Process for preparation of ethylene copolymers

Abstract

The present invention relates to a process for preparing a multimodal ethylene copolymer in a reactor cascade comprising a first polymerization reactor (R-1) and a second polymerization reactor (R-3), the process comprising: a) feeding ethylene, hydrogen and a catalyst component and a first diluent to the first polymerization reactor (R-1), b) feeding at least a portion of the first suspension to a first solid-liquid separator (SLS-1) to obtain a first diluent stream (S-2) enriched in the first diluent and a first particle stream (S-3) enriched in the solid particles, b) feeding at least a portion of the first suspension to a second solid-liquid separator (SLS-1) to obtain a second diluent stream (S-3) enriched in the solid particles, wherein at least a portion (S-5) of the first diluent stream (S-2) is recycled back to the first polymerization reactor (R-1), c) feeding the first particulate stream (S-3) to a flash section comprising a first flash vessel (F-1) and optionally a second flash vessel (F-2) to obtain a hydrogen-enriched vapor stream and a hydrogen-lean stream, wherein at least a portion of the vapor stream is recycled back to the first polymerization reactor (R-1), and d) feeding the hydrogen lean stream, ethylene and comonomer and a second diluent to a second polymerization reactor (R-3) to produce a second suspension of solid particles of the multimodal ethylene copolymer in a second suspension medium.

Description

Technical Field

[0001] The present invention relates to a process for preparing ethylene copolymers. Background Art

[0002] The production of polymer powders by polymerization of monomers in the presence of a catalyst is well known. The commercial use of fluidized bed reactors and suspension polymerization reactors is known.

[0003] In a suspension polymerization reactor, the polymerization is carried out in a stirred tank or a continuous loop reactor, in which a suspension of polymer particles in a suspension medium comprising a hydrocarbon diluent is circulated. During the polymerization, fresh polymer is produced by catalytic polymerization of the monomers, and the polymer product is removed from the reactor by removing a portion of the suspension.

[0004] Cost efficiency and energy efficiency are important factors in polymerization processes. Cost and energy efficiency are improved by recycling components used in the polymerization process. In suspension polymerization processes for preparing ethylene copolymers, various components used in the polymerization, such as ethylene, comonomers, diluents and hydrogen, are typically recycled.

[0005] WO2019243384A1 discloses a method for preparing a multimodal ethylene copolymer in a suspension in a reactor cascade. The method comprises separating the suspension formed in the reactor cascade into multimodal ethylene copolymer particles and a recovered suspension medium, purifying a portion of the recovered suspension medium in a purification section to produce a purified component of the recovered suspension medium, and recycling at least some or a portion of the purified component of the recovered suspension medium to a first polymerization reactor of the reactor cascade, wherein the purified component of the recovered suspension medium (which contains a diluent) recycled to the first polymerization reactor undergoes catalytic hydrogenation and is then introduced into the first polymerization reactor.

[0006] WO2019243384A1 further discloses an embodiment in which a suspension of polyethylene particles taken out from the first polymerization reactor of the reactor cascade is fed into a separator, wherein a portion of the suspension medium is separated from the suspension and recycled to the first polymerization reactor of the reactor cascade, and the concentrated suspension of polyethylene particles is transferred to the next polymerization reactor of the reactor cascade.

[0007] There is a need in the art for a low energy consumption process for the preparation of multimodal ethylene copolymers. Summary of the invention

[0008] It is an object of the present invention to provide a low energy consumption process for the preparation of multimodal ethylene copolymers.

[0009] Accordingly, the present invention provides a process for preparing a multimodal ethylene copolymer in a reactor cascade comprising a first polymerization reactor and a second polymerization reactor connected in series, the process comprising:

[0010] a) feeding ethylene, hydrogen and catalyst components and a first diluent into a first polymerization reactor to prepare a first suspension of solid particles of ethylene homopolymer in a first suspension medium,

[0011] b) feeding at least a portion of the first suspension to a first solid-liquid separator to obtain a first diluent stream enriched in the first diluent and a first particle stream enriched in solid particles, wherein at least a portion of the first diluent stream is recycled back to the first polymerization reactor,

[0012] c) feeding the first particle stream to a flash section comprising a first flash vessel and optionally a second flash vessel to obtain a hydrogen-enriched vapor stream and a hydrogen-depleted stream, wherein at least a portion of the vapor stream is recycled back to the first polymerization reactor, and

[0013] d) feeding a hydrogen-depleted stream, ethylene and comonomer and a second diluent to a second polymerisation reactor to produce a second suspension of solid particles of the multimodal ethylene copolymer in a second suspension medium.

[0014] In the process according to the invention, a first suspension of solid particles of ethylene homopolymer prepared in a first reactor is fed to a first solid-liquid separator to obtain a stream enriched in diluent and a stream enriched in solid particles. At least a portion of the diluent stream is recycled back to the first polymerization reactor. The recycling of the diluent stream back to the first polymerization reactor is carried out without purification to remove comonomers (or in the absence of any additional purification of the unit separation step). At least a portion of the particle stream is fed to a flash section to remove hydrogen, at least a portion of which is recycled back to the first reactor. At least a portion of the hydrogen-poor stream from the flash section is fed to a second polymerization reactor, which is also fed with ethylene and comonomers and a second diluent. Thus, by the process according to the invention, a multimodal ethylene copolymer is obtained in the form of a second suspension of solid particles of a multimodal ethylene copolymer in a second suspension medium.

[0015] The combination of a solid-liquid separator with a subsequent flash vessel reduces the energy consumption in the process.

[0016] Because comonomer is not used in the first polymerization reactor, the diluent extracted by the first solid-liquid separator does not need to be separated from any comonomer and then recycled back to the first reactor. No separation is required to reduce the energy required for recycling diluent. The first diluent stream to be recycled to the first polymerization reactor further comprises hydrogen. Recycling hydrogen back to the first polymerization reactor reduces the amount of fresh hydrogen that needs to be added to the first reactor. In addition, because some amount of diluent and hydrogen have been separated by the first solid-liquid separator, the amount of components (hydrogen, diluent, etc.) to be flashed out by the flash section is reduced. This reduces the energy consumption of hydrogen and diluent from the flash section that need to be cooled and condensed for recycling. Therefore, a huge improvement in the overall energy efficiency of the method is achieved.

