WO2025068073A1 - A gas-phase polymerization reactor and a process for preparing an olefin polymer - Google Patents

Abstract

A gas-phase polymerization reactor for the gas-phase polymerization of olefins; said gas polymerization reactor (1) is a multizone circulating reactor comprising: a first polymerization zone (2), which is configured and arranged for flowing upwards growing polyolefin particles; a second polymerization zone (3), which is configured and arranged 5 for flowing the growing polyolefin particles in a downward direction (DD); an upper connection zone (4) interconnecting top portions of the first and the second polymerization zone (2, 3); a lower connection zone (5) interconnecting lower portions of the first and the second polymerization zone (2, 3); and a valve assembly (6), which is located at the lower portion of the second polymerization zone (3) and comprises a two-10 blade butterfly valve having at least two moveable obstructions (7), each rotatable around a respective rotation axis (A), which are crosswise to said downward direction (DD).

Description

TITLE: “A GAS-PHASE POLYMERIZATION REACTOR AND A PROCESS FOR PREPARING AN OLEFIN POLYMER’’

D E S C R I P T I O N

TECHNICAL FIELD

[0001] The present invention relates to a gas-phase polymerization reactor for the gas-phase polymerization of olefins and a process for preparing an olefin polymer.

BACKGROUND OF THE INVENTION

[0002] Gas-phase polymerization methods are economical processes for the polymerization of olefins such as homopolymerizing ethylene or propylene or copolymerizing ethylene or propylene with other olefins. Suitable reactors for carrying out such gas-phase polymerizations are for example fluidized bed reactors, stirred gas-phase reactors or multizone circulating reactors. These methods are usually carried out in a gas phase comprising monomers and possibly comonomers and often additionally also other gaseous components such as polymerization diluents, for example nitrogen or alkanes, or hydrogen as molecular weight modifier or low-molecular weight reaction products. The obtained products are generally solid polyolefin particles which are formed by polymerization catalyst systems usually comprising particulate catalyst solids.

[0003] Examples of reactors for carrying out such gas-phase polymerizations are disclosed in US2018178180A 1 , US20200031957A 1 and US20110054127A 1.

[0004] US2018178180A1 discloses a process for starting a gas-phase olefin polymerization reaction for producing a particulate polyolefin by polymerizing one or more olefins in a multizone circulating reactor.

[0005] US20200031957A1 discloses a process for the preparation of heterophasic propylene copolymer compositions comprising a random propylene copolymer and an elastomeric propylene copolymer; the process being carried out in a multizone circulating reactor having two interconnected polymerization zones.

[0006] US20110054127A1 discloses a process for the polymerization of olefins in a gas-phase reactor having interconnected polymerization zones.

[0007] The Multi-Zone Circulating Reactor is formed by two zones. The first zone, usually called “riser” operates as fluidized gas reactor in transport conditions. The growing polymer particles leave the riser and enter in the second zone, called “downcomer”, where they fall downwards by gravity in a densified form. [0008] Fluidized bed reactors are reactors in which the polymerization takes place in a bed of polyolefin particles which is maintained in a fluidized state by feeding in a reaction gas mixture at the lower end of a reactor, usually below a gas distribution grid having the function of dispensing the gas flow, and taking off the gas again at the top of the fluidized bed reactor. The reaction gas mixture is then returned to the lower end of the reactor via a recycle line equipped with a centrifugal compressor and a heat exchanger for removing the heat of polymerization. The flow rate of the reaction gas mixture has to be sufficiently high firstly to fluidize the bed of finely divided polymer present in the polymerization zone and secondly to remove the heat of polymerization effectively.

[0009] Multizone circulating reactors are also, for example, described in WO 97/04015 Al and WO 00/02929 Al and have two interconnected polymerization zones, a riser, in which the growing polyolefin particles flow upward under fast fluidization or transport conditions and a downcomer, in which the growing polyolefin particles flow downward in a densified form under the action of gravity. The polyolefin particles leaving the riser enter the downcomer and the polyolefin particles leaving the downcomer are reintroduced into the riser, thus establishing a circulation of polymer between the two polymerization zones and the polymer is passed alternately a plurality of times through these two zones. In such polymerization reactors, a solid/gas separator is arranged above the downcomer to separate the polyolefin and reaction gaseous mixture coming from the riser. The growing polyolefin particles enter the downcomer and the separated reaction gas mixture of the riser is continuously recycled through a gas recycle line to one or more points of reintroduction into the polymerization reactor. Preferably, the major part of the recycle gas is recycled to the bottom of the riser. The recycle line is equipped with a centrifugal compressor and a heat exchanger for removing the heat of polymerization. Preferably, a line for feeding catalyst or a line for feeding polyolefin particles coming from an upstream reactor is arranged on the riser and a polymer discharge system is located in the bottom portion of the downcomer. The introduction of make-up monomers, comonomers, hydrogen and/or inert components may occur at various points along the riser and the downcomer.

