JP2001192377A - Method for producing maleic anhydride - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for safely and efficiently producing maleic anhydride in the method for producing the maleic anhydride comprising a reaction step for reacting a hydrocarbon with an oxygen-containing gas in the presence of a catalyst, a maleic anhydride-collecting step for collecting the maleic anhydride from the reacted gas, a hydrocarbon-recovering step for recovering the unreacted hydrocarbon from the remaining gas, and a recycling step for reusing the recovered hydrocarbon by returning the hydrocarbon to the reactor, while regulating the concentration of the maleic anhydride in the reaction gas so as to be >=2 vol.%. SOLUTION: This method for producing the maleic anhydride satisfies the conditions of D=C/(B+C), E=100A/(100-B-C), wherein, A (vol.%) is the oxygen concentration in the remaining gas after recovering the unreacted hydrocarbon in the hydrocarbon-recovering step; B (vol.%) is the concentration of the hydrocarbon; and C (vol.%) is the concentration of carbon monoxide, and 0<α<10 wherein, α=-10.51+51.22D-35.35D2-E.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing maleic anhydride by catalytically oxidizing hydrocarbons in the gas phase, and more particularly, to recovering unreacted raw material hydrocarbons in a reactor and recovering the recovered hydrocarbons. The present invention relates to a method for producing maleic anhydride by a recycling process, characterized in that recycled hydrocarbons are returned to a reactor and reused.

[0002]

BACKGROUND OF THE INVENTION It is well known to produce maleic anhydride by catalytic oxidation of hydrocarbons in the gas phase. Conventionally, the reaction was carried out using benzene and air as raw materials in the presence of a vanadium pentoxide-based catalyst. Recently, a method using a linear hydrocarbon having 4 carbon atoms, such as butane, butene, and butadiene, has been developed. Among them, a method in which a reaction is carried out using n-butane which is a saturated hydrocarbon as a raw material in the presence of a catalyst containing a vanadium-phosphorus-based composite oxide as an active component has become mainstream. As an active component of this catalyst, divanadyl pyrophosphate ((VO) ₂ P ₂ O ₇ ) is known to exhibit excellent performance, and many documents relating to this compound have been published (for example, Chem). Rev. 88,
p. 55-80 (1988)).

[0003] The reaction is carried out in a fluidized bed system or a fixed bed system, and a reactor is mixed with a raw material hydrocarbon and an oxygen-containing gas, usually air, so that the hydrocarbon concentration of the raw material is about 1.5 to 10%. And react at 300-600 ° C. In the reaction gas flowing out of the reactor, in addition to maleic anhydride,
Contains carbon monoxide, carbon dioxide, water, and other reaction products. Separation and recovery of maleic anhydride from the reaction gas can be performed by cooling the reaction gas to condense maleic anhydride, contacting the reaction gas with water, and collecting maleic anhydride as maleic acid in water. The method is carried out by, for example, a method in which a gas is brought into contact with an organic solvent such as an alkyl ester of phthalic acid or an alkyl ester of hydrogenated phthalic acid and collected in the organic solvent.

[0004] In the production of maleic anhydride, which is currently carried out commercially, the conversion rate of the starting hydrocarbons in the reactor [moles of hydrocarbons consumed in the reaction / moles of hydrocarbons supplied to the reactor × 100 (mol%)] is kept as high as possible. This is a requirement to minimize the amount of raw hydrocarbons required to produce maleic anhydride. Normally, hydrocarbons not reacted in the reactor are incinerated in a waste gas combustion unit.

On the other hand, by lowering the conversion of hydrocarbons, the rate of formation of by-produced carbon monoxide and carbon dioxide is reduced, and the selectivity of maleic anhydride [the number of moles of maleic anhydride formed in the reaction is reduced. / Mol number of hydrocarbons consumed in the reaction × 100 (mol%)] is known to be improved. Utilizing this property, the conversion of hydrocarbons is kept low,
In addition, a recycling process has been proposed in which unreacted raw material hydrocarbons are collected and reused in a reactor.

