Title of Invention	METHOD FOR PRODUCING (METH) ACRYLIC ACID
Abstract	In a production method in which an acrylic acid-containing solution obtained from an acrylic acid-containing gas (or an acrylic acid-containing liquid obtained simply by subjecting the acrylic acid-containing solution to a separation process of reaction by-products) is directly subjected to a crystallization process, the crystallization process can't be carried out smoothly in an unsteady state (when the concentration of water in the acrylic acid-containing solution is high). The present invention aims to provide a method for collecting the purified acrylic acid having high purity in good yield even in an unsteady state. When the acrylic acid-containing solutions 18 and 16 have a concentration of water higher than 10 mass % (unsteady state) , the purified acrylic acid 20 and 22 or the acrylic acid-containing solution 23 containing 10 mass % or less of water is mixed therewith to make the concentration of water 10 mass % or less[a] [b] . Alternatively, when the acrylic acid-containing solutions 18 and 16 have a concentration of water higher than 10 mass %, it is used as an absorption liquid [c]. Thereafter, the crystallization process is carried out.

Title of Invention

METHOD FOR PRODUCING (METH) ACRYLIC ACID

Abstract

In a production method in which an acrylic acid-containing solution obtained from an acrylic acid-containing gas (or an acrylic acid-containing liquid obtained simply by subjecting the acrylic acid-containing solution to a separation process of reaction by-products) is directly subjected to a crystallization process, the crystallization process can't be carried out smoothly in an unsteady state (when the concentration of water in the acrylic acid-containing solution is high). The present invention aims to provide a method for collecting the purified acrylic acid having high purity in good yield even in an unsteady state. When the acrylic acid-containing solutions 18 and 16 have a concentration of water higher than 10 mass % (unsteady state) , the purified acrylic acid 20 and 22 or the acrylic acid-containing solution 23 containing 10 mass % or less of water is mixed therewith to make the concentration of water 10 mass % or less[a] [b] . Alternatively, when the acrylic acid-containing solutions 18 and 16 have a concentration of water higher than 10 mass %, it is used as an absorption liquid [c]. Thereafter, the crystallization process is carried out.

