JP3076650B2 - How to collect polyethylene solids - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for efficiently drying volatile components of polyethylene solids containing volatile components and recovering the solids from a slurry containing polyethylene solids and a diluent with good operability.

[0002]

2. Description of the Related Art In a slurry polymerization method, ethylene and α-
When producing a linear medium- or low-density polyethylene (density of 0.945 g / cm ³ or less) by copolymerizing with an olefin, a large amount of a diluent-soluble polymer is formed, and the polymer is tacky. Therefore, it is difficult to separate the polymer by ordinary separation means. For example, when the diluent is evaporated by a steam tube dryer to separate and recover the polymer, the polymer adheres to the outer wall of the tube, the overall heat transfer coefficient decreases, and the drying becomes insufficient. Also, when the method of drying the solid content of polyolefin in the slurry using a flash dryer is applied to linear low-density polyethylene, the polymer adheres to the flash dryer and closes the dryer. In order to prevent this, for example, JP-A-1-178503 describes that a polymerization catalyst deactivator is fed into a flash dryer. However, if the polymerization catalyst deactivator is fed into the flash dryer, the recovered diluent cannot be fed directly to the polymerization reactor, which is economically undesirable. Further, a propylene polymer containing a volatile component described in JP-A-59-25805 is brought into contact with a gas containing propylene as a main component at a temperature not higher than the melting point of the polymer in a fluidized-bed dryer,
An example of drying a propylene polymer is described.

When a linear low-density polyethylene solid in a slurry is to be dried using a fluidized-bed dryer equipped with a round-hole dispersing plate, which is commonly used, an upstream side of the fluidized-bed dryer is required. When the large lumps generated in the above step enter the fluidized-bed dryer, the lumps are not easily discharged from the fluidized-bed dryer and cause contamination. In addition, the sticky polymer fills the holes of the dispersion plate mounted on the fluidized bed dryer, and as a result, the amount of circulating gas for fluidization decreases, and finally the fluidization state of the bed part becomes poor, and drying occurs. It becomes inadequate and eventually leads to a stoppage of operation.

[0004]

SUMMARY OF THE INVENTION The present invention provides a method for recovering polyethylene solids from a slurry containing polyethylene solids and a diluent with good operability.

[0005]

Means for Solving the Problems As a result of intensive studies to solve the above problems, it has been found that by operating a fluidized bed dryer under specific operating conditions, a remarkable improvement effect on the above problems can be obtained. The inventors have found out that the present invention is exerted, and have completed the present invention. The present invention introduces a slurry containing a polyethylene solid content and a diluent into a fluidized bed drier, separates the polyethylene solid content and the diluent gas, and recovers the polyethylene solid content. Pressure in the bed dryer is 2
The polyethylene solid containing the diluent is brought into contact with a gas containing a diluent as the main component at a temperature not higher than the melting point of the solid while maintaining the pressure at not more than kg / cm ² G, and The gas velocity V blown from the holes of the dispersion plate is 10 to 50 m / sec, and (3) the superficial velocity in the tank in the fluidized bed dryer is 0.2 to 0.8 times the terminal velocity. Is a method for collecting polyethylene solids.

According to the method of the present invention, lumps supplied into the fluidized bed dryer or lumps generated in the vessel are continuously discharged, and the differential pressure across the dispersion plate does not increase. Efficiently dry the volatiles in the polyethylene solids contained,
It has been found that polyethylene solids can be recovered with good operability. The present invention will be described in detail below with reference to the drawings for clarity, but this is merely an example.

