I n d . Eng. C h e m . Res. 1991,30, 1116-1130
1116
preparation of a fluidized bed catalyst starting from this material has to meet certain standards. It has to cope with the interferences that other components, which were added to increase the attrition resistance, impose on the fluidized bed catalyst. In the research done on this subject, a fluidized bed catalyst was developed that revealed good catalytic properties and proved to be attrition resistant in the laboratory-scale unit. Further investigations have to manifest whether this catalyst proves to be equally efficient in pilot-scale fluid beds or solid riser type fluid beds. Registry No. (ZrO)2Pz0,, 12402-10-7; Ti02, 13463-67-7; H(CHZ),H,106-97-8; maleic anhydride, 108-31-6;vanadium oxide,
1314-62-1; phosphoric acid, 7664-38-2.
Literature Cited A Butane-Based Maleic Anhydride Process that Features a Fluidized-Bed Reactor. Chem. Eng. 1979,86 (Oct), 81. Amold, E. W.; Sundaresan, S. Effect of Water Vapor on the Activity and Selectivity Characteristics of a Vanadium Phmphate Catalyst towards Butane Oxidation. Appl. Catal. 1988,41, 225. Amold, S. C.; Suciu, G. D.; Verde, L.; Neri, A. Use Fluid Bed Reador for Maleic Anhydride from Butane. Hydrocarbon Process. 1985, 64 (Sept), 123. Budi, F.; Neri, A.; Stefani, G. Future MA Keys to Butane. Hydrocarbon Process. 1982,61 (Jan), 159. Cavani, F.; Centi, G.; Trifiro, F. The Chemistry of Catalysts Based on Vanadium-Phosphorus Oxides. Appl Catal. 1984, 9, 191. Centi, G.; Trifiro, F.; Poli, G. On the Chemistry of VanadiumPhosphorus Oxides. Appl. &tal. 1985, 19, 225. Chemical Prices. Chem. Mark. Rep. 1976-1986. Chinchen, G.; Davies, P.; Sampson, R. J. Historical Development of Catalytic Oxidation Processes. In Catalysis, Sciences and Technology; Anderson, J. R., Boudart, M., Eds.; Springer Verlag: Berlin, 1987; Vol. 8. Contractor, R. M.; Sleight, A. W. Maleic Anhydride from C-4 Feedstocks Using Fluidized Bed Reactors. Catal. Today 1987, I , 587. Contractor, R. M.; Bergna, H. E.; Horowitz, H. S.; Blackstone, C. M.;
Malone, B.; Torardi, C. C.; Griffiths, B.; Chowdry, U.; Sleight, A. W. Butane Oxidation to Maleic Anhydride over Vanadium Phosphate Catalysts. Catal. Today 1987,1,49. Emig, G.; Hoffmann, U. Planung und Auswertung von Versuchen fih Modelle ersten und zweiten Grades. Chem. Ztg. 1976, 7/8,324. Emig, G.; Martin, F.-G. Economics of Maleic Anhydride Production from C-4 Feedstocks. Catal. Today 1987, I , 477. Gerry,R. T.; Tsuchiya, K. CEH Marketing Research Report 'Maleic Anhydride". SRZ 1986 (June). Griesbaum, K.; Swodenk, W. Forschungs- und Entwicklungstendenzen in der Petrochemie. Erdoel, Erdgas, Kohle 1984,37,103. Hodnett, B. K. Vanadium-Phosphorus Oxide Catalysts for the Selective Oxidation of C-4-Hydrocarbons to Maleic Anhydride. Catal. Rev. Sci. Eng. 1985, 27, 373. Irving-Monshaw, S.; Klein, A. MA soaring. Chem. Eng. 1989, 96 (March), 35. Laguerie, C.; Angelino, H. Extrapolation Test of a Catalytic Fluidized Bed Reactor: Application of Oxidation of Hydrocarbons. Chem. Eng. J. (Lausanne) 1973a, 5 (June), 209. Laguerie, C.; Angelino, H. Catalytic Oxidation of Butane in Maleic Anhydride: Comparison of Fixed Bed with Fluidized Bed. Chem. Eng. J. (Lausanne) 1973b, 5 (Feb), 33. Liebenau, W. Oxidation von C-4 Kohlenwasserstoffen in der Wirbelschicht. Ph.D. Dissertation, T H Aachen, 1979. Martin, F.-G. Entwicklung und kinetische Untersuchung eines Wirbelschichtkatalysators fur die Maleinsaureanhydrid-Herstellung aus n-Butan. Ph.D. Dissertation, Erlangen-Nurnberg, 1989. Martin, F.-G.; Emig, G. Kinetik der n-Butan-Oxidation in der Wirbelschicht. Chem.-Zng.-Tech.1989, 61, 819. Plackett, R. L.; Burman, J. P. The Design of Optimum Multifactorial Experiments. Biometrika 1946, 33, 305. Schneider, P.; Emig, G.; Hofmann, H. Systematic Approach to Development of Catalysts for oxidation Reactions. Ger. Chem. Eng. 1986, 9, 337. Suciu, G. D.; et al. US.Patent 4,510,258, 1985a. Suciu, G. D.; et al. US. Patent 4,511,670, 1985b. Wellauer, T. P. Optimal Policies in Maleic Anhydride Production through Detailed Reactor Modelling. Ph.D. Dessertation, ETH Zurich, 1985. Received for review May 1, 1990 Accepted October 25,1990
Modeling of Thermal Steam Cracking of n -Hexadecane Dominique Depeyre* and Chantal Flicoteaux Laboratoire de GBnie et Znformatique Chimiques, Ecole Centrale Paris, F 92295 Chatenay-Malabry, France
The modeling of experimental thermal steam cracking of pure n-hexadecane in a laboratory-scale tubular quartz reactor a t atmospheric pressure was investigated. Comparison of experimental and simulated data for a weight ratio of steam to hydrocarbon within 2.7-2.9 over the range of temperature from 600 to 750 "C is given. An evolutive kinetic model is presented. For temperatures up to 650 "C, where secondary reactions are still negligible, a kinetic model, based upon 141 radical reactions, 20 molecular species, and 18 radical species, is shown to allow the prediction of gaseous and liquid product concentration distributions as a function of residence time. But, for higher temperatures, it was necessary to take into account the radical reactions of liquid olefin decomposition and molecular Diels-Alder cycloaddition reactions. This complementary model for high conversion involves 357-360 reactions and 31 radical and 22-25 molecular species, depending on the temperature.
