Multi-stage process for olefin polymerization

Abstract

A process for transferring olefin polymer particles from an upstream polymerization reactor to a downstream polymerization reactor or to a downstream low pressure degasser, the process comprising the steps of: (a) withdrawing a reaction material from the first upstream polymerization reactor, the reaction material comprising olefin polymer particles and a process fluid, the process fluid contains a first amount of a molecular weight modifier; (b) feeding the taken-out reaction material into the top of a gas-solid separator, wherein the olefin polymer particles flow downwards under the action of gravity in the gas-solid separator; (c) introducing a gaseous stream free of a molecular weight regulator from the bottom of the gas-solid separator, thereby extracting a process gas containing a molecular weight regulator from the reaction material; and (d) discharging the olefin polymer particles from the bottom of the gas-solid separator into the downstream reactor or into the downstream low pressure degasser.

Description

Technical Field

[0001] This disclosure relates to the field of multi-stage processes for olefin polymerization, and more specifically to the transfer of polymer particles from an upstream polymerization reactor to a downstream polymerization reactor. Background Technology

[0002] Olefin polymerization in two or more reactors connected in series can produce olefin polymers with improved mechanical properties. This is achieved by applying polymerization conditions in a second or subsequent reactor that differ from those present in the upstream reactor. Typically, the olefin polymer is grown on particles containing a catalyst component that continues to exert its catalytic effect after the polymer particles are transferred to the downstream reactor. The polymer produced in the first reactor is transferred to a second reactor, where polymerization continues under different conditions. Thus, by maintaining different monomer concentrations in each reactor, different fractions of the polymer can be grown on the same catalyst particles.

[0003] Examples of polymers that can be produced by multi-stage polymerization processes include bimodal or multimodal polymers obtained by maintaining different concentrations of chain terminators (such as hydrogen) in each reactor. Other examples are random copolymers of propylene obtained by polymerizing different amounts of comonomers in different reactors. Still other examples are multiphase (or impact or block) copolymers of propylene obtained by polymerizing different (co)monomers and / or different amounts of comonomers in different reactors.

[0004] When the polymer is transferred from the first polymerization reactor to the continuous reactor, it contains a significant amount of entrained hydrocarbons that must be recovered. Therefore, the polymer is typically transported in a medium-pressure vessel, optionally preheated to allow the polymerization medium to evaporate, and in this medium-pressure vessel, the polymer is separated from the gaseous hydrocarbons.

[0005] The polymer is then conveyed from the degassing vessel to the downstream reactor. This polymer stream still carries a significant amount of gas with the composition of the upstream reactor, which will be diverted to the downstream reactor. This will prevent the maintenance of different polymerization conditions in the downstream reactor.

[0006] In particular, when it comes to a multi-phase copolymer having a matrix with a relatively low molecular weight (high melt flow rate) and a rubber phase with a high molecular weight (high intrinsic viscosity of the xylene soluble fraction (XSIV)), such as those described in WO 2022 / 017757, the hydrogen content in the entraining gas is so high that the composition of the downstream reactor is altered beyond values tolerable for the quality of the product. Another reason for differentiating the polymerization conditions in the downstream degassing section is that, when producing propylene homopolymers or copolymers, it is necessary to minimize the amount of hydrogen and / or comonomer (e.g. ethylene or 1-butene) carried over for economic or process reasons.

[0007] Therefore, in the above and other cases, there is a need to minimize the contamination of the different reactor environments of the multi-stage polymerization process and / or of the downstream degassing section.

[0008] According to EP-B-192427, the transfer of the polymer from the upstream gas phase reactor to the downstream gas phase reactor is carried out by means of a gas stream comprising a gas mixture from the gas recirculation line of the downstream reactor. Said gas mixture is cooled to a temperature at least 20°C lower than the temperature of the downstream reactor. The transfer device described comprises at least three separate vessels: a discharge vessel connected to the upstream reactor and equipped with valves to prevent excessive withdrawal of polymer from the upstream reactor; a decompression chamber equipped with valves and connected to the discharge vessel; a compression chamber equipped with valves and connected to the downstream reactor. The path of the polymer through this series of vessels and associated valves and pipes is rather tortuous and, in the case where the polymer has a tendency to stick or to compact, then undesired polymer agglomerates can be generated and, eventually, lumps that impair the operation of the plant. Most importantly, the above transfer device has the great disadvantage of not being able to provide a continuous transfer of the polymer from the upstream gas phase reactor to the downstream reactor, as disclosed in column 14, lines 14-19 of EP-B-192427, where it is pointed out that all the operations of withdrawal, decompression, compression, transfer and introduction of the polymer into the downstream reactor are carried out periodically.

[0009] A similar transfer device is disclosed in EP-B-050013. According to this patent, the transfer device comprises a vessel into which the polymer is discharged, said vessel defining an inert gas zone in which inert gas is passed upwards from the bottom to replace a substantial part of the reaction gas mixture from the upstream reactor. Subsequently, the polymer, which is always maintained in the above-mentioned inert gas atmosphere, is transferred to a cell (polymer collection zone) which is connected to the gas reaction mixture from the downstream reactor. The replacement of the gas reaction mixture with inert gas helps to reduce or prevent polymerization in the transfer device, thus eliminating the deposition of polymer on the walls of the transfer device and its clogging. However, the proposed solution has the drawback that the reaction gas mixture in the second polymerization reactor is significantly enriched with said inert gas. This makes it necessary to introduce a large amount of additional olefin monomer in the downstream reactor, which can require an increase in the reactor size or an increase in the total pressure of the gas in this reactor. Moreover, the transfer device described in EP-B-050013 also fails to provide a continuous transfer of polymer from the upstream reactor to the downstream reactor. This is because the cell in which the polymer is collected must first be loaded with polymer and can only continuously discharge polymer therefrom by opening the line connected to the gas reaction mixture from the downstream reactor.

[0010] EP-B-503791 relates to a multi-stage gas phase process for producing a bimodal ethylene polymer blend in a sequence of two fluidized bed reactors. In a first reactor a high molecular weight (HMW) polyethylene is produced, which is subsequently transferred to a second reactor in which a low molecular weight (LMW) polyethylene is produced. The transfer device comprises a discharge tank for collecting the HMW polymer discharged from the first reactor and a transfer hose connected to the second gas phase reactor. Periodically, when a sufficient amount of HMW polymer has been formed in the first reactor, the polymer and catalyst are transferred to the discharge tank, where the reaction gas entrained with the polymer is discharged from the top of the discharge tank. Once the desired amount of polymer has been introduced into the discharge tank, the transfer system to the second reactor is activated by opening a suitable valve to press the HMW polymer into the transfer hose. The transfer hose is thus isolated from the upstream discharge tank and pressurized with reactor recycle gas from the downstream reactor. The transfer device described in EP 503791 effectively prevents the reaction gas of the upstream reactor from entering the downstream reactor, however this transfer device cannot ensure a continuous and reliable transfer of polymer between the two gas phase reactors, since all operations of polymer discharge, degassing, pressurization, transfer and introduction of the polymer powder into the downstream reactor are performed intermittently.

