A system and method for producing hydrogen and helium from BOG

Abstract

The application belongs to the technical field of energy chemical industry, and particularly relates to a system and method for producing hydrogen and helium by using BOG. The system comprises an LNG cold box, which is sequentially connected with a flash evaporation unit, a catalytic reaction unit for methane cracking, a gas-solid separation unit, a membrane separation unit, a pressure swing adsorption unit, an electrochemical hydrogen pump unit and a low-temperature adsorption unit. The flash evaporation unit is also connected with the LNG cold box through an LNG storage unit. The application adds a flash evaporation tank with a pressure higher than normal pressure in front of the LNG storage tank, which can significantly reduce the amount of BOG gas discharged from the top of the flash evaporation tank and sent to the reactor. Based on the reduction of the gas amount, the scale of subsequent treatment such as the catalytic reaction unit, the membrane separation unit and the pressure swing adsorption unit can be reduced, thereby directly reducing the equipment investment cost of the process.

Description

Technical Field

[0001] This invention belongs to the field of energy and chemical technology, specifically relating to a system and method for producing hydrogen and helium using BOG. Background Technology

[0002] Hydrogen, as a clean energy source and strategic chemical raw material, is in high demand in fields such as fuel cells, ammonia synthesis, and semiconductor manufacturing. Traditional hydrogen production processes (such as methane steam reforming, which consumes fossil fuels, and water electrolysis, which consumes a lot of energy) are difficult to meet the requirements of "low-carbon" development. Helium, on the other hand, plays an irreplaceable role in fields such as low-temperature superconductivity, precision manufacturing, and aerospace. However, China has relatively scarce helium resources and urgently needs to develop low-cost, highly adaptable helium recovery technologies.

[0003] Based on current technological levels, the internationally recognized minimum helium content requirement for natural gas helium extraction is greater than 0.2%. However, the helium content in most natural gas in my country is only 0.03% to 0.05%, classifying it as helium-poor natural gas with extremely high extraction costs. But during the natural gas liquefaction process, helium accumulates in the liquefied natural gas (LNG) flash vapor. Therefore, extracting helium from LNG flash vapor (BOG) is economically feasible.

[0004] Currently, research on BOG resource utilization mainly focuses on single-product preparation, such as converting BOG into hydrogen through methane steam reforming or methane cracking. However, these processes do not consider helium recovery. Traditional BOG helium extraction units use flash vapor from LNG storage tanks as feedstock. Due to the near-atmospheric pressure of the flash vapor and the high methane content in the gas source, downstream compression, dehydrogenation, and separation purification costs are high. The methane separated after helium extraction needs to be liquefied and recovered or sent to a flare for combustion, resulting in high energy consumption and low resource utilization. This is a significant reason why some LNG terminal helium extraction units are operating at a loss. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a system and method for producing hydrogen and helium using BOG (Bottle-Off Gas). By adding a flash tank at a pressure higher than atmospheric pressure before the LNG storage tank, the amount of BOG gas discharged from the top of the flash tank can be significantly reduced. Based on this reduction in gas volume, the scale of subsequent units can be reduced, directly lowering the equipment investment cost of the process.

[0006] A system for producing hydrogen and helium using BOG includes an LNG cold box, which is sequentially connected to a flash evaporation unit, a catalytic reaction unit for methane cracking, a gas-solid separation unit, a membrane separation unit, a pressure swing adsorption unit, an electrochemical hydrogen pump unit, and a cryogenic adsorption unit. The flash evaporation unit is also connected to the LNG cold box via an LNG storage unit.

[0007] Preferably, the catalytic reaction unit for methane cracking includes a methane cracking reactor and a catalyst regenerator connected in sequence. The catalyst regenerator is also connected to the methane cracking reactor through a circulation pipeline. The methane cracking reactor is connected to a flash evaporation unit and a gas-solid separation unit, respectively.

[0008] Preferably, the methane cracking reactor is a moving bed reactor or a fluidized bed reactor.

[0009] Preferably, the gas-solid separation unit includes a cyclone separator and a filter connected in sequence, the cyclone separator being connected to the methane cracking reactor, and the filter being connected to the membrane separation unit.

[0010] Preferably, the membrane separation unit is a hollow fiber membrane separation device.

[0011] Preferably, the electrochemical hydrogen pump unit includes a humidifier and an electrochemical hydrogen pump connected in sequence. The anode outlet and cathode outlet of the electrochemical hydrogen pump are respectively connected to a first dryer and a second dryer. The humidifier is connected to a pressure swing adsorption unit, and the first dryer is connected to a low-temperature adsorption unit.

