Abstract:
Provided is a re-liquefying method for a stored liquid which has a simple structure or operation and excellent process efficiency. Since the method does not use separate refrigerant, the structure or operation in the re-liquefying method is significantly simplified. In addition, since a portion of a main stream is separated to form a cycle similar to a refrigerant cycle which cools the mainstream, the process efficiency of the re-liquefying method is significantly improved. 
     The above Abstract is a more accurate literal translation of the abstract from the original priority application than the PCT abstract.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Korean Patent Application No. 10-2012-0121442 filed on Oct. 30, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a re-liquefying method for a stored liquid, and more particularly, to a re-liquefying method for a stored liquid which has a simple structure or operation and excellent process efficiency. 
       BACKGROUND ART 
       [0003]    Gases such as natural gas or carbon dioxide may be liquefied and stored in a storage tank in order to deliver the gases to a desired location, e.g., by a carrying vessel. During such delivery, a portion of a stored gas such as liquefied natural gas or liquefied carbon dioxide may be evaporated, e.g., by external heat to generate boil-off gas (BOG). BOG may be directly discharged to the outside. However, such direct discharge of BOG is economically or environmentally undesired. Thus, technologies of re-liquefying BOG to be re-introduced into a storage tank by using predetermined re-liquefying methods are being variously researched. 
         [0004]    However, re-liquefying devices for re-liquefying BOG are additional parts of storage tanks. Thus, simplicity in structure or operation is a main issue in re-liquefying methods while process efficiency is a main issue in typical liquefying methods. However, since recently researched re-liquefying methods use separate refrigerant, the structure or operation thereof is complicated. In addition, when the structure or operation of re-liquefying methods is simplified, the efficiency of the re-liquefying methods is decreased. 
       SUMMARY OF THE INVENTION 
       [0005]    Accordingly, the present invention aims at providing a re-liquefying method for a stored liquid which has a simple structure or operation and excellent process efficiency 
         [0006]    According to an aspect of the present invention, there is provided a re-liquefying method for a liquid liquefied from a vapor, in which a main stream evaporated from a storage tank storing the liquid is re-liquefied, the method including: a first introduction operation in which the main stream is introduced into a first heat exchange region; a first compression operation in which the main stream is compressed after the first introduction operation; a second introduction operation in which the main stream is introduced into a second heat exchange region after the first compression operation; a third introduction operation in which the main stream is re-introduced into the first heat exchange region after the second introduction operation; a first separation operation in which the main stream is separated into a first sub stream as a vapor and a second sub stream as a liquid after the third introduction operation; a fourth introduction operation in which the first sub stream is introduced into the first heat exchange region; a second separation operation in which the second sub stream is separated into a third sub stream and a fourth sub stream; a first cooling operation in which the main stream is cooled in the second heat exchange region by using the third sub stream; and a storage operation in which at least one portion of the fourth sub stream is stored in the storage tank. 
         [0007]    A re-liquefying method for a stored liquid according to the present invention does not use separate refrigerant. Thus, the structure or operation in the re-liquefying method is significantly simplified. In addition, since a portion of a main stream is separated to form a cycle similar to a refrigerant cycle which cools the mainstream, the process efficiency of the re-liquefying method is significantly improved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a flow diagram illustrating a re-liquefying method for a stored liquid according to a first embodiment of the present invention; 
           [0009]      FIG. 2  is a flow diagram illustrating a first modification of the re-liquefying method of  FIG. 1 ; 
           [0010]      FIG. 3  is a flow diagram illustrating a second modification of the re-liquefying method of  FIG. 1 ; 
           [0011]      FIG. 4  is a flow diagram illustrating a re-liquefying method for a stored liquid according to a second embodiment of the present invention; 
           [0012]      FIG. 5  is a flow diagram illustrating a first modification of the re-liquefying method of  FIG. 4 ; and 
           [0013]      FIG. 6  is a flow diagram illustrating a second modification of the re-liquefying method of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following embodiments. 
