Patent Publication Number: US-2019195536-A1

Title: Fluid cooling apparatus

Description:
TECHNICAL FIELD 
     The present invention disclosed herein relates to a fluid cooling apparatus, and more particularly, to a fluid cooling apparatus that is capable of improving liquefaction efficiency of a gas with low energy by appropriately cooling the gas in various temperature ranges through a simple process. 
     BACKGROUND ART 
     An aqueous mixture extracted from an oil well is separated into water, hydrocarbon-based liquid, and gaseous components in a separator. The gas components separated in the separator forms a natural gas (NG) from which impurities are removed through a pretreatment process of a liquefaction system. The natural gas is supplied to a natural gas liquefaction system and then becomes a liquefied natural gas after a series of processes. Since the natural gas liquefaction system performs liquefaction of the natural gas at a cryogenic temperature, if the natural gas containing heavy hydrocarbon is introduced into the liquefaction system as it is, the natural gas may be frozen to cause failure of the device and also deteriorate liquefaction efficiency of the natural gas. This may be solved before the liquefaction process by a distillation column for removing a low-temperature heavy hydrocarbon. 
     Also, the natural gas is called at a low temperature to produce the liquefied natural gas. Thus, many developments have been made in the liquefaction process cycle used for the above-described process. For example, a double expander cycle has been developed. However, the double expander cycle increases only cooling efficiency of a fluid by using a plurality of compressors and expanders. Thus, in the double expander cycle, an arrangement relationship of the plurality of compressors is complicated, and operation efficiency is not high. 
     SUMMARY 
     Technical Problem 
     The present invention provides a fluid cooling apparatus in which an arrangement relationship between a plurality of compressor and other devices is simplified, refrigerants having the same pressure are discharged from the plurality of compressors, and the discharged refrigerants are mixed in a single flow and then cooled to be used for liquefying a gas, thereby reducing energy consumed in liquefying the gas. 
     The object of the present invention is not limited to the aforesaid, but other objects not described herein will be clearly understood by those skilled in the art from descriptions below. 
     Technical Solution 
     Embodiments of the present invention provide a fluid cooling apparatus including: an expansion unit including a plurality of expanders, which receive refrigerants through a plurality of paths to expand the refrigerants and discharge the expanded refrigerants having different temperatures; a heat exchanger receiving the refrigerants having different temperatures from the expansion unit to cool the fluid in multistages; a precompression unit including a plurality of precompressors, which receive the refrigerants passing through the heat exchanger to compress the refrigerants and discharge the compressed refrigerants at the same pressure; a mixing tube configured to mix the refrigerants discharged from the precompression unit to supply the mixed refrigerant; and a main compression unit connected to the mixing tube to compress the mixed refrigerant and supply the compressed refrigerant to the expansion unit. 
     The expanders of the expansion unit and the precompressors of the precompression unit may operate to be interlocked with each other. 
     The plurality of expanders may include a first expander, a second expander, and a third expander, which expand the refrigerants having different temperatures, and the plurality of compressors may include a first precompressor coaxially connected to the first expander to compress the refrigerant discharged from the first expander, a second precompressor coaxially connected to the second expander to compress the refrigerant discharged from the second expander, and a third precompressor coaxially connected to the third expander to compress the refrigerant discharged from the third expander. 
     The main compression unit may include a plurality of compressors that are connected in series to each other, and the refrigerants supplied to the mixing tube may be compressed by sequentially passing through the plurality of compressors. 
     The fluid cooling apparatus may further include a cooler connected to the mixing tube between the precompression unit and the main compression unit to cool the mixed refrigerant. 
     Advantageous Effects 
     In the fluid cooling apparatus according to the present invention, the arrangement relationship between the plurality of compressors and other devices may be simplified to improve the operation efficiency of the compressors. Also, the refrigerants having the same pressure may be discharged from the plurality of compressors and then mixed with each other and introduced into the compressors while lowering the temperature of the mixed refrigerant to improve the operation efficiency of the compressors. 
     Also, the fluid may be cooled at the various temperature ranges by using the compressed refrigerant so that the fluid is efficiently cooled. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic conceptual view of a fluid cooling apparatus according to an embodiment of the present invention. 
