JP2010222309A - Method for purifying acrylonitrile - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for purifying acrylonitrile, by which the acrylonitrile having low impurity concentration can be obtained in a high yield from a production gas obtained by the ammoxidation reaction of propane. <P>SOLUTION: This method for producing the acrylonitrile, comprising subjecting propane, ammonia and oxygen to an ammoxidation reaction in the presence of a catalyst and then purifying the produced acrylonitrile, is characterized by comprising (a) a process for introducing a gas containing the acrylonitrile into a quenching tower 1 to cool the gas, (b) a process for introducing the cooled gas into an absorbing tower 3 to make the gas into countercurrent-contact with absorbing water, (c) a process for introducing the bottom liquid obtained by countercurrent-contact with the absorbing water into a recovery tower 4 and adding extraction water, (d) a process for condensing and cooling distilled vapor, receiving the distilled and cooled product in a recovery tower decanter 6 to form an organic layer and a water layer, introducing the organic layer into a cyanic acid-removing and water-removing tower decanter 5, and distilling the organic layer, and (e) a process for extracting and cooling a side stream from the cyanic acid-removing and water-removing tower, receiving the cooled product in a cyanic acid-removing and water-removing tower decanter 7 to form an organic layer and a water layer, returning the organic layer to a position below the side stream-extracted position, and extracting the bottom liquid of the cyanic acid-removing and water-removing tower, wherein the temperature of the liquid in the cyanic acid-removing and water-removing tower decanter is controlled to 25 to 45°C. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for purifying acrylonitrile from a product gas obtained by an ammoxidation reaction of propane.

Acrylonitrile is a monomer used for intermediate materials such as synthetic resins such as ABS resin, acrylic fiber, synthetic rubber, acrylamide and the like. In 1960, a method for producing acrylonitrile all at once from propylene and ammonia by an ammoxidation reaction using a fluidized bed reactor was developed. Since then, fluidized bed catalysts and processes have been improved, and the yield of acrylonitrile per unit weight of raw material has been increased and the apparatus has been downsized. At present, the commercial production of acrylonitrile mainly consists of a process comprising a process for ammoxidation of propylene and a process for recovering and purifying acrylonitrile from the gas produced by the reaction.
Propylene ammoxidation reaction products include not only the target product acrylonitrile, but also by-products such as acetonitrile, hydrogen cyanide, acrylic acid, acrolein, acetic acid, oxazole, methacrylonitrile, propionitrile, etc. By separating, acrylonitrile is purified.
In Patent Document 1, in the process for producing acrylonitrile or methacrylonitrile, which is obtained by ammoxidation of propylene or isobutylene, respectively, after the reactor effluent is quenched, the crude acrylonitrile or methacrylonitrile is absorbed in water by an absorption column, After distilling the aqueous solution, the distillate from the distillation column is transferred to the first decanter to form an aqueous layer and an organic layer, and when the organic layer is further distilled, the side stream of the distillation column is changed to the second decanter. And a method comprising purifying acrylonitrile by distillation to form an aqueous layer and an organic layer, and distilling the organic layer. In this method, the internal temperature of the first decanter and the second decanter is maintained at about 32 ° F. to about 75 ° F. (about 0 ° C. to about 24 ° C.).
On the other hand, in recent years, research on methods for producing acrylonitrile using propane, which is easily available and cheaper than propylene, as a raw material has been actively conducted, and development of an ammoxidation catalyst for use in propane ammoxidation is progressing. Patent Document 2 focuses on a method for suppressing the formation of methacrylonitrile, an impurity in acrylonitrile, from the viewpoint of the propane ammoxidation process, and ammoxidizes propane having an i-butane concentration of 3 mol% or less in propane. It is disclosed for use in reactions.

JP-A-10-7639 JP 2003-327571 A

  The present inventors ammoxidized propane by a known propane ammoxidation catalyst and process, and recovered and purified acrylonitrile from the product gas. At this time, the propane ammoxidation product was recovered and purified. Problems have arisen when applying conventional methods. That is, when acrylonitrile is obtained from a reaction gas generated by carrying out a propane ammoxidation reaction in a fluidized bed reactor using a propane ammoxidation catalyst, a conventional method for recovering and purifying acrylonitrile for propylene ammoxidation is used. It has been found that the accuracy of the object cannot be sufficiently increased even if it is simply applied. Even when propane is used as a raw material, the quality of the product acrylonitrile is required to be as high as that derived from propylene. Therefore, in order to put propane ammoxidation into practical use, purification suitable for propane ammoxidation is required. There is an urgent need to significantly improve the accuracy of products by methods.

  In view of the above circumstances, the problem to be solved by the present invention is to purify acrylonitrile, which can obtain acrylonitrile with a low impurity concentration in a high yield when recovering acrylonitrile from a product gas obtained by ammoxidation of propane. Is to provide.

  As a result of intensive studies on a method for recovering and purifying acrylonitrile from a product gas obtained by ammoxidation of propane, the present inventors have found that a small amount of by-product is produced in the case of ammoxidation of propylene and the case of ammoxidation of propane. It has been found that there are differences and this affects the purity of acrylonitrile. Then, when purifying the gas containing acrylonitrile using a quenching tower, an absorption tower, a recovery tower, and a dehydrating acid dehydrating tower, the liquid temperature in the dehydrating acid dehydrating tower decanter into which the side flow of the dehydrating acid dehydrating tower is introduced is set. It has been found that the above problem can be solved by controlling the specific range.

That is, the present invention is as follows.
[1]
A method in which propane, ammonia and oxygen are subjected to an ammoxidation reaction in the presence of a catalyst, and acrylonitrile is purified from the produced gas containing acrylonitrile,
(A) introducing the gas containing acrylonitrile into a quenching tower and cooling it,
(B) introducing the gas cooled in the step (a) into the absorption tower and bringing it into countercurrent contact with the absorption water;
(C) introducing the tower bottom liquid obtained by countercurrent contact with the absorbed water in the step (b) into the recovery tower, adding extracted water and distilling;
(D) After condensing and cooling the distillate vapor obtained in the step (c), it is accommodated in a recovery tower decanter to form an organic layer and an aqueous layer, and the organic layer is introduced into a dehydrating acid dehydration tower, A distillation step,
(E) After extracting and cooling a side stream from the dehydration acid dehydration tower, it is accommodated in a deblue acid dehydration tower decanter to form an organic layer and an aqueous layer, and the organic layer is returned to the lower part from the side stream extraction position. And a step of extracting the bottom liquid of the dehydrating acid dehydration tower,
Including
A method for purifying acrylonitrile, wherein the liquid temperature in the debranching acid dehydration tower decanter is adjusted to 25 to 45 ° C.
[2]
The method for purifying acrylonitrile according to the above [1], wherein the liquid temperature in the recovery tower decanter is adjusted to 25 to 45 ° C.

