阅读说明：本技术 一种天然气液化脱氮系统及工艺 (Natural gas liquefaction denitrification system and process ) 是由 俞徐林 陶敏峰 涂金华 王房杰 周威 谢成杰 何宏杨 黄一飞 梁艳 于 2021-10-22 设计创作，主要内容包括：本发明公开了一种天然气液化脱氮系统,包括干气管路、冷箱、低温分离器、脱氮气塔、增压压缩机和回流罐；干气管路通过冷箱后与低温分离器的原料气入口连通,低温分离器顶部的气相出口与脱氮气塔的上部和脱氮气塔的下部连通；脱氮气塔塔顶的气相出口与与增压压缩机的入口连通,闪蒸汽管路通过冷箱后与增压压缩机的入口连通,增压压缩机的出口与节流管路相连,节流管路经过冷箱后与回流罐连通；回流罐的气相出口与尾气排放管路连通。本发明的有益效果为：配置增压压缩机,脱氮气塔塔顶的气相与来自LNG储罐的闪蒸汽混合后增压节流,通过控制进入脱氮气塔塔顶的气相温度使脱氮气塔塔底的LNG产品合格,且尾气中CH-(4)损失少；本发明所述技术方案流程更加简单,能耗低,系统配置少,投资成本低。(The invention discloses a natural gas liquefaction denitrification system, which comprises a dry gas pipeline, a cold box, a low-temperature separator, a denitrification gas tower, a booster compressor and a reflux tank, wherein the dry gas pipeline is connected with the cold box; a dry gas pipeline is communicated with a raw gas inlet of the low-temperature separator after passing through the cold box, and a gas phase outlet at the top of the low-temperature separator is communicated with the upper part of the denitrogenation gas tower and the lower part of the denitrogenation gas tower; a gas phase outlet at the top of the denitrification gas tower is communicated with an inlet of a booster compressor, a flash steam pipeline is communicated with the inlet of the booster compressor after passing through a cold box, an outlet of the booster compressor is connected with a throttling pipeline, and the throttling pipeline is communicated with a reflux tank after passing through the cold box; the gas phase outlet of the reflux tank is communicated with a tail gas discharge pipeline. The invention has the beneficial effects that: a booster compressor is configured, the gas phase at the top of the denitrogenation gas tower is mixed with flash steam from an LNG storage tank for pressurization and throttling, the LNG product at the bottom of the denitrogenation gas tower is qualified by controlling the temperature of the gas phase entering the top of the denitrogenation gas tower, and CH in tail gas 4 The loss is less; the technical scheme of the invention has simpler process, low energy consumption, less system configuration and low investment cost.)

1. A natural gas liquefaction denitrification system is characterized by comprising a dry gas pipeline, a cold box, a low-temperature separator, a denitrification gas tower, a booster compressor and a reflux tank; the dry gas pipeline is communicated with a raw gas inlet of the low-temperature separator after passing through the cold box, a gas phase outlet at the top of the low-temperature separator is communicated with a first gas phase branch and a second gas phase branch, and the first gas phase branch is communicated with the upper part of the denitrification gas tower after passing through the cold box; the second gas phase branch is communicated with the lower part of the denitrification gas tower; a gas phase outlet at the top of the denitrification gas tower is connected with a gas phase pipeline at the top of the tower, the gas phase pipeline at the top of the tower is communicated with an inlet of a nitrogen/methane booster compressor after passing through a cold box, a flash steam pipeline is communicated with an inlet of the booster compressor after passing through the cold box, an outlet of the booster compressor is connected with a throttling pipeline, and the throttling pipeline is communicated with a reflux tank after passing through the cold box; the gas phase outlet of the reflux tank is communicated with a tail gas discharge pipeline; the liquid phase export of backward flow jar passes through the upper portion intercommunication of pipeline and denitrogenation gas tower, and the liquid phase export at the bottom of the denitrogenation gas tower links to each other with the LNG pipeline, and the LNG pipeline communicates with the LNG storage tank of LNG liquefaction mill behind the cold box.

2. The natural gas liquid nitrogen removal system of claim 1, wherein the liquid phase outlet at the bottom of the cryogenic separator is connected with a heavy hydrocarbon pipeline, and the heavy hydrocarbon pipeline is communicated with the heavy hydrocarbon storage tank after passing through the cold box.

