CA1207986A - Reactor for heterogeneous synthesis and method for its optimisation - Google Patents

Abstract

REACTOR FOR HETEROGENEOUS SYNTHESIS AND METHOD FOR ITS OPTIMISATION

Inventors: Umberto ZARDI and Ettore COMANDINI

Assignee: AMMONIA CASALE SA, Lugano, Switzerland ABSTRACT

The reactor for heterogeneous synthesis, in particular for the catalytic synthesis of ammonia, methanol, and similar substances, consisting of at least an outer shell, at least an internal cartridge formed preferably of modular cartridges, each comprising a catalytic bed consisting of a granular catalyst contained between a closed lower bottom and two con-centric cylindrical walls of which the outer wall is perforated for the whole of its axial length and the inner wall has a perforated length shorter than the outer wall, so that each catalytic bed is run through by a split portion of the reaction gas in a zone with prevalently axial flow and by th? remaining split gas portion in another zone with preva-lently radial flow, the zone with prevalently axial flow of each cataly-tic bed acting also as sealing pad for reaction gas, is characterised by the fact that in the cylindrical space comprised between the inner walls of at least one of the n catalytic beds is inserted a heat exchan-ger lapped on one side by the reacted has coming directly from the relevant catalytic bed and, on the other, run through by water or other liquid fed from outside.
The method is so arranged that heat is removed at a high level in situ so as to obtain at the same time the optimisation of reaction conditions and the reduction of the catalyst volume in each catalytic bed and, in addition, an even more accurate control can be made of the temperature of the gas reacted on one bed and entering the next catalytic bed, by controlling the amount of hot gas reacted on a bed sent to the exchanger associated with the same bed.

Description

~Z0~79~6 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

- 3 ~ ~ 2 ~ 6 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.

_ 4 - ~ Z(~ 9 SU~RY OF THE INVENTION
-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 7~36 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.

` - 6 - ~20~86 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

_ 7 _ ~ 2 ~ ~ 9 ~ 6 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.

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 3L20798~

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 l o 120~9~;

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 -Recovery outside the 7.8190,000 96 reactor .

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Reactor for heterogeneous synthesis and more particu-larly for the catalytic synthesis of ammonia, methanol, fuel, higher alcohols, monomers and similar substances, comprising (a) at least an external shell, an internal cartridge formed by "n" modular cartridges containing n catalytic beds, each having 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 has a perforated length shorter than that of said outer wall;
(b) means for conveying the reaction gas;
(c) means for the extraction of reacted gas; and, (d) means for controlling the temperature of reacted gas, wherein inside the space limited by said concentric walls is inserted a heat exchanger which is lapped externally by hot gas reacted on the bed to which it is associated and which is run through internally by water fed from outside.

2. Reactor according to claim 1, wherein the heat exchanger comprises a tube bundle, with water running inside the tubes which are lapped externally by the hot gas which has run through said catalytic bed inside which is inserted said tube bundle.

3. Reactor according to claim 2, wherein the tube bundle extends substantially along the whole of the perforated axial length of the internal cylindrical wall in each catalytic bed, and is disposed within a cylindrical body with an axial length only slightly shorter than the perforated axial length of the bed's internal wall, said cylindrical body having at is base adjustable bypass openings for the reacted gas.

4. Method for the optimisation of catalytic heterogeneous synthesis, awithin reactors eployiing ammonia, methanol and similar substances, characterised by the fact that the hot gas is reacted first through a prevalently axial flow and then through a radial flow, on a catalytic bed, and then conveyed to a central cylindrical zone inside said bed in a heat exchanging relationship with a fluid flowing in said zone, with which high heat is removed in situ.

5. Method according to claim 4 for the recovery of heat from the gas reacted on a catalytic baskets run through by synthesis gas in a zone with prevalently axial flow and in another zone with prevalently radial flow, characterised by the fact that the fresh gas to be reacted runs through at least one heat exchanger run through by hot reacted gas leaving the reactor, that said fresh gas as so preheated runs through each catalyst basket with a prevalently axial flow and with a prevalently radial flow and is sent to lap a heat exchanger arranged inside a cylindrical body situated inside an internal cylindrical wall with a lesser unperforated length, which is fed, on one side, by water and delivers a mixture of water and steam which is conveyed together with that coming from another tube bundle arranged inside another catalytic basket to a water-steam collector, each cylindrical body outside each exchanger being provided with vents through which can be controlled the amount of hot reacted gas to be sent to one side of the exchanger.