SYSTEM TO IMPROVE THE EFFICIENCY OF REACTORS FOR EXOTHERMIC SYNTHESIS AND MORE PARTICULARLY FOR THE REACTION OF AMMONIA

Abstract: The system to modernize reactors for exothermic heterogeneous synthesis of ammonia, methanol and the like, consisting of a pressure shell (M), of an airspace forming wall (I), of a wall or cartridge delimiting the catalytic beds (C₁,C₂,C₃), and of catalyst-­ containing baskets, now consists in providing: a) a wall (I) forming an airspace (with the shell) in a single piece substantially on the whole axial length of the reactor; b) a bed-forming wall (L) which is entirely distinct from and totally unconnected to the airspace forming wall (I) and is, instead, constituted of independent modules each delimiting at least one catalytic bed, each module resting either on the underlying module or on the extension (20) of the inside gas distribution collector, and being formed by the walls which contain the catalyst and distribute the gas there through; and c) a contact of catalyst portions with section of the airspace forming wall.

Description: Method to revamp (modernize) and improve the efficiency of existing reactors for exothermic heterogeneous synthesis, and more particularly for the synthesis of ammonia and methanol, consisting generally of: a) an external pressure shell (M); b) a single-piece wall (PI) forming an airspace (7) with said shell (M); c) a wall (L) delimiting the catalytic beds (Ci); d) catalytic baskets (Ci); e) feed ducts (5) for the synthesis gas (GS); f) feed ducts (6) for the quench gas (GQ); and g) an internal gas distribution collector (T). Reactors for heterogeneous synthesis and more particularly for the synthesis of ammonia consist generally of a pressure shell, of a wall forming an airspace, and of a cartridge with catalytic beds. In some cases (see Kellogg axial flow and low height/diameter H/D ratio reactors), the wall which with its external side forms the airspace with the shell, acts also with its internal face as wall delimiting the beds. In other cases (Topsoe, for example) the wall forming an airspace is separate and distinct from the wall of the cartridge forming beds, cartridge which in this case is generally in a single piece, and monolithic, that is to say inseparable for the single beds. In a more recent instance (Ammonia Casale system) the external wall forming the airspace is inseparable from the wall of the cartridge forming the beds and is formed by separable modular parts, each containing at least one catalyst bed. Continuing now their research, the Applicants have surprisingly noted that the operating efficiency of these as well as of other reactors can be remarkably improved by making easily and economically achieved structural changes. It has been found, in effect, that the optimal method for modernizing practically all current reactor exception made for conventional "bottle-neck" Kellogg reactors is characterized in that the wall (L) forming beds is entirely distinct and totally unconnected to the wall (PI) forming the airspace (7) and consists of modules (MOi), each module (MO1) delimiting at least one catalytic bed (C1), resting on the underlying module (MO2) and formed by the gas permeable walls (PMFLi) which contain the catalyst and distribute therethrough said synthesis gas (GS). In a preferred embodiment of the invention, said airspace forming wall (PI) has portions of height (1Hi) in contact with the catalyst of at least one bed, at least an other of said beds having a portion of catalyst of height (3Hi) in contact with a portion of said internal gas distribution collector (T). The various aspects and advantages of the invention will be better illustrated by the following description of a preferred reactor embodiment (achieved for example by modernizing a particular type of reactor). In order to define matters straight away and refer to one of the many conventional reactors capable of modernization, Fig. 1 shows in longitudinal cross-section a tipical Topsoe (Series 100) reactor consisting of an outer shell M, of an inner closed shell I with airspace forming wall PI and cover CI, and of the cartridge with catalytic beds C which has, going from the outside to the inside, a solid wall PP and two perforated walls or concentric nets Fe and Fi delimiting the catalytic bed LC which is divided into an upper bed L1 and a lower bed L2, and of a baffle plate separating the beds D3. The synthesis gas GS enters via duct 5, flows along airspace 7, goes into heat exchanger 8, flows upwards along central tube 9 and then runs radially through beds L1 and L2 collecting in annular inner space 10, and feeding, before leaving the reactor, exchanger 8 inside the tubes. From duct 6 quench gas GQ enters, flowing directly from the lid 11 of inner shell I, and mixes with the reacted synthesis gas coming from bed 1. Conversion yields of these reactors are relatively low, with the disadvantage that their energy consumption is rather high. The aim of this invention is a system to modernize in situ any type of conventional reactor (excluding reactors with "bottle-neck" shells) transforming it very easily and simply into an axial-radial reactor preferably with the quenching taking place at the end of the axial flow zone. Another aim of the invention is to modify in situ the structure of these reactors and to increase substantially yields while at the same time reducing energy consumption. A conventional reactor as shown in Fig. 1 can be modernized by removing the inner shell for example ABCD, while leaving unchanged the outer shell and possibly the wall forming an airspace with said shell. Figure 2 shows the optimal reactor achieved by using the system according to the invention, in which the outer shell M and the inner closed shell I, with airspace forming wall sub a) in a single piece PI and cover CI, are maintained. According to an aspect of the invention the wall sub b) forming catalytic beds C1, C2 .... Cn (in the case represented by n=3), PMFL is formed by modules characterised by the fact that each module is formed by wall L which beside delimiting catalyst C and distributing through it synthesis gas GS, now also acts as support for the upper module. As Fig. 2 shows, the wall PMFL1 of upper module MO1 has an appendix or extension 20 which finds support in A1, that is to say on an extension 21 of wall PMFL2 of module MO2. The same applies for supporting in A2 module MO2, resting on module MO3. According to a further remarkable aspect of the invention, portions of height 1Hi, 2Hi, etc. of catalyst C1 and respectively C2 in modules MO1 respectively MO2, are in contact with the airspace forming wall PI. On these portions of catalyst of height 1Hi, 2Hi, etc. the synthesis gas (which enters from the bottom, runs through heat exchanger SC and flows downwards again into the catalytic beds) flows axially whereas in the major portions of catalyst 1H'i-1Hi, 2H'i-2Hi the gas flows radially outwards collecting in the annular areas from 30 where it flows over catalyst C2 of the following module MO2, through which it will flow again axially on the portion of height 2Hi and will flow radially outwards through major portion 2H'i-2Hi. Preferably, in areas 30 and 31 quench gas GQ flows in through duct 10 and annular distributor D1, respectively D2, which are placed directly at the end of the catalytic portion of height 1Hi, and 2Hi respectively. In Fig. 2 two beds have been shown, C1 and C2, in modules MO1 and MO2 "OUTWARDS" with portions of catalyst of height 1Hi, respectively 2Hi, in contact with airspace forming wall PI. Obviously the number of outwards modules may be other than 2, better still if greater than 2. Still in Fig. 2, the third catalytic bed C3 in module MO3 is run through by gas inwardly and the top part of the bed of height 3Hi run through axially by the gas is delimited inside by a portion of the central gas tube T, while the major portion of catalyst C3 of height 3H'i-3Hi is run through inwards radially collecting in annular area 32. Preferably the height of catalytic beds 1H'i, 2H'i, 3H'i, etc., increases on the way towards the lower baskets. The "inwards" module MO3 does not receive any quench gas. For greater details concerning these aspects reference is made also to published European patent application 202454 and US patents No. 4372920 and No. 4405562 in the Applicants' name. According to a preferred embodiment of the invention shown in Fig. 3, the first module MO1 rests on the underlying module by means of extension 20 of the internal gas collector, which rests in A1 on the extension 21 of the internal gas collector of the underlying module MO2. According to a further embodiment of the invention shown in Fig. 4, both modules MO1 and MO2 rest on the underlying module on support A1 (respectively A2) formed by the extension of the internal gas collectors. Furthermore, all three beds are "outwards" and also portion 3Hi of the third bed, run through by the gas axially, is delimited by a portion of the airspace forming wall PI.

