Purification method

Abstract

Provided is a purification method for N-methyl-2-pyrrolidone that makes it possible to appropriately control an addition amount for an alkali compound and also makes it possible to separate and remove water and impurities. According to the present invention, a purification method for purifying a starting material that contains N-methyl-2-pyrrolidone and water and also contains a low-boiling-point amine and a methyl succinimide as impurities includes a pH measurement step for extracting a portion of an in-column liquid from at least one of a first distillation step and a second distillation step as a pH measurement liquid, mixing the pH measurement liquid and water such that the mass ratio of the pH measurement liquid and the water is 0.5–20, and measuring the pH of the mixed liquid, a calculation step for calculating an addition amount for an alkali compound on the basis of the pH results from the pH measurement step, and an alkali addition step for adding the alkali compound to the starting material from before the first distillation step or the like in the addition amount calculated at the calculation step. The amount of water is controlled such that the mass ratio of the pH measurement liquid and the water of the mixed liquid at the pH measurement step is a constant ratio within an error of ±5%.

Description

Purification method

[0001] The present invention relates to a method for purifying a raw material containing N-methyl-2-pyrrolidone, impurities, and water into high-purity N-methyl-2-pyrrolidone.

[0002] N-methyl-2-pyrrolidone (referred to as "NMP") can adequately dissolve polyvinylidene fluoride. Therefore, in the manufacturing process of secondary batteries such as lithium ion batteries, NMP is used as a medium for positive electrode paste containing polyvinylidene fluoride as a binder and a positive electrode active material.

[0003] In the process of drying the positive electrode paste applied to the substrate, NMP evaporates and is removed from the surface of the substrate. At this time, the evaporated NMP is recovered by a gas recovery device, and the recovered NMP is reused. Since NMP can be recovered at a high recovery rate, the amount of recycled NMP used in the manufacturing process of secondary batteries is overwhelmingly greater than the amount of new NMP.

[0004] However, in gas recovery devices, volatile additives used in the production of positive electrode paste and moisture contained in the ambient air are recovered along with NMP. Furthermore, amines may be used as reaction compounds in the production process of NMP itself, and amines may also be used as additives in the production process of secondary batteries. Therefore, NMP often contains amines as impurities. There are many types of amines used as additives, and their boiling points vary. Furthermore, NMP is a highly reactive substance, and exposure to air or heat can produce methylsuccinimide or further amines as decomposition products.

[0005] The presence of amines as impurities in NMP can adversely affect polyvinylidene fluoride, which is used as a binder in the manufacturing process of secondary batteries.Furthermore, the presence of methylsuccinimide as an impurity in NMP can result in methylsuccinimide remaining on the surface of the substrate during the drying process of the positive electrode paste for secondary batteries.

[0006] As described above, impurities and water in NMP cause various problems, and therefore various techniques for removing impurities and water from NMP have been proposed. For example, Patent Document 1 discloses a method for distilling an N-methyl-2-pyrrolidone solution, which is characterized by adding a hydroxide of an alkali metal or alkaline earth metal to the N-methyl-2-pyrrolidone solution and distilling the N-methyl-2-pyrrolidone solution.

[0007] Japanese Patent Application Laid-Open No. 2004-284958

[0008] Patent Document 1 explains that the generation of peroxides can be suppressed by adding a hydroxide of an alkali metal or the like to NMP. The present inventors conducted extensive research into the amount of alkali compound added in the purification of NMP and found that various problems arise unless the amount of alkali compound added is appropriately controlled. Specifically, if the amount of alkali compound added in the purification of NMP is less than the appropriate range, the generation of peroxides is not sufficiently suppressed. Conversely, if the amount of alkali compound added is greater than the appropriate range, the alkali compound reacts with NMP, resulting in an increased amount of impurities. The present inventors have found that, in order to avoid such problems, it is necessary to precisely control the amount of alkali compound added in the purification of NMP.

[0009] Therefore, an object of the present invention is to provide a method for purifying N-methyl-2-pyrrolidone, which can appropriately control the amount of alkaline compound added and can separate and remove impurities and water.

[0010] As a means for solving the above problems, the present invention provides a purification method for purifying a raw material containing N-methyl-2-pyrrolidone and water, and also containing, as impurities, a low-boiling amine having a boiling point lower than that of N-methyl-2-pyrrolidone and methylsuccinimide, the method comprising: a first distillation step in which the raw material is distilled in a first distillation column to separate it into a dehydrated raw material and vapor containing the low-boiling amine; a second distillation step in which the dehydrated raw material is distilled in a second distillation column to separate it into a purified composition in which N-methyl-2-pyrrolidone is concentrated, vapor containing the low-boiling amine, and a solution containing methylsuccinimide; and The method includes a pH measurement step in which a portion of the liquid inside the tower is extracted as a pH measurement liquid, the pH measurement liquid is mixed with water in a mass ratio of the two (= mass of pH measurement liquid / mass of water) of 0.5 to 20, and then the pH of the mixed liquid is measured; a calculation step in which an amount of an alkali compound to be added is calculated based on the pH result of the pH measurement step; and an alkali addition step in which the amount of alkali compound to be added calculated in the calculation step is added to any one of the raw material before the first distillation step, the liquid inside the tower in the first distillation step, and the liquid inside the tower in the second distillation step, and the amount of water mixed is controlled so that the mass ratio of the two in the mixed liquid in the pH measurement step is a constant ratio with an error of ±5% or less.

[0011] In the calculation step, it is preferable to calculate the amount of alkaline compound to be added by feedback control based on the difference between the pH result obtained in the pH measurement step and a target value.

[0012] The mass fraction of the amine in the raw material is 1×10 －６ and it is preferable that the method further comprises, before the first distillation step, an impurity adsorption step of contacting the raw material with an acidic or amphoteric ion exchange resin to adsorb and separate 90 mass % or more of the amines originally contained in the raw material.

[0013] In the impurity adsorption step, the ion exchange resin column packed with an acidic or amphoteric ion exchange resin is passed through at a flow rate SV of 5 to 30 h. －１ It is preferable to treat the raw material by passing it through the above-mentioned process.

[0014] It is preferable that a part of the distillate containing the low-boiling amine distilled from the top of the second distillation column in the second distillation step is passed through the ion exchange resin as the raw material in the impurity adsorption step.

[0015] In the second distillation step, it is preferable that vapor containing a low-boiling amine is distilled from the top of the second distillation column, a solution containing methylsuccinimide is discharged from the bottom of the second distillation column, and a purified composition in which N-methyl-2-pyrrolidone is concentrated is withdrawn from a side cut nozzle of the second distillation column, and the temperatures of the distillation targets in the first distillation step and the second distillation step are controlled to be less than 160°C.

[0016] The mass fraction of the amine in the purified composition is 0.5×10 －６ It is preferable that it is less than 10 ...

[0017] It is preferable that the method further includes a recovery step of recovering the raw material from a gas containing N-methyl-2-pyrrolidone prior to the first distillation step, and that a portion of the distillate containing the low-boiling amine distilled from the first distillation column in the first distillation step is used as recovered water for recovering the gas in the recovery step.

[0018] The amine-containing distillate used as the recovered water preferably has an N-methyl-2-pyrrolidone content of 0.001 to 5% by mass.

[0019] The distillate containing the low-boiling amine used as the recovered water has a mass fraction of N-methyl-2-pyrrolidone of 0.001 × 10 －２ ~5 x 10 －２ It is preferable that:

[0020] The mass fraction of water in the purified composition is 200×10 -６ and the mass fraction of N-methyl-2-pyrrolidone in the purified composition is less than 99.98×10 －２ The purified composition and water are mixed in a mass ratio of 1:1, and the pH of the mixture is 7.2 to 8.0. The mass fraction of methyl succinimide in the purified composition is 10×10 -６Preferably, the purified composition has a colorimetric value of APHA 10 or less after 90 days of sealed storage at room temperature.

[0021] In the second distillation step, it is preferable that distillation is performed in two distillation columns, namely, the second distillation column and a second auxiliary distillation column located upstream of the second distillation column, and that a portion of the purified composition extracted from the second distillation column is returned to the second auxiliary distillation column.

[0022] The second distillation column in the second distillation step is preferably a vertical dividing distillation column, and the interior of the vertical dividing distillation column, excluding the top portion, is divided vertically by a dividing plate, and the distillation target is heated by a reboiler in each of the two bottom portions divided by the dividing plate.

[0023] According to the present invention, when purifying N-methyl-2-pyrrolidone, the amount of alkaline compound added can be appropriately controlled, and impurities and water can be separated and removed.

[0024] 1 is a schematic diagram of a purification system according to a first embodiment; FIG. 2 is a schematic diagram of a purification system according to a second embodiment; FIG. 3 is a schematic diagram of a purification system according to a third embodiment; and FIG. 4 is a schematic diagram of a purification system according to a fourth embodiment.

