I n d . Eng. Chem. Res. 1987,26, 488-494
488
Note that YZmay be approximated by yoover the whole range of Bz covered in this work. Numerical experiments also were performed for several systems having extremely different physical properties, but appreciable deviation of yz from Yo could not be observed. Therefore, the penetration theory appears always to be applicable for mass transfer when the major resistance is in the continuous phase. Conclusion The improved wetted-wall column used in the present work has been shown to have no appreciable end effects. Film-phase mass transfer could be depicted quantitatively in terms of the Beek-Bakker model. The mass-transfer coefficients for the continuous phase were in satisfactory agreement with the penetration theory when the driving force of the solute was evaluated from eq 27. Acknowledgment We are grateful to Dr. K. Ishimi, Department of Chemical Engineering, University of Osaka Prefecture, for helpful discussions. Nomenclature a = velocity gradient at liquid-liquid interface, l / s B = variable, a2DZ/$, cf. eq 21, dimensionless C = concentration of transferring solute, kg/m3 C2,, = concentration of transferring solute in continuous phase defined by eq 27, kg/m3 C2,R= representative concentration of transferring solute in continuous phase, kg/m3 D = diffusivity, m2/s F1(@)-F4(4) = parameters defined by eq 12-15, dimensionless g = gravitational acceleration, m/s2 k = mass-transfer coefficient, m/s M = parameter defined by eq 8, dimensionless N = mass-transfer flux, kg/(m2 s) p = pressure, N/m2 Q = volumetric flow rate of liquid, m3/s R, = radius of inner plane of core circulation region, m Ri = radius of liquid-liquid interface, m Ro = radius of inversion plane of continuous-phase flows, m R1 = radius of wetted-wall rod, m
R2 = radius of inside wall of column, m r = distance in radial direction, m v = velocity, m/s W = total mats-transfer rate, kg/s = variable, k ( Z / u , D ) 1 / 2cf. , eq 22, dimensionless yo= constant 2/7r1I2,dimensionless Y , = variable equal to (3/8)1/3B1/s/I'(4/3),cf. eq 24, dimensionless 2 = film length, m z = distance in axial direction, m Greek Symbols
r(z) = gamma function radius ratio, Ri/R1, dimensionless I.C = viscosity, Pa s p = density, kg/m3 = radius ratio, Ri/R2,dimensionless $ = viscosity ratio, pl/p2, dimensionless 7 =
Subscripts
cir = circulation i = liquid-liquid interface in = inlet out = outlet 1 = film phase 2 = continuous phase Superscript = average value
-
Literature Cited Asai, S.; Hatanaka, J.; Uekawa, Y. J . Chem. Eng. Jpn. 1983, 16,463. Asai, S.; Hatanaka, J.; Kuroi, M. Chem. Eng. J . 1985, 30, 133. Bakker, C. A. P.; Fentener van Vlissingen, F. H.; Beek, W. J. Chem. Eng. Sei. 1967, 22, 1349. Beek, W. J.; Bakker, C. A. P. Appl. Sci. Res. 1961, A10, 241. Byers, C. H.; King, C. J. AIChE J . 1967, 13, 628. Hatanaka, J. Chem. Eng. J . 1984, 28, 127. Hikita, H.; Asai, S.; Himukashi, Y. Kagaku Kogaku 1967, 31,818. Hikita, H.; Asai, S.; Takatsuka, T. Chem. Eng. J. 1976, 11, 131. Hikita, H.; Ishimi, K.; Sohda, N. J . Chem. Eng. Jpn. 1979, 12, 68. Maroudas, N. G.; Sawistowski, H. Chem. Eng. Sci. 1964, 19, 919. Perry, R. H.; Chilton, C. H. Chemical Engineers' Handbook, 5th ed.; McGraw-Hill: New York, 1973.
