Process for transitioning between incompatible catalysts

Abstract

The invention relates to a gas-phase olefin polymerisation process comprising the steps of: (a) operating the process in the presence of a Ziegler-Natta catalyst system (ZN1) in a continuous way, wherein stream (10), comprising unreacted and inert materials, is (partially) circulated back into the reactor; (b) discontinuing the circulation of stream (10) back into the reactor; (c) gradually discontinuing the introduction of ZN1 while at the same time gradually introducing a Ziegler-Natta catalyst system (ZN2) into the reactor so that the polymer production rate is kept at a rate not less than 30% below the production rate as in step (a); and (d) after the introduction of ZN1 is discontinued and the feed of ZN2 is at its full rate, resuming (partially) recirculation of stream (10) back into the reactor; wherein ZN1 comprises an internal electron donor moiety that reduces the polymerization activity of ZN2.

Description

PROCESS FOR TRANSITIONING BETWEEN INCOMPATIBLE CATALYSTS

[0001] This invention relates to a process for transitioning between incompatible polymerization catalysts. Particularly, the invention relates to a process for transitioning between an olefin polymerization utilizing a first Ziegler-Natta catalyst to an olefin polymerization utilizing a second Ziegler-Natta catalyst in a continuous polymerization process.

[0002] It is frequently necessary to transition from one type of catalyst producing polymers having certain properties and characteristics to another catalyst capable of producing polymers of different chemical and / or physical attributes. Transitioning between similar Ziegler-Natta catalysts or compatible catalysts generally takes place easily. Compatible catalysts are those catalysts having similar kinetics of termination and insertion of monomer and co-monomer (s) and / or do not detrimentally interact with each other. However, the process is typically complicated when the catalysts are incompatible or of different types. For example, when transitioning between two incompatible catalysts such as a first Ziegler-Natta catalyst and a second Ziegler-Natta catalyst, it has been found that some of the components of the Ziegler-Natta catalyst can act as poisons to the second Ziegler-Natta catalyst.Consequently, the components of the first Ziegler- Natta catalyst prevent the second Ziegler- Natta catalyst from promoting polymerization.

[0003] Furthermore, particularly in a continuous transition process, the interaction between two incompatible catalysts may lead to the production of high levels of small particles less than about 120 microns that are referred to as "fines". Fines can induce operability problems in the reactor and / or fouling and sheeting incidents.

[0004] In the past, to accomplish an effective transition between incompatible catalysts, the first catalyzed olefin polymerization process was stopped by various techniques known in the art. The reactor was then emptied, recharged and a second catalyst was introduced into the reactor. Such catalyst conversions are time consuming and costly because of the need for a reactor shut-down for an extended period of time during transition and the off-grade material. There have been many attempts to improve the process for transitioning between incompatible catalysts.

[0005] In order to inhibit polymerization of a first incompatible catalyst, it is necessary to interrupt catalyst injection into the reactor. Stopping the first catalyst feed into the reactordoes not immediately stop polymerization occurring within the reactor because the fluidized bed contains catalyst particles which can still polymerize for an extended period of time. Even if one were to allow the polymerization within the reactor to continue for a period of time, the catalyst within the reactor would not be completely deactivated for a considerable period.

[0006] Thus, to at least partially deactivate the first Ziegler-Natta catalyst, polymerization inhibitors or catalyst killers are employed. There are two general types of catalyst killers: reversible catalyst killers and irreversible catalyst killers. Reversible catalyst killers typically initially inhibit catalyst activity and polymerization for a period of time, but, do not irreversibly deactivate the catalyst. In fact, after a period of time under normal polymerization conditions the catalysts reactivate and polymerization will continue. These reversible catalyst killers can be used in any combination or order of introduction in the process. Irreversible catalyst killers irreversibly inactivate a catalyst's ability to polymerize olefins. The use of catalyst killing and / or deactivating agents is disclosed in U.S. Pat. Nos. 5,442,019, 5,753,786, and 6,949,612 B2 to Agapiou et al., U.S. Pat. No. 5,672,666 to Muhle et al., and U.S. Pat. No. 6,858,684 B2 to Burdett et al.

[0007] It would be advantageous to provide a catalyst transitioning process without the need for halting the polymerization, emptying the reactor to rid it of the original catalyst and restarting the polymerization with another catalyst. In addition, it would be advantageous if the process for transitioning could reduce the amount of off-grade material produced during the transition process, reduce the transition time, increase the robustness and stability of the transition process and avoid the need to open the reactor to charge the seed bed. It would further be advantageous to provide a catalyst transitioning process in which fouling of the reactor is prevented.

[0008] It is an object of the present invention to provide a process in which above-described and / or other problems are solved.

[0009] The invention relates to a process for transitioning from a first Ziegler-Natta catalyst system to a second Zeigler-Natta catalyst system in a gas-phase olefin polymerisation unit • wherein the polymerisation unit comprises a polymerisation reactor (8) comprising at least one inlet (11) for supplying a reaction mixture (A) to the polymerisation reactor, at least one outlet (12) for removing polymerisation product (30) from the reactor, and at least one outlet (13) for removal of a stream of unreacted and inert materials (40) from the reactor,• wherein the outlet (13) is connected to inlet (11) by which stream (10) or a portion thereof may be circulated back into the reactor, the process comprising the steps in this order of:(a) operating the polymerisation process in the presence of a first Ziegler-Natta catalyst system (ZN1) in a continuous way, wherein stream (10) or a fraction thereof is circulated back into the reactor;(b) discontinuing the circulation of stream (10) back into the reactor;(c) gradually discontinuing the introduction of ZN1 while at the same time gradually introducing a second Ziegler-Natta catalyst system (ZN2) into the reactor so that the polymer production rate is kept at a rate not less than 30% below the production rate as in step (a); and(d) after the introduction of ZN1 is discontinued and the feed of ZN2 is at its full rate, resuming recirculation of stream (10) or a fraction thereof back into the reactor; wherein ZN1 comprises an internal electron donor moiety that reduces the polymerization activity of ZN2.

[0010] The invention thereby relates for example to a process for transitioning between incompatible catalysts to convert a reactor from producing one type of product to another with minimal reactor down-time. For the purposes of this patent specification and appended claims, the term "incompatible catalysts" are understood as those catalysts that in each other's presence reduce the productivity of at least one of the catalysts by greater than 30 percent.

[0011] Productivity is herein understood as kg of product per kg of catalyst over a certain period of time. Mw is herein understood as the weight average molecular weight as measured using SEC (Size Exclusion Chromatography) using 1 ,2,4-trichlorobenzene as an eluent, and calibrated using linear polyethylene standards. The comonomer incorporation is measured by the analytical temperature rising elution fractionation (aTREF) conducted according to the method described in U.S. Patent No. 4,798,081 and Wilde, L; Ryle, T.R.; Knobeloch, D. C; Peat, LR.; Determination of Branching Distributions in Polyethylene and Ethylene Copolymers, J. Polym. ScL, 20, 441 -455 (1982). The composition to be analyzed is dissolved in 1 ,2-dichlorobenzene of analytical quality filtrated via 0.2 pm filter and allowed to crystallize in a column containing an inert support (Column filled with 150 pm stainless steel beans (volume 2500 pl) by slowly reducing the temperature to 20 degrees centigrade at a cooling rate of 0.1 degrees C / min. The column is equipped with an infrared detector. An ATREF chromatogram curve is then generated by eluting the crystallized polymer samplefrom the column by slowly increasing the temperature of the eluting solvent (1 ,2- dichlorobenzene) from 20 to 130 degrees centigrade at a rate of 1 degrees C / min.

[0012] Preferably, the process according to the invention further comprises the step of (b2) introducing an organometallic compound, especially an organometallic compound that comprises aluminum after step (b) and before step (c). The organometallic compound can bind with the internal electron donor moiety present in the reactor 8, and thereby reduce the deactivating effect of the internal electron donor moiety on the activity of the ZN2.

[0013] Subsequently, the second Ziegler-Natta catalyst ZN2 is introduced into the reactor in step (c). In step (d), a gas composition for the second polymerization with catalyst ZN2 is introduced into the reactor.

[0014] Preferably, the process of the invention is continuous.