[0010] One of the most important operating parameters to keep under control in a Multi-Zone Circulating Reactor (MZCR) is the temperature profile in the downcomer. The temperature profile is critical for reliable and stable MZCR operation, as too high temperature can cause polymer softening and the tackiness between adjacent polymer particles can lead to formation of polymer lumps.

[0011] One of the factors, from which the temperature profile depends, is the solid velocity profile in the section. [0012] The flow of the polymer bed is regulated in two ways: large-scale variations are obtained by acting on the opening of a valve (usually a butterfly valve) positioned at the bottom of the downcomer, while small changes are operated via an L-valve device which uses powder-free aeration gas.

[0013] The reactor can work with a homogeneous gas composition ('monomodal') or with two separate gas mixtures in the two legs ('bimodal'): the latter condition is obtained by feeding a barrier stream of gas (in same cases containing propylene) into the upper part of the downcomer, thereby differentiating the gas compositions of the two legs. During bimodal grade the barrier efficiency depends from the solid flowrate in the downcomer, that is regulated by the butterfly valve on the bottom of the downcomer.

[0014] It has been experimentally observed that with a typical standard butterfly valve consisting of a single blade, especially in production of polymers or another sticky or reactive material, the flow is not uniform across the section whenever the valve is operated in throttled position and there is a risk to have polymer blocked near the center of the reactor section.

[0015] The object of the present invention is to provide a gas-phase polymerization reactor and a process for preparing an olefin polymer, that overcome the drawbacks of the known art at least partially, and which eventually are, at the same time, simple and inexpensive to implement.

SUMMARY

[0016] According to the invention there is provided a gas-phase polymerization reactor and a process for preparing an olefin polymer according to the appended independent claims and, preferably, according to any one of the claims directly or indirectly depending on the independent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention is hereinafter described with reference to the accompanying drawings, which depict some non-limiting embodiments thereof, wherein:

[0018] - figure 1 is schematic and side view of a gas-phase polymerization reactor in accordance with the present disclosure;

[0019] - figure 2 is schematic and side view of a component of the gas-phase polymerization reactor of figure 1 in a first operative configuration;

[0020] - figure 3 is schematic and side view of the component of figure 2 in a second operative configuration; and

[0021] - figure 4 is a plan view of the component of figure 3. DETAILED DESCRIPTION

[0022] In accordance with a first aspect of the present invention it is herein provided gas-phase polymerization reactor 1 (figure 1) for the gas-phase polymerization of olefins. [0023] The gas-phase polymerization reactor 1 is a multizone circulating reactor comprising: a first polymerization zone 2, which is (a riser and is) configured (adapted) and arranged for flowing upwards growing polyolefin particles; a second polymerization zone 3, which is (a downcomer and is) configured (adapted) and arranged for flowing the growing polyolefin particles in a downward direction DD; an upper connection zone 4 interconnecting a top portion of the first polymerization zone 2 and a top portion of the second polymerization zone 3; a lower connection zone 5 interconnecting a lower portion of the first polymerization zone 2 and a lower portion of the second polymerization 3; and a valve assembly 6, which is located at the lower portion of the second polymerization zone 3 and comprises an at least two-blade butterfly valve (figures 2, 3 and 4) having (at least) two moveable obstructions 7, each rotating around a respective rotation axis A, which is (extends) crosswise (in particular, perpendicular) to said downward direction DD.

[0024] Surprisingly, it has been experimentally observed that thanks to such an at least two-blade butterfly valve the formation of polymer lumps is reduced and the jamming of the second polymerization zone 3 (and of the valve assembly 6) is therefore less likely. [0025] Advantageously but not necessarily, at least two-blade butterfly valve is a two- blade butterfly valve having (only) two moveable obstructions 7.

[0026] In particular, rotation axes A of the (at least) two moveable obstructions 7 are substantially at a same height of the polymerization zone 3.

[0027] In particular, rotation axes A of the (at least) two moveable obstructions 7 are parallel to each other.

[0028] In this way the opening and the shutting of the valve assembly 6 can be better controlled and obtained.

[0029] According to non-limiting embodiments, each moveable obstruction 7 has a perimeter edge having an inner stretch 7’ and an outer stretch 7”. The at least two-blade butterfly valve is moveable between a (fully) open configuration (figure 2) and a (fully) closed configuration (figure 3), in which said inner stretches 7’ faces each other and, in particular, said outer stretches 7” face an internal surface IS of the second polymerization zone 3. In particular, the internal surface IS delimits laterally the passage (of the flow) of the growing polyolefin particles at the at least two-blade butterfly valve.

