U.S. patent application number 14/832701 was filed with the patent office on 2016-02-18 for process for reforming hydrocarbons.
The applicant listed for this patent is Haldor Topsoe A/S. Invention is credited to Kim Aasberg-Petersen, Peter Seier Christensen, Thomas Sandahl Christensen.
Application Number | 20160046488 14/832701 |
Document ID | / |
Family ID | 46354305 |
Filed Date | 2016-02-18 |
United States Patent
Application |
20160046488 |
Kind Code |
A1 |
Aasberg-Petersen; Kim ; et
al. |
February 18, 2016 |
PROCESS FOR REFORMING HYDROCARBONS
Abstract
A process for the production of synthesis gas by the use of
autothermal reforming in which tail gas from downstream
Fischer-Tropsh synthesis is hydrogenated and then added to the
autothermal reforming stage.
Inventors: |
Aasberg-Petersen; Kim;
(Allerod, DK) ; Christensen; Peter Seier; (Virum,
DK) ; Christensen; Thomas Sandahl; (Kgs. Lyngby,
DK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Haldor Topsoe A/S |
Kgs. Lyngby |
|
DK |
|
|
Family ID: |
46354305 |
Appl. No.: |
14/832701 |
Filed: |
August 21, 2015 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14129244 |
Jan 31, 2014 |
9162886 |
|
|
PCT/EP2012/061809 |
Jun 20, 2012 |
|
|
|
14832701 |
|
|
|
|
Current U.S.
Class: |
518/703 ;
252/373 |
Current CPC
Class: |
C01B 2203/0244 20130101;
C01B 2203/142 20130101; C01B 2203/0227 20130101; C01B 2203/0233
20130101; C01B 3/38 20130101; C10K 3/02 20130101; B01D 53/72
20130101; C01B 3/34 20130101; C01B 3/36 20130101; C01B 2203/0261
20130101; C01B 2203/143 20130101; C10G 2/31 20130101; C10G
2300/4075 20130101; C10G 2/32 20130101; C01B 2203/1235 20130101;
C01B 2203/1276 20130101; C01B 3/386 20130101; C07C 1/0485 20130101;
C01B 3/382 20130101; C01B 3/24 20130101; C01B 2203/0255 20130101;
C01B 2203/062 20130101; C10G 2/30 20130101; C01B 2203/1258
20130101; C01B 2203/148 20130101 |
International
Class: |
C01B 3/38 20060101
C01B003/38; C10K 3/02 20060101 C10K003/02; C01B 3/34 20060101
C01B003/34; C10G 2/00 20060101 C10G002/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 29, 2011 |
DK |
PA 2011 00485 |
Nov 26, 2011 |
EP |
11009101.4 |
Dec 6, 2011 |
DK |
PA 2011 00947 |
Claims
1. A process for the production of synthesis gas from a hydrocarbon
feedstock with reduced metal dusting in at least the burner parts
of an autothermal reformer (ATR), catalytic partial oxidation
reactor (CPO), or partial oxidation reactor (POx), the process
comprising the steps of: passing said hydrocarbon feedstock through
an ATR, CPO or POx, and withdrawing a stream of hot effluent
synthesis gas from the ATR, CPO or POx; passing tail gas from a
Fischer-Tropsch synthesis stage through a hydrogenation stage to
produce a hydrogenated tail gas; and adding the hydrogenated tail
gas directly to said ATR, CPO or POx.
2. The process according to claim 1, wherein said hydrocarbon
feedstock is a gas that has passed through at least one adiabatic
pre-reforming stage.
3. The process according to claim 1, wherein said hydrocarbon
feedstock is a gas that has passed through at least one steam
reforming stage.
4. The process according to claim 1, wherein said hydrocarbon
feedstock is a gas mixture resulting from dividing a raw
hydrocarbon feed gas into two streams, passing the first stream
through at least one steam reforming stage to form a primary
reformed gas, using the second stream as a by-pass stream to said
steam reforming stage, and subsequently combining said primary
reformed gas with the by-pass stream to form said hydrocarbon
feedstock.
5. The process according to claim 1, comprising dividing a raw
hydrocarbon feed gas into two streams, by which one of the streams
becomes said hydrocarbon feedstock, and passing the other stream
through at least one steam reforming stage to form a reformed
gas.
