CZ725688A3 - Method of reducing packet formation during alpha-olefin polymerization process - Google Patents

Abstract

A method of reducing sheeting during polymsn. of alpha-olefins (I) is a low pressure fluidised bed reactor utilising titanium or vanadium based cpds. as catalysts together with alkyl aluminium cocatalysts, comprises: introducing water into the reactor in a sufficient amt. to maintain the electrostatic levels at the site of possible sheeting without substantially altering the effectiveness of the catalysts. Specifically, the water is introduced into the reactor by passing a pressurised inert gas, esp. nitrogen, at a controlled rate of flow through a temp. controlled container contg. water, directing the gas contg. added water into a mixt. contg. ethylene, and subsequently introducing the water/ethylene mixt. into the reactor. The flow rates of nitrogen and ethylene, and the water temp. are controlled and determined responsive to static levels in the reactor. The water content is 0.1-2 ppm by vol. w.r.t. ethylene. The flow rate of nitrogen is varied between 0-11 lbs/hr. for an ethylene feed of 0-50,000 lbs/hr. The temp. of the water in the container is 10-40 deg.C.

Description

Technical field

The present invention relates to a process for reducing plaque formation during the polymerization of alpha-olefins in a low pressure fluidized bed reactor using a titanium or vanadium catalyst together with an alkyl aluminum cocatalyst.

BACKGROUND OF THE INVENTION

According to the prior art, low density polyethylene is polymerized in thick-walled autoclaves or cylindrical reactors at pressures up to about 350 MPa and temperatures up to about 300 ° C or even higher. The molecular structure of the thus obtained high pressure low density polyethylene (HR-LDPE) is very complex. The permutations in the arrangement of simple building blocks of this molecular structure are essentially infinite. High-pressure low-density polyethylenes are characterized by intricate long chain branched molecular architecture. These long chain branches have a significant effect on the melt rheology of these polymers. High-pressure low-density polyethylenes also have a certain proportion of short-chain branches, which usually contain 1 to 6 carbon atoms. These short-chains have an adverse effect on crystal formation and reduce polymer density.

Later, a technology was developed to produce low density polyethylene using a fluidized bed method at low pressures and low pressures.

temperatures by copolymerizing ethylene with various alpha-olefins. These low-pressure low-density polyethylenes (LP-LDPE) usually have a small number of long-chain branches or do not contain these long-chain branches at all, referred to as linear low-density polyethylene polymers. These polyethylenes have short chain branches, the length and frequency of these short branches being controlled by the type and amount of comonomer used during the polymerization.

It is generally known in the art that these low-pressure and high-pressure low-density or high-density polyethylenes can be produced in fluidized bed reactors using several different catalyst groups to produce a whole range of low-density and high-density polyethylene products. The choice of a suitable catalyst depends in part on the type of product desired, i.e., whether low-density polyethylene, high-density polyethylene, extruded polyethylene suitable for extrusion or film production, and in part other criteria, is to be manufactured.

Various types of catalysts that can be used in the production of polyethylenes in fluidized bed reactors will be discussed below. These catalysts can be classified into the following groups.

Type I

Silylchroman catalysts disclosed in U.S. Patent No. 3,324,101 to Baker and Carrick, et al. 3,324,095 (Carrick, Karapinks and Turbet). These silylchroman catalysts may be characterized by containing groups of the general formula:

R O

Si O-Cr 0

The preferred silylchroman catalysts are bis (triarylsilyl) chromates and especially bis (triphenylsilyl) chromate.

This type of catalyst is used on such support materials as silica, alumina, thorium oxide, zirconia and the like. Carbon black, microcrystalline cellulose and non-sulfonated ion exchange resins can also be used as carrier materials.

^T yp ¹¹

The bis (cyclopentadienyl) chromium compounds disclosed in U.S. Patent No. 3,879,368. These bis (cyclopentadienyl) chromium compounds have the following general formula:

in which it means:

R ¹ and R ^{2 are the} same or different radicals which represent hydrocarbon groups having 1 to 20 carbon atoms, and the n and n are the same or different, with numbers from 0 to 5.

Said hydrocarbon radicals R and R are hydrocarbon groups which may be saturated or unsaturated and which may be aliphatic, alicyclic and aromatic groups, for example methyl, ethyl, propyl, butyl, pentyl, cyclopentyl, cyclohexyl, allyl phenyl, naphthyl.

These catalysts are also used on support materials using the support materials mentioned above.

Type III

Catalysts disclosed in U.S. Patent No. 4,011,382. These catalysts contain chromium and titanium in the form of oxides and optionally fluorine and a support material. These catalysts contain the following components (proportion based on the total weight of carrier and chromium, titanium and fluorine):

about 0.05 to 3.0% by weight of chromium (calculated as Cr), preferably 0.2 to 1.0% by weight;

about 1.5 to 9.0% by weight of titanium (calculated as Ti), preferably about 4.0 to 7.0% by weight;

0.0 to about 2.5% by weight of fluorine (calculated as F), preferably about 0.1 to 1.0% by weight.

Chromium compounds which can be used for type III catalysts include chromium oxide or any other chromium compounds which are oxidizable to chromium oxide under the activation conditions used. At least a portion of the chromium in the supported supported catalyst must be in the hexavalent state. In addition to chromium trioxide, other compounds such as those disclosed in U.S. Patent Nos. 2,825,721 and 3,622,521 can be used, for example chromium acetylacetonate, chromium nitrate, chromium acetate, chromium chloride, chromium sulfate and ammonium chromate.

Titanium compounds which can be used for this type of catalyst include all titanium compounds which are oxidizable to titanium dioxide under the activation conditions used, including those disclosed in U.S. Patent No. 3 No. 622,521 and Dutch Patent Application Publication No. 72-10881.

Fluorine compounds which can be used for this type of catalyst include hydrogen fluoride or any fluorine compound that provides hydrogen fluoride under the activation conditions employed. Other fluorine compounds in addition to hydrogen fluoride are disclosed in the aforementioned Dutch Patent Application Publication No. 72-10881.

The inorganic oxides which can be used to prepare the catalyst of the above type are porous materials having a high surface area, i.e. a surface area in the range of about 50 to about 1000 square meters per gram of material, wherein the porous material is a porous material. The moiety is about 20 to about 200 microns. Inorganic oxides which can be used for this purpose include silica, alumina, thorium oxide, zirconium oxide and other similar inorganic oxides, as well as mixtures of these oxides.

Type IV

Catalysts disclosed in U.S. Patent No. 4,302,566 (F.J. Karol et al.) Entitled Preparation of ethylene copolymers in a fluidized bed reactor. These catalysts comprise at least one titanium compound, at least one magnesium compound, at least one electron donor compound and at least one inert carrier material.

This titanium compound has the general formula

Ti (OR ³ ) _and X _b wherein:

R ^{3 is an} aliphatic or aromatic hydrocarbon radical having 1 to 14 carbon atoms or a COR ⁴ group in which R ⁴ is an aliphatic or aromatic hydrocarbon radical having 1 to 14 carbon atoms,

X is chlorine, bromine or iodine, a is 0 or 1, b is 2 to 4, wherein a + b is 3 or 4.

