A natural gas separation method based on side-line production

Abstract

The present invention relates to the technical field of natural gas liquefaction processing, and in particular to a natural gas separation method based on side-line extraction, which separates helium, nitrogen and finished gas through a single distillation tower; specifically, the raw gas enters the distillation tower from the upper part of the distillation tower for helium extraction treatment, and in the helium extraction process, a side-line stream is extracted from the middle part of the distillation tower for denitrification treatment to obtain a nitrogen stream and an intermediate stream, and the intermediate stream returns from the middle part of the distillation tower to the distillation tower to continue entering the helium extraction treatment, and finally a finished product stream is obtained at the bottom of the distillation tower, and a helium stream is obtained at the top of the distillation tower. The natural gas separation method based on side-line extraction can achieve helium, nitrogen and finished gas through only one distillation tower, thereby greatly reducing the number of equipment and the difficulty of coordinated operation.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas liquefaction processing, and in particular to a natural gas separation method based on side-line production. Background Art

[0002] As a very important strategic resource, helium plays an indispensable role in many important fields, such as: military industry, scientific research, petrochemical, cooling, medical, semiconductors, pipeline leak detection, superconducting experiments, metal manufacturing, deep-sea diving, high-precision welding, optoelectronic product manufacturing, etc.

[0003] Limited by the late development of helium extraction technology and the low molar fraction of helium in natural gas in my country (usually less than 200ppm), most of the existing natural gas production and liquefaction plants in China have not set up special helium extraction equipment in the design stage. Therefore, when there is a demand for helium extraction, new helium extraction equipment is often used to collect helium on the basis of the original LNG liquefaction equipment. The currently used processes are mainly: adding a multi-stage membrane separation device to purify helium to more than 40%; or in the original LNG liquefaction device, by collecting cold box flash gas or storage tank BOG, pressurizing and liquefying again, and then using two or even three distillation towers to separate natural gas, nitrogen and helium respectively, to achieve the collection of crude helium products with a content of more than 40%, and then send them to a special helium purification unit or factory for purification to 99.999% high-purity helium.

[0004] The above systems were not specially equipped with helium extraction devices during the design stage. When the demand for helium extraction increases in the later stage, this system mostly adopts the method of adding helium extraction equipment on the basis of the original LNG liquefaction device (natural gas liquefaction device) to collect helium. There are two main processes that are widely used at present: the first is to extract crude helium with a concentration of more than 40% by adding a multi-stage membrane separation device; the second is to collect cold box flash gas or storage tank BOG in the original LNG liquefaction device, re-liquefy and enrich helium after pressurization, and then send it to a special helium purification unit or factory for purification to 99.999% high-purity helium. However, both methods have disadvantages and shortcomings, as follows:

[0005] The first process has the following disadvantages and shortcomings:

[0006] 1. The stability of the helium mole fraction in the raw gas is required to be high. If the helium mole fraction in the raw gas fluctuates greatly, the product purity and yield will be greatly affected. At the same time, this method is difficult to ensure that the crude helium product purity is above 40% while meeting a yield of more than 95%.

[0007] 2. When the molar fraction of helium in the natural gas raw material is less than 200 ppm, the economic value of using membrane separation technology to extract crude helium is not high.

[0008] 3. The metal membrane for helium extraction is expensive, has a short service life (usually less than 6 years), a low localization rate, and the performance of domestically produced metal membranes is generally low.

[0009] The second process has the following disadvantages and shortcomings:

[0010] 1. This method also has the disadvantages of using a large number of equipment and difficulty in coordinating operations between towers.

[0011] 2. According to the national standard GB / T 38753-2020 "Liquefied Natural Gas", the molar fraction of nitrogen in liquefied natural gas should be less than 1.0%. Therefore, when the molar fraction of nitrogen in the raw natural gas is greater than 1.0%, a denitrification unit is required in the liquefaction process. The denitrification unit usually adopts a low-temperature flash denitrification process. Because helium is easier to gasify than nitrogen, once the denitrification unit is set for flash evaporation, helium will be flashed and discharged, resulting in almost no helium in the BOG storage tank.

[0012] 3. The BOG pressure in the storage tank is generally low (less than 200 kPaA) and needs to be re-pressurized to above 1100 kPaA before it can meet the requirements for re-liquefaction and enrichment of helium, which will consume a lot of extra electricity.

[0013] 4. When no denitrification unit is set, the collected BOG contains a large amount of nitrogen. To re-liquefy and increase the helium concentration to more than 40%, it is necessary to liquefy and remove the nitrogen and methane in the raw gas, such as considering re-pressurizing to more than 1100kPaA for liquefaction and separation. At this pressure, a cold source of at least -170°C is required, but the lowest temperature of the mixed refrigerant cold source used in conventional natural gas liquefaction is generally not less than -170 degrees, so the refrigeration system of a conventional liquefaction plant can only provide a cold source of about -170°C. If this method is used for helium enrichment, it is generally necessary to use liquid nitrogen or add other refrigeration systems to provide a low-temperature cold source, and these methods will greatly increase the operating cost and investment of extracting crude helium.

[0014] 4. This method also has the disadvantages of using a large number of equipment and difficulty in coordinating operations between towers.

[0015] Globally, the supply and demand of helium as a scarce resource is becoming increasingly tense, and my country is one of the major helium consumers. Therefore, it is necessary to strengthen the development and utilization of helium resources and enhance the research and development of helium technology. Summary of the invention

[0016] The purpose of the present invention is to avoid the shortcomings of the prior art and provide a natural gas separation method based on side-line production. The natural gas separation method based on side-line production can produce helium, nitrogen and finished gas through only one distillation tower, thereby greatly reducing the number of equipment and the difficulty of coordinated operation.

[0017] The above-mentioned purpose of the present invention is achieved by the following technical measures:

[0018] A natural gas separation method based on side-line extraction is provided, which uses a single distillation tower to separate helium, nitrogen and finished gas.

[0019] Specifically, the raw gas enters the distillation tower from the upper part of the distillation tower for helium extraction treatment. During the helium extraction process, a side stream is extracted from the middle part of the distillation tower for denitrification treatment to obtain a nitrogen stream and an intermediate stream. The intermediate stream returns to the distillation tower from the middle part of the distillation tower and continues to enter the helium extraction treatment. Finally, a finished product stream is obtained at the bottom of the distillation tower, and a helium stream is obtained at the top of the distillation tower.

[0020] Preferably, the above-mentioned denitrification treatment separates the side stream into gas and liquid through a nitrogen separator to obtain a nitrogen stream and an intermediate stream, wherein the nitrogen stream is discharged from the top of the nitrogen separator, and the intermediate stream is discharged from the bottom of the nitrogen separator and returned to the distillation tower from the middle.

