Patent Publication Number: US-2015082828-A1

Title: Natural gas fractional distillation apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for fractionating natural gas, and more particularly, to an apparatus for fractionating natural gas which may improve energy efficiency. 
     BACKGROUND ART 
     Natural gas is generally collected from a gas well drilled into an underground reservoir. While methane takes most of the natural gas, the natural gas contains a lot of tiny components, for example, water, hydrogen sulfide, carbon dioxide, mercury, nitrogen, and heavier hydrocarbon such as ethane, propane, or butane. 
     Since some of the components such as water, hydrogen sulfide, carbon dioxide, and mercury may work as harmful contaminants in processing of liquefied natural gas (LNG), these contaminants should be removed prior to a natural gas collection process. 
     Also, since the heavier hydrocarbon such as ethane, propane, or butane, which are heavier than methane, has sufficiently qualified as a product, the heavier hydrocarbon is condensed and collected as natural gas liquid and then fractionated generating valuable products. 
     As such, in a natural gas liquid collection process, methane and heavy hydrocarbon are separated from pre-processed natural gas by using a distillation tower and then the methane and heavy hydrocarbon are liquefied. 
     In general, there are many well-known methods of obtaining LNG by liquefying a raw natural gas in a gaseous state. The natural gas not only takes less space when in a liquefied state rather than in a gaseous state, but also does not need to be stored under high pressure in the liquefied state. Accordingly, the natural gas in the liquefied state may be easily stored and transported a far distance, thereby reducing transportation costs. Thus, the natural gas may be liquefied for the purpose of transportation. 
     On the other hand, in a fractional distillation apparatus to obtain LNG, a raw natural gas flows into a distillation tower and is fractionated into an overhead vapor stream including methane and a component-reinforced lower stream that is heavier than methane. 
     A large amount of heat is input to the distillation tower to vaporize the raw natural gas, and a large amount of energy is consumed to cool and condense the vaporized overhead vapor stream. 
     As such, a large amount of energy is consumed for general processes to separate and collect the raw natural gas and thus energy efficiency may be degraded. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problem 
     The present invention provides a natural gas fractional distillation apparatus which may improve energy efficiency by improving a part of a process to reduce energy consumed in fractioning a natural gas that is a raw material. 
     Technical Solution 
     According to an aspect of the present invention, there is provided a natural gas fractional distillation apparatus including: a gas-liquid separator into which a condensed natural gas flows and which separates the condensed natural gas into a vapor stream and a liquid stream; a gas separator which separates the vapor stream separated in the gas-liquid separator into a first vapor stream and a second vapor stream; a first heat exchanger which condenses the first vapor stream separated in the gas separator; a first expander which expands the first vapor stream condensed in the first heat exchanger; a second expander which expands the second vapor separated in the gas separator; and a distillation tower into which the liquid stream separated in the gas-liquid separator, the first vapor stream expanded in the first expander, and the second vapor stream expanded in the second expander flow in order to be divided into an overhead vapor stream containing methane and a component-reinforced lower stream that is heavier than methane, wherein the first heat exchanger exchanges heat among the first vapor stream separated in the gas separator, the first vapor stream expanded in the first expander, and the overhead vapor stream. 
     According to another aspect of the present invention, there is provided a natural gas fractional distillation apparatus including: a gas-liquid separator into which a condensed natural gas flows and which separates the condensed natural gas into a vapor stream and a liquid stream; a gas separator which separates the vapor stream separated in the gas-liquid separator into a first vapor stream and a second vapor stream; a first heat exchanger which condenses the first vapor stream separated in the gas separator; a first expander which expands the first vapor stream condensed in the first heat exchanger; a second expander which expands the second vapor separated in the gas separator; and a distillation tower into which the liquid stream separated in the gas-liquid separator, the first vapor stream expanded in the first expander, and the second vapor stream expanded in the second expander flow in order to be divided into an overhead vapor stream containing methane and a component-reinforced lower stream that is heavier than methane, wherein the first heat exchanger exchanges heat among the first vapor stream separated in the gas separator, the second vapor stream expanded in the second expander, and the overhead vapor stream. 
