An apparatus and method for producing LNG from coke oven gas using a three-tower dual-nitrogen circulation refrigeration system.

Abstract

This invention discloses an apparatus and method for producing LNG from coke oven gas using a three-tower dual-nitrogen cycle refrigeration system. This technology belongs to the field of cryogenic liquefaction separation and aims to solve the problems of low methane recovery rate and high energy consumption in existing coke oven gas-to-LNG processes. The core of the apparatus includes a main heat exchanger, a subcooler, a dehydrogenation tower 1, a dehydrogenation tower 2, a denitrification tower, and a dual-nitrogen cycle refrigeration system. The method employs a two-stage series dehydrogenation process, utilizing a dual-nitrogen cycle to provide cooling at different temperature levels: one stage is a conventional pressure nitrogen cycle, and the other is a deeply throttled low-pressure nitrogen cycle, specifically used to provide an ultra-low temperature cold source for the secondary dehydrogenation stage. This process can reduce the methane content in hydrogen-rich tail gas to below 2000 ppm, significantly improving LNG production and hydrogen purity. This method achieves a higher methane recovery rate, resulting in increased net profit through a substantial increase in LNG production, demonstrating significant economic benefits and industrial application value.

Description

Technical Field

[0001] This invention relates to the field of cryogenic liquefaction separation technology, specifically to an apparatus and method for producing LNG from coke oven gas using a three-tower dual-nitrogen circulation refrigeration system. Background Technology

[0002] Coke oven gas is one of the main byproducts of the coal dry distillation process in coking plants, and its main components include hydrogen, methane, CO, CO2, and nitrogen. Utilizing coke oven gas to produce LNG not only reduces resource waste and environmental pollution but also brings significant economic benefits to the plant.

[0003] The composition of coke oven gas varies greatly depending on the unit. Based on different raw gas compositions, multiple process flows are explored on the existing process routes to increase LNG production (methane recovery rate), reduce energy consumption, and provide customers with higher economic benefits.

[0004] Patent CN114111217A discloses an apparatus and method for producing LNG and synthetic ammonia feedstock gas using liquid nitrogen washing. This method has a high methane yield and low energy consumption, but its disadvantage is that it requires continuous replenishment of liquid nitrogen to wash the methane in the coke oven gas. The replenished liquid nitrogen will also enter the hydrogen-rich gas. When the nitrogen-to-hydrogen ratio in the raw coke oven gas exceeds the 1:3 ratio required for synthetic ammonia, the nitrogen replenished during washing must be vented to meet the requirements of synthetic ammonia for feedstock gas. The consumption of nitrogen reduces the profitability of the apparatus and increases energy consumption. In other words, this method is selective for raw coke oven gas, and the liquid nitrogen washing process is only applicable to apparatuses that use tail gas for synthetic ammonia.

[0005] Patent CN103697659A discloses an apparatus and method for producing liquefied natural gas and hydrogen-rich products from methane-rich gas. This method involves double-tower distillation to dehydrogenate and denitrogenate LNG, hydrogen-rich gas, and nitrogen-rich gas. It is a conventional method for producing LNG and synthetic ammonia feedstock gas from coke oven gas. This method is widely used, but its disadvantage is that it requires higher energy consumption and a larger nitrogen circulation volume to achieve the same methane recovery rate.

[0006] Patent CN102504900A discloses an apparatus and method for producing liquefied natural gas from coke oven gas. This method is a single-tower distillation process for producing LNG by dehydrogenation and denitrogenation. Compared with double-tower distillation, single-tower distillation has higher energy consumption.

[0007] Currently, the mainstream technology for industrial LNG production from hydrogen-containing coke oven gas typically employs cryogenic separation. Common process routes are mostly single-tower dehydrogenation or simple two-tower processes. In these existing technologies, only one dehydrogenation tower is usually installed, coupled with a single refrigeration cycle system (such as a single nitrogen cycle or a mixed refrigerant cycle).

[0008] After pretreatment, the feed gas enters the dehydrogenation tower directly, where methane is condensed and separated at a specific low temperature. Hydrogen-rich tail gas is discharged from the top of the tower, and LNG product is obtained from the bottom of the tower.

[0009] However, the aforementioned existing technologies have the following significant technical defects in practical applications, which limit their economy and efficiency: In single-tower dehydrogenation processes, in order to reduce energy consumption, it is often impossible to lower the tower top temperature to extremely low levels. This results in the hydrogen-rich tail gas still containing a high concentration of methane (typically around 1%, or even higher). This not only causes the loss of valuable LNG resources and reduces methane recovery rates, but also directly reduces the final LNG production because the methane is not fully condensed.

[0010] (2) There is a contradiction between energy consumption and recovery rate. If we try to improve the methane recovery rate by further reducing the temperature in the existing single-tower process, we will face the problem of a sharp increase in energy consumption. In order to obtain a lower condensation temperature, the refrigerant (such as nitrogen) must be throttled to a lower pressure. The pressure ratio of the nitrogen circulation machine will increase, which will lead to a significant increase in compressor power consumption, resulting in a significant increase in the operating cost per unit product and a decrease in economic efficiency.