         FIGS. 2 and 3  are operation diagrams for explaining an operation of the fluid cooling apparatus. 
         FIG. 4  is a graph illustrating a relationship between a temperature and energy while a fluid is liquefied by using a refrigerant in the fluid cooling apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     Advantages and features of the present invention, and implementation methods thereof will be clarified through following embodiments described with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. Further, the present invention is only defined by scopes of claims. Like reference numerals refer to like elements throughout. 
     Hereinafter, a fluid cooling apparatus according to an embodiment of the present invention will be described in detail with reference to  FIG. 1 . 
       FIG. 1  is a schematic conceptual view of a fluid cooling apparatus according to an embodiment of the present invention. 
     A fluid cooling apparatus  1  according to an embodiment of the present invention cools a fluid having a wide temperature range in a three-stage heat exchange loop to improve liquefaction efficiency of the fluid. More specifically, the fluid cooling apparatus  1  is configured so that refrigerants discharged at different temperatures and pressures in the respective stages are discharged as a refrigerant having the same pressure through a precompressors  31  to  33 . Also, the discharged refrigerants are mixed with each other in a mixing tube  40 , cooled again in the cooler  60 , and compressed in a main compression unit  50 . Furthermore, the compressed refrigerant discharged from the main compression unit  50  may be circulated in a heat exchanger  20  to cool the refrigerant in several stages. Particularly, in the fluid cooling apparatus  1 , the fluid may be heat-exchanged at a temperature of about −155° C. to about 40° C., and a liquefaction process, which is involved in a process between precooling and subcooling of the fluid, may be more improved in efficiency of liquefaction. 
     The fluid cooling apparatus  1  may include an expansion unit  10  discharging refrigerants having different temperatures, a heat exchanger  20  connected to one side of the expansion unit  10 , a precompression unit  30  receiving the refrigerants discharged from the heat exchanger  20  to discharge refrigerants having the same pressure, a mixing tube  40  mixing the refrigerants discharged from the precompression unit  30 , and a main compression unit  50  disposed between the mixing tube  40  and the heat exchanger  20 . Furthermore, the fluid cooling apparatus  1  may further include a cooler  60  connected to the mixing tube  40  between the precompression unit  30  and the main compression unit  50 . 
     Hereinafter, components constituting the fluid cooling apparatus  1  will be described in detail. 
     The expansion unit  10  receives the refrigerants having different amounts through a plurality of paths through the heat exchanger  20  to expand the refrigerants having different temperatures and thereby to supply the refrigerants again to the heat exchanger  20 . The expansion unit  10  may include a plurality of expanders, which receive refrigerants having different amounts to discharge refrigerants having different temperatures, i.e., a first expander  11 , a second expander  12 , and a third expander  13 . Here, the expanders  11  to  13  may receive various amounts of refrigerants at different ratios. For example, the first expander  11 , the second expander  12 , and the third expander  13  may receive the refrigerant of about 30% to about 40%, the refrigerant of about 30% to about 45%, and the refrigerant of about 20% to about 30% of the total amount of refrigerant, respectively. Each of the expanders  11  to  13  may adjust a temperature interval between the refrigerant and the fluid to correspond to an amount of supplied refrigerant, thereby controlling a process of liquefying the fluid. For example, the third expander  13  may adjust a temperature interval between the refrigerant and the fluid in a cold region corresponding to a temperature of about −160° C. to about −90° C. by using the amount of supplied refrigerant to control the subcooling, and the second expander  12  may adjust a temperature interval between the refrigerant and the fluid in an intermediate region corresponding to a temperature of about −120° C. to about −80° C. by using the amount of supplied refrigerant to control the liquefaction. Also, the first expander  11  may adjust a temperature interval between the refrigerant and the fluid in a warm region corresponding to a temperature of about −90° C. to room temperature by using the amount of supplied refrigerant to control the precooling. That is, the expansion unit  10  may easily control all of the precooling, the liquefaction, and the subcooling, which are processes of liquefying the fluid. 
