Note: Descriptions are shown in the official language in which they were submitted.
15805-15812/KP/io
'7~2
Haldor Tops0e A/S, Lyngby, Denmark.
A process for converting coal and/or heavy petroleum
fractions into hydrogen or ammonia synthesis gas.
Field of the invention
-
The present invention relates to a process for
converting a raw material of coal and/or heavy petroleum
fractions into hydrogen or an ammonia synthesis gas, so-
called syngas, substantially only consisting of hydrogen and
nitrogen .
Background of the invention
The last step in ammonia synthesis is the conversionof the ultimate syngas of nitrogen and hydrogen, most
frequently in the stoichiometric proportion 1-3, but
sometimes in another proportion and often accompanied by
'7'7B~
small amounts of impurities, notably argon and methane, at
high pressure by the aid of a catalyst, especially consisting
of iron and various promoters, e.g. the ammonia synthesis
catalyst type KM prepared and supplied by Haldor Tops0e A/S.
An example of the preparation of a~monia synthesis
gas from natural gas and air has been described by H.F.A.
Tops~e, H.F. Poulsen and A. Nielsen in Chemical Engineering
Progress, vol. 63, No~ 10, 67,73 (October 1967). A useful
review of ammonia technology is found in the book "Ammonia",
edited by A.V. Slack and G. Russel James, Marcel Dekker IncO,
New York and Basel 1977.
The need of ammonia, not least for fertiliæ~r and
raw material for fertilizers, is heavily increasing. At the
same time, the growing scarcity of energy and increasing cost
of recovering natural gas has rendered it desirable to use
other raw materials for the preparation of the synthesis gas
as well as hydrogen, viz. either coal (including pit coal,
anthracite, lignite, peat and coke including petroleum coke),
or heavy petroleum fractions, whereby they are gasiied by
the aid of oxygen and steam. When using coal the gasification
reactions are:
C + H20 ~ -- CO + H2 (1)
C + ~ 2 > CO (2)
whereby by-products may issue such as methane and possibly
other hydrocarbons and sulfur compounds, notably hydrogen
sulfide and carbonyl sulfide, originating from the sulfur
contaminations of the coal, and possibly small amounts of
nitrogen, argon and possibly tar substances.
-The gasification of mineral oils is more complicated,
i.a. because of the fact that especially the heavy ractions
consist of complex mixtures of higher paraffins, olefins
and aromatics. An ideal example is the general reaction
CnHm + n/2 2~ nCO -~ m/2 H2 (3)
but even a thermal cracking with partial oxidation of the
heavy hydrocarbons ta}Ces place so as to form free carbon:
~ ~ ~'7'i'~
CnHm ~ nC + m/2 H2 (4)
In practice both reactions (3) and (4) take place
when using oxygen since less than the stoichiometric amount
is used in practice. Other possible reactions by
the partial oxidation of mineral oils are, i.a.:
CnHm + (n~m/4)O2 ~ - nCO2 + m/2 H~O (5)
CnHm + nCO2 ~ ~ 2m CO + m/2 H2 (6)
CnHm + m/4 2 ~ j nC + m/2 H20 (7)
and by the partial oxidation of liquid hydrocarbons with
superheated steam: ~
CnHm + nH20 ~--- nCO + (m/2 ~ n~ ~2 (8)
and additionally as secondary reactions reaction (1) and
C + CO2 ~ `-2CO (9)
Since a normal residence time in the gasification reactor is
insufficient to allow reactions (1~ and ~9) to run to
completion, there will always be some soot left in the raw
gas at the conclusion of the gasification; when using heavy
fuel oi~ the amount of soot may be up to 3% of the weight
of the raw material.
The further working up of the raw gas formed by the
gasification into the desired synthesis gas in recent time
mostly takes place via two paths differing in principle;
both have been described by Samuel Strelzoff in an article
in Hydrocarbon Processing, December 1974, pages 79-87.
