Note: Descriptions are shown in the official language in which they were submitted.
3~3~
K 5696 CAN
PROCESS FOR THE PREPARATION OF HYDROCARBONS
The invention relates to a process for the preparation of
hydrocarbons having at least five carbon atoms per molecule.
Hydrocarbons of at least five carbon atoms per molecule
(hereinafter referred to as "Cs~ hydrocarbons") can be prepared
frGm hydrocarbons having at most four carbon ato~s per molecule
(hereinafter referred to as "C4- hydrocarbons") by a two-step
process in which in the first step the C4- hydrocarbons are
converted by steam reforming or partial oxidation into a mixture of
carbon monoxide and hydrogen~ which mixture is contacted in the
second step at elevated tem~erature and pressure with a catalyst and
thus converted into a mixture of hydrocarbons consisting substantial-
ly of Cs~ hydrocarbons. The reaction which takes place in the
second step of the process is known in the literature as the Fischer-
Tropsch hydrocarbon synthesis. Catalysts often used for this reaction
contain one or more metals from the iron group together with one or
more promotors and a carrier material.
A catalyst's usefulness for the preparation of Cs+ hydro-
carbons from H2/CO mixtures is mainly determined by its
activity, Cs~ selectivity and stability, the catalyst being
regarded as more useful according as these parameters have a higher
value. In the preparation of Cs~ hydrocarbons from H2/CO
mixtures according to a single step process the catalyst's Cs+
selectivity and stability draw most emphasis; For according as the
catalyst has a higher Cs+ selectivity, less C4- hydrocarbons
will be formed as by-product and according as the catalyst has
higher stability, the process can be carried out for a longer period
before it becomes necessary to replace the catalyst. It is true
that, according as the catalyst has a lower activity, less of the
H2/CO mixture can be converted per reactor throughput; however,
3o by recirculation of unconverted H2 and CO a higher conversion of
the H2/CO mixture can nevertheless be realised. In the preparation
of Cs~ hydrocarbons from C4- hydrocarbons according to the
afore-mentioned two-step process most emphasis lies on the catalyst's
- 2 ~ 3~
stability. Lower activity can be set off - as in the single-step
~ process - by recirculation of unconverted H2 and CO. Since in the
first step of the two-step process C4- hydrocarbons are converted
into a H2/CO mixture, lower Cs+ selectivity of the catalyst in
the second step can be set off by recirculation of the C~~ hydro-
carbons formed as by-product. On account of the possibilities of
compensating for lower activity and Cs+ selectivity offered by
the two-step process, for carrying out the process on a technical
scale preference will often be given to a catalyst for the second
step which, though it does not have the highest activity and
Cs~ selectivity, is the most stable.
The afore-going observation was made on the assumption that the
H2/CO mixture used as feed for the second step is a relatively
pure synthesis gas, so that the proposed recycling operations can be
carried out without the need of costly separating operations. If
that is the case, the catalyst's stability in the second step is
indeed its most important parameter. The picture is entirely
different when the H2/CO mixture which is used as feed for the
second step is highly contaminated with nitrogen andtor carbon
dioxide. These contaminants may have found their way into the
synthesis gas in different ways. In the first place the feed for the
first step in addition to C4- hydrocarbons may contain more than
20 %v nitrogen and/or carbon dioxide. In this connection it should
be noted that natural gas, which is known to consist mainly of
methane where hydrocarbons are concerned, may contain up to 75 ~v of
said contaminants. When for the first step a feed is used which is
contaminated with nitrogen and/or carbon dioxide, these contaminants
will pass through the reactor unchanged and find their way into the
feed for the second step. And also, when the first step is carried
out by partial oxidation using an oxygen-containing gas mixture
containing more than 50 ~v n-trogen, such as air or oxygen-enriched
air, instead of oxygen, there will be obtained a feed for the second
step which, in addition to hydrogen and carbon monoxide, contains
the nitrogen present in the gas used. If the feed for the first step
_ 3 _ ~2~
contains nitrogen and/or carbon dioxide by nature, then, with
partial oxidation using a nitrogen and oxygen containing gas mixture,
said contaminants will be found in the feed for the second step in
addition the nitrogen from the gas mixture used.
