Purification methods

Abstract

A purification method for N-methyl-2-pyrrolidone is provided, which can appropriately separate and remove impurities and water. A purification method for purifying a feedstock containing N-methyl-2-pyrrolidone and water, and containing low-boiling-point amines and methylsuccinimide as impurities, the purification method comprising the following steps: a first distillation step, in which the feedstock is distilled in a first distillation column (122), causing vapors containing low-boiling-point amines to distill from the top of the column, and dehydrated feedstock to be taken from the bottom of the column; and a second distillation step, in which the dehydrated feedstock is distilled in a second distillation column (131), causing vapors containing low-boiling-point amines to distill from the top of the column, causing liquid containing methylsuccinimide to be discharged from the bottom of the column, and a purified composition concentrated with N-methyl-2-pyrrolidone is taken from a side fraction nozzle, wherein the temperature of the distillate in the first and second distillation steps is controlled to be below 160°C.

Description

Technical Field

[0001] This invention relates to a purification method for purifying raw materials containing N-methyl-2-pyrrolidone, impurities, and water into high-purity N-methyl-2-pyrrolidone. Background Technology

[0002] N-methyl-2-pyrrolidone (appropriately denoted 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 in positive electrode pastes containing polyvinylidene fluoride as a binder, positive electrode active materials, etc.

[0003] During the drying process of the positive electrode paste applied to the substrate, NMP evaporates and is removed from the surface of the substrate. The evaporated NMP is then recovered by a gas recovery unit and reused. Furthermore, NMP is recovered at a high rate; therefore, the amount of NMP used in the secondary battery manufacturing process is overwhelmingly more than the amount of recycled NMP.

[0004] However, in the gas recovery unit, volatile additives used in the manufacture of the positive electrode paste and moisture contained in the ambient air are recovered along with NMP. Furthermore, amines are sometimes used as reactants in the manufacturing process of NMP itself, and are also sometimes used as additives in the manufacturing process of secondary batteries. Therefore, NMP often contains amines as impurities. The types of amines used as additives are numerous, and their boiling points vary widely. Moreover, NMP is a highly reactive substance; upon exposure to air or heating, it can sometimes produce methylsuccinimide, or further produce amines as decomposition products.

[0005] If amines are present as impurities in NMP, they can sometimes adversely affect polyvinylidene fluoride (PVDF) used as a binder in the manufacturing process of secondary batteries. Furthermore, when methylsuccinimide is present as an impurity in NMP, it can sometimes remain on the surface of the substrate during the drying process of the positive electrode paste for secondary batteries.

[0006] As such, impurities and water in NMP become the main causes of various problems. Therefore, various technologies for removing impurities and water from NMP have been proposed to date.

[0007] For example, Patent Document 1 discloses a method for distilling an N-methyl-2-pyrrolidone solution, characterized by adding an alkali metal or alkaline earth metal hydroxide and distilling the N-methyl-2-pyrrolidone solution.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2004-284958 Summary of the Invention

[0011] The problem the invention aims to solve

[0012] Patent Document 1 describes how the generation of peroxides can be suppressed by adding hydroxides of alkali metals, etc., to NMP.

[0013] However, in the prior art, represented by the technology described in Patent Document 1, it cannot be said that the removal of impurities and water from NMP is sufficient; moreover, the required level in this regard has been continuously increasing. Furthermore, it is desirable that even trace amounts of amines, as impurities, do not remain in NMP, and it is desirable to remove as much amine as possible generated during the purification process.

[0014] Therefore, the objective of this invention is to provide a purification method for N-methyl-2-pyrrolidone that can appropriately separate and remove impurities and water.

[0015] Solution for solving the problem

[0016] As a solution to the aforementioned problems, the purification method of the present invention is characterized by purifying a raw material containing N-methyl-2-pyrrolidone and water, and containing a low-boiling-point amine and methylsuccinimide as impurities with a boiling point lower than that of N-methyl-2-pyrrolidone. The method includes the following steps: a first distillation step, in which the raw material is distilled in a first distillation column, vapor containing the low-boiling-point amine is distilled from the top of the first distillation column, and dehydrated raw material is discharged from the bottom of the first distillation column; and a second distillation step, in which the dehydrated raw material is distilled in a second distillation column, vapor containing the low-boiling-point amine is distilled from the top of the second distillation column, liquid containing methylsuccinimide is discharged from the bottom of the second distillation column, and a purified composition concentrated with N-methyl-2-pyrrolidone is taken out from a side distillation nozzle of the second distillation column. The temperature of the distillate in the first and second distillation steps is controlled to be below 160°C, and the mass fraction of the amine in the purified composition taken out in the second distillation step is less than 0.5 × 10⁻⁶. -6 The mass fraction of water is less than 200 × 10⁻⁶ -6 The mass ratio of N-methyl-2-pyrrolidone in the purified composition to the mass of N-methyl-2-pyrrolidone in the raw material, i.e., the recovery rate, is 95% or higher.

[0017] In the first distillation step, it is preferable to carry out distillation in two distillation columns: the first distillation column and a first auxiliary distillation column located upstream of the first distillation column, so that the mass fraction of water in the dehydrated feedstock is 1×10⁻⁶. -2~5×10 -2 Distillation is performed in this manner.

[0018] Preferably, the mass fraction of N-methyl-2-pyrrolidone in the purified composition is 99.98 × 10⁻⁶. -2 The pH of the mixture prepared by mixing the purified composition with water at a mass ratio of 1:1 is 7.2-8.0, and the mass fraction of methylsuccinimide in the purified composition is 10 × 10⁻⁶. -6 The following values ​​for the colorimetric tubes of the purified composition after 90 days of sealed storage at room temperature are below APHA10.

[0019] In the second distillation step, it is preferable to carry out distillation in both the second distillation column and a second auxiliary distillation column located upstream of the second distillation column, so that a portion of the purified composition taken from the second distillation column is returned to the second auxiliary distillation column.

[0020] Preferably, the second distillation column in the second distillation process is a vertically divided distillation column. The interior of the vertically divided distillation column, except for the top, is divided vertically by dividing plates. At the bottom of the two columns separated by the dividing plates, the distillate is heated by reboilers.

[0021] Preferably, the mass fraction of amine in the raw material is 1×10⁻⁶. -6 The above includes an impurity adsorption step before the first distillation step, wherein the liquid flow rate (SV) in the impurity adsorption step is set to 5-30 h. -1 The raw material is passed through an ion exchange resin column filled with acidic or amphoteric ion exchange resin, so that more than 90% by mass of the amine originally contained in the raw material is adsorbed and separated.

[0022] Preferably, a portion of the distillate containing low-boiling-point amines distilled from the top of the second distillation column in the second distillation step is used as the raw material in the impurity adsorption step and is passed through the ion exchange resin column.

[0023] Preferably, the process includes the following steps: a pH measurement step, in which a portion of the liquid in at least one of the first distillation step and the second distillation step is taken out as a pH measurement solution, and the pH measurement solution is mixed with water in a mass ratio of 0.5 to 20 (= mass of pH measurement solution / mass of water), and the pH of the resulting mixture is measured; a calculation step, in which the amount of 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 in the first distillation step, and the liquid in the second distillation step, and the amount of water mixed is controlled so that the mass ratio of the two components in the mixture in the pH measurement step is a constant ratio with an error within ±5%.

[0024] Preferably, the process includes a recovery step prior to the first distillation step to recover the raw material from the gas containing N-methyl-2-pyrrolidone, and a portion of the distillate containing low-boiling-point amine distilled from the first distillation column of the first distillation step is used as recovered water for recovering the gas in the recovery step.

[0025] Preferably, the distillate containing low-boiling-point amines used as the recycled water contains 0.001 × 10⁻⁶ N-methyl-2-pyrrolidone by mass. -2 ~5×10 -2 .

[0026] The effects of the invention

[0027] According to the present invention, impurities and water can be appropriately separated and removed during the purification of N-methyl-2-pyrrolidone. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the purification system according to the first embodiment.

[0029] Figure 2 This is a schematic diagram of the purification system according to the second embodiment.

[0030] Figure 3 This is a schematic diagram of the purification system according to the third embodiment.

[0031] Figure 4 This is a schematic diagram of the purification system according to the fourth embodiment. Detailed Implementation

[0032] Hereinafter, the purification systems and purification methods of the first to fourth embodiments will be described with reference to the accompanying drawings.

[0033] Furthermore, when describing the second to fourth embodiments, descriptions of structures common to the already described embodiments are appropriately omitted, and the descriptions focus on the different structures.

[0034] [First Implementation Method: Purification System]

[0035] First, refer to Figure 1 The purification system 100 of the first embodiment will be described.

[0036] The purification system 100 of the first embodiment includes an impurity adsorption unit 110, a first distillation unit 120, a second distillation unit 130, a pH measurement unit 140, an alkali addition unit 150, and a control unit (not shown).

[0037] The units constituting the purification system 100 of the first embodiment are described below.

[0038] (Impurity adsorption unit)

[0039] The impurity adsorption unit 110 is a unit that adsorbs and separates impurities contained in the raw material.

