CS239968B1 - Process for preparing a supported catalyst - Google Patents

Abstract

The solution relates to a method for producing a supported catalyst for the polymerization of ethylene based on silica impregnated with bis-triphenylsilylchromate (BTFSCH) and organoaluminum compounds while stirring in a suspension in isopentane and finally freed from solvents and dried. Before impregnation on silica, BTFSCH is flooded in a closed vessel with an aliphatic hydrocarbon with 5 to 7 carbon atoms in an amount sufficient to form a well-flowing suspension and at a temperature below the boiling point of the hydrocarbon it is stripped with an inert gas and the thus purified BTFSCH is used to impregnate silica in a suspension in isopentane and the preparation is completed as stated above. The process according to the invention provides a catalyst with higher activity, better flowability and reduces the formation of polymer grain clumps.

Description

The present invention relates to the production of a supported catalyst for ethylene polymerization. In the production of polyethylene by gas phase polymerization at a relatively low overpressure of about 20 bar, a catalyst is used in the form of finely divided silica on which the active ingredients are anchored. The properties of the catalyst greatly affect the production process and the quality of the polymer produced. It requires a high catalyst activity, perfect flowability, uniform properties in parts of one batch, and high reproducibility of the characteristics of each batch. Failure to observe or vary these properties leads to problems such as high ash content in the polymer, fluctuations in polymer molecular weight distribution, undesirable changes in granular polymer granulometry, but also process problems such as clogging of catalyst dosage capillaries or major process deterioration leading to large polymer agglomerates. reduce or even block the throughput of the equipment and force downtime and cleaning with all associated losses.

The preparation of the catalyst, starting from the activation of the silica, the deposition of the components, the storage and the feed into the polymerization reactor, all possible care must be taken to prevent even the slightest traces of atmosphere from reaching the unfinished or finished catalyst. Atmospheric humidity and oxygen deteriorate the quality of the catalyst, either by destroying the active ingredient by chemical reaction or sorption on the oil and by affecting the polymer growth process at the active centers of the catalyst during polymerization. Similarly, care should be taken to ensure the purity of the auxiliary chemicals for catalyst preparation and to avoid irregular or accidental access of other chemicals to the catalyst.

One type of catalyst used for the said ethylene polymerization utilizes organometallic compounds of aluminum and chromium as active compounds. The amount of these active compounds is 239,968, relative to the amount of silica i. The commonly used organometallic bi-chromium compound -TTTfSTTylsilylchromate (hereinafter referred to as BTFSCH), having a molecular weight of about 635 g / mol, is normally a solid, yellowish to brown in air under light and commonly available as a fine powder of different yellowish color, which contains impurities resulting from the photochemical process, as well as components of the atmosphere both sorbed on the surface of the particles and filling the space between the particles.

For example, the prior art process for preparing a catalyst is by adding a batch of powdered BTFSCH free of harmful atmospheric residues to a batch of activated silica suspended in a hydrocarbon in a mixer under a strictly inert atmosphere. This depletion of harmful atmospheric residues is accomplished by pressurizing pure BTFSCH in a closed vessel and then shortly thereafter releasing nitrogen pressure into the off-gases. This overprinting and pressure release is repeated several times. Then the batch of BTFSCH is transferred to the mixer. The residues are flushed with hydrocarbon into the mixer and the prescribed amount of hydrocarbon is added to the mixer. The sorption of BTFSCH on silica takes place under constant agitation and at a temperature of 25 to 45 ° C for several hours. A solution of the organometallic of aluminum in the hydrocarbon is then added and allowed to react while stirring. The final stage of the preparation is the withdrawal of the hydrocarbon with organometallic residues and drying of the impregnated silica at elevated temperature and nitrogen purge for several hours to obtain a dry finished catalyst, as it is known that the catalyst prepared so far in operation often has different activity from batch to batch. the catalyst metering capillaries are clogged, and uneven yellowish discoloured especially in particulate agglomerates has been found on catalyst samples taken in a strictly inert atmosphere. The catalyst prepared in the laboratory has different flow properties and that larger polymer slices of the resulting polymer are present in the polymer.

SUMMARY OF THE INVENTION The present invention provides a process for the preparation of a supported catalyst based on bis-triphenylsilylchromate and an organoaluminum compound for the polymerization of ethylene, wherein the silica is

239, 968, impregnated the bis-triphenylsilylchromate in a 5 to 7 carbon aliphatic hydrocarbon suspension, added an organoaluminum compound solution to the formed intermediate suspension, separated the liquid phase, and dried the resulting catalyst by improving the bis-triphenylsilyl chromate by impregnation on silica, it will flood in a closed vessel with an aliphatic hydrocarbon having 5 to 7 carbon atoms in an amount sufficient to form a well-flowing suspension and stripped at least once with an inert gas at a flow rate of 0.001 to 0.02 »/ s, the largest horizontal cross-section of the vessel up to 5 to 50% vol.

