Note: Descriptions are shown in the official language in which they were submitted.
4~31
SPLIT AXIAL FLOW
CONVERTER IN AMMONIA SYNTHESIS
TECHNICAL FIELD:
_
The present invention is directed to an improved
synthesis process and system for the production of
ammonia. More specifically, the present invention
involves the use of a split axial flow converter in
the low pressure synthesis of ammonia.
_ACKGROUND ART:
U.S. Patent 4,148,866 discloses an improved
synthesis loop for the production of ammonia which has
a synthesis pressure of less than 100 atmospheres, a
water absorption recovery system which utilizes low
level heat recovered from the process for producing the
l~ synthesis gas to distill the ammonia-water mixture and
an ammonia absorption refrigeration system.
U.S. Patent 3,694,169 discloses a reactor for
synthesising ammonia which comprises a pressure-
resisting shell, a catalyst bed of annular section and
a short axial heat exchanger. The reactant gas flows
through the catalyst bed convergently towards an outlet
near one end of the heat exchanger.
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SUMMA~Y OF THE INVENTION:
The present lnvention ls directed to an
improvement in a low pressure ammonia synthesis ~rocess.
More particularly, the ammonia synthesis system of the
S present invention is characterlzed by a simplified
split axlal flow ammonia converter design and a
synthesis pressure in the range of 45 to 80 atmospheres.
BRIEF DESCRIPTION OF THE DRAWINGS:
Fig. 1 is a flow diagram of an ammonia synthesis
Jo loop utilizing the system of the present invention;
Fig. 2 is a schematic diagra~ of a split axial
flow ammonia converter;
Fig. 3 is a cross~sectional view along 3-3 of
Fig. 2 showing details of the outlet collector of the
split axial flow ammonia converter; and
Fig. 4 is a cross-sectional view along 4-4 of
Fig. 3 showing a configuration of the collector rings
in the outlet pipe of the outlet collector.
DETAILED DESCRIPTION OF THE
PREFERRED EMBODIMENTS
~0
The present invention is directed to an ammonia
synthesis system which uses a split axial flow ammonia
converter. According to the present invention, a low
energy ammonia process can be obtalned with a signifi-
cantly simplified ammonia converter design.
The reaction involved in the ~rocess ~or
producing ammonia is:
3H + N _ ~ 2NH (1)
The reaction is an exothermic catalytic reaction which
takes place in an ammonia synthesis converter containing
an ammonia synthesis catalyst~ usually an iron or pro-
moted iron catalyst. S~nce the reaction (1) is not
complete, the desired ammonia is recovered and the
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unreacted hydrogen and nitrogen are recycled. The unit
operational steps of combining the fresh synthesis gas
and the recycled gas, passing the combined gases through
the ammonia synthesis converter, the recovery of the
a~monia from the unreacted gases and the recycle of the
unreacted gases is referred to as an ammonia synthesis
system or loop.
The fresh synthesis gas is a mixture of hydrogen
and nitrogen, u~ually containing three mols of
hydrogen for each mol of nitrogen, and may be prepared
frcm any of the manners known to the art. Conventional
methods may require some compression to provide the
fresh synthesis gas at a pressure suitable for mixing
with the recycled unreacted hydrogen and nitrogen in
I ~ the ammonia synthesis system.
The ammonia synthesis system according to the
present invention preferably comprises a makeup/recir-
culating compressor for compressing the mixed fresh
synthesis gas and recycle gas to a synthesis pressure in
the range of 45 to 80 atmospheres, a pair of split axial
flow converters, and an ammonia recovery system which
recovers the ammonia and recycles the unreacted
hydrogen and nitrogen. In the preferred embodiment,
the ammonia is recovered by scrubbing the product gas
a~ from the ammonia synthesis converter with water. The
unreacted hydrogen and nitrogen from the ammonia-water
absorption system are recycled after purge, as necessary,
and combined with the fresh synthesis gas. The com-
bined gases are dried, preferably by passing through
~O molecular sieves which will remove water before being
introduced to the recirculating compressor.
Referring to Fig. 1, a specific embodiment of the
ammonia synthesis system is illustrated. A fresh
ammonia synthesis gas, which may be produced by one of
~' the common commercial processes such as disclosed in
U.S. 4,148,866, is introduced by line 10 to a
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compressor 12 where the gas is compressed to the
synthesis system or loop pressure and introduced to the
system by llne 14. ~he fresh synthesis gas is mixed
with the recycled gas in line 16. The comblned gases
are passed through a drier 18. The drier 18 com-
prises at least two vessels 20 and 22 which contain
a molecular sieve adaptable to remove water. Valve 24
ln line 25 ls open to permit the combined gases to pass
through vessel 20 while valve 26 in line 27 ls closed.
