JPH07107016B2 - Method for producing acrylic acid - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing acrylic acid from propylene by catalytic gas phase oxidation reaction. More specifically, the present invention comprises a so-called front-rear coupling reaction consisting of a first-stage reaction in which propylene is subjected to catalytic gas-phase oxidation to mainly produce acrolein and a second-stage reaction in which this acrolein is converted to acrylic acid. A method for producing acrylic acid from propylene in a smooth and industrially stable manner, by which the catalyst layer clogging trouble due to the clogging substance contained in the first-stage reaction product gas is prevented.

<Prior Art> In the case of producing acrylic acid from propylene by a catalytic gas phase oxidation method, propylene is once catalytically gas phase oxidized to be converted into acrolein (hereinafter, this reaction is referred to as “pre-stage reaction”,
The catalyst used for this is referred to as "pre-stage catalyst", and then this acrolein is catalytically vapor-phase oxidized to convert it to acrylic acid (hereinafter, this reaction is referred to as "post-stage reaction", and the catalyst used for this is "post-stage catalyst"). A so-called two-step oxidation reaction, which is called a "catalyst", is generally adopted. The catalyst used for the first-stage catalyst is generally a multi-element catalyst oxide containing molybdenum, bismuth and iron. However, when propylene is catalytically vapor-phase oxidized using these catalysts, the main product, acrolein, is used. Besides, a compound having a relatively high boiling point such as maleic acid or terephthalic acid is by-produced, and at the same time, a polymer or a tar-like substance is contained in the produced gas. When the reaction gas containing such a substance is directly subjected to the second-stage reaction, these substances cause blockage in the pipe or in the second-stage catalyst-packed bed, resulting in an increase in pressure loss, a decrease in catalytic activity, and a decrease in acrylic acid. This may cause a decrease in selectivity. Such troubles often occur when the amount of propylene supplied is increased in order to increase the productivity of acrylic acid, that is, when the air volume is increased or the propylene concentration is increased.

Thus, as a method generally adopted to prevent this trouble, the reaction is periodically stopped, and the inert gas filled in the gas inlet side of the latter-stage catalyst to prevent clogging in the catalyst layer and decrease in the activity of the catalyst. A substance, such as a ceramic ball, is extracted and replaced, or acrolein is once separated from the gas produced by the first-stage reaction, and the separated acrolein is supplied again to the second-stage reaction to adopt an optimized oxidation reaction process. A method has been proposed in which the gas concentration is diluted more than necessary and the concentration of by-products is lowered to carry out the reaction. However, all of these methods are unsatisfactory because they are complicated and expensive as industrial methods. Also in both the first-stage reaction and the second-stage reaction, as the reaction gas composition, it is common to recycle the waste gas from the reaction gas as an inert gas in order to suppress the excessive oxidation by reducing the oxygen content as much as possible. A method has been proposed in which the shape of the catalyst used in the second-stage reaction is specified to increase the porosity between the catalysts to suppress clogging of solid matter from the first-stage reactor (Japanese Patent Laid-Open No. 61-221149). Such measures to prevent clogging troubles on the gas inlet side of the post-reactor are becoming more and more important issues as the development of the post-catalyst progresses in the future and it is becoming possible to lower the reaction temperature and increase the load. It's starting.

<Problems to be Solved by the Present Invention> In the present invention, when acrylic acid is produced from propylene by a catalytic gas phase oxidation reaction consisting of a first-stage reaction and a second-stage reaction, a rod-shaped or A plate-shaped insert is provided to prevent clogging of the second-stage catalyst layer due to by-products contained in the first-stage reaction product gas, and to provide a method capable of industrially smoothly and stably carrying out the reaction. It is a thing.

<Means> That is, the present invention is specified as follows.

In the first reactor filled with a catalyst oxide containing bismuth, molybdenum and iron, propylene is catalytically vapor-phase oxidized to mainly produce acrolein, and then the reaction product gas obtained is directly supplied from the first reactor. Separated,
When the catalyst was supplied to a heat conversion type multi-tubular second reactor filled with a catalyst oxide containing molybdenum and vanadium, which was directly connected by a pipe, and when acrolein was catalytically gas-phase oxidized to produce acrylic acid, 2. A two-stage oxidation method, characterized in that a rod-shaped or plate-shaped insert is inserted into the gas inlet space of the catalyst packed tube of the two reactors, and the in-tube porosity of the insert is set to 40 to 99%. Method for producing acrylic acid.

