Ind. Eng. Chem. Res. 1993,32,61-70 Arefeva, E. F.; Tesner, P.A. Kinet. Katal. (Russ.) 1987,28 (l),184. Baker, R. T. K. Carbon 1989,27, 315. Blackwood,J. D.; Cullis, B. D.;McCarthy, D. J. Aust. J. Chem. 1967, 20, 1561. Chaverot, P.; Berthelin, M.; Freund, E. Revue IFP 1986, 41, 529. Ghosh, K. K.; Kunzru, D. Ind. Eng. Chem. Res. 1988,27, 559. Fernandez-Baujin, J. M.; Sundaram, K. M.; Maddock, M. J. AIChE-Meeting, Orlando, FL, March 1990. Froment, G . F. Rev. Chem. Eng. 1990,6 (4), 293. Horak, J.; Baranek, P. Int. Chem. Eng. 1986,26, 547. Hundrots, R. S.; Nighswander, J. A.; Mehrotra, A. K.; Behier, L. A. Chem. Eng. Res. Des. 1989, 67, 632. Kaiser, V.; Clymans, P. J.; Froment, G . F.; Barendregt, S. Oil Gas J . 1984 (Oct. 291, 66. Kopinke, F.-D. Struktur-Reaktivitiits-Beziehungenfiir die Koksbildung unter den Bedingungen der Mitteltemperaturpyrolyse von Kohlenwasserstoffen. Diss. B., Academy of Sciences of the GDR, Leipzig, 1985. Kopinke, F.-D.;Zimmermann, G.; Ondruschka, B. Ind. Eng. Chem. Res. 1987, 26, 2393. Kopinke, F.-D.; Porzel, E.; Bach, G.; Nowak, S.; Zimmermann, G. Erdol Kohle 1981,34, 204.
61
Kopinke, F.-D.;Zimmermann,G.; Nowak, S. Carbon 1988a, 26, 117. Kopinke, F.-D.; Zimmermann, G.; Ondruschka, B.; Dermietzel, J. J . Anal. Appl. Pyrolysis 198813, 13, 259. Kumar, P.; Kunzru, D. Can. J . Chem. Eng. 1985, 63, 598. Kumar, P.; Kunzru, D. Can. J . Chem. Eng. 1987, 65, 280. Mandal, T. K.; Kunzru, D. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 794. Plehiers, P. M.; Reyniers, G. C.; Froment, G . F. Ind. Eng. Chem. Res. 1990, 29, 636. Pramanik, M.; Kunzru, D. Ind. Eng. Chem. Res. 1985, 24, 1275. Sahu, D.; Kunzru, D. Can. J . Chem. Eng. 1988, 66, 808. Sundaram,K. M.; Van Damme, P. S.; Froment, G. F. AlChE J . 1981, 27, 946. Vaish, S.; Kunzru, D.Ind. Eng. Chem. Res. 1989, 28, 1293. Zander, M. Erdol Kohle 1989, 205, 373. Zimmermann, G.; Kopinke, F.-D.;Rehm, R. J. Anal. Appl. Pyrolysis 1985, 7, 195. Zou, R.; Lou, Q.;Liu, H.; Niu, F. Ind. Eng. Chem. Res. 1987,26,2528.
Received for review July 13, 1992 Accepted October 19, 1992
Kinetics of the Fischer-Tropsch Reaction on a Precipitated Promoted Iron Catalyst. 1. Experimental Procedure and Results Egbert S. Loxt and Gilbert F. Froment* Laboratorium voor Petrochemische Techniek, Rijksuniversiteit, Krijgslaan 281, B-9000 Gent, Belgium
The Fischer-Tropsch reaction on a commercial promoted precipitated iron catalyst was studied in a tubular reactor under non-deactivating conditions of temperatures between 523 and 623 K, pressures between 0.6 and 2.1 MPa, hydrogen to carbon monoxide feed ratios between 3.0 and 6.0 mol/mol, and W/Focovalues between 9.2 and 63.0 kg.s/mol. The selectivity for carbon dioxide, methane, and hydrocarbons of different functionality with 2-15 carbon atoms in the molecule was obtained as a function of the carbon monoxide conversion, the reactor temperature, and the total pressure. The initial rate of formation of these products was measured as a function of the total pressure and the partial pressures of hydrogen and carbon monoxide at the reactor inlet. These experiments, combined with the information gained from the catalyst characterization, indicated that carbon dioxide is formed by the water gas shift reaction. Methane, n-paraffins, and l-olefins with two and more carbon atoms in the molecule are all primary products of the Fischer-Tropsch reaction. The composition of the hydrocarbon product fraction, as a function of the number of carbon atoms in the hydrocarbon molecule, could be described by the Schulz-Flory distribution, although it is shown that the latter only approximately holds for the effldent of an integral reactor. 1. Introduction The reaction between carbon monoxide and hydrogen on heterogeneous catalysts yielding hydrocarbons, carbon dioxide, and water has led to intensive research ever since it was discovered by Fischer and Tropsch in 1923. Considerable efforts have been devoted to the understanding of the mechanism of this reaction and, more recently, to the development of more selective catalysts. The major part of the kinetic studies did not go beyond rate expressions of the empirical power law type rate for the reaction rate of carbon monoxide:
Rco = kcoPn2nPcom
(1)
(Vannice, 1976; Dry, 1981;Feimer et al., 1981; for a review see also Huff and Satterfield (1984)). Bub and Baerns (1980) and Grenoble et al. (1981) also expressed the rate of formation of carbon dioxide in terms
* Author to whom correspondence
should be addressed. 'Present address: Inorganic Chemical Product Division, Degussa A.G., P.O. Box 1345, D6450 Hanau-l, Germany.
