MXPA06001255A - Oxidation process and reactor with modified feed system. - Google Patents

Abstract

In an oxidation process in a shell and tube reactor (10), an improvement is disposing a short bed of packing material (30) about the tube (50) inlets. The short bed operates to direct contaminants derived from heat exchange media away from the headspace (20) and thus prevents formation of combustible gas mixtures.

Description

OXIDATION PROCESS. AND REACTOR WITH MODIFIED POWER SUPPLY SYSTEM TECHNICAL FIELD This invention relates to improvements for processes using tubular reactors with a hull. More specifically, the invention relates to the use of a short bed of packaging material to direct the leakage of contaminants, such as heat exchange media and its derivatives, away from the upper space of a tubular reactor, and to prevent the formation of mixtures. combustible gas.

BACKGROUND Tubular reactors are frequently used for exothermic reactions, for example, for the oxidation of propylene to acrylic acid and for the manufacture of maleic anhydride. Typically, the production of acrylic acid is a catalytic oxidation of propylene gas, in two stages. The method employs a first stage reactor with a first stage catalyst to oxidize propylene to acrolein, and a second stage reactor, loaded with a second stage catalyst, suitable for the oxidation of acrolein to acrylic acid. In general, mixed feed reagents, eg, propylene, air and water vapor, used to produce acrylic acid, are not expected to ignite at temperatures below about 450 ° C. However, self-ignition may occur at relatively low temperatures if the feed reagents contain substantial amounts of contaminants. This ignition can damage equipment, consume and waste raw materials, interrupt otherwise continuous reaction cycles, and other consequences. Regulation and temperature suppression of hot spots have been addressed in U.S. Patent No. 5,719,318. In this process for the production of acrylic acid, the hot spots or the heat buildup in the catalyst layers of the reaction tubes are suppressed, using a variable scale of particle sizes, preferably of the catalyst-containing particles. U.S. Patent No. 4,921,681 discloses a method for reducing the loss of ethylene oxide and the risk of localized, uncontrolled burns near the outlet of an ethylene oxide reactor, by packing inert particles in tubes, downstream of the catalysts. U.S. Patent No. 5,080,872 discloses a method for regulating temperature within a reaction vessel, using a bed of solid particles having zones of varying temperature, through which a reactive fluid phase is passed. U.S. Patent No. 6,028,220 describes the oxidation of propylene, during which there is a reduction of hot spots in the catalyst layer by varying the activity of the catalyst; while US Patent No. 6,563,000 discloses a process for producing acrylic acid from acrolein, which includes multiple reaction zones; wherein each of said reaction zones comprises a catalyst with a different level of activity, as compared to an adjacent zone, as is well known. It has been described, in general, the control of the temperature by circulating particulate material; see, for example, U.S. Patent No. 4,594,967, which discloses the use of circulating particles to cool the reaction in a fluidized bed reactor, while possibly converting the calcium sulfide to calcium sulfate. U.S. Patent No. 4,672,918 discloses solids in circulation, at controlled temperature, for controlling the temperature of a fluidized bed. U.S. Patent No. 4,899,695 describes a process for controlling heat transfer and erosion in a fluidized bed combustion reactor by introducing particles into the combustion unit, together, or in the presence of combustion reagents; With which you can recycle some of the particles. In U.S. Patent No. 5,505,907, which discloses a method for controlling the temperature of an incoming gas stream by incorporating solid particles coated in the gas stream, said particles are circulated and separated, and subsequently recirculated for repeated use. U.S. Patent Application No. 2002/0191732 describes the use of suspended solids in circulation to control the temperature; and U.S. Patent Application No. 2002/0048537, which describes a process for the polymerization of olefin, wherein the solid particles are circulated by a compressor. Thus, ceramic spheres have also been used to pack the space between stages, in the two-stage reactors, to act as heat sinks. However, the prior art has not solved the problem of contamination or adulteration of the reactor feed. The removal of contaminants has been described, for example, in US Patent No. 4,029,636, which describes a method for removing molybdenum trioxide from reactor effluent gases, in reactors containing molybdenum-based catalysts, causing the gases effluents pass over a bed of cooled solids, located at the outlet end of the tubular reactor, in which the molybdenum trioxide is deposited. U.S. Patent No. 5,413,699 describes the removal of NOx by forcing the NOx-containing gas through a DeNOx catalyst bed. Finally, U.S. Patent No. 5,538,544 discloses a variable pressure absorption system, by which a gas is introduced into the upper part of the container of said variable pressure system; and it is caused to be evenly distributed over an adsorbent bed, as a result of passing through a bed support system with graduated spheres. The methods described above, to control pollution, require somewhat specialized environments and / or construction and somewhat specialized use, and consequently, are not practical for equipping existing equipment, to limit contamination in the event of reactor decomposition. , for example, when heat exchange fluid can leak and can form derivatives and mix with the reactor feed.

