548
Ind. Eng. Chem. Res. 1994,33, 548-557
Hydrogen-Transferring Pyrolysis of Long-chain Alkanes and Thermal Stability Improvement of Jet Fuels by Hydrogen Donors Chunshan Song,' Wei-Chuan Lai, and Harold H. Schobert Fuel Science Program, 209 Academic Projects Building, The Pennsylvania State University, University Park, Pennsylvania 16802
Hydrogen-transferringpyrolysis refers t o the thermal decomposition of hydrocarbons in the presence of hydrogen donors. Relative to the pyrolysis of pure n-tetradecane (rn = 14 in C,H2m+d at 450 O C , adding 10 vol % of H-donor tetralin suppressed n-C14 conversion by68 76 after 12 min of residence time, by about 66% after 21 min, and by 37% after 30 min. The presence of tetralin not only inhibited the n-C14decomposition, but also altered the product distribution. The decomposition and isomerization of primary radicals are strongly suppressed, leading to a much higher ratio of the l-alkene to n-alkane with m - 2 carbon atoms and slightly higher alkene/alkane ratio for the other product groups. The overall reaction mechanism for the initial stage of hydrogen-transferring pyrolysis is characterized by a one-step @-scissionof secondary radical followed by H-abstraction of the resulting primary radical. Moreover, desirable effects of the H-donor are also observed even after 240 rnin a t 450 "C, especially for inhibiting solid deposition. We also examined the effect of tetralin addition on the deposit formation from a paraffinic jet fuel JP-8 which is rich in C r c 1 6 long-chain alkanes, and an aromatic compound, n-butylbenzene. Adding 10vol % tetralin to a JP-8 jet fuel, n-C14, and n-butylbenzene reduced the formation of deposits by 90% (from 3.1 to 0.3 wt %), 77% (from 3.0 to 0.7 wt %), and 54% (from 5.6 to 2.6 wt %), respectively. These results suggest that, by taking advantage of H-transferring pyrolysis, hydrocarbon jet fuels may be used at high operating temperatures with little or no solid deposition.
Introduction The present work is a part of an on-going research program for developingadvanced jet fuels thermally stable at high temperatures in the pyrolytic (400-500 " C )regime (Song et al., 1992a, 1993; Lai et al., 1992). Until recently, the major concerns of jet fuel stability only involved thermal oxidation stability and storage stability, because the current operating temperature of all the commercial and military jet fuels is below 300 "C, where the fuel degradation is controlled by autoxidation reactions (Morris et al., 1988; Hazlett, 1991). With the renewed interest of developing high-Mach aircraft, thermal stability at high temperatures has become more crucial. The relevant background information on the high-temperature thermal environments of the futurejet fuels for high-Mach aircraft has been described in several recent papers (Roquemore et al., 1989;Lee and Niedzwiecki, 1989;Moler and Steward, 1989; Song et al., 1993). The bulk fuel temperature of interest for long-duration pyrolysis of jet fuels is in the range of 400-500 "C. Lee and Niedzwiecki (1989) and Roquemore et al. (1989) have pointed out that the development of advanced fuels for high-Mach flights requires testing at long residence times in fuel systems where the fuel is subjected to moderate to high heat loads, as well as testing under conditions of very high heat flux at short residence times which fuels encounter in their passage through fuel struts and injectors. The earlier results of Hazlett et al. (1977,1991) and Frankenfeld and Taylor (1980) on the effects of temperature apply to the short residence time (28 s) degradation in a flow tube. In a preceding paper we have discussed the product distribution and reaction mechanisms for the medium- and longduration pyrolytic degradation characteristics of a paraffinicjet fuel component, n-tetradecane (Songet al., 1994). A brief review of previous work on hydrocarbon pyrolysis is given in the preceding paper (Song et al., 1994). Unlike
* Author to whom correspondence should be addressed. 0888-5885/94/2633-0548$04.50/0
the prior pyrolysis work, the hydrogen-transferring pyrolysis reported here involves the thermal decomposition of hydrocarbons in the presence of a hydrogen donor (Hdonor). This work is a continuation of a fundamental study of high-pressure long-duration pyrolysisof saturated hydrocarbons including the long-chain alkanes, alkylcyclohexanes, trans and cis steric isomers of decalin, as well as hydroaromatics such as tetralin, which have been identified as major components of either petroleum- or coal-derived jet fuels (Song and Hatcher, 1992). One of the critical problems in developing thermally stable jet fuels for high-Mach aircraft is the thermal decomposition and formation of solids from hydrocarbon fuels in the pyrolytic regime (Roquemore et al., 1989; Moler and Steward, 1989;Hazlett, 1991;Eser et al., 1992). In studying the pyrolytic degradation of coal- and petroleum-derived jet fuels, it occurred to us that hydrogen transfer from H-donors, such as those present in coal-derived JP-8C jet fuel, could play an important role in suppressing thermal decomposition of jet fuel components and in inhibiting solid formation (Song et al., 1992a, 1993). Little has been reported in the literature on the effects of hydrogen donors on the pyrolysis of long-chain alkanes. Information on such effects is important for developing thermally stable jet fuels, especially coal-based advanced jet fuels. Such information is also needed for coal liquefaction and coal/petroleum resid coprocessing, as these processes often involve both H-donor molecules and the significant amounts of alkanes present in petroleum resids and in some coals (Given, 1984;Ofosu-Asante et al., 1989). In a preliminary communication (Song et al., 1992b), we have reported that the presence of hydrogen donors suppresses the conversion of unstable hydrocarbons. In this paper we will report on (1)the effects of H-donors on the pyrolytic decomposition and product distribution of long-chain alkanes such as n-tetradecane (n-C14) and n-hexadecane (n-Cl6);(2) the effects of H-donors on the solid deposition from n-C14, a real JP-8 jet fuel, and 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 549 n-butylbenzene (n-BB); and (3) the mechanisms of the H-transferring pyrolysis of n-Cl4, together with the effects of reactive compounds on the thermal reactions of H-donor. The experimental conditions used in this work are characterized by condensed or supercritical phases, relatively high pressure, static reactor, and long residence time. Such conditions are distinctly different from those used in most previous alkane pyrolysis work (vapor phase, low pressure, flow reactor, short residence time). The test temperature of 450 "C (842 O F ) used in this work is considered to be close to the anticipated high bulk fuel temperature for future high-Mach aircraft (Storch and Harrison, 1992). However, such a temperature is much lower than those in most literature work on pyrolysis related to industrial pyrolysis processes because the temperature range for industrial reactors is 600-900 "C (Willems and Froment, 1988).
