Process for reliquefying a methane-rich fraction

Abstract

A process for reliquefying a methane-rich fraction, in particular boil-off gas, is described. In this process, a) a methane-rich fraction is compressed to a pressure at least 20% above the critical pressure of the fraction to be compressed, b) liquefied and supercooled, c) depressurized to a pressure in the range from 5 to 20 bar, d) separated into a gaseous nitrogen-rich fraction and a liquid nitrogen-depleted fraction, e) the nitrogen-depleted fraction is depressurized to a pressure in the range from 1.1 to 2.0 bar, f) the gaseous fraction obtained, without being warmed and compressed, is mixed into the methane-rich fraction, and g) the liquid product fraction obtained in the depressurization of the low-nitrogen fraction has a nitrogen content of ≦1.5 mol %.

1 . Process for reliquefying a methane-rich fraction, in particular boil-off gas, wherein
 a) the methane-rich fraction ( 1 ) is compressed (C 1 ) to a pressure which is at least 20% above the critical pressure of the fraction to be compressed,   b) liquefied and supercooled (E 2 ),   c) depressurized (V 1 ) to a pressure in the range from 5 to 20 bar and   d) separated into a gaseous nitrogen-rich fraction ( 4 ) and a liquid nitrogen-depleted fraction ( 7 ) and   e) the nitrogen-depleted fraction ( 7 ) is depressurized (V 2 ) to a pressure in the range from 1.1 to 2.0 bar,   f) where the gaseous fraction ( 8 ) obtained is, without being warmed and compressed, mixed into the methane-rich fraction ( 1 ) and   g) the liquid product fraction ( 9 ) obtained in the depressurization of the low-nitrogen fraction has a nitrogen content of ≦1.5 mol %.   
     
     
         2 . Process according to  claim 1 , where the liquefaction and supercooling (E 2 ) of the methane-rich fraction ( 1 ) are carried out against at least one refrigerant circuit and/or at least one refrigerant mixture circuit and this/these has/have at least one circuit compressor (C 3 ), characterized in that the pressure to which the methane-rich fraction ( 1 ) is compressed (C 1 ), the pressure to which the liquefied and supercooled methane-rich fraction ( 3 ) is depressurized (V 1 ) and the temperature to which the methane-rich fraction ( 2 ) is cooled are selected or varied in such a way that
 the drive power of the compressor (C 1 ) used for compressing the methane-rich fraction ( 1 ) and the drive power of the circuit compressor(s) (C 3 ) are shifted relative to one another without the total power changing by more than ±5% or   the drive power of the compressor (C 1 ) used for compressing the methane-rich fraction ( 1 ) and the drive power of the circuit compressor(s) (C 3 ) are shifted relative to one another in such a way that a division of the total power in the range from 30/70 to 70/30 is achieved.   
     
     
         3 . Process according to  claim 1 , characterized in that the methane-rich fraction ( 1 ) is compressed (C 1 ) to a pressure which is at least 30% above the critical pressure of the fraction to be compressed. 
     
     
         4 . Process according to  claim 1 , characterized in that the liquefied and supercooled methane-rich fraction ( 3 ) is depressurized (V 1 ) to a pressure in the range from 7 to 15 bar. 
     
     
         5 . Process according to  claim 1 , characterized in that the nitrogen-depleted fraction ( 6 ) is depressurized (V 2 ) to a pressure in the range from 1.2 to 1.8 bar.