Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Removing and recycling removed material from an ongoing...
Reexamination Certificate
2001-01-04
2003-07-01
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...
C526S064000, C526S065000, C526S352000, C526S348500, C526S348600
Reexamination Certificate
active
06586537
ABSTRACT:
The present invention relates to a process for manufacturing an olefin polymer composition in at least two reactors connected in series. The invention also relates to a plant used for carrying out the process.
The use of at least two reactors placed in series to manufacture an olefin polymer composition is known from the prior art. The use of reactors in series makes it possible especially to set the polymerization conditions in each reactor and thus to obtain a composition comprising an intimate mixture of polymers having different characteristics, for example in terms of molecular mass and/or comonomer content. In order to be able to vary these characteristics, it is desirable to be able to control the polymerization mixture in each reactor independently and to remove from the polymerization mixture withdrawn from the first reactor the compounds which are undesirable in the subsequent polymerization reactor.
Several methods for treating the polymerization mixture coming from the first reactor have already been described. Patent Application EP-A-0 897 934 describes a process for preparing a polymer composition in which most of the hydrogen present in the suspension withdrawn from the first reactor is removed by relieving the pressure on the suspension before sending it into the second reactor. U.S. Pat. No. 5,639,834 describes a process for preparing a polymer composition in which the comonomer-rich suspension formed in the first reactor is drawn off from this reactor by means of settling legs mounted on the reactor, and then the concentrated suspension is sent to the second reactor in which the amount of comonomer is reduced. However, this process has drawbacks in that the periodic discharging from the reactor is difficult from the safety and reliability standpoint. This is because it is known that the valves isolating the settling legs easily become blocked over time. U.S. Pat. No. 4,692,501 describes a process in which the suspension formed in the first reactor is washed by a liquid countercurrent in an exchange zone before sending it into the second reactor. The drawback of this process is that great precautions have to be taken in order to prevent mixing of the withdrawn suspension and of the liquid added in the exchange zone.
The object of the present invention is to provide a process which does not have the aforementioned drawbacks.
The present invention therefore relates to a process for manufacturing an olefin polymer composition in at least two polymerization reactors connected in series, in which:
(a) at least one olefin is continuously polymerized in a first polymerization reactor in the presence of a diluant (D) and of a catalyst in order to produce a suspension (S) comprising the said diluant (D) and particles of an olefin polymer (A);
(b) some of the suspension (S) is drawn off from the reactor;
(c) the suspension drawn off is sent into a hydrocyclone separator in which, on the one hand, a stream (F) comprising diluant (D) and, on the other hand, a concentrated suspension (CS) of particles of polymer (A) are formed and separated;
(d) the stream (F) is drawn off from the hydrocyclone separator and at least partially recycled to the first polymerization reactor;
(e) the concentrated suspension (CS) is drawn off from the hydrocyclone separator and introduced into a subsequent polymerization reactor in which at least one olefin is polymerized in order to form an olefin polymer (B) and to produce a suspension (S′) comprising the diluant (D) and particles of an olefin polymer composition comprising polymer (A) and polymer (B);
(f) the suspension (S′) is drawn off from the subsequent polymerization reactor and the particles of the olefin polymer composition are separated from the suspension (S′).
The term “olefin polymers” is understood to mean, in the present invention, both the homopolymers of an olefin and the copolymers of an olefin with one or more other olefins or other monomers copolymerizable with the olefin.
The olefin used in polymerization step (a) of the process according to the invention is generally chosen from among olefins comprising from 2 to 12 carbon atoms and mixtures thereof. The olefin is preferably chosen from among 1-olefins comprising from 2 to 8 carbon atoms, more particularly from among ethylene, propylene, 1-butene, 1-methylpentene, 1-hexene, 1-octene and mixtures thereof. It goes without saying that in step (a) several olefins can be used together or that other monomers copolymerizable with the olefins may be used so as to obtain olefin copolymers. Among other monomers copolymerizable with olefins, mention may especially be made of conjugated or unconjugated diolefins.
The diluant (D) used in the process according to the invention may be any diluant which is liquid under the polymerization conditions and in which most of the polymer formed is insoluble under the polymerization conditions. Suitable diluants are hydrocarbons. Aromatic and cyclic aliphatic hydrocarbons containing from 5 to 12 carbon atoms, such as toluene and cyclohexane, are very suitable. Preferred diluants are acyclic aliphatic hydrocarbons containing from 3 to 8 carbon atoms, such as pentane and hexane. Propane and isobutane are particularly preferred.
In one particular case, the diluant may be the olefin itself maintained in the liquid state below its saturation pressure.
In another particular case, the diluant may be maintained in its supercritical state.
The polymerization carried out in step (a) of the process according to the invention is generally carried out in the presence of a catalyst. Any catalyst allowing olefins to be polymerized may be used. By way of examples of such catalysts, mention may be made of catalysts of the Ziegler type, catalysts based on vanadium or chromium, metallocene catalysts and those based on transition metals of Groups 8 to 12 of the Periodic Table of the Elements. These catalysts may be supported on an inorganic or polymeric support.
It goes without saying that, in polymerization step (a), apart from the olefin(s) and the diluant, other compounds may be present, especially cocatalysts and agents for controlling the molecular mass, such as hydrogen.
The polymerization carried out in step (a) of the process may be performed under highly varied temperature and pressure conditions. In general, the polymerization is carried out at a temperature of from 20 to 150° C., preferably from 25 to 130° C. Usually, the polymerization is carried out at a pressure of from 10
5
to 100×10
5
Pa, preferably from 10×10 to 55×10
5
Pa.
According to a variant of the process according to the invention, the polymerization carried out in step (a) is carried out in several polymerization reactors connected in series. In this case, the suspension (S) drawn off from the last polymerization reactor of this series is sent to step (b). The suspensions coming from each reactor preceding this last polymerization reactor may be drawn off and transferred in any known manner to the next reactor. For this purpose, it may also be advantageous to use hydrocyclone separators. If step (a) is carried out in several reactors in series, the stream (F) may be recycled to the last polymerization reactor. According to one advantageous embodiment, the stream (F) is recycled to a polymerization reactor preceding the last reactor.
In step (b) of the process according to the invention, some of the suspension formed in the reactor in step (a) is drawn off from this reactor. The amount of suspension drawn off from the reactor may vary widely.
In general, the amount of suspension drawn off from the reactor is adjusted so as to correspond to the production of polymer.
Apart from the diluant and the particles of olefin polymer (A), the suspension drawn off from the reactor may contain other compounds present or formed in the polymerization reactor. In general, the suspension comprises an amount of unpolymerized olefin.
In step (b), the suspension is preferably drawn off from the polymerization reactor continuously.
In step (c) of the process according to
Marissal Daniel
Walworth Brent
Cheung William
Solvay Polyolefins Europe--Belgium (Societe Anonyme)
Wu David W.
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