Process for controlling the swell ratio of polyethylene compositions

Abstract

A process for controlling the swell ratio of a polyethylene composition having a specific melt flow rate. The polyethylene composition is prepared by a process carried out in a polymerization apparatus having three or more polymerization zones in the presence of a polymerization catalyst and hydrogen as a molecular weight control agent, the polymerization zones having different ratios of hydrogen to ethylene present in the polymerization zones, and three or more polyethylene components having different average molecular weights are formed in the polymerization zones, one of the polyethylene components prepared in one of the polymerization zones having a highest average molecular weight and one of the polyethylene components prepared in another of the polymerization zones having a second-highest average molecular weight.

Description

[Technical Field]

[0001] The present disclosure provides a process for preparing a polyethylene composition, including a multimodal polyethylene. [Background technology]

[0002] Polyethylene is a widely used commercial polymer, and its success is based not only on the fact that it can be produced at relatively low cost, but also on the fact that a wide variety of materials can be made to meet specific product requirements in terms of product properties and processability. A major use of polyethylene is, for example, in the preparation of blow-molded articles.

[0003] An important factor determining the suitability of polyethylene for different applications is the swell ratio, also called the "die swell ratio." If the swell ratio is too high, the product will be heavy. If the swell ratio is too low, it will be difficult to form additional features, such as side handles on containers. Therefore, depending on the desired end product, different requirements must be met, and consumers will only accept products within strict swell ratio ranges.

[0004] Depending on the application, swell ratio requirements may vary. For example, the swell ratio requirements for a blow molding process that produces canisters at high speed in a continuous mode may be different from the requirements for small blow molding applications or large blow molding applications such as large containers or L-ring drums.

[0005] Therefore, there is a need for a configurable process that allows for the production of multimodal polyethylene compositions having swell ratios within a specified range and that can be easily switched to produce multimodal polyethylene compositions having different swell ratios. Summary of the Invention

[0006] The present disclosure provides a process as outlined in claim 1. Specifically, the present disclosure provides a process for controlling the swell ratio of a polyethylene composition having a specific melt flow rate, the polyethylene composition being prepared by a process carried out in a polymerization apparatus including three or more polymerization zones in the presence of a polymerization catalyst and hydrogen as a molecular weight control agent, the polymerization zones having different ratios of hydrogen to ethylene present in the polymerization zones, and three or more polyethylene components having different average molecular weights are formed in the polymerization zones, one of the polyethylene components having a highest average molecular weight and one of the polyethylene components having a second-highest average molecular weight being prepared in another of the polymerization zones, the swell ratio being increased by increasing the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second-highest average molecular weight, and the swell ratio being decreased by decreasing the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second-highest average molecular weight.

[0007] In some embodiments, the step of increasing or decreasing the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second highest average molecular weight is or includes changing the ratio of hydrogen to ethylene in the polymerization zone in which the polyethylene component having the highest average molecular weight is prepared or in the polymerization zone in which the polyethylene component having the second highest average molecular weight is prepared. The ratio of hydrogen to ethylene can be changed, for example, by adjusting the hydrogen supplied to the corresponding polymerization zone.

[0008] In some embodiments, the step of increasing or decreasing the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second highest average molecular weight is or includes changing the temperature in the polymerization zone in which the polyethylene component having the highest average molecular weight is prepared or in the polymerization zone in which the polyethylene component having the second highest average molecular weight is prepared.

[0009] In some embodiments, polymerization conditions in one or more polymerization zones other than the polymerization zone in which the polyethylene component having the highest average molecular weight or the polyethylene component having the second-highest average molecular weight is prepared are modified to increase or decrease the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second-highest average molecular weight, while maintaining the melt flow rate of the polyethylene composition at a particular value.

[0010] In some embodiments, polymerization conditions in the polymerization zone in which the polyethylene component having the second-highest average molecular weight is prepared are modified to increase or decrease the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second-highest average molecular weight, while maintaining the melt flow rate of the polyethylene composition at a particular value.

[0011] In some embodiments, the polymerization conditions in one or more polymerization zones that are not the polymerization zone in which the polyethylene component having the highest average molecular weight or the polymerization zone in which the polyethylene component having the second highest average molecular weight is prepared are maintained constant while the swell ratio is increased or decreased.

[0012] In some embodiments, the polymerization apparatus comprises a series of three subsequent polymerization reactors.

[0013] In some embodiments, the polymerization apparatus comprises a series of a fluidized bed reactor and a multi-zone circulating reactor.

[0014] In some embodiments, the gas fraction is removed from the reaction mixture in a gas separator located between at least two polymerization zones.

[0015] In some embodiments, a process is provided for preparing at least two polyethylene compositions having specific melt flow rates and varying swell ratios, the process comprising polymerizing ethylene in the presence of a polymerization catalyst and hydrogen as a molecular weight control agent in a series of polymerization reactors having three or more polymerization zones, the polymerization zones having different ratios of hydrogen to ethylene present in the polymerization zones, and forming three or more polyethylene components having different average molecular weights in the polymerization zones, one of the polyethylene components having a highest average molecular weight and one of the polyethylene components having a second-highest average molecular weight in another of the polymerization zones, wherein the swell ratio is increased by increasing the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second-highest average molecular weight, and wherein the swell ratio is decreased by decreasing the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second-highest average molecular weight.

[0016] In some embodiments, at least two polyethylene compositions are subsequently prepared without interrupting the process.

[0017] In some embodiments, at least one of the at least two polyethylene compositions, preferably each of the at least two polyethylene compositions, has a viscosity of 0.940 to 0.968 g / cm 3 The high density polyethylene composition has a density of [Brief explanation of the drawings]

[0018] While multiple embodiments are disclosed, still other embodiments will become apparent to those skilled in the art from the following detailed description. As will be apparent, the particular embodiments disclosed herein can be modified in various obvious aspects without departing from the spirit and scope of the claims presented herein. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive.

[0019] [Figure 1] FIG. 1 shows a schematic of an exemplary setup for preparing a polyethylene composition, in which the swell ratio of the polymer composition is controlled. [Figure 2] FIG. 2 shows a schematic of an alternative exemplary setup for preparing a polyethylene composition, in which the swell ratio of the composition is controlled. DETAILED DESCRIPTION OF THE INVENTION

[0020] It is understood that the specific melt flow rate may be a requirement imposed by the application and / or the consumer. The specific melt flow rate may be, for example, about 2.7, about 5.5, about 7.5, about 11, or about 30. As used herein, the term "about" may include a variation of ±25% of the specified value. Specifically, the specific melt flow rate may encompass ranges of 2.0 to 3.0, 4.5 to 6.5, 6.5 to 8.5, 9 to 13, or 23 to 27. Within these ranges, the swell ratio can be controlled from 120% to 250% by applying the difference in average molecular weight between the heaviest and second heaviest components.

