High-pressure pipeline type synthetic natural gas low-temperature catalytic reaction method

Abstract

The invention relates to the technical field of synthetic natural gas preparation, and particularly discloses a high-pressure pipeline type synthetic natural gas low-temperature catalytic reaction method, which comprises the following steps of: activating a specific supported metal catalyst (such as Ni / Al2O3 and the like) in a tubular reactor through hydrogen-nitrogen mixed gas, preheating a synthetic gas raw material, mixing with circulating gas, introducing into a multi-section temperature control tubular reactor, and carrying out high-pressure pipeline type synthetic natural gas low-temperature catalytic reaction. And carrying out methanation reaction under the conditions that the temperature is 180-250 DEG C and the pressure is 10-15 MPa. Liquid byproducts are separated from reaction products through multi-stage condensation, and unreacted gas is recycled. The composition of raw material gas is limited, a honeycomb-shaped guide plate is arranged to optimize mass transfer, and an online analyzer is utilized to adjust the recycle ratio. The method realizes low-temperature efficient reaction, reduces energy consumption, inhibits side reaction, improves the service life of the catalyst and the purity of the synthetic natural gas, can recycle liquid by-products, and has remarkable economic and environmental benefits.

Description

Technical Field

[0001] The present invention relates to the technical field of synthetic natural gas preparation, and specifically to a low-temperature catalytic reaction method for synthetic natural gas in a high-pressure pipeline type. Background Art

[0002] Synthetic natural gas (SNG), as a clean energy source, plays an important role in the transformation of the energy structure. Currently, synthetic natural gas in industry is mainly realized through methanation reaction, and traditional synthetic natural gas production methods mostly adopt high-temperature and high-pressure catalytic reaction processes. However, high-temperature reaction conditions (usually the reaction temperature is higher than 300 °C) not only lead to a significant increase in energy consumption, but also have extremely high requirements for the high-temperature and high-pressure resistance performance of reaction equipment, greatly increasing equipment investment and maintenance costs. At the same time, high temperature is prone to cause side reactions, resulting in problems such as carbon deposition, reducing the activity and service life of the catalyst, and affecting the yield and quality of synthetic natural gas. In addition, problems such as uneven gas mixing and difficult effective control of reaction heat in traditional processes also make the reaction efficiency low, the production cost of synthetic natural gas remain high, and limit the large-scale development and popularization of the synthetic natural gas industry. Therefore, it is urgent to develop a process method that can efficiently and stably produce synthetic natural gas under low-temperature conditions to reduce energy consumption, improve product quality, and reduce equipment investment and operating costs. Summary of the Invention

[0003] (I) Technical Problems to be Solved In view of the deficiencies of the prior art, the present invention provides a low-temperature catalytic reaction method for synthetic natural gas in a high-pressure pipeline type, which solves the problems raised in the above background art.

[0004] (II) Technical Solutions In order to achieve the above object, the present invention discloses a low-temperature catalytic reaction method for synthetic natural gas in a high-pressure pipeline type, and the method includes the following steps: Step A: Place the supported metal catalyst in a tubular reactor, and perform activation treatment with a hydrogen-nitrogen mixed gas. The activation temperature is 300 - 400 °C, the pressure is 5 - 8 MPa, and the activation time is 3 - 5 h; Step B: Pass the syngas raw material into a preheating device, preheat it to 150 - 200 °C, and mix it with the recycle gas at a volume ratio of 1:0.8 - 1.2 to form a reaction mixed gas; Step C: Pass the mixed gas in Step B into the tubular reactor in Step A at a flow rate of 0.5 - 1.2 m / s, control the reaction pressure to be 10 - 15 MPa, and the reaction temperature to be 180 - 250 °C, and perform methanation catalytic reaction; Step D: Gradually cool the reaction product through a multi-stage condensation separator. The first-stage condensation temperature is -30 ~ -10 °C, the second-stage condensation temperature is -80 ~ -50 °C, separate the liquid by-products and collect the gaseous synthetic natural gas; Step E: Return the unreacted gas to Step B through a booster pump to mix with the fresh syngas for cyclic reaction, and the circulation ratio is 20%-40%.

