A method and system for purifying crude acrylonitrile

Abstract

This invention discloses a method and system for purifying crude acrylonitrile. The method includes: (1) distilling the crude acrylonitrile, cooling the top material of the column to obtain condensate, and optionally discharging the bottom material; (2) separating the condensate into a pre-distillate and acrylonitrile product based on the water content; (3) directly collecting the acrylonitrile product and recycling the pre-distillate back to the distillation process in step (1). The required water content in the acrylonitrile product is set to be less than or equal to x. In step (2), the condensate with a water content less than or equal to x is used as the acrylonitrile product, and the condensate with a water content greater than x is used as the pre-distillate. The system includes a distillation column, a condensation unit, a pre-distillate unit, a product unit, a vacuum system, and a reboiler. The entire process is fully automated, offering advantages such as low acrylonitrile raw material loss, minimal acrylonitrile polymerization during batch waiting periods, and safe process operation.

Description

Technical Field

[0001] This invention relates to the purification of crude acrylonitrile, and more particularly to a method and system for purifying crude acrylonitrile. Background Technology

[0002] Acrylonitrile is an important monomer for synthetic fibers, synthetic rubber, and synthetic resins. Polyacrylonitrile fibers, made from acrylonitrile, have properties very similar to wool, hence the name "synthetic wool." Acrylonitrile copolymerized with butadiene produces nitrile-butadiene rubber (NBR), which exhibits good oil resistance, cold resistance, abrasion resistance, and electrical insulation properties, and remains relatively stable under most chemical solvents, sunlight, and heat. Acrylonitrile copolymerized with butadiene and styrene produces ABS resin, which is lightweight, cold-resistant, and has good impact resistance. Acrylonitrile copolymerized with one or more of itaconic acid, acrylic acid, methacrylic acid, methyl acrylate, and methyl methacrylate can be used to prepare polyacrylonitrile dope, which, after oxidation and carbonization, can be used to prepare carbon fibers. High-performance polyacrylonitrile-based precursor fibers are key to the preparation of high-performance carbon fibers, and high-quality spinning dope is the foundation for preparing high-performance precursor fibers; therefore, the purity of acrylonitrile plays a crucial role in the preparation of high-performance carbon fibers.

[0003] In the solution polymerization process for preparing polymers, the composition of solvent and water directly affects the molecular weight and distribution of the polymer, thus influencing the quality of products such as resins, rubbers, or carbon fibers. In particular, during the preparation of carbon fiber spinning solutions, acrylonitrile polymerization primarily employs a batch-method homogeneous solution polymerization. The water content in acrylonitrile has a significant impact on the stability of the spinning solution between different batches. Therefore, during the polymerization reaction, crude acrylonitrile is usually distilled or purified to strictly control the quality of the acrylonitrile raw material during the polymerization reaction, ensuring the stability of the polymerization solution between different batches, reducing the CV value of the spinning solution, and ultimately obtaining high-performance carbon fiber products.

[0004] CN107778197B discloses a method for purifying acrylonitrile, which involves de-peroxide treatment of acrylonitrile. The method is characterized by using an adsorbent combination in a de-peroxide tower, controlling the acrylonitrile feed temperature at 22-25℃, the discharge temperature at 20-25℃, and the residence time at 0.5-2.5 min, to prepare acrylonitrile with a 3-hydroxypropionitrile concentration of less than 50 ppm. The adsorbent combination uses activated alumina balls and molecular sieves. This invention, by selecting a suitable adsorbent combination and adjusting the feed temperature, discharge temperature, and residence time of acrylonitrile in the de-peroxide tower, can control the 3-hydroxypropionitrile generated during the de-peroxide process within a reasonable range. This reduces the toxic effects of 3-hydroxypropionitrile on hydration enzymes during hydration, allowing the hydration enzymes to maintain their hydration activity during hydration, which is beneficial for the efficient production of high-quality acrylamide aqueous solutions.

[0005] CN101665451B discloses an acrylonitrile purification apparatus, including a distillation unit. One or more coolers are connected to the outlet of the distillation unit via pipes. The purification apparatus also includes a coarse nitrile filter filled with an adsorbent for removing oxazole organic compounds during operation. The coarse nitrile filter has an inlet and an outlet connected to the inlet of the distillation unit via a pipe. In this invention, the acrylonitrile purification apparatus includes a coarse nitrile filter before the distillation unit. The coarse nitrile filter can be filled with an adsorbent that removes oxazole organic compounds upon contact with the acrylonitrile feedstock. This ensures that the acrylonitrile feedstock is first de-oxazoleed before entering the distillation unit to remove heavy components, resulting in acrylonitrile feedstock that meets the requirements.

[0006] CN108337889B discloses a method for purifying acrylonitrile, comprising a purification step of distilling an acrylonitrile solution containing acrylonitrile, hydrogen cyanide, and water. The purification step includes: a first step of separating a sidestream fraction drawn from a first position of the distillation column into an organic layer and an aqueous layer, and returning the organic layer to a second position of the distillation column located lower than the first position; and a second step of supplying water from a third position of the distillation column located lower than the second position and higher than the lowest section of the distillation column. This acrylonitrile purification method is a novel approach that, through stabilization in the acrylonitrile manufacturing process using a dehydrocyanic acid dehydration tower, can achieve product quality stabilization and reduce the process load on acrylonitrile manufacturing.