  According to the purification method of the present invention, acrylonitrile having a low impurity concentration can be obtained in a high yield when purifying acrylonitrile from a product gas obtained by an ammoxidation reaction of propane.

It is the figure which showed roughly an example of the refiner | purifier used for the purification method of the acrylonitrile of this Embodiment.

  Hereinafter, a mode for carrying out the present invention (hereinafter referred to as “the present embodiment”) will be described in detail. The present invention is not limited to the following embodiment, and can be implemented with various modifications within the scope of the gist.

The purification method of acrylonitrile of the present embodiment is:
A method in which propane, ammonia and oxygen are subjected to an ammoxidation reaction in the presence of a catalyst, and acrylonitrile is purified from the produced gas containing acrylonitrile,
(A) introducing the gas containing acrylonitrile into a quenching tower and cooling it,
(B) introducing the gas cooled in the step (a) into the absorption tower and bringing it into countercurrent contact with the absorption water;
(C) introducing the tower bottom liquid obtained by countercurrent contact with the absorbed water in the step (b) into the recovery tower, adding extracted water and distilling;
(D) After condensing and cooling the distillate vapor obtained in the step (c), it is accommodated in a recovery tower decanter to form an organic layer and an aqueous layer, and the organic layer is introduced into a dehydrating acid dehydration tower, A distillation step,
(E) After extracting and cooling a side stream from the dehydration acid dehydration tower, it is accommodated in a deblue acid dehydration tower decanter to form an organic layer and an aqueous layer, and the organic layer is returned to the lower part from the side stream extraction position. And a step of extracting the bottom liquid of the dehydrating acid dehydration tower,
Including
It is the purification method of acrylonitrile which adjusts the liquid temperature in the said debrocyaning acid dehydration tower decanter to 25-45 degreeC.

  FIG. 1 schematically shows an example of a purification apparatus used in the acrylonitrile purification method of the present embodiment. Propane, ammonia and oxygen (air) are supplied to a fluidized bed reactor (not shown). The fluidized bed reactor has a raw material gas dispersion pipe and / or a dispersion plate in the lower part, is equipped with a heat removal pipe for removing reaction heat, and collects the catalyst in the reaction gas flowing out from the reactor in the upper part. Have a cyclone.

  The catalyst charged in the fluidized bed reactor is not particularly limited as long as it is an effective catalyst for ammoxidizing propane. Examples of the catalyst used when producing acrylonitrile from propane include, for example, Sb-V catalyst (GB1,366,135), Sb-UW catalyst (USP 3,670,006), Fe-U-Sb catalyst. Catalyst (USP 3,686,295), VP catalyst (Japanese Patent Publication No. 58-5188), Bi-V catalyst (Japanese Patent Laid-Open No. 63-295545), V-Sb catalyst and Fe-Sb catalyst Mixture (Japanese Patent Laid-Open No. 63-295546), a mixture of V-Sb catalyst and Bi-Mo catalyst (Japanese Patent Laid-Open No. 64-38051), Mo-V-Te-Nb catalyst (Japanese Patent Laid-Open No. 2-257) ), Ag-Bi-V-Mo catalyst (JP-A-3-58961), Mo-V-Sb catalyst (JP-A-9-157241), V-Sb-Sn catalyst (JP-A-8-996). ), V-Sb-Ti catalyst (JP No. 8-238428), V-Sb-Li based catalyst (JP-A-8-290058), Nb-Sb-Cr (Japanese Patent Laid-Open No. 10-1465) and the like.

  From the viewpoint of acrylonitrile yield, a metal oxide catalyst containing at least molybdenum, vanadium, niobium, tellurium or antimony as an essential metal is preferable. From the viewpoint of the durability of the catalyst, the metal oxide catalyst contains one or more elements selected from the group consisting of tungsten, titanium, chromium, manganese, iron, tin, and rare earths in addition to the above essential metals. preferable. From the viewpoint of fluidity and strength required for use in a fluidized bed reaction, a silica-supported catalyst is preferable. The particle size of 90% by mass or more of the catalyst particles is 10 to 197 μm, and the crushing strength is 10 MPa or more. Are preferred.

The molar ratio of propane: ammonia: air fed to the fluidized bed reactor is preferably 1: 0.6 to 1.4: 12-18. The reaction temperature is preferably 300 to 500 ° C, more preferably 380 to 480 ° C. The reaction pressure is preferably from ^{0.1~2.0kg / cm 2 G, 0.3~1.5kg /} cm 2 G are preferred. The contact time calculated by the following formula (1) is preferably 0.5 to 10 (sec · kg / L), and more preferably 1 to 6 (sec · kg / L).
Contact time (sec · kg / L) = (W / F) × 273 / (273 + T) (1)
(In formula (1), W = filled catalyst amount (kg), F = raw material mixed gas flow rate (NL / sec) under standard conditions (0 ° C., 1 atm), T = reaction temperature (° C.).)

[Step (a)]
Step (a) in the method for purifying acrylonitrile of the present embodiment is a step of introducing a gas containing acrylonitrile into a quenching tower and cooling it.
The reaction product gas flowing out from the fluidized bed reactor is a gas containing acrylonitrile, and is introduced into the quenching tower 1 from the line 8. The quenching tower 1 is a spray tower, and a sulfuric acid aqueous solution is supplied from a line 9 to a spray provided in the tower. It is preferable that a line (not shown) for adding sulfuric acid is provided in the middle of the line 9. It is preferable that a tray and / or a packing is provided inside the quenching tower 1 because the gas-liquid contact efficiency is improved. Here, as a packing, the thing similar to the packing used for the packed tower mentioned later can be used. The reaction product gas introduced into the quenching tower 1 is cooled with water, and unreacted ammonia contained in the reaction product gas is neutralized with sulfuric acid and separated as ammonium sulfate. The gas from which unreacted ammonia and the like have been separated flows out from the line 11 at the top of the tower, while the separated ammonium sulfate, high-boiling organic substances and water are extracted out of the process from the line 10 and sent to an ammonium sulfate recovery process (not shown). Or incinerated.

