Abstract:
The disclosure relates to a method and apparatus for cooling, preferably liquefying a boil off gas (BOG) stream from a liquefied cargo in a floating transportation vessel, said liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere, said method comprising at least the steps of: -compressing a boil off gas stream ( 01 ) from said liquefied cargo in three or more stages of compression comprising at least a first stage ( 65 ), a second stage ( 70 ) and final stage ( 75 ) to provide a compressed discharge stream ( 06 ), wherein intermediate compressed BOG streams ( 02, 04 ) are provided between consecutive stages of compression; -cooling the compressed discharge stream ( 06 ) to provide a cooled compressed discharge stream ( 07 ); -heat exchanging an expanded, optionally further cooled, portion of the cooled compressed discharge stream ( 07 ), with (i) one or more intermediate compressed BOG streams ( 04 ) from consecutive stages selected from between the second and final stages ( 75 ) of compression to provide one or more cooled intermediate compressed BOG streams ( 05 ) and optionally (ii) one or more portions ( 07   a,    108   a ), optionally after further cooling, of the cooled compressed discharge stream ( 07 ); and -passing the one or more cooled intermediate compressed BOG streams ( 05 ) to the next stage of compression ( 75 ).

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to a method for the cooling, particularly the reliquefaction, of a boil off gas (BOG) from a liquefied cargo, such as liquefied petroleum gas (LPG), on a floating transportation carrier, and an apparatus therefor. 
       BACKGROUND OF THE DISCLOSURE 
       [0002]    Floating transportation carriers, such as liquefied gas carrier vessels and barges, are capable of transporting a variety of cargoes in the liquefied state. In the present context, these liquefied cargoes have boiling points of greater than −110° C. when measured at 1 atmosphere and include liquefied petroleum gas, liquefied petrochemical gases such as propylene and ethylene, and liquefied ammonia. For instance, liquefied petroleum gas is a useful fuel source, such as for heating appliances and vehicles, as well as being a source of hydrocarbon compounds. LPG comprises one or more of propane, n-butane and i-butane, and optionally one or more other hydrocarbons such as propylene, butylenes and ethane. 
         [0003]    Petroleum gases can be extracted from natural gas or produced in the refining of crude oil. As a consequence, petroleum gases normally comprise a plurality of components. It is often desirable to liquefy petroleum gases in a liquefaction facility at or near the source. As an example, petroleum gases can be stored and transported over long distances more readily as a liquid than in gaseous form because they occupy a smaller volume and may not need to be stored at high pressures. Such LPG can be stored at atmospheric pressure if maintained at or below its boiling temperature, such as at −42° C. or below, being the boiling point of the propane component. Alternatively, LPG may be stored at higher temperatures if it is pressurized above atmospheric pressure. 
         [0004]    The long distance transportation of LPG or other liquefied cargo having boiling point of greater than −110° C. when measured at 1 atmosphere may be carried out in a suitable LPG carrier, particularly a carrier vessel, such as an ocean-going tanker having one or more storage tanks to hold the liquefied cargo. These storage tanks may be insulated and/or pressurized tanks. During the loading of the tanks and the storage of liquefied cargo such as LPG in the tanks, petroleum gas may be produced due to the evaporation of the cargo. This evaporated cargo gas such as petroleum gas is known as boil off gas (BOG). In order to prevent the build up of BOG in the tank, a system may be provided on the carrier to re-liquefy the BOG so that it can be returned to the storage tank in a condensed state. This can be achieved by the compression and cooling of the BOG. In many systems, the compressed BOG is cooled and condensed against seawater. 
         [0005]    Liquefied cargoes such as those comprising primarily propane, particularly commercial grade propane, may further comprise relatively high concentrations of lighter components, such as ethane. Such liquefied cargoes, particularly those comprising ethane at or above 3.5 mol %, may require the presence of three stages of compression to provide sufficient compression to facilitate the condensation of the lighter components of the boil off gas. In some circumstances, the compressed BOG stream between the discharge of the first stage of compression and the suction of the second stage of compression may be cooled. The cooling duty may be provided by the compressed and condensed BOG. 
         [0006]    There are many considerations associated with providing systems to reliquefy boil off gas from such liquefied cargoes in floating transportation carriers. The size of the carrier imposes limitations on the space available for the reliquefaction system. This can restrict the number and size of the compressor trains. Furthermore, size restrictions may also preclude the use of a closed refrigeration system to cool the condenser for the compressed BOG stream, such that the cooling duty may be supplied by seawater. When seawater is used, the reliquefaction system is generally designed to operate with seawater temperatures at up to 32° C. 
         [0007]    It would be advantageous to provide an improved method of cooling, particularly re-liquefying, boil off gas from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a floating transportation carrier. In particular, a more efficient method in terms of one or more of reduced power requirements, increased capacity and increased coefficient of performance is desirable. 
       SUMMARY 
       [0008]    The present disclosure utilises a method of heat exchanging one or more intermediate BOG streams between the second and further stages of multiple stages of compression with part of the cooled compressed discharge from the final compressor stage. This can provide an improvement in the co-efficient of performance for the reliquefaction of the boil off gas. 
         [0009]    In a first aspect, there is provided a method of cooling a boil off gas stream from a liquefied cargo in a floating transportation vessel, said liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere, said method comprising at least the steps of:
   compressing a boil off gas stream from said liquefied cargo in three or more stages of compression comprising at least a first stage, a second stage and final stage to provide a compressed discharge stream, wherein intermediate compressed BOG streams are provided between consecutive stages of compression;   cooling the compressed discharge stream to provide a cooled compressed discharge stream;   heat exchanging an expanded, optionally further cooled, portion of the cooled compressed discharge stream, with (i) one or more intermediate compressed BOG streams from consecutive stages selected from between the second and final stages of compression to provide one or more cooled intermediate compressed BOG streams and optionally (ii) one or more portions, optionally after further cooling, of the cooled compressed discharge stream; and   passing the one or more cooled intermediate compressed BOG streams to the next stage of compression.   
 