Experimental Section The reactants used in this work are Aldrich reagentgrade n-tetradecane (n-Cl4; purity 99+ %), n-hexadecane (n-C16; 99+ % ); tetralin (99%), trans-decalin (trans-D; 99%), cis-decalin (cis-D; 99%), decalin (mixture of cis and trans isomers; 99+%); and n-butylbenzene (n-BB; 99+ % ). The so-called hydrogen-donor compounds include tetralin, decalin, cis-decalin, and trans-decalin. We also tested a real JP-8 jet fuel derived from petroleum (JP8P), whose properties are described elsewhere (Song et al., 1993). In a typical run, 5 mL of n-tetradecane (about 3.8 g, 19 mmoL) was charged into a 25-mL 316 stainless steel microreactor. For hydrogen-transferring pyrolysis, the 5-mL mixture of a compound or JP-8P jet fuel and a H-donor agent, usually 10% by volume unless otherwise mentioned, was transferred into the reactor. To minimize any effect of autoxidation, the sealed reactor with the reactant was deaerated by repetitive pressurization with 7 MPa of ultra high purity (UHP; 99.999%) N2 followed by depressurization for six times, and finally pressurized with 0.69 MPa of UHP Nz. A fluidized sand bath preheated to 450 "C was used as heater. The pyrolysis was conducted at 450 "C for 6-240 min under 0.69 MPa of UHP Nz (cold) in 25-mL-tubing bombs using a 5-mL sample. The reactor pressure was monitored after the microautoclave was plunged in the sand bath. The products were separated into gases, liquids, and solid deposits by using the procedure described elsewhere (Song et al., 1993). The gaseous products were analyzed using gas chromatography (GC) with a packed column (Lai et al., 1992). The liquid products were identified by capillary gas chromatography-mass spectrometry (GC-MS) and quantified by capillary GC. To make the quantification as reliable as possible, more than 30 pure compounds were used to calibrate the capillary GC responses of the liquid products, and more than 10 pure compounds were used to calibrate the GC responses of gaseous products. More analytical details may be found elsewhere (Lai et al., 1992; Song et al., 1994).
Results and Discussion Hydrogen-Transferring Pyrolysis of n-Tetradecane. Since closed reactors were used, the pressure changes with time are often characteristic of the reactivity of hydrocarbons toward thermal decomposition. As shown in Figure 1, while the system pressure with tetralin, a hydrogen donor, remained almost constant, that with n-tetradecane (n-Ci4)displayed a sharp increase after the
a
& h
"I
-d- Tewdecane ((214) C14-10% Teualin
e
lo
----C
Te& Niaugen
8
0 0
50
100
150
200
250
Residence Time (min) Figure 1. System pressure change during pyrolysisof n-tetradecane, tetralin, and their mixture (10vol % tetralin-90 vol % n-tetradecane) under intial Nz pressure of 0.69 MPa.
