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Cobalt as an alternative Fischer-Tropsch catalyst to iron for the production of middle distillates
M. de Pontes, R.L. Espinoza, C.P. Nicolaides, J.H. Scholz and M.S. Scurrell (Editors) Natural Gas Conversion IV
207
Studies in Surface Science and Catalysis, Vol. 107 9 1997 Elsevier Science B.V. All rights reserved.
C o b a l t as an a l t e r n a t i v e F i s c h e r - T r o p s c h p r o d u c t i o n of m i d d l e d i s t i l l a t e s .
catalyst
to
iron
for
the
P.J. van Berge Sastech R&D, P.O. Box 1, Sasolburg 9570, South Africa R.C. Everson Department of Chemical Engineering, Potchefstroom, 2520, South Africa
Potchefstroom
University
for
CHE,
1 .INTRODUCTION
With respect to gasoline and light olefin cll production Sasol has successfully implemented and optimized high temperature medium pressure iron based FischerTropsch technology. This technology is commercially operated as the Synthol (circulating fluidized bed) and Sasol Advanced Synthol (SAS) fixed fluidized bed systems (2). Indications are that cobalt, or any other group viii metal, is not an alternative to iron in this application. The disadvantage of cobalt being the hydrogenated nature of the products. The objective of the alternative application of the Fischer-Tropsch reaction is middle distillate production (i.e. wax optimization), commercially operated as the Arge and Sasol Slurry Phase Distillate (SSPD) processes. With respect to this application, cobalt holds the potential of providing a viable alternative to the current iron based catalyst. This notion is supported by published and patented research attention recently paid to cobalt as viable Fischer-Tropsch catalyst c3)~7). A Fischer-Tropsch kinetic/selectivity investigation was therefore undertaken in the slurry phase, in which the spray dried precipitated iron catalyst c8)was compared to a Sastech supported cobalt catalyst c9). 2.EXPERIMENTAL
The Fischer-Tropsch synthesis was performed in a CSTR with a reactor volume of 670m1. The pre-reduced catalyst (20 to 30g) was suspended in -- 300ml molten Arge wax. The syngas flows were controlled by Brooks mass flow meters and use was made of the ampoule-sampling-technique developed by Schulz (15). 3.KINETIC INVESTIGATION
An investigation (1~ into the intrinsic kinetics of the iron catalyst revealed that the following rate equations successfully described the observed activities as measured in a CSTR:
208
rR- = ( k ~ Pco pHO=~ I (Pco + A PH=O)
rwas = kwas [P co - (P co2 P H~) I (Kwes Pa,o) ]
The cobalt catalyst did not s h o w any significant water-gas-shift activity and no water inhibition of the Fisher-Tropsch reaction rate. The published Satterfield rate equation c1~1 could be fitted to the observed kinetic data, viz" rFr = (kFT PCO PH~ I (1 + B Pco)2 As representative reactor temperature for the Fe catalyst, 2 4 0 ~ can be considered. In selecting a suitable temperature for the cobalt alternative, the approach was to keep the temperature as low as possible in order to maximize middle distillate selectivities as supported by thermodynamics ~12). In this regard 2 2 0 ~ was selected, a temperature that also ensured comparable hydrocarbon yields between the t w o catalysts investigated, as can be deduced from figure 1. Figures 1 and 2 were constructed from a CSTR model in which possible masstransfer limitations were eliminated, no gas recycle was considered, and a feed gas of 67 vol% H 2 and 33 vol% CO was assumed. Although space velocities were relativised they are based on a per unit catalyst mass basis and not per unit volume. FIGURE 2. PRODUCTIVITY COMPARISON BETWEEN THE IRON CATALYST (240~ AND THE COBALT CATALYST (220~
FIGURE 1. ACTIVITY COMPARISON BETWEEN THE IRON AND COBALT CATALYSTS AT 20 BAR
(~
~
100 90 \
~.
80 7o
~ ~
60 50
~ ~ ~
40
~
>" 4 t--rm o_ J "' 3 >
'
I,M
= Fe more productive
rat)
3o
Co more productive
14.1
>
c-. 1
2o 10 0
~ .
(.3 < ix. 2
5I . g 0
.
.
.
~"
.
.
i
.
.
g
.
.
.