[0025] The refining system and refining method according to the first to fourth embodiments will be described below with reference to the drawings. In describing the second to fourth embodiments, the description of the components common to the previously described embodiments will be omitted as appropriate, and the description will focus on the components that are different.

[0026] [First Embodiment: Purification System] First, a purification system 100 according to the first embodiment will be described with reference to Fig. 1. The purification system 100 according to the first embodiment includes an impurity adsorption means 110, a first distillation means 120, a second distillation means 130, a pH measurement means 140, an alkali addition means 150, and a control means (not shown). The various means constituting the purification system 100 according to the first embodiment are as follows:

[0027] (Impurity Adsorption Means) The impurity adsorption means 110 adsorbs and separates impurities contained in the raw material. The impurity adsorption means 110 includes a first ion exchange resin column 111 and a second ion exchange resin column 112. The first ion exchange resin column 111 and the second ion exchange resin column 112 allow the raw material to pass through the columns, in other words, the raw material comes into contact with the ion exchange resin in the columns, thereby adsorbing impurities such as amines contained in the raw material onto the ion exchange resin and separating them from the raw material. The first ion exchange resin column 111 and the second ion exchange resin column 112 are arranged in series in the piping t1 to t5 between the raw material tank T11 and the buffer tank T12. The two ion exchange resin columns are configured in a so-called merry-go-round manner. Specifically, (1) the liquid is passed through the first ion exchange resin column 111 and then the second ion exchange resin column 112, (2) when replacing the first ion exchange resin column 111, only the second ion exchange resin column 112 is passed through, (3) after replacing the first ion exchange resin column 111, the liquid is passed through the second ion exchange resin column 112 and then the first ion exchange resin column 111, (4) when replacing the second ion exchange resin column 112, only the first ion exchange resin column 111 is passed through, and (1) is repeated to perform adsorption and separation of impurities. This configuration allows tasks such as replacing the ion exchange resins in the columns to be performed without stopping the adsorption and separation process. The ion exchange resins provided inside the first ion exchange resin column 111 and the second ion exchange resin column 112 may be any known resin capable of adsorbing impurities such as amines, for example, an acidic or amphoteric ion exchange resin.

[0028] (First Distillation Means) The first distillation means 120 is a means for distilling the feedstock, and includes a first auxiliary distillation column 121 disposed upstream and a first distillation column 122 disposed downstream, as well as a reboiler 123 associated with the first auxiliary distillation column 121, and a reboiler 124 and a condenser 125 associated with the first distillation column 122.

[0029] The first auxiliary distillation column 121 is a tray-type distillation column, and raw material stored in buffer tank T12 is supplied to the bottom of the column via pipes t6, t7, reboiler 123, and pipe t8. Vapor rich in low-boiling point components is distilled from the top of the column, and liquid rich in high-boiling point components is discharged from the bottom of the column. The reboiler 123 exchanges heat between steam s and the raw material supplied via pipe t7 or the bottoms supplied via pipe t9, and supplies the heated steam to the first auxiliary distillation column 121 via pipe t8. The steam s used in the reboiler 123 is discharged as drain d. In addition, liquid containing a large amount of non-vaporized non-volatile components (such as alkali compounds described below) accumulates in the liquid holding section (column bottom) of the reboiler 123 and is discharged as waste liquid via pipe t10.

[0030] The first distillation column 122 is a tray-type distillation column, and the distillate vapor from the first auxiliary distillation column 121 is supplied to the feed tray via pipe t11, and water-rich vapor is distilled from the top of the column, and dehydrated raw material is discharged from the bottom of the column. A liquid containing a large amount of impurities (e.g., intermediate-boiling amines described below) having a boiling point lower than that of NMP but higher than that of low-boiling amines is discharged from a discharge tray equipped with a side cut nozzle. The distillate vapor from the first distillation column 122 is supplied to a condenser 125 via pipe t12, the bottoms from the first distillation column 122 are supplied to a reboiler 124 via pipe t13, and the liquid discharged from the side cut nozzle is supplied upstream of the impurity adsorption means 110 via pipes t14 and t33 (pipes t14 and t33 are connected). The feed stage of the first distillation column 122 is not particularly limited, but is generally a stage below a delivery stage provided with a side cut nozzle and above the lowest stage; for example, when the total number of stages of the first distillation column 122 is X, the feed stage is a stage in the range of X×1 / 3 to X×2 / 3 from the bottom, for example, X×1 / 3, X×4 / 10, X×4 / 9, X×1 / 2, X×5 / 9, X×6 / 10, or X×2 / 3 from the bottom. Furthermore, the discharge tray of the first distillation column 122 equipped with a side cut nozzle is not particularly limited, but for example, when the total number of trays of the first distillation column 122 is X, the discharge tray is provided in the range of X×1 / 3 to X×2 / 3 from the top, e.g., X×1 / 3, X×4 / 10, X×4 / 9, X×1 / 2, X×5 / 9, X×6 / 10, or X×2 / 3 from the top (however, the discharge tray is generally a tray above the feed tray). Here, the number of trays of the distillation column described in this specification, including the above description, is expressed in terms of theoretical trays. However, it is quite common for those skilled in the art to understand that in practice, the actual number of trays or, in the case of packing, the packing height is used in accordance with the packing material and tray performance and operating conditions. Such corresponding actual number of trays or packing height is also within the scope of the present technology. The internal pressure of the first distillation column 122 may be, for example, 5 to 15 kPa absolute.

[0031] The reboiler 124, which has a similar configuration to the reboiler 123, exchanges heat between the bottoms supplied via pipe t13 and steam s, and supplies the heated steam to the bottom of the first distillation column 122 via pipe t17. The dehydrated raw material accumulates in the liquid holding section (bottom) of the reboiler 124 and is delivered as a dehydrated raw material via pipe t18. Most of the dehydrated raw material is supplied to the reboiler 132 of the second distillation means 130 via pipes t18 and t19, and a portion is supplied to the pH measurement means 140 via pipes t18 and t20. The condenser 125 liquefies the distillate vapor from the first distillation column 122 by cooling it with a chiller c. A portion of the liquid condensed in the condenser 125 is returned as reflux to the top of the first distillation column 122 via pipe t15, and the remainder is discharged as a water-rich waste liquid via pipe t16.

[0032] (Second distillation means) The second distillation means 130 is a means for distilling the dehydrated raw material dehydrated by the first distillation means 120. The second distillation means 130 includes a second distillation column 131, and is configured to include a reboiler 132, a condenser 133, and a cooler 134 attached to the second distillation column 131.

[0033] The second distillation column 131 is a tray-type distillation column, and the dehydrated feedstock dehydrated in the first distillation means 120 is supplied to the bottom of the column via pipe t19, a reboiler 132, and pipe t21. Vapor containing a large amount of impurities having a boiling point lower than that of NMP (e.g., low-boiling amines) is distilled from the top of the column, liquid containing a large amount of impurities having a boiling point higher than that of NMP (e.g., methyl succinimide) is discharged from the bottom, and a liquid containing concentrated NMP is delivered from the product tray. The distillate vapor from the second distillation column 131 is supplied to a condenser 133 via pipe t22. The bottoms of the second distillation column 131 are supplied to the reboiler 132 via pipe t23. The highly purified NMP delivered from the product tray of the second distillation column 131 is supplied to a cooler 134 via pipe t24. The product stage of the second distillation column 131 is not particularly limited, but for example, when the total number of stages in the second distillation column 131 is 10 or more, it is in the range from the first stage from the top to the fifth stage from the bottom, more specifically, when the total number of stages is 10, it is the stage corresponding to the second to sixth stages from the top, or when the total number of stages is 20, it is the stage corresponding to the third to tenth stages from the top. The internal pressure of the second distillation column 131 may be, for example, 4 to 10 kPa absolute pressure.

[0034] The reboiler 132 is a device having a configuration similar to that of the reboiler 123, and performs heat exchange between the dehydrated raw material supplied via pipe t19 and the bottoms supplied via pipe t23 and steam s, and supplies the heated steam to the bottom of the second distillation column 131 via pipe t21. A liquid containing a large amount of impurities such as methylsuccinimide accumulates in the liquid holding section (bottom) of the reboiler 132, and is returned to the buffer tank T12 via pipe t25. The condenser 133 is a device having a configuration similar to that of the condenser 125, and liquefies the distillate vapor from the second distillation column 131 by cooling it with a chiller c. A portion of the liquid condensed in the condenser 133 is returned as reflux to the top of the second distillation column 131 via pipe t26, and the remainder is supplied upstream of the impurity adsorption means 110 via pipes t27 and t33. The cooler 134 is a device that uses a chiller c to cool and liquefy the vapor delivered via pipe t24 from the product stage of the second distillation column 131. The liquid liquefied in the cooler 134 is then withdrawn as a purified composition (product) via pipe t28.

