Ind. Eng. Chem. Res. 1993,32, 1853-1863
1853
High-pressure Thermal Cracking of n-Hexadecane Farhad Khorasheh and Murray R. Gray* Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6
Thermal cracking of n-hexadecane was carried out in a tubular flow reactor at 380-450 "C, 13.9 MPa, and residence times ranging from 0.06 t o 2.0 h, giving conversions of 1 . 5 1 0 % . Primary reaction products were C1 to c14 n-alkanes and Cp to C15 a-olefins, in agreement with a free-radical chain mechanism. Under high-pressure conditions, radical addition to a-olefins became significant. Addition of parent hexadecyl radicals t o a-olefins resulted in the formation of alkyl hexadecanes in the Cla to CB1 range. Addition of lower primary alkyl radicals t o a-olefins gave higher n-alkanes including n-C15 and d 1 7 . Addition of lower secondary alkyl radicals resulted in the formation of C, to c17 branched alkanes. A simple kinetic model based on a free-radical mechanism was developed to account for the observed product distributions and overall n-Cl6 conversion.
Introduction Thermal cracking reactions play an important role in processes for upgrading petroleum residues, heavy oil, and bitumen. In catalytic hydroprocessing of vacuum bottoms (Miki et al., 1983)and synthetic crudes (Khorashehet al., 1989), the role of catalyst was primarily to hydrogenate aromatic structures, thus suppressing coke formation, and to remove heteroatoms. Cracking reactions such as cleavage of side chains, ring opening of naphthenic and hydroaromatic compounds, and dealkylation of alkyl aromatics are primarily thermal reactions which are responsible for reduction in molecular weight and formation of lighter products. In the absence of a catalyst, however, a high hydrogen pressure is required to suppress coke formation during thermal hydroprocessing of heavy oil and bitumen (Merril et al., 1973). Thermal and catalytic hydrocracking processes are usually carried out at moderate temperatures (400-450 "C) and under relatively high hydrogen pressure (14 MPa). These conditions are different from high temperatures (>600 "C) and low pressures (atmospheric) which are typically employed in hydrocarbon pyrolysis. Hightemperature, low-pressure pyrolysis of hydrocarbons, in particular low-molecular-weight alkanes for production of ethylene and propylene, has been studied extensively in the pyrolysis literature since the early work by Rice, Herzfeld, and Kossiakoff in the 1930s and 1940s (for example, Rice, 1933; Rice and Herzfeld, 1934; Kossiakoff and Rice, 1943). Kossiakoff and Rice (1943) proposed a free-radical chain mechanism for hydrocarbon pyrolysis which, in the case of n-hexadecane, predicts Cp to c15 a-olefins, methane, and ethane as primary products. The product distribution for low-pressure thermal cracking of n-C16 has been verified by several investigators (Voge and Good, 1946; Fabuss et al., 1964; Depeyre et al., 1985). Thermal cracking of high-molecular-weight (C12+)alkanes under high pressures (>lo MPa) and relatively low temperatures (350-450 "C), however, has received much less attention in the literature. At high pressures, bimolecular reactions (radical addition and hydrogen abstraction) are favored over the unimolecular radical decomposition. Radical decomposition reactions also have higher activation energies than bimolecular reactions and are favored at higher temperatures. At high temperatures and low pressures, higher alkyl radicals undergo successive decomposion by @-scission(Rice-Kossiakoff (R-K) mechanism) leading to methane and ethane as the only saturated
* Author for correspondence.
products. A t low temperature and high pressures, however, higher alkyl radicals also participate in bimolecular hydrogen abstraction reactions to give higher alkanes as major reaction products (Fabuss et al., 1964). High-pressure thermal cracking of higher alkanes (C11+) such as n-Cl6 was first studied by Fabuss et al. (1962, 1964). Other investigators such as Doue and Guiochon (1968, 1969) and more recently Shabtai et al. (1979), Mushrush and Hazlett (1984),Blouri et al. (1985), Kissin (19871, Ford (19861, and Zhou et al. (1986, 1987), have reported thermal cracking of higher alkanes at high pressures. Fabuss, Satterfield, and Smith (F-S-S) (1962, 1964) proposed a modification to the R-K mechanism to explain the product distributions for cracking at elevated pressures of 1-7 MPa, and typical pyrolysis temperatures of 550-600 "C. They observed a complete series of n-alkanes in the Cp to c14 range. The formation of these compounds was due to participation of n-alkyl radicals in hydrogen abstraction reactions. To account for these products, they proposed the one-step F-S-S mechanism where the parent radicals undergo a single-step decomposition, and the resulting lower alkyl radicals participate in hydrogen abstraction from the parent alkane to give the corresponding alkanes. For n-Cl6, the chain propagation reactions for the single-step F-S-S mechanism are n-Cl6 + R1'
n-C,,'
-
-+
l-CjHG +
R1H + n-C16'
-
(i + j = 16)
(1)
(2)
n-CiH2i+l*+ n-CiHi+2+ n-cl,' (3) where R1' is an n-alkyl radical and R1H is the corresponding n-alkane. The single-step F-S-S mechanism results in an equimolar distribution of n-alkanes and a-olefins, and very low gas selectivities. As the number of decomposition steps are increased, the product distribution shifts toward lower carbon numbers and lower selectivities for higher n-alkanes. At low pressures and high temperatures, the radical decomposition steps proceed to completion (R-K mechanism). The R-K mechanism (when 0-scission is favored over hydrogen abstraction) and the F-S-S mechanism (when hydrogen abstraction is favored over @scission) are two extremes of a free-radical mechanism for pyrolysis of alkanes. Mushrush and Hazlett (1984) suggested that, for an intermediate pressure of 0.7 MPa, a two-step F-S-S mechanism along with R-K intramolecular hydrogen abstraction adequately represented the observed product distribution for n-Cl6 cracking. Blouri et al. (1985) and Ford (1989) found that the product distribution from liquid-phase thermal cracking
Q88s-5885/93/2632-1853$04.QQlQ 0 1993 American Chemical Society
1854 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993
