Note: Descriptions are shown in the official language in which they were submitted.
~Z0~79~6
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REACTOR FOR HETEROGENEOUS SYNTHESIS AND METHOD FOR ITS OPTIMISATION
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention refers to reactors for heterogeneous synthesis, and
in particular for the catalytic synthesis of ammonia, methanol, fuel,
higher alcohols, monomers and similar substances, consisting of at
least an outer shell, of an internal cartridge preferably formed by
"n" modular cartridges; of n catalytic beds, each consisting of a
granular catalyst arranged between a solid bottom and two concentric
cylindrical walls of which the outer wall is perforated for the
whole of its axial length and the inner wall is perforated for a
shorter axial tength than the outer wall; of means for conveying
reaction gas; of means for extracting the reacted gas.
2. Description of the Prior Art
Reactors of this type have been described in recent Italian patent
applications (Patent Applications no. 24334 A/79, no. 22701 A¦80 and
no. 262~4 A/80) in the name of the Applicant and one of the inventors;
ehey are characterised by the Eact that the inlet gas flow is so
split thae the catalytic beds are run through by a portion of the
split reaction gas in a zone with prevalently axial flow and by the
remaining split gas portion in another zone with prevalently radial
flow, the zone with prevalently axial flow acting also as gas sealing
pad.
It is known that most heterogeneous syntheses are accompanied by a consi-
derable development of heat which is usually recovered outside ehe
reactor by cooling the reacted gas leaving the reactor to produce
energy (steam, for e~ample).
The recovery of heat outside the reactor is certainly disadvantageous
compared with recovery inside the reactor, since the latter, in the
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absence of structural complications, would also permit the obtaining
at the same time of:
a) opeimal adjustment of reaction heat, thus minimising catalyst
volume;
b) maximum yields; and
c) maximum level of temperature of the recovered heat
(for example, steam produced at higher pressure).
These attractive prospectives notwithstanding9 up to the present the
recovery of heat inside the reactor has not found wide application.
In fact, only in exceptional cases has heat been recovered inside
the reactor to produce steam and achieve control of reaction heat
(for example, in the Fauser-Montecatini a~monia reactor, in the
Ammonia Casale reactor with axial gas flow catalytic beds, in the
Lurgi methanol reactor, again with axial flow catalytic bed), but
this has been achieved at the cost of enormous complications in con-
struction, which for the most part have led to the abandonment of
this ~ethod.
Thus it is that in the case of an ammonia reactor according to
modern technology steam is usually produced outside the reactor;
this also applies to methanol reactors, with the exception of the
Lurgi reactor (see E. Supp, "Chemtech", July 1973) and of the Toyo
Engineerin~ reactor (Italian Patent Appl. no. 21172 A/80).
These are, however, very complex methods. In the new radial-axial
flow reactors as described in the above-mentioned recent Italian
paten~ applications, reaction heat is controlled either by gas-gas
exchange or, more generally, by quenching; these systems, however,
do not permit the recovery 'in situ' of reaction heat.
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SU~RY OF THE INVENTION
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In the case of quenching only a part of the quench gas flows through
all the catalytic beds, resulting in lower yields.
Continuing now research in this field, the Applicant has found, not
without surprise, that ins;de its new reactors with flow-splitting and
catalytic layers run through in series by reaction gas, with mixed
axial-radial flow (according to the above-mentioned Italian patent
applications), reaction heat can be advantageously recovered, and that
this internal recovery, with all the other advantages it involves, can
be achieved without complications or complexities.
The reactor according to this invention, for heterogeneous synthesis,
and more particularly for the catalytic synthesis of ammonia, methanol,
fuel, higher alcohols, monomers and similar substances, consisting of
at least an outer shell, of a cartridge preferably internal formed by
"n" modular cartridges; of n catalytic beds each formed by a granular
catalyst arranged between a solid bottom and two concentric cylindrical
walls of which the outer wall is perforated for its full axial length
and the inner wall is perforated for a shorter a~ial Length than that
of the said outer wall; of means for conveying reaction gas; of means
for e~tracting reacted gas; and of means for controlling the temperature
of reacted gas, is characterised by the fact that inside the central
cylindrical space,defined by the internal walls with a shorter perfora~
ted length of at least one of the n catalytic baskets,has been inserted
a heat e~changer which is entered on one side by the gas reacted on the
bed with which it is associated, and on the other side is run through by
water fed from the outside, or by another heat-removing fluid.