[0007] First, a monomer such as ethylene or ethylene and α-olefin,
A slurry containing a polyethylene solid content obtained by feeding a molecular weight regulator such as hydrogen, a catalyst, and a diluent and polymerizing and a diluent is circulated through a slurry circulating line 4 by a slurry circulating pump B. 5 feeds the fluidized bed dryer C. The diluent in the slurry is mostly gasified upon feed and separated from the polyethylene solids, but some diluent remains in the polyethylene solids and reaches the fluidized bed. The gas containing the diluent separated from the polyethylene solids is introduced into the bag filter E through the gas line 6, where the powder is completely separated, and the separated powder passes through the scattered powder recovery line 9 to dry the fluidized bed. The gas collected and separated in the dryer C is sent through the gas line 7 to a part of the recovery process, and sent through the gas recovery line 8, and part of the gas is circulated to the fluidized bed dryer C as a drying gas. Is done. F is a blower for circulating a drying gas, and G is a gas heater for supplementing insufficient heat. In the fluidized bed dryer C, a dispersion plate D is provided on the lower side. The gas fed from the gas circulation line 7 is supplied to the hole 1 in the nozzle 12 in the dispersion plate D.
The polyethylene solids are dispersed and dispersed uniformly in the fluidized bed dryer on a dispersion plate to form a bed portion H, and drying is performed. The dried polyethylene solids are discharged from the fluidized bed dryer through the recovery line 10.

The diluent in the present invention is an inert hydrocarbon medium commonly used in polymerization reactions, such as propane, n-butane, isobutane, n-butane, etc.
-Pentane, isopentane, n-hexane, cyclohexane and mixtures thereof, among which, among them, a saturated hydrocarbon having 4 to 5 carbon atoms such as n-butane, isobutane and isopentane is a main component. preferable.

In the present invention, the solid content of polyethylene in the diluent may be ethylene alone or ethylene and another α-olefin such as propylene, butene-1, hexene-1 in a diluent using a component containing a transition metal such as titanium as a catalyst. , 3
It is obtained by polymerizing a mixture of methylpentene-1 and octene-1.
The present invention is particularly effective for a low-density side, particularly a linear low-density polyethylene having a density of 0.945 g / cm ³ or less.

In the present invention, the pressure in the fluidized-bed dryer is closely related to the reached equilibrium volatile concentration of the polyethylene solid after drying, and the lower the pressure, the lower the volatile concentration after drying. Is 2 kg / cm ² G or less, particularly preferably 1.0 kg / cm ² G or less. When the slurry is fed from the polymerization vessel to the fluidized bed dryer, it is necessary to flash evaporate the diluent. Therefore, the pressure difference between the polymerization vessel and the fluidized bed dryer should be 10 kg / cm ² or more. Is preferred.

When the slurry containing polyethylene solids is introduced into a fluidized bed dryer, the concentration of polyethylene solids in the present invention is not particularly limited, but is preferably 30% by weight or more in terms of drying efficiency. When the slurry is circulated using the slurry circulation line, there is an upper limit to the polyethylene solid concentration in the slurry due to the pump performance. For this reason, it is preferable in terms of drying efficiency to provide a slurry concentration step in which the concentration of polyethylene solids introduced into the fluidized bed dryer is higher than the concentration of polyethylene solids in the reactor. Examples of the slurry concentrator include a settling leg and a liquid cyclone.

In the present invention, the temperature of the inlet of the circulating gas for drying supplied to the fluidized-bed dryer is preferably as high as possible in terms of drying efficiency. It must be:
Since the polyethylene solid content impregnated with the diluent may cause agglomeration of particles even at a temperature lower than the melting point, the inlet temperature of the circulation gas for drying is 1 point lower than the melting point of the polyethylene solid content.
It is preferable to set the temperature lower than 0 ° C.

The circulating gas for drying is uniformly dispersed by a dispersion plate provided on the lower side in the fluidized-bed dryer, rises, and is dried while flowing with the polyolefin solid particles in the fluidized bed. The gas velocity V blown from the holes of the dispersion plate in the present invention is 10 to 50 m / sec. When the speed is 10 m / sec or less, the polyolefin solid particles easily fall below the holes of the dispersion plate, and the fine powder particles fill the holes of the dispersion plate. On the other hand, if the speed is 50 m / sec or more, the pressure difference between the front and rear of the dispersion plate is large, and a circulating gas blower with high power is required, which is not economical. Also, when using a perforated plate with bulk, the fine powder particles containing a large amount of low molecular weight, or the surface of the plate plate,
Since it is strongly hit against the protrusion, a scale is generated in the vicinity thereof.

