Novel BOG replacement zero-emission system for storage tank of LNG receiving station and implementation method

Abstract

The invention relates to a novel LNG receiving station storage tank BOG replacement zero-emission system and an implementation method, and on the basis of the design of an original LNG receiving station, the arrangement of equipment and facilities of the original LNG receiving station is not changed, and BOG replacement is carried out by adopting the operation scheme of the method. And the natural gas in the high-pressure output pipeline is returned to a BOG pipeline of the LNG storage tank through a throttling expansion device on the high-pressure gas supply pipeline, and the natural gas subjected to throttling expansion pressure reduction is introduced into the LNG storage tank from the tank top. Gas in the storage tank passes through the pump cylinder of the low-pressure pump from the bottom of the storage tank to the emptying valve on the pump cylinder connecting pipeline and is emptied outwards. Meanwhile, low-temperature BOG gas is introduced into a discharging pipeline and enters the storage tank through an upper liquid inlet and a spraying pipeline through the discharging pipeline, and the flow is cut off after precooling of the discharging pipeline of the storage tank is completed; the replacement process is simple and convenient, easy to operate and obvious in energy-saving effect.

Description

Technical Field

[0001] This invention relates to the field of liquefied natural gas storage and replacement technology, specifically to a novel BOG replacement zero-emission system for LNG receiving terminal storage tanks and its implementation method. Background Technology

[0002] Liquefied natural gas (LNG) is currently recognized worldwide as one of the cleanest petrochemical energy sources. LNG receiving terminals are an important link in the LNG industry chain, and in recent years, efforts have been made to promote the construction of LNG receiving terminals and the application of LNG.

[0003] LNG storage tanks are the core equipment of LNG receiving terminals, serving as crucial facilities for storing LNG transported from various locations and ensuring its safe and stable supply. Due to their high construction costs, after completion, the tanks require trial production and commissioning by a professional team before being put into operation at the receiving terminal. Trial production of LNG storage tanks is a vital step in ensuring their safe, efficient, and stable operation. The trial production process for LNG storage tanks at LNG receiving terminals typically includes nitrogen purging and replacement, BOG replacement, tank spray pre-cooling, and liquid filling. Currently, BOG replacement at most receiving terminals involves injecting vaporized LNG or cryogenic BOG from the BOG manifold into the storage tank, followed by venting or flaring. This BOG replacement process generates a large amount of mixed nitrogen and methane gas, which cannot be recovered, resulting in significant methane waste.

[0004] Therefore, there is an urgent need for a more efficient BOG replacement system and method that can achieve energy saving and consumption reduction in LNG receiving terminals without changing the original receiving terminal process. Summary of the Invention

[0005] To address the aforementioned problems, the purpose of this invention is to provide a novel BOG replacement zero-emission system and implementation method for LNG receiving terminal storage tanks. The replacement process is simple, easy to operate, and has significant energy-saving effects.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: The novel LNG receiving terminal storage tank BOG replacement zero-emission system of the present invention includes: a storage tank, a main unloading pipe, an upper liquid inlet pipe, a spray pipe, a lower liquid inlet pipe, and a cryogenic BOG inlet pipe; One end of the unloading main pipe is connected to the material source, and the other end is connected to the first end of the upper liquid inlet pipe. The second end of the upper liquid inlet pipe extends into the upper layer of the storage tank. The upper liquid inlet pipe is used to supply liquid to the upper layer of the storage tank. The upper liquid inlet pipe is equipped with a first electric regulating valve and a first manual regulating valve, which are connected in parallel. One end of the spray pipe is connected to the upper liquid inlet pipe, and the other end is connected to the spray pipe installed in the storage tank. The spray pipe is equipped with a first manual switch valve and a second manual regulating valve. One end of the lower inlet pipe is connected to the upper inlet pipe, and the other end extends into the lower part of the storage tank for lower inlet liquid filling of the storage tank. The lower inlet pipe is equipped with a second electric regulating valve and a third manual regulating valve. The cryogenic BOG inlet pipe is connected to the unloading main pipe, and a second manual switch valve is installed on the cryogenic BOG inlet pipe.

