Ind. Eng. Chem. Fundam. 1986, 2 5 , 240-243
240
Liquid-Phase Thermal Decomposition of Hexadecane: Reaction Mechanisms Thomas J. Ford Exxon Research and Engineering Company, Baytown, Texas 77522
Thermal decomposition experiments were performed with n -hexadmane to get a better understanding of liquidphase thermal decomposition. Gas chromatography and mass spectrometry were used to identify the decomposition products. At low conversions, liquid-phase thermal decomposition produced low molecular weight straight-chain alkanes and alkenes. This is in good agreement with the Fabuss, Smith, and Satterfleld thermal cracking mechanism. At conversions above 5 % , liquid-phase thermal decomposition produced more complex products. High molecular weight branched-chain alkanes were formed in addition to the low molecular weight products described above. The branched-chain alkanes were formed by the reaction of straight-chain alkenes with hexadecyi radicals. A liquid-phase thermal decomposition model was developed that accounts for the main products formed.
Research has been undertaken to evaluate the liquidphase thermal decomposition of saturated hydrocarbons. Hexadecane was chosen as a model compound for this research. Considerable work has been reported on the thermal decompositionof saturated hydrocarbons in the gas phase but very little on their liquid-phase decomposition. Rice et al. (1933, 1934, 1943) showed that the main products of the low-pressure gas-phase thermal decomposition of straight-chain alkanes are low molecular weight straightchain alkanes and alkenes. They developed a free-radical mechanism that accounts for the main products formed. Fabuss, Smith, and Satterfield (1964) modified Rice's mechanism to account for the products formed in highpressure gas-phase decompositions. More recently, Doue and Guiochon (1969) and Hazlett (1977) studied the liquid-phase decomposition of straight-chain alkanes; these workers also reported that only straight-chain products are formed. However, they found more alkanes than alkenes, and their material balances were incomplete. In this paper, the thermal decomposition of hexadecane in the liquid phase at 330-420 "C is described, and a more detailed analysis of products than has been previously available is presented. In this work, straight-chain C1 to C14alkanes and Cz to C14 alkenes were found to be formed; however, large amounts of both branched- and straightchain CI8 to (& alkanes were also found. A mechanism is presented that is consistent with these new product data.
Experimental Section Reagents. All chemicals used were of greater than 99% purity. The hexadecane was 99.7% n-hexadecanewith the remainder being C,, isomers. The perdeuteriotetralin contained less than 1% H. Equipment and Procedures. The experiments were performed in a small batch tubing reactor that was fitted with a glass liner (Neavel, 1976). Before each run, about 5 mL of reagents was placed inside the glass-lined tubing bomb. Samples were deaerated by bubbling with argon. After deaeration, the tubing bombs were sealed and placed in a preheated fluidized bath. Within 5 min, the samples were heated to within f 2 "C of the desired temperature. After the run, the samples were rapidly quenched in a water bath. The gas and liquid products were collected and analyzed by gas chromatography and mass spectrometry. Analysis. A gas chromatograph was used to analyze the liquid samples. Two different gas chromatographic
techniques were used. The first determined the carbon number distribution, and the second determined the individual straight-chain alkanes and alkenes. The liquid analysis was obtained by combining the data from the two techniques. In the first technique, the sample with internal standard was injected into a 10% SP-2100 packed column which was programmed from 40 to 350 "C at 14 "C/min. In the second technique, the sample was injected into a SE-54 silica capillary column which was programmed from 50 to 280 "C at 8 "C/min. Peaks were identified by using alkane and alkene standards and further verified by a gas chromatograph/mass spectrometer (GC/MS). Another gas chromatograph was used to analyze the gas samples. The analysis gave C1 to CBstraight-chain alkanes and C2to C8straight-chain alkenes. The gaseous products above C4 made less than 0.1% by weight contribution to products and, thus, were ignored. A GC/MS was used to analyze select samples. The heavy products formed were concentrated by using gas chromatography and then analyzed by GC/MS. This technique was extremely helpful in analyzing the composition of the heavy products formed.