6. The process according to claim 3, wherein the steam reforming
stage is heat exchange reforming, and where at least a portion of
the hot effluent synthesis gas from the ATR, CPO, or POx is used as
heating medium in said heat exchange reforming.
7. The process according to claim 6, wherein said hot effluent
synthesis gas is combined with said reformed gas before, during or
after said hot effluent synthesis gas has delivered heat to the
heat exchange reforming.
8. The process according to claim 7, comprising adding a stream
comprising steam to said hot effluent synthesis gas, to said
reformed gas, or to the combined stream of hot effluent synthesis
gas and reformed gas.
9. The process according to claim 4, wherein at least one adiabatic
pre-reforming stage is conducted prior to dividing said raw
hydrocarbon feed.
10. The process according to claim 1, comprising mixing the
hydrogenated tail gas with the hydrocarbon feedstock prior to
conducting reforming in the ATR, CPO or POx.
11. The process according to claim 1, comprising adding the
hydrogenated tail gas to the ATR, CPO or POx as a separate
stream.
12. The process according to claim 4, comprising mixing the
hydrogenated tail gas with said by-pass stream prior to conducting
reforming in the ATR, CPO or POx.
13. The process according to claim 4, comprising mixing the
hydrogenated tail gas with said primary reformed gas.
14. The process according to claim 1, further comprising converting
the synthesis gas into liquid hydrocarbons via Fischer-Tropsch
synthesis.
Description
[0001] This is a continuation of U.S. application Ser. No.
14/129,244, filed on Jan. 31, 2014, which is a 371 of International
Application No. PCT/EP2012/061809, filed on Jun. 20, 2012, which
claims priority to Denmark Application No. PA 2011 00485, filed on
Jun. 29, 2011, European Application No. 11009101.4, filed on Nov.
26, 2011, and Denmark Application No. PA 2011 00947, filed on Dec.
6, 2011.
FIELD OF THE INVENTION
[0002] The present invention relates to a process for the
production of synthesis gas used for the production of hydrocarbons
by Fischer-Tropsch synthesis. The invention relates in particular
to a process for the production of synthesis gas by the use of
autothermal reforming in which tail gas from a downstream process,
in particular Fischer-Tropsh synthesis, is hydrogenated and then
added to the autothermal reforming. In a more general aspect the
invention encompasses the removal of olefins in a gas to reduce
metal dusting of metal parts in contact with the gas, particularly
for the reduction of metal dusting in ATR, CPO or POx and other
downstream equipment through which the gas is passed.
BACKGROUND OF THE INVENTION
[0003] The production of synthesis gas for Fischer-Tropsch
synthesis is typically obtained by passing a hydrocarbon feed
through primary and secondary reforming stages. The primary
reforming is often conducted in tubular steam reformers or heat
exchange reformers, while the secondary reforming is typically
conducted in autothermal reformers. When combining a heat exchange
reformer with a subsequent autothermal reformer, the hot effluent
gas from the autothermal reformed is usually used as heating medium
in the heat exchange reformer. It is known to recycle tail gas from
Fischer-Tropsch synthesis as part of the hydrocarbon feed used to
produce the synthesis gas in the primary and secondary reforming
stages. Tail gas can be added prior to the primary reforming or to
the primary reformed gas before entering the secondary reforming
(typically autothermal reforming).
[0004] Tail gas from Fischer-Tropsch synthesis contains hydrogen,
carbon monoxide, carbon dioxide as well as light hydrocarbons in
the form of paraffins such as methane, ethane, propane and not
least olefins such as propylene. The tail gas may also include
alcohols and other higher hydrocarbons of both paraffinic and
olefinic nature. It is known that the addition of such tail gas to
the synthesis gas production section enables that there is
sufficient carbon dioxide during the reforming to achieve the
desired H.sub.2/CO molar ratio, typically about 2.0.
[0005] As used herein "tail gas" means off-gas from the
Fischer-Tropsch synthesis stage which is not re-used in said
stage.
[0006] Hydrogenation of tail gas is known in the art. For instance,
in GB 632386 tail gas from Fischer-Tropsch synthesis is
hydrogenated in order to increase the otherwise low heating value
of this gas caused by the presence of i.a. carbon dioxide, carbon
monoxide and hydrogen.
