GB2154242A - Continuous slurry process for producing olefin (co)polymers - Google Patents

Abstract

This invention relates to a continuous process for producing olefin (co)polymers especially high density polyethylene using a slurry process. The catalyst composite used comprises a supported Ziegler catalyst component and a prepolymer component of one or more l-olefins deposited thereon under batch conditions. The process used is the 'particle form' slurry process and yields (co)polyolefins with a reduced content of fines.

Description

SPECIFICATION Continuous slurry process for producing olefin (co)polymers The present invention relates to a continuous process for producing olefin (co)polymers, especially high density polyethylene using a slurry process.

In the slurry process for producing olefin (co)polymers, the olefin is contacted under continuous or batch polymerisation conditions with a suspension of a particulate transition metal-based catalyst in a liquid diluent, for example a hydrocarbon diluent. The polymerisation is conducted under conditions such that the polyolefin is formed as particles suspended in the diluent. The polyolefin particles so produced can be recovered for example by settling or filtration techniques. Avariety of grades of the polyolefin can be produced by controlling the reaction conditions, e.g. the catalyst, temperature and by the use of chain transfer agents. One of the problems with this type of process is the formation of polymer fines, i.e. ultra-fine particles of the polyolefin in the product.The polymer fines not only hinder the separation of the final polymer from the diluent but also give a final polymer product with poor powder flow characteristics.

Moreoever, the fines create a dust problem which is unpleasant to the operatives handling the product and can constitute a dust explosion hazard.

It is also known that during the batch slurry copolymerisation of olefins, especially to produce linear low density polyethylene, some of the problems associated with stickiness in the copolymer particles produced can be mitigated by using a catalyst which has been coated with a prepolymer of one of the olefins to be copolymerised. Such processes are described in GB 1526606 and GB 1532332.

It is an object of the present invention to provide a process for (co)polymerising 1 -olefins under continuous slurry polymerisation conditions and using a transition metal-based catalyst wherein the (co)polyolefin product has a reduced content of fines.

Accordingly, the present invention is a continuous slurry process for (co)polymerising 1-olefins comprising contacting one or more 1-olefinswith a particulate transition metal-based Ziegler catalyst composite under slurry polymerisation conditions in a liquid diluent characterised in that the catalyst composite comprises the product of prepolymerising under batch conditions one or more 1 -olefins with a catalyst comprising (A) and organometallic Ziegler catalyst activator and (B) the solid product obtained by reacting together under anhydrous conditions a halogen containing transition metal compound and an aliphatic alcohol and simultaneously or subsequently impregnating an anhydrous hydroxyl-groupscontaining support material comprising magnesium oxide, magnesium hydroxide or magnesium hydroxychloride with the reaction mixture.

The continuous slurry process referred to herein can be for instance a process in which the reactor comprises a tubular pipeloop around which the catalyst composite particles and the olefins to be polymerised are pumped. Granules of the polyolefin so formed are thereafter withdrawn from the reactor.

The transition metalbased catalysts employed to make the catalyst composite used in the present invention is a supported Ziegler catalyst. The support material is preferably a particulate magnesium oxide or a mixture of magnesium oxide and magnesium hydroxide provided that the support has a hydroxyl content of less than 0.2 OH groups per magnesium atom. The transition metal compound in the supported Ziegler catalyst compound is suitably a metal compound of Groups IVA, VA and VIA of the Periodic Table. Titanium andlor vanadium compounds are preferred. The supported Ziegler catalyst component is activated using a conventional metal alkyl activator, for example triethyl aluminium, tri-n-butyl aluminium or diethyl aluminium chloride.

A particularly preferred transition metal-based catalyst component for preparing the catalyst composite used in the present invention comprises the catalyst disclosed in our GB-A-1553673. The catalyst in GB-A-1553673 is prepared by activating with a conventional organometallic activator a Ziegler catalyst component derived by reacting together under anhydrous conditions a halogen-containing transition metal compound other than a fluorine-containing compound and an aliphatic alcohol and simultaneously or subsequently impregnating an anhydrous hydroxyl groups-containing support material comprising magnesium oxide having a hydroxyl content less than 0.2 OH groups per magnesium atom with the reaction mixture.The transition metal compound is preferably a titanium compound, for example a titanium compound having the empirical formula Ti(OR)nCI4 n wherein n is zero, integral or fractional and is less than 4 and R is an a Ikyl grou p containing 1 to 6 c8rbon atoms. For further details regarding the preparation of this preferred catalyst reference may be made to our GB-A-1553673.

