Method for producing polyethylene using high-pressure centrifugation

Abstract

An embodiment of a process for producing polyethylene includes introducing a monomer feed stream comprising ethylene monomer and optionally a comonomer into a compressor system to produce a high-pressure ethylene stream having a pressure of at least 1,000 bar; introducing the high-pressure ethylene stream to a high-pressure centrifuge unit, which separates solid particles and liquid droplets from the high-pressure ethylene stream to produce an enriched effluent comprising the solid particles and liquid droplets and an upgraded high-pressure ethylene stream comprising the remainder of the high-pressure ethylene stream; removing the enriched effluent from the high-pressure centrifuge unit; and introducing the upgraded high-pressure ethylene stream to a polymerization reactor to produce polyethylene from the upgraded high-pressure ethylene stream.

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED ART) This application claims the benefit of U.S. Provisional Patent Application No. 63 / 385,500, filed November 30, 2022, the entire disclosure of which is incorporated herein by reference.

[0002] FIELD OF THE INVENTION FIELD OF THE INVENTION Embodiments described herein relate generally to the production of polyethylene, and specifically to the production of low density polyethylene (LDPE) involving high pressure centrifugation of solid particles and liquid droplets from a high pressure ethylene monomer feed. [Background technology]

[0003] Polyethylene resins are utilized in a variety of products and may be blended with additives to form polymer blends or compositions. The end use of the resulting polyethylene or polymer blend may limit the purity or quality of the final formed product. While it is preferable for polyethylene resins utilized in the production of polyethylene-based products to be completely pure and free of trace amounts of contaminants, this is not practical in real-world environments. Therefore, current polyethylene polymerization systems attempt to remove contaminants from the ethylene monomer or monomer blends fed to the reactor where polymerization occurs. Such purification is typically performed prior to increasing the pressure of the ethylene monomer stream using conventional separation techniques, such as gravity separation or swirl tubes. However, post-compression purification of the ethylene monomer stream has not previously been achieved, resulting in the presence and transfer of any contaminants introduced during the pressurization process to the reactor where polymerization occurs.

[0004] Thus, there is a continuing need for improved processes for producing polyethylene in which contaminants are removed from the ethylene monomer stream after the ethylene monomer stream is pressurized. Summary of the Invention

[0005]

[0006] Embodiments of the present disclosure fulfill this need for a process for producing polyethylene in which contaminants are removed from the ethylene monomer stream after the ethylene monomer stream is compressed. Specifically, embodiments of the present disclosure accomplish this by utilizing a high-pressure centrifuge unit located after a compressor system that increases the pressure of a monomer feed stream comprising ethylene monomer. Such a location allows for the removal of particles or contaminants introduced into the monomer feed stream from the compressor system.

[0006] According to one embodiment, there is provided a method for producing polyethylene, the process comprising: introducing a monomer feed stream comprising ethylene monomer and optionally a comonomer into a compressor system to produce a high-pressure ethylene stream having a pressure of at least 1,000 bar; introducing the high-pressure ethylene stream into a high-pressure centrifuge unit, which separates solid particles and liquid droplets from the high-pressure ethylene stream to produce an enriched effluent comprising the solid particles and liquid droplets and an upgraded high-pressure ethylene stream comprising the remainder of the high-pressure ethylene stream; removing the enriched effluent from the high-pressure centrifuge unit; introducing the upgraded high-pressure ethylene stream into a polymerization reactor to produce polyethylene from the upgraded high-pressure ethylene stream.

[0007] These and further embodiments are described in more detail below in the Detailed Description of the Invention, taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0008] The following detailed description of certain embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, in which like structure is indicated with like reference numerals and in which: [Figure 1] FIG. 1 is a schematic diagram of the present process for producing polyethylene, in accordance with one or more embodiments of the present disclosure. [Figure 2]FIG. 1 is a schematic diagram of the process for producing polyethylene according to one or more embodiments of the present disclosure, including a bypass. [Figure 3] FIG. 3 is a schematic diagram of the process for producing polyethylene according to one or more embodiments of the present disclosure shown in FIG. 2, further comprising a recycle line. [Figure 4] FIG. 1 is a schematic diagram of the process for producing polyethylene according to one or more embodiments of the present disclosure, in which the accept stream from the high-pressure centrifuge unit is recycled to the compressor system. DETAILED DESCRIPTION OF THE INVENTION

[0009] Certain embodiments of the present application will now be described. These embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the claimed subject matter to those skilled in the art.