In one of the reaction paths the raw gas is cooled
in a waste heat boiler and is thereafter cooled to below
the dew point in a soot removal system. Thereupon sulfur
is removed and the hydrogen sulfide is worked up, e.g. in
a Claus plant. The next step is the conversion of carbon
monoxide into carbon dioxide by the so-called shift process:
77~;Z
CO + H20 ~--- CO2 2 (10)
which when using this reaction path always takes place by
the high-temperature-shift-reaction (at least 330C) by the
aid of high-temperature shift catalysts, optionally followed
by a low-temperature-shift-reaction (down to about 200C)
by the aid of a low-temperature shift catalyst.High-temperature
shift catalysts mostly consist of oxides of Fe and/or Cr. If
this type of shift catalyst is used, the content of CO may
be brought down to about 3.5 mol%. If the reaction is carried
out in two steps using a low-temperature shift catalyst
in the second step, the content of CO may be brought down
to about 0.3 mol%, calculated on the dry gas. The low-
temperature shift catalysts used in such embodiments of the
process always contain Cu and ZnO and optionally Cr2O3 or
~12O3. These catalysts are exceedingly sensitive to
poisoning with sulfur and a very profound removal of all
sulfur compounds from the gas therefore is needed before it
can be conducted to the low-temperature shift reactor. Since
CO is a catalyst poison for ammonia catalysts, the residual CO
remaining after the shift reaction must be removed and this
normally takes place, in this reaction path, in the cryogenic
manner by washing with liquid nitrogen. If a low-temperature
shift catalyst has been used, it is also possible to remove
CO by methanation, i.e. conversion into methane by the
reaction
CO + 3H2 ~ CH~ + H2O (11)
Methanation is economically justified onlv when the content
of carbon monoxide is low and therefore cannot be used when
the shift reaction is solely conducted over a high-temperature
shift catalyst. Before the nitrogen wash or methanation carbon
dioxide must be removed from the gas, which can be done by
a further wash. By wash with liquid nitrogen carbon monoxide,
remaining amounts of methane, argon, and other gaseous
impurities are removed and at the same time part of the
needful nitrogen is added to the synthesis gas. In practice
the nitrogen is proviaed by first fractionating atmospheric
~3t7~7~
air into an oxygen fraction and a nitrogen fraction and
thenusing the former for the partial oxidation and the
latter fox the nitrogen wash.
In the other reaction path the crude gas formed by
the gasification is cooled directly by the so-called
quenching and at the same time soot removal takes place. By
quenching thexe is in the present specification always
meant cooling of the gas by direct contact with liquid water;
it usually has a temperature of 150-250C and is under a
pressure corresponding thereto. The content of water in the
crude gas after the soot removal is sufficient to justify
CO-conversion by the shift-reaction at this stage. Since
the gas still contains sulfur it is necessary to use a
sulfur-resistant shift catalyst and as the sulfur-resistant
shift catalysts known at the time of the development of the
method were all high-temperature shift catalysts, the CO
conversion in this case is normally carried out at high
temperature. This normally takes place in two
steps; thereafter removal of sulfur compounds is carried out,
notably H2S, and of CO2, usually in a single washing process.
The hydrogen ~ulfide is worked up, e.g. in a Claus plant~
The purified gas contains about 4.8 mol% CO and is finally
washed with liquid nitrogen whereby the ammonia synthesis
gas obtains the needful amount of nitrogen.
In both cases there is obtained a synthesis gas which
in known manner is compressed and converted to ammonia.
; Largely, the two processes set the same demands on
energy and require about the same capital investment for
the installation o~ the necessary plants.
When preparing hydrogen the ultimate gas purification
does not take place by wash with liquid N2 but contrariwise
normally with a copper liquor scrubbing, and the CO removed
is recycled to the inlet of the shif-t section.
Swedish patent publication No. 394 192 (Patent No.
