Method of modernising in-situ Braun-type converters for heterogeneous synthesis and converters thus obtained

Abstract: Method for the protection of the internal walls of the shell of Braun type converters, transformed into axial-radial reactors with insertion of a cartridge and a cylindrical perforated wall, as per United States Patent Number 4,935,210, characterized by the fact that cooling gas is circulated (FLU) at temperatures between 250 and 300°C in the airspace (I) between cartridge (C) and (P).

Description: This invention refers to a method to modernize converters for heterogeneous synthesis at high temperatures and pressures, in particular ammonia, having at least one external pressure vessel having an upper majority zone substantially cylindrical and a lower minority zone which is substantially reversed; an internal cartridge that is open at the top, creates a gas passage between the external side and internal side of the shell, and is filled with catalyst through which the synthesis gas rally passes from top to bottom; inlets for the fresh synthesis gases and for the cooling gases; and an outlet for the reacted gases. In particular the invention refers to a method of the said kind which modifies, with an "in-situ" operation, for instance, converters of the preceding type, known as Braun reactors, and protects the shell walls from high temperatures. These converters for the conversion of nitrogen ammonia and hydrogen in the presence of catalyst, function in very severe operating conditions, the shell walls and the feed openings being put under temperatures in the range of 400°C. The construction materials of the reactor must be highly resistant to thermal stresses and are thus made of steel 2,25 Cr 1 Mo. In order to clearly show this idea, Fig. 1 attached hereto shows Fig. 2 of US patent No.4.935.210, the description of which is considered herein. P indicates the shell in which the cartridge (C) (of diameter DC) is introduced, and the cylindrical perforated walls (FE) of diameter (Di) and height (Hi), and the walls (FI) of diameter (D'i) and height (H'i). On the top the closed manhole is shown (but without a reference). The synthesis gas enters from the bottom and crosses the airspace between the shell (P)and the cartridge (C). The conventional Braun-type converters have greatly improved their performances thanks to the modernization according to the United States Patent no. 4,935,210. European Patent application EP-A-0 190 516, on the other hand, discloses a horizontal converter for heterogeneous catalytic synthesis comprising a single catalyst bed within an outer pressure resisting shell, wherein the shell is thermally protected by circulating a cooling gas in an airspace defined between the catalyst bed and the shell inner wall. However, only recently were deep cracks found in the shell walls of many converters which have been in service already many years. The necessary crack repair works are considered very serious, and imply the closure or shutdown of the plant for an undetermined, but lengthy period of time. In addition, even in the case that these expensive repairs should be successful, there is still the risk that the cracks may reappear (the same cracks that were repaired, and/or new ones), since the primary and original cause of the cracks had not been eliminated. This first aim of this invention is to provide a method for the modernization of axial Braun-type reactors, in particular for their transformation into axial-radial reactors, in which the danger of other cracks in the shell walls and feed openings in also prevented. Another aim of this invention is a reactor of the Braun-type transformed into an axial-radial converter type with a reduced volume of catalyst, but of fine granulometry, flushed with a protective gas against the cracks. Another aim of this invention is a converter for heterogeneous synthesis, in particular of ammonia, of the Braun-type with fine catalyst which is axially-radially crossed, and with shell walls protected against high temperatures and without any cracks. These other aims of the invention are achieved by a method and converter defined in the appended claims. According to the invention, the internal shell walls, exposed to severe thermal conditions (400°C) typical of axial Braun-type converters, are protected by investing them with fluxes of feed gas at temperatures from 250°C to 300°C. In particular, the realization of the method as per the preceding U.S. Patent No. 4,935,210 of the Applicants, for the modernization of the Braun converters and for their transformation into axial-radial reactors by means of inserting a cartridge and, inside it, two cylindrical differentially perforated walls, characteristically adopts at least the following steps: rendering the upper end of the cartridge impermeable to external gases; applying a highly thermal insulation