4
RELATIVE SPACE VELOCITY Cobalt catalyst (220~ - - -Iron catalyst (240~
nr"
5
0
10
,
14 ' 18 ' 2'2 ' 2'6 ' 3(3 ' 3~4 ' 3i REACTOR PRESSURE (bar)
From figure 1 it can be concluded that at 20 bar operation, the cobalt catalyst outperforms the iron alternative at low relative space velocities. Figure 2 confirms the notion that cobalt is favoured at low reactor pressures combined with low space velocities, the latter implying conditions favourable for high per pass conversion levels. These observations can be explained in terms of: Cobalt based Fischer-Tropsch kinetics is not inhibited by reaction water. Iron based Fischer-Tropsch kinetics is more sensitive towards the absolute hydrogen partial pressure, whilst cobalt kinetics depend more on the PH21Pco ratio.
209
The activity of this cobalt catalyst does not only compare favourably with iron, but also with patented cobalt alternatives c3~czj, based on a direct comparison of the pre-exponential factors as derived from the selected rate equation. FIGURE 3. ACTIVITYCOMPARISON BETWEEN THE IRON AND COBALT CATALYSTSAT 20 bar (HECO feed ratio = 1)
~. _
.
8
8o 70
60
so 40
oo
o~
30 20 10 0
0
1
2
3
4
5
RELATIVE SPACE VELOCITY - - C o b a l t catalyst - - Iron catalyst
It must be emphasised that the above-mentioned discussion presupposes as H2/CO feed ratio of 2 which should favour cobalt (i.e. usage rate = 2 as a result of relative little CO2 production). A H2/CO feed ratio of 1 would be favourable for iron as can be concluded from figure 3. It must, however, be added that figures 1 and 3 were constructed without taking deactivation as a result of re-oxidation by reaction water into consideration. Iron being more prone to re-oxidation, renders stable operation at high syngas conversion levels unlikely.
4.CATALYST STABILITY (DEACTIVATION) The relative high cost of cobalt demands high catalyst stabilities in order to ensure extended runs. With respect to the cobalt catalyst t w o regimes of deactivation were observed, labelled as A and B in figure 4 (Synthesis conditions: 220~ 20 bar; commercial syngas feed: 50 vol% H 2, 25 vol% CO and 25 vol% inerts; relative space velocity = 0.5). Region A ( - 4 days) can be considered as a conditioning phase and is associated with a reversible deactivation. The build-up of high molecular weight hydrocarbons 80 A B inside the catalyst pores (i.e. surface ~ 70 condensation) is a likely cause ~13~. c 60 o Region B is associated with u 50irreversible deactivation, and is thus ~ 40of critical importance. It was c >" 3 0 t,n determined that the rate of this N 20deactivation is directly proportional 10to the syngas space velocity. This 00 lb 2'0 2'5 30 deactivation is probably due to low level sulphur poisoning. It was T i m e on line ( D a y s ) established that the sulphur level in the particular coal based plant syngas used (i.e. --0.03mgS/m3,), is too high to guarantee run lengths of up to 1 year with the cobalt catalyst. This is expected to be less of a problem for all cases (including natural gas derived syngas) where an adequate sulphur guard system is used.
