Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR PREPARING OLEFINS FROM SYNTHESIS GAS IN A REACTION
COLUMN
The present invention relates to a process for the synthesis of olefins from
synthesis
gas in the presence of at least one Fischer-Tropsch catalyst in a reaction
column.
The preparation of hydrocarbons from synthesis gas, i.e. a mixture of carbon
monoxide
and hydrogen, has been intensively researched for decades. This type of
reaction is
usually referred to as Fischer-Tropsch synthesis. Catalysts which are usually
used in
this reaction usually comprise metals of group VIIIB of the Periodic Table, in
particular
Fe, Co, Ni and/or Ru, as catalytically active metals (e.g. v.d.Laan et al.
Catal. Rev.-Sci.
Eng., 41, 255 (1999)).
Despite the intensive research activities hitherto, there is a need to
optimize Fischer-
Tropsch processes further. It is known that the product compositions, whose
compo-
nent can range from methane through higher alkanes, higher alkenes, etc., to
aliphatic
alcohols can be altered as a function of the chosen reaction conditions, the
catalysts,
etc. In addition, the exothermic nature of the Fischer-Tropsch process makes
handling
and in particular control of the reaction difficult.
If hydrocarbons enriched with olefins, preferably with a-olefins, are to be
prepared, the
catalysts used are generally ones which comprise nickel, cobalt, iron or
ruthenium, in
particular iron, iron and cobalt, iron/cobalt spinel or cobalt/manganese
spinel, and also
copper-promoted cobalt catalysts.
GB 1 512 743, GB 1 553 361, GB 1 553 362 and GB 1 553 363 describe catalytic
processes for the synthesis of unsaturated hydrocarbons from synthesis gas at
from
250 to 350 C and from 10 to 30 bar. The catalysts used here comprise
(a) one or more oxides of the "difficult-to-reduce" metal oxides of group IVB
of the
Periodic Table or a lower oxide of a transition metal of group V or VII of the
Peri-
odic Table; and
(b) one or more metals of group VIII of the Periodic Table.
These catalysts can further comprise an alkali metal (group 1A of the Periodic
Table),
magnesium oxide or zinc oxide as promoters.
US 4,199,523 discloses a Fischer-Tropsch catalyst which comprises at least 60%
of
iron. Furthermore, this catalyst can comprise promoters such as copper, silver
or alkali
metals and/or other additives such as zinc oxide, manganese oxide, cerium
oxide, va-
nadium oxide and chromium oxide.
In US 4,418,155, Chang et al describe a process for the conversion of
synthesis gas
into hydrocarbons enriched with linear a-olefins, by bringing the synthesis
gas at from
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about 260 to 345 C into contact with a catalyst comprising a ZSM-5 type
zeolite on
which metals such as iron, cobalt or ruthenium have been deposited.
Furthermore, US 5,100,856 describes copper/potassium-promoted iron/zinc
catalysts
which display improved activity, selectivity and stability in the synthesis of
a-olefins
from carbon monoxide and hydrogen.
It is likewise known that the composition of the hydrocarbons formed in the
Fischer-
Tropsch process can be strongly influenced by the choice of the catalysts
used, the
types of reactor and the reaction conditions.
WO 02/092216 describes, for example, a Fischer-Tropsch process over a
monolithic
catalyst support in a reactor which is divided into a plurality of reaction
chambers in
which the chemical reaction and the physical separation of the products take
place.
The product streams which are discharged from the various chambers differ in
terms of
their composition. For example, gasoline, kerosene and diesel are discharged
sepa-
rately from the reactor in the present case.
Despite the improvements which have been achieved to date, there continues to
be a
need for improvement of the commercially operated Fischer-Tropsch plants for
the syn-
thesis of olefins having from 4 to 20 carbon atoms.
It is an object of the present invention to provide a process for the
synthesis of olefins,
in particular a-olefins, from synthesis gas.
The object of the present invention is achieved by a process for the synthesis
of olefins
from synthesis gas in the presence of at least one Fischer-Tropsch catalyst in
a reac-
tion column, wherein the synthesis gas is introduced into the reaction column
below a
zone A of the reaction column and the olefins are taken off below the point at
which the
synthesis gas is fed in.
The reaction column used in the process of the invention comprises at least
one top
zone, a zone A and a bottom zone. Top zone, zone A and bottom zone are
arranged in
the stated order from the top downward in the reaction column. The zone A
comprises
at least one reaction zone and a distillation zone. The synthesis gas is
introduced be-
low the zone A but above the bottom zone and the olefins are taken off below
the point
at which the synthesis gas is fed in.
The Fischer-Tropsch catalyst is localized in the reaction zone and the Fischer-
Tropsch
synthesis takes place there.
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The fractional distillation of the products formed in the Fischer-Tropsch
synthesis takes
place in the distillation zone.
However, it can also be the case that the zone of the chemical reaction and
the zone of
the physical separation (fractional distillation) are not physically separate.
In this case,
a combination zone is present. The combination zone is thus a combined
reaction and
distillation zone.
