Note: Descriptions are shown in the official language in which they were submitted.
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T 9024
CATALYST ACTIVATION PROCESS AND CATALYST REACTIVATION
PROCESS
The present invention relates to a process for
activating a catalyst which is used for the conversion of
a mixture of carbon monoxide and hydrogen into hydro-
carbons. The present invention also relates to a process
for reactivating such catalyst after it has been used and
has been at least partially deactivated.
The preparation of hydrocarbons from a mixture of
carbon monoxide and hydrogen at elevated temperature and
pressure in the presence of a suitable catalyst is
generally known as the Fischer-Tropsch hydrocarbon
synthesis. Catalysts used in this hydrocarbon synthesis
are normally referred to as Fischer-Tropsch catalysts and
usually comprise one or more metals from Group VIII of
the Periodic Table of Elements, optionally together with
one or more promoters, and, typically a carrier material.
In order to be suitable in the conversion of a H2/CO
mixture into hydrocarbons the Fischer-Tropsch catalyst is
normally first subjected to an activation treatment.
Activation generally is carried out by contacting the
catalyst with a reducing gas, such as a hydrogen-
contalnlng gas.
For instance, in US-A-4,413,064 a process for
preparing a Fischer-Tropsch catalyst is disclosed,
whereby after the last impregnation step the catalyst is
activated by slowly reducing it in the presence of
hydrogen at a temperature from about 250 C to 400 C.
The hydrogen source may be pure hydrogen or a mixture of
hydrogen and nitrogen. Similarly, in EP-A-0,168,894 a
method for activating a Fischer-Tropsch catalyst is
disclosed which involves contacting the catalyst with a
hydrogen-containing gas at elevated temperature, whereby
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the hydrogen partial pressure is gradually or step-wise
increased from an initial value to an ultimate value
which is at least 5 times as high as said initial value.
The activation procedures according to both
US-A-4,413,064 and EP-A-0,168,894 are conducted on the
catalyst before loading it into a fixed bed.
In situ activation of catalyst, i.e. activation of
catalyst after it has been loaded into the reactor where
the Fischer-Tropsch synthesis will take place, is a known
procedure. It involves passing a stream of reducing gas,
usually a hydrogen-containing gas, through the catalyst
bed in the same direction as the flow of reactant gas
during operation, thereby activating the catalyst
particles. One of the disadvantages of this method is
that the catalyst activity decreases in the direction of
the gas flow. This is mainly caused by the fact that
water is formed as a reaction product in the reduction of
the inactive Group VIII metal compound into its
catalytically active form. For this reason the off-gas
stream, i.e. the gas stream leaving the reactor after the
activation, contains water. The formed water is passed
with the reducing gas through the catalyst bed. However,
without wishing to be bound by a particular theory, it
would appear that water inhibits the reduction of Group
VIII metal compound(s) and as a result the degree of
reduction, and thus the activity of the catalyst,
decreases in the direction of the reducing gas flow.
During normal operation, therefore, the level of
conversion also decreases in the direction of the
reactant gas flow. This can be directly measured by
determining the temperature profile along the catalyst
bed. The Fischer-Tropsch synthesis reaction is strongly
exothermic and accordingly a lot of heat is generated.
The temperature profile shows that the temperature is the
highest in that part of the catalyst bed first contacted
with the reactant gas flow and decreases as the reactant
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gas flow passes further through the catalyst bed. This
shows that the conversion level indeed decreases as the
reactant gas flow passes through the catalyst bed.
In US-A-4,778,826 this problem was also recognised.
It relates to a process for converting a feedstock of
C1-C3 alkane into higher molecular weight hydrocarbons by
first reacting the Cl-C3 alkane with air to form a gas
mixture comprising carbon monoxide, hydrogen and nitrogen
and subsequently converting this gas mixture into the
said higher molecular weight hydrocarbons via a
Fischer-Tropsch synthesis reaction. As a solution to the
above mentioned problem occurring with in situ
activation, it is proposed to perform the Fischer-Tropsch
synthesis reaction by passing the gas mixture through an
elongated reactor packed with a bed of Fischer-Tropsch
catalyst, of which the activity increases from the inlet
to the outlet of the reactor. According to US-A-4,778,826
such activity gradient can be achieved in three ways.