0888-5885/93/2632-0061$04.00/0
of the following empirical power law type rate equation: RCO,
= kC0$COUPH20Y
(2)
In most cases the hydrocarbons were lumped according to the number of carbon atoms in the molecule. Sometimes all the hydrocarbons were simply treated as one lump or split into a paraffm lump and an olefin lump (Zein el Deen et al., 1979; Bub and Baerns, 1980; Dictor and Bell, 1986; Gaube et al., 1982). Some authors derived Langmuir-Hinshelwood-Hougen-Watson (LHHW) rate equations. Dixit and Tavlarides (1983) based their kinetic equations on the "carbide" mechanism, which basically states that the hydrocarbon intermediates on the catalyst surface are formed by polymerization of monomers consisting of carbon and hydrogen. The rate-determining step (RDS) was assumed to be the hydrogenation of a carbon intermediate, C-1: C-1 + X(H-1) CH,-1 + X-1 (3) Various other mechanisms have been assumed. The main difference between these mechanisms is the nature of the monomer. Besides monomers consisting of carbon and 0 1993 American Chemical Society
62 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993
Figure 1. Experimental setup.
hydrogen, which are the backbone of the “carbide” mechanism, monomers consisting of carbon, oxygen, and hydrogen are considered in the ucondensationnmechanism. The common point in the derivation of the respective LHHW rate expressions is the assumption that the elementary step by which the monomer is formed was as1976; Atwood sumed to be the rate-determining step (Dry, and Bennett, 1979; Huff and Satterfield, 1982; Deckwer et al., 1985; Dadyburjor, 1983). Thompson e t d . (1981) and Chen et al. (1983) published detailed LHHW rate expressions for the major reaction paths for carbon monoxide. These authors assumed that the hydrocarbons are formed according to the carbide mechanism and did not make any assumption as to the rate-determining step. Every single olefin, paraffin, and alcohol was considered to be a primary product. They also proposed a LHHW rate expression for the reaction leading to carbon dioxide and assumed that it took place on a different type of active site. Kellner and Bell (1981) and Takoudis (1984) considered only the reaction paths by which carbon monoxide is transformed into hydrocarbons. The elementary steps were based upon the carbide mechanism and, no assumption was made regarding the occurrence of a ratedetermining step. The goal of the present work was to derive detailed LHHW rate expressions for the major reaction paths of the Fischer-Tropsch synthesis on a commercial precipitated iron catalyst. A tubular reactor was set up which allowed the pretreatment of the catalyst under well-defined conditions and the isothermal experimentation required to determine the intrinsic kinetics of the Fischer-Tropsch synthesis. Details on the reactor have been published
before (Lox et al.;1988a). It was also attempted to account to a maximum extent in the kinetic modeling for the information obtained from the characterization of the catalyst (Lox et al., 1988b). Part 1 focuses on the experimental procedure and on the intepretation of the experimental results. The modeling, the model discrimination, and the parameter estimation will be presented in part 2. 2. Experimental Section 2.1. T h e Catalyst. The catalyst is a commercial pre-
cipitated promoted iron catalyst, the preparation of which has been described by Falbe (1977). The catalyst contains 60.3 w t % Fe203,11.1wt % SO2,3.1 wt % CuO, 2.0 w t % K20, and 0.1 w t % Na20, the balance being water. Prior to its use, the catalyst is treated with hydrogen according to the procedure prescribed by the catalyst manufacturer. This treatment essentially leads to a reduction of Fe20, to Fe30,. When the catalyst is contacted with carbon monoxide-hydrogen mixtures, its composition changes again. Besides FeaOI also iron carbides are formed. The detailed characterization of this catalyst before and during use has been published previously (Lox et al., 1988b). 2.2. Experimental Setup. The experimental setup, shown in Figure 1, consists of three sections: the feed section, the reactor section, and the analysis section. In the feed section, carbon monoxide (L’Air Liquide; 99.0 vol %) and hydrogen (L’Air Liquide; 99.8 vol 70)were monitored through thermal mass flow controllers and meters. The gases were used without further purification. The reactor section consisted of a tubular reactor and a heating device. The tubular reactor (length 0.27 m, inside diameter 0.021 m) was loaded with 5 g of catalyst, which
Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 63 Table I. On-Line Gas Chromatographic Analysis column fused silica OV101, 30-m length, 0.32-mm i.d. 10% sorbitol on Chromosorb T 40/60 mesh, l-ft length, 1/8-in. 0.d. 2.7% Carbowax 1540 on Poracil C 80/100 mesh, 2-ft length, 1/8-in. 0.d. 0.4% Carbowax 1500 on Carbopak B, 60/180 mesh, 3-ft length, 1/8-in. 0.d.; + 2.7% Carbowax 1540 on Poracil C 80/100,10-ft length, 1/8-in. 0.d.; + 27.5% bis-2(EE)Ao on Chromosorb P-AW 45/60 mesh, 17-ft lenath, 1/8-in. 0.d. molecular sieve 13X 80/100'mesh, 10-ft iength;l/d-in. 0.d. Porapak Q 50/80 mesh, 7-ft length, 1/8-in. 0.d. a
function separate C1-CI5hydrocarbons separate HzO from main product stream separate alcohols and C4+from main product stream separate C3- and C,-paraffins, olefins, and is0 components separate N P , CO, and CH4 separate COP,C2H4,and C,H,
bis-Z(EE)A = Bis(2-ethoxyethyl) adipate.
was crushed and sieved to particles with mean diameter of 0.4 mm. The catalyst bed was diluted by means of 50 g of steatite in aluminum oxide beads of the same particle size. Heating was achieved either by means of an infrared furnace, used during the catalyst pretreatment, or a salt bath, used during the subsequent kinetic study. A detailed description of this reactor unit has been given elsewhere (Lox et al., 1988a). Nitrogen was mixed with the reactor effluent to serve as an internal standard for the on-line gas chromatographic analysis. The complete analysis was performed on a single, computerized CARLE 500 gas chromatograph. The reaction products were separated both on a capillary column and on a series of packed columns, interconnected by automatic valves. Details of the columns are summarized in Table I. There are three detectors: one flame ionization detector (FID), connected with the capillary column, and two thermal conductivity detectors (TCD's), connected with the packed columns. Furthermore, the gas chromatograph is equipped with a hydrogen-transfer cell, which transfers quantitatively hydrogen from the main eluent stream, in which helium is used as eluent, to a side stream where nitrogen is used as eluent, so that accurate measurement of hydrogen is achieved on the TCD. The integration of the peaks of the gas chromatograph was performed by a Data General S280 process computer, which also runs a peak pattern recognition program. The conversion and mass balance calculations were carried out on line on a Data General Dasher personal computer. The identification of the components eluting from the packed columns was performed on the basis of retention time. For the components eluting from the capillary column, a two-step procedure was adopted, which takes advantage of the unique composition pattern of the hydrocarbon products in the Fischer-Tropsch synthesis on the catalyst used. First, the l-olefin and the n-paraffin of each carbon number were identified on the basis of retention time; their identification was facilitated by the fact that either the l-olefin or the n-paraffin was the major component in each carbon number group. Then, knowing the retention time of the n-paraffin, the retention indices of the remaining components were calculated and compared with the retention index library, which was mainly based upon literature data (Hively and Hinton, 1968; Johansen and Ettre, 1982; Lee and Taylor, 1982). The packed columns allowed the detection of water, carbon monoxide, carbon dioxide, hydrogen, methane, ethane, ethylene, propane, propylene, and nitrogen. On the capillary column, methane and methanol, n-paraffins, isoparaffii, l-olefins, isoolefins, and primary alcohols with 2-15 carbon atoms in the molecule are separated. This elaborate analysis procedure permits a major part of the products to be identified so that accurate mass balances on carbon, hydrogen, and oxygen could be obtained.
I
i
"
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I .
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hi
Figure 2. Conversion of carbon monoxide and hydrogen. Conversion into carbon dioxide and methane as a function of time. Reaction conditions: T = 573 K pt = 1.1 MPa; PH1/F"co= 3 mol/mol; W / P c 0 = 48.0 kg.s/mol.