In the process of the invention, the synthesis gas is introduced into the
reaction column
below the zone A. The synthesis gas then comes into contact with the Fischer-
Tropsch
catalyst and a first hydrocarbon mixture a is formed; unreacted synthesis gas
and vola-
tile components of the hydrocarbon mixture formed then ascend into the next
reaction
zone where a further Fischer-Tropsch reaction takes place and a hydrocarbon
mixture
b is formed; this process is repeated. On the other hand, the volatility of
the hydrocar-
bons formed decreases with increasing chain length and they are then present
in liquid
form and flow downward into the reaction zone(s) located underneath; there,
chain
extension by means of synthesis gas present can again take place; this
process, too, is
repeated- This finally results in a hydrocarbon mixture which can be taken off
below
zone A. This hydrocarbon mixture has, depending on the synthesis gas used, the
Fischer-Tropsch catalyst and the process parameters (e.g. geometry of the
reaction
column, temperature profile of the reaction column, pressure, etc.), a
particular molar
mass distribution and a particular mean molecular weight. This molar mass
distribution
is preferably narrower than those of conventional Fischer-Tropsch
hydrocarbons.
The process of the invention thus makes it possible to set firstly the mean
molecular
weight of the hydrocarbon mixture formed and secondly its molecular weight
distribu-
tion by means of the superposition of the Fischer-Tropsch process on the
distillation
process. Furthermore, a higher selectivity to the desired reaction products,
i.e. to ole-
fins, in particular a-olefins, is achieved.
In one embodiment of the zone A, reaction and distillation zones alternate.
In a further embodiment of the zone A, combination and distillation zones
alternate.
In a further embodiment of the zone A, a single combination zone is present.
In a further embodiment, synthesis gas is introduced at one or more points
within the
zone A in addition to the introduction of synthesis gas below the zone A. In
some
cases, it can be advantageous to carry out the additional introduction(s) into
the distilla-
tion zone(s). However, it is also possible to carry out the introduction(s)
into combina-
tion zone(s).
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In a further embodiment, water in liquid form is fed in above or within the
zone A.
However, it can also be advantageous to feed in water vapor below or within
the zone
A.
In a further embodiment, the Fischer-Tropsch catalyst which is localized in
the reaction
or combination zone forms a fixed bed, a fluidized bed, a suspension or a
bubble col-
umn, preferably a fixed bed or a bubble column.
These embodiments can be implemented in a manner known per se to those skilled
in
the art, by, for example, applying the Fischer-Tropsch catalyst onto trays
having a par-
ticular residence time of the condensate, for example valve trays, bubble cap
trays or
related constructions such as tunnel trays or Thormann trays, or fixing it on
them as a
catalyst bed.
However, it is also possible to introduce the Fischer-Tropsch catalyst into
the column in
the form of packing elements such as Raschig rings, Pall rings, saddle bodies
appro-
priately provided with catalyst. Furthermore, it is possible to use packings
comprising
Fischer-Tropsch catalyst or to use mesh bags filled with Fischer-Tropsch
catalyst,
known as bales or Texas teabags. The packings as such are usually made of
sheet
metal, expanded metal, wire meshes or knitted meshes which preferably have a
cross-
channel structure. In these cases, combination zones are generally formed.
Internais having a distillative separation action are used in the distillation
zones of the
zone A. This can be achieved, for example, by means of trays, for example
valve trays,
bubble cap trays or related constructions, e.g. tunnel trays or Thormann
trays, or sieve
trays. However, it is also possible to use packings which usually comprise
sheet metal,
expanded metal, wire meshes or knitted meshes and preferably have a cross-
channel
structure. Examples are the packings Sulzer MELAPAK, Sulzer BX, Montz B1 types
or
Montz A3 types. However, it is also possible to use disordered packing
elements, e.g.
Raschig rings, Pall rings, saddle bodies, etc..
Preference is given to using reaction columns in which the zone A has from 5
to
150 trays, preferably from 15 to 100 trays, for carrying out the process of
the invention.
A distillation zone usually comprises from 1 to 30 trays, a reaction zone
usually com-
prises one tray and a combination zone usually comprises from 1 to 5 trays.
This ap-
plies particularly when the reaction and distillation zones or the combination
and distil-
lation zones alternate.
In a further embodiment, a combination zone comprises from 20 to 100 trays.
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A similar situation applies to the theoretical plates in the case of other
column internals,
i.e. when packings, etc., are used.
In a further embodiment, a reaction zone which is provided with packings or
with
5 Fischer-Tropsch catalysts in the form of packing elements provided with
catalyst or with
active distillation packings or with mesh bags filled with Fischer-Tropsch
catalyst com-
prises from 20 to 100 theoretical plates.
In a particular embodiment, the zone A comprises from one to three
dilstillation zones
each having from 10 to 100 trays.
In a further particular embodiment, the zone A comprises a combination zone.