Received far review July 22, 1985 Accepted July 2, 1986
Asphaltene Reaction Pathways. 2. Pyrolysis of n -Pentadecylbenzene Phillip E. Savage and Michael T. Klein* Department of Chemical Engineering and Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 19716
The reactions, pathways, kinetics, and mechanisms of alkyl-aromatic moieties likely present in asphaltenes were probed via the thermolysis of n-pentadecylbenzene (PDB) at temperatures from 375 t o 450 "C. The primary reaction pathway led t o two major product pairs, toluene plus 1-tetradecene and styrene plus n-tridecane, respectively. A complete series of n-alkanes, a-olefins, phenylalkanes, and phenylolefins was also formed in lesser yields. PDB thermolysis was demonstrably first-order, and associated Arrhenius parameters of [E* (kcal/mol), log A (s-I)] = [55.45, 14.041 were determined. Reaction in tetralin-d12indicated that the operative PDB thermolysis mechanism was entirely free-radical. This information permitted speculation into the relevance of the modelcompound results t o asphaltene reactions. The current industrial trend toward processing heavy crude oils and residua has focused increased attention on petroleum asphaltenes, the heptane-insoluble and benzene-solublefraction of such feedstocks. Strictly a solubility 0888-5885/87/2626-0488$01.50/0
class, asphaltenes are a chemically ill-defined and complex mixture of heavy hydrocarbons and heteroatom- and metal-containing hydrocarbon oligomers. The complexity and ambiguity of both asphaltenes and their reaction 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 3, 1987 489 product spectra mask the intrinsic kinetics and mechanistic fundamentals that control asphaltene conversion processes. This motivates the use of simpler model compounds,which mimic the important reactive moieties in asphaltene but also allow resolution of reaction pathways, kinetics, and mechanisms. However, this transition from ill-defined to tractable reaction substrates is, ostensibly, at the expense of practical relevance, which makes the transfer of model-compound information to the reactions of real systems a key issue. Herein we probe the thermal reactions of asphaltenes via the pyrolysis of one of its model compounds. The literature concerning asphaltene structure indicates that alkyl chains of up to carbon number C30are bonded to and often connect large peri-condensed aromatic ring systems (Yen, 1981; Speight and Moschopedis, 1981; Takegami et al., 1980; Suzuki et al., 1982; Gould, 1978). These alkyl chains appear to be covalently bound to the asphaltene and thermally scissile with apparent activation energies of about 50 kcal/mol (Savage et al., 1985). The object of the present investigation was more detailed scrutiny of the thermal pathways, kinetics, and mechanisms of a prototypical alkyl-aromatic moiety likely present in asphaltene, namely n-pentadecylbenzene (PDB). Singly substituted alkylbenzenes have been pyrolyzed previously. The pyrolysis products, their formation pathways, observed Arrhenius parameters, and relevant thermochemical data have been reported for compounds with short alkyl chains, such as toluene (Szwarc, 1948; Badger and Spotswood, 1960),ethylbenzene (Szwarc, 1949; Badger and Spotswood, 1960; Crowne et al., 1969; Esteban et al., 1963), n-propylbenzene (Badger and Spotswood, 1960; Esteban et al., 1963) and n-butylbenzene (Leigh and Szwarc, 1952; Badger et al., 1964; Badger and Spotswood, 1960; Esteban et al., 1963). However, only very limited kinetic and mechanistic data exist for the pyrolysis of long-chain alkylbenzenes. Mushrush and Hazlett (1984) pyrolyzed several organic compounds containing long, unbranched alkyl groups, including PDB. Their observed product spectra included long n-alkanes and was explained on the basis of the two-step Fabuss-Smith-Satterfield (1964) theory. These workers noted that a pericyclic mechanism, wherein concerted PDB fragmentation occurs through a six-centered transition state, was also plausible. On the other hand, Blouri et al. (1985) rejected a freeradical mechanism for the liquid-phase pyrolysis of phenyldodecane and, instead, advanced a four-centered molecular mechanism. Clearly, the pyrolysis mechanism remains equivocal. Herein we extend this information through more comprehensive study of the reaction pathways, kinetics, and mechanisms of n-pentadecylbenzene thermolysis.
Experimental Section Pentadecylbenzene (PDB) was pyrolyzed at 375, 400, 425, and 450 "C for batch holding times ranging from 10 to 180 min. The initial PDB concentrations ranged from 0.0043 to 2.3 M. Materials. All chemicals were available commercially and used as received. PDB, 99.4% by GC analysis, was obtained from Alfa Products, and tetralin-d12(99% atomic purity) was obtained from Aldrich. The majority of the pyrolyses were performed in batch tubing bomb reactors comprising a nominal 1/4-in.stainless steel Swagelok port connector and two caps. However, for runs in which gas analyses were performed, larger reactors that could accommodate a higher PDB loading and, hence, facilitate the generation of gas-phase sample sizes convenient for analysis were used. These reactors were assembled from
a nominal 3/s-in. stainless steel Swagelok union tee capped at both ends with a shut-off valve connected to the center port of the tee via a 6-in. length of 1/4-in.tubing. Procedure. Reactors were typically loaded with approximately 60 mg of PDB (2 g for the runs with gas analysis) and 3 mg of diphenylmethane, a thermally stable internal standard, for later chromatographic analyses. Tetralin-dI2was also added in those studies probing the operative reaction mechanisms. After the mixtures were purged with argon to provide an inert environment for reaction, the reactors were sealed and immersed in a Tecam fluidized sandbath that had been preheated to the pyrolysis temperature. The reactors achieved the desired pyrolysis temperature after a heat-up time of roughly 2-3 min (Lawson and Klein, 1985), and the reactor pressure ranged from 1 atm initially to a maximum of 9.8 atm at the most severe pyrolysis conditions (450 "C, 60 min, 2 g of PDB). After the desired reaction time had elapsed, the reactors were removed from the sandbath and immediately plunged into cold water to quench the reaction. Once cool, the larger reactors were vented into a gas sampling valve for GC analysis of the gaseous products and then filled with spectrophotometric grade acetone to extract the remaining reaction products; the smaller reactors were subjected to only the acetone extraction. Gas chromatographic analysis confirmed that three successive acetone extractions were sufficient to remove all acetone-soluble material from the reactors. Analytical Chemistry. A Hewlett-Packard Model 5880A gas chromatograph equipped with a 50-m WCOT SE-54 fused-silica capillary column and a flame ionization detector was used in the analysis of the acetone-soluble products. Gas samples were separated over a 6-ft X l/*-in. column packed with 80/100 silica gel, and the individual constituents were observed by a thermal conductivity detector. Helium served as the carrier gas in both analyses. Reaction products were identified by retention time comparison with standard compounds, by coinjection, and by GC/MS. Relative isotope abundances, used in the quantitation of reaction mechanisms described later, were determined by mass spectral analysis.
