Process for transitioning between incompatible catalysts

Abstract

The invention relates to a process for transitioning from a first continuous polymerization reaction of ethylene and a first comonomer for producing a linear low density polyethylene conducted in the presence of a Ziegler-Natta catalyst in a gas phase reactor to a second continuous polymerization reaction of ethylene and a second comonomer for producing a high density polyethylene conducted in the presence of a chromium-based catalyst in the gas phase reactor, the process comprising: (i) reducing the feed of the first comonomer into the reactor until the ratio of the first comonomer to ethylene in the reactor is at most 0.1; (ii) discontinuing the introduction of the Ziegler-Natta catalyst while the introduction of a co-catalyst of the Ziegler-Natta catalyst is continued and subsequently discontinuing the introduction of the co-catalyst; (iii) maintaining the polymerization conditions in the reactor and permitting polymerization to continue for a time in order to allow the components of the Ziegler-Natta catalyst present in the reactor to consume themselves in the production of additional polymer; (iv) discontinuing the introduction of all feeds into the reactor; (v) depressurizing the reactor; (vi) flow-purging the reactor; (vii) reducing the reactor temperature; (viii) introducing ethylene and H.sub.2 into the reactor to obtain a partial pressure of ethylene and a volume ratio of H.sub.2 to ethylene for the second polymerization reaction, wherein the partial pressure of ethylene is increased to the pressure for the second polymerization reaction at such a speed that the reactor temperature is maintained at a temperature lower than the temperature of the first polymerization conditions; (ix) increasing the reactor temperature to a temperature of the second polymerization conditions; (x) introducing the second catalyst into the reactor and (xi) introducing the second comonomer into the reactor to obtain a reactor composition for the second polymerization reaction.

Claims

1. A process for transitioning from a first continuous polymerization reaction of ethylene and a first comonomer for producing a linear low density polyethylene conducted in the presence of a Ziegler-Natta catalyst in a gas phase reactor to a second continuous polymerization reaction of ethylene and a second comonomer for producing a high density polyethylene conducted in the presence of a chromium-based catalyst in the gas phase reactor, the process comprising: (i) reducing the feed of the first comonomer into the reactor until the ratio of the first comonomer to ethylene in the reactor is at most 0.1; (ii) discontinuing the introduction of the Ziegler-Natta catalyst while the introduction of a co-catalyst of the Ziegler-Natta catalyst is continued and subsequently discontinuing the introduction of the co-catalyst; (iii) maintaining the polymerization conditions in the reactor and permitting polymerization to continue for a time in order to allow the components of the Ziegler-Natta catalyst present in the reactor to consume themselves in the production of additional polymer; (iv) discontinuing the introduction of all feeds into the reactor; (v) depressurizing the reactor; (vi) flow-purging the reactor; (vii) reducing the reactor temperature; (viii) introducing ethylene and H.sub.2 into the reactor to obtain a partial pressure of ethylene and a volume ratio of H.sub.2 to ethylene for the second polymerization reaction, wherein the partial pressure of ethylene is increased to the pressure for the second polymerization reaction at such a speed that the reactor temperature is maintained at a temperature lower than the temperature of the first polymerization conditions; (ix) increasing the reactor temperature to a temperature of the second polymerization conditions; (x) introducing the second catalyst into the reactor and (xi) introducing the second comonomer into the reactor to obtain a reactor composition for the second polymerization reaction.

2. The process according to claim 1, wherein the polymerization conditions in step (iii) comprise a reactor temperature of 83-88 C., a reactor pressure of 15-25 barg, ethylene partial pressure of 5-10 bara, H.sub.2/ethylene ratio of 0.05-0.2.

3. The process according to claim 1, wherein step (iv) is performed by discontinuing the introduction of ethylene while the introduction of H.sub.2 is continued and subsequently discontinuing the introduction of H.sub.2.

4. The process according to claim 1, wherein in step (v), the reactor is depressurized to 5-9 barg.

5. The process according to claim 1, wherein in step (vii), the reactor temperature is decreased to a temperature at least 4 C. lower than the temperature in step (iii).

6. The process according to claim 1, wherein in step (vii), the reactor temperature is decreased to a temperature less than 81 C.

7. The process according to claim 1, wherein step (viii) is performed by increasing the partial pressure of ethylene to an intermediate pressure at a first speed and to the partial pressure of ethylene for the second polymerization reaction at a second speed lower than the second speed.

8. The process according to claim 7, wherein the intermediate pressure is 5-7 bar lower than the pressure for the second polymerization reaction.

9. The process according to claim 1, wherein in step (viii), the introduction of H.sub.2 to the reactor is started after the partial pressure of ethylene for the second polymerization reaction is reached.

