A process for extracting crude helium in the process of liquefying helium-depleted natural gas and a calculation verification method

Abstract

The present invention relates to the technical field of natural gas liquefaction processing, and in particular to a process method and a calculation verification method for extracting crude helium in the process of helium-depleted natural gas liquefaction. The process method of the present invention first performs helium extraction and distillation treatment on the raw gas, and maintains the temperature of the distilled helium outlet at ‑150°C to ‑165°C to obtain a helium gas stream and an intermediate product stream, and then denitrogenates the intermediate product stream to obtain a nitrogen gas stream and a finished product stream; the molar fraction of helium in the raw gas is less than or equal to 0.05%. The beneficial effects of the present invention are as follows: 1. Helium extraction can be completed under the mixed refrigerant cold source conditions used in conventional natural gas liquefaction, without consuming liquid nitrogen or adding other circulating refrigeration units; 2. When the concentration of the natural gas raw gas is as low as 50ppm, helium with a concentration of more than 60% can still be collected, and the extraction and collection rate of helium in the raw gas is as high as 99%, so it has high economic value.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas liquefaction processing, and in particular to a process method and a calculation verification method for extracting crude helium in the process of liquefying helium-depleted natural gas. Background Art

[0002] Although my country has abundant reserves of natural gas resources, the molar fraction of helium in natural gas is relatively low, so the development of helium extraction technology is relatively lagging behind. At present, my country's natural gas production and liquefaction companies mainly use multi-stage membrane separation and low-temperature BOG helium extraction to extract helium from helium-rich natural gas (helium molar fraction above 0.05%). For helium-poor natural gas (molar fraction below 0.05%), advanced technology has not yet been developed to recycle helium, resulting in the waste of some helium resources.

[0003] Due to 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 use Figure 1 The system shown in Figure 1 In the traditional system, after the raw natural gas enters the cold box, it is pre-cooled to -40°C (stream 302) by the MRC refrigerant circulation system, and then the heavy hydrocarbons are removed by the heavy hydrocarbon separation tank (V-100). After the heavy hydrocarbons are removed, the natural gas (the molar fraction of heavy hydrocarbons C4+ is about 1.0%) provides a heat source for the denitrogenation tower reboiler E-106, and then enters the cold box again to be cooled to -155°C (stream 316), and then it is reduced to 1100kPa through the valve (JT-01) and enters the denitrogenation tower (T-1102) to remove nitrogen from the LNG. The molar fraction of nitrogen (stream 321) discharged from the top of the denitrogenation tower is about 75%, and the rest is methane. In this process, most of the helium will be discharged to the flare through this stream, resulting in helium waste. The LNG (stream 322) coming out of the bottom of the tower is deep-cooled to -163°C in the cold box and enters the LNG storage tank (stream 324) for sale.

[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 generally low performance of domestically produced metal membranes. Not only is the investment cost high, but also because the technology is monopolized by foreign countries, there is a risk of supply interruption due to changes in the international situation.

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

[0010] 1. 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.

[0011] 2. 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.

[0012] 3. 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.

[0013] Currently, the following patents belong to the second process:

[0014] Application No. 20211107822.6 discloses a three-tower low-temperature helium extraction system suitable for low-helium natural gas. The patent is applicable to natural gas with a He molar fraction of 300 ppm. The process mainly involves first extracting a mixture of nitrogen and helium from the natural gas, and then denitrifying the mixture. The disadvantage of this method is that an additional low-temperature cooling source is required to ensure that the top condensation temperature is less than -170°C, which has the disadvantages of large investment and high energy consumption.

[0015] Application No. 202222744330.4 discloses a BOG concentration and helium extraction system. The raw material of the helium extraction method of this system is flash gas from a storage tank or cold box, which has a relatively low pressure and requires secondary pressurization before crude helium extraction. The essence of the process of this patent is to first separate nitrogen and helium from LNG, and then separate nitrogen and helium by cooling and liquefying. Therefore, in addition to the need to add a flash gas compressor, it is also necessary to add an additional deep-cold low-temperature cold source (less than -170°C), and there are also disadvantages of large investment and high energy consumption.

[0016] Application No. 202221221820.X discloses a natural gas crude helium co-production LNG device using nitrogen expansion refrigeration. The device is a helium extraction device for the natural gas liquefaction process, but the liquefaction refrigeration process as a whole adopts nitrogen double expansion mechanism refrigeration, and the refrigeration temperature is even as low as -190°C, instead of the current mainstream mixed refrigerant refrigeration process. The refrigerant circulation volume is large and the equipment investment cost is high. It cannot be applied to the transformation of existing domestic devices, and the device does not consider denitrification. When the nitrogen molar fraction in the raw gas is higher than 1%, it cannot produce qualified LNG products and has a small scope of use.

[0017] Application No. 2202111239619.4 discloses a helium extraction device and a method for extracting helium from natural gas. A denitrification unit is added in the patent. However, the patent adopts a method of first removing methane and then separating nitrogen and helium, which results in the need to provide a low-temperature cold source of -185°C to -190°C in the denitrification process. A nitrogen expander is also used in the patent to achieve the above effects. The investment and operating costs are relatively high, and it is difficult to modify the original natural gas liquefaction device.

[0018] 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

[0019] One of the purposes of the present invention is to avoid the shortcomings of the prior art and provide a process for extracting crude helium in the process of liquefying helium-depleted natural gas. The process for extracting crude helium in the process of liquefying helium-depleted natural gas can obtain helium with a purity greater than or equal to 60% when the molar fraction of helium in the raw gas is less than 0.05%, and no additional cooling source is required.

[0020] The above-mentioned purpose of the present invention is achieved by the following technical measures:

[0021] Provided is a process for extracting crude helium in the process of liquefying helium-depleted natural gas. The raw gas is first subjected to helium extraction and distillation treatment, and the temperature of the distilled helium outlet is maintained at -150°C to -165°C to obtain a helium stream and an intermediate product stream; the intermediate product stream is then subjected to denitrogenation treatment to obtain a nitrogen stream and a finished product stream.

[0022] The molar fraction of helium in the raw material gas is less than or equal to 0.05%.

[0023] Preferably, the helium purity of the helium gas stream is greater than or equal to 60%, and the helium extraction rate of the raw gas in the helium extraction and distillation process is greater than or equal to 99%.

[0024] The molar fraction of nitrogen in the raw gas is 1.0% to 4.5%.

[0025] The molar fraction of helium in the raw material gas is 0.005% to 0.02%.