[0021] Preferably, the above denitrification treatment specifically comprises the following steps:

[0022] Step (A1), cooling the side stream to -140°C to -155°C;

[0023] Step (A2), the side stream enters the nitrogen separator from the upper part of the nitrogen separator to undergo gas-liquid separation to obtain a nitrogen stream and an intermediate stream;

[0024] In step (A3), the nitrogen stream is discharged from the top of the nitrogen separator, and the intermediate stream is discharged from the bottom of the nitrogen separator, and then refluxes to the middle of the distillation tower by gravity difference. Preferably, the feed gas is pre-treated to remove heavy hydrocarbons, and then enters the distillation tower from the upper part of the distillation tower for helium extraction.

[0025] Preferably, the above-mentioned heavy hydrocarbon removal pretreatment specifically comprises the following steps:

[0026] Step (B1), precooling the raw gas to -28°C to -45°C;

[0027] Step (B2), conveying the feed gas to a heavy hydrocarbon separation tank for pretreatment to remove heavy hydrocarbons, to obtain a pretreatment stream and a heavy hydrocarbon stream, wherein the pretreatment stream enters the distillation tower from the upper part of the distillation tower;

[0028] Step (B3), decompressing the heavy hydrocarbon stream to 1600 kPaA-1800 kPaA, entering the heavy hydrocarbon secondary flash tank to enter the flash evaporation to recover the light hydrocarbons, and obtaining a recovered stream and a final heavy hydrocarbon stream;

[0029] Step (B4), the recovery stream is discharged from the top of the heavy hydrocarbon secondary flash tank, and then returns to the distillation tower from the bottom of the distillation tower to continue the helium stripping process; the final heavy hydrocarbon stream is discharged from the bottom of the heavy hydrocarbon secondary flash tank.

[0030] Preferably, the liquid phase stream generated by the above-mentioned distillation tower flows through the bottom of the distillation tower to the distillation tower kettle heater for heating, the gas phase stream generated by the distillation tower kettle heater returns to the distillation tower, and the liquid phase stream generated by the distillation tower kettle heater is discharged from the distillation tower kettle heater to obtain the finished product stream.

[0031] Preferably, the pre-treated stream first provides a heat source for the heater located in the distillation tower kettle, then is cooled to -140°C to -150°C and decompressed to 1650kPaA to 1750kPaA, and finally enters the distillation tower from the upper part of the distillation tower.

[0032] Preferably, the pre-treated stream first provides a heat source for the heater located in the distillation tower kettle, then is cooled to -145°C and decompressed to 1700 kPaA, and finally enters the distillation tower from the upper part of the distillation tower;

[0033] In the step (A1), the side stream is cooled to -148°C to -152°C;

[0034] In the step (B1), the raw gas is precooled to -30°C to -40°C;

[0035] In the step (B3), the heavy hydrocarbon stream is depressurized to 1700 kPaA and enters a heavy hydrocarbon secondary flash tank to flash and recover light hydrocarbons, thereby obtaining a recovered stream and a final heavy hydrocarbon stream.

[0036] The natural gas separation method based on side-line production of the present invention maintains the top temperature of the distillation tower above -170°C.

[0037] The molar fraction of helium in the raw gas is less than or equal to 0.02% and the molar fraction of nitrogen is greater than or equal to 2.0%, and the helium purity of the helium gas stream is greater than or equal to 60%.

[0038] Preferably, the distillation tower has 12 theoretical plates.

[0039] The number of theoretical plates of the distillation tower is arranged from 1 to 12 from top to bottom. The raw gas after pretreatment for removing heavy hydrocarbons enters the distillation tower from the first plate, the side stream is extracted from above the sixth plate, and the intermediate stream returns from above the fourth plate.

[0040] The beneficial effects of the present invention are as follows:

[0041] 1. The function of separating three types of materials can be achieved through one distillation tower, thus greatly reducing the investment cost and operating cost of the device.

[0042] 2. Helium extraction can be completed using the traditional MRC refrigerant circulation system and the mixed refrigerant cold source conditions used in conventional natural gas liquefaction, without consuming liquid nitrogen or adding other circulating refrigeration units;

[0043] 3. The side-line extraction scheme is adopted to greatly improve the nitrogen separation effect, and the methane content in the nitrogen gas stream discharged from the system can be reduced to less than 20%;

[0044] 4. It is not sensitive to the helium content in the raw gas. When the helium concentration in natural gas is lower than 0.05%, it can still collect helium with a concentration of more than 60%. Moreover, the extraction and collection rate of helium in the raw gas is as high as 99%, so it has high economic value. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The present invention is further described with reference to the accompanying drawings, but the contents in the accompanying drawings do not constitute any limitation to the present invention.

[0046] Figure 1 The figure is a schematic diagram of a system adopted by a natural gas separation method based on side line production according to the present invention.

[0047] Figure 2 Schematic diagram of a conventional system for first separating methane and nitrogen and then separating helium from nitrogen. DETAILED DESCRIPTION

[0048] The technical solution of the present invention is further described in conjunction with the following embodiments.

[0049] Example 1

[0050] A method for separating natural gas based on side-line extraction, wherein helium, nitrogen and finished gas are separated by a single distillation tower. Specifically, the raw gas enters the distillation tower from the upper part of the distillation tower for helium extraction treatment. During the helium extraction process, a side-line stream is extracted from the middle part of the distillation tower for denitrification treatment to obtain a nitrogen stream and an intermediate stream. The intermediate stream is returned from the middle part of the distillation tower to the distillation tower to continue entering the helium extraction treatment, and finally a finished product stream is obtained at the bottom of the distillation tower, and a helium stream is obtained at the top of the distillation tower. The raw gas is pre-treated to remove heavy hydrocarbons, and then enters the distillation tower from the upper part of the distillation tower for helium extraction treatment. The present invention maintains the top temperature of the distillation tower above -170°C.

[0051] The distillation tower of the present invention has 12 theoretical plates. The theoretical plates of the distillation tower are arranged from 1 to 12 from top to bottom. The raw gas pretreated after removing heavy hydrocarbons enters the distillation tower from the first plate, the side stream is extracted from above the sixth plate, and the intermediate stream returns from above the fourth plate.

[0052] It should be noted that the single-tower side-line extraction of the present invention can be understood as stacking two towers, where the upper section of the distillation tower is used to extract helium; and the lower section of the distillation tower is used to remove nitrogen from the remaining natural gas.

[0053] The reason for the side-line gas extraction of the sixth plate of the present invention is that below this plate, the methane content in the gas phase is relatively high and the nitrogen content is relatively low, while above this plate, the helium content in the gas phase increases significantly.