     The natural gas fractional distillation apparatus may further include a second heat exchanger which discharges the condensed natural gas by exchanging heat between a natural gas and the overhead vapor stream that is discharged after heat exchange in the first heat exchanger. 
     The natural gas fractional distillation apparatus may further include a compressor which compresses the overhead vapor stream that is discharged after heat exchange in the second heat exchanger. 
     According to another aspect of the present invention, there is provided a natural gas fractional distillation apparatus including: a gas-liquid separator into which a condensed natural gas flows and which separates the condensed natural gas into a vapor stream and a liquid stream; a gas separator which separates the vapor stream separated in the gas-liquid separator into a first vapor stream and a second vapor stream; a first heat exchanger which condenses the first vapor stream separated in the gas separator; a first expander which expands the first vapor stream condensed in the first heat exchanger; a second expander which expands the second vapor separated in the gas separator; a distillation tower into which the liquid stream separated in the gas-liquid separator, the first vapor stream expanded in the first expander, and the second vapor stream expanded in the second expander flow in order to be divided into an overhead vapor stream containing methane and a component-reinforced lower stream that is heavier than methane; and a second heat exchanger which discharges the condensed natural gas by exchanging heat among the overhead vapor stream that is discharged from the first heat exchanger after heat exchange with the first vapor stream in the first heat exchanger, the first vapor stream expanded in the first expander, and a natural gas. 
     According to another aspect of the present invention, there is provided a natural gas fractional distillation apparatus including: a gas-liquid separator into which a condensed natural gas flows and which separates the condensed natural gas into a vapor stream and a liquid stream; a gas separator which separates the vapor stream separated in the gas-liquid separator into a first vapor stream and a second vapor stream; a first heat exchanger which condenses the first vapor stream separated in the gas separator; a first expander which expands the first vapor stream condensed in the first heat exchanger; a second expander which expands the second vapor separated in the gas separator; a distillation tower into which the liquid stream separated in the gas-liquid separator, the first vapor stream expanded in the first expander, and the second vapor stream expanded in the second expander flow in order to be divided into an overhead vapor stream containing methane and a component-reinforced lower stream that is heavier than methane; and a second heat exchanger which discharges the condensed natural gas by exchanging heat among the overhead vapor stream that is discharged from the first heat exchanger after heat exchange with the first vapor stream in the first heat exchanger, the second vapor stream expanded in the second expander, and a natural gas. 
     The natural gas fractional distillation apparatus may further include a compressor which compresses the overhead vapor stream that is discharged after heat exchange in the second heat exchanger. 
     According to an aspect of the present invention, there is provided a natural gas fractional distillation apparatus including: a gas-liquid separator into which a condensed natural gas flows and which separates the condensed natural gas into a vapor stream and a liquid stream; a gas separator which separates the vapor stream separated in the gas-liquid separator into a first vapor stream and a second vapor stream; a first heat exchanger which condenses the first vapor stream separated in the gas separator; a first expander which expands the first vapor stream condensed in the first heat exchanger; a third heat exchanger which heats the first vapor stream expanded in the first expander; a second expander which expands the second vapor separated in the gas separator; and a distillation tower into which the liquid stream separated in the gas-liquid separator, the first vapor stream heated in the third heat exchanger, and the second vapor stream expanded in the second expander flow in order to be divided into an overhead vapor stream containing methane and a component-reinforced lower stream that is heavier than methane, wherein the first heat exchanger exchanges heat between the first vapor stream separated in the gas separator and the overhead vapor stream, and the third heat exchanger exchanges heat between the first vapor stream expanded in the first expander and the overhead vapor stream discharged from the first heat exchanger. 
     The natural gas fractional distillation apparatus may further include a second heat exchanger which discharges the condensed natural gas by exchanging heat between a natural gas and the overhead vapor stream that is discharged after heat exchange in the third heat exchanger. 
     The natural gas fractional distillation apparatus may further include a compressor which compresses the overhead vapor stream that is discharged after heat exchange in the second heat exchanger. 
     The gas separator may separate the vapor stream into the first vapor stream and the second vapor stream in a ratio of 2:8 to 1:9. 
