Energy-saving method and system for removing ethyl acetate by differential pressure thermal coupling method

Abstract

The invention relates to the field of vinyl acetate production, in particular to an energy-saving system and method for removing ethyl acetate through a differential pressure thermal coupling method. The method comprises the following steps: carrying out first rectification on a vinyl acetate crude product to obtain a first tower top fraction and a first rectification fraction containing vinyl acetate; carrying out second rectification on the first rectification fraction to obtain a second overhead fraction; the second tower bottom fraction is subjected to third rectification, and an obtained third tower top fraction is extracted as refined vinyl acetate; wherein the temperature of the first rectification is higher than that of the third rectification, the pressure of the first rectification is not less than 0MPa, and the pressure of the third rectification is negative pressure. By means of differential pressure coupling, normal pressure operation of the third rectifying tower is changed into negative pressure operation, a reboiler of the third rectifying tower is combined with a condenser of the first rectifying tower, and it is accidentally found that the change can not only reduce energy consumption and the use amount of water, but also remove trace ethyl acetate in the product vinyl acetate; and an unexpected technical effect is achieved.

Description

Technical Field

[0001] The invention relates to the field of vinyl acetate production, and in particular to an energy-saving system and method for removing ethyl acetate by a differential pressure thermal coupling method. Background Art

[0002] Vinyl acetate is an important chemical raw material, which is widely used in the manufacture of polyvinyl alcohol, vinyl copolymer resin, adhesive, coating, textile processing, paper coating and other aspects. The preparation process of vinyl acetate is mainly divided into a synthesis section and a refining section. The main function of the refining section is to separate and purify the vinyl acetate in the reaction solution. The refined vinyl acetate can be sold directly as a product, or sent to the polymerization section to polymerize the vinyl acetate into polyvinyl acetate, and then through the alcoholysis section to make polyvinyl alcohol, which can be used as a raw material for the production of vinyl acetate, coating, and special rubber. The refining of the crude vinyl acetate plays an extremely important role in subsequent applications. For example, patent CN 1699326A discloses a method for refining a crude vinyl acetate product obtained by oxyacylation gas phase reaction of ethylene, acetic acid and oxygen-containing gas. The method comprises the following steps: the crude vinyl acetate product is pickled with acetic acid through a pickling tower; the first distillation tower is used for distillation to remove by-products of ethyl acetate, acetic acid and water; the second distillation tower is used for distillation to remove by-products of methyl acetate and acetaldehyde; the third distillation tower is used for acetic acid refining; the fourth distillation tower is used for refining again to remove the residual trace amount of ethyl acetate. The method mainly separates ethyl acetate and water from vinyl acetate at the first time of distillation, making the operation of the distillation process easier and more stable, and reducing energy consumption, but there is no reuse of heat in the whole process. Patent CN 105964008A discloses a high-purity vinyl acetate separation system, which uses synthetic crude vinyl acetate as raw material and adopts a double-tower "one evaporation, one condensation" distillation method to obtain refined vinyl acetate with a purity of more than 99.9%, solving the problems of high impurities and high energy consumption in crude vinyl acetate in traditional processes. Although this method reduces the content of product materials in the non-product system, it does not take into account the mutual supplementation and utilization of heat among the streams of the distillation tower.

[0003] The traditional technology of preparing vinyl acetate from ethylene not only has a high ethyl acetate content in the product, but also has high energy consumption. In order to improve product quality, reduce energy consumption of production equipment and enhance the competitiveness of the process, how to provide a new process that can save energy and reduce consumption while removing ppm-level ethyl acetate from vinyl acetate is a technical problem that needs to be solved at present. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides an energy-saving system and method for removing ethyl acetate by differential pressure thermal coupling. The present invention refines the crude vinyl acetate by a first distillation tower, a second distillation tower and a third distillation tower. By differential pressure coupling, the third distillation tower is changed from normal pressure operation to negative pressure operation, and the reboiler of the third distillation tower is combined with the condenser of the first distillation tower. The present invention unexpectedly finds that this change can not only reduce energy consumption and water consumption, but also remove trace amounts of ethyl acetate in the product vinyl acetate, achieving unexpected technical effects.

