Production apparatus and method for low speckled eva resin

Abstract

The application belongs to the technical field of EVA resin production, and specifically discloses a production device and method of low-color-spot-particle EVA resin, wherein ethylene and vinyl acetate are used as raw materials, the pressure of the raw materials is raised by a primary compressor and a secondary compressor after refining, and the raw materials enter a first reactor, a second reactor, a third reactor and a fourth reactor in sequence after preheating by a preheater; the raw materials are initiated by an initiator composition of each reactor to initiate a free radical polymerization reaction, and the reaction product is separated and recycled after cooling; the gas-phase materials separated in a high-pressure separator and a low-pressure separator are circulated to the secondary compressor and the primary compressor respectively, and the liquid-phase molten materials separated are extruded by a granulator to obtain granular products. The application can effectively prevent the risks of carbon black produced by decomposition of ultrahigh-pressure polymerized ethylene, and the risks of adverse reactions such as dehydrogenation, carbonization and color change of the reaction product, and solves the problems of safety and stability in the production of EVA resin by an ultrahigh-pressure polymerization device.

Description

Technical Field

[0001] This invention relates to the field of EVA resin production technology, and in particular to an apparatus and method for producing low-color-spotted EVA resin. Background Technology

[0002] EVA (ethylene-vinyl acetate copolymer) is the fourth largest ethylene series polymer after HDPE, LLDPE, and LDPE. It possesses good flexibility, transparency, adhesion, compatibility with fillers, and heat-sealing properties, and is widely used in photovoltaic films, foamed shoe materials, wires and cables, hot melt adhesives, and functional films. However, the high-temperature, ultra-high-pressure polymerization process for producing EVA is demanding, and the various chemical reactions within the system are complex. Small amounts of material in unstable areas within the system can undergo discoloration, resulting in discolored spots, particles, or even black particles in the EVA product. These unsatisfactory particle appearances negatively impact downstream applications. Taking EVA used in photovoltaic encapsulants as an example, when there are colored spots or particles in the product, the photovoltaic encapsulant produced by the casting machine will have colored spots that affect the transmittance of sunlight in the encapsulant, resulting in a high scrap rate of photovoltaic encapsulant. In addition, if the colored spots are elemental carbon after the high-temperature decomposition of ethylene or the dehydrogenation and carbonization of reaction products, they will affect the performance of photovoltaic modules due to their conductivity, such as poor resistance to PID (potential decay). In other words, excessive colored spots in EVA products restrict their application.

[0003] The particle appearance of resin products is judged according to SH / T 1541.1-2019. Discolored particles refer to particles with pinhead-sized black, yellow, brown, or tan spots. Discolored particles are very common in EVA products; the darker the color and the larger the spots, the greater the impact on product quality. As a high-end polyolefin product, whether it's EVA for high-value photovoltaic films or EVA for wires and cables and foamed shoe materials, product standards have strict requirements on the number of colored particles. GB / T37197-2018 Ethylene-vinyl acetate resin requires that the number of colored particles in superior grade EVA be ≤10 particles / kg. Photovoltaic film companies actually require ≤3 particles / kg for superior grade EVA. If the number of colored particles in the product is 3-10 particles / kg, it can only be judged as a qualified product; if the number exceeds 10 particles / kg, it is a substandard product, i.e., a counterfeit. However, in actual production, the number of colored particles in EVA products can be as high as 20-200 particles / kg (the appearance of colored particles is as follows...). Figure 1As shown in the image, especially when localized trace decomposition of ethylene produces carbon black due to improper temperature or initiator formulation, system contamination can occur, resulting in a persistently excessive number of discolored particles in the product. Discolored particles are a common phenomenon in EVA products produced by ultra-high pressure polymerization, making it difficult to produce high-quality products. Once the number of discolored particles exceeds the acceptable range, they are extremely difficult to eliminate, requiring system shutdown and cleaning, primarily involving cleaning the high-pressure separator, low-pressure separator, and extrusion granulator. If the cause is not identified and rectified based on the actual conditions of the system, the problem of excessive discolored particles will reappear when the system is restarted.

[0004] CN110563871B discloses a production apparatus and preparation method for EVA raw materials used in photovoltaic films. The preparation method includes steps such as feeding compression, polymerization reaction, product separation, and extrusion granulation. It mainly focuses on the preparation method and production apparatus for EVA used in photovoltaic films. The apparatus requires the installation of a chain transfer agent pipeline to inject chain transfer agent into the reaction system. The concentration of peroxide in the initiator composition is 40-50%, including at least three types of peroxides. The reaction temperature of the fourth reactor is higher than that of the first and second reactors. The pressure of the high-pressure separator and the low-pressure separator is relatively high. The product has a VA content of 25-30% and a melt index of 20-30 g / 10 min. It has the characteristics of good transparency, high volume resistivity, and good anti-PID performance.

[0005] CN117209640A discloses an ethylene-vinyl acetate copolymer and its production method. This method, by optimizing the pressure of low-pressure separation, can significantly reduce the volatile content in the obtained ethylene-vinyl acetate copolymer while reducing energy and material consumption.

[0006] CN105732871B discloses a method for preparing an ethylene vinyl acetate copolymer for use as a sealant in solar cells, and the resin obtained by the method, which can provide an EVA copolymer resin with low heat shrinkage and high transparency.

[0007] CN112830988B discloses an ethylene vinyl acetate copolymer and a method for preparing the same. The method is for preparing an ethylene vinyl acetate copolymer for extrusion coating, providing an ethylene vinyl acetate copolymer with excellent processability, and especially excellent necking and sag properties, which are opposite properties.

[0008] CN118681523A discloses a method and apparatus for preparing polyethylene and ethylene-vinyl acetate copolymer in a tubular reactor. The method avoids polymer being entrained by gas in the high-pressure circulation loop and low-pressure circulation loop under unexpected and normal operating conditions by setting pre-separators at the inlet of the low-pressure circulation loop and the high-pressure circulation loop respectively, thereby reducing the difficulty of equipment inspection and maintenance.

[0009] CN118788259A discloses a method and system for cooling polymerization effluent, as well as a polymerization reaction method and system. The method involves mixing and cooling the cooled monomer feedstock with the feedstock after a high-pressure reducing valve, and then mixing and cooling them again in a static mixer. This avoids the generation of temperature hotspots. By suppressing the generation of temperature hotspots, the risk of ethylene decomposition can be reduced. In other words, cold ethylene feedstock is used to enhance the cooling of the high-temperature polymerization effluent, which then enters a separator.

[0010] CN119101185A discloses a method for free radical polymerization of olefins. This method, by matching the initiator decomposition law with the reaction temperature distribution law, can reduce the residual amount of initiator in the product and reduce the content of oxygen-containing functional groups of initiator in the reactor, thereby improving the heat aging resistance and antioxidant properties of the product. This method mainly prevents the residual amount of initiator and improves product performance, without involving the impact of initiator on safe production or product appearance.