[0011] In summary, there is an urgent need for a method for producing LNG from coke oven gas through dehydrogenation that can effectively improve methane recovery rate and increase LNG production, while also reasonably controlling energy consumption and reducing operating costs. Summary of the Invention

[0012] To address the aforementioned problems, this invention provides an apparatus and method for producing LNG using a three-tower dual-nitrogen circulation refrigeration system for coke oven gas.

[0013] Specifically, a three-tower dehydrogenation and denitrification device for producing LNG from coke oven gas includes a main heat exchanger, a subcooler, a dehydrogenation tower 1, a dehydrogenation tower 2, a denitrification tower, and a supporting condenser and compressor system.

[0014] Specifically, the main heat exchanger is equipped with a coke oven gas forward flow channel, an LNG forward flow channel, a hydrogen-rich gas counterflow channel, a high-pressure nitrogen forward flow channel, a low-pressure nitrogen counterflow channel, and an independent low-low-pressure nitrogen counterflow channel.

[0015] Specifically, the subcooler is equipped with a hydrogen-rich forward flow channel, a methane-rich liquid reverse flow channel, and an independent low-pressure reverse flow channel.

[0016] Specifically, the top gas phase outlet of the dehydrogenation tower 1 is connected in sequence to the dehydrogenation tower 1 condenser, the dehydrogenation tower 1 reflux tank, and the dehydrogenation tower 2 via pipelines.

[0017] Specifically, the top gas phase outlet of the dehydrogenation tower 2 is connected to the condenser of the dehydrogenation tower 2 via a pipeline.

[0018] Specifically, the nitrogen outlet of the dehydrogenation tower 2 condenser is directly connected to the inlet of the low-pressure reflux channel of the subcooler via a pipeline.

[0019] Specifically, the outlet of the low-low pressure reflux channel of the subcooler is connected to the inlet of the low-low pressure nitrogen reflux channel of the main heat exchanger via a pipeline.

[0020] Specifically, the outlet of the low-pressure nitrogen reflux channel of the main heat exchanger is connected to the inlet of the low-pressure nitrogen booster system via a pipeline.

[0021] Specifically, the outlet of the low-pressure nitrogen booster system is connected to the inlet pipe of the nitrogen compressor system via a pipeline.

[0022] Specifically, both the main heat exchanger and the subcooler are multi-channel brazed plate-fin heat exchangers or coiled tube heat exchangers.

[0023] Specifically, the low-pressure nitrogen reflux channel and the low-pressure nitrogen reflux channel are physically isolated inside the heat exchanger and are used to circulate nitrogen of different pressure levels respectively.

[0024] Specifically, the device also includes a low-pressure nitrogen pressurization system.

[0025] Specifically, the low-pressure nitrogen booster system is a blower or a low-pressure compressor.

[0026] Specifically, the inlet of the low-pressure nitrogen booster system is connected to the outlet of the low-pressure nitrogen reflux channel of the main heat exchanger via a pipe, and its outlet is connected to the inlet pipe of the nitrogen compressor system.

[0027] Specifically, the flow path connection relationship of the device is as follows: Specifically, the gas phase outlet of the feed gas separator is connected to the inlet of the hydrogen-rich gas forward flow channel of the subcooler.

[0028] Specifically, the outlet of the hydrogen-rich gas reflux channel of the subcooler is connected to the inlet of the hydrogen-rich gas reflux channel of the main heat exchanger.

[0029] Specifically, the bottom liquid phase outlet of dehydrogenation tower 1 and the bottom liquid phase outlet of dehydrogenation tower 2 are both connected to the inlet of the methane-rich liquid reflux channel of the subcooler.

[0030] Specifically, a denitrification tower reboiler is provided at the bottom of the denitrification tower.

[0031] Specifically, the denitrification tower reboiler is equipped with an MR liquid channel, which is connected to the liquid phase outlet of the MR separator via a pipeline.

[0032] Specifically, the dehydrogenation tower 1 condenser and the dehydrogenation tower 2 condenser are respectively equipped with independent liquid nitrogen throttling valves for the dehydrogenation tower 1 condenser and the dehydrogenation tower 2 condenser, which are used to control the flow rate of liquid nitrogen entering the two condensers respectively.

[0033] A method for producing LNG from coke oven gas using a three-tower dehydrogenation and denitrogenation process includes the following steps: S1 Raw Material Pretreatment and Separation: After being cooled in the coke oven gas forward flow channel of the main heat exchanger, the coke oven gas enters the raw material gas separator for gas-liquid separation to obtain a hydrogen-rich gas phase and a methane-rich liquid phase.

[0034] S2 Two-Stage Series Dehydrogenation: The hydrogen-rich gas phase passes sequentially through a subcooler, dehydrogenation tower 1, dehydrogenation tower 1 condenser (E3), dehydrogenation tower 2, and dehydrogenation tower 2 condenser for distillation separation.

[0035] S3 Dual Nitrogen Cycle Refrigeration: High-pressure nitrogen is used as the cold source and is divided into two paths for throttling refrigeration: After throttling, the first path of nitrogen enters the condenser of the dehydrogenation tower 1 and the condenser of the denitrification tower to provide cooling capacity, and after vaporization, it is used as low-pressure nitrogen for rewarming.

