JP4612366B2 - Process for producing α, β-unsaturated carboxylic acid - Google Patents

Description

  The present invention relates to a method for producing an α, β-unsaturated carboxylic acid. Specifically, the present invention relates to a method for producing an α, β-unsaturated carboxylic acid that can be safely operated in the liquid phase oxidation of an α, β-unsaturated aldehyde.

  To oxidize α, β-unsaturated aldehyde in the liquid phase (liquid phase oxidation), molecular oxygen is usually used. However, since there are flammable substances in the gas phase in the reactor, the possibility of explosion and fire cannot be denied, and developing a method that can eliminate the danger is from the viewpoint of operating the plant safely. This is a very important issue.

Although the reaction system is different, Patent Document 1 discloses that a part of the reaction exhaust gas is circulated and supplied as an inert gas and adjusted so that the oxygen concentration in the gas phase is lower than the flammability limit oxygen concentration, while avoiding an explosion. A method for producing an aromatic carboxylic acid by liquid phase oxidation of alkylbenzene is disclosed. Patent Document 2 discloses a method of extracting as much catalyst as possible as a method of producing methacrolein or acrolein with molecular oxygen in methanol to produce the corresponding ester. At this time, it is also described that a method of diluting the supply air with an inert gas such as nitrogen so that the oxygen concentration of the reactor effluent gas does not exceed the flammability limit oxygen concentration is described.
JP 7-258151 A JP 2003-267922 A

  However, the method of Patent Document 1 has a problem that the circulation amount of the reaction exhaust gas has to be increased, which is disadvantageous in terms of cost due to an increase in circulation blower capacity or an increase in power consumption.

  The method of Patent Document 2 also has a problem that a large amount of inert gas such as nitrogen is required. In order to obtain a large amount of nitrogen industrially, a dedicated facility such as a PSA (pressure swing adsorption) device is required, and there is a problem that the construction cost and the running cost are increased.

  Furthermore, in the methods of Patent Documents 1 and 2, since the oxygen concentration at the time of production is low, the speed of the molecular oxygen liquid phase oxidation reaction is reduced, and there is a problem that the production efficiency is low and the cost is disadvantageous. there were.

  The present invention has been made in view of these problems, and its purpose is to set a high oxygen concentration during production without using a large amount of inert gas, and to cause an explosion or fire in the gas phase part of the reactor. It is an object of the present invention to provide a method for producing an α, β-unsaturated carboxylic acid that eliminates such a risk and can be operated safely.

  In order to solve the above-mentioned problems, the present inventors diligently studied the relationship between the gas phase pressure and the presence or absence of an explosion by measuring the pressurized flash point. I found it. It was found that this was caused by the fact that the combustible substance concentration in the gas phase part was less than the lower explosion limit concentration in the oxygen concentration at the time of production, and this was the beginning of the present invention. If the liquid phase oxidation of the α, β-unsaturated aldehyde is performed at a pressure such that the concentration of the combustible substance in the gas phase part is less than the lower explosive limit concentration in the oxygen concentration at the time of production, the production cost can be suppressed safely. The present invention was completed by combining these findings.

That is, the present invention is the presence of a noble metal catalyst, alpha with molecular oxygen in a liquid phase, alpha by oxidizing β- unsaturated aldehydes, a method for producing a β- unsaturated carboxylic acid, at startup of operation A gas having an oxygen concentration lower than the flammability limit oxygen concentration is supplied into the reactor that performs the oxidation, and the concentration of the combustible substance existing in the gas phase portion in the reactor is the lower explosion limit concentration in the oxygen concentration at the time of production. The pressure of the gas phase in the reactor so that the concentration of the combustible material present in the gas phase of the reactor is less than the lower explosive limit concentration in the oxygen concentration at the time of manufacture. Is a method for producing an α, β-unsaturated carboxylic acid.

  According to the method of the present invention, dangers such as explosion and fire can be eliminated, and the plant can be operated safely. In addition, since a large amount of inert gas for dilution, which has been conventionally used to avoid the explosion range, is unnecessary, there is no need to provide an inert gas generator or a circulation facility. Since it can be set high, the manufacturing cost can be greatly reduced.

