Ind. Eng. Chem. Process Des. Dev. 1986, 25, 643-649
643
Kinetic Studies of Fischer-Tropsch Synthesis on Suspended Fe/K Catalyst-Rate Inhibition by C02 and H20 Wolf-Dieter Deckwer, Ryoji Kokuun, E. Sanders, and S. Ledakowicrt Fachbereich Chemie, Universitat Oldenburg, 0-2900Oldenburg, Federal Republic of Germany
FTS was studied in a stirred autoclave on a potassium-promoted precipitated Fe catalyst in the slurry phase. The rate data measured under variations of temperature and flow rate at low H2/C0 inlet gas ratios ( I I0.8) can be fitted with a kinetic law which accounts for competitive adsorption of COP. At higher H2/C0 inlet ratios, only part of the product water is converted by the shift reaction. Under such conditions, rate inhibition by water is observed and the syngas conversion rates can be described fairly well by a rate expression proposed recently.
The FTS in the slurry phase offers a number of advantages over other process modes (Kolbel and Ralek, 1977; Gray et al., 1980; Thompson et al., 1981; Kuo, 1983). In particular, slurry-phase operation permits direct conversion of synthesis gases of low H2/C0 ratio as generated from advanced gasifiers. For the reliable design of FTS slurry reactors, knowledge of the kinetics of this complex reaction system is required. In recent studies (Kuo, 1983; Nettelhoff et al., 1985; Ledakowicz et al., 1985), it was assumed that the hydrocarbon synthesis reaction can be lumped to the following scheme
9
CO + 1 + - Hz
(
-+
CH,
+ HzO
Table I. Kinetic Models for t h e FTS in Slurry Phase rate eq
-~co+H,, mol/@ s)
model
b~ CH2
1+
CO
+ HzO
COZ
+ Hz
~
b2CC02/CC0
blCH,
cco,
rl being the reaction rate of the FTS. Due to the byproduct HzO,the FTS is followed by the consecutive reversible water gas shift reaction
b1
1
(2)
+
b,-
Huff, Satterfield, 1 9 8 4 a Ledakowicz et al., 1 9 8 5
CH" ~~~
1 + b2-
r1
rz
Anderson, 1 9 5 6
b2CH20/CC0 bl
ref
CH,O +bCCO 'CCO CH2
Nettelhoff et al., 1 9 8 5 Ledakowicz e t al., 1 9 8 5 Nettelhoff et al., 1 9 8 5
Huff, Satterfield, 1984a
CH20 CCOCH2
r3
where r2 and r3 are the rates of the forward and backward reactions, respectively. From eq 1 and 2, one can derive
Therefore, irrespective of the extent of the shift reaction, the rate of the FTS follows from the overall synthesis gas (CO H2) consumption rate. On conventional catalysts without shape selective properties, the product slate of the FTS can be approximated by the Schulz-Flory distribution with regard to the C number mass fraction. Hence, the rate of product formation, i.e., products with C number m,is given by rC, = -rCO+H2ZWm (4)
+
where z is a factor which relates the mean mass of the products with the moles synthesis gas consumed (g of products/mol of syngas). W , is the relative mass fraction as predicted from the Schulz-Flory distribution with knowledge of the chain-growing probability. The major kinetic models for syngas consumption rates during FTS are summarized in Table I. Models A, B, and C can be derived from the enol complex theory of Anderson (1956), while model D follows from both the CO
* Author
to whom all correspondence should be addressed. Present address: Chemical Engineering Department, Politechnika Lodzka, Lodz, Poland.
