Rapid activation process and activation treatments for chromium catalysts for producing high melt index polyethylenes
Processes for activating chromium polymerization catalysts, which can use lower maximum activation temperatures and shorter activation times than conventional activation methods, and provide polyethylenes with high melt indices, broader molecular weight distributions, and lower long chain branching content. The activation process can comprise heating a supported chromium catalyst in an inert atmosphere to a first temperature (T 1 ) for a first hold time (t H1 ), followed by allowing the chromium catalyst to attain a second temperature (T 2 ) in the inert atmosphere, then contacting the chromium catalyst with an oxidative atmosphere for a second hold time (t H2 ), in which T 2 can be less than or equal to T 1 . Additional activation treatments and conditioning steps are disclosed which can be used to enhance the melt index potential of Phillips (Cr/silica) catalysts.
1. A process for activating a supported chromium catalyst, the process comprising:
a) heating a supported chromium catalyst in an inert atmosphere to a first temperature (T 1 ) at a first ramp rate (RR 1 );
b) holding the chromium catalyst at the first temperature in the inert atmosphere for a first hold time (t H1 );
c) allowing the chromium catalyst to attain a second temperature (T 2 ) in the inert atmosphere at a second ramp rate (RR 2 ), wherein T 2 is less than or equal to T 1 , followed by exposing the chromium catalyst to an oxidative atmosphere;
d) holding the chromium catalyst at the second temperature in the oxidative atmosphere for a second hold time (t H2 ); and
e) allowing the chromium catalyst to cool to a temperature below the second temperature in an inert atmosphere, in the oxidative atmosphere, or in any sequential combination of an inert atmosphere and an oxidative atmosphere over a cooling time (t C ), to provide an activated chromium catalyst,
wherein the second hold time (t H2 ) is less than 1 hour and greater than about 30 seconds.
2. A process according to claim 1 , wherein T 1 is from about 500° C. to about 1000° C.
3. A process according to claim 1 , wherein the first ramp rate (RR 1 ) is greater than about 1° C./minute and less than about 5° C./minute.
4. A process according to claim 1 , wherein the first hold time (t H1 ) is less than about 15 hours and greater than about 15 minutes.
5. A process according to claim 1 , wherein T 2 is in a range of from about 25° C. lower than T 1 to about 500° C. lower than T 1 .
6. A process according to claim 1 , wherein the activated chromium catalyst comprises, by weight, less than 0.6% Cr(VI).
7. A process according to claim 1 , wherein the process is carried out over a time period of less than 16 hours.
8. A process according to claim 1 , wherein the supported chromium catalyst is a Cr/silica catalyst or a Cr/titania-coated silica catalyst.
9. A process according to claim 1 , wherein step a) of heating the supported chromium catalyst to the first temperature (T 1 ) is preceded by a conditioning process, the conditioning process comprising heating the supported chromium catalyst to a conditioning temperature (T C ) in an oxidative atmosphere at a conditioning ramp rate (RR C ), and holding the chromium catalyst at T C in the oxidative atmosphere for a conditioning time (t C ) to provide a conditioned chromium catalyst.
10. A process according to claim 1 , wherein step a) of heating the supported chromium catalyst to the first temperature (T 1 ) is preceded by a conditioning process, the conditioning process comprising:
i) heating the supported chromium catalyst to a conditioning temperature (T C ) in an inert atmosphere at a conditioning ramp rate (RR C ), and holding the chromium catalyst at T C in the inert atmosphere for a first conditioning time (t C1 );
ii) exposing the supported chromium catalyst to an oxidative atmosphere for a second conditioning time (t C2 );
iii) exposing the supported chromium catalyst to an inert atmosphere for a third conditioning time (t C3 ); and
iv) repeating steps ii) and iii) between 0 and 15 times to provide a conditioned chromium catalyst.
11. A process according to claim 10 , wherein steps ii) and iii) are repeated between 2 and 10 times.
12. A process according to claim 10 , wherein the conditioning temperature (T C ) is from about 200° C. to about 425° C. and wherein the oxidative atmosphere comprises air or oxygen.
13. A supported chromium catalyst prepared according the process of claim 1 .
14. A process for polymerizing olefins using a supported chromium catalyst, the process comprising:
activating the supported chromium catalyst according to the process of claim 1 to provide the activated chromium catalyst; and
under polymerization conditions, contacting in a reaction zone the activated chromium catalyst with at least one C 2 to C 8 mono-1-olefin to form an olefin polymer.
15. A process according to claim 14 , wherein the at least one C 2 to C 8 mono-1-olefin comprises ethylene and at least one C 3 to C 8 mono-1-olefin comonomer.
16. A process for polymerizing olefins according to claim 14 , wherein:
the olefin polymer has a melt index (MI) greater than the melt index of a reference polyolefin (MI R ); and
the reference polyolefin is prepared under identical conditions as the olefin polymer, with the exception that a reference chromium catalyst is used in preparing the reference polyolefin, which is activated by:
heating the supported chromium catalyst in air to an activation temperature T 1 and holding the chromium catalyst at the activation temperature in air for 3 hours; and
allowing the chromium catalyst to cool below the activation temperature in air to provide the activated reference chromium catalyst.
17. A process for polymerizing olefins according to claim 14 , wherein the olefin polymer has a high load melt index (HLMI) greater than about 40, greater than about 50, or greater than about 75.
18. A process for polymerizing olefins according to claim 14 , wherein:
the activated chromium catalyst comprises less than 0.5% Cr(VI) by weight; and
under polymerization conditions, the activated chromium catalyst is contacted in a reaction zone with at least one C 2 to C 8 mono-1-olefin to form an olefin polymer having a melt index (MI) greater than the MI calculated from the following equation:
MI>8×[0.00050124 exp(0.00830795·T)]×[0.0994(PV) 3 −0.7843(PV) 2 +2.0147(PV)−0.8824]×[−0.6102(Ti) 3 +1.2715(Ti) 2 +0.003(Ti)+0.2];
wherein:
T (° C.) is the average temperature used under polymerization conditions;
PV (cc/g) is the pore volume of the activated chromium catalyst and is in a range of from about 0.3 mL/g (cc/g) to about 5.0 mL/g; and
Ti (atoms/nm 2 ) is the titanium concentration in the activated chromium catalyst and is in a range of from 0 atoms/nm 2 to about 5 atoms/nm 2 .
19. A process for polymerizing olefins according to claim 14 , wherein:
the activated chromium catalyst comprises less than 0.5% Cr(VI) by weight; and
under polymerization conditions, the activated chromium catalyst is contacted in a reaction zone with at least one C 2 to C 8 mono-1-olefin to form an olefin polymer having a melt index (MI) greater than the MI calculated from the following equation:
MI>4.8×[0.0994(PV) 3 −0.7843(PV) 2 +2.0474(PV)−0.8824]×[−0.6102(Ti) 3 +1.2715(Ti) 2 +0.003(Ti)+0.2];
wherein:
PV (cc/g) is the pore volume of the activated chromium catalyst and is in a range of from about 0.3 mL/g (cc/g) to about 5.0 mL/g; and
Ti (atoms/nm 2 ) is the titanium concentration in the activated chromium catalyst and is in a range of from 0 atoms/nm 2 to about 5 atoms/nm 2 .