of n-Cl6 at mild temperatures (350-440 "C) could be described by a single-step decomposition. They observed very low gas selectivities (unlike the R-K mechanism) and an equimolar distribution of n-alkanes and a-olefins for liquid products in the Cg to C13 range at low conversions of n-hexadecane. Kissin (1987) and Ford (1986)suggested a single-step (F-S-S) free-radical mechanism for paraffin cracking under high pressures. These investigators also indicated that, under conditions of high pressure, isomerization by intramolecular hydrogen abstraction, which is an important step in the R-K mechanism, was slow compared with bimolecular hydrogen abstractions. In contrast, Blouri et al. (1985)suggesteda molecular scission, rather than a free-radical mechanism for n-C16 thermal cracking at high pressures, on the basis of low gas selectivities and an equilmolar product distribution for n-alkanes and a-olefins. Not surprising, the molecular scission mechanism as proposed by Blouri et al. (1985) predicts a very similar product distribution to the onestep F-S-S mechanism. The validity of such a molecular mechanism for paraffin cracking was questioned by Zhou et al. (19871, who suggested a free-radical mechanism for paraffin cracking regardless of the pressure. The molecular scission mechanism fails to explain the formation of higher-molecular-weight alkylhexadecanes in thermal cracking of n-C16. Formation of compounds heavier than c16 is expected at elevated pressures in the presence of free-radicals, since bimolecular reactions are favored under such conditions. Fabuss et al. (1962,1964) observed these high-molecular-weightcompounds in thermal cracking of n-Cl6 and indicated that they were primarily alkanes in the Cla to CZOrange. The presence of high-molecular-weight compounds in high-pressure thermal cracking of alkanes was also reported by Shabtai et al. (1979),Zhou et al. (1986,1987), and Ford (1986).At low conversions, however, high-molecular-weight compounds were absent in products from high-pressure thermal cracking of alkanes (Blouri et al., 1985;Ford, 1986; Kissin, 1987). These results suggest that the formation of high-molecular-weight compounds is due to participation of the primary cracking products, i.e., a-olefins, in bimolecular reactions since they are present at high concentrations under high-conversion conditions. Zhou et al. (1986,1987)suggested radical combination involving the parent radicals and the lower alkyl radicals produced from 8-scission of the parent radicals, to account for the formation of higher-molecular-weight saturated alkanes. Ford (1986), on the other hand, identified all of the highmolecular-weight products in n-Cl6 cracking as alkylhexadecanes. He suggested that their formation was due to the addition of the parent n-hexadecyl radicals to the terminal carbon of the a-olefins. The resulting alkyl radicals would then stabilize by abstracting a hydrogen from n-C16 to give the corresponding alkane. Ford (1986), however, did not give detailed kinetics, in particular for radical addition reactions, for liquid-phase cracking of n-hexadecane. The purpose of this study was to examine the reaction mechanism and kinetics of thermal cracking of n-C16 (as a representative n-alkane) at high pressures (13.9 MPa) and low temperatures (380-450 "C) with emphasis on modeling the addition reactions that lead to the formation of higher-molecular-weight compounds.
Methods and Materials n-Hexadecane was obtained from Aldrich with 99.85 % purity. Thermal cracking reactions were carried out in a tubular flow reactor. The apparatus was designed after
to Vacuum
(f7
Liquid Sampling
Reactor I
I" Feed Pump
Oven
Figure 1. Schematic diagram of the equipment.
Broderick (1980). A schematic diagram of the equipment is given in Figure 1. The apparatus was designed to feed a liquid, saturated with a dissolved gas, to the reactor for thermal hydrocracking experiments. Saturation was achieved by means of a magnetic circulating pump (Ruska et al., 1970) by recycling the gas from the top of the saturator through the liquid. Once saturation was achieved, the circulating pump was isolated from the saturator and the liquid containing the dissolved gas was pumped to the reactor. In the case of high-pressure thermal cracking of n-Cl6, neat reactant was fed to the 1000-mLsaturator and subsequently pressurized with Nz to a pressure about 1MPa lower than the desired reaction pressure. n-Hexadecane was fed to the reactor by means of a high-pressure pump. The reactor was made up of two 30-cm segments of 114in.-o.d., 4-mm-id., glass-lined stainless steel tubing which were connected in series with 1/16-in.-o.d., 0.7-mm-i.d., glass-linedstainless steel tubing. The feed line to the first segment and the product line from the second segment were 10-cm segments of the 1/16-in. glass-lined tubing. Stainless steel Swagelokfittings were used for connections. Total reactor volume, including the fittings, was 8.0 mL. The fittings were the only metal surface that the reactants were exposed to under reaction conditions. These provisions minimized the contact between the reacting media and stainless steel which may result in some catalytic activity. The reactor was placed inside an air bath oven. The temperature of the oven was monitored and controlled to within f l "C of the desired temperature. The length to diameter ratio for the reactor was approximately 150 and the surface to volume ratio was 1000 m2/m3which would allow for a high surface area for heat transfer to maintain near isothermal conditions with the air bath. Although a thermocouple was not placed inside the reactor, heattransfer calculations (Khorasheh, 1992)indicated that for most experiments the reaction temperature would rapidly increase from room temperature to within 1 "C of the oven temperature before the end of the 10-cm segment of the 1/16-in. line at the reactor inlet. The volume of this inlet line was only 0.5% of total reactor volume. Furthermore, thermal cracking reactions are weakly endothermic ( A H R O = 19.5 kcal/mol) hence eliminating the occurrence of hot spots inside the tubular reactor. Heattransfer calculations indicated that the temperature
Ind. Eng. Chem. Res., Vol. 32, No. 9,1993 1855 T ("C)
600
550
500
450
400
350
10'
- Low pressure 10'
gas phase and liquid phase
0.1 MPa Liquid phose, Ford (1986)
A
- High pressure
4
A
gas phose
1.4 to 7.0 MPa 0.6 to 1.0 MPa
1 o2
-z
10'
h
I
v
3
1 o'
lo-'
lo-'
lo-' 1.1
1.2
1.3
1.4
1.5
1.6
1.7
1/T x 1000 (K -')
Figure 2. Arrhenius plot of first-order rate constants for conversion of n-&.
difference between the reactor contents and the air bath due to the endothermic cracking of n-Cl6 was between 0.05 and 0.3 "C, which is within experimental error of the thermocouple measurements. The products were collected in either of two 500-mL receivers. Transient products were accumulated in receiver 1. Product flow was diverted to receiver 2 when steady state was achieved. The pressure inside the receivers and the reactor was maintained a t a desired level by means of a back-pressure regulator. At the end of the experiment, a sample was withdrawn into an evacuated sampling bomb. The product gas was analyzed for C1 to C6 gases by gas chromatography (GC). The procedure and conditions for gas analysis are outlined elsewhere (Chung, 1982). Liquid products were analyzed by a gas chromatograph equipped with a DB-1 capillary column and a flame-ionizationdetector. The temperature program was as follows; initial temperature of 35 "C for 3 min, up to 300 "C a t 3 "C/min, and held a t 300 OC until elution was completed. A small amount of naphthalene (typically 0.1-0.2% of n-Cl6 by weight) that was added to the feed served as internal standard.