initial heatup. Adding tetralin considerably reduced the pressure increase during n-Ci4 pyrolysis, especially in the first 30 min, where the pressure with the 10% tetralin90% n-Cl4 mixture was lower than with either n-C14 or tetralin alone. This is indicative of the reduced formation of lighter products upon tetralin addition. Table 1presents the detailed analytical results of products from pyrolysis of n-Cu in the presence of tetralin. Adding tetralinaffected not only the extent of decomposition, but also the distribution of products ranging from the lightest molecules such as methane to the heaviest products such as solid deposits. Figure 2 shows the effect of tetralin on the n-Cl4 conversion within the first 30 min. The n-Ci4 conversion in the first 6 rnin is limited to 1 4 mol % . Measurement of the temperature inside the reactor has clearly shown that this period involves mainly heatup from ambient to reaction temperature. After this heatup period, n-Cl4 conversion increased almost linearly with increasing residence time up to 30 min. A substantial inhibiting effect of tetralin on thermal decomposition reactions is apparent from Figure 2. One can quantify such an effect from Table 1 and Figure 2: adding 10 vol % tetralin suppressed the decomposition of n-Cu, by about 68% after 1 2 min of residence time of reactor at 450 "C [(16.9-5.4Y16.9 = 68.0% )I, by about 66 5% after 21 min, and by 37% after 30 min. Relative to the runs of n-Cu alone, the H-transferring pyrolysis gave decreased yields of both alkanes and alkenes. The changes are small within the first 6 min because of the very low conversion, but become remarkable after 12 min. Surprisingly, the molar ratio of 1-dodecene to n-dodecane increased dramatically, whereas the 1-alkene/ n-alkane ratios for the other product groups only rose slightly upon the addition of tetralin, as shown in Figure 3. For n-Cl4 pyrolysis at 450 "C, adding 10 vol % tetralin increased the 1-dodeceneln-dodecaneratio from 5.1 to 9.3 after 12 min, but that of 1-decene to n-decane increased to a much lesser extent, from 1.2 to 1.3 upon tetralin addition. The alkene/alkane ratio decreased with increasing time, and the differences between the values from pyrolysis and H-transferring pyrolysis become smaller. Thus in the 30-min run, adding tetralin increased the ratio of 1-dodecene to n-dodecane from 1.6 to 2.6, and that of 1-undecene to undecane from 0.5 to 0.6. The above results unambiguously show that the formation of n-dodecane decreased much more than that of
550 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 Table 1. Effects of Tetralin (THN) on the Pyrolysis of n-Tetradecane at 450 12 21 21 6 12 6 residence time@(rnin) TS84 MTS32 TS83 MTS3l TS82 MTS30 expt no. no 1 0 % T H N no 1 0 % T H N no 1 0 % T H N with H-donor? 2.64 5.39 31.68 10.84 3.98 16.92 conversionb (mol % ) product yieldb (mol 5%) hydrogen (H2) 0.59 3.90 0.98 0.45 0.38 1.73 methane (C1) 1.43 6.78 2.21 1.04 1.01 3.53 ethane (C2) 0.40 1.62 0.55 0.37 0.22 1.06 ethylene 0.78 3.58 1.22 0.83 0.60 1.95 propane (C3) 0.64 2.70 1.07 0.49 0.41 1.57 propylene 0 0 0 isobutane 0.02 0.49 4.31 1.31 0.35 2.02 n-butane (n-C4) 0.31 0.63 3.63 1.27 1-butene 0.39 1.73 0.34 0.16 0.13 1.17 0.35 3.15 0.73 pentane (n-Cs) 2.13 1.28 0.41 0.69 3.36 0.28 1-pentene 1.21 0.84 0.23 0.18 0.43 2.75 n-hexane 2.10 1.16 0.40 0.60 3.20 0.25 1-hexane 0.44 2.70 1.28 0.93 0.24 0.20 n-heptane (n-C7) 0.50 1.61 1.01 0.35 0.22 2.50 1-heptene 0.39 2.36 0.83 1.13 0.21 0.18 n-octane (n-Cs) 0.47 1.95 0.90 1.36 0.31 0.22 1-octene 0.37 2.11 0.80 n-nonane (n-Cg) 1.04 0.21 0.16 0.44 1.77 1-nonene 0.85 1.23 0.31 0.21 0.34 1.82 0.73 0.92 0.19 0.15 n-decane (n-Clo) 0.43 0.80 1.13 0.29 0.21 1.53 1-decene 0.75 0.20 0.35 1.76 n-undecane (n-Cl1) 0.93 0.16 1.32 0.72 1.00 0.39 0.27 0.19 1-undecene 0.13 0.04 0.45 0.19 0.03 0.02 n-dodecane (n-Cl2) 0.37 1.28 0.70 0.97 0.27 0.18 1-dodecene 0.01 0.26 0 0.05 0.09 0.02 n-tridecane (?&la) 0.10 0.05 0.17 0.22 0.06 0.32 1-tridecene 94.61 68.32 97.36 83.08 89.16 n-tetradecane (n-Cl4) 96.02 0.14 0.02 1-tetradecene 0.09 0.14 0.09 0.24 0.10 n-pentadecane (n-C15) 0.08 0.16 0.06 0.02 n-hexadecane (n-C16) 0.11 0.04 n-heptadecane 0.13 0.15 0.13 0.13 0.13 n-octadecane (n-Cle) 0.13 0.07 n-nonadecane (n-Cle) 0.02 0.06 0.02 n-eicosane (n-Cm) 0.04 0.02 n-heneicosane (n-C~l) 0.03 0.01 n-docosane (n-Cz2) 0.02 0.01 n-tricosane (n-C23) 0.06 0.20 alkylcyclopentanes 0.13 0.57 alkylcyclohexanes alkylbenzenes 94.80 94.83 93.48 tetralinc 0.16 0.09 0.34 1-methylindanc 0.83 0.68 1.36 naphthalenec other polyaromatics solids (wt % c14)