These titanium compounds may be used singly or in combination and may include: TiCl ₃ , TiCl ₄ , Ti (OCH ₃ ) Cl ₃ , Ti (ΗθΗ ₅ ) Cl ₃ , Ti (OCOCH ₃ ) Cl _3, and Ti (OCOC ₆ H ₅ ) C1 ₃ .

Said magnesium compound has the general formula

MgX ₂ in which X represents a chlorine, bromine or iodine atom.

The magnesium compounds may be used singly or in combination with each other, including magnesium chloride, magnesium bromide and magnesium iodide. Anhydrous magnesium chloride is preferably used.

The titanium and magnesium compounds are generally used in a form that facilitates their dissolution in the electron donor compound.

The electron donor compound is an organic compound that is liquid at 25 ° C and in which the titanium compound and the magnesium compound are at least partially or fully soluble. These electron donor compounds are generally known per se or are known as Lewis bases.

The electron donor compound group includes such compounds as alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic esters, cyclic ethers and aliphatic ketones.

Said type of catalyst may be modified with a boron halide compound having the following general formula: ------------------------------------------------------------------------------------------------------------------------- in which it means:

R is an aliphatic or aromatic hydrocarbon radical having 1 to 14 carbon atoms or an OR ⁴ group, wherein R ⁴ also denotes an aliphatic or aromatic hydrocarbon radical having 1 to 14 carbon atoms,

X ¹ represents a halogen selected from the group consisting of chlorine and bromine or mixtures thereof, ac is 0 or 1 when R ³ represents an aliphatic or aromatic hydrocarbon radical, or c is 0, 1 or 2 when R ³ represents an OR ⁴ group.

The boron halide compounds may be used singly or in combination with one another, which may include BC1 ₃ , BBr ₃ , B (C ₂ H ₅ ) C ₂ , b (c ₂ h ₅ ) or ₂ , b (c ₂ h ₅ ) ₂ ci, b (c ₆ h ₅ ) ci ₂ , b (c ₆ h ₅ ) ci ₂ , B (C ₈ H ₁₃ ) Cl ₂ , B (OC ₉ H ₁₃ ) Cl ₂ and B (O C ₉ H ₅ ) ₂ Cl. In a preferred embodiment, the boron halide compound is boron trichloride.

Said activator compound has the following general formula

Al (R ⁵ ) cX ² dH _e wherein:

X ² is a chlorine atom or an OR ⁵ group in which R ⁵ represents a saturated hydrocarbon radical having 1 to 14 carbon atoms,

R is a saturated hydrocarbon radical having 1 to 14 carbon atoms, d is 0 to 1.5;

e jé "Γ or" o "where ´ ............... c + d + e equals 3.

These activator compounds may be used singly or in combination with each other.

The carrier materials for these types of catalysts are solid particulate materials, including inorganic materials such as silicon and aluminum oxides and molecular sieves, and organic materials such as olefin polymers such as polyethylene.

Type V

Catalysts based on vanadium. Catalysts of this type generally contain vanadium as the active ingredient, such catalyst usually comprising a precursor deposited on a support, a cocatalyst, and a promoter. The support precursor essentially consists of a vanadium compound and a modifier which is impregnated on a solid inert support material. . The vanadium compound forming part of the precursor represents the reaction product of vanadium halide and an electron donor. The halogen in this vanadium halide is chlorine, bromine or iodine or a mixture thereof. A particularly preferred vanadium halide is variadium (II) chloride.

The electron donor is a liquid organic Lewis base in which said vanadium halide is soluble.

The electron donor is selected from the group consisting of alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic esters, aliphatic ketones, aliphatic amines, aliphatic alcohols, alkyl ethers and cycloalkyl ethers, and mixtures thereof. Preferred electron donors are alkyl ethers and cycloalkyl ethers, especially tetrahydrofuran. Up to 2 < 3 moles of electron donor, preferably about 1 to 10 moles, and most preferably about 3 moles of electron donor, are complexly bound to each mol of vanadium used.

The modifier used in the precursor has the following general formula

MX _and in which it means:

M either boron or AlR ⁶ ( _{3 and} p wherein each residue)

R ⁶ is independently an alkyl group, with the proviso that the total number of aliphatic carbon atoms in each residue δ

R is at most 14,

X is chlorine, bromine or iodine, and a is 0, 1 or 2, provided that if

M is boron, then a is 3.

Preferred modifiers include alkyl-aluminum monochlorides and alkyl-aluminum dichlorides having 1 to 6 carbon atoms and boron trichloride. A particularly preferred modifier is diethylhiin chloride. About 0.1 to 10 moles, preferably about 0.2 to 2.5 moles of modifier are used per mole of electron donor.

A solid, particulate, porous material which is inert to the polymerization reaction is used as the support material for this type of catalyst. The support material consists essentially of silica or alumina or a mixture thereof. Optionally, the carrier may also contain other materials, such as, for example, zirconium oxyde, thorium dioxide, or other compounds that are chemically inert to the polymerization reaction, or mixtures thereof.

The carrier is used in the form of a dry powder having a mean particle size of about 10 to 250 microns, preferably about 20 to 200 microns, and most preferably about 30 to 100 microns. The porous support has a surface area of greater than or equal to 3 m ² / g, preferably greater than or equal to 50 m ² / g. In a preferred embodiment, as. the support material uses silica whose pore size is greater than or equal to about 8 nm, preferably greater than or equal to 10.0 nm. The support material is pre-dried by heating to remove water, preferably at or above 600 ° C.

The carrier material is used in an amount such that the vanadium content corresponds to about 0.05 to 0.5 moles of vanadium per gram of material (mmol V / g), preferably about 0.2 to 0.5. 0.36 mmol V / g and most preferably about 0.29 mmol V / g.

The support material is usually not pretreated chemically by reaction with an alkyl aluminum compound prior to preparation of the precursor on the support. This treatment results in the formation of aluminum alkoxides chemically bonded to the carrier molecules. In addition, it has been found that this treatment of the carrier material is not only unnecessary, but rather causes undesirable agglomeration when used as a carrier for high density polyethylene (density greater than 0.94 g / cm ³ ), resulting in the formation of a lumpy product which does not have the character of a free-flowing raw material. «

The cocatalyst used for the above-mentioned types

Catalysts IV and V have the following general formula:

air ⁶ 3 in which R ^{6 has the} same meaning as previously defined in connection with the definition of M.

Preferably, the cocatalysts are trialkyl aluminum compounds having 2 to 8 carbon atoms. A particularly preferred cocatalyst is triisobutyl aluminum. About 5 to 500 moles of cocatalyst, preferably about 10 to 50 moles of cocatalyst, are used per mole of vanadium.

The promoter has the following general formula:

^{R 7b CX} (4-b) wherein:

R is hydrogen or unsubstituted or halogen-substituted lower alkyl having up to 6 carbon atoms,

X is halogen, and b is 0, 1 or 2.

About 0.1 to 10 moles of promoter, preferably about 0.2 to 2 moles of promoter are used per mole of cocatalyst.

