Reactor for exothermic heterogeneous synthesis reactions

Synthesis gas is reacted in several catalytic beds having axial-radial or radial flow. Reacted gas is collected at an outlet of a final catalytic bed and is transferred to a reaction heat recovery system situated at a top of a reactor. The reactor includes three catalytic beds, two or more beds having inverted, curved bottoms. A first quenching system is located in the reactor and includes a distributor situated inside a first, upper bed at a location immediately under an unperforated portion of an internal wall of that bed. A gas/gas heat exchanger is located centrally within one or more of two upper beds located within the reactor. A water pre-heater or boiler is located inside an upper bottleneck portion of a shell of the reactor and is fed with reacted gas collected from a lowermost catalytic bed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention concerns a process for exothermic heterogeneous synthesis in 
which the synthesis gas flows over a series of catalytic beds superimposed 
but separate one from the other contained within the same reaction space, 
all the reacted gas collects in the central zone of the last lower 
catalytic bed and from here it flows upwards to the top of the space 
inside which its heat is exchanged and steam is produced. 
The invention also concerns reactors to put this process into effect, 
consisting of a pressure-resistant external shell, of baskets of catalytic 
beds all inside the same shell, of a cartridge and of a heat exchanger. 
2. Description of the Related Art 
In a recent patent application it was pointed out that in ammonia 
production a remarkable amount of heat is developed in the synthesis 
reaction N2+3H2, which is generally recovered for the final purpose of 
producing steam recycled to reduce energy consumption. 
The most advanced technology tends towards maximum recovery of said 
synthesis heat at the highest possible thermic level; synthesis units and 
their main component, the reactor, are therefore designed to this end. 
The reactors used in new plants have several catalytic beds with 
intermediate quenching of the gas by means of indirect exchange through 
heat exchangers; moreover part of the reaction heat is removed with an 
external cooling fluid such as for example water feeding a boiler or by 
means of generating steam before the last reaction stage, and this for the 
purpose of being able to operate at the highest possible temperature (heat 
recovery at maximum thermic level) without any limitations of the greatest 
possible efficiency obtainable. 
Maximum temperature and maximum yield are in fact contrasting needs as 
amply shown by the relevant diagrams which indicate in abscissa the 
concentration of ammonia and in ordinate the temperature of the gas. 
Major synthesis reactor designers in general have favored reactors with 
several catalytic beds in at least two distinct parts in series, in order 
to satisfy the above-mentioned need for the optimal exchange of reaction 
heat (at the highest thermic level) without limiting the maximum yield 
obtainable (Fertilizer Focus October 1987). Where two distinct parts of 
equipment are adopted, the first of the two reaction devices generally 
contains two catalytic beds with indirect intermediate quenching with an 
internal heat exchanger, while the second one generally contains a single 
catalytic bed. 
Heat exchange between the two parts of the installation is carried out by 
introducing a boiler to produce steam. This is the case with the Topsoe 
Series 250 (Series 200 + Series 50) reactor and with the Uhde reactor, 
both of them with radial flow of the gas in the catalytic beds (Fertilizer 
Focus October 1987, pages 36 and 39). 
There are also reactors in three distinct parts, each part containing a 
catalytic bed with axial gas flow as found in the C.F. Braun design 
(Nitrogen Conference, Amsterdam 1986). In this case a steam-producing 
boiler is inserted between the second and the third part of the 
installation (Nitrogen Conference, Amsterdam 1986, Mr. K. C. Wilson, Mr. 
B. J. Grotz and Mr. J. Richez of CdF Chimie). 
According to a recent patent by C. F. Braun (UK Patent Application 
2132501A), the gas/gas exchanger between catalytic beds, usually 
conveniently situated inside the reactors with at least two beds inside a 
single installation, is situated outside the reaction apparatus directly 
connected to the bottom of the shell containing a single catalytic bed. To 
minimize the problems of pipes at a high temperature, the tube connecting 
the above horizontal exchanger with the shell containing the catalytic bed 
is quenched with the fresh gas fed to the reactor. 
