Energy & Fuels 1987,1, 203-210
203
Effects of Poisoning a Fused Magnetite Fischer-Tropsch Catalyst with Dibenzothiophene David K. Matsumoto and Charles N. Satterfield* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received August 18, 1986. Revised Manuscript Received November 5, 1986
Catalyst activity is decreased more if the catalyst is prereduced rather than precarburized before contact with poison. The inhibiting effect of water on the reaction rate decreases more markedly with increasing temperature on a poisoned catalyst than on an unpoisoned catalyst. Formation of methane is significantly less on poisoned catalysts than unpoisoned catalysts. On both poisoned and unpoisoned catalysts, the olefinlparaffin ratio decreases with increased CO conversion and isomerization of cy- to @-olefinsincreases. Isomerization activity is greater on a prereduced/poisoned catalyst than on a precarburized/poisoned catalyst.
Introduction In an earlier paper, we1 reported effects of sulfur poisoning on the Fischer-Tropsch synthesis as carried out on a prereduced fused magnetite catalyst in a slurry reactor. Studies were done with H2Sor dibenzothiophene (DBT). The two poisons behaved quite differently. With HzS, small additions increased catalyst activity up to a maximum at a sulfur loading of 1.3 mg of sulfurlg of Fe, beyond which activity decreased. This effect was not observed with DBT. H2Spoisoning had no significant effects on the C1-C5 product distribution whereas DBT significantly decreased the formation of C1 and C2 products. Both poisons decreased secondary hydrogenation of olefins to paraffins, but the effect was more marked with DBT at higher H2 pressures. The above studies were made a t comparable temperatures, pressures, and sulfur loadings on the catalyst, the catalyst being suspended in an inert liquid. The fact that C1and Cz products were significantly reduced by DBT is of interest since the major use of the Fischer-Tropsch synthesis is in the production of liquid fuels. There was an important difference between the way in which the catalyst was poisoned with the two sulfur compounds. In both cases the catalyst was prereduced to the metallic form. However H2S was contacted with the catalyst only after it had reached steady-state activity in the presence of synthesis gas and was predominantly in the carbidic form. In contrast, in studies with DBT, the sulfur compound was contained in the suspending liquid initially and hence was present while the prereduced catalyst was being converted from the metallic form to the predominantly carbidic form. Of secondary importance, but of considerable significance, is the fact that the suspending liquid was octacosane (CBH5J in the studies with HzS and phenanthrene in the studies with DBT. The present studies were designed to determine the effects of adding DBT to the system after the catalyst had been carburized instead of before, when it was in the reduced form. We are not aware of any previous study which addresses a similar topic. Madon and Shaw2reviewed the effect of sulfur compounds on various kinds of FischerTropsch catalysts. Bartholomew et al.3 discussed sulfur (1) Stenger, H. G., Jr.; Satterfield, C.N. Ind. Eng. Chem. Process Des. Deu. 1985, 24, 415. ( 2 ) Madon, R. J.; Shaw, H. Catal. Reu.-Sci. Eng. 1977, 15, 69.
0887-0624/87/2501-0203$01.50/0
poisoning of metals in general and emphasized the difficulties in interpreting most reports because of experimental methods. In particular, with a packed bed reactor the adsorption of a poison almost inevitably leads to axial poison gradients. All studies here utilized octacosane as the suspending liquid, but some comparisons are made, where possible, with our earlier studies utilizing phenanthrene. Octacosane and phenanthrene, while inert chemically, affect the Fischer-Tropsch synthesis differently in two r e ~ p e c t s .First, ~ in the absence of sulfur poisons, catalyst activity is almost doubled in the presence of phenanthrene compared to octacosane. This is possibly caused by a greater solubility of catalyst deposits or high molecular weight waxes in phenanthrene. Second, phenanthrene is apparently adsorbed onto the catalyst to some degree a t 232 OC but not significantly a t 263 "C. Adsorption of octacosane does not seem to be significant a t any temperature. When the catalyst is unpoisoned, the apparent activation energy in phenanthrene is somewhat higher than in odacosane, since catalyst sites blocked by phenanthrene adsorption a t 232 OC become available at 263 "C. With a substantially poisoned catalyst the activation energy in phenanthrene is the same as in octacosane, indicating that phenanthrene adsorption is reduced. Phenanthrene as such does not seem to affect the primary reaction steps in the Fischer-Tropsch synthesis since a t 263 OC the product distribution as a function of CO partial pressure is the same in both liquids. At lower temperatures, however, phenanthrene reduces secondary reactions such as olefin hydrogenation and olefin isomerization, by competitive adsorption. All studies with the prereducedl poisoned catalyst were made with a Hz/CO inlet ratio of 0.7; all studies with the precarburized/poisoned catalyst were made with a H2/C0 inlet ratio of 1.0. The usage ratio with this catalyst is about 0.7 so with the same feed ratio the H2/C0 ratio in the reactor does not vary significantly over a wide degree of synthesis gas conversion. From previous studies in our laboratory, we have a large amount of data on unpoisoned catalyst at feed ratios of 0.9 and higher, against which we compare the results with precarburizedl poisoned catalyst. (3)Bartholomew, C. H.; Agrawal, P. K.; Katzer, J. R. Adu. Catal. 1982, 31, 135. (4)Stenger, H.G.,Jr.; Satterfield, C.N. Ind. Eng. Chem. Process Des. Deu. 1985, 24,411.
