JP4658104B2 - Acrylic acid production method - Google Patents

Description

  The present invention provides an acrylic acid-containing solution obtained by collecting water to obtain an acrylic acid crystal and a crystal mother liquor through a crystallization process, and circulates the crystal mother liquor to a collection tower to improve the acrylic acid collection rate. The present invention relates to a method for producing acrylic acid with high yield.

  An industrial method for producing acrylic acid is generally a propylene oxidation method in which propylene and / or acrolein is subjected to catalytic gas phase oxidation. When acrylic acid is produced by this propylene oxidation method, impurities such as water, acids such as propionic acid, acetic acid and maleic acid, and aldehydes such as acetone, acrolein, furfural and formaldehyde are by-produced in the propylene oxidation process. . The gas containing these by-products is generally collected as an acrylic acid-containing solution by contacting with the collection solution, and then the collection solution is separated by a method such as distillation.

  For example, a gas containing acrylic acid obtained by catalytic gas phase oxidation is collected with a high boiling point solvent, and then separated into a solvent and crude acrylic acid by distillation, and then high purity acrylic acid is produced by a crystallization process. There is a method (Patent Document 1). In this method, the residual acrylic acid-containing gas that has not been absorbed by the high-boiling solvent is cooled, made into a condensate containing water, formaldehyde, acetic acid, and the like, and discharged out of the system. In addition, the crystal mother liquor is circulated through the collection tower to improve the separation of the co-components by scrubbing the low boiling point material and stripping the low boiling point material.

  There are also methods for producing acrylic acid that directly crystallize the condensate obtained by condensing the acrylic acid-containing gas (Patent Document 2, Patent Document 3, Patent Document 4). If an organic solvent or an extractant is not used, crystals are solidified without precipitation when the solution is cooled in the crystallization step, so an organic solvent has been used in the past. It has been made by finding that acrylic acid can be crystallized directly, and is characterized in that it does not require the addition of auxiliary substances. In the method of Patent Document 2, a part of the crystal mother liquor is circulated in the crystallization process, and in the methods of Patent Documents 3 and 4, each component is separately condensed in contrast to normal condensation. Acrylic acid is separated as a medium-boiling fraction, a low-boiling fraction containing acrylic acid is discarded as waste water, and a portion of the high-boiling fraction containing acrylic acid is recycled to the separation tower and the rest is discarded.

  There is also a method for producing acrylic acid in which crude acrylic acid obtained by quenching and condensing acrylic acid-containing gas with a spray cooler is crystallized (Patent Document 5). The method is made in order to effectively use the acrylic acid oligomer generated in the acrylic acid production process, using the bottom liquid of the separation tower containing the acrylic acid oligomer as a quenching liquid for the acrylic acid-containing gas, A part of the quenching liquid is taken out as an effluent, the acrylic acid oligomer is decomposed into acrylic acid, and the resulting acrylic acid is condensed and used as a quenching liquid.

  It includes a step of distilling the mother liquor obtained when the mixture containing acrylic acid is crystallized to obtain a distillation residue and a top product, and recrystallizing a part of the top product. A method for producing acrylic acid is also disclosed (Patent Document 6). In this method, at least a part of the mother liquor generated in the first crystallization is distilled, the resulting top product is crystallized, the crystals are returned to the first crystallization, and the crystallization residue is removed from the system. The purpose of this is to increase the purity of acrylic acid.

On the other hand, it is economical if an aqueous solution can be used instead of an expensive high-boiling solvent as the acrylic acid collecting liquid. In particular, if a high-concentration acrylic acid-containing solution can be treated, the amount of treatment in the subsequent purification process can be reduced, which is also efficient. Therefore, when collecting the acrylic acid-containing gas obtained by the catalytic gas phase oxidation reaction, the recovered water discharged from the azeotropic dehydration step is supplied to the collection tower, and the resulting acrylic acid-containing solution is supplied to the stripping tower. There is also a method in which an acrylic acid-containing solution containing 70.9% by mass of acrylic acid, 25.6% by mass of water, and 2.0% by mass of acetic acid is obtained from the bottom of the stripping tower (Patent Document 7). In this method, the acrylic acid-containing solution is azeotropically dehydrated and then subjected to a crystallization step to obtain purified acrylic acid.
Japanese Patent Laid-Open No. 9-227446 Special Table 2000-514077 JP-T-2001-516736 Special table 2002-521353 gazette Special Table 2002-539104 Special Table 2002-519402 JP 2001-199931 A

  However, the method described in the above document requires an expensive organic solvent, and is therefore accompanied by an additional operation such as repurification of the used organic solvent (Patent Document 1, Patent Document 7) or usually. It is necessary to carry out a complicated fractional condensation different from the condensation of the above, and the apparatus becomes expensive. In addition, although fractions other than the target component contain a considerable amount of acrylic acid, they are discharged out of the system, so the purification yield is reduced, and a considerable amount of waste treatment such as wastewater treatment of the fraction is required. (Patent Literature 3, Patent Literature 4, Patent Literature 5).

  Under such circumstances, development of a method for producing high-purity acrylic acid with a high yield using a simple apparatus is desired.

  In the acrylic acid production process, the present inventor is able to perform crystallization treatment as a crude acrylic acid as it is with the acrylic acid-containing solution obtained using the collected aqueous solution, and the residual mother liquor obtained in the crystallization process is collected. It was found that the acrylic acid collection rate can be improved by circulating in the tower, and that the acrylic acid loss can be reduced, thus completing the present invention.

  According to the present invention, acrylic acid can be produced by treating an acrylic acid-containing solution collected with water in a crystallization step without performing an azeotropic dehydration step.

  In the present invention, it is particularly suitable for purification of an acrylic acid-containing solution having a high acrylic acid concentration, and such high-concentration acrylic acid is used to collect a distillate obtained by distilling the residual mother liquor obtained in the crystallization step into a collection tower. It is obtained by circulating. ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of acrylic acid which reduces acrylic acid loss, is simple and has high production efficiency is provided. This will be described in detail below.

  The first of the present invention is a method for producing acrylic acid comprising the following steps (a-1), (a-2), (a-3), (a-4), (b) and (c). is there.

(A-1) A step of obtaining an acrylic acid-containing gas obtained by a catalytic gas phase oxidation reaction of an acrylic acid raw material,
(A-2) introducing the acrylic acid-containing gas into a collection tower and bringing it into contact with an aqueous solution for collection to obtain an acrylic acid-containing solution having a content of 80% by mass or more;
(A-3) a step of cooling at least a part of the exhaust gas discharged from the collection tower and then circulating it to the catalytic gas phase oxidation reaction;
(A-4) a step of circulating at least a part of the condensate condensed by cooling to the collection tower;
(B) supplying the acrylic acid-containing solution to a crystallization step and separating the acrylic acid and the residual mother liquor;
(C) A step of subjecting at least a part of the residual mother liquor to a distillation step and circulating the distillate obtained in the distillation step to the collection tower of (a-2).

  The feature of the present invention is that the acrylic acid-containing solution collected in the aqueous solution is crystallized, and the residual mother liquor obtained in the crystallization step is distilled and the obtained distillate is circulated to the collection tower. . Conventionally, as described in Patent Document 4, in order to decompose the water / acrylic acid eutectic mixture containing acrylic acid, a salt is added to the acrylic acid-containing solution, and the crystallization step is performed with the contained water. In order to eliminate the influence of the above, the treatment was performed before the crystallization step, and the crystallization treatment was performed after removing the low-boiling substances and the high-boiling substances in advance. However, the present invention uses an acrylic acid-containing solution obtained by collecting an acrylic acid-containing gas in an aqueous solution, that is, an acrylic acid-containing solution before removing water and high-boiling substances, and is subjected to crystallization treatment as it is. By distilling the residual mother liquor generated in the crystallization step and circulating it in the collection tower, high-purity acrylic acid can be produced in a simple step. Moreover, when the residual mother liquor obtained in the crystallization step is distilled, a distillate containing water, acetic acid and the like can be obtained, and when this is circulated to a collection tower, the collection efficiency of acrylic acid can be improved. . In addition, concentration of high-boiling substances in the collection tower can be prevented, and the acrylic acid concentration in the bottom liquid of the collection tower can be increased. Furthermore, by carrying out the acrylic acid dimer decomposition step, active ingredients such as acrylic acid dimer contained in the residual mother liquor can be decomposed into acrylic acid, and when this decomposition product is circulated to the distillation tower, It can be circulated and finally recovered as an acrylic acid product by crystallization. An example of a preferred embodiment of the present invention will be described with reference to FIG.

