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Method for reducing depositions in polymerization vessels

Method for reducing depositions in polymerization vessels description/claims

This application relates to and claims priority to U.S. Provisional Patent Application Ser. No. 60/969,268 entitled “Method for Reducing Depositions in Polymerization Vessels” which was filed on Aug. 31, 2007, the disclosure of which is incorporated herein by reference.

This disclosure relates to a method for reducing polymer depositions that occur during polymerizations. More specifically, the present disclosure relates to a method for reducing polymer buildup on the interior walls of reaction vessels employing dissolved catalysts. Even more specifically, the present disclosure relates to a method for reducing polymer film depositions on the interior walls of reaction vessels during low temperature polymerization employing dissolved catalysts.

Isoolefin polymers are prepared in carbocationic polymerization processes, generally under low temperatures in the range of 0° C. to −150° C. Heat is usually generated during polymerization, and various methods are used to remove the generated heat. These various methods generally require a large surface area for heat transfer so that the temperature of the polymerization slurry remains constant or nearly constant.

During some polymerizations, there can be a number of issues that arise during the process. First, there is a tendency of the polymer to form or deposit on the reactor surfaces. This manner of polymer formation or deposition occurs when the polymer accumulates directly on the reactor surfaces, and is referred to herein as “film deposition” or “deposition.” As the film deposition accumulates, the heat transfer coefficient between the reactor slurry and the refrigerant decreases, leading to an increase in the polymerization temperature of the reactor slurry. As the reactor slurry temperature increases, the polymerization process becomes less stable since it is more difficult to achieve the desired molecular weight of the polymer product. When this happens, the reactor must generally be taken offline, warmed to above ambient temperatures, and solvent washed before being refilled with feed and chilled back down to polymerization temperatures.

Additionally, during carbocationic polymerization processes, there can be a tendency for the polymer particles in the reactor to agglomerate with each other and to collect on the reactor wall, heat transfer surfaces, impeller(s), and the agitator(s)/pump(s). This is referred to herein as “polymer agglomeration,” “particle agglomeration,” or “agglomeration.” While the rate of polymer film deposition on the reactor surfaces is generally proportional to the rate of polymerization, particle agglomeration depends more on the characteristics of the slurry, flow conditions, particle adhesion, etc. The rate of agglomeration increases rapidly as reaction temperatures rise. Agglomerated particles tend to adhere to and grow and plate-out on all surfaces they contact, such as reactor discharge lines, as well as any heat transfer equipment being used to remove the exothermic heat of polymerization, which is critical since low temperature reaction conditions must be maintained.

As reductions in film deposition and agglomeration may lead to longer periods of time between reactor cleaning/washes, it would be desirable to have a method for reducing film deposition and agglomeration in polymerization vessels. Others have attempted to address these problems in reaction vessels; however, there still remains a need for improved methods for reducing film depositions and agglomeration in polymerization vessels.

U.S. Patent Application Pub. No. 2005/0095176 discloses a loop reactor wherein the goal is to prevent the creation of fine particulates, or fines, during olefin polymerization wherein the process is suitable for the copolymerization of ethylene and a higher 1-olefin. A first polymerization is generated that actually creates a film/coating on the reactor walls so that larger particulates formed during the desired polymerization are not broken or chipped by a non-smooth reactor wall.

US Patent Application Pub. No. 2005/0277748 discloses a method of polymerizing an olefinic monomer system with a catalyst. The olefinic monomer system is comprised of a single monomer or a combination of two or more monomers wherein monomers are defined as ethylene and higher 1-olefins. The polymerization reactor has an inner surface whose arithmetic mean surface roughness of 1.0 μm or less. In the disclosed polymerizations, the agglomeration and film deposition was also avoided by the use of a solid catalyst.

EP Patent Application Pub. No. 0 107 127 A1 discloses a process for olefinic polymerization in which the reaction vessels are finished to a defined surface roughness of 2.5 μm or less. The application further discloses that the polymerization process employs a solid catalyst and specifically teaches that the catalyst must be of a defined size to minimize any buildup on the reaction vessel. Additionally, an agent is added to the vessel to assist in reducing polymer buildup. In the disclosed polymerizations, the monomer systems employ ethylene and higher 1-olefins as monomers.

Additional references of interest include: U.S. Pat. Nos. 3,923,765; 4,049,895; and 4,192934 and U.S. Patent Application Publication No. 2007/0187078.

In one aspect, this disclosure relates to a method for reducing film deposition and agglomeration in a reaction vessel. The method comprises the steps of providing a reaction vessel having polymerization contact surfaces, polishing a majority of the polymerization contact surfaces to have an average percent excess surface areas (SAxs) of 2% or less, introducing a catalyst system and at least one monomer or comonomer mixture in the reaction vessel, and polymerizing the at least one monomer or comonomer mixture. In one embodiment, this invention relates to the reduction of film deposition and agglomeration in polymerization systems employing a dissolved catalyst. In another embodiment, the reduction of film deposition and agglomeration is in low temperature polymerization systems. In some embodiments, a reduction in film deposition and agglomeration of up to 75% may be realized as compared to reaction vessels with SAxs values of greater than 2%.

In another aspect this disclosure relates to a method for producing an isoolefin polymer by polymerization. The method comprises the steps of dissolving a catalyst system, providing at least one monomer or comonomer mixture in a reaction vessel, introducing the dissolved catalyst in the reaction vessel, and polymerizing the at least one monomer or comonomer mixture to produce an isoolefin polymer. A majority of the polymerization contact surfaces in the reaction vessel have an average percent excess surface area (SAxs) of 2% or less. In one embodiment, the method for producing an isoolefin by polymerization occurs at low polymerization temperatures.

In a further aspect, this disclosure relates to a polymerization reaction vessel having polymerization contact surfaces where a majority of the polymerization contact surfaces have an average percent excess surface area of 2% or less. In some embodiments, at least 80% of the polymerization contact surfaces have an average percent excess surface area of less than 2%. The polymerization contact surfaces may include one or more of the interior surfaces/walls of the reaction vessel, or interior/exterior surfaces of the heat exchange tubes in the reaction vessel.

These and other features, aspects, and advantages of the present disclosure will become better understood with regard to the following description and appended claims.

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