Thus while one vessel 20 operates ln the adsorption
cycle, the other vessel 22 undergoes desorption by
heating, cooling and standby for use in the total drying
cycle. More than two ~essels may be employed in the
drying cycle. The dried gases are passed out of
J~ vessels 20 or 22 by lines 28 or 30 respectively to a
recirculating compressor 32.
The recirculatlng compressor 32 and compressor 12
may be separate stages of a single compressor. The
recirculating compressor 32 or final stage compresses
the dried combined gases to a synthesis pressure in the
range of 45 to 80 atmospheres. A preferred range of
pressure is between 50 and 70 atmospheres. The com-
pressed gases are passed from recirculating compressor
32 through line 34 containing an exchanger 36 wherein
a~ the gases are heated to reaction temperature.
The ammonia synthesis gas is then introduced to
the simplified split axial flow ammonia converter 38.
The design of the converter 38 is such that sub-
stantially equal volumes of the synthesis gas enter
line 40 and 42 respectively. Lines 40 and 42 are of
substantially equal piping lengths and sizes for
supplying the two streams to the vessel 44, line 40
introducing one stream to the bottom of the vessel 44
whlle llne 42 introducesthe other stream to the top of
3~ the vessel 44. Within vessel 44 there is a lower
support 46 and an upper restralning 48~ both of
which will be more specifically described hereinafter.
~etween the lower support 46 and upper restrainlng
means 4~ is a continuous bed 50, The bed 50 may be a
continuous bed of ammonla synthesis catalyst. In ~ig. 2,a
S preferred continuous bed 50 is comprised of a lower bed
54 and an upper bed 56 of substantially equal volume of
ammonia synthesis catalyst, a layer 53 of inert material
above support means 46 whlch supports lower bed 54, a
layer 55 of inert material on top of upper bed 56 and
below restraining means 48 and a mid section layer 57
of inert material between lower bed 54 and upper bed 5~.
The layers 53, 55 and 57 of lnert material are of larger
particle size than the catalyst to prevent plugging and
minimize pressure drop. The inert material is used to
prevent the direct impingement of synthesis gas on the
catalyst; however, the use of layers of larger particle
size catalyst may also be used. ~t the mid section of
bed 50 and the vessel 44 is a collector means 58 for
removing the product gases. The details of collector
ao means 58 will be more specifically described hereinafter.
The product gas from the collector means 58 of
ammonia converter 38 is passed through line 60 con-
taining an exchanger 62 wherein the product gases are
cooled and introduced into a second ammonia converter
a~ 64. The second converter is preferrably a second split
axial flow ammonia converter having the sane char-
acteristics as converter 38, accordingly, these details
will not be repeated. The product ~as fro~ converter
64 is passed through line 66 containing an exchanger 68
30 wherein the product gases are cooled and introduced to
an ammonia recovery system 70 which recovers the ammonia
and recycles the unreacted hydrogen and nitrogen.
Specifically, a water scrubbing system 72 may be used
wherein the product gas lntroduced by line 66 is
intimately contacted with water introduced by line 74
whereby the ammonla is absorbed by the water while
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generating con3iderable heat of absorption. In general?
the water scrubbing system 72 ~ay be an absor~tion
system uslng vertica wetted ~all exchanger absorption,
multistage packed/trayed tower absorption, concurrent
S mult~stage heat exchange absorption or a trayed column
with heat transfer area on each tray. The unreacted
hydrogen and nitrogen are removed from the top of the
water scrubbing system 72 by line 76 for recycling and,
after purging, through line 77, mixing with the fresh
~O synthesis gas in llne 16.
The ammonia~water mixture formed in the water
scrubbing system 72 is removed by line 78 to a distil
lation column 80. In distillation column 80, the
ammonia~water mlxture is distilled to recover anhydrous
ammonia overhead in line 82 which may be condensed in
exchanger 84 before recovery through line 86 or
reintroducing a portion by line 88 into distillation
column 80 The distillation may be carried out as
described in U,S. 4,148,866 and the ammonia recovered
at atmospherlc pressure by using the ammonia absorption
system disclosed in U.S, 4,153,673. The water is
removed from the bottom of the distillation column 80
by line 90 wherein the water is cooled in exchanger 92
by the ammonia~water mixture passing to the distillation
column 80.
