Process for producing a polyolefin composition in a multi-stage process

Abstract

The present invention relates to a process and apparatus for producing alpha-olefin polymers in the presence of a polymerization catalyst in a continuously operated multi-stage polymerization sequence, the process comprising: (a) polymerizing an alpha-olefin monomer and, optionally, one or more alpha-olefin comonomers in a slurry phase in the presence of a hydrocarbon diluent or liquid monomer in a slurry reactor to obtain an alpha-olefin polymer, the slurry phase having a first solids concentration; (b) continuously withdrawing a polymer slurry from the slurry reactor, the polymer slurry comprising a polymer and a fluid phase; (c) concentrating at least a portion of the polymer slurry by removing a portion of the fluid phase to provide a first product stream comprising a concentrated slurry having a second solids concentration higher than the first solids concentration, and a second product stream comprising mainly the fluid phase; (d) polymerizing the first product stream in a first gas-phase reactor (GPR1) disposed downstream of the slurry reactor in the presence of an alpha-olefin monomer and, optionally, one or more alpha-olefin comonomers, to obtain a first alpha-olefin product stream; and (e) polymerizing the third product stream withdrawn from the slurry reactor in a second gas-phase reactor (GPR2) in the presence of an alpha-olefin monomer and, optionally, one or more alpha-olefin comonomers, to obtain a second alpha-olefin product stream.

Description

Technical Field

[0001] <Background> The present invention relates to a method and an apparatus for polymerizing an alpha-olefin polymer in a multi-stage process. In particular, the present invention relates to a method and an apparatus for the continuous polymerization of alpha-olefin monomers such as ethylene and / or propylene and other monomers, where the alpha-olefin monomers are polymerized in a continuously operated multi-stage polymerization sequence comprising at least two gas-phase reactors.

[0002] The polymerization of alpha-olefin polymers in a multi-stage process comprising slurry-phase polymerization and one or more gas-phase reactors is well known. Gas-phase reactors are commonly used for the polymerization of alpha-olefins such as ethylene and propylene, as they allow for relatively high flexibility in polymer design and the use of various catalyst systems. A common variant of the gas-phase reactor is the fluidized-bed reactor.

[0003] Such multi-stage processes (known as BORSTAR® technology) developed by Borealis are described, for example, in patent documents such as EP 0 887 379, WO 92 / 12182, WO 2004 / 000899, WO 2004 / 111095, WO 99 / 24478, WO 99 / 24479 or WO 00 / 68315.

[0004] Heterophasic propylene copolymers, also known as impact propylene copolymers, are also well known in the art. Heterophasic propylene copolymers are composed of two phases, namely, a highly crystalline matrix phase and an amorphous elastomeric phase dispersed therein. These two phases are mixed either by in-situ blending via reactors connected in series or by mechanical blending of two components produced separately, but in-situ blending is generally preferred for better mixing quality. In addition, it has been recognized in the art that further improved properties can be obtained by using a bimodal or trimodal matrix phase and / or a bimodal elastomeric phase. When combined with the desired in-situ blend, a plurality of reactors connected in series are required. As yet another option, the use of solution polymerization having two or more reactor trains configured in parallel has been described. However, the separation of the solvent and catalyst in solution polymerization, i.e., the work-up, results in undesirable complexity.

[0005] WO 2013 / 041507 A1 discloses a method for producing heterophasic polypropylene. A three-stage continuous reactor configuration, loop-gas phase reactor 1 - gas phase reactor 2, is used for the production of the matrix phase, which includes the elastomeric phase incorporated by mechanical blending. Higher amounts in excess of 20% by weight can be achieved by mechanical blending.

Background Art

[0006] EP 2 586 823 A1 discloses a method for producing heterophasic polypropylene. A four-stage continuous reactor configuration, loop-gas phase reactor 1 - gas phase reactor 2 - gas phase reactor 3, is applied. The matrix phase is produced in the first three reactors, namely, loop-gas phase reactor 1 - gas phase reactor 2, and the elastomeric phase is produced in gas phase reactor 3. Also in this case, the amount of elastomer prepared in the final reactor is quite limited, which is partially compensated by mechanical blending of a small amount of impact modifier.

[0007] EP 2 174 980 A1 discloses a method for producing heterophasic polypropylene, using a three-stage continuous reactor configuration, a loop-gas phase reactor 1 - gas phase reactor 2, producing the matrix phase only in the loop reactor, and producing the elastomer phase in gas phase reactors 1 and 2. However, such a sequence does not provide bimodality in the matrix.

[0008] WO 2004 / 039847 A1 relates to a method and an apparatus for the continuous polymerization of olefin monomers in a cascade of polymerization reactors. According to the method, the olefin monomer is first polymerized in a slurry phase in an inert hydrocarbon diluent in at least one loop reactor and then subsequently polymerized in the gas phase in at least one gas phase reactor. According to the present invention, the polymer slurry is continuously withdrawn from the loop reactor and optionally concentrated. The concentrated slurry is led to a high-pressure flash unit to remove the remaining liquid phase and is fed to the gas phase reactor. By this method, bimodal polyethylene having good properties can be produced. Due to the continuous operating state, the operation of the process is stable.

[0009] The prior art three-stage or four-stage continuous reactor configurations have certain limitations. On the first aspect, a pressure drop that enables easy transfer of the polymer intermediate powder from one reactor to the subsequent reactor is desirable. On a second and more important aspect, the productivity of the catalyst decreases across the reactors connected in series. The addition of a second catalyst into one or more reactors following the first reactor is conceivable, but usually results in a high level of complexity as it is necessary to deactivate the catalyst used in the preceding reactor. The further introduction of catalyst in the final reactor usually increases the risk of catalyst flushing. On a third and also important aspect, for example, the powder obtained from a preceding reactor assumes an undesirably high volume in the subsequent reactor. On a fourth and also important aspect, the heterogeneity of the individual fractions as produced in the individual reactors is limited to a certain extent. For example, changing the melt flow rate and accordingly the molecular weight by means of hydrogen supply gives rise to certain limitations and / or additional measures to be taken such as degassing steps.

[0010] The drawbacks of existing three- or four-stage continuous reactor configurations impose certain constraints, to an equal extent, in the polymerization process as well as in the resulting polymer products.

[0011] According to standard operating procedures and the hydrodynamics within the reactor, the polymer slurry produced in a slurry reactor is concentrated at the bottom surface of the slurry reactor. Usually, the slurry is withdrawn from its surface and may further condense in the outlet pipe. Thus, based on the reactor residence time and catalyst performance, the solids content in the polymer slurry increases. However, without additional equipment, it is impossible to further increase the solids concentration. Furthermore, if the catalyst performance within the reactor is not as expected, a large amount of diluted slurry is transferred to the downstream side of the gas-phase reactor. This means that a large amount of unreacted monomer is transferred to the gas-phase reactor. This results in inefficient monomer recovery operations and, consequently, a high amount of flaring (incineration of the monomer).

