Patent Publication Number: US-2023160632-A1

Title: Liquefaction and subcooling system and method

Description:
The present invention relates to the storage and transportation of liquefied gases. Specifically, the present invention relates to the management and the re-liquefaction and subcooling of boil off gas of different liquefied gases that are alternatively stored and/or transported in the same facility. 
     BACKGROUND 
     Liquefied gases, such as liquefied ethane or liquefied natural gas (LNG), are typically stored in atmospheric tanks at saturated conditions, when being transported by liquefied gas carriers such as a floating transportation vessel. In the tanks liquefied gas evaporates due to the unavoidably imperfect insulation of the tanks. This evaporated gas is known as boil off gas (BOG). While it is possible to use some of the BOG for powering engines of the gas carrier, normally not all BOG can be utilized in this way. Therefore, liquefied gas carriers have a BOG management system for re-liquefying the BOG, in order to avoid a pressure build-up in the tanks, or venting of BOG into the atmosphere. Such BOG management systems are known; see US 20140102133 A1, for example. 
     For different gases there are large differences in temperature of the liquefied gas stored at saturated conditions in atmospheric tanks. For example, the temperature level in a tank for liquefied ethane is around -90° C., whereas the temperature level in a tank for LNG is around -160° C. Known BOG management systems are generally not able to re-liquefy different gases having a huge difference in the saturation temperatures at atmospheric pressure and requiring a huge difference in the cold temperature of the refrigerant fluid used for re-liquefaction. 
     Currently, large gas carriers being able to transport up to 180,000 m 3  are planned to be built. Surplus ethane, which is a sub-product from shale gas in the US, is transported by such carriers from North America to Europe, for example. However, it is desirable that ethane carriers, whose construction requires a huge investment, are also able to transport LNG to provide a higher degree of flexibility, especially in case of uncertain market conditions. 
     Therefore, an objective of the present invention is to provide a liquefaction and subcooling system that is able to efficiently work at different temperature levels, in particular the liquefaction and subcooling system should be suitable for use in a boil off gas management system for re-liquefying and subcooling boil off gas. 
     SUMMARY 
     Provided are a liquefaction and subcooling system for liquefying and subcooling different first and second gases having different saturation temperatures at a given pressure, a boil off gas management system, and a method for liquefying and subcooling different first and second gases having different saturation temperatures at a given pressure according to the independent claims. Dependent claims and the following descriptions relate to preferred embodiments. 
     The liquefaction and subcooling system for liquefying different first and second gases having different saturation temperatures at a given pressure, comprises a refrigeration device operable to alternatively provide a refrigerant fluid at a first and a second cold temperature that correspond to different liquefaction temperatures of the first and second gases, a subcooling arrangement coupled to the refrigeration device such that the refrigerant fluid is supplied to the subcooling arrangement at its cold temperature, the subcooling arrangement having first and second subcoolers for exchanging heat between a gas to be liquefied and/or subcooled and the refrigerant fluid, wherein, when the gas to be liquefied and/or subcooled is the first gas, the refrigeration device is configured to provide the refrigerant fluid at the first cold temperature and the subcooling arrangement is configured to guide the refrigerant fluid and the gas to be liquefied and/or subcooled through the first subcooler; and, when the gas to be liquefied and/or subcooled is the second gas, the refrigeration device is configured to provide the refrigerant fluid at the second cold temperature and the subcooling arrangement is configured to guide the refrigerant fluid and the gas to be liquefied and/or subcooled through the second subcooler. The term ‘liquefaction temperature’ is used herein to indicate the cryogenic temperature necessary to liquefy a gas at given pressure. After liquefaction (or already after a prior compression stage) a liquid state of the gas is obtained, which may further be cooled below the liquefaction temperature, i.e. which may be subcooled. Thus, generally the process includes liquefaction and/or subcooling. For the sake of simplicity the expression “gas to be liquefied and/or subcooled” is used, with the meaning that the subcooling part of this expression relates to the (re-)liquefied gas, i.e. the part “gas to be subcooled” in the expression means that the (re-)liquefied gas (in the liquid state) is subcooled (cooled below the liquefaction temperature, cooled below the condensation temperature). 