         [0014]    In one embodiment, the method comprises the steps of:
   compressing the boil off gas stream in a first stage of compression to provide a first intermediate compressed BOG stream as an intermediate compressed BOG stream;   compressing the first intermediate compressed BOG stream, optionally after heat exchange to provide a cooled first intermediate compressed BOG stream, in a second stage of compression to provide a second intermediate compressed BOG stream as an intermediate compressed BOG stream.   
 
         [0017]    In a further embodiment, the method further comprises the steps of:
   splitting the cooled compressed discharge stream into a continuing cooled compressed discharge stream and a cooled compressed discharge side stream;   expanding the cooled compressed discharge side stream to provide an expanded cooled discharge stream.   
 
         [0020]    In another embodiment, the method further comprises the step of:
   heat exchanging the expanded cooled discharge stream with an intermediate compressed BOG stream from consecutive stages selected from between the second and final stages of compression to provide a cooled intermediate compressed BOG stream.   
 
         [0022]    In yet another embodiment, the method comprises the further step of:
   flashing the expanded cooled discharge stream to provide an overhead expanded cooled discharge stream;   heat exchanging the overhead expanded cooled discharge stream with an intermediate compressed BOG stream from consecutive stages selected from between the second and final stages of compression to provide a cooled intermediate compressed BOG stream.   
 
         [0025]    In a further embodiment of the method, the intermediate compressed BOG stream may be a second intermediate compressed BOG stream and the cooled intermediate compressed BOG stream may be a cooled second intermediate compressed BOG stream. 
         [0026]    In another embodiment, the method comprises the further step of:
   heat exchanging the expanded cooled discharge stream with the continuing cooled compressed discharge stream to provide a further cooled compressed discharge stream.   
 
         [0028]    In a further embodiment, the method comprises the further steps of:
   splitting the further cooled compressed discharge stream into a continuing further cooled compressed discharge stream and a further cooled compressed discharge side stream;   expanding the further cooled compressed discharge side stream to provide an expanded further cooled discharge stream.   
 
         [0031]    In a still further embodiment, the method comprises the further step of:
   heat exchanging the expanded further cooled discharge stream with an intermediate compressed BOG stream from consecutive stages selected from between the first and final stages of compression to provide a cooled intermediate compressed stream.   
 
         [0033]    In another embodiment, the method comprises the further step of:
   flashing the expanded further cooled discharge stream to provide an overhead expanded further cooled discharge stream;   heat exchanging the overhead expanded further cooled discharge stream with an intermediate compressed BOG stream from consecutive stages selected from between the first and final stages of compression to provide a cooled intermediate compressed BOG stream.   
 
         [0036]    In another embodiment of the method, the intermediate compressed BOG stream is a first intermediate compressed BOG stream and the cooled intermediate compressed BOG stream is a cooled first intermediate compressed BOG stream. 
         [0037]    In a further embodiment, the method comprises the further step of:
   heat exchanging the expanded further cooled discharge stream with the continuing further cooled compressed discharge stream to provide a cooled return stream.   
 
