Triple-nitrogen expansion process for cryogenic liquefaction of natural gas

Abstract

The present invention discloses a triple-nitrogen expansion process for natural gas cryogenic liquefaction. The refrigerant nitrogen undergoes four-stage compression and cooling and then is divided into three streams, which respectively enter the first, second, and third heat flow channels in the cold box. After pre-cooling in the first heat flow channel, it expands and cools down and enters the first cold flow channel; after pre-cooling in the second heat flow channel, it expands and cools down and enters the second cold flow channel; after pre-cooling in the third heat flow channel, it expands and cools down and enters the third cold flow channel. The refrigerant nitrogen output from the second cold flow channel successively undergoes the first and second-stage compression and cooling; the third cold flow channel converges with the refrigerant nitrogen output from the second-stage compression and cooling and undergoes the third-stage compression and cooling; the first cold flow channel converges with the refrigerant nitrogen output from the third-stage compression and cooling and undergoes the fourth-stage compression and cooling, and then is divided into three streams and enters the cold box respectively, continuously circulating to provide cold energy for the cold box. The advantages of the present invention are as follows: greatly reducing the energy consumption of natural gas liquefaction and effectively reducing the compressor failures.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas cryogenic liquefaction, and specifically relates to a nitrogen expansion process. Background Art

[0002] At present, the natural gas expansion liquefaction process mainly realizes it through the refrigeration principle of the reverse Brayton cycle, using the adiabatic expansion of an expander. The gas does work in the expander to achieve the purpose of cooling, and nitrogen is used as the refrigerant.

[0003] It has been found in the long-term production of natural gas liquefaction that: the comprehensive energy consumption of the current single-stage nitrogen expansion liquefaction for providing cooling capacity is relatively high. In order to improve the economy of the overall process of liquefied natural gas, it is necessary to further optimize the production process of liquefied natural gas and further reduce the energy consumption. Summary of the Invention

[0004] The technical problem to be solved by the present invention is: to provide a triple nitrogen expansion process for natural gas cryogenic liquefaction, which not only provides sufficient cooling capacity for natural gas liquefaction, but also greatly reduces the energy consumption of the entire natural gas cryogenic liquefaction process, thereby effectively reducing the cost of natural gas liquefaction.

[0005] To solve the above problems, the technical solution adopted by the present invention is: for the triple nitrogen expansion process of natural gas cryogenic liquefaction, the refrigerant nitrogen is successively compressed and cooled four times to a pressure of 10.0 MpaA and a temperature of 30 °C, and then divided into three streams and respectively enter the first heat flow channel, the second heat flow channel, and the third heat flow channel in the cold box for precooling;

[0006] The refrigerant nitrogen is precooled to -24 °C in the first heat flow channel and then expanded by the first expander, the temperature drops to -123.3 °C, and the pressure is 3.10 MpaA, and then enters the first cold flow channel as a refrigerant; the refrigerant nitrogen is precooled to -78 °C in the second heat flow channel and then expanded by the second expander, the temperature drops to -157.2 °C, and the pressure is 1.40 MpaA, and then enters the second cold flow channel as a refrigerant; the refrigerant nitrogen is precooled to -24 °C in the third heat flow channel and then expanded by the third expander, the temperature drops to -85.05 °C, and the pressure is 3.10 MpaA, and then enters the third cold flow channel as a refrigerant;

[0007] The refrigerant nitrogen gas that has released cold in the second cold flow channel is compressed and cooled for the first time to a pressure of 2.07 MpaA and a temperature of 30 °C; then it is compressed and cooled for the second time to a pressure of 3.0 MpaA and a temperature of 30 °C; the refrigerant nitrogen gas that has released cold in the third cold flow channel converges with the refrigerant nitrogen gas output from the second-stage compression cooling and is then compressed and cooled for the third time to a pressure of 5.5 MpaA and a temperature of 30 °C; the refrigerant nitrogen gas that has released cold in the first cold flow channel converges with the refrigerant nitrogen gas output from the third-stage compression cooling and is then compressed and cooled for the fourth time to a pressure of 10.0 MpaA and a temperature of 30 °C, and then it is divided into three streams and enters the cold box respectively; the above-mentioned refrigerant nitrogen gas circulates continuously to provide cold for the cold box;

[0008] The natural gas to be liquefied enters the cold box for cooling and then is throttled and depressurized by a throttle valve for output.