[0011] Furthermore, literature reports the causes and solutions for excessive colored particles during polyethylene and polypropylene production. Chen Shaozhuang's paper, "Solution and Discussion on the Problem of Excessive Colored Particle Quantity in LLDPE Production," points out that appropriately increasing the melt temperature or feed load of the granulator to ensure high melt fluidity or short residence time can reduce the number of black particles. Tian Yushan's paper, "Analysis and Prevention of Pollution Particles Generated in Polyethylene Granulation Process," points out that the causes of black and yellow spot particles include: dead zones forming in the granulator cavity, long-term resin stagnation leading to oxidation and carbonization; and excessively long heat preservation time after shutdown causing resin carbonization on the screw surface and inner surface of the unit, forming black scale. Zhao Fei's paper, "Analysis and Countermeasures for Colored Particles Affecting Polyethylene Products," points out that colored particles in polyethylene resin are organic resins. One reason is the entry of oxygen into the extrusion granulator system; another reason is the cross-linking of resin during the extrusion process, forming a gel. In general, colored particles mainly originate from the refining reaction system, the additive feeding system, and the extrusion unit. Zhao Wenyang's paper, "Analysis and Countermeasures of 'Colored Particles' Phenomenon in PP Products," describes "colored particles" as abnormally white "stiff particles" or white "expanded particles," not resin colored particles. Fu Xiaosheng's paper, "Analysis and Treatment of Factors Affecting the Long-Term Stable Operation of Extrusion Granulation Units," points out that oxygen entering the system, excessive mixing, or excessively high temperatures can cause colored particles in resin products, and these are black particles that are intolerable to resin products. The above existing technologies mainly address the colored particle problem that occurs in the production process of polyethylene and polypropylene products during extrusion granulation, and these are mostly black particles or black spots that are intolerable to resin products. Compared with low-pressure polymerization, ultra-high-pressure polymerization has more stringent operating conditions such as temperature, pressure, and phase state, making it easier for harmful impurities to form or appear in the reaction system, inducing the generation of colored particles or black spots.

[0012] Existing technologies cannot solve the problem of excessive color spots in EVA products. Therefore, there is an urgent need to provide an EVA resin with low color spot content and its production method for the continuous production of EVA resin products with a color spot content of ≤3 particles / kg, so as to improve the stability of EVA product production in ultra-high pressure polymerization equipment. Summary of the Invention

[0013] The purpose of this invention is to provide a production apparatus and method for low-color-particle EVA resin, in order to solve the problems of excessive color-particles and low yield of superior-grade EVA products produced by ultra-high pressure polymerization equipment. While solving the problem of excessive color-particles, it can also eliminate the safety accidents or system pollution caused by high-temperature decomposition of ethylene or dehydrogenation and carbonization of reaction products.

[0014] To achieve the above objectives, the basic solution provided by this invention is as follows: a production apparatus for low-color-spot EVA resin, comprising a primary compressor, a secondary compressor, a preheater, a first reactor, a second reactor, a third reactor, a fourth reactor, an aftercooler, a high-pressure separator, a low-pressure separator, a granulator, a dryer, and a degassing silo, connected in sequence; the primary compressor is equipped with a purge gas discharge pipeline; the discharge of purge gas reduces the content of harmful impurities such as carbon monoxide, carbon dioxide, acetylene, and acetaldehyde generated by side reactions in the polymerization reaction system, thereby eliminating the harmful impurities that induce color change reactions in the polymer. The outlet of the intermediate pipeline at the tail end of the granulator is equipped with a liquid-sealed tank containing liquid vinyl acetate, and the outlet end of the intermediate pipeline is located below the liquid surface of the liquid-sealed tank. Sudden stops of the granulator can cause backflow of air into the intermediate pipeline. The intermediate pipeline is located at the end opposite to the extrusion direction of the granulator. During normal operation of the granulator, the intermediate pipeline discharges gaseous light hydrocarbons and vinyl acetate overflowing from the extruded melt. Due to the poor production stability of the polymerization reaction, the granulator is prone to forming negative pressure and drawing in air when it stops frequently. This solution prevents backflow of air by setting up a liquid-sealed tank containing liquid vinyl acetate, thus eliminating the color change reaction of the molten material under trace oxygen conditions.

[0015] Furthermore, the feed inlet of the primary compressor is connected to an ethylene (E) feed pipeline, the feed inlet of the secondary compressor is connected to a vinyl acetate (VA) feed pipeline, the vinyl acetate feed pipeline is connected to a molecular sieve refining unit for refining vinyl acetate raw materials, a pulse valve is provided on the connecting pipeline between the fourth reactor and the aftercooler, the feed inlets of the first reactor, second reactor, third reactor and fourth reactor are all connected to an initiator composition tank, the exhaust port of the high-pressure separator is connected to the feed inlet of the secondary compressor via a high-pressure circulating gas recovery pipeline, at least two heat exchangers are connected in series on the high-pressure circulating gas recovery pipeline, a low-molecular wax discharge pipeline is provided on the heat exchanger near the high-pressure separator, the product forming unit includes a granulator, a dryer and a degassing silo connected in sequence, the discharge port of the low-pressure separator is connected to the granulator, the exhaust port of the low-pressure separator is connected to a heat exchanger and a separation tank in sequence, the top of the separation tank is connected to the feed inlet of the primary compressor, and the bottom of the separation tank is connected to the molecular sieve refining unit.

[0016] Furthermore, the molecular sieve refining unit includes a first molecular sieve refining tower and a second molecular sieve refining tower. The first molecular sieve refining tower is used to process fresh vinyl acetate feed, and the second molecular sieve refining tower is used to process recycled vinyl acetate feed. Both the first and second molecular sieve refining towers are connected to the vinyl acetate feed pipeline, and a booster pump is provided on the vinyl acetate feed pipeline. The bottom of the separation tank is connected to the second molecular sieve refining tower.

[0017] A method for producing a low-color-spot EVA resin includes the following steps:

[0018] S1. Raw material pressurization treatment: The primary compressor receives ethylene from the ethylene feed pipeline and the separator, and compresses it to 23.5-24.5 MPa. The pressurized ethylene then enters the secondary compressor. The first molecular sieve refining tower receives fresh vinyl acetate from the fresh vinyl acetate feed pipeline. The first and second molecular sieve refining towers respectively refine the vinyl acetate and then send it to the secondary compressor through the vinyl acetate feed pipeline. The secondary compressor pressurizes the received ethylene and vinyl acetate to 240-265 MPa. After processing by the molecular sieve refining tower, the vinyl acetate has a hydroquinone content of 5-10 ppm, a water content of 1-5 ppm, an acetic acid content of 10-80 ppm, and an acetaldehyde content of 20-60 ppm.