To prepare this catalyst, a supported precursor is first prepared. In one possible variant of this process, the vanadium compound is prepared by dissolving vanadium halide in an electron donor at a temperature in the range of about 20 ° C to the boiling point of the electron donor over a period of several hours. Preferably, this dissolution is carried out with stirring at a temperature of about 65 ° C for about 3 hours or more. The vanadium compound thus prepared is then impregnated on a support material. The impregnation can be carried out by adding the carrier material in the form of a powder or suspension in an electron donor or other inert solvent. The liquid used is then removed by drying at a temperature of less than about 100 ° C for several hours, preferably drying

·'Τ.'Λ? ·· - ^ Λ 'Μ' ⁷ '* ⁷ ·

- 13 carried out at a temperature ranging from 45 to 90 ° C for 3 to 6 hours. The modifier dissolved in an inert solvent, such as a hydrocarbon solvent, is then mixed with the vanadium-impregnated carrier thus obtained. The liquid used is removed by drying at a temperature of less than about 70 ° C for several hours, preferably at a temperature in the range of about 45 to 65 ° C for about 3 hours.

The cocatalyst and promoter are added to the precursor. carriers either prior to the polymerization reaction and / or during the polymerization reaction. During this polymerization, the cocatalyst and the promoter are added either together or separately and either simultaneously or sequentially. Preferably, the cocatalyst and the promoter are added separately during the polymerization in the form of solutions in an inert solvent such as isopentane.

Typically, the above catalysts are fed to the reactor together with the polymerized materials. This reactor has a vertical section above which the expanded zone is. The recirculated gas is introduced into the bottom of the reactor and passes through the bottom upwardly of a fluidized bed gas distribution plate located in the vertical reactor part. Said gas distribution plate serves to disperse gas over the entire cross-section of the reactor and to support the polymer bed in situations where the bed is not floating.

The gas leaving the fluidized bed entrains particles. polymer. Most of these particles are separated from the gas stream.

in the extended zone of the reactor, where the speed of their movement is reduced.

Type IV and V catalysts are used in conjunction with alkyl aluminum cocatalysts to prepare ethylene polymers suitable for certain applications, such as foil production, injection molding or rotary molding. However, attempts to produce ethylene polymers using alkyl aluminum cocatalysts together with the type IV and V catalysts deposited on porous silica in some fluidized bed reactors have not been entirely successful in a particular industrial application. The main reason for the unsatisfactory process of polymerization is the formation of so-called plaques (or shields) that occur in the polymerization reactor after a certain period of operation. Said plaques may be characterized as layers of sintered polymeric material.

The formation and distribution of electrostatic charges in the polymerization reactor has been found to contribute to the formation of said plaques, as a result of which the polymer particles adhere to the walls of the polymerization reactor due to electrostatic forces.

If the adhering particles remain in the reaction space for a sufficient period of time, they will melt and fuse with the other particles due to too high a temperature. There are several reasons for electrostatic charges in the polymerization reactor. These include, in particular, the generation of static electricity due to the friction of various materials, the limited possibility of electrostatic discharge, the introduction of small amounts of prostate reagents, and the excessive activity of the catalyst, between plaque formation of sintered polymer product and the presence of excessive electrostatic charge. , there is a strong correlation. This relationship is evident when sudden changes in the electrostatic charge level occur immediately, *

«H?

- 15 when the reactor wall temperature is changed. These changes in the reactor wall temperature occur towards higher or lower values relative to the normal wall temperature of the reactor. Lower reactor temperatures occur due to the adhesion of the polymer particles to the reactor wall, where the adhering particles isolate the reactor wall 1 from the fluidized bed heat and prevent heating of the part of the reactor wall where the adhering particles are located.

Temperature deviations towards higher values mean that a polymerization reaction occurs where only the heat transfer is detected. As a result, there is usually an apparent disturbance in the fluid flow process, catalyst supply failure and drainage can occur. of product from the reactor due to a drainage path clog. In this way '

In the process, the polymer product formed comprises thin layers of sintered polymer product agglomerates, referred to as plaques.

The size of said plaques varies within a wide range. In many ways, however, these plaques are similar.

They are usually about 0.6 to 1.3 cm thick and about 30 to 150 centimeters long and in some cases longer. The width of these plaques is about 7.6 to 45.7 centimeters and sometimes wider. The core of these plaques is formed by a sintered polymer which is oriented in the longitudinal direction of the plaques, the surface of the plaques being covered with polymer granules fused to the core. φ

The edges of the plaques may have a hairy appearance formed by strands of sintered polymer. 4.

AND

- ^ --—— The essence of the invention ^ —— ~ - - =,

It was therefore an object of the present invention to provide an alpha-olefin polymerization process which substantially avoids or completely eliminates plaque formation that otherwise occurs during low pressure fluidized bed alpha-olefin polymerization using catalysts. based on vanadium or titanium catalysts together with an alkyl aluminum cocatalyst.

The principle of the process of reducing plaque formation during the polymerization of alpha-olefins in a low pressure fluidized bed reactor using a titanium or vanadium catalyst together with the alkyl aluminum cocatalysts of the present invention is to introduce water into the reactor at the point of plaque formation. gas in such a way that the inert gas under pressure is passed at a controlled flow rate through a temperature controlled water-containing container, the inert gas being enriched with water and the mixture of water and inert gas being fed to the reactor, optionally together with the alpha-olefins introduced into polymerization.

Preferably, the alpha-olefin is ethylene and the inert gas is nitrogen.

The flow rate of nitrogen gas, alpha-olefins and the water temperature in the tank are preferably controlled according to the electrostatic conditions in the reactor.

The water content of the mixture entering the reactor is preferably in the range of 0.1 to 2 ppm by volume, based on the amount of ethylene fed, and in an even more preferred embodiment the water content is less than 1 ppm by volume.

The nitrogen flow rate is preferably up to 4.99 kg / h with an alpha olefin flow rate up to 22,700 kg / h.

The temperature of the water in the reservoir is preferably controlled to be in the range of 10 to 40 ° C.

The polyolefins are preferably ethylene linear homopolymers or linear copolymers consisting predominantly of ethylene having a molar content of greater than or equal to 90% and a low content of at least one alpha-olefin having from 3 to 8 carbon atoms corresponding to a molar content of less than or equal to 10%. Preferably said polyolefins are homopolymers or copolymers of propylene, 1-butene,

1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene or

1-octene.

In general, the method of limiting plaque formation during alpha-olefin polymerization in a low pressure reactor with a spheroidal bed using a titanium or vanadium compound as a catalyst together with an alkyl aluminum cocatalyst or by preventing the plaque formation is based on the invention. water is introduced into the polymerization reactor in an amount sufficient to maintain, at the points where plaque formation of the sintered polymer product is possible, an electrostatic level at which plaque formation does not occur or where plaque formation occurs only in the polymerization reactor. however, the introduction of water into the reactor does not substantially alter the efficiency of the catalysts used.