[0015] The first polymerization and the second polymerization may be a continuous polymerization of one or more alfa-olefin monomers of which at least one is ethylene or propylene. Preferred alfa-olefin monomers include for example alfa-olefins having from 4 to 8 carbon atoms. However, small quantities of alfa-olefin monomers having more than 8 carbon atoms, for example 9 to 18 carbon atoms, such as for example a conjugated diene, can be employed if desired. Thus it is possible to produce homopolymers of ethylene or propylene or copolymers of ethylene and / or propylene with one of more alfa-olefin monomers having from 4 to 8 alfa-olefin monomers. Preferred alfa-olefin monomers include but are not limited to 1- butene, isobutene, 1 -pentene, 1 -hexene, hexadiene, isoprene, styrene, 4-methylpent-1-ene, 1 -octene and butadiene. Examples of alfa-olefin monomers having more than 8 carbon atoms that can be copolymerized with an ethylene and / or propylene monomer, or that can be used as partial replacement for alfa-olefin monomers having from 4 to 8 alfa-olefin monomers include but are not limited to 1 -decene and ethylidene norbornene.

[0016] Examples of polyolefins which may be produced, include a wide variety of polymers, for example polyethylene, for example linear low density polyethylene (LLDPE), which may for example be prepared from ethylene and 1 -butene, 4-methylpent-1-ene, 1 -hexene or 1- octene, high density polyethylene (HDPE), which may for example be prepared from ethylene or from ethylene with a small portion of an alfa-olefin monomer having from 4 to 8 carbon atoms, for example 1 -butene, 1 -pentene, 1 hexene or 4-methylpent-1-ene. Other examples include but are not limited to plastomers, elastomers, medium densitypolyethylene, polypropylene homopolymers and polypropylene copolymers, including random copolymers, and block or multi-block copolymer and ethylene propylene rubber (EPR). Preferably, in the process of the invention, the polyolefin produced is a polyethylene, more preferably a linear low density polyethylene, comprising ethylene and one monomer from the group of 1 -butene, 4-methylpent-1-ene, 1 -hexene and 1 -octene.

[0017] When the process of the invention is used for the copolymerization of ethylene and / or propylene with alfa-olefin monomers, the ethylene and / or propylene preferably is used as the major component of the copolymer. For example, the amount of ethylene and / or propylene present in the copolymer is at least 65 percent by weight, for example at least 70 percent by weight, for example at least 80 percent by weight based on the total copolymer.

[0018] With 'continuous polymerization of one or more alfa-olefins' or 'continuous preparation of polyolefin' is meant herein that one or more alfa-olefin monomers of which at least one is ethylene or propylene are fed to the reactor, preferably continuously, and polyolefin thus produced is (semi)-continuously withdrawn through a polymer discharge system connected to the reactor. The continuous polymerization of one or more olefin monomers will produce polyolefins in the form of particles, herein also referred to as 'polyolefin'.

[0019] Ziegler-Natta catalysts may typically comprise a transition metal halide, such as titanium or vanadium halide, and an organometallic compound of a metal of Group 1 , 2 or 3, typically trialkylaluminum compounds, which may serve as a co-catalyst for the transition metal halide. Ziegler-Natta catalyst may further comprise an internal electron donor which may form a complex with the alkyl aluminum and / or the transition metal. The transition metal halide may be supported on a magnesium halide or complexed thereto. Such Ziegler-Natta catalyst may also be impregnated onto an inorganic support such as silica or alumina. For more details on Ziegler-Natta catalysts, see for example, U.S. Pat. Nos. 3,687,920, 4,086,408, 4,376,191 , 5,019,633, 4,482,687.4,101 ,445, 4,560,671 , 4,719,193, 4,755,495, 5,070,055 all of which are herein incorporated by reference.

[0020] Ziegler-Natta catalysts may have different electron donors, which can regulate the activity of the catalyst and selectivity for the polymerization of certain olefins.

[0021] In the present invention the first Ziegler-Natta catalyst comprises an internal electron donor moiety which however has an adverse effect on the activity and / or selectivity of the second Ziegler-Natta catalyst. This internal electron donor moiety will become part of the circulation stream of the gas-phase polymerization reactor and may effect the activity of thesecond Ziegler Natta catalyst upon switching from the polymerisation with the first Ziegler- Natta catalyst to polymerization with the second Ziegler-Natta catalyst.

[0022] Examples of internal electron donor moieties that may affect the second Ziegler-Natta catalyst include certain alcohols, organosilicon compounds, polysiloxanes, phenols, ketones, aldehydes, inorganic acid esters, polycarboxilic acid esters, ethers, acid amides, acid anhydrides, nitrogen-containing compounds, and acid halides.

[0023] Such alcohols may for example be alcohols having 1 to 18 carbon atoms and having an alkyl group, such as methanol, ethanol, propanol, pentanol, hexanol, octanol, 2- ethylhexanol, dodecanol, octadecyl alcohol, benzyl alcohol, phenylethyl alcohol, cumyl alcohol and isopiropylbenzyl alcohol. Such polysiloxanes may for example be polydimethylsiloxanes such as hexamethyldisiloxane. Such phenols may for example be phenols having 6 to 25 carbon atoms, such as phenol, resol, xylenol, ethylphenol, propylphenol, cumylphenol, nonylphenol and naphthol. Such ketones may for example be ketones having 3 to 15 carbon atoms, such as acetone, methyl ethyl ketone, methyl isobutyl ketone, acetophenone and benzophenone. Such aldehydes may for example be aldehydes having 2 to 15 carbon atoms, such as acetaldehyde, propionaldehyde, octylaldehyde, benzaldehyde, tolualdehyde and naphthaldehyde. Such inorganic acid esters may for example be ethyl silicate, butyl silicate, vinyltriethoxysilane, phenyltriethoxysilane or diphenyldiethoxysilane. Such polycarboxylic acid esters may for example be polyethyl acetate, polyvinyl acetate, dibutyl maleate, diethyl butylmalonate, diethyl dibutylmalonate, diethyl 1 ,2-cyclohexanedicarboxylate, di-2-ethylhexyl 1 ,2-cyclohexanedicarboxylate, dimethyl phthalate, diethyl phthalate, dibutyl phthalate, or dioctyl phthalate. Such ethers may for example be ethers having 2 to 20 carbon atoms, such as methyl ether, ethyl ether, isopropyl ether, butyl ether, amyl ether, tetrahydrofuran, anisole and diphenyl ether; acid amides such as acetamide, benzamide and toluamide. Such acid anhydrides may for example be benzoic anhydride or phthalic anhydride. Such nitrogen-containing compounds may for example be ammonia, amines such as methylamine, ethylamine, diethylamine, tributylamine, piperidine, tribenzylamine, aniline, pyridine, picoline and tetramethylethylenediamine, nitriles such as acetonitrile, benzonitrile and tolunitrile, and isocyanates. Such acid halides may for example be acid halides having 2 to 15 carbon atoms, such as acetyl chloride, benzoyl chloride, tolyl chloride, anisoyl chloride and phthaloyl dichloride.

[0024] A preferred example of an internal electron donor moiety is tetrahydrofuran (THF).

[0025] In an embodiment of the invention, an aluminium alkyl compound can be added to the polymerization in order to bind the internal electron donor moiety in the reactor 8 and / or in the recycle stream 10 and thereby prevent or limit deactivation of the second Ziegler-Natta catalyst by the complexation of the internal electron donor moiety.

[0026] The aluminum compound used in the reaction to bind to the internal electron donor moiety may be a single organoaluminum compound or a mixture of two or more different organoaluminum compounds. The aluminum compound of the present invention is preferably a trialkylaluminum or a dialkylaluminumhydride.

[0027] In an embodiment of the present invention the aluminum compound is selected from the group consisting of of tri-methylaluminum, tri-ethylaluminum, tri-propylaluminum, tributylaluminum, tri-isopropylaluminum tri-isobutylaluminum, or di- methylaluminumhydride, diethylaluminumhydride, di-propylaluminumhydride, di- butylaluminumhydride, diisopropylaluminumhydride, di-isobutylaluminumhydride, preferably from the group consisting of triethylaluminium and triisobutylaluminium.

[0028] In a preferred embodiment of the invention, the amounts of aluminum compound and internal electron donor moiety are selected such that the molar ratio of Al to internal electron donor moiety is in the range of 10: 1 to 1 :1 , preferably 5:1 to 2: 1 . .

[0029] Addition of the aluminium compound to bind the internal electron donor moiety can be performed between stap (a) and step (b), during step (b), between step (b) and step (c), and even during step (c). The addition of the aluminium compound to bind the internal electron donor moiety is preferably discontinued during step (d).