Introduction The steam cracking of hydrocarbon cuts, such as naphtha by Kumar and Kunzru (1985a,b), and gas-oil by Hirato et al. (1971),has shown that ethylene and propylene production is greatly affected by the paraffin content of the feed. For a better understanding of the thermal cracking process, a great number of authors have studied
* To whom correspondence should be addressed.
the mechanisms of reactions that occur during steam cracking of n-paraffins, from propane by Sundaram and Froment (1978) to n-tetracosane by Blouri et al. (1981). But, very few detailed descriptions of the kinetics of the product distributions of n-paraffin cracking superior to n-dodecane have been published. n-Hexadecane is particularly suitable for investigation of kinetics and mechanisms of cracking, since its number of carbon atoms correspond to a g a s 4 charge in the range of petroleum fraction. Moreover, the lack of experimental
0888-5885/91/2630-1116$02.50/00 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 1117 data on steam cracking under atmospheric pressure and temperatures above 600 "C, has led to experimentation with the steam cracking of this pure n-paraffin and to a proposal of a mechanistic kinetic model that can predict the evolution of the concentration of the gaseous and liquid products from 600 to 750 "C as a function of residence time. In the modeling of n-hexadecane cracking, the works of Gavalas (1966) and Doug and Guiochon (1968) must be mentioned. These authors established kinetic models based on radical reactions such as initiation, propagation, and termination. Gavalas (1966) emphasized the fact that propagation reactions essentially determine the distribution of the products of cracking and ignored isomerization reactions. Meanwhile, Doug and Guiochon (1968)took into account the isomerization of each radical, the carbon number of which is superior to 4 atoms. The main disadvantages of these models were that they were established for temperatures less than 600 "C and for pressures between 1.5 and 85 bar, which enhanced the production of paraffm. These models neglected secondary reactions that occur at high temperature and high conversion, conditions generally used in industry for the production of light olefins. The aim of this report is to present experimental data and simulated results obtained with an evolutive mechanistic model based upon primary radical reactions for temperatures less than or equal to 650 "C and upon primary and secondary radical and molecular reactions for temperatures greater than or equal to 675 "C, which allowed prediction of the concentrations of molecular and radical species in a large range of conversion as a function of temperature and residence time. 1. Experimental Section
In a previous work of Depeyre et al. (1989, experimental data were obtained on steam cracking of pure n-hexadecane under atmospheric pressure over the temperature range 600-850 "C. It was shown that a steam to hydrocarbon weight ratio near 2.8 enhanced the production of ethylene and propylene and slowed down secondary reactions, leading to aromatics and carbon deposits. Physicochemical characteristics of n-hexadecane, cracking reactor description, analysis methods, and estimation of residence time have also been reported by Depeyre et al. (1985). Consequently only a brief description of the apparatus and the experimental methods used in the present study will be given. 1.1. Apparatus. The tubular quartz reactor consisted of two concentric quartz tubes divided into three sections, as shown in Figure 1. The outer quartz tube of each section was i.d. 22 mm, and the inner quartz tube was 0.d. 15 mm. The three sections were independently regulated and heated by electrical resistors. Water was supplied into the preheating section I (39 cm in length) by a peristaltic pump at a flow rate that could be adjusted from 39 to 179 g h-l. Steam was generated in the preheater. The temperature of the preheating water section was measured by two chromel-alumel thermocouples, AI connected to a temperature controller and A2 linked to a temperature recorder. n-Hexadecane was introduced into the hydrocarbon preheating section I1 (30 cm in length) by syringes connected to a motor-driven unit, at a flow rate that could be adjusted from 13.4 to 64.5 g h-l. The temperature of this section was measured by a thermocouple C linked to a temperature controller. Section 111 (40 cm in length) was used for thermal cracking of n-hexadecane. The reference temperature T,
TemDerahire c o n ti.0 I I r
c- B
eFFluents -l,
9 550 - 1
35
45
55 reactor
65
cm
lengkh
Figure 1. Diagram of the quartz reactor and longitudinal thermal profile of the cracking zone.