to the internal cartridge walls; making a flux of fresh gas flow at temperatures between 250 and 300°C, but preferably at 270°C flow in the interspace between the internal walls of the shell and external walls of the cartridge; use of a new gas inlet feed opening through the manhole at the upper end of the shell. The converters, as per United States Patent No. 4,935,210, are also characterized by a closed cartridge with an upper cover and a layer of insulating material, by an inlet of cooling gas preferably made in the manhole and by a mixing zone of reaction gas, which is preheated to reaction temperature, and cooling gas. The various aspects and advantages of the invention are better shown in the description of a preferred form of embodiment, as illustrated in the attached drawing (Fig. 2), which is a schematic and partial section of the Braun reactor with a plane containing the longitudinal axis of the reactor. As in the United States Patent Number 4,935,210, (P) indicates the reactor shell having an external wall (1) and an internal wall (2) within which a cartridge (C) is placed, in such a way as to create an airspace (I) between the internal wall (2) of (P) and the external wall (3) of (C). Inside (C) the cylindrical perforated walls (FE) are inserted slightly below that of the cartridge (C), and the wall (FI) of diameter (D''i), decisively lower than that of (FE). The said walls are not perforated on the lower conical trunk portion (PR) that develops from point (6) until the feed opening (7), while the internal tubular wall (FI) is not perforated on a lower portion (10), on which the final part of the zone of the conical turn portion (PR) is coupled. Characteristically, the upper end of the cartridge (C) is now closed with a cover (CO'), while its internal wall (4) is equipped with a layer of highly insulating material (IS), for instance, kaowool, the thickness of which is in the order of a few centimeters. According to another aspect of the invention, the upper end of the reactor (in the manhole MH), an inlet (14) is made for the cooling fluid (FLU), which is provided with a pipe (11), extending through the manhole (MH), having a distribution torus (TO) on the upper end of the cartridge (C); the fluid then circulates in the airspace (I) between the shell (P) and the cartridge (C), descending down to the lower end (45) of the cartridge, where is mixed with the flux of synthesis gas (HG) preheated to reaction temperature (340÷440°C, preferably approximately 400°C) in external exchangers (not represented), but of the HEX type in Figure 1. The mixture of HG + FLU ascends towards the top of the airspace (30) between the wall substantially perforated (FE) and the cartridge (C); a minority portion of this mixture passes axially through (As) the upper zone of the catalytic bed (K), while the majority portion of the mixture HG + FLU passes radially through (RA) the zone, which are also a majority zone, of said catalytic mass (CA). The reacted gases (RG) are collected in the tubular wall substantially perforated (FI) and exit at the bottom (arrow 60) in order to be carried to the external heat exchangers (type HEX as shown in Figure 1). It is true that with respect to conventional Braun converters, a certain volume of catalyst is lost, i.e. that which is now occupied by the airspaces (I) and (30) as well as by the delimited space from the pipe (PI); but it is also true that this loss of catalyst volume is largely compensated by the fact that now the catalytic mass (CA) is made of fine granulometric catalyst with a high contact surface, contrary to the catalyst of large granulometric type which was required to be used in conventional converters.

Claims: A method of modernizing in-situ a Braun-type converter for heterogeneous synthesis, the converter comprising an external pressure shell (P), a feed duct (12) for introducing fresh synthesis gas into said shell (P), an outlet tube (T) for withdrawing reacted gases (RG) axially positioned in the feed duct (12), a cartridge (C) inside the shell (P) open on top an filled with catalyst arranged in a catalytic bed (K) through which the synthesis gas axially passes from top to bottom, the cartridge (C) having a diameter (Dc) slightly smaller than that of the external pressure shell (P), said method comprising the steps of delimiting the catalytic bed with: a cylindrical outer wall (Fe) inside and in the vicinity of said cartridge (C), the cylindrical outer wall (Fe) being permeable by gas for at least the major portion of its height (Hi) and having a diameter (Di) only slightly smaller than that of the cartridge (C); a cylindrical internal wall (Fi) also permeable by gas for at least the major portion of its height (H'i), the cylindrical internal wall (Fi) having a diameter (D'i) much smaller than the diameter (Di) of the cylindrical outer wall (Fe) and of the same