14
Ind. Eng. Chem. Res. 1992,31, 14-19
Kinetic Study of Model Reactions in the Gas Phase at the Early Stage of Coke Formation Daisuke Nohara* and Tomoya Sakai Department of Chemical Reaction Engineering, Faculty of Pharmaceutical Sciences, Nagoya City University, Mizuho-ku,Nagoya 467, Japan
The most probable gas-phase reactions at the early stage of coke formation were elucidated by kinetic study on the model reactions adopted for formation of cyclic compounds and growth of ring. It was revealed that the formation and growth of ring proceeded mainly through cycloaddition of butadiene or allyl radicals to unsaturated hydrocarbons at relatively low temperatures (-600 "C),i.e., through a Diels-Alder type reaction. On the other hand, such growth of ring as formation of biphenyl accompanying dehydrogenation from benzene can proceed only a t the higher temperatures. It was also revealed that in the growth of the ring, cycloaddition of butadiene favors a cyclic olefin molecule that possesses a nonconjugated double bond and a nearly planar structure.
Introduction Extremely various and complex reactions should be involved in the process of coke formation. Supposing that this process starts from paraffins or olefins in the gas phase, it is reasonable to state that formation and growth of the ring compound should take place at the early stage. These reactions may be followed, on a simplified model of coke formation, by expansion of the plane of the polycyclic compound in a sheet accompanying partial dehydrogenation and finally by vertical linkage between these planes forming three-dimensional particles. It should be emphasized that the ring formation at the very initial stage is largely attributable to the role of butadiene or the allyl radical. In the pyrolysis of paraffins at 700-84M O C , olefins such as ethylene, propylene, butenes, and 1,3-butadiene and small amounts of methane and hydrogen are formed during the initial stage of reaction. The selectivities of olefins decrease after passing through their maxima as the reaction proceeds. Instead, cycloolefins arise followed by formation of mono- and polynuclear aromatics. In the thermal reaction of olefins such as ethylene or butenes, propylene and butadiene are formed as the initial products and cycloolefins as the secondary products (see, for example, Halstead et al. (1968) and Kunugi et al. (1969)). These fads strongly suggest that the main route of ring formation is due to a Diels-Alder reaction of butadiene with olefins. This mechanism was postulated 61 years ago by Wheeler and Wood (1930) to explain the formation of cycloolefins and aromatics in the pyrolysis of ethylene. Another characteristic feature was exhibited in the thermal reaction of propylene (Simon and Back (1970a,b) and Kunugi et al. (1970a,b)),that is, it was observed that not only ethylene and butadiene but also such C5 cyclyc olefin as methylcyclopentene were produced as the initial product and that the latter product was successively converted to C6 cyclic compounds. This fact implies that the allyl radical which probably originates in the pyrolysis of propylene adds to olefins and cyclizes to produce the C5 cyclic compound. Contribution of the allyl radical to ring formation was proposed by Ruzicka and Bryce (1960). They obtained cyclopentene and cyclopentadiene in the pyrolysis of biallyl by using a static system. Bryce and Ruzicka (1960) also observed the formation of cyclopentenes in the thermal reaction of a gas mixture of biallyl and ethylene, propylene, or 1-butene, although it was not discriminated whether these products were the initial or the secondary ones. Nohara and Sakai (1973) discrimi0888-5885/92/2631-0014$03.00/0
Table I. Relative Rate Constant of Diene Synthesis by Use of Butadiene or Allyl Radicals at 500 O c a butadiene allyl radical re1 rate re1 rate dienophile product const product const
c=c-c=c c=c
0" 0
0.14 0.04
c=c-c-c
0"
cis-C-Ctrans-C-
Q
1.4 x 104
Q
1.3 x 103
c,
4.0 X IO2
0.0023
0.0012
a
c=c-c I
' 0
C
0.056
0.0051
c=c-c C=C
Q
c=c-c
a
a
1.7 x 104 8.5 x 103
( J J J
0.15
4.7 x 104
Concentration . a Concentration of butadiene: 6 X lo4 mol ~ m - ~ of allyl radicals: 8 X lo-* mol dm-3.
nated cyclopentene and cyclopentadiene as the initial product and 1,3-~yclohexadieneand benzene as the secondary products in their study on the thermal reaction of biallyl. Cyclization of the 4-pentenyl radical formed by addition of the allyl radical to ethylene was explained by Watkins and Olsen (1972) and McDonald et al. (1985). It is considered that the formation of these C5cyclic products is attributed to the diene character of the allyl radical, through a mechanism similar to that for the formation of C6 cyclic product by a Diels-Alder reaction between butadiene and olefins. The kinetic features of ring formation by butadiene with olefins were nearly established by Sakai et al. (1970),and those by the allyl radical with unsaturated hydrocarbons 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 15
- 2.0‘ 125
1
I
i
1.30 135 1.40 lo00 K-l T
-
,
“a0 145
Figure 1. Comparison of rate constants for cycloaddition of butadiene or the allyl radical in the thermal reaction of a diallyl oxalate-butadiene mixture between 430 and 510 O C .
have been investigated systematically by Nohara and Sakai (see, for example, 1980a,b). Two model processes for polycyclization or growth of the ring were proposed by Nohara and Sakai (1988a): (1) polycyclization by stepwise cycloaddition of adhesive fragment hydrocarbons such as butadiene or the allyl radical to an unsaturated bond of the olefins or polycyclics; (2) further cycloaddition taking place between congenial polycyclics, sometimes involving participation of a ‘‘fragment hydrocarbon”. Actually, the polycyclization may proceed under a combination of two reactions described above, accompanied or followed by dealkylation and/or dehydrogenation at higher severity. In this paper, the main probable routes for formation and growth of ring are elucidated by a comparative discussion of the rate or equilibrium data obtained in the model reactions adopted for the initial stage of coke formation. These are classified into three groups - for convenience’ sake in the later discussion. (A) Ring formation:
0
‘0
2 4 6 8 1 0 Reaction period I min
Figure 2. Product formation curves in the thermal reaction of cyclohexene with butadiene at 380 “C.
- 0.x)
h
t
i period I min
Figure 3. Product formation curves in the thermal reaction of indene with butadiene at 380 OC.
(C)Ring formation in account of reverse and successive reaction and growth of ring from pure aromatics:
(B)Growth of ring:
Experimental Section Kinetic studies of reactions 1,2, and 7-9 were made by use of a conventional and atmosphericflow-type apparatus (see, for example, Sakai et al. (1970) and Nohara and Sakai (1980a)). For the studies of reaction 8, cycloolefins were used as the starting material (Sakai et al., 1972). Each experimental run caused ita own characteristic temperature profile in the reactor tube owing to employment of a flow-type apparatus. Then, residence time at each run was obtained by calculating the equivalent reactor volume at the designated temperature by use of the method of Hougen and Watson (1943).
16 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 Table 11. Relative Rate Constant of Diene Synthesis by Use of Butadiene at 380 "C" dienophile product re1 rate const
c=c-c=c
1.0
c=c
0.084b
c=c-c
0.0226
trans-C-C=C-C
0.02Bb
0
0.01
0.04
0'
0.06
lLEcZI3 10
5
15
20
25
