Process for the gas-phase polymerization of olefins

Details Process for the gas-phase polymerization of olefins Process for the gas-phase polymerization of olefins

1995-06-07

2001-05-08

Wu, David W. (Department: 1713)

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

Synthetic resins

Polymerizing in two or more physically distinct zones

C526S124200, C526S124300, C526S124900, C526S125300, C526S351000

Reexamination Certificate

active

06228956

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process for the gas-phase polymerization of olefins of the formula CH
2
═CHR in which R is hydrogen or an alkyl or aryl radical with 1 to 8 carbon atoms, which is carried out in one or more reactors having a fluidized or mechanically agitated bed, in the presence of a highly active catalyst comprising a titanium compound supported on active Mg-dihalide.
BACKGROUND INFORMATION
It is known to continuously polymerize one or more olefins, such as ethylene or propylene, in the gas phase in a reactor with a fluidized or mechanically stirred bed, in the presence of a catalyst based on a compound of a transition metal belonging to groups IV, V or VI of the Periodic Table of the Elements, in particular in the presence of a catalyst of the Ziegler Natta type or a catalyst based on chromium oxide.
The polymer particles are kept in the fluidized and/or stirred state in a gaseous reaction mixture containing the olefin(s). The catalyst is introduced continuously or intermittently into the reactor while the polymer constituting the fluidized or mechanically stirred bed is withdrawn from the reactor, also continuously or intermittently. The heat of the polymerization reaction is essentially removed by the gaseous reaction mixture which passes through heat transfer means before being recycled into the reactor. In addition, a liquid stream may be introduced into the gas-phase reactor to enhance heat removal.
When a process for the gas-phase polymerization of an alpha-olefin is carried out in the presence of catalysts of high activity, such as those formed of the product of the reaction of an Al-alkyl compound with a titanium compound supported on active Mg-dihalide, the problem of heat removal is increased due to the low capacity of heat exchange of the gaseous phase.
It has been observed that small variations in the course of the polymerization, resulting, for example, from slight fluctuations in the quality of the catalyst or the olefins used in the reaction, can cause changes in the behavior and the catalytic activity of the polymer particles and have a particularly adverse effect because the small variations can cause an unexpected increase in the amount of heat evolved by the reaction which cannot be removed sufficiently rapidly and efficiently by the gaseous reaction mixture passing through the bed. Hot spots in the bed, as well as agglomerates of molten polymer, can be formed.
When hot spots appear in the bed it is generally too late to prevent the formation of agglomerates. However, if the reaction conditions are corrected sufficiently early, as by lowering the polymerization temperature or pressure, or reducing the rate at which the catalyst is supplied to the reactor in order to avoid the adverse effect of unexpected superactivation, the amount and size of the agglomerates formed can be reduced to some degree. During this period, however, it will not be possible to avoid a drop in the rate of polymer production and a deterioration in the quality of the resulting polymer.
To avoid these disadvantages, the general polymerization conditions are usually chosen with a safety margin such that hot spots and agglomerates do not form. For example, catalysts with reduced activity are used. The application of such conditions, however, either results in a substantial decrease in production or in a deterioration of the quality of the polymer.
To attempt to remedy the above drawbacks, EP 359,444-A1 discloses the introduction into the polymerization reactor of a retarder such as a polymerization inhibitor or a catalyst poison capable of reducing the polymerization rate of the olefin. However, the use of the retarder adversely affects the quality and the properties of the polymer such as the melt index, the melt flow ratio, and/or the stereo regularity of the polymer, as well as decreasing the productivity of the process.
Moreover, in the gas-phase process there is formation of electrostatic charges. Therefore catalyst and resin particles tend to adhere to the reactor walls, as a result of the electrostatic forces. If the polymer remains in a reactive environment for a long time, excess temperature can result in particle fusion with the formation of sheets or layer of thin fused agglomerates in the granular product. There are numerous causes for the formation of electrostatic charges, including generation due to friction of dissimilar materials, limited static dissipation, introduction into the process of minute quantities of prostatic agents, excessive catalyst activities, etc. There is a strong correlation between sheeting and the presence of excessive electrostatic charges (either negative or positive). This is evidenced by sudden changes in electrostatic levels followed closely by deviation in temperature at the reactor wall. The temperature deviations indicate particle adhesion, which causes an insulating effect and poorer heat transfer from the bed temperature. As a result, there generally is disruption in the fluidization patterns, catalyst feed interruption can occur, as well as plugging at the product discharge system.
As discussed in U.S. Pat. No. 4,532,311, the art teaches various processes whereby electrostatic charges can be reduced or eliminated. Processes suitable for use in a fluidized bed include (1) the use of an additive to increase the conductivity of the particles, thus providing a path for electrical discharge, (2) installation of grounding devices in a fluidized bed, (3) ionization of gas or particles by electrical discharge to generate ions to neutralize electrostatic charges on the particles, and (4) the use of radioactive sources to produce radiation that will create ions to neutralize electrostatic charges on the particles. However, the application of such techniques to a commercial scale reactor using a fluidized bed usually is not feasible or practical.
In U.S. Pat. No. 4,803,251 a group of chemical additives is disclosed which generate either positive or negative charges in the reactor and which are fed to the reactor in an amount of a few ppm per part of the monomer in order to prevent the formation of undesired positive or negative charges,. The chemical additives include alcohols, oxygen, nitric oxide, and ketones. Also in this case, however, there is a resulting deterioration in the polymer quality as well as a decrease in the reactor productivity.
The above drawbacks are increased when a gas-phase polymerization process is carried out using a highly active catalyst intended to produce spherical polymers having attractive morphological characteristics (high bulk density, flowability, and mechanical resistance). In this case, only a substantially complete control of the polymerization process enables one to obtain polymer particles having the above desired characteristics. This is particularly true when the gas-phase process is used to produce ethylene polymers, wherein the high polymerization kinetics of ethylene exacerbates the problem.
See also EP 0,416,379-A2, which discloses a process for preparing thermoplastic olefin polymers wherein the polymerization is carried out in at least two reactors using a catalyst based on a titanium halide supported on active MgCl
2
. Reference is made to the possible precontacting of the preformed catalyst with small amounts of an olefin prior to the main polymerization step which is carried out in the liquid or gaseous phase.
SUMMARY OF THE INVENTION
It has now been found that it is possible to carry out a gas-phase polymerization process in a smooth and reliable manner, overcoming or markedly reducing the above difficulties, without a sacrifice in specific productivity and/or a deterioration in polymer quality.
In particular, it has been found that it is possible to obtain ethylene and propylene polymers in the form of high bulk density flowable spherical particles using spheriform catalysts endowed of a high activity. (By “spheriform” we mean substantially spheroidal or spherical particles.)
The process of the invention therefore offers the possibility, particularly

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