Here, the porosity is defined as follows.

Hereinafter, the present invention will be described more specifically.

When the corresponding acrylic acid is produced from propylene by a catalytic gas phase oxidation reaction, a so-called two-step oxidation method is often adopted. Also in this two-stage oxidation method, acrolein is separated from a mixed gas mainly containing acrolein generated in the front catalyst layer and is supplied to the second catalyst layer, and is converted into acrylic acid by the catalytic oxidation method, and acrolein is not separated. There is a method of directly supplying the latter-stage catalyst layer to acrylic acid by a catalytic gas phase method, and in these methods, the combustion gas of the reaction exhaust gas after recovering the active ingredient is circulated or unreacted acrolein is further used. There is a method in which it is mixed with propylene for reuse because it is recycled. By the way, when a multi-component catalyst containing bismuth, molybdenum and iron is adopted as the pre-stage catalyst, formation of a high boiling point compound such as maleic acid or terephthalic acid, a polymer, or a tar-like substance is unavoidable. Further, a polymer, a tar-like substance, or a fume-like solid substance may be produced from the reaction product thermally in the pipe or by collision with the pipe wall. Therefore, when acrylic acid is produced by the above-mentioned process, such a polymer or by-product is circulated in the apparatus in the form of gas or solid fine particles, or fumes, and is not introduced into the reactor. In particular, there are many opportunities to be entrained in the latter-stage reactor. Under such circumstances, the lower the temperature of the mixed gas introduced into the latter-stage reactor, the more easily the by-products such as high-boiling compounds are in a solid state, and the second-stage catalyst inlet side is fixed or adhered to cause clogging. easy.

Furthermore, as the concentration of propylene increases, the amount of by-products such as high-boiling compounds and tar-like substances in the reaction product gas increases accordingly, so that also in this case clogging troubles are likely to occur. As a method of avoiding such a clogging trouble, in addition to the above method, a method of raising the gas temperature before entering the post-reactor, or providing a preheating layer part up to the catalyst inlet part in the post-reactor can be considered. However, each of these measures has its own problems in the usual way. Even if the temperature of the gas supplied to the latter catalyst layer is raised, it is necessary to suppress the autoxidation of acrolein generated in the former catalyst layer, and there is a limit because it is necessary to avoid the explosion range. . If the gas preheating layer portion is provided in an empty cylinder state before entering the catalyst layer, the layer length of the empty cylinder portion needs to be lengthened to some extent, and the reactor needs to be enlarged correspondingly. Also, when an inert carrier is put in the preheating layer portion, there is a possibility that clogging may occur in this portion. Under the circumstances, the inventors of the present invention have studied the method of solving the blocking trouble, and have obtained the following findings. That is, when acrylic acid is produced by the catalytic gas-phase oxidation reaction of propylene, if the two-stage oxidation method is adopted, the product gas in the preceding reactor is usually produced in order to prevent auto-oxidation and to avoid the combustion range. Although the gas is sufficiently cooled, as described above, if the gas temperature is excessively cooled, the by-product becomes solid or fume, and if it is left as it is, it causes blockage at the catalyst layer inlet side of the subsequent reactor. In order to avoid this, the gas temperature is made harmless by raising the gas velocity as quickly as possible and raising the gas temperature as quickly as possible to facilitate overreaction of the by-product from the first-stage reaction on the second-stage catalyst. As a result of further study from the viewpoint that it will be possible to pass through the latter catalyst layer and it is possible to avoid clogging in the latter catalyst layer, the clogging trouble is unavoidable with inert carrier filling or preheating in the latter catalyst layer. Although the avoidance is insufficient, surprisingly, by simply inserting a metal or ceramic rod-shaped or plate-shaped insert into the space on the inlet side of the downstream catalyst, the preheating effect of the downstream reaction gas appears and clogging is avoided. It has been made possible that the reaction can be continued stably for a long period of time, and that the autoxidation of acrolein can be suppressed in the process of temperature rise, and the present invention has been completed.

In this case, the shape of the insert may be linear, zigzag, or twisted when it is rod-shaped. The rod-shaped insert may be cylindrical or cylindrical. Further, in the case of a plate shape, it may be a zigzag shape, a zigzag shape, or a twist shape. The plate shape may be a perfect plate or may be made of wire mesh. Depending on the shape of the insert, the maximum porosity may vary even in the space, and the porosity is suitable for rod-shaped inserts.
It is 40% or more and 99.0% or less, and preferably 50% or more and 99.0% or less for a plate-shaped insert. By adopting this range, the clogging of the insert layer due to the solid matter is prevented, the preheating effect is achieved, and the reaction in the post-catalyst layer can be smoothly carried out. The material for these inserts is preferably a metal having a high thermal conductivity, such as iron, nickel, aluminum and alloys, particularly stainless steel, but may be a metal that has been subjected to surface chemical treatment for rust prevention. As for ceramics,
For example, zirconia or alumina may be used in a sheet form.

Before clarifying the effect of the present invention by Examples, as a preliminary experiment, an experiment of the gas preheating effect by the insert specified in the present invention was conducted. Molten salt was used as a heat source using a steel pipe having an inner diameter of 30 mm and a wall thickness of 2 mm. Then, air preheated to 255 ° C. was used as the gas supplied thereto. The air flow here was 1.9 m ³ per hour in the standard condition. When the temperature of the molten salt is 290 ° C, it is necessary to have a length of approximately 800 to 900 mm in the hollow cylinder to preheat the air temperature to 280 ° C, but with a width of 18 mm made of stainless steel it is zigzag at an angle of about 90 degrees. It was found that 300 mm was sufficient as the insertion length required to raise the temperature up to 280 ° C when a bent plate was inserted into the plate (porosity 98%).

The advantages of the method according to the present invention are that there is no need to forcibly raise the temperature of the gas introduced into the post-reactor, and there is no risk of autooxidation or explosion, and a catalyst that is highly active even at low temperatures can be used as the post-catalyst. , It is not necessary to forcibly lengthen the hollow cylinder part to provide a gas preheating layer, do not need to withdraw the inert carrier due to blockage at the inlet of the latter catalyst layer, and even if propylene has a high concentration. It was found that clogging due to high boiling point compounds and the like could be avoided, and that the yield did not decrease, and continuous operation of acrylic acid production became possible under industrially advantageous conditions.

As the reaction conditions for carrying out the present invention, first, propylene is 1 to 15% by volume, molecular oxygen is 1 to 20% by volume, steam is 1 to 30% by volume, and a raw material gas containing an inert gas such as nitrogen or carbon dioxide is used. Reaction temperature (reactor heat medium temperature) 200-4
It is supplied to a bismuth-molybdenum-iron-containing multi-component pre-stage catalyst which is mainly converted into acrolein at 50 ° C and a space velocity of 300 to 10,000 hr ^-1 (STP). Secondary air and water vapor are added to the front-stage outlet gas as necessary, and the temperature of this mixed gas is preferably maintained at 150 ° C or higher to prevent clogging in the piping and to not enter the auto-oxidation or explosion range. It is supplied to the post-catalyst by controlling the temperature.

Next, the contents of the present invention will be made clearer by showing concrete examples.

Example 1 Preparation of first-stage catalyst 10.3 kg of ammonium molybdate while heating 15 water,
3.2 kg of ammonium paratungstate was added and stirred vigorously. (This is referred to as liquid A). Separately cobalt nitrate 6.
Water made acidic by adding 8 kg to 2 of water, 2.4 kg of ferric nitrate to 2 of water, and 2.9 kg of bismuth nitrate to 0.6 of concentrated nitric acid 3
To the above solution A was added dropwise to the above solution. Then, in terms of silicon dioxide
2.4 kg of silica sol containing 20% by weight and 20.2 g of potassium hydroxide dissolved in 1.5 of water are added, and the suspension thus formed is heated and evaporated and then molded, and the mixture is air-circulated at 450 ° C for 6 hours. The catalyst was prepared by calcining. The composition of elements other than oxygen in this catalyst is Co ₄ Fe ₁ Bi ₁ W ₂ M in atomic ratio.
It was o ₁₀ Si _1.35 K _0.06 .

Preparation of second-stage catalyst While stirring water 60 under heating, 1.3 kg of ammonium paratungstate, 1.1 kg of ammonium metavanadate,
Ammonium molybdate (4.3 g) and ammonium dichromate (150 g) were mixed and dissolved, and copper nitrate (1.1 kg) was added separately.
Was dissolved in 0.72 of water to prepare an aqueous solution, and both solutions were mixed. The mixed solution thus obtained is put into a stainless steel evaporator with a steam heater, the carrier substrate is made of α-alumina, the surface area is 1 m ² / g or less, the porosity is 42%, and the pore diameter is 75 to 250 microns. Volume occupied by pores is 92% of total pore volume
A granular carrier 12 having a diameter of 3 to 5 mm, which occupies 3 to 5 mm, was added to the carrier by evaporation to dryness with stirring to adhere to the carrier, and then calcined at 400 ° C. for 5 hours to prepare a catalyst. The composition of elements other than oxygen excluding the carrier of this catalyst is Mo ₁₂ V _4.6 Cu _2.2 Cr _0.6 W in atomic ratio.
It was _2.4 .

Reaction method The pre-stage catalyst 12.25 is composed of 10 steel reaction tubes having an inner diameter of 25 mm and a length of 3,000 mm, and the shell side is uniformly filled in a multi-tube reactor capable of heat exchange by circulating a molten salt, 33
Heated to 0 ° C.

Separately, the latter catalyst 21.8 is composed of 10 steel reaction tubes having an inner diameter of 25 mm and a length of 7,000 mm, and the shell side is uniformly filled in a multi-tubular reactor capable of heat exchange by circulating a molten salt. Heated to ° C.

The two reactors are connected by a conduit equipped with a nozzle for addition of molecular oxygen-containing gas and steam and equipped with a heat exchanger,
The reaction product gas emitted from the reactor containing the first-stage catalyst was introduced into the reactor containing the second-stage catalyst. At this time, the gas temperature until entering the second-stage reaction tube in the second-stage reactor was maintained at 220 ° C. Furthermore, the upper part of the catalyst layer of the latter stage reaction tube (reaction gas inlet side)
Is a 0.4 mm thick, 17 mm wide SUS304 plate bent in a zigzag pattern at a pitch of 35 mm at an angle of approximately 90 degrees, and a 300 mm long metal plate is placed 200 mm from the reaction tube inlet to the upper end of the post-catalyst. I inserted it to ride. At this time, the porosity of the metal plate insertion portion was 98%.

A mixed gas consisting of 9% by volume of propylene, 76% by volume of air, and 15% by volume of steam was fed to a reactor containing a pre-stage catalyst at 19,600 Nl / hr.
In addition, 2,700 Nl / hr of air is added from the molecular oxygen-containing gas introduction nozzle of the conduit connecting the reactor and the second reactor, and 2,940 Nl / hr of steam is added from the steam introduction nozzle. And the subsequent reaction was carried out. At this time, the pressure difference between the inlet and outlet of the latter stage reactor was 270 mmHg. The reaction was continued for 4,000 hours. During this period, it was necessary to raise the molten salt temperatures of the front and rear reactors by 10 ° C and 5 ° C, respectively.

Table 1 shows the reaction results at the start of the reaction and after 4,000 hours.

Here, the propylene conversion rate was calculated from the amount of propylene consumed at the inlet and outlet of the first-stage reactor, and the acrylic acid single-flow yield was the amount of acrylic acid produced at the outlet of the second-stage reactor and that of the first-stage reactor. The ratio with propylene fed to the reactor is shown.