[0021] As used herein, the term "comprising" has the broad, standard meaning of "including," "comprise," or "contain." This term includes the elements explicitly recited and also allows for the presence of other elements not recited. In addition to this broad inclusive meaning, as used herein, the term "comprising" can also have the restrictive meaning of "consisting of." This means that any aspect or embodiment of the application may be defined as including a particular feature, and also includes the meaning of simply consisting of that feature, whether or not explicitly recited. Additionally, the term "comprising" can also have the meaning of "consisting essentially of."

[0022] As used herein, the term "swell" or "die swell" refers to the expansion of a free-form parison (or annular tube of molten plastic) as it exits any die shape (converging, diverging, linear, etc.) after molten precursor resin is delivered to the die under pressure by a conventional extruder. Die swell is thus an example of a polymer stream being compressed upon entering a die, followed by a partial recovery of the polymer to its previous shape and volume after exiting the die.

[0023] The present disclosure provides a process for controlling the swell ratio of a polyethylene composition having a specific melt flow rate. The polyethylene composition may be, in particular, a multimodal ethylene copolymer. The term "multimodal" herein refers to the modality of the resulting ethylene copolymer, indicating that the ethylene copolymer comprises at least two polymer fractions obtained under different reaction conditions, regardless of whether this modality is recognizable as a separated maximum in a gel permeation chromatography (GPC) curve. The different polymerization conditions can be achieved, for example, by using different hydrogen concentrations and / or by using different comonomer concentrations in different polymerization zones. In one embodiment, the polyethylene composition may be a multimodal ethylene copolymer, having exactly three modalities.

[0024] Ethylene copolymers are prepared by polymerizing ethylene and one or more C3-C12-1 alkenes in the presence of a polymerization catalyst. The C3-C12-1 alkenes may be linear or branched. Preferred C3-C12-1 alkenes are linear C3-C10-1 alkenes such as propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, and 1-decene, or branched C2-C10-1 alkenes such as 4-methyl-1-pentene. A mixture of ethylene and two or more C3-C12-1 alkenes may also be copolymerized. Preferred comonomers are C3-C8-1 alkenes, particularly 1-butene, 1-pentene, 1-hexene, 1-heptene, and / or 1-octene. The amount of units derived from the incorporated comonomer in the prepared multimodal ethylene copolymer is preferably 0.01 to 25% by weight, more preferably 0.05 to 15% by weight, and particularly preferably 0.1 to 12% by weight. Particularly preferred is a process in which ethylene is copolymerized with 0.1 to 12% by weight of 1-hexene and / or 1-butene, in particular 0.1 to 12% by weight of 1-hexene.

[0025] In a preferred embodiment of the present disclosure, the multimodal ethylene copolymer is an ethylene-1-hexene copolymer, i.e., an ethylene copolymer obtained by polymerizing ethylene as the main monomer and 1-hexene as a comonomer.

[0026] The multimodal ethylene copolymer may comprise at least two comonomers, i.e., the multimodal ethylene copolymer may be a terpolymer or a copolymer comprising two or more comonomers. Particularly preferred ethylene copolymers comprise at least 1-hexene and 1-butene as comonomers.

[0027] Polymerization can be carried out using any of the conventional olefin polymerization catalysts. That is, polymerization can be carried out using, for example, chromium oxide-based Phillips catalysts, titanium-based Ziegler or Ziegler-Natta catalysts, single-site catalysts, or mixtures of such catalysts. For purposes of this disclosure, a single-site catalyst is a catalyst based on a chemically uniform transition metal coordination compound. Furthermore, it is also possible to use mixtures of two or more of these catalysts in the polymerization of olefins. Such mixed catalysts are often referred to as hybrid catalysts. The preparation and use of these olefin polymerization catalysts is generally known.

[0028] Preferred catalysts are Ziegler-type catalysts, preferably containing a titanium or vanadium compound, a magnesium compound, and optionally an electron donor compound and / or particulate inorganic oxide as a support material. A large fraction of high-density polyethylene blow molding compositions are made with chromium catalysts. However, it has been discovered that the ability to vary the swell ratio of products made with chromium catalysts is limited. Furthermore, the stress crack resistance of products made with chromium catalysts is inferior to that of multimodal products made with Ziegler-Natta catalysts. Ziegler-type catalysts are usually polymerized in the presence of a cocatalyst. Preferred cocatalysts are organometallic compounds of metals from Groups 1, 2, 12, 13, or 14 of the Periodic Table of the Elements, particularly organometallic compounds of metals from Group 13, especially organoaluminum compounds. Preferred cocatalysts are, for example, organometallic alkyls, organometallic alkoxides, or organometallic halides.

[0029] Preferred organometallic compounds include lithium alkyls, magnesium or zinc alkyls, magnesium alkyl halides, aluminum alkyls, silicon alkyls, silicon alkoxides, and silicon alkyl halides. More preferably, the organometallic compounds include aluminum alkyls and magnesium alkyls. Even more preferably, the organometallic compounds include aluminum alkyls and magnesium alkyls. Even more preferably, the organometallic compounds include aluminum alkyls, most preferably trialkylaluminum compounds, or compounds of this type in which the alkyl group is replaced with a halogen atom, such as chlorine or bromine. Examples of such aluminum alkyls include trimethylaluminum, triethylaluminum, triisobutylaluminum, tri-n-hexylaluminum, diethylaluminum chloride, or mixtures thereof.

[0030] The process of the present disclosure may be a polymerization process carried out in a series of at least two reactors.

[0031] In one embodiment, the polymerization process can be carried out in the gas phase. The process can be carried out in two or more gas-phase polymerization reactors, preferably exactly two gas-phase polymerization reactors arranged in series. The gas-phase polymerization reactors in the series can be any type of gas-phase polymerization reactor, for example, a horizontally or vertically stirred gas-phase reactor, a multi-zone circulating reactor, or a fluidized bed reactor. The gas-phase polymerization reactors in the series can be of the same type, or the series can include different types of gas-phase polymerization reactors. In some embodiments, the gas-phase polymerization in the series of reactors can be carried out before a prepolymerization stage, which is then preferably carried out as a suspension polymerization, preferably in a loop reactor. In an exemplary embodiment of the present disclosure, the polymerization can be carried out in a series of reactors, with a fluidized bed reactor as the first reactor, followed downstream by a multi-zone circulating reactor.

