Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 334784
1 27046-14
Process for the productlon of ammonia from natural gas
The invention relates to a process for the production
of ammonia from natural gas, liquified petroleum gas, naphtha or
hydrogen-bearing gases, using a combined autothermic reformlng
process and feeding a separate oxygen stream and a separate
stream of pre-heated air to the system.
The known steam reforming processes for ammonia
production can be divided into two main groups, i.e. processes
using at least a part of the endothermic steam reforming step in
a fired reactor where flue gas forms also at elevated pressures,
and processes in which an entirely autothermic primary and
secondary steam reforming takes place with the aid of partial
oxidation of the treated gas stream.
The first group also lncludes processes using imported
heated gas, for instance helium instead of the flue gas stream.
Processes in which a partial oxidation with only one
catalytic steam reforming section upstream or downstream of or
parallel to sald oxldation takes place, are not discussed
because their configuration differs considerably from the
processes covered in this application. Typical processes of
this type have been disproved in DE-OS 32 45 088 and 33 43 114.
Processes of the flrst group in whlch at least a part
of the catalytic steam reforming takes place in a fired reactor
where flue gas forms, are for lnstance descrlbed in EP O
093 502. DE-OS 24 12 841 is typical for sald processes using
imported hot gas instead of flue gas.
2 ~ ~47~4 27046-14
The inventlon relates to a process of the second group
in which an entirely autothermic primary and secondary steam
reformlng takes place wlth the ald of partlal oxldatlon of the
treated gas stream.
Other state-of-the-art processes of this type are for
instance described ln GB-A Z 153 382, US patent 4 666 680, DE-OS
35 32 413 and in the paper "Ammonla plant safety", volume 4,
page 64 by Takeshl Mlyasugi et al.
The process descrlbed ln GB 2 153 382 and US patent 4
666 680 uses oxygen or oxygen-rlch alr wlth a mln. 2 content of
25 %, or preferably 35 %, for the generatlon of ammonla synthe-
sls gas. The major economic aspect of thls process is the quan-
tity of oxygen added to the alr whlle malntainlng the requlred
H2/N2 ratlo and the resldual methane content ln the synthesls
gas. Hence, sald quantlty of oxygen and the oxidatlon of a
certain part of the gas stream from the prlmary reforming
sectlon are crucial for the economy of said process. When the
requlred composltlon of the gas leavlng the devlces described ln
the above mentloned patents and the other process parameters are
constant, the oxygen requlrement depends on the followlng:
a) the difference between the temperature of the lnlet
gas mixture containlng hydrocarbons and steam and the
temperature of the reformed gas stream from the above
devlce;
b) the temperature of the oxidlzlng agent admlxed ln the
partlal oxldatlon sectlon.
The temperature dlfference under a) can easlly be
~-~r optlmlzed from the economlc vlew-polnt but the temperature under
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2a 27046-14
b) can only be lnfluenced to a certain extent ln vlew of the
large oxldatlon potentlal (2 25 ./. 40 % by vol.) of the oxldl-
zing agent. Moreover, heatlng requlres expenslve oxygen-compat-
lble materlals. In ~B 2 153 382 lt is suggested that steam be
added to the oxygen-rlch air to overcome these difflcultles but
lt is obvious that make-up nitrogen and heat are withdrawn from
the reactor, thus reducing the heat potential required for high
process temperatures.
The aim of the invention ls to find an economical and
simple process configuration permittlng ammonla productlon wlth
the ald of a comblned autothermlc steam reformlng, thereby
conslderably reduclng the oxygen requlrement and the lnput gas
quantity.
