Ind. Eng. Chem. Res. 1993,32, 1864-1876
1864
High-pressure Thermal Cracking of n-Hexadecane in Aromatic Solvents Farhad Khorasheh and Murray R. Gray' Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Thermal cracking of n-Cl6 in benzene, toluene, and ethylbenzene was carried out in a tubular flow reactor at 398-451 OC and 13.9 MPa. Initial mole fractions of n-C16 in the feed were 0.01,0.03, and 0.05. Thermal cracking of n-Cl6 in benzene gave C1 to CI4n-alkanes and Czt o C16 a-olefins as major reaction products. Biphenyl was also produced as a major product. Low temperatures and high initial concentration of n-C16 resulted in high molar selectivities for C5 to C13 n-alkanes. Increasing reaction temperature and decreasing n-C16 initial concentration resulted in a shift in n-alkane distribution toward lower carbon numbers and an increase in total molar selectivities for a-olefins. increased with increasing The apparent first-order rate constants for overall conversion of ~ - C N initial concentration of n-Cls. Cracking of n-C16 in toluene and ethylbenzene gave higher alkylbenzenes as major reaction products, in addition to lighter n-alkanes and olefins. A kinetic model for cracking in toluene showed that the observed data were consistent with rapid abstraction of benzylic hydrogens, giving benzyl radical as the dominant radical species. T h e kinetic model accurately depicted the changes in reaction selectivity as a function of conversion.
Introduction Liquid-phase cracking of residues at 400-450 "C, 10-20 MPa, involves the simultaneous reactions of a variety of species. Since residues of bitumen and petroleum contain a high concentration of aromatic, alkylaromatic, and aliphatic compounds, the reactions of such compounds, and their interactions with other hydrocarbons in a binary system, are relevant to hydroconversion processes. The product distributions in thermal cracking of n-alkanes under these relatively moderate temperatures and high pressures no longer follow those predicted by the RiceKossiakoff mechanism (Kossiakoff and Rice, 1943)for lowpressure (atmospheric), high-temperature (>600 OC) pyrolysis. The reactant densities in high-pressure supercritical or liquid-phase thermal cracking of n-alkanes can be at least 2 orders of magnitude higher than gas-phase densities under low-pressure,high-temper ature conditions. Hence bimolecular hydrogen abstraction reactions become significant under high-pressure conditions. Unlike the low-pressure pyrolysis of alkanes in which alkyl radicals undergo successive cracking by &scission (multistep cracking), the high-pressure thermal cracking of alkanes has been described in terms of a single-step mechanism (Fabuss et al., 1964; Ford, 1986; Kissin, 1987). At intermediate pressures of 3-7 MPa, a two-step mechanism has been reported for thermal cracking of alkanes (Mushrush and Hazlett, 1984; Zhou et al., 1986, 1987). In a previous paper (Khorashehand Gray, 1993),thermal cracking of pure was investigated at 380-450 "C and 13.9 MPa. Under these conditions, the selectivities for higher n-alkanes (C5 to CIS) were nearly equimolar, consistent with rapid hydrogen abstraction relative to @-scissionfollowing the single-step mechanism of Fabuss et al. (1964). The first objective of the present study was to investigate the effect of reactant concentration on the overall kinetics and product distributions in thermal cracking of n-alkanes. Benzene was chosen as the solvent because hydrogen abstraction from benzene would be limited. Hence, by varying the initial mole fraction of n-Cl6 in the feed, one can vary the relative rate of hydrogen abstraction to radical decomposition by @-scission. Experiments were conducted using low concentrations of n-Cl6 in the feed to minimize addition reactions involving alkyl radicals and a-olefins.
* Author for correspondence.
The second objective of this study was to investigate the reaction mechanism, product distributions, and solvent interactions in thermal cracking of n-Cu in alkylaromatic solvents, namely toluene and ethylbenzene. Benzylic hydrogens (i.e., hydrogens on aliphatic carbon attached to an aromatic ring) are the most readily abstracted hydrogens in alkylaromatic compounds because their abstraction leads to the formation of resonance-stabilized benzyl or benzylic-type radicals. For a long-chain alkylbenzene for example, Billaud et al. (1988) estimated the bond energies to be 110.9, 81.7, and 97.5 kcal/mol for aromatic, benzylic, and B+ C-H bonds, respectively.