Firstly, by dilution of the Fischer-Tropsch catalyst
particles with inert particles, whereby the degree of
dilution decreases from the inlet to the outlet of the
reactor. Secondly, by applying a temperature gradient
along the catalyst bed, whereby the temperature increases
from the inlet to the outlet of the reactor. Finally, by
increasing the concentration of the catalytically active
component in the catalyst particles from the inlet to the
outlet of the reactor. Actual activation of the catalyst
particles is suitably carried out by treatment with a
reducing agent, such as hydrogen, at a temperature of
from 320 to 440 C. This is implemented in practice by
heating the reactor to the appropriate temperature and
passing a stream of hydrogen through the catalyst bed in
a top to bottom direction, i.e. in the same direction as
the flow of reactants during actual operation.
The method according to US-A-4,778,826 evidently
requires additional measures, e.g. careful loading of the
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catalyst bed, in order to ensure the desired activity
gradient to occur. It is one of the main objectives of
the present invention to avoid such additional measures
and still accomplish a more constant conversion level
throughout the entire catalyst bed. More specifically,
the present invention aims to provide an activation
process whereby an activity gradient along the catalyst
bed is obtained with increasing activity in the direction
of the reactant gas flow. Furthermore, the present
invention aims to keep the activation process as simple
as possible and to avoid large capital expenditures for
adapting existing equipment. It will be clear that this
is desirable from both an efficiency and cost perspective
point of view. In fact, it is an objective of the present
invention to provide an activation method, which requires
hardly any adaptation of existing equipment in order to
still achieve the desired activity gradient along the
catalyst bed.
Accordingly, in a first aspect the present invention
relates to a process for activation, preferably in-situ
activation, of a Fischer-Tropsch catalyst packed in a bed
by contacting the catalyst prior to operation, i.e. prior
to operating the catalytic Fischer-Tropsch hydrocarbon
synthesis process, with a reducing gas at a temperature
below 500 C, characterised in that the reducing gas is
passed through the catalyst bed in a direction reversed
to the direction of the flow of reactants during
operation.
Fischer-Tropsch catalysts and methods to prepare them
are known in the art. Usually such catalysts comprise one
or more metals from Group VIII of the Periodic Table of
Elements on a suitable carrier, optionally together with
one or more promoters. Examples of such catalysts and
methods for preparing them are disclosed in
EP-A-0,428,223 and EP-A-0,510,771. Also in the patent
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specifications discussed above, examples of suitable
catalysts are described.
A preferred catalyst to be activated or reactivated
according to the process of the present invention
comprises a cobalt, iron, nickel or ruthenium metal
compound or mixtures thereof. Most preferably, the
catalyst comprises a cobalt metal compound, in particular
a cobalt oxide.
The metal compound is typically supported on a
catalyst carrier. A suitable catalyst carrier may be
chosen from the group of refractory oxides, preferably,
alumina, silica, titania, zirconia or mixtures thereof,
more preferably, silica, silica-zirconia mixtures,
titania or zirconia.
The amount of catalytically active metal present on
the carrier is typically in the range of from 1 to
100 parts by weight, preferably 10 to 50 parts by weight,
per 100 parts by weight of carrier material.
The catalytically active metal may be present in the
catalyst together with one or more metal promoters or co-
catalysts. The promoters may be present as metals or as
the metal oxide, depending upon the particular promoter
concerned. Suitable promoters include oxides of metals
from Groups IIA, IIIB, IVB, VB, VIB and/or VIIB of the
Periodic Table, oxides of the lanthanides and/or the
actinides. Preferably, the catalyst comprises at least
one oxide of an element in Group IVB, VB and/or VIIB of
the Periodic Table, in particular titanium, zirconium,
manganese and/or vanadium. As an alternative or in
addition to the metal oxide promoter, the catalyst may
comprise a metal promoter selected from Groups VIIB
and/or VIII of the Periodic Table. Preferred metal
promoters include rhenium, platinum and palladium.
A most suitable catalyst comprises cobalt as the
catalytically active metal and zirconium as a promoter.
Another most suitable catalyst comprises cobalt as the
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catalytically active metal and manganese and/or vanadium
as a promoter.