     The liquefaction and subcooling system can in particular be used in a boil off gas (BOG) management system of a storage or transportation device for liquid gas at atmospheric temperature, i.e. in a BOG management system for a tank for liquid gas. While in a BOG management system the gas is ‘re-liquefied’, the terms ‘liquefied’, ‘liquefying’, and ‘liquefaction’ are used throughout this application for simplicity and since the prior state of the gas is not essential for carrying out the invention. 
     The liquefaction and subcooling system according to the invention advantageously allows the use of a single cold production system (refrigeration device) that is common for the liquefaction of different gases, in particular for the management of different boil off gases. Specifically, the rotating machinery is common and the refrigerant is common, and switching between the different gases can be done by a simple set of on/off valves. 
     According to this solution, the provision of two different subcoolers allows to liquefy different gases. The liquefaction and subcooling system may simply be switched between the two situations. This implies that a tank facility for liquid gas (e.g. on a gas carrier vessel) can be used for different gases without the need of having to provide different re-liquefaction and BOG management systems for each of the gases, only the subcooling arrangement has to be changed. 
     Preferably, the first subcooler is configured for heat exchange between the first gas to be liquefied and/or subcooled and the refrigerant fluid being at the first cold temperature, and the second subcooler is configured for heat exchange between the second gas to be liquefied and/or subcooled and the refrigerant fluid being at the second cold temperature. Specifically, the subcoolers are selected or designed to enable a large heat exchange at the respective temperature level for each of the gases. This allows to overcome mechanical constraints imposed on the subcoolers (heat exchangers) by the operation at different temperature levels, i.e. to overcome the problem that a heat exchanger designed to operate at a certain temperature may not be suitable to operate effectively at a different temperature. The first and second subcoolers are used alternatively. The first and the second subcoolers are essentially heat exchangers, and may be one of the following types: a shell and tube heat exchanger, a plate in shell heat exchanger, or a plate fin heat exchanger, preferably plate fin heat exchangers are used. 
     According to an embodiment the liquefaction and subcooling system can comprise a multi-stream heat exchanger, in which a heat exchanger of the refrigeration device is formed, and in which the first and second subcoolers are formed by separate conduits. 
     According to an embodiment the subcooling arrangement can include a combined subcooler formed by a heat exchanger in which the first and second subcoolers are formed by separate conduits. 
     According to an embodiment the first and second subcoolers can be separate heat exchangers. Preferably the two separate heat exchangers are of two different types selected from the following types: shell and tube heat exchangers, plate in shell heat exchangers, and plate fin heat exchangers. 
     According to an embodiment the first and/or second subcoolers can be formed by plate fin heat exchangers. 
     According to an embodiment the refrigerant fluid can be selected from helium, nitrogen, methane, ethane, neon, or a combination thereof. According to an embodiment the first cold temperature can be in the range of -120° C. to -85° C., preferably -115° C., and the second cold temperature can be in the range of -183° C. to -155° C., preferably -178° C. Preferably the difference between the first and second cold temperatures is at least 30° C., preferably at least 40° C., most preferably at least 50° C. Preferably the difference between the first and second cold temperatures is in the range of 30° C. - 100° C., more preferably in the range of 40° C. - 80° C., most preferably in the range of 50° C. - 70° C. 
     According to an embodiment the first gas can be ethane and the second gas can be natural gas. The same principles can be applied to ammonia carriers or LPC carriers. 
     The boil off gas management system comprises one of the preceding liquefaction and subcooling systems, wherein the liquefaction and subcooling system is arranged to re-liquefy and/or subcool boil off gas. 
     The method for liquefying different first and second gases having different saturation temperatures at a given pressure comprises providing a refrigeration device operable to alternatively provide a refrigerant fluid at a first and a second cold temperature that correspond to different liquefaction temperatures of the first and second gases, and, when the gas to be liquefied and/or subcooled is the first gas, operating the refrigeration device to provide the refrigerant fluid at the first cold temperature and guiding the gas to be liquefied and/or subcooled through a first subcooler for heat exchange with the refrigerant fluid, and, when the gas to be liquefied and/or subcooled is the second gas, operating the refrigeration device to provide the refrigerant fluid at the second cold temperature and guiding the gas to be liquefied and/or subcooled through a second subcooler for heat exchange with the refrigerant fluid. 