Methods and Materials Benzene (99.9%), toluene (99.9% 1, ethylbenzene (99+ % ), and n-hexadecane (99.9%) were obtained from Aldrich. Thermal cracking reactions were carried out in a tubular flow reactor (Khorasheh and Gray, 1993). The experimental procedures and analytical methods for gas and liquid product analyses were also similar to those described by Khorasheh and Gray (1993). The following temperature programming was used for the capillary gas chromatography (GC) for analysis of liquid products: held at initial temperature of 35 "C for 3 min, increase to 200 OC at 5 OC/min, and held at 200 "C until elution was completed. Thermal cracking of n-Cle in the above solvents was carried out at 398-451 OC and 13.9 MPa. Initial mole fractions of n-Cu in the feed were 0.01,0.03, and 0.05. With the exception of a few high-conversion experiments, feed flow rates were selected to give n-Cl6 conversions below 10'3%.
Results and Discussion Thermal Cracking of n-Cls in Benzene. Product Distributions in Thermal Cracking of n-C16 in Benzene. The major reaction products from thermal cracking of n-Cl6 in benzene included biphenyl, C1 to C14 n-alkanes, and Cp to C16 a-olefins. Minor products included a series of Cq to CIScis- and trans-2-olefins and n-alkylbenzenes from ethylbenzene to pentylbenzene. Both minor series were present in trace amounts. Typical product distributions are presented in Figure 1,where product selectivities are reported in termsof moles of product formed per 100mol of n-C16decomposed. These distributions were quite different from those obtained in
0888-5885/93/2632-1864$04.00/ 0 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 9,1993 1865
-B 8
40 n-alkanes
1
A i 0
5
-
I
0
2
4
6
6 Carbon
10
12
14
16
#
0
mol fraction n-C,,:
.01, T=451 'C
b
mol fraction n-C,,:
.05, T=396 'C
h
0
g 5 4
2
80
N=3 70
30
e EzoL
+ I
a-olefins
\
"A\ 0
\\
0
2
4
6
1 i
0
8 Carbon
10
12
14
16
#
Figure 1. Range of selectivities in thermal cracking of n-Cle in benzene. Solid lines represent simulated product distributions for N-step decomposition of n-Cl&
cracking of pure n-Cl6. Qualitatively, these differences include (1)the absence of higher-molecular-weightaddition products, (2) higher selectivities for C g to C13 a-olefins than corresponding for n-alkanes, and (3) a shift in n-alkane distribution toward lower carbon numbers. One way of describing the product distributions in the thermal cracking of n-alkanes is to consider the number of steps, N, by which an alkyl radical undergoes decomposition by 8-scission before it is stabilized by hydrogen abstraction. In the single-step mechanism (N = 1) proposed by Fabuss et al. (1964),the parent radicals decompose to give a lower alkyl radical and an a-olefin. The lower alkyl radicals participate only in hydrogen abstraction reactions. This single-step mechanism results in equimolar selectivities for n-alkanes and a-olefins. Simulated distributions of n-alkanes and a-olefins in N-step thermal cracking of are illustrated by the solid lines in Figure 1. As N increases, the distribution of n-alkanes shifts toward lower carbon numbers. The total molar selectivity for n-alkanes, however, remains constant at 100 mo1/100 moles of n-Cl6 decomposed. For N = 4,n-alkanes are primarily methane and ethane as the product distribution approaches that predicted by the Rice-Kossiakoff mechanism. The total molar selectivity for a-olefins increases with increasing N mainly due to a substantial increase in the molar selectivities for ethylene and propylene. The value of N, and hence the product distribution in thermal cracking of n-alkanes, depends on the relative rates of the propagation steps in the free-radical chain mechanism. If reaction conditions are such that the rate of hydrogen abstraction reactions is much higher than the
rate of radical decomposition reactions, then the product distribution approaches that for single-step cracking. Hydrogen abstraction reactions are bimolecular and have much lower activation energies than &scission reactions. Hence a single-step product distribution is favored a t low temperatures and high pressures. On the other hand, if reaction conditions are such that the rate of radical decomposition reactions is much higher than that of hydrogen abstraction reactions, then the product distribution approaches that for a multistep cracking at low pressures and high temperatures. The experimental product distributions in Figure 1 represent two extreme conditions employed in this study. The run at the highest temperature and lowest initial mole fraction of n-Cl6 represents conditions at which 8-scission was most favored. On the other hand, the run with the highest initial mole fraction of n-C16 and the lowest temperature represents the most favorable conditions for hydrogen abstraction reactions. The two extreme conditions represent only an increase by approximately a factor of 5 in the initial concentration