In said stripping, the suspension is maintained in circulation and circulated by rising bubbles of pure nitrogen, and after a relatively short time of usually several tens of minutes, the undesirable atmospheric residues and volatile impurities are completely removed.

The thus purified BTFSCH in suspension in the hydrocarbon is discharged and flushed with the hydrocarbon into the mixer and the catalyst preparation is completed in the conventional manner described above. A catalyst of higher polymerization activity, perfect flowability and m / uniform coloration is obtained. In the polymerization / test with this catalyst, a granular polymer with a smaller amount of slices is produced, the reproducibility of the properties of the individual batches of the catalyst being high.

The use of an aliphatic hydrocarbon, in particular C8 to _C7J, is particularly advantageous because these hydrocarbons take up said undesirable BTFSCH impurities and are virtually inert during the preparation of the catalyst in the mixer.

The new catalyst preparation process can also be successfully operated at elevated hydrocarbon temperature - but below its boiling point under particular nitrogen gas pressure conditions. As with temperature, the new process can be operated intermittently within the range of 5 to 50% by volume of the initial amount at a flow rate of v = 0.001 to 0.020 m / s based on the largest horizontal vessel diameter, each time the hydrocarbon replenishes on interruption of stripping to the initial volume or to another known volume.

The following example illustrates the novel method of catalyst preparation and its convenience. % in the example given are by weight.

Example ^{239 968}

Two comparable catalysts X and II were prepared, each containing 3% BTFSCH and 32% isopentane based on silica and at a molar ratio of chromium to aluminum of 1: 4.5, of which the catalyst hereinafter referred to as I was prepared according to the process of the invention. prepared so far. Catalyst I was prepared by pouring 65 g of powdered BTFSCH into a closed, conical-bottomed glass cylindrical vessel (apex angle 60 °, cylindrical diameter and vessel length 60 and 140 mm), then pouring 85g hexane and bubbling at 55 ° C. and pressure around ultrapure nitrogen. Nitrogen was fed through the capillary to the tip of the conical bottom at a rate corresponding to a velocity of v · 0.004 m / s relative to the horizontal cross-section of the cylindrical portion of the container until about 20% of the hexane was stripped off. From this clear BTFSCH suspension, ampoules were collected, each containing approximately 1g BTFSCH. One of the doses was used to determine the concentration of BTFSCH in the suspension. The contents of another ampoule were then discharged and rinsed into a laboratory mixer with the present silica suspension in isopentane in amounts guaranteeing the percentage ratios of the ingredients listed above. The BTFSCH was then adsorbed onto the silica while stirring for 4 hours, then an appropriate amount of organometallic aluminum was added, stirring was continued for 1 hour to react, then the liquid phase was filtered off and the catalyst was completely dried under nitrogen purge for about 5 hours. Catalyst II was prepared by weighing approximately 1g of powdered BTFSCH into a closable cylindrical stainless steel conical bottomed container (bottom angle 60 °, container diameter and length 20 and 70 mm). The vessel was sealed and then 10 times the vessel was pressurized with ultra-pure nitrogen at a pressure of 10 bar and always pressurized to atmospheric off-gas. The BTFSCH thus purified was discharged and flushed into a laboratory mixer with the present suspension of the correct amount of silica and isopentane in the proportions indicated above. The preparation of Catalyst II was then completed as in the case of Catalyst I. The results obtained with the two catalyst preparation processes are shown in the following table (the results are given for each of the four experiments carried out under the same conditions):

Parameter

Catalyst I (according to the invention)

239 968

Catalyst II (comparative)

Catalyst activity

2}

Catalyst flowability

Blocks of polymer grains (1.25 mm sieve residue)

310 to 325 190 to 240 100 ALIGN! cz / 0 60 to 75% 7 to qo / ^a / 0 20 May to 30%

1) The transient laboratory polymerization test was performed with 0.1 g of catalyst in a mechanically fluidized-bed reactor at a volume of 0.0015 ra at 90 ° C and a constant ethylene pressure of 1.8 MPa. The polymerization was carried out for 4 hours. The number indicates the kg of polymer formed per gram of active metal per hour.

2) This is the% of catalyst which is discharged from an inclined glass cylindrical vessel (jacket angle from vertical to 35 °) having a diameter of 60 mm and a length of 85 mm by a 10 mm vertical neck located at the lowest point of the vessel.

Claims (1)

A process for producing a supported catalyst based on bis-triphenylsilyl chromate and an organoaluminum compound for the polymerization of ethylene, in which silica is impregnated with bis-triphenylsilylchromate in an aliphatic hydrocarbon of 5-7 carbon atoms under inert atmosphere. The bis-triphenylsilylchromate is flooded in a closed vessel with an aliphatic hydrocarbon of 5 to 7 carbon atoms in an amount sufficient to form a well-flowable suspension and at a temperature below 50 DEG C. prior to impregnation on the silica. the boiling point of the hydrocarbon is stripped at least once with an inert gas at a flow rate of 0.001 to 0.02 m / s based on the largest horizontal cross-section of the vessel until 5 to 50% by volume of the hydrocarbon is removed.