Compound purification method

Abstract

The present invention pertains to a method for purifying a compound using a purification apparatus having a crystallizer, in which N crystallization tanks are connected in series, and a cleaning column which forcibly transports crystals, the method being characterized by comprising: a step for generating crystals of the compound in the crystallizer; a step for discharging at least part of mother liquid to the outside of the purification apparatus; a step for separating a slurry containing the generated crystals into the mother liquid and a slurry having an increased crystal concentration; a step for returning at least part of the separated mother liquid to the original tank; a step for supplying a liquid to be purified containing the compound to the crystallizer and mixing the liquid to be purified with the slurry in the crystallizer; a step for feeding the slurry to one upstream tank in order from a downstream tank between a plurality of layers included in the crystallizer; and a step for supplying at least part of the slurry from the crystallizer to the cleaning column, wherein the liquid to be purified containing the compound has higher purity than the mother liquid discharged to the outside of the purification apparatus, the purity A1 of the mother liquid in the slurry supplied to the cleaning column is at least 80 mol%, and the difference A1-A2 between A1 and the purity A2 of the discharged mother liquid is at least 5 mol%.

Description

[Technical Field]

[0001] The present invention relates to a purification method that can be suitably used for purifying industrially produced compounds. [Background technology]

[0002] Currently, a wide variety of chemical compounds are produced and used industrially. Industrially produced chemical compounds are required to be high quality products with reduced impurities depending on their applications, and various improved purification technologies are being investigated to achieve this.

[0003] One known purification technique involves connecting multiple cooling crystallization tanks with clarifiers at their tops and vertical purification columns with clarifiers at their tops and heaters at their bottoms in series. Crystals produced in the crystallization tanks are sequentially sent to the crystallization tanks connected to the purification columns. The crystals sent from the crystallization tanks are allowed to settle by gravity within the purification column, while a portion of the crystals heated and melted in a heater at the bottom of the purification column is raised as a reflux liquid and brought into contact with the crystals settling by gravity, thereby washing the crystals (see Patent Documents 1 and 2). Another known purification method involves sending a suspension containing acrylic acid crystals produced in a crystallization tank and a crude acrylic acid melt to a washing column, forcibly transporting the crystals within the washing column, and using the melt obtained by melting the crystals at the bottom of the column as a washing liquid to wash the crystals within the washing column (see Patent Document 3). Another known purification method involves increasing the purity of acrylic acid by repeating suspension crystallization or layer crystallization multiple times from an aqueous solution containing acrylic acid (see Patent Document 4). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 59-66305 [Patent Document 2] Japanese Patent Application Publication No. 6-91103 [Patent Document 3] Special Publication No. 2003-530376 [Patent Document 4] Special Publication No. 2010-501526 Summary of the Invention [Problem to be solved by the invention]

[0005] While various purification techniques have been disclosed as described above, industrial production requires the production of high-purity compounds at high yield and low cost, and further improvements in purification techniques are needed. As a result of the inventors' studies, it became clear that the methods using gravity settling washing columns described in Patent Documents 1 and 2 do not provide industrially sufficient purification effects or production volumes when producing organic compounds that produce crystals with relatively fine particle sizes or when purifying low-purity compound solutions. Furthermore, the method described in Patent Document 3 does not achieve a high yield in the crystallization step, and the operating costs of the process for treating the crystallization residue increase. To achieve a high yield in the crystallization step, the purity of the compound solution used in the crystallization step must be increased, which disadvantageously increases the operating costs of pre-crystallization processes. Furthermore, the method described in Patent Document 4, which involves repeating crystallization multiple times, requires a compound solution with a low purity to be used in the crystallization step, but includes a process of melting the crystals and discharging the mother liquor during the crystallization process, which increases the complexity of the equipment, resulting in increased capital investment and energy consumption, and is therefore disadvantageous. The present invention has been made in view of the above-mentioned current situation, and an object of the present invention is to provide a method for purifying a compound, which enables a highly pure compound to be obtained in high yield at low cost. [Means for solving the problem]

[0006] The present inventors have studied a purification method capable of obtaining a high-purity compound in high yield and at low cost, and have used a purification apparatus having a predetermined configuration, which has a crystallization apparatus having N crystallization or maturation tanks including at least one crystallization tank, and a washing column, and the method comprises the steps of: separating a slurry containing crystals produced in the crystallization apparatus into a mother liquor and a slurry with an increased crystal concentration; returning at least a portion of the separated mother liquor to the original tank; discharging at least a portion of the mother liquor outside the purification apparatus; supplying a liquid to be purified containing the compound to the crystallization apparatus and mixing it with the slurry in the crystallization apparatus; The present inventors have found that in a purification method including a step of sequentially sending a slurry from a downstream tank to the next upstream tank, and a step of supplying at least a portion of the slurry from a crystallizer to a washing column, a highly pure compound can be obtained in high yield and at low cost by making the mother liquor discharged out of the purification apparatus lower in purity than the compound-containing purified liquid supplied to the purification apparatus, and by carrying out purification under conditions such that the purity of the mother liquor in the slurry supplied to the washing column and the difference between the purity of the mother liquor in the slurry and the purity of the mother liquor discharged out of the purification apparatus are within predetermined ranges, which has led to the completion of the present invention.

[0007] That is, the present invention is a method for purifying a compound using a purification apparatus having a crystallizer with a crystallization section and a washing column equipped with a mechanism for forcibly transporting crystals, the crystallizer having N (N≧2) tanks connected in series with the first tank downstream and the Nth tank upstream, at least the first tank is a crystallization tank equipped with a cooling mechanism, the second and subsequent tanks are crystallization tanks or maturation tanks, at least one tank has a line for supplying a solution to be purified containing the compound, the washing column has a line for carrying out the product, a line for returning the mother liquor to the crystallizer, The crystallizer has a washing column, and a line returning the slurry to the crystallizer is connected to at least the Nth tank. The crystallizer has a line for supplying slurry from the Nth tank to the washing column, a line for sending the slurry from the downstream tank to the tank immediately upstream, and a line for sending the mother liquor from the upstream tank to each of the 1st to N-1th tanks. At least one of the lines for sending the slurry from the N-1 downstream tanks to the tank immediately upstream is a line for sending the slurry to the tank immediately upstream via a solid-liquid separator, and a line for returning the mother liquor from which the crystals have been removed by the solid-liquid separator to the original tank. The line for sending the mother liquor from an upstream tank to each of the 1st to N-1th tanks includes at least one of a line for directly sending the mother liquor from the immediately upstream tank and a line for sending the mother liquor from the immediately upstream tank via a solid-liquid separator, and the purification method includes a step of producing crystals of a compound in a crystallizer, a step of discharging at least a portion of the mother liquor to the outside of the purification apparatus, a step of separating a slurry containing the produced crystals into a mother liquor and a slurry with an increased crystal concentration, a step of returning at least a portion of the separated mother liquor to the original tank, a step of returning a liquid to be purified containing the compound to the crystallizer, and a step of separating the liquid to be purified containing the compound from the crystallizer. a step of supplying a liquid to be purified containing the compound to a washing column and mixing it with a slurry in the crystallizer; a step of sending the slurry between multiple layers included in the crystallizer, in order from a downstream tank to the next most upstream tank; and a step of supplying at least a portion of the slurry from the crystallizer to a washing column, wherein the liquid to be purified containing the compound has a higher purity than the mother liquor discharged outside the purification apparatus, the purity A1 of the mother liquor in the slurry supplied to the washing column is 80 mol% or more, and the difference A1-A2 between A1 and the purity A2 of the discharged mother liquor is 5 mol% or more.

[0008] It is preferable to include a step of feeding the mother liquor from at least one of the 2nd to Nth tanks included in the crystallizer to a downstream tank to adjust the liquid level in the tank.

[0009] The temperature inside the crystallization tank included in the crystallizer is preferably 1 to 15° C. lower than the melting point of the pure substance of the compound to be purified.

[0010] Preferably, the wash column is a hydraulic wash column.

[0011] The residence time in the 1st to (N-1th) tanks included in the crystallizer is preferably 0.02 to 6 hours.

[0012] It is preferable to include a step of repurifying at least a part of the mother liquor discharged from the purification apparatus by distillation and / or crystallization.

[0013] The compound is preferably (meth)acrylic acid. [Effects of the Invention]

[0014] By using the method for purifying a compound of the present invention, a highly pure compound can be obtained in high yield and at low cost. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a diagram showing an example of a purification apparatus for carrying out the purification method of the present invention. [Figure 2] 1 is a diagram showing an example of a purification apparatus for carrying out the purification method of the present invention. [Figure 3] 1 is a diagram showing an example of a purification apparatus for carrying out the purification method of the present invention. [Figure 4] 1 is a diagram showing an example of a purification apparatus for carrying out the purification method of the present invention. [Figure 5] 1 is a diagram showing an example of a purification apparatus for carrying out the purification method of the present invention. [Figure 6] 1 is a diagram showing an example of a purification apparatus for carrying out the purification method of the present invention. [Figure 7]1 is a diagram showing an example of a purification apparatus for carrying out the purification method of the present invention. [Figure 8] 1 is a diagram showing an example of a purification apparatus for carrying out the purification method of the present invention. [Figure 9] 1 is a diagram showing an example of a purification apparatus for carrying out the purification method of the present invention. [Figure 10] FIG. 1 is a diagram showing a purification apparatus used to purify a crude acrylic acid solution in Comparative Examples 1 and 2. [Figure 11] FIG. 1 is a diagram showing a purification apparatus used to purify a crude acrylic acid solution in Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention will be described in detail below. In addition, a combination of two or more of the individual preferred embodiments of the present invention described below is also a preferred embodiment of the present invention.

