FI91415B - Method for reducing disc formation during polymerization of alpha-olefins - Google Patents

Description

91415

Method for reducing sheet formation during the polymerization of alpha-olefins For the reduction of alpha-olefins during polymerization of alpha-olefins

The present invention relates to a process for reducing sheet formation in the polymerization of alpha-olefins, and more particularly to a process for reducing sheet formation in the polymerization of polyethylene using titanium-based catalysts or vanadium-based catalysts with alkyl-aluminum cocatalysts.

Traditional low density polyethylene has heretofore been polymerized in thick-walled autoclaves or tubular reactors at pressures as high as 50,000 psi and temperatures as high as 300 ° C or higher. The molecular structure of high pressure low density polyethylene (HP-LDPE) is very complex. The variation in the location of these simple structural units is essentially limitless. HP-LDPE compounds are characterized by a confusing long chain branched molecular structure. These long chain branches dramatically affect the melting rheology of these resins. HP-LDPE compounds also have a number of short chain branches, usually 1 to 6 carbon atoms in length. These short chain branches prevent the formation of crystals and lower the density of the resin.

Some time ago, a technology has been introduced which can produce low density polyethylene by fluidized bed technology at low pressures and temperatures by copolymerizing ethylene with various other alpha-olefins. These low pressure LDPE compounds (LP-LDPE) generally have few, if any, long chain branches and are sometimes referred to as straight chain (linear) LDPE resins. They have short-chain branches, the length and frequency of which are controlled by the type and amount of comonomer used in the polymerization.

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It is well known to those skilled in the art that low pressure, high or low density polyethylenes can now be obtained by a conventional fluidized bed process using numerous families of catalysts to produce a full range of low density and high density products. The appropriate choice of catalysts to be used depends in part on the type of end product desired, i.e., high density, low density, extrusion grade, or film grade resin, as well as other factors.

The different types of catalysts that can be used to produce polyethylenes in fluidized bed reactors can generally be classified as follows:

Type I: Silyl chromate catalysts described in U.S. Patent No. 3,324,101 to Baker and Carrick and U.S. Patent No. 3,324,095 to Carrick, Karapinks and Turbet. These silyl chromate catalysts are characterized by the presence of a group of the following formula: :

-R O

i it --Si-O-Cr-O-—

I II

_R o _ wherein R is a hydrocarbon group having 1 to 14 carbon atoms. Preferred silyl chromate catalysts are bis (triarylsilyl) chromate, corresponding and more preferably bis (triphenylsilyl) chromate.

This catalyst is used on a support such as silica, alumina, thorium oxide, zirconia and the like. Other carriers such as carbon black, microcrystalline cellulose, non-sulfonated ion exchange resins and the like can also be used.

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Type 11: Bis (cyclopentadienyl) chromium (II) compounds described in U.S. Patent 3,879,368. These bis (cyclopentadienyl) chromium (II) compounds have the following formula:. wherein R 'and R "may be the same or different C1-Cao-carbon-hydrogen radical, including limits, and n' and n" may be the same or different integers from 0 to 5, inclusive. R 'and R "hydrocarbon radicals may be saturated or unsaturated and may be mentioned, for example, aliphatic, alicyclic and aromatic radicals such as methyl, ethyl, propyl, butyl, pentyl, cyclopentyl, cyclohexyl, allyl, phenyl and naphthyl radicals.

These catalysts are used on the support described above.

Type III: The catalysts described in U.S. Patent 4,011,382. These catalysts contain chromium and titanium as oxides and optionally fluorine as well as a support. These catalysts contain from about 0.05 to 3.0, preferably from about 0.2 to 1.0% by weight of chromium (calculated as Cr), from about 1.5 to 9.0, preferably from about 4.0 to 7.0% by weight. -% titanium (calculated as Ti), and about 0.0 to 2.5, preferably about 0.1 to 1.0% by weight of fluorine (calculated as F), based on the total weight of the support, chromium, titanium and fluorine.

Compounds useful in Type III catalysts include CrOs or any other chromium compound that can oxidize to the form CrOa under the activation conditions used. At least a portion of the chromium of the activated catalyst bound to the support must be in hexavalent form. Useful chromium compounds other than COa are disclosed in U.S. Patent Nos. 4,825,721 and 3,622,521 and may include chromium (III) acetyl acetate, chromium (III) nitrate, chromium (III} acetate, chromium (III) chloride, chromium (III) sulphate and ammonium chromate.

Useful titanium compounds are all those which can be oxidized to the TiO2 form under the activation conditions used, and can be mentioned, for example, those described in U.S. Pat. No. 3,622,521 and Dutch Patent Application NL 72-10881.

Useful fluorine compounds include, for example, HF or any other fluorine compound that produces hydrogen fluoride under the activation conditions used for HF. Useful fluorine compounds other than HF are described in Dutch patent application NL 72-10881.

Inorganic oxide materials that can be used as supports in catalyst preparations are porous materials having a large surface area of about 50 to 1000 m 2 / g and an average particle size of about 20 to 200 microns. Useful inorganic oxides include, for example, silica, alumina, thorium oxide, zirconium oxide, and other similar inorganic oxides, as well as mixtures of these oxides.

Type IV: These catalysts are described as F. J. Karol et al. U.S. Patent No. 4,302,566 to U.S. Patent No. 4,302,566, entitled "Preparation of Ethylene Copolymers in a Fluid Bed Reactor", assigned to the same applicant as the present application. These catalysts contain at least one titanium compound, at least one magnesium compound at least one electron donating compound, at least one activating compound and at least one inert carrier material.

The titanium compound has the structure:

Ti (OR) aXb 5 91415 wherein R is an aliphatic or aromatic C 1 -C 13 hydrocarbon radical or COR ', wherein R' is an aliphatic or aromatic C 1 -C 13 hydrocarbon radical; X is Cl, Br or I; a is 0 or 1; b is 2-4, inclusive; and a + b is 3 or 4.

Titanium compounds can be used alone or in combination and include TiCl3, TiCl *, Ti (OCH3) Cl3,

Ti (OCeHs) Cla, Ti (OCOCH3) Cl3 and Ti (OCOCeHs) Cl3.

