NZ212052A - Method of reducing sheeting during polymerisation of alpha-olefines in fluidised bed reactor - Google Patents

Description

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£Q8.f iO |corf,C3Qr?C^-I^O" ;nn L/a'iO: ••••»• 'Vi' " v : j.aQ5 Publication usv..-. \G. Journal, NEW ZEALAND PATENTS ACT, 1953 No.: Dite: COMPLETE SPECIFICATION "PROCESS FOR REDUCING SHEETING DURING POLYMERIZATION OF ALPHA-OLEFINS" %IV», UNION CARBIDE CORPORATION , a corporation organized under the laws of the State of New York, United States of America, located at Old Ridgebury Road, Danbury, State of Connecticut 06817, United States of America, hereby declare the invention for which lx/ we pray that a patent may be granted to hook/us, and the method by which it is to be performed, to be particularly described in and by the following statement:- (followed by page la) *7* * -«*> .£" _ I* ,:* I « 13094 2 , • • PROCESS FOR REDUCING SHEETING DURING POLYMERIZATION OF ALPHA-OLEFINS Conventional low density polyethylene has been historically polymerized in heavy walled autoclaves or tubular reactors at pressures aa high as 50,000 psi and temperatures up to 300*C. or higher. The ■olecular structure of high pressure, low density polyethylene (HP-LDPE) is highly complex. The permutations in the arrangement of their almple building blocks are essentially infinite. HP-LDFE's are characterized by an intricate long chain branched molecular architecture. These long chain branches have a dramatic effect on the melt rheology of these resins. HP-LOPE's also possess a spectrum of short chain branches, generally 1 to 6 carbon atoms in length.

These short chain branches disrupt crystal formation and depress resin density.

More recently, technology has been provided whereby low density polyethylene can be produced by fluldized bed techniques at low pressures and temperatures by copolymerizlng ethylene with various alpha olefins. These low pressure LDPE (LP-LDPE) resins generally possess little, if any, long chain branching and are sometimes referred to as linear LDPE resins.

They are ahort chain branched with branch length and la *'13904 • »-J ,~v frequency controlled by the type and amount of cooonomer used during polymerisation.

As la well known to thoae skilled in the art, low pressure, high or low density polyethylenes can now be conventionally provided by a fluldized bed process utilizing several families of cstalysts to produce a full range of low density and high density products. The appropriate selection of cstalysts to be utilized depends in part upon the type of end product desired, I.e., high density, low density, extrusion grade, film grade resins and other criteria.

The various types of catalysts which may be used to produce polyethylenes in fluid bed reactors can generally be typed as follows: Type I. The ailyl chrornate catalysts disclosed in U.S. Patent No. 3,324,101 to Baker and Carrick and U.S. Patent No. 3,324,095 to Carrick, Karaplnka and Turbet. The ailyl chromate catalysts are characterized by the presence therein of a group of the formula: wherein R is a hydrocarbyl group having from 1 to 14 carbon atoms. The preferred ailyl chromate cstalysts are the bisCtriarylsilyl) chromates and more preferably bisCtriphenylsilyl) chromate.

R Si » 0 II Cr y 212052 | i This catalyst is used on a support such •s silica, alumina, thoria, circonia and the lilce. ' | other supports such as carbon black, micro-crystalline cellulose, the non-sulfonated ion exchange resins and the like nay be used.

] Type II. Ths bis(cyclopentadienyl) chromium (II) compounds disclosed in U.S. Patent No. 3,879,368.

• These bis(cyclopentadienyl) chromium (II) compounds have the following formula: wherein R' and R" may be the same or different to C2q< inclusive, hydrocarbon radicals, and n' and n" nay be the same or different and may be 0 or 1 to 5 Inclusive. The R' and R" hydrocarbon radicals may be saturated or unsaturated, and can include aliphatic, alicyclic and aromatic radicals such as methyl, ethyl, propyl, butyl, pentyl, cyclopentyl, cyclohexyl, ailyl, phenyl and naphthyl radicals.

These catalysts are used on a support as heretofore described.

Type III. The catalysts as described in U.S. Patent No. 4,011,382. These catalysts contain chromium and -- " ■ ... .13094 3 13094 2120D2 titanium in the form of oxides and, optionally, fluorine and a support. The catalysts contain, based on the combined weight of the support and the chromium, titanium and fluorine, about 0.05 to 3.0. and preferably about 0.2 to 1.0, weight percent of chromium (calculated as Cr), about 1.5 to 9.0, and preferably about 4.0 to 7.0, weight percent of titanium (calculated as Ti), and 0.0 to about 2.5, and preferably about 0.1 to 1.0 weight percent of fluorine (calculated as F). the Type III catalysts Include CrO^, or any compound of chromium which is oxldizable to CrO^ under the activation conditions employed. At least a portion of the chromium in the supported, activated catalyst oust be in the hexavalent state. Chromium compounds other than CrO^ which may be used are disclosed in U.S. Patent 2,825,721 and U.S. Patent 3,622,521 and include chromic acetyl acetonate, chromic nitrate, chromic acetate, chromic chloride, chromic sulfate, and ammonium chromate. include all those which are oxldizable to T102 under the activation conditions employed, and include those disclosed in U.S. Patent No. 3,622,521. ■ The chromium compounds which may be used for The titanium comppunds which may be used f* ' . V 4 212052 13094 The fluorine compounds which may be used include HF, or any compound of fluorine which will yield HF under the activation conditions employed.

The Inorganic oxide materials which may (2^ be used as a support in the catalyst compositions are porous materials having a high aurface area, that 16, 10 a aurface area In the range of about 50 to about 1000 square meters per gram, and an average particle size of about 20 to 200 microns. The inorganic oxides which may be used Include silica, alumina, thoria, zirconia and other comparable Inorganic oxides, as well as mixtures of such oxides.

Type IV. The catalysts as described in U.S. Patent 4,302,566 and entitled, "Preparation of Ethylene Copolymers in Fluid Bed Reactor".