[0005] Preferably, the supported metal catalyst described in Step A is one of Ni / Al2O3, Ru / ZrO2 or Co-Mo / SiO2. The particle size of the catalyst is 0.5-2 mm, and the specific surface area is 200-400 m² / g.

[0006] Preferably, the activation treatment in Step A specifically includes: a) Purge the reactor with nitrogen until the oxygen content < 50 ppm; b) Heat up to the target temperature at a rate of 2-4 °C / min and keep the temperature constant for activation; c) After the activation is completed, purge with an inert gas until the room temperature is reached.

[0007] Preferably, in Step B, the H2 / CO molar ratio of the syngas raw material is 2.8-3.2, the sulfur content ≤ 0.1 ppm, and the oxygen content ≤ 10 ppm.

[0008] Preferably, the tubular reactor in Step C has a multi-stage temperature control structure. The temperature of the front stage is set at 180-200 °C, the middle stage is 210-230 °C, and the last stage is 240-250 °C. The temperature difference between each stage is adjusted by an external jacket heat exchanger.

[0009] Preferably, in Step D, the first-stage condenser uses an ethylene glycol aqueous solution as the refrigerant, and the second-stage condenser uses liquid nitrogen for indirect cooling.

[0010] Preferably, an on-line analyzer is set in the return path of the recycle gas in Step E to monitor the H2 and CO concentrations in real time and feedback to adjust the circulation ratio.

[0011] Preferably, in Step a, the nitrogen purge flow rate is 10-15 L / min, and the purge time is 30-60 min.

[0012] Preferably, a honeycomb baffle is set in the reactor in Step C, the porosity is 60-80%, and the baffle spacing is 50-100 mm.

[0013] Preferably, in Step D, the separated liquid by-products are recovered for C5+ hydrocarbons through vacuum distillation. The distillation temperature is 80-120 °C, and the pressure is 0.1-0.5 MPa.

[0014] (III) Beneficial technical effects In terms of energy conservation and consumption reduction, by controlling the reaction temperature in the low temperature range of 180 - 250 °C, the energy consumption is significantly reduced compared with the traditional high temperature process, and the energy consumption cost is reduced; for the tubular reactor with a multi-stage temperature control structure, different temperatures are set for the front stage, middle stage and end stage, and the temperature difference between each stage is adjusted by an external jacket heat exchanger, which can more accurately control the reaction process, make the reaction proceed at the most suitable temperature, avoid unnecessary energy consumption caused by too high temperature, and further improve the energy utilization efficiency. In terms of optimizing the catalyst performance, a specific supported metal catalyst (such as Ni / Al2O3, Ru / ZrO2 or Co - Mo / SiO2) is used, and its particle size (0.5 - 2 mm) and specific surface area (200 - 400 m² / g) are limited. With the activation treatment of hydrogen-nitrogen mixed gas at 300 - 400 °C, 5 - 8 MPa and 3 - 5 h, the activity and selectivity of the catalyst are significantly improved, the occurrence of side reactions is effectively inhibited, the carbon deposition phenomenon is reduced, the service life of the catalyst is prolonged, and the catalyst replacement cost is reduced. In terms of improving the reaction efficiency, the syngas raw material and the recycle gas are mixed at a volume ratio of 1:0.8 - 1.2, and with an inlet flow rate of 0.5 - 1.2 m / s, so that the reaction mixture gas can fully contact the catalyst in the tubular reactor, and the reaction rate is increased; the honeycomb-shaped flow guide plate arranged in the reactor has a porosity of 60 - 80% and a flow guide plate spacing of 50 - 100 mm, which optimizes the gas flow path, enhances the gas-solid mass transfer effect, and further improves the reaction efficiency. In terms of ensuring the product quality, the H2 / CO molar ratio (2.8 - 3.2), sulfur content (≤0.1 ppm) and oxygen content (≤10 ppm) of the syngas raw material are strictly controlled, providing ideal reaction conditions for the methanation reaction, reducing the interference of impurities on the reaction, and improving the purity of synthetic natural gas; the multi-stage condensation separator cools the reaction products step by step, the first-stage condensation temperature is - 30~ - 10 °C, and the second-stage condensation temperature is - 80~ - 50 °C, which can effectively separate the liquid by-products, collect the high-purity gaseous synthetic natural gas, and at the same time, the separated liquid by-products are recovered by C5 + hydrocarbons through vacuum distillation, realizing the comprehensive utilization of resources. In terms of process stability and intelligent control, an on-line analyzer is set on the recycle gas return path to monitor the H2 and CO concentrations in real time and feedback to adjust the recycle ratio, so that the reaction process can be automatically adjusted according to the gas concentration change, ensuring that the reaction is always in the best state, and improving the stability and reliability of the process; at the same time, the parameters of each step in this method are clear and adjustable, which is convenient for precise control and large-scale application in industrial production, providing a strong guarantee for the stable and efficient production of synthetic natural gas. Specific embodiments

