Ind. Eng. C h e m . Res. 1993, 32, 71-82
71
Kinetics of the Fischer-Tropsch Reaction on a Precipitated Promoted Iron Catalyst. 2. Kinetic Modeling Egbert S. Lox? and Gilbert F. Froment* Laboratorium uoor Petrochemische Techniek, Rijksuniuersiteit, Krijgslaan 281, B-9000 Gent, Belgium
A detailed kinetic model is derived using Hougen-Watson rate expressions for the Fischer-Tropsch reactions and the water gas shift reaction on a precipitated promoted iron catalyst. The model discrimination and the parameter estimation are performed according to the integral method of kinetic analysis on experiments described in part 1 of this series. In the construction of the kinetic model, the information gained from in situ catalyst characterization is used. The model proposed assumes different active sites for the water gas shift reaction on one side and for the reactions leading to n-paraffins and l-olefins on the other side. The rate expressions for the hydrocarbon-forming reactions are based on elementary reactions corresponding to the carbide mechanism, in the assumption that two kinds of elementary reactions, the ones describing the adsorption of carbon monoxide and these describing the desorption of the hydrocarbon products, are not a t equilibrium. The assumption that the active sites are almost completely occupied with surface hydrocarbon intermediates greatly simplified the kinetic expressions. The rate expression for the water gas shift reaction is based on elementary reactions involving a formate surface intermediate. The two-site reaction describing the formation of the formate intermediate is proposed to be rate determining. The values of the activation energies derived correspond well with data reported in the literature. 1. Introduction The goal of the work reported here is the derivation of detailed Hougen-Watson rate expressions for the major reaction paths in the Fischer-Tropsch synthesis on a commercial precipitated promoted iron catalyst. For this aim, steady-state kinetic experiments were carried out in a tubular reactor under non-deactivating conditions. The catalyst was characterized before, during, and after the reaction, to provide information required in the construction of the kinetic model. The kinetic experiments have been described and discussed in part 1 (Lox and Froment, 1993). The catalyst characterization was published previously (Lox et al., 1988). The results indicated that carbon dioxide, which was one of the major carbon-containing products in the range of experimental conditions studied, was probably formed by the water gas shift reaction: CO H2O s C02 H2 (1)
+
+
which takes place on the iron-oxygen (Fe,O,) sites of the catalyst. Methane, l-olefins, and n-paraffins with two and more carbon atoms in the molecule were found to be primary carbon-containing products of the reaction. These products are probably formed according to the following reactions: n-paraffins: nCO + (2n + l)H2 CnHZn+2+ nH2O l-olefins: nCO + 2nH2 ~i CnH2, + nH2O
(2)
(3) These reactions probably take place on a different kind of active sites than the water gas shift reaction. The active sites for the hydrocarbon-forming reactions could be the iron carbides, which were found to occur in the catalyst during use. It was also found that the composition of the hydrocarbon product fraction could be described by the
* Author to whom correspondence should be addressed.
Present address: Inorganic Chemical Product Division, De-
gussa A.G., P.O. Box 1345, D6450 Hanau-l, Germany.