[0011] WO 2008 / 058839 and WO 2009 / 037080 disclose a multistage olefin polymerization process in which the polymer is discharged from an upstream reactor into a downstream reactor through a transfer device comprising a pair of lockhoppers operating in parallel intermittently. This system operates at low pressure, so that both the separated gas and the polymer need to be compressed and then conveyed separately to the upstream and downstream reactors, which causes a continuous energy consumption.

[0012] The polymer transfer systems disclosed in the above prior art transfer systems have serious drawbacks. If the devices belonging to the transfer system are not working properly or are clogged, it is necessary to shut down the entire polymerization plant. Moreover, from an operational point of view, the discontinuous operation of the polymer transfer, when a batch of polymer product is introduced into the downstream reactor, causes a significant fluctuation of the polymer bed level within the downstream reactor. This fluctuation affects some of the operating parameters in the downstream reactor and can have a considerable impact on the quality of the polymer produced.

[0013] The above technical problems have not been properly solved in the specific case of multistage polymerization in an upstream slurry or gas phase reactor and a downstream gas phase reactor, so that when it is necessary to polymerize a specific olefin monomer in the upstream reactor and not in the downstream reactor, it is desirable to prevent said monomer from entering the downstream reactor. Moreover, when a lower concentration of hydrogen or other molecular weight regulator is required in the downstream reactor, it is desirable to prevent hydrogen from entering the downstream reactor.

[0014] Therefore, it is desirable to provide a method for transferring polymer particles from an upstream reactor to a downstream reactor in an efficient and reliable manner, without causing any undesired change to the composition of the gas present in the downstream reactor. The solution to the above problems is achieved by providing embodiments as described herein below and as characterized in the claims. SUMMARY

[0015] According to one aspect, the present disclosure provides a method for transferring olefin polymer particles from a first upstream polymerization reactor to a second downstream polymerization reactor or to a downstream low-pressure degasser, the method comprising the steps of:

[0016] (a) withdrawing a reaction mass from the first upstream polymerization reactor, the reaction mass comprising olefin polymer particles and a process fluid, the process fluid being gaseous or liquid in the polymer slurry and containing a first amount of a molecular weight regulator;

[0017] (b) passing the withdrawn reaction mass into the top of a gas-solids separator, in which the olefin polymer particles flow downward under the action of gravity;

[0018] (c) introducing a gaseous stream from the bottom of the gas-solids separator containing the reaction mass, wherein the gaseous stream is free of molecular weight regulators and / or free of other reactive comonomers and consists of olefin monomers to be polymerized, thereby extracting at least a portion of the process gas containing molecular weight regulators from the reaction mass; and

[0019] (d) continuously discharging the reaction mass containing a second amount of molecular weight regulators from the bottom of the gas-solids separator into the second downstream reactor or into the downstream low-pressure degasser, wherein the second amount of molecular weight regulators is lower than the first amount of molecular weight regulators.

[0020] According to another aspect, the present disclosure provides a continuous olefin polymerization process comprising the above-described method for transferring olefin polymer particles from a first upstream polymerization reactor to a second downstream polymerization reactor or to a downstream low-pressure degasser. More specifically, the present disclosure provides a continuous olefin polymerization process comprising polymerizing at least one olefin monomer in a series of at least two polymerization reactors, the process comprising a method for transferring olefin polymer particles from a first upstream polymerization reactor to a second downstream polymerization reactor or to a downstream low-pressure degasser, the method comprising the steps of:

[0021] (a) withdrawing a reaction mass from the first upstream polymerization reactor, the reaction mass comprising olefin polymer particles and a process fluid, the process fluid being gaseous or liquid in a polymer slurry and containing a first amount of molecular weight regulators

[0022] (b) passing the withdrawn reaction mass into a top portion of a gas-solids separator, in which the olefin polymer particles flow downward under the influence of gravity;

[0023] (c) introducing a gaseous stream from the bottom of the gas-solids separator containing the reaction mass, wherein the gaseous stream is free of molecular weight regulators and / or free of other reactive comonomers and consists of olefin monomers to be polymerized, thereby extracting at least a portion of the process gas containing molecular weight regulators from the reaction mass; and

[0024] (d) continuously discharging the olefin polymer particles containing a second amount of molecular weight regulators from the bottom of the gas-solids separator into the second downstream reactor or into the downstream low-pressure degasser, wherein the second amount of molecular weight regulators is lower than the first amount of molecular weight regulators.

[0025] According to yet another aspect, the present disclosure provides a continuous olefin polymerization process for producing a heterophasic propylene copolymer, the continuous olefin polymerization process comprising the above described method for transferring olefin polymer particles from a first upstream polymerization reactor to a second downstream polymerization reactor or to a downstream low pressure degasser. More specifically, the present disclosure provides a continuous olefin polymerization process comprising the steps of:

[0026] (A) carrying out a polymerization of propylene in a first upstream reactor, optionally in the presence of up to 5 wt% of a copolymer selected from ethylene or an alpha-olefin having 4 to 10 carbon atoms,

[0027] (B) copolymerizing ethylene with propylene or an alpha-olefin having 4 to 10 carbon atoms in a second downstream reactor in the presence of the polymer produced in the first upstream reactor,

[0028] The process comprises a method for transferring olefin polymer particles from the first upstream polymerization reactor to the second downstream polymerization reactor or to a downstream low pressure degasser, the method comprising the steps of:

[0029] (a) withdrawing a reaction mass from the first upstream polymerization reactor, the reaction mass comprising olefin polymer particles and a process fluid, the process fluid being gaseous or liquid in the polymer slurry and containing a first amount of a molecular weight regulator

[0030] (b) passing the withdrawn reaction mass into a top portion of a gas-solids separator, in which the olefin polymer particles flow downward under the influence of gravity;

[0031] (c) introducing a gaseous stream from a bottom portion of the gas-solids separator containing the reaction mass, wherein the gaseous stream is free of molecular weight regulator and / or free of other reactive comonomers and consists of olefin monomers to be polymerized, thereby extracting at least a portion of the process gas containing the molecular weight regulator from the reaction mass; and

[0032] (d) continuously discharging the olefin polymer particles containing a second amount of a molecular weight regulator from the bottom portion of the gas-solids separator into the second downstream reactor or into the downstream low pressure degasser, wherein the second amount of molecular weight regulator is lower than the first amount of molecular weight regulator. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 A gas-solids separator or flash chamber used in the process of the present disclosure for transferring polymer particles from an upstream polymerization reactor to a downstream polymerization reactor is exemplified.

