A degassing method and degassing system for polymer particles used in a high-pressure polymerization device

Abstract

The present invention discloses a degassing method and degassing system for polymer particles used in a high-pressure polymerization device. The method comprises: (1) delivering the polymer particles to be degassed to a degassing bin via a conveying pipeline; (2) delivering air at a temperature of 0-70°C into the degassing bin from at least two feed positions at different heights at the bottom of the bin; (3) continuously degassing the polymer particles in the degassing bin for 4-96 hours; and (4) after degassing, the particles at the bottom of the degassing bin are sequentially passed through at least a particle circulation three-way valve, a sampling three-way valve, and a dust collector before being sent for packaging. The present invention eliminates the investment costs of a blending bin and a packaging bin, and the operation time required to reduce the VOC content in the product to a target value is short.

Description

Technical Field

[0001] The invention belongs to the field of degassing of polymer particles in a high-pressure polymerization process, and particularly relates to a degassing method and a degassing system for polymer particles used in a high-pressure polymerization device. Background Art

[0002] Low-density polyethylene (LDPE) and ethylene-vinyl acetate copolymer (EVA) are polymer products with a wide range of commercial applications. LDPE typically has a density between 0.915 and 0.935, and offers excellent insulation, fluidity, elasticity, and transparency. It is widely used in extrusion coating, injection molding, agricultural films, and high-transparency films. It is also an important insulating material for the production of wire and cable. Unlike LDPE, EVA incorporates the polar monomer vinyl acetate, resulting in reduced crystallinity and improved flexibility, compatibility, heat sealing, and environmental resistance. It is widely used in foaming, agricultural films, wire and cable, hot-melt adhesives, and photovoltaic cell encapsulation.

[0003] For those skilled in the art, whether it's LDPE or EVA, the volatile components in the polymer product need to be reduced to below standard values ​​before packaging. Existing industrial processes typically utilize separate degassing, blending, and packaging silos. Pellets produced by an extrusion granulator are transported via gas conveying to separate equipment for purification, blending, and packaging. This results in a complex process, high failure rates, and high investment and operating costs. Furthermore, friction between the pellets and the pipe walls during pellet transportation can lead to the formation of fine powder, which can also affect product processing and performance. Summary of the Invention

[0004] The present invention aims to overcome the shortcomings of the prior art and provides a method and system for degassing polymer particles for a high-pressure polymerization device. The present invention first provides a method for devolatilizing polymer particles for a high-pressure polymerization device, the method comprising at least the following steps:

[0005] (1) The polymer particles to be degassed after extrusion granulation are sent to the degassing bin through a conveying pipeline;

[0006] (2) feeding air having a temperature of 0-70° C. into the degassing bin from at least two feed positions at different heights at the bottom of the degassing bin;

[0007] (3) The polymer particles are continuously degassed in the degassing chamber for 4-96 hours to remove low-boiling point components entrained in the polymer;

[0008] (4) After degassing is completed, the particles at the bottom of the degassing bin are discharged through the discharge pipe, and then a portion of the material returns to the top of the degassing bin for circulation through the particle circulation three-way valve, and the remaining material is sent to the dust collector, which removes dust and sends it to the packaging system.

[0009] Wherein, the temperature of the air in step (2) is less than or equal to the Vicat softening point temperature of the polymer, and the air temperature T air (℃), air velocity Ug (m / s) in the degassing chamber and devolatilization time tr (hours) satisfy the following relationship:

[0010]

[0011] The value of A is 10-150, preferably 10-100, and the value of B is 0.05-0.15, preferably 0.05-0.10. It should be noted that if the rise is not met, a too short degassing time will result in the volatile content in the product being too high and not meeting the standard, and a too long degassing time will result in high operating costs. In the degassing process, the combustible gas component at the top outlet of the degassing bin is lower than the explosion limit of 25%, and the bottom of the degassing bin is provided with a high-pressure nitrogen inlet for feeding nitrogen to prevent the combustible gas from exceeding the standard. The packaging system described in the present invention is a mobile packaging line or a fixed packaging line.

[0012] In one embodiment of the present invention, the polymer particles are polyethylene particles obtained by high-pressure polymerization and subjected to upstream extrusion granulation.

[0013] Preferably, the mass flow rate of the polymer particles transported to the degassing bin is 5 to 60 t / h.

[0014] In one embodiment of the present invention, the temperature of the air entering the bottom of the degassing bin is 0-70° C., preferably 0-40° C., more preferably 10-30° C. When the polymer is LDPE, the temperature of the air is more preferably 45-60° C., and when the polymer is EVA, the temperature of the air is more preferably 20-40° C.

