Process for the preparation of high density ethylene polymers in fluid bed reactor

BACKGROUND OF THE INVENTION
The invention relates to the catalytic homopolymerization and copolymerization of ethylene with high activity Mg and Ti containing complex catalysts in a low pressure gas phase process to produce polymers having a density of about .gtoreq.0.95 to .ltoreq.0.97, a melt flow ratio of about .gtoreq.22 to .ltoreq.32 and having a relatively round particle shape and a relatively low level of fines.
DESCRIPTION OF THE PRIOR ART
Ethylene homopolymers having a density of .gtoreq.0.96 and a melt index in the range of about 5 to 50 are useful for injection molding purposes requiring excellent impact strength properties provided that they have a relatively narrow molecular weight distribution. Ethylene polymers having a density of .gtoreq.0.96 and a melt index of .ltoreq.20 can be made with the catalysts and low pressure gas phase processes described in U.S. Pat. Nos. 3,023,203; 4,003,712 and 3,709,853. The polymers made with the supported chromium oxide, silylchromate and chromocene catalysts, however, have a relatively broad molecular weight distribution as evidenced by melt flow ratio values of .gtoreq.35. The polymers of these patents, therefore, have relatively limited use for injection molding applications requiring excellent impact strength properties. It is desirable therefore, to provide catalysts which would enable one to readily prepare, in a gas phase process, ethylene polymers which have a relatively narrow molecular weight distribution.
To be commercially useful in a gas phase process, such as the fluid bed process of U.S. Pat. Nos. 3,709,853; 4,003,712 and 4,011,382, and Canadian Pat. No. 991,798 and Belgian Pat. No. 839,380, the catalyst employed must be a high activity catalyst, that is, it must have a level of productivity of .gtoreq.50,000, preferably .gtoreq.100,000, pounds of polymer per pound of primary metal in the catalyst. This is so because such gas phase processes usually do not employ any catalyst residue removing procedures. Thus, the catalyst residue in the polymer must be so small that it can be left in the polymer without causing any undue problems in the hands of the resin fabricator and/or ultimate consumer. Low catalyst residue contents are also important where the catalyst is made with chlorine containing materials such as the titanium, magnesium and/or aluminum chlorides used in some so-called Ziegler or Ziegler-Natta catalysts. High residual chlorine values in a molding resin will cause pitting and corrosion on the metal surfaces of the molding devices. Cl residues of the order of .gtoreq.200 ppm are not commercially useful.
U.S. Pat. No. 3,989,881 discloses the use of a high activity catalyst for the manufacture, under slurry polymerization conditions, of ethylene polymers having a relatively narrow molecular weight distribution (Mw/Mn) of about 2.7 to 3.1. Attempts were made to use catalysts similar to those described in U.S. Pat. No. 3,989,881 for the purpose of making polyethylene of narrow molecular weight distribution by polymerizing ethylene alone or with propylene in the gas phase in a fluid bed process using apparatus and conditions similar to those employed in U.S. Pat. No. 4,011,382 and Belgian Pat. No. 839,380. These attempts were not successful. In order to avoid the use of the solvents in the slurried catalyst systems of U.S. Pat. No. 3,989,881, the Ti/Mg containing components were dried. However, the dried material, a viscous, gummy, pyrophoric composition, could not be readily fed to the reactor because it was not in a free flowing form. Even when blended with silica to improve its free-flowing properties and then added to the reactor the results were commercially unacceptable. The productivity of the catalyst was poor, or the catalyst was pyrophoric and difficult to handle or the polymer product was produced in the form of needleshaped products which were difficult to fluidize and which exhibited very poor flow properties.
U.S. Pat. Nos. 3,922,322 and 4,035,560 disclose the use of several Ti and Mg containing catalysts for the manufacture of granular ethylene polymers in a gas phase fluid bed process under a pressure .ltoreq.1000 psi. The use of these catalysts in these processes, however, has significant disadvantages. The catalysts of U.S. Pat. No. 3,922,322 provide polymers having a very high catalyst residue content, i.e., about 100 ppm of Ti and greater than about 300 ppm Cl, according to the working example of this patent. Further, as disclosed in the working example of U.S. Pat. No. 3,922,322, the catalyst is used in the form of a prepolymer, and very high volumes of the catalyst composition must be fed to the reactor relative to the volume of polymer made in the reactor. The preparation and use of this catalyst thus requires the use of relatively large sized equipment for the manufacture, storage and transporting of the catalyst.
The catalysts of U.S. Pat. No. 4,035,560 also apparently provide polymers having high catalyst residues and the catalysts compositions are apparently pyrophoric because of the types and amounts of reducing agents employed in such catalysts.
U.S. Pat. No. 4,124,532 discloses the polymerization of ethylene and propylene with high activity catalysts. These catalysts comprise complexes which may contain magnesium and titanium. These complexes are prepared by reacting the halide MX.sub.2 (where M may be Mg) with a compound M'Y (where M' may be Ti and Y is halogen or an organic radical) in an electron donor compound. These complexes are then isolated by either crystallization, by evaporation of the solvent or by precipitation.
Polymerization is carried out with these catalytic complexes and an alkyl aluminum compound.
However, U.S. Pat. No. 4,124,532 does not disclose any special techniques or methods of preparing the catalyst in order to achieve the desirable results described in the present invention. The use of the catalysts described in U.S. Pat. No. 4,124,532, without these special methods, would not lead to a commercial fluid bed process to produce polyethylenes at commercial rates. In addition, the examples in the gas phase, do not describe a practical process of copolymerization to produce copolymers with attractive polymer morphology as described in the present invention.
