WO1999012982A1

WO1999012982A1 - A process for the preparation of polyethylene

Info

Publication number: WO1999012982A1
Application number: PCT/US1998/019185
Authority: WO
Inventors: Jose Fernando Cevallos-Candau; Dale Andrew Wright; Theodore Randolph Engelmann; François Alexandre; Jesus Sergio Tijerina; Stephen Paul Jaker
Original assignee: Union Carbide Chemicals & Plastics Technology Corporation
Priority date: 1997-09-12
Filing date: 1998-09-11
Publication date: 1999-03-18
Also published as: AR017084A1; ZA986048B

Abstract

A process for preparing high density polyethylene in the gas phase comprising contacting ethylene or a mixture comprising ethylene and one or more alpha-olefins with a titanated porous silica supported chromium oxide catalyst wherein the chromium is in the oxidation state of plus 6 in a fluidized bed reactor having a recycle gas line, under polymerization conditions, with the following provisos: (i) the atomic ratio of chromium to titanium is in the range of about 0.008:1 to about 2.8:1; (ii) the amount of catalyst is in the range of about 0.002 to about 0.05 part by weight based on 100 parts by weight of the high density polyethylene; (iii) at least about 70 percent of the pores of the silica are larger than 200 Angstroms; (iv) the partial pressure of ethylene is in the range of about 100 to about 350 psia; (v) oxygen and/or another catalyst poison is introduced into the reactor in the range of about 0.05 to about 0.6 part by volume of catalyst poison per million parts by volume of ethylene; (vi) the molar ratio of alpha-olefin, if present, to ethylene is about 0.001:1 to about 0.004:1; (vii) hydrogen is introduced into the reactor in the range of about 0.1 to about 0.6 mole of hydrogen per mole of ethylene; (viii) the polymerization is carried out at a temperature in the range of about 80 to about 120 degrees C; and (ix) a relatively low boiling inert hydrocarbon is introduced into the recycle gas line where it raises the dew point of the recycle gas, which is comprised of ethylene and other reactor gases, and the recycle gas is partially condensed and recycled to the reactor where it promotes cooling by evaporation.

Description

A Process for the Preparation of Polyethylene

Technical Field

This invention relates to a process for preparing a high density polyethylene, which is particularly useful in blow molding applications.

Background Information

High density polyethylene (HDPE), which spans a density range from 0.940 to 0.960 gram per cubic centimeter, finds application in injection molding, rotational molding, sheet, tubing, hose, pipe, and geomembranes and, of particular interest here, blow molding.

To be competitive, HDPE products for blow molding should have bottle weights in the range of about 65 to about 80 grams according to a bottle weight test. The test is an arbitrary one selected by a resin manufacture to meet customer's specifications. The purpose of the test is to determine a resin's relative swell characteristics by extruding and weighing a bottle made with a standard length parison and a constant extrusion time. Bottle weight is a function of extruded parison wall thickness swell where the parison wall is thicker than the die gap. This type of swell is measured using a bottle weight test where the die gap and parison formation time are held constant. The parison diameter swell occurs when the parison diameter is greater than the overall diameter of the die opening. A detailed definition of parison swell can be found on pages 664 to 668 of the Blow Molding Handbook, edited by Rosato et al, published by Oxford University Press, New York, 1989. Thus, it becomes important to provide a catalyst together with a process for the preparation of blow molding resins, which have those properties necessary for the control of bottle weight (also referred to as diameter swell) during the blow molding process. Further, the HDPE should have consistently good extrusion processability.

Industry is continuously striving to find HDPE's, which, are easily processed by conventional blow molding techniques, and, in product form, reflect desired bottle weights.

Disclosure of the Invention

An object of this invention, therefore, is to provide HDPE resins which, under blow molding conditions, provide desirable bottle weights in the blow molded products, and have a high level of processability. Other objects and advantages will become apparent hereinafter.