[0040] Furthermore, the impurity adsorption unit 110 is composed of a first ion exchange resin column 111 and a second ion exchange resin column 112.

[0041] Regarding the first ion exchange resin column 111 and the second ion exchange resin column 112, by passing the raw material through the column, in other words, by bringing the raw material into contact with the ion exchange resin inside the column, impurities such as amines contained in the raw material can be adsorbed onto the ion exchange resin and separated from the raw material.

[0042] Furthermore, the first ion exchange resin column 111 and the second ion exchange resin column 112 are connected in series in the piping t1~t5 between the raw material tank T11 and the buffer tank T12.

[0043] In addition, the structure of the two ion exchange resin columns adopts a so-called carousel method. In detail, the adsorption and separation of impurities are carried out by repeatedly performing the following operations: (1) passing liquid through the first ion exchange resin column 111 and the second ion exchange resin column 112 in sequence → (2) passing liquid through the second ion exchange resin column 112 only when the first ion exchange resin column 111 is replaced → (3) passing liquid through the second ion exchange resin column 112 and the first ion exchange resin column 111 in sequence after the first ion exchange resin column 111 is replaced → (4) passing liquid through the first ion exchange resin column 111 only when the second ion exchange resin column 112 is replaced → (1) ...

[0044] By adopting this configuration, it is possible to replace the ion exchange resin and other components inside the column without stopping the adsorption and separation process.

[0045] It should be noted that the ion exchange resins placed inside the first ion exchange resin column 111 and the second ion exchange resin column 112 can be any known resins capable of adsorbing impurities such as amines, for example, acidic or amphoteric ion exchange resins.

[0046] (First distillation unit)

[0047] The first distillation unit 120 is a unit that performs distillation processing on the raw materials. Moreover, the first distillation unit 120 includes a first auxiliary distillation column 121 disposed on the upstream side and a first distillation column 122 disposed on the downstream side, and is configured to include a reboiler 123 attached to the first auxiliary distillation column 121 and a reboiler 124 and a condenser 125 attached to the first distillation column 122.

[0048] The first auxiliary distillation column 121 is a plate distillation column. The raw material stored in the 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.

[0049] The reboiler 123 is a device that exchanges heat between the feed supplied via pipe t7, the bottom liquid supplied via pipe t9, and the vapor s, and supplies the heated vapor to the first auxiliary distillation column 121 via pipe t8. The vapor s used in the reboiler 123 is then discharged as wastewater d. Additionally, a large amount of liquid containing unvaporized non-volatile components (such as alkali compounds described later) accumulates in the liquid holding section (bottom of the column) of the reboiler 123 and is discharged as waste liquid via pipe t10.

[0050] The first distillation column 122 is a plate distillation column. The distillate vapor from the first auxiliary distillation column 121 is supplied to the feed section via pipe t11. Vapor containing a large amount of water is distilled from the top of the column, the dehydrated raw material is discharged from the bottom of the column, and liquid containing a large amount of impurities (such as medium-boiling amines, described later) with a boiling point lower than NMP but higher than low-boiling amines is discharged from the discharge section equipped with side fraction nozzles. In addition, the distillate vapor from the first distillation column 122 is supplied to the condenser 125 via pipe t12, the bottom discharge liquid of the first distillation column 122 is supplied to the reboiler 124 via pipe t13, and the liquid discharged from the side fraction nozzles is supplied to the upstream of the impurity adsorption unit 110 via pipes t14 and t33 (pipe t14 and pipe t33 are connected).

[0051] There are no particular restrictions on the supply section of the first distillation column 122. It is usually a section that is lower than the discharge section with side distillation nozzles and higher than the bottom section. For example, when the total number of sections of the first distillation column 122 is set to X, it is a section that falls within 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, there are no particular restrictions on the delivery section of the first distillation column 122 that is equipped with side fraction nozzles. For example, when the total number of stages of the first distillation column 122 is set to X, the delivery section is set in the range of X×1 / 3 to X×2 / 3 from the top, 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 top (however, the delivery section is usually the section above the supply section).

[0052] Including the foregoing description, the number of sections of the distillation column described in this specification is a theoretical number of sections. However, it is extremely common for those skilled in the art to understand that the actual number of sections should correspond to the performance of the packing material, the trays, and the operating conditions, or, in the case of packing material, the filling height. Such actual number of sections or filling height is also included within the scope of this technology.

[0053] It should be noted that the pressure inside the first distillation column 122 can be 5 to 15 kPa using an absolute pressure gauge.

[0054] Reboiler 124 is a device with the same structure as reboiler 123. It exchanges heat between the bottom liquid supplied via pipe t13 and the vapor s, and supplies heated vapor to the bottom of the first distillation column 122 via pipe t17.

[0055] Additionally, the dehydrated raw material accumulates in the liquid holding section (bottom of the column) of reboiler 124 and is sent out as dehydrated raw material via pipe t18. Furthermore, most of the dehydrated raw material is supplied to reboiler 132 of the second distillation unit 130 via pipes t18 and t19, and a portion is supplied to pH measuring unit 140 via pipes t18 and t20.

[0056] Condenser 125 is a device that uses cooler c to cool and liquefy the distillate vapor from first distillation column 122. Furthermore, a portion of the liquid condensed in condenser 125 is returned to the top of first distillation column 122 via pipe t15 in the form of reflux, and the remainder is discharged as waste liquid containing a large amount of water via pipe t16.

[0057] (Second distillation unit)

[0058] The second distillation unit 130 is a unit that performs distillation treatment on the dehydrated raw material that has been dehydrated by the first distillation unit 120. Furthermore, the second distillation unit 130 includes a second distillation column 131 and includes a reboiler 132, a condenser 133 and a cooler 134 attached to the second distillation column 131.

[0059] The second distillation column 131 is a plate distillation column. After dehydration by the first distillation unit 120, the feedstock is fed to the bottom of the column via pipe t19, reboiler 132, and pipe t21. Vapor containing a large amount of impurities with boiling points lower than NMP (e.g., low-boiling amines) is distilled from the top of the column, and liquid containing a large amount of impurities with boiling points higher than NMP (e.g., methylsuccinimide) is discharged from the bottom of the column. Liquid concentrated with NMP is sent from the product section. Furthermore, the distillate vapor from the second distillation column 131 is fed to the condenser 133 via pipe t22, the bottom liquid from the second distillation column 131 is fed to the reboiler 132 via pipe t23, and the high-purity NMP from the product section of the second distillation column 131 is fed to the cooler 134 via pipe t24.

[0060] There are no particular restrictions on the product section of the second distillation column 131. For example, when the total number of sections of the second distillation column 131 is set to 10 or more, it is within the range of sections that are lower than one section from the top and higher than five sections from the bottom. More specifically, for example, when the total number of sections is 10, it is within the range of sections 2 to 6 from the top. For example, when the total number of sections is 20, it is within the range of sections 3 to 10 from the top.

[0061] It should be noted that the pressure inside the second distillation column 131 can be, for example, 4 kPa to 10 kPa using an absolute pressure gauge.

[0062] Reboiler 132 is a device with the same structure as reboiler 123. It is used for heat exchange between the dehydrated feed supplied via pipe t19, the bottom liquid supplied via pipe t23, and the vapor s. The heated vapor is then supplied to the bottom of the second distillation column 131 via pipe t21. In addition, liquid containing a large amount of impurities such as methylsuccinimide accumulates in the liquid holding section (bottom of the column) of reboiler 132 and is returned to the buffer tank T12 via pipe t25.

[0063] Condenser 133 is a device with the same structure as condenser 125, which uses cooler c to cool and liquefy the distillate vapor from second distillation column 131. Furthermore, a portion of the liquid condensed in condenser 133 is returned to the top of second distillation column 131 via pipe t26 in the form of reflux, and the remainder is supplied to the upstream of impurity adsorption unit 110 via pipes t27 and t33.

[0064] Cooler 134 is a device that uses cooler c to cool and liquefy the vapor delivered from the product section of the second distillation column 131 via pipe t24. Furthermore, the liquid liquefied in cooler 134 is taken out as a purified composition (product) via pipe t28.

[0065] (pH measurement unit)

[0066] pH measuring unit 140 is a unit for measuring the pH of a mixture of dehydrated raw material (pH measuring solution) supplied via pipes t18 and t20 and pure water W supplied via pipe t29.

[0067] It should be noted that the pH measurement unit 140 can use a commercially available online pH meter.

[0068] (Alkali Addition Unit)

[0069] The alkali addition unit 150 is a unit that adds alkali compounds stored in the alkali tank T13 to the raw materials in the buffer tank T12 via piping t30 and t31 based on the results of the pH measurement unit 140.

[0070] Alternatively, the alkali addition unit 150 may also be a structure that adds alkali compounds to the feedstock flowing in the pipes t6 and t7 located downstream of the buffer tank T12 and upstream of the reboiler 123 via pipes t30 and t32.

[0071] It should be noted that the alkali addition unit 150 can be a structure for continuously adding alkali compounds or a structure for intermittently adding them.