  The temperature of the gas flowing out from the line 11 at the top of the quenching tower 1 is usually 70 to 95 ° C., and is cooled to 20 to 45 ° C. by water or frozen water as a refrigerant in the heat exchanger 2 at the exit of the quenching tower 1. , Supplied to the absorption tower 3 from the line 15. The refrigerant leading to the heat exchanger 2 is supplied from the line 12 and extracted from the line 13. In the heat exchanger 2, some organic substances and water in the gas are condensed, and the condensate is extracted through the line 14 and sent to the recovery tower 4. The quenching tower may be a single-stage spray tower or a multi-stage spray tower.

[Step (b)]
The step (b) in the acrylonitrile purification method of the present embodiment is a step of introducing the gas cooled in the step (a) into the absorption tower and bringing it into countercurrent contact with the absorption water.
Here, the absorption tower 3 is a plate tower and / or a packed tower. In the case of a shelf tower, the type of shelf is not particularly limited, and a sheave tray, a dual flow tray, or the like can be adopted. In the case of a packed tower, irregular packing and / or regular packing can be used as packing. The type of irregular packing is not particularly limited, and CMR, pole ring, Raschig ring and the like can be used. The type of the regular packing is not particularly limited, and a mesh-structured packing or the like can be used. As materials for these irregular or regular packing materials, those made of magnetic material, metal, plastic or carbon can be used. In addition, it is preferable to provide a liquid redistribution plate at an appropriate height in the packed tower because the gas-liquid contact efficiency can be increased.

  In this step, in order to recover acrylonitrile contained in the reaction product gas in water, 1.80 to 2.25 times the absorbed water in the mass ratio of the gas introduced from the line 15 to the absorption tower 3 supplied from the line 18 is used. And countercurrent contact. The amount of absorbed water (hereinafter also referred to as “absorbed water amount”) is in the range of 23 to 45 times, preferably 25 to 40 times, more preferably 28 to 35 times in terms of mass ratio to acrylonitrile. Here, the mass of acrylonitrile refers to the mass of acrylonitrile in the reaction product gas flowing out from the reactor. When the amount of absorbed water is within the above range, the yield of acrylonitrile increases and the flooding phenomenon tends to be suppressed.

The mass of acrylonitrile in the reaction product gas can be calculated as follows from the supply flow rate of the raw material gas to the reactor and the component analysis of the reaction gas.
The reaction gas is sampled at the inlet line 8 of the quenching tower 1 and components contained in the reaction gas are analyzed by gas chromatography or the like. Analysis component items include acrylonitrile, acetonitrile, hydrogen cyanide, acetic acid, acrylic acid, acrolein, CO, CO ₂ , O ₂ , propylene, propane, and the like. From the gas composition thus obtained and the raw material gas flow rate, the reaction gas flow rate can be calculated.
Usually, since the reaction gas flow rate is not measured with a flow meter, it is convenient if the reaction gas flow rate can be estimated from the raw material gas supply flow rate. According to the experience of the present inventors, it has been found that if the ammoxidation reaction is carried out as usual, the relationship of the following formula (2) is substantially established.
(Reaction gas flow rate, Nm ³ / h) = (propane flow rate Nm ³ / h + ammonia flow rate Nm ³ / h)
+ Air flow rate Nm ³ /h)×1.06 (2)
In Expression (2), “1.06” is an empirically obtained constant for estimating the reaction gas flow rate from the total amount of the supply gas flow rate on the right side. From the reaction gas flow rate thus obtained and the acrylonitrile concentration in the reaction gas, the mass of acrylonitrile in the reaction product gas can be calculated.

  Absorbed water is supplied from the line 18, descends in the absorption tower 3 while coming into countercurrent contact with the reaction product gas, and is taken out from the bottom of the tower. Since the absorption water temperature rises while the absorption water is falling and the gas absorption efficiency is lowered, it is preferable that the absorption water is withdrawn in the middle of the decrease and returned to the absorption tower 3 after cooling. 1-50 degreeC is preferable and the temperature of absorbed water has more preferable 4-40 degreeC. The lower limit of the temperature is set from the viewpoint that freezing does not occur, and the upper limit is set from the viewpoint of maintaining good absorption efficiency.

  From the line 17 at the top of the absorption tower 3, non-condensable gases such as nitrogen, oxygen, carbon dioxide, carbon monoxide, unreacted propane and hydrocarbons, unrecovered trace amounts of acrylonitrile, organic substances such as acetonitrile and hydrocyanic acid And water is extracted, sent to a waste gas incinerator (not shown), and incinerated. From the bottom of the absorption tower 3, an aqueous acrylonitrile solution is withdrawn as a bottom liquid. In the aqueous solution, hydrocyanic acid, acetonitrile and the like are also dissolved.

[(C) Step]
In step (c) in the acrylonitrile purification method of the present embodiment, the column bottom liquid obtained by countercurrent contact with the absorption water in step (b) is introduced into a recovery column, and extracted water is added and distilled. It is a process.

  It is preferable that the bottom liquid of the absorption tower 3 joins with the condensate in the line 14, and is preheated by exchanging heat with a fluid having a temperature higher than itself before being supplied to the recovery tower 4 through the line 16. As a fluid having a higher temperature than itself, a fluid flowing through the line 18 and / or the line 19, separately procured steam, or the like can be used and heated indirectly by a heat exchanger. 60-90 degreeC is preferable and, as for the temperature of the line 16 after a preheating, ie, the feed temperature to the collection tower 4, 70-88 degreeC is more preferable.

  The recovery tower 4 is a tray-type distillation tower, and the number of stages is preferably 80 to 140. The type of shelf is not particularly limited, and a sheave tray, a dual flow tray, or the like can be adopted. Further, the recovery tower 4 may be divided into two towers.

  From the line 19, 8 to 16 times, preferably 9 to 15 times, more preferably 10 to 13 times as much acrylonitrile as the mass ratio is supplied from the line 19 to the upper part of the recovery tower 4, preferably the uppermost stage. Here, the mass of acrylonitrile refers to the mass of acrylonitrile in the reaction product gas flowing out from the reactor. 40-50 degreeC is preferable and, as for the recovery tower feed temperature of extraction water, 43-48 degreeC is more preferable.

