Process for polymerising ethylene

Abstract

A process for polymerising ethylene or for copolymerising ethylene and at least one alpha-olefin comonomer in the presence of a supported single site metallocene catalyst in a multi-stage polymerisation process to obtain an ethylene homo- or copolymer. The process comprises a pre-polymerisation stage wherein the supported single site metallocene catalyst has a D50 from 5 to 20 µm, and wherein the pre-polymerisation stage is carried out at a temperature below 70°C and in the presence of hydrogen at a hydrogen:ethylene ratio of within the range of from 0.05 to 10.0 mol / kmol. The process achieves high catalyst productivity and results in polymers with desirable morphology.

Description

[0001] Process for polymerising ethylene

[0002] Technical Field

[0003] The present invention is concerned with a process for polymerising ethylene or copolymerising ethylene and at least one alpha-olefin comonomer in the presence of a supported single site catalyst in a multi-stage process.

[0004] Background

[0005] Existing commercial polyolefin production processes typically utilise multi-stage reactor configurations in order to provide a multi-modal capability which can be used to manufacture easy-to-process resins with desirable mechanical properties. In such processes, a combination of e.g., slurry loop reactors in series followed by a gas phase reactor may be utilised to produce a wide range of polymer resins.

[0006] A key challenge in multi-stage olefin polymerisation processes is to achieve a desired plant throughput in order to meet the commercial requirements without sacrificing either process performance or desired product requirements. In general, polymerisation start-up conditions can play an important role in achieving smooth process performance in individual polymerisation stages, for example, when single site polyethylene (SSPE) catalysts are used.

[0007] W02014 / 016318 relates to a process for preparing a polyethylene resin having a multimodal molecular weight distribution in at least two loop slurry reactors connected in series, comprising the step of: polymerising ethylene in the presence of at least one supported metallocene catalyst, a diluent, optionally one or more co-monomers, and optionally hydrogen, thereby obtaining the polyethylene resin, wherein the supported metallocene catalyst has a particle size distribution of a span value lower than 2.5 and a D50 value within the range of from 5 pm to 20 pm.

[0008] WO 2020 / 064484 relates to a multi-stage process for producing a C2 to C8 olefin polymer composition in a process comprising at least two reactors, wherein a pre-polymerised solid Ziegler-Natta catalyst is prepared by carrying out an off-line pre-polymerisation of a solid Ziegler-Natta catalyst component with a C2 to C4 olefin monomer before feeding to the polymerisation process, wherein the pre-polymerised solid Ziegler-Natta catalyst has a particle size distribution span (D90 - Dio) / Dso) of below 1.5.

[0009] WO 2020 / 025757 relates to a process for polymerising ethylene or copolymerising ethylene and at least one alpha-olefin comonomer in the presence of a supported polymerisation catalyst in a multi-stage process in which the last polymerisation stage is a gas phase reactor, the use of said process for reducing particle carry-over in the last polymerisation stage and the use of a supported polymerisation catalyst with a certain median particle size to polymerise an ethylene homo- or copolymer in said multi-stage process with a span of its particle size distribution which can be predicted from the median particle size of the catalyst.

[0010] Janicek et al1in their article of 2011 discussed the co-polymerisation of ethylene with co- the monomers oct-1 -ene, dodec-1-ene, octadec-1-ene, and hexacos-1-ene using a zirconocene / MAO catalyst system. The process yielded polymers with different side-chain lengths, and the crystal structure analysed. Co-monomer selection up to C26 is another variable available to polyethylene producers seeking to alter the final material properties. Similar findings were reported by Piel2who analysed the influence of comonomer content and size on the resulting polymers. Further representative examples of ethylene copolymerisation with higher alpha-olefines using metallocene catalysts are mentioned in Lehmus, P et al3and Stadler F.J. et al4and demonstrate that known catalysts are capable of incorporating higher alpha-olefins.

[0011] Brief Description of the Figures

[0012] Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:

[0013] 1Janicek, M.; Cermak, R.; Obadal, M.; Piel, C.; Ponizil, P. Ethylene Copolymers with Crystallizable Side Chains. Macromolecules 2011 , 44 (17), 6759-6766. DOI: 10.1021 / ma201017m.

[0014] 2Piel, C. Polymerization of Ethene and Ethene-co-alpha-Olefin: Investigations on Short- and Long- Chain Branching and Structure-Property Relationships. Dissertation, Department of Chemistry, University of Hamburg, 2005

[0015] 3Lehmus, P.; Kokko, E.; Harkki, O.; Leino, R.; Luttikhedde, H. J. G.; Nasman, J. H.; Seppala, J. V.

[0016] Homo- and Copolymerization of Ethylene and a-Olefins over 1- and 2-Siloxy-Substituted Ethylenebis(indenyl)zirconium and Ethylenebis(tetrahydroindenyl)zirconium

[0017] Dichlorides. Macromolecules 1999, 32 (11), 3547-3552. DOI: 10.1021 / ma981764q

[0018] 4Stadler, F. J.; Piel, C.; Klimke, K.; Kaschta, J.; Parkinson, M.; Wilhelm, M.; Kaminsky, W.; Munstedt, H. Influence of Type and Content of Various Comonomers on Long-Chain Branching of Ethene / a- Olefin Copolymers. Macromolecules 2006, 39 (4), 1474-1482. DOI: 10.1021 / ma0514018 Figures 1a to 1d are micrographs of the polymer particles formed in Examples 1a to 1d ;

[0019] Figure 2 is kinetic profile showing catalyst activity against time for Examples 2a and 2b; and

[0020] Figure 3 is a kinetic profile showing the consumption of ethylene starting material against time for Examples 2c and 2d.