[0007] CN102199105A discloses a method for preparing high-purity acrylonitrile, which mainly solves the problems in the existing technology that the acrylonitrile processing capacity is small when removing various trace impurities such as acetaldehyde, hydrogen cyanide, acetone, acrolein, methacrylonitrile, oxazole, propionitrile, butadiene nitrile, polymerization inhibitors, peroxides, and iron ions from the raw acrylonitrile, or introduces new compounds to increase the new impurities in the raw acrylonitrile, and there is no method to remove multiple trace impurities in the raw acrylonitrile at the same time. This invention employs the following technical solution: a) contacting raw material acrylonitrile with a cation exchange resin to obtain stream I, which has been de-oxazole removed; b) contacting stream I with an anion exchange resin to obtain stream II, which has been de-acrolein removed; c) contacting stream II with alumina to obtain stream III, which has been de-oxided and water removed; d) distilling stream III through a distillation column and extracting it from the vapor side stream at plates 5-15 (counting from bottom to top) of the distillation column to obtain high-purity acrylonitrile with a purity of 99.99% that simultaneously removes acetaldehyde, hydrogen cyanide, acetone, acrolein, methacrylonitrile, oxazole, propionitrile, butadiene nitrile, polymerization inhibitor, peroxide, and iron ions. This solution effectively solves the problem and can be used in the industrial production of high-performance polyacrylonitrile-based carbon fiber raw material purification.

[0008] CN102199104B discloses a purification method for acrylonitrile, which mainly solves the problem in the prior art that it does not simultaneously remove multiple trace impurities such as acetaldehyde, hydrogen cyanide, acetone, acrolein, methacrylonitrile, oxazole, propionitrile, butadiene nitrile, polymerization inhibitors, peroxides, and iron ions from the raw material acrylonitrile. This invention provides a technical solution for obtaining pure acrylonitrile with acetaldehyde, hydrogen cyanide, acetone, acrolein, methacrylonitrile, propionitrile, polymerization inhibitors, and iron ions, and with an oxazole content of less than 3 ppm, by using a distillation column with 30-60 trays or packing structure, operating at a top pressure of 0.026-0.1 MPa(A), a top temperature of 38-77°C, a bottom temperature of 41-79°C, and a reflux ratio of 0.5-10. The raw material acrylonitrile is fed from trays 18-25 (counting from bottom to top), and the gas phase is extracted from trays 5-15 (counting from bottom to top). This solution effectively solves the problem and can be used in the industrial production of high-performance polyacrylonitrile-based carbon fiber raw material purification.

[0009] Existing literature and patent reports mostly focus on acrylonitrile purification processes to reduce impurity content, which are relatively complex. However, there is almost no focus on batch distillation purification processes and control methods for acrylonitrile, especially the use of online moisture analyzers for water content control, which is rarely reported. Summary of the Invention

[0010] To overcome the problems existing in the prior art, the present invention provides a method and system for purifying crude acrylonitrile and the resulting acrylonitrile, mainly solving the problems of large acrylonitrile feedstock loss and easy polymerization of acrylonitrile during batch waiting time in the existing acrylonitrile batch distillation process.

[0011] One objective of this invention is to provide a method for purifying crude acrylonitrile, comprising:

[0012] (1) The crude acrylonitrile is distilled (preferably using a distillation column), the top material is cooled to obtain condensate, and the bottom material is optionally discharged.

[0013] (2) Based on the water content in the condensate, it is divided into pre-distillate and acrylonitrile products;

[0014] (3) The acrylonitrile product is directly extracted, and the pre-distillate material is recycled back to the distillation process in step (1).

[0015] In a preferred embodiment, the crude acrylonitrile contains one or more of oxazole, acetaldehyde, acetone, water, and a polymerization inhibitor.

[0016] In a further preferred embodiment, the water content in the crude acrylonitrile is between 6000 ppm and 1000 ppm, preferably between 4500 ppm and 1000 ppm.

[0017] For example, the water content in the crude acrylonitrile is 6000ppm, 5000ppm, 4000ppm, 3000ppm, 2000ppm or 1000ppm.

[0018] In a preferred embodiment, the distillation process is carried out in a distillation column, preferably a packed column or a plate column.

[0019] In a further preferred embodiment, the crude acrylonitrile enters from the lower or bottom portion of the distillation column; and / or, the fore-distillate enters from the lower or bottom portion of the distillation column.

[0020] In this way, the fore-distillate enters the distillation column and is mixed with crude acrylonitrile. The fore-distillate plays a role in reducing the temperature of the column bottom to a certain extent.

[0021] In a further preferred embodiment, the bottom temperature of the distillation column is 55-70°C (e.g., 55-65°C), preferably 60-65°C; and / or, the operating pressure of the distillation column is 30-70 kPaA, preferably 50-65 kPaA.

[0022] To prevent acrylonitrile polymerization, the temperature of the distillation column reboiler is controlled at the aforementioned low temperature. For example, the reboiler temperature of the distillation column is 55°C, 56°C, 57°C, 58°C, 59°C, 60°C, 61°C, 62°C, 63°C, 64°C, or 65°C; and / or, the operating pressure of the distillation column is 30 kPaA, 40 kPaA, 50 kPaA, 60 kPaA, or 70 kPaA.

[0023] In this invention, the bottom of the tower is discharged intermittently or continuously, with intermittent discharge being preferred.