0 1987 American Chemical Society
Matsumoto and Satterfield
204 Energy &Fuels, Vol. 1, No. 2, 1987
catalyst A
B C D
Table I. Summary of Precarburized/Poisoned ExDeriments total added, mg of amount of DBT present, g time on DBT/g of stream, h before injection after injection at end of period catalyst 430-490 490-670 670-730 730-870
0.0 0.068 0.048 0.23
0.15 0.528 0.568 0.780
0.068 0.048 0.230 0.084
2.3 9.5 17.3 26.8
sulfur loading, mg of S/g of Fe a 0.6 (f20%) 1.1 (&20%) 0.9 (*20%)
Too low for accurate measurement.
All comparisons here of activity and selectivity between phenanthrene and octacosane are for data obtained at 263 "C, 1.48 MPa, and an inlet H2/C0 ratio of 0.7.
Experimental Section The experiments were performed in a well-mixed, continuous flow, slurry-phase reactor. Details of the experimental system have been reported previously.S~6A fused magnetite catalyst was ground and sieved to 52-92 wm (17e270 mesh) and then reduced in a separate vessel and transferred and injected into the reactor under inert gas. The catalyst was the same as that we have used in a number of previous studies, and the reduction procedure has been described? Dibenzothiophene (>98%) was obtained from Fluka. Poisoning of the PrecarburizedCatalyst. The 1-L reactor, about half-full of liquid, was charged with 79 g (unreduced basis) of catalyst and put on stream with 1.0 Hz/CO feed gas a t 0.93 MPa total pressure. Over the first 200 h the CO conversion varied from 30 to 60%, and a series of studies were made, irrelevant for present purposes. After 250 h on stream, steady-state kinetic measurements were obtained at 1.48 MPa and 232,248, and 263 "C on the unpoisoned catalyst. No product composition data were obtained with the unpoisoned catalyst. Poisoning experiments began after 430 h on stream. The procedure for adding DBT to the reactor involved first taking the system off stream. The reactor was blown down to ambient pressure, and helium was purged through the slurry to prevent oxygen from entering the reactor. Dibenzothiophene was injected into the reactor while the system was being purged. The reactor then was placed under 1.48 MPa of helium and stirred for 5-20 h before the system was placed back on stream. DBT has a slight but significant volatility and hence is slowly removed from the reactor by vaporization. Therefore four injections of 0.15, 0.46, 0.52, and 0.55 g of DBT each were made a t 430, 490, 670, and 730 h on stream, respectively, as makeup. The "half-life" of DBT in the reactor will depend on reactor temperature and flow rate of gases leaving the reactor. Table I shows the amount of DBT found to be present in the reactor before and after each injection, as determined by analysis of a small sample of liquid withdrawn from the reactor. The half-lives of 44-53 h calculated from these data are consistent with theoretical calculations based on estimated vapor pressure data. The minimum theoretical value for the half-life of DBT in the reactor is about 20 h for the highest temperature and highest flow rate used; the maximum value is about 90 h. Table I also shows the cumulative amount of DBT added per gram of catalyst and the actual sulfur loading on the catalyst by elemental analysis (Galbraith Labs). Before analysis the catalyst sample was extracted with toluene to remove any suspending liquid and DBT from the catalyst pores. At these very low levels we estimate from the scatter of data that the accuracy of these values is only about 1 2 0 % . With the first addition of DBT, amounting to 2.3 mg of DBT/g of catalyst, the sulfur loading on the catalyst was too low for accurate measurement. Thus product compositions from this catalyst, termed catalyst A, were not used in the results that follow. The data in the table make it evident that only a small fraction of the DBT added became adsorbed (5) Huff, G. A., Jr.; Satterfield, C. N.; Wolf, M. H. Ind. Eng. Chem. Fundam. 1983,22, 259. ( 6 ) Huff, G. A., Jr.; Satterfield,C. N. Znd. Eng. Chem. Fundam. 1982, 21, 479.