According to the present invention, such a process has been discovered. The process is one for preparing high density polyethylene in the gas phase comprising contacting ethylene or a mixture comprising ethylene and one or more alpha-olefins with a titanated porous silica supported chromium oxide catalyst wherein the chromium is in the oxidation state of plus 6 in a fluidized bed reactor having a recycle gas line, under polymerization conditions, with the following provisos:

(i) the atomic ratio of chromium to titanium is in the range of about 0.008:1 to about 2.8:1; (ii) the amount of catalyst is in the range of about 0.002 to about 0.05 part by weight based on 100 parts by weight of the high density polyethylene;

(iii) at least about 70 percent of the pores of the silica are larger than 200 Angstroms;

(iv) the partial pressure of ethylene is in the range of about 100 to about 350 psia;

(v) oxygen and/or another catalyst poison is introduced into the reactor in the range of about 0.05 to about 0.6 part by volume of catalyst poison per million parts by volume of ethylene;

(vi) the molar ratio of alpha-olefin, if present, to ethylene is about 0.001:1 to about 0.004:1;

(vii) hydrogen is introduced into the reactor in the range of about 0J to about 0.6 mole of hydrogen per mole of ethylene;

(viii) the polymerization is carried out at a temperature in the range of about 80 to about 120 degrees C; and

(ix) a relatively low boiling inert hydrocarbon is introduced into the recycle gas line where it raises the dew point of the recycle gas, which is comprised of ethylene and other reactor gases, and the recycle gas is partially condensed and recycled to the reactor where it promotes cooling by evaporation.

Description of the Preferred Embodiment(s)

The HDPE is either a homopolymer of ethylene or a copolymer of ethylene and one or more alpha-olefins. The alpha-olefin can have 3 to 12 carbon atoms, and preferably has 3 to 8 carbon atoms. Examples of the alpha-olefins are propylene, 1-butene, 1-pentene, 1-hexene, 4- methyl-1-pentene, and 1-octene. One or two alpha-olefins are preferred. The most preferred alpha-olefin is 1-hexene. The resin can have a melt index (I2) in the range of about OJ to about 2 grams per 10 minutes, and preferably has a melt index (12) in the range of about 0.2 to about 1.2 gram per 10 minutes. It can also have a melt index (I5) of about 0.5 to about 5 grams per 10 minutes, and preferably has a melt index (I5) of about 0.8 to about 4 grams per 10 minutes. The resin can have a flow index (I21) in the range of about 10 to about 100 grams per 10 minutes, and preferably has a flow index in the range of about 15 to about 70 grams per 10 minutes.

Melt index (I₂) is determined under ASTM D-1238, Condition E. It is measured at 190°C and 2.16 kilograms and reported as grams per 10 minutes. Melt index (I5) can be determined under ASTM D-1238, Condition P. It is measured at 190°C and 5 kilograms and reported as grams per 10 minutes. Flow index (I21) is determined under ASTM D- 1238, Condition F. It is measured at 190°C and 21.6 kilograms, and reported as grams per 10 minutes. Melt flow ratio is the ratio of flow index to melt index.

The melt flow ratio (I21/I2) of the HDPE can be in the range of about 40 to about 150 , and is preferably in the range of about 55 to about 120.

The density of the HDPE can be in the range of 0.940 to 0.967 gram per cubic centimeter, and is preferably in the range of 0.945 to 0.965 gram per cubic centimeter. The polymerization of ethylene or ethylene and alpha-olefin(s) in the presence of a catalyst system comprised of a titanated silica supported chromium oxide wherein the chromium is in the oxidation state of plus 6 is conventional.

The support can be prepared in a conventional manner by modifying the silica with a titanium tetrahydrocarbyloxide compound SLich as titanium tetraisopropoxide. A typical support is a dehydrated, solid, particulate porous material essentially inert to the polymerization. It is used as a dry powder having an average particle size of about 2 to about 250 microns and preferably about 30 to about 180 microns; a surface area of about 100 to about 750 square meters per gram and preferably about 240 to about 550 square meters per gram; and a pore size of about 80 Angstroms to about 300 Angstroms with the proviso that at least 70, and preferably at least 85, percent of the pores are larger than 200 Angstroms.

