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A process comprising polymerizing an olefin monomer optionally together with an olefin comonomer in the presence of a polymerization catalyst in a diluent in a loop reactor which comprises at least 2 horizontal sections and at least 2 vertical sections to produce a slurry comprising solid particulat

A process comprising polymerizing an olefin monomer optionally together with an olefin comonomer in the presence of a polymerization catalyst in a diluent in a loop reactor which comprises at least 2 horizontal sections and at least 2 vertical sections to produce a slurry comprising solid particulate olefin polymer and the diluent wherein the Froude number in at least 20% of the length of the vertical sections of the reactor loop is less than 85% of the Froude number in at least 20% of the length of the horizontal sections of the loop is disclosed.

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The invention claimed is: 1. A process comprising polymerising an olefin monomer optionally together with an olefin comonomer in the presence of a polymerization catalyst in a diluent in a loop reactor which comprises at least 2 horizontal sections and at least 2 vertical sections to produce a slur

The invention claimed is: 1. A process comprising polymerising an olefin monomer optionally together with an olefin comonomer in the presence of a polymerization catalyst in a diluent in a loop reactor which comprises at least 2 horizontal sections and at least 2 vertical sections to produce a slurry comprising solid particulate olefin polymer and the diluent wherein the Froude number in at least 20% of the length of the vertical sections of the reactor loop is less than 85% of the Froude number in at least 20% of the length of the horizontal sections of the loop. 2. A process claimed in claim 1 wherein the average Froude number in the loop reactor is maintained at or below 20. 3. A process as claimed in claim 2 wherein the average Froude number in the loop reactor is maintained in the range 10 to 3. 4. A process as claimed in claim 1 wherein the Froude number in the vertical sections of the loop reactor with upward circulation is maintained at between 30 and 85% of the minimum Froude number used in the horizontal sections. 5. A process as claimed in claim 1 wherein the Froude number in the vertical sections of the loop reactor with downward circulation is maintained at between 15 to 70% of the minimum Froude number used in the horizontal sections. 6. A process as claimed in claim 1 wherein the Froude number in the horizontal sections is maintained below 30. 7. A process as. claimed in claim 1 wherein the Froude number in the vertical sections is maintained below 20. 8. A process as claimed in claim 1 wherein the horizontal sections of the loop reactor consist of no more than 20% of the total reactor length. 9. A process as claimed in claim 1 wherein the total loop pressure drops by less than 1.3 bar. 10. A process as claimed in claim 1 wherein the reactor size is over 50 m3. 11. A process as claimed in claim 1 wherein the average internal diameter of the loop reactor is over 300 millimeters. 12. A process as claimed in claim 11 wherein the average internal diameter of the loop reactor is in the range 600 to 750 millimeters. 13. A process as claimed in claim 1 wherein the ratio of the reactor length to the average internal diameter of the loop reactor is less than 500. 14. A loop reactor of a continuous tubular construction comprising at least two horizontal sections and at least two vertical sections wherein the internal cross sectional area of at least 20% of the vertical sections is at least 5% greater than the largest internal cross sectional area that covers at least 20% of the horizontal sections, wherein the average internal diameter of the horizontal sections is in the range 500-700 millimeters and the average internal diameter of the vertical section is in the range 600-900 millimeters. 15. A process as claimed in claim 13 wherein the ratio of the reactor length to the average internal diameter of the loop reactor is less than 250.