U.S. Pat. No. 4,302,565 in the names of G. L. Goeke et al. and entitled "Impregnated Polymerization Catalyst, Process For Preparing, and Use for Ethylene Copolymerization" discloses that ethylene copolymers, having a density of about .gtoreq.0.91 to .ltoreq.0.94 and a melt flow ratio of .gtoreq.22 to .ltoreq.32 and which have relatively low residual catalyst content and a relatively high bulk density and which provide films of good clarity can be produced at relatively high productivities for commercial purposes by a gas phase process if the ethylene is copolymerized with one or more C.sub.3 to C.sub.6 alpha olefins in the presence of a high activity magnesium-titanium containing complex catalyst prepared under specific activation conditions with an organoaluminum compound and impregnated in a porous inert carrier material.
In the catalyst activation procedure the activation is conducted in at least two stages. In the first stage the precursor composition, impregnated in the silica, is reacted with, and partially activated by, enough activator compound so as to provide a partially activated precursor composition which has an activator compound/Ti molar ratio of about >0 to <10:1 and preferably of about 4 to 8:1. In order to render the partially activated and impregnated precursor composition active for ethylene polymerization purposes, additional activator compound must also be added to the polymerization reactor to complete, in the reactor, the activation of the precursor composition. The additional activator compound and the partially activated impregnated precursor composition are preferably fed to the reactor through separate feed lines. The additional activator compound may be sprayed into the reactor in the form of a solution thereof in a hydrocarbon solvent such as isopentane, hexane, or mineral oil. This solution usually contains about 2 to 30 weight percent of the activator compound.
However, the activity of the impregnated catalysts as prepared according to the procedure as described in U.S. Pat. No. 4,302,565 is at times not high enough to produce ethylene homopolymers or copolymers of high melt index (.gtoreq.1.0) with a density .gtoreq.0.95 and obtain a relatively round particle shape and a relatively low level of fines. Precursor compositions in which q=1.5 m+2 (see below) when activated in a two stage process exhibited lower polymerization activity (Table III).
U.S. patent application Ser. No. 892,037 filed on Mar. 31, 1978 in the names of B. E. Wagner et al. and entitled Polymerization Catalyst, Process For Preparing And, Use For Ethylene Homopolymerization, now abandoned, and continuation-in-part application Ser. No. 014,412, filed Feb. 27, 1979 disclose that ethylene homopolymers having a density range of 0.96 to 0.97 and a melt flow ratio of .gtoreq.22 to .ltoreq.32 and which have a relatively low residual catalyst residue can be produced at relatively high productivities for commercial purposes by a low pressure gas phase process if the ethylene is homopolymerized in the presence of a high activity magnesium-titanium complex catalyst blended with an inert carrier material. However, the polymers produced with this blended catalyst have the disadvantage that the polymer particles formed during the fluid bed polymerization process are irregular in shape and are somewhat difficult to fluidize. Also, the final product contains a relatively high level of fines, i.e., particles having a particle size of .ltoreq.125 microns.
SUMMARY OF THE INVENTION
It has now been unexpectedly found that ethylene homopolymers and copolymers having a density range of about .gtoreq.0.95 to .ltoreq.0.97 and a melt flow ratio of about .gtoreq.22 to .ltoreq.32 can be produced at relatively high productivities at a relatively high bulk density, with relatively low level of fines and with a relatively round particle shape. These polymers can be produced commercially by a low pressure gas phase process if the ethylene is homopolymerized or copolymerized in the presence of a high activity magnesium-titanium complex catalyst prepared as described below, and impregnated in selected amounts in a porous inert carrier material and activated under specific conditions in the polymerization reactor.
An object of the present invention is to provide a process for producing, with relatively high productivities and in a low pressure gas phase process, ethylene homopolymers and copolymers which have a density of about .gtoreq.0.95 to .ltoreq.0.97, a melt flow ratio of about .gtoreq.22 to .ltoreq.32, a relatively low residual catalyst content and a bulk density of about 21 to 32 pounds per cubic foot.
Another object of the present invention is to provide granular ethylene polymers which have a particle shape which is round and wherein the final polymer product contains a relatively low level of fines (particles .gtoreq.125 microns).

BRIEF DESCRIPTION OF THE DRAWING
The drawing shows a gas phase fluid bed reactor system in which the catalyst system of the present invention may be employed.

DESCRIPTION OF THE PREFERRED EMBODIMENT
It has now been found that the desired ethylene homopolymers and copolymers having a density of about .gtoreq.0.95 to .ltoreq.0.97, a low melt flow ratio and relatively high bulk density values can be readily produced with relatively high productivities in a low pressure gas phase fluid bed reaction process if the monomer charge is polymerized under a specific set of operating conditions, as detailed below, and in the presence of a specific high activity catalyst, which is impregnated in an inert porous carrier material in order to achieve high activity and improved polymer particle morphology (including a relatively low level of fines) in producing ethylene polymers. Additionally, elimination of the activation of the precursor composition (where q=1.5 m+2; see below) impregnated in the carrier prior to charging it to a reactor, as described in U.S. Pat. No. 4,302,565, provides for the high activity of the catalyst and at the same time the improved polymer particle morphology of the resulting ethylene polymers.
The Ethylene Polymers
The ethylene polymers have a melt flow ratio of about .gtoreq.22 to .ltoreq.32, and preferably of .gtoreq.25 to .ltoreq.30. The melt flow ratio value is another means of indicating the molecular weight distribution of a polymer. The melt flow ratio (MFR) range of .gtoreq.22 to .ltoreq.32 thus corresponds to a Mw/Mn value range of about 2.7 to 4.1 and the MFR range of .gtoreq.25 to .ltoreq.30 corresponds to a Mw/Mn range of about 2.8 to 3.6.