     It is noted that any features described above in connection with the liquefaction and subcooling system do also apply to the method according to the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by the subsequent description, in which reference is made to the accompanying drawings, wherein: 
         FIG.  1    is a schematic view illustrating the structure and operation of an embodiment of a liquefaction and subcooling system according to the invention; 
         FIG.  2    is a schematic view illustrating the structure and operation of another embodiment of a liquefaction and subcooling system according to the invention; 
         FIG.  3    is a schematic view illustrating the structure and operation of yet another embodiment of a liquefaction and subcooling system according to the invention; 
         FIG.  4    is a schematic view of a boil off gas management system according to an embodiment of the invention; and 
         FIG.  5    is a flow chart depicting an embodiment of the method for liquefaction according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG.  1    there is shown a schematic view of a liquefaction and subcooling system according to said first embodiment of the invention. The liquefaction and subcooling system comprises a refrigeration device  10  (which forms a refrigeration subsystem of the liquefaction and subcooling system) and a subcooling arrangement  50  (which forms a heat exchange subsystem of the liquefaction and subcooling system). The refrigeration device is operable to provide a refrigerant fluid at each of at least two different temperatures, i.e. at a first cold temperature and at a second cold temperature. Generally, the refrigeration device may be able to be operated such that the refrigerant fluid is provided at arbitrary temperatures within a temperature range that depends on the structure or type of the refrigeration device, i.e. on the ability of the refrigeration device to cool the refrigerant fluid to a certain cold temperature. The first cold temperature corresponds to the temperature used to liquefy a first gas (e.g. ethane) and the second cold temperature corresponds to the temperature used to liquefy a second gas (e.g. LNG). The temperatures that are used to liquefy the gases are also denoted as liquefaction temperatures herein. 
     In the refrigeration device a working circuit  12  of the refrigerant fluid is formed. The working circuit  12  comprises a compression section, an expansion section and a heat exchanger section. The compression section is formed by three compressors  14 ,  16 ,  18  which are driven by motors  20 ,  22 ,  24  over driving shafts, for example. More generally any number of compressors may be used, i.e. the compression section may generally comprise any number of compression stages. The compressors  14 ,  16 ,  18  may be of the centrifugal compression type. After each of the compressors  14 ,  16 ,  18  coolers  26 ,  28 ,  30  are included in the working circuit  12  for cooling the refrigerant fluid after each compression stage to ambient temperatures. The coolers  26 ,  28 ,  30  are supplied with a cooling fluid over conduits as indicated by arrows  32 ,  34 ,  36 . Air, water or, on a maritime vessel, sea water may be used as cooling fluid, for example. 
     The expansion section is formed by an expansion turbine  38 , which may be of the centripetal expansion type. More generally it is possible that the expansion section is formed by more than one expansion turbine. The expansion turbine  38  is driven by a motor  24 , which in the embodiment shown in  FIG.  1    is the same motor  24  that also drives a compressor  18 , for example. The expansion turbine  38  and the compressor  18  are both driven over a common driving shaft for efficiency reasons. The refrigerant fluid leaves the expansion turbine  38  at an outlet conduit  40  which is part of the working circuit  12 . At this point (directly after expansion in the expansion turbine  38 ) the refrigerant fluid has reached its lowest temperature, i.e. it has reached the cold temperature. In other words, the refrigerant fluid is provided in the outlet conduit  40  at its cold temperature by the refrigeration device. 
     The heat exchanger section of the refrigeration device  10  is formed by a heat exchanger  42 , in which heat is exchanged between the compressed refrigerant fluid downstream the compression section and upstream the expansion section and the expanded refrigerant fluid downstream of the expansion section and upstream the compression section. In this way the compressed refrigerant fluid that is cooled by cooler  30  to be essentially at ambient temperature is cooled down to a low (cryogenic) temperature by exchanging heat with the refrigerant fluid that downstream the expansion section (expansion turbine  38 ) is at its lowest temperature before being guided through the heat exchanger  42 . To facilitate understanding of the different embodiments, an oval line is drawn around the conduits between which heat is exchanged and a reference sign is attached to the line to indicate the corresponding heat exchanger. This is the case for the heat exchanger  42  of the refrigeration device as well as for the first and second subcoolers  70 ,  72  introduced below. 
     It should be noted that the specific structure of the refrigeration device  10  is not essential for implementing the invention. Key point is that the refrigeration device can be operated such that the refrigerant fluid may be provided at different cold temperatures. In the exemplary embodiment shown in  FIG.  1    this can be achieved by driving the compressors  14 ,  16 ,  18  and the expansion turbine  38  accordingly, i.e. at different speeds and power. 