That is, JP-A-49-81314, JP-A-54-151910, or JP-A-59-296.
In No. 79 etc., a process is proposed in which the selectivity of maleic anhydride is kept high by lowering the conversion of hydrocarbons in the reactor, and at least a part of unreacted hydrocarbons is recovered and returned to the reactor. are doing. However, the reaction conditions that are economically advantageous are significantly different from the operating conditions in the conventional one-pass reaction, so that the explosion safety of the supply gas to the reactor, the product recovery device or the hydrocarbon recovery device, and the exhaust gas from them Gender criteria are also very different. However, although these patents mention the safety of gas supplied to the reactor, they do not mention the safety of the entire recycling process.

In Japanese Patent Application Laid-Open No. 1-165564, in a process of returning unreacted hydrocarbons recovered by a device for selectively separating hydrocarbons to a reactor, a mixture of hydrocarbons and an oxygen-containing gas forms an explosive mixture. Proposals have been made in which the content of the combustion suppressant is regulated so as not to generate it. However, this stipulates the safety of the flow from the reactor through the hydrocarbon recovery unit to the reactor again, and is not sufficient when considering the safety of the entire maleic anhydride production process. That is, it is practically difficult to completely recover hydrocarbons using a hydrocarbon recovery device, and the remaining gas from which hydrocarbons have been recovered is also mixed with a combustible gas such as hydrocarbons or carbon monoxide and oxygen. Since it becomes a gas, the explosion safety of the mixed gas must be considered.

[0008] Particularly high economical, high productivity,
Specifically, the concentration of maleic anhydride in the reaction gas is 2 vol.
%, The concentration of carbon monoxide in the reaction gas also becomes higher than the conventional condition, and therefore, the composition of the remaining gas after the recovery of hydrocarbons is also lower than that of the conventionally known safety. Although it was assumed that the carbon concentration would increase and the explosion range would be wider than before, no specific method for managing explosion safety was known at all.

[0009]

DISCLOSURE OF THE INVENTION The present invention provides a method for producing maleic anhydride by reacting a hydrocarbon and an oxygen-containing gas in the presence of a catalyst, and reacting the produced maleic anhydride from the gas after separation and recovery. Separates and recovers raw hydrocarbons that did not react in the reactor, and recovers hydrocarbons in the hydrocarbon recovery step in a method for producing maleic anhydride by a recycling process in which recovered hydrocarbons are returned to the reactor and reused The purpose is to provide the necessary conditions to ensure the safety of the remaining gas.

[0010]

Means for Solving the Problems In order to achieve the above object, the present inventors have focused on the oxygen concentration, hydrocarbon concentration and carbon monoxide concentration in the remaining gas obtained by recovering hydrocarbons in the hydrocarbon recovery step. The present invention was completed by repeating gas explosion experiments.

That is, according to the present invention, a reaction step of reacting a hydrocarbon with an oxygen-containing gas in the presence of a catalyst, a maleic anhydride recovery step of recovering maleic anhydride from a reaction gas, and an unreacted hydrocarbon from the remaining gas Wherein the maleic anhydride concentration in the reaction gas is 2 vol% or more, and the hydrocarbon recovery step includes a step of recovering the hydrocarbons, and a recycling step of returning the recovered hydrocarbons to the reactor for reuse. The oxygen concentration in the remaining gas from which the unreacted hydrocarbon was recovered in the recovery step was A (vol%), the hydrocarbon concentration was B (vol%), and the carbon monoxide concentration was C (vol%).
%), The following conditions D = C / (B + C), E =
100A / (100-B-C ), α = -10.51 + 51.22D-35.35D 2 -E
Where 0 <α <10, a method for producing maleic anhydride is provided.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. The present invention provides, as described above, a reaction step of reacting a hydrocarbon and an oxygen-containing gas in the presence of a catalyst, a maleic anhydride recovery step of separating and recovering maleic anhydride from a reaction gas, and an unreacted hydrocarbon from the remaining gas. A method for producing maleic anhydride, comprising: a hydrocarbon recovery step of recovering at least a portion of the maleic anhydride including a recycling step of returning the recovered hydrocarbons to the reactor and reusing the remaining hydrocarbons. This is a method for producing maleic anhydride, wherein the oxygen concentration, hydrocarbon concentration and carbon monoxide concentration in the gas satisfy specific conditions.