[0072] (Control unit)

[0073] The control unit (not shown) controls the supply of pH measuring solution and pure water in the pH measuring unit 140, or calculates the amount of alkali compound to be added using the pH result measured by the pH measuring unit 140, or controls the amount of alkali compound to be added from the alkali addition unit 150. Additionally, the control unit reads data from various measuring instruments (such as thermometers, pressure gauges, flow meters, etc.) located throughout the purification system 100, and controls flow rate, reflux ratio, etc., based on this data.

[0074] Furthermore, the control unit is implemented through program execution processing by the CPU (Central Processing Unit) and dedicated circuits. In addition, the storage unit of the control unit can be composed of general storage devices such as RAM (Random Access Memory), ROM (Read Only Memory), HDD (Hard Disk Drive), and flash memory.

[0075] It should be noted that the control method based on the control unit will be described in detail later.

[0076] (Other structures)

[0077] Pump P11 delivers raw materials from raw material tank T11 to impurity adsorption unit 110 via piping t1~t3.

[0078] Pump P12 delivers alkali compounds from alkali tank T13 via pipes t30, t31, and t32 to buffer tank T12 or via pipes t6 and t7 located downstream of buffer tank T12 and upstream of reboiler 123.

[0079] Pump P13 transports the dehydrated raw material from reboiler 124 to the second distillation unit 130 via pipes t18 and t19.

[0080] It should be noted that any pump capable of transporting liquid is acceptable; there are no particular limitations, and any known pump can be used. Furthermore, the pump's location is not limited to any particular location. Figure 1 The location of the pump can be adjusted, and the number of pumps can be increased or decreased as appropriate.

[0081] The purification system 100 may be equipped with valves in each piping as appropriate.

[0082] It should be noted that any valve can adjust the liquid flow rate by adjusting the level of opening and closing (the degree to which the flow path is open / closed), and there are no special restrictions; any known valve can be used.

[0083] Although Figure 1 The purification system 100 is not shown, but it can be appropriately set up in situations where multiple liquids and gases are mixed in the piping, such as at the point where multiple piping flows together. Figure 2 The pipeline mixer M is shown in the diagram.

[0084] [As raw material for the object]

[0085] Next, the raw materials that are the object of the purification method in the first embodiment will be described.

[0086] The raw material in question is a liquid obtained by recovering NMP used in the manufacturing process of secondary batteries using a gas recovery device or similar equipment. Therefore, the raw material contains not only NMP, but also water and impurities.

[0087] Impurities include amines with boiling points lower than NMP (amines with boiling points below 100°C at room temperature: appropriately called "low-boiling-point amines"), methylsuccinimide (an NMP oxidation source with a boiling point above 230°C at room temperature), etc. Moreover, there is no particular limitation on low-boiling-point amines; methylamine can be listed as an example.

[0088] It should be noted that impurities are not specifically limited to the components mentioned above, as long as they are unavoidably present during the reuse of NMP. For example, amines with a boiling point above 100°C and below 200°C at room temperature (appropriately referred to as "medium-boiling amines") can be listed. Moreover, there is no particular limitation on medium-boiling amines, and examples include 2-amino-2-methyl-1-propanol and monoethanolamine.

[0089] There are no specific limits on the content of each component in the raw materials; for example, the mass fraction of water is 5 × 10⁻⁶. -2 ~50×10 -2 The mass fraction of low-boiling-point amines is 1×10⁻⁶. -6 ~1000×10 -6 The mass fraction of methylsuccinimide is 10 × 10⁻⁶. -6 ~10000×10 -6 The total mass fraction of impurities is 0.05 × 10⁻⁶. -2 ~1×10 -2 .

[0090] It should be noted that the "mass fraction" in this specification refers to a value calculated as "mass of the object / total mass". For example, the mass fraction of methyl succinimide in the raw material is 10 × 10⁻⁶. -6 This is synonymous with the content of methyl succinimide in the raw material being 10 ppm (mg / kg).

[0091] [First Implementation Method: Purification Method]

[0092] Next, refer to Figure 1 The purification method of the first embodiment will be described.

[0093] The purification method of 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. Additionally, the purification method of the first embodiment may include a pH measurement step, a calculation step, and an alkali addition step.

[0094] The purification method of the first embodiment comprises the following steps.

[0095] (Impurity adsorption process)

[0096] The impurity adsorption process refers to the process of adsorbing and separating impurities contained in the raw materials.

[0097] In detail, during the impurity adsorption process, the raw material stored in the raw material tank T11 is passed through the first ion exchange resin column 111 and the 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 the buffer tank T12.

[0098] Although the impurity adsorption process is not mandatory, the amine mass fraction in the raw material is 1×10⁻⁶. -6 In the above cases, it is preferable to separate as much amine as possible from the raw material by setting up this process. Furthermore, in the impurity adsorption process, it is preferable to keep the flow rate SV of the raw material in the first ion exchange resin column 111 and the second ion exchange resin column 112 at 5~30 h. -1 Furthermore, in the impurity adsorption process, it is preferable to adsorb and separate at least 90% by mass of the amine originally contained in the raw material, and more preferably at least 99% by mass.

[0099] It should be noted that in this specification, the part referred to as "amine" refers to all amines (compounds formed by replacing the hydrogen atoms of ammonia with hydrocarbon groups). Specifically, it is a concept that combines low-boiling-point amines (methylamine) and medium-boiling-point amines (2-amino-2-methyl-1-propanol, monoethanolamine).

[0100] (First distillation process)

[0101] The first distillation process is a process of distilling raw materials in the first distillation unit 120 (specifically, the first auxiliary distillation column 121 and the first distillation column 122) to obtain dehydrated raw materials.

[0102] Specifically, in the first distillation step, the raw material is heated to produce steam via reboiler 123, and this steam is supplied to the first auxiliary distillation column 121. Then, the raw material supplied to the first auxiliary distillation column 121 is distilled, with steam rich in low-boiling-point components distilled from the top and liquid rich in high-boiling-point components discharged from the bottom. It should be noted that while the bottom liquid of the first auxiliary distillation column 121 is heated in reboiler 123, a large amount of non-volatile components, such as alkali compounds (described later), accumulates in the liquid holding section (bottom) of reboiler 123, and this liquid is discharged as waste liquid.

[0103] Next, the distillate vapor from the first auxiliary distillation column 121 is supplied to the feed section of the first distillation column 122. Then, the feedstock (distillate vapor) supplied to the first distillation column 122 is distilled, causing vapor rich in low-boiling-point components (vapor containing impurities such as low-boiling-point amines and water) to distill from the top of the column, while the dehydrated feedstock is discharged from the bottom. Vapor containing impurities such as medium-boiling-point amines is discharged from the side fraction nozzles located in the discharge section. It should be noted that the distillate vapor from the first distillation column 122 is condensed in the condenser 125, with a portion of it being refluxed back to the first distillation column 122.

[0104] (First distillation process: conditions)

[0105] In the first distillation process, the temperature of the distillate (raw material) in each distillation column is controlled below 160°C. By controlling the temperature of the distillate in this way, the modification and decomposition of NMP can be suppressed, and the amount of newly formed amines and high-boiling-point substances can be reduced.

[0106] It should be noted that the preferred water content in the dehydrated raw material is 5 × 10⁻⁶. -2 Hereinafter, 1×10 is more preferred. -2 ~5×10 -2 1×10 -2 ~3×10 -2 1×10 -2 ~2×10 -2 .

[0107] (Second distillation process)

[0108] The second distillation process refers to the process of distilling the dehydrated raw material in the second distillation unit 130 (specifically, the second distillation column 131) to obtain a purified composition concentrated with NMP.

[0109] Specifically, in the second distillation step, the dehydrated feedstock obtained in the first distillation step is heated in reboiler 132 to generate steam, and the dehydrated steam is fed to the bottom of the second distillation column 131. Then, the dehydrated feedstock fed to the second distillation column 131 is distilled, causing vapor containing low-boiling-point amines to distill from the top of the column, causing liquid containing methylsuccinimide to be discharged from the bottom of the column, and removing liquid concentrated with NMP from the product section. The liquid concentrated with NMP is then cooled using cooler 134 to produce a purified composition (product).

[0110] It should be noted that the distillate vapor from the second distillation column 131 contains components with boiling points lower than NMP. For example, it includes not only low-boiling-point amines and water that were not completely removed in the first distillation step, but also amines produced during the purification process due to the decomposition of NMP. Furthermore, the bottom liquid from the second distillation column 131 contains components with boiling points higher than NMP. For example, it includes not only the aforementioned methylsuccinimide, but also alkali compounds and non-volatile substances that were not completely removed in the first distillation step.

[0111] (Second distillation process: conditions)

[0112] In the second distillation step, similar to the first distillation step, the temperature of the distillate (raw material) in each distillation column is controlled to be below 160°C. In this way, by controlling the temperature of the distillate not only in the first distillation step but also in the second distillation step, the modification and decomposition of NMP can be suppressed more reliably, and the amount of newly generated amines and high-boiling-point substances can be minimized.

[0113] (Second distillation step: purification of the composition)

[0114] The mass fraction of amine in the purified composition obtained in the second distillation step is preferably less than 0.5 × 10⁻⁶. -6 More preferably less than 0.2×10 -6 It should be noted that, as mentioned above, "amine" here is not limited to either low-boiling-point amines or medium-boiling-point amines, but refers to all amines that may be present as impurities (compounds formed by replacing the hydrogen atoms of ammonia with hydrocarbon groups).

