Method for stopping and dealkalizing a coal direct liquefaction plant

Abstract

The application discloses a method for stopping work and withdrawing alkali liquor of a coal direct liquefaction device, and the system comprises a tank area alkali liquor tank, a coal liquefaction device alkali liquor buffer tank, a reaction heating furnace, a first reactor, a second reactor, a high-temperature and high-pressure separator, an alkali liquor discharge buffer tank, a foam cyclone separator and a defoaming device; the alkali liquor is discharged to the coal liquefaction device alkali liquor discharge buffer tank for buffering, steam removal and defoaming, and then is pressurized and transported to the tank area alkali liquor tank for storage through a coal liquefaction device alkali liquor delivery pump, so that the purpose of continuously and massively withdrawing alkali liquor of the coal direct liquefaction device during stopping work is achieved, the stopping work time of the enterprise is saved, and certain economic benefits are brought.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of direct coal liquefaction, and particularly relates to a method for stopping and discharging alkali liquor of a direct coal liquefaction device. BACKGROUND

[0002] China has realized industrial application of direct coal liquefaction technology. Since coal was put into the direct coal liquefaction device, the device has reached a stable, long, high and excellent operation level through scientific research and technical improvement in the past years. Due to engineering development of the second generation of direct coal liquefaction technology, the process route of the second generation of direct coal liquefaction technology makes corresponding adjustment to the previous direct coal liquefaction project, cancels two reactor bottom internal circulation pumps, cancels internal parts such as reactor internal circulation cup and center pipe, adds a high-temperature and high-pressure separator bottom system large circulation pump, and circulates material from the high-temperature and high-pressure separator bottom to the first reactor inlet, thereby forming a large circulation reaction system including the reactor and the high-temperature and high-pressure separator, eliminating the vapor-liquid interface in the reactor, prolonging the operation cycle, and improving the economic benefit of the device.

[0003] At present, in the process of stopping and discharging alkali liquor of the coal liquefaction device, since the tank area storage tank is a non-pressure tank, when a large amount of alkali liquor is discharged from the reactor and passes through the coal slurry heating furnace outlet, the flow is closed at the low point of the reactor to prevent pressure from being transferred to the tank area, and the alkali liquor is discharged to the tank area through the reactor blowdown, which is slow, and the alkali liquor is easy to be gassed when it is discharged to the bottom of the reactor, causing overpressure of the tank area storage tank and affecting the stopping point of the coal liquefaction device.

[0004] Therefore, for the second generation of direct coal liquefaction technology, it is necessary to explore an optimized process for discharging oil and alkali liquor by using the existing process, to ensure that the device stopping meets the requirements of discharging oil and alkali liquor, and meets the needs of the coal liquefaction device stopping. SUMMARY

[0005] The present application aims to provide a system and method for stopping and discharging alkali liquor of a direct coal liquefaction device, to solve the problem of stopping and discharging alkali liquor of the direct coal liquefaction device of the second generation of direct coal liquefaction technology.

[0006] To achieve the above-mentioned purpose, the present application provides a system for stopping and discharging alkali liquor of a direct coal liquefaction device, which adopts the following technical scheme:

[0007] A system for stopping and discharging alkali liquor of a direct coal liquefaction device, comprising a tank area alkali liquor tank, a coal liquefaction device alkali liquor buffer tank, a reaction heating furnace, a first reactor, a second reactor, a high-temperature and high-pressure separator, an alkali liquor discharge buffer tank, a foam cyclone separator and a defoamer, wherein,

[0008] The tank area alkali liquor tank is connected to the coal liquefaction device alkali liquor buffer tank through a tank area alkali liquor conveying pump, so as to send the tank area alkali liquor into the coal liquefaction device alkali liquor buffer tank;

[0009] The coal liquefaction device alkali liquor buffer tank is connected with the reaction heating furnace through a coal liquefaction device alkali liquor pressurizing pump, and the reaction heating furnace is connected with the first reactor, the second reactor and the high-temperature high-pressure separator in sequence, so that the alkali liquor from the coal liquefaction device alkali liquor buffer tank is sequentially sent to the reaction heating furnace, the first reactor, the second reactor and the high-temperature high-pressure separator for alkali washing;