[0035] (pH Measuring Means) The pH measuring means 140 is a means for measuring the pH of a mixed solution obtained by mixing the dehydrated raw material (pH measurement solution) supplied via pipes t18 and t20 with pure water W supplied via pipe t29. Note that a commercially available in-line pH measuring device may be used as the pH measuring means 140.

[0036] (Alkali Addition Means) The alkali addition means 150 is means for adding an alkali compound stored in the alkali tank T13 to the raw material in the buffer tank T12 via pipes t30 and t31 based on the results of the pH measurement means 140. The alkali addition means 150 may also be configured to add an alkali compound via pipes t30 and t32 to the raw material flowing through pipes t6 and t7 located downstream of the buffer tank T12 and upstream of the reboiler 123. The alkali addition means 150 may be configured to add the alkali compound continuously or intermittently.

[0037] (Control Means) The control means (not shown) controls the supply rates of the pH measurement solution and pure water in the pH measurement means 140, calculates the amount of alkali compound added using the pH result measured by the pH measurement means 140, and controls the amount of alkali compound added from the alkali addition means 150. The control means also reads data from various instruments (e.g., thermometers, pressure gauges, flow meters, etc.) installed in various locations in the purification system 100 and controls the flow rate, reflux ratio, etc. based on the data. The control means is realized by program execution processing by a CPU (Central Processing Unit) or a dedicated circuit, etc. The memory unit provided in the control means can be configured with a general memory device such as a RAM (Random Access Memory), a ROM (Read Only Memory), a HDD (Hard Disk Drive), or a flash memory. The control method by the control means will be described in detail later.

[0038] (Other Configurations) Pump P11 delivers raw material from raw material tank T11 to impurity adsorption means 110 via pipes t1 to t3. Pump P12 delivers alkaline compound from alkali tank T13 via pipes t30, t31, and t32 to buffer tank T12 or pipes t6 and t7 located downstream of buffer tank T12 and upstream of reboiler 123. Pump P13 delivers dehydrated raw material from reboiler 124 to second distillation means 130 via pipes t18 and t19. The pump is not particularly limited as long as it is capable of delivering liquid, and any known pump can be used. Furthermore, the installation locations of the pumps are not limited to those shown in FIG. 1 , and the number of pumps may be increased or decreased as appropriate.

[0039] Valves may be provided in each pipe as appropriate in the purification system 100. The valves are not particularly limited as long as they are configured to be able to adjust the flow rate of the liquid by adjusting the level of opening and closing (the degree to which the flow path is open or closed), and known valves can be used.

[0040] Although not shown in the purification system 100 of FIG. 1, when a plurality of liquids or gases are to be mixed in a pipe, such as at a location where a plurality of pipes join together, a line mixer M as shown in FIG. 2 may be installed as appropriate.

[0041] [Target Raw Material] Next, the target raw material of the purification method according to the first embodiment will be described. The target raw material is a liquid obtained by recovering NMP used in the secondary battery manufacturing process using a gas recovery device or the like. Therefore, the raw material contains not only NMP but also water and impurities. Examples of impurities include amines with a boiling point lower than NMP (amines with a boiling point of 100°C or less at room temperature; appropriately referred to as "low-boiling-point amines") and methylsuccinimide (an NMP oxidation product with a boiling point of 230°C or more at room temperature). Low-boiling-point amines include, but are not limited to, methylamine. Impurities are not limited to the aforementioned components, as long as they are components that are inevitably contained when reusing NMP. For example, amines with a boiling point of more than 100°C but less than 200°C at room temperature (appropriately referred to as "medium-boiling-point amines") are included. Examples of medium-boiling-point amines include, but are not limited to, 2-amino-2-methyl-1-propanol and monoethanolamine.

[0042] The content of each component in the raw material is not particularly limited. For example, the mass fraction of water is 5×10 －２ ~50 x 10 －２ and the mass fraction of the low-boiling amine is 1×10 －６ ~1000 x 10 －６ and the mass fraction of methyl succinimide is 10 × 10 －６ ~10000 x 10 －６ and the total mass fraction of impurities is 0.05×10 －２ ~1 x 10 －２ In this specification, the term "mass fraction" refers to a value calculated by "mass of target / total mass." For example, when the mass fraction of methyl succinimide in the raw material is 10×10 －６ This is synonymous with the content of methylsuccinimide in the raw material being 10 ppm (mg / kg).

[0043] [First embodiment: Purification method] Next, a purification method according to the first embodiment will be described with reference to Fig. 1. The purification method according to the first embodiment includes a first distillation step and a second distillation step, and may further include an impurity adsorption step before the first distillation step. The purification method according to the first embodiment may also include a pH measurement step, a calculation step, and an alkali addition step. The steps of the purification method according to the first embodiment are as follows.

[0044] (Impurity Adsorption Step) The impurity adsorption step is a step of adsorbing and separating impurities contained in the raw material. Specifically, in the impurity adsorption step, the raw material stored in the raw material tank T11 is passed through a first ion exchange resin column 111 and a second ion exchange resin column 112. Then, the raw material that has passed through the first ion exchange resin column 111 and the second ion exchange resin column 112 is supplied to a buffer tank T12. Although the impurity adsorption step is not an essential step, it is preferable to use the impurity adsorption step when the mass fraction of amine in the raw material is 1×10 －６ In the above cases, it is preferable to separate as much amine as possible from the raw material by providing this step. In the impurity adsorption step, the raw material flow rate SV to the first ion exchange resin column 111 and the second ion exchange resin column 112 is set to 5 to 30 h. －１ In the impurity adsorption step, it is preferable to adsorb and separate 90% by mass or more of the amines originally contained in the raw material, and more preferably 99% by mass or more. In this specification, the term "amine" simply refers to all amines (compounds in which the hydrogen atoms of ammonia are substituted with hydrocarbon groups), and specifically refers to a concept that combines low-boiling-point amines (methylamine) and medium-boiling-point amines (2-amino-2-methyl-1-propanol, monoethanolamine).

[0045] (First Distillation Step) The first distillation step is a step in which a feedstock is distilled in a first distillation means 120 (specifically, a first auxiliary distillation column 121 and a first distillation column 122) to obtain a dehydrated feedstock. Specifically, in the first distillation step, the feedstock is heated in a reboiler 123 to generate steam, and the steamed feedstock is supplied to the first auxiliary distillation column 121. The feedstock supplied to the first auxiliary distillation column 121 is then subjected to a distillation process, and steam rich in low-boiling point components is distilled from the top of the column, and a liquid rich in high-boiling point components is discharged from the bottom of the column. Note that the bottoms of the first auxiliary distillation column 121 are heated in the reboiler 123, but liquid containing a large amount of non-volatile components such as alkali compounds, which will be described later, accumulates in the liquid hold section (bottom) of the reboiler 123, and this liquid is discharged as waste liquid. Next, the distillate vapor from the first auxiliary distillation column 121 is supplied to the feed stage of the first distillation column 122. The feedstock (distillate vapor) supplied to the first distillation column 122 is then subjected to a distillation process, and vapor rich in low-boiling point components (vapor containing impurities such as low-boiling amines and water) is distilled from the top of the column, the dehydrated feedstock is discharged from the bottom of the column, and vapor containing impurities such as intermediate-boiling amines is discharged from a side cut nozzle provided at the discharge stage. The distillate vapor from the first distillation column 122 is condensed in a condenser 125, and a portion is refluxed to the first distillation column 122.

[0046] (First Distillation Step: Various Conditions) In the first distillation step, the temperature of the distillation target (raw material) in each distillation column is controlled to be less than 160°C. By controlling the temperature of the distillation target in this manner, it is possible to suppress the denaturation and decomposition of NMP and reduce the amount of newly generated amines and high boiling point substances. The mass fraction of water in the raw material after dehydration is 5 x 10 －２ Preferably, 1×10 －２ ~5 x 10 －２ , 1 x 10 －２ ~3 x 10 －２ , 1×10 －２ ~2 x 10 －２ is more preferred.

[0047] (Second Distillation Step) The second distillation step is a step in which the dehydrated feedstock is distilled in a second distillation means 130 (specifically, a second distillation column 131) to obtain a purified composition enriched in NMP. Specifically, in the second distillation step, the dehydrated feedstock obtained in the first distillation step is heated in a reboiler 132 to generate steam, and the dehydrated feedstock in the form of steam is supplied to the bottom of the second distillation column 131. The dehydrated feedstock supplied to the second distillation column 131 is then subjected to a distillation treatment, with steam containing low-boiling-point amines being distilled from the top of the column, a liquid containing methylsuccinimide being discharged from the bottom of the column, and a liquid enriched in NMP being withdrawn from the product tray. The liquid enriched in NMP is then cooled in a cooler 134 to obtain a purified composition (product). The distillate vapor from the second distillation column 131 contains components with a boiling point lower than that of NMP, such as low-boiling amines and moisture that could not be completely removed in the first distillation step, as well as amines generated by decomposition of NMP during the purification process. The bottoms from the second distillation column 131 also contains components with a boiling point higher than that of NMP, such as not only the above-mentioned methylsuccinimide, but also alkali compounds and non-volatile substances that could not be completely removed in the first distillation step.