FIGURE 4. STABILITY RUN ON THE COBALT CATALYST 100 90
210
5.HYDROCARBON
SELECTIVITY
INVESTIGATION
The direct determination of quantitative information regarding high molecular weight Fischer-Tropsch products is difficult on a laboratory scale because of limited wax production. It was, however, established that an adapted version of the published "double (x" Schulz-Flory model (as derived by Donnelly c14)) is suitable for the extrapolation of selectivities. The selectivities of the products C1 to --C~s was determined accurately by means of GC analyses (having utilized the ampoule sampling technique c15) ) of the volatile overhead phase, and this information was extrapolated to the reactor wax (i.e. >C18 ) or the hard wax (i.e. > C37 )cut. Generally observed deviations from the classical Schulz-Flory polymerization model includes: i) An underestimation of C1 selectivities and an overestimation of C2 selectivities ii) A break in experimental Schulz-Flory plots at a carbon number of about 10 The consequence of these experimental deviations is that ideal Schulz-Flory (i.e. a single chain growth probability describing the whole product spectrum) cannot be beaten. By simply comparing catalysts on the basis with which their hard wax selectivities approach ideal Schulz-Flory, care is taken of all likely deviations in one step. An elegant manner of visualizing this comparison is proposed by figure 5. The experimental data points used in the construction of figure 5 represent a large variation in operating conditions from runs performed at Sastech on iron and cobalt. From figure 5 it should be clear that FIGURE 5. HARD WAX (C3e+) SELECTIVITIES AS a cobalt based Fisher-Tropsch FUNCTION OF CHAIN GROWTH PROBABILITY catalyst has selectivities somewhat 100 closer to ideal Schulz-Flory than the 90 iron catalyst. - 80 Ways of manipulating the wax 70 chain growth probability also differs uj ,_J 6 0 between cobalt and iron. In the case uJ 5 0 03 of iron it is best achieved via the .*: E 40 addition of chemical promoters c16~ ~ 30 9 e+* (e.g. potassium) and additional fine u 2(1 tuning can be accomplished through N10 .r. adjustments in reactor temperature Oo~--.-,--,,--::, ,. ......... 9 0.9 and/or syngas HJCO ratios c1~. With WAX CHAIN GROWTH PROBABILITY respect to cobalt based Fischer+ Iron catalyst Tropsch the notion that drastic 9 Co catalyst m Ideal Schulz-Flory selectivity changes in selectivities can be effected by increasing the reactor pressure was already published in 1956 c17). The role of chemical promoters in the case of cobalt was therefore questioned c18). Eri et.al. ~7), however, challenged this premise, but the data on which their conclusions were based reflected differences in conversion levels, thus differences in reactor partial pressures. Their claim of chemical promotion effected by K, Na, Cs and Rb could therefore not be regarded as conclusive. Studies performed by Sastech c9) also confirmed the ineffectiveness of chemical promotion of cobalt Fischer-Tropsch catalysts.
~ !
211
The a p p r o a c h that w a s used during this i n v e s t i g a t i o n w a s to c o m p a r e selectivities at different reactor pressures, w h i l s t keeping the e m p t y Vessel based linear v e l o c i t y constant. A t e m p e r a t u r e of 2 2 0 ~ w a s c o n s i d e r e d for the cobalt c a t a l y s t and 2 4 0 ~ for the iron catalyst. The feed gas c o m p o s i t i o n w a s : 51 v o l % H 2, 27 v o l % CO, 9 v o l % CH, and 13 v o l % Ar. The c o r r e s p o n d i n g w a x chain g r o w t h p r o b a b i l i t y f a c t o r s are d e p i c t e d in figure 6, indicating that cobalt based w a x selectivities indeed s h o w greater s e n s i t i v i t y t o w a r d s reactor pressure. Detailed s e l e c t i v i t y b r e a k - d o w n s FIGURE 6. CHAIN GROWTH PROBABILITY (x2 are p r o v i d e d in figures 7 and 8. For AS FUNCTION OF REACTOR PRESSURE this purpose the following AT CONSTANT SUPERFICIAL VELOCITY c o n v e n t i o n w a s used: 1wax C19 + 0.9 diesel = C~2- C~8 gasoline = C6 - C~1 0.8 LPG = C 3 - C4 8' fuelgas = C~ - C 2 0.7 From figures 7 and 8 the c o n c l u s i o n can be d r a w n that for high e n o u g h 0.6 reactor pressures, w a x selectivities 0o5 of - - 5 0 m a s s % can be o b t a i n e d 0 10 ' 20 ' 3'0 ' 40 w i t h cobalt, w h i c h are c o m p a r a b l e