Cascaded polyolefin slurry polymerization employing disengagement vessel between reactors

Abstract

A disengagement vessel employing a lock hopper effectively reduces concentration of non-polymer-associated components in the polymer product slurry of a first olefin slurry polymerization reactor, allowing cascading of a second slurry polymerization reactor operating at lesser concentration of comonomers, hydrogen, and other components to produce multicompositional polyolefin polymers and / or polymers having a multimodal monomer distribution, e.g. diblock polymers having substantially non-overlapping comonomer contents between the blocks.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention pertains to the use of cascaded slurry reactors to polymerize olefins to produce polyolefin homo- and copolymers of multimodal molecular weight distribution and / or composition. [0003] 2. Background Art [0004] Slurry reactors are in widespread use for production of polyethylene homo- and copolymers. Slurry reactors include stirred tank reactors and water-jacketed tubular reactors arranged in a series of continuous horizontal or vertical loops. A “slurry solvent” in which polyethylene has low solubility constitutes the continuous phase in such reactors, and in the case of slurry loop reactors, is driven around the loop at relatively high speed by one or more rather massive pumps. Ethylene, supported catalyst, comonomers, and processing additives are injected into the loop where polymerization takes place, creating a slurry of polyethylene in solvent. A plurality of settling legs allow polymer par...

AI Technical Summary

Benefits of technology

The patent describes a way to remove hydrogen and other unwanted components from the first step of a polyolefin polymerization process. This is done through a disengagement vessel that collects a slurry with reduced amounts of these components. The slurry is then sent to a subsequent polymerization reactor. The technical effect is to improve the quality of the polyolefin product by removing harmful substances.

Problems solved by technology

One disadvantage of such blended resins is that blending constitutes an additional process step.

The blending operation not only adds additional cost to the resin, but moreover, resins produced by blending have generally inferior physicochemical properties as compared to multimodal resins having been produced by “in situ” routes.

Such products are generally inferior in their processing properties, especially for applications such as film production.

Although this process offers advantages in capital and installed costs relative to multi-reactor processes, the design and synthesis of dual site catalysts is difficult.

An additional process disadvantage is that use of a single reactor reduces the number of process parameters that can be manipulated to control polymer properties.

However, molecular weight is regulated by the presence of hydrogen in both reactors, with the second reactor having higher hydrogen concentration than the first reactor, thus limiting the types of interstitially mixed polymers which may be produced.

Polymerization at lower hydrogen pressure in the second reactor is not possible.

In addition, the polymer formed in each reactor is limited to a specific weight percentage range relative to the weight of the final product.

Continuous takeoff is said to produce inferior products.

These processes do not allow operation of the second reactor at lower hydrogen concentration than the first reactor.

Moreover, limiting olefin comonomer incorporation to only the first reactor limits the types of polymers which may be produced.

The inability to add significant comonomer to the second stage or to increase comonomer incorporation in the first stage detracts from the ability to produce a wide variety of polymers.

Moreover, the catalyst choice is limited to those which consume hydrogen, when a single catalyst is used.

Both the U.S. Pat. Nos. 6,221,982 and 6,291,601 processes, as well that of WO 98 / 58001, are inefficient in both monomer usage and thermal loading, since the hydrogenation reaction consumes ethylene, producing ethane by hydrogenation.

Moreover, in the U.S. Pat. Nos. 6,221,982 and 6,291,601 processes, an additional relatively expensive hydrogenation catalyst which contributes little to polymer production must be added.

Finally, all three processes require substantially homopolymerization in at least the first reactor, thus limiting the types of polymers which may be produced.

However, the patent contains no disclosure of any apparatus suitable for removing hydrogen from the first reactor product stream.

Moreover, the necessity to restrict the first polymerization to homopolymerization is limiting.

In cascaded reactors, it is difficult to obtain a sharp delineation between blocks due to transfer of monomers, catalyst, etc. from the first reactor into the second.

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