3S 7215398-4), claiming priority from British patent
application No. 549~7/71 of 26th November 1971, describes
and claims a process for converting and purifying a raw gas,
obtained by the partial combustion of a fuel and mainly
c`~
containing hydrogen, carbon monoxide and soot particles,
in order to obtain a hydrogen-rich gas stream. The raw gas
is cooled at 220-300C. The cooling may commence by quenching
but is concluded in a waste heat boiler which it must enter
at a temperature of at least 900C. After the cooling the
soot present in the gas may optionally be removed partially
by wash with hot oil or passage through cyklones. C0 is
converted in the soot-containing gas by passing the same
along (not through) catalyst beds in one or more reactors
having a complicated construction with a plurality of wire
mesh discs separated and supported by distance members;
this construction ensures that the soot remains in the gas
instead of being deposited on the catalyst: If a plurality
of shift conversion reactors are used, the gas is cooled
between them at 220-300C, preferably by quenching. When the
C0 conversion has been completed the gas is cooled in a
cooling zone in which soot and ash are removed. The soot
free gas is washed to remove condensate, it is purified for
acid components (H2S and CO2) and the remaining amount of
carbon oxides is methanated. Swedish specification 394192
states that the process is economic because a cooling and
re-heating before shift conversion is not needed and also
says that it is advantageous that soot does not need to be
removed before the shift conversion, and also that the sulfur
compounds may be removed after that conversion.
It may be advantageous in some respects to let the
soot remain in the gas during shift conversion but in other
respects it is highly disadvantageous, viz. because it
necessitates the said complicated construction of the
reactor(s) in order to avoid blocking or destruction of the
catal~st with soot. The fact that - ior the same reason -
the gas must pass along the catalyst bed instead of
passing through it is also a disadvantage because it results
in an inadequate contact between gas and catalyst and
thereby a low conversion per volume of catalyst; this
necessitates capital costs for higher amounts of catalyst
and larger reactors that would otherwise be required.
'7~
It is the object of the invention to provide a
conversion of cheap fuels into hydrogen or ammonia syngas
which avoids the disadvantages just mentioned and which
can be carried out at any desired tem~erature between the
dew point and the highest possible temperature determined
by the thermodynamic equilibrium considerations, and in
which both the energy consumption and the investment costs
for the plant may be reduced. This is obtained by a novel
combination of measures known ~er se,and in this combination,
like according to the abovementioned Swedish publication
shift conversion is carried out before purification for acid
components (H2S and CO2) and is used for CO-removal.
Brief description o_ the invention
The said objective is achieved by the present process
which is characterized by the following combination of steps
in sequence:
(a) gasifying the raw material at an elevated
temperature with an oxygen-containing gas and steam to form
a crude gas,
(b) cocling the crude gas from step (a) by quenching
and/or steam production,
(c) scrubbing the cooled crude gas to completely
remove soot and other possible solid impurities,
(d) re-heating the cooled, soot-free crude gas at a
desired temperature by heat exchange with exit gas from a
subsequent step of converting carbon monoxide,
(e) subjecting the washed, soot-free, re-heated crude
gas to a shift conversion to convert CO into CO2 and
H2 by one or more passages through one or more catalyst
beds of one or more sulfur-resistant shift catal~sts, that
of reaction being utilized for heat exchange with the gas
in step (d),
(f) removing hydrogen sulfide and carbon dioxide from
the shift-converted gas, and
(g) subjecting the gas thus substantially freed of
acid gas components to a catalytic methanation to convert
remaining amounts of carbon oxides into methane.
7~)2
This process differs from that known from the
abovementioned Swedish patent publication in several respects.
Firstly, the present process can utilize simple conventional
reactors and does not need complicated reactors as in the
prior art. Since soot is removed from the gas before the
shift conversion, the gas can pass through the catalyst bed
without risk of depositing soot on the catalyst particles,
whereby the catalyst is utilized much more efficiently. In
contradistinction to the prior art the gas is cooled and re
heated before shift conversion; however as heat e~change is
used to utilize the heat of the exothermal shift reaction
to preheat the gas, this cooling-reheating is not any
noticeable drawback. It is not necessary to limit the use
of quenchin~ in step (b) to quench the gas to a temperature
above 900C; thereby the quenching and cooling by steam
production can be adjusted to each other in any desired
manner, whereby this adjustment can be utilized to add
precizely a desired amount of water (steam) to the gas. The
said prior art prescribes that the gas temperature at the
exit from each shift reactor should preferably be 360-460C,
whereas the present process may be controlled to a gas
temperature of 190-280C at the exit of the last shift
reactor. This lower exit temperature ensures a higher degree
of conversion of CO.