In a particularly advantageous and simple embodiment, the heat exchanger
inserted inside the central cylindrical space defined by the i~mer wall
with the shorter perforated length, is a bundle of tubes inside which
runs water, and which are lapped outside by the hot reacted gas which
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after splitting has run through, with axial flow and with
radial flow, the catalytic bed inside which is inserted the
said tube bundl. According to a remarkable aspect of the
invention, the tube bundl extends alongthewhole of the perfo-
rated axial length of the internal cylindrical wall of each
catalyst basket, and is contained inside a cylindrical body
with an axial extension slightly shorter than the perforated
axial length of the basket's internal wall, said cylindrical
body having at the bottom adjustable by-pass vents for the
reacted gas. The method for optimisingthe reac~or's operating
condition~ consists in removing in situ the very high heat for
exchange between the gas reacted on a bed and wter circulating
from the outside to the central cylindrical part inside the
bed itself, so as to obtain, together with optimal reaction
conditions a reduction in the volume of catalyst in each bed,
as well as the even more accurate control of the temperature
of the gas already reacted on a bed and enteringthe next cata-
lytic bed, by measuring the amount of hot gas reacted on a bed
whichis sent to the internal cylindrical part where heat
exchange takes place.
BRIEF DESCRIPTION OF THE DRAWINGS
The various aspects and advantages of the invention will
better appear fromthe description of some embodiments, gien by
way o example but not by way o limitation, sucha s those
shown in the attached drawings in which:
Figure 1 is the partial and schematic section view o an
radial reactor incorporating a heat recovery system
according to this invention, arranged directly inside
each catalyst layer; and
Figure 2 is a general scheme illustrating more fully the
method o optimisation.
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DETAILED DESCRIPTION OF THE PREFERRED E~IBODI~IENTS
.
In order to give an even clearer illustration, Figure 1 shows schemati-
cally an axial-radial reactor with only two catalyst baskets C1 and C2,
each basket consisting of a support S1 (resp. S2) and of two cylindri-
cal walls T1, T2 (resp. T3 and T4); the outer cylindrical walls T1 and
T3 are perforated for the whole of their a~ial length while inner walls
T2 and T4 have a shorter perforated axial length than said outer walls
T1 and T3.
In effect, as can be seen schematically from Figure 1, the wall portions
T~2, respectively T'4, are unperforated and can consist either of a
solid (unperforated) portion of internal walls T2 respectively T'4, or
of a catalytic layer or of any other unperforated body.
The structure of the unperforated section of T2, resp. T4 can therefore
be constructed in several ways; what matters is that the perforated
axial extension of the inner cylindrical wall T2 (T4) should be smaller
than the fully perforated extension of T1 (T3), so that along the area
defined by the unperforated parts (T'2 resp. T'4) the gas has a preva-
lently a~ial Elow Z1a while in the perforated zone T2 (T4) there is a
prevalently radial flow Z1b This characteristic aspect of axial-radial
reactors has atready been suitably emphasized in the àbove-mentioned
Italian patent applications which must be considered as an integral part
of this description. In general the height T'2 (T'4) defining the
prevalently a~ial flow zone is critical in the sense that it must be
able to act also as a sealing pad for the gas.
It has now been Eound, and this represents the main characteristic of
this invention, thac in the empty cylindrical space limited by the
internal cylindrical wall T2 (resp. T4), it is possible to insert a
heat e~changer SC1 (resp. SC2) which is surrounded by cylindrical body
B81 (BB2) the base of which, B1 (B2)- is fixed to support S1 (S2) of
catalyst basket C1 (C2) in such a manner that substantially all the flow oE
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reacted gas i.e. both Z1a (Z2a) which has flowed axially through the
catalyst in zone Zl, and Z1b (Z2b) which has flowed radially through the
catalyst, flows upwards along the whole of wall BB1 and at the open
top B'l (B'2) of the wall enters exchanger SC1 (resp. SC2) from which
is fed water Erom an outside source SQ1 (SQ2) and which has an outlet at the
top U1 (U2) in which is also present (at very high heat) steam produced
in situ in SCl (SC2) as a result of heat exchange with hot reacted gas
Zla lb
By virtue of this exchange it is possible to maintain the temperature
of the reaction zone at the optimal value (balance temperature), to
produce in situ high-level heat, to obtain high conversion yields and
to reduce the ~olume of catalyst in each basket.
In addition, a remarkable feature of the invention is that the
temperature of gas Gl already reacted on a bed (Cl) and directed
towards the inlet of the following bed (C2) can be adjusted even more
accurately because almost at the bottom B1 of cylindrical body BBt
vents Fl, F2, F3, F6...Fn (i.e., distr;buted all along the
cylindrical surface of BBl) the open part of which can be adjusted by
a closing system (not shown); when perforations F1 ~ Fn are fully clo-
sed the whole Elow of reacted gas Zla + Zlb flows upwards along body
BB~ and enters through B'1 exchanger SC1. In this case gas G1 leaving
bed Ct has the "cold" temperature imposed by exchang;ng heat and enters
therefore the following bed C2 at this temperature which can be ca1.1e~
"lowest". On the other hand, when openings F1 ~ Fn are only partly clo-
sed, a part of the hot reacted gas G'l (for example, a part of Z1b)
will no longer flow upwards along body BBl, but will flow directly
through Fl-Fn into free zone Z2 (between C1 and C2)where it mixes with
gas Gl which by flowing through exchanger SCl has been brought to a
lower temperature.