[0005] The first aspect of the present invention is to provide a method for removing ethyl acetate by a differential pressure thermal coupling method, comprising the following steps:

[0006] (1) subjecting the crude vinyl acetate to a first distillation to obtain a first overhead fraction, and subjecting the water in the first overhead fraction to a first separation from a fraction containing vinyl acetate to obtain a first distillate containing vinyl acetate;

[0007] (2) subjecting the first fraction to a second distillation to obtain a second bottom fraction containing vinyl acetate and a second top fraction containing water and light components;

[0008] (3) subjecting the second bottom fraction to a third distillation to obtain a third bottom fraction and a third top fraction, wherein the third top fraction is extracted as refined vinyl acetate;

[0009] The temperature of the first distillation is higher than the temperature of the third distillation, the pressure of the first distillation is not less than 0 MPa, and the pressure of the third distillation is negative pressure;

[0010] The first tower top fraction is cooled by heat exchange and provides heat for the third distillation.

[0011] The present invention not only can remove ethyl acetate at the ppm level from vinyl acetate through a system method of differential pressure thermal coupling, but also can save energy and reduce consumption, so that energy can be reasonably utilized, and the consumption of low-pressure steam and circulating cooling water can be reduced. After differential pressure thermal coupling, the number of plates, feed position and amount of inhibitor in the third distillation tower are compared with the original operation, but it is surprisingly found that the content of ethyl acetate in the product will be significantly reduced.

[0012] According to the present invention, the specific operating conditions of the differential pressure thermal coupling can be selected within a wide range. In a preferred embodiment of the present invention, the temperature difference between the top temperature of the first distillation tower and the bottom temperature of the third distillation tower corresponding to the differential pressure thermal coupling is not less than 5°C, preferably not less than 10°C, and the pressure difference is not less than 50KPa, preferably not less than 100KPa; further preferably,

[0013] The operating temperature of the first distillation is 70-150°C, preferably 80-140°C, and the operating temperature of the third distillation is 20-80°C, preferably 30-60°C.

[0014] The specific control conditions of the first distillation and the third distillation can be adjusted in a wide range. According to a preferred embodiment of the present invention, the operating pressure of the first distillation is 0.0-0.1 MPaG, and the operating temperature is 70-150° C.; and / or, the operating pressure of the third distillation is 5-95 KPaA, and the operating temperature is 20-80° C. More preferably, the number of plates of the third distillation tower is 30 or more.

[0015] According to a preferred embodiment of the present invention, in terms of percentage by weight, the acetic acid content in the crude vinyl acetate is 50-72wt%, the vinyl acetate content is 15-38wt%, the water content is 4-12wt%, the acetaldehyde / light component content is 0.05-0.5wt%, and the ethyl acetate content is 100-500ppm.

[0016] According to a preferred embodiment of the present invention, the conditions of the first distillation are such that, in the first distillation fraction, the content of vinyl acetate is 95-99.5wt%, the content of water is 0.3-2wt%, the content of acetaldehyde / light components is 0.1-1wt%, and the content of ethyl acetate is 100-1700ppm, calculated by weight percentage;

[0017] The conditions of the second distillation are such that, in the second bottom fraction, the content of vinyl acetate is 99.80 to 99.99 wt % and the content of ethyl acetate is 100 to 1850 ppm, calculated by weight percentage;

[0018] The conditions of the third distillation are such that the content of ethyl acetate in the third top fraction is ≤250 mg / kg.

[0019] According to a preferred embodiment of the present invention, the operating pressure of the first distillation is 0.0-0.1 MPaG, the operating temperature is 70-150°C, and the content of acetic acid in the first distillation feed composition is 50-72wt%, the content of vinyl acetate is 15-38wt%, the content of water is 4-12wt%, the content of acetaldehyde / light components is 0.05-0.5wt%, and the content of ethyl acetate is 100-500ppm.

