Polyethylene slurry conveying method and equipment

Abstract

The invention discloses a polyethylene slurry conveying method and equipment. The polyethylene slurry conveying equipment comprises a polymerization reaction kettle, a flash tank, a discharge pipe and a nitrogen line, a first valve is arranged at the bottom of the polymerization reaction kettle; a second valve is arranged outside the shell of the flash tank; a multi-arc spiral scribed line is arranged on the inner wall of the discharging pipe, the discharging pipe is obliquely arranged, an upper port of the discharging pipe is connected with the first valve, and a lower port of the discharging pipe is connected with the second valve; the nitrogen line is arranged at the position, close to the upper end opening, of the discharging pipe. According to the polyethylene slurry conveying equipment, the conveying efficiency of polyethylene slurry is improved, and deposition and scaling of the polyethylene slurry at the bottom of the polymerization reaction kettle, the discharge pipe and all valves are avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of polyethylene, and particularly relates to a method and equipment for transporting polyethylene slurry, which are particularly applicable to the slurry discharge process of a polyethylene laboratory polymerization device. Background Art

[0002] The polyethylene laboratory polymerization device plays an important role in the research and development of polyethylene catalysts and performance evaluation, the development of new polyethylene products, and the development of polymerization process technologies. Generally, in order to consider conditions such as operation flexibility, construction cost, and space limitation, after polymerization, the polyethylene slurry is directly discharged through the bottom valve of the reaction kettle. During the discharge process, there are risks such as splashing, clogging, air pollution, and flash explosion. Therefore, it is necessary to develop some new discharging modes and methods.

[0003] The slurry after polymerization in the polyethylene reaction kettle contains about 30%-50% (mass fraction) of polyethylene powder particles with a diameter distribution of 75-1000 μm, and the rest are solvents such as hexane. If it is directly and quickly discharged from the bottom of the kettle, there are risks such as splashing, clogging, and flash explosion. At the same time, while the solvent volatilizes and is lost, it pollutes the air; if the discharge is too slow, the solid particles in the slurry settle in the reaction kettle, and the large particles and flocs in the slurry may get stuck at the bottom channel of the kettle. The solid particles accumulate more and more, thus blocking the entire valve, making it impossible to discharge the materials in the kettle and leading to accidents.

[0004] The prior art solves the risks of polyethylene kettle slurry discharging and pipeline clogging by adding a discharge tank and a flash tube, and at the same time improving the layout of the discharge line, and transporting the polyethylene slurry into the flash tank by the action of pressure difference or / and liquid level difference to avoid risks such as splashing, clogging, air pollution, and flash explosion. However, the slurry is still likely to remain at the bottom of the pipeline, and there is still a risk of clogging in the long term.

[0005] Chinese Patent CN111841449A discloses an experimental device for polyethylene slurry discharging, including: a polymerization reactor for storing polyethylene slurry; a discharging tank connected to the polymerization reactor through a feeding pipe, and a first valve is installed on the feeding pipe; a flash tank, the discharging tank is connected to the flash tank through a discharging pipe, and a second valve is installed on the discharging pipe; wherein, one end of the feeding pipe and the discharging pipe far from the polymerization reactor extends obliquely downward; when the first valve is opened, the polymerization reactor conveys polyethylene slurry into the discharging tank under the action of a pressure difference or / and a liquid level difference; when the second valve is opened, the discharging tank conveys polyethylene slurry into the flash tank under the action of a pressure difference or / and a liquid level difference. By arranging a discharging tank between the first valve and the second valve and using the switching of the on-off states of the first valve and the second valve to perform a liquid filling and discharging cycle operation on the discharging tank, the continuous discharging of the slurry in the reactor can be achieved; however, to solve the risks of polyethylene kettle slurry discharging and pipeline blockage problems, this device increases the discharging tank and the flash tank, and at the same time improves the layout of the discharging line, with more components and complex operations.