[0010] The term "polymer" refers to a polymeric compound prepared by polymerizing monomers, whether of the same or different types. Thus, the generic term polymer generally encompasses the term "homopolymer," which refers to a polymer prepared from only one type of monomer, as well as the term "copolymer," which refers to a polymer prepared from two or more different types of monomers. The term "interpolymer" refers to a polymer prepared by polymerization of at least two different types of monomers. Thus, the generic term interpolymer includes copolymers or polymers prepared from more than two different types of monomers, such as terpolymers.

[0011] "Polyethylene" or "ethylene-based polymer" means a polymer containing more than 50 mole percent units derived from ethylene monomers. This includes ethylene-based homopolymers or copolymers (meaning the units are derived from two or more comonomers). Common forms of ethylene-based polymers known in the art include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), ultra-low-density polyethylene (ULDPE), very low-density polyethylene (VLDPE), single-site catalyzed linear low-density polyethylene (m-LLDPE), including both linear and substantially linear low-density resins, medium-density polyethylene (MDPE), and high-density polyethylene (HDPE).

[0012] As used herein, the term "composition" refers to a mixture of materials that comprise the composition, as well as reaction products and decomposition products formed from the materials of the composition.

[0013] The terms "blend," "polymer blend," and the like refer to a composition of two or more polymers. Such blends may or may not be miscible. Such blends may or may not be phase separated. Such blends may or may not contain one or more domain configurations as determined from transmission electron spectroscopy, light scattering, X-ray scattering, and any other method known in the art. A blend is not a laminate, although one or more layers of a laminate may contain the blend. Such blends may be prepared as dry blends formed in situ (e.g., in a reactor), as melt blends, or using other techniques known to those skilled in the art.

[0014] As used herein, the term "supercritical fluid" refers to a substance at a temperature and pressure above its critical point where no distinct liquid and gas phases exist, but below the pressure required to compress it into a solid.

[0015] The terms "comprising," "including," "having," and their derivatives are not intended to exclude the presence of any additional component, step, or procedure, whether or not it is specifically disclosed. For the avoidance of doubt, all compositions claimed through the use of the term "comprising" may include any additional additive, adjuvant, or compound, whether polymeric or otherwise, unless otherwise stated to the contrary. In contrast, the term "consisting essentially of" excludes from the scope of any succeeding description any other component, step, or procedure, except those that are not essential to operability. The term "consisting of" excludes any component, step, or procedure not specifically delineated or listed.

[0016] One particularly useful application of embodiments of the disclosed process for producing polyethylene is the production of cross-linkable polyethylene (XLPE), which is utilized in the manufacture of extra-high voltage cables, such as 500 kilovolt (kV) power transmission cables. The production of such cables typically involves incorporating additives into an LDPE base resin by compounding or liquid dipping, followed by the addition of a vulcanizing agent, such as peroxide, to enable cross-linking of the polyethylene during cable manufacture. Given the extreme operating conditions of such cables, it will be appreciated that even the smallest impurities can cause the cable to fail. Therefore, to avoid such impurities in the final cable, both the base resin and the additive package must be free of impurities. Unexpectedly, however, it has been discovered that when new cylinders or packings are installed as part of routine maintenance of compressor systems for compressing ethylene monomer feed in LDPE production, there is a break-in time during which sharp edges are scraped off the cylinders or packings, creating small metal particles that are carried into the ethylene monomer fed to the polyethylene reactor. These small metal particles must be removed to obtain an LDPE base resin within specifications for extra-high voltage cable production. While this exemplary application and need for post-compression purification of ethylene monomer is directed to XLPE for use in extra-high voltage cables, it will be understood that such a process may also be useful and advantageous for other end products, as polymer resins with no or reduced impurities are generally desirable.

[0017] Embodiments of the inventive process for producing polyethylene will now be described. Referring to system 10 of Figure 1, a monomer feed stream 5 comprising ethylene monomer is fed to a compressor system 20 to produce a high-pressure ethylene stream 22 having a pressure of at least 1000 bar. Although not shown, it is contemplated that in some embodiments, the monomer feed stream 5 may be pressurized before being delivered to compressor system 20. For example, the monomer feed stream 5 may be delivered to compressor system 20 at a pressure of less than 100 bar, or less than 50 bar, or less than 20 bar. All pressure measurements in this disclosure are absolute pressure values.

