Kinetic Study of Thermal Reaction of Benzene Tomoya Sakai', Seiro Wada, and Taiseki Kunugi Department o f Synthetic Chemistry, Faculty o f Engineering, University o f Tokyo, Hongo, Tokyo, Japan Thermal reaction of benzene was conducted under conditions which preclude the decomposition of the aromatic nucleus. Benzene was fed through the silica-glass reactor without any diluent at temperatures ranging from 700" to 850°C and at residence times 0.24 to 3.40 sec. Biphenyl and hydrogen were the main products with small amounts of pyrene and other polycondensed ring compounds. The results almost fit the second-order reversible reaction kinetics. The addition of about 10 mol % of naphthalene to feed benzene decreases the overall rate and yields a- and /3-phenylnaphthalenes, while that of ethylbenzene increased the rate and gave more tar and coke. The paraffinic side chain became the source material for the formation of tar and coke.
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A
systematic study of the thermal reaction of C? C, olefins and unsaturated naphthenes (Sakai et al., 1970) has been made to clarify the reaction scheme of coke and (or) tar formation during pyrolysis of petroleum hydrocarbons. The present paper investigates thermal reaction of benzene kinetically a t temperatures between 700" and 850" C. Many studies have been conducted on pyrolysis of benzene. A survey of the literature allows division of the reaction schemes into two categories, one a t temperatures above 900" C , the other at temperatures below 900" C. Within the lower temperature range the main products are biphenyl and hydrogen, and within the higher range decomposition of the aromatic nucleus occurs to produce acetylene and other gaseous products. Mead and Burk (1935) made a detailed analysis of the reaction a t temperatures between 750" and 85OoC. They proposed the rate equation, assuming the reaction proceeded in terms of a bimolecular surface reaction strongly inhibited by products, as d x / d t = k [ ( a- x ) ' / x ] (1) where a is the starting concentration of benzene, r i a is the fraction decomposed a t time t. There seem to be some deficiencies in their work: The reaction rate becomes infinity at x = 0; the increase in specific surface does not coincide with the increase in the reaction rate despite their kinetics; the analyses of small amounts of by-products are not sufficient. Pease and Morton (1933) and Bauer and Aten (1963) studied the reaction a t 630°, and 420" to 1600"C, respectively, and both proposed the homogeneous first-order kinetics. Hou and Palmer (1965) applied the kinetics with temperatures ranging between 900" and 1200" C, as Equation 2: -dc/dt = krc + kIIC' (2) where c is the concentration of benzene a t time t, h I is the heterogeneous rate constant concerning the decomposition of the nucleus, and kII is the homogeneous rate constant concerning the formation of biphenyl and hydrogen. To whom correspondence should be addressed.
The authors of the present paper carried out the thermal reaction of benzene a t 700" to 850°C and analyzed the results not only to survey several kinetics mentioned above, but also to clarify why the retardation of reaction rate a t high conversion took place. Furthermore, the effects of respective additions of naphthalene and ethylbenzene to benzene were examined to obtain some information on the scheme of formation of polycondensed ring compounds-i.e., tar and coke, in pyrolysis of hydrocarbons. Experimental
A conventional flow-type apparatus was used in this experiment. Reactors made of transparent quartz tube, or copper tube, each 5-mm i.d., were heated electrically through a stainless steel block 350-mm long. The copper tube was treated beforehand with oxygen-free hydrogen at 8 1 P C for 40 min. Commercially available benzene, almost 100% pure by gas chromatographic analysis, was introduced by capillary feeder into the reactor tube and subjected to evaporation as well as reaction a t 700" to 850" C. Reaction temperature and residence time were determined by the method of Hougen and Watson (1943). Outlet gas was led through a water-cooled condenser and a Cottrell precipitator to separate the liquid completely from the gas. Liquid and gas products were analyzed by use of three different gas chromatographs. Solid tar was weighed, and coke was measured as CO, by burning. Results and Discussion
The small amount of coke that accumulated on the reactor wall did not affect the reaction rate. The good reproducibility of the results, therefore, could be attained a t any process period. Reactions conducted below 800" C yielded biphenyl as a sole liquid product. The gas products in this temperature range were mostly Hz, accompanied by small amounts of CH,, C2H4, and C2Hs. The latter may have come from the paraffinic and (or) naphthenic contaminations in the feed benzene. In the experiments at 850°C, however, the small amounts of pyrene and other polycondensed Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971 305
ring compounds were produced additionally. Product yields at 850°C are shown in Figures 1 and 2, and the other typical experimental results of the present work are summarized in Table I. Conversion is defined here as the quotient of the weights of all substances (other than feed materials eluted up to 2OO0C by silicone grease column) to the weights of feed materials. The products symbolized as B and A are unidentified components detected just before and after the peak of pyrene in the gas chromatogram, respectively. Molal sensitivities of unidentified small peaks were estimated from the known peaks in the vicinity. Figure 3 shows the conversion of benzene versus resi-
dence time. The application of Equation 1 to these results settled almost in linear relations, as is shown in Figure 4. Activation energy of the reaction was obtained as 54 kcalimol (Mead and Burk: 50 kcal/mol). Any other rate equations, however, of the form of rate = Zkc", where K and n are arbitrary positive real numbers and c is concentration of benzene, could not explain the retardation of reaction rate at high conversions. Participation of reverse reaction (oXo>+H2-2(0) might be a cause of retardation of reaction at high conversions. The application of second-order reversible reaction
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151 -6 W W Yc
0 v)
W 0
d
E
10
.