     The heat exchanger  20  may receive the refrigerants having different temperatures from the expansion unit  10  to cool the fluid in multistages and then discharge the fluid to the outside and discharge the refrigerants to the precompression unit  30 . In the heat exchanger  20 , a cooling loop for cooling the refrigerants at different temperatures may be formed. That is, in the heat exchanger  20 , a warm loop in which the refrigerant having a temperature of about −100° C. to about −80° C., which is supplied from the expansion unit  10 , is circulated, an intermediate loop in which the refrigerant having a temperature of about −120° C. to about −80° C. is circulated, and a cold loop in which the refrigerant having a temperature of about −160° C. to about −155° C. is circulated may be formed. In the cold loop, the fluids having different temperature ranges may be cooled to improve heat-exchange between the fluid and the refrigerant. 
     The precompression unit  30  may include a plurality of precompressors which respectively receive the refrigerants passing through the heat exchanger  20 , i.e., a first precompressor  31 , a second precompressor  32 , and a third precompressor  33 . Here, the first precompressor  31  may be coaxially connected to the first expander  11  to compress the refrigerant discharged from the first expander  11 , the second precompressor  32  may be coaxially connected to the second expander  12  to compress the refrigerant discharged from the second expander  12 , and the third precompressor  33  may be coaxially connected to the third expander  13  to compress the refrigerant discharged from the third expander  13 . Thus, when the expanders expand the refrigerants, each of the precompressors may compress the refrigerant discharged from each of the expanders in proportion to a degree of expansion of the refrigerant in each of the expanders. Each of the precompressors and each of the expanders may be interlocked with each other to serve as one compander. 
     As described above, the precompression unit  30  may receive and compress the refrigerants passing through the heat exchanger  20  to discharge the refrigerants having the same pressure. The discharged refrigerants may be mixed with each other in the mixing tube  40  and then transferred. Here, in the discharged refrigerants having the same pressure, an inflow temperature of each of the expanders, which are respectively connected to the warm loop, the intermediate loop, and the cold loop, a discharge temperature of each of the expanders, which are respectively connected to the warm loop, the intermediate loop, and the cold loop, and a ratio and a maximum pressure of the refrigerant introduced into each of the warm loop, the intermediate loop, and the cold loop may act as variables. Also, energy of the variables may determine a temperature distribution of the cooler  60  and a pressure state of the refrigerant discharged from the precompression unit  30  when energy balance in the heat exchanger is reached. Also, the variables may also influence a temperature of the liquefied natural gas discharged from the heat exchanger  20  and operations of the expansion unit  10  and the precompression unit  30 . The precompression unit  30  may continuously discharge the refrigerant having a pressure of about 10 barg to about 20 barg through the variables. Also, the precompression unit  30  always discharges the refrigerant with a predetermined pressure so that the first precompressor  31 , the second precompressor  32 , and the third precompressor  33  are always driven in a single operation. Thus, the first precompressor  31  to the third precompressor  33  are simplified in control and improved in operation efficiency. Also, the pressures of the discharged refrigerants are the same to improve compression efficiency of the main compression unit  50 . 
     The mixing tube  40  mixes the refrigerants discharged from the precompression unit  30  to supply the refrigerants to the main compressor  50  and the cooler  60 . Here, the mixing tube  40  may be connected to one end of each of the precompressors  31  to  33  to receive the refrigerants having the same pressure, which are discharged from the precompressor  31  to  33 . Here, the mixing tube  40  may be configured so that the pressure of the refrigerant is constantly maintained. The main compressor  50  is disposed between the mixing tube  40  and the heat exchanger  20  to compress the refrigerant and supply the compressed refrigerant to the heat exchanger  20 . In addition, the main compressor  50  may supply the refrigerant to the expansion unit  10 . The main compression unit  50  may have a structure in which a first compressor  51  and a second compressor  52  are connected in series to each other, a first cooling unit  53  is connected between the first compressor  51  and the second compressor  52 , and a second cooling unit  54  is connected between the second compressor  52  and the heat exchanger  20 . The refrigerant supplied into the mixing tube  40  may be compressed and cooled by passing through the components of the main compression unit  50  having the above-described structure in order of the first compressor  51 , the second cooling unit  52 , the second compressor  52 , and the second cooling unit  54 . 