[0012] Therefore, the current operating procedure desires to shorten the residence time of the catalyst in the slurry reactor by flowing the slurry from the loop to the first gas-phase reactor at a lower solids concentration. A catalyst with a short residence time in the slurry reactor improves productivity in the gas-phase reactor. However, a large amount of unreacted monomer is taken into the gas-phase reactor, and ultimately into the recovery section, together with the slurry phase. At higher production cycles, the current design capacity of the recovery unit is not sufficient to separate and recycle the monomer back to the reactor. As a result, it leads to high monomer losses to the flare. Therefore, increasing the design capacity and polymer throughput of existing gas-phase reactors still pose challenges in existing polymerization systems. Designing a polymerization system that enables the adaptation of flexible polymer designs at higher production rates remains difficult.

Prior Art Documents

Patent Documents

[0013]

Patent Document 1

[0014] Accordingly, an object of the present invention is to improve an existing continuous multi-stage polyolefin polymerization method and provide a method for producing an alpha-olefin polymer in a continuously operated multi-stage polymerization sequence in the presence of a polymerization catalyst, where the catalyst residence time in the slurry reactor is shortened while achieving a high production rate, high productivity, and flexible polymer design. [Means for Solving the Problems]

[0015] To achieve the above object, the present inventors provide a method for producing an alpha-olefin polymer in a continuously operated multi-stage polymerization sequence in the presence of a polymerization catalyst, the method comprising the following steps: (a) In a slurry reactor, in the presence of a hydrocarbon diluent or a liquid monomer, in a slurry phase, polymerize an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain an alpha-olefin polymer, the slurry phase having a first solids concentration, (b) Continuously withdrawing from the slurry reactor a polymer slurry comprising a polymer and a mobile phase, (c) By removing a portion of the mobile phase, concentrating at least a portion of the polymer slurry to provide a first product stream comprising a concentrated slurry having a second solids concentration higher than the first solids concentration, and a second product stream mainly comprising the mobile phase, (d) In a first gas-phase reactor (GPR1) disposed downstream of the slurry reactor, polymerizing the first product stream in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain a first alpha-olefin product stream, (e) In a second gas-phase reactor (GPR2), polymerizing a third product stream withdrawn from the slurry reactor in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain a second alpha-olefin product stream, A method comprising the steps has been found.

[0016] In the method according to the invention, the third product stream may preferably comprise a concentrated slurry having a solids concentration lower than the second solids concentration of the first product stream.

[0017] The above method comprises the following steps: (a) In a slurry reactor, in the presence of a hydrocarbon diluent or a liquid monomer, in a slurry phase, polymerize an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain an alpha-olefin polymer, the slurry phase having a first solids concentration, (b) Continuously withdrawing from the slurry reactor a polymer slurry (1) comprising a polymer and a mobile phase, (c) By removing a part of the mobile phase, at least a part of the polymer slurry (1) is concentrated to provide a first product stream (2) containing a concentrated slurry having a second solid content concentration higher than the first solid content concentration, and a second product stream (3) mainly containing the mobile phase. (d’) A step of dividing the first product stream taken out from the slurry reactor into a first secondary product stream (2a) and a second secondary product stream (2b), where the first product stream contains the concentrated slurry. (e’) In a first gas phase reactor (GPR1) arranged downstream of the slurry reactor, in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers, polymerizing the first secondary product stream (2a) to obtain a first alpha-olefin product stream (4a), and (f) In a second gas phase reactor (GPR2) arranged downstream of the slurry reactor and in parallel with the first gas phase reactor (GPR1), in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers, polymerizing the second secondary product stream (2b) to obtain a second alpha-olefin product stream (4b). may alternatively be included.

[0018] Preferably, in this embodiment of the present invention, in the second gas phase reactor (GPR2), in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers, polymerizing the third product stream (6) taken out from the slurry reactor to obtain a second alpha-olefin product stream (4b).

[0019] Preferably, in this embodiment of the present invention, the solid content concentration in the third product stream (6) is lower than the solid content concentration in the first product stream (2) containing the concentrated slurry.

[0020] Preferably, in the method according to the present invention, step (c) is carried out in a hydrocyclone. Thus, the slurry may be concentrated to provide an underflow containing a concentrated slurry and an overflow rich in hydrocarbons. The overflow may preferably be recycled to the slurry reactor. According to a preferred embodiment of the above method, the first product stream containing the concentrated slurry is taken out from the slurry reactor through a separation vessel, preferably through a hydrocyclone, whereby the solids concentration at the outlet is higher than the solids concentration in the slurry reactor.

[0021] According to a preferred embodiment of the above method of the present invention, the first alpha-olefin product stream obtained in step (d) or step (e'), and the second alpha-olefin product stream obtained in step (e) or step (f) may be integrated to form a combined alpha-olefin product stream (5).

[0022] To achieve the above object, the present inventors have provided a polymerization system according to the present invention for producing an alpha-olefin polymer in the presence of a polymerization catalyst in a continuously operated multi-stage polymerization sequence, as follows: (A) A slurry reactor for polymerizing an alpha-olefin monomer and optionally one or more alpha-olefin comonomers in a slurry phase in the presence of a hydrocarbon diluent or a liquid monomer to obtain an alpha-olefin polymer, the slurry phase having a first solids concentration, the slurry reactor having at least one outlet through which the polymer slurry can be continuously withdrawn; (B) A separation vessel for concentrating at least a part of the polymer slurry withdrawn from the slurry reactor by removing a part of the fluid phase to provide a first product stream (2) containing a concentrated slurry having a second solids concentration higher than the first solids concentration, and a second product stream (3) mainly containing the fluid phase, the separation vessel having an overflow and an underflow; (C) In the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers, polymerize the first product stream (2) to obtain a first alpha-olefin product stream (4a) exiting from the first gas-phase reactor (GPR1), a first gas-phase reactor (GPR1) disposed downstream of the slurry reactor, and (D) In the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers, polymerize the third product stream (6) withdrawn from the slurry reactor to obtain a second alpha-olefin product stream (4b) exiting from the second gas-phase reactor (GPR2), a second gas-phase reactor (GPR2) disposed downstream of the slurry reactor and in parallel with the first gas-phase reactor (GPR1) A polymerization system comprising the above was further found.

[0023] The polymerization system according to the present invention is as follows: (A) A slurry reactor for polymerizing an alpha-olefin monomer and optionally one or more alpha-olefin comonomers in a slurry phase in the presence of a hydrocarbon diluent or a liquid monomer to obtain an alpha-olefin polymer, the slurry phase having a first solids concentration, the slurry reactor having at least one outlet through which the polymer slurry can be continuously withdrawn, (B) By removing a part of the fluid phase, concentrating at least a part of the polymer slurry withdrawn from the slurry reactor to provide a first product stream (2) containing a concentrated slurry having a second solids concentration higher than the first solids concentration, and a second product stream (3) mainly containing the fluid phase, the separation container having an overflow and an underflow, and (G) Respective transfer lines for dividing the first product stream (2) withdrawn from the slurry reactor into a first secondary product stream (2a) and a second secondary product stream (2b), where the first product stream contains the concentrated slurry, May alternatively include, where In a first gas phase reactor (GPR1) arranged downstream of the slurry reactor, in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers, the first secondary product stream (2a) is polymerized to obtain a first alpha-olefin product stream (4a), and In a second gas phase reactor (GPR2) arranged downstream of the slurry reactor and in parallel with the first gas phase reactor (GPR1), in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers, the second secondary product stream (2b) is polymerized to obtain a second alpha-olefin product stream (4b).