The copolymers which may be prepared in the process of the present invention are copolymers of a major mol percent (.gtoreq.98%) of ethylene, and a minor mol percent (.ltoreq.2%) of one (copolymer) or more (ter-, tetrapolymers) C.sub.3 to C.sub.8 alpha olefins. The C.sub.3 to C.sub.8 alpha olefins should not contain any branching on any of their carbon atoms which is closer than the fourth carbon atom.
These alpha olefins include propylene, butene-1 pentene-1, hexene-1, 4-methyl pentene-1, heptene-1 and octene-1. The preferred alpha olefins are propylene, butene-1, hexene-1, 4-methyl pentene-1 and octene-1.
The ethylene polymers have a density of about ;24 0.95 to .ltoreq.0.97, preferably from about 0.955 to 0.970. The density of the copolymer, at a given melt index level for the copolymer, is primarily regulated by the amount of the C.sub.3 to C.sub.8 comonomer which is copolymerized with the ethylene. In the absence of the comonomer, the ethylene would homopolymerize with the catalyst of the present invention to provide homopolymers having a density of about .gtoreq.0.96. 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 C.sub.3 to C.sub.8 comonomers needed to achieve the same result will vary from comonomer to comonomer, under the same conditions.
Thus, to achieve the same results, in the copolymers, in terms of a given density, at a given melt index level, larger molar amounts of the different comonomers would be needed in the order of C.sub.3 >C.sub.4 >C.sub.5 >C.sub.6 >C.sub.7 >C.sub.8.
The melt index of a homopolymer or copolymer is a reflection of its molecular weight. Polymers having a relatively high molecular weight, have a relatively low melt index. Ultra-high molecular weight ethylene polymers have a high load (HLMI) melt index of about 0.0 and very high molecular weight ethylene polymers have a high load melt index (HLMI) of about 0.0 to about 1.0. Such high molecular weight polymers are difficult, if not impossible, to mold in conventional injection molding equipment. The polymers made in the process of the present invention, on the other hand, can be readily molded, in such equipment. They have a standard or normal load melt index of .gtoreq.0.0 to about 100, and preferably of about 0.5 to 80, and a high load melt index (HLMI) of about 11 to about 2000. The melt index of the polymers which are made in the process of the present invention is a function of a combination of the polymerization temperature of the reaction, the density of the copolymer and the hydrogen/monomer ratio in the reaction system. Thus, the melt index is raised by increasing the polymerization temperature and/or by decreasing the density of the polymer and/or by increasing the hydrogen/monomer ratio. In addition to hydrogen other chain transfer agents such as dialkyl zinc compounds may also be used to further increase the melt index of the polymers.
The polymers of the present invention have an unsaturated group content of .ltoreq.1, and usually .gtoreq.0.1 to .ltoreq.0.3, C=C/1000 carbon atoms, and a cyclohexane extractables content of less than about 3, and preferably less than about 2, weight percent.
The polymers of the present invention have a residual catalyst content, in terms of parts per million of titanium metal, of less than 20 parts per million, (ppm) at a productivity level of .gtoreq.50,000. In terms of Cl, Br or I residues, the polymers of the present invention typically have a residue content (Cl, Br or I/Ti=7) of less than about 140 ppm at a productivity of .gtoreq.50,000.
The polymers of the present invention are granular materials which have an average particle size of the order of about 0.02 to about 0.05 inches, and preferably of about 0.02 to about 0.04 inches, in diameter. The particle size is important for the purposes of readily fluidizing the polymer particles in the fluid bed reactor, as described below. These granular materials also have a low level of fines (.ltoreq.4.0 percent of the total polymer product) and these fines are .ltoreq.125 microns. Also, these granular materials exhibit a much more spherical shape (as observed by optical microscopy) than the granular materials described in U.S. Pat. application Ser. Nos. 892,037 and 014,412, supra.
The polymers of the present invention have a bulk density of about 21 to 32 pounds per cubic foot.
High Activity Catalyst
The compounds used to form the high activity catalyst used in the present invention 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, as defined below.
The titanium compound has the structure
Ti(OR).sub.a X.sub.b
wherein R is a C.sub.1 to C.sub.14 aliphatic or aromatic hydrocarbon radical, or COR' where R' is a C.sub.1 to C.sub.14 aliphatic or aromatic hydrocarbon radical,
X is selected from the group consisting of Cl, Br I, or mixtures thereof, a is 0, 1 or 2, b is 1 to 4 inclusive and a+b=3 or 4.
The titanium compounds can be used individually or in combinations thereof, and would include TiCl.sub.3, TiCl.sub.4, Ti(OC.sub.6 H.sub.5)Cl.sub.3, Ti(OCOCH.sub.3)Cl.sub.3 and Ti(OCOC.sub.6 H.sub.5)Cl.sub.3.
The magnesium compound has the structure
MgX.sub.2
wherein X is selected from the group consisting of Cl, Br, I, or mixtures thereof. Such magnesium compounds can be used individually or in combinations thereof and would include MgCl.sub.2 MgBr.sub.2 and MgI.sub.2. Anhydrous MgCl.sub.2 is the particularly preferred magnesium compound.
About 0.5 to 56, and preferably about 1 to 10 moles of the magnesium compound are used per mol of the titanium compound in preparing the catalysts employed in the present invention.
The titanium compound and the magnesium compound should be used in a form which will facilitate their dissolution in the electron donor compound, as described herein below.