     The cooler  60  may be installed between the precompression unit  30  and the main compression unit  50  and be connected to a cooling supply tube  70  having one end connected to the mixing tube  40  and the other end connected to the other end of the precompression unit  30 . The cooler  60  may regularly cool the refrigerant introduced through the mixing tube  40  by using the refrigerant introduced through the cooling supply tube  70  to supply the refrigerant having the constant pressure to the main compression unit  50 . 
     Thus, the cooler  60  may reduce the temperature of the refrigerant, reduce a load generated in the main compression unit  50 , and improve the operating efficiency to efficiently compress the whole refrigerant in the main compression unit  50 . 
     Hereinafter, an operation of the fluid cooling apparatus  1  will be described in more detail with reference to  FIGS. 2 and 3 . 
       FIGS. 2 and 3  are operation diagrams for explaining an operation of the fluid cooling apparatus. 
     In the fluid cooling apparatus  1  according to an embodiment of the present invention, the refrigerant discharged from the plurality of precompressors  31  to  33  may be discharged at the same pressure, and the refrigerants discharged from the mixing tube  40  may be mixed with each other into a single compression process to heat-exchange the refrigerant with the fluid, thereby improving the liquefaction efficiency of the fluid. The refrigerant used in the fluid cooling apparatus  1  may be a medium, which achieves a temperature less than a cooling temperature of a target fluid to be cooled, a single refrigerant. For example, the refrigerant may be nitrogen and hydrocarbon. 
     In this specification, the refrigerant may be, for example, a nitrogen having a pressure of about 10 barg to about 20 barg and a temperature of about 30° C. to about 45° C., which is capable of being maintained in a stable state when compared with other gases. Also, an example in which the fluid cooled by the refrigerant is a natural gas will be described. However, this is merely one example, and the state of nitrogen and the kind of fluid are not limited thereto. 
     Hereinafter, referring to  FIG. 2 , the nitrogen refrigerant having a pressure of about 10 barg to about 20 barg and a temperature of about 30° C. to about 45° C. may be compressed from the outside through the first compressor  51  of the main compression unit  50  and then be discharged as a high-temperature refrigerant having a pressure of about 30 barg to about 40 barg. The discharged refrigerant may pass through the first cooling unit  53  and be cooled to a temperature of about 30° C. while passing through the first cooling unit  53 . Thereafter, the cooled refrigerant is introduced into the second compressor  52 . The second compressor  52  converts the introduced refrigerant into a high-temperature refrigerant having a pressure of about 50 barg to about 60 barg to discharge the converted refrigerant. The discharged refrigerant is cooled to a temperature of about 30° C. again through the second cooling unit  54 . Then, the discharged refrigerant is supplied to the heat exchanger  20 . 
     The refrigerant supplied into the heat exchanger  20  may exchange heat with the natural gas and the refrigerant introduced again through the expansion unit  10  while passing through the heat exchanger  20  and be cooled at a temperature of about 5° C. to about 10° C. in the warm loop and cooled at a temperature of about −20° C. to about −40° C. in the intermediate loop. Also, the refrigerant may be cooled at a temperature of about −90° C. to about −120° C. in the cold loop. 
     As described above, the refrigerant cooled at the different temperatures in the loops may be supplied to the first expander  11  by a ratio of about 30% to about 40%, the second expander  12  by a ratio of about 30% to about 45%, and the third expander  13  by a ratio of about 20% to about 30% through valves disposed between the heat exchanger  20  and the expansion unit  10 . The refrigerant supplied into each of the expanders may be discharged through the first expander  11  at a pressure of about 5 barg to about 10 barge and a temperature of about −100° C. to about −80° C., discharged through the second expander  12  at a pressure of about 8 barg to about 15 barge and a temperature of about −120° C. to about −80° C., and discharged through the third expander  13  at a pressure of about 10 barg to about 20 barge and a temperature of about −160° C. to about −155° C. 