[0017] A first suspension is prepared in a first polymerization reactor at a pressure of P1. A first solid-liquid separator is operated at a pressure of P2. A first flash vessel is operated at a pressure of P3. Typically, P1 ≥ P2 ≥ P3, more typically, P1>P2>P3.

[0018] Preferably, the difference between P1 and P2 is relatively small. Removing liquid from the first suspension at a relatively high pressure significantly reduces the energy required for the operation of the subsequent flash vessel. Preferably, P1-P2 is at most 0.5 bar, such as at most 0.3 bar or at most 0.1 bar. Preferably, the first solid-liquid separator is a centrifugal decanter or a hydrocyclone.

[0019] Preferably, P3 is substantially lower than P2. The first flash vessel operated at a relatively low pressure removes the remaining diluent in addition to removing hydrogen. Preferably, most of the diluent is removed by the flash section, in which case the hydrogen-poor stream obtained takes the form of a filter cake rather than a suspension. The almost complete removal of the diluent occurring by the combination of a solid-liquid separator and a flash section allows the formation of two (almost) independent diluent loops, one for the first polymerization reactor and the other for the second polymerization reactor. Having two independent diluent loops allows the recycling of a large part of the diluent while preventing cross contamination between the first reactor and the second reactor, while particularly preventing mixing of the comonomer with the first diluent.

[0020] Preferably, P2-P3 is at least 1.0 bar, at least 2.0 bar, at least 5.0 bar or at least 10 bar.

[0021] P3 may be, for example, 1.0 to 5.0 bar gauge, for example 1.2 to 3.5 bar gauge. P3 may be, for example, 1.5 x 10 5 Up to 3.5x 10 5 Pa.

[0022] Multimodal ethylene copolymers

[0023] The ethylene copolymers obtained according to the present invention are multimodal. In the context of the present invention, the term "multimodal" shall mean that the ethylene copolymer comprises at least two fractions of polymers obtained under different polymerization conditions. This means that the term "multimodal" as used herein shall also include "bimodal". Different polymerization conditions can be achieved, for example, by using different hydrogen concentrations in different polymerization reactors. The term "copolymer" includes binary copolymers (made from two monomers) and ternary copolymers (made from three monomers).

[0024] Preferably, the ethylene copolymer obtained by the process of the present invention is selected from ethylene copolymers of ethylene and comonomers selected from C3-C12 alpha-olefins. Preferably, the comonomer is selected from 1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene and 1-decene and mixtures thereof. More preferably, the comonomer is 1-hexene and / or 1-butene, most preferably 1-hexene.

[0025] Preferably, the amount of ethylene derived monomer in the copolymer of the present invention is 60 to 99.8 wt% and the amount of comonomer derived monomer is 0.2 to 40 wt%, such as 1 to 35 wt%, 3 to 30 wt%, 5 to 25 wt% or 10 to 20 wt%.

[0026] catalyst

[0027] Polymerization can be carried out using all conventional olefin polymerization catalysts. This means that polymerization can be carried out using Phillips catalysts based on chromium oxide, using Ziegler catalysts or Ziegler-Natta catalysts based on titanium, or using single-site catalysts. For the purposes of the present invention, single-site catalysts are catalysts based on chemically uniform transition metal coordination compounds. Particularly suitable single-site catalysts are those containing bulky σ-bonded or π-bonded organic ligands, such as catalysts based on mono-Cp complexes, catalysts based on bis-Cp complexes (they are generally referred to as metallocene catalysts), or catalysts based on late transition metal complexes, particularly iron-bisimine complexes. In addition, mixtures of two or more of these catalysts can also be used for the polymerization of olefins. Such mixed catalysts are generally referred to as composite catalysts (hybrid catalyst). The preparation and use of these catalysts for olefin polymerization are generally known.

[0028] Preferred catalysts are of the Ziegler type, which preferably comprise a compound of titanium or vanadium, a compound of magnesium and optionally a particulate inorganic oxide as support.

[0029] Further examples of suitable catalysts are those described, for example, in WO2020135939, page 5, lines 22-36, which are incorporated herein by reference.

[0030] The first diluent and the second diluent

[0031] Preferably, each of the first diluent and the second diluent is selected from normal alkanes, isoalkanes and / or branched alkanes and combinations thereof having a carbon number of 3 to 12. Suitable examples include propane, butane, isobutane, normal pentane, isopentane, neopentane, hexane, normal heptane, isoheptane, octane. The first diluent and the second diluent may be of different types, but preferably are of the same type.

[0032] Preferably, each of the first diluent and the second diluent comprises neopentane and / or isobutane.

[0033] Most preferably, each of the first diluent and the second diluent comprises neopentane (also known as 2,2-dimethylpentane). Preferably, the amount of neopentane relative to the sum of the first diluent and the second diluent is at least 50 wt%, more preferably at least 60 wt%, more preferably at least 70 wt%, more preferably at least 80 wt%, more preferably at least 90 wt%, more preferably at least 95 wt%, more preferably at least 99 wt%.

[0034] The present inventors have found that the use of neopentane as diluent advantageously results in a combination of less wax, low pressure required for polymerisation, efficient recovery of the diluent and easy separation of the diluent from the polyethylene particles.

[0035] For some normal alkanes, the vapor pressure at different temperatures and the boiling point at atmospheric pressure are shown below.

[0036]

[0037] It has been found that polyethylene chains have lower solubility in propane and neopentane than in isobutane, pentane and butane. The lower solubility of polyethylene chains results in a smaller amount of low molecular weight polyethylene (wax) being lost by being dissolved in the diluent.

[0038] In addition, it has been recognized that the difference in vapor pressure of normal alkanes results in a difference in pressure required during polymerization. Neopentane has a much lower vapor pressure than propane, so using neopentane as a diluent requires a much lower polymerization pressure than using propane. Therefore, using neopentane as a diluent is also advantageous in terms of the lower pressure required for polymerization.

