Method and apparatus for pre-heating lng boil-off gas to ambient temperature prior to compression in a reliquefaction system

Abstract

A method and apparatus of pre-heating LNG boil-off gas stream flowing from a reservoir in a reliquefaction system, before compression. The method comprises heat exchanging the BOG stream in a first heat exchanger, against a second coolant stream having a higher temperature than the BOG stream, where the second coolant stream is obtained by selectively splitting a first coolant stream into second and third coolant streams, third coolant stream being flowed into a first coolant passage in a reliquefaction system cold box, whereby the BOG has reached near-ambient temperatures prior to compression and the low temperature duty from the BOG is substantially preserved within the reliquefaction system, and thermal stresses in the cold box are reduced. Before the compression step, the BOG is pre-heated to substantially ambient temperatures, by heat exchanging the BOG with said coolant, said coolant prior to the heat exchange having a higher temperature than the BOG.

1 . A method of pre-heating LNG boil-off gas (BOG) stream flowing from a reservoir in a reliquefaction system, prior to compression, the method comprising heat exchanging the BOG stream in a first heat exchanger, against a second coolant stream having a higher temperature than the BOG stream, the method being characterized in that: the second coolant stream is obtained by selectively splitting a first coolant stream into said second coolant stream and a third coolant stream, said third coolant stream being flowed into a first coolant passage in a reliquefaction system cold box, whereby the BOG has reached near-ambient temperatures prior to compression and heat exchange with low temperature BOG is done by optimizing the split of the coolant in the first heat exchanger in order to minimize energy losses, and thermal stresses in the cold box are reduced. 
   
   
       2 . The method of  claim 1 , wherein the selective splitting of the first coolant stream is performed upstream of the first heat exchanger. 
   
   
       3 . A method for cooling an LNG boil-off gas stream in a reliquefaction plant, the BOG flowing from a reservoir, the method comprising:
 compressing the BOG;   heat exchanging the compressed BOG against a coolant in a cold box;   flowing substantially re-liquefied BOG from the cold box to the reservoir;   
     characterized by
 prior to the compression step, pre-heating the BOG to substantially ambient temperatures, by heat exchanging the BOG with said coolant, said coolant prior to the heat exchange having a higher temperature than the BOG. 
 
   
   
       4 . The method of  claim 1 , wherein the necessary duty to heat the BOG prior to compression is transferred from the coolant stream, downstream of a coolant compander aftercooler but upstream of the cold box. 
   
   
       5 . The method of  claim 3 , wherein a portion of the coolant stream to the BOG pre-heater, at a point between the coolant compander and the pre-heater, is routed into a dedicated flow path in the cold-box before it is mixed with the coolant stream flowing from the pre-heater. 
   
   
       6 . The method of  claim 3 , wherein the pressure of the reliquefied BOG between the cold box and the reservoir is controlled independently of the BOG compressor discharge pressure and the reservoir pressure, and the amount of vent gas generated and the vent gas composition thus may be controlled. 
   
   
       7 . An apparatus for cooling an LNG boil-off gas in a reliquefaction system, comprising:
 a closed-loop coolant circuit for heat exchange between a coolant and the BOG;   a BOG compressor having an inlet side fluidly connected to an LNG reservoir;   a cold box having a BOG flowpath with a BOG inlet fluidly connected to the BOG compressor outlet side; said BOG flowpath having outlet for substantially re-liquefied BOG, fluidly connected to the reservoir;   said cold box further comprising coolant flowpaths for heat exchange between the BOG and the coolant, characterized by a first heat exchanger in the fluid connection between the reservoir and the BOG compressor inlet side, said first heat exchanger having a coolant path fluidly connected to the closed-loop coolant circuit, at a point downstream of the coolant circuit's compander aftercooler but upstream of the coolant flow paths in the cold box,   whereby the BOG compressor receives BOG with temperatures near or at the system ambient temperatures.   
   
   
       8 . The apparatus of  claim 7 , further comprising:
 a selector valve in the coolant circuit, in a line downstream of the compander aftercooler, and   a coolant line at one end connected to a first outlet of the selector valve and at the other end connected to the inlet of the coolant passage of the first beat exchanger, and   coolant line, at one end connected to a second outlet of the selector valve and at the other end connected to the inlet of a first coolant passage in the cold box.   
   
   
       9 . The apparatus of  claim 7 , wherein the first heat exchanger coolant path fluid connection further comprises a coolant line at one end connected to the outlet of the coolant passage of the first heat exchanger and at the other end connected to a line fluidly connected to the outlet of the second heat exchanger first coolant passage, and wherein said lines are connected to the inlet of a second coolant passage in the second heat exchanger. 
   
   
       10 . The apparatus of  claim 7 , further comprising a separator in fluid connection with the cold box outlet and with the reservoir, a first valve in the cold box outlet line and a second valve in a line connected to the reservoir, said separator also comprising a vent line, whereby the pressure in the separator may be controlled, and the amount of vent gas and the vent gas composition thus may be adjusted.