INDUSTRIAL AND
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EKGINEERING CHEMISTRY
5 . Intermittent chlorination is economically desirable and requires much less chlorine than continuous chlorination. 6. No detrimental effects, such as corrosion, have been encountered. 7. Manual cleaning of condensers has been considerably reduced. 8. With proper design and location of the chlorine equipment, t h e potential danger which accompanies the handling of chlorine gas may be kept a t a minimum.
Vol. 24, No. 4
LITER.4TURE CITED (1) Boruff, IND. ENQ.CHEM., 22, 1262 (1930). (2) Boruff and Buswell, Illinois State Water Survey, Bull. 28 (1929). (3) Frost and Rippe, W d a c e and Tiernan Co., Tech. Pub. 112 (1930). (4) Morgan and Beck, Sewage Works J . , 1, 46 (1928). (5) National Elec. Light Assoc., Rept. Prime Movers Comm., 1930. ( 6 ) Whipple, "The Microscopy of Drinking Water," p. 341, Wiley, 1927. RECEIVED December 31, 1931
Thermal Decomposition of n-Butane into Primary Products MAXNEUHAUS AND L. F. MAREK R e s e a r c h Laboratory of Applied C h e m i s t r y , Massachusetts I n s t i t u t e of Technology, C a m b r i d g e , Mass.
0
W I N G to the necessity for The pyrolysis of n-butane at 600" and at tion of primary products. Corn6500 was investigated f o r the purpose of deplete p r o d u c t a n a l y s e s were the most profitable utilization of waste gases not a l w a y s c a r r i e d out, and termining the primary products of the thermal from p e t r o l e u m refineries and calculations of yields therefore dissociation. Listed in the Order of their relafrom the fields, as well as t o the involved a s s u m p t i o n s as t o importance of obtaining a knowltive importance, the reaction products were found product compositions. Furtherto be: ( I ) methane, propylene; (2) ethane, more, very few runs a t a paredge of the reaction mechanism ethylene; (3) hydrogen, butylene,. and (4) proticular temperature were made through which t h e paraffin hyby any of the i n v e s t i g a t o r s , drocarbons accurate pane. Neither butadiene nor aromatic hydroand the percentage of n-butane determinations of the primary products of their decomposition carbons were formed as products at these decomposed was rather high in are of both practical and theoretiperatures. Velocity constants for the thermal all cases. Any e s t i m a t i o n of decomposition rates at 609" and 659" C. have trend in the change of prodcal interest. For two principal uct composition with varying reasons the literature, in general, been c&ulated. intensity of cracking as brought can furnish b u t little information about by secondary r e a c t i o n s on primary products. First, in most of the investigations the hydrocarbon under consideration was thus rendered impossible. For these reasons the lack of has been cracked to such an extent t h a t secondary reactions un- agreement among the results is not surprising. doubtedly occurred and obscured the primary reactions. SUMMARY O F YIELDS OBTAINED BY VARIOUS INVESTIQATORS Then, second, accurate methods for the analysis of hydrocarVARIATIONS OBTAINED IN bon mixtures have been developed and brought into general EACH PRODUCT PER use only during the past few years. Owing to the absence of 100 MOLESOF n-BWT.mE RE.ACTIOS PRODUCT REACTINQ complete analytical data, estimations of the products obtained Moles on pyrolysis have actually involved a priori assumptions as to 1 Methane and ropylene 36 to 87 2 Ethane and etgylene 22 to 46 the nature of the dissociation reactions. T h e recent work of 3 Hydrogen and butylene 5 to 20 Schneider and Frolich ( I O ) has involved the thermal dissocia4 Hydrogen and butadiene 0 to 4 tion of propane, propylene, ethylene, and butadiene. The use METHOD OF ATTACK of low-temperature fractionation as a n aid in carrying out the product analyses in this work, and the cracking of t h e several I n order t o determine the primary products formed in the hydrocarbons to various extents permitted satisfactory estimapyrolysis of hydrocarbon, it is necessary either to eliminate tion of t h e primary products. secondary reactions entirely, or to determine in some manner the effect of secondary reactions on the relative proportions of PREVIOUS WORK the products formed. The complete elimination of secondary Pease ( 7 ) , Pease and Durgan ( 8 ) , Hague and Wheeler ( d ) , reactions could possibly be brought about b y cracking the and H u r d a n d Spence ( 6 ) have investigated the pyrolysis of hydrocarbon t o only a very small extent. However, the n-butane. T h e primary decomposition reactions suggested difficulty of separating very small quantities of products from in the above contributions were: the original hydrocarbon and the inherent chances of error involved in the quantitative estimation of such small quantities serve to render this method rather impractical. The effects of the secondary reactions in the pyrolysis may be determined quite simply, however, by cracking the hydroA summary of the yields obtained by the various investigators carbon in varying degree and then by plotting the results in such a manner t h a t extrapolation of the curves t o zero is given in t h e table which follows. This table includes all d a t a available on the pyrolysis of n- per cent reacting is easily carried out. I n this way the butane over the temperature range 600-700" C. Only Hurd analytical difficulties involved in accurately determining and Spence ( 5 ) were directly concerned with the determina- very small amounts of a given constituent are eliminated.