First, a molecular oxygen-containing gas such as air 3, an acrylic acid raw material 1 such as propylene and / or acrolein, and a dilution gas 5 are mixed. In this step, after passing through the acrylic acid collection step, the recycle gas 34 discharged from the top of the collection tower can also be mixed with air, propylene and / or acrolein and a dilution gas. This mixed gas (hereinafter also referred to as source gas) is supplied to the reactor 20 filled with the catalytic gas phase oxidation catalyst 10, and the acrylic acid-containing gas 25 is obtained by the catalytic gas phase oxidation reaction. The gas 25 is supplied to the bottom of the collection tower 30, and the collection aqueous solution 33 is supplied from the top of the collection tower 30 to bring the acrylic acid-containing gas 25 into contact with the collection aqueous solution 33. The collection tower 30 is supplied with a distillate 71 from a distillation tower 70 to be described later and a residual mother liquor from a crystal folding apparatus 50 to be described later. Of the exhaust gas 32 from the top of the collection tower 30, only the recycle gas 34 is introduced into the cooling tower 36, and is cooled by gas-liquid contact with the collection water 33 ′ to be newly supplied into the system for recycling. The condensable substance contained in the gas 34 is condensed and then circulated to the reactor 20. The condensate may be mixed with the collection water 33 ′ and supplied to the collection tower 30 as a collection aqueous solution 33. In the present specification, of the exhaust gas 32 from the top of the collection tower 30, the exhaust gas to be circulated to the reactor is referred to as “recycle gas”, and the gas discharged outside the system is referred to as “waste gas”. . Thus, by circulating the distillation column distillate 71 and cooling the recycle gas 34, an acrylic acid-containing solution 35 containing acrylic acid at a higher concentration than the bottom of the collection tower is obtained.

  The acrylic acid-containing solution 35 is supplied to the acrolein separation tower 31, and the contained acrolein is separated to obtain an acrylic acid-containing solution 35 'in which the amount of acrolein is reduced from the bottom of the tower. In addition, when the tower top distillate of the separation tower 31 is circulated to the bottom of the collection tower 30, acrylic acid distilled together with acrolein can be effectively recovered.

  Next, when the acrylic acid-containing solution 35 ′ is supplied to the crystallizer 50, product acrylic acid 60 is obtained. On the other hand, at least a part of the residual mother liquor from the crystallizer 50 is supplied to the middle stage of the distillation column 70, the contained low boiling point substances and acrylic acid are distilled off from the top of the column, and the distillate 71 is collected in the collection tower. Circulate to 30. The remaining mother liquor is circulated directly to the collection tower 30. Further, since the high boiling point substance contained in the bottom liquid of the distillation tower 70 contains an acrylic acid dimer, it is retained in the thermal decomposition tank 75 via the thin film evaporator 73 and thermally decomposed into acrylic acid. When this acrylic acid is returned to the thin film evaporator 73, it is distilled from the top of the distillation column 70 to form a distillate 71, and when this is circulated to the collection tower 30, it can finally be recovered as product acrylic acid 60. .

  The present invention is characterized in that the acrylic acid-containing solution obtained by collecting the aqueous solution as described above is subjected to crystallization treatment as it is or after removing acrolein by acrolein separation treatment. Is 80% by mass or more, preferably 85% by mass or more, and particularly preferably 87% by mass or more. When the acrylic acid content is less than 80% by mass, the number of crystallizations performed to obtain an acrylic acid crystal by crystallization treatment increases and becomes complicated. The present invention provides a method for producing acrylic acid in a simple and high yield without performing an azeotropic dehydration step and a solvent recovery step on a low-boiling substance such as water and reducing acrylic acid loss. .

  In the present specification, the “low boiling point substance” means a substance having a lower boiling point than acrylic acid in the standard state, and the “high boiling point substance” means a substance having a higher boiling point than acrylic acid in the standard state. . “Condensable substance” means a substance that is liquid at 20 ° C. and atmospheric pressure, and “distillation” is a method of heating a solution to its boiling point to separate volatile components contained therein. Refers to a method in which a diffusion gas is supplied to transfer a target substance in a solution to a gas phase, and “crystallization” is an operation for precipitating crystals from a liquid phase and a gas phase. The “dynamic crystallization process” is a crystallization method in which the liquid phase is moved by forced convection such as a pump during crystallization. The “static crystallization process” is only natural convection without using a pump. Refers to a crystallization method that moves the liquid phase. Hereinafter, the present invention will be described in detail.

(1) Acrylic acid collection step In order to obtain a highly concentrated acrylic acid-containing solution, it is necessary to reduce the amount of water introduced into the system or increase the amount of water discarded outside the system. In order to minimize the acrylic acid loss discharged outside the system, it is preferable to recycle the exhaust gas discharged from the top of the collection tower to the reactor. In the present invention, the recycle gas is cooled and the amount of water contained in the gas is reduced and then recycled to the reactor, thereby reducing the amount of water introduced into the collection tower and reducing acrylic acid loss. A highly concentrated acrylic acid-containing solution can be obtained. However, if the total amount of exhaust gas from the collection tower is cooled and the amount of water and low-boiling substances discarded as gas outside the system is reduced, the collection efficiency of acrylic acid is improved even if the amount of water in the recycled gas is reduced. Rather, it is much lower than if it is not cooled, and low-boiling substances are concentrated in the system. On the other hand, when the condensate obtained by cooling is taken out of the system, it is necessary to treat a large amount of waste liquid. Therefore, it is preferable to cool only the so-called recycle gas which is recycled to the reactor out of the gas discharged from the acrylic acid collection tower. Note that not only moisture but also acid content can be condensed by cooling the recycle gas, the acid content supplied to the reactor can be reduced, and deterioration of the catalyst caused by the acid can also be prevented. Moreover, the condensate condensed by cooling may be returned to the collection tower, or may be taken out of the system without being returned. In both cases, the acrylic acid loss rate does not change so much and the amount of waste liquid is very small. However, if the entire amount is returned to the collection tower, it is advantageous because waste liquid treatment is unnecessary. As a method for reducing the amount of water introduced into the system, the water contained in the molecular oxygen-containing gas supplied to the reactor may be removed and then supplied to the reactor.

  In the present invention, propylene and / or acrolein can be used as the acrylic acid source gas. The reactor 20 is not particularly limited as long as a catalytic gas phase oxidation reaction can be performed, but a multitubular reactor can be preferably used in terms of excellent reaction efficiency. The reactor 20 is filled with a known catalytic gas phase oxidation catalyst 10 and oxidized by bringing a raw material gas into contact with a molecular oxygen-containing gas such as oxygen or air. When propylene is used as the raw material gas, the propylene concentration is 7 to 15% by volume, water is 0 to 10% by volume, and molecular oxygen is propylene: molecular oxygen (volume ratio) of 1: 1.0 to 2.0. The range. Air can be used as a source of molecular oxygen. When the air contains moisture, it is preferable to dehumidify it before supplying it to the reactor. This is because the amount of moisture introduced into the reactor, and hence the amount of moisture introduced into the collection tower, can be reduced. Note that oxygen-enriched air or pure oxygen can be used instead of air. Dilution gas 5 includes nitrogen, carbon dioxide, and other inert gases.