The electron donor compound is an organic compound which is liquid at 25.degree. 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 such compounds as alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic ethers, cyclic ethers and aliphatic ketones. Among these electron donor compounds the preferable ones are alkyl esters of C.sub.1 to C.sub.4 saturated aliphatic carboxylic acids; alkyl esters of C.sub.7 to C.sub.8 aromatic carboxylic acids; C.sub.2 to C.sub.8, and preferably C.sub.3 to C.sub.4, aliphatic ethers; C.sub.3 to C.sub.4 cyclic ethers, and preferably C.sub.4 cyclic mono- or di-ether; C.sub.3 to C.sub.6, and preferably C.sub.3 to C.sub.4, aliphatic ketones. The most preferred of these electron donor compounds would include methyl formate, ethyl acetate, butyl acetate, ethyl ether, hexyl ether, tetrahydrofuran, dioxane, acetone and methyl isobutyl ketone.
The electron donor compounds can be used individually or in combinations thereof.
The activator compound has the structure
AlR.sub.3
wherein the R's are the same or different and are C.sub.1 to C.sub.14 saturated hydrocarbon radicals.
Such activator compounds can be used individually or in combinations thereof and would include Al(C.sub.2 H.sub.5).sub.3, Al(i-C.sub.4 H.sub.9).sub.3, and Al(C.sub.6 H.sub.13).sub.3.
About 10 to 400, and preferably about 15 to 60 mols of the activator compound are used per mol of the titanium compound in activating the catalysts employed in the present invention.
The carrier materials are solid, particulate porous materials which are inert to the other components of the catalyst composition, and to the other active components of the reaction system. These carrier materials would include inorganic materials such as oxides of silicon and/or aluminum. The carrier materials are used in the form of dry powders having an average particle size of about 10 to 250, and preferably of about 50 to 150 microns. These materials are also porous and have a surface area of .gtoreq.3, and preferably of .gtoreq.50, square meters per gram. Catalyst activity or productivity is apparently also improved with silica having pore sizes of .gtoreq.80 Angstrom units and preferably of .gtoreq.100 Angstrom units. The carrier material should be dry, that is, free of absorbed water. Drying of the carrier material is carried out by heating it at a temperature of .gtoreq.600.degree. C. Alternatively, the carrier material dried at a temperature of .gtoreq.200.degree. C. may be treated with about 1 to 8 weight percent of one or more of the aluminum alkyl compounds described above. This modification of the support by the aluminum alkyl compounds provides the catalyst composition with increased activity and also improves polymer particle morphology of the resulting ethylene polymers.
Catalyst Preparation: Formation of Precursor
The catalyst in the present invention is prepared by first preparing a precursor composition from the titanium compound, the magnesium compound, and the electron donor compound, as described below, and then impregnating the carrier material with the precursor composition.
The precursor composition is formed by dissolving the titanium compound and the magnesium compound in the electron donor compound at a temperature of about 20.degree. C. up to the boiling point of the electron donor compound. The titanium compound can be added to the electron donor compound before or after the addition of the magnesium compound, or concurrent therewith. The dissolution of the titanium compound and the magnesium compound can be facilitated by stirring, and in some instances by refluxing these two compounds in the electron donor compound. After the titanium compound and the magnesium compound are dissolved, the precursor composition may be isolated by crystallization or by precipitation with a C.sub.5 to C.sub.8 aliphatic or aromatic hydrocarbon such as hexane, isopentane or benzene.
The crystallized or precipitated precursor composition may be isolated, in the form of fine, free flowing particles having an average particle size of about 10 to 100 microns and a settled bulk density of about 18 to 33 pounds per cubic foot.
Catalyst Preparation
Impregnation of Precursor in the Support
The precursor composition is then impregnated, in a weight ratio of about 0.033 to 1, and preferably about 0.1 to 0.43, parts of the precursor composition into one part by weight of the carrier material.
The impregnation of the dried (activated) support with the precursor composition may be accomplished by dissolving the precursor composition in the electron donor compound, and by then admixing the support with the dissolved precursor composition so as to allow the precursor composition to impregnate the support. The solvent is then removed by drying at temperatures of .ltoreq.70.degree. C.
The support may also be impregnated with the precursor composition by adding the support to a solution of the chemical raw materials used to form the precursor composition in the electron donor compound, without isolating the precursor composition from such solution. This represents the preferred method because of its simplicity. The excess electron donor compound is then removed by drying at a temperature of .ltoreq.70.degree. C.
In the drying process it is necessary to control the level of the electron donor [ED].sub.q to such an extent that the final value of q=1.5 m+2. This procedure involves careful drying of the precursor and considerable care must be exercised to avoid overdrying and hence decomposition of the precursor.
When thus made as disclosed above the impregnated precursor composition has a formula
Mg.sub.m Ti.sub.1 (OR).sub.n X.sub.p [ED].sub.q
Wherein ED is the electron donor compound.
m is .gtoreq.0.5 to .ltoreq.56, and preferably .gtoreq.1.5 to .ltoreq.5,
n is 0, 1 or 2,
p is .gtoreq.2 to .ltoreq.116, and preferably .gtoreq.6 to .ltoreq.14,
q=1.5 m+2
R is a C.sub.1 to C.sub.14 aliphatic or aromatic hydrocarbon radical, or COR' wherein R' is a C.sub.1 to C.sub.14 aliphatic or aromatic hydrocarbon radical and,
X is selected from the group consisting of Cl, Br, I or mixtures thereof.
The subscript for the element titanium (Ti) is the arabic numeral one.