"C 30 240 30 240 240 MTS2l TS85/86 MTS26 TS67 MTS2O no 1 0 % T H N no 1 0 % T H N 25%THN 33.76 97.08 91.98 53.33 95.48 1.17 8.33 13.76 2.21 7.08 5.20 0.03 8.41 6.74 7.00 5.71 5.67 4.96 5.15 3.35 4.44 2.61 3.66 2.31 2.94 1.47 2.69 1.27 0.82 1.17 0.51 0.25
5.26 3.55 5.48 4.99 4.82 3.76 2.27 3.34 2.66 2.24 2.40 1.73 2.05 1.46 1.96 1.23 0.44 1.15 0.21 0.26
46.67
66.24
0.21 0.36 0.26 0.17 0.18 0.09 0.07 0.05 0.04 0.03 0.69 1.71
0.44
2.39 4.55 0.76 2.51 2.08
0.18 0.09 0.08 0.13 0.05 0.06 0.03 0.02 0.02 0.24 0.49 0.29 83.03 4.10 7.29
8.09 70.31 74.48 2.38 46.93 10.52 1.45 17.62 2.83 14.71 2.45 12.65 2.70 9.21 2.36 6.99 0.06 4.93 0.09 3.25 0.03 2.30
28.06 37.40
45.85 56.05 1.41 38.24 6.40 0.92 24.22 2.97 17.70 1.27 14.36 1.36 10.62 2.03 7.95
26.86 8.21 0.42 20.5 3.37 20.06 5.02 18.09 1.59 9.51 3.00 7.05
5.21 0.14 3.70
5.28 0.17 3.90
2.72
3.05
0.81
0.85
0.89
0.40
0.40
0.38
2.92
4.52
8.02
0.05 0.22 0.02 0.03
0.15 0.02 0.04
3.52 5.99 13.19 3.95 6.48 101.85 0.91 0.7
5.22 4.44 6.59 22.47 12.06 61.67 0.94 0.2
0.06
3.52 5.62 21.39 0.36b O.5Bb 0.24 3.0
1.11
Mol % based on the initial molar number of n-tetradecane feed, unless otherwise 0 Reactor residence time in sand bath preheated to 450 "C. mentioned. Yield in mol % based on the initially added amount of H-donor tetralin.
1-dodecene upon tetralin addition. This observation can be rationalized by considering the kinetics of elementary radical reactions. In the initial stage, the m - 2 straightchain paraffinic product from CmH2m+2can only be formed from the @-scission of a primary radical 1-C,H2m+l0 followed by H-abstraction. However, the m - 2 olefinic product is produced from @-scissionof a secondary radical, 4-CmHzm+l*.While adding H-donor has the potential to stabilize both types of radicals, as shown in Scheme 1,the change of 1-dodeceneln-dodecane ratio suggests that the H-abstraction by primary radicals is preferred. The effects of tetralin on product distribution patterns appear to be dependent upon the extents of n-Cl4 decomposition or the residence time. At very low conversion (4 % ), which corresponds to a 6-min residence time of the reactor, the effect of tetralin on product distribution is small. It appeared to have suppressed the alkene
formation (Table 11, but the very low yields and low conversion (14 % ) do not provide a reliable data base for further detailed discussion. Figures 4,5, and 6 show the changes in product distribution upon addition of 10 vol % tetralin to n-Cl4 after 12,21, and 30 min, respectively. When the residence time is extended to 1 2 min, the presence of tetralin altered the pattern of product distribution, as shown in Figure 4. In the absence of a H-donor, there appeared a preferred formation of l-pentene and 1-hexene from n-Cl4 after 1 2 min. The preferential formation of 1-hexene has been attributed to a 1,5shift isomerization of a primary radical generated from substrate molecule or a substrate radical followed by the @-scission;similarly, the preferential formation of l-pentene has been attributed to a 1,4-shift isomerization of a primary radical followed by the P-scission (Song et al., 1994). If these pathways are correct, such isomerization
55 1
-P
50
*
-
C14
RT = 12 min
e C14-10%THN
h
4.0
ep
-
n-Alkane/C14 1-Alkene/ C14
40-
Y
P
30
6 f
-
20-
u 10
-
O! 0
10
30
20
Residence Time (min) Figure 2. Inhibiting effect of tetralin (10 vol %) on decomposition of n-tetradecane during its pyrolysis at 450 "C.
* --*--
8 '8 8 8 '8
.
--e-ClO(lO%THN) 8\8
h
22
30
26
Residence Time (min) Figure 3. Effects of adding tetralin (10 vol %) on the molar ratio of 1-alkene to the corresponding n-alkane.
+
R- H: -CH3
+
m% m%
+
R-CHiCH 3
+.
12
+ n-AlkanelC14 + l-&ne/C14
-
!I
--*-n-Alkane/lO%T"-C 14 1-Alkene/lO%THN-C 14
2.0
0
.
R-CHiCH3
10
I
0.0
Scheme 1. Hydrogen Abstraction from Tetralin by Primary (kl) and Secondary Radicals ( k ~ ) R-CHiCH2*
8
RT = 21 rnin
s
18
6
I " . "
8.0
14
4
C8 --4--C8 (lO%THN)
8
10
2
Carbon Number Figure 4. Effects of adding tetralinon the early stageof n-tetradecane pyrolysis at 450 O C (nominal residence time (RT) = 12 min; real RT at 450 O C I 8 rnin). The molar yields of n-alkanesand 1-alkenes are based on n-tetradecane.