(7) Huff, G . A., Jr.; Satterfield, C. N. Ind. Eng. Chem. Process Des. Deu. 1984, 23, 696.
on the catalyst. Catalysts C and D behaved essentially the same, so in several cases their results are lumped together and ascribed to an average sulfur loading of 1.0 mg of S/g of Fe. Poisoning of the Prereduced Catalyst. The purpose of these studies was to compare these results with our earlier results in which DBT was present in phenanthrene before reduced catalyst was added. A 4.08-g sample of DBT was first added to 407 g of octacosane in the reactor, this giving approximately the same concentration as that of DBT in phenanthrene studied earlier. Reduced catalyst (72 g on an unreduced basis) was then added, and the reactor was pressurized to 0.9 MPa with helium. The system was stirred a t 600 rpm for 6 h before the reactor was put on stream. Experiments were conducted at 232,248, and 263 OC at both 0.79 and 1.48 MPa total pressure, with 0.7 H 2 / C 0 feed gas. No further DBT was added beyond the initial amount. If all the DBT had become adsorbed and poisoned the catalyst, this would have corresponded to about 15 mg of S/g of Fe. The actual sulfur content, by analysis after completion of the studies, was 3.3 of mg S/g of Fe; Le., only a small fraction of the DBT actually acted as a poison. Analysis of the slurry liquid just before startup showed the presence of biphenyl, presumably formed from DBT and equivalent to about 25% of the DBT added. This reaction probably would form H2S or an iron sulfide. The small amount of hydrogen necessary could have been provided by adsorbed H2 on the catalyst or the leakage of some synthesis gas into the reactor. After 50 h on stream, approximately 0.4 g of DBT remained free in the slurry, and after 600 h on stream, about 5 mg remained. This loss was caused by volatilization into the gas stream as described above.
Results: Kinetics Rate Expression. Huff and Satterfield' have found that the data over this fused magnetite catalyst are best correlated by a kinetic expression of the form
The term for H 2 0 is significant only at moderately high conversions. When it is insignificant, eq 1 reduces to a simple form in which the rate is first order in hydrogen and zero order in CO and a is the reaction rate constant. A physical interpretation of b depends upon the model used to derive eq 1. For correlating rate data the above equation can be linearized to PH2 -RH2+C0
= - -1
ab
'HzO pC8H2
1
+ -a
(2)
A plot of the left-hand side vs. PH20/(PcflH2) should give a straight line of slope = l / a b and intercept = l / a from which a and b can be calculated. The data for the precarburized/poisoned catalysts, obtained a t 1.48 MPa, appear to fit the kinetic expression. Figure 1, a plot of the data for catalyst B, is an example. Table I1 contains the calculated values. Even though DBT levels in the reactor were changing with time, catalyst activity was steady at each poisoning level. The data in Table I show that the actual sulfur loading on this catalyst was not very sensitive to the concentration of DBT in the
Poisoning of a Fischer-Tropsch Catalyst 4.0
~
T = 232 T = 248 T = 263
N
Y
N
I
F
-
N
d
0
0.2
0.1
'H20
0.3
'CO
'
0.4
0.5
0.6
A T : 263.0.79 MPa
12 -
.
I
8
00
-
L
w
0.05
0
'CO
'H20
'Ha
Figure 1. Kinetic data for precarburized/poisoned catalyst in octacosane: Catalyst B (0.6 mg of S / g of Fe) at 1.48 MPa.