The promoter, if present in the catalyst, is
typically present in an amount of from 0.1 to 60 parts by
weight, preferably from 0.5 to 40 parts by weight, per
100 parts by weight of carrier material. It will however
be appreciated that the optimum amount of promoter may
vary for the respective elements which act as promoter.
If the catalyst comprises cobalt as the catalytically
active metal and manganese and/or vanadium as promoter,
the cobalt : (manganese + vanadium) molar ratio is
advantageously at least 12:1.
A particularly preferred Fischer-Tropsch catalyst is
a Co/ZrO2/SiO2 catalyst, i.e. cobalt as the catalytically
active metal on a carrier comprising silica admixed with
zirconium oxide. Prior to activation the cobalt is
usually present as cobalt oxide. By reducing the cobalt
oxide the catalytically active cobalt is obtained.
The reducing gas employed in principle may be any gas
having reducing properties. It is, however, preferred to
use a hydrogen-containing gas.
For the purpose of this specification, a hydrogen-
containing gas is a gas containing hydrogen and,
optionally, one or more inert gas components like
nitrogen. A synthesis gas mixture, comprising hydrogen
and carbon monoxide, is not included in the term
hydrogen-containing gas as used herein. It will, however,
be appreciated that a synthesis gas mixture is a reducing
gas in its own right and may be used as such in the
process of the present invention. If the catalyst to be
activated comprises iron, it is preferred to use a
synthesis gas mixture.
When the catalyst is activated by contacting it with
a hydrogen containing gas, water is formed in the
reduction reaction and this water flows along with the
flow of hydrogen-containing gas through the catalyst bed.
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Accordingly, the water content of the reducing gas stream
increases as the reducing gas passes through the catalyst
bed. Since water inhibits the reduction reaction, the
activation of catalyst may increasingly be hampered in
the direction of the flow of reducing gas, if gas rate
and hydrogen content of the reducing gas are kept
constant. In order to minimise this effect, it is
preferred to increase the amount of hydrogen passing
through the catalyst bed during activation in such way
that the water content in the gas stream leaving the
catalyst bed after activation, i.e. the off-gas, is kept
below a certain level. This level may depend on the
catalyst being (re-)activated and can be determined by
routine experimentation. In general, the water content in
the off-gas is preferably kept below or at 60 mbar. More
preferably, the water content of the off-gas is kept
below 50 mbar.
However, catalysts comprising a silica-containing
carrier tend to be sensitive to too high quantities of
steam present during (re-)activation. Thus, if a catalyst
is to be (re-)activated comprising a silica-containing
carrier, the quantity of steam present in the hydrogen-
containing offgas is preferably less than 40 mbar, more
preferably less than 30 mbar. For some titania or
zirconia-containing catalysts, the quantity of steam in
the hydrogen-containing offgas may suitably be higher,
for example in the range from 40-1000 mbar, preferably
from 40-100 mbar.
The amount of hydrogen passing through the catalyst
bed can be increased either by increasing the gas rate of
the reducing gas during activation, or the total
pressure, while keeping the hydrogen content of the
reducing gas at a constant level, or by increasing the
hydrogen content of the reducing gas gradually or
stepwise during activation. It will be clear that a
combination of both may also be applied.
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Alternatively, or in combination with one or more of
the above methods to control the water content in the
off-gas, the temperature of the catalyst bed may be
decreased or any temperature increase temporarily stopped
by decreasing the cooling medium temperature.
The catalytic Fischer-Tropsch hydrocarbon synthesis
process is suitably carried out in a fixed bed operation
and therefore the activation process described above is
also suitably carried out via a fixed bed operation.
However, it will be appreciated that also catalysts for
use in catalyst beds different from fixed beds may be
activated by the process of the present invention.
The activation process itself is most suitably
carried out in a fixed bed of catalysts. However, other
catalyst beds, like moving beds, may also be applied in
the activation process.
The activation process is preferably carried out at a
temperature below 450 C, more preferably below 400 C,
more preferably below 300 C. Typically, the activation
process is carried out at a temperature above 150 C,
preferably above 200 C.
The pressure at which the process is carried out
typically may range from 1 to 150 bar abs., preferably
from 1 to 60 bar abs., more preferably from 1 to 20 bar
abs.