[0007] WO-A-0142175 discloses a process in which tail gas is
hydrogenated in order to saturate any unsaturated hydrocarbons and
is then reformed in a separate steam reformer. The hydrogenation
serves to decrease the tendency towards coking in the subsequent
high temperature treatment of the steam reformer, since the
tendency to coking in said reformer is greater when unsaturated
hydrocarbons are present in the tail gas. The resulting reformed
tail gas may subsequently also be passed to an autothermal
reformer. Accordingly, a steam reformer is used between the
hydrogenation stage and the autothermal reformer.
[0008] EP-A-1860063 discloses a process in which off-gas from
Fischer-Tropsch synthesis where olefins present in the off-gas are
first hydrogenated and then converted to hydrogen by a reforming
process. Olefins are hydrogenated because of carbon deposition or
coking of catalysts used in the hydrogen manufacturing unit and
which form hot spots on the catalyst and the reformer reactor
tubes. Thus, olefins are removed to avoid coking in a steam
reformer having reformer tubes such as a fired reformer.
SUMMARY OF THE INVENTION
[0009] We have now found out that the addition of tail gas to the
autothermal reformer (ATR), or catalytic partial oxidation reactor
(CPO), or non-catalytic partial oxidation reactor (POx), which is
desirable in order to adjust the H.sub.2/CO ratio in the synthesis
gas, has the severe drawback of promoting metal dusting corrosion,
particularly in the burner parts of the ATR or CPO, yet by
hydrogenation of the tail gas prior to its direct addition to the
ATR such metal dusting is significantly reduced. It has come as a
surprise to the applicant that the removal of particularly olefins
in the tail gas via hydrogenation conveys the critical advantage of
significantly reducing the aggressiveness of the tail gas and hence
reducing or eliminating metal dusting in the ATR, or CPO or POx. At
the same time the benefits of using tail gas to adjust the
H.sub.2/CO ratio are maintained.
[0010] The reduction or elimination of metal dusting in an
apparatus, e.g. ATR, CPO or POx, according to the simple,
economical and elegant solution provided by the present invention
translates directly into the reduction or elimination of costly
down-time periods in the plant and reduces thereby maintenance
costs. Metal dusting has otherwise been mitigated through the use
of resistant alloy compositions or metallic coatings that form
protective surfaces under metal dusting conditions, and/or by
operating the reformer at less metal dusting aggressive conditions
but which on the other hand impair the process. Yet even the use of
expensive and otherwise effective alloys against metal dusting such
as Inconel 690 cannot withstand metal dusting attack when exposed
to tail gas from Fischer-Tropsch synthesis.
[0011] Metal dusting is a type of metallic corrosion that may be
encountered when gases containing carbon monoxide come into contact
with metals above ca. 400.degree. C., particularly in the range
400-800.degree. C. Metal dusting conveys the disintegration of
metals to dust and is described extensively in the literature.
[0012] Metal dusting is a highly complex corrosion process which is
not completely understood. However, it is often represented by the
following reaction:
CO+H.sub.2.fwdarw.C+H.sub.2O (1)
[0013] The formed carbon results in corrosion of the construction
material possibly by a mechanism including carbide formation and/or
dissolution of the carbon in the metal material.
[0014] Carbon formation via the exothermic reactions
2CO.fwdarw.C+CO.sub.2 (Boudouard reaction) and
CO+H.sub.2.fwdarw.C+H.sub.2O (CO-reduction) is a precursor for
metal dusting (MD) corrosion. The exothermic reactions are favoured
at low temperatures. However, the reaction rates are higher at
higher temperatures. As a result, the MD potential for a given gas
will be highest in a medium temperature range, typically in the
range of about 400-800.degree. C.
[0015] It has to be appreciated, however, that metal dusting and
coking are two different phenomena. While metal dusting refers to
catastrophic corrosion of metal parts, coking is associated with
the catalyst. Coking refers more specifically to carbon formation
negatively affecting the catalyst of a steam reformer such as a
tubular reformer due to formation of carbonaceous elements that
deposit and dissociate on the nickel surface or support material of
the steam reforming catalyst (typically a nickel-based catalyst).