[0020] According to a preferred embodiment of the present invention, the operating pressure of the second distillation is 0.0-0.1 MPaG, the operating temperature is 50-100°C, and the content of vinyl acetate in the second distillation feed composition, calculated by weight percentage, is 95-99.5wt%, the content of water is 0.3-2wt%, the content of acetaldehyde / light components is 0.1-1wt%, and the content of ethyl acetate is 100-1700ppm.

[0021] According to a preferred embodiment of the present invention, the operating pressure of the third distillation is 5-95 KPaA, the operating temperature is 20-80°C, and the content of vinyl acetate in the third distillation feed composition is 99.8-99.99wt%, and the content of ethyl acetate is 100-1850ppm, calculated by weight percentage.

[0022] The preferred control conditions of each tower in the present invention can further reduce the content of ethyl acetate, and further achieve energy saving and consumption reduction under the process flow of the present invention.

[0023] According to a preferred embodiment of the present invention, each of the first distillation, the second distillation and the third distillation is carried out in the presence of a polymerization inhibitor, and the polymerization inhibitor is a common additive in the art, including but not limited to the polymerization inhibitor selected from p-benzoquinone and / or piperidinol oxide.

[0024] The dosage of the polymerization inhibitor is also the conventional dosage in the art, including but not limited to 0.0001 wt%-0.001 wt% of the reflux amount.

[0025] The second aspect of the present invention is to provide a system for removing ethyl acetate from vinyl acetate by a differential pressure thermal coupling method, such as Figure 1 As shown, it includes a first distillation tower 1, a heat exchanger 2, a first distillation tank 3, a second distillation tower 4, and a third distillation tower 5, wherein:

[0026] The top outlet of the first distillation tower 1 is connected to the heat exchanger 2, the first tower distillation tank 3, and the feed inlet of the second distillation tower 4 in sequence;

[0027] The bottom outlet of the second distillation tower 4 is connected to the feed inlet of the third distillation tower 5;

[0028] The first tower distillation tank 3 is used to separate the water in the first tower top fraction from the fraction containing vinyl acetate and to transport the fraction containing vinyl acetate to the second distillation tower 4;

[0029] The operating temperature of the first distillation tower 1 is higher than the operating temperature of the third distillation tower 5, and the operating pressure of the first distillation tower 1 is not less than 0 MPa, and the operating pressure of the third distillation tower 5 is negative pressure;

[0030] The heat exchanger 2 transfers heat from the top fraction of the first distillation tower 1 to the third distillation tower 5 .

[0031] According to a preferred embodiment of the present invention, the system further comprises a tower condenser 6 , and the tower condenser 6 is disposed between the heat exchanger 2 and a tower distillation tank 3 .

[0032] According to a preferred embodiment of the present invention, the system further comprises a second-tower condenser 7 and a second-tower distillation tank 8 which are connected to the top outlet of the second distillation tower 4 .

[0033] According to a preferred embodiment of the present invention, the system further comprises a three-tower condenser 9 and a three-tower distillation tank 10 connected to the top outlet of the third distillation tower 5 .

[0034] According to a preferred embodiment of the present invention, the system further comprises a reboiler disposed at the lower portion of the first distillation tower 1 and / or a reboiler disposed at the lower portion of the second distillation tower 4 including a first-tower reboiler 11 and a second-tower reboiler 12 .

[0035] According to a preferred embodiment of the present invention, no reboiler is provided at the lower portion of the third distillation tower 5 , and the reboiler is replaced by a heat transfer pipeline connected to the heat exchanger 2 .

[0036] The control conditions of the production system in the present invention are preferably the control conditions described in the first aspect, which will not be repeated here.

[0037] The advantages of the present invention are:

[0038] By adopting the method of the present invention, the ethyl acetate content in the produced VAC is ≤250 mg / kg, and the ethyl acetate content in the VAC is reduced to a relatively low level; the consumption of low-pressure steam and circulating cooling water is greatly reduced.

