Energy & Fuels 1994,8, 507-512
507
Thermal Hydrocracking of n-Hexadecane in Benzene Farhad Khorasheh and Murray R. Gray' Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Received October 5, 1993'
Thermal hydrocracking of n-hexadecane in benzene was carried out in a tubular flow reactor at 400-450 "C and total pressure of 13.9 MPa. Hydrogen concentrations in the feed were 2.3 and 2.9 mol 5%. Mjaor reaction products included biphenyl, C1 to c14 n-alkanes, and C2 to CISa-olefins. The presence of hydrogen shifted the distribution of n-alkanes toward higher carbon numbers and resulted in a decrease in total molar selectivities for a-olefins. Even though a large excess of hydrogen was present, and hydrogenation was thermodynamically favored, selectivities for a-olefins were only reduced 5-10 5% , depending on temperature. Thermal hydrogenation, therefore, was extremely inefficient under the noncatalytic conditions of this study. Unlike thermal cracking of n-Cl6 in benzene, product distributions from thermal hydrocracking of n-Cl6 in benzene were conversiondependent. Total selectivities for n-alkanes were in excess of 100 mol per 100mol n-Cl6 decomposed and increased with increasing n-Cu conversion. Total selectivities for a-olefins were not strongly dependent on n-Cla conversion. Distributions of a-olefins, however, shifted toward lower carbon numbers with increasing n-Cl6 conversion. The presence of hydrogen also inhibited the overall conversion of n-Cl6 at low n-Cl6 conversions (below 3%), but the inhibitory effect disappeared at 545% n-Cl6 conversion.
Introduction Hydrogen-addition processes have long been employed in upgrading of heavy crudes and bitumens. These processes are carried out under hydrogen pressure in the presence of a catalyst or an additive to suppress coke formation. In thermal hydrocracking or hydropyrolysis, free radicals generated from thermal processing of a feedstock can stabilize by abstraction of hydrogen from molecular hydrogen, leading to the formation of hydrogen atoms:
R'
+ H,
H'
+ M-H+
R-H
+ H'
(1) Hydrogen atoms generated from the above reaction can participate in a number of reactions one of which is abstraction of hydrogen by hydrogen atoms: +
H,
+ M'
(2) whichleads to the consumption of the parent hydrocarbon molecules, M-H. Bungerl suggested that these hydrogen abstraction reactions occur at appreciable rates at 400430 OC in hydroprocessingof heavy crudes and bitumens. Hydrogen abstraction involving hydrogen atoms, reaction 2, occurs at faster rates than corresponding reactions involving alkyl radicals, reaction 3.
R' + M-H M'
-
+
R-H
+ M'
R' + products
(3)
(4)
For example, the Arrhenius parameters for reaction 2 involving secondary hydrogens of n-alkanes are A = 1010*6 Author for correspondence. Abstract published in Advance ACS Abstracts, January 1, 1994. (UBunger, J. W. Reactions of Hydrogen During Hydropyrolysis Processing of Heavy Crudes. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1985, 30(4), 658-663.