This may convey also the development of hot spots in the tubes
containing the catalyst. Accordingly, for the skilled person metal
dusting and coking are two different phenomena: while it has been
known for long that the presence of olefins causes coke deposition
in catalyst beds, no one has ever seen nor expected that olefins
are also responsible for causing such a different phenomenon as
metal dusting.
[0016] Accordingly, in a first aspect of the invention we provide a
process for the production of synthesis gas from a hydrocarbon
feedstock with reduced metal dusting potential in at least the
burner parts of an autothermal reformer (ATR), catalytic partial
oxidation reactor (CPO), or partial oxidation reactor (POx)
comprising: passing said hydrocarbon feedstock through an ATR, CPO
or POx, and withdrawing a stream of hot effluent synthesis gas from
the ATR, CPO or POx, passing tail gas from a Fischer-Tropsch
synthesis stage through a hydrogenation stage to produce a
hydrogenated tail gas and adding the hydrogenated tail gas directly
to said ATR, CPO or POx.
[0017] The hydrogenation of the tail gas results in a gas that
protects the ATR, CPO or POx from metal dusting, particularly for
ATR and CPO the burner metal parts located at the inlet of the
reactor and thus upstream the catalyst bed, as it unexpectedly
turns out that the absence of olefins makes a gas less aggressive
with respect to metal dusting corrosion.
[0018] Hence, there is provided in an elegant and simple manner a
solution to the long-standing problem of metal dusting of metal
parts in the ATR, CPO or POx, particularly burner parts of the ATR,
which were encountered when incorporating tail gas from
Fischer-Tropsch synthesis into the process.
[0019] As used herein the term "reduced metal dusting potential in
at least the burner parts of an autothermal reformer (ATR),
catalytic partial oxidation reactor (CPO), or partial oxidation
reactor (POx)" means that the metal dusting potential is reduced in
any metal part within the reactor being in contact with the process
gas fed to the it (ATR, CPO, POx) including burner metal parts,
particularly for ATR or POx. It would be understood by the skilled
person that ATR and POx imply the use of a burner at the top of the
reactor. ATR and CPO use a catalyst bed below the combustion zone.
CPO means a catalytic reactor or catalytic gasifier which does not
always require the use of a burner, but a mixer instead. Further,
in a POx (gasifier) there is no use of catalyst. The term ATR
includes secondary reformers.
[0020] Since the tail gas contains carbon monoxide, carbon dioxide,
hydrogen, various hydrocarbons including olefins and a range of
other components as described above, the gas is converted by
reducing the olefin concentration by hydrogenation according to the
following reaction C.sub.3H.sub.6+H.sub.2C.sub.3H.sub.8. The
reaction is given for propylene hydrogenation but hydrogenation of
other olefins takes place according to a similar reaction.
[0021] Catalysts suitable for selectively hydrogenating the olefins
to saturated hydrocarbons are preferably based on copper, for
instance a Cu/ZnO catalyst, or a combination of copper and a noble
metal, for instance platinum or palladium. A copper based catalyst,
such as Cu/ZnO catalyst, is particularly active in the selective
hydrogenation of olefins to paraffins with reduced formation or
without the formation of methanol or higher alcohols having two or
more carbon atoms in their structure.
[0022] In connection with the above and below embodiments, the
hydrogenation is preferably conducted in a cooled reactor,
particularly at a temperature in the range 100-150.degree. C. or
higher, for instance 185.degree. C. This enables high conversion of
olefins such as C.sub.3H.sub.6 and C.sub.4H.sub.8 while at the same
time avoiding significant formation of methanol or higher alcohols
and other by-products. Alternatively, the hydrogenation is
conducted in an adiabatic reactor in which the inlet temperature is
preferably in the range 70-120.degree. C., more preferably
80-100.degree. C., and the outlet temperature is 120-210.degree.
C., preferably 140-190.degree. C., more preferably 150-185.degree.
C.
[0023] The pressure in the hydrogenation step is in the range 20-70
bar, preferably 20-50 bar, more preferably 20-40 bar.
[0024] In one embodiment of the invention said hydrocarbon
feedstock is a gas that has passed through at least one adiabatic
pre-reforming stage.