[0026] The process for extracting crude helium during the liquefaction of helium-depleted natural gas of the present invention comprises the following steps:

[0027] Step (1), pre-treating the raw gas to remove heavy hydrocarbons to obtain a pre-treated stream;

[0028] Step (2), controlling the pressure of the pretreatment stream at 1280 kPaA to 1320 kPaA and conveying the pretreatment stream to a helium extraction distillation tower for helium extraction distillation treatment, and maintaining the top temperature of the helium extraction distillation tower at -150°C to -165°C, the helium stream is discharged from the top of the helium extraction distillation tower, and the intermediate product stream is discharged from the bottom of the helium extraction distillation tower;

[0029] Step (3), the intermediate product stream is transported to a denitrification tower for denitrification treatment to obtain a nitrogen stream and a finished product stream.

[0030] Preferably, the above step (1) specifically includes:

[0031] Step (1.1), precooling the raw gas to -38°C to -42°C;

[0032] Step (1.2), conveying the feed gas to a heavy hydrocarbon separation tank for pretreatment to remove heavy hydrocarbons, thereby obtaining a first pretreated stream and a pretreated heavy hydrocarbon stream;

[0033] Step (1.3), decompressing the pretreated heavy hydrocarbon stream to 1280 kPaA-1320 kPaA, and entering the heavy hydrocarbon secondary flash tank for flash treatment to obtain a second pretreated stream and a final heavy hydrocarbon stream.

[0034] Preferably, the above step (2) specifically includes:

[0035] Step (2.1), the first pre-treated stream is cooled to -158°C to -153°C and decompressed to 1280 kPaA to 1320 kPaA;

[0036] Step (2.2), the first pretreatment stream enters the helium extraction distillation tower from the upper part of the helium extraction distillation tower, the second pretreatment stream enters the helium extraction distillation tower from the lower part of the helium extraction distillation tower, the first pretreatment stream and the second pretreatment stream are both subjected to helium extraction distillation treatment in the helium extraction distillation tower, the top temperature of the helium extraction distillation tower is maintained at -150°C to -165°C, the gaseous phase stream generated by the helium extraction distillation tower is discharged from the top of the tower to obtain the helium gas stream, the liquid phase stream generated by the helium extraction distillation tower flows to the helium extraction tower reboiler through the bottom of the tower for heating, the gaseous phase stream generated by the helium extraction tower reboiler is returned to the helium extraction distillation tower, the liquid phase stream generated by the helium extraction tower reboiler is discharged from the helium extraction tower reboiler to obtain the intermediate product stream.

[0037] Preferably, the above step (3) specifically includes:

[0038] Step (3.1), the intermediate product stream enters the denitrification tower from the middle of the denitrification tower and then undergoes denitrification treatment, the nitrogen stream is discharged from the top of the denitrification tower, and the finished product stream is discharged from the bottom of the denitrification tower;

[0039] Step (3.2), cooling the finished product stream to -165°C to -160°C and transporting it to the LNG storage tank.

[0040] In the step (2.1), the first pretreatment stream first provides a heat source for the reboiler located in the helium stripping tower and the reboiler located in the denitrification tower, and then is cooled to -158°C to -153°C and depressurized to 1280kPaA to 1320kPaA.

[0041] Preferably, the pressure of the helium gas stream is 1150 kPaA to 1200 kPaA.

[0042] In the step (1.1), the raw gas is precooled to -40°C.

[0043] In the step (1.3), the deheavy hydrocarbon stream is depressurized to 1300 kPaA.

[0044] In the step (2.1), the first pre-treated stream is cooled to -155°C and depressurized to 1300 kPaA.

[0045] In the step (2.2), the top temperature of the helium extraction distillation tower is maintained at -155°C, and the pressure of the helium gas stream is 1180 kPaA.

[0046] In the step (3.2), the finished product stream is cooled to -163°C and then transported to an LNG storage tank.

[0047] In the step (1.2), the heavy hydrocarbon stream is depressurized to 1300 kPaA.

[0048] Preferably, the helium extraction distillation tower has 10 plates.

[0049] Preferably, the denitrification tower has 8 plates.

[0050] In the raw gas, the molar fraction of methane is 80.0% to 98.5%, the molar fraction of ethane is 0.1% to 5.0%, the molar fraction of propane is 0.1% to 5.0%, the molar fraction of n-butane is less than 2.0%, the molar fraction of isobutane is less than 2.0%, the molar fraction of n-pentane is less than 2.0%, and the molar fraction of isopentane is less than 2.0%.

[0051] Another purpose of the present invention is to further explain the gas separation law, explain why a process method for extracting crude helium in the process of liquefying helium-depleted natural gas can overcome the shortcomings of the prior art and obtain more than 60% helium at a higher tower top temperature, and provide a calculation verification method for extracting crude helium in the process of liquefying helium-depleted natural gas. The calculation verification method can prove that only when helium is extracted first and then denitrified, can helium with a purity greater than or equal to 60% be obtained without adding an additional cold source.

[0052] The above-mentioned purpose of the present invention is achieved by the following technical measures:

[0053] A computational verification method for extracting crude helium during the liquefaction of helium-depleted natural gas is provided, which is verified by the following steps:

[0054] Step A, assuming that the above process method is used as the initial conditions for verification, the initial conditions are the composition and composition of the raw gas, the set extraction molar fraction of the helium gas stream, the distillation tower pressure, and the order of helium extraction distillation treatment and denitrification treatment;

[0055] Step B, deriving the Antoine formula, Dalton's partial pressure law and Raoult's law to obtain a calculation formula for the gas phase and liquid phase molar fraction at a specific tower top temperature of the distillation tower;

[0056] Step C, according to the distillation tower pressure, the tower top temperature and the gas phase and liquid phase molar fraction calculation formula obtained in step B, while setting the saturated vapor pressure of pure He substance to infinity, to obtain the calculated molar fraction of the helium gas stream;

[0057] Step D: determining whether the calculated mole fraction of the helium gas stream is equal to the set extracted mole fraction of the helium gas stream.

[0058] Preferably, the above Antoine formula is ln(P)=a+b / (T+c)+ln(T)*d+e*T^f, wherein a, b, c, d, e and f are all constant coefficients, P is the vapor pressure, and T is the temperature.

[0059] Preferably, the above Dalton partial pressure is represented by formula 1-2:

[0060] P i =P·y i ……Formula 1-1;

[0061] Preferably, the above Raoult's law is expressed by formula 1-2:

[0062]

[0063] Preferably, the above gas phase and liquid phase molar fraction calculation formula is shown in Formula 1-3:

[0064]

[0065] The sum of the components of the raw gas phase is shown by Formula 1-4 and Formula 1-5:

[0066] ∑x i =1...Formula 1-4;

[0067]

[0068] in, 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.

