Reactor and plant for manufacturing ammonium salts

A reactor for manufacturing salts which includes a first tubular reaction chamber having at least one acid feed and at least one ammonia feed which are arranged in a first up-stream part of the reactor. The reactor has a second part arranged as an extension of the first and which includes, in a flow direction, a convergent segment, a cylindrical tube and a divergent segment and wherein the second part has at least one second ammonia feed which communicates with the second part in the vicinity of the convergent segment and which is controlled to obtain basic vapors at an outlet of the reactor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the manufacture of ammonium salts used, in 
particular, as fertilizers. 
2. History of the Related Art 
In particular, processes for manufacturing ammonium salts using a tubular 
reactor are disclosed by numerous documents (U.S. Pat. Nos. 2,568,901, 
2,755,176, 2,902,342). According to these processes, the two reactants 
(acid and base) are introduced into a tubular reaction chamber in 
substantially stoichiometric proportions, that is to say mole for mole. 
The reaction chamber proper is extended by a tube of diameter 
substantially similar to that of the chamber, in which the neutralization 
reaction continues. Of the supply conduits of the two reactants, one may 
deliver axially into the reaction chamber and the other tangentially. 
Their mixing is promoted by the presence of elements which can generate 
turbulence, for example venturis, baffles, etc. 
The total length of the reaction chamber and of the tube which extends it 
is in general large (often more than 50 times the diameter of the tube), 
so as to allow a neutralization reaction which is as complete as possible. 
It has been found that, in this type of reactor and process, the high 
temperature resulting from the exothermic reaction between the base and 
the acid (in the case of ammonia and nitric acid, this temperature may be 
between 150 and 220.degree. C.) makes the unneutralized nitric acid 
droplets extremely corrosive. This results in rapid destruction of the 
entire reactor, making it necessary to replace it after a period which may 
be of the order of 6 to 12 months. Longer service could be achieved by 
using, for example, titanium instead of stainless steel, but this would 
make the cost of the reactor prohibitive. 
It is also found in these known processes that, in spite of the 
stoichiometric proportions used and the overall length of the reactor, the 
reaction is not complete and ammonia remains in the vapour which is 
separated from the nitrate solution at the outlet of the reactor. 
The invention forming the subject of this patent proposes mainly to solve 
the first of the two problems mentioned above, namely to reduce or, if 
possible, eliminate the corrosion of the reactor. 
It also proposes to reduce the ammonia losses and consequently improve the 
yield of the neutralization. 
SUMMARY OF THE INVENTION 
To this end, its main subject is a reactor for manufacturing ammonium 
salts, comprising a tubular reaction chamber, at least one acid feed and 
at least one ammonia feed, which are arranged in a first part or upstream 
part of the reactor, wherein said reactor comprises a second part arranged 
in extension of the first and including, in the flow direction: a 
convergent segment, a cylindrical segment and a divergent segment, this 
second part being provided with at least one second ammonia feed which 
delivers into this second part in the vicinity of the convergent segment. 
According to other characteristics: 
the second part of the reactor comprises a single tube; 
this part comprises an outer tube and an inner tube, delimiting between 
themselves an annular chamber communicating, at least at its downstream 
end, with the internal volume of the inner tube; 
in its upstream part, the inner tube includes at least one passage 
communicating with the annular chamber. 
A further subject of the invention is a process for manufacturing ammonium 
salt, in which reactants comprising at least one acid and ammonia are 
introduced in the vicinity of the upstream end of a tubular reactor for 
achieving a neutralization reaction and forming a salt in solution and 
vapour, and the salt in solution is then separated from the vapour leaving 
the reactor in an expansion chamber, wherein between 80 and 99% of the 
total reactant flow is introduced in the upstream part of the reactor, the 
remaining part of the ammonia is introduced in a second part of the 
reactor, lying in extension of the first, so that the vapour separated in 
the expansion chamber has a basic pH, and the neutralization reaction is 
terminated in an additional step, by bringing the basic vapours leaving 
the expansion chamber into contact with the remaining part of the acid. 