[0017] The purification method of the present invention is a method for purifying a compound using a purification apparatus of a predetermined configuration having a crystallization apparatus having N crystallization or aging tanks including at least one crystallization tank, and a washing column equipped with a mechanism for forcibly transporting crystals, and the purification method includes a step of producing crystals of the compound in the crystallization apparatus, a step of discharging at least a part of the mother liquor outside the purification apparatus, a step of separating a slurry containing the produced crystals into the mother liquor and a slurry with an increased crystal concentration, a step of returning at least a part of the separated mother liquor to the original tank, a step of supplying a liquid to be purified containing the compound to the crystallization apparatus, and a step of separating the mother liquor from the mother liquor. The method includes a step of mixing the compound with a slurry in a crystallizer, a step of transferring the slurry from a downstream tank to a tank one tank upstream among multiple tanks included in the crystallizer, and a step of supplying at least a portion of the slurry from the crystallizer to a washing column, wherein the liquid to be purified containing the compound has a higher purity than the mother liquor discharged outside the purification apparatus, and purification is carried out under conditions such that the purity A1 of the mother liquor in the slurry to be supplied to the washing column is 80 mol% or more, and the difference between the purity A1 of the mother liquor and the purity A2 of the mother liquor discharged outside the purification apparatus is 5 mol% or more.

[0018] A mother liquor is separated from a slurry containing crystals of a compound in a crystallizer, and the slurry with an increased crystal concentration until the mother liquor purity A1 of the slurry is 80 mol% or more is sent to a washing column. Since the amount of impurities contained in the slurry sent to the washing column is low, the amount of washing liquid used in the washing column can be reduced, thereby increasing the yield of crystals recovered from the purification device and reducing variable costs.

[0019] Furthermore, when the difference between the purity A1 of the mother liquor in the slurry supplied to the washing column and the purity A2 of the mother liquor discharged outside the purification apparatus is 5 mol % or more, it can be said that crystals of the compound are sufficiently produced from the liquid to be purified in the crystallizer, and impurities are concentrated in the mother liquor and discharged outside the purification apparatus, and therefore a larger amount of the compound can be obtained as a product in a higher yield from the liquid to be purified containing the compound that is supplied to the purification apparatus.

[0020] The purity A1 of the mother liquor in the slurry supplied to the washing column may be 80 mol % or more, preferably 82 mol % or more, more preferably 83 mol % or more, and even more preferably 84 mol % or more.

[0021] The difference between the purity A1 of the mother liquor in the slurry supplied to the washing column and the purity A2 of the mother liquor discharged from the purification apparatus may be 5 mol% or more, preferably 6.5 mol% or more, more preferably 8 mol% or more, and even more preferably 10 mol% or more.

[0022] The purity of the mother liquor in the present invention refers to the ratio of the number of moles of the compound to be purified to the total number of moles of the compound to be purified and other impurities (such as by-products, solvents, and polymerization inhibitors) contained in the mother liquor.

[0023] The purification apparatus used in the purification method of the present invention includes a crystallizer having N (N≧2) tanks connected in series and a washing column for forcedly transporting crystals. At least the first tank included in the crystallizer is a crystallization tank equipped with a cooling mechanism, and the second and subsequent tanks are crystallization tanks or aging tanks. The Nth tank is connected to a washing column, and slurry is sent from the downstream tank to the next upstream tank, and from the Nth tank to the washing column. Furthermore, at least one of the lines sending slurry from the downstream tank to the next upstream tank is a line sending slurry to the upstream tank via a solid-liquid separator, and has a line returning at least a portion of the mother liquor discharged from the solid-liquid separator to the original tank. Furthermore, in the purification apparatus of the present invention, all of the 1st to (N−1th) tanks included in the crystallization apparatus have at least one of a line sending mother liquor directly from the next upstream tank and a line sending mother liquor from the next upstream tank via a solid-liquid separator, and a line sending mother liquor to the outside of the purification apparatus.

[0024] By connecting multiple crystallization or aging tanks in series and sending a slurry, which is a suspension of compound crystals and mother liquor, to the next upstream tank, and sending the mother liquor from the upstream tank to the downstream tank while it is in countercurrent contact with the crystals, the purity of the crystals and mother liquor can be improved the further upstream. Here, by using as many solid-liquid separators as possible and concentrating the slurry through the solid-liquid separators before sending it to the next upstream tank, the purity of the crystals and mother liquor can be more effectively improved.

[0025] Therefore, in the purification apparatus of the present invention, the line for sending the mother liquor outside the purification apparatus is preferably a line for sending the mother liquor from the most downstream tank to the outside of the purification apparatus, which allows a smaller amount of low-purity mother liquor (crystallization residue) containing concentrated impurities to be discharged from the most downstream tank, thereby enabling a high-purity compound to be obtained in high yield.

[0026] In the washing column, the purity of the crystals is increased by using a portion of the liquid obtained by heating and melting the purified crystals as a washing liquid (reflux liquid) and bringing it into countercurrent contact with the crystal bed. However, the amount of this washing liquid can be reduced by supplying high-purity crystals and mother liquor from the Nth tank to the washing column.

[0027] That is, the step of producing crystals of a compound in a crystallizer in the purification method of the present invention is a step of producing crystals of the compound in a crystallization tank provided in the crystallizer, and the step of separating a slurry containing crystals into a mother liquor and a slurry with an increased crystal concentration is a step of separating a slurry containing crystals into a mother liquor and a slurry with an increased crystal concentration in a solid-liquid separator.

[0028] The purification method of the present invention also includes a step of using a crystallization apparatus in which multiple crystallization or aging tanks, including at least one crystallization tank, are connected in series, and transferring a slurry removed from a downstream tank to the immediately upstream tank, at least one of which is a step of transferring the slurry removed from a tank to the immediately upstream tank via a solid-liquid separator. The step of supplying at least a portion of the slurry from the crystallization apparatus to a washing column is a step of supplying at least a portion of the slurry from the most upstream tank included in the crystallization apparatus to the washing column.

[0029] Furthermore, in the purification apparatus used in the purification method of the present invention, at least one of the lines for sending the slurry from the downstream tank to the tank one tank upstream has a line for sending the slurry to the tank one tank upstream via a solid-liquid separator. This prevents crystals from being sent to the downstream tank, efficiently increases the purity in the upstream tank, and facilitates purification in the next step (washing column). Of the N-1 lines that transfer slurry from a downstream tank to the tank one level upstream, the proportion of lines that transfer slurry to the tank one level upstream via a solid-liquid separator is preferably 60% or more. Most preferably, this proportion is 100%, i.e., all of the N-1 lines that transfer slurry from a downstream tank to the tank one level upstream are lines that transfer slurry to the tank one level upstream via a solid-liquid separator.

[0030] Among the tanks included in the crystallization apparatus, those having a line for sending the slurry to the immediately upstream tank via a solid-liquid separator also have a line for returning at least a portion of the mother liquor discharged from the solid-liquid separator to the original tank, so that the slurry containing concentrated crystals separated in the solid-liquid separator is sent to the immediately upstream tank, and at least a portion of the remaining mother liquor is returned to the original tank.

[0031] Furthermore, in the crystallization apparatus, each of the 1st to (N-1th) tanks has at least one line for feeding the mother liquor from the upstream tank to the tank, either a line for feeding the mother liquor directly from the upstream tank or a line for feeding the mother liquor from the upstream tank via a solid-liquid separator. By providing these lines, the liquid level in the tanks can be kept constant. As described above, the method for purifying a compound of the present invention includes a step of adjusting the liquid level in each of the 2nd to Nth tanks included in the crystallizer by transferring the mother liquor to the tank immediately downstream.

[0032] In the purification apparatus of the present invention, at least one of the 1st to (N-1th) tanks included in the crystallization apparatus preferably has a line for directly transferring the mother liquor from the tank immediately upstream, and more preferably, the (N-1th) tank has a line for directly transferring the mother liquor from the tank immediately upstream (the Nth tank).

[0033] That is, a preferred embodiment of the method for purifying a compound of the present invention includes a step of directly transferring the mother liquor from at least one of the 2nd to Nth tanks included in the crystallizer to the tank immediately downstream, and it is more preferred to include a step of directly transferring the mother liquor from the Nth tank to the tank immediately downstream. The reason why it is preferable to have a line for directly transferring the mother liquor from the Nth tank to the (N-1)th tank will be described later.

[0034] A known method for directly transferring the mother liquor from the tank is to provide a crystal settling area above the tank and transfer the crystal-free mother liquor (hereinafter sometimes referred to as clear mother liquor) by overflow. This method has the advantage of eliminating the need for a liquid transfer pump and facilitating the adjustment of the liquid level in each tank.

[0035] However, this method requires a settling zone for the crystals at the top of the tank, which makes the tank structure complicated. Furthermore, when purifying compounds that produce fine crystals or when purifying low-purity compound solutions, the settling speed of the crystals is slow, so the tank itself must be oversized to accommodate the settling zone. Furthermore, if the crystals are too fine and the settling zone is not properly formed, the crystals may be transported downstream, reducing the purification effect of the device.

[0036] If the tank included in the crystallization apparatus has a line that sends mother liquor from the tank one tank upstream via a solid-liquid separator, the liquid level in the tank can be kept constant while effectively preventing crystals from being sent downstream from the tank one tank upstream by solid-liquid separation. This allows the purification efficiency of the purification apparatus to be maintained high even when used to purify compounds that produce fine crystals and have a slow settling rate. Furthermore, since there is no need to provide a crystal settling area in the tank itself to collect the mother liquor, the size of the tank itself can be reduced.

[0037] Therefore, it is preferable that at least one of the lines for sending the mother liquor from the upstream tanks to each of the 1st to N-1th tanks is a line for sending the mother liquor from the tank immediately upstream via a solid-liquid separator.

[0038] That is, a preferred embodiment of the method for purifying a compound of the present invention includes a step of transferring the mother liquor from at least one of the 2nd to Nth tanks included in the crystallizer to the tank one tank downstream via a solid-liquid separator.

[0039] When a line for transferring the mother liquor from a tank included in a crystallizer to a tank immediately downstream via a solid-liquid separator is provided, it is preferable to share the solid-liquid separator provided on the line for transferring the slurry to the tank immediately upstream from the crystallizer in view of the cost of the purification apparatus itself and the operating cost of the apparatus, which allows the number of equipment such as solid-liquid separators and liquid transfer pumps to be reduced.