of n-Cl6 and a temperature range of 53 "C. This rather small range of conditionsresulted in n-alkane distributions with the value of N between 1and 2 for one extreme and between 2 and 3 for the other. The distributions of a-olefins were also affected by the reaction conditions with the most significant changes in the c2 to c6 range. For a given temperature and initial concentration of n-Cl6, the product distribution was not affected by the conversion. This insensitivity to converdegree of sion, along with the lack of alkylbenzenes in the products, indicated that radical addition to a-olefins was negligible under dilute n-Cl6 concentrations. Product distributions at 416-418 "C are presented in Figure 2 as a function of initial n-Cl6 mole fractions. As the initial concentration decreased, the molar selectivities for CZ to (26 a-olefins increased while the distribution of n-alkanes shifted toward lower carbon numbers. These trends in product distributions with increasing initial mole fraction of were consistent with trends reported by Doue and Guiochon (1968,1969) in product distributions with increasing pressure (from at 507 O C . 0.15 to 8.3 MPa) in thermal cracking of An equivalent trend toward lighter n-alkanes was observed when temperature was increased. This trend was consistent with a shift toward &scission reactions relative to hydrogen abstraction. The total molar selectivity for n-alkanes was approximately 100 mol/100 mol of n-Cl6 decomposed for all experiments. The total molar selectivity for a-olefins increased with increasing reaction temperature, and also increased with decreasing initial mole fraction. For example, the ratio of a-olefins to n-alkanes from 0.05 mole fraction n-Cl6 increased from 1.35 to 1.55 as reaction temperature was increased from 398 to 434 "C. This increase was primarily due to increases in selectivities for C2 to c6 a-olefins (Figures 1 and 2). This result suggests that the primary alkyl radicals generated by the decomposition of hexadecyl radicals isomerized to secondaryalkyl radicals via 1-4 and 1-5 internal hydrogen abstraction prior to decompositionto a smaller radical and an a-olefin. In the absence of such radical isomerizationreactions, only ethylene would have been produced from the decomposition of lower alkyl radicals. Overall Kinetics of n-Cls Decomposition in Benzene. The Arrhenius plot of the apparent first-order rate constants for the overall conversion of n-Cl6 is presented in Figure 3. The apparent activation energy for overall
1866 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 n-C,,
Temperature ("C)
0 .Ol, 418
mol fraction, Temperature ('C).
A
io
.03. 418 .OS. 416
440
420
400
380
2
a-olefins
1
4
25
-
h
C
5 -
I
d
-
1
1
0 '
4
A
;1
12
;1
16 0.1
Carbon Number
0
>02
r
v
v)
35
I
1
n-alkanes
A
0.01 1.36
1.38
1.40
1.42
1.44
1.46
1/T x 1000 (K
0
2
4
6
8
10
12
14
16
Carbon Number
Figure 2. Distributions of products in thermal cracking of n-Cl6 in benzene a~ a function of initial n-C~emole fraction.
conversion of n-Cl6 was about 65 kcal/mol for each initial n-Cl6 mole fraction, which was slightly higher than the corresponding value of 61.2 kcal/mol obtained from thermal cracking of pure n-C16 (Khorasheh and Gray, 1993). The increase in apparent first-order rate constants with increasing initial mole fraction of n-C16 indicated that the overall reaction order with respect to n-Cl6 was greater than 1. The effect of pressure on the overall first-order rate constants for thermal cracking of some alkanes has been reported in the literature. For example, the first-order rate constants for thermal cracking of propane and n-Butane at 10 and 17 MPa (Hepp and Frey, 1953) and for n-hexadecane at 2 MPa (Voge and Good, 1949) were higher than the corresponding rate constants at atmospheric pressure. Fabuss et al. (1962) reported that the first-order rate constants for thermal cracking of n-hexadecane at 1.5-7 MPa were nearly independent of pressure. Fabuss et al. (1964) summarized the early literature on thermal cracking of alkanes and found that the first-order rate constants increased with increasing pressure up to a maximum of about 10-30 MPa. Beyond this pressure range, further increases in pressure resulted in a decrease in the first-order rate constants. Simulation results have been reported for very high pressure (>lo0 MPa) thermal cracking of n-butane (Mallinson et al.,1992)and n-hexane (Domine et al.,1990) at low temperature (less than 200 "C). Under these conditions, Domine et al. (1990) reported that the firstorder rate constants for thermal cracking of n-hexane would decrease with increasing pressure. Mallinson et al. (1992)reported two different regimes for thermal cracking of n-butane: a high-temperature regime where first-order rate constants increase with increasing pressure and a low-
1.48
1.50
1.52
1.54
-l)
Figure 3. Arrhenius plot of firsborder rate constants for conversion of n-Cle.
