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A Comparison Of Forced Feed Cycling Of The Fischer-Tropsch Synthesis Over Iron And Cobalt Catalysts
191
S. Kaliaguine and A. Mahay (Editors), Catalysis on the Energy Scene © 1984 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
A COMPARISON OF FORCED FEED CYCLING OF THE FISCHER-TROPSCH SYNTHESIS OVER IRON AND COBALT CATALYSTS
A.A. Adesina, P.L. Silveston and R.R. Hudgins Chemical
Engineering Department,
University
of Waterloo,
Waterloo,
Ontario,
Canada N2L 3Gl
INTRODUCTION In
recent
years,
a
substantial
effort
has
been
devoted
to
detai led
examination of the effect of periodic operation on chemical processes. the
possible
improvement
in
time-average
reaction
rates
under
Besides periodic
operation (forced cycling), enhanced selectivity to desired products is also a potential
benefit
laboratory,
to
Feimer
be
[1]
sought
was the
from first
complex to
reaction
study
the
systems.
effect
of
In
forced
our feed
concentration cycling on both reaction rate and product selectivity in the FT synthesis
for
hydrocarbon
polymerisation reaction. led to improvement in gasoline range
production
over
an
iron
He observed that cyclical
catalyst
-
a
complex
feed composition changes
the methanation rates whereas the formation rates for
hydrocarbons
remained
practically
unaffected.
Consequently,
selectivity under periodic operation dropped below its steady-state value. This
paper
compares
results
over
the
cobalt
catalyst
for
similar
experiments.
EXPERIMENTAL The experimental system and product analysis procedure for these two studies on iron and cobalt were similar. of
the preset temperature.
pre-purified Hz and
The oil bath was controlled to within + 1°C
A11 feed gases were predri ed and premi xed from
CP grade CO.
Detai ls
of
generating
square wave feed
concentrations are given elsewhere [2,3]. The i ron catalyst promoted with copper and potassium was provi ded by Exxon Corporation
while
the
kieselguhr-supported
United Catalysts Inc., Louisville, Kentucky. about 0.25 mn and 0.15
Jml
respectively.
cobalt
catalyst was
supplied
by
Catalyst particle diameters were
In each case, the catalyst bed was
192 diluted with about 5:1 with quartz beads to ensure uniform temperature within the bed [4 J. The reactor, a simple fixed bed with thermocouples placed within the bed and entrance to the reactor, also had a preheater coil for the reactant gas and the Differential conversions « 10%) were
whole assembly immersed in the oil bath. maintained in all runs.
Various diagnostic criteria and experiments were used
to demonstrate that the kinetic data were free from transport intrusions. The
literature
[5J
suggests
that
iron
FT catalysts
have
much
higher
stabi 1ity and product i vity with operating pressure in the range 3-10 atm whil e optimal performance for the cobalt catalyst is found in the interval 1-3 atm. Both, however, thrive in the normal In
our
studies,
these
FT reaction temperature range (450-600K).
considerations
combined
with
equipment
limitations
necessitated the use of 519K and 384 kPa pressure for the iron catalyst and experimental conditi ons for the cobalt catalyst were 473K and 115 kPa.
Data
from cyclic operation were obtained under cycle-invariant states. Hydrocarbons were analysed using a Carle gas chromatograph, AGC-211 equipped with a flame ionisation detector. to C9 components were monitored. Fluorad
-
were
used
to
Because of the low conversions, only the C1 Two separate parallel columns - Poracil C and
separate
the
C1
to
C5
and
C6
to
C9
fractions
respecti vely. The infrared
(IR) spectrophotometer (Beckmann Acculab 2) was set at 2360
cm- 1 wave number to provide continuous monitoring of the COz concentration in the product.
Water
product was removed
over
a Dri eri te fi 1ter.
Further
details are given by Feimer [IJ and Adesina [3J.
RESULTS AND DISCUSSION Steady-state results for the iron and cobalt catalysts were obtained over
< YH z < 1.
the feed composition range t ,e, 0 olefin
formers,
maxima
in
ratios.
the
except
at
steady-state
high
Hz
curves
feed
were
Both catalysts were primarily
concentrations
heavily
skewed
(YH > 0.8). The z towards high Hz/CO
However, cobalt appears to be a better olefin former than iron.
agrees with the findings of
Bell
[6J.
