Methods for purifying methacrylic acid and methods for producing methacrylic acid.

Abstract

A method for purifying methacrylic acid, comprising mixing raw material methacrylic acid with methanol to produce a mixed solution, causing the precipitation of crystals of methacrylic acid from the mixed solution, and separating the crystals from a mother liquor, wherein the raw material methacrylic acid is mixed with methanol in such a manner that the concentration of methanol in the mixed solution can become 3.0 to 3.75% by mass and the precipitation of the crystals of methacrylic acid from the mixed solution is carried out in a cooled crystallization chamber.

Description

Method for purifying methacrylic acid and method for producing the same The present invention relates to a method for purifying methacrylic acid and a method for producing the same. As a method for producing methacrylic acid, there is a method in which isobutylene, tertiary butyl alcohol, methacrolein or isobutyraldehyde is catalytically gas-phase oxidized in one or two steps with molecular oxygen. In the product obtained by this method, in addition to the target methacrylic acid (boiling point 161 ° C / 760 mmHg, melting point 15 ° C), for example, carboxylic acids such as formic acid, acetic acid, propionic acid, maleic acid, citraconic acid, benzoic acid, toluic acid, terephthalic acid, acrylic acid, and aldehydes such as formaldehyde, acetaldehyde, propionaldehyde, methacrolein, benzaldehyde, tolualdehyde, furfural are contained as by-products. Most of these impurities are separated by purification means such as extraction operation, distillation operation, or a combination thereof, and methacrylic acid of a certain purity can be obtained. In addition, as a method for purifying methacrylic acid, there is a crystallization method in addition to the extraction method and the distillation method. Patent Document 1 describes a method for obtaining purified methacrylic acid by adding methanol, ethanol, propanol, butanol, etc. as a second component to crude methacrylic acid, crystallizing methacrylic acid from this solution, and separating the precipitated crystals and the mother liquor. Patent Document 2 also describes a method for crystallizing a solution obtained by adding methanol as a second component to crude methacrylic acid using a continuous crystallization apparatus in which a first crystallization tank and a second crystallization tank are connected, and circulating a part of the suspension slurry in the second crystallization tank to the first crystallization tank. A method for obtaining purified methacrylic acid by crystallizing methacrylic acid and separating the precipitated crystals and the mother liquor in a purification column is described. Patent Document 3 also describes a method for obtaining methacrylic acid purified stably for a long period by performing crystallization using a scraper unit having a structure that effectively suppresses the growth of the crystal layer on the cooling surface. International Publication No. 99 / 06348 JP-A-2011-219376 JP-A-2012-246263 Aldehydes contained as impurities in methacrylic acid are absorbed in the ultraviolet region, so there has been a problem of coloration in methacrylic acid products in which a large amount of aldehydes remain. With ordinary extraction and distillation methods, it is difficult to sufficiently remove aldehydes, and it has been difficult to solve such a problem of coloration. As a means of removing impurities that are difficult to reduce by distillation, a crystallization method described in Patent Document 1 has been proposed. However, in this method, the concentration range of the second component added to crude methacrylic acid is very wide, being 1 to 35% by mass or 3 to 30% by mass, and the operating conditions under which purification can be performed more efficiently than the described operating conditions have not been clearly shown. In addition, the second component added to crude methacrylic acid, like other impurities, ultimately needs to be removed. Therefore, it is preferable that the content of the second component in the crystal group separated by solid-liquid separation after the crystallization operation is low, but no method for efficiently achieving this has been shown either. Regarding the method described in Patent Document 2, it has been shown that by circulating the suspension slurry in the second crystallization tank to the first crystallization tank, the cooling loads of both tanks can be equalized, and scaling in the first crystallization tank can be suppressed, thereby enabling long-term operation, but nothing is shown regarding the quality of methacrylic acid. Regarding the method described in Patent Document 3, it has been shown that long-term operation can be achieved using a scraper unit, but nothing is shown regarding the quality and productivity of methacrylic acid. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a purification method and a production method for methacrylic acid that can increase the purity of the crystal group after solid-liquid separation and can achieve high productivity. According to one aspect of the present invention, raw material methacrylic acid and methanol are mixed, crystals of methacrylic acid are precipitated from the mixed solution containing the raw material methacrylic acid and methanol, and a purification method for methacrylic acid in which the crystals and the mother liquor are separated, wherein methanol is mixed so that the concentration of methanol in the mixed solution becomes 3.0 to 3.75% by mass. A method for purifying methacrylic acid is provided, in which crystals of methacrylic acid are precipitated from the mixed solution in a cooling crystallization tank. According to another aspect of the present invention, a step of forming raw material methacrylic