The magnesium compound has the structure:

MgXa where X is Cl, Br or I. Such magnesium compounds may be used singly or in combination and include, for example, MgCl 2, MgBra and Mgla. Anhydrous MgCl 2 is a preferred magnet.

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

The electron donating compound is an organic compound which is liquid at a temperature of 25 ° C and in which the titanium compound and the magnesium compound are partially or completely soluble. Electron donating compounds are known per se or are known as Lewis bases.

Electron donating compounds include, for example, compounds such as alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic ethers, cyclic ethers and aliphatic ketones.

The catalyst can be modified with a boron halide compound having the structure: BRoX 's-ο wherein R is an aliphatic or aromatic hydrocarbon radical having 1 to 14 carbon atoms or OR', wherein R 'is also an aliphatic or aromatic hydrocarbon radical having 1 to 14 carbon atoms; X 'is selected from chlorine Cl and bromine Br or mixtures thereof; and c is 0 or 1 if R is an aliphatic or aromatic hydrocarbon, and c is 0, 1 or 2 if R is OR '.

The boron halide compounds can be used alone or in combination and include, for example, BC13 / BBr3 / B (C2H =) C12, B (OC2Hs) C12 / B (OC2Hs) 2Cl, B (CsHs) Cl2, B (OCeHs) Cl2, B (CeHi3) Cl2, B (C6H13) 2Cl and B (OCeH5) 2Cl. Boron trichloride is a particularly preferred boron compound.

The activating compound has the structure: Al (R ") oX 'a.H · wherein X' is Cl or ORi; R1 and R" are the same or different group and are saturated C1-Cm hydrocarbon radicals; d is 0 to 1, 5, e is 1 or 0 and c + d + e = 3.

Such activating compounds may be used singly or in combination.

The carrier materials are solid particulate materials and include, for example, inorganic materials such as oxides and molecular sieves of silicon and aluminum, and organic materials such as olefin polymers such as polyethylene.

Type V: Vanadium-based catalysts. Catalysts of this type generally contain vanadium as their active component, a catalyst of this type generally comprising a support-bound precursor, a cocatalyst and a promoter. The carrier-bound precursor consists essentially of a vanadium compound and a modifier with which a solid, inert carrier is impregnated. The vanadium precursor compound is the reaction product between the vanadium trihalide and the electron donor. Vanadium tri halogen is halogen is chlorine, bromine or iodine or a mixture thereof. A particularly preferred vanadium trihalide is vanadium trichloride VCl3.

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The electron donor is a liquid organic Lewis base in which vanadium trihalide 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 and cycloalkyl ethers, and mixtures thereof. Preferred electron donors are alkyl and cycloalkyl ethers, especially tetrahydrofuran. For each mole of vanadium used, about 1 to 20, preferably about 1 to 10, most preferably about 3 moles of electron donor are complexed.

The modifier used in the precursor has the formula: MX.

wherein: M is either boron or AlR 0 -,> each symbol R independently represents alkyl such that the total number of aliphatic carbon atoms in any group R does not exceed 14; X is chlorine, bromine or iodine; and a is 0, 1 or 2 provided that when M is boron, then a is 3.

Preferred modifiers are C 1 -C 6 alkylaluminum mono- and dichlorides and boron trichloride. A particularly preferred modifier is diethylaluminum chloride. About 0.1 to 10, preferably about 0.2 to 2.5, moles of modifier are used per mole of electron donor.

The carrier is a solid porous particulate material that is inert to polymerization. The support is essentially silica or alumina or a mixture thereof. The carrier may optionally contain other materials such as zirconia, thorium oxy, or other compounds or mixtures thereof that are chemically inert to the polymerization.

The carrier is used as a dry powder having a particle size of about 10 to 250, preferably about 20 to 200, most preferably about 30 to 8,100 microns. The surface area of the porous support is equal to or greater than about 3, preferably equal to or greater than about 50 m 2 / g. The preferred support is silica having a pore size equal to or greater than about 80, preferably equal to or greater than about 100 Angstroms. The support is preheated by heating to remove water, preferably at a temperature equal to or greater than about 600 ° C.

The amount of carrier used is such that the vanadium content is about 0.05 to 0.5 millimoles of vanadium / g (mmol V / g), preferably about 0.2 to 0.35 mmol V / g, most preferably about 0.29 mmol V / g g.

The support is not usually chemically treated by reacting it with an alkylaluminum compound prior to forming the support-bound precursor. Such treatment results in the formation of aluminum alkoxides chemically bonded to the support molecules. It has been found that the use of such a treated support in a catalyst suppressor and process is not only irrelevant but also leads to undesired agglomeration when used in the production of high density polyethylene (> 0.94 g / cm 3), resulting in a lumpy, non-free flowing product.

The cocatalyst that can be used in type IV and V catalysts has the formula: AI Rs where R is as defined above in connection with the symbol M. Preferred cocatalysts are, for example, Ca-C 1 trialkylaluminum compounds. A particularly preferred cocatalyst is triisobutylaluminum. About 5 to 500, preferably about 10 to 50 moles of cocatalyst are used per mole of vanadium.

The promoter has the formula: R '* »CX' 9 91415 wherein: R 'is hydrogen or unsubstituted or halogen-substituted lower, i.e. alkyl having up to 6 carbon atoms; X 'is halogen and b is 0, 1 or 2.

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

The catalyst is prepared by first preparing a support-bound precursor. In one embodiment, the vanadium compound is prepared by dissolving the vanadium trihalide in its electron donor at a temperature of about 20 ° C to the boiling point of the electron donor with stirring for a few hours. Stirring is preferably performed at a temperature of about 65 ° C for about 3 hours or longer. The support is then saturated with the vanadium compound thus obtained. Impregnation can be accomplished by adding the carrier as a dry powder or as a slurry in an electron donor or other inert solvent. The liquid is removed by drying at a temperature below about 100 ° C for a few hours, preferably at a temperature of about 45 ° C for about 3-6 hours. The modifier dissolved in an inert solvent such as a hydrocarbon is then mixed with a vanadium-saturated support. The liquid is removed by drying for a few hours at a temperature of less than about 70 ° C, preferably at a temperature of about 45-65 ° C for about 3 hours.

The cocatalyst and promoter are added to the support-bound precursor either before or during the polymerization reaction. The cocatalyst and promoter are added either together or separately, * and either simultaneously or sequentially during the polymerization. The cocatalyst and promoter are preferably added separately dissolved in an inert solvent such as isopentane during the polymerization.