temperature regime where the first-order rate constants would initially decrease with increasing pressure (up to about 10MPa) and then increase with increasing pressure. These studies indicate that, depending on the reaction condition, the overall reaction order with respect to the parent alkane may be greater than, approximately equal to, or less than 1. Free-radical chain mechanisms frequently result in fractional order kinetics. Thermal decomposition of certain classes of hydrocarbons, for example 1,3-diarylpropanes (Poutsma and Dyer, 1982;Gilbert and Gajewski, 1982; Smith and Savage, 1991b), is represented by 3/2order kinetics. In thermal cracking of n-alkanes, the overall reaction order with respect to the concentration of the parent n-alkane depends on the order of the initiation reaction and on the type of radical combinations for chain termination reactions. For a first-order initiation reaction, the overall reaction order can vary between 1/2- and 3/2order depending on the concentrations of @ and p radicals, which depend on the relative magnitude of the two propagation steps, namely @-scissionand hydrogen abstraction reactions. 1/2-0rder kinetics arise at low temperatures and high alkane concentrations (high pressure) where parent radicals ( p ) are the dominant radical species. 3/2-0rder kinetics arises at high temperatures and low alkane concentrations where @ radicals are dominant radical species. Complex free-radical chain mechanisms often lead to rate expressions that cannot be represented by power-law kinetics over a wide range of conditions. For example, Savage and Klein (1987,1989) found variations in the first-order rate constants for pyrolysis of n-pentadecylbenzene with changes in the initial reactant concentrations. Concentration dependence of decomposition rates and product selectivities have also been reported for pyrolysis of 1-dodecylpyrene (Smith and Savage, 1991a) and benzyl phenyl ether (Wu et al., 1990).
Ind. Eng. Chem. Res., Vol. 32, No. 9,1993 1867 Product selectivities and decomposition rates for thermal cracking of n-C16 in benzene were also dependent on the initial n-Cl6 concentrations. For a complexfree-radical mechanism for thermal cracking of n-Cl6 in benzene, especially with solvent interactions, a simple power-law rate expression is not adequate to represent the observed kinetics over the range of conditions employed in this study. Selectivity for Biphenyl in Thermal Cracking of n-Cj6 in Benzene. It is interesting to note the presence of biphenyl as a major reaction product in thermal cracking of n-Cl6 in benzene. The average selectivities for biphenyl were 45, 16, and 9 mol/100 mol of n-Cu decomposed at 0.01,0.03, and 0.05 n-Cl6initial mole fractions, respectively. The high selectivities for biphenyl suggested the presence of phenyl as a major radicalspecies. Phenyl radicals would be generated primarily by hydrogen abstraction from benzene. Formation of biphenyl could be either due to combination of two phenyl radicals in a termination step, or due to addition of a phenyl radical to benzene and subsequent elimination of a hydrogen atom from the resulting cyclohexadienyl type radical (Dasgupta and Maiti, 1986; Poutsma, 1990). The presence of biphenyl as a major reaction product indicated that benzene could not be considered as an inert solvent. Benzene has been known to act as a chain transfer agent in free-radical polymerization a t relatively low temperatures (60-100 "C) and affect the degree of polymerization (Walling, 1957). In some recent hydrocarbon pyrolysis studies (Savage and Korotney, 1990; Smith and Savage, 1991a,b), however, benzene was suggested to be an inert diluent. In