BRIEF DESCRIPTION OF THE INVENTION The invention is based, in part, on the discovery that a short bed of packing material, in the vicinity of the inlets of the reactor tubes of a tubular reactor with a hull, can restrict the migration of decomposition gases (ie say, Nov) from the means of heat exchange towards the upper space of the reactor. It has been found that said bed placement virtually eliminates self-ignition problems that are based on leaks from the heat exchange medium. It has been discovered that a short bed is enough to improve the polluting problems, without the need for a deeper bed and its pressure drop and the associated material cost. In general, the invention relates to an improved process for the high temperature oxidation of a gaseous reactant in a hollow tubular reactor, of the kind having a plurality of reactor tubes, where the reactor tubes are immersed in a means of heat exchange contained within the housing and the inner volume of the reactor tubes is thus isolated from the heat exchange medium. Typically, the inlets inside a reactor tube are in communication with a feed plenum, or headspace, having a characteristic cross-sectional area in the vicinity of the inlets of the reactor tubes, generally free of obstruction; so that the velocity of a gaseous feed mixture at the inlets of the reactor tubes is the volumetric flow rate of the feed gaseous mixture divided by the characteristic cross-sectional area of the plenum, in the vicinity of the inlets to the reactor tubes. The process is also of the kind in which the feed gas mixture is fed from the plenum to the reactor tubes. The improvement of the present invention includes disposing a short bed of packing material, adjacent to the inlets of the reactor tubes. The short bed can include an empty space of about 0.3 to about 0.75, to increase the speed of the feed gas mixture, in the vicinity of the inlets to the reactor tubes, thereby controlling contamination of the plenum. of feed by the decomposition gases of the heat exchange medium, in case there is a breakdown or rupture of the reactor in the vicinity of the inlets to the reactor tubes, as could occur when the heat exchange medium leaks between the tubes and the end plate, due to corrosion. Typically, the short bed occupies less than 20 percent of the upper space volume and, preferably, less than about 10 percent of the upper space volume. Preferably the packaging material comprises spherical macroparticles having diameters of about 3.17 mm (0.125") to about 10.16 cm (4"). The most preferred are ceramic particulates that have diameters of less than about 5.08 cm (2") .Alternate form of packing material can be selected from among pellets, disks, rods and plates of various shapes DENSTONE® spheres , obtainable from Norton (Akron, OH, USA) are those that are particularly preferred.The process and apparatus of the invention can be used in connection with the manufacture of methacrylic acid, maleic anhydride, acrylic acid and potentially other partial oxidations that could occur in the manufacture of ethylene oxide or vinyl acetate monomer, for example In another aspect of the invention an improvement is provided to a tubular reactor with a hull having tubes submerged in a heat exchange medium, to a temperature from 200 to 400 ° C, which includes adding a short bed of packing material around the inlets to the reactor tubes, as described below. a depth of about 25.4 to 63.5 cm (10"to 25"), while the reactor tubes have an inner diameter of about 19.05 mm (0.75") to 5.08 cm (2") in the preferred embodiments. The improved reactor is properly used in the manufacture of acrylic acid, as described later. The control of contaminants, for example, of oxidants, such as nitrogen oxides, can control flammability and undesirable spontaneous self-ignition. The proper flow rate prevents temperature excursions when the feed gas mixture and contaminants are combined to form a mixture of increasing flammability. Thus, the placement of the bed could also prevent the undesirable migration of contaminants to the upper space, whether the contaminants increase flammability or not. For example, a pollutant could be a poison for the catalyst; thus, it is necessary to restrict the contaminant to localized regions of the reactor, instead of being everywhere in the upper space, so that it is fed to all the tubes. Other aspects and advantages of the invention will be apparent in general from the following detailed description and from the claims.

DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram illustrating the process and apparatus of the present invention. Figure 2 is a graph illustrating the spontaneous ignition of feed gas stream in the presence of 0.2 percent NO. The same reference symbols, in the different drawings, indicate similar elements.

DETAILED DESCRIPTION Typically, tubular steel hull reactors with a heat exchange medium are used in exothermic reactions to remove heat from the reaction. In very high temperature processes, salts are used as heat exchange media to remove heat from the reaction. Without wishing to be bound by theory, the applicants believe that the anions of the salts present in the heat exchange medium can react with the iron oxide formed in the reactor tubes. Anions, for example nitrates and nitrites, can be decomposed to generate nitrogen oxides in the presence of iron oxide, in the area of a leak, as described by J. C. Casanova, Thermal decomposition of sodium nitrate; Part I - Thermogravimetric study, with data, of the reaction of nuclear oxide with sodium oxide. Part II - Systematic analytical study of the reaction in the presence of ox oxide, Bull. Soc. Chim., France (1959), pages 429-440, which is incorporated herein in its entirety by means of this reference. Nitrogen oxides can include nitric oxide (NO), nitrous oxide (N20) and nitrogen dioxide (N02), among others. The presence of nitrogen oxides at low levels, for example, from 10 to 9000 ppm, can act as an oxidant, and decrease the ignition temperature of the gas feed mixture, as can occur when the heat exchange medium is changed. flight to the upper space. For example, a gaseous feed mixture of 7 percent propylene / 60 percent air / 30 percent vapor is stable at a temperature of about 450 ° C. The presence of 5,000 ppm of nitrogen oxides can lower the ignition temperature, for example, at 300 ° C. The presence of a short bed of packing material, adjacent to the reactor tube inlets, can prevent the migration of contaminants into the headspace, or shut down the autoignition, in the presence of nitrogen oxides, or decrease the residence time of the fuel mixture in the reactor, or confine the contaminants to the area where the leak occurred or, alternatively, alter the temperature profile of the reactor. Regardless of the theory, it has been found that the short bed is remarkably effective in improving self-ignition problems. As used herein, "macroparticle" is any three-dimensional, solid object having a volume of at least about 0.015 mL or more, preferably more than about 0.1 mL or more. For reference, note that a spherical particle with 6.35 mm diameter (1/4") has an approximate volume of 0.13 mL As used here," empty space "is the volumetric ratio of the interstitial spaces in a bed of material , to the total volume of the bed (here material plus free space) With reference to the drawings, Figure 1 shows a tubular reactor 10, which includes a head or upper part 15 defg a plenum or upper space 20, a bed short of packing material 30, a heat exchanger hull 40 and a plurality of reactor tubes 50, disposed in the reactor The feed plenum includes a distributor 60 for mixing the reactants into a gas feed mixture. feed 20 is in communication with the plurality of reactor tubes 50 in a heat exchanger 40, through an end plate 55. The dimensions of the feed plane or top space 20 may vary with The cross-sectional area of the reactor tubes in the reactor 10. For example, the plenum can be about 1.52 m (5 ') to about 4.26 m (14') high, with a diameter of the feeding plenum of about 0.60 m (2 ') to about 6.09 m (20'). The plurality of reactor tubes 50 with the reactor tube inlets 70 are surrounded by a heat exchanger means 80. For example, heat exchanger means 80 may be a salt. Typically the salt refrigerant may include mixtures of salts. Suitable salts include: potassium nitrate, potassium nitrite, sodium nitrite and / or sodium nitrate, or metals having a low melting point, for example: sodium, mercury, or alloys of different metals. The temperature of the heat exchange medium may be less than 45 ° C, more preferably, about 420 ° C. Specifically, the HITEC salt of DuPont, which includes about 53 percent potassium nitrate, about 40 percent sodium nitrite and about 7 percent sodium nitrate, can be used. The pollutants formed by the decomposition of the anions in the salt coolant can leak through the area 65 of the end plate towards the feed plenum 20, in the event of a rupture of the reactor. It is believed that the salt leaks first and then decomposes in the presence of the oxide and the oxidation catalyst. The packed bed 30 is disposed adjacent to the reactor tube inlets 70, which extend horizontally in the plenum. The short bed 30 includes discrete inert macroparticles, for example, of ceramic material. The short bed of packaging material 30 may vary in its dimensions. The proper depth H of the short bed can be less than 60.96 cm (24") more or less, but at least 12.7 cm (5"); typically 30.48 cm (1 ') more or less. The shape of said inert macroparticles is not critical. For example, the particulates can be granulated, such as in the form of spheres, pellets, discs, hollow tubes, spherical, cylindrical, in the form of rings, or can have the form of bars, plates and wire network, or in the form of aggregates of these forms. Suitable macroparticles can be spheres. When granulated substances or other inert substances are used, their size does not necessarily have to be uniform. However, it is preferred when an inert spherical substance is used, that the diameter of the sphere is about 1.58 mm (1/16") to about 5.08 cm (2"); preferably, about 6.35 mm (0.25"). It will be appreciated that the size of the particulates most preferably is not greater than the diameter of the reactor tubes (approximately 2.54 cm (1")) in the reactor, so as not to occlude the reactor tubes. The short bed 30 has a substantial void space so as not to cause too much pressure drop or pressure differential during the passage of the feed gas mixture to the reactor tubes. The void space of the particulates in the short bed 30 may be about 0.25 to about 0.75, preferably about 0.3 to 0.5, and, most preferably, 0.4. The packing density of the particulates can be about 1,120.7 g / L to about 1,761.1 g / L (70 to 110 Ibs / ft3), I feel somewhat typical around 1,280.8-1,440.9 g / L (80-90 Ibs / foot3). Specifically, the spheres may be, for example, DENSTONE® spheres, which are commercially available from Norton Chemicals (Akron, OH, E. U. A.). In various modalities, the DENSTONE® spheres can be, for example, DENSTONE® 57, DENSTONE® 2000 or DENSTONE® 99. The macroparticles can have a composition of ceramic, alumina, silica or clay.

