Ethylene-vinyl acetate copolymer continuous production apparatus based on autoclave reactor and method thereof

Abstract

The application discloses a kind of based on autoclave type reactor's ethylene-vinyl acetate copolymer continuous production device and method thereof, it is related to polymer material synthesis technical field, including sequentially connected primary compressor, secondary compressor, autoclave type reactor, high-pressure separator, medium-pressure separator, low-pressure separator and melt extrusion granulator;Gas phase outlet of high-pressure separator is connected with secondary compressor;The gas phase outlet of medium-pressure separator is connected with primary compressor, vinyl acetate recovery tank and rectifying column respectively;The gas phase outlet of low-pressure separator is connected with tail gas recovery system, and tail gas recovery system is connected with primary compressor and rectifying column respectively;Autoclave type reactor is equipped with five feed inlets.The application is directed to the chemical sensitivity that ethylene-vinyl acetate is extremely easy to hydrolyze, self-polymerize or oxidize in circulation process, proposes a kind of based on pressure and component characteristic hierarchical recovery and accurate reuse strategy, realizes the efficient, stable, green production of EVA product.

Description

Technical Field

[0001] This invention relates to the field of polymer material synthesis technology, specifically to a continuous production apparatus and method for ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor. Background Technology

[0002] Ethylene-vinyl acetate copolymer (EVA), as an important high-performance thermoplastic elastomer, is widely used in photovoltaic films, foamed shoe materials, hot melt adhesives, wires and cables, and high-end packaging due to its excellent flexibility, transparency, impact resistance, heat-sealing properties, and good compatibility with inorganic fillers. The vinyl acetate (VA) content is a key indicator determining the performance and applications of EVA. For example, products with a VA content of 15-20% are suitable for foamed materials, 25-30% are used for photovoltaic encapsulation films, while high-polarity EVA with a VA content exceeding 30% is the core raw material for high-end hot melt adhesives, with a significantly higher added value than general-purpose polyethylene products.

[0003] Currently, industrial EVA is mainly produced using high-pressure free radical continuous bulk polymerization, which can be divided into tubular and batch processes based on reactor type. The tubular process has a high single-pass conversion rate (25-35%), but due to limitations in reactor heat transfer and mixing characteristics, it is difficult to stably synthesize products with a VA content exceeding 30%, and the resulting polymer has a narrow molecular weight distribution and fewer long-branched structures, limiting its overall performance. In contrast, the batch process, with its excellent temperature control and backmixing characteristics, can produce high-end EVA products with a VA content as high as 40%, a wide molecular weight distribution, abundant long branches, and excellent mechanical and optical properties, and has become the mainstream process route for photovoltaic-grade and hot melt adhesive-grade EVA.

[0004] However, the batch process has an inherent problem of low single-pass conversion rate (usually only 1-20%), resulting in more than 80% of unreacted monomers needing to be recycled. Traditional processes generally adopt a crude circulation strategy of "mixing and depressurizing - unified recompression", which means that all unreacted gases discharged from the separator are combined and reduced to atmospheric pressure, then condensed and separated before being sent back to the inlet of the primary compressor to be repressurized. This method has three fatal flaws: (1) Extremely high energy consumption: A large amount of high-pressure gas (such as 160MPa) is unnecessarily reduced to atmospheric pressure and then returned to the reaction pressure after multiple stages of compression, resulting in huge waste of compression work; (2) Deterioration of vinyl acetate quality: Vinyl acetate is prone to hydrolysis, self-polymerization or oxidation during the circulation process, generating impurities such as acetaldehyde and acetic acid, which can easily trigger chain termination or cross-linking side reactions, resulting in increased yellowing index, increased gel content, increased melt index fluctuations, and serious impact on batch stability; (3) Poor operational flexibility: A single circulation path cannot be differentiated according to the characteristics of the components, making it difficult to meet the dual requirements of high VA content products for monomer purity and heat removal of reaction.

[0005] While existing technologies have attempted to improve recycling efficiency by adding separation stages or optimizing compression processes, they have not designed selective recovery and purification mechanisms for components specific to the chemical sensitivity of vinyl acetate. Especially under conditions of low conversion rates and high recycling loads in batch processes, current technologies still cannot effectively prevent impurity accumulation, thus limiting the quality stability and energy efficiency of high-VA-content EVA products.