[0024] Preferably, this embodiment of the polymerization system of the present invention is as follows: (D’) A transfer line for withdrawing a third product stream (6) from the slurry reactor and transferring it to the second gas phase reactor (GPR2) to polymerize the third product stream (6) in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain the second alpha-olefin product stream (4b) exiting the second gas phase reactor (GPR2), further comprises.

[0025] The method and system of the present invention provide important advantages. Thus, it is possible to produce a multimodal alpha-olefin polymer composition having high properties, preferably a heterophasic or random polypropylene composition. There is no harmful carry-over of reactants from the first polymerization stage to the second and / or third polymerization stages. Costly overdesign of process elements can be avoided. Due to continuous operation, the operation of the process is stable. The transition and startup times can be shortened. An improvement in the polymer throughput (production rate) can be achieved and monomer losses can be reduced. Furthermore, by operating two gas phase reactors in parallel, two different polymers can be produced simultaneously in each reactor. By integrating these two independent streams, a unique product with product characteristics tailored to the purpose can be produced in a subsequent extrusion process.

Brief Description of the Drawings

[0026] [Figure 1] Figure 1 is a schematic diagram of a prior art reactor configuration. [Figure 2] Figure 2 is a schematic diagram of a reactor configuration according to the first embodiment of the present invention. [Figure 3] Figure 3 is a schematic diagram of a reactor configuration according to the second embodiment of the present invention. [Figure 4] Figure 4 is a schematic diagram of a further reactor configuration according to the third embodiment of the present invention.

Mode for Carrying Out the Invention

[0027] <Detailed Description of the Invention> An alpha-olefin homopolymer or copolymer may be produced in a continuously operated multi-stage polymerization process comprising at least two gas phase reactors (GPR1) and (GPR2). The polymerization apparatus may preferably include a prepolymerization reactor (PR), a first polymerization reactor (R1) and a second polymerization reactor (R2), a third polymerization reactor (R3) and optionally a fourth polymerization reactor (R4). The term "polymerization reactor" indicates that the main polymerization is carried out.

[0028] At least two of the polymerization reactors are gas phase reactors (GPR). More preferably, the second polymerization reactor (R2), the third polymerization reactor (R3) and any fourth polymerization reactor (R4) are gas phase reactors (GPRs), namely the first gas phase reactor (GPR1) and the second gas phase reactor (GPR2), and optionally the third gas phase reactor (GPR3). The gas phase reactor (GPR) according to the present invention is preferably a fluidized bed reactor, a high speed fluidized bed reactor, or a sedimentation bed reactor, or any combination thereof.

[0029] The polymerization in the gas-phase reactor is preceded by slurry polymerization in a slurry reactor, preferably a loop reactor. In the slurry reactor (SR), in the presence of a hydrocarbon diluent or a liquid monomer, in the slurry phase, an alpha-olefin monomer and optionally one or more alpha-olefin comonomers are produced to obtain an alpha-olefin polymer. The slurry phase has a first solids concentration.

[0030] Accordingly, the slurry reactor (SR) may be any continuously stirred or simply stirred batch tank reactor or loop reactor operating in bulk or in slurry. Bulk means polymerization in a reaction medium containing at least 60% (w / w) monomer. According to the present invention, the slurry reactor (SR) is preferably a (bulk) loop reactor (LR).

[0031] The operating temperature in the first polymerization reactor (R1) is preferably in the range of 62 to 85 °C, more preferably in the range of 65 to 82 °C, and even more preferably in the range of 67 to 80 °C.

[0032] In the slurry reactor, the alpha-olefin monomer, preferably propylene, may be homopolymerized or copolymerized with at least one C 2 and C 4 ~C 10 alpha-olefins. The polymerization is carried out in an inert hydrocarbon diluent or a liquid monomer, preferably a C 3 ~C 5 hydrocarbon diluent, more preferably in a propane or isobutane diluent.

[0033] Suitable catalysts that can be used to polymerize the alpha-olefin monomer are, for example, Ziegler-Natta catalysts, single-site catalysts, multi-site catalysts containing one or more single-site catalyst components, or combinations or mixtures thereof.

[0034] The alpha-olefin polymer is continuously withdrawn from the slurry reactor in a polymer slurry that includes a polymer and a fluid phase and further includes a hydrocarbon and optionally hydrogen. The term "continuously withdrawing the alpha-olefin polymer from the slurry reactor" encompasses the option of withdrawing the polymer slurry with or without a concentration step before introducing the slurry into any further downstream polymerization reactor.

[0035] At least a portion of the polymer slurry may preferably be concentrated by removing a portion of the fluid phase to provide a first product stream and a second product stream that mainly includes the fluid phase. The first product stream includes the concentrated slurry. The concentrated slurry has a (second) solids concentration that is higher than the (first) solids concentration of the slurry phase circulating within the slurry reactor. The concentration step for concentrating at least a portion of the polymer slurry withdrawn from the slurry reactor may be carried out in a separation vessel, preferably a hydrocyclone.

[0036] The concentration step is preferably carried out in a hydrocyclone. Hydrocyclones are known and are described, for example, in WO 2004 / 039847 A1. A hydrocyclone typically produces two mass streams (a lower (underflow) and an upper (overflow)). Generally, the underflow is a high-density or coarse fraction having a higher solids concentration than the overflow, while the overflow includes a fluid phase mainly containing a major portion of the hydrocarbon. Stable operating conditions of the hydrocyclone can be achieved by recycling a portion of the slurry from the lower (underflow) of the hydrocyclone to the slurry reactor. By using such a concentration step, the solids concentration in the concentrated slurry can be increased up to a maximum of 55 wt%, preferably 60 wt%, more preferably 65 wt% based on the total mass of the slurry.

[0037] The first product stream containing the concentrated slurry is introduced into a first gas-phase reactor (GPR1) disposed downstream of the slurry reactor. In the first gas-phase reactor (GPR1), in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers and optionally hydrogen, the first product stream containing a major portion of the alpha-olefin polymer produced in the slurry reactor is polymerized to obtain a first alpha-olefin product stream.

[0038] In GPR1, preferably the following conditions are adjusted:

[0039] [Table 1]

[0040] The above conditions may be changed as occasion demands. Specifically, not only may propylene be polymerized, but other alpha-olefins such as ethylene, 1-butene, 1-hexene or 1-octene etc. may also be produced. As comonomers, not only may ethylene be copolymerized, but other alpha-olefins such as propylene, 1-butene, 1-hexene or 1-octene etc. may be used.

