Considerable work has been dunr on the thermal decomposition of cyclohexane, and under proper conditions the reaction is reasonably clean-cut to yield hydrogen, ethylene, and butadiene ( I , 3 , 4 , 5 ) ,presumably by the reaction:
LLOYD BERG, GEORGE L. S U M N E R , J R . , A N D C.W. MONTGOMERY ,GULF RESEARCH & DEVELOPMENT COMPANY. PITTSBURGH. PA.
J A M E S COULL UNIVERSITY OF PITTSBURGH P I T T S B U R G H , PA,
A
CONSIDERABLE awourit of work has been done on the Experimental data, as well as thermodynamic considerations, indicate butadiene production to be favored by low pressure. At higher pressures it has been shown that the pyrolysis of cyclohexane yields little or no butadiene ( 3 ) . One cheap and abundant Source of cyclohexane is the Cg Eraction of a naphthenic petroleum Such a fraction from a properly selected petroleum often contains more than 50% naphthenes. Unfortunately the naphthenes in this fraction are scldom cyclohexane exclusively, but invaiiably comprise methylcyclopentane as well as cyclohexane. Figure 1 shows the results of a precision fractionation of a naphthenic gasoline, and the large amount of methylcyclopentane as well as cyclohexane in the Ce fraction can be seen from the refractive index curve. The boiling point and the refractive index at 20" C. are: for cyclohexane, 80.8" C. and 1 4262; for methylcyclopentane, 71.8" C. and 1.4098. Little previous woilr has been reported on the pyrolgiir of methylcyclopentane. KamnskiY and Plate reported that at 650' C. the gaseous products consisted chiefly of propylene and. some isobutylene (9). The two principal reactions appeared
pyrolysis of normally liquid hydrocarbons at conditions conducive to the formation of butadiene. Under suitable conditions of temperature and contact time, usually above 13M)'F. and below 1second, all normally liquid hydrocarbons yield some butadiene upon thermal decomposition. The nature of the hydrocarbon decomposed has a marked effect upon the yield of butadiene. Other conditions being equal, the yield of butadiene increases for different charge stocks in the order: aromatics, raraffins, olefins, naphthenes, unsaturated naphthenes. Conbidering the decomposition of normally liquid pure hydrocarbons, cyclohexene appears t o give the highest yield of butadiene on thermal decomposition. Follovang in decreasing order of yield, other normally liquid pure hydrocarbons Khich give high yields of butadiene upon thermal cracking are: 3-methylcyclohexene, cyclohexane, ethylcyclohexane, methylcyclohexane, 1,4-,1,2-, and 1,3-dimethylcyclohexanes,and decalin. If the compounds not wadily available are eliminatpd from this list, cyclohexane appears to be the best of the normally liquid hydrocarbons for the production of butadiene by thermal cracking.
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OPERATION OP EQUIPMENT
-1I.5200 - I.5100 - 1.5000 - 1.4900 - 1.4000 - 1.4700
Figure 2 is a diagram of the equipment. Xaphthene and water, when a diluent was used, were fed from graduated separatory funnels into slurry pumps equipped with neoprene tubes. Slurry pumps normally deliver the feed with a pulsating flow. To eliminate this pulsation, which might have caused - 1.4600 unevenness of flow through the reactor, REFRACTIVE INDEX - 1.4500 a valve was introduced into the line % a beyond the pump, and a back pressure - 1.4400 of about 5 pounds was maintained on - 1.4x)O 1: 0 the pump. The naphthene (and water - 1.4200 if used) was discharged into a Pyrex - 1.4100 glass vaporizer. The vaporizer was filled 6 with '/2-inch Berl saddles and sur1.4000 d L rounded by an electrical heater. Tlie 1.3800 a vapors passed out through an electri- l.3000 cally heated Pyrex tube, which acted as a preheater, to a Vitreosil quartz reac- 1.3700 tion tube, 70 cm. long and 1.3 cm. inside - 1.3600 diameter and having a volume of 93 cc. - 19500 Three indentations, evenly spaced, were made in the quartz tube and chromcl- I.3400 alumel thermocouples inserted in them. The quartz reaction tube was fitted IO 20 30 40 50 60 70 80 90 100 VOLUME PERCENT OF CHARGE DISTILLED into a 1-inch i.d. refractory combustion tube which was heated with a 2000Figure 1. Precision Fractionation of a Naphthenic Straight-Run Gasoline watt Nichrome winding. The temperature was maintained so that, when plotted against reaction tube length, the areas above and to be, first, splitting into two niolecules of propylene and, secbelow the average temperature balanced. The highest temond, splitting into a molecule of ethylene and a molecule of isoperature which occurred near the middle of the tube was about butylene. 15-30' C. above the average. The apparatus may yield some This paper is concerned with a laboratory-scale investigation error in the absolute value of temperature; however, the temof the thermal noAcatalxtic decomposition of the common sixperature differentials, from which the conclusions were drawn, are carbon naphthenes under conditions conducive to the formation accurate. At the exit end of the reaction tube, distilled water of butadiene. The work was confined to ranges of temperature was supplied a t a constant rate of 70 ml. per minute by means of and contact time which give a maximum yield of butadiene. another slurry pump to act as a quench for the hot gases emitted Each of these variables was studied independently so that peak from the reaction tube. The products were condensed by passing yields of butadiene were obtained and optimum conditions thus through Liebig condensers. The liquid product was collected in established. The effect of a diluent was investigated by carrya cooled., jacketed receiver where the water was separated and ing out a series of thermal decompositions in the presence of an" continuously recycled to the quench pump. The hydrocarbon proximately 4 moles of steam per mole of naphthene.
