Znd. Eng. Chem. Res. 1988, 27, 915-920
energy values have been determined. These, along with k h e n i u s correlations for the rate constants, are presented in Table V. The calculated activation energy values are consistent with the low values normally observed with free-radical reactions. Conclusions Quantitative and selective oxidation of styrene to benzaldehyde was achieved with Wilkinson complex. The reaction initiation period was independent of temperature. The reaction induction period strongly depended upon the catalyst concentration below the critical catalyst concentration. Co-oxidation of aldehyde to acid was found to be significant above a temperature of 75 "C and a styrene conversion of 25%. The optimum reaction conditions have been determined as (C/So) = 5.0 X lo4, (So/T) = 0.5, and T = 75 OC. A second-order kinetic model has been found to fit satisfactorily the experimental kinetic data both in the presence and absence of co-oxidation reactions. Nomenclature AI, A2,A3 = constants (B) = concentration of benzaldehyde, mol/L (C) = concentration of the catalyst, mol/L (C/So) = mole ratio of the catalyst to initial styrene kf = first-order rate constant, l / h k'f = pseudo-first-order rate constant, l / h
915
k, = second-order rate constant, L/(mol h)
(S)= styrene concentration, mol/L (So) = initial concentration of styrene, mol/L (So/T) = mole ratio of initial styrene to toluene (T) = toluene concentration, mol/L T = temperature, K t = time, h r = [(So) - (S)]/(So) = mole fraction of styrene converted xB = (B)/(So)= mole ratio of benzaldehyde to initial styrene Registry No. RhCl(PPh3)3, 14694-95-2; benzaldehyde, 10052-1; styrene, 100-42-5.
Literature Cited Arai, H.; Halpern, J. Chem. Commun. 1971, 13, 1571. Blum, J.; Rosenman, H.; Bergman, E. D. Tetrahedron Lett. 1967,38, 3665. Choch, P.; Halpern, J. J. Am. Chem. SOC.1966, 88, 3511. Gorokhovatski, Ya. B. Kinet. Catal. 1973, 14, 62. Harris, W. E.; Kratochoil, B. Chemical Separation and Measurement; Saunders: Philadelphia, 1974. Martell, A. E.; Khan, T. M. M. Homogenous Catalysis by Metal Complexes; Academic: New York, 1974: Vols. I and 11. Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J. J.Chem. SOC.A 1966, 1711. Takao, K.; Wayaku, M.; Fujwara, Y.; Imanaka, T.; Teranishi, S. Bull. Chem. SOC.Jpn. 1970,43, 3898. Young, J. F. Chem. Commun. 1965, 7, 131.
Received for review February 24, 1987 Revised manuscript received September 4, 1987 Accepted September 16, 1987
Kinetics of the Pyrolysis of Cyclohexane Using the Pulse Technique D. Stan Aribike* and Alfred A. Susu Department of Chemical Engineering, University of Lagos, Lagos, Nigeria
T h e kinetics and product distributions of the thermal cracking of cyclohexane were investigated in a stainless steel annular reactor using the pulse method. Experiments were conducted a t 1-atm pressure and with excessive nitrogen dilution. Data were obtained at temperatures of 700-860 OC and space times of 0.40-1.14 s. A kinetic analysis of the conversion-space time data revealed that conversion was autocatalytic at 700-800 "C,while at higher temperatures (820-860 O C ) it was governed by first-order kinetics. T h e activation energies for the two kinetic regimes were 192.5 and 240.0 k J mol-', respectively. T h e autocatalysis was ascribed to the participation of methylallyl radicals in a new bimolecular propagation sequence. The product distributions and the first-order kinetic rate parameters were consistent with those obtained in the continuous-flow mode. 1. Introduction
Most pyrolysis studies conducted to date have been devoted to light paraffins (Green et al., 1975; McConnell and Head, 1983; Corcoran, 1983). On the other hand, little or no work has been done on the thermal cracking of naphthenes and high molecular weight paraffins; also their mechanisms are still not well understood. Current trends in laboratory studies and the industry are toward the use of heavier feeds including whole crudes (Kunii and Kunugi, 1975; Zdonik et al., 1975; Rebick, 1983; Mushrush and Hazlett, 1984; Nowak and Guenschel, 1983). Research studies on the pyrolyses of hydrocarbons still continue as the design of pyrolysis plants based on laboratory data has not been entirely successful. Before the 1970s, most gas-phase pyrolyses of hydrocarbons in the laboratory were carried out by the batch reactor or static method (Come, 1983);the use of open readom for the same purpose is relatively recent, but their use is common in industrial practice. An experimental method that will give good and reliable data and lead to substantial cost and 0888-5885/88/2627-0915$01.50/0