of the cracking zone was measured by a thermocouple B connected to a temperature controller. The longitudinal thermal profile of the reactor Ti was measured for each experiment by a moving thermocouple D sliding along the wall surface of the inner quartz tube and linked to a temperature recorder. The effluent products were quenched in two ice water traps, where liquids were condensed. Gas was collected in a Mariotte flask. 1.2. Experimental Methods and Results. n-Hexadecane was preheated at the temperature of 480 "C and was vaporized in section 11. Steam was preheated to the temperature of the cracking zone to improve the heat transfer from the tube to the cracking gas and was used to lower the partial pressure of the hydrocarbon. Different residence times were obtained by varying the n-hexadecane feed flow rate over the range 13.4-64.5 g h-' and the steam flow rate over the range 39-179 g h-l, while a steam to hydrocarbon weight ratio within 2.7-2.9 was maintained. After steady state was achieved, the effluent products were collected during 20 min and then analyzed by gas chromatography. The reference temperature T,of the cracking zone was varied from 600 to 700 "C with an increment of 25 "C, and additional experiments were conducted at 750 "C. Tables I-IV give some of the experimental results performed in the quartz reactor between 600 and 750 "C, where the C and H balances were evaluated as follows.
C balance error wt % = loo[ (MfloNCo) -
(MfljNCj)] /MaoNCo (1)
j=l
H balance error wt % = 4
1OO[(M&"NHo) - C (MfljNHj)l /MhNoNHo (2) j=l
The accuracy of chromatographic analysis was within 1-370 for molecular species present in high concentrations in the effluent products and 13% for molecular species present in low concentrations. The reproductibility of the injection feed was 2%. The temperature profiles Ti were
1118 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 Table I. Experimental (ER, mol 5%) and Simulated Results (SR, mol %) of n-Hexadecane Steam Cracking at T = 600 OC HC, R h-l
13.4 39.5 1.23 30.13
H 2 0 , g h-l t,8 wt % conversion
wt % C bal error wt % H bal error
17.9 49.4 0.98 26.01
30.0 82.4 0.56 19.63
38.0 103.0 0.44 15.86
53.8 147.0 0.38 14.65
64.5 179.0 0.29 11.88
ER
SR
ER
SR
ER
SR
ER
SR
ER
SR
ER
SR
2.9 8.8 3.0 19.3 0.6 7.5 2.5 0.6 6.8 3.0 1.9 1.5 1.4 1.0 0.8 0.8 0.6 0.5 0.3 36.1
2.8 9.8 1.9 20.9 0.6 9.4 3.4 1.3 2.7 2.1 1.7 1.3 1.1 0.9 0.7 0.6 0.5 0.3 0.2 37.8