I
850 'C
-E W c
1.50
/" 1.25
I /
I
I/ 0
I
A
Pyrene
1 2 3 Residence time t , sec
Residence time t
,
sec
Figure 2. Yields of gaseous products
Figure 1. Yields of main liquid products
Table I. Typical Experimental Data
Feed stock
Benzene
Benzene'
Temp, "C
700 750 800 800 800 850 810 810 810
Residence time, sec
2.14 0.94 0.27 0.42 2.07 1.45 0.41 1.81 10.4
Conversion, wt 9'0
3.80 5.70 4.30 7.10 18.3 25.6 2.11 8.16 18.6
Product, mole per 100 moles of feed
B"
Biphenyl
0.0 0.0 0.0 0.0 0 .o 0.20 0.0 0.05 0.13
1.75 2.62 1.83 3.25 8.24 10.40 0.98 3.31 5.96
Pyrene
A"
Tar B coke*
H2
CHI
CZH~
C2H6
0.09 0.06 0.16 0.10 0.04 0.04 0.03 0.02 0.0
0.03 0.02 0.01 0.01 0.03 0.02 0.01 0.02 0.04
0.0 0.0 0.0 0.0 0.4 0.9 0.0 0.25 0.66
0.0 0.0 0.0 0.0 0.2 0.6 0.0 0.18 0.32
0.3 0.5 0.6 0.6 0.9 0.9 0.2 0.3 0.5
1.62 2.55 2.59 2.81 5.20 8.70 2.02 4.13 8.25
0.22 0.16 0.08 0.11 0.14 0.16 0.07 0.12 0.31
0.0 0.19 0.23
0.0 0.10 0.09
1.0 1.3 1.4
3.1 5.8 6.5
0.14 0.24 0.28
0.12 0.05 0.03
0.01 0.03 0.03
0.20' 0.33 0.42
0.11 0.22 0.27
5.4 5.7 4.9
6.8 9.2 10.8
1.60 1.90 2.20
1.60 1.90 1.20
0.15 0.20 0.20
Phenylnaphthalene
Benzene
+
naphthalene
Benzene
+
ethylbenzene
810 810 810
810 810 810
0.29 1.03 1.77
0.21 0.92 1.95
4.74 10.4 11.2
15.1 21.6 24.5
1.43 3.52 3.85
2.87 5.90 6.15
a-
P-
0.19 0.38 0.40
0.25 0.44 0.47
Toluene
Stvrene
Phenanthrene
1.02 1.34 1.00
1.62 1.18 0.82
0.09 0.27 0.37
"Molecular weight of A or B was assumed as that of pyrene or phenanthrene, respectively. copper tube reactor.