acid, A method for producing methacrylic acid is provided, which includes the step of purifying the raw material methacrylic acid by the above purification method. Also, according to another aspect of the present invention, a step of forming raw material methacrylic acid, a step of purifying the raw material methacrylic acid by the above purification method, A method for producing methacrylic acid is provided, which includes the step of solid-liquid separating the suspension slurry formed in the above purification step and purifying the obtained solid content by a high-purity operation. According to the present invention, a method for purifying methacrylic acid and a method for producing methacrylic acid can be provided, which can increase the purity of the crystal group after solid-liquid separation and achieve high productivity. It is a graph showing the relationship between the methanol concentration and the purity of the crystal group. It is a graph showing the relationship between the methanol concentration and the crystal precipitation rate. As a result of intensive studies by the present inventors, by setting the solvent concentration in the raw material and the temperature in the crystallization tank within a specific range, it is possible to increase the crystal precipitation rate while maintaining a high purity of the crystal group after solid-liquid separation. Furthermore, by using a specific crystallization apparatus, it has been found that the crystal precipitation rate can be further increased while maintaining a high purity of the crystal group after solid-liquid separation, and the present invention has been completed. The temperature in the crystallization tank refers to the temperature of the suspension slurry in the crystallization tank. The crystal precipitation rate is the amount of crystals generated per unit time. The method according to an embodiment of the present invention is a method for purifying methacrylic acid, which mixes raw material methacrylic acid and methanol, precipitates crystals of methacrylic acid from the mixed solution containing the raw material methacrylic acid and methanol, and separates the crystals and the mother liquor, wherein methanol is mixed so that the concentration of methanol in the mixed solution is 3.0 to 3.75% by mass, and crystals of methacrylic acid are precipitated from the mixed solution in a cooling crystallization tank. In the above purification method, it is preferable to control the temperature inside the crystallization tank of the cooling type crystallization tank within the range of 4 to 8 °C. By controlling the temperature within this range, crystals can be sufficiently precipitated. In the above purification method, after mixing the raw material methacrylic acid and methanol so that the concentration of methanol in the mixed solution becomes 3.0 to 3.75% by mass, it is preferable to supply the mixed solution to the cooling type crystallization tank. For example, the raw material methacrylic acid and methanol can be mixed in advance to prepare the mixed solution in which the concentration of methanol is 3.0 to 3.75% by mass, and this mixed solution can be supplied to the cooling type crystallization tank. In the above purification method, it is preferable to continuously supply the mixed solution to the cooling type crystallization tank. On the other hand, it is preferable to extract the slurry containing the precipitated crystals and the mother liquor from the cooling type crystallization tank continuously or intermittently. At this time, it is preferable to extract the slurry so that the position of the liquid level in the cooling type crystallization tank becomes substantially constant. In the above purification method, it is preferable to supply the mixed solution at 9 to 20 °C to the cooling type crystallization tank. When the temperature of the mixed solution is higher than 20 °C, it is preferable to cool the temperature of the mixed solution to the range of 9 to 20 °C and then supply it. In the above purification method, the cooling type crystallization tank preferably includes a cylindrical container with a baffle plate equipped with a cooling jacket, and inside the cylindrical container, a scraper unit for scraping the crystals deposited on the inner surface of the cylindrical container, a rotating shaft, and an anchor-shaped scraper arm connecting between the rotating shaft and the scraper unit. Further, the scraper unit preferably includes a scraper blade for scraping the crystals deposited on the inner surface of the cylindrical container, a blade support portion for supporting the scraper blade, and a scraper blade holder having an arm connecting portion fixed to the scraper arm, and a spring type pressing device for pressing the scraper blade against the inner surface of the cylindrical container via the blade support portion. It is preferable that the blade support portion and the arm connecting portion, the blade support portion and the spring type pressing device, and the spring type pressing device and the scraper arm are each movably connected. Furthermore, the lower part of the scraper arm is a flat paddle wing part, and it is preferable that the length of the flat paddle wing part in the axial direction of the cylindrical container has a portion that gradually shortens as it goes from the rotating shaft toward the side surface of the cylindrical container. The purification method according to the embodiment of the present invention can be preferably implemented using, for example, the crystallization device described in Patent No. 5708257 (Japanese Patent Application Laid-Open No. 2012-246263). As this crystallization device, the following crystallization device can be used. A crystallization device that can be preferably used in the purification method according to the embodiment