[0010] The feed inlet of the foam cyclone separator is connected with the reaction heating furnace, the first reactor, the second reactor and the high-temperature high-pressure separator respectively, so as to receive the alkali liquor discharged from the reaction heating furnace, the first reactor, the second reactor and the high-temperature high-pressure separator and perform foam separation;

[0011] The alkali liquor discharge buffer tank is connected with the bottom outlet of the foam cyclone separator, so as to receive the separated alkali liquor;

[0012] The defoaming device is used for eliminating the foam from the top outlet of the foam cyclone separator, and the defoamed material is sent back to the alkali liquor discharge buffer tank;

[0013] The bottom of the alkali liquor discharge buffer tank is connected with the tank area alkali liquor tank through a coal liquefaction device alkali liquor conveying pump, so as to send the alkali liquor in the alkali liquor discharge buffer tank back to the tank area alkali liquor tank.

[0014] To achieve the above-mentioned purpose, the coal direct liquefaction device shutdown and alkali liquor withdrawal system provided by the present application adopts the following technical solution:

[0015] A coal direct liquefaction device shutdown and alkali liquor withdrawal method, the method comprising the following steps:

[0016] S1) The alkali liquor in the tank area is conveyed to the coal liquefaction device alkali liquor buffer tank through a tank area alkali liquor conveying pump, and is sequentially conveyed to the reaction heating furnace, the first reactor, the second reactor and the high-temperature high-pressure separator through a coal liquefaction device alkali liquor pressurizing pump for alkali washing;

[0017] S2) After the alkali washing is completed, the tank area alkali liquor conveying pump and the coal liquefaction device alkali liquor pressurizing pump are stopped to stop the alkali liquor conveying;

[0018] S3) The alkali liquor in the reaction heating furnace, the first reactor, the second reactor and the high-temperature high-pressure separator is separated from the foam through a foam cyclone separator and is discharged to an alkali liquor discharge buffer tank; at the same time, a defoaming device is used to eliminate the foam separated from the foam cyclone separator and send the defoamed material into the alkali liquor discharge buffer tank;

[0019] S4) After the alkali liquor discharge is completed, the alkali liquor in the alkali liquor discharge buffer tank is pressurized through a coal liquefaction device alkali liquor conveying pump and is then externally sent to a tank area alkali liquor tank for storage.

[0020] Compared with the prior art, the present application has the following advantages:

[0021] The present application is aimed at the non-pressure tank of the tank area lye tank, and the use of nitrogen pressurization to transport lye is easy to cause overpressure of the tank area lye tank and easy to produce foam in the tank area lye tank due to the surfactant contained in the lye for alkali washing, thereby further slowing down the speed at the end of lye withdrawal and affecting the shutdown node of the coal liquefaction device. An innovative idea is adopted to discharge the lye to the lye discharge buffer tank of the coal liquefaction device for buffering, steam removal and defoaming, and then pressurized transportation to the tank area lye tank for storage by the lye transportation pump of the coal liquefaction device.

[0022] The present application achieves the purpose of the method for continuous and large amount of lye withdrawal of the coal direct liquefaction device, saves the shutdown time for the enterprise and brings certain economic benefits.

[0023] The present application solves the problem of overpressure of the tank area lye tank caused by the use of nitrogen pressurization to transport lye.

[0024] The present application provides buffering, steam removal and defoaming of the lye after alkali washing, prevents the tank area lye tank from foaming to further slow down the speed at the end of lye withdrawal and affect the shutdown node of the coal liquefaction device, and has low investment cost, high economic benefits, small safety risk and simple operation.

[0025] The present application has carried out a large amount of improvement and innovation work in design, production, commissioning and the like, and proposes a series of control indexes to ensure buffering, steam removal and defoaming of the lye and avoid overpressure of the tank area lye tank. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 It is a flowchart of an embodiment of the system for lye withdrawal of the coal direct liquefaction device of the present application;

[0027] ATTACHED Figure 1 The middle mark indicates:

[0028] 101, tank area lye tank; 102, coal liquefaction device lye buffer tank; 103, reaction heating furnace; 104, first reactor; 105, second reactor; 106, high temperature and high pressure separator; 107, lye discharge buffer tank; 108, foam cyclone separator; 109, defoamer;