[0039] Compared with the production system of removing ethyl acetate from vinyl acetate in the prior art, the production system of the present invention not only does not require the addition of additional equipment, but also can reduce the number of devices (reboilers), thereby further reducing production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is the process flow of removing ethyl acetate from vinyl acetate in the prior art;

[0041] The first rectifying tower 1, the first tower distillation tank 3, the second rectifying tower 4, the third rectifying tower 5, the first tower condenser 6, the second tower condenser 7, the second tower distillation tank 8, the third tower condenser 9, the third tower distillation tank 10, the first tower reboiler 11, the second tower reboiler 12, the third tower reboiler 13

[0042] Figure 2 This is the process flow of removing ethyl acetate from vinyl acetate in the present invention.

[0043] The first distillation tower 1, the heat exchanger 2, the first tower distillation tank 3, the second distillation tower 4, the third distillation tower 5, the first tower condenser 6, the second tower condenser 7, the second tower distillation tank 8, the third tower condenser 9, the third tower distillation tank 10, the first tower reboiler 11, the second tower reboiler 12 DETAILED DESCRIPTION

[0044] The present invention is described in detail below in conjunction with specific drawings and embodiments. It is necessary to point out that the following embodiments are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made to the present invention by those skilled in the art based on the content of the present invention still fall within the scope of protection of the present invention.

[0045] The method for removing ethyl acetate by differential pressure thermal coupling method comprises the following steps:

[0046] (1) subjecting the crude vinyl acetate to a first distillation to obtain a first overhead fraction, and subjecting the water in the first overhead fraction to a first separation from a fraction containing vinyl acetate to obtain a first distillate containing vinyl acetate;

[0047] (2) subjecting the first fraction to a second distillation to obtain a second bottom fraction containing vinyl acetate and a second top fraction containing water and light components;

[0048] (3) subjecting the second bottom fraction to a third distillation to obtain a third bottom fraction and a third top fraction, wherein the third top fraction is extracted as refined vinyl acetate;

[0049] The temperature of the first distillation is higher than that of the third distillation, and the pressure of the first distillation is not less than 0 MPa, and the pressure of the third distillation is negative pressure; the first tower top fraction is cooled by heat exchange and heat is provided to the third distillation.

[0050] like Figure 2 As shown: the reaction liquid is sent to the first distillation tower 1 from the reaction section. The first distillation tower 1 is a normal pressure or pressurized distillation, and the heavy component acetic acid is returned to the reaction system from the bottom of the tower, and the top distillate is vinyl acetate, water, ethyl acetate and light components. After the top gas is condensed, the water phase and the oil phase are separated, and vinyl acetate and trace ethyl acetate and light components enter the second distillation tower 4. The second distillation tower 4 is a normal pressure tower, and the light component is removed from the top of the tower. The top distillate is water, light components and entrained vinyl acetate, and the bottom of the tower is vinyl acetate, ethyl acetate and inhibitor. The bottom material of the second distillation tower 4 enters the third distillation tower 5. In order to achieve the purpose of differential pressure thermal coupling, the third distillation tower 5 is optimized as a vacuum tower. The tower pressure is adjusted to 5-95KPaA, and the bottom of the third distillation tower 5 is thermally coupled with the top heat exchanger 2 of the first distillation tower 1, which can significantly reduce the energy consumption of the third distillation tower 5. At the same time as the superheat coupling energy saving, it was unexpectedly found that the purity of the product distilled from the top of the third distillation tower 5 was greatly improved, and ethyl acetate was removed from the bottom of the tower.

[0051] In the following examples, the pressure range is the pressure from the top of the tower to the bottom of the tower; the corresponding temperature range is the temperature from the top of the tower to the bottom of the tower.

