Olefin polymerization processes
A process for reducing the level of hydrogen in certain polymerization effluent and recycle streams containing unreacted propylene monomers and hydrogen by contacting the streams with a hydrogenation catalyst so as to convert at least part of the propylene to the corresponding alkane. The process is particularly applicable to the effluent from a slurry polymerization reactor which has been used to produce a polypropylene homopolymer or copolymer having a first molecular weight and at least part of the effluent is to be supplied to a slurry polymerization reactor to produce a polypropylene homopolymer or copolymer having a second, higher molecular weight.
1. A process for producing a polypropylene homopolymer or copolymer, the process comprising:
(a) contacting a feed comprising at least propylene monomer with a polymerization catalyst in a first polymerization reactor operating in the slurry phase under conditions effective to polymerize at least part of the at least one monomer and produce a first effluent containing polypropylene homopolymer or copolymer having a first molecular weight and unreacted monomer;
(b) contacting at least part of the first effluent, at least propylene monomer and hydrogen with a polymerization catalyst in a second polymerization reactor operating in the slurry phase fluidly connected in series to the first polymerization reactor to produce a second effluent containing a polypropylene homopolymer or copolymer having a second molecular weight lower than the first molecular weight, unreacted monomer and hydrogen;
(c) contacting at least part of a second effluent with a hydrogenation catalyst under conditions effective to hydrogenate at least part of the unreacted monomer and produce a third effluent comprising a lower level of hydrogen than the second effluent; and
(d) recycling at least part of the third effluent to the contacting step (b).
2. The process of claim 1 , wherein the hydrogenation catalyst comprises a titanocene and an aluminum alkyl.
3. The process of claim 2 , further comprising combining the titanocene and aluminum alkyl with at least part of the second effluent.
4. The process of claim 2 , further comprising separately adding the titanocene and aluminum alkyl to at least part of the second effluent.
5. The process of claim 1 , wherein the second effluent also comprises an aluminum alkyl, wherein the hydrogenation catalyst comprises a titanocene, and the titanocene is added to at least part of the second effluent.
6. The process of claim 1 , wherein the polymerization catalyst of steps (a) and (b) comprises a Ziegler-Natta catalyst.
7. The process of claim 1 , wherein the monomers in the contacting step (a) is the same as the propylene in the contacting step (b) and the contacting step (b) produces a polypropylene homopolymer or copolymer having a bimodal molecular weight distribution.
8. The process of claim 1 , further comprising:
(e) recovering at least part of the polypropylene homopolymer or copolymer from the second effluent or the third effluent prior to the recycling step (d).
9. The process of claim 1 , wherein a step of venting an effluent stream to remove hydrogen is absent.
10. A process for producing a polypropylene homopolymer or copolymer, the process comprising:
(a) contacting a first feed comprising at least propylene and hydrogen with a polymerization catalyst in a polymerization reactor operating in the slurry phase under conditions effective to polymerize at least part of the at least propylene and produce a first effluent containing a polypropylene homopolymer or copolymer having a first molecular weight, unreacted monomer and hydrogen;
(b) contacting at least part of the first effluent with a hydrogenation catalyst under conditions effective to hydrogenate at least part of the unreacted monomer and produce a second effluent comprising a lower level of hydrogen than the first effluent;
(c) recycling at least part of the second effluent to the polymerization reactor operating in the slurry phase; and
(d) contacting the recycled second effluent and a second feed comprising at least propylene with a polymerization catalyst in the polymerization reactor operating in the slurry phase under conditions effective to polymerize at least part of the propylene and the recycled monomer to produce a third effluent containing a polypropylene homopolymer or copolymer having a second molecular weight higher than the first molecular weight.
11. The process of claim 10 , wherein the hydrogenation catalyst comprises a titanocene and an aluminum alkyl.
12. The process of claim 11 , further comprising combining the titanocene and aluminum alkyl with at least part of the first effluent.
13. The process of claim 11 , further comprising separately adding a titanocene and an aluminum alkyl to at least part of the second effluent.
14. The process of claim 10 , wherein the first effluent also comprises an aluminum alkyl, wherein the hydrogenation catalyst comprises a titanocene, and the titanocene is added to at least part of the first effluent.
15. The process of claim 11 , wherein the aluminum alkyl has the formula AlR 3 , where R is a C1 to C12 alkyl.
16. The process of claim 10 , wherein the polymerization catalyst comprises a Ziegler-Natta catalyst.
17. The process of claim 10 , wherein a step of venting an effluent stream to remove hydrogen is absent.
18. The process of claim 10 , further comprising:
(e) recovering at least part of the polypropylene homopolymer or copolymer having a first molecular weight from the first effluent or the second effluent prior to the recycling step (c).