[0021] Definitions

[0022] An ethylene polymer denotes a polymer derived from at least 50 mol-% ethylene monomer units and additional comonomer units.

[0023] An ethylene homopolymer denotes a polymer consisting essentially of ethylene monomer units. Due to the requirements of large-scale polymerisation it may be possible that the ethylene homopolymer includes minor amounts of comonomer units. This is usually below 0.1 mol%, preferably below 0.05 mol%, most preferably below 0.01 mol% of the ethylene homopolymer.

[0024] An ethylene copolymer denotes a polymer consisting of ethylene monomer units and comonomer units in an amount of at least 0.1 mol%. In an ethylene random copolymer the comonomer units are randomly distributed in the polymer chain.

[0025] A multi-stage polymerisation process is a polymerisation process in which two or more polymerisation reactors are connected in series.

[0026] The particle size and particle size distribution can be used to measure the size of the supported single site catalyst particles, and / or the size of polymer particles produced in the process described herein.

[0027] The D-values (Dw, Dso and D90) represent the intercepts for 10 volume %, 50 volume % and 90 volume %, respectively, of the total volume of the sample. D-values are well-known in the art and are typically used to provide an indication of the size distribution of particles in a sample. For example, the Dw value is indicative of the particle size at which 10 volume % of the sample is comprised of particles with a size less than the D value. Dso is indicative of the threshold where 50% of a sample's volume has a smaller particle size that the Dso value, and 50% of a sample's volume has a larger particle size than the Dso value. D90 is indicative of the size at which 90% of the sample's volume is comprised of particles with a size less than the D90 value. The Dso value is also called median particle size. From laser diffraction measurements according to ASTM 13320-1 the volumetric D-values are obtained, based on the volume distribution. The D-values based on mass (weight) can be determined from the density of the particles. Provided that particle density does not change with particle size then the volume-based PSD and the mass-based PSD are the same, which means that the characteristic values of Dw, D50 and D90 are the same for the volume and mass-based particle size distributions (PSDs). Thus, weight-based Dw, Dsoand D90 values are generally interchangeably with volume-based Dw, D50 and D90 which can be determined by laser diffraction measurements according to ASTM 13320-1.

[0028] The distribution span of the particle size distribution is calculated from the d-values according to the formula: (D90 - Dw) / Dso, whereby 90 volume % of the particles of the supported single site catalyst have a diameter of Dgo or less; 50 volume % of the particles of the supported single site catalyst have a diameter of D50 or less; and 10 volume % of the particles of the supported single site catalyst have a diameter of Dw or less, relative to the total volume of the particles of the supported single site catalyst.

[0029] Summary of Invention

[0030] A first aspect of the invention provides a process for polymerising ethylene or for copolymerising ethylene and at least one alpha-olefin comonomer with 3 to 12 carbon atoms in the presence of a supported single site metallocene catalyst (polymerisation catalyst) in a multi-stage polymerisation process comprising a pre-polymerisation stage and at least one further polymerisation stage downstream of the pre-polymerisation stage, to obtain an ethylene homo- or copolymer. The process comprises contacting ethylene with particles of the supported single site metallocene catalyst in a pre-polymerisation stage. The catalyst has a volume-based median particle size D50 within the range of from 5 to 20 pm. The D50 may be derived from a volume-based median particle size determined by laser diffraction measurements according to ASTM 13320-1. The pre-polymerisation stage is carried out at a temperature of 70°C or less. The pre-polymerisation stage is carried out in the presence of hydrogen at a hydrogen:ethylene ratio within the range of from 0.05 to 10 mol / kmol.

[0031] Preferably, the pre-polymerisation stage is carried out in the presence of hydrogen at a hydrogen:ethylene ratio within the range of from 0.05 to 10 mol / kmol, preferably from 0.1 to 8 mol / kmol, more preferably from 0.3 to 6 mol / kmol, yet more preferably from 0.5 to 4 mol / kmol, for example, from 1 to 3 mol / kmol.

[0032] The purpose of the pre-polymerisation is to polymerise a small amount of polymer onto the supported single site metallocene catalyst at a low temperature and / or a low monomer concentration. It has been found that, prior art catalyst particles having Dso value greater than 25 to 40 pm used in prior art prepolymerisation can result in mass and heat transfer phenomena which can lead to a decrease in the polymerisation activity, uncontrollable kinetics (e.g. an undesired production split) and / or other operability issues related to agglomeration. Moreover, given the relatively low catalyst productivity, the economics of the polymer plants can be compromised.

[0033] However, in operations using smaller sized supported single site metallocene catalysts, the initial polymerisation rate and particle growth rate are higher than for comparative standardsized supported single site catalyst. However, this fast start-up can lead to an uncontrolled catalyst fragmentation e.g. the formation of fines and polymer particles with irregular shapes, as well as the formation of agglomerates. This poor control of the morphology can cause operational issues in the subsequent polymerisation stages, such as poor powder flowability, low bulk density and poor fluidisation performance in downstream reactor(s), such as downstream gas phase reactor(s).