10

The catalysts described above are generally placed together with the materials to be polymerized in a reactor with an expanded section above the straight side section. The recirculated gas is introduced into the reactor at its bottom and passes upwards through the gas distribution plate to a fluidized bed located in the straight side part of the reaction vessel. The function of the gas distribution plate is to ensure the proper distribution of the gas and to support the resin bed when the gas flow is stopped.

The gas leaving the fluidized bed carries resin particles with it. Most of these particles differ from the gas as it passes through the expanded portion, thereby reducing its velocity.

In order to meet the requirements for certain final applications of ethylene resins, such as film application, injection molding and rotational molding applications, type IV and V catalysts have been used in the polymerization together with alkylaluminum cocatalysts. However, some attempts to produce certain ethylene resins using alkylaluminum cocatalysts in combination with type IV and V catalysts bound to the surface of a porous silica substrate have not been entirely satisfactory in practice from a commercial point of view. This is mainly due to the formation of "plates" in the reactor after a period of operation. These "sheets" can be characterized as a fused polymeric material.

It has been found that the static mechanism contributes to the plate formation phenomenon in which catalyst and resin particles adhere to the reactor walls as a result of static forces. If excessive temperatures are allowed to exist for a sufficient time in the reaction environment, they can result in the particles fusing together. There are numerous reasons for static charges. These include electrification due to abrasion between different materials, limited static decomposition, small amounts of static promoters entering the process, excessive activity of catalysts, 11 91415, etc. A strong dependence is found between the plate support and the presence of excessive static negative or positive charges. This is seen in the sudden changes in static concentrations and the temperature deviations close to them near the reactor wall. These temperature deviations are either large or small. Low temperatures indicate particle adhesion, which has an insulating effect on the temperature of the bed. Large deviations indicate that reactions occur in zones where heat transfer is limited. As a result, Lei entrapment disorders are usually evident, catalyst feed interruptions can occur, the product removal system becomes clogged, and thin fused agglomerates (plates) are observed in the granular product.

The size of the plates varies greatly, but they are similar in many respects. Their thickness is usually about 1/4. . . 1/2 inch (about 0, 95... 1, 3 cm) and their length is about 1.. . 5 feet (approx. 0.3 ... 1.5 m), with some plates even longer. Their width is about 3.. . 18 inches (about 7, 6... 45, 7 cm) or more. The sheets have a core of molten polymer oriented parallel to the length of the sheets, and their surface is covered with a granular resin fused to the core. The edges of the sheets may be "hairy" due to the filaments of molten polymer.

Thus, it is an object of the present invention to provide a process for substantially reducing or eliminating the sheet formation that occurs when polymerizing alpha-olefins in a low pressure fluidized bed using titanium-based compounds or vanadium-based compounds in combination with alkylaluminum as cocatalysts.

Another object of the invention is to provide a process for reducing sheet formation in fluidized bed reactors used in the preparation of polyolefin resins, in which titanium or vanadium-based catalysts are used in combination with alkylamine coca-cytes.

These and other objects of the invention will be apparent from the following description and the accompanying drawing, which shows a generally typical gas phase fluidized bed polymerization process for the production of high density and low density polyolefins, which process has been slightly modified in accordance with the present invention.

The invention is defined in the appended claims. Thus, the invention provides a process for reducing sheet formation in the polymerization of alpha-olefins in a low pressure fluidized bed reactor using titanium or vanadium-based compounds as catalysts in combination with alkylaluminum cocatalysts. which avoid sheet formation without substantially altering the efficiency of the catalysts.

The amount of water fed to the reactor depends on the static voltage in the reactor and can generally be 0, 1. . 2 ppm based on the ethylene fed. The nitrogen flow is generally adjusted so that the nitrogen flow is about 0 to 11 lb / h (about 0 to 5 kg / h) when the ethylene supply is in the range of about 0 to 50,000 lb / h (about 0 lb / h). ..22680 kg / h). The temperature of the water cylinder in the O'Brien box can usually be about 10 ... 40 * C. The nitrogen overpressure can usually vary in the range of about 200 ... 400 psi (pressure about 1, 48 ... 2, 86 MPa) , preferably in the range of about 320 to 370 psi (pressure about 2.31 to 2.65 MPa).

The critical magnitude of the static voltage for sheet formation depends in a complex manner on the sintering temperature of the resin, the operating temperature, the resistance forces in the fluidized bed, the particle size distribution of the resin, and the composition of the recycled gas. The static voltage can be reduced by a number of techniques such as treating the surface of the reactor to reduce the generation of static electricity by injecting an antistatic agent to increase the electrical conductivity of the surface of the particles, which promotes the discharge of electrical charge from the particles; installing appropriate equipment connected to the walls of the reactor to facilitate the discharge of electrical charges by creating areas with a high local field strength; and by neutralizing charges by injecting or providing ion pairs, ions, or charged particles of opposite polarity to the resin bed.

According to the present invention, the addition of water to the low pressure polyethylene process as a gas phase helps to reduce the formation of agglomerates in the fluidized bed. This is due to a decrease in the positive static voltage, as a result of which the adhesion forces of the particles in the reaction system decrease.

As can be seen from the sole figure of the accompanying drawing, a conventional fluidized bed reaction system for the polymerization of alpha-olefins, which system has been slightly modified to add water, comprises a reactor 10 consisting of a reaction zone 12 and a rate-reducing zone 14.

The reaction zone 12 contains a bed of growing polymer particles, formed polymer particles and a small amount of catalyst particles, which is fluidized by continuously passing polymerizable and shaping gaseous components and recyclable gas through the reaction zone as a continuous replacement feed. In order to maintain a usable fluidized bed, the mass flow rate of the gas through the bed is considered to be greater than the minimum possible flow required for fluidization, preferably about 1.5. 10 times Gmf and more preferably about 3.. . 6 times Gmf. G »t is an accepted abbreviation for the minimum gas flow required for fluidization, see C. Y. Wen and 14 Y. H. Yu," Mechanics of Fluidization, "Chemical Engineering Progress Symposium Series, Vol. 62, pp. 100-111 (1966).

