Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Removing and recycling removed material from an ongoing...
Reexamination Certificate
2002-12-26
2003-12-02
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Removing and recycling removed material from an ongoing...
C526S086000, C526S901000, C526S348000, C422S139000, C422S143000, C422S151000
Reexamination Certificate
active
06657021
ABSTRACT:
The present invention relates to a nozzle for the injection of liquid and to its use for the introduction of condensed liquid into a reactor for the gas-phase (co-) polymerization of ethylene and/or propylene in a fluidized bed.
It is known to polymerize one or more monomers in a gas phase at a pressure greater than atmospheric pressure in a fluidized-bed reactor in which polymer particles being formed are maintained in the fluidized state by means of a reaction gas mixture containing the monomer or monomers to be polymerized and flowing in an ascending stream. The polymer thus manufactured, in powder form, is generally withdrawn from the reactor so as to maintain the bed at a more or less constant volume. A preferred industrial-scale process uses a fluidization grid which delivers the reaction gas mixture through the bed and which serves as a support for the bed should the ascending gas flow be cut off. The reaction gas mixture leaving via the top of the fluidized-bed reactor is recycled into the base of the latter, beneath the fluidization grid via an external circulation pipe equipped with a compressor.
The polymerization of the monomers is an exothermic reaction. It is therefore necessary to provide a means suitable for cooling the bed so as to extract the heat of polymerization therefrom. The preferred method for the polymerization of ethylene and/or propylene in a fluidized bed consists in cooling the reaction gas mixture below the polymerization temperature, thereby making it possible, when this fluidization gas passes through the bed, to compensate for the excess heat generated by the polymerization. Thus, during its return, the reaction gas mixture is generally cooled with the aid of at least one heat exchanger placed in the external circulation pipe so as to remove the heat produced by the polymerization reaction and to maintain the polymerization temperature at the desired level.
Attempts have been made, particularly in recent years, to optimize the gas-phase polymerization process so as to increase the polymer production in the existing plants. The process is consequently analysed in terms of polymer production output, namely in terms of weight yield of polymer produced per unit volume of the reactor and per unit time (kg/h/m
3
). In the commercial fluidized-bed reactors of the abovementioned type, it is known that the production output depends directly on the rate of extraction of the heat generated in the reactor. This extraction rate can be increased, for example by increasing the velocity of the fluidization gas and/or by reducing the temperature of the fluidization gas and/or by increasing the thermal capacity of the fluidization gas.
For example, Patent Application WO 94/28032 describes a gas-phase olefin polymerization process in which the recycle gas stream is cooled to a temperature low enough to form a liquid and a gas. By separating the liquid from the gas and by introducing the liquid directly into the fluidized bed, by means of a nozzle, it is possible to increase the total amount of liquid introduced into the fluidized-bed reactor, thereby allowing better cooling of the bed by evaporation and therefore allowing higher productivity levels to be reached. Many methods for introducing a liquid into a fluidized bed and many nozzles have already been described.
French Patent 2,215,802 describes a process using one or more spray nozzles, the orifice of which is provided with an external non-return valve. The use of such a mechanical means in a nozzle for injecting a liquid very often has the drawback of being limited in reliability, this being due to fatigue of the joints with the moving part(s), to the possibility of the moving part(s) becoming blocked and to possible leaks through the gaps between the moving part(s) and the actual structure of the nozzle.
Patent Applications WO 96/20780 and EP 0,876,202 describe nozzles for the injection of liquid into a gas-phase polymerization reactor. These nozzles are provided with a purge gas which, should the injection of liquid be interrupted, is able to prevent them from being blocked by powder from the fluidized bed. The drawback with this technique is that it causes entrainment or partial atomization of the liquid, particularly during the transient phases when starting or stopping the liquid injection.
The nozzles for injecting a liquid which are described in the prior art, particularly in the aforementioned patents, are often ill-suited for the transient, stopping and starting phases but also throughout the duration of the shutdown of the liquid injection. This is because, during these transient or steady-state phases, the following problems may arise:
(i) blockage of the orifices for expelling the liquid with solid particles,
(ii) contamination of the internal parts of the nozzle with solid particles, with liquid and/or with gas,
(iii) entrainment or atomization of the liquid by a purge gas.
The abovementioned problems may arise, especially in the case of the use of the nozzles described in the prior art for introducing a condensed liquid into a reactor for the gas-phase (co)polymerization of ethylene and/or propylene in a fluidized bed, and more particularly for the transient, stopping and starting phases but also throughout the duration of the shutdown of the liquid injection. The above mentioned problems may arise as follows:
(i) the blockage of the orifices for expelling the liquid with solid particles, and this is all the more prejudicial when these particles are reactive and can stick together and to the nozzle;
(ii) the contamination of the internal parts of the nozzle with solid particles, with liquid and/or gas may generate chemical reactions within the nozzle, resulting in blockages;
(iii) the entrainment or atomization of the liquid by a purge gas may result in partial or complete vaporization of the liquid inside the nozzle, thus limiting its action of cooling the fluidized bed. Another consequence is that the contact area between the gas and the liquid is increased in such a way that the active particles which are in the liquid phase could react with the purge gas (the latter not necessarily being inert) and could coarsen until reaching the sedimentation or blockage limit of the nozzle.
An apparatus consisting of a nozzle for injecting a liquid under pressure has now been found which makes it possible:
(1) to prevent any backflow of solid, liquid or gas, from the medium into which the liquid is injected, towards the inside of the nozzle,
(2) to prevent the entrainment or atomization of the injected liquid by a purge gas, and
(3) to maintain high reliability.
Thus, the present invention is a nozzle for the injection of a liquid under pressure, comprising a vertical feed pipe surmounted by a hollow head, the liquid under pressure being conducted between the outer wall of the vertical feed pipe and an inner tube, the upper part of the nozzle comprising at least one lateral orifice for expelling the liquid under pressure, characterized in that the upper end of the inner tube of the vertical feed pipe emerges at a height above the lateral orifice(s), thereby making it possible, when stopping the injection of the liquid under pressure, to introduce a gas under pressure via the inner tube of the vertical feed pipe, which generates an overpressure in the upper part of the nozzle in order, on the one hand, to lower the level of the liquid under pressure to below the lateral orifice(s) and, on the other hand, to prevent any backflow of solids, liquids and/or gases, from the medium into which the liquid under pressure is injected, towards the inside of the nozzle.
A non-limiting example of the device of the invention is shown schematically in
FIGS. 1 and 2
.
According to the present invention, the nozzle comprises a vertical feed pipe which is used to conduct the liquid under pressure from the base of the said pipe towards the upper part of the nozzle. Preferably, the feed pipe has a cylindrical shape.
Hereafter, the term “outer wall” of the vertical feed pipe will refer to the pipe itself without its inter
Chamayou Jean-Louis
Chinh Jean-Claude
BP Chemicals Limited
Cheung William
Finnegan, Henderson Farabow, Garrett and Dunner L.L.P.
Wu David W.
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