Patent Publication Number: US-11662140-B2

Title: Raw material gas liquefying device and method of controlling this raw material gas liquefying device

Description:
TECHNICAL FIELD 
     The present invention relates to a raw material gas liquefying device which liquefies a raw material gas to be liquefied at a cryogenic temperature, such as a hydrogen gas, and a method of controlling this raw material gas liquefying device. 
     BACKGROUND ART 
     For example, a raw material gas liquefying device which liquefies a raw material gas to be liquefied at a cryogenic temperature, such as a hydrogen gas, is conventionally known. Patent Literature 1 discloses this technique. 
     The raw material gas liquefying device disclosed in Patent Literature 1 has been conceived by the inventors of the present application, and is a prior art of the present application.  FIG.  9    shows a conventional raw material gas liquefying device  200  disclosed in Patent Literature 1. As shown in  FIG.  9   , the raw material gas liquefying device  200  disclosed in Patent Literature 1 includes a feed line  1  which flows therethrough a raw material gas (e.g., hydrogen gas), and a refrigerant circulation line  3  which flows therethrough a refrigerant (e.g., hydrogen gas) for cooling the raw material gas. The raw material gas liquefying device  200  includes heat exchangers  81  to  86  which exchange heat between the raw material gas in the feed line  1  and the refrigerant in the refrigerant circulation line  3 , and a cooler  88  which cools the raw material gas with a liquefied refrigerant stored (reserved) in a liquefied refrigerant storage tank  40 . 
     The feed line  1  passes through the heat exchangers  81  to  86 , the cooler  88 , and a feed system Joule-Thomson valve (hereinafter will be referred to as “feed system JT valve  16 ”) in this order. The raw material gas which has been compressed (whose pressure has been increased) by a compressor or the like (not shown) and has a high pressure is introduced into the feed line  1 . In the feed line  1 , the raw material gas is cooled by the heat exchangers  81  to  86  and the cooler  88  while flowing through them, and is liquefied by Joule-Thomson (isenthalpic) expansion at the feed system JT valve  16 . In this way, the liquefied raw material gas is produced. 
     The refrigerant circulation line  3  includes two circulation flow paths which partially overlap with each other, which are a refrigerant liquefaction route  41  and a cryogenic (cold) energy generation route  42 . The refrigerant liquefaction route  41  passes through a low-pressure-side compressor (hereinafter will be referred to as “low-pressure compressor  32 ”), a high-pressure-side compressor (hereinafter will be referred to as “high-pressure compressor  33 ”), the heat exchangers  81  to  86 , a circulation system Joule-Thomson valve (hereinafter will be referred to as “circulation system JT valve  36 ”, the liquefied refrigerant storage tank  40 , and the heat exchangers  86  to  81  in this order and returns to the low-pressure compressor  32 . In the refrigerant liquefaction route  41 , the refrigerant is compressed by the compressors  32 ,  33 , is cooled by the heat exchangers  81  to  86 , is liquefied by Joule-Thomson expansion at the circulation system JT valve  36 , and thereafter flows into the liquefied refrigerant storage tank  40 . The temperature of a boil-off gas of the liquefied refrigerant, which is generated in the liquefied refrigerant storage tank  40 , is raised while the boil-off gas is flowing through the heat exchangers  81  to  86 . Then, the boil-off gas returns to an entrance of the low-pressure compressor  32 . The cryogenic energy generation route  42  passes through the high-pressure compressor  33 , the heat exchangers  81 ,  82 , a high-pressure-side expansion unit (hereinafter will be referred to as “high-pressure expansion unit  37 ”), the heat exchanger  84 , a low-pressure-side expansion unit (hereinafter will be referred to as “low-pressure expansion unit  38 ”), and the heat exchangers  85  to  81 , in this order, and thereafter returns to the high-pressure compressor  33 . The refrigerant liquefaction route  41  and the cryogenic energy generation route  42  share the flow paths in a range from the high-pressure compressor  33  to the heat exchanger  82  at a second stage. A portion of the refrigerant, which exits the heat exchanger  82  at the second stage, flows to the cryogenic energy generation route  42 . In the cryogenic energy generation route  42 , the refrigerant flows through the expansion units  37 ,  38 , and is changed into a low-temperature gas. The temperature of this low-temperature gas is raised while the low-temperature gas is flowing through the heat exchangers  85  to  81 . Thereafter, the gas returns to an entrance of the high-pressure compressor  33 . 
     The process of the raw material gas liquefying device  200  is controlled by a controller  6 . The controller  6  obtains process data (e.g., flow rates, pressures, and temperatures of the raw material gas and the refrigerant, a liquid level in the liquefied refrigerant storage tank  40 , rotation speeds and the like of the compressors  32 ,  33  and the expansion units  37 ,  38 ) in the feed line  1  and the refrigerant circulation line), and controls opening rates (opening degrees) of a bypass valve  34  and the JT valves  16 ,  36 , based on the process data. 
     In the raw material gas liquefying device  200  disclosed in Patent Literature 1, the opening rate (opening degree) of the feed system JT valve  16  is adjusted so that the temperature of the refrigerant at an exit side of the low-pressure expansion unit  38  reaches a predetermined set value, to control the amount of raw material gas to be liquefied. In this way, the refrigerant temperature and the amount of raw material gas to be liquefied are controlled to keep a balance so that cryogenic energy insufficiency and excessive cooling for the raw material gas do not take place. In the raw material gas liquefying device  200  disclosed in Patent Literature 1, a bypass flow path  31   b  which bypasses the high-pressure compressor  33  is provided, and the bypass valve  34  is provided on the bypass flow path  31   b . The opening rate of the bypass valve  34  is adjusted so that the detected pressure of the refrigerant at an exit side of the high-pressure compressor  33  reaches a predetermined pressure. This makes it possible to control the amount of the refrigerant circulated through the refrigerant circulation line  3 . 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese-Laid Open Patent Application Publication No. 2016-176654 
     SUMMARY OF INVENTION 
     Technical Problem 
     In general, in the Joule-Thomson valve, a liquefaction yield changes depending on the entrance temperature or the entrance pressure (specifically, temperature and pressure at which isenthalpic expansion is initiated). As the entrance temperature is lower, the liquefaction yield is higher. In the raw material gas liquefying device  200  disclosed in Patent Literature 1, if the entrance pressure or the entrance temperature of the circulation system JT valve  36  changes, the liquefaction yield in the circulation system JT valve  36  changes. With the change of the liquefaction yield in the circulation system JT valve  36 , it is difficult to stabilize the liquid level in the liquefied refrigerant storage tank  40 , which causes a disorder of a cycle balance. Once the cycle balance is disordered, it is not easily restored. Patent Literature 1 does not specifically describe a control for the opening rate of the circulation system JT valve  36  and a control for the liquid level in the liquefied refrigerant storage tank  40 . 
     An object of the present invention is to realize stable production of a liquefied raw material gas by keeping a good cycle balance while stabilizing a liquid level in a liquefied refrigerant storage tank in a raw material gas liquefying device. 