[0039] Furthermore, the condensation point of neopentane allows for efficient recovery of evaporated diluent by simple condensation with cooling water at low to moderate pressures without the need for high energy input unit operations such as compressors and / or chillers.

[0040] Furthermore, the boiling point of neopentane allows separation from the polyethylene particles by simple flash distillation and stripping with nitrogen without the need to use drying operations such as rotary dryers, or fluidized bed dryers or steam dryers.

[0041] Reactor cascade

[0042] The process according to the invention is carried out in a reactor cascade comprising a first polymerization reactor and a second polymerization reactor connected in series.

[0043] The process of the present invention can be carried out in a series of only two reactors (first and second polymerization reactors). However, there may also be additional reactors connected to the upstream and / or downstream of these reactors. The presence of additional reactors has a positive influence on the uniformity of the ethylene copolymer particles obtained.

[0044] There is no limitation on the number of additional reactors, for example 1, 2, 3 or 4, preferably 1 or 2. In some embodiments, the reactor cascade comprises one additional polymerization reactor upstream of the first polymerization reactor (the total number of reactors is 3). In some embodiments, the reactor cascade comprises one additional polymerization reactor downstream of the second polymerization reactor (the total number of reactors is 3). In some embodiments, the reactor cascade comprises one additional polymerization reactor upstream of the first polymerization reactor and one additional polymerization reactor downstream of the second polymerization reactor (the total number of reactors is 4).

[0045] Preferably, these reactors are horizontal or vertical loop reactors or stirred tank reactors, particularly preferably continuous stirred tank reactors. The reactor can be operated as a complete liquid or under boiling conditions. The reactor can have different types, but preferably has the same type. In some preferred embodiments, all reactors in the reactor cascade are horizontal loop reactors operated as full liquid reactors. In other preferred embodiments, all reactors in the reactor cascade are continuous stirred tank reactors operated under boiling conditions.

[0046] The reactor cascade may include a first group of reactors and a second group of reactors, wherein the first group of reactors includes a first polymerization reactor and (one or more) other polymerization reactors upstream of the first polymerization reactor, and the second group of reactors includes a second polymerization reactor and (one or more) other polymerization reactors downstream of the second polymerization reactor. The reactor in the first group produces ethylene homopolymers, and the reactor in the second group produces ethylene copolymers. The reactor in the second group is preferably operated at a lower temperature than the reactor in the first group. The hydrogen content in the reactor in the second group is typically less than the hydrogen content in the reactor in the first group.

[0047] The further polymerisation reactor (reactor in addition to the first and second polymerisation reactor) is a suspension polymerisation reactor, preferably of the same type as the suspension reactor used in the first and second reactor.

[0048] Suspension polymerization processes comprising two or more suspension polymerization reactors connected in series are known for many years. One of the reasons for frequently using a cascade of two or more polymerization reactors is that different reaction conditions can be set in the polymerization reactor, thereby for example widening the molecular weight distribution. Ethylene polymers with a wide molecular weight distribution are generally used for a variety of applications because they show advantages in product properties and processability. Such polymers are also commonly referred to as bimodal or more generally as multimodal polyolefin polymers because they are polymerized under different reaction conditions in a cascade of two or more polymerization reactors. The term "multimodal" as used herein and also frequently used in the art should include "bimodal".

[0049] In such case of polymerization in a cascade of polymerization reactors, the polymerization catalyst is fed together with the monomers to the first reactor, the produced polymer still containing the active polymerization catalyst is transferred to a second polymerization reactor having different reactor conditions, and the polymerization is continued using the polymerization catalyst still contained in the polymer particles. Typically, the different reaction conditions in the different polymerization reactors are set by using different hydrogen concentrations, hydrogen being usually used as a molecular weight regulator.

[0050] Preferably, the solids concentration in the reactor (defined as the weight of polymer in the total weight of the suspension) is from 15 to 60 wt%, preferably from 25 to 55 wt%, and most preferably from 35 to 50 wt%.

[0051] The proportion of polymer produced in each of the polymerization reactors is not particularly limited. For example, when the number of polymerization reactors in the reactor cascade is 4, 10 to 50 wt. % or 20 to 40 wt. % of polymer may be produced in each of the polymerization reactors relative to the total amount of polymer produced by the reactor cascade. The weight ratio of ethylene homopolymer to ethylene copolymer produced in the reactor cascade is not particularly limited.

[0052] Step a)

[0053] In step a), ethylene, hydrogen and catalyst components and a first diluent are fed to a first polymerization reactor. Ethylene is polymerized. A first suspension of solid particles of ethylene homopolymer in a first suspension medium is prepared.

[0054] Preferably, the polymerization of the first polymerization reactor is carried out at a pressure of 1 to 50 barg, for example 1 to 15 barg, 2 to 20 barg or 20 to 50 barg. The pressure may be selected, for example, based on the type of the first diluent.

[0055] Preferably, the polymerization of the first polymerization reactor is carried out at a temperature of 65 to 110° C., more preferably 65 to 90° C., and particularly preferably 75 to 85° C. The temperature in the first polymerization reactor is lower than the melting point of the polymer in the first reactor.

[0056] The first suspension medium forming the liquid phase or supercritical phase of the first suspension is composed of a first diluent and various components dissolved in the first suspension medium.

[0057] The first suspension medium comprises ethylene dissolved in the first suspension medium and the residue of the catalyst component. The first suspension medium further comprises hydrogen dissolved in the first suspension medium. During the polymerization, low molecular weight hydrocarbon reaction products are also formed, which may include polyethylene wax, oligomers, alkanes (C4, C6, C8, C10, C12, C14...) produced by polymerization; olefin isomers (e.g. 2-hexene).

[0058] The first suspension medium is substantially free of comonomer. Preferably, the amount of comonomer in the first suspension medium is at most 0.1 wt. % of the first suspension medium, more preferably the amount of comonomer in the first suspension medium is at most 0.01 wt. % (or 100 ppm) of the first suspension medium.