[0018] In one or more embodiments, the monomer feed stream 5 may further comprise one or more comonomers in combination with the ethylene monomer. Suitable comonomers include ethylenically unsaturated monomers, particularly C 3~20 These may include, but are not limited to, α-olefins, diolefins, polyenes, and polar comonomers, including those with carboxylic acid, acrylate, or acetate functionality, such as, but not limited to, methacrylic acid, acrylic acid, vinyl acetate, methyl acrylate, isobutyl acrylate, n-butyl acrylate, glycidyl methacrylate, and the monoethyl ester of maleic acid.

[0019] Referring again to FIG. 1 , compressor system 20 may include one or more compressors in parallel or series. As shown in FIG. 1 , compressor system 20 may include a primary compressor 24 and a secondary compressor 26 downstream from primary compressor 24. Primary compressor 24 may compress monomer feed stream 5 such that partially compressed feed 28 to secondary compressor 26 has a pressure of at least 200 bar. In one or more embodiments, primary compressor 24 may compress monomer feed stream 5 to a pressure of 200 to 1000 bar, or 300 to 900 bar. To achieve this compression, primary compressor 24 may include one or more compression stages.

[0020] The secondary compressor 26, sometimes referred to as a hyper compressor, compresses the partially compressed feedstock 28 to a pressure of at least 1000 bar, or at least 2000 bar, or at least 2500 bar, or at least 3000 bar. Like the primary compressor 24, the secondary compressor 26 may include one or more compression stages. In one or more embodiments, the secondary compressor 26 may comprise a plunger reciprocating compressor and may consist of single or multiple compression stages.

[0021] In various embodiments, the high pressure ethylene stream 22 exiting the compressor system 20 has a pressure of at least 1000 bar, or at least 2000 bar, or at least 2500 bar, or at least 3000 bar. Furthermore, in various embodiments, the high pressure ethylene stream 22 exiting the compressor system 20 may alternatively be defined as having a pressure such that the high pressure ethylene stream 22 is a supercritical fluid.

[0022] 1 , high-pressure ethylene stream 22 exits compressor system 20 and travels to high-pressure centrifuge unit 30. High-pressure centrifuge unit 30 separates solid particles and liquid droplets from high-pressure ethylene stream 22 to produce an enriched effluent 34 comprising the solid particles and liquid droplets, and an upgraded high-pressure ethylene stream 32 comprising the remainder of the high-pressure ethylene stream. In accordance with terminology understood in the art, enriched effluent 34 may be considered a waste product or waste stream, and upgraded high-pressure ethylene stream 32 may be considered an acceptor product or acceptor stream. Accordingly, enriched effluent 34 may be removed from high-pressure centrifuge unit 30 and further processed or disposed of.

[0023] It will be appreciated that due to incomplete separation within the high-pressure centrifuge unit 30, the enriched effluent 34 may contain ethylene or other liquid or gaseous components in addition to solid particles and liquid droplets. For example, in a continuous operation, the enriched effluent may contain more than 99% by weight of ethylene, while in a batch or semi-batch operation, the enriched effluent may contain up to 50% by weight of ethylene.

[0024] In one or more embodiments, the solid particles and liquid droplets are collected for periodic removal in a separate section of the high-pressure centrifuge unit 30. Specifically, the enriched effluent 34 is directed to a storage vessel provided as part of the high-pressure centrifuge unit 30, where it is sequestered for periodic drainage or removal.

[0025] In one or more embodiments, solid particles and liquid droplets are removed from the high-pressure centrifugation unit 30 via a first stream to drive the enriched effluent 34 away from the high-pressure centrifugation unit 30. Specifically, the enriched effluent 34, and the solid particles and liquid droplets contained within the enriched effluent 34, may be transported away from the high-pressure centrifugation unit 30 for disposal or further processing. For example, the first stream may be an underflow that removes the solid particles and liquid droplets semi-continuously or continuously.

[0026] According to one or more embodiments, multiple high-pressure centrifuge units 30 are provided. The multiple high-pressure centrifuge units 30 may be provided in series or in parallel according to alternative embodiments. In various embodiments, the system 10 may include one, two, three, four, five, six, or more separate high-pressure centrifuge units 30. It will be appreciated that multiple high-pressure centrifuge units 30 in series enable the linear purification of the improved high-pressure ethylene stream 32 through the continuous removal of solid particles and liquid droplets. Similarly, it will be appreciated that multiple high-pressure centrifuge units 30 in parallel enable the increased production rate of the improved high-pressure ethylene stream 32 by utilizing multiple high-pressure centrifuge units 30 to simultaneously remove solid particles and liquid droplets.