0196-4305/86/1125-0643$01 SO10
insertion mechanism (Pichler and Schulz, 1970) and the carbide theory (Rofer-DePoorter, 1981) by making appropriate assumptions and simplifications. This was shown by Huff and Satterfield (1984a). The kinetics of the FTS in the slurry phase was investigated by Huff and Satterfield (1984a) by employing a reduced fused magnetite catalyst-a conventional catalyst for ammonia synthesis. These authors report that their comprehensive rate data covering a wide range of experimental conditions can be described excellently by model D. Nettelhoff et al. (1985) studied the FTS in the slurry phase on a precipitated Fe catalyst and a fused magnetite catalyst as well. For the unpromoted precipitated catalyst, rate inhibition by product water probably due to competitive adsorption was observed. However, the rate data did not permit to discriminate among rival mechanistic views, i.e., model A and D, respectively. The magnetite catalyst was triply promoted with KzO, CaO, and A1,03. It was assumed that as a result of this promotion, this catalyst would have a high activity with regard to the water gas shift (WGS) reaction. Therefore, at low H 2 / C 0 inlet ratio, the water concentration was low and an inhibition by water could not be observed. Instead, the rate data indicated that weak adsorption of CO, may take place and thus the reaction rate decreases. Already Brotz and Rottig (1952) speculated on an inhibitory action of COz in as far as these authors assumed a rate resistance caused by the sum of the oxidizing gases (HzO + CO,). 0 1986 American Chemical Society
644
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986
Ledakowicz et al. (1985) studied the FTS in the slurry phase on a K-promoted precipitated Fe catalyst. In this study, the Hz/CO inlet ratio was kept low, Le., I S 0.8. Owing to the low inlet ratio and the high WGS activity of the K-promoted catalyst, the water concentration in the reaction mixture was very low and could be neglected, approximately. However, even under such conditions, the rate data did not follow a first-order law in H2 as expected from models A and D. Instead, rate inhibition by competitive chemisorption of CO, was introduced which describes the measured data well. This is in agreement with the more qualitative conclusion of Nettelhoff et al. (1985) for the promoted fused Fe catalyst. Therefore, these authors as well as Ledakowicz et al. (1985) assumed that the rate of the FTS in the slurry phase may be inhibited by chemisorption of both water and C02, i.e., model C. Model A of Anderson (1956) has not been shown to be valid unambiguously for the FTS in the slurry phase. However, its applicability to fixed bed studies has been demonstrated by several investigations (Dry, 1976; Atwood and Bennett, 1979; Thompson et al., 1980). Model C has not been proven experimentally, either. The purpose of the present study was to continue the work of Ledakowicz et al. (1985) with a K-promoted precipitated Fe catalyst. These authors reported only on a limited number of data points for one batch of catalyst as strong wax formation filled up the autoclave vessel. In addition, Ledakowicz et al. (1985) varied the reaction rate merely by the temperature and gas flow rate but not by syngas composition. Therefore, the aims of this work were to confirm the results of Ledakowicz et al. (1985) and to check the reproducibility of FTS rate data by using various batches of catalyst from two precipitations as well as to study the inhibitory effect of water by using H2/C0 inlet ratios from 0.5 to 2 and by introducing additional water into the reaction mixture. It was thought that on the basis of the measured data, a discrimination among the models of Table I would be possible and a uniform data description can be found, possibly by a model which involves chemisorption of both H 2 0 and COz, i.e., model C.
Table 11. Summary of Experimental Runs batch of catalyst (precipitation) varied conditions I(A), II(A), III(B), IV(A), V(A)," VI(A)," 493 5 T 5 533 K 0.5 6 VO, 5 4 L/min WB) I 5 0.8 493 6 T 6 533 K VII(A) 0.5 5 I6 2 VOc = 1.5 L/min VIII(A) 493 5 T 5 533 K 0.5 5.I 6 2 1 5 vOG 5 2.5 L/min Some runs with water in feed gas.