Full Text	BACKGROUND OF THE INVENTION Field of the Invention [0001] The present invention relates to a method for producing (meth)acrylic acid, particularly to a method for producing acrylic acid in good yield without any special operation even in an unsteady state which arises at startup or due to some accident or the like. Description of the Related Art [0002] Acrylic acid and methacrylic acid (occasionally referred to as (meth) acrylic acid in this specification and claims) are useful raw materials widely used as raw materials for chemical products, and various production methods thereof are proposed. For example, as methods for producing acrylic acid industrially, a method in which propylene and/or acrolein are subjected to the catalytic gas phase oxidation is commonly used, and a method in which an acrylic acid-containing gas obtained by the catalytic gas phase oxidation method is condensed or absorbed in an absorption liquid to make an acrylic acid-containing solution followed by being subjected to distillation and crystallization to obtain a purified acrylic acid is proposed. [0003] As a purification process, in addition to the method described above, for example, in Japanese Patent Publication No. 2005-15478A, a method in which an acrylic acid-containing gas is brought in contact with an aqueous solution for absorption in an absorption column to obtain an . acrylic acid-containing solution, and the acrylic acid-containing solution as a crude acrylic acid is directly subjected to a crystallization process, and a distillate obtained by distilling a residual mother liquid discharged in the crystallization process is circulated in the absorption column is proposed. Such circulation to introduce a high-concentration acrylic acid-containing solution in the crystallization process has an effect of obtaining a highly purified acrylic acid with good absorption efficiency, and even more, the production process is easy and simple. [0004] In Japanese Patent Publication No. 2003-530376A, the purification of acrylic acid using a cleaning column having a function of forcible transport of crystal for the crystallization of acrylic acid is proposed. SUMMARY OF THE INVENTION [0005] In accordance with the methods described in Japanese Patent Publication No. 2005-15478A and Japanese Patent Publication No. 2003-530376A, it is possible to obtain acrylic acid having high purity by easy and simple processes, but both of the above methods are only effective when the purification process of acrylic acid is already put in a steady state, and when it is in an unsteady state, for example, when the concentration of water in the acrylic acid-containing solution becomes higher at startup or due to some accident, the crystallization process is not smoothly carried out and it will be difficult to obtain a purified acrylic acid having high purity and the yield will be lower; in a worse case, it will be difficult to continue the operation itself. [0006] Therefore, when the concentration of water in the acrylic acid-containing solution is high, the acrylic acid-containing solution has to be disposed of without being subjected to crystallization process, or has to be subjected newly to the water separation operation to lower the concentration of water before being supplied to a device for the crystallization process. As a result, the yield of acrylic acid is lowered in the entire process, and the cost increases due to the additional water separation operation. [0007] The problem described above does not occur in a method in which the crystallization is performed after an acrylic acid-containing solution is distilled, and as described above, it is a new problem found in a production method in which an acrylic acid-containing solution obtained by condensing an acrylic acid-containing gas or bringing it in contact with an absorption liquid is directly subjected to a crystallization process. There is also a case in which the acrylic acid-containing liquid obtained from the acrylic acid-containing gas is subjected to a separation process step followed by a crystallization process to separate reaction by-products, but regardless of whether or not the separation process step is carried out, the crystallization process can't be carried out smoothly when the acrylic acid-containing solution to be subjected to the crystallization process has a high concentration of water. [0008] The present invention has been accomplished in view of the above circumstances. The object of the present invention is, even when the (meth) acrylic acid-containing solution which is a raw liquid material in a crystallization process has a high concentration of water, to enable the collection of the purified (meth) acrylic acid having high purity and to increase the yield of (meth)acrylic acid throughout the entire process, without increasing the cost too much. [0009] The present inventor intensively examined the above problems, and accomplished the present invention with discoveries that (1) when the acrylic acid-containing solution to be subjected to the crystallization process contains more than 10 mass % of water, purity and yield of the resultant purified acrylic acid are lowered; (2) by mixing an acrylic acid-containing solution having sufficiently low concentration of water or a purified acrylic acid in the acrylic acid-containing solution containing more than 10 mass % of water, the concentration of water thereof can be made 10 mass % or less, and if subjecting the resultant solution to the crystallization process, the purified acrylic acid having high purity can be obtained in good yield; or (3) if the acrylic acid-containing solution contains more than 10 mass % of water, it is to be used as an absorption liquid in an absorption process step, and to be circulated like this so that an acrylic acid-containing solution containing 10 mass % or less of water can be obtained, and if subjecting the resultant solution to the crystallization process, the purified acrylic acid having high purity can be obtained in good yield; in addition, with respect to methacrylic acid, the purified methacrylic acid having high purity can also be obtained in good yield in a same manner as acrylic acid. [0010] Namely, the method for producing (meth)acrylic acid according to the present invention comprises obtaining a (meth)acrylic acid-containing gas from a raw material of (meth)acrylic acid by a catalytic gas phase oxidation reaction and obtaining a (meth) acrylic acid-containing solution from the (meth)acrylic acid-containing gas followed by subjecting the (meth)acrylic acid-containing solution to a crystallization process or subjecting the (meth)acrylic acid-containing solution obtained after separating the reaction by-products therefrom to a crystallization process, thereby producing the purified (meth) acrylic acid; in the method, it is characterized that the (meth)acrylic acid-containing solution or the (meth)acrylic acid-containing solution upstream thereof (for example, a (meth)acrylic acid-containing solution prior to a separation process in a production method including a step of separation process of reaction by-products) is mixed with a purified (meth)acrylic acid and/or a (meth)acrylic acid-containing solution containing 10 mass % or less of water to make the concentration of water 10 mass % or less followed by carrying out a crystallization process when the concentration of water in the (meth)acrylic acid-containing solution to be subjected to the crystallization process is higher than 10 mass %. A method for obtaining a (meth) acrylic acid-containing solution from a (meth) acrylic acid-containing gas includes a method condensing the (meth)acrylic acid-containing gas and a method obtaining the (meth)acrylic acid-containing solution by bringing the (meth)acrylic acid-containing gas in contact with an absorption liquid. The absorption liquid includes water and high boiling point solvents. [0011] As the purified (meth)acrylic acid to be mixed in the (meth)acrylic acid-containing solution having a concentration of water higher than 10 mass %, a purified (meth) acrylic acid which has been obtained separately may be used (for example, a purified (meth)acrylic acid produced during an operation in a steady state in the production line may be stored for use later, or a purified (meth) acrylic acid obtained in another production line (in addition -to the one obtained by a crystallization process, it may be the one obtained by a distillation process) may be used), or a purified (meth)acrylic acid obtained in the crystallization process may be circulated for use while the production process is in operation. [0012] When the purified (meth) acrylic acid to be mixed is the one obtained by the same production method (namely, the purified (meth)acrylic acid obtained by subjecting a (meth)acrylic acid-containing solution produced from a (meth)acrylic acid-containing gas obtained by the catalytic gas phase oxidation reaction to a crystallization process or subjecting the (meth)acrylic acid-containing solution obtained after separating the reaction by-products therefrom to a crystallization process) , it is preferable because impurities included in a slight amount therein do not affect the production method. Additionally, there are advantages when the follows are used as the purified (meth) acrylic acid. Namely, [1] : initially purified (meth)acrylic acid obtained at the onset of the operation is discharged while cleaning each part such as a pipe and a tank, so that it contains various dirt. Therefore, although the initially purified (meth)acrylic acid has to be disposed of under ordinary circumstances, it can be repurified by bringing it back to the previous step for use just as the present invention, and accordingly, the ratio of loss in the whole process can be reduced. [2] : when the production line is brought to an emergency stop and the like for some reason, the purified (meth)acrylic acid is not transported to a predetermined tank (namely, it is not subjected to a polymerization-prevention treatment), but stored in the production line. When the accumulation time (stopping time) becomes long, a polymer and the like may be formed to lower the purity of the purified (meth) acrylic acid. However, by bringing it back to the previous step for use without disposing of it just as the present invention, the ratio of loss in the whole process can be reduced. [0014] Additional, as the (meth)acrylic acid-containing solution containing 10 mass % or less of water to be mixed in the (meth)acrylic acid-containing solution containing more than 10 mass % of water, the (meth)acrylic acid-containing solution containing 10 mass % or less of water obtained in a process producing (meth)acrylic acid may be stored for use. [0015] In addition, the method for producing (meth) acrylic acid according to the present invention comprises obtaining a (meth)acrylic acid-containing gas from a raw material of (meth)acrylic acid by the catalytic gas phase oxidation reaction and obtaining a (meth)acrylic acid-containing solution from the (meth)acrylic acid-containing gas followed by subjecting the (meth)acrylic acid-containing solution to a crystallization process or subjecting the (meth)acrylic acid-containing solution obtained after separating the reaction by-products therefrom to a crystallization process, thereby producing the purified (meth)acrylic acid; in the method, it is characterized that the (meth)acrylic acid-containing solution or the (meth) acrylic acid-containing solution upstream thereof is used as an absorption liquid when the (meth)acrylic acid-containing solution to be subjected to the crystallization process contains more than 10 mass % of water. [0016] In this case, as an absorption liquid used for absorption, the (meth)acrylic acid-containing solution containing more than 10 mass % of water may be used; alternatively, the (meth) acrylic acid-containing solution containing more than 10 mass % of water may be concomitantly used with a usual absorption liquid. In addition to a use for absorption by being circulated during an operation of the production process, the (meth)acrylic acid-containing solution containing more than 10 mass % of water can be used as an absorption liquid after temporarily being stored. Further, in a case wherein it is used as an absorption liquid after being temporarily stored as described above, it may be used as an absorption liquid after the (meth) acrylic acid-containing solution containing 10 mass % or less of water can be obtained in a stable manner, (in this case, it is preferred to use a necessary amount of the liquid little by little to an extent that the (meth)acrylic acid-containing solution obtained in the production process can maintain the concentration of water of 10 mass % or less) , or it may be used as an absorption liquid in a state where only a (meth)acrylic acid-containing solution containing more than 10 mass % of water can be obtained. [0017] As the method for supplying the (meth)acrylic acid-containing solution containing more than 10 mass % of water to the absorption column, a supply line of a (meth) acrylic acid-containing solution containing more than 10 mass % of water may be connected to a supply line of a usual absorption liquid so as to supply the solution to the absorption column; alternatively, a supply line of the (meth)acrylic acid-containing solution containing more than 10 mass % of water alone may be disposed in the absorption column (in a manner that the supply line has an opening at a position higher than a gas feed inlet of the absorption column) so as to supply the solution directly to the absorption column. [0018] In addition, the method for producing (meth) acrylic acid according to the present invention comprises obtaining a (meth)acrylic acid-containing gas from a raw material of (meth)acrylic acid by the catalytic gas phase oxidation reaction, and obtaining a (meth)acrylic acid-containing solution from the (meth)acrylic acid-containing gas, followed by subjecting the (meth)acrylic acid-containing solution to a crystallization process or subjecting the (meth)acrylic acid-containing solution obtained after separating the reaction by-products therefrom to a crystallization process, thereby producing the purified (meth)acrylic acid; in the method, it is characterized that when the (meth)acrylic acid-containing solution contains more than 10 mass % of water, it is temporarily stored, and when the (meth)acrylic acid-containing solution containing 10 mass % or less of water can be obtained in a stable manner, the stored (meth)acrylic acid-containing solution containing more than 10 mass % of water is mixed therein to an extent that the concentration of water is maintained at 10 mass % or less, followed by being subjected to the crystallization process. [0019] Namely, in a case where the (meth) acrylic acid-containing solution contains more than 10 mass % of water, it is temporarily stored, and when the purification process comes to a steady state, the (meth)acrylic acid-containing solution containing more than 10 mass % of water which has been stored is mixed in a (meth)acrylic acid-containing solution containing 10 mass % or less of water. However, mixing is made to an extent that the concentration of water is retained 10 mass % or less, and the resultant (meth)acrylic acid-containing solution containing 10 mass % or less of water is to be supplied to the step of the crystallization process. [0020] With respect to the production methods, methods for obtaining a (meth)acrylic acid-containing solution from a (meth)acrylic acid-containing gas include a method condensing a(meth)acrylic acid-containing gas and a method obtaining a (meth)acrylic acid-containing solution by bringing the (meth)acrylic acid-containing gas in contact with an absorption liquid and the like. [0021] Further, in the present invention, each of the above methods may be used in combination. [0022] In the method for producing (meth) acrylic acid, when the method obtaining a (meth)acrylic acid-containing solution by bringing a (meth) acrylic acid-containing gas in contact with an absorption liquid is employed in which water is used as the absorption liquid, the concentration of water tends to be high so that it is particularly useful to apply the present invention thereto. [0023] The reaction by-products include acrolein, methacrolein (hereinafter occasionally referred to as (meth) acrolein) and the like. Additionally, a case in which the (meth)acrylic acid-containing solution to be subjected to the crystallization process contains 10 mass % or less of water (when it contains 10 mass % or less of water even without carrying out a measure according to the present invention) is called a steady state, while a case wherein the (meth) acrylic acid-containing solution to be subjected to the crystallization process contains more than 10 mass % of water(when it contains more than 10 mass % of water without carrying out a measure according to the present invention) is called an unsteady state. Brief Description of the Drawings [0024] FIG.l is a block diagram showing an example of a method for producing acrylic acid according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT [0025] FIG.l is a block diagram showing an example of the method for producing acrylic acid. First, a method for producing acrylic acid (purification process) in a steady state will be explained with reference to FIG.l. [0026] In a reactor 11, an acrylic acid-containing gas 12 is generated from a raw material of acrylic acid 13 by the catalytic gas phase oxidation reaction, and the acrylic acid-containing gas 12 is introduced to an absorption column 14 (line 32) . In the absorption column 14, an absorption liquid 15 and the acrylic acid-containing gas 12 are brought in contact to obtain an acrylic acid-containing solution 16 (absorption step). The methods of the contact include cross-flow contact, counter-flow contact and the like. In the present embodiment, an absorption method wherein the absorption liquid is supplied from outside the system as a method for obtaining the acrylic acid-containing solution 16 will be described; however, for example, in a condensing method, the absorption liquid is not to be added from outside the system. [0027] Next, the acrylic acid-containing solution 16 is introduced to an acrolein separation column 17 (line 36), and acrolein is subjected to a separation process (acrolein separation process step). Methods for the separation process include distillation, diffusion or the like, and may be suitably selected depending on the concentrations of acrylic acid and acrolein contained. The step of the acrolein separation process can be skipped when the acrylic acid-containing solution 16 does not contain acrolein. Furthermore, if the acrylic acid-containing solution 16 is subjected to a crystallization process (crystallizer 19) via a step of the acrolein separation process, lowering of the concentration of water can be expected in the step of the acrolein separation process. In this view, in a method wherein the acrylic acid-containing solution 16 obtained by condensing the acrylic acid-containing gas 12 or bringing it in contact with an absorption liquid is directly (i.e., without going through a step of the acrolein separation process) subjected to a crystallization process, it is preferred to obtain an acrylic acid-containing liquid having a high concentration of acrylic acid in an absorption step compared with that going through a step of the acrolein separation process. [0028] An acrylic acid-containing solution 18 obtained by the separation process or the acrylic acid-containing solution 16 is supplied to the crystallizer 19 (line 38) to obtain the purified acrylic acid 20. The crystallization method at this time is not limited, and either a continuous type or a batch-type crystallizer may be used, and the crystallizer is not limited to one-stage, but may be two or more stages for use in implementing the method. For example, as a continuous type crystallizer, a column-shaped one having a crystallization part, a solid-liquid separation part and a crystal purification part all united in one can be used. In the figure, reference numeral 21 denotes a residual mother liquid discharged from the crystallizer 19. In Japanese Patent Publication No. 2005-15478A, at least a part of the residual mother liquid 21 is distilled, and the resultant distillate is to be supplied to the absorption column 14. [0029] Next, a method for producing acrylic acid (purification process) in an unsteady state will be explained. When the acrylic acid-containing solution 18 (the acrylic acid-containing solution 16 when the acrolein separation process is not carried out) contains more than 10 mass % of water, any one of following [a] to [d] or a combination of two or more thereof and the like (hereinafter occasionally referred to as measures [a] to [d] ) are employed so that the concentration of water in the acrylic acid-containing solution to be supplied to the crystallizer 19 is made 10 mass % or less. [0031] [a] : The purified acrylic acid 20 discharged from the crystallizer 19, or the purified acrylic acid 22 obtained in another production process is mixed in the acrylic acid-containing solution 18 (the acrylic acid-containing solution 16 when the acrolein separation process is not carried out) of the line 38, and the mixture is supplied to the crystallizer 19. Alternatively, the purified acrylic acid 20 or 22 is mixed in the acrylic acid-containing solution 16 of the line 36 followed by subjecting the mixture to the acrolein separation process, and the resultant acrylic acid-containing solution 18 is supplied to the crystallizer 19. [0032] [b]: An acrylic acid-containing solution having a low concentration of water obtained from the absorption column 14 in a steady state (hereinafter referred to as extracted liquid) is stored in a tank 25 used for the extracted liquid (line 34) in advance, and when the acrylic acid-containing solution 16 containing more than 10 mass % of water is discharged from the absorption column 14, the extracted liquid 23 is mixed in the acrylic acid-containing solution 16 (line 33). If necessary, the mixture is subjected to the acrolein separation process followed by being supplied to the crystallizer 19. [0033] [c] : The acrylic acid-containing solution 18 or 16 having a high concentration of water is supplied from an absorption liquid feed inlet of the absorption column 14 (namely, the acrylic acid-containing solution 18 or 16 having a high concentration of water is supplied to the absorption column 14 from an intermediate tank discharge line 26 via an absorption liquid supply line 27) or from a feed inlet for the return of the acrylic acid-containing solution (the feed inlet for the return of the acrylic acid-containing solution is disposed at a position higher than the gas feed inlet) connected with the absorption column 14 (namely, the acrylic acid-containing solution 18 or 16 having a high concentration of water is supplied to the absorption column 14 from the intermediate tank discharge line 28) . In this manner, the acrylic acid-containing solutions 18 and 16 having a high concentration of water are circulated as absorption liquids, thereby the acrylic acid component is condensed so that the acrylic acid-containing solution 16 having a low concentration of water can be obtained. If the amount of the acrylic acid-containing solution 18 or 16 having a high concentration of water is more than that needed in an absorption liquid in the absorption column 14, it is preferable that the acrylic acid-containing solution 18 or 16 is stored temporarily in an intermediate tank 24, and an amount needed for absorption in the absorption column 14 is supplied little by little. When the amount of the acrylic acid-containing solution 18 or 16 having a high concentration of water is sufficiently smaller than that needed in the absorption liquid in the absorption column 14, the acrylic acid-containing solution 18 or 16 may be supplied to the absorption liquid feed inlet without going through the intermediate tank 24. Further, as a timing for supplying an acrylic acid-containing solution having a high concentration of water in the intermediate tank 24 to the absorption column 14, it may be when the concentration of water in the acrylic acid-containing solution 16 discharged from the absorption column 14 becomes 10 mass % or less, or before the concentration of water becomes 10 mass % or less (when it is higher than 10 mass %). [0034] [d] : When the acrylic acid-containing solution 18 or 16 having a high concentration of water is temporarily stored in the intermediate tank 24 (line 37) and the purification process of acrylic acid comes to a steady state, the solution is brought back from the intermediate tank 24 through the line 35 to the original line (line 38), thereafter is mixed with the acrylic acid-containing solution 18 or 16 in a steady state, and the resultant mixture is supplied to the crystallizer 19. However, in order that the concentration of water in the acrylic acid-containing solution after being mixed does not become higher than 10 mass %, the amount of the acrylic acid-containing solution supplied from the intermediate tank 24 is to be adjusted. [0035] In accordance with [a] to [d] described above, by making the acrylic acid-containing solution to be supplied to the crystallizer 19 contain 10 mass % or less of water, a step for crystallization can be carried out smoothly without any special operation, and the purified acrylic acid with high purity can be produced in good yield. [0036] As a timing for switching from an operation mode implementing the measures [a] to [d] (a method using any one of [a] to [d], or a combination of two or more thereof) (an operation mode in which measures for an unsteady state are implemented) to an operation mode for a steady state (steady operation), the operation can be switched to the steady operation not only immediately after the concentration of water in the acrylic acid-containing solution 18 (the acrylic acid-containing solution 16 in a case where an acrolein separation process is not carried out) becomes 10 mass % or less, but also at a stage where the concentration of water in the acrylic acid-containing solution 18 (or the acrylic acid-containing solution 16) becomes 8 mass % or less, and further, 5 mass % or less. [0037] Even though the acrylic acid-containing solution 18 (16) contains 10 mass % or less of water, it cannot be said that the process is in a sufficiently stable condition as soon as the concentration of water becomes 10 mass % or less, so that there will possibly be a need to switch to the operation mode implementing the measures [a] to [d] again in a short time. Accordingly, in view of complication in switching, it is preferred to switch to the steady operation after the concentration of water becomes 8 mass % or less, or, further, 5 mass % or less, which is a more stable condition. Additionally, by implementing the measures [a] to [d], it becomes possible to introduce an acrylic acid-containing solution having a lower concentration of water to the crystallizer, so the crystallization process of the solution having a lower concentration of water can be carried out more smoothly, and product specifications will also become higher. Therefore, from a viewpoint of the concentration of water, it is preferable to switch to the steady operation after the concentration of water in the acrylic acid-containing solution 18 (16) becomes 8 mass % or less, or, further, 5 mass % or less. [0038] On the other hand, in order to lower the concentration of water, it takes longer to implement the above measures [a] to [d] (compared with a case wherein the concentration of water becomes 10 to 8 mass %, it takes longer for the concentration of water to become 8 to 5 mass %, and it takes further longer for the concentration of water to become 5 mass % or less); therefore, an operation mode