Method for separating all or some of the compounds from a biogas in the liquid state or in the two-phase state

A method for separating all or some of the compounds from a biogas in a liquid state or in a two-phase liquid/vapor state, containing methane, carbon dioxide wherein a first separation for separating the methane from the biogas is carried out by cryogenic distillation in a first distillation column comprising a column top brought to the condensation temperature of the methane at a given pressure, the first separation being carried out by injecting into the first distillation column:

• in a primary feed, the biogas liquefied at an equilibrium temperature making it possible to obtain a two-phase mixture ensuring the separation of the various compounds,
• in a secondary feed, a liquefying agent in the liquid state composed of a hydrocarbon or a mixture of hydrocarbons from the C3 to C7 family, the liquefying agent being injected at the top of the first distillation column, above a biogas inlet, at a temperature T1 lower than or equal to the carbon dioxide desublimation temperature at a given pressure of the first distillation column and in an amount proportional to the vapor flow rate of the carbon dioxide ascending to the top of the first distillation column,
• and wherein a second separation of the compounds of the liquid mixture recovered at the end of the first separation and comprising carbon dioxide and the liquefying agent is carried out by cryogenic distillation in a second distillation column after expansion of the liquid mixture to reach a temperature and an equilibrium pressure allowing separation of the carbon dioxide in the form of condensate at the top of the second distillation column and the liquefying agent in liquid residue at the bottom of the second distillation column.