WO2022263389A1

WO2022263389A1 - A method for controlling the initial fragmentation time of a porous polymerization catalyst in a process for polymerizing alpha-olefins

Info

Publication number: WO2022263389A1
Application number: PCT/EP2022/066066
Authority: WO
Inventors: Alexandra Romina ALBUNIA; Vasileios TOULOUPIDIS; Vasileios KANELLOPOULOS; Antonio DE NICOLA; Giuseppe Milano
Original assignee: Borealis Ag
Priority date: 2021-06-14
Filing date: 2022-06-14
Publication date: 2022-12-22
Also published as: TW202307013A

Abstract

The present invention relates to method for controlling the initial fragmentation time of a porous polymerization catalyst in a process for polymerizing alpha-olefin polymers, the method comprising: • Determining the initial fragmentation time of a specific porous polymerization catalyst for a given process for polymerizing a specific alpha-olefin polymer in the presence of said porous polymerization catalyst; and • Controlling the initial fragmentation time of said porous polymerization catalyst in said reactor during said polymerization process, and a process for polymerizing an alpha-olefin polymer, thereby following said method.

Description

A method for controlling the initial fragmentation time of a porous polymerization catalyst in a process for polymerizing alpha-olefins

The present invention relates to a method for controlling the initial fragmentation time of a porous polymerization catalyst in a process for polymerizing alpha-olefins.

Technical background

Polyolefins, such as polyethylene, are polymerized in low pressure polymerization processes in the presence of polymerization catalysts.

In order to adjust the properties of the resultant polyolefins, multistage processes are often in use in which a number of polymerization reactors are connected in series and fractions of the polyolefin are polymerized in the presence of the polyolefin fractions, which are polymerized in earlier polymerization stages. Due to different polymerization conditions as to e.g. temperature, pressure, monomer and comonomer content and/or chain transfer agent content polyolefin fractions with different properties are obtained in the different polymerization stages. Well establish licensing multistage polymerization processes for polymerizing polyolefins are, amongst others, e.g. Borstar™ from Borealis AG or Spheripol™ from LyondellBasell.

Usually the same polymerization catalyst is transferred through all stages of the multistage process. One well established category of polyolefin polymerization catalysts are porous polymerization catalysts. A porous polymerization catalyst is a particulate catalyst with catalyst particles, which includes pores. The catalytic centres of these catalysts are distributed over the whole surface of the catalyst including inside the pores of the catalyst. Therefore, during polymerization the polymeric chain initially not only grows on the outer surface of the polymer particles but also inside the pores. As a consequence of the growing polymeric chain inside the pores the pressure within the catalyst particles grows to such an extent that the structural integrity of the catalyst particles deteriorates and the porous catalyst particles starts to break into catalyst fragments (also known as microparticles). Said catalyst fragmentation especially occurs during the earliest phase of the polymerization process until the structure of the catalyst fragments can sustain the internal pressure due to growing polymeric chains in the remaining pores. In multistage processes catalyst fragmentation of porous polymerization catalysts, especially those that exhibit high polymerization activity, is often conducted in a so-called prepolymerization reactor, which precedes the first polymerization stage, in order to avoid uncontrolled fragmentation in said first polymerization stage.

Uncontrolled fragmentation of the catalyst particles results in fines formation and active catalyst fragments with a broad variety of different morphologies.

The catalyst particles follow a replication pattern, meaning that the growing particle maintains the morphological characteristics of the initial particle. Consequently, catalyst particles with a uniform spherical morphology yield in polyolefin particles with spherical morphology and narrow particle size distribution and hence high bulk densities, which is important for the efficient operability of the polymerization reactors, and for reaching higher production rates Thus, it is important to avoid inhomogeneous fragmentation during the earliest stage (i.e. the first minutes) of polymerization in order to obtain active catalyst fragments with a spherical morphology, which is as uniform as possible.

Additionally catalyst lot variation in terms of particle size distribution, porosity, and pore size distribution and reaction performance further complicates the selection of polymerization conditions in the earliest stage of the polymerization for ensuring a smooth fragmentation.

Therefore, it is crucial to understand the fragmentation mechanism, and predict the time needed for the expected initial fragmentation to happen, adjusting accordingly the polymerization conditions in the earliest stage of the polymerization and residence time. The present invention is based on a new model together with an engineering method for the description of the fragmentation mechanism and the prediction of the polymerization time needed for the initial fragmentation to take place.

As a consequence the polymerization conditions in the earliest stage of the polymerization can be adapted as to control the initial fragmentation rate of a porous polymerization catalyst in order to obtain catalyst fragments with uniform morphology and in the following polymerization process polyolefin particles with spherical morphology and narrow particle size distribution and hence high bulk densities, which ensure smooth and efficient operability of the polymerization process and high production rates.

Summary of the invention

The present invention relates to a method for controlling the initial fragmentation time of a porous polymerization catalyst in a process for polymerizing alpha-olefin polymers, the method comprising:

• Determining the initial fragmentation time of a specific porous polymerization catalyst for a given process for polymerizing a specific alpha-olefin polymer in the presence of said porous polymerization catalyst comprising the steps of: a) Determining the initial total pore volume (VO) of the porous polymerization catalyst; b) Determining the number of alpha-olefin monomers in form of polymerized alpha-olefin monomer units in the alpha-olefin polymer needed to fill the initial pores of the porous polymerization catalyst (n_V0) depending on the determined initial pore volume (VO); c) Determining the number of alpha-olefin monomers in form of polymerized alpha-olefin monomer units in the alpha-olefin polymer needed to start fragmentation of the porous polymerization catalyst into catalyst particles (n total) from the determined number of alpha-olefin monomers needed to fill the initial pores of the porous polymerization catalyst (n_V0); d) Determining the initial polymerization rate of the alpha-olefin polymer in the presence of the porous polymerization catalyst in a reactor of the process for polymerizing alpha-olefin polymers; and e) Determining the initial fragmentation time of said porous polymerization catalyst in the reactor depending on the number of alpha-olefin monomers needed to start fragmentation of the porous polymerization catalyst into fragmented polymerization catalyst particles (n total) and the initial polymerization rate of the alpha-olefin polymer ; and

• Controlling the initial fragmentation time of said porous polymerization catalyst in said reactor during said polymerization process comprising the steps of: f) Adjusting the polymerization rate of the alpha-olefin polymer in the presence of the porous polymerization catalyst by selecting the polymerization conditions of the alpha-olefin polymer in the reactor as such that the initial fragmentation time as calculated from process steps a) to e) is in the range of from 1 s to 60 s.