implementing the measures [a] to [d] (unsteady operation) needs to be continued for a long period of time. Therefore, from this viewpoint, it is preferred to switch to the steady operation at a stage in which the concentration of water in the acrylic acid-containing solution 18 (16) is not lowered so much (for example, a concentration of water of 8 to 5 mass %, preferably 10 to 8 mass %). [0039] Here, an absorption method for obtaining an acrylic acid-containing solution from an acrylic acid-containing gas using the absorption liquid described above will be described. [0040] As an aqueous solution for absorption, a wide range of aqueous solutions may be used as long as they can absorb acrylic acid, and a condensate obtained by cooling and condensing a gas discharged from the top of the absorption column (which is discharged in order to minimize the loss of acrylic acid discharged outside the system) is preferably used as an aqueous solution for absorption. The condensate often contains acrylic acid, so if it is used as the absorption aqueous solution, the acrylic acid-containing solution having a high concentration of acrylic acid can be obtained. It is also a preferred embodiment that the' gas discharged from the top of the absorption column is partially or fully recycled in the reactor. [0041] The aqueous solution for absorption is preferably introduced to the absorption column at a temperature of 0 to 50°C, more preferably 10 to 40°C. [0042] A ratio of mass flow rate of an aqueous solution for absorption to be newly supplied from outside to an acrylic acid-containing gas may be suitably selected in accordance with an objective acrylic acid concentration, and preferably 0.1 to 1.5 times the mass flow rate of acrylic acid included in the acrylic acid-containing gas, more preferably 0.1 to 1.0 times, even more preferably 0.15 to 0.8 times. Then an aqueous solution for absorption having this mass flow rate is subjected to counter-flow contact with an acrylic acid-containing gas to absorb acrylic acid. If the ratio of mass flow rate is lower than 0.1 times, efficiency of the acrylic acid absorption column may be extremely lowered. [0043] In addition, since acrylic acid is a polymerizable material,- for a purpose to prevent the polymerization, one or more compounds selected from the group consisting of a N-oxyl compound, a phenol compound, a manganese salt such as manganese acetate and the like, a dialkyldithiocarbamate copper salt such as dibutylcarbamate copper and the like, a nitro compound, an amine compound and phenothiazine may be included in the aqueous solution for absorption (refer to Japanese Patent Publication No. 2001-348360A, Japanese Patent Publication No. 2001-348358A, Japanese Patent Publication No.2001-348359A and the like). [0044] The acrylic acid absorption column is generally operated under a normal pressure or above. During the steady operation, the column-top pressure (gauge pressure) is preferably at 0 to 0.4 MPa, more preferably at 0 to 0.1 MPa, even more preferably at 0 to 0.03 MPa. If it is lower than 0 MPa (gauge pressure), a decompressor becomes necessary and it costs equipment and utilities expenses, while if it is higher than 0.4 MPa (gauge pressure),the temperature of the absorption column needs to be increased considerably in order to discharge low boiling point substances, so that the absorption efficiency may be lowered. [0045] A column top temperature of an absorption column is generally from 30 to 85°C, preferably from 40 to 80°C. [0046] Under the absorption condition described above, in the present embodiment, an acrylic acid-containing solution with a following blending ratio can be obtained: acrylic acid :80 to 98 mass %, water:1 to 19 mass %,other impurities (acids such as acetic acid, maleic acid, propionic acid and the like, aldehydes such as furfural, formaldehyde and the like) : 1 to 10 mass %. [0047] With respect to the method for producing methacrylic acid (purification process), the crystallization step can also be carried out smoothly without any special operation in a same manner as the acrylic acid, thereby a purified methacrylic acid with high purity can be produced in good yield. [0048] The concentration of water in the acrylic acid-containing solution (or the methacrylic acid-containing solution) can be determined by Karl Fischer's method. The acrylic acid concentration in the acrylic acid-containing solution (the methacrylic acid concentration in the methacrylic acid-containing solution) can be determined by any one of gas chromatography analysis, electrical conductivity measurement and neutralization titration. [0049] Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not restricted by the following examples and can be suitably modified within the scope described above or below and such modifications are also included in the technical scope of the present invention. [0050] The acrylic acid-containing gas 12 obtained by the catalytic gas phase oxidation method was introduced to the absorption column 14 to be brought in counter-flow contact with the absorption liquid 15, thereby the acrylic acid-containing solution 16 was obtained. The liquid composition of the acrylic acid-containing solution 16 comprised 65.0 mass % of acrylic acid, 20.0 mass % of water, and 15.0 mass % of other impurities. [0051] The purified acrylic acid 20 (the composition of the purified acrylic acid comprised 99.4 mass % of acrylic acid, 0 . 004 mass % of water and 0 . 6 mass % of other impurities) obtained in advance during an operation in a steady state was mixed in the acrylic acid-containing solution 16 to give a mixture thereof. The mixing mass ratio of the acrylic acid-containing solution to the purified acrylic acid was 2:5. The above mixture contained 5.7 mass % of water, 89.6 mass % of acrylic acid, and 4.7 mass % of other impurities. [0052] The above mixture was supplied to the crystallizer 19 to be subjected to crystallization. The resultant acrylic acid had a purity of 99.4 mass %. The purification yield of acrylic acid was 99.9 %. [0053] In this Example 1, although no special operation other than mixing the purified acrylic acid was carried out, purified acrylic acid having high purity could be produced in good yield as described above. [0054] The acrylic acid-containing gas 12 obtained by the catalytic gas phase oxidation method was brought in counter-flow contact with the absorption liquid 15 in the absorption column 14 to obtain the acrylic acid-containing solution 16. The liquid composition of the acrylic acid-containing solution 16 comprised 75.0 mass % of acrylic acid, 20.0 mass % of water, and 5.0 mass % of other impurities. [0055] On the other hand, during an operation in a steady state, an acrylic acid-containing solution containing 10 mass % or less of water discharged from the absorption column 14 was extracted and stored in advance (the acrylic acid-containing solution is referred to as extracted liquid) (tank 25 for the extracted liquid) , and the extracted liquid 23 was mixed with the acrylic acid-containing solution 16 of the line 36 (a mixture). The mixing mass ratio of the acrylic acid-containing solution to the extracted liquid was 1:20. The composition of the extracted liquid 23 comprised 88.7 mass % of acrylic acid, 4.4 mass % of water, and 6.9 mass % of other impurities. The liquid composition of the above mixture comprised 88.0 mass % of acrylic acid, 5.1 mass % of water, and 6.8 mass % of other impurities. [0056] The mixture was supplied to the crystallizer 19 to be subjected to crystallization. The resultant acrylic acid had a purity of 99.4 mass %. The purification yield of acrylic acid was 99.9 %. In this Example 2, too, although no special operation other than mixing the acrylic acid-containing solution having a low concentration of water was carried out, the purified acrylic acid having high purity could be produced in good yield as described above. [0058] The acrylic acid-containing gas 12 obtained by the -catalytic gas phase oxidation method was subjected to counter-flow contact with the absorption liquid 15 in the absorption column 14 to obtain the acrylic acid-containing solution 16. The liquid composition of the acrylic acid-containing solution 16 comprised 75.0 mass % of acrylic acid, 15.0 mass % of water and 10.0 mass % of other impurities. [0059] The acrylic acid-containing solution 16 was stored in the intermediate tank 24 (the stored acrylic acid-containing solution is occasionally referred to as a stored acrylic acid-containing solution), and from the tank, the acrylic acid-containing solution was supplied from an absorption liquid feed inlet of the absorption column 14 in a manner that it was included in an amount of 20 mass % relative to the amount of the absorption liquid (line 27 via line 26) . Then, this operation was continued until the concentration of water in the acrylic acid-containing solution 16 (line 36) discharged from the absorption column 14 became 10 mass % or less. [0060] After that, it was confirmed that the acrylic acid-containing solution 16 discharged from the absorption column 14 had a sufficiently low concentration of water which can be obtained in a stable manner (specifically, the liquid composition comprised 86.0 mass % of acrylic acid, 8.0 mass % of water, and 6.0 mass % of other impurities) , and the acrylic acid-containing solution 16 having a low concentration of water was supplied to the crystallizer 19 for crystallization. The resultant acrylic acid showed a purity of 99.4 mass %, and the purification yield of acrylic acid was 99.9 %. [0062] In this Example 3, although no special operation was carried out except that the acrylic acid-containing solution containing more than 10.0 mass % of water was brought back to the absorption column as the absorption liquid, the purified acrylic acid having high purity could be produced in good yield as described above. [0063] The acrylic acid-containing gas 12 obtained by the catalytic gas phase oxidation method was subjected to counter-low contact with the absorption liquid 15 in the absorption column 14 to obtain the acrylic acid-containing solution 16. Next, the acrylic acid-containing solution 16 was supplied to the acrolein separation column 17 for subjecting acrolein and the like to separation by distillation, thereby the acrylic acid-containing solution 18 with acrolein and the like removed was obtained. The liquid composition of the acrylic acid-containing solution 18 comprised 67.0 mass % of acrylic acid, 20.0 mass % of water and 13.0 mass % of other impurities. [0064] The purified acrylic acid 20 (the composition of the purified acrylic acid comprised 99.4 mass % of acrylic acid, 0.004 mass % of water, and 0.6 mass % of other impurities) obtained in advance during an operation in a steady state was mixed in the acrylic acid-containing solution 18 (line 38) to be supplied to the crystallizer 19. The mixing mass ratio of the acrylic acid-containing solution to the purified acrylic acid was 2: 5. The mixture contained 5.7 mass % of water, 90.1 mass % of acrylic acid, and 4.1 mass % of other impurities. [0065] The acrylic acid obtained by crystallizing the mixture had a purity of 99.4 mass %. The purification yield of acrylic acid was 99.9 %. [0066] In the Example 4, too, although no special operation other than mixing the purified acrylic acid was carried out, the purified acrylic acid having high purity could be produced in . good yield as described above. [0067] (Comparative Example) The acrylic acid-containing gas 12 obtained by the catalytic gas phase oxidation method was subjected to counter-flow contact with the absorption liquid 15 in the absorption column 14 to obtain the acrylic acid-containing solution 16. The liquid composition of the acrylic acid-containing solution 16 comprised 65.0 mass % of acrylic acid, 20 . 0 mass % of water, and 15 . 0 mass % of other impurities . [0068] The acrylic acid-containing solution 16 was supplied to the crystallizer 19. In the crystallization process, the above Examples 1 to 4 showed an acrylic acid crystallization speed equivalent to that at a time of a steady operation, compared with this, the crystallization speed of acrylic acid in Example 5 is slow so that the crystal amount obtained in an equivalent time to , that in Examples 1 to 4 was very small. The crystallization speed became slow probably because the melting point of the acrylic acid-containing solution was lowered due to the high concentration of water in the acrylic acid-containing solution 16 so that the heat exchange efficiency with the refrigerant became poor. Further, a change in the solution component accompanied with the progress of crystallization led to further lowering of the melting point so that it became difficult to continue the crystallization step and thus the operation was stopped. [0069] (Comparative Example) The acrylic acid-containing gas 12 obtained by the catalytic gas phase oxidation method was subjected to counter-flow contact with the absorption liquid 15 in the absorption column 14, thereby the acrylic acid-containing solution 16 was obtained. The liquid composition of the acrylic acid-containing solution 16 comprised 65.0 mass % of acrylic acid, 20.0 mass % of water, and 15.0 mass % of other impurities. [0070] On the other hand, the purified acrylic acid 20 (the composition of the purified acrylic acid comprised 99.4 mass % of acrylic acid, 0.004 mass % of water and 0.6 mass % of other impurities) obtained in advance during an operation in a steady state was mixed in the acrylic acid-containing solution 16 in a manner that the mass ratio of the acrylic acid-containing solution to the purified acrylic acid became 2: 1, and the resultant mixture was supplied to the crystallizer 19. The mixture contained 13.3 mass % of water, 7 6.5 mass % of acrylic acid, and 10.2 mass % of other impurities. [0071] In the above crystallizer 19, only 60 mass % of the acrylic acid included in the mixture could be crystallized. It is probably because the melting point of the acrylic acid-containing solution was lowered since the concentration of water in the acrylic acid-containing solution supplied to the crystallizer 19 was high, resulting in the lower crystallization rate. Therefore, it became difficult to continue the crystallization process, and thus the operation was stopped. [0072] The acrylic acid-containing gas 12 obtained by the catalytic gas phase oxidation method was subjected to counter-flow contact with the absorption liquid 15 in the absorption column 14 to obtain the acrylic acid-containing solution 16. The liquid composition of the acrylic acid-containing solution 16 comprised 65.0 mass % of acrylic acid, 20.0 mass % of water, and 15.0 mass % of impurities. The purified acrylic acid 20 (the composition of the purified acrylic acid comprised 99.4 mass % of acrylic acid, 0.004 mass % of water, and 0.6 mass % of other impurities) obtained in advance during an operation in a steady state was mixed in the acrylic acid-containing solution 16 in a manner that the ratio of the acrylic acid-containing solution to the purified acrylic acid became 1:1, thereafter supplied to the crystallizer 19. The mixture contained 10.0 mass % of water, 82.2 mass % of acrylic acid, and 7.8 mass % of other impurities. [0074] The resultant acrylic acid showed a purity of 99.4 mass %. The purification yield of acrylic acid was 96.0 %. [0075] In this Example 7, although no special operation other than mixing the purified acrylic acid was carried out, the purified acrylic acid having high purity could be produced as described above. [0076] The acrylic acid-containing gas 12 obtained by the catalytic gas phase oxidation method was subjected to counter-flow contact with the absorption liquid 15 in the absorption column 14 to obtain the acrylic acid-containing solution 16. Since the acrylic acid-containing solution 16 had a high concentration of water up to 20.0 mass %, the acrylic acid-containing solution 16 was stored in the intermediate tank 24 (the stored acrylic acid-containing solution) . The operation for accumulation was continued until the concentration of water in the acrylic acid-containing solution 16 became 10 mass % or less. Then, it was confirmed that the concentration of water in the acrylic acid-containing solution 16 discharged from the absorption column 14 became 10 mass % or less and could be obtained in a stable manner. The liquid composition of the acrylic acid-containing solution (the stored acrylic acid-containing solution) stored in the intermediate tank 24 thus far comprised 77.0 mass % of acrylic acid, 15.0 mass % of water and 8.0 mass % of other impurities. [0077] Next, the stored acrylic acid-containing solution in the intermediate tank 24 was supplied from the line 28 to the absorption column 14. The amount of the stored acrylic acid-containing solution supplied then was 20 mass % of a total amount of the absorption liquid, and a usual absorption liquid was used for the rest of the total amount of the absorption liquid. [0078] The liquid composition of the acrylic acid-containing solution 16 thus obtained from the absorption column 14 comprised 89.0 mass % of acrylic acid, 6.0 mass % of water and 5.0 mass % of other impurities. The acrylic acid-containing solution 16 was supplied to the crystallizer 19 for crystallization. [0079] The resultant acrylic acid showed a purity of 99.4 mass %, and the purification yield of acrylic acid was 99.9 %. [0080] In this Example 8, although no special operation was carried out except that the acrylic acid-containing solution containing more than 10.0 mass % of water was brought back to the absorption column as the absorption liquid, the purified acrylic acid having high purity could be produced in good yield as described above. [0081] The acrylic acid-containing gas 12 obtained by the catalytic gas phase oxidation method was subjected to counter-flow contact with the absorption liquid 15 in the absorption column 14 to obtain the acrylic acid-containing solution 16. Since the acrylic acid-containing solution 16 had a high concentration of water up to 20.0 mass %, the acrylic acid-containing solution 16 was stored in the intermediate tank 24 (the stored acrylic acid-containing solution). This operation for accumulation was continued until the concentration of water in the acrylic acid-containing solution 16 became 8 mass %. The liquid composition of the acrylic acid-containing solution (the stored acrylic acid-containing solution) thus stored in the intermediate tank 24 comprised 75.0 mass % of acrylic acid, 14.0 mass % of water and 11.0 mass % of other impurities. [0082] Next, the stored acrylic acid-containing solution in the intermediate tank 24 was mixed from the line 35 in the acrylic acid-containing solution 16 (containing 8 mass % or less of water) of the line 38. The mixing mass ratio of the acrylic acid-containing solution to the stored acrylic acid-containing solution was 2:1. The liquid composition of the mixture comprised 81.7 mass % of acrylic acid, 10.0 mass % of water, and 8.3 mass % of other impurities. [0083] The mixture was supplied to the crystallizer 19 for crystallization. The resultant acrylic acid showed a purity of 99.4 mass %, and the purification yield of acrylic acid was 96.0 %. [0084] In this Example 9, although no special operation was carried out except that the acrylic acid-containing solution containing more than 10. 0 mass % of water was temporarily stored, and that the stored acrylic acid-containing solution was mixed in the acrylic acid-containing solution 16 containing 8 mass % or less of water, the purified acrylic acid having high purity could be produced as described above. [0085] In summary of the above results of experiments, separation and purification by crystallization could not be carried out smoothly in Examples 5 and 6, while could be carried out smoothly in the crystallizer 19 to obtain the purified acrylic acid having high purity in Examples 1 to 4 and 7 to 9. With respect to the yield, a high yield was obtained in Examples 1 to 4 and 8, but in Examples 7 and 9 in which an acrylic acid-containing solution having a concentration of water of 10.0 mass % was crystallized, the yield was somewhat inferior to that of Examples 1 to 4 and 8. From the results of the experiments, it is found that by making the concentration of water 10 mass % or less using the above measures [a] to [d] when the acrylic acid-containing solution contains more than 10 mass % of water, the crystallization process can be progressed smoothly, and even more, a high yield can be obtained. [0086] Although the above examples showed methods for producing acrylic acid, the present invention can also be applied to methacrylic acid in a same manner. [0087] According to the present invention, even if the (meth)acrylic acid-containing solution contains more than 10 mass % of water, which is an unsteady state, (meth) acrylic acid can be collected from the (meth)acrylic acid-containing solution without any special operation, and the purified acrylic acid having high purity can be obtained at high yield with less loss in the (meth) acrylic acid production process as a whole. Moreover, the steps are easy and simple, and elevation of cost arising on the occasions such as disposal or the additional water separation operation as described above is not induced. This application is based on Japanese Patent Application No. 2006-254955 filed on September 20, 2006, the contents of which are hereby incorporated by reference. We Claim : 1. A method for producing (meth)acrylic acid comprising the steps of: obtaining a (meth)acrylic acid-containing gas from a raw material for producing (meth)acrylic acid by a catalytic gas phase oxidation reaction; obtaining a (meth)acrylic acid-containing solution from the (meth)acrylic acid- containing gas; followed by subjecting the (meth)acrylic acid-containing solution to a crystallization process, or followed by subjecting the (meth)acrylic acid-containing solution obtained after separating a reaction by-product therefrom to a crystallization process, thereby producing a purified (meth)acrylic acid, wherein, when the (meth)acrylic acid-containing solution to be subjected to the crystallization process contains more than 10 mass % of water, the concentration of water is made 10 mass % or less by mixing a purified (meth)acrylic acid and/or a (meth)acrylic acid-containing solution containing 10 mass % or less of water in the (meth)acrylic acid-containing solution containing more than 10 mass % of water or in the (meth)acrylic acid-containing solution upstream thereof, followed by carrying out the crystallization process. 2. The method for producing (meth)acrylic acid as claimed in Claim 1 comprising the steps of: obtaining the (meth)acrylic acid-containing solution from the (meth)acrylic acid- containing gas obtained by the catalytic gas phase oxidation reaction; and subjecting the (meth)acrylic acid-containing solution to the crystallization process or subjecting the (meth)acrylic acid-containing solution obtained after separating the reaction by-product therefrom to the crystallization process, thereby obtaining the purified (meth)acrylic acid, wherein the purified (meth)acrylic acid is used as the purified (meth)acrylic acid to be mixed in the (meth)acrylic acid-containing solution containing more than 10 mass % of water. 3. The method for producing (meth)acrylic acid as claimed in Claim 1 or 2, wherein a part of the (meth)acrylic acid-containing solution containing 10 mass % or less of water obtained in the production process of (meth)acrylic acid is stored in advance and then mixed in the (meth)acrylic acid-containing solution containing more than 10 mass % of water. 4. A method for producing (meth)acrylic acid comprising the steps of: obtaining a (meth)acrylic acid-containing gas from a raw material for producing (meth)acrylic acid by a catalytic gas phase oxidation reaction; bringing the (meth)acrylic acid-containing gas in contact with an absorption liquid to obtain a (meth)acrylic acid-containing solution; followed by subjecting the (meth)acrylic acid-containing solution to a crystallization process, or followed by subjecting the (meth)acrylic acid-containing solution obtained after separating a reaction by-product therefrom to a crystallization process, thereby producing a purified (meth)acrylic acid, wherein, when the (meth)acrylic acid-containing solution to be subjected to the crystallization process contains more than 10 mass % of water, the (meth)acrylic acid- containing solution containing more than 10 mass % of water or the (meth)acrylic acid- containing solution upstream thereof is used as the absorption liquid. 5. The method for producing (meth)acrylic acid as claimed in Claim 4, wherein, when the (meth)acrylic acid-containing solution to be subjected to the crystallization process contains more than 10 mass % of water, the (meth)acrylic acid-containing solution containing more than 10 mass % of water or the (meth)acrylic acid-containing solution upstream thereof is circulated to be used as the absorption liquid in the step in which the (meth)acrylic acid-containing gas is brought in contact with the absorption liquid. 6. The method for producing (meth)acrylic acid as claimed in Claim 5, wherein after being circulated as the absorption liquid, the (meth)acrylic acid-containing solution containing 10 mass % or less of water obtained by the absorption is subjected to the crystallization process. 7. A method for producing (meth)acrylic acid comprising the steps of: obtaining a (meth)acrylic acid-containing gas from a raw material for producing (meth)acrylic acid by a catalytic gas phase oxidation reaction; obtaining a (meth)acrylic acid-containing solution from the (meth)acrylic acid- containing gas; followed by subjecting the (meth)acrylic acid-containing solution to a crystallization process, or followed by subjecting the (meth)acrylic acid-containing solution obtained after separating a reaction by-product therefrom to a crystallization process, thereby producing a purified (meth)acrylic acid, wherein, when the (meth)acrylic acid-containing solution contains more than 10 mass % of water, it is temporarily stored, and when the (meth)acrylic acid-containing solution containing 10 mass % or less of water can be obtained in a stable manner, the stored (meth)acrylic acid-containing solution containing more than 10 mass % of water is mixed therewith to an extent of maintaining the concentration of water 10 mass % or less, followed by carrying out the crystallization process. 8. The method for producing (meth)acrylic acid as claimed in any one of Claim 1 to 7 comprising the step of bringing the (meth)acrylic acid-containing gas in contact with an absorption liquid to obtain the (meth)acrylic acid-containing solution, wherein water is used as the absorption liquid. In a production method in which an acrylic acid-containing solution obtained from an acrylic acid-containing gas (or an acrylic acid-containing liquid obtained simply by subjecting the acrylic acid-containing solution to a separation process of reaction by-products) is directly subjected to a crystallization process, the crystallization process can't be carried out smoothly in an unsteady state (when the concentration of water in the acrylic acid-containing solution is high). The present invention aims to provide a method for collecting the purified acrylic acid having high purity in good yield even in an unsteady state. When the acrylic acid-containing solutions 18 and 16 have a concentration of water higher than 10 mass % (unsteady state) , the purified acrylic acid 20 and 22 or the acrylic acid-containing solution 23 containing 10 mass % or less of water is mixed therewith to make the concentration of water 10 mass % or less[a] [b] . Alternatively, when the acrylic acid-containing solutions 18 and 16 have a concentration of water higher than 10 mass %, it is used as an absorption liquid [c]. Thereafter, the crystallization process is carried out.

