An unconventional natural gas liquefaction and denitrification system and its working method

Abstract

The present invention discloses an unconventional natural gas liquefaction and denitrification system, which is composed of a liquefaction system, a separation system and a cryogenic system. On the one hand, the present invention cools the liquefaction system through the cryogenic system, processes its liquefaction process in sections, collects the liquid phase obtained in the intermediate process, and simultaneously obtains gas phases with different temperatures and different nitrogen contents. On the other hand, a number of feed ports are arranged at different tray heights of the distillation column in the separation system. By leading out the raw gas with different temperatures and nitrogen contents in the liquefaction system and introducing it into the trays at different positions of the distillation column according to its different nitrogen content or temperature, the denitrification efficiency of the overall system is improved, and the purity and recovery rate of the liquefied product are ensured. The present invention sets two working modes in the liquefaction system, and the nitrogen-rich tail gas can be used as fuel gas or directly discharged according to the on-site needs.

Description

Technical Field

[0001] The present invention relates to a natural gas liquefaction and denitrification system, and more particularly to an unconventional natural gas liquefaction and denitrification system and its working method. Background Art

[0002] In the process of unconventional natural gas liquefaction, it is necessary to denitrify the raw gas and minimize the loss of methane as much as possible during the liquefaction and denitrification process. Generally, in the prior art, most of the liquefaction and recovery processes of unconventional natural gas follow the existing systems and devices for conventional natural gas liquefaction and recovery. However, when the existing system is used to liquefy and process unconventional natural gas following the conventional natural gas liquefaction treatment system, the following deficiencies exist.

[0003] On the one hand, the existing natural gas liquefaction treatment system has the problem of low denitrification efficiency. Conventional natural gas liquefaction and denitrification mostly adopt the method of directly introducing the cooled raw gas into the rectification tower for rectification separation, with low denitrification efficiency, resulting in large specifications and sizes of the rectification tower.

[0004] On the other hand, due to the fluctuation of nitrogen concentration in the raw gas of unconventional natural gas, while the processing capacity of the conventional natural gas liquefaction and recovery system is relatively stable, that is, the methane content in the nitrogen-rich tail gas during the processing fluctuates with the fluctuation of methane concentration in the raw gas of unconventional natural gas, and too high methane content in the nitrogen-rich tail gas will cause serious environmental and resource waste problems.

[0005] When the nitrogen content in the raw gas of unconventional natural gas is high and the methane content is low, the methane content in the nitrogen-rich tail gas in the recovery and treatment system is very low. In this case, the nitrogen-rich tail gas can be directly discharged. When the nitrogen content in the raw gas of unconventional natural gas is low and the methane content is high, the methane content in the nitrogen-rich tail gas in the recovery and treatment system is relatively high. In this case, directly discharging the nitrogen-rich tail gas is very uneconomical. Considering that unconventional natural gas wells are often located in remote mountainous areas, when the methane content in the nitrogen-rich tail gas is relatively high, its calorific value is relatively large, and this part of the tail gas can be directly used as fuel gas for small and medium-sized facilities driven by electric motors.

[0006] How to optimize the unconventional natural gas liquefaction and denitrification system to further improve its denitrification efficiency and improve the adaptability of the system to the fluctuation of nitrogen concentration in the raw gas during the processing is an urgent problem to be solved at present. Summary of the Invention

[0007] The purpose of this invention patent is to solve the problems and deficiencies existing in the above-mentioned prior art, and provide an unconventional natural gas liquefaction and denitrification system and its working method.

[0008] On the one hand, the present invention cools the liquefaction system through a low-temperature system, processes its liquefaction process in segments, collects the liquid phase obtained in the intermediate process, and simultaneously obtains gas phases with different temperatures and different nitrogen contents. On the other hand, a number of feed ports are provided at different tray heights of the rectification column in the separation system. By leading out the raw material gas at different temperatures and different nitrogen contents in the liquefaction system and introducing it into different trays of the rectification column according to the difference in its nitrogen content or temperature, the denitrification efficiency of the overall system is improved, and the purity and recovery rate of the liquefied product are ensured. The working method of the present invention sets two working modes in the liquefaction system, and the nitrogen-rich tail gas can be used as fuel gas or directly discharged according to the on-site needs.

[0009] To achieve the above object, the present invention is implemented by the following technical solutions.

[0010] An unconventional natural gas liquefaction and denitrification system, characterized in that it is composed of a liquefaction system, a separation system and a low-temperature system.

[0011] The liquefaction system is composed of: the raw material gas is connected to the left-end inlet 2c of the first heat exchanger 2 through the first compressor 1, the right-end outlet 2d of the first heat exchanger 2 is connected to the left-end inlet 3c of the second heat exchanger 3, and the pipeline at the right-end outlet 3d of the second heat exchanger 3 is branched into two pipelines through the three-way valve 8. The first pipeline is connected to the inlet of the first gas-liquid separation tank 9, and the second pipeline is connected to the left-end inlet 7c of the third heat exchanger 7. The right-end outlet 7d of the third heat exchanger 7 is connected to at least one inlet at the lower part of the rectification column 15 through a pipeline; the gas-phase outlet of the first gas-liquid separation tank 9 is connected to the left-end inlet 11c of the fourth heat exchanger 11, the right-end outlet 11d of the fourth heat exchanger 11 is connected to the inlet of the second gas-liquid separation tank 10, the gas-phase outlet of the second gas-liquid separation tank 10 is connected to the left-end inlet 12c of the fifth heat exchanger 12, and the right-end outlet 12d of the fifth heat exchanger 12 is connected to at least one inlet at the upper part of the rectification column 15 through a pipeline; the pipelines at the liquid-phase outlet of the second gas-liquid separation tank 10, the liquid-phase outlet of the first gas-liquid separation tank 9 and the liquid-phase outlet of the reboiler 17 converge and are jointly connected to the inlet of the LNG storage tank 18. The gas-phase outlet of the LNG storage tank 18 is connected to the left-end inlet 13c of the sixth heat exchanger 13 through the second compressor 14, and the right-end outlet 13d of the sixth heat exchanger 13 is connected to at least one inlet at the middle part of the rectification column 15 through a pipeline.