Further, the present invention relates to a process for polymerizing an alpha-olefin polymer comprising the steps of

(i) Polymerizing alpha-olefin monomers onto a porous polymerization catalyst in a first reactor thereby following the method as described above or below to produce fragmented polymerization catalyst particles;

(ii) Transferring the fragmented polymerization catalyst to a second reactor; and (iii) Polymerizing alpha-olefin monomers in the presence of the fragmented polymerization catalyst particles in the second reactor to produce a reaction mixture comprising an alpha-olefin polymer. Definitions

A porous polymerization catalyst is a particulate catalyst with catalyst particles, which includes pores. The catalytic centres (also known as catalyst active sites) of these catalysts are distributed over the whole surface of the catalyst including inside the pores of the catalyst.

The initial total pore volume of the porous polymerization catalyst reflects the volume of all pores of the catalyst particles before start of the polymerization reaction.

The initial pores of the porous polymerization catalyst are the pores of the catalyst particles before start of fragmentation.

The initial polymerization rate of an alpha-olefin polymer is the polymerization rate of said alpha-olefin polymer at the earliest stage of the polymerization process in which the initial pores of the porous polymerization catalyst are filled with alpha- olefin polymer.

The initial fragmentation time of a porous polymerization catalyst is the time from the start of the polymerization reaction until the first disintegration and fragmentation of the particles of the porous polymerization catalyst into fragmented polymerization catalyst particles.

“Specific porous polymerization catalyst”, “specific alpha-olefin polymer” and “given process” means that the method according to the invention is applied for an existing polymerization process in which the porous polymerization process and the resulting alpha-olefin polymer are known. The method can be used for different catalytic processes for polymerizing alpha-olefin polymers of different variety as long as the polymerization catalyst is porous.

A multistage polymerization process is a process for polymerizing monomers in which two or more polymerization reactors are connected in series.

An ethylene based polymer is a polymer with a molar amount of ethylene monomer units of more than 50 mol%. Brief description of the figures

Figure 1 shows a schematic model of the catalyst pore volume and the active centre column used in the model for determining the number of alpha-olefin monomers in form of polymerized alpha-olefin monomer units in the alpha-olefin polymer needed to fill the initial pores of the porous polymerization catalyst (n_V0) depending on the determined initial pore volume (VO).

Figure 2 shows indicative examples of catalyst centres dispersion (100%, 50% and 4%) in the catalyst pores used in the model for determining the number of alpha- olefin monomers in form of polymerized alpha-olefin monomer units in the alpha- olefin polymer needed to fill the initial pores of the porous polymerization catalyst (n_V0) depending on the determined initial pore volume (V0).

Figure 3 shows exemplary electron micrographs of the SEM/EDS analysis for two different lots of Lynx 200 polymerization catalyst after 10 min pre-polymerization. Figure 4 shows the reconstructed activity profile of the catalyst of Example 1.

Figure 5 shows the reconstructed activity profile of the catalyst of Example 2. Figure 6 shows the reconstructed activity profile of the catalyst of Example 3.

Detailed description of the invention

Method In a first aspect the present invention relates to a method for controlling the initial fragmentation time of a porous polymerization catalyst in a process for polymerizing alpha-olefin polymers, the method comprising:

The inventive method is based on the finding that a smooth and efficient operability of an olefin polymerization process resulting in polyolefin particles with spherical morphology, narrow particle size distribution and high bulk densities is dependent on the initial fragmentation behaviour of the porous polymerization catalyst.

Instantaneous and uncontrollable initial fragmentation of the porous polymerization catalyst results in catalyst fragments of uneven morphology and catalyst centre distribution which results in uneven polymerization patterns. As a consequence, polyolefin particles with low spherical morphology are obtained which cause operability issues and limitations in the polymerization process associated to fines generation, segregation phenomena, severe external mass and heat transfer limitations, increased back mixing conditions, poor fluidization and excessive sheeting and chunking. By controlling the initial fragmentation time of the porous polymerization catalyst, catalyst fragments are obtained which are sufficiently encapsulated by the initially produced polyolefin chains and allow smooth and efficient operability of an olefin polymerization process resulting in polyolefin particles with spherical morphology, narrow particle size distribution and high bulk densities.

It has been found that the initial fragmentation time of the porous polymerization time depends on time in which the initial pores of the porous polymerization catalyst are filled with alpha-olefin polymer to such an extent that the pressure inside the catalyst particles causes disintegration and fragmentation of the porous polymerization catalyst.

Said time depends on the initial pore volume of the porous polymerization catalyst (VO), the number of alpha-olefin monomers in form of polymerized alpha-olefin monomer units in the alpha-olefin polymer needed to fill the initial pores of the porous polymerization catalyst (n_V0), the number of alpha-olefin monomers in form of polymerized alpha-olefin monomer units in the alpha-olefin polymer needed to start fragmentation of the porous polymerization catalyst into catalyst particles (n total), and the polymerization rate of the alpha-olefin polymer in the presence of the porous polymerization catalyst.

The initial fragmentation time of the porous polymerization catalyst can then be adjusted by adjusting the polymerization rate of the alpha-olefin polymer by selecting the polymerization conditions of the alpha-olefin polymer in the reactor. In a first step a) of the inventive method the initial pore volume (VO) of the porous polymerization catalyst is determined.

The initial pore volume (VO) is the volume of all pores of the catalyst particle and can be determined by any suitable method such as e.g. mercury pressure porosimetry, gas porosimetry, capillary flow porometry, X-ray refraction, and other suitable methods.