10. The process according to claim 1, wherein the partial pressure of ethylene for the second polymerization reaction is 13-17 bara and the volume ratio of H.sub.2 to ethylene for the second polymerization reaction is 0.01-0.015.

11. The process according to claim 1, wherein the first comonomer is 1-butene or 1-hexene.

12. The process according to claim 1, wherein the second comonomer is 1-hexene.

13. The process according to claim 1, wherein the Ziegler-Natta catalyst is prepared by (a) contacting a dehydrated support having hydroxyl groups with a magnesium compound having the general formula MgR.sup.1R.sup.2, wherein R.sup.1 and R.sup.2 are the same or different and are independently selected from the group comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and alkadienylaryl group; (b) contacting the product obtained in step (a) with modifying compounds (A), (B) and (C), wherein: (A) is at least one compound selected from the group consisting of carboxylic acid, carboxylic acid ester, ketone, acyl halide, aldehyde and alcohol; (B) is a compound having the general formula R.sup.11.sub.f(R.sup.12O).sub.gSiX.sub.h, wherein f, g and h are each integers from 0 to 4 and the sum of f, g and h is equal to 4 with a proviso that when h is equal to 4 then modifying compound (A) is not an alcohol, Si is a silicon atom, O is an oxygen atom, X is a halide atom and R.sup.11 and R.sup.12 are the same or different and are independently selected from the group comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and alkadienylaryl group; (C) is a compound having the general formula (R.sup.13O).sub.4M, wherein M is a titanium atom, a zirconium atom or a vanadium atom, O is an oxygen atom and R.sup.13 is selected from the group comprising an alkyl group, alkenyl group, alkadienyl group, aryl group, alkaryl group, alkenylaryl group and alkadienylaryl group; and (c) contacting the product obtained in step (b) with a titanium halide compound having the general formula TiX.sub.4, wherein Ti is a titanium atom and X is a halide atom.

14. The process according to claim 4, wherein in step (v), the reactor is depressurized to 6-8 barg.

Description

EXAMPLES

(1) The gas phase reactor system as schematically shown in FIG. 1 was used for the transition process. The gaseous feed streams are mixed together in a mixing tee arrangement and enters the reactor from the bottom, and passes through a perforated distribution plate. The unreacted gas stream is separated from the entrained polymer particles, and is then compressed, cooled, and recycled back into the reactor. Product properties are controlled by adjusting reaction conditions (temperature, pressure, flow rates, etc.).

(2) The polymerizations were conducted in a continuous gas phase fluidized bed reactor having an internal diameter of 45 cm and a reaction zone height of 228 cm. The fluidized bed was made up of polymer granules. The reactor was filled with a bed of about 50 kg of dry polymer particles that was vigorously agitated by a high velocity gas stream. The bed of polymer particles in the reaction zone was kept in a fluidized state by a recycle stream that works as a fluidizing medium as well as a heat dissipating agent for absorbing the exothermal heat generated within reaction zone.

(3) The individual flow rates of ethylene, hydrogen and comonomer were controlled to maintain fixed composition targets. The ethylene concentration was controlled to maintain a constant ethylene partial pressure. The hydrogen/ethylene flow ratio was well controlled to maintain a relatively steady melt index of the final resin. The concentrations of all the gases were measured by an on-line gas chromatograph to ensure relatively constant composition in the recycle gas stream.

(4) The solid catalyst was injected directly into the fluidized bed using purified nitrogen as a carrier. Its rate was adjusted to maintain a constant production rate of about 10-15 kg/hr.

(5) The reacting bed of growing polymer particles was maintained in a fluidized state by the continuous flow of the make-up feed and recycle gas through the reaction zone. A superficial gas velocity of 0.40 m/sec was used to achieve this. The reactor was operated at a pressure and temperature as shown in below tables. To maintain a constant reactor temperature, the temperature of the recycle gas is continuously adjusted up or down to accommodate any changes in the rate of heat generation due to the polymerization.

(6) The fluidized bed was maintained at a constant height by withdrawing a portion of the bed at a rate equal to the rate of formation of particulate product. The product was removed semi-continuously via a series of valves into a fixed volume chamber. The so obtained product was purged to remove entrained hydrocarbons and treated with a small steam of humidified nitrogen to deactivate any trace quantities of residual catalyst.

(7) The properties of the polymer were determined by the following test methods:

(8) TABLE-US-00001 TABLE 1 Melt Index ASTM D-1238 - Condition E (190 C., 2.16 kg) Flow Index ASTM D-1238 - Condition F (190 C., 21.6 kg) Density ASTM D-1505 Bulk Density The resin is poured in a fixed volume cylinder of 400 cc. The bulk density is measured as the weight of resin divided by 400 cc to give a value in g/cc. Average Particle Size The particle size is measured by determining the weight of material collected on a series of U.S. Standard sieves and determining the weight average particle size based on the sieve series used. Fines The fines are defined as the percentage of the total distribution passing through a 120 mesh standard sieve. This has a particle size equivalent of 120 microns.