       Embodiment 1 
       [0015]      FIG. 1  is a flow diagram illustrating a re-liquefying method for a stored liquid according to a first embodiment of the present invention. A re-liquefying method according to the first embodiment is applied to a method of re-liquefying gas evaporated from a storage tank  210 . A low temperature stored liquid to which such a re-liquefying method is applied may be liquefied natural gas or liquefied carbon dioxide. However, the application of the re-liquefying method is not limited to liquefied natural gas or liquefied carbon dioxide. Hereinafter, the re-liquefying method will now be described in more detail with reference to  FIG. 1 . 
         [0016]    A main stream evaporated from the storage tank  210  is introduced through a conduit  111  into a first heat exchange region  161  in which heat exchange is performed (a first introduction operation). The first heat exchange region  161  may be disposed in a typical heat exchanger. A second heat exchange region, which will be described later, may also be disposed in a typical heat exchanger. The main stream introduced into the first heat exchange region  161  through the conduit  111  exchanges heat with other streams introduced into the first heat exchange region  161  through conduits  115  and  122 . 
         [0017]    After that, the main stream is introduced into a first compression member  171  through a conduit  112  and is compressed (a first compression operation). The first compression member  171  may be a typical compressor and a multi-stage compressor. Other compression members to be described later may also be a typical compressor and a multi-stage compressor. The main stream compressed as described above is introduced into a cooling member  182  through a conduit  113  and is cooled (a second cooling operation). The cooling member  182  may be a water-cooled cooler or an air-cooled cooler. A cooling member  183  to be described later may be a water-cooled cooler or an air-cooled cooler. The cooling member  182  may be removed. That is, the cooling member  182  may be used when cooling of the main stream is needed after the main stream is compressed by the first compression member  171 . 
         [0018]    After the main stream is cooled as described above, the main stream is introduced through a conduit  114  into a second heat exchange region  162  (a second introduction operation). The main stream is cooled in the second heat exchange region  162  by a third sub stream to be described later. To this end, the third sub stream forms a cooling loop to be described later. After the main stream is cooled as described above, the main stream is re-introduced through the conduit  115  into the first heat exchange region  161  (a third introduction operation). The main stream re-introduced into the first heat exchange region  161  exchanges heat with other streams in the first heat exchange region  161 . 
         [0019]    After that, the main stream is introduced into a first expansion member  191  through a conduit  116  and is expanded (a first expansion operation). Accordingly, the temperature of the main stream decreases. To this end, the first expansion member  191  may be constituted by a Joule-Thomson (J-T) valve. Other expansion members to be described later may also be constituted by a J-T valve. When a stream expands through a J-T valve, the pressure and temperature of the stream may be decreased by a J-T effect. 
         [0020]    After the main stream is expanded as described above, the main stream is introduced through a conduit  117  into a separation member  201  and is separated into a first sub stream as a vapor and a second sub stream as a liquid (a first separation operation). The separation member  201  may be a typical vapor-liquid separator. For reference, the first expansion member  191  before the separation member  201  may be removed. That is, the first expansion member  191  may be used when a temperature decrease of the main stream is needed for vapor-liquid separation. 
         [0021]    After the main stream is separated as described above, the first sub stream is introduced into a second expansion member  192  through a conduit  121  and is expanded (a second expansion operation). Accordingly, the temperature of the first sub stream decreases. Then, the first sub stream may cool other steams through heat exchange in the first heat exchange region  161 . To this end, after being expanded, the first sub stream is introduced into the first heat exchange region  161  through the conduit  122  (a fourth introduction operation). After that, the first sub stream is discharged to the outside through a conduit  123 . Accordingly, a portion of impurities may be discharged to the outside. For reference, the second expansion member  192  may be removed. 
         [0022]    The second sub stream is separated into the third sub stream and a fourth sub stream (a second separation operation). To this end, a conduit  126  is divided into two conduits (refer to a conduit  131 ). After the separation of the second sub stream, the fourth sub stream is recovered as a liquid into the storage tank  210  (a storage operation). 
