Note: Descriptions are shown in the official language in which they were submitted.
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TREATMENT OF COKE OVEN GAS
This invention relates to the treatment of coke oven
gases and in particular to reducing the propensity to carbon
formation of such gases.
BACKGROUND OF THE INVENTION
-
Coke oven gas has been used in the United Kingdom
and in Europe as a town gas~ The gas is generally produced by
heating rough coal to a temperature in the range 1000 to 1200C
to yield the coke oven gas together with coke. The ~as is sub-
jected to several stages of cooling and purification to remove ;
impurities such as tar, naphthalene, ammonia and sulphur and
the resulting gas is stable and easily transportable. Coke oven
gases normally have a composition within the Eollowing ranges:
H2 ~---- 51 to 56%~
CO ......................................... 5 to 10%,
C2 ~ ....................................... 2 to 5%,
CH4 ....................................... 25 to 30%, ~ ;
C2H4 ........................................ 2 to 3%,
N2 ~ 5 to 10%,
2 ~ ~ to 2.0%.
The extension of the use of natural gas has led to
the gradual elimination of coke oven gas from domestic use.
Large volumes of coke oven gas are still produced, however, as
a by~produc-t in the preparation of metallurgical coke. This
gas has found few convenient outlets.
A recent development of a direct reduction process
for the production of substitute scrap for electric arc furnaces
has provided a possible opening for coke oven gas as a source
of reducing gas. In this process a reducing gas is reacted
with ferric oxide in a shaft furnace at a temperature in the
range 500 to 900C to produce iron. The iron ln the form o-E
pellets may then be used in electric arc furnaces.
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In the preparation of a suitable reducing gas, coke
oven gas i.s preheated to a temperature in the range 400 to 530C
and .introduced into a steam reformer ~here it reacts with steam
in the presence of a catalyst such as nickel on a refractory
support, The hydrocarbons in the coke oven gas are converted
to hydrogen and carbon dioxide and -the carbon monoxide to
carbon dioxide. The disadvantage with this process however is
that the preheating performed by conventional heaters is liable ~.
to cause carbon formation in accordance with the Boudouard
a reaction:
2C0.~...._. _..:..> C + C02
since the equilibrium favours carbon formation at this .
temperature ranye. This carbon formation is highly undesirable.
It is an object of the present invention to treat
coke oven gas to reduce this formation of carbon.
BRIEE' SUMMARY OF THE INVENTION ~
Therefore according to the invention there is ~:
provided a method of preparing a reducing gas suitable for use
in the direct reduction of ferric oxide from a source gas
stream, comprising from 5 to 12% by volume of carbon monoxide,
1.8 to 7% by volume of carbon dioxide and 50 to 65% by volume
of hydrogen, in whi.ch the gas stream is treated to reduce its
carbon monoxide content therein hy a process selected from
(l) passing the gas stream at an initial temperature of 250 to
375C and a-t elevated pressure over a catalyst to convert car-
bon oxides to methane, and (2) mixing the gas stream with steam
and passing the mixture at an initial temperature of 200 to
270C over a low temperature shift catalyst to conver-t the
carbon monoxide to carbon dioxide, and thereaEter subjecting
the resulting gas to steam reforming in the presence of a cata-
lyst to give the desired reducing gas.
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The resulting gas has a low carbon formation
propensity when subjected to the steam reforming step. Thus
the gas may be reformed in a conventional steam reformer to
produce a reducïng gas without undue carbon formation.
In one embodiment of the invention the carbon oxides
in the coke oven gas are reacted with the hydrogen in the
gas to form methane and water in the presence of a catalyst
such as nickel on a refractory support. Suitable catalysts
are well known in the art and are commercially available,
for example, from Imperial Chemical Industries Limited or
from C.C.E. The methanation reaction is favaured by a low
temperature in the range 250 to 375C and elevated pressure,
although the pressure effect is not very pronounced. The
reaction however, is exothermic and the temperature rise
must be moderated, for example, by inclusion of a relatively
inert component. Generally it has been found that the total
temperature rise for a single reactor should be limited to
about 200C.