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1 3~47~4 27046-l4
The inventlon Provldes a process for produclng ammonia
from natural gas, llqulfled petroleum gas, naphtha or a hydrogen-
hearlng gas ln a comblned autothermlc reformlng process whlch
process provldes that ln addltlon to an atmospherlc oxygen-
contalning alr stream, a separate oxygen stream wlth a mlnlmum
oxygen content of 50% ls admlxed, the separate oxygen stream belng
pre-heated to a maxlmum temperature of 250C and the atmospherlc
alr stream to approxlmately 450-900C, and the H2/N2 ratlo
requlred at the outlet of the reformlng sectlon belng ad~usted
wlth the ald of the atmospherlc alr stream and/or make-up oxygen
stream at the start of reformatlon.
In varlous preferred embodlments of the lnventlon:
(a) the dlfference between the lnlet temperature of a mlxture of
steam and hydrocarbons and the outlet temperature of the reformed
stream ls set to a value of c 150C;
(b) supply of the concentrated oxygen stream ls controlled as a
functlon of the content of lmpurltles measured at the outlet of
the flnal process step;
(c) desulphurlzatlon of feedstock, converslon of C0 to C02, C02
separatlon, separatlon of ammonla or hydrogen from synthesls gas
and return to a related maln stream can be lmplemented upstream or
downstream of autothermlc reformatlon;
~d) the temperature of the atmosphere alr stream ls malntalned at
a constant value of approxlmately 700C and the oxygen stream at
amblent temperature;
(e) methane content ls controlled at the outlet of a reformlng
sectlon ln order to obtaln 0.2-3% by volume (preferably 1.3%);
t
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(f) the H2/N2 ratio in synthesis gas is set to 2.1 - 2.9;
(g) imported fuel gas is fed to a partial oxidation
section of a reforming stage;
(h) the H2O/C ratio of all streams fed to a reformer is
maintained at a value of < 2.75, further steam being added
to the gas stream flowing from a reforming section;
(i) the amount of N2 fed to a reformer is smaller than
the quantity required stoichiometrically for NH3 formation in
synthesis gas, an oxygen-bearing nitrogen stream being added in
a selective CO oxidation section upstream of CO separation in
order to adjust the required H2/N2 ratio;
/q rqe ~~
, (j) the amount of N2 fed to a reformer is sm~lcr than
the quantity required stoichiometrically for NH3 formation in
synthesis gas, the required H2/N2 ratio being adjusted ~s a low
temperature purification section.
The preferred embodiments offer further advantages.
For instance, the make-up oxygen stream which contains more
than 50~ oxygen depending on the oxygen source, is heated to a
maximum temperature of 250 C. Said temperature should preferably
correspond to the compressor outlet temperature but this oxygen
stream may also be pre-heated with the aid of steam condensat-
on .
The air is preferably heated at 450 - 900 C which
is higher than the temperature of the reformed gas at the outlet
of the autothermic section. The air can be heated by various
methods but preferably by burning synthesis waste or tail gas in
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a superheater.
This high pre-heating temperature of the air stream
which is voluminous compared with the oxygen stream, permits
a substantial reduction of the overall oxygen requirement for
the process and, consequently, it leads to savings in the supply
of a concentrated oxygen stream and to lower input quantities
of hydrocarbons.
It is known that a very high pre-heating temperature
of the air stream may necessitate an overall supply of concentrat-
ed oxygen of less than 17% compared with approximately 40%
in the case of a higher overall oxygen requirement (for ammonia
production: GB 2 153 382). Particularly when using NH3 synthesis
catalysts of the new generation operating at a synthesis pre-
ssure of < 120 bar, the concentrated 2 stream may be omitted
because of the lower H2/N2 ratio required and the residual
methane content at the outlet of the
6 1 334784 27046-14
steam reformlng sections exclusively controlled with the aid of
the temperature of the pre-heated air at a constant H2/N2 ratio.
A further advantage of the process configuration
according to the invention is that the control of the two ma~or
process parameters, i.e. H2/N2 ratio and residual methane
content, can be managed with systems of simple deslgn and lower
degree of integration. The amount of concentrated 2 and the
temperature of the pre-heated air stream can be used indepen-
dently as control parameter for the residual methane content
while the amount of air is primarlly suitable for the control of
the H2/N2 ratlo.
