Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DESCRIPTION
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BACRGROUND QF THE INVENTEON
F1eld of the inventl-n
This invention refers to a system to increase the conversion
yield and the reduce the energy consumptions in the reactors
of heterogeneous synthesis reactors, in particular ammonia
reactor~, consisting of an external shell (P), usually in one
piece, and of an internal cartridge ~C~ having a diameter ~Dc)
and containing granular catalyst arranged in one or more
catalytic beds tK) that have an axial height (Hi) and that are
closed between one external cylindrical wall made up of
sections of sald cartridge tC) having an internal diameter
(Dc) and a height (Hi), an internal cylindrical wall made up
Or sections of an internal feed pipe for the quench gas (T)
having a diameter (Dt) (small with respect to Dc) and a height
(Hii, the upper transversal face of each bed being open and
invested axially by synthesis gas.
As already known, the reactors for catalytic synthesis under
pressure, particularly for the catalytic synthesis of ammonia,
methanol, higher alcohols, etc., consist of an external shell,
generally in one piece and an inner cartrdige shell containing
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a granular catalyst arranged in one or more layers (catalytic
beds)
The synthesis gas that crosses the various catalytic beds
is generally cooled between one bed and the other, with the
aim of obtaining optimal temperature conditions in the
various beds by means of the injection of fresh gas (quench
reactor) or by means of a direct interchange with cold inlet
gas.
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D _cri~ti_n f _h_ Pr_or A__
Recently reactors have been proposed with gas radial flow in
the catalytic beds (Lummus, Topsoe, Kellogg, Patents U.S.A.
No. 3918918 and no. 4181701, --------- ~ ~
or with axial-radial flow (Ammonia Casale Patent
U.S.A. no. 4372920 and 4405562) that represent an important
progress in comparision to reactors with axial flow,
particularly when a large volume of catalyst must be
exploited, permitting the reduction of the charge loss reduced
across the catalytic beds, and therefore, of the energY
consumptions. The axial flow of the gas in these cases means
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the adoption of reactors developed in width (low length-
diameter ratio of the equipment) with high costs of the
machinery and high energy consumption.
The Ammonia Casale patents U.S.A. no. 4372920 and U.S.A. no.
44D5562 have allowed a considerable simplification of the
reactor's cartridge design favouring the obtainment of an
internal reactor structure, with easy access for the
maintenance and the loading and the substitution of the
catalyst, and at the same time with low charge losses.
According to the above patents, each catalytic bed is made up
of a perforated external cylindrical wall, of a perforated
internal cylindrical wall and of only one closed bottom ~the
upper wall Or the basket is in fact completely open); an
unperforated UppQr portion of the above internal cylindrical
wall (or Or both the cylindrical walls), in continuation with
the open cross section between the upper edges of the two
cylindrical walls arranged on a plane approximately
perpendicular to the longitudinal axis of said perforated
walls, forms a passage where A minor portion Or the gas
crosses the bed with a prevailingly axial flow, whereas the
remaining major portion of the gas crosses with radial flow
the major cross section of the catalytic bed arranged in the
perforated zone of the two cylindrical bed walls.
According to the patented inventlon, the gas portion that
crosses the bed with prevailingIy axial flow is controled by
the ~ height of the portion of the unperforated internal (or
internal or external) cylindrical wall, that is in all cases,
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a minor portion of the total height of the said wall.
Reactors with dimensions corresponding to a height-diameter
ratio higher than 10 (f.i. with diameter-height ratio lower
than 0.1) are preferablely realized according to the said
patented invention.
In the present world-wide economic situation, the market for
the modernization of existing plants is very important, the
major part of which especially, in the case of synthesis
processes (f.i. ammonia synthesis) use reactors with axial gas
flow in the catalytic beds taxial reactors~, characterized by
a lo;Y height-diameter ratio of the equipment, due to the ahove
mentioned necqssity to reduce the charge loss within the
machinery.