A reducing gas prepared from coke oven gas should
have a reducing ratio defined at (H2 + C0)/(H20-~ C02)
equal to about 9.0 if the gas is to be used for direct re-
duction of iron ore. This means that in a steam reEormer
for converting coke oven gas that the steam to hydrocarbon
molar ratio should be in the range of 1.2 to 1.5. It is
therefore convenient to use steam as an inert component
to modify the temperature of the methanation reaction in
the process of the invention since the resulting gaseous
product may then be passed directly for steam reforming.
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In a preferred embodiment o-F the invention coke
oven gas and steam are passed through a reactor having a
plurality of react;on zones containing the catalyst in such
a way that a portion of the coke oven gas to be reformed is
mixed with at least a major portion oE the steam required for
steam reforming and this mixture is passed through the firs-t
reaction zone of the reac-tor. The mixture leaving the first
reaction zone is mixed with a cold second portion of coke
oven gas thereby lowering the temperature of the mixture and
the gases passed through a second reaction zone. This quenching
operation can be repeated several times. The resulting
steam/treated coke oven gas mixture may be passed on to a
steam reformer for subsequent conversion to a reducing gas
with negligible carbon formation.
Other methods of controlling the temperature rise
during the methanation reaction include recycling some of the
product gas to increase the inert component content in the
mixture and utilising the temperature rise to convert water
to steam.
In a second method of reducing carbon formation
from coke oven gas in accordance with the invention, coke
oven gas is mixed with steam and reacted at a temperature
in the range 200 to 270C in the presence of a low temperature
shift catalyst to convert carbon monoxide to carbon dioxide.
This reaction is also exothermic and similar arrangements to
those illustrated above in the conversion of carbon oxides to
methane may be used to control the temperature rise. The
resulting gaseous product may be used directly in a steam
reforming reactor. Suïtable catalysts are similar to the
~, .
methanation catalysts and are commercially available from
Imperial Chemical Industries Limited and C.C~E.
The process of the invention is not limi-ted to the
treatment of coke oven gas and similar gas streams containing
hydrogen and carbon oxides may also be trea-ted. Such gas
streams include, for example, those having compositions
within the ranges:
5 to 12% by volume carbon monoxide,
2 to 7% by volume of carbon dioxide, and
50 to 65% by volume hydrogen.
The process of the invention is however ideal for
coke oven gas treatment and may be incorporated as a stage in
the puriflcation and preparation of coke oven gas for use as
a reducing gas. Thus suitable purification and processing stages
would include in the following order de-tarring, ammonia removal,
benzol removal, H2S removal, preheating organic sulphur removal,
reduction of carbon monoxide content, preheating and direct
reduction by steam reforming.
BRIEF DESCRIP _ON OF THE DRAWINGS
The invention will now be illustrated by the following
Examples with reference to the accompanying drawings in which
Figures 1, 2 and 3 represent flow diagrams of the reactions
in Examples 1, 2 and 3, respectively.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
-
Example 1
100 Moles of coke oven gas to be steam reformed with
50.99 moles of steam, which is equivalen-t to a steam to hydro-
carbon mole ratio of 1.~ were treated by the methanation
process in accordance with the invention.
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Figure 1 of the accompanying drawlngs represen~s
a flow diagram of the system used. It includes a methanation
reactor 2 having a nun~er of stages 2a to 2_.
The coke oven gas stream was divided such that one
fifth was fed to the methanator reactor 2 at the inlet 4 with
the major portion o~ process- steam. The amount of steam
fed was 37.6 moles the remaining 13.39 moles required was
produced in the methanator reactor and fed in as an additional
moderator.
rrhe feed to the first stage 2a was:
moles mole ~ drymole ~ wet
CO 1.02 5.1 1.77
H2 12.3 61.5 21~39
C~145.14 25.7 8.94
C2H60.52 2.6 0.90
C2 0.36 1.8 0.63
2 0.10 0.5 0.17
N2 0.56 2.8 0.97 ~;
H2O37.6 65.23.