[0032] A fluidized-bed reactor is a reactor in which polymerization occurs within a layer of polyolefin particles. The reaction gas mixture is fed to the bottom of the reactor, usually under a gas distribution grid with a gas distribution function, and the gas is withdrawn again at the top of the reactor, maintaining the layer of polyolefin particles in a fluidized state. The reaction gas mixture is then returned to the bottom of the reactor via a recycle line containing a centrifugal compressor and a heat exchanger for removing the heat of polymerization. The flow rate of the reaction gas mixture must be sufficiently high to first fluidize the fine polymer layer present in the polymerization zone and, second, to effectively remove the heat of polymerization.

[0033] Multi-zone circulation reactors, such as those described in WO 97 / 04015 A1 and WO 00 / 02929 A1, have two interconnected polymerization zones: a riser through which grown polyolefin particles flow upward under rapid flow or transport conditions, and a downcomer through which grown polyolefin particles flow downward in a compact form by gravity. The polyolefin particles exiting the riser enter the downcomer, and are reintroduced into the riser, thereby establishing a polymer circulation between the two polymerization zones through which the polymer alternates multiple times. The polymerization reactor of the present disclosure has a solid-gas separator located above the downcomer to separate the polyolefin from the reaction gas mixture from the riser. The grown polyolefin particles enter the downcomer, and the separated reaction gas mixture from the riser is continuously recycled to one or more points where it is reintroduced into the polymerization reactor via a gas recycle line. Preferably, the majority of the circulation gas is recycled to the lower part of the riser. The recycle line includes a centrifugal compressor and a heat exchanger for removing heat of polymerization. Preferably, the catalyst supply line or the polyolefin particle supply line from the upstream reactor is located in the riser, and the polymer discharge system is located at the bottom of the downcomer. Introduction of make-up monomer, comonomer, hydrogen, and / or inert components can occur at various points along the riser and downcomer.

[0034] An exemplary embodiment of the setup is shown in FIG. 1, which comprises a series of fluidized bed reactors (1) arranged upstream of a multi-zone circulation reactor (21).

[0035] Ethylene can be polymerized in a fluidized-bed reactor (1), preferably in the presence of propane as an inert diluent and hydrogen as a molecular weight control agent. The fluidized-bed reactor (1) comprises a polyethylene particle fluidized bed (2), a gas distribution grid (3), and a velocity reduction zone (4). The diameter of the velocity reduction zone (4) is generally increased compared to the diameter of the fluidized-bed portion of the reactor. Upward gas supplied through the gas distribution grid (3) located at the bottom of the reactor (1) maintains the polyethylene layer in a fluidized state. The reaction gas stream discharged from the top of the velocity reduction zone (4) via a recycle line (5) is compressed by a compressor (6) and sent to a heat exchanger (7), where it is cooled and then recycled to a point below the gas distribution grid (3) at point (8) in the bottom of the fluidized-bed reactor (1). Supplemental monomers, molecular weight control agents such as hydrogen, and propane as an inert diluent may be supplied to the reactor (1) at various points via a line (9) upstream of the compressor (4). The catalyst may be fed into the fluidized bed reactor (1) via a line (12) preferably located below the fluidized bed (2).

[0036] The polyethylene particles obtained in the fluidized bed reactor (1) may be discontinuously discharged via line (11) and fed to a solid / gas separator (12) in order to avoid the gas mixture from the fluidized bed reactor (1) entering the second gas phase reactor. The gas leaving the solid / gas separator (12) is discharged as off-gas from the reactor via line (13), and the separated polyolefin particles are fed to the second gas phase reactor via line (14).

[0037] The second gas-phase reactor may be a multi-zone circulating gas-phase reactor (21) with two reaction zones, a riser (22) and a downcomer (23), through which the polyethylene particles repeatedly pass. In the riser (22), the polyethylene particles flow upward in the direction of arrow (24) under fast fluidization conditions. In the downcomer (23), the polyethylene particles flow downward by gravity in the direction of arrow (25). The riser (22) and downcomer (23) are interconnected by an upper interconnecting bend (26) and a lower interconnecting bend (27).

[0038] After flowing through the riser (22), the polyethylene particles and gas mixture leave the riser (22) and are conveyed through an upper interconnecting bend (26) to a solid / gas separation zone (28). This solid / gas separation can be accomplished using conventional separation means, such as a centrifugal separator, e.g., a cyclone. From the separation zone (28), the polyethylene particles enter the downcomer (23).

[0039] The gas mixture leaving the separation zone (28) is recycled to the riser (22) via a recycle line (29) equipped with a compressor (30) and a heat exchanger (31). Downstream of the heat exchanger (31), the recycle gas is conveyed via line (33) to the lower part of the riser (22) where rapid fluidization conditions are established.

[0040] The polyethylene particles from the first gas phase reactor enter the multi-zone circulating gas phase reactor (21) through line (14) at the lower interconnecting bend (27) at point (34).

[0041] The polyethylene particles obtained in the multi-zone circulation reactor (21) can be continuously discharged from the bottom of the downcomer (23) via the discharge line (35).

[0042] To prevent the reactant gas mixture in the riser (22) from entering the downcomer (23), a liquid stream may be supplied to the upper part of the downcomer (23) via line (40) as a barrier fluid. The liquid for creating the barrier may be produced by partially condensing the recycle gas mixture and separating the liquid and gas components in a separation vessel (62), e.g., a column. The separation vessel (62) is supplied with compressed recycle gas via line (61), which branches the recycle line (29) between the compressor (30) and the heat exchanger (31). The separated gas phase may be reintroduced into the recycle line via line (63), and the liquid fraction may be withdrawn from the separation vessel (62) via line (64) and supplied to the downcomer (23) via lines (40), (41), (42), and (43) by pump (44). Supplemental monomer, supplemental comonomer, and, optionally, inert gas and / or process additives may be introduced into lines (41), (42), and (43) through lines (45), (46), and downcomer (47), respectively, and then fed to (23) at monomer feed points (48), (49), and (50).

[0043] Make-up monomer, make-up comonomer, and, optionally, inert gas and / or process additives may also be introduced into recycle line (29) via line (51).

[0044] The polymerization apparatus according to Figure 1 comprises three polymerization zones: a first polymerization zone (65) formed by a fluidized bed reactor (1), a second polymerization zone (66) formed by an uprising pipe (22), and a third polymerization zone (67) formed by a downcomer (23). Each polymerization zone has a different ratio of hydrogen to ethylene, and thus produces three polyethylene components with different average molecular weights. The first polymerization zone (65) produces the polyethylene component with the lowest molecular weight. The second polymerization zone (66) produces the polyethylene component with the middle molecular weight, and the third polymerization zone (67) produces the polyethylene component with the highest molecular weight.