[0034] Figure 2A combination of equipment for a continuous olefin polymerization process including the polymer particle transfer process of the present disclosure is schematically depicted, the combination of equipment including an upstream slurry loop reactor and a downstream fluidized bed gas phase reactor and a downstream low pressure degasser.

[0035] Figure 3 A combination of equipment for a continuous olefin polymerization process including the polymer particle transfer process of the present disclosure is schematically depicted, the combination of equipment including an upstream slurry loop reactor and a downstream fluidized bed gas phase reactor and a downstream low pressure degasser.

[0036] The embodiments illustrated in the above-mentioned figures are provided for illustrative purposes and are not for limiting purposes. DETAILED DESCRIPTION

[0037] According to some non-limiting embodiments, the molecular weight adjusting agent is hydrogen.

[0038] According to some non-limiting embodiments, the gaseous stream introduced in the gas-solids separator, consisting of olefin monomers to be polymerized, free of molecular weight adjusting agent such as hydrogen, can optionally contain an inert gas such as nitrogen and / or a lower alkane such as ethane, propane, butane, pentane or hexane. By "consisting of olefin monomers to be polymerized" is meant comprising at least 95 wt%, preferably at least 97 wt%, more preferably at least 99 wt% of olefin monomers to be polymerized.

[0039] The material discharged from the bottom of the gas-solids separator includes the discharged olefin polymer particles and a second amount of molecular weight adjusting agent. In some embodiments, the second amount of molecular weight adjusting agent in the discharged material including the olefin polymer particles can consist of a detectable or non-detectable trace amount (e.g., the discharged olefin polymer is discharged in a manner that can not contain a detectable molecular weight adjusting agent). In some embodiments, the discharged material includes olefin polymer particles and molecular weight adjusting agent in an amount equal to or less than 0.1 volume percent (vol%) based on the total weight of the contents discharged with the olefin polymer particles, preferably equal to or less than 0.01 vol%. In some embodiments, the discharged material includes 0.01 vol% to 0.00001 vol% of molecular weight adjusting agent.

[0040] In some embodiments, the gaseous stream introduced from the bottom of the gas-solids separator extracts 50 wt% or more, preferably 90% or more, more preferably 99% or more of the process gas containing the molecular weight adjusting agent from the reaction material to produce a material discharge including olefin polymer particles containing the molecular weight adjusting agent in an amount equal to or less than 0.1 vol%, or equal to or less than 0.01 vol%, or 0.01 vol% to 0.00001 vol%.

[0041] The gas-solid separator typically operates at a pressure of 10 to 30 barg, preferably 13 to 23 barg,

[0042] The downstream low pressure degasser typically operates at a pressure of 0 to 5 barg, preferably 0.2 to 2 barg,

[0043] The present disclosure provides a process for the polymerization of olefins, such as 1-olefins (i.e. hydrocarbons having a terminal double bond), but not limited thereto. Typical 1-olefins are linear or branched 1-olefins having 2 to 12 carbon atoms, in particular linear 1-olefins having 2 to 10 carbon atoms (such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-decene), or branched 1-olefins having 2 to 10 carbon atoms (such as 4-methyl-1-pentene), conjugated and non-conjugated dienes (such as 1,3-butadiene, 1,4-hexadiene or 1,7-octadiene), or vinylaromatics (such as styrene or substituted styrenes). Mixtures of various 1-olefins can also be polymerized. Olefins that can be polymerized with the process of the present disclosure include olefins in which the double bond is part of a cyclic structure which can have one or more ring systems. Examples are cyclopentene, norbornene, tetracyclododecene or methyl norbornene or dienes such as 5-ethylidene-2-norbornene, norbornadiene or ethyl norbornadiene. Mixtures of two or more olefins can also be polymerized.

[0044] According to some non-limiting embodiments, the process can be used for the homo- or copolymerization of ethylene or for the homo- or copolymerization of propylene. According to one embodiment, the comonomer for the polymerization of ethylene is an a-olefin having 3 to 8 carbon atoms, such as 1-butene, 1-pentene, 1-hexene and / or 1-octene, in an amount of up to 20 wt%, or 0.01 to 15 wt%, or 0.05 to 12 wt%. According to another embodiment, the comonomer for the polymerization of propylene is ethylene and / or 1-butene and / or 1-hexene, in an amount of up to 40 wt%, or 0.5 to 35 wt%. According to yet another embodiment, the process can be used for the production of a heterophasic propylene copolymer comprising a propylene homopolymer or copolymer matrix and an elastomeric ethylene copolymer phase. The optional comonomer of the matrix is selected from ethylene or an a-olefin having 4 to 10 carbon atoms and is present in an amount of up to 5 wt%. The comonomer of the elastomeric ethylene copolymer phase is selected from propylene or an a-olefin having 4 to 10 carbon atoms.

[0045] The process of the present disclosure allows the preparation of any type of common olefin polymer. According to some non-limiting embodiments, the prepared olefin polymer can be a broad molecular weight olefin polymer, and in particular a multimodal olefin polymer, wherein the term "multimodal" refers to the modality of the molecular weight distribution. As used in the art and as used herein, multimodal shall include bimodal. Such polymers can be obtained by polymerizing the olefin under different reaction conditions in a cascade of two or more polymerization reactors or in different zones of a multi-zone reactor. Thus, "modality" indicates how many different polymerization conditions are used to prepare the polyolefin, independently indicating whether the modality of this molecular weight distribution can be considered as a separated maximum in the gel permeation chromatography (GPC) curve. In addition to the molecular weight distribution, the olefin polymer can also have a comonomer distribution. According to one embodiment, the average comonomer content of the polymer chains having a higher molecular weight is higher than the average comonomer content of the polymer chains having a lower molecular weight. However, it is also possible to employ the same or very similar reaction conditions in all the polymerization reactors of the reaction cascade, thereby preparing a narrow molecular weight or unimodal olefin polymer.

[0046] The polymerization of the olefin can be carried out using a conventional olefin polymerization catalyst. This means that the polymerization can be carried out using a titanium-based Ziegler-Natta catalyst, a Phillips catalyst based on chromium oxide or a single-site catalyst. For the purposes of the present disclosure, a single-site catalyst is a catalyst based on a chemically homogeneous transition metal coordination compound. Furthermore, it is also possible to use a mixture of two or more of these catalysts for the polymerization of the olefin. Such a mixed catalyst can be referred to as a composite catalyst.

[0047] According to some non-limiting embodiments, the catalyst used in the process of the present disclosure is a Ziegler-Natta catalyst comprising:

[0048] (i) a solid catalyst component comprising Mg, Ti, halogen and an electron donor compound (internal donor),

[0049] (ii) an aluminum alkyl compound, and

[0050] (iii) optionally an electron donor compound (external donor).