time savings in the laboratory is most preferred. One such method is the pulse kinetic technique; it has been used considerably in catalytic studies (Bassett and Habgood, 1964; Kumar and Sarkar, 1982; Blanton et al., 1968). Besides, there is only one reported work in which this technique was applied in studying hydrocarbon (liquefied petroleum gas) pyrolysis (Nand and Sarkar, 1979). In this method, a detailed and accurate mass balance suitable for the modeling of the hydrocarbon pyrolysis reactions being studied is achieved (Aribike, 1986). Furthermore, a lot of information can be collected with relatively few experiments and minimal material consumption. The major limitation of this technique lies in the fact that its application is restricted to simple reactions. Cyclohexane was chosen as a feedstock because it is the most studied of all the naphthenes. Furthermore, naphthas and gas oils may contain a high percentage of naphthenes (Rebick, 1983). The present work reports on the high-temperature pyrolysis of cyclohexane at 1 atm with excessive nitrogen 0 1988 American Chemical Society
916 Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988 .
1
.
~
f14 - --
..13
1-
?-Pi61 - n*
f.Ti
L__---"
~
la
b7
5 L L - L -_011; -1---LpL2Li
~
~
Figure 1. Diagram of experimental setup: 1, nitrogen gas cyclinder, 2, injection port (for reaction); 3, thermowell; 4, furnace reactor; 5, liquid nitrogen cold trap; 6, injection port (for blank runs); 7, flow split valve; 8, Nitachi GC (FID); 9, air compressor; 10, hydrogen gas cylinder; 11, air compressor; 12, Carle GC (FID); 13, Carle GC (TCD); 14,effluent vent; 15, thermocouples; 16, cylinder pressure regureactant and product lator; 17, pressure gauges; 18, valves; (---I flow.
dilution using the pulse kinetic method. Efforts are made to compare the product distributions and the kinetics of the pyrolysis of cyclohexane obtained by using this method with those obtained by using continuous-flow experimentation reported previously. 2. Experimental Work 2.1. Instrumentation. Figure 1 shows the experimental setup used in the present work. It was adapted from the continuous-flow system used previously for the pyrolysis of cyclohexane (Aribike et al., 1981; Susu and Ogunye, 1979). It consisted essentially of an annular tubular reactor (304 stainless steel), an electric furnace (Stanton Redcroft, U.K.), and three gas chromatographs, each of which had an integrating facility. Others were a liquid nitrogen cold trap and a split flow valve. The annular reactor consisted of inner (i.d. 2.3 mm, 0.d. 3.2 mm) and outer (0.d. 6.4 mm) tubes and had a surface-to-volume ratio ( S / V ) of 16.3 cm-'. The reactor temperature was monitored in the same way as had been done in previous studies (Aribike et al., 1981; Susu and Ogunye, 1979). 2.2. Materials. Cyclohexane (99.5% purity; Hopkins and William, Essex, U.K.) was used as the reactant. Its purity was determined by the GCs in this work. The major impurities of the reactant are benzene and cyclohexene. Pure nitrogen and hydrogen gases purchased from Industrial Gases Limited, Lagos, Nigeria, were used without further purification. Liquid nitrogen was also obtained locally. 2.3. Product Analysis. The three GCs were used on-line in analyzing the products of cyclohexane pyrolysis. A Carle GC with a flame ionization detector (FID) was used for the analysis of gaseous products (C,-C,) using a modified alumina column (length 1.9 m; i.d. 2.3 mm) for separation; the column temperature was in the range 60-70 "C. Another Carle GC with a thermal conductivity detector (TCD) was employed for the analysis of Hz, COz, and CO gases using a 5A molecular sieve column (length 1.9 m; i.d. 2.3 mm); the column temperature was maintained a t 60 "C. A Hitachi GC provided with FID was used for the analysis of liquid hydrocarbons (C5+)using a squalane-on-Chromsorb P 80/ 100-mesh column (length 1.9 m; i.d. 2.3 mm) for separation; this column was maintained a t 100 "C. The squalane column not only separated the C5+products but also lumped all the gases (Cl-C4) as one single peak. Consequently, it was possible
-
-
-
Conversion % Figure 2. Product selectivities v8 conversion in the pyrolysis of 700 "C; ( 0 )720 OC; (e)740 "C; (0)760 "C; (0)780 cyclohexane: (0) "C; (4) 800 "C; (0) 820 "C; (+) 840 "C; ( 6 -) 860 "C.