[0054] The denitrification method of the present invention adopts side-line gas production, re-condensation, and finally gas-liquid separation to achieve the denitrification method. Therefore, the helium component in the side-line stream will enter the evacuated nitrogen-rich gas. If the helium concentration in this part of the gas is too high, it will affect the helium yield. At the same time, if the gas-phase nitrogen content in the side-line stream is insufficient, the denitrification efficiency will be reduced. As the distillation tower moves upward, the temperature continues to decrease, and the ratio of nitrogen and helium in the gas phase will continue to increase, but the ratio of helium will increase faster in a lower temperature range. The above is a complex model, and it is necessary to find the best extraction point through repeated debugging (taking into account the removal of enough nitrogen and reducing helium loss at the same time). The present invention is to verify the extraction position of the side-line stream and the return position of the intermediate stream through the Hysys simulation software.

[0055] The denitrification treatment separates the side stream into gas and liquid through a nitrogen separator to obtain a nitrogen stream and an intermediate stream. The nitrogen stream is discharged from the top of the nitrogen separator, and the intermediate stream is discharged from the bottom of the nitrogen separator and returned to the distillation tower from the middle.

[0056] The denitrification process of the present invention specifically comprises the following steps:

[0057] Step (A1), cooling the side stream to -140°C to -155°C;

[0058] Step (A2), the side stream enters the nitrogen separator from the upper part of the nitrogen separator to undergo gas-liquid separation to obtain a nitrogen stream and an intermediate stream;

[0059] In step (A3), the nitrogen stream is discharged from the top of the nitrogen separator, and the intermediate stream is discharged from the bottom of the nitrogen separator, and then refluxes to the middle of the distillation tower by gravity difference.

[0060] It should be noted that the reason for cooling the side stream in step (A1) is that nitrogen needs to be removed from the gas phase of the side stream, so a heat exchanger needs to be used to condense components with relatively high boiling points, so that components with relatively low boiling points are retained more in the gas phase, which is conducive to separating gas phase nitrogen in the nitrogen separator. The gravity difference reflux of the present invention refers to reflux with the help of the gravity of the liquid phase, because the side stream extracted from the side line of the 6th plate of the distillation tower is all gas phase (stream 313), and the pressure is 1589kPaA. In the actual device, the gas of the stream can use pressure to enter the shell side of the top heat exchanger (specifically to reduce the pressure drop and go through the shell side), and the pressure drop should be below 5kPa. At this time, a liquid phase is generated after cooling. Assuming the pressure drop is 5kPa, the pressure of the liquid phase is the same as the pressure of the 4th plate of the distillation tower, but because the liquid phase is located at the top of the tower, it can be directly refluxed with the help of its static liquid column pressure (because the liquid density is large, when the gas pressure is the same, it will flow from high to low).

[0061] The heavy hydrocarbon removal pretreatment of the present invention specifically comprises the following steps:

[0062] Step (B1), precooling the raw gas to -28°C to -45°C;

[0063] Step (B2), conveying the raw gas to a heavy hydrocarbon separation tank for pretreatment to remove heavy hydrocarbons, to obtain a pretreatment stream and a heavy hydrocarbon stream, wherein the pretreatment stream enters the distillation tower from the upper portion of the distillation tower; the pretreatment stream first provides a heat source for the heater located in the kettle of the distillation tower, and then is cooled to -140°C to -150°C and reduced in pressure to 1650kPaA to 1750kPaA, and finally enters the distillation tower from the upper portion of the distillation tower.

[0064] Step (B3), decompressing the heavy hydrocarbon stream to 1600 kPaA-1800 kPaA, entering the heavy hydrocarbon secondary flash tank to enter the flash evaporation to recover the light hydrocarbons, and obtaining a recovered stream and a final heavy hydrocarbon stream;

[0065] Step (B4), the recovery stream is discharged from the top of the heavy hydrocarbon secondary flash tank, and then returns to the distillation tower from the bottom of the distillation tower to continue the helium stripping process; the final heavy hydrocarbon stream is discharged from the bottom of the heavy hydrocarbon secondary flash tank.

[0066] It should be noted that the heavy hydrocarbon removal pretreatment of the present invention can remove C4+ components, and the total molar fraction of C4+ in the obtained pretreated stream is less than 0.1%. In the heavy hydrocarbon removal pretreatment, a heavy hydrocarbon secondary flash tank is added to recover light hydrocarbons, and the obtained recovery stream is merged with the bottom discharge stream of the distillation tower, thereby increasing the recovery rate of light hydrocarbons and helium in natural gas.

[0067] Generally, for a distillation tower, it is most ideal to feed the gas phase at the dew point temperature and the liquid phase at the bubble point temperature. For the same substance, the dew point temperature of the gas phase is higher than the bubble point temperature of the liquid phase, so the feed point temperature of the gas phase is generally higher, so the feed position is lower. Even if the dew point and bubble point feeding are not completely followed, generally speaking, high-temperature materials should be fed from the bottom and low-temperature materials should be fed from the top to avoid reversal of the temperature gradient in the tower. From the perspective of mass transfer, the gas phase is fed from near the bottom and the liquid phase is fed from near the top, so that the gas moves upward and the liquid moves downward, which can achieve a better contact mass transfer effect in the tower. For the present invention, the finished product stream is a gas phase after flash evaporation, and the pre-treated stream is first cooled to a liquid phase, so that the pre-treated stream of the present invention enters from the upper part of the distillation tower, and the recovery stream enters from the lower part of the distillation tower.

[0068] The liquid phase stream generated by the distillation tower of the present invention flows through the bottom of the distillation tower to the distillation tower kettle heater for heating, the gas phase stream generated by the distillation tower kettle heater returns to the distillation tower, and the liquid phase stream generated by the distillation tower kettle heater is discharged from the distillation tower kettle heater to obtain the finished product stream.

[0069] It should be noted that the function of the distillation tower kettle heater of the present invention is to increase the bottom temperature of the distillation tower, so that a temperature gradient is formed in the distillation tower, that is, the temperature from the bottom to the top of the tower becomes lower and lower, so that the light components in the stream are vaporized at the bottom of the distillation tower.

[0070] The separation tank, distillation tower bottom heater, heavy hydrocarbon separation tank and heavy hydrocarbon secondary flash tank are all common equipment in the field. Those skilled in the art should be aware of their working principles. Those skilled in the art can choose specific models according to actual conditions. The separation tank, distillation tower bottom heater, heavy hydrocarbon separation tank and heavy hydrocarbon secondary flash tank that can only be applied to the present invention fall within the protection scope of the present invention.