The packed bed 30 provides an increase in the velocity of the feed gas mixture to the reactor tube inlets, because the cross section available for the flow decreases by the area occupied by the particulates. Said increase in the speed of the feed gas mixture scavenges contaminants, such as oxidants, for example, nitrogen oxides, from openings in the endplate and passing to the reactor tubes 50, before migrating to the plenum. in general. The proper flow velocity and aerodynamic flow path can reduce flammability in the feed plane 20, where the feed gas mixture and contaminants can be mixed to cause spontaneous potential ignition of the feed gas mixture. Spontaneous autoignition can be controlled by passing the feed gas mixture to the reactor tubes in a time that is less than the time required for autoignition. In general, a method for producing acrylic acid from propylene, in a catalytic oxidation in two stages, using a tubular heat exchanger type reactor has been described. See, for example, U.S. Patent Nos. 6,545,178, 6,482,981 and 6,069,271, which are incorporated herein in their entirety by means of this reference. Referring again to Figure 1, in a process for preparing said products, the distributor 60 conveys a gaseous mixture for feeding the reagents to the plenum or upper space 20. The gaseous feed mixture expands in and through the full feed 20 to the short bed of packed material 30. The surface velocity of the feed gas mixture in the plenum can be in the range of 0.91 to 3.04 meters per second (3 'to 10' feet / sec). For example, the gaseous feed mixture may include 7 percent propylene / 60 percent air / 30 percent water vapor. The gaseous feed mixture enters the packed bed 30, adjacent the inlets 70 of the reactor tubes of the reactor. The optimum packing material can be determined by the size of the tubular reactor, the combination of the gas flow, the flow through the inlet plenum, the desired pressure drop and the velocity profile of the short bed. In another embodiment of the invention, with reference to Figure 1, a gaseous feed mixture including n-butane and air passes through the distributor 60 to the feed plenum 20. The feed gas mixture is evenly distributed over the short bed of packaging material 30 and passes to reactor tubes 50. Upon passing through reactor 10, n-butane reacts with oxygen in the air to produce maleic anhydride. In another embodiment of the invention, with reference to Figure 1, a gaseous feed mixture including isobutylene and air passes through the distributor 60 to the feed plenum 20. The gas feed mixture is evenly distributed over the short bed of the feed. packaging material 30 and passes to reactor tubes 50. When passing through the reactor tubes, isobutylene reacts with oxygen from the air to produce methacrylic acid. Other additional products, such as vinyl acetate or ethylene oxide can be prepared according to the present invention. The aspects of the present invention are further illustrated by the following examples, which are given to illustrate the invention, and are not intended to be limitation thereto.