to superimpose the analysis of C1-C4products by the Carle GC (FID) on that of C5and higher products by the Hitachi GC (FID). This procedure allows the calculation of the material balance based on the analysis of all products on one single column (Aribike, 1986). The C1-C4 gaseous and C5+liquid products were identified by table matching of chromatograms with standard ones produced using modified alumina and squalane-on-ChromosorbP, respectively. 2.4. Procedures. Initially, the diluent pressure was set to 1 atm and allowed to flow through the entire system (Figure 1). The reactor furnace was also set to a known temperature, and the carrier gas flow rates through the three GCs and the vent were measured. Pulses of cyclohexane (size = 2.0 pL) were injected into the system for cracking runs after the operating conditions have stabilized. Experimental runs were performed in the temperature range 700-860 "C and space times of 0.40-1.14 s. The results were highly reproducible; conversions were reproducible to within *2%. Furthermore, mass balance of each run was equal to or better than 99.0%. These two conditions were taken as the criteria of reliability. Consequently, experiments which did not meet these criteria were ignored. 3. Results and Discussion The following definitions used by the present authors in previous studies (Aribike, 1986; Aribike et al., 1981)were adopted in the present work: space time ( V / F ,s) = effective volume of reactor/flow rate of reactant at reaction conditions; yield (moles of products per 100 mol of feed); selectivity (moles of products per 100 mol of hydrocarbon decomposed). 3.1. Product Distributions. The product distributions of the thermal cracking of an hydrocarbon depend largely on temperature, space time, and hydrocarbon partial pressure. Furthermore, conversion had been found to be independent of reactant pulse size (Aribike, 1986). This behavior significantly ensured linear chromatography in the pulse kinetic method. The thermal cracking of cyclohexane a t temperatures of 700-860 "C and space times of 0.40-1.14 s gave hydrogen, methane, ethylene, propylene, and 1,3-butadiene as the major products; traces of benzene, acetylene, and coke were also formed. Similar major products were obtained by other workers (Aribike et al., 1981; Susu and Ogunye, 1979; Levush et al., 1969; Pease and Morton, 1933) who also worked on the pyrolysis of cyclohexane, but in continuous-flow tubular reactors. Figures 2 and 3 show the plots of product selectivities of cyclohexane pyrolysis against conversion. According to these plots, ethylene,
Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988 917
-F 0 LA
2
f
20
'
20
40
Figure 5. Conversion of cyclohexane as a function of V I R (1)700 "C; (2) 720 "C; (3) 740 "C; (4) 760 "C; (5) 780 "C;(6) 800 "C; (7) 820 "C; (8) 840 "C; (9) 860 "C.
60 80 1oG Conversion %
I I
O'
2b
4b
60
80
Conversion 'A
7
I
4
100
Figure 3. (a, top) Product selectivities vs conversion in the pyrolysis of cyclohexane: (0) 700 "C; (e)720 OC; (e)740 "C; ( 0 )760 "C; ( 0 ) 780 "C; 800 "C;(0) 820 "C; 840 "C; (G) 860 "C.(b, bottom) H2selectivity vs conversion: (0) 700 OC; ( 0 )720 OC; (€3) 740 OC; ( 0 ) 760 "C;( 0 )780 "C; 800 "C; (0) 820 "C; (4) 840 "C; (+) 860 "C.
(4)
(0)
Propylene
0.2
0.4
06
08 1.0 V/F,s.
1.2
02
04
06
08
10
12
(0)
'
o
o
l
V
I
conversion % Figure 4. Gas production as a function of the conversion of cyclohexane: (0) 700 OC; (9) 720 "C; (e)740 "C; 760 "C; (Q) 780 "C;(0) 800 "C;(+) 820 "C; ( 0 )840 "C; ( 0 )860 "C.