As regards the catalysts which are eligible for use in the
second step of the afore-mentioned two-step process in which the
feed for the second step is heavily contaminated by nitrogen and/or
carbon dioxide as a result of the use of a feed for the first step
containing more than 20 %v nitrogen and/or carbon dioxide, and/or
the use in the first step of partial oxidation using an oxygen-
containing gas mixture containing more than 50 ~v nitrogen, the
following may be observed. If, in order to set off too low activity
and/or selectivity of the catalyst in the second step, it should
be contemplated to recirculate as indicated hereinbefore, unconverted
H2 and C0 and/or C4- hydrocarbons formed it should be taken
into account that when a H2/C0 mixture thus contaminated with
nitrogen and/or carbon dioxide i5 used as feed for the second step,
this is possible only after said contaminants have been removed from
the recirculation streams. Such removal entails considerable expenses,
and for that reason it is not suitable for use on a technical scale.
This involves that for carrying out the process on a technical scale
the two possibilities of compensation mentioned hereinbefore cease
to be applicable, so that in the second step a catalyst will necessarily
have to be used which not only has the high stability mentioned
hereinbefore, but whose activity and Cs+ selectivity in the
presence of nitrogen and carbon dioxide are sufficiently high for
the process to be realised on a technical scale without recirculation.
In order to get a fair knowledge of the influence of nitrogen
and carbon dioxide on the performance of Fischer-Tropsch catalysts
an investigation was carried out in which these catalysts were used
for the conversion of gas mixtures some of which in addition to H2
and C0, contained nitrogen or carbon dioxide some not. It was found
that the presence of nitrogen and carbon dioxide in the H2/C0
mixture lessens the activity of these catalysts, the decrease
becoming larger as the mixture contained more nitrogen and carbon
_ 4 _ ~ ~3~3~
dioxide. It is true that by increasing the severity of the reaction
- conditions - notably raising the temperature and/or pressure - in
the presence of nitrogen and carbon dioxide an activity level could
be attained which corresponded with that of a nitrogen and carbon
dioxide-free operation, but this was accompanied by a loss of the
catalysts' stability, which became larger as severer reaction
conditions were used. It was further found that the Cs+ selectivity
of these catalysts was barely influenced by the presence of nitrogen
and/or carbon dioxide in the H2/C0 mixture. As regards the stability
the investigation produced a surprising finding. In contrast with
other Fischer-Tropsch catalysts whose stability - as well as Cs+
selectivity - was barely influenced by the presence of nitrogen
and/or carbon dioxide, there was a certain group of cobalt catalysts
of which the stability was found to be considerably increased by the
presence of nitrogen and/or carbon dioxide, the increase being
larger according as the mixture contained mOFe nitrogen and carbon
dioxide. The Fischer-Tropsch caealysts displaying this surprising
behaviour comprise silica, alumina or silica-alumina as carrier
material and cobalt together with zirconium, titanium and/or chromium
as catalytically active metals in such quantities that in the
catalysts there are present 3-60 pbw cobalt and 0.l-l00 pbw zirconium~
titanium,ruthenium and/or chromium per l00 pbwcarriermaterial. The catalysts
were prepared by depositing the metals concerned by kneading and/or
impregnation on the carrier material. For further information on the
preparation of these catalysts by kneading and/or impregnation
reference is made to the Canadian patent application no. 453,317
recently filed in the name of the Applicant.
When a cobalt catalyst belonging to the afore-mentioned class
is used for the conversion of a H2/C0 mixture containing no
nitrogen or carbon dioxide, under the given reaction conditions this
catalyst is seen to have not only high stability and Cs+ select-
ivity, but also very high activity. When the same catalyst is used
under similar reaction conditions for the conversion of a gas
mixture which, in addition to H2 and C0, contains nitrogen and/or
carbon dioxide, a decrease in activity is seen as ~ell as a consider-
able increase in stability, as was re-narked hereinbefore. In view of
~Y~
~3~3~
-- 5 --
the very high activity level of the present cobalt catalysts, some
~ loss of activity in return for a considerable increase in stability
is quite acceptable for an operation carried out on a technical
scale. Another option is to raise the activity to its original level
by increasing the severity of the reaction conditions; this i9
coupled with some loss of stability. However, it has surprisingly
been found that this loss of stability is amply compensated for by
the increase in stability due to the presence of nitrogen and/or
carbon dioxide. This means that when the cobalt catalysts belonging
to the above-mentioned class are used for converting a H2/C0
mixture which is heavily contaminated with nitrogen and/or carbon
dioxide, a level of activity can be realised which is very similar
to that seen in the nitrogen and carbon dioxide free operation,
whilst the stability is higher. These special properties render the
cobalt catalysts eminently suitable for use in the second step of
said two-step process in which the feed for the second step is
heavily contaminated with nitrogen and/or carbon dioxide. As to the
carbon dioxide which may occur in the feed for the second step as
contaminant, the following should be remarked. Depending on the
reaction conditions used in the steam reforming and the partial
oxidation, minor quantities of carbon dioxide may be formed as
by-product in the first step as a result of side reactions. This
carbon dioxide, together with carbon dioxide which may already have
been present in the feed for the first step, finds its way into the
feed for the second step as contaminant. In view of the amounts of
contaminants that land in the feed for the second step as a result
of the use of a feed for the first step containing more than 20 %v
nitrogen and/or carbon dioxide, and/or as a result of the use of the
partial oxidation of an oxygen-containing gas containing more than
50 %v nitrogen, the amount of carbon dioxide which, when the steam
reforming or partial oxidation is carried out in a normal operation,
may find its way into the feed for the second step as a result of
said side reactions, is of secondary importance. Therefore the
presence of contaminants in the feed for the second step is mainly
, . .