The olefinic prepolymer deposited on the Ziegler catalyst component is suitably derived by polymerising an olefin selected from ethylene, propylene and 1-butene.

The prepolymerisation is preferably carried out at a temperature which is at or below 110C preferably 0-80"C. The precise temperature will depend upon the nature of the catalyst and the reactants used e.g.

olefin, hydrogen and the like. The prepolymerisation is suitably conducted at pressures ranging from atmospheric to 50 bars or more. The molecular weight of the olefin prepolymer can be controlled to the extent desired by conventional techniques e.g. by controlling the temperature or by using adjuvants such as hydrogen.

It is preferable to control the aforementioned parameters e.g. diluents, temperature, pressure and molecular weight during the prepolymerisation so that the catalyst composite so formed contains less than 2 g of the prepolymer per gram of the Ziegler catalyst component. The prepolymer present in the catalyst composite prepared under the range of conditions specified above suitably has a viscosity average molecular weight of 1000-100,000.

The 1-olefins that may be polymerised in a continuous slurry process using the catalyst composite as described above include ethylene, propylene, butenes, pentenes, hexenes, heptenes, octenes and mixtures thereof. The pentenes, hexenes, heptenes and octenes include all terminally unsaturated polymerisable isomers. Preferred 1-olefins are one or more of the C2-C4 olefins. The C5-C8 olefins, if used, are suitably used as comonomers during the copolymerisation of a feedstock comprising at least one of the C2-C4 olefins. The monomer employed in the process of the present invention is preferably ethylene or a mixture of ethylene with up to 40% w/w of one or more of the other 1 -olefins specified above which are copolymerisable under the reaction conditions employed.The process is particularly suitable for producing granular or particulate high density polyethylene and linear low density polyethylene.

Without wishing to limit in any way the scope of the continuous slurry process referred to herein, the general conditions and process for producing high density polyethylene especially in particulate form is described below.

The polymerisation process conditions of the present invention are the so-called "particle form" process conditions which are well known in the art. In the "particle form" process the momomeric 1-olefin is contacted with a suspension of the catalyst particles in a fluid medium under conditions such that the polymeric 1-olefin forms as solid particles suspended in orfluidised in the fluid medium. The fluid medium can be for example a liquid hydrocarbon. Examples of suitable liquid hydrocarbons are isobutane and n-pentane.

In the present invention the continuous (co)polymerisation of 1-olefins is preferably carried out using "particle form" process under conditions in which the liquid diluent is preferably a liquid hydrocarbon.

Examples of suitable liquid hydrocarbons are paraffins or cycloparaffins having from 3-30 carbon atoms per molecule, for example isopentane, isobutane, cyclohexane. Most preferably the liquid medium is isobutane.

The concentration of monomer in the liquid diluent used is suitably in the range 2-15 wt % although concentrations outside this range can be employed.

When using a liquid phase "particle form" process the polymerisation temperature is suitably in the range 50 to 112"C, preferably 80 to 110or. The polymerisation pressure is preferably in the range 2 to 100 bar. The residence or reaction time can vary from a few minutes to several hours and is generally in the range 15 minutes to 3 hours.

Preferred apparatus for conducting the reaction under continuous "particle form" process conditions in a liquid medium is described in UK Patent Specification 899,156.

For further details of examples of particle form process conditions and apparatus which can suitably be employed in the present invention, reference may be made to UK Patent Specification Nos 899156, 886784 and 853414.

If desired, the polymerisation process of the present invention can be conducted in the presence of hydrogen gas as a chain transfer agent to lower the average molecular weight of the polymer produced.

Methods of recovering polyolefins produced by "particle form" polymerisation processes are well known in the art.

The present invention is illustrated in the following Examples and Comparative Tests.

Preparation of Catalyst Composite 2.24 kg (37.3 moles) isopropanol were mixed with 5 litres of dry cyclohexane in a dry, nitrogen purged, 20 litre round bottomed flask equipped with stirrer, heating mantle and reflux condenser. 3.35 kg (17.7 moles) TiCI4 were added, with stirring, over a period of approximately 2 hours. After addition of the TiCI4 was complete, the mixture was heated under reflux, with stirring, for 3 hours. The mixture was allowed to cool and a slurry of 5009 (12.4 moles) magnesia (Maglite K, Merck and Co) in dry cyclohexane was added. The resulting mixture was heated under reflux, with stirring, for 2 hours, allowed to cool and then filtered and washed with fresh, dry cyclohexane. The catalyst was handled as a slurry and stored under N2.The dry catalyst powder contained 5.6 wt% Ti, 19.1 wt% Mg and 35.4 wt% Cl.