[0024] In a preferred embodiment, the distillation process is carried out in a distillation column, which uses a reboiler to heat the bottom material, and the reboiler uses hot water and / or low-pressure steam as the heating medium.

[0025] The hot water refers to water at 80℃ to 95℃, and the low pressure refers to 0.15KPaA to 0.45KPaA.

[0026] In a further preferred embodiment, the temperature of the reboiler is controlled by controlling the flow rate of the heating medium, wherein the flow rate of the heating medium is controlled by a temperature controller located in the reboiler of the distillation column.

[0027] This allows for precise control of the temperature in the distillation column reboiler, ensuring the stability of the distillation process and preventing excessive temperature fluctuations in the reboiler from causing acrylonitrile polymerization and substandard product quality at the top of the column.

[0028] In a further preferred embodiment, when the temperature of the column bottom is higher than the set value, the flow rate of the heating medium is reduced; when the temperature of the column bottom is lower than the set value, the flow rate of the heating medium is increased.

[0029] In a preferred embodiment, the pressure of the distillation column is controlled by adding inert gas into the column.

[0030] In a further preferred embodiment, when the pressure of the distillation column is higher than the set value, the flow rate of the inert gas entering the distillation column is reduced; when the pressure of the distillation column is lower than the set value, the flow rate of the inert gas entering the distillation column is increased.

[0031] In a preferred embodiment, the distillation in step (1) is a batch distillation, wherein when the bottom liquid level of the crude acrylonitrile added to the distillation column reaches a set value, the feed of crude acrylonitrile is cut off; after the batch distillation is completed, the fore-distillate stream and the crude acrylonitrile stream are introduced into the distillation column until the bottom liquid level reaches the set value.

[0032] In a further preferred embodiment, the set value of the liquid level in the column bottom is 0.4 to 0.9H, preferably 0.5 to 0.8H, where H is the height of the straight section of the column bottom.

[0033] In a further preferred embodiment, the liquid level in the distillation column reboiler is controlled by a liquid level detector installed in the distillation column reboiler.

[0034] By monitoring the liquid level in the column reboiler, it is possible to prevent the level from becoming too low or too high. If the level is too low, the required quantity for a single batch cannot be met; if the level is too high, it can lead to insufficient product purity and wasted energy.

[0035] In a preferred embodiment, in step (1), the temperature is cooled to 10–40°C, preferably to 15–25°C.

[0036] For example, cooling to 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, or 40°C.

[0037] In a further preferred embodiment, in step (1), the cooling process is performed under a vacuum or inert atmosphere.

[0038] Acrylonitrile has a boiling point of 77.3℃ under normal pressure and is easy to polymerize. Vacuum is used to reduce the saturated vapor pressure of acrylonitrile and avoid high-temperature polymerization. An inert atmosphere is used because acrylonitrile is a flammable and explosive substance.

[0039] In a preferred embodiment, non-condensable gases are optionally discharged during the cooling process.

[0040] In a preferred embodiment, the water content of the desired acrylonitrile product is set to be less than or equal to x. In step (2), the condensate with a water content less than or equal to x is used as the acrylonitrile product, and the condensate with a water content greater than x is used as the pre-distillate material.

[0041] Where x can take different values, which are mainly determined according to actual needs. For example, if an acrylonitrile product with a water content of less than or equal to 500 ppm is required, then x is taken as 500 ppm. In this invention, it is preferred, but not limited to, that x takes any value from 200 to 800 ppm. For example, x can take 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, or 800 ppm.

[0042] In a further preferred embodiment, an online moisture analyzer is used to detect the water content in the condensate.

[0043] In a preferred embodiment, the distillation process and the cooling process are carried out in the presence of a vacuum or an inert gas (e.g., nitrogen).

[0044] In a preferred embodiment, (periodic analysis of the composition of the column bottom material) when the content of heavy components in the column bottom material is too high (preferably higher than 5 wt%), the column bottom material is discharged before the next acrylonitrile feed to avoid the accumulation of heavy components, which could lead to product defects.

[0045] The heavy components include polymers, polymerization inhibitors, etc., and the content of heavy components is usually measured by the weight loss method.

[0046] In a further preferred embodiment, the bottom material includes one or more of acrylonitrile, polyacrylonitrile, water, and polymerization inhibitors.

[0047] The purity of the crude acrylonitrile used in this invention is, for example, but not limited to, greater than or equal to 99.4 wt%, preferably, for example, but not limited to, greater than or equal to 99.5 wt%. The purity will be further improved after processing by the method described in this invention.

[0048] A second objective of this invention is to provide a system for purifying crude acrylonitrile, preferably for carrying out the method described in one objective of this invention. The system includes a distillation column, a condenser, a pre-distillation unit, a product unit, and a vacuum system.

[0049] In a preferred embodiment, the system further includes a crude acrylonitrile feed line, a top material conveying line, a non-condensable gas discharge line, a condensate conveying line, a front fraction conveying line, an acrylonitrile product conveying line, a front fraction discharge line, and a bottom material discharge line.

