Process for polymerising alpha-olefin monomers

1. A process for polymerising alpha-olefin monomers in a loop reactor comprising the steps of:

(i) introducing a main feed stream ( 2 ) comprising at least one alpha-olefin monomer into the loop reactor ( 1 );

(ii) introducing a polymerisation catalyst into the loop reactor ( 1 );

(iii) polymerising the at least one alpha-olefin monomer in the presence of the polymerisation catalyst in the loop reactor ( 1 ) to produce a slurry comprising polyolefin particles;

(iv) withdrawing an outlet stream ( 4 ) comprising at least a portion of the slurry from the loop reactor ( 1 );

(v) adding a first feed stream ( 9 ) comprising the at least one alpha-olefin monomer and/or hydrogen to the outlet stream ( 4 ) to form a concentrator inlet stream ( 8 );

(vi) introducing the concentrator inlet stream ( 8 ) into a concentrator ( 5 );

(vii) withdrawing from the concentrator ( 5 ) an overflow stream ( 6 ) comprising the polyolefin particles, wherein the concentration of the polyolefin particles in the overflow stream ( 6 ) is smaller than in the concentrator inlet stream ( 8 );

(viii) withdrawing from the concentrator ( 5 ) a bottom outlet stream ( 12 ) comprising the polyolefin particles, wherein the concentration of the polyolefin particles in the bottom outlet stream ( 12 ) is greater than in the concentrator inlet stream ( 8 );

(ix) returning the overflow stream ( 6 ) into the loop reactor ( 1 ) in an area different from that from which the outlet stream ( 4 ) is withdrawn.

2. The process according to claim 1 , wherein the bottom outlet stream ( 12 ) is returned into the loop reactor ( 1 ) in an area different from that from which the outlet stream ( 4 ) is withdrawn or passed to a subsequent loop reactor or passed to a subsequent gas phase reactor.

3. The process according to claim 1 , further comprising adding a second feed stream ( 7 ) comprising the at least one alpha-olefin monomer and/or hydrogen to the overflow stream ( 6 ) before returning the combined second feed stream/overflow stream into the loop reactor ( 1 ).

4. The process according to claim 2 , further comprising adding a third feed stream ( 10 ) comprising the at least one alpha-olefin monomer and/or hydrogen to the bottom outlet stream ( 12 ) before returning the combined third feed stream/bottom outlet stream into the loop reactor ( 1 ).

5. The process according to claim 1 , further comprising the step of withdrawing a slurry stream ( 3 ) from the loop reactor ( 1 ).

6. The process according to claim 1 , wherein a product withdrawal stream ( 11 ) is split off from the bottom outlet stream ( 12 ).

7. The process according to claim 6 , wherein the slurry stream ( 3 ) is combined with the product withdrawal stream ( 11 ) to form a combined loop reactor product stream.

8. The process according to claim 1 , wherein the overflow stream ( 6 ) enters the loop reactor ( 1 ) at a distance of 30 to 60% of the total loop reactor length downstream of the inlet of the main feed stream ( 2 ).

9. The process according to claim 2 , wherein the bottom outlet stream ( 12 ) enters the loop reactor ( 1 ) at a distance of 65 to 90% of the total loop reactor length downstream of the inlet of the main feed stream ( 2 ).

10. The process according to claim 1 , wherein the at least one alpha-olefin monomer is ethylene or propylene.

11. The process according to claim 1 , wherein the molar ratio of hydrogen to alpha-olefin monomer in the first feed stream ( 9 ) is 0 to 10000 mol/kmol.

12. The process according to claim 1 , wherein a pre-polymerization step or a polymerization step precedes step (i).

13. The process according to claim 1 , wherein the polymerization catalyst is a Ziegler-Natta catalyst or a metallocene catalyst.

14. A loop reactor for the process according to claim 1 , comprising

a concentrator ( 5 ),

a first conveying pipe connecting a first area of the loop reactor ( 1 ) with a middle section of the concentrator ( 5 ) to withdraw an outlet stream ( 4 ) from the loop reactor ( 1 ) and to introduce a concentrator inlet stream ( 8 ) into the concentrator ( 5 ),

a second conveying pipe connecting an upper section of the concentrator ( 5 ) with a second area of the loop reactor ( 1 ) to withdraw an overflow stream ( 6 ) from the concentrator ( 5 ) and to introduce said overflow stream ( 6 ) into the loop reactor ( 1 ),

a first feed pipe connected to the first conveying pipe for introducing a first feed stream ( 9 ) into the first conveying pipe to form a concentrator inlet stream ( 8 ),

and a third conveying pipe connecting a lower section of the concentrator ( 5 ) with a third area of the loop reactor.

15. A process for polymerising alpha-olefin monomers in a loop reactor comprising the steps of:

(i) introducing a main feed stream ( 2 ) comprising at least one alpha-olefin monomer into the loop reactor ( 1 );

(ii) introducing a polymerisation catalyst into the loop reactor ( 1 );

(iii) polymerising the at least one alpha-olefin monomer in the presence of the polymerisation catalyst in the loop reactor ( 1 ) to produce a slurry comprising polyolefin particles;

(iv) withdrawing an outlet stream ( 4 ) comprising at least a portion of the slurry from the loop reactor ( 1 );

(v) adding a first feed stream ( 9 ) comprising the at least one alpha-olefin monomer and/or hydrogen to the outlet stream ( 4 ) to form a concentrator inlet stream ( 8 );

(vi) introducing the concentrator inlet stream ( 8 ) into a concentrator ( 5 );

(vii) withdrawing from the concentrator ( 5 ) an overflow stream ( 6 ) comprising the polyolefin particles, wherein the concentration of the polyolefin particles in the overflow stream ( 6 ) is smaller than in the concentrator inlet stream ( 8 );

(viii) withdrawing from the concentrator ( 5 ) a bottom outlet stream ( 12 ) comprising the polyolefin particles, wherein the concentration of the polyolefin particles in the bottom outlet stream ( 12 ) is greater than in the concentrator inlet stream ( 8 );

(ix) returning the overflow stream ( 6 ) into the loop reactor ( 1 ) in an area different from that from which the outlet stream ( 4 ) is withdrawn;

wherein the loop reactor comprises:

the concentrator ( 5 ),

a first conveying pipe connecting a first area of the loop reactor ( 1 ) with a middle section of the concentrator ( 5 ) to withdraw the outlet stream ( 4 ) from the loop reactor ( 1 ) and to introduce the concentrator inlet stream ( 8 ) into the concentrator ( 5 ),

a second conveying pipe connecting an upper section of the concentrator ( 5 ) with a second area of the loop reactor ( 1 ) to withdraw the overflow stream ( 6 ) from the concentrator ( 5 ) and to introduce the overflow stream ( 6 ) into the loop reactor ( 1 ),

a first feed pipe connected to the first conveying pipe for introducing the first feed stream ( 9 ) into the first conveying pipe to form a concentrator inlet stream ( 8 ),

and a third conveying pipe connecting a lower section of the concentrator ( 5 ) with a third area of the loop reactor.

16. The process of claim 1 , wherein the loop reactor is connected in series with at least one additional reactor.

17. The process of claim 16 , wherein the at least one additional reactor comprises a slurry reactor.

18. The process of claim 16 , wherein the at least one additional reactor comprises a gas phase reactor.

19. The process of claim 16 , wherein the at least one additional reactor comprises a gas phase reactor downstream of the loop reactor.

20. The process of claim 19 , wherein the at least one additional reactor further comprises a slurry reactor upstream of the loop reactor.