These catalysts comprise at ^ least one titanium compound, at least one magnesium compound, at least one electron donor compound, at least one activator compound and at least one inert carrier material.

The titanium compound has the structure °\ Ti(OR)aXh ■ - " wherein R is a Cx to C1A aliphatic or aromatic ti iI ii hydrocarbon radical, or COR* where R* is a C^ to Cj^ aliphatic or aromatic hydrocarbon radical; X la CI, Br, or I; a ic 0 or 1; b ia 2 to U Inclusive; and a + b • 3 or 4.

The titanium compounds can be used Individually or in combination thereof, and would Include TiCl3, T1C14. T1(0CH3)C13. Ti(OC6H5)Cl3. Ti(OCOCH3)Cl3 and Ti(OCOC6H5)Cl3.

The magnesium compound has the structure HgX2 wherein X la CI, Br, or 1. Such magnesium compounds can be used individually or in combinations thereof and would include MgC^, "HgBrj and Mg^- Anhydrous MgCl2 is the preferred magnesium compound.

The titanium compound and the magnesium compound are generally used in a form which will facilitate their dissolution in the electron donor compound.

The electron donor compound is an organic compound which is liquid at 25*C and in which the titanium compound and the magnesium compound are partially or completely soluble. The electron donor compounds are known as such or as Lewis bases.

The electron donor compounds would include auch compounds as alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic ethers, cyclic ethers and aliphatic ketones. ■hn. -• - • ,1 2 l 13U94 • ^0; The catalyst Bay be modified with a boron fcallde compound having the structure brcx'3.c wherein R is an aliphatic or aromatic hydrocarbon radical containing from 1 to 14 carbon atoms or OR' , wherein R' is also an aliphatic or aromatic hydro- & carbon radical containing from 1 to 14 carbon atoms, X' is selected from the group consisting of CI and Br, or mixtures thereof, and ^ c Is 0 or 1 when R is an aliphatic or aroma tic hydrocarbon and 0, 1 or 2 when R is OR'.

The boron halide compounds can be used individually or in combination thereof, and would include BC13> BBr^, B(C2H5)C12. B(0C2H5)C12, b(oc2h5)2ci, b(c6h5)ci2, b(oc6h5)ci2, b(c6h13)ci2, B(OCgHjj)Cl2, and B(0C^H^)2C1. Boron trichloride is the particularly preferred boron compound.

The activator compound has the structure Al(r")cX'dhe wherein X' is CI or OR^; R^ and R" are the same or different and are C^ to Cj^ saturated hydrocarbon radicals, d is 0 to 1.5, a is 1 or 0, and c + d + e - 3.

Such activator compounds can be used individually or in combinations thereof.

The carrier materials are solid, particulate materials and would Include Inorganic materials such as oxides of silicon and aluminum and molecular aleves, and organic materials such as olefin polymers, e.g., polyethylene. -j « 7 2 1 205 130% t I In general, the above catalysts are Intro-duced together with the polymerlzable materials, into a reactor having an expanded aection above a straight-aided aectlon. Cycle gas antfrs the -bottom of the reactor and passes upward through a gas distributor plate Into a fluldized bed located In the straight-aided section of the vessel. The gas distributor plate serves to ensure proper gas distribution and to support the resin bed when gas flow Is stopped.

Cas leaving the fluldized bed entrains resin particles. Most of these particles are dis-•ngaged as the gas passes through the expanded section where its velocity is reduced.

The operating difficulties associated with the utilization of catalyst types I through III in the above described reactors have been substantially eliminated, resulting in the economic and efficient production of low pressure, low or high density polyethylene resins which have a wide variety of uses.

In order to satisfy certain end use applications for ethylene resins, such as for film, Injection molding and roto molding applications, catalyst type IV has been used. However, attempts to produce certain ethylene resins utilizing the type IV catalysts supported on a porous silica substrate in 8 2 1 205 2 13094 ■ > certain fluid bed reactors, have not been entirely satisfactory from a practical cotonerclal standpoint.

This is primarily due to the formation of "aheets" in the reactor after a brief period of operation. The "aheets" can be characterized as constituting a fused polymeric material.

The aheets vary widely in else, but are aimilar in most respects. They are usually about 1/4 to 1/2 inch thick and are from about one to five feet 10 long, with a few specimens even longer. They hsve a width of about 3 Inches to more than 16 inches. The # aheeta have a core composed of fused polymer which is oriented in the long direction of the sheets and their aurfaces are covered with granular resin which has fused to the core. The edges of the sheets have a hairy appearance from strands of fused polymer.

After a relatively short period of time during polymerization, aheets begin to appear in the reactor, and these sheets plug product discharge systems 20 forcing shutdown of the reactor.

Accordingly, it will be seen that there presently exists a need to improve the polymerization techniques necessary for the production of poly-olefin products utilizing titanlun based catalysts in fluldized bed reactors. y f &- ^ J30& U 5 ^ Ic Is therefore an object of the present invention to provide a process to substantially reduce or eliminate the amount of sheeting which occurs during the low pressure fluldized bed polymerization of alpha olefins utilizing titanium based compounds as catalyst.

Another object is to provide a process'for treating fluldized bed reactors utilized for the production of polyolefin resins utilizing titanium based catalyst or other catalysts vhich result In similar sheeting phenomena.

These and other objects will become readily apparent from the following description taken in conjunction with the accompanying drawing which generally Indicates a typical gas phase fluldized bed polymerization process for producing high density and low density polyoleflns.