Ledakowicz et al. (1985) which were included in the data analysis of this study. In the runs with batches I-VI and IX as well, the H,/CO inlet ratio was kept low in the range of 0.7-0.8 as this is the range where application of FT slurry reactors appear particularly promising. However, in series VI1 and VIII, the Hz/CO inlet ratio was varied from 0.5 to 2. In order to detect the inhibitory action of water, some runs of serious V and VI were carried out with water vapor in the inlet gas flow. This was done by saturating the inlet gas with water at different temperatures (Kokuun, 1985). All experiments were done at a pressure of 1MPa. The stirrer speed was kept at 600 rpm, where any mass-transfer limitation can be neglected (Ledakowicz et al., 1984). The synthesis gas was composed of Hz, CO, and Nz (~10% by volume). Inert Nz was added to account for the volume contraction. The gas composition at the autoclave inlet and outlet was determined by gas chromatography. The synthesis gas conversion was calculated from the measured inlet and outlet mole fractions of H2,C02,and N2 Yk, YE0 + Y k , (5) X C O + H 2 = 1 -Yk, Y80 + YOH, The rate of synthesis gas consumption (in mol/(g of catalyst s)) follows therefrom as QXCO+H2
(6) "tVM60 The water mole fraction at the reactor outlet was calculated from the relation given by Ledakowicz et al. (1985). As outlined by Ledakowicz et al. (1985),mass balances over the experimental setup were checked after finishing a series of runs with one batch of catalyst in order to account for the higher hydrocarbons (wax) which accumulated in the slurry phase. Balances agreed within 1.4%. As the reaction takes place on the catalyst surface in the slurry phase, the rate laws are formulated with concentrations, which requires knowledge of Henry constants. Ledakowicz et al. (1984) have shown that the solubilities of Hz and CO reported by Peter and Weinert (1955) can be applied to the liquid wax (Vestowax SH 105) used in this study. It is therefore assumed that also the solubilities of H 2 0 and COz can be calculated from Peter and Weinert's data using the measured densities of the liquid wax used in this study. -rCO+H2
Experimental Section The measurements were carried out in a 1-L stirred autoclave unit with continuous flow of the gas as described by Ledakowicz et al. (1985). Also the analytical devices and techniques were the same as used by Ledakowicz et al. More experimental details are reported by Nettelhoff (1985) and Kokuun (1985). Two precipitations of the Fe catalyst promoted with 1.3 w t % potassium (referred to Fe) were employed in this study. The catalyst was precipitated in a continuous-flow apparatus as described by Deckwer et al. (1982). One hundred grams of unreduced catalyst (550 pm) was suspended in 350 g of molten wax and activated in the slurry phase (Ledakowiczet al., 1985). The catalyst employed produced large amounts of higher hydrocarbons, i.e., wax. As withdrawal of the wax without simultaneous withdrawal of the suspended catalyst fines turned out to be very difficult, the product wax was allowed to accumulate in the autoclave. Therefore, only a limited number of runs could be performed with one batch of catalyst. On the whole, eight batches of catalyst (11-IX) from the two precipitations (A and B) have been applied. The reaction rate was altered by variations of the temperature (493-533 K), syngas flow rate (0.5-4 nL/min), and Hz/CO inlet ratio. Table I1 summarizes the conditions which were varied in each series of runs with one batch of catalyst. Batch I in Table I1 refers to the data already reported by
=
Experimental Results In Figures 1-5, some representative results are shown. Syngas conversions for different catalyst batches and four temperatures are given as a function of the inlet flow rate in Figure 1. The syngas consumption rates derived therefrom are shown in Figure 2. As expected, the rates increase with rising flow rate, while the conversions decrease. At low flow rates, high conversions are observed; however,
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986 645 1.0 X
0.8
0,6
0.L -
3
2
1
L
Vi
, NI lmin
Figure 1. Syngas conversion vs. inlet flow rate (symbols as given in Figure 2).
8 I2
'
10
,
I
0.6
-
1
I
1
I
FelK ( V l )
. bl
m
z,
n
z
-6 *
4
FoIK
e
v m , 533 K 0 V I , 533 K 6
OV.523K
m n,523~ X
L
V , 513 K
-
0 I V , 513 K
1
A VI , 503 K
2
3
2
\
V', . N Itmin
r
1 Fe/K (VIII)
:V
= 1 . 5 Nllmin.
J
L
5 ' 1
2
I H, IC0
H2 IC0 I,
Figure 5. Mole fraction of COz in the outlet gas as function of the Hz/CO inlet ratio.
L
Figure 2. Syngas consumption rates vs. inlet flow rate for different batches and temperatures.