[0027] In one or more embodiments, the high-pressure ethylene stream 22 is introduced into the high-pressure centrifugal unit 30 in a pulsating manner. Specifically, as will be appreciated by those skilled in the art, the high-pressure ethylene stream 22 exiting the compressor system 20 need not be continuous. For example, a reciprocating compressor can generate a stream having a pulsating or unsteady flow rate. Accordingly, the high-pressure centrifugal unit 30 can be configured to handle the pulsating introduction of the high-pressure ethylene stream 22. It will be appreciated that by considering the compressor frequency in light of the residence time of the high-pressure ethylene stream 22 within the high-pressure centrifugal unit 30, the volume of the high-pressure centrifugal unit 30 relative to the inlet flow pulsation can be used to adjust for the impact of the pulsating flow.

[0028] Referring again to Figure 1, the upgraded high-pressure ethylene stream 32 exiting the high-pressure centrifuge unit 30 passes to a polymerization reactor 40 to produce polyethylene from the upgraded high-pressure ethylene stream 32. While any high-pressure polymerization reactor can be used in accordance with the present disclosure, examples of specific reactors and reactor types are provided to fully illustrate the process for producing polyethylene, but should not be considered limiting. In one or more embodiments, the polymerization reactor 40 may be a free-radical polymerization reactor. Additionally, in one or more embodiments, a polymerization initiator 44 may be added to the polymerization reactor 40.

[0029] The polymerization reactor 40 may include one or more autoclave reactors or tubular reactors. The pressure in each autoclave or tubular reactor zone may be 1000 to 4000 bar, or 1500 to 3600 bar, or 2000 to 3200 bar. The polymerization temperature in each tubular reactor zone may be 100°C to 400°C, or 150°C to 360°C, or 180°C to 340°C. The polymerization temperature in each autoclave reactor zone may be 150 to 300°C, more typically 165 to 290°C, and even more typically 180 to 280°C.

[0030] 1, reactor effluent 42 from polymerization reactor 40 includes polyethylene produced within polymerization reactor 40 as well as unreacted components of upgraded high-pressure ethylene stream 32. It will be appreciated that polymerization initiator 44, if provided to polymerization reactor 40, may also be present in reactor effluent 42.

[0031] 2 , in one or more embodiments, high-pressure ethylene stream 22 is provided to bypass control valve 50 for controllably directing high-pressure ethylene stream 22 to high-pressure centrifuge unit 30 or to bypass line 52 for directing high-pressure ethylene stream 22 directly to polymerization reactor 40. When bypass control valve 50 is positioned in a separation mode configuration, high-pressure ethylene stream 22 is directed to high-pressure centrifuge unit 30 via separator feed line 54, and the upgraded high-pressure ethylene stream 32 from high-pressure centrifuge unit 30 is directed to polymerization reactor 40 via separator effluent line 56 connected to reactor inlet line 36. Conversely, when bypass control valve 50 is positioned in a bypass mode configuration, high-pressure ethylene stream 22 is directed to polymerization reactor 40 via bypass line 52 and reactor inlet line 36, directing high-pressure ethylene stream 22 directly to polymerization reactor 40. In one or more embodiments, the system 10 may further include a backflow control valve 60 at the junction of the bypass line 52 and the separator effluent line 56 to prevent backflow through the bypass line 52 in the separation mode configuration and to prevent backflow through the separator effluent line 56 in the bypass mode configuration.

[0032] With continued reference to FIG. 2 , it will be understood that bypass control valve 50, and, if present, backflow control valve 60, may be configured to allow for the diversion of high-pressure ethylene stream 22 such that a portion of high-pressure ethylene stream 22 passes through high-pressure centrifuge unit 30 and the remainder of high-pressure ethylene stream 22 passes directly to polymerization reactor 40.

[0033] 2, system 10 may be operated in a separation mode in which high-pressure ethylene stream 22 is passed through high-pressure centrifuge unit 30 for a period of time following replacement of the cylinder or packing, when metal particle generation during the break-in period is currently anticipated. After the break-in period, system 10 may be converted to a bypass mode configuration in which high-pressure centrifuge unit 30 is omitted from the process flow, as metal particle generation from the break-in of the cylinder or packing is no longer anticipated. However, it will be appreciated that system 10 may still be operated in a separation mode configuration if other solid particles or wax are anticipated to be present in high-pressure ethylene stream 22 from one or more different sources.