[0019] S2. Polymerization Reaction: The gas, after being pressurized by the secondary compressor, enters the preheater and is heated to 135–147°C. The initiator composition is injected through the feed inlets of the first, second, third, and fourth reactors. After the initiator composition is injected, the polymerization reaction begins. The reaction temperature of the four reactors is controlled by the amount of initiator composition injected. The reaction temperature of the first, second, and third reactors is 200–250°C, and the reaction temperature of the fourth reactor is 185–230°C. The reaction pressure of the four reactors is 240–265 MPa. The pressure oscillation generated by the pulse valves in the four reactors reduces material adhesion to the walls. The pulse depth of the pulse valves is 170–200 MPa, the duration is 1.3–1.8 s / cycle, and the interval is 50–80 s. The initiator composition consists of 2–5 peroxides, with isododecane as the diluent, and the dilution ratio is 3–25%. The dilution ratio of the peroxides in the initiator composition added in the fourth reactor is 1 / 3 to 1 / 2 of the peroxides in the first, second, or third reactors.

[0020] S3. Separation Processing: The molten material generated by the polymerization reaction is cooled by an aftercooler and then enters a high-pressure separator, where it is cooled to 180-200°C. The separated gaseous material enters the secondary compressor along the high-pressure circulating gas recovery pipeline, while the molten material enters a low-pressure separator, where its temperature is reduced to 175-182°C. The upper limit of the molten material temperature is determined through high-temperature baking and thermo-oxidative stability tests of EVA resin. The liquid level in the low-pressure separator is controlled at 42-48%. The gaseous material separated by the low-pressure separator is heat-exchanged by a heat exchanger and then enters a separation tank for further separation. The gaseous ethylene is recycled to the primary compressor, while the liquid vinyl acetate enters the second molecular sieve purification tower. The purified vinyl acetate then enters the vinyl acetate feed pipeline. The molten liquid material at the bottom of the low-pressure separator enters the product molding unit for further processing.

[0021] S4. Product Forming Process: The molten liquid material discharged from the low-pressure separator enters the granulator for granulation, where the cylinder temperature of the granulator is 160-180℃. Then, the EVA granules are sent to the dryer for drying. After drying, the EVA granules are sent to the degassing silo to remove unreacted ethylene or vinyl acetate from the material, thus solving the problems of odor and volatile matter in the product, thereby obtaining the desired EVA product.

[0022] Furthermore, the inlet pressure of the primary compressor is 30-70 kPa, and the discharge volume of the primary compressor purge gas is 2-4% of the ethylene feed volume. Taking a 200,000-ton / year EVA unit as an example, the purge gas discharge volume is 0.41-0.73 tons / hour, while the conventional purge gas discharge volume is about 0.3 tons / hour.

[0023] Further, the initiator composition is selected from 2 to 5 of the following: dioctyl peroxide dicarbonate, tert-butyl peroxypentanoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-3,5,5-trimethylhexanoate, and di-tert-butyl peroxide, and isododecane is used as a diluent. In the initiator composition of the fourth reactor, the peroxide is selected from at least two of the following: tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-3,5,5-trimethylhexanoate, and di-tert-butyl peroxide.

[0024] Furthermore, the dilution ratios of peroxide in the initiator compositions of the first, second, third, and fourth reactors are 12–22%, 10–20%, 10–20%, and 5–9%, respectively. By increasing the dilution ratio of peroxide, the dispersibility of the initiator composition in the reaction system is improved, eliminating the strong exothermic reaction of micro-local polymerization caused by peroxide enrichment or residue, and initiating the decomposition of ethylene to produce carbon black particles.

[0025] Furthermore, the hot water temperature inside the jackets of the four reactors and the aftercooler is 125–145°C, and the temperature of the molten material after pressure reduction via the pulse valve is 210–250°C. According to the anti-Joule-Thomson effect, the temperature rise of the molten material from the outlet of the fourth reactor after pressure reduction is 30–40°C. For example, the molten material temperature at the outlet of the fourth reactor is 195°C, 200°C, and 205°C, and the molten material temperature after pressure reduction via the pulse valve is 225°C, 230°C, and 240°C. By lowering the reaction temperature of the fourth reactor to 185–230°C and the peroxide concentration in the initiator composition to 5–9%, the temperature of the molten material is simultaneously further reduced to 210–250°C by the aftercooler. This eliminates the residual peroxide in the molten material and continues to initiate a strongly exothermic polymerization reaction, generating high-temperature points in micro-local areas that trigger the decomposition of ethylene to produce carbon black and methane. The carbon black disperses into the reaction products, resulting in colored specks, and black particles appear in the discharged low-molecular-weight wax.

[0026] Furthermore, the low-pressure separator is opened 8-15 hours after nitrogen purging during shutdown and maintenance, and the wax removal cycle of the heat exchanger is 8-16 hours. During shutdown and maintenance, the absence of gaseous flow within the low-pressure separator distorts temperature monitoring values. Premature introduction of air into the low-pressure separator during shutdown and maintenance can cause the molten material at higher temperatures to prematurely react with oxygen and undergo discoloration. This solution, by opening the low-pressure separator 8-15 hours after nitrogen purging, prevents the influence of trace amounts of oxygen on the molten material. During long-term operation, low-molecular-weight wax and other materials accumulating in the high-pressure circulating gas recovery pipeline can induce discoloration reactions due to prolonged high-temperature retention. The discolored material enters the reaction system through the high-pressure circulating gas recovery pipeline, thus contaminating the product. This solution controls product discoloration by reducing the long-term retention of molten material, including low-level control of the low-pressure separator.

[0027] Compared with the prior art, the advantages of this invention are:

[0028] 1. The EVA resin produced by the method of the present invention has a VA content of 18-30%, a melt index of 1-30 g / 10 min, and a number of colored particles of 0-3 per kg.

[0029] 2. By reducing the reaction temperature of the fourth reactor and the concentration of peroxide in the initiator composition, controlling the temperature of the molten material in the aftercooler, and reducing the content of harmful impurities such as carbon monoxide, acetylene, and acetaldehyde in the raw materials and reaction system, the risk of carbon black being generated by the decomposition of ultra-high pressure polymerized ethylene and adverse reactions such as dehydrogenation, carbonization, and discoloration of the reaction products is effectively prevented, thus solving the safety and stability problems existing in the production of EVA resin by ultra-high pressure polymerization equipment.

[0030] 3. By blocking trace amounts of oxygen in the molten reaction products, reducing long-term retained materials, and exploring the upper limit of temperature, the factors that induce color changes in the reaction products are eliminated, effectively controlling the number of colored particles in EVA resin products. The equipment has a high yield of superior EVA resin, good economic benefits, and low application risks. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the colored particles in EVA resin.