[0012] Preferably, the low-temperature adsorption unit is a low-temperature Dewar.

[0013] A method for producing hydrogen and helium using a BOG (Bottle-Off Gas) system, employing the aforementioned system, is detailed below: (i) Natural gas enters the LNG cold box and is liquefied into LNG, and then enters the flash unit. The pressure of the flash unit is adjusted to 0.12~0.45MPa and the temperature is kept consistent with the temperature of the LNG cold box. Gas-liquid separation occurs, and helium-rich BOG and LNG are obtained respectively. (ii) Helium-rich BOG is discharged from the top of the flash unit and enters the catalytic reaction unit; the LNG obtained by gas-liquid separation enters the LNG storage unit. In the LNG storage unit, part of the LNG generates helium-lean BOG, which is discharged from the top of the LNG storage unit and returned to the LNG cold box; the remaining LNG is stored in the LNG storage unit. (III) In the catalytic reaction unit, the reaction pressure is adjusted to atmospheric pressure and the reaction temperature is 600~1000℃. Methane is cracked to produce carbon materials and hydrogen, which enter the gas-solid separation unit. Some carbon materials are removed by the cyclone separator. The reaction gas from the cyclone separator enters the filter to further remove the remaining carbon materials, and then enters the membrane separation unit. (iv) In the membrane separation unit, the permeate gas generated after separation by the membrane enters the pressure swing adsorption unit, and the residual gas is discharged. (v) In the pressure swing adsorption unit, impurities are adsorbed and separated to obtain a mixture of hydrogen and helium, which enters the electrochemical hydrogen pump unit. The humidity is adjusted to 50-80% and the temperature is adjusted to 30-70℃ by a humidifier. Then, the hydrogen and helium are separated by the electrochemical hydrogen pump. The hydrogen is dried by the second dryer and discharged. The helium is dried by the first dryer and enters the low-temperature adsorption unit to remove impurities, obtaining helium product gas. The temperature of the low-temperature adsorption unit is -198 to -190℃.

[0014] Preferably, the LNG cold box has a pressure of 4~6MPa and a temperature of -164~-160℃.

[0015] Preferably, the pressure of the LNG storage unit is atmospheric pressure, and the temperature is consistent with the temperature of the LNG cold box.

[0016] Advantages of this invention: 1. This invention overcomes the technical limitations of "single utilization" of BOG (Boiler Gas), taking "high proportion of methane" (hydrogen source) and "trace amount of helium" (scarce resource) in BOG as synergistic recovery targets, realizing "full component utilization" of methane and helium in BOG, solving the problem of idle or wasted components under single process, and greatly improving the resource utilization efficiency of BOG; compared with the traditional independent process of "extracting helium first and then processing methane" or "only producing hydrogen", this invention reduces the overall equipment investment and operating costs; 2. The present invention adds a flash tank with a pressure higher than atmospheric pressure before the LNG storage tank, which can significantly reduce the amount of BOG gas discharged from the top of the flash tank and sent to the reactor; based on the reduction of gas volume, the scale of subsequent treatments such as catalytic reaction unit, membrane separation unit and pressure swing adsorption unit can be reduced, directly reducing the equipment investment cost of the process. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the system for producing hydrogen and helium using BOG as described in this invention. Among them, 1-LNG cold box, 2-flash evaporation unit, 3-LNG storage unit, 4-catalytic reaction unit, 5-gas-solid separation unit, 6-membrane separation unit, 7-pressure swing adsorption unit, 8-electrochemical hydrogen pump unit, and 9-low temperature adsorption unit. Detailed Implementation

[0018] Example 1: A system for producing hydrogen and helium using BOG includes an LNG cold box 1, wherein the LNG cold box 1 is sequentially connected to a flash evaporation unit 2, a catalytic reaction unit 4 for methane cracking, a gas-solid separation unit 5, a membrane separation unit 6, a pressure swing adsorption unit 7, an electrochemical hydrogen pump unit 8, and a cryogenic adsorption unit 9. The flash evaporation unit 2 is also connected to the LNG cold box 1 through an LNG storage unit 3.