After having pre-heated the fresh feed gas, the gas leaving the catalytic 
bed leaves from the exchanger and feeds the device containing the second 
catalytic bed (C. F. Braun reactor with several reaction devices as shown 
in FIG. 5 of the Wilson, Grotz, Richez report of the above-mentioned 
reference and at page 48 of Fertilizer Focus, October 1987). 
The problem solved in the C. F. Braun patent mentioned above, i.e. avoiding 
contact between high temperature gas and the tubes connecting shell and 
exchanger, does not affect reactors with several catalytic beds within a 
single piece of apparatus since, as described above, the gas/gas exchanger 
is inserted directly inside the reactor itself. 
Even according to C. F. Braun the problem of optimal heat exchange is 
solved in a complex way by introducing a boiler connected by means of 
complex piping to the reactor itself (see FIG. 5 of the C. F. Braun 
presentation, Nitrogen '86 and Fertilizer Focus October 1987, page 48). 
All the above plans, although resolving the thermodynamic problem, are very 
complex, hence very expensive. Ammonia synthesis reactors operate in fact 
at high pressure, generally not below 80 bar, and more often between 130 
and 250 bar, and at a high temperature (400.degree..div.500.degree. C.). 
The connecting tubes for the various pieces of equipment necessary 
according to the drawings described above (as shown schematically in the 
above-mentioned references), operate under critical conditions (high 
temperature of the gas between the various reaction beds) and must 
therefore be made of special material and with long runs to minimize the 
mechanical stress resulting from thermic dilation. The situation is 
particularly complex in reactors according to C. F. Braun, in spite of the 
measures taken according to the C. F. Braun patent application, UK No. 
2132501A. 
In the above-mentioned UK patent application the Applicants have suggested 
a process and a reactor with several catalytic beds which do not suffer 
from the drawbacks described above, can be produced in a single piece, and 
permit the easy removal of reaction heat between catalytic beds, and more 
particularly before the last catalytic bed, so as to achieve maximum 
recovery of reaction heat at the highest thermic level, such heat being 
exchanged, for example, to pre-heat boiler water or to produce steam 
directly. 
The hot gas reacted in the last catalytic bed but one is transferred, 
through a duct generally situated along the axis of the vertical reactor, 
directly to the heat exchange system (pre-heater or boiler), returning 
then directly to the reactor through a duct, either internal or external 
to the above-mentioned transfer duct, creating an airspace for the gas to 
run through, returning to the reactor, said gas feeding then directly the 
last catalytic bed with an axial-radial or radial flow either centrifugal 
or centripetal. Said gas, after reacting in the last catalytic bed, is 
transferred once again to the central or external part of the reactor, and 
leaves then from the bottom of the reactor. 
This system works very well with reactors with a cylindrical shell with a 
substantially constant diameter, but would meet some difficulties with 
reactors having a graduated diameter shell. 
SUMMARY OF THE INVENTION 
Continuing in their research and experiments, the Applicants have now found 
that, especially when using and modernizing bottleneck-type reactors, it 
is advantageous to introduce the pre-heater or boiler inside said neck, 
collect the reacted gas in the central zone of the last bed, remove it and 
send it upwards axially and centrally to the top or neck where its heat is 
recovered.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
To facilitate comprehension of the system, subject of this invention, it is 
now described with reference to the illustration representing a 
cross-section by way of example of the converter according to a preferred 
embodiment of the invention. 
The reactor consisting of shell (1) and of cartridge (2) forming the 
catalytic beds (in this case 3, namely 6, 7 and 13) is fed by the fresh 
gas (3) entering from the bottom of the reactor and running through from 
bottom to top of the airspace (4) between the shell (1) internal wall and 
the cartridge (2) external wall, for the purpose of reducing to a minimum 
the temperature of the shell. 
