Ind. Eng. Chem.
PrOC8SS
McAlllster, R. A. et al. Chem. Eng. Sci. 1958, 9 , 25. Miller, R. H. Ph.D. Thesls, Unhrersity of Mlchlgan, 1959. Nutter, D. E., 88th National Meeting of AIChE, Houston, Paper No. 49C, Feb 1971. Porter, K. E.; Wong, P. F. Y. Inst. Chem. Eng. Symp. Ser. 1080, 32,2:22. Ram, V. C.; Pavlov, V. P. Khim. from. 1968, &(IO), 776. Rush, F. E., Jr.; Stirba, C. AIChE J . 1957, 3(3), 336. Sterbacek, 2 . Brn. Chem. Eng. 1987, 12(10), 1577. Takahashi. T. et al. J. Chem. Ena. J m . 1973, 6(1). 38. Thomas, W. J.; Campbell, M. Trans. inst. Chem.'€ng. 1987, 45, T53.
Des. Dev. 1981, 20,307-313
307
Thomas, W. J.; Ogboja, 0. Ind. Eng. Chem. Process Des. D e v . 1978, 17, 429. Unno, H.; Inoue, I. J . Chem. Eng. Jpn. 1978, 9(2), 92.
Received for review February 29, 1980 Accepted December 19, 1980
Presented at the AIChE Annual Meeting, San Francisco, Calif., Nov 1979.
Steam Cracking of High-Molecular-Weight Hydrocarbons Byouk Blourl, Jean Glraud, Sirous Nourl, and Danielle Herault Laboratoire de G n i e et hformatique Chimiques, €Cole centrale des Arts et Manufactures, 9229O-ChEitenay Malabry, France
The steam cracking of n-tetracosane, 6-methyleicosane,and dodecylbenzene was studied between 550 and 700 OC in a tubular stainless steel reactor whose walls had been passivated by chromaluminization. The kinetic data on the cracking of these heavy hydrocarbons are similar to those previously reported for light hydrocarbons. The order of the reactions is close to 1; the activation energies are 58.2 kcal for tetracosane, 56.3 kcal for 6methyleicosane, and 45 kcal for dodecylbenzene. The composition of the cracked products of n-tetracosane, in particular for low advance rates, agrees with the prediction of Rice's free-radical theory, while those of 6methyleicosane and dodecylbenzene are different: there is a slight formation of cracked gas and a larger conversion of the heavy treated hydrocarbons into light fluid ones.
The thermal cracking of hydrocarbons has been the object of much work. In 1931 Rice proposed his classical rules involving the formation of free radicals for the decomposition of these hydrocarbons. For light hydrocarbons (cI-c6),the theory of Rice and Kossiakoff explains the formation of reaction products, in particular when cracking occurs under relatively mild operative conditions: low temperatures (500-650 "C) and low conversion rates. As mentioned earlier, the cracking of heavier hydrocarbons, such as naphtha or gas-oil, is much more complex (in particular for the preparation of light olefins requiring a higher conversion rate). We studied (Blouri and Giraud, 1977) the steam-cracking of high-molecular-weight hydrocarbons. In the present paper are reported the experimental results of a kinetic and chemical study of the cracking of three high-molecular-weighthydrocarbons (with more than 17 carbon atoms): n -tetracosane, 6-methyleicosane, and dodecylbenzene. The composition of the obtained cracked products is compared with the predictions of KossiakoffRice theory (1943). The results are applicable to the increasing use of high-molecular-weight hydrocarbons of petroleum and to preparation of raw materials in the petrochemical industry. Previous Work There is much work available on the cracking of molecules of higher molecular weight. Kunzru et al. (1972) studied the pyrolysis of n-nonane and Illes et al. (1973) the cracking of n-octane, isooctane, cyclohexane, and (&-branched hydrocarbons. According to their results, the distribution of the reaction products is predicted to within 20% by the theory of Rice and Kossiakoff, but this mechanism was not tested on molecules representative of a fuel oil by their molecular weight. Experimental Section Apparatus. The dynamic reactor used consisted of three coaxial tubes arranged as shown in Figure 1. In order to better differentiate the vaporization and pre0196-4305/81 I 1 120-0307$01.25/0
