Polymerisation control process

Abstract

The present invention relates to a process for controlling the gas-phase co-polymerisation of olefins in a fluidised bed reactor. The present invention futher relates to a method for the continuous gas-phase (co-)polymerisation of olefins in a fluidised bed reactor in the presence of a polymerisation catalyst wherein the density SPAN of the polymer powder particles leaving the reactor is maintained below certain values throughout the polymerisation.

1 . Process for controlling a continuous fluidised bed reactor (co-)polymerisation process which comprises: 
 (i) withdrawing from the reactor a hot recycle stream comprising a principal monomer and at least another unreacted reactant,    (ii) cooling part or all of said recycle stream withdrawndfrom said reactor, and    (iii) recycling part or all of said cooled recycle stream comprising the principal monomer and the unreacted reactant(s) through the polymerisation zone in said reactor in the presence of a polymerisation catalyst under reactive conditions, characterised in that the said controlling process consists in controlling the density SPAN of.the polymer powder particles.    
     
     
         2 . Process according to  claim 1  wherein the density SPAN of the polymer powder particles means the absolute value of the density difference in g/cm3 between the average density of the material. with particle size above the X% cumulative volume of the volume/particle size distribution of the (aggregate) polymer particles exiting the reactor and the average density of all the said (aggregate) polymer particles exiting the reactor, X being comprised between 51 and 99, preferably 80 and 99, more preferably 90 and 99, and Y beingcomprised between 49 and 1, preferably 20 and 1, more preferably 10 and 1.  
     
     
         3 . Process according to  claim 1  wherein the density SPAN of the polymer powder particles means the absolute value of the density difference in g/cm3 between the average density of the material with particle size. below the Y% cumulative volume of the volume/particle size distribution of the (aggregate) polymer particles exiting the reactor and the average density of all the said (aggregate) polymer particles exiting the reactor, X being comprised betveen 51 and 99, preferably 80 and 99, more preferably 90 and 99, and Y being comprised between 49 and 1, preferably 20 and 1, more preferably 10 and 1.  
     
     
         4 . Process according to  claim 1  wherein the density SPAN of the polymer powder particles means the absolute value of the density difference in g/cm3 betwveen the average density of the material with particle size above the X% cumulative volume of the volume/particle size distribution of the (aggregate) polymer particles exiting the reactor and the average density of of the material with particle size below the Y% cumulative volume of the volume/particle size distribution of the (aggregate) polymer particles exiting the reactor, X being comprised between 51 and 99, preferably 80 and 99, more preferably 90 and 99, and Y being comprised between 49 and 1, preferably 20 and 1, more preferably 10 and 1.  
     
     
         5 . Process according to any of the preceding claims wherein the density SPAN of the polymer powder particles is maintained below 0.0028, preferably below 0.0025, more preferably below 0.0023.  
     
     
         6 . Process according to any of the preceding claims wherein the catalyst used produces a density SPAN of the polymer powder particles higher than 0.0035, preferably higher than 0.004, under the following polymerisation conditions: 
 a. Ethylene-hexene-1 copolymer production in a continuous gas phiase fluidised bed reactor    b. polymerization operating temperature of 72° C.    c. polymerization operating pressure of 20 bars    d. no condensation rate    e. melt index (2.16) of the polymer produced equal to 1.2 (as measured according to ASTM-D-1238/method B    f. density of the polymer produced equal to 0.918 g per cm3 (annealed/as measured according to ASTM-D-2839).    
     
     
         7 . Process according to any of the preceding claims wherein the catalyst is a metallccene.  
     
     
         8 . Process according to any of the preceding claims wherein the polymerisation production rates are equal to or higher than 100 kg of polymer per hour, preferably higher than 150 kg/hour.  
     
     
         9 . Process according to any of the preceding claims wherein the control of the density SPAN is obtained through the control of the condensation rate and/or of the polymerisation temperature.  
     
     
         10 . Process according to  claim 9  wherein the density SPAN reduction is obtained by increasing the condensation rate and/or by increasing the polymerisation temperature.  
     
     
         11 . Process according to any of the preceding claims wherein the principal monomer is ethylene or propylene, preferably ethylene.