[0041] A third product stream circulating in the slurry reactor and containing a concentrated slurry may be introduced into a second gas-phase reactor (GPR2) disposed downstream of the slurry reactor and in parallel with the first gas-phase reactor (GPR1). In a preferred embodiment of the present invention, the solids concentration in the first product stream is the same as or higher than the solids concentration in the third product stream.

[0042] In the second gas-phase reactor (GPR2), in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers and optionally hydrogen, the third product stream may be polymerized to obtain a second alpha-olefin product stream.

[0043] In GPR2, preferably the following conditions are adjusted:

[0044]

Table 2

[0045] The above conditions may be changed as the opportunity arises. Specifically, not only propylene may be polymerized, but other alpha-olefins such as ethylene, 1-butene, 1-hexene, or 1-octene may be produced. As the comonomer, not only ethylene may be copolymerized, but other alpha-olefins such as propylene, 1-butene, 1-hexene, or 1-octene may be used.

[0046] According to the present invention, the method may preferably be carried out such that the first product stream obtained after step (c) and withdrawn from the slurry reactor is divided into two product streams, namely a first secondary product stream (2a) and a second secondary product stream (2b).

[0047] The weight ratio of the streams (2a):(2b), i.e., the weight ratio of the first secondary product stream:the second secondary product stream, may preferably be controlled in the range of 15:85 to 85:15 via a flow control valve in the outlet stream.

[0048] In this embodiment of the present invention, the first secondary product stream (2a) is introduced into a first gas-phase reactor (GPR1) disposed downstream of the slurry reactor, and in the first gas-phase reactor (GPR1), in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers, the second secondary product stream (2b) is polymerized to obtain a first alpha-olefin product stream.

[0049]

Table 3

[0050] Preferably, the above-described conditions described for GPR1 may be similarly applied to the polymerization of the first secondary product stream (2a) in GPR1. On the other hand, the second secondary product stream (2b) may be introduced into a second gas-phase reactor (GPR2) disposed downstream of the slurry reactor and arranged in parallel with the first gas-phase reactor (GPR1). Thereafter, in the second gas-phase reactor (GPR2), the second secondary product stream (2b) is polymerized in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain a second alpha-olefin product stream.

[0051]

Table 4

[0052] Preferably, the above-described conditions described for GPR2 may be similarly applied to the polymerization of the second secondary product stream (2b) in GPR2.

[0053] Instead of or in addition to the previous paragraph, the operating temperature in the second polymerization reactor (GPR1) and the third reactor (GPR2) and optionally the fourth reactor (preferably GPR3) is preferably in the range of 70 to 95 °C, more preferably 75 to 85 °C.

[0054] In this alternative embodiment of the invention, after step (c), the first product stream obtained and withdrawn from the slurry reactor is provided and divided into two product streams, namely the first secondary product stream (2a) and the second secondary product stream (2b). The transfer of the third product stream (6) from the slurry reactor to the second gas-phase reactor (GPR2) for polymerizing the third product stream (6) in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers is optional. However, as shown in Figure 4, in a more preferred embodiment of the invention, the transfer may be provided.

[0055] In this alternative embodiment of the present invention, the transfer of the third product stream (6) from the slurry reactor to the second gas-phase reactor (GPR2) may be provided directly, or the third product stream (6) may be integrated with the second secondary product stream (2b), and both streams may be transferred together to the second gas-phase reactor (GPR2) to carry out the polymerization of the second alpha-olefin product stream (4b). Figure 4 takes into account these two options by means of a dashed line indicating the transfer of the third product stream (6). However, only one of these two options designated by the dashed line is implemented simultaneously.

[0056] When providing the third product stream (6) in any embodiment of the present invention, the solids concentration in the third product stream (6) may preferably be lower than the solids concentration in the first product stream (2) containing the concentrated slurry.

[0057] In a preferred embodiment of the present invention, the solids concentration of the first secondary product stream (2a) is higher than the solids concentration of the second secondary product stream (2b). This can be achieved in the concentration step, for example, by removing a part of the mobile phase in order to provide a concentrated first secondary product stream (2a) and a fifth product stream mainly containing the mobile phase. The fifth product stream may be recycled to the slurry reactor.

[0058] Preferably, the first alpha-olefin product stream obtained in the above step (d) or step (e'), and the second alpha-olefin product stream obtained in the above step (e) or step (f) may be integrated to form a combined alpha-olefin product stream (5). This is particularly preferred in the production of multimodal polymers that enable single-line downstream processing. Equipment and manufacturing costs can be significantly reduced.

[0059] The above method may include blending methods such as mechanical blending including mixing blends and melt blending methods and any combination thereof, similar to in-situ blending during the polymerization method. These can be carried out by methods known to those skilled in the art.

[0060] The method of the present invention may be carried out in a polymerization system as defined in claim 13. Preferred embodiments are defined in claims 18 to 20. These embodiments are shown in Figures 2 and 3 respectively. These polymerization systems are adapted to carry out the present method of the invention as described above.

[0061] The polymerization system according to the present invention may preferably further include respective transfer lines for integrating a first alpha-olefin product stream exiting a first gas-phase reactor and a second alpha-olefin product stream exiting a second gas-phase reactor.

[0062] Alternatively, the polymerization system according to the present invention may be defined according to claim 15. Thus, according to this alternative, the polymerization system includes respective transfer lines for dividing a first product stream withdrawn from a slurry reactor into a first secondary product stream (2a) and a second secondary product stream (2b), where the first product stream includes the concentrated slurry, where the first secondary product stream (2a) is introduced into a first gas-phase reactor (GPR1) and the second secondary product stream (2b) is introduced into a second gas-phase reactor (GPR2).

[0063] The polymerization system according to the above alternative embodiment of the present invention preferably withdraws a third product stream (6) from the slurry reactor and transfers it to the second gas-phase reactor (GPR2) for polymerizing the third product stream (6) in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain a second alpha-olefin product stream (4b) exiting the second gas-phase reactor (GPR2), and may further include a transfer line for this purpose.

[0064] In this alternative embodiment of the present invention, the solids concentration in the third product stream (6) may preferably be lower than the solids concentration in the first product stream (2) containing the concentrated slurry.

[0065] The polymerization system preferably further includes a flash unit (not shown) provided downstream of the slurry reactor and upstream of the first and second gas phase reactors to remove essentially all of the remaining mobile phase from the first product stream and provide a fourth product stream containing a suspension of polymer solids and gas that is transferred to at least one of the first and second gas phase reactors.

