Optimization of a process for denitrogenation of a natural gas stream

Abstract

A process for the separation of 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, including introducing the gas mixture to be treated into a first distillation column thereby producing, at the column top, a first gas stream enriched in methane and, at a level lower than that of the top of the column, another gas stream; and introducing the other gas stream resulting from step a) into a second distillation column, at a level lower than that of that of the top of the second column, thereby producing, at the top of this column, a second gas stream rich in methane.

1 .- 8 . (canceled) 
     
     
         9 . A process for the separation of 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) introducing the gas mixture to be treated into a first distillation column thereby producing, at the column top, a first gas stream enriched in methane and, at a level lower than that of the top of said column, another gas stream;   b) introducing said other gas stream resulting from step a) into a second distillation column, at a level lower than that of that of the top of said second column, thereby producing, at the top of this column, a second gas stream rich in methane,   wherein the nitrogen content of said second gas stream is at least 1.5 times lower than the nitrogen content of the first gas stream and in that from 5 mol % to 30 mol % of the methane initially present in the gas mixture to be treated is comprised in the second gas stream.   
     
     
         10 . The process as claimed in  claim 9 , further comprising:
 c) introducing said first gas stream enriched in methane resulting from step a) into a denitrogenation unit in order to separate the nitrogen.   
     
     
         11 . The process as claimed in  claim 10 , wherein the second gas stream resulting from step b) is not treated by the denitrogenation unit. 
     
     
         12 . The process as claimed in  claim 9 , further comprising, prior to step a), 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 from said two-phase mixture into said first demethanization column at a first injection stage;   injecting the vapor phase from said two-phase mixture into said first demethanization column at an injection stage different from said first stage.   
     
     
         13 . The process as claimed in  claim 9 , wherein the gas stream, extracted from the first distillation column in step a), comprises at most half of the amount of hydrocarbons having more than two carbon atoms present in the feed gas. 
     
     
         14 . The process as claimed in  claim 9 , 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. 
     
     
         15 . The process as claimed in  claim 9 , wherein said gas stream resulting from step b) is extracted directly from said second distillation column at a pressure of greater than 20 bara and comprises 95 mol % of methane. 
     
     
         16 . The process as claimed in  claim 9 , wherein a portion of the second gas stream rich in methane and depleted in nitrogen resulting from step b) at the outlet of the top of the second distillation column is compressed and then condensed in order to be introduced, for one portion, into the upper part of the first distillation column and, for the other portion, into the upper part of the second distillation column in order to carry out the reflux of said distillation columns.