Method for cooling a hydrocarbon-rich fraction

Abstract

A method is described for cooling a hydrocarbon-rich fraction, in particular of natural gas, wherein the hydrocarbon-rich fraction ( 1 ) is cooled against at least one coolant circuit ( 10 - 15 ), the coolant has at least nitrogen, carbon dioxide, methane and/or C 2+ hydrocarbons, the coolant is compressed by means of at least one turbocompressor (C 1 ) having one or more gas-lubricated sliding-ring seals, and as primary seal gas, a substream of the coolant and/or an external gas or gas mixture having substantially nitrogen and/or methane is fed to the turbocompressor (C 1 ), and as secondary seal gas, nitrogen is fed. According to the invention, at least one nitrogen-rich stream ( 21 ) is withdrawn at least at times from the coolant circuit ( 10 - 15 ).

1 . A method for cooling a hydrocarbon-rich fraction, in particular of natural gas, wherein
 the hydrocarbon-rich fraction is cooled against at least one coolant circuit,   the coolant has at least nitrogen, carbon dioxide, methane and/or C 2+  hydrocarbons,   the coolant is compressed by means of at least one turbocompressor having one or more gas-lubricated sliding-ring seals, and   as primary seal gas, a substream of the coolant and/or an external gas or gas mixture having substantially nitrogen and/or methane is fed to the turbocompressor, and as secondary seal gas, nitrogen is fed,   characterized in that at least one nitrogen-rich stream is withdrawn at least at times from the coolant circuit.   
     
     
         2 . The method according to  claim 1 , characterized in that the nitrogen-rich stream is withdrawn at the cold end of the coolant circuit. 
     
     
         3 . The method according to  claim 1 , characterized in that the nitrogen-rich stream that is withdrawn has a fraction of C 2+  hydrocarbons of less than 2 mol%. 
     
     
         4 . The method according to  claim 1 , characterized in that the nitrogen-rich stream is withdrawn as soon as the nitrogen and/or methane content of the coolant circulating in the coolant circuit exceeds a preset threshold value. 
     
     
         5 . The method according to  claim 1 , wherein the hydrocarbon-rich fraction is liquefied, fed to at least one storage container and from this a boil-off gas fraction is taken off, characterized in that the nitrogen-rich stream that is withdrawn is at least partially added to the boil-off gas fraction and/or is flared off. 
     
     
         6 . The method according to  claim 1 , wherein the coolant circulating in the coolant circuit is compressed in a single or multistage manner, characterized in that the seal gas mixture that is taken off from the turbocompressor via the primary outlet line of the gas seal is fed to the compressor or the first compressor stage by elevating the seal gas pressure on the suction side. 
     
     
         7 . The method according to  claim 1 , wherein, as primary seal gas, an external gas or gas mixture having substantially nitrogen and/or methane is fed to the turbocompressor. characterized in that this gas or gas (mixture) is produced from a fraction present and/or occurring within the cooling process. 
     
     
         8 . The method according to  claim 1 , characterized in that the coolant is expanded at the cold end of the coolant circuit in two stages, wherein the coolant that is expanded in the first stage is separated in a separator into a liquid C 2+  hydrocarbon-rich fraction and into a gaseous fraction substantially containing solely nitrogen and methane and the liquid C 2+  hydrocarbon-rich fraction is then expanded to the evaporation pressure of the coolant. 
     
     
         9 . The method according to  claim 2 , characterized in that the nitrogen-rich stream that is withdrawn has a fraction of C 2+  hydrocarbons of less than 0.5 mol %. 
     
     
         10 . The method according to  claim 7 , characterized in that the gas or gas mixture is produced from a substream of the compressed boil-off gas fraction  24  and/or the process nitrogen.