2.3. Experimental Procedure. Isothermal steadystate experiments were carried out under non-deactivating conditions in a fixed bed flow reactor. The catalytic activity depends on the state of the iron in the catalyst, which is determined by the pretreatment of the catalyst and the way the reactor is started up (Dry, 1981; Blanchard et al., 1982; Reymond et al., 1982; Madon and Taylor, 1981; Krebs et al., 1981; Kikuchi et al., 1984; Dictor and Bell, 1986). Furthermore, the composition of the catalyst can change during the course of the reaction, depending on the reaction conditions (Dry, 1981). The pretreatment proposed by the catalyst manufacturer was applied. It consists essentially of a reduction of Fe203into Fe304by means of hydrogen. The catalyst is extremely active in this state and care has to be taken to avoid hot spots upon admission of carbon monoxide. These would lead to uncontrolled carbon formation on the catalyst. During start up the catalyst was kept in a hydrogen stream. Then the carbon monoxide content was gradually raised, keeping the reactor at 523 K. Care was taken to ensure that the hydrogen to carbon monoxide feed ratio always exceeded the value of 2 mol/mol. Then, the reactor pressure and temperature were slowly raised to pt = 1.1MPa and T = 623 K. The catalyst reached a stable activity after about 12 h. During this lining out, the composition of the catalyst changed from FeaOl to a mixture of Fe304and iron carbides. A comparison of the Mossbauer spectrum of the catalyst after completion of the lining out with the Mossbauer spectrum of the catalyst used during 200 h of reaction at various reaction conditions indicated that the major phase transformations of the iron took place during the pretreatment, so that the subsequent kinetic analysis was performed on a relatively stable catalyst. (Lox et al., 1988b). The conversion of carbon monoxide and hydrogen and the conversions into the various products were followed over a period of 300 h at 573 K, pt = 1.1 MPa, hydrogen to carbon monoxide feed ratio 3 mol/mol, and W / F c o= 48 kg.s/mol. From the results, shown in Figure
64 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 Table 11. Total Conversion of Carbon Monoxide and Hydrogen and Conversion of Carbon Monoxide into Carbon Dioxide and Methane as a Function of the Catalyst Particle size at T = 623 K,p t = 2.1 MPa, FaH2/FoC0 = 2.0 mol/mol and W / F a c o= 4.8 kg*s/mol particle diam, mm X C O ,90 X H ~7', XCO,, 70 X C H ~%, 0.15-0.17 0.4-0.70 1.0-1.20 2.5-3.0
61.5 82.4 82.1 54.3
19.1 32.8 33.4 19.8
20.9 26.8 26.7 17.55
19.3 20.4 20.1 20.7
Table 111. Experimental Conditions
FoH2/FoC0, W/Foco, temp, K
press., MPa
623 573 553 523
0.6, 1.1, 1.6, 2.1 0.6, 1.1, 2.1 0.6, 1.1, 2.1 1.1, 2.1
mol/mol 3.0, 3.0, 3.0, 3.0,
4.0, 6.0 6.0 6.0 6.0
kg.s/mol
no. of points
9.2-63.0 7.9-82.0 12.0-63.0 17.0-63.0
33 25 19 13
2, the conclusion was drawn that eventual further solidphase transformations did not affect the catalytic activity in'a measurable way. These results also indicate that no catalyst deactivation occurred under these conditions. Thermodynamic calculations according to the procedures of Cairns and Tevebaugh (1964) and Manning and Reid (1978) indicated that the formation of free carbon was thermodynamically not feasible in the temperature range 525-623 K and in the pressure range 0.1-2.0 MPa, as long as the hydrogen to carbon monoxide feed ratio exceeds the value of 2 mol/mol. Therefore, to avoid deactivation by free carbon, the hydrogen to carbon monoxide feed ratio was never lower than 2 mol/mol. These calculations revealed also that, in this range of reaction conditions, iron carbides (Fe,C) and Fe304are the iron phases in equilibrium with the gas phase, in agreement with the experimental results. In the course of the kinetic experiments, each change in reaction conditions was followed by a stabilization period of at least 5 h. The criteria of Weisz and Prater and Mears (see Froment and Bischoff (1990)) were calculated at T = 623 K, pt = 2.1 MPa, hydrogen to carbon monoxide feed ratio 2 mol/mol, and W/FOco= 4.8 kg.s/mol, to ensure that the true intrinsic kinetics were measured. Under these reaction conditions, the measured reaction rate of carbon monoxide was 125 mmol/kg.s and that of hydrogen 117 mmol/kgs. These were the highest rates observed in the experimental program. It was taken into account in these calculations that the catalyst pores are filled with a hydrocarbon wax during the reaction. It was reported previously that this hydrocarbon wax mainly consisted of l-olefins and n-paraffins with up to 100 carbon atoms in the molecule (Lox et al., 1988b). The activity of catalysts with different particle sizes, pretreated with hydrogen and stabilized during 12 h with synthesis gas, was measured at T = 623 K, pt = 2.1 MPa, hydrogen to carbon monoxide feed ratio 2 mol/mol, and W / P c o = 4.8 kg-s/mol. The results of these experiments, summarized in Table 11, indicate that no diffusional limitations occur for the catalyst particle size used in the kinetic study. The lower activity measured for the smallest particle size is believed to be the result of a further reduction of the catalyst (see also Reymond et al. (1980)). The criterion of Caldwell and Van Vuuren (1986) indicated that product condensation did not occur in the reactor at 523 K, therefore that single phase flow prevailed. The experimental program is outlined in Table 111. A total of 90 different runs were carried out. Under these conditions, the carbon monoxide conversion ranged from