Carbon Number c12 c 1 2 (10%THN) --o- c10
'8,
0
2
4
6
8
10
12
Carbon Number Figure 5. Effects of adding tetralin on the molar yields of n-alkanes and 1-alkenes from tetradecane after RT of 21 min (real RT at 450 OC 5 17 min).
a
should be selectively suppressed by adding H-donor, because H-abstraction by primary radicals is preferred. In fact this is the case. Adding tetralin strongly suppressed the preferential formation of 1-pentene and 1-hexene, giving a flat distribution pattern of C3-Clll-alkenes. This trend essentially continued up to 21 min residence time, as can be seen from Figure 5. With further increase in residence time, the peak carbon number of olefinic products shifts from 5-6 toward 4 and finally toward 3, but using tetralin also retarded this shift (Figure 6, Table 1). It is also apparent from Table 1 and Figures 4-6 that formation of light alkanes, including methane, ethane, and propane, from n-C14 was suppressed most significantly by adding tetralin. The extent of decrease in CH4 yields, relative to pure n-Cl4 pyrolysis, also increased with increasing amount of tetralin added from 5 to 50 vol % In addition, as can be seen from Figures 4-6, for the 1230-min runs, the yields of 1-alkenes from n-C14 also
.
decreased with increasing carbon number from 5 to 13. On the other hand, the ratios of alkane yields with and without tetralin addition was fairly constant a t 12 and 21 min for C1-C11 alkanes (ratio of about 3). Therefore, the reduction of paraffinic decomposition products was significant but not selective for C1-C11 alkanes within the first 30 rnin of reaction. On the basis of the compositional differences of the products and the mechanisms of c14 pyrolysis discussed in the preceding paper (Song et al., 19941, it seems that the alteration of product distribution pattern with tetralin is mainly due to its differentiation between different radicals, being more effective in stabilizing the pri-R' than the sec-R' radical. The radical stability decreases in the order of tertiary > secondary > primary > CH3' radical. In other words, under the conditions used, pri-R' can abstract hydrogen from tetralin more rapidly than sec-R'; this leads to decreased yields of both alkanes and alkenes but with increased ratio of alkene/alkanes upon tetralin addition in the short-duration runs. The decreased yields of decomposition products are due to substantially reduced
552 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 60
1"
'
RT = 30 min
e n-Akane/C14 e - -
Figure 9. Carbon number distributionof n-alkanes from pyrolysis of n-Cld alone at 450 OC.
petroleum-derived jet fuels and some intermediate products formed during the transformation from paraffinic jet fuels to solid deposits (Peng et al., 1992a,b; Song et al., 1992b). In the absence of H-donor, the amounts of deposits formed were 5.6 w t % from n-BB, 3.1 wt % from JP-BP, and 3.0 w t % from n-Ci4. It is clear from Figure 10 that adding a small amount of tetralin significantly reduced the deposit formation from all these compounds. As for the efficiency of H-donor, adding 10 vol 9% tetralin to JP8, n-C14, and n-BB reduced the formation of deposits by 90% (from 3.1 to 0.3 wt %), 77% (from 3.0 to 0.7 wt %), and54% (from5.6 to2.6wt %),respectively. These results clearly demonstrated a reduction in solids deposition from jet fuel at high temperature upontetralin addition in static reactors, which suggests useful application of H-transferring pyrolysis to high-Mach aircraft fuel system. Multi-ring cyclic alkanes such as decalin may also serve as H-donors at high temperatures, although decalin is not as active as tetralin for inhibiting solid formation (Song et al., 1991). Table 2 shows that, a t 450 "C, decalin can also suppress the deposit formation from JP-8P jet fuel, n-Ci4, and n-BB. In fact, adding both trans- and cis-D by 50 vol % almost eliminated solid formation from n'Cl4, JP-8P, and n-BB. Since decalin and n-Cl.4 are also
--
--
cycloalkenes polyaromatics
solids