[0006] Chinese Patent CN202245148U discloses a crystal slurry screw conveyor device. It includes a cylinder, and a screw conveyor rod driven by a screw driving device is axially arranged in the cylinder; the cylinder is respectively communicated with the outlet of the upstream device and the inlet of the downstream device. This device mainly solves the technical problem that when the existing conveying device is in use, when the crystal thickens to a high concentration, it is extremely easy to be blocked in the round pipe section when flowing through the valve for adjusting the flow rate. It is applicable to conveying crystal slurries of various concentrations. However, this device adds a screw driving equipment and reduces the pipeline diameter. If polymerization blockage occurs at the screw, it is difficult to repair and it is not applicable to the field of polyethylene slurry. Summary of the Invention

[0007] One technical problem to be solved by the present invention is that when conveying polyethylene slurry using a pressure difference and / or a liquid level difference, the conveying of polyethylene slurry is slow, affecting the efficiency. Another technical problem that the present invention can also solve is the problem that scale deposits at the bottom of the polymerization reactor, the discharging pipeline and the valve, and the reactor needs to be disassembled and cleaned multiple times, affecting the normal operation of the equipment.

[0008] To solve the above technical problems, the present invention provides a polyethylene slurry conveying device, which includes:

[0009] A polymerization reactor with a first valve provided at the bottom;

[0010] A flash tank with a second valve provided outside the shell;

[0011] The discharge pipe has multi-arc spiral grooves on its inner wall. The discharge pipe is inclined. The upper port of the discharge pipe is connected to the first valve, and the lower port of the discharge pipe is connected to the second valve; and

[0012] The nitrogen line is arranged near the upper port of the discharge pipe.

[0013] Furthermore, the depth of the multi-arc spiral grooves is 1 / 15 to 1 / 20 of the wall thickness of the discharge pipe.

[0014] Furthermore, the outer wall of the discharge pipe is provided with solvent lines.

[0015] Furthermore, the number of the solvent lines is two, namely the first solvent line and the second solvent line; the first solvent line is close to the first valve, and the flushing direction of the first solvent line is to flush upward to the polymerization reactor; the second solvent line is close to the second valve, and the flushing direction of the second solvent line is to flush downward to the flash tank.

[0016] Furthermore, the first solvent line and the second solvent line flush alternately, further avoiding the risk of blockage of each pipeline.

[0017] Furthermore, the diameter of the solvent line is 1 / 3 - 1 / 2 of the diameter of the discharge pipe.

[0018] Furthermore, a large-diameter ball valve is provided on the solvent line near the discharge pipe.

[0019] Furthermore, when the first valve and the second valve are opened simultaneously, the polymerization reactor conveys polyethylene slurry into the flash tank under the action of a pressure difference and / or a liquid level difference. The polyethylene slurry is spirally accelerated and discharged into the flash tank under the conveying action of the multi-arc spiral grooves on the inner wall of the discharge pipe and the nitrogen in the nitrogen line.

[0020] Furthermore, a pressure reducing valve is provided on the nitrogen line, and the pressure of the nitrogen conveyed in the nitrogen line is lower than the pressure of the polymerization reactor and higher than the pressure of the flash tank.

[0021] Furthermore, the nitrogen line is tangent to the multi-arc spiral grooves; the number of the nitrogen lines is two, and the two nitrogen lines are respectively arranged on both sides of the discharge pipe; the diameter of the nitrogen line is 1 / 3 - 1 / 4 of the diameter of the discharge pipe, and the feeding direction of the nitrogen is from top to bottom.

[0022] Furthermore, a needle-type regulating valve is arranged on the nitrogen line near the discharge pipe, and the nitrogen line can freely adjust the air intake under the action of the needle-type regulating valve.

[0023] Further, the discharge pipe is connected to the pipeline at the first valve through a flange, and the discharge pipe is connected to the pipeline at the second valve through a flange.

[0024] Further, the first valve and the second valve are large-diameter ball valves.

[0025] Further, the first valve and the second valve are pneumatic ball valves or electric ball valves.

[0026] The use of large-diameter ball valves or pneumatic ball valves can achieve cut-off / connection operations. The large diameter allows the polyethylene slurry in the discharge pipe to flow through the subsequent processes by gravity, simplifies the process, prevents blockages, and ensures the smoothness of the entire pipeline.

[0027] Further, the second valve is arranged in the upper middle part outside the shell of the flash tank.

[0028] The present invention also provides a method for transporting polyethylene slurry, which includes the following steps:

[0029] (1) Open the first valve and the second valve, and the polymerization reactor transports polyethylene slurry into the flash tank through a discharge pipe with multi-arc spiral grooves on the inner wall;

[0030] (2) Open the nitrogen line to transport nitrogen, and the polyethylene slurry in the discharge pipe accelerates and is transported into the flash tank until all the polyethylene slurry in the polymerization reactor is discharged, then close the first valve, the second valve and the nitrogen line.