[0015] For the convenience of understanding the present invention, the present invention will be described more comprehensively below. Preferred embodiments of the present invention are given below. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, these embodiments are provided to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0016] Example 1: Step A: Place the supported metal catalyst Ni / Al2O3 in a tubular reactor. First, purge the reactor with nitrogen at a flow rate of 10 L / min for 60 min until the oxygen content < 50 ppm; then heat it to 300 °C at a rate of 2 °C / min and keep it at a constant temperature and activate it for 3 h under a pressure of 5 MPa; after activation, purge it with an inert gas until it reaches room temperature. The particle size of this catalyst is 0.5 mm and the specific surface area is 200 m² / g.

[0017] Step B: Feed the syngas raw material with an H2 / CO molar ratio of 2.8, a sulfur content ≤ 0.1 ppm, and an oxygen content ≤ 10 ppm into a preheating device, preheat it to 150 °C, and mix it with the recycle gas at a volume ratio of 1:0.8 to form a reaction mixture gas.

[0018] Step C: Feed the mixture gas in Step B into the tubular reactor in Step A at a flow rate of 0.5 m / s. The tubular reactor has a multi-stage temperature control structure. The temperature of the front stage is set at 180 °C, the middle stage is 210 °C, and the last stage is 240 °C. The temperature difference between each stage is adjusted by an external jacket heat exchanger, and the reaction pressure is controlled at 10 MPa to carry out the methanation catalytic reaction. A honeycomb deflector is arranged in the reactor, with a porosity of 60% and a deflector spacing of 50 mm.

[0019] Step D: Gradually cool the reaction products through a multi-stage condensation separator. The first-stage condensation temperature is -30 °C, and an ethylene glycol aqueous solution is used as the refrigerant; the second-stage condensation temperature is -80 °C, and indirect cooling with liquid nitrogen is used to separate the liquid by-products and collect the gaseous synthetic natural gas.

[0020] Step E: Return the unreacted gas to Step B through a booster pump to be mixed with the fresh syngas for cyclic reaction. An on-line analyzer is set on the return path of the recycle gas to real-time monitor the concentrations of H2 and CO and feedback to adjust the recycle ratio, and the recycle ratio is set at 20%. The separated liquid by-products are recovered for C5+ hydrocarbons through vacuum distillation, with a distillation temperature of 80 °C and a pressure of 0.1 MPa.

[0021] Example 2: Step A: Place the supported metal catalyst Ru / ZrO2 in a tubular reactor. Purge the reactor with nitrogen at a flow rate of 12 L / min for 45 min to make the oxygen content < 50 ppm. Then, heat it up to 350 °C at a rate of 3 °C / min and keep it at a constant temperature under a pressure of 6 MPa for 4 h for activation. After activation, purge it with an inert gas until it reaches room temperature. The particle size of this catalyst is 1 mm and the specific surface area is 300 m² / g.

[0022] Step B: Feed the syngas raw material with an H2 / CO molar ratio of 3.0, a sulfur content ≤ 0.1 ppm, and an oxygen content ≤ 10 ppm into a preheating device, preheat it to 175 °C, and mix it with the recycle gas at a volume ratio of 1:1 to obtain a reaction mixture gas.

[0023] Step C: Feed the mixture gas from Step B into the tubular reactor in Step A at a flow rate of 0.8 m / s. Set the temperature of the front section of the tubular reactor to 190 °C, the middle section to 220 °C, and the end section to 245 °C. Control the reaction pressure to be 12 MPa and carry out the methanation catalytic reaction. The porosity of the honeycomb baffle in the reactor is 70% and the distance between the baffles is 75 mm.