these studies, evidence for possible solvent interactions was not detected since biphenyl was added to the feed to serve as an internal standard for chromatographic analysis. Thermal Cracking of n-C16 in Alkylaromatic Solvents. Thermal Cracking of Toluene by Itself. The stability of the benzyl radical, resulting from the abstraction of benzylic hydrogens in toluene, was the basis for the development of the toluene carrier gas technique for hydrocarbon pyrolysis in the classical work by Szwarc in the late 1940s and 1950s (Szwarc, 1950). This pyrolytic method was used extensively by Szwarc and other investigators to estimate bond dissociation energies and the heats of formation of radicals (for example Szwarc, 1949; Leigh and Szwarc, 1952a-c). Pyrolysis of toluene proceeds via a free-radical mechanism (Szwarc, 1948;Blades et al., 1954;Takahasi, 1960a) initiated by the cleavage of the benzylic C-H bond (reaction 1):
In the present study, thermal cracking of toluene by itself at 440 "C and 13.9 MPa resulted in the formation of a number of two-ring compounds. The dominant compound was 1,Zdiphenylethane resulting from the combination of benzyl radicals. Other two-ring compounds were also present and were identified by GC-MS as
dimethylbiphenyls
rnelhylbiphenyls
methyldiphenylmethanes
diphenylrnethane
16
16
14
14
n-alkanes
a-olefins
T (T), 12
%Conversion 435.5, 9.86 0 397.0, 10.05
12
10
10 h
0
8
8
8
2 8 6
4 2
6
0
4
:
-
4
0
E
2
n
0
2
4
6
8
10 12 14 16-
Carbon number
0
t
i
2
4
6
8
10 12 14 16 I 4 CISand frons 2-olefins
> 8
-o 5 .-D f -3
2
n
-0 8 4
2
4 6 8 10 12 14 16 Carbon number
10 12 14 16 18 20 22 24 ,
I
I
I
/
I
I
n-alkylbenzenes
0
1 2
8%_,12 01
"
'
*
"
'
L J
Figure 4. Distributions of producta in thermal cracking of n-Cle in toluene.
The isomers shown are arbitrary. These two-ring compounds are formed by termination reactions and radical displacement reactions (for example, reaction 2)
involving benzyl and methylphenyl radicals which are generated in hydrogen abstraction reactions (Blades and Steacie, 1954; Takahasi, 1960b; Cher et al., 1966; Errede and Cassidy, 1960; Badger and Spotswood, 1960). Thermal Cracking of n-Crs in Toluene. In thermal cracking of n-Cu in toluene, the fractional conversion of n-Cle was approximately 70 times greater than the fractional conversion of toluene. Typical product distributions from thermal cracking of n-Cl6 in toluene are presented in Figure 4. The major reaction products were a complete series of n-alkanes in the C1 to C14 range and a complete series of a-olefins in the CZto CISrange. Also present in the products were a complete series of n-alkylbenzenes with the alkyl chain containing 3-16 carbon atoms and a series of cis- and trans-2-olefins in the C4 to CISrange. Experimental methane selectivities were higher than expected selectivities for methane from thermal cracking of n-C16 due to the formation of methyl radicals from toluene. A lower bound on methane selectivity from n-Cl6 decomposition was estimated on the basis of the s u m of selectivities for l-C1~,cis- and trans-2-Cl{s, and n-hexadecylbenzene, which was formed from the addition of benzyl radical to the terminal carbon of 1-c~. The molar selectivities for the Yexcess"methane from toluene were
1868 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 mol froction n-C,6:
120
I
.01
mol fraction n-C,6:
,
I
mol fraction n-C,6: .05
.03
I
T ("C) 397 407 v 417 v 427 0 437 446 0
cis a n d t r a n s
2-olefins
-
1
01
I
I
I
2olw j
c
30
-1
1
I
alkylbenzenes
10
I
0
4
8 12 % Conversion
160
I
I
8 12 Z Conversion 4
I 160
2 4 6 % Conversion
8
Figure 5. Molar selectivities for olefins and alkylbenzenes in thermal cracking of n-Cls in toluene.