         [0039]    In another embodiment, the method comprises the further step of:
   expanding the cooled return stream to provide an expanded cooled return stream.   
 
         [0041]    In yet another embodiment of the method, the liquefied cargo is LPG, such as LPG comprising at least 3.5 mol % ethane. 
         [0042]    In another embodiment of the method, the compressed discharge stream can be cooled against a water stream, such as a seawater stream, to provide the cooled compressed discharge stream. Typically, the water stream has a temperature of +36° C. or below, more typically +32° C. or below. 
         [0043]    In a further embodiment of the method, the first stage, second stage and final stage of compression are stages of a multi-stage compressor. 
         [0044]    In a second aspect, there is provided an apparatus to cool a boil off gas stream from a liquefied cargo in a floating transportation vessel, said liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere, said apparatus comprising at least:
   a multiple stage compression system to compress a boil off gas stream from a liquefied cargo, said compression system comprising three or more stages of compression comprising at least a first stage, a second stage and final stage to provide a compressed discharge stream, wherein intermediate compressed BOG streams are provided between consecutive stages of compression,   a first heat exchanger to cool the compressed discharge stream to provide a cooled compressed discharge stream; and   one or more further heat exchangers to heat exchange an expanded, optionally further cooled, portion of the cooled compressed discharge stream with (i) one or more intermediate compressed BOG streams from consecutive stages selected from between the second and final stages of compression to provide one or more cooled intermediate compressed BOG streams to the next stage of compression and optionally (ii) one or more portions, optionally after further cooling, of the cooled compressed discharge stream.   
 