of the present invention is a crystallization device that crystallizes methacrylic acid by indirect cooling crystallization from a raw material containing methacrylic acid to obtain a crystal slurry, The crystallization device is As a crystallization tank, a cylindrical container with a baffle plate equipped with a cooling jacket, The cylindrical container includes a motor-driven rotating shaft penetrating in the axial direction of the cylindrical container, and a bearing that supports the rotating shaft and is disposed on the bottom surface of the cylindrical container, Inside the cylindrical container, A scraper unit for scraping the crystals deposited on the inner surface of the cylindrical container, An anchor-shaped scraper arm connecting the rotating shaft and the scraper unit, A methacrylic acid crystallization device including a ring support attached to the scraper arm for reinforcing the scraper arm. In the above crystallization apparatus, the scraper unit is A scraper unit that is attached to a scraper arm and used, A scraper blade for scraping crystals deposited on the inner surface of the crystallization tank, A scraper blade holder having a blade support portion for supporting the scraper blade and an arm connection portion fixed to the scraper arm, A spring-type pressing device that presses the scraper blade against the inner surface of the crystallization tank via the blade support portion, and is provided with The scraper unit for a methacrylic acid crystallization apparatus, characterized in that the blade support portion, the arm connection portion, the blade support portion and the spring-type pressing device, and the spring-type pressing device and the scraper arm are each movably connected. In the above crystallization apparatus, The lower part of the anchor-shaped scraper arm is a flat paddle wing part, and it is preferable that the length of the flat paddle wing part in the axial direction of the cylindrical container gradually decreases as it goes from the rotating shaft toward the side surface of the cylindrical container. Using the above crystallization apparatus, and the stirring power required per unit volume in the crystallization tank is 0.15 kW / m ３ or more and 0.7 kW / m ３ or less is preferable. As a specific structure of such a crystallization apparatus, the structures shown in FIGS. 1, 2, 3, and 4 of Patent No. 5708257 (Japanese Patent Application Laid-Open No. 2012-246263) and the description of these figures can be adopted. In the above purification method, it is preferable that the obtained suspension slurry is further subjected to solid-liquid separation and a purification operation. Hereinafter, the purification method according to the embodiment of the present invention will be described in detail, but the present invention is not limited thereto. In an embodiment of the present invention, crude methacrylic acid or methacrylic acid purified from crude methacrylic acid by a method such as precision distillation or crystallization can be used as a raw material for producing crystals. The raw material methacrylic acid is methacrylic acid containing impurities to be removed by the purification method according to the present invention. Even if it is methacrylic acid purified by other purification methods such as distillation or crystallization, those containing impurities to be removed by the method of the present invention are included in the raw material methacrylic acid to be purified by the method of the present invention. Hereinafter, crude methacrylic acid or methacrylic acid purified therefrom will be referred to as "raw material methacrylic acid" as a raw material for purification according to the present invention. The raw material methacrylic acid can be produced by various methods such as, for example, the direct oxidation method or the ACH (acetone cyanohydrin) method. As a method for producing such raw material methacrylic acid, for example, a condensate obtained by condensing a reaction gas obtained by catalytic gas-phase oxidation of at least one compound selected from the group consisting of isobutylene, tertiary butyl alcohol, methacrolein, and isobutyraldehyde with molecular oxygen in one or two steps, or adding water to the condensate of the reaction gas, or extracting methacrylic acid from an aqueous methacrylic acid solution obtained by absorbing the reaction gas in water using an organic solvent, and removing the organic solvent and non-volatile components by distillation to obtain raw material methacrylic acid. At this time, as the organic solvent, n-heptane, dimethylcyclohexane, ethylcyclohexane, toluene, ethylbenzene, chlorobenzene, xylene, diethyl ketone, diisopropyl ketone, methyl propyl ketone, methyl isobutyl ketone, methyl-t-butyl ketone, n-propyl acetate, n-butyl acetate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, vinyl acrylate, n-propyl acrylate, allyl acetate, isopropenyl acetate, vinyl propionate, propyl propionate, methyl crotonate, methyl valerate, ethyl butyrate, dibutyl ether, and mixtures thereof, or a solvent containing at least one selected from them can be used. Further, a method of separating by-produced methacrylic acid in the ACH method by extraction or distillation to obtain raw material methacrylic acid can be mentioned. As the purity of the raw material methacrylic acid, 99.00% by mass or more is preferable, 99.00 to 99.99% by mass is more preferable, and 99.00 to 99.90% by mass is further preferable. In the purification method according to the embodiment of the present invention, a mixed solution obtained by adding a solvent to raw material methacrylic acid can be used as a crystallization raw material. As this solvent, polar substances such as methanol, ethanol, propanol, and water can be used, but it is preferable to use methanol in terms of improving the fluidity of the