Broadly contemplated, the present invention provides an improvement in the method for polymerization of alpha olefins in a fluid bed reactor utilizing titanium based catalysts or other catalysts prone to cause sheeting during said polymerization, the improvement comprising maintaining the static electric charge in said reactor at the site of possible sheet formation below static voltage levels which would otherwise cause sheet formation. 2 0 The critical static voltage level for sheet formation la a complex function of resin sintering temperature, operating tempersture, drag forces in the fluid bed, resin particle size distribution and recycle gas composition. The static voltage can be reduced by a variety of techniques such as by treating the reactor surface to reduce static electric generation, by Injection of an antistatic agent to increase particle surface electrical conductivity thus promoting particle discharging; by installation of appropriate devices connected to the reactor vails vhich are designed to promote •lectrical discharging by creating areas of high localized field strength, and by neutralization of charges by the injection or creation of ion pairs, ions or charged particles of the opposite polarity from the resin bed.

A particularly preferred technique generally involves treating the reactor vessel prior to polymerization by Introducing a chromium containing compound into the reaction vessel in a non-reacting atmosphere.

Referring particularly to the sole figure of the drawing, a conventional fluldized bed reaction system for polymerizing alpha-olefins Includes a reactor 10 which consists of a reaction zone 12 and a velocity reduction zone 14. 11 2 1205 13094 The reaction zone 12 includes a bed of growing polymer particles, formed polymer particles and a minor amount of catalyst particles fluldized by the continuous flow of polymerlzable and modifying gaseous components In the form of make-up feed and recycle gas through the reaction zone. To maintain a viable fluldized bed, the mass gas flow rate through the bed is normally maintained above the minimum flow required for fluldization, and preferably from about 1.5 to about 10 times Gmf and more preferably from about 3 to about 6 times Gmf. is used in the accepted form as the abbreviation for the minimum gas flow required to achieve fluldization, C. Y. Ven and Y. H. Yu, "Mechanics of Fluldization", Chemical Engineering Progress Symposium Series, Vol. 62, p. 100-111 (1966).

It is highly desirable that the bed always contains particles to prevent the formation of localized "hot spots" and to entrap and distribute the particulate catalyst throughout the reaction zone. On start up, the reactor is usually charged with a base of particulate polymer'particles before gas flow is initiated. Such particles may be identical in nature 12 2 120' 13094 to the polymer to be formed or different therefrom. When different, they ere withdrawn with the desired formed polymer particles me the first product. Eventually, a fluldized bed of the desired polymer particles aupplants the start-up bed.

The appropriate catalyst used In the fluldized bed la preferably stored for service in a reservoir 16 under a blanket of a gas vhich is inert to the stored material, such as nitrogen or argon.

Fluldization is achieved by a high rate of gas recycle to and through the bed, typically in the order of about 50 times the rate of feed of make-up gas. The fluldized bed has the general appearance of a dense mass of viable particles in possible free-vortex flow as created by the percolation of gas 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. It is thus dependent on the geometry of the reactor.

Make-up gas is fed to the bed at a rate equal to the rate at vhich particulate polymer product la withdrawn. The composition of the make-up gas is determined by a gas analyzer 18 positioned above the bed. The gas analyzer .determines the composition of the gas being recycled and the composition of the make-up gas is adjusted accordingly to maintain an 13 ^ 1 IP ip 13094 essentially steady state gaseous composition within the reaction cone.

To insure complete fluldization, the recycle gas and. where desired, part or all of the make-up gas are returned to the reactor at base 20 below the bed. Cas distribution plate 22 positioned above the point of return ensures proper gas distribution and O «l»o supports the resin bed when gas flow is stopped.

The portion of the gas stream which does not 10 react in the bed constitutes the recycle gas which is removed from the polymerization zone, preferably by w passing it into velocity reduction zone 14 above the bed where entrained particles are given an opportunity to drop back into the bed.

The recycle gas is then compressed in a compressor 24 and thereafter passed through a heat exchanger 26 wherein it is stripped of heat of reaction before it is returned to the bed. By constantly removing heat of reaction, no noticeable temperature gradient appears 20 to exist within the upper portion of the bed. A temperature gradient will exist in the bottom of the bed in a layer of about 6 to 12 Inches, between the temperature of the inlet gas and the temperature of the remainder of the bed. Thus, it has been observed that the bed acts to almost inanedistely adjust the temperature of the recycle gas above this bottom layer of the bed zone to make It conform to the temperature of the remainder of the bed thereby maintaining Itself at an essentially constant temperature under steady conditions. The recycle is then returned 14 7 i n 13094 ' ^ to the reactor at ltc bate 20 and to the fluldized bed through distribution plate 22. The compressor 24 can also be pieced downstream of heat exchanger 26.

Hydrogen nay be used as a chain transfer agent for conventional polymerization reactions of the types contemplated herein. In the case where ethylene is used as a monomer the ratio of hydrogen/ethylene employed will vary between about 0 to about 2.0 noles of hydrogen per nole of the monomer in the gas atream.

Any gas Inert to the catalyst and reactants can also be present in the gas stream. The cocatalyst is added to the gas recycle stream upstream of its connection with the reactor as from dispenser 28 through line 30.

As is well known, it is essential to operate the fluid bed reactor at a temperature below the sintering temperature of the polymer particles. Thus to Insure that sintering will not occur, operating temperatures below sintering temperature are desired. For the production of ethylene polymers an operating temperature of from about 90 to 100*C is preferably used to prepare products having a density of about 0.94 to 0.97 while a temperature of about 75* to 95*C is preferred for products having a density of about .91 to .94.

Normally the fluid bed reactor is operated a pressures of up to about 1000 psi, and is preferably 13094 052 operated at a pressure of from about 150 to 350 psl. with operation at the higher pressures In such ranges favoring heat transfer since an Increase In pressure Increases the unit volume heat capacity of the gas.

The catalyst is injected Into the bed at a rate equal to Its consumption at a point 32 vhich is above the distribution plate 22. A gas vhich 1$ Inert to the catalyst such as nitrogen or argon Is used to carry the catalyst Into the bed. Injecting the catalyst at a point above distribution plate 22 Is an Important feature. Since the cstalysts normally used are highly active, injection into the area below the distribution plate may cause polymerization to begin there and eventually cause plugging of the distribution plate. Injection into the viable bed, instead, aids in distributing the catalyst throughout the bed and tends to preclude the formation of localized spots of high catalyst concentration vhich may result in the formation of "hot spots".