[0019] The method for producing hydrogen and helium using the aforementioned system via BOG is as follows: (i) The raw material natural gas enters the LNG cold box 1 and is liquefied into LNG, and then enters the flash unit 2. The pressure and temperature of the flash unit 2 are adjusted to achieve gas-liquid separation, and helium-rich BOG and LNG are obtained respectively. (ii) Helium-rich BOG is discharged from the top of flash unit 2 and enters catalytic reaction unit 4; LNG obtained by gas-liquid separation enters LNG storage unit 3. In LNG storage unit 3, part of LNG generates helium-poor BOG, which is discharged from the top of LNG storage unit 3 and returned to LNG cold box 1; the remaining LNG is stored in LNG storage unit 3. (iii) In the catalytic reaction unit 4, the reaction pressure is adjusted to atmospheric pressure and the reaction temperature is 600-1000℃. Methane is cracked to produce carbon materials and hydrogen, which enter the gas-solid separation unit 5. After the carbon materials are removed, the gas enters the membrane separation unit 6. (iv) In the membrane separation unit 6, after the separation by the membrane, the generated permeate enters the pressure swing adsorption unit 7, and the residual permeate is discharged to the flare system. (v) In the pressure swing adsorption unit 7, impurities are adsorbed and separated to obtain a mixture of hydrogen and helium containing trace impurities. This mixture enters the electrochemical hydrogen pump unit 8 to separate the hydrogen and helium. The hydrogen obtained is discharged as a product. The separated helium flows into the low-temperature adsorption unit 9 at a temperature of -198 to -190°C to remove impurities and obtain helium product gas.

[0020] Example 2: Based on Example 1, the catalytic reaction unit 4 for methane cracking includes a methane cracking reactor and a catalyst regenerator connected in sequence. The catalyst regenerator is also connected to the methane cracking reactor via a circulation pipeline. The methane cracking reactor is connected to a flash evaporation unit 2 and a gas-solid separation unit 5, respectively. Catalysts with reduced activity due to carbon buildup in the methane cracking reactor enter the catalyst regenerator to burn off the coke on the coked catalyst. The heat released during coke combustion raises the catalyst temperature. The hot catalyst is returned to the methane cracking reactor via the circulation pipeline, providing heat for the methane cracking reaction. Preferably, the methane cracking reactor is a moving bed reactor or a fluidized bed reactor; The gas-solid separation unit 5 includes a cyclone separator and a filter connected in sequence. The cyclone separator is connected to the methane cracking reactor, and the filter is connected to the membrane separation unit 6. The membrane separation unit 6 is a hollow fiber membrane separation device; The electrochemical hydrogen pump unit 8 includes a humidifier and an electrochemical hydrogen pump connected in sequence. The anode outlet and cathode outlet of the electrochemical hydrogen pump are respectively connected to a first dryer and a second dryer. The humidifier is connected to the pressure swing adsorption unit 7, and the first dryer is connected to the low temperature adsorption unit 9. The low-temperature adsorption unit 9 is a low-temperature Dewar; The flash evaporation unit 2 is a flash evaporation tank; The methane cracking reactor is filled with a catalyst for methane cracking, including nickel-based catalysts, iron-based catalysts or noble metal catalysts in the prior art. The pressure swing adsorption unit 7 is a pressure swing adsorption tower device, which is filled with molecular sieve adsorbent to adsorb impurities in the permeate gas of the membrane separation unit 6. The LNG storage unit 3 is an LNG storage tank.

[0021] A method for producing hydrogen and helium using a BOG (Bottle-Off Gas) system, implemented using the system described in this embodiment, is as follows: (i) Natural gas enters LNG cold box 1 and is liquefied into LNG, and then enters flash unit 2. The pressure of flash unit 2 is adjusted to 0.12~0.45MPa and the temperature is kept consistent with the temperature of LNG cold box 1, and gas-liquid separation occurs, yielding helium-rich BOG and LNG respectively. (ii) Helium-rich BOG is discharged from the top of flash unit 2 and enters catalytic reaction unit 4; LNG obtained by gas-liquid separation enters LNG storage unit 3. In LNG storage unit 3, part of LNG generates helium-poor BOG, which is discharged from the top of LNG storage unit 3 and returned to LNG cold box 1; the remaining LNG is stored in LNG storage unit 3. (iii) In the catalytic reaction unit 4, the reaction pressure is adjusted to atmospheric pressure and the reaction temperature is 600-1000℃. Methane is cracked to produce carbon materials and hydrogen, which enter the gas-solid separation unit 5. After passing through the cyclone separator, some carbon materials are removed. The reaction gas from the cyclone separator enters the filter to further remove the remaining carbon materials, and then enters the membrane separation unit 6. (iv) In the membrane separation unit 6, after the separation effect of the membrane, the generated permeate gas enters the pressure swing adsorption unit 7, and the residual permeate gas is discharged. (v) In the pressure swing adsorption unit 7, impurities are adsorbed and separated to obtain a mixture of hydrogen and helium, which enters the electrochemical hydrogen pump unit 8. After passing through a humidifier, the humidity is adjusted to 50-80% and the temperature is adjusted to 30-70℃. Then, the hydrogen and helium are separated by the electrochemical hydrogen pump. The hydrogen is dried by the second dryer and then discharged. The helium is dried by the first dryer and then enters the low temperature adsorption unit 9 at a temperature of -198~-190℃ to remove impurities, and helium product gas is obtained. The pressure of the LNG cold box 1 is 4-6 MPa and the temperature is -164~-160℃; The pressure of the LNG storage unit 3 is atmospheric pressure, and the temperature is consistent with that of the LNG cold box 1.

