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
2002-12-03
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, C526S348200, C526S348500, C526S348600, C422S131000
Reexamination Certificate
active
06489409
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a process for manufacturing olefin polymers in suspension in a diluent. The invention also relates to a plant used for carrying out the process.
Patent Application EP 0 891 990 describes a continuous olefin polymerization process in which a suspension comprising up to 52% by weight of polyethylene particles in isobutane is formed in the polymerization reactor and is continuously withdrawn therefrom. This process has the disadvantage that the suspension withdrawn from the reactor still contains a large amount of diluent and of other reactants, such as the monomer, which it is then necessary to subsequently separate from the polymer particles and to treat for the purpose of reusing it in the reactor.
Several techniques used for further concentrating a suspension of polymer particles which has been withdrawn from a polymerization reactor have been described. On an industrial scale, it is general practice to use several settling legs which are mounted directly on the polymerization reactor and are periodically filled and discharged in a sequential manner. However, this process has drawbacks in that the periodic discharging from the reactor is difficult from the safety and reliability standpoints. This is because it is known that the valves isolating the settling legs readily become blocked over time. U.S. Pat. No. 3,816,383 and GB 1 147 019 describe continuous olefin polymerization processes in which a suspension of polyethylene particles, withdrawn from a reactor, is concentrated by means of a hydrocyclone separator, and then a fraction of the concentrated suspension is drawn off for the purpose of recovering the polyethylene produced, the other fraction being recycled to the reactor. These processes have the drawback that the suspension drawn off contains relatively large amounts of diluent and of monomer compared with the amount of polymer produced, these having to be separated and subsequently treated.
The object of the present invention is to provide a process which does not have the aforementioned drawbacks and which makes it possible, in particular, to considerably reduce the amounts of diluent and other reactants to be separated from the polymer particles. The invention therefore relates to a process for manufacturing olefin polymers, in which:
(a) at least one olefin is continuously polymerized in a polymerization reactor in the presence of a diluent (D) in order to produce a suspension (S) comprising the said diluent (D) and olefin polymer particles;
(b) some of the said suspension (S) is drawn off from the reactor;
(c) the suspension drawn off is diluted by means of a liquid (L) so as to obtain a dilute suspension (DS);
(d) the dilute suspension (DS) is sent into a hydrocyclone separator in which, on the one hand, a concentrated suspension (CS) of polymer particles and, on the other hand, a stream (F) comprising diluent (D) are formed and separated;
(e) the stream (F) is drawn off from the hydrocyclone separator and at least partially recycled to the polymerization reactor;
(f) the concentrated suspension (CS) is drawn off from the hydrocyclone separator; and
(g) the olefin polymer particles are separated from the concentrated suspension (CS).
It has been found, surprisingly, that diluting the suspension (S) coming from the polymerization reactor by means of a liquid (L) makes it possible to considerably increase the efficiency of the hydrocyclone separator while at the same time allowing very concentrated suspensions of polymer particles to be obtained at the outlet of the hydrocyclone separator. In the present invention, the term “liquid (L)” is understood to mean any compound which is liquid under the temperature and pressure conditions used in the polymerization reactor and in which most of the olefin polymer formed is insoluble under these polymerization conditions.
According to a first method of implementing the process according to the invention, the liquid (L) added in step (c) to the suspension (S) drawn off from the polymerization reactor consists essentially of diluent (D). The amount of diluent added as liquid (L) in this first method of implementing the process may vary widely. In general, the amount of diluent added to the suspension is at least 0.1 1, preferably at least 0.5 1, per kg of polymer particles present in the suspension (S) drawn off from the reactor. The amount of diluent added generally does not exceed 51, and preferably does not exceed 3 1, per kg of polymer particles present in the suspension (S) drawn off from the reactor.
This first method of implementing the process has the additional advantage that the concentration of residual olefin in the concentrated suspension (CS) drawn off downstream of the hydrocyclone separator decreases strongly compared with its polymer particle content and its diluent content. Consequently, the purification and repressurization of the diluent, for the purpose of reusing it in the reactor, are greatly simplified and more economic. In addition, the amount of olefin to be recovered from the concentrated suspension is greatly decreased and therefore the recovery is also simplified and more economic.
According to a second method of implementing the process according to the invention, the liquid (L) added in step (c) to the suspension (S) drawn off from the polymerization reactor consists essentially of at least part of the stream (F). In this second method of implementing the process according to the invention, the amount of stream (F) added to the suspension drawn off from the reactor during step (c) may vary widely. In general, the amount of stream (F) thus added to the suspension is at least 0.1 1, preferably at least 1 1, per kg of polymer particles present in the suspension (S) drawn off from the reactor. The amount of stream (F) added generally does not exceed 201, preferably does not exceed 101, per kg of polymer particles present in the suspension (S) drawn off from the reactor. The fraction of the stream (F) used as liquid (L) is in general between 10 and 99% by weight, the balance being recycled to the polymerization reactor. Preferably, the fraction of the stream (F) used as liquid (L) is at least 25% by weight. Preferably, the fraction of the stream (F) used as liquid (L) does not exceed 95% by weight.
This second method of implementing the process has the additional advantage that the efficiency of the hydrocyclone separator can be varied by modifying the fraction of the stream (F) used as liquid (L). In fact, increasing the fraction of the stream (F) recycled as liquid (L) allows the efficiency of the hydrocyclone separator to be improved. Decreasing the fraction of the stream (F) recycled as liquid (L) reduces the separating power of the hydrocyclone separator so that certain fine polymer particles are entrained by the stream (F) and therefore recycled to the polymerization reactor, this having the effect of increasing the productivity of the catalyst. Another advantage of this second method of implementing the process resides in the fact that controlling the amount of the stream (F) recycled as liquid (L) allows the concentration of polymer particles present in the polymerization reactor to be adjusted. Thus, it is possible to work at relatively low polymer concentrations in the polymerization reactor while maintaining a high polymer concentration at the output end of the process.
According to a third method of implementing the process according to the invention, the liquid (L) added in step (c) to the suspension (S) drawn off from the polymerization reactor consists essentially of diluent (D) and at least part of the stream (F). In this third method of implementing the process according to the invention, the amount of stream (F) and the amount of diluent (D) are generally those described above with respect to the first and the second method of implementing the process, respectively.
The olefin used in polymerization step (a) of the process according to the invention is generally chosen from among olefins containi
Marissal Daniel
Walworth Brent
Cheung William K
Solvay Polyolefins Europe--Belgium (Societe Anonyme)
Wu David W.
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