     Solution to Problem 
     According to an aspect of the present invention, a raw material gas liquefying device comprises a feed line in which a raw material gas flows through a raw material flow path of a heat exchanger, a liquefied refrigerant storage tank which stores a liquefied refrigerant therein, and a feed system Joule-Thomson valve in this order; a refrigerant circulation line including a refrigerant liquefaction route and a cryogenic energy generation route, wherein in the refrigerant liquefaction route, a refrigerant flows through a compressor, a high-temperature-side refrigerant flow path of the heat exchanger, a circulation system Joule-Thomson valve, the liquefied refrigerant storage tank, and a first low-temperature-side refrigerant flow path of the heat exchanger in this order, and returns to the compressor, while in the cryogenic energy generation route, the refrigerant flows through the compressor, an expansion unit, and a second low-temperature-side refrigerant flow path of the heat exchanger in this order, and returns to the compressor; a temperature sensor which detects a temperature of the refrigerant at an exit side of the high-temperature-side refrigerant flow path of the heat exchanger or a temperature of the raw material gas at an exit side of the raw material flow path of the heat exchanger; a liquid level sensor which detects a refrigerant storage tank liquid level which is a liquid level in the liquefied refrigerant storage tank; and a controller which determines whether or not the refrigerant storage tank liquid level is within a predetermined allowable range, manipulates an opening rate of the feed system Joule-Thomson valve to control the temperature detected by the temperature sensor so that the temperature reaches a predetermined temperature set value in a case where the refrigerant storage tank liquid level is within the predetermined allowable range, and manipulates the opening rate of the feed system Joule-Thomson valve to control the refrigerant storage tank liquid level so that the refrigerant storage tank liquid level falls into the predetermined allowable range in a case where the refrigerant storage tank liquid level is outside the predetermined allowable range. 
     According to an aspect of the present invention, there is provided a method of controlling a raw material gas liquefying device including: a feed line in which a raw material gas flows through a raw material flow path of a heat exchanger, a liquefied refrigerant storage tank which stores a liquefied refrigerant therein, and a feed system Joule-Thomson valve in this order; and a refrigerant circulation line including a refrigerant liquefaction route and a cryogenic energy generation route, wherein in the refrigerant liquefaction route, a refrigerant flows through a compressor, a high-temperature-side refrigerant flow path of the heat exchanger, a circulation system Joule-Thomson valve, the liquefied refrigerant storage tank, and a first low-temperature-side refrigerant flow path of the heat exchanger in this order, and returns to the compressor, while in the cryogenic energy generation route, the refrigerant flows through the compressor, an expansion unit, and a second low-temperature-side refrigerant flow path of the heat exchanger in this order, and returns to the compressor, the method comprising: manipulating an opening rate of the feed system Joule-Thomson valve to control a refrigerant storage tank liquid level which is a liquid level in the liquefied refrigerant storage tank so that the refrigerant storage tank liquid level falls into a predetermined allowable range, in a case where the refrigerant storage tank liquid level is outside the predetermined allowable range; and manipulating the opening rate of the feed system Joule-Thomson valve to control a temperature of the refrigerant at an exit side of the high-temperature-side refrigerant flow path of the heat exchanger or a temperature of the raw material gas at an exit side of the raw material flow path of the heat exchanger so that the temperature reaches a predetermined temperature set value, in a case where the refrigerant storage tank liquid level is within the predetermined allowable range. 
     In accordance with the raw material gas liquefying device and the method of controlling the raw material gas liquefying device, described above, the refrigerant storage tank liquid level is controlled to fall into the predetermined allowable range in a case where the refrigerant storage tank liquid level is outside the predetermined allowable range. In brief, the refrigerant storage tank liquid level is preferentially controlled to fall into the predetermined allowable range in a case where the refrigerant storage tank liquid level is outside the predetermined allowable range. This allows the refrigerant storage tank liquid level to quickly fall into the predetermined allowable range irrespective of the initial position of the refrigerant storage tank liquid level. Thus, the refrigerant storage tank liquid level is easily stabilized. 
     Also, in accordance with the above-described raw material gas liquefying device and the method of controlling the raw material gas liquefying device, described above, in a case where the refrigerant storage tank liquid level is within the predetermined allowable range, the temperature of the refrigerant at the exit side of the heat exchanger or the temperature of the raw material gas at the exit side of the heat exchanger is controlled so that the temperature is held at the temperature set value, and the temperature of the refrigerant at the exit side of the heat exchanger is stabilized. This makes it possible to stabilize the temperature at the entrance of the circulation system Joule-Thomson valve, and stabilize the liquefaction yield in the circulation system Joule-Thomson valve. As a result, the refrigerant storage tank liquid level can be stabilized. In this way, to realize a good cycle balance, the cryogenic (cold) energy generated in the cryogenic energy generation route is distributed to the refrigerant liquefaction route and the feed line. Therefore, a good cycle balance can be kept while stabilizing the liquid level in the liquefied refrigerant storage tank, which leads to stable production of the liquefied raw material gas. 
     Advantageous Effects of Invention 
     In accordance with the present invention, in a raw material gas liquefying device, production of a liquefied raw material gas can be stabilized by keeping a good cycle balance while stabilizing a liquid level in a liquefied refrigerant storage tank. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a view showing the overall configuration of a raw material gas liquefying device according to one embodiment of the present invention. 
         FIG.  2    is a block diagram showing the configuration of a control system of the raw material gas liquefying device. 
         FIG.  3    is a view for explaining a flow of processing performed by a circulation system JT valve opening rate control section. 
         FIG.  4    is a view for explaining a flow of processing performed by a feed system JT valve opening rate control section. 
         FIG.  5    is a graph showing a relation between a load factor (load rate) set value and a set temperature of a refrigerant. 
         FIG.  6    is a graph showing a relation between a liquid level in a liquefied refrigerant storage tank and a set temperature compensation amount. 
         FIG.  7    is a view showing the overall configuration of a raw material gas liquefying device according to Modified Example 1. 
         FIG.  8    is a view showing the overall configuration of a raw material gas liquefying device according to Modified Example 2. 
         FIG.  9    is a view showing the overall configuration of a conventional raw material gas liquefying device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, the embodiment of the present invention will be described with reference to the drawings.  FIG.  1    is a view showing the overall configuration of a raw material gas liquefying device  100  according to one embodiment of the present invention.  FIG.  2    is a block diagram showing the configuration of a control system of the raw material gas liquefying device  100 . The raw material gas liquefying device  100  according to the present embodiment is configured to cool and liquefy a raw material gas supplied to the raw material gas liquefying device  100  to generate a liquefied raw material gas. In the present embodiment, a high-purity hydrogen gas is used as the raw material gas. As the liquefied raw material gas, liquid hydrogen is generated. However, the raw material gas is not limited to the hydrogen gas so long as the raw material gas is in a gaseous state at a room temperature and a normal pressure and its boiling temperature is lower than that (minus 196 degrees C.) of a nitrogen gas. As the raw material gas, for example, there are the hydrogen gas, a helium gas, and a neon gas. 
     As shown in  FIGS.  1  and  2   , the raw material gas liquefying device  100  includes a feed line  1  which flows the raw material gas therethrough, a refrigerant circulation line  3  which circulates a refrigerant therethrough, and a controller  6  which controls the operation of the raw material gas liquefying device  100 . The raw material gas liquefying device  100  includes heat exchangers  81  to  86  at multiple stages, which exchange beat between the raw material gas flowing through the feed line  1  and the refrigerant flowing through the refrigerant circulation line  3 , and coolers  73 ,  88 . 
     [Configuration of Feed Line  1 ] 
     The feed line  1  is a flow path which flows the raw material gas therethrough. The feed line  1  includes high-temperature-side flow paths (raw material flow paths) inside the heat exchangers  81  to  86 , flow paths inside the coolers  73 ,  88 , a feed system Joule-Thomson valve (hereinafter will be referred to as “feed system JT valve  16 ”), flow paths inside pipes connecting them to each other, and the like. The raw material gas with a room temperature and a normal pressure, which has been compressed (whose pressure has been increased) by a compressor (not shown) or the like, is fed to the feed line  1 . 