[0030] Step (c) is preferably performed in such a way that the introduction of the first Ziegler- Natta catalyst is progressively reduced over a period of time, for example over a period of time of 0.5 to 2 hours or 0.5 to 4 hours, until it is discontinued, while at the same time the second Ziegler-Natta catalyst is progressively increased over a period of time.

[0031] During step (b) the circulation of stream (10) is discontinued, and preferably the stream is removed form the polymerisation unit. In an embodiment stream (10) can be subjected to a condensation step, or any step designed to remove the internal electron donor moiety from stream (10).

[0032] In step (d) the recirculation of stream (10) is resumed, in a preferred embodiment under conditions wherein the amount of internal electron donor moiety is below a value of 5 ppm, more preferably below 4, 3, 2 or even below 1 ppm.

[0033] The second Ziegler-Natta catalyst ZN2 is preferably an advanced Ziegler-Natta catalyst (AZN), wherein the Ziegler-Natta catalyst is produced in a process comprising the steps of: a) contacting a dehydrated solid support having hydroxyl groups with a magnesium compound having the general formula MgR'R", wherein R' and R" are the same or different and are independently selected from the group comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and alkadienylaryl group; b) contacting the product obtained in step (a) with modifying compounds (A), (B) and (C), wherein: compound (A) is at least one compound selected from the group consisting of carboxylic acid, carboxylic acid ester, ketone, acyl halide, aldehyde and alcohol; compound (B) is a compound having the general formula R1a(R2O)bSiY1c, wherein a, b and c are each integers from 0 to 4 and the sum of a, b and c is equal to 4 with a proviso that when c is equal to 4 then modifying compound (A) is not an alcohol, Si is a silicon atom, O is an oxygen atom, Y1is a halide atom and R1and R2are the same or different and are independently selected from the group comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and alkadienylaryl group; compound (C) is a compound having the general formula (R11O)4M1, wherein M1is a titanium atom, a zirconium atom or a vanadium atom, O is an oxygen atom and R11is selected from the group comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and alkadienylaryl group; and c) contacting the product obtained in step (b) with a titanium halide compound having the general formula TiY4, wherein Ti is a titanium atom and Y is a halide atom.

[0034] The Advanced Ziegler-Natta catalyst may for example be produced in process comprising a first step (a) of contacting a support, preferably a dehydrated solid support having hydroxyl (OH) groups, with a magnesium compound to form a solid magnesium containing support material.

[0035] The support may for example be any material containing hydroxyl groups. Suitable examples of such materials include inorganic oxides, such as silica, alumina, magnesia, thoria, zirconia and mixtures of such oxides. Preferably, porous silica is used as the support as higher bulk densities and higher catalyst productivities are obtained therewith. Silica may be in the form of particles having a mean particle diameter of 1 micron to 500 microns, preferably from 5 microns to 150 microns and most preferably from 10 microns to 100 microns. Silica with a lower mean particle diameter produces a higher level of polymer fines and silica with a higher mean particle diameter reduces polymer bulk density. The silica may have a surface area of 5 m2 / g to 1500 m2 / g, preferably from 50 m2 / g to 1000 m2 / g and a pore volume of from 0.1 cm3 / g to 10.0 cm3 / g, preferably from 0.3 cm3 / g to 3.5 cm3 / g, as higher catalyst productivity is obtained in this range.

[0036] The dehydrated solid support may for example be obtained by drying the support in order to remove physically bound water and to reduce the content of hydroxyl groups to a level which may be of from 0.1 mmol to 5.0 mmol hydroxyl groups per gram of support, preferably from 0.2 mmol to 2.0 mmol hydroxyl groups per gram of support, as this range allows sufficient incorporation of the active catalyst components to the support, determined by the method as described in J. J. Fripiat and J. Uytterhoeven, J. Phys. Chem. 66, 800, 1962 or by applying H NMR spectroscopy. The hydroxyl groups content in this range may be achieved by heating and fluidizing the support at a temperature of from 150 degrees C to 900 degrees C for a time of 1 hour to 15 hours under a nitrogen or air flow. The dehydrated support can be slurried, preferably by stirring, in a suitable hydrocarbon solvent in which the individual catalyst components are at least partially soluble. Examples of suitable hydrocarbon solvents include n-pentane, isopentane, cyclopentane, n-hexane, isohexane, cyclohexane, n-heptane, isoheptane, n-octane, isooctane and n-decane. The amount of solvent used is not critical, though the solvent should be used in an amount to provide good mixing of the catalyst components.

[0037] The magnesium compound is represented by the general formula MgR'R", wherein R' and R" are the same or different and are independently selected from a group comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and an alkadienylaryl group, and may for example have from 1 to 20 carbon atoms. Suitable examples of the magnesium compound include dimethylmagnesium, diethylmagnesium, ethylmethylmagnesium, di-n- propylmagnesium, diisopropylmagnesium, n- propylethylmagnesium, isopropylethylmagnesium, di-n-butylmagnesium, diisobutylmagnesium, n- butylethylmagnesium, n-butyl-n-propylmagnesium, n- butylisopropylmagnesium, isobutylethylmagnesium, isobutyl-n-propylmagnesium,isobutylisopropylmagnesium, di-n-pentylmagnesium, diisopentylmagnesium, n- pentylethylmagnesium, n-pentyl-n-propylmagnesium, n-pentylisopropylmagnesium, n-pentyl- n-butylmagnesium, n-pentylisobutylmagnesium, di-n-hexylmagnesium, diisohexylmagnesium, n-hexylethylmagnesium, n-hexyl-n-propylmagnesium, n- hexylisopropyl magnesium, n-hexyl-n-butylmagnesium, n-hexylisobutylmagnesium, isohexylethylmagnesium, isohexyl-n-propylmagnesium, isohexylisopropyl magnesium, isohexyl-n-butylmagnesium, isohexylisobutylmagnesium, di-n- octylmagnesium, diisooctylmagnesium, n-octylethylmagnesium, n-octyl-n- propylmagnesium, n- octylisopropylmagnesium, n-octyl-n-butylmagnesium, n- octylisobutyl magnesium, isooctylethylmagnesium, isooctyl-n-propylmagnesium, isooctylisopropylmagnesium, isooctyl- n-butylmagnesium, isooctylisobutyl magnesium, dicyclopentylmagnesium, cyclopentylethylmagnesium, cyclopentyl-n- propylmagnesium, cyclopentylisopropylmagnesium, cyclopentyl-n-butylmagnesium, cyclopentylisobutylmagnesium, dicyclohexylmagnesium, cyclohexylethylmagnesium, cyclohexyl-n-propylmagnesium, cyclohexylisopropyl magnesium, cyclohexyl-n- butylmagnesium, cyclohexylisobutylmagnesium, diphenylmagnesium, phenylethylmagnesium, phenyl-n-propylmagnesium, phenyl- n-butylmagnesium and mixtures thereof.

[0038] Preferably, the magnesium compound is selected from the group comprising di-n- butylmagnesium, n-butylethylmagnesium and n-octyl-n- butylmagnesium.

[0039] The magnesium compound may for example be used in an amount ranging from 0.01 to 10.0 mmol per gram of solid support, preferably from 0.1 to 3.5 mmol per gram of support and more preferably from 0.3 to 2.5 mmol per gram of support as by applying this range the level of polymer fines of the product is reduced and higher catalyst productivity is obtained. The magnesium compound may be reacted, preferably by stirring, with the support at a temperature of 15 degrees C to 140 degrees C during 5 minutes to 150 minutes, preferably at a temperature of 20 degrees C to 80 degrees C for a duration of 10 minutes to 100 minutes.

[0040] The molar ratio of Mg to OH groups in the solid support may for example be in the range of 0.01 to 10.0, preferably of from 0.1 to 5.0 and more preferably of from 0.1 to 3.5, as the level of polymer fines of the product is reduced and higher catalyst productivity is obtained.

[0041] The modifying compound (A) is at least one compound selected from the group consisting of carboxylic acids, carboxylic acid esters, ketones, acyl halides, aldehydes and alcohols. The modifying compound (A) may be represented by the general formula R3COOH, R4COOR5, R6COR7, R8COY2, R9COH or R10OH, wherein Y2is a halide atom and R3, R4, R5, R6, R7, R8, R9and R10are independently selected from a group of compounds comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and an alkadienylaryl group and may have from 1 to 20 carbon atoms.