3. The natural gas liquid nitrogen removal system of claim 1, wherein the exhaust gas discharge line is in communication with the atmosphere after passing through the cold box.

4. The natural gas liquid nitrogen removal system according to claim 1, wherein a thermometer is provided at each of the positions of the inlet refrigerant tank and the outlet refrigerant tank.

5. The natural gas liquid nitrogen removal system of claim 1, wherein a reflux pump is further added to the nitrogen removal system, an inlet of the reflux pump is communicated with a liquid phase outlet of the reflux tank through a pipeline, and an outlet of the reflux pump is communicated with the upper part of the nitrogen removal gas tower through a reflux pipeline.

6. The system for liquefied nitrogen removal of natural gas as claimed in claim 1, wherein a safety line is provided at the top of the nitrogen removal column, and a safety valve is provided on the safety line.

7. A natural gas denitrification process based on the denitrification system as claimed in any one of claims 1 to 6, characterized in that the process comprises the following steps:

firstly, carrying out upstream deep decarburization and dehydration process treatment on dry gas, and then condensing the dry gas through a cold box;

step two, the condensed dry gas enters a low-temperature separator for gas-liquid separation, the gas phase in the low-temperature separator is divided into two strands, one is 90% of the gas phase flowing into a first gas phase pipeline, the gas phase is condensed into LNG through a cold box, and the LNG throttles to enter the upper part of a nitrogen removal tower; the other stream is 10% gas phase flowing into the path of the second gas phase, directly throttled to enter the lower part of the nitrogen removal tower to be used as a heat source, and rectified and separated in the nitrogen removal tower;

thirdly, the liquid phase in the low-temperature separator is condensed heavy hydrocarbon, and the heavy hydrocarbon passes through a cold box through a heavy hydrocarbon pipeline and enters a heavy hydrocarbon storage tank of an LNG liquefaction plant after being reheated;

step four, respectively mixing the gas phase at the top of the denitrogenation gas tower and flash steam from an LNG storage tank after passing through a cold box, and pressurizing the nitrogen/methane mixed gas in a booster compressor;

step five, cooling the pressurized nitrogen/methane mixed gas through a cold box, throttling the cooled nitrogen/methane mixed gas, then feeding the throttled nitrogen/methane mixed gas into a reflux tank at the top of a nitrogen removal tower, feeding a liquid phase in the reflux tank into the nitrogen removal tower for washing, reheating a gas phase in the reflux tank to obtain nitrogen-containing tail gas, and then emptying the nitrogen-containing tail gas through the cold box;

and step six, the liquid phase at the bottom of the denitrification gas tower is the product LNG, and the LNG enters an LNG storage tank of an LNG liquefaction plant after being cooled and throttled.

8. The denitrification process for natural gas of claim 7, wherein in step one, the dry gas is condensed to-55 ℃ by a cold box.

9. The denitrification process for natural gas according to claim 7, wherein in the fourth step, the nitrogen/methane mixture enters a booster compressor and then is pressurized to 1.1-1.3 MPa.

10. The denitrification process for natural gas according to claim 7, wherein in the fifth step, the temperature of the nitrogen/methane mixture after throttling is-184 ℃ to-180 ℃.

Technical Field

The invention relates to a denitrification technology, in particular to a natural gas liquefied denitrification system and a natural gas liquefied denitrification process.

Background

LNG (liquefied natural gas) is a clean and efficient energy source, and it must play an important role as a transition energy source under the national 'double-carbon' goal. The nitrogen content of the block is too high, and if the nitrogen content is not removed, the nitrogen content of the LNG product can exceed the standard. The liquefied natural gas is a liquid obtained by compressing and cooling natural gas to the boiling temperature of the natural gas, and is stored in a low-temperature storage tank. The main component of the raw gas of the liquefied natural gas is methane and also comprises nitrogen, the content of the nitrogen in the raw gas exploited by part of gas fields is too high, and if the nitrogen is not effectively removed, the LNG product is unqualified.