According to other characteristics of this process: 
the acid and the ammonia which are introduced in the upstream part of the 
tubular reactor are kept in substantially stoichiometric proportions; 
the ammonia flow introduced in the second part of the reactor is controlled 
so as to obtain the desired excess of ammonia in the downstream part of 
the reactor, and so as, after separation in the expansion chamber, to 
obtain vapours having a basic pH, preferably greater than 9; 
the additional step during which the neutralization reaction is terminated 
is carried out by neutralizing the basic vapours using a washing solution 
formed by a mixture of recycled ammonium salt and acid. 
Lastly, another subject of the invention is a plant for manufacturing 
ammonium salt, for implementing the process defined above and comprising, 
in particular, a reactor according to the invention. 
More precisely, this plant comprises a tubular reactor, at least one 
ammonia feed and at least one acid feed delivering into a first part, or 
upstream part, of the reactor, this reactor delivering to an expansion 
chamber in which the salt in solution is separated from the vapour, and it 
is characterized in that the reactor is as designed according to the 
invention and an additional stage is provided in which the vapour from the 
expansion chamber is brought into contact with the remaining part of the 
acid, so as to obtain complete neutralization.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 represents a tubular reactor R comprising three parts segments 
R.sub.1, R.sub.2, R.sub.3. The first part, or upstream part, R.sub.1 
constitutes a reaction chamber 110 into which, on the one hand, an axial 
ammonia gas feed conduit 111 and, on the other hand, a radial or 
preferably tangential acid feed conduit 112 deliver, though the positions 
of these two feeds may be reversed or modified. 
The second part comprises a tube 121, of diameter less than that of the 
tube delimiting the chamber 110, extended at its upstream part by a 
convergent 122 and at its downstream part by a divergent 123. An ammonia 
gas feed conduit 124 delivers, preferably tangentially, into a short 
cylindrical tube segment arranged immediately upstream of the convergent 
122 and of diameter substantially equal to that of the tube delimiting the 
chamber 110. For ease of manufacture, the ammonia inlet conduit 124 may be 
arranged upstream of the flanged connection between the segments R.sub.1 
and R.sub.2 of the reactor. In this case, the axial positions, in FIG. 1, 
of the flanges 116, 126, on the one hand, and of the conduit 124, on the 
other hand, are substantially reversed. 
The reactor is supplemented by a third tube 130 which is fixed to the 
downstream end of the segment R.sub.2 and the diameter of which is close 
to or optionally slightly less than that of the tube 110, while its length 
may be between 10 and 50 times its diameter. However, preferably and by 
virtue of the improvement provided by the invention, the length of this 
third part or segment R.sub.3 may be kept as short as possible and less 
than it must be in known arrangements. 
In the embodiment shown in FIG. 2, the elements similar to those in FIG. 1 
are denoted by the same numerical references, the first part includes an 
axially gas feed 211 and a tangential acid feed 212 with 100 added. The 
second part comprises an outer tube 220, of diameter substantially equal 
to that of the segment 210, and an inner tube 221, of smaller diameter, 
including a convergent 222 at its upstream part and a divergent 223 at its 
downstream part. The inner tube is here cantilevered by being fixed at its 
upstream end by a flange 225 clamped between two flanges 216, 226 secured 
to the segment 210 and the outer tube 220, respectively. A third tube 230 
is connected to tube 220. 
A chamber 227, of substantially annular shape, is delimited between the 
outer tube and the inner tube, and an ammonia gas feed conduit 224 
delivers in the vicinity of the upstream part of this chamber. 
The reactor represented in FIG. 3 is very similar in its design to the one 
represented in FIG. 2. The reactor includes a first chamber with axial gas 
feed 310, tangential acid feed 312, a second part with a convergent 
segment 322, outer tube 320 and divergent segment 323 and third part 330. 
However, in its upstream region, the cylindrical inner tube 321 includes a 
plurality of series of orifices 328 connecting the interior of this tube 
to the annular chamber 327. These orifices are located in the vicinity of 
the ammonia supply constituted by the tangential conduit 324. 
Furthermore, at its downstream end the inner tube is guided in the outer 
tube by means of at least three spacers 329. 
In the variant represented in FIG. 4, a reactor R in three parts R.sub.1, 
R.sub.2, R.sub.3 is seen. As in the previous examples, the upstream part 
comprises a tube 410 into which the respective axial 411 and tangential 
412 reactant feed conduits deliver. 