L/(mol-s) and E = 8 kcal/mol and for reaction 3 involving methyl radicals and secondary hydrogens of n-alkanes are A = logL/(mol-s)and E = 10 kcal/moL2 Hence hydrogen can act as a homogeneous catalyst in propagation steps 1-4 if the rate of reactions 1 + 2 is greater than that of reaction 3. This would lead to an increase in the overall cracking rate of the parent hydrocarbon in the presence of added hydrogen.3~~ For example,pyrolysis of propylene at 750-850 "C and 101 kPa was enhanced in the presence of hydrogens as less reactive allyl radicals gave rise to hydrogen atoms via reaction 1. In high-temperature (700960 "C) pyrolysis of various crude oil fractions at atmospheric6 and elevated pressures up to 3.1 MPa? the rate of pyrolysis was enhanced in hydrogen compared with steam or nitrogen as diluent. In thermal cracking of n-alkanes, however, the rate of decomposition was either suppressed7or accelerated899 in the presence of hydrogen depending on experimental conditions. The presence of hydrogen can also affect product selectivities in hydrocarbon pyrolysis. Hydrogen atoms generated in abstraction reactions (reaction 1) can participate in radical addition reactions with olefins: (2)Kerr, J. A., Moss, S. J., Eds. CRC Handbook of Bimoleculur and Termolecular Gas Reactiom; CRC Preee, Inc.: Boca Raton, FL, 1981. (3) Taniewski, M.; Lachowicz, A,; Skutil, K.; Maciejko, D. Hydropyrolysis of Hydrocarbons. Znd. Eng. Chem. Prod. Res. Dev. 1981,20,746752. (4)Rebick, C. Pyrolysisof Heavy Hydrocarbons. InPyro1yai.x Theory and Industrial Practice; Academic Press: New York, 1983;pp 69-87. (5)Ammo, A.; Uchiyama, M. Thermal Hydrogenolyeis of Propylene. J . Phys. Chem. 1963,67,1242-1247. (6)Kunugi, T.; Tominaga, H.; Abiko, S. Pyrolysis of Hydrocarbons in the Presence of Hydrogen. Proc. World Pet. Congr., 7th 1967,6, 239245. (7)Zhou, P.; Crynes, B. L. Thermolytic Reactions of Dodecane. Ind. Eng. Chem. Process Des.. Dev. 1986,26,508-514. (8)Shabtai,J.;,R.;Oblad, A. G. Hydropyrolysisof Model Compounds. In Thermal Hydrocarbon Chemistry, Advancea in Chemistry Series No.183;American Chemical Society: Waehington, DC, 1979; pp 297-328. (9)Rybin, V. M.; Yampolakii, Y. P. Effect of Hydrogen on Hexane Pyrolysis. Pet. Chem. USSR 1976,16, 167-176.
0887-0624/94/2508-0507$04.50/00 1994 American Chemical Society
Khorasheh and Gray
508 Energy & Fuels, Vol. 8, No. 2, 1994
-
I
I
I
I
I
I
I
30
5
E
30
I
I
I
l
I
I
I
v
a-olefins
.5 v)
- 20
0,
20 -
a-olefins
c 0
-m 0
VI
-2
i
10-
0
I
0
-
H' + olefins R' (5) The resulting radical could abstract a hydrogen to give the corresponding saturated compound. For example, in thermal hydrocracking of n-propylbenzene at 450-725 "C and 4.8 MPa,lOand n-butylbenzene at 460-500 "C and 8.1 MPa," olefins were rapidly hydrogenated and the ratio of saturates to olefins increased with increasing partial pressure of hydrogen. Hydrogen atoms can also promote dealkylation of alkylaromaticsll and a-ring opening of hydroaromatics12 via a hydrodealkylation mechanism. In high-pressure thermal hydrocracking, molecular hydrogen provides additional source of hydrogen for abstraction (reaction 1)thereby stabilizing radicals that could otherwise participate in addition reactions with olefins, leading to the formation of high-molecular-weight compounds. Radical stabilization by molecular hydrogen and hydrogenation of a-olefins eliminated or significantly suppressed the formation of high-molecular-weight compounds in thermal cracking of n-C168 and n-C12' in the presence of hydrogen. The main objective of this study was to investigate the effect of added hydrogen on the overall kinetics and product selectivities, in particular those for a-olefins, in thermal cracking of n-Cle in benzene. Hydrocracking ~~~
~
~
(10) Moore, R. N.; Engelbrecht, R. M.; Hill, J. C.; Spillane, L. J. The
Thermal Hydrocrackingof Hydrocarbons: A Study of Model Compounds. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1967,12(3), 101-116. (11)Slotboom,H. W.;Penninger, J. M. L. Reactions of n-Butylbenzene in Thermal Hydrocracking. ErdolKohle, Erdgas, Petrochim. 1974,27(8), 410-412. (12)Penninger, J.M. L. Newhpectsofthe Mechanism for the Thermal Hydrocracking of Indan and Tetrali. Int. J. Chem. Kinet. 1982, 14, 761-780.
