FEBRUARY, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
It now appears possible by efficient fractionation to prepare fractions constituting 5 or more volume per cent of the over-all naphtha that may contain upwards of 50 per cent of a particular definite hydrocarbon, or other 5 per cent fractions that may contain a total of only three or four particular hydrocarbons as their principal components. These definite hydrocarbons, as far as virgin naphthas are concerned, are: 2-methylbutane, n-pentane, 2-methylpentane, cyclopentane, n-hexane, methylcyclopentane, cyclohexane, n-heptane, methylcyclohexane, toluene, dimethylcyclohexanes, n-octane, ethylbenzene and the xylenes, n-nonane, 9-carbon-atom aromatics, arid n-decane. In some cases fractional distillation alone may suffice to produce in two distillations in columns with 30 to 100 theoretical plates, hydrocarbons ranging in purity from 80 to 95 per cent. This was done in the case of all the normal paraffins listed above and for cyclohexane, methylcyclohexane, and ethylcyclohexane. Despite statements in the literature, no true constant-boiling mixture of hydrocarbons was found It was again noted,
169
however, that when aromatic and nonaromatic hydrocarbons were distilled together, they exhibited unusual distillation relationships. For this reason it is practical to use a third component-for example, tert-butyl alcohol-in the separation of n-hexane from small amounts of benzene (4). A great chemical industry has been developed for hydrocarbons from coal tar. It should be possible to develop similar fields for normal paraffins, isoparaffins, cyclopentane hydrocarbons, and cyclohexane hydrocarbons from petroleum.
Literature Cited (1) Bruun, J. H., and Hicks-Bruun, M. M., Bur. Standards J . Research, 10, 465 (1933). (2) Hicks-Bruun. M. M.. and Bruun. J. H.. Ibid.. 7. 799 (1931). . , (3) Tongberg, C. O . , Fenske, M. R., and Nickels, J. E., IND.ENQ. CHEM.,29, 70 (1937). (4) Tongberg, C. O., and Johnston, F., Ibid., 25, 733 (1933). (6) White, J. D., and Rose, F. W., Bur. Standards J. Research, 13, 799 (1934).
RECEIVED October 8, 1937.
Olefins and Conjugated Dienes from Gas Oil HANS TROPSCH,l CHARLES L. THOMAS, GUSTAV EGLOFF, AND a. c. MORRELL Universal Oil Products Company, Chicago, Ill.
Gas oil can be made to yield useful products corresponding to 73 weight per cent of the charge (83 weight per cent of the reacted gas oil) by pyrolysis at 950"C. and 175 mm. pressure with a contact time of 0.05 second. The weight per cent yields of products interesting for chemical synthesis are: ethylene, 28.2 per cent; propene, 19.7 per cent: butadiene, 6.6 per cent; butenes, 8.3 per cent: penta-
T
HAT small amounts of butadiene are formed in the pyrolysis of various hydrocarbons ( 2 ) and other organic compounds ( 5 ) is well known. Higher diolefins are formed in certain cracking operations using petroleum fractions as the charging stocks (6). Although conjugated dienes have been shown to be present in the reaction products of a large number of thermal reactionf, the conditions favorable to the formation of such compounds and the maximum yields have not been determined. Because of their value in organic syntheses it would be desirable to determine the maximum yields of these compounds as well as of the other olefins of actual and potential value which are produced simultaneously. The mechanism of the formation of the conjugated dienes has not, been established. It has been postulated (1, 4 ) that the dienes are the precursors of aromatic hydrocarbons in hydrocarbon pyrolyses; and although Groll (3) and others 1
Deceased October 8, 1935.
diene, 3.1 per cent, and higher conjugated dienes, 7.2 per cent based on the gas oil reacted. I n addition, 10.8 per cent of a high antiknock gasoline is produced. Under the conditions studied the butadiene yield is not greatly influenced by temperature (800"to 1000"C.) or pressure (50 to 500 mm.), provided the oil is gasified to the extent-of 50 to 75 per cent of the charge. consider it doubtful that aromatic formation passes through the conjugated open-chain molecule stage, this mechanism has been used as a valuable tool in directing the present work. Assuming for the moment that conjugated dienes are the intermediates between other hydrocarbons and aromatics, four useful conclusions concerning diene formation are reached 1 1. Since aromatics are produced from almost any hydrocarbon source, conjugated dienes should be also. On this basis a gas oil should prove a convenient and cheap source. 2. Aromatics are produced at temperatures usually above 600' C. in vapor-phase cracking. Similar temperatures should be used for diene formation. 3. Since conjugated dienes are highly reactive at these temperatures, the reaction time should be as short as is practical. 4. The reaction t o produce aromatics from most conjugated dienes is at least bimolecular. Low- pressures would hinder such bimolecular reactions.