T=523K
2 (
I ,075-0 8
'A
10
Figure 3. Consumption rate of synthesis gas as a function of the H2/C0 inlet ratio.
the differences in the rates measured at various temperatures become small. Therefore, more reliable rate data are obtained at higher flow rates. As can be discerned from Figures 1 and 2, data reproducibility with different batches from different precipitations is good. Only the data of batches I1 and V differ slightly at T = 523 K which will be explained later. The effect of the H 2 / C 0 inlet rate is demonstrated in Figures 3-5. With an increasing inlet ratio, the rate decreases as shown in Figure 3. The decrease of the overall rate is accompanied by a drop of the syngas conversion plotted in Figure 4. Also shown are the conversions of H2 and CO. While Xco rises slightly with an increasing inlet
ratio, a strong reduction of XH2is observed. At (Hz/CO)o = 0.5, the conversions of CO and Hz are almost the same. This indicates that product water is converted by the WGS reaction to a large extent at low inlet ratios. This is also evident from Figure 5, where the COPoutlet mole fraction is plotted vs. the Hz/CO inlet ratio. The high WGS activity of the catalyst used in this study at low H2/C0 inlet ratios was already analyzed and discussed in more detail by Ledakowicz et al. (1985). By comparing the synthesis gas consumption rate with the rate of COz production, Ledakowicz et al. especially concluded that the rate of the shift reaction is larger than the rate of the FTS at low values of the inlet ratio ( I < 0.8). In general, alkali-promoted Fe catalysts show high activities for the WGS reaction. This observation led Kolbel and Engelhardt (1951,1952) to the development of a hydrocarbon synthesis from CO and HzO alone. In those runs with batches V and VI, where water vapor was in the feed gas ( I I0.8),the water feed rate was changed from 0.002 to 0.01 mol/min, while the water generated in the synthesis reaction varied from 0.007 to 0.015 mol/min. It was observed that 7042% of the sum of the feed and product water was shifted by the catalyst. Hence, the WGS reaction takes place to a large extent but not all the water is converted, which leads to water mole fractions up to 0.14 in the reaction mixture. Kinetic Analysis The rate data were analyzed by nonlinear regression involving models A-D. The optimization procedure of
646
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986
Table 111. Summary of Results from Rate Data Analysis data from batch I(A), III(B); IV(A), II(A), IX(B) VIII(A) VIII(A) (precipitation) V(A); VI(A), VII(A) varied conditions T, VOG, I S 0.8 T , VOG, I 5 0.8 T,VO,, I 5 0.8 T , I 2 0.8 no. of exptl data point fit of data by model mean error, (70 blo, cm3/(g s)
EA, kJ/mol b20a 9Adsi
kJ/mol
77 B 8.3 2.583 X 10'O
34
B 10.7 6.795
X
lo9
11 B 5.9 5.22
V(A)*, VI(A)*; VII(A)**
* T ,I 5 0 . 8 but water in feed **T, I 2 0.8 25 D 12.9 (8.3) 0.715 X 10' (1.138 X
21 D X
10"
12.2 1.155 X
105 0.126
105 0.0515
105 0.189
80.4 3.052
0
0
0
54.9
X
Huff and Satterfield, 1984a
lo8
D 0.277 X lo8
106) 80.4 (63.0) 2.065 X lo-'' (1.28 X 10-9) 54.9 (35.3)
lo-"
77.9 9.17
X
82.8
b20 is dimensionless in model B; in model D, bzo is in mol/cm3.
/ 1i
Model
B
T
- 533 K
i
L93
01
3
E
Model 8 Series II and IX 1 I L93 - 533 K
12
P .J
P
i
IO
-8 '
0
8
0 0
d
6
#Q
A 4
2
5
IO
-10'
15 rC0.H2
c.,c
./t
/
7."' 2
6
L
mol / g s
8
10 -".PI,
12
li
sa1c ,mo' ' g
Figure 6. Parity plot of measured and calculated rates, (7) (experimental conditions given in Tables I1 and 111).
