Slurry phase polymerisation process

1. Process for making a multimodal ethylene polymer in which a low molecular weight (LMVV) polymer is made in one reactor, and a high molecular weight (HMW) polymer is made in another reactor, which process comprises polymerizing in a loop reactor of a continuous tubular construction an olefin monomer optionally together with an olefin comonomer in the presence of a polymerization catalyst in a diluent to produce a slurry comprising solid particulate olefin polymer and the diluent, wherein the average internal diameter of at least 50% of the total length of the continuous tubular loop reactor is at least 700 mm and the total pressure drop in the loop of the reactor is less than 1.3 bar, the Froude number is maintained in at least one reactor of the multiple reactor system at or below 30, and the HMW polymer is produced in a reactor upstream of the LMW polymer reactor and the ratio of the average internal diameter of the HMW reactor to the average internal diameter of the LMW reactor is between 0.8 and 1.4, and further wherein the process has a polymer production rate greater than 25 tones per hour.

2. Process according to claim 1 , wherein the solids concentration in the continuous tubular loop reactor is at least 20 volume %.

3. Process according to claim 1 , wherein the average internal diameter of at least 50% of the total length of the continuous tubular loop reactor is at least 750 mm.

4. Process according to claim 1 , wherein greater than 70% of the total length of the continuous tubular loop reactor has an internal diameter at least 700 mm.

5. Process according to claim 4 , wherein greater than 70% of the total length of the continuous tubular loop reactor has an internal diameter at least 750 mm.

6. Process according to claim 1 , wherein the Froude number is maintained in at least one reactor of the multiple reactor system between 1 and 20.

7. Process according to claim 1 , wherein at least 30 wt % of the total polymer produced in the multiple reactor system is made in said continuous tubular loop reactor.

8. Process according to claim 1 , wherein the density span of the polymer powder particles (defined as the absolute value of the density difference in g/cm 3 between the average density of the polymer particles exiting the loop reactor with particle size above D90 and the average density of the material with particle size below D10) is below 0.005, in which D10 and D90 are the diameters under which 10% and 90% by weight respectively of the particles are collected.

9. Process according to claim 1 , wherein the particle size distribution of the polymer particles is such that (D90-D10)/D50 is less than 2, in which D10, D50 and D90 are the diameters under which 10%, 50% and 90% by weight respectively of the particles are collected.

10. Process according to claim 1 , wherein D95 is less than 2000 μm, in which D95 is the diameter under which 95% by weight of the particles are collected.

11. Process according to claim 1 , wherein either:

the multimodal ethylene polymer has a density greater than 940 kg/m 3 and a melt flow index MI 5 of 0.05 to 50 g/10 min, said ethylene polymer comprising—

from 30 to 70 wt %, based on the total weight of the ethylene polymer, of a first polyethylene fraction having a density of at least 950 kg/m 3 and a melt flow index MI 2 of at least 10 g/10 min, and

from 70 to 30 wt %, based on the total weight of the multimodal ethylene polymer, of a second polyethylene fraction comprising units of ethylene and optionally up to 5 mol % of at least one other alpha-olefin containing from 3 to 12 carbon atoms, and a melt flow index MI 2 of less than 10 g/10 min; or

the multimodal ethylene polymer has a density between 900 and 930 kg/m 3 and a melt flow index MI 2 of 0.1 to 20 g/10 min, said ethylene polymer comprising:

from 30 to 70 wt %, based on the total weight of the ethylene polymer, of a first polyethylene fraction having a density of less than 950 kg/m 3 and a melt flow index MI 2 of at least 10 g/10 min, and

from 70 to 30 wt %, based on the total weight of the multimodal ethylene polymer, of a second polyethylene fraction comprising units of ethylene, from 0.1 to 20 mol % of an alpha-olefin containing from 3 to 12 carbon atoms, and a melt flow index MI 2 of less than 10 g/10 min.

12. Process according to claim 2 , wherein the solids concentration in the continuous tubular loop reactor is at least 25 volume %.

13. Process according to claim 2 , wherein the solids concentration in the continuous tubular loop reactor is at least 30 volume %.

14. Process according to claim 3 , wherein the average internal diameter of at least 50% of the total length of the continuous tubular loop reactor is at least 850 mm.

15. Process according to claim 1 , wherein the total pressure drop in the loop of the reactor is less than 1 bar and the polymer production rate is greater than 45 tonnes per hour.

16. Process according to claim 1 , wherein D95 is less than 1000 μm.