Method and apparatus for cooling down a cryogenic heat exchanger

Abstract

The present invention relates to a method for cooling down a liquefaction system for liquefying a hydrocarbon-containing gas stream. The method for cooling down the liquefaction system comprises: a) performing a pre-cool-down procedure to cool down the pre-cooling stage, b) performing a cryogenic cool-down procedure to cool down the main cooling stage. B) comprises forming a main cool-down stream (201) that meets a predetermined Cn+specification. The main cool-down stream (201) is formed out of at least one auxiliary stream not being the pre-cooled hydrocarbon containing gas stream, and the cryogenic cool down procedure and the pre-cool down procedure are at least partially performed simultaneously.

1 . A method for cooling down a liquefaction system for liquefying a hydrocarbon-containing gas stream, the liquefaction system comprising a pre-cooling stage comprising one or more pre-cooling heat exchangers and a separator, a main cooling stage comprising one or more cryogenic heat exchangers and the liquefaction system further comprising at least one LNG storage tank, the pre-cooling stage being arranged to generate a pre-cooled hydrocarbon containing gas stream by pre-cooling the hydrocarbon-containing gas stream and passing the pre-cooled hydrocarbon containing gas stream to the main cooling stage, the method for cooling down the liquefaction system comprising:
 a) performing a pre-cool-down procedure to cool down the pre-cooling stage, wherein the pre-cool-down procedure comprises feeding the hydrocarbon-containing gas stream to the pre-cooling stage, and   b) performing a cryogenic cool-down procedure to cool down the main cooling stage, wherein the cryogenic cool-down procedure comprises feeding a main cool-down stream to the main cooling stage,   wherein b) comprises forming the main cool-down stream and passing the main cool-down stream to the main cooling stage, the main cool-down stream meeting a predetermined C n   + -specification, the C n   + -specification specifying a first maximum amount of molecules having n or more carbon molecules,   wherein the main cool-down stream is formed out of at least one auxiliary stream not being the pre-cooled hydrocarbon containing gas stream, and   wherein the cryogenic cool down procedure and the pre-cool down procedure are at least partially performed simultaneously.   
     
     
         2 . The method according to  claim 1 , wherein the at least one auxiliary stream meets a second C n   + -specification, the second C n   + -specification specifying a second maximum amount of molecules having n or more carbon molecules, the second maximum amount being lower than the first maximum amount. 
     
     
         3 . The method according to  claim 1 , wherein the predetermined C n   + -specification is C 5   + <0.15 mol %. 
     
     
         4 . The method according to  claim 1 , wherein the at least one auxiliary stream comprises one or more of the following:
 boil-off gas obtained from the at least one LNG storage tank,   liquid natural gas obtained from the at least one LNG storage tank and   nitrogen obtained from a N 2 -source.   
     
     
         5 . The method according to  claim 1 , wherein the hydrocarbon-containing gas stream is a lean hydrocarbon-containing gas stream. 
     
     
         6 . The method according to  claim 1 , wherein the one or more cryogenic heat exchangers comprises refrigerant lubes, hydrocarbon tubes and a shell side and b) comprises feeding the main cool down stream to the refrigerant tubes, hydrocarbon lubes and the shell side. 
     
     
         7 . The method according to  claim 1 , wherein b) comprises
 obtaining a pre-cooled stream from the pre-cooling stage, the pre-cooled stream being derived from the hydrocarbon-containing gas stream,   obtaining one or more auxiliary streams and   mixing the pre-cooled stream and the one or more auxiliary streams to obtain the main cool-down stream.   
     
     
         8 . The method according to  claim 1 , wherein b) comprises
 obtaining a pre-cooled stream from the pre-cooling stage, the pre-cooled stream being derived from the hydrocarbon-containing gas stream,   obtaining an auxiliary stream obtained from a boil-off stream obtained from the at least one LNG storage tank and   mixing the pre-cooled stream and the auxiliary stream to obtain the main cool-down stream.   
     
     
         9 . The method according to  claim 1 , wherein b) comprises
 obtaining a pre-cooled stream from the pre-cooling stage, the pre-cooled stream being derived from the hydrocarbon-containing gas stream,   obtaining an auxiliary stream from a nitrogen-source (N2), the auxiliary stream mainly consisting of nitrogen and   mixing the pre-cooled stream and the auxiliary stream from the nitrogen-source (N2) to obtain the main cool-down stream.   
     
     
         10 . The method according to  claim 1 , wherein the method comprises obtaining a temperature indication (T MCHE ) of the main cooling stage and determine a ratio (R) of the mass How rate of the pre-cooled stream and the mass flow rate of the auxiliary streams combined based on the temperature indication (T MCHE ). 
     
     
         11 . The method according to  claim 1 , wherein b) comprises
 obtaining a first auxiliary stream by taking a side-stream from the pressurized boil-off gas stream   obtaining a second auxiliary stream by taking a side-stream from the healed or cooled and pressurized boil-off gas stream and   mixing the first and second auxiliary streams to obtain the main cool-down stream.   
     
     
         12 . The method according to  claim 1 , wherein b) comprises
 obtaining a first auxiliary stream obtained from a boil-off stream obtained from the at least one LNG storage tank,   obtaining a second auxiliary stream by taking a side-stream from the hydrocarbon-containing gas stream upstream of the pre-cooling stage, and   mixing the first and second auxiliary streams to obtain the main cool-down stream.   
     
     
         13 . The method according to  claim 1 , wherein b) comprises
 obtaining a first auxiliary stream being a liquid natural gas stream from the at least one LNG storage tank,   obtaining a second auxiliary stream by taking a side-stream from the hydrocarbon-containing gas stream upstream of the pre-cooling stage, and   mixing the first and second auxiliary streams to obtain the main cool-down stream.   
     
     
         14 . The method according to  claim 1 , wherein b) is performed until the one or more cryogenic heat exchangers have reached a predetermined first temperature or first temperature profile, subsequently the method continuing with
 feeding a pressurized main cool-down stream to the main cooling stage to further cool-down the main cooling stage, the main cool-down stream having a pressure above 40 barg.   
     
     
         15 . A liquefaction system for liquefying a hydrocarbon-containing gas stream, the liquefaction system comprising a pre-cooling stage comprising one or more pre-cooling heat exchangers and a separator, a main cooling stage comprising one or more cryogenic heat exchangers and the liquefaction system further comprising at least one LNG storage tank, the pre-cooling stage being arranged to generate a pre-cooled hydrocarbon containing gas stream by pre-cooling the hydrocarbon-containing gas stream and passing the pre-cooled hydrocarbon containing gas stream to the main cooling stage, the system comprising a control unit being arranged to cool down the liquefaction system by:
 a) performing a pre-cool-down procedure to cool down the pre-cooling stage, wherein the pre-cool-down procedure comprises feeding the hydrocarbon-containing gas stream to the pre-cooling stage,   b) performing a cryogenic cool-down procedure to cool down the main cooling stage, wherein the cryogenic cool-down procedure comprises feeding a main cool-down stream via a main cool-down stream feed to the main cooling stage, wherein the main cool-down stream feed is arranged to receive at least one auxiliary stream not being the pre-cooled hydrocarbon containing gas stream, and
 wherein the control unit is arranged to perform the cryogenic cool down procedure and the pre-cool down procedure at least partially performed simultaneously.