Detailed description of the preferred embodiments
The pressure during the gasification may be any
desired pressure between atmospheric pressure and ~00 bar.
Gasification of coal will in many cases take place at
atmospheric pressure whereas when gasifying heavy petroleum
fractions one will fre~uently employ elevated pressure. If
the process is to be used for the preparation of ammonia
synthesis gas it will often be expedient to have the crude
gas leave the gasification step at a pressure within a
comparatively high pressure range, which may be maintained
fairly unaltered during the entire conversion of the crude
gas to syngas whereby the pressure increase of the latter
~7~
with a view to its final conversion to ammonia, which
normally takes place at a pressure of, e.g., 150-250 bar,
can be carried out in a single compression step. Thus, it
may be advantageous to obtain the crude gas with a pressure
in the range of 30-125 bar, preferably 50-80 bar.
Cooling in step (b) may take p]ace solely by the
generation of steam or solely by quenching, whereby
part of the soot removal is caused already at this stage.
But according to the invention the cooling
conveniently is carried out by a combination of quenching
and cooling by the production of high pressure steam. By
the combined method of cooling it is obtained that the
content of water-vapour may be predetermined at a desired
level by choosing the correct ratio of the amounts of heat
removed partly by the quenching, partly by the steam
production. This is particularly valuable for the subsequent
shift conversion because there in this process step is
optimally used a steam content which is lower than that
normally obtained by quenching alone.
In many cases it is advantageous to quench the crude
gas at a temperature in the range of 400-800C and to carry
out the remainder of the cooling by production of steam.
Even though part of the solids are removed by the
quenching, a soot removal is carried out by scrubbing with
water.,The soot removal may take place in any conventional
manner.
Before the CO conversion by the shift reaction the
soot-freed gas is heated at the desired temperature by heat
exchange between inflow and outflow for the CO conversion
system. In itself, the reaction may take place at any
temperature between the dew point and the maximum temperatures
determined by the equilibrium temperature. As mentioned
hereinbefore, it is desirable for thermodynamic reasons to
use the lowest possible conversion temperatures whereas the
temperatures on the other hand are limited by the dew point,
for example with an addition of about 30C but also the
temperature limits for the activity of the catalyst must be
taken into consideration.
77~2
The shift conversion may take place in one or more
shift reactors and with one or more sulfur-resistant shift
catalysts. In principle it is a low-temperature shift
reaction, at least in the last part of the shift process,
and there is used a special catalyst which is able to
function, in a sulfur-containing atmosphere, both as a high-
temperature shift catalyst and as a low-temperature shift
catalyst at temperatures down to about 190C. The
availability of such a catalyst is an important prerequisite
for the invention.
A particularly suitable catalyst has been described in
~erman patent publication No. 1928389. According to the
invention there is therefore conveniently emp~oyed a catalyst
with or without catalyst support and consisting o~ (a~ at
least one alkali metal compound prepared from an acid having
a dissociation constant below 1 x 10 3, and (b~ a
hydrogenation-dehydrogenation component of at least one
element belonging to group VB (vanadium, niobium, tantalum~,
VIB (chromium, molybdenum, tungsten) and VIII (iron, cobalt,
nickel, the noble metals) in the Periodic Table, or a
compound thereof, the ratio a:b being 1:0.001 to 1:10. The
catalyst may be sulfided. Component (a) advantageously may
be a potassium salt or cesium sulfide. As component (b) there
is preferably employed a combination of metals or metal
compounds, particularly conveniently nickel and tungsten, or
molybdenum, cobalt and molybdenum, or iron and chromium.
As ~entioned the shift conversion takes place under
conditions, especially with respect to pressure, temperature
and steam content and possible nitrogen content in the gas,
so as to obtain a final temperature only a little above the
dew point of the gas. More precizely the conditions according
to the invention may be adjusted so as to obtain the lowest
possible end temperature which is sufficiently much above
the dew point of the gas to protect the catalyst against
condensation of water vapour~ The shift conversion
particularly conveniently takes place under conditions so
as to obtain a final temperature of 30-60C above the dew
point of the gas, preferably about 40C above the dew point.
~'7'7~
11
In practice it is usually possible to carry out the shift
conversion, or its last part, at-temperatures in the range
of 190-280C.
