878
INDUSTRIAL _ AND _ ENGINEERING CHEMISTRY
Vol. 23, No. 8
Velocity Constants for the Thermal Dissociation of Ethane and Propane' L. F. Marek and W. B. McCluer RESEARCH LABORATORY OF APPLIED CHEMISTRY. DEPARTMENT 08 CHEMICAL ENGINEERING. MASSACHUSETTS INSTITUTEOB TECHNOLOGY, CAMBRIDGB, MASS.
This paper presents the results of an investigation HE industrial i m p o r A reactor, designed to allow to determine the velocity constants for the dissociation tance of commercial dissipation of the heat of reacof ethane and propane and the correlation of these oil- and gas-cracking tion and a peak temperature data with values calculated from data on the pyrolysis operations is recognized genof 160' C., gave complete conof these hydrocarbons as obtained from the literature. erally although very little is version without the formaThe pure gases were quickly preheated to reaction known of the m e c h a n i s m s tion of methane. -4slight temperature and passed through a copper reactor at t,hrough which these reactions excess of ethylene was used carefully controlled temperatures and short times of occur. This condition is due and later removed by absorpcontact. The extent of cracking in the reactor was primarily to the fact that the tion in b r o m i n e water foldetermined by the difference in composition of the majority of the a v a i l a b l e lowed by scrubbing with conpreheater and reaction coil samples and values for the data have been obtained from centrated caustic s o l u t i o n . velocity constants were calculated from the general the c r a c k i n g of exceedingly The purity of the resultant equation for unimolecular homogeneous gas reactions. complex oil and gas mixtures gas was determined by voluIn the temperature range investigated, the following where the results are so commetric analyses and density equations serve to reproduce satisfactorily the values plicated that no a c c u r a t e measurements and found to of the velocity constants as calculated from the experianalysis of the mechanisms or be better than 99.5 per cent. mental data: rates of dissociation of the The propane was prepared separate components can be by fractionating Pyrofax in a 15,970 log ~ c =~ 15.12 H ~- __ made. It is proposed, therepressure still. The top and T fore, to study the thermal bebottom f r a c t i o n s were re13,500 havior of pure components in jected and the middle cut was log ~ c ~ = H ,13.44 - T order that the results may be used a f t e r s c r u b b i n g with interpreted a c c u r a t e l y and concentrated sulfuric a c i d serve as a basis for the analysis of the data previously ob- to remove small amounts of propylene. The purity of the final tained on the cracking of gas and oil mixtures. product was determined by liquid air fractionation and denSeveral difficulties are encountered in working with hydro- sity measurements to be approximately 99 per cent. carbons of high molecular weight. The difficulty of preparing Apparatus and Procedure a high molecular weight hydrocarbon in a pure state is not inconsiderable and the ordinary methods of analyzing the liquid The complete apparatus is shown schematically in Figure 1. and gaseous products are decidedly unsatisfactory. Even if satisfactory methods of purifying the hydrocarbon and analyz- The hydrocarbon gas was stored in the reservoir, forced out b y ing the reaction products were available, it would be difficult practically air-free water, dried by calcium chloride, metered to interpret the results accurately unless the thermal stability by a calibrated flowmeter, passed into the preheater, and of the reaction products and the rate of secondary cracking then into the reaction coil. Samples of the exit gas from the reactions were known. For this reason, it is advisable to preheater and from the reaction coil were collected in aspirator study the thermal decomposition of the simpler hydrocarbons bottles a t atmospheric pressure, and the exit rates of gas flow were determined by the time required for the collection of before going to the more complex. The lower molecular weight hydrocarbons may be pre- two liters of the saturated zinc sulfate confining solution. The preheater and reaction coil are shown in Figure 2. pared in a pure state