Under a given set of operating conditions, the fluldized bed is maintained at essentially a constant height by vithdrawing a portion of the bed as product at a rate equal to the rate of formation of the particulate polymer product. Since the rate of heat generation is directly related to product formation, a measurement of the temperature rise of the lb 2 12052 13094 I" •cross the resctor (the difference between inlet gas temperature and exit gas temperature) is determinative of the rate of particulate polymer formation.st a constant gas velocity.

The particulate polymer product is preferably withdrawn at a point 34 at or close to distribution plate 22. The particulate polymer product is conveniently and preferably withdrawn through the sequential operation of a pair of timed valves 36 and 38 defining a segregation zone 40. Vhile valve 38 is closed, valve 36 is opened to emit a plug of gas 4 and product to the zone 40 between it and valve 36 which is then closed. Valve 38 is then opened to deliver the product to an external recovery zone and after delivery, valve 38 is then closed to await the next product recovery operation.

Finally, the fluldized bed reactor is equipped with an adequate venting system to allow venting the bed during the start up and shut down. The reactor does not require the use of stirring means and/or wall scraping means.

The reactor vessel is normally constructed of carbon steel and is designed for the opersting conditions stated above. 17 21205, 13094 In order to better Illustrate the problems Incident to the utilization of the type IV catalysts, reference is again made to the drawing. The titanium based catalyst (type IV) is Introduced Into the reactor 10 at point 32. Under conventional operations on certain resins, after a brief period of time, i.e. in the order of about 36 to 72 hours, sheets begin to form In reactor 10, at a site in the reactor proximate the wall of the reactor and located about a distance of one-half the reactor diameter up from the base of the fluid bed. The sheets of fused resin begin to appear in segregation cone 40, rapidly plugging the system, causing the reactor to be ahut down. More characteristically the sheeting begins after production equivalent to 6 to 10 times the weight of the bed of resin In reactor 10.

Many possible causes were investigated in attempting to discover and eliminate the sheeting.

In the course of the Investigation, thermocouples were Installed just Inside the reactor walls at elevations of 1/4 and 1/2 reactor diameter above the gas distributor plate. Under conventional operations , "skin" thermocouples indicate temperatures equal to the temperature of the fluldized bed.

When sheeting occurs, these thermocouples Indicate temperature excursions of up to 20°C above the temperature of the fluldized bed thus providing 18 3 13094 - ' reliable indication of the occurrence of sheeting. In addition, an electrostatic voltmeter was used to measure voltage on a 1/2 inch spherical electrode located in the fluid bed 1 Inch radially from the reactor wall and 1/2 reactor diameter above the gas distributor plate. The location was selected because sheet formation was observed to initiate in a band ranging from 1/4 to 3/4 reactor diameter in elevation above the base of the fluid bed. As is well known for deep fluldized beds, this corresponds to the region of least mixing intensity near the wall, i.e. a null zone where particle motion near the wall changes from generally upward to generally downward. The possible causes investigsted included factors affecting mixing in the fluldized bed, reactor operating conditions, catalyst and resin particle size, particle size distribution, and others. A correlation was found between sheeting and buildup of static electric charge on the resin particles proximate the reactor walls. When the static voltage level of resin particles at particular sites proximate the reactor wall in a fluldized bed reactor is low, the reactor runs normally and no sheets are formed. Vhen the static voltage level exceeds a critical level at those sites, uncontrolled sheeting occurs and the reactor must be shut down.

Surprisingly sheeting had not occurred to any significant degree on any resin utilizing the type iv catalysts in reactors which had previously utilized type Ccatalyats or in reactors that had utilized type i through III catalysts. 19 13094 ^ .. u It was further discovered that sheeting could b« substantially reduced ..tid In some cases •ntlrely eliminated by controlling static voltage In the fluldlsed bed at a site proximate the reactor walls below the critical level for sheet formation.

This critical level for sheet formation Is not a fixed value, but is a complex function dependent on variables including resin sintering temperature, operating temperature, drag forces in the fluid bed, resin particle size distribution and recycle gas composition.

The critical voltage level Vc for sheeting of ethylene homopolymers and ethylene-butene copolymers is primarily a function of the resin sintering temperature, the reactor bed temperature and the concentration of hydrogen in the recycle gas. The relationship can be expressed as: Vc - -8000 - 50 Ts + 90[H2] + 150 To where Vc ■ voltage below which sheeting will not occur, volts; Ts ■ sintering temperature of resin under reactor operating conditions, in *C; To - temperature of reactor, in *C and [ H^I ■ mole percent hydrogen in recycle gas The sintering temperature of the resin under reactor operating conditions is the temperature at which a settled bed of resin in contact with a gas having the same composition as the reactor recycle gas used in producing the resin will sinter and form 13094 aggolomerates vhen refluldizatlon is attempted efcer allowing the bed to remain settled for fifteen ainutes. The sintering temperature is decreased by decreasing the resin density, by increasing the Belt index and by increasing the amount of dissolved sonomers.

The constants in the equation were determined from data collected during reactor operation vhen the reactor just began to exhibit sheeting symptoms through •kin thermocouple temperature excursions above the bed temperature. The voltage Indicated on the voltage probe described earlier varies vith time due to the random nature of a fluldized bed. Thus the critical voltage, Vc, is expressed as a time averaged voltage. Voltage measurements are difficult to interpret because additional static electric charge is generated vhen a sheet, formed because of a static charge, separates from the reactor vail. In addition, the sheeting phenomena can start as a very local phenomenon and spread further clouding interpretation of voltage readings.