[0029] 201, tank area lye transportation pump; 204, coal liquefaction device lye pressurization pump; 202, coal liquefaction device feed pump; 203, coal liquefaction device lye transportation pump; 301, coal liquefaction device lye buffer tank liquid level control valve; 302, coal liquefaction device lye pressurization pump outlet flow control valve; 303, coal liquefaction device lye transportation pump outlet flow control valve; 304, lye discharge buffer tank vent control valve; 305, lye discharge buffer tank nitrogen control valve;

[0030] 401, caustic tank inlet zone caustic tank valve; 402, caustic tank zone caustic tank extraction valve; 403, caustic tank zone caustic water pump inlet valve; 404, caustic tank zone caustic water pump outlet valve; 405, coal liquefaction device caustic buffer tank extraction valve; 406, coal liquefaction device caustic pressurizing pump inlet valve; 407, coal liquefaction device caustic pressurizing pump outlet valve; 408, coal liquefaction device feed pump inlet valve; 409, coal liquefaction device feed pump outlet valve; 410, coal liquefaction device caustic delivery pump outlet valve; 416, coal liquefaction device caustic delivery pump inlet valve; 411, caustic discharge buffer tank extraction valve; 412, reaction heating furnace outlet caustic discharge valve; 413, first reactor caustic discharge valve; 414, second reactor caustic discharge valve; 415, high temperature and high pressure separator caustic discharge valve; 417, coal liquefaction device feed pump outlet nitrogen valve; 418, caustic inlet defoamer valve;

[0031] 501, caustic tank zone caustic delivery pump inlet filter; 502, coal liquefaction device caustic pressurizing pump inlet filter; 503, coal liquefaction device feed pump inlet filter; 504, coal liquefaction device caustic delivery pump inlet filter; 601, caustic tank zone caustic delivery pump outlet check valve; 602, coal liquefaction device caustic pressurizing pump outlet check valve; 603, coal liquefaction device feed pump outlet check valve; 604, coal liquefaction device caustic delivery pump outlet check valve;

[0032] Figure 2 is a schematic diagram of a foam hydrocyclone;

[0033] attached Figure 2 middle mark explanation: 119, foam inlet, 129,

[0034] Figure 3 is a schematic diagram of a defoamer structure;

[0035] attached Figure 3 middle mark explanation: 119, foam inlet, 129, DETAILED DESCRIPTION

[0036] The present application will be described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0037] In the following, the technical solutions in the embodiments of the present application will be described clearly and completely with reference to the drawings in the embodiments of the present application in order to make the technical personnel in the technical field better understand the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by the ordinary skilled in the art without creative work should belong to the scope of protection of the present application.

[0038] It should be noted that the terms "first", "second" and the like in the description and claims of the present application and the above drawings are used to distinguish similar objects, and do not necessarily indicate a specific order or sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented. In addition, the terms "comprise" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device comprising a series of steps or units does not necessarily limit to those clearly listed steps or units, but can include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0039] As shown in Figure 1 The system for stopping and withdrawing alkali liquor of the coal direct liquefaction device of the present application comprises a tank area alkali liquor tank 101, a coal liquefaction device alkali liquor buffer tank 102, a reaction heating furnace 103, a first reactor 104, a second reactor 105, a high-temperature and high-pressure separator 106, an alkali liquor discharge buffer tank 107, a foam cyclone separator 108 and a defoamer 109.

[0040] The tank area alkali liquor tank 101 is used to store alkali liquor, and is connected to the coal liquefaction device alkali liquor buffer tank 102 through a tank area alkali liquor delivery pump 201, so as to send the tank area alkali liquor into the coal liquefaction device alkali liquor buffer tank 102.

[0041] The coal liquefaction device alkali liquor buffer tank 102 is connected to the reaction heating furnace 103 through a coal liquefaction device alkali liquor pressurizing pump 204, and the reaction heating furnace 103 is connected in series with the first reactor 104, the second reactor 105, and the high-temperature high-pressure separator 106 in sequence, so as to sequentially send the alkali liquor from the coal liquefaction device alkali liquor buffer tank into the reaction heating furnace, the first reactor, the second reactor, and the high-temperature high-pressure separator for alkali washing; the specific connection is well known in the art, for example, the outlet of the reaction heating furnace is connected to the feed port at the bottom of the first reactor, the outlet at the top of the first reactor is connected to the feed port at the bottom of the second reactor, and the outlet at the top of the second reactor is connected to the feed port of the high-temperature high-pressure separator. It is well known in the art that, when the coal direct liquefaction reaction is performed, the reaction heating furnace is used to heat the oil slurry to a predetermined temperature so as to enter the first reactor and the second reactor for coal direct liquefaction reaction, and the coal liquefaction reaction product is separated in the high-temperature high-pressure separator from the second reactor. For details, please refer to CN 108998068 A.