[0036] The second stream of nitrogen is deeply throttled to 1.0~1.8 barg and enters the condenser of the dehydrogenation tower 2 to provide low-temperature cooling. After vaporization, it is reheated as low-pressure nitrogen and then pressurized by the low-pressure nitrogen booster system.

[0037] S4 Mixed Refrigerant (MR) Refrigeration Cycle: MR gas is compressed by the MR compressor system and then split into two paths: The first path is cooled by the main heat exchanger and throttled by the upper throttle valve of the MR before entering the MR flash tank; The second stream, after being cooled by the main heat exchanger, enters the MR separator. The separated liquid phase passes through the MR liquid forward flow channel and the denitrification tower reboiler for heat exchange, and then enters the MR flash tank through the throttling valve. The separated gas phase passes through the MR gas forward flow channel and the MR lower throttling valve for throttling, and then enters the lower part of the MR flash tank. S5 Denitrification and Product Output: After the methane-rich liquid at the bottom of dehydrogenation tower 1 and dehydrogenation tower 2 merges, it enters the denitrification tower through throttling for denitrification. LNG product is obtained at the bottom of the tower, and nitrogen-rich gas is reheated and discharged from the top of the tower.

[0038] Specifically, in step S2, the operating pressure of the dehydrogenation tower 1 is controlled at 20~25 barg, and the top temperature of the tower is controlled at -179℃ to -181℃.

[0039] Specifically, the operating pressure of dehydrogenation tower 2 is controlled at 18~23 barg, and the tower top temperature is controlled at -190℃ to -192℃, so that the methane molar content in the hydrogen-rich gas at the outlet of the dehydrogenation tower 2 reflux tank is less than 2000 ppm.

[0040] Specifically, in step S3, after the low-pressure nitrogen is reheated in the low-pressure reflux channel of the subcooler and the low-pressure nitrogen reflux channel of the main heat exchanger, it enters the low-pressure nitrogen pressurization system and is pressurized to 2.5~3.5 barg, and then flows into the nitrogen compressor system.

[0041] It has the following beneficial effects: (1) High methane recovery rate, significantly increasing LNG production. By setting up a two-stage dehydrogenation tower and using dual nitrogen circulation to provide a lower condensation temperature, the methane content in hydrogen-rich gas can be further reduced to below 2000 ppm (current technology is usually around 1%). The methane lost with the tail gas will be effectively recovered and converted into LNG products, directly increasing LNG production.

[0042] (2) Significantly reduced energy consumption. A "dual nitrogen cycle" refrigeration strategy is adopted. Most of the nitrogen is maintained in a conventional pressure cycle (first-stage cycle), and only a small portion of the nitrogen is deeply throttled to provide low-temperature cooling (second-stage cycle). At the same output, compared with single-stage dehydrogenation, this scheme can save approximately 119 kW·h of energy (energy saving ratio exceeding 20%). Calculated based on 8,000 hours of operation per year, it can save approximately RMB 476,000 in electricity costs annually.

[0043] (3) Improved quality of hydrogen-rich tail gas. Due to the deep removal of methane, the hydrogen molar fraction in the hydrogen-rich tail gas can be increased to over 90%. High-purity hydrogen tail gas is more conducive to subsequent use as feedstock for ammonia synthesis or for hydrogen purification, thus increasing the economic value of the by-product.

[0044] (4) Excellent balance between equipment investment and operating costs. While increasing LNG production, the low-pressure nitrogen circulation system can use a blower instead of a high-pressure compressor for pressurization due to the small pressure difference, avoiding the investment in expensive high-pressure compressor equipment. The overall equipment investment is not much different from that of conventional processes, but the economic benefits are greatly improved. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0046] Figure 1 This is a schematic diagram of the structure of a device for producing LNG from coke oven gas using a three-tower dual-nitrogen circulation refrigeration system according to the present invention. Figure reference numerals: T1 - Dehydrogenation tower 1, T2 - Dehydrogenation tower 2, T3 - Denitrification tower, C1 - MR compressor system, C2 - Low-pressure nitrogen booster system, C3 - Nitrogen compressor system, E1 - Main heat exchanger, E2 - Subcooler, E3 - Condenser of dehydrogenation tower 1, E4 - Condenser of dehydrogenation tower 2, E5 - Condenser of denitrification tower, E6 - Reboiler of denitrification tower, S1 - Feed gas separator, S2 - Reflux tank of dehydrogenation tower 1, S3 - Reflux tank of dehydrogenation tower 2, S4 - Reflux tank of denitrification tower, S5 - Upper part of MR flash tank, S6 - Middle part of MR flash tank, S7 - MR flash tank Below the tank, S8-MR separator, A1-coke oven gas forward flow channel, A2-LNG forward flow channel, A3-hydrogen-rich gas reflux channel, A4-nitrogen-rich gas reflux channel, A5-high-pressure nitrogen forward flow channel, A6-low-pressure nitrogen reflux channel, A7-low-pressure nitrogen reflux channel, A8-high-pressure MR liquid forward flow channel, A9-high-pressure MR gas forward flow channel, A10-MR liquid forward flow channel, A11-MR gas forward flow channel, A12-low-pressure MR reflux channel, A21-hydrogen-rich gas forward flow channel, A22-methane-rich liquid reflux channel, A... 23-Hydrogen-rich reflux channel, A24-Nitrogen-rich reflux channel, A25-Nitrogen forward flow channel, A26-Low-pressure nitrogen reflux channel, A27-Low-low pressure reflux channel, A31-Hydrogen-rich channel for condenser of dehydrogenation tower 1, A32-Nitrogen channel for condenser of dehydrogenation tower 1, A33-Hydrogen-rich channel for condenser of dehydrogenation tower 2, A34-Nitrogen channel for condenser of dehydrogenation tower 2, A35-Nitrogen-rich channel for condenser of denitrification tower, A36-Nitrogen channel for condenser of denitrification tower, A37-LNG channel for reboiler of denitrification tower, A38-MR liquid channel for reboiler of denitrification tower. V1 - Bottom level control valve of feed gas separator; V2 - Level control valve of dehydrogenation tower 1; V3 - Level control valve of dehydrogenation tower 2; V4 - LNG take-out valve; V5 - Liquid nitrogen throttling valve of condenser in dehydrogenation tower 1; V6 - Liquid nitrogen throttling valve of condenser in dehydrogenation tower 2; V7 - Liquid nitrogen throttling valve of condenser in denitrification tower; V8 - Liquid nitrogen bypass throttling valve; V9 - Dehydrogenation pressure control valve; V10 - Denitrification pressure control valve; V11 - Upper throttling valve of MR; V12 - Middle throttling valve of MR; V14 - Lower throttling valve of MR; V12 - Bypass throttling valve of reboiler in denitrification tower. Detailed Implementation

