Process for the preparation of polyethylene

Details Process for the preparation of polyethylene Process for the preparation of polyethylene

1999-07-14

2003-09-09

Harlan, Robert D. (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S124200, C526S153000, C526S160000, C526S348600, C526S352000, C502S104000, C502S106000, C502S132000

Reexamination Certificate

active

06617405

ABSTRACT:

TECHNICAL FIELD
This invention relates to a process for the preparation of polyethylene with a focus on catalyst activation.
BACKGROUND INFORMATION
Polyethylene has long been prepared with Ziegler-Natta catalyst systems, which typically include a catalyst precursor made up of transition metal(s) and an electron donor, and a cocatalyst. In order to obtain the high level of catalyst activity essential to commercial polymerization processes, it is necessary to provide for essentially complete activation of the precursor. It should be noted, however, that catalyst activity in these systems is also affected by a number of other variables including the method of catalyst manufacture or formation, the use of internal electron donors, the chemical composition of the internal electron donor, and the total amount of internal electron donor.
Once the basic catalyst is formed, it is necessary to remove internal electron donor from the vicinity of the active site and (if necessary) chlorinate and/or reduce the valence site of the active metal. Electron donor (ED) is complexed or reacted away from the active site by either activator compound (defined as an Al compound which is capable of reducing TiCl
4
to Ti
+3
valency; such compounds can be generically described as R
n
AlX
3−n
wherein X is a halogen, typically Cl; n ranges from 1 to 3 with preferred values ranging from 1.5 to 3; and R is preferably CH
3
, C
2
H
5
, iC
4
H
9
, nC
4
H
9
, nC
6
H
13
, or nC
8
H
17
) or by a Lewis acid (typically R
n
MX
m
wherein n+m=the valency of M; R is an alkyl or alkoxy or aryloxy group; X is a halogen; and M=B, Al, or Si). The formula of a non-activator Lewis Acid can be R
x
BX
3−x
wherein x=0 to 2. It is desirable to supply sufficient activator or Lewis acid to remove at least 90 percent and preferably close to 100 percent of the ED compound from the active site. Due to chemical equilibria, it may be necessary to add a greater than stoichiometric amount of activator compound to electron donor to fully activate the catalyst. Activation can be accomplished by partial activation before introduction of the precursor into the reactor and completion of the activation in the reactor by means of the cocatalyst, or full activation prior to introduction of the precursor into the reactor.
The disadvantages of partial activation lie in the requirement for additional process steps and equipment to provide the partially activated precursor followed by final activation in the reactor, which requires excessive amounts of activator compound, i.e., cocatalyst (typically, aluminum alkyl compounds), to be added to the reactor so that an adequate concentration of activator compound is present at the active polymerization site. As noted above, the function of this activator compound is to extract electron donor compound from the potential active site and to activate the active site either by alkylation (if the active site is already at the correct valence state) or by reduction and alkylation if the active site valence state requires reduction, for example, from Ti
+4
to Ti
+3
. In addition to being wasteful, this excess activator compound can cause operational problems or detriment to the final product.
Typical preactivated catalysts are described in U.S. Pat. Nos. 4,482,687; 4,508,842; and 5,290,745. Preactivated catalysts of the prior art can be formulated with sufficient activator compound to fully activate the precursor composition; however, putting large amounts of activator compound directly onto these catalysts can also result in deactivation, catalysts which are hazardous to handle due to pyrophoricity, or catalysts which have poor flow properties. Some of these disadvantages can be addressed by dispersing the catalyst in a carrier such as an inert hydrocarbon; however, the disadvantage of needing multiple operations to produce the catalyst and the inability to adjust the final catalyst composition to account for variations in monomer quality are serious limitations for commercial operation. A further limitation for commercial operation is the need to dilute the catalyst sufficiently with the inert hydrocarbon to prevent deactivation of the catalyst due to high concentration of activator compound in the catalyst slurry.
The use of excess amounts of cocatalyst in Ziegler-Natta catalyzed polymerizations is standard practice in the art. Typical Al/Ti molar ratios used in prior art processes are usually greater than 20:1, and many are in the 50:1 to 100:1 range. In the description of these processes, lower values are mentioned, but clearly are not preferred. In other prior art, very low levels of added aluminum alkyl ranging from 0.1:1 to about 10:1 are suggested, but the catalyst systems involved here are devoid of electron donor type compounds, and so are not relevant to the processes under discussion in this specification. A typical non-ED system is described in U.S. Pat. No. 5,077,358.
Prepolymerization systems are also mentioned in the prior art. Typical examples of this kind of catalyst system are mentioned in U.S. Pat. Nos. 5,077,358 and 4,990,479. These systems also do not use internal electron donors, and addition of extra aluminum alkyl activator component can lead to catalyst productivity increases to such an extent that severe operational instabilities are observed. These catalysts are typically activated in the prepolymer preparation with low amounts of an aluminum alkyl activator compound and frequently with the use of an external electron donor. While “free” aluminum alkyl may be removed from the prepolymer prior to use in the main polymerization reactor, the prepolymer catalysts are fully activated in the reactor with large excess amounts of cocatalyst.
European Patent Application 783 007 discloses a process for production of polyethylene using reduced amounts of aluminum alkyl feed; however, this disclosure focuses exclusively on the use of external addition of additional activator compound, i.e., separate activator and catalyst feeds, and overall Al/Ti molar ratios, which are at the lower end of the higher ratios recited above for electron donor bearing catalysts. Operating in this mode introduces other specific problems, such as the need for exceptionally precise control of cocatalyst feed rates to avoid reaction runaway and agglomerate formation. A further disadvantage of “starved” activator feed is that the hydrogen response of the catalyst, i.e. the relative hydrogen:ethylene mole ratio required to achieve a given molecular weight at otherwise constant reaction conditions, is dramatically effected requiring larger amounts of hydrogen to achieve the same given molecular weight or melt index.
Although good polymerization activity is achievable by adding external activator compound, the fact that catalyst and activator compound are added separately requires excessive amounts of activator compound to keep the required amount of activator compound at the desired level at the active polymerization site. Excess activator compound also can cause formation of oils, can over-activate some sites thus forming highly branched materials which are undesirable, and can, in the extreme, actually cause catalyst activity decreases.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a process for preparing polyethylene, which overcomes the deficiencies associated with separate and/or excessive addition of cocatalyst aluminum alkyl. Other objects and advantages will become apparent hereinafter.
According to the present invention, such a process has been discovered. The process entails contacting ethylene per se or ethylene and one or more comonomers in one or more fluidized bed reactors, under polymerization conditions, with a catalyst system comprising (i) a supported or unsupported magnesium/titanium based precursor in slurry form, said precursor containing an electron donor; and (ii) an activator in an amount sufficient to complete the activation of the precursor. The invention lies in an improvement to this process comprisin

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