The amount of support used is generally that which will provide about 3.5 to about 105 grams of support per millimole of chromium and preferably about 7 to about 52 grams of support per millimole of chromium, and about 0.8 to about 10 grams of support per millimole of titanium and preferably about 0.95 to about 1.9 grams of support per millimole of titanium. Most preferred is about 15 grams of support per millimole of chromium and about 0.78 gram of support per millimole of titanium.

An example of the catalyst and the sLipport and the preparation of the catalyst system follows: The catalyst has three components: chromium (III) acetate (chrome) on silica having an average particle size of about 50 microns; a surface area of about 300 (silica I) or 250 (silica II) square meters per gram; and a percentage of pores with a diameter greater than 200 Angstroms of 45 (silica I) or 85 (silica II); and tetraisopropyl titanate (50 percent by weight concentration in n-hexane). The chrome on silica is fluidized and heated under dry nitrogen to 150 degrees C at 50 degrees C per hour, and then held at 150 degrees C for 4 hours. The dried chrome on silica is then slurried in isopentane at a 2.8:1 weight ratio of isopentane to chrome on silica. The tetraisopropyl titanate is added at 55 degrees C and is allowed to react for 2 hours. The mix is then dried at 80 degrees C until less than 1 percent by weight organics remain on the solid. The chromium is activated to provide a chromium (VI) compound, i.e., CrO,₃, for 6 hours at 860 degrees C. Undesirable reactions are reduced with high air/catalyst ratios and allowing water to escape at lower temperatures. A chromium catalyst system is described in United States patent 3,622,521.

The polymerization is conducted in the gas phase using a fluidized process. It is preferably carried out in the continuous mode. A typical fluidized bed reactor is described in United States patent 4,482,687.

The most important process parameters are as follows:

(i) the atomic ratio of chromium to titanium can be in the range of about 0.008:1 to about 2.8:1, and is preferably in the range of about 0.04:1 to about 0.26:1. Most preferred is a ratio of about 0.09:1. (ii) the amount of catalyst can be in the range of about 0.002 to about 0.05 part by weight based on 100 parts by weight of high density polyethylene, and is preferably in the range of about 0.005 to about 0.015 part by weight of high density polyethylene. Typically, the amount of catalyst is about 0.01 part by weight.

(iii) at least about 70 percent of the pores of the silica are larger than 200 Angstroms, and preferably at least about 85 percent of the pores are larger than 200 Angstroms;

(iv) the partial pressure of ethylene can be in the range of about 100 to about 350 psia (pounds per square inch absolute), and is preferably in the range of about 150 to about 300 psia. Most preferred is a partial pressure in the range of about 200 to about 250 psia.

(v) oxygen can be introduced into the reactor in the range of about 0.05 to about 0.6 part by volume per million parts by volume of ethylene feed, and is preferably added in the range of about 0.08 to about 0.35 part by volume per million parts by volume of ethylene feed. The most preferred amount is in the range of about 0.18 to about 0.25 ppmv (part by volume per million parts by volume) of ethylene feed. This can be referred to as "Oxygen Add-Back", which is a dilute system of oxygen in nitrogen added to the reactor in a controlled flow relative to the feed rate of ethylene to the reactor to achieve the desired oxygen add-back level in ppmv. As an alternative to or in combination therewith, other catalyst poisons can be used to produce the same effect. These catalyst poisons can be introduced from external sources just as the oxygen or they can be present as impurities in the ethylene feed or other gases or liquids added to the re actor. Examples of useful and preferred catalyst poisons, in addition to oxygen, are acetone and other oxygen bearing compounds, methanol and other hydroxyl bearing compounds, and water. Various nitrogen, phosphorus, sulfur, arsenic and halogen bearing compounds can also be useful in this respect, but are less commonly encountered as impurities in ethylene feed or other gases or liquids added to the reactor. One of the effects of the catalyst poison is to lower the molecular weight of the polymer. This is reflected in increased melt and flow indices. In any case, oxygen is the preferred catalyst poison.

(vi) the molar ratio of alpha-olefin, if present, to ethylene can be about 0.001:1 to about 0.004:1, and is preferably about 0.0012:1 to about 0.003:1. Most preferred is a molar ratio of about 0.0015:1 to about 0.0025:1.

