Note: Descriptions are shown in the official language in which they were submitted.
SYNTHESIS GAS FOR AMMONIA PRODUCTION
This invention relates to a method for producing
a gas containing hydxogen and nitrogen which is
particularly suitable for use as an ammonia synthesis gas.
Commercial production of hydrogen is frequently
carried out by a succession of process steps which
essentially comprise:
~i) the production of a gas containing carbon-
oxides and hydrogen as its principal
constituents by reaction of the hydrocarbon
feedstock with oxygen and/or air and/or
steam,
(ii) oxidation of the carbon monoxide with steam
to carbon dioxide and hydrogen ('shift
conversion'),
(iii) removal of carbon dioxide, leaving a
substantially pure hydrogen stream,
(iv) final purification as appropriate to remove
residual impurities.
Two major variants of this process sequence currently in
use are:
A. Catalytic Steam Reforming
This process is presently restricted by the
availability of suitabla catalysts to use with natural
gas, naphtha and similar light feedstocks. The catalysts
.~
are sulphur-sensitive and accordingly the hydrocarbon
must be rigorously desulphurised prior to contact with
the catalyst. The desulphurised feedstock is mixed
with 2 to 4 moles steam/atom carbon and then passed
over the catalyst, leaving at high temperature as a
mixture containing chiefly hydrogen, carbon oxides,
residual methane and unreacted steam. The heat needed
to raise the reactants temperature to the exit temperature
and to provide the endothermic heat of reaction is
supplied by enclosing the catalyst in tubes which are
heated externally in a furnace.
The steam reEorming process can alternatively
be carried out wholly or partially autothermically,
by admission of air and/or oxygen to allow combustion
within the catalytic reactor. Specif:ically in the
production of ammonia synthesis gas from natural gas
indirect heat supply to the reactants in the primary
reformer is supplemented by internal combustion of air
i~ the secondary reformer which supplies inter alia
the nitrogen requirement of the ammonia synthesis process.
In another steam-reforming process all the high
temperature heating necessary is provided by the auto-
thermic combustion of oxygen or oxygen-enriched air
in the catalyst zone and there is n.o indirectly heated
reformer at all.
=4=
The reformer product gas is subjected to CO
shift conversion, CO2 removal and ~inal purification
such as methanation in accordance with the requirements
of individual applications.
B. Partial Oxidation
The partial oxidation processes are based on the
combustion of the hydrocarbon feed in a restricted
supply of oxygen or air. ~xamples include some such as
the Texaco and Shell processes that are capable of
accepting the full range of hydrocarbons from natural
gas to coal and others such as the Koppers Totzek and
Lurgi Processes that are specific to coal.
Since no catalyst is used in these processes,
the sulphur content of the feed hydrocarbon is not
critical.
The product gases from the partial oxidation
processes contain hydrogen, carbon oxides, residual
methane and steam in various proportions, with sulphur
compounds, chiefly hydrogen sulphide, to the extent
that sulphur is present in the feed and other trace
impurities. Particularly in the case of the Lurgi and
other processes in which coal is maintained in the
gasifier at relatively low temperatures, the product gases
can contain substantial amounts of high molecular weight
organic material such as benzole and tars.
~.~
8~
=5=
The desirability of freeing the product gas of
trace impurities, combined with the difficulty in
operating a low temperature (about 200 to 250C) carbon
monoxide shift catalyst on sulphur-laden gases, has
frequently led to the choice of nitrogen wash for final
gas purification after shift and carbon dioxide and
hydrogen sulphide removal cf. the use of methanation
with steam reforming.
It will be recognised that in the application of
the partial oxidation processes and of the autothermic
steam reforming processes as outlined above, the employ-
ment of air as the internal oxidant is restricted by
the degree that the resultant nitrogen present is
acceptable in the product gas.