Referring to Figs. 2, 3 and 4, the split axial
flow converter (38 or 64) of the present invention is
illustrated in more detail. With specific reference to
converter 38, the vessel 44 comprises a bottom head 100
3o and a top head 102, both of which may be hemispherical.
An inlet means 104 is in bottom head 100 and an inlet
means 106 is ln top head 102. Inlet means 104 and 106
may both be designed as manholes such that a man can
enter ~essel 44, otherwise a manhole (not shown) is
provided in one or both heads 100 and 102, and are
adapted to connect to inlet lines 40 and 42~ respectively.
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The inlet means 104 and 106 serve not only as connectors
to inlet lines 40 and 42 but as distributors to the
lncoming gases. Inlet means 104 includes a distributor
108 and inlet means 106 includes a distributor 110.
Distributor 108 and 110 may be comprised of a series of
ring-shaped baffles 112 and a solid circular baffle 114
connected to head 100 and 102, respectlvely.
A lower support means 46 is positioned within
lower head 100 to support bed 50. Lower support means
46 comprlses a support grating 116 whlch may be
supported by a rlng 118 and beams 120 connected to head
100. A screen 122 covers the grating 116. The con-
tinuous bed 50 extends upward from the screen 122 of
lower support means 46. Preferrably, the continuous bed
50 is a lower bed 54 and an upper bed 56 of ammonia
synthesis catalyst with layers 53, 55 and 57 of inert
balls, e.g, 1/2-1~ alumina balls having a quality to
withstand the high temperatures and having minimum
impurities, A high quality alumina ball may be
specified as 94% minimum A1203 and 0.2% maximum S~ 2-
The vessel 44 is easily filled by introducing the inertballs and catalyst sequentially in predetermined amounts.
The layer 53 and 55 of lnert balls may extend between 6
and 18 inches in depth and likewise extend above and
below collector means 58 about the same depth. The
upper restraining means 48 maintains the bed 50 in a
fixed position within vessel 44. Upper restraining
means 48 comprises a screen 124 and a grating 126 which
may be fixed to upper head 102. The inert balls are
used to prevent the screens 122 and 124 from plugging.
The continuous bed 50 extends to the wall of vessel 44
and the gases are uniformly distributed throughout the
cross~sectional area of vessel 44. The vessel 44 may be
easily emptied by opening discharge pipe(s) 127.
The ammonia synthesis catalyst making up the con-
tinuous bed 50 or lower bed 54 and upper bed 56 is
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preferably iron or promoted iron catalysts whlch are
commercially available. Details of a~monia s~nthesis
catalyst and ammonia operations are set forth in
Ammonia, Part III, edited by A. V. Slack and G. Russell
5 James, 1977, Marcel Dekker, Inc., New York. When
catalyst is used to make up the entire continuous
50, a larger particle size would be used in layers 53
55 and 57 so as to prevent screens 122 and 124
plugging as well as screen 128 which surrounds collector
~o means 58 which removes the product gases from the vessel
44.
Referring to Fig. 3, the collector means 58 com--
prises an outlet pipe 130 and two concentric collector
rings, inner ring 132 and outer ring 134. ~oth inner
rings 132 and outer ring 134 are pipes perforated by holes
136 around the entire periphery. ~ach of ring 132 and
ring 134 pass through the outlet pipe 130 and as shown
in Fig. 4 have a slot 138 cut in the pipe internal of
outlet pipe 130. Each of rings 132 and 134 of the
~O collector means 58 may be covered by a wire screen 128
to prevent the holes 136 from becoming clogged.
The ammonia synthesis system of the present
invention utilizes a synthesis pressure in the range cf
~5 to 80 atmospheres. This range of pressures is
d~ significantly lower than pressures presently commercially
employed (i.e, in excess of 130 atmospheres). With the
lower pressure and water recovery, an energy efficient
system is obtained. The main advantage of the split
axial flow converter of the present invention is the
simple and less expensive construction. The
construction eliminates the vessel within a vessel
design of the prior art. Further, the converter pro-
vides a design for large volumes of catalyst without
costly heat removal systems while maintaining low
~S' pressure drops within the converter.
The nature and obJects of the present invention
having been completely describecl and illustrated and the
best mode thereof set forth, what we wish to claim as
new and useful and secure by Letters Patent is:
