JP2010077087A - Method for producing acrylic acid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing acrylic acid, which can be suitably used for suppressing formation and deposition of solids stemming from impurities or the like contained in a reaction feedstock gas to enable the pressure rise in the reaction tubes to be suppressed. <P>SOLUTION: The method for producing acrylic acid includes the following process: when acrolein contained in a reaction feedstock gas is subjected to a vapor-phase catalytic oxidation reaction using a multitubular fixed bed reactor, a catalytically inert packing material layer including an inert packing material(s) (e.g. ceramics based on silica and/or alumina) is formed on the upstream side of the vapor-phase oxidative catalyst layer; and the temperature at the boundary between the vapor-phase oxidative catalyst layer and the inert packing material layer is set within the temperature range between the set temperature of a heat transfer medium (e.g. a molten salt) being in contact with the outer side of the reaction tubes and a temperature lower than the set temperature by 20°C (preferably, lower than the set temperature by 5-15°C). <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing acrylic acid. More specifically, the present invention relates to the production of a compound resulting from impurities contained in the raw material gas and by-products produced during the reaction during the production of acrylic acid, and the compound adhered to the packed catalyst and the inner wall of the reaction tube, Since it is possible to suppress the accumulation and solidification, the increase in pressure loss in the reaction tube and the increase in temperature can be suppressed, the activity of the catalyst does not decrease and the reaction can be continued easily. The present invention relates to a method for producing acrylic acid having a high yield.

  Generated due to impurities contained in the raw material gas or by-products generated in the first stage propylene oxidation reaction in a method of producing acrylic acid by propylene as a starting material by a two-stage gas phase catalytic oxidation reaction It is known that a compound is deposited and solidified in a gas phase oxidation catalyst layer at the second-stage reaction gas inlet. Then, due to the adhesion and deposition of this solid matter, the pressure loss of the reaction tube increases, the reaction yield decreases, and in some cases, the reaction tube may be blocked, making it difficult to continue the reaction. It may interfere with yield maintenance.

  In order to suppress the occurrence of the above-mentioned problems, the solid organic substances and carbides deposited on the gas passages in the reactor including the catalyst layer by the continuation of the reaction are treated with air containing steam. By doing so, a method of removing a solid organic substance that removes part or all of the same is known (see, for example, Patent Document 1). Further, there is known a gas phase catalytic oxidation method in which a treatment agent is arranged for removing solid organic substances and carbides, and the treatment agent is exchanged at least once a year (for example, see Patent Document 2).

Japanese Patent Application Laid-Open No. 6-263689 JP 2008-24644 A

  However, in the removal method described in Patent Document 1, since the reaction is stopped and the treatment is performed at a high temperature for a long time, there is a concern that the performance and life of the catalyst may be adversely affected. There is a problem that the effect of processing is not constant. Further, according to the gas phase catalytic oxidation method described in Patent Document 2, there is an advantage that the adverse effect on the catalyst performance due to the high temperature treatment can be avoided, but at least once a year, preferably twice a year. It is necessary to replace the entire part or the entire amount. When refilling with a new treatment agent, there is a considerable cost. When using a recycled treatment agent, heat treatment and cleaning with a solvent are required. There are problems such as requiring complicated processing.

  The present invention has been made in view of the above-described problems of the prior art, and at the time of acrylic acid production, the compounds contained in the raw material gas, the generation of compounds due to by-products generated during the reaction, and the like were filled. Since the compound adheres to and deposits on the catalyst and the inner wall of the reaction tube, and the generation of solidified organic substances and / or carbides thereof can be suppressed, and the increase in pressure loss in the reaction tube can be suppressed. It is an object of the present invention to provide a method for producing acrylic acid which does not decrease in activity, can be easily continued, and has a high yield.

The present invention is as follows.
1. In the method for producing acrylic acid in which acrolein contained in the reaction raw material gas is subjected to gas phase catalytic oxidation reaction using a multitubular fixed bed reactor, the inertness having no catalytic activity upstream of the gas phase oxidation catalyst layer An inert packing layer formed by packing a packing is formed, and the temperature of the boundary between the gas phase oxidation catalyst layer and the inert packing layer is set to a heat medium in contact with the outside of the reaction tube. A method for producing acrylic acid, characterized in that the temperature ranges from a temperature to a temperature 20 ° C. below the set temperature.
2. The above-mentioned 1. in which the superficial velocity of the reaction tube is 0.6 to 2.5 m / sec and the actual velocity is 1.0 to 6.0 m / sec. A method for producing acrylic acid as described in 1. above.
3. Propylene is oxidized in the first stage reactor to produce the acrolein, and then the outlet gas from the first stage reactor is introduced into the second stage reactor as the reaction raw material gas, and the second stage reactor The above-mentioned 1. forming the inert packing layer at the inlet of the reaction raw material gas. Or 2. A method for producing acrylic acid as described in 1. above.
4). The above-mentioned 1. using a ceramic mainly composed of silica and / or alumina as the inert filler. To 3. The manufacturing method of acrylic acid of any one of these.

According to the method for producing acrylic acid of the present invention, when acrylic acid is produced by subjecting acrolein contained in a reaction raw material gas to a gas phase catalytic oxidation reaction, an inert filler is filled on the inlet side of the reaction raw material gas. The temperature of the boundary portion between the inert packing layer and the gas phase oxidation catalyst layer is determined from the set temperature of the heat medium in contact with the outside of the reaction tube. The reaction source gas, impurities contained in the reaction source gas, or by-product produced by the gas phase catalytic oxidation reaction, and the catalyst. It is possible to sufficiently prevent the compound from adhering to, depositing and solidifying on the particles, reaction tube wall and the like. Therefore, an increase in pressure loss or the like can be suppressed, the reaction can be easily continued, and acrylic acid can be produced with a high yield.
Further, when the superficial velocity of the reaction tube is 0.6 to 2.5 m / sec and the actual velocity is 1.0 to 6.0 m / sec, formation of a compound and adhesion to a catalyst, Accumulation and solidification can be prevented more sufficiently, and even when adhered, the adhered matter is easily peeled off, and an increase in pressure loss is suppressed.
Further, acrolein is generated by oxidizing propylene in the first stage reactor, and then the outlet gas from the first stage reactor is introduced into the second stage reactor as a reaction source gas, and the reaction source gas of the second stage reactor is introduced. When an inert packing layer is formed at the inlet of the reactor, the reaction raw material gas containing impurities generated in the first stage reactor is introduced into the second stage reactor. However, in the method of the present invention, adhesion, deposition, and solidification of a compound etc. on the catalyst, etc. An increase or the like can be sufficiently suppressed, the continuation of the reaction is easy, and acrylic acid can be produced in a high yield.
In addition, when a ceramic mainly composed of silica and / or alumina is used as an inert filler, adhesion, deposition, and solidification of a compound or the like to a catalyst or the like can be more sufficiently suppressed, and the reaction is continued. Is easier and acrylic acid can be produced in higher yields.

The present invention will be described in detail below.
The method for producing acrylic acid according to the present invention is a method for producing acrylic acid in which acrolein contained in a reaction raw material gas is subjected to a gas phase catalytic oxidation reaction using a multi-tube fixed bed reactor. Forming an inert packing layer filled with an inert packing having no catalytic activity on the side, and setting the temperature at the boundary between the gas phase oxidation catalyst layer and the inert packing layer to the outside of the reaction tube The temperature range is from the set temperature of the contacting heat medium to a temperature 20 ° C. below this set temperature.

  The “multi-tubular fixed bed reactor” is not particularly limited, and a normal reactor used for production of acrylic acid can be used. In the gas phase catalytic oxidation reaction using this reactor, a raw material gas containing acrolein was introduced into the reactor (reaction tube), and acrolein was subjected to a gas phase catalytic oxidation reaction to produce acrylic acid, and then produced. Acrylic acid is recovered, and other components, that is, a by-product gas containing nitrogen as a main component and containing oxygen, unreacted propylene, and the like (hereinafter abbreviated as off-gas) is detoxified by a predetermined method, A so-called single flow method for discharging out of the system may be used, or a so-called off-gas recycle method in which a part or all of the off-gas described above is reused as a reaction raw material gas. Also, the structure of the reactor is not particularly limited, and for example, a so-called tandem reactor in which an independent first-stage reactor and a second-stage reactor are connected by piping can be employed.

  The reaction conditions are not particularly limited, and general reaction conditions when propylene is oxidized to acrylic acid by a two-stage gas phase catalytic oxidation reaction can be employed. For example, the raw material gas is propylene 1 to 12% by volume, preferably 4 to 10% by volume, molecular oxygen 3 to 20% by volume, preferably 4 to 18% by volume, water vapor 0 to 60% by volume, preferably 1.6. Acrolein is produced by circulating a mixed gas consisting of -50% by volume and 20-80% by volume, preferably 30-60% by volume of inert gas (nitrogen, carbon dioxide, etc.) through the first stage reaction tube. Let

After that, molecular oxygen, inert gas, etc. are added to the outlet gas as necessary, and the above-mentioned “reaction raw material gas” is supplied to the second stage reaction tube to oxidize acrolein contained in the reaction raw material gas and Acid can be produced. The reaction temperature can be 250 to 450 ° C., particularly 250 to 350 ° C., the reaction pressure can be 20 to 100 kPa, particularly 25 to 80 kPa, and the space velocity of the mixed gas and the reaction raw material gas (gas flow rate / filled catalyst only is applied Capacity) can be 300-5000 hr ⁻¹ .

  In the gas phase catalytic oxidation reaction of propylene for producing acrolein, a general catalyst used for this purpose can be used without any particular limitation. As this catalyst, a composite metal oxide containing molybdenum, bismuth, nickel and the like as essential components can be used. Examples of such a catalyst include a catalyst having a composition represented by the formula (1). It is done.

_{Mo 12 Bi a Ni b Co c} Fe d Y e Z f O g (1)
[In the above formula (1), Mo, Bi, Ni, Co, Fe and O represent molybdenum, bismuth, nickel, cobalt, iron and oxygen, respectively. Y is at least one element selected from the group consisting of tin, zinc, tungsten, chromium, manganese, magnesium, cerium, antimony and titanium, and Z is at least one selected from the group consisting of potassium, rubidium, thallium and cesium. Represents a seed element. Further, a, b, c, d, e, f, and g represent atomic ratios of the respective elements. With respect to the molybdenum atom 12, a is 0.1 ≦ a ≦ 7, and b + c is 0.5 ≦ b + c ≦. 20, d is 0.5 ≦ d ≦ 8, e is 0 ≦ e ≦ 2, and f is 0 ≦ f ≦ 1. G is a numerical value determined by the oxidation state of each element. ]

  The “gas phase oxidation catalyst layer” is formed by filling a reaction tube with a catalyst. As this catalyst, a general catalyst used for a gas phase catalytic oxidation reaction of acrolein can be used without any particular limitation. As this catalyst, a composite metal oxide containing molybdenum, vanadium, copper, antimony and the like as essential components can be used. As such a catalyst, for example, a catalyst having a composition represented by the formula (2) Is mentioned.

_{Mo 12 V h W i Cu j} Sb k X l Y m Z n O o (2)
[In the above formula (2), Mo, V, W, Cu, Sb and O represent molybdenum, vanadium, tungsten, copper, antimony and oxygen, respectively. X is at least one element selected from the group consisting of alkali metals and thallium, Y is at least one element selected from the group consisting of magnesium, calcium, strontium, barium and zinc, Z is niobium, cerium, tin Represents at least one element selected from the group consisting of chromium, manganese, iron, cobalt, samarium, germanium, titanium and arsenic. Furthermore, h, i, j, k, l, m, n, and o represent the atomic ratio of each element. For molybdenum atom 12, h is 0 <h ≦ 10, i is 0 ≦ i ≦ 10, j is 0 <j ≦ 6, k is 0 <k ≦ 10, l is 0 ≦ l ≦ 0.5, m is 0 ≦ m ≦ 1, and n is 0 ≦ n ≦ 6. Further, o is a numerical value determined by the oxidation state of each element. ]

  The catalyst is formed into a predetermined shape so that it can be easily used in a multitubular fixed bed reactor. The shape of the catalyst is not particularly limited, and may be any of a spherical shape, a flat shape with a circular cross section, a columnar shape, a cylindrical shape, a prismatic shape, and the like. Examples of the method for molding the catalyst include various methods such as tableting, extrusion, and granulation.

The “inert packing layer” is formed by filling the reaction tube with an inert packing. The “inert filler” is not particularly limited as long as it is a substance that has substantially no reaction activity with respect to acrolein and acrylic acid during the gas phase catalytic oxidation reaction. As the inert filler, oxides and composite oxides of various metal elements can be used. Examples of the oxide include alumina (α-alumina and the like, which may be an alundum which is an abrasive), silica, zirconia, silicon carbide (which may be a carborundum which is an abrasive) and the like. Examples of the composite oxide include silicon / aluminum composite oxide (mullite), silicon / titanium composite oxide, silicon / zirconium composite oxide, aluminum / titanium composite oxide, and aluminum / zirconium composite oxide. Furthermore, metals such as stainless steel and aluminum can be used as the inert filler. As the inert filler, oxides such as alumina, silica and zirconia, and composite oxides such as mullite are preferable, and inert fillers mainly composed of alumina and / or silica are particularly preferable. Only one inert filler may be used, or two or more inert fillers may be used in combination.
The above “main component” means that the total content of alumina and silica is 85% by mass or more when the total amount of the inert filler is 100% by mass.

  The shape of the inert filler is not particularly limited, and may be any of a spherical shape, a ring shape, a linear shape, a strip shape, and other shapes. However, considering the workability at the time of extraction in replacement of the inert filler, etc., it is spherical. It is preferable that Moreover, although the maximum dimension of an inert packing is based also on the internal diameter of the reaction tube used for a gaseous-phase catalytic oxidation reaction, it is preferable that it is 2-10 mm. Furthermore, the porosity of the inert filler layer [(volume of the space portion of the filler layer / volume of the entire filler layer) × 100] is preferably 30 to 95%.

  The inert packing layer is formed and arranged on the upstream side of the gas phase oxidation catalyst layer. The upstream side means the inlet side of the reaction tube into which the reaction raw material gas is introduced, and the inert packing layer is usually an upper part of the reaction tube erected in the vertical direction (the flow direction of the reaction raw material gas is It is formed and arranged at the lower part (when introduced so that the flow direction of the reaction raw material gas becomes upflow).

  In the present invention, the temperature at the boundary between the gas phase oxidation catalyst layer and the inert packing layer is a temperature range from the set temperature of the heat medium contacting the outside of the reaction tube to a temperature 20 ° C. below this set temperature. In this way, the gas phase catalytic oxidation reaction is continued. It is preferable that the temperature of this boundary part is a temperature range which falls below the set temperature of the heat medium by 2 to 18 ° C, particularly 5 to 15 ° C. By maintaining the temperature of the boundary portion so as to be in the temperature range from the set temperature of the heat medium to a temperature that is 20 ° C. below this set temperature, the inert filler layer is heated by the heat medium and heated up, The compounds contained in the reaction raw material gas are transferred to the downstream gas phase oxidation catalyst layer without adhering to, depositing, and solidifying on the inert packing, and from the set temperature of the heat medium due to the heat generated by the oxidation reaction. It is presumed that the gas phase oxidation catalyst layer at a high temperature burns and becomes a gas and is discharged from the reaction tube. Therefore, an increase in pressure loss in the reaction tube can be suppressed, a stable reaction can be easily continued over a long period of time, and acrylic acid can be produced with a high yield.

  The heat medium that contacts the outside of the reaction tube is not particularly limited as long as the inside of the reaction tube can be maintained at a predetermined reaction temperature. For example, when the reaction temperature is 250 to 450 ° C., the temperature of the heat medium is preferably in the same temperature range. Therefore, a molten salt having a wide use temperature range, a large heat capacity, and a relatively low viscosity is often used as the heat medium. Although this molten salt is not specifically limited, An alkali molten salt etc. can be used.

Furthermore, in this gas phase catalytic oxidation reaction, the superficial velocity in the reaction tube is preferably maintained at 0.6 to 2.5 m / sec. When the superficial velocity is less than 0.6 m / sec, a compound or the like may adhere to and deposit on the inert packing and solidify. On the other hand, when the superficial velocity exceeds 2.5 m / sec, the differential pressure of the catalyst layer increases and the yield of acrylic acid may decrease, which is not preferable. Moreover, it is more preferable that the superficial velocity is 0.6 to 2.5 m / sec and the actual velocity is 1.0 to 6.0 m / sec. If the superficial velocity and the actual velocity are within this range, the adhesion, deposition, and solidification of the compound or the like to the inert packing can be sufficiently suppressed, and the gas phase catalytic oxidation reaction can easily proceed, and the acrylic acid can be collected. The rate can be further improved.
Note that the superficial velocity and the actual velocity are defined as in Equation (3) and Equation (4), respectively.
Superficial velocity (m / sec) = Passing gas flow rate per reaction tube (Nm ³ / sec)
× (Set temperature of heat medium (° C.) + 273.15) /273.15
× 101.3 / (Reactor inlet pressure (kPaG) +101.3)
÷ Cross sectional area of catalyst per reaction tube (m ² ) (3)
Actual speed (m / sec) = superficial velocity (m / sec) ÷ porosity (%) x 100 (4)

Hereinafter, the present invention will be specifically described by way of examples.
In the following examples and comparative examples, “part” means part by mass, “%” means mass% unless otherwise specified, and propylene conversion rate and acrylic acid yield are the following formulas (5), It is defined as (6).
Propylene conversion rate (mol%) = [(mol number of reacted propylene) / (mol number of supplied propylene)] × 100 (5)
Acrylic acid yield (mol%) = [(mol number of generated acrylic acid) / (mol number of supplied propylene)] × 100 (6)

(1) Preparation of propylene oxidation catalyst (1) Aqueous solution (A) was prepared by dissolving 423.8 g of ammonium molybdate and 2.02 g of potassium nitrate while heating and stirring 3000 ml of distilled water. Separately, 302.7 g of cobalt nitrate, 162.9 g of nickel nitrate, and 145.4 g of ferric nitrate were dissolved in 1000 ml of distilled water to prepare an aqueous solution (B). Further, 164.9 g of bismuth nitrate was dissolved in 200 ml of distilled water acidified by adding 25 ml of concentrated nitric acid to prepare an aqueous solution (C). Then, aqueous solution (B) and (C) were mixed and this liquid mixture was dripped, stirring vigorously to aqueous solution (A).

  Next, the resulting suspension was dried using a spray dryer and pre-baked at 440 ° C. for 3 hours to prepare 570 g of pre-baked powder. Thereafter, 200 g of this pre-fired powder and 10 g of crystalline cellulose as a molding aid were mixed to obtain a mixture. Next, 300 g of an alumina carrier having an average particle size of 3.5 mm was put into a rolling granulator, and then the above mixture and 90 g of a 33% glycerin aqueous solution as a binder were added simultaneously to carry the mixture on the carrier. 40% active ingredient-supported particles were obtained. Next, the active component-carrying particles were dried at room temperature for 15 hours, and then calcined at 560 ° C. for 5 hours under air flow to prepare a propylene oxidation catalyst (1). The obtained catalyst had an average particle size of 4.0 mm, and the composition excluding oxygen as a catalytically active component had an atomic ratio of Mo; 12, Bi; 1.7, Ni; 2.8, Fe; 1.8. , Co; 5.2, K; 0.1.

(2) Preparation of propylene oxidation catalyst (2) 300 g of the pre-fired powder obtained in the preparation of the propylene oxidation catalyst (1) and 15 g of crystalline cellulose as a molding aid were mixed to obtain a mixture. Thereafter, 300 g of an alumina carrier having an average particle diameter of 3.5 mm was put into a rolling granulator, and then the above mixture and 135 g of 33% glycerin aqueous solution as a binder were added simultaneously to carry the mixture on the carrier. Active component-carrying particles with a rate of 50% were obtained. Thereafter, the active component-carrying particles were dried at room temperature for 15 hours, and then calcined at 520 ° C. for 5 hours under air flow to prepare a propylene oxidation catalyst (2). The average particle diameter of the obtained catalyst was 4.1 mm, and the composition excluding oxygen of the catalytically active component was Mo: 12, Bi; 1.7, Ni; 2.8, Fe; 1.8 in atomic ratio. , Co; 5.2, K; 0.1.

(3) Preparation of Acrolein Oxidation Catalyst 600 parts of deionized water at 95 ° C. and 16.26 parts of ammonium tungstate were added to a preparation tank (A) equipped with a stirring motor and stirred. Thereafter, 18.22 parts of ammonium metavanadate and 110 parts of ammonium molybdate were dissolved, and then 7.75 parts of antimony acetate was added. On the other hand, 15.56 parts of copper sulfate was dissolved in a preparation tank (B) charged with 96 parts of deionized water, and this solution was added to the preparation tank (A) to obtain a slurry solution. Subsequently, the amount of liquid feeding was adjusted so that the outlet temperature of a spray dryer might be about 100 degreeC, and the slurry solution was dried.

  Then, the obtained granule was accommodated in a heating furnace, the temperature of the heating furnace was increased from room temperature to 390 ° C. at about 60 ° C./hour, and pre-baked at 390 ° C. for about 5 hours. Next, this pre-fired granule was pulverized with a ball mill to obtain a pre-fired powder, and using a tumbling granulator, 36 parts of an alundum carrier having a porosity of 40 volume%, a water absorption of 19.8%, and an average particle diameter of 4 mm. Then, 12 parts of pre-fired powder was supported while 2.4 parts of a 20% aqueous solution of glycerin was sprayed. Thereafter, the temperature of the heating furnace was raised from room temperature to about 390 ° C. at about 70 ° C./hour, and the resulting molded product was calcined at 390 ° C. for 5 hours to prepare an acrolein oxidation catalyst. The composition of the obtained acrolein oxidation catalyst excluding oxygen as an active component was Mo: 12, V; 3, W; 1.2, Cu; 1.2, Sb; Further, the friability of this catalyst was 0.1% or less.

Example 1
A propylene oxidation catalyst (2) is filled to a height of 250 cm in a reaction tube (inner diameter 25 mm, length 460 mm) of a first stage reactor for propylene oxidation having an aftercooler attached to the outlet, and further propylene oxidation is performed thereon. Catalyst (1) was packed to a height of 100 cm. As a result, the total length of the first stage propylene oxidation catalyst layer is 350 cm. On the other hand, the reaction tube (inner diameter 27 mm, length 350 cm) of the second stage reactor for acrolein oxidation was filled with an acrolein oxidation catalyst at a height of 300 cm, and further a spherical ceramic packing was filled at a height of 30 cm. . As a result, the total filling length in the reaction tube of the second stage reactor is 330 cm.

After filling the catalyst and the like as described above, the first-stage reactor is connected so that the flow direction of the reaction raw material gas introduced into the second-stage reactor is downflow, and in terms of molar ratio, propylene / air / A raw material gas having a composition of water vapor = 1 / 8.1 (converted to oxygen becomes 1.7) / 3 was introduced and reacted under the condition of a space velocity of 1210 h ⁻¹ with respect to the second stage catalyst charge. The temperature of the heating medium in the first stage reaction bath at the start of the reaction was 335 ° C., and the temperature of the heating medium in the second stage reaction bath was 267 ° C. The temperature at the boundary between the gas phase oxidation catalyst layer and the inert packing layer in the reaction tube of the second-stage reactor is 259 to 262 ° C., and 5 to 8 ° C. from the set temperature (267 ° C.) of the heat medium. It was low. Further, the inlet pressure of the second stage reactor was 52 kPa, and the outlet pressure of the second stage reactor was 34 kPa.

  The reaction results when the reaction was carried out as described above were propylene conversion rate 97.4%, unreacted acrolein 1.05%, and acrylic acid yield 87.5%. The temperature of the heating medium in the first stage reaction bath after continuing the reaction for 16000 hours was 343 ° C., and the temperature of the heating medium in the second stage reaction bath was 277 ° C. Further, the temperature at the boundary between the gas phase oxidation catalyst layer and the inert packing layer in the reaction tube of the second stage reactor is 267 to 270 ° C., which is 7 to 10 ° C. from the set temperature (277 ° C.) of the heat medium. It was low. Furthermore, the inlet pressure of the second stage reactor was 61 kPa, and the outlet pressure of the second stage reactor was 34 kPa. As described above, even if the reaction was continued for a long time, the reaction temperature did not increase significantly and the pressure loss did not increase significantly. The reaction results after continuing the reaction for a long time were a propylene conversion of 98.4%, an unreacted acrolein of 0.55%, and an acrylic acid yield of 86.5%. As described above, even after the reaction was continued for a long time, a decrease in the reaction result was slight and a stable reaction could be continued.

Example 2
The reaction tube (inner diameter: 27 mm, length: 350 cm) of the reactor for acrolein oxidation was filled with a spherical ceramic filler at a height of 30 cm, and further, an acrolein oxidation catalyst was filled at a height of 300 cm. As a result, the total filling length in the reaction tube of the second stage reactor is 330 cm. Thereafter, the reaction was conducted in the same manner as in Example 1 except that the reaction raw material gas introduced into the acrolein oxidation reactor was connected so as to be up-flowed.

  In the above reaction, the temperature of the heating medium in the first stage reaction bath at the start of the reaction was 338 ° C., and the temperature of the heating medium in the second stage reaction bath was 270 ° C. The temperature at the boundary between the gas phase oxidation catalyst layer and the inert packing layer in the reaction tube of the second stage reactor is 260 to 264 ° C., and 6 to 10 ° C. from the set temperature (270 ° C.) of the heat medium. It was low. Furthermore, the inlet pressure of the second stage reactor was 50 kPa, and the outlet pressure of the second stage reactor was 34 kPa.

  The reaction results when the reaction was carried out as described above were propylene conversion 97.6%, unreacted acrolein 0.70%, and acrylic acid yield 87.7%. The temperature of the heating medium in the first stage reaction bath after continuing the reaction for 16000 hours was 344 ° C., and the temperature of the heating medium in the second stage reaction bath was 277 ° C. The temperature at the boundary between the gas phase oxidation catalyst layer and the inert packing layer in the reaction tube of the second stage reactor is 266 to 270 ° C., and 7 to 11 ° C. from the set temperature (277 ° C.) of the heat medium. It was low. Furthermore, the inlet pressure of the second stage reactor was 58 kPa, and the outlet pressure of the second stage reactor was 34 kPa. As described above, even if the reaction was continued for a long time, the reaction temperature did not increase significantly and the pressure loss did not increase significantly. The reaction results after the long-time reaction were as follows: propylene conversion 98.5%, unreacted acrolein 0.62%, acrylic acid yield 86.5%. As described above, even after the reaction was continued for a long time, a decrease in the reaction result was slight and a stable reaction could be continued.

  INDUSTRIAL APPLICABILITY The present invention can be used regardless of reaction conditions such as the form of a reactor, the type of catalyst, temperature, pressure, etc., when propylene is used as a starting material to produce acrylic acid by a two-stage oxidation reaction. .

Claims (4)

In a method for producing acrylic acid, in which acrolein contained in a reaction raw material gas is subjected to a gas phase catalytic oxidation reaction using a multitubular fixed bed reactor,
Forming an inert packing layer filled with an inert packing having no catalytic activity on the upstream side of the gas phase oxidation catalyst layer, and a boundary portion between the gas phase oxidation catalyst layer and the inert packing layer; The acrylic acid is produced in a temperature range from the set temperature of the heat medium in contact with the outside of the reaction tube to a temperature 20 ° C. below the set temperature.   The method for producing acrylic acid according to claim 1, wherein a superficial velocity in the reaction tube is 0.6 to 2.5 m / sec and an actual velocity is 1.0 to 6.0 m / sec.   Propylene is oxidized in the first stage reactor to produce the acrolein, and then the outlet gas from the first stage reactor is introduced into the second stage reactor as the reaction raw material gas, and the second stage reactor The method for producing acrylic acid according to claim 1 or 2, wherein the inert packing layer is formed at the inlet of the reaction raw material gas.   The method for producing acrylic acid according to any one of claims 1 to 3, wherein a ceramic mainly composed of silica and / or alumina is used as the inert filler.