  Substantially most of the acetonitrile and propionitrile are withdrawn in gaseous form from the line 20 of the recovery column 4. It is efficient to set the position of the line 20 at a position where the acetonitrile concentration of the steam in the recovery tower becomes maximum, and it is preferable to set it at a position of 1/4 to 2/5 from the bottom of the recovery tower. The extraction amount of the line 20 is adjusted so that the acrylonitrile in the extraction steam is preferably 300 to 3000 mass ppm, more preferably 500 to 2500 mass ppm. Moreover, it adjusts so that it may become 0.50-8.0% of the line 16 flow rate by mass ratio, More preferably, it is 1.0-5.0%. The extraction amount of the line 20 is measured by a flow meter attached to the line 20.

  The absorption water (line 18) and the extraction water (line 19) are preferably drawn from a position where the acrylonitrile concentration and acetonitrile concentration of the liquid in the recovery tower are substantially 0 or low, from the bottom of the recovery tower to the bottom of the recovery tower. It is good to extract from the range of 1/10.

  Absorbed water and extracted water may be extracted from different positions of the recovery tower 4. In this case, the absorption water (line 18) is preferably extracted from a position where the concentration of acrylonitrile in the liquid in the recovery tower is substantially 0 or low, and from a range of 1/5 from the bottom of the recovery tower to the bottom of the recovery tower. It is good to extract. The extracted water (line 19) is preferably extracted from a position where the acetonitrile concentration of the liquid in the recovery tower is low, and is preferably extracted from a range of 1/10 from the bottom of the recovery tower to the bottom of the recovery tower. Further, the take-out position of the line 19 is preferably below the line 18.

  From the line 21, high-boiling substances, water, and the like are extracted out of the system and supplied to a wastewater treatment facility (not shown).

  The amount of heat applied to the recovery tower 4 is provided by indirectly heating the tower bottom liquid with a reboiler (not shown).

[Step (d) and Step (e)]
In step (d) in the acrylonitrile purification method of the present embodiment, the distillate vapor obtained in step (c) is condensed and cooled, and then accommodated in a recovery tower decanter to form an organic layer and an aqueous layer. In this step, the organic layer is introduced into a dehydrating acid dehydration tower and distilled.
The step (e) in the acrylonitrile purification method of the present embodiment, after extracting and cooling a side stream from the dehydrating acid dehydration tower, accommodated in a dehydrating acid dehydration tower decanter to form an organic layer and an aqueous layer, In this step, the organic layer is returned to the lower part from the side stream extraction position, and the bottom liquid of the dehydrating acid dehydration tower is extracted.

As a result of intensive studies on the liquid temperatures in the recovery tower decanter and the deblue acid dehydration tower decanter in the steps (d) and (e), the present inventors have found the following.
In the conventional acrylonitrile production process by ammoxidation of propylene, in order to minimize the amount of peroxide (peroxide) that is a causative substance that suppresses polymerization of acrylonitrile, which is the final product, in the product. The liquid temperature in the decanter is maintained at 0 ° C to 24 ° C. On the other hand, in the acrylonitrile production | generation reaction by the ammoxidation of propane, it discovered newly that there was little generation | occurrence | production of a peroxide and it did not become a problem. Therefore, it was noted that it is not necessary to minimize the peroxide by controlling the temperature of the decanter. Furthermore, in the propane ammoxidation process, while the generation of peroxide is small as described above, the byproduct rate of acetonitrile and propionitrile is high, and their separation becomes an obstacle to obtaining high-quality acrylonitrile. there is a possibility. From the study of these inventors, if the liquid temperature in the decanter is set to a temperature higher than the conventional temperature (25 ° C. or higher), the concentrations of acetonitrile and propionitrile in the organic layer can be reduced, and finally It was found that the purity of acrylonitrile obtained can be improved.

  Further, as conventionally used in the ammoxidation process of propylene, in order to cool the liquid temperature in the decanter to 0 ° C. to 24 ° C., it is almost essential to use frozen water as a refrigerant to be supplied to the cooling device. The energy consumption as power of the refrigerator is large. Further, since a low-temperature liquid is fed to the dehydrating acid dehydration tower, the amount of heat required for the dehydrating acid dehydration tower is large, and accordingly, a large reboiler is required. On the other hand, when the liquid temperature is higher than 45 ° C., the amount of water dissolved in acrylonitrile increases exponentially, so that the amount of liquid fed to the dehydrating acid dehydration tower increases, resulting in the large size of the apparatus. It is forced to become. In the case where the decanter temperature is maintained at 25 ° C. to 45 ° C., there is an advantage that a refrigerator is not necessary and the size of each device is not required to be increased.

  The recovery tower decanter and the dehydration acid dehydration tower decanter both have the function of reducing the amount of by-products in the acrylonitrile product, but are close to the final product and usually have a large amount of fluid entering and leaving the decanter. The temperature control of the dehydrator decanter is more important. By controlling the liquid temperature in the decanter to 25 to 45 ° C., the effect of improving the purity of the target product becomes remarkable. Of course, in order to maintain the purity of the target product higher, it is preferable to maintain the liquid temperature in both decanters at 25 ° C to 45 ° C.

Hereinafter, the step (d) will be described.
In step (d), the distillate vapor obtained in step (c) is condensed and cooled, and then accommodated in a recovery tower decanter to form an organic layer and an aqueous layer. Introducing and distilling.
The distillate vapor of the recovery tower 4 obtained in the step (c) is withdrawn from the line 22 and acrylonitrile is recovered together with hydrocyanic acid and water. The distillate vapor is condensed by the cooling device A30 and then sent to the recovery tower decanter 6 to form an organic layer and an aqueous layer. The organic layer has a specific gravity smaller than that of the aqueous layer, and overflows the weir in the recovery tower decanter 6. The organic layer is introduced into the dehydrating acid dehydration tower 5 through a line 24. On the other hand, the water layer is extracted from the line 23, joined to the line 16 or 19, and recycled.

  The cooling device A30 is a heat exchanger system, and includes a condenser and, if necessary, a cooler. As the refrigerant of the heat exchanger system, cooling water having a temperature of usually 20 ° C. to 45 ° C. and / or frozen water having a temperature of usually 0 ° C. to 15 ° C. are used. When these are used in combination, the temperature is first lowered with cooling water and then further cooled with frozen water to lower the temperature. The amount of refrigerant communicated with the heat exchanger system is adjusted so that the liquid temperature of the recovery tower decanter 6 becomes a predetermined temperature. The thermometer for measuring the liquid temperature of the recovery tower decanter 6 may be of a type used in a normal chemical plant such as a thermocouple or a side temperature resistor, and can be used to measure the liquid temperature of the aqueous layer and / or the organic layer. Installed. In the acrylonitrile purification method of the present embodiment, the liquid temperature of the recovery tower decanter 6 is preferably adjusted to 25 ° C to 45 ° C, more preferably 26 ° C to 40 ° C. The temperatures of the organic layer and the aqueous layer formed in the recovery tower decanter 6 are basically the same temperature, but when there is a temperature difference, it is preferable to adjust the temperature of each layer to 25 ° C. to 45 ° C., respectively. .