When using commercial catalysts the initial pore volume (VO) can also be calculated from the catalyst particle dimensions and the porosity data disclosed in their technical data sheets. It is preferred that the initial pore volume (VO) of the porous polymerization catalyst is determined from the initial particle size and the initial porosity of the porous polymerization catalyst suitably determined as described above. In a further step b) the number of alpha-olefin monomers in form of polymerized alpha-olefin monomer units in the alpha-olefin polymer needed to fill the initial pores of the porous polymerization catalyst (n_V0) is determined depending on the determined initial pore volume (VO).

Thereby, n_V0 depends on the three-dimensional structure of the alpha-olefin polymer as well as on the initial pore volume (VO).

The dimensions of the alpha-olefin polymer are usually dependent on the alpha- olefin monomers and the lamellarity of the resultant alpha-olefin polymer. An alpha- olefin polymer usually has a three-dimensional structure of lamellas of the polymeric chains. The dimensions of the lamellaric structures thereby are dependent on the crystallinity and thus the density of the alpha-olefin polymer as such that the higher the density and crystallinity of the alpha-olefin polymer the higher is the lamellarity of alpha-olefin polymer. An alpha-olefin polymer with low crystallinity and density, e.g. by means of a higher number of side chains tends to aggregate in a more loose three-dimensional structure.

These principles are well known in the art and can be read about in any handbooks dealing with the structure of polyolefins such as polyethylene or polypropylene. Consequently, the number of alpha-olefin monomers needed to fill the pores of the porous polymerization catalyst (n_V0) is determined depending on the density of the alpha-olefin polymer.

In a further step c) of the inventive method the number of alpha-olefin monomers in form of polymerized alpha-olefin monomer units in the alpha-olefin polymer needed to start fragmentation of the porous polymerization catalyst into catalyst particles (n total) is determined from the determined number of alpha-olefin monomers needed to fill the initial pores of the porous polymerization catalyst (n_V0) in addition to the number of additional monomer units beyond n_V0 required for the pressure built-up until fragmentation (n breakup), i.e. n total = n_V0 + n breakup.

In practice, n breakup can be estimated using molecular dynamics modelling.

As discussed above, the fragmentation of the porous polymerization catalyst usually starts at a certain pressure inside the catalyst particle. Said inner pressure is typically increased by the polymerization of the polymeric chain in the initial pores of the porous polymerization catalyst. Thereby, usually the lamellarity of the alpha-olefin polymer as discussed above plays a role.

The above discussed observations regarding method steps a) to c) can be incorporated into a model for describing the fragmentation mechanism of a porous polymerization catalyst.

Said proposed model takes into account the dispersion of the active centres within the catalyst particle and thus, it can be further employed in reverse mode in order to define the centres dispersion (in terms of area coverage) or distribution (in terms of local concentration), using experimental information derived from bench-scale polymerization.

Said preferred model can be described as follows:

The catalyst particle exhibits a porosity value corresponding to the volume fraction of the pores. The total catalyst pore volume is represented as a parallelepiped with volume equal to the pore volume and area equal to the catalyst area as shown in Figure 1. The average pore size (d_p0re) is a result of the given volume and area according to the formula

The catalyst centres, located on the pore surface (the area of the pore walls), are the positions where polymerization takes place, forming additional building blocks to the polymer chain. The pore surface (where the catalyst centres are dispersed) is discretized. It is assumed that the minimum area for a catalyst centre to exist and react and develop crystalline lamellas is a square of 4x4nm².

As the polymerization reaction takes place, new polymer mass is formed and gradually fills the pore volume. Depending on the crystallinity of the polymer produced, two extreme flow behaviours are foreseen: (i) amorphous polymer behaves as a liquid and it is able to flow, and,

(ii) crystalline polymer behaves as a solid and crystallizes in a strict pattern over the catalyst centre forming a stiff column of lamellas of dimension of 4x4nm².

The catalyst centres may cover the whole pore surface (perfect dispersion) or being partially dispersed over the pore area (covering a fraction of the available area) as illustrated in Figure 2. Figure 2 shows indicative examples of catalyst centres dispersions of 100% (left), 50% (middle) and 4% (right). The total concentration of the active centres is proportional to the chemical composition of the catalyst active metal; however, this amount may be dispersed in different ways (i.e., evenly, partly evenly or unevenly). Moreover, even for partial surface coverage it can be considered a homogeneous or not distribution to the covered area. For simplicity, homogeneous distribution of the catalyst centres to the fraction covered is assumed. The homogeneous distribution and 100% dispersion represents the ideal condition. Nevertheless, various SEM-EDS measurements indicate that the chemical components are not homogeneously nor fully dispersed over the catalyst surface.

The catalyst centre distribution can be determined for each lot of the porous polymerization catalyst by SEM-EDS measurements. A suitable method is described below in the experimental section. The initial catalyst fragmentation time is affected by the flow behaviour of the polymer produced and the catalyst centres dispersion:

1. In the case of amorphous polymer produced, the whole pore volume has to be filled. Since amorphous polymer flows as liquid the volume of the formulated polymer has to be equal to the volume of the pore. In the amorphous polymer case, the catalyst centres dispersion will not affect the time needed for the initial fragmentation.

2. In the case of crystalline polymer only a single column of crystalline polymer having a dimension of 4x4nm² and height equal to the pore size (width) would be enough. As it can be understood, lower pore surface coverage, leading to higher local concentration of catalyst centres will lead to faster initial fragmentation times. Thus, in the case of the alpha-olefin polymer being a non-amorphous alpha-olefin polymer, such as a non-amorphous ethylene based polymer, the number of alpha- olefin monomers, preferably ethylene monomers, needed to start fragmentation of the porous polymerization catalyst into catalyst particles (n total) is preferably inversely proportional to the total concentration of the number of active catalytic centres distributed over the pore surface of the initial pores of the porous polymerization catalyst.

In the case of the alpha-olefin polymer being an amorphous alpha-olefin polymer, such as an amorphous ethylene based polymer, the number of alpha-olefin monomers, preferably ethylene monomers, needed to start fragmentation of the porous polymerization catalyst into fragmented polymerization catalyst particles (n total) is preferably independent from the total concentration of the number of active catalytic centres distributed over the pore surface of the initial pores of the porous polymerization catalyst.