suspension slurry and suppressing scaling on the cooling heat transfer surface. As a method for supplying raw methacrylic acid and methanol to the crystallization tank, there is no particular limitation, but examples include methods of continuously or intermittently supplying a predetermined amount to the crystallization tank respectively. When continuously supplying raw methacrylic acid and methanol respectively, it is preferable because the methanol concentration in the crystallization tank can be controlled to be constant by making the respective mass flow rates constant. On the other hand, when methanol is supplied intermittently, it is necessary to sample the slurry in the crystallization tank and analyze the concentration of methanol as the solvent, which is time-consuming. Therefore, it is preferable to continuously supply a predetermined amount of methanol as described above, and it is more preferable to supply it continuously as follows. That is, after mixing raw methacrylic acid and methanol so that the methanol reaches a predetermined concentration, it is preferable to continuously supply the mixed solution to the crystallization tank. For example, a mixed solution of raw methacrylic acid and methanol can be prepared in advance so that the methanol reaches a predetermined concentration, and this mixed solution can be continuously supplied to the crystallization tank. The mixed solution of raw methacrylic acid and methanol may be prepared using a container such as a tank, or may be mixed in a pipe while measuring their respective mass flow rates and then directly supplied to the crystallization tank. The mass flow rate may be measured with a mass flow meter, or the volume flow rate may be measured and converted to the mass flow rate using the specific gravity of the liquid. The methanol concentration in the mixed solution of raw methacrylic acid and methanol is preferably 3.0 to 3.75% by mass. The methanol concentration of this mixed solution is defined by formula (1). Methanol concentration [% by mass] = (mass of methanol added as solvent) / (mass of raw methacrylic acid + mass of methanol added as solvent) × 100 (1) When the methanol concentration of this mixed solution is 3.0% by mass or more, the particle size of the methacrylic acid crystals in the suspension slurry becomes smaller, the fluidity of the suspension slurry is improved, and not only troubles such as clogging in the extraction piping and crystal retention in the crystallization tank are eliminated, but also the adhesion force of the crystals to the cooling heat transfer surface decreases, and scaling is suppressed. From this viewpoint, the methanol concentration of this mixed solution is preferably 3.0% by mass or more, more preferably 3.2% by mass or more, and even more preferably 3.5% by mass or more. On the other hand, when the methanol concentration of this mixed solution is 3.75% by mass or less, the purity of the crystal group after solid-liquid separation of the suspension slurry can be maintained high. This is because when the concentration exceeds 3.75% by mass, the methacrylic acid crystals in the suspension slurry become smaller than necessary, and as a result, the specific surface area becomes larger, so that the amount of mother liquor adhering to the crystal surface increases. Furthermore, since the methanol concentration in the mother liquor is high, it is considered that the amount of methanol adhering to the crystal surface also increases. Therefore, it is desirable to keep the methanol concentration of the mixed solution not exceeding 3.75% by mass so as not to make the crystal size smaller than necessary. From this viewpoint, the methanol concentration of this mixed solution is preferably 3.75% by mass or less, more preferably 3.74% by mass or less, and even more preferably 3.70% by mass or less. When methanol is present in the raw material methacrylic acid in advance, the methanol in the raw material methacrylic acid can also be considered as the methanol added as a solvent. In the purification method according to the embodiment of the present invention, the crystallization operation of the raw material methacrylic acid is performed using a cooling type crystallization tank. As the cooling type crystallization tank, a cooler having a cooling coil for circulating a cooling heat medium in the tank, or a cooler having a cooling jacket for bringing the cooling heat medium into contact with the outer peripheral surface of the tank from the outside, or a cooler equipped with both of them can be used. Among these, those that can cool the inside of the tank by heat exchange using the peripheral surface of the crystallization tank as a heat transfer surface are preferable. Also, a configuration having both a cooling coil and a cooling jacket may be adopted. In the purification method according to the embodiment of the present invention, the crystallization operation is preferably of a continuous type in that the conditions inside the crystallization tank, such as the temperature inside the crystallization tank, suspension density, and mother liquor composition, which affect productivity and crystal quality, are kept constant. Note that the suspension density is the mass concentration of crystals in the suspension slurry inside the crystallization tank. The temperature inside the cooling crystallization tank is preferably 4 to 8°C. By setting the temperature inside the cooling crystallization tank to 4°C or higher, the above-described effects such as improvement in the fluidity of the suspension slurry and suppression of scaling can be sufficiently obtained. From this point, the temperature inside the cooling crystallization tank is preferably 4°C or higher, more preferably 5°C or higher. Further, according to the study by the inventors, it has been clarified that the lower the temperature