[0050] In a further preferred embodiment:

[0051] The crude acrylonitrile feed line is connected to the lower part or bottom of the distillation column; and / or,

[0052] The material conveying pipeline at the top of the column is located between the upper part or top of the distillation column and the condensation unit; and / or,

[0053] The non-condensable gas discharge pipeline is installed on the cooling device and is preferably connected to a vacuum system; and / or,

[0054] Along the material flow direction, a condensate conveying pipeline, a pre-distillate conveying pipeline, and an acrylonitrile product conveying pipeline are provided after the condensation device. Preferably, the pre-distillate conveying pipeline and the acrylonitrile product conveying pipeline are connected in parallel, thus forming a Y-shaped structure after connection; and / or,

[0055] A pre-distillation unit discharge control valve is installed on the pre-distillation unit discharge line; and / or,

[0056] The bottom material discharge pipeline is connected to the bottom of the distillation column, and preferably a bottom material discharge control valve is installed on the bottom material discharge pipeline.

[0057] In a further preferred embodiment, the pre-distillation unit is connected to the pre-distillation delivery pipeline; and / or, the product unit is connected to the acrylonitrile product delivery pipeline; and / or, an inert gas input pipeline is provided on the non-condensable gas discharge pipeline, preferably, an inert gas feed regulating valve is provided on the inert gas input pipeline; and / or, a liquid level regulating valve is provided on the crude acrylonitrile feed pipeline.

[0058] In a preferred embodiment, the distillation column is a packed column or a plate column.

[0059] In a further preferred embodiment, a pressure detector is provided at the top of the distillation column; and / or, a level detector is provided at the bottom of the distillation column.

[0060] In a further preferred embodiment, the pressure detector is connected to the inert gas feed regulating valve; and / or, the liquid level detector is connected to the liquid level regulating valve.

[0061] Specifically, when the pressure in the distillation column is higher than the set value, the inert gas feed regulating valve is closed; when the pressure in the distillation column is lower than the set value, the inert gas feed regulating valve is opened.

[0062] In a preferred embodiment, an online moisture detector is installed on the condensate delivery pipeline.

[0063] In a further preferred embodiment, a feed regulating valve for the pre-distillation unit is provided on the pre-distillation unit conveying pipeline, and a feed regulating valve for the product unit is provided on the acrylonitrile product conveying pipeline. Preferably, both the feed regulating valve for the pre-distillation unit and the feed regulating valve for the product unit are connected to the online moisture analyzer.

[0064] The online moisture detector is used to control the feed regulating valve of the pre-distillation unit and the feed regulating valve of the product unit, thereby controlling whether the condensate enters the pre-distillation unit or the product unit.

[0065] Preferably, the pre-distillation unit is a pre-distillation tank, and the product unit is a product tank.

[0066] In this invention, when the water content is above 200 ppm, the feed regulating valve of the pre-distillation unit is opened and the feed regulating valve of the product unit is closed, so that the condensate enters the pre-distillation unit; when the water content is below 200 ppm, the feed regulating valve of the pre-distillation unit is closed and the feed regulating valve of the product unit is opened, so that the condensate enters the product unit.

[0067] In a preferred embodiment, the system further includes a reboiler disposed in the bottom of the distillation column for heating the bottom material, wherein a heating medium inlet line and a heating medium outlet line are disposed on the reboiler.

[0068] In a further preferred embodiment, a heating medium inlet regulating valve is provided on the heating medium inlet pipeline, and a heating medium outlet regulating valve is provided on the heating medium outlet pipeline.

[0069] In a further preferred embodiment, a temperature detector is further provided in the bottom of the distillation column, and the heating medium inlet regulating valve is connected to the temperature detector.

[0070] Specifically, when the temperature of the bottom of the column is higher than the set value, the regulating valve for the heating medium inlet is closed; when the temperature of the bottom of the column is lower than the set value, the regulating valve for the heating medium inlet is opened.

[0071] Preferably, the reboiler uses hot water and / or low-pressure steam as the heating medium, and the reboiler is a thermosiphon type or a forced circulation type.

[0072] In this invention, the pre-distillate from the pre-distillation unit is fed into the reboiler of the distillation column after each batch of distillation. Crude acrylonitrile is added to the reboiler before each batch of distillation begins. The two streams are mixed and distilled under vacuum or inert gas conditions. The pre-distillate enters the pre-distillation unit, while qualified acrylonitrile is sent to the product unit. The entire process is fully automated, offering advantages such as minimal acrylonitrile feedstock loss, reduced polymerization risk during batch waiting periods (the reboiler is cooled using the pre-distillate after each batch, resulting in a shorter high-temperature period compared to traditional natural cooling, further reducing polymerization risk), and safer operation. Furthermore, acrylonitrile is a highly hazardous medium (Class I), and the fully automated process avoids potential harm to personnel during on-site operation.

[0073] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values; these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. In the following, various technical solutions can, in principle, be combined with each other to obtain new technical solutions, which should also be considered as specifically disclosed herein.

[0074] Compared with the prior art, the present invention has the following advantages: the entire process is fully automated, with advantages such as low acrylonitrile raw material loss, low acrylonitrile polymerization during batch waiting time, and safe process operation. Attached Figure Description

[0075] Figure 1 A schematic diagram of the system described in this invention is shown.