order of magnitude of diameter (Dt) of the outlet tube (T), the top (SO) of said cylindrical internal wall (Fi) being closed by a cover (CO) possibly permeable by gas at least in part; a lower wall (PR) having a diameter (D'') decreasing towards the bottom starting from the diameter (Di) of the cylindrical outer wall (Fe) towards the diameter (D'i) of the cylindrical internal wall (Fi); characterized in that the method further comprises the steps of: providing said cartridge (C) with: i) a top cover (CO'); ii) a substantially cylindrical lower end (45), said lower end (45) extending downwardly into said feed duct (12) between an internal cylindrical wall thereof and said outlet tube (T); iii) a lower portion (CL), substantially parallel to said lower wall (PR) of the catalytic bed (K), having a diameter decreasing starting fom the diameter (Dc) of the cartridge (C) towards the diameter of said substantially cylindrical lower end (45); applying a highly insulating material (IS) to the internal walls (4) of the cartridge (C); providing an inlet (14) through the shell (P) for feeding a cooling fluid (FLU) between the cartridge (C) and the shell (P) down to said lower end (45) of the cartridge (C). A method according to claim 1, characterized in that the inlet (14) of said cooling fluid (FLU) is provided on top of the shell (P) through a manhole (MH). A method according to claim 2, characterized in that it further comprises the step of providing said inlet (14) of the cooling fluid (FLU) with a pipe (11) extending through said manhole (MH), the pipe (11) having a distribution torus (TO) positioned inside the shell (P) on the upper end of the cartridge (C). A Braun-type converter for heterogeneous synthesis comprising: an external pressure shell (P), a feed duct (12) for introducing fresh synthesis gas into said shell (P), an outlet tube (T) for withdrawing reacted gases (RG) axially positioned in the feed duct (12), a cartridge (C) inside the shell (P), having a diameter (Dc) slightly smaller than that of the external pressure shell (P), filled with catalyst arranged in a catalytic bed (K) through which the synthesis gas axially passes from top to bottom, a cylindrical outer wall (Fe) inside and in the vicinity of said cartridge (C), the cylindrical outer wall (Fe) being permeable by gas for at least the major portion of its height (Hi) and having a diameter (Di) only slightly smaller than that of the cartridge (C); a cylindrical internal wall (Fi) also permeable by gas for at least the major portion of its height (H'i), the cylindrical internal wall (Fi) having a diameter (D'i) much smaller than the diameter (Di) of the cylindrical outer wall (Fe) and of the same order of magnitude of diameter (Dt) of the outlet tube (T), the top (SO) of said cylindrical internal wall (Fi) being closed by a cover (CO) possibly permeable by gas at least in part; a lower wall (PR) having a diameter (D'') decreasing towards the bottom starting from the diameter (Di) of the cylindrical outer wall (Fe) towards the diameter (D'i) of the cylindrical internal wall (Fi); characterized in that the converter further comprises: a top cover (CO') provided on said cartridge (C), a substantially cylindrical lower end (45) extending downwardly from said cartridge (C) into said feed duct (12) between an internal cylindrical wall thereof and said outlet tube (T); a cartridge lower portion (CL), substantially parallel to said lower wall (PR) of the catalytic bed (K), having a diameter decreasing starting fom the diameter (Dc) of the cartridge (C) towards the diameter of said substantially cylindrical lower end (45); a highly insulating material (IS) applied to the internal walls (4) of the cartridge (C); an inlet (14) through the shell (P) for feeding a cooling fluid (FLU) between the cartridge (C) and the shell (P) down to said lower end (45) of the cartridge (C). A converter according to claim 4, further comprising a zone proximate to said lower end (45) of the cartridge (C) for mixing synthesis preheated gas (HG) with said cooling fluid (FLU). A converter according to claim 4, characterized in that the inlet (14) of said cooling fluid (FLU) is provided on top of the shell (P) through a manhole (MH). A converter according to claim 6, characterized in that said inlet (14) of the cooling fluid (FLU) is provided with a pipe (11) extending through said manhole (MH), the pipe (11) having a distribution torus (TO) positioned inside the shell (P) on the upper end of the cartridge (C). A converter according to claim 4, characterized in that the catalyst within said bed (K) has a fine granulometry with a high contact surface.

Inventors: PAGANI GIORGIO, ZARDI UMBERTO, PAGANI, GIORGIO, ZARDI, UMBERTO
Applicants: AMMONIA CASALE SA, ZARDI UMBERTO, AMMONIA CASALE S.A., ZARDI, UMBERTO
Application year: 1993
Main classification: B01J8
IPC codes: B01J8, C01C1