[0059] The average residence time of the slurry in the first polymerization reactor may be, for example, from 2 to 4 hours or from 1 to 3 hours.

[0060] Step b)

[0061] In step b), at least a portion of the first suspension from the first polymerization reactor is fed to a first solid-liquid separator to obtain a first diluent stream enriched in the first diluent and a first particle stream enriched in solid particles of ethylene homopolymer.

[0062] There may be a container between the first polymerization reactor and the first solid-liquid separator. The container receives at least a portion (preferably all) of the first suspension from the first polymerization reactor, and the first solid-liquid separator receives at least a portion (preferably all) of the first suspension from the container. The container allows for smoother operation of the first solid-liquid separator. Preferably, the container is operated at substantially the same pressure as the first polymerization reactor and at the same or lower temperature as the first polymerization reactor. The average residence time in the container may be, for example, 2 to 20 minutes, preferably 5 to 10 minutes. Some amount of steam may be removed from the first suspension in the container, but preferably not. The temperature in the container may be, for example, 35°C to 95°C, preferably 35°C to 75°C, and most preferably 35°C to 55°C.

[0063] The first solid-liquid separator can be any suitable device, such as a centrifugal decanter, a hydrocyclone, a filter or a combination thereof. Preferably, the first solid-liquid separator is a centrifugal decanter. Preferably, the first solid-liquid separator is operated at substantially the same pressure as the first polymerization reactor and the above-mentioned optional container (same as the pressure of the first polymerization reactor or at most 0.5 bar lower).

[0064] Preferably, the first solid-liquid separator is operated such that the first particle stream does not comprise a major portion of the hydrogen in the first suspension. Preferably, the solid-liquid separator is operated such that the first particle stream comprises at most 30 wt %, more preferably at most 15 wt %, more preferably at most 8 wt % of the hydrogen in the first suspension.

[0065] Preferably, the first solid-liquid separator is operated such that the first diluent stream comprises a majority of the first diluent in the first suspension. Preferably, the solid-liquid separator is operated such that the first diluent stream comprises at least 70 wt %, more preferably at least 85 wt %, more preferably at least 92 wt % of the first diluent in the first suspension.

[0066] Preferably, the solid-liquid separator is operated such that the first diluent stream is substantially free of dissolved polymer or wax.

[0067] At least a portion of the first diluent stream is recycled back to the first polymerization reactor. The transfer of at least a portion of the first diluent stream may be direct or may be via an optional container present between the first solid-liquid separator and the first polymerization reactor. The average residence time in the container may be, for example, 5 to 45 minutes, preferably 5 to 30 minutes, most preferably 5 to 20 minutes. Some amount of vapor may be removed from the container, but preferably, the pressure and temperature in the container are such that no vapor leaves the container.

[0068] A portion of the first diluent stream may be sent to the post-treatment section described in relation to optional step f). However, it is preferred to recycle as much of the first diluent stream as possible back to the first polymerization reactor. This results in low energy consumption.

[0069] Preferably, at least 75 wt%, more preferably at least 95 wt%, more preferably at least 99.5 wt% of the first diluent fed to the first polymerization reactor is fed as a recycle stream from the first solid-liquid separator.

[0070] Step c)

[0071] In step c), the first particle stream enriched with solid particles is fed to a flash section, in which a vapor stream enriched with hydrogen and a hydrogen-depleted stream (a hydrogen-depleted suspension or a hydrogen-depleted filter cake) are obtained. At least a portion of the vapor stream is recycled back to the first polymerization reactor.

[0072] The recycling of hydrogen is beneficial because hydrogen is required in the first polymerization reactor for preparing ethylene homopolymer, while the presence of hydrogen is not required in the second polymerization reactor. The molecular weight of the ethylene copolymer prepared in the second polymerization reactor can be controlled by the amount of hydrogen fed to the second polymerization reactor. In order to obtain high molecular weight ethylene copolymers, a smaller amount of hydrogen needs to be fed to the second polymerization reactor.

[0073] The flash section may include only one flash vessel (the first flash vessel) or multiple flash vessels (the first flash vessel and the second flash vessel and any additional (one or more) flash vessels) connected in series. The second flash vessel operates at a lower pressure than the first flash vessel. The use of multiple flash vessels increases the flash efficiency and reduces the energy in recovering the flashed components.

[0074] Preferably, the flash section is operated so that the hydrogen-depleted stream fed from the flash section to the second polymerization reactor contains substantially no hydrogen. Preferably, the flash section is operated so that at least 99 wt.%, more preferably at least 99.5 wt.%, more preferably at least 99.9 wt.%, more preferably at least 99.95 wt.%, more preferably at least 99.99 wt.% of the hydrogen in the first particle stream is removed to obtain a hydrogen-depleted stream. A higher level of depletion of hydrogen in the hydrogen-depleted stream allows obtaining higher molecular weight ethylene copolymers in the second polymerization reactor.

[0075] The flashing in the flash section causes a large portion of the first diluent to eventually enter the vapor stream. Thus, the hydrogen-enriched vapor stream obtained by the flash section comprises hydrogen and the first diluent. Preferably, the flash section is operated so that the hydrogen-depleted stream is substantially free of the first diluent. Preferably, the flash section is operated so that no more than 10%, preferably no more than 5%, and most preferably no more than 1% of the first diluent in the first particle stream is present in the hydrogen-depleted stream. Preferably, the flash section is operated so that no more than 10% by weight, preferably no more than 5% by weight, and most preferably no more than 1% by weight of the first diluent in the first particle stream is present in the hydrogen-depleted stream.

[0076] When at least a portion of the vapor stream is recycled back to the first polymerization reactor, at least a portion of the hydrogen in the vapor stream and at least a portion of the first diluent in the vapor stream are recycled back to the first polymerization reactor. Preferably, at least 50 wt %, more preferably at least 60 wt %, more preferably at least 70 wt %, more preferably at least 80 wt %, more preferably at least 90 wt %, more preferably at least 95 wt %, more preferably at least 99 wt % of the first diluent in the first particle stream is recycled back to the first polymerization reactor.