[0066] The flash unit typically consists of a heated flash pipe and a receiving vessel. The slurry entering the flash unit may preferably have a solids concentration of 30 to 60% by volume. In the flash unit, the remaining hydrocarbons are removed from the polymer. The flash pipe is preferably heated by, for example, steam or water. When water is used for heating, the heated water can advantageously be obtained from the jacket of the slurry reactor. The temperature is selected such that the fluid essentially evaporates depending on the composition of the hydrocarbon fluid. The expression "essentially removing the mobile phase" means that the major fraction of the mobile phase is removed and only the amount of fluid filling the volume between the polymer particles and the pores within the polymer particles remains with the polymer. Typically, the temperature in the receiving vessel may be 50 to 100°C, preferably 60 to 90°C, particularly 70 to 90°C, and the pressure may be preferably 10 to 30 bar, more preferably 12 to 27 bar, particularly 14 to 24 bar. To enable smooth transfer of the polymer to the gas phase reactor, the pressure is preferably higher than the pressure in the gas phase reactor. Advantageously, the pressure is at least 0.05 bar higher than in the gas phase reactor. At least a part of the overhead flow from the receiving vessel of the flash unit is directed to a recovery system for recycling to at least one of the slurry reactor and the gas phase reactor.

[0067] Before introduction into the gas-phase reactor, the product flow from the flash receiver vessel can be passed to a gas exchange zone (not shown), where the product flow is countercurrent to an essentially hydrogen-free gas fraction from diluent recovery or a pure hydrocarbon, preferably propane, to reduce the amount of hydrogen carry-over to the gas-phase reactor. The gas exchange zone may be designed as described in WO 2004 / 039847.

Example

[0068] <Example> In all reactor configurations, the following components were used.

[0069] <Reactor Configuration 1 (comparison)> A loop-gas phase (GPR) reactor configuration was employed for both reactors connected in series. A portion of the slurry circulating within the first slurry (loop) reactor (SR) was introduced as slurry stream (1) into a separation vessel (SV) such as a hydrocyclone to concentrate the solids fraction of the slurry, and then the concentrated slurry (2) was introduced as the product stream (underflow) into the downstream gas-phase reactor (GPR). The overflow stream (3) exiting the hydrocyclone, which contained a lower solids concentration than the concentrated underflow and was rich in hydrocarbons, was recycled to the slurry (loop) reactor (SR). The product stream (4) exits the gas-phase reactor (GPR) for further processing and end use.

[0070] Particularly at first, in the loop reactor, polymerization was carried out at 70 °C and a pressure of 55 bar. Liquefied propylene acting as a solvent and a monomer was continuously supplied to the reactor. A prepolymerized Ziegler-Natta catalyst was supplied to the loop reactor. Ethylene was added to the reactor as a comonomer. The polymer produced in the loop reactor was taken out of the loop reactor as a slurry having a solids concentration of up to 32% by weight and supplied to a hydrocyclone. The bottom flow of the hydrocyclone was concentrated to up to 44% by weight and transferred to a fluidized bed gas phase reactor (GPR) arranged downstream of the loop reactor. The top flow of the hydrocyclone was returned to the loop reactor. Polymerization was further carried out in the gas phase reactor (GPR), and fresh gaseous propylene, ethylene and hydrogen were supplied to the reactor to achieve the target polymer properties. The operating conditions of the GPR were a maximum of 85 °C and a maximum of 21 bar. The production rate ratio (split) between the loop reactor and the gas phase reactor was controlled to satisfy the target product properties in the final polymer. The gaseous unreacted monomers and comonomers were recovered from the product tank. The gas stream was filtered, compressed, flowed into the recovery system and circulated back to the reactor. The polymer powder from the product tank was transferred to a degassing device and further pelletized.

[0071] <Reactor configuration 2 (Example 1 of the present invention)> A loop reactor-gas phase reactor (GPR1)-gas phase reactor (GPR2) configuration was adopted, whereby, as described herein, the reactors GPR1 and GPR2 were configured in parallel downstream of the loop reactor. In a similar manner as described for Reactor configuration 1, a separation vessel (SV) such as a hydrocyclone was used, whereby the concentrated slurry (2) obtained from the underflow of the hydrocyclone was introduced into the inlet of GPR1, and the slurry (6) circulating in the loop reactor (SR) having a solid concentration lower than the solids concentration of the concentrated slurry (2) was introduced into the inlet of GPR2. In each of GPR1 and GPR2, polypropylene products (4a, 4b) were produced under the conditions shown below.

[0072] Particularly at first, in the loop reactor, polymerization was carried out at 70 °C and a pressure of up to 55 bar. Liquefied propylene acting as a solvent and a monomer was continuously supplied to the reactor. A prepolymerized Ziegler-Natta catalyst was supplied to the loop reactor. Ethylene was added to the reactor as a comonomer. The polymer produced in the loop reactor was taken out of the loop reactor as a slurry having a solids concentration of up to 32% by weight and supplied to a hydrocyclone. The bottom flow of the hydrocyclone was concentrated to up to 44% by weight and transferred to a fluidized bed gas phase reactor (GPR1) arranged downstream of the loop reactor. The top flow of the hydrocyclone was returned to the loop reactor. At the same time, another slurry stream was taken out from the bottom of the loop reactor and directly transferred to a second fluidized bed reactor (GPR2) arranged downstream of the loop reactor and in parallel with GPR1. The solids concentration in this slurry stream was up to 38% by weight due to the condensation behavior at the outlet valve. Polymerization was further carried out in the gas phase reactors (GPR1 and GPR2) arranged in parallel downstream of the slurry reactor. Fresh gases of propylene, ethylene and hydrogen were simultaneously supplied to both gas phase reactors in order to obtain the target polymer properties. The operating conditions of GPR1 and GPR2 were up to 85 °C and up to 21 bar. The production rate ratio (split) between the loop reactor and the gas phase reactors was controlled so as to satisfy the target product properties in the final polymer. The unreacted monomer and comonomer gases were recovered from the product tank. The gas stream was filtered, compressed, flowed into the recovery system and circulated back to the reactor. The polymer powder from the product tank was transferred to a degassing device and further pelletized.

[0073] Optionally, both product streams (4a, 4b) may be integrated into a combined product stream (5). However, this is not essential.

[0074] <Reactor Configuration 3 (Example 2 of the Present Invention)> The loop reactor - gas phase reactor (GPR1) - gas phase reactor (GPR2) configuration was adopted, whereby reactors GPR1 and GPR2 were configured in parallel downstream of the loop reactor as described herein. In a similar manner as described for Reactor Configuration 1, a separation vessel (SV) such as a hydrocyclone was used, whereby the concentrated slurry obtained from the underflow of the hydrocyclone was split into two product streams (2a) and (2b), with product stream (2a) introduced into the inlet of GPR1 and product stream (2b) introduced into the inlet of GPR2. In each of GPR1 and GPR2, polypropylene products (4a, 4b) were produced under the conditions shown below.