[0024] Step D: The reaction products pass through a multi-stage condensation separator. Set the first-stage condensation temperature to -20 °C and cool it with an ethylene glycol aqueous solution; the second-stage condensation temperature is -70 °C and indirectly cool it with liquid nitrogen to separate the liquid by-products and collect the gaseous synthetic natural gas.

[0025] Step E: The unreacted gas returns to Step B through a booster pump. The on-line analyzer in the recycle gas return path monitors the H2 and CO concentrations in real time and adjusts the recycle ratio to 30%. The liquid by-products are recovered by vacuum distillation of C5+ hydrocarbons. The distillation temperature is 90 °C and the pressure is 0.2 MPa.

[0026] Example 3: Step A: Put the supported metal catalyst Co-Mo / SiO2 into a tubular reactor. Purge the reactor with nitrogen at a flow rate of 15 L / min for 30 min to make the oxygen content meet the standard. Heat it up to 400 °C at a rate of 4 °C / min and activate it under a pressure of 8 MPa for 5 h. After completion, purge it with an inert gas until it reaches room temperature. The particle size of this catalyst is 2 mm and the specific surface area is 400 m² / g.

[0027] Step B: Preheat the syngas raw material with an H2 / CO molar ratio of 3.2 and sulfur and oxygen contents meeting the requirements to 200 °C, and mix it with the recycle gas at a volume ratio of 1:1.2 to form a reaction mixture gas.

[0028] Step C: Feed the mixed gas from Step B into the tubular reactor of Step A at a flow rate of 1.2 m / s. The temperature at the front section of the tubular reactor is 200 °C, 230 °C in the middle section, and 250 °C at the end section. The reaction pressure is 15 MPa to carry out the methanation catalytic reaction. The porosity of the honeycomb baffle in the reactor is 80%, and the distance between the baffles is 100 mm.

[0029] Step D: The reaction products are condensed in multiple stages. The first stage is cooled by an ethylene glycol aqueous solution at -10 °C, and the second stage is indirectly cooled by liquid nitrogen at -50 °C to separate the liquid by-products and collect the gaseous synthetic natural gas.

[0030] Step E: The unreacted gas is returned to Step B, monitored and adjusted by an on-line analyzer, and the recycle ratio is 40%. The liquid by-products are vacuum distilled at a temperature of 120 °C and a pressure of 0.5 MPa to recover C5+ hydrocarbons.

[0031] Example 4: Step A: Use a Ni / Al2O3 catalyst. The operations such as purging with nitrogen are the same as in Example 1. The activation temperature is 320 °C, the pressure is 7 MPa, the activation time is 3.5 h, the catalyst particle size is 0.8 mm, and the specific surface area is 250 m² / g.

[0032] Step B: The molar ratio of H2 / CO in the syngas feed is 2.9. Other conditions are the same as in Example 2. It is preheated to 160 °C and mixed with the recycle gas at a volume ratio of 1:0.9.

[0033] Step C: The mixed gas is fed into the reactor at a flow rate of 0.6 m / s. The temperature at the front section is 185 °C, 215 °C in the middle section, and 242 °C at the end section. The pressure is 11 MPa. The porosity of the baffle in the reactor is 65%, and the distance is 60 mm.

[0034] Step D: The first-stage condensation temperature is -25 °C, and the second stage is -75 °C. The cooling method is the same as in Example 2.

[0035] Step E: The recycle ratio is 25%. The distillation temperature of the liquid by-products is 85 °C, and the pressure is 0.15 MPa.

[0036] Example 5: Step A: Use a Ru / ZrO2 catalyst. Purging with nitrogen and other operations are similar to Example 2. The activation temperature is 360 °C, the pressure is 7 MPa, and the activation time is 4.5 h. The catalyst particle size is 1.2 mm, and the specific surface area is 320 m² / g.

[0037] Step B: The molar ratio of H2 / CO in the syngas feed is 3.1. It is preheated to 180 °C and mixed with the recycle gas at a volume ratio of 1:1.1.