In the method of the present invention, a reaction gas containing maleic anhydride is generated in a reaction step of reacting a hydrocarbon and an oxygen-containing gas in the presence of a catalyst (reaction step). In this reaction step, as the hydrocarbon used as a raw material,
Hydrocarbons having 4 carbon atoms such as butane, butene, and butadiene are preferred, and among them, n-butane which is a saturated hydrocarbon having 4 carbon atoms is particularly preferred. As the oxygen-containing gas, air is usually used, but air diluted with an inert gas,
Air or the like enriched with oxygen can also be used.

The catalyst to be used is preferably a catalyst containing a composite oxide containing vanadium and phosphorus as main constituents (hereinafter sometimes abbreviated as "vanadium-phosphorus composite oxide catalyst"). . These catalysts are commonly used and known per se. For example, US Pat. Nos. 4,520,127 and 4,472,527
And JP-A-7-68179.

The reactor used may be a commonly used fixed bed reactor or a fluidized bed reactor known per se. However, in the reaction conditions for achieving high productivity, it is preferable to use a fluidized-bed reactor that does not have a problem of explosion safety of a gas supplied to the reactor or a risk of generating a high-temperature portion called a hot spot due to reaction heat. . The fluidized bed reactor used in this case has a gas dispersion plate at the bottom part that defines the lower end of the catalyst fluidized bed, a raw material gas supply port in the lower region of the catalyst fluidized bed, and fine particles such as cyclones and filters. A conventional recovery device may be provided at the top of the reactor or at the outlet of the reactor. Further, an indirect heat exchange device for removing heat of the reaction product gas at a position where a catalyst fluidized bed is to be formed, for example, a device having a heat removal coil is suitable.

The optimum reaction temperature varies depending on the composition of the gas supplied to the reactor, the flow rate of the supplied gas, the reaction pressure, the amount of the catalyst, or the performance of the catalyst, but is usually about 330 to 500 ° C., preferably about 360 to 460 ° C. The range is appropriate. The reaction product gas contains various concentrations of unreacted oxygen and raw material hydrocarbons, as well as by-produced carbon dioxide, water, carbon monoxide, and the like, in addition to maleic anhydride, which is a target substance. Maleic anhydride is separated and recovered from the reaction product gas (maleic anhydride recovery step).

The separation and recovery of maleic anhydride can be carried out by a commonly used method known per se, for example, a method of condensing maleic anhydride by cooling the reaction gas, or a method of contacting the reaction gas with water to convert maleic anhydride to maleene anhydride. A method of trapping in water as an acid, the reaction gas is contacted with an organic solvent such as dialkyl phthalate or an alkyl ester of hydrogenated phthalic acid such as tetrahydrophthalic acid or hexahydrophthalic acid, and collected in an organic solvent. A method or the like can be used.

[0018] At least part, preferably most, of the raw hydrocarbons not reacted in the reactor is recovered from the remaining gas after the maleic anhydride is separated and recovered from the gas flowing out of the reactor (hydrocarbon recovery). Step), the hydrocarbon gas recovered in this hydrocarbon recovery step is returned to the reactor and reused (recycle step). At this time, each of the oxygen-containing gas, the hydrocarbon and the other gas is supplied to the reactor or the reactor so that the total supply gas amount to the reactor and the oxygen concentration and the hydrocarbon concentration in the total supply gas are maintained at predetermined values. Supplied anywhere during the recycling process.

In the hydrocarbon recovery step, a part of the remaining gas after separating and recovering maleic anhydride from the reaction product gas is extracted as a discharge stream, and the remaining is recovered, returned to the reactor and reused. At this time, if gases other than the raw material hydrocarbons are recovered, their components are accumulated by recycling, and as a result, the amount of air that can be used as an oxygen source to be supplied to the reactor is significantly restricted, and expensive pure A problem arises in that the amount of oxygen used increases.

This problem can be solved by using a method for selectively separating and recovering hydrocarbons as a method for recovering raw hydrocarbons. The term selective here means that most of the hydrocarbons (about 90% or more) are recovered, but about half or more of the other gases are not recovered. Such selective separation and recovery of hydrocarbons makes it possible to maximize the use of inexpensive air as a source of oxygen to be supplied to the reactor.