         [0048]    In a further embodiment said apparatus can be present on the floating transportation vessel. 
         [0049]    In a further embodiment, the apparatus of the second aspect can be operated using the method of the first aspect. 
         [0050]    The apparatus and method disclosed herein are applicable to any floating transportation vessel for a liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere, such as an LPG carrier. The apparatus and method disclosed herein may be utilized in floating transportation vessels where the liquefied cargo storage tanks are fully refrigerated to maintain the cargo in liquid phase at approximately atmospheric pressure by lowering the temperature, as well as in those vessels in which the cargo in the storage tanks is maintained in the liquid phase by a combination of reduced temperature and increased pressure. 
         [0051]    The liquefied cargo may be selected from the group comprising liquefied petroleum gas, liquefied petrochemical gas and liquefied ammonia. The apparatus and method disclosed herein are of particular benefit for a liquefied cargo, such as LPG, comprising light components, particularly ethane at or above 3.5 mol %. For compositions with higher concentrations of light components, additional compression stages may be required for cooling, particularly to effect condensation of the compressed discharge stream of BOG against seawater. 
         [0052]    The method and apparatus disclosed herein utilizes three or more stages of compression. Heat exchangers such as economizers can be placed between consecutive stages of compression selected between the second and final stages. For instance, an economizer can be situated between at least the second and third stages, optionally in combination with an economizer between the first and second stages. In such an economizer, an expanded, optionally further cooled, portion of the cooled compressed discharge stream can be heat exchanged with (i) an intermediate compressed BOG stream and/or (ii) an optionally further cooled continuing portion of the cooled compressed discharge stream. This leads to further improvements in the coefficient of performance and increased cooling, particularly reliquefaction, capacity. 
         [0053]    As used herein, the term “multiple stages of compression” defines two or more stages of compression in series in a compression system. Each stage of compression may be achieved by one or more compressors. The one or more compressors of each compression stage may be independent from those of the other stages of compression, such that they are driven separately. Alternatively, two or more of the stages of compression may utilize compressors which are linked, typically powered by a single driver and drive shaft, with optional gearing. Such linked compression stages may be part of a multi-stage compressor. 
         [0054]    The method and apparatus disclosed herein requires at least three stages of compression. After the first stage of compression, each subsequent stage provides an increased pressure compared to the pressure at the discharge of a previous stage. The term “consecutive stages” refers to pairs of adjacent stages of compression i.e. a stage (n) and the next (n+1) stage where ‘n’ is a whole number greater than 0. Consequently, consecutive stages are, for instance, first and second stages or second and third stages or third and fourth stages. Intermediate compressed streams (and cooled intermediate compressed streams) refer to those streams connecting consecutive stages of compression. The term “next stage of compression” used in relation to the cooled intermediate compressed stream refers to the numerically higher number (and higher pressure stage) of the two consecutive stages defining the intermediate stream. 
         [0055]    The heat exchange steps may be indirect, where the two or more streams involved in the heat exchange are not in direct contact. Alternatively, the heat exchange may be direct, in which case the two or more streams involved in the heat exchange can be mixed, thereby producing a combined stream. 
         [0056]    Other aspects, features, and advantages will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, which are a part of this disclosure and which illustrate, by way of example, principles of any inventions disclosed. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0057]    The accompanying drawings facilitate an understanding of the various embodiments. 
           [0058]      FIG. 1  shows a schematic diagram of one possible known system of reliquefying boil off gas from a cargo tank in an LPG carrier; 
           [0059]      FIG. 2  shows a schematic diagram of a system of cooling, particularly reliquefying, boil off gas from a liquefied cargo in a floating transportation vessel in accordance with this disclosure; and 
           [0060]      FIG. 3  shows a schematic diagram of a system for cooling, particularly reliquefying, boil off gas from a liquefied cargo in a floating transportation vessel in accordance with this disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0061]    Shipboard LPG reliquefaction systems based on the open cycle refrigeration principle draw LPG vapor, also known as boil off gas, from one or more storage tanks, pass the boil off gas to a compressor in which it is compressed such that the compressed vapor can be cooled and condensed using sea water as the heat sink/refrigerant. Typically, the LPG is kept in the storage tank under one or both of reduced temperature (versus ambient) and increased pressure (versus atmospheric). 
         [0062]      FIG. 1  shows a schematic diagram of a known system for reliquefying boil off gas in an LPG carrier vessel. Liquefied petroleum gas (LPG) is stored in a tank  50  which may be insulated and/or pressurized in order to maintain the petroleum gas in a liquefied state. Vaporization of the LPG in the tank, for instance due to imperfect thermal insulation, will result in the formation of petroleum gas in the overhead space of the tank  50 . In order to prevent the build-up of this gas, it is removed from the tank  50  as a boil off gas stream  01 . The removed boil off gas is normally compressed and cooled to condense it before it is returned to the tank  50 . 