[0012] The separation system is constituted by connecting the gas-phase outlet of the rectification column 15 to the gas-phase inlet on the shell side of the condenser 16, connecting the liquid-phase outlet on the shell side of the condenser 16 to the top reflux port of the rectification column 15, connecting the gas-phase outlet of the condenser 16 to the upper inlet 2a of the first heat exchanger 2, connecting the lower outlet 2b of the first heat exchanger 2 to the outside, connecting the inlet and outlet on the tube side of the condenser 16 to the inlet and outlet of the external cold source respectively, connecting the liquid-phase outlet of the rectification column 15 to the inlet on the shell side of the reboiler 17, connecting the gas-phase outlet of the reboiler 17 to the bottom reflux port of the rectification column 15, and connecting the inlet and outlet on the tube side of the reboiler 17 to the inlet and outlet of the external heat source.

[0013] The low-temperature system is constituted by connecting the outlet of the third compressor 4 to the left-end inlet 5c of the seventh heat exchanger 5, connecting the right-end outlet 5d of the seventh heat exchanger 5 to the inlet of the expander 6, connecting the outlet of the expander 6 to the upper inlet 7a of the third heat exchanger 7, connecting the lower outlet 7b of the third heat exchanger 7 to the lower inlet 13b of the sixth heat exchanger 13, connecting the upper outlet 13a of the sixth heat exchanger 13 to the lower inlet 12b of the fifth heat exchanger 12, connecting the upper outlet 12a of the fifth heat exchanger 12 to the upper inlet 11a of the fourth heat exchanger 11, connecting the lower outlet 11b of the fourth heat exchanger 11 to the upper inlet 3a of the second heat exchanger 3, and connecting the lower outlet 3b of the second heat exchanger 3 to the inlet of the third compressor 4.

[0014] Further preferably, a first stop valve 49 is provided in the pipeline connecting the raw material gas to the first compressor 1; a fourth stop valve 40 is provided at the inlet of the second gas-liquid separation tank 10; a fifth stop valve 44 is provided at the liquid-phase outlet of the second gas-liquid separation tank 10.

[0015] Further preferably, two pipelines are connected in parallel at the right-end outlet 11d of the fourth heat exchanger 11. The first pipeline is connected to the inlet of the second gas-liquid separation tank 10 through the fourth stop valve 40; the second pipeline converges with the pipeline at the right-end outlet 12d of the fifth heat exchanger 12 after passing through the second stop valve 39, and then is connected to at least one inlet at the upper part of the rectification column 15 through the provided first check valve 41; the right-end outlet 13d of the sixth heat exchanger 13 is connected to at least one inlet at the middle part of the rectification column 15 through the provided second check valve 42; the right-end outlet 7d of the third heat exchanger 7 is connected to at least one inlet at the lower part of the rectification column 15 through the provided third check valve 43.

[0016] Further preferably, a ninth thermometer 27 is also provided on the outlet pipeline of the second stop valve 39, and a tenth thermometer 28 is also provided on the pipeline of the right - hand outlet 12d of the fifth heat exchanger 12. After the two pipelines converge, two pipelines are arranged in parallel again. The first path is sequentially connected to the first feed inlet 15a of the rectification column 15 through a tenth stop valve 19, a sixth one - way valve 38, and a first thermometer 30; the second path is sequentially connected to the second feed inlet 15b of the rectification column 15 in the separation system through a third stop valve 20, a first one - way valve 41, and a second thermometer 31;

[0017] Further preferably, at the right - hand outlet 13d of the sixth heat exchanger 13, three pipelines are arranged in parallel by setting an eleventh thermometer 29. The first path is sequentially connected to the third feed inlet 15c of the rectification column 15 through a sixth stop valve 21, a second one - way valve 42, and a third thermometer 32; the second path is sequentially connected to the fourth feed inlet 15d of the rectification column 15 through an eleventh stop valve 24, a seventh one - way valve 46, and a fourth thermometer 33; the third path is sequentially connected to the fifth feed inlet 15e of the rectification column 15 through a twelfth stop valve 23, an eighth one - way valve 47, and a fifth thermometer 34;

[0018] Further preferably, at the right - hand outlet 7d of the third heat exchanger 7, three pipelines are arranged in parallel by setting a twelfth thermometer 50. The first path is sequentially connected to the sixth feed inlet 15f of the rectification column 15 through a seventh stop valve 22, a third one - way valve 43, and a sixth thermometer 35; the second path is sequentially connected to the seventh feed inlet 15g of the rectification column 15 through an eighth stop valve 25, a fourth one - way valve 48, and a seventh thermometer 36; the third path is sequentially connected to the eighth feed inlet 15h of the rectification column 15 through a ninth stop valve 26, a fifth one - way valve 45, and an eighth thermometer 37.