Although the sheeting mechanism is not fully understood, it is believed that static clectriclty generated in the fluid bed charges resin particles. Vhen the charge on the particles reaches the level vhere the electrostatic forces trying to hold the charged particle near the reactor vail exceed the drag forces in the bed trying to move the particle avay from the vail, a layer of catalyst containing, polymerizing resin particles formsa non-fluldlzed layer 21 130% near the raactor wall. Heat removal from this layer is not aufficient to remove the heat of polymerization because the non-fluldized layer near the vail has less contact vith the fluldizing gas than do particles in the fluldized portion of the bed. The heat of polymerization increases the temperature of the non-fluidized layer near the reactor vail until the particles melt and fuse. At this point other particles from the fluldized bed will stick to the fused layer and it vlll grow In size until it comes loose from the reactor vail. The separation of a dielectric from a conductor (the sheet from the reactor vail) is known to generate additional static electricity thus accelerating subsequent sheet formation.

The art teaches various processes whereby static voltage can be reduced or eliminated. These comprise (1) reducing the rate of charge generation, (2) Increasing the rate of discharge of electrical charge, and (3) neutralization of electrical charge. Some processes suited for use in a fluldized bed comprise (1) use of an additive to Increase the conductivity of the particles thus providing a path for discharging, (2) installation of grounding devices in a fluldized bed to provide additional area for discharging electrostatic 22 13094 T ?Q charges to ground, (3) Ionization of gas or particles by electrical discharge to generate ions to neutralize •lectrostatic charges on the particles, and (4) the use of radioactive sources to produce radiation that will create ions to neutralize electrostatic charges on the particles. The application of these techniques to a conmercial scale, fluldized bed, polymerization reactor may not be feasible or practical. Any additive uaed must not act as a poison to the polymerization catalyst and must not adversely affect the quality of the product. Thus water, the nost widely used additive to reduce static on particles, cannot be used since it is a aevere catalyst poison. The installation of grounding devices may actually generate additional electrostatic charge aince the friction of resin particles on metal surfaces creates electrostatic charges on the resin particles. The use of ion generators and radiation sources pose severe problems of scale. The Ions generated by electric discharge or radiation will be attracted to the reactor walls and other grounded objects and will travel only a limited distance before contacting a grounded object. Thus the ions may not travel far enough from the site of ion generation to discharge the region of the bed where sheeting occurs. Generation of ions within the fluid bed is severely limited by the quenching effect of the cloud of charged particles which form around 23 130% an Ion generator. Thus the number of ion generation sources required may be high adding to the complexity and danger of radiation sources or electrical discharge generators In or near a pressurised, hydrocarbon containing reactor. In the course of the Investigation, it vas discovered that an effective process for treating the vails of the reactor vessel to reduce static charge generation comprises operation of the reactor for a short, i.e. two veek, period utilizing a chromium containing catalyst (type I through III) vhere the chromium is in the 2 or 3 valence state during at least part of its residence time in the reactor.

Surprisingly, however, it vas also discovered that if the vails of the reactor vessel are treated prior to the commencement of polymerization with a chromium containing compound wherein the chromium is present in the reactor at a valence of 2 or 3, then the formation of sheeting during polymerization is substantially reduced and in some cases entirely eliminated.

The chromium containing compounds contemplated for use In the present Invention are as explained previously those in vhich the chromium is present in the reactor at a valence of 2 or 3.

Merely as illustrative the following compounds would be suitable for the present invention; 24 13094 212052 The bis(cyclopentadienyl) chromium (II) compounds having the following formula: n" wherein R' and R" may be the same or different to O ^20' hydrocarbon radicals, and n' and n." be the tame or different and may be 0 or l to 5., inclu sive. The R' and R" hydrocarbon radicals can be saturated or unsaturated, and can Include aliphatic, alicycllc and aromatic radicals auch as methyl, ethyl, propyl, butyl, pentyl, cyclopentyl, cyclohexyl, ailyl, phenyl and naphthyl radicals. Other specific compounds which are suitable Include chromic acetyl acetonate, chromic nitrate, chromous or chromic acetate, chromous or chromic chloride, chromous or chromic bromide, chromous or chromic fluoride, chromous or chromic sulfate, and 20 polymerization catalysts produced from chromium com pounds where the chrome is in the plus 2 or 3 valence state.

^ Bis(cyclopentadienyl) chromium(chromocene) is the preferred chromium containing compound because of the excellent results achieved.

In general, the chromium containing compound is introduced into the reactor prior to polymerization and can be Introduced in any manner such that the surface of the walls of the reactor is contacted with the .! chromium compoiind.

If "8 JAN 1988 mjj XPj i \Je- the ""05 In a preferred technique, the chromium compound is dissolved in a suitable solvent and is introduced into the reactor in an inert or non-reactive atmosphere. A resin bed may be employed to help disperse the chromium compound through the reactor.

Suitable solvents for this purpose include but are not limited to benzene, toluene, isopentane, hexane and vater. The choice and use of a solvent is dependent on the form of the chrome containing compound and the method of application selected. The function of the solvent is to carry and aid in the dispersion of the chrome containing compound. Suitable inert or non-reactive gases include but are not limited to nitrogen, carbon dioxide, methane, ethane and air.

The amount of chromium compound utilized in the process should be sufficient to effect the desired result, and the amount can be generally determined by one skilled in the art. In general, however, an amount of at least 3,5x10"^ pound moles chromium per square foot of surface to be treated, preferably 1.0x10"^ to about 5x10 ^ pound moles per square foot of surface to be treated is preferred.

The polymers .to which the present invention is primarily directed and which cause the sheeting problems above referred to in the presence of titanium catalysts are linear homopolymers of ethylene or linear copolymers of a major mol percent 901) of ethylene, and a minor mol percent (- 10X) of 1 oiw. - if 26 13094 one or sore to Cg alpha olefins. The C3 to C8 alpha olefins should not contain any branching on any of their carbon atoms vhich is closer than the fourth carbon atom. The preferred to Cg alpha olefins are propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-l, heptene-1 and octene-1. This description is not intended to exclude the use of, this invention with alpha olefin hotnopolyiner and copolymer resins in vhich ethylene is not a monomer.