[0034] The present inventors have found that, by operating the pre-polymerisation at a temperature of 70°C or less, and with a hydrogen:ethylene ratio of between 0.05 and 10 mol / kmol, the morphology of the polymer produced can be improved.

[0035] The pre-polymerisation stage may be carried out at no greater than 70, 65, or 60 °C . The pre-polymerisation stage may be carried out at no less than 30, 35 or 40°C. The pre- polymerisation stage may be carried out at a temperature within the range of from 30°C to 69°C, particularly from 35°C to 65°C, or optionally, from 40°C to 60°C. The supported single site metallocene catalyst may have a span value (i.e. relative span) between 0.1 to 1.3. The span may be defined as:

[0036] The volume based D90, D50, and D10 values may be determined by laser diffraction measurements according to ASTM 13320-1. For example, 90 volume % of the particles of the supported single site metallocene catalyst may have a diameter of D90 or less; 50 volume % of the particles of the supported single site metallocene catalyst may have a diameter of D50 (median particle size) or less; and 10 volume % of the particles of the supported single site metallocene catalyst may have a diameter of D10 or less, relative to the total volume of the particles of the supported single site metallocene catalyst.

[0037] The particles of the supported single site metallocene catalyst may have a particle size distribution that has a relative span of less than 1.3, 1.2, 1.1 or 1.0. The particles of the supported single site metallocene catalyst may have a particle size distribution that has a relative span of more than 0.1 , 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 or 0.8. The particles may have a span within the range of from 0.1 to 1.3, such as from 0.1 to 1.2, preferably from 0.2 to 1.1 , more preferably from 0.3 to 1.0, yet more preferably from 0.4 to 0.9 and most preferably from 0.5 to 0.9.

[0038] The supported single site metallocene catalyst may comprise a metallocene catalyst comprising a support, an activator (herein also referred to as cocatalyst), and / or a transition metal complex precursor.

[0039] The pre-polymerisation stage may be carried out at an ethylene concentration within the range of from 1 mol% to 16 mol%, more preferably from 2 mol% to 12 mol%, most preferably from 1 mol% to 10 mol% based upon the total amount of inert diluent (such as propane), ethylene, optional comonomer and hydrogen present during the pre-polymerisation stage.

[0040] The mean residence time of the supported single site metallocene catalyst in the pre- polymerisation stage may be at least 5, 10, 15, 20, or 25 minutes. The mean residence time of the supported single site metallocene catalyst in the pre-polymerisation stage may be less than 60, 55, 50, 45, 40 or 35 minutes. The mean residence time of the supported single site metallocene catalyst in the pre-polymerisation stage may be within the range of from 5 to 60 minutes. In some embodiments, the mean residence time of the supported single site metallocene catalyst in the pre-polymerisation stage may be from 10 to 55, from 15 to 50, or from 20 to 45 minutes.

[0041] The pre-polymerisation stage may be carried out at a pressure greater than 10, 15, 20, 25, 30, 35, 40, or 45 bar. The pre-polymerisation stage may be carried out at a pressure less than 70, 65, 60, 55 or 50 bar. The pre-polymerisation stage may be carried out at a pressure within the range of from 10 to 70 bar, for example, from 15 to 69 bar, for example, from 20 to 68 bar.

[0042] The pre-polymerisation stage is preferably carried out in a slurry phase reactor. The slurry phase reactor may comprise a loop reactor.

[0043] The process may comprise copolymerising ethylene and at least one alpha-olefin comonomer with 3 to 12 carbon atoms, the amount of comonomer in the pre-polymerisation stage may be less than 20, 18, 16, 15, 14, 12, 10, 8, 6, 5, 4, 2, or 1 mol per kmol of ethylene. In some embodiments, the amount of comonomer in the pre-polymerisation stage may be more than 0, 1 , 2, 4, 5, 6, 8, 19, 12, 14, 15, 16, or 18 mol per kmol of ethylene. In some embodiments, the amount of comonomer in the pre-polymerisation stage may be within the range of from 0 to 20 mol per kmol of ethylene. In some embodiments, the process may comprise no comonomer during the pre-polymerisation stage.

[0044] Suitable alpha olefin comonomers include propene, 1 -butene, 1 -pentene, 1 -hexene, 1- heptene, 1 -octene, 1 -nonene and 1 -decene. In one series of embodiments, the comonomer comprises one or more of: propene, 1 -butene, 1 -hexene, and 1 -octene. One or more alpha comonomers may be employed.

[0045] The pre-polymerisation stage may be carried out in the presence of an inert diluent. The inert diluent may comprise a C3 to C10 alkane or mixtures thereof. The inert diluent may comprise a hydrocarbon diluent such as methane, ethane, propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes etc., or their mixtures. Preferably the diluent is a low-boiling hydrocarbon having from 1 to 4 carbon atoms or a mixture of such hydrocarbons. In a series of embodiments, the inert diluent is propane, possibly containing minor amount of methane, ethane and / or butane.