[0115] Furthermore, the water mass fraction of the purified composition obtained in the second distillation step is preferably less than 200 × 10⁻⁶. -6 More preferably 50×10 -6 the following.

[0116] In this way, according to the purification method of the first embodiment, it is possible to obtain a purified composition with the proportion of water and amine as impurities reduced as much as possible; in other words, it is possible to obtain a purified composition with NMP concentrated at a very high level.

[0117] Furthermore, the ratio of the mass of NMP in the purified composition to the mass of NMP in the raw material, i.e., the recovery rate (NMP recovery rate), is preferably 95% or more, more preferably 99% or more.

[0118] In this way, the purification method according to the first embodiment can recover NMP with a very high recovery rate.

[0119] The NMP mass fraction of the purified composition obtained in the second distillation step is preferably 99.98 × 10⁻⁶. -2 The above is preferred, with 99.99×10 being even better. -2 above.

[0120] Furthermore, in the purified composition obtained in the second distillation step, the mass fraction of methylsuccinimide is preferably 10 × 10⁻⁶. -6 The following is more preferably 5×10 -6 the following.

[0121] Furthermore, the pH of the mixture obtained from the second distillation process and water at a mass ratio of 1:1 is preferably 7.2 to 8.0.

[0122] In addition, the colorimetric value of the purified composition obtained in the second distillation process after being stored in a sealed container at room temperature for 90 days is preferably below APHA10, and more preferably below 5.

[0123] Thus, the purified composition obtained according to the purification method of the first embodiment can satisfy the above-mentioned technical features.

[0124] (Second distillation process: determination and calculation methods for the purified composition)

[0125] The mass fractions of NMP, amine, and methylsuccinimide in the purified composition can be determined, for example, by gas chromatography or ion chromatography. Additionally, the mass fraction of water in the purified composition can be determined, for example, using a Karl Fischer moisture meter.

[0126] It should be noted that the method for determining the mass fraction of each component in the raw material and the dehydrated raw material is the same as the method described above.

[0127] Regarding the recovery rate of NMP, firstly, the "mass of NMP contained in the total purified composition taken out" is calculated from the total amount of purified composition taken out and the concentration of NMP contained in the purified composition. Then, the "mass of NMP contained in the total raw material to be purified" is calculated from the total amount of raw material to be purified and the concentration of NMP contained in the raw material. Finally, the NMP recovery rate can be calculated using the formula: "mass of NMP contained in the total purified composition taken out / mass of NMP contained in the total raw material to be purified × 100".

[0128] The pH of the mixture can be measured using a commercially available pH meter.

[0129] The APHA value of the purified composition can be determined, for example, by visual inspection or by a UV-Vis spectrophotometer.

[0130] (pH measurement procedure)

[0131] The pH measurement process involves taking out a portion of the liquid from the column in the first distillation process as the pH measurement solution, mixing the pH measurement solution with water at a mass ratio of 1 to 40:2, or in other words, mixing them in a mass ratio of 0.5 to 20 (mass of pH measurement solution / mass of water), and then measuring the pH of the resulting mixture.

[0132] Specifically, in the pH measurement process, a portion of the dehydrated raw material (liquid inside the column) discharged from the first distillation column 122 is extracted as a pH measurement solution. This pH measurement solution is then mixed with water in a predetermined mass ratio (a pre-set constant mass ratio). The pH of the mixed solution is then measured by the pH measurement unit 140.

[0133] pH measurements performed by the pH measuring unit 140 can be performed continuously, or at predetermined time intervals (e.g., every 1 hour to 24 hours). Furthermore, the preparation of the mixture to be measured can also be performed continuously (by continuously supplying and mixing the pH measuring solution and water at predetermined amounts), or in conjunction with the aforementioned predetermined time intervals.

[0134] In addition, the pH measurement solution preferably uses a portion of the liquid in the column from the first distillation step, but a portion of the liquid in the column from the second distillation step can also be used, or both can be used.

[0135] (pH measurement procedure: mixing ratio)

[0136] In the pH measurement process, the pH measurement solution and water are preferably mixed in a mass ratio (mass of pH measurement solution / mass of water) of 0.5 to 20. Furthermore, it is preferable to control the mixing amount so that the mass ratio of the two in the mixture in the pH measurement process (a predetermined constant mass ratio) is within a constant range with an error within ±5%. By keeping the mass ratio within the specified range and the error of this mass ratio within the specified range, the appropriate amount of alkali compound to be added can be calculated in the calculation process described later.

[0137] Furthermore, "controlling the mixing volume of the pH measuring solution and the mixing volume of water so that the mass ratio of the two in the mixture during the pH measuring process is a constant ratio with an error within ±5%" specifically means consistently confirming, or confirming at specified time intervals (e.g., every 1 hour to 24 hours) the flow rate of the pH measuring solution in pipe t20 and the flow rate of water in pipe t29, and controlling the flow rates of both so that the mass ratio of the two is within a constant range. Additionally, the error of the aforementioned mass ratio being within ±5% means, for example, that when the mass ratio of the two (= mass of pH measuring solution / mass of water) is set to 2, the mixing volume of the two can be controlled in a manner that conforms to the range of 1.9 to 2.1 (= 2 × 0.95 to 2 × 1.05). It should be noted that the control of the mixing volume can be applied to both (pH measuring solution and water). For example, if the mixing volume of the pH measuring solution remains unchanged, it can be achieved by controlling only the mixing volume of water.

[0138] The mass ratio of pH measurement solution to water is not particularly limited as long as the above conditions are met. The supply rate of pH measurement solution to pH measurement unit 140 is, for example, 20~40 ml / min, typically about 30 ml / min, and the supply rate of water to pH measurement unit 140 is, for example, 5~15 ml / min, typically about 10 ml / min.

[0139] (Calculation process)

[0140] The calculation process is based on the pH result from the pH measurement process to determine the amount of alkali compound to be added.

[0141] In detail, the amount of alkali compound to be added is calculated through feedback control based on the difference between the pH result obtained in the pH measurement process and the target value.

[0142] It should be noted that the target value refers to the preferred pH value of the mixture, which is achieved by appropriately suppressing the generation of peroxides and other impurities such as new amines caused by the addition of alkali compounds. This value is set through prior experiments. Furthermore, the feedback control is specifically implemented using the well-known PID control (Proportional-Integral-Differential Controller).

[0143] (Alkali addition process)

[0144] The alkali addition process is the process of adding the amount of alkali compound calculated in the calculation process to the raw material or dehydrated raw material.

[0145] Furthermore, any alkaline compound can be a substance exhibiting alkalinity; specifically, hydroxides of alkali metals or alkaline earth metals are preferred, and potassium hydroxide is more preferred. Potassium hydroxide has the advantage of good solubility in raw materials, but it also has the disadvantage of high reactivity with NMP and the tendency to generate impurities such as amines when added in large quantities. However, according to the purification method of the first embodiment, the amount of potassium hydroxide added can be appropriately controlled, thus allowing the advantages of potassium hydroxide to be enjoyed while suppressing the formation of impurities such as amines.

[0146] It should be noted that there are no particular restrictions on the method of adding the alkali compound; it can be added in the form of an aqueous solution.

[0147] Furthermore, the calculation process and the alkali addition process can be carried out at a specified time interval, such as an interval of 1 hour to 24 hours (preferably 3 hours to 6 hours).

[0148] (Alkali addition process: Added object)

[0149] like Figure 1 As shown, in the purification method of the first embodiment, the alkali compound added in the alkali addition step is either the raw material stored in the buffer tank T12 or the raw material flowing between the buffer tank T12 and the distillation column 121.

[0150] It should be noted that the alkali compound added in the alkali addition step can be any of the raw material before the first distillation step, the liquid inside the column of the first distillation step, or the liquid inside the column of the second distillation step, preferably the raw material before the first distillation step. This is because the effect of suppressing peroxide generation through the addition of the alkali compound can be fully utilized, and the remaining alkali compound can be appropriately removed in the first distillation step (reboiler 123).

[0151] The purification method of the first embodiment is a so-called continuous process (a process in which raw materials are supplied at a constant flow rate and the purified composition is taken out at a constant flow rate). Therefore, in the purification method of the first embodiment, an impurity adsorption step, a first distillation step, and a second distillation step are performed on the raw materials, and a pH measurement step, a calculation step, and an alkali addition step are performed in parallel.

[0152] [Second Implementation Method: Purification System]

[0153] Next, refer to Figure 2 The purification system 200 of the second embodiment will be described.

[0154] The purification system 200 of the second embodiment includes an impurity adsorption unit 210, a first distillation unit 220, a second distillation unit 230, a pH measurement unit 240, an alkali addition unit 250, and a control device (not shown).

[0155] The units constituting the purification system 200 of the second embodiment are described below.

[0156] (Impurity adsorption unit)

[0157] The impurity adsorption unit 210 of the purification system 200 has a structure that is substantially the same as that of the purification system 100, so the description is omitted.