The present invention is a method for producing an α, β-unsaturated carboxylic acid by oxidizing an α, β-unsaturated aldehyde with molecular oxygen in a liquid phase in the presence of a noble metal catalyst . A gas having an oxygen concentration less than the flammable limit oxygen concentration is supplied into the reactor for oxidation, and the concentration of the flammable substance existing in the gas phase in the reactor is less than the lower explosion limit concentration in the oxygen concentration at the time of production. after the pressure becomes, so that the concentration of the flammable materials present in the gas phase portion in the reactor is less than lower explosion limit concentration in the oxygen concentration during manufacturing, the pressure of the gas phase portion of the reactor It is something to control.

  Examples of the α, β-unsaturated aldehyde used in the present invention include acrolein, methacrolein, crotonaldehyde (β-methylacrolein), cinnamaldehyde (β-phenylacrolein) and the like. The α, β-unsaturated carboxylic acid produced is an α, β-unsaturated carboxylic acid in which the aldehyde group of the α, β-unsaturated aldehyde is changed to a carboxyl group. For example, the α, β-unsaturated carboxylic acid obtained when the raw material is acrolein is acrylic acid, and the α, β-unsaturated carboxylic acid obtained when the raw material is methacrolein is methacrylic acid. Suitable for liquid phase oxidation using acrolein or methacrolein. The raw α, β-unsaturated aldehyde may contain a small amount of saturated hydrocarbon and / or lower saturated aldehyde as impurities.

  As the gas containing molecular oxygen used in the liquid phase oxidation reaction, air is preferable from the viewpoint of its oxygen concentration and economy, but pure oxygen or pure oxygen is necessary if it is produced at a higher oxygen concentration or the like. A mixed gas of air, nitrogen, carbon dioxide, water vapor and the like can also be used. A gas containing molecular oxygen that has passed through the liquid phase in the reactor can be circulated and used again for the reaction. Note that the gas containing molecular oxygen that has passed through the liquid phase in the reactor can be directly released into the atmosphere. In addition, low-concentration raw materials and solvents are present in the gas containing molecular oxygen that has passed through the liquid phase in the reactor. From the viewpoint of preventing air pollution or cost, it is preferable that these are recovered and then diffused into the atmosphere. Examples of the recovery method include an absorption method and an adsorption method.

  The reaction solvent used for the liquid phase oxidation reaction is not particularly limited, and water, alcohols, ketones, organic acids, organic acid esters, hydrocarbons, and the like can be used. Examples of alcohols include tertiary butanol and cyclohexanol. Examples of ketones include acetone, methyl ethyl ketone, and methyl isobutyl ketone. Examples of the organic acids include acetic acid, propionic acid, n-butyric acid, iso-butyric acid, n-valeric acid, iso-valeric acid and the like. Examples of the organic acid esters include ethyl acetate and methyl propionate. Examples of hydrocarbons include hexane, cyclohexane, toluene, and the like. Among these, organic acids having 2 to 6 carbon atoms, ketones having 3 to 6 carbon atoms, and tertiary butanol are preferable, and acetic acid, n-valeric acid, and tertiary butanol are particularly preferable. A solvent may be individual or 2 or more types of mixed solvents may be sufficient as it. Further, when at least one compound selected from the group consisting of alcohols, ketones, organic acids and organic acid esters is used as a solvent, it is preferable to use a mixed solvent of this compound and water. The amount of water at that time is not particularly limited, but is preferably 2% by mass or more, more preferably 5% by mass or more, based on the mass of the mixed solvent. Moreover, 70 mass% or less is preferable, More preferably, it is 50 mass% or less. Although the solvent is desirably uniform, it may be used in a non-uniform state.

  In the present invention, the noble metal catalyst prepared by the following method can be suitably used, but a commercially available product may be used.

  The noble metal catalyst can be prepared by dissolving a noble metal compound in a solvent and reducing with a reducing agent. The target noble metal catalyst is precipitated by this reduction. A supported catalyst in which a noble metal is supported on a carrier can be used, but a non-supported catalyst may also be used. The reduction can be performed in the gas phase, but is preferably performed in the liquid phase. Hereinafter, a liquid phase reduction method for reducing a noble metal compound in a liquid phase will be described.