- 6 - ~ 3~
due to the use of a contaminated feed for the first step and/or to
the use of partial oxidation with an oxygen-containing gas containing
nitrogen.
The present patent application therefore relates to a process
for the preparation of Cs+ hydrocarbons from C4- hydrocarbons,
in which in the first step C4- hydrocarbons are converted by
steam reforming or partial oxidation into a mixture of carbon
monoxide and hydrogen, which mixture is subsequently, in a second
step, converted into a mixture of hydrocarbons consisting mainly of
Cs+ hydrocarbons by contacting it at elevated temperature and
pressure with a catalyst comprising 3-60 pbw cobalt and O.l-lO0 pbw
of at least one other metal chosen from the group formed by zirconium,
titanium~ruthenium andchromium perlOOpbw silica, ~umina o~silica-alumina,
which catalyst has been prepared by kneading and/or impregnation, in
which the feed for the second step contains nitrogen and/or carbon
dioxide as contaminants, the presence therein of these contaminants
having been caused substantially by ehe fact that the feed for the
first step contained more than 20 %v nitrogen and/or carbon dioxide
and/or the fact that the first step was carried out by partial
oxidation using an oxygen-containing gas mixture containing more
than 50 ~v nitrogen.
In the process according to the invention the starting material
may be a feed consisting substantially of one or more C4- hydro-
carbons or a feed which, in addition to C4- hydrocarbons,
contains nitrogen and/or carbon dioxide. Examples of C4- hydro-
carbons which may occur in the feed individually or in admixture are
methane, ethane, propane, butane and isobutane. Preference is given
to carrying out the process with a feed in which the C4- hydro-
carbons consist mainly of methane. Special preference is given to
natural gas as feed.
In the process according to the invention in the first step steam
reforming or partial oxidation is used to convert the C4- hydro-
carbons into a mixture of carbon monoxide and hydrogen. The steam
reforming is usually carried out by contacting the hydrocarbons
to be converted together with steam at a temperature of 500-12004C,
a pressure of 2-40 bar and a steam/hydrocarbon ratio of l-lO g mol
H20/g atom C with a catalyst containing one or more metals from
the iron group supported on a carrier. The steam reforming is prefer-
ably carried out at a temperature of 700-1000C, a pressure of
2 25 bar and a steam/hydrocarbon ratio of 1.5-5 g mol H20/g atom C
and by using a nickel-containing catalyst. In order to prevent
deposition of coke on the catalyst and also to remove coke already
deposited from the catalyst by conversion into C0, it is preferred
to use a catalyst containing an alkali metal, in particular potassium.