[0025] Adiabatic pre-reforming is preferably conducted in a fixed
bed reactor containing a reforming catalyst, thereby converting all
higher hydrocarbons into a mixture of carbon oxides, hydrogen and
methane. This endothermic process is accompanied by the
equilibration of exothermic methanation and shift reactions.
Removal of higher hydrocarbons allows a higher preheat temperature
to the subsequent steam reforming.
[0026] In another embodiment of the invention said hydrocarbon
feedstock is a gas that has passed through at least one steam
reforming stage. The steam reforming stage may for instance be
tubular reforming (steam methane reforming, SMR) or heat exchange
reforming (convective reforming).
[0027] In yet another embodiment, the invention encompasses also a
process wherein said hydrocarbon feedstock is a gas mixture
resulting from dividing a raw hydrocarbon feed gas into two
streams, passing the first stream through at least one steam
reforming stage to form a primary reformed gas, using the second
stream as a by-pass stream to said steam reforming stage, and
subsequently combining said primary reformed gas with the by-pass
stream to form said hydrocarbon feedstock.
[0028] According to this embodiment, steam reforming is arranged in
series with the ATR, CPO or POx.
[0029] In a separate embodiment, an arrangement where steam
reforming is arranged in parallel with the ATR, CPO or POx, is also
provided. Hence, the process comprises dividing a raw hydrocarbon
feed gas into two streams, by which one of the streams formed
becomes said hydrocarbon feedstock, and passing the other stream
through at least one steam reforming stage to form a reformed
gas.
[0030] In another embodiment in combination with anyone of the
above or below embodiments, there is provided a process wherein the
steam reforming stage is heat exchange reforming, and where at
least a portion of the hot effluent synthesis gas from the ATR, or
CPO, or POx stage is used as heating medium in said heat exchange
reforming.
[0031] Hence, one preferred embodiment is a process in which a heat
exchange reformer is arranged upstream and in series with an ATR or
CPO, preferably an ATR. The raw hydrocarbon feed, for example
desulphurised natural gas, is mixed with steam and the resultant
mixture is directed to the catalyst side of the heat exchange
reformer. In the heat exchange reformer, the gas is then steam
reformed according to the reactions:
CH.sub.4+H.sub.2O.revreaction.CO+3H.sub.2 and
CO+H.sub.2O.revreaction.CO.sub.2+H.sub.2. The gas leaving the heat
exchange reformer is close to chemical equilibrium for the
reactions above. Typically, the exit temperature is 600-850.degree.
C. or preferably 675-775.degree. C. The primary reformed gas
leaving the heat exchange reformer is passed to the ATR or CPO. In
the reactor (ATR or CPO) also oxygen and in some cases a small
amount of steam is added. Synthesis gas is formed by a combination
of steam reforming and partial oxidation in the reactor. The gas
leaving the reactor is free of oxygen and generally the above
reactions are close to chemical equilibrium. The temperature of
this hot effluent gas from e.g. an autothermal reformer is between
950 and 1100.degree. C., typically between 1000 and 1075.degree.
C.
[0032] This hot effluent gas leaving the reactor comprises carbon
monoxide, hydrogen, carbon dioxide, steam, residual methane, and
various other components including nitrogen and argon. This
synthesis gas is passed to the non-catalytic side of the heat
exchange reformer, where it is cooled by supplying heat to the
catalytic side of the heat exchange reformer by indirect heat
exchange. The exit temperature from this side of the heat exchange
reformer would typically be in the range from 500-800.degree.
C.
[0033] It also follows that in another preferred embodiment a heat
exchange reformer is arranged in parallel with an ATR, CPO or POx,
preferably an ATR, and hot effluent synthesis gas from the ATR, CPO
or POx is used to provide heat for the endothermic reforming
reactions in the heat exchange reformer.
[0034] In the parallel arrangement said hot effluent synthesis gas
is combined with said reformed gas before, during or after said hot
effluent synthesis gas has delivered heat to the heat exchange
reforming. Preferably, said hot effluent synthesis gas is combined
with said reformed gas before it has delivered heat to the heat
exchange reforming.
[0035] In yet another embodiment in combination with one of the
above or below embodiments, the process comprises also adding a
stream comprising steam to said hot effluent synthesis gas, said
reformed gas, or the combined stream of hot effluent synthesis gas
and reformed gas.