  The noble metal contained in the noble metal catalyst used in the present invention refers to palladium, platinum, rhodium, ruthenium, iridium, gold, silver, rhenium, osmium, among which palladium, platinum, rhodium, ruthenium, iridium and gold are preferred, and palladium is preferred. Particularly preferred. The noble metal compound used for the production of the catalyst is not particularly limited. For example, noble metal chlorides, oxides, acetates, nitrates, sulfates, tetraammine complexes, and acetylacetonato complexes are preferable. Noble metal chlorides, Oxides, acetates, nitrates and sulfates are more preferred, and noble metal chlorides, acetates and nitrates are particularly preferred.

  First, the above noble metal compound is dissolved in a solvent to obtain a noble metal compound solution. As this solvent, one or two or more solvents selected from the group consisting of water, alcohols, ketones, organic acids and hydrocarbons can be used. The concentration of the noble metal compound is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, particularly preferably 0.5% by mass or more, and preferably 20% by mass or less, more preferably 10% by mass. Hereinafter, it is particularly preferably 7% by mass or less.

  Next, a reducing agent is added to this to reduce the noble metal in the noble metal compound. The reducing agent to be used is not particularly limited, and examples thereof include hydrazine, formaldehyde, sodium borohydride, hydrogen, formic acid, formic acid salts, ethylene, propylene, and isobutylene.

  The temperature of the system and the reduction time during the reduction vary depending on the reduction method, the precious metal compound used, the solvent, the reducing agent, etc., but cannot be generally stated. In the case of the liquid phase reduction method, the reduction temperature is preferably 0 to 100 ° C. The time is preferably 0.5 to 24 hours.

  When preparing a supported catalyst, it may be reduced in the same manner as when the support is not used, except that the support is dispersed in the noble metal compound solution. Examples of the carrier include activated carbon, carbon black, silica, alumina, magnesia, calcia, titania and zirconia. Among them, activated carbon is preferably used. The loading ratio of the noble metal is preferably 0.1% by mass or more, more preferably 1% by mass or more, still more preferably 2% by mass or more, particularly preferably 4% by mass or more, based on the mass of the carrier before loading. It is preferably at most mass%, more preferably at most 30 mass%, further preferably at most 20 mass%, particularly preferably at most 15 mass%.

  The precipitate deposited by reduction is preferably separated by a method such as filtration or centrifugation. The separated catalyst is appropriately dried. The drying method is not particularly limited, and various methods can be used. The physical properties of the prepared catalyst can be confirmed by BET surface area measurement, XRD measurement, CO pulse adsorption method, TEM measurement, and the like.

  In the present invention, the amount of α, β-unsaturated aldehyde used as a raw material is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, with respect to 100 parts by mass of the solvent. Moreover, 50 mass parts or less are preferable, More preferably, it is 30 mass parts or less.

  The amount of molecular oxygen used is preferably at least 0.1 mol, more preferably at least 0.3 mol, particularly preferably at least 0.5 mol, per mol of α, β-unsaturated aldehyde. . Moreover, 30 mol or less is preferable, More preferably, it is 25 mol or less, Especially preferably, it is 20 mol or less.

  The noble metal catalyst is preferably used in a state of being suspended in a reaction solution for liquid phase oxidation, but may be used in a fixed bed. The amount of the noble metal catalyst in the reaction solution is preferably 0.01 parts by mass or more as the noble metal catalyst present in the reactor with respect to 100 parts by mass of the solution present in the reactor performing liquid phase oxidation. Is 0.2 parts by mass or more. Moreover, 60 mass parts or less are preferable, More preferably, it is 50 mass parts or less.

  The temperature at which liquid phase oxidation is performed is appropriately selected depending on the solvent and the raw material used. The reaction temperature is preferably 60 ° C or higher, more preferably 70 ° C or higher, preferably 200 ° C or lower, more preferably 150 ° C or lower. The reaction pressure for performing liquid phase oxidation is set by the method described later.