It is highly desirable that the bed always contain particles to prevent the formation of local hot spots and to retain and distribute the particulate catalyst throughout the reaction zone. At start-up, the reactor is usually filled with particulate polymer particles before starting the gas flow. Such particles may be similar or different in nature to the polymer to be formed. If different, they are removed with the desired polymer particles formed as the first product. Eventually, a fluidized bed of the desired polymer particles replaces the starting pad.

A suitable catalyst for use in a fluidized bed is preferably stored prior to use in the tank 16 under a layer of gas which is inert to the material to be stored and which is, for example, nitrogen or argon.

Fluidization is accomplished by circulating gas at and through the bed at a high flow rate, this flow rate typically being about 50 times the feed rate of the replacement gas. The general appearance of the fluidized bed is a dense mass of moving particles in the most turbulent flow produced by the gas 1 penetrating through the bed. The pressure drop through the bed is equal to or slightly greater than the mass of the bed divided by the cross-sectional area. Thus, it depends on the geometry of the reactor.

The replacement gas is fed to the reactor at a rate equal to the rate at which the particulate polymer product is removed. The composition of the replacement gas is determined by a gas analyzer 18 located above the bed. The gas analyzer determines the composition of the recycled gas and the composition of the replacement gas is changed accordingly to keep the composition of the gas substantially unchanged in the reaction zone.

To ensure complete fluidization, the recirculated gas and, if desired, some or all of the replacement gas is returned to the reactor from its bottom below 20 beds. A gas distribution plate 22 located above the inflection point ensures proper distribution of the gas and also supports the resin bed when the gas flow stops.

The part of the gas stream which does not react in the bed forms a recyclable gas which is removed from the polymerization zone, preferably leading to a speed-reducing zone 14 above the bed, where the entrained particles are allowed to fall back into the bed.

The recirculated gas is then compressed in a compressor 24 and then passed through a heat exchanger 26, where the heat of reaction is removed from the gas before it is returned to the bed. There does not appear to be a detectable temperature gradient at the top of the bed when the heat of reaction is continuously removed from the gas. At the bottom of the bed, in a layer about 6 to 12 inches (about 15 to 30 cm) thick, there is a temperature gradient that varies between the temperature of the incoming gas and the temperature of the rest of the bed. Thus, it has been found that the bed sets the temperature of the gas to be recycled almost immediately in the bed zone above this bottom layer to correspond to the temperature of the rest of the bed, whereby the bed itself remains at a substantially constant temperature under established conditions. The recirculated gas is then returned to the reactor from its bottom 20 and to the fluidized bed through the distribution plate 22. Compressor 24 may also be located after heat exchanger 26.

Hydrogen can be used as a chain transfer agent in the case of the conventional polymerization reactions discussed below. In the case where ethylene is used as the monomer, the ratio of hydrogen to ethylene used varies in the range of about 0.2 moles of hydrogen per mole of monomer in the gas stream.

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Any gas inert to the catalyst and reactants may also be present in the gas stream. The cocatalyst is added to the recycle gas stream before the gas stream joins the reactor, for example from dispenser 28 along line 30.

As is well known in the art, it is essential to operate a fluidized bed reactor at a temperature lower than the sintering temperature of the polymer particles. Therefore, to ensure that no sintering occurs, operating temperatures lower than the sintering temperatures are desirable. In the production of ethylene polymers, it is preferable to operate at operating temperatures of about 90 to 100 ° C, aiming for products with a density of about 0.94. . 0, 97, while about 75.. . A temperature of 95 ° C is preferred for products with a density of about 0.91 to 0.94.

The fluidized bed reactor is normally operated at pressures up to about 80 ... 110 psi (about 0, 55 ... 0, 76 MPa) for high density products and 65 ... 95 (about 0.45 ... 0 , 66 MPa) psi for low and medium density products.

The catalyst is injected into the bed in an amount corresponding to its consumption in an amount from a point 32 above the distribution plate 22. A gas inert to the catalyst, such as nitrogen or argon, is used to transport the catalyst to the bed. Injecting the catalyst from a point above the distribution plate 22 is an important feature. Because the catalysts normally used are highly active, injection into the area below the distribution plate can cause polymerization to start in that area, which can ultimately lead to clogging of the distribution plate. In contrast, injection into a moving bed promotes the distribution of the catalyst throughout the bed, thereby avoiding the formation of local dots at high catalyst concentrations. Such a high concentration of 17 91415 can lead to the formation of hot spots.

Under certain operating conditions, the height of the fluidized bed is maintained substantially constant by removing a portion of the bed as a product at a rate equal to the rate of formation of the particulate polymer product. Since the amount of heat generated depends directly on the formation of the product, the rate of particulate polymer formation while the gas rate remains constant can be determined from the measured increase in gas temperature across the reactor (difference between inlet gas temperature and outlet gas temperature).

The particulate polymer product is preferably removed at a point 34 located at or near the distribution plate 22. The particulate polymer product is expediently and preferably removed by successively using two timed valves 36 and 38 defining a separation zone 40. When the valve 38 is closed, the valve 36 is opened to provide a gas dose and product to the zone 40 between valves 38 and 36, after which the valve 36 is closed. Valve 38 is then opened to direct the product to the external recovery zone, and after this delivery, valve 38 is closed to await the next product recovery operation.

Finally, the fluidized bed reactor is equipped with a suitable ventilation system to ventilate the bed during start-up and shut-down. It is not necessary to use a stirrer and / or a wall scraper in the reactor.

The reactor vessel is normally made of carbon steel and is designed for the operating conditions described above.

The problems associated with the use of type IV catalysts can be better illustrated by drawing. A titanium-based catalyst (type IV) is charged to the reactor, 18 out of 32 points. During conventional operation using certain resins, after a period of time, reactor 10 begins to form plates at a point near the reactor wall, which is further equidistant above the bottom of the fluidized bed. Plates of fused resin begin to appear in the separation zone 40 and quickly clog the system, causing the reactor to be shut down. More typically, sheet formation begins after the amount of product produced corresponds to about 6 to 10 times the weight of the resin bed in reactor 10.