[0158] (First distillation unit)

[0159] The first distillation unit 220 includes a preheater 221 disposed on the upstream side and a first distillation column 222 disposed on the downstream side, and is configured to include a reboiler 223 and a condenser 224 attached to the first distillation column 222.

[0160] Preheater 221 is a device that uses the heat from the drain water d to heat the raw material. It should be noted that the drain water d used in preheater 221 can be the drain water d discharged from reboilers 223, 233, and 234.

[0161] The supply section of the first distillation column 222, which supplies the raw material heated by the preheater 221, is not particularly limited. For example, when the total number of sections of the first distillation column 222 is set to X, it is within 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.

[0162] It should be noted that the pressure inside the first distillation column 222 can be set to 5 to 101.3 kPa using an absolute pressure gauge.

[0163] (Second distillation unit)

[0164] The second distillation unit 230 includes a second auxiliary distillation column 231 disposed on the upstream side and a second distillation column 232 disposed on the downstream side, and is configured to include a reboiler 233 attached to the second auxiliary distillation column 231, a reboiler 234 attached to the second distillation column 232, a condenser 235 and a cooler 236.

[0165] Furthermore, although the second distillation unit 230 of the purification system 200 has a structure known as the so-called "modified Petlyuk method", it has been modified from the previous structure to be suitable for NMP purification.

[0166] The second auxiliary distillation column 231 is a packed distillation column with a regular packing. The dehydrated feedstock (specifically, a crudely dehydrated feedstock pre-dehydrated to approximately 1% to 5% by mass) from the first distillation unit 220 is fed to a height near the middle. Vapor, from which impurities with boiling points higher than NMP have been removed, is distilled from the top of the column, while liquid, from which impurities with boiling points lower than NMP have been removed, is discharged from the bottom. The distillate vapor from the second auxiliary distillation column 231 is then fed to the feed section of the second distillation column 232. Furthermore, a portion of the bottom liquid from the second auxiliary distillation column 231 is supplied to the bottom of the second distillation column 232 via pipe t1b. This is consistent with the existing Petrovich process (where...) Figure 2 The dotted line in the diagram shows a different piping (t1a). By configuring it in this way, the concentration of succinimide can be increased, and consequently, the product recovery rate can be improved.

[0167] It should be noted that the supply position of the second auxiliary distillation column 231 to the dehydrated raw material (near the aforementioned intermediate height) is only required to be between the upper packing layer 231a and the lower packing layer 231b.

[0168] In addition, the pressure inside the second auxiliary distillation column 231 can be, for example, 5 to 20 kPa using an absolute pressure gauge.

[0169] The second distillation column 232 is a plate distillation column, which is supplied with the distillate vapor and the bottom liquid of the second auxiliary distillation column 231. The vapor containing a large amount of impurities with a lower boiling point than NMP (such as low-boiling-point amines, residual small amounts of water, etc.) is distilled from the top of the column, and the liquid containing a large amount of impurities with a higher boiling point than NMP (such as methyl succinimide, etc.) is discharged from the bottom of the column. The vapor concentrated with NMP is sent out from the product section.

[0170] Then, a portion of the liquid withdrawn from the product section in the second distillation column 232 is returned to the top of the second auxiliary distillation column 231. In the conventional Petliuk configuration, the liquid and vapor are returned from the feed section of the second distillation column 232 to the second auxiliary distillation column 231. Figure 2 The pipe t2a shown by the dashed line in the diagram is common, but in the feed section, water and NMP are mixed. In actual distillation column operation, the concentrations of water and NMP vary slightly. Therefore, when the liquid / vapor returns from the feed stage to the second auxiliary distillation column 231, the latent heat difference between water and NMP is large, and thus, due to the effect of concentration changes, the heat that must be applied to the second auxiliary distillation column 231 also varies significantly. As a result, the distillation control in the purification system 200 becomes unstable. Therefore, by returning the almost anhydrous liquid from the product section instead of from the feed section of the second distillation column 232 to the second auxiliary distillation column 231, the effects of compositional changes can be suppressed, and the purification system 200 can be stabilized. Ultimately, the quality of the purified composition can be stabilized.

[0171] For example, when the total number of sections in the second distillation column 232 is X, the supply section of the second distillation column 232 (the section that supplies steam from the second auxiliary distillation column 231) is not particularly restricted and is a section ranging from X×1 / 4 to X×3 / 4 from the bottom, such as sections ranging from X×1 / 4, X×1 / 3, X×1 / 2, X×2 / 3, and X×3 / 4 from the bottom. Furthermore, for example, when the total number of sections below the aforementioned supply section of the second distillation column 232 is X, the product section of the second distillation column 232 is a section ranging from X×1 / 5 to X×4 / 5 downwards from the aforementioned supply section, such as sections ranging from X×1 / 5, X×2 / 5, X×1 / 2, X×3 / 5, and X×4 / 5 downwards.

[0172] It should be noted that the pressure inside the second distillation column 232 can be, for example, 10 kPa to 40 kPa using an absolute pressure gauge.

[0173] (pH measurement unit, alkali addition unit, control unit)

[0174] The pH measurement unit 240, alkali addition unit 250, and control unit (not shown) of the purification system 200 have a structure that is substantially the same as that of the purification system 100, so descriptions are omitted.

[0175] [Second Implementation Method: Purification Method]

[0176] Next, refer to Figure 2 The purification method of the second embodiment will be described.

[0177] The purification method of the second embodiment is described below.

[0178] (Impurity adsorption process)

[0179] The impurity adsorption step in the purification method of the second embodiment is the same as that in the purification method of the first embodiment, so the description is omitted.

[0180] (First distillation process)

[0181] The first distillation process is a process of obtaining dehydrated raw materials by distilling the raw materials using the first distillation unit 220 (specifically, the first distillation column 222).

[0182] Specifically, in the first distillation step, the feedstock is heated using a preheater 221. Next, the preheated feedstock is fed to the feed section of the first distillation column 222. Then, the feedstock supplied to the first distillation column 222 is distilled, causing vapor rich in low-boiling-point components (vapor containing impurities such as low-boiling-point amines and water) to distill from the top of the column, while the dehydrated feedstock (specifically, crudely dehydrated feedstock with a water content of approximately 5% by mass) is discharged from the bottom of the column. Furthermore, the distillate vapor from the first distillation column 222 is condensed in a condenser 224, with a portion of it being refluxed back to the first distillation column 222.

[0183] Furthermore, in the first distillation step, distillation is performed using the first distillation column 222, reducing the moisture content of the crudely dehydrated feedstock to approximately 5% by mass (1-5% by mass). By pre-setting the moisture content of the crudely dehydrated feedstock obtained in the first distillation step to below a specified value, the reflux ratio in the subsequent second distillation step can be reduced, thereby achieving energy savings as a whole.

[0184] It should be noted that energy saving can be achieved by preheating the raw materials using the preheater 221 in the first distillation process.

[0185] (Second distillation process)

[0186] The second distillation process involves distilling the crudely dehydrated raw material using a second distillation unit 230 (more specifically, a second auxiliary distillation column 231 and a second distillation column 232) to obtain a purified composition concentrated with NMP.

[0187] Specifically, in the second distillation step, the crude dehydrated feedstock obtained in the first distillation step is fed to the second auxiliary distillation column 231. Then, the crude dehydrated feedstock fed to the second auxiliary distillation column 231 is distilled, causing vapor with impurities having a boiling point higher than NMP removed to distill from the top of the column, and liquid with impurities having a boiling point lower than NMP removed to be discharged from the bottom of the column. The distillate vapor is then fed to the feed section of the second distillation column 232, and the bottom discharge is fed to the bottom of the second distillation column 232.

[0188] Next, the distillate vapor and bottom liquid supplied to the second distillation column 232 are distilled, so that the vapor containing low-boiling-point amines is distilled from the top of the column, and the liquid containing methylsuccinimide is discharged from the bottom of the column. The liquid concentrated with NMP is then taken out from the product section. Then, the liquid concentrated with NMP is cooled by cooler 236 to prepare a purified composition (product).

[0189] In addition, a portion of the liquid concentrated with NMP is returned from the product section of the second distillation column 232 to the second auxiliary distillation column 231.

[0190] (pH measurement process, calculation process, alkali addition process)

[0191] The pH measurement, calculation, and alkali addition steps of the purification method in the second embodiment are the same as those in the purification method of the first embodiment, so the description is omitted.

[0192] [Third Implementation Method: Purification System]

[0193] Next, refer to Figure 3 The purification system 300 of the third embodiment will be described.

[0194] The purification system 300 of the third embodiment includes an impurity adsorption unit 310, a first distillation unit 320, a second distillation unit 330, a pH measurement unit 340, an alkali addition unit 350, and a control unit (not shown).

[0195] The units constituting the purification system 300 of the third embodiment are described below.

[0196] (Impurity adsorption unit, first distillation unit)

[0197] The impurity adsorption unit 310 and the first distillation unit 320 of the purification system 300 have a structure that is substantially the same as that of the purification system 200, so the description is omitted.