2.3 10.9 4.1 19.7 0.7 9.7 2.4 0.7 1.8 2.2 1.1 1.0 1.1 1.2 1.1 1.2 0.8 0.6 0.3 37.2
2.5 8.7 1.7 18.4 0.7 8.3 3.0 1.3 2.4 1.9 1.5 1.2 1.0 0.8 0.6 0.5 0.4 0.3 0.2 44.6
2.2 7.1 2.9 13.1 0.4 5.2 1.7 0.3 1.9 2.4 1.6 1.4 1.4 1.6 1.3 1.3 0.9 0.8 0.5 52.2
1.8 5.9 1.3 12.8 0.9 5.8 2.1 1.1 1.7 1.3 1.0 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 61.2
2.2 7.5 3.6 14.5 0.8 5.4 2.2 0.3 2.5 1.3 0.8 0.8 1.0 0.8 0.9 0.8 0.7 0.6 0.3 53.2
1.6 5.0 1.2 11.3 0.9 5.7 2.1 1.1 1.7 1.1 0.9 0.7 0.6 0.5 0.4 0.3 0.2 0.2 0.1 66.0
1.7 5.6 3.2 12.4 0.5 4.4 1.7 0.2 3.1 1.8 1.3 1.1 1.o 1.0 0.8 0.7 0.6 0.5 0.2 58.2
1.4 4.4 1.0 9.9 1.0 4.5 1.7 0.9 1.3 1.1 0.8 0.6 0.5 0.4 0.4 0.3 0.2 0.2 0.1 69.2
1.5 4.8 2.6 11.1 0.3 3.5 1.5 0.2 2.8 1.6 1.0 0.8 0.8 0.9 0.7 0.7 0.6 0.5 0.2 64.4
1.2 3.4 0.8 7.7 1.0 3.5 1.3 0.8 1.1 0.8 0.6 0.5 0.4 0.3 0.3 0.2 0.2 0.1 0.1 75.8
8.4 8.2
3.0 3.0
0.9 2.7
1.9 1.3
1.3 3.4
2.3 2.2
Table 11. Experimental (ER, mol %) and Simulated Results (SR, mol % ) of n -Hexadecane Steam Cracking at T = 650 "C" HC, g h-l 13.4 17.9 30.0 38.0 53.8 64.5 HzO, g h-' 39.5 49.4 82.4 103.0 147.0 179.0 1.07 0.85 0.53 0.40 0.36 0.25 t, 5 w t % conversion 62.98 60.77 45.60 43.70 37.96 33.86
ER H,
CBHIZ ClH14
C8H16
C9Hl8 ClOHlo C11Hzz ClZH24 C13H29
CllH29 ClSH30 C16H34
benzene toluene wt % C bal error wt % H bal error a
5.4 12.9 3.5 33.1 0.8 14.8 5.5 2.3 4.3 1.7 1.2 1.2 0.8 0.7 0.6 0.5 0.4 0.3 0.2 9.7 0.2
SR 4.5 13.8 2.6 34.4 0.3 15.5 4.9 2.3 3.8 3.0 2.3 1.8 1.5 1.2 0.9 0.7 0.5 0.4 0.2 5.5
ER
SR
ER
SR
ER
SR
ER
SR
ER
SR
4.9 12.6 3.9 35.9 0.8 14.5 4.8 2.0 3.0 0.9 0.9 0.9 0.8 0.8 0.8 0.6 0.5 0.4 0.2 10.6 0.2
4.4 13.4 2.6 33.5 0.4 15.1 4.7 2.2 3.7 2.9 2.3 1.8 1.4 1.2 0.9 0.7 0.5 0.4 0.2 7.6
5.4 12.4 4.0 28.6 0.8 12.1 5.0 1.7 3.8 1.2 0.8 0.9 0.9 1.1 0.9 0.8 0.6 0.5 0.3 18.4
3.9 12.1 2.4 30.2 0.7 13.7 4.4 2.1 3.4 2.7 2.1 1.6 1.3 1.1 0.9 0.7 0.5 0.4 0.2 15.6
5.7 12.4 4.2 25.2 1.0 12.1 5.3 1.8 4.4 0.7 0.8 1.o 1.3 1.1 1.1 0.7 0.6 0.4 0.3 20.1
3.6 11.0 2.2 27.4 0.9 12.5 4.0 2.1 3.1 2.5 2.0 1.5 1.2 1.0 0.8 0.6 0.5 0.3 0.2 22.7
3.4 10.5 2.1 26.4 0.9 12.0 3.9 2.1 3.1 2.4 1.9 1.5 1.2 1.o 0.8 0.6 0.5 0.3 0.2 25.3
3.6 9.2 4.1 27.1 0.5 9.2 3.9 1.3 3.5 1.2 1.1 1.2 1.2 1.2 1.0 0.8 0.7 0.6 0.2 28.4 0.05 0.0
3.0 9.2 1.9 23.4 1.2 10.7 3.5 1.9 2.8 2.1 1.7 1.3 1.1 0.9 0.7 0.6 0.4 0.3 0.2 28.5
T
T
T
T
0.0
0.0
4.0 11.2 4.5 27.3 1.2 9.8 3.9 1.2 4.1 1.4 1.1 1.1 1.1 1.1 0.9 0.7 0.6 0.5 0.2 24.2 0.05 0.0
4.4 3.1
5.5 4.0
2.7 0.5
4.8 2.3
2.9 0.8
1.9 1.3
T, traces.