[0038] Step C: The mixed gas is introduced into the reactor at a flow rate of 0.9 m / s, with the temperature of the first section being 195 °C, the middle section being 225 °C, the last section being 248 °C, the pressure being 13 MPa, the porosity of the baffle being 72%, and the spacing being 80 mm.

[0039] Step D: The condensation temperature is -15 °C in the first stage and -65 °C in the second stage, and the cooling method remains unchanged.

[0040] Step E: The recycle ratio is 35%, the distillation temperature of the liquid by-product is 100 °C, and the pressure is 0.3 MPa.

[0041] Example 6: Step A: The Co-Mo / SiO2 catalyst is purged with nitrogen as in Example 3. The activation temperature is 380 °C, the pressure is 6 MPa, the activation time is 4 h, the catalyst particle size is 1.5 mm, and the specific surface area is 350 m² / g.

[0042] Step B: The molar ratio of H2 / CO in the syngas raw material is 3.0, it is preheated to 190 °C, and mixed with the recycle gas at a volume ratio of 1:1.

[0043] Step C: The mixed gas is introduced into the reactor at a flow rate of 1.0 m / s, with the temperature of the first section being 200 °C, the middle section being 230 °C, the last section being 250 °C, the pressure being 14 MPa, the porosity of the baffle being 75%, and the spacing being 90 mm.

[0044] Step D: The condensation temperature is -20 °C in the first stage and -70 °C in the second stage, and the cooling method is the same as before.

[0045] Step E: The recycle ratio is 32%, the distillation temperature of the liquid by-product is 110 °C, and the pressure is 0.4 MPa.

[0046] Comparative Example 1: Step A: The supported metal catalyst Ni / Al2O3 is placed in a tubular reactor without activation treatment.

[0047] Step B: The syngas raw material with an H2 / CO molar ratio of 2.8, a sulfur content of ≤0.1 ppm, and an oxygen content of ≤10 ppm is introduced into a preheating device, preheated to 150 °C, and mixed with the recycle gas at a volume ratio of 1:0.8 to form a reaction mixed gas.

[0048] Step C: The mixed gas from Step B is introduced into the tubular reactor in Step A at a flow rate of 0.5 m / s. The tubular reactor has a multi-stage temperature control structure. The temperature of the first section is set at 180 °C, the middle section is 210 °C, and the last section is 240 °C. The temperature difference between each section is adjusted by an external jacket heat exchanger, and the reaction pressure is controlled at 10 MPa to carry out the methanation catalytic reaction. There is no honeycomb baffle in the reactor.

[0049] Step D: Gradually cool the reaction products through a multi-stage condensation separator. The first-stage condensation temperature is -30°C, and an ethylene glycol aqueous solution is used as the refrigerant; the second-stage condensation temperature is -80°C, and indirect cooling with liquid nitrogen is adopted to separate the liquid by-products and collect the gaseous synthetic natural gas.

[0050] Step E: Return the unreacted gas to Step B through a booster pump to be mixed with the fresh synthesis gas for cyclic reaction. An on-line analyzer is not set in the return path of the recycle gas, and the recycle ratio is set at 20%. The separated liquid by-products are recovered of C5+ hydrocarbons through vacuum distillation at a distillation temperature of 80°C and a pressure of 0.1 MPa.

[0051] Comparative Example 2: Step A: Place the supported metal catalyst Ru / ZrO2 in a tubular reactor, purge the reactor with nitrogen at a flow rate of 12 L / min for 45 min to make the oxygen content < 50 ppm; then heat it up to 350°C at a rate of 3°C / min and keep it at a constant temperature for 4 h under a pressure of 6 MPa; after the activation is completed, purge it with an inert gas to room temperature. The particle size of this catalyst is 1 mm and the specific surface area is 300 m² / g.

[0052] Step B: Feed the synthesis gas raw material with an H2 / CO molar ratio of 3.0, a sulfur content ≤ 0.1 ppm, and an oxygen content ≤ 10 ppm into a preheating device, preheat it to 175°C without mixing with the recycle gas.