[0048] (Second Distillation Step: Conditions) In the second distillation step, as in the first distillation step, the temperature of the distillation target (raw material) in each distillation column is controlled to be less than 160° C. By controlling the temperature of the distillation target not only in the first distillation step but also in the second distillation step in this way, it is possible to more reliably suppress the denaturation and decomposition of NMP and to reduce the amounts of newly produced amines and high boiling point substances as much as possible.

[0049] (Second Distillation Step: Purified Composition) The purified composition obtained in the second distillation step has an amine mass fraction of 0.5 × 10 －６ Preferably, it is less than 0.2×10 －６ It is more preferable that the mass fraction of water is less than 200×10. As described above, the term "amine" used here is not limited to low-boiling point amines or medium-boiling point amines, but refers to all amines (compounds in which the hydrogen atoms of ammonia are substituted with hydrocarbon groups) that may be contained as impurities. In addition, the purified composition obtained in the second distillation step has a mass fraction of water of 200×10. －６Preferably, it is less than 50×10 －６ It is more preferable that the NMP recovery rate is 95% or less. As described above, the purification method according to the first embodiment can obtain a purified composition in which the content of water and amine impurities is reduced as much as possible, in other words, a purified composition in which NMP is highly concentrated. The recovery rate (NMP recovery rate), which is the ratio of the mass of NMP in the purified composition to the mass of NMP in the raw material, is preferably 95% or more, and more preferably 99% or more. As described above, the purification method according to the first embodiment can recover NMP at a very high recovery rate.

[0050] The purified composition obtained in the second distillation step has a mass fraction of NMP of 99.98 × 10 －２ It is preferably 99.99 x 10 or more. －２ The purified composition obtained in the second distillation step preferably has a mass fraction of methyl succinimide of 10×10 or more. －６ Preferably, it is 5×10 or less. －６ It is more preferable that the pH of the mixture obtained by mixing the purified composition obtained in the second distillation step with water in a mass ratio of 1:1 is 7.2 to 8.0. It is also preferable that the purified composition obtained in the second distillation step has a colorimetric value of APHA 10 or less, more preferably 5 or less, after 90 days of sealed storage at room temperature. In this way, according to the purification method of the first embodiment, the obtained purified composition can satisfy each of the above-mentioned constituent requirements.

[0051] (Second Distillation Step: Measurement and Calculation Methods for Purified Composition) The mass fractions of NMP, amine, and methylsuccinimide in the purified composition can be measured, for example, by gas chromatography or ion chromatography. The mass fraction of water in the purified composition can be measured, for example, by a Karl Fischer moisture meter. The methods for measuring the mass fractions of each component in the raw material and the raw material after dehydration are the same as the above-mentioned measurement methods.

[0052] The recovery rate of NMP is determined by first calculating the "mass of NMP contained in all extracted purified compositions" from the total amount of the extracted purified compositions and the concentration of NMP contained in the purified compositions, and then calculating the "mass of NMP contained in all raw materials to be purified" from the total amount of the raw materials to be purified and the concentration of NMP contained in the raw materials. The recovery rate of NMP can then be calculated by "mass of NMP contained in all extracted purified compositions / mass of NMP contained in all raw materials to be purified x 100". The pH of the mixed solution can be measured using a commercially available pH meter. The value (APHA) of the colorimetric tube of the purified composition can be measured, for example, visually or using an ultraviolet-visible spectrophotometer.

[0053] (pH Measurement Step) In the pH measurement step, a portion of the liquid inside the column in the first distillation step is extracted as a pH measurement liquid, and the pH measurement liquid and water are mixed in a mass ratio of 1 to 40:2, in other words, the mass ratio of the two (= mass of pH measurement liquid / mass of water) is 0.5 to 20, and the pH of the resulting mixture is measured. Specifically, in the pH measurement step, a portion of the dehydrated raw material (column liquid) discharged from the bottom of the first distillation column 122 is extracted as a pH measurement liquid, and this pH measurement liquid and water are mixed in a predetermined mass ratio (a predetermined constant mass ratio). The pH of the resulting mixture is then measured by pH measurement means 140. The pH measurement by the pH measurement means 140 may be performed continuously or at predetermined time intervals (e.g., every 1 to 24 hours). The preparation of the mixture to be measured for pH may be carried out continuously (the pH measurement liquid and water are constantly supplied at predetermined amounts and mixed), or may be carried out at the predetermined time intervals described above. Note that, although it is preferable to use a part of the liquid in the column in the first distillation step as the pH measurement liquid, a part of the liquid in the column in the second distillation step may also be used, or both may be used.

[0054] (pH Measurement Step: Mixing Ratio) In the pH measurement step, the pH measurement solution and water are preferably mixed so that the mass ratio between them (= mass of pH measurement solution / mass of water) is 0.5 to 20. Furthermore, it is preferable to control the amounts of the two components mixed together so that the mass ratio between them (a predetermined constant mass ratio) in the mixed solution in the pH measurement step is within a certain range with an error of ±5% or less. By keeping the mass ratio between them within a predetermined range and keeping the error in this mass ratio within a predetermined range, an appropriate amount of alkaline compound to be added can be calculated in the calculation step described below. "Controlling the amounts of the pH measurement solution and the water mixed together so that the mass ratio between them in the mixed solution in the pH measurement step is within a certain range with an error of ±5% or less" specifically means checking the flow rates of the pH measurement solution in the pipe t20 and the water in the pipe t29 constantly or at predetermined time intervals (e.g., every 1 to 24 hours) and controlling the flow rates of both components so that the mass ratio between them is within a certain range. Furthermore, the mass ratio of the two components within a ±5% error means, for example, that if the mass ratio of the two components (= mass of pH measurement solution / mass of water) is set to 2, the amounts of the two components mixed together can be controlled so that the mass ratio falls within the range of 1.9 to 2.1 (= 2 × 0.95 to 2 × 1.05). The mixed amounts may be controlled for both components (pH measurement solution and water), but, for example, in a configuration in which the mixed amount of the pH measurement solution does not fluctuate, it is also possible to control only the mixed amount of water. As long as the mass ratio of the pH measurement solution to water satisfies the above requirements, there are no particular limitations. The supply rate of the pH measurement solution to the pH measurement means 140 is, for example, 20 to 40 ml / min, typically about 30 ml / min, and the supply rate of water to the pH measurement means 140 is, for example, 5 to 15 ml / min, typically about 10 ml / min.

[0055] (Calculation Step) The calculation step is a step of calculating the amount of alkaline compound to be added based on the pH result of the pH measurement step. Specifically, the amount of alkaline compound to be added is calculated by feedback control based on the difference between the pH result obtained in the pH measurement step and the target value. The target value is a preferred pH value of the mixed solution when the addition of the alkaline compound can appropriately suppress the generation of peroxides and the like and can suppress the generation of new impurities such as amines due to the addition of the alkaline compound, and is a value set by prior testing or the like. Specifically, the feedback control may be performed using a known PID (Proportional-Integral-Differential) controller.

[0056] (Alkali Addition Step) The alkali addition step is a step of adding the alkali compound in the amount calculated in the calculation step to the raw material or the dehydrated raw material. The alkali compound may be any substance that exhibits basicity. Specifically, hydroxides of alkali metals or alkaline earth metals are preferred, and potassium hydroxide is more preferred. Potassium hydroxide has the advantage of being highly soluble in the raw material, but has the disadvantage of being highly reactive with NMP and easily generating impurities such as amines when added in large amounts. However, according to the purification method of the first embodiment, the amount of potassium hydroxide added can be appropriately controlled, thereby suppressing the generation of impurities such as amines while enjoying the benefits of potassium hydroxide. The alkali compound may be added in any manner, but may be added as an aqueous solution. The calculation step and the alkali addition step may be performed at a predetermined time interval, for example, at intervals of 1 to 24 hours (preferably 3 to 6 hours).

[0057] (Alkali Addition Step: Addition Target) As shown in Figure 1, in the purification method according to the first embodiment, the alkali compound is added to the raw material stored in the buffer tank T12 or the raw material flowing between the buffer tank T12 and the distillation column 121. The alkali compound may be added to any of the raw material prior to the first distillation step, the liquid in the column of the first distillation step, and the liquid in the column of the second distillation step; however, the alkali compound is preferably added to the raw material prior to the first distillation step. This is because the effect of suppressing the generation of peroxides due to the addition of the alkali compound can be fully enjoyed, and excess alkali compound can be appropriately removed in the first distillation step (reboiler 123).

[0058] The purification method according to the first embodiment is a so-called continuous process (a process in which a raw material is supplied at a constant flow rate and a purified composition is withdrawn at a constant flow rate), and therefore, in the purification method according to the first embodiment, the raw material is subjected to an impurity adsorption step, a first distillation step, and a second distillation step, and in parallel, a pH measurement step, a calculation step, and an alkali addition step.