[0045] It has been found that the swell ratio of the final polyethylene composition can be controlled by manipulating the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second highest average molecular weight. To increase the swell ratio, the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second highest average molecular weight can be increased, and to decrease the swell ratio, the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second highest average molecular weight can be decreased.

[0046] In one embodiment, the step of increasing or decreasing the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second highest molecular weight may include changing the ratio of hydrogen to ethylene in the polymerization zone in which the polyethylene component having the highest average molecular weight is prepared or in the polymerization zone in which the polyethylene component having the second highest average molecular weight is prepared. The ratio of hydrogen to ethylene in the polymerization zone in which the polyethylene component having the highest average molecular weight is prepared may be adjusted to, for example, 0.002 to 0.500. The ratio of hydrogen to ethylene in the polymerization zone in which the polyethylene component having the second highest average molecular weight is prepared may be adjusted to, for example, 0.020 to 1.000. The ratio of hydrogen to ethylene can be adjusted by manipulating the hydrogen supplied to the corresponding polymerization zone.

[0047] 1, the ratio of hydrogen to ethylene feed to the downcomer (23) can be manipulated by changing the column conditions, for example, by increasing the heat introduced to the bottom of the column. The composition leaving the column can be analyzed, and the flow rate of the heating fluid to a heat exchanger (not shown) in the bottom of the column can be adjusted, either manually or automatically, until the desired hydrogen content is reached.

[0048] Alternatively, or in addition, the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second highest average molecular weight may include changing the temperature in the polymerization zone in which the polyethylene component having the highest average molecular weight is prepared or in the polymerization zone in which the polyethylene component having the second highest average molecular weight is prepared.

[0049] It is particularly preferred to increase or decrease the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second highest average molecular weight while maintaining the melt flow rate of the polyethylene composition at a particular value. The melt flow rate can be maintained constant by adjusting the polymerization conditions in any of the polymerization zones while increasing or decreasing the swell ratio.

[0050] In one embodiment, the polyethylene composition can be maintained at a particular value by adjusting the polymerization conditions in a polymerization zone that does not include the polyethylene component having the highest average molecular weight or the polyethylene component having the second highest average molecular weight.

[0051] By adjusting the polymerization conditions in the first polymerization zone (65), i.e., the polymerization zone in which neither the polyethylene component having the highest average molecular weight nor the polyethylene component having the second-highest average molecular weight is prepared, the melt flow rate of the polyethylene composition can be maintained at a particular value while increasing or decreasing the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second-highest average molecular weight.

[0052] Alternatively, or in addition, the reaction conditions in a polymerization zone other than the polymerization zone in which the polyethylene component having the highest average molecular weight or the polymerization zone in which the polyethylene component having the second highest average molecular weight is prepared, in this example the first polymerization zone (65), can be maintained constant. The melt flow rate of the polyethylene composition can be further adjusted by manipulating the hydrogen-ethylene ratio in the second polymerization zone, i.e., the polymerization zone in which the polyethylene component having the second highest average molecular weight is prepared.

[0053] The process can further include analyzing the hydrogen-ethylene ratio in each polymerization zone. The ratio can be analyzed, for example, by chromatography, such as gas chromatography.

[0054] In one embodiment, the process further comprises analyzing the final polyethylene composition and adjusting the hydrogen supply to the corresponding polymerization zone until the polyethylene composition has a specific melt flow index within a predetermined range and a specific swell ratio within a predetermined range. This step may be implemented by a computer in one embodiment. Alternatively, this step may be accomplished manually.

[0055] In the process of the present disclosure, the gaseous stream withdrawn from the polymerization apparatus may be fed to an analytical device. The analytical device may be a gas chromatograph, It may be a Raman probe, an infrared detector, a mass spectrometer, or a thermal conductivity detector. Preferably, the analytical device is a gas chromatograph.

[0056] Before being introduced into the analytical instrument, the gaseous stream may be passed through a bed of particulate solids having chemical groups on their surface that react with the organometallic compound.

[0057] In a preferred embodiment of the present disclosure, the particulate solid having chemical groups on its surface that react with organometallic compounds is a porous material such as talc, layered silicates, inorganic oxides, and the like.

[0058] Suitable inorganic oxides for use as particulate solids can be found among the oxides of elements of Groups 2, 3, 4, 5, 13, 14, 15, and 16 of the Periodic Table. Oxides or mixed oxides of calcium, aluminum, silicon, magnesium, or titanium are preferred. Other inorganic oxides can be used alone or in combination with the above-mentioned oxides, such as ZrO2 or BO3. Preferred oxides are silica, especially in the form of silica gel or pyrogenic silica, alumina, or silicon-aluminum mixed oxide. Preferred mixed oxides are, for example, calcined hydrotalcite. It is also preferred to use silica of the formula SiO2·Al2O3, where a is 0 to 2, preferably 0 to 0.5. The particles of the particulate solid may be in granular or spray-dried form, and are composed of smaller primary particles, for example, with an average particle size of 5 nm to 5 μm.

[0059] The particulate solid having on its surface a chemical group reactive with an organometallic compound preferably has an average particle size of 50 μm to 10 mm, more preferably 200 μm to 5 mm. The specific surface area of ​​the particulate solid, measured by gas adsorption using the BET method specified in ISO 9277:2010, is preferably in the range of 200 m / g to 1000 m / g, more preferably 500 m / g to 800 m / g.

[0060] The chemical groups on the surface of the particulate solid which react with the organometallic compound are preferably OH groups, adsorbed water or strained Si-O-Si bridges, especially of the calcined solid.

[0061] In a preferred embodiment of the present disclosure, the particulate solid having chemical groups on its surface that react with organometallic compounds is a silica gel equipped with a humidity indicator.

[0062] In a preferred embodiment of the present disclosure, the gaseous stream withdrawn from the polymerization apparatus is a continuous gaseous stream withdrawn from the polymerization apparatus at a flow rate of 1 Nl / h to 500 Nl / h, preferably 5 Nl / h to 350 Nl / h, and in particular 10 Nl / h to 250 Nl / h. The unit "Nl" should be understood as standard liter, which is the amount of gas per liter of volume under standard conditions of 101325 Pa (=1.01325 bar) and 0°C. As used herein, 1 g / h corresponds to approximately 11.9 Nl / h.

[0063] In a preferred embodiment of the present disclosure, the analyzer is provided with samples of the gaseous stream at regular intervals.