without considerable difficulty and the reaction products analyzed with reasonable ease by the usual The preheater consisted of a copper block electrically heated gasometric analytical methods. Considerable data are avail- to a temperature somewhat higher than reaction coil temperaable on the pyrolysis of these hydrocarbons, although greater ture. Frey and Smith (a) have shown that copper has a emphasis has been given to the experimental difficulties in- negligible catalytic effect on the thermal decomposition of volved in studying the mechanism of decomposition than propane. The gas entered a t the top and passed through a to the rate of dissociation. The rate factor seems to be just hole drilled the full length of the block. d quartz thermoas important as studies of mechanism for interpreting com- couple well occupied the center of the hole and the annular merical oil- and gas-cracking operations and therefore deserves space was filled with short segments of copper wire to give accurate investigation. For the purpose of obtaining these turbulent flow to the gas and to improve the heat transfer. data, a study of the hydrocarbons of lower molecular weight The true temperature of the gas stream was measured by a a t atmospheric pressure in an apparatus permitting accurate thermocouple as the gas left the preheater, the wall temcontrol of temperature and time of contact has been initiated. perature a t the point of measurement being that of the lead This paper presents the results of the work on ethane and bath. A sampling tube was provided a t the bottom of the preheater just before the reaction coil so that the composition propane. The ethane used in these experiments was prepared by of the gas entering the reaction coil was exactly known. The reaction coil was formed from a 12-foot (3.65-meter), passing pure ethylene (specified for use in anesthesia) and hydrogen over a reduced nickel catalyst supported on pumice. copper-lined steel tube having an internal volume of 30 cubic centimeters. This coil was immersed in an electrically heated 1 Received March 17, 1931. Presented before the Division of Pelead bath, the temperature of which was measured by a troleum Chemistry at the 81st Meeting of the American Chemical Society, thermocouple. During a run the temperature of the preIndianapolis, Ind., March 30 to April 3, 1931.
T
I S D USTRIAL A X D ENGINEERING CHEMISTRY
August, 1931
879
Discussion The results of a number of recent investigators (d,S, 4 , 5 , 7 ) have indicated rather definitely that the decomposition reactions involved in the pyrolysis of the low molecular weight hydrocarbons are unimolecular and homogeneous. On the basis of these data, the velocity constants for the thermal decomposition of ethane and propane have been calcu!ated from the experimental data obtained by the usual rate equation for first order homogeneous gas reactions. This literature shows further that velocity constants have been calculated on the basis of both change in concentration and percentage decomposition. In this investigation, the change in concentration of the hydrocarbon gas occurring in the reaction coil has been used to calculate values for the velocity constant, since this basis simplified the calculations required in correcting for the reverse reaction. It was considered probable that secondary reactions such as the demethanization and polymerization of ethylene might constitute a significant experimental difficulty. Experiments were made, however, with nitrogen-ethylene mixtures c to determine the extent of ethylene decomposition under the experimental conditions used and these data showed the loss of ethylene from this source to be slight. The accepted mechanisms for the thermal decomposition of propane consist of simultaneous dehydrogenation to propylene and demethanization to ethylene which give a product composed of methane, ethylene, hydrogen, propylene, and undecomposed propane. Since methane is relatively stable a t the temperatures employed and loss of ethylene through secondary reactions was Figure 1 -Flow Sheet of Hydrocarbon-Gas Cracking Apparatus p r e v i o u s l y shown to be slight, the only probable exA more complicated method of analysis was required in the perimental