Activation of the Impregnated Precursor Composition
In order to render the impregnated precursor composition active for ethylene polymerization purposes, activator compound is added to the polymerization reactor to activate the precursor composition. The activator compound and the impregnated precursor composition are preferably fed to the reactor through separate feed lines. The activator compound may be sprayed into the reactor in the form of a solution thereof in a hydrocarbon solvent such as isopentane, hexane, or mineral oil. This solution usually contains about 2 to 30 weight percent of the activator compound. The activator compound is added to the reactor in such amounts as to provide a total Al/Ti molar ratio of 10 to 400 and preferably of about 15 to 60. The activator compound added to the reactor, reacts with, and activates the titanium compound in the reactor.
In a continuous gas phase process, such as the fluid bar process disclosed below, discrete portions of the precursor composition impregnated on the support are continuously fed to the reactor, with discrete portions of activator compound needed to activate the precursor composition, during the continuing polymerization process in order to replace active catalyst sites that are expended during the course of the reaction.
The Polymerization Reaction
The polymerization reaction is conducted by contacting a stream of the monomer(s), in a gas phase process, such as in the fluid bed process described below, and substantially in the absence of catalyst poisons such as moisture, oxygen, carbon monoxide, carbon dioxide and acetylene, with a catalytically effective amount of the completely activated precursor composition (the catalyst) impregnated on a support at a temperature and at a pressure sufficient to initiate the polymerization reaction.
A fluidized bed reaction system which can be used in the practice of the process of the present invention is illustrated in FIG. 1. With reference thereto the reactor 10 consists of a reaction zone 12 and a velocity reduction zone 14.
The reaction zone 12 comprises a bed of growing polymer particles, formed polymer particles and a minor amount of catalyst particles fluidized by the continuous flow of polymerizable and modifying gaseous components in the form of make-up feed and recycle gas through the reaction zone. To maintain a viable fluidized bed, the mass gas flow rate through the bed must be above the minimum flow required for fluidization, and preferably from about 1.5 to about 10 times G.sub.mf and more preferably from about 3 to about 6 times G.sub.mf. G.sub.mf is used in the accepted form as the abbreviation for the minimum mass gas flow required to achieve fluidization, C. Y. Wen and Y. H. Yu, "Mechanics of Fluidization", Chemical Engineering Progress Symposium Series, Vol. 62, p. 100-111 (1966).
It is essential 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 to the polymer to be formed or different therefrom. When different, they are withdrawn with the desired formed polymer particles as the first product. Eventually, a fluidized bed of the desired polymer particles supplants the start-up bed.
The impregnated precursor composition used in the fluidized bed is preferably stored for service in a reservoir 32 under a blanket of a gas which is inert to the stored material, such as nitrogen or argon.
Fluidization 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 fluidized 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 which particulate polymer product is withdrawn. The composition of the make-up gas is determined by a gas analyzer 16 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 essentially steady state gaseous composition within the reaction zone.
To insure complete fluidization, the recycle gas and, where desired, part of the make-up gas are returned to the reactor at point 18 below the bed. There exists a gas distribution plate 20 above the point of return to aid fluidizing the bed.
The portion of the gas stream which does not react in the bed constitutes the recycle gas which is removed from the polymerization zone, preferably by passing it into a velocity reduction zone 14 above the bed where entrained particles are given an opportunity to drop back into the bed. Particle return may be aided by a cyclone 22 which may be part of the velocity reduction zone or exterior thereto. Where desired, the recycle gas may then be passed through a filter 24 designed to remove small particles at high gas flow rates to prevent dust from contacting heat transfer surfaces and compressor blades.
The recycle gas is then compressed in a compressor 25 and then 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 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 immediately 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 to the reactor at its base 18 and to the fluidized bed through distribution plate 20. The compressor 25 can also be placed upstream of the heat exchanger 26.
The distribution plate 20 plays an important role in the operation of the reactor. The fluidized bed contains growing and formed particulate polymer particles as well as catalyst particles. As the polymer particles are hot and possibly active, they must be prevented from settling, for if a quiescent mass is allowed to exist, any active catalyst contained therein may continue to react and cause fusion. Diffusing recycle gas through the bed at a rate sufficient to maintain fluidization at the base of the bed is, therefore, important. The distribution plate 20 serves this purpose and may be a screen, slotted plate, perforated plate, a plate of the bubble cap type and the like. The elements of the plate may all be stationary, or the plate may be of the mobile type disclosed in U.S. Pat. No. 3,298,792. Whatever its design, it must diffuse the recycle gas through the particles at the base of the bed to keep them in a fluidized condition, and also serve to support a quiescent bed of resin particles when the reactor is not in operation. The mobile elements of the plate may be used to dislodge any polymer particles entrapped in or on the plate.
Hydrogen may be used as a chain transfer agent in the polymerization reaction of the present invention. The ratio of hydrogen/ethylene employed will vary between about 0 to about 2.0 moles of hydrogen per mole of the monomer in the gas stream.
Any gas inert to the catalyst and reactants can also be present in the gas stream. The activator compound is preferably added to the gas recycle system at the hottest portion thereof. Addition into the recycle line, therefore, downstream from the heat exchanger is preferred, as from dispenser 27 thru line 27A.
Compounds of the structure Zn(R.sub.a)(R.sub.b), wherein R.sub.a and R.sub.b are the same or different C.sub.1 to C.sub.14 aliphatic or aromatic hydrocarbon radicals, may be used in conjunction with hydrogen, with the catalysts of the present invention as molecular weight control or chain transfer agents, that is, to increase the melt index values of the copolymers that are produced. About 0 to 50, and preferably about 20 to 30 moles of the Zn compound (as Zn) would be used in the gas stream in the reactor per mol of titanium compound (as Ti) in the reactor. The zinc compound would be introduced into the reactor preferably in the form of a dilute solution (2 to 10 weight percent) in a hydrocarbon solvent or absorbed on a solid diluent material, such as silica, in amounts of about 10 to 50 weight percent. These compositions tend to be pyrophoric. The zinc compound may be added into the recycle gas stream from a feeder adjacent to feeder 27.
