How to separate and recover methane during liquid nitrogen washing
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The raw gas fed into the liquid nitrogen washing system contains a high level of methane, which must be recovered and separated. How is CH4 separation achieved during the liquid nitrogen washing process? Thank you.
The gas impurities that need to be removed during liquid nitrogen washing are methane, carbon monoxide, and argon; the carrier gas is hydrogen, and the solvent is liquid nitrogen. The designed temperature for liquid nitrogen washing is -196 degrees. The physical properties of various gases are as follows: Methane is a colorless and odorless gas; its solubility is such that it is slightly soluble in water, but soluble in alcohol and ether. Density (at standard conditions): 0.717 grams per liter; boiling point: -161.5°C; melting point: -182.48°C. Under normal conditions, carbon monoxide is a colorless, odorless, tasteless, and toxic gas with a melting point of –199°C and a boiling point of –191.5°C. Ar, argon, atomic number: 18, atomic weight: 39.95, melting point: -189.2, boiling point: -185.7. Under standard conditions, the density of hydrogen is 0.0899 g/L, making it the gas with the lowest density of all gases. At a pressure of 1.01×105 Pa, the boiling point of colorless liquid hydrogen is –252.80°C (20.2 K), and the melting point of snow-like solid hydrogen is –259.14°C (13.86 K). Molecular formula: N2, molecular weight: 28.01, CAS number: 7727-37-9, properties: melting point -210°C. Boiling point -196°C. If the methane content is high, and given that methane’s melting point is -182.48°C, will solid methane form? If solid methane is formed, liquid nitrogen washing cannot be carried out properly. If no solid methane is formed, can liquid nitrogen washing completely remove methane, carbon monoxide, and argon? The gas emerging from the top of the flash tank inside the liquid nitrogen cryogenic tank in our plant is mainly hydrogen, which is sent to the recycle gas compressor for low-temperature methanol washing. The washed combustible gas is sent to the fuel gas pipeline, where it is used in the blowair boiler to generate medium-pressure steam.
Generally, low-temperature heat is used to recover cooling capacity, which is then supplied to the fuel gas system!
The melting point of methane is -182.48°C; this value applies at 101.325 KPA, and the melting point is even lower in pressurized mixed gases. So there’s no need to worry about the solidification issue; otherwise, the liquid nitrogen washing technique cannot be used in processes with high CH4 contents.