[0007] The novel BOG replacement zero-emission system for LNG receiving terminal storage tanks preferably further includes a low-pressure pump cylinder outlet vent pipe; a low-pressure pump cylinder is installed inside the storage tank, with the bottom of the low-pressure pump cylinder serving as the bottom inlet and the top as the top outlet; the low-pressure pump cylinder outlet vent pipe is connected to the low-pressure pump cylinder, and a third manual switch valve is installed on the low-pressure pump cylinder outlet vent pipe.

[0008] The novel LNG receiving terminal tank BOG replacement zero-emission system preferably further includes a high-pressure gas replenishment pipeline, a tank BOG pipeline, and a BOG main pipeline; one end of the high-pressure gas replenishment pipeline is connected to a high-pressure gas source, and the other end is connected to the tank BOG pipeline and the BOG main pipeline respectively, with the tank BOG pipeline extending into the upper part of the tank; the high-pressure gas replenishment pipeline is sequentially equipped with a first manual ball valve, a second manual ball valve, a throttling expansion unit, and a first electric shut-off valve from upstream to downstream; the BOG main pipeline is equipped with a second electric shut-off valve.

[0009] In the novel LNG receiving terminal storage tank BOG replacement zero-emission system, preferably, the throttling expansion unit includes a PCV regulating valve and a reducer, which are connected in series on the high-pressure gas replenishment pipeline.

[0010] This invention also provides a method for implementing a novel BOG replacement zero-emission system for LNG receiving terminal storage tanks, comprising the following steps: The BOG replacement operation of the storage tank can be carried out simultaneously with the BOG precooling of the unloading riser. Connect the low-temperature BOG inlet pipe to the unloading main pipe, open the second manual switch valve and control the flow rate, open the first manual switch valve, slowly open the second manual regulating valve, and slowly open the first manual regulating valve on the bypass of the upper liquid inlet pipe to introduce the low-temperature BOG gas into the unloading main pipe. The gas enters the storage tank from the upper space through the spray pipe and the upper liquid inlet pipe. During this period, control the storage tank pressure to avoid fluctuations. Open the third manual switch valve on the low-pressure pump cylinder outlet vent pipe and release the gas in the tank into the atmosphere from the bottom of the storage tank through the low-pressure pump cylinder outlet vent pipe. Open the first manual ball valve, the second manual ball valve, and the first electric shut-off valve on the high-pressure gas supply pipeline. Adjust the valve opening of the PCV regulating valve in the throttling expansion unit. Close the second electric shut-off valve on the BOG main pipe. Introduce the ambient temperature methane gas in the high-pressure external pipeline into the storage tank through the storage tank BOG pipeline and the top of the storage tank. After the unloading main pipe and spray pipe cool down to the set temperature, the pre-cooling of the pipes is completed. Then, close the second manual switch valve and the first manual switch valve, and keep the first manual regulating valve open to balance the pipe pressure. Continue introducing ambient temperature methane gas into the storage tank through the high-pressure gas supply pipeline until the methane content at the vent outlet of the low-pressure pump cylinder reaches the set value. Then, close the first manual ball valve, the second manual ball valve, and the first electric shut-off valve on the high-pressure gas supply pipeline, close the PCV regulating valve in the throttling expansion unit, open the second electric shut-off valve on the BOG main pipe, and connect the pipeline of the trial production storage tank with the original BOG processing system of the receiving station to complete the BOG replacement operation of the storage tank.