[0059] [Second Embodiment: Purification System] Next, a purification system 200 according to a second embodiment will be described with reference to Fig. 2. The purification system 200 according to the second embodiment includes an impurity adsorption means 210, a first distillation means 220, a second distillation means 230, a pH measurement means 240, an alkali addition means 250, and a control means (not shown). The respective means constituting the purification system 200 according to the second embodiment are as follows.

[0060] (Impurity Adsorption Means) The impurity adsorption means 210 of the refining system 200 has substantially the same configuration as that of the refining system 100, and therefore a description thereof will be omitted.

[0061] (First Distillation Means) The first distillation means 220 includes a preheater 221 disposed upstream and a first distillation column 222 disposed downstream, as well as a reboiler 223 and a condenser 224 associated with the first distillation column 222. The preheater 221 is a device that heats a raw material using the heat of drain d. The drain d used in the preheater 221 may be the drain d discharged from the reboilers 223, 233, and 234. The feed stage of the first distillation column 222 to which the raw material heated by the preheater 221 is supplied is not particularly limited, but may be, for example, a stage in the range of X×1 / 3 to X×2 / 3 from the bottom, where X is the total number of stages in the first distillation column 222, e.g., X×1 / 3, X×4 / 10, X×4 / 9, X×1 / 2, X×5 / 9, X×6 / 10, or X×2 / 3 from the bottom. The internal pressure of the first distillation column 222 may be, for example, 5 to 101.3 kPa absolute pressure.

[0062] (Second Distillation Means) Second distillation means 230 includes a second auxiliary distillation column 231 located upstream and a second distillation column 232 located downstream, as well as a reboiler 233 associated with second auxiliary distillation column 231, and a reboiler 234, condenser 235, and cooler 236 associated with second distillation column 232. Second distillation means 230 of purification system 200 has a so-called "modified Petriuk system" configuration, but has been improved from the conventional configuration to be suitable for purifying NMP.

[0063] The second auxiliary distillation column 231 is a packed distillation column equipped with structured packing. The dehydrated feedstock obtained by the first distillation means 220 (specifically, the roughly dehydrated feedstock pre-dehydrated to a water content of approximately 1 to 5% by mass) is fed to the column near its midpoint. Vapor from which impurities with a boiling point higher than NMP have been removed is distilled from the column top, and liquid from which impurities with a boiling point lower than NMP have been removed is discharged from the column bottom. The distillate vapor from the second auxiliary distillation column 231 is supplied to the feed stage of the second distillation column 232. Unlike the conventional Petriuk system configuration (pipe t1a indicated by the dotted line in FIG. 2 ), a portion of the bottoms from the second auxiliary distillation column 231 is supplied to the bottom of the second distillation column 232 via pipe t1b. This configuration increases the concentration rate of succinimide, thereby increasing the product recovery rate. The feed location (near the midpoint) of the second auxiliary distillation column 231 to which the dehydrated feedstock is supplied may be between the upper packed bed 231a and the lower packed bed 231b. The internal pressure of the second auxiliary distillation column 231 may be, for example, 5 to 20 kPa absolute.

[0064] The second distillation column 232 is a tray-type distillation column. The distillate vapor and bottoms from the second auxiliary distillation column 231 are supplied to the second distillation column 232. The top of the column distills vapor containing impurities with lower boiling points than NMP (e.g., low-boiling amines and small amounts of residual water), the bottoms of the column distillate liquid containing impurities with higher boiling points than NMP (e.g., methyl succinimide), and the product column delivers vapor containing concentrated NMP. The second distillation column 232 returns a portion of the liquid withdrawn from the product column to the top of the second auxiliary distillation column 231. Conventional Petriuk-type distillation columns typically return liquid and vapor from the feed column of the second distillation column 232 to the second auxiliary distillation column 231 (pipe t2a shown by the dotted line in FIG. 2 ). However, water and NMP are mixed in the feed column. During actual operation of the distillation column, the concentrations of water and NMP fluctuate slightly. Therefore, when liquid or vapor is returned from the feed stage to the second auxiliary distillation column 231, the difference in latent heat between water and NMP is large, and therefore the amount of heat that must be provided to the second auxiliary distillation column 231 also fluctuates significantly due to the influence of concentration fluctuations. As a result, distillation control in the purification system 200 becomes unstable. Therefore, by returning a liquid containing almost no water from the product stage to the second auxiliary distillation column 231 rather than from the feed stage of the second distillation column 232, the influence of composition fluctuations can be suppressed, and the purification system 200 can be stabilized. Ultimately, the quality of the purified composition that is purified can be stabilized.

[0065] The feed stage of the second distillation column 232 (the stage to which vapor is supplied from the second auxiliary distillation column 231) is, for example, when the total number of stages of the second distillation column 232 is X, but is not particularly limited thereto, and is a stage in the range of X×1 / 4 to X×3 / 4 from the bottom, for example, X×1 / 4, X×1 / 3, X×1 / 2, X×2 / 3, and X×3 / 4 from the bottom. Furthermore, when the total number of stages below the aforementioned feed stage of the second distillation column 232 is X, the product stages of the second distillation column 232 are, for example, stages in the range of X×1 / 5 to X×4 / 5 from the aforementioned feed stage downward, for example, X×1 / 5, X×2 / 5, X×1 / 2, X×3 / 5, and X×4 / 5 downward. The internal pressure of the second distillation column 232 may be, for example, 10 to 40 kPa absolute pressure.

[0066] (pH Measuring Means, Alkali Adding Means, Controlling Means) The pH measuring means 240, alkali adding means 250 and controlling means (not shown) of the purification system 200 have substantially the same configurations as those of the purification system 100, and therefore description thereof will be omitted.

[0067] [Second embodiment: Purification method] Next, a purification method according to a second embodiment will be described with reference to Fig. 2. Each step of the purification method according to the second embodiment is as follows.

[0068] (Impurity Adsorption Step) The impurity adsorption step of the purification method according to the second embodiment is the same as that of the purification method according to the first embodiment, and therefore a description thereof will be omitted.

[0069] (First Distillation Step) The first distillation step is a step in which a feedstock is distilled in a first distillation means 220 (specifically, a first distillation column 222) to obtain a dehydrated dehydrated feedstock. Specifically, in the first distillation step, the feedstock is heated in a preheater 221. Next, the feedstock heated in the preheater 221 is supplied to the feed stage of the first distillation column 222. The feedstock supplied to the first distillation column 222 is then subjected to a distillation process, and vapor rich in low-boiling point components (vapor containing impurities such as low-boiling point amines and water) is distilled from the top of the column, and the dehydrated dehydrated feedstock (specifically, a crude dehydrated feedstock pre-dehydrated to a moisture content of approximately 5% by mass) is discharged from the bottom of the column. In addition, the distilled vapor from the first distillation column 222 is condensed in a condenser 224, and a portion of it is refluxed to the first distillation column 222. In the first distillation step, distillation is performed using the first distillation column 222, and the moisture content of the roughly dehydrated feedstock is adjusted to approximately 5% by mass (1 to 5% by mass). By keeping the moisture content of the roughly dehydrated feedstock obtained in the first distillation step at a predetermined value or less, the reflux ratio in the second distillation step (described below) can be reduced, thereby achieving overall energy savings. Note that in the first distillation step, energy savings can be achieved by preheating the feedstock using the preheater 221.

[0070] (Second Distillation Step) The second distillation step is a step in which the crudely dehydrated feedstock is distilled in a second distillation means 230 (specifically, a second auxiliary distillation column 231 and a second distillation column 232) to obtain a purified composition enriched in NMP. Specifically, in the second distillation step, the crudely dehydrated feedstock obtained in the first distillation step is supplied to the second auxiliary distillation column 231. The crudely dehydrated feedstock supplied to the second auxiliary distillation column 231 is then subjected to a distillation treatment, and vapor from which impurities with a boiling point higher than that of NMP have been removed is distilled from the top of the column, and liquid from which impurities with a boiling point lower than that of NMP have been removed is discharged as a bottom product from the bottom of the column. The distilled vapor is then supplied to the feed stage of the second distillation column 232, and the bottom product is supplied to the bottom of the second distillation column 232. Next, the distillate vapor and bottoms supplied to the second distillation column 232 are subjected to a distillation treatment, with vapor containing low-boiling-point amines being distilled from the top of the column, a liquid containing methylsuccinimide being discharged from the bottom of the column, and a liquid enriched in NMP being withdrawn from the product tray. The liquid enriched in NMP is then cooled in a cooler 236 to produce a purified composition (product). In addition, a portion of the liquid enriched in NMP is returned from the product tray of the second distillation column 232 to the second auxiliary distillation column 231.

[0071] (pH Measuring Step, Calculating Step, and Alkali Adding Step) The pH measuring step, calculating step, and alkali adding step of the purification method according to the second embodiment are the same as those of the purification method according to the first embodiment, and therefore description thereof will be omitted.