difficulty which work with propane than with ethane. Each gas sample was m i g h t b e encountered in first separated with a carbon dioxide-snow column and the work with propane would be lighter constituents further fractionated on :t liquid-air through the further decomcolumn. In operation, cuts were made a t predetermined position and polymerization temperatures, which were close to the boiling point of the of t h e i n i t i a l l y f o r m e d next higher constituent, in order to minimize the column propylene. The extent of holdup. The distillation products were collected in inverted such secondary r e a c t i o n s burets connected to the column by a manifold and the was not investigated experiseparate fractions were then analyzed. Propylene and m e n t a l l y , since data obethylene were determined by absorption in 87 per cent sulfuric tained by Hurd and Meinert acid followed by absorption in bromine water. Davis and (3) on t h e p y r o l y s i s of Quiggle ( 1 ) have shown that these olefins may be separated propylene h a d i n d i c a t e d effectively in this manner. The remainder of the analysis that loss from this source was made by the usual methods. would be negligible when small amounts of propylene Results were initially formed. It The analyses of the gas samples as determined on the w a s concluded, therefore, Burrell apparatus showed negligible amounts of methane that the loss of reaction formed during the cracking of ethane. However, in order p r o d u c t s t h r o u g h these to indicate this more definitely, the coil sample of run 19, in probable mechanisms was which the greatest amount of cracking occurred, was fraction- negligible. The above considerations ated in a liquid-air column and a methane content of only 2.0 per cent was found. The polymerization tendency of do not pertain to the possible ethylene under the conditions of this investigation was deter- loss of unsaturated hydromined by making runs in the usual manner with ethylene- c a r b o n s i n t h e reaction nitrogen mixtures. At 675" C. and a time of contact of product by reversal of the Figure 2-Apparatus for Cracking Methane and Propane approximately 14 seconds, the ethylene concentration de- d e c o m p o s i t i o n reaction. creased from 19.1 to 18.0 per cent, and a t 700" C. and a time Values f o r t h e f o r w a r d of contact of 7 seconds, the ethylene concentration decreased velocity constant may be corrected for this effect when the from 9.1 to 8.8 per cent. equilibrium constant for the reaction is known. Also, The results of the work with ethane are summarized in cracking the gas to a slight degree only for the purpose of Table I. The velocity constant, as shown in this table, minimizing secondary reactions largely eliminated the neceshas been calculated both with and without correction for sity of applying this correction. However, since Pease and the reverse reaction. Table I1 shows the data obtained in Durgan (6) had previously determined the equilibrium conthe work with propane. The terms Za and Z b designate the stant for mixtures of ethane, ethylene, and hydrogen, the summation of the gaseous constituents, other than the un- values of the velocity constant for the decomposition of cracked gas, in the preheater and reaction coil, respectively. ethane were corrected largely for the purpose of determining
heater block was regulated so that the temperature of the gas leaving the preheater and entering the coil was the same as that of the lead bath. I n the work with ethane, analyses were made on a Burrell gas-analysis apparatus, hydrogen being determined by combustion over copper oxide a t 320" C. and ethylene by absorption in bromine water. The reaction coil sample of one run, in which the percentage cracking was high, was fractionated in a liquid air column to determine the amount of methane formed. Runs with mixtures of ethylene and nitrogen were made in precisely the same way as with ethane, the gas mixture being made up in the gas reservoir.
C L TUBE
COIL
INDUSTRIAL A N D ENGINEERING CHEMISTRY
880
Table I-Thermal
Vol. 23, No. 8
Decomposition of Ethane
MOLEFRACTIONS RUN
TEMP.
PREHEATER
log
REACTION COIL
Za
Hz
CZH4
HZ
Zb
E 1 - Zb
TIMEOF CONTACT
c.
VELOCITY CONSTANT
Uncor.
Cor.
Sec.
