Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in two or more physically distinct zones
Patent
1995-12-08
1997-03-11
Weber, Thomas R.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in two or more physically distinct zones
526 78, 526901, C08F 234
Patent
active
056102445
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a continuous process for the gas phase polymerization of olefins of formula CH.sub.2 .dbd.CHR, in which R is hydrogen or an alkyl, cycloalkyl or aryl radical having 1 to 12 carbon atoms, carried out in one or more reactors having a fluidised bed, in the presence of a high activity catalyst preferably comprising a titanium compound having at least one Ti-halogen bond supported on magnesium dichloride in active form.
Processes for the continuous polymerization of one or more olefins, such as ethylene or propylene, carried out in gas phase in fluidised bed reactors are well known in the art. The polymerization is generally carried out in the presence of a catalyst based on a transition metal compound belonging to the groups IV, V or VI of the periodic table, in particular in the presence of a Ziegler-Natta type catalyst or a chromium oxide based catalyst.
The reactor generally consists of a reaction zone, in which the polymer particles are maintained in a fluidised state by passing a gaseous reaction mixture containing olefin(s) and optionally an inert gas through a bed of polymer particles, and a gas velocity reduction zone, where most of the polymer particles entrained in the fluidisation gas fall in the underlying reaction zone. The catalyst is introduced in the reactor continuously and the polymer constituting the fluidised bed is also removed continuously.
A gas distribution grid placed in the lower part of the reactor under the reaction zone is the means through which the fluidisation gas is sent through the polymer bed and is used to support the bed itself when the polymerization is discontinued.
The gaseous mixture, comprising monomers, comonomers, inert gas and molecular weight regulators, leaving the top of the reactor is sent to the reactor at a point below the gas distribution grid through a recycling line. Devices for the compression and cooling of the gases are generally arranged on said recycling line.
Make-up monomers are usually fed in the gas recycling line in such a way to have a certain homogeneity of the gaseous mixture inside the reactor.
It is in fact known that small variations in the operating conditions during the polymerization, resulting for example from small variations in the quality of the catalyst or of the olefin used in the reaction or from the dishomogeneity in the composition and in the flow rate of the gaseous mixture, can bring about changes in behaviour and catalytic activity of the polymer particles and produce a particularly adverse effect on the gas phase polymerization process. These small variations may cause an unexpected rising of the amount of heat produced in the reaction, which can not be removed in a sufficiently quick and efficient manner by the gaseous reaction mixture passing through the bed.
As a result, hot spots can be generated in the bed with the consequent formation of aggregates of melted polymer.
When these hot spots are formed in the bed, it is in general too late to prevent the formation of aggregates. Nevertheless if the reaction conditions are promptly corrected, especially by reducing the temperature or the pressure of polymerization, or by reducing the velocity at which the catalyst is fed into the reactor in order to avoid the negative effect of undesirable superactivity, the quantity and size of the aggregates could be reduced to a certain extent.
In the industrial practice these operations are not generally carried out, in that they bring about a reduction in the polymer production and a deterioration of the quality of the obtained polymer.
In order to avoid these drawbacks, general polymerization conditions are usually selected with a safety margin such to not allow local rising of the temperature and the consequent formation of aggregates. For example catalysts are used having a reduced activity.
The use of these conditions inevitably results either in a substantial reduction in the production or in a deterioration of the quality of the polymer produced.
U.S. Pat. No. 3,709,853 describes a process for the
REFERENCES:
patent: 3709853 (1971-04-01), Karapinka
Govoni Gabriele
Penzo Giuseppe
Rinaldi Roberto
Montell North America Inc.
Montell Technology Company BV
Weber Thomas R.
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