     The feed line  1  passes through the heat exchanger  81  at a first stage, the cooler  73  for preliminary cooling, the heat exchangers  82  to  86  at second to sixth stages, the cooler  88 , and the feed system JT valve  16  in this order. In the heat exchangers  81  to  86 , heat exchange between the raw material gas and the refrigerant takes place. In this way, the raw material gas is cooled. 
     The feed line  1  passes through the heat exchanger  81  at the first stage and then through the cooler  73 , before it enters the heat exchanger  82  at the second stage. The cooler  73  for preliminary cooling includes a liquid nitrogen storage tank  71  storing liquid nitrogen therein, and a nitrogen line  70  which externally feeds the liquid nitrogen to the liquid nitrogen storage tank  71 . The feed line  1  extends through the inside of the liquid nitrogen storage tank  71 . The cooler  73  for preliminary cooling cools the raw material gas to a temperature that is almost equal to that of the liquid nitrogen. 
     The feed line  1  passes through the heat exchanger  86  at the sixth stage and then through the cooler  88 , before it enters the feed system JT valve  16 . The cooler  88  includes a liquefied refrigerant storage tank  40  which stores therein a liquefied refrigerant generated by liquefying the refrigerant in the refrigerant circulation line  3 . The feed line  1  extends through the inside of the liquefied refrigerant storage tank  40 . The cooler  88  cools the raw material gas to a temperature that is approximately equal to a temperature (specifically, cryogenic temperature) of the liquefied refrigerant, with the liquefied refrigerant stored in the liquefied refrigerant storage tank  40 . 
     The raw material gas with the cryogenic temperature exits the cooler  88  and then flows into the feed system JT valve  16 . At the feed system JT valve  16 , the raw material gas with the cryogenic temperature is liquefied to liquid with a low temperature and a normal pressure, by Joule-Thomson expansion. The raw material gas (liquefied raw material gas) liquefied in this way is sent to a storage tank (not shown) and stored therein. The generation amount (liquefaction amount) of the liquefied raw material gas is adjusted according to the opening rate (opening degree) of the feed system JT valve  16 . 
     [Configuration of Refrigerant Circulation Line  3 ] 
     The refrigerant circulation line  3  is a closed flow path which circulates the refrigerant therethrough. The refrigerant circulation line  3  includes flow paths inside the heat exchangers  81  to  86 , flow path inside the cooler  73 , two compressors  32 ,  33 , two expansion units  37 ,  38 , a circulation system Joule-Thomson valve (hereinafter will be referred to as “circulation system JT valve  36 ”), the liquefied refrigerant storage tank  40 , flow paths inside pipes connecting them, and the like. 
     A filling line (not shown) for filling the refrigerant is connected to the refrigerant circulation line  3 . In the present embodiment, hydrogen is used as the refrigerant. However, the refrigerant is not limited to hydrogen and may be any substance which is in a gaseous state at a room temperature and a normal pressure, and whose boiling temperature is equal to or lower than that of the raw material gas. As the refrigerant, for example, there are hydrogen, helium, and neon. 
     The refrigerant circulation line  3  includes two circulation flow paths (closed loop) which are a refrigerant liquefaction route  41  and a cryogenic energy (cold energy) generation route  42  which partially share flow paths. 
     The refrigerant liquefaction route  41  passes through the low-pressure-side compressor (hereinafter will be referred to as “low-pressure compressor  32 ”), the high-pressure-side compressor (hereinafter will be referred to as “high-pressure compressor  33 ”), a high-temperature-side refrigerant flow path of the heat exchanger  81  at the first stage, the cooler  73  for preliminary cooling, high-temperature-side refrigerant flow paths of the heat exchangers  82  to  86  at the second to sixth stages, the circulation system JT valve  36 , the liquefied refrigerant storage tank  40 , and low-temperature-side refrigerant flow paths of the heat exchangers  86  to  81  at the sixth to first stages in this order, and then returns to the low-pressure compressor  32 . 
     A low-pressure flow path  31 L is connected to the entrance of the low-pressure compressor  32 . The exit of the low-pressure compressor  32  and the entrance of the high-pressure compressor  33  are connected to each other by a medium-pressure flow path  31 M. The refrigerant in the low-pressure flow path  31 L is compressed by the low-pressure compressor  32  and discharged to the medium-pressure flow path  31 M. The exit of the high-pressure compressor  33  and the entrance of the circulation system JT valve  36  are connected to each other via a high-pressure flow path  31 H. The refrigerant in the medium-pressure flow path  31 M is compressed by the high-pressure compressor  33  and discharged to the high-pressure flow path  31 H. 
     The low-pressure flow path  31 L and the medium-pressure flow path  31 M are connected to each other via a first bypass flow path  31   a  which does not pass through the low-pressure compressor  32 . The first bypass flow path  31   a  is provided with a first bypass valve  30 . The medium-pressure flow path  31 M and the high-pressure flow path  31 H are connected to each other via a second bypass flow path  31   b  which does not pass through the high-pressure compressor  33 . The second bypass flow path  31   b  is provided with a second bypass valve  34 . 
     The refrigerant in the high-pressure flow path  31 H flows through the high-temperature-side refrigerant flow path of the heat exchanger  81  at the first stage, the cooler  73  for preliminary cooling, and the high-temperature-side refrigerant flow paths of the heat exchangers  82  to  86  at the second to sixth stages, in this order, and is cooled. Then, the refrigerant flows into the circulation system JT valve  36 . The refrigerant is liquefied by Joule-Thomson expansion at the circulation system JT valve  36 . The liquefied refrigerant flows into the liquefied refrigerant storage tank  40 . The generation amount (liquefaction amount) of the liquefied refrigerant is adjusted according to the opening rate (opening degree) of the circulation system JT valve  36 . 
     In the liquefied refrigerant storage tank  40  which stores the liquefied refrigerant therein, a boil-offgas is generated. This boil-off gas flows into the low-pressure flow path  31 L connecting the exit of the liquefied refrigerant storage tank  40  to the entrance of the low-pressure compressor  32 . The low-pressure flow path  31 L passes through the heat exchangers  81  to  86  at the first to sixth stages in an order which is the reverse of the order in which the high-pressure flow path  31 H passes. Specifically, the low-pressure flow path  31 L passes through the heat exchanger  86  at the sixth stage to the heat exchanger  81  at the first stage in this order. The temperature of the refrigerant in the low-pressure flow path  31 L is increased while flowing through the low-temperature-side refrigerant flow paths of the heat exchangers  86  to  81 . Then, the refrigerant returns to the entrance of the low-pressure compressor  32 . 
     The cryogenic energy generation route  42  passes through the high-pressure compressor  33 , the high-temperature-side refrigerant flow paths of the heat exchangers  81 ,  82  at the first and second stages, the high-pressure-side expansion unit (hereinafter will be referred to as “high-pressure expansion unit  37 ”), the heat exchanger  84  at the fourth stage, the low-pressure-side expansion unit (hereinafter will be referred to as “low-pressure expansion unit  38 ”), and the heat exchangers  85  to  81  at the fifth to first stages in this order, and then returns to the high-pressure compressor  33 . 