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( U p p e r ) HIGH-PRESSURE REACTION ROOM
(Lower) TWO-COILPILOT-PLANT CRACKINQ
UNIT
was found that about 15 cm. were heated to reaction temperature within +20" C. This 15-cm. distance was used in estimating the contact time. The condensing and collecting system consisted of watercooled, dry-ice-cooled, o r liquid-nitrogencooled traps. The vacuum pump (Kelson. Berkeley, Calif.) permitted the collection of the gaseous products and was filled with glycerol. The charging stock was Pennsylvania gas oil with a density of 0.8275 and an 9.P. I. gravity of 39.5 a t 60" F. (15.6" C.). The Engler d i s t i l l a t i o n was as follows : Initial b. p.
5%
70 % 90 %
End point
c.
F.
213 261 280 294 306 326 357
415 502 537 56 1 583 619 674
Sinety-nine per cent went over; 1 per cent was bottoms. The average molecular weight was 245.
Gas Analysis I n the preliminary work the exit gases were analyzed by determining the density of the gas and the butadiene content as determined by the maleic a n h y d r i d e method (7). In this way it was possible to follow the yields and concentrations of the conjugated dienes in the gas. This method was used in calculating the early results (Tables I and 11). Obviously if butadiene were the only conjugated diene produced. these results would be correct; but if other dienes were present, they might well be partly in the gas as vapor. The latter was shown to be the case. To obtain the correct analysis of the mixture, the methods used are described in the individual cases.
Effect of Packing
In the actual experiments 800" to 1000" C. and 50 to 500 mm. pressure were utilized. At low pressures and short contact times, high temperatures are necessary to give appreciable decomposition to gas.
Apparatus The apparatus (Figure 1) was arranged so that the gas oil from a charging buret could be passed through a capillary feed control to a flash pot (400" C.) where the gas oil was vaporized. Leaving the flash pot, the vapors passed into a 1-cm. i. d. silica reaction tube packed with 6-10 mesh porcelain chips. The reaction tube was heated in a 30-cm. Burrell high-temperature furnace (silicon carbide resistance rods, heated electrically). Of the total 30-cm. furnace, it
I n the preliminary experiments better yields of butadiene were obtained in a reaction tube containing a granulated packing than in a n empty tube. This effect was thought to be due to the increased turbulence and consequently improved heat transfer. This may not be the sole effect, however, because different packings give different results, although these differences may be due partly to different thermal conductivities. The results can be seen in Table -I which gives the concentration of butadiene in the gas, the per cent gasification and the butadiene yield based on the amount of the charge converted to gas. The pressure was 175 mm., and a temperature range of 800" to 950" c. was studied. The temperature was varied to see whether there was some particular range in which one of the packings might be especially active. In the 800-950" C. range studied there
FEBRUARY, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
was no noticeable effect on the butadiene yield provided the feed rate was adjusted so that the percentage of the charge converted to gas remained substantially constant. TABLEI. EFFECT OF PACKING Packing Carborundum Petroleum coke Activated alumina Electrical porcelain Coors porcelain
Gasification
CiH6 Concn., % 3.4-5.8 4.8-6.0 2.2-4.9 5 .'8-6.5 6,6-8.0
%
64-75 62-67 63-75 76-84 67-77
C4He, Wt. % of Yield 7.8-12.0 9 ,8-11.5 5,8-9.2 9.5-10.4 13,2-15.4
IN BUTADIENE YIELDDURING ONE EXPERITABLE11. CHANGE MENT
GasiC4H6, fica- CaHe Wt. % Time, tion, Cono., of Hr. % % Yield 0 5 67 6.6 13.2
Time, Hr. 2.5
Gasification,
%
77
C4H6, C ~ H BWt. Conc., of % Yield 7.8 15.4 8 15.4
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Effect of Temperature, Pressure, and Reaction T'ime Definite efforts were made to establish the effect of these variables without clear-cut success. In these experiments a t high temperatures and short contact time it is difficult to study the effect of one variable a t a time. When the pressure is changed, for example, the pressure drop through the apparatus, heat transfer, and turbulence all change. Similar difficulties are encountered with the other variables. From the evidence available it would seem that, over the range studied, the butadiene yield is independent of the temperature and pressure, provided the percentage decomposition to gas is from 50 to 75 per cent of the charge. For the time being this conclusion must be limited to the apparatus used in this work. The butadiene yield dropped off as the percentage gasification dropped below 50 per cent, and above about 75 per cent gasification the carbon formation increased rapidly.