[0069] The beneficial effects of the present invention are as follows:

[0070] 1. By extracting helium first and then removing nitrogen, helium extraction can be completed under the conditions of the traditional MRC refrigerant circulation system and the mixed refrigerant cold source used in conventional natural gas liquefaction, without consuming liquid nitrogen or adding other circulating refrigeration units;

[0071] 2. When the helium concentration in natural gas is lower than 0.05%, helium with a concentration of more than 60% can still be collected, and 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

[0072] 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.

[0073] Figure 1 Schematic diagram of the traditional system.

[0074] Figure 2 The present invention is a schematic diagram of a system used in a process for extracting crude helium during the liquefaction of helium-depleted natural gas.

[0075] Figure 3 This is a schematic diagram of the system of Scheme 2 in Example 6.

[0076] Figure 4 This is a schematic diagram of the system of Scheme 1 in Example 6. DETAILED DESCRIPTION

[0077] The technical solution of the present invention is further described in conjunction with the following embodiments.

[0078] Example 1

[0079] A process for extracting crude helium in the process of liquefying helium-depleted natural gas, wherein the raw gas is first subjected to helium extraction and distillation treatment, and the temperature of the distilled helium outlet is maintained at -150°C to -165°C to obtain a helium stream, and an intermediate product stream is obtained at the same time, and then the intermediate product stream is subjected to denitrogenation treatment to obtain a nitrogen stream and a finished product stream. The molar fraction of helium in the raw gas is less than or equal to 0.05%, the molar fraction of nitrogen in the raw gas is 1.0% to 4.5%, and the molar fraction of methane is 80.0% to 98.5%.

[0080] The process for extracting crude helium during the liquefaction of helium-depleted natural gas of the present invention comprises the following steps:

[0081] Step (1), pre-treating the raw gas to remove heavy hydrocarbons to obtain a pre-treated stream;

[0082] Step (2), controlling the pressure of the pretreatment stream at 1280 kPaA to 1320 kPaA and conveying the pretreatment stream to a helium extraction distillation tower for helium extraction distillation treatment, and maintaining the top temperature of the helium extraction distillation tower at -150°C to -165°C, the helium stream is discharged from the top of the helium extraction distillation tower, and the intermediate product stream is discharged from the bottom of the helium extraction distillation tower;

[0083] Step (3), the intermediate product stream is transported to a denitrification tower for denitrification treatment to obtain a nitrogen stream and a finished product stream.

[0084] The step (1) specifically includes:

[0085] Step (1.1), precooling the raw gas to -38°C to -42°C;

[0086] Step (1.2), conveying the feed gas to a heavy hydrocarbon separation tank for pretreatment to remove heavy hydrocarbons, thereby obtaining a first pretreated stream and a pretreated heavy hydrocarbon stream;

[0087] Step (1.3), decompressing the pretreated heavy hydrocarbon stream to 1280 kPaA-1320 kPaA, and entering the heavy hydrocarbon secondary flash tank for flash treatment to obtain a second pretreated stream and a final heavy hydrocarbon stream.

[0088] It should be noted that the pretreatment for removing heavy hydrocarbons in step (1) 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 step (1.3), the light hydrocarbon stream obtained by adding a heavy hydrocarbon secondary flash tank to recover light hydrocarbons is combined with the bottom discharge stream of the helium extraction distillation tower after the helium extraction distillation treatment, thereby increasing the recovery rate of light hydrocarbons and helium in natural gas.

[0089] Wherein step (2) specifically includes:

[0090] Step (2.1), the first pre-treated stream is cooled to -158°C to -153°C and decompressed to 1280 kPaA to 1320 kPaA;

[0091] In step (2.2), the first pretreatment stream enters the helium extraction distillation tower from the upper part of the helium extraction distillation tower, and the second pretreatment stream enters the helium extraction distillation tower from the lower part of the helium extraction distillation tower. Both the first pretreatment stream and the second pretreatment stream are subjected to helium extraction distillation treatment in the helium extraction distillation tower, and the top temperature of the helium extraction distillation tower is maintained at -150°C to -165°C. The gas phase stream generated by the helium extraction distillation tower is discharged from the top of the tower to obtain a helium gas stream. The liquid phase stream generated by the helium extraction distillation tower flows to the helium extraction tower reboiler through the bottom of the tower for heating. The gas phase stream generated by the helium extraction tower reboiler returns to the helium extraction distillation tower. The liquid phase stream generated by the helium extraction tower reboiler is discharged from the helium extraction tower reboiler to obtain a finished product stream, wherein the pressure of the helium gas stream is 1150kPaA to 1200kPaA.

[0092] 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 first pretreatment stream is cooled to a liquid phase, so that the first pretreatment stream of the present invention enters from the upper part of the helium extraction distillation tower, and the second pretreatment stream enters from the lower part of the helium extraction distillation tower.

[0093] Wherein step (3) specifically includes:

[0094] Step (3.1), the intermediate product stream enters the denitrification tower from the middle of the denitrification tower and then undergoes denitrification treatment, the gas phase stream generated by the denitrification tower is discharged from the top of the tower to obtain a nitrogen gas stream, the liquid phase stream generated by the denitrification tower flows through the bottom of the tower to the denitrification tower reboiler for heating, the gas phase stream generated by the denitrification tower reboiler returns to the denitrification tower, and the liquid phase stream of the denitrification tower reboiler is discharged from the denitrification tower reboiler to obtain a finished product stream;

[0095] Step (3.2), cooling the finished product stream to -165°C to -160°C and transporting it to the LNG storage tank.

[0096] It should be noted that the function of the helium extraction tower reboiler and the denitrogenation tower reboiler of the present invention is to increase the bottom temperature of the helium extraction distillation tower and the denitrogenation tower, so that a temperature gradient is formed in the helium extraction distillation tower and the denitrogenation tower, that is, the temperature from the bottom to the top of the tower becomes lower and lower, and the function of this is to vaporize the light components in the stream at the bottom of the helium extraction distillation tower and the bottom of the denitrogenation tower.

[0097] The helium extraction tower reboiler, denitrogenation tower reboiler, 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 select specific models according to actual conditions. The helium extraction tower reboiler, denitrogenation tower reboiler, 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.