The inlet temperature was 250C.
The outlet gas composition from the first stage
was 2a was:
moles
H2 7.6
CH4 6.52
C2H6 0.52 ~-
N2 0.56
H2O 39.44.
The outlet temperature was 416C.
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The gas leaving the first stage 2a of methanation
was quenched with a further one fifth of the incoming co]ce oven
gas added through an inlet 6 such that the mixture temperature
was 319C and the gas composition was:
moles :
CO 1.02
H2 19.90
CH4 11.66
C2H6 1.04
CO2 0.36
2 0~50 :
N2 1.12
H2O 39~44
The gas was fed into a second stage 2b of methanation
and the outlet gas from that stage had the following compositions:
moles
H2 15.20
CH4 13.04
C2H6 1.04
N2 1.12
H2O 41.38.
The temperature of the gas at the outlet from the
second stage 2b was 441C.
The gas leaving the second stage 2a was again
quenched with a further one fifth of the feed coke oven gas
introduced through inlet 8 and the temperature of the
mixture was 362C and its composition was: .
.,,1
,: ~
2~:
moles
CO 1.02 ;.
H2 27.50 ~
CH4 18.18 ::
C2H6 1.56
C2 ~.36
2 0.10 :~
N2 1.68
H2O 41.38.
The gas mixture was fed into the third s-tage 2c of
the methanation reactor and the resulting gas left it at 458C
and the gas composl-tion was as follows: -
moles
H2 22.80 ;
CH4 19.S6
C2H6 1.56
N2 1.68
H2O 43.32.
The gas was then again mixed with a further one fifth
20 of coke oven gas introd~ced through an .inlet 10 and the remaining
process steam of 5.73 moles to give a mixture temperature of
371C. The gas was then fed into the fourth stage 2d o:E the
methanation reactor to give a product gas of the following
composition at 452C:
moles
H2 30 40
CH4 26.08
C2H6 2.08 .
N2 2.24
~2 50 99
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This gas was then mixed with the remaining portion
of coke oven gas feed passing along a line 11 and the resulting
gas had the following composition:
moles mole % wet mole % dry
CO 1.02 0.77 1026
H242.70 32O44 52.84
CH431.22 23.69 38.64
C2H62.60 1.97 3.22
C2 0.36 0.27 0.45
2 0.10 0.06 0.12
N2 2.80 2.12 3.47
H2O50.99 38.70
The resulting gas had a low carbon monoxide content
and did not produce large carbon deposits when subjected to
steam reforming to give a reducing gas.
Example 2
Figure 2 represents a flow diagram of the reaction
system used in this Example. This Example describes the use
of an isothermal reactor. '
Coke oven feed gas of composition shown below was
fed into a multitube reactor 12 in which the tubes are packed
with catalyst and the tubes were surrounded by water at its
boiling point at a pressure of about 24 ky/cm absolute (220C).
The coke oven gas composition was:
moles
CO10.20 ;
H256.20
CH426.20
C2H62.50
C2 3.20
2 0 50
N2 1.10
H2O44.84.
_g_
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A portion of steam re~uired for the subsecluent steam
reforming reaction was also fed into the reactor.
The heat of reaction was suEficient to produce 736
kg of steam at 24 kg/cm absolute.
The gas leaving the reactor had the following
composition:
molesmole ~ wet mole % dry
H2 11.80 10.05 12.00
CH4 39.60 33.72 72.00
C2H6 2.50 2.13 4.55
N2 1.10 0.94 2.00 ;
H2O 62.44 53.16
and were suitable for being subjected to steam reforming to
give a reducing gas with large carbon deposition.