Products from Gas Oil Cracking ~~~
The effect of the packing is still more complicated, for the packing was covered with a thin deposit of carbon in the first few minutes of the reaction. After this initial deposit the amount of carbon apparently did not increase further. All of the packing materials tried gave the same result. Thus it is apparently the type of carbon deposited on the packing that influences the reaction rather than the packing per se, except as the latter influences the type of carbon deposited. A packing which has been used and is giving certain yields loses part of its activity if the carbon is burned off but regains it as a new run proceeds and the carbon is replaced. This effect is illustrated in Table I1 where a packing of Coors chemical porcelain was used. This same packing had been used in the same tube in a large number of previous experiments. The carbon deposit was burned off just before the present experiment was begun. The experiment was made a t 950" C. and 175 mm. average pressure. Although experimental difficulties prevented the peroentage gasification from being kept as constant as was desired, there was a definite trend upward in butadiene concentrations and yield. In this last experiment a total of 1042 cc. of gas oil was charged to the 1-cm. i. d. reaction tube, and over 700 cc. were gasified. This result will serve to emphasize the minuteness of the amount of carbon formed under these conditions, especially because the pressure drop through the apparatus increased only 5 mm. during the run-i. e., from 50 to 55 mm. of mercury.
I n order to determine the products of the reaction in some detail and to eliminate possible contamination by passage through the vacuum pump, a liquid nitrogen trap was inserted in place of the fog precipitator. In this way all the products of the reaction were condensed except methane, hydrogen, and part of the Czand Ca hydrocarbons. In one experiment the contents of the liquid nitrogen traps were warmed to -80" C. and then brominated in chloroform solution at this temperature. Although this method was not particularly useful for analyzing all the unsaturated products, it did permit the isolation of the butadiene tetrabromides. The amount of the butadiene tetrabromides indicated that a t least 8 per cent by weight of the charged gas oil was converted into butadiene. It was confirmed @) that the high-melting and low-melting forms of butadiene tetrabromide are formed in equal amounts. Both were actually isolated and used in calculating the butadiene yield. In another experiment the contents of the liquid nitrogen trap were warmed to 30" C., and the gases evolved were passed through a weighed amount of molten maleic anhydride. The effluent gas was condensed and repassed through the maleic anhydride. Here again the material combined with the maleic anhydride to an extent which indicated that 8 per cent by weight of the gas oil*charged was converted into butadiene. The gas which did not combine with the maleic anhydride was brominated a t -80" C. Besides the expected bromides of ethylene, propene, and butenes, a bromide
FIGURE 1. DIAGRAM OF APPARATUS Constant-head charging buret Feed regulator, a glass capillary containing a closely fitted Nichrome wire C. Manometer D. Lead bath and flash pot E. Furnace and reaction tube F. ThermocouDle. in contact with but outside of reaction tube ' G. Water-cooled condenser H. Vacuum gage I. Ice trap J. Dry ice trap K. Glass-wool-packed fog precipitator L. Liquid nitroqen trap used in position occupied by fop precipitator M. Manometer N. Pressure regulating valve 0. Glycerol-filled Nelson vacuum pump P. Gas sampling outlet 0. Wet test meter R . $Waste or t o dryer and dry ice trap A. B.
