Method Of Cooling Boil Off Gas And An Apparatus Therefor

Abstract

A method of cooling a boil off gas stream (01) from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a liquefied cargo tank (50) in a floating transportation vessel, said method comprising at least the steps of: compressing a boil off gas stream (01) from said liquefied cargo in two or more stages of compression comprising at least a first compression stage (65) and a final compression stage (75) to provide a compressed BOG discharge stream (06), wherein said first compression stage (65) has a first stage suction pressure and said final compression stage (75) has a final stage suction pressure; cooling the compressed BOG discharge stream (06) against one or more first coolant streams (202, 302) to provide a first cooled compressed BOG stream (08); providing a gaseous vent stream (51) from the first cooled compressed BOG stream (08); cooling the first cooled compressed BOG stream (08) against a second coolant stream (33) to provide a second cooled compressed BOG stream (35); expanding a portion of the second cooled compressed BOG stream (35) to the first stage suction pressure or below to provide a first expanded cooled BOG stream (33); using the first expanded cooled BOG stream (33) as the second coolant stream to provide a first expanded heated BOG stream (38); and cooling the gaseous vent stream (51) against the second coolant stream (33) to provide a cooled vent stream (53), wherein cooling of the first cooled compressed BOG stream (08) and cooling of the gaseous vent stream (51) occurs in a heat exchanger located adjacent to the liquefied cargo tank (50).

1 . (canceled) 
     
     
         2 . (canceled) 
     
     
         3 . A method of cooling a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a floating transportation vessel, said method comprising the steps of:
 compressing a boil off gas stream from said liquefied cargo in at least two stages of compression comprising at least a first compression stage and a second compression stage to provide a compressed BOG discharge stream, wherein said first compression stage has a first stage suction pressure and a first stage discharge pressure, and said second compression stage has a second stage suction pressure; 
 cooling the compressed BOG discharge stream against at least one first coolant streams at a first cooling stage to provide a first cooled compressed BOG stream; 
 cooling the first cooled compressed BOG stream against a second coolant stream at a second cooling stage to provide a second cooled compressed BOG stream; 
 cooling the second cooled compressed BOG stream against a third coolant stream at a third cooling stage to provide a third cooled compressed BOG stream; 
 providing a gaseous vent stream from the first cooled compressed BOG stream; 
 cooling the gaseous vent stream against the second coolant stream at the second cooling stage to provide a cooled vent stream; 
 cooling the cooled vent stream against the third coolant stream at the third cooling stage to provide a cooled vent stream; 
 expanding a portion of the second cooled compressed BOG stream at the second cooling stage to a pressure between that of the first stage discharge pressure and the second stage suction pressure to provide a first expanded cooled BOG stream; 
 expanding a portion of the third cooled compressed BOG stream at the third cooling stage to the first stage suction pressure or below to provide a second expanded cooled BOG stream; 
 using the first expanded cooled BOG stream as the second coolant stream to provide a first expanded heated BOG stream; and 
 using the second expanded cooled BOG stream as the third coolant stream to provide a second expanded heated BOG stream. 
 
     
     
         4 . The method as in  claim 3 , wherein the liquefied cargo is selected from the group comprising:
 ethane, liquefied petroleum gas, liquefied petrochemical gas such as propylene and ethylene, and liquefied ammonia.   
     
     
         5 . The method as in  claim 3  wherein the liquefied cargo is ethane. 
     
     
         6 . The method as in  claim 5  wherein the liquefied ethane cargo comprises >0.1 mol % methane. 
     
     
         7 . The method as in  claim 6  wherein the liquefied ethane cargo comprises >0.5 mol % methane. 
     
     
         8 . The method as in  claim 3  comprising at least three stages of compression. 
     
     
         9 . The method as in  claim 3  comprising the further step of:
 separating the further cooled vent stream to provide a vent discharge stream and a cooled vent BOG return stream. 
 
     
     
         10 . The method as in  claim 9  comprising the further steps of:
 expanding the cooled vent BOG return stream to provide an expanded cooled vent BOG return stream; and 
 passing the expanded cooled vent BOG return stream to a storage tank. 
 
     
     
         11 . The method as in  claim 3  wherein the step of cooling the compressed BOG discharge stream against at least one first coolant stream to provide a first cooled compressed BOG stream comprises:
 pre-cooling the compressed BOG discharge stream against a pre-cooling coolant stream as a first coolant stream to provide a pre-cooled compressed BOG stream; and 
 cooling the pre-cooled compressed BOG stream against a first refrigerant stream as a first coolant stream to provide the first cooled compressed BOG stream. 
 