[0006] Therefore, providing a continuous EVA production apparatus and method is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] To address the problems of low single-pass conversion rate, low unreacted monomer recycling efficiency, high energy consumption, large operational fluctuations, and poor batch stability in existing batch-process EVA, this invention provides a continuous production apparatus and method for ethylene-vinyl acetate copolymer based on a high-pressure batch reactor. Addressing the chemical sensitivity of vinyl acetate during recycling, which is highly susceptible to hydrolysis, self-polymerization, or oxidation, this invention proposes a staged recovery and precise reuse strategy based on pressure gradients and component characteristics, thereby achieving efficient, stable, and environmentally friendly production of high-end EVA products.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A continuous production apparatus for ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor includes a primary compressor, a secondary compressor, a high-pressure autoclave reactor, a high-pressure separator, a medium-pressure separator, a low-pressure separator, and a melt extrusion granulator connected in sequence. The gas phase outlet of the high-pressure separator is connected to the inlet of the secondary compressor via a high-pressure circulating gas cooler; The gas phase outlet of the medium-pressure separator is connected to the medium-pressure circulating gas cooler. After component separation, the ethylene is returned to the inlet of the primary compressor, a portion of the vinyl acetate is sent to the vinyl acetate recovery tank, and after being pressurized and vaporized by the vinyl acetate injection unit, it is reinjected to the inlet of the secondary compressor. The remaining portion is sent to the vinyl acetate distillation column or the vinyl acetate recovery tank. The gas phase outlet of the low-pressure separator is connected to the tail gas recovery system. The ethylene separated by the tail gas recovery system is pressurized by the booster compressor and returned to the medium-pressure circulating gas cooler and finally enters the inlet of the primary compressor. The separated vinyl acetate is sent to the vinyl acetate distillation column. The high-pressure autoclave reactor is provided with five feed inlets. The first feed inlet is located at the top of the high-pressure autoclave reactor, the second, third and fourth feed inlets are located at the upper, middle and lower parts of the side line of the high-pressure autoclave reactor in sequence, and the fifth feed inlet is located at the bottom of the high-pressure autoclave reactor.

[0010] Preferably, the inlet of the primary compressor is connected to the gaseous ethylene storage tank, and an ethylene flow control device is provided on the pipeline between the two. The inlet of the secondary compressor is connected to the vinyl acetate injection unit, which pressurizes and vaporizes liquid vinyl acetate and then merges it with the ethylene from the outlet of the primary compressor. A vinyl acetate flow control device is provided on the outlet pipeline of the vinyl acetate injection unit to regulate the flow rate of vinyl acetate entering the secondary compressor.

[0011] Preferably, the five feed inlets are connected to the outlet of the secondary compressor to inject the high-pressure ethylene-vinyl acetate mixture into the high-pressure autoclave reactor from different heights. The first feed inlet is also connected to the initiator storage tank; The third feed inlet is also connected to the chain transfer agent storage tank; The feed line of the fifth feed inlet is equipped with a high-pressure cooler, which is used to cool the high-pressure mixture of ethylene and vinyl acetate before injection.

[0012] This invention addresses the unique chemical sensitivity of vinyl acetate during recycling, which is highly susceptible to hydrolysis, self-polymerization, or oxidation. It achieves precise component separation of ethylene and vinyl acetate at the medium-pressure stage, and implements independent purification and precise low-temperature reinjection of vinyl acetate. Combined with a five-stream functionalized feed design, it effectively prevents the accumulation of impurities in the recycling system. As a result, the total monomer recovery rate is increased to >99.5%, the melt index fluctuation of high VA content (5-40%) EVA products is controlled within ±5%, and overall energy consumption is reduced by more than 25%. This technology significantly outperforms industry standards, overcoming the technical bias that high VA content inevitably leads to high energy consumption and low stability. It solves the long-standing problem of the difficulty in synergistically improving energy efficiency and quality in high-end EVA production, demonstrating outstanding substantive features and significant progress.

[0013] Preferably, the melt extrusion granulator is also connected to a blending system.

[0014] A continuous production method for ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor, using the aforementioned apparatus, specifically includes the following steps: (1) The gaseous ethylene in the gaseous ethylene storage tank is compressed by a primary compressor. At the same time, the liquid vinyl acetate in the vinyl acetate storage tank is pressurized and completely vaporized by the vinyl acetate injection unit. The flow rate is adjusted by the flow regulating device and combined with the compressed gaseous ethylene. The mixture is then further pressurized by a secondary compressor to obtain a mixture of ethylene and vinyl acetate. (2) The mixture of ethylene and vinyl acetate, the initiator and the chain transfer agent are fed into the five feed ports of the high-pressure autoclave and reacted to obtain a mixture. (3) The mixture is fed into a high-pressure separator, a medium-pressure separator and a low-pressure separator for stepwise pressure reduction and separation to obtain ethylene-vinyl acetate copolymer melt; The mixture separated by the high-pressure separator is cooled by a high-pressure circulating gas cooler and then returned to the inlet of the secondary compressor for recycling. The mixture separated by the medium-pressure separator is cooled by a medium-pressure circulating gas cooler, and the separated ethylene is returned to the inlet of the primary compressor. The separated vinyl acetate is sent to a vinyl acetate recovery tank or a distillation column for purification. The mixture separated by the low-pressure separator is sent to a tail gas recovery system. The ethylene separated by the tail gas recovery system is pressurized and returned to the outlet pipeline of the medium-pressure circulating gas cooler, and finally enters the inlet of the primary compressor with the material flow. The separated vinyl acetate is sent to a distillation column, purified, and then sent to the inlet of the secondary compressor. (4) The ethylene-vinyl acetate copolymer melt is fed into a melt extrusion granulator for extrusion granulation, and then conveyed by air to the blending and packaging system.

[0015] Preferably, in step (1), the primary compressor pressurizes the pressure to 16-20 MPa, and the secondary compressor pressurizes the pressure to 155-175 MPa; The mass ratio of ethylene to vinyl acetate in the mixture of ethylene and vinyl acetate is 67-82:18-33.