[0019] Further preferably, the first heat exchanger 2, the second heat exchanger 3, the third heat exchanger 7, the fourth heat exchanger 11, the fifth heat exchanger 12, the sixth heat exchanger 13, and the seventh heat exchanger 5 are all shell - and - tube heat exchangers, fin - tube heat exchangers, or double - pipe heat exchangers.

[0020] Further preferably, the expander 6 is a scroll expander or a screw expander.

[0021] Further preferably, the first compressor 1, the second compressor 14, and the third compressor 4 are all piston compressors, screw compressors, centrifugal compressors, or linear compressors.

[0022] A working method of an unconventional natural gas liquefaction and denitrification system, characterized in that: it is respectively the working methods of the liquefaction system, the separation system, and the cryogenic system,

[0023] I. The working method of the liquefaction system includes two working modes:

[0024] 1. When the nitrogen-rich tail gas is used as fuel gas: Open the first shut-off valve 49 and the second shut-off valve 39, and close the fourth shut-off valve 40 and the fifth shut-off valve 44. After the tail gas is pressurized by the first compressor 1, the cold energy of the nitrogen-rich tail gas is recovered in the first heat exchanger 2, cooled in the second heat exchanger 3, and then branched by the three-way valve 8. One part enters the first gas-liquid separation tank 9 for gas-liquid separation. The separated liquefied natural gas enters the LNG storage tank 18 for storage. The gas phase is further cooled in the fourth heat exchanger 11 and then enters the second feed port 15b of the rectification column 15 successively through the second shut-off valve 39 and the first one-way valve 41. The other part branched by the three-way valve 8 is cooled in the third heat exchanger 7 and then enters the sixth feed port 15f of the rectification column 15 through the third one-way valve 43. The flash gas of the LNG storage tank 18 is pressurized by the second compressor 14, cooled in the sixth heat exchanger 13, and enters the third feed port 15c of the rectification column 15 through the second one-way valve 42.

[0025] 2. When the nitrogen-rich tail gas is directly discharged: Open the first shut-off valve 49, the fourth shut-off valve 40 and the fifth shut-off valve 44, and close the second shut-off valve 39. The gas phase obtained from the first gas-liquid separation tank 9 is further cooled in the fourth heat exchanger 11 and then enters the second gas-liquid separation tank 10 for further gas-liquid separation through the fourth shut-off valve 40. The obtained liquid phase enters the LNG storage tank 18 for storage, and the obtained gas phase is cooled in the fifth heat exchanger 12 and then enters the second feed port 15b of the rectification column 15 through the first one-way valve 41.

[0026] II. Working method of the separation system: Fluids entering from different feed port positions are separated in the rectification column 15. The gas phase at the top of the column is condensed by the condenser 16. The nitrogen-rich tail gas is discharged after recovering the cold energy in the first heat exchanger 2. The condensed recovered liquid phase flows back to the rectification column 15. The liquid phase at the bottom of the column is reboiled by the reboiler 17. The obtained gas phase flows back to the rectification column 15, and the liquid phase enters the LNG storage tank 18.

[0027] III. Working method of the cryogenic system: The refrigerant is compressed by the third compressor 4, cooled by the cold source in the seventh heat exchanger 5, expanded and cooled in the expander 6, and then heated successively in the third heat exchanger 7, the sixth heat exchanger 13, the fifth heat exchanger 12, the fourth heat exchanger 11 and the second heat exchanger 3 and then returns to the third compressor 4 for compression to complete the cycle.

[0028] Advantages and beneficial effects of the present invention:

[0029] 1. Different from directly introducing unconventional natural gas feed gas into the rectification column for rectification in the prior art, an unconventional natural gas liquefaction and denitrification system of the present invention performs split and staged liquefaction treatment on the liquefaction process in the liquefaction system. The liquefied natural gas obtained from the previous liquefaction is separated, and the unliquefied gas phase is introduced into the rectification column for rectification. Its separation efficiency is higher, the size of the rectification column can be greatly reduced, and thus the problem of low denitrification efficiency in the natural gas liquefaction treatment system can be well improved. Through the HYSYS simulation results, for the feed gas with the same components, when the separation effects of the prior art and the present invention are close, that is, when the nitrogen content in the top gas of the prior art reaches 0.9151 and the nitrogen content in the top gas of the present invention reaches 0.9039, the number of trays in the rectification column in the prior art should be set to 12 trays, while the number of trays in the rectification column in the present invention only needs 6 trays, greatly reducing the size of the rectification column and significantly improving the denitrification efficiency of the overall system.

[0030] 2. According to the on-site requirements, the present invention divides the working method of the liquefaction system into two working modes. When the nitrogen-rich tail gas is used as fuel gas, the feed gas enters the rectification column for rectification separation after one-stage liquefaction separation. When the nitrogen-rich tail gas is directly discharged, the feed gas enters the rectification column for separation after two-stage liquefaction separation. This ensures the power demand of small and medium-sized liquefied natural gas facilities driven by motors on-site during actual operation. Brief Description of the Drawings

[0031] Figure 1 is the structural schematic diagram of Embodiment 1 of the present invention;

[0032] Figure 2 is the structural schematic diagram of Embodiment 2 of the present invention; Detailed Description of the Invention

[0033] In order to enable those of ordinary skill in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be further clearly and completely described below with reference to the accompanying drawings.

[0034] Embodiment 1:

[0035] As Figure 1 shown, an unconventional natural gas liquefaction system is composed of a liquefaction system, a separation system, and a cryogenic system.