approximately 8.4, 2.8, and 1.8 mole/100 mol of n-C16 decomposed for 0.01, 0.03, and 0.05 n-C16 initial mole fraction, respectively. 1,2-Diphenylethanewas also present as a significant reaction product, suggesting the presence of benzyl as a major radical species. Molar selectivities for 1,2-diphenylethane were approximately 9.2, 4.2, and 2.8 mol/100 mol of n-Cu decomposed for 0.01,0.03, and 0.05 n-Cl6 initial mole fraction. The molar ratio of 1,2diphenylethane to all other two-ring compounds was approximately 1:2. Product distributions in Figure 4 illustrate some key features of thermal cracking of n-Cu in toluene. The initial mole fraction of mC16 and the overall conversion of n-C16 were similar in both experiments, but the product distributions were significantly different. The high molar selectivities for Cq to Clan-alkanes for both experiments suggest that alkyl radicals generated from the decomposition of hexadecylradicals tended to stabilize by hydrogen abstraction rather than by @-scission.The distribution of the higher alkanes was nearly equimolar for the experiment at 397 "C, while for the experiment at 435.5 "C, the distribution of n-alkanes was slightly shifted toward lower carbon numbers. The experiment at 397 "C also resulted in lower molar selectivities for a-olefins and higher selectivities for n-alkylbenzenes and internal olefins compared with the experiment at 435.5 "C. The overall selectivities for all experiments are summarized in Figure 5 for a-olefins, alkylbenzenes, and internal olefins. For a given reaction temperature and n-C16 initial mole fraction, total molar selectivities for a-olefins decreased while total molar selectivities for internal olefins and alkylbenzenes increased with increasing n-Cl6 conversion. Figure 5 also indicates that the consumption of a-olefins to n-alkylbenzenes in radical addition reactions became more significant with decreasing reaction temperature. Other than participating in termination reactions, benzyl radicals generated by hydrogen abstraction from toluene could either stabilize by hydrogen abstraction or participate in radical addition reactions to
a-olefins. Addition reactions have lower activation energies than hydrogen abstraction reactions and are more favorable at lower temperatures, resulting in the observed decrease in the molar selectivities for alkylbenzenes with increasing temperature. Total molar selectivities for n-alkanes were approximately 100mo1/100mol of n-Cl6 decomposedand remained relatively unchanged with changes in the reaction temperature or n-Cl6 conversion. Total product selectivities, however, increased slightly with increasing reaction temperature (from approximately 200 mo1/100 mol of n-Cl6 decomposed a t 396 "C to 215 at 447 "C) as radical decomposition by 8-scission was favored at higher temperatures. Thermal Cracking of Ethylbenzene by Itself. Pyrolysis of ethylbenzene has also been studied extensively (Szwarc, 1949; Esteban et al., 1963; Crowne et al., 1969; Clark and Price, 1970; Davis, 1983). In the present study, thermal cracking of ethylbenzene by itself at 442 OC and 13.9 MPa resulted in the formation of a variety of two-ring compounds including diphenylmethane, 1,2-diphenylethane, and 2,3-diphenylbutane (the most dominant compound). Other products included methane, ethane, benzene, and toluene. Styrene (present in the feed as an impurity) was not formed as a major product. The absence of styrene as a major reaction product suggested that, under highpressure and low-temperature conditions employed in this study, the l-phenyleth-l-yland 2-phenyleth-l-yl radicals resulting from abstraction of a- and 8-hydrogens from ethylbenzene, respectively, do not undergo unimolecular decomposition to styrene and a hydrogen atom to any significant extent. The large number of two-ring compounds resulting from thermal cracking of ethylbenzene also suggested that, similar to thermal cracking of toluene, hydrogen abstraction involving aromatic ring hydrogen was also significant. These two-ring compounds were not identified individually. An analogy with toluene suggests that 18different compounds can be formed in various radical combination
Ind. Eng. Chem. Res., Vol. 32, No. 9,1993 1869 mol froctlon n-C,6:
loo
.01
mol fraction n-C,6: .03
mol fraction
n-C,6: .OS
7 a-olefins
mpl
T ("C) 0 401
c i s and trans
z-olefins
411 42' v 431 0 441 451
olkylbenzenes
- 0 0
4 8 12 % Conversion
160
4 8 12 % Conversion
160
4 8 12 % Conversion
16
Figure 6. Molar selectivities for olefins and alkylbenzenes in thermal cracking of n-Cle in ethylbenzene.