[0075] First, in the loop reactor, polymerization was carried out at 70 °C and a pressure of up to 55 bar. Liquefied propylene acting as a solvent and monomer was continuously fed to the reactor. A prepolymerized Ziegler - Natta catalyst was fed to the loop reactor. Ethylene was added to the reactor as a comonomer. The polymer produced in the loop reactor was taken out of the loop reactor as a slurry having a solids concentration of up to 38 wt% and fed to a hydrocyclone. The bottom flow of the hydrocyclone was concentrated to up to 44 wt% and split into two vapors in a 50:50 ratio. One was transferred to the first fluidized bed gas phase reactor (GPR1) arranged downstream of the loop reactor, and the other was transferred to the second fluidized bed gas phase reactor (GPR2) arranged in parallel with GPR1 and downstream of the loop reactor. The top flow of the hydrocyclone was returned to the loop reactor. In the gas phase reactors (GPR1 and GPR2), polymerization was further carried out. Fresh gases of propylene, ethylene, and hydrogen were simultaneously fed to both gas phase reactors to achieve the target polymer properties. The operating conditions of GPR1 and GPR2 were up to 85 °C and up to 21 bar. The production rate ratio (split) between the loop reactor and the gas phase reactors was controlled to meet the target product properties in the final polymer. Unreacted monomer and comonomer gases were recovered from the product tank. The gas stream was filtered, compressed, flowed into the recovery system, and circulated back to the reactor. The polymer powder from the product tank was transferred to a degassing device and further pelletized.

[0076] Optionally, the two product streams (4a, 4b) may be integrated into a combined product stream (5). However, this is not essential.

[0077] The results are shown in the table provided below.

[0078] [Table 5]

[0079] From the above examples, it can be seen that the method of the present invention achieves a higher production rate, which is partially due to less carry-over of reactants to the GPR. Example 1 produced additional polymer by the parallel arrangement of GPR2, and as a result, significantly improved the total production rate of the polymer. Since a slurry stream with a high concentration (44 wt% / 38 wt% solids concentration) was fed to the gas-phase reactor, the carry-over of reactants to the gas-phase reactor could be reduced.

[0080] Similar advantages are also apparent from Example 2 compared with the comparative examples. Furthermore, Example 2 has an additional effect on Example 1 in that the concentrated slurry stream (solids concentration 44 wt%) obtained downstream of the hydrocyclone is divided into two secondary streams that are fed in parallel to the gas-phase reactors GPR1 and GPR2. Thereby, the carry-over of reactants to the gas-phase reactor could be further reduced. Preferred embodiments of this specification include at least the following: [1] A method for producing an alpha-olefin polymer in a continuously operated multi-stage polymerization sequence in the presence of a polymerization catalyst, comprising the following steps: (a) In a slurry reactor, in the presence of a hydrocarbon diluent or a liquid monomer, polymerizing an alpha-olefin monomer and optionally one or more alpha-olefin comonomers in a slurry phase to obtain an alpha-olefin polymer, the slurry phase having a first solids concentration; (b) Continuously withdrawing from the slurry reactor a polymer slurry (1) comprising a polymer and a fluid phase; (c) By removing a portion of the fluid phase, concentrating at least a portion of the polymer slurry (1) to provide a first product stream (2) comprising a concentrated slurry having a second solids concentration higher than the first solids concentration, and a second product stream (3) mainly comprising the fluid phase; (d) In a first gas-phase reactor (GPR1) disposed downstream of the slurry reactor, polymerizing the first product stream (2) in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain a first alpha-olefin product stream (4a); (e) In a second gas-phase reactor (GPR2), polymerizing a third product stream (6) withdrawn from the slurry reactor in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain a second alpha-olefin product stream (4b). A method comprising the above steps. [2] The method according to [1], wherein the third product stream comprises a concentrated slurry having a solids concentration lower than the second solids concentration of the first product stream. [3] A method for producing an alpha-olefin polymer in a continuously operated multi-stage polymerization sequence in the presence of a polymerization catalyst, comprising the following steps: (a) In a slurry reactor, in the presence of a hydrocarbon diluent or a liquid monomer, polymerizing an alpha-olefin monomer and optionally one or more alpha-olefin comonomers in a slurry phase to obtain an alpha-olefin polymer, the slurry phase having a first solids concentration; (b) Continuously withdrawing from the slurry reactor a polymer slurry (1) comprising a polymer and a fluid phase; (c) By removing a part of the mobile phase, at least a part of the polymer slurry (1) is concentrated to provide a first product stream (2) containing a concentrated slurry having a second solids concentration higher than the first solids concentration, and a second product stream (3) mainly containing the mobile phase. (d’) Splitting the first product stream (2) into a first secondary product stream (2a) and a second secondary product stream (2b), where the first product stream (2) contains the concentrated slurry. (e’) In a first gas-phase reactor (GPR1) disposed downstream of the slurry reactor, polymerizing the first secondary product stream (2a) in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain a first alpha-olefin product stream (4a), and (f) In a second gas-phase reactor (GPR2) disposed downstream of the slurry reactor and arranged in parallel with the first gas-phase reactor (GPR1), polymerizing the second secondary product stream (2b) in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain a second alpha-olefin product stream (4b). A method comprising the above steps. [4] The method according to [3], wherein in the second gas-phase reactor (GPR2), the third product stream (6) taken out from the slurry reactor is polymerized in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain a second alpha-olefin product stream (4b). [5] The method according to any one of [1] to [4], wherein the solids concentration in the third product stream (6) is lower than the solids concentration in the first product stream (2) containing the concentrated slurry. [6] The method according to any one of [1] to [5], wherein step (c) is carried out in a hydrocyclone. [7] The method according to [6], wherein the polymer slurry (1) is concentrated to provide an underflow containing a concentrated slurry (2) and a hydrocarbon-rich overflow (3). [8] The method according to [7], wherein the overflow (3) is recycled to the slurry reactor. [9] The method according to any one of [1] to [8], wherein the first alpha-olefin product stream (4a) obtained in step (d) of [1] or step (e') of [3], and the second alpha-olefin product stream (4b) obtained in step (e) of [1] or step (f) of [3] are integrated to form a combined alpha-olefin product stream (5).

[10] The method according to any one of [1] to [9], wherein the first product stream (2) containing the concentrated slurry is taken out from the slurry reactor, whereby the solids concentration at the outlet is higher than the solids concentration in the slurry reactor.

[11] The method according to any one of [3] to

[10] , wherein the solids concentration in the first secondary product stream (2a) is higher than the solids concentration in the second secondary product stream (2b).

[12] After step (c) of [1], the first product stream (2) containing the concentrated slurry is transferred to a flash unit to remove substantially all of the remaining mobile phase to provide a modified product stream containing a suspension of polymer solids and gas, and the modified product stream is polymerized in a first gas-phase reactor (GPR1). The method according to any one of [1] to

[11] .