[0076] Figure 1 Marker explanation:

[0077] 101. Crude acrylonitrile feedstock

[0078] 102. Top Stream of Distillation Column

[0079] 103. Non-condensable gas stream from the top of a distillation column

[0080] 104. Streaming of condensate from the top of a distillation column

[0081] 105. Feed Flow to the Fore-Distillation Tank

[0082] 106. Product tank feeding logistics

[0083] 107. Flow of the fore-distillate tank back to the distillation column

[0084] 108. Product tank discharge logistics

[0085] 109. Inert gases

[0086] 110. Reboiler heating medium feed stream

[0087] 111. Reboiler heating medium discharge material flow

[0088] 112. Reboiler feed material

[0089] 113. Reboiler discharge material flow

[0090] 114. Distillation column bottoms

[0091] T101, Distillation Column

[0092] E101, Tower Top Condenser

[0093] E102, Reboiler

[0094] S101, Forward Distillation Tank

[0095] S102, Product Can

[0096] PA101, Vacuum System

[0097] V1, Liquid Level Control Valve

[0098] V2, Reboiler Feed Control Valve

[0099] V3, Fore-Distillation Tank Feed Control Valve

[0100] V4, Product Tank Feed Regulating Valve

[0101] V5, Inert Gas Feed Regulating Valve

[0102] V6, Heating medium inlet regulating valve

[0103] V7, Forward Distillation Tank Discharge Control Valve

[0104] V8, Tower Boiler Material Discharge Control Valve

[0105] LC, liquid level detector

[0106] PC, pressure detector

[0107] TC, temperature monitor

[0108] CC, Online Water Analyzer

[0109] Figure 1In the process, crude acrylonitrile is fed into the reboiler of distillation column T101 via the crude acrylonitrile feed line. The level control valve V1 on the crude acrylonitrile feed line is connected to the reboiler level detector LC. When the reboiler level reaches the set value, the feed valve V1 automatically closes. Under vacuum conditions, the light component distills from the top of the column and is condensed in condenser E101. The condensate is analyzed by the online moisture analyzer CC. If the material is substandard, the feed control valve V3 of the fore-distillation tank is opened, and the substandard material is sent to the fore-distillation tank S101. If the material is qualified, the feed control valve V3 of the fore-distillation tank is closed, and the feed control valve V4 of the product tank is opened, and the qualified product is sent to the product tank S102. When the vacuum of the top pressure detector PC is too high, the inert gas feed control valve V5 is increased; when the vacuum of the top pressure detector PC is too low, the inert gas feed control valve V5 is decreased to ensure stable pressure in the distillation column. The bottom feed stream 112 is heated via reboiler feed regulating valve V2 through reboiler E102. The bottom feed temperature detector TC is connected to the heating medium inlet regulating valve V6 on the reboiler heating medium feed stream 110 feed line. When the bottom temperature is too low, the opening of the heating medium inlet regulating valve V6 is increased; when the bottom temperature is too high, the opening of the heating medium inlet regulating valve V6 is decreased. At the end of distillation, the fore-distillate tank discharge control valve V7 is opened to send the cold stream from the fore-distillate tank into the bottom of the distillation column, lowering the bottom temperature. The composition of the bottom feed stream is analyzed periodically. When the content of heavy components is too high, the bottom feed stream is collected via distillation column bottom effluent 114 before the next acrylonitrile feed to prevent the accumulation of heavy components and the resulting product defects. Detailed Implementation

[0110] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0111] It should also be noted that the various specific technical features described in the following embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the various possible combinations will not be described separately in this invention.

[0112] Furthermore, various embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention. The resulting technical solutions are part of the original disclosure of this specification and also fall within the protection scope of the present invention.

[0113] The examples and comparative examples use a 100-ton-level carbon fiber production unit as an example. The crude acrylonitrile raw material is sent from the acrylonitrile tank area, where the acrylonitrile purity is 99.4 wt% and the water content is 0.5 wt%.

[0114]

Example 1

[0115] like Figure 1 As shown, the distillation column operates at a pressure of 50 kPaA. The reboiler heating medium is 90°C hot water, and the feed flow rate of the heating medium is adjusted by the heating medium inlet regulating valve V6 to control the reboiler material temperature at 57.1°C. The top condenser uses chilled water for condensation, and the condensate outlet temperature is controlled at 20°C. The online moisture detection instrument is set to 500 ppm. When the moisture content is greater than 500 ppm, the feed regulating valve V3 of the fore-distillation tank is opened, and the feed regulating valve V4 of the product tank is closed. When the moisture content is less than or equal to 500 ppm, the feed regulating valve V3 of the fore-distillation tank is closed, and the feed regulating valve V4 of the product tank is opened. After distillation is complete, the discharge control valve V7 of the fore-distillation tank is opened to send the 20°C cold solution from the fore-distillation tank S101 into the distillation column reboiler. Crude acrylonitrile is introduced into the distillation column. When the bottom liquid level of the crude acrylonitrile added to the distillation column reaches the set value, the feed of crude acrylonitrile is cut off. After the distillation of this batch is completed, the fore-distillate stream and the crude acrylonitrile stream are introduced into the distillation column until the bottom liquid level reaches the set value. In this embodiment 1, the set value of the bottom liquid level is 0.7H, where H is the height of the straight section of the bottom column.

[0116] After seven intermittent runs, before the next fresh acrylonitrile feedstock was introduced, samples of the bottom column material were analyzed. The content of heavy components (polymers, polymerization inhibitors, etc.) was 2.19%, of which the polymer content was 0.12%, and the water content was 2.63%. The batch-to-batch CV value of the acrylonitrile product was less than 1%, and the acrylonitrile product purity was 99.98 wt%. The CV value refers to the coefficient of variation; the smaller the value, the better the batch-to-batch stability. When the heavy component content in the bottom column material was detected to be greater than 5 wt%, the heavy components were discharged.