0888-5885 193f 2632-0071$04.00/0
“Schulz-Flory” distribution. This observation has profound implications on the choice of the mechanism of the hydrocarbon-forming reactions, as well as on the rate and the value of the kinetic parameters, as will be shown below. The kinetic modeling was performed according to a strict methodology. Once the primary products of the reaction were identified, the functional relationships between the products and between the reactants and the produds were fixed in a number of reaction paths. Then, for each of the reaction paths, several sets of elementary reactions were chosen on the basis of literature data. An elementary reaction was considered to be either a reaction in which one bonding is made or destroyed, or a reaction in which a bonding is made while another is destroyed (Boudart, 1968). To model the rate rj along a reaction path j , several simplifying assumptions are discussed and applied when appropriate. The assumption of steady-state conditions for the catalyst composition and for the concentration of the reactive intermediates on the catalyst surface was generally applied. However, the assumption that one of the elementary reactions in each of the reaction paths is rate determining in the whole range of experimental conditions is discussed, especially for the hydrocarbon-forming reactions. Finally, kinetic expressions for the rate of formation were derived for all the products considered. The estimation of the value of the kinetic parameters in the respective models was performed according to the integral method of kinetic analysis (Froment, 1975; Froment and Bischoff, 1990). This methodology was applied first to the experimental data at each of the reaction temperatures separately and finally to the complete set of experimental data. The discrimination between the rival models is based upon the goodness of fit, supplemented with statistical tests on the parameter values and the physicochemical meaningfulness of the estimated parameter values. 2. Reaction Networks I t was mentioned above already that carbon dioxide is formed through the water gas shift reaction (1). Water is formed in the hydrocarbon-producing reactions (2) and (3). The water gas shift reaction is a parallel-consecutive reaction path for carbon monoxide consumption relative to the reaction paths leading to the hydrocarbon products, 0 1993 American Chemical Society
72 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 Hq
f
+
COq
t
Figure 1. Reactions of carbon monoxide and hydrogen. CO
+
Hq
CH3-I
CH4 C2H4 C2H6 CnH2n CnH2n+2 Figure 2. Reaction paths leading to the hydrocarbon products.
as illustrated in Figure 1. For the reaction paths leading to the various hydrocarbons, the scheme shown in Figure 2 was retained. These relationships were chosen on the basis of the observation that methane as well as n-paraffins and 1-olefins with two and more carbon atoms in the molecule and 1-olefins with two and more carbon atoms in the molecule are primary products of the FischerTropsch synthesis. Furthermore, they take into account that the Schulz-Flory distribution can only be observed when these primary products do not participate in secondary reactions other than hydrogenation of the olefin to the corresponding paraffin or isomerization reactions of the olefins and the paraffins. Novak et al. (1981) and Novak and Madon (1984), for example, have calculated that when ethylene participates in secondary reactions other than the ones mentioned or when cracking of the primary products occurs, the composition of the hydrocarbon product fraction deviates from the Schulz-Flory distribution. From the experimental data presented in part 1, it follows that the partial pressure of hydrogen and carbon monoxide influences the initial rate of formation of paraffins and olefins almost independently of the number of carbon atoms in the respective molecules. Also, the selectivity for the olefins with three and more carbon atoms in the molecule as well as the selectivity for the paraffins increased with increasing carbon monoxide conversion in a very similar fashion, independent of the number of carbon atoms in the molecule. Furthermore, the initial rate of formation of a paraffin and the initial rate of formation of an olefin are influenced in a similar way by the partial pressure of carbon monoxide and by the partial pressure of hydrogen. These observations support the assumption that paraffins and olefins with varying number of carbon atoms in the molecule are formed along parallel reaction paths. It should be stressed that, although the hydrocarbon intermediates are built up consecutively by addition of one carbon atom at a time, the overall functional relationships between carbon monoxide and hydrogen on one side and the various hydrocarbons on the other side operate in parallel. 3. Kinetic Models 3.1. Nature of the Active Sites. Several authors proposed that the iron-oxygen phase (Fe,O,) of the catalyst is the most active phase for the water gas shift reaction (Madon and Taylor, 1981; Schulz et al., 1984; Grenoble et al., 1981; Oki and Mezaki, 1973a,b). According to Madon and Taylor (1981) the water gas shift reaction could also