REACTOR PRESSURE (bar) w i t h that of iron. Iron based w a x + Iron catalyst 9 Co catalyst selectivities are not m u c h a f f e c t e d by reactor pressure, but it does seem as if a m a x i m u m is reached at --- 25 bar. If diesel p r o d u c t i o n is the o b j e c t i v e in the case of cobalt based F i s c h e r - T r o p s c h , high pressure operation is also advised. The reason for this being t w o - f o l d , viz: l o w pressure o p e r a t i o n ( < 9 bar) is associated w i t h high b r a n c h i n g degrees c19~(i.e. as high as S y n t h o l ) , w h i l s t m e d i u m pressure o p e r a t i o n is associated w i t h b r a n c h i n g degrees on par w i t h iron based l o w t e m p e r a t u r e F i s c h e r - T r o p s c h (-- 5 m a s s % c19~ ). Fisher-Tropsch w a x boiling a b o v e 3 5 0 ~ can easily be h y d r o c r a c k e d to e x t i n c t i o n , yielding -- 8 0 % diesel w i t h a cetane n u m b e r of at least 70. =
FIGURE 7. MASS% PRODUCT DISTRIBUTION AS FUNCTION OF REACTOR PRESSURE AT CONSTANT SUPERFICIAL VELOCITY FOR THE IRON SLURRY PHASE CATALYST 100 z o 90 r-- 80
(n 70 o a. 60 :~ 50 O u 40 30 (f} 20 < 10 3~ 0
FIGURE 8. MASS% PRODUCT DISTRIBUTION AS FUNCTION OF REACTOR PRESSURE AT CONSTANT SUPERFICIAL VELOCITY FOR THE COBALT CATALYST 100
~
90
rJo
80 70
o
SFuelgas
_
~ o l i n e
.~ 6o
o 50 ~ m <
10 20 30 40 REACTOR PRESSURE (bar)
2~
40 30 20 10 0
10 20 30 REACTOR PRESSURE (bar)
40
212
6.CONCLUSION In conclusion it can be stated that cobalt based catalysts (despite higher initial cost) can be considered as an alternative to iron based Fischer-Tropsch catalysts for the production of middle distillates utilizing a syngas w i t h a H2/CO ratio of 2. The strong advantage of the cobalt option is to be found in the application w h e r e high per pass syngas conversion levels are desired. If a comparable net conversion is to be achieved w i t h iron, it will require gas recycle in order to knock out water, implying added compression costs. In order to help c o m p e n s a t e for the relative high cost of cobalt, extended runs will have to be guaranteed. This prerequisite increases the importance of catalyst stability, and the presence of l o w level sulphur catalyst poisoning will seriously disadvantage commercial cobalt application. 7.NOMENCLATURE Specific Fischer-Tropsch reaction rate Fischer-Tropsch rate c o n s t a n t Specific water-gas-shift reaction rate Water-gas-shift rate c o n s t a n t Water-gas-shift equilibrium constant Partial reactor pressure of c o m p o u n d x Symbols used to represent reaction rate variables Chain g r o w t h probability factor (i = 1 for l o w molecular w e i g h t products and i = 2 for high molecular w e i g h t products)
rFT kFT FWGS kwGs KWGS Px A,B Oli
REFERENCES
10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
M.E. Dry, Catalysis-Science and Technology, editors: J.R. Anderson and M. Boudart, Springer-Verlag, 1(1981 ). B. Jager, M.E. Dry, T. Shingles and A.P. Steynberg, Catalysis Letters, 7 (1990) 293-302. E. Iglesia, S.L. Soled and R.A. Fiato, US Patent No. 4 794 099 (1988). H. Beuther, C.L. Kibby, T.P. Kobylinski and R.B. Pannell, US Patent No. 4 413 064 (1983). P. Chaumette and C. Verdon, UK Parent No. GB 2 258 414 A (1993). M.F.M. Post and S.T. Sie, European Patent No. 0 167 215 A2 (1986). S. Eri, J.G. Goodwin, G. Marcilin and T. Riis, US Patent No 4 880 763 (1989). B. Jager and R. Espinoza, Catalysis Today, 23 (1995) 17-28. R.L. Espinoza, J.L. Visagie, P.J. van Berge and F.H.A. Bolder, RSA Provisional Patent No 95/2903 (1995). P.J. van Berge, PhD thesis, Potchefstroom University for CHE, South Africa, (1994). C.N. Satterfield, I.C. Yates and C.A. Chanenchuk, Energy and Fuels, 5 (1991) 168-173 and 847-855. S. Oliv~ and G. Henrici-Oliv~, "The Chemistry of the Catalyzed Hydrogenation of CO", Springer-Verlag, Berlin - Heidelberg - New York - Tokyo (1984). E. Iglesia, S.L. Soled, R.A. Fiato and G.H. Via, Journal of Catalysis, 143 (1993) 345-368. T.J. Donnelly, C.N. Satterfield and I.C. Yates, Energy and Fuels, 2(1988) 734-739. H Schulz and A. Geertsema, Erd61 und Kohle, 30 (1977) 31 3. M.E. Dry, Brennstoff-Chemie, 50 (1969) 193-224. R.B. Anderson, Catalysis, editor: P.H. Emmett, 4 (1956). R.W. Joyner, Vacuum, 38 (1988) 309-315. H. Pichler, H. Schulz and D. K(ihne, Brennstoff-Chemie, 49 (1968) 344-351.