Full Text

BACKGROUND OF THE INVENTION
Field of the Invention
[0001]
The present invention relates to a method for producing
(meth)acrylic acid, particularly to a method for producing
acrylic acid in good yield without any special operation even
in an unsteady state which arises at startup or due to some
accident or the like.
Description of the Related Art
[0002]
Acrylic acid and methacrylic acid (occasionally referred
to as (meth) acrylic acid in this specification and claims) are
useful raw materials widely used as raw materials for chemical
products, and various production methods thereof are proposed.
For example, as methods for producing acrylic acid industrially,
a method in which propylene and/or acrolein are subjected to
the catalytic gas phase oxidation is commonly used, and a method
in which an acrylic acid-containing gas obtained by the
catalytic gas phase oxidation method is condensed or absorbed
in an absorption liquid to make an acrylic acid-containing
solution followed by being subjected to distillation and
crystallization to obtain a purified acrylic acid is proposed.
[0003]
As a purification process, in addition to the method
described above, for example, in Japanese Patent Publication
No. 2005-15478A, a method in which an acrylic acid-containing
gas is brought in contact with an aqueous solution for
absorption in an absorption column to obtain an . acrylic
acid-containing solution, and the acrylic acid-containing
solution as a crude acrylic acid is directly subjected to a
crystallization process, and a distillate obtained by