[0064] In a preferred embodiment of the present disclosure, two or more, e.g., two, three, four, or five, gaseous streams are withdrawn from the polymerization apparatus at different locations, and some or all samples of the two or more gaseous streams are provided to a single analyzer for analyzing the samples, or some or all of the two or more gaseous streams have a dedicated analyzer that is provided with a sample of only one gaseous stream withdrawn from the polymerization apparatus.

[0065] To analyze the material composition in a polymerization apparatus according to the present disclosure, a gaseous stream is withdrawn from the polymerization apparatus. The gaseous stream is preferably delivered into a sampling loop located adjacent to or preferably within an analytical device, such as a gas chromatograph. The sampling loop delivers a predetermined volume of gaseous sample, which is then transported into an analytical unit, e.g., by an inert carrier gas. Analytical devices are typically calibrated so that the sum of all measured components equals 100%. However, when a gaseous stream is withdrawn from a polymerization apparatus in which olefins are polymerized in the presence of a polymerization catalyst and an organometallic compound, this calibration value becomes unstable, and the measured sum of all components continues to decrease. The further the measured sum deviates from 100%, the less accurate the measurement becomes. Furthermore, while recalibration of the analytical device can restore the measured sum to 100%, each recalibration takes less and less time before unacceptable deviations are reached. It has been found that accumulation of fine solid particles in the sample loop slowly shrinks the volume of the sample loop, thereby reducing the volume added to the analytical device and distorting the analytical value. After recalibrating the analyzer once or twice, cleaning the sample loop and sampling valve is inevitable.

[0066] By passing the gaseous stream withdrawn from the polymerization reactor through a bed of particulate solids having chemical groups on their surface that react with organometallic compounds, the accumulation of fine solid particles in the sampling device of the analyzer is prevented, and the measured total of all components is kept stable. By passing the gaseous stream through a bed of particulate solids having chemical groups on their surface that react with organometallic compounds, the concentration of the organometallic compounds in the gaseous stream leaving the bed of particulate solids is preferably less than 99%, more preferably less than 99.5%, more preferably less than 99.8%, and especially less than 99.9% of the concentration of the organometallic compounds in the gaseous stream withdrawn from the polymerization reactor.

[0067] The bed of particulate solid having chemical groups on its surface that react with organometallic compounds is preferably contained in a vessel having a volume of 50 cm3 to 10,000 cm3, preferably 100 cm3 to 5,000 cm3, more preferably 200 cm3 to 2,500 cm3.

[0068] The results obtained from analyzing the gaseous stream samples provide information about the conditions within the polymerization equipment. These data can be used to determine the polymerization conditions for a particular grade or condition, or to adapt the measured polymerization conditions to predetermined values. This adaptation can be performed manually by an operator or automatically. In a preferred embodiment of the present disclosure, the results obtained from analyzing the gaseous stream samples are supplied as measurement signals to a controller for controlling the olefin polymerization process.

[0069] A gas separator may be provided between at least two polymerization zones. As used herein, a gas separator is understood to be any structure that separates the reaction mixture into a gas fraction and a solid and / or liquid fraction, the gas fraction being able to be removed from the remainder of the reaction mixture. Such a gas separator may be, for example, a flash vessel, a gas lock hopper, a barrier, a cyclone, etc.

[0070] At least two different polyethylene compositions having a particular melt flow rate and varying swell ratios can then be prepared. In particular, a first type of polyethylene composition can be prepared over a particular time frame and without process interruption, i.e., shutdown, etc., and a further type can be prepared having the same melt flow rate but different swell ratios.

[0071] In an alternative embodiment, the process of the present disclosure may be a polymerization process carried out in suspension. The process of the present disclosure may be carried out in a series of one first polymerization reactor and one or more subsequent polymerization reactors, each of which forms a polymerization zone. Such suspension polymerization, also called slurry polymerization, is carried out in a medium that is liquid or supercritical under the conditions in the corresponding polymerization reactor, in which the ethylene produced is insoluble and forms solid particles, a so-called suspension medium. The solid content of the suspension is typically in the range of 10 to 80% by weight, preferably 20 to 40% by weight.

[0072] The suspension medium that forms the liquid or supercritical phase of the suspension typically contains a diluent as the primary component, but may also contain additional components, such as dissolved monomers or comonomers, dissolved cocatalysts or scavengers (e.g., aluminum alkyls), dissolved reaction aids (e.g., hydrogen), or dissolved polymerization reaction products (e.g., oligomers or waxes). Suitable diluents must be inert, i.e., not decompose under the reaction conditions. Examples of such diluents include hydrocarbons having 3 to 12 carbon atoms, particularly saturated hydrocarbons such as isobutane, butane, propane, isopentane, pentane, hexane, or octane, or mixtures thereof. In a preferred embodiment, the diluent is a hydrocarbon mixture. To produce a hydrocarbon mixture from a raw material, less separation of the raw material components is required than to produce a specific hydrocarbon, making the hydrocarbon mixture economically more attractive as a diluent, but exhibiting the same diluent performance as a specific hydrocarbon. However, the hydrocarbon mixture may have a range of boiling points.

[0073] The diluent preferably has a boiling point significantly different from that of the monomers and comonomers used, so that these starting materials can be recovered from the mixture by distillation. Such diluents are, for example, hydrocarbons with boiling points above 40° C. or above 60° C., or mixtures containing a high proportion of these hydrocarbons. Thus, in a preferred embodiment of the present disclosure, the polymerization is carried out in a liquid suspension medium containing at least 50% by weight of saturated hydrocarbons having a boiling point above 60° C. at 0.1 MPa, or at least 80% by weight of saturated hydrocarbons having a boiling point above 60° C. at 0.1 MPa.

[0074] The slurry polymerization may be carried out at a reactor temperature of 60°C to 95°C, preferably 65°C to 90°C, more preferably 70°C to 85°C, and at a reactor pressure of 0.15 MPa to 3 MPa, preferably 0.2 MPa to 2 MPa, more preferably 0.25 MPa to 1.5 MPa.

[0075] The catalyst is fed to a polymerization reactor along with a diluent, an aluminum alkyl, ethylene, and optionally a comonomer and hydrogen, and the fed components react to form a polyethylene product suspended in a slurry that also contains diluent, unreacted ethylene, and wax. In the polyethylene product, polymer forms around the catalyst particles as a result of the polymerization reaction, thereby making the catalyst part of the polyethylene itself.

[0076] Preferably, the slurry polymerization is carried out in a multiple reactor cascade, where the reactors are operated in series and the catalyst remains active in the polymer as it flows from reactor to reactor. More preferably, the slurry polymerization is carried out in three reactors in series. In this configuration, the slurry from the first reactor in the series flows to the second reactor, and the slurry from the second reactor flows to the third reactor.