In a further step d) of the inventive method the initial polymerization rate of the alpha-olefin polymer in the presence of the porous polymerization catalyst in a reactor of the process for polymerizing alpha-olefin polymers is determined.

The initial polymerization rate thereby usually depends on the polymerization conditions such as temperature, pressure, alpha-olefin monomer and comonomer concentrations, chain transfer agent concentrations and the catalyst activity.

The initial polymerization rate can be determined by experimental measurement or theoretically using kinetic models and simulation tools.

One suitable kinetic model which can be applied in the method of the invention is described in V. Touloupidis, A. Albrecht, J. B. P. Soares, Macromol. Reac. Eng. 2018, 12, 2, 1700056 and V. Touloupidis, G. Rittenschober, C. Paulik, Macromol. React. Eng. , 2021, D01:0.1002/mren.202000028.

In a further step e) of the method of the invention the initial fragmentation time of said porous polymerization catalyst in the reactor is determined depending on the number of alpha-olefin monomers needed to start fragmentation of the porous polymerization catalyst into fragmented polymerization catalyst particles (n total) and the initial polymerization rate of the alpha-olefin polymer. Thereby, preferably the catalyst reaction kinetic parameters, the catalyst morphological characteristics (particle size, average porosity, pore size distribution and catalyst centres distribution), the reaction conditions and the density of the produced polymer. For these parameters the above described model can be applied.

In one embodiment the number of alpha-olefin monomers needed to start fragmentation of the porous polymerization catalyst into fragmented polymerization catalyst particles (n total) is preferably additionally dependent on the overflow of alpha-olefin monomers in form of polymerized alpha-olefin monomer units in the alpha-olefin polymer in the initial pores of the porous polymerization catalyst ((l/b)*DP.

In this regard, DP is the difference of the pressure inside to outside of the porous polymerization catalyst in the presence of the alpha-olefin polymer in the initial pores of the porous polymerization catalyst.

(1/b) is a parameter depending on the alpha-olefin monomers and the density of the alpha-olefin polymer in the initial pores of the porous polymerization catalyst and optionally on one or both of the alpha-olefin comonomers in form of polymerized alpha-olefin comonomer units in the alpha-olefin polymer and/or chain transfer agent in the initial pores of the porous polymerization catalyst.

(1/b) is preferably an empiric parameter b is expressed as [bar nm²/n ].

For example for an amorphous ethylene polymer, b is 25-45, such as 41. For example for a semi-crystalline ethylene polymer, b is 100-300, such as 271.

The value of b is estimated via Molecular Dynamics Simulation, and depends on both the polymer structure (for example crystallinity) and the reactor media composition (for example monomer, comonomer, chain transfer agent, diluent). In this embodiment even distribution of the catalyst active sites in the catalyst particles is considered. The number of alpha-olefin monomers needed to start fragmentation of the porous polymerization catalyst into fragmented polymerization catalyst particles (n total) in this embodiment is calculated according to the following formula: n_total = n_V0 + (l/b)*DP.

In this embodiment n total not only depends on the number of alpha-olefin monomers in form of polymerized alpha-olefin monomer units in the alpha-olefin polymer needed to fill the initial pores of the porous polymerization catalyst (n_V0) but also on the factor (l/b)*DP.

However, in has been found that for determining n total in most models, such as the model as described above, said factor (l/b)*DP can be neglected.

For ensuring controlled catalyst fragmentation it has been found that the fragmentation time is in the range of from 1 s to 60 s, preferably from 2 s to 30 s, most preferably from 3 s to 20 s.

Thus, in the case that for a process for polymerizing alpha-olefin polymers the initial polymerization rate is too high or too low the initial fragmentation time of said porous polymerization catalyst in the reactor can be adjusted in step f) of the inventive method by adjusting the polymerization rate of the alpha-olefin polymer in the presence of the porous polymerization catalyst.

The polymerization rate of the alpha-olefin polymer in the presence of the porous polymerization catalyst is adjusted by selecting the polymerization conditions of the alpha-olefin polymer in the reactor. Suitable polymerization conditions to be selected in this regard are temperature, pressure, alpha-olefin monomer and comonomer concentrations and chain transfer agent concentrations.

It is preferred that the polymerization temperature is in the range of from 30 to 90°C. Further, the pressure is preferably in the range of from 20 to 70 bar.

Additionally, the alpha-olefin monomer concentration is preferably in the range of from 1 to 20 mol%.

In the case that a comonomer is present, the comonomer is preferably selected from alpha olefins having 4 or 6 carbon atoms, i.e. 1 -butene or 1 -hexene. The molar ratio of the comonomer to alpha-olefin monomer is preferably in the range of from 0 to 600 mol/kmol.

Further, the molar ratio of the chain transfer agent to alpha-olefin monomer is preferably in the range of from 0 to 500 mol/kmol. The chain transfer agent is preferably hydrogen.

The method of the invention is suitable for controlling the initial fragmentation time of a porous polymerization catalyst in a process for polymerizing alpha-olefin polymers. The porous polymerization catalyst is preferably a particulate polymerization catalyst, which can be supported or is self-supporting.

The method is applicable for any kind of porous polymerization catalyst suitable for polymerizing alpha-olefin polymers, such as porous Ziegler-Natta catalysts, porous single-site catalysts or others.

The process for polymerizing alpha-olefin polymers is preferably conducted in one or more polymerization reactors. It is preferred that the polymerization reactor is the first reactor of a multistage polymerization process for polymerizing alpha-olefin polymers in the presence of a porous polymerization catalyst.

The process for polymerizing alpha-olefin polymers is preferably a multistage polymerization process in which two or more reactors, preferably two to six reactors, such as two, three, four, five or six reactors are connected in series. The two or more reactors are usually selected from slurry phase reactors, such as loop reactors, and gas phase reactors, such as fluidized bed gas phase reactors. Typical multistage polymerization processes in which the method of the invention is applicable are amongst others, e.g. Borstar™ from Borealis AG or Spheripol™ from LyondellBasell.

In one embodiment the first reactor of the multistage polymerization process is the first polymerization reactor followed by subsequent polymerization reactors.