0.
experiments were performed using a dilute solution of nCIS in benzene that had been previously saturated with dissolved hydrogen. The concentration of hydrogen in the feed was limited by the solubility of hydrogen in benzene a t room temperature. The data from hydrocracking experiments were compared with previous work on thermal crackingof n-C16 in benzene in the same apparatus in the absence of hydrogen.13J4
Methods and Materials Benzene (99.9%) and n-& (99.9%) were obtained from Aldrich. The equipment,experimentalprocedures,and analytical methods were similar to those described by Khorasheh and Gray.13J4 Thermal hydrocracking experimentswere carried out in a tubular flow reactor at 13.9 MPa and 400-450 "C using a feed containing 0.01 mole fraction n-Cl8 in benzene. The feed was saturated with hydrogen (ultra high purity grade) using a circulating pump. The hydrogen solubility in the above feed at 13.9 MPa and room temperature was about 3 mol %. Gas and liquid products were analyzed by gas chromatography. Results and Discussion Two sets of hydrocracking experimentswere performed. The measured hydrogen concentrations for set A and B were 2.3 and 2.9 mol 9% , respectively. These amounts of dissolved hydrogen present in the feed were in excess of hydrogen requirements for complete hydrogenation of a-olefins produced from the decomposition of wC16 at (13)Khoraeheh, F.;Gray, M. R. High-pressure Thermal Cracking of n-Hexadecane. Ind. Eng. Chem. Res. 1993,32,1853-1863. (14)Khoraeheh, F.;Gray, M. R. High Pressure Thermal Cracking of n-Hexadecane in Aromatic Solvents. Ind. Eng. Chem. Res. 1993,32, 1864-1876.
Thermal Hydrocracking of n-Hexadecane in Benzene
low conversions. Considering set A, for example, the feed composition (mol % ) was 2.27,0.98, and 96.75for hydrogen, n-C16, and benzene, respectively. Assuming that total molar selectivities for a-olefins were at most 200 mol per 100 mol n-C16 decomposed,14 the hydrogen requirement for complete hydrogenation of a-olefins at 10% n-Cl6 conversion would be 0.2 mol 5%. Hydrogenation of olefins to alkanes was thermodynamically favored at the reaction temperatures, hydrogen concentration, and total pressure employed in this study. Product Distributions. Low-pressure (atmospheric) pyrolysis of n-Cl6 proceeds by a free-radical chain mechanism1"l7 in which higher alkyl radicals undergo successive (multistep)decomposition to give methane and ethane as the only saturated products and C2 to c15 a-olefins.lg-m Fabuss, Satterfield, and SmithmP2lproposed a single-step mechanism for alkanes pyrolysis at high pressures (1-7 MPa) where parent radicals undergo a single-stepdecomposition. The single-stepdecomposition of n-alkanes is characterized by nearly equimolar distribution of n-alkanes and a-olefins in the products.13~22 Mushrush and H a ~ l e tsuggested t~~ a two-step mechanism for an intermediate pressure of 0.7 MPa. In high-pressure thermal cracking of n-C16 in benzene,14 the product distributions were characterized by 2-3 decomposition steps depending on the reaction temperature and initial n-CI6 concentration. Product distributions from thermal hydrocracking of n-C16 in benzene were qualitatively similar to those reported for thermal cracking of n-Cl6 in benzene.14 Typical product distributions are illustrated in Figure 1. Major reaction products included biphenyl, C1 to c14 n-alkanes, and C2 to C15 a-olefins. Internal olefins were present only as minor products with a total molar selectivity of less than 3 mol per 100 mol of n-Cl6 decomposed. Similar to thermal cracking of n-Cl6 in benzene,14 product distributions from thermal hydrocracking of n-Cu in benzene were characterized by high selectivities for C1 to C4 products and quite low selectivities for C5 and higher n-alkanes. As illustrated in Figure la, an increased in reaction temperature resulted in a shift in the distribution of n-alkanes toward lower carbon numbers. This shift was accompanied by increasing selectivities for a-olefins, in particular in the C2 to c6 range, consistent with higher activation energies for &scission compared with hydrogen abstraction reactions. In thermal cracking of n-Cl6 in benzene,14 product distributions were not affected by the overall conversion (15) Rice, F. 0. The Thermal Decomposition of Organic Compounds from the Standpoint of Free Radicals. 