The homopolymers and copolymers have a density ranging from about 0.97 to 0.91. The density of the copolymer, at a given melt index level is primarily regulated by the amount of the C^ to Cg comonomer vhich lis copolymerized with the ethylene. Thus, the addition of progressively larger amounts of the comonomers to the copolymers results in a progressive lowering of the density of the copolymer. The amount of each of the various C3 to Cg comonomers needed to achieve the same result vill vary from monomer to monomer, under the same reaction conditions. In the absence of the comonomer, the ethylene would homopolymerize.

The melt index of a homopolymer or copolymer Is a reflection of its molecular velght. Polymers having a relatively high molecular weight, have relatively high viscosities and low melt Index. 27 13094 .N c; ' j' In a typical Bode of utilizing the subject Invention to reduce sheeting, a reactor vessel such as shown in Figure 1 and which is susceptible to sheeting problems by the polymerization of the above described aaterlals utilizing type IV catalysts is partially filled with granular polyethylene resin which is purged with a non-reactive gas such as nitrogen and is fluldized by circulating said non-reacting gas through the reactor at a velocity above the minimum fluidizing velocity (Gmf) of the granular polyethylene and preferably at 3 to 5 Gmf. It is to be understood that the use of a fluldized bed of resin is a convenience in said process and Is not essential to the process. While the non-reactive gas is being circulated, a chromium-containing compound such as chromocene either neat or preferably dissolved in an inert solvent such as toluene is introduced into the reactor. The concentration of the chromium-containing chemical in the inert aol-vent is not critical to the process but ean be aelected by one akilled in the art so as to assure that the chromium-containing chemical Is completely dissolved in the solvent. For the preferred case, a solution containing 6 to 8 percent by weight of chromocene in -toluene is typical. Approximately 4.0x10' pound moles of the chromium-containing chemical is injected into the reactor for every square foot of aurface to be treated. The non-reacting gas is circulated to bring the chromium-containing chemical in contact with the metal surfaces in the system. The 28 13094- ^ p r treatnent is carried out for sufficient tine to achieve the desired result, typically several hours to several days. In other nodes of treatnent, the chemical solution could be applied to the nstsl surfaces by painting, spraying, or other application nethods familiar to one skilled in the art. After treatnent the reactor is now ready to begin poly-nerization in the usual nanner.

Having set forth the general nature of the invention, the following examples illustrate some specific embodiments of the invention. It is to be understood, however, that this invention is not limited to the examples, since the invention nay be practiced by the use of various modifications.

Examples 1-8 were conducted in a fluldized bed reactor as described in Figure 1. The catalyst used was a Ziegler type, titanium based catalyst supported on porous silica produced as described earlier as type IV. The cocatalyst used was triethyl aluminum. The products nade in the examples were copolymers of ethylene and 1-butene. Hydrogen was used as a chain transfer agent to control the melt index of the polymer. The reactors of Example 1 and 2, had not been used to produce polyethylene with any catalyst except those of the type described earlier as type IV. 29 1309<r 120 5^ EXAMPLE 1 A fluldized bed reaqtor vas started up at operating conditions designed to produce • film grade low density ethylene copolymer product having a density of 0.918, a aelt index of 1.0, and a sticking temperature of 104*C. The reaction vas started by feeding catalyst to- a reactor precharged with a bed of granular resin similar to the product to be aade. The catalyst vas a mixture of 5.5 parts titanium tetrachloride, 8.5 parts magnesium chloride and 14 parts tetrahydrofuran deposited on 100 parts Jfcvlson grade 952 silica vhich had been dehydrated at 800*C and treated vlth four parts trlethylalumlnum prior to deposition and vas activated vlth thirty five parts tri-n-hexyl aluminum subsequent to deposition.

# Prior to starting catalyst feed, the reactor and resin bed were brought up to the operating temperature of 85"C, were purged of impurities by circulating nitrogen through the resin bed. Ethylene, butene and hydrogen concentrations were established at 53, 24, and 111 respectively. Cocatalyst was fed at a rate of 0.3 parts trlethylamunlmum per part of catalyst.

Reactor start-up vas normal. After producing product for 29 hours and equivalent to 6-1/2 times the velght of the fluldized bed, temperature excursions of 1 to 2*C above bed temperature vere observed using thermocouples located just inside the reactor vail at an elevation of 1/2 reactor diameter 13094 7 •; above the gas distributor plate. Prior experience had shown that such tempersture excursions are a positive indication thst sheets of resin are being formed in the fluldized bed. Concurrently, bed voltage (measured using an electrostatic voltmeter connected to a 1/2 inch diameter spherical electrode located one inch from the reactor wall at an elevation of 1/2 reactor diameter above the gas distributor plate) increased from a reading of approximately +1500 to -4*2000 volts to a reading of over +5000 volts and then dropped back to +2000 volts over a 3 minute period. Temperature and voltage excursions continued for approximately 12 hours and increased in frequency and magnitude. During this period, sheets of fused polyethylene resin began to show up in the resin product. Evidence of sheeting became more severe, i.e. temperature excursions increased to as high as 20*C above bed temperature and stayed high for extended periods of time and voltage excursions also became more frequent. The reactor was shut down because of the extent of sheeting. example 2 The fluldized bed reactor used in Example 1 vas started up and operated to produce a linear low density ethylene copolymer suitable for extrusion or rotational molding and having a density of 0.934, a melt index of 5 and a sticking temperature of 118*C. The reaction was started by feeding catalyst similar 31 2 1 205Z 130% to tbt catalyst In txaople 1 except activated with 2B parts trl-n-hexylsluminun, to the reactor pre-charged with a bed of granular resin similar to the product to be nade. Prior to atartlng catalyst feed the reactor and resin bed were brought up to the operating tenperature of 85*C,. and yere purged of lapurltles with nitrogen. The concentrations of ethylene (521). butene (141), hydrogen (211) were introduced into the reactor. Cocatalyst triethyl-alumlnua was fed at 0.3 parts per pert of catalyst. The reactor was operated continuously for 48 hours and (hiring that period produced resin equivalent to 9 times the amount of resin contained in the bed.