(9) A Ziegler-Natta (ZN) catalyst was used to produce 1-butene-copolymerized LLDPE having a density of 918 kg/m.sup.3 and a melt index of 1.0. The LLDPE obtained is suitable for processing by blown film extrusion process. The ZN catalyst was the catalyst prepared according to example 1 of WO2012069157:

(10) 2.5 g of Sylopol 955 silica which had been dehydrated at 600 C. for 4 hours under a nitrogen flow was placed in a 40 cm3 flask. 15 cm3 of isopentane was added to slurry the silica, then 2.5 mmol of di-n-butylmagnesium was added to the flask and the resultant mixture was stirred for 60 minutes at a temperature of 35 C. Then, 3.5 mmol of methyl n-propyl ketone was added to the flask and the resultant mixture was stirred for 60 minutes at a temperature of 35 C. Then, 0.25 mmol of tetraethoxysilane was added to the flask and the resultant mixture was stirred for 30 minutes at a temperature of 35 C. Next, 0.25 mmol of titanium tetraethoxide was added to the flask and the resultant mixture was stirred for 30 minutes at a temperature of 35 C. Subsequently, 1.50 mmol of titanium tetrachloride was added to the flask and the resultant mixture was stirred for 30 minutes at a temperature of 35 C. Finally, the slurry was dried using a nitrogen purge at 70 C. for 60 minutes to yield a free-flowing solid product.

(11) The conditions for this first polymerization are shown in Table 2.

(12) TABLE-US-00002 TABLE 2 Reactor Conditions Bed temperature ( C.) 85 Reactor pressure (barg) 20.7 C2 partial pressure (bara) 7.0 Bed level (mbar) 35 Superficial velocity (m/s) 0.40 H.sub.2/C2 volume ratio 0.140 C4/C2 volume ratio 0.40 TEAL flow (kg/h) 0.08

(13) After running the reactor at steady state for producing the LLDPE, the feed of 1-butene flow to the reactor was stopped until the C4/C2 volume ratio was down to 0.1 or less.

(14) Subsequently the feeding of the ZN catalyst was stopped. After the ZN catalyst feed was stopped, the feeding of the co-catalyst TEAL was maintained for an additional 30 minutes before it was stopped. The reactor conditions, i.e. the conditions of Table 2 except that C4/C2 volume ratio and TEAL flow were zero, were maintained until the reaction died. All feeds were stopped when the reaction died by first stopping the ethylene feed and subsequently stopping H.sub.2 feed within 5-10 minutes after the ethylene feed was stopped.

(15) The reactor was depressurized to 7 barg. Subsequently the reactor was flow-purged with N2 for 4 hours at a reactor temperature of 85 C. The bed temperature was subsequently decreased to 80 C.

(16) The C2 partial pressure was increased to the target pressure in two steps. The C2 partial pressure was gradually increased up to 10 bara by feeding 10 kg/hr flow rate of C2 while maintaining the bed temperature to 80 C. In the second step, the C2 partial pressure was gradually increased up to 15 bara by feeding 5 kg/hr flow rate of C2 while maintaining the bed temperature to 80 C. When the target C2 partial pressure of 15 bara was reached, H.sub.2 was introduced to reach the H.sub.2/C2 volume ratio of 0.02.

(17) Subsequently, the bed temperature was gradually increased to 95 C. by increasing 2 C. per hour.

(18) The chromium-based catalyst was charged to the reactor. The chromium-based catalyst comprises a chromium oxide on a silica support, which was titanated by drying and then treating with tetra-isopropyltitanate prior to activation of the catalyst.

(19) This was followed by the introduction of 1-hexene. The final reactor composition for the polymerization using the chromium-based catalyst is as listed below in Table 3.

(20) TABLE-US-00003 TABLE 3 Reactor Conditions Target Bed temperature ( C.) 100 Reactor pressure (barg) 20.7 C2 partial pressure (bara) 15 Bed level (mbar) 35 Superficial velocity (m/s) 0.40 H.sub.2/C2 volume ratio 0.02 C6/C2 volume ratio 0.0015 TEAL flow (kg/h) 0

(21) HDPE having a density of 0.952 g/cm3 and a flow index (measured at 21.6 kg) of 10 g/10 min was successfully obtained.

(22) A successful transitioning was achieved from a Ziegler-Natta catalyst to a chromium-based catalyst.