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Production Of Dual Reactor Polyethylene With Increased Molecular Weight Distributions And/Or Increased Short Chain Branching Distributions

IP.com Disclosure Number: IPCOM000186088D
Publication Date: 2009-Aug-07
Document File: 2 page(s) / 11K

Publishing Venue

The IP.com Prior Art Database

Abstract

Dual reactor copolymer polyethylenes made by the solution process in plants that recycle the reactor effluent after recovering the polymer currently have an upper limit in short-chain branching distributions (SCBD) and molecular weight distribution (MWD) due to residual alpha-olefin and/or hydrogen in the recycle stream. The recycle stream sets a minimum concentration for both of them in a given reactor, which depends on the concentrations in the total reactor effluent. Eliminating or reducing the concentration of comonomer in the recycle stream to a reactor, specifically the reactor that produces the fraction with the lower short chain branching content, would enable production of a resin with a broader SCBD. Similarly, eliminating or reducing the hydrogen concentration in the recycle stream to a reactor, specifically the one that produces the higher molecular weight fraction, will enable the production of resins with a broader MWD. Increasing SCBD and / or MWD is desirable for certain applications, e.g. pipe.

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  PRODUCTION OF DUAL REACTOR POLYETHYLENE WITH INCREASED MOLECULAR WEIGHT DISTRIBUTIONS AND/OR INCREASED SHORT CHAIN BRANCHING DISTRIBUTIONS

Dual reactor copolymer polyethylenes made by the solution process in plants that recycle the reactor effluent after recovering the polymer currently have an upper limit in short-chain branching distributions (SCBD) and molecular weight distribution (MWD) due to residual alpha- olefin and/or hydrogen in the recycle stream. The recycle stream sets a minimum concentration for both of them in a given reactor, which depends on the concentrations in the total reactor effluent. Eliminating or reducing the concentration of comonomer in the recycle stream to a reactor, specifically the reactor that produces the fraction with the lower short chain branching content, would enable production of a resin with a broader SCBD. Similarly, eliminating or reducing the hydrogen concentration in the recycle stream to a reactor, specifically the one that produces the higher molecular weight fraction, will enable the production of resins with a broader MWD. Increasing SCBD and / or MWD is desirable for certain applications, e.g. pipe.

A heterogeneous catalyst, e.g. palladium on alumina, can be used to hydrogenate the comonomer in the recycle solvent by addition of sufficient hydrogen, or to consume residual hydrogen in the recycle solvent by partially hydrogenating dissolved monomers, i.e. ethylene or comonomer, present in the recycle stream.

For the broader SCBD case, the recycle solvent going to the reactor that produces the fraction with the lower short chain branching content, can be treated by passing it over a hydrogenation catalyst in the presence of excess hydrogen to hydrogenate dissolved monomers in the solvent. This allows polymer with a lower short chain branching content (usually polymer with a higher density) to be produced in the target reactor in comparison to untreated solvent.

For the broader MWD case the recycle solvent going to the target reactor can be treated by passing it over a hydrogenation catalyst bed, or vessel, which partially hydrogenates monomer and comonomer present in the recycle stream with the residual dissolved hydrogen. Through this the residual hydrogen concentration, added for Mw control in the reactor section, can be driven to a small fraction of what it was pre-treatment. This allows for higher molecular weight polymer to be produced in a target reactor, or reactor stage, with the treated recycle solvent. This reactor or reactor stage may then in turn be placed in parallel or series with a reactor that produces low Mw polymers, through hydrogen addition. Thus, the treatment of the whole recycle stream provides enhanced Mw split flexibility in comparison to a process setup that does not treat recycle solvent. The above two cases can even be combined in one plant to make a polymer with both a broader SCBD and a broader MWD.

A hydrogenatio...