[0098] The beneficial effects of the process for extracting crude helium in the process of liquefying helium-depleted natural gas are as follows:

[0099] 1. By extracting helium first and then removing nitrogen, helium extraction can be completed under the conditions of the traditional MRC refrigerant circulation system and the mixed refrigerant cold source used in conventional natural gas liquefaction, without consuming liquid nitrogen or adding other circulating refrigeration units;

[0100] 2. When the helium concentration of natural gas is lower than 0.05%, it can still collect helium with a concentration of more than 60%, and the extraction and collection rate of helium in the raw gas is as high as 99%, so it has high economic value;

[0101] 3. By adding a heavy hydrocarbon secondary flash tank, the recovery rate of light hydrocarbons in natural gas can be increased, the loss of light hydrocarbons in the process of heavy hydrocarbon separation during natural gas liquefaction can be reduced, and the output of LNG products can be increased;

[0102] 4. The present invention adopts the process of first extracting helium and then removing nitrogen based on the traditional cold box design. By modifying the existing natural gas liquefaction process and adding equipment, the demand for crude helium extraction is met. There is no need to change the flow channel design of the original cold box, and the energy consumption of the original natural gas liquefaction is not increased. Only a helium extraction distillation tower and a heavy hydrocarbon secondary flash tank need to be added. Therefore, the investment is small and the operating cost is low.

[0103] Example 2

[0104] A process for extracting crude helium during the liquefaction of helium-depleted natural gas, the other features of which are the same as those of Example 1, except that:

[0105] The molar fraction of helium in the raw gas is 0.005% to 0.02%, the molar fraction of nitrogen is 1.0% to 4.5%, the molar fraction of methane is 80.0% to 98.5%, the molar fraction of ethane is 0.1% to 5.0%, the molar fraction of propane is 0.1% to 5.0%, the molar fraction of normal butane is less than 2.0%, the molar fraction of isobutane is less than 2.0%, the molar fraction of normal pentane is less than 2.0%, and the molar fraction of isopentane is less than 2.0%. The total molar fraction of helium, nitrogen, methane, ethane, propane, normal butane, isobutane, normal pentane and isopentane in the raw gas is 100%.

[0106] In step (1.1), the feed gas is precooled to -40°C. In step (1.3), the deheavy hydrocarbon stream is depressurized to 1300 kPaA.

[0107] In step (2.1), the first pre-treated stream is cooled to -155°C and depressurized to 1300 kPaA.

[0108] In step (2.2), the top temperature of the helium extracting distillation tower is maintained at -155°C, and the pressure of the helium gas stream is 1180 kPaA.

[0109] In step (3.2), the finished product stream is cooled to -163°C and transported to the LNG storage tank.

[0110] In step (1.2), the heavy hydrocarbon stream is depressurized to 1300 kPaA.

[0111] The present invention is simulated by chemical process simulation software ASPEN HYSYS V14 (using Peng-Robinson physical property package), and the present embodiment is compared with the traditional process ( Figure 1 The results are shown in Table 1:

[0112] Compare Projects Raw gas flow LNG production Refrigerant compressor shaft power <![CDATA[Product energy consumption kWh / Nm 3 NG]]> Traditional crafts 92.5t / h 81.0t / h 30370kW 0.2962 The present invention 92.5t / h 82.6t / h 30370kW 0.2900

[0113] From the above table, it can be seen that the present invention can increase the LNG production by about 2% and can also reduce the unit energy consumption of natural gas liquefaction, thereby having significant economic benefits.

[0114] Example 3

[0115] A system for a process for extracting crude helium in a helium-depleted natural gas liquefaction process based on Example 1 or 2, such as Figure 2 shown.

[0116] Among them Figure 2 T-1102 is a denitrification tower, T-1101 is a helium extraction distillation tower, E-102 is a denitrification tower top condenser, E-106 is a denitrification tower reboiler, E-105 is a nitrogen extraction tower reboiler, V-100 is a heavy hydrocarbon separation tank, V-101 is a primary outlet separator, V-102 is a secondary outlet separator, V-103 is an LNG product separation tank, V-104 is a heavy hydrocarbon secondary 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, and LNG-JT01 and VLV-101 are both valves. Figure 2 The intermediate raw gas is stream 301 and stream 302; the first pretreatment stream is stream 303, stream 304, stream 305, stream 308, stream 312, stream 315, stream 316 and stream 317; the pretreated heavy hydrocarbon stream is stream 306 and stream 307; the second pretreatment stream is stream 309; the final heavy hydrocarbon stream is stream 310 and stream 311; the helium stream is stream 318; the intermediate product stream is stream 319; the nitrogen stream is stream 321 and stream 327; and the finished product stream is stream 322.

[0117] The helium extraction distillation tower of this embodiment has 10 plates, and the first pretreatment stream enters from above the first plate above the helium extraction distillation tower. Specifically, the tower diameter of the helium extraction distillation tower of this embodiment is 1300 mm, the height of the packing section is 4000 mm, and the packing type is MELLAPAK 125Y.

[0118] The denitrogenation tower has 8 plates, including 3 plates in the rectifying section and 5 plates in the stripping section. The intermediate product stream enters from the top of the denitrogenation tower to the top of the 4th plate. Specifically, the packing height of the rectifying section of the denitrogenation tower in this embodiment is 1200 mm, and the packing type is MELLAPAK 750Y; the packing height of the rectifying section is 2000 mm, and the packing type is MELLAPAK 125Y.

[0119] It should be noted that, according to the actual situation, a condenser can be added to the gas phase outlet of the helium extraction distillation tower, so as to stabilize the temperature control of the helium gas stream. For the distillation tower system, when the temperature and pressure of the components entering the distillation tower are determined, the components of the distillation tower discharge are also determined. However, in actual operation, the changes in temperature and pressure are often linked, that is, when the temperature rises, the pressure often increases because the gas phase in the tower increases. 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.

[0120] Example 4

[0121] The system of Example 3 is adopted, and the molar fraction of the raw gas is specifically shown in Table 2 below, and the raw gas pressure is 8100 kPaA; the raw gas flow rate is 92.5 t / h; the refrigerant circulation volume is 500 t / h; the refrigerant compressor inlet pressure is 510 kPaA; the refrigerant compressor outlet pressure is 4900 kPaA; the refrigerant compressor shaft power is 30370 kW, wherein the refrigerant composition and molar fraction are: 18.0% nitrogen, 25.3% methane, 25.5% ethylene, 16.9% propane, 14.3% isopentane.

[0122] Table 2: Composition of the raw gas in this embodiment

[0123] Components Nitrogen Methane Ethane Propane Helium n-Butane Isobutane n-Pentane Isopentane total Mole fraction (%) 4.50 88.035 3.80 2.00 0.015 0.50 0.50 0.55 0.10 100

[0124]

[0125]

[0126]

[0127]

[0128] The data in Tables 3 and 4 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 3 and 4 that in this embodiment, the temperature of stream 318 is -151.25°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 0.015%, and the helium extraction rate of the raw gas is greater than 99%.