0.15
0.10 '
0.20
'H2
Figure 2. Kinetic data for prereduced/poisoned catalyst in octacosane (3.3 mg of S/g of Fe). 10.00
Table 11. Values of Parameters B and b for Poisoning ExDeriments a , mmol of H2+ CO min-' T,"C MPa-' (g of catalyst)-' b, MPa-'
I
5.00
I
I
I
I
I
-
Catalyst A 263
1.92
232 248 263
Catalyst B 0.446 f 0.006 0.787 f 0.010 1.46 f 0.08
263
1.19
0.126 f 0.007 0.802 i 0.083 3.54 f 1.08
Catalyst C 1.51
Catalyst D 232 248 263
0.285 0.476 f 0.059 1.12 i 0.16
232 248 263
Prereduced Catalyst 0.0676 i 0.0009 0.148 f 0.004 0.279 i 0.032
0,02
0.034 0.335 f 0.377 1.51 f 1.37
reactor. Insufficient data were obtained with catalysts A and C to calculate b. The data in Figure 2 obtained with the prereducedl poisoned catalyst scatter somewhat, and this may be due to the difficulty in getting good measurements because of the low activity of the catalyst. It does appear, however, that kinetics are essentially first order in hydrogen, a t least a t 0.79 MPa. There are not enough data to obtain both a and b for this system. The rate constant a was calculated from the data a t 0.79 MPa with the assumption that the kinetics were first order in hydrogen. Figure 3 is an Arrhenius plot of rate constant a, giving activation energies in kilojoules per mole for precarburized/poisoned catalysts a t two sulfur loadings and the prereduced/poisoned catalyst. Also presented are the plots for unpoisoned catalyst in octacosane and phenanthrene and prereduced/poisoned catalyst in phenanthrene, all from our previous studies. The activation energies for the precarburized/poisoned catalyst at the two sulfur loadings here agree within experimental error with the value obtained by Huff and Satterfield' for unpoisoned catalyst and with that obtained by Stenger and Satterfieldl for the prereduced/poisoned catalyst in phenanthrene. The higher activation energy for the prereduced/poisoned catalyst in octacosane may be an artifact caused by the
.Procarburized,0.6 mg S/g Fa/ Octacosane 0Pracarbur1zad.10 m g S/g Fa/Octacosana - A P r e r a d u c a d , 7 8 m g S/g FalPhananthrana V P r a r a d u c e d . 3 3 mg S/g Fa/Octacosana
-
1
5.00 -
O
n
I c \
d O.1°
\ I
t 0
Unpoisonad I Octacosana P r a c a r b u r i z e d . 0 . 6 ~S/gFa/ O c t a c W
0.01 0.00186
0,00190
1/T
0,00194
0,00198
(l/K)
Figure 4. van't Hoff plot of adsorption parameter b.
first-order kinetic assumption. Figure 4 is a van't Hoff plot of b for the precarburized/poisoned catalysts at two S loadings plus data from Huff and Satterfield7 for the unpoisoned system. The
Matsumoto and Satterfield
206 Energy & Fuels, Vol. I, No. 2, 1987 1 . 0 C l
I
I
A P r a r o d u c a d / Octacosana
-
I
I
1
I
I
PracarburizadIOctacosana V Praraducad / Phananthrana
-
-
0.6
Table 111. Gas Phase Composition as a Function of CO Conversion
co
conversion, % 24-33 46-54 73-80
H, 37-46 31-51 22-40
mol 5% CO COz HzO 40-52 7-10 0.6-1.5 32-43 13-22 0.4-1.7 22-29 29-43 0.4-3.0
CH, 0.4-0.6 0.6-1.5 1.8-2.3
H2/CO
exit ratio
0.7-1.1 0.8-1.6 0.9-1.8
-4
W
z
Q W
I
; w 0
1
2
3
4
5
6
7
0 Unpoisonad, P h a n a n t h r a n a O P r a r e d u c a d , 7 B m g S/g F a f r o m DBT/ Phananthrana 30 0 Unpoisonad, Octacosane P r e r a d u c a d , 3.3 m g S/g F e f r o m DBT / Octacosana 2E
Unpoisonad
-
6
SULFUR LOADING, mg S /g FQ
Figure 5. Effect of sulfur loading from dibenzothiopheneon reduction of catalyst activity.