12 to 100%. The detailed results are reported elsewhere (Lox, 1987). The conversion of the reactants carbon monoxide and hydrogen is defined as
xco =
FOCO - F " C 0 x 100 poco
(4)
and
The conversion into the produts is defined as
F": XI = ' x 100 poco The selectivity for any product j , represented by S I , is calculated from SI = (X,/XC") x 100 (70) ~
In addition, 12 experiments were performed in order to study the influence of the partial pressure of carbon monoxide and hydrogen at the reactor inlet on the initial reaction rates. For this purpose the reactor was operated in a differential mode, with Xco < 15%. In that case the initial reaction rates of the reactants are obtained from
ROC0 = X c o / ( W / F o c o ) X 100
(8)
= XH,(FOH,/FOCO)/(W/FOCO) X 100
(9)
and R O H ~
The initial rate of formation of the products is calculated from R', = X,/(W/FOco) X 100 (10) The influence of the partial pressure of hydrogen on the initial reaction rates was studied at 523 K, 0.2-MPa partial pressure of carbon monoxide at the reactor inlet, and W / F o c o = 14.4 kgs/mol by varying the partial pressure of hydrogen at the reactor inlet between 0.46 and 1.0 MPa. The experimental program was started at the highest hydrogen partial pressure. The influence of the partial pressure of carbon monoxide at the reactor inlet on the initial reaction rates was also studied at 523 K, 0.9-MPa partial pressure of hydrogen at the reactor inlet, and W/Foco = 14.4 kgs/mol by varying the partial pressure of carbon monoxide at the reactor inlet between 0.133 and 1.55 MPa. The experimental program was started at the lowest carbon monoxide partial pressure. 3. Results and Discussion 3.1. Influence of Process Variables on Initial Rates. 3.1.a. Initial Rates of Disappearance of Carbon Monoxide and Hydrogen. The initial reaction rate of carbon monoxide and hydrogen increases more than linearly with increasing hydrogen partial pressure, as is shown in Figure 3. The difference between the reaction rate of hydrogen and the reaction rate of carbon monoxide also increases with increasing hydrogen partial pressure. The partial pressure of carbon monoxide has a completely different influence on the initial reaction rate of carbon monoxide, as can be seen from Figure 4: the initial reaction rate of carbon monoxide goes through a maximum with increasing partial pressure of carbon monoxide. The initial reaction rate of hydrogen continuously decreases in
R, T
523 K
POc o
0 2 MPO
lWIF:ol
1 4 L k g SlmOl
/
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Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 65 Immollkgk.rl
T
523 X
Pro
0 2 MPO
lWlk~ol
144 kg rim01
10-
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, R ,
.
1'0
05
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Figure 3. Initial reaction rate of carbon monoxide and hydrogen a a function of partial pressure of hydrogen. Reaction conditions: T = 523 K; poco = 0.2 MPa; W/Foco = 14.4 kgs/mol. R,
4 lmrnollkg
523 K
PI
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:
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.
.
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Figure 4. Initial reaction rate of carbon monoxide and hydrogen as a function of partial pressure of carbon monoxide. Reaction conditions: T = 523 K; poH2= 0.9 MPa; W/Foco = 14.4 kgs/mol.
Figure 6. Initial rate of methane and carbon dioxide formation as a function of partial pressure of carbon monoxide. Reaction conditions: T = 523 K; p o ~ =2 0.9 MPa; W/Foco = 14.4 kg-s/mol.
the range of carbon monoxide partial pressures studied. The difference between the initial rate of reaction of hydrogen and the initial rate of reaction of carbon monoxide is rather constant for partial pressures of carbon monoxide exceeding 0.4 MPa. Below this value, the difference between the respective rates increases. This indicates that there is more than one reaction path along which carbon monoxide reacts, and that the rates along these reaction paths are influenced in a different way by the carbon monoxide partial pressure. I t was stated in the Introduction that several authors fitted a empirical power law type rate expression for the reaction rate of carbon monoxide. For iron-based catalysts the exponent of the partial pressure of hydrogen in eq 1, n, was comprised between 0.5 and 1.6, whereas for the exponent of the partial pressure of carbon monoxide, m, values between -0.25 and 0.5 were found. This corresponds qualitatively with the results of the present work. 3.1.b. Initial Rate of Formation of Carbon Dioxide. The initial rate of formation of carbon dioxide first increases with increasing hydrogen partial pressure and then becomes independent of it, as can be seen from Figure 5. A similar influence is observed for the influence of the carbon monoxide partial pressure, as follows from Figure 6. It was also observed that the initial rate of formation of carbon dioxide goes through a maximum with increasing total pressure.
