CLIAMS:1. A process of controlling particle size distribution of a Ziegler-Natta catalyst composition; said process comprising the following steps:
a. reacting magnesium metal with an alcohol mixture comprising methanol and at least one alcohol selected from the group consisting of ethanol, propanol and butanol using iodine as an initiator at a temperature in the range from 40 to 120 °C to obtain a slurry containing a mixture of magnesium alkoxides and unreacted alcohol; wherein, the ratio of the amount of magnesium to the amount of iodine is in the range from 10:1 to 100:1;
b. separating the unreacted alcohol from the slurry to obtain a mixture of magnesium alkoxides having methoxy content less than 2 % on mass basis;
c. chlorinating the mixture of magnesium alkoxides with a mixture of TiCl4 and chlorobenzene in a hydrocarbon medium to obtain a Ziegler-Natta pro-catalyst;
d. mixing with stirring the Ziegler-Natta pro-catalyst, a co-catalyst and at least one electron donor to obtain a Ziegler-Natta catalyst composition having the particle size distribution in the range of 5 to 15 µm.
2. The process as claimed in claim 1, wherein the co-catalyst is at least one selected from the group consisting of triethylaluminium, tridecylaluminium, tri-n-butylaluminium, tri-isopropylaluminium, tri-isoprenylaluminium, tri-isobutylaluminium, ethyl aluminium sesquichloride, diethylaluminium chloride, di-isobutyl aluminium chloride, triphenylaluminium, tri-n-octylaluminium and tri-n-decylaluminium.

3. The process as claimed in claim 1, wherein the hydrocarbon medium is at least one selected from the group consisting of hexane, decane, heptane, chlorobenzene, toluene and cyclohexane.