     The refrigerant liquefaction route  41  and the cryogenic energy generation route  42  share the flow paths in a range from the high-pressure compressor  33  to the heat exchanger  82  at the second stage. A branch part  31   d  is provided at the high-pressure flow path  31 H at a location that is between the exit of the heat exchanger  82  at the second stage and the entrance of the heat exchanger  83  at the third stage. The upstream end of a cryogenic energy generation flow path  31 C is connected to the branch part  31   d . The downstream end of the cryogenic energy generation flow path  31 C is connected to the medium-pressure flow path  31 M. 
     In a range from the branch part  31   d  to the medium-pressure flow path  31 M, the cryogenic energy generation flow path  31 C passes through the high-pressure expansion unit  37 , the heat exchanger  84  at the fourth stage, the low-pressure expansion unit  38 , and the low-temperature-side refrigerant flow paths of the heat exchangers  85  to  81  at the fifth to first stages. A most part of the refrigerant which has passed through the heat exchanger  82  at the second stage in the high-pressure flow path  31 H flows to the cryogenic energy generation flow path  31 C by the operation of the high-pressure expansion unit  37 , and the remaining refrigerant flows to the heat exchanger  83  at the third stage. 
     The refrigerant which has flowed into the cryogenic energy generation flow path  31 C and has a temperature lower than that of the liquid nitrogen and a high pressure, is expanded by the high-pressure expansion unit  37  so that its pressure and temperature are reduced, flows through the heat exchanger  84  at the fourth stage, and is expanded by the low-pressure expansion unit  38  so that its pressure and temperature are further reduced. The refrigerant with a cryogenic temperature exits the low-pressure expansion unit  38 , and then flows through the heat exchanger  85  at the fifth stage to the heat exchanger  81  at the first stage in this order (in other words, cools the raw material gas and the refrigerant in the high-pressure flow path  31 H), and joins the refrigerant in the medium-pressure flow path  31 M. 
     Note that in the feed line  1  and the refrigerant circulation line  3 , a section including the heat exchangers  81  to  86  at the first to sixth stages, the cooler  73  for preliminary cooling, the cooler  88 , and the expansion units  37 ,  38  is constructed as a liquefier  20 . 
     [Configuration of Control System of Raw Material Gas Liquefying Device  100 ] 
     The feed line  1  and the refrigerant circulation line  3  are provided with sensors for detecting process data in the raw material gas liquefying device  100 . The refrigerant circulation line  3  is provided with a flow rate sensor  51  which detects a flow rate F 1  of the refrigerant flowing through the refrigerant circulation line  3 , at a location that is upstream of the heat exchanger  86  at the first stage in the high-pressure flow path  31 H and where the refrigerant liquefaction route  41  and the cryogenic energy generation route  42  share the flow paths. A flow rate sensor  52  which detects a flow rate F 2  of the refrigerant at the entrance of the high-pressure expansion unit  37  is provided at an upstream portion of the cryogenic energy generation flow path  31 C. In brief, the flow rate F 1  is a sum of the flow rate of the refrigerant flowing through the refrigerant liquefaction route  41  and the flow rate of the refrigerant flowing through the cryogenic energy generation route  42 , while the flow rate F 2  is the flow rate of the refrigerant flowing through the cryogenic energy generation route  42 . 
     In the high-pressure flow path  31 H, a temperature sensor  53  which detects a refrigerant temperature T at the exit side of the high-temperature-side refrigerant flow paths of the heat exchangers  81  to  86  is provided at the exit side of the high-pressure-side refrigerant flow paths of the heat exchangers  81  to  86 . It is sufficient that the temperature sensor  53  is provided in a flow path connecting the exit of the heat exchanger  86  at a final stage (sixth stage in the present embodiment) to the entrance of the circulation system JT valve  36 . The temperature sensor  53  may detect a refrigerant temperature at the entrance of the circulation system JT valve  36 , instead of the refrigerant temperature T at the exit side of the high-temperature-side refrigerant flow paths of the heat exchangers  81  to  86 . 
     The liquefied refrigerant storage tank  40  is provided with a liquid level sensor  54  which detects a liquid level (hereinafter will be referred to as “refrigerant storage tank liquid level L”) of the liquefied refrigerant stored (reserved) in the liquefied refrigerant storage tank  40 . The cryogenic energy generation flow path  31 C is provided with a pressure sensor  55  which detects a pressure P of the refrigerant at the entrance of the high-pressure expansion unit  37 . The flow rate sensor  51 , the flow rate sensor  52 , the temperature sensor  53 , the liquid level sensor  54 , and the pressure sensor  55  are connected via wires or wirelessly to the controller  6  so that these sensors can transmit detection values to the controller  6 . 
     The controller  6  controls the opening rates of the first bypass valve  30 , the second bypass valve  34 , the circulation system JT valve  36 , and the feed system JT valve  16 . The controller  6  includes a feed system JT valve opening rate control section  61  which controls the opening rate (opening degree) of the feed system JT valve  16 , a circulation system JT valve opening rate control section  62  which controls the opening rate of the circulation system JT valve  36 , and a bypass valve opening rate control section  63  which controls the opening rates of the first bypass valve  30  and the second bypass valve  34 . The controller  6  is a computer. The controller  6  is configured to execute pre-stored programs to operate as the feed system JT valve opening rate control section  61 , the circulation system JT valve opening rate control section  62 , and the bypass valve opening rate control section  63 . These functional blocks are configured to derive the opening rate of the valve based on the obtained process data, and output an opening rate command to this valve. 
     [Processing Performed by Bypass Valve Opening Rate Control Section  63 ] 
     In the raw material gas liquefying device  100  with the above-described configuration, when a pressure in the refrigerant circulation line  3  changes, an entrance pressure in the circulation system JT valve  36  changes. For this reason, a liquefaction yield in the circulation system JT valve  36  becomes unstable, and the liquid level in the liquefied refrigerant storage tank  40  may tend to become unstable. To avoid this, the bypass valve opening rate control section  63  controls the opening rates (opening degrees) of the first bypass valve  30  and the second bypass valve  34  based on the detection value of the pressure sensor (not shown) which measures the refrigerant pressure in the high-pressure flow path  31 H so that the refrigerant pressure in the high-pressure flow path  31 H reaches a predetermined pressure. 
     [Processing Performed by Circulation System JT Valve Opening Rate Control Section  62 ] 
     In the raw material gas liquefying device  100 , when a ratio of the refrigerant flowing to the cryogenic energy generation flow path  31 C which branches off from the high-pressure flow path  31 H, with respect to the refrigerant flowing through the high-pressure flow path  31 H, in the refrigerant circulation line  3  (or a flow rate ratio of the flow rate in the cryogenic energy generation route  42 , with respect to a sum of the flow rate in the refrigerant liquefaction route  41  and the flow rate in the cryogenic energy generation route  42  in the refrigerant circulation line  3 ) changes, the amount of cryogenic energy (cold energy) generated in the cryogenic energy generation route  42  changes. When the amount of cryogenic energy generated in the refrigerant liquefaction route  41  changes, the entrance temperature in the circulation system JT valve  36  changes. As a result, the liquefaction yield in the circulation system JT valve  36  becomes unstable, and the liquid level in the liquefied refrigerant storage tank  40  tends to become unstable. To avoid this, the circulation system JT valve opening rate control section  62  controls the opening rate of the circulation system JT valve  36  so that the amount of cryogenic energy generated in the cryogenic energy generation route  42  becomes constant. 
