PREVENTION OF POST OXIDATION OF METHACROLEIN

Abstract: A method is disclosed for the prevention of post oxidation of methacrolein produced by vapor-phase oxidation of isobutylene, tertiary butanol or methallyl alcohol with a molecular-oxygen-containing gas in a reaction tube 3 containing a catalyst. The method comprising injecting at 5 and mixing with the reaction product gas, immediately downstream of the reaction tube outlet 4, a gas optionally mixed with air and selected from an inert gas, recirculated reaction gas and mixtures thereof.

Description: This invention relates to a method for the prevention of post oxidation of methacrolein. Methacrylic acid is generally produced by a process, which comprises a former-stage reaction and a latter-stage reaction. In the former-stage reaction, isobutylene, tertiary butanol, methallyl alcohol or the like is used as a starting material. In the presence of an oxidation catalyst of the molybdenum-bismuth-iron system, the starting material is subjected at 300-450°C to vapor-phase oxidation with a molecular oxygen-containing gas so that methacrolein is obtained primarily. In the latter-stage reaction, in the presence of a multi-element molybdate catalyst, the methacrolein obtained by the former-stage reaction is subjected at 250-400°C to vapor-phase oxidation with a molecular-oxygen-containing gas as in the former-stage reaction, whereby methacrylic acid is obtained. In the former-stage reaction, a reaction product gas flowing out of a reaction tube at a high temperature of at least 300°C as mentioned above is abruptly reduced in linear velocity in an empty column portion provided at an outlet of the reaction tube, and oxidation of methacrolein with unreacted oxygen, which may be regarded as a post reaction, takes place to form carbon monoxide, carbon dioxide and the like. The oxidation (hereinafter referred to as "post oxidation") of methacrolein with the unreacted oxygen leads to a decrease in the yield of methacrolein, whereby the yield of methacrylic acid from isobutylene, tertiary butanol or methallyl alcohol is lowered. It is hence necessary to prevent or inhibit the post oxidation. For the prevention of the post reaction, it has been known effective to lower the temperature of a product gas immediately after the product gas has flowed out of the reaction tube. It has therefore been known to provide a cooler immediately after an outlet of a reaction tube or to spray water to an outlet portion of a reaction tube so as to cool the same (Japanese Patent Laid-Open No. 54317/1974). When the cooler is provided at a location immediately downstream of the outlet of the reaction tube, the cooler is however assembled as a unitary element with the reactor so that the production facilities become complex and large and replacement of catalyst or the like is rendered complex. In the spraying of water to the outlet portion of the reaction tube, the reaction product gas is prone to excessive over-cooling partially or locally so that high boiling substances, for example, terephthalic acid and trimellitic acid contained in the reaction product gas are caused to deposit, thereby causing their sticking on line walls or blocking of lines. An object of this invention is to provide an improved method for the prevention of post oxidation of methacrolein. The invention provides a method for the inhibition of post oxidation of methacrolein in a gas stream also containing oxygen, characterised in that the gas is cooled sufficiently to substantially inhibit the post oxidation by injection of a gas, optimally mixed with air, and selected from an inert gas, recirculated reaction gas and mixtures thereof. The invention also provides a method for the prevention of post oxidation of methacrolein produced by vapor-phase oxidation of isobutylene, tertiary butanol or methallyl alcohol with a molecular-oxygen-containing gas in a reaction tube containing a catalyst, characterised by injecting and mixing with the reaction product gas, immediately downstream of the reaction tube outlet, a gas optionally mixed with air and selected from an inert gas, recirculated reaction gas and mixtures thereof. In embodiments of the present invention, methacrolein and methacrylic acid can be obtained efficiently by a method, which does not require complicated and scaled-up facilities and does not cool the reaction product gas abruptly, while preventing any substantial reduction in the yield of methacrolein and improving the yield in the production of methacrylic acid from isobutylene, tertiary butanol or methallyl alcohol as a raw material. A fixed-bed oxidation column, which is employed commonly in processes for the production of methacrylic acid from isobutylene, tertiary butanol or methallyl alcohol as a raw material, can be used advantageously as a reactor in embodiments of the present invention. As the fixed-bed oxidation reactor, a reactor in the form of a shell-and-tube heat exchanger may be used by way of example. Each reaction tube packed with a catalyst is also packed at both end portions thereof with alundum (alumina balls) which is inert to the catalyst, feed gas and reaction product gas. The inert gas used includes for example, nitrogen, carbon dioxide, steam or the like. The recirculated reaction gas may be for example a first gas which is a portion of a gas obtained after removing reaction products such as methacrolein and/or methacrylic acid from a former-stage and/or latter-stage reaction gas and is to be fed back to the