Results and Discussion The results are discussed in terms of the product identities and yields, product interrelationships and reaction pathways, kinetics, mechanisms, and implications to asphaltene reactivity. Within the first of these sections, an illustrative but detailed discussion of experimental results is presented only for PDB pyrolysis at 400 "C. However, a complete listing of the yields of major products and product groups at all four temperatures is presented in Table I. All pyrolysis product yields are expressed as the molar percent of the original moles of PDB. PDB Thermolysis Products' Yields at 400 "C. Pyrolysis of PDB at 400 "C led to an ultimate PDB conversion of 73% at 180 min. The identified products index (IPI), herein defined as the total weight of all identified products divided by the initial PDB weight, ranged from 59.5% to 104% and averaged 80% at this temperature. The molar balance on aromatic rings ranged from 73.8% to 105% with an average of 85%. These are both consistent with the observed formation of char at this temperature. Major products observed at 400 "C were toluene, 1tetradecene, and n-tridecane. Styrene and ethylbenzene were present in lower yields. Also appearing in yet lower yields were series of n-alkanes and a-olefins containing 6-14 carbon atoms as well as l-phenylalkanes and phenylolefins (a-phenylalk-w-enes) with alkyl chains con-
490 Ind. Eng. Chem. Res., Vol. 26, No. 3, 1987 Table I. Molar Yields ( % ) of Products and Product Groups time (min) at 375 "C time (min) a t 400 "C 15 30 60 90 120 150 180 10 20 30 45 60 90 120 150 180 2.55 3.84 5.36 6.98 7.58 1.39 3.09 4.68 8.48 11.5 15.0 21.0 21.6 27.0 toluene 0.70 1.88 2.56 1.90 2.14 2.33 2.62 1.11 2.04 2.80 2.93 3.10 2.84 2.23 2.09 1.60 1-tetradecene 0.56 1.38 1.10 1.28 2.08 2.72 2.79 0.65 1.40 2.26 3.57 4.38 5.17 6.43 6.84 8.13 n-tridecane 0.32 0.45 0.18 0.33 0.37 0.32 0.45 0.36 0.57 0.67 0.66 0.74 0.75 0.59 0.54 0.53 styrene 0.16 0.32 0.52 0.38 0.45 1.07 0.65 0.13 0.28 0.40 1.20 1.63 2.05 3.55 4.88 4.33 ethylbenzene 0.04 0.20 94.0 101. 91.7 73.6 64.1 54.5 54.1 50.2 35.5 27.0 93.7 91.0 87.8 78.0 PDB 97.8 93.1 phenylalkanes phenylolefins n-alkanes a-olefins IPI ring balance
1.15 0.35 0.66 0.79 103. 99.3
3.01 0.91 1.59 2.35 90.9 97.0
4.65 0.79 1.97 2.63 101. 99.1
5.44 1.09 2.58 3.04
8.96 15.2 1.40 1.65 3.77 6.92 3.58 5.17
10 15 20 6.59 9.60 14.5 3.43 3.93 4.21 2.89 3.27 5.05 1.47 2.21 1.82 0.87 1.29 1.97 87.0 71.5 67.8
phenylalkanes phenylolefins n-alkanes a-olefins
11.8 15.9 24.7 31.3 42.9 3.73 4.90 4.91 4.19 4.49 5.74 7.27 11.4 14.0 18.7 7.03 9.90 10.7 10.1 12.1 101. 102.