[0040] In this case, the system may be configured such that one or more additional lines for sending the mother liquor from the solid-liquid separator provided on the line for sending the slurry to the tank one tank upstream to the line for returning the mother liquor to the original tank are further provided, and the additional lines are connected to a tank one tank downstream from the original tank. The additional lines may also be further branched, connecting to two or more tanks further downstream from the original tank in addition to the tank one tank downstream. The additional lines may also be connected to the outside of the purification system.

[0041] A crystallization apparatus having such a configuration, i.e., at least one of the solid-liquid separation devices provided in the line for transferring the slurry from the downstream tank to the upstream tank, has one or more additional lines for transferring the mother liquor in addition to a line for returning the mother liquor to the original tank, and the additional lines are connected to a downstream tank and / or to the outside of the purification apparatus relative to the original tank, is one preferred embodiment of a purification apparatus for performing the purification method of the present invention.

[0042] Therefore, a preferred embodiment of the method for purifying a compound of the present invention includes a step of transferring the mother liquor from at least one of the 2nd to Nth tanks included in the crystallizer to the original tank and a tank one tank downstream, or to the tank one tank downstream and two or more tanks further downstream, or to the outside of the purification apparatus via a solid-liquid separation device.

[0043] Of the solid-liquid separation devices provided in the line for transporting the slurry from the downstream tank to the upstream tank, the proportion of those having such an additional line is preferably 30% or more, more preferably 60% or more, and even more preferably 100%.

[0044] The solid-liquid separation device can be a commonly used device such as a basket centrifuge, a decanter centrifuge, a liquid cyclone, a filter, a centrifugal separator, etc. Examples of basket centrifuges include the Escher-Wyss extrusion centrifuge manufactured by Tsukishima Kikai Co., Ltd., and examples of decanter centrifuges include the Bird decanter centrifuge manufactured by Tsukishima Kikai Co., Ltd. and the screw decanter centrifuge manufactured by IHI Corporation.

[0045] When a basket centrifuge is used, the crystal concentration in the cake after solid-liquid separation is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. When a decanter centrifuge is used, the crystal concentration after concentration is preferably 40% or more, more preferably 50% or more, and even more preferably 60% or more.

[0046] When using a liquid cyclone, the crystal concentration in the concentrated slurry is preferably 25% or more, more preferably 30% or more, and even more preferably 35% or more. If the slurry concentration is too high, the fluidity decreases and the risk of pipe clogging increases, so the concentrated slurry concentration is preferably 55% or less, more preferably 50% or less, and even more preferably 45% or less.

[0047] When a basket centrifuge or decanter centrifuge is used as a solid-liquid separator, the initial investment and operating costs are high, but the advantage is that the efficiency of concentrating the slurry (crystals) is high, which improves the purification efficiency of the compound. On the other hand, when a liquid cyclone is used, the efficiency of concentrating the slurry (crystals) is low, and multiple tanks must be installed in the crystallizer to achieve a sufficient purification effect, but the advantages are that the initial investment and operating costs are reduced and problems caused by the rotating machine do not occur.

[0048] The crystallization apparatus includes multiple crystallization tanks or aging tanks, including at least one crystallization tank. The total number of crystallization tanks and aging tanks is not particularly limited, but from the viewpoint of sufficiently improving the purity of the crystals and mother liquor, it is preferable that the number be two or more when a basket-type centrifuge or a decanter-type centrifuge is used as the solid-liquid separation apparatus, and three or more in other cases. The greater the number of tanks, the greater the effect of improving the purity of the crystals and mother liquor. However, using too many tanks increases the capital investment and the power consumption of pumps, agitators, and other equipment associated with the tanks, which are disadvantageous. Therefore, the total number of crystallization tanks and aging tanks is preferably six or less, regardless of the type of solid-liquid separation apparatus. It is more preferable that the number be five or less.

[0049] As long as the crystallizer includes at least one crystallization tank, the other tanks may be either crystallization tanks or aging tanks, but the number of aging tanks included in the crystallizer is preferably 0 to 2, and more preferably 0 to 1.

[0050] The tanks included in the crystallizer preferably have a structure that allows a clear mother liquor layer to form at the top, and when any of the 1st to (N-1th) tanks included in the crystallizer has a line for directly transferring the mother liquor from the tank immediately upstream, the line is preferably a line for directly transferring the clear mother liquor layer at the top of the tank immediately upstream by overflow. In addition, in one preferred embodiment of the purification apparatus for performing the purification method of the present invention, the line for discharging the mother liquor to the outside of the purification apparatus is a line for discharging the mother liquor to the outside of the purification apparatus by overflow.

[0051] The purification apparatus of the present invention preferably has a line for sending the mother liquor from the first tank, which is the most downstream tank included in the crystallization apparatus, to the outside of the purification apparatus. In one preferred embodiment of the purification apparatus of the present invention, the line is a line for sending the clear mother liquor layer above the first tank included in the crystallization apparatus to the outside of the purification apparatus by overflow.

[0052] Therefore, a preferred embodiment of the method for purifying a compound of the present invention includes a step of directly transferring the clear mother liquor layer in the upper part of at least one of the second to Nth tanks included in the crystallization apparatus to a downstream tank by overflow, and a preferred embodiment of the method for purifying a compound of the present invention also includes a step of directly discharging the clear mother liquor layer in the upper part of the first tank, the most downstream tank, by overflow outside the purification apparatus.

[0053] If the tank included in the crystallization apparatus does not form a clear mother liquor layer at the top, a solid-liquid separator can be installed, and the mother liquor and crystals can be separated from the slurry removed from the tank using the solid-liquid separator, and the mother liquor can be sent to one or more downstream tanks. Alternatively, the mother liquor separated by the solid-liquid separator can be discharged outside the purification apparatus. A preferred embodiment of the purification apparatus for carrying out the purification method of the present invention is one in which the mother liquor is discharged from the first tank included in the crystallization apparatus via a line that discharges the mother liquor from the first tank to the outside of the purification apparatus via the solid-liquid separator.

[0054] Furthermore, the method for purifying a compound of the present invention preferably includes a step of repurifying at least a portion of the mother liquor discharged from the purification apparatus by distillation and / or crystallization. By repurifying at least a portion of the mother liquor discharged from the purification apparatus and recovering the compounds contained in the discharged mother liquor, it is possible to reduce the amount of compounds contained in the residue discharged from the purification apparatus. The recovered compounds may be reintroduced into the purification apparatus together with the liquid to be purified that is supplied to the purification apparatus, or may be returned to a previous step for obtaining the liquid to be purified. Therefore, a preferred embodiment of the purification method of the present invention includes a step of reintroducing the compounds recovered by the repurification step into the purification apparatus, or a step of returning the recovered compounds to a previous step for obtaining the liquid to be purified.

[0055] Of the 1st to (N-1th) tanks included in the crystallizer, preferably 30% or more, more preferably 60% or more, and most preferably all (N-1th) tanks are configured to send the mother liquor to the next downstream tank (or to two or more downstream tanks) via a solid-liquid separator and / or discharge it to the outside of the crystallizer. This allows the tank size to be reduced, thereby reducing investment costs and construction area. As mentioned above, from a cost perspective, it is also preferable to share the solid-liquid separator in this case with the solid-liquid separator installed on the line that sends the slurry to the next upstream tank.

[0056] However, in the Nth tank, in order to hold / age the slurry to be sent to the washing column, Because the capacity of the Nth tank is preferably larger than that of the downstream tanks, and because the purity of the slurry is improved and crystals grow larger than in the downstream tanks, it is relatively easy to design the crystal settling region in the upper part of the tank, i.e., the clear mother liquor layer. Therefore, it is preferable that the Nth tank be designed so that the mother liquor is sent directly to the downstream tanks by overflow.

[0057] Therefore, a preferred embodiment of the method for purifying a compound of the present invention includes a step of directly transferring the clear mother liquor layer at the top of the Nth tank contained in the crystallizer to a downstream tank by overflow.

[0058] The crystallization tank included in the crystallizer is not particularly limited as long as it is equipped with a cooling mechanism and can cool a compound solution to precipitate crystals and generate a slurry containing the crystals and mother liquor. Broadly speaking, it can be broadly classified into a system in which a cooling jacket is attached to the tank itself to directly cool the tank interior to generate crystals, and a system in which the cooling mechanism is separate from the tank and connected by piping to circulate the cooling / crystallization system.

[0059] Although the method in which the cooling jacket is attached to the tank itself has the advantage of requiring fewer pieces of equipment, the tank itself must be made larger to increase the heat transfer area.When high production capacity is required, the tank size becomes excessively large, which has disadvantages in terms of initial investment and site area.

[0060] Therefore, when the size of the tank itself is limited or when purifying a compound that requires high production capacity, a system in which the contents of the tank are cooled outside the tank is preferred. In this way, if the tank and the cooling mechanism are connected by piping, and a portion of the compound solution (or slurry containing crystals) in the tank is sent to the cooling mechanism to produce crystals in the cooling mechanism, and the slurry containing the produced crystals is returned to the tank, the heat transfer area can be easily increased by adding more cooling mechanisms, and the crystallization tank can be easily scaled up.

[0061] In this case, the cooling mechanism is not particularly limited as long as it can cool the compound solution and precipitate crystals. However, it is preferable to use a shell-and-tube heat exchanger, a spiral heat exchanger, or the like, which can ensure a large heat transfer area, or a cooling disk crystallizer or a scraping-type cooling crystallizer, which performs crystallization while scraping the cooling surface.

[0062] The cooling disk crystallizer may be any device that cools a solution of a compound to precipitate crystals and scrapes off the precipitated crystals. For example, it may be a device that is composed of a tube and a number of cooling plates that separate the tube, where crystals are formed on the wall surfaces of the cooling plates, and where stirring blades with wipers are rotated inside the tube to scrape off the crystals.