       FIG.  3    is a view for explaining a flow of the processing performed by the circulation system JT valve opening rate control section  62 . As shown in  FIG.  3   , the circulation system JT valve opening rate control section  62  of the controller  6  includes a divider  75 , a circulation system flow rate control unit  76  corresponding to the flow rate ratio, and a switch  77 . 
     The divider  75  obtains the flow rate F 1  of the refrigerant at the entrance of the heat exchanger  81  at the first stage in the high-pressure flow path  31 H, and the flow rate F 2  of the refrigerant at the entrance of the high-pressure expansion unit  37  in the cryogenic energy generation flow path  31 C, and calculates a ratio of the refrigerant flowing to the cryogenic energy generation route  42  with respect to the refrigerant flowing through the refrigerant circulation line  3 , based on the flow rate F and the flow rate F 2 . Specifically, the divider  75  calculates the flow rate ratio R in which the flow rate F 1  is a denominator and the flow rate F 2  is a numerator, and outputs the flow rate ratio R to the circulation system flow rate control unit  76 . The flow rate ratio R refers to a ratio of the refrigerant flowing to the cryogenic energy generation route  42  with respect to the refrigerant flowing through the refrigerant circulation line  3 . 
     The circulation system flow rate control unit  76  obtains a flow rate ratio set value R′ which is pre-stored and the flow rate ratio R, derives the opening rate (manipulation amount) of the circulation system JT valve  36  so that a deviation between the flow rate ratio R and the flow rate ratio set value R′ becomes zero, and outputs this opening rate. 
     The switch  77  switches an opening rate command for the circulation system JT valve  36  based on whether the load factor of the liquefier  20  is constant or changing. Note that the load factor may be regarded as constant when a changing magnitude of the load factor of the liquefier  20  is a predetermined threshold or less, and may be regarded as changing when the changing magnitude is more than the predetermined threshold. 
     The load factor [%] is proportional to the refrigerant pressure at the entrance of the high-pressure expansion unit  37 . For example, in a case where the entrance pressure in the high-pressure expansion unit  37  corresponding to the load factor of 50% is P50, the entrance pressure in the high-pressure expansion unit  37  corresponding to the load factor of 100% is P100, and the entrance pressure in the high-pressure expansion unit  37  which is detected by the pressure sensor  55  is P, the load factor x can be derived according to the following formula (equation):
 
 x =[( P− 2× P 50+ P 100)×50]/( P 100− P 50)
 
     In a case where the load factor is constant, a present (current) opening rate command for the circulation system JT valve  36  is output as the opening rate command for the circulation system JT valve  36 . In other words, in a case where the load factor in the liquefier  20  is constant, the opening rate of the circulation system JT valve  36  is fixed so that a pressure change does not occur in the refrigerant circulation line  3 . 
     On the other hand, in a case where the load factor is changing, the command output from the circulation system flow rate control unit  76  is output as the opening rate command for the circulation system JT valve  36 . For example, in a case where the flow rate ratio R is higher than the flow rate ratio set value R′, the amount of generation of cryogenic energy in the cryogenic energy generation route  42  is excessive and cooling is excessive. In light of this, in the above-described control, the flow rate in the refrigerant liquefaction route  41  is increased, namely, the opening rate of the circulation system JT valve  36  is increased so that the flow rate ratio R approaches (gets close to) the flow rate ratio set value R′. For example, in a case where the flow rate ratio R is lower than the flow rate ratio set value R′, the amount of generation of cryogenic energy in the cryogenic energy generation route  42  is insufficient and cooling is insufficient. In light of this, the flow rate in the refrigerant liquefaction route  41  is reduced, namely, the opening rate of the circulation system JT valve  36  is reduced so that the flow rate ratio R approaches (gets close to) the flow rate ratio set value R′. 
     In accordance with the above-described processing performed by the circulation system JT valve opening rate control section  62 , even in a case where the load factor changes, the ratio (flow rate ratio) of the refrigerant flowing to the cryogenic energy generation route  42  is held (kept) at the predetermined value. This makes it possible to stabilize the amount of generation of cryogenic energy (cold energy) in the refrigerant circulation line  3 . 
     [Processing Performed by Feed System JT Valve Opening Rate Control Section  61 ] 
       FIG.  4    is a view for explaining a flow of the processing performed by the feed system JT valve opening rate control section  61 . As shown in  FIG.  4   , the feed system JT valve opening rate control section  61  of the controller  6  includes a control method determination unit  90 , a set temperature calculator  91 , a set temperature compensation amount calculator  92 , an adder  93 , a liquefaction amount control unit  94  associated with the temperature, a liquefaction amount control unit  95  associated with the temperature, and a switch  96 . 
     The control method determination unit  90  determines whether to execute a liquid level control in which a priority is given to the refrigerant storage tank liquid level L or to execute a temperature control in which a priority is given to a cycle balance, as the control for the opening rate of the feed system JT valve  16 . As shown in  FIG.  6   , an allowable (permissible) range of the refrigerant storage tank liquid level L is set. The allowable range of the liquid level is set to a lower limit value L1 [m] or more and an upper limit value L4[m] or less. Note that the allowable range of the liquid level includes a proper range of the liquid level. The proper range of the liquid level is set to a lower limit value L2 [m] or more and an upper limit value L3[m] or less (L1&lt;L2&lt;L3&lt;L4). The lower limit value L2 [m] may be equal to the upper limit value L3[m], and thus the proper range of the liquid level may be uniquely defined. 
     The control method determination unit  90  determines whether or not the refrigerant storage tank liquid level L is outside the allowable range. In a case where the control method determination unit  90  determines that the refrigerant storage tank liquid level L is outside the allowable range (L&lt;L1, L4&lt;L), the control method determination unit  90  outputs a command (signal ON) directing the liquid level control. On the other hand, in a case where the control method determination unit  90  determines that the refrigerant storage tank liquid level L is within the allowable range (L1≤L≤L4), the control method determination unit  90  outputs a command (signal OFF) directing the temperature control. The command output from the control method determination unit  90  is input to the switch  96 . The switch  96  selects which of the liquefaction amount control unit  94  associated with the temperature and the liquefaction amount control unit  95  associated with the liquid level outputs the opening rate command to the feed system JT valve  16 . 
     (Liquid Level Control for Opening Rate of Feed System JT Valve  16 ) 
     Initially, the liquid level control performed for the opening rate of the feed system JT valve  16  will be described. In a case where the refrigerant storage tank liquid level L is outside the allowable range (L&lt;L1, L4&lt;L), the feed system JT valve opening rate control section  61  manipulates the opening rate of the feed system JT valve  16  to control the refrigerant storage tank liquid level L so that the refrigerant storage tank liquid level L quickly falls into the allowable range. 
     Specifically, the liquefaction amount control unit  95  associated with the liquid level obtains the refrigerant storage tank liquid level L and a liquid level set value L′, derives the opening rate (manipulation amount) so that a deviation between the refrigerant storage tank liquid level L and the liquid level set value L′ becomes zero, and outputs the opening rate command to the feed system JT valve  16 . The liquid level set value L′ is a value (L1≤L′≤L4) within the allowable range of the liquid level, preferably, a value (L2≤L′≤L3) within the proper range of the liquid level. 