0
"/e
extent to which b has theoretical significance depends upon the mechanism assumed in the derivation of (1)and the extent to which this reflects reality. Equation 1 is consistent with a version of the carbide theory in which b is a product of two equilibrium constant^.^ The difference between the values of ( m a d s for the two reactions from Figure 4 is 243-277 kJ/mol for the poisoned system compared to 100 kJ/mol for the unpoisoned system. The physical significance is that the inhibiting effect of water on the reaction rate decreases with increasing temperature more markedly in the presence of poison than in its absence. Within this form of the carbide theory the change would be attributed to an increase in ( m a d , for water and/or a decrease in ( m a d a for carbon monoxide. Effect of Sulfur Loading on Catalyst Activity. Figure 5 presents the reduction in catalyst activity a t 263 "C relative to unpoisoned catalyst, caused by DBT poisoning to specified sulfur loadings. Data are shown for the precarburized catalysts B, C, and D and the prereduced catalyst. Also shown are data from two studies we previously published for prereducedl poisoned catalysts suspended in phenanthrene. Interestingly, with a prereduced catalyst that is heavily poisoned, the percent reduction relative to the initial unpoisoned activity is the same in the presence of either octacosane or phenanthrene, although the rate of reaction without poisoning is about twice as great in phenanthrene as in octacosane.s The precarburized/poisoned catalysts in octacosane show a much lower percent loss of activity than a prereduced/poisoned catalyst in phenanthrene a t the same sulfur loading. On an absolute basis, however, the two lightly poisoned catalysts would show about the same activity. It is probable that the reduction in activity with both the prereduced and precarburized catalysts was due to formation of some kind of iron sulfide on the catalyst surface. Catalysts were subjected to X-ray photoelectron spectroscopy (XPS) examination, but it was not possible to characterize the form of sulfur present, in part because of the very low concentrations.
Results: Selectivity Three product classes were examined methane, olefins, and oxygenates. The product selectivities reported are plotted against percent carbon monoxide conversion. (8)Satterfield,C. N.; Stenger, H. G.,Jr. Ind. Eng. Chem. Process Des. Deu. 1986, 24, 407.
40
20
CO
60
80
100
CONVERSION
Figure 6. Plot showing lowering of methane selectivity with poisoned catalysts. Conditions: 263 "C; 1.48 MPa; (H2/CO)fed = 0.7.
These data were collected a t various temperatures, pressures, and space velocities, and a number of variables were tried as a basis for comparison. When plotted against CO conversion, selectivities for a given catalyst lay closer together in a single band than they did in other attempts at correlation. This brought together results covering large differences in operating temperature or pressure. In order to present the large amount of data obtained, the %CO conversion scale was chosen for convenience and consistency. For general guidance, Table I11 gives the extremes of the range of concentrations observed here in the gas phase for selected compounds from unpoisoned, prereduced/poisoned, and precarburized/poisoned catalysts in three CO conversion ranges. Note that the inlet H2/C0 ratios varied from 0.7 to 1.0. In general, as CO conversion increases, hydrogen concentration decreases slightly while COz,HzO, and methane concentrations increase. There is also a moderate increase in the Hz/CO ratio in the reactor. Iron catalysts are active for the water gas shift reaction. The equilibrium is more favorable toward H2 at lower temperatures, but the rate of reaction toward equilibrium increases at higher temperatures. The H 2 0 ranges shown here sum up countervailing influences. When the water gas shift goes to completion, about half of the CO reacted forms COP. Methane. Methane selectivity for two prereduced/ poisoned catalysts is presented in Figure 6 as mole percent based on total organic Fischer-Tropsch products from C1 to C,, including oxygenates. Data for studies of DBT in phenanthrene are from the work of Stenger and Satterfield.' For comparison data from Stengergfor unpoisoned systems at high conversions are also given; the data were obtained a t 263 "C and 1.48 MPa. At this temperature, 263 "C, methane selectivity with an unpoisoned catalyst is the same in phenanthrene and octacosane. Methane selectivities in both poisoned systems were significantly lower than those in the unpoisoned cases. The lower methane selectivity in phenanthrene was probably caused by the higher sulfur loading rather than the different liquid. (9) Stenger, H. G., Jr., Sc.D. Thesis, MIT, 1984.
Poisoning of a Fischer-Tropsch Catalyst T.232,
Energy & Fuels, Vol. 1, No. 2, 1987 207 0 Unpoisonod, Octacosana
0.79MPa
P r a r a d u c a d , 3.3mg S/g Fo
z
A T ~ 2 6 3 .0.79 M P a
Q
I
0 0 0
2
bAa
t-
w V cc
w
w =
I
w I
0 0 20! 1 5
0
0
I *
0 0
B
.
20 10 CO
40 60 CONVERSION
80
100
Figure 7. Plot showing methane selectivity increasing with CO conversion for prereduced/poisoned catalyst (3.3 mg of S/g of Fe). (H2/CO)fd= 0.7.