[0011] The present invention has the following advantages due to the adoption of the above technical solutions: (1) This invention does not change the conventional LNG receiving terminal equipment, but only changes the operating steps to achieve zero BOG emissions at the receiving terminal; (2) In this invention, methane gas is introduced into the storage tank through a high-pressure gas supply pipeline. After the high-pressure outgoing gas undergoes throttling and expansion, the gas temperature decreases. It is calculated that the gas temperature entering the storage tank after throttling and expansion from a 20°C outgoing gas is approximately -20°C, at which point the gas density is 0.7 kg / m³. 3 Previous operating procedures introduced low-temperature BOG gas, with a gas temperature of approximately -120 to 140°C and a gas density of 1.4 to 1.5 kg / m³. 3 The invention produces a gas with low density, forming a dense, cap-shaped gas layer that is uniformly pushed downwards, creating a good piston effect and ensuring maximum nitrogen discharge from the storage tank, thus reducing methane waste. (3) This invention does not affect the pre-cooling of the unloading pipeline while performing BOG replacement in the storage tank, thus accelerating the commissioning process of the storage tank. 4. This invention accelerates the BOG replacement process, achieves zero BOG emissions, effectively reduces resource utilization efficiency, saves energy, and is easy to implement and highly scalable. Attached Figure Description

[0012] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. In the drawings: Figure 1 This is a schematic diagram of the novel BOG replacement zero-emission system for LNG receiving station storage tanks according to the present invention; Figure 2 This is a schematic diagram of gas flow according to the present invention.

[0013] The labels for the attached figures are as follows: 1-High-pressure gas replenishment pipeline; 2-Storage tank BOG pipeline; 3-Low-pressure pump cylinder outlet vent pipeline; 4-Unloading main pipe; 5-Upper liquid inlet pipeline; 6-Spray pipeline; 7-BOG main pipe; 8-Lower liquid inlet pipeline; 9-Throttle expansion unit; 10-Low-pressure pump cylinder bottom inlet; 11-Cryogenic BOG inlet pipeline; 101-First manual ball valve; 102-Second manual ball valve; 103-First electric shut-off valve; 104-Second electric shut-off valve; 105-First manual on / off valve; 106-Second manual regulating valve; 107-Second electric regulating valve; 108-Third manual regulating valve; 109-First electric regulating valve; 110-First manual regulating valve; 111-Third manual on / off valve; 112-Second manual on / off valve. Detailed Implementation

[0014] Exemplary embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0015] This invention provides a novel zero-emission BOG replacement system and implementation method for LNG receiving terminal storage tanks. Based on the existing LNG receiving terminal design, without altering the original equipment and facility layout, this method is used for BOG replacement. Natural gas from the high-pressure export pipeline is returned to the BOG pipeline of the LNG storage tank via a throttling expansion device on the high-pressure replenishment pipeline. The throttled and depressurized natural gas is then introduced into the LNG storage tank from the top. Gas inside the storage tank is vented from the bottom of the tank through a low-pressure pump cylinder to a vent valve on the pump cylinder connection pipeline. Simultaneously, cryogenic BOG gas is introduced into the unloading pipeline, and then enters the storage tank through the unloading pipeline, the upper liquid inlet, and the spray pipeline. After pre-cooling the unloading pipeline, this process is shut off. The replacement process is simple, easy to operate, and has significant energy-saving effects.

[0016] like Figure 1 As shown, the novel LNG receiving terminal storage tank BOG replacement zero-emission system provided by the present invention includes: a storage tank, a discharge main pipe 4, an upper liquid inlet pipe 5, a spray pipe 6, a lower liquid inlet pipe 8, and a cryogenic BOG inlet pipe 11. One end of the unloading main pipe 4 is connected to the material source, and the other end is connected to the first end of the upper liquid inlet pipe 5. The second end of the upper liquid inlet pipe 5 extends into the upper layer of the storage tank. The upper liquid inlet pipe 5 is used to supply liquid to the upper layer of the storage tank. The upper liquid inlet pipe 5 is equipped with a first electric regulating valve 109 and a first manual regulating valve 110, which are connected in parallel. One end of the spray pipe 6 is connected to the upper liquid inlet pipe 5, and the other end is connected to a spray pipe installed in the storage tank. The spray pipe 6 is equipped with a first manual switch valve 105 and a second manual regulating valve 106. One end of the lower liquid inlet pipe 8 is connected to the upper liquid inlet pipe 5, and the other end extends into the lower layer of the storage tank. It is used to supply liquid to the lower layer of the storage tank. The lower liquid inlet pipe 8 is equipped with a second electric regulating valve 107 and a third manual regulating valve 108. The cryogenic BOG inlet pipe 11 is connected to the unloading main pipe 4, and the cryogenic BOG inlet pipe 11 is equipped with a second manual switch valve 112.