At present, the nitrogen content is not particularly high (N)₂Less than or equal to 5 percent) of raw material gas can be treated by the following method: performing a flash evaporation in the LNG storage tank, and discharging the flash evaporation gas (BOG) from the LNG storage tank to the system, thereby reducing the nitrogen content in the LNG product and the N in the LNG product₂Less than or equal to 1 percent. The treatment method requires that the nitrogen content in the feed gas is not too high, and the nitrogen content in the feed gas is preferably not too highOver 5% (as can be determined by process simulation). However, when the nitrogen content in the feed gas is higher and exceeds 5%, the BOG flash evaporation amount is larger along with the increase of the nitrogen content, and the N in the LNG product cannot be ensured₂Less than or equal to 1 percent, and the quality of the obtained LNG product is difficult to ensure.

In the prior art, the following two processes are mostly adopted for the natural gas raw material gas with the nitrogen content of more than 5 percent:

1. a heating, rectifying and denitrifying process for the bottom of a denitrifying gas tower is provided with a rectifying tower, a reboiler is arranged at the bottom of the tower, a heat source is heated by adopting a low-temperature separator at the temperature of minus 45 to minus 50 ℃, simultaneously, the gas phase of the low-temperature separator is divided into two strands, one 90% of the gas phase enters a cold box to be condensed into LNG, and the LNG is throttled to enter the middle upper part of a denitrifying gas tower; one 10 percent of the mixture is directly throttled and enters the middle lower part of the nitrogen removal tower. Rectifying and separating the liquid-phase LNG in the tower by using the material flow as a heat source, and heating the liquid-phase LNG by using a reboiler at the bottom of the tower; the process can effectively remove the nitrogen content in the feed gas, so that the nitrogen content in the LNG product is less than or equal to 1 percent, but the generated nitrogen-containing tail gas has high CH4 content and large CH4 loss.

2. A washing tower is arranged in the process, meanwhile, the gas phase of a low-temperature separator is divided into two strands, one 90% of the gas phase enters a cold box to be condensed into LNG, the LNG is throttled and enters the middle upper part of a nitrogen removal tower, the other 10% of the gas phase directly throttled and enters the middle lower part of the nitrogen removal tower, and the material flow is used as a heat source to form rectification separation in the tower; meanwhile, the mixed refrigerant is divided into one strand, a small cooling box is added, the mixed refrigerant is used for refrigeration, the gas phase at the top of the denitrification gas tower is condensed, the liquid phase in a reflux tank at the top of the denitrification gas tower is pressurized to 0.6MPa by using a low-temperature shielding pump and flows back to the top of the denitrification gas tower, the gas phase is released to the air after being reheated in the cooling box, and the refrigerant is condensed and flows back; but this process is costly in investment.

Disclosure of Invention

The invention aims to provide a natural gas liquefied denitrification system and a natural gas liquefied denitrification process with good denitrification effect and low cost aiming at the defects of the prior art.

The technical scheme adopted by the invention is as follows: a natural gas liquefaction denitrification system comprises a dry gas pipeline, a cold box, a low-temperature separator, a denitrification gas tower, a booster compressor and a reflux tank; the dry gas pipeline is communicated with a raw gas inlet of the low-temperature separator after passing through the cold box, a gas phase outlet at the top of the low-temperature separator is communicated with a first gas phase branch and a second gas phase branch, and the first gas phase branch is communicated with the upper part of the denitrification gas tower after passing through the cold box; the second gas phase branch is communicated with the lower part of the denitrification gas tower; a gas phase outlet at the top of the denitrification gas tower is connected with a gas phase pipeline at the top of the tower, the gas phase pipeline at the top of the tower is communicated with an inlet of a nitrogen/methane booster compressor after passing through a cold box, a flash steam pipeline is communicated with an inlet of the booster compressor after passing through the cold box, an outlet of the booster compressor is connected with a throttling pipeline, and the throttling pipeline is communicated with a reflux tank after passing through the cold box; the gas phase outlet of the reflux tank is communicated with a tail gas discharge pipeline; the liquid phase export of backward flow jar passes through the upper portion intercommunication of pipeline and denitrogenation gas tower, and the liquid phase export at the bottom of the denitrogenation gas tower links to each other with the LNG pipeline, and the LNG pipeline communicates with the LNG storage tank of LNG liquefaction mill behind the cold box.

According to the scheme, the liquid phase outlet at the bottom of the low-temperature separator is connected with the heavy hydrocarbon pipeline, and the heavy hydrocarbon pipeline is communicated with the heavy hydrocarbon storage tank after passing through the cold box.