inside the crystallization tank, the lower the purity of the crystal group obtained by solid-liquid separation of the suspension slurry. From this point as well, it is important to keep the temperature inside the crystallization tank at this temperature or higher. On the other hand, if the temperature inside the crystallization tank exceeds 8°C, the suspension slurry concentration significantly decreases, leading to a significant reduction in productivity, which is not preferable. From this viewpoint, the temperature inside the cooling crystallization tank is preferably 8°C or lower, more preferably 7°C or lower, and even more preferably 6°C or lower. When supplying the mixed solution of raw methacrylic acid and methanol to the cooling crystallization tank, if the temperature of this mixed solution is higher than 20°C, it is preferable to cool this mixed solution to the range of 9 to 20°C (more preferably the range of 9 to 15°C) and then supply it to the cooling crystallization tank. Thereby, the cooling load in the crystallization tank can be reduced. Note that the freezing point of raw methacrylic acid depends on its purity, but is around 15°C under normal pressure. Therefore, when only raw methacrylic acid is supplied to the crystallization tank, when cooling it before supply, it is necessary to set the temperature higher than the freezing point, that is, higher than 15°C. In this temperature range, it becomes difficult to obtain a significant cooling load reduction effect in the crystallization tank. In such a case, it is preferable to mix raw methacrylic acid and methanol in advance to form a mixed solvent. Thereby, the freezing point of the mixed solvent can be lowered, and the cooling load in the crystallization tank can be sufficiently reduced. The cooling crystallization tank is provided with a cylindrical container with a baffle plate and a cooling jacket. Inside the cylindrical container, it preferably includes a scraper unit for scraping the crystals deposited on the inner surface of the cylindrical container, a rotating shaft, and an anchor-shaped scraper arm connecting between the rotating shaft and the scraper unit. By providing a baffle plate in the crystallization tank (cylindrical container), precipitation of crystals at the bottom of the crystallization tank can be suppressed. Also, the raw material concentration distribution and temperature distribution inside the tank can be made uniform, and a local increase in supersaturation can be suppressed. Further, the scraper unit preferably includes a scraper blade for scraping the crystals deposited on the inner surface of the cylindrical container, a blade support portion for supporting the scraper blade, and a scraper blade holder having an arm connecting portion fixed to the scraper arm, and a spring-type pressing device for pressing the scraper blade against the inner surface of the cylindrical container via the blade support portion. By using the scraper unit, even when operating at a relatively high rotation aiming at mixing and stirring in the crystallization tank, the rotation shaft does not vibrate and scraping can be performed uniformly. Furthermore, the lower part of the scraper arm is a flat paddle wing portion, and it is preferable that the length of the flat paddle wing portion in the axial direction of the cylindrical container has a portion that gradually shortens as it goes from the rotating shaft toward the side surface of the cylindrical container. By using the flat paddle wing portion, not only can a flow for winding up the crystals deposited at the bottom of the crystallization tank (cylindrical container) to the upper part be formed, and good mixing inside the tank can be easily performed, but also poor extraction at the discharge nozzle can be easily suppressed. The suspension slurry obtained in the cooling crystallization tank is withdrawn from the crystallization tank and then subjected to solid-liquid separation. The method of solid-liquid separation is not particularly limited, and for example, known solid-liquid separation devices such as filtration devices and centrifugal separation devices, and combinations thereof can be used. In the crystal group, which is the solid content obtained after solid-liquid separation, mother liquor containing impurities and methanol remain. In order to separate and remove these, a purification operation can be performed. As an operation for further purifying the obtained crystal group, purification including an operation for purifying in the crystal state can be performed. Specifically, purification including an operation of raising the crystal temperature to release impurities (for example, mother liquor and methanol) in the crystal to the outside of the crystal by utilizing a phenomenon called "sweating" of the crystal can be performed. As a specific example of a device for performing such a purification operation, for example, the KCP (Kureha Crystal Purifier), a continuous crystal purification device manufactured by Kureha Environment Co., Ltd., which is described on page 49 of "Purification of Organic Compounds by Kureha Continuous Crystal Purification Device" by Tadao Shimizu, Chemical Engineering, Vol. 27, No. 3 (1982), can be mentioned. The separation operation can be either a batch operation or a continuous operation. Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples. In the examples and comparative examples, as the crystallization device, a stainless steel continuous jacket-cooled crystallization tank (volume 5 L) equipped with a stirring mechanism by a flat paddle blade, a scraper unit, and baffle plates was used to perform the crystallization operation. As the heat medium, an aqueous solution of 40% by mass ethylene glycol was used. For the measurement of the component concentration, gas chromatography (main body: GC-17A (product name), manufactured by Shimadzu Corporation, analytical column: HP-FFAP (trade name), manufactured by Agilent Technologies) was used. (Example 1) Tertiary butyl alcohol was catalytically oxidized with molecular oxygen, and methacrylic acid was extracted from the aqueous methacrylic acid solution obtained by absorbing the resulting reaction gas in water using n-heptane. The extract was distilled to remove the organic solvent and non-volatile components, obtaining crude methacrylic acid. The crude methacrylic acid was purified by crystallization operation to obtain purified methacrylic acid (purity: 99.98% by mass). In this example, this purified methacrylic acid was used as the raw material methacrylic acid. Methanol at 25 °C was mixed with the raw material methacrylic acid maintained at 22 °C as a solvent so that the concentration became 3.74% by mass, preparing a mixed solution of raw material methacrylic acid and methanol. After cooling this mixed solution to 10 °C, 3.4 L of it was charged into a crystallization tank, and a heat medium at 10 °C was supplied to the jacket of the crystallization tank under stirring conditions of 125 rpm. Thereafter, the temperature of the heat medium was decreased at 2.0 °C / hr. When the heat medium temperature reached 4.5 °C, the mixed solution cooled to 10 °C was started to be supplied to the crystallization tank at a flow rate of 1.1 kg / hr, and the slurry in the tank was intermittently withdrawn so that the position of the liquid level in the crystallization tank became constant. The withdrawn slurry was transferred to a glass column (inner diameter: 70 mm, height: 400 mm) equipped with a filter at the bottom and subjected to gravity filtration. The composition of the obtained mother liquor and the temperature inside the crystallization tank were confirmed. This continuous operation was continued for 4 hours, and from the change over time of the composition of the obtained mother liquor and the temperature inside the crystallization tank, it was confirmed that the inside of the tank was in a steady state. Table 1 shows the purity of the obtained crystal group, and the temperature, suspension density, and crystal precipitation rate inside the crystallization tank in the steady state. Also, the relationship between the methanol concentration and the purity of the crystal group, and the relationship between the methanol concentration and the crystal precipitation rate are shown in FIG. 1 and FIG. 2, respectively. Subsequently, a high-purity purification operation was performed on the obtained crystal group so that the methacrylic acid purity was 99.98% by mass or more. For the high-purity purification operation, KCP (Kureha Crystal Purifier), a continuous crystal purification device manufactured by Kureha Environment Co., Ltd., was used. Table 2 shows the productivity ratio of the obtained methacrylic acid. Note that the purity of the crystal groups shown in the examples and comparative examples was calculated taking into account methanol contained in the crystal groups as an impurity as well. The suspension density was calculated by the following formula (1) after measuring the total mass of the sampled slurry, separating the solid and liquid by a method such as filtration, and measuring the mass of the crystals. Suspension density [mass%] = mass of crystals [kg] / mass of slurry [kg] × 100 (1) The crystal precipitation rate was calculated by the following formula (2). Crystal precipitation rate [g / min] = (feed rate of raw material or withdrawal rate of slurry [g / min]) × (suspension density difference between raw material and withdrawn slurry [mass%]) / 100 (2) The productivity ratio was calculated by the following formula (4) based on the value of productivity represented by the following formula (3) with the value of Comparative Example 1 as a reference. Productivity = amount of purified methacrylic acid obtained by the purification operation [kg] (3) Productivity ratio = productivity in each example and comparative example / productivity in Comparative Example 1 (4) (Example 2) A purification operation was carried out in the same manner as in Example 1 except that the methanol concentration in the mixed solution of raw material methacrylic acid and methanol was 3.60% by mass. The purity of the crystal group, and the temperature, suspension density, and crystal precipitation rate in the crystallization tank were measured in the same manner as in Example 1. The results are shown in Table 1. Also, the relationship between the methanol concentration and the purity of the crystal group, and the relationship between the methanol concentration and the crystal precipitation rate are shown in FIGS. 1 and 2, respectively. Subsequently, a purification operation was carried out on the obtained crystal group in the same manner as in Example 1. The productivity ratio of the obtained methacrylic acid is shown in Table 2. (Example 3) A purification operation was carried out in the same manner as in Example 1 except that the methanol concentration in the mixed solution of raw material methacrylic acid and methanol was 3.10% by mass. The purity of the crystal group, and the temperature, suspension density, and crystal precipitation rate in the crystallization tank were measured in the same manner as in Example 1. The results are shown in Table 1. Also, the relationship between the methanol concentration and the purity of the crystal group, and the relationship between the methanol concentration and the crystal precipitation rate are shown in FIGS. 1 and 2, respectively. Subsequently, the obtained crystal group was subjected to a purification operation in the same manner as in Example 1. The productivity ratio of the obtained methacrylic acid is shown in Table 2. (Comparative Example 1) A purification operation was carried out in the same manner as in