[0031] Further, after completing step (2), it also includes:

[0032] (3) Open the first solvent line and the first valve close to the polymerization reactor, and use the solvent to wash the residual slurry in the polymerization reactor and the discharge pipe. After the washing is completed, close the first valve and the first solvent line;

[0033] (4) Repeat steps (1) and (2) to discharge the residual slurry;

[0034] (5) Open the second solvent line and the second valve close to the flash tank, and use the solvent to wash the residual slurry in the discharge pipe and the second valve. After the washing is completed, close the second valve and the second solvent line.

[0035] The present invention uniquely adopts a discharge pipe with multi-arc spiral grooves on the inner arm and a nitrogen line, which can perform spiral acceleration on the polyethylene slurry in the discharge pipe, improve the transportation efficiency of the polyethylene slurry, effectively scour the pipeline, effectively prevent the solid particles in the polyethylene slurry from settling and causing blockages in each pipeline, ensure the smoothness of the pipeline, improve the efficiency, and solve the problems of slow transportation and easy blockage of polyethylene slurry when the prior art uses pressure difference and / or liquid level difference to transport polyethylene slurry.

[0036] Further preferably, a solvent line is also provided on the outer wall of the discharge pipe of the polyethylene slurry conveying equipment of the present invention. After the polyethylene slurry is discharged, the polymerization reactor, the discharge pipe and the relevant valves are rinsed, and the discharge line is rinsed again, further avoiding the deposition and scaling of the polyethylene slurry at the bottom of the polymerization reactor, the discharge pipe and each valve, greatly reducing the number of times of disassembling the reactor, improving the utilization rate of the reactor, and solving the problem of easy deposition and scaling at the bottom of the polymerization reactor, the discharge pipeline and each valve in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a schematic structural diagram of the polyethylene slurry conveying equipment according to an embodiment of the present invention.

[0038] Figure 1 In the figure, reference numerals:

[0039] 1, polymerization reactor; 2, flash tank; 3, first valve; 4, second valve; 5, discharge pipe; 6 and 7, nitrogen line; 8, first solvent line; 9, second solvent line.

[0040] Figure 2 It is a schematic structural diagram of an embodiment of the discharge pipe of the present invention.

[0041] Figure 3 It is for the polyethylene slurry discharge experimental device in Figure 1 Chinese Patent CN111841449A.

[0042] Figure 3 In the figure, reference numerals:

[0043] 201, polyethylene polymerization reactor; 202, discharge tank; 221, feed pipe; 222, discharge pipe; 223, flange; 224, exhaust pipe; 203, flash tank; 204, first valve; 205, second valve; 206, third valve. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] The present invention will be further described below with reference to the specific drawings, but the present invention is not limited to the following content.

[0045] Figure 1 It is a schematic structural diagram of the polyethylene slurry conveying equipment according to an embodiment of the present invention. The polyethylene slurry conveying equipment includes: a polymerization reactor 1, a flash tank 2, a discharge pipe 5 and a nitrogen line.

[0046] A first valve 3 is provided at the bottom of the polymerization reactor 1. The polymerization reactor 1 is used for polymerization reaction to generate polyethylene slurry.

[0047] A second valve 4 is provided outside the shell of the flash tank 2.

[0048] Figure 2 The structure schematic diagram of an embodiment of the discharge pipe of the present invention is shown. Please refer to Figure 1 and Figure 2 simultaneously. There are multi-arc spiral grooves on the inner wall of the discharge pipe 5. The discharge pipe 5 is inclined. The upper port of the discharge pipe 5 is connected to the first valve 3, and the lower port of the discharge pipe 5 is connected to the second valve 4. The flash tank 2 and the polymerization reactor 1 are connected through the first valve 3, the second valve 4 and the discharge pipe 5.

[0049] Furthermore, the depth of the multi-arc spiral grooves is 1 / 15 to 1 / 20 of the wall thickness of the discharge pipe 5, preferably 1 / 18.