It is essential to operate the fluid bed reactor at a temperature below the sintering temperature of the polymer particles. To insure that sintering will not occur, operating temperatures below the sintering temperature are desired. For the production of ethylene polymers in the process of the present invention an operating temperature of about 90.degree. to 105.degree. C. is preferably used to prepare products having a density of about 0.95 to 0.97.
The fluid bed reactor is operated at pressures of up to about 1000 psi, and is preferably operated at a pressure of from about 150 to 350 psi, 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 impregnated precursor composition is injected into the bed at a rate equal to its consumption at a point 30 which is above the distribution plate 20. A gas which is inert to the impregnated precursor composition such as nitrogen or argon is used to carry the composition into the bed. Injecting the precursor composition at a point above the distribution plate is an important feature of this invention. Since the catalysts formed from the impregnated precursor composition used in the practice of the invention 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 which may result in the formation of "hot spots".
Under a given set of operating conditions, the fluidized bed is maintained at essentially a constant height by withdrawing 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 gas across the reactor (the difference between inlet gas temperature and exit gas temperature) is determinative of the rate of particulate polymer formation at a constant gas velocity.
The particulate polymer product is preferably continuously withdrawn at a point 34 at or close to the distribution plate 20 and in suspension with a portion of the gas stream which is vented before the particles settle to preclude further polymerization and sintering when the particles reach the ultimate collection zone. The suspending gas may also be used, as mentioned above, to drive the product of one reactor to another reactor.
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. While valve 38 is closed, valve 36 is opened to emit a plug of gas 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. Valve 38 is then closed to await the next product recovery operation.
Finally, the fluidized 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 highly active supported catalyst system of this invention appears to yield a fluid bed product having an average particle size between about 0.02 to about 0.05 inches and preferably about 0.02 to about 0.04 inches wherein catalyst residue is unusually low. The polymer particles have a relatively round particle shape and a relatively low level of fines (<125 microns) i.e., .ltoreq.4 percent by weight.
The feed stream of gaseous monomer, with or without inert gaseous diluents, is fed into the reactor to achieve a space time yield of about 2 to 10 pounds/hour/cubic foot of bed volume.
The term virgin resin or polymer as used herein means polymer, in granular form, as it is recovered from the polymerization reactor.
The following Examples are designed to illustrate the process of the present invention and are not intended as a limitation upon the scope thereof.
The properties of the polymers produced in the Examples were determined by the following test methods:
______________________________________Density A plaque is made and conditioned for one hour at 120.degree. C. to approach equilibrium crystallinity and is then quickly cooled to room temper- ature. Measurement for density is then made in a density gradient column.Melt Index (MI) ASTM D-1238 - Condition E -- Measured at 190.degree. C. - reported as grams per 10 minutes.Flow Rate (HLMI) ASTM D-1238 - Condition F -- Measured at 10 times the weight used in the melt index test above. ##STR1##Productivity A sample of the resin product is ashed, and the weight percent of ash is determined; since the ash is essentially composed of the catalyst, the productivity is thus the pounds of polymer produced per pound of total catalyst consumed. The amount of Ti, Mg and Cl in the ash are determined by elemental analysis.Bulk Density The resin is poured via 3/8" diameter funnel into a 100 ml graduated cylinder to 100 ml line without shaking the cylinder, and weighed by difference.Average Particle Size This is calculated from sieve analysis data measured according to ASTM-D-1921 Method A using a 500 g. sample. Calculations are based on weight fractions retained on the screens.______________________________________
IA. PREPARATION OF IMPREGNATED PRECURSOR
In a 12 liter flask equipped with a mechanical stirrer are placed 41.8 g (0.439 mol) anhydrous MgCl.sub.2 and 2.5 l tetrahydrofuran (THF). To this mixture, 27.7 g (0.184 mol) TiCl.sub.4 is added dropwise over 1/2 hour. It may be necessary to heat the mixture to 60.degree. C. for about 1/2 hour in order to completely dissolve the material.
The precursor composition may be isolated by recovering it from the solution in THF by crystallization or precipitation. It may also be analyzed at this point for Mg and Ti content since some of the Mg and/or Ti compound may have been lost during the isolation of the precursor composition.
The empirical formulas used herein in reporting the precursor compositions are derived by assuming that the Mg and the Ti still exist in the form of the compounds in which they were first added to the electron donor compound. The amount of electron donor is determined by chromatography.
The impregnated precursor composition is prepared by adding 500 g of porous silica, dehydrated at 800.degree. C., optionally treated with 4 to 8 wt.% triethyl aluminum to the solution prepared above and the mixture is stirred for 1/4 hour. The mixture is dried with a N.sub.2 purge at 60.degree. C. for about 3-5 hours to provide a dry free flowing powder having the particle size of the silica. The absorbed precursor composition has the formula:
TiMg.sub.3.0 Cl.sub.10 (THF).sub.6.7
[In this formula the calculated value of q is 6.5 in excellent agreement with the 6.7 value measured experimentally.]
This procedure can be used to prepare the impregnated precursor on silica treated with triethyl aluminum. Silica, previously dehydrated at 800.degree. C. is treated with triethyl aluminum to produce a silica modified with 4 weight percent triethyl aluminum.