6
600
0.002
0.004
0.003
0.009
0.006
0.012
0.0026
7.68
0.00078
0.00079
1 9 10 16
625
0.008 0.006 0.008 0.011
0.008 0.006 0 006 0.021
0.016 0.013 0.021 0.031
0.014 0.016 0.019 0.041
0.016 0,012 0.014 0.032
0.030 0,029 0.040 0.072
0.0062 0.0075 0.0116 0.0183
6.85 10.38 14.80 15.00
0.00208 0.00166 0 00180 0.00281
0.00209 0.00167 0.00181 0.00285
0.013 0 012 0.010 0.013 0.014
0.007 0.015 0 014 0 013 0 015
0 033 0 021 0.021 0.043
0.030 0.030 0.025
0.020 0.027
0.056
0.043 0.055
0.029 0.026 0.029
0.063 0.051 0.046 0 086 0 111
0.0195 0.0108 0.0099 0.0276 0.0383
6.66 3.96 5.16 9.70 13.18
0.00674 0.00628 0.00442 0.00655 0.00669
0 00678 0.00634 0.00444 0.00663 0.00682
0.025
0.018
0.062
0.057
0.043
0 119
0.0359
6.22
0.0133
0.0136
0.082 0.119 0 088
0.040
0.076 0.055
0 166 0 233 0.178
0.0611 0.0809 0.0606
6.50 9.20 4.35
0.0217 0.0203 0.0321
0.0223 0.0217 0.0333
0.106 0.145 0.214
0.094 0,132 0.206
0 223 0 292 0.417
0.0667 0.0885 0.1342
4.15 6.08 11.70
0.0370 0 0335 0.0264
0.0386
4 5
7 8
1
2
668
13 14 15
675
0.020 0.032 0.0255
0.020 0.044 0,029
0.084 0.114 0.0905
17 18 19
700
0.047 0.064 0.097
0.047 0.068 0.109
0.117 0.147 0.203
the magnitude of this factor. In the majority of the data, this correction amounted to less than 5 per cent and it seemed probable, therefore, that the velocity constants for the decomposition of propane, which could not be corrected owing to lack of equilibrium data, were accurate within similar limits.
0.0364 0 0333
the corrected velocity constant for the decomposition of ethane. In this equation K is the equilibrium constant for the reaction ClHs = CzH4 Hz; 0 has the same designation as above; and a and b are the mole fractions of ethylene or hydrogen in the preheater and reaction coil gases, respectively. In Figure 3 the different values of the velocity constant for the decomposition of ethane and propane have been plotted against the reciprocal of the absolute temperature. Also, values of the velocity constant as calculated from data in the literature have been plotted in this figure. The straight lines represent the effect of temperature on the average value of constants obtained. Although there are few data from this investigation on propane, the values obtained are in agreement with those calculated from the literature. In fact, the constants calculated from the literature decrease regularly with increasing time of contact and, when extrapolated to zero time of contact, fall on or very slightly above the line as drawn in Figure 3. The different data on ethane are not in such close agreement, but the data of this investigation are rather extensive and the results are believed sufficient to substantiate the curve as drawn in the plot. From these lines equations may be derived to represent the effect of temperature on the velocity constant for the decomposition of ethane to ethylene and hydrogen:
+
15 970 log k = 15.12 - T
and of propane to propylene, hydrogen, ethylene, and methane : logK = 13.44
Figure 3-Velocity
Constant vs. Reciprocal of Absolute Temperature
The general rate equation for unidirectional firsborder homogeneous gas reactions may be written in the integrated form as follows: k = -2.303 1%- 1 - ZU
1 - Zb where 0, 1-Za, and 1- Z b represent the time of contact expressed in seconds, and the initial and iinal concentration of the hydrocarbon gas, respectively. The following equation 2.303K
b
+K + d w K a i - K - d K T K +K - d K q ) ( a +K+ d K v
ed=log(b ) was obtained by integrating the differential equation which accounts for the reverse reaction and was used in calculating
13,500 -T
These equations indicate that the values of the velocity constant double every 15' C. for ethane and every 18" C. for propane. From the values of the velocity constant for the decomposition of ethane and those for the equilibrium constant, the velocity constant for the hydrogenation of ethylene was determined. The values at 600', 650°, and 700' C. were found t o be 0.021, 0.079, and 0.25, respectively, and show a considerably higher rate of reaction for the hydrogenation of ethylene than for the dehydrogenation of ethane. The effect of temperature on the hydrogenating reaction may be expressed by the equation log k'
8.79
- 9140 T
which indicates that the rate of hydrogenation in this temperature range doubles approximately every 25' C.