306 Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971
Weight % on feed. 'By use of
I
A
30
s x-
20
c
.-0
z W
s
0
10
u
0 Residence time t , sec
Residence time t , sec Figure 5 . Application of equation dx/df = k { ( l - Xjx' - 2 x + 1 I X = (1 - X,)/X,, x r = 0.355
Figure 3. Conversion of benzene
-+
Y
c
Residence time t
,
sec
Figure 4. Application of equation d x / d f = k [ ( l - xj'/x]
kinetics below, by use of the equilibrium constant reported by Trevissoi (1955), gave a fairly good linearity as is shown in Figure 5 :
log [l - (1 - X)X]/[l - (1 + X ) x ] = 2 hXt
(3)
where x is conversion of benzene a t time t , X = (1 x , ) / X , . The equilibrium conversion of benzene, x?, was estimated to be 35.5% at 850°C. The heterogeneous nature of the reaction, however, should not be neglected. Some experimental tests were conducted in this connection. The first test is the reaction by use of 400-mm long silica rods, 2-mm in diam, inserted along the axis of the reacdr. Results obtained showed an increase in the conversion of benzene in the case of large specific surface area, but the increase was not so large as to be proportional to the area, as was reported
by Mead and Burk. The second test is the reaction by use of the copper surface, which resulted in considerable retardation in rate as shown in Figure 6. Rate constant h in Equation 1 was 4.64 times smaller using copper tube than that by silica tube a t 810" C. Also, the catalytic effect of copper surface has been found to retard the reaction rate in pyrolysis of n-heptane. Moreover, as is shown in Figure 7 , 10.0 mol Z naphthalene in benzene inhibited the rate a t almost the same degree as product biphenyl did. This shows that the heterogeneous part of the reaction is inhibited by biphenyl as well as by naphthalene, supposedly by'the mechanism of Mead and Burk, and the homogeneous part is retarded by the reverse reactions of biphenyl to two benzenes and phenylnaphthalene to benzene plus naphthalene. In considering the heterogeneity of the reaction and linear relations shown in Figures 4 and 5, the authors conclude that the thermal reaction of benzene a t temperatures around 800° C proceeds partly as a heterogeneous reaction strongly inhibited by high-molecular-weight compounds, and partly as homogeneous reversible reaction. To obtain more information on coke and tar formation from benzene, mixed feeds of 10.0 and 8.8 mol % of naphthalene and ethylbenzene, respectively, with benzene were examined. The results obtained are shown in Figures 7 and 8 and Table I . The mixed feed of naphthalene with benzene resulted in retardation of the overall reaction rate, although 4 0 5 conversion of naphthalene itself was attained a t 810"C. The solid lines in Figure 7 represent the experimental facts. The dashed line ( I ) represents the calculated yield of biphenyl from Equation 1 when a = 0.900, that is, naphthalene works as an inert diluent. The other dashed line (11) shows the result of calculations when naphthalene works as the product-Le., biphenyl-to inhibit the reaction by Equation 1. The latter coincides fairly well with the experimental results. The reverse reaction scheme could not be applied to this result because of the lack in thermodynamical data on phenylnaphthalenes. The mixed feed of ethylbenzene with benzene was found Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971
307
I
/
Residence time t , sec Residence time t , sec Figure 6. Mixed feed of 10.0 mol Yo naphthalene and benzene
+ 0
0
1 2 Residence time t, sec
Figure 7. Mixed feed of 8.8 mol % ethylbenzene and benzene to react in a more complicated way. Conversion of ethylbenzene reached almost 100%. Yields of CHI, C2H4, and C2Hs increased more than ten times compared to those from benzene feed a t 800°C (Table I). The black liquid obtained contained biphenyl, pyrene, A, phenanthrene, toluene, styrene, fluorene, naphthalene, B, xylenes, and traces of many unidentified substances, in the order of decreasing quantity. Styrene and toluene may be produced from ethylbenzene itself. The production of phenanthrene and the increased yield of pyrene are considered to be related to a larger concentration of ethylene. Generally, the most characteristic feature of this experiment is that tar yield (the 308 Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971
Figure 8. Comparison of several reactions
result of C12and higher hydrocarbons other than biphenyl) is very large. When this tar is assumed as biphenyl, yields of the obtained biphenyl and higher hydrocarbons are much larger than that calculated from Equation 1 (a = 0.912), as is shown by the lower dashed line in Figure 8. (See also Figure 6.) Namely, the ethyl group in ethylbenzene works effectively to produce tar or polycondensed ring compounds. I n Figure 6, comparison of the several experiments above is made in terms of conversion of feed versus residence time at 810°C. From the figure, the authors consider that the paraffinic chain (or perhaps olefins) is mainly responsible for the coke and tar formation, that aromatic compounds with no paraffinic or olefinic side chains are very refractory at the temperatures around 8OO0Cc,and that dimerization of aromatic rings are strongly retarded by product itself or by similar high-molecular-weight compounds. I t is, therefore, another conclusion of the present work that bare aromatic compounds, benzene, naphthalene, etc., cannot be any intermediate compounds for the formation of coke and (or) tar in pyrolysis of petroleum hydrocarbons, at temperatures around 800”C. Literature Cited
Bauer, S. H., Aten, C. F., J . Chem. Phys., 39, 1253 (1963). Hou, K. C., Palmer, H. B., J . Phys. Chem., 69, 863 (1965). Hougen, D. A., Watson, K. M., “Chemical Process Principles,” Wiley, New York, N. Y., 1943, p 884. Mead, F. C., Jr., Burk, R. E., Ind. Eng. Chem., 27, 299 (1935). Pease, R. N., Morton, J. M., J . Am. Chem. SOC.,55, 3190 (1933). Sakai, T., Soma, K., Sasaki, Y., Tominaga, H., Kunugi, T., “Secondary Reactions of Olefins in Pyrolysis of Petroleum Hydrocarbons” in “Advances in Chemistry Series 97,” pp 68-91, ACS, Washington, D. C., 1970. Trevissoi, C., Ann. Chim. (Rome), 45, 943 (1955). RECEIVED for review April 27, 1970 ACCEPTED February 9, 1971