Figure 7. Comparison of experimental and predicted rates. Runs with batches I1 and IX described by (8).
Marquardt was used (Kuester and Mize, 1973). At first, the experimental data of a series of runs with one batch of catalyst and a constant temperature were taken into account to determine the model parameters bl, b2, and bS. By inspection of the temperature behavior of the optimized parameters and consideration of the average error between observed and predicted rates, a discrimination among the rival models could be achieved. Model B successfully describes the rate data obtained from various batches at low H2/C0 inlet ratio ( I I0.8). However, not all the data of different batches of catalyst can be matched with one set of parameters bl and bz. Indeed, the data fall into three groups. The experimental data of series I, 111,IV, V, VI (V and VI without those data with water in the feed gas), and VI1 as well (as long as I I0.8) fall into one group which can be fitted by 2.583 X 1010e-12 569/TC HZ
energy was practically the same as observed for the other batches (I and 111-VI). Therefore, the same value of 105 kJ/mol was used. The regression then leads to
-
1
+ 0.126Cc0,/Cc0
(7)
A parity plot is given in Figure 6. The 77 data points can be fitted by (7) with a mean error of 8.3%. It should also be noticed that the fit includes catalyst batches from different precipitations. The optimized parameters in (7) are very close to those values obtained from batch I by Ledakowicz et al. (1985). The range of conditions covered in the measurements to generate the rate data used to establish (7) and the following correlations can be taken from Tables I1 and 111. The data of series I1 and IX can also be fitted best with model B. However, the activity of these two batches from precipitations A and B is smaller, though the activation
=
6.795 X 569/TC H2 1 -t o.o515C~o,/cco
(8)
A parity plot of experimental and predicted rates for these two series (I1 and IX) is given in Figure 7. The 34 data points are fitted with an average error of 10.7%. Equation 8 shows that not only the preexponential factor has been reduced in comparison to (7) but also the value of b2 is smaller. Model B could fit again those data of series VI11 carried out at a low H2/C0 inlet ratio under variation of the gas flow rate and temperature. However, another set of parameters was necessary to give a good fit. Using the same activation energy as in the other series fitted with model B, an excellent description of the experimental data could be obtained with 5.22 X 10'0e-12569/TC HZ -rCo+Hz
=
1
+ 0.189Cc0,/Cc0
(9)
By comparison with (7) and (S), it is seen that this batch of catalyst (VIII) has a higher activity. Also the ratio of the adsorption constants (b,) is enlarged. However, it was not possible to obtain a reasonable description of those data of series VI1 and VI11 measured at higher Hz/CO inlet ratios by using model B. It was believed that model C, which can be regarded as a combination of models A and B, would be appropriate to de-
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986 647
*1
I
Fe / K
vni
Runs with vorioble(H2 /CO),
1.9
2.0
2
-106rc0.H2,C.,. , m o l / g 5
Figure 8. Arrhenius diagram for optimized values of bl and bp.