[0016] Preferably, the material injected into the five feed inlets in step (2) is a high-pressure mixture of ethylene and vinyl acetate with the same composition; the feed inlets are distributed according to the total feed amount as follows: The first feed inlet accounts for 35-45 wt% of the total feed from the five feed inlets, and the initiator is added therein; wherein the concentration of the initiator is 60-100 ppm, based on the total mass of the ethylene-vinyl acetate high-pressure mixed gas in the first feed inlet; The second feed inlet accounts for 15-25 wt% of the total feed volume from the five feed inlets. The third feed inlet accounts for 15-20 wt% of the total feed from the five feed inlets, and the chain transfer agent is added thereto; wherein, the concentration of the chain transfer agent is 80-150 ppm, based on the total mass of the ethylene-vinyl acetate high-pressure mixed gas in the third feed inlet; The fourth feed inlet accounts for 10-20 wt% of the total feed volume of the five feed inlets. The fifth feed inlet accounts for 5-15 wt% of the total feed volume of the five feed inlets, and the temperature of the high-pressure ethylene-vinyl acetate mixture in the fifth feed inlet is 40-50℃.

[0017] The five feed inlets are used to inject materials with different functions: the first inlet is located at the top of the reactor and is used to introduce the main reactant containing organic peroxide initiator; the second to fifth inlets are located at the top, middle, bottom and bottom of the reactor, respectively, and are used to enhance material backmixing, improve temperature distribution uniformity, and synergistically remove reaction heat through the injection of low-temperature mixed gas at the bottom.

[0018] Preferably, the reaction conditions in step (2) are: reaction pressure of 155-175 MPa, reaction temperature of 190-210℃, and average residence time of the material in the high-pressure autoclave reactor of 80-100 s.

[0019] Preferably, the initiator in step (2) includes at least one of benzoyl peroxide, tert-butyl peroxide, and dicumyl peroxide; The chain transfer agent includes at least one of propionaldehyde and acetone.

[0020] Preferably, the pressure of the high-pressure separator in step (3) is 150-170 MPa, the pressure of the medium-pressure separator is 25-35 MPa, and the pressure of the low-pressure separator is 2.0-3.0 MPa; The reaction conditions in step (2) are: reaction pressure of 155-175 MPa, reaction temperature of 190-210℃, and average residence time of the material in the high-pressure autoclave reactor of 80-100 s.

[0021] Compared with the prior art, the present invention has the following beneficial effects: (1) Under the premise of maintaining the typical single-pass conversion rate of the batch process, the present invention constructs a three-stage closed-loop circulation architecture of high pressure direct return + medium pressure diversion + low pressure deep mining, realizing the efficient graded utilization of unreacted monomers, and fundamentally avoiding the high energy consumption problem caused by mixing and depressurization-unified recompression in the traditional process. (2) The high-pressure separator of the present invention directly returns the gas phase to the inlet of the secondary compressor, eliminating the unnecessary pressure reduction process, shortening the heating process of vinyl acetate, and effectively preventing hydrolysis and self-polymerization; (3) In this invention, the pressure component achieves precise separation of ethylene and vinyl acetate. Ethylene is returned to the primary compression system, while vinyl acetate enters the distillation and purification process independently, fundamentally blocking the accumulation of impurities, ensuring the purity of the feed monomer, and controlling the batch fluctuation of the melt index (MI) of high VA content (such as 28-33%) EVA products within ±5%, significantly improving quality stability and batch consistency. (4) The present invention uses low-pressure tail gas for deep recovery treatment, ethylene is pressurized and reinjected once for compression, and vinyl acetate is purified by distillation and co-injected with fresh vinyl acetate. The total recovery rate of monomers exceeds 99.5%, which greatly reduces raw material consumption and production costs. (5) Through the synergistic effect of the five functional feed design and the three-stage pressure gradient recovery system, the present invention enables the device to have a wide range of product adaptability. It can flexibly switch the production of EVA with different VA contents on the same production line to meet the diversified needs of high-end applications such as photovoltaic film, foamed shoe material, hot melt adhesive, and agricultural film for high-performance EVA resin. Through systematic process innovation targeting the chemical sensitivity of vinyl acetate, the present invention has successfully solved the long-standing technical problem of the difficulty in synergistic improvement of energy efficiency and quality in the production of high-end EVA, and provides a practical and feasible technical path for the green, continuous and stable manufacturing of high-performance EVA. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. The drawings in this description are merely embodiments of the present invention.

[0023] Figure 1 This is a schematic diagram of a continuous production apparatus for ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor according to the present invention. Detailed Implementation

[0024] Embodiments of the present invention are described below, examples of which are shown in the accompanying drawings. The embodiments described with reference to the drawings are exemplary and intended to explain the present invention, but are not to be construed as limiting the present invention.