As a device for selectively separating and recovering hydrocarbons, there are generally used membrane separation devices known per se; PSA (pressure swing adsorption device), VSA
An adsorption-separation type device such as (vacuum swing adsorption device) or TSA (temperature swing adsorption device) can be used. When an adsorption-separation type apparatus is used, an adsorbent capable of selectively separating and recovering hydrocarbons, for example, zeolites described in JP-A-8-325256, U.S. Pat. No. 4,987,239, etc. A method using an adsorbent such as silicalite can be employed.

In such a recycling process, it is important to efficiently produce maleic anhydride by minimizing the loss of hydrocarbons, and the recovery rate of hydrocarbons [the number of moles of hydrocarbons recovered by the recovery apparatus] / The number of moles of hydrocarbon supplied to the recovery device × 100 (%)] is 90% or more, preferably 95% or more, and more preferably 98% or more.

Separation of maleic anhydride from the reaction product gas
It is also possible to return a part of the remaining gas after recovery directly to the reactor without passing through a device for separating and recovering hydrocarbons. In this case, a gas such as carbon dioxide, carbon monoxide or nitrogen coexisting in addition to the hydrocarbon is also circulated to the reactor, and the hydrocarbon separation / recovery device can be downsized.

In the above method for producing maleic anhydride, the unreacted hydrocarbon gas is returned to the reactor and reused.
In order to produce maleic anhydride more economically than before, the points to focus on are how to minimize the consumption of raw material hydrocarbons, and the reaction process-maleic anhydride recovery process-hydrocarbon recovery process-recycling. It is how to keep the amount of gas circulating in the process small.

As a means for achieving this, the conversion in the reactor is controlled to 80% or less, preferably 60% or less, more preferably 50% or less, and the selectivity of maleic anhydride is improved to improve the carbonization of the raw material. At the same time as suppressing the consumption of hydrogen, the concentration of hydrocarbons and oxygen in the whole gas to be subjected to the reaction is increased more than before, and the concentration of maleic anhydride in the reaction product gas is 2.0 vol% or more, preferably 2.5 vol.
%, More preferably at least 3.0 vol%, it is effective to reduce the amount of gas circulating in the recycling process.

In a recycling process satisfying such conditions, maleic anhydride is separated from the reaction product gas.
From the remaining gas after recovery, the remaining gas from which unreacted hydrocarbons have been recovered in the hydrocarbon recovery step is a mixed gas containing carbon monoxide, hydrocarbons, oxygen, carbon dioxide, and nitrogen. When hydrocarbons are selectively separated and recovered under preferable conditions, the combustible gas in the remaining gas contains carbon monoxide as a main component and a trace amount of hydrocarbons.

In general, the explosion safety of a mixed gas containing oxygen and a plurality of combustible gas components is determined by the oxygen concentration, the concentration of each combustible gas component, and the temperature and pressure. However, a method of accurately predicting, for example, the explosion safety of a mixed gas of butane and carbon monoxide has not been known so far. On the other hand, the composition of the supply gas to the hydrocarbon recovery process fluctuates according to the change in the reaction conditions or the reaction results, and further, the remaining gas obtained by recovering the hydrocarbons in the hydrocarbon recovery process due to the change in the hydrocarbon recovery conditions or the recovery results. May vary. In particular, when an adsorption-separation-type hydrocarbon recovery device is used, the composition of the exhaust gas from the device will periodically fluctuate.
It is essential to confirm by some means that the gas composition is within a safe composition range.

Generally, as a method for ensuring the explosion safety of a mixed gas of an oxygen-containing gas and a combustible gas, a method of keeping the concentration of a combustible gas component below the lower explosive limit concentration, a method of keeping the concentration of a combustible gas component above the upper explosion limit concentration. And a method of keeping the oxygen concentration below the limit oxygen concentration.

