Method for activation of chromium containing catalyst precursor for polymerization and the resulting polymerization catalyst

Abstract

Method for activation of chromium containing catalyst precursor for polymerization and improved polymerization catalyst resulting The instant invention relates to an activation of a polymerization catalyst precursor by heat treatment comprising a support material and a catalyst precursor deposited thereon in a fluidized bed activator and to the use of the activated polymerization catalyst in the manufacture of polyolefins. The Method is performed in a cylindrical activator (1) arranged vertically comprising tubular activator walls, a fluidization gas inlet (2) near the bottom, a fluidization gas outlet (3) near the top and a heat exchange jacket (4) outside the tubular activator walls, wherein the heating jacket covers the complete outer surface of the walls of the activator. The catalyst prepared by that method is improved with respect to its minor content of impurities and causes less interruptions during gas-phase polymerization or slurry polymerization either in stirred vessel or loop.

Claims

1. A method for preparing a Cr-containing polymerization catalyst comprising: heat-treating a reaction mixture, wherein the reaction mixture comprises a chromium containing catalyst precursor and residual liquid in a fluidized bed, wherein the chromium catalyst precursor comprises a support material, wherein the heat-treating step is performed in an activator in the presence of oxygen at a temperature in the range of from 300 to 1000 C. over a time period of from 0.5 to 48 hours, wherein the activator has an inner activator chamber surrounded by generally-vertically shaped metallic walls having an inner surface and an outer surface, a fluidization gas inlet near the bottom portion of the activator, and a fluidization gas outlet near the top portion of the activator, the outer surface of the metallic walls are completely covered by a heating jacket, and the heating jacket has a heating medium inlet and a heating medium outlet, and feeding a heating medium to the heating medium inlet of the heating jacket, wherein the heating medium inlet is near the top portion of the activator, and recovering the heating medium from the heating medium outlet of the heating jacket, wherein the heating medium outlet is near the bottom portion of the activator.

2. The method of claim 1, wherein the heating jacket is a double heating jacket.

3. The method of claim 1, wherein the resulting Cr-containing polymerization catalyst does not comprise Cr.sub.2O.sub.3.

4. The method of claim 3, wherein the Cr-containing polymerization catalyst is used in a gas-phase polymerization or a slurry polymerization in a stirred vessel or loop reactor.

5. The method of claim 4, further comprising a step of polymerizing olefins in a gas-phase or a slurry polymerization process.

6. The method of claim 5, wherein the olefins are selected from the group consisting of ethylene, propylene, butene, pentene, hexene and mixtures thereof.

7. The method of claim 1, wherein fluidization gas is fed to the fluidization gas inlet in a gaseous state, the fluidization gas flows up to the fluidization gas outlet, and the fluidization gas remains in the gaseous state while in the activator.

8. The method of claim 7, wherein the activator is free of residual liquid.

9. The method of claim 7, wherein the fluidization gas is air.

10. The method of claim 7, wherein the resulting Cr-containing polymerization catalyst does not comprise Cr.sub.2O.sub.3.

11. The method of claim 9, wherein air is used to oxidize Cr(III) to Cr(VI).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a temperature/time diagram illustration the method of the instant invention.

(2) FIG. 1 shows how the temperature of the reactor goes up to the temperature of treatment and then is kept at that temperature for the whole time period for treatment. After the time period for treatment, the temperature goes down, whereby the oxygen is replaced by nitrogen.

(3) FIG. 2 shows schematically a cut through an activator with an outer heating jacket suitable for the method of the present invention.

(4) FIG. 2 shows the cylindrical activator (1) arranged vertically comprising an inner activator chamber surrounded by tubular activator walls. A fluidization gas inlet (2) is arranged at the bottom of the activator (1) and a fluidization gas outlet (3) is arranged near the top of the activator (1). Also the heat exchange jacket (4) is to see covering the complete outer surface of the tubular activator walls. The heating jacket (4) comprises a heating medium inlet (5) on top and a heating medium outlet (6) at the bottom. A cyclone (7) is arranged in the upper part of the activator (1) inside the chamber.