[0030] According to some advantageous but non limiting embodiments, in the (fully) closed configuration, there is a distance of 0.5 to 10 cm (in particular, from 1 to 5 cm) between the outer stretches 7” and the internal surface IS. [0031] Advantageously but not necessarily, the shapes of the inner stretches 7’ of perimeter edges of said obstructions 7 are (in particular, have shapes) (substantially) complementary to each other.

[0032] Unexpectedly, also in this way the opening and the shutting of the valve assembly 6 can be even better controlled and obtained.

[0033] In particular, both said inner stretches 7’ are linear.

[0034] According to non-limiting embodiments, the at least two-blade butterfly valve comprises two rotation stem ST, each of which extends along a respective one of said rotational axes A. Each moveable obstruction 7 is solidly connected to a respective rotation stem ST (so that the rotational movement of each stem ST is transferred to the respective obstruction 7) and has a cross-section tapering from the rotation stem ST to the perimeter edge, in particular, to the inner stretch 7’ of the perimeter edge and, in particular, also to the outer stretch 7” of the perimeter edge.

[0035] Surprisingly, it has been experimentally observed that because of this shape of the obstructions 7 it is less likely that lumps of polymer are formed on the surfaces of the obstructions 7. It has been supposed that, thanks to the tapering of the obstructions 7 it seems that the polymer particles tend to slide more on the surface of the obstructions 7.

[0036] Advantageously but not necessarily, each moveable obstruction 7 has a first portion FP extending from the rotation axis A to the inner stretch 7’ of the perimeter edge and a second portion SP, which extends from the rotation axis A to the outer stretch 7” of the perimeter edge and is longer than said first portion FP.

[0037] Surprisingly, it has been experimentally observed that, in this, way the formation of polymer lumps is further reduced. It has been supposed that, in this way, the central part of the second polymerization zone 3 is less hindered and the heat distribution is more homogeneous. The radial thermal profile is more uniform.

[0038] In particular, the second portion SP is from 1. 1 to 5 times (in particular, 1.5 to 5 times; more in particular, from 2 to 5 times) longer than the first portion FP. In other words, the maximum distance between the outer stretch 7” and the rotation axis A is from 1.1 to 5 times (in particular, 1.5 to 5 times; more in particular, from 2 to 5 times) the maximum distance between the inner stretch 7’ and the rotation axis A. Specifically, the ratio of the length of said first portion FP, extending from the rotation axis A to the inner stretch 7’ of the perimeter edge, to said second portion SP, which extends from the rotation axis A to the outer stretch 7” of the perimeter edge, is in the range of from 1: 1.1 to 1:5, preferably from 1: 1.5 to 1:5, more preferably from 1:2 to 1:5 ,

[0039] Advantageously but not necessarily, the internal cross section of the second polymerization zone 3 at the at least two-blade butterfly valve is substantially circular. In this case, the outer stretch 7” of the perimeter edge of each moveable obstruction 7 has substantially the shape of a circular arch; in particular, in said closed configuration the combined shape of said outer stretches 7” is substantially circular.

[0040] Advantageously but not necessarily, the first polymerization zone 2 is a riser, which is configured (adapted) and arranged for flowing upwards growing polyolefin particles under fast fluidization (or transport) conditions; the second polymerization zone 3 is a downcomer, which is configured (adapted) and arranged for flowing the growing polyolefin particles in the downward direction DD in a densified form; the upper connection zone 4 interconnecting the top portion of the first polymerization zone 2 and the top portion of the second polymerization zone 3 so that polyolefin particles leaving the first polymerization zone 2 enter the second polymerization zone 3; the lower connection zone 5 interconnecting the lower portion of the first polymerization zone 2 and the lower portion of the second polymerization zone 3 so that polyolefin particles leaving the second polymerization zone 3 enter the first polymerization zone 2, thus establishing a circulation of polyolefin particles through the first polymerization zone 2 and the second polymerization zone 3.

[0041] In some non-limiting embodiments and in the first polymerization zone (riser - figure 1), fast fluidization conditions are established by feeding a gas mixture made from or containing one or more alpha-olefins at a velocity higher than the transport velocity of the polymer particles. In some embodiments, the velocity of the gas mixture is between 0.5 and 15 m/s, alternatively between 0.8 and 5 m/s. As used herein, the terms “transport velocity” and “fast fluidization conditions” are as defined in “D. Geldart, Gas Fluidisation Technology, page 155 et seq., J. Wiley & Sons Ltd., 1986”.