[0046] The multi-stage polymerisation process may comprise at least one further polymerisation stage downstream of the pre-polymerisation stage. For example, the at least one further polymerisation stage may be carried out in at least one slurry phase reactor. The slurry phase reactor may comprise a loop reactor. In some embodiments, the multi-stage polymerisation process comprises a first and second polymerisation stage. The first and second polymerisation stage may each be carried out in slurry phase reactors e.g. loop reactors. In some embodiments, one or more polymerisation stages may be carried out in a gas phase reactor. In some embodiments, a final polymerisation stage may be carried out in a gas phase reactor. In a preferred series of embodiments, the multi-stage polymerisation process comprises a pre-polymerisation stage, a first polymerisation stage, a second polymerisation stage, and a third polymerisation stage. The first and second polymerisation stages may be carried out in slurry phase reactors e.g. loop reactors. The third polymerisation stage may be carried out in a gas phase reactor.

[0047] Prepolymerisation

[0048] As mentioned above, the purpose of the prepolymerisation is to polymerise a small amount of polymer onto the supported single site metallocene catalyst at a low temperature and / or a low monomer concentration. By prepolymerisation, it may be possible to improve the performance of the catalyst in subsequent polymerisation stages. The pre-polymerisation is performed as part of the herein disclosed multi-stage polymerisation process. Thus, the herein disclosed multi-stage polymerisation process is advantageously devoid of any offline prepolymerisation of the supported single site metallocene catalyst.

[0049] The supported single site metallocene catalyst components are preferably all introduced to the prepolymerisation step. Thus, the supported single site metallocene catalyst used in the prepolymerisation stage may be transferred to each downstream reactor(s) in sequence. Thus, this supported single site metallocene catalyst may be used to catalyse more than one, for example, two or three, and more preferably all the polymerisation stages in the proves.

[0050] It is understood within the scope of the invention that the amount or polymer produced in the prepolymerisation lies within 1 to 7 wt% in respect to the final multimodal (co)polymer. Polymer produced in the prepolymerisation stage may not be detectable in the final polymer product.

[0051] Polymerisation The process may be a process whereby any polymer produced in the prepolymerisation reactor is passed down into at least one of the downstream reactors. The process may be a process whereby polymerisation in any given downstream reactor takes place in the presence of polymer produced in one or more (e.g., each) of the upstream reactor(s).

[0052] The process for polymerising ethylene in the multistage polymerisation process configuration may be a homopolymerisation or copolymerisation process. For example, polymerisation in each reactor may be independently selected from a homopolymerisation or copolymerisation stage. Accordingly, the process may produce a polyethylene that is a homopolymer, copolymer, or terpolymer of ethylene.

[0053] At least one reactor may be present downstream of the prepolymerisation reactor. As noted above, at least two reactors may be present downstream of the prepolymerisation reactor. In some embodiments at least three reactors may be present downstream of the prepolymerisation reactor. The reactor immediately downstream of the prepolymerisation reactor may be a slurry reactor. In some embodiments slurry loop reactors may be present downstream of the prepolymerisation reactor.

[0054] Where more than one downstream reactor is used, at least the most downstream of the downstream reactors may be a gas phase reactor.

[0055] Preferably, a product stream comprising polymer and unreacted olefin monomer may be withdrawn from the gas phase reactor. This product stream may be recovered from any gas phase reactor where more than one gas phase reactor is employed. Preferably, where more than one downstream gas phase reactor is used, the product stream is recovered from the most downstream of the downstream gas phase reactors.

[0056] It is preferred to conduct at least one stage of the polymerisation in a slurry loop reactor. In such reactors the slurry is circulated with a high velocity along a closed pipe by using a circulation pump. Loop reactors are generally known in the art and examples are given, for instance, in US A-4582816, US-A-3405109, US-A-3324093, EP-A-479186, and US-A- 5391654. The slurry may be withdrawn from the reactor either continuously or intermittently. A preferred way of intermittent withdrawal is the use of settling legs where slurry is allowed to concentrate before withdrawing a batch of the concentrated slurry from the reactor. The use of settling legs is disclosed, among others, in US-A-3374211 , US-A-3242150 and EP- A-1310295. Continuous withdrawal is disclosed, among others, in EP-A-891990, EP-A- 1415999, EP-A-1591460 and WO-A-2007 / 025640. The continuous withdrawal may advantageously combined with a suitable concentration method, as disclosed in EP-A- 1310295, EP-A-1591460, and EP3178853B1.

[0057] The product stream from the most downstream reactor may be withdrawn from a reactor outlet and introduced to a separation unit. In the separation unit, the polymer may be separated from gases, including unreacted olefin monomers that may be adsorbed or contained within the polymer. Unreacted olefin monomer may be recovered from the separated gases, e.g., by distillation. The recovered unreacted olefin monomer may be recycled to the process, preferably to a gas phase reactor as recycled monomer feed.

[0058] The temperature in any polymerisation slurry reactor or reactors downstream of the prepolymerisation reactor may be within the range of from 60 to 100 °C, from 65 to 95 °C, preferably from 70 to 90 °C or more preferably, from 80°C to 90°C. The pressure may be within the range of from 4 to 7 MPa, preferably from 4.5 to 6.9 MPa, more preferably from 5 to 6.7 MPa. In embodiments comprising two or more polymerisation reactors, the temperatures in each reactor may be different.

[0059] The residence time in the any polymerisation slurry reactor(s) downstream of the prepolymerisation reactor may be within the range of from 0.15 h to 3.0 h, preferably from 0.20 h to 2.0 h and in particular from 0.30 to 1 .5 h.