[0077] Preferably, at least 50 wt.-%, more preferably at least 60 wt.-%, more preferably at least 70 wt.-%, more preferably at least 80 wt.-%, more preferably at least 90 wt.-%, more preferably at least 95 wt.-%, more preferably at least 99 wt.-% of the first diluent fed to the first polymerization reactor in step a) is fed as a recycle stream from the first solid-liquid separator (at least a portion of the first diluent stream is recycled back to the first polymerization reactor in step b)) and as a recycle stream from the flash section (at least a portion of the vapor stream is recycled back to the first polymerization reactor (step c)).

[0078] The vapor stream may be fed to a heat exchanger to obtain a liquid stream of the first diluent, which may be recycled back to the first polymerization reactor. The heat exchanger may use cooling water or a chiller condenser.

[0079] In some embodiments, the flash section includes a first flash vessel and a second flash vessel, the vapor stream from the first flash vessel is fed to a heat exchanger using cooling water, and the vapor stream from the second flash vessel and the uncondensed stream leaving the cooling water heat exchanger are fed to a heat exchanger using a cooler condenser.

[0080] A heater may be present between the first solid-liquid separator and the first flash vessel. The heater is configured to receive the first particle stream from the first solid-liquid separator and heat it if necessary before transferring it to the first flash vessel.

[0081] Preferably, the first flash vessel is an adiabatic flash vessel. In this case, a heater is not necessary or its heat load is equal to zero.

[0082] In another embodiment of the invention, the first flash vessel is not insulated and a heater is used to heat the first particle stream to a temperature below the melting temperature of the polymer.

[0083] Step d)

[0084] In step d), a hydrogen-depleted stream as well as ethylene and comonomers are fed to a second polymerization reactor. In addition, a second diluent is fed to the second polymerization reactor. The monomers are polymerized. A second suspension of solid particles of a multimodal ethylene polymer in a second suspension medium is prepared.

[0085] Preferably, the polymerization in the second polymerization reactor is carried out at a pressure of 1 to 50 barg, for example 1 to 15 barg, 2 to 20 barg or 20 to 50 barg. The pressure may be selected, for example, based on the type of the second diluent.

[0086] Preferably, the polymerization in the second polymerization reactor is performed at a temperature of 65 to 110° C., more preferably 65 to 90° C., and particularly preferably 75 to 85° C. The temperature in the second polymerization reactor is lower than the melting temperature of the polymer in the second reactor.

[0087] The second suspension medium forming the liquid phase or supercritical phase of the second suspension is formed from the medium of the hydrogen-depleted stream from the flash vessel and the second diluent and the various components dissolved in the second suspension medium.

[0088] The second suspension medium comprises ethylene, comonomers and residues of the catalyst components dissolved in the second suspension medium. The second suspension medium may contain hydrogen, but its level is low. Similar to the polymerization in the first reactor, low molecular weight hydrocarbon reaction products are also formed during the polymerization, which may include polyethylene wax, oligomers, alkanes (C4, C6, C8, C10, C12, C14...) produced by polymerization; olefin isomers (e.g. 2-hexene).

[0089] Preferably, the amount of comonomer in the second suspension medium is from 0.1 to 10 wt%, more preferably from 1.0 to 8.0 wt%, more preferably from 2.0 to 6.0 wt% of the second suspension medium.

[0090] Preferably, no fresh hydrogen is fed to the second polymerization reactor.The amount of hydrogen in the second polymerization reactor is controlled by the pressure setting in the flash section.

[0091] The average residence time of the slurry in the second reactor may be, for example, 0.5 to 3 hours or 1 to 2 hours.

[0092] Step e)

[0093] The method according to the invention may further comprise (after step d)):

[0094] e) feeding at least a portion of the second suspension to a second solid-liquid separator to obtain a second diluent stream enriched in the second diluent and a second particle stream enriched in solid particles, wherein at least a portion of the second diluent stream is recycled back to the second polymerization reactor.

[0095] There may be a container between the second polymerization reactor and the second solid-liquid separator. The container receives at least a portion (preferably all) of the second suspension from the second polymerization reactor, and the second solid-liquid separator receives at least a portion (preferably all) of the second suspension from the container. The container allows for smoother operation of the second solid-liquid separator. Preferably, the container is operated at substantially the same pressure as the second polymerization reactor and at the same or lower temperature as the second polymerization reactor. The average residence time in the container may be, for example, 2 to 20 minutes, preferably 5 to 10 minutes. Some amount of steam may be removed from the second suspension in the container, but preferably not. The temperature in the container may be, for example, 35 to 95°C, preferably 35 to 75°C, and most preferably 35 to 55°C.

[0096] The second solid-liquid separator can be any suitable device, such as a centrifugal decanter, a hydrocyclone, a filter or a combination thereof. Preferably, the second solid-liquid separator is a centrifugal decanter. Preferably, the second solid-liquid separator is operated at substantially the same pressure as the second polymerization reactor and the above-mentioned optional container (same as the pressure of the first polymerization reactor or at most 0.5 bar lower).

[0097] Preferably, the second solid-liquid separator is operated such that the second diluent stream comprises a majority of the second diluent in the second suspension. Preferably, the second solid-liquid separator is operated such that the second diluent stream comprises at least 70 wt %, more preferably at least 85 wt %, more preferably at least 92 wt % of the second diluent in the second suspension.

[0098] Preferably, the second solid-liquid separator is operated such that the second diluent stream is substantially free of dissolved polymer or wax.

[0099] At least a portion of the second diluent stream is recycled back to the second polymerization reactor. The transfer of at least a portion of the second diluent stream may be direct or may be via an optional container present between the second solid-liquid separator and the second polymerization reactor. The average residence time in the container may be, for example, 5 to 45 minutes, preferably 5 to 30 minutes, most preferably 5 to 20 minutes. Some amount of vapor may be removed from the container, but preferably, the pressure and temperature in the container are such that no vapor leaves the container.

