JPH0374242B2 - - Google Patents

Description

Detailed Description of the Invention The present invention relates to a method for recovering unreacted olefin remaining in polyolefin powder after polymerization, and more specifically, recovering unreacted olefin by bringing a specific gas into contact with polyolefin powder. Regarding the method.

Recently, attempts to produce solid polyolefins by gas phase polymerization have progressed in conjunction with the development of highly active catalysts and advances in powder engineering technology. In this case, the polyolefin powder containing unreacted olefin discharged from the gas phase polymerization reactor is usually first sent to a flash tank where it is separated into unreacted olefins in the polyolefin powder, and the separated unreacted olefins are subjected to gas phase polymerization. It is recycled into containers for reuse. The polyolefin powder separated in the flash tank still has 0.7 to 2.5% (weight, dry basis, same below) of unreacted olefin dissolved, so it is further sent to a surge tank to separate the dissolved olefin. The olefin is also recycled to the gas phase polymerizer and reused. The polyolefin powder discharged from the surge tank after such a process still contains a small amount of dissolved olefin, and olefin remains in the voids between the powders as so-called space olefin. The amount of these remaining dissolved olefins and spatial olefins is 0.3~
Since this amount reaches as much as 1.5%, it is important whether or not to collect and reuse this as it affects the basic unit of expensive olefin.

On the other hand, many methods have been known for removing unreacted polyolefin remaining in solid polyolefin. One method is to contact solid polyolefin with steam or hot water to expel the remaining olefin (Japanese Patent Publication No. 36-18794,
(Japanese Patent Publication No. 49-106580, etc.) Another method is to contact solid polyolefin with a nonflammable inert gas such as nitrogen to drive out the remaining olefin (Japanese Patent Application Laid-Open No. 76006/1983). Another method has been disclosed in which solid polyolefin is passed through an extruder with a vent and volatile components are removed by suction under heating and melting conditions (Japanese Patent Laid-Open Nos. 47-6031 and 51).
-47078, JP-A-51-11882, JP-A-53-
18681 etc.).

However, when these methods are used to remove and recover dissolved olefins and space olefins remaining in the polyolefin powder obtained in the surge tank of the gas phase polymerization method, various problems occur as described below.

In other words, in the method of removing residual olefin by bringing steam or hot water into contact with polyolefin powder, water is mixed into the removed olefin, so unless the water is completely separated and purified, it must be recycled to the gas phase polymerizer. cannot be used. In addition, the residual catalyst in the polyolefin powder reacts with water and decomposes, generating hydrogen chloride gas that is harmful to polymerization and contaminating the olefin, so these harmful components must also be separated and removed. Corrosion-resistant and expensive equipment is required. Furthermore, since polyolefin powder also contains water, it must be dried.

Next, the most commonly used method is to bring a nonflammable inert gas such as nitrogen into contact with the polyolefin powder to remove residual olefin, but this method removes unreacted olefin from the polymer powder. Although there is not much of a problem in this respect, it is extremely difficult to efficiently recover only olefin from the recovered olefin-inert gas mixture with a low olefin concentration. This is because high-cost processes such as a compression strengthening method of a low-pressure mixed gas, a freezing liquefaction method, or an adsorption method using an adsorbent are required.

Furthermore, the method of heating and melting polyolefin powder in a vented extruder to remove residual olefin not only requires expensive equipment, but also requires the use of an extruder to prevent the dangers that may arise from applying a polymer containing flammable gas to an extruder. Complete measures are required and are not very desirable.

In view of the above circumstances, the present inventors investigated various methods for efficiently recovering olefin remaining in polyolefin powder, and as a result, discovered the method of the present invention.

That is, in the process of producing a polyolefin polymer, the present invention uses polyolefin powder containing a trace amount of unreacted residual olefin to produce an aliphatic saturated polyolefin having the same number of carbon atoms as the olefin containing a lower concentration of olefin than in the polymerization vessel. This method of recovering olefin remaining in polyolefin powder is characterized by connecting a gas containing hydrocarbon as a main component as a purge gas, and recycling the purge gas to the olefin concentration step for polymerization.

According to the method of the present invention, unreacted olefin gas can be almost completely recovered by simply installing relatively inexpensive equipment, so it not only has the effect of reducing the consumption of raw material olefin, but also has the effect of reducing unreacted olefin gas. This prevents the release of residual olefin into the atmosphere, and also prevents residual olefin from being carried into the product powder or pellets, so excellent effects can be expected from the environmental safety and product quality aspects.

The polyolefin powders targeted in the present invention include crystalline polypropylene, crystalline polyethylene, crystalline polybutene-1, and their crystalline copolymers, but also include amorphous polymers such as ethylene propylene rubber and polybutadiene. However, it can be applied to processes where powdered polymers are obtained.