V/F,5 Figure 6. (a, top) Product yields of the pyrolysis of cyclohexane vs space time: (1) 740 "C; (2) 760 "C; (3) 780 "C; (4) 800 "C; (5) 820 "C; (6) 840 "C. (b, bottom) Yield of 1,3-butadiene formed in the pyrolysis of cyclohexane vs V I R (1)740 "C; (2) 760 "C; (3) 780 "C; (4) 800 "C; (5) 820 "C; (6) 840 "C.
propylene, and l,&butadiene were the primary products, while methane was a secondary product. Levush et al. (1969) also found ethylene, propylene, and 1,3-butadiene to be the main primary products of cyclohexane pyrolysis; they also observed acetylene as a primary product at high temperatures (1000-1300 "C). The total gas weight yield increased with conversion (Figure 4). Cyclohexane conversion also increased with space time a t all the temperatures of investigation (Figure 5). Furthermore, the plots a t 700-800 "C exhibited the S shape which is characteristic of autocatalytic reactions. We are
not aware of any study on the pyrolysis of cyclohexane or any other pure naphthene in which autocatalysis was reported. A detailed discussion on these observations is provided in section 3.2. Furthermore, yields of ethylene and propylene generally increased with space time at all temperatures of investigation (Figure 6a); 1,3-butadiene yield also increased with space time at 740-800 "C, passed through a maximum at 820 "C, and decreased at 840 OC (Figure 6b). Levush et al. (1969) made similar observations in respect of these major products at temperatures of 900-1300 "C. Table I
A 80 100
(x)
918 Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988 Table I. Comparison of Main Product Yields of Cvclohexane Pyrolysis (1 atm) Virk this Aribike Susu and et al., work et al., Ogunye, 1979" 1979" (pulse) 1981" 830 900 860 temp, "C 860 0.03 0.48 0.03 space time, s 0.50 diluent N2 N2 H, steamb S / V , cm-' 16.3' 16.3' 21.5c notCgiven vroduct yields 172.0 52.8 167.4 15.1 22.6
H O
CH, C2H4 CSH, 1,3-C,H6
15.8 128.7 12.7 34.6
74.0 128.6 34.8 20.4
62.5 29.3 96.4 13.2 29.1
c a 0 s
Levush et al.,
a
1969"
E
900 0.001
0 LJ
100.
N2 notd given 20.8 4.2 23.8 4.2 19.9
1
Continuous-flow mode. bPressure = 1.78 atm. CStainlesssteel reactor. dProcelain tube reactor.
0
0.2
04
06
0.8
1.0
VIF
1.2
I
Figure 8. Product selectivities of the pyrolysis of cyclohexane vs VfF; 820 O C .
J
P
20. 101
CL
0
I
0.2
a4
0.6
08
1.0
viF
12
Figure 7. Product selectivities of the pyrolysis of cyclohexane vs VIF; 760 "C.
shows the comparison of the main product yields of cyclohexane pyrolysis in the present work with those reported previously in the literature. Though this comparison is not completely valid (due to the different cracking conditions employed), it would be useful in highlighting the apparent similarities and differences between the product distributions of this work and those of other workers who used continuous-flow reactors in their studies (Aribike et al., 1981; Susu and Ogunye, 1979; Levush et al., 1969; Virk et al., 1979). The product yields obtained in this work were fairly consistent with those of Aribike et al. (1981), Susu and Ogunye (1979), and Virk et al. (1979), though there were small differences. The consistency among the product yields may be attributed to the stainless steel reactors used in these studies. On the other hand, the major product yields obtained in the present work were considerably higher than those reported by Levush et al. (1969); this may be explained by the relatively very short space times used in the latter studies. Figures 7 and 8 show typical plots of the variations of the selectivities of methane, ethylene, propylene, and 1,3-butadiene with space time. The selectivities of ethylene and methane generally increased with space time, while those of propylene and 1,3-butadiene passed through a broad maximum and decreased respectively. Aribike et al. (1981) reported previously similar observations in the pyrolysis of cyclohexane in a continuous-flow reactor.