[0100] A portion of the second diluent stream may be sent to the post-treatment section described in relation to optional step f). However, it is preferred that as much of the second diluent stream as possible is recycled back to the second polymerization reactor. This results in low energy consumption.

[0101] Preferably, at least 75 wt%, at least 95 wt%, at least 99.5 wt% of the second diluent fed to the second polymerization reactor is fed as a recycle stream from the second solid-liquid separator.

[0102] The recycling of the first diluent and the recycling of the second diluent are performed independently, thereby preventing the comonomer fed to the second polymerization reactor from mixing with the first diluent to be recycled to the first polymerization reactor. Thus, the second diluent stream is not mixed with the stream to be recycled back to the first polymerization reactor. The second diluent stream is not fed to the flash section.

[0103] Step f)

[0104] The method according to the invention may further comprise (after step e)):

[0105] f) feeding at least a portion of the second diluent stream to a diluent post-treatment section to obtain a third diluent stream enriched in the second diluent and a comonomer stream enriched in comonomer, wherein at least a portion of the third diluent stream is recycled back to the first polymerization reactor and / or the second polymerization reactor.

[0106] Preferably, at most 20 wt%, preferably at most 10 wt%, more preferably at most 6.5 wt% of the total amount of the first diluent and the second diluent fed to the first and second polymerization reactors is fed to the post-treatment section.

[0107] Preferably, at most 0.5 tonnes, preferably at most 0.25 tonnes, more preferably at most 0.12 tonnes of diluent per tonne of ethylene copolymer produced are fed to the post-treatment section.

[0108] Preferably, the diluent work-up section comprises a distillation column or a series of distillation columns which separate a heavy components-enriched stream through the bottoms, a comonomer-enriched side stream, a substantially pure third diluent stream and a light purge.

[0109] Preferably, the comonomer content in the diluent-rich stream is less than 10,000 ppm by weight, preferably less than 1,000 ppm by weight, and more preferably less than 250 ppm by weight.

[0110] The heavy component-enriched stream may be purged out of the process. The comonomer-enriched stream is preferably recycled back to the second polymerization reactor. The third diluent stream is preferably recycled back to the first polymerization reactor and / or the second polymerization reactor. The light stream at the top of the distillation column is purged out of the process or fed to a nitrogen-hydrocarbon separation unit.

[0111] The diluent post-treatment section may also be fed with a portion of the first diluent stream.

[0112] The diluent post-processing section may additionally include a wax separation unit upstream of the distillation column. Such a unit may be made of unit operations such as evaporators, filters, rectifying towers / distillation towers, drying belts, decanters, or any combination thereof. Preferred unit operations are evaporators, preferably double-effect evaporators, and most preferably triple-effect evaporators. As a result of the wax separation unit, a purified wax stream and a lean wax stream enriched in diluent and having comonomers and dissolved monomers are produced. The purified wax stream is sent out of the process and may be sold for commercial purposes, burned for energy production, or any other suitable industrial use.

[0113] In some preferred embodiments, the diluent post-treatment section does not include a wax separation unit, i.e. the process according to the invention does not include a step of removing wax from the first diluent stream or the second diluent stream. This improves energy efficiency and can be achieved in particular by using neopentane as the first and second diluents. This is particularly suitable when neopentane is used as the diluent.

[0114] Step g)

[0115] The method according to the invention may further comprise (after step e)):

[0116] g) purifying the second particle stream by flashing and purging to obtain solid particles of the multimodal ethylene copolymer.

[0117] The solid particles of the multimodal ethylene copolymer thus obtained can be subjected to typical operations, such as pelletization, in a known manner.

[0118] In step g), the second particle stream is subjected to purification to obtain pure polymer particles. The purification may involve the use of a flash vessel and a purge vessel.

[0119] Purification by use of a flash vessel can be performed in a manner similar to that described with respect to step c). Preferably, flashing of the second particle stream is performed so as to remove the entirety of the second diluent. This eliminates the necessity for purification by use of a dryer involving heating. This improves process efficiency. Therefore, in some preferred embodiments, step g) does not involve the use of a dryer involving heating.

[0120] In the purge vessel, residual diluent, unreacted monomer and inert gases are purged out. This can be accomplished by using a stripping agent, such as nitrogen in countercurrent to the polymer stream passing through the purge vessel.

[0121] The components removed from the second particle stream by purification can be recycled in the process in a manner known to those skilled in the art. Specific implementation plan

[0122] In some embodiments, at least 50 weight percent of the first diluent and the second diluent is neopentane, and each of the first polymerization reactor and the second polymerization reactor is a continuously stirred tank reactor operating at boiling conditions and a pressure of 2 to 20 barg, preferably 3 to 14 barg.

[0123] In some embodiments, at least 50 weight percent of the first diluent and the second diluent is isobutane, and each of the first polymerization reactor and the second polymerization reactor is a horizontal slurry loop operated under supersaturated conditions and a pressure of 20 to 50 barg, preferably 35 to 45 barg.

[0124] In some embodiments, at least 50 weight percent of the first diluent and the second diluent are hexane isomers, and each of the first polymerization reactor and the second polymerization reactor is a continuously stirred tank reactor operating at boiling conditions and a pressure of 1 to 15 barg, preferably 2 to 13 barg.

[0125] In some embodiments, at least 50 weight percent of the first diluent and the second diluent are heptane isomers, and each of the first polymerization reactor and the second polymerization reactor is a continuously stirred tank reactor operated at boiling conditions and a pressure of 1 to 15 barg, preferably 2 to 13 barg.

[0126] In some embodiments, an antistatic agent is added to some or all polymerization reactors in a reactor cascade to avoid reactor fouling. In other embodiments, no antistatic agent is added to the polymerization reactors in a reactor cascade.

[0127] Preferably, none of the first diluent stream, the second diluent stream and the third diluent stream undergoes catalytic hydrogenation before being recycled back to the first polymerization reactor or the second polymerization reactor.