It was found that it is possible to perform the cata-
lytic steam reforming at a H2O/C ratio which causes a deficit of
steam in the product gas stream which is subsequently treated in
a catalytic CO conversion, i.e. a deficit of steam for the
conversion. The consequences of said deficit are undesirable
secondary reactions which, inter alia, cause a formation of
hydrocarbons re-converted in the catalyst bed and a major pres-
sure drop in the conversion section, said phenomena impairing
the ammonia production.
Another advantage of the process configuration accor-
ding to the lnventlon ls that a low H2O/C ratlo ls adjusted ln
the autothermlc reformlng sectlon, thus favourably affectlng the
oxygen requirement, and that the additional amount of steam
required for the conversion ls added prlor to the conversion.
Therefore another advantage of this process is that no
high-temperature waste gas streams are avallable on the process
gas and flue gas sldes as ln the case of the conventional
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7 27046-14
primary and secondary steam reformlng. In fact, the waste heat
from the reforming section, conversion and synthesis is suffi-
cient to generate steam but superheating of steam with the aid
of process waste heat cannot be performed on an economical basis
for turbines.
It is of course possible to burn fossil fuels and/or
imported fuel gas in order to ensure efficient steam generation
and supply for the compressor in the process plant. A combined
steam and gas-turbine system is an alternative provided adequate
fuels are available. The process according to the invention
will be superior to any other process of this group if cheap
electrical energy can be used.
Said process permits low-cost production of saturated
steam which can be used as indicated below:
a) Installation of an absorption refrigerating system,
using the cryogenic potential for
- reducing the compressor capacity requirement by
cooling the gases to be compressed;
- operating a physical C02 separation;
- drying the gases;
- gas fractionation by the low-temperature method.
b) Partial or complete absorption of the ammonia in the
loop gas with the aid of water and single- or multi-
stage desorption using steam, the loop gas which
leaves the absorber and contains c1% by vol. NH3
being pre-cooled and then fed to a zeolite-operated
dryer prior to re-heating and recycling to the
converter. In this case, the loop-gas compressor is
- 8 l 334784 27046-14
installed between absorber and dryer, the dryers being
regenerated with a part stream of the dried loop gas.
All ammonia-bearlng streams are returned to the
absorption/desorption system.
Accordlng to a speclal embodlment of the lnventlon, it
is posslble to use part of the process heat for evaporating and
superheating at least a part stream of the ammonla liquor from
the absorber and to feed this stream to a turbine, the waste
steam from said turbine being piped to the ammonia separation
unit described under b). This turbine should be coupled to a
generator or, if required, to the loop compressor and/or NH3
compressor.
It is of course possible to use process steam directly
for desorption. Accordlng to another embodlment, the compressed
process alr ls also sultable for burning gas not obtained in the
primary steam reforming sectlon. If said gas is burnt outside
the partial oxidation section lt ls recommended that the process
steam be enriched by the necessary amount of oxygen prior to
burning the make-up gas and that the amount be selected so as to
permit the required pre-heating of the air. The product leaving
the combustion chamber thus has an oxygen content which approxi-
mates that of the ambient air. If purge gas from the synthesis
loop is used in this case, the synthesis gas has a higher argon
content which ls regarded as favourable for argon recovery.
The overall oxygen requirement in the autothermic
steam reforming sectlon can be further reduced by the followlng
method: The amount of nltrogen entrained into this section can
be decreased by reduclng the alr feed rate below the value
8a l 334784 27046-14
required for the specified H2/N2 ratio in the product synthesis
gas. Said ratlo would for lnstance be ad~usted with the aid of
a selective catalytic CO oxidation (SELECTOXO process) upstream
of the CO2 separatlon from the synthesls gas, an oxygen-bearlng
nitrogen stream belng partlcularly sultable in thls case. The
N2/H2 ratlo may also be ad~usted during the low-temperature
purlflcatlon of the synthesls gas.
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