(vii) hydrogen can be introduced into the reactor in a molar ratio of about 0.1 to about 0.6 mole of hydrogen per mole of ethylene, and is preferably introduced in a molar ratio of about 0.15:1 to about 0.3:1. Most preferred is a molar ratio of about 0.25:1.

(viii) the polymerization can be carried out at a temperature in the range of about 80 to about 120 degrees C, and is preferably carried out at a temperature in the range of about 108 to about 116 degrees C. Most preferred is a temperature in the range of about 110 to about 113 degrees C.

(ix) a relatively low boiling inert hydrocarbon can be introduced into the reactor and passed through the recycle line where it is vaporized when it is introduced upstream of the condenser. The hydrocarbon is introduced in an amount sufficient to raise the dew point temperature of the cycle gas. The cycle gas is partially condensed in the cycle gas cooler, and is recycled to the reactor to promote cooling by evaporation. The hydrocarbon can also be introduced into the recycle gas line downstream from the condenser. This condensing mode technique is discussed in more detail below.

The pressure, i.e., the total pressure in the reactor, can be in the range of about 250 to about 515 psia (pounds per square inch absolute) and is preferably in the range of about 300 to about 415 psia. The ethylene partial pressure is set as noted above. The balance of the total pressure is provided by alpha-olefin(s) and/or an inert gas such as nitrogen.

Nolumetric production rates are reported in pounds per hour per cubic foot. This is also referred to as Space/TimeΥield (STY). The STY for the process of the invention can be about 5 to about 22 lbs/hr/ft³ (pounds per hour per cubic foot), and is preferably about 6 to about 14 lbs/hr/ft³.

A typical fluidized bed reactor can be described as follows and is also described in United States patent 4,482,687.

The bed is usually made up of the same granular resin that is to be produced in the reactor. Thus, during the course of the polymerization, the bed comprises formed polymer particles, growing polymer particles, and catalyst particles fluidized by polymerization and modifying gaseous components introduced at a flow rate or velocity sufficient to cause the particles to separate and act as a fluid. The fluidizing gas is made up of the initial feed, make-up feed, and cycle (recycle) gas, i.e., ethylene, other alpha-olefins, if any, and/or an inert carrier gas, and other reactor gases. As noted, a low boiling inert hydrocarbon is also added to the reactor. When added upstream of the cycle gas cooler, it vaporizes and becomes a part of the cycle gas. This hydrocarbon generally boils (normal boiling point at atmospheric pressure) at a temperature in the range of about minus 10 to about plus 100 degrees C. Examples of these hydrocarbons are isobutane, isopentane, hexane, and heptane. Isopentane and hexane are preferred. Isopentane can beintroduced into the recycle line in an amount of about 2.5 to about 25 parts by volume per 100 parts by volume of cycle gas. Hexane can be introduced in an amount of about 1 to about 10 parts by volume per 100 parts by volume of cycle gas. The hydrocarbons can be introduced in greater amounts to achieve higher STYs.

The essential parts of the reaction system are the vessel, the bed, the gas distribution plate, inlet and outlet piping, a cycle or recycle gas line, a compressor, cycle gas cooler, and a product discharge system. In the vessel, above the bed, there is a velocity reduction zone, and, in the bed, a reaction zone. Both are above the gas distribution plate. The inert hydrocarbon enters the recycle line, raises the dew point temperature of the cycle (or recycle) gas, is partially condensed in the cycle gas cooler (condenser) when it is added upstream of the cooler, and then passes into the reactor where, along with other condensed cycle gas, it vaporizes (flashes off) and cools the exothermic polymerization reaction. This is a feature of the condensing mode technique described, for example, in United States patents 4,543.399 and 4,588,790. The condensed level of the cycle gas at the cycle gas cooler outlet can be about 2.5 to about 25 percent by weight based on the weight of the cycle gas, and is preferably about 5 to about 20 percent by weight. Most preferred is about 7.5 to about 15 weight percent. The condensing mode is considered to increase the production rate by providing additional cooling in the reaction zone. The gaseous feed streams of ethylene, other alpha-olefins (if any), hydrogen, and oxygen are preferably fed to the reactor recycle line as well as liquid alpha-olefins (if any) and catalyst. The catalyst can be fed as a solid or a mineral oil slurry or in another hydrocarbon medium. Optionally, the catalyst can be fed directly to the fluidized bed. The product composition can be varied by changing the molar ratios of the alpha-olefins introduced into the fluidized bed. The product is continuously discharged in granular or particulate form from the reactor as the bed level builds up with polymerization. The production rate is controlled by adjusting the catalyst feed rate. The reactor temperature and/or oxygen add-back can be adjusted to control average molecular weights.