reactor or a second gas which is obtained by mixing the first gas with the thus-separated methacrolein and is to be fed to the reactor. The oxygen concentration in the mixed gas of the inert gas and/or recirculated reaction gas and air is for example up to 13 mole %, preferably 10 mole % so as to prevent the post oxidation of methacrolein. In embodiments of the invention, the inert gas and/or recirculated reaction gas or the mixture of inert gas and/or recirculated reaction gas and air (hereinafter referred to as "cooling gas"), which is fed to a point immediately downstream of the outlet of the reaction tube, is injected into and mixed with the reaction product gas, flowing out of the reaction tube, through injection nozzles (hereinafter referred to as "sparger") provided in an empty column portion arranged immediately downstream of the outlet of the reaction tube. As a result, the flow of the reaction product gas is disturbed so that its temperature is lowered evenly and the possible post oxidation of methacrolein can be prevented. The linear velocity (injection velocity) of the cooling gas injected from the sparger may preferably be at least twice the linear velocity of the reaction product gas (i.e., the linear velocity of the reaction product gas as expressed in terms of its velocity in the empty column portion. If the former linear velocity does not reach twice the latter linear velocity, the flow of the reaction product gas may not be disturbed to a sufficient extent to achieve uniform cooling effects. An unduly high linear velocity of the cooling gas however may waste energy unnecessarily and moreover, cause a back flow of the reaction product gas to the reactor column and also influence the pressure to the subsequent step (latter-stage reaction). Although the linear velocity of the cooling gas may be determined suitably in accordance with operational conditions of the process, a linear velocity 4-8 times the linear velocity of the reaction product gas is preferably employed. Regarding the volumetric flow rate of the cooling gas: apparatus, piping and the like for the subsequent step or steps must be enlarged when the flow rate is too great while the disturbing effects for the flow of the reaction product gas are reduced when the flow rate is too small. The preferable flow rate ratio of the cooling gas to the reaction product gas is in a range of 0.1-3.0, with 0.3-1.5 being particularly preferred. The temperature of the cooling gas to be injected into the reaction product gas may be adjusted in such a way that the temperature of the resulting mixed gas falls within a range of 200-300°C. If the temperature of the cooling gas is too low to allow the resulting mixed gas to be at least 200°C, high boiling substances contained in the reaction product gas, for example, terephthalic acid, trimellitic acid and the like may deposit on the wall of the empty column portion and cause blocking of piping and the like. On the other hand, any temperature higher than 300°C may result in acceleration of oxidation of methacrolein in the mixed gas. The shape and injection angle of the sparger provided in the empty column portion at a location immediately downstream of the outlets of the reaction tubes may be determined depending for example on the flow rate and linear velocity of the reaction product gas, the number of reaction tubes, so as to permit efficient injection and mixing of the cooling gas for the attainment of the preferable temperature range of the mixed gas, for example, by arranging the sparger in the form of a ring, a plus sign (+) or a minus sign (-) inside the inner periphery of the empty column or by arranging a plurality of spargers. In particular, we believe good results may be obtained by injecting the cooling gas countercurrently against the flow of the reaction product gas. One embodiment of this invention will hereinafter be described in detail with reference to the accompanying drawings. This description is given by way of example of the invention only and not by way of limitation thereof. In the accompanying drawings: FIG. 1 is a vertical cross-section illustrating a reactor equipped with a sparger; FIG. 2 is an enlarged fragmentary view of parts of the reactor; and FIG. 3 is a plan view from above of the sparger. A feed gas, which has been supplied to the feed gas inlet 1 provided in an upper empty portion 2 of the reactor and contains an inert gas, passes through the upper empty column portion 2 and then reaction tubes 3 packed with a catalyst and controlled in temperature by circulation of a heating medium from an inlet 8 to an outlet 9, so that the feed gas is subjected to an oxidation reaction to obtain a reaction product gas containing methacrolein. In the lower empty column portion 6 of the reactor, the flow of the reaction product gas is disturbed by a cooling gas jetted out from the sparger 5 provided in the lower empty column portion 6 at a location immediately downstream of outlets of reaction tubes. The temperature of the reaction product gas is lowered, whereby the loss of methacrolein due to its post oxidation is minimized. A portion of the resulting mixed gas which contains methacrolein in a high concentration is then recirculated as a cooling gas from the reaction product gas outlet 7 to the sparger 5. An an alternative, the resulting mixed gas is delivered in its entirety to the next step. In addition, the thermometers 10, 11 are provided inside the lower empty column 6 at a position between the