  In the present invention, the recycle gas may be cooled to condense the condensable material and then introduced into the reactor. When such a recycle gas is used, the moisture concentration in the raw material gas supplied to the reactor The water in the recycle gas is removed in advance so that is 0 to 10% by volume, more preferably 0 to 7% by volume, and particularly 0 to 6% by volume. If it exceeds 10% by volume, the acrylic acid loss rate may increase due to moisture supplied to the collection tower via the reactor. The total acid concentration is 0 to 0.2% by volume, more preferably 0 to 0.1% by volume. If the total acid concentration exceeds 0.2% by volume, deterioration due to oxidation of the catalyst may be promoted. The recycled gas contains unreacted propylene, acrolein, oxygen, dilution gas, and the like in addition to moisture and acid components. Calculate the amount of water contained in the recycle gas and the amount blended in the source gas so that the water concentration and total acid concentration in the source gas are within the above optimal ranges, and calculate the propylene concentration and oxygen concentration contained in the recycle gas. If the propylene concentration and the amount of air to be newly supplied to the reactor are determined, the propylene, oxygen, water concentration, and total acid concentration can be easily adjusted. The “total acid” is a compound having a carboxyl group, and the recycled gas includes acrylic acid, formic acid, acetic acid and the like.

  In the case of using propylene as a raw material, the catalytic gas phase oxidation reaction is usually performed in two stages, and two types of catalytic gas phase oxidation catalysts 10 are used. The first stage catalyst can mainly produce acrolein by gas phase oxidation of the raw material gas containing propylene, and the second stage catalyst can mainly produce acrylic acid by vapor phase oxidation of the raw material gas containing acrolein. Is. Examples of the first stage catalyst include a composite oxide containing iron, molybdenum and bismuth, and examples of the second stage catalyst include a catalyst containing vanadium as an essential component.

  Although FIG. 1 shows a mode in which the above two-stage reaction is performed in a single reactor, it may be performed in tandem in which two different reactors are connected. The acrylic acid-containing gas 25 obtained by the catalytic gas phase oxidation reaction includes 5 to 14% by volume of acrylic acid, 0.1 to 2.5% by volume of acetic acid, 0.5 to 3% by volume of molecular oxygen, and 5 to 36% of water. The other components are unreacted components in the raw material gas and reaction by-products such as propionic acid, maleic acid, acetone, acrolein, furfural, formaldehyde, and COx.

  In the acrylic acid collection tower 30, a known contact method can be used as a method of contacting the acrylic acid-containing gas and the aqueous solution for collection. For example, a bubble bell tray, a uniflat tray, a perforated plate tray, a jet tray Cross flow contact using bubble tray and venturi tray; turbo grid tray, dual flow tray, ripple tray, kittel tray, gauze type, sheet type, regular packing of grit type, countercurrent contact using irregular packing, etc. Can be mentioned.

  In the present invention, when the acrylic acid-containing gas 25 is brought into contact with the collection aqueous solution 33 to collect acrylic acid, a distillate 71 from the distillation tower 70 described later and a crystal folding device 50 described later are collected. It is preferable to supply the residual mother liquor (hereinafter referred to as circulating liquid) to the middle stage of the collection tower 30. The circulating fluid contains acetic acid and can be used as an acetic acid-containing solution. In the present invention, “other than the top” refers to a range of 2 to 100 theoretical plates when the top of the distillation column or the collection column is 1 theoretical plate and the bottom is 100 theoretical plates. Acrylic acid is absorbed by the collection aqueous solution 33 falling from the top of the tower, but it has been found that when acetic acid is introduced from a location other than the top of the collection tower, the collection efficiency of acrylic acid is improved. Although the principle is not clear, when acetic acid is introduced from any of the collection towers 30, an acetic acid gas layer is formed in the vicinity of the acetic acid supply position. This is probably because the gas distribution in the collection tower changes so that a boiling point gas layer is formed, and acrylic acid migrates to the bottom of the tower. In particular, it is preferable to introduce the acetic acid-containing solution from the middle stage of the collection tower having 2 to 100 theoretical plates, more preferably 25 to 100 plates, and particularly 50 to 100 plates. At the top of the column, the effect of improving the collection efficiency of acrylic acid due to the formation of an acetic acid gas layer is little. Rather, the acrylic acid collection rate is reduced, so the concentration of acrylic acid at the bottom of the column is lowered, and a highly concentrated acrylic acid-containing solution is obtained. I can't.

  The concentration of acetic acid contained in the circulating liquid is preferably 2% by mass or more, more preferably 2 to 20% by mass, and particularly preferably 3 to 15% by mass. If it is less than 2% by mass, the effect of changing the gas phase distribution in the collection tower is low.

  In particular, the amount of acetic acid to be introduced is 0.005 to 0.2 times the mass flow rate of acrylic acid contained in the acrylic acid-containing gas, more preferably 0.008 to 0.15 times, and particularly preferably 0.01 to 0.00. The amount of circulating fluid is adjusted so that the mass flow rate is 1 time. In some cases, fresh acetic acid can be added. If it is less than 0.005 times, there is no effect of improving the collection efficiency of acrylic acid. On the other hand, if it exceeds 0.2 times, the concentration of acetic acid in the collected solution will increase significantly, so a highly concentrated acrylic acid-containing solution It becomes difficult to obtain.

  The collection aqueous solution 33 used in the present invention can be widely used as long as it is an aqueous solution capable of collecting acrylic acid, but a condensed liquid condensed by cooling the recycle gas is used as the collection aqueous solution. You can also. Since the condensate often contains acrylic acid, it is preferably reused as an aqueous solution for collection. The temperature of the aqueous solution for collection is 0 to 50 ° C, preferably 10 to 40 ° C.

  The mass flow ratio of the collection water 33 ′ newly supplied from the outside to the acrylic acid-containing gas can be appropriately selected depending on the target acrylic acid concentration, and is 0 of the mass flow rate of acrylic acid contained in the acrylic acid-containing gas. 0.1 to 1.5 times, preferably 0.1 to 1.0 times, and particularly 0.15 to 0.8 times the mass flow rate is brought into countercurrent contact to collect acrylic acid. When the mass flow rate ratio is less than 0.1 times, the acrylic acid collection tower may be extremely reduced in efficiency. On the other hand, when it exceeds 1.5 times, it becomes difficult to obtain a highly concentrated acrylic acid-containing solution. In addition, in order to prevent polymerization of a polymerizable substance such as acrylic acid, N-oxyl described in JP-A Nos. 2001-348360, 2001-348358, 2001-348359 and the like is used as the collection water. One or more compounds selected from the group consisting of a compound, a phenol compound, a manganese salt such as manganese acetate, a dialkyldithiocarbamate copper salt such as copper dibutylthiocarbamate, a nitroso compound, an amine compound and phenothiazine may be contained.

  The acrylic acid collection tower is generally operated at atmospheric pressure or higher. In the present invention, the tower top pressure (gauge pressure) is 0 to 0.4 MPa, preferably 0 to 0.1 MPa, particularly 0 to 0.03 MPa. If the pressure is lower than 0 MPa (gauge pressure), a decompression device is required, which increases equipment costs and utility costs. On the other hand, if the pressure is higher than 0.4 MPa (gauge pressure), the temperature of the collection tower is reduced in order to discharge low-boiling substances from the top of the tower. It may be necessary to increase the efficiency, and the collection efficiency may decrease. Moreover, as tower | column top temperature, generally it is 30-85 degreeC, It is especially preferable that it is 40-80 degreeC. In the present invention, acrylic acid: 80 to 98% by mass, water: 1 to 19% by mass, and other impurities (acids such as acetic acid, maleic acid, propionic acid, and furfural, formaldehyde, etc.) Aldehydes, etc.): 1 to 10% by mass of acrylic acid-containing solution 35 is obtained.