[0129] Example 5

[0130] The system of Example 3 is adopted, and the molar fraction of the raw gas is specifically shown in Table 5 below, and the raw gas pressure is 8100 kPaA; the raw gas flow rate is 92.5 t / h; the refrigerant circulation volume is 500 t / h; the refrigerant compressor inlet pressure is 510 kPaA; the refrigerant compressor outlet pressure is 4900 kPaA; the refrigerant compressor shaft power is 30370 kW, wherein the refrigerant composition and molar fraction are: 18.0% nitrogen, 25.3% methane, 25.5% ethylene, 16.9% propane, 14.3% isopentane.

[0131] Table 5: Composition of the raw gas in this embodiment

[0132] Components Nitrogen Methane Ethane Propane Helium n-Butane Isobutane n-Pentane Isopentane total Mole fraction (%) 4.50 88.045 3.80 2.00 0.005 0.50 0.50 0.55 0.10 100

[0133]

[0134]

[0135]

[0136]

[0137] The data in Tables 6 and 7 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 6 and 7 that in this embodiment, the temperature of stream 318 is -151.25°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 0.005%, and the helium extraction rate of the raw gas is greater than 99%.

[0138] Example 6

[0139] The system of Example 3 is adopted, and the molar fraction of the raw gas is specifically shown in Table 8 below, and the raw gas pressure is 8100 kPaA; the raw gas flow rate is 92.5 t / h; the refrigerant circulation volume is 500 t / h; the refrigerant compressor inlet pressure is 510 kPaA; the refrigerant compressor outlet pressure is 4900 kPaA; the refrigerant compressor shaft power is 30370 kW, wherein the refrigerant composition and molar fraction are: 18.0% nitrogen, 25.3% methane, 25.5% ethylene, 16.9% propane, 14.3% isopentane.

[0140] Table 8: Composition of the raw gas in this example

[0141] Components Nitrogen Methane Ethane Propane Helium n-Butane Isobutane n-Pentane Isopentane total Mole fraction (%) 4.50 88.030 3.80 2.00 0.02 0.50 0.50 0.55 0.10 100

[0142]

[0143]

[0144]

[0145]

[0146] The data in Tables 9 and 10 are simulated and calculated by the chemical process simulation software ASPEN HYSYS V14 (using the Peng-Robinson physical property package). It can be seen from Tables 9 and 10 that in this embodiment, the temperature of stream 318 is -151.25°C, which is much lower than the lowest temperature of MRC. Therefore, a traditional 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. About 61% of helium can be extracted from a raw gas with a helium molar fraction of only 0.02%, and the helium extraction rate of the raw gas is greater than 99%.

[0147] Example 7

[0148] The system of Example 3 is adopted, and the molar fraction of the raw gas is specifically shown in Table 8 below, and the raw gas pressure is 8100 kPaA; the raw gas flow rate is 92.5 t / h; the refrigerant circulation volume is 500 t / h; the refrigerant compressor inlet pressure is 510 kPaA; the refrigerant compressor outlet pressure is 4900 kPaA; the refrigerant compressor shaft power is 30370 kW, wherein the refrigerant composition and molar fraction are: 18.0% nitrogen, 25.3% methane, 25.5% ethylene, 16.9% propane, 14.3% isopentane.

[0149] Table 11. Composition of the raw gas in this example

[0150] Components Nitrogen Methane Ethane Propane Helium n-Butane Isobutane n-Pentane Isopentane total Mole fraction (%) 4.50 88.000 3.80 2.00 0.05 0.50 0.50 0.55 0.10 100

[0151]

[0152]

[0153]

[0154]

[0155] 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 318 is -151.25°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 0.05%, and the helium extraction rate of the raw gas is greater than 99%.

[0156] Example 8

[0157] A calculation verification method for extracting crude helium during the liquefaction of helium-depleted natural gas is disclosed. Specifically, the calculation verification method is used to demonstrate the applicable range of the tower top temperature in the process for extracting crude helium during the liquefaction of helium-depleted natural gas in Examples 1 and 2.

[0158] The calculation verification method for extracting crude helium in the helium-depleted natural gas liquefaction process of this embodiment is verified by the following steps:

[0159] Step A, assuming that the process methods of Examples 1 to 2 are used as initial conditions for verification, the initial conditions are the composition and composition of the feed gas, the set extraction molar fraction of the helium gas stream, the distillation tower pressure, and the order of the helium extraction distillation treatment and the denitrification treatment;

[0160] Step B, deriving the Antoine formula, Dalton's partial pressure law and Raoult's law to obtain a calculation formula for the gas phase and liquid phase molar fraction at a specific tower top temperature of the distillation tower;

[0161] Step C, according to the distillation tower pressure, the tower top temperature and the gas phase and liquid phase molar fraction calculation formula obtained in step B, while setting the saturated vapor pressure of pure He substance to infinity, to obtain the calculated molar fraction of the helium gas stream;

[0162] Step D: determining whether the calculated mole fraction of the helium gas stream is equal to the set extracted mole fraction of the helium gas stream.

[0163] In order to illustrate that under the conditions of raw natural gas with a high N2 mole fraction (about 5%) and a low He mole fraction (set as 0.05% as follows), crude helium is first extracted from the raw gas, and then the remaining LNG is denitrogenated to a N2 mole fraction of less than 1%. Compared with the prior art of first denitrogenating the raw gas to a N2 mole fraction of less than 1%, and then extracting crude helium of the same concentration from the removed nitrogen-rich gas, there is no need for an ultra-low temperature cold source at the top of the tower (a cold source below -170 degrees).

[0164] This embodiment is introduced by comparing two solutions as follows:

[0165] Solution 1: Using the method of the present invention, the raw gas is first subjected to helium extraction and distillation treatment to obtain a helium stream and an intermediate product stream, and then the intermediate product stream is subjected to denitrification treatment to obtain a nitrogen stream and a finished product stream. Figure 3 ,exist Figure 3 In the process, the raw gas (stream 1) is separated from helium by the helium extraction distillation tower T-101, and the crude helium product is directly extracted from the top of the tower. The bottom material is separated from nitrogen and methane by the denitrification tower T-102. When the tower pressure of the helium extraction distillation tower T-101 is 1200kPaA, the molar fraction of helium in the crude helium product (stream 3) at the top of the tower is required to be greater than 60%; at the same time, the molar fraction of methane in the nitrogen (stream 4) at the top of the T-102 tower is required to be less than 25%, and the molar fraction of N2 in stream 5 is ≤1%.