[13] A polymerization system for producing an alpha-olefin polymer in the presence of a polymerization catalyst in a continuously operated multi-stage polymerization sequence, comprising: (A) A slurry reactor for polymerizing an alpha-olefin monomer and optionally one or more alpha-olefin comonomers in a slurry phase in the presence of a hydrocarbon diluent or a liquid monomer to obtain an alpha-olefin polymer, the slurry phase having a first solids concentration, the slurry reactor having at least one outlet through which the polymer slurry can be continuously withdrawn; (B) A separation vessel for concentrating at least a portion of the polymer slurry withdrawn from the slurry reactor by removing a portion of the mobile phase to provide a first product stream (2) containing a concentrated slurry having a second solids concentration higher than the first solids concentration, and a second product stream (3) mainly containing the mobile phase, the separation vessel having an overflow and an underflow; (C) A first gas-phase reactor (GPR1) disposed downstream of the slurry reactor for polymerizing a first product stream (2) in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain a first alpha-olefin product stream (4a) exiting the first gas-phase reactor (GPR1), and (D) A second gas-phase reactor (GPR2) disposed downstream of the slurry reactor and in parallel with the first gas-phase reactor (GPR1) for polymerizing a third product stream (6) withdrawn from the slurry reactor in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain a second alpha-olefin product stream (4b) exiting the second gas-phase reactor (GPR2) A polymerization system comprising.

[14] The polymerization system according to

[13] , wherein the third product stream (6) comprises a concentrated slurry having a solids concentration lower than the second solids concentration of the first product stream (2).

[15] A polymerization system for producing an alpha-olefin polymer in the presence of a polymerization catalyst in a continuously operated multi-stage polymerization sequence, comprising: (A) A slurry reactor for polymerizing an alpha-olefin monomer and optionally one or more alpha-olefin comonomers in a slurry phase in the presence of a hydrocarbon diluent or a liquid monomer to obtain an alpha-olefin polymer, the slurry phase having a first solids concentration, the slurry reactor having at least one outlet through which the polymer slurry can be continuously withdrawn, (B) A separation vessel for concentrating at least a portion of the polymer slurry withdrawn from the slurry reactor by removing a portion of the fluid phase to provide a first product stream (2) comprising a concentrated slurry having a second solids concentration higher than the first solids concentration and a second product stream (3) mainly comprising the fluid phase, the separation vessel having an overflow and an underflow, and (G) Respective transfer lines for dividing the first product stream (2) withdrawn from the slurry reactor into a first secondary product stream (2a) and a second secondary product stream (2b), the first product stream (2) comprising the concentrated slurry, Including, where In a first gas-phase reactor (GPR1) arranged downstream of the slurry reactor, in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers, polymerize a first secondary product stream (2a) to obtain a first alpha-olefin product stream (4a), and A polymerization system, in a second gas-phase reactor (GPR2) arranged downstream of the slurry reactor and in parallel with the first gas-phase reactor (GPR1), polymerize a second secondary product stream (2b) in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain a second alpha-olefin product stream (4b).

[16] (D’) A transfer line for withdrawing a third product stream (6) from the slurry reactor and transferring it to the second gas-phase reactor (GPR2) in order to polymerize the third product stream (6) in the presence of an alpha-olefin monomer and optionally one or more alpha-olefin comonomers to obtain the second alpha-olefin product stream (4b) exiting the second gas-phase reactor (GPR2). The polymerization system according to

[15] , further comprising

[17] The polymerization system according to

[15] or

[16] , wherein the solid content concentration in the third product stream (6) is lower than the solid content concentration in the first product stream (2) containing the concentrated slurry.

[18] The polymerization system according to any one of

[13] to

[17] , wherein the separation vessel is a hydrocyclone.

[19] (H) Each transfer line for integrating the first alpha-olefin product stream (4a) exiting the first gas-phase reactor (GPR1) and the second alpha-olefin product stream (4b) exiting the second gas-phase reactor (GPR2) to form a combined alpha-olefin product stream (5). The polymerization system according to any one of

[13] to

[18] , further comprising

[20] (I) A flash unit provided downstream of the slurry reactor and upstream of one or both of the first gas-phase reactor and the second gas-phase reactor to remove substantially all of the remaining mobile phase from the first product stream (2) and / or the third product stream (6) and provide a modified product stream containing a suspension of polymer solids and gas to be transferred to at least one of the first gas-phase reactor and the second gas-phase reactor. The polymerization system according to any one of

[13] to

[19] , further comprising

Claims

1. A process for producing alpha-olefin polymers in the presence of a polymerization catalyst in a continuously operated multi-stage polymerization sequence comprising the steps of: (a) polymerizing an alpha-olefin monomer and, optionally, one or more alpha-olefin comonomers in a slurry phase in the presence of a hydrocarbon diluent or liquid monomer in a slurry reactor to obtain an alpha-olefin polymer, the slurry phase having a first solids concentration; (b) continuously removing a polymer slurry (1) from the slurry reactor, the polymer slurry (1) comprising a polymer and a fluid phase; (c) concentrating at least a portion of the polymer slurry (1) by removing a portion of the fluid phase to provide a first product stream (2) comprising a concentrated slurry having a second solids concentration greater than the first solids concentration, and a second product stream (3) comprising primarily the fluid phase; (d) polymerizing the first product stream (2) in the presence of alpha-olefin monomers and, optionally, one or more alpha-olefin comonomers in a first gas phase reactor (GPR1) located downstream of said slurry reactor to obtain a first alpha-olefin product stream (4a); (e) polymerizing in a second gas phase reactor (GPR2) the third product stream (6) withdrawn from the slurry reactor in the presence of alpha-olefin monomers and, optionally, one or more alpha-olefin comonomers to obtain a second alpha-olefin product stream (4b); A method comprising:

2. 10. The method of claim 1, wherein the third product stream comprises a concentrated slurry having a solids concentration lower than the second solids concentration of the first product stream.

3. A process for producing alpha-olefin polymers in the presence of a polymerization catalyst in a continuously operated multi-stage polymerization sequence comprising the steps of: (a) polymerizing an alpha-olefin monomer and, optionally, one or more alpha-olefin comonomers in a slurry phase in the presence of a hydrocarbon diluent or liquid monomer in a slurry reactor to obtain an alpha-olefin polymer, the slurry phase having a first solids concentration; (b) continuously removing a polymer slurry (1) from the slurry reactor, the polymer slurry (1) comprising a polymer and a fluid phase; (c) concentrating at least a portion of the polymer slurry (1) by removing a portion of the fluid phase to provide a first product stream (2) comprising a concentrated slurry having a second solids concentration greater than the first solids concentration, and a second product stream (3) comprising primarily the fluid phase; (d') splitting the first product stream (2) into a first secondary product stream (2a) and a second secondary product stream (2b), wherein the first product stream (2) comprises said concentrated slurry; (e') polymerizing a first secondary product stream (2a) in the presence of alpha-olefin monomers and, optionally, one or more alpha-olefin comonomers in a first gas phase reactor (GPR1) located downstream of said slurry reactor to obtain a first alpha-olefin product stream (4a); and (f) polymerizing a second secondary product stream (2b) in the presence of alpha-olefin monomers and, optionally, one or more alpha-olefin comonomers in a second gas phase reactor (GPR2) located downstream of the slurry reactor and arranged in parallel with the first gas phase reactor (GPR1) to obtain a second alpha-olefin product stream (4b); where the split (production rate ratio between the slurry reactor and the gas phase reactor) in the first gas phase reactor (GPR1) is 40% or more and the split in the second gas phase reactor (GPR2) is 25% or less; the first alpha-olefin product stream (4a) and the second alpha-olefin product stream (4b) are different from each other; method.