[0034] 3, the upgraded high-pressure ethylene stream 32 is at least partially recycled to the compressor system 20. Specifically, in one or more embodiments, at least a portion of the upgraded high-pressure ethylene stream 32 is recycled back to the compressor system 20 via recycle line 38, and the remainder of the upgraded high-pressure ethylene stream 32 is transferred to the polymerization reactor 40 via separator effluent line 56.

[0035] In one or more embodiments, the entire upgraded high-pressure ethylene stream 32 is recycled back to compressor system 20 for at least a period of time. It will be appreciated that by recycling at least a portion of the upgraded high-pressure ethylene stream 32 back to compressor system 20, such portion of the upgraded high-pressure ethylene stream 32 is directed through high-pressure centrifugal separator 30 for at least an additional period of time.

[0036] Recycling at least a portion of the upgraded high-pressure ethylene stream 32 back to compressor system 20 allows for repeated separation of solid particles and liquid droplets from high-pressure ethylene stream 22 to achieve a desired purity or reduction in concentration of solid particles and liquid droplets in the upgraded high-pressure ethylene stream 32 when fed to polymerization reactor 40.

[0037] 4, the entire upgraded high-pressure ethylene stream 32 may be recycled back to compressor system 20 via recycle line 38. Additionally, bypass control valve 50 controls the portion of high-pressure ethylene stream 22 that is fed to high-pressure centrifuge unit 30 and the portion that is fed directly to polymerization reactor 40. It will be understood that the embodiment of FIG. 4 is a variation of the embodiment of FIG. 3.

[0038] It will be appreciated that locating the high-pressure centrifuge unit 30 after the compressor system 20 but before the polymerization reactor 40 allows for the removal of any solid particles and liquid droplets introduced in the early stages of the process. However, such removal is also accomplished prior to polymerization in the polymerization reactor 40, thereby avoiding the challenges associated with separation from the polymer product.

[0039] solid particles According to one or more embodiments, the solid particles removed in the enriched effluent 34 are metals. For example, the solid particles may be copper, bronze, steel, iron, zinc, aluminum, or any other metallic species. In one or more specific embodiments, the solid particles may be filings or other wear products from one or more unit operations that process components of the monomer feed stream before it is introduced into the high-pressure centrifuge unit 30. For example, the solid particles may include copper or bronze particles that have worn off components of the compressor system 20.

[0040] In one or more embodiments, the solid particles have a longest dimension of less than 1,000 microns. In various further embodiments, the solid particles have a longest dimension of less than 500 microns, less than 200 microns, less than 150 microns, or less than 100 microns. It will be appreciated that smaller particle sizes present unique challenges in their removal or separation from the stream, which are exacerbated by the high pressures of current systems. However, it will also be appreciated that the process discussed in this disclosure addresses such challenges in a unique and novel way through the implementation of a high-pressure centrifuge unit 30 between the compressor system 20 and the polymerization reactor 40.

[0041] droplet According to one or more embodiments, the removed droplets in the enriched effluent 34 include wax. Typically, wax is a low molecular weight polyethylene that forms droplets or small particles in the ethylene stream.

[0042] polymer In one or more embodiments, polymerization reactor 40 specifically produces low density polyethylene (LDPE).

[0043] In one embodiment, the LDPE is prepared according to ASTM D4703 and has a viscosity of 0.914 to 0.930, more typically 0.916 to 0.930, and even more typically 0.918 to 0.926 grams per cubic centimeter (g / cc or g / cm), measured according to ASTM D792, Method B, within one hour of sample pressing. 3 In one embodiment, the LDPE has a melt index, I2, of 0.1 to 40 g / 10 min, or 0.2 to 25 g / 10 min, measured according to ASTM D-1238 (Method B) at 190°C and a 2.16 kg load. In some embodiments, the LDPE may have a lower I2, of 0.1 to 10 g / 10 min, or 0.1 to 1 g / 10 min. Alternatively, the LDPE may have a higher melt index (I2), of 5 to 40 g / 10 min, or 10 to 25 g / 10 min, or 15 to 25 g / 10 min.