measured within f5 OC and were used in the calculation of residence times. Experimental results were then compared with values obtained from our kinetic models. 2. Simulation of Thermal Steam Cracking of n -Hexadecane Prediction of gaseous and liquid product concentrations as a function of residence time at constant temperature requires the knowledge of the cracking temperature, the choice of an appropriate reaction scheme with the activation energy and the frequency factor relative to each reaction, the initial concentration of the molecular and
radical species, the limit residence time of the study, and the use of a computer program. Margchal (1977) established a mathematical kinetic model and a program of simulation based upon radical mechanisms. The continuity equations for the rate of formation of species in an isothermal reactor with plug flow have been described by Snow (1966) and Sundaram and Froment (1978). The rate of production of a radical or a molecular species is the s u m of its rates in each reaction. 2.1. Mathematical Model and Computational Methods. The mathematical model consisted of two systems: a nonlinear algebraic radical system linearized by Raphson-Newton methods and a nonlinear differential
Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 1119 Table 111. Experimental HC,g h-l
(ER,mol %) and Simulated Results (SR, mol
HpO, g h-' t, 8 wt % conversion
HP
CH, CZG
Cab C4H8
C4b c a 1 0 C6H12 c7H14
C&lB CoHl8
ClOHKI CllHzz C12Hu
Cis% cl4HZe Cl&W Cl8HS4
benzene toluene
C bal error
H bal error
CH,
T 700 "C
53.8 147.0 0.30 71.83
64.5 179.0 0.29 66.88
SR
ER
SR
ER
SR
ER
SR
ER
SR
ER
SR
5.3 16.8 2.5 40.6 0.1 19.2 4.7 7.4 1.1 0.7 0.2 0.1 0.05 0.03 0.01 0.008 0.005 0.003 0.001 0.03 0.9 0.2
6.8 16.8 3.1 40.7 1.4 16.3 5.0 3.7 2.3 0.3 0.2 0.2 0.2 0.2 0.1 0.1 0.07 0.06 0.03 1.7 0.6 0.1 1.7 1.8
4.6 16.6 2.5 40.9 0.1 19.2 4.8 7.2 1.5 1.1 0.3 0.2 0.1 0.06 0.04 0.02 0.01 0.007 0.003 0.1 0.6 0.1
7.5 14.4 3.5 37.7 1.2 16.2 5.9 3.5 3.4 0.5 0.4 0.3 0.4 0.4 0.3 0.3 0.2 0.2 0.09 3.4 0.2 0.06 1.6 1.4
3.7 16.1 2.5 40.6 0.3 18.6 4.9 6.4 2.2 1.7 0.7 0.5 0.3 0.2 0.1 0.1 0.05 0.04 0.02 0.9 0.2 0.05
6.3 15.1 3.8 36.6 1.2 16.8 6.2 3.7 3.4 0.2 0.2 0.2 0.3 0.4 0.3 0.3 0.2 0.2 0.1 4.4 0.06 0.03 7.2 4.2
3.5 15.7 2.5 40.1 0.4 18.3 4.9 6.0 2.4 1.9 0.9 0.6 0.4 0.3 0.2 0.1 0.1 0.05 0.03 1.6 0.1 0.03
5.1 14.1 4.3 39.9 1.1 15.0 5.4 3.3 3.3 0.2 0.2 0.3 0.4 0.5 0.4 0.3 0.2 0.2 0.06 5.8 0.08 0.03 2.0 0.8
3.3 15.1 2.4 39.3 0.7 17.8 4.8 5.3 2.6 2.1 1.1 0.7 0.5 0.4 0.3 0.2 0.1 0.1 0.04 3.0 0.08 0.02
5.0 12.1 4.5 39.1 0.6 14.5 5.5 3.6 3.2 0.5 0.5 0.5 0.5 0.5 0.5 0.4 0.4 0.3 0.1 7.6 0.09 0.02 2.6 1.3
3.3 15.1 2.4 39.3 0.7 17.8 4.8 5.3 2.6 2.1 1.1 0.7 0.5 0.4 0.3 0.2 0.1 0.1 0.04 3.0 0.08 0.02
39.2 1.2 17.4 5.2 3.7 2.3 0.4 0.3 0.2 0.2 0.2 0.2 0.2 0.4 0.1 0.04 1.5 0.7 0.2 1.2 2.0
(SR, mol %) of Ir-Hexadecane Steam Cracking at T = 750 OC0 30.0 38.0 53.8 64.5 82.4 103.0 147.0 179.0 0.41 0.34 0.24 0.20 87.57 85.22 96.18 95.5
ER
SR
ER
SR
ER
SR
ER
SR
ER
SR
ER
SR
9.7 21.1 2.8 43.6 0.3 1.3 14.0 2.0 3.3 0.1 0.2 0.02 0.04 0.01 0.002 0.006 0.01
7.4 16.2 3.5 45.4 0.4 0.05 14.9 4.4 4.5 0.3 0.05 0.004 0.002
7.7 17.6 2.5 46.7 0.4 1.3 14.9 2.3 3.9 0.3 0.1 0.03 0.01 0.02 0.02 0.02 0.03
6.1 16.2 3.5 46.1 0.4 0.06 15.0 4.5 5.2 0.5 0.1 0.02 0.009 0.004 0.003 0.001 0.001
10.0 18.4 3.3 40.5
0.4 0.8 0.3 0.1 0.03 0.05 2.8
0.008 1.7 0.3 0.1 0.2
3.1 15.6 3.8 46.9 0.2 0.2 15.1 4.7 5.9 1.6 0.8 0.3 0.2 0.1 0.09 0.06 0.04 0.03 0.02 0.02 0.5 0.5 0.08 0.04 0.05
2.4 14.9 3.9 46.0 0.1 0.5 14.6 4.7 5.5 2.2 1.2 0.6 0.4 0.3 0.2 0.2 0.1 0.09 0.06 0.04 1.7 0.3 0.04 0.02 0.03
6.2 13.5 4.1 43.5
T T T
7.6 18.3 3.4 40.7 1.7 0.2 16.4 4.4 4.4 1.1 0.07 0.04 0.04 0.03 0.04 0.04 0.06 0.02 0.03 0.02 0.6 0.4 0.1 0.01 0.05 3.3 0.9
6.0 15.0 3.6 45.2
T T T