[0022] Example 3: A method for producing hydrogen and helium using BOG, employing the system described in Example 2, as follows: (i) Natural gas enters LNG cold box 1 and is liquefied into LNG, wherein the pressure of LNG cold box 1 is 5 MPa and the temperature is -160℃; the molar composition of LNG in LNG cold box 1 is 97.48% methane, 2.08% nitrogen, 0.27% ethane, 0.1% helium, 0.01% hydrogen and trace amounts of impurities such as argon and neon; then it enters flash evaporation unit 2, the pressure of flash evaporation unit 2 is adjusted to 0.15 MPa and the temperature is kept consistent with the temperature of LNG cold box 1, gas-liquid separation occurs in flash evaporation unit 2, and helium-rich BOG and LNG are obtained respectively, wherein the molar composition of helium-rich BOG gas is: 81% methane, 6.5% helium, 11.2% nitrogen and trace amounts of hydrogen, ethane, argon and neon; (ii) The helium-rich BOG is discharged from the top of the flash unit 2 and enters the catalytic reaction unit 4. The methane cracking reactor in the catalytic reaction unit 4 is a fluidized bed reactor. The LNG obtained through gas-liquid separation enters LNG storage unit 3, which is at atmospheric pressure and at the same temperature as the LNG cold box 1. Inside LNG storage unit 3, some of the LNG generates lean helium BOG, which is discharged from the top of LNG storage unit 3 and returned to LNG cold box 1. The remaining LNG is stored inside LNG storage unit 3. (iii) In catalytic reaction unit 4, the methane cracking reactor is packed with a methane cracking catalyst (specifically, a Fe / Al2O3 catalyst with a Fe to Al2O3 molar ratio of 1:1), and the mass hourly space velocity of the helium-rich BOG is 0.049 h⁻¹. -1 The reaction pressure in the methane cracking reactor is adjusted to atmospheric pressure and the reaction temperature to 700℃. Methane cracking produces carbon materials and hydrogen. The molar composition of the products is 53.4% ​​hydrogen, 26.6% carbon materials, 11.4% methane, 3% helium, 5.7% nitrogen, and trace amounts of impurities such as argon and neon. The products enter the gas-solid separation unit 5. After passing through a cyclone separator, most of the solid carbon materials are removed. The reaction gas from the cyclone separator enters a filter to further remove the remaining carbon materials. The molar composition of the decarbonized gas is: 72.7% hydrogen, 15.5% methane, 7.6% nitrogen, 4.1% helium, and trace amounts of impurities such as argon and neon. This decarbonized gas enters the membrane separation unit 6. The catalyst with reduced activity due to carbon buildup in the methane cracking reactor enters the catalyst regenerator to burn off the coke on the coked catalyst. The coke combustion releases heat, raising the catalyst temperature. The hot catalyst is returned to the methane cracking reactor through a circulation pipeline to provide heat for the methane cracking reaction. (iv) In the membrane separation unit 6, after separation by the hollow fiber membrane, helium and hydrogen are enriched on the permeate side, while methane, nitrogen and other gases are enriched on the residual gas side. The permeate enters the pressure swing adsorption unit 7, and the residual gas is discharged to the boundary flare system. The molar composition of the permeate is 91.8% hydrogen, 5.2% helium, 2% methane, 1% nitrogen, and trace impurities such as argon and neon. (V) In the pressure swing adsorption unit 7, the pressure swing adsorption tower is filled with 13X molecular sieve adsorbent. Trace amounts of methane and nitrogen impurities in the gas are adsorbed and separated to obtain a high-purity hydrogen and helium mixture with the following molar composition: 94.6% hydrogen, 5.4% helium, and trace amounts of methane, nitrogen, argon, and helium impurities. This hydrogen and helium mixture enters the electrochemical hydrogen pump unit 8, where the humidity is adjusted to 80% and the temperature is adjusted to 50°C by a humidifier. Then, the hydrogen and helium are separated by the electrochemical hydrogen pump. The hydrogen is dried by a second dryer and discharged to obtain high-purity hydrogen with a purity of 99.999%. The helium is dried by a first dryer to obtain helium with a purity of 99.8%. Then, it enters a low-temperature Dewar at a temperature of -196°C to further remove trace amounts of argon and neon to obtain helium product gas with a purity of 99.999%.