C
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firaction was found which t a n e n u m b e r is highb o i 1e d a b o v e t h e butene namely, 90. The density dibromides and which had of t h e g a s o l i n e is 0.837. a, d e n s i t y of 2.33. Since Certainly the possibility of the olefin dibromide boilconverting gas oil into such ing in this r a n g e s h o u l d yields of useful and potenh a v e a d e n s i t y of 1.9, it tially useful products is inmust be a higher bromide, t e r e s t i n g , both from the possibly a t e t r a b r o m i d e . t h e o r e t i c a l and practical Because of the instability point of view. and powerful l a c r i m a t o r y Maleic Anhydride properties of this fraction, Addition Products its constituents could not b e i d e n t i f i e d . It was Earlier in this article it roughly equivalent to 1-2 was mentioned that both p e r c e n t of t h e g a s oil the Cg f r a c t i o n ( b o i l i n g charged. point, 1040" C.) and the . Also in this experiment g a s o l i n e fraction (boiling a liquid hydrocarbon fracpoint, 40-200" C.) contain tion boiling above the C d hydrocarbons which unite hydrocarbons remained in with maleic a n h y d r i d e . t h e l i q u i d nitrogen trap. This is a characteristic propThis was f o u n d a l s o t o e r t y of conjugated dienes contain conjugated dienes. and it has been assumed, I n a third experiment the in the absence of other data, p y r o l y s i s products were that the hydrocarbons are separated into four fracconjugated dienes. Further, tions: (a) the gases boiling s i n c e m a n y of t h e c o n lower than 10" C., ( b ) a C j jugated diene-maleic anhyf r a c t i o n boiling from dride a d d i t i o n p r o d u c t s 10-40" C., (c) a g a s o l i n e (tetrahydrophthalic a n h y BATTERY OF LABORATORY PUMPSWHICHDELIVERS FROM 7 b o i l i n g from 40-200" C., dride homologs) are solids, TO 1000 Cc. OF LIQUIDPER HOTJR and (ci) a residue b o i l i n g there seemed to be a posThey are a n extraordinarily useful tool in research work. above 200°C. which was sibility of identifying the r e g a r d e d as unconverted hydrocarbons in this way. gas oil. By using low-temperature fractionation on the gases Unfortunately the addition products were thick liquids from , in addition to the methods used in the first two experiments, which only a small amount of solid material could be sepathe yields of a number of the products were determined rated. This solid seemed to be a mixture for it began to (Table 111). The yield of butadiene is somewhat lower in liquefy a t about 40" and some solid remained a t 55" C. The this experiment than was found in the first two experiments. amount of the solid was so smaIl that it could not be further The cause for this difference is not known, although it is pospurified. sible that butadiene was not the only conjugated diene deterSome of the addition products were distilled in vacuum mined in the earlier experiments. so that the excess maleic anhydride could be removed. From several experiments about 50 grams of an addition product were obtained which boiled at 125-126" C. under 1 mm. TABLE111. YIELDSOF PRODUCTS FROM PENNSYLVANIA GASOIL pressure. The cryoscopic molecular weight of this product (950' C., 175 mm. pressure, 0.05 second ooctact time) in benzene was 170; titration with alcoholic potassium Yield, hydroxide gave 162. The theoretical molecular weight of a Yield Wt. % of Wt. % bf G ~ oil S pentadiene addition product is 166. Since all three of the Product Charge Cracked conjugated pentadienes (isoprene, piperylene, and cyclopenHydrogen 0.6 0.7 3.0 s .4 Methane tadiene) addition products are solids, the product obtained 24.6 28.2 Ethvlene 9 2 10 5 Ethane here is probably a mixture of two or more of them. 17 2 19 7 Propene As yet the higher boiling addition products have not been 0 9 1 0 Propane Butadiene 5 8 6 6 satisfactorily separated so that appreciable material has a Butenes 7 3 8 3 0 3 0.3 Butanes sufficiently narrow boiling range to indicate pure compounds CS: or a simple mixture. Conjugated dienes 2.7 3.1 Other hydrocarbons Gasoline : Conjugated dienes Other hydrocarbons Residue a t 200' C., b. p. Unaccounted for
3.2
3.7
6.3 6 2
7 2 7 1
12 5 0 2
0 2
Table I11 shows that 7 3 per cent of the charge (83 per cent of the reacted gas oil) is converted into usable olefins or gasoline. The yields of conjugated dienes besides butadiene are extremely interesting. About 50 per cent of the liquid hydrocarbons boiling below 200" C. consists of conjugated dienes. As would be expected, the gasoline has a pronounced tendency to form gum and become discolored, and the oc-
Literature Cited (1) Davidson, J. IND.ENG.CHEM.,10, 901 (1918). (2) Egloff, "Reactions of Pure Hydrocarbons," A. C. S. Monograph 73, New York, Reinhold Publishlng Corm, 1937. (3) Groll, IXD.ENG.CHEW,25, 784 (19331; 26, 697 (1934). (4) Hague and Wheeler, $. Chem. SOC.,1929, 378; Fuel, 8 , 560 (1929) ; IND. ENG.CHEM.,26, 697 (1934!; ( 5 ) Hurd, "Pyrolysis of Carbon Compounds, A. C. S. Monograph 50, New York, Chemical Catalog Co., 1929. (6) Thomas and Carmody, INDEXG.CHEM.,24, 1125 (1932). (7) Tropsch and Mattox, I b i d . , Anal. Ed., 6 , 104 (1934). (8) Willstdtter and Bruce, Ber., 40, 3987 (1907). RECEIVED December 3, 1937.