3.7 15.9 3.7 47.0 0.2 0.2 15.2 4.6 5.9 1.3 0.6 0.2 0.1 0.07 0.05 0.03 0.02 0.02 0.01 0.008 0.2 0.8 0.1 0.06 0.08
2.2 14.3 4.0 45.1 0.08 0.8 14.3 4.7 5.2 2.4 1.4 0.8 0.5 0.4 0.3 0.2 0.2 0.1 0.09 0.07 2.6 0.2 0.03 0.01 0.02
T T T toluene styrene ethylbenzene wt % C bal error wt % H bal error
38.0 103.0 0.36 77.94
8.0 15.9
Table IV. Experimental (ER,mol %) and Simulated Results HC,g h-l 13.4 17.9 HpO,g h-' 39.5 49.4 0.87 0.69 48 wt % conversion 97.76 96.87
HZ
%) of a-Hexadecane Steam Cracking at
30.0 82.4 0.44 81.26
17.9 49.4 0.73 89.34
ER 3.0
C2H4 CSH8
wt % wt %I
13.4 39.5 0.94 90.44
0.3 1.0 0.2 0.07 0.03 5.2 1.6
T T T T T T T T 2.2 0.3 0.2 0.2
T 1.5 15.1 4.2 4.7 0.9 0.1 0.05 0.04 0.04 0.04 0.04 0.04 0.03 0.02 0.01 0.5 0.3 0.1 0.07 0.01 1.2 4.9
T 1.1
14.9 5.0 4.4 2.2 0.05 0.05 0.1 0.2 0.2 0.1 0.1 0.08 0.06 0.02 2.0 0.1 0.05 0.0 0.02 2.1 0.7
T 0.7 16.0 5.1 5.3 1.7 0.09 0.08 0.1 0.2 0.2 0.2 0.2 0.1 0.1 0.04 2.5 0.09 0.05 0.0 0.01 1.6 0.4
~~
T, traces.
molecular system solved by Euler's method. To overcome mathematical difficulties, steady state for the radical concentration was assumed. The inputs of the nonlinear algebraic system were the concentrations of the molecular species, and the outputs were the concentrations of the radical species. The inputs of the nonlinear differential system were the radical concentrations and the initial concentrations of the
molecular species, and the outputs were the concentrations of the molecular species. The general system was a closed-loop system, of which the inputs were the initial concentrations of the molecular species and the outputs were the concentrations of the molecular species evolving with the residence time for a given temperature of cracking. Flow charts of the closed-loopgeneral system and of the calculations are given
1120 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991
in Figures 2 and 3, where aR, = kie-Ei/RT(BR.. 1J - AR-) LJ
i = 1, m ; j = I , p
(3)
aCij = kje-Ei/RT(BC.. 51 - ACij)
i = 1, m ; j = 1, q
(4)
p
Algebraic system
radical
q
i
initial concentrations
of the molecular lpecies
concentrations i I\ \(
P R ~= fi(Rjwj)
i = 1, m
j=l
(5)
I
differential system
q
t
2.2. Remarks on the Development of Various Models. To establish reliable models, experimentally observed product distributions were used to limit the number of reactions, and different reaction schemes were studied with their kinetic parameters. When discrepancies between experimental and simulated results were observed, two possibilities were considered to minimize these deviations: either by adding or discarding reactions in the scheme or by adjusting the values of kinetic parameters. A great number of kinetic parameters published in the literature by Allara and Edelson (1975), Sundaram and Froment (1978), and Allara and Shaw (1980) were relative to the cracking of light n-paraffins smaller than n-hexane, and some of them were evaluated at low temperature and low conversion. For long-chain hydrocarbons, it was necessary to adjust these parameters for high temperature and high conversion until the simulated product distribution matched experimental results. Some parameters that were lacking were estimated by analogy with similar reactions of small radical species. 2.3. Various Reaction Schemes. The different sets of reactions implied in the six models studied are presented below. primary radical reaction
P
1. initiation 2. isomerization
Ri
Ri
3. propagation
Ri
4.
Ri'
+ R;
-+ -+
-+
0
7.
0
I
Estimate of vector PC, J/
Salve Algebraic system .L,
0
R;
0
R;'
I 1
r"-V convergence ?
I
I
Salve differential system v
I
I---.
1
&-.-Limit
+ Ri +Pz + R;
R=' + Ri
residence time ?
.cy
+ Ri R=' + P
a.
R,"
9.
R:'
0
R,"
R;
DO
10. dehydrogenation R'
R;'
H,
Figure 3. Flow chart of the calculations.