[0128] It is to be noted that the present invention relates to the subject matter defined in the independent claims, alone or in combination with any possible combination of features described herein, with particular preference given to those combinations of features present in the claims. It will therefore be appreciated that all combinations of features relating to the composition according to the invention; all combinations of features relating to the method according to the invention as well as all combinations of features relating to the composition according to the invention and all combinations of features relating to the method according to the invention are described herein.

[0129] It is also noted that the term "comprising" does not exclude the presence of other elements. However, it is also understood that a description of a product / composition comprising certain components also discloses a product / composition consisting of these components. A product / composition consisting of these components may be advantageous because it provides a simpler and more economical method for preparing the product / composition. Similarly, it is also understood that a description of a method comprising certain steps also discloses a method consisting of these steps. A method consisting of these steps may be advantageous because it provides a simpler and more economical method.

[0130] When the values ​​of the lower limit and the upper limit of a parameter are mentioned, it should also be understood that the range formed by the combination of the value of the lower limit and the value of the upper limit is disclosed. BRIEF DESCRIPTION OF THE DRAWINGS

[0131] The present invention will now be described with reference to the accompanying drawings, in which:

[0132] Figure 1 schematically illustrates an embodiment of a system for carrying out the method of the present invention; and

[0133] Figure 2 Another embodiment of a system for carrying out the method of the present invention is schematically shown. DETAILED DESCRIPTION

[0135] Figure 1 An embodiment of a schematic diagram of a process according to the invention is shown. The system in this embodiment has a reactor cascade of two polymerization reactors and the flash section between the two polymerization reactors has one flash vessel.

[0136] Ethylene, hydrogen and catalyst components are fed to the first polymerization reactor R-1 together with a first diluent. Ethylene polymerization is carried out and a first suspension S-25 of solid particles of a first ethylene polymer in a first suspension medium is prepared.

[0137] The first suspension S-25 is withdrawn from the first polymerization reactor R-1 and fed to a receiving vessel V-3 and fed as S-7 to the first solid-liquid separator SLS-1. The first solid-liquid separator SLS-1 separates the first suspension S-7 into a first diluent stream S-2 and a particle-enriched stream S-3.

[0138] The first diluent stream S-2 is transferred to the receiving vessel V-1. Vapor S-6 is removed from the first diluent stream S-2 in the receiving vessel V-1, and the remaining diluent stream S-5 is then recycled back to the first polymerization reactor R-1.

[0139] The first particle stream S-3 is transferred to a flash vessel F-1, which flashes off hydrogen, a first diluent and inerts (vapor stream S-34) to obtain a hydrogen-depleted stream S-26. The vapor stream S-34 from the flash vessel F-1 is fed to a heat exchanger HE-1 to obtain a liquid stream S-20 of the first diluent, which is recycled back to the first polymerization reactor R-1. The remaining non-condensed components S-38 from the heat exchanger HE-1 are fed to a nitrogen-hydrocarbon separation unit MS-1.

[0140] The hydrogen-depleted stream S-26 from the flash vessel F-1 is transferred to the second polymerization reactor R-3 for further polymerization. The polymer particles in the hydrogen-depleted stream S-26 are reslurried by the diluent S-8 recycled from the post-processing section D-1 described later to obtain a stream S-27. Ethylene and comonomers are fed to the second polymerization reactor R-3 together with a second diluent. Copolymerization of ethylene and comonomers occurs and a second suspension S-28 of solid particles of ethylene copolymer in a second suspension medium is prepared.

[0141] The second suspension S-28 is taken out from the second polymerization reactor R-3 and fed to the receiving vessel V-4 and fed to the second solid-liquid separator SLS-2 as S-29. The second solid-liquid separator SLS-2 separates the second suspension S-29 into a second diluent stream S-10 and a particle-enriched stream S-11.

[0142] The second diluent stream S-10 is transferred to receiving vessel V-2. Vapor S-12 is removed from the second diluent stream S-10 in receiving vessel V-2 to obtain stream S-30.

[0143] Part S-8 of stream S-30 is then recycled back to the second polymerization reactor R-3. Part S-33 of stream S-30 is fed to diluent post-treatment section D-1.

[0144] The diluent post-processing section D-1 separates the diluent stream fed thereto into a heavy component-enriched stream S-21, a comonomer-enriched side stream S-22, a substantially pure diluent third diluent stream S-23, and a light purge S-24.

[0145] The second particle stream S-11 is transferred to a flash vessel F-3, which flashes off hydrogen, ethylene, a second diluent and inerts (vapor stream S-35). The suspension S-12 from the flash vessel F-3 is transferred to a purge vessel PV-1, in which the remaining amounts of diluent, unreacted monomers and organic inert gases present in the suspension are removed. A stripping agent S-14 (e.g. nitrogen) is used in countercurrent to the polymer flow. Stream S-13 provides sufficient supplementary stripping agent to compensate for the losses occurring in the process. Free-flowing particles of the ethylene copolymer S-15 are obtained without the use of a dryer involving heating. The free-flowing particles of the ethylene copolymer S-15 can be transferred directly from the purge vessel PV-1 to, for example, a granulator (not shown).

[0146] The vapor leaving the purge vessel PV-1 is fed to the nitrogen-hydrocarbon separation unit MS-1 together with the vapor leaving the heat exchangers HE-1 and HE-3. The nitrogen-hydrocarbon separation unit MS-1 may consist of the unit operations required for separating the feed into a nitrogen-enriched stream and a nitrogen-depleted stream S-17. In a preferred embodiment of the present invention, the nitrogen-hydrocarbon separation unit MS-1 is a membrane separation package.

[0147] Figure 2 An embodiment of a schematic diagram of a method according to the invention is shown. Figure 1 of Figure 2 The components in the system are composed of Figure 1 The same reference numerals are used to represent the same reference numerals as in the figure. Figure 1 Different elements of the embodiments.

[0148] In this system, the reactor cascade includes two ethylene homopolymerization reactors R-1 and R-2 and two ethylene copolymerization reactors R-3 and R-4. There are heat exchangers HE-2, HE-5, HE-4, HE-6, HE-8 and HE-7. The flash section between polymerization reactor R-2 and polymerization reactor R-3 has two flash vessels F-1 and F-2. The vapor stream S-1 is fed to the heat exchanger HE-1.