[0047] It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0048] The following detailed description of the implementation method of the present invention is in conjunction with the accompanying drawings. The description is only a partial embodiment and not all embodiments. For clarity, representations and descriptions unrelated to the present invention are omitted in the drawings and description.

[0049] To provide a clearer understanding of the technical features, objectives, and beneficial effects of this invention, the following detailed description of the technical solution is provided. Obviously, the described embodiments are only a portion of the embodiments of this invention, not all of them, and should not be construed as limiting the scope of implementation of this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of this invention.

[0050] Example 1 This embodiment provides an apparatus for producing LNG using a three-tower dual-nitrogen circulation refrigeration system for coke oven gas.

[0051] like Figure 1 As shown, the device for producing LNG from coke oven gas using a three-tower dual-nitrogen circulation refrigeration system includes a main heat exchanger (E1), a subcooler (E2), a dehydrogenation tower 1 condenser (E3), a dehydrogenation tower 2 condenser (E4), a denitrification tower condenser (E5), a denitrification tower reboiler (E6), a feed gas separator (S1), a dehydrogenation tower 1 reflux tank (S2), a dehydrogenation tower 2 reflux tank (S3), a denitrification tower reflux tank (S4), an upper MR flash tank (S5), a middle MR flash tank (S6), a lower MR flash tank (S7), an MR separator (S8), a dehydrogenation tower 1 (T1), a dehydrogenation tower 2 (T2), a denitrification tower (T3), an MR compressor system (C1), a low-pressure nitrogen booster system (C2), and a nitrogen compressor system (C3). The main heat exchanger (E1) contains a coke oven gas compressor. The gas forward flow channel (A1), LNG forward flow channel (A2), hydrogen-rich gas reverse flow channel (A3), nitrogen-rich gas reverse flow channel (A4), high-pressure nitrogen forward flow channel (A5), low-pressure nitrogen reverse flow channel (A6), low-low-pressure nitrogen reverse flow channel (A7), high-pressure MR liquid forward flow channel (A8), high-pressure MR gas forward flow channel (A9), MR liquid forward flow channel (A10), MR gas forward flow channel (A11), and low-pressure MR reverse flow channel (A12) are provided. The subcooler (E2) is equipped with a hydrogen-rich forward flow channel (A21), a methane-rich liquid reverse flow channel (A22), a hydrogen-rich reverse flow channel (A23), a nitrogen-rich reverse flow channel (A24), a nitrogen forward flow channel (A25), a low-pressure nitrogen reverse flow channel (A26), and a low-low-pressure reverse flow channel (A27).

[0052] In this embodiment, the dehydrogenation tower 1 condenser is provided with a hydrogen-rich gas channel (A31) and a nitrogen channel (A32), the dehydrogenation tower 2 condenser is provided with a hydrogen-rich gas channel (A33) and a nitrogen channel (A34), the denitrification tower condenser is provided with a nitrogen-rich gas channel (A35) and a nitrogen channel (A36), and the denitrification tower reboiler is provided with an LNG channel (A37) and an MR liquid channel (A38).

[0053] In this embodiment, the following valves are provided: bottom level control valve (V1) of the feed gas separator, level control valve (V2) of dehydrogenation tower 1, level control valve (V3) of dehydrogenation tower 2, LNG take-out valve (V4), liquid nitrogen throttling valve (V5) of the condenser of dehydrogenation tower 1, liquid nitrogen throttling valve (V6) of the condenser of dehydrogenation tower 2, liquid nitrogen throttling valve (V7) of the condenser of denitrification tower, liquid nitrogen bypass throttling valve (V8), dehydrogenation pressure control valve (V9), denitrification pressure control valve (V10), upper throttling valve (V11) of MR, middle throttling valve (V12) of MR, lower throttling valve (V14) of MR, and MR bypass throttling valve (V12) of the reboiler of denitrification tower.