  In addition, there is no restriction | limiting in the cooling method of recycle gas, What is necessary is just an apparatus which can condense the condensable substance contained in recycle gas. For example, multi-tube heat exchangers, finned tube heat exchangers, air-cooled heat exchangers, double-tube heat exchangers, coil heat exchangers, direct contact heat exchangers, plate heat exchangers, etc. Can be used.

  However, since the condensate often contains a polymerizable substance such as acrylic acid, as shown in FIG. 1, a cooling method combining a cooling tower 36 and a cooler 39 facilitates the use of a polymerization inhibitor. Since it can supply, it is preferable.

  Moreover, although there is no restriction | limiting in the cooling temperature of recycle gas, the water concentration in the raw material gas whole quantity supplied to a reactor will be 0-10 volume%, More preferably, it will be 0-7 volume%, Especially 0-6 volume%. Thus, more preferably, the total acid concentration is further condensed by cooling so that the total acid concentration is 0 to 0.2% by volume, more preferably 0 to 0.1% by volume. When air is used as the molecular oxygen-containing gas, the air contains moisture. The amount of water after cooling the recycle gas is determined from the preferable amount of water and the above-mentioned preferable water concentration and amount of the raw material gas, and cooled to achieve the water concentration. In the present invention, the temperature of the recycle gas is cooled to 1 to 50 ° C, more preferably 2 to 40 ° C, and particularly preferably 3 to 30 ° C lower than the temperature of the waste gas.

  As described above, a very high concentration collection tower bottom liquid having an acrylic acid concentration of 80% by mass or more can be obtained.

(2) Acrolein separation The acrylic acid-containing solution 35 may contain acrolein, which is an acrylic acid raw material. In order to remove this, it may be supplied to the acrolein separation tower 31 to separate the acrolein.

  The separation tower is not particularly limited as long as it can separate acrolein, but a packed tower, a plate tower (tray tower), or the like can be used. The conditions for the separation tower can be appropriately selected from methods such as distillation and diffusion depending on the concentration of acrylic acid and the concentration of acrolein contained. In the case of distillation, the column top pressure (absolute pressure) is preferably 20 to 800 hPa, preferably 40 to 600 hPa, and particularly preferably 60 to 400 hPa. When the pressure is lower than 20 hPa (absolute pressure), the tower, the condenser, and the vacuum apparatus are increased in size, which is disadvantageous in terms of equipment costs. On the other hand, if the pressure is higher than 800 hPa (absolute pressure), the temperature in the separation column becomes high, which is disadvantageous because the risk of polymerization increases. Moreover, generally tower top temperature is 30-100 degreeC, Especially 40-80 degreeC. On the other hand, the bottom temperature is generally 40 to 110 ° C, particularly 50 to 90 ° C. In the case of emission, acrolein can be separated by a conventionally known method. Under such separation conditions, the amount of acrolein can be reduced, and an acrylic acid-containing solution having an acrylic acid concentration of 80% by mass or more can be obtained.

(3) Crystallization step of acrylic acid-containing solution In the present invention, the acrylic acid-containing solution 35 or 35 ′ is supplied to the crystallizer 50 to obtain purified acrylic acid 60.

There is no restriction | limiting in the crystallization method to be used, either a continuous type or a batch type may be sufficient, and it can implement by 1 step | paragraph or 2 steps | paragraphs or more. Examples of the continuous crystallizer include a tower type BMC (Backmixing Column Crystallizer) crystallizer (Nippon Chemical Co., Ltd .: Japan), a crystallizing unit, a solid-liquid separation unit, and a crystal purification unit. For example, a CDC (Cooling Disk Crystallizer) crystallizer (GOUDA: Netherlands) and the like, a solid-liquid separation unit such as a belt filter or a centrifuge, and a crystal purification unit such as a KCP (Kureha Crystal Purifier) purification device (Kureha). It is possible to use a crystallizer combined with Techno Engineering (Japan).

  Examples of the crystallization apparatus 50 include a combination of a crystallization apparatus, a solid-liquid separation apparatus, and a crystal purification apparatus, and one advantageous embodiment in the present invention is a method using a continuous crystallization apparatus. As the crystallization section, for example, an apparatus in which two crystallization apparatuses (CDC) described in “Chemical apparatus, July 2001, p77-78” are arranged can be used. In each crystallization apparatus (1), (2), the inside of the horizontal crystallization tank is partitioned by several cooling plates that pass through the lower part and have a gap between the passages. Through these cooling plates, cooling and crystallization are performed. Is done. The stirring shaft that passes through the center of the cooling plate is provided with a stirring blade and a wiper for renewing the cooling transmission surface. The liquid supplied from the raw material liquid inlet is sequentially supplied from the path under the cooling plate to the other side by the stirring shaft. Move to the end of the. For example, when an acrylic acid aqueous solution is supplied to the crystallizer (1), the acrylic acid is crystallized here, and a belt filter is used as a solid-liquid separation unit, and the crystal and mother liquor are separated, and then the crystallizer is crystallized. It can be supplied to (2) to crystallize acrylic acid, and the crystals and mother liquor can be separated with a belt filter. Next, the crystals obtained by these crystallization apparatuses (1) and (2) are introduced into the crystal purification section. The crystallization amount of acrylic acid obtained by adding the crystallization devices (1) and (2) is 20% by mass or more of the amount of acrylic acid supplied to the crystallization device (1), preferably 30 to 90% by mass, In particular, the content is preferably 40 to 90% by mass. Thereby, the acetic acid concentration in the residual mother liquor can be set to 2% by mass or more. As the crystal refining section, for example, a crystal refining apparatus (KCP) described in “Chemical apparatus, July 2001 issue p76-77” or Japanese Patent Publication No. 47-40621 can be used. For example, a screw conveyor is provided at the center of a metal tube, a melting machine for melting the crystal at the top and a product take-out port after melting, a take-out port for the residual liquid at the bottom, and a crystal supply to the side of the lower column The thing provided with the mouth can be illustrated. Crystals are conveyed to the top of the purification tower by a screw conveyor and melted by a melting machine. The molten liquid is taken out from the product outlet and part of it is dropped from the upper part of the purification tower. With this falling liquid, the crystal carried from the screw conveyor is washed and sweated, and the liquid falling from the upper part is extracted from the lower residue outlet. The amount of the falling liquid can be appropriately selected depending on the purity of the target acrylic acid, but is 1 to 60% by mass, more preferably 2 to 40% by mass, and particularly 5 to 35% by mass of the amount of the melt. preferable. When the amount is less than 1% by mass, the effect of washing crystals and sweating with the falling liquid is low. On the other hand, if it exceeds 60% by mass, the effects of washing and sweating are no longer improved, and in some cases, the operation of the crystal purification tower becomes difficult. The taken-out residue can be circulated to the crystallization apparatus and / or the crystallization residual mother liquor, and a part thereof may be taken out of the system.

  Another advantageous embodiment of the present invention is a method using a batch crystallizer. As such an apparatus, for example, a layer crystallization apparatus (dynamic crystallization apparatus) manufactured by Sulzer Chemtech or a static crystallization apparatus manufactured by BEFS PROKEEM can be used.

  In a batch system, the number of crystallization stages required depends on how much purity is required. In the present invention, in order to produce high-purity acrylic acid, the purification step (dynamic crystallization step) is performed 1 to 6 times, preferably 2 to 5 times, more preferably 2 to 4 times, and a stripping step ( The dynamic crystallization step and / or the static crystallization step) are performed 0 to 5 times, preferably 0 to 3 times. A part of the removed residue may be taken out of the system.