[0077] An exemplary embodiment of a reactor cascade (100) comprising three reactors (102) operated in series is shown schematically in FIG.

[0078] Each reactor (102) of the reactor cascade (100) may form a single polymerization zone (104). As shown in FIG. 2, the reactors (102) may be continuous stirred tank reactors. The diluent for the olefin polymerization in the first polymerization reactor (102) is fed to the reactor via a feed line (106), while the other components of the reaction mixture, such as catalyst, monomer, possible comonomer, and polymerization aid (such as hydrogen), are fed to the reactor via feed lines (108), (109), and (110). As a result of the polymerization in reactor (1), a slurry of solid polyolefin particles in a suspension medium is formed. This slurry is fed via line (112) to the second polymerization reactor (102), where further polymerization takes place. A gas separator (114) may be provided between at least two subsequent reactors (102) of the reactor cascade (100). The gas separator (114) can separate the reactor slurry into a vapor stream flowing in line (116) and a liquid slurry product flowing in line (118).

[0079] The liquid slurry product flows through line (118) to subsequent reactors (102), with the second reactor receiving the liquid slurry product from the first reactor and the third reactor receiving the liquid slurry product from the second reactor. The ethylene, diluent, and optionally hydrogen and comonomer are routed to a second polymerization reactor, where a polymerization reaction is carried out in the slurry to form additional polyethylene. The reactor slurry from the second polymerization reactor is transferred to a second flash vessel, where gases are separated from the reactor slurry, and the separated liquid slurry product is routed to a third polymerization reactor. The ethylene, diluent, and optionally hydrogen and comonomer are routed to a third polymerization reactor, where a polymerization reaction is carried out in the slurry to form additional polyethylene. The reactor slurry from the third polymerization reactor is transferred to a third flash drum, where gases are separated from the reactor slurry, and the liquid slurry product is forwarded for solid / liquid separation and further processing of the polymer. The nature and amount of comonomer(s) and hydrogen in the different reactors can be the same or different.

[0080] The first reactor produces a polyethylene component with a low average molecular weight, the second reactor produces a polyethylene component with an intermediate average molecular weight, and the third reactor produces a polyethylene component with a maximum average molecular weight. To control the swell ratio of the final polyethylene, the difference in average molecular weight between the polyethylene components in the second and third reactors may be manipulated, for example, by varying the hydrogen-ethylene ratio in the second and third reactors and / or by varying the reaction temperature. If a gas fraction is removed between the two reactors, the hydrogen-ethylene content may be independently adjusted for each reactor. While FIG. 2 shows a three-reactor system, it should be understood that the process can also include a reactor system including four or more reactors. Example

[0081] The following analytical methods are used to characterize the polymer compositions. density

[0082] Measured at 23°C according to ISO 11831-1. Melt Flow Rate (MFR)

[0083] Measured according to ISO 1133 at 190°C and a specified load. flow rate ratio

[0084] The flow rate ratio FRR is the ratio of MFR21.6 / MFR2.16. Rheological measurements and calculations

[0085] Rheological measurements were performed according to ASTM 4440-95a, which measures dynamic rheological data in a frequency sweep mode (plate diameter 50 mm) in a nitrogen environment to minimize sample oxidation / decomposition with a parallel plate geometry of 1.2-1.4 mm and a gap at 10% strain amplitude. The frequency ranged from 0.0251 to 398.1 rad / s.

[0086] ER is determined by the method of R. Shroff and H. Mavridis, "A New Measurement of Polydispersity from Rheological Data of Polymer Melts," J. Applied Polymer Science 57 (1995) 1605 (see also U.S. Pat. No. 5,534,472, column 10, lines 20-30). The storage modulus (G') and loss modulus (G'') are measured. A linear equation is fitted by least squares regression to logG' and logG'' using the nine lowest frequency points (five points per decade of frequency). ER is then calculated as follows: ER=(1.781*10-3)*G' where G'' = 5,000 dyn / cm 2 is the value.

[0087] As will be appreciated by those skilled in the art, a minimum G'' value of 5,000 dyn / cm 2 If greater, determining ER involves extrapolation. The calculated ER value then depends on the degree of nonlinearity in the log G'-log G' plot. The temperature, plate diameter, and frequency range are selected to yield a minimum G'' value close to or less than 5000 dyne / cm2, within the resolution of the rheometer. Swell Ratio

[0088] The swell ratio of the polymer compositions was measured at T=190°C using a capillary rheometer GOTTFERT Rheotester 2000 and Rheograph 25 equipped with a commercial 30 / 2 / 2 / 20 die (total length 30 mm, effective length = 2 mm, diameter = 2 mm, L / D = 2 / 2, and 20° approach angle) and an optical device (GOTTFERT laser diode) to measure the thickness of the extruded strand. The sample was melted in the capillary barrel at 190°C for 6 minutes and extruded at a piston speed corresponding to a die shear rate of 1440 s-1.

[0089] When the piston reaches a position 96 mm from the die entrance, the extrudate is cut (by an automatic cutting device from GOTTFERT) at a distance of 150 mm from the die exit. The extrudate diameter is measured as a function of time using a laser diode at a distance of 78 mm from the die exit. The maximum value corresponds to the D extrudate. The swell ratio is determined according to the following formula:

[0090]

number

[0091] The environmental stress crack resistance of polymer samples is measured in aqueous surfactant solutions according to the international standard ISO 16770 (FNCT). Polymer samples are compression-molded into 10 mm thick sheets. Square cross-section bars (10 x 10 x 100 mm) are notched on all four sides perpendicular to the stress direction using a razor blade. Sharp notches 1.6 mm deep are produced using the notching device described by M. Fleissner in Kunststoffe 77 (1987), p. 45.

[0092] The applied load was calculated by dividing the tension by the initial ligament area. The ligament area is the remaining area = total cross-sectional area of ​​the specimen minus the notch area. For FNCT specimens: 10x10mm 2 - 4 times the trapezoidal notch area = 46.24 mm 2 (Remaining cross section for fracture process / crack propagation). The specimens were loaded with a constant load of 4 MPa at 80°C or 6 MPa at 50°C in a 2% (by weight) aqueous solution of the non-ionic surfactant ARKOPAL N100 under the standard conditions suggested by ISO 16770. The time until the specimens failed was determined. Cast Film Measurement

[0093] Film measurements of the gels were carried out on an OCS extruder type me 23. Example 1

[0094] As shown in Figure 1, polyethylene compositions were produced in a series of fluidized bed reactors and a multi-zone circulating reactor (MZCR) with two interconnected reaction zones.