Hereinafter, the step (e) will be described.
In step (e), after the side stream is extracted from the dehydration acid dehydration tower and cooled, the organic layer and the aqueous layer are formed by being accommodated in a deblue acid dehydration tower decanter, and the organic layer is removed from the side stream extraction position. This is a step of returning to the lower part and extracting the bottom liquid of the dehydrating acid dehydration tower.
In the step (d), the organic layer introduced into the dehydrating acid dehydration tower 5 is distilled, hydrocyanic acid is separated from the line 25 at the top of the dehydrating tower 5, and the side stream extracted from the line 26 in the middle of the dehydrating tower 5 is cooled. After cooling with the apparatus B31, it is sent to a dehydration acid dehydration tower decanter 7 to form an organic layer and an aqueous layer. The organic layer has a specific gravity smaller than that of the aqueous layer, and overflows the weir inside the dehydrating acid dehydration tower decanter 7. The organic layer is returned to a position below line 26 by line 28. The line 28 may be provided with a heat exchanger for heating, and the liquid may be heated and then returned to the dehydride dehydration tower 5. On the other hand, the water layer is extracted from the line 27 and joined to the line 23 or the like.

  The cooling device B31 is a heat exchanger system and includes one or more coolers. As the refrigerant of the heat exchanger system, cooling water having a temperature of usually 20 ° C. to 45 ° C. and / or frozen water having a temperature of usually 0 ° C. to 15 ° C. is used. The amount of refrigerant communicated with the heat exchanger system is adjusted so that the liquid temperature of the dehydration acid dehydration tower decanter 7 becomes a predetermined temperature. The thermometer for measuring the liquid temperature of the dehydrating acid dehydration tower decanter 7 may be of the type used in ordinary chemical plants, and is installed at a position where the temperature of the aqueous layer and / or the organic layer can be measured. In the purification method of acrylonitrile of the present embodiment, the liquid temperature in the deblue acid dehydration tower decanter 7 is adjusted to 25 ° C. to 45 ° C., preferably 26 ° C. to 40 ° C. The organic layer and the aqueous layer formed in the dehydrating acid dehydration tower decanter 7 are basically at the same temperature, but even if there is a temperature difference, as long as the respective temperatures are maintained at 25 ° C. to 45 ° C., Category.

The analysis of acetonitrile and propionitrile in the internal liquids of the recovery tower decanter 6 and the dehydration acid dehydration tower decanter 7 is performed by sampling the internal liquid and performing gas chromatography.
The apparatus and conditions for gas chromatography can be as follows.
Shimadzu GC-17A is used as the gas chromatography apparatus, TC-FFAP 60 m × 0.32 film thickness 0.25 μm is used as the column, FID is used as the detector, and helium is used as the carrier gas.

The bottom liquid extracted from the line 29 is distilled in a distillation tower (not shown) to obtain an acrylonitrile product. The impurity concentration in the product acrylonitrile is measured by gas chromatography. The apparatus and conditions for gas chromatography are the same as described above. Hydrogen cyanide is measured as H ₂ O ₂ by a silver nitrate titration method described in ASTM E1178, and peroxide is measured by a spectrophotometer described in ASTM E1784.

  This embodiment will be further described with reference to examples and comparative examples. However, this embodiment is not limited to the following examples. Meters and attached equipment are normally used and are within the normal error range. Further, as the purification apparatus, the same apparatus as that shown in FIG. 1 was used.

The reaction product yield and the propane conversion were calculated from the analytical data obtained by sampling the reaction gas and measuring by gas chromatography.
Propane conversion (%) = (moles of propane reacted) / (moles of propane fed) × 100
Acrylonitrile yield (%) = (Mole number of acrylonitrile produced) / (Mole number of supplied propane) × 100
Acetonitrile yield (%) = (number of moles of acetonitrile produced) × 2/3 / (number of moles of propane fed) × 100

For each concentration analysis in the fluid, the sampled fluid was analyzed as follows.
Acetonitrile: Gas Chromatography: Shimadzu GC-17A, Column: TC-FFAP 60 m × 0.32, Film thickness 0.25 μm, Detector: FID, Carrier gas: Helium Propionitrile: Similar to acetonitrile Oxazole: Similar to acetonitrile Peroxidation Material: Method according to ASTM E1784

[Example 1]
(1) Preparation of catalyst (1-i) Preparation of niobium mixed solution A niobium mixed solution was prepared by the following method.
To 552 g of water, 352 g of niobic acid containing 80% by mass as Nb ₂ O ₅ and 1344 g of oxalic acid dihydrate [H ₂ C ₂ O ₄ .2H ₂ O] were mixed. The molar ratio of the charged oxalic acid / niobium was 5.03, and the charged niobium concentration was 0.50 (mol-Nb / kg-solution).
This solution was heated and stirred at 95 ° C. for 1 hour to obtain a mixed solution in which niobium was dissolved. The mixed solution was allowed to stand, ice-cooled, and then the solid was separated by suction filtration to obtain a uniform niobium mixed solution. . This operation was repeated several times, and the liquid was collected and mixed. The molar ratio of oxalic acid / niobium in the obtained niobium mixed solution was 2.52 according to the following analysis.

10 g of this niobium mixed solution was precisely weighed in a crucible, dried overnight at 95 ° C., and then heat-treated at 600 ° C. for 1 hour to obtain 0.8228 g of Nb ₂ O ₅ . From this result, the niobium concentration was 0.618 (mol-Nb / kg-solution).
3 g of this niobium mixture was precisely weighed into a 300 mL glass beaker, 200 mL of hot water at about 80 ° C. was added, and then 10 mL of 1: 1 sulfuric acid was added. The obtained mixed liquid was titrated with 1 / 4N KMnO ₄ under stirring while maintaining the liquid temperature at 70 ° C. on a hot stirrer. The end point was a point where a faint pale pink color by KMnO ₄ lasted for about 30 seconds or more. The concentration of oxalic acid was 1.558 (mol-oxalic acid / kg) as a result of calculation according to the following formula from titration.
2KMnO ₄ + 3H ₂ SO ₄ + 5H ₂ C ₂ O ₄ → K ₂ SO ₄ + 2MnSO ₄ + 10CO ₂ + 8H ₂ O
The obtained niobium mixed solution was used as the following niobium mixed solution (B0) for catalyst preparation.