  Further, pre-purification may be performed in advance before the crystallization purification step. Prepurification is carried out by crystallization. Therefore, the pre-purification step and the subsequent purification step can be regarded as a series of crystallization steps. For the pre-purification, a pre-purification apparatus combining a crystallization apparatus and a solid-liquid separation apparatus can be used. A CDC crystallization apparatus, a tank crystallization apparatus, or the like can be used as the crystallization apparatus, and a belt filter, a centrifuge, or the like can be used as the solid-liquid separation apparatus.

  One advantageous embodiment including pre-purification in the present invention includes a tank crystallization device and a centrifuge as the pre-purification device, and the tank crystallization device comprises two tanks arranged in series. Each is equipped with a stirrer, the surface is composed of a double jacket, and this jacket is controlled to be a constant temperature which is a thermostat. First, an acrylic acid aqueous solution is supplied to the first crystallization tank to crystallize acrylic acid, and after the crystal and the mother liquor are separated by a centrifugal separator, the crystal is washed with the melt. Next, the obtained mother liquor and washing liquid are supplied to the second crystallization tank to crystallize acrylic acid, and after the crystals and mother liquor are separated by a centrifuge, the crystals are washed using the melt. The total amount of acrylic acid crystallized in the first crystallization tank and the second crystallization tank is 20% by mass or more of acrylic acid supplied to the first crystallization tank, preferably 30 to 90% by mass. In particular, the content is preferably 40 to 90% by mass. As a result, the concentration of acetic acid in the residual mother liquor can be increased to 2% by mass or more. The obtained crystals are melted, and the pre-purified acrylic acid-containing solution is further purified.

  Purification is, for example, a multistage fractional crystallization method, which includes a tubular crystallizer equipped with a temperature control mechanism for crystallization, sweating, and melting, a tank for collecting the mother liquor after sweating, and an acrylic in the crystallizer. A dynamic crystallization process using a dynamic crystallization apparatus comprising a circulation pump for supplying an acid and capable of transferring acrylic acid from a reservoir disposed at the lower part of the crystallizer to the upper part of the crystallizer tube by a circulation pump, or Static crystal with a tubular crystallizer equipped with a temperature control mechanism for performing crystallization, sweating and melting, with an extraction valve at the bottom, and a tank for collecting the mother liquor after sweating Crystallization can be performed by a combination of a dynamic crystallization process and a static crystallization process, which simultaneously use a crystallization apparatus.

  The crude acrylic acid-containing solution is introduced into the crystallizer as a liquid phase, and then a solid phase having a composition different from that of the introduced liquid phase is solidified on the cooling surface. The remaining liquid phase is separated and removed as soon as the amount of acrylic acid used in the range of 40 to 90% by mass, preferably 50 to 80% by mass, solidifies. The remaining phase is separated and removed by pumping (dynamic crystallization process) or flowing (static crystallization process). Subsequently, for example, so-called crystal layer cleaning or so-called perspiration, that is, partial melting and removal of the impurity crystal region can be further performed.

  When dynamic and static crystallization are carried out in a multistage process, it is advantageously carried out by the countercurrent principle, the material crystallized in each process being separated from the residual mother liquor after crystallization and this crystallisation. The material is fed to the next step with higher purity, while the crystallization residue is fed to the next step with lower purity.

  In general, the entire process in which an acid having a higher purity than the supplied crude acid solution is obtained is known as a purification process, and the other processes are known as stripping processes. The stripping step is performed to recover the acrylic acid in the mother liquor from the purification step. In dynamic crystallization, if the purity of acrylic acid is low, crystallization becomes difficult. On the other hand, static crystallization is easy even if the purity of acrylic acid is lower than that of dynamic crystallization. Therefore, in order to increase the recovery rate of acrylic acid, the final mother liquor of dynamic crystallization is further crystallized by static crystallization.

  The number of crystallization stages required depends on how much purity is required, but in order to produce high purity acrylic acid, the purification step (dynamic crystallization step) is 1-5 times, Preferably 2-3 times, stripping step (dynamic crystallization step) 1-5 times, preferably 1-4 times, stripping step (static crystallization step) 1-5 times, preferably 1-5 times. It is preferable to perform three times.

  The residue taken out from the final step of the static crystallization step may be circulated to the pre-purified crystallization residual mother liquor or taken out of the system.

(4) Distillation of acrylic acid-containing solution Residual mother liquor from the crystallizer 50 contains high-concentration acrylic acid, low-boiling substances such as acetic acid and water, and high-boiling substances such as acrylic acid dimers and polymerization inhibitors. It is included. In the present invention, in order to effectively use these residual mother liquors, at least a part of the residual mother liquor is supplied to the distillation column 70, and high-boiling substances are distilled from the tower bottom, and low-boiling substances and acrylic acid are distilled from the tower top. As the distillation column 70 used for such a purpose, a packed column, a plate column (tray column) or the like can be used.

  Distillation conditions are distilled under the condition of distilling low boiling point substances such as water and acetic acid and acrylic acid, and can be appropriately selected depending on the acrylic acid concentration, water concentration, acetic acid concentration, etc. of the residual mother liquor to be introduced. Generally, the tower top pressure (absolute pressure) is 10 to 400 hPa, preferably 15 to 300 hPa, particularly 20 to 200 hPa. When the pressure is lower than 10 hPa (absolute pressure), the tower, the condenser, and the vacuum apparatus are increased in size, which is disadvantageous in that the equipment cost is increased. On the other hand, if it is higher than 400 hPa (absolute pressure), the temperature in the distillation column 70 becomes high, which is disadvantageous because the risk of polymerization increases. The tower top temperature is generally 30 to 70 ° C, particularly 40 to 60 ° C. On the other hand, the tower bottom temperature is generally 50 to 140 ° C, particularly 60 to 120 ° C. Under such distillation conditions, a distillate 71 having a higher acetic acid concentration than the residual mother liquor can be obtained. The distillate 71 is circulated through the collection tower 30.

  In order to prevent polymerization of a polymerizable substance such as acrylic acid during distillation, a polymerization inhibitor can be added to the reflux liquid. As such a polymerization inhibitor, various polymerization inhibitors that can be added to the collection aqueous solution described above can be used.

(5) Decomposition of acrylic acid dimer The bottom liquid of the distillation tower 70 contains acrylic acid dimer, which can be decomposed to recover acrylic acid. Here, the acrylic acid dimer decomposition is not particularly limited as long as the acrylic acid dimer is decomposed and recovered as acrylic acid. For example, the acrylic acid dimer decomposition and the acrylic acid distillation may be carried out simultaneously (see Japanese Patent Publication No. 61-35977, Japanese Patent Publication No. 61-36501, etc.). Examples thereof include those using an acrylic acid recovery tower equipped with a thin film evaporator provided with a stage tower (see JP-A-11-12222, etc.).

  The above-mentioned acrylic acid dimer decomposition apparatus can be newly used separately from the distillation tower 70, but can preferably be implemented in a form in which a pyrolysis tank is provided in addition to the distillation tower 70 provided with a thin film evaporator. .

  That is, the tower bottom liquid of the distillation tower 70 (the bottom of the thin film evaporator 73) is introduced into the thermal decomposition tank 75 to decompose the acrylic acid dimer contained therein. The decomposition tank 75 decomposes the acrylic acid dimer at a temperature in the range of 120 to 220 ° C., and the residence time (pyrolysis tank dissolved amount / waste oil amount) varies depending on the thermal decomposition temperature, but is usually 0.1 to 60. Time. After the acrylic acid dimer is decomposed into acrylic acid, the acrylic acid can be recovered from the top of the distillation column 70 by circulating it into the thin film evaporator.

  In addition to the distillation tower 70, when using an acrylic acid recovery tower equipped with a thin film evaporator newly provided with a pyrolysis tank and a stage tower, the recovered acrylic acid obtained from the top of the acrylic acid recovery tower is Can be circulated to the distillation column 70 and / or the collection column 30.