[0032] Figure 2 A schematic diagram demonstrating the color change on the surface of EVA material during high-temperature baking and its resistance to thermo-oxidative stability tests.

[0033] Figure 3 This is a schematic diagram of the low-molecular-weight wax (containing carbon black particles produced by the localized minor decomposition of ethylene) discharged from the heat exchanger.

[0034] Figure 4 This is a schematic diagram of the material inside the low-pressure separator after it changes color upon exposure to oxygen at high temperatures.

[0035] Figure 5 This is a simplified process flow diagram of the EVA device manufacturing process of the present invention. Detailed Implementation

[0036] The present invention will be further described in detail below through specific embodiments:

[0037] The reference numerals in the accompanying drawings include: primary compressor 1, vent gas discharge line 2, ethylene feed line 3, secondary compressor 4, vinyl acetate feed line 5, high-pressure circulating gas recovery line 6, preheater 7, initiator composition tank 8, first reactor 9, second reactor 10, third reactor 11, fourth reactor 12, pulse valve 13, aftercooler 14, high-pressure separator 15, heat exchanger 16, low molecular weight wax discharge line 17, low-pressure separator 18, intermediate body line 19, liquid seal tank 20, granulator 21, dryer 22, degassing silo 23, separation tank 24, second molecular sieve refining tower 25, jacket 26, booster pump 27, first molecular sieve refining tower 28, and fresh vinyl acetate feed line 29.

[0038] like Figure 5 The apparatus shown is for the production of low-color-spot EVA resin, comprising, in sequence, a primary compressor 1, a secondary compressor 4, a preheater 7, a first reactor 9, a second reactor 10, a third reactor 11, a fourth reactor 12, an aftercooler 14, a high-pressure separator 15, a low-pressure separator 18, a product molding unit, and a liquid-sealed tank 20 containing liquid vinyl acetate. The primary compressor 1 is equipped with a purge gas discharge pipe 2, and its inlet is connected to an ethylene feed pipe 3. The inlet of the secondary compressor 4 is connected to a vinyl acetate feed pipe 5. A pulse valve 13 is installed on the connecting pipe between the fourth reactor 12 and the aftercooler 14. The inlets of the first reactor 9, second reactor 10, third reactor 11, and fourth reactor 12 are all connected to an initiator composition tank 8. The exhaust port of the high-pressure separator 15 is connected to the inlet of the secondary compressor 4 via a high-pressure circulating gas recovery pipe 6. At least two heat exchangers 16 are connected in series on the high-pressure circulating gas recovery pipe 6, near the high-pressure separator. The heat exchanger 16 of unit 15 is equipped with a low-molecular-weight wax discharge pipe 17. The product forming unit includes a granulator 21, a dryer 22, and a degassing silo 23 connected in sequence. The discharge port of the low-pressure separator 18 is connected to the granulator 21. The outlet end of the middle body pipeline 19 at the tail of the granulator 21 is located below the liquid level of the liquid seal tank 20. The exhaust port of the low-pressure separator 18 is connected to the heat exchanger 16 and the separation tank 24 in sequence. The top of the separation tank 24 is connected to the feed port of the primary compressor 1. The vinyl acetate feed pipe 5 A molecular sieve refining unit is connected, comprising a first molecular sieve refining tower 28 and a second molecular sieve refining tower 25. The first molecular sieve refining tower 28 processes fresh vinyl acetate feedstock, and the second molecular sieve refining tower 25 processes recycled vinyl acetate feedstock. Both the first molecular sieve refining tower 28 and the second molecular sieve refining tower 25 are connected to a vinyl acetate feed pipeline 5, which is equipped with a booster pump 27. The bottom of the separation tank 24 is connected to the second molecular sieve refining tower 25. Fresh vinyl acetate feedstock enters the first molecular sieve refining tower 28 for processing through a fresh vinyl acetate feed pipeline 29.

[0039] like Figure 5 The following is a method for producing low-color-particle EVA resin, characterized by comprising the following steps:

[0040] S1. Raw material pressurization treatment: The primary compressor 1 receives ethylene from the ethylene feed line 3 and the separator 24, and compresses it to 23.5-24.5 MPa. The pressurized ethylene then enters the secondary compressor 4. The inlet pressure of the primary compressor 1 is 30-70 kPa, and the discharge volume of the primary compressor 1 is 2-4% of the ethylene feed volume. The first molecular sieve purification tower 28 receives fresh vinyl acetate feed from the fresh vinyl acetate feed line 29. The ethylene fed into the ethylene feed line 3 and the vinyl acetate feed line 29 are then... The mass ratio of fresh vinyl acetate to the raw material is 70-82:18-30. After the first molecular sieve refining tower 28 and the second molecular sieve refining tower 25 are processed, the vinyl acetate is transported to the secondary compressor 4 through the vinyl acetate feed pipeline 5. The secondary compressor 4 pressurizes the received ethylene and vinyl acetate to 240-265 MPa. After the vinyl acetate is processed by the molecular sieve refining tower, the hydroquinone content is 5-10 ppm, the water content is 1-5 ppm, the acetic acid content is 10-80 ppm, and the acetaldehyde content is 20-60 ppm.

[0041] S2. Polymerization Reaction: The gas, pressurized by the secondary compressor 4, enters the preheater 7, where it is heated to 135–147°C. The initiator composition is injected through the inlets of the first reactor 9, second reactor 10, third reactor 11, and fourth reactor 12. The polymerization reaction begins after the initiator composition is injected. The reaction temperature of the four reactors is controlled by the amount of initiator composition injected. The reaction temperature of the first reactor 9, second reactor 10, and third reactor 11 is 200–250°C, and the reaction temperature of the fourth reactor 12 is 185–230°C. The reaction pressure of the four reactors is 240–265 MPa. The four reactors are controlled by pulse valve 13. Pressure oscillation is used to reduce material adhesion to the wall. The pulse depth of pulse valve 13 is 170-200 MPa, the duration is 1.3-1.8 s / time, and the interval is 50-80 s. The hot water temperature in the jacket 26 of the four reactors and the aftercooler 14 is 125-145℃. The temperature of the molten material after pressure reduction by pulse valve 13 is 210-250℃. The initiator composition consists of 2-5 peroxides, and isododecane is used as a diluent with a dilution ratio of 3-25%. The dilution ratio of the peroxides in the initiator composition added in the fourth reactor 12 is 1 / 3 to 1 / 2 of the peroxides in the first reactor 9, the second reactor 10, or the third reactor 11.