     Advantageous Effects 
     According to one or more embodiments of the present invention, since the first vapor stream and the overhead vapor stream discharged from the gas separator and the first vapor stream expanded in the first expander or the second vapor stream expanded in the second expander are heat-exchanged in the first heat exchanger, the natural gas and the overhead vapor stream, and the first vapor stream expanded in the first expander or the second vapor stream expanded in the second expander are heat-exchanged in the second heat exchanger, or the overhead vapor stream discharged from the first heat exchanger and the first vapor stream discharged from the first expander are heat-exchanged in the third heat exchanger that is additionally installed, energy that is consumed in fractioning natural gas that is a raw material is reduced and thus energy efficiency may be improved. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a configuration of a natural gas fractional distillation apparatus according to an embodiment of the present invention. 
         FIG. 2  is a view illustrating a configuration of a natural gas fractional distillation apparatus according to another embodiment of the present invention. 
         FIG. 3  is a view illustrating a configuration of a natural gas fractional distillation apparatus according to another embodiment of the present invention. 
         FIG. 4  is a view illustrating a configuration of a natural gas fractional distillation apparatus according to another embodiment of the present invention. 
         FIG. 5  is a view illustrating a configuration of a natural gas fractional distillation apparatus according to another embodiment of the present invention. 
     
    
    
     BEST MODE 
     The attached drawings for illustrating preferred embodiments of the present invention are referred to in order to gain a sufficient understanding of the present invention, the merits thereof, and the objectives accomplished by the implementation of the present invention. 
     Hereinafter, the present invention will be described in detail by explaining preferred embodiments of the invention with reference to the attached drawings. Like reference numerals in the drawings denote like elements. 
     The natural gas stream that is described below is defined to be all hydrocarbon compounds including methane, ethane, propane, or butane from which water, hydrogen sulfide, carbon dioxide, mercury, and nitrogen are removed. 
     A natural gas fractional distillation apparatus  100  according to an embodiment of the present invention is described below. 
       FIG. 1  is a view illustrating a configuration of a natural gas fractional distillation apparatus  100  according to an embodiment of the present invention. 
     Referring to  FIG. 1 , the natural gas fractional distillation apparatus  100  according to the present embodiment includes a gas-liquid separator  110  into which a condensed natural gas flows and which separates the condensed natural gas into a vapor stream and a liquid stream, a gas separator  120  which separates the vapor stream separated in the gas-liquid separator  110  into a first vapor stream and a second vapor stream, a first heat exchanger  130  which condenses the first vapor stream separated in the gas separator  120 , a first expander  140  which expands the first vapor stream condensed in the first heat exchanger  130 , a second expander  150  which expands the second vapor separated in the gas separator  120 , a distillation tower  160  into which the liquid stream separated in the gas-liquid separator  110 , the first vapor stream expanded in the first expander  140 , and the second vapor stream expanded in the second expander  150  flow in order to be divided into an overhead vapor stream containing methane and a component-reinforced lower stream that is heavier than methane, a second heat exchanger  170  which discharges a condensed natural gas by exchanging heat between the natural gas and the overhead vapor stream discharged after the heat exchange in the first heat exchanger  130 , and a compressor  180  which compresses the overhead vapor stream discharged after the heat exchange in the second heat exchanger  170 . 
     In the natural gas fractional distillation apparatus  100  according to the present embodiment, the first vapor stream decompressed and cooled in the first expander  140  is used as a coolant of the first heat exchanger  130  and thus energy applied to the distillation tower  160  and energy used in the compressor  180  which condenses and cools the overhead vapor stream discharged from the distillation tower  160  may be reduced. 
     After having passed through a pre-treatment process, the natural gas flows into the second heat exchanger  170  along a first flow pass  101 . The second heat exchanger  170  changes the state of natural gas into a condensed natural gas by condensing the natural gas in a gaseous state after having passed through the pre-treatment process. 
     The natural gas condensed in the second heat exchanger  170  is discharged along a second flow pass  102 . In the second heat exchanger  170 , the natural gas exchanges heat with the overhead vapor stream that is cooled after having sequentially passed through the distillation tower  160  and the first heat exchanger  130 . 