[0051] Component (i) can be prepared by contacting a magnesium halide, a titanium compound having at least one Ti-halogen bond and optionally an electron donor compound. The magnesium halide can be MgCl2in its active form, which is widely known in the patent literature as a support for Ziegler-Natta catalysts. The titanium compound can be TiCl4or TiCl3. It is also possible to use a compound of formula Ti(OR) n--y X yTiXnyR(3-n), wherein n is the valence of titanium, y is a number between 1 and n-1, X is halogen, and R is a hydrocarbon group having 1 to 10 carbon atoms.

[0052] The electron donor compounds used for the preparation of Ziegler type catalysts are, for example, alcohols, diols, esters, ketones, amines, amides, nitriles, alkoxysilanes and aliphatic ethers. These electron donor compounds can be used alone or in mixture with other electron donor compounds.

[0053] Other solid catalyst components that can be used are based on chromium oxide supported on a refractory oxide, such as silica, and activated by heat treatment. The catalysts obtainable by these components consist of chromium trioxide (VI) chemically fixed on silica gel. These catalysts are prepared by heating silica gel doped with a chromium (III) salt (precursor or pre-catalyst) under oxidizing conditions. During this heat treatment, chromium (III) is oxidized to chromium (VI), which is fixed, while the hydroxyl groups of the silica gel are removed in the form of water.

[0054] Still other solid catalyst components that can be used are single-site catalysts supported on a support, such as metallocene catalysts comprising:

[0055] (i) at least one transition metal compound containing at least one n bond; and

[0056] (ii) at least one co-catalyst selected from aluminoxanes or compounds capable of forming an alkyl metallocene cation.

[0057] According to some non-limiting embodiments, when the catalyst comprises an aluminum alkyl compound, such as in a Ziegler-Natta catalyst, the molar ratio of component (a) to the aluminum alkyl compound introduced into the polymerization reactor is from 0.05 to 3 or from 0.1 to 2 or from 0.5 to 1.

[0058] The catalyst can optionally be subjected to a prepolymerization before being fed to the polymerization reactor. According to one embodiment, the prepolymerization occurs in a loop reactor. The prepolymerization of the catalyst system can be carried out at low temperature in the range of 0 °C to 60 °C.

[0059] The process of the present disclosure can be carried out in any polymerization plant comprising two or more liquid phase and / or gas phase polymerization reactors. Examples of liquid phase reactors are loop reactors and continuous stirred tank reactors (CSTR). Examples of gas phase reactors include fluidized bed reactors, stirred bed reactors and reactors with two interconnected polymerization zones as described in EP 0782587 and EP 1012195. The process of the present disclosure can be carried out in two or more cascaded reactors, resulting in a sequential multi-stage polymerization process. For example, a fluidized bed reactor can be used to make a first polymer component, which is continuously fed into a gas phase reactor with two interconnected polymerization zones to make a second polymer component and a third polymer component. Thus, an olefin polymer with a multimodal molecular weight distribution can be obtained, as well as an olefin copolymer comprising two or more components with different comonomer contents.

[0060] According to some non-limiting embodiments, the polymerization process is carried out as a gas phase polymerization, i.e. a process by which a solid polymer is obtained from the gas phase of one or more monomers. Such a gas phase polymerization can be carried out at a pressure of 1 to 200 barg or 5 to 100 barg or 10 to 50 barg and a polymerization temperature of 40 to 150 °C or 65 to 125 °C.

[0061] The gas phase polymerization reactor can be, for example, a horizontal or vertical stirred reactor, a fluidized bed gas phase reactor or a multi-zone circulating reactor.

[0062] The fluidized bed polymerization reactor is a reactor in which the polymerization takes place in a bed of polymer particles which is kept in a fluidized state by feeding a gas at the lower end of the reactor, for example below a gas distribution grid which has the function of distributing the gas flow, and discharging it again at its upper end. The reactor gas is then returned to the lower end of the reactor via a recirculation line equipped with a compressor and a heat exchanger. The circulating reactor gas is a mixture of, for example, the olefins to be polymerized, an inert gas such as nitrogen and / or a lower alkane such as ethane, propane, butane, pentane or hexane, and optionally a molecular weight regulator such as hydrogen. According to one embodiment, nitrogen or propane can be used as inert gas, if appropriate in combination with a further lower alkane. The velocity of the reactor gas must be sufficiently high to fluidize the mixed bed of finely divided polymer present in the tubes used as polymerization zone, first of all, and to remove the polymerization heat effectively, secondly. The polymerization can also be carried out in a condensed or super-condensed mode, in which a portion of the circulating reaction gas is cooled below the dew point and returned to the reactor as a liquid phase and a gas phase, respectively, or together as a two-phase mixture, to additionally use the evaporation enthalpy to cool the reaction gas.

[0063] A multi-zone circulating reactor is a gas phase reactor in which two polymerization zones are connected to each other and the polymer alternately passes multiple times through both zones. Such reactors are described, for example, in WO 97 / 04015 A1 and WO 00 / 02929 A1 and have two interconnected polymerization zones, a riser in which the growing polymer particles flow upwards under fast fluidization or transport conditions and a downcomer in which the growing polymer particles flow in a densified form under the action of gravity. The polymer particles leaving the riser enter the downcomer and the polymer particles leaving the downcomer are reintroduced into the riser, thus establishing a circulation of the polymer between the two polymerization zones and the polymer alternately passes multiple times through both zones. It is also possible to operate the two polymerization zones of one multi-zone circulating reactor with different polymerization conditions by establishing different polymerization conditions in its riser and its downcomer. For this purpose, it is possible to partially or completely prevent the gas mixture leaving the riser and entraining polymer particles from entering the downcomer. This can be achieved, for example, by feeding a spacer fluid in the form of a gas and / or liquid mixture into the downcomer. According to one embodiment, in the upper part of the downcomer, the spacer fluid should have a suitable composition that is different from the gas mixture present in the riser. The amount of spacer fluid added can be adjusted in such a way that an upward gas flow counter-current to the polymer particle flow is generated (in particular at its top) to act as a spacer for the gas mixture entrained in the particles from the riser. In this way, it is possible to obtain two different gas composition zones in one multi-zone circulating reactor. Furthermore, according to one embodiment, it is also possible to introduce additional monomers, comonomers, molecular weight regulators such as hydrogen and / or inert fluids at any point in the downcomer below the spacer feed point. Thus, it is also possible to easily produce different monomer concentrations, comonomer concentrations and hydrogen concentrations along the downcomer, leading to further differences in the polymerization conditions.

[0064] Reference is made to Figure 1 A two-phase stream containing vaporized monomer and polymer particles from an upstream reactor enters a gas-solids separator or flash chamber 1 via line 2 in the upper part of the flash chamber. The operating pressure of the flash chamber is lower than the pressure of the upstream reactor. The solid polymer particles fall under the action of gravity to the bottom of the flash chamber, while the gaseous monomer flows upwards to the top of the chamber, where it is collected and conveyed via line 3 to a monomer recovery section. Fresh gaseous hydrogen-free propylene is fed to the flash chamber via line 4 placed in the lower part of the flash chamber. Alternatively, a gaseous hydrogen-free propylene / propane stream from the reaction environment is fed to the flash chamber via line 4. Polymer is discharged from line 5 placed on the bottom of the flash chamber.