[0063] The scraping-type cooling crystallizer may be any device that cools a solution of a compound to precipitate crystals and scrapes off the precipitated crystals, but it may also be a device that is composed of a double-structured tube, in which a refrigerant flows through the outer tube and a solution of the compound (or a slurry containing the crystals) in a tank flows through the inner tube to form crystals on the wall of the inner tube, and a shaft with a scraping blade rotates inside the inner tube to scrape off the crystals.

[0064] Therefore, in a preferred embodiment of the method for purifying a compound of the present invention, the step of producing crystals of the compound in a crystallizer is a step of sending a part of the solution of the compound (or a slurry containing the crystals) in a tank to a cooling mechanism to produce crystals in the cooling mechanism, and returning the slurry containing the produced crystals to the tank.

[0065] The crystallizer may or may not have an aging tank, but preferably has one. In the present invention, the aging tank is a tank that does not have a cooling mechanism for precipitating crystals, but rather grows compound crystals by holding the compound crystals for a certain period of time. By growing the crystals to be as uniform as possible and then sending the crystals to the washing column, impurities can be efficiently removed in the washing column, making it possible to obtain a compound with a higher purity and in a higher yield. Therefore, it is preferable that the tank that sends the crystals to the washing column, i.e., the Nth tank, is an aging tank.

[0066] Therefore, the method for purifying a compound of the present invention preferably includes a step of maturing a slurry containing crystals of the compound in an aging tank. When a crystallization apparatus having N tanks is used, the method preferably includes a step of maturing a slurry containing crystals of the compound in an aging tank that is the Nth tank most upstream of the crystallization apparatus.

[0067] The aging tank is not particularly limited as long as it can maintain crystals of the compound in a suspended state within the tank. By maintaining the crystals for a certain period of time, fine crystals melt due to Ostwald ripening, and larger crystals grow further, narrowing the crystal size distribution and further improving the purification efficiency in the washing column. Furthermore, even in a crystallization tank, the same effect as that of an aging tank can be expected by maintaining the crystals for a certain period of time.

[0068] A washing column included in the purification apparatus for carrying out the purification method of the present invention receives a slurry containing compound crystals from a crystallizer, washes the crystals, and obtains high-purity crystals of the compound as a product. In a preferred form of washing column of the present invention, if the specific gravity of the crystals is greater than that of the mother liquor, the crystals move downward within the column to form a crystal bed. The crystal bed is then scraped off and heated and melted at the bottom of the column, and a portion of the resulting melt is withdrawn as product. The remaining melt is brought into countercurrent contact with the crystal bed as a washing liquid and used to increase the crystal purity. In addition, the mother liquor and washing liquid in the washing column are returned to the crystallizer through a line that returns the liquid to the crystallizer. If the specific gravity of the crystals is less than that of the mother liquor, the crystals move upward within the column, and the crystal bed is suspended, melted, and the product is withdrawn at the top of the column. In the particularly preferred embodiment of the purification apparatus of the present invention described above, the line for returning the mother liquor to the crystallizer is connected to at least the Nth tank, but may also be connected to a tank further downstream. Also, a line for returning a portion of the mother liquor to the wash column may be provided.

[0069] Therefore, when a crystallizer having N tanks is used, the method for purifying a compound of the present invention preferably includes a step of sending a slurry containing crystals of the compound from the Nth tank of the crystallizer to a washing column. The method also preferably includes a step of withdrawing some of the crystals from the bottom of the washing column and melting them by heating, withdrawing a portion of the resulting melt as a product, and returning the remaining melt to the washing column and bringing it into countercurrent contact with the crystal bed in the column to wash the crystals.

[0070] The method for purifying a compound of the present invention preferably includes a step of returning the mother liquor from the wash column to a crystallizer, and when a crystallizer having N tanks is used, the method preferably includes a step of returning the mother liquor from the wash column to the Nth tank of the crystallizer or to a tank further downstream. The method may further include a step of returning a portion of the mother liquor removed from the wash column to the wash column again.

[0071] The washing column in the present invention is a column that forcibly transports a crystal bed. Specific examples include a mechanical washing column, which compresses the crystals with a piston to form / transport a crystal bed, and a hydraulic washing column (water pressure washing column), which pumps a slurry into the column and extracts the mother liquor from a filter placed inside the column to form / transport a bed. The operating principles of these washing columns are described in the book Melt Crystallization (Ed.). ted by Joachim Ulrich, Heike Glade, Shaker Verlag, Aachen 2003).

[0072] The wash column is not particularly limited as long as it is capable of washing the crystals, and may be either a mechanical wash column or a hydraulic wash column. Mechanical wash columns are characterized by high operational stability and high compound purification efficiency. On the other hand, hydraulic wash columns are characterized by high production capacity per column cross-sectional area and fewer moving parts within the wash column, resulting in fewer equipment-related problems. When purifying an easily polymerizable substance, the use of a hydraulic wash column with fewer moving parts may be more effective in suppressing the generation of polymers within the wash column.

[0073] A preferred form of washing column is one having a mechanical mechanism for scraping off the crystal bed (see U.S. Patent No. 3,872,009A). In washing columns having a forced crystal bed transport mechanism, the purified crystal bed is scraped off with a scraper or the like, resuspended, and then melted.

[0074] Another preferred form of washing column is one that does not have a mechanical mechanism for scraping off the crystal bed (see U.S. Patent No. 7,425,273 B2). In this system, the crystal bed is scraped off by the dynamic pressure of the circulating liquid. Since there are no sliding surfaces such as shaft seals, the generation of polymers due to liquid stagnation and sliding heat can be suppressed when purifying easily polymerizable substances.

[0075] The solution of the compound to be purified may be supplied to any tank included in the crystallizer in which the purification method of the present invention is carried out, but from the viewpoint of purification efficiency, it is preferable to supply it to the second or subsequent tank.

[0076] Therefore, when a crystallizer having multiple tanks is used, the method for purifying a compound of the present invention preferably includes a step of supplying the compound-containing solution to be purified to the second or subsequent tank. The optimum location varies depending on the feed liquid composition, crystallization yield, and the concentration efficiency of the crystals in the solid-liquid separator, and can be selected appropriately.

[0077] The crystallization temperature in the crystallizer in the purification method of the present invention may be adjusted appropriately depending on the type of compound to be purified, but is generally in the range of -1 to -15°C relative to the melting point of the pure substance, preferably -1.5 to -13.5°C, more preferably -3.5 to -12.5°C, and even more preferably -5 to -11.5°C. When the compound to be purified is (meth)acrylic acid, the temperature is preferably 0 to 12°C, more preferably 1 to 10°C, and even more preferably 2 to 8.5°C. Therefore, the step of producing crystals of the compound in the crystallizer in the compound purification method of the present invention is preferably carried out at these temperatures.

[0078] If the temperature of the crystallization tank is high, crystals with high purity are produced, but for example, when the crystallization tank is a scraping-type cooling crystallizer described below, problems such as the need for a lot of power to scrape the crystals in the crystallization tank may occur. Furthermore, if the temperature difference between the refrigerant and the inside of the crystallization tank is too high, problems such as blocking of the scraping scraper may occur when the crystallization tank is a scraping-type cooling crystallizer, and continued operation may become difficult.

[0079] Therefore, when the temperature of the crystallization tank is high, it is necessary to reduce the temperature difference between the refrigerant and the inside of the crystallization tank and reduce the amount of crystals produced per heat transfer area. Although the purity of the crystals produced decreases when the temperature of the crystallization tank is low, if the crystallization tank uses a scraping-type cooling crystallizer, less power is required to scrape the crystals in the crystallization tank, and scraper blocking is less likely to occur even if the temperature difference between the refrigerant and the inside of the crystallization tank is increased. As a result, it is possible to increase the temperature difference between the refrigerant and the inside of the crystallization tank and increase the amount of crystals produced per heat transfer area. However, if the crystallization temperature is too low, the particle size of the produced crystals will be small and the crystals will tend to be difficult to settle.

[0080] The residence time of the compound in the crystallization tank and aging tank can also be adjusted appropriately depending on the type of compound to be purified. However, taking into consideration the yield of the compound obtained after purification, the purification efficiency, and the capital investment cost, the residence time in the 1st to (N-1th) tanks included in the crystallization apparatus is generally 0.02 to 6 hours.

[0081] In the Nth tank, the residence time is preferably longer than a certain time in order to adjust the particle size distribution of the slurry sent to the washing column and reduce the reflux ratio (flow rate of washing liquid / flow rate of purified acrylic acid) in the washing column, preferably 0.5 to 6 hours, more preferably 1 to 5 hours, and even more preferably 1.2 to 4.5 hours.

[0082] Furthermore, since the 1st to (N-1th) tanks are not connected to a washing column, a long residence time is not necessarily required. A shorter residence time allows the size of the tank itself to be reduced, which is advantageous in terms of capital investment costs. Therefore, the residence time for the 1st to (N-1th) tanks is preferably 0.03 to 4 hours, more preferably 0.04 to 3 hours, even more preferably 0.05 to 2 hours, and most preferably 0.1 to 1.5 hours.

[0083] The residence time of a compound in a crystallization tank here means the residence time in the tank and in the cooling mechanism outside the tank when the crystallization tank is of the type in which the contents of the tank are cooled outside the tank, as described below. The residence time of each tank is calculated as the total volume of the tank and the cooling mechanism outside the tank divided by the sum of the flow rate at which the slurry is supplied from the tank to an upstream tank or washing column and the flow rate at which the liquid is sent / discharged from each tank to a lower tank or outside the purification device.

[0084] The purification method of the present invention may be used to purify any compound, but as described above, it can also be suitably used to purify crystals that have poor sedimentation properties, and therefore it is suitable for purifying (meth)acrylic acid. Therefore, one of the preferred embodiments of the present invention is that the compound purified by the purification method of the present invention is (meth)acrylic acid.