[0042] Suitable examples of the carboxylic acids include acetic acid, propionic acid, isopropionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, caproic acid, isocaproic acid, enanthic acid, isoenanthic acid, caprylic acid, isocaprylic acid, pelargonic acid, isopelargonic acid, capric acid, isocapric acid, cyclopentanecarboxylic acid, benzoic acid and mixtures thereof.

[0043] Suitable examples of carboxylic acid esters include methyl acetate, ethyl acetate, n- propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, isoamyl acetate, ethyl butyrate, n-butyl butyrate and / or isobutyl butyrate.

[0044] Suitable examples of ketones include dimethyl ketone, diethyl ketone, methyl ethyl ketone, di-n-propyl ketone, di-n-butyl ketone, methyl n-propyl ketone, methyl isobutyl ketone, cyclohexanone, methyl phenyl ketone, ethyl phenyl ketone, n-propyl phenyl ketone, n-butyl phenyl ketone, isobutyl phenyl ketone, diphenyl ketone and mixtures thereof.

[0045] Suitable examples of acyl halides include ethanoyl chloride, propanoyl chloride, isopropanoyl chloride, n-butanoyl chloride, isobutanoyl chloride, benzoyl chloride and mixtures thereof.

[0046] Suitable examples of aldehydes include acetaldehyde, propionaldehyde, n- butyraldehyde, isobutyraldehyde, n-pentanaldehyde, isopentanaldehyde, n- hexanaldehyde, isohexanaldehyde, n-heptanaldehyde, benzaldehyde and mixtures thereof. Suitable examples of alcohols include methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol, tert-butanol, cyclobutanol, n- pentanol, isopentanol, cyclopentanol, n-hexanol, isohexanol, cyclohexanol, n- octanol, isooctanol, 2-ethylhexanol, phenol, cresol, ethylene glycol, propylene glycol and mixtures thereof.

[0047] Preferably, the modifying compound (A) is at least one compound selected from the group comprising methyl n-propyl ketone, ethyl acetate, n-butyl acetate, acetic acid,isobutyric acid, isobutyraldehyde, ethanoyi chloride, ethanol and sec- butanol, and more preferably from methyl n-propyl ketone, n-butyl acetate, isobutyric acid and ethanoyi chloride as higher catalyst productivity and higher bulk density of the products are obtained and these compounds can be used to vary molecular weight distribution of the product.

[0048] The molar ratio of modifying compound (A) to magnesium in the solid support may for example be in a range of from 0.01 to 10.0, preferably of from 0.1 to 5.0, more preferably of from 0.1 to 3.5 and most preferably of from 0.3 to 2.5, as higher catalyst productivity and higher bulk density of the products are obtained. The modifying compound (A) may be added to the reaction product obtained in step (a), preferably by stirring, at a temperature of 15 degrees C to 140 degrees C for a duration of 5 minutes to 150 minutes, preferably at a temperature of 20 degrees C to 80 degrees C for a duration of 10 minutes to 100 minutes.

[0049] The modifying compound (B) is a silicon compound represented by the general formula R1a(R2O) bSiY1c, wherein a, b and c are each integers from 0 to 4 and the sum of a, b and c is equal to 4 with a proviso that when c is equal to 4 then modifying compound (A) is not an alcohol, Si is a silicon atom, O is an oxygen atom, Y1is a halide atom and R1and R2are the same or different. R1and R2are independently selected from the group of compounds comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and an alkadienylaryl group. R1and R2may for example have from 1 to 20 carbon atoms.

[0050] Suitable silicon compounds include tetramethoxysilane, tetraethoxysilane, tetra-n- propoxysilane, tetraisopropoxysilane, tetra-n-butoxysilane, tetraisobutoxysilane, methyltrimethoxysilane, ethyltrimethoxysilane, n- propyltrimethoxysilane, isopropyltrimethoxysilane, n-butyltrimethoxysilane, isobutyltrimethoxysilane, n- pentyltrimethoxysilane, n-hexyltrimethoxysilane, n- octyltrimethoxysilane, isooctyltrimethoxysilane, vinyltrimethoxysilane, phenyltrimethoxysilane, dimethyldimethoxysilane, diethyldimethoxysilane, isobutylmethyldimethoxysilane, diisopropyldimethoxysilane, diisobutyldimethoxysilane, diisobutyldimethoxysilane, isobutylisopropyldimethoxysilane, dicyclopentyldimethoxysilane, cyclohexylmethyldimethoxysilane, phenylmethyldimethoxysilane, diphenyldimethoxysilane, trimethylmethoxysilane, triethylmethoxysilane, methyltriethoxysilane, ethyltriethoxysilane, n- propyltriethoxysilane, isopropyltriethoxysilane, n-butyltriethoxysilane, isobutyltriethoxysilane, n- pentyltriethoxysilane, n-hexyltriethoxysilane, n-octyltriethoxysilane, isooctyltriethoxysilane, vinyltriethoxysilane, phenyltriethoxysilane, dimethyldiethoxysilane, diethyldiethoxysilane, isobutylmethyldiethoxysilane, diisopropyldiethoxysilane, diisobutyldiethoxysilane,isobutylisopropyldiethoxy silane, dicyclopentyldiethoxysilane, cyclohexylmethyldiethoxysilane, phenylmethyldiethoxysilane, diphenyldiethoxysilane, trimethylethoxysilane, triethylethoxysilane, silicon tetrachloride, methyltrichlorosilane, ethyltrichlorosilane, n-propyltrichlorosilane, isopropyltrichlorosilane, n-butyltrichlorosilane, isobutyltrichlorosilane, n-pentyltrichlorosilane, n-hexyltrichlorosilane, n-octyltrichlorosilane, isooctyltrichlorosilane, vinyl Itrichlorosilane, phenyltrichlorosilane, dimethyldichlorosilane, diethyl dichlorosilane, isobutylmethyldichlorosilane, diisopropyldichlorosilane, diisobutyldichlorosilane, isobutylisopropyldichlorosilane, dicyclopentyldichloro silane, cyclohexylmethyldichlorosilane, phenylmethyldichlorosilane, diphenyldichlorosilane, trimethylchlorosilane, triethylchlorosilane, chloro trimethoxysilane, dichlorodimethoxysilane, trichloromethoxysilane, chloro triethoxysilane, dichlorodiethoxysilane and / or trichloroethoxysilane. Preferably, the modifying compound (B) used is tetraethoxysilane, n- propyltriethoxysilane, isobutyltrimethoxysilane, dimethyldichlorosilane, n-butyltrichlorosilane and silicon tetrachloride, and more preferably isobutyltrimethoxysilane, tetraethoxysilane, n- propyltriethoxysilane, n-butyltrichlorosilane and silicon tetrachloride as higher catalyst productivity and higher bulk density are obtained with the ability to vary the molecular weight distribution of the product by employing these preferred compounds.

[0051] The molar ratio of modifying compound (B) to magnesium may be in a range of from 0.01 to 5.0, preferably from 0.01 to 3.0, more preferably from 0.01 to 1 .0 and most preferably from 0.01 to 0.3, as higher catalyst productivity and higher bulk density are obtained. The modifying compound (B) may be added to the reaction product obtained in step (a), preferably by stirring, at a temperature of 15 degrees C to 140 degrees C during 5 minutes to 150 minutes, preferably at a temperature of 20 degrees centigrade to 80 degrees centigrade during 10 minutes to 100 minutes. The modifying compound (C) is a transition metal alkoxide represented by the general formula (R11O)4M1wherein M1is a titanium atom, a zirconium atom or a vanadium atom, O is an oxygen atom and R11is a compound selected from the group of compounds comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and an alkadienylaryl group. R11may have from 1 to 20 carbon atoms.