     The refrigerants discharged at the different pressures and temperatures as described above may be introduced again into the heat exchanger  20  to exchange heat with nitrogen introduced from the outside. Here, the nitrogen may be changed to a constant temperature so as to be supplied into each of the expanders  11  to  13 . Also, the refrigerant that is treated as described above may be supplied to each of the precompressors  31  to  33 , which are interlocked with the expanders  11  to  13 , and then be discharged at the same pressure. The discharged refrigerants may be mixed with each other in the mixing tube  40  to form one refrigerant. 
     Referring to  FIG. 3 , the mixed refrigerant is cooled through the cooler  60  and lowered to a predetermined temperature. Then, the refrigerant is compressed and cooled by sequentially passing through the first compressor  51 , the first cooling unit  53 , the second compressor  52 , and the second cooling unit  54  and then is introduced into the heat exchanger  20 . 
     Here, the mixed refrigerant is entirely compressed in two stages through the main compression unit  50  and introduced into the heat exchanger  20 . Thereafter, the refrigerant continues to cool the fluid in a single stream. 
     As described above, the flowing refrigerant may be liquefied at a cryogenic temperature of about −160° C. to about −155° C. by the precooling, the liquefaction and the subcooling of the natural gas heat-exchanged with the refrigerant in the heat exchanger  20 . 
     Hereinafter, an operation of the fluid cooling apparatus  1  will be described in more detail with reference to  FIG. 4 . 
       FIG. 4  is a graph illustrating a relationship between a temperature and energy while the fluid is liquefied by using the refrigerant in the fluid cooling apparatus. 
     In the graph, an x-axis represents an amount of heat generated in the heat exchanger through a heat flow of each of the expanders and compressors, and a y-axis represents a temperature of the heat. Also, an upper composite curve represents a hot composite of a fluid, and a lower composite curve represents a cold composite of a refrigerant. 
     The fluid cooling apparatus  1  of the present invention is constituted by a warm loop, an intermediate loop, and a cold loop. Each of the loops operates in various temperature ranges in consideration of the temperature curves. For example, in the cold loop, the refrigerant may be circulated, and the cold loop may operate to be cooled until reaching a temperature of about 25° C. to about 45° C. after cooled up to a temperature of −160° C. to about −155° C. In the intermediate loop, the refrigerant may be circulated, and the intermediate loop may operate to be cooled until reaching a temperature of about 25° C. to about 45° C. after cooled up to a temperature of −120° C. to about −80° C. Also, in the warm loop, the refrigerant may be circulated, and the warm loop may operate to be cooled until reaching a temperature of about 25° C. to about 45° C. after cooled up to a temperature of −100° C. to about −80° C. The change in amount or ratio of refrigerant circulated through each of the loops may have a significant effect on the temperature curve. In more detail, the change in amount of refrigerant circulated through the cold loop may have a significant effect on a subcooling region between about −160° C. and about −90° C., and the variation in amount of refrigerant circulated through the intermediate loop may have a significant effect on a liquefaction region between about −120° C. and about −80° C. Also, the variation in amount of refrigerant circulated through the warm loop may mainly have an effect on a temperature of about −90° C. or more. 
     As described above, the fluid cooling apparatus  1  may adjust the amount of refrigerant circulated through each of the loops to control the temperature of each of the loops, thereby effectively reducing the temperature curve interval between the fluid and the refrigerant in the temperature range period that is mainly occupied in each of the loops. Also, since the refrigerants discharged from the precompression unit  30  are mixed with the same pressure and then introduced into the main compression unit  30 , the refrigerant may be improved in compression efficiency. 
     That is, the fluid cooling apparatus  1  may improve the efficiency of the liquefaction of the fluid by improving the compression efficiency of the refrigerant through the simple process and effectively cooling the fluid to reduce the energy consumed for liquefying the fluid. 
     Although the embodiment of the inventive concept is described with reference to the accompanying drawings, those with ordinary skill in the technical field of the inventive concept pertains will be understood that the present disclosure can be carried out in other specific forms without changing the technical idea or essential features. Therefore, the above-disclosed embodiments are to be considered illustrative and not restrictive.