COMPARATIVE EXAMPLE In a process for the oxidation of propylene to acrylic acid a tubular reactor was operated which included a plurality of reactor tubes having a cross-sectional area (approximately 5.52 m2 (60 ft2) of open tube area, approximately 18.40 m2 (200 ft2) of upper space area) and a length of 6.09 m (20 ') (including the cooling zone), at a temperature of 326 ° C (620 ° F) and a pressure of about 1.1 baria gauge (16 psig), with a gaseous feed mixture composition that was about 7 percent propylene, about 60 percent air and approximately 30 percent water vapor. The system was operated so as to obtain a gas flow of approximately 33,979 cubic meters per hour at normal temperature and pressure (1200 MSCFH). It appeared that the reactor had leakage problems when contaminants from the heat exchange medium in the upper reactor space changed the flammability of the feed. The reactor was stopped due to autoignition episodes of the gas feed mixture in the headspace. The reactor tubes were cooled by means of a salt cooling bath, of the HITEC salt of DuPont, which was believed to be a source of contamination.

EXAMPLE 1 The reactor of the comparative example was charged with a short bed of packing material formed by 1/4"DENSTONE® spheres.The depth of the short bed of packing material was 30.48 cm (1 '). The reactor conditions were selected so as to prevail in the reactor a temperature of about 326 ° C (620 ° F) and a pressure of about 1.1 baria gauge (16 psig), with a composition of the feed gas mixture that was about 7 percent propylene, about 60 percent air, and about 30 percent water vapor, the system was operated so that a circulating volumetric gas flow of about 33,979 cubic meters per hour was obtained Normal temperature and pressure (1200 MSCFH) Virtually the self-ignition of the feed was eliminated by the placement of the short bed around the inlets of the reactor tubes, while the yields remained unchanged. and conversions.

EXAMPLE 2 The spontaneous ignition of a gas mixture of propylene feed / air / water in the presence of nitrogen oxides was evaluated, using a modified AST G72-82 method (reapproved in 1996). A stainless steel container was loaded from a book, with circumferential heaters, with a sample of the gas mixture feeding 6.7 percent propylene, 61.3 percent air, 31.8 percent water vapor and 0.2 percent NO. . As shown in Figure 2, temperature (° C) was monitored as a function of time (minutes) and pressure (barias). With reference to Figure 2, the result shows that spontaneous ignition occurred at approximately 280 ° C, in contrast to a gaseous feed mixture without NO, which was not flammable at 450 ° C. Other embodiments are within the scope of the following claims.

Claims (22)

1. - In a process for the high temperature oxidation of a gaseous reactant in a hull tubular reactor of the kind having a plurality of reactor tubes, where the reactor tubes are submerged in a heat exchange medium contained within the hull , and the inner volume of the reactor tubes is thus isolated from the heat exchange medium; and where the inlets inside the reactor tubes are in communication with a feed plenum having a characteristic cross-sectional area in the vicinity of the inlets to the reactor tubes, generally free of obstructions, so that the speed of a gaseous feed mixture at the inlets of the reactor tubes is the volumetric flow rate of the feed gaseous mixture divided by the characteristic cross-sectional area of the plenum in the vicinity of the reactor tubes; being generally the process of the kind in which the gaseous feed mixture is fed from the plenum to the reactor tubes; the improvement characterized in that it comprises: arranging a short bed of packing material adjacent to the inlets to the reactor tubes; and where the short bed occupies less than 20 percent of the volume of the feeding plenum; and where the short bed has an empty space of around 0.3 to 0.75, and in that way it is operative to increase the speed of the gas feed mixture, in the vicinity of the inlets to the reactor tubes, through which the contamination of the feed plenum by the heat exchange medium is controlled in the event of a rupture in the reactor, in the vicinity of the inlets of the reactor tubes.