According to the scheme, the tail gas discharge pipeline is communicated with the atmosphere after passing through the cold box.

According to the scheme, thermometers are respectively arranged at the positions of the cold inlet box and the cold outlet box.

According to the scheme, the denitrification system is additionally provided with a reflux pump, the inlet of the reflux pump is communicated with the liquid phase outlet of the reflux tank through a pipeline, and the outlet of the reflux pump is communicated with the upper part of the denitrification gas tower through a reflux pipeline.

According to the scheme, the top of the nitrogen removal tower is provided with a safety pipeline, and a safety valve is arranged on the safety pipeline.

The invention also provides a natural gas denitrification process based on the denitrification system, which comprises the following steps:

firstly, carrying out upstream deep decarburization and dehydration process treatment on dry gas, and then condensing the dry gas through a cold box;

step two, the condensed dry gas enters a low-temperature separator for gas-liquid separation, the gas phase in the low-temperature separator is divided into two strands, one is 90% of the gas phase flowing into a first gas phase pipeline, the gas phase is condensed into LNG through a cold box, and the LNG throttles to enter the upper part of a nitrogen removal tower; the other stream is 10% gas phase flowing into the path of the second gas phase, directly throttled to enter the lower part of the nitrogen removal tower to be used as a heat source, and rectified and separated in the nitrogen removal tower;

thirdly, the liquid phase in the low-temperature separator is condensed heavy hydrocarbon, and the heavy hydrocarbon passes through a cold box through a heavy hydrocarbon pipeline and enters a heavy hydrocarbon storage tank of an LNG liquefaction plant after being reheated;

step four, respectively mixing the gas phase at the top of the denitrogenation gas tower and flash steam from an LNG storage tank after passing through a cold box, and pressurizing the nitrogen/methane mixed gas in a booster compressor;

step five, cooling the pressurized nitrogen/methane mixed gas through a cold box, throttling the cooled nitrogen/methane mixed gas, then feeding the throttled nitrogen/methane mixed gas into a reflux tank at the top of a nitrogen removal tower, feeding a liquid phase in the reflux tank into the nitrogen removal tower for washing, reheating a gas phase in the reflux tank to obtain nitrogen-containing tail gas, and then emptying the nitrogen-containing tail gas through the cold box;

and step six, the liquid phase at the bottom of the denitrification gas tower is the product LNG, and the LNG enters an LNG storage tank of an LNG liquefaction plant after being cooled and throttled.

According to the scheme, in the step one, dry gas is condensed to-55 ℃ through a cold box.

According to the scheme, in the fourth step, the nitrogen/methane mixed gas enters the booster compressor and then is boosted to 1.1-1.3 MPa.

According to the scheme, in the fifth step, the temperature of the nitrogen/methane mixed gas is-184 ℃ to-180 ℃ after throttling.

The invention has the beneficial effects that: the invention is provided with the booster compressor, the gas phase at the top of the denitrogenation gas tower is pressurized and throttled after being mixed with flash steam (BOG) from an LNG storage tank, the LNG product at the top of the denitrogenation gas tower can be qualified by controlling the temperature of the gas phase entering the top of the denitrogenation gas tower, and CH in tail gas₄The loss is less; compared with the prior art, the technical scheme of the invention has simpler process, low energy consumption, less system configuration and low investment cost.

Drawings

Fig. 1 is a schematic structural diagram of an embodiment of the present invention.

Wherein: 1. a cold box; 2. a cryogenic separator; 3. a nitrogen removal tower; 4. a reflux pump; 5. a reflux tank; 6. a booster compressor; 7. a dry gas line; 8. a first gas phase branch; 9. a second gas phase branch; 10. a top gas phase line; 11. a flash steam pipeline; 12. a throttling pipeline; 13. a tail gas discharge pipeline; 14. a return line; 15. an LNG pipeline; 16. a heavy hydrocarbon pipeline.

Detailed Description

For a better understanding of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings.

