soluble products which are soluble therein, excluding only carbon a n d gas. T h e figures given are in a way representative of t h e stability of t h e various compounds, their resistance t o conversion into carbon
T H E J O U R N A L OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
I022
Vol. 7 , No.
12
......
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4 c c o .
0 * 0 . 0 " O m
U
.. .. .. .. .. .. . . . . . . . . .
N*m
W
:::.++ .
... ... ... ... ... .....
m-
.. .. .. .. .. .. .. .. .. .. .. .. .. .. m
.
.
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.
.
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86.". egng++ N
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gt.0
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T H E J O C R S A L O F I , V D L - S T R I A L A S D E,YGI,YEERI-VG C H E M I S T R Y
Dec., 191;
from both xylene and cymene, moit readily from t h e former. The amount which can be produced from benzene is negligible, t h a t from naphthalene and anthracene nil. The effects of temperature a n d pressure seem rather irregular, very possibly on account of experimental error. The general tendency seems t o be for maximum yields t o occur with decreasing temperature as pressure increases. T h u s in t h e xylene figures in Section B, Table T'd, a maximum appears a t 800' for vacuum runs, a t j 2 j ' for runs a t I atmosphere and at 6 j o for t h e runs at 1 2 atmospheres. Beyond this point t h e curve seems t o swing the other way. A like tendency will be noted among cymene figures if the obviously wrong bracketed value is omitted from consideration. ( e ) XYLESE FoRuaTIox-The figures in Table Lre show t h a t while xylene may be formed in moderate quantity from cymene, the amounts produced from
and gas being indicated b y t h e amount of liquid product obtained. I t appears t h a t for the four liquid hydrocarbons, t h e order of decreasing stability is benzene, toluene, xylene and cymene. This is as would be expected on t h e grounds of t h e theory t h a t with similar molecular form t h e more unwieldy units of matter are t h e more easily broken down. (b)
PERCEXTAGE
OF
ORIGISAL
HYDROCARBOS
1023
RE-
is another index of t h e stability of the compounds cracked. I t represents total resistance t o transformation while t h e percentage yield of liquid products represents resistance t o conversion into carbon a n d gas. I t appears t h a t t h e order of stability is the same as t h a t indicated in ( a ) b u t t h a t there arc greater differences. The effects of temperature and pressure are analogous t o those indicated in ( a ) . (c) B E N Z E N E FoRxATIoN-It appears from t h e figures in Table Vc t h a t benzene is formed in apCOVERED
TABLE V-ANALYSIS O F Q U A N T I T A T I V E RESULTS (a) YIELD ( b ) RECOVERY (%) O F P E R C E N T A G E FORMATION O F LIQVIDAROMATIC HYDROCARBONS (%) OF LIQCID ORIGISAL (6) BENZENE ( d ) TOLUENE ( e ) XYLENE ( j ) CYNENE REACTIOX PRODUCTS HYDROCARBON FORMATION FORMATION FORMATION FORMATION :
83.0 95.0 98.0 90.0 62.0 69.0 98.3 86.6 12 53.3 66.0 94.8 87.5 40.8 64.3 60.8 73.1 18 725 Vac. 52.0 72.0 85.0 94.0 1 40.060.064.290.0 12 2i.5 5 4 . 2 4 8 . 3 58.3 18 25.0 46.6 18.0 53.3 800 Vac. 49.0 66.0 66.0 87.0 1 17.6 27.1 32 5 62.0 12 15.8 31.7 34.2 22.5 18 0.0 11.5 1 1 . 7 18.3 Average , , . . . . , . . , . . . . . 3 8 . 8 5 4 . 2 5 9 . 3 6 5 . 5 1
B Tzmp,
Hydrocarbon Cymene , . . . .
c
, .
Xylene , . , , . . . . . Toluene . . . . . . . . . Benzene
650 725 800 ?SO ;25 800 650 725 800 650 725 800
6
$
. E
2
12
.
5