(1-ii) Preparation charged composition formula of the catalyst was prepared by an oxide catalyst represented by _{Mo 1 V 0.21 Nb 0.09 Sb 0.25} Ce 0.005 O n /45.0wt%-SiO 2 as follows.
921.4 g of ammonium heptamolybdate [(NH ₄ ) 6 Mo ₇ O ₂₄ .4H ₂ O], 127.4 g of ammonium metavanadate [NH ₄ VO ₃ ], and antimony trioxide [Sb ₂ O ₃ ] in 3800 g of water 189.8 g and cerium nitrate hexahydrate [Ce (NO ₃ ) ₃ .6H ₂ O] were added, and the mixture was heated at 90 ° C. for 2 hours 30 minutes with stirring to obtain a mixed solution A. .

105.8 g of hydrogen peroxide containing 30 wt% as H ₂ O ₂ was added to 754.7 g of the niobium mixed solution (B0), and the mixture was stirred and mixed at room temperature for 10 minutes to prepare a mixed solution B. After cooling the obtained mixed liquid A to 70 ° C., 1843 g of silica sol containing 29.3 wt% as SiO ₂ was added, and further, 220.4 g of hydrogen peroxide containing 30 wt% as H ₂ O ₂ was added. And stirring was continued at 50 ° C. for 1 hour. Next, the mixed solution B was added. Furthermore, a liquid in which 360 g of fumed silica having an average primary particle size of 12 nm was dispersed in 5040 g of water was added to obtain a raw material preparation liquid. The obtained raw material mixture was supplied to a centrifugal spray dryer and dried to obtain a microspherical dry powder. The dryer inlet temperature was 210 ° C. and the outlet temperature was 120 ° C. The above operation was repeated to collect dry powder to obtain about 20 kg. Next, firing was performed using a continuous kiln having a diameter of 127 mm and a length of 1150 mm. The obtained dry powder was supplied at 220 g / Hr, and pre-baked at 360 ° C. for 2 hours under a nitrogen flow of 3.6 NL / min counter-current to obtain a pre-baked product. Next, the pre-stage calcined product was supplied at 130 g / Hr, and calcined at 660 ° C. for 2 hours under a nitrogen flow of 2.3 NL / min countercurrent to obtain a catalyst.

(2) Production of acrylonitrile Mo-V-Sb-Nb-Ce supported catalyst having a particle size of 10 to 100 µm and an average particle size of 55 µm obtained as described above in a fluidized bed reactor having an inner diameter of 8 m and a length of 20 m. Was packed so that the height of the stationary layer was 2.2 m.
Propane 4300 Nm ³ / h, ammonia 4300 Nm ³ / h and air 64500 Nm ³ / h were supplied to the fluidized bed reactor to carry out ammoxidation of propane. The reaction temperature was 440 ° C. and the pressure was 0.75 kg / cm ² G.
From the analysis of the product gas, the acrylonitrile yield was 52.1%, the acetonitrile yield was 3.4%, and the propane conversion was 89.4%. Accordingly, the mass of acrylonitrile in the reaction product gas was 5307 kg / h.
The reaction product gas flowing out from the reactor was introduced into the quenching tower 1 shown in FIG. Ammonia in the product gas was neutralized with sulfuric acid and separated from the tower bottom as ammonium sulfate. The temperature of the gas flowing out from the top of the quenching tower 1 was 80 ° C. Subsequently, the gas was cooled with cooling water in the heat exchanger 2 at the exit of the quenching tower 1. The gas temperature at this time was 33 ° C. The condensate produced in the heat exchanger 2 was extracted from the line 14 and joined to the bottom liquid line 16 of the absorption tower 3.
The gas 81.44 t / h flowing out from the heat exchanger 2 was introduced into the absorption tower 3 through the line 15. The absorption tower 3 was a tower filled with stainless steel CMR, and the total height of the packed bed was 25 m. The gas introduced into the absorption tower 3 was brought into countercurrent contact with the absorption water 164.5 t / h supplied from the line 18. The absorbed water temperature was 5.5 ° C. At this time, the acrylonitrile and acetonitrile concentrations in the absorption water supplied from the line 18 were 1 and 3 mass ppm, respectively.
The acrylonitrile mass calculated from the acrylonitrile concentration analysis of the liquid of the line 16 and the liquid flow rate was 5270 kg / h.
The liquid in the line 16 was preheated to 75 ° C. by indirect heat exchange with the liquid in the line 18 and the liquid in the line 19, and supplied to the 64th stage from the bottom of the recovery tower 4 composed of a 95th sieve tray. Distillation was performed by applying heat to the bottom liquid of the recovery tower 4 with a reboiler. The heat source used for the reboiler was 4 kg / cm ² G saturated steam, and the flow rate was 30.3 t / h. Extracted water 63.7 t / h was supplied from the line 19 to the top (95th stage) of the recovery tower 4. A steam flow of 4.6 t / h was extracted from the line 20 installed at the 35th stage counting from the bottom of the recovery tower 4. Water was extracted from the line 21 to maintain the bottom liquid level of the recovery tower 4.
Absorbed water and extracted water were extracted from the first stage counted from the bottom of the recovery tower 4 and fed through lines 18 and 19, respectively.
The distillate vapor of the recovery tower 4 was extracted from the line 22, condensed and cooled with cooling water in the cooling device A30, and introduced into the recovery tower decanter 6 to form an organic layer and an aqueous layer. The liquid temperature in the recovery tower decanter 6 was 35 ° C.
The aqueous layer of the recovery tower decanter 6 was extracted from the line 23 at 1350 kg / h and joined to the line 16.
The organic layer of the recovery tower decanter 6 was extracted from the line 24 at 6050 kg / h, and was introduced into the dehydration acid dehydration tower 5.
The dehydrating acid dehydration tower 5 was a 62-stage tray tower, the 26th stage counted from the bottom of the tower was a chimney tray, and the others were sieve trays. By distilling the organic layer introduced from the recovery tower decanter 6, the hydrocyanic acid is separated from the line 25 at the top of the tower, and the side stream extracted from the line 26 attached to the 26th stage is cooled with cooling water by the cooling device B31. Thereafter, an organic layer and an aqueous layer were formed in the decanter 7 for dehydrating acid dehydration tower. The liquid temperature in the dehydration acid dehydration tower decanter 7 was 37 ° C.
The water layer of the dehydration acid dehydration tower decanter 7 was extracted from the line 27 at 545 kg / h and joined to the line 23.
The organic layer of the dehydration acid dehydration tower decanter 7 was extracted from the line 28 at 15070 kg / h and returned to the 25th stage of the deblue acid dehydration tower 5.
The liquid extracted from the line 29 at the bottom of the dehydrating acid dehydration tower 5 is distilled in a distillation tower (not shown), high-boiling substances are separated from the bottom of the distillation tower, and acrylonitrile product 5200 kg / h ( A value obtained by dividing the total production amount of 8000 hours by the total time).
The obtained acrylonitrile could be used as a synthetic resin such as ABS resin, an intermediate raw material such as acrylic fiber, synthetic rubber and acrylamide. Table 1 shows the concentrations of acetonitrile, propionitrile, oxazole and peroxide as examples of impurities contained in the acrylonitrile product.