In another embodiment the first reactor of the multistage polymerization process is a pre-polymerization reactor followed by the first polymerization reactor and subsequent polymerization reactor(s).

A pre-polymerization step is required in some polymerization processes in order to properly condition the catalyst before entering the harsh reaction conditions of the first loop reactor. This takes place in an additional pre-polymerization reactor in the series, which is usually a slurry phase reactor and typically smaller in size than the following first polymerization reactor. The reaction conditions are milder in terms of temperature and concentrations, targeting to lower polymerization rate, enabling a smoother particle growth and controlled fragmentation. The pre-polymerization step can be necessary since the polymerization conditions in the loop reactors are such that the local reaction rates are quite high, and fresh catalyst particles, especially the highly active ones used in this process, would not maintain their integrity without pre-polymerization. The pre-polymerization step is carried out in continuous mode and it ensures good morphology of the final polymer and makes up a few percent of the total polymerization production.

Typically the amount of alpha-olefin polymer polymerized in the pre-polymerization reactor makes up from 1.0 to 7.0 wt% of the total alpha-olefin polymer polymerized in the polymerization process.

Preferably the process for polymerizing alpha-olefin polymers is a continuous process. It is especially preferred that the process for polymerizing alpha-olefin polymers is a continuous process in said first reactor is continuous.

The average residence time of the porous polymerization catalyst in the reactor, preferably the first reactor is in the range of from 10 min to 60 min, preferably from 20 min to 45 min and most preferably from 25 min to 35 min.

The alpha-olefin monomers polymerized in the polymerizing alpha-olefin polymers are preferably selected from one or more alpha-olefin monomers having from 2 to 12 carbon atoms, more preferably from 2 to 10 carbon atoms, still more preferably from 2 to 8 carbon atoms, such as ethylene, propylene, 1 -butene, 1 -hexene and 1-octene. The alpha-olefin monomers preferably comprise ethylene monomers, more preferably ethylene monomers as major molar monomer component.

The alpha-olefin monomers can comprise ethylene monomers as single sort of alpha- olefin monomers.

Alternatively, the alpha-olefin monomers can comprise ethylene monomers as major molar monomer component and alpha-olefin monomers having from 2 to 12 carbon atoms, more preferably from 2 to 10 carbon atoms, still more preferably from 2 to 8 carbon atoms, such as propylene, 1 -butene, 1 -hexene and 1-octene, as minor molar monomer component. The alpha-olefin polymer is preferably a polymer based on an alpha-olefin having from 2 to 12 carbon atoms, more preferably from 2 to 10 carbon atoms, still more preferably from 2 to 8 carbon atoms, such as ethylene, propylene, 1 -butene, 1 -hexene and 1-octene.

It is preferred that the alpha-olefin polymer is an ethylene based polymer. The ethylene based polymer can be an ethylene homopolymer or a copolymer of ethylene and comonomer units selected from 2 to 12 carbon atoms, more preferably from 2 to 10 carbon atoms, still more preferably from 2 to 8 carbon atoms, such as propylene,

1 -butene, 1 -hexene and 1-octene, as minor molar monomer component.

In the case of a copolymer of ethylene and comonomer units selected from 2 to 12 carbon atoms the initial fragmentation time can be adjusted, amongst others, by adjusting the concentration of comonomer units selected from 2 to 12 carbon atoms.

The initial fragmentation time can be adjusted, amongst others, by adjusting the concentration of chain transfer agent.

A suitable chain transfer agent is hydrogen.

The method of the invention preferably has the following features and benefits:

• Estimation of the time of the initial fragmentation considering catalyst centres coverage.

• Define catalyst lot variation with regards to centres dispersion. · Support the selection of the pre-polymerization operating conditions for ensuring a mild fragmentation process.

• Decrease the amount of fines as a result of not controlled fragmentation within the pre-polymerization reactor. • Uniform catalyst particles growth rates leading to narrow particle size distribution.

• Less tendency for particle segregation due to narrower particle size distribution.

• Less risk for sheeting and chunking in subsequent polymerization stages, especially in gas phase polymerization reactors.

Further, the inventive method is able to estimate the time of the initial catalyst fragmentation considering the catalyst reaction kinetic parameters, the catalyst morphological characteristics (particle size, average porosity, pore size distribution and catalyst centres distribution), the reaction conditions and the density of the produced polymer. This information is crucial for the selection of the pre polymerization operating conditions.

Moreover, the method can be employed in reverse mode using bench scale polymerizations in order to estimate the catalyst centres dispersion and define catalyst lot variation with regard to its reaction performance and catalyst centres dispersion.

Process

In a further aspect the present invention relates to process for polymerizing an alpha- olefin polymer comprising the steps of

(ii) Transferring the fragmented polymerization catalyst to a second reactor; and (iii) Polymerizing alpha-olefin monomers in the presence of the fragmented polymerization catalyst particles in the second reactor to produce a reaction mixture comprising an alpha-olefin polymer. The process preferably is a multistage polymerization process in which two or more reactors, preferably two to six reactors, such as two, three, four, five or six reactors are connected in series. The two or more reactors are usually selected from slurry phase reactors, such as loop reactors, and gas phase reactors, such as fluidized bed gas phase reactors. Typical multistage polymerization processes in which the method of the invention is applicable are amongst others, e.g. Borstar™ from Borealis AG or Spheripol™ from LyondellBasell.

In one embodiment the first reactor is the first polymerization reactor and the second reactor is the second polymerization reactors.

In another embodiment the first reactor is a pre-polymerization reactor and the second reactor is the first polymerization reactor. The second reactor can be succeeded by further reactors which are connected in series and in which further reactors alpha-olefin monomers are polymerized in the presence of the fragmented polymerization catalyst particles and the alpha-olefin polymer polymerized in the previous reactor(s). The second reactor and optional further reactors are selected from slurry phase reactor(s), preferably loop reactor(s), and/or gas phase reactor(s), preferably fluidized bed reactor(s).