     The condensed natural gas flows into the gas-liquid separator  110  along the second flow pass  102 . The gas-liquid separator  110  separates the condensed natural gas into a vapor stream in a gaseous state and a liquid stream in a liquefied state. 
     The liquid stream flows into a lower position of the distillation tower  160  along a third flow pass  113 . 
     The vapor stream in the gaseous state separated from the gas-liquid separator  110  flows into the gas separator  120  along a fourth flow pass  111 . The gas separator  120  separates the vapor stream into a first vapor stream and a second vapor stream according to a preset ratio. 
     The first vapor stream and the second vapor stream may be separated in a ratio of 2:8 to 1:9. This is to reduce energy used by the compressor  180  to condense and cool the overhead vapor stream and improve energy efficiency of the whole natural gas fractional distillation apparatus  100  by separating the vapor stream flowing into the distillation tower  160  into the first vapor stream and the second vapor stream in the above ratio and cooling the overhead vapor stream discharged from the distillation tower  160  by using the first vapor stream. 
     The second vapor stream separated in the gas separator  120  flows into the second expander  150  along a fifth flow pass  122 . The second expander  150  expands the second vapor stream and lowers the temperature of the second vapor stream. 
     As the temperature of the second vapor stream expanded in the second expander  150  is lowered, the second vapor stream may be changed from a gaseous state to a liquefied state or may exist in both gaseous and liquefied states. 
     The second vapor stream that has passed through the second expander  150  flows into the distillation tower  160  along a sixth flow pass  124 . The sixth flow pass  124  is connected to the distillation tower  160  at an upper position compared to the position of the third flow pass  113  such that the second vapor stream that has passed through the sixth flow pass  124  flows into the distillation tower  160  at an upper position compared to the position of the liquid stream that has passed through the third flow pass  113 . 
     This is to reduce cold energy input to cool the overhead vapor stream by cooling the overhead vapor stream that is vaporized in the distillation tower  160  and then discharged from the top of the distillation tower  160  by using the second vapor stream that flows into the distillation tower  160  along the sixth flow pass  124 . 
     On the other hand, the first vapor stream flows into the first heat exchanger  130  along a seventh flow pass  121 , and then, the first vapor stream discharged from the first heat exchanger  130  flows into the first expander  140  along an eighth flow pass  123 . 
     The first vapor stream rapidly expands and is decompressed in the first expander  140  and the temperature of the first vapor stream is rapidly lowered. Accordingly, the first vapor stream may be changed from a gaseous state to a liquefied state or may exist in both gaseous and liquefied states. The first vapor stream that has passed through the first expander  140  maintains the lowest temperature in the present embodiment. 
     On the other hand, the first vapor stream discharged from the first expander  140  flows again into the first heat exchanger  130  along a ninth flow pass  125 . The overhead vapor stream discharged from the top portion of the distillation tower  160  along an eleventh flow pass  161  flows into the first heat exchanger  130 . 
     As such, the first heat exchanger  130  exchanges heat among the first vapor stream flowing in along the seventh flow pass  121 , the first vapor stream flowing in again along the ninth flow pass  125 , and the overhead vapor stream flowing in along the eleventh flow pass  161 . 
     The first vapor stream that flows in along the ninth flow pass  125  functions as a coolant. Accordingly, the first vapor stream that flows in along the seventh flow pass  121  and the overhead vapor stream that flows in along the eleventh flow pass  161  both are condensed and discharged from the first heat exchanger  130 . The first vapor stream that flows in along the ninth flow pass  125  is heated and discharged from the first heat exchanger  130 . 
     As such, since the overhead vapor stream that has passed through the first heat exchanger  130  is cooled and discharged from the first heat exchanger  130 , the energy used by the compressor  180  to condense and cool the overhead vapor stream may be reduced. Also, since the first vapor stream that has passed through the first expander  140  has an increased temperature and then flows into the distillation tower  160 , the energy used in the distillation tower  160  may be reduced. 