207
Studies in Surface Science and Catalysis, Vol. 107 9 1997 Elsevier Science B.V. All rights reserved.
C o b a l t as an a l t e r n a t i v e F i s c h e r - T r o p s c h p r o d u c t i o n of m i d d l e d i s t i l l a t e s .
catalyst
to
iron
for
the
P.J. van Berge Sastech R&D, P.O. Box 1, Sasolburg 9570, South Africa R.C. Everson Department of Chemical Engineering, Potchefstroom, 2520, South Africa
Potchefstroom
University
for
CHE,
1 .INTRODUCTION
With respect to gasoline and light olefin cll production Sasol has successfully implemented and optimized high temperature medium pressure iron based FischerTropsch technology. This technology is commercially operated as the Synthol (circulating fluidized bed) and Sasol Advanced Synthol (SAS) fixed fluidized bed systems (2). Indications are that cobalt, or any other group viii metal, is not an alternative to iron in this application. The disadvantage of cobalt being the hydrogenated nature of the products. The objective of the alternative application of the Fischer-Tropsch reaction is middle distillate production (i.e. wax optimization), commercially operated as the Arge and Sasol Slurry Phase Distillate (SSPD) processes. With respect to this application, cobalt holds the potential of providing a viable alternative to the current iron based catalyst. This notion is supported by published and patented research attention recently paid to cobalt as viable Fischer-Tropsch catalyst c3)~7). A Fischer-Tropsch kinetic/selectivity investigation was therefore undertaken in the slurry phase, in which the spray dried precipitated iron catalyst c8)was compared to a Sastech supported cobalt catalyst c9). 2.EXPERIMENTAL
The Fischer-Tropsch synthesis was performed in a CSTR with a reactor volume of 670m1. The pre-reduced catalyst (20 to 30g) was suspended in -- 300ml molten Arge wax. The syngas flows were controlled by Brooks mass flow meters and use was made of the ampoule-sampling-technique developed by Schulz (15). 3.KINETIC INVESTIGATION
An investigation (1~ into the intrinsic kinetics of the iron catalyst revealed that the following rate equations successfully described the observed activities as measured in a CSTR:
208
rR- = ( k ~ Pco pHO=~ I (Pco + A PH=O)
rwas = kwas [P co - (P co2 P H~) I (Kwes Pa,o) ]
The cobalt catalyst did not s h o w any significant water-gas-shift activity and no water inhibition of the Fisher-Tropsch reaction rate. The published Satterfield rate equation c1~1 could be fitted to the observed kinetic data, viz" rFr = (kFT PCO PH~ I (1 + B Pco)2 As representative reactor temperature for the Fe catalyst, 2 4 0 ~ can be considered. In selecting a suitable temperature for the cobalt alternative, the approach was to keep the temperature as low as possible in order to maximize middle distillate selectivities as supported by thermodynamics ~12). In this regard 2 2 0 ~ was selected, a temperature that also ensured comparable hydrocarbon yields between the t w o catalysts investigated, as can be deduced from figure 1. Figures 1 and 2 were constructed from a CSTR model in which possible masstransfer limitations were eliminated, no gas recycle was considered, and a feed gas of 67 vol% H 2 and 33 vol% CO was assumed. Although space velocities were relativised they are based on a per unit catalyst mass basis and not per unit volume. FIGURE 2. PRODUCTIVITY COMPARISON BETWEEN THE IRON CATALYST (240~ AND THE COBALT CATALYST (220~
FIGURE 1. ACTIVITY COMPARISON BETWEEN THE IRON AND COBALT CATALYSTS AT 20 BAR
(~
~
100 90 \
~.
80 7o
~ ~
60 50
~ ~ ~
40
~
>" 4 t--rm o_ J "' 3 >
'
I,M
= Fe more productive
rat)
3o
Co more productive
14.1
>
c-. 1
2o 10 0
~ .
(.3 < ix. 2
5I . g 0
.
.
.
~"
.
.
i
.
.
g
.
.
.
