Process and reactor for exothermic heterogeneous synthesis with several catalytic beds and heat exchange

Abstract: In a process for exothermic heterogeneous synthesis, for example of ammonia, in which the synthesis gas is reacted in several catalytic beds with axial-radial or just radial flow, the reacted gas is collected at the outlet from the last catalytic bed and is transferred to a reaction heat recovery system situated at the top of the reactor. An improved reactor comprises three catalytic beds (6, 7, 13) with at least two inverted curve bottoms, a first quenching system (Q1) with its distributor situated inside the first upper bed, immediately under the unperforated portion of the internal wall of said bed, a gas/gas exchanger (5) central to at least one of the two upper beds, a water pre-heater or a boiler (RC) inside the shell upper bottleneck, and an internal central tube (X3) to take to the top the reacted gas collected in the third bed.

Description: This invention concerns a process for exothermic heterogeneous synthesis with improved heat recovery, comprising the steps of: reacting gaseous reactants in a plurality of superimposed catalytic beds (6, 7, 13) separate from one another and contained within a reaction space (1), the gas leaving one bed being made to flow through the next bed; cooling the reacted gazes leaving at least one of said catalytic beds (6, 7, 13) by means of indirect gas/gas heat exchange; collecting the hot reacted gas leaving the lowermost catalytic bed (13) at the lower end (X3) of said reaction space (1); transferring said hot reacted gases to the upper end (COL) of said space (1); removing the hot reacted gases from said reaction space (1). The invention also concerns reactors to put this method into effect, consisting of a pressure-resisting outer shell (1) provided with opposite upper (COL) and lower (X3) ends and having inlets for feeding unreacted fresh gas (3, 20, Q1), a plurality of catalytic beds (6, 7, 13), a gas/gas heat exchanger (5) for cooling the reacted gases leaving at least one of said catalytic beds (6, 7, 13) and at least a quench means, a centrally located tube (T) within said catalytic beds (6, 7, 13) connecting said lower (X3) and upper (COL) ends of the shell (1) together. 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 thermal level; synthesis units and their main component, the reactor, are therefore designed to this end. EP-A-0 297 474, for example, discloses a heterogeneous synthesis reactor of the so-called bottleneck type having threee catalyst beds, wherein the reaction heat is recovered by means of indirect cooling with an exchanger placed between the second and the third bed. 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 the concentration of ammonia versus the temperature of the gas. Major synthesis reactor designers in general have favoured 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 thermal 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 application GB-A-2 132 501 by C.F. Braun, the gas/gas exchanger between catalytic beds, usually conveniently positioned inside the reactors with at least two beds inside a single installation, is placed 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 application 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 (400-500°C). The connecting tubes for the various pieces- of equipment necessary according to 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 GB-A-2 132 501. In EP-A-0 355 303, 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 thermal 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. Continuing in their research and experiments, the Applicants have now found that, especially when using and modernizing bottleneck-type reactors (for instance of the type described in EP-A-0 297 474), 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. The present invention therefore provides a process for exothermic heterogeneous synthesis with improved heat recovery which is characterized in that it comprises the step of recovering the heat of the hot reacted gas leaving said reaction space (1) by generating steam in a boiler (RC) located internally said upper end (COL) of the reaction space (1). 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. 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 9.12.1988 (EP-A-0 376 000). 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. 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). The upper end (COL) of (RC) is integral 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 US-A-4 372 920 and US-A-4 405 562 by the Applicants carrying out their modernization in situ according to the system found in US-A-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°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°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.

Claims: A process for exothermic heterogeneous synthesis with improved heat recovery, comprising the steps of: reacting gaseous reactants in a plurality of superimposed catalytic beds (6, 7, 13) separate from one another and contained within a reaction space (1), the gas leaving one bed being made to flow through the next bed; cooling the reacted gases leaving at least one of said catalytic beds (6, 7, 13) by means of indirect gas/gas heat exchange; collecting the hot reacted gas leaving the lowermost catalytic bed (13) at the lower end (X3) of said reaction space (1); transferring said hot reacted gases to the upper end (COL) of said space (1); removing the hot reacted gases from said reaction space (1); characterized in that it further comprises the step of recovering the heat of the hot reacted gas leaving said reaction space (1) by generating steam in a boiler (RC) located internally said upper end (COL) of the reaction space (1). A process according to claim 1, characterized in that the reaction gas flows through each of the catalytic beds (6, 7, 13) with an inwardly directed axial-radial flow. A process according to the preceding claims 1 and 2, characterized in that a first portion (3) of said gaseous reactants are cooled by quenching with fresh unreacted gases (20) before entering the first catalytic bed (6). A process according to claim 3, characterized in that the reacted gases leaving the first catalytic bed (6) are cooled by quenching with fresh unreacted gases (Q1). A process according to claim 1, characterized in that the reacted gases leaving the second catalytic bed (7) are cooled by indirect gas/gas heat exchange with said first portion (3) of the gaseous reactants. A reactor for carrying out exothermic heterogeneous synthesis reactions according to the process of any of the preceding claims, consisting of a pressure-resisting outer shell (1) provided with opposite upper (COL) and lower (X3) ends and having inlets for feeding unreacted fresh gas (3, 20, Q1), a plurality of catalytic beds (6, 7, 13) superimposed but separate from one another, a gas/gas heat exchanger (5) for cooling the reacted gases leaving at least one of said catalytic beds (6, 7, 13) and at least one quench means, a centrally located tube (T) within said catalytic beds (6, 7, 13) connecting said lower (X3) and upper (COL) ends of the shell (1) together, characterized in that a steam generating boiler (RC) is located inside said upper end (COL) of the shell (1), whereby the heat of the hot reacted gases leaving said lower end (X3) of the shell (1) is recovered. A reactor according to claim 6 characterized in that it comprises at least two superimposed catalytic beds (6, 7) contained inside a cartridge (4) within said shell (1) separate from one another and provided with bottoms having an inverted curve profile. A reactor according to claim 6 characterized in that said at least one quench means is supported on top of the first bed (6). A reactor according to claim 8 characterized in that it further comprises a second quench means between the first and second catalytic beds (6, 7). A reactor according to claim 6 characterized in that said gas/gas heat exchanger (5) is located within at least one of the two upper beds (6, 7). Reactor according to claim 6, characterized in that said upper end (COL) of the shell (1) presents a diameter smaller than the diameter of the remainder portion of the shell (1).

Inventors: PAGANI GIORGIO, ZARDI UMBERTO, PAGANI, GIORGIO, ZARDI, UMBERTO
Applicants: AMMONIA CASALE SA, ZARDI UMBERTO, AMMONIA CASALE S.A., ZARDI, UMBERTO
Application year: 1990
Main classification: B01J8
IPC codes: C01C1, B01J8