[0071] The beneficial effects of the natural gas separation method based on side-line production are as follows:

[0072] 1. The function of separating three types of materials can be achieved through one distillation tower, thus greatly reducing the investment cost and operating cost of the device.

[0073] 2. Helium extraction can be completed using the traditional MRC refrigerant circulation system and the mixed refrigerant cold source conditions used in conventional natural gas liquefaction, without consuming liquid nitrogen or adding other circulating refrigeration units;

[0074] 3. The side-line extraction scheme is adopted to greatly improve the nitrogen separation effect, and the methane content in the nitrogen gas stream discharged from the system can be reduced to less than 20%;

[0075] 4. It is not sensitive to the helium content in the raw gas. When the helium concentration in natural gas is lower than 0.05%, it can still collect helium with a concentration of more than 60%. Moreover, the extraction and collection rate of helium in the raw gas is as high as 99%, so it has high economic value.

[0076] Example 2

[0077] A natural gas separation method based on side-line production, the other features of which are the same as those of Example 1, except that: the present invention is adaptable to the requirements of different working conditions in all natural gas liquefaction processing industries, the present invention is adaptable to the requirements of different working conditions in all natural gas liquefaction processing industries, especially when the molar fraction of helium in the raw gas is less than or equal to 0.02% and the molar fraction of nitrogen is greater than or equal to 2.0%.

[0078] The pretreatment stream first provides a heat source for the heater in the distillation tower kettle, then is cooled to -145°C and decompressed to 1700 kPaA, and finally enters the distillation tower from the upper part of the distillation tower;

[0079] In the step (A1), the side stream is cooled to -148°C to -152°C;

[0080] In the step (B1), the raw gas is precooled to -30°C to -40°C;

[0081] In the step (B3), the heavy hydrocarbon stream is depressurized to 1700 kPaA and enters a heavy hydrocarbon secondary flash tank to flash and recover light hydrocarbons, thereby obtaining a recovered stream and a final heavy hydrocarbon stream.

[0082] The present invention is simulated and calculated by chemical process simulation software ASPEN HYSYS V14 (using Peng-Robinson physical property package). For the convenience of calculation, this paper will illustrate the liquefaction of natural gas with a domestic 3 million m3 / d liquefaction plant and the following raw gas components, wherein the raw gas pressure is 8000 kPaA and the raw gas flow rate is 92.5 t / h.

[0083] In order to meet the requirements of natural gas liquefaction and helium extraction of the above components, the present invention adopts the commonly used mixed refrigerant refrigeration process (MRC) in China, and the required MRC refrigerant circulation system parameters and components are as follows: refrigerant circulation volume: 497t / h, refrigerant compressor inlet pressure: 510kPaA; refrigerant compressor outlet pressure: 4900kPaA; refrigerant compressor shaft power: 29580kW. The refrigerant components are shown in Table 1.

[0084] Table 1. Refrigerant composition

[0085] Components Nitrogen Methane Ethylene Propane Isopentane total content 16.5％ 26.0％ 26.5％ 16.5％ 14.5％ 100％

[0086] Example 3

[0087] A system for separating a natural gas produced by a side-line according to Embodiment 1 or 2, such as Figure 1 As shown. Figure 1 E-301 is a nitrogen condenser, E-302 is a distillation tower top condenser, T-1101 is a distillation tower, E-303 is a distillation tower kettle heater, V-303 is a nitrogen separator, V-301 is a heavy hydrocarbon separation tank, V-302 is a heavy hydrocarbon flash tank, LNG-100 is a cold box, C-101 is an MRC compressor, E-101 is a primary outlet cooler, E-102 is a secondary outlet cooler, V-101 is a primary separation tank, V-102 is a secondary separation tank, and LNGV-101 and LNGV-102 are both valves.

[0088] exist Figure 1 The intermediate raw gas is stream 301 and stream 302; the pretreatment streams are stream 303, stream 304, stream 308, stream 309 and stream 310; the pretreatment heavy hydrocarbon stream is stream 304 and stream 305; the recovery stream is stream 307; the final heavy hydrocarbon stream is stream 306; the helium stream is stream 317; the side stream is stream 313 and stream 314; the nitrogen stream is stream 315 and stream 318; the intermediate stream is stream 316; and the finished product stream is stream 311 and stream 312.

[0089] The diameter of the distillation tower in this embodiment is 1500 mm, the height of the packing section is 4800 mm, and the height of the distillation tower is 6000 mm; the packing type is MELLAPAK 125X.

[0090] The heat load of nitrogen condenser E-301 is 800KW~900KW; the heat load of top condenser E-302 is 2.5KW~5.0KW; the heat load of kettle reboiler E-303 is 3000KW~4000KW.

[0091] It should be noted that the present invention can add a condenser corresponding to the gas phase outlet of the distillation tower according to actual conditions, so as to stably control the temperature control of the helium gas stream. When the temperature and pressure of the components entering the distillation tower are determined for the distillation tower system, the components of the distillation tower discharge are also determined, but in actual operation, the changes in temperature and pressure are often linked, that is, when the temperature rises, because the gas phase in the tower becomes more, the pressure often also rises. Therefore, by directly controlling the distillation tower extraction flow and tower pressure, the temperature gradient and tower top temperature of the distillation tower can be indirectly controlled.

[0092] Example 4

[0093] The system of Example 3 is adopted, and the overall system parameters of this embodiment are as shown in Table 2:

[0094]

[0095]

[0096] The composition of the raw gas in this embodiment is shown in the following table:

[0097] Table 3: Composition of the raw gas in this embodiment

[0098] Components Nitrogen Methane Ethane Ethylene Propane Isobutane n-Butane n-Pentane Isopentane Helium total Mole fraction (%) 4.50 88.045 3.80 0.0 2.00 0.50 0.50 0.55 0.100 0.005 100

[0099]

[0100]

[0101]

[0102]

[0103]

[0104] The data in Tables 4 and 5 are obtained through simulation calculations using the chemical process simulation software ASPEN HYSYS V14 (using the Peng-Robinson property package). It can be seen from Tables 4 and 5 that in this embodiment, the temperature of stream 317 is -155°C, which is much higher than the lowest temperature of MRC. In this embodiment, a conventional natural gas liquefaction process refrigeration system (MRC) can be used to provide a corresponding cold source without consuming an additional liquid nitrogen cold source or a circulating refrigeration system. 61% of helium can be extracted from a raw gas with a helium molar fraction of only 0.005%, and the helium extraction rate of the raw gas is 99.78%.