4
RELATIVE SPACE VELOCITY Cobalt catalyst (220~ - - -Iron catalyst (240~
nr"
5
0
10
,
14 ' 18 ' 2'2 ' 2'6 ' 3(3 ' 3~4 ' 3i REACTOR PRESSURE (bar)
From figure 1 it can be concluded that at 20 bar operation, the cobalt catalyst outperforms the iron alternative at low relative space velocities. Figure 2 confirms the notion that cobalt is favoured at low reactor pressures combined with low space velocities, the latter implying conditions favourable for high per pass conversion levels. These observations can be explained in terms of: Cobalt based Fischer-Tropsch kinetics is not inhibited by reaction water. Iron based Fischer-Tropsch kinetics is more sensitive towards the absolute hydrogen partial pressure, whilst cobalt kinetics depend more on the PH21Pco ratio.
209
The activity of this cobalt catalyst does not only compare favourably with iron, but also with patented cobalt alternatives c3~czj, based on a direct comparison of the pre-exponential factors as derived from the selected rate equation. FIGURE 3. ACTIVITYCOMPARISON BETWEEN THE IRON AND COBALT CATALYSTSAT 20 bar (HECO feed ratio = 1)
~. _
.
8
8o 70
60
so 40
oo
o~
30 20 10 0
0
1
2
3
4
5
RELATIVE SPACE VELOCITY - - C o b a l t catalyst - - Iron catalyst
It must be emphasised that the above-mentioned discussion presupposes as H2/CO feed ratio of 2 which should favour cobalt (i.e. usage rate = 2 as a result of relative little CO2 production). A H2/CO feed ratio of 1 would be favourable for iron as can be concluded from figure 3. It must, however, be added that figures 1 and 3 were constructed without taking deactivation as a result of re-oxidation by reaction water into consideration. Iron being more prone to re-oxidation, renders stable operation at high syngas conversion levels unlikely.
4.CATALYST STABILITY (DEACTIVATION) The relative high cost of cobalt demands high catalyst stabilities in order to ensure extended runs. With respect to the cobalt catalyst t w o regimes of deactivation were observed, labelled as A and B in figure 4 (Synthesis conditions: 220~ 20 bar; commercial syngas feed: 50 vol% H 2, 25 vol% CO and 25 vol% inerts; relative space velocity = 0.5). Region A ( - 4 days) can be considered as a conditioning phase and is associated with a reversible deactivation. The build-up of high molecular weight hydrocarbons 80 A B inside the catalyst pores (i.e. surface ~ 70 condensation) is a likely cause ~13~. c 60 o Region B is associated with u 50irreversible deactivation, and is thus ~ 40of critical importance. It was c >" 3 0 t,n determined that the rate of this N 20deactivation is directly proportional 10to the syngas space velocity. This 00 lb 2'0 2'5 30 deactivation is probably due to low level sulphur poisoning. It was T i m e on line ( D a y s ) established that the sulphur level in the particular coal based plant syngas used (i.e. --0.03mgS/m3,), is too high to guarantee run lengths of up to 1 year with the cobalt catalyst. This is expected to be less of a problem for all cases (including natural gas derived syngas) where an adequate sulphur guard system is used.
FIGURE 4. STABILITY RUN ON THE COBALT CATALYST 100 90
210
5.HYDROCARBON
SELECTIVITY
INVESTIGATION
The direct determination of quantitative information regarding high molecular weight Fischer-Tropsch products is difficult on a laboratory scale because of limited wax production. It was, however, established that an adapted version of the published "double (x" Schulz-Flory model (as derived by Donnelly c14)) is suitable for the extrapolation of selectivities. The selectivities of the products C1 to --C~s was determined accurately by means of GC analyses (having utilized the ampoule sampling technique c15) ) of the volatile overhead phase, and this information was extrapolated to the reactor wax (i.e. >C18 ) or the hard wax (i.e. > C37 )cut. Generally observed deviations from the classical Schulz-Flory polymerization model includes: i) An underestimation of C1 selectivities and an overestimation of C2 selectivities ii) A break in experimental Schulz-Flory plots at a carbon number of about 10 The consequence of these experimental deviations is that ideal Schulz-Flory (i.e. a single chain growth probability describing the whole product spectrum) cannot be beaten. By simply comparing catalysts on the basis with which their hard wax selectivities approach ideal Schulz-Flory, care is taken of all likely deviations in one step. An elegant manner of visualizing this comparison is proposed by figure 5. The experimental data points used in the construction of figure 5 represent a large variation in operating conditions from runs performed at Sastech on iron and cobalt. From figure 5 it should be clear that FIGURE 5. HARD WAX (C3e+) SELECTIVITIES AS a cobalt based Fisher-Tropsch FUNCTION OF CHAIN GROWTH PROBABILITY catalyst has selectivities somewhat 100 closer to ideal Schulz-Flory than the 90 iron catalyst. - 80 Ways of manipulating the wax 70 chain growth probability also differs uj ,_J 6 0 between cobalt and iron. In the case uJ 5 0 03 of iron it is best achieved via the .*: E 40 addition of chemical promoters c16~ ~ 30 9 e+* (e.g. potassium) and additional fine u 2(1 tuning can be accomplished through N10 .r. adjustments in reactor temperature Oo~--.-,--,,--::, ,. ......... 9 0.9 and/or syngas HJCO ratios c1~. With WAX CHAIN GROWTH PROBABILITY respect to cobalt based Fischer+ Iron catalyst Tropsch the notion that drastic 9 Co catalyst m Ideal Schulz-Flory selectivity changes in selectivities can be effected by increasing the reactor pressure was already published in 1956 c17). The role of chemical promoters in the case of cobalt was therefore questioned c18). Eri et.al. ~7), however, challenged this premise, but the data on which their conclusions were based reflected differences in conversion levels, thus differences in reactor partial pressures. Their claim of chemical promotion effected by K, Na, Cs and Rb could therefore not be regarded as conclusive. Studies performed by Sastech c9) also confirmed the ineffectiveness of chemical promotion of cobalt Fischer-Tropsch catalysts.
~ !
211
The a p p r o a c h that w a s used during this i n v e s t i g a t i o n w a s to c o m p a r e selectivities at different reactor pressures, w h i l s t keeping the e m p t y Vessel based linear v e l o c i t y constant. A t e m p e r a t u r e of 2 2 0 ~ w a s c o n s i d e r e d for the cobalt c a t a l y s t and 2 4 0 ~ for the iron catalyst. The feed gas c o m p o s i t i o n w a s : 51 v o l % H 2, 27 v o l % CO, 9 v o l % CH, and 13 v o l % Ar. The c o r r e s p o n d i n g w a x chain g r o w t h p r o b a b i l i t y f a c t o r s are d e p i c t e d in figure 6, indicating that cobalt based w a x selectivities indeed s h o w greater s e n s i t i v i t y t o w a r d s reactor pressure. Detailed s e l e c t i v i t y b r e a k - d o w n s FIGURE 6. CHAIN GROWTH PROBABILITY (x2 are p r o v i d e d in figures 7 and 8. For AS FUNCTION OF REACTOR PRESSURE this purpose the following AT CONSTANT SUPERFICIAL VELOCITY c o n v e n t i o n w a s used: 1wax C19 + 0.9 diesel = C~2- C~8 gasoline = C6 - C~1 0.8 LPG = C 3 - C4 8' fuelgas = C~ - C 2 0.7 From figures 7 and 8 the c o n c l u s i o n can be d r a w n that for high e n o u g h 0.6 reactor pressures, w a x selectivities 0o5 of - - 5 0 m a s s % can be o b t a i n e d 0 10 ' 20 ' 3'0 ' 40 w i t h cobalt, w h i c h are c o m p a r a b l e REACTOR PRESSURE (bar) w i t h that of iron. Iron based w a x + Iron catalyst 9 Co catalyst selectivities are not m u c h a f f e c t e d by reactor pressure, but it does seem as if a m a x i m u m is reached at --- 25 bar. If diesel p r o d u c t i o n is the o b j e c t i v e in the case of cobalt based F i s c h e r - T r o p s c h , high pressure operation is also advised. The reason for this being t w o - f o l d , viz: l o w pressure o p e r a t i o n ( < 9 bar) is associated w i t h high b r a n c h i n g degrees c19~(i.e. as high as S y n t h o l ) , w h i l s t m e d i u m pressure o p e r a t i o n is associated w i t h b r a n c h i n g degrees on par w i t h iron based l o w t e m p e r a t u r e F i s c h e r - T r o p s c h (-- 5 m a s s % c19~ ). Fisher-Tropsch w a x boiling a b o v e 3 5 0 ~ can easily be h y d r o c r a c k e d to e x t i n c t i o n , yielding -- 8 0 % diesel w i t h a cetane n u m b e r of at least 70. =