The gas rate, that is the Gas Hourly Space Velocity
may typically range from 100 to 3000 Nl/l/h, preferably
from 200 to 1500 Nl/l/h.
The activation process is typically carried out for a
period sufficient to substantially activate the catalyst.
It will be appreciated that this period may vary,
depending on the composition of the catalyst, the average
reaction temperature, the gas rate, and the reducing gas
partial pressure. Typically, the catalyst is contacted
with the reducing gas for 0.5 to 150 hours, preferably
for 8 to 120 hours, more preferably for 16 to 96 hours.
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According to a preferred embodiment, the catalyst is
contacted with the reducing gas until at least 25% by
weight, preferably at least 50 % by weight, more
preferably at least 80% by weight, of the Group VIII
metal compound is reduced to the metallic state.
The quantity of the Group VIII metal compound that
has been reduced can suitably be monitored by measurement
of the cumulative water production during the process.
Other methods known to those skilled in the art include
Thermogravimetric Analysis and Temperature Programmed
Reduction.
As set out above, the temperature, gas rate (GHSV)
and the content (partial pressure) of the reducing gas
may be varied in order to control the activation process.
It will be appreciated that it belongs to the skill of
the skilled person to select the most appropriate way to
control the activation process for a particular catalyst
by routine experimentation. According to one typical
activation process scenario, the temperature, total
pressure and total gas rate are kept constant and the
reducing gas content, preferably the hydrogen content, is
gradually or step-wise increased from 1~ up to e.g. 85
by volume or higher, preferably up to 100% by volume.
According to another embodiment, the temperature is
continuously or step-wise increased from at least 150 C
up to e.g. at most 400 C at a rate in the range from 0.5
to 5 C/min.
In US-A-4,605,676 and US-A-4,670,414 methods for
activating a Fischer-Tropsch catalyst are disclosed
involving the successive steps of reduction in hydrogen,
oxidation in an oxygen-containing gas and activation of
the catalyst by reduction in hydrogen. All steps are
typically performed at temperatures between about 100 and
450 C. The activation method is referred to as the "ROR
treatment".
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The activation process according to the present
invention as described above can very suitably be applied
as the activation step and/or the first reduction step in
the ROR treatment. Thus, according to a further aspect,
the present invention relates to a process for activation
of a Fischer-Tropsch process by contacting the catalyst
successively with
(a) a reducing gas;
(b) an oxidising gas; and
(c) a reducing gas,
wherein step (a) and/or step (c), preferably step (c), is
carried out as described hereinbefore. Preferably, at
least step (c) is carried out in-situ, more preferably
steps (a) to (c).
It will be appreciated that it is also possible to
conduct step (b) such that the direction of flow of
oxidising gas is reversed to the direction of flow of
reactants during operation.
After the Fischer-Tropsch catalyst has at least
partially been deactivated after operation, it may be re-
activated for repeated use. Suitably, the activation
process of the present invention is used to re-activate
the catalyst. Thus, according to a further aspect the
present invention relates to a process for re-activation
of an at least partially deactivated Fischer-Tropsch
catalyst packed in a bed by contacting the catalyst with
a reducing gas at a temperature below 500 C, wherein the
reducing gas is passed through the catalyst bed in a
direction reversed to the direction of the flow of
reactants during operation.
It has been found that the ROR treatment is also very
suitable for re-activation of at least partially
deactivated Fischer-Tropsch catalyst. nithOut wishing to
be bound by a particular theory, it would appear that the
ROR treatment, when used for re-activation, works as
follows.
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The first step of the ROR treatment basically
involves stripping with hydrogen to remove heavy wax
and/or carbonaceous particles, which have precipitated
onto the catalyst particles during operation, and slow
reduction of the catalyst. In the subsequent oxidation
step any carbonaceous particles still present on the
catalyst are oxidised into carbon dioxide and water and
the catalytically active metal is oxidised. Finally, in
the activation step, the oxidised catalyst is converted
into its active form by reduction and hence is ready
again for operation.