[0166] Solution 2: For the existing technology (such as publication numbers CN20211107822.6 and 202222744330.4), the initial conditions are that the order of helium extraction distillation treatment and denitrification treatment is to first extract a mixed stream of nitrogen and helium from the raw gas, and then denitrify the mixed stream. Figure 4 As shown, the raw gas (stream 6) is separated into qualified LNG products (nitrogen molar fraction ≤ 1.0%) at the bottom of the distillation tower T-103, and the top of the tower (stream 12) is separated into a mixed stream of nitrogen and helium (methane molar fraction is required to be ≤ 25%). At this time, the composition of the top of the tower (stream 12) is set to: 74% nitrogen; 25% methane; 1% helium, and the N2 molar fraction of stream 8 is ≤1%. Then stream 12 enters the denitrification tower T-104 for denitrification, and the crude helium concentration is required to be 60% (stream 9).

[0167] 1. The temperature of the top of T-101 and T-104 towers is required to be controlled at -165°C, so as to prove the feasibility of extracting 60% crude helium with two schemes at this temperature.

[0168] In order to simplify the calculation process, the following initial conditions are set in this embodiment, which are the composition and composition of the raw gas, the set extraction molar fraction of the helium gas stream, and the distillation tower pressure. The details are as follows:

[0169] (1) The raw gas consists only of the following components: 5% N2; 94.95% CH4; 0.05% He;

[0170] (2) The pressure of towers T-101 and T-104 is set to 1200 kPaA, and the pressure drop in the entire tower is ignored;

[0171] (3) The vapor pressure is calculated using the Antoine formula, which is ln(P)=a+b / (T+c)+ln(T)*d+e*T^f, where a, b, c, d, e and f are all constant coefficients, P is the vapor pressure, and T is the temperature.

[0172] (4) To improve the yield of natural gas liquefaction, the CH4 molar fraction in the nitrogen-rich gas removed from the top of the T-103 tower in Scheme 2 is ≤25%;

[0173] (5) CH4, N2 and He are all considered as ideal gases for calculation;

[0174] (6) At the same time, because the critical temperature of He is -268 °C, the temperature is always much higher than its critical temperature during the entire separation process. Therefore, the liquid three-component system can be regarded as He gas dissolved in a mixed solution of N2 and CH4, rather than He liquefied and mixed with liquid N2 and CH4.

[0175] It should be emphasized that because the ternary mixed components also involve binary interactions, etc., this embodiment is manually calculated, all gases are regarded as ideal gases, and binary interactions are not considered. Therefore, there may be a small amount of error in the actual calculation results. However, this embodiment can reflect the basic law of distillation separation of this ternary mixed component.

[0176] Antoine is used to calculate the saturated vapor pressure of nitrogen and methane. The Antoine equation with 6 constants used by Aspen Hysys software is used. Compared with the common 3-constant equation, the 6-constant Antoine equation is more accurate. The temperature T is Kelvin temperature, unit is K.

[0177] According to the Aspen Hysys database, the coefficients of the corresponding gas applied to the Antoine equation are obtained, as shown in Table 8:

[0178] Table 14. Coefficients of Antoine equation

[0179] coefficient <![CDATA[N2]]> <![CDATA[CH4]]> a 3.54113××10 3.13500×10 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 Temperature application range (K) 62～226 91～190.4

[0180] According to Antoine's formula, when the tower top temperature is -165°C (127.15K), the vapor pressure of methane is 76.01Kpa and the vapor pressure of nitrogen is 1312Kpa. According to the design principle of the distillation tower, the tower top temperature is the gas phase dew point temperature.

[0181] According to Dalton's pressure divider law, as shown in formula 1-1:

[0182] P i =P·y i ……Formula 1-1;

[0183] And Raoult's law, as shown in formula 1-2:

[0184]

[0185] According to formula 1-1 and formula 1-2, we can get x i The expression is shown in formula 1-3:

[0186]

[0187] Since the sum of the components of the raw gas is 1, as shown in the following formula 1-4:

[0188] ∑x i =1...Formula 1-4;

[0189] According to formula 1-3 and formula 1-4, we can get formula 1-5:

[0190]

[0191] 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.

[0192] It should be noted that for pure gas, if the pressure continues to increase after reaching the saturated vapor pressure, it will begin to liquefy, because the critical temperature of He is -268°C. When the pure substance is above this temperature, no matter how high the pressure is, it will not begin to liquefy. Therefore, the saturated vapor pressure of the pure substance can be approximately considered to be infinite (+∞). Because the He molar fraction is required to reach 60%, the sum of the N2 and CH4 molar fractions is 40%.

[0193] Therefore, formula 1-6 is obtained, which is as follows:

[0194]

[0195] Solve equation 1-6, where the first term of equation 1-6 is 0, and calculate therefore According to formula 1-3, the components of the liquid phase at the top of the tower can be solved: In addition, a small amount of helium is dissolved in the liquid phase.

[0196] When the He concentration in the gas phase is required to be 60% constant, and the CH4 liquid phase molar fraction in the top liquid phase is lower than 67.3% (for example, assuming that the CH4 molar fraction in the top liquid phase is 50%, that is, ), can be calculated from formula 1-3 Substitute into equation 1-6, the details are as follows:

[0197]

[0198] Because the result of equation 1-6 is less than 1, that is, the gas phase mixture of N2 and CH4 is not saturated at this time, When N2 is further gasified, the sum of the molar fractions of N2 and CH4 will exceed 40%. Should meet the following:

[0199]

[0200] Solved So we get the following:

[0201]

[0202] Then the He concentration at this time is: Therefore, at -165°C, when the CH4 liquid phase molar fraction in the top liquid phase is lower than 67.3%, He with a concentration of 60% cannot be obtained.

[0203] For Scheme 1, the tower top temperature requirement is -165°C, and the verification process of obtaining 60% crude He is as follows:

[0204] The molar fraction of methane in the material entering the distillation tower is 94.95%, and CH4 is the component with the highest mass boiling point among He and N2. Therefore, in the distillation tower, the molar fraction of CH4 in the top liquid phase component must be lower than that in the feed liquid phase, because 67.3% < 94.95%. Therefore, in the scheme 1 under the above specified conditions, when the top temperature of T-101 is set to -165℃, a crude helium gas with a concentration of 60% can be obtained.

[0205] For Scheme 2, the tower top temperature requirement is -165°C, and the verification process of obtaining 60% crude He is as follows:

[0206] The methane molar fraction of the material entering the T-104 distillation tower is about 24%, and the methane molar fraction of the liquid phase at the top of the tower must be lower than 24%, and therefore must also be lower than the minimum methane molar fraction of 67.3% calculated previously. Therefore, in Scheme 2 under the above-mentioned conditions, when the tower top temperature is -165°C, 60% crude He cannot be obtained.