[0042] S3. Separation Processing: The molten material generated by the polymerization reaction is first transported to the aftercooler 14 for cooling, and then enters the high-pressure separator 15. The high-pressure separator 15 cools the molten material to 180-200°C. The separated gaseous material enters the secondary compressor 4 along the high-pressure circulating gas recovery pipeline 6, while the molten material enters the low-pressure separator 18, where the temperature of the molten material is reduced to 175-182°C. The liquid level in the low-pressure separator 18 is controlled at 42-48%. The gaseous material separated by the low-pressure separator 18 enters the separation tank 24 after heat exchange in the heat exchanger 16. The gaseous ethylene is recycled to the primary compressor 1, and the liquid vinyl acetate enters the second molecular sieve purification tower 25. The purified vinyl acetate enters the vinyl acetate feed pipeline 5, while the molten liquid material enters the product forming unit for processing.

[0043] S4. Product forming process: The molten liquid material discharged from the low-pressure separator 18 enters the granulator 21 for granulation, wherein the cylinder temperature of the granulator 21 is 160-180℃; then the EVA particles are sent to the dryer 22 for drying. After drying, the EVA particles are sent to the degassing silo 23, where unreacted ethylene or vinyl acetate is removed from the material, thereby obtaining the desired EVA product.

[0044] The initiator composition in step S2 above is selected from 2 to 5 of the following: dioctyl peroxide dicarbonate, tert-butyl peroxypentanoate, tert-butyl peroxide-2-ethylhexanoate, tert-butyl peroxide-3,5,5-trimethylhexanoate, and di-tert-butyl peroxide. Isododecane is used as a diluent. The dilution ratios of peroxide in the initiator compositions of the first reactor 9, the second reactor 10, the third reactor 11, and the fourth reactor 12 are 12-22%, 10-20%, 10-20%, and 5-9%, respectively. Among them, the peroxide in the initiator composition of the fourth reactor 12 is selected from two of the following: tert-butyl peroxide-2-ethylhexanoate, tert-butyl peroxide-3,5,5-trimethylhexanoate, and di-tert-butyl peroxide.

[0045] In step S3 above, the low-pressure separator 18 is opened 8-15 hours after nitrogen purging is completed during shutdown and maintenance. The wax removal cycle of heat exchanger 16 is 8-16 hours. The molten material temperature of 175-182℃ in the low-pressure separator in step S3 above is obtained through a high-temperature baking thermo-oxidative stability test of EVA resin. The surface of the molten material that has been retained in the low-pressure separator for a long time gradually changes color under high temperature and trace oxygen conditions, ranging from pale yellow, brown, wine red, to black. The critical temperature is 183-185℃. The demonstration of the color change of the molten material surface with temperature and time is shown below. Figure 2 As shown.

[0046] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0047] Example 1

[0048] S1. Raw material pressurization treatment: The primary compressor 1 receives ethylene from the ethylene feed pipeline 3 and the separator 24, and compresses it to 23.8 MPa. The ethylene feed rate from the ethylene feed pipeline 3 to the primary compressor 1 is 20.3 tons / hour. The pressurized ethylene then enters the secondary compressor 4. The inlet pressure of the primary compressor 1 is 37 kPa, and the discharge rate of the purge gas from the primary compressor 1 is 0.63 tons / hour. The molecular sieve purification tower purifies vinyl acetate and then sends it to the secondary compressor 4 through the vinyl acetate feed pipeline 5. The feed rate of vinyl acetate raw material in the secondary compressor 4 is 12.1 tons / hour. After purification, the vinyl acetate content is 10 ppm hydroquinone, 2 ppm water, 73 ppm acetic acid, and 54 ppm acetaldehyde. The secondary compressor 4 pressurizes the received ethylene and vinyl acetate to 248 MPa.

[0049] S2. Polymerization Reaction: The gas, pressurized by the secondary compressor 4, enters the preheater 7, where it is heated to 144°C. The preheated gas then sequentially enters the first reactor 9, the second reactor 10, the third reactor 11, and the fourth reactor 12. An initiator composition is injected into the inlet of each reactor, initiating the polymerization reaction. The reaction temperature of the four reactors is controlled by the amount of initiator composition injected. The reaction temperature of the first reactor 9, the second reactor 10, and the third reactor 11 is 245°C, and the reaction temperature of the fourth reactor 12 is 230°C. Pressure oscillations generated by pulse valves 13 reduce material adhesion to the reactor walls. The pulse depth of pulse valve 13 is 200 MPa, the duration is 1.3 s / cycle, and the interval is 70 s. The temperature of the molten material after depressurization by pulse valve 13 is 242°C. The hot water temperature in the reactor jacket 26 is 144°C. The molten material produced by the reaction is cooled by after-cooler 14 after depressurization by pulse valve 13. The initiator composition consists of peroxide... The initiator composition of the first reactor 9, along with isododecane diluent, comprises the following peroxides: dioctyl peroxide dicarbonate, tert-butyl peroxypentanoate, tert-butyl peroxy-2-ethylhexanoate, and tert-butyl peroxy-3,5,5-trimethylhexanoate, with the four peroxides accounting for 20% of the initiator composition. The initiator composition of the second reactor 10 also comprises the following peroxides: dioctyl peroxide dicarbonate, tert-butyl peroxypentanoate, tert-butyl peroxy-2-ethylhexanoate, and tert-butyl peroxy-3,5,5-trimethylhexanoate, with the four peroxides accounting for 20% of the initiator composition. The proportion of peroxides in the initiator composition is 17%; in the initiator composition of the third reactor 11, the peroxides are tert-butyl peroxypentanoate, tert-butyl peroxy-2-ethylhexanoate, and tert-butyl peroxy-3,5,5-trimethylhexanoate, and the proportion of the three peroxides in the initiator composition is 14%; in the initiator composition of the fourth reactor, the peroxides are tert-butyl peroxy-2-ethylhexanoate and di-tert-butyl peroxide, and the proportion of the two peroxides in the initiator composition is 7%, and the ratio of tert-butyl peroxy-2-ethylhexanoate and di-tert-butyl peroxide is 1:1.

[0050] S3. Separation Processing: After the molten material is conveyed to the aftercooler 14, where the hot water temperature in the jacket 26 of the aftercooler 14 is 144℃, it then enters the high-pressure separator 15. The high-pressure separator 15 cools the molten material to 185℃. The separated gaseous material enters the secondary compressor 4 along the high-pressure circulating gas recovery pipeline 6. The gaseous material separated by the high-pressure separator 15 is condensed and dewaxed in the heat exchanger 16 on the high-pressure circulating gas recovery pipeline 6. The dewaxing cycle of the heat exchanger 16 is 12 hours. The gaseous material dewaxed is then transported along the high-pressure circulating gas recovery pipeline 6. The gas recovery pipeline 6 enters the secondary compressor 4, while the molten material separated by the high-pressure separator 15 enters the low-pressure separator 18, reducing the temperature of the molten material to 178°C. The liquid level in the low-pressure separator 18 is controlled at 48%. The gaseous material separated by the low-pressure separator 18 enters the separation tank 24 after heat exchange in the heat exchanger 16. The gaseous ethylene is recycled to the primary compressor 1, and the liquid vinyl acetate enters the second molecular sieve purification tower 25. The purified vinyl acetate enters the vinyl acetate feed pipeline 5, while the molten liquid material enters the product forming unit for processing.