  The present invention can be carried out either batchwise or continuously. In the case of a continuous type, there is no restriction as long as a gas-liquid solid reaction can be carried out, but for example, a packed tower reactor, a bubble tower reactor, a stirred tank reactor, a spray tower reactor, a plate tower reactor, etc. Among these, bubble column reactors and stirred tank reactors are preferably used.

  In the liquid phase oxidation according to the raw materials and conditions as described above, the reaction is performed so that the concentration of the combustible substance existing in the gas phase portion of the reactor is less than the lower explosion limit concentration in the oxygen concentration at the time of production. The pressure in the gas phase section in the vessel is controlled.

  The explosive limits of flammable substances are described in the “New Solvent Pocket Book” (edited by the Society for Synthetic Organic Chemistry). According to this, the explosion limit occurs because if the amount of combustible gas or oxygen in the mixture is insufficient, the amount of heat generated by the combustion reaction and the amount of heat necessary to maintain combustion cannot be balanced. This is because combustion cannot be maintained. Therefore, this limit exists both on the side where there is too little combustible gas and on the side where there is too much combustible gas (the side where there is relatively too little oxygen). In addition, the concentration range in which the explosion is possible, that is, the explosive concentration range is called an explosion region.

  FIG. 1 schematically shows the relationship between the flammable substance concentration and the oxygen concentration in the vicinity of the explosion region. A shaded portion in FIG. 1 represents an explosion region, and the other portions represent non-explosion regions. The boundary between the explosion region and the non-explosion region represents the explosion limit.

  If the oxygen concentration is assumed to be concentration c, the explosion limit is intersected at two points. The concentration a at this time corresponds to the lower explosion limit concentration of the combustible substance, and the concentration b corresponds to the upper explosion limit concentration of the combustible substance. Therefore, when the oxygen concentration is the concentration c, the flammable substance concentration becomes an explosion region in the range of the concentrations a to b. Further, when the oxygen concentration is lowered to the concentration e, the lower explosion limit concentration and the lower explosion limit concentration of the combustible substance coincide with each other at the concentration d, and the explosion region disappears. The oxygen concentration e at this time is called the flammability limit oxygen concentration.

  In the conventional method, since the combustible substance concentration is carried out under a condition higher than the lower explosion limit concentration a at the oxygen concentration c, in order to avoid the explosion, it is diluted with an inert gas or the like, and the oxygen concentration is set to the flammable limit oxygen. It was necessary to make the concentration less than e. That is, as a result, the oxygen concentration at the time of production is set low, so that the speed of the liquid phase oxidation reaction is reduced, and the production efficiency is poor and the cost is disadvantageous. The present invention is characterized in that the flammable substance concentration is performed under conditions such that the oxygen concentration c is less than the lower explosion limit concentration a. That is, the oxygen concentration at the time of production can be set high, and the production efficiency is increased.

  Here, the concentration of the combustible substance in the gas phase is expressed by the following formula (1).

Cg = P _Vap / π × 100 (1)
Here, Cg represents the concentration of the combustible substance [vol%] in the gas phase, P _Vap represents the vapor pressure of the combustible substance at the measurement temperature, and π represents the test pressure (total pressure). According to the equation (1), for example, the vapor pressure of acetic acid at 51 ° C. is 60 mmHg (according to “Chemical Handbook Basic Edition Revised 3rd Edition” (edited by the Chemical Society of Japan), so the gas phase at atmospheric pressure (760 mmHg) The acetic acid concentration of the part is 7.89 vol%. P _Vap is generally a function of temperature. If the temperature is constant, P _Vap is constant, and by increasing π, the combustible substance concentration can be reduced. That is, by setting the pressure higher than a certain value, the combustible substance concentration can be made lower than the lower explosion limit concentration in the oxygen concentration at the time of manufacture, and explosion can be avoided. The combustible substance concentration can also be measured by analysis such as gas chromatography.

  The lower explosion limit concentration can be determined by measuring the pressurized flash point. Pressurized flash point measurement is performed at a constant temperature and constant pressure by forming a squealing gas in an explosion container and igniting it, and determining explosion and non-explosion from the rate of pressure increase expressed by equation (2). In the determination, the following determination criteria are used.