[0044] Purpose The polyethylene formed according to the process of the present disclosure may be used in a variety of conventional thermoplastic manufacturing processes to produce useful articles including, for example, films; molded articles such as blow-molded, injection-molded, or rotationally molded articles; foams; wire and cable, fibers, extrusion coatings, and woven or nonwoven fabrics. [Example]

[0045] To demonstrate the process improvement achieved by incorporating a high-pressure centrifuge unit 30 between the compressor system 20 and the polymerization reactor 40, the reduction of solid particles in the improved high-pressure ethylene stream 32 entering the polymerization reactor 40 was modeled. The model utilized minimum particles of 50 μm length, 20 μm width, and 2 μm height, corresponding to a spherical diameter of 15.6 μm, at a concentration of less than 0.1 volume percent of the feed stream. Comparative Example 1 did not have a separator between the compressor system 20 and the polymerization reactor 40; Comparative Example 2 had a gravity separator between the compressor system 20 and the polymerization reactor 40; and Inventive Example 3 had a high-pressure centrifuge unit 30 between the compressor system 20 and the polymerization reactor 40.

[0046] The simulated separation results for each of Comparative Examples 1 and 2 and Inventive Example 3 are provided in Table 1. Note that cut size is presented as a d50 value, representing the size particle at which 50% particle separation efficiency is achieved. Additionally, fouling resistance was quantified based on the predicted impact of fouling accumulation on performance characteristics within a typical operating cycle of such a unit. Performance characteristics considered include pressure drop, cut size, and safe operation of the unit, with "high" fouling resistance representing no or negligible impact on performance, as represented by an increase in pressure drop and / or cut size of less than 5%.

[0047] [Table 1]

[0048] It should be noted that Inventive Example 3 exhibited a 98% improvement in particle separation efficiency compared to the base case of Comparative Example 1, minimal pressure drop, and high fouling resistance.

[0049] It will be apparent that modifications and variations are possible without departing from the scope of the present disclosure, as defined in the appended claims. More specifically, although some aspects of the present disclosure may be identified herein as preferred or particularly advantageous, it is intended that the present disclosure not necessarily be limited to those aspects.

Claims

1. 1. A process for producing polyethylene, comprising: introducing a monomer feed stream comprising ethylene monomer and optionally a comonomer into a compressor system to produce a high pressure ethylene stream having a pressure of at least 1,000 bar; introducing the high-pressure ethylene stream into a high-pressure centrifuge unit, wherein the high-pressure centrifuge unit separates solid particles and liquid droplets from the high-pressure ethylene stream to produce an enriched effluent comprising the solid particles and the liquid droplets and an upgraded high-pressure ethylene stream comprising the remainder of the high-pressure ethylene stream; removing the enriched effluent from the high-pressure centrifuge unit; introducing said upgraded high-pressure ethylene stream into a polymerization reactor to produce said polyethylene from said upgraded high-pressure ethylene stream.

2. 2. The process of claim 1, wherein the high pressure ethylene stream has a pressure of at least 2,000 bar.

3. 3. The process of claim 1 or 2, wherein the high pressure ethylene stream is a supercritical fluid.

4. 4. The process of any of claims 1 to 3, wherein the high-pressure ethylene stream is provided to a bypass control valve for controllably directing the high-pressure ethylene stream to the high-pressure centrifuge unit, or to a bypass line for introducing the high-pressure ethylene stream directly into the polymerization reactor.

5. The process of any one of claims 1 to 4, wherein the upgraded high pressure ethylene stream is at least partially recycled to the compressor system.

6. The process of any one of claims 1 to 5, wherein the solid particles and the liquid droplets are collected in separate parts of the high-pressure centrifuge unit for periodic removal.

7. 7. The process of any one of claims 1 to 6, wherein the solid particles and the liquid droplets are removed from the high-pressure centrifuge unit via a first stream to drive the enriched effluent away from the high-pressure centrifuge unit.

8. The process according to any one of claims 1 to 7, wherein a plurality of high-pressure centrifugation units are provided in series or in parallel.

9. The process of any one of claims 1 to 8, wherein the solid particles are metal.

10. 10. The process of claim 9, wherein the solid particles have a longest dimension of less than 1,000 microns.

11. 10. The process of claim 9, wherein the solid particles have a longest dimension of less than 200 microns.

12. The process of any preceding claim, wherein the droplets comprise wax.

13. 13. The process of any of claims 1 to 12, wherein the high pressure ethylene stream is introduced into the high pressure centrifuge unit in a pulsating manner.

14. The process of any one of claims 1 to 13, wherein the polyethylene is low density polyethylene (LDPE).

15. The process of any of claims 1 to 14, wherein the monomer feed stream introduced into the compressor comprises the ethylene monomer and the comonomer.

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