[0051] S4. Product Forming Process: The molten liquid material discharged from the low-pressure separator 18 enters the granulator 21 for granulation. The cylinder temperature of the granulator 21 is 164℃. The outlet of the middle body pipeline 19 at the tail of the granulator 21 is sealed by liquid vinyl acetate. Then, the EVA particles are sent to the dryer 22 for drying. After drying, the EVA particles are sent to the degassing silo 23. Unreacted ethylene or vinyl acetate in the material is removed in the degassing silo 23 to obtain the desired EVA product.

[0052] The granular EVA product obtained in Example 1 above has a VA content of 28.4%, a melt index of 26.6 g / 10 min, and a maximum number of colored particles of 1 per kg. The product is a superior grade.

[0053] Example 2

[0054] Compared with the scheme in Example 1, the difference is as follows: the inlet pressure of the primary compressor 1 is 55 kPa, the primary compressor 1 compresses ethylene to 24 MPa, the discharge rate of purge gas is 0.73 tons / hour, the preheater 7 heats the gas to 135°C, the secondary compressor 4 pressurizes ethylene and vinyl acetate to 254 MPa, the reaction temperature of the first reactor 9, the second reactor 10 and the third reactor 11 is 205°C, the reaction temperature of the fourth reactor 12 is 195°C, the hot water temperature in the jacket 26 of the four reactors and the aftercooler 14 is 125°C, the pulse depth of the pulse valve 13 is 170 MPa, and the temperature of the molten material after depressurization by the pulse valve 13 is 212°C.

[0055] The peroxides in the initiator composition of the fourth reactor 12 are tert-butyl peroxide-2-ethylhexanoate and tert-butyl peroxide-3,5,5-trimethylhexanoate in a ratio of 1:1.2. The peroxides in the initiator compositions of the first reactor 9, the second reactor 10, the third reactor 11 and the fourth reactor 12 are 14%, 12%, 12% and 5%, respectively.

[0056] The heat exchanger 16 has a wax removal cycle of 8 hours. The high-pressure separator 15 cools the molten material to 198°C, and the low-pressure separator 18 reduces the temperature of the molten material to 178°C. The liquid level of the low-pressure separator 18 is controlled at 45%, and the cylinder temperature of the granulator 21 is 173°C.

[0057] Example 2 yielded a granular EVA product with a VA content of 27.7%, a melt index of 6.5 g / 10 min, and 0 colored particles / kg, classifying it as a superior product.

[0058] Example 3

[0059] Compared with Example 1, the difference is that after the vinyl acetate raw material is refined, the hydroquinone content is 6 ppm, the water content is 4 ppm, the acetic acid content is 42 ppm, and the acetaldehyde content is 28 ppm.

[0060] The purge gas discharge rate is 0.44 tons / hour. The primary compressor 1 receives 19.1 tons / hour of ethylene feed from the ethylene feed line 3, and the secondary compressor 4 receives 9.4 tons / hour of vinyl acetate feed. The preheater 7 heats the gas to 147°C, and the secondary compressor 4 pressurizes the ethylene and vinyl acetate to 260 MPa. The reaction temperature of the first reactor 9, the second reactor 10, and the third reactor 11 is 232°C, and the reaction temperature of the fourth reactor 12 is 220°C. The hot water temperature in the jacket 26 of the four reactors and the aftercooler 14 is 130°C. The pulse depth of the pulse valve 13 is 190 MPa, the duration is 1.7 s / time, and the interval is 80 s. The temperature of the molten material after being depressurized by the pulse valve 13 is 213°C.

[0061] The proportions of peroxide in the initiator compositions of the first reactor 9, the second reactor 10, the third reactor 11, and the fourth reactor 12 are 16%, 14%, 14%, and 7%, respectively; the wax removal cycle of the heat exchanger 16 is 16 hours; the high-pressure separator 15 cools the molten material to 198°C; and the cylinder temperature of the granulator 21 is 180°C.

[0062] The granular EVA product obtained in Example 3 had a VA content of 22.0%, a melt index of 4.5 g / 10 min, and 0 colored particles / kg, making it a superior product.

[0063] Example 4

[0064] Compared with Example 1, the differences are as follows: the inlet pressure of the primary compressor 1 is 68 kPa, the primary compressor 1 compresses ethylene to 24.4 MPa, the purge gas discharge rate is 0.56 tons / hour, the primary compressor 1 receives 18.6 tons / hour of ethylene feed from the ethylene feed pipeline 3, and the feed rate of vinyl acetate feed into the secondary compressor 4 is 6.0 tons / hour; the preheater 7 heats the gas to 144°C, and the secondary compressor 4 pressurizes the ethylene and vinyl acetate to 265 MPa. The reaction temperature of reactor 9, reactor 10 and reactor 11 is 241℃, the reaction temperature of reactor 12 is 231℃, the hot water temperature in the jacket 26 of the four reactors and aftercooler 14 is 135℃, the pulse depth of pulse valve 13 is 170MPa, the duration is 1.7s / time, the interval is 80s, and the temperature of the molten material after pressure reduction by pulse valve 13 is 228℃. The low-pressure separator 18 is opened 12 hours after nitrogen purging is qualified when it is shut down for maintenance.

[0065] The peroxides in the initiator composition of the second reactor 10 are dioctyl peroxydicarbonate, tert-butyl peroxypentanoate, and tert-butyl peroxy-2-ethylhexanoate. The peroxides in the initiator composition of the third reactor 11 are tert-butyl peroxypentanoate and tert-butyl peroxy-2-ethylhexanoate. The proportions of peroxides in the initiator compositions of the first reactor 9, the second reactor 10, the third reactor 11, and the fourth reactor 12 are 16.5%, 18%, 20%, and 7.5%, respectively.

[0066] The high-pressure separator 15 cools the molten material to 198°C, and the low-pressure separator 18 reduces the temperature of the molten material to 182°C. The liquid level in the low-pressure separator 18 is controlled at 42%. The cylinder temperature of the granulator 21 is 180°C.

[0067] The granular EVA product obtained in Example 4 has a VA content of 18.2%, a melt index of 2.2 g / 10 min, and a maximum number of colored particles of 2 per kg. The product is classified as a superior grade.