Results and Discussion Thermal cracking of n-Cl6 was carried out at 13.9 MPa and 38&450 "C with feed flow rates selected to give conversions of 1.5-10 5%. The Arrhenius plot of first-order rate constants for overall conversion of n-C16 is presented in Figure 2. The rate constants were estimated using residence times based on reactor inlet conditions. The molar volume of n-Cl6 at reaction temperature and pressure was estimated using the Peng-Robinson equation of state (Peng and Robinson, 1976). The estimated activation
energy of approximately 61.2 kcal/mol is quite close to the accepted value of 60 kcal/mol for higher paraffins (Fabuss et al., 1964; Voge and Good, 1949; Shabtai et al., 1979). The lines in Figure 2 represent literature data from lowpressure and high-pressure gas-phase as well as liquidphase thermal cracking of n-Cl6 reported by other investigators and compiled by Ford (1986). The rate constants and the associated activation energy obtained in this study are in good agreement with available literature data. The first-order rate constants from this study are lower (approximately by a factor of 2) than values reported for gas-phase thermal cracking of n-Cl6 a t similar temperatures but much lower pressures. They are, however, in excellent agreement with first-order rate constants for liquid-phase thermal cracking of n-Cl6 reported by Ford (1986) over a similar range of temperatures. This result suggests that the assumption of isothermal conditions inside the reactor is reasonable. The products obtained in thermal cracking of n-Cl6 fall into two general categories; those with molecular weight lower than n-C16 and those with higher molecular weight. Among the lower-molecular-weight products was a complete series of n-alkanes from C1 to C14 and a complete series of a-olefins from CZ to C15. These were major reaction products. n-Cls was also present, but only as a minor reaction product. These n-alkanes and a-olefins were easily identified from the chromatogram of the liquid products by their retention times. Also appearing in smaller quantities between successive n-alkane and a-olefin peaks were isomeric products representing branched alkanes and internal olefins. Of these isomeric products, only a series of cis- and trans-2-olefins were identified. cis- and trans-2-butene and cis- and trans-2-pentene, which were also present in the product gas, were identified by comparison of their retention times in the GC analysis of the product gas with retention times from a calibration mixture containing the above compounds. c6 and C7 cisand trans-2-olefins were also available as calibration compounds for liquid GC. All compounds appearing between n-C, and n-C,+1 in the chromatogram of the liquid products with the exception of l-C,+1 and cis- and trans-2-Cm+1 olefins were lumped together as Cm+l branched alkanes. The higher-molecular-weight compounds were previously identified by Ford (1986)and represented a complete series of alkylhexadecanes and corresponding n-alkanes in Cle to Csl range. The alkyl branches appear on carbons 2 to 8 of a hexadecane backbone. These compounds were identified on the basis of their retention order as reported by Ford (1986). n-C17 was also present as a minor product. Selected product distributions are presented in Figure 3 (for a reaction temperature of 402 "cand different n-Cl6 conversions) and Figure 4 (for 6.6% conversion and different reaction temperatures) in which molar selectivities are presented in moles of products formed per 100 mol of n-C16 decomposed. Overall product selectivities for all experiments are summarized in Figure 5. The most striking feature of the product distributions is the very high and nearly equimolar selectivities for n-alkanes in the Ca to C13 range, which is consistent with the single-step F-S-Splechanism. At high pressures (>10 MPa) and relatively moderate temperatures (400 "C), free radicals generated from decomposition of parent radicals are stabilized much faster by hydrogen abstraction than by decomposition via 8-scission. The ratio of the rate of saturation of primary butyl or higher radicals by hydrogen abstraction to the rate of decomposition by 8-scission can be estimated using rate parameters suggested by Ranzi et
1856 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993
n-alkanes
a-olefins
branched alkanes
t
0
2
6
4
8
10 12
Carban
14 16
0
2
4
#
6
8
10 12 14 16
Carbon
I
-1
1
..................................................................
,
,
*
,
,
,
16 18 20 22 2 4 26 28 30 32
#
Carbon
#
conversion.
Figure 3. Distributions of products in thermal cracking of n-Cle at 402 O C as a function of
n-alkanes
% Conversion:
a-olefins
branched alkones
0
402, 6.62
o 434, 6.61
Ii I
......
0
2
4
6
8
Carbon
10 12
14 16
0
2
4
#
6
8
Carbon
10 12 14 16
................................................................
16 18 20 2 2 24 26 28 30 32
#
Figure 4. Distributions of products in thermal cracking of n-Cle as a function of temperature at 6.6%
al. (1983). In the case of n-hexadecane, for example, at 600 OC and 0.1 MPa, 400 OC and 0.1 MPa, and 400 OC and 13.9 MPa, these ratios are 0.03:1, 0.91, and 1101, respectively. Under conditions employed in this study, the rates of hydrogen abstraction reactions were much faster than j3-scission reactions resulting in equimolar distribution of n-alkanes and low selectivities for gases. Note in particular the low selectivities for ethylene, which is a major product in low-pressure pyrolysis of alkanes. Some modifications, however, are required to the simple single-step F-S-S mechanism to explain the following observations: (1)formation of higher-molecular-weight alkylhexadecanes; (2) formation of lower-molecular-weight branched alkanes, n-Cu, and n-Cl7; (3) decreasing molar selectivities for a-olefins with increasing conversion. The product distributions from thermal cracking of n-Cp3, in particular molar selectivities for a-olefins and
Carban
#
n-cpjconversion.
alkylhexadecanes, were highly dependent on the overall n-C16 conversion. This observation was in spite of the low-conversion conditions employed in this study. Product distributions presented in Figure 3 show a substantial decrease in the molar selectivities for a-olefins over the entire carbon number range with increasing n-Cl6 conversion. For n-alkanes, molar selectivities also decreased with increasing conversion but not to the same extent as the a-olefins. On the other hand, the molar selectivities for branched alkanes and alkylhexadecanes increased with increasing conversion. The results of this study confirm Ford's suggestion that the formation of high-molecular-weight compounds is due to addition of alkyl radicals to a-olefins and not radical combination reactions. As summarized in Figure 5, for a given reaction temperature, as the conversion is increased, molar selectivities for a-olefins are decreased, molar
Ind. Eng. Chem. Res., Vol. 32, No. 9,1993 1857
-;
200
d
180
2
0
P
X Conversion 4 6
X Convarsion
8
1 0 0
ii
2
4
6
8
1
1
1
1
Y)
P
1.8 E
-
E
I)
0
5!