[0025] like Figure 1 The present invention provides a continuous production apparatus for ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor, comprising a primary compressor 2, a secondary compressor 3, a high-pressure autoclave reactor 6, a high-pressure separator 7, a medium-pressure separator 9, a low-pressure separator 12, and a melt extrusion granulator 16 connected in sequence. The gas phase outlet of the high-pressure separator 7 is connected to the inlet of the secondary compressor 3 through the high-pressure circulating gas cooler 8; The gas phase outlet of the medium-pressure separator 9 is connected to the medium-pressure circulating gas cooler 10. After component separation, the ethylene is returned to the inlet of the primary compressor 2, a portion of the vinyl acetate is sent to the vinyl acetate recovery tank 11, and after being pressurized and vaporized by the vinyl acetate injection unit 19, it is reinjected to the inlet of the secondary compressor 3. The remaining portion is sent to the vinyl acetate distillation column 15 or the vinyl acetate recovery tank 11. The gas phase outlet of the low-pressure separator 12 is connected to the tail gas recovery system 13. The ethylene separated by the tail gas recovery system 13 is pressurized by the booster compressor 14 and returned to the outlet pipeline of the medium-pressure circulating gas cooler 10. It then enters the inlet of the primary compressor 2 along with the material flow. The separated vinyl acetate is sent to the vinyl acetate distillation column 15. The high-pressure autoclave reactor 6 is provided with five feed inlets. The first feed inlet 101 is located at the top of the high-pressure autoclave reactor. The second feed inlet 102, the third feed inlet 103 and the fourth feed inlet 104 are located at the upper, middle and lower parts of the side line of the high-pressure autoclave reactor 6 in sequence. The fifth feed inlet 105 is located at the bottom of the high-pressure autoclave reactor 6. Preferably, the inlet of the primary compressor 2 is connected to the gaseous ethylene storage tank 1, and an ethylene flow control device is provided on the pipeline between the two. The inlet of the secondary compressor 3 is connected to the vinyl acetate injection unit 19. The vinyl acetate injection unit is used to pressurize and vaporize liquid vinyl acetate and then combine it with the ethylene at the outlet of the primary compressor 2. A vinyl acetate flow control device is provided on the outlet pipeline of the vinyl acetate injection unit to regulate the flow rate of vinyl acetate entering the secondary compressor. Preferably, the five feed inlets are connected to the outlet of the secondary compressor 3 to inject the high-pressure ethylene-vinyl acetate mixture into the high-pressure autoclave 6 from different heights; Among them, the first feed inlet 101 is also connected to the initiator storage tank 4; The third feed inlet 103 is connected to the chain transfer agent storage tank 5; A high-pressure cooler 18 is provided on the feed pipeline of the fifth feed inlet 105, which is used to cool the high-pressure mixture of ethylene-vinyl acetate before injection. Preferably, the melt extrusion granulator 16 is also connected to the blending system 17.

[0026] Example 1 Adopting such Figure 1 The present invention provides a continuous production method for ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor (for a 150,000-ton / year photovoltaic-grade EVA project), specifically including the following steps: (1) After the gaseous ethylene in the gaseous ethylene storage tank is pressurized to 18MPa by a primary compressor, it is combined with vinyl acetate at a mass ratio of 72:28 and then pressurized to 165MPa by a secondary compressor to obtain a high-pressure mixture of ethylene-vinyl acetate. (2) The obtained mixture of ethylene and vinyl acetate is divided into five streams according to the total feed amount and injected into the five feed ports of the high-pressure autoclave. Under the action of the organic peroxide initiator dicumyl peroxide (DCP), a free radical copolymerization reaction is carried out at 200°C for a reaction residence time of 90s, generating a mixture of EVA melt, unreacted ethylene, and unreacted vinyl acetate; the specific configuration of each feed stream is as follows: First feed inlet: accounting for 40 wt% of the total feed, containing ethylene and vinyl acetate in a mass ratio of 72:28, with an initiator concentration of 80 ppm (based on the total mass of the ethylene-vinyl acetate high-pressure mixture in the first feed inlet). The second feed inlet accounts for 20 wt% of the total feed, with an ethylene to vinyl acetate mass ratio of 72:28, and is used to supplement the concentration of the upper monomer. The third feed inlet accounts for 15 wt% of the total feed, with an ethylene to vinyl acetate mass ratio of 72:28 and a chain transfer agent propionaldehyde concentration of 120 ppm (based on the total mass of the ethylene-vinyl acetate high-pressure mixture in this feed inlet). The fourth feed inlet accounts for 15 wt% of the total feed, with a mass ratio of ethylene to vinyl acetate of 72:28, maintaining a uniform vinyl acetate concentration at the bottom. The fifth feed inlet accounts for 10 wt% of the total feed volume. The mass ratio of ethylene to vinyl acetate is 72:28. The material is injected after being cooled to 40°C by a high-pressure cooler and is used as an internal cooling medium to remove the heat of reaction. Each feed stream is regulated by a mass flow controller to adjust the total flow distribution. The temperature inside the reactor is controlled by the fifth cooling material flow rate and the jacket cooling water in a coordinated manner, with a fluctuation range of ≤±2℃. (3) The mixture after reaction is sequentially fed into a high-pressure separator (operating pressure 160MPa), a medium-pressure separator (operating pressure 30MPa) and a low-pressure separator (operating pressure 2.5MPa) for step-by-step pressure reduction and separation to obtain EVA melt; The gas-phase mixture separated by the high-pressure separator mainly contains ethylene and vinyl acetate, and basically contains no polymers. Under typical operating conditions, due to the high proportion of vinyl acetate in the raw material, the molar fraction of vinyl acetate in the circulating gas is also relatively high, about 20-30%, and the molar fraction of ethylene is 70-80%. After being cooled to 40-60°C by the high-pressure circulating gas cooler, the mixture is directly returned to the inlet of the secondary compressor for recycling. The mixed gas separated by the medium-pressure separator (mainly containing ethylene, vinyl acetate, and trace amounts of inert gases) is sent to the medium-pressure circulating gas cooler in the component separation unit. Utilizing the differences in volatility or condensation characteristics between ethylene and vinyl acetate, efficient separation is achieved to obtain high-purity ethylene (purity ≥99.5%) and a stream enriched with vinyl acetate. The obtained high-purity ethylene is returned to the inlet of the primary compressor. Part of the stream enriched with vinyl acetate is temporarily stored in the vinyl acetate recovery tank for process reuse, while the other part is sent to the vinyl acetate distillation column for deep purification. Under typical operating conditions, the tail gas separated by the low-pressure separator has a relatively high proportion of vinyl acetate due to the high VA content of the product. It contains approximately 60-70 mol% ethylene, approximately 30-40 mol% vinyl acetate, and trace amounts of inert gases. This tail gas is sent to the tail gas recovery system. This system separates the tail gas into high-purity ethylene and vinyl acetate components through multi-stage condensation combined with adsorption or distillation processes. The separated ethylene is pressurized by a booster compressor and returned to the medium-pressure circulation system, while the vinyl acetate component is sent to the distillation column. After distillation purification, the obtained vinyl acetate has a purity of ≥99.9%, and is mixed with fresh vinyl acetate and injected into the inlet of the secondary compressor. (4) The separated EVA melt is fed into a melt extrusion granulator for extrusion granulation, and then blown to the blending system. Qualified products are sent to the finished product packaging process. The obtained EVA product has a VA content of 28% and a melt index (MI) of 25g / 10min, which meets the standard for EVA resin for photovoltaic films.