The terminal velocity used in the present invention is defined as a value when the sedimentation velocity of a particle in a stationary fluid in a gravitational field becomes equal to the gravitational force and the resistance force, and the particle operates at a constant velocity. Speed. In the present invention, the superficial velocity in the tank in the fluidized-bed dryer is 0.2 to 0.8 times the terminal velocity, particularly preferably 0.3 to 0.7 times.
If it is 0.2 times or less, a uniform fluidized bed cannot be obtained,
If the ratio is 0.8 times or more, the solid polyethylene particles scattered from the fluidized bed increase, and this increases the load on the bubble filter, which is not preferable.

The bed height of the bed made of polyethylene solid particles in the fluidized-bed dryer is not particularly limited, but if it is too low, the fed slurry directly reaches the dispersion plate,
Contaminating the dispersion plate, scale adheres to the dispersion plate surface,
Alternatively, the pores of the dispersion plate are more likely to be closed, so that it is preferably 50 cm or more. Density 0.945g / cm ³
When attempting to produce the following low-density polyethylene, since low-density polyethylene contains a large amount of low-molecular-weight polymer, scale tends to be generated at the entrance of a fluidized-bed dryer, on a wall, on a dispersion plate, etc. Large lumps of polymer tend to accumulate in the dryer. In order to transport the large lumps in the fluidized bed dryer and discharge them out of the system, kinetic energy for moving the large lumps is required.

In the present invention, the above-mentioned mass is dried in a fluidized bed.
To discharge from the dryer to the outside of the system, blow out from the hole of the dispersion plate
The gas velocity in the horizontal direction of powder discharge of gas is V (m /
Second) and the total area of the holes per unit area of the dispersion plate is A (m^Two/ M
^Two), The gas density is G (kg / m ^ThreeV when expressed as^Two
× A × G value is 10 kg / m · sec^TwoPreferably
Good.

In the present invention, the dispersion plate of the fluidized-bed dryer has a large number of holes through which gas passes. In order to prevent clogging of the holes, it is not preferable that the gas blowout direction of the holes is directed directly upward, and for example, a bulky dispersion plate is preferable. In order to discharge large lumps of polymer out of the system, it is preferable to deflect the gas flow from the holes so that kinetic energy acts in the discharge direction. For example, in the case of a bulky dispersion plate,
Preferably, the nozzle holes are not drilled in all directions of the nozzle, but only in the discharge direction.

[0018]

The present invention will be described in more detail with reference to the following examples and reference examples, but the present invention is not limited to these examples. The melt index (MFI) and density in the examples are based on JIS-K-676.
0 was used.

[0019]

Example 1 In the polymerization process shown in FIG. 1, isobutane was used as a diluent, and the MFI of the polymer was 8 g / 10 min.
Copolymerization of ethylene and hexene-1 was performed so that the density became 0.925 g / cm ³ . Further, isobutane is supplied to a downward extraction pipe called a settling leg so that the weight ratio of the diluent to the polymer is 6: 4, and the polymer is supplied to the downward extraction pipe.
It was withdrawn at a throughput of g / hour into a fluid bed dryer with an inner diameter of 0.7 m. The polymerization temperature was 70 ° C. and the polymerization pressure was 20 kg / c.
m ² G. The nozzle of the dispersing plate was of the type shown in FIG. 2 and had three holes in the direction of 0 ° and ± 45 ° with respect to the direction of discharging polyethylene solids per nozzle. Further, the total area A of the dispersion plate per unit area of the holes was 0.0115m ² / m ^2. The temperature of the circulating gas at the inlet of the fluidized bed dryer is 60 ° C., the pressure in the system is 0.5 kg / cm ² G, the superficial velocity is 0.345 m / sec,
The gas velocity V blown from the holes of the dispersion plate is 30 m / sec, and V ²
× A × G is 16kg / m · sec, bed height of bed is 70c
The operating conditions of the fluidized-bed dryer were controlled so as to obtain m.