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The aPPlicants have already
described a system to reduce energy consumptions of
heterogeneous catalytic synthesis reactors under pressure, in
particular a~ reactors with dimensions~corresponding to low
height-diameter ratios tlower than 10); in particular a
system is described wh~ch is a~ ideal system to be utilized in
the modernization of existing reactors with axial gas flow, in
particular in Rellogg-type.reactors which are used in numerous
large-scale plants t800-1500 MTD) mostlg built towards the end
of the 1970's. The main characteristics of the said plants,
(besides the high capacity already cited), is the use of
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centrifugal machinery for the compression Or gas operated with
vapour produced in the plant itself, according to an
integrated cycle between the process vapour and the vapour
used to operate the machinery, and the use of the said axial
Kellogg-type reactors.
The said plants are also characterized by the uncritically
high energy consumptions of those years in which energy was
availa~le at a low cost, the axial-type reactor being the
cause of said high consumptions.
In the continuation of their researches the Applicants have
found a naw system that is now also easily applied for the
modernization of "Chemico"-type reactors.
~-mQa~y- Q_ t h_ ~y_~t_Qn
The invRntion, concerns therefore a system specified in the
introduction of the description and of the main claim, which
i~ now characterized by the fact that in order to delimit each
catalytic bed the following is inserted therein (see fig. 2):
a) inside and near said external wall which forms the
cartridge (C), an external c~ylindrical wall (Fe) that is
; at least partially perforated on its height tHi~ and has a
diameter (Di) slightly smaller than that of the cartridge
( C);
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b) an internal cylindrical wall (Fi) showing, on at least one
major portion (H'i) of the height (Hi) of the catalytic
bed, a diameter (D'i) higher than that (Dt) of the
internal transfer pipe (T) and perforations, and
c) a cover (CO) on the top of the internal wall (Fi) whereby
the catalytic bed has an annular a structure Or diameter
(Di-D'i) on the said height (H'i) and structure
substantially cylindrical, more precisely o~ annular
: diameter (Di-Dt) on the minor height (Hi-H'i).
__talled _eSQEl~tl_n _f the Pref____d Emb dme_t_
In one Or the preferred embodiments Or the invention, the flow
of the synthesis gas that axially invests the said upper open
face of each bed i~ so divided into a radial flow along the
; perrorated haight (H'i) of (Fi) and, eventually, into an axial
flow along the height (Hi-H'i). The flow that crosses
(axially and radially) the catalytic bed is collected in the
annular zone between the said transfer pipe (T) of diameter
(Dt) and the portion of the internal wall (Fi) having height
(H'i) and diameter (Di). In this zone it is mixed with quench
gas exiting from holes in the pipes (T, T').
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: ~ ~ Th- external face of the perforated wall (Fe) forms a conduct
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for the gas with the internal cartridge face tC~. Since the
height (Hi) varies in the various catalytic beds, the height
(H'i) can be regulated in order to have the optimal ratios on
each bed between the radial flow and the axial flow (the
latter which can be negligible and even null).
The ratio between the portions of gas (with prevailingly axial
flow and radial flow) will result differently in the various
catalytic beds Or the cartridge in the case that the same are
of a variable height (ratio height-diameter of variable
baskets), as in the application Or this invention for the
transformation of the above mentioned "Chemico" reactors, in
which the height (Hi) of the catalytic beds of the internal
cartridge increases in the various beds arranged in series (3
to 4 beds), and even in the case that the described hereafter
should be applied which forsees the use of a diaphram that
reduces the cross section arranged on the upper edges of the
external cylindrical wall of the bed, lying directly upon the
upper surface of the catalytic bed itself.
As said above according to another embodiment Or the
invention, the portion of gas that crosses the beds with axial
flow in the upper beds of minor height can be reduced, at
least equal to that of the following beds of major height, by
means of a diaphram red~ucing the open cross section, contained
between the upper edges of the external cylindrical wall of
the beds, provided with suitable openings tholes, grooves or
other) for the reduced passage of the gas, said diaphram lying
directl~ on top of the upper surface of the catalytic bed
it~elf, and showing accordlng to a prererred e-bodiment, an
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opening, the total cross section of which increases r~dially
from the inside to the outside of the bed (see European patent
application no. 86104911.2). - ~ -
According to another embodiment of the invention, especiallyin the case that the ~atio between the height of the catalytic
bed and the radial distance Or the cylindrical walls of the
same is much lower ~lower than 3), the diaphram reducing the
open cross section mentioned above can be a solid wall
~without openings), alwa~s lying on the upper surface Or the
bed.