         [0063]    The boil off gas stream  01  can be passed to a compression system  60 , such as the three stage compressor shown in  FIG. 1  which comprises a first compression stage  65 , a second compression stage  70  and a third compression stage  75 . The three-stage compressor  60  produces a compressed discharge stream  06  which can be passed to a condenser  100 , in which the compressed discharge stream  06  is cooled against seawater. The condenser  100  produces a cooled compressed discharge stream  07 , such as an at least partially, typically fully, condensed compressed discharge stream and a warmed seawater stream (not shown). 
         [0064]    The cooled compressed discharge stream  07  may be optionally passed to a discharge stream gas/liquid separation device, such as a knock-out drum or accumulator to allow the separation of uncondensed components from the cooled compressed discharge stream  07 . The uncondensed components would normally be vented, while the condensed compressed discharge stream is passed from the gas/liquid separation device for further cooling. 
         [0065]    The cooled compressed discharge stream  07  can be passed to a discharge stream pressure reduction device  120 , such as an expander or Joule-Thomson valve, where it is expanded to provide an expanded cooled discharge stream  17 . The expanded cooled discharge stream  17  can then be passed to a first stage heat exchanger  80 , to provide a cooled return stream  08 , which is typically a partially, more typically a fully condensed stream. 
         [0066]    The cooled return stream  08  may then be passed to a return expansion device  130 , such as an expander or Joule-Thomson valve, to provide an expanded cooled return stream  10 . The expanded cooled return stream  10  may be a sub-cooled condensed return stream. Typically, the return expansion device  130  will reduce the pressure of the cooled return stream  08  to close to that of the LPG and BOG in the tank  50 , such as a pressure just above that of the BOG in the tank, sufficient to ensure an adequate flow of the expanded cooled return stream  10  to the tank  50 . 
         [0067]    The first stage  65  of the three-stage compressor  60  provides a first intermediate compressed BOG stream  02 , which is passed to first stage heat exchanger  80 . The first intermediate compressed BOG stream  02  can be heat exchanged against the expanded cooled discharge stream  17  in the first stage heat exchanger  80  to provide a cooled first intermediate compressed BOG stream  03 . It will be apparent that the discharge stream pressure reduction device  120  should reduce the pressure of the cooled compressed discharge stream  17  to at or near that of the first intermediate compressed BOG stream  02 . The cooled compressed discharge stream  17  and the first intermediate compressed BOG stream  02  are mixed in the shell side of the first stage heat exchanger  80 . A vapor stream can be withdrawn from the first stage heat exchanger  80  as cooled first intermediate compressed BOG stream  03 . 
         [0068]    The cooled first intermediate compressed BOG stream  03  can then be passed to the suction of the second stage  70  of the three-stage compressor  60 . The second stage  70  compresses the cooled first intermediate compressed BOG stream  03  to provide a second intermediate compressed BOG stream  04 . 
         [0069]    The second intermediate compressed BOG stream  04  can then be passed to the suction of a third stage  75  of the three-stage compressor  60 , where it is compressed to provide the compressed discharge stream  06 . 
         [0070]    The method and apparatus disclosed herein seeks to improve the system of reliquefying BOG according to  FIG. 1 . An embodiment of the method and apparatus according to the present disclosure is given in  FIG. 2 . Where appropriate, identical stream and component names and reference numerals to that of  FIG. 1  have been used for corresponding streams and components in  FIG. 2 . 
         [0071]      FIG. 2  shows a liquefied cargo storage tank  50  in a floating transportation vessel, such as an LPG carrier. The liquefied cargo may be LPG and the boil off gas may be petroleum gas. In order to cool, particularly reliquefy, evaporated cargo from the storage tank  50 , a boil off gas stream  01 , comprising evaporated cargo, is passed to a compression system  60  having three or more stages of compression. The boil off gas stream  01  may have a pressure (the “BOG pressure”) in the range of from above 0 to 500 kPa gauge. The compression system  60  may be a multi-stage compressor comprising three of more stages. By “multi-stage compressor” it is meant that each compression stage in the compressor is driven by the same drive shaft. Alternatively, the compression system  60  may comprise independently driven compressors for each of the stages of compression. When the compression system  60  is a multi-stage compressor, it is typically a reciprocating compressor. 
         [0072]    The embodiment of  FIG. 2  shows a compression system  60  having a first stage  65 , a second stage  70  and a third stage  75 , although the method and apparatus described herein is also applicable to compressors having four or more stages. The first stage, second and third stages  65 ,  70 ,  75  provide low, intermediate and high pressure streams respectively at their discharge. 
         [0073]    The compression system  60  compresses the boil off gas stream  01  to provide a compressed discharge stream  06 . The compressed discharge stream  06  may have a pressure (the “third stage pressure”) in the range of from 2.0 to 3.5 MPa gauge. The compressed discharge stream  06  can be passed to a discharge stream heat exchanger  200 , such as a condenser. The compressed discharge stream  06  is cooled against a heat exchange fluid, such as seawater, to provide a cooled compressed discharge stream  07  and warmed heat exchange fluid (not shown). Typically, the seawater used as the heat exchange fluid would have a temperature of +36° C. or below, more typically +32° C. or below. 
         [0074]    The cooled compressed discharge stream  07  is typically a partially, more typically a fully condensed, compressed discharge stream. It is preferred that the cooled compressed discharge stream  07  is further cooled. This can be achieved by passing the cooled compressed discharge stream  07  to one or more further heat exchangers  190 ,  180 , such as an intermediate stage economizer for cooling the intermediate BOG streams. These are discussed in more detail below. 