[0105] Example 5

[0106] The system of Example 3 is adopted, and the overall system parameters of this embodiment are as shown in Table 6:

[0107]

[0108] The composition of the raw gas in this embodiment is shown in the following table:

[0109] Table 7. Composition of the raw gas in this embodiment

[0110] Components Nitrogen Methane Ethane Ethylene Propane Isobutane n-Butane n-Pentane Isopentane Helium total Mole fraction (%) 4.50 88.04 3.80 0.0 2.00 0.50 0.50 0.55 0.100 0.010 100

[0111]

[0112]

[0113]

[0114]

[0115]

[0116] The data in Tables 8 and 9 are obtained through simulation calculations using the chemical process simulation software ASPEN HYSYS V14 (using the Peng-Robinson physical property package). It can be seen from Tables 8 and 9 that in this embodiment, the temperature of stream 317 is -155.00°C, which is much lower than the lowest temperature of MRC. Therefore, a conventional natural gas liquefaction process refrigeration system (MRC) can be used to provide a corresponding cold source without consuming an additional liquid nitrogen cold source or a circulating refrigeration system. Approximately 61% of helium can be extracted from a raw gas having a helium molar fraction of only 0.010%, and the helium extraction rate of the raw gas is 99.81%.

[0117] Example 6

[0118] The system of Example 3 is adopted, and the overall system parameters of this embodiment are as shown in Table 10:

[0119]

[0120] The composition of the raw gas in this embodiment is shown in the following table:

[0121] Table 11. Composition of the raw gas in this example

[0122] Components Nitrogen Methane Ethane Ethylene Propane Isobutane n-Butane n-Pentane Isopentane Helium total Mole fraction (%) 4.50 88.000 3.80 0.0 2.00 0.50 0.50 0.55 0.100 0.050 100

[0123]

[0124]

[0125]

[0126]

[0127]

[0128] The data in Tables 12 and 13 are obtained through simulation calculations using the chemical process simulation software ASPEN HYSYS V14 (using the Peng-Robinson property package). It can be seen from Tables 12 and 13 that in this embodiment, the temperature of stream 317 is -155.00°C, which is much lower than the lowest temperature of MRC. Therefore, a conventional natural gas liquefaction process refrigeration system (MRC) can be used to provide a corresponding cold source without consuming an additional liquid nitrogen cold source or a circulating refrigeration system. Approximately 60.0% of helium can be extracted from a raw gas having a helium molar fraction of only 0.050%, and the helium extraction rate of the raw gas is greater than 99.84%.

[0129] Example 7

[0130] The system of Example 3 is adopted, and the overall system parameters of this embodiment are as shown in Table 14:

[0131]

[0132] The composition of the raw gas in this embodiment is shown in the following table:

[0133] Table 15. Composition of the raw gas in this example

[0134]

[0135]

[0136]

[0137]

[0138]

[0139]

[0140] The data in Tables 12 and 13 are obtained through simulation calculations using the chemical process simulation software ASPEN HYSYS V14 (using the Peng-Robinson property package). It can be seen from Tables 12 and 13 that in this embodiment, the temperature of stream 317 is -155.00°C, which is much lower than the lowest temperature of MRC. Therefore, a conventional natural gas liquefaction process refrigeration system (MRC) can be used to provide a corresponding cold source without consuming an additional liquid nitrogen cold source or a circulating refrigeration system. 60.9% of helium can be extracted from a raw gas with a helium molar fraction of only 0.200%, and the helium extraction rate of the raw gas is 99.84%.

[0141] Example 8

[0142] Regarding the natural gas separation method based on side-line extraction of the present invention, the best effect of extracting the side-line stream from above the 6th tower plate and returning the intermediate stream from above the 4th tower plate was verified as follows.

[0143] Among methane, helium and nitrogen, helium has the lowest boiling point, followed by nitrogen, and methane has the highest boiling point. Therefore, as the tower plate moves up, the temperature continues to decrease, the proportion of methane in the liquid phase continues to increase, and the proportion in the gas phase continues to decrease, while the proportion of nitrogen and helium components in the gas phase continues to increase. Therefore, the gas phase is extracted from a higher tower plate for denitrification. Due to the lower temperature, more helium will be extracted in the gas phase, and then discharged together with the rich nitrogen during the denitrification process, resulting in helium loss and reducing the helium yield. But at the same time, because the proportion of methane in the gas phase will be lower, when the extracted gas phase is cooled and flashed, less methane will be lost from the rich nitrogen.

[0144] Because this is a complex model of multivariate distillation, with side-line extraction and side-line reflux, it is difficult to calculate the result manually. It is necessary to repeatedly debug the simulation software to find the best extraction point (taking into account the discharge from the nitrogen-rich gas while reducing the helium loss). This embodiment uses Aspen Hysys software simulation to determine the best extraction point.

[0145] Now, the key parameters when the gas phase is extracted from the 4th, 5th, 6th and 7th plates are compared when the molar fraction of helium in the raw material is 0.2% (i.e., 2000 ppm).

[0146]

[0147]

[0148]

[0149]

[0150]

[0151]

[0152]

[0153] It can be seen from Tables 14 to 21 that below the 6th plate, the proportion of helium in the gas phase increases slowly as the plates go up; while above the 6th plate, it increases rapidly as the plates go up. Below the 6th plate, the proportion of methane in the gas phase decreases rapidly as the plates go up; while above the 6th plate, the proportion of methane in the gas phase decreases slowly as the plates go up. Therefore, after the side-line gas phase removes most of the nitrogen and refluxes from the 4th plate, the proportion of methane above the 4th plate increases instead.

[0154] As the extraction tray moves up, the proportion of helium increases, resulting in more helium being extracted from the denitrification tower, causing the total helium yield to decrease. However, from the 7th tray to the 6th tray, the yield only decreases by 0.01%, which is reflected in the pure helium output of only 0.0025 kg / h (about 0.014 Nm 3 / h), but can reduce 47.2kg / h (about 65.83Nm 3 / h) of pure methane emissions, its value is much greater than the value of 0.0025kg / h of helium. When it is extracted from the 5th tray, the yield drops from 99.84% to 99.75%, and the helium output reaches 40.066kg / h, which is 0.0363kg / h less than the output when it is extracted from the 6th tray, about 0.2Nm3 / h, and the value is equivalent to the 25.8kg (about 35.98Nm3 / h) of reduced emissions. 3 When the extraction tray is adjusted from the 5th tray to the 4th tray, the helium yield drops to 98.57%, corresponding to a pure helium output of 39.591 kg / h, which is 0.475 kg / h less than the 5th tray, about 2.66 Nm 3 / h, while the amount of methane discharged from the nitrogen flash gas was only 12.3kg / h (about 17.15Nm 3 / h), the value of the lost nitrogen is far greater than the value of the reduced methane emissions.