Pressure increase rate = (pressure after ignition) / (test pressure) × 100 [%] (2)
(Criteria)
Over 10% Explosion 10% Explosion limit Less than 10% No explosion, that is, the lower explosive limit concentration is the combustible substance concentration when the rate of pressure increase is 10%. If the combustible substance concentration with a pressure increase rate of 10% is not directly determined by measurement in the pressurized flash point measurement, the pressure increase rate is over 1% and less than 10% and over 10% and less than 50%. It can be determined by an interpolation method from at least two points, preferably at least two points of a pressure increase rate of more than 5% and less than 10% and more than 10% and less than 20%. It can also be determined by extrapolation from at least two measurement points in a pressure increase rate of more than 10% and less than 50%, preferably in a range of pressure increase rate of more than 10% and less than 20%.

  In the present invention, explosion can be avoided by controlling the pressure in the gas phase in the reactor so that the combustible substance concentration is less than the lower explosion limit concentration. Specifically, the lower limit of the pressure in the gas phase where the combustible substance concentration is less than the lower explosion limit concentration in the oxygen concentration during production is estimated in advance, and the pressure in the gas phase in the reactor exceeds the pressure. Thus, it is possible to adopt a method of manufacturing while managing.

  In the present invention, a gas having an oxygen concentration less than the flammability limit oxygen concentration is supplied into the reactor at the start-up of operation, and the concentration of the combustible substance existing in the gas phase portion in the reactor is the oxygen at the time of production. It is preferable to set the pressure inside the reactor to the oxygen concentration at the time of production after the pressure becomes less than the lower explosion limit concentration. The gas supplied to the reactor first is an inert gas such as nitrogen, carbon dioxide or argon, or a mixed gas obtained by diluting an oxygen-containing gas such as air with an inert gas to a concentration lower than the flammability limit oxygen concentration. Can be used. It is preferable to use an inert gas because of its availability. If the pressure in the gas phase part in the reactor is the atmospheric pressure and the oxygen concentration at the time of production is reached, the gas enters the explosion region of the combustible substance existing in the gas phase part in the reactor. Therefore, using a gas having an oxygen concentration less than the flammable limit oxygen concentration, the concentration of the combustible substance present in the gas phase portion in the reactor is supplied to a pressure that is less than the lower explosion limit concentration in the oxygen concentration at the time of production, After that, by making the oxygen concentration in the gas phase in the reactor the oxygen concentration at the time of production, it is possible to avoid passing through the explosion area of combustible substances at the start-up of the operation, and the operation can be started up safely. .

  In the present invention, it is preferable to set the pressure in the gas phase in the reactor to atmospheric pressure after the oxygen concentration in the gas phase in the reactor is less than the flammability limit oxygen concentration when the operation is shut down. As a method of setting the oxygen concentration in the gas phase portion in the reactor to less than the flammability limit oxygen concentration, the oxygen concentration in the gas phase portion in the reactor is replaced with a gas having an oxygen concentration less than the flammability limit oxygen concentration. It can be performed by a method such as supplying an inert gas until the concentration is less than the concentration. It is preferable to replace with a gas having an oxygen concentration lower than the flammable limit oxygen concentration for ease of management. As the gas having an oxygen concentration less than the flammability limit oxygen concentration, an inert gas such as nitrogen, carbon dioxide or argon, or a mixed gas obtained by diluting an oxygen-containing gas such as air with an inert gas to a concentration less than the flammability limit oxygen concentration Etc. can be used. It is preferable to use an inert gas because of its availability. A method of replacing the gas phase in the reactor with an inert gas is more preferable. Since the gas phase part in the reactor at the time of production is normally in a pressurized state, if the pressure of the gas phase part in the reactor is lowered to atmospheric pressure as it is, flammability existing in the gas phase part in the reactor Enter the explosion area of the material. Therefore, after reducing the oxygen concentration in the gas phase part in the reactor to below the flammability limit oxygen concentration as described above, the pressure in the gas phase part of the reactor is changed to atmospheric pressure, so that it is flammable when the operation is shut down. Passing through the explosion area of the material can be avoided and the operation can be shut down safely.