[0042] In some non-limiting embodiments (figure 1), in the second polymerization zone 3 (downcomer), the polymer particles flow under the action of gravity in a densified form, thereby achieving the high values of density of the solid (mass of polymer per volume of reactor) and approaching the bulk density of the polymer. As used herein, the term “densified form” of the polymer indicates that the ratio between the mass of polymer particles and the reactor volume is higher than 80% of the “poured bulk density” of the polymer. In some non-limiting embodiments and in the downcomer, the polymer flows downward in a plug flow and small quantities of gas are entrained with the polymer particles.

[0043] According to non-limiting embodiments, the two interconnected polymerization zones 2 and 3 are operated such that the gas mixture coming from the riser is totally or partially prevented from entering the downcomer by introducing into the upper part of the downcomer a liquid or gas stream, denominated “barrier stream”, having a composition different from the gaseous mixture present in the riser. In some embodiments, one or more feeding lines for the barrier stream are placed in the downcomer close to the upper limit of the volume occupied by the polymer particles flowing downward in a densified form.

[0044] In particular, the gas-phase polymerization reactor 1 comprises a gas/solid separator 9, located at the top portion of the second polymerization zone 3 and configured (adapted) and arranged for (at least partially) separating unreacted monomers from the polymer particles (in particular, so that the unreacted monomers are - at least partially - removed from the second polymerization zone 3).

[0045] In some non-limiting embodiments, the feed of the barrier stream causes a difference in the concentrations of monomers (olefins) or a molecular weight regulator (e.g. hydrogen) inside the riser and the downcomer, thereby producing a bimodal polymer.

[0046] In some non-limiting embodiments, the gas-phase polymerization process involves a reaction mixture made from or containing the gaseous monomers, inert polymerization diluents and chain transfer agents to regulate the molecular weight of the polymeric chains. In some embodiments, hydrogen is used to regulate the molecular weight. In some non-limiting embodiments, the polymerization diluents are selected from C2-C8 alkanes, alternatively propane, isobutane, isopentane and hexane. In some nonlimiting embodiments, propane is used as the polymerization diluent in the gas-phase polymerization.

[0047] In this text, unless otherwise specified, “C_x-C_y” refers to a group and/or a compound which is intended having x to y carbon atoms.

[0048] More precisely but not necessarily, the barrier stream is made from or contains: i. from 10 to 100% by mol of propylene, based upon the total moles in the barrier stream; ii. from 0 to 80% by mol of ethylene, based upon the total moles in the barrier stream; iii. from 0 to 30% by mol of propane, based upon the total moles in the barrier stream; and iv. from 0 to 5% by mol of hydrogen, based upon the total moles in the barrier stream.

[0049] In particular, the separated unreacted monomers, optionally together with polymerization diluents, flow up to the top of separator 9 and are successively recycled to the bottom of the riser 1 via the recycle line 10.

[0050] Olefins which may be polymerized in the gas-phase polymerization reactors of the present disclosure are especially 1-olefms, i.e. hydrocarbons having terminal double bonds, without being restricted thereto. Preference is given to nonpolar olefinic compounds. Particularly preferred 1-olefms are linear or branched C2 -Ci2-l-alkenes, in particular linear C2 -Cio-l-alkenes such as ethylene, propylene, 1-butene, 1-pentene, 1- hexene, 1-heptene, 1-octene, 1-decene or branched C2 -Cio-l-alkenes such as 4-methyl-l - pentene, conjugated and nonconjugated dienes such as 1,3-butadiene, 1,4-hexadiene or 1,7- octadiene. It is also possible to polymerize mixtures of various 1-olefms. Suitable olefins also include ones in which the double bond is part of a cyclic structure which can have one or more ring systems. Examples are cyclopentene, norbomene, tetracyclododecene or methylnorbomene or dienes such as 5-ethylidene-2-norbomene, norbomadiene or ethylnorbomadiene. It is also possible to polymerize mixtures of two or more olefins.

[0051] In particular, the gas-phase polymerization reactor is configured (adapted) for the homopolymerization or copolymerization of ethylene and/or propylene and is especially preferred for the homopolymerization or copolymerization of propylene. Preferred comonomers in propylene polymerization are up to 40 wt.% of ethylene, 1 -butene and/or 1-hexene, preferably from 0.5 wt.% to 35 wt.% of ethylene, 1-butene and/or 1- hexene. As comonomers in ethylene polymerization, preference is given to using up to 20 wt.%, more preferably from 0.01 wt.% to 15 wt.% and especially from 0.05 wt.% to 12 wt.% of Cs-Cs-l -alkenes, in particular 1-butene, 1 -pentene, 1-hexene and/or 1 -octene. Particular preference is given to polymerizations in which ethylene is copolymerized with from 0. 1 wt.% to 12 wt.% of 1-hexene and/or 1-butene.

