Process for manufacturing a polyolefin resin

Details Process for manufacturing a polyolefin resin Process for manufacturing a polyolefin resin

2001-01-04

2003-07-22

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, C526S348000, C422S132000

Reexamination Certificate

active

06596823

ABSTRACT:

The present invention relates to a continuous process for manufacturing a polyolefin resin of improved homogeneity. The invention also relates to a device for carrying out the process.
The manufacture of polyolefin resins by continuous polymerization in a diluent has been known for a long time. Generally, continuous processes comprise the continuous introduction of an olefin, a catalyst and a diluent into a polymerization reactor and the continuous removal from this reactor of a suspension comprising polyolefin particles and the diluent. These continuous manufacturing processes generally lead to resins whose polymer particles have a certain size distribution as a function of the residence time in the polymerization reactor. The particles of different sizes often also have different properties, such as, for example, the melt flow index and the content of catalytic residues. These properties consequently differ from the average properties of the polyolefin resin obtained, which may pose problems during segregation of resins during transportation or storage. Furthermore, the known manufacturing processes generally give an appreciable fraction of fine particles, which may be detrimental to the storage and to the subsequent use of the resins.
The present invention is directed towards providing a process which does not have the abovementioned drawbacks.
The invention consequently relates to a continuous process for manufacturing a polyolefin resin of improved homogeneity, according to which:
(a) in a polymerization reactor (R) comprising a diluent (D), an olefin is polymerized continuously in the presence of a catalyst to produce a suspension (S) comprising the said diluent (D) and polyolefin particles,
(b) a portion of the suspension (S) is taken from the reactor (R),
(c) the suspension taken is transferred into a hydrohydrocyclone separator in which a flow (F) comprising diluent (D) and polyolefin particles, on the one hand, and a concentrated suspension (CS) of polyolefin particles, on the other hand, are formed and separated,
(d) the flow (F) is taken from the hydrohydrocyclone separator and recycled into the polymerization reactor (R), into step (a),
(e) the concentrated suspension (CS) is taken from the hydrohydrocyclone separator and introduced into a subsequent polymerization reactor (R′), fed with olefin to continue the polymerization and to produce a suspension (S′) comprising the diluent (D) and polyolefin particles,
(f) a portion of the suspension (S′) is taken from the subsequent polymerization reactor (R′),
(g) the suspension taken is transferred to a hydrocyclone separator in which a flow (F′) comprising diluent (D) and polyolefin particles, on the one hand, and a concentrated suspension (CS′) of polyolefin particles, on the other hand, are formed and separated,
(h) the flow (F′) is taken from the hydrocyclone separator and recycled into the polymerization reactor (R), into step (a),
(i) the suspension (CS′) is taken from the hydrocyclone separator and the polyolefin particles are separated from the suspension (CS′).
In the present invention, the term “polyolefin” is intended to denote both the homopolymers of an olefin and the copolymers of an olefin with one or more other olefins or other comonomers that are copolymerizable with the olefin.
The olefin used in the polymerization steps (a) and (e) of the process according to the invention is generally chosen from olefins containing from 2 to 12 carbon atoms, and mixtures thereof. The olefin is preferably chosen from 1-olefins containing from 2 to 8 carbon atoms, more particularly from ethylene, propylene, 1-butene, 1-methylpentene, 1-hexene and 1-octene, and mixtures thereof. The olefin used in the subsequently polymerization reactor (R′), in step (e), is the same as that used in the polymerization reactor (R), in step (a).
It goes without saying that in the polymerizations in steps (a) and (e) of the process according to the invention, besides the olefin, at least one other comonomer that is copolymerizable with the olefin may be used so as to manufacture copolymers. The comonomers are usually chosen from conjugated or unconjugated olefins and diolefins, containing from 2 to 12 atoms. The 1-olefins containing from 2 to 8 carbon atoms as described above give good results. When one or more comonomers are used, it is preferred to use the same comonomer(s) in the polymerization reactors in steps (a) and (e). The amounts of comonomer used in steps (a) and (e) are preferably controlled such that the comonomer/olefin molar ratio in the subsequent polymerization reactor (R′), in step (e), is between 80% and 120% by weight of the comonomer/olefin molar ratio in the polymerization reactor (R), in step (a).
The diluent (D) used in the process according to the invention may be any diluent which is liquid under the polymerization conditions and in which most of the polymer formed is insoluble under the polymerization conditions. Hydrocarbons are suitable diluents. Aromatic and aliphatic cyclic hydrocarbons containing from 5 to 12 carbon atoms, such as toluene and cyclohexane, are suitable. Preferred diluents are acyclic aliphatic hydrocarbons containing from 3 to 8 carbon atoms, such as pentane and hexane. Propane and isobutane are particularly preferred.
According to one particular case, the diluent may be the olefin itself maintained in liquid form under its saturation pressure.
In another particular case, the diluent may be maintained in its supercritical state.
The polymerization carried out in step (a) of the process according to the invention is carried out in the presence of a catalyst. Any catalyst allowing the polymerization of olefins may be used. Examples of such catalysts which may be mentioned are catalysts of the Ziegler type, vanadium-based or chromium-based catalysts, metallocene catalysts and catalysts based on transition metals from groups 8 to 12 of the Periodic Table of the Elements. These catalysts may be supported on an inorganic or polymeric support. Good results have been obtained with a chromium catalyst supported on a support comprising silica.
The polymerization carried out in step (e) of the process according to the invention is preferably carried out without any fresh catalyst being added to the reactor. Specifically, the concentrated suspension (CS) of polyolefin particles introduced into the subsequent polymerization reactor (R′) generally still contains enough active catalyst from the reactor (R) to continue the polymerization.
It goes without saying that, in the polymerizations in steps (a) and (e) of the process, besides the olefin(s) and the diluent, other compounds may be present, in particular cocatalysts and agents for regulating the molecular mass, such as hydrogen.
When a cocatalyst is added, it is preferably added only into the polymerization reactor (R), in step (a).
When a regulating agent is added, it is preferable to add the same regulating agent into the polymerization reactors in steps (a) and (e) of the process. The amounts of molecular mass regulating agent are advantageously adjusted such that the regulating agent/olefin molar ratio in the subsequent polymerization reactor (R′), in step (e), is between 80% and 120% of the regulating agent/olefin molar ratio in the polymerization reactor (R), in step (a). Preferably, the regulating agent/olefin molar ratio is substantially the same in the polymerization reactors in steps (a) and (e).
The polymerization carried out in steps (a) and (e) of the process may take place under very variable temperature and pressure conditions. The polymerization is generally carried out at a temperature of from 20° C. to 150° C., preferably from 25° C. to 130° C. Usually, the polymerization is carried out at a pressure of from 10
5
Pa to 100×10
5
Pa, preferably from 10×10
5
Pa to 55×10
5
Pa.
Usually, the polymerization temperature in step (e) of the process is between 95% and 105% of the polymerization temperature in step (a) of the process according to th

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