c.
April, 1932
I K D U S T R I A L A K 1> E S G I X E E R I N G C H E M I S T R Y
The latter method employed by Schneider and Frolich ( I O ) was also used in carrying out this work. The data were plotted as moles of each product formed per 100 moles of n-butane reacting us. percentage of n-butane reacting, the curves so obtained being extrapolated to zero per cent cracked. -4clear distinction between secondary and primary products was hereby obtained, in that the curves for the secondary products passed definitely through the origin, while those for primary products cut the ordinate axis a t points representing the proportions of the products formed when the butane just began to decompose. That is to say, if the propylene curve, for example, intersected the ordinate axis a t 50, then 50 moles of propylene were formed initially in the cracking of 100 moles of n-butane. It must be remembered, h o w e v e r , that although this method d i f f e r e n t i a t e s between s e c o n d a r y and primary products, it does not permit the detection of u n s t a b l e intermediates, such as free radicals.
40 1
carbon dioxide could be obtained. The explosion data sufficed for those fractions which contained only two saturated hydrocarbons, but the presence of a third prevented the determination of the composition of the mixture from the explosion data alone. I t was possible, however, to determine the concentration of the third saturated hydrocarbon in some independent manner whenever this condition arose. For example, the propane-propylene fraction was always found to contain small amounts of n-butane and ethane, as well as propane. The amount of butane present was estimated from the fractionation curve obtained in making the cut, and the quantities of the other components present were then calculated from the explosion data. As the noncondensed gas also contained three saturated hydrocarbons, the explosion data alone did not serve to fix its composition. Since no fractionation of this gas could be carried out with the equipment available, it was necessary to find some other method of estimating the third hydrocarbon. It was seen that the ratio of moles of ethane to moles of ethylene in the liquid could be calculated from the analyses of the Davis-column fractions. As the concentration of the ethylene in the noncondensed gas was already known from absorption data, it seemed possible that the percentage of ethane in the gas could be calculated by simple proportion, provided that the relative volatility, in butane solution, of ethane with respect to ethylene could be obtained. The data compiled by Copson and Frolich ( 1 ) EXPERIMENTALprovided vapor pressures for the two compounds; and, by PROCEDURE assuming a Raoult's law deviation of -10 per cent for the ethylene, the relative volatility was calculated to be 0.525. T h e n - b u t a n e Estimation of the ethane by this method and calculation of the used in this investi- percentages of methane and butane from the explosion data gation was obtained were found to yield consistent results. by refractionation I n those fractions in which the olefin concentration was PER CWT OF N-BUTANE REPCTING of a b u t a n e c u t small, titration of the gas samples with 0.02 N potassium FIGURE1. CRACKIKG OF 72-BUTANE f 11r n i s h e d by t h e bromide-potassium bromate solution was employed. The AT 600' C . Humble Oil and Re- method used was a modification of that described by Franfining Company. As used, the ?A-butane contained 1.6 per cis (a'). cent isobutane, no propane, and no pentane. RESULTS .4XD DISCUSSION The apparatus in which the thermal decomposition of the The data obtained in the pyrolysis of n-butane are shown n-butane was carried out was a duplicate of that described by graphically in Figures 1 and 2. Extrapolation of the curves Schneider and Frolich ( I O ) . drawn through the experimental points to zero per cent In making a run, the n-butane was passed through a flowmeter, scrubbed %-ithconcentrated sulfuric acid, and dried over cracked indicate that the primary products formed in the calcium chloride. The gas was then passed through a 24 X 1 pyrolysis of n-butane a t 600" and a t 650" C. were as shown in inch (61 X 2.5 cm.) fused silica preheater maintained at a tem- Table I. perature of 540' C., and from the preheater it entered directly TABLE I. DISTRIBUTION O F PRIM.4RY PRODUCTS FROM into a 24 X 0.5 inch (61 X 1.25 cm.) fused silica reactor which THERMAL DISSOCIATION O F n-BnTkvE was held a t the desired cracking temperature. The true temperature of the gas leaving the reactor wm measured by