[0054] In this embodiment, the coke oven gas forward flow channel (A1) and the raw material gas separator (S1) of the main heat exchanger are connected according to the coke oven gas flow direction. According to the hydrogen-rich gas flow direction, the raw material gas separator (S1), the hydrogen-rich gas forward flow channel (A21) of the subcooler, the dehydrogenation tower 1 (T1), the hydrogen-rich gas channel of the dehydrogenation tower 1 condenser (A31), the dehydrogenation tower 1 reflux tank (S2), the dehydrogenation tower 2 (T2), the hydrogen-rich gas channel of the dehydrogenation tower 2 condenser (A33), the dehydrogenation tower 2 reflux tank (S3), the hydrogen-rich gas counterflow channel of the subcooler (A23), and the hydrogen-rich gas counterflow channel of the main heat exchanger (A3) are connected sequentially. According to the methane-rich liquid flow direction, the liquid level control valve (V2) of dehydrogenation tower 1 / liquid level control valve (V3) of dehydrogenation tower 2, the methane-rich liquid counterflow channel of the subcooler (A22), the denitrification tower (T3), the LNG forward flow channel (A2) of the main heat exchanger, and the LNG outlet valve (V4) are connected sequentially. According to the nitrogen flow direction, the high-pressure nitrogen forward flow channel of the main heat exchanger (A5), the nitrogen forward flow channel of the precooler (A25), the nitrogen throttling valve (V5 / V7 / V8), the nitrogen channel of the dehydrogenation tower 1 condenser (A32) / the nitrogen channel of the denitrification tower condenser (A34), the low-pressure nitrogen backflow channel of the subcooler (A26), the low-pressure nitrogen backflow channel of the main heat exchanger (A6), and the nitrogen compressor system (C3) are connected in sequence.

[0055] In this embodiment, the flow path sequence of low-pressure nitrogen is as follows: nitrogen throttle valve (V6), nitrogen passage of dehydrogenation tower 2 condenser (A34), low-pressure reflux passage of subcooler (A27), low-pressure reflux passage of main heat exchanger (A7), nitrogen blower system (C2), and nitrogen compressor system (C3). According to the MR flow direction, the main heat exchanger low-pressure MR reflux channel (A12), MR compressor system (C1), main heat exchanger MR liquid forward flow channel (A8), throttle valve (V11), MR flash tank upper (S5), and low-pressure MR reflux channel (A12) are connected in sequence. The MR compressor system (C1), high-pressure MR gas forward flow channel (A9), MR separator (S8), MR liquid forward flow channel (A10), denitrification tower reboiler MR channel (A38), throttle valve (V13) / bypass valve (V12), MR flash tank middle (S6), and low-pressure MR reflux channel (A12) are connected in sequence. The MR separator (S8), MR gas forward flow channel (A11), throttle valve (V14), MR flash tank lower (S7), and low-pressure MR reflux channel (A12) are connected in sequence.

[0056] Example 2 The apparatus for producing LNG from coke oven gas using a three-tower dual-nitrogen circulation refrigeration system as described in Example 1 can be prepared using the parameters for purifying the raw coke oven gas as shown in the table below. The parameters and composition of the resulting hydrogen-rich gas, nitrogen-rich gas, and LNG are shown in Table 1 below. Table 1 Based on the raw material coke oven gas parameters in Table 1, the equipment parameters used in the nitrogen circulation system are shown in Table 2: Table 2 In this embodiment, based on theoretical calculations, the methane content at the bottom of dehydrogenation tower 2 (T2) is 86 kg / h. The low-pressure nitrogen compressor's power is increased by approximately 100 kW. Assuming an LNG price of 3500 yuan / ton, electricity consumption of 0.5 yuan / kW, and 8000 hours of operation per year, the direct economic benefit is approximately 2.008 million yuan / year. The above calculations do not consider equipment investment, but the economic benefits are significant. Detailed investment and economic benefit comparisons can be made for specific projects.