[0198] (Second distillation unit)

[0199] The second distillation unit 330 includes a second distillation column 331, and is configured to include reboilers 332 and 333, a condenser 334, and a cooler 335 attached to the second distillation column 331.

[0200] The second distillation column 331 is a plate distillation column and a vertically divided distillation column. Furthermore, in the second distillation column 331, the interior of the column, except for the top, is vertically divided by dividing plates 331a into an upstream distillation section 331b and a downstream distillation section 331c. Moreover, at the bottom of the upstream distillation section 331b, the distillate is heated by a reboiler 332, and at the bottom of the downstream distillation section 331c, the distillate is heated by a reboiler 333.

[0201] Additionally, the supply unit 331d is located inside the second distillation column 331 and near the top of the column. This supply unit 331d is a structure that supplies a portion of the distillate from the second distillation column 331, which is condensed in the condenser 334, to both the upstream distillation section 331b and the downstream distillation section 331c.

[0202] For example, in the total number of sections of the upstream distillation section 331b ( Figure 3 When the total number of sections (located on the left side of the dividing plate 331a) is set to X, the supply section of the second distillation column 331, which is supplied with the crude dehydrated raw material, is not particularly limited. For example, it can be a section ranging from 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, or X×2 / 3 from the bottom. Furthermore, for example, when the total number of sections (located on the left side of the dividing plate 331a) is set to X, the supply section of the second distillation column 331 is not particularly limited. Figure 3 When the total number of segments (the segments located on the right side of the dividing plate 331a) is set to X, the product segments of the second distillation column 331 are not particularly limited. For example, they may be segments ranging from X×1 / 5 to X×4 / 5 from the bottom, such as segments of X×1 / 5, X×2 / 5, X×1 / 2, X×3 / 5 and X×4 / 5 from the bottom.

[0203] It should be noted that the pressure inside the second distillation column 331 can be, for example, 10 kPa to 40 kPa using an absolute pressure gauge.

[0204] (pH measurement unit, alkali addition unit, control unit)

[0205] The pH measurement unit 340, alkali addition unit 350, and control unit (not shown) of the purification system 300 have roughly the same structure as those of the purification systems 100 and 200, so descriptions are omitted.

[0206] [Third Implementation Method: Purification Method]

[0207] Next, refer to Figure 3 The purification method of the third embodiment will be described.

[0208] The purification method of the third embodiment is described below.

[0209] (Impurity adsorption process, first distillation process)

[0210] The impurity adsorption step and the first distillation step of the purification method in the third embodiment are the same as those in the purification method in the second embodiment, so the description is omitted.

[0211] (Second distillation process)

[0212] The second distillation process refers to the process of distilling the crudely dehydrated raw material using the second distillation unit 330 (more specifically, the second distillation column 331) to obtain a purified composition concentrated with NMP.

[0213] Specifically, in the second distillation step, the crude dehydrated feedstock obtained in the first distillation step is fed into the second distillation column 331. Then, the crude dehydrated feedstock fed into the second distillation column 331 is distilled, causing vapor containing low-boiling-point amines and a small amount of residual water to distill off from the top of the column, while liquid containing methylsuccinimide is discharged from the bottom. Liquid concentrated with NMP is then removed from the product section. The liquid concentrated with NMP is then cooled using cooler 335 to produce a purified composition (product).

[0214] (pH measurement process, calculation process, alkali addition process)

[0215] The pH measurement, calculation, and alkali addition steps of the purification method in the third embodiment are the same as those in the purification methods of the first and second embodiments, so the description is omitted.

[0216] [Fourth Implementation Method: Purification System]

[0217] Next, refer to Figure 4 The purification system 400 of the fourth embodiment will be described.

[0218] The purification system 400 of the fourth embodiment includes a first distillation unit 420, a second distillation unit 430, a pH measurement unit 440, an alkali addition unit 450, and a control device (not shown).

[0219] The units constituting the purification system 400 of the fourth embodiment are described below.

[0220] (Impurity adsorption unit)

[0221] Purification system 400 differs from purification systems 100, 200, and 300 in that it does not have an impurity adsorption unit.

[0222] When the mass fraction of amine in the raw material is low (e.g., less than 1×10⁻⁶), -6 In the case of purification system 400, a configuration without an impurity adsorption unit can also be adopted.

[0223] (First distillation unit)

[0224] Unlike purification system 100, the first distillation unit 420 of purification system 400 is a configuration that does not include the first auxiliary distillation column 121.

[0225] (Second distillation unit, pH measurement unit, and control device)

[0226] The second distillation unit 430, pH measurement unit 440, and control unit (not shown) of the purification system 400 have a structure that is substantially the same as that of the purification system 100, so descriptions are omitted.

[0227] (Alkali Addition Unit)

[0228] The alkali addition unit 450 differs from the purification systems 100, 200, and 300 in that the alkali compound is added at the bottom of the feed tank T41 or the first distillation column 421.

[0229] [Fourth Implementation Method: Purification Method]

[0230] Although the purification method of the fourth embodiment does not include the impurity adsorption step, the other steps are largely the same as those of the purification method of the first embodiment, so the description is omitted.

[0231] [First to Fourth Implementation Methods: Recycling Process]

[0232] The purification methods of the first to fourth embodiments may include a recovery step of recovering raw materials from a gas containing NMP before the first distillation step.

[0233] In the recovery process, preferably a portion of the distillate containing low-boiling-point amines distilled from the first distillation column of the first distillation process (specifically, via...) is used. Figure 1 The liquid supplied through piping t16 in the pipeline is used as recycled water for the recovered gas. Furthermore, the mass fraction of NMP in the distillate containing low-boiling-point amines used as recycled water is preferably 0.001 × 10⁻⁶. -2 ~5×10 -2 .

[0234] It should be noted that gas recovery devices for recovering raw materials from gases containing NMP transfer NMP from the gas to the recovery water by contacting the gas with the recovery water, and recover the NMP-concentrated recovery water as a raw material. For example, there is a known gas recovery device described in Japanese Patent No. 6351403. It should also be noted that the relevant content described in Japanese Patent No. 6351403 is incorporated into this specification in connection with it.

[0235] [Variation Example]

[0236] The purification systems and purification methods of the first to fourth embodiments have been described above, but the embodiments are not limited thereto, and for example, the following modifications are possible.

[0237] Figure 1 The first auxiliary distillation column 121 of the purification system 100 shown can be a demister or a packed distillation column with regular packing material and internal components.

[0238] Figures 1-4 The first and second distillation columns in the purification systems 100-400 shown can also be packed distillation columns with regular packing material and internal components.

[0239] from Figure 2 The liquid returned to the second auxiliary distillation column 231 from the second distillation column 232 in the purification system 200 shown can be a liquid that is mostly water condensed in the condenser 235 at the top of the column.

[0240] [Effect]

[0241] The purification system and purification method according to the above embodiments can achieve the following effects.

[0242] In the purification methods of the first to fourth embodiments, since the temperature of the distillation object in the first and second distillation steps is controlled to be below 160°C, the modification and decomposition of NMP during the purification process can be suppressed, and the amount of newly generated amines and high-boiling-point substances can be reduced.

[0243] In the purification methods of the first to fourth embodiments, vapor containing low-boiling-point amines is distilled from the top of the second distillation column in the second distillation step, liquid containing methylsuccinimide is discharged from the bottom of the column, and the purified composition is taken out from the side fraction nozzle. Therefore, impurities such as low-boiling-point amines, methylsuccinimide, and water can be appropriately separated and removed.

[0244] In the purification methods of the second and third embodiments, since distillation is carried out using a first distillation column in the first distillation step, the mass fraction of water in the raw material after crude dehydration can be reduced. As a result, the reflux ratio in the second distillation step can be reduced, thus achieving energy saving.

[0245] In the purification method of the second embodiment, distillation is carried out using two distillation columns, a second auxiliary distillation column and a second distillation column, in the second distillation step. The liquid, which contains almost no water, is returned from the product section of the second distillation column to the second auxiliary distillation column. Therefore, by suppressing the effects caused by compositional changes, quality stabilization can be achieved, and energy-saving effects can be achieved through the conventional Petliuk method.

[0246] In the purification method of the third embodiment, since a vertically divided distillation column is used in the second distillation step, distillation is carried out in both the upstream distillation section and the downstream distillation section of the vertically divided distillation column, thus enabling more reliable separation and removal of impurities and water.

[0247] In the purification methods of the first to third embodiments, since impurities are adsorbed and separated from the raw materials in advance during the impurity adsorption step, a purified composition with a lower mass fraction of impurities can be obtained.

[0248] In the purification methods of the first to third embodiments, a portion of the distillate containing low-boiling-point amine distilled from the top of the second distillation column in the second distillation step is returned as raw material to the upstream of the ion exchange resin column in the impurity adsorption step, thereby improving the NMP recovery rate.

[0249] The purification methods of the first to fourth embodiments include a pH measurement step, a calculation step, and an alkali addition step. Furthermore, the mass ratio of water in the mixed solution during the pH measurement step is controlled in a manner with virtually no error, thus allowing the addition of an optimal amount of alkali compound. As a result, not only can the effect of suppressing peroxide formation through the addition of alkali compound be reliably achieved, but the generation of new impurities such as amines due to excessive addition of alkali compound can also be avoided.