[0095] 11.7 g / h of Ziegler-Natta catalyst prepared according to Example 1a of WO 2014 / 202420 A1 with an electron donor / Ti molar feed ratio of 8 was fed using 0.7 kg / h of liquid propane to a first stirred pre-contact vessel, into which triisobutylaluminum (TIBA) and diethylaluminum chloride (DEAC) were added. The weight ratio of triisobutylaluminum to diethylaluminum chloride was 7:1. The weight ratio of aluminum alkyl to catalytic solids was 5:1. The first pre-contact vessel was maintained at 50°C with a residence time of 30 minutes. The catalyst suspension from the first pre-contact vessel was continuously transferred to a second stirred pre-contact vessel, also maintained at 50°C, with a residence time of 30 minutes. The catalyst suspension was then continuously transferred via line (10) to the fluidized bed reactor (1).

[0096] In the fluidized bed reactor (1), ethylene was polymerized in the presence of propane as an inert diluent, using hydrogen as a molecular weight regulator. 47.5 kg / h of ethylene and 185 g / h of hydrogen were fed to the fluidized bed reactor (1) via line 3. No comonomer was added. The polymerization was carried out at a temperature of 80°C and a pressure of 3.0 MPa. The selected feed rates resulted in an ethylene concentration of 10.8 vol.% and a hydrogen concentration of 29.4 vol.% in the reactor. The ratio of hydrogen to ethylene present in the fluidized bed reactor (1) was therefore 2.7.

[0097] The polyethylene component obtained in the fluidized bed reactor (1) had an MIE of 2.16 of 84 g / 10 min and a density of 0.967 g / cm 3 .

[0098] The polyethylene components obtained in the fluidized bed reactor (1) were subsequently transferred to a multi-zone circulation reactor (21), which was operated at a pressure of 2.5 MPa and a temperature of 85°C, measured at the beginning of line (29), where the reaction gas mixture leaves the separation zone (28). The riser (22) had an internal diameter of 200 mm and a length of 19 m. The downcomer (23) had a total length of 18 m, with the upper half divided into 5 m sections and having an internal diameter of 300 mm, and the lower half divided into 13 m sections and having an internal diameter of 150 mm. The final polymer was discharged discontinuously via line (35).

[0099] To prevent the reaction gas mixture in the riser (22) from entering the downcomer (23), a 330 kg / h liquid stream was fed as a barrier fluid to the top of the downcomer via line (40). The liquid for producing the barrier was obtained by partially condensing the recycle gas mixture in a heat exchanger (37) under operating conditions of 55°C and 2.6 MPa, followed by separation of the liquid and gaseous components in a separation vessel (38). The liquid barrier fluid had a composition obtained by evaporating 5.9 vol.% of the barrier fluid, 0.26 vol.% of hydrogen, 0.47 vol.% of 1-hexene, and 93.4 vol.% of propane to form a gas. To introduce additional monomer into the downcomer (23), 55 kg / h of additional barrier fluid obtained in the heat exchanger (37) was fed as input gas to three monomer feed points (48), (49), and (50) below the barrier. The total amount of fresh monomers fed to the downcomer through the monomer feed points (48), (49), and (50) was 18 kg / h of ethylene and 0.93 kg / h of 1-hexene.

[0100] To reach the target composition of the reaction gas in the riser, 5 kg / h of propane, 27.8 kg / h of ethylene and 17 g / h of hydrogen were fed through line (51) to the recycle line (29).

[0101] Of the final polyethylene composition prepared in a series of the fluidized bed reactor (1) and the multi-zone circulating reactor (21), 50% by weight was produced in the first reactor and 50% by weight was produced in the second reactor.

[0102] The polymerization conditions in the riser (22) and downcomer (23) of the multi-zone circulation reactor (21) are given in Table 1. Table 1 also reports the properties of the final polyethylene composition leaving the multi-zone circulation reactor (21). Example 2

[0103] The polymerization of Example 1 was carried out under the same conditions, except that the operating conditions of column 62 for producing the barrier fluid were varied, for example, by varying the amount of heat entering the column, so that the gas produced by evaporating the barrier fluid had a composition with a hydrogen concentration of 0.21%. Furthermore, the composition contained 5.9 vol% ethylene, 0.52 vol% 1-hexene, and 93.4 vol% propane.

[0104] The total amounts of fresh monomers fed to the downcomer through the monomer feed points (48), (49), and (50) were 9 kg / h of ethylene and 1.025 kg / h of 1-hexene.

[0105] To reach the target composition of the reaction gas in the riser, 5 kg / h of propane, 27.8 kg / h of ethylene and 21 g / h of hydrogen were fed through line (51) to the recycle line (29).

[0106] The polymerization conditions in the riser (22) and downcomer (23) of the multi-zone circulation reactor (21) are given in Table 1. Table 1 also reports the properties of the final polyethylene composition leaving the multi-zone circulation reactor (21). Example 3

[0107] The polymerization of Example 2 was carried out under the same conditions, except that the operating conditions of the column (62) for producing the barrier fluid were changed so that the gas produced by evaporating the barrier fluid had a composition in which the liquid barrier fluid had a hydrogen concentration of 0.17%. Furthermore, the composition contained 6.1 vol% ethylene, 0.52 vol% 1-hexene, and 93.2 vol% propane.

[0108] The total amounts of fresh monomers fed to the downcomer through the monomer feed points (48), (49), and (50) were 9 kg / h of ethylene and 1.025 kg / h of 1-hexene.

[0109] To reach the target composition of the reaction gas in the riser, 5 kg / h of propane, 27.8 kg / h of ethylene and 30 g / h of hydrogen were fed through line (51) to the recycle line (29).

[0110] The polymerization conditions in the riser (22) and downcomer (22) of the multi-zone circulation reactor (21) are given in Table 1. Table 1 also reports the properties of the final polyethylene composition leaving the multi-zone circulation reactor (21). Example 4

[0111] The polymerization of Example 3 was carried out under the same conditions, except that the operating conditions of the column (62) for producing the barrier fluid were changed so that the gas produced by evaporating the barrier fluid had a composition with a hydrogen concentration of 0.10%. Furthermore, the composition contained 5.8 vol% ethylene, 0.49 vol% 1-hexene, and 93.6 vol% propane.

[0112] The total amounts of fresh monomers fed to the downcomer (23) through the monomer feed points (48), (49), and (50) were 9 kg / h of ethylene and 1.025 kg / h of 1-hexene.

[0113] To reach the target composition of the reaction gas in the riser, 5 kg / h of propane, 27.8 kg / h of ethylene and 33 g / h of hydrogen were fed through line (51) to the recycle line (29).