[0036] The composition and connection method of the liquefaction system are as follows:

[0037] The raw material gas is successively connected to the left-end inlet 2c of the first heat exchanger 2 through the first stop valve 49 and the first compressor 1. The right-end outlet 2d of the first heat exchanger 2 is connected to the left-end inlet 3c of the second heat exchanger 3. The pipeline at the right-end outlet 3d of the second heat exchanger 3 is branched into two pipelines through the three-way valve 8. The first branch is connected to the inlet of the first gas-liquid separation tank 9, and the second branch is connected to the left-end inlet 7c of the third heat exchanger 7. The right-end outlet 7d of the third heat exchanger 7 is connected to the sixth feed port 15f of the distillation column 15 through the third one-way valve 43. The gas-phase outlet of the first gas-liquid separation tank 9 is connected to the left-end inlet 11c of the fourth heat exchanger 11. There are two parallel pipelines at the right-end outlet 11d of the fourth heat exchanger 11. The first branch is connected to the inlet of the second gas-liquid separation tank 10 through the fourth stop valve 40, and the second branch converges with the pipeline at the right-end outlet 12d of the fifth heat exchanger 12 after passing through the second stop valve 39 and is then connected to the second feed port 15b of the distillation column 15 in the separation system through the first one-way valve 41. The gas-phase outlet of the second gas-liquid separation tank 10 is connected to the left-end inlet 12c of the fifth heat exchanger 12, and the liquid-phase outlet of the second gas-liquid separation tank 10 is connected to the inlet of the fifth stop valve 44. The pipelines at the outlet of the fifth stop valve 44, the liquid-phase outlet of the first gas-liquid separation tank 9, and the liquid-phase outlet of the reboiler 17 converge and are jointly connected to the inlet of the LNG storage tank 18. The gas-phase outlet of the LNG storage tank 18 is connected to the left-end inlet 13c of the sixth heat exchanger 13 through the second compressor 14, and the right-end outlet 13d of the sixth heat exchanger 13 is connected to the third feed port 15c of the distillation column 15 through the second one-way valve 42.

[0038] The working method of the liquefaction system includes two working modes:

[0039] When the nitrogen-rich tail gas is used as fuel gas: Open the first stop valve 49 and the second stop valve 39, and close the fourth stop valve 40 and the fifth stop valve 44; After the tail gas is pressurized by the first compressor 1, the cold energy of the nitrogen-rich tail gas is recovered in the first heat exchanger 2, cooled in the second heat exchanger 3, and then branched through the three-way valve 8. A part enters the first gas-liquid separation tank 9 for gas-liquid separation. The liquefied natural gas obtained from the separation is stored in the LNG storage tank 18. The gas phase is further cooled in the fourth heat exchanger 11 and then enters the second feed port 15b of the distillation column 15 successively through the second stop valve 39 and the first one-way valve 41. Another part branched through the three-way valve 8 is cooled in the third heat exchanger 7 and then enters the sixth feed port 15f of the distillation column 15 through the third one-way valve 43. The flash gas of the LNG storage tank 18 is pressurized by the second compressor 14, cooled in the sixth heat exchanger 13, and enters the third feed port 15c of the distillation column 15 through the second one-way valve 42;

[0040] When the nitrogen-rich tail gas is directly discharged: Open the first cut-off valve 49, the fourth cut-off valve 40 and the fifth cut-off valve 44, and close the second cut-off valve 39. The gas phase obtained from the first gas-liquid separation tank 9 is further cooled by the fourth heat exchanger 11, and then undergoes further gas-liquid separation in the second gas-liquid separation tank 10 through the fourth cut-off valve 40. The obtained liquid phase enters the LNG storage tank 18 for storage, and the obtained gas phase is cooled by the fifth heat exchanger 12 and then enters the second feed port 15b of the rectification column 15 through the first one-way valve 41.

[0041] The composition and connection mode of the separation system are as follows:

[0042] The gas phase outlet of the rectification column 15 is connected to the shell-side gas phase inlet of the condenser 16. The shell-side liquid phase outlet of the condenser 16 is connected to the top reflux port of the rectification column 15. The gas phase outlet of the condenser 16 is connected to the upper inlet 2a of the first heat exchanger 2. The lower outlet 2b of the first heat exchanger 2 is connected to the outside. The tube-side inlet and outlet of the condenser 16 are respectively connected to the outside cold source inlet and outlet. The liquid phase outlet of the rectification column 15 is connected to the shell-side inlet of the reboiler 17. The gas phase outlet of the reboiler 17 is connected to the bottom reflux port of the rectification column 15. The tube-side inlet and outlet of the reboiler 17 are connected to the outside heat source inlet and outlet.

[0043] The working method of the separation system: Fluids entering from different feed port positions are separated in the rectification column 15. The gas phase at the top of the column is condensed by the condenser 16. The nitrogen-rich tail gas is discharged after recovering cold energy through the first heat exchanger 2. The condensed and recovered liquid phase flows back to the rectification column 15. The liquid phase at the bottom of the column is reboiled by the reboiler 17. The reboiled gas phase flows back to the rectification column 15, and the liquid phase enters the LNG storage tank 18.

[0044] The composition and connection mode of the low-temperature system are as follows:

[0045] The outlet of the third compressor 4 is connected to the left-end inlet 5c of the seventh heat exchanger 5. The right-end outlet 5d of the seventh heat exchanger 5 is connected to the inlet of the expander 6. The outlet of the expander 6 is connected to the upper inlet 7a of the third heat exchanger 7. The lower outlet 7b of the third heat exchanger 7 is connected to the lower inlet 13b of the sixth heat exchanger 13. The upper outlet 13a of the sixth heat exchanger 13 is connected to the lower inlet 12b of the fifth heat exchanger 12. The upper outlet 12a of the fifth heat exchanger 12 is connected to the upper inlet 11a of the fourth heat exchanger 11. The lower outlet 11b of the fourth heat exchanger 11 is connected to the upper inlet 3a of the second heat exchanger 3. The lower outlet 3b of the second heat exchanger 3 is connected to the inlet of the third compressor 4.