Example 3
This Example illustrates how gas treated by the
present invention may be used as an inert COmpQnent to control
the reaction temperature.
Figure 3 of the accompanying drawings represents
a flow diagram of the system used.
Coke oven gas of composition as in Example 2 was
mixed with product gas resulting from the methanation of coke
oven gas and passed into a reactor 20. The product gas left the
reactor at a temperature of 392C and was passed through at
heat exchanger 22 in which it gives up some heat to recycled
product gas. After the heat exchanger the product gas was
cooled in a cooler 24 and the water content removed in a water
separator 26. A portion of the resulting produGt gas~as then recycled
via a compressor 28, heat exchanger 22 and heater 30 to provide
an inert component to help to moderate the methanation of Eresh
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incoming coke oven gas.
The inlet gas mixture -to the reactor 20 was as
follows:
moles
CO 10.20
H2 127.00
CH4 263.80
C2H6 85.00
C2 3.20
lO 2 0-50
N2 7~70 ~-
H2O 4.92.
The gas was heated to 250C by heat exchange with the
product yases leaving the reactor 20. The produc-t gas left the
reactor at 392C and had the following composition:
moles
H2 82.60
CH4 277.20
C2H6 85.00
N2 7 70 ;
2 22.52.
Example 4
This Example illustrates carbon monoxide conversion
to carbon dioxide and hydrogen to give a gas suitable for steam
reforming to a reducing gas without significant carbon deposi-tion.
A portion of gas of composition as in Example l was
preheated to 227C together with 48.06 moles of steam and was
passed through the first stage oE the low temperature catalytic
shift reactor.
The lnlet composition was:
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moles
CO 2.55
H2 30 75
CH4 12.85
C2H6 1.30
CO2 090
2 0.25
N2 1.40 :
H2O 48.06.
The gas at the outlet from the reactor had the following
composition at a temperature of 287C.
moles
CO 0.15
H232.65
CH412.85
C2H61.30
C2 3 30
N2 1.40
H2O46.16.
The yas was quenched with the remaining portion oE the
coke oven feed gas and the temperature of the mixture was 207 C
and its composition was~
moles
CO 2.70
H263.40
CH425.70
C2~62.60
C2 4.20
2 0.25
N2 2.80
H2O46.16.
The mixture was passed through the second stage of the
'low temperature shift' reactor and composition of the resulting
gas a-t a temperature of 237C was:
moles
CO 0.30
H265.30
CH425.70
C2H62.60
C2 6.60
N2 2.80
H2O44~26O
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3~2
The steam to hydrocarbon carbon ration was 1.43
and the ratio of oxidising components to hydrocarbon carbon
i.e. H2O + 0.5 CO2 to carbon, was 1.54. The gas was suitable
for steam reforming.
Example_5
Gas of eomposition as in Example 2 was preheated
together with steam to 226C and passed through a tubular
reaetor, in which the tubes were paeked with the 'low
temperature shift' eatalyst and were surrounded by boiling
water at approximately 15 kg/em2 absolute.
The gas was of the following composition:
moles
CO 10.20
H2 56.20
CH4 26.20
C2H6 2.50
C2 3.20
2 0.50
N2 1.10
H2O 46.00.
The resulting gas has left the.reaetor at 226C and the
gas eomposition was:
moles
CO 0.30
H~ 65.10
CH4 26.20
C2H6 2.50
C2 13.10
N2 1.10
H2O 37.10.
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The heat of reaction evapora-ted 148 kg of water
at 15 kg/cm2 absolute.
~ he ratio of oxidising components to hydrocarbon
~arbon tlle resulting g~s was 1~4 the gas was suitable for
steam reforming.
A latitude of modification,change and substitution
is intended in the foregoing disclosure and in some
instances some features of the invention will be employed
without a corresponding use oE other features. Accordingly
it is appropriate that the appended claims be construed
broadly and in a manner consistent wi-th the spirit and scope
of the invention herein.
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