         [0075]    For instance, the cooled compressed discharge stream  07  can be cooled against an expanded portion of the cooled compressed discharge stream in the first further heat exchanger  190 . In the embodiment shown in  FIG. 2 , a discharge stream splitting device  110  divides the cooled compressed discharge stream  07  into a continuing cooled compressed discharge stream  07   a  and a cooled compressed discharge side stream  09 . The cooled compressed discharge side stream  09  can be passed to a first discharge stream pressure reduction device  120 , such as an expander or Joule-Thomson valve, where it is expanded to provide an expanded cooled discharge side stream  19 , which can then be heat exchanged against the continuing compressed discharge stream  07   a  to provide a further cooled compressed discharge stream  108 . 
         [0076]    The first further heat exchanger  190 , which can be a second intermediate stage economizer, may be a shell and tube or a shell and coil heat exchanger in which the cooled compressed discharge stream  07  is passed through one or more tubes or coils  195  ( FIG. 2  shows a coil) in which it is cooled against the expanded cooled discharge side stream  19  injected into the shell side of the first heat exchanger. The cooled compressed discharge side stream  09  can be expanded to a pressure close to the pressure of the discharge of the second stage of the multi-stage compressor, as is discussed in more detail below. 
         [0077]    In a further embodiment not shown in  FIG. 2 , the discharge stream splitting device  110  can be provided downstream of the first further heat exchanger  190 , such that the fluid providing the cooling duty is obtained by the expansion of a portion of the further cooled compressed discharge stream  108 , rather than the expansion of a portion of the cooled compressed discharge stream  07 . 
         [0078]    The further cooled compressed discharge stream  108  can then be passed to a second further heat exchanger  180 , such as first intermediate stage economizer, typically of the shell and tube or shell and coil type. For instance, the further cooled compressed discharge stream  108  can be cooled against an expanded portion of the further cooled compressed discharge stream. In the embodiment shown in  FIG. 2 , a further cooled discharge stream splitting device  210  divides the further cooled compressed discharge stream  108  into a continuing further cooled compressed discharge stream  108   a  and a further cooled compressed discharge side stream  11 . The further cooled compressed discharge side stream  11  can be passed to a second discharge stream pressure reduction device  220 , such as an expander or Joule-Thomson valve, where it is expanded to provide an expanded further cooled discharge side stream  21 , which can then be heat exchanged against the continuing further cooled compressed discharge stream  108   a  to provide a cooled return stream  08 , typically as a sub-cooled stream. Typically, this heat exchange is carried out by injecting the expanded further cooled discharge side stream  21  into the shell side of the second further heat exchanger  180 , with the continuing further cooled compressed discharge stream  108   a  present in one or more second further heat exchanger tubes or coils  185  (a coil is shown in  FIG. 2 ) within the shell of the heat exchanger. 
         [0079]    In an embodiment not shown in  FIG. 2 , the stream providing the cooling duty to the second further heat exchanger  180  may be drawn as a side stream from the cooled return stream  08 , and then expanded and injected into the second further heat exchanger  180 . In this case, the splitting device would be provided in the cooled return stream  08 , rather than in the further cooled compressed discharge stream  108 . 
         [0080]    In a similar manner to the scheme of  FIG. 1 , the cooled return stream  08  can then be passed to a return expansion device  130 , such as an expander or Joule-Thomson valve, to provide an expanded cooled return stream  10 , which may be a sub-cooled condensed return stream. This can then be returned to the storage tank  50 . 
         [0081]    Returning to the first and second further heat exchangers  190  and  180 , as well as cooling continuing compressed discharge stream  07   a  and continuing further cooled compressed discharge stream  108   a,  they can also cool intermediate compressed streams from the first and second compressor stages  65 ,  70 . In such an embodiment, the first and second heat exchangers  190 ,  180  can be economizers. This heat exchange can lead to an increased coefficient of performance. 
         [0082]    In particular, the boil off gas stream  01  can be compressed by first stage  65  to a first intermediate compressed BOG stream  02  at a first stage pressure. The first intermediate compressed BOG stream  02  can then be heat exchanged against the expanded further cooled discharge side stream  21  to provide a cooled first stage compressed BOG stream  03 . This heat exchange can be carried out in second further heat exchanger  180 , which is typically a first intermediate stage economizer. When the first intermediate stage economizer is of the shell and tube type, the first intermediate compressed BOG stream  02  and the expanded further cooled discharge side stream  21  can both be injected into the shell-side of the heat exchanger. This is known as liquid subcooling. During the heat exchange process, these streams will mix such that the cooled first stage compressed BOG stream  03  will be a combination of these streams. It will be apparent that the further cooled compressed discharge side stream  11  should therefore be expanded to a pressure at or slightly above that provided by the discharge of the first stage  65 , namely the first stage pressure. This will provide an acceptable pressure balance within the second further heat exchanger  180 . 
         [0083]    The cooled first stage compressed BOG stream  03  can then be passed to the suction of the second stage  70  of the compression system  60 , where it is compressed to provide a second intermediate compressed BOG stream  04  at a second stage pressure. In order to provide the benefits of the method and apparatus disclosed herein, the second stage compressed BOG stream  04  should be cooled prior to passing it to the suction of the third stage  75  of the multi-stage compressor  60 . The cooling of the steam  04  therefore leads to a reduction in the temperature of the stream provided at the discharge of the third stage  75 . This can enable a reduction in the size of the discharge stream heat exchanger  200 , which can be a condenser. 