[0250] The purification methods of the first to fourth embodiments include a recovery step, and a portion of the distillate containing low-boiling-point amine distilled from the first distillation column in the first distillation step is used as recovery water for the recovered gas, thus reducing the amount of recovery water that needs to be prepared, thereby achieving the effect of cost reduction.

[0251] Example

[0252] Next, embodiments of the present invention will be described.

[0253] [Example 1]

[0254] (Example 1: Raw material as the object)

[0255] The raw material used has the following composition: water content: 19%~21% by mass, amine content: 300ppm~400ppm, impurities other than amine: 30ppm~60ppm, and NMP content: 79%~81% by mass.

[0256] Here, the content of amines is specifically the total amount of the three substances: methylamine, 2-amino-2-methyl-1-propanol, and monoethanolamine.

[0257] It should be noted that the methods for determining the content of each component in the embodiments are implemented by the methods described in the first embodiment.

[0258] (Example 1: Test Conditions)

[0259] Example 1 uses Figure 1 The purification system 100 of the first embodiment shown was tested according to the purification method of the first embodiment. It should be noted that the detailed test conditions are as follows.

[0260] The ion exchange resins used in the ion exchange resin columns 111 and 112 of the impurity adsorption unit 110 are strongly acidic ion exchange resins. Furthermore, the flow rate (SV) of the raw material in the ion exchange resin columns 111 and 112 is 10 h⁻¹. -1 .

[0261] The first auxiliary distillation column 121 is a plate distillation column with a total of 5 sections, and the pressure inside the column is controlled at 11 kPa using an absolute pressure gauge. The first distillation column 122 is a plate distillation column with a total of 32 sections. The supply section connected to pipe t11 is the sixteenth section from the bottom, and the discharge section connected to pipe t14 is the tenth section from the top. The pressure inside the column is controlled at 11 kPa using an absolute pressure gauge. The second distillation column 131 is a plate distillation column with a total of 24 sections. The product section connected to pipe t24 is the fifth section from the top, and the pressure inside the column is controlled at 8 kPa using an absolute pressure gauge.

[0262] Then, the temperature of the distillation objects (raw materials, dehydrated raw materials) in the first and second distillation processes is controlled at 150°C.

[0263] Regarding the dehydrated raw material (dehydrated raw material supplied via pipe t20) and water (water supplied via pipe t29) supplied to the pH measuring unit 140, the supply rates are set to 30 ml / min and 10 ml / min respectively, with a mass ratio of 3:1 (the mass ratio of the two is 3). Moreover, the water supply rate is strictly controlled within the range of 9.9 to 10.1 ml / min to ensure that the error in the mass ratio of the two is within ±1%.

[0264] The addition of potassium hydroxide using the alkali addition unit 150 is carried out in an aqueous solution (concentration of 48%), and the amount of potassium hydroxide to be added is calculated and added every 3 hours. Furthermore, potassium hydroxide is added to the feed at the bottom of the first auxiliary distillation column 121. It should be noted that the calculation of the amount of potassium hydroxide added is based on PID control (target pH value: set to 8.0).

[0265] (Example 1: Experimental Results)

[0266] When the raw material supplied to the first auxiliary distillation column 121 is measured downstream of the impurity adsorption unit 110, the amine content is 0.5 ppm. That is, through the impurity adsorption unit 110, 99.83 to 99.88% by mass (= (300-0.5) / 300×100 to (400-0.5) / 400×100) of the amine originally contained in the raw material can be adsorbed and separated.

[0267] The liquid obtained by condensing the distillate vapor from the first distillation column 122 in the condenser 125 contains 0.015% by mass of NMP and is mostly water. Furthermore, the bottom liquid (dehydrated feedstock) from the first distillation column 122 contains 99.97% by mass of NMP, 0.01% by mass of water, less than 0.00008% by mass of amines, and 0.02% by mass of other impurities. Therefore, the dehydrated feedstock obtained through the first distillation unit 120 can adequately remove water and also remove impurities such as amines.

[0268] The purified composition taken from the product section of the second distillation column 131 has less than 0.003% by mass of water, less than 0.00005% by mass of amine, and more than 99.98% by mass of NMP. Furthermore, the NMP recovery rate is 99%. Therefore, according to Example 1, it is possible to appropriately separate impurities such as amines and water, and to purify a purified composition containing NMP at a very high concentration.

[0269] Furthermore, the pH of the mixture obtained by mixing the purified composition taken from the product section of the second distillation column 131 with water at a mass ratio of 1:1 is 7.5. Moreover, the purified composition after being stored at room temperature in a sealed container for 90 days is colorless and transparent, and the colorimetric tube value is APHA < 5 (less than 5).

[0270] This result can be attributed to the fact that by keeping the temperature of the distillate in the first and second distillation steps below 160°C, the modification and decomposition of NMP are suppressed, and by optimizing the amount of alkali compound added, excessive addition is prevented, thereby avoiding the formation of new impurities such as amines.

[0271] In addition, the alkali compound concentrated in reboiler 123 needs to be taken out as drain via pipe t10, but in Example 1, the amount of alkali compound added is optimized, so the amount of drain can be suppressed to less than 1% of the total amount of raw material supplied.

[0272] [Comparative Example 1]

[0273] (Comparative Example 1: As the raw material of the object)

[0274] The raw materials used in Comparative Example 1 were the same as those used in Example 1.

[0275] (Comparative Example 1: Experimental Conditions)

[0276] The test conditions for Comparative Example 1 were basically the same as those for Example 1, but differed in the following aspects.

[0277] In Comparative Example 1, the pH measurement and calculation procedures were not performed. Furthermore, similar to Example 1, potassium hydroxide was added every 3 hours, but the amount added was set to three times the average amount added in Example 1.

[0278] (Comparative Example 1: Experimental Results)

[0279] In Comparative Example 1, the addition of a sufficient amount of alkali compound effectively suppressed the formation of peroxides caused by the addition of the alkali compound.

[0280] However, in Comparative Example 1, due to the excessive addition of alkali compound, more alkali compound was concentrated in reboiler 123, resulting in the amount of drain liquid taken out via pipe t10 being 3 times that of Example 1.

[0281] In addition, in Comparative Example 1, due to the excessive addition of the alkali compound, the alkali compound reacted with NMP to generate impurities such as amines.

[0282] Ultimately, according to Comparative Example 1, the recovery rate of NMP was 3% lower than that of Example 1.

[0283] [Comparative Example 2]

[0284] (Comparative Example 2: Raw materials for the object)

[0285] The raw materials used in Comparative Example 2 were the same as those used in Example 1.

[0286] (Comparative Example 2: Experimental Conditions)

[0287] The test conditions for Comparative Example 2 were basically the same as those for Example 1, but differed in the following aspects.

[0288] In Comparative Example 2, the pH measurement and calculation steps were not performed. Furthermore, potassium hydroxide was added every 3 hours, similar to Example 1, with the amount added being the same as the average amount added in Example 1.

[0289] (Comparative Example 2: Experimental Results)

[0290] In Comparative Example 2, unlike Example 1, the amount of alkali compound added was not controlled, resulting in instances of insufficient or excessive alkali compound. Therefore, purification was performed in the presence of impurities such as peroxides due to insufficient alkali compound and amines due to excessive alkali compound. As a result, the pH of the purified composition of Comparative Example 2 (a mixture of the purified composition and water at a mass ratio of 1:1) was 6.2, lower than that of Example 1. Furthermore, after being stored in a sealed container at room temperature for 90 days, it turned yellow, and the colorimetric tube reading was APHA30.

[0291] [Comparative Example 3]

[0292] (Comparative Example 3: As the raw material of the object)

[0293] The raw materials used in Comparative Example 3 were the same as those used in Example 1.

[0294] (Comparative Example 3: Experimental Conditions)

[0295] The test conditions for Comparative Example 3 were basically the same as those for Example 1, but differed in the following aspects.

[0296] In Comparative Example 3, the amount of water supplied to pH measuring unit 140 was not strictly controlled.

[0297] (Comparative Example 3: Experimental Results)

[0298] In Comparative Example 3, unlike Example 1, the amount of water supplied to the pH measuring unit 140 was not controlled. As a result, the error in the amount of water supplied (the mass ratio of the two) frequently exceeded 5%, and the measured pH value was not accurate. Therefore, the amount of alkali added sometimes became excessive or insufficient. Consequently, the same as Comparative Examples 1 and 2, the results were not excellent.

[0299] [Example 2]

[0300] (Example 2: Raw material as the object)

[0301] The raw materials used in Example 2 are the same as those used in Example 1.

[0302] (Example 2: Test Conditions)

[0303] Example 2 uses Figure 2The purification system 200 of the second embodiment shown was tested according to the purification method of the second embodiment. The detailed test conditions are as follows.

[0304] The ion exchange resin and liquid flow rate SV of the impurity adsorption unit 210 are the same as in Example 1.