[0042] The feed port of the foam cyclone separator 108 is connected to the reaction heating furnace 103, the first reactor 104, the second reactor 105, and the high-temperature high-pressure separator 106, respectively, so as to receive the alkali liquor discharged from the reaction heating furnace, the first reactor, the second reactor, and the high-temperature high-pressure separator and perform foam separation; the specific connection is well known in the art, for example, the feed port 118 of the foam cyclone separator is connected to the outlet of the reaction heating furnace and the bottom of the first reactor, the second reactor, and the high-temperature high-pressure separator through a pipeline, respectively, so as to receive the alkali liquor. Of course, in order to facilitate the control of alkali liquor discharge, a valve can also be provided on each pipeline, which is well known in the art.

[0043] As shown in Figure 2 The foam cyclone separator 108 can be a cyclone separator well known in the art, in which the light foam is separated from the top and the alkali liquor is discharged from the bottom. The specific structure is well known in the art, and will not be described here.

[0044] The defoamer 109 is used to eliminate the foam from the top outlet of the foam cyclone separator 108 and send the defoamed material back to the alkali liquor discharge buffer tank 107. The defoamer can be a defoaming device commonly used in the art.

[0045] In one embodiment, as shown in Figure 3As shown, the defoamer 109 comprises a foam inlet 119, a spray liquid inlet pipe 129 and a shell, the upper end of the shell is closed, the lower end is open as the bottom outlet 179 of the defoamer, and the shell is sequentially a first cylinder segment 139, a reduced diameter segment 149, a second cylinder segment 159 and an expanded diameter segment 169 from top to bottom, the foam inlet 119 is arranged on the side wall of the first cylinder segment 139, the spray liquid inlet pipe 129 passes through the first cylinder segment 139 from top to bottom and extends into the second cylinder segment 159 so as to send in spray liquid, and the end of the spray liquid inlet pipe is a reduced diameter end 189, for example, reduced to 1 / 3-2 / 3 such as 1 / 2 of the original, that is, the end diameter of the reduced diameter end 189 can be 1 / 3-2 / 3 of the diameter of the circular tube-shaped part of the spray liquid inlet pipe 129. In operation, the spray liquid such as lye enters the spray liquid inlet pipe and is sprayed from the reduced diameter end, the foam enters from the foam inlet and is mixed and defoamed in the second cylinder segment and the expanded diameter segment, and then is discharged from the bottom outlet of the defoamer and enters the lye discharge buffer tank. Wherein, the reduced diameter means that the diameter decreases from top to bottom, and the expanded diameter means that the diameter increases from top to bottom; the length and the inclination angle (that is, the angle between the side wall and the axial direction) of the reduced diameter segment and the expanded diameter segment can be adjusted according to actual conditions, for example, the length of the reduced diameter segment can be 1 / 3-2 / 3 such as 1 / 2 of the first cylinder segment, the inclination angle thereof can be 30°-60° such as 45°, the length of the expanded diameter segment can be 2 / 3-4 / 3 such as 1 of the first cylinder segment, and the inclination angle thereof can be 10°-20° such as 15°. The defoamer adopted in the present application has simple structure and good lye defoaming effect.

[0046] The lye discharge buffer tank 107 is connected with the bottom outlet of the foam cyclone separator 108 so as to receive the separated lye; and the bottom of the lye discharge buffer tank 107 is connected with the tank area lye tank by a coal liquefaction device lye conveying pump 203, so that after the lye discharge is completed, the lye in the lye discharge buffer tank 107 can be sent back to the tank area lye tank 101.

[0047] In one embodiment, the top of the caustic discharge buffer tank 107 is respectively provided with a first caustic inlet and a second caustic inlet, wherein the first caustic inlet is connected to the bottom outlet 138 of the foam cyclone separator to receive the separated caustic; the foam inlet 119 of the defoamer 109 is connected to the top outlet 128 of the foam cyclone separator, the bottom outlet of the defoamer is connected to the second caustic inlet below, and the second caustic inlet extends into the caustic discharge buffer tank to send the material from the defoamer below the liquid surface for better defoaming; the bottom of the caustic discharge buffer tank is provided with a caustic outlet, and the spray liquid inlet pipe 129 of the defoamer 109 and the tank area caustic tank 101 are respectively connected by the coal liquefaction device caustic delivery pump 203 to deliver caustic to the defoamer and tank area caustic tank; and the top of the caustic discharge buffer tank is also provided with an exhaust pipe, and a nitrogen pipe can also be provided here to send nitrogen, thereby facilitating the control of the pressure in the caustic discharge buffer tank.