89.3 92.3
92.2 97.4
2.74 0.93 1.39 1.89
37.0 97.4 85.8 103. 104. 97.5 98.2 94.8 107. 105. time (min) a t 425 "C 30 40 60 90 120 18.1 26.2 34.5 47.4 44.7 1.64 0.73 0.65 3.30 2.93 6.40 5.17 4.23 5.68 6.29 1.18 0.49 0.58 1.44 1.64 8.49 18.7 15.2 3.35 4.62 47.0 33.0 179.7 7.06 5.45
toluene 1-tetradecane n-tridecane styrene ethylbenzene PDB
IPI ring balance
11.4 1.48 4.85 4.44
74.2 82.5
66.6 80.4
7.96 13.5 18.8 24.8 34.5 37.4 44.5 1.81 1.94 2.35 2.47 2.34 2.30 2.30 4.04 5.67 8.27 11.2 15.0 17.3 19.3 4.36 4.50 6.10 6.46 5.73 5.60 4.10
5.35 1.50 2.71 3.42 98.4 98.6
83.1 83.4
150 180 10 20.9 50.4 48.2 3.80 0.40 0.22 6.33 4.99 3.39 2.96 0.40 0.34 3.65 17.1 19.7 4.62 1.05 2494
57.0 79.8 72.2 81.3 79.0 3.38 1.62 1.73 1.24 0.95 24.4 32.3 27.8 33.3 29.8 10.8 8.15 7.94 7.09 5.22 56.5 78.1
Table 11. Representative Gas Yields (mol % ) from PDB time (min) a t 400 "C product 10 45 90 0.32 0.55 CH4 0.09 0.86 0.03 0.50 C2H6 C2H4 0.03 0.15 0.18 0.30 0.45 C3H8 0.02 0.28 0.36 C3H6 0.02 0.08 0.12 C4H10
TIME (MINUTES)
TIME (MINUTES)
Figure 1. PDB thermolysis at 400 "C: (a, left) temporal variations of major products; (b, right) temporal variations of product groups.
Figure l b shows that phenylalkanes were produced in the highest yields and phenylolefins in the lowest. The phenylolefin yield exhibited a maximum of 2.5% at 90 min. Olefins and paraffins were formed in roughly equal yields initially, but the olefin yield decreased from a maximum of 6.5% at 90 min, while the alkane yield increased to 19.3% at 180 min. In summary, the present results show that the major products from PDB thermolysis at low conversion were toluene, 1-tetradecene, n-tridecane, styrene, and ethylbenzene. A complete series of n-alkanes, a-olefins, phenylalkanes, and phenylolefins were also produced but in
Ind. Eng. Chem. Res., Vol. 26, No. 3, 1987 491 Table 111. PDB Thermolysis Rate Parameters parameters a t temp, (see Figure 3) 375 400 1.1 x 10-4 k,, s-l 2.1 x 10-5 8.2 x 10-4 k,, s? 2.5 x 10-4 1.2 x 10-3 123, s-' 7.5 x 10-4 1.9 x 104 k4, s-l 1.5 x 10-9 0.312 Y1 0.370 0.117 Y2 0.128 0.571 Y3 0.502
O C
lesser yields. These results are in substantial agreement with those of Mushrush and Hazlett (1984). These studies differ on only one substantial issue. On the one hand, Mushrush and Hazlett found styrene to be thermally stable at their reaction conditions (450 "C and 600 kPa), and ethylbenzene underwent secondary decomposition reactions. On the other hand, the present results generated at 450 O C and about 230 kPa showed facile secondary decomposition of styrene, and ethylbenzene's yield increased steadily with time. PDB Thermolysis Pathway. The object of the present section is resolution between the primary and secondary reactions involved in PDB thermolysis. The discussion will therefore focus on the low-temperature (375 and 400 "C) experiments wherein low-conversion data are available. The positive initial slopes associated with the toluene, 1-tetradecene,styrene, and n-tridecane yields in Figure l a indicated that these products evolved directly from the reactant and were therefore all primary products. Ethylbenzene was also a minor primary product that was produced mainly via secondary reactions. These assignments are confirmed through scrutiny of Figure 2, a plot of the reactant's selectivity (herein defined as a product's molar yield divided by the reactant conversion) to each product vs. conversion. The primary products, toluene, 1-tetradecene, styrene, and n-tridecane, each possess a positive, non-zero selectivity at zero conversion. Furthermore, extrapolation of the selectivity data of Figure 2a to zero conversion shows that the selectivity of PDB to both toluene and 1-tetradecene was about 0.35, suggesting that these two products formed in the same reaction pathway. The selectivity to toluene is nearly constant, but the 1-tetradecene selectivity decreases with conversion, suggesting that it either undergoes secondary reactions or that the overall PDB network comprises pathways of different reaction order. A similar decline in selectivity with conversion was observed for styrene. Figure 2b suggests that the styrene and n-tridecane selectivities were nearly equal (about 0.12) at zero PDB conversion, which indicates that their formation was via a common pathway. Finally, although ethylbenzene was a major product at high conversions, its yields were small at low conversions. The selectivity to ethylbenzene was about 0.02 at zero conversion, but it increased with conversion, indicating that this minor primary product was a major secondary product of PDB thermolysis. The foregoing observations are summarized in the apparent thermolysis pathways presented in Figure 3. This network comprises a single major primary reaction of PDB to the product pairs, toluene plus 1-tetradecene, and styrene plus n-tridecane, and also to the numerous minor products, respectively. Styrene, 1-tetradecene, and ntridecane all follow secondary decomposition pathways as well. The reaction order for PDB disappearance was deduced through PDB pyrolyses at 400 "C and 10 different initial
425 4.4 x 10-4 3.6 x 10-3 1.9 x 10-3 1.6 X lo4 0.454 0.117 0.429
450 2.0 x 10-3 1.0 x 10-2 2.8 x 10-3 6.8 X 0.510 0.108 0.382
E*, kcal/mol
log A, s-l 14.0 12.3 2.5 53.9
55.5 47.3 16.6 18.8
.
