GB2122102A - Reactor for heterogeneous catalytic synthesis and method for its operation - Google Patents

Abstract

A reactor for the catalytic production of e.g. ammonia, methanol and similar substances has at least one catalyst basket (C1, C2) composed of an outer perforated wall (T1, T3), an inner perforated wall (T2, T4) and a base (S1, S2); the whole of the axial length of the outer wall is perforated, the inner wall is perforated over a smaller axial length than the outer wall, and within the inner wall is located a heat exchanger (SC1, SC2). Reactant gas enters the top of the reactor, passes axially and radially through the catalyst in the basket (C1, C2) and reacts exothermically therein. Heat generated in the reaction in one catalyst basket (e.g. C1) is removed in the heat exchanger (SC1) associated with that basket before the gas passes onto a further catalyst basket (e.g. C2) in which further reaction takes place. <IMAGE>

Description

SPECIFICATION Reactor for heterogeneous synthesis and method for its operation 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 length 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 British published Application No. 2,055,606 and Italian Application No. 26294 A/80; they are characterised by the fact that the inlet gas flow is so split that a portion of the split reaction gas passes through the catalyst bed in a zone with prevalently axial flow and the remaining split gas portion passes through 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 considerable development of heat which is usually recovered outside the reactor by cooling the reacted gas leaving the reactor to produce energy (steam, for example).

The recovery of heat outside the reactor is certainly disadvantageous compared with recovery inside the reactor, since the latter, in the absence of structural complications, would also permit the obtaining at the same time of: a) optimal 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 notwithstanding, 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 ammonia 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 construction, which for the the most part led to the abandonment of this method.

Thus it is that in the case of an ammonia reactor according to modern technology, steam is usualiy 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 Engineering reactor (Italian Patent Appl.

No. 211 72/A/80). These are, however, very complex methods. In the new radial-axial flow reactors as described in the above-mentioned recent British and Italian patent 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.

SUMMARY 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, we have found, not without surprise, that inside reactors with flow-splitting and catalytic layers through which reaction gas passes in series with mixed axialradial flow (according to the above-mentioned British and 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 reaction 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 axial length than that of the said outer wall; of means for conveying reaction gas; of means for extracting 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 perforated length of at least one of the n catalytic baskets, has been inserted a heat exchanger 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 inside the central cylindrical space defined by the inner wall with the shorter perforated length, is a bundle of tubes inside which runs water, and which are contacted on their outer surfaces by the hot reacted gas which after splitting has passed through, with axial flow and with radial flow, the catalytic bed inside which is inserted the said tube bundle. According to a remarkable aspect of the invention, the tube bundle may extend along the whole of the perforated axial length of the internal cylindrical wall of each catalyst basket, and may be 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 optionally at the bottom adjustable by-pass vents which allow reacted gas to by-pass the heat exchanger.The method for operating the reactor consists in removing in situ heat from the gas reacted on a bed by heat exchange with water circulating from the outside to the heat exchanger inside the bed itself, so as to obtain, optimal reaction conditions, a reduction in the volume of catalyst needed in each bed, as well as more accurate control of the temperature of the gas already reacted on a bed and entering the next catalytic bed. A portion only of hot gas reacted on a bed may be 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 from the description of some embodiments, given by way of example but not by way of limitation, such as those shown in the attached drawings in which: FIGURE 1 is the partial and schematic sectional view of an axial-radial reactor for example according to British published patent application No. 2 055 606 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 of optimisation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to give an even clearer illustration, Figure 1 shows schematically an axial-radial reactor with only two catalyst baskets C, and C2, each basket having a support S, and, respectively, S2 and two cylindrical walls T,, T2 (resp. T3 and T4); the outer cylindrical walls T, and T3 are perforated for the whole of their axial length while inner walls T2 and T4 have a shorter perforated axial length than said outer walls T, 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 construed 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 T, (T3), so that along the area defined by the unperforated parts (T'2 resp. T'4) the gas has a prevalently axial flow Zia while in the perforated zone T2 (T4) there is a prevalently radial flow Zib. This characteristic aspect of axial-radial reactors has already been suitably emphasized in the abovementioned British and Italian patent applications which should be read with this description. In general the height T'2 (T'4) defining the prevalently axial 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 found, and this represents the main characteristic of this invention, that in the empty cylindrical space limited by the internal cylindrical wall T2 (resp. T4), it is possible to insert a heat exchanger wall SC, (resp. SC2) which is surrounded by cylindrical body BB (BB2) the base of which, B1 (B2), is fixed to support S, (S2) of catalyst basket C, (C2) in such a manner that substantially all the flow of reacted gas i.e. both Z,a (Z2a) which has flowed axially through the catalyst in zone Z1, and Z ib(Zbb) which has flowed radially through the catalyst, flows upwards along the whole of wall BB, and at the open top B', (B'2) of the wall enters exchanger SC, (resp.SC2) through which is fed water from an outside source SQ, (SQ2) and which has an outlet at the top U, (U2) in which is also present (at a very high temperature) steam produced in situ in SC, (SC2) as a result of heat exchange with hot reacted gas Zia + Z.b (Z2a + Z2b).