means PRODUCT AMOUNT FORXED P E R 100 h f O L E s n-BUTANE R E4CTINQ of a compensated thermocouple. Upon leaving the reactor, At 600' C. At 650' C. the decomposition products and the remaining n-butane then Moles Moles passed into a cold trap immersed in a mixture of solid carMethane 48.5 48.0 bon dioxide and acetone. The gas not condensing in the Propylene 48.5 48.0 cold trap was collected in a calibrated gas bottle by displaceEthane 34.5 36.7 Eth lene 34.5 38.7 ment of saturated zinc sulfate solution. This gas contained Hy Aogen 16.0 12.3 methane, ethane, ethylene, propylene, and n-butane, as well as Butylenes 16.0 12.3 all the hydrogen formed in the pyrolysis. The liquid condensing Propane 0 1.0 (7) in the cold trap contained most of the undecomposed n-butane, propylene, propane, ethylene, ethane, and methane. The As the weighing of the cold-trap condensate obtained in quantity of propane formed was never so great as to cause an these runs could well have brought about loss of methane (the appreciable amount of it to be present in the noncondensed as. most volatile component present in the liquid, particularly in Upon completion of a run, the liquid which had collectefi in the cold trap was fractionated in a Davis column (2). The those cases in which the degree of cracking was high), the various fractions were collected over saturated zinc sulfate propylene curve was given full weight in the estimation of solution in displacement burets. With the exception of those primary products a t 650" C. The propane determinations tractions in which the percentage of olefins was inordinately low, analyses were carried out in a Williams gas-analysis apparatus, were probably not as accurate as were the determinations of in which the usual absorbents were employed. Hydrogen was the other constituents, because of the fact that only very small determined by combustion over copper oxide at 300' C., and quantities of propane were formed in the pyrolysis. At 600" saturated hydrocarbons were estimated by explosion with oxygen. C. the propane curve seems to pass definitely through the Explosions were carried out n-ith small gas samples with oxygen origin, indicating that propane is a secondary product at this present in excess. temperature. At 650" C., however, the indications are that From three to six explosions were made in each case in order approximately 1 mole of propane is primarily formed per 100 that accurate average values for the ratio of contraction to moles of n-butane reacting. It is not believed, however, that
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the determinations of propane in such small amounts were sufficiently accurate to warrant a positive statement as to whether or not propane was formed as a primary product. The rather rapid decrease in the relative amount of ethane found in the products as a degree of cracking increased is worthy of note. I n order to determine the nature of the butylenes formed in the pyrolysis, a long-time run was made a t 650" C. The butane-butylene fraction was brominated a t 0" C., the bromination temperature being kept low in order to prevent substitution reactions. The fractionation of the bromides and the refractive indices of the fractions so obtained indicated that approximately 20 per cent of the butylene formed in the pyrolysis was 1-butene, while the remainder was 2butene. The absence of butadienetetrabromide in the brominated product indicated that no butadiene had been formed.
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products of n-butane, the velocity constants for the over-all decomposition have been calculated from the present data for their practical and theoretical interest. The general first-order rate equation was used: 2.303 log (1/1
where z
- 3)
=
kt
fraction of n-butane uiidergoing thermal dissociation t av. time of contact, seconds k = velocity constant I n calculating the rate constants, it was assumed that no cracking took place in the preheater. It was assumed further that, for the purpose of calculating the time of contact for each run, the temperature of the gas entering the reactor was 540" C., the temperature of the preheater wall. The gas temperature used in calculating time of contact was the arithmetic mean of this temperature and the temperature of the exit gas as measured by a compensated thermocouple. Reverse reactions were neglected in making the calculations, since the extent of decomposition was small in most cases. The calculated data are shown in Table 11. = =
TABLE11. RATE O F DECOMPOSITION O F n-BUTANE TEXP.OF CALCD. .&V. EXITGAE TEMP.
a .