4. Polyethylene having particle size distribution in the range from 180 to 320 µm is obtained by subjecting ethylene to polymerization in the presence of the Ziegler-Natta catalyst composition claimed in claims 1 to 3. ,TagSPECI:FIELD
The present invention relates to a process of controlling particle size distribution of Ziegler-Natta catalyst.
BACKGROUND
Ultra High Molecular Weight Polyethylene (UHMWPE) has high density and molecular weight ranging from 3 to 6 million. UHMWPE is used for a wide range of applications. UHMWPE does not melt or flow as a molten liquid. Therefore, for industrial applications, UHMWPE needs to be in powder form. UHMWPE particles with narrow particle size distribution are desirable, since UHMWPE particles with narrow particle size distribution can be molded in different shapes using different processing methods like injection molding and blow molding. Hence, the particle size distribution of UHMWPE plays an important role in UHMWPE manufacturing.
Producing UHMWPE components is a three step process consisting of polymerization, consolidation and machining. During olefin polymerization the olefin gas is fed into a reactor where temperature, pressure and catalysts produce polyethylene. The processing conditions in the reactor determine the average molecular weight and most importantly its distribution as well as the powder particle size and shape.
One type of Ziegler-Natta pro-catalyst comprises a magnesium chloride supported titanium chloride species. The magnesium alkoxide acts as a precursor for the synthesis of the Ziegler-Natta pro-catalyst. Currently the catalyst used for the production of UHMWPE is prepared using pure magnesium ethoxide having PSD in the range from 600 to 700 µm.
It has been suggested that, if the particle size of the magnesium alkoxide is higher, it generates more fines during catalyst synthesis. Based on this phenomenon many technologies use particle size adjustment to achieve desired particle size distribution of the catalyst and hence for UHMWPE production. Irregular shapes of fines also affect the plant throughout and create problems of flow ability and chocking of compressors.
The morphology of the catalyst precursor is replicated in the polyethylene resin via the catalyst which is synthesized from said precursor. A controlled particle size and morphology of the catalyst produces a desired particle size of the polymer due to replication of precursor morphology to the catalyst and eventually to the polymer. This gives better flowability and hence results into better plant throughout.
Therefore, there is felt a need to provide a Ziegler-Natta pro-catalyst for the preparation of a Ziegler-Natta catalyst composition that provides a polyolefin with an improved particle size distribution.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows:
An object of the present disclosure is to provide a process of controlling particle size distribution of Ziegler-Natta catalyst composition.
Still another object of the present disclosure is to provide a Ziegler-Natta catalyst composition for the olefin polymerization.
Other objects and advantages of the present disclosure will be more apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit the scope of the present disclosure.
SUMMARY
In accordance with one aspect of the present disclosure, there is provided a process of controlling particle size distribution of polyethylene. The process comprises reacting magnesium metal with an alcohol mixture comprising methanol and at least one alcohol selected from the group consisting of ethanol, propanol and butanol and using iodine as an initiator. The ratio of the amount of magnesium and the amount of iodine ranges from 10:1 to 100:1. The resulting mixture of magnesium alkoxides has methoxy content of less than 2 % on mass basis. The Ziegler-Natta pro-catalyst, a co-catalyst and at least one electron donor are mixed to obtain a Ziegler-Natta catalyst composition having the particle size distribution in the range from 5 to 15 µm. Ethylene polymerized in the presence of said Ziegler-Natta catalyst composition to obtain polyethylene having the particle size distribution of the polyethylene is in the range from 180 to 320 µm.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
The process for size controlled Ziegler-Natta catalyst for ethylene polymerization will now be described with the help of the accompanying drawings, in which:
Figure 1 illustrates the graphical representation of the effect of recovered solvent on the composition of the mixture of magnesium alkoxides.
Figure 2 illustrates the graphical representation of the effect of initiator concentration on particle size distribution of pro-catalyst.
Figure 3 illustrates the graphical representation of the effect of methoxy content in the mixture of magnesium alkoxides on the particle size distribution of the pro-catalyst and the Ziegler-Natta catalyst composition.
Figure 4 illustrates the graphical representation of the performance of the Ziegler-Natta catalyst composition of the present disclosure.
Figure 5 illustrates the graphical representation of the particle size distribution of the polyethylene prepared in presence of the Ziegler-Natta catalyst composition of the present disclosure.
DETAILED DESCRIPTION
The present disclosure provides a process of controlling particle size distribution of the Ziegler-Natta catalyst composition.
The particle size distribution of the Ziegler-Natta catalyst composition depends upon the particle size distribution of the pro-catalyst. The particle size distribution of the pro-catalyst of the present disclosure is controlled by maintaining the methoxy content of less than 2% on mass basis in the mixture of magnesium alkoxides used in the preparation of the pro-catalyst.
The present invention discloses the effect of initiator on particle size distribution of the Ziegler-Natta catalyst composition. The mean particle size of the Ziegler-Natta catalyst composition increases with increase in initiator concentration.
In accordance with one aspect of the present disclosure, there is provided a process of controlling a particle size distribution of a Ziegler-Natta catalyst composition; said process comprising the following steps:
In the first step, a magnesium metal is reacted with an alcohol mixture comprising methanol and at least one alcohol selected from the group consisting of ethanol, propanol and butanol and using iodine as an initiator at a temperature in the range from 40 to 120 °C to obtain a slurry containing a mixture of magnesium alkoxides and unreacted alcohol.
In the second step, the unreacted alcohol is separated from the slurry to obtain a mixture of magnesium alkoxides.
In the third step, the mixture of magnesium alkoxides is chlorinated using a mixture of TiCl4 and chlorobenzene in a hydrocarbon medium to obtain a Ziegler-Natta pro-catalyst.
In the fourth step, the Ziegler-Natta pro-catalyst is mixed and stirred with a co-catalyst and at least one electron donor to obtain a Ziegler-Natta catalyst composition having the particle size distribution in the range of 5 to 15 µm.
The ratio of the amount of magnesium to the amount of iodine is in the range from 10:1 to 100:1.
In accordance with one embodiment of the present disclosure, the ratio of the amount of magnesium to the amount of iodine is 100:3.
The methoxy content of the mixture of the magnesium alkoxides is less than 2 % on mass basis.
The magnesium metal is added to the alcohol mixture at a temperature in the range from 30 to 60 °C.
In accordance with one embodiment of the present disclosure, the magnesium metal is added to the alcohol mixture at a temperature of 40 °C.
The co-catalyst can be at least one selected from the group consisting of triethylaluminium, tridecylaluminium, tri-n-butylaluminium, tri isopropylaluminium, tri-isoprenylaluminium, tri-isobutylaluminium, ethyl aluminium sesquichloride, diethylaluminium chloride, di-isobutyl aluminium chloride, triphenylaluminium, tri-n-octylaluminium and tri-n-decylaluminium.
In accordance with one embodiment of the present disclosure the co-catalyst is triethylaluminium.
The hydrocarbon medium is at least one selected from the group consisting of hexane, decane, heptane, chlorobenzene, toluene and cyclohexane.
In accordance with one embodiment of the present disclosure, the hydrocarbon medium is hexane.
The chain transfer agent can be hydrogen.
The polyethylene obtained by the process as discussed in the present disclosure has the particle size in the range from 180 to 320 µm.
It was observed that the particle size distribution for Ziegler-Natta catalyst composition can be controlled by controlling methoxy content of the mixture of magnesium alkoxides used for the preparation of the pro-catalyst.
The particle size distribution of the Ziegler-Natta catalyst composition is in the range from 5 to 15 µm when it is prepared from the mixture of magnesium alkoxides with methoxy content of less than 2% on mass basis. When the methoxy content was greater than 2% on mass basis, the particle size distribution of the Ziegler-Natta catalyst composition was higher. Further, the particle size distribution of the Ziegler-Natta catalyst compositions prepared from a pro-catalyst without magnesium methoxide content is also higher.
Pure magnesium ethoxide is highly reactive and generates uncontrolled heat of reaction which is responsible for the fragmentation of catalyst particles formed during the reaction.
The Ziegler-Natta catalyst compositions of the present disclosure were used for the polymerization of ethylene. The Ziegler-Natta catalyst composition, a chain transfer agent and ethylene are introduced in a hydrocarbon medium to attain a predetermined pressure. The olefin is subjected to polymerization at a predetermined temperature under continuous agitation to obtain polyethylene.
Polyethylene having an average particle size in the range from 180 to 320 µm was produced in the presence of the Ziegler-Natta catalyst compositions of the present disclosure having particle size distribution in the range from 5 to 15 µm.
The particle size distribution of the Ziegler-Natta catalyst composition of the present disclosure can be controlled by controlling the concentration of initiator during the preparation of the Ziegler-Natta catalyst composition. The mean particle size of the Ziegler-Natta catalyst composition increases with increase in the concentration of the initiator.
The Ziegler-Natta pro-catalyst was synthesized using the solvent recovered from the step of separating the unreacted alcohol mixture from the slurry of magnesium alkoxides. It was observed that the composition of the mixture of the magnesium alkoxides is not affected by using the recovered alcohol. Thus, the invention provides an economical method for the synthesis of the Ziegler-Natta pro-catalyst using recovered solvent.
The productivity of the polyethylene prepared in the presence of the Ziegler-Natta catalyst composition of the present disclosure obtained was high and the productivity did not show significant difference for the catalyst prepared using recovered alcohol.
The polyethylene products prepared using the Ziegler-Natta catalyst composition of the present disclosure showed very low MFI indicating that the polyethylene has higher molecular weight.
The present disclosure is further illustrated herein below with the help of the following examples. The examples provided herein are intended merely to facilitate an understanding of the ways in which the embodiments herein may be practiced and to further enable those of skilled in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
Example 1a: Preparation of the mixture of magnesium alkoxides
A reactor was charged with 250 mL of mixed alcohol comprising methanol and ethanol. 10 g of magnesium metal was added to the reactor under nitrogen atmosphere to obtain a mixture. The mixture was stirred at 300 rpm using a magnetic stirrer. After stirring for 20 minutes, 0.3 g of iodine was added to the reactor. This was immediately accompanied by the evolution of hydrogen gas indicating the initiation of the reaction. The hydrogen gas evolution was monitored visually and the temperature was increased in step wise manner from 40 to 80 °C. After the evolution of hydrogen gas ceased, the mixture of alcohols was evaporated at higher temperature to obtain a white powder of the mixture of magnesium ethoxide and magnesium methoxide.
Magnesium content of the mixture of magnesium alkoxides was determined by volumetric method. Composition of magnesium alkoxides (Ethoxy/methoxy content) was determined by Gas chromatographic method. Particle size distribution of the mixture of magnesium alkoxides was determined using CILAS-1190D instrument. The results are shown in Figure-1.
Table 1.
Magnesium alkoxide Mg (wt%) Ethoxy (wt%) Methoxy (wt%) D10 D50 D90 Mean PSD
A 21-22.5 70.5-74.0 1-2 13-20 µ 27-32 µ 41-51 µ 27-33 µ
B
C
D