[0065] Stream 2 enters the preferably cylindrical portion of the top of the flash chamber, maximizing the separation between gas and solids. Preferably, the inlet is tangential. The dimensions of the top portion of the flash chamber are designed so as to maximize the separation between solids and gas, and minimize the solid particles entrained with the gas (stream 3) exiting the flash chamber through the top. The separated solids fall under the action of gravity to the bottom of the flash chamber, where they form a downward flowing packed moving bed, and are discharged through stream 5 by means of a suitably designed device that allows to control the level of solids in the bottom portion of the flash chamber. The bottom portion of the flash chamber where the packed bed is present is preferably of cylindrical shape ending at the bottom with a conical reducing section. Stream 4 is fed into the polymer packed bed (below the polymer level), its feeding position being such that the gas discharged from the upstream reactor with the polymer has a high separation efficiency. Preferably, stream 4 is fed through one or more specially designed nozzles at the bottom of the cylindrical portion. Preferably, the diameter of the bottom portion is smaller than the diameter of the top portion, and is designed to prevent the gas fed through stream 4 from fluidizing the polymer particles. Preferably, the connection between the top and bottom portions is performed through a conical reducing section. The dimensions of both the conical reducing section between the top and bottom portions of the flash chamber and the conical reducing section below the bottom portion of the flash chamber are designed so as to have a good flowability of the polymer particles. In particular, the inclination of these portions is such that the polymer particles have a good flowability.

[0066] Reference will now be made to the drawings Figure 2 The detailed description represents a cascade polymerization process according to one embodiment of the present disclosure, which is a schematic diagram and must be considered as illustrative and not as a limitation of the scope of the invention.

[0067] In such an embodiment, the slurry polymerization of liquid propylene is carried out in a loop reactor 6. Schematically, catalyst components, co-catalysts and propylene are introduced into the loop reactor, as indicated by arrow 7. In case a Ziegler / Natta catalyst comprising a solid component supported on active MgCl2is used, the solid component can be fed as such or in the form of a pre-polymer.

[0068] The loop reactor 6 can be the first polymerization reactor of the process, or other reactors can be present upstream of the reactor 6. For the purposes of the present description, the reactor 6 can receive from line 7 the polymer or pre-polymer produced in other upstream reactors and / or the polymerization catalyst or catalyst components. In the simplified diagram, the feed lines of catalyst, monomers, molecular weight regulators and other possible ingredients are omitted for simplicity. Figure 2 In the simplified diagram, the feed lines of catalyst, monomers, molecular weight regulators and other possible ingredients are omitted for simplicity.

[0069] Most of the polymer slurry is continuously recycled in the loop reactor 6, but a portion is continuously discharged to a transfer line 8 connected to the flash chamber 1. The transfer line 8 includes a pipe 9 equipped with heating elements 10, such as a steam-jacketed pipe. Discharge into the transfer line involves a pressure drop, causing the liquid propylene to evaporate under pressure conditions lower than those inside the loop reactor by the heating elements distributed along the appropriate length of the pipe, creating turbulence containing polymer and gaseous monomers.

[0070] At the outlet of jacketed pipe 9, a two-phase flow containing evaporated monomer and polymer particles is conveyed via pipe 2 into the upper part of flash chamber 1, which operates at a lower pressure than the upstream reactor. Solid polymer particles fall to the bottom of flash chamber 1 under gravity, countercurrent to the hydrogen-free gas flow introduced via pipe 4, while gaseous monomer flows upward to the top of flash chamber 1, where it is collected and conveyed via pipe 3 to a monomer recovery section, which includes a condenser 11, a monomer replenishment unit 12, and a pump 13. Fresh propylene supplied as indicated by arrow 14 and recycled propylene from flash chamber 1 are fed via pipe 15 into loop reactor 6 for continuous polymerization.

[0071] Propylene polymer discharged from flash tank 1 via line 5 can be transferred via line 5a to a low-pressure degasser (not shown), or via line 5b to a fluidized bed gas-phase reactor 16, in which propylene copolymers, such as ethylene-propylene elastomer copolymers, are produced on homopolymer PP particles from loop reactor 6. Reactor 16 can operate at pressures between 10 and 30 bar and temperatures between 50 and 110°C. Schematically, fresh monomer 17 is fed into reactor 16 via line 18, unreacted monomer is recycled via line 19 equipped with compressor 20 and a heat exchanger 21 located downstream of compressor 20, and multiphase copolymers or shock PP are discharged from line 22. Such products can be the final product of the polymerization process; therefore, they are transferred to the finishing section of the facility, or they can be transferred to a second gas-phase reactor (not shown) to enrich copolymer fractions.

[0072] Now refer to the appendix Figure 3 The detailed description illustrates another embodiment of the cascade polymerization process of this disclosure, which includes a gas-phase multi-zone circulating reactor having two interconnected polymerization zones (a riser and a downcomer). The figure is schematic and should be considered illustrative rather than limiting the scope of the invention.

[0073] Figure 3The illustrated multi-zone circulating reactor comprises a first polymerization zone 23 (riser) in which the polymer particles flow upwards in the direction of arrow A under fast fluidization conditions, and a second polymerization zone 24 (downcomer) in which the polymer particles flow downwards in the direction of arrow B under the effect of gravity.

[0074] The upper part of the riser 23 is connected to a solid / gas separator 25 by an interconnection section 26. The separator 25 removes most of the unreacted monomers from the polymer particles, and the polymer withdrawn from the bottom of the separator 25 enters the top part of the downcomer 24. The separated unreacted monomers, optionally together with a polymerization diluent such as propane, flow upwards to the top of the separator 25 and are subsequently recycled via a recycle line 27 to the bottom of the riser 23.

[0075] The mixture comprising one or more olefin monomers, hydrogen as a molecular weight regulator, propane as a polymerization diluent, is fed via one or more lines M into the polymerization reactor, the one or more lines being suitably placed along the gas recycle line 27, in accordance with the knowledge of the skilled person.

[0076] The catalyst component is introduced continuously into the riser 23 via line 28, optionally after a prepolymerization step. The produced polymer can be discharged from the reactor via line 29, which can be placed in the lower part of the downcomer 24, so that the amount of gas entrained by the discharged polymer is minimized due to the packed flow of densified polymer. By inserting a control valve (not shown) on the polymer discharge line 29, the flow rate of the polymer produced by the polymerization reactor can be controlled continuously. A further polymer discharge line with respect to line 29 can be placed at the bottom of the downcomer. Figure 3

[0077] The two-phase stream containing gaseous monomers and polymer particles from line 29 is conveyed in the upper part of a flash chamber 1, the operating pressure of which is lower than the pressure of the upstream reactor. The solid polymer particles fall under the effect of gravity towards the bottom of the flash chamber 1, while the gaseous monomers flow upwards to the top of the flash chamber 1, where they are collected and conveyed via line 3 to the multi-zone reactor, after being cooled and compressed for continued polymerization. A gaseous hydrogen-free stream is introduced in the lower part of the flash chamber 1 via line 4, to replace the reaction gas, which can contain a relatively high amount of hydrogen and / or comonomer, discharged from the upstream reactor with the polymer.