     Additionally, a by-pass valve  44  may be provided that allows the refrigerant fluid to by-pass the heat exchanger  42  via a conduit. 
     In the subcooling arrangement  50  a flow of the gas to be liquefied and/or subcooled is provided at an inlet  52 , passed through the subcooling arrangement for cooling, and leaving the subcooling arrangement at an outlet  54  preferably in a fluid state. The subcooling arrangement  50  has two different flow paths for the gas to be liquefied and/or subcooled, a first flow path  56  and a second flow path  58 . The flow path taken by the gas to be liquefied and/or subcooled can be chosen by switching valves accordingly. Specifically, the gas to be liquefied and/or subcooled flows through the first flow path  56 , when valves  60 ,  62  are open and valves  64  and  66  are closed. The gas to be liquefied and/or subcooled flows through the second flow path  58 , when valves  64 ,  66  are open and valves  60 ,  62  are closed. 
     The gas to be liquefied and/or subcooled will typically be provided in compressed form and may at least partially be in a liquid state. In particular the BOG will normally be guided through a compression stage before being fed to the inlet  52  by a BOG management system. 
     In the first flow path  56  a first subcooler  70  is provided and in the second flow path  58  a second subcooler  72  is provided. The first subcooler  70  is configured to efficiently exchange heat between the first gas to be liquefied and/or subcooled and the refrigerant fluid being at the first cold temperature. The second subcooler  70  is configured to efficiently exchange heat between the second gas to be liquefied and/or subcooled and the refrigerant fluid being at the second cold temperature. The first and the second subcoolers  70 ,  72  are heat exchangers, preferably plate fin heat exchangers. 
     Specific to the embodiment shown in  FIG.  1    is that the heat exchanger  42  of the refrigeration device, the first subcooler  70 , and the second subcooler  72  are formed in a single multi-stream heat exchanger  100  having different conduits. The multi-stream heat exchanger  100  is provided downstream of the expansion section (expansion turbine  38 ) and is supplied by outlet conduit  40  with the refrigerant fluid being at the cold temperature. The multi-stream heat exchanger  100  has two conduits  102 ,  104 : one conduit  102  which guides the refrigerant fluid through the heat exchanger  42  of the refrigeration device, and another conduit  104  which guides the refrigerant fluid through the first and second subcooler  70 ,  72 . The first and second subcoolers  70 ,  72  are formed by different conduits in the multi-stream heat exchanger  100 . Valves  106 ,  108 ,  110 ,  112  are provided that control the flow of the refrigerant fluid through the conduits  102 ,  104  of the multi-stream heat exchanger  100 . 
     When the gas to be liquefied and/or subcooled is the first gas, the refrigerant fluid is provided at the first cold temperature and the gas is guided through the first flow path  56 , which includes the conduits in the multi-stream heat exchanger  100  that form the first subcooler  70 . When the gas to be liquefied and/or subcooled is the second gas, the refrigerant fluid is provided at the second cold temperature and the gas is guided through the second flow path  58 , which includes the conduits in the multi-stream heat exchanger  100  that form the second subcooler  72 . In both cases a valve  108  should be open, wherein this valve  108  controls the flow of the refrigerant fluid to the conduit  104  which is guided through the first and second subcooler  72 . 
     Additionally, a by-pass valve  68  may be provided that allows to directly connect the inlet  52  with the outlet  54 , which, for example, is helpful when performing maintenance of the multi-stream heat exchanger  100 . 
     Obviously, further structures or elements may be included in the working circuit  12  and/or the first and second flow paths  56 ,  58 , such as a surge valve, flow meters, pressure sensors and the like. 
       FIG.  2    shows a liquefaction and subcooling system according to another embodiment of the invention. In order to avoid repetitions, the description of structures common with the embodiment shown in  FIG.  1    is not repeated in the following and common elements are indicated with the same reference signs. Therefore, unless indicated otherwise, the description of  FIG.  1    applies for these elements, differences will be pointed out in the following. 
     The difference between the embodiments of  FIG.  1    and  FIG.  2    lies in the different arrangement of the heat exchangers, particularly of the subcooling arrangement  50 ; specifically of the heat exchanger  42  of the refrigeration device  10  and of the first and second subcoolers  70 ,  72  of the subcooling arrangement  50 . 