-
0.50
t
CONVERSION
0.20 0.24 0.16
i
.
n-TRIDECANE
0.08 0.04
~.
a
0.00 0.0
I
STYRENE
0.2
0.4
0.6
0.15 0.12-
5-
0.09
-
+
ETHYLBENZENE
0.06 VI
0.03
-
0.00 _r
Figure 3. PDB thermolysis pathway.
reactant concentrations ranging from 0.0043 to 2.29 M. Apparent first-order rate constants were calculated at each initial concentration, and the resulting concentration dependence of these rate constants provided an estimate of the overall reaction order. These analyses are summarized in Figure 4, which provides a global reaction order of 1.08 for PDB thermolysis. Having determined the overall rate of PDB disappearance to be essentially first-order, a sequential Simplex search routine was used to determine the stoichiometric coefficients and rate constants shown in Figure 3. The best-fit kinetic parameters are listed in Table 111, along with the associated Arrhenius parameters. At 375 and 400 "C, the results of the parameter estimation routine were relatively insensitive to the value of k,, the rate constant
492 Ind. Eng. Chem. Res., Vol. 26, No. 3, 1987
I
7
.C
E
10-*t
v
= $
I 1 .. 1
t-
0
.'L .
.
9
t
u
$
tW
t 10-3
L c
i. , , , , , , ) / I , , , , ,
, , , , , ,,,, , , ,
lo-' 100 PDB INITIAL CONC (M)
10'
10-2
10-3
Figure 4. PDB thermolysis reaction order.
+Ph - CH, CH, CH-CH, 2,' R H * Ph-(CH,)I,hi,-Ph-
(CH,);a
+
CH,.CH,
i , i ,+PRODUCTS +
Figure 6. Possible PDB thermolysis mechanisms: (a, top) concerted retro-ene mechanism; (b, middle) four-centered molecular mechanisms; (c, bottom) free-radical mechanism. I
0
20
40
005-
2
003-
140
160
180
200
I
1
0 0 6 41
9
80 100 120 TIME (MINUTES)
vn
0 074
w
60
n-TRIDECANE
'
1 -TETRADECENE
001SVRENE
ooot 0
~
, , , , 20
40
, , , , , , , , , , , , , , 60
80 1 0 0 120 TIME (MINUTES)
140
160
180 200
Figure 5. Comparison of experimental and calculated product yields for PDB thermolysis a t 425 "C.