[0048] The method for coal direct liquefaction device shutdown and caustic discharge by using the above system comprises the following steps S1-S4:

[0049] S1) The caustic in the tank area is delivered to the coal liquefaction device caustic buffer tank by the tank area caustic delivery pump, and is sequentially delivered to the reaction heating furnace, the first reactor, the second reactor and the high temperature and high pressure separator for caustic washing by the coal liquefaction device caustic pressurizing pump;

[0050] Specifically, in step S1, the caustic in the tank area can be first delivered to the coal liquefaction device caustic buffer tank by the tank area caustic delivery pump; then the caustic in the coal liquefaction device caustic buffer tank is delivered to the reaction heating furnace for heating by the coal liquefaction device caustic pressurizing pump, and then is sent into the first reactor, the second reactor and the high temperature and high pressure separator for caustic washing. In some embodiments, the tank area caustic temperature is 60-70°C; the outlet pressure of the tank area caustic delivery pump is 1.0-1.5 MPa, which is more conducive to sending back to the tank area and defoaming.

[0051] S2) After the caustic washing is completed, the tank area caustic delivery pump and the coal liquefaction device caustic pressurizing pump are stopped to stop the caustic delivery;

[0052] Specifically, in step S2, after the caustic washing is completed, the delivery of caustic to the coal liquefaction device caustic buffer tank by the tank area caustic delivery pump is stopped; then after the caustic in the coal liquefaction device caustic buffer tank is completely output, the coal liquefaction device caustic pressurizing pump is stopped. In some embodiments, the coal liquefaction device caustic buffer tank temperature is 50-60°C; the outlet pressure of the coal liquefaction device caustic pressurizing pump is 2.0-2.5 MPa.

[0053] S3) The alkaline solution in the reaction heating furnace, the first reactor, the second reactor and the high temperature and high pressure separator is discharged to the alkaline solution discharge buffer tank after the foam is separated by the foam cyclone separator; at the same time, the foam separated from the foam cyclone separator is eliminated by the defoamer and the defoamed material is sent to the alkaline solution discharge buffer tank.

[0054] Specifically, in step S3, during alkali discharge, the alkali solution from the reaction furnace, the first reactor, the second reactor, and the high-temperature and high-pressure separator can be discharged into the alkali discharge buffer tank after foam separation by a foam cyclone separator; simultaneously, a defoamer is used to eliminate the foam separated from the foam cyclone separator, and the defoamed material is sent into the alkali discharge buffer tank. In some embodiments, the temperature of the alkali discharge buffer tank of the coal liquefaction unit is 50-60°C; the pressure of the alkali discharge buffer tank of the coal liquefaction unit is 0.1-0.15 MPa.

[0055] S4) After the alkaline solution is discharged, the alkaline solution in the alkaline solution discharge buffer tank is pressurized by the alkaline solution transfer pump of the coal liquefaction unit and then sent to the alkaline solution tank in the tank area for storage.

[0056] In some embodiments, the pressure after being pressurized by the alkaline solution transfer pump of the coal liquefaction unit is 1.0-1.5 MPa.

[0057] The present invention is further illustrated below with reference to the embodiments:

[0058] Example 1

[0059] like Figures 1-3 As shown, when preparing for alkaline washing, the relevant valves are first opened to transfer the alkaline solution in the alkaline solution tank of the tank area to the alkaline solution buffer tank 102 of the coal liquefaction unit via the alkaline solution transfer pump 201 (the alkaline solution temperature in the tank area is 60-70℃; the outlet pressure of the alkaline solution transfer pump in the tank area is 1.0-1.5MPa). Then, the alkaline solution in the alkaline solution buffer tank of the coal liquefaction unit is transferred to the reaction heating furnace (heating can be carried out if necessary to control the temperature) via the alkaline solution pressurization pump 204 of the coal liquefaction unit (the alkaline solution buffer tank temperature in the coal liquefaction unit is 50-60℃; the outlet pressure of the alkaline solution pressurization pump in the coal liquefaction unit is 2.0-2.5MPa). Then, it is sequentially sent to the first reactor, the second reactor and the high temperature and high pressure separator for alkaline washing.