Table 1 depicts the mean PSD of magnesium alkoxides synthesized with methoxy content of 1-2 wt %.
Example 1b: Preparation of a Ziegler-Natta pro-catalyst
A reactor was charged with hexane and the mixture of magnesium methoxide and magnesium ethoxide obtained in Example 1a was added to the reactor under nitrogen atmosphere. TiCl4 was added to the mixture over the time period of 5.5 hours at 85 °C. The reaction was tempered at 120 °C for 60 hours to obtain a crude Ziegler-Natta pro-catalyst. The Ziegler-Natta pro-catalyst was further washed with hexane at 70 °C to obtain the pro-catalyst.
Example 2: Effect of plant recovered solvent on magnesium alkoxides (precursor) composition.
To study the effect of recovered fluid medium on composition of the mixture of magnesium alkoxides (precursor), experiment was carried out at laboratory scale using recovered alcohol (2 wt% methanol and 98 wt% ethanol). The experiment was performed as per the procedure given in Example 1a.
The results indicate that composition of the mixture of magnesium alkoxides is not affected using the recovered alcohol as methoxy content of the mixture of magnesium alkoxides was found in the range from 1 to 2 % on mass basis. The mean particle size of the mixture of magnesium alkoxides was observed to be in the range from 27µm to 33µm.
Example 3: Effect of the initiator on particle size distribution of pro-catalyst.
To study the effect of the initiator on particle size distribution of magnesium alkoxide, synthesis of magnesium alkoxide was carried out at laboratory scale using pure ethanol. The experimental procedure given in Example 1a was followed at varying initiator concentrations. The results are displayed in Figure-2.
Table 2.
Magnesium alkoxide Iodine (mg) Mg (wt%) Ethoxy (wt%) D10 D50 D90 Mean PSD
E 300 21-22 76-77.5 14-25 µ 25-37 µ 40-54 µ 26
F 600 31
G 700 35
H 700 37