     In accordance with the above-described control, in a case where the refrigerant storage tank liquid level L is lower than the lower limit value L1 [m] of the allowable range, the opening rate command for reducing the opening rate of the feed system JT valve  16  is output. In response to this, the flow rate (liquefaction amount) in the feed line  1  is reduced, and thus the cryogenic energy is provided to the refrigerant circulation line  3 . In this way, the liquefaction yield (cooling ability) of the refrigerant circulation line  3  can be increased, and the refrigerant storage tank liquid level L can fall into the allowable range. On the other hand, in a case where the refrigerant storage tank liquid level L is higher than the upper limit value L4 [m] of the allowable range, the opening rate command for increasing the opening rate of the feed system JT valve  16  is output. In response to this, the liquefaction yield (cooling ability) of the refrigerant circulation line  3  is reduced, and thus the cryogenic energy is provided to the feed line  1 . In this way, the flow rate (liquefaction amount) in the feed line  1  can be increased and the refrigerant storage tank liquid level L can fall into the allowable range. 
     (Temperature Control for Opening Rate of Feed System JT Valve  16 ) 
     Next, the temperature control performed for the opening rate of the feed system JT valve  16  will be described. In a case where the refrigerant storage tank liquid level L is within the allowable range, the feed system JT valve opening rate control section  61  manipulates the opening rate of the feed system JT valve  16  so that the cryogenic energy with a certain amount generated in the cryogenic energy generation route  42  is distributed to the feed line  1  and the refrigerant liquefaction route  41  of the refrigerant circulation line  3  so as to stabilize the cycle balance. The cryogenic energy provided to the feed line  1  is the cryogenic energy (namely, heat energy (calories) transferred from the raw material gas to the refrigerant in the low-temperature-side refrigerant flow paths) shifted to the raw material gas in the high-temperature-side raw material flow paths of the heat exchangers  81  to  86 . The cryogenic energy provided to the refrigerant liquefaction route  41  is the cryogenic energy (namely, heat energy (calories) transferred from the refrigerant in the high-temperature-side refrigerant flow paths to the refrigerant in the low-temperature-side refrigerant flow paths) shifted to the refrigerant in the high-temperature-side refrigerant flow paths of the heat exchangers  81  to  86 . There is a relation between the cryogenic energy provided to the feed line  1  and the cryogenic energy provided to the refrigerant liquefaction route  41 , in which when one of them reduces, the other increases. 
     Specifically, the set temperature calculator  91  obtains a specified load factor set value in the liquefier  20 , derives the set temperature of the refrigerant temperature T at the exit side of the heat exchangers  81  to  86  based on the load factor set value, and outputs the set temperature to the adder  93 . In the present embodiment, “the refrigerant temperature T at the exit side” is defined as the temperature at the exit side of the high-temperature-side refrigerant flow paths of the heat exchangers  81  to  86  which cool the raw material gas (and the refrigerant) by utilizing the cryogenic energy generated in the cryogenic energy generation route  42  of the refrigerant circulation line  3 . In the present embodiment, “the refrigerant temperature T at the exit side” is the temperature of the refrigerant having flowed through all of the high-temperature-side refrigerant flow paths of the heat exchangers  81  to  86  at six stages (the refrigerant temperature at the entrance of the circulation system JT valve  36 ). 
     A relation between the load factor and the set temperature (e.g., formula, map, or table) for uniquely calculating the set temperature from the load factor is pre-stored in the set temperature calculator  91 . The graph of  FIG.  5    represents the relation between the load factor and the set temperature of the refrigerant. In this graph, a vertical axis indicates the set temperature and a horizontal axis indicates the load factor. The set temperature of the refrigerant temperature at the exit side of the heat exchangers  81  to  86  is T2 [degrees C.] and constant in a range in which the load factor is from zero to D1[%], decreases from T2 [degrees C.] to T1 [degrees C.] in a linear function manner in a range in which the load factor is from D1[%] to 100[%], and is T1 [degrees C.] and constant in a range in which the load factor is more than 100[%] (T1&lt;T2). 
     While the opening rate of the feed system JT valve  16  is manipulated based on the refrigerant temperature T at the exit side of the heat exchangers  81  to  86 , the refrigerant storage tank liquid level L changes. In light of this, the above-described set temperature is compensated with the set temperature compensation amount associated with the refrigerant storage tank liquid level L to keep the refrigerant storage tank liquid level L within the allowable range. By associating the refrigerant storage tank liquid level L with the liquefaction amount in the feed system JT valve  16  in the control, a good cycle balance between the feed line  1  and the refrigerant circulation line  3  is kept. 
     Specifically, the set temperature compensation amount calculator  92  obtains the refrigerant storage tank liquid level L, derives the set temperature compensation amount based on the refrigerant storage tank liquid level L, and outputs the set temperature compensation amount to the adder  93 . A relation between the set temperature compensation amount and the refrigerant storage tank liquid level L (e.g., formula, map, or table) for uniquely calculating the set temperature compensation amount from the refrigerant storage tank liquid level L, is pre-stored in the set temperature compensation amount calculator  92 . The graph of  FIG.  6    represents the relation between the set temperature compensation amount and the refrigerant storage tank liquid level L. In this graph, a vertical axis indicates the set temperature compensation amount and a horizontal axis indicates the refrigerant storage tank liquid level L. The set temperature compensation amount is C1 [degrees C.] when the refrigerant storage tank liquid level L is L1 [m], increases from C1 [degrees C.] to 0 [degree C.] in a linear function manner in a range in which the refrigerant storage tank liquid level L is from L1 [m] to L2[m], is 0 [degree C.] in the proper range in which the refrigerant storage tank liquid level L is from L2[m] to L3[m], increases from 0 [degree C.] to C2 [degrees C.] in a linear function manner in a range in which the refrigerant storage tank liquid level L is from L3[m] to L4[m], and is C2 [degrees C.] when the refrigerant storage tank liquid level L is L4[m] (C1&lt;0&lt;C2). 
     The adder  93  outputs a sum of the set temperature and the set temperature compensation amount as a temperature set value T′ to the liquefaction amount control unit  94  associated with the temperature. In a case where the refrigerant storage tank liquid level L is within the proper range, the set temperature is the temperature set value T′. The liquefaction amount control unit  94  obtains the refrigerant temperature (the refrigerant temperature at the entrance of the circulation system JT valve  36 ) T at the exit side of the heat exchangers  81  to  86 , derives the opening rate (manipulation amount) of the feed system JT valve  16  so that a deviation between the refrigerant temperature T and the temperature set value T′ becomes zero, and outputs the opening rate command directing this opening rate to the feed system JT valve  16 . 
     In the above-described control, in a case where the refrigerant storage tank liquid level L is within the proper range (L2≤L≤L3), the set temperature compensation amount is zero, and the opening rate of the feed system JT valve  16  is decided so that the refrigerant temperature T at the exit side of the heat exchangers  81  to  86  reaches the set temperature corresponding to the load factor of the liquefier  20 . In a case where the refrigerant storage tank liquid level L exceeds the proper range (L3&lt;L≤L4), the opening rate command for increasing the opening rate of the feed system JT valve  16  is output. In response to this, the cooling ability (liquefaction yield) of the refrigerant circulation line  3  is reduced, the corresponding cryogenic energy is provided to the feed line  1  to increase the flow rate (liquefaction amount) in the feed line  1 , and the refrigerant storage tank liquid level L falls into the proper range. In a case where the refrigerant storage tank liquid level L is less than the proper range (L1≤L&lt;L2), the opening rate command for reducing the opening rate of the feed system JT valve  16  is output. In response to this, the flow rate (liquefaction amount) in the feed line  1  is reduced, the corresponding cryogenic energy is provided to the refrigerant circulation line  3 , and the refrigerant storage tank liquid level L falls into the proper range. 