0
0
1.6
160
0
2 E
v
0
i 140
1.4
p
-
7c
r
L
.
-
0
120
0
a
100 0 382 0
0
392 "C
1.2
i
Total products
0
402 "C 412 'C
a-olefins
A
422 432
C,,+
-0
2
4
6
X Conversion
8
1 0 0
'c 'c
0
442 "C
branched alkanes
B
s
%
1 .o A
I
,
2
4
Z
I
6
8
1"; 5
combination reactions with the parent radicals. In view of the radical addition mechanism, the absence of m 1 alkanes is expected since ethylene is the smallest a-olefin. Recent studies in thermal cracking of n-hexane at very high pressures (21-1560 MPa) and low temperatures (290365 "C) (Domine, 1989; Domine et al., 1990) also confirm the formation of higher-molecular-weight saturated alkanes via radical addition reactions. Radical addition reactions have quite low activation energies (2-8 kcal/mol) and are favored at low temperatures. As temperature is increased, the molar selectivities for a-olefins increased while molar selectivities for highermolecular-weight products decreased (see Figure 4 for constant conversion). The effect of conversion level on selectivities for a-olefins and total products was most pronounced at low temperatures. As illustrated in Figure 5, plots of molar selectivitiesversus conversiongave smaller slopes as temperature was increased from 382 to 422 O C . It is interesting to note the presence of mC15 and n-C17 as reaction products. n-C15 is not a primary product in thermal cracking of n-Cl6, and n-C17 is not a product resulting from addition of n-hexadecylradicals to a-olefiis. The presence of n-C15and n-C17as minor reaction products suggests that the primary lower alkyl radicals, which are produced by j3-scission of the parent hexadecyl radicals, compete with the parent radicals for addition to a-olefins. Addition of these primary lower alkyl radicals to the terminal carbon of a-olefins results in the formation of higher secondary n-alkyl radicals, which then abstract hydrogen from n-Cl6 to form the corresponding n-alkane. For example, addition of primary n-butyl radicals to the terminal carbon of l-Cn and 14213results in the formation of secondary n-Cl5 and n-C17 radicals, respectively. Addition reactions leading to the formation of higher n-alkanes mainly involve methyl to n-butyl primary radicals. Primary pentyl and higher radicals produced from decomposition of parent radicals can isomerize rapidly to secondary radicals by 1-4 and 1-5 internal hydrogen abstraction. Addition of these secondary lower alkyl radicals to the terminal carbon of a-olefins resulted in the formation of a large number branched alkyl radicals, which subsequently abstracted hydrogen from n-Cl6 to give the corresponding branched alkanes. For high-pressure thermal cracking of paraffins, some investigators, for example Ford (1986) and Kissin (1987), suggested that radical isomerization by internal hydrogen abstraction was absent. Other investigators, for example Mushrush and Hazlett (1984)and Zhou and Crynes (1986), however, have suggested radical isomerization during highpressure cracking of paraffiis. Under conditions employed in this study, hydrogen abstraction reactions were much faster than radical decomposition reactions giving rise to nearly equimolar distribution of n-alkanes and a-olefins. Hence it is difficult to confirm or rule out radical isomerization from the distribution of primary reaction products, namely a-olefins and n-alkanes. The presence of branched alkanes in the products, however, suggests that primary alkyl radicals do undergo isomerization. More conclusive evidence for radical isomerization under highpressure conditions is presented in a subsequent study on high-pressure thermal cracking of n-Cl6 in aromatic solvents (Khorasheh and Gray, 1993). The above discussions can be summarized in terms of a free-radical mechanism as illustrated in Figure 6. This simplified mechanism is appropriate only for low n-C16 conversions where secondary reactions, in particular those involving hydrogen abstraction from primary products and
+
10 2.0
j
10-s
Convarslon
F@re 5. Overall productselectivitiesin thermal cracking of n-Cl6.
selectivities for alkylhexadecanes are increased, the ratio of n-alkanes to a-olefins is increased, and total product selectivities are decreased. These results are consistent with radical addition reactions which become more significant at higher conversions where a-olefins are present at higher concentrations. With the single-step F-S-S mechanism, the molar selectivities for both n-alkanes and a-olefins are 100 mol/ 100 mol of n-Cl6 decomposed. If radical combination involvingparent hexadecyl radicals and lower alkyl radicals was responsible for the formation of higher alkanes, then for every x mol of higher alkanes produced from 100 mol of n-Cl6 decomposed, the total selectivity for n-alkanes would be 100- 2%mol, and the total selectivity for a-olefins would be 100 - x mol per 100 mol of n-C16 decomposed because the parent radical could not complete its normal decomposition to an a-olefin and a lower alkyl radical. In other words, one would see a greater decrease in the molar selectivities for n-alkanes than for a-olefins. For such a mechanism, the ratio of n-alkanes to a-olefins would have been less than 1, and it would have decreased with increasing conversion. The opposite trend was observed in this study (Figure 5). It is also interesting to note that Fabuss et al. (1962, 1964) and Zhou and Crynes (1986) reported that among the higher-molecular-weight compounds produced in thermal cracking of n-alkanes of chain length m, the m 1alkanes were missing. For example, Zhou and Crynes (1986) reported that, in thermal cracking of n-dodecane a t temperatures up to 440 O C and pressures in the range of 9.2-10.3 MPa, C13 compounds were missing among the C12 + (up to C22's) producta. They used a stirred batch reactor with liquid and gas phases present a t the reaction conditions, and N2 as an inert gas. They suggested that the absence of m + 1alkanes was due to methyl radicals escaping to the gas phase and not participating in
+
1858 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 n-CI6
R'
+
-
+%
Ri
n-C16
(1)
+
R-H
n-Ci6
(2)
n4i6 cn4i6
(isomer)
(3)
"-Cis
+
(4)
-
wle6n
primary n-alkyl radical
primary n-alkyl radical
secondary n-alkyl radical
+
n-Ci6
a-dle6n
--
secondary n-alkyl radical R' Ri
+ Ri
-
wle6n
+
(5)
(isomer)
(6)
higheralkylradical
+ cr-ole6n + a-dle6n
primary n-alkyl radical
(isomer)
= secondary n-alkyl radical
-
secondary n-alkyl radical
(7)
higher n-alkyl radical
(8)
higher branched alkyl radical
primary n-alkyl radical
(9) (10)
products
(11)
Figure 6. Reaction mechanism for high-pressure thermal cracking of n-&. + "C16
H-abrlmtion
RoduQl
+ Level I
Addition to oldms
I
I
1
I
lsancriration
Level I
+
R,'
eLevel1
Level I
n q 6 + Level I
I I I Isaneriution
= *
Level II
Level II
+
H-abstraction
+ nCI6 Decanpxilion
Level II
+ R,CH,CH=CH,
.--, R,H
+ R,CHCH=CH,