In order to avoid sintering of the catalyst, it is moreover preferred
to use a catalyst containing an alkaline earth metal, in particular
calcium. If the C4- hydrocarbons in the feed consist completely
or to a considerable extent of hydrocarbons containing two or more
carbon atoms per molecule, if it preferred to use a catalyst having
cracking activity. The catalyst can be invested ~ith cracking
activity by the use of a silica-alumina as carrier. In the process
according to the invention the conversion of the C4- hydrocarbons
into a mixture of carbon monoxide and hydrogen can also be carried
out by partial oxidation instead of by steam reforming. This partial
oxidation is usually carried out by contacting the hydrocarbons to be
converted, together with oxygen or an oxygen-containing gas, at a
temperature of 500-1750C, a pressure of 5-100 bar and an oxygen/
hydrocarbon ratio of 0.2-0.9 g mol 02/g atom C with a catalyst
containing one or more metals from the iron group supported on a
carrier. If desired, the partial oxidation can also be conducted in
the absence of a catalyst. The partial oxidation is preferably
carried out at a temperature of 1100-1500C, a pressure of 10-60 bar
and an oxygen/hydrocarbon ratio of 0.35-0.75 g mol 02/g atom C and
by using a nickel-containing catalyst. To the other components which
3Q may be present in the~e catalysts the same preference applies as
stated for the steam reforming catalysts. Th~ partial oxidation is
carried out by using oxygen or a gas which, in addition to oxygen,
contains one or more other co~ponents, in particular nitrogen. If
for the partial oxidation an oxygen/nitrogen mixture is used, it is
preferred to choose air or oxygen-enriched air for the purpose. If
-- 8 --
in the process according to the invention partial oxidation is used
in the first step, it is preferred in order to increase the H2/C0
molar ratio of the synthesis gas to be prepared, to add to the
mixture to be subjected to partial oxidation a molar quantity of
steam smaller than the molar quantity of oxygen used.
The process according to the invention is of particular
importance for the preparation of Cs+ hydrocarbons from a
natural gas which is contaminated with nitrogen and/or carbon
dioxide, in particular a natural gas containing more than 30 %v of
these contaminants. Such contaminated natural gases are amply
available in nature. When the process according to the invention is
used for preparing Cs~ hydrocarbons from natural gas which is
contaminated with nitrogen and/or carbon dioxide, the first step is
preferably carried out either by steam reforming, or by partial
oxidation using oxygen.
If in the process according to the invention the starting
material is a feed with a relatively high H/C ratio, such as natural
gas which is contaminated by carbon dioxide, and if the first step
of the process is carried out by steam reforming, the process offers
the additional advantage that hydrocarbon selectivities above lO0
can be realised. (In the present two-step process hydrocarbon
selectivity should be taken to be the number of g atom C present in
the hydrocarbon product of the second step, calculated on the number
of g atom C present in the part of the hydrocarbon feed for the
first step whish is converted in the first step). This is mainly a
result of the fact that feeds with a relatively high H/C ratio, such
as methane, when steam reformed yield a synthesis gas having a
H2/C0 molar ratio higher than 2, whilst the H2/C0 consumption
ratio of the cobalt catalysts used in the second step is not higher
3o than about 2. When during the steam refor~ing carbon dioxide is
present, part thereof will react with part of the excess of hydrogen
produced by the reverse C0-shift reaction C2 + H2 ~ C0 + H20.
As a consequence9 a synthesis gas is obtained whose H2/C0 molar
ratio is more in keeping with the H2/C0 consumption ratio of
the cobalt catalysts, so that a hi8her conversion of the synthesis
gas int~ hydrocarbons can be obtained.
~3~
g
In the process of the invention it is preferred to use in the
second step the cobalt catalysts which form the subject matter of
Canadian patent ap~lication No. 453,317. These are catalys-ts
which satisfy the relation
(3 + 4 R) > - > (0.3 + 0.4 R), wherein
S
L = the total quantity of cobalt present on the catalyst, expressed
as mg Co/ml catalyst,
S = the surface area of the catalyst, expressed as m~/ml catalyst,
and
R = the weighc.ratio of the quantity of cobalt deposited on the
catalyst by kneading to the total quantity of cobalt present on
the catalyst.
The preparation of the cobalt catalysts which are used in the
second step of the process of the invention is preferably carried out
according to one of the three procedures mentioned hereinafter:
a) first cobalt is deposited in one or more steps by impregnation
and subsequently the other metal is deposited in one or more
steps, also by impregnation,
b) first the other metal is deposited in one or more steps by
impregnation and subsequently the cobalt is deposiCed in one or
more steps, also by impregnation, and
c) first cobalt is deposited in one or more steps by kneading and
. subsequently the other metal is deposited in one or more steps by
impregnation.
In the process according to the invention preference is given
to the use of cobalt catalysts containing i5-50 pbw cobalt per
lO0 pbw carrier. The preferred quantity of other metal present in
the cobalt catalysts depends on the way in which this metal has been
deposited. In the case of catalysts where first cobalt has been
deposited on the carrier, followed by the other metal, prefer-
ence is given to catalysts containing 0.1-5 pbw other metal per
lO0 pbw carrier. In the case of catalysts where first the other
metal has been deposited on the carrier, followed by the cobalt,
.