[0155] In summary, if the side gas phase extraction point is lowered to the 7th plate and below, the loss of helium in the denitrogenated gas due to the reduction of helium in the gas phase is small, and the pure helium output increases slightly as the plate moves down, but due to the high methane content in the gas phase, more methane will be lost from the denitrogenated gas. When it rises to the 4th plate and above, the helium yield will drop rapidly. Through the above quantitative comparison, it can be seen that extraction from the 5th and 6th plates are both preferred options, so the present invention selects the 6th plate as the extraction plate.

[0156] Regarding reflux, after the produced gas is cooled and condensed for gas-liquid separation and denitrification, the reflux tray should be set above the produced tray for the following two reasons:

[0157] a. There is a pressure drop from bottom to top in the distillation tower. The side line production pipeline and the gas-liquid separator used for denitrification also have a pressure drop. Without considering the pressurization of the reflux liquid phase, the reflux liquid phase can only enter the distillation tower from a position higher than the production tower plate, that is, a position with lower pressure.

[0158] b. The temperature of the reflux liquid phase is relatively low (-149.8℃), and a large amount of it will be vaporized after refluxing into the distillation tower. If the reflux tray is arranged below the gas phase extraction tray, the reflux liquid will be vaporized and extracted again.

[0159] At the same time, because the temperature of the reflux liquid phase is low, the temperature at the reflux plate will further drop. As discussed in the previous article, above the 6th plate, the proportion of helium components in the gas phase on the pedal increases rapidly as the plate moves up. When a certain plate is selected as the reflux plate, a large amount of helium has been removed from the refluxed low-temperature liquid phase, and at the same time, it has a lower temperature and cannot be completely vaporized on the plate. Therefore, the liquid will dissolve a very small part of the helium in the plate component and bring it to the plate below the tower. This part of the helium dissolved in the low-temperature reflux liquid phase will precipitate in the high-temperature plate below and be extracted from the 6th plate, which will cause a slight decrease in the helium yield. The process flow was simulated by Aspen Hysys. When the gas phase was extracted from the side line of the 6th tray and condensed for denitrification and refluxed to the 2nd, 3rd, 4th and 5th trays, the pure helium production was 40.1003kg / h, 40.1014kg / h, 40.1020kg / h and 40.1026kg / h, respectively. Although the reflux from the 2nd, 3rd and 4th trays has a relatively slight effect on the helium yield and other parameters of the distillation tower, the present invention is based on the principle of increasing the helium yield as much as possible, and selects the lower tray as the reflux tray as much as possible.

[0160] In summary, when selecting a reflux tray instead of using a pump for pressurization, a tray above the side gas phase extraction tray should be selected so that there is a certain pressure difference between the extraction and reflux trays. The higher the tray, the greater the pressure difference. Taking into account the appropriate pressure difference between the extraction and reflux trays and the principle of giving priority to helium yield, the present invention preferably uses the fourth tray as the reflux tray after the side gas phase denitrogenation.

[0161] Example 9

[0162] A calculation verification method for extracting crude helium in a system based on a side-line natural gas separation method is disclosed. Specifically, the applicable range of the tower top temperature in the side-line natural gas separation system of Example 1 is demonstrated through the calculation verification method.

[0163] The system calculation verification method of the natural gas separation method based on side-line production in this embodiment is verified by the following steps:

[0164] Specifically, the analysis is now conducted based on the situation that the gas phase extracted from the 6th tray is condensed and the nitrogen-rich gas is removed through gas-liquid separation, and the liquid phase is refluxed to the 4th tray. Since the refluxed liquid phase mainly removes part of the nitrogen and components with a lower boiling point than helium, such as helium, the proportion of methane in the remaining components is higher than before denitrification. (Refer to the data in the material balance table, for example, the material balance table for the case where the helium in the feed gas is 2000ppm, that is, Example 7, in which the molar fraction of methane in stream 314 is 42.9%, and the molar fraction of methane in stream 316 is 54.9%).

[0165] Therefore, compared with the conventional method of first using a distillation tower to separate methane and nitrogen in the raw gas (because the boiling point of helium is higher than that of nitrogen, most of the helium will also be removed during denitrification in the first distillation tower), and then using a second distillation tower to separate helium from the nitrogen-rich gas, this method requires a cold source with a temperature lower than -170°C to condense nitrogen from the nitrogen-helium mixture to separate the crude helium. However, when the present invention separates helium in the tower top condenser, the molar fraction of methane in the gas and liquid phases is higher, which is actually beneficial to the separation of helium, and a cold source with a temperature above -170°C can be used to achieve the effect of separating crude helium of the same concentration.

[0166] The following proves the above conclusion:

[0167] According to the design principles of distillation towers, the top temperature is the gas phase dew point temperature.

[0168] According to Dalton's pressure divider law, as shown in formula 1-1:

[0169] P i =P·y i ……Formula 1-1;

[0170] And Raoult's law, as shown in formula 1-2:

[0171]

[0172] According to formula 1-1 and formula 1-2, we can get x i The expression is shown in formula 1-3:

[0173]

[0174] Since the sum of the components of the raw gas is 1, as shown in the following formula 1-4:

[0175] ∑x i =1...Formula 1-4;

[0176] According to formula 1-3 and formula 1-4, we can get formula 1-5:

[0177]

[0178] In Formula 1-1 to Formula 1-5: P i s is the saturated vapor pressure of the pure substance, in kPa; P i is the partial pressure of component i in the gas phase, in kPa; x i is the molar fraction of component i in the liquid phase; y i is the molar fraction of component i in the gas phase.

[0179] For pure substances, after the gas reaches the saturated vapor pressure, if the pressure continues to increase, it will begin to liquefy. Since the critical pressure of He is -268 degrees, the pure substance will not begin to liquefy when the temperature is higher than this, no matter how high the pressure is. Therefore, the saturated vapor pressure of the pure substance can be approximately considered to be infinite (+∞). It is also known that the molar fraction of He is required to reach 60%, so the molar fraction of other components should be 40%.

[0180] It can be seen from Tables 22 and 23 that for the process flow of the present invention, in the top condenser, the molar fraction of nitrogen is 0.2751284, methane is 0.1148702, helium is 0.61, and other components are trace amounts; in the liquid phase components, the molar fraction of nitrogen is 0.1286836, methane is 0.8676053, nitrogen is 0.003, and other components are less.