[0036] Hence, regardless of whether the heat exchange reformer is
arranged in series or in parallel with the ATR, CPO or POx, steam
is introduced to the gas from the ATR, CPO or POx delivering heat
to the heat exchange reformer. This enables reduction of metal
dusting in the metal parts, particularly the shell side, of the
heat exchange reformer, particularly where the heat exchange
reformer is in series arrangement with the ATR, CPO or POx. This
stream comprising steam contains preferably more than 90 vol % of
steam (H.sub.2O in the vapour phase), more preferably more than
95%, and most preferably more than 99%. Preferably, the temperature
of the hot effluent synthesis gas is 950 to 1050.degree. C., more
preferably 1025.degree. C., while the steam added is preferably at
271.degree. C. at 55 barg, thus resulting in a temperature of the
mixed stream, i.e. hot effluent synthesis gas combined with stream
comprising steam, of 900 to 990.degree. C.
[0037] In a further embodiment in combination with anyone of the
above or below embodiments, the at least one adiabatic
pre-reforming stage is conducted prior to dividing said raw
hydrocarbon feed. Hence, prior to dividing the raw hydrocarbon feed
gas in separate streams in the series or parallel arrangements,
adiabatic pre-reforming of the raw hydrocarbon feed (typically
comprising methane and higher hydrocarbons) is conducted.
[0038] In an another embodiment in combination with anyone of the
above or below embodiments, the process comprises also mixing the
hydrogenated tail gas with the hydrocarbon feedstock prior to
conducting reforming in the ATR, CPO or POx; or alternatively,
adding the hydrogenated tail gas to the ATR, CPO or POx as a
separate stream.
[0039] In connection with the operation of the series arrangement
as described above, there is also provided a process comprising
mixing the hydrogenated tail gas with said by-pass stream prior to
conducting reforming in the ATR, CPO or POx; or alternatively,
mixing the hydrogenated tail gas with said primary reformed
gas.
[0040] In yet a further embodiment in combination with anyone of
the above embodiments, the process further comprises of converting
the synthesis gas into liquid hydrocarbons, particularly diesel via
Fischer-Tropsch synthesis.
[0041] In a second aspect the invention encompasses the use of
hydrogenated tail gas from a Fischer-Tropsch synthesis stage as
means for reduction of metal dusting in an autothermal reformer
(ATR), catalytic partial oxidation reactor (CPO), or partial
oxidation reactor (POx).
[0042] Hence, according to this aspect the invention encompasses
the use of a known substance (hydrogenated tail gas) to obtain the
surprising technical effect of reduced metal dusting in an ATR, CPO
or POx. Alternative expensive methods such as the provision of
resistant alloy compositions or metallic coatings that form
protective surfaces under metal dusting conditions are thus
avoided.
[0043] Tail gas from Fischer-Tropsch synthesis is hydrogenated,
thereby converting olefins (alkenes) into alkanes, and thus
unexpectedly results in reduction of metal dusting in at least the
burner parts of the reactor compared to a situation where the tail
gas is added directly, without being hydrogenated. Since the use of
tail gas is desirable in order to adjust the H.sub.2/CO ratio in
the synthesis gas, this is now possible without risking expensive
downtime periods and maintenance costs in the ATR, CPO or POx due
to metal dusting issues.
[0044] The hydrogenated tail gas contains preferably less than 1
mol % olefins, more preferably less than 1 mol %, most preferably
below 0.5 mol %, such as less than 0.2 mole %, or less than 0.1
mole %.
[0045] The hydrogenated tail gas is added directly to the ATR, CPO
or POx, as illustrated in the enclosed Figures. The term "directly"
means without any intermediate processes which change the chemical
composition of the hydrogenated tail gas, e.g. without a steam
reformer between said hydrogenation stage and said ATR, CPO or
POx.
[0046] In a broader aspect the invention encompasses also a method
for reducing metal dusting in an apparatus, said apparatus
containing an off-gas, said method comprising the removal of
olefins from said off-gas. In particular, olefins are removed by
hydrogenation thereof. The method is particularly useful for the
reduction of metal dusting in ATR, CPO or POx and other downstream
equipment through which off-gas is passed. The invention
encompasses a method for the reduction of metal dusting in an ATR,
CPO or POx and further downstream equipment by removing the content
of olefins in an off-gas to be passed through the ATR, CPO or
POx.