[0078] ​The propylene polymer discharged from the flash tank 1 via line 5 can be transferred to a low pressure degasser (not shown) via line 5a, or to a fluidized bed gas phase reactor 16 via line 5b, in which a propylene copolymer, for example an ethylene propylene elastomeric copolymer, is produced on the homopolymer PP particles coming from the multi-zone reactor. The reactor 16 can operate at a pressure comprised between 10 and 30 bar and at a temperature comprised between 50 and 110°C. Fresh monomers 17 are fed to the reactor 16 by line 18, unreacted monomers are recycled by line 19 equipped with a compressor 20 and a heat exchanger 21 placed downstream of the compressor 20, and the heterophasic copolymer or impact PP is discharged from line 22. Such product can be the final product of the polymerization process; therefore, it is transferred to the finishing section of the plant, or it can be transferred to a second gas phase reactor (not shown) to enrich the copolymer fraction.

[0079] The polymerization reactor also comprises a transport section 30 connecting the bottom of the downcomer 24 with the lower region of the riser 23. The bottom of the downcomer 24 converges into a slight flow restriction 31. A control valve 32 with adjustable opening can be placed within the flow restriction 31. The flow rate Fp of the polymer continuously circulating between the downcomer 24 and the riser 23 is regulated by the opening level of the control valve 32. The control valve 32 can be a mechanical valve, such as a butterfly valve, a ball valve, etc. A metering gas stream is fed to the lower part of the downcomer 24 by means of a line 33 placed at a short distance above the flow restriction 31. The metering gas introduced by line 33 can be taken from the recycle line 27. In summary, the flow rate Fp of the polymer particles circulating between the downcomer 24 and the riser 23 can be regulated by varying the opening of the control valve 32 at the bottom of the downcomer and / or by varying the flow rate of the metering gas entering the downcomer via line 33. The flow rate of the metering gas is regulated by a control valve 34 suitably arranged on line 33.

[0080] The transport section 30 is designed as a bend descending from the bottom of the downcomer 24 to the lower region of the riser 23. Moreover, a carrier gas is introduced at the inlet of the transport section 30 via line 35. The flow rate of the carrier gas is regulated by a control valve 36 suitably arranged on line 35.

[0081] Moreover, the carrier gas is also taken from the gas recycle line 27. In particular, the gas recycle stream of line 27 is first compressed by a compressor 37 and a small fraction of the recycle stream passes through line 35, thus entering the transport section 30 and diluting the solid phase of the polymer flowing through the transport section 30. Downstream of the compressor 37, the major fraction of the recycle stream is cooled in a heat exchanger 38 and continuously introduced at high velocity at the bottom of the riser 23 via line 39, to ensure fast fluidization conditions in the polymer bed flowing along the riser 23.

[0082] After exiting the slits of the gas distribution grid 40, the carrier gas merges with the densified polymer from the downcomer 24 at the inlet portion of the transport section 30. In the illustrated embodiment, the top end of the distribution grid 40 coincides with the inlet of the transport section 30, and the distribution grid 40 extends along the curvature of the transport section 30 by an angle a = 60°. The gas distribution grid 40 is formed by a plurality of trays that are fixed to the transport section 30 in such a way that slits are formed in the overlapping area of adjacent trays. Figure 3

[0083] After exiting the slits of the gas distribution grid 40, the carrier gas merges with the densified polymer from the downcomer 24 at the inlet portion of the transport section 30. In the illustrated embodiment, the top end of the distribution grid 40 coincides with the inlet of the transport section 30, and the distribution grid 40 extends along the curvature of the transport section 30 by an angle a = 60°. The gas distribution grid 40 is formed by a plurality of trays that are fixed to the transport section 30 in such a way that slits are formed in the overlapping area of adjacent trays. Figure 3

[0084] Depending on the olefin (co)polymer to be produced, the polymerization reactor can be operated by appropriately adjusting the polymerization conditions and the monomer concentration in the riser and the downcomer in order to produce various bimodal homopolymers and random copolymers. For this purpose, the gas mixture entraining polymer particles and coming from the riser can be partially or completely prevented from entering the downcomer in order to polymerize two different monomer compositions in the riser and the downcomer. This effect can be achieved by feeding a gas and / or liquid barrier stream through a line placed in the upper part of the downcomer. The barrier stream should have a different composition than the gas present in the riser. The flow rate of the barrier stream can be adjusted such that an upward gas flow (in particular at the top of the downcomer) is generated countercurrent to the flow of polymer particles, thus acting as a barrier for the gas mixture coming from the riser. For further details regarding this barrier effect at the top of the downcomer, reference is made to the disclosure of EP 1012195 A1.

[0085] If desired, different or identical polymerization processes can also be connected in series and thus form a polymerization cascade. A parallel arrangement of reactors using two or more different or identical processes is also possible.

[0086] According to some non-limiting embodiments, the gas phase polymerization process according to the present disclosure is carried out in the presence of an alkane having from 3 to 5 carbon atoms as polymerization diluent, for example in the presence of propane.

[0087] Example

[0088] The following examples are given for the purpose of illustration and not for the purpose of limitation.​​

[0089] Test Method

[0090] Melt flow rate (MFR "L") - as determined according to ISO 1133 (230°C, 2.16 Kg)

[0091] Ethylene content in the copolymer - The ethylene comonomer content was determined by infrared spectroscopy by collecting the IR spectrum of the sample versus an air background with a Fourier Transform Infrared Spectrometer (FTIR). The instrument data acquisition parameters were:

[0092] - Purge time: minimum 30 seconds

[0093] - Collection time: minimum 3 minutes

[0094] - Apodization: Happ-Genzel

[0095] - Resolution: 2 cm"1.

[0096] Sample Preparation - Using a hydraulic press, a thick sheet was obtained by pressing about 1 g of sample between two aluminum foils. A small piece was cut from the sheet to mold a film. The recommended film thickness range is between 0.02 and 0.05 cm (8 to 20 mils). The pressing temperature was 180 ± 10°C (356°F), the pressure was about 10 kg / cm2(142.2 PSI), and the time was about 1 minute. The pressure was released, the sample was removed from the press and cooled to room temperature.