By virtue of this heat 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 volume of catalyst in each basket. In addition, a remarkable feature of the invention is that the temperature of gas G, already reacted on a bed (C,) and directed towards the inlet of the following bed (C2) can be adjusted even more accurately because almost at the bottom B, of cylindrical body BB, there are vents F1, F2, F3, F6... Fn (i.e. distributed all along the cylindrical surface of BB) the open part of which can be adjusted by a closing system (not shown); when vents F, -- F, are fully closed the whole flow of reacted gas Z18 + Zib flows upwards along body BB1 and enters through B', exchanger SC,. In this case gas G, leaving bed C, has the "cold" temperature imposed by exchanging heat and enters therefore the following bed C2 at this temperature which can be called "lowest". On the other hand, when openings F, -- F, are only partly closed, a part of the hot reacted gas G, (for example, a part of Zib) will no longer flow upwards along body BB,, but will flow directly through F, -- F, into free zone Z2 (between C1 and C2) where it mixes with gas G, which by flowing through exchanger SC, has been brought to a lower temperature.

By controlling, therefore, the degree of opening or closing of vents F1 - F2 it is possible to measure out the amount (smaller) of hot gas G, which by-passes exchanger SC, and arrives hot in Z2 where it mixes with the flow (greater) G, of colder gas which has transferred heat to the water from SQ, circulating in exchanger SC,. In this way, i.e. by inserting exchangers SC" SC etc. etc. in 1' 2 tc. etc. in several catalytic beds C1, C2 etc. etc. and with by-pass system F, -- F, at the bottom of body BB,, 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 Figure 1 exchangers SC, 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 P, (P',) 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 (to steam, for example) permitting a better recovery of heat in situ at the highest possible heat level. Devices not using tube bundles are known in themselves and the use of such heat exchangers must be considered available to the expert in the art.

In Figure 2 a more generalised scheme is shown, for an optimisation process and plant, particularly suitable for methanol synthesis. The methanol reactor RME is drawn here with four catalytic beds Cl, C2, C3, C4; the three exchangers Scar, 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.

The fresh synthesis gas GSI is brought to main line 12 and through lines 1 2'-1 2" flows, for instance, into two exchangers 1 5 and 1 6 in which circulates counter-currently the hot reacted gas GRC leaving from bottom 30 through line 20 and distribution lines 20' and 21'.Synthesis gas GSI' which has flowed through exchangers 1 5 and 1 6 and coilected, partly preheated, in 1 7, arrives through lines 18 and 1 9 at the top of reactor RME and enters as gas MSI the first free zone ZX, 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 reacted gas flows upwards along body BB, and then spirals down exchanger SC from whence it leaves as a flow of cooled gas G, (or G, -- G', if there is a partial by-pass of SC, through the partial opening of vents F, -- F,) which enters the second catalytic bed C2, flows through it axially and radially, flows upwards along BB2, flows downwards again along exchanger SC2, flows as G2 (or G2 + G'2 if there is a partial by-pass of SC2 through opening vents F', -- F'.) 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 vents F"1 - F11) finally it flows through bed C4 (without exchanger), and leaves the reactor at 30 and flows through lines 20, 20', 21, 21' and 22 to final condenser CO.