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r
I
THE PYROLYSIS of t h e naphthenes, cyclohexane, and methylcyclopentane, has been studied over a range of temperat u r e and contact times favorable t o t h e formation of butadiene. Cyclohexanegivesa good yield of butadiene, while methylcyclopentane i s a relatively poor source. Maximum yields of butadiene from cyclohexane were obtained when temperature and contact t i m e were regulated t o give 35 pounds of 3-carbon and lighter compounds per 100 pounds of cyclohexane charged. The pict u r e shows t h e northwest portion of Neches Butane Products Company a t Port Neches, Texas (courtesy, The Lummus Company)
PRESSURE RELEASE PLUG
BACK PRESSURE VAL-
H.C. FEED
...
Figure 2.
Diagram of Equipment
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with maleic anhydride, isobutylene by reaction with anhydrous hydrogen chloride, and the noncondensable gases (hydrogen and air) by Orsat analysis. The amount of gas dissolved in the liquid product was determined in a low-temperature Podbielniak column and analyzed as above. The gas density was determined by comparison with air in an Acme specific gravity balance. Analysis of the 1-acetylenes showed that, even under the most drastic conditions here reported, the amount did not exceed 0.5% of the gaseous product. It appears, then, that the total acetylenes are very small under these conditions. RESULTS
In the following discussion the word "conversion" refers t o the pounds of product obtained per 100 pounds of naphthene charged-that is, the products per pass. The word "yield" refers to the pounds of product obtained per 100 pounds of naphthene decomposed, or the ultimate recycle yield. A large number of runs was made over a temperature range of 1300" to 1700" F. and at contact times of 0.1 to 1.2 seconds. The results of these runs are summarized in Figures 3,4, and 5. Figure 3 is a plot of the conversion to gaseous products against temperature a t a relatively constant contact time of 0.4 * 0.1 second. Thegaseous 1300 1400 1500 1600 1400 I500 1600 products from both cyclohexane and methylTEMPERATURE - DEGREES F cyclopentane are presented on the same plot Figure 3. Effect of Temperature on Conversion t o Gaseous Products to emphasize the difference in decomposition of the two naphthenes. Under these conditions gases passed through a saturated salt water trap in a cooled jackthe pyrolysis products of both naphthenes were highly olefinic. eted receiver where, a t higher reaction temperature or high flow Methane and a small amount of ethane were the only rate, some hydrocarbon liquid condensed. The uncondensed paraffins formed. The 3-carbon a;"d 4-carbon fractions gases passed through a gallon surge bottle into a wettest gas meter for measurement. From there they either passed out the blowdown line or were switched to a gas bottle and collected over saturated salt mater for analysis. The system was put together with ground-glass joints. A ground-glass joint with a sealed tltbe on the vaporizer held by rubber bands acted as a pressure release. The condensing system, cold trap, and jacketed receivers mere connected in series, and cold methanol was circulated by means of a small centrifugal pump. The methanol was kept at 0-15" C. by circulation through a copper coil immersed in a trichloroethylene-dry ice l o - I I 1 lo1 1 I I 1 bath. '3 oil 0.3 0.5 0.7 0.9 0.1 03 0.5 0.7 0.9 W The operating times and amounts of naphthene and __ FROM METHYLCYCLOPENTANE 3 --- FROM CYCLOHEXANE TEMPERATURE = 1500 OF water introduced were noted, and the rate of input was I 1 I I I I I I calculated for each run. Rate of hydrocarbon input varied from 1.0 to 55 ml. per 2 5 minute. The contact time was determined by an arith0 v) metic average of the contact times, calculated from d w 4 the entrance and exit gas volumes. The liquid product > z was separated from the water and weighed. The total 0 v3 gas evolved mas measured by the gas meter. ANA LYSIS
The liquid product was rectified in a &foot glass fractionating column packed with '/(-inch stainlesssteel carding teeth to determine gas, naphthene, and residue. The gas was rectified in a low-temperature microfractionating column. The unsaturates in each fraction were determined by bromination-butadiene by reaction
2
1
0 0.1
0.3
05
CONTACT
Figure 4.