[0097] The spectrum of the pressed film sample was recorded in absorbance versus wavenumber (cm"1). The following measurements were used to calculate the ethylene content:

[0098] - Area of the combined absorption bands (At) between 4482 and 3950 cm -1 for spectral normalization for film thickness;

[0099] - Area of the absorption band (AC2) between 750 and 700 cm -1 after two appropriate consecutive spectral subtractions of the isotactic non- added polypropylene spectrum followed by the reference spectrum of the ethylene-propylene random copolymer in the range of 800 to 690 cm -1

[0100] - Height of the absorption band (DC4) at 769 cm -1 after two appropriate consecutive spectral subtractions of the isotactic non- added polypropylene spectrum followed by the reference spectrum of the ethylene-propylene random copolymer in the range of 800 to 690 cm -1

[0101] ​​To calculate the ethylene content, an ethylene calibration straight line is needed, obtained using known amounts of ethylene samples, by plotting AC2 / At versus ethylene molar percentage (%C2m). The slope GC2 is calculated by linear regression.

[0102] The spectrum of the unknown sample is recorded and then the (At), (AC2) and (DC4) of the unknown sample are calculated. The ethylene weight content is obtained from the ethylene content of the sample (% molar fraction C2m) and is calculated as follows:

[0103]

[0104] Xylene solubles (XS) - is determined as follows: 2.5 g of polymer and 250 ml of xylene are introduced into a glass flask equipped with a refrigerated and magnetic stirrer. The temperature is raised to the boiling point of the solvent in 30 minutes. The resulting clear solution is then kept under reflux and stirred for another 30 minutes. The closed flask is then kept in a thermostatic water bath at 25°C for 30 minutes. The solid thus formed is filtered on a fast filter paper. 100 ml of the filtered liquid are poured into a pre-weighed aluminum container, heated on a hot plate under a nitrogen stream to remove the solvent by evaporation. The container is then kept in an oven at 80°C under vacuum until a constant weight is obtained. The weight percentage of polymer soluble in xylene at room temperature is then calculated.

[0105] Inherent viscosity - the sample is dissolved in tetralin at 135°C and then poured into a capillary viscometer. The viscometer tube (Ubbelohde type) is surrounded by a cylindrical glass jacket. This setup allows temperature control with a circulating thermostated liquid. The downward passage of the meniscus is timed by a photoelectric device. The passage of the meniscus in front of the upper lamp starts a counter with a quartz crystal oscillator. When passing through the lower lamp, the meniscus stops the counter and the efflux time is recorded: this is translated into an intrinsic viscosity value by the Huggins' equation (Huggins, M.L., J. Am. Chem. Soc., 1942, 64, 2716), provided that the flow time of the pure solvent is known under the same experimental conditions (same viscometer and same temperature). A single polymer solution is used to determine the intrinsic viscosity.

[0106] Example 1

[0107] Preparation of the Ziegler-Natta solid catalyst component

[0108] An initial quantity of microspheres MgCl2*2.8 C2H5OH adduct was prepared according to the method described in Example 2 of W098 / 44009, but operating on a larger scale. Then the adduct thus obtained was subjected to partial dealcoholation under a stream of nitrogen until the alcohol content reached a value of 50% by weight based on the total weight of the adduct.

[0109] TiCI4, 300 ml, was introduced into a 500 ml round bottom flask, equipped with a mechanical stirrer, a cooler and a thermometer, under a nitrogen atmosphere at room temperature. After cooling to 0°C, diisobutylphthalate (internal donor) and 9.0 g of the adduct prepared as described above were added to the flask, in succession, while stirring. The amount of internal donor added was such as to satisfy a Mg / donor molar ratio of 9. The temperature was raised to 100°C and maintained for 2 hours. After this time, stirring was stopped, the solid product was allowed to settle and the supernatant was siphoned off at 100°C. After removal of the supernatant, further fresh TiCI4was added to reach again the initial liquid volume. The mixture was then heated at 120°C and maintained at this temperature for 1 hour. Stirring was again stopped, the solid was allowed to settle and the supernatant was siphoned off. The solid was washed six times with anhydrous hexane with a temperature gradient down to 60°C and once at room temperature. Further fresh TiCI4was then added to reach again the initial liquid volume. The mixture was then heated again at 120°C and maintained at this temperature for 1 hour. Stirring was again stopped, the solid was allowed to settle and the supernatant was siphoned off. The solid was washed six times with anhydrous hexane with a temperature gradient down to 60°C and once at room temperature. The solid obtained was then dried under vacuum and analyzed.

[0110] Catalyst activation and prepolymerization

[0111] The solid catalyst component prepared as described above was contacted with triethylaluminium (TEAL) and cyclohexylmethyldimethoxysilane (donor-C) under the conditions reported in Table 1, before introducing it into the polymerization reactor.

[0112] The activated catalyst discharged from the activation vessel was continuously fed, together with liquid propylene, into a prepolymerization loop reactor operating at a temperature of 30°C and with a residence time of 10 minutes.

[0113] Polymerization

[0114] The polymerization run was carried out in a series of two reactors, equipped with means for transferring the product from the first reactor to the second reactor, in continuous mode. The first reactor was a multi-zone reactor and the second reactor was a fluid bed gas phase reactor, as described in WO2016 / 197334. Figure 3The prepolymerization catalyst is discharged from the prepolymerization reactor and continuously fed to the multi-zone reactor. In the multi-zone reactor propylene homopolymer is produced, while in the gas phase reactor ethylene copolymer is produced in the presence of the propylene homopolymer from the first stage. The polypropylene powder in the presence of residual reaction gas is discharged from the multi-zone reactor and the polypropylene and residual unreacted gas stream continuously flows through the transfer line 29, is transferred to the flash chamber 1, where the gaseous reaction gas is separated from the polymer particles. The tangential entry of the above stream ensures the gas / solid separation by centrifugal effect. A gaseous stream of fresh or recycled hydrogen-free propylene is fed to the flash chamber via line 4 placed in the lower part of the flash chamber. The flash chamber 1 operates at a pressure of 20 barg. The solid polymer particles fall under the action of gravity to the bottom of the tank, while the gas phase that flows from the top is transferred to the monomer recovery section. The polypropylene particles are discharged from the bottom of the flash chamber 1 and are transported to the downstream gas phase reactor 16 to produce a binary copolymer. The gas phase reaction environment (mainly propylene, ethylene and hydrogen) is continuously analyzed via gas chromatography. At the end of the run, the powder is discharged, transported to a low-pressure degasser and then subjected to steam treatment to remove reactive monomers and volatile substances and finally dried under a stream of nitrogen. The main polymerization conditions and polymer characteristics are reported in Table 1.

[0115] Example 2C (comparative)

[0116] Example 1 is repeated, with the difference that no gaseous fresh or recycled hydrogen-free propylene is fed to the flash chamber 1 via line 4 placed in the lower part of the flash chamber, and the same homopolymer produced in the multi-zone reactor and the same binary copolymer content and composition produced in the gas phase reactor are used. Since no gaseous fresh or recycled hydrogen-free propylene is fed to the flash chamber 1 via line 4, the hydrogen concentration in the second gas phase reactor is higher compared to Example 1, which is reflected in the final properties of the product, in particular the melt flow index and the XSIV, which become much lower than the XSIV of the product of Example 1.