[0072] [Third Embodiment: Purification System] Next, a purification system 300 according to a third embodiment will be described with reference to Fig. 3. The purification system 300 according to the third embodiment includes an impurity adsorption means 310, a first distillation means 320, a second distillation means 330, a pH measurement means 340, an alkali addition means 350, and a control means (not shown). The various means constituting the purification system 300 according to the third embodiment are as follows:

[0073] (Impurity Adsorption Means, First Distillation Means) The impurity adsorption means 310 and the first distillation means 320 of the purification system 300 have substantially the same configuration as those of the purification system 200, and therefore a description thereof will be omitted.

[0074] (Second Distillation Means) The second distillation means 330 includes a second distillation column 331, as well as reboilers 332 and 333, a condenser 334, and a cooler 335 associated with the second distillation column 331. The second distillation column 331 is a tray-type distillation column, a vertically divided distillation column. The interior of the second distillation column 331, excluding the top, is vertically partitioned by a dividing plate 331a, dividing the column into an upstream distillation section 331b and a downstream distillation section 331c. The distillation target is heated by a reboiler 332 at the bottom of the upstream distillation section 331b, and by a reboiler 333 at the bottom of the downstream distillation section 331c. A supply means 331d is provided inside the second distillation column 331 near the top. This supply means 331d is configured to supply a portion of the distillate from the second distillation column 331 condensed in the condenser 334 to both the upstream distillation section 331b and the downstream distillation section 331c.

[0075] For example, when the total number of stages in the upstream distillation section 331b (the total number of stages provided on the left side of the dividing plate 331a in FIG. 3 ) is X, the feed stages of the second distillation column 331 to which the raw material after rough dehydration is supplied are not particularly limited, but are, for example, stages in the range of X×1 / 3 to X×2 / 3 from the bottom, such as X×1 / 3, X×4 / 10, X×4 / 9, X×1 / 2, X×5 / 9, X×6 / 10, and X×2 / 3 from the bottom. Furthermore, for example, when the total number of stages in the downstream distillation section 331c (the total number of stages provided to the right of the dividing plate 331a in FIG. 3) is X, the product stages of the second distillation column 331 are, but are not particularly limited to, stages in the range of X×1 / 5 to X×4 / 5 from the bottom, such as X×1 / 5, X×2 / 5, X×1 / 2, X×3 / 5, and X×4 / 5 from the bottom. The internal pressure of the second distillation column 331 may be, for example, 10 to 40 kPa absolute pressure.

[0076] (pH Measuring Means, Alkali Adding Means, Controlling Means) The pH measuring means 340, alkali adding means 350 and controlling means (not shown) of the purification system 300 have substantially the same configurations as those of the purification systems 100 and 200, and therefore description thereof will be omitted.

[0077] [Third embodiment: Purification method] Next, a purification method according to a third embodiment will be described with reference to Fig. 3. Each step of the purification method according to the third embodiment is as follows.

[0078] (Impurity Adsorption Step, First Distillation Step) The impurity adsorption step and the first distillation step of the purification method according to the third embodiment are the same as those of the purification method according to the second embodiment, and therefore, description thereof will be omitted.

[0079] (Second Distillation Step) The second distillation step is a step in which the crudely dehydrated feedstock is distilled in a second distillation means 330 (specifically, a second distillation column 331) to obtain a purified composition enriched in NMP. Specifically, in the second distillation step, the crudely dehydrated feedstock obtained in the first distillation step is supplied to the second distillation column 331. The crudely dehydrated feedstock supplied to the second distillation column 331 is then subjected to a distillation treatment, and vapor containing low-boiling-point amines and small amounts of remaining water is distilled from the top of the column, a liquid containing methylsuccinimide is discharged from the bottom of the column, and a liquid enriched in NMP is withdrawn from the product tray. The liquid enriched in NMP is then cooled in a cooler 335 to obtain a purified composition (product).

[0080] (pH measurement step, calculation step, alkali addition step) The pH measurement step, calculation step, and alkali addition step of the purification method according to the third embodiment are the same as those of the purification method according to the first embodiment and the purification method according to the second embodiment, and therefore will not be described here.

[0081] [Fourth Embodiment: Purification System] Next, a purification system 400 according to a fourth embodiment will be described with reference to Fig. 4. The purification system 400 according to the fourth embodiment includes a first distillation means 420, a second distillation means 430, a pH measurement means 440, an alkali addition means 450, and a control means (not shown). The respective means constituting the purification system 400 according to the fourth embodiment are as follows.

[0082] (Impurity Adsorption Means) The purification system 400 is configured without an impurity adsorption means, unlike the purification systems 100, 200, and 300. When the mass fraction of amine in the raw material is low (for example, 1×10 －６In the case where the impurity adsorption means is not provided, the impurity adsorption means may be omitted, as in the configuration of the purification system 400.

[0083] (First Distillation Means) Unlike the purification system 100 , the first distillation means 420 of the purification system 400 does not include the first auxiliary distillation column 121 .

[0084] (Second Distillation Means, pH Measuring Means, Control Means) The second distillation means 430, pH measuring means 440, and control means (not shown) of the purification system 400 have substantially the same configurations as those of the purification system 100, and therefore description thereof will be omitted.

[0085] (Alkali Addition Means) The alkali addition means 450 differs from the purification systems 100, 200, and 300 in that the alkali compound is added to the raw material tank T41 or the bottom of the first distillation column 421.

[0086] [Fourth Embodiment: Purification Method] Although the purification method according to the fourth embodiment does not include the impurity adsorption step, the other steps are substantially the same as those of the purification method according to the first embodiment, and therefore a description thereof will be omitted.

[0087] [First to Fourth Embodiments: Recovery Step] The purification methods according to the first to fourth embodiments may include a recovery step of recovering a raw material from a gas containing NMP before the first distillation step. In the recovery step, it is preferable to use a part of the distillate containing the low-boiling-point amine distilled from the first distillation column in the first distillation step (more specifically, the liquid sent via the pipe t16 in FIG. 1 ) as recovered water for recovering the gas. The distillate containing the low-boiling-point amine used as recovered water has an NMP mass fraction of 0.001 × 10 －２ ~5 x 10 －２ The gas recovery apparatus for recovering a raw material from a gas containing NMP is an apparatus that transfers NMP in the gas to the recovered water by contacting the gas with recovered water, and recovers the recovered water in which NMP is concentrated as a raw material, and is, for example, a known gas recovery apparatus such as that described in Japanese Patent No. 6351403. The relevant content of Japanese Patent No. 6351403 is incorporated herein by reference.

[0088] [Modifications] While the purification systems and purification methods according to the first to fourth embodiments have been described above, the embodiments are not limited thereto and can be modified, for example, as follows. The first auxiliary distillation column 121 in the purification system 100 shown in FIG. 1 may be a mist eliminator or a packed distillation column equipped with structured packing or internals. The first distillation column and the second distillation column in the purification systems 100 to 400 shown in FIGS. 1 to 4 may be packed distillation columns equipped with structured packing or internals. In the purification system 200 shown in FIG. 2, the liquid returned from the second distillation column 232 to the second auxiliary distillation column 231 may be a liquid consisting mostly of water condensed in the condenser 235 at the top of the column.

[0089] [Effects] The purification systems and purification methods according to the above-described embodiments can achieve the following effects. The purification methods according to the first to fourth embodiments include a pH measurement step, a calculation step, and an alkali addition step, and can add an optimal amount of alkali compound by controlling the mass ratio of water in the mixed solution in the pH measurement step so that there is almost no error. As a result, not only can the addition of alkali compound effectively suppress the generation of peroxides, but it can also avoid situations where the excessive addition of alkali compound results in the generation of new impurities such as amines.

[0090] The purification methods according to the first to third embodiments can obtain a purified composition having a lower mass fraction of impurities because impurities are pre-adsorbed and separated from the raw material in the impurity adsorption step.The purification methods according to the first to third embodiments can increase the recovery rate of NMP because a portion of the distillate containing the low-boiling amine distilled from the top of the second distillation column in the second distillation step is returned as a raw material upstream of the ion exchange resin column in the impurity adsorption step.

[0091] In the purification methods according to the first to fourth embodiments, the temperature of the distillation target in the first distillation step and the second distillation step is controlled to be less than 160°C, thereby suppressing denaturation and decomposition of NMP during purification and reducing the amount of newly generated amines and high-boiling substances. In the purification methods according to the first to fourth embodiments, in the second distillation step, vapor containing low-boiling amines is distilled from the top of the second distillation column, a liquid containing methylsuccinimide is discharged from the bottom of the column, and the purified composition is withdrawn from the side-cut nozzle, thereby enabling appropriate separation and removal of impurities such as low-boiling amines and methylsuccinimide, as well as water.