     The heat exchanger  42  of the refrigeration device is formed by a regular counter current heat exchanger  200 . The first and second subcoolers  70 ,  72  of the subcooling arrangement  50  are formed as a combined subcooler (combined heat exchanger)  202 . The combined subcooler  202  is basically a heat exchanger which combines heat exchangers that constitute the first and second subcoolers  70 ,  72 . The combined subcooler  202  is separate from the counter current heat exchanger  200 , i.e. the combined subcooler  202  is dedicated for subcooling. Several valves  204 ,  206 ,  208  are arranged in the working circuit  12  for controlling the flow of the refrigerant fluid to and from the combined subcooler  202  and the counter current heat exchanger  200 . 
     The combined subcooler  202  is provided in the working circuit  12  between the expansion and heat exchanger sections of the refrigeration device  10 , i.e. downstream of the expansion turbine  38  and upstream of the counter current heat exchanger  200 . The refrigerant fluid is guided from the outlet conduit  40  of the expansion section through valve  204  into a single inlet  210  for refrigerant fluid of the combined subcooler  202 . The first and second subcoolers  70 ,  72  are formed by means of two separate conduits in the combined subcooler  202 . These two separate conduits are arranged to exchange heat with the refrigerant fluid which is guided through the combined subcooler  202  by another conduit. The two separate conduits form part of the first and second flow paths  56 ,  58 , respectively. 
     When the gas to be liquefied and/or subcooled is the first gas, the refrigerant fluid is provided at the first cold temperature and the gas to be liquefied and/or subcooled is guided through the first flow path  56 , which includes the conduits in combined subcooler  202  that form the first subcooler  70 . When the gas to be liquefied and/or subcooled is the second gas, the refrigerant fluid is provided at the second cold temperature and the gas to be liquefied and/or subcooled is guided through the second flow path  58 , which includes the conduits in the combined subcooler  202  that form the second subcooler  72 . In both cases a valve  204  should be open, wherein this valve  204  controls the flow of the refrigerant fluid to the inlet  210  for refrigerant fluid of the combined subcooler  202 . 
       FIG.  3    shows a liquefaction and subcooling system according to yet another embodiment of the invention. As already mentioned in connection with  FIG.  2   , in order to avoid repetitions, the description of structures common with the embodiment shown in  FIG.  1    is not repeated in the following and common elements are indicated with the same reference signs. Therefore, unless indicated otherwise, the description of  FIG.  1    applies for these elements, differences will be pointed out in the following. 
     The difference between the embodiments of  FIG.  1    and  FIG.  3    lies in the different arrangement of the heat exchangers, particularly of the subcooling arrangement  50 ; specifically of the heat exchanger  42  of the refrigeration device  10  and of the first and second subcoolers  70 ,  72  of the subcooling arrangement  50 . 
     The heat exchanger  42  of the refrigeration device is formed by regular counter current heat exchanger  300 . The first and second subcoolers  70 ,  72  of the subcooling arrangement are formed as two separate subcoolers (heat exchangers) that are also separate from the counter current heat exchanger  300 . That is, the first subcooler  70  is formed by first heat exchanger  302  and the second subcooler  72  is formed by a second heat exchanger  304 , the second heat exchanger  304  is separate from the first heat exchanger  302 . Several valves  306 ,  308 ,  310 ,  312 ,  314  are arranged in the working circuit  12  for controlling the flow of the refrigerant fluid to and from first and second heat exchangers  302 ,  304  and the counter current heat exchanger  300 . 
     The first and second heat exchangers  302 ,  304  are provided in parallel in the working circuit  12  between the expansion and heat exchanger sections of the refrigeration device  10 , i.e. downstream the expansion turbine  38  and upstream the counter current heat exchanger  300 . A valve  306  controls the flow of refrigerant fluid to the first heat exchanger  302  and another valve  308  controls the flow of refrigerant fluid to the second heat exchanger  304 . 