.~
3~
-- 10 --
preference is given to catalysts containing 5-40 pbw of the other
metal per ~00 pbw carrier. Preference is given to zirconium as
other metal and to silica as carrier material. In order to be
suitable for use the cobalt catalysts should first be reduced. This
reduction may suitably be carried out by contacting the catalyst at
a temperature between 200 and 350C with a hydrogen-containing gas.
In the process according to the invention the second step is
preferably carried out at a temperature of 125-350 C and a pressure
of 5-100 bar. Special preference is given to the use of a temperature
10 of 175-275C and a pressure of 10-75 bar in the second step.
In addition to their afore-mentioned surprising increase in
stability in the presence of nitrogen and/or carbon dioxide the
cobalt catalysts used in the second step have the special property
of yielding a product in which only very minor quantities of olefins
and oxygen-containing organic compounds occur and in which the
organic part consists virtually completely of unbranched paraffins
a considerable part of which boil above the middle distillate
- range. In the present patent application middle distillates are
taken to be hydrocarbon mixtures whose boiling range corresponds
substantially with that of the kerosine and gas oil fractions
obtained in the conventional atmospheric distillation of crude
mineral oil. l'he middle distillate range lies substantially between
about 150 and 360C, the fractions boiling between about 200 and
360C generally being refered to as gas oils. Owing to the high
normal paraffins/isoparaffins ratio and the low content of olefins
and oxygen-containing organic compounds of the product prepared over
the cobalt catalysts, the gas oil present therein has a very high
cetane number. It has been found that by hydrocracking in the
presence of a catalyst containing one or more noble metals from
Group VIII supported on a carrier the high-boiling part of said
product can be converted in high yield into middle distillate. As
feed for the hydrocracking at least the part of the product is
chosen whose initial boiling point lies above the final boiling
point of the heaviest middle distillate desired as end product. The
hydrocracking, which is characterized by a very low hydrogen consumpt-
ion, yields a product in which, owing to the high normal paraffins/
3f~ 3~i
isoparaffins ratio, the gas oil has a very high cetane number.
Although in the preparation of middle distillates from the product
obtained over the cobalt catalyst the part of the product whose
initial boiling point lies above the final boiling point of the
heaviest middle distillate desired as end product will do as hydro-
cracking feed, for this purpose it is preferred to use the total
Cs~ fraction of the product prepared over the cobalt catalyst
because it has been found that the catalytic hydrotreatment leads to
enhanced quality of the gasoline, kerosine and gas oil fractions
present therein.
The hydrocracking catalyst used by preference is a catalyst
containing 0.1-2 %w and in particular 0.2-l ~w of one or more noble
metals from Group VIII supported on a carrier. Preference is given
to catalysts comprising platinum or palladium as Group VIII noble
metal and silica-alumina as carrier. The hydrocracking in which the
feed, together with added hydrogen, is passed over the noble metal
catalyst is preferably carried out at a temperature of 200-400~C and
in particular of 250-350C and a pressure of 5-lO0 bar and in
particular of 10-75 bar.
If the two-step process according to the invention is combined
with a hydrocracking treatment as a third step for the preparation
of middle distillates, the second and third step can be carried out
in 'series-flow', provided that the reaction product of the second
step still contains sufficient unconverted hydrogen for carrying out
the hydrocracking. It is a matter of common knowledge that carrying
out a multi-step process in 'series-flow' comprises using the total
reaction product - without any components being removed therefrom or
added thsreto - of a certain step as feed for the following step,
which is carried out substantially at the same pressure as the
preceding step. If desired, the whole three-step process can be
carried out in 'series-flow'.
The invention is now illustrated with the aid of the following
example.
- 12 -
Example
Feed 1: A natural gas consisting substantially of methane.
Feed 2: A natural gas consisting substantial].y of a mixture of
methane and carbon dioxide in a volume ratio l:l.
Catalyst 1: Ni/Ca/K/A12O3 catalyst containing 13 phw nickel,
12 pbw calcium and 0.2 pbw potassium per 100 pbw alumina.
Catalyst 2: Fe/K/SiO2 catalyst containing 50 pbw iron and 4 pbw
potassium per lO0 pbw silica. Catalyst 2 had been prepared by
three-step co-impregnation of a silica carrier with a solution of
potassium nitrate and ferric nitrate in water.