Carbon dioxide can be formed from carbon monoxide via two reactions. The first reaction is the Boudouard reaction:
2co + cop + c
(11)
This reaction would, however, imply that free carbon is formed on the catalyst surface, whereas thermodynamic calculations indicate that the formation of free carbon does not occur in the range of the experimental conditions used. The second reaction is the water gas shift reaction: CO + H2O
COP + H2
(12)
Since water is present in the products and FeaOl is known to catalyze the water gas shift reaction (Madon and Taylor, 1981; Shultz et al., 1955; Grenoble et al., 1981; Oki and Mezaki, 1973; Rethwisch et al., 1985), it is concluded that the water gas shift reaction is the main reaction by which carbon monoxide is transformed into carbon dioxide. 3.l.c. Initial Rate of Formation of Hydrocarbons. The second major group of observed carbon-containing products is the hydrocarbons. Among the hydrocarbons, methane was always the product with the highest concentration. It was already shown in Figure 5 that the initial rate of formation of methane increases almost linearly with increasing hydrogen partial pressure. It was also shown in Figure 6 that the carbon monoxide partial pressure has
66 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993
%
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623 I
inno l k q S
Nn?':c *
3
!4tFt2
'9-2.
'9
5
r n ~
/
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Figure 7. Initial rate of methane formation as a function of temperature and total pressure a t PH,/Foc0= 3 mol/mol. 1
Figure 9. Initial rate of formation of 1-hexene, n-hexane, and i s 0 4 6 components as a function of partial pressure of carbon monoxide. = 0.9 MPa; W/Foco = 14.4 Reaction conditions: T = 523 K; kgs/mol.
523 K
Figure 8. Initial rate of formation of I-hexene, n-hexane, and isc& components as a function of partial pressure of hydrogen. Reaction conditions: T = 523 K; poco = 0.2 MPa; W/Foco = 14.4 kgs/mol.
Figure 10. Initial rate of formation of 1-hexene, n-hexane, and the sum of all c6 hydrocarbons as a function of total reactor pressure. Reaction conditions: T = 623 K; FoH,/FoCo = 3 mol/mol; W/Foco = 7.9-24 kgs/mol.
a completely different influence on the initial rate of methane formation. Figure 7 shows that the initial rate of methane formation increases almost linearly with increasing total pressure over the total range of temperature studied. Besides methane, a spectrum of other hydrocarbons was observed: 1-olefins, n-paraffm, isoolefm, and isoparaffins with 2-15 and more carbon atoms in the molecule. In most of the experiments, however, the concentration of the hydrocarbons with more than 10 carbon atoms per molecule was found to be too low to permit accurate on-line determination. The influence of the hydrogen and carbon monoxide partial pressure on the initial reaction rate of the various hydrocarbons will be demonstrated for hydrocarbons with six carbon atoms in the molecule. The initial reaction rate of 1-hexene, n-hexane, isohexenes, and isohexanes increases with increasing partial pressure of hydrogen, as is shown in Figure 8. This increase is nonlinear, with the nonlinearity more pronounced for n-hexane and for the is0 components than for 1-hexene. The initial rates of formation of these Components goes through a maximum with increasing carbon monoxide partial pressure, as is shown in Figure 9. The effect is almost identical for n-hexane, 1-hexene, and the is0 components. This tendency was observed for all the hydrocarbons with three and more carbon atoms in the molecule. Upon increasing the total pressure, the initial reaction rates of n-hexane and 1-hexene increase in a nonlinear way, as is shown in Figure 10.