for the decomposition of n-tridecane. The routine typically converged to the initial input value of k4, provided it was sufficiently small. Thus, at these lower temperatures, the value of K4 shown in Table I11 should be taken as an indication of a very slow secondary reaction and not as an accurate quantitative measure of the rate constant. Finally, the experimental data could be compared with the model of Figure 3 through numerical integration of the implied rate equations with the best-fit rate constants and stoichiometric coefficients as parameters. Figure 5 , which shows the experimental molar yields of major primary products a t 425 "C, along with the curves based on the best-fit rate constants, demonstrates that the proposed pathway is consistent with the experimental results. PDB Thermolysis Mechanisms. Three possible PDB thermolysis mechanisms are illustrated in Figure 6. Figure 6a depicts a six-center, intramolecular retro-ene reaction wherein PDB fragmentation to 1-tetradecene and iso-
toluene occurs in a single, rate-determining concerted step; rearrangement of isotoluene to toluene is fast. This type of pericyclic reaction has been proposed previously for pyrolysis of a-olefins (Rebick, 1979; Miller, 1963) and phenethyl phenyl ether (Klein and Virk, 1983). The four-center intramolecular mechanisms favored by Blouri et al. (1985) are shown in Figure 6b. The intermolecular free-radical reaction depicted in Figure 6c comprises a sequence of elementary steps including radical initiation, hydrogen abstraction, @-scission,and termination. Similar free-radical chains have been suggested to be controlling the pyrolyses of dibenzyl ether (Simmons and Klein, 1985; Schlosberg et al., 1981) and lI3-diphenylpropane and 1,4-diphenylbutane (Gilbert and Gajewski, 1982; Poutsma and Dyer, 1982). The presently observed product spectra and kinetics allow limited scrutiny of these mechanisms. Either unimolecular mechanism would predict the observed firstorder kinetics and the high selectivity to equimolar amounts of toluene and 1-tetradecene at low conversion. Note that the pericyclic mechanism alone fails to account for the formation of all other observed products. In contrast, the free-radical formed from PDB via hydrogen abstraction at the y- and a-carbons can undergo @-scission to produce a toluene precursor plus 1-tetradecene and styrene plus a n-tridecane precursor, respectively. Hydrogen abstraction at other positions can account for the formation of the minor products. However, superficial inspection of the free-radical mechanism might suggest hydrogen abstraction a t the a-carbon to be favored and, hence, formation of styrene to be predominant. Since the yield of toluene was typically an order of magnitude larger than that for styrene, more detailed examination of the mechanism was in order. Thermolyses in deuteriated tetralin allowed quantitation of the proportions of any concerted (six-centered and four-centered) or free-radical component of PDB pyrolysis. This is because the benzyl radical chain carrier of the latter
Ind. Eng. Chem. Res., Vol. 26, No. 3, 1987 493
1
o.2
0.0
1 f
I
I
0.0
I
I
0.4
0.2
I
I
I
I
0.8
0.6
I
1 .o
1/R (moles PDB/mole tetralin-d12)
Figure 7. Deuterium incorporation as a measure of free-radical mechanisms.
mechanism could, in general, abstract hydrogen in the form of protium from PDB or deuterium from tetralin-d12. Irrespective of the relative rate constants for abstraction of deuterium to protium, in the limit of infinite PDB dilution in tetralin (tetralin-d12/PDB = R m ) , the chain carrier would always abstract deuterium, and thus, all toluene produced from the free-radical mechanism would be singly deuteriated. In contrast, the unimolecular mechanism would always lead to fully protonated toluene. The free-radical fraction of PDB thermolysis could thus be estimated as the fraction of singly deuteriated toluene (DI) that formed as R m. The relevant experimental results are shown in Figure 7, where DI is plotted vs. 1/R for reactions a t 400 "C and conversion I20%. Extrapolation of the data in Figure 7 to the limit of 1/R 0 suggests that PDB thermolysis is fully free-radical. Having determined the reaction mechanism to be freeradical, the preponderance of toluene in the product spectrum requires more careful rationalization, since, ostensibly, PDB contains 15 carbons with abstractable hydrogen, 14 of which being roughly equal in reactivity (C-H bond energy 95 kcal/mol) (Benson, 1976) and the acarbon being significantly more reactive (C-H bond energy 85 kcal/mol). However, preferential hydrogen abstraction at the a-carbon would lead to styrene and an n-tridecane radical upon @-scissionof the a-PDB radical. Hydrogen abstraction at the other positions should be roughly equivalent, and if hydrogen abstraction alone were controlling, the toluene should be a minor product found in roughly equimolar proportion with the products from @-scissionof all non-a-PDB radicals. The overall reaction of PDB is clearly a function of both hydrogen abstraction and &scission steps, however. The y-PDB radical is unique among the 15 PDB radical positions in that its @-scissionleads to the resonance-stabilized benzyl radical; p-scission of all other PDB radicals leads to a primary radical plus an olefin. Thus, possible combinations of bond energetics are as follows: hydrogen abstraction at the y-position yields a secondary alkyl radical which decomposes to produce a primary benzyl radical and 1-tetradecene; hydrogen abstraction at the a-position produces a secondary benzyl radical which undergoes @-scissionto a primary alkyl radical and styrene. Abstraction at all other positions leads to a secondary alkyl