[Examples 2 to 6]
The propane ammoxidation reaction was carried out under the same reaction conditions as in Example 1 except that the liquid temperatures in the recovery tower decanter 6 and the dehydration acid dehydration tower decanter 7 were changed, and acrylonitrile was recovered and purified in the same equipment. It was.
Under any of the conditions of Examples 2 to 6, no trouble on the equipment occurred, and the production planned value of 5200 kg / h was achieved. The obtained acrylonitrile could be used as a synthetic resin such as ABS resin, an intermediate raw material such as acrylic fiber, synthetic rubber and acrylamide. Table 1 shows the concentrations of acetonitrile, propionitrile, oxazole and peroxide as examples of impurities contained in the acrylonitrile product.

[Example 7]
(1) Preparation compositional formula of the catalyst was prepared by an oxide catalyst represented by _{Mo 1 V 0.33 Te 0.22 Nb 0.12} O n /30.0wt%-SiO 2 as follows.
To 720 g of water, 164.31 g of ammonium heptamolybdate [(NH ₄ ) ₆ Mo ₇ O ₂₄ · 4H ₂ O], 36.05 g of ammonium metavanadate [NH ₄ VO ₃ ] and telluric acid [H ₆ TeO ₆ ] 47. 01 g was added and dissolved by heating to 60 ° C. with stirring to obtain an aqueous mixture.
Meanwhile, to 170 g of water, 19.53 g of niobic acid having an Nb ₂ O ₅ content of 76.6% by mass and 38.0 g of oxalic acid dihydrate [H ₂ C ₂ O ₄ .2H ₂ O] were added and stirred. The solution was heated to 60 ° C. and dissolved, and then cooled to 30 ° C. to obtain a niobium-oxalic acid aqueous solution. 167.2 g of 5 mass% hydrogen peroxide solution was added to the niobium-oxalic acid aqueous solution to obtain a niobium-hydrogen peroxide aqueous solution.
While stirring the aqueous mixture, 286 g of silica sol having a silica content of 30% by mass was added and cooled to 30 ° C., and then an aqueous niobium-hydrogen peroxide solution was added to obtain an aqueous raw material mixture. The aqueous raw material mixture was dried under conditions of an inlet temperature of 240 ° C. and an outlet temperature of 145 ° C. using a centrifugal spray dryer to obtain a fine spherical dry powder. The dried powder was pre-calcined at 240 ° C. for 2 hours in an air atmosphere to obtain a catalyst precursor. The obtained catalyst precursor was filled into a quartz container, placed in a furnace, and calcined at 600 ° C. for 2 hours under a nitrogen gas flow of 600 mL / min while rotating the container to obtain an oxide catalyst. The oxygen concentration of the nitrogen gas used was 1 ppm as a result of measurement using a trace oxygen analyzer (306WA type, manufactured by Teledyne Analytical Instruments, USA).

(2) Production of acrylonitrile The Mo-V-Te-Nb supported catalyst having a particle size of 10 to 100 µm and an average particle size of 55 µm obtained as described above is stationary in a fluidized bed reactor having an inner diameter of 8 m and a length of 20 m. The layer height was 2.3 m.
Propane 4300 Nm ³ / h, ammonia 4300 Nm ³ / h and air 64500 Nm ³ / h were supplied to the fluidized bed reactor to carry out ammoxidation of propane. The reaction temperature was 440 ° C. and the pressure was 0.75 kg / cm ² G.
Analysis of the product gas revealed that the acrylonitrile yield was 51.3%, the acetonitrile yield was 3.8%, and the propane conversion was 89.0%. From this, the mass of acrylonitrile in the reaction product gas was 5225 kg / h.
The reaction product gas flowing out from the reactor was introduced into the quenching tower 1 shown in FIG. Ammonia in the product gas was neutralized with sulfuric acid and separated from the tower bottom as ammonium sulfate. The temperature of the gas flowing out from the top of the quenching tower 1 was 80 ° C. Subsequently, the gas was cooled with cooling water in the heat exchanger 2 at the exit of the quenching tower 1. The gas temperature at this time was 33 ° C. The condensate produced in the heat exchanger 2 was extracted from the line 14 and joined to the line 16 of the bottom liquid of the absorption tower 3.
The gas 81.25 t / h flowing out from the heat exchanger 2 was introduced into the absorption tower 3 through the line 15. The absorption tower 3 was a packed tower filled with stainless steel CMR, and the total height of the packed bed was 25 m. The gas introduced into the absorption tower 3 was brought into countercurrent contact with the absorption water 164.1 t / h supplied from the line 18. The temperature of the absorption water was 5.5 ° C. At this time, the acrylonitrile and acetonitrile concentrations in the absorption water supplied from the line 18 were 1 and 3 mass ppm, respectively.
The acrylonitrile mass calculated from the acrylonitrile concentration analysis of the liquid of the line 16 and the flow volume of the liquid was 5185 kg / h.
The liquid in the line 16 was preheated to 75 ° C. by indirect heat exchange with the liquid in the line 18 and the liquid in the line 19, and supplied to the 64th stage from the bottom of the recovery tower 4 consisting of a 95th sieve tray. Distillation was performed by applying heat to the bottom liquid of the recovery tower 4 with a reboiler. The heat source used for the reboiler was 4 kg / cm ² G saturated steam, and the flow rate was 30.2 t / h. Extracted water 62.1 t / h was supplied from line 19 to the top (95th stage) of recovery tower 4. A 4.7 t / h vapor stream was extracted from the line 20 installed at the 35th stage counting from the bottom of the recovery tower 4. Water was extracted from the line 21 to maintain the bottom liquid level of the recovery tower 4.
Absorbed water and extracted water were extracted from the first stage counted from the bottom of the recovery tower 4 and fed through lines 18 and 19, respectively.
The distillate vapor of the recovery tower 4 was extracted from the line 22, condensed and cooled with cooling water in the cooling device A30, and introduced into the recovery tower decanter 6 to form an organic layer and an aqueous layer. The liquid temperature in the recovery tower decanter 6 was 35 ° C.
The aqueous layer of the recovery tower decanter 6 was extracted from the line 23 at 1380 kg / h and joined to the line 16.
The organic layer of the recovery tower decanter 6 was extracted from the line 24 at 6025 kg / h and introduced into the dehydrating acid dehydration tower 5.
The dehydrating acid dehydration tower 5 was a 62-stage tray tower, the 26th stage counted from the bottom of the tower was a chimney tray, and the others were sieve trays. By distilling the organic layer introduced from the recovery tower decanter 6, the hydrocyanic acid is separated from the line 25 at the top of the tower, and the side stream extracted from the line 26 attached to the 26th stage is cooled with cooling water by the cooling device B31. Thereafter, an organic layer and an aqueous layer were formed in the decanter 7 for dehydrating acid dehydration tower. The liquid temperature in the dehydration acid dehydration tower decanter 7 was 37 ° C.
The aqueous layer of the dehydrating acid dehydration tower decanter 7 was extracted from the line 27 at 557 kg / h and joined to the line 23.
The organic layer of the dehydration acid dehydration tower decanter 7 was extracted from the line 28 at 15045 kg / h and returned to the 25th stage of the deblue acid dehydration tower 5.
The liquid extracted from the line 29 at the bottom of the dehydrating acid dehydration tower 5 is distilled in a distillation tower (not shown), high-boiling substances are separated from the bottom of the distillation tower, and acrylonitrile product 5115 kg / h (from the top of the tower) A value obtained by dividing the total production amount of 8000 hours by the total time).
The obtained acrylonitrile could be used as a synthetic resin such as ABS resin, an intermediate raw material such as acrylic fiber, synthetic rubber and acrylamide. Table 1 shows the concentrations of acetonitrile, propionitrile, oxazole and peroxide as examples of impurities contained in the acrylonitrile product.