(5) A further advantage of the reactor suitable for the instant invention is that the separator used can advantageously be a cyclone, i.e. effective and reliable discharge of very fine material is facilitated without having to accept material losses due to material discharge during the fluidisation process. The disadvantages of the filter elements employed in the processes usually used, which are located in the fluidization gas outlet (3), have been described in the introduction. The cyclone (7) separator used in all cases serves to remove very fine particles and to retain the catalyst in the reactor.

(6) The catalyst prepared by the process according to the invention is employed, in particular, in the polymerization of olefins. For that purpose, the catalyst is generally fed to a polymerization reactor in the form of solid particles. The active components used are chromium or titanium, preferably chromium. Examples of support materials have been mentioned before in terms of their chemical composition, their pore volume and the particle diameters. Further possible support materials are fluorinated silica, fluorinated alumina, fluorinated silica-alumina, boron oxides or mixtures thereof.

(7) The catalyst prepared by the method according to the invention is employed, in particular, in the polymerization of ethylene or in the copolymerization of ethylene with other 1-olefins having from 3 to 10 carbon atoms, preferably with propylene, 1-butene or 1-hexene. Such polymerization may be performed in gas-phase or slurry, whereby for the slurry polymerization stirred vessel reactors or loop reactors are very useful.

(8) During the activation, in addition to the fluidization gas introduced through the fluidization gas inlet (2), additional gases and, in addition to the originally introduced particles of catalyst precursor, additional solid can also be introduced into the fluidized bed. This introduction can take place at any time during the process and through feed points installed at any desired locations. Examples of suitable additional gases are oxygen, carbon dioxide or steam, while examples of additional solids which can be employed are ammonium hexafluorosilicate, untreated support material or catalysts having a different physical/chemical structure. In addition, liquids, for example water, can be sprayed into the fluidized bed. Thus, liquids, additional solids and/or additional gases can also be introduced into the activator.

(9) The treatment by the method according to the invention is described in greater detail below by means of the working examples.

WORKING EXAMPLE 1

(10) 400 kg of a catalyst precursor having a bulk density of 250 kg/m.sup.3 and a chromium content of 0.3 wt.-% were activated in a steel activator having an overall height of 8 m and a cylindrical diameter of 1 m. The outer heating jacket was covering the full length of the activator.

(11) The activator was heated from ambient temperature to 630 C. and kept there for 10 hours, with air being used as fluidizing gas and as oxidizing agent. Thereafter the activator was allowed to cool down. During the cooling phase the fluidizing gas was switched to nitrogen at 300 C. After the end of the process, the fluidizing gas was turned off and the catalyst was discharged.

COMPARATIVE EXAMPLE

(12) 125 kg of a catalyst precursor having a bulk density of 250 kg/m.sup.3 and a chromium content of 0.3 wt.-% were activated in a steel activator having an overall height of 8.5 m and a cylindrical diameter of 0.6 m. The outer heating jacket was covering only the lower half of the full length of the activator.

(13) The activator was heated from ambient temperature to 630 C. and kept there for 10 hours, with air being used as fluidizing gas and as oxidizing agent. Thereafter the activator was allowed to cool down. During the cooling phase the fluidizing gas was switched to nitrogen at 300 C. After the end of the process, the fluidizing gas was turned off and the catalyst was discharged.

(14) Polymerization

(15) Subsequent polymerization of ethylene in a gasphase reactor in the presence of catalyst produced according the examples were evaluated. The settings for the gas phase polymerization were 20 bars pressure, 112 C. temperature with a cyclegas composition of 57 vol.-% ethylene, 0.45 vol.-% 1-hexene, 5 vol.-% n-hexane and nitrogen as the rest. The throughput in both cases was 25 t/h.

(16) With the catalyst prepared according to working example 1 steady production without any disturbances whatsoever did occur. The quality of polyethylene produced was always within the specification and accordingly never poor film notes due to catalyst residues could be observed.

(17) With catalysts prepared according the comparative example regularly dosing problems (upto blocked dosing lines) did occur due to small solid particles encountered. The film notes of the products were worse since catalyst residues (analysis revealed always Cr.sub.2O.sub.3) were continuously present.