[0057] In this embodiment, the specific implementation method and steps are as follows: S1, after being purified to remove impurities (H2O, CO2, etc.) and containing hydrogen, methane, ethane, propane, butane, pentane, nitrogen, etc., coke oven gas enters the coke oven gas positive flow channel A1 in the main heat exchanger (E1), is cooled to ~-162℃, and enters the raw material gas separator (S1) for gas-liquid separation. The methane-rich liquid at the bottom is throttled and depressurized to 6~7 barg by the liquid level control valve V1 before entering the middle of the denitrification tower (T3). The hydrogen-rich gas (5.4% methane content) at the top of the feed gas separator (S1) first enters the subcooler (E2) and is cooled to -175°C in the hydrogen-rich gas positive flow channel A21 before entering the bottom of dehydrogenation tower 1 (T1). The hydrogen-rich gas at the top of dehydrogenation tower 1 (T1) then enters the dehydrogenation tower 1 condenser (E3) and is cooled to -179~-181°C in the hydrogen-rich gas channel A31 before entering the dehydrogenation tower 1 reflux tank (S2) for gas-liquid separation. The liquid is returned to dehydrogenation tower 1 (T1) as rectification reflux liquid, while the top gas (1% methane content) enters the bottom of dehydrogenation tower 2 (T2) for further rectification. The gas phase at the top of the dehydrogenation tower 2 (T2) enters the hydrogen-rich gas channel A33 of the dehydrogenation tower 2 condenser (E4) and is cooled to -190~-192℃. Then it enters the dehydrogenation tower 2 reflux tank (S3) for gas-liquid separation. The liquid returns to the dehydrogenation tower 2 (T2) as the rectification reflux liquid. The gas at the top (methane content <0.2% mole) enters the hydrogen-rich gas reflux channel A23 in the subcooler (E2) and is reheated to -165~-170℃. It then enters the hydrogen-rich gas reflux channel A3 of the main heat exchanger (E1) and is reheated to room temperature before being taken out through the pressure control valve V9. The methane-rich liquid at the bottom of the S3 feed gas separator (S1) enters the middle of the denitrification tower (T3) through the level control valve V1. The methane-rich liquid at the bottom of dehydrogenation tower 1 (T1) and the methane-rich liquid at the bottom of dehydrogenation tower 2 (T2) are controlled by level control valves V2 and V3, respectively, and then enter the methane-rich liquid reflux channel A22 of the subcooler (E2) for reheating to -165~-170℃ before also entering the middle of the denitrification tower (T3). The liquid at the bottom of the denitrification tower (T3) first enters the LNG channel of the denitrification tower reboiler (E6) for heating to -133℃, ensuring that the nitrogen content of the LNG liquid exiting the bottom of the denitrification tower is ≤1%. The LNG exiting the bottom of the denitrification tower (T3) enters the LNG forward flow channel in the main heat exchanger (E1) for subcooling to -162℃, and then passes through the LNG outlet valve V4 to control the LNG temperature and output. The nitrogen-rich gas at the top of the S4 denitrification tower (T3) first enters the nitrogen-rich gas channel A35 of the denitrification tower condenser (E5) and is cooled to -176~-178℃. Then it enters the denitrification tower reflux tank (S4) for gas-liquid separation. The liquid returns to the denitrification tower (T3) as the distillation reflux liquid. The top gas (methane content ≤0.5% mole) enters the nitrogen-rich gas reflux channel A24 of the subcooler (E2) to be reheated to -165~-170℃. Then it enters the nitrogen-rich gas reflux channel A4 of the main heat exchanger (E1) to be reheated to room temperature and then taken out through the pressure control valve V10. Nitrogen gas with a pressure higher than 22 barg first enters the high-pressure nitrogen forward flow channel A5 of the main heat exchanger (E1) and is cooled to -162°C. Then, it enters the high-pressure nitrogen forward flow channel A25 of the subcooler (E2) for further subcooling to -175°C. Afterward, the high-pressure liquid nitrogen is divided into four parts. The first part is throttled to 2.8 barg by the throttling valve V5 and enters the nitrogen channel of the dehydrogenation tower 1 condenser (E3) to provide cooling for the dehydrogenation tower 1 condenser (E3). The third part is throttled to 2.8 barg by the throttling valve V7. The nitrogen gas enters the nitrogen passage of the denitrification tower condenser (E5) to provide cooling capacity to the dehydrogenation tower 2 condenser (E5). The fourth part is throttled to 2.8 barg by the throttle valve V8 as a valve to regulate the cooling capacity of the subcooler. The low-pressure and low-temperature nitrogen gas from the throttle valves V6, V7 and V8 merges and enters the low-pressure nitrogen reflux passage A26 of the subcooler (E2) to be reheated to -165~-170℃. Then it enters the nitrogen reflux passage A6 of the main heat exchanger (E1) to be reheated to room temperature and then enters the nitrogen compression system (C3). The second part of the liquid nitrogen in S6, which is divided into four parts, first enters the liquid nitrogen throttling valve V6 to reduce its pressure to 1.3 barg. Then it enters the nitrogen channel of the dehydrogenation tower 2 condenser (E4) to provide cooling capacity for the dehydrogenation tower 2 condenser (E4). After that, it enters the low-pressure nitrogen reflux channel A27 of the subcooler (E2) to be reheated to -165~-170℃. Then it enters the low-pressure nitrogen reflux channel A7 of the main heat exchanger (E1) to be reheated to room temperature. After that, it enters the low-pressure nitrogen boosting system (C2) to be boosted to 2.5 barg. Then it merges with the low-pressure nitrogen after being reheated by the main heat exchanger and enters the nitrogen boosting system (C3) to be boosted to 23 barg. The high-pressure room-temperature nitrogen then enters the high-pressure nitrogen channel A5 of the main heat exchanger (E1) for cooling. This cycle of compression and refrigeration continues. The high-pressure room-temperature liquid from the MR compressor system first enters the high-pressure MR liquid channel A8 in the main heat exchanger (E1) and is cooled to -60°C. Then it enters the throttling valve V11 and is throttled to 2.7 barg before entering the MR flash tank (S5). After that, the gas phase and liquid phase enter the MR reflux channel respectively. High-pressure, ambient-temperature gas from the MR compressor system (S8) first enters the high-pressure MR gas channel A9 in the main heat exchanger (E1) and is cooled to -60°C. It then enters the MR separator (S8) for gas-liquid separation. The liquid from the separator continues to enter the MR liquid forward flow channel A10 in the main heat exchanger and is cooled to ~-100°C. It then enters the MR channel A38 of the denitrification tower reboiler (E6) to provide heat for the reboiler. The MR liquid exiting the reboiler is at ~-130°C. It then enters the MR throttling valve V13 to reduce its pressure to -2.9 barg before entering the MR flash tank (S6). The gas and liquid phases from the separator then enter the main heat exchanger's MR reactor, respectively. In the MR gas flow channel A12, the gas phase from the MR separator enters the main heat exchanger (E1). The MR gas is cooled to -162°C in the forward flow channel A11, and then enters the throttling valve V14. After being throttled to 3.0 barg, it enters the MR flash tank (S7) for gas-liquid separation. The gas and liquid phases then enter the main heat exchanger (E1) respectively. In the MR reverse flow channel A12, the ambient temperature MR gas phase of about 2.2 barg exits the main heat exchanger (E1) and enters the MR compression system (C1). After being pressurized, cooled, and separated, it is divided into gas and liquid phases and enters the main heat exchanger (E1) respectively. This cycle of compression and refrigeration provides cooling capacity to the main heat exchanger (E1).