Table 2 depicts the effect of initiator concentration on mean PSD of magnesium alokoxides.
The results clearly indicate that mean particle size of the mixture of magnesium alkoxides increases from 26 µm to 37 µm with the increase in the amount of the initiator (Iodine) from 300 mg to 700 mg in 250 mL of alcohol.
Example 4: Effect of methoxy content in the mixture of magnesium alkoxides on the particle size distribution of the pro-catalyst and the Ziegler-Natta catalyst composition
The experiments were carried out using experimental procedure of Example 1a by varying the methoxy content of the mixture of magnesium alkoxides from 0 to 7 wt%. The effect of variation of methoxy content of the mixture of magnesium alkoxides on particle size distribution of the pro-catalyst and the Ziegler-Natta catalyst composition is shown in self-explanatory Figure-3.
A: PSD of magnesium alkoxide, B: PSD of pro-catalyst and C: PSD of Ziegler-Natta catalyst composition.
It was observed that desired particle size was achieved with the mixture of magnesium alkoxide having methoxy content less than 2 % on mass basis. However, the particle size distribution increases with increase in methoxy content of the mixture of magnesium alkoxides.
Composition analysis of the catalysts was carried out using different analytical techniques. Amount of magnesium was determined using volumetric titration and % chloride using titro-processor. Amount of titanium was analyzed using spectrophotometer. Particle size distribution (PSD) of magnesium alkoxide was analyzed using CILAS1190 instrument.
Example 5: Polymerization using the Ziegler-Natta Catalyst compositions
The Ziegler-Natta catalyst composition in hexane was taken in a reactor. Hydrogen gas was charged in the reactor. Ethylene gas was introduced in the reactor till the ethylene pressure was 6 kg/cm2. The olefin was subjected to polymerization at 80 °C under continuous agitation at 400 rpm for 2 hours.
The performance of the Ziegler-Natta catalyst composition is demonstrated in Figure-4 and the particle size distribution is demonstrated in Figure-5.
Table 3.
Al/Ti Productivity (kgPE/g cat) MFI (g/10 min) BD-UT (g/CC) BD-T (g/cc)
50 5 0.7 0.32 0.35
50 4.9 0.61 0.31 0.36
25 2.4 0.4 0.26 0.33
25 2.3 0.5 0.25 0.34

Table 3 depicts the catalytic activity of the Ziegler-Natta catalyst composition of the present disclosure.
Table 4.
Al/Ti D10 D50 D90 Average particle size
50 84 276 454 271
50 84 267 450 267
25 69 160 378 191
25 48 154 414 195

Table 4 depicts the particle size of the polyethylene prepared using Ziegler-Natta catalyst composition of the present disclosure.
The productivity obtained was 4.9 and 2.3 kgPE/g catalyst for the catalyst having Al/Ti molar ratio of 50 and 25. The productivity did not show significant difference for the catalyst prepared using recovered alcohol.
MFI of the polyethylene prepared in the presence of the Ziegler-Natta catalyst composition of the present disclosure was determined at 230 °C and 21.6 kg load. The polyethylene products prepared using Al/Ti ratio of 50 and 25 showed very low MFI indicating that the polyethylene has higher molecular weight.
The average particle size (APS) of the polyethylene prepared in the presence of the Ziegler-Natta catalyst composition of the present disclosure was in the range of 191 µm to 271 µm for the Ziegler-Natta catalyst composition with Al/Ti ratio of 50 and 25.

TECHNICAL ADVANCEMENT
The technical advancements offered by the present disclosure include the realization of:
• The particle size distribution of the catalyst is governed by methoxy content of the mixture of magnesium alkoxides which in turn acts as a precursor for the synthesis of a Ziegler-Natta pro-catalyst.
• The process for the synthesis of polyethylene having particle size distribution in the range from 180 to 320 µm.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results. While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Variations or modifications to the formulation of this invention, within the scope of the invention, may occur to those skilled in the art upon reviewing the disclosure herein. Such variations or modifications are well within the spirit of this invention.
The numerical values given for various physical parameters, dimensions and quantities are only approximate values and it is envisaged that the values higher than the numerical value assigned to the physical parameters, dimensions and quantities fall within the scope of the invention unless there is a statement in the specification to the contrary.
While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.