sparger 5 and the outlets 4 of the reaction tubes and at another position near the outlet 7 for the reaction product gas, respectively. The temperature of the cooling gas is controlled on the basis of the thus-detected temperatures of the mixed gas, so that the resulting mixed gas can be controlled within a suitable temperature range. Example 1 : Employed as a reactor was a vertical shell-and-tube reactor equipped with 44 reaction tubes having a length of 4.0 m and an inner diameter of 21.4 mm as illustrated in FIG. 1. The inner diameter and length of an upper empty column portion were 340 mm and 300 mm respectively, while those of a lower empty column portion were 340 mm and 1,000 mm respectively. A sparger was provided at a location 100 mm the way down from outlets of reaction tubes. Each reaction tube was packed with alundum, a former-stage reaction catalyst and alundum over 400 mm, 3,500 mm and 100 mm successively in order from an inlet thereof. The sparger had such a structure that as illustrated in FIG. 2, a cooling gas could be evenly jetted out against a reaction product gas flowing out of the outlets of the reaction tubes. The sparger was provided with 12 injection holes of 15 mm in diameter, which were formed through a semi-cylindrical portion of the sparger on the side of the outlets of the reaction tubes. Thermometers were provided near the outlet of one of the reaction tubes and the outlet for the reaction product gas respectively, thereby making it possible to measure the temperature of the mixed gas at both locations. A feed gas containing isobutylene as a raw material and oxygen, steam and nitrogen as inert gases at a molar ratio of 1:2.5:5:15 was fed to the reactor, whose temperature was controlled at 360°C, to give an hourly space velocity of 1,800 hr⁻¹, whereby the reaction was conducted. Nitrogen and air, which had been heated to 150°C, were mixed in proportions of 40 Nm³/hr and 30 Nm³/hr and injected through the sparger. The linear velocity of the reaction product gas at the outlet of each reaction tube was 2.09 m/sec. Results are shown in Table 1. Example 2 : A reaction was conducted under similar apparatus, packing and reaction conditions as for Example 1. A recirculated reaction gas, which was composed of 88.0 mole % of nitrogen, 6.0 mole % of oxygen, 4.5 mole % of carbon dioxide, 1.5 mole % of steam, and air were mixed in proportions of 56 Nm³/hr and 14 Nm³/hr, preheated to 150°C, and then injected through the sparger. Results are shown in Table 1. Example 3 : A reaction was conducted under similar apparatus, packing and reaction conditions as for Example 1. A recirculated reaction gas, which was composed of 55.0 mole % of nitrogen, 4.5 mole % of oxygen, 4.5 mole % of carbon dioxide, 31.5 mole % of steam and 4.5 mole % of methacrolein, and air were mixed in proportions of 50 Nm³/hr and 20 Nm³/hr, preheated to 150°C, and then injected through the sparger. Results are also shown in Table 1. Comparative Example 1 : A reaction was conducted in the same manner as in Example 1 except that the cooling gas (150°C) from the sparger was composed of 15 Nm³/hr of nitrogen and 10 Nm³/hr of air. Results are shown in Table 1. Post oxidation is believed to have taken place in view of the temperature increase in the empty column portion. The yield of methacrolein was reduced, while the yields of carbon monoxide and carbon dioxide increased. Comparative Example 2 : A reaction was conducted in the same manner as in Example 1 except that the injection of the cooling gas from the sparger was stopped. Results are shown in Table 1. Marked post oxidation is observed in view of the temperature increase in the empty column portion. The yield of methacrolein was reduced significantly.

Claims: A method for the inhibition of post oxidation of methacrolein in a gas steam also containing oxygen, characterised in that the gas is cooled sufficiently to substantially inhibit the post oxidation,by injection of a gas, optionally mixed with air, and selected from an inert gas, recirculated reaction gas and mixtures thereof. A method for the prevention of post oxidation of methacrolein produced by vapor-phase oxidation of isobutylene, tertiary butanol or methallyl alcohol with a molecular-oxygen-containing gas in a reaction tube (3) containing a catalyst, characterised by injecting (at 5) and mixing with the reaction product gas, immediately downstream of the reaction tube outlet (4), a gas, optionally mixed with air,and selected from an inert gas, recirculated reaction gas and mixtures thereof. A method according to claim 2, wherein the linear velocity of the injected gas is, at the point of injection thereof, at least twice the linear velocity of the reaction product gas as expressed in terms of its velocity downstream of the reaction tube outlet (4). A method according to claim 2 or claim 3, wherein the volumetric flow rate of the injected gas is 0.1-3.0 times the flow rate of the reaction product gas. A method according to any one of claims 2, 3 and 4, wherein the temperature of the reaction product gas after the injection is in the range of 200°C-300°C. A method according to any one of claims 2-5, wherein the gas for injection contains air and its oxygen content is up to 13 mole %.

Inventors: ISOBE, TAKESHI, IWASAKI, KOZO, KOYAMA, YOSHIO, KURAGANO, MORIMASA, SEGAWA, HIROZO, YOGUCHI, KATSUJI
Applicants: KURARAY CO., LTD., MITSUI TOATSU CHEMICALS, INC.
Application year: 1988
Main classification: C07C47
IPC codes: C07C45, B01J8, C07C47, C07C67