Even t~ough
This
the mechanism of the FT
reaction is not fully understood, models based on the oxygenated intermediate mechani sm seem to descri be the methane steady state data for both catalysts reasonably well [1,3J. Response to step-change feed concentrations [2J gave overshoots higher than maximum attainable steady-state rates during transients on both catalysts (see Figure 1).
Even so, the magnitudes of the overshoots for some hydrocarbons and
193
2
T
a
473 K
pa 115 kPa
240
Fig. 1. Alkene Production Following Step-Changes In Feed Concentration Over Co Catalyst.
1.4 10.1
Q~
~~
1.2
....I~
c[> 2c[ Q::'
010.1 1.0
%2
i= 0.8
0
_~E
~-
Cz
4
8
/2
PERIOD (min)
Fig. 2. Normalized Time-Average Rate as a Function of Period for C to C l 3 Over Fe Catalyst.
194
the relaxation times to the steady-state were substantially different on iron and cobalt.
These differences are symptomatic of the dissimilarities between
the catalysts under periodic operation.
Nevertheless, for both iron [2J and
cobalt [3J catalysts, there was evidence that carbon, probably as carbide, diffused into and out of storage in the solid catalyst bulk during transient operations. Rate improvements through peri odi c operation are to be expected when the cycle periods approximate the relaxation times of the overshoots observed in step-change experiments.
Gradually, the relaxati on times found
for cobalt
(approx. 40 minutes) were about twice those for the iron catalyst.
As a
result, studies on cycling over cobalt were done in a range of cycle-periods about double those for iron. Figure 2 shows the effect of cycling period on normalized time-average rates for the C1 to C3 products over iron [1J.
Only the methanation rate was
improved above the steady-state value (=1.0).
Results are not shown for the
C4 + species since they also exhibit no rate improvement. However, over cobalt catalyst for symmetrical cycles having a time-average mole fraction of hydrogen in the feed, m=O.825, the C1 to C7 formation rates were improved, the optimum period being 40 minutes.
Figure 3 is typical of
the general behavi our. Although the ranges of cycle periods differ, time-average rates for both cata lysts approach the quasi -steady-state va1ue « 1.0).
(The quas i-steady
state is simply a mixed output of the steady-state rates correspondi ng to the feed compositions used in each part of the cycle).
In this case, the feed
duri ng one hal f-cycle was pure H2 , correspondi ng to a zero steady-state rate. Thus, the improvement in time-average rates is simply a manifestation of the overshoot that results from forced repetition. For the iron catalyst, the overshoot arising from the diffusion of a bulk phase carbide inventory enhances the surface reaction rate. times found
for cobalt suggest that the bulk-diffusi onal
than with iron.
Longer relaxation process is slower
Since the FT synthesis is a polymerisation, it would appear
that the weaker overshoot for the C2 + species on i ron is probably due to the relaxation to steady-state concentrations of surface relaxes to C2 + species before any appreciable chain construction occurs.
Thus, the chain propagation
would only be due to the contribution of surface carbon atoms.
The longer
relaxation times found for cobalt suggest that there was probably enough time for chain growth.
Consequently, the contribution by carbon from within the
bulk and surface capacitance made possible the occurrence of overshoots higher than
the
maximum
attainable
steady
state.
The
result
was
to
improve
195
o
PROPANE ETHANE a ETHENE METHANE a OVERALL l!. PROPENE • •
2
55
OUASI- STEADY-STATE LINE FOR OVERALL RATE OF REACTION
oL - -_ _......L o 20
..L.-
.-...Jl.-
40
60
-.L:-__ 80
CYCLE PERIOD (min)
Fig. 3. Effect of Cycling on Rate with Cycle-Splat of 0.5 and Amplitude of 0.125 for C1 to C3; Co Catalyst.
"z
FEIllER (I I l!. CYCLING A S5 ADESINA 119831 o CYCLING • S5
~
.<:
D ...
....• '0
E E
... ~
II::
O.ZO
a
.n 0.10 0.08
0.04
O.OZ
0.01
I
3
5
7
,
.83
CARBON NUMBER
Fig. 4. Comparison of Flory Plots for Steady-State and Cycling over Co and Fe Catalysts.
196 time-average production for C1 to C7 under cycling.