R;' DO H' secondary molecular reaction 12. Miller reaction O1 O2 + C3H6 13. Diels-Alder reaction 0 + DO aromatic + H2 Model I. As shown in Table V, model I consisted of 141 reactions, 20 molecular species, and 18 radical species. The mOk!CUlar species were H2, CHI, C&, C2H4, C3H8, C3H6, C4H8, C4H6, C5H10, C6H12, C7H14, C8H16, CBH18, C10H20, CllH22, C12H24, C13H28, CUH28, C15H30, and C16H34. The radical species were H', CH3', CzH5*,C3H7*,C4Hg',C4H,*, 11.
Estimate of matrix
Record concentrations
-+ - + - + - +
-+
Reaction scheme. T,Ei,ki
of the molecular and radical species and the residence time
secondary radical reaction 6.
Initial concentrations of molecular and radical species
Error messages
Ri'
5. hydrogen transfer P
concentrations
of the molecular species
-
-
C5Hii*, C6H13.r GHiti', GHi7', CgHig*, CioH2!', C1&5*, C13H27*, C14H29.9 C15H31', and C16H33
.
CiiH23',
This model gave simulated results from 600 to 750 "C. Only initiation, propagation, hydrogen transfer, and termination reactions were used. Dehydrogenation reactions were employed to explain the presence of C4H, in effluent products. For temperatures greater than 650 O C , the simulated concentrations of liquid olefins superior to pentene were too high in comparison to experimental data. For this reason model I must be employed for temperatures less than 675 "C, the range where the secondary reactions are negligible. In order to verify the validity of this model, values of kinetic parameters, frequency factor, and activation energy of the global rate of n-hexadecane decomposition were calculated from simulated results. The experimental
Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 1121 Table V. Abstract of Different Models (I-V) Studiedn
I no. of molecular species no. of radical species no. of reactions different type of reaction used
I1
Vn
22 20 20 31 19 31 268 357 157 *T 1,3,5,10-12 1,3,5,7-11 1,3,5,6-11,13,*B,
I, *B = benzene; *T= toluene; *St = styrene; *EtB = ethylbenzene. values k = 0.96 X 1014s-' and E = 57 kcal-mol-l were in reasonable agreement with the simulated values k = 0.78 X 1014 s-l and E = 58 kcal-mol-'. Model 11. The isomerization of all radicals greater than C4Hg' was considered. This model, composed of 20 molecular species, 71 radical species, and 236 reactions, gave satisfying simulated results up to 675 "C, but did not work at 700 "C since the program did not converge. Moreover, the resolution of the radical system for 71 radicals required a long calculation time on the computers. The simulated concentrations of CH4were in deficit by comparison to the experimental data; meanwhile C2H6, C6H12, and C13H26 were in excess. Model 111. In this model, the Miller reactions (1963) for olefin decomposition were added to the 141 reactions of model I. This model consisted of 20 molecular species, 19 radical species with a new radical C3H13. Dente. M.: Ranzi. E. In Mathematical modelina of hydrocarbon pyrolysis reactions. Pyrolysis, Theory and In2ustrihl practice; Albright, L. F., Crynes, B. L., Concoran, W. H., Eds.; Academic
1130
Ind. Eng. Chem. Res. 1991,30,1130-1138
Press: New York, 1983,Chapter 7, pp 133-175. Depeyre, D.; Flicoteam, C.; Chardaire, C. Pure n-hexadecane thermal steam cracking. Ind. Eng. Chem. Process Des. Dev. 1988,24, 1251-1258. Depeyre, D.; Flicoteaux C.; Zabaniotou A. Modeling of thermal steam cracking of an atmospheric gas oil. Ind. Eng. Chem. Res. 1989, 28,967-976. Doub, F.; Guiochon, G. Etude thborique et experimentale de la cinetique de dbcomposition thermique du n-hexadbcane, de son mecanisme et de la composition du melange des produits obtenus. J. Chim. Phys. Phys.-Chim. Biol. 1968,64,395-409. Gavalas, G. R. The long chain approximation in free radical reaction systems. Chem. Eng. Sci. 1966,21,133-141. Hirato, M.; Yoshioka, S., Tanaka, M. Gas-oil pyrolysis by tubular reactor and ita simulation model of reaction. Hitachi Rev. 1971, 20 (8),326-334. Kumar, P.; Kunzru, D. Modeling of naphtha pyrolysis. Ind. Eng. Chem. Process Des. Dev. 1985a,24, 774-782. Kumar, P.; Kunzru, D. Kinetics of coke deposition in naphta pyrolysis. Can. J. Chem. Eng. 1985b,63,598-604. Kunugi, T.; Soma, K.; Sakai, T. Thermal reaction of propylene. Ind. Eng. Chem. Fundam. 1970,9,319-324. MarCchal, T. Contribution B 1Ptude de la modelisation des