111. The Calculation of the Products from ParaffiiHydrocarbons. J. Am. Chem. SOC.1933,55,30353040. (16) Rice, F. 0.;Henfeld, K. F. The Thermal Decomposition of Organic Compounds from the Standpoint of Free Radicale. IV. The Mechanism of some Chain Reactions. J. Am. Chem. SOC.1934,56, 284-289. (17) Koesiakoff, A,; Rice, F. 0.Thermal Decomposition of Hydrocarbons, Resonance Stabilization and Isomerization of Free Radicals. J. Am. Chem. SOC.1943,65, 590-595. (18) Voge, H. H.; Good, G. M. Thermal Cracking of Higher Paraffins. J. Am. Chem. SOC.1949, 71,593-597. (19) Depeyre, D.; Flicoteaux, C.; Chardaire, C. Pure n-Hexadecane Thermal Steam Cracking. Znd. Eng. Chem. Process Des. Diu. 1985,24, 1251-1258. (20) Fabuss, B. M.; Smith, J. 0.; Satterfield, C. N. Thermal Cracking of Pure Saturated Hydrocarbons. Adu. Pet. Chem. Ref. 1964,9,157-201. (21) Fabuee,B. M.; Smith, J. 0.;Lait, R. I.;Boreanyi,A. S.;Satterfield, C. N. Rapid Thermal Cracking of n-Hexadecane at Elevated Pressures. Znd. Eng. Chem. Process Des. Diu. 1962,1, 293-299. (22) Ford, T. J. Liquid Phase Thermal Decomposition of Hexadecane: Reaction Mechanisms. Znd. Eng. Chem. Fundam. 1986,25,240-243. (23) Mushrush, G.W.; Hazlett, R. N. Pyrolysis of Organic Compounds Containing Long Unbranched Alkyl Groups. Znd. Eng. Chem. Fundam. 1984,23, 288-294.
Energy & Fuels, Vol. 8,No. 2,1994 SO9 Temperature ('C):
0 407
A 426 0 436
417
447
398 A
120
I
,
8
115
110
105
I80
100
..........................
..-"" .........
....................................
a-olefins
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0'
.............. A .....................................................
.,,,,,
A
A"'
160
A....................A .....................................
..*..........
0
I50
0..............
3
140 A
cis and trans 2-olefins 2
1
a 0
0
2
4
6
8
10
X Conversion
Figure 2. Overall product selectivitiesin hydrocracking(set B) of n-Cls in benzene.
of n-C16. In the presence of hydrogen, however, product distribution were conversion-dependent, indicating that a-olefins were participating in secondary reactions. As illustrated in Figure 2, total molar selectivities for n-alkanes, in particular in the C1 to C4 range, increased with increasing conversion. Distributions of n-alkanes from thermal hydrocracking of n-Cle in benzene were also different from those obtained from thermal cracking of in benzene. Figure 3 indicates that selectivities for C5 and higher n&anes were higher in the presence of hydrogen. In thermal cracking Of n-Cl6 in benzene," total molar selectivities for n-alkanes were approximately 100 mol per 100 mol of decomposed. The addition of hydrogen resulted in an increase in the rate of hydrogen abstraction by alkyl radicals relative to @-scissionwhich resulted in an increase in the molar selectivities for higher n-alkanes and a decrease in total molar selectivities for a-olefins (Figure 4). Depending on the reaction temperature, total molar selectivities for a-olefins were 5 1 0 % lower than corresponding values for thermal cracking. Selectivities for n-alkanes could also increase by addition of hydrogen atoms to a-olefins to give an alkyl radical which could subsequently abstract a hydrogen to give the corresponding n-alkane. Total molar selectivities for a-olefins increased with increasing reaction temperature but did not change significantly with n-Cl6 conversions (Figure 2). Distributions of a-olefiis, however, were conversion-dependent.
Khorasheh and Gray
510 Energy & Fuels, Vol. 8, No. 2, 1994 0 thermal cracking 0
0
8n
hydrocracking t a t B
10 T = 3 9 8 'C
T = 4 1 7 "C
....................................................................... ._
.....................................................................
...............................................
...........................