distilling a residual mother liquid discharged in the
crystallization process is circulated in the absorption column
is proposed. Such circulation to introduce a high-concentration
acrylic acid-containing solution in the crystallization
process has an effect of obtaining a highly purified acrylic
acid with good absorption efficiency, and even more, the
production process is easy and simple.
[0004]
In Japanese Patent Publication No. 2003-530376A, the
purification of acrylic acid using a cleaning column having a
function of forcible transport of crystal for the
crystallization of acrylic acid is proposed.
SUMMARY OF THE INVENTION
[0005]
In accordance with the methods described in Japanese
Patent Publication No. 2005-15478A and Japanese Patent
Publication No. 2003-530376A, it is possible to obtain acrylic
acid having high purity by easy and simple processes, but both
of the above methods are only effective when the purification
process of acrylic acid is already put in a steady state, and
when it is in an unsteady state, for example, when the
concentration of water in the acrylic acid-containing solution
becomes higher at startup or due to some accident, the
crystallization process is not smoothly carried out and it will
be difficult to obtain a purified acrylic acid having high
purity and the yield will be lower; in a worse case, it will
be difficult to continue the operation itself.
[0006]
Therefore, when the concentration of water in the acrylic
acid-containing solution is high, the acrylic acid-containing
solution has to be disposed of without being subjected to
crystallization process, or has to be subjected newly to the
water separation operation to lower the concentration of water
before being supplied to a device for the crystallization
process. As a result, the yield of acrylic acid is lowered in

the entire process, and the cost increases due to the additional
water separation operation.
[0007]
The problem described above does not occur in a method
in which the crystallization is performed after an acrylic
acid-containing solution is distilled, and as described above,
it is a new problem found in a production method in which an
acrylic acid-containing solution obtained by condensing an
acrylic acid-containing gas or bringing it in contact with an
absorption liquid is directly subjected to a crystallization
process. There is also a case in which the acrylic
acid-containing liquid obtained from the acrylic
acid-containing gas is subjected to a separation process step
followed by a crystallization process to separate reaction
by-products, but regardless of whether or not the separation
process step is carried out, the crystallization process can't
be carried out smoothly when the acrylic acid-containing
solution to be subjected to the crystallization process has a
high concentration of water.
[0008]
The present invention has been accomplished in view of
the above circumstances. The object of the present invention
is, even when the (meth) acrylic acid-containing solution which
is a raw liquid material in a crystallization process has a high
concentration of water, to enable the collection of the purified
(meth) acrylic acid having high purity and to increase the yield
of (meth)acrylic acid throughout the entire process, without
increasing the cost too much.
[0009]
The present inventor intensively examined the above
problems, and accomplished the present invention with
discoveries that (1) when the acrylic acid-containing solution
to be subjected to the crystallization process contains more
than 10 mass % of water, purity and yield of the resultant
purified acrylic acid are lowered; (2) by mixing an acrylic
acid-containing solution having sufficiently low

concentration of water or a purified acrylic acid in the acrylic
acid-containing solution containing more than 10 mass % of
water, the concentration of water thereof can be made 10 mass %
or less, and if subjecting the resultant solution to the
crystallization process, the purified acrylic acid having high
purity can be obtained in good yield; or (3) if the acrylic
acid-containing solution contains more than 10 mass % of water,
it is to be used as an absorption liquid in an absorption process
step, and to be circulated like this so that an acrylic
acid-containing solution containing 10 mass % or less of water
can be obtained, and if subjecting the resultant solution to
the crystallization process, the purified acrylic acid having
high purity can be obtained in good yield; in addition, with
respect to methacrylic acid, the purified methacrylic acid
having high purity can also be obtained in good yield in a same
manner as acrylic acid.
[0010]
Namely, the method for producing (meth)acrylic acid
according to the present invention comprises obtaining a
(meth)acrylic acid-containing gas from a raw material of
(meth)acrylic acid by a catalytic gas phase oxidation reaction
and obtaining a (meth) acrylic acid-containing solution from the
(meth)acrylic acid-containing gas followed by subjecting the
(meth)acrylic acid-containing solution to a crystallization
process or subjecting the (meth)acrylic acid-containing
solution obtained after separating the reaction by-products
therefrom to a crystallization process, thereby producing the
purified (meth) acrylic acid; in the method, it is characterized
that the (meth)acrylic acid-containing solution or the
(meth)acrylic acid-containing solution upstream thereof (for
example, a (meth)acrylic acid-containing solution prior to a
separation process in a production method including a step of
separation process of reaction by-products) is mixed with a
purified (meth)acrylic acid and/or a (meth)acrylic
acid-containing solution containing 10 mass % or less of water
to make the concentration of water 10 mass % or less followed

by carrying out a crystallization process when the
concentration of water in the (meth)acrylic acid-containing
solution to be subjected to the crystallization process is
higher than 10 mass %. A method for obtaining a (meth) acrylic
acid-containing solution from a (meth) acrylic acid-containing
gas includes a method condensing the (meth)acrylic
acid-containing gas and a method obtaining the (meth)acrylic
acid-containing solution by bringing the (meth)acrylic
acid-containing gas in contact with an absorption liquid. The
absorption liquid includes water and high boiling point
solvents.
[0011]
As the purified (meth)acrylic acid to be mixed in the
(meth)acrylic acid-containing solution having a concentration
of water higher than 10 mass %, a purified (meth) acrylic acid
which has been obtained separately may be used (for example,
a purified (meth)acrylic acid produced during an operation in
a steady state in the production line may be stored for use later,
or a purified (meth) acrylic acid obtained in another production
line (in addition -to the one obtained by a crystallization
process, it may be the one obtained by a distillation process)
may be used), or a purified (meth)acrylic acid obtained in the
crystallization process may be circulated for use while the
production process is in operation.
[0012]
When the purified (meth) acrylic acid to be mixed is the
one obtained by the same production method (namely, the purified
(meth)acrylic acid obtained by subjecting a (meth)acrylic
acid-containing solution produced from a (meth)acrylic
acid-containing gas obtained by the catalytic gas phase
oxidation reaction to a crystallization process or subjecting
the (meth)acrylic acid-containing solution obtained after
separating the reaction by-products therefrom to a
crystallization process) , it is preferable because impurities
included in a slight amount therein do not affect the production
method.

Additionally, there are advantages when the follows are
used as the purified (meth) acrylic acid. Namely, [1] : initially
purified (meth)acrylic acid obtained at the onset of the
operation is discharged while cleaning each part such as a pipe
and a tank, so that it contains various dirt. Therefore,
although the initially purified (meth)acrylic acid has to be
disposed of under ordinary circumstances, it can be repurified
by bringing it back to the previous step for use just as the
present invention, and accordingly, the ratio of loss in the
whole process can be reduced. [2] : when the production line is
brought to an emergency stop and the like for some reason, the
purified (meth)acrylic acid is not transported to a
predetermined tank (namely, it is not subjected to a
polymerization-prevention treatment), but stored in the
production line. When the accumulation time (stopping time)
becomes long, a polymer and the like may be formed to lower the
purity of the purified (meth) acrylic acid. However, by bringing
it back to the previous step for use without disposing of it
just as the present invention, the ratio of loss in the whole
process can be reduced.
[0014]
Additional, as the (meth)acrylic acid-containing
solution containing 10 mass % or less of water to be mixed in
the (meth)acrylic acid-containing solution containing more
than 10 mass % of water, the (meth)acrylic acid-containing
solution containing 10 mass % or less of water obtained in a
process producing (meth)acrylic acid may be stored for use.
[0015]
In addition, the method for producing (meth) acrylic acid
according to the present invention comprises obtaining a
(meth)acrylic acid-containing gas from a raw material of
(meth)acrylic acid by the catalytic gas phase oxidation
reaction and obtaining a (meth)acrylic acid-containing
solution from the (meth)acrylic acid-containing gas followed
by subjecting the (meth)acrylic acid-containing solution to a

crystallization process or subjecting the (meth)acrylic
acid-containing solution obtained after separating the
reaction by-products therefrom to a crystallization process,
thereby producing the purified (meth)acrylic acid; in the
method, it is characterized that the (meth)acrylic
acid-containing solution or the (meth) acrylic acid-containing
solution upstream thereof is used as an absorption liquid when
the (meth)acrylic acid-containing solution to be subjected to
the crystallization process contains more than 10 mass % of
water.
[0016]
In this case, as an absorption liquid used for absorption,
the (meth)acrylic acid-containing solution containing more
than 10 mass % of water may be used; alternatively, the
(meth) acrylic acid-containing solution containing more than 10
mass % of water may be concomitantly used with a usual absorption
liquid. In addition to a use for absorption by being circulated
during an operation of the production process, the
(meth)acrylic acid-containing solution containing more than
10 mass % of water can be used as an absorption liquid after
temporarily being stored. Further, in a case wherein it is used
as an absorption liquid after being temporarily stored as
described above, it may be used as an absorption liquid after
the (meth) acrylic acid-containing solution containing 10 mass %
or less of water can be obtained in a stable manner, (in this
case, it is preferred to use a necessary amount of the liquid
little by little to an extent that the (meth)acrylic
acid-containing solution obtained in the production process can
maintain the concentration of water of 10 mass % or less) , or
it may be used as an absorption liquid in a state where only
a (meth)acrylic acid-containing solution containing more than
10 mass % of water can be obtained.
[0017]
As the method for supplying the (meth)acrylic
acid-containing solution containing more than 10 mass % of
water to the absorption column, a supply line of a (meth) acrylic

acid-containing solution containing more than 10 mass % of
water may be connected to a supply line of a usual absorption
liquid so as to supply the solution to the absorption column;
alternatively, a supply line of the (meth)acrylic
acid-containing solution containing more than 10 mass % of
water alone may be disposed in the absorption column (in a manner
that the supply line has an opening at a position higher than
a gas feed inlet of the absorption column) so as to supply the
solution directly to the absorption column.
[0018]
In addition, the method for producing (meth) acrylic acid
according to the present invention comprises obtaining a
(meth)acrylic acid-containing gas from a raw material of
(meth)acrylic acid by the catalytic gas phase oxidation
reaction, and obtaining a (meth)acrylic acid-containing
solution from the (meth)acrylic acid-containing gas, followed
by subjecting the (meth)acrylic acid-containing solution to a
crystallization process or subjecting the (meth)acrylic
acid-containing solution obtained after separating the
reaction by-products therefrom to a crystallization process,
thereby producing the purified (meth)acrylic acid; in the
method, it is characterized that when the (meth)acrylic
acid-containing solution contains more than 10 mass % of water,
it is temporarily stored, and when the (meth)acrylic
acid-containing solution containing 10 mass % or less of water
can be obtained in a stable manner, the stored (meth)acrylic
acid-containing solution containing more than 10 mass % of
water is mixed therein to an extent that the concentration of
water is maintained at 10 mass % or less, followed by being
subjected to the crystallization process.
[0019]
Namely, in a case where the (meth) acrylic acid-containing
solution contains more than 10 mass % of water, it is temporarily
stored, and when the purification process comes to a steady
state, the (meth)acrylic acid-containing solution containing
more than 10 mass % of water which has been stored is mixed in

a (meth)acrylic acid-containing solution containing 10 mass %
or less of water. However, mixing is made to an extent that the
concentration of water is retained 10 mass % or less, and the
resultant (meth)acrylic acid-containing solution containing
10 mass % or less of water is to be supplied to the step of the
crystallization process.
[0020]
With respect to the production methods, methods for
obtaining a (meth)acrylic acid-containing solution from a
(meth)acrylic acid-containing gas include a method condensing
a(meth)acrylic acid-containing gas and a method obtaining a
(meth)acrylic acid-containing solution by bringing the
(meth)acrylic acid-containing gas in contact with an
absorption liquid and the like.
[0021]
Further, in the present invention, each of the above
methods may be used in combination.
[0022]
In the method for producing (meth) acrylic acid, when the
method obtaining a (meth)acrylic acid-containing solution by
bringing a (meth) acrylic acid-containing gas in contact with an
absorption liquid is employed in which water is used as the
absorption liquid, the concentration of water tends to be high
so that it is particularly useful to apply the present invention
thereto.
[0023]
The reaction by-products include acrolein, methacrolein
(hereinafter occasionally referred to as (meth) acrolein) and
the like. Additionally, a case in which the (meth)acrylic
acid-containing solution to be subjected to the crystallization
process contains 10 mass % or less of water (when it contains
10 mass % or less of water even without carrying out a measure
according to the present invention) is called a steady state,
while a case wherein the (meth) acrylic acid-containing solution
to be subjected to the crystallization process contains more
than 10 mass % of water(when it contains more than 10 mass %