FIGURE 7. MASS% PRODUCT DISTRIBUTION AS FUNCTION OF REACTOR PRESSURE AT CONSTANT SUPERFICIAL VELOCITY FOR THE IRON SLURRY PHASE CATALYST 100 z o 90 r-- 80
(n 70 o a. 60 :~ 50 O u 40 30 (f} 20 < 10 3~ 0
FIGURE 8. MASS% PRODUCT DISTRIBUTION AS FUNCTION OF REACTOR PRESSURE AT CONSTANT SUPERFICIAL VELOCITY FOR THE COBALT CATALYST 100
~
90
rJo
80 70
o
SFuelgas
_
~ o l i n e
.~ 6o
o 50 ~ m <
10 20 30 40 REACTOR PRESSURE (bar)
2~
40 30 20 10 0
10 20 30 REACTOR PRESSURE (bar)
40
212
6.CONCLUSION In conclusion it can be stated that cobalt based catalysts (despite higher initial cost) can be considered as an alternative to iron based Fischer-Tropsch catalysts for the production of middle distillates utilizing a syngas w i t h a H2/CO ratio of 2. The strong advantage of the cobalt option is to be found in the application w h e r e high per pass syngas conversion levels are desired. If a comparable net conversion is to be achieved w i t h iron, it will require gas recycle in order to knock out water, implying added compression costs. In order to help c o m p e n s a t e for the relative high cost of cobalt, extended runs will have to be guaranteed. This prerequisite increases the importance of catalyst stability, and the presence of l o w level sulphur catalyst poisoning will seriously disadvantage commercial cobalt application. 7.NOMENCLATURE Specific Fischer-Tropsch reaction rate Fischer-Tropsch rate c o n s t a n t Specific water-gas-shift reaction rate Water-gas-shift rate c o n s t a n t Water-gas-shift equilibrium constant Partial reactor pressure of c o m p o u n d x Symbols used to represent reaction rate variables Chain g r o w t h probability factor (i = 1 for l o w molecular w e i g h t products and i = 2 for high molecular w e i g h t products)
rFT kFT FWGS kwGs KWGS Px A,B Oli
REFERENCES
10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
M.E. Dry, Catalysis-Science and Technology, editors: J.R. Anderson and M. Boudart, Springer-Verlag, 1(1981 ). B. Jager, M.E. Dry, T. Shingles and A.P. Steynberg, Catalysis Letters, 7 (1990) 293-302. E. Iglesia, S.L. Soled and R.A. Fiato, US Patent No. 4 794 099 (1988). H. Beuther, C.L. Kibby, T.P. Kobylinski and R.B. Pannell, US Patent No. 4 413 064 (1983). P. Chaumette and C. Verdon, UK Parent No. GB 2 258 414 A (1993). M.F.M. Post and S.T. Sie, European Patent No. 0 167 215 A2 (1986). S. Eri, J.G. Goodwin, G. Marcilin and T. Riis, US Patent No 4 880 763 (1989). B. Jager and R. Espinoza, Catalysis Today, 23 (1995) 17-28. R.L. Espinoza, J.L. Visagie, P.J. van Berge and F.H.A. Bolder, RSA Provisional Patent No 95/2903 (1995). P.J. van Berge, PhD thesis, Potchefstroom University for CHE, South Africa, (1994). C.N. Satterfield, I.C. Yates and C.A. Chanenchuk, Energy and Fuels, 5 (1991) 168-173 and 847-855. S. Oliv~ and G. Henrici-Oliv~, "The Chemistry of the Catalyzed Hydrogenation of CO", Springer-Verlag, Berlin - Heidelberg - New York - Tokyo (1984). E. Iglesia, S.L. Soled, R.A. Fiato and G.H. Via, Journal of Catalysis, 143 (1993) 345-368. T.J. Donnelly, C.N. Satterfield and I.C. Yates, Energy and Fuels, 2(1988) 734-739. H Schulz and A. Geertsema, Erd61 und Kohle, 30 (1977) 31 3. M.E. Dry, Brennstoff-Chemie, 50 (1969) 193-224. R.B. Anderson, Catalysis, editor: P.H. Emmett, 4 (1956). R.W. Joyner, Vacuum, 38 (1988) 309-315. H. Pichler, H. Schulz and D. K(ihne, Brennstoff-Chemie, 49 (1968) 344-351.