     The first vapor steam that has passed through the first heat exchanger  130  flows into the distillation tower  160  along a tenth flow pass  127  at an upper position of the distillation tower  160  compared to the position of the second vapor stream that has passed through the sixth flow pass  124 . In other words, the tenth flow pass  127  is connected to the distillation tower  160  at an upper position compared to the position of the sixth flow pass  124 . 
     This is to reduce cold energy input to cool the overhead vapor stream by sequentially cooling the overhead vapor stream that is vaporized in the distillation tower  160  and then discharged from the top of the distillation tower  160  by using the second vapor stream that flows into the distillation tower  160  along the sixth flow pass  124  and the first vapor stream that flows into the distillation tower  160  along the tenth flow pass  127 . 
     As such, the liquid stream, the first vapor stream, and the second vapor stream that flow into the distillation tower  160  are heated and vaporized by a reboiler  154  provided adjacent to the distillation tower  160  along a circulation flow pass provided in a lower portion of the distillation tower  160 , and then flow again into the distillation tower  160 . 
     The distillation tower  160  separates a raw natural gas into a methane-reinforced overhead vapor stream and the component-reinforced lower stream that is heavier than methane. 
     The overhead vapor stream is discharged from the top of the distillation tower  160  and cooled by passing through the first heat exchanger  130  along the eleventh flow pass  161 , and is discharged from the first heat exchanger  130  along a twelfth flow pass  163  to flow into the second heat exchanger  170 . 
     The second heat exchanger  170  exchanges heat between the raw natural gas that underwent the pre-treatment process and the overhead vapor stream that flows in along the twelfth flow pass  163 . As described above, the natural gas that has passed through the second heat exchanger  170  is condensed and discharged along the second flow pass  102 . 
     The overhead vapor stream discharged from the second heat exchanger  170  flows into the compressor  180  along a thirteenth flow pass  165  to be compressed and condensed therein, and then flows along a fourteenth flow pass  167  and is stored in a reservoir (not shown). 
     On the other hand, the component-reinforced lower stream that is heavier than methane, which is discharged from the distillation tower  160 , may flow along a discharge flow pass  162  connected to the lower portion of the distillation tower  160  and may be stored outside. 
     In the following description, a natural gas fractional distillation apparatus  100   a  according to another embodiment of the present invention is described. 
       FIG. 2  is a view illustrating a configuration of the natural gas fractional distillation apparatus  100   a  according to another embodiment of the present invention. 
     Referring to  FIG. 2 , the natural gas fractional distillation apparatus  100   a  according to the present embodiment includes a gas-liquid separator  110   a  into which a condensed natural gas flows and which separates the condensed natural gas into a vapor stream and a liquid stream, a gas separator  120   a  which separates the vapor stream separated in the gas-liquid separator  110   a  into a first vapor stream and a second vapor stream, a first heat exchanger  130   a  which condenses the first vapor stream separated in the gas separator  120   a , a first expander  140   a  which expands the first vapor stream condensed in the first heat exchanger  130   a , a second expander  150   a  which expands the second vapor separated in the gas separator  120   a , a distillation tower  160   a  into which the liquid stream separated in the gas-liquid separator  110   a , the first vapor stream expanded in the first expander  140   a , and the second vapor stream expanded in the second expander  150   a  flow in order to be divided into an overhead vapor stream containing methane and the component-reinforced lower stream that is heavier than methane, a second heat exchanger  170   a  which discharges a condensed natural gas by exchanging heat between the natural gas and the overhead vapor stream discharged after the heat exchange in the first heat exchanger  130   a , and a compressor  180   a  which compresses the overhead vapor stream discharged after the heat exchange in the second heat exchanger  170   a.    
     In the natural gas fractional distillation apparatus  100   a  according to the present embodiment, the second vapor stream decompressed and cooled in the second expander  150   a  is used as a coolant of the first heat exchanger  130   a  and thus energy applied to the distillation tower  160   a  and energy used in the compressor  180   a  which condenses and cools the overhead vapor stream discharged from the distillation tower  160   a  may be reduced. 