[0027] Example 2 Adopting such Figure 1 The present invention provides a continuous production method for ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor (for a 100,000-ton / year photovoltaic-grade EVA project), specifically including the following steps: (1) After the gaseous ethylene in the gaseous ethylene storage tank is pressurized to 17MPa by a primary compressor, it is combined with vinyl acetate at a mass ratio of 82:18 and then pressurized to 160MPa by a secondary compressor to obtain a mixture of ethylene and vinyl acetate. (2) The obtained ethylene and vinyl acetate mixture is divided into five streams according to the total feed amount and injected into the five feed ports of the high-pressure autoclave. Under the action of the organic peroxide initiator tert-butyl peroxide (TBP), a free radical copolymerization reaction is carried out at 190°C with a reaction residence time of 100s, generating a mixture of EVA melt, unreacted ethylene, and unreacted VAC. The specific configuration of each feed stream is as follows: First feed inlet: accounting for 45 wt% of the total feed, containing ethylene and vinyl acetate in a mass ratio of 82:18, with an initiator concentration of 60 ppm (based on the total mass of the ethylene-vinyl acetate high-pressure mixture in the first feed inlet). The second feed inlet accounts for 20 wt% of the total feed, with an ethylene to vinyl acetate mass ratio of 82:18, and is used to supplement the concentration of the upper monomer. The third feed inlet accounts for 15 wt% of the total feed, with an ethylene to vinyl acetate mass ratio of 82:18 and a chain transfer agent propionaldehyde concentration of 100 ppm (based on the total mass of the ethylene-vinyl acetate high-pressure mixture in this feed inlet). The fourth feed inlet accounts for 12 wt% of the total feed, with an ethylene to vinyl acetate mass ratio of 82:18, maintaining a uniform VAC concentration at the bottom. The fifth feed inlet accounts for 8 wt% of the total feed volume. The mass ratio of ethylene to vinyl acetate is 82:18. The material is injected after being cooled to 50°C by a high-pressure cooler and is used as an internal cooling medium to remove the heat of reaction. Each feed stream is regulated by an independent mass flow controller. The temperature inside the reactor is controlled by the flow rate of the fifth feed inlet and the jacket cooling water, with a fluctuation range of ≤±3℃. (3) The mixture after reaction is sequentially fed into a high-pressure separator (operating pressure 155MPa), a medium-pressure separator (operating pressure 25MPa) and a low-pressure separator (operating pressure 2.0MPa) for step-by-step pressure reduction and separation to obtain EVA melt; The gas-phase mixture separated by the high-pressure separator mainly contains ethylene and vinyl acetate, and basically contains no polymers. Under typical operating conditions, since ethylene is the dominant component of the raw material, the molar fraction of ethylene in the circulating gas is relatively high, about 80-90%, and the molar fraction of vinyl acetate is 10-20%. The mixed gas is cooled to 40-60°C by the high-pressure circulating gas cooler and then directly returned to the inlet of the secondary compressor for recycling. The mixed gas separated by the medium-pressure separator (mainly containing ethylene, vinyl acetate, and trace amounts of inert gases) is sent to the medium-pressure circulating gas cooler in the component separation unit. Utilizing the differences in volatility or condensation characteristics between ethylene and vinyl acetate, efficient separation is achieved to obtain high-purity ethylene (purity ≥99.5%) and a stream enriched with vinyl acetate. The obtained high-purity ethylene is returned to the inlet of the primary compressor. Part of the stream enriched with vinyl acetate is temporarily stored in the vinyl acetate recovery tank for process reuse, while the other part is sent to the vinyl acetate distillation column for deep purification. Under typical operating conditions, the tail gas separated by the low-pressure separator mainly consists of ethylene, containing approximately 70-80 mol% ethylene, approximately 20-30 mol% vinyl acetate, and trace amounts of inert gases, due to the low VA content of the product. This tail gas is sent to the tail gas recovery system. After separation, the ethylene component is pressurized by the booster compressor and returned to the medium-pressure circulation system, while the vinyl acetate component is sent to the distillation column for purification. The purified vinyl acetate has a purity of ≥99.9%, and after being mixed with fresh vinyl acetate, it is injected into the inlet of the secondary compressor. (4) The separated EVA melt is fed into a melt extrusion granulator for extrusion granulation, and then blown to the blending system. Qualified products are sent to the finished product packaging process. The obtained EVA product has a VA content of 18% and a melt index (MI) of 2.0 g / 10 min, which meets the requirements for shoe material foaming applications.