3v
' 0
2
4
6
8
IO
1 2
14
I6
18
20
22
PER CENT OF N-BUTANE REACTING
I
OF B BUTANE FIGURE 2. CRACKING
AT
650' C.
Seither tars nor aromatic hydrocarbons were formed in appreciable amounts in any of these runs. The fractionation data, as well as analyses of the gases remaining in the still and column a t the end of the distillation, failed to indicate the presence of any hydrocarbon having a molecular weight greater than that of n-butane. This latter fact seems to indicate that propane is not formed in the pyrolysis through a bimolecular reaction of the type found by Schneider and Frolich (IO)to take place in the thermal dissociation of propane, in which ethane and butane were formed. A similar reaction in the case of n-butane would yield propane and pentane. Because of the relative thermal instability of pentane, the proportions of pentane in the product, if it were formed by such a reaction, would decrease as the extent of reaction increased. It is probable that the analytical methods employed would not permit detection of the small amounts of pentane that could form in this way. It is interesting to note that the cracking mechanism proposed by Rice (9) on the basis of free-radical formation gives a ratio of the extent of occurrence of the two major reactions (1and 2) which approximates very closely that obtained in the present work:
+
TEMP., C. 600 650
ETHANE ETHYLENE (REACTION 2) METHANE (REACTION 1) .~~ 4-PROPYLENE Predicted by Experimentally Rice determined 0.75 0.71 0.79 0.79
However, the primary formation of butylenes and hydrogen in the pyrolysis of n-butane is not accounted for by this proposed mechanism. Although the principal object of this work was to determine the distribution of the primary thermal-decomposition
n-BuTANm RATEOF REACTINQ FEED" O c. c. % Cc./sec. 2.14 600 570 4.4 4.4 2.39 2.42 4.4 ... 3.3 3.92 4.31 2.7 5.34 2.6 ... 2.2 6.12 ... ... 595 14.6 3.79 650 5.51 9.7 ... ... 6.08 6.8 ... *.. 5.3 ... 12.60 3.4 18.77 At 0' C. and 1 atm. pressure.
... ... ...
... ... ... ...
.... ..
...
.
.
I
VELOCITY
CONSTANT k
0,00457 0.00500 0.00518 0.00624 0.00564 0.00673 0.00668 0.0200 0.0268 0.0215 0.0346 0.0344
The velocity constants shown in Table I1 for the 600" C. runs a t low feed rate and for the 650" C. runs a t intermediate feed rates approximate those obtained for n-butane a t 570" and 595" C., respectively, in a separate, unpublished investigation, The thermal-decomposition rates of the lower members of the paraffin hydrocarbon series are being investigated in a special apparatus (6) designed to give an accurate knowledge of temperature and time of contact. CONCLUSIONS The primary products of the thermal decomposition of nbutane a t 600" and 650" C. were found to be as follows: PRODUCT
AMOUNT FORMED P E R 100 MOLES n-BUTANE REACTINQ At 600' C. At 650' C . Moles Moles
Over the temperature range investigated and with the times of contact used, neither butadiene nor aromatic hydrocarbons were formed. No hydrocarbons of molecular weight higher than that of butane were detected in the reaction products.
ACKBOWLEDGMENT The authors wish to acknowledge the assistance given by the Humble Oil and Refining Company toward this work. LITERaTURE
CITED
(1) Copson and Frolich, IND.ENQ.CREM.,21, 1116 (1929). (2) Davis, Ibid., Anal. Ed., 1, 61 (1929). (3) Francis, IND. ENQ.CHEM.,18, 821 (1926). (4) Hague and Wheeler, J. Chem. SOC..1929, 378. (5) Hurd and Spence, J. Am. Chem. Soc., 51, 3353 (1929). (6) M a r e k and McCluer, IND.ENQ.CHEM.,23, 878 (1931). (7) Pease, J. Am. Chem. SOC.,50, 1779 (1928). (8) Pease and Durgan, Ibid., 52, 1262 (1930). (9) Rice, Ibid., 53, 1959 (1931). (10) Schneider and Frolich, IND.EXG.CHEM.,23, 1406 (1931). RECEIVED December 28, 1931