[Comparative Example 1]
The same as in Example 1 except that refrigerated water was supplied to the cooling devices 1 and 2 so that the liquid temperature in the recovery tower decanter 6 and dehydration acid dehydration tower decanter 7 was 20 ° C., and the decanter temperatures were controlled. In this apparatus and method, propane ammoxidation was performed, and acrylonitrile was recovered and purified.
The amount of water layer extracted from the recovery tower decanter 6 was 1392 kg / h, and the amount of organic layer extracted was 6008 kg / h.
The amount of water layer extracted from the decanter 7 was 576 kg / h, and the amount of organic layer extracted was 15037 kg / h.
It was necessary to add a calorie of 0.46 T / h (about 160 T per year) in terms of steam to the reboiler attached to the de-blue acid dehydration tower 5. In addition, the power consumption of the cooling devices A and B increased by about 33%.
In addition, since the heat load on the dehydration acid dehydration tower 5 was increased, signs of flooding appeared. Judging that the operation state of the dehydration tower was unstable, the production rate was reduced and stabilized. The obtained acrylonitrile yield and impurity concentration are shown in Table 1.
The obtained acrylonitrile was 5035 kg / h (a value obtained by dividing the total production amount of 8000 hours by the total time). Further, the impurity concentration was increased as compared with the example.

[Comparative Examples 2 to 4]
The propane ammoxidation reaction was carried out under the same reaction conditions as in Example 1 except that the liquid temperatures in the recovery tower decanter 6 and the dehydration acid dehydration tower decanter 7 were changed, and acrylonitrile was recovered and purified in the same equipment. It was.
Since the low-temperature liquid was supplied to the dehydration acid dehydration tower 5, in any case, it was necessary to increase the heat load of the deblue acid dehydration tower reboiler as compared to Example 1.
In addition, since the heat load on the dehydration acid dehydration tower 5 was increased, signs of flooding appeared. Judging that the operation state of the dehydration tower was unstable, the production rate was reduced and stabilized. The obtained acrylonitrile yield and impurity concentration are shown in Table 1.

  As is clear from the results in Table 1, in Examples 1 to 7 using the acrylonitrile purification method of the present embodiment, it was possible to obtain an acrylonitrile product with a low impurity concentration in a high yield.

  The purification method of the present invention has industrial applicability in a method for purifying acrylonitrile from a product gas obtained by an ammoxidation reaction of propane.

DESCRIPTION OF SYMBOLS 1 Quenching tower 2 Heat exchanger 3 Absorption tower 4 Recovery tower 5 Dehydrating acid dehydration tower 6 Recovery tower decanter 7 Dehydrating acid dehydration tower decanter 8-29 Line 30 Cooling device A
31 Cooling device B

Claims (2)

A method in which propane, ammonia and oxygen are subjected to an ammoxidation reaction in the presence of a catalyst, and acrylonitrile is purified from the produced gas containing acrylonitrile,
(A) introducing the gas containing acrylonitrile into a quenching tower and cooling it,
(B) introducing the gas cooled in the step (a) into the absorption tower and bringing it into countercurrent contact with the absorption water;
(C) introducing the tower bottom liquid obtained by countercurrent contact with the absorbed water in the step (b) into the recovery tower, adding extracted water and distilling;
(D) After condensing and cooling the distillate vapor obtained in the step (c), it is accommodated in a recovery tower decanter to form an organic layer and an aqueous layer, and the organic layer is introduced into a dehydrating acid dehydration tower, A distillation step,
(E) After extracting and cooling a side stream from the dehydration acid dehydration tower, it is accommodated in a deblue acid dehydration tower decanter to form an organic layer and an aqueous layer, and the organic layer is returned to the lower part from the side stream extraction position. And a step of extracting the bottom liquid of the dehydrating acid dehydration tower,
Including
A method for purifying acrylonitrile, wherein the liquid temperature in the debranching acid dehydration tower decanter is adjusted to 25 to 45 ° C.   The method for purifying acrylonitrile according to claim 1, wherein the liquid temperature in the recovery tower decanter is adjusted to 25 to 45 ° C.

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