heating zones from the reaction zone, water and hydrocarbons were vaporized and preheated in zones I and 11, respectively. Zone 111was the furnace proper. To improve the thermal profile, the furnace was plunged into a fluidized sand bed (particle diameter, 150 pm; 20 L / h of nitrogen preheated at 500 "C). Each outer tube was 360 mm in length and 22 and 25 mm in diameter. Each internal tube was 350 mm in length and 14 and 16 mm in diameter. The three sections of the apparatus were heated by electrical resistors (- 20 a) with adjustable pitch. The temperatures were read regulated with the aid of chromel-alumel thermocouples and regulators. The thermal profile of the operating furnace was registered by the mobile thermocouple 4. The thermal isolation was provided by a kaolin layer and the fluid pressure was measured at the top (tube 5). The hydrocarbon was introduced into section I by means of a syringe and of an electrical push-syringe at a flow rate of about 16 g/h. The preheating temperature was 500 "C. In our operating conditions, the treated hydrocarbons have no significant reaction in the preheater. Water was pumped by a peristaltic pump at a flow rate varying from 20 to 150 g/h at the same preheating temperature. The reaction products evacuated through tubing 6 were tempered by a refrigerant cooled with tap water and collected in a vessel plunged into melting ice. The noncondensed gaseous products were collected in a Mariotte bottle. The effluents were collected when the apparatus was in equilibrium, i.e., 15 mn after the beginning of each run. The apparatus had been previously flushed out with circulating water for 1h ( E 50 9). After each run the possible carbon deposits where burned out by the passage of air heated at 600 "C. Wall Effects. The catalytic effect of the walls on the pyrolysis reactions, in particular in the case of stainless steel tubes, has been pointed out in many publications (Albright, 1975). The three sections of our apparatus were made of stainless steel 18/8 protected from oxidation and carburation up to 1200 OC by a 200 pm thick deposit of @ 1981 American Chemical Society
308
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Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981
and dodecylbenzene with 97% purity are commercial products. The hydrocarbon purity was controlled by gas chromatography. Results (1) Kinetic Analysis. The experiments were carried out between 550 and 750 "C at a small hydrocarbon partial pressure (e&%), and at the maximal flow rates and temperatures, the Reynolds number was Re N 230. The flow was thus laminar. In the following a plug flow was assumed. The furnace was rendered nearly isothermal by the use of a fluidized bed. However, to determine more accurately the reaction volume, the equivalent reactor volume concept as developed by Hougen and Watson (1947) was used. Since the conversion rate in the equivalent reactor is the same as in the real one, it is easy to deduce for a reference temperature T,the expression for the equivalent volume V , as a function of the real volume V
..... . .
, '.
. . ... ...
'.
_'
Figure 1. Experimental device.
chromium and aluminum (chromaluminization). A previous set of runs during which a mixture of nparaffins in C,-CB was cracked between 800 and 950 "C in the presence of water showed that a tube of steel 18/8 passivated in this way was stable and led to the same results as a quartz tube, quartz being known to produce small catalytic effects (Pines and Arrizo, 1957). After 60 runs of about 1 h each alternating with 100 h of regenerations by the passage of air at 600 L/h (about 1.5 h for each) the results were still perfectly reproducible, contrary to numerous other alloy steels which give more or less rapidly significant amounts of carbon and hydrogen (Partovi et al., 1978). Analysis. The