  The present invention will be described in more detail with reference to the following examples. However, these examples show the outline of the present invention, and the present invention is not limited to these examples.

(Vapor pressure measurement 1)
0.5 liter explosion container (Co., Ltd.) equipped with stirrer, injection container, pressure transducer (9.8 MPa-abs), low-pressure pressure transducer (0.2 MPa-abs), and φ0.5 Pt wire heating wire ignition source After evacuating Mitsubishi Chemical Science and Technology Research Center), the original valve of the low-pressure pressure transducer was closed. As Sample 1, 20 mL of acetic acid (manufactured by Wako Pure Chemical Industries, Ltd.) was introduced into the explosion container via the injection container. The temperature was raised with a heater, and the explosion container and contents were sufficiently stabilized at 90 ° C. After stirring the inside of the explosion container for 10 minutes, the stirring was stopped and then allowed to stand for 10 minutes. After adjusting the zero point of the low pressure pressure transducer, the main valve was opened and the pressure indication (1) at that time was read and recorded. The pressure instruction (2) after evacuating the explosion container and the injection container was read and recorded. The difference between the pressure instruction (1) and the pressure instruction (2) was used as the vapor pressure of the sample 1 at the measurement temperature (90 ° C.). As a result, the vapor pressure of Sample 1 was 41 kPa-abs.

(Pressure flash point measurement 1)
After a 0.5 liter explosion vessel used for vapor pressure measurement was subjected to an airtight test under vacuum, the sample 1 was put into the explosion vessel via a 20 mL injection vessel. The temperature was raised with a heater, and the container and contents were sufficiently stabilized at 90 ° C. Air was transferred from the compressed air cylinder to the explosion container until a predetermined gas phase pressure was reached. After stirring for 10 minutes, the stirring was stopped, and then allowed to stand for 10 minutes. Then, the ignition source was activated and exploded, and pressure data at that time was recorded. Thereafter, the pressure flash point was measured in the same manner by varying the gas phase pressure in various ways. The results are shown in Table 1. In addition, the combustible substance density | concentration in a gaseous phase was computed by Formula (1), and the pressure increase rate was calculated | required from Formula (2). In addition, the presence or absence of an explosion is based on the criteria shown above.

この結果より、気相部圧力１．６ＭＰａ−ａｂｓを超える圧力では不爆であることが確認された。すなわち、気相部を空気で加圧したときの可燃性物質の爆発下限界濃度は２．６ｖｏｌ％である。

From this result, it was confirmed that there was no explosion at pressures exceeding the gas phase pressure of 1.6 MPa-abs. That is, the lower explosion limit concentration of the combustible substance when the gas phase part is pressurized with air is 2.6 vol%.

(Vapor pressure measurement 2)
The result of measuring the vapor pressure in the same manner as in the vapor pressure measurement 1 for the sample 2 which is a mixture of 15% by mass of methacrolein, 75% by mass of acetic acid and 10% by mass of water (containing 200 ppm of paramethoxyphenol as a polymerization inhibitor). The vapor pressure of sample 2 was 103 kPa-abs.

(Pressure flash point measurement 2)
For the sample 2, the pressurized flash point was measured in the same manner as the pressurized flash point measurement 1. The results are shown in Table 2.

この結果より、気相部圧力５．３ＭＰａ−ａｂｓを超える圧力では不爆であることが確認された。すなわち、気相部を空気で加圧したときの可燃性物質の爆発下限界濃度は２．０ｖｏｌ％である。

From this result, it was confirmed that there was no explosion at pressures exceeding the gas phase pressure of 5.3 MPa-abs. That is, the lower explosion limit concentration of the combustible substance when the gas phase part is pressurized with air is 2.0 vol%.

(Gas phase pressure setting)
From the results of the above vapor pressure measurement and pressurized flash point measurement, the lower explosive limit concentration (C _gLL [vol%]) of the combustible substance when the gas phase is pressurized with air and the vapor pressure of the sample (P _Vap [KPa-abs]) is in a relationship represented by Expression (3).