[0052] Suitable transition metal alkoxide compounds include titanium tetramethoxide, titanium tetraethoxide, titanium tetra-n-propoxide, titanium tetraisopropoxide, titanium tetra-n- butoxide, titanium tetraisobutoxide, titanium tetra-n-pentoxide, titanium tetraisopentoxide, titanium tetra-n-hexoxide, titanium tetra-n-heptoxide, titanium tetra-n-octoxide, titanium tetracyclohexoxide, titanium tetrabenzoxide, titanium tetraphenoxide, zirconium tetramethoxide, zirconium tetraethoxide, zirconium tetra-n-propoxide, zirconiumtetraisopropoxide, zirconium tetra-n-butoxide, zirconium tetraisobutoxide, zirconium tetra-n- pentoxide, zirconium tetraisopentoxide, zirconium tetra-n-hexoxide, zirconium tetra-n- heptoxide, zirconium tetra-n-octoxide, zirconium tetracyclohexoxide, zirconium tetrabenzoxide, zirconium tetraphenoxide, vanadium tetramethoxide, vanadium tetraethoxide, vanadium tetra-n-propoxide, vanadium tetraisopropoxide, vanadium tetra-n- butoxide, vanadium tetraisobutoxide, vanadium tetra-n-pentoxide, vanadium tetraisopentoxide, vanadium tetra-n-hexoxide, vanadium tetra-n-heptoxide, vanadium tetra-n- octoxide, vanadium tetracyclohexoxide, vanadium tetrabenzoxide, vanadium tetraphenoxide or mixtures thereof. Preferably, titanium tetraethoxide, titanium tetra-n-butoxide and zirconium tetra-n-butoxide are used because higher catalyst productivity and higher bulk density are obtained with the ability to vary the molecular weight distribution of the product by employing these preferred compounds.

[0053] The molar ratio of the modifying compound (C) to magnesium may be in the range of from 0.01 to 5.0, preferably from 0.01 to 3.0, more preferably from 0.01 to 1 .0 and most preferably from 0.01 to 0.3, as higher catalyst productivity, higher bulk density and improved hydrogen response in polymerization are obtained. The modifying compound (C) may be reacted, preferably by stirring, with the product obtained in step (a) at a temperature of 15 degrees C to 140 degrees C for a duration of 5 minutes to 150 minutes, preferably at a temperature of 20 degrees C to 80 degrees C for a duration of 10 minutes to 100 minutes. The modifying compounds (A), (B) and (C) can be contacted in any order or simultaneously with the solid magnesium containing support obtained in step (a).

[0054] Preferably, (A) is added first to the reaction product obtained in step (a) and then (B), followed by the addition of (C) as higher catalyst productivity and higher product bulk density are obtained by employing this order of adding the modifying compounds. Pre- mixtures of the individual catalyst components can also be effectively utilized.

[0055] Preferably, when modifying compound (A) is methyl n-propyl ketone and modifying compound (C) is titanium tetraethoxide, a further increase of molecular weight distribution is obtained when modifying compound (B) is selected in the following order from the group consisting of isobutyltrimethoxysilane, n-propyltriethoxysilane, tetraethoxysilane, n- butyltrichlorosilane and silicon tetrachloride, at the same levels of titanium halide compound.

[0056] In the preferred case when the modifying compound (B) is silicon tetrachloride and modifying compound (C) is titanium tetraethoxide, a further improved combination of catalyst productivity and bulk density is obtained when modifying compound (A) is selected in thefollowing order from the group consisting of isobutyraldehyde, ethyl acetate, n-butyl acetate, methyl n-propyl ketone and isobutyric acid, at the same levels of titanium halide compound.

[0057] The titanium halide compound is represented by the general formula TiY4, wherein Ti is a titanium atom and Y is a halide atom.

[0058] Suitable titanium halide compounds include titanium tetrachloride, titanium tetrabromide, titanium tetrafluoride or mixtures thereof. The preferred titanium halide compound is titanium tetrachloride, as higher catalyst productivity is obtained. The molar ratio of the titanium halide compound to magnesium may be in the range of 0.01 to 10.0, preferably from 0.01 to 5.0 and more preferably from 0.05 to 1 .0, as a better balance of high catalyst productivity and high bulk density is obtained.

[0059] The titanium halide compound may be added to the reaction mixture obtained by applying step (a) and step (b) in any conventional manner, such as by stirring, at a temperature of 15 degrees C to 140 degrees C for a duration of 5 minutes to 150 minutes, preferably at a temperature of 20 degrees C to 80 degrees C for a duration of 10 minutes to 100 minutes. The reaction mixture may be then dried using a nitrogen purge and / or by vacuum at a temperature from 15 degrees C to 140 degrees C, preferably 30 degrees C to 100 degrees C and most preferably 50 degrees C to 80 degrees C to yield the Advanced Ziegler-Natta catalyst component. The total molar ratio of the modifying compound (C) and the titanium halide compound to magnesium may be in the range of from 0.01 to 10.0, preferably of from 0.01 to 5.0 and more preferably of from 0.05 to 1.0, as a better balance of high catalyst productivity and high bulk density is obtained.

[0060] The total molar ratio of the modifying compound (C) and the titanium halide compound to hydroxyl (OH) groups in the support after dehydration may be in the range of from 0.01 to 10.0, preferably of from 0.01 to 5.0 and more preferably of from 0.05 to 1.0, as a better balance of high catalyst productivity and high bulk density is obtained. Higher levels would produce high catalyst productivity though with reduced bulk density, especially in a gas phase polymerization processes. Further, applying these amounts eliminates the requirement to conduct solvent decanting, solvent filtering, solvent washing steps in catalyst preparation and hence eliminates generation of highly hazardous solvent waste material.

[0061] In one embodiment the Advanced Ziegler-Natta catalyst system can comprise a catalyst component and a co-catalyst. The co-catalyst is typically an organometallic compound such as aluminum alkyls, aluminum alkyl hydrides, lithium aluminum alkyls, zincalkyls, calcium alkyls, magnesium alkyls or mixtures thereof. Preferred co-catalysts are represented by the general formula R12nAIY33-n, wherein Y3represents a halide atom; n represents an integer from 0 to 3; and R12is selected from a group of compounds comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and alkadienylaryl group. R12may have from 1 to 20 carbon atoms. Suitable examples of the cocatalyst include trimethylaluminum, triethylaluminum, tri-isobutylaluminum, tri-n- hexylaluminum, tri-n-octylaluminum, diethylaluminum chloride, diisobutylalumium chloride, ethylaluminium dichloride, isobutyl laluminum dichloride and mixtures thereof. Preferably, the cocatalyst is trimethylaluminum, triethylaluminum and / or tri- isobutylaluminum; and more preferably, the cocatalyst is triethylaluminum.

[0062] The cocatalyst may be used in a molar ratio of aluminum in the co-catalyst to titanium in the solid catalyst component of from 1 to 500, more preferably from 10 to 250, as high catalyst productivity is obtained.

[0063] The Ziegler-Natta catalysts may be fed as a suspension in a solvent or as a dry catalyst.Fluidized bed

[0064] The process of this invention can be used in any polymerization process in a gas phase reactor. The gas phase reactor may be any reactor suitable for gas phase polymerizations and may e.g. be vertically, horizontally mechanically agitated reactor or a fluidized bed reactor. A gas phase polymerization process in a fluidized bed reactor is preferred. In a typical continuous gas fluidized bed polymerization process for the production of polymer from monomer, a gaseous stream comprising monomer is passed through a fluidized bed reactor in the presence of a catalyst under reactive conditions.

[0065] Conventionally, gas phase polymerization processes typically run continuously, therefore the temperature of the fluidized bed reactor is controlled to an essentially isothermal level through continuously removing the heat of polymerization by circulating the gas exiting from the fluidized bed to a condenser / heat exchanger outside the reactor and recirculating the cooled gas stream back into the reactor. When the temperature of the recirculating stream introduced or recycled into the fluidized bed polymerization reactor is above the dew point temperature, substantially no liquid is present. This process is known as the "dry mode" process. One method to maximize the ability of heat removal is, throughoutthe operation, to reduce to the lowest possible value the temperature of the gaseous feed stream into the reactor. According to the "condensed mode" process a two phase mixture comprising liquid and gas is used into the fluidized bed as a fluidizing medium, the liquid portion of which vaporizes when it is exposed to the heat of the reactor. Fluid can be formed by cooling the recycle stream withdrawn from the reactor below the dew point temperature, thereby converting a portion of the gas into a liquid, and the cooled recycle stream can then be reintroduced into the fluidized bed polymerization reactor. The objective here is to take advantage of the cooling effect brought about by the vaporization, i.e., by bringing the temperature of the fluidized bed down to a point where degradation of the polymer and the catalyst can be avoided and agglomeration of the polymer and chunking can be prevented. The liquid phase / portion is provided by a portion of the recycle gases, which includes monomers and low boiling liquid hydrocarbons, inert to the reaction conditions needed for polymerization, and condensation. Condensed mode fluidized bed reactor polymerization processes are disclosed in for example in US 4543399 and US 4588790. These publications describe the introduction of an inert liquid into the recycle stream to increase the dew point temperature of the recycle stream and allow the process to operate at levels of up to 17.4 percent liquid by weight, based on the total weight of the cooled recycle stream. A condensed mode process is advantageous because its ability to remove greater quantities of heat generated by polymerization increases the polymer production capacity of a fluidized bed polymerization reactor. A common liquid hydrocarbon used in the liquid phase / portion is isopentane, which boils at about 27 degrees centigrade, and consequently becomes a vapor in the recycle line in view of the heat present in the recycle gases. The recycle gases leave the reactor, are cooled, and then condensed to the extent that a vapor phase / portion and liquid phase / portion are formed. The velocity of the recycled gas / liquid mixture should be sufficient to support the fluidized bed, but slow enough to avoid excessive entrainment of fines. The cooling capacity should be sufficient to improve the production rate in terms of space / time / yield.