[0017] In the above embodiments, preferably, the present invention further includes a low-pressure pump cylinder outlet vent pipe 3; a low-pressure pump cylinder is provided inside the storage tank, the bottom of the low-pressure pump cylinder is the low-pressure pump cylinder bottom inlet 10, and its top is the low-pressure pump cylinder top outlet; the low-pressure pump cylinder outlet vent pipe 3 is connected to the low-pressure pump cylinder, and a third manual switch valve 111 is provided on the low-pressure pump cylinder outlet vent pipe 3.

[0018] In the above embodiments, preferably, the present invention further includes a high-pressure gas supply pipeline 1, a storage tank BOG pipeline 2, and a BOG main pipe 7; one end of the high-pressure gas supply pipeline 1 is connected to a high-pressure gas source, and the other end is connected to the storage tank BOG pipeline 2 and the BOG main pipe 7 respectively, with the storage tank BOG pipeline 2 extending into the upper part of the storage tank; a first manual ball valve 101, a second manual ball valve 102, a throttling expansion unit 9, and a first electric shut-off valve 103 are sequentially arranged on the high-pressure gas supply pipeline 1 from upstream to downstream; a second electric shut-off valve 104 is arranged on the BOG main pipe 7.

[0019] In the above embodiments, preferably, the throttling expansion unit 9 includes a PCV regulating valve and a reducer, which are connected in series on the high-pressure gas supply line 1.

[0020] This invention also provides a method for implementing a novel BOG replacement zero-emission system for LNG receiving terminal storage tanks, comprising the following steps: (1) The BOG replacement operation of the storage tank can be carried out simultaneously with the BOG precooling of the unloading riser. The low-temperature BOG inlet pipe 11 is connected to the unloading main pipe 4. The second manual switch valve 112 is opened and the flow rate is controlled. The first manual switch valve 105 is opened, the second manual regulating valve 106 is opened slowly, and the first manual regulating valve 110 of the bypass on the upper liquid inlet pipe 5 is opened slowly. The low-temperature BOG gas is introduced into the unloading main pipe and enters the storage tank from the upper space of the storage tank through the spray pipe 6 and the upper liquid inlet pipe 5. During this period, the pressure of the storage tank is controlled to avoid fluctuations. The third manual switch valve 111 on the low-pressure pump cylinder outlet vent pipe 3 is opened, and the gas in the tank is released into the atmosphere from the bottom of the storage tank through the low-pressure pump cylinder outlet vent pipe. (2) Open the first manual ball valve 101, the second manual ball valve 102 and the first electric shut-off valve 103 on the high pressure gas supply line 1, adjust the valve opening of the PCV regulating valve in the throttling expansion unit 9, close the second electric shut-off valve 104 on the BOG main pipe 7, and introduce the room temperature methane gas in the high pressure external transmission pipeline through the storage tank BOG pipeline 2 and through the top of the storage tank into the storage tank; (3) After the unloading main pipe and spray pipe are cooled to the set temperature (-120℃), the pipe precooling is completed. Then, close the second manual switch valve 112 and the first manual switch valve 105, and keep the first manual regulating valve 110 open to balance the pipe pressure. (4) Continue to introduce room temperature methane gas into the storage tank through the high pressure gas supply pipeline 1 until the methane content at the vent of the low pressure pump cylinder outlet vent pipeline 3 reaches the set value (5%). Then close the first manual ball valve 101, the second manual ball valve 102 and the first electric shut-off valve 103 on the high pressure gas supply pipeline, close the PCV regulating valve in the throttling expansion unit 9, open the second electric shut-off valve 104 on the BOG main pipe 7, connect the trial production storage tank and the original BOG processing system pipeline of the receiving station, and complete the BOG replacement operation of the storage tank.

[0021] The overall gas flow diagram for BOG replacement using this method is shown below. Figure 2 As shown.