[0052] Advantageously but not necessarily, the gas-phase polymerization reactor 1 comprises one or more lines M, configured to feed a gaseous mixture comprising one or more olefin monomers, optionally the molecular weight regulator (such as hydrogen) and optionally a polymerization diluent to the gas recycle line 10, according to the knowledge of the person skilled in art.

[0053] According to some non-limiting embodiments, the gas-phase polymerization reactor 1 comprises a line 11 for continuously introducing catalyst components into the first polymerization zone 2 (riser). In particular, line 11 is connected to the lower portion of the first polymerization zone 2.

[0054] Advantageously but not necessarily, the gas-phase polymerization reactor 1 also comprises discharge line 8, which is configured (adapted) for discharging the produced polymer from reactor 1 itself and, in particular, connected to the second polymerization zone 3 (more in particular, in the area of the lower portion thereof).

[0055] More precisely but not compulsorily, the discharge line 7 is arranged between the top portion of the second polymerization zone 3 and the valve assembly 6 (above the valve assembly 6).

[0056] Advantageously but not necessarily, the discharge line 7 comprises a control valve (not shown in Fig. 1) for continuously controlling the flow rate of polymer produced through discharge line 7.

[0057] In some cases, additional polymer discharge lines with respect to discharge line 7 can conveniently be placed in the bottom part of the downcomer.

[0058] Advantageously but not necessarily, the gas-phase polymerization reactor 1 also comprises a dosing line 12, which is located between the valve assembly 6 and the top portion of the second polymerization zone 3 (in particular, between the valve assembly 6 and the discharge line 7) and is configured (adapted) to feed a dosing gas into (the lower part of) the downcomer (i.e. the second polymerization zone 3).

[0059] According to some non-limiting embodiments, said dosing line 12 can be conveniently split into multiple lines that can suitably be arranged around a section of the reactor, preferably in an even number (e.g. two, four, six, eight). The dosing gas to be introduced through dosing line 12 is conveniently taken from the recycle line 10. The flow rate of dosing gas is adjusted by means of one or more control valves suitably arranged on dosing line 12.

[0060] In particular (in actual use), the flow of polymer particles circulated between downcomer 3 and riser 2 is conveniently adjusted by varying the opening of the valve assembly 6 (at the bottom of the downcomer) and/or by varying the flow rate of said dosing gas entering the downcomer via dosing line 12.

[0061] Advantageously but not necessarily, the gas-phase polymerization reactor 1 also comprises a carrier line 13 for introducing a carrier gas at the lower connection zone 5. In particular, the flow rate of carrier gas is adjusted by means of a control valve 14, which is suitably arranged on line 13.

[0062] In some non-limiting embodiments, the carrier gas is taken from the gas recycle line 10. In particular, the gas recycle stream of line 5 is first subjected to compression by a compressor 15 and a minor percentage of the recycle stream passes through a line 16, thereby entering the lower connection zone 5 and diluting the solid phase of polymer flowing through the lower connection zone 5. The major part of the recycle stream, downstream the compressor 15, is subjected to cooling in a heat exchanger 16 and successively introduced via line 17 at the bottom of the riser 2 (i.e. the first polymerization zone) at a high velocity, thereby ensuring fast fluidization conditions in the polymer bed flowing along the riser 2.

[0063] More precisely but not necessarily, the carrier gas merges with the densified polymer coming from downcomer 3 at the inlet portion of the lower connection zone 5, after exiting the slits of a gas distribution grid (not shown). In particular, the top end of the distribution grid is coincident with the inlet of the lower connection zone 5, and the distribution grid. The gas distribution grid is formed by a plurality of trays fixed to lower connection zone 5 to form slits in the overlapping area of adjacent trays. More precisely, The gas distribution grid is as described in Patent Cooperation Treaty Publication No. WO 2012/031986.

[0064] In some non-limiting embodiments, the polymerization reactor is operated by adjusting the polymerization conditions and the concentration of monomers and hydrogen in the riser and in the downcomer, thereby tailoring the RAHECO (heterophasic copolymer compositions). In some heterophasic propylene copolymer compositions embodiments, the gas mixture entraining the polymer particles and coming from the riser is partially or totally prevented from entering the downcomer, thereby polymerizing two different monomers compositions in the riser and the downcomer. In some non-limiting embodiments, a gaseous or liquid barrier stream is fed through a line placed in the upper portion of the downcomer. In some non-limiting embodiments, the barrier stream has a composition different from the gas composition present inside the riser. In some non-limiting embodiments, the flow rate of the barrier stream is adjusted such that an upward flow of gas counter-current to the flow of the polymer particles is generated. In some e non-limiting embodiments, the counter-current is at the top of the downcomer, thereby acting as a barrier to the gas mixture coming from the riser. In some embodiments, the barrier effect at the top of the downcomer occurs as described in European Patent Application No. EP 1012195 Al.