[0052] Specifically, it is described by the following examples:

[0053] Example 1

[0054] The reboiler of the third distillation tower is combined with the condenser of the first distillation tower through differential pressure coupling. Specifically, the conditions are as follows:

[0055] The production scale of vinyl acetate (VAC) is 300,000 tons / year. The operating pressure of the first distillation tower is 0.03-0.08MPaG, the operating temperature is 74-132°C, and the reaction liquid feed flow rate is 176615kg / h. The composition includes acetic acid content of 71.1wt%, vinyl acetate content of 21.4wt%, water content of 7.2wt%, acetaldehyde / light component content of 0.07wt%, and ethyl acetate content of 350ppm. After the first distillation tower, all the heavy acetic acid is returned from the tower bottom to the reaction section, and the heat load of the top condenser is 29.9MW. The operating pressure of the second distillation tower is 0.0-0.05MPaG, the operating temperature is 70-85°C, and the feed composition includes vinyl acetate content of 98.1wt%, water content of 1.3wt%, acetaldehyde / light component content of 0.3wt%, and ethyl acetate content of 1600ppm. The operating pressure of the third distillation tower is 20-60KPaA, the operating temperature is 25-60℃, the content of vinyl acetate in the feed composition is 99.8wt%, and the content of ethyl acetate is 1800ppm. The number of plates is 58, and the number of feed plates is 30. The heat load required by the tower bottom reboiler is 28.9MW. The first stage distillation and the third stage distillation are differential pressure thermally coupled. After differential pressure thermal coupling, the third distillation tower does not need to provide an additional heat source, saving a lot of steam. At the same time, it was unexpectedly found that the ethyl acetate content at the top of the third distillation tower was reduced to 136ppm.

[0056] Example 2

[0057] The production scale of vinyl acetate (VAC) is 500,000 tons / year. The operating pressure of the first distillation tower is 0.06-0.11MPaG, the operating temperature is 80-138°C, and the reaction liquid feed flow rate is 294538kg / h. The composition includes acetic acid content of 70.7wt%, vinyl acetate content of 20.8wt%, water content of 8.3wt%, acetaldehyde / light component content of 0.07wt%, and ethyl acetate content of 350ppm. After the first distillation tower, all the heavy acetic acid is returned from the tower bottom to the reaction section, and the heat load of the top condenser is 50.6MW. The operating pressure of the second distillation tower is 0.0-0.05MPaG, the operating temperature is 71-82°C, and the feed composition includes vinyl acetate content of 98.0wt%, water content of 1.3wt%, acetaldehyde / light component content of 0.3wt%, and ethyl acetate content of 1460ppm. The operating pressure of the third distillation tower is 30-70KPaA, the operating temperature is 35-63°C, and the content of vinyl acetate in the feed composition is 99.8wt%, and the content of ethyl acetate is 1632ppm. The number of plates is 64, and the number of feed plates is 36. The heat load required by the tower bottom reboiler is 47.6MW. The first stage distillation and the third stage distillation are differential pressure thermally coupled. After differential pressure thermal coupling, the third distillation tower does not need to provide an additional heat source, saving a lot of steam. At the same time, it was unexpectedly found that the ethyl acetate content at the top of the third distillation tower was reduced to 116ppm.

[0058] Comparative Example 1

[0059] The production scale of vinyl acetate (VAC) is 500,000 tons / year. The operating pressure of the first distillation tower is 0.06-0.11MPaG, the operating temperature is 80-138°C, and the reaction liquid feed flow rate is 294538kg / h. The composition includes acetic acid content of 70.7wt%, vinyl acetate content of 20.8wt%, water content of 8.3wt%, acetaldehyde / light component content of 0.07wt%, and ethyl acetate content of 350ppm. After the first distillation tower, all the heavy acetic acid is returned from the tower bottom to the reaction section, and the heat load of the top condenser is 50.6MW. The second distillation tower is thermally coupled with the first distillation tower. The operating pressure of the second distillation tower is 30-0.05MPaG, the operating temperature is 38-58°C, the vinyl acetate content in the feed composition is 98.0wt%, the water content is 1.3wt%, the acetaldehyde / light component content is 0.3wt%, the ethyl acetate content is 1460ppm, and the heat load required by the tower bottom reboiler is 5.3MW. The operating pressure of the third distillation tower is 0-0.3MPaG, the operating temperature is 67-83℃, and the content of vinyl acetate in the feed composition is 99.8wt%, and the content of ethyl acetate is 1656ppm. The number of plates is 64, and the number of feed plates is 36. Through differential pressure thermal coupling, the second distillation tower does not need to provide an additional heat source, which can save a certain amount of steam, but the heat at the top of the first distillation tower is not fully utilized. At the same time, the temperature of the kettle of the third distillation tower is higher than the temperature at the top of the first distillation tower, and the heat of the first distillation tower cannot be used. The ethyl acetate content at the top of the third distillation tower is 520ppm.