It is apparent from Table 1 that the cyclization and aromatization reactions during n-Ci4 pyrolysis are inhibited significantly in the presence of tetralin. The polyaromatics indicated as the intermediates in the above pathway from long-chain alkanes to solid deposits should be relatively reactive ones. Although tetralin is converted to naphthalene after H-donation, this does not contribute to increased solid formation under the conditions employed. On the basis of the foregoing, the enhanced stability of hydrocarbons and the reduced solid formation in H-transferring pyrolysis can be attributed to the stabilization of the reactive radicals via hydrogen abstraction from H-donors such as tetralin, as shown in Scheme 3. The pathways for H-transfer from some other hydroaromatics such as those present in coal-derived jet fuels and middle distillates are considered to be similar to that from tetralin. Stock (1985) has published an extensive review on H-transfer from various hydroaromatic compounds in coal liquefaction. When hydroaromatics are not readily available or their concentrations become very low, polycyclic hydrocarbons such as decalin can also act as hydrogen donors. The final product of such H-transfer reactions is naphthalene, and it is one of the major products from thermal stressing of samples containing either tetralin or decalin (Song et al., 1992a,c). H-donation from such compounds contributes mainly to inhibiting the radical reactions and suppressing solid formation (Tables 1 and 2), but the effectiveness of H-transfer is substantially higher with tetralin than with decalin, probably because the benzylic radical from tetralin is more stable than the tertiary radical formed from the H-abstraction from decalin. Pyrolysis of Tetralin with and without Reactive Compounds. Table 3 presents the detailed results of products from tetralin pyrolysis. Tetralin is relatively stable when stressed alone at 450 "C. The major products from pyrolysis of pure tetralin are 1-methylindan and naphthalene, as well as a small amount of n-butylbenzene. Figure 11 shows the temporal variations of the major products from tetralin. Even after 8 h of pyrolysis at 450 "C, the yields of n-butylbenzene and the total gas products were still very low, within 1 wt %, indicating the ringopening cracking and subsequent dealkylation reactions were very limited with tetralin. We also noted the isomerization of cis-decalin to trans-decalin (both of them are present in tetralin originally as impurities, 0.1 % transdecalin and 0.3 % cis-decalin) and the increase in transdecalinlcis-decalin ratio with residence time from 0.52 in 30 min, to 1.06 in 1 h, and to 3.75 after 8 h. This is consistent with both the results of Bockrathand Schroeder (1981) in their coal liquefaction work using tetralin which contained decalins as minor impurities and our results on
554 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 I
67
h
5
2
-?4 c) .d
n 0
a3 0
n z
0
2
VJ
1
0
0
20
40
60
8 0 100
0
20
40
60
8 0 100
0
20 4 0
60
80 1 0 0
Yo1 x Tetralin Vol % Tetralin Vol X Tetralin Figure 10. Effect of tetralin addition on the formation of solid deposita from n-tetradecane (left), a petroleum-derived JP-8 jet fuel (middle), and n-butylbenzene (right) at 450 O C for 4 h.
Table 2. Deposit Formation and Liquid Depletion during H-Transferring Pyrolysis of Hydrocarbons and a Petroleum-DerivedJP-8 Jet Fuel feedstocks condition products (wt %) sample + vol % H-Donor temp, O C time, h CI-C~gas >Cg liquid solid deposita recovered deposit* 450 4.0 38.3 58.8 3.0 1.9 n-tetradecane 19.5 80.3 0.1 0 tetradecane + 50% cis-decalin 450 4.0 18.2 81.7 0.1 0 tetradecane + 50% trans-decalin 450 4.0 27.3 72.0 0.7 0.2 tetradecane + 10% tetralin 450 4.0 9.1 90.8 0.1 0 tetradecane + 50% tetralin 450 4.0 450 4.0 5.4 94.6 0 0 cis-decalin 450 4.0 0.8 99.2 0 0 trans-decalin 450 4.0 0.7 99.4 0 0 tetralin 450 4.0 17.2 77.2 5.6 5.0 n-butylbenzene (n-BB) 14.3 85.9 0 0 n-BB + 50% cis-decalin 450 4.0 450 4.0 12.1 87.9 0 0 n-BB + 50% trans-decalin 450 4.0 15.4 82.0 2.6 2.5 n-BB + 10% tetralin 450 4.0 9.8 90.2 0 0 n-BB + 50% tetralin 450 4.0 26.8 70.2 3.1 1.9 JP-8 jet fuel JP-8 + 50% trans-decalin 450 4.0 13.3 86.6 0.1 0 450 4.0 20.0 79.7 0.3 0.1 JP-8 + 10% tetralin 450 4.0 7.9 92.0 0.1 0 JP-8 + 50% tetralin 450 0.5 5.9 94.1 0 0 n-tetradecane 2.3 97.7 0 0 n-tetradecane + 10% tetralin 450 0.5 450 0.5 5.1 94.9 0 0 n-butylbenzene 450 0.5 5.4 94.6 0 0 n-BB + 10% tetralin 450 0.5 0.1 99.9 0 0 tetralin 0
Solid deposit on the reactor wall determined by measuring weight gain of the microreactor after the stressing, pentane washing,and drying.
* Solid deposit recovered from the reactor wall.