The process is preferably suitable for polymerizing an alpha-olefin polymer which is preferably a polymer based on an alpha-olefin having from 2 to 12 carbon atoms, more preferably from 2 to 10 carbon atoms, still more preferably from 2 to 8 carbon atoms, such as ethylene, propylene, 1 -butene, 1 -hexene and 1-octene. It is preferred that the alpha-olefin polymer is an ethylene based polymer. The ethylene based polymer can be an ethylene homopolymer or a copolymer of ethylene and comonomer units selected from 2 to 12 carbon atoms, more preferably from 2 to 10 carbon atoms, still more preferably from 2 to 8 carbon atoms, such as propylene, 1 -butene, 1 -hexene and 1-octene, as minor molar monomer component.

The alpha-olefin monomers are preferably selected from one or more alpha-olefin monomers having from 2 to 12 carbon atoms, more preferably from 2 to 10 carbon atoms, still more preferably from 2 to 8 carbon atoms, such as ethylene, propylene, 1 -butene, 1 -hexene and 1-octene.

The alpha-olefin monomers preferably comprise ethylene monomers, more preferably ethylene monomers as major molar monomer component.

Alternatively, the alpha-olefin monomers can comprise ethylene monomers as major molar monomer component and alpha-olefin monomers having from 2 to 12 carbon atoms, more preferably from 2 to 10 carbon atoms, still more preferably from 2 to 8 carbon atoms, such as propylene, 1 -butene, 1 -hexene and 1-octene, as minor molar monomer component.

The alpha-olefin monomers can be polymerized in the presence of a chain transfer agent such as hydrogen. All embodiments of features described for the method of invention in the first aspect also apply of the process of the invention in the second aspect. The process of the invention shows a smooth and efficient operability resulting in polyolefin particles with spherical morphology, narrow particle size distribution and high bulk densities. Examples

The following Examples are included to demonstrate certain aspects and embodiments of the invention as described in the claims. It should be appreciated by those of skill in the art, however, that the following description is illustrative only and should not be taken in any way as a restriction of the invention.

Reactor setup

The polymerization experiments were carried out in a 5 L batch reactor constructed for a maximum operating pressure of 60 bar. A helical stirrer ensures good mixing conditions in slurry and in gas phase polymerizations. A jacket heating system is used for temperature control.

Ethylene and hydrogen can be introduced in the reactor either by batch-dosing or continuously with thermal mass flow controllers. During polymerization experiments, ethylene is continuously fed in order to maintain the pressure at the desired set-point, counterbalancing the pressure decrease due to monomer consumption. The monomer inflow rate can be used for precise monitoring of the polymerization activity. Comonomer (in this instance 1 -butene) can be added to the reactor with a syringe pump.

Lynx 200, Ziegler-Natta catalyst supplied by Grace, is prepared offline in a glovebox and is then transferred into the reactor by an injection unit consisting of a pneumatically driven cylinder. The resulting pressure spikes are minor and stable process conditions are reached shortly after the injection. The temperature, reactor pressure and mass flow values are continuously recorded during the experiment by a data acquisition/control unit.

The average catalyst particle size (dso) was 10pm and the span ((d9o - dio) / dso) was 0.8.

The particle volume of the catalyst was 5.236e ¹⁶ m³.

The average porosity (volume fraction, porous volume/particle volume) of the catalyst particle was 20% and the average pore size was 400nm.

The catalyst density was equal to 2000kg/m³. VO was 1.047 m³.

Determination of the catalyst centre distribution by SEM/EDS analysis

SEM/EDS analysis is performed to prepolymerised catalyst samples using FEI Quanta 200F Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectrometer (EDS). In this method, the prepolymerised catalyst particles are cut with microtome blade and attached on a sample holder with carbon conductive adhesive. The cut specimens are sputter-coated with Pd/Au in a Polaron sputter coater to make them conductive. The thus obtained samples are analysed by SEM/EDS to analyse the aluminium, carbon, chlorine, magnesium, oxygen and titanium contents in the cut sample surfaces as it is known in the art.

Exemplary electron micrographs obtained are shown in Figure 3 for the elements Al, C, Cl, Mg, O and Ti, as well as an overall SEM picture of the particles included in the SEM analysis.

Figure 3 shows the electron micrographs for two different lots of Lynx 200 polymerization catalyst after 10 min pre-polymerization using the following prepolymerization conditions:

The reactor is operated at a temperature of 70°C and the pressure set-point is selected to be 20 barg (typical Borstar™ pre-polymerization process conditions). More specifically, at room temperature the reactor was filled with 800 g propane, TEA, and 10 g ethylene and lOOOOnml hydrogen were further batch-fed, the temperature was increased to 70°C and propane and catalyst was introduced until the pressure reaches the set-point value. Ethylene was continuously fed to maintain the set-point value. The polymerization time for the experiment was set to 10 minutes.

Example 1 (Comparative)

In Example 1, the reactor is operated at a temperature of 95°C and the pressure set- point is selected to be 50 barg (typical Borstar™ slurry phase (i.e. loop reactor) process conditions). More specifically, the reactor was filled with 1.0kg propane, and 41.5g ethylene and lOOOOnml hydrogen were further batch-fed until the pressure reaches the set-point value. 90mg of catalyst and 5e ⁴ mol TEA were added. The polymerization time for the experiment was set to 60 minutes. The crystallinity of the polymer produced was estimated equal 78% based on DSC measurement. This crystallinity would translate to a polymer density of 942kg/m³ under the above defined conditions.

According to the inventive method: [Catalyst particle volume, m³] = (4*3.14159/3)*([Catalyst particle radius, m]³)

[Pore volume, m³] = [Catalyst particle volume, m³]* [Porosity, fraction]

[Pore mass, kg] = [Pore volume, m³]* [Polymer density, kg/m³]

These calculations estimate (per particle): n_V0(mass) = 9.86e ^u g, n_V0(mol) = 3.52e ¹² mol, whereby n_V0(mol)= n_V0(mass) / MW_ethylene, where MW_ethylene = 28.05 g/mol, n_V0 = 2.12e¹² (number of monomers) whereby n_V0 = n_V0(mol)*NA, where Avogadro’s number is: NA= 6.023e²³ mol ¹) The measured activity profile of the catalyst was reconstructed using the polymer reaction engineering method described in Touloupidis V., Rittenschober G., Paulik C., 2020, An Integrated PRE Methodology for Capturing the Reaction Performance of Single- and Multi-site Type Catalysts Using Bench-Scale Polymerization Experiments, Macromol. React. Eng., DOI: 10.1002/mren.202000028.