[0058] Example 3 This embodiment is based on the same feed gas conditions (flow rate 40,000 Nm³). 3 (H, hydrogen content 31.32%, methane content 57.64%), comparing the performance of the two-stage dehydrogenation process of this invention with that of the conventional one-stage dehydrogenation process in LNG production.

[0059] The specific feed gas conditions are: flow rate: 40,000 Nm³ 3 / h, Pressure: 23.00 barA, Temperature: 20.00 ℃ Composition: Hydrogen 31.32%, Methane 57.64%, Nitrogen 11.03%; Operating parameters: Annual operating time: 8,000 hours; Electricity price: RMB 0.5 / kW·h; LNG unit price: RMB 3,500 / ton. A detailed comparison of process performance is shown in Table 3, and a comparison of economic benefits is shown in Table 4. Table 3 Table 4 In summary, under the same feed gas conditions, although the energy consumption of the two-stage dehydrogenation process of this invention is increased (total power 443 kW vs 377 kW), the LNG production is increased by 114.36 Nm³ due to the significantly improved methane recovery rate. 3 The annual net income reaches RMB 2.0232 million, demonstrating significant industrial application value and economic benefits.

[0060] The above description is merely a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the concept described herein through the above teachings or related technologies or knowledge. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A device for producing LNG from coke oven gas using a three-tower dual-nitrogen circulation refrigeration system, comprising a main heat exchanger (E1), a subcooler (E2), a dehydrogenation tower 1 (T1), a dehydrogenation tower 2 (T2), a denitrification tower (T3), and a matching condenser and compressor system, characterized in that: The main heat exchanger (E1) is internally equipped with a coke oven gas forward flow channel (A1), an LNG forward flow channel (A2), a hydrogen-rich gas counterflow channel (A3), a high-pressure nitrogen forward flow channel (A5), a low-pressure nitrogen counterflow channel (A6), and an independent low-low-pressure nitrogen counterflow channel (A7); the subcooler (E2) is internally equipped with a hydrogen-rich gas forward flow channel (A21), a methane-rich liquid counterflow channel (A22), and an independent low-low-pressure counterflow channel (A27); the top gas phase outlet of the dehydrogenation tower 1 (T1) is connected in sequence to the dehydrogenation tower 1 condenser (E3), the dehydrogenation tower 1 reflux tank (S2), and the dehydrogenation tower 2 (T2) via pipelines; the top gas phase outlet of the dehydrogenation tower 2 (T2) is connected to... The nitrogen outlet of the dehydrogenation tower 2 condenser (E4) is connected to the inlet of the low-pressure reflux channel (A27) of the subcooler (E2) via a pipeline; the outlet of the low-pressure reflux channel (A27) of the subcooler (E2) is connected to the inlet of the low-pressure nitrogen reflux channel (A7) of the main heat exchanger (E1) via a pipeline; the outlet of the low-pressure nitrogen reflux channel (A7) of the main heat exchanger (E1) is connected to the inlet of the low-pressure nitrogen booster system (C2) via a pipeline; and the outlet of the low-pressure nitrogen booster system (C2) is connected to the inlet pipeline of the nitrogen compressor system (C3) via a pipeline.

2. The apparatus for producing LNG from coke oven gas using a three-tower dual-nitrogen circulation refrigeration system according to claim 1, characterized in that: The main heat exchanger (E1) and subcooler (E2) are both multi-channel brazed plate-fin heat exchangers or coiled tube heat exchangers; the low-pressure nitrogen reflux channel (A7) and the low-pressure nitrogen reflux channel (A6) are physically isolated inside the heat exchanger and are used to circulate nitrogen of different pressure levels respectively.

3. The apparatus for producing LNG from coke oven gas using a three-tower dual-nitrogen circulation refrigeration system according to claim 1, characterized in that, The device also includes a low-pressure nitrogen booster system (C2); the low-pressure nitrogen booster system (C2) is a blower or a low-pressure compressor; the inlet of the low-pressure nitrogen booster system (C2) is connected to the outlet of the low-pressure nitrogen reflux channel (A7) of the main heat exchanger (E1) through a pipe, and its outlet is connected to the inlet pipe of the nitrogen compressor system (C3).