liquids were analyzed by gas chromatography in a chromatograph fitted with a flame ionization detector (the peak surface was determined with an integrator) by means of 3 m long, in. diameter s t a i n l a steel column packed with SE 30 on Chromsorb W/AW, 60-80 mesh. The temperature was programmed from 50 to 210 "C at a linear rate of 10 "C/mn. The gases were analyzed in a chromatograph fitted with a flame ionization detector by means of a stainless steel column which was 4 m in length and in. in diameter and contained Porasil B 80-100 mesh. The hydrogen/ methane ratio was determined by means of a third chromatograph fitted with a catharometer and of a 2 m lon in. diameter stainless steel column packed with 5 molecular sieves. Treated Hydrocarbon. The 6-methyleicosane was prepared in our laboratory by allowing heptanone-2 to react with a magnesium derivative of bromo-1 n-tetradecane. By hydrolyzing the resulting complex 6-methyl-6hydroxyeicosane was obtained, whose hydrogenation under 150 kg/cm2 in the presence of a modified Adkin's catalyst gave 6-methyleicosane with 97.5% purity. Tetracosane
k
The activation energy E is set equal to 60 kcal. The volume V, calculated with it gives a residence time t leading to a rate constant K and, therefore, to a new value E of the activation energy which replaces the preceding value E. The method had a rapid convergence. The order of reaction was determined using the relation established by Kershembaum and Martin (1967) for constant pressure and temperature profiles log (-Af) = n log X + log K The rate constants were determined by assuming from numerous studies that the order of the hydrocarbon cracking kinetics was close to or equal to 1. When this assumption is verified, the constant K of the reaction can be determined using Benton's formula (1931) A
k ---.+
vB
hence 1 - (v - l)X l-x Since Y is Micult to determine when studying the cracking of high-molecular-weight hydrocarbons, we have established the kinetics by studying the variation of
k t = v ln-
1
log -= f(t) l-ff
as a function of the residence time t , where a is the hydrocarbon conversion rate. (a) Order of the Reactions. Figures 2, 3, and 4 illustrate the application of the Kershenbaum-Martin relation to n-tetracosane, 6-methyleicosane, and dodecylbenzene, respectively. The values of n deduced from these graphs are listed in Table I. The order of the three reactions is close to 1and not very sensitive to a significant increase in temperature. For n-tetracosane, the order increases with the temperature; for dodecylbenzene, it decreases with increasing temperature. (b) Rate Constants. Figures 5, 6, and 7 show the variation of the logarithm of the 1/1- a ratio as a function of the residence time (calculated after determination of the equivalent volume). The values found for the rate constants k are listed in Table 11.
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 309 L8
(-An
Logl-:F) 600'
-5
-4
-3
-2
- -
-4
6
-5
1
Figure 2. Logarithm of rate of transformation vs. logarithm of average mole fraction of n-Czr
1
-4
Log ( A )
Log ( F )
-i
Figure 4. Logarithm of rate of transformation vs. logarithm of average mole fraction of dodecylbenzene.
I - z €1
1
Residence t m e s
A
600'
n124
-4
-3
-2
720'
-1
1
1.0
-5
-4
-6
Log
tT)
Figure 3. Logarithm of rate of transformation vs. logarithm of average mole fraction of 6-methyleicosane.
Log-
3.0
2.0
1-4
Figure 5. Log [ l / ( l- a)]vs. residence time. Table I1
Table I order of reaction, n temp, "C
600 630 650 660 690 720
6-methyln-tetracosane eicosane 1.07 1.10
1.06
dodecylbenzene 1.18 1.12
1.16
1.17 1.18 1.13
1.11 1.11
rate constants. k. s-I 6-methyltemp, "C n-tetracosane eicosane 550 0.14 600 0.57 1.19 630 1.59 650 3.33 660 4.10 690 7.32 920 12.50 "_
dodecylbenzene 0.55 1.38 2.82 4.90
310
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 ~ ~ s l d e n ct e ime s .