Claims: Method to revamp (modernize) and improve the efficiency of existing reactors for exothermic heterogeneous synthesis, and more particularly for the synthesis of ammonia and methanol, consisting of: a) an external pressure shell (M); b) a single-piece wall (PI) forming an airspace (7) with said shell (M); c) a wall (L) delimiting the catalytic beds (Ci); d) catalytic baskets (Ci); e) feed ducts (5) for the synthesis gas (GS); f) feed ducts (6) for the quench gas (GQ); and g) an internal gas distribution collector (T), characterized by the fact that the wall (L) forming beds is entirely distinct and totally unconnected to the wall (PI) forming the airspace (7) and consists of modules (MOi), each module (MO1) delimiting at least one catalytic bed (C1), resting on the underlying module (MO2) and formed by the gas permeable walls (PMFLi) which contain the catalyst and distribute therethrough said synthesis gas (GS). Method according to claim 1 in which said airspace forming wall (PI) has portions of height (1Hi) in contact with the catalyst of at least one bed, at least an other of said beds having a portion of catalyst of height (3Hi) in contact with a portion of said internal gas distribution collector (T).

Inventors: PAGANI, GIORGIO, ZARDI, UMBERTO
Applicants: AMMONIA CASALE S.A., ZARDI, UMBERTO
Application year: 1988
Main classification: B01J8
IPC codes: C01C1, B01J8, C07C29