Process for removing nitrogen from natural gas

Abstract

A process for separating the components of a gas mixture comprising methane, nitrogen, and at least one hydrocarbon having at least two carbon atoms, or a mixture of these hydrocarbons, including the following steps: a) demonization of the gas mixture with at least one demethenization column; b) extraction of a liquid comprising at least 85 mol % of the hydrocarbons having at least two carbon atoms, partial condensation of a gas mixture in order to obtain a liquid, at least a portion of which is treated in order to be extracted as denitrogenated natural gas product and a second gas; d) introduction of the second gas and/or the gas mixture into a nitrogen removal column, obtained from which are a gas and a liquid, e) treatment of the gas in a nitrogen removal system in order to produce a gas stream comprising 5 mol % at most of nitrogen.

1 .- 9 . (canceled) 
     
     
         10 . A process for separating the components of a gas mixture to be treated comprising methane, nitrogen and at least one hydrocarbon having at least two carbon atoms, or a mixture of these hydrocarbons, comprising the following steps:
 a) removing methane from said gas mixture using at least one methane removal column;   b) extracting from the methane removal column a liquid comprising at least 85 mol % of hydrocarbons having at least two carbon atoms initially present in the mixture to be treated;   c) partially condensing a gas mixture extracted from the methane removal column in order to obtain a liquid, at least one portion of which is treated in order to be extracted as denitrogenated natural gas product, and a second gas;   d) introducing said second gas and/or the gas mixture into a nitrogen removal column obtained from which are a gas and a liquid, at least one portion of which is treated in order to be extracted as denitrogenated natural gas product;   e) treating said gas from step d) in a nitrogen removal system in order to produce a gas stream comprising 5 mol % at most of nitrogen and a gaseous nitrogen stream comprising at most 8 mol % of methane;   wherein the operating temperature between steps b) and c) does not exceed −50° C. and the gas is reheated to a temperature above −10° C. before being cooled to a temperature below −50° C. in said nitrogen removal system.   
     
     
         11 . The process as claimed in  claim 10 , wherein step a) comprises the following steps:
 at least partially condensing said gas mixture to be treated in order to obtain a two-phase mixture;   injecting the liquid phase of said two-phase mixture into a methane removal column at a first injection stage;   injecting the vapor phase of said two-phase mixture into said methane removal column at an injection stage different from said first stage.   
     
     
         12 . The process as claimed in  claim 10 , wherein the gas mixture, extracted from the methane removal column, condensed in step c) comprises at most half of the amount of hydrocarbons having more than two carbon atoms present in the feed gas. 
     
     
         13 . The process as claimed in  claim 10 , wherein step e) of treating said gas from step d) in a nitrogen removal system produces a gas stream comprising 5 mol % at most of nitrogen and a gaseous nitrogen stream comprising at most 2 mol % of methane. 
     
     
         14 . The process as claimed in  claim 10 , wherein the gas from step d) comprises between 10 mol % and 90 mol % of nitrogen. 
     
     
         15 . The process as claimed in  claim 10 , wherein the liquid extracted from the methane removal column during step b) comprises at least 90 mol % of the hydrocarbons having at least two carbon atoms and preferably at least 95 mol %. 
     
     
         16 . The process as claimed in  claim 10 , wherein said gas mixture to be treated comprises 70 mol % of methane, at least 4 mol % of nitrogen and 2 mol % of hydrocarbons having at least two carbon atoms. 
     
     
         17 . The process as claimed in  claim 16 , wherein said gas mixture to be treated comprises at least 0.05 mol % of helium. 
     
     
         18 . The process as claimed in  claim 17 , further comprising an additional step f) following step e) of producing a stream comprising at least 20 mol % of helium from said nitrogen removal system.