R,CHCH=CH, + R,CH=CHCH,
H-abbrlraction
".CIS-
Level II
produced by an addition reaction does not participate in a subsequent addition reaction. The assumption that level I11 radicals do not decompose or isomerize and then decompose is also reasonable since, under conditions employed in this study, hydrogen abstraction reactions are much faster than radical decomposition. The products from thermal cracking of n-hexadecane included 28 major components, namely the C1 to C14 n-alkanes and Cp to C15 a-olefins, and over 200 minor products resulting from isomerization and addition reactions. In order to simplify the computations, branched isomers of the same carbon number were grouped as a single species. For example, branched CZOcompounds included all of the 2-butyl-to 8-butylhexadecanes (addition of level I radicals to 1-butene)and all other C:!Ocompounds from addition of level I1 radicals to a-olefins. Cis and trans isomers of 2-olefins were also grouped as single species. A total of 84 products were considered. These included n-alkanes from C1 to c31, a-olefins from C:!to C15, cis- and trans-Zolefins from C4 to c15, and branched alkanes from C5 to C31. Shabtai et al. (1979)suggested the followingmechanism for the formation of 2-olefins during thermal cracking of n-Cls:
Producis c Levell
R,CH=CHcH,
+ n-C,,
+
R2CH=CHCH3
(12) (13)
+ n-C,;
(14) For simplicity, a first-order reaction was included in the model to account for isomerization of a-olefins to 2-olefins as follows:
OlefmI
kk7-2,
a-olefin cis- and trans-2-olefin (15) It was assumed that initiation reactions only involved the cleavage of the C-C bonds of the parent molecule. Furthermore, it was assumed that hydrogen was abstracted only from the parent molecule. These two assumptions are valid at low conversions where the parent compound is predominant. At high conversions, initiation and hydrogen abstraction from products would become significant. Chain termination included all possible radical combinations. Products from termination reactions, however, were not included in the model. Under typical conditions employed in this study, kinetic chain length was approximately lo3. The reactor used in this study was a tubular flow reactor. For each molecular and radical species, therefore, one may write the following differential equation: -+
Addition to olefur
the molecular retro-ene mechanism for decomposition of a-olefins (Rebick, 1979,19831,are assumed to be negligible. Description of the Kinetic Model To incorporate the proposed reaction mechanism (Figure 6) in the kinetic model, three levels of radicals were considered. These levels were differentiated on the basis of the propagation reactions in which they participate. The reaction network involving the three levels of radicals is illustrated in Figure 7. Level I radicals are the parent radicals which can participate in isomerization, hydrogen abstraction, addition to a-olefins, and @-scissionreactions. Level I1 radicals are the lower alkyl radicals which are formed from @-scissionof level I radicals. Level I1 radicals can participate in isomerization, addition to a-olefins, hydrogen abstraction, and @-scissionreactions. Level I11 radicals are those produced in addition reactions involving the a-olefins and level I and level I1 radicals. It was assumed that level I11 radicals would only participate in hydrogen abstraction reactions. This assumption was necessary to simplify the reaction network and limit the number of radical species. A very large number of radical species would otherwise be present due to successive addition of radicals to a-olefins. The assumption used in the model is that a radical which is
dFi/dV = net rate of formation of species i (16) where Fi is the molar flow rate of species i, and V is the reactor volume. In order to facilitate numerical integration of eq 16, the pseudo-steady-state approximation may be used for radical species which were present at very low concentrations. Thus equations of the form given by eq 16 for radical species become algebraic equations. The approach taken in the model was to apply the pseudosteady-state approximation to increments of reactor volume. At each incremental reactor volume,one can write the following equation for radical species: net rate of formation of radicali = 0 (17) Equations of the form given by eq 17 for radical species are a system of linear algebraic equations which can be
Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 1859 solved to obtain the radical concentrations. This system of equations contained a large number of equations, so a sequential method was used for computations. First the concentrations of level I radicals were obtained. For each level I radical RIj (where j refers to the radical position) one may write, for example for j = 1,
Table I. Summary of Rate Parameters in Thermal CraCkilng Of D - c ~ t
E4 log A* 18.7 11.0 k(iDO)(l4)@#) 11.7 10.2 k(boWM8,P) 23.0 11.0 k(bo)(14)(8,P) 16.0 10.2 k(h)(lA)(ia) 20.7 11.0 k(i.o)(ld)(ia) 13.7 10.2 8-scissiond k(dec)@-p) 30.0 14.0 k(dec)(rp) 31.0 14.0 k(dec)(rMe.) 33.0 14.0 kdec)WMv) 32.0 14.0 H-abstractione k(.b)@,*l 14.0 8.6 k(.b)(S.P) 16.0 7.5 k(.b)(l#) 13.7 7.5 k(.b)@#) 11.7 8.6 isomerizationf kb-2) 61.0 12.2 addition kadd 7.0 g ki 67.0 12.6 kt 0 10.0 E , activation energy in kcallmol. b A, preexpnoential factor in s-l or L mol-' s-l. 1-4 and 1-5 refer to type of isomerization; (ij) refers to the type of radical, i, and the type of hydrogen,j . p, primary; s, secondary. For &scission (i-j) refers to reacting radical, i, and resultingradical,j . e (ij)refers to radical type, i, and hydrogen type, j . f (-2) refers to isomerization of a-olefins to cis- and trans-2olefins. g Rate constant for addition reactions depended on radical size; see text. reaction isomerization'
rate constant k(W(l4)W
(I
where the first term in eq 18 refers to isomerization of RIi to RI1, the second term refers to isomerization of RI1 to RIi, the third term refers to disappearance of RI1 via addition to a-olefins, the fourth term refers to disappearance of RI1 via 8-scission, the fifth term refers to disappearance of RI1 in hydrogen abstraction from parent molecule resulting in the formation of RIi radicals (i # l), the sixth term refers to the formation of RI1 from hydrogen abstraction reactions involving other parent radicals RIi (i # l),the seventh and eighth terms refer to formation of RI1 radicals from hydrogen abstraction reactions involving primary and secondary radicals, respectively, of level I1 and I11 radicals, and the last term refers to disappearance of RI1 by termination reactions. To solve eq 18 for all parent radicals, estimates were required for the concentration of a-olefins and parent molecule, the total radical concentration, ROT, and the fraction of level I1 and I11 radicals which were primary,
#.