[0047] Preferably, said step of removing the content of olefins is
a hydrogenation stage.
[0048] As used herein the term "further downstream equipment" means
waste heat boiler and/or steam superheater located downstream the
ATR, CPO or POx and which are used for cooling the synthesis gas
under the production of steam.
[0049] As used herein the term "removing the content of olefins"
means reducing the content of olefins in the gas to less than 0.2
mole %, preferably less than 0.1 mole %.
[0050] As used herein the term "off-gas" means any gas containing
hydrocarbons and olefins, which has to be reformed in the ATR, CPO
or POx to form a synthesis gas comprising hydrogen and carbon
monoxide. The off-gas is preferably tail gas from Fischer-Tropsch
synthesis or tail gas from downstream process for production of
gasoline, such as a process in which gasoline is produced from
oxygenates comprising methanol and dimethyl ether, for instance via
the so-called TIGAS process as disclosed in U.S. Pat. No. 4,520,216
and U.S. Pat. No. 4,481,305.
[0051] The invention encompasses also the use of a gas free of
olefins as means for the reduction of metal dusting of the metal
parts of apparatus in direct contact with the gas. Preferably the
apparatus in direct contact with the gas is an ATR, CPO or POx.
Preferably, the gas is an off-gas; i.e. the waste gas from an
industrial process such as gasoline synthesis as defined above.
[0052] As used herein and in accordance above the term "gas free of
olefins" a gas with less than 0.2 mole %, preferably less than 0.1
mole % olefins.
[0053] As used herein the term "in direct contact with the gas"
means that the gas free of olefins is added directly to the
equipment or to a separate hydrocarbon feedstock without first
being passed through an intermediate stage of reforming, such as
steam reforming.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] The invention is further illustrated by reference to the
accompanying figures.
[0055] FIG. 1 shows a schematic view of the invention when using a
stand-alone autothermal reformer yet including a pre-reformer.
[0056] FIG. 2 shows heat exchange reforming and autothermal
reforming in series with hydrogenated tail gas addition to the
primary reformed gas.
[0057] FIG. 3 shows a process with by-pass of the primary reforming
stage, with addition of hydrogenated tail gas to the by-pass
stream, or to the combined stream of primary reformed gas and
by-pass stream.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0058] The accompanying FIG. 1 shows a general schematic view of an
embodiment for the production of synthesis gas for Fischer-Tropsch
synthesis using a stand-alone autothermal reformer. Clean (free of
sulphur and other poisons to reforming catalysts) hydrocarbon feed
gas 100 such as natural gas or other hydrocarbon containing gas
source is mixed with process steam 110, optionally partly via
saturator/humidifier. The mixture is preheated and pre-reformed
adiabatically in pre-reformer 300 in order to convert any higher
hydrocarbons into H.sub.2, CO, CO.sub.2 and CH.sub.4. This
resulting hydrocarbon feedstock mixture 120 is fed to the
autothermal reformer 400 together with oxygen 130, protection steam
140 and hydrogenated tail gas 150. From the autothermal reformer
400 at hot effluent of synthesis gas 160 is withdrawn and further
processed to form the synthesis gas feed to the downstream
Fischer-Tropsch section 500. Liquid hydrocarbons 170 are produced
and tail gas recycle stream 180 is passed through hydrogenating
stage 600 prior to entering the autothermal reformer 400. Notably,
hydrogenated tail gas is added directly from the hydrogenator to
the autothermal reformer 400.
[0059] In FIG. 2, a mixture of the raw hydrocarbon feed gas and
steam 10 is passed to heat-exchange reformer 25 where it is
catalytically steam reformed and thereafter leaves the
heat-exchange reformer as stream 30. The primary reformed gas
stream 30 is mixed with hydrogenated tail gas 65 from
Fischer-Tropsch section 150 forming the ATR feed stream 70. The
mixed stream 70 is fed to an autothermal reformer 75 with oxidant
80 and protection steam (not shown) also being supplied. The
primary reformed gas is partially combusted and brought towards
equilibrium over reforming catalyst in the autothermal reformer 75.