The second part R.sub.2 of the reactor comprises an outer tube 420 of 
diameter substantially equal to that of the tube 410. 
This outer tube contains, on the one hand, a convergent 440 consisting of a 
tube segment, of frustoconical shape, fixed by a flange 441 between the 
tubes 410 and 420 and, on the other hand, an inner tube fixed in the 
vicinity of its downstream end by a flange 426, between the tube 420 and 
the tube 430 constituting the third part R.sub.3 of the reactor. 
This inner tube comprises a cylindrical tube segment 421, a convergent 422 
located at its upstream end, with an entry diameter greater than the exit 
diameter of the convergent 440, and a divergent 423 arranged at its 
downstream end. The inner tube preferably includes stiffening ribs 421a 
extending over its entire length, and spacers 429 for guiding it relative 
to the outer tube 420. 
An annular chamber 427 is delimited between the outer tube 420, on the one 
hand, and the convergent 440 and the inner tube 421, on the other hand, 
and an ammonia gas feed conduit 424 delivers in the vicinity of the 
upstream part of this chamber. 
The downstream end of the inner tube includes a plurality of openings 428 
connecting its internal volume to the annular chamber 427 (FIGS. 4 and 5). 
In the example represented, the third part or segment R.sub.3 of the 
reactor comprises an intermediate connecting segment 431 and a tube 432, 
slightly smaller in diameter than the tube 420 and with a length which may 
be between 10 and 50 times its diameter, but it is preferable to keep this 
in the lower part of this range or even below. 
With reference to FIG. 6, a description will now be given of a plant for 
manufacturing ammonium salt, for example ammonium nitrate, with which a 
reactor R according to the invention, for example in the form of one or 
other of the variants described above, is incorporated. 
In addition to this reactor R, this plant comprises an expansion chamber D 
and a treatment column T. 
The expansion chamber consists of a tank 500 into which the end of the 
reactor R delivers, preferably tangentially, so as efficiently to separate 
the nitrate solution flowing downwards from the vapour which follows an 
upward movement and escapes via a conduit 501. In its upper part, it may 
advantageously include a droplet separator device, of woven mat, bubble 
plate or packing type, or any other equivalent means, which retains any 
droplets which may be entrained by the vapour. In its lower part, it has a 
conical shape and is connected to a conduit 502 for removing the nitrate 
solution. 
The treatment or washing column T includes, for example, from the bottom 
upwards, the following sections: 
a liquid tank 510; 
a device 511 for injecting the vapour leaving the expansion tank via the 
conduit 501; 
a first region 512 which includes packing elements of the ring, plate or 
other type; 
a circuit 513 for recirculating liquid leaving the lower part of the 
column, including a pump 514; a level control in the tank 510 makes it 
possible to remove excess salt solution and send it to the tank 500 via a 
line 515; 
a second region 516, including packing elements or a series of bubble-cap 
plates or equivalents; 
an inlet 517 for clean water and/or for process condensates treated 
beforehand; 
a droplet separator 518 of the woven mat, bubble plate or packing element 
type, etc.; 
a vapour discharge line 519. 
As a variant, the column T may be replaced by a series of independent 
elements, placed above one another and fulfilling substantially the same 
functions as the various stages of the column T. Similarly, the 
neutralization region 512 containing the packing elements may be replaced 
by a venturi reactor or similar equivalent means. 
The reactant feed circuits comprise: 
a circuit for nitric acid at the appropriate concentration, comprising two 
lines 521, 522, a line 521 for feeding the reactor R and a line 522 which 
delivers into the nitrate solution recirculation circuit 513 associated 
with the treatment column; this line 522 could, as a variant, deliver 
directly into the treatment column T; 
an ammonia feed circuit, also comprising two lines 523, 524, respectively 
feeding the upstream R.sub.1 and intermediate R.sub.2 segments of the 
reactor. As a variant, the main ammonia feed 523 may be distributed 
between two conduits, one delivering upstream and one delivering 
downstream of the point where the line 524 providing the controlled 
additional amount of ammonia enters the reactor. 
As is known, the nitric acid and the ammonia may be preheated in suitable 
heat exchangers (not shown), preferably supplied with the vapour produced 
in the plant, so as to improve the thermal efficiency of the reaction. 