[0066] "Super condensed mode" fluidized bed reactor polymerization processes operate with above 17.4 percent liquid by weight in the cooled recycle stream as described for example in US 5352749. These must be confined under certain more specific and restrictive conditions within a limited and known range of operating conditions to avoid destabilizing the fluidized bed, thereby halting the process.

[0067] Gas fluidized bed polymerization process generally employ a continuous gas cycle. In one part of the cycle, in a reactor a cycling gas stream is heated by the heat of polymerization. This heat is removed in another part of the cycle by a cooling systemexternal to the reactor. In one embodiment the cycle gas stream is cooled to form a gas and a liquid phase mixture that is then introduced into the reactor. A polymer product is withdrawn from the reactor. For a detailed description of a gas phase process see U.S. Pat. Nos. 4,543,399 and 4,588,790 herein fully incorporated by reference.

[0068] Using a fluidized bed polymerization process substantially reduces the energy requirements as compared to other polymerization processes and most importantly reduces the capital investment required to run such a polymerization process. In preferred embodiments, the fluidized bed is maintained in a fluidized condition during the process of this invention.

[0069] Figure 1 represents a schematic overview of a fluidized bed reactor.

[0070] There are many types of fluidized bed reactors, among which a bubbling fluidized bed reactor, a circulating fluidized bed reactor, an annular fluidized bed reactor, a multi- zone fluidized bed reactor and a flash reactor.

[0071] With 'fluidized bed' as used herein is meant that an amount of solid and / or polymer particles (in this case preferably the solid catalyst and / or the solid catalyst to which the monomer is attached) in a solid / fluid mixture acts as a fluid. This may be achieved by placing the amount of solid and / or polymer particles under appropriate conditions, for instance by the introduction of a fluid through the solid and / or polymer particles at a velocity enough to suspend the solid and / or polymer particles and causing them to behave as a fluid.

[0072] An example of a process using a fluidized bed for producing polyolefins is disclosed in U.S. Patent No. 4,882,400. Other examples of processes using a fluidized bed for producing polyolefins are described in, for example, U.S. Patent Nos. 3,709,853; 4,003,712; 4,01 1 ,382; 4,302,566; 4,543,399; 4,882,400; 5,352,749; 5,541 ,270; 7,122,607, and 7,300,987. The bottom of a fluidized bed reactor (FBR) can comprise for example an inlet connected to a feeder for the reaction composition such as ethylene, nitrogen (N2), hydrogen (H2), comonomer, tri-isobutylamine (TIBAL)-amine, and triethylaluminium (TEAL). The middle zone in the reactor above the distribution plate comprises an inlet for the polymerization catalyst that can be fed to the reactor in combination with nitrogen (N2). The middle zone of the reactor also comprises an outlet to the product discharge tank. The top zone of the reactor comprises an outlet for a top recycle stream, wherein the outlet for the top recycle stream is connected to an inlet of the compressor. The compressor comprises an outlet for compressed fluids and the outlet of the compressor is connected to an inlet forcompressed fluids of the cooling unit. The cooling unit comprises an outlet for providing a bottom recycle stream, which outlet of the cooling unit is connected to the inlet at the bottom of the reactor.

[0073] The fluidized bed reactor may be of a multi-zone fluidized bed reactor (FBR) system as shown in Figure 2. The multi-zone reactor of this example is a multi-zone reactor operable in condensed mode, which multi-zone reactor comprises a first zone, a second zone, a third zone, a fourth zone and a distribution plate, wherein the first zone is separated from the second zone by the distribution plate, wherein the multi-zone reactor is extended in the vertical direction, wherein the second zone of the multi-zone reactor is located above the first zone and wherein the third zone of the multi-zone reactor is located above the second zone, and wherein the fourth zone of the multi-zone reactor is located above the third zone wherein the second zone contains an inner wall, wherein at least part of the inner wall of the second zone is either in the form of a gradually increasing inner diameter or a continuously opening cone, wherein the diameter or the opening increases in the vertical direction towards the top of the multi-zone reactor, wherein the third zone contains an inner wall, wherein at least part of the inner wall of the third zone is either in the form of a gradually increasing inner diameter or a continuously opening cone, wherein the diameter or the opening increases in the vertical direction towards the top of the multi-zone reactor, wherein the largest diameter of the inner wall of the third zone is larger than the largest diameter of the inner wall of the second zone.

[0074] The multi-zone reactor of this example can operate in a so-called "condensing mode" or "condensed mode" which is effective for removal of the heat produced during the exothermic polymerization. In this mode, heat removal is achieved by cooling the gaseous recycle stream to a temperature below its dew point, resulting in the condensation of at least a part of the recycle stream to form a bottom recycle stream containing liquid and gas. The thus formed bottom recycle stream is then introduced into the fluidized bed polymerization reactor, where the liquid portion will vaporize upon exposure to the heat of the reactor, which vaporization will remove heat from the reactor and enables feeding of one or more very highly active catalysts. Details of the FBR which operates in a condensing mode are further described in Application no. EP 13195141.0, incorporated herein by reference.

[0075] Figure 2 illustrates an FBR system comprising a multi-zone reactor (8), a compressor (400) and a cooling unit (5).

[0076] The multi-zone reactor (8) of this example is extended in the vertical direction and comprises four reaction zones (1), (2), (3) and (4). Zone (4) can preferably be located abovezone (3), zone (3) can be located above zone (2) and zone (2) can located above zone (1) in the vertical direction toward the top of the reactor.

[0077] The first zone (1) comprises a first inlet for receiving a bottom recycle stream (10) and the first zone (1) is separated from the second zone (2) by a distribution plate (6). The second zone (2) comprises a first inlet for receiving a solid polymerization catalyst (20). The third zone (3) comprises a first outlet for providing polyolefin (30). This outlet can also be located in the second zone (2). At least one of the second zone (2) or the third zone (3) can comprise at least one section where the inner wall of the reactor is either in the form of a gradually increasing inner diameter or a continuously opening cone in the vertical direction towards the top of the reactor. Here both the second zone (2) and the third zone (3) comprise such sections designated respectively by 2A and 3A. In the second zone (2), at least one section where the inner wall of the reactor is either in the form of a gradually increasing inner diameter or a continuously opening cone in the vertical direction towards the top of the reactor (2A) can preferably be located immediately above the distribution plate (6). Immediately above the distribution plate (6) can thereby preferably mean so that pooling of liquid can be reduced or avoided. Moreover, at least one of the second zone (2) or the third zone (3) can comprise at least one section where the inner wall of the reactor is either in the form of a cylinder. Here both the second zone (2) and the third zone (3) comprise such sections designated respectively by 2B and 3B. The fourth zone (4) comprises a first outlet for a top recycle stream (40) which is connected to a first inlet of the compressor (400) via a first connection means (AA). The fourth zone is thereby a disengagement zone, which may be designed so that polymer particles preferably do not reach that zone or do not stay as little as possible in that zone but rather return to the third zone (3) or the second zone (2), especially for example to allow avoiding the clogging of the compressor (400). The connection means (AA) comprise a first inlet for receiving a feed (60). The compressor (400) comprises a first outlet for compressed fluids (50) which is connected to a first inlet for compressed fluids of the cooling unit (5) via a second connection means (BB). The second connection means (BB) comprise an inlet for receiving a feed (70). The cooling unit (5) comprises a first outlet for providing the bottom recycle stream (10) which is connected to the first inlet of the first zone (1).

[0078] The FBR system may further comprise a polymer withdrawal system, a polymer degassing system and a vent gas recovery system (not shown in Fig. 1). The outlet for the recovered components (in liquid form) (80) from the vent gas recovery system may be transported to the first inlet (70) of the second connection means (BB) by means of pump (7).

[0079] This system can suitably be used for a process for continuous polymerization comprising:• supplying the second zone (2) with a solid polymerization catalyst using the first inlet for receiving the solid polymerization catalyst (20);• supplying a feed (60) comprising an olefin monomer to the first connection means (AA)• optionally supplying a feed (70) comprising condensable inert components to the second connection means (BB)• withdrawing the polyolefin (30) using the first outlet of the second zone (2) and / or the third zone (3) and• circulating fluids from the first outlet of the fourth zone (4) to the first inlet of the first zone wherein the fluids are circulated by• compressing the feed (60) and the top recycle stream (40) using the compressor (400) to form the compressed fluids (50)• subsequently cooling the compressed fluids (50) using the cooling unit (5) to below the dew point of the compressed fluids to form the bottom recycle stream (10) and• feeding the bottom recycle stream (10) to the first zone of the multi-zone reactor (8) via the inlet for receiving the bottom recycle stream of the first zone, and• wherein the superficial gas velocity in this process is in the range of 0.5 to 5 m / s.

[0080] The feed (60) may for example comprise a chain transfer agent, for example hydrogen and may further comprise gaseous olefin monomers and insert gaseous components, for example nitrogen.

[0081] The feed (70) may for example comprise condensable inert components, for example a condensable inert component selected from the group of alkanes having 4 to 20 carbon atoms, preferably 4 to 8 carbon atoms, and mixtures thereof, for example propane, n-butane, isobutene, n-pentane, isopentane, neopentane, n-hexane, isohexane or other saturated hydrocarbons having 6 C-atoms, n-heptane, n-octane and other saturated hydrocarbons having 7 or 8 C-atoms and any mixtures thereof; and may further comprise condensable alfa- olefin monomers, alfa-olefin comonomers and / or mixtures thereof.

[0082] The above FBR system has the advantage that introduction of higher amounts of liquid is allowed without causing destabilization of the fluidized bed.Figures

[0083] Figure 1 shows a typical gas-phase polymerization setup.

[0084] Figure 2 shows a multi-zone fluidized bed polymerisation process setup.

[0085] Figure 3 shows the activity of the catalyst system ZN2 in the presence of THF (30 ppmw) as a function of time (example 1).

[0086] Figure 4 shows the effect of addition of 30 ppmw THF on the sensitivity of the ZN2 catalyst on hydrogen (example 1).

[0087] Figure 5 shows the activity of the catalyst system ZN2 in the presence of THF (45-55 ppmw) as a function of time (example 2), and as a function of the addition of addition TEAL.

[0088] Figure 6 shows the effect of addition of 45-55 ppmw THF on the sensitivity of the ZN2 catalyst on hydrogen (example 2), and as a function of the addition of addition TEAL.

[0089] The invention will now be illustrated by the following non-limiting examples.Preparation of a supported Ziegler Natta catalyst on silica (Catalyst I):

[0090] Under a dry nitrogen atmosphere, a Schlenk flask was charged with 33 grams silica (Davison 955,) previously calcined at 600°C for 4 hours, and 19 ml of 1 M of triethylaluminum (TEAL) in isopentane was added to the silica to form a slurry. The slurry was kept at 37°C for 1 .0 hour. Then, 4 g of MgCI2 and 2.46 g of TiCI3 were dissolved in 1 .5 liter of THF (100%) at 75°C for 2 hours in a three nick bottom flask (Mg / Ti molar ratio of 3.1). After dissolving MgCI2-TiCI3 precursor in THF the solution was transferred into a Schlenk flask with TEAL on Silica and mixed for another hour at 75°C. Drying took place at 105°C then 100°C under nitrogen purge to reach 14.0 weight % THF in the final dried catalyst powder. Finally, 4.1 g of neat DEAC was added to the previously dried powder and mixed for 20 minutes followed by the addition of 4.8 g of TnHAL for 30 minutes then drying took place at 65°C to obtain the free flowing catalyst.Preparation of a supported Ziegler Natta catalyst on silica (Catalyst II):

[0091] In a glovebox a 3-L, 3-necked flask was fitted with a mechanical stirrer and charged with 150g of silica followed by 1.2L of dry isopentane. The necks were stoppered with septaand the mixture stirred at 350 rpm until the silica was fully suspended. Dibutylmagnesium (108 ml_, 1 .0 M in heptane) was added via syringe over a 15 min period and the suspension was then stirred for a further 45 minutes. The stirrer speed was increased to 450 rpm and Et1 .5AICI1 .5 (40.2 mmol) was added slowly over a 10 minute period. Upon addition of Et 1 .5AICI1 .5 the suspension became “gel-like” and this suspension was stirred for another 20 minutes. Tetrachlorosilane (30 mmol) was added over 10 minutes and the still gel-like suspension was stirred for a further 20 minutes. 2-Pentanone (210 mmol) was added slowly upon which the suspension quickly lost its “gel-like” nature and the stirrer speed was reduced to 350 rpm to avoid splashing. Titanium tetraethoxide (10.8 mmol) was added slowly over a 10 minute period upon which the suspension became pale yellow. Stirring was continued for a further 20 minutes. Finally, titanium tetrachloride (24 mmol) was added over a 10 minute period and the suspension slowly became darker in colour. Stirring was continued for a further 20 minutes. The suspension was then purged with nitrogen until it became almost solid at which point the stirring was stopped and the nitrogen purge continued until a free- flowing, beige powder was formed.

[0092] Processes to prepare catalyst systems to be applied for producing polyethylene and its copolymers are generally known in the art. The essential components used in the preparation of such catalysts typically include a supported magnesium compound, a titanium compound supported thereon and an organoaluminium compound. Such catalysts are generally referred to in the art as supported Ziegler-Natta catalysts and an overview of such catalyst types is for example given by T. Pullukat and R. Hoff in Catal. Rev. - Sci. Eng. 41 , vol. 3 and 4, 389-438, 1999.

[0093] Two examples of simulating the process of on-line transitioning between Catalyst I to Catalyst II in a commercial scale reactor are provided below. The simulated examples were conducted in in a continuous gas phase fluidized bed reactor having an internal diameter of 45 cm and a reaction zone height of 140 cm. The bed of polymer particles in the reaction zone is kept in a fluidized state by a recycle stream that works as a fluidizing medium as well as a heat-dissipating agent for absorbing the exothermal heat generated within reaction zone. The reactor was kept at a constant temperature of about 86 °C and at a constant pressure of about 21 .7 bar. Ethylene and 1 -butene were used as the raw materials for polymerization. These materials form a make-up stream. A co-catalyst was mixed with the make-up stream as a 5% by weight solution in isopentane carrier solvent.

[0094] The solid catalyst composition was injected directly in the reaction zone of the fluidized bed using purified nitrogen as a carrier gas. The injection rate was adjusted tomaintain a constant production rate of about 10 kg / hr. The produced polymer was discharged from the reaction zone semi-continuously via a series of valves into a fixed volume chamber.Example I:

[0095] Based on a typical Catalyst I productivity of around 4000 kg PE resin per kg catalyst in pilot plant scale, the corresponding THF concentration in the reactor bed is around 30 ppmw. It is important to note that free THF in the gas phase was not considered in the above estimation. The test was carried out by running the reactor with Catalyst II catalyst to produce LLDPE film grade. Then, a stream of THF was mixed with the make-up stream as a 0.15% by weight solution in isopentane carrier solvent. Once THF was injected to the reactor, the system was allowed to reach steady state where THF concentration in the bed reached the targeted value of 30 ppmw.