3.2. Product Distribution as a Function of Carbon Monoxide Conversion. The selectivity for carbon dioxide is practically independent of the carbon monoxide conversion, as shown in Figure 11. The selectivity for carbon dioxide increases slightly with increasing temperature, but remains almost unaffected by the total pressure and the hydrogen to carbon monoxide feed ratio. When carbon monoxide conversion is extrapolated to zero, the selectivity for carbon dioxide differs from zero for the whole range of temperature studied. Also, the selectivity for methane was found to be practically independent of the carbon monoxide conversion, as shown in Figure 12. It tends to increase with increasing temperature and increasing hydrogen to carbon monoxide ratios in the feed, but remains unaffected by the total pressure. When carbon monoxide conversion is extrapolated to zero, the selectivity for methane also differs from zero, indicating that methane is a primary product. The selectivity for ethane increases with increasing carbon monoxide conversion, as shown in Figure 13. This trend is independent of the total reactor pressure. These results indicate that ethane is not consumed in secondary reactions. When carbon monoxide conversion is extrapolated to zero, the selectivity for ethane differs from zero, so that ethane is a primary product. In contrast, the selectivity for ethylene decreases with increasing carbon monoxide conversion,as is shown in Figure 14. This trend is independent of the totd pressure and also of the reactor temperature, as shown in Figure 15. These results indicate
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that ethylene is probably involved in secondary reactions, confirming conclusions reached by Novak et al. (1981), Novak and Madon (1984), and Dry (1981). When carbon monoxide conversion is extrapolated to zero, the selectivity for ethylene differs from zero, indicating that ethylene is also a primary product. The selectivities for propane and propylene both increase with increasing carbon monoxide conversion, as shown in Figure 16. This trend is independent of the total pressure, indicating that neither propane nor propylene participates in secondary reactions. When carbon monoxide conversion is extrapolated to zero, the selectivity for propylene clearly differs from zero, whereas the picture is not so clear for propane. The same conclusions can be drawn for the selectivity for hydrocarbons with more than
30 ln
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" O V
20
0
40
so
00
100
*
x c o (-1.) Figure 12. Selectivity for methane formation as a function of carbon monoxide conversion.
three carbon atoms in the molecule. 3.3. Hydrocarbon Product Distributions. In most of the work published on Fischel-Tropsch synthesis, the authors found that the composition of the hydrocarbon
68 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993
I 1
1 1w
50
4
Xcd%l
Figure 13. Selectivity for ethane formation as a function of carbon monoxide conversion.
1w
50
LoglMnl
O
f
0
T
523
K
P+
21
MPa
iH2/CO)" W/Ft0
I
10
30
20
1
I
I
40
50
60
10
I
,
80
90
I
( */.
I
100yco
Figure 14. Selectivity for ethylene formation as a function of carbon monoxide conversion and total pressure.
I
Y(
I %I
Figure 16. Selectivity for propylene and propane as a function of carbon monoxide conversion and total reactor pressure.
-2
3 mollmol 36 kg simoi
I
D 553
Figure 17. Mole fraction of hydrocarbons with n carbon atoms in the molecule as a function of number of carbon atoms.
10
20
30
LO
50
60
70
80
90
100 Ycol%!
Figure 15. Selectivity for ethylene formation as a function of carbon monoxide conversion and temperature.
fraction follows the Schulz-Flory distribution. This distribution basically implies that the ratio between the rate of formation of the hydrocarbons with n carbon atoms and the rate of formation of the hydrocarbons with n - 1carbon atoms is independent of the number of carbon atoms in the molecule. This ratio is called the growth probability, in analogy with the terminology used in polymerization: RCn/RCn-l
= a(T!pC0,pH2)
(13)
The mole fraction, M,, of hydrocarbons with n carbon atoms in the total amount of hydrocarbons formed can then be calculated from M n
= Rc,/CRc,
(14)
By substitution of (13) in (14) it follows that
M , = a"-'(l - a) or, by taking the logarithm of both sides of (15): log (M,) = n log (a)+ log((1 - a ) / a )
(15)
(16)
which means that a plot of log (M,) versus n should be linear. This was also observed in the present experiments, as shown in Figure 17. It should be noted that (13) does not guarantee the Schulz-Flory distribution for the hydrocarbon products at the outlet of an integral reactor. Indeed, since the growth probability factor, a,is the ratio of two rates of formation, it depends on the temperature and the partial pressure of carbon monoxide and hydrogen (see also Kellner and Bell (1981), Baerns (19831, and Bub and Baerns (1980)). What is measured at the outlet of an integral reactor is a weighted average of Schulz-Flory distributions, which does not necessarily correspond to the Schulz-Flory distribution. A reactor simulation using the kinetic model of Kellner and Bell (1981) proved that the hydrocarbon product distribution at the outlet of an integral reactor is indeed different from the Schulz-Flory distribution, but that the deviation becomes pronounced only for the mole fraction of hydrocarbons with more than ten carbon a t o m (Lox,1987). Further, if the Schulz-Flory functionalism is accepted, the growth probability factor corresponds very well with the weighted average of the growth probability factors a t each point in the reactor. In the present experimental work, it was found that the growth probability factor was almost independent of the hydrogen partial pressure, as shown in Figure 18, but increased with increasing carbon monoxide partial pressure, as illustrated in Figure 19. These trends correspond to
Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 69
'I
523 K 0 2 MPa
Po
CO
0 55
IWIF&)
lk'. k g S l m O l
1 I
I
05
oa
0:
06
09
10
P i InPo1
-
2
Figure 18. Growth probability as a function of partial pressure of hydrogen.