[0207] 2. Verification process of the highest tower top temperature when 60% helium can be separated from the tower top in Scheme 2

[0208] Furthermore, for the second scheme, the molar fraction of methane at the top of the T-104 tower must be lower than 24%. In order to obtain the highest tower top temperature at which 60% helium can be separated from the tower top, the molar fraction of methane in the liquid phase at the top of the tower is set to the limit value of 24%, while the molar fraction of nitrogen in the liquid phase at the top of the tower is about 75%. Moreover, the molar fractions of methane and nitrogen in the gas phase at the top of the tower can be obtained by formula 1-3, where 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 directly borrow it. Therefore, this embodiment uses the trial and error method to determine the maximum temperature range for 60% helium to leave the tower top.

[0209] When the tower top temperature is -170℃, the vapor pressure of CH4 can be obtained from the Antoine equation to be 47.8kPa, and the vapor pressure of N2 is 959.74kPa. The vapor pressure of CH4 and N2 can be obtained from equation 1-3. At this time, the concentration of crude He extracted is 1-0.009559-0.599837=0.390604. Since 0.390604<0.6, 60% crude He cannot be obtained.

[0210] When the tower top temperature is further reduced to -175℃, the saturated vapor pressure of CH4 and N2 can be obtained as 28.61kPa and 681.44kPa respectively by the Antoine equation. At this time, the concentration of crude He extracted is 1-0.005721-0.425901=0.568378. Since 0.568378<0.6, when the tower top temperature is -175°C, 60% crude He cannot be obtained.

[0211] When the tower top temperature is further reduced to -176℃, the saturated vapor pressure of CH4 and N2 can be obtained from the Antoine equation as 25.64kPa and 633.72kPa, respectively. At this time, the concentration of crude He extracted is 1-0.005128-0.396076=0.5988. Since 0.5988≈0.6, 60% crude He can be obtained only when the temperature at the top of the tower is -176°C or below.

[0212] It can be seen from this that in the case of Option 2, if the tower pressure is the same, in order to extract 60% or more of the crude helium, the tower top temperature must be lower than -176°C, because the lowest temperature of the mixed refrigerant cold source used in conventional natural gas liquefaction is generally not lower than -170°C. Therefore, it can be proved that Option 2 must additionally use ultra-low temperature cold sources such as liquid nitrogen to continue condensing the tower top to keep the tower top temperature below -176°C.

[0213] 3. Demonstration process of obtaining the highest concentration of helium at the top of the tower when the tower top temperature is between -150℃ and -180℃

[0214] Because the above processes are all fixed tower top temperature, and then the demonstration process of deriving tower top component requirements. The following is the highest tower top helium concentration obtained by calculating and demonstrating the tower top temperature from -150℃ to -180℃ under two tower top components at a tower pressure of 1200kPa based on the different tower top components of Scheme 1 and Scheme 2.

[0215] It should be noted that, because methane is a relatively high-boiling-point component, the methane concentration at the top of the tower cannot be higher than the methane concentration in the feed. It has been verified above that the higher the methane molar fraction, the greater the helium concentration that can be obtained at the same tower top temperature. Therefore, for the calculation of the maximum helium concentration that can be obtained at the tower top, the distillation tower feed component can be directly taken as the tower top component.

[0216] For Scheme 1, the molar fraction of CH4 in the liquid phase at the top of the tower is 0.9495, and that of N2 is 0.05. If 0.05% He dissolution is not considered, and only the liquid phase mixture of CH4 and N2 is considered, the molar fractions of the two components in the liquid phase are:

[0217]

[0218]

[0219] From formula 1-3, we get formula 1-8, which is as follows:

[0220]

[0221] When the tower top temperature is -150°C, -155°C, -160°C, -165°C, -170°C, -175°C, -180°C and -185°C, the saturated vapor pressures of CH4 and N2 are calculated according to the Antoine formula, as shown in Table 9:

[0222] Table 15. Saturated vapor pressure of CH4 and N2 when the tower top temperature is between -150℃ and -185℃

[0223]

[0224] Substituting the data in Table 15 into equation 1-8 respectively, we can obtain the maximum helium concentration at each temperature point, as shown in Table 16:

[0225] Table 16. Maximum helium concentration when the tower top temperature of Scheme 1 is between -150℃ and -185℃

[0226]

[0227] It can be seen from Table 16 that in Scheme 1, when the tower top temperature is at -150°C, crude helium with a concentration exceeding 60% can be obtained, thus verifying the simulation results of the present invention.

[0228] Similarly, for Scheme 2, since the T-103 denitrification tower mainly removes nitrogen and helium, the molar fraction of methane in the nitrogen-rich gas is relatively low. According to the Aspen Hysys simulation results, it is about 24%-25%. Therefore, and

[0229] Similarly, it is calculated that when the tower top temperature is -150℃, -155℃, -160℃, -165℃, -170℃, -175℃ and -180℃, the maximum helium concentration can be obtained at each temperature point, as shown in Table 17:

[0230] Table 17. Maximum helium concentration when the tower top temperature of Scheme 2 is between -150℃ and -185℃

[0231]

[0232] It can be seen from Table 17 that when the tower top temperature is -150°C, -155°C and -160°C, the He molar fraction of the gas phase is negative, that is, the feed will be completely vaporized into gas, and helium cannot be extracted at the above temperature. When it is necessary to obtain crude helium with a concentration of 60%, the tower top temperature needs to be below -175°C.

[0233] In summary, the calculation verification of this embodiment proves that when the mixed stream of nitrogen and helium is extracted together first and then denitrogenated in the mixed stream (Scheme 2), under the condition of raw natural gas with a low He molar fraction, when it is necessary to extract crude helium with a concentration of 60%, an additional liquid nitrogen cold source or a circulating refrigeration system must be used to further reduce the tower top temperature to -175°C or below. However, when the raw gas is first subjected to helium extraction and distillation treatment to obtain a helium stream and an intermediate product stream, and then the intermediate product stream is denitrogenated to obtain a nitrogen stream and a finished product stream (Scheme 1), under the same conditions, when it is necessary to extract crude helium with a concentration of 60%, the tower top temperature is only -150°C, so only the traditional natural gas liquefaction process refrigeration system (MRC) can be used.