It is also apparent that
the lack of improvement for the C8 and C9 products could be attributed to the slow polymerisation process that dampens the overshoot phenomenon. Selectivity to higher molecular weight
products
in
the FT synthesis
is
usually measured in terms of the chain growth probability, a. Under steady-state reaction conditions the rate is related to a by means of the Schulz-Flory model:
A semilog plot of r versus n, carbon number, yields a straight line with slope In a.
Figure 4 is an extension of the normal plot for the products over iron
and cobalt to include both steady and cycling operation. at
steady
state
gasoline-range
the
cobalt
hydrocarbons
selectivity seemed to fall
catalyst
than
the
produced
iron
a
It is evident that
higher
catalyst.
In
fraction
addition,
of
while
slightly (5%) under periodic operation for iron,
there was a slight improvement (5%) in the selectivity for cobalt.
ACKNOWLEDGEMENTS Support Engineering
for
this
Research
work
was
Council
provided (NSERC)
through
grant.
a
Natural
Thanks
are
Sciences due
to
and Exxon
Corporation and United Catalysts Inc , , Louisville, Kentucky for the iron and cobalt catalysts respectively.
One of us (A.A.A.) also appreciates the study
leave granted by the University of Port Harcourt, Nigeria.
REFERENCES 1 2 3 4 5
J.L. Feimer, "A Study of the Forced-Feed Composition Cycling in the Fischer-Tropsch Synthesis", Ph.D. Thesis, University of Waterloo, Waterloo, Ontario, Canada, 19B2. J.L. Feimer, P.L. Silveston and R.R. Hudgins, "Influence of Forced Cycling on the Fi scher-Tropsch Synthes is, Part I. Response to Feed Concent rat ion Step-Changes", Can. J. Chern. Eng., in press, 1984. A.A. Adesina, "Transient Studies of the Fischer-Tropsch Reaction", M.A.Sc. Thesis, University of Waterloo, Waterloo, Ontario, Canada, 1984. D.E. Mears, "Diagnostic Criteria for Heat Transport Limitations in Fixed Bed Reactors", J. Cata1., 20, 127-131 (1971). Storch, H.H., N. Goulombic and R.B. Anderson, "Fischer-Tropsch and Related Syntheses", John Wiley, New York, 1951.
S. Kaliaguine and A. Mahay (Editors), Catalysis on the Energy Scene © 1984 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
A COMPARISON OF FORCED FEED CYCLING OF THE FISCHER-TROPSCH SYNTHESIS OVER IRON AND COBALT CATALYSTS
A.A. Adesina, P.L. Silveston and R.R. Hudgins Chemical
Engineering Department,
University
of Waterloo,
Waterloo,
Ontario,
Canada N2L 3Gl
INTRODUCTION In
recent
years,
a
substantial
effort
has
been
devoted
to
detai led
examination of the effect of periodic operation on chemical processes. the
possible
improvement
in
time-average
reaction
rates
under
Besides periodic
operation (forced cycling), enhanced selectivity to desired products is also a potential
benefit
laboratory,
to
Feimer
be
[1]
sought
was the
from first
complex to
reaction
study
the
systems.
effect
of
In
forced
our feed
concentration cycling on both reaction rate and product selectivity in the FT synthesis
for
hydrocarbon
polymerisation reaction. led to improvement in gasoline range
production
over
an
iron
He observed that cyclical
catalyst
-
a
complex
feed composition changes
the methanation rates whereas the formation rates for
hydrocarbons
remained
practically
unaffected.
Consequently,
selectivity under periodic operation dropped below its steady-state value. This
paper
compares
results
over
the
cobalt
catalyst
for
similar
experiments.
EXPERIMENTAL The experimental system and product analysis procedure for these two studies on iron and cobalt were similar. of
the preset temperature.
pre-purified Hz and
The oil bath was controlled to within + 1°C
A11 feed gases were predri ed and premi xed from
CP grade CO.
Detai ls
of
generating
square wave feed
concentrations are given elsewhere [2,3]. The i ron catalyst promoted with copper and potassium was provi ded by Exxon Corporation
while
the
kieselguhr-supported
United Catalysts Inc., Louisville, Kentucky. about 0.25 mn and 0.15
Jml
respectively.
cobalt
catalyst was
supplied
by
Catalyst particle diameters were
In each case, the catalyst bed was
192 diluted with about 5:1 with quartz beads to ensure uniform temperature within the bed [4 J. The reactor, a simple fixed bed with thermocouples placed within the bed and entrance to the reactor, also had a preheater coil for the reactant gas and the Differential conversions « 10%) were
whole assembly immersed in the oil bath. maintained in all runs.