     After having passed through a pre-treatment process, the natural gas flows into the second heat exchanger  170   a  along a first flow pass  101   a . The second heat exchanger  170   a  changes the state of natural gas into a condensed natural gas by condensing the natural gas in a gaseous state after having passed through the pre-treatment process. 
     The natural gas condensed in the second heat exchanger  170   a  is discharged along a second flow pass  102   a . In the second heat exchanger  170   a , the natural gas exchanges heat with the overhead vapor stream that is cooled after having sequentially passed through the distillation tower  160   a  and the first heat exchanger  130   a.    
     The condensed natural gas flows into the gas-liquid separator  110   a  along the second flow pass  102   a . The gas-liquid separator  110   a  separates the condensed natural gas into a vapor stream in a gaseous state and a liquid stream in a liquefied state. 
     The liquid stream flows into a lower position of the distillation tower  160   a  along a third flow pass  113   a.    
     The vapor stream in the gaseous state separated from the gas-liquid separator  110   a  flows into the gas separator  120   a  along a fourth flow pass  111   a . The gas separator  120   a  separates the vapor stream into a first vapor stream and a second vapor stream according to a preset ratio. 
     The first vapor stream and the second vapor stream may be separated in a ratio of 2:8 to 1:9. This to reduce energy used by the compressor  180   a  to condense and cool the overhead vapor stream and improve energy efficiency of the whole natural gas fractional distillation apparatus  100  by separating the vapor stream flowing into the distillation tower  160   a  into the first vapor stream and the second vapor stream in the above ratio and cooling the overhead vapor stream discharged from the distillation tower  160   a  by using the first vapor stream. 
     The first vapor stream separated in the gas separator  120   a  flows into the first heat exchanger  130   a  along a seventh flow pass  121   a . The first vapor stream discharged from the first heat exchanger  130   a  flows into the first expander  140   a  along an eighth flow pass  123   a.    
     As the first vapor stream rapidly expands and is decompressed in the first expander  140   a  and the temperature of the first vapor stream is rapidly lowered, the first vapor stream may be changed from a gaseous state to a liquefied state or may exist in both gaseous and liquefied states. The first vapor stream that has passed through the first expander  140   a  maintains the lowest temperature in the present embodiment. 
     On the other hand, the first vapor stream discharged from the first expander  140   a  flows into the distillation tower  160   a  along a ninth flow pass  125   a . The first vapor stream flows into the distillation tower  160   a  at an upper position compared to the position of the second vapor stream that has passed through a tenth flow pass  127   a . In other words, the ninth flow pass  125   a  is connected to the distillation tower  160   a  at an upper position compared to the position of the tenth flow pass  127   a.    
     This is to reduce cold energy input to cool the overhead vapor stream by sequentially cooling the overhead vapor stream that is vaporized in the distillation tower  160   a  and then discharged from the top of the distillation tower  160   a  by using the second vapor stream that flows into the distillation tower  160   a  along the tenth flow pass  127   a  and the first vapor stream that flows into the distillation tower  160   a  along the ninth flow pass  125   a.    
     On the other hand, the second vapor stream separated in the gas separator  120   a  flows into the second expander  150   a  along a fifth flow pass  122   a . The second expander  150   a  expands the second vapor stream and lowers the temperature of the second vapor stream. The second vapor stream expanded in the second expander  150   a  may be changed from a gaseous state to a liquefied state as the temperature of the second vapor stream is lowered or may exist in both gaseous state and liquefied states. 
     The second vapor stream that has passed through the second expander  150   a  flows into the first heat exchanger  130   a  along a sixth flow pass  124   a . The overhead vapor stream discharged from the top of the distillation tower  160   a  flows into the first heat exchanger  130   a  along an eleventh flow pass  161   a.    
     As such, the first heat exchanger  130   a  exchanges heat among the first vapor stream flowing in along the seventh flow pass  121   a , the second vapor stream flowing in along the sixth flow pass  124   a , and the overhead vapor stream flowing in along the eleventh flow pass  161   a.    
     The second vapor stream that flows in along the sixth flow pass  124   a  functions as a coolant. Accordingly, the first vapor stream that flows in along the seventh flow pass  121   a  and the overhead vapor stream that flows in along the eleventh flow pass  161   a  both are condensed and discharged from the first heat exchanger  130   a . The second vapor stream that flows in along the sixth flow pass  124   a  is heated and discharged from the first heat exchanger  130   a.    
