Method and unit for processing a gas mixture containing nitrogen and methane

A method for processing a gas mixture containing nitrogen and methane,

• wherein the gas mixture is at least partly liquefied using a mixed refrigerant circuit, expanded, and introduced into a storage tank,
• wherein a liquid phase, which is depleted in nitrogen and enriched with methane relative to the gas mixture, and a vapor phase, which is enriched with nitrogen and depleted in methane relative to the gas mixture, are formed in the storage tank,
• wherein at least some of the vapor phase is withdrawn from the storage tank, compressed, at least partly liquefied using a mixed refrigerant circuit, and subjected to low-temperature rectification,
• wherein a top fraction rich in nitrogen and lean in methane and a bottom liquid lean in nitrogen and rich in methane are formed in the low-temperature rectification, and the top fraction is withdrawn from the low-temperature rectification, and
• wherein the liquefaction of the gas mixture containing nitrogen and methane and the at least partial liquefaction of the vapor phase are both performed using a single, mixed refrigerant circuit,
• wherein
• the liquefaction of the gas mixture containing nitrogen and methane is performed in a first heat exchanger and the at least partial liquefaction of the vapor phase is performed in a first second exchanger wherein said first heat exchanger and said second heat exchanger are separate heat exchangers,
• a partial stream of the bottom liquid is withdrawn from the low-temperature rectification, at least partly vaporized by heat exchange with a top gas drawn off from the low-temperature rectification, and the top gas is at least partially condensed and supplied to the low-temperature rectification as a return stream,
• wherein, in the mixed refrigerant circuit, the mixed refrigerant is split into a first refrigerant gas phase, a first refrigerant liquid phase, a second refrigerant gas phase, and a second refrigerant liquid phase,
• wherein a first partial stream of the first refrigerant liquid phase is combined with a first partial stream of the second refrigerant gas phase and used as refrigerant in the first heat exchanger, and a second partial stream of the first refrigerant liquid phase is combined with a second partial stream of the second refrigerant gas phase and used as refrigerant in the second heat exchanger, and
• the top fraction withdrawn from the low-temperature rectification has contains inert component(s) comprising nitrogen, and the concentration of all inert components, including nitrogen, in the withdrawn top fraction is at least 99 mol %.