[0046] After being compressed by the third compressor 4, the refrigerant is cooled by the cold source in the seventh heat exchanger 5, expands and cools in the expander 6, and then sequentially heats up in the third heat exchanger 7, the sixth heat exchanger 13, the fifth heat exchanger 12, the fourth heat exchanger 11, and the second heat exchanger 3, and then continues to return to the third compressor 4 for compression to complete the cycle.

[0047] In the first embodiment, the raw material gas is shunted. The first gas stream is obtained by shunting through the three-way valve 8, cooled by the third heat exchanger 7 to obtain the gas stream with the highest methane content, and enters the sixth feed port 15f of the rectification column 15. The second gas stream is the flash gas above the LNG storage tank 18, and its methane content is lower than that of the first gas stream. It is introduced into the third feed port 15c of the rectification column 15. The third gas stream is the gas stream at the gas phase outlet of the second gas-liquid separation tank 10 (when the nitrogen-rich tail gas is used as fuel gas) or the first gas-liquid separation tank 9 (when the nitrogen-rich tail gas is directly discharged), and its methane content is the lowest. It is introduced into the sixth feed port 15f of the rectification column 15. On the one hand, by segmenting the liquefaction process, the liquid phase obtained in the intermediate process is collected, and at the same time, gas phases with different temperatures and different nitrogen contents are obtained. On the other hand, a number of feed ports are arranged at different tray heights of the rectification column in the separation system. By introducing the raw material gas at different temperatures and different nitrogen contents in the liquefaction system, and introducing them into different trays of the rectification column according to their different nitrogen contents or temperatures, the denitrification efficiency of the overall system is improved, and the purity and recovery rate of the liquefied product are ensured. Compared with the prior art in which all the raw material gas is introduced from one feed port of the rectification column for rectification, the denitrification efficiency of the present invention is higher, the number of trays of the rectification column is smaller, and the size is smaller.

[0048] Next, the process in the first embodiment is simulated by Aspen Hysys. The number of trays of the rectification column is set to 6, and the reflux ratio is 2.5.

[0049] Table 1 Feed Parameters

[0050]

[0051]

[0052] Table 2 Stream Parameters

[0053]

[0054] In the existing liquefaction and denitrification technology, the raw material gas at the right outlet 3d of the second heat exchanger 3 is not subjected to segmented liquefaction treatment, and all the fluid is introduced into the rectification column 15 for separation. The number of trays is 12, the reflux ratio is 3.9, the feed tray is 10, and the overhead gas and liquefied product obtained by rectification are obtained. The simulation results are shown in Table 3.

[0055] Table 3 Comparison between the Prior Art and the First Embodiment

[0056]

[0057] It can be seen that in the present invention, the liquefaction system is cooled by a low-temperature system, and its liquefaction process is segmented. The liquid phase obtained in the intermediate process is collected, and at the same time, gas phases with different temperatures and different nitrogen contents are obtained. By introducing the raw material gas with different temperatures and nitrogen contents in the liquefaction system and introducing them into different trays of the rectification column according to their different nitrogen contents or temperatures, the specifications and dimensions of the rectification column can be significantly reduced, and the denitrification efficiency of the overall system can be improved.

[0058] Example Two:

[0059] In Example One, the separation efficiency of the rectification column is improved by allowing the raw material gas to enter at different trays of the rectification column according to different nitrogen contents. In actual operation, if the nitrogen content of the raw material gas cannot be measured, or if measuring elements are arranged in each feed gas pipeline and the components of the fluid in the pipeline are analyzed one by one, which is not economical, the temperature can be used as an index to determine the feed position of the raw material gas in the rectification column. This example details the method of using temperature as an index to determine the feed position of the raw material gas in the rectification column.

[0060] As Figure 2 shown, an unconventional natural gas liquefaction system is composed of a liquefaction system, a separation system, and a low-temperature system.

[0061] Among them, the composition and connection mode of the separation system and the low-temperature system are the same as those in Example One.

[0062] The composition and connection mode of the liquefaction system are as follows:

[0063] The raw material gas is successively connected to the left-end inlet 2c of the first heat exchanger 2 through the first stop valve 49 and the first compressor 1. The right-end outlet 2d of the first heat exchanger 2 is connected to the left-end inlet 3c of the second heat exchanger 3. The pipeline at the right-end outlet 3d of the second heat exchanger 3 is branched into two pipelines through the three-way valve 8.

[0064] The first branch after being branched by the three-way valve 8 is connected to the inlet of the first gas-liquid separation tank 9, and the second branch is connected to the left-end inlet 7c of the third heat exchanger 7. The right-end outlet 7d of the third heat exchanger 7 is connected to the twelfth thermometer 50. There are three parallel pipelines at the outlet of the twelfth thermometer 50. The first pipeline is successively connected to the sixth feed port 15f of the rectification column 15 through the seventh stop valve 22, the third one-way valve 43, and the sixth thermometer 35. The second pipeline is successively connected to the seventh feed port 15g of the rectification column through the eighth stop valve 25, the fourth one-way valve 48, and the seventh thermometer 36. The third pipeline is successively connected to the eighth feed port 15h of the rectification column through the ninth stop valve 26, the fifth one-way valve 45, and the eighth thermometer 37.