The residence time of the mixture of reactants including gaseous and liquid reactants, catalyst, and resin in the fluidized bed reactor can be in the range of about 1 to about 4 hours and is preferably in the range of about 1.25 to about 3 hours.

The HDPE resin can be extruded into various products in a conventional extruder adapted for the particular product desired. Extruders and processes for extrusion are described in United States patents 4,814,135; 4,857,600; 5,076,988; and 5,153,382. A typical single screw type extruder can be described as one having a hopper at its upstream end and a die at its downstream end. The hopper feeds into a barrel, which contains a screw. At the downstream end, between the end of the screw and the die, is a screen pack and a breaker plate. The screw portion of the extruder is considered to be divided up into three sections, the feed section, the compression section, and the metering section, and multiple heating zones from the rear heating zone to the front heating zone, the multiple sections and zones running from upstream to downstream. If it has more than one barrel, the barrels are connected in series. The length to diameter ratio of each barrel is in the range of about 16:1 to about 30:1. The extrusion can take place at temperatures in the range of about 160 to about 270 degrees C, and is preferably carried out at temperatures in the range of about 180 to about 240 degrees C.

Of particular interest here is the blow molding process. A description of a typical blow molding apparatus and process can be found in the Blow Molding Handbook referred to above. Typical conditions are described at pages 530 to 535.

The advantages of the invention lie mainly in the blow molded products which are produced from the HDPE made from the process of this invention. In the case of bottles, the bottle weight is easily controlled within the range of about 10 to about 400 grams. The HDPE also shows consistently good extrudability and the products, better trimability.

Conventional additives, which can be introduced into HDPE's, are exemplified by antioxidants, ultraviolet absorbers, anti-static agents, pigments, dyes, nucleating agents, fillers, slip agents, fire retardants, plasticizers, processing aids, lubricants, stabilizers, smoke inhibitors, viscosity control agents, and crosslinking agents, catalysts, and boosters, tackifiers, and anti-blocking agents. Aside from the fillers, the additives can be present in the blend in amounts of about 0.05 to about 5 parts by weight of additive for each 100 parts by weight of polymer blend. Fillers can be added in amounts up to 20 parts by weight and more for each 100 parts by weight of the blend.

Generally, these products contain a primary antioxidant, a secondary antioxidant, and, in many cases, a processing aid. An example of a primary antioxidant is IRGANOX™ 1010, and an example of a secondary antioxidant is IRGAFOS™ 168. Examples of processing aids are calcium stearate, zinc stearate, and fluoroelastomers. A preferred additive system includes IRGχA.NOX™ 1010 and IRGAFOS™ 168 antioxidants. In parts per million by weight, preferred amounts are about 500 to about 2,000 ppmw for each of the primary and secondary antioxidants. and about 200 to about 800 ppmw for the processing aid. The ppmw are based on a million parts by weight of ethylene.

Patents mentioned in this specification are incorporated by reference herein.

The invention is illustrated by the following examples.