[0053] Step C: Feed the synthesis gas from Step B into the tubular reactor of Step A at a flow rate of 0.8 m / s. The temperature at the front section of the tubular reactor is set at 190°C, the middle section is 220°C, and the end section is 245°C. Control the reaction pressure at 12 MPa to carry out the methanation catalytic reaction. The porosity of the honeycomb baffle in the reactor is 70% and the distance between the baffles is 75 mm.

[0054] Step D: The reaction products pass through a multi-stage condensation separator. The first-stage condensation temperature is set at -20°C and cooled with an ethylene glycol aqueous solution; the second-stage condensation temperature is -70°C, and indirect cooling is carried out with the help of liquid nitrogen to separate the liquid by-products and collect the gaseous synthetic natural gas.

[0055] Step E: The step of recycling the unreacted gas is not set. The liquid by-products are recovered of C5+ hydrocarbons through vacuum distillation at a distillation temperature of 90°C and a pressure of 0.2 MPa.

[0056] Comparative Example 3: Step A: Place the supported metal catalyst Co-Mo / SiO2 into a tubular reactor, purge the reactor with nitrogen at a flow rate of 15L / min for 30min to make the oxygen content reach the standard; heat to 400℃ at a rate of 4℃ / min, activate at a pressure of 8MPa for 5h, and purge with inert gas to room temperature after completion. The catalyst has a particle size of 2mm and a specific surface area of ​​400m² / g.

[0057] Step B: The synthesis gas raw material with a H2 / CO molar ratio of 3.2 and a sulfur content and an oxygen content that meet the requirements is preheated to 200°C and mixed with the circulating gas at a volume ratio of 1:1.2 to form a reaction mixed gas.

[0058] Step C: The mixed gas of step B is passed into the tubular reactor of step A at a flow rate of 1.2 m / s. The tubular reactor does not use multi-stage temperature control, the temperature is constant at 260°C, the reaction pressure is 15 MPa, and the methanogenic catalytic reaction is carried out. The porosity of the honeycomb guide plate in the reactor is 80%, and the guide plate spacing is 100 mm.

[0059] Step D: The reaction product is subjected to multi-stage condensation, the first stage is cooled by -10°C ethylene glycol aqueous solution, the second stage is indirectly cooled by -50°C liquid nitrogen, and the liquid by-products are separated to collect gas-phase synthetic natural gas.

[0060] Step E: The unreacted gas is returned to step B. The online analyzer of the circulating gas return path is not monitored and adjusted in real time, and the circulation ratio is fixed at 40%. The liquid by-product is distilled under reduced pressure at a temperature of 120°C and a pressure of 0.5MPa to recover C5+ hydrocarbons.

[0061] Comparison table of reaction effects of embodiments and comparative examples:

[0062] From the above table data, it can be seen that in Examples 1-6, due to the reasonable activation treatment of the catalyst, the mixing of circulating gas and the multi-stage temperature control, the methane yield is high and the by-product content is relatively low. However, in Comparative Example 1, the catalyst was not activated, resulting in insufficient catalyst activity, a significant decrease in methane yield, and an increase in by-products; in Comparative Example 2, the circulating gas was not used, and the synthesis gas could not fully react, resulting in a significant decrease in methane yield and an increase in by-product content; in Comparative Example 3, a constant temperature was used, which was not conducive to the optimization of the reaction at different stages, thereby affecting the methane yield and increasing the by-product content.

[0063] Comparison table of performance of different catalysts:

[0064] From the table data, it can be seen that the Ru / ZrO2 catalyst is slightly higher than the Ni / Al2O3 and Co-Mo / SiO2 catalysts in terms of the average methane yield, and the average by-product content is relatively low. This indicates that in this reaction system, the Ru / ZrO2 catalyst has certain advantages in comprehensive performance. However, the performance differences among the three catalysts are not particularly significant, and all can meet the reaction requirements for synthetic natural gas to a certain extent.

[0065] Comparison table of reaction effects at different recycle ratios:

[0066] It can be analyzed from this table that within a certain range, as the recycle ratio increases, the methane yield shows a trend of first increasing and then slightly decreasing. When the recycle ratio is 30%, the methane yield is relatively high and the by-product content is relatively low. This shows that an appropriate recycle ratio can enable unreacted gases to fully participate in the reaction and improve the utilization rate of syngas. However, too high a recycle ratio may lead to some adverse factors and affect the reaction effect.