To operate the exchangers SC1, SC2, SC3, the main source of water SO feeds lines 42, 43 and 44 and pump P1, which passes water through line 45 and the three lines 46, 47 and 48 into the tubes associated with SC1, SC2 and SC3, whose outlets U1, U2 and U3 are taken by a single line U4 which feeds a collector RC in the top part of which is steam ST (produced in individual exchangers SC1, SC2 and SC3) which goes to utilisation ST'.

At the bottom of collector RC collects water SO' which is recycled together with the fresh water from SQ. It has been found that for a 1000 MTD plant with an operating pressure of 80 bar and a recovery of saturated stream at about 1 8 bar, the catalyst volume required is shown in the following table for the cases in which heat is recovered (a) inside the reactor according to this invention (the scheme shown in Fig. 2) and (b) is recovered externally according to previous methodology.

Production 1000 t/d CH3OH at 80 bar. Steam recovery at 1 8 bar with 4 catalytic beds Kcal per ton Volume in m3 of Total MM (Kcal/h) methanol catalyst over 4 beds Recovery inside the reactor 17 410,000 85 according to the invention Recovery outside the reactor 7.8 190,000 96

Claims (8)

1. Reactor for heterogeneous synthesis and more particularly for the catalytic synthesis of ammonia, methanol, fuel, higher alcohols, monomers and similar substances, concisting of at least an external shell, 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 has a perforated axial portion smaller than that of said outer wall; of means of conveying the reaction gas; of means for the extraction of reacted gas; and of means of controlling the temperature of reacted gas, characterised by the fact that inside the cylindrical central space limited by the internal walls with the smaller perforated extent of at least one of the n catalytic baskets, is inserted a heat exchanger which is contacted on one side by the hot gas reacted on the bed to which it is associated and through which it has passed and, on the other side, by water fed from the outside.

2. Reactor according to claim 1, characterised by the fact that the exchanger inserted in the cylindrical central space defined by the internal wall with a lesser area of perforations consists of a tube bundle with water running inside the tubes which are contacted externally by the hot gas which has passed through, either in a prevalently axial flow or in a prevalently radial flow, the said catalytic bed inside which the tube bundle is located.

3. Reactor according to either of the preceding claims characterised by the fact that the tube bundle extends substantially along the whole of the perforated axial length of the internal cylindrical wall of each catalytic basket, and comprises a cylindrical body with an axial length only slightly shorter than the perforated axial length of the basket's internal wall, said cylindrical body having at its base adjustable by-pass openings for the reacted gas,

4.Method of catalytic heterogeneous synthesis, in particular of ammonia, methanol and similar substances, effected with reactors according to any one of the preceding claims, characterised by the fact that the hot gas reacted first through a prevalently axial flow and then through a radial flow in a catalytic bed, are conveyed to a central cylindrical zone inside said bed in a heat exchanging relationship with a fluid flowing in said zone, from which heat is removed in situ.

5. Method according to claim 4 for the recovery of heat from the gas reacted in a catalytic basket through which passes synthesis gas in one zone with prevalently axial flow and in another zone with prevalently radial flow, such baskets comprising reactors of the type according to any one of claims 1 to 3, characterised by the fact that the fresh gas to be reacted passes through at least one heat exchanger through which passes the hot reacted gas leaving the reactor, that said fresh gas so preheated runs through each catalyst basket with a prevalently axial flow and with a prevalently radial flow and contacts a heat exchanger arranged inside a cylindrical body situated inside the internal cylindrical wall with the lesser unperforated length, which is fed, on one side, by water and produces a mixture of water and steam which is conveyed together with that coming from the other heat exchangers arranged inside the other catalytic baskets 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 contact the exchanger.

6. A reactor for heterogeneous catalytic synthesis, which comprises an external shell, at least one modular cartridge located within the shell having two perforated concentric nested cylindrical walls and a base, which together define an annular basket for holding catalyst material, the outer wall being perforated along the whole of its axial length and the inner wall being perforated over a lesser axial length that the outer wall, and a heat exchanger located in the cylindrical space within the inner cylindrical wall, the arrangement being such that, in operation, reacted gas from the catalyst bed passes through the inner perforated wall and encounters the heat exchanger.

7. A reactor substantially as hereinbefore described with reference to, and as shown in, Figures 1 or2.

8. A method substantially as hereinbefore described with reference to Figures 1 or 2.