Many possible causes were investigated in an effort to detect and eliminate plate formation. During the studies, thermocouples were installed just inside the reactor walls so that they were located in the reactor 1/4. . . At a height corresponding to 1/2-diameter above the gas distribution plate. During traditional operation, surface-to-skin thermocouples show temperatures equal to or slightly less than the fluidized bed temperature. During plate formation support, these thermocouples can indicate that the temperature has risen up to 20 ° C above the fluidized bed temperature, which is a reliable indication of the presence of plate formation. To supplement this, an electrostatic voltmeter was used to measure the voltage at 1/2 inch (approx.

1.27 cm) from a spherical electrode placed in a fluidized bed radially 1 inch (about 2.54 cm) from the reactor wall and usually 5 to 6 feet (about 1.5 to 1.8 m) above the gas distribution plate . This location was chosen because the plate formation support was found to start in a zone located at a height corresponding to 1/4 to 3/4 of the diameter of the reactor above the bottom of the fluidized bed. As is well known in the case of deep fluidized beds, this corresponds to the area where the mixing efficiency is lowest in the vicinity of the wall, i.e. the zero zone where the movement of particles in the vicinity of the walls generally changes from an upward movement to a generally downward movement. Possible causes investigated included factors influencing mixing in the fluidized bed, reactor operating conditions, catalyst and resin particle size, particle size distribution, and other causes. A correlation was found between plate formation and static electricity charge in resin particles in the vicinity of the reactor walls. When the static voltage of the resin particles in the vicinity of the reactor walls at certain points in the fluidized bed reactor is low, the reactor operates normally and no plates are formed. When this static voltage exceeds a critical level at these points, plate formation occurs in the reactor that cannot be controlled and the reactor must be stopped.

It was further found that sheet formation can be substantially reduced and in some cases completely eliminated by adjusting the static voltage in the fluidized bed in the vicinity of the reactor walls below a level critical for sheet formation. This level critical for sheet formation is not a fixed value, but is a complex function depending on variables such as resin sintering temperature, operating temperature, fluidized bed resistance, resin particle size distribution, and recyclable gas composition.

The critical voltage Vc for sheet formation in the case of ethylene homopolymers and ethylene-butene and ethylene-hexene copolymers depends mainly on the sintering temperature of the resin, the temperature of the reactor bed and the concentration of hydrogen in the recycle gas.

The sintering temperature of the resin under reactor operating conditions is the temperature at which the settled resin in contact with a gas having the same composition as the gas used to make the resin in the reactor sinks. The bed sinteres and forms agglomerates when it is attempted to re-fluidize the bed after 15 minutes. The sintering temperature decreases as the density of the resin decreases, the melt index increases, and the amount and type of monomer dissolved increases.

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The constants in the equation were determined from the data obtained during the operation of the reactor, just as plate formation began to occur in the reactor based on the temperatures measured by the surface thermocouple above the bed temperature. The voltage indicated by the voltage probe described above varied as a function of time as a result of the random nature of the fluidized bed. Thus, the critical voltage Vc is expressed as a time-averaged voltage. Interpretation of voltage measurements is difficult because static electrical charges are still generated as the plate formed as a result of static charge separates from the reactor wall. In addition, plate formation can begin as a very local phenomenon and spread further, making it difficult to interpret voltage readings.

Although the mechanism of plate formation is not fully understood, it is assumed that the static electricity generated in the fluidized bed charges the resin particles. When the charge of the particles reaches a level such that the electrostatic forces tending to hold the charged particles in the vicinity of the reactor wall exceed the resistive forces acting in the bed tending to move the particle away from the reactor wall, the catalyst-containing layer of polymerizable resin particles Heat transfer from this layer is not sufficient to remove the heat of polymerization because the non-fluidized layer in the vicinity of the wall is in less contact with the fluidizing gas than the particles in the fluidized portion of the bed. The heat of polymerization raises the temperature of the non-fluidized bed in the vicinity of the reactor wall until the particles melt and fuse together. At this point, the other particles in the fluidized bed adhere to the fused bed and increase in size until it detaches from the reactor wall. It is known that the separation of the insulation from the conductor (separation of the plate from the reactor wall) generates more static electricity, which accelerates the formation of the next plate.

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Numerous methods have been described in the art for reducing or eliminating static voltage. These methods (1) reduce the rate of charge generation, (2) increase the rate of discharge of the electric charge, and (3) neutralize the electric charge. Some methods that can be applied to a fluidized bed (2) use additives to improve the conductivity of the particles to provide a discharge path, (2) install grounding means in the fluidized bed to provide an additional area for discharging electrostatic charges to the ground, (3) ionize gas or particles by electric discharge, to generate ions that neutralize the electrostatic charges of the particles, and (4) radioactive sources are used to produce radiation whose ions generated by the radiation neutralize the electrostatic charges of the particles. The application of these techniques to commercial fluidized bed polymerization reactors may not be profitable or practical. Not all additives used may poison the polymerization catalyst and may not adversely affect the quality of the product. Until now, it has been assumed that water, which is the most widely used additive to reduce static electricity in particles, cannot be used because it is a potent catalyst poison.

However, it has been found within the scope of the invention that in certain specific reactions, for example when type IV and V catalysts are used in combination with alkylaluminum cocatalysts in a fluidised polymerization process, the addition of controlled small amounts of water to the reactor dramatically reduces the occurrence of sheet formation. The amount of water fed to the reactor depends on the static charges present in the reactor.

Water can be added by simply modifying traditional procedures. Referring again to Figure 1, 22, an inert gas such as dry nitrogen from a nitrogen source is introduced to a point commonly known in the art as an "O'Brien box", designated 42 in the figure. A 'Brien box generally comprises one or more distilled water tank containing water, and is equipped with temperature and flow control devices, none of which are shown. Nitrogen is bubbled through one water tank, which is a one-liter stainless steel cylinder containing distilled water, the temperature of the jacket of which can be adjusted. The flow of water-saturated nitrogen leaving O'Brien's box 42 along line 44 is then controlled and passed along a line comprising a temperature sensor to an olefin, e.g., ethylene, feed line 46 and further to the reaction cycle. The resulting water content in ethylene is generally less than one part per million by volume. The addition of water can be ensured by a moisture analyzer 48 in the ethylene supply line 46. The nitrogen flow is typically adjusted to about 0.. . 11 lb / h (approx. 0... 5 kg / h) with an ethylene feed of 0.. . 50,000 lb / h (approx. 0... 22680 kg / h). When the temperature of the water cylinder is 20 ° C and the nitrogen overpressure is 350 pe (pressure approx. 2.5 MPa), the water content is 0 ... 0.3 ppm. This concentration can be varied as desired by changing the water temperature or nitrogen pressure.