         [0023]    Unlike this, the third sub stream forms a cooling loop to cool the main stream in the second heat exchange region  162  (a first cooling operation). In particular, the third sub stream is introduced into the second heat exchange region  162  through a conduit  141  (a fifth introduction operation). After that, the third sub stream is introduced into a second compression member  172  through a conduit  142  and is compressed (a second compression operation). After that, the third sub stream is introduced into the cooling member  183  through a conduit  143  and is cooled (a third cooling operation). 
         [0024]    After that, the third sub stream is introduced through a conduit  144  into a separation member  202  and is separated into a fifth sub stream as a vapor and a sixth sub stream as a liquid (a third separation operation). After that, the fifth sub stream is discharged to the outside through a conduit  145 . Accordingly, a portion of impurities may be discharged to the outside. Unlike this, the sixth sub stream is introduced into a third expansion member  193  through a conduit  146  and is expanded (a third compression operation). After that, the sixth sub stream is mixed with the third sub stream to be introduced into the second heat exchange region  162  through the conduit  141  (a first mixing operation). According to the first mixing operation, the sixth sub stream as a portion of the third sub stream flows with the third sub stream. Accordingly, the third sub stream may form a cooling loop for cooling the main stream. 
         [0025]    A re-liquefying device for re-liquefying the main stream evaporated from the storage tank  210  may be an additional part of the storage tank  210 . Thus, simplicity in structure or operation is a main issue in the re-liquefying method while process efficiency is a main issue in typical liquefying methods (for example, a method of liquefying natural gas). As a result, a use of refrigerant for re-liquefying a main stream as in typical liquefying methods is inappropriate for the re-liquefying method. This is because when refrigerant is used, members for compressing, condensing, and expanding the refrigerant are provided, which complicate structure or operation in the re-liquefying method. For reference, the complicated operation complicates control of the re-liquefying method. 
         [0026]    However, when refrigerant is not used, process efficiency is significantly decreased. Thus, the re-liquefying method needs a member for improving process efficiency without using refrigerant. To this end, the third sub stream forms a separate cooling loop. That is, although the re-liquefying method does not use separate refrigerant for forming a refrigerant cycle, the third sub stream cools the main stream in the second heat exchange region  162  by forming a cycle similar to a refrigerant cycle. 
         [0027]    Thus, since the re-liquefying method does not use separate refrigerant, the structure or operation in the re-liquefying method is significantly simplified. In addition, since a portion of the main stream is separated to form a cycle similar to a refrigerant cycle which cools the mainstream, the process efficiency of the re-liquefying method is significantly improved. For reference, each stream may be a vapor or a liquid in each of the locations thereof according to thermodynamic characteristics of the stream. 
         [0028]    The re-liquefying method illustrated in  FIG. 1  may be changed to a re-liquefying method illustrated in  FIG. 2 .  FIG. 2  is a flow diagram illustrating a first modification of the re-liquefying method of  FIG. 1 . In the re-liquefying method according to the first modification, the third sub stream is forcibly transferred by a pump  220  between the second separation operation and the first cooling operation. That is, referring to  FIG. 2 , the third sub stream is not naturally introduced into the cooling loop and is forcibly introduced thereinto by the pump  220 . In this case, a pressure of the cooling loop formed by the third sub stream is further increased, thereby increasing a re-liquefaction amount and decreasing consumed power. 
         [0029]    The re-liquefying method illustrated in  FIG. 1  may also be changed to a re-liquefying method illustrated in  FIG. 3 .  FIG. 3  is a flow diagram illustrating a second modification of the re-liquefying method of  FIG. 1 . In the re-liquefying method according to the second modification, a portion of the fourth sub stream is used for cooling the main stream, instead of just storing the fourth sub stream, thereby improving the process efficiency. In addition, a pump is not used in the re-liquefying method according to the second modification. In particular, the fourth sub stream is introduced into a fourth expansion member  194  through a conduit  1361  after the separation for the fourth sub stream and is expanded. After that, the fourth sub stream is introduced through a conduit  1362  into a separation member  203  and is separated into a seventh sub stream as a vapor and an eighth sub stream as a liquid. After that, the seventh sub stream is introduced into a fifth expansion member  195  through a conduit  1363  and is expanded. After that, when the seventh sub stream is mixed with the first sub stream to be introduced into the first heat exchange region  161  through the conduit  122 . After that, the seventh sub stream and the first sub stream cool the main stream in the first heat exchange region  161 . Finally, the eighth sub stream is recovered as a liquid into the storage tank  210 . 