After this 48 hour period of smooth operation, sheets of fused resin began to come out of the reactor with the normal, granular product. - At this tine voltages measured 1/2 reactor diameter above the distributor plate averaged +2000 volts and ranged from 0 to +10,000 volts, while the skin thermocouples at the same elevation Indicated excursions of >15*C above the bed temperature. Two hours after the first sheets were noted in the product from the reactor, It was necessary to stop feeding catalyat and cocatalyst to the reactor to reduce the resin production rate because sheets were plugging the resin discharge system. One hour later, catalyst and cocatalyst feeds were 32 - i 205 13094 restarted. The production of sheets continued and after Cvo hours catalyst and cocatalyst feed were again •topped end the reaction was terminated by injecting carbon monoxide. The voltage at this tine was>+12,000 volts and the thermocouple excursions continued until the poison was Injected. In total, the reactor was operated for 53 hours and produced 10-1/2 bed volumes of resin before the reaction was stopped due to •heeclng. example 3 The reactor of Examples 1 and 2 was treated as follows: The treatment comprised charging a bed of granular resin and purging and drying the bed with high purity nitrogen to a vapor water concentration of less than 10 ppmv. The bed was thereafter fluldized by circulating nitrogen. Chromocene [bis(cyclopentadlenyl)chromium] in toluene solution was injected into the bed. 4,3x10"^ pound moles of chromocene were added for each aquare foot of steel surface in the system. The bed was heated to 92*C and nitrogen was circulated for 24 hours. After the treatment was completed, the bed was cooled to 40*C and 20 standard cubic feet of air was Injected for each pound of chromocene In the system to oxidize the chromocene before removing the resin from the reactor. 33 - ; 2 0 5 13094 V The treated raactor waa than '.-;.^-ged with • bad of raain similar to thac described in Example 1. The bad was brought up to 85'C, purged,and the ethylene, butane, hydrogen, and cocatalyst concentrations were •stablished at the aame concentrations of Example 1, prior to injection of the aaac catalyst as Example 1. The raactor atarted up at operating conditions designed to produce a film grade low density polyethylene copolymer product having a density of 0.918, a melt index of 1.0, and a sintering temperature of 10A*C as in Example 1. The raactor ran for 90 hours, producing approximately 3 times as much product as in Example 1, after which It was shut down for routine Inspection and maintenance. No temperature excursions were noted and no resin sheets were formed. -At the and of the run. the voltage measured near the wall at an elevation 1/2 reactor diameter above the gas distributor plate had stabilized at about -100 volts and no major voltage excursions were observed at any time during the run.

EXAMPLE 4 The reactor utilized for Example 3 was aubsequently charged with a bed of resin similar to that described in Example 2. The bed was heated to 90*C, purged and the ethylene (511), butene (131) and hydrogen (181) and cocatalyst (0.3 parts per part of catalyst) were established prior to Injection of catalyst. The reaction started smoothly and produced linear low-density polyethylene resin with a density of 0.934, a melt Index of 5, and a sintering temperature of 118*C. 34 - - L 13094 The reactor operated continuously for 80 houra and produced resin equivalent to twenty times the weight of the resin bed before it was transitioned to another product grade. The thermocouples located near the surface of the reactor wall 1/4 and 1/2 the reactor diameter above the distributor shoved a few brief (1 minute) temperature excursions. Voltage measured near the wall at an elevation of 1/2 reactor diameter above the gas distributor plate averaged 4*1200 volts and allowed voltage oscillation from 0 to as high as +8.000 volts. Some pieces of resin typically 1/4 by 1 by 1 inch with the appearance of alntered fine particles appeared In the product discharge tank and constituted <0.01 percent of the resin produced. These did not reduce the production rate of the reaction system nor did they harm the quality of the resin produced.

As vill be seen from the above, the following data corresponds to the Vc formula expressed previously: Vc " *8000 -50 (sintering temp.) +90 (hydrogen concentration) +150 (operating temperature) - -8000 - .50(118*C) +90(181) +150(90*C) ■ +1220 volts ilti. ^ k _ 1205 13094 EXAMPLES 3-8 Four rune were made utilizing the reactor and procedure of Examples 1 & 2 to determine critical voltage. Various ethylene, butene-1 copolymers and/or ethylene homopolymers were used for each run as shown In Table 1.

The critical voltages, Vc. were the voltage level aeasured near the reactor wall (one half Che reactor diameter above the distributor plate) when the reactor ahoved symptons of Initiation of sheeting (normally small akin thermocouple excursions above bed temperature). The atleking temperatures were estimated from tests in which a reaction was terminated, the bed allowed to settle for 15 minutes and then refluidized.

The results are indicated in Table I below. 36 €; o o TABLE t 6 7 8 h3i« x 11 u 21 Ethylene Butene Coneentra- Concentra-tlow Mole x tion Mole x 53 51 50 65 24 23 7 0 Catalyst of Exawple 1 1 2 2 Realn Melt Reain Sintering Operating Vc Index Penalty Tewp. *C Twp. *C Volt» 1.0 .918 104 85 +200 to +1000 2.0 .918 102 85 +200 to +1000 12. .926 108 85 +2100 7.5 .965 125+ no +4100 to o en - 20 130% As will be noted from Table Z, for Example 5, sheeting begins to occur over +1000 volts. Moreover, from the above Table I it will be seen that the critical voltage is dependent on the resin sintering temperature, the operating temperature and the hydrogen concentration in the recycle gas. 38

Claims (22)

212052 130% wff/VT u/e claim tv. WHAT IS CLAIMED 15.

1. An improvement in the method for polymerization of alpha-olefins in a fluidired bed reactor utilizing titanium based catalysts or other catalyats prone to cause sheeting during aaid polymerization, the improvement comprising maintaining the static electric charge in said reactor at the site of possible sheet formation -below static voltage levels vhich would otherwise cause sheet formation.