For the sake of clarity only, the following is a way to calculate the concentration obtained by adding water to the reactor: 1. The water vapor pressure at the temperature in question in the O 'Brien box is determined (Phso).

2. The nitrogen flow is determined using the full orifice (WNa).

3. The nitrogen pressure is determined as the sum of the hydrostatic pressure (D * »a) at the bottom of the reactor and the pressure drop in the line.

4. The ethylene flow to the reactor is determined (Hczh *).

5. Nitrogen is assumed to be saturated with water: Ρη »ο x Wm2 x _1_8 = Water flow (WHao)

Pm2 28 23 91415 WHao x 28 x 10 * * ppm HaO in ethylene feed

HcaH4 18 at 20 ° C PHa © = 0.339 psia (approx. 2.34 kPa)

Pn2 = 325 psia (approx. 2.24 MPa) when the reactor pressure is 300 psig (pressure approx. 2, 17 MPa)

Wua = 0 ... 11.36 ppm typical flow at 3 ppm W © 2h4 * = 18,000 pph

Water content = (0.333) (3) (10) = 0.174 ppm 325 18,000

Static voltages in the range of -15,000 to +15,000 V can be monitored in the reactor in the vicinity of its wall by one or more static voltage indicating devices 50 located in the reactor bed about five feet (about 1.5 m) above the distribution plate. As the reaction proceeds, the change of the static voltage from neutral to positive can be resisted by supplying moist nitrogen to the ethylene stream. If this is not done, then the imminent formation of agglomerations is likely to lead to process disturbances. Care must be taken when adding water to avoid excessive water concentrations, which can lead to undesirably high negative static voltages.

The system is operated by a number of flow and shut-off valves which are common in the art and which are therefore not shown. In addition, line 44 is preferably insulated and steam is determined prior to joining the gas supply line 46.

The polymers which are the main object of the present invention, which involve the problematic sheet formation described above in the presence of titanium catalysts, are linear homopolymers of ethylene or linear copolymers of ethylene and one or more Ca ... C * alpha-olefins with a high molar content of ethylene (* 90%). and the molar fraction of other alpha-olefins is small (£ 10%). Cs. . . No carbon atom of the Ce-alpha-olefins should comprise any branch chain closer than the fourth carbon atom. Preferred Cs. . . Ce-alpha-olefins include propylene, butene-i, pentene-1, hexene-1,4-methyl-pentene-1, heptene-1 and octene-1. This description is not intended to preclude the application of this invention to alpha-olefin homopolymer and copolymer resins in which ethylene is not a monomer.

These homopolymers and copolymers have a density of about 0.97 to 0.91. In the case of a certain suiting index, the density of the copolymer is mainly controlled by Cs copolymerized with ethylene. . . Amount of Ce comonomer. Thus, the addition of gradually increasing amounts of comonomers to the copolymer results in a gradual decrease in the density of the copolymer. Each of the numerous Cs. . . The amount of Ce comonomer required to achieve the same result varies from one monomer to another under the same reaction conditions. In the absence of comonomer, ethylene homopolymerizes.

The suiting index of a homopolymer or copolymer depends on its molecular weight. Polymers with a relatively high molecular weight have a relatively high viscosity and a low viscosity.

A typical way to use the present invention to reduce sheet formation is, for example, that the reactor vessel of Figure 1, which is prone to sheet formation problems when polymerizing the materials described above using type IV and V catalysts with an alkylaluminum cocaine catalyst, is partially filled with granular polyethylene resin. with a non-reactive gas such as nitrogen and fluidized by circulating said non-reactive gas through the reactor at a rate greater than the minimum rate (Gmf) required to fluidize the granular polyethylene, preferably 3-5 Gmf. The reactor is brought to operating temperature by means of a gas and the reaction is started by placing a catalyst and a cocatalyst in the reactor. For the time of reaction 25,91415, the static voltage values approach the values leading to plate formation, after which the pressure, temperature, and flow control in the O'Brien box is increased to saturate the nitrogen with water. Nitrogen saturated with water is then fed to the gas supply line and further to the reactor. Voltage changes in the reactor as a result of the aqueous gas supply are monitored and this procedure is continued until the values of static voltages are obtained in areas that do not cause plate formation.

The general nature of the invention has thus been illustrated and specific embodiments thereof are illustrated in the following examples. It should be noted, however, that the invention is not limited to these examples, but various modifications may be made.

Examples 1 and 2 were carried out in a conventional bed reactor. The catalyst used was a Ziegler-type titanium-based catalyst bound to the surface of porous silica as previously described in connection with type IV. The cocatalyst used was triethylaluminum. The products prepared in the examples were copolymers of ethylene and 1-butene. Hydrogen was used as a chain transfer agent to control the suffocation index of the polymer.

Example 1

The fluidized bed reactor was started under operating conditions for the production of a film-grade low-density ethylene copolymer product having a density of 0.918, a melt index of 1.0 and a tack temperature of 104 ° C. The reaction was started by feeding the catalyst to a reactor. a bed of granular resin. The catalyst was a mixture of 5.5 parts of titanium tetrachloride, 8.5 parts of magnesium chloride and 14 parts of tetrahydrofuran coated with 100 parts of silica (Davison grade 955). The silica was dried at 600 ° C and treated with four parts of triethylaluminum before coating, and the catalyst was activated with 35 parts of tri-n-hexylaluminum after coating. Prior to starting the catalyst feed, the reactor and resin pipette were brought to an operating temperature of 85 ° C and the impurities were flushed out by circulating nitrogen through the resin bed. Ethylene, butene, and hydrogen concentrations were set at 53, 24, and 11 percent, respectively. The cocatalyst was fed at a rate of 0.3 parts of tri e style aluminum a per part of catalyst.