  It should be noted that a decomposition catalyst such as an alkali metal salt, an alkaline earth metal salt, or an N-oxyl compound described in JP-A-2003-89672 may be added to the thermal decomposition tank during the acrylic acid dimer decomposition. it can. When the N-oxyl compound is used as a polymerization inhibitor in the collection step or distillation step, it also acts as a decomposition catalyst for the acrylic acid dimer.

  Hereinafter, examples of the present invention will be described in detail.

(Catalyst production example)
A molybdenum-bismuth catalyst was prepared according to the description in Example 1 of JP-A No. 2000-325795. This is designated as catalyst (I). Also, a molybdenum-vanadium catalyst was prepared according to the description in Example 1 of JP-A-8-206504. This is referred to as catalyst (II).

Example 1
Acrylic acid was produced using the apparatus shown in FIG.

  A heat medium circulation jacket is provided on the outer periphery, a reaction tube with an inner diameter of 25 mm and a length of 7,000 mm is accommodated in the inside, and a 75 mm thick hole that divides the heat medium jacket into two parts vertically at a position of 3,500 mm from the bottom of the jacket A reactor equipped with a perforated tube plate was used. The lower part of the reactor (first reaction zone) and the upper part (second reaction zone) each circulate a heat medium to control the temperature, and from the bottom to the top of the reaction tube (1) ceramic balls with an average diameter of 5 mm, (2 A mixture of catalyst (I) and ceramic balls having an average diameter of 5 mm in a volume ratio of 70:30, (3) catalyst (I), (4) an outer diameter of 5 mm, an inner diameter of 4.5 mm, and a length of 6 mm Stainless steel Raschig rings, (5) a mixture of catalyst (II) and ceramic balls having an average diameter of 5 mm in a volume ratio of 75:25, and (6) catalyst (II) in the order of 250 mm, 700 mm, 2 , 300 mm, 500 mm, 600 mm, and 1,900 mm.

In the first reaction zone of the reactor, propylene, air (water concentration 2% by mass), and part of the exhaust gas (recycle gas) from the collection tower are circulated, propylene: 8.0% by volume, O ₂ : 14.4% by volume, H ₂ O: 2.0% by volume (the rest are N ₂ , propane, COx, acrylic acid, acetic acid, etc.), and the space velocity of the first reaction zone is 1,250 hr ⁻¹ (STP). Each flow rate and the cooling temperature of the recycle gas were adjusted so as to be supplied.

  In the second reaction zone, the first reaction zone and the second reaction zone are adjusted so that the propylene conversion rate is 97 ± 0.5 mol% and the acrolein yield is 1 ± 0.5 mol% at an outlet pressure of 0.15 MPa (absolute pressure). The heating medium temperature of each reaction zone was adjusted to obtain an acrylic acid-containing gas containing 16.62% by mass of acrylic acid at 18.77 kg / hour.

  Next, after the obtained acrylic acid-containing gas was cooled to 200 ° C. with a precooler, it was led to collection of acrylic acid and collected as an acrylic acid-containing solution.

  The collection tower is a packed tower packed with a regular packing, and has 21 theoretical theoretical plates. The supply port for the acrylic acid-containing gas and the extraction port for the collected liquid are collected at the bottom of the tower, and the collection is performed at the top of the tower. Aqueous solution inlet and gas outlet, tower side (theoretical stage 19th stage) is provided with a supply pipe for the top liquid from the distillation tower, and a part of the gas discharged from the top of the tower is cooled. The cooler is provided.

  As water for collection, 1.01 kg / hour of water containing hydroquinone corresponding to 200 mass ppm with respect to the amount of acrylic acid in the acrylic acid-containing gas introduced into the collection tower was supplied.

  The top temperature of the acrylic acid collection tower was 66.9 ° C., the top pressure was 0.11 Mpa (absolute pressure), the recycle gas cooling temperature was 40.6 ° C., and the recycle rate was 29.0%. Note that the entire amount of the condensate obtained by cooling the recycle gas was circulated to the collection tower.

  From the column side, acrylic acid 74.8% by mass, water 8.6% by mass, acetic acid 5.1% by mass, maleic acid 2.1% by mass, furfural 0.2% by mass, benzaldehyde 0.8% by mass, 1.90 kg / hr of circulating liquid consisting of distillate and residual mother liquor of a distillation column having a composition of 0.2% by mass of formaldehyde, 4.2% by mass of acrylic acid dimer, and 4.0% by mass of other impurities Supplied with.

  The absorption efficiency of acrylic acid in the collection tower at this time was 98.22%.

  Further, this acrylic acid-containing solution is supplied to the top of a packed tower having an inner diameter of 100 mm and a packed height of 5 m, while heating so that the tower top pressure is 265 hPa (absolute pressure) and the tower bottom temperature is 70 ° C. Acrolein and the like were separated by distillation. Thereby, from the tower bottom part, acrylic acid 89.0 mass%, water 3.2 mass%, acetic acid 1.9 mass%, maleic acid 1.1 mass%, furfural 0.07 mass%, benzaldehyde 0.3 mass% Acrylic acid aqueous solution containing 0.06% by mass of formaldehyde, 2.3% by mass of acrylic acid dimer, and 2.07% by mass of other impurities was obtained at 5.10 kg / hour. The gas obtained from the top of the tower was supplied to the lower part of the acrylic acid collection tower.

  Next, this acrylic acid aqueous solution was supplied to a crystallizer and crystallized. Here, the crystallization apparatus is one in which two crystallization apparatuses are arranged in series as described in “Chemical apparatus, July 2001 issue P77-78”. In each crystallization apparatus, the inside of a horizontal crystallization tank is partitioned by several cooling plates (a clearance is formed in the lower part), and cooling and crystallization are performed through the cooling plates. . The stirring shaft penetrating the center of the cooling plate is provided with a stirring blade and a wiper for renewing the cooling transmission surface. The liquid supplied from the raw material liquid input at one end passes through the cooling plate by the stirring shaft. It is structured to move sequentially from one to the other end. First, when an acrylic acid aqueous solution is supplied to the first crystallizer, acrylic acid is crystallized here, and the crystals and mother liquor are separated by a belt filter, and then the obtained mother liquor is supplied to the second crystallizer to obtain acrylic acid. And crystal and mother liquor are separated with a belt filter. The temperature of the cooling plate of the first crystallizer is 0 ° C., the temperature of the cooling plate of the second crystallizer is −7 ° C., and the crystallization amount of acrylic acid is the sum of the first and second crystallizers. Was adjusted to 68% by mass of the amount of acrylic acid supplied to the first crystallizer.

  Next, the crystals obtained by the first and second crystallization apparatuses were supplied to the crystal purification apparatus and purified.

  The apparatus conforms to the crystal apparatus described in “Chemical apparatus, July 2001 issue P76-77”, Japanese Patent Publication No. 47-40621. That is, a screw conveyor is provided at the center of a metal tube, a melting machine for melting the crystal at the top and a product take-out port after melting, a take-out port for residual liquid at the bottom, and a crystal feed port at the bottom side It is equipped with. The crystals were transported to the top of the refiner by a screw conveyor and melted by a melting machine. The melt was taken out from the product outlet, and 10% by mass of the melt was dropped from the top of the apparatus. With this falling liquid, the crystals carried from the screw conveyor were washed and sweated. The liquid falling from the upper part was extracted from the lower residue outlet.

  As a result, acrylic acid having a purity of 99.94% by mass was obtained at 3.12 kg / hour. At this time, water 40 mass ppm, acetic acid 515 mass ppm, maleic acid 4 mass ppm, furfural 0.1 mass ppm, benzaldehyde 0.5 mass ppm, formaldehyde 0.0 mass ppm, acrylic acid dimer 40 mass ppm. It was.