The plant is supplemented by a control device, schematized at 531, 532, 
533, 534, which controls the flow rates in the various lines of the plant 
as a function of set-point values and measured values of a number of 
parameters. 
This control device acts on valves placed respectively on the various 
lines, only the main ones 541, 542, 543, 544 and 545 of which have been 
represented in the drawings. 
In general, this device will not be described in detail, and merely the 
main functions fulfilled will be indicated, the means of effecting such 
control being well-known to the person skilled in the art. 
A reactor and a plant according to the invention operate as follows: 
Most of the neutralization reaction takes place in the reactor R. The acid 
and ammonia flow rates in part R.sub.1 are controlled so as to keep a 
ratio which is as close as possible to the stoichiometric ratio, for 
example to within .+-.1%, and preferably with a very slight excess of 
ammonia. Between 80 and 99%, and preferably between 92 and 99%, for 
example 98%, of the total reactant flow is introduced in the upstream 
reaction chamber R.sub.1 of the reactor. The nitric acid has a 
concentration of between 50 and 70%, and preferably between 55 and 63%. In 
the case of sulphuric acid and phosphoric acid, the concentration selected 
may be between 70 and 99%, and between 52 and 70%, respectively. The 
ammonia is in anhydrous gas form. 
The remaining percentage of ammonia is introduced in the second segment 
R.sub.2. 
In the example in FIG. 1, the fact that an additional amount of ammonia is 
introduced in the central part R.sub.2 of the reactor has the effect of 
improving the yield of the reaction and of limiting the corrosion in this 
central part. Furthermore, the tube 121, 122, 123 which constitutes the 
reactor part liable to suffer the largest degree of corrosion may be 
considered as a consumable part, the cost of which is substantially less 
than that of the entire reactor. 
In the embodiment in FIG. 2, the presence of an inner tube in the central 
part R.sub.2 of the reactor, and the fact that an additional amount of 
ammonia is introduced in the annular chamber 227, limits the corrosion 
substantially to just the inner element 221, 222, 223 of this central 
part. 
In the embodiment in FIGS. 3 and 4, the presence of orifices 328, in the 
first case, and the provision of convergents 422 and 440, in the second 
case, causes the ammonia leaving via the conduit 324, 424, to be sucked in 
the direction of the inner tube. Depending on the operating conditions of 
the reactor, a flow may be set up in the annular chamber 327, 427, either 
in the downstream direction or in the opposite direction. In the two 
examples, this circulation guarantees that nitrate cannot stagnate in this 
intermediate region, which could represent a danger in view of the 
explosive nature of this product. 
In general, as a variant, the tubes 121; 221; 321; 421, 440, of small 
length (1 m to 1.8 m, for example) may be made of titanium. 
According to the invention, the ammonia flow rate in the line 524 is 
controlled so as to obtain the desired excess of ammonia in the parts 
R.sub.2 and R.sub.3, and to obtain, in the expansion tank 500, vapours 
which have a basic pH, for example greater than 9. The device 532 
therefore controls the opening of the valve 544 as a function of the value 
of the pH measured on the line 501. 
In known fashion, the nitrate solution and the vapour separate in the tank 
500, the solution being collected in the lower part of the latter, while 
the vapour is removed in the upper part. 
The pressure in this tank is preferably kept between 1 and 8 bar, the 
reactor being, for its part, preferably fed with ammonia and nitric acid 
at pressures of between 5 and 10 bar, in order to allow sufficient 
expansion in the tank. 
The basic vapours which enter the treatment column T are neutralized 
therein by the addition of a washing solution formed by a mixture of 
ammonium nitrate, recycled from the bottom of this column, and nitric 
acid, the quantity of nitric acid injected via the line 522 into the 
circuit 513, or into the column T, being conditioned by the pH of the 
ammonium nitrate solution recycled to the column. The set-point of the pH 
meter 533 is fixed so as to obtain a washing solution which is 
sufficiently acidic to neutralize the remaining ammonia. This pH meter 
acts on the control valve 542 of the line 522 via which the nitric acid 
arrives. The concentration of the acidic washing solution is 
advantageously between 5 and 20%, but may be increased to 40%. 
The presence of this column has the result that complete neutralization and 
substantially ammonia-free vapours are obtained. The two problems 
mentioned at the start of this document are therefore indeed resolved by 
the invention.