Process for the polymerisation of ethylene or the copolymerisation of ethylene and alpha-olefins in a fluidised bed in the presence of a chromium based catalyst

Abstract

The present invention relates to a process for the polymerisation of ethylene or copolymerisation of ethylene and at least one other alpha-olefin, in the gas phase in a fluidised bed reactor, in the presence of a catalyst comprising chromium compounds associated with a granular support comprising a refractory oxide and activated by thermal treatment, which process is characterised in that the catalyst is used in the form of active particles of prepolymer, obtained by bringing the said catalyst in contact with ethylene, alone, or in a mixture with at least one higher alpha-olefin, in such a way that the prepolymer contains from 4×10 -5 to 3 milligram atoms of chromium in the catalyst per gram.

We claim: 
     
       1. Process for the polymerisation of ethylene, or the copolymerisation of ethylene and at least one higher alpha-olefin comprising 3 to 12 carbon atoms, in the gas phase in a fluidised bed reactor, in the presence of a catalyst comprising a chromium oxide associated with a granular refractory oxide support which catalyst has been activated by thermal treatment at a temperature of at least 250° C. and at most equal to the temperature at which the granular support commences to sinter, the activation being carried out in a non-reducing atmosphere, which process is characterised in that the catalyst is used in the polymerisation or copolymerisation in the form of dry catalytically active particles of prepolymer which are obtained during a separate prepolymerisation operation comprising bringing the said catalyst into contact with ethylene or, with a mixture of ethylene and at least one higher alpha-olefin comprising 3 to 12 carbon atoms, in suspension in a liquid hydrocarbon medium during a period of time such that the produced prepolymer contains from 4×10 -5  to 3 milligram atoms of chromium per gram and separating the prepolymer from the unreacted quantity of ethylene or the mixture of ethylene with at least one higher alpha-olefin and from the liquid hydrocarbon medium.[...]..Iadd., and said prepolymer has a content of polymer or copolymer soluble in n-hexane at 60° C. of less than or equal to 2.0% by weight..Iaddend. 
     
     
       2. Process according to claim 1, characterised in that the content by weight of chromium in the catalyst is .[.comprised.]. between 0.05 and 30%. 
     
     
       3. Process according to claim 1, characterised in that the prepolymerisation is performed at a temperature lower than the temperature at which the particles of the prepolymer begin to soften and to form agglomerates. 
     
     
       4. Process according to claim 1, characterised in that the prepolymerisation is performed in the presence of an organo-metallic compound (a) of a metal of groups I to III of Periodic Table of Elements, in a quantity such that the atomic ratio of the quantity of metal in the said organo-metallic compound (a) to the quantity of chromium in the catalyst is comprised between 0.01 to 30. 
     
     
       5. Process according to claim 1, characterized in that the prepolymer is brought into contact with ethylene or a mixture of ethylene and at least one higher alpha-olefin under conditions of polymerisation or copolymerisation in the gas phase in a fluidised bed reactor, in the presence of an organo-metallic compound (b) of a metal of groups I to III of the Periodic Table of Elements, in a quantity such that the atomic ratio of the quantity of metal in the said organo-metallic compound (b) to the quantity of chromium in the catalyst contained in the prepolymer is at most equal to 100. 
     
     
       6. Process according to claim 1, characterised in that the polymerisation or copolymerisation in the gas phase in the fluidised bed reactor is performed using a gas comprising ethylene or a mixture of ethylene and at least one higher alpha-olefin, and circulating at a fluidisation velocity .[.comprised.]. between 40 and 120 cm/sec, under a total pressure .[.comprised.]. between 0.5 and 4 MPa, at a temperature of at least 50° C. and lower than the temperature at which the particles of the polymer or of the copolymer begin to soften and to form agglomerates. 
     
     
       7. Process according to claim 6, characterised in that the gas mixture comprises hydrogen in a quantity such that the molar ratio of hydrogen to ethylene and any higher alpha-olefin is .[.comprised.]. between 0 and 5. 
     
     
       8. Process according to claim 1, wherein the activation is carried out in an oxidising atmosphere; and wherein the produced prepolymer contains from 10 -3  to 10 -1  milligram atoms of chromium per gram. 
     
     
       9. Process according to claim 2, characterised in that the content by weight of chromium in the catalyst is .[.comprised.]. between 0.1 and 3%. 
     
     
       10. Process according to claim 1, characterised in that the prepolymer occurs in the form of a powder consisting of particles having a mean diameter by mass .[.comprised.]. between 80 and 500 microns. 
     
     
       11. Process according to claim 3, characterised in that the prepolymerisation is performed at a temperature .[.comprised.]. between 40 and 115° C. 
     
     
       12. Process according to claim 4, characterised in that the atomic ratio of the quantity of metal in the said organo-metallic compound (a) to the quantity of chromium in the catalyst is .[.comprised.]. between 0.1 and 1. 
     
     
       13. Process according to claim 5, characterised in that the atomic ratio of the quantity of metal in the said organo-metallic compound (b) to the quantity of chromium in the catalyst contained in the prepolymer is .[.comprised.]. between 0.1 and 20. 
     
     
       14. Process according to claim 6, characterised in that the fluidisation velocity is .[.comprised.]. between 50 and 100 cm/sec, and the temperature is at least 50° C. and lower than 115° C. 
     
     
       15. Process according to claim 7, characterised in that the molar ratio of hydrogen to ethylene and any higher alpha-olefin is .[.comprised.]. between 0.1 and 1. 
     
     
       16. Process according to claim 5, wherein the organo-metallic compound (b) is a trialkyl aluminium, an alkyl aluminium hydride, or an alkyl aluminium alcoholate.