Method of Cooling Boil-Off Gas and Apparatus Therefor

Abstract

There is provided a method of cooling a boil-off gas (BOG) stream from a liquefied gas tank using a single mixed refrigerant (SMR) comprising at least the step of heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system to provide a cooled BOG stream, wherein the SMR is provided in an SMR recirculating system comprising at least the steps of: (a) compressing the SMR using at least one centrifugal compressor to provide a post-compression SMR stream; (b) passing the post-compression SMR stream into the liquefaction heat exchanger system to cool the post-compression SMR stream and provide a cooled first SMR vapour stream; (c) withdrawing the cooled first SMR vapour stream from the liquefaction heat exchanger system; (d) separating the cooled first SMR vapour stream to provide a liquid-phase SMR stream and a light SMR vapour stream; (e) passing the light SMR vapour stream through the liquefaction heat exchanger system to provide a condensed SMR stream; and (f) expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream to pass through the liquefaction heat exchanger system for heat exchange against the BOG stream.

1 . A method of cooling a boil-off gas (BOG) stream from a liquefied gas tank using a single mixed refrigerant (SMR) comprising the steps of:
 heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system to provide a cooled BOG stream, wherein the SMR is provided in an SMR recirculating system comprising the steps of   (a) compressing the SMR using at least one centrifugal compressor to provide a post-compression SMR stream;   (b) passing the post-compression SMR stream into the liquefaction heat exchanger system to cool the post-compression SMR stream and provide a cooled first SMR vapour stream;   (c) withdrawing the cooled first SMR vapour stream from the liquefaction heat exchanger system;   (d) separating the cooled first SMR vapour stream to provide a liquid-phase SMR stream and a light SMR vapour stream;   (e) passing the light SMR vapour stream through the liquefaction heat exchanger system to provide a condensed SMR stream; and   (f) expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream to pass through the liquefaction heat exchanger system for heat exchange against the BOG stream.   
     
     
         2 . The method as claimed in  claim 1  wherein the BOG is from a liquefied cargo tank in a floating vessel. 
     
     
         3 . The method as claimed in  claim 1  wherein the liquefaction heat exchanger system comprises a single liquefaction heat exchanger. 
     
     
         4 . The method as claimed in  claim 1  further comprising in step (e) the step of passing the light SMR vapour stream partly through a single liquefaction heat exchanger. 
     
     
         5 . The method as claimed in  claim 1  further comprising in step (e) the step of passing the light SMR vapour stream fully through a single liquefaction heat exchanger. 
     
     
         6 . The method as claimed in  claim 1  wherein the liquefaction heat exchanger system comprises a multi-unit liquefaction heat exchange comprising at least two heat exchange units, and the BOG stream and the expanded lowest-temperature SMR stream pass through each of the at least two heat exchange units. 
     
     
         7 . The method as claimed in  claim 1  further comprising the steps of passing the post-compression SMR stream into a first heat exchange unit of the at least two heat exchange units, and passing the light SMR vapour stream into a second heat exchanger unit of the at least two heat exchange units. 
     
     
         8 . The method as claimed in  claim 1  further comprising the steps of passing the post-compression SMR stream into a first heat exchange unit, and passing the light SMR vapour stream into both the first heat exchange unit and a second heat exchange unit. 
     
     
         9 . The method as claimed in  claim 1  wherein the liquefaction heat exchanger system comprises a multi-unit liquefaction heat exchange comprising two multi-stream heat exchangers. 
     
     
         10 . The method as claimed in  claim 1  wherein the liquefaction heat exchanger system comprises a multi-unit liquefaction heat exchange comprising one multi-stream heat exchanger and a plurality of two-stream heat exchangers. 
     
     
         11 . The method as claimed in  claim 1  further comprising the step of ambient-cooling the post-compression SMR stream prior to step (b). 
     
     
         12 . The method as claimed in  claim 1  further comprising the steps of expanding the liquid-phase SMR stream, and passing the expanded liquid-phase SMR stream into the liquefaction heat exchanger system. 
     
     
         13 . The method as claimed in  claim 1  further comprising the step of combining the expanded liquid-phase SMR stream with the expanded lowest-temperature SMR stream in the liquefaction heat exchanger system. 
     
     
         14 . The method as claimed in  claim 1  wherein the liquefaction heat exchanger system comprises a multi-unit liquefaction heat exchanger system, and further comprising the step of combining the expanded liquid-phase SMR stream with the expanded lowest-temperature SMR stream between two units of the multi-unit liquefaction heat exchanger system. 
     
     
         15 . The method as claimed in  claim 1  further comprising the step of combining the expanded liquid-phase SMR stream with the expanded lowest-temperature SMR stream after the liquefaction heat exchanger system 
     
     
         16 . The method as claimed in  claim 1  wherein step (f) provides a post-cooling vapour SMR stream for recirculation or reuse as part of the SMR recirculating system. 
     
     
         17 . (canceled) 
     
     
         18 . The method as claimed in  claim 1  wherein the post-compression SMR stream of step (a) does not undergo any external refrigerant cooling prior to step (d). 
     
     
         19 . The method as claimed in  claim 1  wherein the BOG stream does not undergo any external refrigerant cooling prior to passing through the liquefaction heat exchanger. 
     
     
         20 . The method as claimed in  claim 1  wherein the liquefaction heat exchanger system comprises one or more plate-fin heat exchangers. 
     
     
         21 . The method as claimed in  claim 1  wherein the expanded lowest-temperature SMR stream provides the cooling of the first SMR vapour stream. 
     
     
         22 . A recirculating system for use with a method of cooling a boil-off gas (BOG) stream from a liquefied gas tank using a single mixed refrigerant (SMR), the system comprising:
 a liquefaction heat exchanger system for heat exchanging the BOG stream with the SMR to provide a cooled BOG stream, and   an SMR recirculating system comprising the steps of:
 (a) compressing the SMR using at least one centrifugal compressor to provide a post-compression SMR stream; 
 (b) passing the post-compression SMR stream into the liquefaction heat exchanger system to cool the post-compression SMR stream and provide a cooled first SMR vapour stream; 
 (c) withdrawing the cooled first SMR vapour stream from the liquefaction heat exchanger system; 
 (d) separating the cooled first SMR vapour stream to provide a liquid-phase SMR stream and a light SMR vapour stream; 
 (e) passing the light SMR vapour stream through the liquefaction heat exchanger system to provide a condensed SMR stream; and 
 (f) expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream to pass through the liquefaction heat exchanger system for heat exchange against the BOG stream. 
   
     
     
         23 . The recirculating system as claimed in  claim 22  for use with cooling BOG from a liquefied gas cargo tank in a floating LNG cargo tank. 
     
     
         24 - 26 . (canceled) 
     
     
         27 . An apparatus for cooling a boil-off gas (BOG) stream from a liquefied gas tank comprising: a single mixed refrigerant (SMR) recirculating system as defined in  claim 22  and a liquefaction heat exchanger system for heat exchange against the BOG stream.