Method of polymerizing olefins using unsupported catalyst...

Details Method of polymerizing olefins using unsupported catalyst... Method of polymerizing olefins using unsupported catalyst...

1999-12-10

2002-07-30

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S348000, C526S901000, C526S943000, C526S119000, C526S158000, C502S152000

Reexamination Certificate

active

06426394

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods of polymerizing olefins in the gas phase using unsupported catalyst compositions. At least one olefin monomer is polymerized in a gas phase reactor in the presence of an unsupported liquid catalyst composition whereby the method reduces reactor fouling by maintaining the partial pressure of the primary olefin monomer that is polymerized below about 240 psi. in the reactor. The invention is particularly useful for, but is not limited to polymerizing propylene or copolymerizing propylene with olefins having two or more carbon atoms. The method described herein produces a polymer product having controlled, uniform particle size, narrow molecular weight distribution, high bulk density and stereoregularity, and prevents and/or inhibits reactor fouling during catalyst delivery and during polymerization.
2. Description of Related Art
Gas phase polymerization of olefin monomers to produce polyolefins is well known in the art. Various polyolefins can be produced including homopolymers, copolymers and terploymers of &agr;-olefins and optionally including dienes, aromatic compounds with vinyl unsaturation and/or carbon monoxide. A catalyst typically is required to initiate polymerization of one or more of the &agr;-olefin monomers, and the optional dienes, etc. Typical catalysts include, but are not limited to, coordinated anionic catalysts, cationic catalysts, free-radical catalysts, anionic catalysts and the like. As described more fully, inter alia, in U.S. Pat. Nos. 3,779,712, 3,876,602 and 3,023,203, these known catalysts are introduced to the reaction zone as solid particles whereby the active catalyst material is supported on an inert support typically made of alumina, silica and the like. It was generally known in the art that delivering conventional catalysts to a gas phase reactor that were unsupported would result in numerous problems in catalyst delivery, as well as undesirable polymer properties.
Recent developments in the industry, however, have led to the discovery of a class of unsupported catalysts, some of which are typically referred to as metallocenes, or single site catalysts. Delivery of liquid, unsupported catalysts to a gas phase reactor was first described in Brady et al., U.S. Pat. No. 5,317,036, the disclosure of which is incorporated herein by reference in its entirety. Brady recognized disadvantages of supported catalysts including, inter alia, the presence of ash, or residual support material in the polymer which increases the impurity level of the polymer, and a deleterious effect on catalyst activity because not all of the available surface area of the catalyst comes into contact with the reactants. Brady further described a number of advantages attributable to delivering a catalyst to the gas phase reactor in liquid form.
These advantages included a cost savings since there were no costs associated with providing the support material, and processing the support so as to impregnate the active catalyst thereon. In addition, a high catalyst surface area to volume ratio was achieved thereby resulting in improved catalytic activity. Moreover, it was more efficient since the catalytic solid no longer needed to be separated and processed (filtered, washed, dried, etc.), and then handled and transported.
Despite these advantages, the solid catalytic material still needed to be dissolved in a suitable solvent and delivered to the gas phase reactor in the solvent. Many, if not all, of the single site metallocene catalysts which may polymerize olefins, and especially propylene isotactically, such as metallocene dichlorides, are difficult to use because they are insoluble in hydrocarbon solvents such as alkanes. Other unsupported catalysts that may polymerize olefins also are not readily soluble in hydrocarbon solvents, or require significant amounts of hydrocarbon to dissolve the unsupported catalysts. Solvents such as toluene and methylene chloride, although capable of solvating such catalysts, are undesirable because they are toxic in nature and leave undesirable residues. Even in these types of solvents, however, solubilities still can be very low, typically less than 21 mmol/l in concentration at room temperature. In addition, feeding unsupported catalysts to a gas phase reactor using large quantities of solvents (hydrocarbon or otherwise) can cause reactor fouling to occur, as described, for example, in Burkhardt, et al., U.S. Pat. No. 5,240,894, the disclosure of which is incorporated by reference herein in its entirety.
As stated therein, unsupported metallocene and organoaluminum catalyst systems suffer from the limiting disadvantage of producing polymer which sticks to the reactor walls during the polymerization process or polymer having small particle size and low bulk density which limit their commercial utility. Typically, polymer particle size and bulk density are determined by the morphological properties of the catalyst solid component (i.e., an inert carrier or support media). Poor particle size of the final polymer product can often result without a solid component in the polymerization media. Likewise, maintaining commercially acceptable levels of catalyst activity with minimal levels of reactor fouling occurring during polymerization, is also a problem.
Burkhardt also teaches that low catalytic activity and reactor wall fouling which occurs during polymerization may be due to several factors. When methyl alumoxane (MAO) is used as cocatalyst in the polymerization at temperatures about or greater than 40° C., the MAO dissolves and extracts the metallocene catalyst from the support and forms a soluble catalyst in the polymerization medium, or if an unsupported catalyst is employed, the catalyst already is soluble in the MAO solution. This soluble catalyst easily deposits polymer onto the reactor walls and/or generates very small particles of low bulk density which are undesirable in a commercial reactor.
In addition, when a liquid catalyst is employed in gas phase polymerization, several phenomena can occur. First, the soluble or liquid catalyst tends to deposit on the resin or polymer forming the fluidized bed which in turn leads to accelerated polymerization on the surface of the particles of the bed. As the coated resin particles increase in size, they are exposed to a higher fraction of catalyst solution or spray because of their increased cross-sectional dimensions. If too much catalyst is deposited on the polymer particles, they can grow so large that they cannot be fluidized thereby causing reactor shut down.
Second, using liquid catalyst under conditions of high catalyst activity, e.g., a liquid metallocene catalyst, the initial polymerization rate is often so high that the newly formed polymer or resin particles can soften or melt, adhering to larger particles in the fluidized bed. This needs to be avoided or minimized to avert reactor shutdown.
On the other hand, if the polymer particle size is too small, entrainment can occur resulting in fouling of the recycle line, compressor, and cooler and increased static electricity can occur leading to sheeting,—and ultimately, reactor shut down.
SUMMARY OF THE INVENTION
Thus, there exists a need to develop a mechanism by which unsupported catalysts can effectively be delivered to a gas phase polymerization reactor without causing reactor fouling, and without causing polymer agglomeration. There also exists a need to develop a method of polymerizing at least one olefin monomer using an unsupported catalyst, where the polymerization process can proceed smoothly and produce polymer in high yield. It is therefore an object of the invention to provide a method of polymerization that does not suffer from the aforementioned problems, and that satisfies the needs discussed above.
In accordance with these and other objects of the present invention, there is provided a method of making a polymer in a gas phase polymerization reactor comprising contacting an olefin monomer, preferably propylene, with an unsupporte

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