IB. PREPARATION OF IMPREGNATED PRECURSOR FROM PREFORMED PRECURSOR COMPOSITION
In a 12 liter flask equipped with a mechanical stirrer, 146 g of precursor composition is dissolved in 2.5 liters dry THF. The solution may be heated to 60.degree. C. in order to facilitate dissolution. 500 g of porous silica, dehydrated at 800.degree. C., is added and the mixture is stirred for 1/4 hour. The mixture is dried with a N.sub.2 purge at .ltoreq.60.degree. C. for about 3-5 hours to provide a dry free flowing powder having the particle size of the silica.
II. ACTIVATION PROCEDURE
The activator compound is fed to the polymerization reactor for the purpose of activating the precursor composition. It is fed into the reactor as a dilute solution in a hydrocarbon solvent such as isopentane. These dilute solutions contain about 5 to 30% by volume of the activator compound.
The activator compound is added to the polymerization reactor so as to maintain the Al/Ti ratio in the reactor at a level of about .gtoreq.10 to 400:1 and preferably of 15 to 60:1.
EXAMPLES 1 TO 6
Ethylene was homopolymerized in each of this series of Examples with catalyst formed as described above. The silica impregnated precursor composition contained 20 to 35 percent by weight (percent loading) of precursor composition. The activation of the precursor composition in the polymerization reactor was accomplished with triethyl aluminum in a five weight percent isopentane solution so as to provide the activated catalyst in the reactor with an Al/Ti mol ratio of 15-40.
Each of the polymerization reactions was continuously conducted for >1 hour after equilibrium was reached and under a pressure of about 300 psig and a gas velocity of 3 to 4 times G.sub.mf in a fluid bed reactor system at a space time yield of about 3 to 6 lbs/hr/ft.sup.3 of bed space. The reaction system was as described in the drawing above. It has a lower section 10 feet high and 131/2 inches in (inner) diameter, and an upper section which is 16 feet high and 231/2 inches in (inner) diameter.
Table I below lists the percent loading of the precursor composition, type of silica, reaction temperature, H.sub.2 /C.sub.2 mol ratio as well as melt index, melt flow ratio, density, residual titanium, average particle size, bulk density, and fines of the granular virgin resins made in these Examples.
TABLE I__________________________________________________________________________Reaction Conditions Using Catalyst of the Present Invention FinesEx- Percent Loading of Silica treated Residual Average Bulk <125am- the Precursor with triethyl Temp. H.sub.2 /C.sub.2 MI Density Ti Particle Density micronsple Composition aluminum (wt. %) (.degree.C.) Ratio dg/min MFR g/cc ppm Size, in. lbs/ft.sup.3 wt.__________________________________________________________________________ %1 20 Yes (8) 95 0.56 13 26 0.970 10 0.031 28 1.62 20 Yes (8) 105 0.37 9.3 24 0.970 9 0.030 26 1.63 25 Yes (4) 103 0.38 6.8 25 0.968 5 0.024 25 3.54 30 Yes (4) 103 0.45 8.4 26 0.969 7 0.026 26 3.85 30 No 103 0.42 9.9 25 0.970 12 0.031 22 2.26 35 Yes (4) 103 0.42 9.2 28 0.970 17 0.037 22 1.2__________________________________________________________________________
COMPARATIVE EXAMPLES A AND B
A catalyst was prepared according to the procedure as found in U.S. Pat. application Ser. Nos. 892,037 014,412, supra, as follows:
III. PREPARATION OF PRECURSOR COMPOSITION
In a 5 liter flask equipped with a mechanical stirrer, 16.0 g (0.168 Mol) of anhydrous MgCl.sub.2 was mixed with 850 ml of pure tetrahydrofuran under nitrogen. The mixture was stirred at room temperature (.about.25.degree. C.) while 13.05 g (0.069 Mol) of TiCl.sub.4 was added dropwise. After complete addition, the contents of the flask were heated to reflux for about 1/2 to 1 hour to dissolve the solids. The system was cooled to room temperature and 3 liters of pure n-hexane was slowly added over a period of 1/4 hour. A yellow solid precipitated. The supernatant was decanted and the solids were washed with 3X one liter of n-hexane. The solids were filtered and dried in a rotating evaporating flask at 40.degree.-60.degree. C. to give 55 g of solid precursor composition.
IV. Activation Procedure
The activation is conducted in such a way that the precursor composition is only partially activated prior to the introduction thereof into the polymerization reactor, and then the remainder of the activation process is completed within such reactor.
400 grams of silica is charged to a mixing vessel or tank. The silica is then admixed with sufficient amounts of isopentane to provide a slurry system. This usually requires about 4 to 7 ml of diluent per gram of inert carrier. 100 g. of the precursor composition is then charged to the mixing vessel and thoroughly admixed with the slurry composition. The precursor composition has an elemental titanium content of 1.+-.0.1 millimole of Ti per gram of precursor composition. About 5.0 equivalent of triethyl aluminum is added to the contents of the mixing vessel so as to partially activate the precursor composition. The triethyl aluminum is added to the mixing vessel in the form of a solution which contains 20 weight percent of the triethyl aluminum in hexane. The activation is accomplished by thoroughly mixing and contacting the triethyl aluminum compound with the precursor composition. All of the operations described above are conducted at room temperature, and at atmospheric pressure in an inert atmosphere.
The resulting slurry is then dried under a purge of dry inert gas such as nitrogen or argon, at atmospheric pressure at a temperature of .ltoreq.60.degree. C. to remove the hydrocarbon diluent. This process usually requires about 3 to 5 hours. The resulting product is in the form of a dry free-flowing particulate material wherein the activated precursor composition is uniformly blended with the inert carrier. The dried non-pyrophoric product is stored under an inert gas.
The partially activated precursor composition is fed to the polymerization reactor and completely activated in the reactor by the activator compound. The activator compound is fed into the reactor as a dilute solution in a hydrocarbon solvent such as isopentane. These dilute solutions contain about 5 to 30% by volume of the activator compound.