The most suitable method for producing these polyolefin powders is gas phase polymerization, but other processes such as bulk polymerization, solution polymerization, etc. can also be used as long as the powder can be obtained. The catalyst system is not particularly limited.

The particle size of the powder with which preferable results are obtained by applying the method of the present invention is about 0.1 to 4 mm.

Next, to explain the method of the present invention in more detail based on the diagram, the polyolefin powder containing unreacted olefin under high pressure discharged from the polymerization vessel 2 of the gas phase method is first heated at 20 to 80°C in an amount of 1 to 2 kg/kg. It is introduced into a flash tank 3 which is maintained at a low pressure of about cm ² G. Most of the unreacted dissolved olefin is separated in this flash tank 3 and recycled to the polymerization vessel via a pipe 11. In this way, the unreacted olefin powder, which usually still contains about 0.7 to 2.5% of residual unreacted olefin, is then transferred to the surge tank 4. The surge tank is kept at a temperature of 20°C to 20°C below the melting point of polyolefin, for example 40 to 100°C for polypropylene. The surge tank may be either horizontal or vertical, but it is preferable that the inside of the tank be able to be stirred mechanically. This is because the rate of dissolution of olefin is faster than in a non-stirring method, and continuous treatment of sticky copolymer powder is possible. The powder is usually retained in the surge tank for 5 minutes to 5 hours, and then the powder is transferred to the purge tank 5. Unreacted olefin separated in the surge tank is also recycled to the polymerizer 2 via the pipe 13. The polyolefin powder transferred to the purge tank 5 still contains about 0.3 to 1.5% of dissolved residual olefin. Here, a purge gas containing raw material olefin at a lower concentration than in the polymerization vessel and whose main component is an aliphatic saturated hydrocarbon having the same number of carbon atoms as the olefin is introduced into the purge tank and brought into contact with the polyolefin powder to remove the remaining powder. Dissolved olefins are also removed at the same time as the interbody space olefins are replaced with purge gas. In this case, if the olefin concentration in the purge gas is higher than that in the polymerization vessel, it is difficult to completely recover the remaining olefin even if catalytic replacement with nonflammable gas is performed in the subsequent step. Specifically, it is preferable to set the olefin gas concentration to 50 mol% or less. In addition, although the purpose of removing and recovering residual olefins can be achieved using only saturated hydrocarbon gas that does not contain raw material olefins, the efficiency of olefin recovery is poor, and saturated hydrocarbon gases that do not contain olefins are generally expensive. There is no industrial significance in using things. To give specific examples of suitable purge gases that can be used in practice, in the case of polyethylene powder, the bottom liquid (residual content) of the ethylene/ethane rectification column of the ethylene production facility is used, and in the case of polypropylene powder, the bottom liquid (residual content) of the ethylene/ethane rectification column of the ethylene production facility is used. Economically advantageous are the bottom liquid of a propylene/propane rectification column, and in the case of polybutene powder, the spent liquid of a butene-1/butane purification facility. The amount of purge gas used is 0.1 to 100 per kg of polyolefin powder (standard condition,
(the same applies below). In addition, when the olefin content in the purge gas is low, the olefin content in the discharged purge gas is not so high either, so only a part of it is returned to the rectification column 1, and the rest is returned to the purge gas supply pipe through the pipe 25. It can be reused as purge gas. The temperature inside the purge tank is 20℃ or more, same as the surge tank.
The temperature is kept at 20℃ lower than the melting point of polyolefin, for example 40-100℃ for polypropylene.
kept at ℃. The purge tank may be of either horizontal or vertical type, but it is preferable that the inside be able to be stirred mechanically. The residence time of the purge tank will naturally vary depending on its size and other conditions, but
It usually ranges from 1 to 120 minutes. The purge gas discharged from the purge tank is recirculated to the olefin concentration step through a pipe 16, and the olefin gas therein is recovered. The olefin concentration process is equipment for producing high-purity olefins used for producing polyolefins.

The above explanation mainly concerns the case of a homopolymer; however, in the case of a copolymer, a mixed gas corresponding to its components may be used as the purge gas, but only the monomer olefin and its corresponding saturated hydrocarbon, which are the main components, can be used as the purge gas. Even if you use , you can achieve most of your goals.

As described above, the polyolefin powder from which unreacted olefin has been removed in the purge tank contains about 0.1 to 1% of hydrocarbons (saturated and a small amount of unsaturated), but is transferred to the receiver tank and at the same time filled with non-flammable gas such as nitrogen and argon. An inert gas is introduced to replace and remove the purge gas accompanying the powder. Otherwise, if oxygen is mixed in, the residual hydrocarbons will form a gas with a composition within the explosive limit, which is dangerous.

In this way, the polyolefin powder has a hydrocarbon content of about 10 to 1000 ppm (0.001 to 0.1%),
The catalyst residue is sent to a process of detoxification or stabilization as in a normal process, or it is used as a product as it is.