     
     
         12 . The method as in  claim 11  wherein the pre-cooling coolant stream is selected from the group comprising: a seawater stream, an ambient air stream and a refrigerant stream. 
     
     
         13 . The method as in  claim 11  wherein the first refrigerant stream is selected from the group comprising propane and propylene. 
     
     
         14 . The method as in  claim 3  wherein stages of compression are the compression stages of a multi-stage compressor. 
     
     
         15 . An apparatus to cool a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a liquefied cargo tank in a floating transportation vessel, said apparatus comprising:
 a compression system to compress the boil off gas stream comprising at least two stages of compression comprising at least a first compression stage and a final compression stage to provide a compressed BOG discharge stream, 
 at least one first heat exchanger to cool the compressed BOG discharge stream to provide a first cooled compressed BOG stream; 
 at least one second heat exchanger adjacent to the liquefied cargo tank to further cool the first cooled compressed BOG stream and a gaseous vent stream against a coolant stream to provide a second cooled compressed BOG stream and a cooled vent stream. 
 
     
     
         16 . An apparatus to cool a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a liquefied cargo tank in a floating transportation vessel comprising, the apparatus comprising:
 a compression system to compress the boil off gas stream comprising at least two compression trains, each train comprising at least two stages of compression comprising at least a first compression stage and a second compression stage to provide a compressed BOG discharge stream, wherein said first compression stage has a first stage suction pressure and a first stage discharge pressure, and said second compression stage has a second stage suction pressure; 
 a first cooling stage comprising at least one first heat exchanger to cool the compressed BOG discharge stream to provide a first cooled compressed BOG stream; 
 a second cooling stage comprising at least one second heat exchanger to cool the first cooled compressed BOG stream to provide a second cooled compressed BOG stream using a second coolant stream having a pressure between that of the first stage discharge pressure and the second stage suction pressure; and 
 a third cooling stage comprising at least one third heat exchanger to cool the second cooled compressed BOG stream to provide a third cooled compressed BOG stream using a third coolant stream having first stage suction pressure or below. 
 
     
     
         17 . The apparatus as in  claim 16  further comprising:
 a first passage to pass a first expanded heated BOG stream provided from the passage of the second coolant stream through the second cooling stage into a second compression train, and 
 a second passage to expand a second expanded heated BOG stream provided from the passage of the third coolant stream through the third cooling stage to the first stage suction pressure or below, and to pass the further expanded heated BOG stream into a first compression train. 
 
     
     
         18 . The apparatus as in  claim 17  further comprising:
 a third passage to expand at least a portion of a first expanded heated BOG stream provided from the passage of the second coolant stream through the second cooling stage, to a pressure between that of the first stage discharge pressure and the second stage suction pressure, and to pass the further expanded heated BOG stream into a first compression train, and 
 a fourth passage to pass at least a portion of a second expanded heated BOG stream provided from the passage of the third coolant stream through the third cooling stage into a second compression train. 
 
     
     
         19 . (canceled) 
     
     
         20 . (canceled) 
     
     
         21 . The apparatus as in  claim 20  further comprising: a controller able to control the passage of the first expanded heated BOG stream from the second cooling stage into the first compression train or the second compression train or both, and to control the passage of the second expanded heated BOG stream after the third cooling stage into the first compression train or the second compression train or both. 
     
     
         22 . (canceled) 
     
     
         23 . (canceled) 
     
     
         24 . (canceled) 
     
     
         25 . The method of cooling a boil of gas stream as in  claim 3 , wherein the cooling of the first cooled compressed BOG stream and cooling of the gaseous vent stream occurs in a heat exchanger located adjacent to a tank retaining the liquefied cargo. 
     
     
         26 . The method of cooling a boil of gas stream as in  claim 25 , wherein cooling of the first cooled compressed BOG stream and the cooling of the gaseous vent stream take place in the heat exchanger. 
     
     
         27 . A method of cooling a boil off gas stream as in  claim 3  wherein
 the pressure of the second coolant stream is between that of the first stage discharge pressure and the second stage suction pressure at a second cooling stage within the step of cooling of the first cooled compressed BOG stream against a second coolant stream, and 
 the third coolant stream has a first stage suction pressure within the step of cooling the second cooled compressed BOG stream against the third coolant stream the third cooling stage; and 
 further comprising the step of 
 passing the first expanded heated BOG stream and the second expanded heated BOG stream into at least one compression stage at a pressure that is between the first stage discharge pressure and the second stage suction pressure, that is at or below the first stage suction pressure, or that is between the first stage discharge pressure and the second stage suction pressure and also at or below the first stage suction pressure.