[0009] Further, in the above-mentioned triple-nitrogen expansion process for deep cryogenic liquefaction of natural gas, the refrigerant nitrogen gas undergoes four-stage compression and cooling by a four-stage compression and cooling mechanism. The four-stage compression and cooling mechanism includes a first-stage compressor assembly, a second-stage compressor assembly, a third-stage compressor assembly, and a fourth-stage compressor assembly that are connected in sequence; the first-stage compressor assembly includes a first-stage compressor and a first-stage cooler that are connected in sequence; the second-stage compressor assembly includes a second-stage compressor and a second-stage cooler that are connected in sequence, the third-stage compressor assembly includes a third-stage compressor and a third-stage cooler that are connected in sequence, and the fourth-stage compressor assembly includes a fourth-stage compressor and a fourth-stage cooler that are connected in sequence.

[0010] Further, in the above-mentioned triple-nitrogen expansion process for deep cryogenic liquefaction of natural gas, the refrigerant nitrogen gas that has released cold in the third cold flow channel and the refrigerant nitrogen gas output from the second-stage cooler enter the third-stage compressor in the third-stage compressor assembly for compression after being mixed by a first mixer.

[0011] Even further, in the above-mentioned triple-nitrogen expansion process for deep cryogenic liquefaction of natural gas, the refrigerant nitrogen gas that has released cold in the first cold flow channel and the refrigerant nitrogen gas output from the third-stage compressor assembly enter the fourth-stage compressor in the fourth-stage compressor assembly for compression after being mixed by a second mixer.

[0012] Even further, in the above-mentioned triple-nitrogen expansion process for deep cryogenic liquefaction of natural gas, the refrigerant nitrogen gas that has been compressed and cooled to a pressure of 10.0 MpaA and a temperature of 30 °C is divided into three streams by a diverter and enters the first heat flow channel, the second heat flow channel, and the third heat flow channel in the cold box respectively for precooling. The setting of the diverter can distribute and adjust the amount of refrigerant nitrogen gas entering the first heat flow channel, the second heat flow channel, and the third heat flow channel, so as to adjust the temperature in the cold box according to the actual production situation and ensure that the overall process energy consumption can be further optimized while the natural gas is liquefied.

[0013] Furthermore, in the aforementioned triple-nitrogen expansion process for natural gas cryogenic liquefaction, the split molar ratio of refrigerant nitrogen in the first heat flow channel, the second heat flow channel, and the third heat flow channel is: 0.1:0.2:0.7.

[0014] Further, in the aforementioned triple-nitrogen expansion process for natural gas cryogenic liquefaction, the natural gas to be liquefied enters the natural gas cooling channel in the cold box and is cooled to -157°C, and then is throttled and liquefied through a throttle valve to form liquefied natural gas at -163°C and a pressure of 1.0 atmosphere for output.