[0085] In this case, the solution of the compound subjected to the purification method of the present invention is an aqueous (meth)acrylic acid solution or a crude (meth)acrylic acid solution. The aqueous (meth)acrylic acid solution refers to a solution in which (meth)acrylic acid is dissolved in water. The crude (meth)acrylic acid solution refers to a solution consisting of (meth)acrylic acid and containing impurities such as by-products produced during the production of (meth)acrylic acid. These can be obtained, for example, by collecting in an absorption tower and, if necessary, distilling the gas of a compound that is a reaction product obtained by the gas-phase oxidation reaction of propylene and isobutylene. However, they are not limited to those synthesized by the company itself and may be procured from other sources. The aqueous (meth)acrylic acid solution or the crude (meth)acrylic acid solution can be cooled, for example, to obtain a slurry containing crystals of (meth)acrylic acid.

[0086] Examples of the by-products include acids such as propionic acid, acetic acid, maleic acid, benzoic acid, and acrylic acid dimer, aldehydes such as acrolein, furfural, formaldehyde, and glyoxal, acetone, and protoanemonin. In addition, solvents such as toluene and methyl butyl ketone may also be contained. In this specification, (meth)acrylic acid refers to acrylic acid and / or methacrylic acid.

[0087] Examples of the purification apparatus used in the purification method of the present invention are shown in FIGS. Figure 1 shows a crystallization apparatus having one crystallization tank and one aging tank. A line is installed to directly transfer the mother liquor from the aging tank, which is the tank immediately upstream of the crystallization tank, to the crystallization tank, and a line is installed to directly discharge the residue (mother liquor) from the crystallization tank, which is the most downstream tank. Solution 1 of the compound to be fed to the purification system is introduced into aging tank 21. It is cooled in crystallization tank 11, which is equipped with a cooling mechanism, and the slurry containing the precipitated crystals is sent to solid-liquid separator 31 via line 51. In solid-liquid separator 31, the slurry is separated into mother liquor and concentrated crystal slurry. The concentrated crystal slurry is sent to the adjacent aging tank 21 via line 52, and the mother liquor is returned to crystallization tank 11 via line 61. Residue 2 is discharged from crystallization tank 11 to the outside of the purification system via line 71, and the liquid level in crystallization tank 11 is adjusted. After crystals are grown in aging tank 21, the crystal slurry is sent to mechanical washing column 41 via line 53. To adjust the liquid level in aging tank 21, mother liquor is sent directly from aging tank 21 to crystallization tank 11 via line 72. In the mechanical washing column 41, the crystals are compressed by a piston to form a crystal bed. Then, at the bottom of the column, the crystal bed is scraped off, suspended in a circulating liquid, and heated to melt. A portion of the circulating liquid containing the resulting melt is discharged as a high-purity compound 3. A portion of the remaining circulating liquid (washing liquid) is returned to the mechanical washing column 41, where it is brought into countercurrent contact with the crystal bed to wash the crystals. The mother liquor in the washing column is returned to the aging tank 21 through line 75, which returns the mother liquor to the crystallizer. In this way, the compound is purified and a high-purity compound is obtained.

[0088] Figure 2 shows a crystallization apparatus with one crystallization tank and one aging tank. A line is installed to send mother liquor directly from the aging tank, which is the tank one tank upstream, to the crystallization tank, and a line is installed to discharge the residue (mother liquor) from the crystallization tank, which is the most downstream tank, via a solid-liquid separator. A hydraulic washing column is also used. Below, only the differences from the purification apparatus in Figure 1 are explained. The crystallization is cooled in crystallization tank 11, which is equipped with a cooling mechanism, and the slurry containing the precipitated crystals is sent to solid-liquid separator 31 via line 51. In solid-liquid separator 31, the slurry is separated into mother liquor and concentrated crystal slurry, and the concentrated crystal slurry is sent to the adjacent aging tank 21 via line 52. A portion of the mother liquor is returned to crystallization tank 11 via line 61, and the remainder is discharged to the outside of the purification apparatus as residue 2 via line 101, and the liquid level in crystallization tank 11 is adjusted. In the hydraulic washing column 43, the crystals move downward to form a crystal bed. The crystal bed is then scraped off at the bottom of the column, suspended in a circulating liquid, and heated and melted. This hydraulic washing column 43 does not have a mechanical mechanism for scraping off the crystal bed; instead, the crystal bed is scraped off by the dynamic pressure of the circulating liquid.

[0089] Figure 3 shows a crystallization apparatus with one crystallization tank and one aging tank. A line is installed to send mother liquor directly from the aging tank, which is the upstream tank, to the crystallization tank, and a line is installed to directly discharge the residue (mother liquor) from the crystallization tank, which is the most downstream tank. In addition, the crystallization tank is a type in which the contents of the tank are cooled outside the tank. Only the parts that are different from the purification apparatus in Figure 1 will be explained below. Crystallization tank 11 is composed of tank 11A and cooling mechanism 11B outside the tank, which are connected by lines 111 and 121. The compound solution (or slurry containing compound crystals) sent from tank 11A to cooling mechanism 11B via line 111 is cooled in cooling mechanism 11B, and the slurry containing the precipitated crystals is sent to tank 11A via line 121. A portion of the slurry containing compound crystals from tank 11A is sent to cooling mechanism 11B via line 111, and the remainder is sent to solid-liquid separator 31 via line 51.

[0090] Figure 4 shows a crystallization apparatus having two crystallization tanks. A line is installed to send mother liquor directly from crystallization tank 12, which is the tank one tank upstream, to crystallization tank 11, and a line is installed to directly discharge the residue (mother liquor) from crystallization tank 11, which is the most downstream tank. In addition, a hydraulic washing column is used. Only the parts that differ from the purification apparatus in Figure 1 will be explained below. A solution 1 of a compound to be supplied to a purification apparatus is introduced into a crystallization tank 12 . Crystallization tank 11 is composed of tank 11A and cooling mechanism 11B outside the tank, which are connected by lines 111 and 121. The compound solution (or slurry containing compound crystals) sent from tank 11A to cooling mechanism 11B via line 111 is cooled in cooling mechanism 11B, and the slurry containing the precipitated crystals is sent to tank 11A via line 121. A portion of the slurry containing compound crystals from tank 11A is sent to cooling mechanism 11B via line 111, and the remainder is sent to solid-liquid separator 31 via line 51. Similarly, crystallization tank 12 is composed of tank 12A and an external cooling mechanism 12B, which are connected by lines 112 and 122. A portion of the slurry containing compound crystals from tank 12A is sent to cooling mechanism 12B via line 112 and returned to tank 12A via line 122. The crystal slurry is sent from the crystallization tank 12 to the hydraulic washing column 42 via line 53. At the bottom of the hydraulic washing column 42, the crystal bed is scraped off by a mechanical mechanism (scraper), and the crystals are heated and melted while suspended in the circulating liquid. A portion of the circulating liquid containing the resulting melt is discharged as high-purity compound 3. A portion of the remaining circulating liquid (washing liquid) is returned to the hydraulic washing column 42, where it is brought into countercurrent contact with the crystal bed to wash the crystals. The mother liquor in the washing column is returned to the crystallization tank 12 via line 75, which returns the mother liquor to the crystallizer.

[0091] Figure 5 shows a crystallization apparatus with two crystallization tanks and one aging tank. Between the three tanks, a line is installed to directly send mother liquor from the upstream tank, and a line is installed to directly discharge the residue (mother liquor) from the most downstream tank. In addition, the washing column is hydraulic and has a mechanical mechanism for scraping off the crystal bed. Below, only the parts that are different from the purification apparatus in Figure 1 will be explained. The crystallization tank 11 is cooled, and the slurry containing the precipitated crystals is sent via line 51 to the solid-liquid separator 31. In the solid-liquid separator 31, the slurry is separated into a mother liquor and a concentrated crystal slurry. The concentrated crystal slurry is sent via line 52 to the adjacent crystallization tank 12, and the mother liquor is returned via line 61 to the crystallization tank 11. In addition, the residue 2 is discharged from the crystallization tank 11 to the outside of the purification apparatus via line 71, and the liquid level in the crystallization tank 11 is adjusted. In the crystallization tank 12, the same operation as in the crystallization tank 11 is performed, and the slurry containing the crystals is sent via line 53 from the crystallization tank 12 to the solid-liquid separator 32. In the solid-liquid separator 32, the slurry is separated into a mother liquor and a concentrated crystal slurry. The concentrated crystal slurry is sent via line 54 to the adjacent aging tank 21, and the mother liquor is returned via line 62 to the crystallization tank 12. To adjust the liquid level in the crystallization tank 12, the mother liquor is sent directly from the crystallization tank 12 to the crystallization tank 11 through a line 72. After crystals are grown in the aging tank 21, the crystal slurry is sent to the hydraulic washing column 42 through a line 55. To adjust the liquid level in the aging tank 21, the mother liquor is sent directly from the aging tank 21 to the crystallization tank 12 through a line 73 connecting the aging tank 21 and the crystallization tank 12.

[0092] At the bottom of the hydraulic washing column 42, the crystal bed is scraped off by a mechanical mechanism (scraper), and the crystals are withdrawn while suspended in the circulating liquid and heated to melt, and a portion of the circulating liquid containing the resulting melt is discharged as high-purity compound 3. A portion of the remaining circulating liquid (washing liquid) is returned to the hydraulic washing column 42 and brought into countercurrent contact with the crystal bed to wash the crystals.