The activator compound is added to the polymerization reactor so as to maintain the Al/Ti ratio in the reactor at a level of about 10 to 400:1 and preferably of 15 to 60:1.
Ethylene was homopolymerized with catalyst formed as described in procedure I and II, of the present invention, and also with catalyst formed as described in procedure III and IV, as found in Ser. Nos. 892,037 014,412. The silica impregnated precursor composition formed by procedure I and II contained 20 percent by weight of precursor composition. This precursor composition as well as the partially activated precursor composition formed in procedure III and IV were activated in the polymerization reactor with triethyl aluminum as a five weight percent isopentane solution so as to provide the activated catalyst in the reactor with an Al/Ti mol ratio of 30 to 40. Each of the polymerization reactions was continuously conducted as described previously.
Table II below lists the melt index, bulk density, particle size, percent fines and plasticizing time of the granular virgin resin produced with the catalyst of the prior art (Example A) and the catalyst produced by the process of the present invention (Example B).
TABLE II______________________________________Comparison of Properties of Polyethylene Prepared WithCatalyst of the Prior Art and the Present Invention EXAMPLE A B______________________________________MI, dg/min 8.4 8.4Density, g/cc 0.968 0.970Bulk Density lb/ft.sup.3 30 25.6Particle size, inches 0.022 0.026Percent fines, <125 microns 10.4 3.8Plasticizing time.sup.(1), sec. 17.5 12.0______________________________________ .sup.(1) A measure of time necessary to prepare molten polymer from the resin in a standard injection molding machine.
The data of this table show that materials produced with the present invention contain less fines and exhibit lower plasticizing times.
COMPARATIVE EXAMPLES C AND D
A catalyst was prepared according to the procedure as found in U.S. Pat. No. 4,302,565 supra, as follows:
V. PREPARATION OF IMPREGNATED PRECURSOR COMPOSITION
The impregnated precursor composition was prepared by the method as described in procedure Ia., supra.
VI. ACTIVATION PROCEDURE
The desired weights of impregnated precursor composition and activator compound are added to a mixing tank with sufficient amounts of anhydrous aliphatic hydrocarbon diluent such as isopentane to provide a slurry system.
The activator compound and precursor compound are used in such amounts as to provide a partially activated precursor composition which has an Al/Ti ratio of <0 to .ltoreq.10:1 and preferably of 4 to 8:1.
The contents of the slurry system are then thoroughly mixed at room temperature and at atmospheric pressure for about 1/4 to 1/2 hour. The resulting slurry is then dried under a purge of dry inert gas such as nitrogen or argon, at atmospheric pressure and at a temperature of 65.degree..+-.10.degree. C. to remove the hydrocarbon diluent. This process usually requires about 3 to 5 hours. The resulting catalyst is in the form of a partially activated precursor composition which is impregnated within the pores of the silica. The material is a free flowing particulate material having the size and shape of the silica. It is not pyrophoric unless the aluminum alkyl content exceeds a loading of 10 weight percent. It is stored under a dry inert gas such as nitrogen or argon prior to future use. It is now ready for use by being injected into, and fully activated within, the polymerization reactor.
When additional activator compound is fed to the polymerization reactor for the purpose of completing the activation of the precursor composition, it is fed into the reactor as a dilute solution in a hydrocarbon solvent such as isopentane. These dilute solutions contains about 5 to 30% by volume of the activator compound.
The activator compound is added to the polymerization reactor so as to maintain the Al/Ti ratio in the reactor at a level of about .gtoreq.10 to 400:1 and preferably of 15 to 60:1.
Ethylene was homopolymerized in each of Examples C and D with catalyst found as described by Procedure V and VI, above. The silica impregnated precursor composition (q calculated=6.5; q measured=6.5) contained 15 (Example C) and 20 (Example D) percent by weight (percent loading) of precursor composition. The partially activated precursor composition (Al/Ti=5) was completely activated in the polymerization reactor with triethyl aluminum so as to provide the activated catalyst in the reactor with an Al/Ti mol ratio of 30 to 40. Each of the polymerization reactions was conducted for >1 hour after equilibrium was reached and under a pressure of about 300 psig, and temperature of 105.degree. C., according to the procedure as previously described.
Table III below lists percent loading of the precursor composition, H.sub.2 /C.sub.2 mol ratio as well as melt index, melt flow ratio, density, residual titanium, average particle size and bulk density of the granular virgin resins made in these Examples.
TABLE III______________________________________Properties of Polyethylene Prepared With Catalystsof the Prior Art EXAMPLE C D______________________________________Percent loading of theprecursor composition 15 20H.sub.2 /C.sub.2 ratio 0.358 0.381MI, dg/min. 6.2 7.1MFR 26.7 25.8Density, g/cc 0.969 0.969Residual Ti, ppm 25 28Average particle size, in. 0.019 0.02Bulk density lbs/cu ft. 28.7 28.1______________________________________
The data of Table III show that the polyethylene produced with the catalysts of the prior art contains such higher residual titanium than the polyethylene produced with the catalyst of the present invention.

Number	Name	Date
3787384	Stevens et al.	Jan 1974
3790550	Miller	Feb 1974
4035560	Caumartin et al.	Jul 1977
4086409	Karol et al.	Apr 1978
4098979	Maemoto et al.	Jul 1978
4113933	Schweier et al.	Sep 1978
4124532	Giannini et al.	Nov 1978
4243619	Fraser et al.	Jan 1981
4302566	Karol et al.	Nov 1981

Process for the preparation of high density ethylene polymers in fluid bed reactor

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