The natural gas liquefaction denitrification system shown in fig. 1 comprises a dry gas pipeline 7, a cold box 1, a low-temperature separator 2, a denitrification gas tower 3, a booster compressor 6 and a reflux tank 5 of a reflux pump 4; a dry gas pipeline 7 is communicated with a raw gas inlet of the low-temperature separator 2 after passing through the cold box 1, a gas phase outlet at the top of the low-temperature separator 2 is communicated with a first gas phase branch 8 and a second gas phase branch 9, and the first gas phase branch 8 is communicated with the upper part of the denitrification gas tower 3 after passing through the cold box 1; the second gas phase branch 9 is communicated with the lower part of the denitrification gas tower 3; a gas phase outlet at the top of the nitrogen-removing tower 3 is connected with a gas phase pipeline 10 at the top of the tower, the gas phase pipeline 10 at the top of the tower is communicated with an inlet of a nitrogen/methane booster compressor 6 after passing through a cold box 1, a flash steam pipeline 11 is communicated with an inlet of the booster compressor 6 after passing through the cold box 1, an outlet of the booster compressor 6 is connected with a throttling pipeline 12, and the throttling pipeline 12 is communicated with a reflux tank 5 after passing through the cold box 1; a gas phase outlet of the reflux tank 5 is communicated with a tail gas discharge pipeline 13; the liquid phase export of reflux tank 5 passes through the upper portion intercommunication of pipeline and denitrogenation gas tower 3, and the liquid phase export at the bottom of the 3 towers in denitrogenation gas tower links to each other with LNG pipeline 15, and LNG pipeline 15 communicates with the LNG storage tank of LNG liquefaction mill behind cold box 1.

Preferably, the liquid phase outlet at the bottom of the low-temperature separator 2 is connected with a heavy hydrocarbon pipeline 16, and the heavy hydrocarbon pipeline 16 is communicated with a heavy hydrocarbon storage tank after passing through the cold box 1.

Preferably, the exhaust gas discharge pipeline 13 is communicated with the atmosphere after passing through the cold box 1.

Preferably, a thermometer is disposed at each of the positions of the cooling inlet box 1 and the cooling outlet box 1.

Preferably, the denitrification system is additionally provided with a reflux pump 4, the inlet of the reflux pump 4 is communicated with the liquid phase outlet of the reflux tank 5 through a pipeline, and the outlet of the reflux pump 4 is communicated with the upper part of the denitrification gas tower 3 through a reflux pipeline 14.

Preferably, a safety pipeline is arranged at the top of the nitrogen removal tower 3, and a safety valve is arranged on the safety pipeline.

In the invention, the cold box 1, the low-temperature separator 2, the denitrogenation gas tower 3, the booster compressor 6, the reflux tank 5, the reflux pump 4 and other equipment are all in the existing structure; the flash steam pipeline 11 is from an LNG storage tank; each pipeline is respectively provided with a valve.

A natural gas denitrification process based on the denitrification system comprises the following steps:

step one, after the dry gas is subjected to upstream deep decarburization and dehydration process, the dry gas is condensed by a cold box 1. In the invention, before the dry gas passes through the cold box 1, CO₂The content of the water is less than or equal to 50ppm, and the water content is less than or equal to 1 ppm; condensing the dry gas to-55 to-60 ℃ through a cold box 1;

step two, the condensed dry gas enters a low-temperature separator 2 for gas-liquid separation, the gas phase in the low-temperature separator 2 is divided into two strands, one is 90% of the gas phase flowing into a first gas phase pipeline, the gas phase is condensed into LNG through a cold box 1, and the LNG is throttled to enter the upper part of a nitrogen removal tower 3; the other stream is 10 percent of gas phase flowing into the path of the second gas phase, directly throttled and enters the lower part of the nitrogen removal tower 3 to be used as a heat source, and rectified and separated in the nitrogen removal tower 3;

thirdly, the liquid phase in the low-temperature separator 2 is condensed heavy hydrocarbon, and the heavy hydrocarbon passes through the cold box 1 through the heavy hydrocarbon pipeline 16 and enters a heavy hydrocarbon storage tank of the LNG liquefaction plant after being reheated;

and step four, mixing the gas phase at the top of the denitrogenation gas tower 3 and flash steam (BOG) from an LNG storage tank after passing through a cold box 1 respectively, and pressurizing the nitrogen/methane mixed gas in a booster compressor 6. In the invention, nitrogen/methane mixed gas enters a booster compressor 6 to be boosted to 1.1-1.3 MPa;

and step five, cooling the pressurized nitrogen/methane mixed gas through a cooling box 1, throttling the cooled nitrogen/methane mixed gas, then feeding the throttled nitrogen/methane mixed gas into a reflux tank 5 at the top of a nitrogen removal tower 3, feeding a liquid phase in the reflux tank 5 into the nitrogen removal tower 3 for washing, wherein a gas phase in the reflux tank 5 is nitrogen-containing tail gas, and discharging the nitrogen-containing tail gas after reheating through the cooling box 1. In the invention, the temperature of the nitrogen/methane mixed gas after throttling is-184 ℃ to-180 ℃.