2. The method according to claim 1, further characterized in that the packaging material comprises macroparticles.

3. The method according to claim 1, further characterized in that each macropartic has a diameter of between 3.17 mm (0.125") and about 10.16 cm (4").

4. - The method according to claim 3, further characterized in that each macroparticulate is less than about 5.08 cm (2") in diameter

5. - The method according to claim 1, further characterized in that the particulates comprise macroparticles

6. The method according to claim 2, further characterized in that the macroparticles are substantially spherical in shape and have an average diameter of about 3.17 mm (0.125") to about 10.16 cm (4"). The method according to claim 2, further characterized in that the macroparticles of a group consisting of spheres, pellets, disks, hollow tubes, rods and plates are selected 8. The method according to claim 7, characterized also because the particulates are spheres 9. - The method according to claim 8, further characterized in that the spheres are spheres DENSTON E. 10. The method according to claim 9, further characterized in that the DENSTONE spheres are selected from the group consisting of DENSTONE 57, DENSTONE 2000 and DENSTONE 99. 11. The method according to claim 1, further characterized because the oxidation reaction comprises the oxidation of isobutylene to methacrylic acid. 12. - The method according to claim 1, further characterized in that the oxidation reaction comprises the oxidation of butane to maleic anhydride. 13. - The method according to claim 1, further characterized in that the oxidation reaction comprises the oxidation of propylene. 14. - The method according to claim 1, further characterized in that the heat exchange medium is a molten salt coolant. 15. - The method according to claim 14, further characterized in that the salt is a HITEC salt. 16. - The method according to claim 15, further characterized in that the salt is about 53 percent potassium nitrate, about 40 percent sodium nitrate and about 7 percent sodium nitrate. 1

7. - The method according to claim 1, further characterized in that the short bed occupies less than about 10 percent of the volume of the feed plenum. 1

8. - In an apparatus for high temperature oxidation of a gaseous reactant in a hull tubular reactor, configured to flow a gaseous feed mixture to the tubular reactor through a distributor, and direct the gaseous feed mixture from a plenum of feeding to a plurality of reactor tubes, through their entrances that communicate with the plenum of feeding; the reaction tubes being submerged in a heat exchange medium at a temperature of about 200 ° C to 400 ° C, the improvement characterized in that it comprises: a short bed of packing material, adjacent to the inlets to the reactor tubes, where the short bed has an empty space of around 0.3 to 0.75, so that contamination of the feed plenum by any decomposition gas of the heat exchange medium is controlled in the event of a rupture of the reactor in the vicinity of the inlets to the reactor tubes; where the short bed occupies less than about 20 percent of the volume of the plenum. 1

9. The apparatus according to claim 18, further characterized in that the diameter of each reaction tube is about 1.90 cm (0.75") to about 5.08 cm (2"). 20. The apparatus according to claim 18, further characterized in that the depth of the short bed of packing material is approximately 25.4 cm (10") to 63.5 cm (25"). 21. - The apparatus according to claim 20, further characterized in that the depth of the short bed is at least 25.4 cm (10") 22. - In a method for manufacturing acrylic acid in a tubular reactor with a hull, for oxidizing propylene, which comprises flowing a feed gas mixture to a feed plenum through a distributor, and directing the feed gas mixture from the feed plenum to a plurality of reactor tubes disposed in the hull tubular reactor, the reactor tubes being submerged in a molten salt cooler, at a temperature of about 200 ° C to 400 ° C, the improvement characterized in that it comprises: providing a short bed of packing material adjacent to the inlets to the reactor tubes, where the short bed occupies less than about 20 percent of the volume of the feed plenum, and contact the gaseous feed mixture with the short bed.