The ethylene concentration in the slurry phase was estimated using Aspen software, equal to 0.32mol/lt. A set of three kinetic parameters, namely activation, propagation and deactivation describe the activity profile characteristics (shape and polymerization rate values in time). The estimated kinetic parameter values are: · Kactivation 1.2e lt/mol/s

• Kpropagation 112.0 lt/mol/s

• Kdeactivation ^— 2.2e ³ lt/mol/s

The reconstructed activity profile of the catalyst in form of the initial polymerization rate of the ethylene homopolymer on the catalyst is presented in Fig. 3.

In order to have the first fragmentation, the pore volume needs to be filled with polymer. For crystalline polymer, we distinguish different expected fragmentation times, affected by the percentage of the active site coverage of the catalyst area. For 100% coverage, the results imply that the whole pore volume should be filled for the first fragmentation. For lower coverage values only this pore volume fraction needs to be filled with polymer for the first fragmentation. According to the above calculations, for each catalyst particle, in order to fill the pore volume with the estimated pore mass, under the given activity profile, the following fragmentation times are required:

• 2.99 seconds <=> 100% coverage

• 2.12 seconds <=> 50% coverage · 0.29 seconds <=> 10% coverage

Since we have no information regarding the actual coverage values (we can only reverse engineer it based on the method described), we expect the first fragmentation to happen within the presented time span.

A polymer sample was collected from the reactor and the polymer particles exhibited irregular shape, poor morphology and large fraction of fines.

Example 2 (Inventive) In Example 2, the reactor is operated at a temperature of 95°C and the pressure set- point is selected to be 50 barg (typical Borstar™ slurry phase (i.e. loop reactor) process conditions). More specifically, the reactor was filled with 1.0kg propane, and 44. Og ethylene, 20. Og 1-butene and lOOOOnml hydrogen were further batch-fed until the pressure reaches the set-point value. 45mg of catalyst and 2.5e ⁴ mol TEA were added. The polymerization time for the experiment was set to 60 minutes.

The crystallinity of the polymer produced was estimated equal 65% based on DSC measurement. This crystallinity would translate to a polymer density of 932kg/m³ under the above defined conditions.

The calculations defined above in Example 1 estimate (per particle): n_V0(mass) = 9.76e ^u g, n_V0(mol) = 3.48e ¹² mol whereby n_V0(mol) = n_VO(mass)/MW_ethylene, where MW_ethylene =2 8.05g/mol, n_V0 = 2.10e¹² (number of monomers) whereby n_V0 = n_V0(mol)*NA, where Avogadro’s number is: NA= 6.023e²³ mol^-1.

The ethylene and 1 -butene concentration in the slurry phase was estimated using Aspen software, equal to 0.44 and O.lmol/lt, respectively. A set of three kinetic parameters, namely activation, propagation and deactivation describe the activity profile characteristics (shape and polymerization rate values in time). The estimated kinetic parameter values are:

• Kactivation 2.0e lt/mol/s

• Kpropagation ^— 91.8 lt/mol/s

• Kdeactivation ^— 2.2e ³ lt/mol/s

The reconstructed activity profile of the catalyst in form of the initial polymerization rate of the ethyl ene/1 -butene copolymer on the catalyst is presented in Fig. 4.

According to the above calculations, for each catalyst particle, in order to fill the pore volume with the estimated pore mass, under the given activity profile, the following fragmentation times are required:

• 2.27 seconds <=> 100% coverage

• 1.55 seconds <=> 50% coverage

• 0.22 seconds <=> 10% coverage

The estimated fragmentation times may seem similar to the corresponding values of Example 1; however, since the polymer produced exhibits lower crystallinity values and it is expected to behave similar to a rather amorphous polymer in terms of flowability, the coverage effect would rather be minimized. Thus, a fragmentation time closer to the 100% coverage is expected.

A polymer sample was collected from the reactor and the polymer particles exhibited more regular shape compared to Example 1 polymer, better morphology and smaller fraction of fines.

Example 3 (Inventive)

In Example 3, the reactor is operated at a temperature of 70°C and the pressure set- point is selected to be 28.3 barg (typical Borstar™ prepolymerization (slurry phase) process conditions). More specifically, the reactor was filled with 1.0kg propane, and 10. Og ethylene and 9700nml hydrogen were further batch-fed till the pressure reaches the set-point value. 90mg of catalyst and 5e ⁴ mol TEA were added. The polymerization time for the experiment was set to 10 minutes.

The crystallinity of the polymer produced was estimated equal 78% based on DSC measurement. This crystallinity would translate to a polymer density of 942kg/m³ under the above defined conditions. The calculations defined above in Example 1 estimate (per particle): n_V0(mass) = 9.86e ^u g, n_V0(mol) = 3.52e ¹² mol whereby n_V0(mol) = n_V0(mass)/MW_ethylene, where MW_ethylene =2 8.05g/mol, n_V0 = 2.12e¹² (number of monomers) whereby n_V0 = n_V0(mol)*NA, where Avogadro’s number is: NA= 6.023e²³ mol^-1. The ethylene concentration in the slurry phase was estimated using Aspen software, equal to 0.11 mol/lt. A set of three kinetic parameters, namely activation, propagation and deactivation describe the activity profile characteristics (shape and polymerization rate values in time). The estimated kinetic parameter values are: · Kactivation 1.25 e lt/mol/s

• Kpropagation 34.0 lt/mol/s

• Kdeactivation ^— 6.7e ^ lt/mol/s

The reconstructed activity profile of the catalyst in form of the initial polymerization rate of the ethylene homopolymer on the catalyst is presented in Fig. 5.

• 24.11 seconds <=> 100% coverage · 17.03 seconds <=> 50% coverage

• 2.40 seconds <=> 10% coverage

Since we have no information regarding the actual coverage values, we expect the first fragmentation to happen within the presented time span. In any case, these fragmentation values are at one order of magnitude higher than the estimated values of Example 1.