Various diagnostic criteria and experiments were used
to demonstrate that the kinetic data were free from transport intrusions. The
literature
[5J
suggests
that
iron
FT catalysts
have
much
higher
stabi 1ity and product i vity with operating pressure in the range 3-10 atm whil e optimal performance for the cobalt catalyst is found in the interval 1-3 atm. Both, however, thrive in the normal In
our
studies,
these
FT reaction temperature range (450-600K).
considerations
combined
with
equipment
limitations
necessitated the use of 519K and 384 kPa pressure for the iron catalyst and experimental conditi ons for the cobalt catalyst were 473K and 115 kPa.
Data
from cyclic operation were obtained under cycle-invariant states. Hydrocarbons were analysed using a Carle gas chromatograph, AGC-211 equipped with a flame ionisation detector. to C9 components were monitored. Fluorad
-
were
used
to
Because of the low conversions, only the C1 Two separate parallel columns - Poracil C and
separate
the
C1
to
C5
and
C6
to
C9
fractions
respecti vely. The infrared
(IR) spectrophotometer (Beckmann Acculab 2) was set at 2360
cm- 1 wave number to provide continuous monitoring of the COz concentration in the product.
Water
product was removed
over
a Dri eri te fi 1ter.
Further
details are given by Feimer [IJ and Adesina [3J.
RESULTS AND DISCUSSION Steady-state results for the iron and cobalt catalysts were obtained over
< YH z < 1.
the feed composition range t ,e, 0 olefin
formers,
maxima
in
ratios.
the
except
at
steady-state
high
Hz
curves
feed
were
Both catalysts were primarily
concentrations
heavily
skewed
(YH > 0.8). The z towards high Hz/CO
However, cobalt appears to be a better olefin former than iron.
agrees with the findings of
Bell
[6J.
Even t~ough
This
the mechanism of the FT
reaction is not fully understood, models based on the oxygenated intermediate mechani sm seem to descri be the methane steady state data for both catalysts reasonably well [1,3J. Response to step-change feed concentrations [2J gave overshoots higher than maximum attainable steady-state rates during transients on both catalysts (see Figure 1).
Even so, the magnitudes of the overshoots for some hydrocarbons and
193
2
T
a
473 K
pa 115 kPa
240
Fig. 1. Alkene Production Following Step-Changes In Feed Concentration Over Co Catalyst.
1.4 10.1
Q~
~~
1.2
....I~
c[> 2c[ Q::'
010.1 1.0
%2
i= 0.8
0
_~E
~-
Cz
4
8
/2
PERIOD (min)
Fig. 2. Normalized Time-Average Rate as a Function of Period for C to C l 3 Over Fe Catalyst.
194
the relaxation times to the steady-state were substantially different on iron and cobalt.
These differences are symptomatic of the dissimilarities between
the catalysts under periodic operation.
Nevertheless, for both iron [2J and
cobalt [3J catalysts, there was evidence that carbon, probably as carbide, diffused into and out of storage in the solid catalyst bulk during transient operations. Rate improvements through peri odi c operation are to be expected when the cycle periods approximate the relaxation times of the overshoots observed in step-change experiments.
Gradually, the relaxati on times found
for cobalt
(approx. 40 minutes) were about twice those for the iron catalyst.
As a
result, studies on cycling over cobalt were done in a range of cycle-periods about double those for iron. Figure 2 shows the effect of cycling period on normalized time-average rates for the C1 to C3 products over iron [1J.
Only the methanation rate was
improved above the steady-state value (=1.0).
Results are not shown for the
C4 + species since they also exhibit no rate improvement. However, over cobalt catalyst for symmetrical cycles having a time-average mole fraction of hydrogen in the feed, m=O.825, the C1 to C7 formation rates were improved, the optimum period being 40 minutes.
Figure 3 is typical of
the general behavi our. Although the ranges of cycle periods differ, time-average rates for both cata lysts approach the quasi -steady-state va1ue « 1.0).