         [0084]    The second intermediate compressed BOG stream  04  can be heat exchanged against the expanded further cooled discharge side stream  19  to provide a cooled second intermediate compressed BOG stream  05 . This heat exchange can be carried out in first further heat exchanger  190 , which is typically a second intermediate stage economizer. When the second intermediate stage economizer is of the shell and tube type, the second intermediate compressed BOG stream  04  and the expanded cooled discharge side stream  19  can both be injected into the shell-side of the heat exchanger. During the heat exchange process, these streams will mix such that the cooled second stage compressed BOG stream  05  will be a combination of these streams to provide liquid subcooling of the second intermediate compressed BOG stream  04 . It will be apparent that the cooled compressed discharge side stream  110  should therefore be expanded to a pressure at or slightly above that provided by the discharge of the second stage  70 , namely the second stage pressure. This will provide an acceptable pressure balance within the first further heat exchanger  190 . 
         [0085]    In an alternative embodiment of the method and apparatus disclosed herein, rather than the use of liquid subcooling in which the discharge vapor from a previous compressor stage is passed into the further heat exchanger where it mixes with the vapor before being passed to the suction of the next stage of the compressor as shown in  FIG. 2 , a flash liquid subcooling process may be used. In the flash liquid subcooling process, the discharge vapor from the previous compressor stage, is not passed through the further heat exchanger but is mixed with the vapor produced in the heat exchanger at or before the suction to the next stage of the compression cycle. 
         [0086]    This embodiment is shown in relation to a first further heat exchanger  190 ′, such as a second intermediate stage economizer which can be of the shell and tube type, in  FIG. 3 . The second intermediate compressed BOG stream  04 , is not passed through the first further heat exchanger  190 ′ as it is in the embodiment of  FIG. 2 , but is mixed with the vapor produced in the second intermediate stage economizer at or before the suction to the next stage of the compression cycle. 
         [0087]    In particular with regard to  FIG. 3 , the expanded cooled discharge side stream  19  can be injected into the first further heat exchanger  190 ′, such as the second intermediate stage economizer, to provide an overhead expanded cooled discharge stream  31  which is withdrawn from the heat exchanger. The overhead expanded cooled discharge stream  31  can be produced by flashing the expanded cooled discharge side stream  19  into the shell of the first further heat exchanger  190 ′. 
         [0088]    The overhead expanded cooled discharge stream  31  is then mixed with, typically by comingling, the second intermediate compressed BOG stream  04  to provide a cooled second intermediate compressed BOG stream  05 , which is then passed to the suction of the third stage  75  of the compression system  60 . 
         [0089]    The continuing cooled compressed discharge stream  07   a  can be cooled in the first further heat exchanger  190 ′ by heat exchange with the expanded cooled discharge side stream  19  in a similar manner to the embodiment of  FIG. 2 . 
         [0090]    Although not shown in  FIG. 3 , a similar flash liquid subcooling process can be carried out in the second further heat exchanger, which can be a first intermediate stage economizer, typically of the shell and tube or shell and coil type. Thus, rather than passing the first intermediate compressed BOG stream  02  to the second further heat exchanger  180 , the overhead stream (an overhead expanded further cooled discharge stream) withdrawn from the second further heat exchanger produced by flashing the expanded further cooled discharge side stream  21  into the shell side of the second further heat exchanger  180  can be mixed with the first intermediate compressed BOG stream  02  to provide a cooled first intermediate compressed BOG stream. 
         [0091]    In a further alternative embodiment not shown in the Figures, the cooled compressed discharge stream  07  can, instead of being passed through the further heat exchangers  190 ,  180  in series in the order of those operating at the highest pressure (first further heat exchanger  190 ) to lowest pressure (second further heat exchanger  180 ), be provided to all the heat exchangers in parallel. In such a situation, the cooling duty would be supplied by cooled compressed discharge side stream  09  or drawn as side streams from the cooled return streams, after expansion to pressures appropriate for each heat exchanger. 
       EXAMPLE 
       [0092]    This Example provides a hypothetical calculation of the power requirements, cooling capacity and coefficient of performance of two LPG re-liquefaction systems, namely a system according to the present disclosure in which flash liquid subcooling is carried out on the first and second intermediate compressed BOG streams (i.e. the embodiment of  FIG. 3  in which flash liquid subcooling is also carried out in the first intermediate stage economizer) and a comparative system in which flash liquid subcooling is only carried out on the first compressed intermediate stream (i.e.  FIG. 2  without a first further heat exchanger/second intermediate stage economizer). 
         [0093]    Compression system data was based upon a three-stage compressor (Burckhardt Compression AG, Winterthur, Switzerland). The liquefied petroleum gas cargo comprised 5.0 mol % ethane and 95.0 mole % propane in the liquid phase. The vapor phase composition of the boil off gas at a tank storage pressure of 0.4 bar gauge was calculated to be 24.23 mole % ethane and 75.77 mole % propane, based upon Peng Robinson Stryjek-Vera equations of state. 
         [0094]    Table 1 shows the calculated suction and discharge pressures and temperatures at the three stages of compression of the comparative Example. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Suction 
                 Discharge 
               