[0060] When the caustic washing is completed, the valves 412, 413, 414, and 415 are opened to discharge the caustic solution (temperature: 50-60°C, pressure: 0.5-1.0 MPa) from the outlet of the reaction heater, the first reactor, the second reactor, and the high-temperature high-pressure separator to the foam cyclone separator 108 for foam separation. The separated caustic solution is buffered, de-aerated, and defoamed in the caustic solution discharge buffer tank 107. The foam and the caustic solution as a spray liquid are defoamed in the defoamer 109 and then enter the caustic solution discharge buffer tank 107 (temperature: 50-60°C, pressure: 0.1-0.15 MPa). A portion of the caustic solution in the caustic solution discharge buffer tank 107 is supplied to the defoamer 109 as a spray liquid via the coal liquefaction device caustic solution pump. The pressure of the caustic solution discharge buffer tank 107 is controlled by the caustic solution discharge buffer tank nitrogen control valve 305 and the caustic solution discharge buffer tank vent control valve 304.

[0061] When the caustic solution is returned, the relevant valves are adjusted to open and close. After being filtered by the coal liquefaction device caustic solution pump inlet filter 504, the caustic solution is pressurized by the coal liquefaction device caustic solution pump 203 (coal liquefaction device caustic solution pump outlet temperature: 50-60°C, pressure: 1.0-1.5 MPa). The flow rate is controlled by the coal liquefaction device caustic solution pump outlet flow control valve 303. The caustic solution is then introduced into the tank area caustic solution tank 101 through the opened caustic solution tank valve 401 for storage.

Claims

1. A system for shutdown and dealkalization of a coal direct liquefaction plant, the system comprising a tank farm caustic tank, a coal liquefaction plant caustic buffer tank, a reaction heating furnace, a first reactor, a second reactor, a high temperature and high pressure separator, a caustic discharge buffer tank, a foam cyclone separator and a defoamer; wherein, the tank farm caustic tank is connected to the coal liquefaction plant caustic buffer tank by a tank farm caustic pump to send tank farm caustic to the coal liquefaction plant caustic buffer tank; the coal liquefaction plant caustic buffer tank is connected to the reaction heating furnace by a coal liquefaction plant caustic pump, and the reaction heating furnace is connected in series to the first reactor, the second reactor and the high temperature and high pressure separator to send caustic from the coal liquefaction plant caustic buffer tank to the reaction heating furnace, the first reactor, the second reactor and the high temperature and high pressure separator in sequence for caustic washing; the foam cyclone separator is connected to the reaction heating furnace, the first reactor, the second reactor and the high temperature and high pressure separator respectively to receive caustic discharged from the reaction heating furnace, the first reactor, the second reactor and the high temperature and high pressure separator and to perform foam separation; the caustic discharge buffer tank is connected to a bottom outlet of the foam cyclone separator to receive the separated caustic; the defoamer is used to eliminate foam from a top outlet of the foam cyclone separator and to send the defoamed material back to the caustic discharge buffer tank; a bottom of the caustic discharge buffer tank is connected to the tank farm caustic tank by a coal liquefaction plant caustic pump to send caustic from the caustic discharge buffer tank back to the tank farm caustic tank.

2. The system according to claim 1, wherein, the defoamer comprises a foam inlet, a spray liquid inlet pipe and a housing, the housing is closed at an upper end and open at a lower end to serve as a bottom outlet of the defoamer, and the housing comprises, from top to bottom, a first cylindrical section, a reduced diameter section, a second cylindrical section and an expanded diameter section in sequence and in communication with each other, the foam inlet is provided on a side wall of the first cylindrical section, the spray liquid inlet pipe passes through the first cylindrical section from top to bottom and extends into the second cylindrical section, and a distal end of the spray liquid inlet pipe is a reduced diameter end.