[0093] Figure 6 shows a crystallization apparatus with three crystallization tanks and one aging tank. Between the four tanks, a line is installed to directly send the mother liquor from the upstream tank, and a line is installed to directly discharge the residue (mother liquor) from the most downstream tank. The washing column is hydraulic and has a mechanical mechanism for scraping off the crystal bed. Below, only the differences from the purification apparatus in Figure 5 are explained. Solution 1 of the compound to be fed to the purification unit is introduced into crystallization tank 13. From crystallization tank 12, the second most downstream tank, a slurry containing crystals is sent via line 53 to solid-liquid separator 32. In solid-liquid separator 32, the slurry is separated into mother liquor and concentrated crystal slurry. The concentrated crystal slurry is sent via line 54 to the adjacent crystallization tank 13, and the mother liquor is returned via line 62 to crystallization tank 12. To adjust the liquid level in crystallization tank 12, the mother liquor is sent directly from crystallization tank 12 to crystallization tank 11 via line 72. In crystallization tank 13, the same operation as in crystallization tank 12 is performed, and the slurry containing crystals is sent from crystallization tank 13 to solid-liquid separator 33 via line 55. In solid-liquid separator 33, the slurry is separated into mother liquor and concentrated crystal slurry. The concentrated crystal slurry is sent via line 56 to the adjacent aging tank 21, and the mother liquor is returned via line 63 to crystallization tank 13. To adjust the liquid level in the crystallization tank 13, the mother liquor is sent directly from the crystallization tank 13 to the crystallization tank 12 through a line 73. After crystals are grown in the aging tank 21, the crystal slurry is sent to the hydraulic washing column 42 through a line 57. To adjust the liquid level in the aging tank 21, the mother liquor is sent directly from the aging tank 21 to the crystallization tank 13 through a line 74 connecting the aging tank 21 and the crystallization tank 13.

[0094] Figure 7 shows a crystallization apparatus having two crystallization tanks and one aging tank, with a line between the three tanks that sends the mother liquor from the upstream tank via a solid-liquid separator, and a line that discharges the residue from the most downstream tank via a solid-liquid separator. Below, only the differences from the purification apparatus in Figure 5 will be explained. In place of a line for directly discharging the residue, the crystallization tank 11 in FIG. 7 is provided with a solid-liquid separator 33 for separating the residue from the slurry in the crystallization tank. The slurry removed from the crystallization tank 11 is sent to the solid-liquid separator 33 via a line 81, the residue 2 separated in the solid-liquid separator 33 is discharged outside the purification apparatus, and the remaining crystals are returned to the crystallization tank 11, and the liquid level in the crystallization tank 11 is adjusted. Instead of a line for directly sending the mother liquor to the crystallization tank 11, a solid-liquid separator 34 is installed in the crystallization tank 12. The slurry removed from the crystallization tank 12 is sent to the solid-liquid separator 34 via a line 83. The mother liquor separated in the solid-liquid separator 34 is sent to the crystallization tank 11 for adjusting the liquid level, and the remaining crystals are returned to the crystallization tank 12. In place of the line for directly sending the mother liquor to the crystallization tank 12, a solid-liquid separator 35 is installed in the aging tank 21. The slurry removed from the aging tank 21 is sent to the solid-liquid separator 35 via a line 85, the mother liquor separated in the solid-liquid separator 35 is sent to the crystallization tank 12 for liquid level adjustment, and the remaining crystals are returned to the aging tank 21. The wash column 43 is hydraulic and does not have a mechanical mechanism for scraping the crystal bed.

[0095] Figure 8 shows a crystallization apparatus having two crystallization tanks and one aging tank, with a line between the three tanks for sending mother liquor from the tank one upstream via a solid-liquid separator, and a line from the most downstream tank for discharging residue via a solid-liquid separator.The solid-liquid separator for separating mother liquor from the slurry extracted from the second crystallization tank and sending it to the most downstream (first) crystallization tank, and the solid-liquid separator for separating residue discharged from the most downstream (first) crystallization tank to the outside of the purification apparatus are shared with the solid-liquid separator installed on the line for sending slurry to the one upstream tank.Only the parts that are different from the purification apparatus in Figure 7 will be described below. In the apparatus of Figure 8, the slurry containing crystals that have been cooled in crystallization tank 11 is sent to solid-liquid separation apparatus 31 via line 51. In solid-liquid separation apparatus 31, the slurry is separated into mother liquor and concentrated crystal slurry, and the concentrated crystal slurry is sent to the adjacent crystallization tank 12 via line 52. A portion of the mother liquor separated in solid-liquid separation apparatus 31 is returned to crystallization tank 11 via line 61, and the remainder is discharged outside the purification apparatus via additional line 101 connected to line 61. The slurry containing the crystals cooled in crystallization tank 12 is sent via line 53 to solid-liquid separator 32. In solid-liquid separator 32, the slurry is separated into mother liquor and concentrated crystal slurry, and the concentrated crystal slurry is sent via line 54 to the adjacent aging tank 21. A portion of the mother liquor separated in solid-liquid separator 32 is returned via line 62 to crystallization tank 12, and the remainder is sent via additional line 102 connected to line 62 to crystallization tank 11. In the purification system of Figure 8, line 51 → solid-liquid separator 31 → lines 61 and 101 correspond to line 81 → solid-liquid separator 33 → lines 82 and 91 in the system of Figure 7, and instead of installing solid-liquid separator 33, the solid-liquid separator 31 provided on the line that sends the slurry to the upstream tank is shared, thereby reducing the number of pieces of equipment. Similarly, line 53 → solid-liquid separator 32 → lines 62 and 102 correspond to line 83 → solid-liquid separator 34 → lines 84 and 92 in the system of Figure 7, and instead of installing solid-liquid separator 34, the solid-liquid separator 32 provided on the line that sends the slurry to the upstream tank is shared, thereby reducing the number of pieces of equipment.

[0096] Figure 9 shows a crystallization apparatus with two crystallization tanks and one aging tank. It includes a line for sending mother liquor from the second crystallization tank to the most downstream (first) crystallization tank via a solid-liquid separator, a line for discharging residue from the most downstream tank via a solid-liquid separator, and a line for sending mother liquor directly from the aging tank to the second crystallization tank. The solid-liquid separator for separating mother liquor from the slurry extracted from the second crystallization tank and sending it to the most downstream crystallization tank, and the solid-liquid separator for separating residue discharged from the most downstream crystallization tank to the outside of the purification apparatus are also used as solid-liquid separators on the line for sending slurry to the tank one upstream. The crystallization tanks are designed to cool the contents of the tanks outside the tank. Below, only the differences from the purification apparatus in Figure 8 are described. In the apparatus of Figure 9, crystallization tank 11 is composed of tank 11A and cooling mechanism 11B outside the tank, which are connected by lines 111 and 121. The compound solution (or slurry containing compound crystals) sent from tank 11A to cooling mechanism 11B via line 111 is cooled in cooling mechanism 11B, and the slurry containing the precipitated crystals is sent to tank 11A via line 121. A portion of the slurry containing compound crystals from tank 11A is sent to cooling mechanism 11B via line 111, and the remainder is sent to solid-liquid separation device 31 via line 51. Similarly, crystallization tank 12 is composed of tank 12A and an external cooling mechanism 12B, which are connected by lines 112 and 122. A portion of the slurry containing compound crystals is sent from tank 12A to cooling mechanism 12B via line 112, and the remainder is sent to solid-liquid separator 32 via line 53. The apparatus of FIG. 9 has a line 73 that sends the mother liquor directly from the aging tank 21 to the crystallization tank 12A, instead of the line that sends the mother liquor from the aging tank 21 to the crystallization tank 12 via the solid-liquid separator 35 that is used in the apparatus of FIG. [Example]

[0097] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, "parts" means "parts by weight" and "%" means "% by mass."

[0098] Manufacturing Example 1 An acrylic acid-containing gas obtained by catalytic vapor phase oxidation of propylene was supplied to an absorption column and absorbed with water containing a polymerization inhibitor to produce a crude acrylic acid solution having the following composition: <Composition of crude acrylic acid solution> Acrylic acid 94.1% Acetic acid 2.0% Furfural 800ppm Maleic acid 5000ppm Benzaldehyde 350 ppm Water 2.2% Hydroquinone 500ppm Other 1.0%

[0099] Example 1 The crude acrylic acid solution obtained in Production Example 1 was supplied to a purification apparatus having the same structure as that shown in FIG. 2, and acrylic acid was continuously purified. The crude acrylic acid solution was continuously supplied to the second tank 21 at 10 kg / h, and acrylic acid crystals were generated using a cooling mechanism (heat exchanger) installed in the first crystallization tank. A slurry consisting of the crystals and mother liquor was sent to the upstream tank 21 via line 52, while the crystals were concentrated in a solid-liquid separator 31. Crystal-free mother liquor was sent (72) directly from the second tank 21 to the first tank 11 by overflow, and the mother liquor was discharged (101) from the first tank 11 to the outside of the crystallization apparatus via the solid-liquid separator 31, thereby adjusting the liquid level in each tank to a constant level. A basket-type centrifuge was used as the solid-liquid separator 31. The crystal concentration in the concentrated slurry was 90%, and the crystal concentration in the slurry held in the second tank 21 was 25%. The slurry from the second tank 21 was continuously pumped to a hydraulic washing column 43 to purify and melt the crystals. A portion of the melt was used as a washing liquid to wash the crystal bed in the washing column, and the remainder was used as purified acrylic acid at a rate of 6 kg / h. The amount of washing liquid was adjusted so that the furfural concentration in the product was less than 1 ppm. Mother liquor was also extracted from the top of the washing column and returned to the second tank 21 (75). The flow rate of the mother liquor (crystallization residue) discharged from the first tank 11 to the outside of the purification system via the solid-liquid separator 31 was 4.0 kg / h. The yield of the crystallizer during steady-state operation (purified acrylic acid flow rate / crude acrylic acid flow rate x 100) was 60%, and the reflux ratio in the washing column (washing liquid flow rate / purified acrylic acid flow rate) was 0.43. The composition of the purified acrylic acid was as follows: The acrylic acid concentration in the mother liquor in the tanks (purity of the mother liquor) was 73.4 mol% in tank 11 and 90.2 mol% in tank 21, and the temperatures in each tank were 3.3°C in tank 11 and 9.8°C in tank 21. The residence time in each tank was 1.5 hours in tank 11A and 3.1 hours in tank 21. Through this series of purification operations, it was possible to obtain high-purity purified acrylic acid in high yield and with low energy consumption (low reflux ratio). <Composition of purified acrylic acid> Acrylic acid 99.8% Acetic acid 1500 ppm Furfural 0.8 ppm Maleic acid 4.5 ppm Benzaldehyde 0.3 ppm Water 25ppm Hydroquinone 0.4 ppm Other 900ppm