Examples 1 to 5

A polymerization is carried out in a typical fluidized bed reactor as described above using as a catalyst a titanated silica supported chromium oxide wherein the chromium is in the oxidation state of plus 6 and the support is modified; at least 85 percent of the pores of the silica are larger than 200 Angstroms (except in examples 1 and 5 where only 45 percent of the pores are larger than 200 Angstroms); and the atomic ratio of chromium to titanium is 0.12:1. The preparation of the catalyst is described above. The catalyst is introduced into the reactor in an amount of 0.008 to 0.03 part by weight per 100 parts by weight of high density polyethylene product. The feed into the recycle line is comprised of ethylene, hexane, hydrogen, and oxygen. This is the cycle gas. The balance of the total pressure is made up with nitrogen. Variables and results are set forth in the Table. Hexane is introduced in an amount of 4 to 6 parts by volume based on 100 parts by volume of cycle gas. The HDPE product is recovered and blow molded on an improved IMPCO™B-15 reciprocating screw, single head blow molding machine using 1.625 inch diameter converging tooling.

Table

Example 1 2 3

T deg C 112 112 110.5

Total psia 310 310 307.6 pressure

C₂PP psia 185 185 177.2

H2/C2 molar 0.35 0.35 0.266

O2/C2 ppmv 0.0056 0.0056 0.3475

Res Time hours 2.7 3 2.14

MI(I₂) g/lOmin 0.72 0.64 0.88

FKI21) g/lOmin 46.6 44.4 68.7

MFR 64.8 69.3 69.6

Level of % by wt 0 0 0 condensing

Bottle grams 73.4 71.9 75 Weight

Expected grams about 74 about 70 about 74 bottle to 72 weight (dry mode)

Density g/cc 0.9605 0.9607 0.960

Catalyst lbs/lb of 11,400 11,600 3840 average catalyst productivity

Extrud- good good good ability Table (continued)

Example 4 5 6

T deg C 109.5 111.2 113

Total psia 310 307 302 pressure

C₂PP psia 195.3 178 230.4

H2/C2 molar 0.09 0.24 0.0387

O2/C2 ppmv 0.403 0.279 0.339

Res Time hours 1.87 2.1 2.8

MI(I₂) g/lOmin 0.94 1.02 1.01

FI(I₂₁) g/lOmin 66.5 63.9 62.9

MFR 71 62.6 62.2

Level of % by wt 7 to 9 9 0 condensing

Bottle grams 71.8 72.2 73.7

Weight

Expected grams 75 to 76 See Notes about 7' bottle Below to 74 weight

(dry mode)

Density g/cc 0.961 0.961 0.9604

Catalyst lbs/lb of 3250 5800 7800 average catalyst productivity

Extrud- good good good ability Notes to Table:

T = temperature in degrees C.

Total pressure = total pressure in psia of gases in reactor, i.e., ethylene, hydrogen, oxygen, nitrogen, hexane vapor

C₂PP = ethylene partial pressure in pounds per square inch absolute.

H2/C2 = molar ratio of hydrogen to ethylene.

O2/C2 = part by volume of oxygen per million parts by volume of ethylene feed to the reactor. In example 2, the process takes advantage of another catalyst poison present in the ethylene feed in an amount of at least 0.005 ppmv.

Res Time = residence time in hours.

Mlfø) = melt index at 2.16 kilograms in grams per 10 minutes.

FI(∑2i) = flow index at 21.6 kilograms in grams per 10 minutes.

MFR(I₂ι/l2) = FI(I₂ι) / MI(I₂) = melt flow ratio

Level of condensing in % by wt = percent by weight of cycle gas, which condenses in the cycle gas cooler and vaporizes in the reactor

Bottle weight = a function of the extruded parison wall thickness swell where the parison wall is thicker than the die gap.

Bottle weight test = As noted above, the test is an arbitrary one selected by a resin manufacture to meet customer's specifications. The purpose of the test is to determine a resin's relative swell characteristics by extruding and weighing a bottle made with a standard length parison and a constant extrusion time. Expected bottle weight in dry mode = the bottle weight, which is expected when the reactor is not run in the condensing mode. In example 5, the catalyst used nullified the effect of the condensing mode. Under same conditions, but in dry mode, the expected bottle weight would be 3 grams more.

Density in gram per cubic centimeter.

Extrudability is a subjective determination based on the extrusion line operator's experience.

