Catalyst deactivation

Details Catalyst deactivation Catalyst deactivation

1998-12-12

2001-01-30

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S082000, C526S128000, C526S352000, C528S485000, C528S489000

Reexamination Certificate

active

06180730

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a method for deactivating and pacifying halogen-containing catalyst residues in a medium pressure solution process to polymerize ethylene. The deactivator is an insoluble particulate material which is added to the process as a suspension.
BACKGROUND OF THE INVENTION
Thermoplastic polyethylene is commercially produced by three important classes of catalysts, namely free radical catalysts (such as the peroxides and/or hydroperoxides which are typically used in the so-called “high pressure” polymerization process); chromium catalysts (such as the supported chromium oxides which are used in the so-called “Phillips” polymerization process); and “Ziegler-Natta” type catalysts which are typically used in “gas phase” processes and “medium pressure solution processes”.
The polymer solution emerging from the reactor system in the medium pressure polymerization process still contains unreacted monomers and active catalyst which would continue an uncontrolled polymerization reaction in the process equipment down-stream from the polymerization reactor system and thus compromise the quality of the desired commercial polymer. Therefore the catalyst has to be deactivated.
There are many deactivators known including various amines (see, for example, U.S. Pat. No. 4,803,259 to Zboril et al; alkali or alkaline earth metal salts of carboxylic acid (especially calcium stearate, per U.S. Pat. No. 4,105,609 to Machon et al); water (U.S. Pat. No. 4,731,438 to Bernier et al); and hydrotalcites (or synthetic clays) as disclosed in U.S. Pat. No. 4,379,882. In fact most polar compounds will deactivate a Ziegler catalyst at the typical temperature at the reactor exit.
However, most Ziegler-Natta catalysts contain halogens (typically chlorine) which remain in the polyethylene and may cause undesirable reactions (especially corrosion of metals which come into subsequent contact with the polyethylene). In a solution polymerization process, these undesirable reactions may occur in process vessels which are immediately downstream of the polymerization reactor so there is a need to employ an effective “deactivator” either in, or downstream from, the polymerization reactor.
Preferred deactivators should also satisfy the following requirements: a deactivator must deactivate the catalyst rapidly; must not deposit on the equipment (particularly on heater surfaces); must not generate color or odor and must be safe and non-toxic. This limits the types of useful deactivators and dictates the way they are added to the reactor effluent. Accordingly, the selection of optimal deactivators and the method of their use depends upon the process in question.
The method of adding the deactivator is affected by the form of the polyethylene product and the type of polymerization reactor. In general, it is not particularly difficult to add a deactivator to the solid product from a gas phase or slurry polymerization process. (See, for example, the aforementioned U.S. Pat. No. 4,731,438 which discloses that water may be added to the solid product from a gas phase polymerization process by simply spraying the water into a purge bin.) Likewise, it is not particularly difficult to deactivate the molten solution product which emerges from a high pressure, plug flow tubular reactor—as the deactivator may be added directly to the end of the reactor tube. (See, for example, U.S. Pat. No. 4,634,744; Huang et al). However, the direct addition of a deactivator at the exit of a back-stirred reactor (such as a CSTR) would kill the reaction.
Conversely, the addition of a particulate deactivator to the polyethylene solution at a point downstream of the reactor is not trivial—particularly with respect to the problem of achieving fast deactivation by adequate dispersion of the deactivation throughout the solution.
Often, it is advantageous to separate the catalyst deactivation and passivation. Thus a soluble deactivator such as methanol may be added first, and a suspension of a passivator second. Methanol mixes well and deactivates quickly, but the so-deactivated catalyst must also be passivated.
SUMMARY OF THE INVENTION
The present invention provides an improved method to add a particulate deactivator to a polyethylene solution which is produced in a medium pressure solution polymerization process. More particularly the improvement comprises a process to prepare a polyethylene solution wherein ethylene is polymerized with a halogen-containing Ziegler-Natta catalyst system, at a pressure of from 3 to 35 mega Pascals (“MPa”) and a temperature of from 100 to 320° C. in the presence of a solvent for said polyethylene, the improvement which comprises the injection of a particulate deactivator to said polyethylene solution subsequent to the discharge of same from said stirred reactor further characterized in that said deactivator is selected from a metal carbonate and hydrotalcite.
In a preferred embodiment, a soluble secondary deactivator is also used. In a particularly preferred embodiment, the deactivator suspension contains a polymeric “suspension enhancer” which is preferably a mixture of isobutylene polymer and a long chain carboxylic acid.
DETAILED DESCRIPTION
This invention specifically relates to a unit operation of a medium pressure solution process for ethylene polymerization. The term “medium pressure solution” will, in general, be well understood by those skilled in the art of ethylene polymerization and is widely described in the literature.
A brief description of the medium pressure solution polymerization process follows.
The polymerization takes place in a solvent for the resulting polyethylene, at a temperature which is sufficient to maintain polymer solubility. Suitable solvents include C
5-20
alkanes, cycloalkanes, aromatics and mixtures thereof. Non-limiting examples include hexane, methyl pentane, cyclohexane, and commercially available solvents (such as the mixed alkane solvents sold under the trademarks “Exxsol®” and “Isopar®” by Exxon). The lower temperature limit is fixed by polymer solubility (100° C. is generally a practical minimum) and by pressure consideration at the upper limit (about 320° C. is a practical maximum).
The term “medium pressure” refers to a pressure which is sufficiently high to allow economic monomer correlations but low enough to avoid the use of expensive high pressure reactions. In practical terms, this fixes pressures at about 35 MPa.
Ethylene may be either homopolymerized or copolymerized with at least one copolymerizable olefin. Suitable alpha olefin comonomers include propene, butene, pentene, hexene, heptene, norbornene and octene with butene and octene being particularly preferred. In copolymerization, the resulting copolymer contains comonomer units so as to produce thermoplastic polyethylene of reduced crystallinity.
The resulting thermoplastic copolymers may be high density polyethylene (having a density of at least 0.935 grams/cubic centimeter (“g/cc”) which is produced using small amounts of alpha olefin comonomer); low density polyethylene (having a density of from 0.910 to 0.934 g/cc, which is produced using larger amounts of alpha olefin comonomer) or very low/ultra low density polyethylene (having a density of from about 0.88 g/cc to 0.909 g/cc, produced with still larger amounts of comonomer).
The polymerization is catalyzed by a halogen-containing catalyst system which includes a transition metal catalyst and a cocatalyst. The transition metal catalyst is typically provided in the form of a metal halide (especially a metal chloride) and the cocatalyst is typically an aluminum alkyl which may also contain a halide ligand. Preferred catalyst systems comprise a titanium or vanadium halide.
A magnesium alkyl or magnesium alkyl halide may also be part of the catalyst component.
The term “Ziegler-Natta” is widely used in the literature to describe such transition metal catalyst/aluminum alkyl cocatalyst systems.
The polymerization is preferably conducted in a continuous flow stirred reactor. This means that solvent/catalyst/monomers are pumpe

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