[0100] Example 2 The crude acrylic acid solution obtained in Production Example 1 was supplied to a purification apparatus having the same structure as that shown in FIG. 9, and acrylic acid was continuously purified. The crude acrylic acid solution was continuously supplied to the third tank 21 at 10 kg / h, and acrylic crystals were generated in cooling mechanisms (heat exchangers) 11B and 12B provided outside the first and second tanks. The slurry consisting of the crystals and mother liquor was sent (52, 54) to the upstream tanks while the crystals were concentrated in hydrocyclones 31 and 32. The crystal-free mother liquor was sent (73) directly from the third tank 21 to the second tank 12A by overflow, and the mother liquor was sent (102) from the second tank 12A to the first tank 11A via solid-liquid separator 32. The mother liquor was then discharged (101) from tank 11A to the outside of the crystallizer via solid-liquid separator 31, thereby controlling the liquid level in each tank to a constant level. The crystal concentration in the slurry concentrated in each hydrocyclone was 40%, and the crystal concentration in the slurry retained in the third tank 21 was 25%. The slurry from the third tank 21 was continuously pumped to a hydraulic washing column 43 to purify and melt the crystals. A portion of the melt was used as a washing liquid to wash the crystal bed in the washing column, and the remainder was obtained as purified acrylic acid at a rate of 6 kg / h. The amount of washing liquid was adjusted so that the furfural concentration in the product was less than 1 ppm. Mother liquor was extracted from the top of the washing column and returned to the third tank 21 (75). The flow rate of the mother liquor (crystallization residue) discharged from the first tank 11A was 4.0 kg / h. The yield of the crystallizer during steady-state operation (purified acrylic acid flow rate / crude acrylic acid flow rate×100) was 60%, and the reflux ratio in the washing column (washing liquid flow rate / purified acrylic acid flow rate) was 0.67. The composition of the purified acrylic acid was as follows: The acrylic acid concentration in the mother liquor in the tanks (purity of the mother liquor) was 73.4 mol% in tank 11A, 81.0 mol% in tank 12A, and 86.4 mol% in tank 21, and the temperatures in each tank were 3.3°C in tank 11A, 6.3°C in tank 12A, and 8.3°C in tank 21. The residence time in each tank was 1.0 hour in tank 11A, 0.4 hour in tank 12A, and 1.8 hours in tank 21. Through this series of purification operations, it was possible to obtain high-purity purified acrylic acid in high yield and with low energy consumption (low reflux ratio). <Composition of purified acrylic acid> Acrylic acid 99.7% Acetic acid 1800 ppm Furfural 0.9 ppm Maleic acid 6.0 ppm Benzaldehyde 0.4 ppm Water 30ppm Hydroquinone 0.6 ppm Other 1100ppm

[0101] Example 3 A crude acrylic acid solution obtained in the same manner as in Production Example 1 was supplied to a purification apparatus having the same structure as that shown in FIG. 5, where acrylic acid was continuously purified. The crude acrylic acid solution was continuously supplied to the third tank 21 at 10 kg / h, and instead of sending the mother liquor from the second tank 12 to the first tank 11 via a solid-liquid separator, the crystal-free mother liquor was sent by direct overflow (72). Instead of discharging the mother liquor from the first tank 11 to the outside of the crystallizer via a solid-liquid separator, the crystal-free mother liquor was discharged by direct overflow (71). Purification was carried out in the same manner as in Example 2, with the exception of this, and purified acrylic acid was obtained at a rate of 6 kg / h. The yield of the crystallizer during steady-state operation (purified acrylic acid flow rate / crude acrylic acid flow rate × 100) was 60%, and the reflux ratio in the wash column (washing liquid flow rate / purified acrylic acid flow rate) was 0.69. The composition of the purified acrylic acid was as follows: The acrylic acid concentration in the mother liquor in the tanks (purity of the mother liquor) was 72.8 mol% in tank 11, 80.7 mol% in tank 12, and 86.1 mol% in tank 21. The temperature of each tank was 3.1°C in tank 11, 6.0°C in tank 12, and The temperatures were 8.1°C in tank 11 and 8.2°C in tank 21. The residence times in each tank were 1.0 hour in tank 11, 0.4 hour in tank 12, and 1.8 hours in tank 21. During operation, the mother liquor discharged and sent by overflow from tanks 11 and 12 contained small amounts of crystals. Through this series of purification operations, it was possible to obtain high-purity purified acrylic acid in high yield and with energy savings (low reflux ratio). <Composition of purified acrylic acid> Acrylic acid 99.7% Acetic acid 1800 ppm Furfural 0.9 ppm Maleic acid 6.2 ppm Benzaldehyde 0.5 ppm Water 30ppm Hydroquinone 0.7 ppm Other 1100ppm

[0102] Comparative Example 1 A crude acrylic acid solution obtained in the same manner as in Production Example 1 was supplied to a purification apparatus having a structure equivalent to that shown in Figure 10, where acrylic acid was continuously purified. The crude acrylic acid solution was continuously supplied to tank 11 at 10 kg / h, and acrylic crystals were generated in a heat exchanger installed in tank 11. The refrigerant temperature was adjusted so that the crystal concentration in the slurry in the tank was 25%. The slurry from tank 11 was continuously sent to a hydraulic washing column 43, where the crystals were purified and melted. A portion of the melt was used as a washing liquid to wash the crystal bed in the washing column, and the remainder was obtained as purified acrylic acid (3). The amount of washing liquid was adjusted so that the furfural concentration in the product was less than 1 ppm. In addition, mother liquor was extracted from the top of the washing column and returned to tank 11 (75), and crystal-free mother liquor was discharged from the tank as crystallization residue by overflow (71) so that the liquid level remained constant. The flow rate of purified acrylic acid / flow rate of slurry sent to the washing column was adjusted so that the concentration of acrylic acid in the mother liquor in the tank 11 was 86 mol%. The yield of the crystallizer during steady operation (flow rate of purified acrylic acid / flow rate of crude acrylic acid x 100) was 16%, and the reflux ratio in the washing column (flow rate of washing liquid / flow rate of purified acrylic acid) was 0.67. The flow rate of purified acrylic acid was 1.6 kg / h, and the composition was as follows: The crystallization residue flow rate was 8.4 kg / h. The concentration of acrylic acid in the mother liquor in the tank 11 (purity of the mother liquor) was 86.4 mol %, the temperature in the tank 11 was 8.3° C., and the residence time in the tank 11 was 2.0 hours. Although a highly purified acrylic acid was obtained through a series of purification operations, the crystallization yield was very low and a sufficient amount of purified acrylic acid could not be obtained. <Composition of purified acrylic acid> Acrylic acid 99.7% Acetic acid 1800 ppm Furfural 0.9 ppm Maleic acid 6.1 ppm Benzaldehyde 0.4 ppm Water 30ppm Hydroquinone 0.7 ppm Other 1100ppm

[0103] Comparative Example 2 A crude acrylic acid solution obtained in the same manner as in Production Example 1 was supplied to a purification apparatus having a structure equivalent to that shown in Figure 10, where acrylic acid was continuously purified. The crude acrylic acid solution was continuously supplied to tank 11 at 10 kg / h, and acrylic crystals were generated in a heat exchanger installed in tank 11. The refrigerant temperature was adjusted so that the crystal concentration in the slurry in the tank was 25%. The slurry from tank 11 was continuously sent to a hydraulic washing column 43, where the crystals were purified and melted. A portion of the melt was used as a washing liquid to wash the crystal bed in the washing column, and the remainder was obtained as purified acrylic acid at 6 kg / h (3). The amount of washing liquid was adjusted so that the furfural concentration in the product was less than 1 ppm. In addition, mother liquor was withdrawn from the top of the washing column and returned to tank 11 (75), and crystal-free mother liquor was discharged from the tank by overflow (71) as crystallization residue so that the liquid level remained constant. The yield of the crystallizer (purified acrylic acid flow rate / crude acrylic acid flow rate×100) was 60%, and the reflux ratio in the washing column 41 (washing liquid flow rate / purified acrylic acid flow rate) was 4.0. The composition of the purified acrylic acid was as follows: The flow rate of the crystallization residue was 4 kg / h. The concentration of acrylic acid in the mother liquor in the tank 11 (purity of the mother liquor) was 73.4 mol %, the temperature in the tank 11 was 3.3° C., and the residence time in the tank 11 was 1.5 hours. Although a series of purification operations resulted in the production of highly purified acrylic acid, the reflux ratio in the washing column was high, and a lot of energy was required to obtain the product. <Composition of purified acrylic acid> Acrylic acid 99.6% Acetic acid 2500 ppm Furfural 0.9 ppm Maleic acid 6.2 ppm Benzaldehyde 0.5 ppm Water 30ppm Hydroquinone 0.7 ppm Other 1100ppm