[0022] The low-temperature BOG gas introduced through the low-temperature BOG introduction pipeline 11 gradually heats up during the heat exchange process of precooling the unloading main pipe 4 and the spray pipe 6, and enters the storage tank. After the pipeline precooling is completed, a small amount of low-temperature BOG gas enters the storage tank from the top and slowly diffuses. After this operation is completed, the ambient temperature natural gas that enters the storage tank through the high-pressure gas replenishment pipeline 1 diffuses in the upper space of the storage tank due to its low density, forming a dense cap-shaped natural gas layer (1). During the continuous injection of gas, it is pushed downward. The nitrogen layer (2) at the bottom of the storage tank is vented out through the low-pressure pump cylinder vent port located at the bottom of the storage tank into the low-pressure pump cylinder outlet vent pipe 3. At this time, the vented gas is all nitrogen.

[0023] Example 1: An LNG receiving terminal is an expansion project with four operational storage tanks and two newly built tanks, all of which are scheduled for trial production. Because the terminal's BOG (Break-off Gas) system is interconnected, the newly built tanks lack a separate flare connection, making individual flare venting of the new tanks impossible. Furthermore, it is crucial to ensure that the nitrogen content in the pipelines during downstream gas export does not exceed 5%. Therefore, this method is required for zero-emission BOG replacement.

[0024] During the precooling process of the unloading main pipe 4 and the spray pipe 6, the valve opening of the second manual switch valve 112 on the low-temperature BOG inlet pipe 11 under the tank is adjusted, the valve opening of the third manual switch valve 111 on the low-pressure pump outlet vent pipe is adjusted, and the speed of the low-temperature BOG gas entering the unloading main pipe 4 and the gas flow rate at the vent outlet are adjusted to control the precooling speed of the pipe. The actual cooling rate is about 2℃ / h. During this process, the tank pressure is maintained at about 3kPaG, the tank temperature does not change significantly, and the methane content at the vent outlet is monitored to be 0%.

[0025] After the pipeline precooling is completed, the second manual switch valve 112 on the cryogenic BOG inlet pipeline 11 is closed, and BOG replacement is carried out only through the high-pressure gas replenishment pipeline 1. At the same time, the tank pressure is strictly controlled during the replacement process. Each tank is equipped with 3 low-pressure pumps, and each low-pressure pump has two vent ports: one for removing the RS short section at the nitrogen injection port of the low-pressure pump barrel and the other for removing the RS short section on the nitrogen purging pipeline of the low-pressure pump outlet. A total of 6 2” vent ports are provided for each tank, which increases the venting effect and accelerates the BOG replacement process.

[0026] During BOG replacement using high-pressure gas supply line 1, the tank pressure was maintained at 10-12 kPaG by adjusting the opening of the PCV regulating valve on line 1. Both tanks showed similar methane content trends at the vent outlets during BOG replacement. In the first 40 hours of replacement, the methane content at the vent outlets was low, around 0.01%. From the 40th to the 50th hour of replacement, the methane content at the vent outlets showed a sudden increase, indicating that the methane in the tanks was concentrated and the BOG replacement piston effect was well achieved. The local vent valves were closed, and the newly commissioned tank was connected to the existing tank's BOG system. The nitrogen concentration in the exported mixed gas was measured and found to be below 5%, meeting the export requirements. Throughout the entire process, no flare venting was used, resulting in a good replacement effect and achieving zero-emission BOG replacement.

[0027] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A novel BOG (Body Gas) replacement zero-emission system for LNG receiving terminal storage tanks, characterized in that, include: Storage tank, unloading main pipe, upper liquid inlet pipe, spray pipe, lower liquid inlet pipe, and cryogenic BOG inlet pipe; One end of the unloading main pipe is connected to the material source, and the other end is connected to the first end of the upper liquid inlet pipe. The second end of the upper liquid inlet pipe extends into the upper layer of the storage tank. The upper liquid inlet pipe is used to supply liquid to the upper layer of the storage tank. The upper liquid inlet pipe is equipped with a first electric regulating valve and a first manual regulating valve, which are connected in parallel. One end of the spray pipe is connected to the upper liquid inlet pipe, and the other end is connected to the spray pipe installed in the storage tank. The spray pipe is equipped with a first manual switch valve and a second manual regulating valve. One end of the lower inlet pipe is connected to the upper inlet pipe, and the other end extends into the lower part of the storage tank for lower inlet liquid filling of the storage tank. The lower inlet pipe is equipped with a second electric regulating valve and a third manual regulating valve. The cryogenic BOG inlet pipe is connected to the unloading main pipe, and a second manual switch valve is installed on the cryogenic BOG inlet pipe.