[0060] By comparison, the differential pressure thermal coupling of the first and third distillation stages can fully utilize the heat at the top of the first distillation tower, and unexpectedly achieve the purpose of product purification. The thermal coupling of the second distillation tower with the first distillation tower can achieve a certain energy-saving effect, but the energy at the top of the first distillation tower cannot be fully utilized, and the trace amount of ethyl acetate in the product cannot be removed.

[0061] It should be noted that the embodiments described above are only used to explain the present invention and do not constitute any limitation to the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory words, rather than restrictive words. The present invention may be modified as specified within the scope of the claims of the present invention, and the present invention may be revised without departing from the scope and spirit of the present invention. Although the present invention described therein relates to specific methods, materials and embodiments, it does not mean that the present invention is limited to the specific examples disclosed therein, on the contrary, the present invention can be extended to all other methods and applications with the same functions.

[0062] All publications, patent applications, patents and other references mentioned in this specification are incorporated herein by reference. Unless otherwise defined, all technical and scientific terms used in this specification have the meanings conventionally understood by those skilled in the art. In the event of a conflict, the definition in this specification shall prevail.

[0063] When this specification uses the prefix "well-known to those skilled in the art", "prior art" or similar terms to introduce materials, substances, methods, steps, devices or components, etc., the objects introduced by the prefix cover those commonly used in the art when this application is filed, but also include those that are not commonly used at present but will become recognized in the art as being suitable for similar purposes.

[0064] The endpoints and any values ​​of the scope disclosed in the present application document are not limited to the precise scope or value, and these scopes or values ​​should be understood to include values ​​close to these scopes or values. For numerical ranges, between the endpoint values ​​of each scope, between the endpoint values ​​of each scope and a separate point value, and between separate point values, one or more new numerical ranges can be combined with each other, and these numerical ranges should be considered as specifically disclosed in this article. Hereinafter, in principle, each technical solution can be combined with each other to obtain a new technical solution, which should also be considered as specifically disclosed in this article.

[0065] In the context of the present specification, any matters or items not mentioned are directly applicable to those known in the art without any changes except those explicitly stated.

[0066] Moreover, any embodiment described in this document may be freely combined with one or more other embodiments described in this document, and the technical solutions or technical ideas formed thereby are deemed to be part of the original disclosure or original record of the present invention, and should not be regarded as new content that has not been disclosed or anticipated in this document, unless a person skilled in the art considers that the combination is obviously unreasonable.

Claims

1. A method for removing ethyl acetate by differential pressure thermal coupling method, The following steps are involved: (1) subjecting the crude vinyl acetate to a first distillation to obtain a first overhead fraction, and subjecting the water in the first overhead fraction to a first separation from a fraction containing vinyl acetate to obtain a first distillate containing vinyl acetate; (2) subjecting the first fraction to a second distillation to obtain a second bottom fraction containing vinyl acetate and a second top fraction containing water and light components; (3) subjecting the second bottom fraction to a third distillation to obtain a third bottom fraction and a third top fraction, wherein the third top fraction is extracted as refined vinyl acetate; The temperature of the first distillation is higher than the temperature of the third distillation, the pressure of the first distillation is not less than 0 MPa, and the pressure of the third distillation is negative pressure; The first tower top fraction is cooled by heat exchange and provides heat for the third distillation.

2. The method according to claim 1, Features: The temperature difference between the top temperature of the first distillation tower and the bottom temperature of the third distillation tower corresponding to the differential pressure thermal coupling is not less than 5°C, preferably not less than 10°C, and the pressure difference is not less than 50KPa, preferably not less than 100KPa.

3. The method according to claim 2, Features: The operating pressure of the first distillation is 0.0-0.1 MPaG, and the operating temperature is 70-150° C.; and / or, The operating pressure of the third distillation is 5-95 KPaA, and the operating temperature is 20-80°C.