2$
0 T= 263,0.6 mg .AT=248. 0.6 mg - OT.232.06mg eT.263.11 m g .T=263, 0.9mg 3 0 1 AT:248,0,9mg O T = 2 3 2 , 0.9 mg
w
251
I
w
A
0
mm
25
100
Figure 9. Three sets of studies on methane selectivity.
S/g Fa S/g F a S/gFe Slg F a S/gFo S/g Fa S/g Fa
AoI
0
0
20 40 60 80 PERCENT CO CONVERSION
0
A
E
"
3
\
lo
\
w J 0
I
z
LL LL
2
4
L
zLL
ad
0 Unpoisoned, Octacosane ' Prareducad, 3.3 m g S/g Fa from DBT/Octocosan
0 Unpoisoned, Phenanthrene 0 Prereduced,ZBmg S/gFa f r o m
w J 0 0 1 0
I
1
20
1
I
I
I
% CO
I
60
40
I
1
80
I 100
CONVERSION
Figure 10. C5 olefin/paraffin ratios on poisoned and unpoisoned
catalysts.
0 T:
232, 1.48 MPa 0 T= 2 4 8 . 0.79 MPo 0 T.248, 1.48 MPa A T = 2 6 3 . 0 7 9 MPa A T.263; 1.48 M b mT.232, 0.79 MPa
P
z
a AA
a
3
A
0
20
40
60
80
100
% CO CONVERSION
Figure 11. C5 olefin/paraffin ratio decreasing with CO conversion for prereduced/poisoned catalyst (3.3mg of S/g of Fe). (HZ/CO)fd = 0.7.
(10)Huff, G . A., Jr. Sc.D. Thesis, MIT, 1982.
nature of the liquid carrier on the ratio. The O / P ratio is higher on the more heavily poisoned catalyst, presumably reflecting decreased hydrogenation functionality. The O / P ratio decreases with increased CO conversion on both poisoned catalysts. I t may rise slightly at very high CO conversions on an unpoisoned catalyst. Figure 11 for a prereduced/poisoned catalyst shows that the decreasing ratio of O / P with increased conversion is not affected by
Matsumoto and Satterfield
208 Energy & Fuels, Vol. 1, NO.2, 1987 LL
0 Unpoisonod, Phanonthrene
3
T= 163, 0.9 m g s / g FU
LL
4
fLL
w
i
e
20
0
0
Proreducad, f r o m DBT i Phenonthrano 7.8 mg Slg Fe
e
M P r o r e d u c e d , 3 . 3 ~S / g F e f r o m DBT / O c t o c o s o n a
'
O
O
60
0
40 % CO
60
80
100
t
w
CONVERSION
Figure 12. C5olefii/paraffi ratio decreasing with CO conversion for precarburized/poisoned catalyst. (H2/CO)fed= 1.0.
d
Y
01 0
I
I
1
20
I
1
I
40
1
I
1
80
60
I 100
% CO CONVERSION 0 Unpotsonad
V
P r e c a r b u r 1 z e d , 0 6 , 1 0 q S / g Fa APrereducod. 3 . 3 S/g ~ Fe 1,4M VPreroducod,
A
A
0 kLI
Figure 14. Percent olefin (C,) as the a-isomer decreases with CO conversion. Conditions: 263 OC; 1.48 MPa; (Hn/CO)fed= 0.7. VI
V A T = 232 ,0.79 = 232. 1.48 T = 248,0.79 0 T = 2 4 8 , 1.48 A T: 2 6 3 , 1.48 A T = 2 6 3 , 0.79
60 7
0T
I
I !
e
-
I-
z
f
w
D
60
MPo MPo MPo MPa MPa MPa
i
2-
i 01t 0
I
I
20
I
I
I
I
I
I
40 60 00 PERCENT CO CONVERSION
I
1
o.AA
j
100
A
Figure 13. Three sets of studies on the C4 olefin/paraffin ratio.