phCnom&nescinbtiques caract6ristiques des ophations de craquage thermique p€troliers. Ph.D. Dissertation, Universite Pierre et Marie Curie, Paris, 1977. Miller, D. B. Higher alpha omega-dienes in paraffin and olefin pyrolyzates. Ind. Eng. Chem. Prod. Res. Dev. 1963,2(3),22+223. Ranzi, E.; Dente, M.; Pierucci, S.; Biardi, G. Initial product distributions from pyrolysis of normal and branched paraffins. Ind. Eng. Chem. Fundam. 1983,22,132-139. Snow, R. H. A chemical kinetics computer program for homogeneous and free radical systems of reactions. J. Phys. Chem. 1966,70, 2780-2786. Sundaram, M.; Froment, G. F. Modeling of thermal cracking kinetics. 3. Radical mechanisms for the pyrolysis of simple paraffins, olefins, and their mixtures. Ind. Eng. Chem. Fundam. 1978,17, 174-182. Vermeulen, J. Contribution B l'etude de l'analyse des sysGmes reactionneb complexes. Application au craquage thermique d'hydrocarbures. Ph.D. Dissertation, Ecole Centrale Paris, Chatenay-Malabry, France, 1980. Received for review April 11, 1990 Revised manuscript received October 17, 1990 Accepted October 26, 1990
Study of the Mechanism of Higher Alcohol Synthesis on Cu-Zn0-A1203 Catalysts by Catalytic Tests, Probe Molecules, and Temperature Programmed Desorption Studies A. Kiennemann,* H. Idriss, and R. Kieffer Laboratoire de Chimie Organique Appliquie, EHICS, UA au CNRS 469, I, rue Blaise Pascal, 67008 Strasbourg Cedex, France
P. Chaumette* and D. Durand Institut Francais d u Pitrole, 1 et 4, Avenue de Bois Priau, B.P. 311, 92506 Rueil-Malmaison Cedex, France
In addition to catalytic tests, the addition of probe molecules (methanol, formaldehyde, ethylene glycol, ethanol, acetone, propionaldehyde) and thermoprogrammed-desorption studies (formaldehyde, acetaldehyde, ethanol, acetone) have been used to investigate the mechanism of higher alcohol synthesis on Cu-Zn0-A1203 catalysts. More attention is given to the first carbon-carbon bond and branched-product formation. An overall mechanism is proposed.
Introduction Important researches have been performed by companies on the direct higher alcohol synthesis from synthesis gas in the past years (Chaumette and Hughes, 1985;Xu et al., 1987). Although petroleum prices have dropped, this synthesis remains an interesting challenge for both industrial applications (good octane blending values and compatibilizing properties) (Chaumette and Courty, 1987) and fundamental research (understanding of the first carbon-carbon bond formation) (v. d. Lee and Ponec, 1987). The large amount of natural gas reserves is a major incentive to pursue researches in that field. Catalysts for higher alcohol synthesis can be divided into three categories: (a) The first is modified Fischer-Tropsch catalysts, baaed on cobalt or iron. Copper-cobalt catalysts show a productivity of 0.1-0.15 g-g of catalySt-'-h-' and a selectivity to alcohols of 7 M % for a higher alcohol content (C2+OH) of 35-40 wt ?% in smooth conditions (270-320 "C, 6-10 MPa) (Chaumette and Courty, 1987;Courty et al., 1987). (b) A second category of catalysts for higher alcohol synthesis is methanol synthesis catalysts modified by addition of alkali metals. These catalyste are either alkalinized Zn(H=r2OSformulas operating at high pressure and temperature (350-400 "C, 26 MPa) or low-pressure cop-
per-baaed catalysts, which may attain higher productivities (up to 0.45 g g of catalyst-l-h-l) but suffer from selectivity drawback because of the formation of various oxygenates (ketones, esters, etc.) in large quantities (Smith and Anderson, 1984;Vedage et al., 1985). (c) The third category includes other catalysts such as supported rhodium or molybdenum ones. Rhodium-based catalysts are very selective toward C2 oxygenated products (ethanol, acetaldehyde, acetic acid) (Ichikawa, 1982; Ichikawa et al., 19W,Fukushima et al., 1985;Kiennemann et al., 1987). Acetic acid should be avoided in case of gasoline blendmg. Molybdenum-based catalysts give C& alcohols, but sulfur addition (H2S) in the feed seems necessary to maintain their activity, and an another drawback is the important amount of C02 formed during the reaction (Tatsumi et al., 1986; Inoue et al., 1987; Quarderer, 1984). In the case of cobalt or iron catalysts, a classical Anderson-Schulz-Flory distribution is observed for alcohols which is similar to that observed for hydrocarbons. It seems now well established on Co-Cu, MoS2, or Co-Mo catalysts that the chain growth proceeds through the reaction of a C1 oxygenated entity with a CH, species and that methanol does not take part in the propagation step (Kiennemann et al., 1989;Tatsumi et al., 1989).
0888-5885/91/2630-1130$02.50/00 1991 American Chemical Society