The reactor was started normally. After the product 011 was produced for 29 hours, 6.5 times the weight of the fluidized bed, the temperature was found to rise 1 ... 2 * C above the temperature of the bed with thermocouples placed just inside the reactor wall, at a height corresponding to half the diameter of the reactor in the gas distribution plate. in relation to. Previous experience had shown that such temperature deviations are a positive indication that resin sheets are formed in the fluidized bed. At the same time, the bed voltage (measured at a 1/2 inch (about 1.7 cm) diameter spherical electrode located one inch (about 2.54 cm) from the reactor wall above the gas distribution plate, at a height corresponding to the reactor side diameter) electrostatic voltmeter) increased by about + 1500.. . +2000 V above +5000 V and then fell back to +2000 V over a 3 minute period. The change in temperature and voltage continued for an estimated 12 hours, with the frequency and magnitude of the changes increasing. During this period, sheets of fused polyethylene resin appeared in the resin product. Signs of plate formation intensified, i.e., temperature deviations grew as large as 20 ° C above the bed temperature and the deviations remained greatest over long periods of time, while voltage changes became the most common. The reactor was shut down due to the intensity of plate formation.

27 91415

Example 2

The fluidized bed reactor used in Example 1 was started and prepared into a linear, low density ethylene copolymer that could be extruded or rotoformed having a density of 0.934, a melt index of 5, and an adhesion temperature of 118 ° C. The reaction was initiated by feeding a catalyst as described in Example 1, except that it was activated with 28 parts of tri-n-hexylaluminum, into a reactor formed with a bed of granular resin similar to the product to be prepared in advance. Before starting the catalyst feed, the reactor and resin bed were brought to an operating temperature of 85 ° C and the impurities were purged with nitrogen. The concentrations of ethylene, butene and hydrogen fed to the reactor were 52%, 14% and 21%, respectively. Triethylaluminum used as a cocatalyst was fed at 0.3 parts per part of catalyst. The reactor was allowed to operate continuously for 48 hours, during which time 9 times the amount of resin containing the bed was produced. After this 48 hours of trouble-free operation, plates formed of resin fused with the normal granular product began to leave the reactor. In this case, the voltages measured above the distribution plate at a height corresponding to the reactor side diameter were +2000 V, with a range of 0 ... + 10,000 V, while surface thermocouples at the same height showed deviations of 15 ° C above the bed temperature. Two hours after the first plates were detected in the product from the reactor, the feed of catalyst and cocatalyst to the reactor had to be stopped to reduce the rate of resin formation because the plates blocked the resin removal system. Catalyst and cocatalyst feed was restarted one hour later. The formation of plates continued and after two hours the feed of catalyst and cocatalyst was stopped again and the reaction was stopped by injecting carbon monoxide. The voltage was then +12,000 V and the deviations measured by the thermocouple occurred until toxin was injected into the reactor. The reactor was operated for a total of 53 hours and produced 10.5 volumes of resin before stopping the reaction due to plate formation.

28

The following example illustrates the prevention of plate formation by adding water to the gas feed when high voltages are present in the reactor.

Example 3

The reactor of Examples 1 and 2 was modified as shown in Figure 1 to continuously produce a high density film grade polyethylene resin having a density of 0.946, a flow index of 7.5 and a tack temperature of 124 ° C. The product was an ethylene-hexene copolymer having a vanadium-based catalyst together with an aluminum alkyl cocatalyst and a halogen extender were used for the polymerization. The catalyst contained 0.29 millimoles of vanadium per gram of precursor and 1.2% aluminum added to diethyl aluminum chloride, and the catalyst was bound to supported 30-30 micron Davison silica. The reaction was allowed to proceed under a bed temperature of 98 ° C and 76% ethylene, 1.6% hydrogen, 1.2% hexene were present at an absolute pressure of 315 psi (about 2.17 MPa) in the reactor. the remainder being inert gases such as nitrogen, methane, isopentane, etc. The cocatalyst was adjusted by feeding triethylaluminum so that the concentration remained at 200 ppm (v) in the resin produced. Freon was fed as a promoter by feeding 0.7 moles of freon for each mole of triethylaluminum. The production rate was kept substantially at 20,000 pph, giving a space-time yield of 5 mlbs / h / ft3 petit volume.

After 18 hours of stable production, the static voltage, measured 5 feet (about 1.5 m) above the distribution plate, near the reactor wall, began to increase gradually during production. Voltage development was noted 1.. . Every 5 minutes as small, short-lived static peaks with a voltage of 0 ... 100-300 V. The trend continued upward as the brown line shifted from 0 volts to 1000 ... 5000 volts and the static peaks increased by 10,000. . 15,000 volts as their frequency increased all the time. These static peaks were associated with deviations in surface temperatures measured 3 to 6 feet (approximately 0.9 to 1.8 m) above the distribution plate, from the reactor wall. These deviations were generally negative, indicating the insulating effect of the resin accumulation adhering to the wall. If the reaction is allowed to continue, sheet formation is observed in the reactor, which ultimately necessitates stopping the reactor as a result of clogged exhaust systems or a clogged distribution plate, such clogging eventually causing large agglomerations to form during Lei entrenching.

In this situation, water was added to the reactor by passing a nitrogen stream of about 5 lb / h (about 2.27 kg / h) at a temperature of 20 ° C and a pressure of 350 psig (pressure about 2.51 MPa) first through a cylinder containing distilled water. and then further to the ethylene feed line. The resulting water content in the ethylene feed was 0.2 ppm (v). The addition was adjusted to bring the static level close to zero. In this case, care was taken that the amount of water fed was not too large, which too much water can lead to undesired negative static deviations, which can also lead to plate formation.

The static level was adjusted to close to zero, plate formation was avoided, and the operation of the reactor remained stable without undesired downtime due to plate formation.

Example 4

The modified reactor of Example 3 was used to prepare a linear, low density film resin. The resin produced having a density of 0.917 and a melting index of 2.7 was an ethylene-hexene copolymer having an adhesion temperature of 102 ° C. The catalyst used was titanium-based and bound to a supported silica. The titanium content was 0.25 mmol per gram of precursor. Magnesium chloride, diethylaluminum chloride and tri-normal hexylaluminum were added so that the molar ratios based on the titanium content were 3, 0.02 and 0.02, respectively. The silica base was a Davison 955 with a size range of 10 to 80 microns. The reaction was carried out at a bed temperature of 76 ° C in the presence of 29% ethylene, 11% hydrogen, 5% 1-hexene, with a final concentration of inert nitrogen, ethane, methane and isopentane. The cocatalyst was fed to give a triethylaluminum content of 300 ppm (w) in the resin. The productivity of the catalyst under these conditions was 2200 kg of polyethylene produced per kg of catalyst. Production rates were 18,000 lb / h (about 8170 kg / h), i.e. the space-time yield was 4.5.