of water without carrying out a measure according to the present
invention) is called an unsteady state.
Brief Description of the Drawings
[0024]
FIG.l is a block diagram showing an example of a method
for producing acrylic acid according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025]
FIG.l is a block diagram showing an example of the method
for producing acrylic acid. First, a method for producing
acrylic acid (purification process) in a steady state will be
explained with reference to FIG.l.
[0026]
In a reactor 11, an acrylic acid-containing gas 12 is
generated from a raw material of acrylic acid 13 by the
catalytic gas phase oxidation reaction, and the acrylic
acid-containing gas 12 is introduced to an absorption column
14 (line 32) . In the absorption column 14, an absorption liquid
15 and the acrylic acid-containing gas 12 are brought in contact
to obtain an acrylic acid-containing solution 16 (absorption
step). The methods of the contact include cross-flow contact,
counter-flow contact and the like. In the present embodiment,
an absorption method wherein the absorption liquid is supplied
from outside the system as a method for obtaining the acrylic
acid-containing solution 16 will be described; however, for
example, in a condensing method, the absorption liquid is not
to be added from outside the system.
[0027]
Next, the acrylic acid-containing solution 16 is
introduced to an acrolein separation column 17 (line 36), and
acrolein is subjected to a separation process (acrolein
separation process step). Methods for the separation process
include distillation, diffusion or the like, and may be suitably
selected depending on the concentrations of acrylic acid and

acrolein contained. The step of the acrolein separation process
can be skipped when the acrylic acid-containing solution 16 does
not contain acrolein. Furthermore, if the acrylic
acid-containing solution 16 is subjected to a crystallization
process (crystallizer 19) via a step of the acrolein separation
process, lowering of the concentration of water can be expected
in the step of the acrolein separation process. In this view,
in a method wherein the acrylic acid-containing solution 16
obtained by condensing the acrylic acid-containing gas 12 or
bringing it in contact with an absorption liquid is directly
(i.e., without going through a step of the acrolein separation
process) subjected to a crystallization process, it is
preferred to obtain an acrylic acid-containing liquid having
a high concentration of acrylic acid in an absorption step
compared with that going through a step of the acrolein
separation process.
[0028]
An acrylic acid-containing solution 18 obtained by the
separation process or the acrylic acid-containing solution 16
is supplied to the crystallizer 19 (line 38) to obtain the
purified acrylic acid 20. The crystallization method at this
time is not limited, and either a continuous type or a batch-type
crystallizer may be used, and the crystallizer is not limited
to one-stage, but may be two or more stages for use in
implementing the method. For example, as a continuous type
crystallizer, a column-shaped one having a crystallization part,
a solid-liquid separation part and a crystal purification part
all united in one can be used. In the figure, reference numeral
21 denotes a residual mother liquid discharged from the
crystallizer 19. In Japanese Patent Publication No. 2005-15478A,
at least a part of the residual mother liquid 21 is distilled,
and the resultant distillate is to be supplied to the absorption
column 14.
[0029]
Next, a method for producing acrylic acid (purification
process) in an unsteady state will be explained.

When the acrylic acid-containing solution 18 (the acrylic
acid-containing solution 16 when the acrolein separation
process is not carried out) contains more than 10 mass % of water,
any one of following [a] to [d] or a combination of two or more
thereof and the like (hereinafter occasionally referred to as
measures [a] to [d] ) are employed so that the concentration
of water in the acrylic acid-containing solution to be supplied
to the crystallizer 19 is made 10 mass % or less.
[0031]
[a] : The purified acrylic acid 20 discharged from the
crystallizer 19, or the purified acrylic acid 22 obtained in
another production process is mixed in the acrylic
acid-containing solution 18 (the acrylic acid-containing
solution 16 when the acrolein separation process is not carried
out) of the line 38, and the mixture is supplied to the
crystallizer 19. Alternatively, the purified acrylic acid 20
or 22 is mixed in the acrylic acid-containing solution 16 of
the line 36 followed by subjecting the mixture to the acrolein
separation process, and the resultant acrylic acid-containing
solution 18 is supplied to the crystallizer 19.
[0032]
[b]: An acrylic acid-containing solution having a low
concentration of water obtained from the absorption column 14
in a steady state (hereinafter referred to as extracted liquid)
is stored in a tank 25 used for the extracted liquid (line 34)
in advance, and when the acrylic acid-containing solution 16
containing more than 10 mass % of water is discharged from the
absorption column 14, the extracted liquid 23 is mixed in the
acrylic acid-containing solution 16 (line 33). If necessary,
the mixture is subjected to the acrolein separation process
followed by being supplied to the crystallizer 19.
[0033]
[c] : The acrylic acid-containing solution 18 or 16 having a high
concentration of water is supplied from an absorption liquid
feed inlet of the absorption column 14 (namely, the acrylic

acid-containing solution 18 or 16 having a high concentration
of water is supplied to the absorption column 14 from an
intermediate tank discharge line 26 via an absorption liquid
supply line 27) or from a feed inlet for the return of the acrylic
acid-containing solution (the feed inlet for the return of the
acrylic acid-containing solution is disposed at a position
higher than the gas feed inlet) connected with the absorption
column 14 (namely, the acrylic acid-containing solution 18 or
16 having a high concentration of water is supplied to the
absorption column 14 from the intermediate tank discharge line
28) . In this manner, the acrylic acid-containing solutions 18
and 16 having a high concentration of water are circulated as
absorption liquids, thereby the acrylic acid component is
condensed so that the acrylic acid-containing solution 16
having a low concentration of water can be obtained. If the
amount of the acrylic acid-containing solution 18 or 16 having
a high concentration of water is more than that needed in an
absorption liquid in the absorption column 14, it is preferable
that the acrylic acid-containing solution 18 or 16 is stored
temporarily in an intermediate tank 24, and an amount needed
for absorption in the absorption column 14 is supplied little
by little. When the amount of the acrylic acid-containing
solution 18 or 16 having a high concentration of water is
sufficiently smaller than that needed in the absorption liquid
in the absorption column 14, the acrylic acid-containing
solution 18 or 16 may be supplied to the absorption liquid feed
inlet without going through the intermediate tank 24. Further,
as a timing for supplying an acrylic acid-containing solution
having a high concentration of water in the intermediate tank
24 to the absorption column 14, it may be when the concentration
of water in the acrylic acid-containing solution 16 discharged
from the absorption column 14 becomes 10 mass % or less, or before
the concentration of water becomes 10 mass % or less (when it
is higher than 10 mass %).
[0034]
[d] : When the acrylic acid-containing solution 18 or 16 having

a high concentration of water is temporarily stored in the
intermediate tank 24 (line 37) and the purification process of
acrylic acid comes to a steady state, the solution is brought
back from the intermediate tank 24 through the line 35 to the
original line (line 38), thereafter is mixed with the acrylic
acid-containing solution 18 or 16 in a steady state, and the
resultant mixture is supplied to the crystallizer 19. However,
in order that the concentration of water in the acrylic
acid-containing solution after being mixed does not become
higher than 10 mass %, the amount of the acrylic acid-containing
solution supplied from the intermediate tank 24 is to be
adjusted.
[0035]
In accordance with [a] to [d] described above, by making
the acrylic acid-containing solution to be supplied to the
crystallizer 19 contain 10 mass % or less of water, a step for
crystallization can be carried out smoothly without any special
operation, and the purified acrylic acid with high purity can
be produced in good yield.
[0036]
As a timing for switching from an operation mode
implementing the measures [a] to [d] (a method using any one
of [a] to [d], or a combination of two or more thereof) (an
operation mode in which measures for an unsteady state are
implemented) to an operation mode for a steady state (steady
operation), the operation can be switched to the steady
operation not only immediately after the concentration of water
in the acrylic acid-containing solution 18 (the acrylic
acid-containing solution 16 in a case where an acrolein
separation process is not carried out) becomes 10 mass % or less,
but also at a stage where the concentration of water in the
acrylic acid-containing solution 18 (or the acrylic
acid-containing solution 16) becomes 8 mass % or less, and
further, 5 mass % or less.
[0037]
Even though the acrylic acid-containing solution 18 (16)

contains 10 mass % or less of water, it cannot be said that the
process is in a sufficiently stable condition as soon as the
concentration of water becomes 10 mass % or less, so that there
will possibly be a need to switch to the operation mode
implementing the measures [a] to [d] again in a short time.
Accordingly, in view of complication in switching, it is
preferred to switch to the steady operation after the
concentration of water becomes 8 mass % or less, or, further,
5 mass % or less, which is a more stable condition. Additionally,
by implementing the measures [a] to [d], it becomes possible
to introduce an acrylic acid-containing solution having a lower
concentration of water to the crystallizer, so the
crystallization process of the solution having a lower
concentration of water can be carried out more smoothly, and
product specifications will also become higher. Therefore, from
a viewpoint of the concentration of water, it is preferable to
switch to the steady operation after the concentration of water
in the acrylic acid-containing solution 18 (16) becomes 8 mass %
or less, or, further, 5 mass % or less.
[0038]
On the other hand, in order to lower the concentration
of water, it takes longer to implement the above measures [a]
to [d] (compared with a case wherein the concentration of water
becomes 10 to 8 mass %, it takes longer for the concentration
of water to become 8 to 5 mass %, and it takes further longer
for the concentration of water to become 5 mass % or less);
therefore, an operation mode implementing the measures [a] to
[d] (unsteady operation) needs to be continued for a long period
of time. Therefore, from this viewpoint, it is preferred to
switch to the steady operation at a stage in which the
concentration of water in the acrylic acid-containing solution
18 (16) is not lowered so much (for example, a concentration
of water of 8 to 5 mass %, preferably 10 to 8 mass %).
[0039]
Here, an absorption method for obtaining an acrylic
acid-containing solution from an acrylic acid-containing gas

using the absorption liquid described above will be described.
[0040]
As an aqueous solution for absorption, a wide range of
aqueous solutions may be used as long as they can absorb acrylic
acid, and a condensate obtained by cooling and condensing a gas
discharged from the top of the absorption column (which is
discharged in order to minimize the loss of acrylic acid
discharged outside the system) is preferably used as an aqueous
solution for absorption. The condensate often contains acrylic
acid, so if it is used as the absorption aqueous solution, the
acrylic acid-containing solution having a high concentration
of acrylic acid can be obtained. It is also a preferred
embodiment that the' gas discharged from the top of the
absorption column is partially or fully recycled in the reactor.
[0041]
The aqueous solution for absorption is preferably
introduced to the absorption column at a temperature of 0 to
50°C, more preferably 10 to 40°C.
[0042]
A ratio of mass flow rate of an aqueous solution for
absorption to be newly supplied from outside to an acrylic
acid-containing gas may be suitably selected in accordance with
an objective acrylic acid concentration, and preferably 0.1 to
1.5 times the mass flow rate of acrylic acid included in the
acrylic acid-containing gas, more preferably 0.1 to 1.0 times,
even more preferably 0.15 to 0.8 times. Then an aqueous solution
for absorption having this mass flow rate is subjected to
counter-flow contact with an acrylic acid-containing gas to
absorb acrylic acid. If the ratio of mass flow rate is lower
than 0.1 times, efficiency of the acrylic acid absorption column
may be extremely lowered.
[0043]
In addition, since acrylic acid is a polymerizable
material,- for a purpose to prevent the polymerization, one or
more compounds selected from the group consisting of a N-oxyl
compound, a phenol compound, a manganese salt such as manganese

acetate and the like, a dialkyldithiocarbamate copper salt such
as dibutylcarbamate copper and the like, a nitro compound, an
amine compound and phenothiazine may be included in the aqueous
solution for absorption (refer to Japanese Patent Publication
No. 2001-348360A, Japanese Patent Publication No. 2001-348358A,
Japanese Patent Publication No.2001-348359A and the like).
[0044]
The acrylic acid absorption column is generally operated
under a normal pressure or above. During the steady operation,
the column-top pressure (gauge pressure) is preferably at 0 to
0.4 MPa, more preferably at 0 to 0.1 MPa, even more preferably
at 0 to 0.03 MPa. If it is lower than 0 MPa (gauge pressure),
a decompressor becomes necessary and it costs equipment and
utilities expenses, while if it is higher than 0.4 MPa (gauge
pressure),the temperature of the absorption column needs to be
increased considerably in order to discharge low boiling point
substances, so that the absorption efficiency may be lowered.
[0045]
A column top temperature of an absorption column is
generally from 30 to 85°C, preferably from 40 to 80°C.
[0046]
Under the absorption condition described above, in the
present embodiment, an acrylic acid-containing solution with
a following blending ratio can be obtained: acrylic acid :80
to 98 mass %, water:1 to 19 mass %,other impurities (acids such
as acetic acid, maleic acid, propionic acid and the like,
aldehydes such as furfural, formaldehyde and the like) : 1 to 10
mass %.
[0047]
With respect to the method for producing methacrylic acid
(purification process), the crystallization step can also be
carried out smoothly without any special operation in a same
manner as the acrylic acid, thereby a purified methacrylic acid
with high purity can be produced in good yield.
[0048]
The concentration of water in the acrylic acid-containing

solution (or the methacrylic acid-containing solution) can be
determined by Karl Fischer's method. The acrylic acid
concentration in the acrylic acid-containing solution (the
methacrylic acid concentration in the methacrylic
acid-containing solution) can be determined by any one of gas
chromatography analysis, electrical conductivity measurement
and neutralization titration.
[0049]
Hereinafter, the present invention will be described in
more detail with reference to examples, but the present
invention is not restricted by the following examples and can
be suitably modified within the scope described above or below
and such modifications are also included in the technical scope
of the present invention.
[0050]