scribe the data. However, the temperature behavior of the optimized b2 and b3 in this model was chaotic. In some cases, b2 or b3 became negative. Therefore, this model (C) as well as model B was discarded. This leaves only models A and D. By comparison of the fits obtained for data at constant T under variation of the inlet ratio, model D turned out to be better than model A. In addition, a reasonable temperature dependency was observed for the parameters bl and b2 in model D. This is shown in Figure 8 for the values of bl and b2 obtained from runs of series VIII. From the Arrhenius diagram, an activation energy ( E A ) of 80.4 kJ/mol is obtained for bl, which is again in the reasonable range of rate constants for the FTS. b2 can be interpreted as a function of adsorption constants which decreases with temperature. From the data shown in Figure 8, an apparent heat of adsorption (qA&) of 54.9 kJ/mol can be derived. When these values of E A and q A & are used, the data of series VI11 were resubjected to regression. The best fit is given by -'CO+Hz
1.155 x 108e-9667/~ H2
=
1
CH20
(10)
+ 3.052 X 10-11e6600/TCCOCH2
As model D proved to give a reasonable description of the experimental rates of series VIII, a new attempt was undertaken to reanalyze the data of series VI1 and those of series V and VI with water vapor in the inlet gas on the basis of model D. In this regression analysis, all four constants, i.e., the activation energy, an apparent heat of adsorption, and both preexponential factors, were fitted simultaneously by applying transformation of variables to reduce the interaction of parameters (Himmelblau, 1970). Out of 28 experimental points obtained from these 3 batches of catalyst, 25 data could be fitted to -rCO+H,
1.138 X 106e-7581/TC H2
=
1
8
6
L
1 0 ' / ~ , K-'
CHzO + 1.28 X 10-9e4242/TCCOCH2
(11)
The average error is 8.3% and the parity plot shown in Figure 9 illustrates that a fair description could be achieved for the data when rate inhibition by product water is predominant. In Figure 10, the data are plotted as to a linearized version of model D. For the same temperature, the experimental data from the three batches of catalyst can be approximated by straight lines. The activation
Figure 9. Fit of experimental data of series V and VI with water in inlet gas and series VI1 by (11).
/A
8
503 K
o v
5 "
A
VI
0
VI1
6
L
2
I 10
20
30
50
LO
10.'
c,lo , cm'/ CH,
60
mol
Cm
Figure 10. Plot of data as to linearized version of model D.
energy is only 63 kJ/mol, which is at the lower limit of values expected for the FTS. It is important to point out that the same 25 data of series V, VI, and VII, where water adsorption is relevant, can also be described fairly well by setting the activation energy and the apparent heat of adsorption to the values obtained from Figure 8 for the data of series VI11 and used to develop the rate law given by (10). In this case, one obtains -rCO+Hz
0.715 x 108e-9667/~ H2
= 1
(12)
CH20
+ 2.065 X 10-11e6600/r-
CHZCCO
Equation 12 fits the water-inhibited rate data of series V, VI, and VI1 by a mean error of 12.9%. Though the average error is larger compared to the fit obtained by (ll),it is thought that (12) may be a more reasonable fit from the physical point of view as the overall consistency of the water-inhibited rate data is improved, which is evident by comparing (10) and (12). Discussion and Conclusions Unfortunately, the rate data measured in this study could not be described by a uniform kinetic model. In addition, the nine batches of K-promoted Fe catalysts from two precipitations (including batch I of Ledakowicz et al.
648
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986
(1985)) did not give the same activity. The various rate laws proposed to fit the experimental data of this study are summarized in Table 111. For the case of low H2/C0 inlet ratios ( I I0.8), all the experimental data could be described by a macrokinetic law of equal structure, i.e., model B which accounts for rate inhibition by COz adsorption. All the data from six batches (I, 111, and IV-VI1 as well, provided I I0.8) can be comprehended by the same rate law, i.e., (7). Two batches (I1 and IX) showed less activity, but the data of both series of runs can again be fitted by the same rate law, namely (8). A higher activity was observed only with batch VIII, as can be discerned from the value of the rate constant of (9). As the series of batches with medium activity (eq 7) and low activity (eq 8) contain batches from both precipitations, the activity discrepancy cannot be attributed to the preparation procedure of the catalyst precursor. It can therefore only be speculated that little but undetected differences in the activation of the precursor