The method of keeping the concentration of the combustible gas component below the lower explosive limit concentration has been conventionally used in the maleic anhydride production process. That is, the lower explosive limit concentration of the mixed gas is L which is widely known to those skilled in the art.
e Since it can be predicted by using the Chateller equation or the like (for example, see “Revised Safety Engineering Handbook” edited by the Safety Engineering Association, Corona, P439 (1980)), the concentrations of hydrocarbons and carbon monoxide are monitored. And Le Chate
The lower explosive limit concentration of the composition is calculated using the Lier equation or the like, and by comparing the two, a constant safety margin can always be secured. This method has not only been used to ensure the safety of residual gas obtained by recovering maleic anhydride from the reaction gas, but has also been applied to the safety of the remaining gas obtained by recovering hydrocarbons from the hydrocarbon recovery process in the recycling process. Can be performed depending on the conditions.

However, as a result of the study by the present inventors, the concentration of combustible gas (the sum of the concentration of hydrocarbons and the concentration of carbon monoxide) in the remaining gas from which hydrocarbons have been recovered in the hydrocarbon recovery process is often lower than the lower explosive limit concentration. It has been found that it can be high. In particular, under the condition where the concentration of maleic anhydride in the reaction gas is high, under the condition of high productivity where the concentration of maleic anhydride is 2 vol% or more, the method of keeping the concentration of flammable gas below the lower explosive limit concentration is practically It becomes difficult. This is because the concentration of carbon monoxide by-produced in the reaction step also increases, and a small amount of unrecovered hydrocarbons is mixed into the mixture, so that the flammable gas concentration becomes equal to the lower explosive limit concentration. In addition to this, if there is a change in the composition over time, especially a periodic or unexpected change in the hydrocarbon concentration in the adsorption separation type recovery apparatus, in order to ensure a sufficient safety margin at all times, the reaction process must be performed. By keeping the concentration of maleic anhydride low, it is necessary to reduce the concentration of by-produced carbon monoxide, or to recover hydrocarbons at an extremely high recovery rate in the hydrocarbon recovery step. At the same time, it is necessary to constantly monitor the gas composition accurately and without delay in the measurement time.

The remaining gas obtained by recovering hydrocarbons in the hydrocarbon recovery step has a very high explosive limit concentration since carbon monoxide is a main component as a combustible gas, and keeps the combustible gas concentration at or above the explosive limit concentration. It is practically difficult.

On the other hand, the method of keeping the oxygen concentration in the residual gas from which the hydrocarbons are recovered in the hydrocarbon recovery step at or below the limit oxygen concentration is a sufficient safety management method even under such high conditions of maleic anhydride productivity. It turned out to be usable.

The oxygen concentration in the gas can be controlled by controlling the conditions of the reaction step and controlling the oxygen concentration at the outlet of the reactor in the same manner as in the prior art, even under the reaction conditions with high productivity of maleic anhydride. It is possible to maintain the oxygen concentration below the limit oxygen concentration easily.

The limit oxygen concentration is also somewhat affected by changes in the flammable gas composition, but the effect is small compared to the change in the lower explosive limit, and the target to be directly monitored is not the flammable gas concentration with large fluctuations. Even in an adsorption-separation type apparatus, the composition is small, and the oxygen concentration is easy to control.

Thus, the present inventors have determined by experiments the limit oxygen concentration of a mixed gas of carbon monoxide and hydrocarbon, which has not been known hitherto, and manages explosion safety based on this. Reached. That is, in the present invention, the oxygen concentration in the remaining gas from which unreacted hydrocarbons are recovered in the hydrocarbon recovery step is A (vol%),
%), And when the concentration of carbon monoxide is C (vol%), the following conditions must always be satisfied in order to ensure the safety of the gas.

The safety coefficient α is a numerical value calculated by the following equation (1) α = −10.51 + 51.22D−35.35D ² −E (1), where D is the following equation (2) D = C / (B + C) indicates a numerical value calculated by (2), E indicates a numerical value calculated by the following equation (3) E = 100A / (100−BC) (3), and 0 <α <10. To meet.