5I
4
6
8
12
10
14
164
Carbon Number
6
8
10
12
164
14
6
Carbon Number
8
10
12
14
16
Carbon Number
Figure 3. Distributions of n-alkanes in thermal cracking and hydrocracking of 0.01 mole fraction n-Cla in benzene. selectivities from thermal cracking hydrocracking temperature (T):
300
0
-
140
P
II D
.-e
120
A
407 A 417
0
426 436 447
4
-
0
v
0 398
-
thermal cracking hydrocrocking ret A 0 hydrocracking ret B v
..
I
250
0
2
4
6
8
10
Z Conversion
Figure 5. Total product selectivities in thermal cracking and hydrocracking of 0.01 mole fraction n-Cla in benzene. Horizontal linea indicate the selectivities from thermal cracking. radicals (reaction 9) could isomerize to secondary alkyl radicals (reaction 7). Alkyl radicals could also decompose via @-scissionto give an a-olefin and a smaller radical (reaction 10) or stabilize by hydrogen abstraction from n-Cl6, benzene, or hydrogen, reactions 8, 11, and 13, respectively, to give the corresponding n-alkanes. The presence of hydrogen would enhance the rate of hydrogen abstraction by alkyl radicals relative to 8-scission, which would result in lower selectivities for a-olefins (Figure 4). Other effectswould include a shift in n-alkanes distribution toward higher carbon numbers (Figure 31, and lower selectivities for total products at low n-Cl8 conversions (Figures 5). Hydrogen atoms generated from reactions 13 and 14 could either abstract a hydrogen from n-Cl6 or benzene, reactions 15 and 16, respectively, or add to an a-olefin, reaction 17, to give the corresponding alkyl radical. These addition reactions would become more significant with increasing n-Cl6 conversion as a-olefin concentrations increase.
Energy & Fuels, Vol. 8, No.2, 1994 fill 1 0 hydrocrocklng ret A 0 hydrocracking ret E .......... thermal crocking
......
__.e-
......”.”” .-. ..x..... . .. ........... 436
_
............
.
......0 ...... .......................
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*c
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............................. ..............................
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-436 *c
....0
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.................................................................................................................................
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...................................................................................................
P h + Ph-H
it
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........................................................................................................................................
t H’
407 1
~
....598
*C
398 ‘C 0.01
Ph=@ 2 Convrrrion
Figun, 6. Reaction mechanhm for thermal hydrocracking of n-Cv in benzene.
Figure 7. Apparent fiitorder rate constants for thermal crackingand hydrocracking of 0.01 mole fractionn-Cle in benzene.
Termination reactions could involve assorted radical combinations (reaction 18)including the combination of two phenyl radicals to give biphenyl. Biphenyl, which was present as a major reaction product, could also be formed by addition of phenyl radical to benzene followed by elimination of a hydrogen atom (reaction 19). Alkylradicalegenerated by reaction 17could (1)stabilize by hydrogen abstraction to give the corresponding n-alkane, or (2) decompose by @-scissionto a smaller radical and an a-olefin. In the first case, total selectivities for n-alkanes would increase at the expense of a-olefins while total product selectivities would remain unchanged with increasing n-Cle conversion. These trends were not observed under the conditions employed in this study. The second case, however, does explain the trends. Total selectivities for n-alkanes, in particular selectivities for lower n-alkanes (Figure lb), would increase while total selectivities for a-olefins would remain unchanged with increasing n-Cls conversion (Figure 2). The distributions of a-olefins would shift toward lower carbon numbers (Figure lb), and selectivities for total products would increase (Figure 5)with increasing wC16 conversion. The fate of alkyl radicals generated in reaction 17 depends on the relative rate of hydrogen abstraction to @-scissionbut the observed product distributions are consistent with @-scissionbeing predominant. In thermal cracking of n-Cl6 in tetralin,uradicals generated from addition of hydrogen atoms to a-olefins were primarily stabilized by hydrogen abstraction (case 1) from tetralin. Selectivitiesfor biphenyl decreased from 45 f 5 mol per 100 mol n-Cl6 decomposed for thelplal cracking to 30 f 4 mol per 100 mol n-Cl6 decomposed for thermal hydro-