[0068] Comparative Example 1

[0069] Compared to Example 1, the difference lies in the following: the fresh vinyl acetate feedstock has a high impurity content and is unrefined; the water content in the vinyl acetate feedstock is 35 ppm and the acetic acid content is 130 ppm. After 36 days of operation, the low-molecular-weight wax discharged from heat exchanger 16 in the circulating pipeline contains yellow particles, and the highest number of colored particles in the EVA product is 8 per kg, with yellow and black spots (the appearance of the colored particles is as follows). Figure 1 As shown in the image, the product is a qualified product.

[0070] Comparative Example 2

[0071] Compared to Example 1, the difference lies in the following: the hydroquinone content in the fresh vinyl acetate feedstock was 58 ppm, while the hydroquinone content in the refined vinyl acetate feedstock was 32 ppm. After 14 days of operation, the highest number of colored particles in the EVA product was 52 particles / kg, with a brownish-red color, indicating that the product was a secondary grade.

[0072] Comparative Example 3

[0073] Compared with Example 1, the differences are as follows: the discharge rate of the purge gas is 0.06 tons / hour; after 34 days of production and operation of the unit, the methane content in the purge gas is 0.44%, the carbon dioxide content is 0.9%, and the acetylene content is 14 ppm; the low molecular weight wax discharged from the heat exchanger 16 on the high pressure circulating gas recovery pipeline 6 contains black particles, the highest value of EVA product color spots is 27 particles / kg, the color spots are black, and the product is a secondary brand.

[0074] Comparative Example 4

[0075] Compared with Example 1, the differences are as follows: the inlet pressure of primary compressor 1 is 115 kPa, primary compressor 1 compresses ethylene to 26.2 MPa, high-pressure separator 15 cools the molten material to 212°C, low-pressure separator 18 reduces the temperature of the molten material to 200°C, the amount of low molecular weight wax discharged in heat exchanger 16 on high-pressure circulating gas recovery pipeline 6 is relatively large, the gas-liquid separation effect in high-pressure separator 15 and low-pressure separator 18 is poor, and a large amount of reaction products are entrained in the gas phase material; after 19 days of production operation, the low molecular weight wax discharged from heat exchanger 16 contains yellow particles, the highest value of EVA product color spots is 12 particles / kg, the color spots are yellow, and some products are secondary brands.

[0076] Comparative Example 5

[0077] Compared with Example 1, the difference lies in the proportion of peroxide in the initiator compositions of the first reactor 9, the second reactor 10, the third reactor 11, and the fourth reactor 12, which are 20%, 17%, 14%, and 14%, respectively. After 23 days of operation, the highest monitored methane content in the purge gas was 0.6%, the acetylene content was 9 ppm, and the temperature of the molten material after depressurization by pulse valve 13 was 279°C; the low molecular weight wax discharged from heat exchanger 16 on the high-pressure circulating gas recovery pipeline 6 contained a large number of black particles (such as...). Figure 3 As shown), the low molecular weight wax contained 0.13% elemental carbon according to thermogravimetric analysis (TGA), which is suspected to be due to a localized trace decomposition reaction of ethylene in the reaction system. The highest number of colored particles in the EVA product was 119 / kg, and the colored particles were black. The product was a secondary brand, and the unit was shut down for cleaning.

[0078] Comparative Example 6

[0079] Compared to Example 1, the difference lies in that: the low-pressure separator 18 reduces the temperature of the molten material to 200°C, the cylinder temperature of the granulator 21 is 200°C, and the outlet of the central pipeline 19 at the tail of the granulator 21 is directly connected to the atmosphere without liquid sealing measures. Within 3 to 20 days of operation, the polymerization reaction was interrupted four times due to malfunctions in the compression unit or the initiator composition injection system, and the granulator 21 stopped 10 minutes after each interruption. After 27 days of operation, brownish-red and black speckled particles appeared in the EVA product, with the highest number of speckled particles reaching 12 per kg; some products were secondary grades.

[0080] Comparative Example 7

[0081] Compared to Example 4, the difference lies in the following: The low-pressure separator 18 lowers the temperature of the molten material to 200°C. Within 46 days of operation, the granulator 21 frequently stopped due to large lumps interlocking with the pelletizing blade. The liquid level fluctuation range of the low-pressure separator 18 was 36-76%. Yellow and black speckled particles appeared in the EVA product, with the highest number of speckled particles reaching 156 per kg. The product was a secondary grade. During the shutdown and maintenance, the thickness of the wall-adhering lumps on the wall of the low-pressure separator 18 was 12 cm. The wall-adhering lumps were dark yellow with localized brownish-black hues, while conventional wall-adhering lumps are transparent materials with a thickness of 0.3-1.0 cm.

[0082] Comparative Example 8

[0083] Compared with Example 4, the difference is that: after 82 days of production operation, the equipment was shut down for maintenance. After the low-pressure separator 18 was successfully purged with nitrogen for 3 hours, its top cover was opened. During cleaning, once the system temperature had dropped to room temperature, the material surfaces at the low-pressure separator 18, the screw of the granulator 21, and the die of the granulator 21 were all light wine-red (e.g., ...). Figure 4 (As shown). Within 3 days of the unit resuming production, the highest number of colored particles in the EVA product was 15 particles / kg, with the color spots being brownish-red, and some products were secondary brands; the colored particles disappeared after 3 days of production operation.

[0084] Comparative Example 9

[0085] Compared with Example 4, the difference is that the wax removal cycle of heat exchanger 16 is 48 hours, and the liquid level control of low-pressure separator 18 is 55%. After 35 days of production and operation, the low molecular weight wax discharged from heat exchanger 16 in the circulation pipeline contains yellow particles. The highest value of EVA product color spot particles is 5 particles / kg, the color spot is yellow, and the product is qualified. After 43 days of production and operation, the highest value of color spot particles is 12 particles / kg, which exceeds the standard.

[0086] The comprehensive examples and comparative examples show that the production method of low-color-particle EVA resin of the present invention eliminates polymer side reactions caused by harmful impurities such as acetylene, aldehydes, acids, and trace amounts of water, controls occasional localized trace decomposition of ethylene due to unreasonable dilution concentration of peroxide in the initiator composition, eliminates the introduction of trace amounts of oxygen and long-term retention of molten materials in the equipment system, and cuts off the source of color change in the reaction products. Through a series of combined measures, the safety and stability problems existing in the production of EVA resin in ultra-high pressure polymerization equipment are solved, the color-particles of EVA resin are effectively controlled, the yield of superior products is high, and the production of high-pressure polymerization equipment is economical.