[0065] The second path shunted by the three-way valve 8 is connected to the inlet of the first gas-liquid separation tank 9. The gas-phase outlet of the first gas-liquid separation tank 9 is connected to the left-end inlet 11c of the fourth heat exchanger 11. At the right-end outlet 11d of the fourth heat exchanger 11, there are two parallel pipelines. The first path is connected to the inlet of the second gas-liquid separation tank 10 through the fourth stop valve 40. The gas-phase outlet of the second gas-liquid separation tank 10 is connected to the left-end inlet 12c of the fifth heat exchanger 12. The right-end outlet 12d of the fifth heat exchanger 12 is connected to the tenth thermometer 28. After the second path passes through the second stop valve 39 and the ninth thermometer 27 in sequence and converges with the pipeline at the outlet of the tenth thermometer 28, there are again two parallel pipelines, which are respectively: the first path is connected to the first feed port 15a of the distillation column 15 through the tenth stop valve 19, the sixth one-way valve 38, and the first thermometer 30 in sequence; the second path is connected to the second feed port 15b of the distillation column 15 in the separation system through the third stop valve 20, the first one-way valve 41, and the second thermometer 31 in sequence.

[0066] The liquid-phase outlet of the second gas-liquid separation tank 10 is connected to the inlet of the fifth stop valve 44. The pipelines at the outlet of the fifth stop valve 44, the liquid-phase outlet of the first gas-liquid separation tank 9, and the liquid-phase outlet of the reboiler 17 converge and are jointly connected to the inlet of the LNG storage tank 18. The gas-phase outlet of the LNG storage tank 18 is connected to the left-end inlet 13c of the sixth heat exchanger 13 through the second compressor 14. The right-end outlet 13d of the sixth heat exchanger 13 is connected to the inlet of the eleventh thermometer 29. There are three parallel pipelines at the outlet of the eleventh thermometer 29, which are respectively: the first path is connected to the third feed port 15c of the distillation column 15 through the sixth stop valve 21, the second one-way valve 42, and the third thermometer 32 in sequence; the second path is connected to the fourth feed port 15d of the distillation column 15 through the eleventh stop valve 24, the seventh one-way valve 46, and the fourth thermometer 33 in sequence; the third path is connected to the fifth feed port 15e of the distillation column 15 through the twelfth stop valve 23, the eighth one-way valve 47, and the fifth thermometer 34 in sequence.

[0067] In the second embodiment, the feed position of the distillation column is optimized. According to the temperatures of different feed fluids, the feed positions of the distillation column are further matched to further optimize the operating conditions of the distillation column.

[0068] A working method of an unconventional natural gas liquefaction system.

[0069] The working methods of the separation system and the cryogenic system are the same as those described in the first embodiment.

[0070] The working method of the liquefaction system specifically includes:

[0071] When the nitrogen-rich tail gas is used as fuel gas: at startup, operate according to the method described in the first embodiment. After the system runs stably, make the following adjustments to the feed port of the distillation column 15:

[0072] Compare the temperature value T9 measured by the ninth thermometer 27 with the temperature value T1 measured by the first thermometer 30 and the temperature value T2 measured by the second thermometer 31 respectively. If |T9 - T1| < |T9 - T2|, open the tenth stop valve 19 and close the third stop valve 20 to allow the raw material gas to enter the rectification column 15 through the first feed port 15a of the rectification column 15; otherwise, close the tenth stop valve 19 and open the third stop valve 20 to allow the raw material gas to enter the rectification column 15 through the second feed port 15b of the rectification column 15.

[0073] Compare the temperature value T11 measured by the eleventh thermometer 29 with the temperature value T3 measured by the third thermometer 32, the temperature value T4 measured by the fourth thermometer 33, and the temperature value T5 measured by the fifth thermometer 34 respectively. If |T11 - T3| < |T11 - T4| < |T11 - T5| or |T11 - T3| < |T11 - T5| < |T11 - T4|, close the eleventh stop valve 24 and the twelfth stop valve 23 and open the sixth stop valve 21. If |T11 - T4| < |T11 - T3| < |T11 - T5| or |T11 - T4| < |T11 - T5| < |T11 - T3|, close the sixth stop valve 21 and the twelfth stop valve 23 and open the eleventh stop valve 24. If |T11 - T5| < |T11 - T4| < |T11 - T3| or |T11 - T5| < |T11 - T3| < |T11 - T4|, close the sixth stop valve 21 and the eleventh stop valve 24 and open the twelfth stop valve 23.

[0074] Compare the temperature value T12 measured by the twelfth thermometer 50 with the temperature value T6 measured by the sixth thermometer 35, the temperature value T7 measured by the seventh thermometer 36, and the temperature value T8 measured by the eighth thermometer 37 respectively. If |T12 - T6| < |T12 - T7| < |T12 - T8| or |T12 - T6| < |T12 - T8| < |T12 - T7|, close the eighth stop valve 25 and the ninth stop valve 26 and open the seventh stop valve 22. If |T12 - T7| < |T12 - T6| < |T12 - T8| or |T12 - T7| < |T12 - T8| < |T12 - T6|, close the seventh stop valve 22 and the ninth stop valve 26 and open the eighth stop valve 25. If |T12 - T8| < |T12 - T6| < |T12 - T7| or |T12 - T8| < |T12 - T7| < |T12 - T6|, close the seventh stop valve 22 and the eighth stop valve 25 and open the ninth stop valve 26.