cracking. This decrease is consistent with stabilization of phenyl radicals by abstraction reaction 14,which would in turn inhibit reaction 19. Overall Kinetics. The apparent first-order rate constants for the overall conversion of n-Cl6 in thermal hydrocracking of n-Cle in benzene are presented in Figure 7 as a function of n-Cl6 conversion. The results indicated that, at n-Cr6 conversions below 3%, first-order rate constants for conversion of n-Cl6 in the presence of hydrogen were lower than the corresponding values in the absence of hydrogen (dotted lines in Figure 7). Furthermore, these first-order rate constants for hydrocracking of n-Cl6 increased with increasing n-Cl6 conversion and, at conversions above 8%, they were greater than corresponding values for thermal cracking in the absence of hydrogen. The presence of hydrogen has been reported to either suppress or enhance the overall decomposition rate in the pyrolysis of n-alkanes. In those studies that reported an enhancement effect, experiments were conducted under high-temperature,high-conversionconditions.as@The only study which reported an inhibition effect by hydrogen: was conducted under low-temperature, low-conversion conditions. Zhou and Crynes’ suggested that radical capping by hydrogen was responsible for the observed inhibition. Reaction temperatures in the present study were higher than the 350“C employed by Zhou and Crynes’ and lower than those employed by Shabtai et aL8 and Rybin and Yamp0lskii.B At the intermediate temperatures in this study, inhibition was observed at low n-Cl6 conversions and enhancementwas observed at conversions over 5-6 % . The exact cause of the conversion dependence of firstorder rate constants for hydrocracking of n-Cl6 in benzene is unclear. One possible explanation is the high reactivity
(24) Kborarheh, F.; Gray, M. R. Hsh Prmure Thermal Cracking of n-Hexadecane in Tetralin. Energy h e & 1993,7,980.
512 Energy &Fuels, Vol. 8, No. 2, 1994
Khorasheh and Gray
Conclusions
1. The presence of hydrogen enhanced the rate of hydrogen abstraction reactions by alkyl radicals relative to j3-scission, which resulted in lower selectivities for total a-olefins and an increase in the selectivities for higher n-alkanes. 2. a-Olefins were only partially hydrogenated when hydrogen atoms participated in addition reactions with a-olefins to give the corresponding alkyl radicals. Decomposition of these radicals resulted in an increase in total selectivities for n-alkanes and a shift in a-olefins distribution toward lower carbon numbers. 3. Thermal hydrogenation of a-olefins, which was thermodynamically favored, was extremely inefficient in spite of a large excess of hydrogen. In the presence of hydrogen, total molar selectivities for a-olefins were only 5-10% lower than corresponding values for thermal cracking. 4. The presence of hydrogen inhibited the overall conversion of n-Cl6 at low n-Cl6 conversions (below 3%). The apparent first-order rate constants for overall conversion of n-Cl6, however, increased with increasing n-C16 conversion.
The presence of hydrogen in thermal hydrocracking of 0.01 mole fraction n-C16 in benzene affected both the product distributions and the overall conversion of n-C16 compared with thermal cracking in the absence of hydrogen. The following conclusions can be made from this study:
Acknowledgment. Financial support was provided by Alberta Oil Sands Technology and Research Authority (AOSTRA) under agreemenb 521 and 781, and by Esso Petroleum Canada under University Research Grants Program.
of hydrogen atoms in termination reactions. In thermal cracking Of n-C16 in benzene,14phenyl radicals were major radical species as evident by the presence of biphenyl as a major reaction product. In the presence of hydrogen, phenyl radicals would be stabilized (reaction 14) to give hydrogen atoms which are more reactive in termination reactions. Note also that this would lead to lower selectivities for biphenyl as was observed. An increase in the rate of termination reactions would decrease the total radical concentration and suppress the rate of n-Cl6 decomposition. As conversion increases, addition of hydrogen atoms to a-olefins (reaction 17) becomes significant and the more reactive hydrogen atoms are converted to less reactive alkyl radicals. This mechanism would result in a decrease in the rate of termination reactions, thus increasingtotal radical concentration and the rate Of n-C16 decomposition. Since termination reactions have low (zero) activation energies, the inhibition effect would be more pronounced at low temperatures. Under high-temperature conditions, the inhibition effect may be absent or undetected at high conversions.