     As described above, the raw material gas liquefying device  100  of the present embodiment includes the feed line  1 , the refrigerant circulation line  3 , and the controller  3 . In the feed line  1 , the raw material gas whose boiling temperature is lower than that of the nitrogen gas, flows through the raw material flow paths of the heat exchangers  81  to  86 , the liquefied refrigerant storage tank  40  which stores therein the liquefied refrigerant, and the feed system JT valve  16  in this order. The refrigerant circulation line  3  includes the circulation flow paths which are the refrigerant liquefaction route  41  and the cryogenic energy generation route  42  which partially share the flow paths. In the refrigerant liquefaction route  41 , the refrigerant flows through the compressors  32 ,  33 , the high-temperature-side refrigerant flow paths of the heat exchangers  81  to  86 , the circulation system JT valve  36 , the liquefied refrigerant storage tank  40 , and the first low-temperature-side refrigerant flow paths of the heat exchangers  86  to  81 , in this order, and returns to the compressor  32 . In the cryogenic energy generation route  42 , the refrigerant flows through the compressor  33 , the expansion units  37 ,  38 , and the second low-temperature-side refrigerant flow paths of the heat exchangers  85  to  81 , in this order, and returns to the compressor  33 . The raw material gas liquefying device  100  is provided with the temperature sensor  53  which directly or indirectly detects the refrigerant temperature T at the exit side of the high-temperature-side refrigerant flow paths of the heat exchangers  81  to  86 , and the liquid level sensor  54  which detects the liquid level (the refrigerant storage tank liquid level L) in the liquefied refrigerant storage tank  40 . 
     In the raw material gas liquefying device  100 , the controller  6  determines whether or not the refrigerant storage tank liquid level L is within the predetermined allowable range, manipulates the opening rate of the feed system JT valve  16  to control the temperature (the refrigerant temperature T at the exit side of the high-temperature-side refrigerant flow paths of the heat exchangers  81  to  86 ) detected by the temperature sensor  53  so that this temperature reaches the predetermined temperature set value, in a case where the refrigerant storage tank liquid level L is within the predetermined allowable range, and manipulates the opening rate of the feed system JT valve  16  to control the refrigerant storage tank liquid level L so that the level L falls into the predetermined allowable range, in a case where the refrigerant storage tank liquid level L is outside the predetermined allowable range. 
     In the method of controlling the raw material gas liquefying device  100  of the present embodiment, the opening rate of the feed system JT valve  16  is manipulated so that the refrigerant storage tank liquid level L which is the liquid level in the liquefied refrigerant storage tank  40  falls into the predetermined allowable range, in a case where the refrigerant storage tank liquid level L is outside the predetermined allowable range, and the opening rate of the feed system JT valve  16  is manipulated so that the refrigerant temperature T at the exit side of the high-temperature-side refrigerant flow paths of the heat exchangers  81  to  86  reaches the predetermined temperature set value, in a case where the refrigerant storage tank liquid level L is within the predetermined allowable range. 
     In accordance with the raw material gas liquefying device  100  and the control method thereof (therefor), described above, in a case where the refrigerant storage tank liquid level L is outside the predetermined allowable range, the refrigerant storage tank liquid level L is preferentially caused to fall into the predetermined allowable range. This allows the refrigerant storage tank liquid level L to quickly fall into the predetermined allowable range irrespective of the initial position of the refrigerant storage tank liquid level L. Thus, the refrigerant storage tank liquid level L is easily stabilized. On the other hand, in a case where the refrigerant storage tank liquid level L is within the predetermined allowable range, the opening rate of the feed system JT valve  16  is manipulated so that the refrigerant temperature T at the exit side of the heat exchangers  81  to  86  reaches the predetermined temperature set value. The predetermined temperature set value is set to a value at which a cycle balance between the feed line  1  and the refrigerant circulation line  3  is stabilized. Therefore, in accordance with the above-described control, the cryogenic energy generated in the refrigerant circulation line  3  can be distributed to the feed line  1  and the refrigerant circulation line  3  so that the cycle balance is stabilized. Since the temperature of the refrigerant flowing into the circulation system JT valve  36  is stabilized, the liquefaction amount in the feed system JT valve  16  is stabilized, and hence the refrigerant storage tank liquid level L is easily stabilized. Since the cycle balance between the feed line  1  and the refrigerant circulation line  3  can be kept while stabilizing the refrigerant storage tank liquid level L, the liquefied raw material gas can be stably produced. 
     In accordance with the raw material gas liquefying device  100  and the control method thereof (therefor), according to the above-described embodiment, the temperature set value is associated with the load factor so that the temperature set value decreases as the load factor increases, and the temperature set value derived based on the set value of the load factor is used. 
     Thus, the temperature set value derived based on the set value of the load factor, with which a good cycle balance can be obtained, is used in the control. 
     In accordance with the raw material gas liquefying device  100  and the control method thereof (therefor), according to the above-described embodiment, the set temperature compensation amount is associated with the refrigerant storage tank liquid level L so that the set temperature compensation amount is zero in a case where the refrigerant storage tank liquid level L is within the predetermined proper range included in the predetermined allowable range, is a negative value in a case where the refrigerant storage tank liquid level L is lower than the predetermined proper range, and is a positive value in a case where the refrigerant storage tank liquid level L exceeds the predetermined proper range, and the temperature set value is compensated with the set temperature compensation amount derived based on the refrigerant storage tank liquid level L. 
     Thus, by use of the set temperature compensation amount, the temperature set value is compensated to be increased in a case where the refrigerant storage tank liquid level L is higher than the predetermined proper range (namely, the cryogenic energy in the refrigerant circulation line  3  is excessive), and is compensated to be reduced in a case where the refrigerant storage tank liquid level L is lower than the predetermined proper range (namely, the cryogenic energy in the refrigerant circulation line  3  is insufficient). Therefore, the refrigerant storage tank liquid level L can be kept within the predetermined allowable range while controlling the refrigerant temperature T at the exit side of the heat exchangers  81  to  86  so that the refrigerant temperature T reaches the temperature set value. 
     In accordance with the raw material gas liquefying device  100  and the control method thereof (therefor), according to the above-described embodiment, the opening rate of the circulation system JT valve  36  is fixed (made constant), in a case where the load factor changes within the predetermined range, and is manipulated to control the flow rate of the refrigerant flowing to the cryogenic energy generation route  42  so that the ratio of the refrigerant flowing to the cryogenic energy generation route  42  with respect to the refrigerant flowing through the refrigerant circulation line  3  reaches the predetermined value, in a case where the load factor changes in a range outside the predetermined range. The raw material gas liquefying device  100  is provided with the flow rate sensors  51 ,  52  to detect the ratio of the refrigerant flowing to the cryogenic energy generation route  42  with respect to the refrigerant flowing through the refrigerant circulation line  3 . 
     As described above, in a case where the load factor changes, the opening rate (liquefaction amount) of the circulation system JT valve  36  is manipulated so that the ratio of the refrigerant flowing to the cryogenic energy generation route  42  is kept at the predetermined value. This makes it possible to stabilize the amount of the cryogenic energy generated in the cryogenic energy generation route  42  even in a case where the load factor changes. 
     The load factor and the pressure of the refrigerant flowing into the expansion unit  37  are associated with each other so that the load factor and the pressure are proportional to each other. The load factor derived based on the pressure of the refrigerant flowing into the expansion unit  37  is used in the control. To derive the load factor, the raw material gas liquefying device  100  is provided with a pressure sensor  55  which detects the pressure of the refrigerant flowing into the expansion unit  37 . 