4. The method according to any one of claims 1 to 3, Features: In terms of percentage by weight, the crude vinyl acetate contains 50-72 wt% acetic acid, 15-38 wt% vinyl acetate, 4-12 wt% water, 0.05-0.5 wt% acetaldehyde / light components, and 100-500 ppm ethyl acetate.

5. The method according to any one of claims 1 to 3, Features: The conditions of the first distillation are such that, in the first distillation fraction, the content of vinyl acetate is 95-99.5 wt %, the content of water is 0.3-2 wt %, the content of acetaldehyde / light components is 0.1-1 wt %, and the content of ethyl acetate is 100-1700 ppm, calculated by weight percentage; The conditions of the second distillation are such that, in the second bottom fraction, the content of vinyl acetate is 99.80 to 99.99 wt % and the content of ethyl acetate is 100 to 1850 ppm, calculated by weight percentage; The conditions of the third distillation are such that the content of ethyl acetate in the third top fraction is ≤250 mg / kg.

6. The method according to any one of claims 1 to 3, Features: The operating pressure of the first distillation is 0.0-0.1 MPaG, the operating temperature is 70-150° C., and the content of acetic acid in the first distillation feed composition is 50-72wt%, the content of vinyl acetate is 15-38wt%, the content of water is 4-12wt%, the content of acetaldehyde / light components is 0.05-0.5wt%, and the content of ethyl acetate is 100-500ppm; and / or, The operating pressure of the second distillation is 0.0-0.1 MPaG, the operating temperature is 50-100° C., and the content of vinyl acetate in the second distillation feed composition is 95-99.5wt%, the content of water is 0.3-2wt%, the content of acetaldehyde / light components is 0.1-1wt%, and the content of ethyl acetate is 100-1700ppm; and / or, The operating pressure of the third distillation is 5-95 KPaA, the operating temperature is 20-80°C, and the content of vinyl acetate in the third distillation feed composition is 99.8-99.99wt%, and the content of ethyl acetate is 100-1850ppm in terms of weight percentage.

7. The method according to any one of claims 1 to 3, Features: The first distillation, the second distillation and the third distillation are each carried out in the presence of a polymerization inhibitor, preferably, the polymerization inhibitor is selected from p-benzoquinone and / or piperidinol oxide; and / or, The dosage of the polymerization inhibitor is 0.0001wt%-0.001wt% of the reflux amount.

8. A system for removing ethyl acetate from vinyl acetate by differential pressure thermal coupling method, comprising a first distillation tower, a heat exchanger, a distillation tank, a second distillation tower, and a third distillation tower. in, The top outlet of the first distillation tower is connected to the heat exchanger, the first tower distillation tank, and the feed inlet of the second distillation tower in sequence; The bottom outlet of the second distillation tower is connected to the feed inlet of the third distillation tower; The first tower distillation tank is used to separate the water in the first tower top fraction from the fraction containing vinyl acetate and to transport the fraction containing vinyl acetate to the second distillation tower; The operating temperature of the first distillation tower is higher than the operating temperature of the third distillation tower, and the operating pressure of the first distillation tower is not less than 0 MPa, and the operating pressure of the third distillation tower is negative pressure; The heat exchanger transfers heat from the top fraction of the first distillation column to the third distillation column.

9. The system according to claim 8, Features: The system further comprises a tower condenser, wherein the tower condenser is disposed between the heat exchanger and a tower distillation tank; and / or, The system further comprises a second-tower condenser and a second-tower distillation tank connected to the top outlet of the second distillation tower; and / or, The system also includes a three-tower condenser and a three-tower distillation tank which are connected to the top outlet of the third distillation tower.

10. The system according to claim 8 or 9, Features: The system further comprises a reboiler arranged at the lower part of the first distillation tower and / or a reboiler arranged at the lower part of the second distillation tower; and / or, The lower part of the third distillation tower is not provided with a reboiler, and the reboiler is replaced by a heat transfer pipeline connected to the heat exchanger.

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