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By controlling, therefore, the degree of opening or closing of perfo-
rations F1-Fn it is possible to measure out the amount (smaller) of hot
gas G1 which by-passes exchanger SC1 and arrives hot in Z2 where it mixes
with the flow (greater) G1 of colder gas which has transferred heat
eO the water from SQ1 circulating in exchanger SC1. In this way, i.e.
by inserting exchangers SC1, SC2 etc. etc. in several catalytic beds
C1, C2 etc. etc. and with by-pass system F1-Fn at the bottom of body
BB1, it is possible not only to optimise reaction conditions in each
single bed, but also to obtain the flow of gas from one bed to the other
at optimal temperatures. In Fig. 1 exchangers SC1 and SC2 are shown
schematically in their simplest form, i.e. as a tube bundle 1 (1'),
2 (2'), 3 (3') etc. etc. inserted between a lower plate P1 (P'1) and an
upper collecting plate P2 (P'2)- It is evident that the exchanger can
be of any other type known "per se" and can simply heat any fluid
(water, for example) or transform it (steam, for example) permitting a
better recovery of heat in situ at the highest possible heat`level.
Devices replacing the tube bundle are known in themselves and their
replacement must be considered available to the expert in the art.
In Fig. 2 a more generalised scheme is shown, for an optimisation pro
cess and plant, particularly suitable for methanol synthesis. The
methanol reactor ~ is drawn here with four catalytic beds C1 3 C2,
C3, C4; the three exchangers SC1, SC2, SC3 have been inserted only in
the internal cylindrical central part of the first three catalytic beds,
the last bed C4 being without exchanger.
~he fresh synthesis gas GSI is brought to main line 12 and through
lines 12'-12" lows, for instance, into two exchangers 15 and 16 in
which circulates counter-currently the hot reacted gas GRC leaving from
bottom 30 through line 20 and distribution lines 2n' and 21'.
Synthesis gas GSI' which has flown through exchangers 15 and 16 and
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collected, partly preheated, in 17 arrives through lines 18 and 19 at
the top of reactor R~E and enters as gas MSI the first free zone Z1,
where MSI is split into a first portion which flows axially and into a
second portion which flows radially in the first catalytic bed C1,
subsequently flows upwards along body BB1 and then spirals down
exchanger SC1 from which leaves a flow of cooled gas G1 (or G1-G'1 if
there is a partial by-pass of SC1 througb the partial opening of perfo-
rations F1-Fn) which enters the second catalytic bed C2, flows through
it axially and radially, flows upwards along BB2, flows downwards again
along e~changer SC2, flows as G2 (or G2+G'2 if there is a partial by-pass
of SC2 through opening perforations F'1-F'n) into bed C3 running through
it first axially and then radially, flows along BB3 into SC3 which it
leaves as cooled flow G3 (or as flow G3 + G'3 by partial by-pass due to
the incomplete closing of perforations F"1-F"n) finally to flow through
bed C4 (without exchanger), leave from 30 and, through lines 20, 20',
21, 21' and 22 be sent to final condenser CO.
To operate the e~changers according to the invention SC1, SC2,SC3, the
main source of water SQ feeds through lines 42, 43 and 44 pump P1, which
through line 45 and the three lines 46, 47 and 48 circulates water in the
tubes associated with SC1, SC2 and SC3, whose outlets U1, U2 and U3
are borne by a single line Uc which feeds a collector RC in the top
part of which is steam ST (produced in individual e~changers SC1, SC2
and SC3) which ~oes to utilization ST'.
At the bottom of ~ollector RC collects water SQ' which is recycled
together with the fresh water from SQ.
It has been found that by adopting a scheme of the type shown in Fig. 2
for a 1000 `~TD plant with an operating pressure of 80 bar, the recovery
of saturated steam at about 18 bar, and the catalyst volume assume the
values shown in the following table when heat is recovered inside the
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reactor according to this invention, or is recovered externally accor-
ding to previous methodology.
Production lO00 t/d CH30H at 80 bar. Steam recovery
at 18 bar with 4 catalytic beds.
Total ~1 Kcal per ton . Volume in m3 of
Kcal/h methanol catalyst over
4 beds
Recovery inside
the reactor 17410,000 85
according to
the invention
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Recovery
outside the 7.8190,000 96
reactor
.