[0114] The polymerization conditions in the riser (22) and downcomer (23) of the multi-zone circulation reactor (21) are given in Table 1. Table 1 also reports the properties of the final polyethylene composition leaving the multi-zone circulation reactor (21). Table 1 [Table 1]

[0115] Examples 1 to 4 are based on polyethylene compositions containing three polyethylene components with different average molecular weights. They demonstrate that the swell ratio of the polyethylene composition can be controlled by varying the difference between the average molecular weight of the polyethylene component with the highest average molecular weight and the average molecular weight of the polyethylene component with the second-highest average molecular weight. The polyethylene compositions obtained in Examples 1 to 4 all have substantially the same melt flow rate MFR21, and are characterized in that the polyethylene component with the lowest average molecular weight in a fluidized bed reactor is also the same. As a result, when varying the average molecular weight of the polyethylene component with the highest average molecular weight or the average molecular weight of the polyethylene component with the second-highest average molecular weight, in order to maintain the same melt flow rate in the final polyethylene composition, it is necessary to also vary the average molecular weight of the other polyethylene component with the highest or second-highest average molecular weight to balance the first variation.

[0116] In Examples 1-4, the polyethylene component obtained in the downcomer (23) is the polyethylene component having the highest average molecular weight, and the polyethylene component obtained in the riser (22) is the polyethylene component having the second highest average molecular weight. Going from the conditions of Example 1, through the conditions of Examples 2 and 3, to the conditions of Example 4, the hydrogen / ethylene ratio in the downcomer decreases, i.e., the hydrogen concentration in the downcomer decreases, and therefore the average molecular weight of the polyethylene component having the highest average molecular weight increases. To balance this variation in order to maintain the melt flow rate of the polyethylene composition, the hydrogen / ethylene ratio in the riser increases, lowering the average molecular weight of the polyethylene component having the second highest average molecular weight, and therefore increasing the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second highest average molecular weight.

[0117] The data in Table 1 show that, when the melt flow rate of the final polyethylene composition remains unchanged, increasing the difference in molecular weight between the highest average molecular weight polyethylene component and the second highest average molecular weight polyethylene component has the effect of increasing the swell ratio from 150% to 197% without significantly affecting the further property profile of the polyethylene composition.

[0118] The reverse is also true; the data in Table 1 also shows that, going from Example 4 to Example 1, the effect of decreasing swell ratio can occur when the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second highest average molecular weight decreases.

[0119] It has been found that during a continuous process, it is possible to switch between product grades having essentially the same melt flow rate and / or density but different swell ratios without having to shut down and restart the process. Rather, the processes can be run continuously, switching between each other, by adjusting the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the next highest average molecular weight, e.g., by modifying the hydrogen to ethylene ratio and / or temperature in the corresponding polymerization zones.

Claims

1. 1. A process for controlling the swell ratio of a multimodal polyethylene composition having a particular melt flow rate, the polyethylene composition being prepared by a process carried out in a polymerization apparatus comprising three or more polymerization zones in the presence of a polymerization catalyst and hydrogen as a molecular weight control agent, the polymerization zones having different ratios of hydrogen to ethylene present in the polymerization zones, wherein three or more polyethylene components having different average molecular weights are formed in the polymerization zones, one of the polyethylene components having a highest average molecular weight and one of the polyethylene components having a second highest average molecular weight and prepared in another of the polymerization zones. to increase the swell ratio, by increasing the difference between the average molecular weight of the polyethylene component having the largest average molecular weight and the average molecular weight of the polyethylene component having the second largest average molecular weight; A controlled process in which the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second highest average molecular weight is reduced to reduce the swell ratio.

2. 2. The process of claim 1, wherein the step of increasing or decreasing the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second highest average molecular weight is or includes the step of changing the ratio of hydrogen to ethylene in the polymerization zone in which the polyethylene component having the highest average molecular weight is prepared or in the polymerization zone in which the polyethylene component having the second highest average molecular weight is prepared.

3. 2. The process of claim 1, wherein the step of increasing or decreasing the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second highest average molecular weight is or includes a step of changing the temperature in the polymerization zone in which the polyethylene component having the highest average molecular weight is prepared or in the polymerization zone in which the polyethylene component having the second highest average molecular weight is prepared.

4. 2. The process of claim 1, wherein polymerization conditions in one or more polymerization zones other than the polymerization zone in which the polyethylene component having the highest average molecular weight or the polymerization zone in which the polyethylene component having the second-highest average molecular weight is prepared are modified to increase or decrease the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second-highest average molecular weight, while maintaining the melt flow rate of the polyethylene composition at a particular value.

5. 2. The process of claim 1, wherein polymerization conditions in the polymerization zone in which the polyethylene component having the second-highest average molecular weight is prepared are varied to increase or decrease the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second-highest average molecular weight, while maintaining the melt flow rate of the polyethylene composition at a particular value.

6. 6. The process of claim 5, wherein polymerization conditions in one or more polymerization zones other than the polymerization zone in which the polyethylene component having the highest average molecular weight and the polymerization zone in which the polyethylene component having the second highest average molecular weight is prepared are maintained constant.

7. 10. The process of claim 1, wherein the polymerization apparatus comprises a series of at least three subsequent polymerization reactors.

8. 10. The process of claim 1, wherein the polymerization apparatus comprises a series of a fluidized bed reactor and a multi-zone circulating reactor.

9. 10. The process of claim 1, wherein a gas separator is provided between at least two polymerization zones, said gas separator removing a gas fraction of said reaction mixture between said at least two polymerization zones.

10. 1. A process for preparing at least two polyethylene compositions having specific melt flow rates and varying swell ratios, the process comprising polymerizing ethylene in the presence of a polymerization catalyst and hydrogen as a molecular weight control agent in a series of polymerization reactors having three or more polymerization zones, the polymerization zones having different ratios of hydrogen to ethylene present in the polymerization zones, wherein three or more polyethylene components having different average molecular weights are formed in the polymerization zones, one of the polyethylene components having a highest average molecular weight and one of the polyethylene components having a second-highest average molecular weight, the process comprising: increasing the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second-highest average molecular weight to increase the swell ratio; and decreasing the difference between the average molecular weight of the polyethylene component having the highest average molecular weight and the average molecular weight of the polyethylene component having the second-highest average molecular weight to decrease the swell ratio.

11. 11. The process of claim 10, wherein the at least two polyethylene compositions are prepared subsequently.

12. At least one of the at least two polyethylene compositions has a viscosity of 0.940 to 0.968 g / cm ３ 12. The process of claim 10 or 11, wherein the high density polyethylene composition has a density of

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