[0181] Table 22 Proportions of gas phase components on each plate of the distillation tower in the present invention

[0182]

[0183]

[0184] Table 23 The proportion of each component of the liquid phase on each plate of the distillation tower in the present invention

[0185] Nitrogen Methane Ethane Propane Isobutane n-Butane n-Pentane Isopentane Helium Condenser 0.1286836 0.8676053 4.89E-04 7.96E-06 2.07E-07 7.76E-08 2.68E-09 1.33E-09 0.0032136 Plate #1 0.0611906 0.8706218 3.58E-02 1.77E-02 4.08E-03 3.87E-03 3.31E-03 6.57E-04 0.0026915 Plate #2 0.1131783 0.8274351 3.21E-02 1.59E-02 3.65E-03 3.47E-03 2.96E-03 5.88E-04 0.0007093 Plate #3 0.128292 0.8140679 3.15E-02 1.56E-02 3.59E-03 3.40E-03 2.91E-03 5.77E-04 5.561E-05 Plate #4 0.1479405 0.7983133 2.94E-02 1.45E-02 3.34E-03 3.17E-03 2.71E-03 5.38E-04 4.153E-06 Plate #5 0.1462834 0.8000108 2.94E-02 1.45E-02 3.34E-03 3.17E-03 2.71E-03 5.38E-04 5.552E-07 Plate #6 0.1404325 0.8059367 2.94E-02 1.45E-02 3.34E-03 3.16E-03 2.71E-03 5.37E-04 2.639E-07 Plate #7 0.1303163 0.8161935 2.93E-02 1.45E-02 3.33E-03 3.16E-03 2.70E-03 5.35E-04 2.52E-07 Plate #8 0.1144174 0.8323649 2.92E-02 1.44E-02 3.31E-03 3.14E-03 2.68E-03 5.32E-04 2.536E-07 Plate #9 0.0925806 0.8546887 2.89E-02 1.42E-02 3.28E-03 3.11E-03 2.66E-03 5.27E-04 2.568E-07 Plate #10 0.0673087 0.880685 2.86E-02 1.40E-02 3.23E-03 3.06E-03 2.62E-03 5.19E-04 2.601E-07 Plate #11 0.0430937 0.9057396 2.81E-02 1.38E-02 3.17E-03 3.01E-03 2.57E-03 5.10E-04 2.615E-07 Plate #12 0.0236826 0.9252823 2.82E-02 1.37E-02 3.14E-03 2.97E-03 2.54E-03 5.04E-04 2.593E-07 Reboiler 0.01 0.9192521 3.88E-02 1.91E-02 4.39E-03 4.16E-03 3.55E-03 7.05E-04 1.187E-08

[0186] In order to verify the Aspen Hysys simulation results manually, except for helium, methane and nitrogen, the other components are ignored in the following calculations due to their low molar fractions. The principle of the top condenser itself is to use gas-liquid equilibrium for separation, so the gas phase is saturated.

[0187] The Antoine equation can be used to calculate the saturated vapor pressure of nitrogen and methane. Here, the Antoine equation with 6 constants used by Aspen Hysys software is used, which is more accurate than the common 3 constants. The temperature T is Kelvin temperature, unit K.

[0188] Ln(p)=a+b / (T+c)+d*ln(T)+e*T f

[0189] According to the Aspen Hysys database, the coefficients of the corresponding gases applied to the Antoine equation are shown in Table 24:

[0190] coefficient <![CDATA[N2]]> <![CDATA[CH4]]> a 3.54113×10 3.13500E×10 b <![CDATA[-9.66234×10 2 ]]> <![CDATA[-1.30752×10 3 ]]> c 0 0 d -4.31849 -3.26134 e <![CDATA[7.93190×10 -5 ]]> <![CDATA[2.94180×10 -5 ]]> f 2.00000 2.00000 Scope of application 62K~226K 91K~190.4K

[0191] According to the design principle of distillation tower, the temperature of the top condenser is the dew point temperature of the gas phase and the gas phase is in a saturated state.

[0192] According to Dalton's pressure divider law, as shown in formula 1-1:

[0193] P i =P·y i ……Formula 1-1;

[0194] And Raoult's law, as shown in formula 1-2:

[0195] P i =P i S ·x i ……Formula 1-2;

[0196] According to formula 1-1 and formula 1-2, we can get x i The expression is shown in formula 1-3:

[0197]

[0198] Since the sum of the components of the raw gas is 1, as shown in the following formula 1-4:

[0199] ∑x i =1...Formula 1-4;

[0200] According to formula 1-3 and formula 1-4, we can get formula 1-5:

[0201]

[0202] In Formula 1-1 to Formula 1-5: P i s is the saturated vapor pressure of the pure substance, in kPa; P i is the partial pressure of component i in the gas phase, in kPa; x i is the molar fraction of component i in the liquid phase; y i is the molar fraction of component i in the gas phase.

[0203] For pure substances, after the gas reaches the saturated vapor pressure, if the pressure continues to increase, it will begin to liquefy. Since the critical temperature of He is -268 degrees, the pure substance will not begin to liquefy when the temperature is higher than this, no matter how high the pressure is. Therefore, the saturated vapor pressure of the pure substance can be approximately considered to be infinite (+∞).

[0204] The critical temperature of nitrogen is -147.05℃, and the critical temperature of methane is -82.6℃. Therefore, the saturated vapor pressure can be calculated at -155℃ (the top condenser temperature is -155℃). The saturated vapor pressures of nitrogen and methane at -155℃ can be calculated by the Antoine equation to be 2282kPa and 169kPa respectively.

[0205] If 60% helium is required, the sum of the molar fractions of nitrogen and methane in the gas phase of the top condenser should be 40%. The pressure in the top condenser is 1580 kPa, so for the gas phase in the condenser,

[0206]

[0207] Solving the equation, we get Therefore According to formula 1-3, the components of the liquid phase at the top of the tower can be solved: In addition, there are small amounts of other components in the liquid phase, as well as a small amount of helium dissolved in the liquid phase.

[0208] As the top liquid phase components When, from formula 1-3 we can know For example, when Then, from equation 1-3, we can get Assume that the helium concentration in the gas phase is y He , then

[0209]

[0210] Substitution Solved

[0211] Therefore, when the molar fraction of methane in the liquid phase component of the top condenser is 50%, only crude helium with a concentration of 22.43% can be obtained.

[0212] Conventional methods first use a distillation tower to separate methane and nitrogen from the raw gas (because helium has a higher boiling point than nitrogen, most of the helium will also be removed when denitrifying in the first distillation tower), and then use a second distillation tower to separate the helium from the nitrogen-rich gas. The step of extracting crude helium is mainly completed in the second distillation tower, which is referred to as Scheme 2 below. Figure 2 .