[0096] Figure 2 and Figure 3 summarizes the outcome of the first pilot plant test. Figure 2 shows Ash and APS results for the produced resin where the red shaded area in the plot represent the period at which THF concentration was 30 ppmw of reactor bed. One can look at Ash and APS results as two independent productivity measure. With this in mind and looking at the two trends, it can be seen clearly that there is a significant poisonous effect of THF towards Catalyst I. At a concentration of 30 ppmw of THF, there was a drop in Catalyst I productivity by around 25%. Figure 3 shows Ml response to H2 / C2 molar ratio in the reactor. It can be seen that with the presence of THF, Ml response to H2 is declining.Example II:

[0097] In this test, free THF present in the gas phase was estimated in addition to absorbed THF in the resin. In a commercial scale gas phase process utilizing Catalyst I, gaseous components exiting the reactor through Product Discharge System are mostly recovered in Vent Gas Recovery unit and recycled back to the reaction system. This means that THF will accumulate in the reaction system until it reach a steady state concentration. The concentration of free and absorbed THF is considerably higher than what was estimated in the above first test. The targeted THF concentration in the reactor bed for this test is in the range of 45-55 ppmw. Similar to the first test, the reactor was running with Catalyst II to produce LLDPE film grade. Then, a stream of THF was mixed with the make-up stream as a 0.75% by weight solution in isopentane carrier solvent. Once THF was injected to thereactor, the system was allowed to reach steady state where THF concentration in the bed reached the targeted value of 45-55 ppmw.

[0098] Figure 4 and Figure 5 summarizes the outcome of the second pilot plant test. Figure 4 shows Ash and APS results for the produced resin where this time there are two shaded areas in the plot of interest, red and yellow. The red shaded area represents the period at which THF concentration was in the range of 45-55 ppmw of reactor bed while the yellow shaded area represents the period where THF concentration was the same, except that Teal concentration was increase by around 30%. Similar to the first test, it can be seen clearly that that there is a significant poisonous effect of THF towards Catalyst II. At a concentration in the range of 45-55 ppmw of THF, there was a drop in Catalyst II productivity by around 45%. However, the yellow shaded area of the plot suggests that the negative impact of THF was reduced to some extent. It is important to highlight that this second test was designed to test if increasing Teal concentration in reactor bed would reduce the poisonous effect of THF on Catalyst II, which happened to be the case. This result is expected since Teal and THF would act as Lewis acid and Lewis base, respectively, and will form a Lewis adduct. This is confirmed by the slight increase in APS results by 16% by volume which indicate an increase in productivity. Similar to the first test, Figure 5 shows Ml response to H2 / C2 molar ratio in the reactor. It can be seen that with the presence of THF, Ml response to H2 is declining. Table 2 shows process parameters for Example II.

[0099] The properties of the polymer were determined by the following test methods:Table 1Table 2

Claims

Claims1 . Process for transitioning from a first Ziegler-Natta catalyst system to a second Zeigler- Natta catalyst system in a gas-phase olefin polymerisation unit• wherein the polymerisation unit comprises a polymerisation reactor (8) comprising at least one inlet (11) for supplying a reaction mixture (A) to the polymerisation reactor, at least one outlet (12) for removing polymerisation product (30) from the reactor, and at least one outlet (13) for removal of a stream of unreacted and inert materials (40) from the reactor,• wherein the outlet (13) is connected to inlet (11) by which stream (10) or a portion thereof may be circulated back into the reactor, the process comprising the steps in this order of:(a) operating the polymerisation process in the presence of a first Ziegler-Natta catalyst system (ZN1) in a continuous way, wherein stream (10) or a fraction thereof is circulated back into the reactor;(b) discontinuing the circulation of stream (10) back into the reactor;(c) gradually discontinuing the introduction of ZN1 while at the same time gradually introducing a second Ziegler-Natta catalyst system (ZN2) into the reactor so that the polymer production rate is kept at a rate not less than 30% below the production rate as in step (a); and(d) after the introduction of ZN1 is discontinued and the feed of ZN2 is at its full rate, resuming recirculation of stream (10) or a fraction thereof back into the reactor; wherein ZN1 comprises an internal electron donor moiety that the reduces the polymerization activity of ZN2.

2. The process according to claim 1 , wherein the polymerisation process involves the polymerisation of ethylene, optionally with one or more comonomers selected from propylene, 1 -butene, 4-methyl-1 -pentene, 1 -hexene, and 1 -octene.

3. The process according to any one of claims 1-2, wherein a trialkylaluminium cocatalyst is added to the reactor in step C to bind the internal electron donor moiety of ZN1 .

4. The process according to claim 3, wherein the molar ratio between the trialkylaluminium and internal electron donor moiety ranges between 10:1 and 1 :1.5 The process according to any one of claims 1-4, wherein the internal electron donor moiety that is present in ZN1 is selected from alcohols, organosilicon compounds,polysiloxanes, phenols, ketones, aldehydes, inorganic acid esters, polycarboxilic acid esters, ethers, acid amides, acid anhydrides, nitrogen-containing compounds, and acid halides, preferably wherein the internal electron donor moiety is tetrahydrofuran.

6. Process according to any one of claims 1-5, wherein ZN1 is a supported catalyst system, preferably wherein the support is selected from inorganic oxides, such as silica, alumina, magnesia, thoria, zirconia and mixtures of such oxides, more preferably, the support is porous silica.

7. Process according to any one of claims 1-6, wherein ZN2 is produced in a process comprising the steps of: a) contacting a dehydrated solid support having hydroxyl groups with a magnesium compound having the general formula MgR'R", wherein R' and R" are the same or different and are independently selected from the group comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and alkadienylaryl group; b) contacting the product obtained in step (a) with modifying compounds (A), (B) and (C), wherein: compound (A) is at least one compound selected from the group consisting of carboxylic acid, carboxylic acid ester, ketone, acyl halide, aldehyde and alcohol; compound (B) is a compound having the general formula R1a(R2O)bSiY1c, wherein a, b and c are each integers from 0 to 4 and the sum of a, b and c is equal to 4 with a proviso that when c is equal to 4 then modifying compound (A) is not an alcohol, Si is a silicon atom, O is an oxygen atom, Y1is a halide atom and R1and R2are the same or different and are independently selected from the group comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and alkadienylaryl group; compound (C) is a compound having the general formula (R11O)4M1, wherein M1is a titanium atom, a zirconium atom or a vanadium atom, O is an oxygen atom and R11is selected from the group comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and alkadienylaryl group; and c) contacting the product obtained in step (b) with a titanium halide compound having the general formula TiY4, wherein Ti is a titanium atom and Y is a halide atom.

8. Process according to any one of claims 3-7, wherein the trialkylaluminum compounds are selected from tri-methylaluminum, tri-ethylaluminum, tri-propylaluminum, tributylaluminum, tri-isopropylaluminum tri-isobutylaluminum, di- methylaluminumhydride,di-ethylaluminumhydride, di-propylaluminumhydride, di- butylaluminumhydride, diisopropylaluminumhydride, and di-isobutylaluminumhydride, preferably from triethylaluminium and triisobutylaluminium.9 Process according to any one of claims 1-6, wherein ZN2 is a supported catalyst system, preferably wherein the support is selected from inorganic oxides, such as silica, alumina, magnesia, thoria, zirconia and mixtures of such oxides, more preferably, the support is porous silica.

10. Process according to any one of claims 1-9, wherein during steps (b) and (c), the stream (10) is removed from the polymerisation unit.

11. Process according to any one of claims 1-10, wherein the stream (10) is, prior to recirculation back into the polymerisation reactor via inlet (11) subjected to a condensation.

12. Process according to any one of claims 1-11 , wherein the polymerisation reactor is a fluid-bed gas-phase polymerisation reactor.

13. Process according to claim 12, wherein the polymerisation process is performed a multi-zone reactor, the polymerisation process involving:• supplying a second zone (2) with a solid polymerization catalyst using a first inlet for receiving a solid polymerization catalyst (20);• supplying a feed (60) comprising an olefin monomer to a first connection means (AA)• optionally supplying a feed (70) comprising condensable inert components to a second connection means (BB)• withdrawing a polyolefin (30) using a first (12) outlet of the second zone (2) and / or a third zone (3) and• circulating fluids from a first outlet (13) of a fourth zone (4) to a first inlet (11) of a first zone (1) wherein the fluids are circulated by• compressing the feed (60) and a top recycle stream (40) using a compressor (400) to form compressed fluids (50)• subsequently cooling the compressed fluids (50) using a cooling unit (5) to below the dew point of the compressed fluids to form a bottom recycle stream (10) and• feeding the bottom recycle stream (10) to the first zone of the multi-zone reactor (8) via the inlet (11) for receiving the bottom recycle stream of the first zone, and• wherein the superficial gas velocity in this process is in the range of 0.5 to 5 m / s.

14. Process according to any one of claims 1-13, wherein the resumption of recirculation of step (d) occurs when the content of internal electron donor moieties in the stream (10) is reduced to 5 ppm by weight or less.

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