The safety coefficient α is a coefficient relating to the limiting oxygen concentration of the oxygen-containing combustible gas mixture. A larger value indicates that the current composition is farther from the limiting oxygen concentration and the safety is higher. Therefore, it can be said that it is safe if the safety coefficient α shows a positive value, but it is preferable to have a certain margin in consideration of an analysis error of each component and a measurement time delay. On the other hand, in order to increase the safety coefficient α, it is not realistic to take an excessive margin because it is necessary to sacrifice productivity, the life of the catalyst, or the operating cost.
Therefore, it is desirable to maintain a preferable range of 0 <α <10, more preferably, a range of 1 <α <5.

The above equation (1) is an empirical equation obtained from explosion experiments under various conditions. The prerequisite temperature and pressure are as follows: temperature is from normal temperature to 100 ° C., and pressure is from normal pressure to 0.1 MPa. Gauge, range. Further, the value of D in the above equation (2) in which this equation is substantially effective is 0.7 ≦ D ≦ 1.0 from the range in which the explosion experiment was performed.

In the hydrocarbon recovery step, the oxygen concentration in the remaining gas obtained by recovering the hydrocarbons not reacted in the reaction step is continuously measured using a commonly known oxygen analysis means such as an on-line oxygen meter. It is preferable to measure. It is preferable that the concentration of hydrocarbons and the concentration of carbon monoxide in the gas are continuously measured using a commonly used online monitoring means such as an infrared analyzer which is known per se. In addition, these gas compositions can be measured using a discontinuous measuring means such as a gas chromatograph. However, when the composition of the gas fluctuates over time, a preferable means for safety management is used. It can not be said.

In particular, when an adsorption / separation device is used as the hydrocarbon recovery device, the composition of the remaining gas after the recovery of the hydrocarbon fluctuates periodically. Therefore, an analyzer capable of continuously measuring each concentration is used. , The safety coefficient α is calculated in real time, and it is desirable to confirm by calculation that the safety coefficient α is within a predetermined range.

When the composition change of the gas is not large, for example, the hydrocarbon concentration and the carbon monoxide concentration use the latest values obtained by a discontinuous measuring means such as a gas chromatograph, and the oxygen concentration is used for the oxygen concentration. It is also possible to calculate the safety coefficient α by using continuous measurement values obtained from an online oxygen meter, but in this case, a large value is used as the safety coefficient α in anticipation of a change in the limiting oxygen concentration due to a composition change. It is desirable to secure

As a method of managing explosion safety using this safety coefficient α, when the calculated safety coefficient α is likely to fall below a predetermined safety margin, the operating condition of the reactor is changed to change the oxygen concentration at the outlet of the reactor. Reduce the oxygen concentration by changing the operating conditions of the hydrocarbon separation device,
Alternatively, the safety coefficient α can be increased by taking a method such as lowering the oxygen concentration by adding an inert gas such as nitrogen to the hydrocarbon separator supply stream. Methods for reducing the oxygen concentration at the reactor outlet include reducing the supply of oxygen-containing gas to the reactor, increasing the supply of hydrocarbons to the reactor, and increasing the reactor volume, as is well known to those skilled in the art. The reaction temperature can be increased.

On the other hand, when the safety coefficient α is too large than the predetermined margin, the safety coefficient α is reduced to a desired range by performing the operation completely opposite to the above method in consideration of the economics of the process. It is also possible. Needless to say, the safety coefficient α can be used not only for the discharge stream from the hydrocarbon recovery device, but also for the explosion safety management of a mixed gas of carbon monoxide and hydrocarbon having the same composition. For example, the exhaust gas from the maleic anhydride recovery step in this process is also a mixed gas containing carbon monoxide and hydrocarbons as the main flammable gas, and is safe if the temperature, pressure, and composition are within the applicable range. Safety management can be performed by the coefficient α.

[0044]

EXAMPLES The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to the following examples. Example 1 A mixed gas of n-butane, carbon monoxide, oxygen and nitrogen having the composition shown in Table 1 was prepared and introduced into a 1-liter preheated explosion vessel. The explosion container was ignited by a 15 kV alternating current spark (0.01 second, spark gap: 3 mm), and the presence or absence of an explosion was determined based on the pressure increase in the container.
The temperature of the explosion vessel was 80 ° C. Table 1 shows the results.