Concentrations of the parent alkane and a-olefins were obtained from the mole fraction of these species and the molar volume of the mixture as determined from the PengRobinson equation of state (Peng and Robinson, 1976). Critical constants for individual compounds were taken from Reid et al. (19771, when available, and were extrapolated for higher-molecular-weightcompounds. The critical constants for branched alkanes were assumed to be the same as the corresponding n-alkanes. Binary interaction parameters were assumed to be zero. An initial estimate for R'T was obtained using eq 19 assuming that initiation involved only the parent molecules:
where ki is the rate constant for initiation involvingasingle C-C bond, kt is the rate constant for termination reactions, and the factor of 15 indicates the existence of 15 C-C bonds for the parent molecule. To handle the nonlinear term (last term) of eq 18, initial estimates for the concentration of all radicals were required. These were set to zero initially and subsequently updated. Parameter # was guessed initially and was subsequently obtained by the Gauss-Seidel iterative procedure. For an initial guess for #, equations of the form given by eq 18 were solved simultaneously by a
Gaussian elimination algorithm to obtain the concentrations of level I radicals. The concentrations of level I1and level I11 radicals were then calculated. Parameter # was updated and the procedure was repeated until # converged. The convergence criteria for # was chosen such that the difference in #between successive iterations was less than 1 X 1o-S. Once convergence of # was obtained, R'T was updated using the following equation:
where the summation extends over all radical species i. Once convergence of R*Twas obtained, equations of the form given by eq 16 corresponding to molecular species were solved numerically using the fourth-order RungeKutta algorithm. The integration step was 0.1 mL, and the total reactor volume was 8.0 mL.
Model Parameters and Results Activation energies, E, and preexponential factors, A, for various reactions considered in the model are summarized in Table I. The kinetic parameters used for isomerization, 8-scission, and hydrogen abstraction were based on values reported by Ranzi et al. (1983). For these reactions, the values of A and E depend on the type of radical involved and on the resulting products. There are, however, the following minor differences between parameters adapted in this work and those reported by Ranzi et al. (1983). Activation energies for &scission reactions leading to the formation of methyl radicals were assumed to be 2.0 kcal/mol higher than the corresponding reactions leading to the formation of primary radicals. A value of 1.5 kcal/mol was suggested by Ranzi et al. (1983). This slight adjustment was necessary to obtain a good agreement between predicted and experimental selectivities for methane and 1415 in the present study and in thermal cracking Of n-C16 in aromatic solvents (Khorasheh and Gray, 1993). Values of 8.6 and 7.5 were used for log A for hydrogen abstraction reactions involving primary and secondary radicals, respectively. The value suggested
1860 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993
by Ranzi et al. (1983) for both reactions was 8.3. This adjustment was necessary in order to get good agreement between predicted and experimental distributions of n-alkanes and a-olefins in thermal cracking of n-C16 and thermal cracking Of n-Cl6in aromaticsolvents (Khorasheh and Gray, 1993). A value of 1O'O was chosen for the preexponential factor of termination reactions. The activation energy for termination reactions was chosen to be 0, as expected for radical recombination. The activation energy for most initiation reactions involving the cleavage of C-C bonds is about 80 kcal/mol which is very close to the aliphatic C-C bond energy. For pyrolysis of n-Cl6, activation energies between 60 and 80 kcal/mol for initiation have been reported in the literature. For example, Depeyre and Flicoteaux (1991) suggested a value of 59 kcal/mol while Doue and Guiochon (1968) suggested an activation energy of 80 kcaVmol for initiation. Rate parameters for initiation reactions have a significant effect on the total radical concentration (eq 20) and thus on the overall conversion of n-hexadecane. In this study, rate parameters for initiation reactions were estimated to match the predicted conversions of n-Cl6 with experimental values. The activation energy and log A for cleavage of a C-C bond of n-C16 were estimated to be 67 kcal/mol and 12.6, respectively. The lower activation energy for initiation compared with C-C bond energy suggested a heterogeneous initiation involving the reactor wall. In thermal cracking of alkanes, the reaction order with respect to the parent alkane could vary between 1/2-order and 3/2-order depending on the reaction conditions. Under high-pressure, low-temperature conditions employed in this study, hydrogen abstraction reactions were much faster that @-scissionreactions and an approximate rate expression is given by
where M is the parent alkane, E* is the overall activation energy, and kdW, ki,and kt refer to the rate constants for @-scission,initiation, and termination, respectively. If one assumes zero activation energy for termination reactions, 30 kcal/mol as a typical activation energy for @-scission, and 80 kcal/mol for homogeneous initiation, the expected overall activation energy is 70 kcal/mol. The lower observed activation energy (61.2 kcal/mol) for overall conversion Of n-Cl6 was due to the heterogeneous initiation with lower activation energy (67 kcal/mol). Radical addition reactions, which become favorable at high pressures and low temperatures, had a profound effect on the product distributions. Product selectivities were highly conversion dependent as addition reactions became more significant at higher conversions due to higher concentrations of a-olefins. In liquid-phase thermal cracking of n-hexadecane (Ford, 1986), radical addition reactions involved parent radicals exclusively, giving rise to n-alkanes and alkylhexadecanes in the CISto Csl range as addition products. One explanation for this observation is that, under low-temperature liquid-phase conditions, the total radical concentration is dominated by parent hexadecyl radicals. Another explanation is that, under high-density conditions, addition reactions involving lower alkyl radicals are suppressed due to a cage effect as summarized by the following sequence of reactions:
-
n-C16* primary alkyl radical + a-olefin
primary alkyl radical + n-C,,
+
n-C16' a-olefin
-
-
(4)
alkane + n-cl,' (2)
higher alkyl radical
(7)
where lower alkyl radicals generated from the decomposition of hexadecyl radicals are stabilized by hydrogen abstraction from n-C16 to regenerate parent radicals which participate in @-scissionand addition reactions. Predicted free-radical concentrations in thermal cracking of mC16 are presented in Figure 8 for conditions employed in this study (5" = 382-450 "C, P = 13.9 MPa). These concentrations were predicted using kinetic parameters reported in Table I. Under the above conditions, total radical concentrations were dominated by parent radicals. The ratio of the concentration of parent radicals to total radical concentration, however, decreased with increasing reaction temperature (as @-scissionof parent radicals became more significant) and with increasing conversion (as addition of parent radicals to a-olefins became more significant). The presence of parent hexadecyl radicals as the most dominant radical speciesresulted in the formation of alkylhexadecanes as major addition products. Radical addition reactions involving lower alkyl radicals, however, were also significant resulting in the formation of branched alkanes and n-C15 and n-Cl,. If the rate constant for addition reactions was independent of the size of the radical, addition products would primarily be alkylhexadecanes due to the high concentrations of parent radicals. To account for the formation of branched alkanes resulting from addition of lower alkyl radicals to a-olefins, the following empirical treatment was used. The activation energy for all addition reactions was assumed to be 7.0 kcal/mol, which is typical of addition reactions involving alkyl radicals (Dente and Ranzi, 1983; Domine et al., 1990). It was then assumed that the logarithm of the preexponential factor (log A ) for addition reactions increases by an amount AA when the radical size is decreased by one carbon. This approach is in agreement with the general observation that the rate constant for radical addition reactions decreases with increasing radical size (Dente and Ranzi, 1983). It was also assumed that, for a given radical size, the log A for addition of primary radical is greater than those corresponding to addition of secondary radicals by an amount AAp. This treatment was necessary to account for the relatively high fraction (about 10-15%) of the n-alkane isomer among alkylhexadecane isomers. Secondary hydrogens from n-C16 are more readily abstracted than primary hydrogens. Primary hexadecyl radicals can also quickly isomerize to secondary radicals. Hence, parent radicals are dominated by secondary radicals. If the rate constants for addition reactions involving primary and secondary alkyl radicals were the same, the n-alkane isomer would be minor among alkylhexadecane isomers. The assumed relationships between the radical type and size and log A for addition reactions were purely empirical treatments adapted to describe the observed product distributions. The followingvalues gave a good fit between predicted and experimental product distributions: logA = 7.15 (for addition of secondary hexadecyl radicals) AA = 0.14
AA, = 0.8
Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 1861 1.00
1.00
,
b) 1
0.95 T=382 'C, ~ 3 . 6 1 5h
0.90 T=422 'C, r=.272 h
- 0.85
1:;:
0.80 T=450 T, +=.059 h
0.75
0.70 380
0.70 0
1
2
3
4
5
6
7
1
8
,
,
400
420
440
Cumulative reactor volume ( m l )
0
1
2
3
4
5
6
1
,
460
Reaction Temperature ('C)
7
8
380
P
400
420
440
460
1. T = 4 5 0 'C,
E
~=.059h
v E
1 o-8
c
e
t
T=422 'C, r=.272 h
-
I ............................
T = z ~ . . ~ ~ ~ = . ~ . 6 ...................... 15..h
0
.I -+ 10-9
L
IO-'
Figure 8. Predicted free-radical concentrations: (a) as a function of cumulative reactor volume; (b) at zero conversion. T
=
402 "C
, 9.
Conversion
= 6.62
2-Olefins
- Predictad o
a-Olefins
Experimental
. .. Predicted Experimental
0
1
2
3 % n-C,,
2
3
5
4
7
6
1
I
I
8
9
10
Conversion (Experimental) 4
5
6
7
6
9
1
0
8
0
2
4
6
8
10
12 14 16 18 20 22 Carbon Number
24
0
2
4
6
8
10
12
14
16
18 20
28
30
32
Experimental
0
Branched alkanes
26
o
22
Experimental
24
26
28
30
32
Figure LO. Experimental and predicted conversions of n-Cls.
Carbon Number
Figure 9. Experimental and predicted product selectivities in thermal cracking of n-Cls(2' = 402 OC,T = 1.158 h).
The rate parameters for addition reactions using the above relationships are in good agreement with those reported by Domine et al. (1990). Experimental and predicted product selectivities from high-pressure thermal cracking of n - C l 6 are presented in Figure 9 for a typical experiment. In all cases the agreement between predicted and experimental selectivities was quite satisfactory. Predicted and experimental conversions of n - C l 6 for all experiments are presented in
Figure 10. All experiments were performed under lowconversion conditions (n-Cle conversions below 10% ). Under these low conversions, the predicted conversion of n - C 1 6 was within f 5 % of the experimental value for most cases. The model predictions, however, are expected to become poor at higher conversions because of some inherent assumptions in the model. The results obtained in this study suggest that the product selectivitiesin thermal cracking of n-alkanes under high-pressure conditions are quite different than those from low-pressure pyrolysis. The reactant densities in
1862 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 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 high-temperature, low-pressure pyrolysis conditions. Hence bimolecular reactions, namely hydrogen abstraction and radical addition, become significant under high-pressure conditions. The fast rates for hydrogen abstraction reactions result in a high and nearly equimolar distribution of n-alkanes which is typical of a single-step decomposition mechanism. Radical addition reactions, on the other hand, result in the formation of high-molecular-weight compounds as a-olefins are consumed. Furthermore, radical addition reactions alter the product distribution as parent radicals participate in addition reactions instead of completing their normal decomposition to an a-olefin and a smaller alkyl radical. This type of side reaction results in product distributions that are dependent on the degree of conversion. Radical addition reactions are therefore key reactions which must be considered in modeling of highpressure thermal cracking of hydrocarbons.