[0050] Please continue to refer to Figure 1 and Figure 2 simultaneously. The nitrogen line is arranged near the upper port of the discharge pipe 5, that is, it is arranged near the polymerization reactor 1. The nitrogen transported in the nitrogen line can increase the transportation capacity of the polyethylene slurry in the discharge pipe 5. The present invention does not particularly limit the number of nitrogen lines. Although Figure 2 only one nitrogen line is shown, the number of nitrogen lines of the present invention is not limited thereto. In other embodiments, the number of nitrogen lines can be set to 2, 3 or more than 3. Preferably, it is set to 2, which are respectively Figure 1 the nitrogen lines 6 and 7 shown. The two nitrogen lines 6 and 7 are respectively arranged on both sides of the discharge pipe 5.

[0051] Furthermore, both the nitrogen lines 6 and 7 are tangent to the multi-arc spiral grooves; the diameters of the nitrogen lines 6 and 7 are 1 / 3 - 1 / 4 of the diameter of the discharge pipe 5, and the feeding direction of the nitrogen in the nitrogen lines 6 and 7 is from top to bottom.

[0052] Furthermore, needle valves are arranged on the nitrogen lines 6 and 7 near the discharge pipe 5. The nitrogen intake of the nitrogen lines 6 and 7 can be freely adjusted under the action of the needle valves.

[0053] Furthermore, a solvent line is arranged on the outer wall of the discharge pipe.

[0054] The present invention does not particularly limit the number of solvent lines, which can be one, two or more than two. Preferably, it is two, which are respectively the first solvent line 8 and the second solvent line 9; the first solvent line 8 is close to the first valve 3, and the flushing direction of the first solvent line 8 is from bottom to top to flush to the polymerization reactor 1; the second solvent line 9 is close to the second valve 4, and the flushing direction of the second solvent line 9 is from top to bottom to flush to the flash tank 2.

[0055] Furthermore, the first solvent line 8 and the second solvent line 9 flush alternately, further avoiding the risk of blockage of each pipeline.

[0056] Further, the diameters of the first solvent line 8 and the second solvent line 9 are 1 / 3 - 1 / 2 of the diameter of the discharge pipe 5, preferably 3 / 8, and the included angles between the first solvent line 8, the second solvent line 9 and the discharge line 5 are 30°.

[0057] Further, large-diameter ball valves are provided at the first solvent line 8 and the second solvent line 9 near the discharge pipe 5.

[0058] Further, when the first valve 3 and the second valve 4 are opened simultaneously, the polymerization reactor 1 conveys the polyethylene slurry into the flash tank 3 under the action of the pressure difference and / or the liquid level difference. The polyethylene slurry is spirally and accelerated to be discharged into the flash tank 3 under the conveying action of the multi-arc spiral engraving line on the inner wall of the discharge pipe 5 and the nitrogen in the nitrogen line.

[0059] Further, pressure reducing valves (not shown in the figure) are also provided on the nitrogen lines 6 and 7. The pressure of the nitrogen conveyed in the nitrogen lines 6 and 7 is slightly lower than the pressure of the polymerization reactor 1 and slightly higher than the pressure of the flash tank 3, so as to avoid the problem that the polyethylene slurry is pushed back into the polymerization reactor 1 due to excessive pressure of the nitrogen conveyed in the nitrogen lines 6 and 7.

[0060] Further, the pipeline at the first valve 3 and the discharge pipe 5 is connected by a flange, and the pipeline at the second valve 4 and the discharge pipe 5 is connected by a flange.

[0061] Further, the first valve 3 and the second valve 4 are large-diameter ball valves.

[0062] Further, the first valve 3 and the second valve 4 are pneumatic ball valves or electric ball valves.

[0063] The use of large-diameter ball valves or pneumatic ball valves can achieve the cut-off / connection operation. The large diameter enables the polyethylene slurry in the discharge pipe 5 to flow through the subsequent process by gravity, simplifies the process, prevents blockage, and ensures the smoothness of the entire pipeline.

[0064] Further, the second valve 4 is arranged at the upper middle part outside the shell of the flash tank 2.

[0065] The present invention also provides a method for transporting a polyethylene slurry, and the method includes the following steps:

[0066] (1) Open the first valve 3 and the second valve 4, and the polymerization reactor 1 conveys the polyethylene slurry into the flash tank 2 through the discharge pipe 5 with multi-arc spiral engraving lines on the inner wall;

[0067] (2) Open the nitrogen lines 6 and 7 to transport nitrogen, and the polyethylene slurry in the discharge pipe 5 is accelerated to be transported into the flash tank 2 until all the polyethylene slurry in the polymerization reactor 1 is discharged, and then close the first valve 3, the second valve 4 and the nitrogen lines.