[0015] The advantages of the present invention are as follows: First, three parallel nitrogen expansion cycles provide sufficient cooling capacity for natural gas liquefaction. The refrigerant nitrogen in the first cold flow channel, the second cold flow channel, and the third cold flow channel in the cold box provides sufficient cooling capacity for natural gas liquefaction; then, through the first heat flow channel, the second heat flow channel, and the third heat flow channel, the reflux and precooling of the refrigerant nitrogen are realized, greatly reducing the energy consumption. Second, the refrigerant nitrogen in the first cold flow channel, the second cold flow channel, and the third cold flow channel is respectively provided after being expanded and cooled by the first expander, the second expander, and the third expander. Three expanders are used to provide refrigerant nitrogen in three different temperature zones respectively, which makes the temperature difference between the hot end and the cold end in the cold box relatively uniform, the temperature gradient between the hot and cold load curves is small, and the irreversible heat loss is reduced, that is, the overall energy consumption of the natural gas liquefaction system is greatly reduced. Third, four-stage compression is adopted. The refrigerant nitrogen that has released cooling capacity in the second cold flow channel is successively cooled by the first-stage compression and the second-stage compression. The refrigerant nitrogen that has released heat in the third cold flow channel converges with the refrigerant nitrogen output from the second-stage compression cooling and then enters the three-stage compression assembly. The refrigerant nitrogen that has released heat in the first cold flow channel converges with the refrigerant nitrogen after the third-stage compression cooling and then enters the four-stage compression assembly. That is, four compressors are used for compression, so as to reasonably distribute the load of the compressors, greatly reducing the overall energy consumption of the compressors, effectively reducing the occurrence of failures caused by compressor overheating, and also being able to distribute the refrigerant nitrogen with different cooling energies to different compressor sections, thereby further greatly reducing the overall energy consumption of the compressors. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a schematic diagram of the principle structure of the triple-nitrogen expansion process for natural gas cryogenic liquefaction described in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention will be further described in detail below with reference to the drawings and preferred embodiments.

[0018] As Figure 1As shown, in the triple-nitrogen expansion process for natural gas cryogenic liquefaction, the refrigerant nitrogen gas successively undergoes four-stage compression and cooling to a pressure of 10.0 MpaA and a temperature of 30 °C, and then is divided into three streams and enters the first heat flow channel 11, the second heat flow channel 12, and the third heat flow channel 13 in the cold box respectively for precooling.

[0019] In this embodiment, the refrigerant nitrogen gas undergoes compression and cooling through four-stage compression and cooling mechanisms. The four-stage compression and cooling mechanisms include a first-stage compressor assembly 2, a second-stage compressor assembly 3, a third-stage compressor assembly 4, and a fourth-stage compressor assembly 5 that are connected in sequence. The first-stage compressor assembly 2 includes a first-stage compressor 21 and a first-stage cooler 22 that are connected in sequence; the second-stage compressor assembly 3 includes a second-stage compressor 31 and a second-stage cooler 32 that are connected in sequence, the third-stage compressor assembly 4 includes a third-stage compressor 41 and a third-stage cooler 42 that are connected in sequence, and the fourth-stage compressor assembly 5 includes a fourth-stage compressor 51 and a fourth-stage cooler 52 that are connected in sequence.

[0020] After being precooled to -24 °C in the first heat flow channel 11, the refrigerant nitrogen gas expands through the first expander 100, the temperature drops to -123.3 °C, and the pressure is 3.10 MpaA, and then enters the first cold flow channel 14 as a refrigerant.

[0021] After being precooled to -78 °C in the second heat flow channel 12, the refrigerant nitrogen gas expands through the second expander 200, the temperature drops to -157.2 °C, and the pressure is 1.40 MpaA, and then enters the second cold flow channel 15 as a refrigerant.

[0022] After being precooled to -24 °C in the third heat flow channel 13, the refrigerant nitrogen gas expands through the third expander 300, the temperature drops to -85.05 °C, and the pressure is 3.10 MpaA, and then enters the third cold flow channel 16 as a refrigerant.

[0023] The refrigerant nitrogen gas that has released cold in the second cold flow channel 15 undergoes the first-stage compression and cooling, that is, it is successively compressed by the first-stage compressor 2 to a pressure of 2.07 MpaA and cooled to a temperature of 30 °C by the first-stage cooler 22; then it undergoes the second-stage compression and cooling, that is, it is successively compressed by the second-stage compressor 32 to a pressure of 3.0 MpaA and cooled to a temperature of 30 °C by the second-stage cooler.