-
-
-
-
-
radical that undergoes @-scissionto a primary alkyl radical and a stable product. The most favored combination involves the y-carbon, which explains the high yields of toluene observed. The absence of phenylbutene in the product spectrum can also now be rationalized. A y-PDB radical will likely undergo rapid @-scissionat the a-position, where the C-C bond energy is about 69 kcal/mol (Benson, 1976),to form a benzyl radical and 1-tetradecene rather than at the 6position, where the C-C bond energy is about 82 kcal/mol, to form phenylbutene and a primary undecyl radical. This 13 kcal/mol difference in bond strengths, if directly analogous to activation energy differences, indicates that the relative rate constants for @-scissiondiffer by roughly lo4 at 400 "C. Thus, toluene will be produced at the expense of phenylbutene. Implications to Asphaltene Reactivity. Perhaps the most obvious difference between polymers and low-molecular-weightmodel compounds is molecular weight. This difference suggests that one consideration in the extension of these model compound results to asphaltene reactivity would involve species mobility. The aromatic portions of petroleum asphaiteneu, mimicked by PDB, likely contain more than a single aromatic ring. For example, a hypothetical average structure based on 12 condensed aromatic rings has been proposed for Athabasca asphaltene (Speight, 1970). Furthermore, several of these large aromatic sheets might be joined together in an oligomeric fashion by aliphatic, naphthenic, or heteroatomic linkages (Yen, 1974; Ignasiak et al., 1981; Gould, 1978). With the PDB free-radical chain considered as the intrinsic chemistry of asphaltenic PDB analogues, it is reasonable to suspect that the reactions of asphaltenic alkyl-aromatic radicals would be affected by diffusion because the asphaltenic species would be markedly larger and considerably less mobile than the benzyl radical. Since the PDB results indicate that the y-carbons are the important sites for fragmentation chemistry, the initial rates of decomposition of PDB-like moieties in asphaltenes will be slower than the model compound kinetics because their molecular-weight-induced diminished mobility will trap the radicals near their point of origin and, thus, constrain them to react with nearby atoms that are not necessarily y to an aromatic ring. These likely diffusional limitations will be most significant at low asphaltene conversion, when few bonds have broken. The increased mobility of radicals at higher conversions may lead to apparent autocatalytic asphaltene decomposition kinetics. Finally, it is only in the limit of complete conversion that the pyrolysis kinetics of the asphaltenic moieties will approach those of their model compound analogues.
Conclusions 1. Pentadecylbenzene thermolysis proceeds through a series of free-radical reaction steps involving hydrogen abstraction, @-scission,and termination. Hydrogen abstraction at the y-carbon is especially important as it leads to a thermochemically favorable @-scissionreaction. Evidence of concerted reactions was absent in the experimental results. 2. The formation of high yields of n-alkanes from PDB suggests that long-chain alkyl-aromatic moieties likely present in asphaltenes may be responsible for the paraffins produced from asphaltene thermolysis. 3. It is likely that the intrinsic kinetics provided by relevant model compounds may be masked by diffusional limitations during the fragmentation of the model-compound related moieties in asphaltenes. This effect could
Ind. Eng. Chem. Res. 1987,26, 494-501
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lead to apparent autocatalytic kinetics for asphaltene decomposition. Registry No. PDB, 2131-18-2; l-tetradecene, 1120-36-1; styrene, 100-42-5; n-tridecane, 629-50-5; ethylbenzene, 100-41-4; toluene, 108-88-3.
Literature Cited Badger, G. M.; Kimber, R. W. L.; Novotny, J. Aust. J . Chem. 1964, 17, 78. Badger, G. M.; Spotswood, T. M. J. Chem. SOC. 1960,4420. Benson, S. W. Thermochemical Kinetics; Wiley: New York, 1976. Blouri, B.; Handam, F.; Herault, D. Znd. Eng. Chem. Process Des. Dev. 1985, 24, 30. Crowne, C. W. P.; Grigulis, V. J.; Throssell, J. J. Trans. Faraday SOC. 1969, 65, 1051. Esteban, G. L.; Kerr, J. A.; Trotman-Dickenson, A. F. J. Chem. SOC. 1963, 3873. Fabuss, B. M.; Smith, J. 0.;Satterfield, C. N. In Aduances in Petroleum Chemistry and Refining; McKetta, J. J., Jr., Ed.; Wiley: New York, 1964; Vol. 9. Gilbert, K. E.; Gajewski, F. J. J. Org. Chem. 1982, 47, 4899. Gould, K. A. Fuel 1978,57, 756. Ignasiak, T.; Bimer, J.; Samman, N.; Montgomery, D. S.; Strausz, 0. P. In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; Advances in Chemistry Series No. 195; American Chemical Society: Washington, DC, 1981; p 183. Klein, M. T.; Virk, P. S. Znd. Eng. Chem. Fundam. 1983, 22, 35. Lawson, J. R.; Klein, M. T. Znd. Eng. Chem. Fundam. 1985,24,203.
Leigh, C. H.; Szwarc, M. J. Chem. Phys. 1952, 20, 407. Miller, D. B. Znd. Eng. Chem. Prod. Res. Dev. 1963, 2, 220. Mushrush, G. W.; Hazlett, R. N. Ind. Eng. Chem. Fundam. 1984,23. 288.