may have a crucial effect on the activity pattern. It is, however, interesting to notice that the temperature dependency of the rate constant of the rate-limiting step is the same for all batches. All the data could be fitted fairly well with an activation energy of 105 kJ/mol. The analysis of the data of this experimental study confirms the conclusions of Ledakowicz et al. (1985) that the syngas consumption rate of the FTS can be inhibited by competitive adsorption of CO and C02. All the runs carried out at low H2/C0 inlet ratios ( I I0.8) can be interpreted by accounting for C02 adsorption. Obviously, at low values of I , most of the product water is converted by the WGS reaction and inhibition by water adsorption on the surface sites is negligible. On the basis of the enol complex theory, the constant b, in the various kinetic models of Table I can be interpreted as the ratio of the adsorption equilibrium constants of C02and CO (Kco,/ Kco). During data fitting, there was no need to introduce any temperature dependency for b2. Obviously, b2 is independent of T which is in accordance with the findings of Dry et al. (1969), who observed almost the same heats of chemisorption for CO and C02on Fe catalysts promoted with potassium. As indicated by the optimized value of b2 = KCO2/KCO, adsorption of C02 is weaker than that of CO. However, with increasing catalyst activity as expressed by the constant of the rate-limiting step ( b l ) ,also the adsorption capability of COz increases, as compared to that of CO, cf. (71, (81, and (9). If higher H2/C0 inlet ratios are used and if water vapor is introduced with the inlet feed gas into the reaction medium at a low inlet ratio ( I I0.8), rate inhibition by water adsorption comes into play as only part of the water produced by FTS and/or fed to the reaction system is shifted. However, under such conditions, it is not model C which applies, but model D which gives a better and more consistent description of the data of series V-VIII. Again, batch VI11 turned out to be of higher activity as was already found when employing model B to match the data obtained at low H2/C0 inlet ratios. The rate data from series V, VI, and VI1 can be described by one rate expression, i.e., (12). The same result was obtained when applying model B. It was not possible to fit the rate data by a kinetic model which involves simultaneous adsorption of both C 0 2 and H 2 0 (in addition to that of CO); though C02adsorption has to be considered at low H2/C0 ratios, it appears negligible at larger H 2 / C 0 feed ratios, where water adsorption is predominant. Physical interpretation of this result is difficult, as derivation of mechanistic conclusions from kinetic data alone usually is not allowed.
Table IV. Comparison of Kinetic Parameters with Literature Data this study, (5) Huff and Satterfield, 198413 bl, 105b2, K b l , cm3/(g s) 105b2,mol/cm3 cm3/(g s) mol/cm3 493 0.154 5.50 0.352 1.99 513 0.324 2.50 0.756 1.18 533 0.643 1.21 1.534 0.73
r,
However, the findings of this kinetic analysis indicate that none of the leading mechanisms of the FTS can be excIuded. In addition, one can speculate that at low H,/CO ratios, enol complex theory accompanied by COz adsorption prevails, while at larger H2/C0 ratios the CO insertion mechanism and/or carbide theory, respectively, seem to be more appropriate. The result that in the presence of water a better description of the data can be achieved by model D rather than by model A or C is in agreement with the conclusions of Huff and Satterfield (1984a), who studied a fused magnetite catalyst in the slurry phase, as well. Huff and Satterfield report for their catalyst an activation energy of 82.5 kJ/mol for the rate constant of the limiting step. For the catalyst used in this study, the activation energy of the same constant is found to be 80.4 kJ/mol. The rate law given by Huff and Satterfield (1984a) is formulated with partial pressures. Hence, the activation energy includes the temperature dependency of H2solubility. If one corrects for that, the activation energy reported by Huff and Satterfield reduces to 77.9 kJ/mol which agrees well with the value of this study. The constant b2 (in mol/cm3) of model D corresponds with the value of 1/b’ (in kPa) of the work of Huff and Satterfield (1984a). These authors found for their catalyst (1984b) b2/ = 1.053 X 10-7e12066/T = l/b’
in kPa. Converting from kPa to mol/cm3 by considering Henry’s constants of H2,CO, and H20gives for the catalyst of Huff and Satterfield b, = 9.17 X 10-14e9965/T