[0024] The refrigerant nitrogen that has released the cooling capacity in the third cold flow channel 16 converges with the refrigerant nitrogen output from the second-stage compression cooling and then undergoes the third-stage compression cooling. In this embodiment, the refrigerant nitrogen that has released the cooling capacity in the third cold flow channel 16 and the refrigerant nitrogen output from the secondary cooler 32 enter the third-stage compressor 41 in the three-stage compressor assembly after being mixed by the first mixer 7 for compression until the pressure reaches 5.5 MpaA and the temperature reaches 30°C. The refrigerant nitrogen that has released the cooling capacity in the first cold flow channel 14 converges with the refrigerant nitrogen output from the third-stage compression cooling and then undergoes the fourth-stage compression cooling. In this embodiment, the refrigerant nitrogen that has released the cooling capacity in the first cold flow channel 14 and the refrigerant nitrogen output from the three-stage compressor assembly enter the fourth-stage compressor 51 in the four-stage compressor assembly 5 after being mixed by the second mixer 8 for compression to a pressure of 10.0 MpaA and cooled to a temperature of 30°C by the four-stage cooler 52.

[0025] The refrigerant nitrogen with a pressure of 10.0 MpaA and a temperature of 30°C output from the four-stage compressor assembly 5 is divided into three streams by the diverter 6 and enters the first hot flow channel 11, the second hot flow channel 12, and the third hot flow channel 13 in the cold box 1 respectively for precooling. In this embodiment, the split molar ratio of the refrigerant nitrogen in the first hot flow channel 11, the second hot flow channel 12, and the third hot flow channel 13 is 0.1:0.2:0.7. The above-mentioned refrigerant nitrogen circulates continuously to provide cooling capacity for the cold box 1.

[0026] The natural gas to be liquefied enters the natural gas cooling channel 17 in the cold box 1 and is cooled to -157°C, and then throttled and liquefied by the throttle valve 171 to reduce the pressure to -163°C, and the liquefied natural gas with a pressure of 1.0 atmospheric pressure is output.

[0027] As shown in Table 1, compared with the single-nitrogen expansion, the overall energy consumption of the triple-nitrogen cycle expansion in this embodiment is reduced by 43.7% and the efficiency is increased by 77.5%. Compared with the double-nitrogen expansion, the energy consumption is reduced by 13.5% and the efficiency is increased by 15.6%.

[0028] 。

[0029] As can be seen from the above, the advantages of the present invention are as follows: First, three parallel nitrogen expansion cycles provide sufficient cooling capacity for natural gas liquefaction. The refrigerant nitrogen in the first cold stream channel, the second cold stream channel, and the third cold stream channel in the cold box provides sufficient cooling capacity for natural gas liquefaction; then through the first hot stream channel, the second hot stream channel, and the third hot stream channel, the reflux and precooling of the refrigerant nitrogen are realized, greatly reducing the energy consumption. Second, the refrigerant nitrogen in the first cold stream channel, the second cold stream channel, and the third cold stream channel is provided after being expanded and cooled by the first expander, the second expander, and the third expander respectively. Three expanders are used to provide refrigerant nitrogen in three different temperature zones respectively, which makes the temperature difference from the hot end to the cold end in the cold box relatively uniform, the temperature gradient between the hot and cold load curves is small, and the irreversible heat loss is reduced, that is, the overall energy consumption of the natural gas liquefaction system is greatly reduced. Third, four-stage compression is adopted. The refrigerant nitrogen that has released cooling capacity in the second cold stream channel is successively cooled by the first-stage compression and the second-stage compression. The refrigerant nitrogen that has released heat in the third cold stream channel converges with the refrigerant nitrogen output from the second-stage compression cooling and then enters the three-stage compression assembly. The refrigerant nitrogen that has released heat in the first cold stream channel converges with the refrigerant nitrogen after the third-stage compression cooling and then enters the four-stage compression assembly. That is, four compressors are used for compression, so as to reasonably distribute the load of the compressors, greatly reduce the overall energy consumption of the compressors, effectively reduce the occurrence of failures caused by compressor overheating, and can also distribute the refrigerant nitrogen with different cooling energies to different compressor sections, thereby further greatly reducing the overall energy consumption of the compressors.