Example 1, except that the methanol concentration in the mixed solution of raw material methacrylic acid and methanol was 3.80% by mass. The purity of the crystal group, as well as the temperature in the crystallization tank, the suspension density, and the crystal precipitation rate were measured. The results are shown in Table 1. Also, the relationship between the methanol concentration and the purity of the crystal group, and the relationship between the methanol concentration and the crystal precipitation rate are shown in FIGS. 1 and 2, respectively. Subsequently, the obtained crystal group was subjected to a purification operation in the same manner as in Example 1. The productivity ratio of the obtained methacrylic acid is shown in Table 2. (Comparative Example 2) A purification operation was carried out in the same manner as in Example 1, except that the methanol concentration in the mixed solution of raw material methacrylic acid and methanol was 6.32% by mass, and the crystallization raw material (the mixed solution) was supplied to the crystallization tank at 1.4 kg / hr. The purity of the crystal group, as well as the temperature in the crystallization tank, the suspension density, and the crystal precipitation rate were measured. The results are shown in Table 1. Also, the relationship between the methanol concentration and the purity of the crystal group, and the relationship between the methanol concentration and the crystal precipitation rate are shown in FIGS. 1 and 2, respectively. Subsequently, the obtained crystal group was subjected to a purification operation in the same manner as in Example 1. The productivity ratio of the obtained methacrylic acid is shown in Table 2. (Comparative Example 3) A purification operation was carried out in the same manner as in Comparative Example 2, except that the crystallization raw material (the mixed solution) was supplied to the crystallization tank when the temperature of the heat medium reached 1.5°C. The purity of the crystal group, as well as the temperature in the crystallization tank, the suspension density, and the crystal precipitation rate were measured. The results are shown in Table 1. Also, the relationship between the methanol concentration and the purity of the crystal group, and the relationship between the methanol concentration and the crystal precipitation rate are shown in FIGS. 1 and 2, respectively. Subsequently, the obtained crystal group was subjected to a purification operation in the same manner as in Example 1. The productivity ratio of the obtained methacrylic acid is shown in Table 2. (Comparative Example 4) The same operations as in Example 1 were carried out except that the methanol concentration in the mixture of raw methacrylic acid and methanol was 2.90% by mass. However, two hours after the start of the supply of the crystallization raw material (the above-mentioned mixed solution), the flow state of the suspension slurry deteriorated, clogging of the extraction pipe occurred, and scale formed on the cooling heat transfer surface, so the operation was stopped. (Comparative Example 5) The methanol concentration in the mixed solution of raw methacrylic acid and methanol was 3.80% by mass, and a purification operation was carried out in the same manner as in Example 1 using a device equipped with only a stirring mechanism in the crystallization apparatus. The purity of the crystal group, as well as the temperature in the crystallization tank, the suspension density, and the crystal precipitation rate were measured in the same manner as in Example 1. The results are shown in Table 1. Also, the relationship between the methanol concentration and the purity of the crystal group, and the relationship between the methanol concentration and the crystal precipitation rate are shown in FIGS. 1 and 2, respectively. Subsequently, the same high-purity operation as in Example 1 was carried out on the obtained crystal group. The productivity ratio of the obtained methacrylic acid is shown in Table 2. In Examples 1, 2, and 3 with methanol concentrations of 3.74% by mass, 3.60% by mass, and 3.10% by mass, respectively, the crystal precipitation rates were 7%, 14%, and 23% higher, respectively, than in Comparative Example 1 with a methanol concentration of 3.8% by mass. Also, the purity of the crystal group after solid-liquid separation in Examples 1, 2, and 3 was 0.07 point, 0.19 point, and 0.22 point higher, respectively, than the purity in Comparative Example 1. Furthermore, the productivity ratios in Examples 1, 2, and 3 were improved to 1.10, 1.17, and 1.26, respectively, compared to Comparative Example 1 (1.0). On the other hand, in Comparative Example 2 where the methanol concentration was increased to 6.32% by mass, not only did the crystal precipitation rate decrease significantly, but the purity of the crystal group after solid-liquid separation also decreased, and as a result, the productivity ratio also deteriorated significantly. Also, in Comparative Example 3 where the methanol concentration was increased to 6.32% by mass and the temperature in the crystallization tank was lowered to about 2°C, an improvement in the crystal precipitation rate similar to that in Example 1 was observed, but the purity of the crystal group after solid-liquid separation was about 1 point lower than in Examples 1, 2, and 3. Although the productivity ratio improved slightly, it did not reach the results of Examples 1, 2, and 3. Also, in Comparative Example 4 where the methanol concentration was reduced to 2.9% by mass, not only did the fluidity of the suspension slurry deteriorate, but scale also formed on the cooling heat transfer surface, making stable operation impossible. In Comparative Example 5 where the methanol concentration was the same as in Comparative Example 1, 3.8% by mass, and the crystallization