Figure 9. First-order plots of cyclohexane pyrolysis. Table 11. Rate Constants of Cyclohexane Pyrolysis at t"60
oc
k,,s-'
temn "C
2.576 3.735 5.258
820 840 860
3.2. Kinetic Analysis. It is clear from the kinetic data presented in Figure 5 that the kinetic analysis of the thermal decomposition of cyclohexane must account for autocitalytic conversions at temperatures between 700 and 800 OC. A t higher temperatures (820-860 "C),the autocatalysis disappears and the thermal decomposition reaction is describable by a first-order rate equation (eq 1). -[In (1 = klT (1)
x)]
Figure 9 shows the first-order plots of the thermal decomposition of cyclohexane. The rate constants estimated from Figure 9 increased with cracking temperature and hence with conversion (Table 11).
Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988 919 Table 111. Comparison of Arrhenius Parameters of Cyclohexane Pyrolysis (First Order) this worka Aribike et al., 1981* Levush et al., 1969* temp range, O C 820-860 730-860 900-1300 240.0 activation energy, E, kJ mol-' 163.5 288.9 preexponential factor, A , 8-l 8.14 X 10" 1.86 X lo8 3.24 x 1014
Pease and Morton 1933* 526-578 288.0 2.57 x 1015
Pulse mode. * Continuous-flow mode.
I
201
I
l
o! 1st -order reactlon i820 - &O°C l
e
-7 080
\
\ 1.00
%YT
1
1.20
Figure 10. Arrhenius plot of the pyrolysis of cyclohexane.
The fact that the estimated rate constants did not decrease with increasing conversion confirms that the decomposition reaction is first order at the temperature range 820-860 "C. The Arrhenius plot is shown in Figure 10. The kinetic rate parameters estimated for the first-order process are preexponential factor, A = 8.14 X 10" s-l; and activation energy, E = 240.0 kJ mol-l. The rate parameters estimated for the first-order conversion of cyclohexane were compared with previously reported values (Table 111);they lie in the ranges of values found in the literature. In particular, the A and E values were lower than those determined by Levush et al. (1969) and Pease and Morton (1933), but higher than those reported by Aribike et al. (1981). The obvious discrepancies in the E values may be due to a change in mechanism which is probable considering the wide temperature range of the pyrolysis data used in the comparison (586-1300 "C). Autocatalysis. Autocatalysis in hydrocarbon pyrolysis has been reported for methane (Chen et al., 1976; Palmer et al., 1968) and n-eicosane (Susu and Kunugi, 1980). In the case of methane pyrolysis, autocatalysis has been shown to occur in the homogeneous phase and not to be catalyzed by carbon deposited on the reactor wall (Back and Back, 1983). Although, the mechanism of autocatalysis in the homogeneous phase is still unclear, biomolecular reactions involving the participation of unsaturated products offer the most probable explanation. Susu (1982) attributed the autocatalytic behavior of n-eicosane pyrolysis to the participation of allyl radicals in a new bimolecular propagation sequence. The actual mechanism for the autocatalytic conversions of cyclohexane a t lower temperatures may be attributed to the same phenomenon described above. It is quite conceivable that the initial reaction was first order. The reaction may then be accelerated in a second-order process involving the participation of unsaturated free radicals generated in the initial propagation reaction. The second-order reaction would then occur with a lower activa-
Table IV. Second-Order Rate Constants of Cyclohexane Pyrolysis at 700-800 OC k, L mol-' s-l temp, "C k , L m o 1 - l ~ ~ temp, "C 0.584 700 2.000 760 0.896 720 3.372 780 1.300 740 5.716 800
tion energy. I t appears that the new chain-propagating step may involve methylallyl radicals formed in a new initiation step as follows: H' + CIH, C4H7 This is suggested by the presence of relatively high concentration of hydrogen and the large drop in 1,3-butadiene selectivity from 100 at 10% conversion to 66 at 50% conversion (see Figure 3). Fortunately, the determination of the rate parameters for the autocatalysis does not depend on the definition of the mechanism of the new chain sequence. As described earlier (Boudart, 1968; Susu and Kunugi, 19801, the conversion-space time data at 700-800 "C can be subjected to an Ostwald-type analysis. According to Ostwald, the maximum rate in an autocatalytic reaction occurs at the inflection point and is given by k/4(1 + p)*, where p = k l / k (k, = first-order rate constant of decomposition reaction and k = bimolecular rate constant of the second-order reaction). Autocatalysis will occur only if p is very small ( p