The amount of water introduced into the reactor depends on the electrostatic voltage in the reactor and can generally be 0.1 µL in ethylene. Typically, the nitrogen supply is controlled in such a way that the flow rate is about 0 to about 4.99 kg / h, the ethylene flow rate being about 0 to about 22,700 kg / h. The temperature of the water in the O'Brian tank (a water tank that will be described in detail below with reference to the figure) is usually in the range of about 10 to about 40 ° C. The nitrogen pressure is in the range of about 1.4 to 2.8 MPa, preferably in the range of about 2.24 MPa to about 2.59 MPa.

The critical electrostatic voltage level for plaque formation of the sintered polymer product is a complex function of temperature at which polymer agglomeration, operating temperature, fluidized bed drag forces, polymer particle size distribution, and recirculated gas composition occur. The electrostatic voltage level in the reactor can be reduced in various ways, including: special treatment of the internal surface of the reactor resulting in reduced electrostatic charge in the reactor by injecting an antistatic agent to increase the surface electrical conductivity of the particles, thereby accelerating particulate removal? the installation of a suitable device for discharging the electric charge connected to the reactor walls, thereby accelerating the discharge of the electrostatic charge by creating places with a highly localized field strength; and neutralizing the charge by spraying or forming ion pairs, ions, or charged particles having opposite polarity to the fluidized bed particles.

According to the present invention, the introduction of water into a low-pressure polyethylene production process carried out by means of a & quot ^; in the gas phase, it achieves a reduction in agglomerate formation in the fluidized bed. This achieves a reduction in the level of positive electrostatic stress and thus a reduction in the adhesive forces in the reaction system.

In order to identify the causes and eliminate plaque formation, a thorough investigation of the phenomenon was carried out. In this survey, thermocouples were installed immediately next to the reactor walls. These thermocouples were placed above the gas distribution plate and spaced upward at intervals of one quarter to one half of the reactor diameter. Under normal conditions, these surface thermocouples indicate temperatures that are equal to or slightly lower than the fluidized bed temperature. However, when said plaques are formed, said thermocouples indicate temperature deviations of up to 20 ° C. Thus, at controlled locations, the temperature is up to about 20 ° C higher than the fluidized bed temperature, and this temperature increase is therefore a reliable indication of plaque formation. In addition, an electrostatic voltmeter was used to measure the voltage on a 1.27 cm spherical electrode placed in a spherical electrode. fluid bed at a radial distance of 2.54 cm from the reactor wall and usually about 152-183 cm above the gas distribution plate. This location of the electrostatic voltmeter was chosen because it was observed that plaque formation began in the range from one quarter to three quarters of the reactor diameter away from the base of the fluidized bed. As is well known, this zone corresponds to a region with a highly fluidized bed in which the lowest agitation intensity near the reactor wall occurs, which is referred to as the zero zone, where the direction of movement of the fluidized bed particles changes. The particles stop moving upwards and start moving downwards. Possible causes for this phenomenon include factors affecting fluidized bed mixing, reactor operating conditions, catalyst and polymer particle size, particle size distribution, and the like. In carrying out this research, a relationship has been found between plaque formation and electrostatic charge formation on polymer particles located near the reactor wall. If the electrostatic voltage level of the polymer particles at locations near the reactor wall is low, then the reactor runs normally and no plaque formation occurs. However, if said electrostatic voltage level of the polymer particles at locations near the reactor walls exceeds a certain critical level at said location, sintered polymer plaques are uncontrolled and the reactor operation must be stopped.

Furthermore, it has been found that plaque formation can be reduced or even completely eliminated by maintaining the electrostatic voltage level in; fluidized bed at sites near the reactor wall below said critical plaque formation level. This critical level for plaque formation is not a constant value, but is a complex function depending on the temperature at which the resin agglomerates, on the operating temperature of the polymerization reactor, on the fluidized bed driving forces, on the ... ... the particle size of the polymer and the composition of the recycle gas.

Said critical electrostatic voltage level V _c for plaque formation of ethylene homopolymers, ethylene-butene and ethylene-hexene copolymers is mainly a function of polymer agglomeration temperature, reactor fluidized bed temperature and hydrogen concentration in the recycle gas.

The temperature at which the polymer agglomerates (or sinters) under reactor operating conditions is defined as the temperature at which a settled bed of polymer in contact with a gas of the same composition as the gas recycled to the reactor during polymer preparation will form agglomerates of sintered polymer particles at the time an attempt is made to float the settled bed after it has been deposited on the gas distribution plate for fifteen minutes. This agglomeration temperature can be reduced by decreasing the density of the polymer, increasing the melt index of the polymer, and increasing the amount of dissolved monomers.

The constants in this relationship were determined from the data collected during the reactor operation at the time. when the sintered polymer plaques have just started to form in the reactor, as determined from data *.

temperature deviations measured by the above-mentioned surface thermocouples from the fluidized bed temperature. The voltage measured by the voltage probes described above varies with time as the fluidized bed character changes randomly. Therefore, the critical stress V _{c is} expressed as an average value over a period of time. Interpretation of the measured values of said voltage is very difficult due to the fact that at the moment of separation of the plaque, the formation of which was caused by electrostatic charge, an additional electrostatic charge occurs. In addition, plaque formation can only begin at a particular location of the fluidized bed and from there later spread to other parts of the fluidized bed, making it difficult to correctly interpret the measured voltage values.

Although the mechanism of plaque formation is not fully understood, it is believed that the static electricity generated in the fluidized bed charges the polymer particles. When the charge on the particles reaches a level after 1 minute, the forces (attracting charged particles) to the reactor wall outweigh the fluidized bed forces, which in turn tend to pull the charged particles away from the reactor wall, a layer of polymerizing particles comprising a catalyst in the form of a non-fluidized layer is formed near the reactor wall. Heat dissipation from the non-fluidized bed is not sufficient to dissipate the heat generated during the polymerization, since the non-fluidized bed located near the reactor wall has less contact with the fluidizing gas than the particles in the fluidized bed. The heat of polymerization increases the temperature of the non-fluid layer near the reactor wall until the particles melt and sinter. Thereafter, the other particles of the fluidized bed are gradually adhered to the molten layer and this plaque is increased until it is released from the reactor wall. As is known, the separation of the dielectric from the conductor (separation of the plaque from the reactor wall) generates additional electrostatic electricity, which in turn accelerates further plaque formation.

Several ways are known to reduce or eliminate the generation of static electricity. These include in particular:

(a) reducing the rate of electrostatic charge formation; (b) increasing the rate of electrostatic charge discharge; (c) neutralizing the electrostatic charge.

Some good practices for fluid bed use include:

(b) installing grounding means in the fluidized bed to provide a sufficient surface for discharging the electrostatic charge contained in the fluidized bed; (c) ionizing the gas or particles by electrical means. and (d) using radioactive sources to generate radiation that produces ions capable of neutralizing the electrostatic charge of the fluidized bed particles.

However, the use of these methods is often not practicable or practicable in the industrial scale operation of a fluidized bed polymerization reactor. Any such additive must not act as a poison for the polymerization catalyst and must not adversely affect the quality of the polymer produced. It has previously been suggested to use water as an additive that reduces the electrostatic charge level of particles, but this possibility has been rejected because water acts as a strong catalytic poison.