C _gLL = −0.00968P _Vap +2.997 (3)
Therefore, the concentration of methacrolein to be supplied, the average residence time, the amount of catalyst, etc. are reacted in advance to determine the composition of the reaction solution from the relationship between these factors and methacrolein conversion, and the reaction solution at 90 ° C. After obtaining the vapor pressure by actual measurement or estimation, the gas phase pressure is adjusted so that the concentration of the combustible substance existing in the gas phase determined by the equation (1) is less than the lower explosion limit concentration determined by the equation (3). It was set.

(Catalyst preparation)
1 part of commercially available palladium acetate was dissolved in 55 parts of 88% by mass n-valeric acid aqueous solution. This solution was transferred to an autoclave, 5 parts of activated carbon with a specific surface area of 790 m ² / g produced from coal raw material was added, the autoclave was sealed, and the gas phase part in the autoclave was replaced with nitrogen while stirring the liquid phase part. Thereafter, it was cooled to 5 to 10 ° C. Propylene was introduced until the pressure in the autoclave reached 0.5 MPa-gauge pressure, and then stirred at 50 ° C. for 1 hour. Then, stirring was stopped and the pressure in the reactor was released, and then the reaction solution was taken out. The precipitate was filtered off from the reaction solution obtained under a nitrogen stream to obtain a palladium metal supported catalyst. The palladium loading rate of this catalyst was 10% by mass.

<Example 1>
The reaction apparatus used was a stainless steel bubble column reactor (liquid capacity: 1.7 L) having an inner diameter of 41.2 mm and a height of 1,375 mm. The raw material methacrolein was supplied from the lower part of the reactor, and the reaction liquid was filtered with a cross flow filter (laminated sintered wire mesh filter; manufactured by Nichidai Co., Ltd.) while keeping the liquid surface of the liquid phase constant. It is structured to be pulled out of the system.

(Operation start-up and steady operation)
Into the reactor, 55 g of palladium metal supported catalyst and 88 mass% acetic acid aqueous solution were charged in advance so as to reach the control liquid level. Nitrogen gas was supplied to the reactor, and the supply was stopped when the gas phase pressure reached 3.2 MPa-gauge pressure. In this case, when the gas phase part is pressurized with air, the lower limit of the pressure at which the concentration of the combustible substance existing in the gas phase part is less than the lower explosion limit concentration is 1.7 MPa-gauge pressure (lower explosion limit concentration is 2.6 vol. %), A margin was set to 3.2 MPa-gauge pressure. The liquid phase temperature was raised to 90 ° C. and stabilized for about 10 minutes, and then a raw material solution prepared by adding 3 parts by mass of methacrolein to 100 parts by mass of an 88% by mass acetic acid aqueous solution was used. Continuously fed for 6 hours. Next, the reaction was started by continuously supplying air from the compressed air cylinder so that the superficial velocity was 2.7 cm / s while maintaining the gas phase pressure. After reacting in this state for 6 hours, the gas phase gas was sampled and the combustible concentration was measured. Gas analysis was performed using gas chromatography [device name: HP6850 Series (FID detector), column; CARBOWAX20M (manufactured by Quadrex), length 50 m, film thickness 3 μm, inner diameter 0.32 mm], column temperature 70 ° C., The test was carried out under conditions of an inlet temperature of 200 ° C. and a detector temperature of 250 ° C. As a result of analysis of the reaction solution, methacrolein conversion was 77% and methacrylic acid selectivity was 74%. The methacrolein concentration in the reaction solution was 0.7% by mass.

  The gas phase gas was blown out from the back pressure valve to atmospheric pressure, then led to a 20 L absorption tank filled with 15 L of water as an absorbing solution, subjected to absorption treatment, and then diffused into the atmosphere.

(End of driving)
At the end of operation, first, the supply of air was stopped, then nitrogen gas was supplied for 10 minutes, and the gas phase portion in the reactor was replaced with nitrogen gas. Next, cooling was started and when the liquid temperature reached 30 ° C., the reactor pressure was returned to atmospheric pressure.