4. 4. The process according to claim 3, wherein the third product stream (6) withdrawn from the slurry reactor is polymerized in a second gas phase reactor (GPR2) in the presence of alpha-olefin monomers and optionally one or more alpha-olefin comonomers to obtain a second alpha-olefin product stream (4b).

5. 5. The method of claim 1, 2 or 4, wherein the solids concentration in the third product stream (6) is lower than the solids concentration in the first product stream (2) comprising the concentrated slurry.

6. The method according to any one of claims 1 to 5, wherein step (c) is carried out in a hydrocyclone.

7. 7. The process according to claim 6, wherein the polymer slurry (1) is concentrated to provide a first product stream (2), which is an underflow comprising the concentrated slurry, and a second product stream (3), which is a hydrocarbon-rich overflow.

8. 8. The process of claim 7, wherein the second product stream (3), which is the overflow, is recycled to the slurry reactor.

9. 9. The process according to any of claims 1 to 8, wherein the first alpha-olefin product stream (4a) obtained in step (d) of claim 1 or step (e') of claim 3 and the second alpha-olefin product stream (4b) obtained in step (e) of claim 1 or step (f) of claim 3 are combined to form a combined alpha-olefin product stream (5).

10. 10. The method of any of claims 1 to 9, wherein a first product stream (2) comprising a concentrated slurry is removed from a slurry reactor, whereby the solids concentration at the outlet is higher than the solids concentration in the slurry reactor.

11. The method according to any of claims 3 to 10, wherein the solids concentration in the first secondary product stream (2a) is higher than the solids concentration in the second secondary product stream (2b).

12. 12. The process according to any one of claims 1 to 11, wherein after step (c) of claims 1 and 3, a first product stream (2) comprising the concentrated slurry is transferred to a flash unit to remove essentially all of the remaining fluid phase and provide a reformed product stream comprising a suspension of polymer solids and gas, and said reformed product stream is polymerized in a first gas phase reactor (GPR1).

13. 1. A polymerization system for producing alpha-olefin polymers in the presence of a polymerization catalyst in a continuously operated multi-stage polymerization sequence, comprising: (A) a slurry reactor for polymerizing an alpha-olefin monomer and, optionally, one or more alpha-olefin comonomers in a slurry phase in the presence of a hydrocarbon diluent or liquid monomer to obtain an alpha-olefin polymer, the slurry phase having a first solids concentration, the slurry reactor having at least one outlet through which a polymer slurry can be continuously removed; (B) a separation vessel for concentrating at least a portion of the polymer slurry removed from the slurry reactor by removing a portion of the fluid phase to provide a first product stream (2) comprising a concentrated slurry having a second solids concentration higher than the first solids concentration, and a second product stream (3) comprising primarily the fluid phase, the separation vessel having an overflow and an underflow. (C) a first gas phase reactor (GPR1) located downstream of said slurry reactor for polymerizing the first product stream (2) in the presence of alpha-olefin monomers and, optionally, one or more alpha-olefin comonomers to obtain a first alpha-olefin product stream (4a) exiting the first gas phase reactor (GPR1); and (D) a second gas phase reactor (GPR2) located downstream of the slurry reactor and arranged in parallel with the first gas phase reactor (GPR1) for polymerizing the third product stream (6) removed from the slurry reactor in the presence of alpha-olefin monomers and, optionally, one or more alpha-olefin comonomers to obtain a second alpha-olefin product stream (4b) exiting the second gas phase reactor (GPR2). A polymerization system comprising:

14. 14. The polymerization system of claim 13, wherein the third product stream (6) comprises a concentrated slurry having a solids concentration lower than the second solids concentration of the first product stream (2).

15. 1. A polymerization system for producing alpha-olefin polymers in the presence of a polymerization catalyst in a continuously operated multi-stage polymerization sequence, comprising: (A) a slurry reactor for polymerizing an alpha-olefin monomer and, optionally, one or more alpha-olefin comonomers in a slurry phase in the presence of a hydrocarbon diluent or liquid monomer to obtain an alpha-olefin polymer, the slurry phase having a first solids concentration, the slurry reactor having at least one outlet through which a polymer slurry can be continuously removed; (B) a separation vessel for concentrating at least a portion of the polymer slurry removed from the slurry reactor by removing a portion of the fluid phase to provide a first product stream (2) comprising a concentrated slurry having a second solids concentration greater than the first solids concentration, and a second product stream (3) comprising primarily the fluid phase, the separation vessel having an overflow and an underflow; and (G) respective transfer lines for splitting a first product stream (2) removed from the slurry reactor into a first secondary product stream (2a) and a second secondary product stream (2b), the first product stream (2) comprising said concentrated slurry; where polymerizing a first secondary product stream (2a) in the presence of an alpha-olefin monomer and, optionally, one or more alpha-olefin comonomers in a first gas phase reactor (GPR1) located downstream of said slurry reactor to obtain a first alpha-olefin product stream (4a); polymerizing the second secondary product stream (2b) in the presence of alpha-olefin monomers and, optionally, one or more alpha-olefin comonomers in a second gas phase reactor (GPR2) located downstream of the slurry reactor and arranged in parallel with the first gas phase reactor (GPR1) to obtain a second alpha-olefin product stream (4b); The split (production rate ratio between the slurry reactor and the gas phase reactor) in the first gas phase reactor (GPR1) is 40% or more and the split in the second gas phase reactor (GPR2) is 25% or less; and the first alpha-olefin product stream (4a) and the second alpha-olefin product stream (4b) are different from each other; Polymerization system.

16. (D') a transfer line for removing the third product stream (6) from the slurry reactor and transferring it to the second gas phase reactor (GPR2) for polymerizing the third product stream (6) in the presence of alpha-olefin monomers and, optionally, one or more alpha-olefin comonomers to obtain a second alpha-olefin product stream (4b) exiting the second gas phase reactor (GPR2); The polymerization system of claim 15 further comprising:

17. 17. The polymerization system of claim 16, wherein the solids concentration in the third product stream (6) is lower than the solids concentration in the first product stream (2) comprising said concentrated slurry.

18. The polymerization system according to any one of claims 13 to 17, wherein the separation vessel is a hydrocyclone.

19. (H) transfer lines combining a first alpha-olefin product stream (4a) from the first gas phase reactor (GPR1) and a second alpha-olefin product stream (4b) from the second gas phase reactor (GPR2) to form a combined alpha-olefin product stream (5); The polymerization system according to any one of claims 13 to 18, further comprising:

20. (I) a flash unit downstream of the slurry reactor and upstream of one or both of the first and second gas phase reactors for removing essentially all of the remaining mobile phase from the first product stream (2) and / or the third product stream (6) and providing a reformed product stream comprising a suspension of polymer solids and gas which is transferred to at least one of the first and second gas phase reactors; The polymerization system according to any one of claims 13 to 19, further comprising:

Images