             
          
           
               
                   
                 Pressure 
                 Temperature 
                 Pressure 
                 Temperature 
               
               
                 Stage no. 
                 (bar abs) 
                 (° C.) 
                 (bar abs) 
                 (° C.) 
               
               
                   
               
             
          
           
               
                 1 
                 1.40 
                 −20.0 
                 4.73 
                 46.2 
               
               
                 2 
                 4.73 
                 26.5 
                 10.82 
                 67.3 
               
               
                 3 
                 10.82 
                 67.3 
                 24.00 
                 103.8 
               
               
                   
               
             
          
         
       
     
         [0095]    Table 2 shows the suction and discharge pressures and temperatures at the three stages of compression of the Example according to the present disclosure. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Suction 
                 Discharge 
               
             
          
           
               
                   
                 Pressure 
                 Temperature 
                   
                 Temperature 
               
               
                 Stage no. 
                 (bar abs) 
                 (° C.) 
                 Pressure (bar abs) 
                 (° C.) 
               
               
                   
               
             
          
           
               
                 1 
                 1.40 
                 −20.0 
                 4.33 
                 42.8 
               
               
                 2 
                 4.33 
                 30.9 
                 11.09 
                 75.3 
               
               
                 3 
                 11.09 
                 65.5 
                 24.00 
                 101.3 
               
               
                   
               
             
          
         
       
     
         [0096]    In both the comparative Example and the Example according to the present disclosure, a third stage discharge pressure of 24 bar abs provides a condensing temperature of +40° C. 
         [0097]    Table 3 shows the calculated power, reliquefaction capacity and coefficient of performance of the system according to the comparative Example and the Example according to the present disclosure. 
         [0098]    It is apparent from Table 3 that the introduction of a flash liquid heat exchange step between the second and third stages of the three-stage compressor results in a reduction in overall compressor driver power, together with an increase in cooling capacity and coefficient of performance, compared to a system in which there is no cooling of the discharge from the second stage compressor in a second stage economizer. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Power (kWE) 
                 Capacity (kWR) 
                 C.O.P. 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Comparative 
                 381.1 
                 394.4 
                 1.035 
               
               
                   
                 Present new 
                 375.7 
                 422.6 
                 1.125 
               
               
                   
                 disclosure 
               
               
                   
                   
               
             
          
         
       
     
         [0099]    The person skilled in the art will understand that any invention disclosed herein can be carried out in many various ways without departing from the scope of the appended claims. For instance, an invention may encompass the combination of one or more of the optional or preferred features disclosed herein. For example, it may not be required that there be the presence of a heat exchanger such as an economizer between the first and second stages of a multi-stage compressor. Alternatively or additionally, the present disclosure can be applicable to multi-stage compressors comprising more than three stages. Heat exchangers such as economizers may be placed between more than at least one of the second and further stages of the multi-stage compressor. For instance, in a four stage compressor, such heat exchangers may be placed between one or both of the second and third stages and third and fourth stages, as well as optionally between the first and second stages. 
         [0100]    Also, the various embodiments described above may be implemented in conjunction with other embodiments, e.g., aspects of one embodiment may be combined with aspects of another embodiment to realize yet other embodiments. Further, each independent feature or component of any given assembly may constitute an additional embodiment. 
         [0101]    In the foregoing description of certain embodiments, specific terminology has been resorted to for the sake of clarity. However, the disclosure is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes other technical equivalents which operate in a similar manner to accomplish a similar technical purpose. Terms such as “left” and right“, “front” and “rear”, “above” and “below” and the like are used as words of convenience to provide reference points and are not to be construed as limiting terms. 
         [0102]    In this specification, the word “comprising” is to be understood in its “open” sense, that is, in the sense of “including”, and thus not limited to its “closed” sense, that is the sense of “consisting only of”. A corresponding meaning is to be attributed to the corresponding words “comprise”, “comprised” and “comprises” where they appear.