  As described above, the pressure of the gas phase part in the reactor is set so that the concentration of the combustible substance existing in the gas phase part of the reactor performing the oxidation is less than the lower explosion limit concentration in the oxygen concentration at the time of production. By performing the liquid phase oxidation reaction while controlling, it was possible to operate safely in each process of startup, steady operation and shutdown.

<Example 2>
As the reaction apparatus, a jacketed stirred tank reactor having an inner diameter of 126 mm and a capacity of 4 liters was used. The raw material methacrolein is supplied from the upper part of the reactor, and the reaction liquid is structured to be extracted from the system after filtering the catalyst with a cross flow filter (same as in Example 1) while keeping the liquid surface part constant. ing.

(Operation start-up and steady operation)
Into the reactor, 176 g of a palladium metal supported catalyst and an 88% by mass acetic acid aqueous solution were charged in advance so as to reach the control liquid level (the liquid level was adjusted so that the liquid volume was 3 liters). Nitrogen gas was supplied from the top of the reactor, and the supply was stopped when the gas phase pressure reached 8.0 MPa-gauge pressure. In this case, when the gas phase part is pressurized with air, the lower limit of the pressure at which the combustible substance concentration present in the gas phase part is less than the lower explosion limit concentration is 5.4 MPa-gauge pressure (lower explosion limit concentration is 2.0 vol. %), The margin was set to 8.0 MPa-gauge pressure. The liquidus temperature was raised to 90 ° C. and stabilized for about 10 minutes, and then prepared by containing 22% by mass of methacrolein, 69% by mass of acetic acid, 9% by mass of water, and 200 ppm of paramethoxyphenol as a polymerization inhibitor. The raw material liquid was continuously supplied to the reactor at 0.91 liter / hr. At this time, the average residence time was 3.3 hours. Next, the reaction was started by continuously supplying air from a compressed air cylinder through a sparger at 0.7 kg / hr to the liquid phase part of the reactor while maintaining the gas phase part pressure. After reacting in this state for 6 hours, the gas phase gas was sampled and the combustible concentration was measured. As a result, it was 1.2 vol%, which was less than the lower explosion limit concentration. As a result of analyzing the reaction solution, the methacrolein conversion rate was 41% and the methacrylic acid selectivity was 61%. The methacrolein concentration in the reaction solution was 13.2% by mass. The gas phase gas was blown out from the back pressure valve to atmospheric pressure, and then treated in the same manner as in Example 1.

(End of driving)
At the end of the operation, the same operation as in Example 1 was performed, and the reactor pressure was returned to atmospheric pressure.

  As described above, the pressure of the gas phase part in the reactor is set so that the concentration of the combustible substance existing in the gas phase part of the reactor performing the oxidation is less than the lower explosion limit concentration in the oxygen concentration at the time of production. By performing the liquid phase oxidation reaction while controlling, it was possible to operate safely in each process of startup, steady operation and shutdown.

It is a figure which shows typically the relationship between the combustible substance density | concentration and oxygen concentration in the explosion area vicinity.

Claims (2)

The presence of a noble metal catalyst, alpha with molecular oxygen in a liquid phase, alpha by oxidizing β- unsaturated aldehydes, a method for producing a β- unsaturated carboxylic acid, at startup of the operation, the reaction for performing oxidation A gas having an oxygen concentration less than the flammable limit oxygen concentration is supplied to the reactor, and the pressure is such that the concentration of the flammable substance existing in the gas phase in the reactor is less than the lower explosion limit concentration in the oxygen concentration at the time of production. After that, the pressure of the gas phase portion in the reactor is controlled so that the concentration of the combustible substance existing in the gas phase portion of the reactor is less than the lower explosion limit concentration in the oxygen concentration at the time of manufacture. A method for producing an α, β-unsaturated carboxylic acid.   2. The pressure of the gas phase in the reactor is set to atmospheric pressure after the oxygen concentration in the gas phase in the reactor is made less than the flammability limit oxygen concentration when the operation is shut down. The manufacturing method of the alpha, beta-unsaturated carboxylic acid of description.

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