Results Kinetics of Liquid-Phase Thermal Decomposition. Kinetic data were obtained for the liquid-phase thermal decomposition of hexadecane. Figure 1shows three concentrations vs. time plots. The straight lines indicate that hexadecane liquid-phase thermal decomposition is firstorder. The first-order rate constants are 0.27 X 1.5 X and 7.5 X h-' for 330, 350, and 370 "C, respectively. Figure 2 gives the rate data for both liquidphase and gas-phase thermal decomposition of n-hexadecane (Fabuss et al., 1962; Voge and Good, 1949; Tilicheev and Zimina, 1956; Doue and Guiochon, 1968; Panchenkov and Baranov, 1958; Groenendyk et al., 1970). The activation energy is about 57 kcal/mol over a range of rate constants of 7 orders of magnitude. Excellent agreement between liquid- and gas-phase data was seen. Hexadecane Liquid-Phase Therinal Decomposition Products. In one run, meant to model the gas-phase conditions, hexadecane was heated at 350 " C for 4 h to a 0.3% conversion. In this run, like in the gas phase, only C1to C1, straight-chain alkanes and C2to CI4straight-chain alkenes were formed (Table I, first column). One mole of hexadecane led to about two moles of straight-chain products in agreement with the Fabuss, Smith, and Satterfield mechanism.
0196-4313/86/1025-0240$01.50/0Q 1986 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 25, No. 2, 1986 I
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Table I. Product Distribution for Liquid Phase
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Thermal Decomposition of Hexadecane a t 350 "C
100 0
hexadecane, % tetralin, % run time, h conversion, %
100 0 100
4 0.6
C no. of products
20.9
mol of product/100 mol of hexadecane decomposed
Straight-Chain Alkenes LOG ( C I C i ) HEXADECANE
4 5 6 7 8 9 10 11 12 13 14 15 25
0
75
50
125
100
1 2
Figure 1. Plots of log C/Ci, relative concentration, vs. time for hexadecane liquid-phase thermal decompositionat 330,350, and 370 OC.
104
600
550
500
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3 4 5 6 7 8 9 10
300
350 I
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GAS PHASE A VOGE AND GOOD 0 FABUSS ET AL. V TlLlCHEEV AND ZIMINA
103
V
11
-
12 13 14 15
DOUE AND GUIOCHON PANCHENKOV AND BARANOV
-
A
VOGE AND GOOD 0 GROENENDVK ET AL.
102
r
-
K
r 101 I-.
5 5 $
1
-
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-
W
d
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total
lo-'
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1.2
1.3
I
1.4
I
I
1.5
1.6
I
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1.7
1.8
TIT, ( K ) x 103
Figure 2. First-order rate constants for hexadecane gas-phase and liquid-phase thermal decomposition vs. reciprocal temperature.
At longer times, the product mixture is more complex. Figure 3 shows a packed column chromatogram of a run exposed at 350 O C for 100 h and in which 20.9% of the hexadecane was converted (Table I, second column). Special care was used in this chromatographic analysis to obtain base line resolution up to about Cm As expected,
1.0 0.9 0.9 0.8 0.8 0.9 0.9 0.9 1.3 0
llO3 I
3.8
8.8 9.4 9.5 9.6 9.6 9.4 9.3 8.1 8.0 0.5 0
Branched- and Straight-Chain Alkanes 17 0 2.6 18 0 6.2 19 0 3.5 20 0 4.1 21 0 3.5 22 0 3.3 23 0 3.2 24 0 3.0 25 0 2.9 26 0 2.7 27 0 2.5 28 0 2.4 29 0 1.3 30 0 1.1 31+ 0 1.5 187 107
product H/C ratio
10-2
8.6 9.2 8.6 8.8 8.7 8.3 8.2 8.0 7.2 7.2 0
Straight-Chain Alkanes
150
TIME (HOURS)
TEMPERATURE,'% 400 450
}11.3
2.119
2.119
the main products were the C1to CI4straight-chain alkanes and the C2to C14straight-chain alkenes; however, a significant amount of C18 to C30material was also found. Figure 4 shows a capillary chromatogram of these materials. Notice that the area represented by the CISto C3,, compounds in the capillary column appears to be much smaller than the area for the corresponding compounds in the packed column. This results because of the split discrimination which occurs in the capillary injection system. Figure 5 shows a reconstructed ion chromatogram of the CZ4to Czsregion from the complete GC/MS spectra. By examination of the fragmentation patterns, the structure of the specific products could be identified. Analysis
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Ind. Eng. Chem. Fundam., Vol. 25, No. 2, 1986 Scheme I C16H34
+
2R*
1111
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3b5 6
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9
10
1
1
1
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R. + C16H33, ______)
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11 12 1 3 14 15 16
18
PRODUCTS
(VI
30
Hexadecane Liquid-PhaseThermal Decomposition. At low conversions (