  The purification yield of acrylic acid was 99.9%.

  30% of the residual mother liquor obtained from the second crystallization apparatus and the residue taken out from the crystal purification apparatus were fed to the middle stage of the acrylic acid dimer decomposition distillation column. The acrylic acid dimer decomposition distillation column has a structure in which a distillation column having a 15-stage sieve tray is combined with a thermal decomposition tank and a thin film evaporator for decomposing the acrylic acid dimer, Thermal decomposition is performed under the conditions of a pyrolysis tank temperature of 145 ° C. and a residence time of 4 hours, the thin film evaporator is controlled so that the tower bottom temperature is 85 ° C., the tower top pressure is 33 hPa, the reflux ratio is 0.1, It was operated by adding 4H-TEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidineoxyl equivalent to 200 ppm) from the refluxing liquid as a stabilizer to the acrylic acid supplied to the distillation column. In the thermal decomposition tank, 0.04% by mass (converted to NaOH) was added to the residual mother liquor supplying a 20% by mass sodium hydroxide aqueous solution as an acrylic acid dimer decomposition catalyst.

  Acrylic acid containing 82.9% by mass of acrylic acid, 9.6% by mass of water, and 5.7% by mass of acetic acid is recovered from the top of the column at 0.51 kg / hr, and the remaining second crystallizer remains. Along with the mother liquor and the rest of the crystal purifier, it was circulated to the side of the collection tower. The composition of the circulating fluid was as follows.

Acrylic acid 74.8 wt%, water 8.6 wt%, acetic acid 5.1 wt%, maleic acid 2.1 wt%, furfural 0.2 wt%, benzaldehyde 0.8 wt%, formaldehyde 0.2 wt% , 4.2% by mass of acrylic acid dimer, 4.0% by mass of other impurities
(Example 2)
Acrylic acid was produced using the apparatus shown in FIG.

  From the side of the tower, as a circulating liquid composed of the distillate from the distillation tower and the residual mother liquor, acrylic acid 77.5% by mass, water 8.5% by mass, acetic acid 4.8% by mass, maleic acid 2.1% by mass A circulating fluid having a composition of 0.2% by mass of furfural, 0.6% by mass of benzaldehyde, 0.2% by mass of formaldehyde, 2.5% by mass of acrylic acid dimer, and 3.6% by mass of other impurities was supplied. Except for the above, the same operation as in Example 1 was performed, and from the bottom of the packed column, acrylic acid 89.9% by mass, water 3.2% by mass, acetic acid 1.9% by mass, maleic acid 1.1% by mass, 5.05 kg of aqueous acrylic acid solution containing 0.06% by mass of furfural, 0.2% by mass of benzaldehyde, 0.06% by mass of formaldehyde, 1.6% by mass of acrylic acid dimer, and 1.98% by mass of other impurities / Hour. The gas obtained from the top of the tower was supplied to the lower part of the acrylic acid collection tower.

  The collection efficiency of acrylic acid in the collection tower at this time was 98.21%.

  Next, this acrylic acid aqueous solution was supplied to a pre-purification apparatus consisting of a crystallization tank and a centrifuge to cause crystallization. The crystallization tank consists of two tanks arranged in series, each equipped with a stirrer, the surface is composed of a double jacket, and this jacket is controlled to a constant temperature which is a thermostat. First, an acrylic acid aqueous solution was supplied to the first crystallization tank. Here, acrylic acid was crystallized, and the crystal was separated from the mother liquor with a centrifuge, and then the crystal was washed with the melt. Next, the obtained mother liquor and washing liquid were supplied to a second crystallization tank to crystallize acrylic acid, and after separating the crystals and mother liquor with a centrifuge, the crystals were washed with the melt. The jacket temperature of the first crystallization tank was −5 ° C., the jacket temperature of the second crystallization tank was −14 ° C., and the cleaning liquid was 0.16 kg / hour. 98.9% by mass of acrylic acid, 0.2% by mass of water, 0.3% by mass of acetic acid, 0.3% by mass of maleic acid, 0.003% by mass of furfural, 0.003% by mass of furfural from the first and second crystallization tanks. Acrylic acid containing 01% by mass, formaldehyde 0.002% by mass, and acrylic acid dimer 0.4% by mass was obtained at 3.15 kg / hour.

  30% of the residual mother liquor obtained from the crystallization tank 2 was supplied to the middle stage of the acrylic acid dimer decomposition distillation column. The acrylic acid dimer decomposition distillation column was the same as in Example 1, and was operated under the same conditions as in Example 1.

  From the top of the column, a distillate containing 84.0% by mass of acrylic acid, 9.3% by mass of water and 5.3% by mass of acetic acid is recovered at 0.50 kg / hr, and the tower of the collection tower is collected together with the remaining residual mother liquor. Circulated to the side. The composition of the circulating fluid was as follows.

  Acrylic acid 77.5 mass%, water 8.5 mass%, acetic acid 4.8 mass%, maleic acid 2.1 mass%, furfural 0.2 mass%, benzaldehyde 0.6 mass%, formaldehyde 0.2 mass% And 2.5% by mass of acrylic acid dimer and 3.6% by mass of other impurities.

  Next, acrylic acid obtained from the crystallization tank was supplied to another crystallizer and purified by two dynamic crystallization steps. Furthermore, the crystallization residue of this purification step was treated with three dynamic crystallization steps and two static crystallization steps.

  The dynamic crystallization was performed with a crystallization purification apparatus according to the crystallization apparatus described in JP-B-53-41637. That is, a metal pipe having a length of 6 m and an inner diameter of 70 mm provided with a reservoir at the lower part, the liquid in the reservoir is transferred to the upper part of the pipe by a circulation pump, and the liquid is allowed to flow on the inner wall surface of the pipe in a falling film form. Be able to. The surface of the tube is composed of a double jacket, which is a thermostat and is controlled to reach a certain temperature. One dynamic crystallization was performed by the following procedure.

  1. Crystallization: Acrylic acid was supplied to the reservoir, and it was allowed to flow in the form of a falling film on the wall surface of the tube by a circulation pump. The temperature of the jacket was lowered below the freezing point, and about 60 to 80% by mass was crystallized on the wall surface.

2. Sweating: Stop circulation pump, raise jacket temperature to near freezing point, about 2
~ 5% by weight was perspired. After sweating, the residual melt was pumped out.

  3. Melting: The temperature of the jacket was raised above the freezing point, the crystals were melted and pumped out.

  In the above operation, the temperature and the freezing point depended on each process to be performed.

  Static crystallization is a tube with an inner diameter of 90 mm and a length of 1 m with an extraction cock at the bottom, and the tube surface is composed of a double jacket, which is a thermostat and controlled to reach a certain temperature. ing. One static crystallization was performed by the following procedure.

  Static crystallization is a tube with an inner diameter of 90 mm and a length of 1 m with an extraction cock at the bottom, and the tube surface is composed of a double jacket, which is a thermostat and controlled to reach a certain temperature. ing. A single static crystallization procedure was performed.

  1. Crystallization: Acrylic acid was supplied into the tube, the temperature of the jacket was lowered below the freezing point, and about 60 to 80% by mass was crystallized.

  2. Sweating: The residual mother liquor after crystallization was withdrawn from the lower part of the tube, the temperature of the jacket was raised near the freezing point, and about 15 to 25% by mass was perspired. After sweating, the sweat solution was extracted.

  3. Melting: The temperature of the jacket was raised above the freezing point, and the crystals were melted and extracted. As a result, high-purity acrylic acid having a purity of 99.95% by mass was obtained at 3.07 kg / hour.

  At this time, water 10 mass ppm, acetic acid 475 mass ppm, maleic acid 2 mass ppm, furfural 0.1 mass ppm, benzaldehyde 0.3 mass ppm, formaldehyde 0.0 mass ppm, acrylic acid dimer 12 mass ppm. It was.