Claims

1. A process for preparing high density polyethylene in the gas phase comprising contacting ethylene or a mixture comprising ethylene and one or more alpha-olefins with a titanated poro╧ä╬╣s silica supported chromium oxide catalyst wherein the chromium is in the oxidation state of plus 6 in a fluidized bed reactor having a recycle gas line, under polymerization conditions, with the following provisos:

(i) the atomic ratio of chromium to titanium is in the range of about 0.008:1 to about 2.8:1;

(ii) the amount of catalyst is in the range of about 0.002 to about 0.05 part by weight based on 100 parts by weight of the high density polyethylene;

(vii) hydrogen is introduced into the reactor in the range of about 0.1 to about 0.6 mole of hydrogen per mole of ethylene;

(viii) the polymerization is carried out at a temperature in the range of about 80 to about 120 degrees C; and (ix) a relatively low boiling inert hydrocarbon is introduced into the recycle gas line where it raises the dew point of the recycle gas, which is comprised of ethylene and other reactor gases, and the recycle gas is partially condensed and recycled to the reactor where it promotes cooling by evaporation.

2. The process defined in claim 1 wherein

(i) the atomic ratio of chromium to titanium is in the range of about 0.04:1 to about 0.26:1;

(ii) the amount of catalyst is in the range of about 0.005 to about 0.015 part by weight based on 100 parts by weight of high density polyethylene;

(iii) at least about 85 percent of the pores of the silica are larger than 200 Angstroms;

(iv) the partial pressure of ethylene is in the range of about 150 to about 300 psia;

(v) oxygen is introduced into the reactor in the range of about 0.08 to about 0.35 part by volume per million parts by volume of ethylene;

(vi) the molar ratio of alpha-olefin, if present, to ethylene is in the range of about 0.0012:1 to about 0.003:1;

(vii) hydrogen is introduced into the reactor in a ratio of about 0.15 to about 0.3 mole of hydrogen per mole of ethylene; and

(viii) the polymerization is carried out at a temperature in the range of aboiit 108 to about 116 degrees C.

3. The process defined in claim 2 wherein

(i) the atomic ratio of chromium to titanium is about 0.09:1;

(ii) the partial pressure of ethylene is in the range of about 200 to about 250 psia;

(iii) oxygen is introduced into the reactor in the range of about 0.18 to about 0.25 part by volume per million parts of ethylene;

(v) hydrogen is introduced into the reactor in a molar ratio of about 0.25 mole of hydrogen per mole of ethylene; and

(vi) the polymerization is carried out at a temperature in the range of about 110 to about 113 degrees C.

4. The process defined in claim 1 wherein, in proviso 8, the condensed level of the inert hydrocarbon is about 2.5 to about 25 percent by weight based on the weight of the gases passing through the recycle gas line.

5. The process defined in claim 1 wherein the STY is about 5 to about 22 pounds of high density polyethylene per hour per cubic foot of reactor.

6. The process defined in claim 1 wherein the high density polyethylene prepared by the process has the following properties:

(a) density = 0.940 to 0.967 gram per cubic centimeter;

(b) flow index (I21) = about 10 to about 100 grams per 10 minutes; (c) melt index (I2) = about 0.1 to about 2 grams per 10 minutes; and

(d) melt flow ratio (I21/I2) = about 40 to about 150.

7. A process for preparing high density polyethylene in the gas phase comprising contacting ethylene or a mixture comprising ethylene and one or more alpha-olefins with a titanated porous silica supported chromium oxide catalyst wherein the chromium is in the oxidation state of plus 6 in a fluidized bed reactor having a recycle gas line, under polymerization conditions, with the following provisos:

(ii) the amount of catalyst is in the range of about 0.005 to about 0.015 part by weight based on 100 parts by weight of the high density polyethylene;

(v) oxygen and/or another catalyst poison is introduced into the reactor in the range of about 0.08 to about 0.35 part by volume of catalyst poison per million parts by volume of ethylene;

(vi) the molar ratio of alpha-olefin, if present, to ethylene is about 0.0012:1 to about 0.003:1;

(vii) hydrogen is introduced into the reactor in the range of about 0.15 to about 0.3 mole of hydrogen per mole of ethylene; (viii) the polymerization is carried out at a temperature in the range of about 108 to about 116 degrees C; and