Poutsma, M. L.; Dyer, C. W. J. Org. Chem. 1982, 47, 4903. Rebick, C. In Thermal Hydrocarbon Chemistry; Oblad, A. G., Davis, H. G., Eddinger, R. T., Eds.; Advances in Chemistry No. 170; American Chemical Society: Washington, DC, 1979; p 1. Savage, P. E.; Klein, M. T.; Kukes, S. G. Ind. Eng. Chem. Process Des. Deu. 1985, 24, 1169. Schlosberg, R. H.; Ashe, T. R.; Pancirov, R. J.; Donaldson, M. F u d 1981, 60, 201. Simmons, M. B.; Klein, M. T. Znd. Eng. Chem. Fundam. 1985, 25, 55. Speight, J. G. Fuel 1970, 49, 76. Speight, J. G.; Moschopedis, S. M. In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; Advances in Chemistry Series No. 195; American Chemical Society: Washington, DC, 1981; p 1. Suruki, T.; Itoh, M.; Takegami, Y.; Watanabe, Y. Fuel 1982,61,402. Szwarc, M. J . Chem. Phys. 1948, 16, 128. Szwarc, M. J . Chem. Phys. 1949, 17, 431. Takegami, Y.; Watanabe, Y.; Suzuki, T.; Mitsudo, T.; Itoh, M. Fuel 1980, 59, 253. Yen, T. F. Energy Sources 1974,1,447. Yen, T. F. In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; Advances in Chemistry Series No. 195; American Chemical Society: Washington, DC, 1981; p 39. Received for review August 12, 1985 Accepted July 18, 1986
HDS Kinetic Studies on Greek Oil Residue in a Spinning Basket Reactor Jamal M. Ammus and George P. Androutsopoulos* Department of Chemical Engineering, National Technical University of Athens, GR 106 82 Athens, Greece
A spinning basket reactor was used to investigate the hydrodesulfurization (HDS) kinetics of Greek (Thasos) atmospheric residue. The effects of temperature (285-395 "C), pressure (30-70 bar), and particle size (L, = 5.67 X 10-3-58.9 X cm) upon the intrinsic reaction rate were determined. Two commercial HDS supported catalysts (Co-Mo/y-Al,03) differing in their chemical composition and physical structure were employed. It was shown that the overall HDS reaction rate was kinetically controlled when the pulverized catalyst was rotated a t a rate of 2540 rpm. Parameters defined by a general lumped kinetic equation, incorporating a hydrogen inhibition term, were obtained for both catalysts. The following kinetic equations were developed: (i) catalyst HT-400 E, R1 = 2.799 X 1O1O e~p(-3497O/RT)P~,C&~/ (1 0.0036PH,); (ii) catalyst ICI-41-6, R2 = 4.962 X 1O1O exp(-32590/ R7')PH,C$."/(1 + 0.0050P~,).Evaluations of effectiveness factors and effective diffusion coefficients reported in this paper and associated with the use of particular catalyst pellet sizes were based on HDS standard experiments.
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The recent developments in residuum hydroprocessing reflect the worldwide trends in product demands and crude oil quality. The challenge of the 1980s for the petroleum industry is to convert more of the heaviest portion of crude oil into more valuable fuels. Residuum hydroprocessing will contribute in a decisive way to the new refinery requirements and the widespread needs for oil products of an improved quality. One of the most important methods of hydrotreating an oil residue is the direct catalytic hydrodesulfurization because of its effectiveness in reducing sulfur, nitrogen, metals, and oxygen contents and also in improving the quality of the hydrodesulfurized feedstocks. A considerable number of studies on residue HDS have appeared in the literature. Gates et al. (1979), Crynes (1977), and Schuman and Shalit (1970) reviewed the general aspects of the hydrodesulfurization process. Hirotsugu et al. (1980) and Cecil et al. (1968) discussed the HDS of different Middle East atmospheric residues in terms of the plug flow 0888-5885/87/2626-0494$01.50/0
assumption, with a second-order kinetics with respect to sulfur concentration in both a microreactor and a pilot trickle bed reactor. Beuther and Schmidt (1963) reported a mechanism and kinetics of HDS of high sulfur Middle East reduced crude. They proposed that the simple second-order reaction rate model was adequate for expressing the kinetics of HDS processing. Papayannakos and Marangozis (1984) studied HDS of Thasos atmospheric residue by using a fix bed recycle reactor and deduced a n = 2.5 kinetic order. The nature of the sulfur compounds present in petroleum residuum is discussed by Schuit and Gates (1973), Corbet (1969), Dickie and Teh (1967), Drushell (1972), and Richardson and Alley (1975). Hirotsugu et al. (1982) used the gel permeation chromatography (GPC) technique to investigate the molecular size distribution of both sulfur and organometallic metal compounds in various Middle East atmospheric residues. They concluded that the major portion of these atmosphere residues consisted of compounds of a molecular weight below 700 0 1987 American Chemical Society