It is advantageous that the shift conversion thus
may be conducted at least partly and at least so far as its
last part is concerned by low temperatures beca~se this
involves a higher degree of conversion carbon monoxide
into carbon dioxide than by using higher
temperatures since reaction (10) is thermodynamically
favoured towards the right by lower and toward the left
by higher temperatures. Instead of a content of about 3.5%
CO it is possible by the present process when using a low-
temperature active shift catalyst even by the presence of
gaseous sulfur compounds to come down to a content of about
0.5~ CO or lower. Hereby there is obtained a higher yield
of hydrogen. It involves the further advantage that the
remaining amount of CO may be removed by methanation by
reaction (11). Methanation of such small amounts of CO is
possible in an economical manner and there is obtained the
~0 advantage that one avoids capital costs, operation costs and
energy consumption for the cryo-unit with cooling at
temperatures between -175 and -200C necessary when removing
carbon oxides by nitrogen wash. As mentioned it isnot Fossible
to remove so high amounts of CO as 3.5~ in an economic manner
by methanation.
As will be understood from the statement hereinbefore
on the known processes for preparing ammonia synthesis gas
from petroleum fractions, it is most advantageous to carry
out the shift conversion before the removal of sulfur because
then one can remove sulfur (particularly in the form of
hydrogen sulfide) simultaneously with carbon dioxide in one
washing process for removal of acidic gases. Hitherto is
has not been possible to use low-temperature shift catalysts
in the presence of sulfur because they were all sulfur
sensitive and very rapidly became sulfur-poisoned. The
abovementioned sulfur-resistant catalysts not only are sulfur
resistant but even require the presence of a certain minimum
amount of H2S.
'7~
12
An advantage of the conversion process used is that
even by low water/dry gas ratios it causes no significant
methanation according to the formulae (11) and (12).
Substantial ~ethanation would involve loss of synthesis
hydrogen. It is true that the methane formed could be
utilized in the process as fuel but too large-amounts of
methane will influence the energy economy in adverse direction,
firstly because it is compressed to high pressure but used
under low pressure, secondly because the very removal ~e.g.
in a cryogenic extraction unit for purge gas) requires
supply of energy.
The conversion of the carbon monoxide o~ the crude
gas so as to form hydrogen as stated usually takes place
by the aid of a catalyst usable in the shift reaction both
at low and high temperature, e.g. at 190-480C and which
allows or even requires the presence of sulfur. The flow
rate of the gas is not a critical factor and it is possible
to use the space velocities that are comrnon in such reactions,
e.g. in the range of 300-30,000 volumes of gas per volume
of catalyst per hour (Nm3/m3/h). Even if the shift reaction
may take place at high temperature, for the reasons stated
it is most advantageous to conclude it at so low a
temperature as possible, yet above the dew point, as
condensation of liquid water on the catalyst may damage it.
Because of the sulfur tolerance of the catalyst the
acid gases, i.e. mainly carbon dioxide and hydrogen sulfide,
may be removed after the shift reaction which for the
stated reasons of energy economy is the most advantageous.
The removal may occur in known manner and a number of
suitable methods are described in a paper "Merits of acid-
gas removal processes !- by K.G. Christensen and W.J. Stupin,
Hydrocarbon Processing, February 1978, pages 125-130. As
examples may be mentioned here absorption in a solvent
; containing polyethyleneglycol-dimethyl ether (~he "Selexol"
process), absorption in a promoted, hot potassium carbonate
solution (the Benfield process), absorption in potassium
salt solutions (the "Catacarb" process), absorption in
methanol (the "Rectisol" process), absorption in diglycol
amine, and absorption with various other amines.
77~2
13
The low temperature at the completion of the shift
reaction allows as stated a high degree of conversion of CO
so that the residual content of the carbon oxides, CO and
CO2, after elimination of the acid gases is suf~iciently
low to allow them to be removed advantageously by methanation.
~hereby one avoids a nitrogen wash which is expensive
because of the large consumption of cooling energy.