Claims

1. A process for preparing a multimodal ethylene copolymer in a reactor cascade, said reactor cascade comprising a first polymerization reactor (R-1) and a second polymerization reactor (R-3), said process comprising: a) feeding ethylene, hydrogen and catalyst components and a first diluent into the first polymerization reactor (R-1) to prepare a first suspension of solid particles of ethylene homopolymer in a first suspension medium, b) feeding at least a portion of the first suspension to a first solid-liquid separator (SLS-1) to obtain a first diluent stream (S-2) enriched in the first diluent and a first particle stream (S-3) enriched in the solid particles, wherein at least a portion (S-5) of the first diluent stream (S-2) is recycled back to the first polymerization reactor (R-1), c) feeding said first particle stream (S-3) to a flash section comprising a first flash vessel (F-1) and optionally a second flash vessel (F-2) to obtain a hydrogen-enriched vapor stream and a hydrogen-depleted stream, wherein at least a portion of said vapor stream is recycled back to said first polymerization reactor (R-1), and d) feeding said hydrogen-depleted stream, ethylene and comonomer and a second diluent to said second polymerization reactor (R-3) to produce a second suspension of solid particles of said multimodal ethylene copolymer in a second suspension medium.

2. The process according to claim 1, wherein the first suspension is prepared in the first polymerization reactor at a pressure of P1, the first solid-liquid separator is operated at a pressure of P2, and the first flash vessel is operated at a pressure of P3, wherein P1-P2 is at most 0.5 bar and P2>P3, preferably wherein P1 is 1.0-50 bar gauge.

3. The method according to any one of the preceding claims, wherein the first solid-liquid separator is a centrifugal decanter or a hydrocyclone.

4. The method according to any one of the preceding claims, further comprising e) feeding at least a portion of the second suspension to a second solid-liquid separator (SLS-2) to obtain a second diluent stream (S-10) enriched in the second diluent and a second particle stream (S-11) enriched in the solid particles, wherein at least a portion (S-8) of the second diluent stream (S-10) is recycled back to the second polymerization reactor (R-3).

5. The method according to claim 4, further comprising f) feeding at least a portion of the second diluent stream (S-10) to a diluent post-treatment section (D-1) to obtain a third diluent stream (S-23) enriched in the second diluent and a comonomer stream (S-22) enriched in the comonomer, wherein at least a portion of the third diluent stream (S-23) is recycled back to the first polymerization reactor (R-1) and / or the second polymerization reactor (R-3), preferably wherein the process does not include a step of removing wax from the first diluent stream or the second diluent stream.

6. The process according to claim 5, wherein at most 20 wt. %, preferably at most 10 wt. %, more preferably at most 6.5 wt. % of the total amount of the first diluent and the second diluent fed to the first polymerization reactor and the second polymerization is fed to the post-treatment section (D-1).

7. The method according to any one of claims 4 to 6, further comprising g) purifying said second particle stream (S-11) by flashing and purging to obtain solid particles of said multimodal ethylene copolymer (S-15), preferably without using a dryer involving heating.

8. The method according to any one of the preceding claims, wherein the ethylene copolymer is selected from ethylene copolymers of ethylene and C3-C12 α-olefins, preferably selected from copolymers of ethylene and 1-hexene, copolymers of ethylene and 1-butene and copolymers of ethylene, 1-hexene and 1-butene, and / or each of the first diluent and the second diluent is selected from normal alkanes, isoalkanes and / or branched alkanes having a carbon number of 3 to 12 and combinations thereof, preferably selected from propane, butane, isobutane, normal pentane, isopentane, neopentane, hexane, normal heptane, isoheptane, octane and combinations thereof.

9. The process according to any one of the preceding claims, wherein each of the first diluent and the second diluent comprises neopentane, preferably wherein the amount of neopentane relative to the sum of the first diluent and the second diluent is at least 50 wt%, more preferably at least 60 wt%, more preferably at least 70 wt%, more preferably at least 80 wt%, more preferably at least 90 wt%, more preferably at least 95 wt%, more preferably at least 99 wt%.

10. The process according to any of the preceding claims, wherein the reactor cascade comprises one further polymerization reactor upstream of the first polymerization reactor and / or one further polymerization reactor downstream of the second polymerization reactor.

11. The process according to any one of the preceding claims, wherein the first solid-liquid separator is operated such that the first particle stream comprises at most 30 wt.-%, more preferably at most 15 wt.-%, more preferably at most 8 wt.-% of the hydrogen in the first suspension.

12. The process according to any one of the preceding claims, wherein the first solid-liquid separator is operated such that the first diluent stream comprises at least 70 wt.-%, more preferably at least 85 wt.-%, more preferably at least 92 wt.-% of the first diluent in the first suspension.

13. The method according to any one of the preceding claims, wherein the flash section comprises the first flash vessel (F-1) and the second flash vessel (F-2), the vapor stream from the first flash vessel (F-1) being fed to a heat exchanger (HE-1) using cooling water, and the vapor stream from the second flash vessel (F-2) being fed to a heat exchanger (HE-2) using a cooler condenser.

14. The process according to any one of the preceding claims, wherein the flash section is operated such that at least 99 wt.%, more preferably at least 99.5 wt.%, more preferably at least 99.9 wt.%, more preferably at least 99.95 wt.%, more preferably at least 99.99 wt.% of the hydrogen in the first particle stream is removed to obtain the hydrogen-depleted stream, and / or wherein the flash section is operated such that no more than 10%, preferably no more than 5% and most preferably no more than 1% of the first diluent in the first particulate stream is present in the hydrogen-depleted stream.

15. A process according to any one of the preceding claims, wherein at least 50 wt. %, more preferably at least 60 wt. %, more preferably at least 70 wt. %, more preferably at least 80 wt. %, more preferably at least 90 wt. %, more preferably at least 95 wt. %, more preferably at least 99 wt. % of the first diluent in the first particle stream is recycled back to the first polymerization reactor.

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