     When the gas to be liquefied and/or subcooled is the first gas, the refrigerant fluid is provided at the first cold temperature and the gas to be liquefied and/or subcooled is guided through the first flow path  56 , which includes the first heat exchanger  302  that forms the first subcooler  70 . When the gas to be liquefied and/or subcooled is the second gas, the refrigerant fluid is provided at the second cold temperature and the gas to be liquefied and/or subcooled is guided through the second flow path  58 , which includes the second heat exchanger  304  that forms the second subcooler  72 . In the first case (first gas to be liquefied and/or subcooled), the valve  306  controlling the flow of refrigerant fluid to the first heat exchanger  302  is open while the valve  308  controlling the flow of refrigerant fluid to the second heat exchanger  304  is closed, and vice versa for the second case (second gas to be liquefied and/or subcooled). 
       FIG.  4    depicts an exemplary boil off gas (BOG) management system  400  for a tank  402  for a liquefied gas  404 . In the tank  402  BOG  408  occurs above a surface  406  of the liquefied gas  404  stored in the tank. The BOG is pumped over a conduit to the inlet  52  of a liquefaction and subcooling system  412  according to the invention, which may for example be any one of the liquefaction and subcooling systems shown in  FIGS.  1 - 3   . After liquefaction the (re-)liquefied and subcooled BOG is pumped back from the outlet  54  of the liquefaction and subcooling system  412  and reintroduced into the tank  402  in liquid form. Between the tank  402  and the inlet  52  of the liquefaction and subcooling system  412  there can be included a compressor  410  (as shown for example in  FIG.  4   ) in the conduit that compresses the BOG prior to being (re-)liquefied. 
       FIG.  5    depicts a flow chart of a method for liquefaction according to an embodiment. The method includes the step  510  of providing a refrigeration device operable to alternatively provide a refrigerant fluid at a first and a second cold temperature that correspond to different liquefaction temperatures of the first and second gases. As the refrigeration device is able to be operated at first and second cold temperatures, the cold temperature at which the refrigeration device is operated can be chosen depending on which gas is to be liquefied. This choice is made in step  520  of selecting whether the first or the second gas is to be liquefied. When the gas to be liquefied and/or subcooled is the first gas, the method continues with step  530  of operating the refrigeration device to provide the refrigerant fluid at the first cold temperature and step  540  of guiding the gas to be liquefied and/or subcooled through a first subcooler for heat exchange with the refrigerant fluid. On the other hand, when the gas to be liquefied and/or subcooled is the second gas, the method continues with step  550  of operating the refrigeration device to provide the refrigerant fluid at the second cold temperature and step  560  of guiding the gas to be liquefied and/or subcooled through a second subcooler for heat exchange with the refrigerant fluid. For embodiments of this method, reference is made to the embodiments of the system according to the invention, which accordingly apply to the method according to the invention. 
     While the invention has been described in terms of embodiments and examples in the preceding specification, the scope of the present invention is restricted by the appended claims not by specific embodiments of the specification. It should be noted that elements of the different embodiments may be combined even if not explicitly stated. 
     LIST OF REFERENCE SIGNS 
     
         
           10  refrigeration device 
           12  working circuit 
           14  compressor 
           16  compressor 
           18  compressor 
           20  motor 
           22  motor 
           24  motor 
           26  cooler 
           28  cooler 
           30  cooler 
           32  supply of cooling fluid 
           34  supply of cooling fluid 
           36  supply of cooling fluid 
           38  expansion turbine 
           40  outlet conduit 
           42  heat exchanger 
           44  by-pass valve 
           50  subcooling arrangement 
           52  inlet 
           54  outlet 
           56  first flow path 
           58  second flow path 
           60  valve 
           62  valve 
           64  valve 
           66  valve 
           68  by-pass valve 
           70  first subcooler 
           72  second subcooler 
           100  multi-stream heat exchanger 
           102  conduit 
           104  conduit 
           106  valve 
           108  valve 
           110  valve 
           112  valve 
           200  counter current heat exchanger of refrigeration device 
           202  combined subcooler of the subcooling arrangement 
           204  valve 
           206  valve 
           208  valve 
           210  inlet for refrigerant fluid 
           300  counter current heat exchanger of refrigeration device 
           302  first heat exchanger 
           304  second heat exchanger 
           306  valve 
           308  valve 
           310  valve 
           312  valve 
           314  valve 
           400  boil off gas management system 
           402  tank 
           404  liquefied gas 
           406  surface of liquefied gas 
           408  boil off gas 
           410  compressor 
           412  liquefaction and subcooling system