[0117]

Example 2

[0118] like Figure 1As shown, the distillation column operates at a pressure of 52 kPaA. The reboiler heating medium is 90°C hot water, and the feed flow rate of the heating medium is adjusted by the heating medium inlet regulating valve V6 to control the reboiler material temperature at 58.0°C. The top condenser uses chilled water for condensation, and the condensate outlet temperature is controlled at 20°C. The online moisture detection instrument is set to 500 ppm. When the moisture content is greater than 500 ppm, the feed regulating valve V3 of the fore-distillation tank is opened, and the feed regulating valve V4 of the product tank is closed; when the moisture content is less than or equal to 500 ppm, the feed regulating valve V3 of the fore-distillation tank is closed, and the feed regulating valve V4 of the product tank is opened. After distillation is complete, the discharge control valve V7 of the fore-distillation tank is opened to send the 20°C cold solution from the fore-distillation tank S101 into the distillation column reboiler. Crude acrylonitrile is introduced into the distillation column. When the bottom liquid level of the crude acrylonitrile added to the distillation column reaches the set value, the feed of crude acrylonitrile is cut off. After the distillation of this batch is completed, the fore-distillate stream and the crude acrylonitrile stream are introduced into the distillation column until the bottom liquid level reaches the set value. In this embodiment 2, the set value of the bottom liquid level is 0.7H, where H is the height of the straight section of the bottom column.

[0119] After seven intermittent runs, before the next fresh acrylonitrile feedstock was introduced, samples were taken from the bottom of the column for analysis. The content of heavy components (polymers, polymerization inhibitors, etc.) was 2.22% (at this point, the bottom material was not discharged). The polymer content was 0.16%, and the water content was 2.64%. The batch-to-batch water content CV value of the acrylonitrile product was less than 1%, and the acrylonitrile product purity was 99.99 wt%. The CV value refers to the coefficient of variation; a smaller value indicates better batch-to-batch stability. When the heavy component content in the bottom material exceeded 5 wt%, the heavy components were discharged.

[0120]

Example 3

[0121] like Figure 1As shown, the distillation column operates at a pressure of 55 kPaA. The reboiler heating medium is 90°C hot water, and the feed flow rate of the heating medium is adjusted by the heating medium inlet regulating valve V6 to control the reboiler material temperature at 59.2°C. The top condenser uses chilled water for condensation, and the condensate outlet temperature is controlled at 20°C. The online moisture detection instrument is set to 500 ppm. When the moisture content is greater than 500 ppm, the feed regulating valve V3 of the fore-distillation tank is opened, and the feed regulating valve V4 of the product tank is closed. When the moisture content is less than or equal to 500 ppm, the feed regulating valve V3 of the fore-distillation tank is closed, and the feed regulating valve V4 of the product tank is opened. After distillation is complete, the discharge control valve V7 of the fore-distillation tank is opened to send the 20°C cold solution from the fore-distillation tank S101 into the distillation column reboiler. Crude acrylonitrile is introduced into the distillation column. When the bottom liquid level of the crude acrylonitrile added to the distillation column reaches the set value, the feed of crude acrylonitrile is cut off. After the distillation of this batch is completed, the fore-distillate stream and the crude acrylonitrile stream are introduced into the distillation column until the bottom liquid level reaches the set value. In this embodiment 3, the set value of the bottom liquid level is 0.7H, where H is the height of the straight section of the bottom column.

[0122] After seven intermittent runs, before the next fresh acrylonitrile feedstock was introduced, samples of the bottom column material were analyzed. The content of heavy components (polymers, polymerization inhibitors, etc.) was 2.29%, of which the polymer content was 0.21%, and the water content was 2.62%. The batch-to-batch water content CV value of the acrylonitrile product was less than 1%, and the acrylonitrile product purity was 99.99 wt%. The CV value refers to the coefficient of variation; the smaller the value, the better the batch-to-batch stability. When the heavy component content in the bottom column material was detected to be greater than 5 wt%, the heavy components were discharged.

[0123]

Example 4

[0124] like Figure 1As shown, the distillation column operates at a pressure of 60 kPaA. The reboiler heating medium is 90°C hot water, and the feed flow rate of the heating medium is adjusted by the heating medium inlet regulating valve V6 to control the reboiler material temperature at 61.8°C. The top condenser uses chilled water for condensation, and the condensate outlet temperature is controlled at 20°C. The online moisture detection instrument is set to 500 ppm. When the moisture content is greater than 500 ppm, the feed regulating valve V3 of the fore-distillation tank is opened, and the feed regulating valve V4 of the product tank is closed. When the moisture content is less than or equal to 500 ppm, the feed regulating valve V3 of the fore-distillation tank is closed, and the feed regulating valve V4 of the product tank is opened. After distillation, the discharge control valve V7 of the fore-distillation tank is opened to send the 20°C cold solution from the fore-distillation tank S101 into the distillation column reboiler. Crude acrylonitrile is introduced into the distillation column. When the bottom liquid level of the crude acrylonitrile added to the distillation column reaches the set value, the feed of crude acrylonitrile is cut off. After the distillation of this batch is completed, the fore-distillate stream and the crude acrylonitrile stream are introduced into the distillation column until the bottom liquid level reaches the set value. In this embodiment 4, the set value of the bottom liquid level is 0.7H, where H is the height of the straight section of the bottom column.