     As such, since the overhead vapor stream that has passed through the first heat exchanger  130   a  is cooled and discharged from the first heat exchanger  130   a , the energy used by the compressor  180   a  to condense and cool the overhead vapor stream may be reduced. Also, since the second vapor stream that has sequentially passed through the second expander  150   a  and the first heat exchanger  130   a  has an increased temperature and then flows into the distillation tower  160   a , the energy used in the distillation tower  160   a  may be reduced. 
     The second vapor steam that has passed through the first heat exchanger  130   a  flows into the distillation tower  160   a  along the tenth flow pass  127   a  at a lower position of the distillation tower  160  compared to the position of the first vapor stream that has passed through the ninth flow pass  125   a . In other words, the tenth flow pass  127   a  is connected to the distillation tower  160   a  at a lower position compared to the position of the ninth flow pass  125   a.    
     This is to reduce cold energy input to cool the overhead vapor stream by sequentially cooling the overhead vapor stream that is vaporized in the distillation tower  160   a  and then discharged from the top of the distillation tower  160   a  by using the second vapor stream that flows into the distillation tower  160   a  along the tenth flow pass  127   a  and the first vapor stream that flows into the distillation tower  160   a  along the ninth flow pass  125   a.    
     As such, the liquid stream, the first vapor stream, and the second vapor stream that flow into the distillation tower  160   a  are heated and vaporized by a reboiler  164   a  provided adjacent to the distillation tower  160   a  along a circulation flow pass provided in a lower portion of the distillation tower  160   a , and then flow again into the distillation tower  160   a.    
     The distillation tower  160   a  separates a raw natural gas into the methane-reinforced overhead vapor stream and the component-reinforced lower stream that is heavier than methane. 
     The overhead vapor stream is discharged from the top of the distillation tower  160   a  and cooled by passing through the first heat exchanger  130   a  along the eleventh flow pass  161   a , and is discharged from the first heat exchanger  130   a  along a twelfth flow pass  163   a  to flow into the second heat exchanger  170   a.    
     The second heat exchanger  170   a  exchanges heat between the raw natural gas that underwent the pre-treatment process and the overhead vapor stream that flows in along the twelfth flow pass  163   a . As described above, the natural gas that has passed through the second heat exchanger  170   a  is condensed and discharged along the second flow pass  102   a.    
     The overhead vapor stream discharged from the second heat exchanger  170   a  flows into the compressor  180   a  along a thirteenth flow pass  165   a  to be compressed and condensed therein, and then flows along a fourteenth flow pass  167   a  and is stored in a reservoir (not shown). 
     On the other hand, the component-reinforced lower stream that is heavier than methane, which is discharged from the distillation tower  160   a , may flow along a discharge flow pass  162   a  connected to the lower portion of the distillation tower  160   a  and may be stored outside. 
     In the following description, a natural gas fractional distillation apparatus  100   b  according to another embodiment of the present invention is described. 
       FIG. 3  is a view illustrating a configuration of the natural gas fractional distillation apparatus  100   b  according to another embodiment of the present invention. 
     Referring to  FIG. 3 , the natural gas fractional distillation apparatus  100   b  according to the present embodiment includes a gas-liquid separator  110   b  into which a condensed natural gas flows and which separates the condensed natural gas into a vapor stream and a liquid stream, a gas separator  120   b  which separates the vapor stream separated in the gas-liquid separator  110   b  into a first vapor stream and a second vapor stream, a first heat exchanger  130   b  which condenses the first vapor stream separated in the gas separator  120   b , a first expander  140   b  which expands the first vapor stream condensed in the first heat exchanger  130   b , a second expander  150   b  which expands the second vapor separated in the gas separator  120   b , a distillation tower  160   b  into which the liquid stream separated in the gas-liquid separator  110   b , the first vapor stream expanded in the first expander  140   b , and the second vapor stream expanded in the second expander  150   b  flow in order to be divided into an overhead vapor stream containing methane and the component-reinforced lower stream that is heavier than methane, a second heat exchanger  170   b  which discharges a condensed natural gas by exchanging heat between the overhead vapor stream discharged from the first heat exchanger  130   b  after the heat exchange with the first vapor stream in the first heat exchanger  130   b , the first vapor stream expanded in the first expander  140   b , and the natural gas, and a compressor  180   b  which compresses the overhead vapor stream discharged after the heat exchange in the second heat exchanger  170   b.    