[0028] Example 3 Adopting such Figure 1 The present invention provides a continuous production method for ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor (for an 80,000-ton / year photovoltaic-grade EVA project), specifically including the following steps: (1) After the gaseous ethylene in the gaseous ethylene storage tank is pressurized to 19 MPa by a primary compressor, it is combined with vinyl acetate at a mass ratio of 67:33 and then pressurized to 170 MPa by a secondary compressor to obtain a mixture of ethylene and vinyl acetate. (2) The obtained ethylene and vinyl acetate mixture is divided into five streams according to the total feed rate and injected into the five feed ports of the high-pressure autoclave. Under the action of the organic peroxide initiator benzoyl peroxide (BPO), a free radical copolymerization reaction is carried out at 210°C for a reaction residence time of 80s, generating a mixture of EVA melt, unreacted ethylene, and unreacted VAC. The specific configuration of each feed stream is as follows: First feed inlet: accounting for 35 wt% of the total feed, containing ethylene and vinyl acetate in a mass ratio of 67:33, with an initiator concentration of 100 ppm (based on the total mass of the ethylene-vinyl acetate high-pressure mixture in the first feed inlet). The second feed inlet accounts for 20 wt% of the total feed, with an ethylene to vinyl acetate mass ratio of 67:33, and is used to supplement the concentration of the upper monomer. The third feed inlet accounts for 20 wt% of the total feed, with an ethylene to vinyl acetate mass ratio of 67:33 and a chain transfer agent propionaldehyde concentration of 140 ppm (based on the total mass of the ethylene-vinyl acetate high-pressure mixture in this feed inlet). The fourth feed inlet accounts for 15 wt% of the total feed, with an ethylene to vinyl acetate mass ratio of 67:33, maintaining a uniform VAC concentration at the bottom. The fifth feed inlet accounts for 10 wt% of the total feed volume. The mass ratio of ethylene to vinyl acetate is 67:33. The material is injected after being cooled to 40°C by a high-pressure cooler and is used as an internal cooling medium to remove the heat of reaction. Each feed stream is regulated by an independent mass flow controller. The temperature inside the reactor is controlled by the flow rate of the fifth feed inlet and the jacket cooling water, with a fluctuation range of ≤±3℃. (3) The mixture after reaction is sequentially fed into a high-pressure separator (operating pressure 168MPa), a medium-pressure separator (operating pressure 28MPa) and a low-pressure separator (operating pressure 3.0MPa) for step-by-step pressure reduction and separation to obtain EVA melt; The gas-phase mixture separated by the high-pressure separator mainly contains ethylene and vinyl acetate, and basically contains no polymers. Under typical operating conditions, since the proportion of vinyl acetate in the raw material is the highest, the molar fraction of vinyl acetate in the circulating gas is also the highest, about 25-35%, and the molar fraction of ethylene is 65-75%. This mixed gas is cooled to 40-60°C by the high-pressure circulating gas cooler and then directly returned to the inlet of the secondary compressor for recycling. The mixed gas separated by the medium-pressure separator (mainly containing ethylene, vinyl acetate, and trace amounts of inert gases) is sent to the medium-pressure circulating gas cooler in the component separation unit. Utilizing the differences in volatility or condensation characteristics between ethylene and vinyl acetate, efficient separation is achieved to obtain high-purity ethylene (purity ≥99.5%) and a stream enriched with vinyl acetate. The obtained high-purity ethylene is returned to the inlet of the primary compressor. Part of the stream enriched with vinyl acetate is temporarily stored in the vinyl acetate recovery tank for process reuse, while the other part is sent to the vinyl acetate distillation column for deep purification. Under typical operating conditions, the tail gas separated by the low-pressure separator has the highest VA content and the highest degree of vinyl acetate enrichment, containing approximately 55-65 mol% ethylene, approximately 35-45 mol% vinyl acetate, and trace amounts of inert gases. This tail gas is sent to the tail gas recovery system. After separation, the ethylene component is pressurized by the booster compressor and returned to the medium-pressure circulation system, while the vinyl acetate component is sent to the distillation column for purification. The purified vinyl acetate has a purity of ≥99.9% and is mixed with fresh vinyl acetate before being injected into the inlet of the secondary compressor. (4) The separated EVA melt is fed into a melt extrusion granulator for extrusion granulation, and then blown to the blending system. Qualified products are sent to the finished product packaging process. The obtained EVA product has a VA content of 33% and a melt index (MI) of 400g / 10min. It has excellent adhesion and fluidity and is suitable for the field of high-end hot melt adhesives.