pressure or temperature. The data for a precarburizedl poisoned catalyst show the same trend (Figure 12). Figure 13 brings together all studies here with poisoned catalysts, together with data from Hufflo on unpoisoned catalysts at a variety of temperatures and pressures, the latter all obtained with a H2/C0 feed ratio of 0.9. Results here, for the C4 fraction, are similar to those for the C5 fraction in Figures 10-12. The level of sulfur contamination within the present studies or the method of poisoning seems to be unimportant. For poisoned and unpoisoned catalysts, the O/P ratio drops with increased CO conversion. Olefin Isomerization. The olefins formed initially are the a-isomer. These may isomerize to the @-formby a secondary reaction, the extent of which is indicated by the @/a ratio in the product. The degree of isomerization increases with CO conversion but is much less on unpoisoned catalysts than on prereduced/poisoned catalysts (Figure 14). With the precarburized/poisoned catalysts here slightly less isomerization occurs on the catalyst with the lower sulfur loading (Figure 16). With reference to Figure 14, on the basis of our earlier studies with unpoisoned catalyst, we would expect no significant difference between phenanthrene and octacosane a t 263 "C. The slightly less isomerization on the prereduced catalyst with very high sulfur loading may be associated in some way with poisoning occurring in the presence of phenanthrene. For a given sulfur loading, there seems to be little effect of pressure or temperature (figure 15),but the effect of CO conversion on the a/@ ratio is markedly different for the precarbur-
A 0
I
0
I
20 "/e
I
I
80
100
I
40 60 CO CONVERSION
Figure 15. Percent olefin (C,) as the a-isomer decreases with CO conversion for prereduced/poisoned catalyst (3.3 of mg S/g of Fe). (Hz/CO)fed= 0.7.
..
'
'
A
r t L
m
6 0 1
E
-
w
2
O T = 2 6 3 , 0.6mg S/g AT:248. 0.6mgS / g Fe Fa
A
-
o T = 2 3 2 , 0 . 6 m g S l g Fa *T= 2 6 3 , 1.1 mg sig FU VI Q . ~ = 2 6 3 , 0 9 m g S/g Fo Ln A Tz 2 4 8 , 0.9 mg S/g Fe 2 0 - e T : 2 3 2 , 0.9mg SlgFe 40
'
-
LL
w
2
0
20
40
60
80
100
PERCENT CO CONVERSION
Figure 16. Percent olefin (C,) as the a-isomer decreases markedly a t high CO conversion for precarburized/poisoned catalyst, (H2/CO)fed= 1.0.
ized/poisoned catalyst in contrast to the prereducedl poisoned catalyst. With the precarburized/poisoned catalyst, little isomerization occurs until CO conversion reaches about 70%, beyond which isomerization increases markedly (Figure 16).
Poisoning of a Fischer-Tropsch Catalyst
Energy & Fuels, Vol. I, No. 2, 1987 209
t
1
00
* * 60
A
** 40
301
F20/OTa263:, A T = 2 4 8 . 0 ..66mm ;S/;F gu s l g FO
I
,o-;
~l
I
0"
O T = 232.0.6 mg S/g Fo *T= 263, 1.1 mg S/gFo Z 8 T= 263, 0.9 mg S/g Fo lo AT: 248, 0.9 mg S/gFo OT: 232, 0.9 m g S/g Fo +
5
**
a**
r
-I
n
* .
0 0
40
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Figure 20. Effect of CO conversion on Cz oxygenates for precarburized/poisoned catalyst. (Hz/CO)fed = 1.0. 0 Unpasonud, Phenanthreno 0 Prereduced.7Bmg S/g Fe fromDBT/Phemnthrono
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Figure 21. Effect of CO conversion on C4oxygenates. Conditions: 263 "C; 1.48 MPa; (H2/CO)fed = 0.7. 20
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Figure 18. Effect of CO conversion on Cz oxygenates. Conditions: 263 "C; 1.48 MPa; (HZ/COlfd = 0.7. 401
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Figure 19. Effect of CO conversion on Cz oxygenates for prereduced/poisoned catalyst (3.3 mg of S/g of Fe). (Hz/CO)fed = 0.7.
+
Figure 17 brings together all studies here with poisoned catalysts, together with data from Hufflo on unpoisoned catalyst, in a manner analogous to Figure 13, again here for the C4 fraction. For a given CO conversion, isomerization activity is about the same on a precarburizedl poisoned catalyst and an unpoisoned catalyst but is much greater on a prereduced/poisoned catalyst. Oxygenates. The oxygenate selectivities are presented as the mole percent a t a given carbon number, based on total organic product formed. Figures 18-20 show data for the C2 fraction, and Figures 21-23 show data for C4. There is more scatter in these data than in previous plots.