According to the present invention, it has been found that in some specific reactions, i.e., processes where polymerization uses type IV or V catalysts together with alkyl aluminum cocatalysts, the addition of controlled small amounts of water to the reactor significantly reduces the occurrence of plaque formation without this addition of water had an adverse effect on the catalysts used. The amount of water supplied to the reactor depends on the level of electrostatic charge present in the reactor.

DETAILED DESCRIPTION OF THE INVENTION

In the following, the process of reducing plaque formation during alpha-olefin polymerization in a low pressure fluidized bed reactor using a titanium or vanadium compound catalyst along with an alkyl aluminum cocatalyst will be illustrated with the aid of the accompanying drawing showing a commonly used industrial polymerization reactor. for the manufacture of low-density and high-density polyolefins modified for carrying out the process according to the invention.

A reactor zone 12 and a reduced flow rate zone 14 are formed in the reactor 10. In the reaction zone 12 a bed of growing polymer particles and already formed polymer particles is formed. This bed is held up by a continuous stream of gas fed to the polymerization and modifying gaseous components in the form of a newly fed gas and gas recycled to the reaction zone. In order to keep the bed fluid (or fluidized), it is necessary to maintain the mass flow rate of the gas passing through the bed above a certain minimum mass flow rate of gas necessary for the fluidization of the bed, preferably about 1.5 times to about 10 -ti times the value of G _mf and more preferably the value 2 in the range from about 3 times to about 6 times the value of G _m f.

Note: The designation G _m f is a commonly used abbreviation for the minimum gas flow rate required to achieve a fluidized state; CY Wen and YH Yu, Mechanics of Fludization, Chemical Engineering Progress Symposium Series, Vol. 62, pp. 100-111 (1966).

When carrying out this process, it is highly preferred that the bed always contain particles, thereby avoiding the formation of local hot areas and achieving trapping and distribution of catalyst particles throughout the reaction zone. Upon start-up, a certain proportion of the polymer product particles are charged into the reactor before the stream of fluidizing gas begins to flow therethrough. This proportion of particles of the radius of the material to be made in the reactor can be chemically identical to or different from the polymer to be produced. If a polymer different from the polymer to be produced in the polymerization process is used to start the particles, then a certain amount of the product to be produced must be separated as the so-called first

the proportion of product having a composition different from that produced in the next stage of the process. This product consists of a mixture of the starting polymer and the polymer produced. After a short period of fluid bed operating time, the fluidized bed of the desired polymer produced displaces the starter polymer bed at an early stage of production and later the blend.

starting polymer and produced polymer.

The catalyst prepared for introduction into the fluidized bed is preferably stored in a reservoir 16 under an a-phospho-pyrene which is inert to the stored material. The inert gas may be, for example, nitrogen or argon.

Fluidization of the bed is accomplished by passing a high proportion of gaseous recycle to and through the bed, typically constituting a proportion of the recycled gas

_{Χ4; ϊ,;} , based on the amount of newly introduced polymerization gas about 50%. The fluidized bed thus formed generally has the appearance of a dense mass of moving particles swirling in a vortex created by gas percolation through the bed. The pressure drop between the front and rear of the bed is equal to or slightly higher than the weight of the bed divided by the cross-sectional area. The pressure drop is thus dependent on the reactor geometry.

Up gas is fed to the bed ^'ΐ speed which is equal to the rate of discharge of particulate polymer product from the polymerization reactor. The composition ^{41 of the} newly supplied gas is determined on the basis of the data from the gas analyzer of the placement of the gas by means of the gas (

By means of this gas analyzer 18 the composition of the gas to be recycled is determined and the composition of the newly supplied gas is adjusted on the basis of the data obtained so as to maintain substantially the same composition of the gaseous reaction mixture in the actual reaction zone.

In order to achieve perfect fluidization of the bed, the recycled gas and possibly part of the newly supplied gas are returned to the bottom 20 of the reactor at a point below the bed. The gas distribution plate 22 located above the recycled gas returning point ensures that the gas is dispersed throughout the reactor cross section and at the same time carries the bed in a state where gas supply to the reactor is stopped.

Part of the gas stream that does not react in the fluidized bed is a recycled gas stream that is discharged from the polymerization zone, preferably through a reduced velocity zone 14 located above the fluidized bed in which entrained particles fall back into the fluidized bed. due to the reduction of the off-gas velocity due to the expanded reactor cross-section at these locations.

The recycled gas is then compressed in a compressor 24 and then passed to a heat exchanger 26 where it transfers the heat of reaction before it is reintroduced into the fluidized bed. By periodically removing the heat of reaction from the recycle gas, there is no significant thermal gradient in the upper part of the fluidized bed. This temperature gradient occurs at the bottom of the fluidized bed in a layer about 15 to 30 cm thick. It is a temperature gradient between the temperature of the gas entering the fluidized bed and the temperature in the remaining part of the fluidized bed. Indeed, it has been observed that the fluidized bed functions to immediately adjust the temperature of the recycle gas at the points located above the very narrow bottom of the fluidized bed to the temperature that is in the remainder of the fluidized bed. In this way, the temperature of the fluidized bed is kept constant and a stable equilibrium condition is created. The recycle gas is then fed into the reactor through the bottom 20 towards the gas distribution plate 22, which disperses the gas throughout the fluidized bed space. The compressor 24 can also be placed downstream of the heat exchanger 26 downstream of the recycle gas to the reactor.

Hydrogen can be used as a chain transfer agent for the conventional polymerization reactions of the above type. In the case where ethylene is used as the monomer, the hydrogen / ethylene ratio used can range from about 0 to about 2.0 moles hydrogen per . mole of monomer in the feed gas stream.

Any gas inert to the C-1 catalyst and reactants may also be present in the feed gas stream. The cocatalyst is added to the recycle gas stream before the recycle gas stream is introduced into the reactor. The cocatalyst from the feeder 28 is added to the recycled gas stream.

via.

As is well known, it is important to operate the fluidized bed reactor at a temperature that is below the sintering temperature of the polymer produced. Thus to insure that sintering does not part ic_ produced _po.lymeru ,,., _{R =} _nutno, work př.i _=, ^. = Temperature which is lower than the temperature at which sintering occurs produced polymer. In the production of ethylene polymers, operating temperatures in the range of about 90 to about 100 ° C are used in the preparation of products having a density in the range of about 0.94 to about 0.97 g / cm ³ , while

operating temperatures in the range of about 75 ° C to about 95 ° C are used in the production of polymers having a density of about 0.91 to about 0.94 g / cm 2.

The fluidized bed reactor typically operates at a pressure in the range of about 0.56 to about 0.77 MPa to produce high density products at these pressures. In the manufacture of medium and low-density products, pressures in the range of about 0.5 to about 5 atmospheres are used.