2. The novel LNG receiving terminal storage tank BOG replacement zero-emission system according to claim 1, characterized in that, It also includes the vent pipe at the outlet of the low-pressure pump cylinder; The storage tank is equipped with a low-pressure pump cylinder, with the bottom of the low-pressure pump cylinder being the bottom inlet and the top being the top outlet. The vent pipe at the outlet of the low-pressure pump barrel is connected to the low-pressure pump barrel, and a third manual switch valve is provided on the vent pipe at the outlet of the low-pressure pump barrel.

3. The novel LNG receiving terminal storage tank BOG replacement zero-emission system according to claim 2, characterized in that, It also includes high-pressure gas supply pipelines, storage tank BOG pipelines, and BOG main pipes; One end of the high-pressure gas supply pipeline is connected to the high-pressure gas source, and the other end is connected to the storage tank BOG pipeline and the BOG main pipe respectively. The storage tank BOG pipeline extends into the upper part of the storage tank. The high-pressure gas supply pipeline is equipped with a first manual ball valve, a second manual ball valve, a throttling expansion unit, and a first electric shut-off valve in sequence from upstream to downstream. A second electrically operated shut-off valve is installed on the BOG main pipe.

4. The novel LNG receiving terminal storage tank BOG replacement zero-emission system according to claim 3, characterized in that, The throttling expansion unit includes a PCV regulating valve and a reducer, which are connected in series on the high-pressure gas supply line.

5. A method for implementing a novel LNG receiving terminal storage tank BOG replacement zero-emission system based on any one of claims 1 to 4, characterized in that, Includes the following steps: The BOG replacement operation of the storage tank can be carried out simultaneously with the BOG precooling of the unloading riser. Connect the low-temperature BOG inlet pipe to the unloading main pipe, open the second manual switch valve and control the flow rate, open the first manual switch valve, slowly open the second manual regulating valve, and slowly open the first manual regulating valve on the bypass of the upper liquid inlet pipe to introduce the low-temperature BOG gas into the unloading main pipe. The gas enters the storage tank from the upper space through the spray pipe and the upper liquid inlet pipe. During this period, control the storage tank pressure to avoid fluctuations. Open the third manual switch valve on the low-pressure pump cylinder outlet vent pipe and release the gas in the tank into the atmosphere from the bottom of the storage tank through the low-pressure pump cylinder outlet vent pipe. Open the first manual ball valve, the second manual ball valve, and the first electric shut-off valve on the high-pressure gas supply pipeline. Adjust the valve opening of the PCV regulating valve in the throttling expansion unit. Close the second electric shut-off valve on the BOG main pipe. Introduce the ambient temperature methane gas in the high-pressure external pipeline into the storage tank through the storage tank BOG pipeline and the top of the storage tank. After the unloading main pipe and spray pipe cool down to the set temperature, the pre-cooling of the pipes is completed. Then, close the second manual switch valve and the first manual switch valve, and keep the first manual regulating valve open to balance the pipe pressure. Continue introducing ambient temperature methane gas into the storage tank through the high-pressure gas supply pipeline until the methane content at the vent outlet of the low-pressure pump cylinder reaches the set value. Then, close the first manual ball valve, the second manual ball valve, and the first electric shut-off valve on the high-pressure gas supply pipeline, close the PCV regulating valve in the throttling expansion unit, open the second electric shut-off valve on the BOG main pipe, and connect the pipeline of the trial production storage tank with the original BOG processing system of the receiving station to complete the BOG replacement operation of the storage tank.

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