Comparative Example 1 In Example 1, the post-catalyst was packed as in Example 1 without inserting an insert into the post-catalyst inlet side of the post-reactor so that the upper surface of the catalyst was located 500 mm from the reactor inlet side. Otherwise, the reaction was carried out according to the method of Example 1. The starting performance of this reaction is shown in Table 1. This reaction was continued for a long time, but the pressure between the inlet and outlet of the latter stage reactor was 390 after about 3,000 hours.
It rose to mmHg (Comparative Example 1-1). When the reaction was stopped and the post-reactor was inspected, it was found that the catalyst layer on the post-catalyst inlet side was blocked with a polymer or the like. Also, in order to avoid this blockage, the length of the empty cylinder part on the inlet side of the latter catalyst layer is set to 1,50
It was set to 0 mm. The results are shown in Table 1 at the start of the reaction and after the reaction was continued for 4,000 hours. The reaction method was otherwise the same as in Example 1 (Comparative Example 1-2.). In this case, as is apparent from the table, the low yield was due to the length of the hollow portion and the occurrence of autoxidation. Because of the increase (carbon monoxide and acetic acid increase), and the pressure loss increased slightly after 4,000 hours.

Example 2 An insert having a structure in which a metal plate having the same material and dimensions as used in Example 1 was wound in a spiral shape was used. The total length of this insert was 300 mm (porosity 97.5%). The reaction method was according to Example 1. The results of this reaction are shown in Table 1.

Example 3 Instead of the insert used in Example 1, the material is stainless steel.
A 300 mm long insert was used in which a cylindrical gold rod having an outer diameter of 5 mm was 304 bent in a zigzag shape at an angle pitch of approximately 90 degrees to approximately 35 mm. The porosity at this time was almost 96%. The reaction method was according to Example 1. The results are shown in Table 1.

Comparative Example 2 In Example 3, the outer diameter of the cylindrical rod was made thin so that the porosity of the insertion portion in the reaction tube was 99.3%. The rod-like insertion and reaction were according to Example 3 (Comparative Example 2-1). The reaction results are shown in Table 1. The preheating effect was not sufficient and the pressure loss increased with the reaction.

On the other hand, in Example 3, the outer diameter of the cylindrical rod was increased so that the porosity was 36%. The rod-like insertion and reaction method were in accordance with Example 3 (Comparative Example 2-2). The results are shown in Table 1. As a result, the pressure loss increased at the beginning of the reaction due to the insertion of the thick rod, but the pressure loss increased as the reaction continued. When the reaction was stopped and the latter-stage reactor was inspected, a part where solid matter was deposited and blocked between the insert and the inner wall of the reaction tube was observed.

Comparative Example 3 Instead of the insert of Example 1, a reactor having a wall thickness of 0.4 mm and a Raschig ring made of stainless steel of 10 mmφ × 10 mmL was filled in the latter stage reactor. The filling method is 200 mm in the empty cylinder on the reactor inlet side.
And a layer height of the above-mentioned Raschig ring insert was 300 mm below that, and a post-stage catalyst was filled thereunder. The porosity of the Raschig ring packed layer was 91%. The reaction method was according to Example 1. The results are shown in Table 1.

Along with the reaction, the pressure loss at the inlet and outlet of the latter reactor increased.
After 3,000 hours, the reaction was stopped and the latter-stage reactor was inspected, and it was found that a considerable amount of solid matter such as a polymer was deposited in the Raschig ring packed bed in a blocked state. It was found that the plugged state greatly depends on the shape of the filling material even though the porosity is large.

Example 4 A sheet made of alumina having a thickness of 0.4 mm, a width of 17 mm and a length of 300 mm was inserted according to the method of Example 1.

At this time, the porosity at the alumina sheet insertion portion was 98.3%. The reaction method and the catalyst filling method were the same as in Example 1, and the results shown in Table 1 were obtained.

Claims (1)

[Claims] 1. Propylene is catalytically gas-phase oxidized in a first reactor filled with a catalyst oxide containing bismuth, molybdenum and iron to mainly produce acrolein,
Then, the reaction product gas obtained as it is was separated from the first reactor and directly supplied to a heat exchange type multi-tubular second reactor filled with a catalyst oxide containing molybdenum and vanadium, which was directly connected by a pipe. Therefore, when acrolein is catalytically vapor-phase oxidized to produce acrylic acid, a rod-shaped or plate-shaped insert is inserted into the gas inlet space of the catalyst filling tube of the second reactor, and the porosity in the tube is 40 to 40 A method for producing acrylic acid by a two-step oxidation method, characterized by comprising 99%.