in m o l / c d . All the kinetic parameters of Huff and Satterfield (1984b) are also given in Table I11 in the dimensions used in this work. Table IV compares numerical values of bl and b2 from the two studies with different catalysts for three temperatures. bl can be interpreted as the rate constant of the limiting step. The bl of this study is greater by a factor of about 2. This is not unexpected as precipitated catalysts usually have a higher activity than fused catalysts. b2 can be interpreted as a function of adsorption constants. The b2 values of Huff and Satterfield are larger than those of this study. A more sophisticated interpretation of b2 on the basis of various mechanisms is not unambiguous. Indeed, b2 should be regarded as a fitting parameter as long as adsorption data on suspended catalysts, particularly for water, are not available. On the whole, the analysis of the water inhibited rates measured in this study at I 5 0.8 confirms the results and conclusions reported by Huff and Satterfield (1984a). In addition, the kinetic parameters of both studies with different catalysts show a surprising agreement. Acknowledgment
We gratefully acknowledge support from Deutsche Forschungsgemeinschaft and German Federal Ministry of Research and Technology. S. L. thanks Alexander von
Ind. Eng. Chem. Process Des. Dev. 1086, 25, 649-654
Humboldt-Foundation for granting a research fellowship. Nomenclature
bl, b2, b3 = fitting parameters C = concentration, mol/cm3 E A = activation energy, kJ/mol I = H2/C0 inlet ratio mcat = mass of catalyst, g qAds
= apparent heat of adsorption, kJ/mol
rc, = rate of formation of products with C number m,g of
product/(g of catalyst s) = synthesis gas consumption rate, mol/(g of catalyst
-rCO+& S)
R = gas constant, cm3 kPa/(mol K) T = temperature, K V , = molar volume, L/mol V G = gas flow rate, nL/min X = conversion y = mol fraction in gas phase z = conversion factor, g of products/mol of synthesis gas Indexes 0 = inlet 1 = outlet G = gas phase Registry No. CO, 630-08-0; HzO,7732-18-5; COz, 124-38-9; Fe, 7439-89-6; K, 7440-09-7. L i t e r a t u r e Cited Anderson, R. B. I n "Catalysis"; Emmett, P. H., Ed.; Rheinhold: New York, 1956; Vol. 4. Atwood, H. E.; Bennett, C. 0. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 162. Brotz, W.; Rottig, W. 2.Elektrochem. 1952, 56, 969.
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Received for review April 18, 1985 Revised manuscript received September 7, 1985 Accepted October 24, 1985
Predicting the Effect of Operating Temperature on the Minimum Fluidization Velocity Mlloslav Hartman' and Karel Svoboda Institute of Chemical Process Fundamentals, Czechoslovak Academy of Science, 765 02 Prague -Suchdo/,
Czechoslovakia
The minimum fluidization velocities of lime particles formed by the thermal decomposition of a reactive, commercial limestone were measured at temperatures ranging from 20 to 870 ' C . A modified Ergun equation was developed to fit the data. The specific constant at the kinetic term is expressed as a function of temperature in the proposed correlation. Using the derived curves, the Wen-Yu and Broadhurst-Becker equations are tested with predictions of the modified Ergun equation. Experimental data for a variety materials, including brown coal and sand, are also compared with the Wen-Yu and Broadhurst-Becker predictions.
Fluidized-bed combustion is one of the most promising methods for processing of low-grade coal. The coal usually burns in a bed containing primarily inert particles such as ash and sand or SOz sorbents such as lime. This work is part of our investigation of the general hydrodynamic characteristics of the fluidized bed of limestone, lime, and ash particles for removal of sulfur dioxide from flue gas. Experimental measurements of the fluidization characteristics of the above materials were reported (Pata and Hartman, 1978, 1980; Svoboda and Hartman 1981a, 1981b). Experimental measurements of the minimum fluidization velocity at high temperatures are scant in the literature (e.g.: Botterill et al., 1982; Pattipati and Wen, 1981;
Stubington et al., 1984). Additional references can be found in the recent review of ours (Hartman and Svoboda, 1985). The experiments are often performed with wide size fractions of particles which makes the analysis of results difficult. The minimum fluidization velocity was found to increase with increasing temperature for large particles and decrease for small particles with temperature. The present work explores the influence of temperature on the state of incipient fluidization of the lime particles used in a recent study of ours on sulfur dioxide removal in a batch fluidized-bed reactor (Hartman et al., 1984). Two generalized correlations are also tested with the data of ours covering a wide range of temperatures and flow conditions.
0196-4305/~ I 125-0649$01.5010 0 1986 American Chemical Society
