Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in reactor of specified material – or in reactor...
Reexamination Certificate
2001-01-24
2002-01-01
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in reactor of specified material, or in reactor...
C526S074000, C422S137000, C422S241000, C422S240000
Reexamination Certificate
active
06335402
ABSTRACT:
The present invention relates to solids reactors for carrying out reactions in the gas phase and to plant components for handling fluidized solids, whose inner wall is coated with a thin antistatic layer consisting essentially of a poly-&agr;-olefin and a nonvolatile antistatic agent. It further relates to a process for coating such reactors and plant components, the use of such reactors for the polymerization and copolymerization of a &agr;-olefins and to a process for the polymerization of &agr;-olefins using a coated reactor.
In the polymerization of a &agr;-olefins in the gas phase, deposits are frequently formed on the walls of the reactor. It is known that this deposit formation is at least partly attributable to electrostatic charging, as disclosed, for example, by WO 86/07065. Owing to electrostatic forces, catalyst and polymer particles adhere to the wall of the reactor and finally bake together under the action of the heat of polymerization liberated to form solid deposits. These deposits can fall off and lead to blocking of the product discharge system. They thus cause problems in the continuous operation of such polymerization plants, increase the need for cleaning and can make more frequent stopping of the plant necessary. In addition, in the gas-phase fluidized-bed process, the fluidization behavior of the bed is adversely affected.
Problems with electrostatic charging are also known in the handling of poly-&agr;-olefins. Thus, for example, electrostatic charging can easily occur when conveying poly-&agr;-olefins in pneumatic conveying systems or when filling and emptying silos and this can lead to wall deposits and blockages. Electrostatic charging can also be the cause of dust explosions.
Electrostatic charging is influenced in a complex way by numerous system parameters of the gas-phase polymerization process, for example by the particle size distribution of the polymer and of the catalyst, the chemical composition of the catalyst, the internal reactor temperature, the reaction pressure and the composition of the circulating gas.
To solve this problem, it has been proposed that the polymerization be carried out in the presence of various antistatic agents. Thus, for example, U.S. Pat. No. 4,855,370 discloses the use of water as antistatic agent, U.S. Pat. No. 5,026,795 discloses mixtures of polysulfone copolymers/polyamines and a sulfonic acid, EP-A 364 759 discloses Kerostat® (mixture of chromium stearylanthranilate, calcium medialanate and di-t-butylphenol), EP-A 584 574 discloses mixtures of alcohol phosphate salts and quaternary ammonium salts, EP-A 653 441 discloses the use of naphthoquinone dimer compounds and EP-A 636 636 discloses metal salts of anthranilic acid. The use of Stadis® 450 (EP-A 803 514, polyamine/polysulfone) or of particularly suitable amines (EP-A 811 638) has been proposed specifically for &agr;-olefin polymerization using metallocene catalysts. Furthermore, U.S. Pat. No. 4,803,251 has proposed measuring the electrostatic potential in the reactor during the polymerization and, depending on the presence of excess positive or negative charge, using exactly the correct amount of either methanol or methyl isobutyl ketone to neutralize the respective charge. However, this process is complicated in terms of measurement and regulation.
Although the problems in respect of deposits on the reactor wall can be largely solved by means of the processes described, they all have the disadvantage that the antistatic agent or its solvent, e.g. propanol, introduced into the reaction space can reduce the activity of the catalysts used. Only low catalyst productivities are therefore achieved. Metallocene catalysts in particular are extremely sensitive to polar components in the antistatic agent. Furthermore, polar components can modify the catalysts and thus change the product properties.
It has therefore also been proposed (WO 86/7065) that the reactor wall be treated with a chromocene compound in order to reduce the electrostatic charging of the reactor. However, this treatment takes from a number of hours to a number of days. In addition, chromocene is complicated and expensive to prepare and is very sensitive to impurities and therefore difficult to handle. Furthermore, the antistatic action does not last long.
It has also been proposed (RD 23803 (1984)) that the inner wall of the reactor be sprayed with a composition comprising an aromatic polyimide, Teflon and pigments such as chromium oxides or iron oxides and the mixture be crosslinked. The coating formed in this way has a thickness of 1-3 mm. However, a thick, Teflon-containing layer of this type is very expensive. In addition, the spraying and crosslinking of such a layer can only be carried out with the reactor open, so that the coating procedure or any necessary repairs to the coating can only be carried out after the production plant has been shut down.
Coating with comparatively soft polymers is generally problematical since it has to be feared that they might be abraded by hard constituents of the fluidized bed, e.g. catalyst particles.
It is an object of the present invention to find an antistatic coating for solids reactors for carrying out reactions in the gas phase which is cheap, simple and quick to apply and repair, which has good durability and, in particular, is not abraded under the conditions for the polymerization of &agr;-olefins in the gas phase.
We have found that this object is achieved by a coating comprising polyolefins and nonvolatile antistatic agents.
The present invention accordingly provides reactors for carrying out reactions in the gas phase and plant components for handling fluidized solids whose inner wall is coated with a thin antistatic coating having a thickness of 0.1-800 &mgr;m and consisting essentially of a poly-&agr;-olefin and a nonvolatile antistatic agent. We have also found a process for coating such reactors and plant components, the use of such reactors for the polymerization and copolymerizaton of &agr;-olefins and a process for the polymerization of &agr;-olefins using coated reactors.
The reactors of the present invention can be all types of reactors which can be used for carrying out reactions in the gas phase. For the purposes of the present invention, the term reaction is not restricted to chemical reactions but also includes other chemical engineering operations which can be carried out in the gas phase, for example drying or classification in the gas phase. The reactors of the present invention are preferably used for reactions of organic solids, in particular for the polymerization of &agr;-olefins in the gas phase. Possible reactor types are, in particular, stirred autoclaves, fluidized-bed reactors, stirred fluidized-bed reactors, fluidized-bed reactors with a circulating fluidized bed or flow tubes.
Furthermore, reactors for other gas-phase operations used in chemical engineering can be treated according to the present invention, for example reactors which can be used for drying fluidized solids, in particular organic solids, e.g. fluidized-bed driers or spray driers.
The plant components treated according to the present invention can be any components of chemical plants in which solids, in particular organic solids and very particularly preferably poly-&agr;-olefins, are fluidized, for example pipes and other components of pneumatic conveying systems or silos.
The inner wall is coated with a layer having an antistatic action. Preference is given to coating the entire reaction space, but it is also possible to coat only those parts of the wall of the reactor on which deposits are preferentially formed during the course of the reaction. The thickness of the antistatic layer on the reactor wall is from 0.1 to 800 &mgr;m, in particular from 1 to 100 &mgr;m, particularly preferably from 5 to 10 &mgr;m.
The antistatic layer consists essentially, i.e. to an extent of at least 90% by weight based on the sum of all constituents, of a poly-&agr;-olefin and a nonvolatile antistatic agent.
Preferred poly-&agr;-olefins are polymers of &agr;-olefins h
Lange Armin
Mihan Shahram
Rohde Wolfgang
Basell Polyolefine
Cheung William K.
Wu David W.
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