Thus in the usual natural gas based ammonia
process, the amount of air admitted to the secondary
reformer is limited to the supply of nitro~en required
for the ammonia synthesis step. Also in the partial
oxidation and autothermic reforming operations, recourse
to at least partial supply of the oxidant in the form
of substantially pure oxygen is usually necessary,
except when the process is to be used only to produce
a low grade fuel gas. The necessity for the supply of
substantially pure oxygen means that an air separatlon
plant must be provided. The additional capital and
i, ~
=6~ 4~
running costs incurred thereby results in such processes
appearing less attractive as a means of producing hydrogen
rich gases except when the feed hydrocarbon is very cheap
or complete flexibility of feedstock source is desired.
One exception to this restriction is in the Braun
"Purifier" Process for the manufacture of ammonia by steam
reforming, according to which a larger quantity of air
than that necessary to supply the synthesis nitrogen is
introduced into a secondary reformer and the resulting excess
of nitrogen over synthesis requirement is condensecl out
downstxeam of the reformer~
It is an ob~ect of the present invention to provide a
method of producing a gas stream sui-table for the synthesis
of ammonia.
lS In one broad aspect, the invention comprises a process
for producing a feed gas stream for the synthesis of ammonia
which process comprises la) partia:lly oxidizing a substance
selected from the group consisting of oil, coal, natural gas
or any combination thereof in the presence of air at a pressure
of 15 to 150 bar and a-t a temperature of 300 to 2,000C, to
produce a raw gas stream containing hydrogen and nitrogen,
with a stoichiometric excess of nitrogen based upon that
needed for ammonia synthes.is, together with carbon oxides,
methane and hydrogen sulphide if sulphur was present in the
oil, coal or gas. The process also includes (b) treating
the raw gas stream from step (a) to remove substantially all
componen-t gases other than hydrogen and nitrogen, (c) clrying
the raw gas stream from step ~b) if wa-ter is present, and
d) subjecting the raw gas stream from step (c) at a pressure
of lS to 100 bar to a separation stage to separate (1) a
hydrogen-nitrogen feed gas stream at a pressure of 15 to
:ln0 bar, the raw gas stream having a pxedetermined nitrogen:
hydrogen ratio suitable for ammonia syn-thesis and (2) a
. ~ ~
`.~;;~ nitrogen-rich gas stream at a pressure of S to 50 bar.
=7=
(e) The hydrogen~nitrogen feed gas stream from step (d~
is injected into a reactor for ammonia synthesis, (f) the
nitrogen-rich gas stream from step (d) is heated, still at
a pressure of 5 to 50 bar, to a temperature of 500 to
2,000C, and (g) the high pressure nitrogen-rich gas stream
from step (f) is expanded in a turbine to generate power.
The invention further comprehends a process for producing
a feed gas stream Eor the synthesis of ammonia which process
comprises ta) partially oxidizing a substance selected from
the group consisting of oil, coal, natural gas, or any
combination thereof in the presence of air at a pressure
of 15 to 150 bar and at a temperature of 300 to 2,000C,
to produce a raw gas stream containing hydrogen and nitrogen
with a stoichiometric excess of nitrogen based upon that
needed for amrnonia synthesis, together with carbon oxides,
rnethane and hydrogen sulphide if sulphur was present in
the oil, coal or gas. The process further includes (b~
passing the raw gas stream from step ~a) over a shift
catalyst and reacting the raw gas stream with steam at elevated
temperatures to convert the carbon monoxide present to carbon
dioxide and hydrogen, (c) treating the raw gas stream from
step Ib) to remove carbon dioxide in a wash solution,
(d) treating the raw gas stream from step (c~ to rernove sub-
stantially all component gases other than hydrogen and
~5 nitrogen, (e) drying the raw gas strearn from step (d) if water
is present, and ~f) subjecting the raw gas stream from step
(e) at a pressure of 15 to 100 bar to a cryogenic separation
stage to separate (1) a hydrogen-nitrogcn feed gas stream
at a pressure of lS to 100 bar, the feed gas stream havin~
a predetermined nitrogen:hydrogen rat.io suitable for
ammonia synthesis, and (2) a nitrogen-r:ich gas stream at a
pressure oE 5 to 50 bar. (g) The hydrogen-nitrogen feed gas
=8=
stream from step (f) is injected into a reactor for a~nonia
synthesis, (h) with the nitrogen-rich gas stream produced by
the cryogenic separator in step (f) still at a pressure oE
5 to 50 bar, being used to strip a substantial part of the
carbon dioxide from the wash solution produced in step (c).