A polymer sample was collected from the reactor and the polymer particles exhibited good shape, good morphology and no presence of fines.

Claims

1. A method for controlling the initial fragmentation time of a porous polymerization catalyst in a process for polymerizing alpha-olefin polymers, the method comprising:

• Determining the initial fragmentation time of a specific porous polymerization catalyst for a given process for polymerizing a specific alpha-olefin polymer in the presence of said porous polymerization catalyst comprising the steps of: a) Determining the initial total pore volume (VO) of the porous polymerization catalyst; b) Determining the number of alpha-olefin monomers in form of polymerized alpha-olefin monomer units in the alpha-olefin polymer needed to fill the initial pores of the porous polymerization catalyst (n_V0) depending on the determined initial pore volume (VO); c) Determining the number of alpha-olefin monomers in form of polymerized alpha-olefin monomer units in the alpha-olefin polymer needed to start fragmentation of the porous polymerization catalyst into catalyst particles (n total) from the determined number of alpha-olefin monomers needed to fill the initial pores of the porous polymerization catalyst (n_V0); d) Determining the initial polymerization rate of the alpha-olefin polymer in the presence of the porous polymerization catalyst in a reactor of the process for polymerizing alpha-olefin polymers; and e) Determining the initial fragmentation time of said porous polymerization catalyst in the reactor depending on the number of alpha-olefin monomers needed to start fragmentation of the porous polymerization catalyst into fragmented polymerization catalyst particles (n total) and the initial polymerization rate of the alpha-olefin polymer; and • Controlling the initial fragmentation time of said porous polymerization catalyst in said reactor during said polymerization process comprising the steps of: f) Adjusting the polymerization rate of the alpha-olefin polymer in the presence of the porous polymerization catalyst by selecting the polymerization conditions of the alpha-olefin polymer in the reactor as such that the initial fragmentation time as calculated from process steps a) to e) is in the range of from 1 s to 60 s.

2. The method according to claim 1, wherein the reactor is the first reactor of a multistage polymerization process for polymerizing alpha-olefin polymers in the presence of a porous polymerization catalyst.

3. The method according to claims 1 or 2, wherein the process for polymerizing alpha-olefin polymers is a multistage polymerization process in which at least one polymerization reactor is preceded by a pre-polymerization reactor as first reactor.

4. The method according to any one of the preceding claims, wherein the initial pore volume (VO) of the porous polymerization catalyst is determined from the initial particle size and the initial porosity of the porous polymerization catalyst.

5. The method according to any one of the preceding claims, wherein the number of alpha-olefin monomers needed to fill the pores of the porous polymerization catalyst (n_V0) is determined depending on the density of the alpha-olefin polymer.

6. The method according to any one of the preceding claims, wherein the number of alpha-olefin monomers needed to start fragmentation of the porous polymerization catalyst into fragmented polymerization catalyst particles (n total) is additionally dependent on the overflow of alpha-olefin monomers in form of polymerized alpha-olefin monomer units in the alpha-olefin polymer in the initial pores of the porous polymerization catalyst ((l/b)*DP, wherein DP is the difference of the pressure inside to outside of the porous polymerization catalyst in the presence of the alpha-olefin polymer in the initial pores of the porous polymerization catalyst and (1/b) is a parameter depending on the alpha- olefin monomers and the density of the alpha-olefin polymer in the initial pores of the porous polymerization catalyst and optionally on one or both of the alpha- olefin comonomers in form of polymerized alpha-olefin comonomer units in the alpha-olefin polymer and/or chain transfer agent in the initial pores of the porous polymerization catalyst.

7. The method according to any one of the preceding claims, wherein the alpha- olefin monomers are ethylene monomers and the alpha-olefin polymer is an ethylene based polymer.

8. The method according to claim 7, wherein the alpha-olefin polymer is a non-amorphous ethylene based polymer and the number of alpha-olefin monomers needed to start fragmentation of the porous polymerization catalyst into catalyst particles (n total) is inversely proportional to the total concentration of the number of active catalytic centres distributed over the pore surface of the initial pores of the porous polymerization catalyst.

9. The method according to claim 7, wherein the alpha-olefin polymer is an amorphous ethylene based polymer and the number of alpha-olefin monomers needed to start fragmentation of the porous polymerization catalyst into fragmented polymerization catalyst particles (n total) is independent from the total concentration of the number of active catalytic centres distributed over the pore surface of the initial pores of the porous polymerization catalyst.

10. The method according to any one of the preceding claims, wherein the polymerization conditions of the alpha-olefin polymer in the reactor are selected as such that the polymerization temperature is in the range of from 30 to 90°C, the pressure is in the range of from 20 to 70 bar, the alpha-olefin monomer concentration is in the range of from 1 to 20 mol%, the optional comonomer is selected from alpha olefins having 4 or 6 carbon atoms, the molar ratio of the comonomer to alpha-olefin monomer is in the range of from 0 to 600 mol/kmol and the molar ratio of the chain transfer agent to alpha-olefin monomer is in the range of from 0 to 500 mol/kmol.

11. The method according to any one of the preceding claims, wherein the initial fragmentation time is from 2 s to 30 s, most preferably from 3 s to 20 s.

12. A process for polymerizing an alpha-olefin polymer comprising the steps of

(i) Polymerizing alpha-olefin monomers onto a porous polymerization catalyst in a first reactor thereby following the method according to any one of the preceding claims to produce fragmented polymerization catalyst particles; (ii) Transferring the fragmented polymerization catalyst particles to a second reactor; and (iii) Polymerizing alpha-olefin monomers in the presence of the fragmented polymerization catalyst particles in the second reactor to produce a reaction mixture comprising an alpha-olefin polymer.

13. The process according to claim 12, wherein the first reactor is a pre polymerization reactor and the second reactor is the first polymerization reactor.

14. The process according to claim 12 or 13, wherein the second reactor is succeeded by further reactors which are connected in series and in which further reactors alpha-olefin monomers are polymerized in the presence of the fragmented polymerization catalyst particles and the alpha-olefin polymer polymerized in the previous reactor(s).

15. The process according to any one of claims 12 to 14 for producing an ethylene-based polymer.