[0060] Where employed, the temperature of any gas phase polymerisation may within the range of be 60 to 100°C, preferably from 65 to 90 °C, more preferably from 70 to 85 °C. The pressure may be from 1 to 3 MPa, preferably from 1 .5 to 2.5 Mpa, more preferably from 1 .8 to 2.2 Mpa. The residence time of any gas phase polymerisation may be from 1 .0 h to 4.5 h, preferably from 1.5 h to 4.0 h and in particular from 2.0 to 3.5 h. The residence time in the gas phase polymerisation may be longer than the residence time in the first reactor (e.g., slurry reactor) by at least 30%, preferably at least 40% or at least 50%.

[0061] An example of a suitable downstream gas phase polymerisation reactor may comprise three zones: a) a bottom zone where fluidization gas is introduced into the reactor; b) a middle zone, which may have a generally cylindrical shape, where olefin monomer(s) present in the fluidization gas are polymerised to form the polymer particles; and c) a top zone, where fluidization gas is withdrawn from the reactor. A fluidization grid (also named distribution plate) may be employed to separate the bottom zone from the middle zone. The top zone may also act as a disengaging or entrainment zone in which, due to its expanding diameter compared to the middle zone, the fluidization gas can expand and disengage from the polyolefin powder.

[0062] Catalyst

[0063] The catalyst components are preferably all introduced to the pre-polymerisation step. Preferably, the reaction product of the pre-polymerisation step is introduced to the polymerisation in which the first polymer component is produced.

[0064] Preferably, the same catalyst is used in each step and may be transferred from pre- polymerisation to subsequent polymerisation steps in sequence. Where more than one downstream reactor is employed, the catalyst may be transferred in sequence from the pre- polymerisation reactor, the first reactor and to each downstream reactor in sequence. Where more than one downstream reactor is employed, the catalyst may be transferred in sequence from the pre-polymerisation reactor to the first reactor and then to each downstream reactor in sequence. Any polymer produced in each reactor may also be transferred in sequence to any reactor located downstream.

[0065] In some instances, catalyst and a cocatalyst can be fed separately into the pre- polymerisation stage. Here, it is possible that only a part of the cocatalyst is introduced into the pre-polymerisation stage and the remaining part into subsequent polymerisation stages. Also, in such cases it may not be necessary to introduce as much cocatalyst into the pre- polymerisation stage to achieve sufficient polymerisation therein.

[0066] The supported single site metallocene catalyst may be any supported singe site metallocene catalyst which can produce the desired olefin polymer. Suitable supported single site metallocene catalysts comprise a transition metal complex, a co-catalyst or activator and a carrier (support).

[0067] The transition metal complex may be a metallocene complex comprising a Group 3-12 transition metal. Preferably, the transition metal complex is a Group 4 metal metallocene. More preferably, the transition metal complex is a hafnium or zirconium complex.

[0068] The co-catalyst or activator may be a boron containing compound, an aluminium containing compound or a combination of both. Preferably, the boron containing compound is a borate compound. Preferably, the aluminium containing compound is an aluminoxane such as methylaluminoxane.

[0069] Metallocene catalysts comprise a transition metal compound which contains a cyclopentadienyl, indenyl or fluorenyl ligand. Preferably the catalyst contains two cyclopentadienyl, indenyl or fluorenyl ligands, which may be bridged by a group preferably containing silicon and / or carbon atom(s). Further, the ligands may have substituents, such as alkyl groups, aryl groups, arylalkyl groups, alkylaryl groups, silyl groups, siloxy groups, alkoxy groups or other heteroatom groups or the like. Suitable metallocene catalysts are known in the art and are disclosed, among others, in WO-A-95 / 12622, WO-A-96 / 32423, WO-A-97 / 28170, WO-A-98 / 32776, WO-A-99 / 61489, WO-A-03 / 010208, WO-A-03 / 051934, WO-A-03 / 051514, WO-A-2004 / 085499, EP-A-1752462, EP-A-1739103 W02005 / 105863, WO 2006097497, W02007 / 116034, W02007 / 107448, W02009 / 027075,

[0070] W02009 / 054832, WO 2012 / 001052, and EP 2532687.

[0071] A metallocene catalyst is generally used together with an activator, also called cocatalysts. Suitable activators are metal alkyl compounds and especially aluminium alkyl compounds known in the art. Especially suitable activators used with metallocene catalysts are alkylaluminium oxy-compounds, such as methylalumoxane (MAO), tetraisobutylalumoxane (TIBAO) or hexaisobutylalumoxane (HIBAO).

[0072] Typically, triethylaluminium (TEAI) or tri-isobutylaluminium (TIBA) may be used as scavenger in such amount that the molar ratio of aluminium to the transition metal, like Al / Zr, is from 1 to 1000, preferably from 3 to 100 and in particular from about 5 to about 30 mol / mol.

[0073] The catalyst employed in the process of the present disclosure is a supported catalyst. The catalyst support is a particulate support. The particulate support can be an inorganic oxide support, such as silica, alumina, titania, silica-alumina and silica-titania. Preferably, the support is silica.

[0074] The particles of the support may have a median particle size, Dso, within the range of 5 to 20 pm determined by laser diffraction measurements according to ASTM 13320-1. The particles of the support may have particle size distribution that has a relative span within the range of from 0.1 to 1.3. The relative span is defined by the equation: relative span = (D90 - Dio) / Dso, whereby 90 volume % of the particles of the support have a diameter of D90 or less; 50 volume % of the particles of the support have a diameter of D50 or less; and 10 volume % of the particles of the support have a diameter of D or less, relative to the total volume of the particles of the support. Preferably, the particles of the support have a particle size distribution that has a relative span within the range of from 0.1 to 1 .2, more preferably from 0.2 to 1 .1 , more preferably from 0.3 to 1 .0, yet more preferably from 0.4 to 0.9 and most preferably between 0.5 to 0.9. The D90, D50, and Dm values may be determined by laser diffraction measurements according to ASTM 13320-1.