[0234] 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 process for extracting crude helium during the liquefaction of helium-depleted natural gas, characterized in that: The raw gas is first subjected to helium extraction and distillation treatment, and the temperature of the distilled helium outlet is maintained at -150°C to -165°C to obtain a helium stream and an intermediate product stream; the intermediate product stream is then subjected to denitrification treatment to obtain a nitrogen stream and a finished product stream; The molar fraction of helium in the raw gas is less than or equal to 0.05%, and the helium purity of the helium gas stream is greater than or equal to 60%; The specific steps include: Step (1), pre-treating the raw gas to remove heavy hydrocarbons to obtain a pre-treated stream; Step (2), controlling the pressure of the pretreatment stream at 1280 kPaA to 1320 kPaA and conveying the pretreatment stream to a helium extraction distillation tower for helium extraction distillation treatment, and maintaining the top temperature of the helium extraction distillation tower at -150°C to -165°C, the helium stream is discharged from the top of the helium extraction distillation tower, and the intermediate product stream is discharged from the bottom of the helium extraction distillation tower; Step (3), the intermediate product stream is transported to a denitrification tower for denitrification treatment to obtain a nitrogen stream and a finished product stream; The step (1) specifically includes: Step (1.1), precooling the raw gas to -38°C to -42°C; Step (1.2), conveying the feed gas to a heavy hydrocarbon separation tank for pretreatment to remove heavy hydrocarbons, thereby obtaining a first pretreated stream and a pretreated heavy hydrocarbon stream; Step (1.3), decompressing the pretreated heavy hydrocarbon stream to 1280 kPaA-1320 kPaA, entering the heavy hydrocarbon secondary flash tank for flash treatment, and obtaining a second pretreated stream and a final heavy hydrocarbon stream; The step (2) specifically includes: Step (2.1), the first pre-treated stream is cooled to -158°C to -153°C and decompressed to 1280 kPaA to 1320 kPaA; Step (2.2), the first pretreatment stream enters the helium extraction distillation tower from the upper part of the helium extraction distillation tower, the second pretreatment stream enters the helium extraction distillation tower from the lower part of the helium extraction distillation tower, the first pretreatment stream and the second pretreatment stream are both subjected to helium extraction distillation treatment in the helium extraction distillation tower, the top temperature of the helium extraction distillation tower is maintained at -150°C to -165°C, the gaseous phase stream generated by the helium extraction distillation tower is discharged from the top of the tower to obtain the helium gas stream, the liquid phase stream generated by the helium extraction distillation tower flows to the helium extraction tower reboiler through the bottom of the tower for heating, the gaseous phase stream generated by the helium extraction tower reboiler is returned to the helium extraction distillation tower, the liquid phase stream generated by the helium extraction tower reboiler is discharged from the helium extraction tower reboiler to obtain the intermediate product stream.

2. The process for extracting crude helium during the liquefaction of helium-depleted natural gas according to claim 1, characterized in that: The helium extraction rate in the raw gas during the helium extraction and distillation process is greater than or equal to 99%; The molar fraction of nitrogen in the raw gas is 1.0% to 4.5%; The molar fraction of helium in the raw material gas is 0.005% to 0.02%.

3. The process for extracting crude helium in the process of liquefying helium-depleted natural gas according to claim 1, characterized in that: The step (3) specifically includes: Step (3.1), the intermediate product stream enters the denitrification tower from the middle of the denitrification tower and then undergoes denitrification treatment, the gas phase stream generated by the denitrification tower is discharged from the top of the tower to obtain the nitrogen gas stream, the liquid phase stream generated by the denitrification tower flows to the denitrification tower reboiler through the bottom of the tower to be heated, the gas phase stream generated by the denitrification tower reboiler returns to the denitrification tower, and the liquid phase stream of the denitrification tower reboiler is discharged from the denitrification tower reboiler to obtain the finished product stream; Step (3.2), cooling the finished product stream to -165°C to -160°C and transporting it to the LNG storage tank.

4. The process for extracting crude helium during the liquefaction of helium-depleted natural gas according to claim 3, characterized in that: In the step (2.1), the first pretreatment stream first provides a heat source for the helium stripping tower reboiler and the denitrification tower reboiler, and then is cooled to -158°C to -153°C and depressurized to 1280 kPaA to 1320 kPaA; The pressure of the helium gas stream is 1150 kPaA-1200 kPaA.

5. The process for extracting crude helium during the liquefaction of helium-depleted natural gas according to claim 4, characterized in that: In the step (1.1), the raw gas is precooled to -40°C; In the step (1.3), the decompression of the heavy hydrocarbon stream is reduced to 1300 kPaA; In the step (2.1), the first pretreated stream is cooled to -155°C and depressurized to 1300 kPaA; In the step (2.2), the top temperature of the helium extraction distillation tower is maintained at -155°C, and the pressure of the helium gas stream is 1180 kPaA; In the step (3.2), the finished product stream is cooled to -163°C and then transported to an LNG storage tank; In the step (1.2), the heavy hydrocarbon stream is depressurized to 1300 kPaA; In the raw gas, the molar fraction of methane is 80.0% to 98.5%, the molar fraction of ethane is 0.1% to 5.0%, the molar fraction of propane is 0.1% to 5.0%, the molar fraction of n-butane is less than 2.0%, the molar fraction of isobutane is less than 2.0%, the molar fraction of n-pentane is less than 2.0%, and the molar fraction of isopentane is less than 2.0%.

6. A computational verification method for extracting crude helium during the liquefaction of helium-depleted natural gas, characterized in that: Verify by following the steps below: Step A, assuming that the process method according to any one of claims 1 to 5 is used as the initial conditions for verification, the initial conditions are the composition and composition of the feed gas, the set extraction molar fraction of the helium gas stream, the distillation tower pressure, and the order of the helium extraction distillation treatment and the denitrification treatment; Step B, deriving the Antoine formula, Dalton's partial pressure law and Raoult's law to obtain a calculation formula for the gas phase and liquid phase molar fraction at a specific tower top temperature of the distillation tower; Step C, according to the distillation tower pressure, the tower top temperature and the gas phase and liquid phase molar fraction calculation formula obtained in step B, while setting the saturated vapor pressure of the pure He substance to infinity, to obtain the calculated molar fraction of the helium gas stream; Step D: determining whether the calculated mole fraction of the helium gas stream is equal to the set extraction mole fraction of the helium gas stream.

7. The method for calculating and verifying the extraction of crude helium in the process of liquefying helium-depleted natural gas according to claim 6, characterized in that: The Antoine formula is ln(P)=a+b / (T+c)+ln(T)*d+e*T^f, where a, b, c, d, e and f are constant coefficients, P is the vapor pressure, and T is the temperature; The Dalton partial pressure is expressed by formula 1-2: P i =P·y i ……Formula 1-1; The Raoult's law is expressed by equation 1-2: The gas phase and liquid phase molar fraction calculation formula is shown in Formula 1-3: The sum of the components of the raw gas phase is shown by Formula 1-4 and Formula 1-5: ∑x i =1...Formula 1-4; 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.

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