CL5
I
:
0 5'3
0
x
_h_ $5
$5
20
P,~VPO
Figure 20. Growth probability as a function of total pressure and temperature.
513 K
'
u 1
0
:
:
:
, 05
,
,
,
.
. .O
,
;
,
: 15
-
P?oIHPoI
Figure 19. Growth probability as a function of partial pressure of carbon monoxide.
those observed by Baerns (1983) for an iron-vanadium Fischer-Tropsch catalyst and by Bub and Baerns (1980) for an iron-manganese catalyst. The growth probability factor decreases with increasing reactor temperature, but remains unaffected by the total pressure, as shown in Figure 20. The Schulz-Flory distribution for the composition of the hydrocarbon products and the dependency of the growth probability factor on the temperature, the total pressure, and the partial pressures of carbon monoxide and hydrogen are important criteria in the kinetic modeling, as will be shown in part 2 of this work. 4. Conclusions From the work described above, the following conclusions can be drawn: 1. The major carbon-containing products of the Fischer-Tropsch synthesis on a precipitated promoted iron catalyst are carbon dioxide and hydrocarbons containing from 1to 15 and even more carbon atoms in the molecule. 2. The selectivity for carbon dioxide and for the various hydrocarbons is different from zero at zero carbon monoxide conversion. This phenomenon wm observed in the whole range of reaction conditions studied. 3. The initial rates of formation of carbon dioxide and the various hydrocarbons depend in a completely different way upon the partial pressure of carbon monoxide and hydrogen. The reactions leading to hydrocarbons and carbon dioxide are of a different nature. The reaction leading to carbon dioxide could be the water gas shift reaction, taking place mainly on the magnetite phase of the catalyst.
4. The initial rates of formation of the 1-olefins and of the n-paraffins with more than two carbon atoms are influenced in a very similar way by the partial pressure of carbon monoxide and hydrogen. It is likely that these products are formed according to the same mechanism. 5. The composition of the hydrocarbon fraction corresponds to the Schulz-Flory distribution, although the latter is strictly speaking only approximated in the case of integral reactors. 6. Application of the Schulz-Flory distribution functionalism to the actual hydrocarbon product distribution at the outlet of an integral reactor leads to a value of the growth probability which is a weighted average of the growth probability factors at every point in the integral reactor. 7. The growth probability was found to be almost independent of the total pressure and the hydrogen partial pressure, but to increase with increasing carbon monoxide partial pressure and to decrease with increasing temperature.
Acknowledgment
E.S.L. gratefully acknowledges the "Diensten voor Programmatie van het Wetenschapsbeleid" for financial support during the period 1982-1986. Nomenclature cy = growth probability factor in the Schulz-Flory distribution F O , = molar flow of reactant i at the reactor inlet (mol/s) PI = molar flow of component j at the reactor exit (mol/$ kco = rate constant in the homogeneous expression for the reaction rate of carbon monoxide (mol/ (kg.s.Pa"+")) kcO, = rate constant in the homogeneous expression for the rate of formation of carbon dioxide (mol/(kg.s.Pa"+Y)) m = exponent of the carbon monoxide partial pressure in the homogeneous rate expression (1) M,,= mole fraction of hydrocarbons with n carbon atoms in the molecule on the total amount of hydrocarbons formed n = exponent of the hydrogen partial pressure in the homogeneous rate expression (1);also number of carbon atoms in a hydrocarbon molecule p o l = partial pressure of reactant i at the reactor entrance (MPa) pt = total reactor pressure (MPa) R1 = rate of reaction of component j (mol/(kg.s)) RocO= initial rate of reaction of carbon monoxide (mol/(kgd) RoH2= initial rate of reaction of hydrogen (mol/(kg.s)) R O , = initial rate of formation of product j (mol/(kgs)) SI = selectivity for product j (mol formed/100 mol of CO converted)
70 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993
T = temperature (K) u = exponent of the carbon monoxide partial pressure in the
power law rate expression for carbon dioxide W = mass of catalyst in the reactor (kg) X c o = conversion of carbon monoxide (mol of CO converted/100 mol of CO fed) XHp= conversion of hydrogen (mol of H2converted/100 mol of H2fed) Xj= conversion to product j (mol of j formed/100 mol of CO fed) y = exponent of the water partial pressure in the power law rate expression for carbon dioxide
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Received for review March 25, 1992 Revised manuscript receiued September 16, 1992 Accepted October 5, 1992