4. The apparatus for producing LNG from coke oven gas using a three-tower dual-nitrogen circulation refrigeration system according to claim 1, characterized in that, The flow path connections of the device are as follows: the gas phase outlet of the feed gas separator (S1) is connected to the inlet of the hydrogen-rich forward flow channel (A21) of the subcooler (E2); the outlet of the hydrogen-rich reverse flow channel (A23) of the subcooler (E2) is connected to the inlet of the hydrogen-rich reverse flow channel (A3) of the main heat exchanger (E1); the bottom liquid phase outlet of the dehydrogenation tower 1 (T1) and the bottom liquid phase outlet of the dehydrogenation tower 2 (T2) are both connected to the inlet of the methane-rich liquid reverse flow channel (A22) of the subcooler (E2).

5. The apparatus for producing LNG from coke oven gas using a three-tower dual-nitrogen circulation refrigeration system according to claim 1, characterized in that, The denitrification tower (T3) is equipped with a denitrification tower reboiler (E6) at the bottom; the denitrification tower reboiler (E6) is equipped with an MR liquid channel (A38), which is connected to the liquid phase outlet of the MR separator (S8) through a pipe.

6. The apparatus for producing LNG from coke oven gas using a three-tower dual-nitrogen circulation refrigeration system according to claim 1, characterized in that, The dehydrogenation tower 1 condenser (E3) and dehydrogenation tower 2 condenser (E4) are respectively equipped with independent liquid nitrogen throttling valves (V5) for dehydrogenation tower 1 condenser and (V6) for dehydrogenation tower 2 condenser, which are used to control the flow rate of liquid nitrogen entering the two condensers respectively.

7. A method for producing LNG from coke oven gas using a three-tower dual-nitrogen cycle refrigeration system, implemented based on the coke oven gas three-tower dehydrogenation and denitrogenation LNG production apparatus according to any one of claims 1-6, characterized in that... Includes the following steps: S1. Raw Material Pretreatment and Separation: After being cooled in the coke oven gas forward flow channel (A1) of the main heat exchanger (E1), the coke oven gas enters the raw material gas separator (S1) for gas-liquid separation, yielding a hydrogen-rich gas phase and a methane-rich liquid phase; S2. Two-Stage Series Dehydrogenation: The hydrogen-rich gas phase sequentially passes through the subcooler (E2), dehydrogenation tower 1 (T1), dehydrogenation tower 1 condenser (E3), dehydrogenation tower 2 (T2), and dehydrogenation tower 2 condenser (E4) for rectification separation; S3. Dual Nitrogen Circulation Refrigeration: High-pressure nitrogen is used as the cold source and is divided into two streams for throttling: the first stream of nitrogen is throttled and enters the dehydrogenation tower 1 condenser (E3) and the denitrification tower condenser (E5) to provide cooling capacity, and is then vaporized as low-pressure nitrogen for rewarming; the second stream of nitrogen is deeply throttled to 1.0~1.8 barg and enters the dehydrogenation tower 2 condenser (E4) to provide low-temperature cooling capacity, and is then vaporized as low-pressure nitrogen for rewarming, and is further pressurized by the low-pressure nitrogen pressurization system (C2); S4. Mixed Refrigerant (MR) Refrigeration Cycle: MR gas is compressed by the MR compressor system (C1) and divided into two paths: the first path is cooled by the main heat exchanger (E1) and throttled by the upper MR throttling valve (V11) before entering the upper MR flash tank (S5); the second path is cooled by the main heat exchanger (E1) before entering the MR separator (S8). The separated liquid phase is cooled by the MR liquid forward flow channel (A10) and the denitrification tower reboiler (E6) before entering the MR flash tank through the throttling valve (S6); the separated gas phase is throttled by the MR gas forward flow channel (A11) and the lower MR throttling valve (V14) before entering the lower MR flash tank (S7); S5 Denitrification and Product Output: The methane-rich liquid at the bottom of dehydrogenation tower 1 (T1) and dehydrogenation tower 2 (T2) merges and enters the denitrification tower (T3) for denitrification after throttling. LNG product is obtained at the bottom of the tower, and the nitrogen-rich gas at the top of the tower is reheated and discharged.

8. The method for producing LNG from coke oven gas using a three-tower dual-nitrogen cycle refrigeration system according to claim 7, characterized in that, In step S2, the operating pressure of dehydrogenation tower 1 (T1) is controlled at 20~25 barg, and the top temperature is controlled at -179℃ to -181℃; the operating pressure of dehydrogenation tower 2 (T2) is controlled at 18~23 barg, and the top temperature is controlled at -190℃ to -192℃, so that the methane molar content in the hydrogen-rich gas at the outlet of the reflux tank (S3) of dehydrogenation tower 2 is less than 2000 ppm.

9. The method for producing LNG from coke oven gas using a three-tower dual-nitrogen cycle refrigeration system according to claim 7, characterized in that, In step S3, after the low-pressure nitrogen is reheated in the low-pressure reflux channel (A27) of the subcooler (E2) and the low-pressure nitrogen reflux channel (A7) of the main heat exchanger (E1), it enters the low-pressure nitrogen booster system (C2) and is boosted to 2.5~3.5 barg, and then flows into the nitrogen compressor system (C3).

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