2. The improvement according to claim 1 vhereln aaid static voltage levels are maintained below critical voltage levels as expressed by the following formula:. Vc - -8000 - 50 Ts + 90 [H2J + 150 To where Vc - voltage below which sheeting will not occur, in volts; Ts ■ sintering temperature of resin under reactor operating conditions, in *C; To ■ temperature of reactor, in *C and 1^] ■ mole percent hydrogen in recycle gas

3. The improvement according to claim 1 wherein said static electric charge in said reactor is maintained below static voltage levels vhich would otherwise cause sheet formation by injection into said fluldized bed or creation in said fluldized 39 -8 JAN 1988"-,.j 13094 212052 bed^ion pairs, ions or charged partlclas of opposite polarity from that of the comDonents of the sairi fluidized bed. k.

The improvement according to claim 1 therein aaid atatic electric charge in said reactor is maintained belov static voltage levela which would otherwise cause sheet formation by devices connected to said reactor vails, said devices being designed to promote electrical discharging to ground by creating areas of high localized field strength.

5. The improvement according to claim 1 wherein aaid atatic electric charge in said reactor is maintained belov atatic voltage levels vhich would otherwise cause sheet formation by introducing a chromium containing compound into said reactor, aaid chromium containing compound being present in a valence state of 2 or 3.

6. A process for reducing sheeting during production of polyolefins by polymerization of alpha-olefins in.a.fluidized bed reactor utilizing titanium based doI vmerizatit/* catalysts which comprises contacting the furfaces of the reaction vessel in vhich said polymerization takes place with a chromium containing compound, said chromium being present in said compound at a valence state of 2 or 3, said chromium containing compound bejng-lused in an ~i amount sufficient to reduce the amount formed during said polymerization. 40 ? -8 JAM 1988 2) V\ j 212052 13094

7. A process according to dala 6 in which said chromium containing compound is contained In an Inert solvent.

8. A process according to claia 7 wherein •aid inert solvent is toluene.

9. A process according to claia 7 wherein aaid Inert solvent is introduced into said reaction vessel in an Inert atmosphere.

10. A process according to claim 6 wherein ••id chromium containing compound ia bisCcyclopentadienyl) chromium.

11. A process according to claim 6 wherein •aid polyolefins are linear homopolymers of ethylene or linear copolymers of a major mol percent 901) of ethylene, and a minor mol percent (* 101) of one or aore to Cg alpha olefins.

12. A process according to claim 11 wherein said C3 to polyolefins are homopolymers or copolymers of propylene, butene-1, pentene-1, . hexene-1, 4-methylpentene-l, heptene-1 pr octene-1.

13. A process for reducing sheeting during polymerization of alpha-olefins in a fluidized bed reactor to produce homopolymers of ethylene or linear copolymers of a major > v/-' -8 JAN!98u"! 13094 212052 mol percent (* 901) of ethylene, end a minor aol percent (* 101) of one or more to Cg elphe olefins, utilizing titanium based polymerization catalysts vhich comprises introducing Into the reaction vessel prior to polymerization, e chromium containing coopound, said chromium being present in said compound at a valence state of 2 or 3, aaid chromium containing compound being dissolved in an inert solvent end being introduced into said reactor in an inert atmosphere

14. A process according to claim 13 In vhich aaid titanium catalyst comprises at least one titanium compound, at least one magnesium compound, at least en electron donor compound^ at least one activator compound and at least one inert carrier material.

15. A process according to claim 14 in vhich said titanium compound has the structure: TKOR)^ vhereln R is a to C14 aliphatic or aromatic hydrocarbon radical., or COR' vhere R' is a to aliphatic or aromatic hydrocarbon radical; X is CI, Br, or I; a is 0 or 1; b is 2 to A inclusive; and e + b ■ 3 or 4. 42 -8 JANi988 212052 13094

16. A process according to claim 15 wherein said chromium containing compound has the following formula: <rv & wherein R' and R" say be the same or different to C2q> inclusive, hydrocarbon radicals, and n' and n" «ay be the same or different and may be 0 or 1 to 5, inclusive.

17. A process according to claim 16 wherein said chromium containing compound is bis(cyclopentadienyl) chromium. IB.

A process for reducing sheeting during polymerization of alpha-olefins in a fluidized bed reactor to produce linear homopolymers of ethylene or linear copolymers of a major mol percent (* 901) of ethylene, and a minor mol percent (* 101) of one or more to Cg alpha olefins, utilizing a titanium based catalyst, said catalyst comprising at least one titanium . - 8 JAN ^j lawT" 212052 * 13094 compound, at lease one magnesium compound, at least one electron donor compound, et least one activator compound, end at lease one inert carrier ■attrial for the polymerisation vhich comprises introducing into aa£d- reaction vessel prior to polymerization bis Ccyclopentadisnyl) chromium in an inert solvent, said solvent being introduced into said reactor in sn inert atmosphere.

19. A process according to claim 18 Wherein said titanium compound has the structure Ti(OR)^Xb wherein 1 is i to aliphatic or aromatic hydrocarbon radical, or COR' where R' Is a C^ to Cj^ aliphatic or aromatic hydrocarbon radical; X is CI, Br, or 1; a is 0 or 1; b is 2 to 4 inclusive; and a + b - 3 or 4.

20. The improvement as claimed in any one of claims 1 to 5 substantially as hereinbefore described with or without reference to the accompanying drawing^— ^ e N i J/Q .% L 3, " .8 "■ / 2 t 205 7: Q i 45 -

21. A process as claimed in any one of claims 6 to 19 when performed substantially as hereinbefore described with or without reference to the accompanying drawing.

22. Sheeting produced using a process as claimed in any one of claims 6 to 19 and 21. DATED THIS ^W\DAY OF (V\CW( 19*35 a. s0n PER *o AGENTS FOR THE applicants k • —■—- •; itow* *vi.BM*W