` According to this modified embodiment the cylindrical walls
may result perforated along their entire height tlacking the
upper zone of the unperforated walls).
Other characteristics of the invention are specified in the
claim~ from 2 to 6.
DQs_ri~tiQ~ of _he erawing_
The different aspects and advantages Or the invention better
appear from the following description of the embodiment
represented in Fig. ~ 2, as compared to the Prior A~t
represented in Fig. 1, these two figures being schematic,
longltudinal cross-sections. In particular the following
exemplifying description of a preferred embodiment
(modification of the Chemico re~actor with axial Plow) should
better illustrate the characteristics of the invention.
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The embodiment described is referring to the modification of a
Chemico ammonia reactor with several catalytic beds (f.i. rour
beds) crossed by gas with axial flow, and cooling of the gas
in the various beds by means Or an injection of fresh gas
(quench reactor represented in fig. 1 of the English patent
no. 11281244 to which it is referred).
The reaction gas enters from the top of the reactor in 1 and
axially penetrates on a first catalytic bed (K1) open at the
top and delimited by a portion o~ the internal wall (Ci~ of
the cartridge ~C), by the corresponding portion of the
internal face o~ the feed pipe (T) of the quench gas (GQ) and
by a closed bottom (FO).
The raacted gases that have crossed the catalytic layer (K1)
are mixed by various systems, with the quench gas (GA) exiting
rrom the holes (F~ in the pipe ~T) and, cooled in this manner
they pass through the next catalytic bed ~K2). Even on the
lower part of this bed, the reacted~gases are mixed and the
quench gases (GQ') fed by a pipe (T'). In fig. 1 the shell i5
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not represented.
In rig~ 2 the system according to the invention is
represented, which forsees the insertion of an external
cylindrical~wall ~Fe) at least partially perforated, having a
height (Hi) and of diameter (Di) slightly lo~er than the
1'1
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internal diameter (Dc) of the cartridge (Cj; the insertion of
a second internal wall (Fi) of a height ~H~i < Hi) and of a
diameter (D'i < Di), but higher than the external diameter
(Dt) of the feed pipe (T) of the quench gas (G~); and the
application of a cover (C0) on the upper end of (Fi) that
creates in this waY a reversed cylinder closed at the top and
open at the bottoM (A). As can be seen, on each basket the
catalyst assumes a prevailingly annular structure in diameter
(Di - D'i~ and in height (zone of radial flow ZR) and a minor
structure substanially cylindrical in height (Hi - H'i) (zone
of axial flow ZA). Actually the zone (ZA) is also annular in
diameter (Di - Dt) and since (Dt) is small with respect to
~Di), it can be considered practically cylindrical. The
reaction gases enter from the top into 1, the reacted gases
exit from the bottom into UR, IGR, and UGR indicate the
entrance and exit of the cooling gas respectively Or the
internal wall of the shell (P) and of the external wall of the
cartridge tC), f..i. of the airspace (I). T and T' indicate
r. i. two feed pipes of the quench gas GQ and GQ', that can
however be even more according to the number of catalytic
baskets (K). Preferably, and in accordance with what
described above in the preceding European patent application
No. 86104911.2, the icatalytic beds now have heights (Hi)
differing one from the other, even (H'i) shall be varible,
r. i. increasing Erom the top to the:bottom, proportionally or
no:t, to the incrementation of Hi.~: In other words, the ratio
(Hi~H'i) can vary from one basket to another, which allows a
better regulation of the valu~s of axial flows (and therefore,
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radial flows) on the slngle beds Hith respect to these and
other aspects of the invention, it is ~orthwhile to refer to
the above mentioned U.S. patents nos. 4372920 and 4405562.
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In accordance to what is foreseen in the said European patent
application on at least some of the upper surfaces of the
catalytic beds tK), as the reducinq ~or annullator~) diaphrams
(DR1 - DRn) of axial flow are placed. They ars provided with
adjustable ~penings (at the most, closable), preferably in
sections varying from the inside to the outside.
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Even i~ the invention has been described with reference to the
preferred embodiment represented in Fig. 2 and 2A, it is
susceptible to variations, modifications, substitutions and
the like, that is iust because they are obvious to the
technician skilled in this field, are to be considered
automatically falling within the scope and the spirit of this
invention.
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