[0117] Example 3C (comparative)

[0118] Example 1, except that no gaseous fresh or recycled hydrogen-free propylene was fed to the flash chamber 1 via line 4 placed in the lower part of the flash chamber, and the same final product melt index was targeted as the product produced in Example 1. Thus, the homopolymer produced in the multi-zone reactor had a lower melt index, and the H2 / C3 gas produced was lower. The binary copolymer content and composition produced in the gas phase reactor was kept the same as the product of Example 1. Similar to Example 2C, since no gaseous fresh or recycled hydrogen-free propylene was fed to the flash chamber 1 via line 4, the hydrogen concentration in the second gas phase reactor was higher compared to Example 1 (although lower than Example 2C), which was reflected in the final properties of the product, in particular the XSIV, which became much lower than the XSIV of the product of Example 1 (although slightly higher than the XSIV of the product of Example 2C).

[0119] Table 1

[0120]

[0121] Note: C2 - = ethylene; C3 - = propylene; H2 = hydrogen; split ratio = amount of polymer produced in the relevant reactor (relative to total weight); yield = amount of polymer obtained per unit of catalyst added.

Claims

1. A method for transferring olefin polymer particles from a first upstream polymerization reactor to a second downstream polymerization reactor or to a downstream low pressure degasser, the method comprising the steps of: a) withdrawing an olefin reaction material from the first upstream polymerization reactor, the olefin reaction material comprising polymer particles and a process fluid, the process fluid being gaseous or liquid in the polymer slurry and containing a first amount of molecular weight adjusting agent; b) passing the withdrawn reaction material into the top of a gas-solids separator, in which the olefin polymer particles flow downward under the influence of gravity; c) introducing a gaseous stream from the bottom of the gas-solids separator containing the reaction material, wherein the gaseous stream is free of molecular weight adjusting agent and / or free of other reaction comonomers and consists of olefin monomers to be polymerized, thereby extracting at least a portion of the process gas containing molecular weight adjusting agent from the reaction material; and d) continuously discharging the reaction material containing a second amount of molecular weight adjusting agent from the bottom of the gas-solids separator into the second downstream reactor or into the downstream low pressure degasser, wherein the second amount of molecular weight adjusting agent is lower than the first amount of molecular weight adjusting agent.

2. The method according to claim 1, wherein the molecular weight adjusting agent is hydrogen.

3. The method according to claim 1 or 2, wherein the second amount of molecular weight adjusting agent is equal to or less than 0.1 volume percent (vol%) based on the total weight of the contents discharged with the olefin polymer particles, preferably equal to or less than 0.01 vol%.

4. The method according to any one of claims 1 to 3, wherein the second amount of molecular weight adjusting agent is from 0.01 vol% to 0.00001 vol%.

5. The method according to any one of claims 1 to 4, wherein the gaseous stream introduced from the bottom of the gas-solids separator extracts 50 wt% or more, preferably 90% or more, more preferably 99% or more of the process gas containing molecular weight adjusting agent from the reaction material.

6. The method according to any one of claims 1 to 5, wherein the gaseous stream introduced into the gas-solids separator contains an inert gas (such as nitrogen) and / or a lower alkane (such as ethane, propane, butane, pentane or hexane).

7. The method according to any one of claims 1 to 6, wherein the upstream polymerization reactor is a slurry loop reactor.

8. The method according to any one of claims 1 to 6, wherein the upstream polymerization reactor is a gas phase multi-zone circulating reactor.

9. The method according to any one of claims 1 to 8, wherein the downstream polymerization reactor is a fluidized bed gas phase reactor.

10. The method according to any one of claims 1 to 9, wherein the gas-solids separator is operated at a pressure of from 10 to 30 barg.

11. The method according to any one of claims 1 to 10, wherein the downstream low pressure degasser is operated at a pressure of from 0 to 5 barg. ​ 12. A continuous olefin polymerization process comprising polymerization of at least one olefin monomer in a series of at least two polymerization reactors, the process comprising a method for transferring olefin polymer particles from a first upstream polymerization reactor to a second downstream polymerization reactor or to a downstream low pressure degasser, the method comprising the steps of: a) withdrawing a reaction material from the first upstream polymerization reactor, the reaction material comprising olefin polymer particles and process fluid, the process fluid being gaseous or liquid in the polymer slurry and containing a first amount of molecular weight regulator b) passing the withdrawn reaction material into the top of a gas-solids separator, in which the olefin polymer particles flow downward under the influence of gravity; c) introducing a gaseous stream from the bottom of the gas-solids separator containing the reaction material, wherein the gaseous stream is free of molecular weight regulator and / or free of other reactive comonomers and consists of the olefin monomers to be polymerized, thereby extracting at least a portion of the process gas containing molecular weight regulator from the reaction material; and d) continuously discharging the reaction material containing a second amount of molecular weight regulator from the bottom of the gas-solids separator into the second downstream reactor or into the downstream low pressure degasser, wherein the second amount of molecular weight regulator is lower than the first amount of molecular weight regulator.

13. A continuous olefin polymerization process comprising the steps of: A) carrying out polymerization of propylene, optionally in the presence of up to 5 wt% of a copolymer selected from ethylene or alpha-olefins having 4 to 10 carbon atoms, in a first upstream reactor, B) copolymerizing ethylene with propylene or alpha-olefins having 4 to 10 carbon atoms in a second downstream reactor in the presence of the polymer produced in the first upstream reactor, the process comprising a method for transferring olefin polymer particles from the first upstream polymerization reactor to the second downstream polymerization reactor or to a downstream low pressure degasser, the method comprising the steps of: a) withdrawing a reaction material from the first upstream polymerization reactor, the reaction material comprising olefin polymer particles and process fluid, the process fluid being gaseous or liquid in the polymer slurry and containing a first amount of molecular weight regulator b) passing the withdrawn reaction material into the top of a gas-solids separator, in which the olefin polymer particles flow downward under the influence of gravity; c) introducing a gaseous stream from the bottom of the gas-solids separator containing the reaction material, wherein the gaseous stream is free of molecular weight regulator and / or free of other reactive comonomers and consists of the olefin monomers to be polymerized, thereby extracting at least a portion of the process gas containing molecular weight regulator from the reaction material; and d) continuously discharging the reaction material containing a second amount of molecular weight regulator from the bottom of the gas-solids separator into the second downstream reactor or into the downstream low pressure degasser, wherein the second amount of molecular weight regulator is lower than the first amount of molecular weight regulator. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ d) continuously discharging from the bottom of the gas-solids separator into the second downstream reactor or into the downstream low-pressure degasser the reaction mass containing a second amount of molecular weight regulator, wherein the second amount of molecular weight regulator is lower than the first amount of molecular weight regulator.

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