CNOOC Datong SNG Methanation Unit

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The methanation process technology for the methanation unit adopts the CRG methanation process from British company Davy. Domestic technology is used for natural gas compression and drying. (1) Description of the methanation process: The purified syngas enters the gas-liquid separation tank for feed gas, to prevent the presence of liquids from the acid gas removal system in the feed. Thereafter, the syngas is heated by the partially cooled product gas in the feed/SNG heat exchanger, and then sent to the desulfurization tank, which provides protection in case of sulfur overflow from the upstream acid gas removal system; otherwise, the excess sulfur would poison the CGR catalyst. The purified syngas is then preheated in the feed preheating heat exchanger by the outlet gas from Reactor 1# for supplemental methanation. The syngas is roughly divided equally between Reactor 1 and the second methanation reactor. The syngas sent to Reactor 1# for methanation is first mixed with the recycle gas, and then together they flow through the fixed adiabatic bed of Reactor 1#, where the exothermic methanation reaction takes place. The hot exhaust gas is used to generate high-pressure steam in the 1# methane reactor boiler and to superheat the steam in the steam superheater. The gas leaving the steam superheater is mixed with fresh feed gas that has not passed through Reactor #1 for methanation, and then enters Reactor #2 for further methanation. The heat in the gas leaving the 2# methanation reactor is used to generate steam in the boiler of the 2# methanation reactor, which is then used to preheat the recycle gas in the recycle heat exchanger. The hot gas leaving the circulating heat exchanger is divided into two parts; one part is first cooled by the circulating boiler feedwater heater before being circulated back to Reactor 1# via the circulating compressor. The resulting condensate is separated in the circulating gas-liquid separation tank and sent outside the system. Another portion of the hot gas leaving the circulating heat exchanger is sent to Reactor 1# for supplementary methanation to undergo further reactions. The hot gas leaving this methanation section is used to preheat the fresh feed gas entering the unit in the feed preheater, and then is further cooled successively by the methanation boiler feedwater heater, the low-pressure waste boiler of Reactor No. 1, and finally by the desalinated water heater. The resulting condensate is separated in the vapor-liquid separator of Reactor 1# for methanation and sent outside the plant. The cooled gas is first heated by the outlet gas from Reactor 2 for supplementary methanation in the heat exchanger of that reactor, and then undergoes the final methanation reaction in Reactor 2 for supplementary methanation. The outlet gas from the 2# supplementary methanation reactor is already product gas that meets the specifications; this product gas is cooled in the heat exchanger of the 2# supplementary methanation reactor. The product gas is also used to preheat the fresh feed gas in the feed/SNG heat exchanger, and then it is cooled by cooling water in the SNG cooler. The process condensate is separated in the vapor-liquid separator of Reactor No. 2 for methanation and sent outside the plant. 2# The overhead stream from the gas-liquid separator of the methanation reactor is supplemented and exported as SNG product outside the plant. Outside the facility, the SNG product is further dried and/or compressed in accordance with transportation requirements. A nitrogen/hydrogen circulation system is required when driving and reducing the catalysts in each unit. A nitrogen compressor drives nitrogen to a directly flame-heated nitrogen startup heater; the hot nitrogen output from this heater can be supplied to various catalyst beds and reactors. The hot nitrogen gas flowing through the catalyst bed is cooled in the startup cooler, and then returns to the inlet of the startup compressor via the startup gas-liquid separator. During startup, a small amount of syngas is fed into the startup pressure swing adsorption system to produce hydrogen, which is then added to the circulating nitrogen to reduce the catalyst. The carbon-rich gas from the startup pressure swing adsorption system can be used to provide part of the fuel gas for the startup heater. (2) Description of the natural gas compression process: The synthetic natural gas produced via methanation is first compressed to 10.0 MPa and 40°C by a natural gas compressor driven by a steam turbine. After water is separated out using a water separator, it enters a triethylene glycol dehydration unit to ensure that the dew point of the product gas is at -30°C. The separated water is sent back to the process unit for reuse along with the methanation condensate. (3) Description of the gas drying process: The triethylene glycol TEG dehydration process mainly consists of two stages: glycol absorption and regeneration. The aqueous natural gas enters the absorption tower, where the feed gas flows from bottom to top through various trays, coming into countercurrent contact with the anhydrous glycol solution flowing from the top of the tower to absorb the water vapor in the natural gas. The dehydrated natural gas flows out from the top of the tower. Glycerol enriched with water flows out from the bottom of the tower, then enters the flash tank through the condenser coils to be flashed; the flash vapor is used as fuel gas for sulfur recovery. Then the glycol flows into the glycol heat exchanger and enters the regenerator section (distillation tower) of the dehydration unit. The water absorbed by glycol through heating is removed at atmospheric pressure. The regenerated glycol is cooled in a glycol heat exchanger, and then pumped to the absorption tower using a glycol pump for reuse. A natural gas transmission station to which natural gas is sent after drying to meet the standard dew point requirements.