The activation process according to the present
invention as described above can very suitably be applied
as the activation step in the ROR treatment. Accordingly,
the present invention also relates to a process for the
reactivation of an at least partially deactivated
Fischer-Tropsch catalyst packed in a bed comprising the
successive steps of:
(a) contacting the catalyst with a reducing gas, in
particular a hydrogen-containing gas;
(b) contacting the catalyst with an oxidising gas; and
(c) contacting the catalyst with a reducing gas (that is,
reducing the catalyst),
characterised in that step (c) is performed according to
the activation process described above as an aspect of
the present invention.
It will be appreciated that it is also possible to
conduct step (b) such that the direction of flow of
oxidising gas is reversed to the direction of flow of
reactants during operation. In fact in the process for
re-activation of at least partly deactivated Fischer-
Tropsch catalysts this may be preferred if for example
relatively high amounts of water are produced as a result
of oxidation of carbonaceous particles. The presence of
high amounts of water may e.g. induce formation of metal-
support compounds. Therefore, according to an embodiment,
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the above ROR process is further characterised in that
the oxidising gas of step (b) is passed through the
catalyst bed in a direction reversed to the direction of
the flow of reactants during operation. According to one
S embodiment of the invention the oxidation step is carried
out such that the amount of water present in the off-gas
is kept within the limits as discussed above, but higher
or lower amounts may also be preferred.
In principle it is also possible to conduct step (a)
such that the direction of flow of reducing gas is
reversed to the direction of flow of reactants during
operation. However, as step (a) in the ROR process for
re-activation of an at least partly deactivated Fischer-
Tropsch catalyst, mainly comprises removal of
carbonaceous particles and heavy wax, no substantial
improvement will be encountered when operating step (a)
in this way.
All steps are preferably carried out at a temperature
between 150 and 400 C, more preferably between 200 and
300 C.
As regards steps (a) and (b) the conditions as
described in the before-mentioned two U.S. patents are
applicable. It is preferred to use a hydrogen-containing
gas as the reducing gas in step (a) and to use an
oxygen-containing gas as the oxidising gas in step (b).
An example of a suitable oxygen-containing gas is diluted
air, i.e. air diluted with an inert gas such as nitrogen.
Preferably, the oxygen-containing gas contains from 0.1
to 10~ by volume of oxygen, more preferably from 0.2 to
5~ by volume.
The amount of oxygen is preferably kept within the
above range as a way to control the oxidation step. The
operating conditions for conducting step (b) are
preferably in the same range as set out hereinbefore with
respect to the activation (reduction) process. In
principle, it is also possible to use an oxygen-
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containing gas containing a higher amount of oxygen, such
as air. It will be appreciated by those skilled in the
art that in order to control the oxidation reaction,
operating conditions may then have to be adapted.
As regards step (a), preferably the operating
conditions are within the same range as set out
hereinbefore with respect to the activation (reduction)
process, being step (c) of the ROR treatment. It will
however be appreciated that if the ROR treatment is used
for re-activation of an at least partially deactivated
catalyst, a high percentage of Group VIII metal on the
catalyst will already be in the metallic state.
Nevertheless, it is preferred to contact the catalyst
with the reducing gas for 0.5 to 150 hours, more
preferably for 8 to 120 hours, most preferably for 16 to
96 hours. Further, according to one preferred embodiment,
the hydrogen content in the hydrogen-containing gas, and
other operating conditions, like the temperature, are
kept constant during step (a) of the ROR treatment, when
used for re-activation of an at least partially
deactivated catalyst. The hydrogen partial pressure in
step (a) of the treatment is preferably less than
15 bar abs., more preferably less than 10 bar abs.
The invention is further illustrated by the following
examples.
Example 1
The catalyst used was an 1.7 mm trilobe Fischer-
Tropsch catalyst comprising 23~ by weight Co, 10% by
weight of ZrO2 and 56~ by weight of SiO2 based on fully
oxidised catalyst. The experiments were carried out in a
single tube pilot plant equipped with two reactors
connected in series. Each reactor had a length of 4
meters. The volume of each catalyst bed was lg50 ml.