[0065] In some non-limiting embodiments and by feeding hydrogen in the riser, the molecular weight of the RACO (random copolymer) component is lowered, thereby yielding a RAHECO with high melt flow rate. In some embodiments and at the same time, feeding a barrier stream with little or no hydrogen yields a high molecular weight BIPO (elastomeric copolymer) component in the downcomer.

[0066] In some non-limiting embodiments, the gas-phase polymerization reactor 1, having the two interconnected polymerization zones, is placed upstream or downstream one or more other polymerization reactors based on liquid- or gas-phase technologies, thereby giving rise to a sequential multistage polymerization process. For instance, a fluidized bed reactor is used to prepare a first polymer component, which is successively fed to the gas-phase polymerization reactor 1 of the present disclosure to prepare a second polymer component and a third polymer component.

[0067] In some non-limiting embodiments, the process is carried out by using olefin polymerization catalysts, alternatively titanium-based Ziegler-Natta-catalysts, Phillips catalysts based on chromium oxide, or single-site catalysts. As used herein, single-site catalysts are catalysts based on chemically uniform transition metal coordination compounds, such as metallocene catalysts. In some embodiments, mixtures of two or more different catalysts are used. In some embodiments, the mixed catalyst systems are designated as hybrid catalysts.

[0068] In some embodiments, the process is carried out in the presence of Ziegler- Natta catalysts made from or containing: i. a solid catalyst component made from or containing Mg, Ti, a halogen and an electron donor compound (internal donor), ii. an alkylaluminum compound, and iii. optionally, an electron-donor compound (external donor).

[0069] In some non-limiting embodiments, component (i) is prepared by contacting a magnesium halide, a titanium compound having at least a Ti-halogen bond, and optionally an electron donor compound. In some non-limiting embodiments, the magnesium halide is MgCb in active form as a support for Ziegler-Natta catalysts. In some non-limiting embodiments, the titanium compounds are TiCfi. TiCh. or Ti-haloalcoholates of formula Ti(OR)n-yX_y , where n is the valence of titanium, y is a number between 1 and n-1, X is a halogen and R is a hydrocarbon radical having from 1 to 10 carbon atoms, can also be used. [0070] In some non-limiting embodiments, electron donor compounds for preparing Ziegler type catalysts are selected from the group consisting of alcohols, glycols, esters, ketones, amines, amides, nitriles, alkoxysilanes and aliphatic ethers. In some non-limiting embodiments, these electron donor compounds are used alone or in mixtures with other electron donor compounds.

[0071] In some non-limiting embodiments, other solid catalyst components used are based on a chromium oxide supported on a refractory oxide, such as silica, and activated by a heat treatment. Catalysts obtainable from those components consist of chromium (VI) trioxide chemically fixed on silica gel. These catalysts are produced under oxidizing conditions by heating the silica gels that have been doped with chromium (III) salts (precursor or precatalyst). During this heat treatment, the chromium (III) oxidizes to chromium (VI), the chromium (VI) is fixed and the silica gel hydroxyl group is eliminated as water.

[0072] In some non-limiting embodiments, other solid catalyst components used are single-site catalysts supported on a carrier, such as metallocene catalysts, made from or containing: i. at least a transition metal compound containing at least one n bond; and ii. at least a cocatalyst selected from an alumoxane or a compound able to form an alkylmetallocene cation.

[0073] In some non-limiting embodiments, when the catalyst includes an alkylaluminum compound, such as in Ziegler-Natta catalysts, the molar ratio of solid catalyst component to alkylaluminum compound introduced into the polymerization reactor is in the range from 0.05 to 3, alternatively from 0.1 to 2, alternatively from 0.5 to 1.

[0074] In some non-limiting embodiments, the catalysts are subjected to prepolymerization before being fed to the polymerization reactor. In some embodiments, the prepolymerization occurs in a loop reactor, In some non-limiting embodiments, the prepolymerization of the catalyst system is carried out at a low temperature, alternatively in a range of from 0° C. to 60° C. [0075] In some non-limiting embodiments, additives, fillers and pigments are added. In some embodiments, the additional components are selected from the group consisting of nucleating agents, extension oils, mineral fillers, and other organic and inorganic pigments. In some embodiments, the inorganic fillers are selected from the group consisting of talc, calcium carbonate and mineral fillers and affect mechanical properties, such as flexural modulus and HDT. In some embodiments, talc has a nucleating effect.