[0045]

[Table 1]

Atmosphere oxygen at which no explosion is observed when the total combustible gas concentration [the total amount of the n-butane concentration and the carbon monoxide concentration (vol%)] is changed from below the assumed lower explosive lower limit concentration to above. The maximum value of the concentration [oxygen concentration ÷ (oxygen concentration + nitrogen concentration) × 100 (vol%)] is the limit oxygen concentration of the mixed gas having the carbon monoxide / butane ratio. Therefore, from the results in Table 1, each carbon monoxide /
The critical oxygen concentration of the mixed gas having the butane ratio is determined as shown in Table 2.

[0047]

Table 2 CO / BTA (vol / vol) limit oxygen concentration (vol%) 95/5 6.4 87/13 7.3 70/30 8.1

Example 2 An example of the embodiment of the present invention is calculated and shown below. Here, the material balance for producing 1,000 kg of maleic anhydride per hour was determined. As an example of the embodiment of the present invention, the concentration of butane in the total gas supplied to the reactor is set to 15 vol.
1%, oxygen concentration 25 vol%, butane conversion rate 3
The composition and flow rate of each part of the recycling process were determined by setting the selectivity of maleic anhydride to 63 mol% and 5%. For the recovery of unreacted butane, the recovery rate of PSA (pressure swing adsorption device) (butane 95%, carbon dioxide 84%, other 20%) described in US Pat. No. 4,987,239 was employed. In this case, the gas composition or the amount of gas generated at each part was as shown in Table 3.

[0049]

[Table 3]

[0050]

From the composition of the remaining gas from which the hydrocarbon was recovered in the hydrocarbon recovery step shown in Table 3, the safety factor α was calculated to be 0.1.
It becomes 32. That is, it can be seen that this composition has an atmospheric oxygen concentration lower than the limit oxygen concentration and is outside the explosion range. By the way, the lower explosive limit concentration of the gas is LeCh
Calculated by the atelier equation, it is about 6.6 vol%, and the total combustible gas concentration of 10.8 vol% is higher than the lower explosive limit concentration. Therefore, it is clear that in order to manage the explosion safety of this composition, it is necessary to adopt the method of the present invention in which the atmospheric oxygen concentration is kept below the limiting oxygen concentration.

[0052]

According to the method of the present invention, maleic anhydride is produced by gas-phase oxidation of hydrocarbons in the presence of a catalyst, and maleic anhydride is involved in a recycling process for recovering and reusing unreacted hydrocarbons. In the method for producing an acid, it is possible to provide a safe and efficient method for producing maleic anhydride by eliminating the danger of explosion of residual gas from which unreacted hydrocarbons have been recovered in the hydrocarbon recovery step.

Claims (6)

[Claims] 1. A reaction step of reacting a hydrocarbon with an oxygen-containing gas in the presence of a catalyst, a maleic anhydride recovery step of recovering maleic anhydride from a reaction gas, and a hydrocarbon recovering unreacted hydrocarbon from the remaining gas In the method for producing maleic anhydride including a recovery step and a recycling step of returning and recovering the recovered hydrocarbons to the reactor, the concentration of maleic anhydride in the reaction gas is 2 vol% or more, and unreacted in the hydrocarbon recovery step. A (vol%) is the oxygen concentration in the remaining gas from which the hydrocarbons are recovered, B (vol%) is the hydrocarbon concentration,
Assuming that the concentration of carbon monoxide is C (vol%), the following conditions D = C / (B + C), E = 100 A / (100−B−)
C), α = -10.51 + 51.22D−35.35D ² −E
A method for producing maleic anhydride, wherein 0 <α <10 is satisfied. 2. The method according to claim 1, wherein the reaction between the hydrocarbon and the oxygen-containing gas is carried out using a fluidized-bed reactor. 3. The method according to claim 1, wherein the hydrocarbon is a saturated hydrocarbon having 4 carbon atoms. 4. The method according to claim 1, wherein the catalyst is a phosphorus-vanadium-based catalyst. 5. The method according to claim 1, wherein hydrocarbons are separated and recovered from the remaining gas by a hydrocarbon recovery device and returned to the reactor. 6. The method according to claim 5, wherein the hydrocarbon recovery device is a membrane separation device, a pressure swing adsorption device, a vacuum swing adsorption device or a temperature swing adsorption device.