3. The system of claim 2, wherein, the caustic discharge buffer tank is provided with a first caustic inlet and a second caustic inlet at a top portion, wherein the first caustic inlet is connected to a bottom outlet of the foam cyclone separator to receive the separated caustic; the foam inlet of the defoamer is connected to a top outlet of the foam cyclone separator, the bottom outlet of the defoamer is connected to the second caustic inlet below, and the second caustic inlet extends into the caustic discharge buffer tank to send the material from the defoamer below the liquid surface; the bottom of the caustic discharge buffer tank is provided with a caustic outlet, and is connected to the spray liquid inlet pipe of the defoamer and the tank farm caustic tank by a coal liquefaction plant caustic pump to send caustic to the defoamer and the tank farm caustic tank; and the top of the caustic discharge buffer tank is further provided with an exhaust pipe.

4. A method for coal direct liquefaction plant shutdown and caustic liquid withdrawal using the system according to any one of claims 1-3, the method comprising the following steps: S1) delivering caustic liquid in the tank area to a coal liquefaction plant caustic liquid buffer tank via a tank area caustic liquid delivery pump, and sequentially delivering the caustic liquid to a reaction heating furnace, a first reactor, a second reactor, and a high-temperature high-pressure separator via a coal liquefaction plant caustic liquid pressurizing pump for caustic washing; S2) stopping the tank area caustic liquid delivery pump and the coal liquefaction plant caustic liquid pressurizing pump to stop caustic liquid delivery after the caustic washing is completed; S3) discharging caustic liquid in the reaction heating furnace, the first reactor, the second reactor, and the high-temperature high-pressure separator to a caustic liquid discharge buffer tank after the caustic liquid is separated from foam by a foam cyclone separator, and simultaneously eliminating foam separated from the foam cyclone separator by a defoaming device and feeding the defoamed material into the caustic liquid discharge buffer tank; S4) pressurizing caustic liquid in the caustic liquid discharge buffer tank via the coal liquefaction plant caustic liquid delivery pump and then externally delivering the caustic liquid to a caustic liquid tank in the tank area for storage after the caustic liquid discharge is completed.

5. The method of claim 4, wherein, In step S1, caustic liquid in the tank area is first delivered to a coal liquefaction plant caustic liquid buffer tank via a tank area caustic liquid delivery pump, and then caustic liquid in the coal liquefaction plant caustic liquid buffer tank is delivered to a reaction heating furnace via a coal liquefaction plant caustic liquid pressurizing pump for heating, and then fed into a first reactor, a second reactor, and a high-temperature high-pressure separator for caustic washing.

6. The method of claim 5, wherein, The temperature of the caustic liquid in the tank area is 60-70℃, and the outlet pressure of the tank area caustic liquid delivery pump is 1.0-1.5MPa.

7. The method according to claim 5 or 6, characterized in that, In step S2, after the caustic washing is completed, caustic liquid is stopped from being delivered to the coal liquefaction plant caustic liquid buffer tank via the tank area caustic liquid delivery pump, and then the coal liquefaction plant caustic liquid pressurizing pump is stopped after the caustic liquid in the coal liquefaction plant caustic liquid buffer tank is completely output.

8. The method of claim 7, wherein, The temperature of the coal liquefaction plant caustic liquid buffer tank is 50-60℃, and the outlet pressure of the coal liquefaction plant caustic liquid pressurizing pump is 2.0-2.5MPa.

9. The method of claim 7, wherein, In step S3, caustic liquid in the reaction heating furnace, the first reactor, the second reactor, and the high-temperature high-pressure separator is discharged to a caustic liquid discharge buffer tank after the caustic liquid is separated from foam by a foam cyclone separator, and simultaneously, foam separated from the foam cyclone separator is eliminated by a defoaming device and the defoamed material is fed into the caustic liquid discharge buffer tank.

10. The method of claim 8, wherein, In step S3, caustic liquid in the reaction heating furnace, the first reactor, the second reactor, and the high-temperature high-pressure separator is discharged to a caustic liquid discharge buffer tank after the caustic liquid is separated from foam by a foam cyclone separator, and simultaneously, foam separated from the foam cyclone separator is eliminated by a defoaming device and the defoamed material is fed into the caustic liquid discharge buffer tank.

11. The method according to claim 9 or 10, characterized in that, The temperature of the caustic liquid discharge buffer tank is 50-60℃, and the pressure of the caustic liquid discharge buffer tank is 0.1-0.15MPa. The pressure of the caustic liquid after being pressurized by the coal liquefaction plant caustic liquid delivery pump is 1.0-1.5MPa.

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