[0075] The particles of the support may have a median particle size, D50, within the range of 5 to 20 pm and a relative span of within the range of from 0.1 to 1.3, particularly from 0.1 to 1.2, more preferably from 0.2 to 1.1 , more preferably from 0.3 to 1.0, yet more preferably from 0.4 to 0.9 and most preferably from 0.5 to 0.9.

[0076] The support particles of the supported single site metallocene catalyst may have a skeletal density of at least 1.3 g / ml, more preferably at least 1.4 g / ml and most preferably at least 1.5 g / ml. Suitably the skeletal density of the support is 3.0 g / ml or less, preferably 2.8 g / ml or less, more preferably 2.5 g / ml or less. The skeletal density may be 1.3 g / ml to 3.0 g / ml, preferably 1.4 g / ml to 2.8 g / ml, more preferably 1.5 to 2.5 g / ml.

[0077] The skeletal density (also termed absolute, true, or real density) of the support is the density of the material used to form the support particles. Accordingly, this density is obtained when the volume of a particle measured excludes the pores, cavities as well as the void spaces between the particles within the bulk sample. The skeletal powder density or absolute powder density is defined according to British Standards Institution as the mass of powder per unit of absolute powder volume whereby the absolute powder volume is defined as the volume occupied by a powder excluding all pores and voids. The skeletal density can be measured by using water or another liquid which is expected to fill the pores in the sample, thus removing their volume from the measurement. The skeletal density can also be measured by employing an apparatus, called Pycnometer, distributed by Micromeritics, Norcross (USA) that utilizes helium or any other inert gas. See also US12 / 043687B2. Where the particulate support is formed of silica, for example, the skeletal density of the particulate support is the skeletal density of silica. Experiments and Examples

[0078] Skeletal Density Measurement

[0079] In the present measurement skeletal density was measured as the difference of the masses of a dry sample under current conditions and after filling the pores with n-heptane.

[0080] Analysis:

[0081] Firstly, the density of n-heptane under current conditions was measured by taking a 10 ml bottle and filling the bottle with a pre-determined volume of dry n-heptane. The weight of the n-heptane was measured, and the density of the n-heptane was calculated using the formula density = mass / volume.

[0082] In a second step, the weight of a new dry clean bottle was measured. Approximately 2 g catalyst was added into the bottle, and the weight of the sample was measured accurately using a high precision balance placed in a glove box.

[0083] A bottle containing the sample was placed in a vacuum impregnator in the glove box. A tube was inserted into the cap of the bottle and a vacuum was applied.

[0084] Vacuum was applied on the bottle such that a pressure of 150 mbar was obtained within 30 seconds. The pressure of 150 mbar was maintained for 5 min. While maintaining the vacuum on the bottle, n-heptane was carefully introduced into the bottle by opening the valve in order to cover the sample fully with n-heptane.

[0085] Afterwards, the valve was closed to prevent further n-heptane influx and the vacuum was turned off.

[0086] The bottle was filled to a pre-determined mark with n-heptane. The weight of the filled bottle was measured and then the skeletal density was calculated.

[0087] Particle Size, Particle Size Distribution

[0088] Particle size distribution [volume percent] and particle sizes were determined by laser diffraction measurements by use of, for example, Mastersizer or Coulter instruments. The particle size and particle size distribution are measures for the size of the particles. The D-values (Dw (or dw), Dso (or dso) and D90 (or dgo)) represent the intercepts for 10%, 50% and 90% of the cumulative volume of sample. The D-values can be thought of as the diameter of the sphere which divides the sample’s volume into a specified percentage when the particles are arranged on an ascending volume basis. For example, the D10 is the diameter at which 10% of the sample's volume is comprised of particles with a diameter less than this value. The D50 is the diameter of the particle where 50% of a sample's volume is smaller than and 50% of a sample's volume is larger than this value. The D90 is the diameter at which 90% of the sample's volume is comprised of particles with a diameter less than this value. The D50 value is also called median particle size. From laser diffraction measurements according to ASTM 13320-1 the volumetric D-values are obtained, based on the volume distribution

[0089] The distribution width or span of the particle size distribution (PSD) is calculated from the D-values Dw, D50 and D90 according to the below formula:

[0090] D90 - D10 Span= DSO

[0091] The mean particle size corresponds to the average particle size. From laser diffraction measurements according to ASTM 13320-1 the volume based mean particle size is obtained and calculated as follows: wherein D = the average or mean particle size

[0092] (p-q) = the algebraic power of 13pq, whereby p>q Dj = the diameter of the ith particle

[0093] Z = the summation of DjPor Djqrepresenting all particles in the sample

[0094] Only in symmetric particle size distributions the mean particle size and the median particle size D50 have the same value.