(The quas i-steady
state is simply a mixed output of the steady-state rates correspondi ng to the feed compositions used in each part of the cycle).
In this case, the feed
duri ng one hal f-cycle was pure H2 , correspondi ng to a zero steady-state rate. Thus, the improvement in time-average rates is simply a manifestation of the overshoot that results from forced repetition. For the iron catalyst, the overshoot arising from the diffusion of a bulk phase carbide inventory enhances the surface reaction rate. times found
for cobalt suggest that the bulk-diffusi onal
than with iron.
Longer relaxation process is slower
Since the FT synthesis is a polymerisation, it would appear
that the weaker overshoot for the C2 + species on i ron is probably due to the relaxation to steady-state concentrations of surface relaxes to C2 + species before any appreciable chain construction occurs.
Thus, the chain propagation
would only be due to the contribution of surface carbon atoms.
The longer
relaxation times found for cobalt suggest that there was probably enough time for chain growth.
Consequently, the contribution by carbon from within the
bulk and surface capacitance made possible the occurrence of overshoots higher than
the
maximum
attainable
steady
state.
The
result
was
to
improve
195
o
PROPANE ETHANE a ETHENE METHANE a OVERALL l!. PROPENE • •
2
55
OUASI- STEADY-STATE LINE FOR OVERALL RATE OF REACTION
oL - -_ _......L o 20
..L.-
.-...Jl.-
40
60
-.L:-__ 80
CYCLE PERIOD (min)
Fig. 3. Effect of Cycling on Rate with Cycle-Splat of 0.5 and Amplitude of 0.125 for C1 to C3; Co Catalyst.
"z
FEIllER (I I l!. CYCLING A S5 ADESINA 119831 o CYCLING • S5
~
.<:
D ...
....• '0
E E
... ~
II::
O.ZO
a
.n 0.10 0.08
0.04
O.OZ
0.01
I
3
5
7
,
.83
CARBON NUMBER
Fig. 4. Comparison of Flory Plots for Steady-State and Cycling over Co and Fe Catalysts.
196 time-average production for C1 to C7 under cycling.
It is also apparent that
the lack of improvement for the C8 and C9 products could be attributed to the slow polymerisation process that dampens the overshoot phenomenon. Selectivity to higher molecular weight
products
in
the FT synthesis
is
usually measured in terms of the chain growth probability, a. Under steady-state reaction conditions the rate is related to a by means of the Schulz-Flory model:
A semilog plot of r versus n, carbon number, yields a straight line with slope In a.
Figure 4 is an extension of the normal plot for the products over iron
and cobalt to include both steady and cycling operation. at
steady
state
gasoline-range
the
cobalt
hydrocarbons
selectivity seemed to fall
catalyst
than
the
produced
iron
a
It is evident that
higher
catalyst.
In
fraction
addition,
of
while
slightly (5%) under periodic operation for iron,
there was a slight improvement (5%) in the selectivity for cobalt.
ACKNOWLEDGEMENTS Support Engineering
for
this
Research
work
was
Council
provided (NSERC)
through
grant.
a
Natural
Thanks
are
Sciences due
to
and Exxon
Corporation and United Catalysts Inc , , Louisville, Kentucky for the iron and cobalt catalysts respectively.
One of us (A.A.A.) also appreciates the study
leave granted by the University of Port Harcourt, Nigeria.
REFERENCES 1 2 3 4 5
J.L. Feimer, "A Study of the Forced-Feed Composition Cycling in the Fischer-Tropsch Synthesis", Ph.D. Thesis, University of Waterloo, Waterloo, Ontario, Canada, 19B2. J.L. Feimer, P.L. Silveston and R.R. Hudgins, "Influence of Forced Cycling on the Fi scher-Tropsch Synthes is, Part I. Response to Feed Concent rat ion Step-Changes", Can. J. Chern. Eng., in press, 1984. A.A. Adesina, "Transient Studies of the Fischer-Tropsch Reaction", M.A.Sc. Thesis, University of Waterloo, Waterloo, Ontario, Canada, 1984. D.E. Mears, "Diagnostic Criteria for Heat Transport Limitations in Fixed Bed Reactors", J. Cata1., 20, 127-131 (1971). Storch, H.H., N. Goulombic and R.B. Anderson, "Fischer-Tropsch and Related Syntheses", John Wiley, New York, 1951.