[0068] Further, after completing step (2), it further includes:

[0069] (3) Open the first solvent line 8 and the first valve 3 near the polymerization reactor 1, and use the solvent to flush the residual slurry in the polymerization reactor 1 and the discharge pipe 5. After flushing is completed, close the first valve 3 and the first solvent line 8;

[0070] (4) Repeat steps (1) and (2) to discharge the residual slurry;

[0071] (5) Open the second solvent line 8 and the second valve 4 near the flash tank 2, and use the solvent to flush the residual slurry in the discharge pipe 5 and the second valve 4. After flushing is completed, close the second valve 4 and the second solvent line 8.

[0072] Example 1

[0073] Adopt Figure 1 the polyethylene slurry conveying equipment shown in

[0074] for the research and development of polyethylene catalysts and the performance evaluation. In the polymerization reactor, a polymerization reaction occurs to generate polyethylene slurry. Open the first valve and the second valve, and open the nitrogen line to convey nitrogen. The polyethylene slurry in the polymerization reactor is quickly conveyed to the flash tank until all the polyethylene slurry in the polymerization reactor is discharged, and then close the first valve, the second valve, and the nitrogen line. The entire conveying process is fast and there is no pipeline blockage.

[0075] The polyethylene slurry conveying equipment continuously conducts the performance evaluation of polyethylene catalysts 10 times. The equipment still operates smoothly and there is no pipeline blockage. Open the first solvent line and the first valve near the polymerization reactor, and use the solvent to flush the residual slurry in the polymerization reactor and the discharge pipe. After flushing is completed, close the first valve and the first solvent line, discharge the residual slurry, open the second solvent line and the second valve near the flash tank, and use the solvent to flush the residual slurry in the discharge pipe and the second valve. After flushing is completed, close the second valve and the second solvent line.

[0076] Comparative Example 1

[0077] Adopt Figure 3 the polyethylene slurry discharging experimental device disclosed in Chinese Patent CN111841449A shown in

[0078] Open the first valve, and the polyethylene polymerization reactor conveys the polyethylene slurry into the discharge tank until the pressure difference and / or liquid level difference in the discharge tank is the same as that in the polyethylene polymerization reactor. Then close the first valve, open the second valve, and the discharge tank conveys the polyethylene slurry into the flash tank; when all the polyethylene slurry in the discharge tank is completely conveyed into the flash tank, close the second valve. The entire conveying process is slow and the waiting time is long.

[0079] After the polyethylene catalyst performance was evaluated 5 times with the polyethylene slurry discharging experimental device, the discharging pipe was severely deposited and fouled, resulting in blockage. The device stopped running, and the device was disassembled and the dirt was cleaned up.

[0080] The present invention uniquely adopts a discharging pipe with multi-arc spiral lines on the inner wall, together with a nitrogen line, which can perform spiral acceleration on the polyethylene slurry in the discharging pipe, improve the conveying efficiency of the polyethylene slurry, effectively scour the pipeline, and effectively prevent the solid particles in the polyethylene slurry from settling and causing blockage of each pipeline, ensuring the smoothness of the pipeline, improving the efficiency, and solving the problems of slow polyethylene slurry conveying and easy blockage when the prior art uses pressure difference and / or liquid level difference to convey polyethylene slurry.

[0081] Further preferably, a solvent line is also provided on the outer wall of the discharging pipe of the polyethylene slurry conveying equipment of the present invention. After the polyethylene slurry is discharged, it flushes the polymerization reactor, the discharging pipe and related valves, and flushes the discharging line again, further avoiding the deposition and fouling of the polyethylene slurry at the bottom of the polymerization reactor, the discharging pipe and each valve, greatly reducing the number of times of disassembling the reactor, improving the utilization rate of the reactor, and solving the problem of easy deposition and fouling at the bottom of the polymerization reactor, the discharging pipeline and each valve in the prior art.

[0082] Although the present invention has been disclosed as above by way of examples, it is not intended to limit the present invention. Any person with common general knowledge in the technical field can make some modifications and refinements without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the scope defined by the appended claims.