[0095] Unless specifically otherwise defined, the percentage numbers (Dw, D50 and D90) used herein refer to percentage by volume. The volume-based fraction of each size class (d), of the particle size distribution is providing by the following formula,

[0096] The corresponding mass-based fraction of each size class (d), of the distribution can be calculated by the following formula considering the density of the size class, ( / ?;) and the overall density of the material (ptot), i.e. ,

[0097] Provided that the pt is equal to ptot, (assuming particle density does not change with particle size) then the volume-based PSD and the mass-based PSD are the same, that in turn means that the characteristic values of D10, D50 and D90 are the same for the volume and mass-based PSDs.

[0098] Catalyst preparation

[0099] The catalysts used in the examples were silica-based single-site catalysts comprising rac- dimethylsilanediylbis{2-(5-(trimethylsilyl)furan-2-yl)-4,5-dimethylcyclopentadien-1- yljzirconium dichloride as metallocene component and methyl aluminoxane (MAO) as cocatalyst. The catalysts were prepared following the preparation procedure as disclosed in the Catalyst Example 2 (C-IE2) of WO 2022 / 268961 .

[0100] Catalyst A (comparative catalyst)

[0101] The silica used as above catalyst support was ES757 supplied by Ecovyst. This silica is spray-dried silica calcined at a temperature of 600 °C. The silica batch had a volume based median particle size (D50) of about 26 pm (in accordance with supplier’s data sheet) and a skeletal density of 2.2 g / ml (determined by the above disclosed method). The span (i.e. (D90 - D10) / D50) of the silica was calculated to be 1.5 as shown in Table 1. It may be noted that there may be variations in measured D-values between different batches of the same silica product. The span of the catalyst is assumed to be equivalent to the span of the silica. The median particle size (D50) of the catalyst is assumed to be equivalent to the median particle size (D50) of the silica.

[0102] Catalyst B

[0103] The silica used as above catalyst support was ES731-6 supplied by Ecovyst. This silica batch had a volume based median particle size (D50) of about 13 pm (in accordance with supplier’s data sheet) and a skeletal density of 2.2 g / ml (determined by the above disclosed method. The span (i.e. (D90 - D10) / D50) of the silica was calculated to be 0.9 as shown in Table 1.

[0104] The span of the catalyst is assumed to be equivalent to the span of the silica. The median particle size (D50) of the catalyst is assumed to be equivalent to the median particle size (D50) of the silica.

[0105] Table 1

[0106] * volume based D values determined by laser diffraction in accordance with supplier’s data sheet

[0107] Experiment 1 - Initial investigations

[0108] A small scale polymerisation reactor was used to carry out a series of polymerisation reactions according to the conditions shown in Table 1 below. The reactions were indicative of pre-polymerisation stages in multi-stage polymerisation processes, and all used the small sized supported single site metallocene Catalyst B having a D50 of about 13 pm. The results of each reaction are shown in Figures 1a to 1d. Table 2

[0109] As shown in Figures 1a and 1 b, the polymer agglomerates produced exhibited poor morphology and had a ‘fluffy’ texture. Such results lead to poor further processing steps in multi-stage polymerisation processes. As shown by examples 1a and 1 b, the reduction in temperature alone was ineffective at resolving the morphology issues.

[0110] Examples 1c and 1d utilised both the lower operating temperature and a small addition of hydrogen to the reaction. The resulting polymer particles formed, as shown in Figures 1c and 1d, exhibit significantly improved morphology and would be suitable for further polymerisation in a multi-stage polymerisation process.

[0111] Additionally, pre-polymerisation mean residence time was found to be a significant operating condition which ensures that the catalyst particle has grown sufficiently (the prepolymerisation degree) and while avoiding potential morphology issues, before the transition of the particles to the further polymerisation reactors (e.g. a loop reactor).

[0112] It was also surprisingly discovered that lowering ethylene concentration, in combination with extending the mean residence time of the polymer particles in the pre-polymerisation reactor, resulted in further improvement of particle morphology. More specifically, it was found that selecting an ethylene concentration in the liquid phase of the pre-polymerisation reactor ranging from 1 to 16 mol% and adjusting the mean residence time from 5 to 60 min assured proper morphology of the polymer particles for the subsequent polymerisation stages.

[0113] Experiment 2 - Two-stage polymerisation tests

[0114] A small-scale polymerisation reactor was employed to carry out a series of ethylene / 1- butene (C2 / C4) co-polymerisations in a propane diluent in a two-stage polymerisation process (i.e. comprising a prepolymerisation stage and one polymerisation stage in a slurry reactor) using Catalyst A and Catalyst B, respectively. The process conditions and catalyst sizes used are given in Table 3 below:

[0115] Table 3

[0116] Triethylaluminium (TEAI) was used as a scavenger.

[0117] Example 2a and 2c are comparative examples utilising the standard-size single site Catalyst A having a median particle size (Dso) of about 26 microns. Examples 2b and 2d show the same reactions carried out using single site Catalyst B having a median particle size of about 13 microns.

[0118] Figure 2 is a graph of the kinetic profile showing catalyst activity against time for Examples 2a and 2b. Figure 3 is a graph of the kinetic profile showing the consumption of ethylene starting material against time for Examples 2c and 2d. In both cases, the reactions incorporating the smaller size catalysts show a significant increase in catalyst activity over the processes utilising the catalysts comprising standard sized carriers. Based on the findings of the results of examples 2a to 2d, it has been found that selection of the pre-polymerisation and slurry reactor conditions result in good catalyst performance, and it has been further shown that the small size silica carrier catalyst has higher activity compared to the standard size silica carrier, therefore showing its suitability to be employed in single site polyethylene catalyst technology.