0.7
TIME
0.9 0.1
-
0.3
03
0.7
0.9
SECONDS
Effect of Contact T i m e on Conversion t o Gaseous Products
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show, however, that hydrogen comprises 15 to 30 mole per cent of the gaseous product. The residue shown on Figure 5 represents all the liquid or solid products except the original naphthene. It includes all products from the light liquids to the heavy tars and coke. Thus, in methylcyclopentane pyrolysis, the minimum residue observed a t about 1500’ F. or 0.5-second contact time merely denotes maximum gasification a t those conditions. At the more drastic conditions methylcyclopentane showed an increase in the amount of tar and coke formation, whereas cyclohexane tended toward more complete gasification. For both naphthenes above 1500” F. the coke lay-down became appreciable; below that temperature, except a t the longest contact times, coke formation was CYCLMXANE TEMPERATURE =1500 METHYLCYCLOPENTANE- TEMPERATURE~15W°F. not an important factor. The use of diluent w&s investigated by cracking the hydrocarbon in the presence of 4 moles of steam. Little difference in results was observed between the runs with and without steam. A small increase in the optimum amount of butadiene from cyclohexane was noted. The yield was about 12 pounds per 100 pounds of charge with steam and about 9 pounds in the absence of a diluent. The optimum conditions are the same, either with or without steam, a maximum conversion to butadiene being obtained a t 1500-1600” F. and 0.3-0.5 second contact time. The advantage of using a diluent appears to be principally as a means of attaining the high temperature quickly and the deCONTACT TIME SECONDS sired short contact time. A relation between contact time and temperaFigure 5. Ultimate Yield of Gaseous Products on a Cumulative Basis ture may be obtained by considering the depth of cracking-namely, the pounds of C8 and lighter products per 1 0 0 pounds-of charge. The data show were always 100% olefinic over this range of conditions. Figure that a maximum amount of butadiene is produced from cyclo3 shows that cyclohexane cracks predominantly to hydrogen, hexane when the conditions of temperature and contact time ethylene, and butadiene. From methylcyclopentane, however, are so regulated as to produce about 35 pounds of Ca and no such simple cracking reaction appears to take place. Propyllighter per 100 pounds of cyclohexane charged, regardless of ene is the most abundant product except at the highest temwhether a diluent is used. perature, where the Ca and Cq products as well as the methylThe results of the pyrolysis of cyclohexane indicate that cyclopentane appear t o be decomposing. This would indicate shorter contact times favor a higher percentage of butadiene in considerable cracking of the methylcyclopentane molecule into the Cd fraction. They also show that, at constant contact time two Cs molecules. The Cq products are about equally divided in the temperature range 1400-1600° F., lower temperatures among 1- and 2-butenes, butadiene, and isobutylene, an indicayield higher percentages of butadiene in this fraction. These tion that carbon-carbon fission takes place anywhere in the statements apply only to the quality of the Ca fraction, not t o the methylcyclopentane molecule. The fact that 1- and 2-butenes quantity of this fraction or actual amount of butadiene produced. plus butadiene exceed isobutylene is in accord, since methylcyclopentane can be split in several ways to yield a normal 4-carbon chain, but in only one way t o form isobutylene. The ACKNOWLEDGMENT considerable quantity of methane produced from methylcyclopenThe authors wish to acknowledge the cooperation of D. H. tane even a t relatively mild conditions indicates cracking t o CI Liohtenfels and V. N. Hurd for gas and liquid analyses, respecfragments as well. tively, and of W. E. Barr for the fabrication of special quartz At 1600” F. the gaseous product appears to be resolving into equipment. hydrogen, methane, and ethylene only; the other gaseous products tend t o diminish. At longer contact times this is even more LITERATURE CITED pronounced, as Figure 4 shows. Figure 4 is analogous to Figure (1) Frolich, P. K., IND. ENG.CHEM.,22,240 (1930). 3 except that the conversion to gaseous products is plotted against (2) Haensel, V., and Ipatieff, V. N., IND. ENG.CHEM.,35, 632 (1943). contact time, the temperature being held constant a t about 1500°F. (3) Kazanskil, B. A., and Plate, A. F., Ber., 67, 1023 (1934). The similarity in the shape and trends of the curves of Figure 4 and Katz, M., IND. ENG.CHEM., 25, 1388 (1933). (4) Whitby, G. S., and Figure 3 shows that increased temperature and lengthened (5) Zelmskii, N. D., Mikhailov, B. M., and Arbuaov, Y.A., J . Gew. Chem. (U.S.S.R.), 4 , 8 5 6 (1934). contact time have essentially the same effect upon the pyrolysis products. Figure 5 shows the ultimate yield of gaseous products us. PRESENTED in two parts (entitled “Pyrolysis of Cyclohexane” and “Pyrolysis of Methylcyclopentane”) before the Division of Industrial and Engineertemperature and contact time plotted on a cumulative basis. ing Chemistry a t the 107th and 108th Meetings of the AMERICAN CHEMICAL Thus, the distance from the abscissa to the upper butadiene or SOCIETYi n Cleveland, Ohio, and New York, N. Y., respectively. Based 1- and 2-butene line represents the total gaseous product. Since upon a thesis submitted b y George L. Sumner, Jr., t o the faculty of the Unithe curves are plotted on a weight rather than a volume or molar versity of Pittsburgh, in partial fulfillment of the requirements for the degree of Master of Science in Chemical Engineering. basis, the amount of hydrogen is not readily apparent. The data
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