reactions involving 1-and 2-phenyleth-1-yl radicals, the three ethylphenyl radicals resulting from abstraction of aromatic ring hydrogens, and the benzyl radical produced by the cleavage of j3 C-C bond of ethylbenzene. Other two ring compounds could be produced in radical displacement reactions. Thermal Cracking of n-C16inEthylbenzene. In thermal cracking of n-Cl6 in ethylbenzene, the fractional conversion of n-Cl6 was approximately 30 times greater than the fractional conversion of ethylbenzene. The product distributions from thermal cracking of n-C16 in ethylbenzene were qualitatively similar to those obtained from thermal cracking of n-Cl6in toluene. Major reaction products were C1 to c14 n-alkanes, CZto C15 a-olefins, C4 to C15 cis- and trans-2-olefins, and two series of alkylbenzenes with the alkyl chain containing 4-17 carbons. In thermal cracking of n-Cl6 in ethylbenzene, two major solvent radicals were present as hydrogens from both aand &carbons of the ethyl group were abstracted. Radical addition reactions involving 1- and 2-phenyleth-1-yl radicals and a-olefins resulted in the formation of l-phenyl1-methylalkanes and n-alkylbenzenes, respectively. The former series was more abundant since hydrogen abstraction from the benzylic (i.e., a) position was favored. Experimental selectivities for methane and ethane from decomposition of n-Cl6 were adjusted as excess amounts of these compounds were produced from ethylbenzene. The presence of excess methane was due to the formation of methyl radicals from the cleavage of the weak 0 C-C bond in ethylbenzene while excess ethane was due to the formation of ethyl radicals in radical displacement reactions. The trends in product distributions from thermal cracking of n-Cl6 in ethylbenzene (Figure 6) with reaction temperature and n-C16 conversion were similar to those observed in thermal cracking of n-C16 in toluene. As was the case with toluene as solvent, the selectivitiesfor internal olefins and alkylbenzenes increased with increasing conversion at the expense of a-olefins. Furthermore, isomer-
ization and addition reactions leading to the consumption of a-olefins had low activation energies and became more favorable with decreasing temperature. Molar selectivities for total n-alkanes and total products were approximately 100 and 200 mol/100 mol of n-Cl6 decomposed, respectively. The two-ring compounds from cracking of ethylbenzene were major products. Molar selectivity for 2,3-diphenylbutane (which was the major two-ring compound) was 40 mol/100 mol of n-Cl6 decomposed at 400 "C and 0.01 initial n-C16 mole fraction. The molar ratio of 2,3diphenylbutane to all other two-ring compounds was approximately 1:2.5. Molar selectivities for two-ring compounds decreased with increasing n-Cl6 initial mole fraction. For a given n-Cl6 initial mole fraction, however, molar selectivities for two-ring compounds decreased with increasing temperature and were unchanged with increasing n-Cl6 conversion. Overall Kinetics for Decomposition of n-Cl6 in Alkylaromatic Solvents. The Arrhenius plots of the apparent first-order rate constants for the overall conversionof n-Cl6 in toluene and ethylbenzene are presented in Figure 7. For both solvents, the first-order rate constants were lower than the corresponding values for thermal cracking of pure n-C16 and thermal cracking of ? d l 6 in benzene. For comparison, the Arrhenius plots of the first-order rate constants for 0.01 mole fraction n-Cu in benzene are also presented in Figure 7. The observed decreases in the firstorder constants could be explained in terms of the stability of the benzyl radical, with toluene as solvent, and the benzylic-type radical (1-phenyleth-1-yl)with ethylbenzene as solvent. In thermal cracking of n-Cl6 in toluene, for example, alkyl radicals generated from the decomposition of parent hexadecyl radicals would participate in hydrogen abstraction primarily from the solvent. Benzylic hydrogens of toluene are readily abstractable and were present at high concentrations. Hence in the followingpropagation steps, where R' is an alkyl radical, reaction 4 would be much faster than reaction 3. The formation of benzyl
t
give n-alkylbenzene radicals, which would subsequently abstract a hydrogen to give the corresponding n-alkylbenzene.
i (0)
mol fraction n-C,,
+ a-olefin
-
-
-
Y
0.1
r
0.01 r
1
mal fraction n-C16 0 01
-
.01 in benzens , .h I
3.1
f, A!
I
I
I
I
I
I
I
I
3.01
1 / T x 1000 (K -')
Figure 7. Arrhenius plot of apparent first-order rate constants for overall conversion of n-Cle in thermal cracking of d (a) and ethylbenzene (b). R*
+
n-C16-
n-alkylbenzene radical
(6)
01 in benzene
I
5
---t
RH + n-Cl