Under the above operating conditions, the operation was stopped after one week of continuous operation, and the inside of the fluidized-bed dryer was inspected. As a result, the dispersion plate was very clean without scale adhesion. For the polymer lump discharge test, a lump having a diameter of about 1.5 cm made by melting polyethylene was placed on a dispersion plate before the operation, and the lump was also discharged. During operation, the pressure difference before and after the dispersion plate hardly increased, and the concentration of volatile components in the solid polyethylene content at the outlet of the fluidized-bed dryer was 1 wt% or less.

[0021]

Comparative Example 1 In Example 1, the hole diameter of the nozzle of the dispersion plate was used.
And changing the total area A of the holes per unit area of the dispersion plate to 0.0
69m^Two/ M^TwoThe gas velocity V blown out of the hole is 5 m /
Seconds, V ^Two× A × G at 3kg / m · sec^TwoFluidized bed dryer
I drove. As a result, after the start of operation,
It gradually rises and the set value of circulating gas depends on the performance of the circulating blower.
, And operation was stopped in three days. Also flowing
When the inside of the bed dryer was inspected,
Particles are falling from the holes of the dispersion plate to the bottom of the dispersion plate,
In many cases, the nozzle holes were closed. Before driving
Immediately after being placed on the dispersion plate
Lumps remained without being discharged.

[0022]

Comparative Example 2 In Example 1, the total area A of the holes per unit area of the dispersion plate was changed to 0.00 by changing the hole diameter of the nozzle of the dispersion plate.
21 m ² / m ² , the gas velocity V blown out of the hole is 70 m / m ²
The fluidized-bed dryer was operated at 87 kg / m · sec ² at V ² × A × G for ² sec. As a result, after the start of operation, the differential pressure across the dispersion plate gradually increased, and the set value of the circulating gas could not be maintained due to the performance of the circulating blower, and the operation was stopped in six days. When the inside of the fluidized bed dryer was inspected, it was presumed that the flow velocity of the gas blown out from the nozzle holes of the dispersion plate was high and that the polyolefin solids were strongly hit against the surface of the dispersion plate and the back of other nozzles. Hard scale was attached near the place. In addition, a diameter of about 1.5 placed on the dispersion plate before operation
cm of polyethylene lumps had been discharged.

[0023]

According to the present invention, the volatile matter of the polyethylene solid containing volatile components is efficiently dried, and the polyethylene solid is recovered from the slurry containing the polyethylene solid and the diluent with good operability. Can be. In particular,
The lumps supplied during the fluidized bed drying or the lumps generated in the vessel are continuously discharged, and almost no increase in the differential pressure before and after the dispersion plate due to scaling on the dispersion plate, blockage of the holes on the dispersion plate, etc. Not recognized, the load on the bag filter is small, and long-term continuous operation is possible.

[Brief description of the drawings]

FIG. 1 is a flow sheet for explaining the method of the present invention.

FIG. 2 is a partial detailed view of a dispersion plate.

[Explanation of symbols]

Reference Signs List 1 Feed line for α-olefins such as ethylene and propylene and molecular weight regulators such as hydrogen 2 Catalyst feed line 3 Diluent feed line 4 Slurry circulation line 5 Slurry feed line 6 Gas line 7 Gas circulation line 8 Gas recovery line 9 Dispersion Powder recovery line 10 Polyolefin solids recovery line 11 Plate 12 Nozzle 13 holes A Polymerizer B Slurry circulation pump C Fluidized bed dryer D Dispersion plate E Bag filter F Blower G Gas heater H Bed section

Claims (1)

(57) [Claims] 1. A slurry containing polyethylene solids and a diluent is introduced into a fluidized bed drier to separate the polyethylene solids and diluent gas to recover polyethylene solids. While maintaining the pressure in the fluidized-bed dryer at 2 kg / cm ² G or less, the polyethylene solid containing the diluent is brought into contact with a gas containing the diluent as a main component at a temperature not higher than the melting point of the solid, and (2) The gas velocity V blown out from the hole of the dispersion plate of the fluidized bed dryer is 10 to 50 m / sec, and (3) The superficial velocity in the tank in the fluidized bed dryer is 0.1% of the terminal velocity.
A method for recovering polyethylene solids, which is 2 to 0.8 times.