[0104] Comparative Example 3 A crude acrylic acid solution obtained in the same manner as in Production Example 1 was supplied to a purification apparatus using a gravity settling column having the same structure as shown in FIG. 11, and acrylic acid was continuously purified. The crude acrylic acid solution was continuously supplied to the third tank 21 at 10 kg / h, and acrylic crystals were generated in cooling mechanisms 11B and 12B provided outside the first and second tanks. A slurry consisting of the crystals and mother liquor was sent (52, 54) to the upstream tanks while the crystals were concentrated in liquid cyclones 31 and 32. The crystal-free mother liquor was sent (73) directly from the third tank 21 to the second tank 12A by overflow, and the mother liquor was sent (102) from the second tank 12A to the first tank 11A via the solid-liquid separator 32. The mother liquor was then discharged (101) from the first tank 11A to the outside of the crystallizer via the solid-liquid separator 31, thereby controlling the liquid level in each tank to a constant level. The crystal concentration in the slurry concentrated in the liquid cyclone was 40%, and the crystal concentration in the slurry held in the third tank 21 was 25%. The slurry was continuously sent from the third tank 21 to a gravity settling washing column 44 to purify and melt the crystals. A portion of the melt was used as a washing liquid to wash the crystal bed in the washing column, and the remainder was obtained as purified acrylic acid at a rate of 6 kg / h (3). In addition, mother liquor was extracted from the top of the washing column and returned to the third tank 21 (75). The mother liquor (crystallization residue) discharged by overflow from the first tank 11A was 4.0 kg / h. The yield of the crystallizer during steady-state operation (purified acrylic acid flow rate / crude acrylic acid flow rate × 100) was 60%. The reflux ratio in the washing column (washing liquid flow rate / purified acrylic acid flow rate) was adjusted to make the furfural concentration in the purified acrylic acid less than 1.0 ppm, but even when the reflux ratio was increased to 9.0, the composition of the purified acrylic acid was as shown below, and it was not possible to obtain a product of the desired quality. The acrylic acid concentration in the mother liquor in the tanks (purity of the mother liquor) at this time was 73.4 mol% in tank 11A, 82.6 mol% in tank 12A, and 88.7 mol% in tank 21, and the temperatures in the tanks were 3.3°C in tank 11A, 6.8°C in tank 12A, and 9.2°C in tank 21. The residence time in each tank was 1.2 hours in tank 11A, 0.4 hours in tank 12A, and 1.9 hours in tank 21. The purified acrylic acid thus obtained contains a large amount of furfural, a polymerization retardant, and hydroquinone, a polymerization inhibitor, making it unsuitable for use as a raw material for superabsorbent resins and the like. <Composition of purified acrylic acid> Acrylic acid 99.7% Acetic acid 1500 ppm Furfural 19 ppm Maleic acid 120 ppm Benzaldehyde 8 ppm Water 500ppm Hydroquinone 12 ppm Other 1100ppm

[0105] Comparative Example 4 A crude acrylic acid solution obtained in the same manner as in Production Example 1 was supplied to a purification apparatus having the same structure as shown in FIG. 9, and acrylic acid was continuously purified. The crude acrylic acid solution was continuously supplied to the third tank 21 at 10 kg / h, and acrylic crystals were generated in cooling mechanisms 11B and 12B provided outside the first and second tanks. A slurry consisting of the crystals and mother liquor was sent (52, 54) to the upstream tanks while the crystals were concentrated in hydrocyclones 31 and 32. The crystal-free mother liquor was sent (73) directly from the third tank 21 to the second tank 12A by overflow, and the mother liquor was sent (102) from the second tank 12A to the first tank 11A via solid-liquid separator 32. The mother liquor was then sent (101) from the first tank 11A to the outside of the crystallizer via solid-liquid separator 31, thereby controlling the liquid level in each tank to a constant level. The crystal concentration in the slurry concentrated in the liquid cyclone was 14%, and the crystal concentration in the slurry held in the third tank 21 was 10%. The slurry was continuously pumped from the third tank 21 to a hydraulic washing column 43 to purify and melt the crystals. A portion of the melt was used as a washing liquid to wash the crystal bed in the washing column, and the remainder was used as purified acrylic acid at a rate of 6 kg / h (3). The amount of washing liquid was adjusted so that the furfural concentration in the product was less than 1 ppm. The mother liquor and washing liquid were withdrawn from the top of the washing column and returned to the third tank. The flow rate of the mother liquor (crystallization residue) discharged from the first tank 11A was 4.0 kg / h. The yield of the crystallizer during steady-state operation (purified acrylic acid flow rate / crude acrylic acid flow rate x 100) was 60%, and the reflux ratio in the washing column (washing liquid flow rate / purified acrylic acid flow rate) was 3.0. The composition of the purified acrylic acid was as follows: The acrylic acid concentration in the mother liquor in the tanks (purity of the mother liquor) was 73.4 mol% in tank 11A, 76.5 mol% in tank 12A, and 79.3 mol% in tank 21. The temperature of each tank was 3.3°C in tank 11A, 3.3°C in tank 12A, and 3.3°C in tank 21. The temperature was 5.3°C in tank 2A ​​and 6.8°C in tank 21. The residence time in each tank was 0.5 hours in tank 11A, 0.2 hours in tank 12A, and 1.8 hours in tank 21. Although a series of purification operations resulted in the production of highly purified acrylic acid, the reflux ratio in the washing column was high, and a lot of energy was required to obtain the product. <Composition of purified acrylic acid> Acrylic acid 99.7% Acetic acid 1600 ppm Furfural 0.9 ppm Maleic acid 5.9 ppm Benzaldehyde 0.4 ppm Water 30ppm Hydroquinone 0.6 ppm Other 1100ppm [Explanation of symbols]

[0106] 1: Compound solution 2:Residue 3: High purity compounds 11-13: Crystallization tank with cooling mechanism 11A, 12A: Tank 11B, 12B: Cooling mechanism 21: Aging tank 31~35: Solid-liquid separator 41: Mechanical cleaning column 42: Hydraulic washing column (with a mechanical mechanism for scraping off the crystal bed) 43: Hydraulic washing column (without mechanical mechanism for scraping off the crystal bed) 44: Gravity settling washing column 51-57: Lines for sending slurry (or crystals) from downstream tanks to upstream tanks or washing columns 61-63: Lines that return the mother liquor separated from the slurry by the solid-liquid separator to the original tank 71: A line that discharges the residue (mother liquor) directly from the most downstream tank to the outside of the refining equipment 72-74: Lines that directly transfer mother liquor from the upstream tank to the next downstream tank 75: Line returning mother liquor from the wash column to the crystallizer 81-86: Lines that separate crystals from the slurry taken out of the tank using a solid-liquid separator and return it to the original tank 91: A line that discharges the residue (mother liquor) separated from the slurry taken out of the tank by the solid-liquid separator outside the refining equipment. 92, 93: Lines that send the mother liquor separated from the slurry taken out of the tank by the solid-liquid separator to the tank one step downstream 101: An additional line for discharging a portion of the mother liquor separated from the slurry taken out of the most downstream tank by the solid-liquid separator to the outside of the refining unit. 102: An additional line for sending a portion of the mother liquor separated by a solid-liquid separator from the slurry taken out of the second tank to the most downstream tank. 111, 121, 112, 122: Lines connecting the crystallization tank and the cooling mechanism in a type of tank in which the contents of the tank are cooled outside the tank

Claims

1. A method for purifying a compound using a purification apparatus having a crystallizer having a crystallization section that cools a solution of the compound to precipitate crystals, and a washing column equipped with a mechanism for forcibly transporting the crystals of the compound, comprising: The washing column is either a mechanical washing column in which the crystals are compressed with a piston to form / transport a crystal bed, or a hydraulic washing column in which a slurry is sent to the column with a pump and mother liquor is extracted from a filter disposed in the column to form / transport a bed, The crystallization apparatus has N (N≧2) tanks, which are connected in series with a first tank downstream and an Nth tank upstream, with the direction of a slurry flow being the upstream side, and at least the first tank is a crystallization tank equipped with a cooling mechanism, and the second and subsequent tanks are crystallization tanks or aging tanks, a line for supplying a compound-containing liquid to be purified from outside the purification apparatus to at least one tank; the washing column has a line for carrying out a product obtained by melting crystals of a high-purity compound to the outside of the purification apparatus, and a line for returning the mother liquor to the crystallization apparatus, the line for returning the mother liquor to the crystallization apparatus being connected to at least the Nth tank; the crystallizer includes a line for supplying a slurry from the Nth tank to the washing column, a line for delivering the slurry from the downstream tank to the tank immediately upstream, and lines for delivering mother liquor from the upstream tank to each of the 1st to (N-1th)th tanks; at least one of the lines for transferring the slurry from the N-1 downstream tanks to the tank one tank upstream is a line for transferring the slurry to the tank one tank upstream via a solid-liquid separator, and has a line for returning the mother liquor from which the crystals have been removed in the solid-liquid separator to the original tank; the line for feeding the mother liquor from an upstream tank to each of the 1st to N-1th tanks includes at least one of a line for directly feeding the mother liquor from the immediately upstream tank and a line for feeding the mother liquor from the immediately upstream tank via a solid-liquid separator; The purification method includes the steps of producing crystals of the compound in a crystallizer; Discharging at least a portion of the mother liquor from the first tank in the purification apparatus outside the purification apparatus; a step of separating the slurry containing the produced crystals into a mother liquor and a slurry with an increased crystal concentration; returning at least a portion of the separated mother liquor to the original vessel; a step of supplying a liquid to be purified containing the compound to a crystallizer through a line for supplying the liquid to be purified containing the compound from outside the purification apparatus and mixing the liquid with the slurry in the crystallizer; a step of transferring the slurry from a downstream tank to a tank one level upstream among a plurality of tanks included in the crystallizer; feeding at least a portion of the slurry from the Nth crystallizer vessel to a wash column; The method includes a step of melting the crystals of the high-purity compound and transporting the resulting product out of the refining device, a step of supplying at least a portion of the slurry from the Nth tank of the crystallization apparatus to a washing column, wherein the purity (compound concentration) A1 of the mother liquor in the slurry supplied to the washing column is 80 mol% or more, and the difference A1 - A2 between A1 and the purity (compound concentration) A2 of the mother liquor discharged from the first tank to the outside of the purification apparatus is 5 mol% or more.

2. The method for purifying a compound according to claim 1, further comprising a step of feeding the mother liquor from at least one of the second to Nth tanks included in the crystallizer to a downstream tank to adjust the liquid level in the tank.

3. 3. The method for purifying a compound according to claim 1, wherein the temperature inside the crystallization tank included in the crystallization apparatus is 1 to 15° C. lower than the melting point of the pure substance of the compound to be purified.

4. The method for purifying a compound according to any one of claims 1 to 3, wherein the washing column is a hydraulic washing column.

5. The method for purifying a compound according to any one of claims 1 to 4, wherein the residence time in the 1st to (N-1th) tanks included in the crystallizer is 0.02 to 6 hours.

6. 6. The method for purifying the compound according to claim 1, further comprising a step of repurifying at least a portion of the mother liquor discharged from the purification apparatus by distillation and / or crystallization.

7. The method for purifying a compound according to any one of claims 1 to 6, wherein the compound is (meth)acrylic acid.

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