Scheme 3. Radical Stabilization via Hydrogen Transfer from Tetralin and Decalin
decalin pyrolysis (Song et al., 1992~). I t should be mentioned that we also detected small amounts of CO and COZin the gas products from tetralin. Detailed GCMS and capillary GC analysis of the original reagent tetralin (with nominal purity of 99%) revealed the presence of 1.2 mol % 3,4-dihydro-l-naphthaleneone, and 0.6 mol 5% 1,2,3,4-tetrahydro-l-naphthalenol (Table 3). Probably they were formed by oxidation during storage. However, the presence of these impurities had little effect in enhancing tetralin decomposition, probably due to its radical-scavenging ability. In other words, tetralin can stabilize itself. In the H-transferring pyrolysis of n-Cu and n-BB, however,the distribution of produck from tetralin displays
a significantly different pattern, as shown in Scheme 4. The product distribution for pyrolysis of n-BB alone has been reported recently (Peng et al., 1992a,b). Compared to the runs of tetralin alone at 450 "C within 30 min, the presence of a long-chain paraffinic compound such as n-C14 or n-Cls slightly enhanced both the dehydrogenation and the ring-contraction isomerization. For the 30-min run in the presence of n-BB, tetralin mainly underwent dehydrogenation to form naphthalene. Further increasing the residence time from 30 to 240 min increased mainly tetralin dehydrogenation. After 4 h, more than 90% of tetralin has been dehydrogenated in the case of its mixture and its isomerization was reduced significantly as compared to the run of tetralin alone. For 30-min runs, the radicals from n-BBwere more active in dehydrogenating tetralin, although its conversion was lower than that of n-Cl4 under this condition. For the mixtures of 25 vol 5% tetralin with n-Clr, however, the yield of 1-methylindan from tetralin increased significantly after the 240-min run, the value of which is close to that from pure tetralin. The quantification of n-BB product formed from tetralin was difficult
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 555 Table 3. Products from Pyrolysis of Tetralin at 450 O C under UHP NZ 30 60 150 240 480 TS79 TS36 TS56 TS6 TS69 5.42 6.88 13.73 19.92 35.62
residencetime (min) none expt no. origfeed 3.27 conversion (mol %) yield (mol 7% ) Hz methane ethane ethylene propane propylene n-butane 1-butene toluene 0.05 ethylbenzene o-xylene n-propylbenzene 2-ethyltoluene 0.11 trans-decalin n-butylbenzene 0.25 cis-decalin 2-methylindan 0.08 1-methylindan 96.73 tetralin (THN)a 1,2-DHNa 0.65 0.31 naphthalene 0.57 a-tetralolb 1.18 a-tetralonec
0.73 0.01 0.07
0.03 0.05
3.55 0.23 0.18 0.06 0.16 0.09 0.13 0.05 0.13 0.11
0.11 0.03 0.12 0.18 0.19 0.22 0.23 0.17 0.06 0.10 1.81 2.87
0.08 0.01 0.37 0.55 0.17 0.23 6.11
0.02 0.04 0.02 0.03 0.07
2.37 0.12 0.09 0.01 0.04 0.02 0.01
0.11
3.73 0.41 0.21 0.01 0.27 0.06 0.23 0.03 0.13 0.16 0.02 0.10 0.02 0.20 0.48 0.11 0.47 12.68
9.39 1.96 0.88 0.04 0.32 0.18 0.14 0.01 0.56 0.49 0.05 0.12 0.05 0.30 1.01 0.08 0.71 20.71
94.58 93.12 86.57 80.08 64.38
0.12 1.56 0.05 0.79
0.12 1.63
0.15 3.24
0.12 3.98
0.14 8.83
0.74
0.52
0.22
0.07
1,2-Dihydronaphthalene (DHN) and 1,2,3,4-tetrahydronaphthalene (THN). 1,2,3,4-Tetrahydro-l-naphthol.3,4-Dihydro-lnaphthalenone. Pyrolysis of THN
-P
i2
* THNConv
%
1-MI
30
Naphthalene
v
n-BB
5:
E
s u , '0 E (D
3 0)
$
10
0 0
100
200
300
400
500
Residence Time (min) Figure 11. Product distribution from pyrolysis of tetralin (THN) alone at 450 "C.
in the case of H-transferring pyrolysis of n-C14, but its yield is apparently lower than that of 1-methylindan. These results clearly indicate that, for the 240-min runs, when the amount of H-donor tetralin is small (10 vol %), the reactive radicals generated from pyrolysis of n-Cl4 and n-BB accelerate the tetralin dehydrogenation via H-abstraction, as shown in Scheme 3, but such radicals do not promote the ring-contraction isomerization as significantly as the dehydrogenation, as can be seen from Scheme 4. When more tetralin (25 vol %5 ) is available, the radicals from also promote the isomerization, but such an increase was not significant with the radicals from n-BB. On the basis of the findings of Benjamin et al. (1979) and Franz et al. (1980),the isomerization proceeds mainly through the 2-tetralyl radical to form 1-indanylmethyl radical, as shown in Scheme 5. In the presence of unstable
Scheme 4. Products from Tetralin for Pyrolysis of Itself and Its Mixture with n-C14 or n-BB Me
'
450'C/30 min 1.7 450W4 h 12.7
aI @ ; >@yJ ; 10%Telralin-C14 450'C/30 min 450'C/4 h I OXTelralin-BB 450'C/30 min 450'C/4 h 25%Tetralin-C14 450'C/4 h ZSxTetralin-BB 450'C/4 h
1.3
0.2 mol%
4.0
0.5 mol%
mol% mol%
5.9 90.9
4.5
27.1 97.0
3.6
2.3
mol% mol%
64.1
12.6
mol%
92.3
5.6
mol%
5.7
Scheme 5. Isomerization of Tetralin to 1-Methylindan and Dehydrogenation of 2-Tetralyl Radical in the Presence of &active Compounds
I