[0305] Preheater 221 uses 150°C drain water to heat the feedstock to 90°C. The first distillation column 222 is a 20-section plate distillation column; the feedstock heated by preheater 221 is supplied to the sixth section from the bottom, and the pressure inside the column is controlled at 13 kPa using an absolute pressure gauge. The second distillation column 232 is a 40-section plate distillation column; the feed section is the twenty-first section from the top, and the product section is five sections lower than the feed section; the pressure inside the column is controlled at 9 kPa using an absolute pressure gauge.

[0306] Then, the temperature of the distillation objects (raw materials, dehydrated raw materials) in the first and second distillation processes is controlled at 145°C.

[0307] The mass ratio of the dehydrated raw material to water supplied to the pH measuring unit 240, the supply amounts of both, and the control of the water supply amount are the same as in Example 1. Furthermore, the addition of potassium hydroxide using the alkali addition unit 250 and the calculation of the amount of potassium hydroxide added are also the same as in Example 1.

[0308] (Example 2: Experimental Results)

[0309] The liquid obtained by condensing the distillate vapor from the first distillation column 222 in condenser 224 contains 0.001% by mass of NMP and 0.0001% of other impurities, with the remainder being water. Therefore, this liquid can be used as recycled water for recovering feedstock from gas containing NMP.

[0310] The bottom liquid (dehydrated feed) of the first distillation column 222 contains 95% by mass NMP, 5% by mass water, 0.00005% by mass amine, and 0.01% by mass other impurities. Therefore, in the crude dehydrated feed obtained through the first distillation unit 220, water can be appropriately removed, and impurities such as amines can also be removed.

[0311] The purified composition taken from the product section of the second distillation column 232 has less than 0.003% by mass of water, less than 0.5 ppm of amine, and more than 99.98% by mass of NMP. Furthermore, the NMP recovery rate is 99%. Therefore, according to Example 2, it is possible to appropriately separate impurities such as amines and water, and to purify a purified composition with NMP concentrated at a very high level.

[0312] Furthermore, the pH of the mixture obtained by mixing the purified composition taken from the product section of the second distillation column 232 with water at a 1:1 mass ratio is 7.6. Moreover, the purified composition is colorless and transparent after being stored at room temperature in a sealed container for 90 days, and the colorimetric tube value is APHA < 5 (less than 5).

[0313] Furthermore, in Example 2, compared to Example 1, a large amount of water remains in the bottom liquid (dehydrated feedstock) of the first distillation column 222. This reduces the amount of water discharged from the condenser 224 and the reflux flow, thus significantly reducing the heat load on the reboiler 223. It should be noted that, unlike Example 1, the main component of the distillate vapor from the second distillation column 232 in Example 2 is water. Therefore, energy is required to vaporize the water, and this energy also serves as the energy for purifying NMP. Furthermore, in Example 2, unlike the conventional Petrolky method, a configuration is made to return the reflux liquid from the product section rather than from the feed section of the second distillation column 232 to the second auxiliary distillation column 231 (the vapor is returned via pipe t2b instead of pipe t2a). Therefore, it is possible to avoid large compositional changes while fully utilizing the energy-saving effects achieved by the conventional Petrolky method. As a result, Example 2 reduces energy costs by approximately 50% compared to Example 1.

[0314] [Comparative Example 4]

[0315] (Comparative Example 4: As the raw material of the object)

[0316] The raw materials used in Comparative Example 4 were the same as those used in Example 1.

[0317] (Comparative Example 4: Experimental Conditions)

[0318] The test conditions for Comparative Example 4 were basically the same as those for Example 2, but differed in the following aspects.

[0319] Unlike Example 2, in Comparative Example 4, the reflux liquid is returned from the supply section of the second distillation column 232 to the second auxiliary distillation column 231 (the reflux liquid is returned via piping t2a: the conventional Petliuk method).

[0320] (Comparative Example 4: Experimental Results)

[0321] In Comparative Example 4, the reflux liquid was returned from the feed section of the second distillation column 232 to the second auxiliary distillation column 231. Therefore, this reflux liquid contained a relatively large amount of water. As a result, the impact of the compositional change was greater, not only failing to fully realize the energy-saving effect achieved by the conventional Petrovich method, but also resulting in a throughput of 10% less of the previously anticipated purified composition. Therefore, Comparative Example 4 resulted in the following: even considering only the reduction in throughput, the heat per unit throughput increased by 11% compared to Example 2.

[0322] Explanation of reference numerals in the attached figures

[0323] 100, 200, 300, 400 purification systems

[0324] 110, 210, 310 impurity adsorption units

[0325] 120, 220, 320, 420 First Distillation Unit

[0326] Second distillation units 130, 230, 330, and 430

[0327] 140, 240, 340, 440 alkali addition units

[0328] 150, 250, 350, 450 alkali supply units

Claims

1. A purification method, characterized in that, It is a purification method for purifying raw materials containing N-methyl-2-pyrrolidone and water, and containing low-boiling-point amines and methylsuccinimide as impurities with boiling points lower than N-methyl-2-pyrrolidone, including the following steps: In the first distillation step, the raw material is distilled in a first distillation column, and vapor containing low-boiling-point amines is distilled off from the top of the first distillation column, while the dehydrated raw material is discharged from the bottom of the first distillation column; and In the second distillation step, the dehydrated raw material is distilled in a second distillation column. Vapor containing low-boiling-point amines is distilled from the top of the second distillation column, and liquid containing methylsuccinimide is released from the bottom of the second distillation column. A purified composition concentrated with N-methyl-2-pyrrolidone is then taken out from a side fraction nozzle of the second distillation column. The temperature of the distillate in the first and second distillation processes is controlled to be below 160°C. The mass fraction of amine in the purified composition taken out in the second distillation step is less than 0.5 × 10⁻⁶. -6 The mass fraction of water is less than 200 × 10⁻⁶ -6 , The ratio of the mass of N-methyl-2-pyrrolidone in the purified composition to the mass of N-methyl-2-pyrrolidone in the raw material, i.e., the recovery rate, is above 95%.

2. The purification method according to claim 1, characterized in that, The mass fraction of N-methyl-2-pyrrolidone in the purified composition is 99.98 × 10⁻⁶. -2 above, The pH of the mixture prepared by mixing the purified composition with water at a mass ratio of 1:1 is 7.2-8.

0. The mass fraction of methylsuccinimide in the purified composition is 10 × 10⁻⁶. -6 the following, The colorimetric value of the purified composition after 90 days of storage at room temperature in a sealed container is below APHA10.

3. The purification method according to claim 1 or 2, characterized in that, In the first distillation step, distillation is carried out in two distillation columns: the first distillation column and a first auxiliary distillation column located upstream of the first distillation column, so that the mass fraction of water in the dehydrated feedstock is 1×10⁻⁶. -2 ~5×10 -2 Distillation is performed in this manner.

4. The purification method according to claim 1 or 2, characterized in that, In the second distillation process, distillation is carried out in two distillation columns: the second distillation column and a second auxiliary distillation column located upstream of the second distillation column, so that a portion of the purified composition taken from the second distillation column is returned to the second auxiliary distillation column.

5. The purification method according to claim 1 or 2, characterized in that, The second distillation column in the second distillation process is a vertically segmented distillation column. The interior of the vertically divided distillation column, except for the top, is divided vertically by dividing plates. At the bottom of the two columns separated by the dividing plate, the distillate is heated by a reboiler.

6. The purification method according to claim 1, characterized in that, The amine in the raw material has a mass fraction of 1×10⁻⁶. -6 above, The purification method includes an impurity adsorption step before the first distillation step, wherein the flow rate (SV) is set to 5-30 h. -1 The raw material is passed through an ion exchange resin column filled with acidic or amphoteric ion exchange resin, so that more than 90% by mass of the amine originally contained in the raw material is adsorbed and separated.

7. The purification method according to claim 6, characterized in that, A portion of the distillate containing low-boiling-point amines distilled from the top of the second distillation column in the second distillation step is passed through the ion exchange resin column as the raw material in the impurity adsorption step.

8. The purification method according to claim 1, characterized in that, The process includes the following steps: In the pH measurement process, a portion of the liquid in the column of at least one of the first distillation process and the second distillation process is taken out as the pH measurement solution. The pH measurement solution is mixed with water in a mass ratio of 0.5 to 20 (mass of pH measurement solution / mass of water), and the pH of the resulting mixture is measured. The calculation step, based on the pH result from the pH measurement step, calculates the amount of alkali compound to be added; and In the alkali addition step, for any one of the raw materials 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, an alkali compound is added in the amount calculated in the calculation step. The amount of water mixed is controlled so that the mass ratio of the two components in the mixture during the pH measurement process is a constant ratio with an error within ±5%.

9. The purification method according to claim 1, characterized in that, This includes a recovery step prior to the first distillation step, in which the feedstock is recovered from a gas containing N-methyl-2-pyrrolidone. A portion of the distillate containing low-boiling-point amines distilled from the first distillation column of the first distillation step is used as recycled water for the recovered gas in the recovery step.

10. The purification method according to claim 9, characterized in that, The distillate containing low-boiling-point amines used as the recycled water has a mass fraction of 0.001 × 10⁻⁶ N-methyl-2-pyrrolidone. -2 ~5×10 -2 .

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