[0076] In some non-limiting embodiments, the nucleating agents are added in quantities ranging from 0.05 to 2% by weight, alternatively from 0.1 to 1% by weight, with respect to the total weight.

[0077] The operating parameters, such as temperature and pressure, are those that are usual in gas- phase catalytic polymerization processes. For example, in both riser and downcomer the temperature is generally comprised between 60°C and 120°C, while the pressure can range from 5 to 50 bar.

[0078] In accordance with a second aspect of the present disclosure, it is also herewith provided a process for preparing an olefin polymer, comprising homopolymerizing a first olefin or copolymerizing the first olefin and one or more second olefins (in particular, at temperatures of from 20 to 200 °C and pressures of from 0.5 to 10 MPa) in the presence of a polymerization catalyst, wherein the polymerization is carried out in the gas-phase polymerization reactor 1 as above described in accordance with the first aspect of the present disclosure. In particular, the homopolymerizing a first olefin or copolymerizing the first olefin and one or more second olefins is carried at a temperature comprised between 60°C and 120°C (and a pressure from 5 to 50 bar.

[0079] Advantageously but not necessarily, the process comprises feeding the first olefin with or without one or more the second olefin/s into the gas-phase polymerization reactor 1, contacting the olefins and the polymerization catalyst under reaction conditions and discharging the polymer product from the gas-phase polymerization reactor. The growing polymer particles flow upward through the first polymerization zone 2 under fast fluidization or transport conditions, leave the first polymerization zone 2, pass through the gas/solid separator 9 and enter the second polymerization zone where the polymer particles flow downward under the action of gravity, leave the second polymerization zone 3 and are at least partially reintroduced into the first polymerization zone 2, thus establishing a circulation of polymer between the first polymerization zone 2 and the second polymerization zone 3.

[0080] In some non-limiting embodiments, the second polymerization zone 3 comprises a bed of densified polymer particles.

[0081] The gas-phase polymerization process of the invention allows the preparation of a large number of polyolefins. Examples of polymers that can be obtained are: high- density polyethylenes (HDPEs having relative densities higher than 0.940) including ethylene homopolymers and ethylene copolymers with a-olefins having 3 to 12 carbon atoms; linear polyethylenes of low density (LLDPEs having relative densities lower than 0.940) and of very low density and ultra low density (VLDPEs and ULDPEs having relative densities lower than 0.920 down to 0.880) consisting of ethylene copolymers with one or more a-olefins having 3 to 12 carbon atoms; elastomeric terpolymers of ethylene and propylene with minor proportions of diene or elastomeric copolymers of ethylene and propylene with a content of units derived from ethylene of between about 30 and 70% by weight; isotactic polypropylene and crystalline copolymers of propylene and ethylene and/or other a-olefins having a content of units derived from propylene of more than 85% by weight; isotactic copolymers of propylene and a-olefins, such as 1 -butene, with an a- olefin content of up to 30% by weight; impact-resistant propylene polymers obtained by sequential polymerisation of propylene and mixtures of propylene with ethylene containing up to 30% by weight of ethylene; atactic polypropylene and amorphous copolymers of propylene and ethylene and/or other a-olefins containing more than 70% by weight of units derived from propylene.

[0082] In particular, gas-phase polymerization reactor 1 (with the exception of the structure of the valve assembly 6) and its functioning are as disclosed in one or more of the following patent applications: US2018178180A1, US20200031957A1,

US20110054127A1, WO 97/04015 Al, WO 00/02929 Al, US2020031957A1 and US20200031957.

[0083] Unless expressly indicated to the contrary, the contents of the references (articles, books, and patent applications etc.) cited in this text are herein recalled in full. In particular, the above-mentioned references are herein incorporated by reference.

Claims

CLAIMS What is claimed is:

1. A gas-phase polymerization reactor for the gas-phase polymerization of olefins; said gas-phase polymerization reactor (1) is a multizone circulating reactor comprising: a first polymerization zone (2), which is a riser and is configured and arranged for flowing upwards growing polyolefin particles; a second polymerization zone (3), which is a downcomer and is configured and arranged for flowing the growing polyolefin particles in a downward direction (DD); an upper connection zone (4) interconnecting a top portion of the first polymerization zone (2) and a top portion of the second polymerization zone (3); a lower connection zone (5) interconnecting a lower portion of the first polymerization zone (2) and a lower portion of the second polymerization (3); and a valve assembly (6), which is located at the lower portion of the second polymerization zone (3) and comprises an at least two-blade butterfly valve having at least two moveable obstructions (7), each rotatable around a respective rotation axis (A), which is crosswise to said downward direction (DD), wherein each moveable obstruction (7) has a perimeter edge having an inner stretch (7’) and an outer stretch (7”); the at least two-blade butterfly valve is moveable between an open configuration and a closed configuration, in which said inner stretches (7’) faces each other, and wherein each moveable obstruction (7) has a first portion (FP) extending from the rotation axis (A) to the inner stretch (7’) of the perimeter edge and a second portion (SP), which extends from the rotation axis (A) to the outer stretch (7”) of the perimeter edge and is longer than said first portion (FP).