[0087] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific structures and characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. An apparatus for producing a low speckle EVA resin, characterized by comprising: It includes in turn one compressor (1), secondary compressor (4), preheater (7), first reactor (9), second reactor (10), third reactor (11), fourth reactor (12), after-cooler (14), high-pressure separator (15), low-pressure separator (18), granulator (21), dryer (22) and degassing bin (23); the one compressor (1) is provided with a purge gas discharge pipeline (2); the outlet of the middle body pipeline (19) at the tail of the granulator (21) is provided with a liquid seal tank (20) containing liquid vinyl acetate, and the outlet end of the middle body pipeline (19) is below the liquid level of the liquid seal tank (20); the inlet of the one compressor (1) is communicated with an ethylene feed pipeline (3), the inlet of the secondary compressor (4) is communicated with a vinyl acetate feed pipeline (5), the vinyl acetate feed pipeline (5) is communicated with a molecular sieve refining unit for refining vinyl acetate raw material, a pulse valve (13) is arranged on the connecting pipeline between the fourth reactor (12) and the after-cooler (14), the inlets of the first reactor (9), the second reactor (10), the third reactor (11) and the fourth reactor (12) are all connected with an initiator composition tank (8), the exhaust port of the high-pressure separator (15) is communicated with the inlet of the secondary compressor (4) through a high-pressure cycle gas recovery pipeline (6), at least two heat exchangers (16) are connected in series on the high-pressure cycle gas recovery pipeline (6), a low-molecular wax discharge pipeline (17) is arranged on the heat exchanger (16) close to the high-pressure separator (15), the discharge port of the low-pressure separator (18) is connected with the granulator (21), the exhaust port of the low-pressure separator (18) is connected in turn with a heat exchanger (16) and a separation tank (24), the top of the separation tank (24) is communicated with the inlet of the one compressor (1), and the bottom of the separation tank (24) is communicated with the molecular sieve refining unit.

2. The apparatus for producing a low-scratch-particle EVA resin according to claim 1, wherein The molecular sieve refining unit includes a first molecular sieve refining tower (28) and a second molecular sieve refining tower (25), the first molecular sieve refining tower (28) is used for treating fresh vinyl acetate, the second molecular sieve refining tower (25) is used for treating recycled vinyl acetate, and the first molecular sieve refining tower (28) and the second molecular sieve refining tower (25) are both communicated with the vinyl acetate feed pipeline (5), a booster pump (27) is arranged on the vinyl acetate feed pipeline (5), and the bottom of the separation tank (24) is communicated with the second molecular sieve refining tower (25).

3. A production method of low speckle EVA resin using the production apparatus according to claim 2, characterized by, It includes the following steps: S1, raw material pressure treatment: the first compressor (1) receives ethylene from the ethylene feed pipeline (3) and the separation tank (24) and compresses it to 23.5-24.5 MPa, and then the pressurized ethylene enters the second compressor (4); the first molecular sieve refining tower (28) receives fresh vinyl acetate from the fresh vinyl acetate feed pipeline (29), and the first molecular sieve refining tower (28) and the second molecular sieve refining tower (25) respectively refine the vinyl acetate and deliver it to the second compressor (4) through the vinyl acetate feed pipeline (5), and the second compressor (4) pressurizes the received ethylene and vinyl acetate to 240-265 MPa, wherein the content of hydroquinone in the vinyl acetate treated by the molecular sieve refining unit is 5-10 ppm, the water content is 1-5 ppm, the acetic acid content is 10-80 ppm, and the acetaldehyde content is 20-60 ppm; S2, polymerization reaction: the gas after the pressure increase of the second compressor (4) enters the preheater (7), and the gas is heated to 135-147℃, the initiator composition is injected through the feed ports of the first reactor (9), the second reactor (10), the third reactor (11) and the fourth reactor (12), and the polymerization reaction is initiated after the injection of the initiator composition, and the reaction temperature of the four reactors is controlled by the injection amount of the initiator composition, wherein the reaction temperature of the first reactor (9), the second reactor (10) and the third reactor (11) is 200-250℃, and the reaction temperature of the fourth reactor (12) is 185-230℃, and the reaction pressure of the four reactors is 240-265 MPa; the initiator composition is composed of 2-5 kinds of peroxides, and isododecane is used as a diluent, and the dilution ratio is 3-25%, wherein the dilution ratio of the peroxide in the initiator composition added in the fourth reactor (12) is 1 / 3-1 / 2 of the peroxide in the first reactor (9), the second reactor (10) and the third reactor (11); S3, separation treatment: the molten material generated by the polymerization reaction is cooled by the after-cooler (14) and enters the high-pressure separator (15), the molten material is cooled to 180-200℃, the separated gas phase material enters the second compressor (4) along the high-pressure circulating gas recovery pipeline (6), and the molten material enters the low-pressure separator (18), the temperature of the molten material is reduced to 175-182℃, the liquid level control of the low-pressure separator (18) is 42-48%, and the gas phase material separated by the low-pressure separator (18) is heated by the heat exchanger (16) and enters the separation tank (24) for separation, the gaseous ethylene is recycled to the first compressor (1), and the liquid vinyl acetate enters the second molecular sieve refining tower (25), the refined vinyl acetate enters the vinyl acetate feed pipeline (5); and the molten liquid phase material at the bottom of the low-pressure separator enters the product forming unit for treatment; S4, product forming treatment: the molten liquid phase material discharged from the low pressure separator (18) enters the granulator (21) for granulation, wherein the barrel temperature of the granulator (21) is 160-180℃; then the EVA particles are sent to the dryer (22) for drying treatment, and after drying, the EVA particles are sent to the degassing bin (23) to remove unreacted ethylene or vinyl acetate in the material, thereby obtaining the required EVA product.

4. The production method of low speckle EVA resin according to claim 3, characterized by, The inlet pressure of the primary compressor (1) is 30-70KPa, and the discharge amount of the primary compressor (1) is 2-4% of the ethylene feed amount.

5. The method of producing a low-scratching EVA resin according to claim 3, characterized by, The initiator composition is selected from 2-5 kinds of dioctyl peroxydicarbonate, tert-butyl peroxypivalate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-3,5,5-trimethylhexanoate and di-tert-butyl peroxide, and isododecane is used as a diluent, wherein the peroxide in the initiator composition of the fourth reactor (12) is selected from at least two of tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-3,5,5-trimethylhexanoate and di-tert-butyl peroxide.

6. The production method of low speckle EVA resin according to claim 5, characterized by, The dilution ratio of peroxide in the initiator composition of the first reactor (9), the second reactor (10), the third reactor (11) and the fourth reactor (12) is 12-22%, 10-20%, 10-20% and 5-9%, respectively.

7. The production method of low speckle EVA resin according to claim 3, characterized by, In step S3, the jacket (26) of the four reactors and the aftercooler (14) is heated to a temperature of 125-145℃, and the temperature of the molten material after pressure reduction by the pulse valve (13) is 210-250℃.

8. The production method of low speckle EVA resin according to claim 3, characterized by, The low pressure separator (18) is opened 8-15h after nitrogen replacement is qualified, and the wax discharge period of the heat exchanger (16) is 8-16h.

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