compounds, the thermally generated reactive radicals can also lead the 2-tetralyl radical to dihydronaphthalene and subsequently naphthalene, as also shown in Scheme 5. This explains why the dehydrogenation reaction of tetralin is dominant. When the H-donor concentration is relatively low, the radicals from n-Cl4 and n-BB mainly abstract benzylic hydrogen to yield 1-tetralylradical. In such case, the formation of 2-tetralyl radical is very limited and hence the isomerization is not very important. This also confirms that the reactions via 1-tetralyl radical shown in Scheme 3 are major pathways for hydrogen donation. When tetralin is present in high levels, however, the probability for radicals to abstract hydrogens from the 2-position increases. As a result, n-Cu not only promotes tetralin dehydrogenation, but also enhances the isomerization to form 1-methylindan in the presence of excess tetralin, although the former is still the dominant reaction. Some Practical Aspects. The present results are pertinent to pyrolytic degradation at high temperatures in the absence of air. The fuel degradation in real situations, of course, may incorporate both autoxidation reactions a t low temperatures and pyrolytic reactions at high temperatures. Although we did not examine the autoxidation reactions under hydrogen-transferring conditions, some useful information has been generated in the past. Russell (1955) studied the oxidation of tetralin and cumene and found that small quantities of tetralin added to cumene would markedly lower the rate of oxidation, even though neat tetralin oxidized 10 times as fast as cumene at the same temperature and initiation rate. Many years ago tetralin was thought to have certain positive impact on carbon deposits in internal combustion engines (Lucas, 1935; Ray, 1947). Taylor (1969) found that under thermal oxidation conditions (121 "C)tetralin reduced deposits by more than 80% in air-saturated blends. In fact, tetralin was one of the most effective inhibitors studied by Taylor (19691, presumably because it possesses four benzylic hydrogen atoms which inhibit oxidation reactions involving hydrogen abstraction by
556 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994
peroxy radicals (Frankenfeld and Taylor, 1980). In deoxygenated fuels, tetralin was only slightly inhibitory at relatively low temperatures. Kubo et al. (1991) and Coleman et al. (1992) have shown that, under oxidative atmosphere, benzyl alcohol, tetralin, and other hydroaromatics can act as deterioration inhibitors for hydrocarbon fuels stressed at elevated temperatures (350-425 "C). More recently, Kubo et al. (1992)have demonstrated that tetralin has radical-scavenging ability in the presence of oxygen at temperatures as low as 50 "C. The present results also have a bearing on the behavior of long-chain paraffinic species in the presence of H-donor solvents during coal liquefaction, coprocessing of coal/ petroleum resids, and upgrading of heavy oils and coal liquids. Our results suggest that H-donors could retard the pyrolytic decomposition of long-chain alkanes at conventional coal liquefaction and coprocessing temperatures (400-470 "C). However, H-donors may have different effects for the reactions of aromatic compounds. McMillen and Malhotra have reported that, in the H-donor solvent-mediated coal liquefaction, a bimolecular hydrogen transfer of hydrogen from cyclohexadienyl-type radicals (of either coal or solvent origin) to the ipso positions of linkages to aromatic systems can result in cleavageof strong bonds between aromatic and aliphatic carbons (C,-Cai) (McMillen et al., 1987; Malhotra and McMillen, 1990).
Summary It appears from the above results and discussion that application of H-donors to improve jet fuel thermal stability is promising. Relative to the pyrolysis of pure paraffinic jet fuel components, two of the salient features of the hydrogen-transferring pyrolysis are the much lower rate of decomposition of the starting materials, and the substantially reduced amountsof solid deposits. Hydrogen donors act like radical scavengers by donating hydrogen to the radicals from reactive compounds. On the other hand, the radicals from reactive compounds, via H-abstraction, enhance the dehydrogenation reaction of Hdonors, tetralin or decalin. The results of H-transferring pyrolysis of reactive compounds in the presence of H-donors provided some very important information. Adding H-donors such as tetralin or decalin to the conventional jet fuel significantly reduced or even eliminated the formation of carbonaceous deposits at 450 "C and decreased the extents of fuel decomposition and gas formation, which led to substantial increase in the fuel stability. The enhanced stability can be attributed to the stabilization of the reactive radicals via H-abstraction from tetralin- or decalin-type compounds, which contribute mainly to inhibiting the radical decomposition, cyclization, aromatization, and condensation reactions. The present results have several important implications to the development of advanced jet fuels. First, jet fuels from hydrotreating of coal-derived liquids are rich in H-donors and cycloalkanes, which make them good candidates for high-temperature applications. Second, blending of conventional petroleum-based jet fuels with small quantities of coal-derived jet fuels may significantly enhance their high-temperature thermal stability. Finally, specialty fuels may be made based on aliphatic compounds or their mixtures which are highly resistant to solid deposition. Examples of such "model" fuels are decalin and its mixtures with long-chain alkanes. Finally, the above results and discussion suggest that, by taking advantage of hydrogen-transferring pyrolysis,hydrocarbon
jet fuels may be used at high operating temperatures with little or no solid deposition.
Acknowledgment This project was supported by the US.Department of Energy, Pittsburgh Energy Technology Center, and the Air Force Wright Laboratory/ Aero Propulsion and Power Directorate, Wright-Patterson AFB. Funding was provided by the U.S. DOE under Contract DE-FG2292PC92104. We wish to thank Mr. W. E. Harrison I11and Dr. D. M. Storch of WL, and Dr. S. Rogers of PETC for their support. We are pleased to acknowledge Dr. M. M. Coleman for his encouragement and helpful discussions, and Ms. Y. Peng for helpful discussions on the solid formation from neat n-butylbenzene. We are grateful to Mr. R. Copenhaver for the fabrication of the microreactors.
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