Catalyst 3: Co/ZriSiO2 catalyst containing 25 pbw cobalt and
0.9 pbw zirconium per lO0 pbw silica. Catalyst 3 had been prepared
by one-step impregnation of a silica carrier with a solution of
cobalt nitrate in water, followed by one-step impregnati,on of the
cobalt-loaded carrier with a solution of zirconium nitrate in water.
For Catalyst 3 L was 98 mg/ml and S was 96 m2/ml, and therefore
L/S was 1.02 mg/m2.
In the preparation of Catalysts 2 and 3 a quantity of solution
was used in each impregnation step whose volume corresponded substant-
ially with the pore volume of the carrier. After each impregnation
step the material was dried and then calcined at 500C.
Seven two-step experiments (Experiments 1-7) were carried out
in which in the first step Feeds l and 2 were subjected to steam
reforming or partial oxidation in the presence of Catalyst l to be
converted into H2 and C0 containing gas mixtures. From the gas
mixtures prepared by the first step of Experiments 1, 3, 6 and 7 the
carbon dioxide formed was removed. Moreover, by selective removal of
hydrogen from the gas mixtures prepared by the first step of Experi-
ments l and 3 the H2/CO molar ratio of these mixtures was reduced
to 2, corresponding with the H2/CO molar ratio of gas mixtures
prepared by the first step of Experiments 2 and 4-7. Subsequently
the gas mixtures were contacted in the second step at a temperature
of 220C and at various space velocities with Catalysts 2 and 3
which had previously been subjected to reduction at 250C in a
hydrogen-containing gas.
- 13 ~ 6
In Experiments 1-5 the first step was carried out by steam
reforming at a tempeature of 930C, a pressure of 22 bar and a
steam/hydrocarbon ratio of 2.5 g mol H20/g atom C. In Experiments 6
and 7 the first step was carried out by partial oxidation with air
and by adding steam at a temperature of 1225C, a pressure of 30 bar,
an oxygen/hydrocarbon ratio of 0.60 g mol 02/g atom C and a
steam/hydrocarbon ratio of 0.27 g mol ~20/g atom C. The results of
Experiments 1-7, as well as the conditions under which the second
step was carried out in each of these experiments, are given in the
appending Table.
Of Experiments 1-7 only Experiments 4-7 are experiments
according to the invention. In the first step of Experiments 4 and 5
a carbon-dioxide-contaminated synthesis gas was prepared by stea~
reforming of a heavily carbon-dioxide-contaminated natural gas. In
the first step of Experiments 6 and 7 a nitrogen-contaminated
synthesis gas was prepared by partial oxidation with air of a
natural gas subs-tantially consisting of methane. In Experiments 4-7
the second step was carried out by using a cobalt catalyst belonging
to the class described hereinbefore. Experiments 1-3 fall outside
the scope of the invention. They have been included in the patent
application for comparison. In the first step of Experiment 1 and 3
no contaminated synthesis gas as expressed in the present patent
application was prepared. In the second step of Experiments 1 and 2
use was made of an iron catalyst.
As regards the results mentioned in the Table the following
may be observed.
1) Comparison of the results of Experiments 1 and 2 shows that when
an iron catalyst is used, contamination of the synthesis gas with
15 %v carbon dioxide results in a decrease in activity, with
unchanging stability.
2) Comparison of the results of Experiments 3 and 4 shows that when
a cobalt catalyst belonging to the afore-described class is used,
contamination of the synthesis gas with 15 %v carbon dioxide also
leads to a decrease in ativity (which by the way is considerably
smaller than in the case of the iron catalyst) J but an increase
in stability.
- 14 ~ 3~
.
3) Comparison of the results of Experiments 3 and 6 shows that when
a cobalt catalyst is used, contamination of the synthesis gas
with 45 %v nitrogen also leads to a decrease in activity and an
increase in stability.
4) Comparison of the results of Experiments 3 and 5, and of 3 and 7
shows that by raising the pressure in the experiments with
contaminated synthesis gas the activity can be raised to the
original level of the nitrogen and carbon dioxide free operation,
but that in the latter the stability obtained is considerably
higher. As regards the hydrocarbon selectivity (not mentioned in
the Table) it may further be remarked that in Experiment 5
(according to the invention) a hydrocarbon selectivity of 116%
was achieved, whereas in Experiment 3 (not according to the
invention) it was only 83%.
- 15~ 9.~j
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