Claims

1. A system for producing hydrogen and helium using a BOG (Bottle-Off Gas) container, comprising an LNG cold box, characterized in that: The LNG cold box is sequentially connected to a flash evaporation unit, a catalytic reaction unit for methane cracking, a gas-solid separation unit, a membrane separation unit, a pressure swing adsorption unit, an electrochemical hydrogen pump unit, and a cryogenic adsorption unit. The flash evaporation unit is also connected to the LNG cold box through an LNG storage unit.

2. The system for producing hydrogen and helium using a BOG according to claim 1, characterized in that: The catalytic reaction unit for methane cracking includes a methane cracking reactor and a catalyst regenerator connected in sequence. The catalyst regenerator is also connected to the methane cracking reactor via a circulation pipeline. The methane cracking reactor is connected to a flash evaporation unit and a gas-solid separation unit, respectively.

3. The system for producing hydrogen and helium using a BOG according to claim 1, characterized in that: The methane cracking reactor is a moving bed reactor or a fluidized bed reactor.

4. The system for producing hydrogen and helium using a BOG according to claim 2, characterized in that: The gas-solid separation unit includes a cyclone separator and a filter connected in sequence. The cyclone separator is connected to the methane cracking reactor, and the filter is connected to the membrane separation unit.

5. The system for producing hydrogen and helium using a BOG according to claim 1, characterized in that: The membrane separation unit is a hollow fiber membrane separation device.

6. The system for producing hydrogen and helium using a BOG according to claim 4, characterized in that: The electrochemical hydrogen pump unit includes a humidifier and an electrochemical hydrogen pump connected in sequence. The anode outlet and cathode outlet of the electrochemical hydrogen pump are respectively connected to a first dryer and a second dryer. The humidifier is connected to a pressure swing adsorption unit, and the first dryer is connected to a low-temperature adsorption unit.

7. The system for producing hydrogen and helium using a BOG according to claim 1, characterized in that: The low-temperature adsorption unit is a low-temperature Dewar.

8. A method for producing hydrogen and helium using BOG, characterized in that: The system described in claim 6 is used, specifically as follows: (i) Natural gas enters the LNG cold box and is liquefied into LNG, and then enters the flash unit. The pressure of the flash unit is adjusted to 0.12~0.45MPa and the temperature is kept consistent with the temperature of the LNG cold box. Gas-liquid separation occurs, and helium-rich BOG and LNG are obtained respectively. (ii) Helium-rich BOG is discharged from the top of the flash unit and enters the catalytic reaction unit; The LNG obtained through gas-liquid separation enters the LNG storage unit. Inside the LNG storage unit, some of the LNG is converted into lean helium BOG, which is discharged from the top of the LNG storage unit and returned to the LNG cold box; the remaining LNG is stored inside the LNG storage unit. (III) In the catalytic reaction unit, the reaction pressure is adjusted to atmospheric pressure and the reaction temperature is 600~1000℃. Methane is cracked to produce carbon materials and hydrogen, which enter the gas-solid separation unit. Some carbon materials are removed by the cyclone separator. The reaction gas from the cyclone separator enters the filter to further remove the remaining carbon materials, and then enters the membrane separation unit. (iv) In the membrane separation unit, the permeate gas generated after separation by the membrane enters the pressure swing adsorption unit, and the residual gas is discharged. (v) In the pressure swing adsorption unit, impurities are adsorbed and separated to obtain a mixture of hydrogen and helium, which enters the electrochemical hydrogen pump unit. The humidity is adjusted to 50-80% and the temperature is adjusted to 30-70℃ by a humidifier. Then, the hydrogen and helium are separated by the electrochemical hydrogen pump. The hydrogen is dried by the second dryer and discharged. The helium is dried by the first dryer and enters the low-temperature adsorption unit to remove impurities, obtaining helium product gas. The temperature of the low-temperature adsorption unit is -198 to -190℃.

9. The method for producing hydrogen and helium using BOG according to claim 8, characterized in that: The LNG cold box has a pressure of 4~6MPa and a temperature of -164~-160℃.

10. The method for producing hydrogen and helium using BOG according to claim 8, characterized in that: The LNG storage unit is at atmospheric pressure and its temperature is consistent with that of the LNG cold box.

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