f
n-tetracosane 6-methyleicosane dodecy lbenzene
6-Methyleicosane
1.6 x 1014 1.2 x 10l4 1.2 x 10"
58.2 56.3 45.0
1 0
/
Dodecylbenzene
,530"
For n-tetracosane, the values of k are independent of the conversion rate a. For dodecylbenzene, they slightly increase and for 6-methyleicosane they sensibly decrease with increasing a. (c) Activation Energy. From the graphs of the functions
we could deduce the values for the global activation energies (E,)and the DreexDonential factors (A) as shown in"Tab1e %I. Since no data were available on the global activation energies of cracking of such hydrocarbons, it was not posssible to compare these values with other results. The activation energy of n-paraffins is close to 60 kcal. The cracking of ramified hydrocarbons requires a slightly lower energy activation, that of aromatic hydrocarbons requires a slightly lower energy activation, and that of aromatic hydrocarbons requires a much higher energy. (2) Composition of the Cracked Products. (a) nTetracosane. Table IV gives the molar distribution of
2 r
LOCL
I-
CH(CH2)7CH3
-
6"'
+
CH2=CH(CH217CH3
This is more probable as the binding energy of the C, - C, carbons is very small as compared to the other bonds (67 kcal instead of 93 or 94 kcal)
The weakness of the C,-C, bond and its homolytical cleavage explains the great number of benzyl radicals 4-CH2, The high stability of these radicals due to the conjugation of an odd electron with the ring electrons still favors their concentration. The duplication of the benzyl radicals leads to 1,Zdiphenylethane. Since the hydrogen and carbon a-bondings are weaker than the other C-H bonds (95 kcal instead of 94 kcal), the
formation of styrene by abstraction and subsequent Bcleavage is more probable. The amount of styrene is actually very high for small progression rates of reaction. The a-olefins and a-olefins with a phenyl group are formed according to the free radical mechanism and to the rule of @-cleavageof the Rice-Kossiakoff hypothesis. Conclusions The experimental study of the cracking of three highmolecular-weight hydrocarbons belonging to three types of hydrocarbons-normal type (n-tetracosane), branched type (6-methyleicosane), and aromatic type (dodecylbenzene)-has revealed some common characteristics of the cracking reactions: nearly first order kinetics; activation energies close to those reported in the literature for lighter hydrocarbons (nonane, hexadecane); and fundamental difference in the cracking mechanism. For a normal paraffin (n-tetracosane C24H50)the cracking mechanism of the radical type and the composition of the cracked products formed exclusively of gases and a-olefins agree fairly well with the predictions of Rice-Kossiakoff s theory. For an hparaffii (6-methyleicosane C21H44)the reaction mostly develops according to a radical mechanism, but there also occurs an intramolecular cleavage at the branching which favors the liquid formation at the expense of the cracked gases and the composition of the products obtained differs from the Rice-Kossiakoff predictions. For a long chain aromatic hydrocarbon (6-methyleicosane) simultaneous cleavages occur according to radical and intramolecular mechanisms with an important formation of cracked liquids a t the expense of the cracked gases and the composition of the resulting products is sensibly different from the theoretical one. From a practical point of view, the cracking of normal paraffins gives the best results in the preparation of gases rich in ethylene, which is the basic product in the petrochemical industry. By selective cleavages at the ramification of high-molecular-weight isoparaffins one can obtain more cracked liquids of low molecular weight which can be regarded as light fuels. The presence of an aromatic ring on a long chain highly modifies the cracking mechanism of the hydrocarbon treated. So, by pyrolyzing dodecylbenzene, one can obtain very little gases and a large amount of liquids rich in styrene, a-olefins and a-olefins with an end phenyl group, which are very precious intermediates in the petrochemical industry. Literature Cited Albright, L. F. Chem. Eng. News 1976, 53, 29, 8-12. Benton, A. F. J . Am. Chem. Soc. 1931, 5 3 , 2984. Blouri, B.; Giraud, J. Inf. Chim. 1977, 171, 229. Hougen, 0. A.; Watson, K. M. "Chemical Process Prlncipales". Vol. 111, WC ley: New York, 1947; p 884. Illes, V.; Welther, K.; Fleskats, I. Acta Chlm. (Budapest) 1973, 78 (4), 357. Kershembaum, L. S.; Martin, J. J. AIChE J. 1987, 13(1). 148. Kosslakoff, A.; Rice, F. 0. J. Am. Chem. Soc. 1943, 85, 520. Kunzru, D.; Shah, Y. T.; Stuart, E. B. Ind. Eng. Chem. Process Des. Dev. 1972, 11, 605. Miller. D. B. Ind. Eng. Chem. Process Des. D e v . 1983, 2. 220. Partovi, A. R.; Giraud, J.; Blouri, B. Inf. Chlm. 1978, 184, Ill. Pines, H.; Arrlzo. S. T. J. Am. Chem. Soc. 1957. 78, 4958. Rice, F. 0. J . Am. Chem. Soc. 1931, 53, 1959.
Received for review April 29, 1980 Accepted October 15, 1980