[0092] The purification methods according to the first to fourth embodiments include a recovery step, and a portion of the distillate containing the low-boiling-point amine distilled from the first distillation column in the first distillation step is used as recovered water for recovering the gas. This reduces the amount of recovered water that needs to be prepared, thereby achieving a cost-cutting effect.

[0093] In the purification methods according to the second and third embodiments, the distillation is performed in the first distillation column in the first distillation step, so that the mass fraction of water in the crude dehydrated feedstock can be reduced, which makes it possible to reduce the reflux ratio in the second distillation step and thereby achieve energy savings.

[0094] In the purification method according to the second embodiment, distillation is carried out in two distillation columns, the second auxiliary distillation column and the second distillation column, in the second distillation step, and liquid containing almost no water is returned from the product stage of the second distillation column to the second auxiliary distillation column. This suppresses the effects of composition fluctuations, thereby stabilizing quality and also achieving the energy-saving effects of the conventional Petriuk system.

[0095] The purification method according to the third embodiment uses a vertical dividing distillation column in the second distillation step, and therefore distillation is carried out in both the upstream distillation section and the downstream distillation section of the vertical dividing distillation column, making it possible to more reliably separate and remove impurities and water.

[0096] Next, examples of the present invention will be described. [Example 1] (Example 1: Target Raw Material) The target raw material had a composition of water content: 19 to 21 mass %, amine content: 300 to 400 ppm, content of impurities other than amine: 30 to 60 ppm, and NMP content: 79 to 81 mass %. Here, the amine content specifically refers to the total amount of three types of amine: methylamine, 2-amino-2-methyl-1-propanol, and monoethanolamine. Note that the methods for measuring the content of each component in the examples were the same as those described in the first embodiment.

[0097] (Example 1: Test Conditions) In Example 1, the purification system 100 according to the first embodiment shown in FIG. 1 was used, and a test was carried out according to the purification method according to the first embodiment. The detailed test conditions are as follows. The ion exchange resin packed in the ion exchange resin columns 111 and 112 of the impurity adsorption means 110 was a strongly acidic ion exchange resin. The flow rate SV of the raw material through the ion exchange resin columns 111 and 112 was 10 h －１The first auxiliary distillation column 121 was a tray distillation column with a total of 5 plates, and the column internal pressure was controlled to 11 kPa absolute pressure. The first distillation column 122 was a tray distillation column with a total of 32 plates, and the feed plate to which pipe t11 was connected was the 16th plate from the bottom, and the discharge plate to which pipe t14 was connected was the 10th plate from the top, and the column internal pressure was controlled to 11 kPa absolute pressure. The second distillation column 131 was a tray distillation column with a total of 24 plates, and the product plate to which pipe t24 was connected was the 5th plate from the top, and the column internal pressure was controlled to 8 kPa absolute pressure. The temperatures of the distillation targets (feedstock, dehydrated feedstock) in the first distillation step and the second distillation step were controlled to 150°C. The dehydrated feedstock (the dehydrated feedstock supplied via pipe t20) and water (the water supplied via pipe t29) supplied to the pH measurement means 140 were supplied at rates of 30 ml / min and 10 ml / min, respectively, so that the mass ratio was 3:1 (the mass ratio of the two was 3). The water supply rate was strictly controlled within the range of 9.9 to 10.1 ml / min so that the error in the mass ratio of the two was within ±1%. Potassium hydroxide was added by the alkali addition means 150 in the form of an aqueous solution (concentration: 48%), and the potassium hydroxide addition amount was calculated and added every three hours. Potassium hydroxide was added to the feedstock at the bottom of the first auxiliary distillation column 121. The potassium hydroxide addition amount was calculated based on PID control (target pH: set to 8.0).

[0098] (Example 1: Test Results) When the feedstock downstream of the impurity adsorption means 110 and before being supplied to the first auxiliary distillation column 121 was measured, the amine content was found to be 0.5 ppm. In other words, the impurity adsorption means 110 adsorbed and separated 99.83 to 99.88 mass% (= (300-0.5) / 300×100 to (400-0.5) / 400×100) of the amine originally contained in the feedstock. The liquid obtained by condensing the distillate vapor from the first distillation column 122 in the condenser 125 had an NMP content of 0.015 mass%, mostly water. The bottoms of the first distillation column 122 (feedstock after dehydration) contained 99.97 mass% NMP, 0.01 mass% water, 0.00008 mass% or less amine, and 0.02 mass% other impurities. Therefore, the dehydrated feed obtained by the first distillation means 120 had water appropriately removed, as well as impurities such as amines. The purified composition withdrawn from the product tray of the second distillation column 131 had a water content of 0.003 mass% or less, an amine content of 0.00005 mass% or less, and an NMP content of 99.98 mass% or more. The recovery rate of NMP was 99%. Thus, according to Example 1, impurities such as amines and water were appropriately separated, and a purified composition with a very high concentration of NMP was obtained. Furthermore, the pH of the mixture obtained by mixing the purified composition withdrawn from the product tray of the second distillation column 131 with water in a 1:1 mass ratio was 7.5. After 90 days of sealed storage at room temperature, the purified composition was colorless and transparent, and the colorimeter reading was APHA <5 (less than 5). These results were obtained because the temperature of the distillation target in the first distillation step and the second distillation step was set to less than 160°C, thereby suppressing the denaturation and decomposition of NMP, and because the amount of alkali compound added was optimized, preventing excessive addition and thereby avoiding the generation of new impurities such as amines. Furthermore, although the alkali compound concentrated in the reboiler 123 needs to be withdrawn as a waste liquid via the pipe t10, in Example 1, the amount of the alkali compound added was optimized, so that the amount of the waste liquid could be suppressed to less than 1% of the total amount of the raw material supplied.

[0099] [Comparative Example 1] (Comparative Example 1: Target Raw Material) The target raw material of Comparative Example 1 was the same as that of Example 1. (Comparative Example 1: Test Conditions) The test conditions of Comparative Example 1 were basically the same as those of Example 1, but differed in the following respects. In Comparative Example 1, the pH measurement step and calculation step were not performed. Potassium hydroxide was added every three hours as in Example 1, but the amount added each time was three times the average amount added in Example 1.

[0100] (Comparative Example 1: Test Results) In Comparative Example 1, a sufficient amount of alkaline compound was added, and thus the effect of suppressing the generation of peroxides due to the addition of the alkaline compound was exerted. However, in Comparative Example 1, an excessive amount of alkaline compound was added, resulting in a large amount of alkaline compound concentrated in the reboiler 123. As a result, the amount of waste liquid extracted via the pipe t10 was three times that of Example 1. Furthermore, in Comparative Example 1, the excessive addition of the alkaline compound caused the alkaline compound to react with NMP, resulting in the production of impurities such as amines. Finally, in Comparative Example 1, the recovery rate of NMP was 3% lower than that of Example 1.

[0101] [Comparative Example 2] (Comparative Example 2: Target Raw Material) The target raw material of Comparative Example 2 was the same as that of Example 1. (Comparative Example 2: Test Conditions) The test conditions of Comparative Example 2 were basically the same as those of Example 1, but differed in the following respects. In Comparative Example 2, the pH measurement step and calculation step were not performed. Potassium hydroxide was added every 3 hours as in Example 1, but the amount added was the same as the average amount added in Example 1.

[0102] (Comparative Example 2: Test Results) In Comparative Example 2, unlike Example 1, the amount of alkaline compound added was not controlled, and therefore there were times when the amount of alkaline compound was insufficient and times when it was excessive. Therefore, the purification process was carried out in the presence of impurities such as peroxides generated by a shortage of alkaline compound and amines generated by an excess of alkaline compound. As a result, the purified composition of Comparative Example 2 (a mixed liquid obtained by mixing the purified composition and water at a mass ratio of 1:1) had a pH of 6.2, which was lower than that of Example 1, and after 90 days of sealed storage at room temperature, it turned yellow, and the value on the colorimeter was APHA 30.

[0103] [Comparative Example 3] (Comparative Example 3: Target Raw Material) The target raw material of Comparative Example 3 was the same as that of Example 1. (Comparative Example 3: Test Conditions) The test conditions of Comparative Example 3 were basically the same as those of Example 1, but differed in the following respects: In Comparative Example 3, the amount of water supplied to the pH measurement means 140 was not strictly controlled.

[0104] (Comparative Example 3: Test Results) In Comparative Example 3, unlike Example 1, the amount of water supplied to the pH measurement means 140 was not controlled. As a result, the error in the amount of water supplied (mass ratio of both) was frequently 5% or more, and the measured pH was not an accurate value, so the amount of alkali added was sometimes excessive or insufficient. Ultimately, as with Comparative Examples 1 and 2, excellent results were not obtained.