[0075] When the nitrogen-rich tail gas is directly discharged: When starting up, operate according to the method described in Embodiment 1. After the system runs stably, make the following adjustments to the feed port of the rectification column 15:

[0076] Compare the temperature value T10 measured by the tenth thermometer 28 with the temperature value T1 measured by the first thermometer 30 and the temperature value T2 measured by the second thermometer 31 respectively. If |T10 - T1| < |T10 - T2|, open the tenth shut-off valve 19 and close the third shut-off valve 20 to allow the raw material gas to enter the rectification column 15 through the first feed port 15a of the rectification column 15; otherwise, close the tenth shut-off valve 19 and open the third shut-off valve 20 to allow the raw material gas to enter the rectification column 15 through the second feed port 15b of the rectification column 15.

[0077] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should be regarded as the scope protected by the claims of the present invention.

Claims

1. An unconventional natural gas liquefaction and denitrification system, characterized in that It is composed of a liquefaction system, a separation system and a cryogenic system. In the liquefaction system, the raw material gas is connected to the left-end inlet (2c) of the first heat exchanger (2) via the first compressor (1). The right-end outlet (2d) of the first heat exchanger (2) is connected to the left-end inlet (3c) of the second heat exchanger (3). The pipeline at the right-end outlet (3d) of the second heat exchanger (3) is branched into two pipelines through a three-way valve (8). The first pipeline is connected to the inlet of the first gas-liquid separation tank (9), and the second pipeline is connected to the left-end inlet (7c) of the third heat exchanger (7). The right-end outlet (7d) of the third heat exchanger (7) is connected to at least one inlet at the lower part of the distillation column (15) through a pipeline. The gas-phase outlet of the first gas-liquid separation tank (9) is connected to the left-end inlet (11c) of the fourth heat exchanger (11). The right-end outlet (11d) of the fourth heat exchanger (11) is connected to the inlet of the second gas-liquid separation tank (10). The gas-phase outlet of the second gas-liquid separation tank (10) is connected to the left-end inlet (12c) of the fifth heat exchanger (12). The right-end outlet (12d) of the fifth heat exchanger (12) is connected to at least one inlet at the upper part of the distillation column (15) through a pipeline. The pipelines at the liquid-phase outlet of the second gas-liquid separation tank (10), the liquid-phase outlet of the first gas-liquid separation tank (9), and the liquid-phase outlet of the reboiler (17) converge and are jointly connected to the inlet of the LNG storage tank (18). The gas-phase outlet of the LNG storage tank (18) is connected to the left-end inlet (13c) of the sixth heat exchanger (13) via the second compressor (14). The right-end outlet (13d) of the sixth heat exchanger (13) is connected to at least one inlet at the middle part of the distillation column (15) through a pipeline. In the separation system, the gas-phase outlet of the distillation column (15) is connected to the shell-side gas-phase inlet of the condenser (16). The shell-side liquid-phase outlet of the condenser (16) is connected to the top reflux port of the distillation column (15). The gas-phase outlet of the condenser (16) is connected to the upper-end inlet (2a) of the first heat exchanger (2). The lower-end outlet (2b) of the first heat exchanger (2) is connected to the outside. The tube-side inlet and outlet of the condenser (16) are respectively connected to the outside cold source inlet and outlet. The liquid-phase outlet of the distillation column (15) is connected to the shell-side inlet of the reboiler (17). The gas-phase outlet of the reboiler (17) is connected to the bottom reflux port of the distillation column (15). The tube-side inlet and outlet of the reboiler (17) are connected to the outside heat source inlet and outlet. The low-temperature system is formed by connecting the outlet of the third compressor (4) to the left-end inlet (5c) of the seventh heat exchanger (5), connecting the right-end outlet (5d) of the seventh heat exchanger (5) to the inlet of the expander (6), connecting the outlet of the expander (6) to the upper-end inlet (7a) of the third heat exchanger (7), connecting the lower-end outlet (7b) of the third heat exchanger (7) to the lower-end inlet (13b) of the sixth heat exchanger (13), connecting the upper-end outlet (13a) of the sixth heat exchanger (13) to the lower-end inlet (12b) of the fifth heat exchanger (12), connecting the upper-end outlet (12a) of the fifth heat exchanger (12) to the upper-end inlet (11a) of the fourth heat exchanger (11), connecting the lower-end outlet (11b) of the fourth heat exchanger (11) to the upper-end inlet (3a) of the second heat exchanger (3), and connecting the lower-end outlet (3b) of the second heat exchanger (3) to the inlet of the third compressor (4). A first stop valve (49) is provided in the pipeline connected to the first compressor (1) for the raw material gas; a fourth stop valve (40) is provided at the inlet of the second gas-liquid separation tank (10); a fifth stop valve (44) is provided at the liquid-phase outlet of the second gas-liquid separation tank (10). Two pipelines are connected in parallel at the right-end outlet (11d) of the fourth heat exchanger (11). The first pipeline is connected to the inlet of the second gas-liquid separation tank (10) via the fourth stop valve (40); the second pipeline converges with the pipeline at the right-end outlet (12d) of the fifth heat exchanger (12) after passing through the second stop valve (39), and then is connected to at least one inlet at the upper part of the distillation column (15) via the provided first check valve (41); the right-end outlet (13d) of the sixth heat exchanger (13) is connected to at least one inlet at the middle part of the distillation column (15) via the provided second check valve (42); the right-end outlet (7d) of the third heat exchanger (7) is connected to at least one inlet at the lower part of the distillation column (15) via the provided third check valve (43).