The process and the catalyst used moreover have the
advantage that there is obtained a more complete conversion
of carbonyl sulfide to CO2 and H2S, which are easily removed
by the elimination of the acid gases, whereby the washed
gas will not contain sulfur compounds disturbing subsequent
processes such as for instance the ammonia synthesis.
Purification of the shift-converted gas for acidic
gases, i.e. notably H2S and CO2, may be carried out by a
number of known methods. After removal of the acidic gases
still a little CO remains and possibly also a little CO2
may remain in the gas. Both are almost completely removed
by the said methanation treatment which ta~es part, so far
as CO is concerned, by reac'ion (11), whereas CO2 is
methanated according to the scheme
C2 + 4H2 ~ CH4 2 (12)
If the process is used for preparing hydrogen i-t is
hereby completed. If it is used for the preparation of
ammonia synthesis gas, nitrogen must be added. This can take
place by the direct addition of N2 but according to the
invention it may very advantageously take place by using
oxygen-enriched air as oxygan source for the gasification
of the raw material, whereby the nitrogen will be present
in the gas during the entire sequence of reactions from
(b) to (g).
This has the advantage that there is thereby obtained
a dilution of the crude gas whereby the equilibrium content
of methane in the crude gas, under conditions of equal total
pressure and equal temperature, will become lower than under
conditions where the gasification is carried out with pure
oxygen. The presence of nitrogen in the crude gas also gives
'7~
14
an increase of the heat capacity f the gas and a
decrease of the dew point, which again involves that the
shift conversion may be carried out at a lower temperature,
whereby there is obtained a favourable thermodynamic
equilibrium of the conversion, viz. a higher conversion into
H2 than otherwise would be the case. By using oxygen-enriched
air in the gasification there is finally achieved a saving
in the purchase of pure oxygen or the erection of a plant
for oxygen production.
In the conventional nitrogen wash in connection with,
e.g. the above known processes there is obtained a content
of N2 in the purified gas of about 10%, after which more N2
is added to obtain the stoichiometric ratio H2 to ~ for the
ammonia synthesis. Gasification with oxygen-enxiched air
accordingly is not advantageous in connection with nitrogen
wash because a content of N2 beyond about 10% will merely
condense in the washing column, resulting in a considerable
increase of the needful cooling energy.
The process of the invention is particularly usable
when using heavy petroleum fractions as starting material.
The crude gas is formed by the gasification of the
hydrocarbon material either with pure oxygen, if hydrogen
is to be prepared, or, for the preparation of ammonia syngas,
preferably with oxygen-enriched air, in both cases while
adding steam according to the exothermal reactions ~3) and
(8). Typically the temperature hereby increases to 1300-
1400C. The crude gas formed by the gasification is cooled,
; and for the reasons explained hereinbefore preferably
by a combination of quenching and a further cooling in a
boiler. During the cooling one can thus partly utili~e the
heat content of the crude gas in a waste heat boiler to
prepare high pressure steam. After thecooling the gas is
cooled further in a soot scrubber to remove soot and
possible other solids in known mannerO The cooling takes
place to a temperature near the dew point of the crude gas.
The temperature obtained and the concentration of steam
obtained in the gas depends upon the total pressure and of
the relative proportion of the amounts of heat removed by
~ ~'7~2
the quenching and steam production, respectively. Expediently
these parameters are so adjusted mutually that the steam
concentration becomes precizely that which is optimum ~or
the subsequent shift conversion.
If hydrogen is to be produced by the present process
there is also used pure oxygen for the gasification. If
ammonia synthesis gas is to be producedl it is instead
advantageous to use oxygen-enriched air obtained by mixing
atmospheric air and oxygen. The proportion may be calcu]ated
in such a way that the synthesis gas obtained after the
methanation process and an optional admixture of a hydrogen~
rich fraction, obtained either by the process of the
invention from a crude gas formed by gasification ~ith
steam and pure oxygen or obtained from an extraction unit
for purge gas, contains substantially the stoichiometrical
amount of nitrogen for formation of ammonia, i.e. a ratio
H2:N2 of about 3:1. However, also other ratios hydrogen to
nitrogen can come into question.
The process of the invention will now be illustrated
by a calculated Example of the preparation of ammonia
synthesis gas from a heavy oil which is gasified with
enriched air.