The rapid increase in ethylene content led to a clear improvement in catalyst activity. The static voltage in the vicinity of the reactor wall increased from almost zero to 6000 volts over a period of 10 minutes. Wall surface temperatures were found to rise, indicating rapid formation of polymer sheets on the reactor wall at a height of 6 feet (about 1.8 m) above the distribution plate. If the reaction was allowed to proceed, shutdown of the reactor was inevitable due to product clogging.

The addition of water was started by passing a stream of nitrogen of 4 lb / h (about 1.81 kg / h) through a water cylinder with a cylinder jacket temperature adjusted to 20 ° C. The resulting water content in ethylene was less than 0.2 ppm. The static voltage quickly returned to near zero. Reactor surface temperature deviations decreased in ten minutes and normal reactor operation was restored.

k

Claims (22)

91415 The patent was claimed by et al A process for reducing sheet formation during the polymerization of alpha-olefins in a low pressure fluidized bed reactor using compounds based on titanium or vanadium as catalysts in combination with alkylaluminum cocatalysts, characterized in that water is introduced into said reactor in an amount corresponding to about 0.1 .. 2 ppm based on the ethylene feed and in an amount sufficient to maintain the electrostatic levels at the potential point of sheet formation at values that avoid sheet formation without substantially altering the efficiency of said catalysts. Process according to Claim 1, characterized in that one of the alpha-olefins is ethylene. A method according to claim 2, characterized in that water is added to the reactor by passing a pressurized inert gas at a controlled flow rate through an aqueous tank whose temperature is controlled to add water to this inert gas, after which the aqueous inert gas is directed from the temperature controlled then this mixture is introduced into the reactor. Process according to Claim 3, characterized in that the inert gas is nitrogen. A method according to claim 4, characterized in that the nitrogen gas flow rate, the ethylene flow rate and the temperature of the water contained in said tank are controlled and determined by the static levels present in the reactor. Process according to Claim 5, characterized in that the water content of the mixture fed to the reactor is less than one part per million by volume, based on the ethylene feed. Process according to Claim 5, characterized in that the water content of the mixture introduced into the reactor is approximately 0, 1 by volume. . 2 ppm of ethylene feed. Process according to Claim 5, characterized in that the nitrogen flow rate varies in the range from 0 to 11 lb / h (approx. 0.... 5 kg / h) with an ethylene feed of about 0.. . 50,000 lb / h (approx. 0 ... 22680 kg / h). 9. A process for reducing sheet formation in the production of polyolefins by polymerizing alpha-olefins in a low pressure fluidized bed reactor using titanium or vanadium-based compounds as catalysts in combination with a stream of water-controlled alkylated aluminum catalysts, characterized in that water is added to said reactor through which the temperature of the tank is controlled to add water to this inert gas, the water-containing inert gas is directed from the temperature controlled tank and mixed with alpha-olefins, and this mixture is introduced into the reactor, and in which water is added to said reactor in an amount corresponding to about 0.1. . .2 ppm based on the ethylene feed and in an amount sufficient to maintain electrostatic levels at the point where the sheets are formed at values that avoid sheet formation without substantially altering the efficiency of said catalysts. A method according to claim 9, characterized in that said inert gas is nitrogen. Process according to Claim 10, characterized in that the flow rate of nitrogen gas, the flow rate of α1-olefins and the temperature of the water contained in said tank are adjusted and adjusted on the basis of the static levels present in the reactor. Process according to Claim 9, characterized in that the water content of the mixture fed to the reactor is less than one part per million by volume, based on the ethylene feed. Process according to Claim 9, characterized in that the water content of the mixture fed to the reactor is approximately 0.1 to 2 ppm by volume, based on the ethylene feed. Process according to Claim 10, characterized in that the nitrogen flow rate varies in the range from 0 to 11 lb / h (approximately 0... 5 kg / h) with an alpha-olefin feed of about 0 to 50,000 lb / h (approx. 0 ... 22680 kg / h). A method according to claim 9, characterized in that the temperature of the water in the tank is regulated in the range of about 10 to 40 ° C. Process according to Claim 9, characterized in that the polyolefins are linear homopolymers of ethylene or linear copolymers containing a high molar proportion (s 90%) of ethylene and a low molar proportion (s 10%) of one or more Cs. . . Ce alpha-olefin. Process according to Claim 16, characterized in that the polyolefins are homopolymers or copolymers of propylene, butene-1, pentene-1, hexene-1,4-methylpentene-1, heptene-1 or octene-1. 18. A process for reducing sheet formation in the production of polyolefins by polymerizing alpha-olefins in a low pressure fluidized bed reactor using titanium or vanadium-based compounds as catalysts in combination with alkylaluminum cocatalysts. At a flow rate of 11 lb / h (about 0... 5 kg / h) when the alpha-olefin feed rate is about 0 ... 50,000 lb / h (about 0 ... 22680 kg / h), about 10. ..40 ° C through an aqueous tank whose temperature is controlled to add water to this nitrogen gas, the aqueous nitrogen gas is directed from the temperature controlled tank and mixed with alpha-olefins, and then this mixture is introduced into the reactor, whereby water is added to said to the reactor in an amount of less than about one part per million by volume of ethyl from the feed. Process according to Claim 18, characterized in that the nitrogen gas flow rate, the alpha-olefin flow rate and the temperature of the water contained in the tank are controlled and adjusted on the basis of the static levels present in the reactor. Process according to Claim 18, characterized in that the water content of the mixture introduced into the reactor is from about 0.1 to 2 ppm by volume, based on the ethylene feed. Process according to Claim 18, characterized in that the polyolefins are linear homopolymers or linear copolymers of ethylene which contain a high molar proportion (έ90%) of ethylene and a low molar proportion (1010%) of one or more Cs. . . Cs alpha-olefin. Process according to Claim 21, characterized in that the polyolefins are homopolymers or copolymers of propylene, butene-1, pentene-1, hexene-1,4-methylpentene-1, heptene-1 or octene-1. 91415