Example 1 The activated catalyst component (1009) prepared as described above was added as a 10g/100 ml slurry in heptane to an autoclave (2 litres) and heated to 40"C with stirring. Hydrogen (75 psi) was added to the reactor and then ethylene (50g) was added over a period of 20 mins. The resultant catalyst composite containing 0.59 polymerig catalyst was removed in slurry form and stored under dry nitrogen.

Example 2 The activated catalyst component (759) prepared as described above was added as a 7.5g/1 00 ml slurry in a mixture of cyclohexane and isobutane to a 2-litre autoclave under nitrogen and was stirred and heated to 75"C. Hydrogen (75 psi) was added to the reactor and then ethylene (75g) was added over 95 mins. to produce a catalyst composite containing 1 .0g of polymers of catalyst.

Example 3 The activated catalyst component (100g) prepared as described above was added as a 10g1100 ml slurry in a mixture of cyclohexane and heptane to an autoclave (2 litres) under nitrogen and was stirred and heated to 55"C. Hydrogen (70 psi) was added to the reactor and then ethylene (100g) was added over 105 mins. to produce a catalyst composite with 1.0g of polymer of catalyst.

Slurry polymerisation The catalyst composites were fed to a pilot scale pipe loop reactor in a continuous process to polymerise ethylene under the conditions specified below. The polymerisation reactions were repeated using the parent Ziegler catalysts alone without the prepolymer in Tests 1-3.

Conditions Process : continuous Concentration in reactor* (Mole %) C2H4 10.1-12.3 H2 : 0.81-1.08 H2:C2H4 Ratio 0.080-0.091 Product Melt Index : 12.2-14.5 at (216 kg load) *Measured from reactor off gases.

The amount of polymer fines formed in each case is tabulated below.

Catalyst Employed wlw % Polymer Fines less than 106 microns Test 1 Ziegler Catalyst alone 7 Example 1 Prepolymercomposite 1 Test2 Ziegler Catalyst alone 7 Example 2 Prepolymer composite 2 Test3 Ziegler Catalyst alone 4 Example 3 Prepolymercomposite Trace The results show that the prepolymericatalyst composite gives rise to a small amount of polymer fines than the parent Ziegler catalyst used alone.

Claims (12)

1. A continuous slurry process for (co)polymerising 1-olefins comprising contact one or more 1-olefins with a particulate transition metal-based Ziegler catalyst composite under slurry polymerisation conditions in a liquid diluent characterised in that the catalyst composite comprises the product of prepolymerising under batch conditions one or more 1-olefins with a catalyst comprising (A) an organometallic Ziegler catalyst activator and (B) the solid product obtained by reacting together under anhydrous conditions a halogen containing transition metal compound and an aliphatic alcohol and simultaneously or subsequently impregnating an anhydrous hydroxylgroups-containing support material comprising magnesium oxide, magnesium hydroxide or magnesium hydroxychloride with the reaction mixture.

2. A process according to claim 1 wherein the transition metal compound in the supported Ziegler catalyst component is titanium and/orvanadium.

3. A process according to claim 1 or 2 wherein the supported Ziegler catalyst component is activated using a metal alkyl activator.

4. A process according to claim 3 wherein the metal in the activator is aluminium.

5. A process according to any one of the preceding claims wherein the prepolymerisation is carried out at a temperature which is at or below 110"C and a pressure from atmospheric to 50 bars.

6. A process according to any one of the preceding claims wherein the catalyst composite contains less than 29 of the prepolymer per gram of the Ziegler catalyst component.

7. A process according to any one of the preceding claims wherein the prepolymer in the catalyst composite has a viscosity average molecular weight of 1000-100,000.

8. A process according to any one of the preceding claims wherein the 1-olefin being (co)polymerised is selected from ethylene, propylene, butenes, pentenes, hexenes, heptenes, octenes and mixtures thereof.

9. A process according to any one of the preceding claims wherein the liquid diluent is a paraffin or cycloparaffin having from 3-30 carbon atoms per molecule.

10. A process according to any one of the preceding claims wherein the concentration in the liquid diluent of the monomer(s) to be (co)polymerised is in the range of 2-15% w/w.

11. A process according to any one of the preceding claims wherein the (co)polymerisation is carried out at a temperature in the range 50-1 1 2"C and a pressure in the range of 2-100 bar.

12. A process for producing (co)polymers of 1 -olefins as claimed in claim 1 and as described hereinbefore with reference to the Examples.