The purification yield of acrylic acid was 98.5%.
(Example 3)
Acrylic acid was produced using the apparatus shown in FIG.

From the side of the tower, as a circulating liquid composed of the distillate from the distillation tower and the residual mother liquor, acrylic acid 83.5% by mass, H ₂ O 6.7% by mass, acetic acid 3.1% by mass, maleic acid 1.3% by mass %, Furfural 1.4% by mass, benzaldehyde 0.5% by mass, formaldehyde 0.2% by mass, acrylic acid dimer 2.6% by mass and other impurities 0.7% by mass Except that, the same operation as in Example 1 was performed, and 90.2% by mass of acrylic acid, 3.4% by mass of water, 1.6% by mass of acetic acid, 1.0% by mass of maleic acid from the bottom of the packed column, 6.36 kg of acrylic acid aqueous solution containing furfural 0.7 mass%, benzaldehyde 0.3 mass%, formaldehyde 0.09 mass%, acrylic acid dimer 2.4 mass%, and other impurities 0.31 mass% / Hour. The gas obtained from the top of the tower was supplied to the lower part of the acrylic acid collection tower.

  At this time, the collection efficiency of acrylic acid in the collection tower was 98.20%.

  Next, this aqueous acrylic acid solution was purified by the dynamic crystallization process four times in the same manner using the same dynamic crystallization apparatus as in Example 2.

  As a result, high-purity acrylic acid having a purity of 99.94% by mass was obtained at 3.09 kg / hour.

  At this time, water 100 mass ppm, acetic acid 450 mass ppm, maleic acid 3 mass ppm, furfural 0.4 mass ppm, benzaldehyde 0.1 mass ppm, formaldehyde 0.0 mass ppm, and acrylic acid dimer 30 mass ppm. It was.

  The purification yield of acrylic acid was 99.0%.

45% of the residual mother liquor obtained by the dynamic crystallization process was fed to the middle stage of the acrylic acid dimer decomposition distillation column. The acrylic acid dimer decomposition distillation column was the same as in Example 1, and was operated under the same conditions as in Example 1.
From the top of the column, a distillate containing 86.4% by mass of acrylic acid, 7.0% by mass of water, and 3.2% by mass of acetic acid is recovered at 1.42 kg / hr, and the tower of the collection tower is collected together with the remaining residual mother liquor. Circulated to the side. The composition of the circulating fluid was as follows.

Acrylic acid 83.5% by mass, H ₂ O 6.7% by mass, acetic acid 3.1% by mass, maleic acid 1.3% by mass, furfural 1.4% by mass, benzaldehyde 0.5% by mass, formaldehyde 0.2% by mass %, Acrylic acid dimer 2.6 mass%, and other impurities 0.7 mass%.

(Comparative Example 1)
The same operation as in Example 1 was carried out except that the circulating liquid consisting of the distillate from the distillation column and the residual mother liquor was not circulated to the collection tower, and 90.0% by mass of acrylic acid and water 4. 7% by weight, acetic acid 2.8% by weight, maleic acid 0.5% by weight, furfural 0.03% by weight, benzaldehyde 0.03% by weight, formaldehyde 0.01% by weight, acrylic acid dimer 1.0% by weight Acrylic acid aqueous solution containing 0.93% by mass of other impurities was obtained at 3.46 kg / hour. The gas obtained from the top of the tower was supplied to the lower part of the acrylic acid collection tower. In Comparative Example 1, the collection efficiency of acrylic acid in the collection tower was 97.90%.

  The aqueous acrylic acid solution and the mother liquor from the crystallization step (acrylic acid 43.6% by mass, water 27.1% by mass, acetic acid 15.1% by mass, maleic acid 2.9% by mass, furfural 0.1% by mass, 80% 2.40 kg / hr of benzaldehyde 0.1% by mass, formaldehyde 0.5% by mass, acrylic acid dimer 5.8% by mass, and other impurities 4.8% by mass. And purified by the same crystallographic apparatus.

  Thereby, the purity of the obtained acrylic acid was 99.71% by mass, and the purification yield was 91.7%.

(result)
From Example 1 and Comparative Example 1, the acrylic acid collection rate was reduced from 98.21% to 97.90% when the circulating liquid was not supplied to the collection tower. Further, when 80% of the mother liquor was circulated to the crystallization process and 20% was discharged out of the system, the purity and purification yield were lowered.

It is process drawing which shows an example of the preferable aspect of this invention. It is a flowchart of acrylic acid manufacture used in Example 2 of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Acrylic acid raw material, 3 ... Air, 5 ... Dilution gas, 10 ... Catalytic gas phase oxidation catalyst, 20 ... Reactor, 25 ... Acrylic acid containing gas, 30 ... Acrylic acid collection tower, 31 ... acrolein separation tower, 32 ... collection tower top discharge gas, 33 ... collection aqueous solution, 33 '
... Collection water, 34 ... Recycle gas, 35, 35 '... Acrylic acid-containing solution, 36 ... Cooling tower, 39 ... Cooler, 50 ... Crystallizer, 53 ..Pre-purification separation device, 55 ... crystallizer, 60 ... product acrylic acid, 70 ... distillation tower, 71 ... distillate, 73 ... thin film evaporator, 75 ... Pyrolysis tank.

Claims (10)

The manufacturing method of acrylic acid including the process of the following (a-1), (a-2), (a-3), (a-4), (b) and (c).
(A-1) A step of obtaining an acrylic acid-containing gas obtained by a catalytic gas phase oxidation reaction of an acrylic acid raw material,
(A-2) introducing the acrylic acid-containing gas into a collection tower and bringing it into contact with a collection aqueous solution to obtain an acrylic acid-containing solution having an acrylic acid content of 80% by mass or more;
(A-3) a step of cooling to at least a part of the exhaust gas discharged from the collection tower and circulating to the catalytic gas phase oxidation reactor;
(A-4) a step of circulating at least a part of the condensate condensed by cooling to the collection tower;
(B) supplying the acrylic acid-containing solution to a crystallization step and separating the acrylic acid and the residual mother liquor;
(C) A step of subjecting at least a part of the residual mother liquor to a distillation step and circulating the distillate obtained in the distillation step to the collection tower of (a-2).   The method for producing acrylic acid according to claim 1, wherein the cooling in the step (a-4) is direct cooling.   The method for producing acrylic acid according to claim 1 or 2, wherein in the step (a-4), all of the condensate condensed by cooling is circulated to the collection tower.   In the step (a-3), the cooling of the exhaust gas is cooled to a temperature lower by 1 to 50 ° C than the temperature of the exhaust gas, The acrylic acid according to any one of claims 1 to 3, Production method.   The method for producing acrylic acid according to any one of claims 1 to 4, wherein in the step (c), the distillation column distillate is circulated to a place other than the top of the collection tower.   Furthermore, the manufacturing method of acrylic acid as described in any one of Claims 1-5 including the process of supplying the tower bottom liquid of the distillation process of said (c) to an acrylic acid dimer decomposition | disassembly apparatus.   The method for producing acrylic acid according to any one of claims 1 to 6, wherein the residual mother liquor (b) has an acetic acid content of 2% by mass or more.   The method for producing acrylic acid according to any one of claims 1 to 7, wherein an acrylic acid concentration in the solution in the step (a-2) of obtaining the acrylic acid-containing solution is 85% by mass or more.   The method for producing acrylic acid according to any one of claims 1 to 8, wherein the residual mother liquor (b) has an acetic acid content of 3 to 15% by mass.   The acrolein contained in the acrylic acid-containing solution obtained in the above (a) is separated, and then the above-mentioned (b) is performed using the obtained acrylic acid-containing solution. The manufacturing method of acrylic acid as described in any one.

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