2. The unconventional natural gas liquefaction and denitrification system according to claim 1, characterized in that A ninth thermometer (27) is further provided on the outlet pipeline of the second stop valve (39), and a tenth thermometer (28) is further provided on the pipeline of the right-end outlet (12d) of the fifth heat exchanger (12). After the two pipelines converge, two pipelines are connected in parallel again. The first pipeline is sequentially connected to the first feed inlet (15a) of the distillation column 15 via the tenth stop valve (19), the sixth check valve (38), and the first thermometer (30); the second pipeline is sequentially connected to the second feed inlet (15b) of the distillation column (15) in the separation system via the third stop valve (20), the first check valve (41), and the second thermometer (31). At the right - hand outlet (13d) of the sixth heat exchanger (13), there are three parallel pipelines on the pipeline, and an eleventh thermometer (29) is installed. The first pipeline is connected to the third feed inlet (15c) of the distillation column 15 successively through a sixth stop valve (21), a second check valve (42), and a third thermometer (32); the second pipeline is connected to the fourth feed inlet (15d) of the distillation column (15) successively through an eleventh stop valve (24), a seventh check valve (46), and a fourth thermometer (33); the third pipeline is connected to the fifth feed inlet (15e) of the distillation column (15) successively through a twelfth stop valve (23), an eighth check valve (47), and a fifth thermometer (34); At the right - hand outlet (7d) of the third heat exchanger (7), there are three parallel pipelines on the pipeline, and a twelfth thermometer (50) is installed. The first pipeline is connected to the sixth feed inlet (15f) of the distillation column (15) successively through a seventh stop valve (22), a third check valve (43), and a sixth thermometer (35); the second pipeline is connected to the seventh feed inlet (15g) of the distillation column successively through an eighth stop valve (25), a fourth check valve (48), and a seventh thermometer (36); the third pipeline is connected to the eighth feed inlet (15h) of the distillation column successively through a ninth stop valve (26), a fifth check valve (45), and an eighth thermometer (37).

3. The unconventional natural gas liquefaction and denitrification system according to claim 1, wherein The first heat exchanger (2), the second heat exchanger (3), the third heat exchanger (7), the fourth heat exchanger (11), the fifth heat exchanger (12), the sixth heat exchanger (13), and the seventh heat exchanger (5) are all shell - and - tube heat exchangers, fin - tube heat exchangers, or double - pipe heat exchangers.

4. The unconventional natural gas liquefaction and denitrification system according to claim 1, characterized in that, The expander (6) is a scroll expander or a screw expander.

5. The unconventional natural gas liquefaction and denitrification system according to claim 1, characterized in that, The first compressor (1), the second compressor (14), and the third compressor (4) are all piston compressors, screw compressors, centrifugal compressors, or linear compressors.

6. A working method of the unconventional natural gas liquefaction and denitrification system as described in claim 1 or 2, characterized in that: They are respectively the working methods of the liquefaction system, the separation system, and the cryogenic system. I. The working method of the liquefaction system includes two working modes: (1) When the nitrogen - rich tail gas is used as fuel gas: Open the first stop valve (49) and the second stop valve (39), and close the fourth stop valve (40) and the fifth stop valve (44); after the tail gas is pressurized by the first compressor (1), the cold energy of the nitrogen - rich tail gas is recovered in the first heat exchanger (2), cooled in the second heat exchanger (3), and then split by the three - way valve (8). A part enters the first gas - liquid separation tank (9) for gas - liquid separation. The liquefied natural gas obtained from the separation enters the LNG storage tank (18) for storage. The gas phase is further cooled in the fourth heat exchanger (11), and then enters the second feed inlet (15b) of the distillation column (15) successively through the second stop valve (39) and the first check valve (41). Another part split by the three - way valve (8) is cooled in the third heat exchanger (7) and then enters the sixth feed inlet (15f) of the distillation column through the third check valve (43). The flash gas of the LNG storage tank (18) is pressurized by the second compressor (14), cooled in the sixth heat exchanger (13), and enters the third feed inlet (15c) of the distillation column through the second check valve (42); When the nitrogen-rich tail gas is directly discharged: Open the first shut-off valve (49), the fourth shut-off valve (40), and the fifth shut-off valve (44), and close the second shut-off valve (39). After the gas phase obtained from the first gas-liquid separation tank (9) is further cooled by the fourth heat exchanger (11), it undergoes further gas-liquid separation in the second gas-liquid separation tank (10) through the fourth shut-off valve (40). The obtained liquid phase enters the LNG storage tank (18) for storage, and the obtained gas phase enters the second feed port (15b) of the rectification column (15) through the fifth heat exchanger (12) and then through the first check valve (41) after being cooled; II. Working method of the separation system: Fluids entering from different feed port positions are separated in the rectification column (15). The gas phase at the top of the column is condensed by the condenser (16). The nitrogen-rich tail gas is discharged after recovering cold energy through the first heat exchanger (2). The condensed and recovered liquid phase is refluxed to the rectification column (15). The liquid phase at the bottom of the column is reboiled by the reboiler (17), and the obtained gas phase is refluxed to the rectification column (15), while the liquid phase enters the LNG storage tank (18); III. Working method of the cryogenic system: The refrigerant is compressed by the third compressor (4), cooled by a cold source in the seventh heat exchanger (5), expanded and cooled in the expander (6), and then heated successively in the third heat exchanger (7), the sixth heat exchanger (13), the fifth heat exchanger (12), the fourth heat exchanger (11), and the second heat exchanger (3), and then returns to the third compressor (4) for compression to complete the cycle.

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