[0029] Comparative Example 1 Compared with Example 1, the difference is that: instead of using a staged recovery system, all unreacted gases discharged from the high-pressure separator, medium-pressure separator, and low-pressure separator are combined and depressurized to atmospheric pressure, and then returned to the inlet of the primary compressor for recycling after condensation and phase separation; all other conditions are the same as in Example 1.

[0030] Comparative Example 2 Compared with Example 1, the difference is that: instead of using five-stream feed, a single-stream top feed is used (ethylene:vinyl acetate mass ratio is 72:28, and the amount of initiator added is the same as in Example 1), and there is no side stream supplementation or internal cooling; the other conditions (including the staged recovery system, operating pressure, and temperature) are the same as in Example 1.

[0031] Comparative Example 3 Compared with Example 1, the difference is that the medium-pressure separator is removed, and only the high-pressure separator (160MPa) and the low-pressure separator (2.5MPa) are retained, so the medium-pressure separation of ethylene and vinyl acetate is not achieved; all other conditions are the same as in Example 1.

[0032] Comparative Example 4 Compared with Example 1, the difference is that the operating pressure of the medium-pressure separator is set to 10 MPa (lower than the preferred range of 25-30 MPa of the present invention), which results in the inability to effectively separate vinyl acetate and ethylene; the other conditions are exactly the same.

[0033] Performance testing The EVA products obtained by the continuous production process of Comparative Examples 1-4 and Examples 1-3 were tested using standard methods, and the results are shown in Table 1.

[0034] Table 1. Performance comparison of different batches of EVA products in Examples 1-3 and Comparative Examples 1-4

[0035] As shown in Table 1: Comparative Example 1 suffered from the lack of graded recovery, resulting in the accumulation of vinyl acetate impurities, large fluctuations in MI, and high gel content. Comparative Example 2 had a lower VA content and uneven composition due to single-stream feeding, resulting in a lower pass rate. Comparative Example 3 lacked medium-pressure separation, resulting in a significant decrease in monomer recovery rate; In Comparative Example 4, the pressure was too low, resulting in separation failure and severe yellowing of the product.

[0036] This invention achieves high stability, high recovery rate, and low gelation in the production of high-end EVA with a synergistic design of five functional feed streams and a three-stage gradient closed-loop recovery system, with significantly better technical performance than existing processes.

[0037] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A continuous production apparatus for ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor, characterized in that, It includes a primary compressor, a secondary compressor, a high-pressure autoclave reactor, a high-pressure separator, a medium-pressure separator, a low-pressure separator, and a melt extrusion granulator connected in sequence; The gas phase outlet of the high-pressure separator is connected to the inlet of the secondary compressor via a high-pressure circulating gas cooler; The gas phase outlet of the medium-pressure separator is connected to the medium-pressure circulating gas cooler. After component separation, the ethylene is returned to the inlet of the primary compressor, a portion of the vinyl acetate is sent to the vinyl acetate recovery tank, and after being pressurized and vaporized by the vinyl acetate injection unit, it is reinjected to the inlet of the secondary compressor. The remaining portion is sent to the vinyl acetate distillation column or the vinyl acetate recovery tank. The gas phase outlet of the low-pressure separator is connected to the tail gas recovery system. The ethylene separated by the tail gas recovery system is pressurized by the booster compressor and returned to the medium-pressure circulating gas cooler and finally enters the inlet of the primary compressor. The separated vinyl acetate is sent to the vinyl acetate distillation column. The high-pressure autoclave reactor is provided with five feed inlets. The first feed inlet is located at the top of the high-pressure autoclave reactor, the second, third and fourth feed inlets are located at the upper, middle and lower parts of the side line of the high-pressure autoclave reactor in sequence, and the fifth feed inlet is located at the bottom of the high-pressure autoclave reactor.

2. The continuous production apparatus for ethylene-vinyl acetate copolymer according to claim 1, characterized in that, The inlet of the primary compressor is connected to the gas phase ethylene storage tank, and an ethylene flow control device is installed on the pipeline between the two. The inlet of the secondary compressor is connected to the vinyl acetate injection unit, and a vinyl acetate flow control device is provided on the outlet pipeline of the vinyl acetate injection unit.