[0015] In one embodiment of the present invention, the feeding time of the polymer particles in the degassing silo is shorter than the degassing time. Degassing begins when the feeding begins, and degassing continues while the particles are being fed. The feeding time of the polymer particles in the degassing silo is 1-12 hours, preferably 2-8 hours. In one embodiment of the present invention, the degassing time of the polymer particles in the degassing silo is 4-96 hours, preferably 6-72 hours.

[0016] In one embodiment of the present invention, the flow rate of the gas in the degassing silo is less than the initial fluidization velocity of the polymer particles, preferably less than 0.6 times the initial fluidization velocity of the polymer particles.

[0017] The present invention also provides a polymer degassing system for a high-pressure polymerization device, comprising

[0018] Surge hopper, used to receive upstream polymer particles and store them;

[0019] A degassing chamber connected to the buffer hopper is used to reduce the content of low-boiling point components entrained in the polymer to below 1000 ppmw; at least two devolatilization feed ports are provided at different heights at the bottom of the degassing chamber;

[0020] The particle circulation three-way valve has its inlet connected to the particle outlet at the bottom of the degassing bin, its first outlet connected to the dust collector, and its second outlet connected to the top of the degassing bin through a conveying pipeline;

[0021] Dust collector for polymer removal with particle diameter less than 100 microns;

[0022] The fan is arranged on the conveying pipeline and is used to convey polymer particles.

[0023] In one embodiment of the present invention, the degassing system includes at least one buffer hopper and at least two degassing bins, wherein the degassing bins include a head, a straight cylinder and a bottom, and the bottom of the degassing bins is a cone, a frustum or a combination of a cone and a straight cylinder.

[0024] In one embodiment of the present invention, the height-to-diameter ratio of the straight tube of the degassing bin is 1 to 20, preferably 2 to 8, more preferably 3 to 6. The angle formed by two generatrixes at the bottom of the degassing bin is 20-120°.

[0025] A discharge valve is provided on the bottom particle outlet of the degassing bin, the inlet of the particle circulation three-way valve and the outlet of the discharge valve are connected through a discharge pipeline, and the first outlet of the particle circulation three-way valve is connected to the dust collector through the discharge pipeline; a sampling three-way valve is provided on the discharge pipeline for sampling the material.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] (1) The degassing method and system proposed in the present invention have the functions of degassing and blending, eliminating the original blending bin and packaging bin. The process is simple, the degree of automation is high, and the operation is easy, which can significantly reduce the investment cost and operating cost of the devolatilization system.

[0028] (2) In the degassing method proposed in the present invention, the particles at the bottom of the degassing bin are directly sent to the packaging system after passing through the discharge valve, the particle circulation three-way valve, the sampling three-way valve and the dust collector. The polymer particles at the bottom of the degassing bin are easy to discharge and not prone to clogging.

[0029] (3) In the present invention, a dust collector is provided at the bottom of each degassing bin, so that less fine powder and filamentous material are entrained in the product, and the product quality is better. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of a polymer particle degassing system according to a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0031] The present invention will be further described and illustrated below in conjunction with specific embodiments. The technical features of each embodiment of the present invention may be combined accordingly, provided that there is no conflict between them.

[0032] Figure 1 The following is a schematic diagram of a polymer particle degassing system, which includes:

[0033] Buffer hopper 1, used for temporarily storing polymer particles;

[0034] Degassing chamber 2, used to reduce the content of low-boiling point components entrained in the polymer to below 1000 ppmw;

[0035] The dust collector 3 is used to remove polymer particles with a diameter of less than 100 microns.

[0036] Delivery pipeline 4-14.

[0037] In a preferred embodiment of the present invention, the extruded polymer pellets enter the buffer hopper 1 through a first pipeline 4. The polymer pellets then pass through the buffer hopper discharge pipeline 5, mix with the gas in the second pipeline 6, and then enter the degassing bin 2 through a third pipeline 7. Devolatilization gas from devolatilization gas pipelines 11 and 12 enters the bottom of the degassing bin 2 in a continuous or intermittent feeding manner, and removes any residual unreacted materials such as ethylene, propylene, and propionaldehyde from the polymer pellets. After degassing, the polymer particles are discharged through the bottom discharge pipeline 8 of the degassing bin. A portion of the particles are entrained by the conveying gas in the fourth pipeline 13 and returned to the top of the degassing bin via pipeline 10. The remaining material passes through the dust collector 3 and is then sent to packaging through the fifth pipeline 9.

[0038] In a preferred embodiment of the present invention, a necessary discharge valve and a sampling three-way valve are provided between the degassing bin 2 and the dust collector 3, which are respectively used for discharging materials from the bottom of the degassing bin.

[0039] In a preferred embodiment of the present invention, a particle circulation three-way valve is provided on the discharge pipe 8 at the bottom of the degassing bin, so that a portion of the material is mixed with the conveying gas from the fourth pipe 13 and then returned to the degassing bin 2. The conveying gas passes through the degassing bin 2 and is discharged from the fifth pipe 14.