Example
The following streams are assumed to be gasified:
Fuel oil: 32.2 t/h (C 8503%, H 10.5~, S ~.0%, N 0.2~)
Enriched air: 55,281 Nm3/h (2 ~3~9%~ N2 55%~ ~r 1.1%
85 kg/cm2g, 600C, prepared by admixing
98.5% 2 with air.
Steam 13.0 t/h
Hereby there is formed a crude gas having the
following composition (leaving water vapour out of
consideration):
7~2
16
Mol%
H2 34.0
N2 24.2
CO 37O0
C2 3-
Ar 0.5
CH4 0.25
H2S 0-7
COS O . 0 1
Temperature 1365 C, pressure 80 kg/cm g, flow rate
125,920 Nm3/h calculated as dry and 135,360 Nm3/h
calculated as wet gas.
The exit gas is quenched to 650C and is cooled
further in a boiler at 340C. Thereby is formed about 42 t/h
saturated steam at 115 bar.
After the boiler the gas is further quenched in a
scrubber at a dew point of 240C whereby there is achieved a
ratio water to dry gas of 0.78. The total amount of quenching
water is 71,830 kg/h.
CO conversion
The purified gas leaving the soot-scrubber is heated
at 280C ~y heat exchange between inflow and outflow and is
conveyed through three CO-converters after each other, all
being reactors having downwards flow and fixed catalyst bed.
The c ~ osition of the dry yas (mol%) after the converters
~ and other data are seen in the following Table:
Converter 1 Converter 2 Converter 3
H2 49.2 51.2 51.4
N2 18.6 17.8 17.8
30 CO 5.2 1.1 0.7
C2 25.9 28.8 29.0
Ar 0.4 0.4 0.4
CH4 0.2 0.2 0.2
H2S 0.5 0.5 0.5
35 COS100 ppm 21 ppm 13 ppm
7~
17
Converter 1 Converter 2 Converter_3
Discharge
stream Qf dry
gas, Nm /h 164,000 170,710 171,410
Temp. in/out
5C) 280/441 250/280 245/24
H O/dry
gas, inlet 0.78 0.37 0.32
The heat of reaction is utilized to generate high
pressure steam to preheat boiler feed water in so far as
possible. Optionally there may be used an absorption cooling
unit to win heat down to 120C.
Removal of acid gases
The process gas is con~eyed to a plant for removing
acid gases, e~g. a Selexol unit, where CO2 and H2S are
removed and separated off, whereby there is obtained an
H2S-rich stream which is utilized in a Claus plant.
The gas thus purified is thereupon methanated to
remove remaining CO and CO2.
The composition (mol~) after the elimination of acid
gases and methanation as well as other data are seen in
the following Table:
After elimination After
of acid gasesmethanation
2 73.0 72.04
N2 25.2 26.06
CO 1.0
C2 ~.1
Ar 0.5 0.52
CH4 3 0.25 1.38
Discharge of dry gas, Nm /h 120,440 116,400
Temp. in/out ~C~ 320/389
Pressure 70 kg/cm g
q,i~;
18
Ammonia synthesis
The methanated gas is mixed w.ith an H2-rich gas
coming from a purge gas recovery unit and after
compression the synthesis gas obtained is conveyed to the
synthesis loop.
The synthes:is converter, which operates at a pressure
of 160 kg/cm g, is a converter as described in DK patent
application No. 1041/77 with steam boiler and preheater for
boiler feed water for chilling the exit gas from the converter.
The purge gas is conveyed to a cryogenic purge
gas recovery unit in which the major part of ~r and CH4
are removed by condensation and the hydrogen-rich fraction
is recycled to the suction side of the compressor.
Gas composition (mol~) and amounts of flows are as
follows:
Hydrogen-rich Make up gas Convertex
gas from residual for synthesis inflow outflow
gas extraction
unit
H2 89.96 73.57 64.6252.97
N2 8.39 24.55 21.5517.67
NH3 3.82 17.99
Ar 0.98 0.56 3.00 3.40
CH4 0.67 1.32 7.01 7.97
dry gas
stream Nm3/h 10,890127,290 457,270 402,350