Hereinafter, the method of the present invention will be explained in more detail using polypropylene as an example.

Example Raw material olefin containing 95 mol% propylene and 5 mol% propane was rectified in rectification column 1, and propylene 99%
The refined propylene containing 1 mol% of propane is sent to the gas phase polymerizer 2 (horizontal mechanical stirring type). A highly active titanium trichloride-organoaluminum catalyst was introduced into the polymerization vessel as a polymerization catalyst, and the temperature was raised to 70°C.
Continuous gas phase polymerization was carried out under the condition of 20 kg/cm ² G.
The obtained polypropylene powder had an MFR of 8.0 and an average particle size (median diameter) of 0.4 to 0.5 mm, and was discharged into a flash tank 3. The flash tank is
The flashed propylene gas maintained at 40° C. and 1.2 kg/cmG was recirculated to the polymerization vessel 2 via a pipe 11 and a compressor. The polypropylene powder extracted from the bottom of the flash tank contains approximately 1.5% of unreacted propylene, but is dropped into surge tank 4 (horizontal mechanical stirring type, rotation speed 50 rpm), where it is kept under normal pressure.
It was kept at 60°C for 60 minutes. During this time, the de-dissolved propylene is recirculated to the polymerizer 2 via the pipe 13 and the compressor 7. The polypropylene powder was then transferred to purge tank 5 (horizontal mechanical stirring type, rotation speed 50 rpm), at which point it contained about 0.7% unreacted propylene. The purge tank is maintained at 60℃ and normal pressure.
Purge gas is supplied from the pipe 17 to the lower part of the powder. In this example, propane gas containing 3 mol % of propylene obtained by gasifying the bottom liquid of the propylene/propane rectification column 1 was used as the purge gas.
The amount of purge gas supplied was set at a ratio of 10 to 1 kg of polypropylene powder. Also, the average residence time was set to 60 minutes. The exhaust gas discharged from the purge tank is recirculated to the rectification column 1 via a pipe 16. The propylene concentration in the exhaust gas had increased to 45 mol%. In this case, since the propylene component has not yet grown so large, it is possible to return only one part to the rectification column and use the remaining part as part of the purge gas again via piping 25.The polypropylene powder in the purge tank is Then, it is transferred to receiver tank 6 (horizontal mechanical stirring type, 50 rpm). At this point the polypropylene powder contained approximately 0.5% propane and traces of propylene. In the receiver tank, nitrogen gas was supplied from the bottom of the tank at a rate of 50% per 1 kg of polypropylene powder under conditions of 60°C, normal pressure, and a residence time of 30 minutes, to drive out residual propane, and the discharged gas was combusted. The polypropylene powder obtained in this way is approximately 20 ppm
(0.002%) of hydrocarbons (mostly propane), and the rest was sent to the normal catalyst residue detoxification process.

As described above, according to the method of the present invention, it was possible to recover and reuse almost all of the approximately 0.7% propylene that accompanied the polypropylene powder when it was transferred to the purge tank.

[Brief explanation of drawings]

FIG. 1 is a process diagram showing the method of the present invention. 1... Rectification column, 2... Polymerization vessel, 3... Flash tank, 4... Surge tank, 5... Purge tank, 6... Receiver tank, 7... Compressor,
8... Raw olefin supply piping, 9... Purified olefin extraction piping, 10, 12, 14, 15, 1
9,20,23...control valve, 11,
13...Unreacted olefin discharge pipe, 16...Purge gas discharge pipe, 17...Purge gas supply pipe, 21...Inert gas supply pipe, 22...Inert gas discharge pipe, 24...Polyolefin powder extraction pipe .

Claims (1)

[Claims] 1. In the process of producing a polyolefin polymer, polyolefin powder containing a small amount of unreacted olefin is treated with the same number of carbon atoms as the olefin containing the raw material olefin at a lower concentration than in the polymerization vessel. 1. A method for recovering unreacted olefin remaining in polyolefin powder, the method comprising contacting a gas containing an aliphatic saturated hydrocarbon as a main component as a purge gas, and recycling the purge gas to an olefin concentration step for polymerization. 2. The purge gas is a saturated hydrocarbon containing 2 to 50 mol% of olefin, and the purge gas is heated at a rate of 0.1 to 100% (standard state) per 1 kg of polyolefin powder at 20°C or 20°C above the melting point of the polyolefin.
The olefin recovery method according to claim 1, wherein the discharged polyolefin powder is brought into contact with the discharged polyolefin powder at a low temperature range. 3. The olefin recovery method according to claim 1 or 2, wherein the olefin is propylene, and therefore the saturated hydrocarbon is propane. 4. The olefin recovery method according to claim 1 or 2, wherein the olefin is ethylene, and therefore the saturated hydrocarbon is ethane. 5. Claim 1 or 2, wherein the olefin is butene-1, and therefore the saturated hydrocarbon is butane.
Olefin recovery method described in section.