[0067] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. All equivalent changes and improvements made in accordance with the scope of the present invention application shall still fall within the scope covered by the patent of the present invention.

Claims

1. A low-temperature catalytic reaction method for synthetic natural gas in a high-pressure pipeline, characterized in that The method includes the following steps: Step A: Place the supported metal catalyst in a tubular reactor and perform activation treatment with a hydrogen-nitrogen mixture. The activation temperature is 300 - 400 °C, the pressure is 5 - 8 MPa, and the activation time is 3 - 5 h. Step B: Pass the syngas raw material into a preheating device, preheat it to 150 - 200 °C, and mix it with the recycle gas at a volume ratio of 1:0.8 - 1.2 to form a reaction mixture gas. Step C: Pass the mixture gas in Step B into the tubular reactor in Step A at a flow rate of 0.5 - 1.2 m / s, control the reaction pressure to be 10 - 15 MPa, and the reaction temperature to be 180 - 250 °C to carry out the methanation catalytic reaction. Step D: Gradually cool the reaction product through a multi-stage condensation separator. The first-stage condensation temperature is -30 ~ -10 °C, and the second-stage condensation temperature is -80 ~ -50 °C. Separate the liquid by-products and collect the gaseous synthetic natural gas. Step E: Return the unreacted gas to Step B through a booster pump to mix with the fresh syngas for cyclic reaction, and the recycle ratio is 20% - 40%. The supported metal catalyst described in Step A is one of Ni / Al2O3, Ru / ZrO2, or Co-Mo / SiO2. The catalyst particle size is 0.5 - 2 mm, and the specific surface area is 200 - 400 m² / g. The activation treatment in Step A specifically includes: a) Purge the reactor with nitrogen until the oxygen content < 50 ppm. b) Heat up to the target temperature at a rate of 2 - 4 °C / min and keep it at a constant temperature for activation. c) After the activation is completed, purge with an inert gas until it reaches room temperature.

2. The low-temperature catalytic reaction method for synthetic natural gas in a high-pressure pipeline according to claim 1, characterized in that, In Step B, the H2 / CO molar ratio of the syngas raw material is 2.8 - 3.2, the sulfur content ≤ 0.1 ppm, and the oxygen content ≤ 10 ppm.

3. The low-temperature catalytic reaction method for synthetic natural gas in a high-pressure pipeline according to claim 1, characterized in that, In Step C, the tubular reactor has a multi-stage temperature control structure. The temperature of the front stage is set to 180 - 200 °C, the middle stage is 210 - 230 °C, and the last stage is 240 - 250 °C. The temperature difference between each stage is adjusted by an external jacket heat exchanger.

4. The low-temperature catalytic reaction method for synthetic natural gas in a high-pressure pipeline according to claim 1, characterized in that, In Step D, the first-stage condenser uses an ethylene glycol aqueous solution as the refrigerant, and the second-stage condenser uses liquid nitrogen for indirect cooling.

5. The low-temperature catalytic reaction method for synthetic natural gas in a high-pressure pipeline according to claim 1, characterized in that, In Step E, an on-line analyzer is set in the return path of the recycle gas to monitor the H2 and CO concentrations in real time and feedback to adjust the recycle ratio.

6. The low-temperature catalytic reaction method for synthetic natural gas in a high-pressure pipeline according to claim 1, characterized in that, In Step a, the nitrogen purge flow rate is 10 - 15 L / min, and the purge time is 30 - 60 min.

7. The low-temperature catalytic reaction method for synthetic natural gas in a high-pressure pipeline according to claim 1, characterized in that, In Step C, a honeycomb baffle is arranged in the reactor, the porosity is 60 - 80%, and the baffle spacing is 50 - 100 mm.

8. The low-temperature catalytic reaction method for synthetic natural gas in a high-pressure pipeline according to claim 1, wherein, In Step D, the separated liquid by-products are recovered for C5+ hydrocarbons through vacuum distillation. The distillation temperature is 80 - 120 °C, and the pressure is 0.1 - 0.5 MPa.

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