[0040] During the degassing process of polymer pellets produced through high-pressure polymerization, a blending silo is used to homogenize multiple batches of product, improving product quality stability. The presence of a packaging silo can reduce the number of processes along the conveying and packaging line. However, this traditional process separates degassing, blending, and packaging, resulting in a complex process. The present invention eliminates the need for the traditional blending and packaging silos, allowing the degassing silo to function as both. This significantly reduces equipment investment costs and reduces the total operating time for degassing, blending, and packaging. Specific examples and comparative examples are provided below.

[0041] Example 1

[0042] exist Figure 1 The process flow shown here degasses low-density polyethylene (LDPE). The total flow rate of polyethylene and volatiles in conveying line 4 is 40 t / h, with an ethylene content of 0.0012 wt%. The Vicat softening temperature of LDPE is 92°C. This 40 t / h of LDPE is fed into a degassing bin 2 for an 8-hour feed period. Two streams of air are introduced at the bottom of the bin as the devolatilizing gas, with an average air velocity of 0.2 m / s and a temperature of 50°C. After 10 hours of continuous devolatilization, the ethylene content of the LDPE product is reduced to 50 ppm. The material is discharged from the bottom of the bin through discharge line 8. After passing through a particle recirculation three-way valve, a portion of the material returns to the top of the bin. The remaining material enters a dust collector for dust removal before being sent to the packaging system.

[0043] The cone angle at the bottom of the degassing bin is 60°, the height-to-diameter ratio of the degassing bin is 5, and the dust collector filter can separate more than 99% of fine powder larger than 50μm from LDPE.

[0044] Compared with Comparative Example 1, Example 1 does not require a blending silo and a packaging silo, thus saving 33% of the investment cost of the air conveying packaging system and shortening the devolatilization time by 38 hours.

[0045] Example 2

[0046] exist Figure 1 The process flow shown here degasses low-density polyethylene (LDPE). The total flow rate of polyethylene and volatiles in conveying line 4 is 30 t / h, with an ethylene content of 0.0015 wt%. The Vicat softening temperature of LDPE is 92°C. This 30 t / h of LDPE is fed into a degassing bin 2 for an 8-hour feed period. Two streams of air are introduced at the bottom of the bin as the devolatilizing gas, with an average air velocity of 0.15 m / s and a temperature of 50°C. After 12 hours of continuous devolatilization, the ethylene content of the LDPE product is reduced to 50 ppm. The material is discharged from the bottom of the bin through discharge line 8. After passing through a particle recirculation three-way valve, a portion of the material returns to the top of the bin. The remaining material enters a dust collector for dust removal before being sent to the packaging system.

[0047] The cone angle at the bottom of the degassing bin is 60°, the height-to-diameter ratio of the degassing bin is 5, and the dust collector filter can separate more than 99% of fine powder larger than 50μm from LDPE.

[0048] Compared with Comparative Example 1, Example 2 saves 30% of the investment cost of the air conveying packaging system and shortens the devolatilization time by 36 hours.

[0049] Example 3

[0050] exist Figure 1 The process flow shown here degasses low-density polyethylene (LDPE). The total flow rate of polyethylene and volatiles in conveying line 4 is 30 t / h, with an ethylene content of 0.0015 wt%. The Vicat softening temperature of LDPE is 92°C. This 30 t / h of LDPE is fed into degassing bin 2 for an 8-hour feed period. Two air streams are positioned at the bottom of the bin, with an average air velocity of 0.35 m / s and a temperature of 50°C. After 8 hours of continuous devolatilization, the ethylene content of the LDPE product is reduced to 50 ppm. The material is discharged from discharge line 8 at the bottom of the bin. After passing through a particle recirculation three-way valve, a portion of the material returns to the top of the bin. The remaining material enters a dust collector for dust removal before being sent to the packaging system.

[0051] The cone angle at the bottom of the degassing bin is 60°, the height-to-diameter ratio of the degassing bin is 5, and the dust collector filter can separate more than 99% of fine powder larger than 50μm from LDPE.

[0052] Compared with Comparative Example 1, Example 3 saves 28% of the investment cost of the air-conveying packaging system and saves 40 hours of devolatilization time.

[0053] Comparative Example 1

[0054] Compared to Example 1, this Comparative Example 1 utilizes an existing industrial process (separately equipped degassing, blending, and packaging silos). The devolatilized pellets are conveyed by air to the blending and packaging silos, where they are then discharged from the bottom of the packaging silo and sent to the packaging line. The Vicat softening temperature of LDPE is 92°C, while the average air velocity in the degassing silo is 0.35 m / s, the temperature is 50°C, and the continuous devolatilization time is 48 hours. Furthermore, compared to the Examples, the time required to transport the pellets from the degassing silo to the packaging machine in Comparative Example 1 is 8 hours longer, resulting in longer operation time and higher investment costs.