(i) The combined nitrogen and carbon dioxide stream from
step (h) ls heated still at a pressure of 5 to 50 bar, to
a temperature of 500 to 2,000C, and (j) expanding the
combined nitrogen and carbon dioxide stream from step (i)
in a turbine to generate power.
The invention is based on the fact that hydrogen and
nitrogen mi~tures can be separated with ease in view of
the large difference in their properties and hence the
nitrogen content of such mixtures may be accurately control-
led. The simplest method o~ separating the gases is bycryogenic treatment al-though other separation metho~s which
rely on the difEerence in molecular size of the gases, e.g.
differential adsorption methods or di~fusivity, may also be
used. The invention allows any gas which predominantly
contains hydrogen and nitroyen to be used and the source gas
may therefore be derived from the partial oxidation of oil,
coal or gas in air.
In a preferred embodiment the desired amount of nitrogen
is separated in a cryogenic separator. The separator may use
Joule Thomson cooling and regenerative heat exchanye, low
temperature work expanders, supplementary refrigeration or
any combination thereof. Suitable cryogenic separators
are well known and commercially available. T~le separated
hydrogen containing the desired quantlty of nitrogen normally
leaves the cryogenlc separator at a slightly lower pressure
than its inlet pressure and is injected into a system for
ammonia synt~esis.
=9=
The nitrogen stream normally leaves the cryogenic
separator at a somewhat lower pressure than its inlet
pressure but nevertheless may still give useful power when
heated and passed through an expansion turbine.
The invention will now be described with
reference to the accompanying drawings, in which:
Figure 1 represents a flow diagram of a process
in accordance with the invention,
Figure 2 represents a flow diagram of an alter-
native process in accordance with the invention, and
Figure 3 represents a diagram of a high temperature
open cycle gas turbine suitable for use in the process of
the invention.
Referring to Figure 1, natural gas, oil or coal
or a combination thereof is partially oxidised with air or
oxygen enriched air which is generally preheated and
pressurised. The resulting gas stream contains hydrogen,
nitrogen, carbon oxides, methane and hydrogen sulphide i~
sulphur is present, the nitrogen:hydrogen ratio being in
excess of that required for ammonia synthesis. The partial
oxidation process is conducted at a pressure up to 150 bar
generally 15 to 150 bar, preferably 30 to 100 bar and a
temperature of 300 to 2000C generally up to 1000C. The
oxidation may be conducted at atmospheric pressure in
which case the yas stream may be pressurised at a later
stage in the treatment process.
The resulting gas stream is passed over a shift
catalyst, e.g. iron oxide or cobalt molybdate, generally
at a temperature in the range 200 to 500C to convert the
-10=
carbon monoxide present to carbon dioxide and hydrogen.