3. The continuous production apparatus for ethylene-vinyl acetate copolymer according to claim 1, characterized in that, The five feed inlets are connected to the outlet of the secondary compressor, and are used to inject the high-pressure ethylene-vinyl acetate mixture into the high-pressure autoclave from different heights. The first feed inlet is also connected to the initiator storage tank; The third feed inlet is also connected to the chain transfer agent storage tank; The feed line of the fifth feed inlet is equipped with a high-pressure cooler, which is used to cool the high-pressure mixture of ethylene and vinyl acetate before injection.

4. The continuous production apparatus for ethylene-vinyl acetate copolymer according to claim 1, characterized in that, The outlet of the melt extrusion granulator is connected to a blending and packaging system.

5. A continuous production method for ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor, characterized in that, The apparatus according to any one of claims 1-4 specifically includes the following steps: (1) The gaseous ethylene in the gaseous ethylene storage tank is compressed by a primary compressor. At the same time, the liquid vinyl acetate in the vinyl acetate storage tank is pressurized and completely vaporized by the vinyl acetate injection unit. The flow rate is adjusted by the flow regulating device and combined with the compressed gaseous ethylene. The mixture is then further pressurized by a secondary compressor to obtain a mixture of ethylene and vinyl acetate. (2) The mixture of ethylene and vinyl acetate, the initiator and the chain transfer agent are fed into the five feed ports of the high-pressure autoclave and reacted to obtain a mixture. (3) The mixture is fed into a high-pressure separator, a medium-pressure separator and a low-pressure separator for stepwise pressure reduction and separation to obtain ethylene-vinyl acetate copolymer melt; The mixture separated by the high-pressure separator is cooled by a high-pressure circulating gas cooler and then returned to the inlet of the secondary compressor for recycling. The mixture separated by the medium-pressure separator is cooled by a medium-pressure circulating gas cooler, and the separated ethylene is returned to the inlet of the primary compressor. The separated vinyl acetate is sent to a vinyl acetate recovery tank or a distillation column for purification. The mixture separated by the low-pressure separator is sent to a tail gas recovery system. The ethylene separated by the tail gas recovery system is pressurized and returned to the outlet pipeline of the medium-pressure circulating gas cooler, and finally enters the inlet of the primary compressor with the material flow. The separated vinyl acetate is sent to a distillation column, purified, and then sent to the inlet of the secondary compressor. (4) The ethylene-vinyl acetate copolymer melt is fed into a melt extrusion granulator for extrusion granulation, and then conveyed by air to the blending and packaging system.

6. The continuous production method of ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor according to claim 5, characterized in that, In step (1), the primary compressor pressurizes the pressure to 16-20 MPa, and the secondary compressor pressurizes the pressure to 155-175 MPa; The mass ratio of ethylene to vinyl acetate in the mixture of ethylene and vinyl acetate is 67-82:18-33.

7. The continuous production method of ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor according to claim 5, characterized in that, The mixture injected into the five feed inlets in step (2) is a high-pressure mixture of ethylene and vinyl acetate of the same composition; the feed inlets are distributed according to the total feed amount as follows: The first feed inlet accounts for 35-45 wt% of the total feed from the five feed inlets, and the initiator is added therein; wherein the concentration of the initiator is 60-100 ppm, based on the total mass of the ethylene-vinyl acetate high-pressure mixed gas in the first feed inlet; The second feed inlet accounts for 15-25 wt% of the total feed volume from the five feed inlets. The third feed inlet accounts for 15-20 wt% of the total feed from the five feed inlets, and the chain transfer agent is added thereto; wherein, the concentration of the chain transfer agent is 80-150 ppm, based on the total mass of the ethylene-vinyl acetate high-pressure mixed gas in the third feed inlet; The fourth feed inlet accounts for 10-20 wt% of the total feed volume of the five feed inlets. The fifth feed inlet accounts for 5-15 wt% of the total feed volume of the five feed inlets, and the temperature of the high-pressure ethylene-vinyl acetate mixture in the fifth feed inlet is 40-50℃.

8. The continuous production method of ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor according to claim 7, characterized in that, The reaction conditions in step (2) are: reaction pressure of 155-175 MPa, reaction temperature of 190-210℃, and average residence time of the material in the high-pressure autoclave reactor of 80-100 s.

9. The continuous production method of ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor according to claim 8, characterized in that, The initiator mentioned in step (2) includes at least one of benzoyl peroxide, tert-butyl peroxide, and dicumyl peroxide; The chain transfer agent includes at least one of propionaldehyde and acetone.

10. The continuous production method of ethylene-vinyl acetate copolymer based on a high-pressure autoclave reactor according to claim 5, characterized in that, The pressure of the high-pressure separator in step (3) is 150-170 MPa, the pressure of the medium-pressure separator is 25-35 MPa, and the pressure of the low-pressure separator is 2.0-3.0 MPa; The high-pressure circulating gas cooler is used to cool the mixed gas separated by the high-pressure separator to 40-60°C, and the medium-pressure circulating gas cooler is used to cool the mixed gas separated by the medium-pressure separator to 20-35°C.

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