The hot effluent synthesis gas 110 from the autothermal reformer is
passed through the heat exchange reformer 25. The synthesis gas is
cooled by heat exchange with the gas undergoing reforming over the
catalyst in the heat-exchange reformer 25. The thus cooled
synthesis gas leaves the heat exchange reformer as stream 120 and
is further processed to form the synthesis gas feed to the
Fischer-Tropsch section 150 downstream. Liquid hydrocarbon products
140 are withdrawn together with a tail gas recycle stream 60. The
tail gas recycle stream 60 passes through hydrogenator 160 to form
hydrogenated tail gas stream 65 before being combined with primary
reformed gas 30. Notably, hydrogenated tail gas is added directly
from the hydrogenator to the autothermal reformer 400.
[0060] In FIG. 3, a mixture of raw hydrocarbon feed gas (10) is
divided into two streams 20 and 40. The first stream 20 is fed to
the heat-exchange reformer 25 where it is catalytically steam
reformed and thereafter leaves the heat-exchange reformer as
primary reformed gas 30. The second stream 40 is preheated in a
heat exchanger 45 and bypasses the heat exchange reformer. The
primary reformed gas 30 is mixed with the preheated second stream
50. Hydrogenated tail gas 65 is added to this mixed stream or to
the preheated second stream 50 thus forming the ATR feed stream 70.
The ATR feed stream is fed to the autothermal reformer 75 to which
oxidant 80 and protection steam (not shown) are also supplied. The
ATR feed stream is partially combusted and brought towards
equilibrium over reforming catalyst in the autothermal reformer 75.
The hot effluent synthesis gas 110 is passed through the heat
exchange reformer 25. The mixture stream is cooled by heat exchange
with the gas undergoing reforming over the catalyst in the
heat-exchange reformer 25. The thus cooled synthesis gas leaves the
heat exchange reformer as stream 120 and is further processed to
form the synthesis gas feed to the Fischer-Tropsch section 150
downstream. Liquid hydrocarbon products 140 are withdrawn together
with a tail gas recycle stream 60. The tail gas recycle stream 60
passes through hydrogenator 160 to form hydrogenated tail gas
stream 65 which is then combined with primary reformed gas 30 or
by-pass stream 50. Alternatively, the hydrogenated tail gas 65 may
also be added to the primary reformed stream 30. Notably,
hydrogenated tail gas is added directly from the hydrogenator to
the autothermal reformer 400.
EXAMPLE
[0061] Two tests were made in the same experimental setup: An 800
mm long sample of Inconel 690 was placed in a reactor. The reactor
was placed in an oven with three heating zones. The temperature of
the Inconel 690 sample varied with the position in the oven. The
sample temperatures were 200 to 640.degree. C. The sample was
exposed to a continuous flow of gas with the composition given in
Table 1 as Test 1. The flow rate was 100 Nl/h. The pressure was 29
barg. The conditions were kept for 626 hours. The sample was
examined after the test using stereo microscope and scanning
electron microscope. The sample was attacked by metal dusting
corrosion.
[0062] The second test was made analogous to the first test, with
the exceptions that the gas composition used was as given in Table
1 as Test 2 and the conditions were kept for 672 hours. Examination
of the sample after the test showed that the sample was not
attacked by metal dusting corrosion.
TABLE-US-00001 TABLE 1 Gas compositions (mole %) Component Test 1
Test 2 Hydrogen 12.1 12.1 Water 22.6 22.6 Carbon 6.9 6.9 monoxide
Carbon 7.8 7.8 dioxide Ethylene 0.14 0 Ethane 0 0.14 Methane 49.8
49.8 Propane 0.45 0.45 1-Butene 0.21 0 Butane 0 0.21
[0063] The two gas compositions in the two tests are identical with
the exception that the gas in test 1 contains the olefins
(alkenes), whereas the gas in test 2 contains the corresponding
alkanes. Metal dusting attack occurs in Test 1 but not in Test 2,
which is of longer duration.
[0064] The presence of alkenes makes a gas more aggressive with
respect to metal dusting corrosion. Thus, the use of a hydrogenated
tail gas conveys the reduction or elimination of metal dusting
compared to a situation where tail gas is used without being
hydrogenated.
* * * * *