The acrylic acid-containing gas 12 obtained by the
catalytic gas phase oxidation method was introduced to the
absorption column 14 to be brought in counter-flow contact with
the absorption liquid 15, thereby the acrylic acid-containing
solution 16 was obtained. The liquid composition of the acrylic
acid-containing solution 16 comprised 65.0 mass % of acrylic
acid, 20.0 mass % of water, and 15.0 mass % of other impurities.
[0051]
The purified acrylic acid 20 (the composition of the
purified acrylic acid comprised 99.4 mass % of acrylic acid,
0 . 004 mass % of water and 0 . 6 mass % of other impurities) obtained
in advance during an operation in a steady state was mixed in
the acrylic acid-containing solution 16 to give a mixture
thereof. The mixing mass ratio of the acrylic acid-containing
solution to the purified acrylic acid was 2:5. The above mixture
contained 5.7 mass % of water, 89.6 mass % of acrylic acid, and
4.7 mass % of other impurities.
[0052]
The above mixture was supplied to the crystallizer 19 to
be subjected to crystallization. The resultant acrylic acid had

a purity of 99.4 mass %. The purification yield of acrylic acid
was 99.9 %.
[0053]
In this Example 1, although no special operation other
than mixing the purified acrylic acid was carried out, purified
acrylic acid having high purity could be produced in good yield
as described above.
[0054]

The acrylic acid-containing gas 12 obtained by the
catalytic gas phase oxidation method was brought in
counter-flow contact with the absorption liquid 15 in the
absorption column 14 to obtain the acrylic acid-containing
solution 16. The liquid composition of the acrylic
acid-containing solution 16 comprised 75.0 mass % of acrylic
acid, 20.0 mass % of water, and 5.0 mass % of other impurities.
[0055]
On the other hand, during an operation in a steady state,
an acrylic acid-containing solution containing 10 mass % or less
of water discharged from the absorption column 14 was extracted
and stored in advance (the acrylic acid-containing solution is
referred to as extracted liquid) (tank 25 for the extracted
liquid) , and the extracted liquid 23 was mixed with the acrylic
acid-containing solution 16 of the line 36 (a mixture). The
mixing mass ratio of the acrylic acid-containing solution to
the extracted liquid was 1:20. The composition of the extracted
liquid 23 comprised 88.7 mass % of acrylic acid, 4.4 mass % of
water, and 6.9 mass % of other impurities. The liquid
composition of the above mixture comprised 88.0 mass % of
acrylic acid, 5.1 mass % of water, and 6.8 mass % of other
impurities.
[0056]
The mixture was supplied to the crystallizer 19 to be
subjected to crystallization. The resultant acrylic acid had
a purity of 99.4 mass %. The purification yield of acrylic acid
was 99.9 %.

In this Example 2, too, although no special operation
other than mixing the acrylic acid-containing solution having
a low concentration of water was carried out, the purified
acrylic acid having high purity could be produced in good yield
as described above.
[0058]

The acrylic acid-containing gas 12 obtained by the
-catalytic gas phase oxidation method was subjected to
counter-flow contact with the absorption liquid 15 in the
absorption column 14 to obtain the acrylic acid-containing
solution 16. The liquid composition of the acrylic
acid-containing solution 16 comprised 75.0 mass % of acrylic
acid, 15.0 mass % of water and 10.0 mass % of other impurities.
[0059]
The acrylic acid-containing solution 16 was stored in the
intermediate tank 24 (the stored acrylic acid-containing
solution is occasionally referred to as a stored acrylic
acid-containing solution), and from the tank, the acrylic
acid-containing solution was supplied from an absorption liquid
feed inlet of the absorption column 14 in a manner that it was
included in an amount of 20 mass % relative to the amount of
the absorption liquid (line 27 via line 26) . Then, this
operation was continued until the concentration of water in the
acrylic acid-containing solution 16 (line 36) discharged from
the absorption column 14 became 10 mass % or less.
[0060]
After that, it was confirmed that the acrylic
acid-containing solution 16 discharged from the absorption
column 14 had a sufficiently low concentration of water which
can be obtained in a stable manner (specifically, the liquid
composition comprised 86.0 mass % of acrylic acid, 8.0 mass %
of water, and 6.0 mass % of other impurities) , and the acrylic
acid-containing solution 16 having a low concentration of water
was supplied to the crystallizer 19 for crystallization.

The resultant acrylic acid showed a purity of 99.4 mass %,
and the purification yield of acrylic acid was 99.9 %.
[0062]
In this Example 3, although no special operation was
carried out except that the acrylic acid-containing solution
containing more than 10.0 mass % of water was brought back to
the absorption column as the absorption liquid, the purified
acrylic acid having high purity could be produced in good yield
as described above.
[0063]

The acrylic acid-containing gas 12 obtained by the
catalytic gas phase oxidation method was subjected to
counter-low contact with the absorption liquid 15 in the
absorption column 14 to obtain the acrylic acid-containing
solution 16. Next, the acrylic acid-containing solution 16 was
supplied to the acrolein separation column 17 for subjecting
acrolein and the like to separation by distillation, thereby
the acrylic acid-containing solution 18 with acrolein and the
like removed was obtained. The liquid composition of the acrylic
acid-containing solution 18 comprised 67.0 mass % of acrylic
acid, 20.0 mass % of water and 13.0 mass % of other impurities.
[0064]
The purified acrylic acid 20 (the composition of the
purified acrylic acid comprised 99.4 mass % of acrylic acid,
0.004 mass % of water, and 0.6 mass % of other impurities)
obtained in advance during an operation in a steady state was
mixed in the acrylic acid-containing solution 18 (line 38) to
be supplied to the crystallizer 19. The mixing mass ratio of
the acrylic acid-containing solution to the purified acrylic
acid was 2: 5. The mixture contained 5.7 mass % of water, 90.1
mass % of acrylic acid, and 4.1 mass % of other impurities.
[0065]
The acrylic acid obtained by crystallizing the mixture
had a purity of 99.4 mass %. The purification yield of acrylic

acid was 99.9 %.
[0066]
In the Example 4, too, although no special operation other
than mixing the purified acrylic acid was carried out, the
purified acrylic acid having high purity could be produced in .
good yield as described above.
[0067]
(Comparative Example)
The acrylic acid-containing gas 12 obtained by the
catalytic gas phase oxidation method was subjected to
counter-flow contact with the absorption liquid 15 in the
absorption column 14 to obtain the acrylic acid-containing
solution 16. The liquid composition of the acrylic
acid-containing solution 16 comprised 65.0 mass % of acrylic
acid, 20 . 0 mass % of water, and 15 . 0 mass % of other impurities .
[0068]
The acrylic acid-containing solution 16 was supplied to
the crystallizer 19. In the crystallization process, the above
Examples 1 to 4 showed an acrylic acid crystallization speed
equivalent to that at a time of a steady operation, compared
with this, the crystallization speed of acrylic acid in Example
5 is slow so that the crystal amount obtained in an equivalent
time to , that in Examples 1 to 4 was very small. The
crystallization speed became slow probably because the melting
point of the acrylic acid-containing solution was lowered due
to the high concentration of water in the acrylic
acid-containing solution 16 so that the heat exchange
efficiency with the refrigerant became poor. Further, a change
in the solution component accompanied with the progress of
crystallization led to further lowering of the melting point
so that it became difficult to continue the crystallization step
and thus the operation was stopped.
[0069]
(Comparative Example)
The acrylic acid-containing gas 12 obtained by the
catalytic gas phase oxidation method was subjected to

counter-flow contact with the absorption liquid 15 in the
absorption column 14, thereby the acrylic acid-containing
solution 16 was obtained. The liquid composition of the acrylic
acid-containing solution 16 comprised 65.0 mass % of acrylic
acid, 20.0 mass % of water, and 15.0 mass % of other impurities.
[0070]
On the other hand, the purified acrylic acid 20 (the
composition of the purified acrylic acid comprised 99.4 mass %
of acrylic acid, 0.004 mass % of water and 0.6 mass % of other
impurities) obtained in advance during an operation in a steady
state was mixed in the acrylic acid-containing solution 16 in
a manner that the mass ratio of the acrylic acid-containing
solution to the purified acrylic acid became 2: 1, and the
resultant mixture was supplied to the crystallizer 19. The
mixture contained 13.3 mass % of water, 7 6.5 mass % of acrylic
acid, and 10.2 mass % of other impurities.
[0071]
In the above crystallizer 19, only 60 mass % of the acrylic
acid included in the mixture could be crystallized. It is
probably because the melting point of the acrylic
acid-containing solution was lowered since the concentration
of water in the acrylic acid-containing solution supplied to
the crystallizer 19 was high, resulting in the lower
crystallization rate. Therefore, it became difficult to
continue the crystallization process, and thus the operation
was stopped.
[0072]

The acrylic acid-containing gas 12 obtained by the
catalytic gas phase oxidation method was subjected to
counter-flow contact with the absorption liquid 15 in the
absorption column 14 to obtain the acrylic acid-containing
solution 16. The liquid composition of the acrylic
acid-containing solution 16 comprised 65.0 mass % of acrylic
acid, 20.0 mass % of water, and 15.0 mass % of impurities.

The purified acrylic acid 20 (the composition of the
purified acrylic acid comprised 99.4 mass % of acrylic acid,
0.004 mass % of water, and 0.6 mass % of other impurities)
obtained in advance during an operation in a steady state was
mixed in the acrylic acid-containing solution 16 in a manner
that the ratio of the acrylic acid-containing solution to the
purified acrylic acid became 1:1, thereafter supplied to the
crystallizer 19. The mixture contained 10.0 mass % of water,
82.2 mass % of acrylic acid, and 7.8 mass % of other impurities.
[0074]
The resultant acrylic acid showed a purity of 99.4 mass %.
The purification yield of acrylic acid was 96.0 %.
[0075]
In this Example 7, although no special operation other
than mixing the purified acrylic acid was carried out, the
purified acrylic acid having high purity could be produced as
described above.
[0076]

The acrylic acid-containing gas 12 obtained by the
catalytic gas phase oxidation method was subjected to
counter-flow contact with the absorption liquid 15 in the
absorption column 14 to obtain the acrylic acid-containing
solution 16. Since the acrylic acid-containing solution 16 had
a high concentration of water up to 20.0 mass %, the acrylic
acid-containing solution 16 was stored in the intermediate tank
24 (the stored acrylic acid-containing solution) . The operation
for accumulation was continued until the concentration of water
in the acrylic acid-containing solution 16 became 10 mass % or
less. Then, it was confirmed that the concentration of water
in the acrylic acid-containing solution 16 discharged from the
absorption column 14 became 10 mass % or less and could be
obtained in a stable manner. The liquid composition of the
acrylic acid-containing solution (the stored acrylic
acid-containing solution) stored in the intermediate tank 24
thus far comprised 77.0 mass % of acrylic acid, 15.0 mass % of

water and 8.0 mass % of other impurities.
[0077]
Next, the stored acrylic acid-containing solution in the
intermediate tank 24 was supplied from the line 28 to the
absorption column 14. The amount of the stored acrylic
acid-containing solution supplied then was 20 mass % of a total
amount of the absorption liquid, and a usual absorption liquid
was used for the rest of the total amount of the absorption
liquid.
[0078]
The liquid composition of the acrylic acid-containing
solution 16 thus obtained from the absorption column 14
comprised 89.0 mass % of acrylic acid, 6.0 mass % of water and
5.0 mass % of other impurities. The acrylic acid-containing
solution 16 was supplied to the crystallizer 19 for
crystallization.
[0079]
The resultant acrylic acid showed a purity of 99.4 mass %,
and the purification yield of acrylic acid was 99.9 %.
[0080]
In this Example 8, although no special operation was
carried out except that the acrylic acid-containing solution
containing more than 10.0 mass % of water was brought back to
the absorption column as the absorption liquid, the purified
acrylic acid having high purity could be produced in good yield
as described above.
[0081]