And step six, the liquid phase at the bottom of the nitrogen removal gas tower 3 is the product LNG, and the LNG enters an LNG storage tank of an LNG liquefaction plant after being cooled and throttled. In the invention, LNG is cooled to-159 ℃ through the cold box 1, throttled to 120kPa and cooled to-162 ℃ and then enters an LNG storage tank of an LNG liquefaction plant.

The denitrification process utilizes the principle that throttling is carried out after nitrogen/methane pressurization to form a gas phase with a lower temperature of-180 to-184 ℃, the gas phase enters the top of a denitrification tower 3, the gas phase rises in the middle of the tower and contacts in a counter-current mode, the similar phase dissolution principle is adopted, the lower the reflux temperature at the top of the denitrification tower is, and the gas phase CH flows out of the top of the denitrification tower₄The lower the content; control of CH in nitrogen-containing tail gas by using pressure and throttling temperature of nitrogen methane pressurization₄The content is controlled to be 1.1-1.2 MPa by process simulation, the temperature is controlled to be-184 ℃ to-180 ℃ after the flow of the azomethine is throttled, the energy consumption of a denitrification device is lowest, and the nitrogen-containing tail gas CH is₄Content is less than or equal to 5 percent, CH₄The loss amount is small.

Example (b):

the dry gas after the treatment of the upstream deep decarburization and the dehydration process is subjected to the denitrification treatment of the feed gas by the following three denitrification processes, respectively, as shown in table 1.

The process A comprises the following steps: the invention relates to a denitrification method. And a process B: a heating, rectifying and denitrifying process at the bottom of the nitrogen removal tower 3. And (4) a process C: and pumping the mixed refrigerant to the top of the denitrogenation gas tower 3 for condensation and reflux. The system configuration of process B and process C is as described in the background.

TABLE 1 comparison table of three denitrification processes

Serial number	Denitrification process	A	B	C
					1	The content of N2 in the LNG product is less than or equal to 1 percent	Can realize that	Can realize that	Can realize that
2	Refrigerant compressor power	2500kW	2200kW	2600kW
					3	BOG compressor power	160kW	35kW	35 kW
4	Content of tail gas CH4	5% ~10% (controllable)	25%~30%	5% ~10% (controllable)
					5	BOG flash gas amount	1.8ⅹ104Nm3/d	0.6ⅹ104Nm3/d	0.6ⅹ104Nm3/d
6	Whether to add equipment	Whether or not	Whether or not	Adding a small cold box 1 (considering integration into the main cold box 1 according to scale)
					7	Investment (Wan/Yuan)	3321	3140	3579
8	Benefit of	LNG production increases by about 7.5m3/d (3.4 t/d), increasing income by 360 ten thousand yuan/year	Basic scheme	LNG production is increased by about 4.6m3/d (2.08 t/d) and a 220 ten thousand standard of harvest is increased And (5) year.
					9	Recommendation scheme	√

As can be seen from Table 1, the three denitrification processes can control the nitrogen content in the LNG product to be less than or equal to 1%; but CH in the nitrogen-containing tail gas treated by the denitrification process B₄The content is very high and reaches 25% -30%, and CH₄The loss is large; and the denitrification process B has the advantages of more configuration equipment, high investment cost and low benefit.

It should be noted that the above-mentioned embodiments are only preferred embodiments of the present invention, and the present invention is not limited thereto, and although the present invention has been described in detail with reference to the embodiments, it will be apparent to those skilled in the art that modifications can be made to the technical solutions described in the above-mentioned embodiments, or equivalent substitutions of some technical features, but any modifications, equivalents, improvements and the like within the spirit and principle of the present invention shall be included in the protection scope of the present invention.