apparatus was equipped only with a stirring mechanism, scaling on the cooling heat transfer surface in the tank progressed and the heat removal capacity decreased, so the productivity ratio did not reach the result of Comparative Example 1. From the above results, according to the purification method according to the embodiment of the present invention, it is possible to improve the crystal precipitation rate directly related to productivity and the purity of the crystal group after solid-liquid separation, and as a result, increase productivity as well. In addition, in this Example and Comparative Examples, since the purified methacrylic acid (purity: 99.98% by mass) used for crystallization was used as the raw material methacrylic acid, the main impurity component is methanol, and from the above results, it can be seen that the residual amount of methanol mixed in the crystals of the raw material methacrylic acid can be reduced. In the purification method according to the embodiment of the present invention, as described above, crude methacrylic acid can be used as the raw material methacrylic acid, and the content of impurities in this crude methacrylic acid can be reduced by the purification method according to this embodiment. When crude methacrylic acid is used as the raw material methacrylic acid, the fact that impurities can be reduced by the above purification method will be specifically described with reference to reference examples. (Reference Example 1) Purification operations were performed in the same manner as in Example 1 except that crude methacrylic acid was used as the raw material methacrylic acid, and the concentration of impurities other than methanol in the crystal group was measured. For the measurement of the component concentration, gas chromatography (main body: GC-2014AFsc (product name), manufactured by Shimadzu Corporation, analytical column: DP-FFAP (trade name), manufactured by Agilent Technologies) was used. Table 3 shows the impurity concentration in the raw material methacrylic acid and the impurity concentration in the crystal group obtained in Reference Example 1. Using crude methacrylic acid as the raw material methacrylic acid, by performing the crystallization operation shown in this example using a mixed solution of this crude methacrylic acid and methanol, a crystal group with reduced impurities in the raw material can be obtained.

Claims

Page 1 of 2 pages of Claims 1. Method for the purification of methacrylic acid consisting of: mixing of methacrylic acid as a feedstock and methanol; precipitation of methacrylic acid crystals from the mixed solution consisting of methacrylic acid as a feedstock and methanol; and separation of crystals and culture medium where methacrylic acid as a feedstock and methanol are mixed, thus the concentration of methanol in the mixed liquid is 3.0 to 3.75% by mass, and methacrylic acid crystals are precipitated from the mixed solution in a cooled crystallization vessel.

2. Method for the purification of methacrylic acid according to claim 1 where the temperature in the cooled crystallization vessel is controlled in the range of 4 to 8 degrees Celsius.

3. The method for purifying methacrylic acid according to claim 1 or 2 involves mixing methacrylic acid as the raw material and methanol, so that the concentration of methanol in the mixed solution is 3.0 to 3.75% by mass, and then the mixed solution is delivered to a cooling crystallization vessel.

4. The method for purifying methacrylic acid according to claim 3 involves continuously delivering the mixed solution to a cooling crystallization vessel. 5.Methods for purifying methacrylic acid according to Reservience 3 or 4 where the mixed solution at a temperature of 9 to 20 degrees Celsius is delivered to a cooling crystallization vessel.

6. Methods for purifying methacrylic acid according to Reserviences 1 to 5 where the cooling crystallization vessel consists of a cylindrical vessel fitted with a plate fitted with a cooling jacket, and the cooling crystallization vessel consists of an inner cylindrical vessel, a scraping unit for scraping the precipitated crystals onto the inner surface of the cylindrical vessel, a rotating shaft, and an anchor-shaped scraping arm connecting the rotating shaft and the scraping unit.

7. Methods for purifying methacrylic acid according to Reservience 6 where the scraping unit consists of: a scraping blade for scraping the precipitated crystals onto the inner surface of the cylindrical vessel (page 2 of 2), a scraping blade holder consisting of a blade support that supports the scraping blade, and an anchor-shaped arm connecting the scraping arm and a spring-loaded compression device for pressing the scraping blade against the inner surface of the cylindrical vessel through the blade support. 8.The method for purifying methacrylic acid according to claim 7, where the lower part of the scraping arm is incorporated with the blade portion of the flat stirring blade, and the lower part of the scraping arm is incorporated with the portion where the length of the blade portion of the flat stirring blade in the axial direction of the cylindrical container is gradually shortened from the rotating shaft to the direction of the lateral surface of the cylindrical container.

9. The method for producing methacrylic acid, which includes the steps of: formation of the methacrylic acid raw material and purification of the methacrylic acid raw material by any one of the methods according to claims 1 through 8.

10. The method for producing methacrylic acid according to claim 9, which is further incorporated with the steps of introduction, solid-liquid separation path, the formation of a viscous suspension in the purification step of the methacrylic acid raw material, and purification of the obtained solid by high-purity operation.