[0125] After seven intermittent runs, before the next fresh acrylonitrile feedstock was introduced, samples of the bottom column material were analyzed. The content of heavy components (polymers, polymerization inhibitors, etc.) was 2.38%, of which the polymer content was 0.27%, and the water content was 2.62%. The batch-to-batch water content CV value of the acrylonitrile product was less than 1%, and the acrylonitrile product purity was 99.99 wt%. The CV value refers to the coefficient of variation; the smaller the value, the better the batch-to-batch stability. When the heavy component content in the bottom column material was detected to be greater than 5 wt%, the heavy components were discharged.

[0126]

Example 5

[0127] like Figure 1As shown, the distillation column operates at a pressure of 65 kPaA. The reboiler heating medium is 90°C hot water, and the feed flow rate of the heating medium is adjusted by the heating medium inlet regulating valve V6 to control the reboiler material temperature at 64.2°C. The top condenser uses chilled water for condensation, and the condensate outlet temperature is controlled at 20°C. The online moisture detection instrument is set to 500 ppm. When the moisture content is greater than 500 ppm, the feed regulating valve V3 of the fore-distillate tank is opened, and the feed regulating valve V4 of the product tank is closed. When the moisture content is less than or equal to 500 ppm, the feed regulating valve V3 of the fore-distillate tank is closed, and the feed regulating valve V4 of the product tank is opened. After distillation, the discharge control valve V7 of the fore-distillate tank is opened to send the 20°C cold solution from the fore-distillate tank S101 into the distillation column reboiler. Crude acrylonitrile is introduced into the distillation column. When the bottom liquid level of the crude acrylonitrile added to the distillation column reaches the set value, the feed of crude acrylonitrile is cut off. After the distillation of this batch is completed, the fore-distillate stream and the crude acrylonitrile stream are introduced into the distillation column until the bottom liquid level reaches the set value. In this embodiment 5, the set value of the bottom liquid level is 0.7H, where H is the height of the straight section of the bottom column.

[0128] After seven intermittent runs, before the next fresh acrylonitrile feedstock was introduced, samples of the bottom column material were analyzed. The content of heavy components (polymers, polymerization inhibitors, etc.) was 2.41%, of which the polymer content was 0.31%, and the water content was 2.65%. The batch-to-batch water content CV value of the acrylonitrile product was less than 1%, and the acrylonitrile product purity was 99.98 wt%. The CV value refers to the coefficient of variation; the smaller the value, the better the batch-to-batch stability. When the heavy component content in the bottom column material was detected to be greater than 5 wt%, the heavy components were discharged.

[0129]

Example 6

[0130] like Figure 1As shown, the distillation column operates at a pressure of 50 kPaA. The heating medium for the reboiler is 151.9°C low-pressure steam. The feed flow rate of the heating medium is adjusted by the heating medium inlet regulating valve V6, controlling the reboiler material temperature at 57.1°C. The top condenser uses chilled water for condensation, controlling the condensate outlet temperature at 20°C. The online moisture meter is set to 500 ppm. When the moisture content is greater than 500 ppm, the feed regulating valve V3 of the fore-distillate tank is opened, and the feed regulating valve V4 of the product tank is closed. When the moisture content is less than or equal to 500 ppm, the feed regulating valve V3 of the fore-distillate tank is closed, and the feed regulating valve V4 of the product tank is opened. After distillation, the discharge control valve V7 of the fore-distillate tank is opened, sending the 20°C cold solution from the fore-distillate tank S101 into the distillation column reboiler. Crude acrylonitrile is introduced into the distillation column. When the bottom liquid level of the crude acrylonitrile added to the distillation column reaches the set value, the feed of crude acrylonitrile is cut off. After the distillation of this batch is completed, the fore-distillate stream and the crude acrylonitrile stream are introduced into the distillation column until the bottom liquid level reaches the set value. In this embodiment 6, the set value of the bottom liquid level is 0.7H, where H is the height of the straight section of the bottom column.

[0131] After seven intermittent runs, before the next fresh acrylonitrile feedstock was introduced, samples of the bottom column material were analyzed. The content of heavy components (polymers, polymerization inhibitors, etc.) was 2.61%, of which the polymer content was 0.45%, and the water content was 2.63%. The batch-to-batch water content CV value of the acrylonitrile product was less than 1%, and the acrylonitrile product purity was 99.98 wt%. The CV value refers to the coefficient of variation; the smaller the value, the better the batch-to-batch stability. When the heavy component content in the bottom column material was detected to be greater than 5 wt%, the heavy components were discharged.

[0132]

Comparative Example 1

[0133] like Figure 1 As shown, the process of Example 1 is repeated, except that the foredistillate is not recycled back to the distillation column reboiler: the distillation column operating pressure is 50 kPaA, the heating medium for the reboiler is 90°C hot water, and the heating medium feed flow rate is adjusted by the heating medium inlet regulating valve V6 to control the reboiler material temperature at 57.1°C. Chilled water is used for condensation in the top condenser, and the condensate outlet temperature is controlled at 20°C. The online moisture meter is set to 500 ppm. When the moisture content is greater than 500 ppm, the foredistillate tank feed regulating valve V3 is opened and the product tank feed regulating valve V4 is closed; when the moisture content is less than or equal to 500 ppm, the foredistillate tank feed regulating valve V3 is closed and the product tank feed regulating valve V4 is opened.