The catalyst is injected into the fluidized bed at a rate that corresponds to its rate of consumption. This catalyst is introduced into the fluidized bed reactor via line 32 located above the gas distribution plate 22. A catalyst inert gas is used to transport the catalyst into the fluidized bed, for example nitrogen or argon. Injection molding. The catalyst to the reactor at the feed point 32, located above the gas distribution plate 22, is an important feature of this process. Because used; The catalysts are highly active, injecting the catalyst into the reactor at locations below the gas distribution plate 22 would result in polymerization already occurring here, possibly resulting in clogging of the gas distribution plate 22. Injection of the catalyst into the bed, instead, aids in distributing the catalyst throughout the fluidized bed volume and ¹ - - tends to preclude the formation of localized spots of high catalyst concentration which could create t. Vol. fluidized bed hot spots.

Under the given conditions, substantially the same fluidized bed height is maintained by draining the portion of the fluidized bed that represents the product to be produced at a rate corresponding to the rate of particle formation of the polymer product. Since the generation of reaction heat is directly proportional to the formation of the product, the measurement of the temperature rise of the gas as it passes is measured. fluid bed (temperature difference of the inlet and outlet gas) is an indicator of the rate of formation of the polymer product particles at a constant gas velocity.

The polymer product particles are preferably discharged through an outlet 34 located at or near the gas distribution plate 22. This proportion of polymer product particles is preferably discharged from the reactor by alternating opening and closing of a pair of time-controlled valves 36 and 38, which delimit the separation zone 40. When valve 38 is closed, valve 36 is opened, thereby expelling the gas cylinder and % of the product into the separation zone 40, after which the valve 36 is closed. The product 38 valve is then opened to drain the product to the outer collection zone, and after this product product removal, the valve 38 is closed again until the next removal operation begins.

The fluidized bed reactor is also equipped with a corresponding degassing system allowing the bed to be degassed during start-up and shutdown of the fluidized bed. Said reactor does not require the use of mixing means and / or means for wiping the walls of the reactor.

The reactor vessel is usually made of carbon steel and is designed to meet the above conditions.

The titanium-based catalyst (type IV) is fed to the reactor 10 through a supply 32 and 2a, under normal operating conditions, the plaques described above will begin to form in the reactor 10 after some time in the preparation of some polymers. Plaques are formed in close proximity to the reactor walls, the plaque forming site being located about one-half the reactor diameter from the base of the fluidized bed. The formed plaques soon begin to enter the separation zone 40 and immediately clog the drainage system, causing the reactor to stop operating. The characteristic plaque formation begins to occur after the amount of polymer product corresponding to about 6 to 10 times the bed weight in the reactor 10 has been produced.

As already mentioned, it has been found according to the invention that if type IV or V catalysts are used together with alkyl aluminum cocatalysts in the polymerization of the aforementioned type, plaques are significantly reduced in controlled small water additions to the reactor, without this having a detrimental effect. used catalysts. The amount of water introduced into the reactor depends on the level of electrostatic charge in the reactor. The addition of water to the polymerization reactor can be achieved by simply modifying the conventional polymerization process. It is apparent from the attached figure that an inert gas, such as dry nitrogen, is fed from the nitrogen source 41 to a unit called the O'Brian reservoir. This O '.4 Brienův Tray 2 _"t usually consists of one or more chambers ™ containing distilled water, which is equipped with temperature and flow control (these control means are not shown in the figure). Nitrogen is bubbled through one of these chambers. This chamber consists of a 1 liter stainless steel cylinder which is filled with water, and the chamber is enclosed in a temperature-controlled jacket. The water-saturated nitrogen is discharged from this O &apos; Brian tank 42 through line 44, and then the mixture is passed under controlled flow through a thermally insulated line to line 46 for feeding ethylene to the reactor. The achieved concentration of water in ethylene is less than 1 ppm vol. To confirm the presence of added water, a moisture analyzer 48 can be used, which is connected to the ethylene feed line 46 to the polymerization reactor.

Typically, the amount of nitrogen is in the range of about 0 to about 4.9 kg / h and the amount of ethylene fed is in the range of 0 to 22,700 kg / h. At a water temperature of 20 DEG C. and a nitrogen pressure of 2.45 MPa, a water concentration of from 0 to 0.3 ppm by volume can be achieved. The final water concentration can also be adjusted by adjusting the water temperature and nitrogen pressure.

By way of illustration, the calculation of the above concentration of added water will be given in the following section:

(1) Determination of water vapor pressure at given temperature in Ο'-Brian tank (P _H0 ).

(2) Determination of nitrogen flow rate through integral orifice (W · ^).

(3) Determination of the nitrogen pressure from the pressure < ^d n ₂ ) in the front of the reactor bottom plus the pressure drop in the pipeline.

(4) Determination of ethylene flow rate to reactor (5) Assuming nitrogen saturation with water:

HoO

Nx w _N2 x water flow (W _{H 0} )

χ 10 ⁶ = ppm Η ₂ Ο in the ethylene stream

At 20 ° CP _R θ = 2.373 kPa

P 2 ⁼ 2.27 MPa at reactor pressure of 2.1 MPa

W _N = 0 to 11.36 ppm, usually 3 ppm ^W C ₂ H ₄ = ^{8 163 to} 9 / ^h

Water concentration = 0.174 ppm.

The electrostatic voltage may be monitored near the reactor wall by one or more electrostatic charge indicators 50 inserted into the reactor bed approximately 1.5 meters above the gas distribution plate, operating in the range of -15,000 to +15,000 volts.

During the polymerization process, the change of electrostatic charge from neutral to positive can be eliminated by introducing nitrogen containing moisture into the ethylene stream. If this introduction of steam saturated with nitrogen into the reactor is not carried out, it is necessary to stop the operation of the polymerization reactor due to the occurring plaque formation in the fluidized bed. Care must be taken to avoid excess water being introduced into the reactor. the result of undesired negative electrostatic charge.

In the polymerization system, sensors and control and non-return valves are used, which are common to systems of this kind and have therefore not been shown in the attached figure. It should further be noted that line 44 is isolated and controlled for leakage of water vapor prior to entry into line 46 for injecting the reaction gas. This insulation and monitoring is not necessary, only advantageous.

The polymers to which the process according to the invention relates in particular and which exhibit the above-mentioned problems when using titanium catalysts

- with the formation of 'β-acetyl' are —flarary — homopolymers — ethylTemt — or non-linear copolymers having a predominantly (i.e. greater than or equal to 90%) ethylene content and having a low (i.e. less than or equal to 10%) ) of at least one C 3 -C 8 alpha-olefin. Said alpha-olefins having 3 to 8 carbon atoms should not contain any branching on a carbon atom which, as indicated, is less than the fourth. Preferred alpha-olefins having 3 to 8 carbon atoms to which the process of the invention is applicable are propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene and 1-okten. +

Said homopolymers and copolymers have a density in the range of about 0.97 to about 0.91 g / cm ³ . The density of the copolymer at a given melt index is mainly controlled by the amount of the C 3 -C 8 comonomer that is copolymerized with ethylene. Addition of progressively larger, the amount of the comonomers to the kopolvmerům has rias1edek after the ^ lable ^- with RFI Z van i ^- density 'in the harvest kayas CII RCT copolymer Iýmeřu7 amount of each comonomer from the group of comonomers comprising from 3 to 8 carbon atoms which is necessary to achieve the same result varies from one monomer to another comonomer under the same reaction conditions.