Conclusions Thermal cracking of n-hexadecane at high pressures (13.9 MPa) proceeds via a free-radical chain mechanism resulting in C1 to C14 n-alkanes and CZto C16 a-olefins as the primary products. Under high-pressure conditions, radical addition reactions become important resulting in product distributions that are highly conversion-dependent. Addition of the parent hexadecyl radicals to the a-olefins results in the formation of Cle to C B alkanes. ~ Addition of lower primary alkyl radicals to a-olefinsresults in the formation of higher n-alkanes, including n-C15 and n-Cl7. The lower alkyl radicals can also undergo isomerization prior to addition to a-olefins to give branched alkanes, mainly in the C7 to C1, range. A simple kinetic model based on a free-radical mechanism was developed to account for the observed product distributions and overall n-Cls conversion. The fundamental kinetic parameters proposed by Ranzi et al. (1983) for low-pressure pyrolysis of n-alkanes were used with only minor adjustments. Under the relatively low-conversion conditions employed in this study, agreement between predicted and experimental results was quite satisfactory.
Financial support was provided by Alberta Oil Sands Technology and Research Authority (AOSTRA) under Agreements 521 and 781, and by Esso Petroleum Canada under University Research Grants Program.
Nomenclature A = preexponential factor, s-1 or L-mol-1.s-1 E = activation energy, kcal/mol
s-1
[RII = concentration of a level I radical, mol/L ROT= total concentration of radicals, mol/L T = temperature, K V = reactor volume, L = fraction of level I1 and I11 radicals that are primary
+
Subscripts abs = hydrogen abstraction
Literature Cited Blouri, B.; Hamdan, F.; Herault, D. Mild Hydrocracking of High Molecular Weight Hydrocarbons. Znd. Eng. Chem. Process Des. Dev. 1985,24,30-37. Broderick, D. H. High-pressure Reaction Chemistry and Kinetics Studies of Hydrodesulfurizationof Dibenzothiophene Catalyzed by Sulfided CoO-MoO&-Al20~. Ph.D. Thesis, University of Delaware, 1980. Chung, S. Y. K. Thermal Hydroprocessingof Heavy Gas Oils. M.Sc. Thesis, University of Alberta, 1982. Dente, M. E.; Ranzi, E. M. Mathematical Modeling of Hydrocarbon PyrolysisReactions. InPyrolysis: Theory andlndustrial Practice; Academic Press: New York, 1983;pp 133-175. Depeyre, D.; Flicoteaux, C. Modeling of Thermal Steam Cracking of n-Hexadecane. Znd.Eng. Chem.Process Des.Dev. 1991,30,11161130.
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Domine, F.-Kineticsof Hexane Pyrolysis at Very High Pressures. 1. Experimental Study. Energy Fuels 1989,3,89-96. Domine, F.; Marquaire, P. M.; Muller, C.; Come, G. M. Kinetics of Hexane Pyrolysis at Very High Pressures. 2. Computer Modeling. Energy Fuels 1990,4,2-10. Doue, F.; Guiochon, G. Etude Theorique et Experimentale de la Cinetique de DecompositionThermique du n-Hexadecane,de son Mechanisme et de la Composition du Melange des Produita Obtenus. J. Chim. Phys. 1968,65,395-409. Doue, F.; Guiochon, G. The Fcjrmation of Alkanes in the Pyrolysis of n-Hexadecane: Effect o k p Inert Gas on the Decomposition of Alkyl Radicals. Can.!J. Chem. 1969,47,3477-3480. Fabuss, B. M.; Smith, J. 0.;Lait, R. I.; Borsanyi, A. S.; Satterfield, C. N. Rapid Thermal Cracking of n-Hexadecane at Elevated Pressures. Znd. Eng. Chem. Process Des. Dev. 1962,l (4),293299.
Fabuss, B. M.; Smith, J. 0.;Satterfield, C. N. Thermal Cracking of Pure Saturated Hydrocarbons. Adv. Pet. Chem. Refin. 1964,9, 157-201.
Ford, T. J. Liquid Phase Thermal Decomposition of Hexadecane: Reaction Mechanisms. Znd. Eng. Chem. Fundam. 1986,25,240243.
Acknowledgment
F = molar flow rate, mol/s k = reaction rate constant, s-l, h-l, or L-mol-1.s-l ki = rate constant for initiation, s-1 kt = rate constant for termination reaction, mol-' [MI= concentration of n-Cle, mol/L R = hydrocarbon group
add = addition dec = &scission i = initiation is0 = isomerization i, j , I , m = counters in summation equations p = primary radical s = secondary radical t = termination a+2 = isomerization of a-olefins to 2-olefins
Khorasheh, F. High Pressure Thermal Cracking of n-Hexadecane. Ph.D. Thesis, University of Alberta, 1992. Khorasheh, F.; Gray, M. R. High-pressure Thermal Cracking of n-Hexadecanein Aromatic Solvents. Ind. Eng. Chem. Res. 1993, following paper in this issue. Khorasheh, F.; Gray, M. R.; Rangwala, H. A.; Dalla Lana, 1. G. Interactions Between Thermal and Catalytic Reactions in Mild Hydrocracking of Gas Oil. Energy Fuels 1989,3,716-722. Kissin, Y. V. Free Radical Reactions of High Molecular Weight Isoalkanes. Znd. Eng. Chem. Res. 1987,26,1633-1638. Kossiakoff,A.; Rice,F. 0.Thermal Decompositionof Hydrocarbons, Resonance Stabilization and Isomerization of Free Radicals. J. Am. Chem. SOC.1943,65,590-595. Merrill, W. H.; Logie, R. B.; Denis,J. M. "A Pilot Scale Investigation of Thermal Hydrocracking of Athabasca Bitumen"; Department of Energy, Mines and Resources, Ottawa, Mines Branch Report R-281,1973.
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HighPressureand-200to+400OCApplication. Rev. Sci.Znstrum. 1970,41 (lo), 1444-1446. Shabtai,J.;Ramakrishnan, R.;Oblad, A. G. Hydropyrolysisof Model Compounds. In Thermal Hydrocarbon Chemistry; Advances in Chemistry Series 183; American Chemical Society: Washington, DC, 1979; pp 297-328. Voge, H. H.; Good, G. M. Thermal Cracking of Higher Paraffins. J. Am. Chem. SOC.1949, 71,593-597. Zhou, P.; Crynes, B. L. Thermolytic Reactions of Dodecane. Znd. Eng. Chem. Process Des. Dev. 1986,25, 508-514. Zhou, P.; Hollis, 0.L.; Crynes, B. L. Thermolysis of Higher Molecular Weight Straight-Chain Alkanes. Znd. Eng. Chem. Res. 1987,26, 846-852.
Received for review December 15, 1992 Accepted May 6, 1993