The reversed flow activation was conducted as
follows. A reducing gas was passed through the catalyst
beds at 250 C and 4 bar in the direction reversed
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compared to the direction of the gas flow during normal
operation. The reducing gas was a nitrogen/hydrogen
mixture and during activation the hydrogen partial
pressure was raised in such a way that the water content
in the off-gas stayed below 5000 ppmv. The maximum
hydrogen content of the reducing gas stream was
75% volume. The gaseous hourly space velocity (GHSV) was
600 Nl/l/hour. Total reduction time was 48 hours. Exact
conditions are listed in table 1.
A CO/H2 containing gas was subsequently passed over
the activated catalyst obtained in the way described
above. The conditions applied and the results as
determined after 50 hours of operation are listed in
table 3, while the temperature profile along the catalyst
bed as measured after 50 hours of operation is depicted
in figure 1.
Comparative Example 1
The same catalyst as used in Example 1 was activated
by passing the reducing gas through the catalyst bed in
the same direction as the gas flow during normal
operation (normal flow activation).
The conditions are listed in table 2.
A CO/H2 containing gas was subsequently passed over
the activated catalyst obtained in the way described
above. The conditions applied and the results as
determined after 50 hours of operation are listed in
table 3, while the temperature profile along the catalyst
bed as measured after 50 hours of operation is depicted
in figure 1.
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TABLE 1 Reversed flow activation
Time Total Gas H2 content H2O content
flow N2/H2 reducing gas off-gas
(h) (Nl/h) (volume %) (ppmv)
0.0 2010 0.5
1.0 2020 1.0 926
2.0 2020 1.0 3878
4.0 2020 1.5 4331
6.0 2040 2.0 4123
8.0 2000 8.0 4746
10.0 2200 40.0 3707
25.0 2400 75.0 1973
32.5 2400 75.0 1037
48.0 2400 75.0 670
TABLE 2 Normal flow activation
Time Total Gas H2 content H2O content
flow N2/H2 reducing gas off-gas
(h) (Nl/h) (volume %) (ppmv)
0.0 2010 0.5
1.0 2020 1.0 946
2.0 2020 1.0 2886
4.0 2030 1.5 3076
6.0 2000 4.0 3978
8.0 2000 16.0 3356
10.0 2200 40.0 3162
24.0 2200 75.0 1795
32.0 2200 75.0 909
48.0 2200 75.0 558
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TABLE 3 Fischer-Tropsch Synthesis
Example 1 Comparative Example 1
GHSV(Nl/l/h) H~ 479 466
CO 434 435
Inerts 218 242
Total 1131 1143
Inlet pressure (bar) 37.8 38.3
H2/CO at inlet (v/v) 1.10 1.07
Liquid velocity (mm/s) 1.5 1.5
Temp. coolant ( C) 210 209
WABT ( C) 213 216
STY 105.1 101.4
(g/l/h)
C5+ selectivity (% wt)92.2 89.4
From Table 3 it can be seen that the reversed flow
activation according to the present invention results in
a better catalyst performance. At an even lower weight
average bed temperature (WABT), namely, both C1+ yield
and C5+ selectivity of the catalyst activated via
reversed flow activation are higher than the catalyst
activated via normal activation. C1+ yield is indicated
as Space Time Yield (STY), that is the amount of
hydrocarbons containing two or more carbon atoms in grams
produced per litre catalyst per hour.
In figure 1 the temperature profiles along the two
4 m reactors as measured after 50 hours of operation is
given for both reversed activated catalyst and normally
activated catalyst. Along the vertical axis the
difference between the temperature of the coolant and the
temperature within the reactor during the hydrocarbon
synthesis reaction is indicated, while along the
horizontal axis the distance from the top of the upper
reactor is indicated.
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From figure 1 it can be seen that the temperature
profile along the reactors containing reversed activated
catalyst (RAC reactors) is more flat than the temperature
profile along the reactors containing normally activated
catalyst (NAC reactors) during operation, which implies
that the temperature within the RAC reactors during the
hydrocarbon synthesis reaction is more constant than the
temperature within the NAC reactors. This in return
indicates that the activity of the reversed activated
catalyst is more constant along the entire length of the
reactors, as a result of which the conversion level also
fluctuates less. As can be seen from table 3, this also
positively influences the overall conversion level: the
final space time yield of the reaction in the RAC reactor
is higher than that of reaction in the NAC reactor, at a
higher Cs+ selectivity.