         [0030]    The re-liquefying method according to the second modification may be an improved modification of the re-liquefying method according to the first modification. In particular, in the re-liquefying method according to the first modification as illustrated in  FIG. 2 , the third sub stream has the same pressure as that of the storage tank  210  before the third sub stream is forcibly transferred by the pump  220 . However, in the re-liquefying method according to the second modification as illustrated in  FIG. 3 , the third sub stream (refer to the conduit  131 ) has the same pressure as a pressure (in the conduit  1361 ) before the fourth expansion member  194 . The pressure before the fourth expansion member  194  is decreased to the pressure of the storage tank  210  by the fourth expansion member  194 . 
         [0031]    That is, the pressure of the third sub stream in the re-liquefying method according to the second modification is higher than the pressure of the third sub stream in the re-liquefying method according to the first modification. Thus, a separate pump is unnecessary in the re-liquefying method according to the second modification. Furthermore, since the re-liquefying method according to the second modification recovers cold energy through the seventh sub stream, the process efficiency thereof is higher than that of the re-liquefying method according to the first modification. Accordingly, a re-liquefaction amount in the re-liquefying method according to the second modification is greater than that in the re-liquefying method according to the first modification, and power consumed in the former is less than that in the latter. 
       Embodiment 2 
       [0032]      FIG. 4  is a flow diagram illustrating a re-liquefying method for a stored liquid according to a second embodiment of the present invention. Referring to  FIG. 4 , a re-liquefying method according to the second embodiment has a configuration that is similar to that of the re-liquefying method according to the first embodiment. However, the re-liquefying method according to the second embodiment is different from the re-liquefying method according to the first embodiment in a flow of the third sub stream after the separation for the third sub stream. For reference, parts, which are the same as (or correspond to) the previously-described parts, are denoted by the same (or corresponding) reference numerals, and a detailed description thereof will be omitted. 
         [0033]    Referring to  FIG. 4 , the third sub stream is not introduced into the second heat exchange region  162  and is introduced into the separation member  202  after the separation for the third sub stream in the re-liquefying method according to the second embodiment. In this case, the simplicity in operation of the re-liquefying method can be further improved. That is, the re-liquefying method can be more efficiently controlled. This is because an amount of a stream to be separated into the fifth sub stream and the sixth sub stream at the separation member  202  can be more efficiently determined. The amount of the stream to be separated may be determined through liquid level control at the separation member  202 . 
         [0034]    The re-liquefying method illustrated in  FIG. 4  may be changed to a re-liquefying method illustrated in  FIG. 5 .  FIG. 5  is a flow diagram illustrating a first modification of the re-liquefying method of  FIG. 4 . In the re-liquefying method according to the first modification, the third sub stream is forcibly transferred by a pump  2201  between the second separation operation and the first cooling operation. That is, referring to  FIG. 5 , the third sub stream is not naturally introduced into the cooling loop and is forcibly introduced thereinto by the pump  2201 . 
         [0035]    The re-liquefying method illustrated in  FIG. 4  may also be changed to a re-liquefying method illustrated in  FIG. 6 . FIG.  6  is a flow diagram illustrating a second modification of the re-liquefying method of  FIG. 4 . In the re-liquefying method according to the second modification, a portion of the fourth sub stream is used for cooling the main stream, instead of just storing the fourth sub stream, thereby improving the process efficiency. In addition, a pump is not used in the re-liquefying method according to the second modification. This is described in detail in the re-liquefying method illustrated in  FIG. 3 .