[0134] After seven intermittent runs, samples were taken from the bottom of the column before the next fresh acrylonitrile feedstock was introduced. The heavy component content was 3.64%, of which the polymer content was 1.52% and the water content was 2.63%. The batch-to-batch water content CV value of the acrylonitrile product was less than 1%. In Comparative Example 1, the fore-distillate was not recycled back to the bottom of the column, and the bottom of the column relied on natural cooling, resulting in a very slow cooling of the bottom of the column. Acrylonitrile was kept at a high temperature for a long time, which made it easy for it to polymerize.

[0135] [Comparative Example 2]

[0136] like Figure 1 As shown, the distillation column operates at a pressure of 50 kPaA, and the reboiler heating medium is 90°C hot water. The feed flow rate of the heating medium is adjusted by the heating medium inlet regulating valve V6, controlling the reboiler material temperature at 57.1°C. The top condenser uses chilled water for condensation, controlling the condensate outlet temperature at 20°C. Offline sampling and analysis of the condensate are performed, and the feed regulating valves V3 and V4 of the fore-distillate tank and product tank are manually switched based on the water content. After distillation is complete, the fore-distillate tank outlet control valve V7 is opened, sending the 20°C cold solution from the fore-distillate tank S101 into the distillation column reboiler.

[0137] After seven intermittent runs, samples were taken from the bottom of the tower before the next fresh acrylonitrile feedstock was introduced. The heavy component content was 2.21%, of which the polymer content was 0.14% and the water content was 2.64%. The batch-to-batch water content CV value of the acrylonitrile product was 3.2%. Operators need to frequently visit the plant to take samples, which increases the risk of exposure to the highly hazardous medium (Class I) acrylonitrile. Furthermore, product stability varies between different batches due to differences in operators.

[0138] The present invention has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for purifying crude acrylonitrile, comprising: (1) The crude acrylonitrile is distilled and the top material is cooled to obtain condensate. The bottom material is discharged arbitrarily. (2) Based on the water content in the condensate, it is divided into pre-distillate and acrylonitrile products; (3) The acrylonitrile product is directly extracted, and the pre-distillate is recycled back to the distillation process in step (1); The distillation process is carried out in a distillation column, with the crude acrylonitrile entering from the bottom or lower part of the distillation column, and the fore-distillate entering from the bottom or lower part of the distillation column.

2. The method of claim 1, wherein, The crude acrylonitrile contains one or more of the following: oxazole, acetaldehyde, acetone, water, and polymerization inhibitor.

3. The method of claim 2, wherein, The water content in the crude acrylonitrile is between 6000 ppm and 1000 ppm.

4. The method according to claim 2, characterized in that, The water content in the crude acrylonitrile is between 4500 ppm and 1000 ppm.

5. The method according to claim 1, characterized in that, The bottom temperature of the distillation column is 55~70℃; and / or the operating pressure of the distillation column is 30~70 kPaA.

6. The method according to claim 1, characterized in that, The distillation process is carried out in a distillation column, which uses a reboiler to heat the bottom material. The reboiler uses hot water and / or low-pressure steam as the heating medium.

7. The method according to claim 1, characterized in that, The temperature of the column reboiler is controlled by controlling the flow rate of the heating medium, wherein the flow rate of the heating medium is controlled by a temperature controller installed in the reboiler of the distillation column.

8. The method according to claim 1, characterized in that, When the temperature of the bottom column is higher than the set value, the flow rate of the heating medium is reduced; when the temperature of the bottom column is lower than the set value, the flow rate of the heating medium is increased.

9. The method according to claim 1, characterized in that, The distillation process is carried out inside a distillation column, and the pressure of the distillation column is controlled by adding inert gas into the column.

10. The method according to claim 9, characterized in that, When the pressure in the distillation column is higher than the set value, the flow rate of the inert gas entering the distillation column is reduced; when the pressure in the distillation column is lower than the set value, the flow rate of the inert gas entering the distillation column is increased.

11. The method according to claim 1, characterized in that, The distillation in step (1) is a batch distillation, wherein when the bottom liquid level of the crude acrylonitrile added to the distillation column reaches the set value, the feed of crude acrylonitrile is cut off; after the batch distillation is completed, the fore-distillate stream and the crude acrylonitrile stream are introduced into the distillation column until the bottom liquid level reaches the set value.

12. The method according to claim 11, characterized in that, The set value for the bottom liquid level is 0.4~0.9H, where H is the height of the straight section of the bottom column.

13. The method according to claim 11, characterized in that, The set value of the liquid level in the bottom of the column is 0.5~0.8H, where H is the height of the straight section of the bottom of the column.

14. The method according to claim 1, characterized in that, In step (1), the temperature is cooled to 10~40°C; and / or, In step (1), the distillation process and the cooling process are carried out in the presence of a vacuum or an inert gas.

15. The method according to claim 1, characterized in that, In step (1), the temperature is cooled to 15~25°C; and / or, In step (1), the distillation process and the cooling process are carried out in the presence of nitrogen.

16. The method according to any one of claims 1 to 15, characterized in that, Set the required acrylonitrile product to have a water content of less than or equal to x In step (2), when the water content is less than or equal to x The condensate, when used as an acrylonitrile product, has a water content greater than [missing information]. x The condensate at that time is used as the fore-distillate; and / or, When the content of heavy components in the bottom material of the tower exceeds 5 wt%, the bottom material of the tower should be discharged before the next acrylonitrile feed.

17. The method according to claim 16, characterized in that, The water content in the condensate was detected using an online moisture analyzer.

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