Claims

1. A polyethylene slurry conveying device, characterized in that, it includes: a polymerization reactor with a first valve provided at the bottom; a flash tank with a second valve provided outside the shell; a discharge pipe with multi-arc spiral grooves provided on the inner wall, the discharge pipe is inclined, the upper port of the discharge pipe is connected to the first valve, and the lower port of the discharge pipe is connected to the second valve; and a nitrogen line provided near the upper port of the discharge pipe.

2. The polyethylene slurry conveying device according to claim 1, characterized in that, the depth of the multi-arc spiral grooves is 1 / 15 to 1 / 20 of the wall thickness of the discharge pipe.

3. The polyethylene slurry conveying device according to claim 1, characterized in that, a solvent line is provided on the outer wall of the discharge pipe.

4. The polyethylene slurry conveying device according to claim 3, characterized in that, the number of the solvent lines is two, namely a first solvent line and a second solvent line; the first solvent line is close to the first valve, and the flushing direction of the first solvent line is to flush upward to the polymerization reactor; the second solvent line is close to the second valve, and the flushing direction of the second solvent line is to flush downward to the flash tank.

5. The polyethylene slurry conveying device according to claim 4, characterized in that, the first solvent line and the second solvent line flush alternately.

6. The polyethylene slurry conveying device according to claim 3, characterized in that, the diameter of the solvent line is 1 / 3 - 1 / 2 of the diameter of the discharge pipe.

7. The polyethylene slurry conveying device according to claim 3, characterized in that, a large-diameter ball valve is provided on the solvent line near the discharge pipe.

8. The polyethylene slurry conveying device according to claim 1, characterized in that, when the first valve and the second valve are opened simultaneously, the polymerization reactor conveys polyethylene slurry into the flash tank under the action of a pressure difference or / and a liquid level difference, and the polyethylene slurry is spirally accelerated and discharged into the flash tank under the conveying action of the multi-arc spiral grooves on the inner wall of the discharge pipe and the nitrogen in the nitrogen line.

9. The polyethylene slurry conveying device according to claim 1, characterized in that, a pressure reducing valve is provided on the nitrogen line, and the pressure of the nitrogen conveyed in the nitrogen line is lower than the pressure of the polymerization reactor and higher than the pressure of the flash tank.

10. The polyethylene slurry conveying device according to claim 1, characterized in that, the nitrogen line is tangent to the multi-arc spiral grooves; the number of the nitrogen lines is two, and the two nitrogen lines are respectively provided on both sides of the discharge pipe; the diameter of the nitrogen line is 1 / 3 - 1 / 4 of the diameter of the discharge pipe, and the feeding direction of the nitrogen is from top to bottom.

11. The polyethylene slurry conveying device according to claim 1, characterized in that, a needle-type regulating valve is provided on the nitrogen line near the discharge pipe, and the nitrogen line freely adjusts the air intake under the action of the needle-type regulating valve.

12. The polyethylene slurry conveying device according to claim 1, characterized in that, The discharge pipe is connected to the pipeline at the first valve through a flange, and the discharge pipe is connected to the pipeline at the second valve through a flange.

13. The polyethylene slurry conveying device according to claim 1, characterized in that the first valve and the second valve are large-diameter ball valves.

14. The polyethylene slurry conveying device according to claim 1, characterized in that the first valve and the second valve are pneumatic ball valves or electric ball valves.

15. The polyethylene slurry conveying device according to claim 1, characterized in that the second valve is arranged at the upper middle part outside the shell of the flash tank.

16. A method for conveying polyethylene slurry, characterized in that it includes the following steps: (1) Open the first valve and the second valve, and the polymerization reactor conveys polyethylene slurry into the flash tank through a discharge pipe with multi-arc spiral grooves on the inner wall; (2) Open the nitrogen line to convey nitrogen, and the polyethylene slurry in the discharge pipe is accelerated to be conveyed into the flash tank until all the polyethylene slurry in the polymerization reactor is discharged, and then close the first valve, the second valve and the nitrogen line.

17. The method for conveying polyethylene slurry according to claim 16, characterized in that after completing step (2), it further includes: (3) Open the first solvent line and the first valve close to the polymerization reactor, and use the solvent to flush the residual slurry in the polymerization reactor and the discharge pipe. After flushing, close the first valve and the first solvent line; (4) Repeat steps (1) and (2) to discharge the residual slurry; (5) Open the second solvent line and the second valve close to the flash tank, and use the solvent to flush the residual slurry in the discharge pipe and the second valve. After flushing, close the second valve and the second solvent line.

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