     Thus far, the preferred embodiment of the present invention has been described. The specific structures and/or the details of the function of the above-described embodiment may be changed within the scope of the invention. For example, the configuration of the raw material gas liquefying device  100  can be changed as follows. 
     In the above-described embodiment, the balance between the amount of cryogenic energy provided to the feed line  1  and the amount of cryogenic energy provided to the refrigerant liquefaction route  41  is adjusted by use of the refrigerant temperature T at the exit side of the high-temperature-side refrigerant flow paths of the heat exchangers  81  to  86 . The temperature sensor  53  is provided on the flow path at the exit side of the heat exchangers  81  to  86  which cool the raw material gas by utilizing the cryogenic energy generated in the refrigerant liquefaction route  41  of the refrigerant circulation line  3 , to be precise, the flow path at the exit side of the heat exchanger  86  at the final stage (sixth stage). Alternatively, the balance between the amount of cryogenic energy provided to the feed line  1  and the amount of cryogenic energy provided to the refrigerant liquefaction route  41  may be adjusted by use of the refrigerant temperature at the exit side or entrance side of any one of the high-temperature-side refrigerant flow paths of the heat exchangers  83  to  85  other than the heat exchanger  86  at the final stage (sixth stage) so long as the refrigerant temperature is at a location that is downstream of the branch part  31   d  of the high-pressure flow path  31 . 
     For example, in a raw material gas liquefying device  100 A according to Modified Example 1 shown in  FIG.  7   , a temperature sensor  53 A is provided between the heat exchanger  85  at the fifth stage and the heat exchanger at the sixth stage in the refrigerant liquefaction route  41  of the refrigerant circulation line  3 . This temperature sensor  53 A detects the refrigerant temperature at the exit side of the high-temperature-side refrigerant flow path of the heat exchanger  85  at the fifth stage (or the refrigerant temperature at the entrance side of the high-temperature-side refrigerant flow path of the heat exchanger  86  at the sixth stage). The controller  6  of the raw material gas liquefying device  100 A manipulates the opening rate of the feed system JT valve  16  based on the detection value of the temperature sensor  53 A and the temperature set value corresponding to this detection value to control the refrigerant temperature at the exit side of the high-temperature-side refrigerant flow path of the heat exchanger  85  at the fifth stage, as in the above-described embodiment. 
     In the raw material gas liquefying device  100  according to the above-described embodiment, the balance between the amount of cryogenic energy provided to the feed line  1  and the amount of cryogenic energy provided to the refrigerant liquefaction route  41  is adjusted by use of the temperature (the refrigerant temperature T at the exit side of the high-temperature-side refrigerant flow paths of the heat exchangers  81  to  86 ) of the refrigerant flowing through the refrigerant liquefaction route  41 . The cryogenic energy with a certain amount generated in the cryogenic energy generation route  42  is distributed to the feed line  1  and the refrigerant liquefaction route  41 . Therefore, the balance between the amount of cryogenic energy provided to the feed line  1  and the amount of cryogenic energy provided to the refrigerant liquefaction route  41  may be adjusted by use of the temperature of the raw material gas flowing through the feed line  1 . 
     For example, in a raw material gas liquefying device  100 B according to Modified Example 2 shown in  FIG.  8   , a temperature sensor  53 B is provided on the feed line  1 , to detect the temperature of the raw material gas at the exit side of the raw material flow paths of the heat exchangers  81  to  86 . Specifically, in the feed line  1 , the temperature sensor  53 B which detects the temperature of the raw material gas is provided at a location that is between the heat exchanger  86  at the final stage (sixth stage) and the cooler  88 . The controller  6  of the raw material gas liquefying device  100 B manipulates the opening rate of the feed system JT valve  16  to control the temperature of the raw material gas which is detected by the temperature sensor  53 B so that this temperature reaches a predetermined temperature set value, by use of the detection value of the temperature sensor  53 B and the temperature set value corresponding to this detection value, as in the above-described embodiment. 
     In the raw material gas liquefying device  100  according to the above-described embodiment, the ratio of the refrigerant flowing to the cryogenic energy generation route  42  with respect to the refrigerant flowing through the refrigerant circulation line  3 , is detected by use of the flow rate sensor  51  provided at the entrance of the heat exchanger  81  at the first stage in the high-pressure flow path  31 H of the refrigerant circulation line  3  and the flow rate sensor  52  provided at the entrance of the high-pressure expansion unit  37  of the cryogenic energy generation flow path  31 C. Alternatively, the ratio of the refrigerant flowing to the cryogenic energy generation route  42  with respect to the refrigerant flowing through the refrigerant circulation line  3 , may be detected by use of a flow rate sensor provided at another location. 
     For example, in a raw material gas liquefying device  100 B according to Modified Example 2 shown in  FIG.  8   , the flow rate sensor  51  is provided at the entrance of the heat exchanger  81  at the first stage in the high-pressure flow path  31 H, and a flow rate sensor  52 B is provided at a location that is downstream of the branch part  31   d  of the high-pressure flow path  31 H. In this case, the controller  6  can derive the ratio of the refrigerant flowing to the cryogenic energy generation route  42  with respect to the refrigerant flowing through the refrigerant circulation line  3 , based on the detection values of the flow rate sensors  51 ,  52 B. Alternatively, a flow rate sensor may be provided at the entrance of the high-pressure expansion unit  37  of the cryogenic energy generation flow path  31 C, a flow rate sensor may be provided at a location that is downstream of the branch part  31   d  of the high-pressure flow path  31 H, and the ratio of the refrigerant flowing to the cryogenic energy generation route  42  with respect to the refrigerant flowing through the refrigerant circulation line  3 , may be derived based on the detection values of these sensors. 
     In the raw material gas liquefying device  100  according to the above-described embodiment, two compressors  32 ,  33 , and two expansion units  37 ,  38  are provided. The number of them depends on performance of the compressors  32 ,  33 , and the expansion units  37 ,  38  and is not limited to two of the above-described embodiment. Further, although the raw material gas liquefying device  100  according to the above-described embodiment includes the heat exchangers  81  to  86  at six stages, the number of the heat exchangers  81  to  86  is not limited to this. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  feed line 
               3  refrigerant circulation line 
               6  controller 
               16  feed system JT valve 
               20  liquefier 
               30 ,  34  bypass valve 
               31 C cryogenic energy generation flow path 
               31 H high-pressure flow path 
               31 L low-pressure flow path 
               31 M medium-pressure flow path 
               31   a ,  31   b  first bypass flow path 
               31   d  branch part 
               32 ,  33  compressor 
               36  circulation system JT valve 
               37 ,  38  expansion unit 
               40  liquefied refrigerant storage tank 
               41  refrigerant liquefaction route 
               42  cryogenic energy generation route 
               51 ,  52  flow rate sensor 
               53  temperature sensor 
               54  liquid level sensor 
               55  pressure sensor 
               61  feed system JT valve opening rate control section 
               62  circulation system JT valve opening rate control section 
               63  bypass valve opening rate control section 
               70  nitrogen line 
               73  cooler for preliminary cooling 
               75  divider 
               76  circulation system flow rate control unit 
               77  switch 
               81  to  86  heat exchanger 
               88  cooler 
               90  control method determination unit 
               91  set temperature calculator 
               92  set temperature compensation amount calculator 
               93  adder 
               94  liquefaction amount control unit 
               95  liquefaction amount control unit 
               96  switch