In a further embodiment, low boilers can be taken off via the top zone of the
reaction
column. These low boilers generally comprise inert gases such as nitrogen
which may
be present in the synthesis gas and also any carbon dioxide formed, low-
boiling paraf-
fins, in particular methane, low-boiling olefins such as ethene, etc.
In a further embodiment, low boilers formed, which comprise, for example, any
low-
boiling paraffins formed, low-boiling olefins and/or water, are taken off from
zone A via
a side offtake. The liquid product taken off via the side offtake can consist
of two
phases. It is possible for a phase separation to be carried out and the
organic phase to
be recirculated to the column. In this way, water can be specifically removed
from the
reaction zone.
In a further embodiment of the reaction column, the hydrocarbon mixture formed
is
removed from the reaction column below the point at which the synthesis gas is
fed in.
This can be achieved via a side offtake. However, it is also possible to take
off the hy-
drocarbon mixture formed via the bottom of the column.
In a further embodiment, part of the hydrocarbon mixture formed is taken off
from zone
A via a side offtake and the other part of the hydrocarbon mixture formed is
taken off
below the point at which the synthesis gas is fed in.
In a further embodiment, the reaction column used comprises a top zone, a zone
A and
a bottom zone.
In a further embodiment, the reaction column used comprises a top zone, a zone
A and
a bottom zone and also a distillation zone B which is localized between the
zone A and
the bottom zone. Internals having a distillative separation action can be
installed or
packings can be comprised in this distillation zone. The embodiments of the
internals
or packings are analogous to those of the distillation zones of the zone A.
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In a further embodiment, the reaction column used comprises a top zone, a zone
A and
a bottom zone and also a distillation zone C which is localized between the
top zone
and the zone A. Internals having a distillative separation action can be
installed or
packings can be comprised in this distillation zone. The embodiments of the
internals
or packings are analogous to those of the distillation zones of the zone A.
In a further embodiment, the reaction column used comprises a top zone, a zone
A and
a bottom zone and also a distillation zone B which is localized between the
zone A and
the bottom zone and a distillation zone C which is localized between the top
zone and
the zone A. Internals having a distillative separation action or packings can
be com-
prised in these distillation zones B and C. The embodiments of the internals
or pack-
ings are analogous to those of the distillation zones of the zone A.
The synthesis gas used in the process of the invention can be produced by
generally
known processes (as described, for example, in Weissermel et al., Industrial
Organic
Chemistry, Wiley-VCH, Weinheim, 2003, 15-24), for example reaction of coal or
meth-
ane with steam or by comproportionation of methane with carbon dioxide. It
usually has
a ratio of carbon monoxide to hydrogen of from 3:1 to 1:3. Preference is given
to using
a synthesis gas which has a mixing ratio of carbon monoxide to hydrogen of
from 1:0.5
to 1:2.5.
As catalysts, use is made of those Fischer-Tropsch catalysts which
preferentially cata-
lyze the formation of olefins, in particular a-olefins. Possible catalysts
here are, in par-
ticular, Fischer-Tropsch catalysts comprising iron, iron and cobalt,
iron/cobalt spinel or
cobalt/manganese spinel and also copper-promoted cobalt Fischer-Tropsch
catalysts.
In particular, the catalysts described in GB 1 512 743, GB 1 553 361, GB 1 553
362,
GB 1 553 363, US 4,199,523, US 4,418,155, US 5,100,856 are incorporated by
refer-
ence into the present invention.
The process of the invention is usually carried out at from 150 to 350 C. The
pressure
here is from 1 to 60 bar, preferably from 10 to 50 bar.
The GHSV (gas hourly space velocity) is generally from 100 to 30 000 parts by
volume
of feed stream per part by volume of catalyst and hour (1/1=h).
The product obtained in the process of the invention, which is removed from
the reac-
tion column below the point at which the synthesis gas is fed in, is a mixture
of a plural-
ity of hydrocarbons. This mixture has a particular mean molar mass and a
particular
molecular weight distribution. This product preferably comprises at least 50%
by
weight of olefins, preferably a-olefins.
The olefins obtained generally have from 4 to 20 carbon atoms, preferably from
5 to 14.
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In a particular embodiment, a product comprising at least 50% by weight of
olefins hav-
ing from 5 to 7 carbon atoms, of which in turn at least 50% by weight is made
up of one
or more a-olefins, in particular 1-pentene and 1-hexene, is obtained.
In a further particular embodiment, a product comprising at least 50% by
weight of ole-
fins having from 8 to 14 carbon atoms, of which in turn at least 50% by weight
is made
up of one or more a-olefins, is obtained. These products obtained by the
process of the
invention are novel.
In a further particular embodiment, a product comprising at least 50% by
weight of ole-
fins having from 15 to 20 carbon atoms, of which in turn at least 50% by
weight is made
up of one or more a-olefins, is obtained. These products obtained by the
process of the
invention are novel.
Furthermore, it can be advantageous to introduce an a-olefin or a mixture of a-
olefins
whose number of carbon atoms is at least 1 less than that of the olefin mainly
formed
which can be separated off below the zone A into the zone A of the reaction
column
during start-up of the process.