As amply described in other patent applications by the Applicants, the 
synthesis gas leaving from the top of the airspace (4) runs with an axial 
flow through the smaller upper portion of the first bed (6) (defined by 
height "h", unperforated, of the internal wall Pi1), and with radial flow 
the greater portion defined by perforated height "H". 
The gas reacted on the first bed (6) collects in the annular central zone 
X1 and from here goes to penetrate the second bed (7) through which it 
flows axially and radially. 
The gas reacted on the second bed collects in internal annular zone X2. 
From here, after exchanging heat with fresh gas (Q2) in exchanger (5) it 
goes on to the third and last bed through which it flows both axially and 
radially collecting in zone X3. More particularly, the embodiment shown in 
the illustration is substantially of the type described and claimed in 
Swiss Patent Application No. 04551/88-8 of Sept. 12, 1988. It comprises, 
besides the three catalytic beds (6, 7 and 13), quenching Q1 at the top of 
the first bed and a heat exchanger (5) which is situated centrally through 
the first and second bed (6 and 7) and is fed with fresh gas Q2. As 
described in said patent application the bottoms of the two catalytic 
baskets (6 and 7) have an inverted curve as compared to the curve of the 
bottom of the third bed (13). 
According to the main feature of this invention, the gas reacted on the 
third bed 13 collects in central space X3 and from here is sent through 
tube T to the upper end (COL) of shell (1), where the heat exchanger (RC) 
for said reacted gas is situated. RC can be a pre-heater for water 
(introduced for example from A) or a boiler generating steam (at a high 
level) leaving from (V). Reacted gas exited the heat exchanger (RC) exits 
from the reactor from (V). 
The upper end (COL) of (RC) is solid with the shell (1) of which it is an 
extension while the cartridge (2) is closed at 4' on the lower part of RC. 
The structure of a reactor with boiler incorporated in the upper part of 
the reactor has proved in itself (not unsurprisingly) the ideal solution 
to achieve maximum heat recovery in new high-yield reactors. The same 
solution has proved a winner when modernizing in situ the more reliable 
and generally used old reactors still in operation at the present time, 
i.e. Kellogg bottleneck type reactors. It has been found that with a few 
marginal modifications old-type reactors with high energy consumption can 
be transformed in situ into high-yield and minimum energy consumption 
reactors with axial-radial flow such as for example the reactors according 
to U.S. Pat. Nos. 4,372,920 and 4,405,562 by the Applicants carrying out 
their modernization in situ according to the system found in U.S. Pat. No. 
4,755,362, again by the Applicants. 
In carrying out the transformation according to this invention, the typical 
outline of the Kellogg bottleneck reactor is maintained; inside the 
reactor three catalytic beds (6, 7 and 13) are introduced with quenching 
(Q1) and an exchanger (5) (alternatively, two exchangers), a boiler (for 
example a bayonet or hairpins type) is installed in the neck (COL), the 
width dimensions of the old and bulky Kellogg reactor (for example ID=2946 
mm) are maintained, and reversed bottoms are given to the first two beds 
to achieve maximum pressure and efficiency of the catalyst with a small 
granulometry. 
By way of example, it has been found that with a reactor according to this 
invention, with a capacity of 1000 MTD, at a pressure of 140 bar abs, feed 
gas at 218.degree. C. and a volume of catalyst (with granulometry between 
1.5 and 3 mm) of 70 m3, heat recovery in the pre-heater BFW and in the 
boiler (RC) can be achieved of 634'000 Kcal/MT of ammonia (equal to a 
production of about 1170 kg/MT of steam at 110 ata, starting from BFW at 
105.degree. C. 
As indicated above, together with the advantages resulting from heat 
recovery there is also the further advantage arising from the possibility 
of being able to maintain the configuration and layout of bottleneck 
reactors, well known for their simplicity, reliability, efficiency and low 
costs. 
Although the present invention has been described in connection with a 
preferred embodiment thereof, many other variations and modifications will 
now become apparent to those skilled in the art without departing from the 
scope of the invention. It is preferred therefore, that the present 
invention not be limited by the specific disclosure herein, but only by 
the appended claims.