[0105] [Example 2] (Example 2: Target Feedstock) The target feedstock for Example 2 was the same as that for Example 1. (Example 2: Test Conditions) In Example 2, a test was conducted using the purification system 200 according to the second embodiment shown in FIG. 2 , and in accordance with the purification method according to the second embodiment. The detailed test conditions are as follows. The ion exchange resin and the liquid flow rate SV of the impurity adsorption means 210 were the same as those in Example 1. The preheater 221 used drain water d at 150°C, and heated the feedstock to 90°C. The first distillation column 222 was a tray-type distillation column with a total of 20 stages, and the feed stage to which the feedstock heated by the preheater 221 was supplied was the sixth stage from the bottom, and the pressure inside the column was controlled to an absolute pressure of 13 kPa. The second distillation column 232 was a tray-type distillation column with a total of 40 plates, with the feed plate being the 21st plate from the top and the product plate being the plate five plates below the feed plate. The column internal pressure was controlled to 9 kPa absolute. The temperatures of the distillation targets (feedstock, dehydrated feedstock) in the first distillation step and the second distillation step were controlled to 145°C. The mass ratio of the dehydrated feedstock to water supplied to the pH measurement means 240, the feed rates of both, and the control of the water feed rate were the same as in Example 1. The addition of potassium hydroxide by the alkali addition means 250 and the calculation of the amount of potassium hydroxide added were also the same as in Example 1.

[0106] (Example 2: Test Results) The liquid obtained by condensing the distillate vapor from the first distillation column 222 in the condenser 224 contained 0.001% NMP by mass, 0.0001% other impurities, and the remainder was water. Therefore, this liquid could be used as recovered water for recovering a raw material from a gas containing NMP. The bottoms (dehydrated raw material) from the first distillation column 222 contained 95% NMP by mass, 5% water by mass, 0.00005% amines by mass, and 0.01% other impurities by mass. Therefore, the crude dehydrated raw material obtained by the first distillation means 220 was properly purified from water and impurities such as amines. The purified composition extracted from the product tray of the second distillation column 232 contained 0.003% water by mass or less, 0.5 ppm amines or less, and 99.98% NMP by mass or more. The NMP recovery rate was 99%. Thus, in Example 2, impurities such as amines and water were appropriately separated, and a purified composition with a very high concentration of NMP was produced. Furthermore, the pH of a mixture of the purified composition withdrawn from the product tray of the second distillation column 232 and water, mixed at a mass ratio of 1:1, was 7.6. The purified composition after 90 days of sealed storage at room temperature was colorless and transparent, with an APHA reading of <5 (less than 5) on the colorimeter. In Example 2, compared to Example 1, a large amount of water remained in the bottoms (dehydrated feedstock) of the first distillation column 222, and the amount of water withdrawn from the condenser 224 and the reflux amount could be reduced, significantly reducing the heat load on the reboiler 223. Unlike Example 1, Example 2 requires energy to vaporize water because the main component of the distillate vapor from the second distillation column 232 is water. However, this energy also served as energy for purifying NMP. In addition, Example 2 differs from the conventional Petriuk system in that reflux liquid is returned to second auxiliary distillation column 231 from the product stage rather than from the feed stage of second distillation column 232 (vapor is returned via pipe t2b rather than pipe t2a), thereby making it possible to fully utilize the energy-saving effects of the conventional Petriuk system while avoiding large composition fluctuations. As a result, Example 2 achieved a reduction in energy costs by approximately 50% compared to Example 1.

[0107] Comparative Example 4 (Comparative Example 4: Target Feedstock) The target feedstock for Comparative Example 4 was the same as that for Example 1. (Comparative Example 4: Test Conditions) The test conditions for Comparative Example 4 were basically the same as those for Example 2, but differed in the following respects. Unlike Example 2, Comparative Example 4 was configured to return the reflux liquid from the feed stage of second distillation column 232 to second auxiliary distillation column 231 (a configuration in which the reflux liquid is returned via pipe t2a: a conventional Petriuk system configuration).

[0108] (Comparative Example 4: Test Results) In Comparative Example 4, the reflux liquid was returned to the second auxiliary distillation column 231 from the feed tray of the second distillation column 232, and therefore the reflux liquid contained a relatively large amount of water. As a result, the impact of composition fluctuations was significant, and not only was the energy-saving effect of the conventional Petriuk system not fully realized, but the throughput of the extracted purified composition was 10% less than the amount previously estimated. Therefore, even considering only the reduction in throughput, the heat quantity per unit throughput of Comparative Example 4 increased by 11% compared to Example 2.

[0109] 100, 200, 300, 400 Purification system 110, 210, 310 Impurity adsorption means 120, 220, 320, 420 First distillation means 130, 230, 330, 430 Second distillation means 140, 240, 340, 440 Alkali addition means 150, 250, 350, 450 Alkali supply means

Claims

1. A method for purifying a raw material containing N-methyl-2-pyrrolidone and water, and also containing a low-boiling amine having a boiling point lower than that of N-methyl-2-pyrrolidone and methylsuccinimide as impurities, comprising: a first distillation step of distilling the raw material in a first distillation column to separate the dehydrated raw material and vapor containing the low-boiling amine; a second distillation step of distilling the dehydrated raw material in a second distillation column to separate the dehydrated raw material into a purified composition in which N-methyl-2-pyrrolidone is concentrated, vapor containing the low-boiling amine, and a solution containing methylsuccinimide; a pH measurement step of extracting a part of the liquid inside the column in at least one of the first distillation step and the second distillation step as a pH measurement liquid, mixing the pH measurement liquid and water in a mass ratio of the two (= mass of pH measurement liquid / mass of water) of 0.5 to 20, and then measuring the pH of the mixed liquid; and a calculation step of calculating the amount of an alkali compound to be added based on the pH result of the pH measurement step. an alkali addition step of adding an alkali compound in an amount calculated in the calculation step to any one of the raw material before the first distillation step, the liquid in the column of the first distillation step, and the liquid in the column of the second distillation step, wherein the amount of water mixed is controlled so that the mass ratio of the two in the mixed liquid in the pH measurement step is a constant ratio within an error of ±5%.

2. The purification method according to claim 1, characterized in that in the calculation step, the amount of alkaline compound to be added is calculated by feedback control based on the difference between the pH result obtained in the pH measurement step and the target value.

3. The mass fraction of the amine in the raw material is 1×10 －６ and an impurity adsorption step of contacting the raw material with an acidic or amphoteric ion exchange resin prior to the first distillation step to adsorb and separate 90 mass % or more of the amines originally contained in the raw material.

4. In the impurity adsorption step, the amount of solution passed through an ion exchange resin column filled with an acidic or amphoteric ion exchange resin is kept at SV for 5 to 30 h. －１ 4. The method according to claim 3, wherein the raw material is treated by passing it through a purifying solution.

5. A purification method as described in claim 3, characterized in that a portion of the distillate containing low-boiling amines distilled from the top of the second distillation column in the second distillation step is passed through the ion exchange resin as the raw material in the impurity adsorption step.

6. The purification method according to claim 1 or 2, characterized in that in the second distillation step, vapor containing low boiling point amines is distilled from the top of the second distillation column, a solution containing methylsuccinimide is discharged from the bottom of the second distillation column, and a purified composition in which N-methyl-2-pyrrolidone is concentrated is extracted from a side cut nozzle of the second distillation column, and the temperature of the distillation target in the first distillation step and the second distillation step is controlled to be less than 160°C.

7. The mass fraction of the amine in the purified composition is 0.5×10 －６ 3. The purification method according to claim 1 or 2, wherein the concentration is less than 100%.

8. A purification method according to claim 1 or 2, characterized in that it further comprises a recovery step of recovering the raw material from a gas containing N-methyl-2-pyrrolidone prior to the first distillation step, and in that a portion of the distillate containing low-boiling amines distilled from the first distillation column in the first distillation step is used as recovery water for recovering the gas in the recovery step.

9. The distillate containing low boiling point amines used as the recovered water has a mass fraction of N-methyl-2-pyrrolidone of 0.001 x 10 －２ ~5×10 －２ The purification method according to claim 8, 10. The mass fraction of water in the purified composition is 200 x 10 -６ The mass fraction of N-methyl-2-pyrrolidone in the purified composition is less than 99.98×10 －２ or more, the pH of a mixture of the purified composition and water in a mass ratio of 1:1 is 7.2 to 8.0, and the mass fraction of methylsuccinimide in the purified composition is 10×10 -６ The purification method according to claim 1 or 2, characterized in that the purified composition has a colorimetric value of APHA 10 or less after 90 days of sealed storage at room temperature.

11. A purification method as described in claim 1 or 2, characterized in that in the second distillation step, distillation is performed in two distillation towers, namely, the second distillation tower and a second auxiliary distillation tower located upstream of the second distillation tower, and a portion of the purified composition extracted from the second distillation tower is returned to the second auxiliary distillation tower.

12. The purification method according to claim 1 or 2, characterized in that the second distillation column in the second distillation step is a vertically divided distillation column, the interior of the vertically divided distillation column except for the top of the column is divided vertically by a dividing plate, and the distillation target is heated by a reboiler in each of the two column bottoms separated by the dividing plate.

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