Claims

1. A triple-nitrogen expansion process for cryogenic liquefaction of natural gas, characterized in that: The refrigerant nitrogen undergoes four stages of compression and cooling in sequence until the pressure reaches 10.0 MpaA and the temperature reaches 30°C, and then it is divided into three streams and enters the first heat flow channel, the second heat flow channel, and the third heat flow channel in the cold box respectively for precooling; the refrigerant nitrogen is precooled to -24°C in the first heat flow channel and then expanded by the first expander, the temperature drops to -123.3°C, and the pressure is 3.10 MpaA, and then it enters the first cold flow channel as a refrigerant; The refrigerant nitrogen is precooled to -78°C in the second heat flow channel and then expanded by the second expander, the temperature drops to -157.2°C, and the pressure is 1.40 MpaA, and then it enters the second cold flow channel as a refrigerant; The refrigerant nitrogen is precooled to -24°C in the third heat flow channel and then expanded by the third expander, the temperature drops to -85.05°C, and the pressure is 3.10 MpaA, and then it enters the third cold flow channel as a refrigerant; The refrigerant nitrogen that has released cold in the second cold flow channel is compressed and cooled by the first stage to a pressure of 2.07 MpaA and a temperature of 30°C; then it is compressed and cooled by the second stage to a pressure of 3.0 MpaA and a temperature of 30°C; the refrigerant nitrogen that has released cold in the third cold flow channel converges with the refrigerant nitrogen output from the second stage of compression and cooling and is then compressed and cooled by the third stage to a pressure of 5.5 MpaA and a temperature of 30°C; the refrigerant nitrogen that has released cold in the first cold flow channel converges with the refrigerant nitrogen output from the third stage of compression and cooling and is then compressed and cooled by the fourth stage to a pressure of 10.0 MpaA and a temperature of 30°C, and then it is divided into three streams and enters the cold box respectively; the above refrigerant nitrogen circulates continuously to provide cold for the cold box; the natural gas to be liquefied enters the cold box for liquefaction and cooling and then is throttled and depressurized by a throttle valve for output; the refrigerant nitrogen undergoes compression and cooling through four compression and cooling mechanisms, and the four compression and cooling mechanisms include a first-stage compressor assembly, a second-stage compressor assembly, a third-stage compressor assembly, and a fourth-stage compressor assembly connected in sequence; the first-stage compressor assembly includes a first-stage compressor and a first-stage cooler connected in sequence; the second-stage compressor assembly includes a second-stage compressor and a second-stage cooler connected in sequence, the third-stage compressor assembly includes a third-stage compressor and a third-stage cooler connected in sequence, and the fourth-stage compressor assembly includes a fourth-stage compressor and a fourth-stage cooler connected in sequence; the refrigerant nitrogen that has released cold in the third cold flow channel and the refrigerant nitrogen output from the second-stage cooler enter the third-stage compressor in the third-stage compressor assembly after being mixed by the first mixer for compression; the refrigerant nitrogen that has released cold in the first cold flow channel and the refrigerant nitrogen output from the third-stage compressor assembly enter the fourth-stage compressor in the fourth-stage compressor assembly after being mixed by the second mixer for compression; the refrigerant nitrogen that has been compressed and cooled to a pressure of 10.0 MpaA and a temperature of 30°C is divided into three streams by a diverter and enters the first heat flow channel, the second heat flow channel, and the third heat flow channel in the cold box respectively for precooling.

2. The triple-nitrogen expansion process for natural gas cryogenic liquefaction according to claim 1, characterized in that: The split molar ratio of the refrigerant nitrogen in the first heat flow channel, the second heat flow channel, and the third heat flow channel is: 0.1:0.2:0.

7.

3. The triple-nitrogen expansion process for natural gas cryogenic liquefaction according to claim 1, characterized in that: The natural gas to be liquefied enters the natural gas cooling channel in the cold box and is cooled to -157°C. After that, it is throttled and depressurized by a throttle valve to form liquefied natural gas at -163°C and a pressure of 1.0 atmosphere for output.

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