2. The gas-phase polymerization reactor of claim 1, wherein said rotation axes (A) of the at least two moveable obstructions (7) are parallel to each other and substantially at a same height of the polymerization zone 3.

3. The gas-phase polymerization reactor of claim 1 or 2, wherein the shapes of the inner stretches (7’) of perimeter edges of said moveable obstructions (7) have shapes substantially complementary to each other.

4. The gas-phase polymerization reactor of any of claims 1 to 3, wherein said at least two-blade butterfly valve comprises two rotation stem (ST), each of which extends along a respective one of said rotational axes (A); each moveable obstruction (7) is solidly connected to a respective rotation stem (ST) and has a cross-section tapering from the rotation stem (ST) to the perimeter edge, in particular, to the inner stretch (7’) of the perimeter edge and, in particular, also to the outer stretch (7”) of the perimeter edge.

5. The gas-phase polymerization reactor of any of claims 1 to 4, wherein the second portion (SP) is from 1.1 to 5 times longer than the first portion (FP).

6. The gas-phase polymerization reactor of any of claims 1 to 5, wherein the second polymerization zone (3) has an internal surface (IS); in the closed configuration, there is a distance of 0.5 to 10 cm between the outer stretches (7”) and the internal surface (IS).

7. The gas-phase polymerization reactor of any of claims 1 to 6, wherein the internal cross section of the second polymerization zone (3) at the at least two-blade butterfly valve is substantially circular; the outer stretch (7”) of the perimeter edge of each moveable obstruction (7) has substantially the shape of a circular arch.

8. The gas-phase polymerization reactor of any of claims 1 to 7, and comprising a dosing line (12) for feeding a dosing gas into the lower part of the second polymerization zone (3) at one or more positions above the at least two-blade butterfly valve.

9. The gas-phase polymerization reactor of any of claims 1 to 8, wherein: the first polymerization zone (2) is a riser, which is configured and arranged for flowing upwards growing polyolefin particles under fast fluidization conditions; the second polymerization zone (3) is a downcomer, which is configured and arranged for flowing the growing polyolefin particles in the downward direction (DD) in a densified form; the upper connection zone (4) interconnecting the top portion of the first polymerization zone (2) and the top portion of the second polymerization zone (3) so that polyolefin particles leaving the first polymerization zone (2) enter the second polymerization zone (3); the lower connection zone (5) interconnecting the lower portion of the first polymerization zone (2) and the lower portion of the second polymerization zone (3) so that polyolefin particles leaving the second polymerization zone (3) enter the first polymerization zone (2), thus establishing a circulation of polyolefin particles through the first polymerization zone (2) and the second polymerization zone (3).

10. The gas-phase polymerization reactor of any of claims 1 to 9, and comprising a gas/solid separator (9) configured and arranged for at least partially separating unreacted monomers from the polymer particles; the gas/solid separator is located at the top portion of the second polymerization zone (3) so that the unreacted monomers are at least partially removed from the second polymerization zone (3).

11. A process for preparing an olefin polymer, comprising homopolymerizing a first olefin or copolymerizing the first olefin and one or more second olefins at temperatures of from 20 to 200 °C and pressures of from 0.5 to 10 MPa in the presence of a polymerization catalyst, wherein the polymerization is carried out in the gas-phase polymerization reactor (1) of anyone of claims 1 to 10.

12. The process of claim 11, comprising feeding the first olefin with or without one or more the second olefin/s into the gas-phase polymerization reactor (1), contacting the olefins and the polymerization catalyst under reaction conditions and discharging the polymer product from the gas-phase polymerization reactor, wherein the growing polymer particles flow upward through the first polymerization zone (2) under fast fluidization or transport conditions, leave the first polymerization zone (2), pass through the gas/solid separator (9) and enter the second polymerization zone (3) where the polymer particles flow downward under the action of gravity, leave the second polymerization zone (3) and are at least partially reintroduced into the first polymerization zone (2), thus establishing a circulation of polymer between the first polymerization zone (2) and the second polymerization zone (3).

13. The process of claim 12, wherein the second polymerization zone comprises a bed of densified polymer particles.