[0055] Comparative Example 2

[0056] Compared to Example 1, Comparative Example 2 employed an existing industrial process (separately equipped degassing, blending, and packaging silos). The devolatilized pellets were conveyed by air to the blending and packaging silos, where they were then discharged from the bottom of the packaging silo and sent to the packaging line. The Vicat softening temperature of LDPE is 93°C, while the average air velocity in the degassing silo was 0.50 m / s and the temperature was 20°C. The continuous devolatilization time was 120 hours. Furthermore, compared to Comparative Example 1, the time required to discharge the pellets from the degassing silo to the packaging machine increased by an additional 12 hours.

[0057] The above-described embodiments merely represent several implementation methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A method for degassing polymer particles used in a high-pressure polymerization device, characterized in that: The steps include: (1) The polymer particles to be degassed after extrusion granulation are sent to the degassing bin through a conveying pipeline; (2) feeding air with a temperature of 0-70°C into the degassing bin from at least two feed positions at different heights at the bottom of the degassing bin; (3) The polymer particles are continuously degassed in the degassing chamber for 4-96 hours to remove low-boiling point components entrained in the polymer; (4) After degassing is completed, the particles at the bottom of the degassing bin are discharged through the discharge pipe, and then a portion of the material is returned to the top of the degassing bin for circulation through the particle circulation three-way valve, and the remaining material is sent to the dust collector, which removes dust and sends it to the packaging system; Wherein, the temperature of the air in step (2) is less than or equal to the Vicat softening point temperature of the polymer, and the air temperature T air , in °C, the air velocity Ug in the degassing chamber, in m / s and the devolatilization time tr, in hours, satisfy the following relationship: The value of A is 10-150, and the value of B is 0.05~0.15; The polymer particles are polyethylene particles obtained by high-pressure polymerization and subjected to upstream extrusion granulation; During the degassing process, the combustible gas components at the top outlet of the degassing chamber are lower than the explosion limit of 25%, and there is a high-pressure nitrogen inlet at the bottom of the degassing chamber for feeding nitrogen to prevent the combustible gas from exceeding the standard.

2. The method according to claim 1, characterized in that The value of A is 10-100, and the value of B is 0.05-0.

10.

3. The method according to claim 1, characterized in that The mass flow rate of the polymer particles transported to the degassing bin is 5 to 60 t / h, and the flow rate of air in the degassing bin is less than the initial fluidization velocity of the polymer particles.

4. The method according to claim 3, characterized in that The flow rate of the air in the degassing chamber is lower than 0.5 times of the initial fluidization velocity, and the temperature of the air is 5-70° C. lower than the Vicat softening point of the polymer.

5. The method according to claim 1, wherein The feeding time of the polymer particles in the degassing silo is shorter than the degassing time. Degassing starts when the feeding starts, and degassing occurs while feeding. The feeding time of the polymer particles in the degassing silo is 1-12 hours.

6. The method according to claim 1, wherein the method is implemented using a polymer degassing system, characterized in that: The polymer degassing system comprises Surge hopper, used to receive upstream polymer particles and store them; A degassing chamber connected to the buffer hopper is used to reduce the content of low-boiling point components entrained in the polymer to below 1000 ppmw; at least two devolatilization feed ports are provided at different heights at the bottom of the degassing chamber; The particle circulation three-way valve has its inlet connected to the particle outlet at the bottom of the degassing bin, its first outlet connected to the dust collector, and its second outlet connected to the top of the degassing bin through a conveying pipeline; Dust collector for polymer removal with particle diameter less than 100 microns; The fan is arranged on the conveying pipeline and is used to convey polymer particles.

7. The method according to claim 6, characterized in that There are multiple degassing bins, which are connected in parallel. The degassing bins include a head, a straight cylinder and a bottom. The bottom of the degassing bin is a cone, a frustum or a combination of a cone and a straight cylinder.

8. The method according to claim 7, characterized in that The height-to-diameter ratio of the straight cylinder of the degassing bin is 1-20.

9. The method according to claim 6, characterized in that A discharge valve is provided on the bottom particle outlet of the degassing bin, the inlet of the particle circulation three-way valve and the outlet of the discharge valve are connected through a discharge pipeline, and the first outlet of the particle circulation three-way valve is connected to the dust collector through the discharge pipeline; a sampling three-way valve is provided on the discharge pipeline for sampling the material.

10. The method according to claim 7, characterized in that The angle formed by the two generatrixes at the bottom of the degassing bin is 20-120°.

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