proceed on the iron carbides, but with a much lower rate than on the iron oxide. Zhang and Schrader (1985) concluded from a laser Raman spectroscopic study that carbon monoxide adsorbed both on the iron carbide and on Fe304,but that the adsorption on Fe304did not lead to dissociation of carbon monoxide. These authors therefore concluded that the hydrocarbon-forming reactions would mainly proceed on the iron carbide. In the present experimental work, it was shown that an iron-oxygen phase (Fe304)coexists with various iron carbides on the catalyst under working conditions (Lox et al., 1988). During the course of the kinetic modeling, models based upon one single active site were also derived. These models failed completely in describing the experimental data. Therefore, it was assumed for the further kinetic modeling that the water gas shift reaction and the hydrocarbon-forming reactions proceed on different active sites. 3.2. Kinetic Models for the Hydrocarbon Formation. 3.2.1. Discussion of Elementary Reactions. Numerous publications have dealt with the mechanism of the hydrocarbon-forming reactions (Pijolat, 1983; Deluzarche et al., 1982; Rofer-DePoorter, 1981; Biloen and Sachtler, 1981; Dry, 1981; Kummer et al., 1948). In the present work, the so-called "carbide" mechanism, proposed by Fischer and Tropsch, was chosen as the basis for the selection of the elementary reactions (Fischer and Tropsch, 1923). The key feature of this mechanism is that the hydrocarbons are built by successive addition of building unita with one carbon atom and no oxygen in the molecule. Five groups of elementary reactions are discussed. The first group of elementary steps describes the adsorption of carbon monoxide. The adsorption of carbon monoxide was supposed to be molecular, proceeding either on a free active site: co + 12 e co=12 (4) or on an active site which already contains a hydrocarbon intermediate: CO + 12-CnH2n+le CO=12-C,H2,+1 (n = 0, ...) (5) (Chen et al., 1983; Barrault et al., 1982; Bell, 1981; Vannice, 1976; Joyner and Roberts, 1974; Blyholder, 1964). The second group of elementary reactions deals with the reaction of hydrogen. It was supposed that hydrogen reacta either in the molecular state or via dissociative adsorption (Zhang and Schrader, 1985; Nahon et al., 1979; Ponec and van Barneveld, 1979; Vannice, 1976). I t was assumed that the dissociative adsorption takes place on two adjacent free active sites: H2 + 212 s 2H-12 (6) For the reaction of molecular hydrogen the following alternatives were considered: (i) reaction with a hydrocarbon intermediate: H2 + 12-CnH2,+i * H-12 + CnH2,+2 (n = 0, ...) (7) (ii) reaction with adsorbed carbon monoxide: 12=C=O
+
H2 Sr I2=C-O
I
H
I
( 8)
H
and 12=C-O
I
H
I
-t H2
12=CH2
+ H20
(9)
H
(iii) reaction with adsorbed oxygen: l2=0 H2 s l2 + H 2 0
+
(10)
Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 73 Table I. Sets of Elementary Reactions for the Reaction Paths Leading to the Hydrocarbon Products set number elementary reaction set number elementary reaction HCI HC1 C O + L F ? CO-1, HCIV HCl(n) CO + H-12 s H-lz-CO HC2 HC3 HCZ(n) HC4 HC5(n)
HCV
HC3(n) HC4(n) HC5(n) HC6(n) HC7(n) HC8 HC9 HCl(n)
HCVI
HC2(n) HC3(n) HC4(n) HC5(n) HC6(n) HC7 HC8 HCl(n)
HC6 HC7 HC8(n)
HCII
HC9(n) HClO HC1 HC2 HC3 HC4(n) HC5 HC6 HC7(n)
HCIII
HC8(n) HC9 HCl(n) HC2(n) HC3(n) HC4(n) HC5(n) HC6(n) HC7
HCVII
The third group of elementary reactions describes the formation of the hydrocarbon building block "CH2". It is supposed that the hydrocarbon building block is formed by a sequence of dual site reactions, proceeding on free active sites: 1 2 4 0 + 12 F? 1 2 4 + 12=0 (11)
+ 12-H 1 2 4 H + 12 1 2 4 H + 12-H * 12=CH2 + 12
(12) (13)
1 2 4
or on active sites already containing a hydrocarbon intermediate: CnHzn+1-12+0 + 12 F? CnH2,+1-12=C + 12=0 (n = 0,...) (14) CnH2,+1-12=C CnH2,+1-12=CH
+ 12-H
F?
+ 12-H
CnH2,+1-124H + 12 (n = 0,...) (15) CnH2,+1-124H2 + 12 (n = 0,...) (16)
Alternatively, it is also considered that the hydrocarbon building block could be formed by a sequence of single-site reactions, proceeding either on free active sites according to reactions 8 and 9, or on active sites already containing an hydrocarbon intermediate: C,4+,7,7-I2=CO
+ H2
Cl)l2n+i--Iz=C-O
I
H
I
H
( n = 0,...)
(17)