In the absence of comonomer, ethylene is homopolymerized.

The melt index of a homopolymer or copolymer is a reflection of its molecular weight. Polymers of relatively high molecular weight also have relatively high viscosities and a relatively high melt index.

In the most common method of applying a process for reducing plaque formation of a sintered polymer product during the polymerization of alpha-olefins of the present invention, a reactor vessel of the same type as in the attached figure (and thus prone to the above described plaque formation problems in polymerizing the above materials) using type IV and type V catalysts together with alkyl aluminum cocatalysts), partially filled with granulated polyethylene, which is purged with a non-reactive gas such as nitrogen, and then this particulate layer is fluidized by circulating said non-reactive gas through the reactor at a higher rate than the minimum fluidizing velocity (G f _m) for the granular polyethylene and preferably at a speed which corresponds

3 to 5 times the minimum fluidization rate. The reactor is then brought to operating temperature by supplying the reaction gas and the reaction is started by introducing the catalyst and cocatalyst into the reactor. During the reaction, the electrostatic voltage level approaches the level at which the sintered polymer product is formed, whereupon the pressure, temperature and inert gas flow in the 0 ' Brianian vessel are increased to allow saturation of the inert gas. nitrogen vapor. The steam-saturated nitrogen is then introduced into the polymerized gas supply line and is then fed to the polymerization reactor. The drop in the electrostatic voltage level is monitored until the level drops to a level where plaque is no longer formed.

The general process of the present invention will be illustrated by the following non-limiting examples.

Examples 1 and 2 describe the operation of a commonly used industrial fluidized bed reactor. The catalyst used was a Ziegler-type catalyst based on titanium, which was deposited on a porous silica as previously described in connection with a type IV catalyst. The cocatalyst was triethyl aluminum. The products made according to these examples were copolymers of ethylene and 1-butene. Hydrogen was used as a chain transfer agent to control the melt index of the polymer.

Example 1

In this embodiment, the fluidized bed reactor was started to conditions suitable for the preparation of low density, ethylene copolymers of suitable film quality, with a desired density of 0.918 g / cm ³ and a melt index of 1.0 dg / minute. The temperature at which the product began to adhere was 104 ° C. The reaction was initiated by introducing the catalyst into the reactor into which it had been charged. a granulated polymer having similar characteristics to the product produced. The catalyst used was a mixture of 5.5 parts of titanium tetrachloride, 8.5 parts of magnesium chloride and 14 parts of tetrahydrofuran, and the catalyst was applied to 100 parts of silica (Davison grade)

955), which was previously dehydrated at 600 [deg.] C. and treated with four parts of triethyl aluminum before the catalyst component was applied and 35 parts of tri-n-hexyl aluminum were activated after the catalyst component was applied. Prior to the catalyst feed, the reactor and the fluidized bed were brought to an operating temperature of 85 ° C and purged and cleaned of impurities by circulating nitrogen through the fluidized bed. The ethylene, butene and hydrogen concentrations were determined as follows: ethylene 53%, butene 24% and hydrogen 11%. The cocatalyst was fed to the reactor in an amount corresponding to 0.3 part triethyl aluminum per part catalyst.

The reactor was started in a conventional manner. After a reactor run of 29 hours, producing 6.5 times the amount of product in excess of the weight of the fluidized bed, temperature deviations of 1 to 2 were observed by thermocouples located in the immediate vicinity of the reactor wall at half the reactor diameter above the gas distribution plate. ° C above the fluidized bed temperature. Experience has shown that these temperature variations are indicative of plaque formation of the sintered polymer product in the fluidized bed. At the same time, an increase in the electrostatic voltage of the bed (measured by electrostatic voltmeters associated with a 12.7 mm diameter spherical electrode, located 25.4 mm from the reactor wall at a distance corresponding to half the reactor diameter from the gas distribution plate) was observed. about +1500 to +2000 volts to over +5000 volts, which then dropped to +2000 volts in three minutes. Temperature and voltage variations were then continued for approximately 12 hours, with the magnitude and frequency of these variations increasing steadily. During this time, plaques began to appear in the polymer product

- 37 sintered polymer. Over time, plaque formation became more and more apparent, as variations in fluidized bed temperatures were up to 20 ° C and these variations lasted longer than at the beginning of the process. . Also, the voltage variations were. increasingly. team .......

more numerous. Finally, the reactor operation was stopped due to the large number of plaque polymer plaques in the polymer product produced.

^Example-2: According to this embodiment was the same fluidized bed reactor as in the previous example, the reactor was started up and operated to produce a linear low density ethylene copolymer suitable for extrusion or rotational molding and having a density of 0, 93 4 g / cm @ 2, melt flow index:

dg / minute and the temperature at which the product starts 1'epit 118 ’C. The reaction was initiated by feeding a catalyst that was the same as the one in Example 1 except that 28 parts of tri-n-hexyl aluminum was activated to a reactor in which a bed of polymer having similar characteristics to the polymer produced was previously placed. Prior to catalyst introduction into the reactor, the reactor and the fluidized bed were brought to a temperature of 85 ° C and purged and cleaned of impurities by circulating nitrogen through the bed. The ethylene, butene, and hydrogen concentrations fed to the reactor were determined as follows: ethylene 52%, butene 14% -and hydrogen 21%. —Triethyl aluminum cocatalysts were fed to the reactor in an amount of 0.3 parts per part of the catalyst. The reactor was operated continuously for 48 hours, during which time a 9-fold amount of the desired product was made than the weight of the fluidized bed. After 48 hours of trouble-free reactor operation, sintered polymer plaques began to appear along with the polymer product particles at the reactor exit. At this time of reactor operation, the electrostatic voltage variations, measured at a distance equal to half the reactor diameter above the gas distribution plate, were from an average of up to +10,000 volts, while surface thermocouples indicated deviations from the fluidized bed temperature above 15 ° C. Two hours after the first plaques were discovered in the polymer product at the exit of the reactor, it was necessary to shut off the catalyst and cocatalyst feed to the reactor in order to reduce the polymer formation rate as the formed sintered polymer plaques began to clog the product discharge system. reactor. An hour later, the catalyst and cocatalyst feeds were resumed. The formation of sintered polymer plaques continued and after a further two hours the feed of catalyst and cocatalyst to the reactor was interrupted again and the reaction was terminated by injection of carbon monoxide.

The electrostatic voltage was greater than +12,000 volts at this point, with temperature variations measured by the thermocouples being observed until catalytic poison was injected into the reactor. The total reactor operating time was 53 hours, producing 10.5 times the desired product than the volume of the fluidized bed polymer before stopping the reaction due to the formation of sintered polymer plaques.

The following example illustrates a method of preventing plaque formation of a fused polymer product by adding water to the gas fed to the polymerization reactor when high deviations from the average electrostatic voltage value occur in the reactor.