The acrylic acid-containing gas 12 obtained by the
catalytic gas phase oxidation method was subjected to
counter-flow contact with the absorption liquid 15 in the
absorption column 14 to obtain the acrylic acid-containing
solution 16. Since the acrylic acid-containing solution 16 had
a high concentration of water up to 20.0 mass %, the acrylic
acid-containing solution 16 was stored in the intermediate tank
24 (the stored acrylic acid-containing solution). This

operation for accumulation was continued until the
concentration of water in the acrylic acid-containing solution
16 became 8 mass %. The liquid composition of the acrylic
acid-containing solution (the stored acrylic acid-containing
solution) thus stored in the intermediate tank 24 comprised 75.0
mass % of acrylic acid, 14.0 mass % of water and 11.0 mass %
of other impurities.
[0082]
Next, the stored acrylic acid-containing solution in the
intermediate tank 24 was mixed from the line 35 in the acrylic
acid-containing solution 16 (containing 8 mass % or less of
water) of the line 38. The mixing mass ratio of the acrylic
acid-containing solution to the stored acrylic acid-containing
solution was 2:1. The liquid composition of the mixture
comprised 81.7 mass % of acrylic acid, 10.0 mass % of water,
and 8.3 mass % of other impurities.
[0083]
The mixture was supplied to the crystallizer 19 for
crystallization. The resultant acrylic acid showed a purity of
99.4 mass %, and the purification yield of acrylic acid was
96.0 %.
[0084]
In this Example 9, although no special operation was
carried out except that the acrylic acid-containing solution
containing more than 10. 0 mass % of water was temporarily stored,
and that the stored acrylic acid-containing solution was mixed
in the acrylic acid-containing solution 16 containing 8 mass %
or less of water, the purified acrylic acid having high purity
could be produced as described above.
[0085]
In summary of the above results of experiments,
separation and purification by crystallization could not be
carried out smoothly in Examples 5 and 6, while could be carried
out smoothly in the crystallizer 19 to obtain the purified
acrylic acid having high purity in Examples 1 to 4 and 7 to 9.
With respect to the yield, a high yield was obtained in Examples

1 to 4 and 8, but in Examples 7 and 9 in which an acrylic
acid-containing solution having a concentration of water of
10.0 mass % was crystallized, the yield was somewhat inferior
to that of Examples 1 to 4 and 8. From the results of the
experiments, it is found that by making the concentration of
water 10 mass % or less using the above measures [a] to [d] when
the acrylic acid-containing solution contains more than 10
mass % of water, the crystallization process can be progressed
smoothly, and even more, a high yield can be obtained.
[0086]
Although the above examples showed methods for producing
acrylic acid, the present invention can also be applied to
methacrylic acid in a same manner.
[0087]
According to the present invention, even if the
(meth)acrylic acid-containing solution contains more than 10
mass % of water, which is an unsteady state, (meth) acrylic acid
can be collected from the (meth)acrylic acid-containing
solution without any special operation, and the purified
acrylic acid having high purity can be obtained at high yield
with less loss in the (meth) acrylic acid production process as
a whole. Moreover, the steps are easy and simple, and elevation
of cost arising on the occasions such as disposal or the
additional water separation operation as described above is not
induced. This application is based on Japanese Patent
Application No. 2006-254955 filed on September 20, 2006, the
contents of which are hereby incorporated by reference.

We Claim :
1. A method for producing (meth)acrylic acid comprising the steps of:
obtaining a (meth)acrylic acid-containing gas from a raw material for producing
(meth)acrylic acid by a catalytic gas phase oxidation reaction;
obtaining a (meth)acrylic acid-containing solution from the (meth)acrylic acid-
containing gas; followed by
subjecting the (meth)acrylic acid-containing solution to a crystallization process,
or followed by subjecting the (meth)acrylic acid-containing solution obtained after
separating a reaction by-product therefrom to a crystallization process, thereby
producing a purified (meth)acrylic acid,
wherein, when the (meth)acrylic acid-containing solution to be subjected to the
crystallization process contains more than 10 mass % of water, the concentration of
water is made 10 mass % or less by mixing a purified (meth)acrylic acid and/or a
(meth)acrylic acid-containing solution containing 10 mass % or less of water in the
(meth)acrylic acid-containing solution containing more than 10 mass % of water or in the
(meth)acrylic acid-containing solution upstream thereof, followed by carrying out the
crystallization process.
2. The method for producing (meth)acrylic acid as claimed in Claim 1 comprising the
steps of:
obtaining the (meth)acrylic acid-containing solution from the (meth)acrylic acid-
containing gas obtained by the catalytic gas phase oxidation reaction; and
subjecting the (meth)acrylic acid-containing solution to the crystallization process
or subjecting the (meth)acrylic acid-containing solution obtained after separating the
reaction by-product therefrom to the crystallization process, thereby obtaining the
purified (meth)acrylic acid,
wherein the purified (meth)acrylic acid is used as the purified (meth)acrylic acid
to be mixed in the (meth)acrylic acid-containing solution containing more than 10 mass
% of water.

3. The method for producing (meth)acrylic acid as claimed in Claim 1 or 2, wherein a
part of the (meth)acrylic acid-containing solution containing 10 mass % or less of water
obtained in the production process of (meth)acrylic acid is stored in advance and then
mixed in the (meth)acrylic acid-containing solution containing more than 10 mass % of
water.
4. A method for producing (meth)acrylic acid comprising the steps of:
obtaining a (meth)acrylic acid-containing gas from a raw material for producing
(meth)acrylic acid by a catalytic gas phase oxidation reaction;
bringing the (meth)acrylic acid-containing gas in contact with an absorption liquid
to obtain a (meth)acrylic acid-containing solution; followed by
subjecting the (meth)acrylic acid-containing solution to a crystallization process,
or followed by subjecting the (meth)acrylic acid-containing solution obtained after
separating a reaction by-product therefrom to a crystallization process, thereby
producing a purified (meth)acrylic acid,
wherein, when the (meth)acrylic acid-containing solution to be subjected to the
crystallization process contains more than 10 mass % of water, the (meth)acrylic acid-
containing solution containing more than 10 mass % of water or the (meth)acrylic acid-
containing solution upstream thereof is used as the absorption liquid.
5. The method for producing (meth)acrylic acid as claimed in Claim 4, wherein, when
the (meth)acrylic acid-containing solution to be subjected to the crystallization process
contains more than 10 mass % of water, the (meth)acrylic acid-containing solution
containing more than 10 mass % of water or the (meth)acrylic acid-containing solution
upstream thereof is circulated to be used as the absorption liquid in the step in which the
(meth)acrylic acid-containing gas is brought in contact with the absorption liquid.
6. The method for producing (meth)acrylic acid as claimed in Claim 5, wherein after
being circulated as the absorption liquid, the (meth)acrylic acid-containing solution
containing 10 mass % or less of water obtained by the absorption is subjected to the
crystallization process.

7. A method for producing (meth)acrylic acid comprising the steps of:
obtaining a (meth)acrylic acid-containing gas from a raw material for producing
(meth)acrylic acid by a catalytic gas phase oxidation reaction;
obtaining a (meth)acrylic acid-containing solution from the (meth)acrylic acid-
containing gas; followed by
subjecting the (meth)acrylic acid-containing solution to a crystallization process,
or followed by subjecting the (meth)acrylic acid-containing solution obtained after
separating a reaction by-product therefrom to a crystallization process, thereby
producing a purified (meth)acrylic acid,
wherein, when the (meth)acrylic acid-containing solution contains more than 10
mass % of water, it is temporarily stored, and when the (meth)acrylic acid-containing
solution containing 10 mass % or less of water can be obtained in a stable manner, the
stored (meth)acrylic acid-containing solution containing more than 10 mass % of water is
mixed therewith to an extent of maintaining the concentration of water 10 mass % or
less, followed by carrying out the crystallization process.
8. The method for producing (meth)acrylic acid as claimed in any one of Claim 1 to 7
comprising the step of bringing the (meth)acrylic acid-containing gas in contact with an
absorption liquid to obtain the (meth)acrylic acid-containing solution, wherein water is
used as the absorption liquid.

In a production method in which an acrylic
acid-containing solution obtained from an acrylic
acid-containing gas (or an acrylic acid-containing liquid
obtained simply by subjecting the acrylic acid-containing
solution to a separation process of reaction by-products) is
directly subjected to a crystallization process, the
crystallization process can't be carried out smoothly in an
unsteady state (when the concentration of water in the acrylic
acid-containing solution is high). The present invention aims
to provide a method for collecting the purified acrylic acid
having high purity in good yield even in an unsteady state.
When the acrylic acid-containing solutions 18 and 16 have
a concentration of water higher than 10 mass % (unsteady state) ,
the purified acrylic acid 20 and 22 or the acrylic
acid-containing solution 23 containing 10 mass % or less of
water is mixed therewith to make the concentration of water 10
mass % or less[a] [b] . Alternatively, when the acrylic
acid-containing solutions 18 and 16 have a concentration of
water higher than 10 mass %, it is used as an absorption liquid
[c]. Thereafter, the crystallization process is carried out.

Documents:

01295-kol-2007-abstract.pdf

01295-kol-2007-claims.pdf

01295-kol-2007-correspondence others.pdf

01295-kol-2007-description complete.pdf

01295-kol-2007-drawings.pdf

01295-kol-2007-form 1.pdf

01295-kol-2007-form 2.pdf

01295-kol-2007-form 3.pdf

01295-kol-2007-form 5.pdf

01295-kol-2007-pa.pdf

1295-KOL-2007-ASSIGNMENT 1.1.pdf

1295-KOL-2007-ASSIGNMENT.pdf

1295-KOL-2007-CORRESPONDENCE 1.1.pdf

1295-KOL-2007-CORRESPONDENCE 1.2.pdf

1295-KOL-2007-CORRESPONDENCE OTHERS 1.1.pdf

1295-KOL-2007-CORRESPONDENCE.pdf

1295-KOL-2007-EXAMINATION REPORT.pdf

1295-KOL-2007-FORM 13 1.1.pdf

1295-KOL-2007-FORM 13 1.3.pdf

1295-KOL-2007-FORM 13.pdf

1295-KOL-2007-FORM 18.pdf

1295-KOL-2007-FORM 3-1.1.pdf

1295-KOL-2007-FORM 3.pdf

1295-KOL-2007-FORM 5.pdf

1295-KOL-2007-GRANTED EP AND USA PATENT.pdf

1295-KOL-2007-GRANTED-ABSTRACT.pdf

1295-KOL-2007-GRANTED-CLAIMS.pdf

1295-KOL-2007-GRANTED-DESCRIPTION (COMPLETE).pdf

1295-KOL-2007-GRANTED-DRAWINGS.pdf

1295-KOL-2007-GRANTED-FORM 1.pdf

1295-KOL-2007-GRANTED-FORM 2.pdf

1295-KOL-2007-GRANTED-SPECIFICATION.pdf

1295-KOL-2007-OTHERS.pdf

1295-KOL-2007-PA 1.1.pdf

1295-KOL-2007-PA.pdf

1295-KOL-2007-PETITION UNDER RULE 137.pdf

1295-KOL-2007-PRIORITY DOCUMENT.pdf

1295-KOL-2007-REPLY TO EXAMINATION REPORT.pdf

1295-KOL-2007-TRANSLATED COPY OF PRIORITY DOCUMENT 1.1.pdf

1295-KOL-2007-TRANSLATED COPY OF PRIORITY DOCUMENT.pdf

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Patent Number

251947

Indian Patent Application Number

1295/KOL/2007

PG Journal Number

16/2012

Publication Date

20-Apr-2012

Grant Date

18-Apr-2012

Date of Filing

17-Sep-2007

Name of Patentee

NIPPON SHOKUBAI CO., LTD.

Applicant Address

1-1, KORAIBASHI 4-CHOME, CHUO-KU, OSAKA-SHI, OSAKA

Inventors:

#	Inventor's Name	Inventor's Address
1	NAKAGAWA SATOSHI	C/O NIPPON SHOKUBAI CO., LTD of 992-1, AZA NISHIOKI, OKINOHAMA, ABOSHI-KU,, HIMEJI-SHI, HYOGO

PCT International Classification Number

C07C67/08; C07C67/58; C07C69/54; C07C67/

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	JP2006-254955	2006-09-20	Japan