The gas stream is then treated to remove carbon dioxide
and hydrogen sulphide impurities. There are many types
of process for such gas removal including scrubbing with
hot potassium carbonate e.g. at 70 to llO~C and the
Rectisol process. The sulphur content of the gas may be
removed at any prior stage. Any residual carbon oxides
present may be removed by methanation, generally at 250
to 450C. The resultant gas comprises a mixture of hydrogen
and nitrogen, with methane, inert gases such as argon and
water vapour as the chief impurities. This gas is then dried
by cooling initially and subsequently by contact with drying
medium e.g. molecular sieve adsorbent (which would also
remove any remaining traces of carbon dioxide). The dried
gas is then passed to a cryogenic nitrogen/hydrogen
separator, e.g. which uses Joule Thomson cooling and
regenerative heat exchange. The gas is contacted with heat
exchange elements which cool the gas to about 100 K. In
the cryogenic condenser, the nitrogen content of the
hydrogen is reduced to the level required ~or the ammonia
synthesis gas, typically 25~ N2 for ammonia synthesis. The
cryogenic nitrogen condensation will result in partial
depletion of the methane and argon and content of the inlet
gas, the impurities removed appearing in the waste nitrogen
stream. The hydrogen-nitrogen stream which leaves the
condensor at a pressure slightly less than the inlet pressure
of the gas strec~m is injected into an ammorlia synthesis system.
~,
~ `.j`~ !
=11=
In the embodiment depicted in Figure 2, the
nitrogen condenser incorporates a form of liquid nitrogen
washing to remove residual carbon monoxide to a level
acceptable for ammonia synthesis. This expedient enables
methanation to be dispensed with and allows the convenient
use of a higher CO level from the shift conversion
resulting in a final synthesis gas substantially free of
CH4 and inert gases. The pure nitrogen needed for the
washing may conveniently be obtained from the condensed
nitrogen in the cryogenic separator, thus there would be
no dependence on an external source of liquid nitrogen
as in the classical nitrogen wash plants.
It is also possible for the cryogenic nitrogen
condensation to be placed upstream of methanation in
Figure 1~
In all applications it is advantageous for the
waste nitrogen to be discharged from the cryogenic
condenser at near ambient temperature and at an elevated
pressure up to 50 bar, generally 5 to 10 bar since it
may then be heated to a representative inlet temperature
for a high-temperature turbine and expanded therein to
near atmospheric pressure, thus generating a useful
proportion of the power needed for pressuxing the gas
stream, e.g. to compress the process air for the partial
oxidation or steam reforming steps.
~1
=12=
The waste nitrogen may be heated to the turbine
inlet temperature, e.g. 500 to ~000C, generally 500
to 1000C, by indirect heat exchange and/or by direct
combustion of its combustible content, i.e~ traces of
methane, hydrogen, carbon monoxide with supplementary
air and additional fuel if required upstream of the
turbine.
In the arrangement of the expansion turbine shown
in Figure 3 the hot gas expansion turbine is the turbine
element of an open cycle gas turbine. The nitrogen is
mixed with supplementary fuel and fed to the combustion
chamber of the gas turbine as fuel. At the same time the
process air requirement for the partial oxidation or
reforming processes is bled from the gas turbine compressor
discharge. By this expedient approximate parity is
maintained between the mass flows in the compressor and
expander sections of the gas turbine and an efficie~t
means of compression and expansion provided using
developed industrial equipment designs.
Alternatively the waste nitrogen may be expanded
at a low temperature, e.g. ambient temperature to generate
power and may be used for refrigeration of desirable parts
of the ammonia synthesis plant.
~7
=13=
If the waste carbon dioxide from the acid gas
removal plant may be discharged to the atmosphere in
impure form it is expedient to use the waste nitrogen
under pressure from the cryogenic separator to strip
a substantial part of the carbon dioxide from the wash
solution and then to pass the combined nitrogen and
carbon dioxide stream still at high pressure to heating
and work expansion.
In summary, the following advantages are offered
for the air oxidation/nitrogen condensation/nitrogen
expansion system described over current practice involving
oxidation of feedstock:
l. elimination of air separation plant, oxygen
compressors, pipework, etc.
2. reduction in gross installed power of plant
compressors,
3. taking into account the high potential
efficiency of the waste nitrogen containing
gas expansion and conventional associated
heat recovery,
a substantial reduction in the total energy
requirement for the whole plant.
The pressures referred to herein are gauge pressures.
~,