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A 0 T.263. 0.6 mg S/g Fo A T.248.0.6 mg S/g Fe 0 T=232,06 mg S/g Fo *T:263,1.1 mg Slg Fo 8 T.263, 0.9 m g S/g Fo A T:248,0.9 mg S/g Fe T = 2 3 2 , 0 9 mg S/g Fe
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210 Energy & F u e l s , Vol. 1, No. 2, 1987
they may originate from several different pathways and disappear by various secondary reactions. The oxygenate concentration in isolated products is the highest for the C2 products, and ethanol is the dominant species present. There is a general pattern of drop in oxygenate concentrations with increased CO conversion; however, with the precarburized/poisoned catalyst the drop occurs markedly in the region of about 90% CO conversion (Figure 20) whereas with the prereducedl poisoned catalyst significant drop occurs a t much lower CO conversions (Figure 19). Poisoned and unpoisoned prereduced catalysts are compared in Figure 18, but because of scatter in the data and the fact that those for the unpoisoned catalyst are predominantly a t high CO conversions, it is not clear whether or not there is a significant difference between poisoned and unpoisoned catalyst at a specified CO conversion. Comparison of Figures 19 and 20 suggests that a predominant effect here is the nature of the poisoning. The data for the C4oxygenate fraction (figures 21-23) show the same kind of behavior relative to CO conversion and method of sulfurization as those for the C2oxygenate fraction. In our previous studies with unpoisoned catalysts, at a temperature of 232 "C oxygenate concentrations were substantially higher in phenanthrene than in octacosane under comparable conditions, suggesting that oxygenates disappear by secondary reactions. These are partly blocked by competitive adsorption from phenanthrene.
Discussion A fused magnetite catalyst is more readily poisoned with respect to activity by dibenzothiophene (DBT) if the catalyst is prereduced rather than precarburized. The DBT is much more readily adsorbed onto the prereduced catalyst and, for a given sulfur loading on the catalyst, appears to cause greater loss in activity than if the same sulfur loading is achieved by adsorption of H2S. Thus, a t a sulfur loading of 3.3 mg of S/g of Fe, activity was reduced to about 10% of its original value, whereas from Figure 1 of Satterfield and Stenger,l when this loading was achieved from H2S,activity reduction was only about 50%.
Matsumoto and Satterfield
(The earlier study was at 0.79 MPa, instead of 1.48 MPa, but both were a t 263 OC.) With the precarburized catalyst, repeated dosing with DBT caused no further increase in sulfur loading on the catalyst above about 1mg of S/g of Fe. This suggests that some plateau in degree of adsorption may exist for the precarburized catalyst with a corresponding activity of about 50% of original (Figure 5). Elsewhere well report studies of the behavior of this catalyst in the absence of poisons during startup after prereduction. The activity rises steadily during the first 15-20 h to a degree 50-100% greater than the initial activity. Mossbauer spectroscopy studies of the catalyst during this same period (at 248 "C) showed that the catalyst, initially 100% a-Fe, is converted largely to an iron carbide (x-Fe,C2),which is believed to be the most active phase. Some a-Fe remains, but no iron oxide was detected. The DBT may well interfere with this carburizing process as well as covering a substantial fraction of active sites. The kinetic data from the precarburized/poisoned catalyst experiments fit the form of the expression proposed by Huff and Satterfield' and yielded about the same activation energy, suggesting that the same basic Fischer-Tropsch mechanism is occurring in both poisoned and unpoisoned systems. The large increase in the apparent adsorption energy of b for the poisoned systems indicates a higher heat of adsorption for water and/or lower heat of adsorption for carbon monoxide. In absolute terms b drops for the more highly poisoned catalyst, meaning that water vapor inhibition is more significant at the higher sulfur loading.
Acknowledgment. This work was supported in part by the Office of Fossil Energy, US.Department of Energy, under Grant No. DE-FG-81PC40771 and Contract No. DE-AC22-85-PC80015. We also gratefully acknowledge support in the form of a fellowship (to D.K.M.) from the Fannie and John Hertz Foundation. Registry No. DBT, 132-65-0; Fe304,1317-61-9; CO, 630-08-0; CHI, 74-82-8; magnetite, 1309-38-2. (11) Satterfield, C. N.; Hanlon, R. T.; Tung, S. E.;Zuo, Z.-m.; Papaefthymiou, G. C. Ind. Eng. Chem. Prod. Res. Deu. 1986, 25, 407.