[0119] Experiment 3 - Multi-stage polymerisation tests A further series of tests were conducted on an ethylene / 1-butene / 1 -hexene three stage polymerisation process comprising a pre-polymerisation stage, a split loop stage, and a final gas phase reactor stage. Catalyst B was used in this set of experiments. The polymerisation conditions for all three examples were maintained constant, except for prepolymerisation temperature. The full process parameters are set out in Table 4 below. Table 4

[0120] The polymer powder morphology and reactor operability during collection of the three examples was excellent. The total catalyst productivity clearly shows a strong dependence on prepolymerisation conditions.

[0121] Examples 3a to 3c also show that the metallocene catalysts used and the processing parameters were able to incorporate hexene into the polymer products without requiring any changes to the process. Without wishing to be bound by theory, it is understood that the size and chemistry of the metallocene catalyst used makes it highly suitable for incorporating higher alpha-olefins e.g. C12, C18 or higher, within the present monomer to co-monomer ratios without requiring any changes to the polymerisation process. More specifically, the higher alpha-olefins under the above described process conditions would be in liquid form and could be simply fed to the various process stages as ordinary liquids. Due to different molecular weight of the various suitable co-monomers, the comonomer feed can be simply adjusted to for higher alpha-olefins to achieve the same incorporation into the polymer (comonomer response).

[0122] In summary, the present inventors surprisingly found that when a small size single site catalyst, i.e. within the range of from 5 to 20 pm, is employed under a specific range of operating pre-polymerisation conditions, the performance of the catalyst in terms of activity, kinetics, and morphology is improved compared to standard size single site catalyst. More specifically, the absence or a low concentration of comonomer (e.g. 1 -butene), the presence of small amounts of hydrogen (e.g. wherein the initial H2 / C2 ratio in the liquid phase is within the range of from 0.05 to 10 mol / kmol), and an operating temperature of 70°C or less, particularly within the range from 40°C to 50°C, result in improved particle morphology in terms of controlled catalyst particle fragmentation and growth.

Claims

23CLAIMS1 . A process for polymerising ethylene or for copolymerising ethylene and at least one alpha-olefin comonomer with 3 to 12 carbon atoms in the presence of a supported single site metallocene catalyst in a multi-stage polymerisation process comprising a pre-polymerisation stage and at least one further polymerisation stage downstream of the pre-polymerisation stage, to obtain an ethylene homo- or copolymer, the process comprising: contacting ethylene with particles of the supported single site metallocene catalyst in a pre-polymerisation stage; wherein the supported single site metallocene catalyst has a volume-based median particle size Dso within the range of from 5 to 20 pm measured according to ASTM 13320-1 ; and wherein the pre-polymerisation stage is carried out at a temperature of 70°C or less and in the presence of hydrogen at a hydrogemethylene ratio within the range of from 0.05 to 10.0 mol / kmol.

2. The process of claim 1 , wherein the pre-polymerisation stage is carried out at a temperature within the range of from 30°C to 69°C.

3. The process of any one of the preceding claims, wherein the supported single site metallocene catalyst has a span value within the range of from 0.1 to 1.3, wherein the span is defined as:D90-D10 span = -D50 wherein 90 volume % of the particles of the supported single site metallocene catalyst have a diameter of Dgo or less;50 volume % of the particles of the supported single site metallocene catalyst have a diameter of Dso (median particle size) or less; and10 volume % of the particles of the supported single site metallocene catalyst have a diameter of D or less, relative to the total volume of the particles of the supported single site catalyst.

4. The process of any one of the preceding claims, wherein the single site metallocene catalyst comprises a metallocene catalyst comprising a support, an activator, and a transition metal complex precursor.

5. The process of any one of the preceding claims, wherein the mean residence time of the supported single site catalyst in the pre-polymerisation stage is within the range of from 5 to 60 minutes.

6. The process of any one of the preceding claims, wherein the pre-polymerisation stage is carried out at a pressure within the range of from 10 to 70 bar.

7. The process of any one of the preceding claims, wherein the pre-polymerisation stage is carried out in a slurry phase reactor.

8. The process of any one of the preceding claims, wherein the process comprises copolymerising ethylene and at least one alpha-olefin comonomer with 3 to 12 carbon atoms, and wherein the amount of comonomer in the pre-polymerisation stage is within the range of from 0 to 20 mol per kmol of ethylene.

9. The process of claim 8, wherein the comonomer comprises an alpha-olefin comonomer with 3 to 8 carbon atoms.

10. The process of claim 9, wherein the comonomer comprises one or more of: propene, 1 -butene, 1 -hexene, and 1 -octene.

11. The process of any one of the preceding claims, wherein the pre-polymerisation stage is carried out in the presence of an inert diluent, such as a C3 to C10 alkane or mixtures thereof.

12. The process of claim 11 , wherein the pre-polymerisation stage is carried out at an ethylene concentration within the range of from 1 to 16 mol% based upon the total amount of inert diluent, ethylene, optional comonomer and hydrogen present during the pre-polymerisation stage.

13. The process of any one of the preceding claims, wherein the multi-stage polymerisation process comprises at least one polymerisation stage carried out in at least one slurry phase reactor, such as loop reactor.

14. The process of claim 13, wherein a final polymerisation stage is carried out in a gas phase reactor.

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