Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in reactor of specified material – or in reactor...
Reexamination Certificate
2000-02-22
2001-06-26
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...
C526S059000, C526S061000, C526S074000, C526S088000, C526S113000, C526S228000, C526S901000, C526S921000
Reexamination Certificate
active
06252015
ABSTRACT:
The present invention relates to a process for the catalytic production of polymer and, particularly, to a process for the production of (&agr;-olefin polymers.
Such polymerisation processes can be carried out by bringing the monomers or their mixtures into contact with a catalyst comprising a compound of a transition metal and a suitable cocatalyst or activator. The processes can be suitably performed in a gas phase apparatus wherein the catalysts, usually supported on solid particles, and the monomers are fed to a fluidised bed made up by growing polymer particles and sustained by an upwardly flowing gas stream, essentially composed of unreacted monomers and, optionally, of suitable inert compounds. Said gas stream is introduced typically into the fluidised bed through a gas distribution plate of suitable conformation, placed below the bed. An enlargement of the cross section of the reactor (gas velocity reduction zone) above the bed reduces the velocity of the gas stream, so that entrained solid particles are separated from the stream and fall back into the bed. The gas leaving the fluidised bed is sent through a recycle line to a compressor and then recycled to the fluidised bed. On the said recycle line is generally placed a cooling means, to remove the heat generated by the polymerisation reaction. The produced polymer particles are withdrawn from the fluidised bed at such a rate to keep the level of the bed substantially constant. Other gas phase processes are carried out in circulated fluidised bed reactors as described in WO 97/04015. Other gas phase processes foresee the circulation of a monomer containing gas through a mechanically stirred bed of growing solid polymer particles. Still other processes used are those whereby the growing polymer particles are suspended in a liquid medium essentially composed of the monomers and, optionally, of suitable solvents or diluents.
It is well known that a problem of the above mentioned processes, more precisely those in which the polymer produced is in the form of a suspended solid in particle form, is the agglomeration of the polymer particles and their deposition on the reactor's walls, with formation of sheets of polymer. This problem is more serious when ethylene is polymerised. This phenomenon is probably due to the presence on the polymer particles of electrostatic charges of both positive and negative sign.
Various methods have been proposed to reduce this drawback in polymerisation processes. Such methods include the introduction into the polymerisation reactor of compounds capable of neutralising the electrostatic charge of the polymer particles, or, more generally, to avoid the occurrence of sticking. Other methods are based on monitoring the electrostatic potential within the reactor by means of probes of various kinds. The conditions of reaction are varied during the process in order to keep the measured value within levels at which the occurrence of sheeting is not expected.
U.S. Pat. No. 4,855,370 suggests the introduction of given amounts of water into the gas stream entering a gas phase polymerisation reactor prior to the formation of polymer sheets, detected by variations of the static potential in the reactor.
According to U.S. Pat. No. 5,034,479, the presence of water, as well as that of oxygen, as an impurity in the monomer, or in the molecular weight regulator, or, more generally, in the gaseous stream fed to a polymerisation reactor, is deemed to be a sheeting promoting factor. In order to avoid this drawback, it is taught to feed the gaseous stream into the reactor's recycle stream at a point prior to the cooler.
U.S. Pat. No. 4,532,311 teaches that sheeting can be avoided by introducing into the reactor a chromium compound in such a way that the reactor's walls are contacted by the said compound, before starting the polymerisation with a Ziegler-Natta type catalyst.
U.S. Pat. No. 5,648,581 describes a method for reducing sheeting based on measuring the current flow due to electrostatic charges transferred from the polymer particles to an electrode placed on the wall of the reactor. The measured flow is then adjusted to positive values close to zero by acting on operative parameters such as monomer or comonomer concentration or temperature of polymerisation.
EP A 811 638 teaches the use of an amine-containing antistatic agent in the polymerisation of ethylene with a metallocene catalyst. A serious drawback of the use of antistatic agents, however, is that these substances have typically the property of depressing the activity of the catalyst or to alter the characteristics of the produced polymer. Water, for instance, is a well known poison for many catalysts commonly used in polymerisation processes.
Methods comprising the variation of the polymerisation conditions can force to operate under conditions which are not optimal for the process or for obtaining the target product. Moreover, it may be impossible to control the parameters influencing the formation of sheets and chunks in a reliable way.
It is known that the tendency to agglomerate of granular particles of polymer, due to electrostatic charges of both positive and negative sign, can be reduced or eliminated by allowing the granular solid to be contacted with a gas subjected to a corona discharge. In Chemie Ing. Techn. 42 (1970) Nr. 5, 294-299 it is described an apparatus suitable for neutralising the static charge of PVC particles, which charge had been created by subjecting the polymer to pneumatic conveying. The neutralisation of the static charges can be due to the fact that the gaseous means in which the solid is suspended is made conductive through ionisation, or that the particles undergo an intense electrical field.
This principle is presently utilised in operations of handling granular solids, for example silo storage.
There is a strong prejudice against the use of gas ionising devices in polymerisation reactors, as can be read in U.S. Pat. No. 4,532,311. According to this patent, it is expected that ionised gas does not travel for a sufficient distance to be effective in neutralising the solid static charges everywhere needed in the reactor.
Contrarily to the teachings of the prior art, it has now been found that it is possible to carry out a polymerisation process, wherein the polymer formed is in the form of solid particles, without incurring in sheeting and formation of particle agglomerates (chunks) simply by subjecting at least part of the reaction fluid or suspension phase to a corona discharge.
Therefore, broadly contemplated, the present invention provides a process for the production of polymers by means of a catalytic polymerisation, wherein the produced polymer is in the form of solid particles and the monomer(s) is (are) contained in a gaseous or liquid reaction medium, characterised in that a corona discharge is established in at least a portion of the space occupied by said reaction medium or by a fluid feed stream sent to the polymerisation zone.
Preferably the solid polymer particles are suspended in said reaction medium, which is preferably in the gaseous state, optionally containing also a suspended liquid phase, which, as well as the gaseous phase, can contain monomers, comonomers, and inert components. More preferably, the solid particles form a fluidised bed kept suspended by a gas stream continuously recirculated through the said bed.
Preferably, the corona discharge is established in a region occupied by the fluid reaction medium. When a fluidised bed process is used, the corona discharge is preferably generated at the upper limit of said fluidised bed. In the case of a gas-phase process carried out in a circulated fluidised bed reactor as described in WO 97/04015, the corona discharge may be generated in the zone where fast-fluidisation conditions are established or in the -as-solid separator or in the gas recycle line; preferably, the corona discharge is generated in the gas recycle line at a point just before the introduction of the gas stream into the zone where fast-fluidisation conditions ar
Arletti Arrigo
Vincenzi Paolo
Cheung William
Montell Technology Company BV
Wu David W.
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