[0213] In this conventional process, since most of the methane has been separated from the bottom of the tower (stream 8) in the first distillation tower (T-103), the material entering the helium extraction tower is nitrogen-rich gas (whose methane molar fraction is about 24%), and the methane molar fraction of the liquid phase at the top of the tower is lower than 24%. Therefore, the pressure at the top of the tower is consistent with that of the distillation tower in the present invention (1580 kPaA), and when the top distillation device does not use an ultra-low temperature cold source below -170 degrees, 60% crude He cannot be obtained.

[0214] Since the molar fraction of methane at the top of the tower in Scheme 2 must be lower than 24%, in order to obtain the maximum tower top condenser temperature at which 60% helium can be separated from the tower top, the molar fraction of methane in the tower top liquid phase can be set to the limit value of 24%, and the molar fraction of nitrogen in the tower top liquid phase is about 75%. The molar fractions of methane and nitrogen in the tower top gas phase can be calculated using Formula 1-3.

[0215] in The Antoine equation can be transformed into a function of temperature. Substituting this into equation 1-6 will result in a complex one-variable equation. Although there is a solution, it is difficult to solve it directly. Therefore, the trial and error method is used to determine the maximum temperature range.

[0216] When the tower top temperature is -170℃, the saturated vapor pressure of CH4 can be obtained from the Antoine equation to be 47.8kPa, and the saturated vapor pressure of N2 is 959.74kPa. The crude He concentration extracted at this time is 1-0.007261-0.461647=0.531092<0.6

[0217] When the tower top temperature is further reduced to -175℃, the vapor pressure of CH4 and N2 can be obtained from the Antoine equation to be 28.61kpa and 681.44kpa, respectively. The crude He concentration extracted at this time is 1-0.004346-0.327781=0.667873>0.6

[0218] It can be seen from this that in the case of Scheme 2, if the second distillation tower wants to extract 60% or more of crude helium, the temperature of the top condenser should be between -170℃ and -175℃, because the lowest temperature of the mixed refrigerant cold source used in conventional natural gas liquefaction is generally not lower than -170 degrees. Therefore, if Scheme 2 wants to obtain crude helium with a concentration of more than 60% when the tower pressure is the same as that of this scheme, it is necessary to use additional ultra-low temperature cold sources such as liquid nitrogen.

[0219] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A method for separating natural gas based on side-line production, characterized in that: Separation of helium, nitrogen and product gas by a single distillation column; Specifically, the raw gas enters the distillation tower from the upper part of the distillation tower for helium extraction treatment, and in the helium extraction process, a side stream is extracted from the middle part of the distillation tower for denitrification treatment to obtain a nitrogen stream and an intermediate stream, and the intermediate stream is returned from the middle part of the distillation tower to the distillation tower to continue to enter the helium extraction treatment, and finally a finished product stream is obtained at the bottom of the distillation tower, and a helium stream is obtained at the top of the distillation tower; The number of theoretical plates of the distillation tower is 12; The theoretical plates of the distillation tower are arranged from 1 to 12 from top to bottom, the feed gas after the pretreatment of removing heavy hydrocarbons enters the distillation tower from the first plate, the side stream is produced from above the sixth plate, and the intermediate stream returns from above the fourth plate; After the raw gas is pre-treated to remove heavy hydrocarbons, it enters the distillation tower from the upper part of the distillation tower for helium extraction treatment; The heavy hydrocarbon removal pretreatment specifically comprises the following steps: Step (B1), precooling the raw gas to -28°C to -45°C; Step (B2), conveying the feed gas to a heavy hydrocarbon separation tank for pretreatment to remove heavy hydrocarbons, to obtain a pretreatment stream and a heavy hydrocarbon stream, wherein the pretreatment stream enters the distillation tower from the upper part of the distillation tower; Step (B3), decompressing the heavy hydrocarbon stream to 1600 kPaA-1800 kPaA, entering the heavy hydrocarbon secondary flash tank to enter the flash evaporation to recover the light hydrocarbons, and obtaining a recovered stream and a final heavy hydrocarbon stream; Step (B4), the recovery stream is discharged from the top of the heavy hydrocarbon secondary flash tank, and then returns to the distillation tower from the bottom of the distillation tower to continue the helium extraction process; The final heavy hydrocarbon stream is discharged from the bottom of the heavy hydrocarbon secondary flash tank; The liquid phase stream generated by the distillation tower flows through the bottom of the distillation tower to the distillation tower kettle heater for heating, the gas phase stream generated by the distillation tower kettle heater returns to the distillation tower, and the liquid phase stream generated by the distillation tower kettle heater is discharged from the distillation tower kettle heater to obtain the finished product stream; The pretreatment stream first provides a heat source for the heater located in the distillation tower kettle, and then is cooled to -140°C to -150°C and decompressed to 1650kPaA to 1750kPaA, and finally enters the distillation tower from the upper part of the distillation tower.

2. The method for separating natural gas based on side-line production according to claim 1, characterized in that: The denitrification treatment separates the side stream into gas and liquid through a nitrogen separator to obtain a nitrogen stream and an intermediate stream. The nitrogen stream is discharged from the top of the nitrogen separator, and the intermediate stream is discharged from the bottom of the nitrogen separator and returned to the distillation tower from the middle.

3. The method for separating natural gas based on side-line production according to claim 2, characterized in that: The denitrification treatment specifically comprises the following steps: Step (A1), cooling the side stream to -140°C to -155°C; Step (A2), the side stream enters the nitrogen separator from the upper part of the nitrogen separator to undergo gas-liquid separation to obtain a nitrogen stream and an intermediate stream; In step (A3), the nitrogen stream is discharged from the top of the nitrogen separator, and the intermediate stream is discharged from the bottom of the nitrogen separator, and then refluxes to the middle of the distillation tower by gravity difference.

4. The method for separating natural gas based on side-line production according to claim 3, characterized in that: The pretreatment stream first provides a heat source for the heater located in the distillation tower kettle, then is cooled to -145°C and decompressed to 1700 kPaA, and finally enters the distillation tower from the upper part of the distillation tower; In the step (A1), the side stream is cooled to -148°C to -152°C; In the step (B1), the raw gas is precooled to -30°C to -40°C; In the step (B3), the heavy hydrocarbon stream is depressurized to 1700 kPaA and enters a heavy hydrocarbon secondary flash tank to flash and recover light hydrocarbons, thereby obtaining a recovered stream and a final heavy hydrocarbon stream.

5. The method for separating natural gas based on side-line production according to any one of claims 1 to 4, characterized in that: The top temperature of the distillation tower is maintained above -170°C.

6. The method for separating natural gas based on side-line production according to any one of claims 1 to 4, characterized in that: The molar fraction of helium in the raw gas is less than or equal to 0.02% and the molar fraction of nitrogen is greater than or equal to 2.0%, and the helium purity of the helium gas stream is greater than or equal to 60%.

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