Example 3

In this embodiment, the reactor used in Examples 1 and 2 was modified as shown in the attached drawing to produce a polyethylene film product having a density of 0.946 g / cm ³ , a melt index of 7.5 dg / min and a temperature at The product produced was an ethylene-hexene copolymer and was produced using a vanadium-based catalyst together with an aluminum alkyl-C-catalyst and a halogen polymerization promoter. The catalyst contained 0.29 millimoles of vanadium per gram of precursor and 1.2% of aluminum added as diethyl aluminum chloride, and the catalyst was supported on silica (Davison species) with a particle size in the range of 30 to 130 microns. The reaction was carried out at a fluid bed temperature of 98 ° C. The concentrations of ethylene, hexene and hydrogen were determined as follows: 76% ethylene, 1.2% hexene, and 1.6% hydrogen, the remainder being inert gases, i.e.. nitrogen, methane, isopentane, etc. The operating pressure in the reactor was 2,205 MPa. The amount of triethyl aluminum fed as cocatalyst was controlled to maintain a concentration of 200 ppm by volume in the polymer produced. The promoter used was a freon corresponding to a ratio of 0.7 mol of freon per mole of triethyl aluminum as a cocatalyst. The amount of polymer produced corresponded to approximately 9080 kg / h.

About 18 hours after the stabilization of the operating conditions in the polymerization reactor, an electrostatic voltage, which was measured about 150 cm above the gas distribution plate at the reactor wall, gradually began to increase. Every 1 minute to 5 minutes this voltage was increased in small increments corresponding to 100 to 300 volts. This trend has continued, with the shift from baseline 0 to 1000-5000 volts, with voltage peaks extending from this base of up to 10,000-15,000 volts * - and their frequency has steadily increased. Simultaneously with the increase in electrostatic voltage in the reactor, there were deviations from the average fluidized bed temperature as measured by surface thermocouples at a height of 90-180 cm above the gas distribution plate in the immediate vicinity of the reactor wall. These deviations were usually negative, indicating that the reactor wall was isolated at the point of formation of polymer agglomerates adhered to the reactor wall. Further, if it was allowed to continue to form the sintered polymer plaques, this could result in the polymerization reactor being stopped due to clogging of the product discharge system or blockage of the gas distribution plate, resulting in the formation of large polymer particle agglomerates due to fluidization limitation. fluidized bed.

to

At this critical point, water was added to the reactor in such a way that a nitrogen stream of 2.27 kg / h was fed to a chamber containing distilled water at 20 ° C and a pressure of 2.45 MPa and then a nitrogen stream so saturated with water was introduced into the chamber. piping for ethylene feed to the reactor. The achieved water concentration in the ethylene feed was 0.2 ppm by volume.

... ..... to keep the electrostatic voltage level at. , "A value close to zero. Care should also be taken not to feed too much water into the reactor, which could result in undesirable negative deviations from the electrostatic voltage value, which would also lead to plaque formation of the sintered polymer product.

By keeping the electrostatic voltage level close to zero, stable reactor operation without plaque formation of the sintered polymer product was achieved, which otherwise would inevitably lead to a cessation of the polymerization reactor operation.

Example 4

The same reactor as in the previous Example 3 was used, and a linear low density polymer suitable for film production was produced. The polymer produced was an ethylene-hexene copolymer having a density of 0.917 g / cm ³ , a melt index of 2.7 dg / min and a temperature at which the product began to adhere to 102 ° C. The catalyst used was a titanium-based catalyst which was deposited on silica. Titanium was used in an amount of 0.25 millimoles per gram of precursor. Due to the titanium content, the amounts of magnesium chloride, diethyl aluminum chloride and tri-n-hexyl aluminum used corresponded to the following molar ratios: magnesium chloride / titanium = 3, diethyl aluminum chloride / titanium = 0.02 and tri-n-hexyl aluminum / titanium = 0.02. The silica (type ¹ Davison 955) carrier material had a particle size distribution in the range of 10 to 80 microns. The polymerization reaction was carried out at 76 ° C. The concentrations of ethylene, hexene, and hydrogen were determined as follows, 29% ethylene, 5% hexene and 11% hydrogen, the remainder being inert gases, i.e. nitrogen, ethane, methane, and isopentane. The cocatalyst was added in an amount corresponding to a concentration of 300 ppm by weight. of triethyl aluminum in the polymer. The catalyst productivity under these conditions corresponded to 2200 kg / h of polyethylene produced per kilogram of catalyst used. The amount of polyethylene produced corresponded to 8 172 kg / h.

A sudden increase in ethylene concentration resulted in increased catalytic activity. The electrostatic voltage near the reactor wall increased from zero to 6000 volts in ten minutes. The temperature measured in the immediate vicinity of the reactor wall began to rise, indicating a sudden formation of sintered polymer plaques on the reactor wall at a height of about 180 cm above the gas distribution plate. If this plaque formation were allowed to continue, it would be necessary to shut down the reactor soon due to clogging of the product removal system.

At this point, water was added to the reactor in such a way that the water-containing chamber, which was placed in a 20 ° C temperature controlled jacket, was fed with 1.816 kg / h of nitrogen enriched with water. steam was then fed into the ethylene stream. The water concentration in the ethylene stream was less than 0.2 ppm. The electrostatic voltage level dropped rapidly to zero. The deviations of the temperature near the reactor wall from the average fluidized bed temperature disappeared within ten minutes and the reactor operation returned to normal.

DOT cii i 1, Žfta & SS

Tv Kfi - gt

Claims (10)

PATENT CLAIMS A process for reducing plaque formation in the course of alpha-olefin polymerization in a low pressure fluidized bed reactor using a titanium or vanadium compound ^J catalyst together with an alkyl-aluminum cocatalyst, characterized in that water is introduced into the reactor at the plaque formation site. inert gas such that the inert gas is passed under pressure at a controlled rate of flow through a temperature-controlled reservoir containing water, the inert gas being enriched with water and the mixture of water and inert gas being fed to the reactor, optionally together with alpha. olefins introduced into the polymerization. The method of claim 1, wherein said alpha-olefin is ethylene. Process according to claims 1-2, characterized in that the inert gas is nitrogen. The process according to claims 1-3, characterized in that the flow rate of nitrogen gas, the flow rate of alpha-olefins and the water temperature in the tank are controlled according to the electrostatic ratios in the reactor. Process according to claims 1-4, characterized in that the water content of said mixture entering said reactor. -is- in the range of 0, T- to -2-ppm- in the amount of "" "" "" - "-" - "· · · · · · · · · · · · · · · · · · · · · · · · · 6. The process of claim 5, wherein the water content of said mixture fed to the reactor is less than 1 ppm by volume based on the amount of ethylene fed. Process according to claims 1-6, characterized in that the flow rate of nitrogen is up to 4.99 kg / h with an alpha-olefin flow rate of up to 22,700 kg / h. Method according to claims 1-7, characterized in that the temperature of the water in the tank is regulated in the range of 10 to 40 ° C. The process according to claims 1-8, wherein said polyolefins are linear ethylene homopolymers or linear copolymers consisting predominantly of ethylene having a molar content of greater than or equal to 90% and a low content of at least one C 3 -C 8 alpha-olefin. less than or equal to 10%. The process according to claims 1-9, wherein said polyolefins are homopolymers or copolymers of propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene and 1-octene.