     In the natural gas fractional distillation apparatus  100   b  according to the present embodiment, the first vapor stream decompressed and cooled in the first expander  140   b  is used as a coolant of the second heat exchanger  170   b  and thus energy applied to the distillation tower  160   b  and energy used in the compressor  180   b  which condenses and cools the overhead vapor stream discharged from the distillation tower  160   b  may be reduced. 
     After having passed through a pre-treatment process, the natural gas flows into the second heat exchanger  170   b  along a first flow pass  101   b . The second heat exchanger  170   b  changes the state of natural gas into a condensed natural gas by condensing the natural gas in a gaseous state after having passed through the pre-treatment process. 
     The second heat exchanger  170   b  exchanges heat between the overhead vapor stream that is cooled after having sequentially passed through the distillation tower  160   b  and the first heat exchanger  130   b  and the first vapor stream that is decompressed and cooled by passing through the first expander  140   b.    
     The natural gas condensed in the second heat exchanger  170   b  is discharged along a second flow pass  102   b . The condensed natural gas flows into the gas-liquid separator  110   b  along the second flow pass  102   b . The gas-liquid separator  110   b  separates the condensed natural gas into a vapor stream in a gaseous state and a liquid stream in a liquefied state. 
     The liquid stream flows into a lower position of the distillation tower  160   b  along a third flow pass  113   b.    
     The vapor stream in the gaseous state separated from the gas-liquid separator  110   b  flows into the gas separator  120   b  along a fourth flow pass  111   b . The gas separator  120   b  separates the vapor stream into a first vapor stream and a second vapor stream according to a preset ratio. 
     The first vapor stream and the second vapor stream may be separated in a ratio of 2:8 to 1:9. This to reduce energy used by the compressor  180   b  to condense and cool the overhead vapor stream and improve energy efficiency of the whole natural gas fractional distillation apparatus  100   b  by separating the vapor stream flowing into the distillation tower  160   b  into the first vapor stream and the second vapor stream in the above ratio and cooling the overhead vapor stream discharged from the distillation tower  160   b  by using the first vapor stream. 
     The second vapor stream separated in the gas separator  120   b  flows into the second expander  150   b  along a fifth flow pass  122   b . The second expander  150   b  expands the second vapor stream and lowers the temperature of the second vapor stream. 
     As the temperature of the second vapor stream that has expanded in the second expander  150   b  is lowered, the second vapor stream may be changed from a gaseous state to a liquefied state or may exist in both gaseous and liquefied states. 
     The second vapor stream discharged from the second expander  150   b  flows into the distillation tower  160   b  along a sixth flow pass  124   b . The sixth flow pass  124   b  is connected to the distillation tower  160   b  at an upper position compared to the position of the third flow pass  113   b  such that the second vapor stream that has passed through the third flow pass  113   b  flows into the distillation tower  160   b  at an upper position compared to the position of the liquid stream that has passed through the third flow pass  113   b.    
     This is to reduce cold energy input to cool the overhead vapor stream by cooling the overhead vapor stream that is vaporized in the distillation tower  160   b  and then discharged from the top of the distillation tower  160   b  by using the second vapor stream that flows into the distillation tower  160   b  along the sixth flow pass  124   b.    
     On the other hand, the first vapor stream flows into the first heat exchanger  130   b  along a seventh flow pass  121   b , and then, the first vapor stream discharged from the first heat exchanger  130   b  flows into the first expander  140   b  along an eighth flow pass  123   b.    
     As the first vapor stream rapidly expands and is decompressed in the first expander  140   b  and the temperature of the first vapor stream is rapidly lowered, the first vapor stream may be changed from a gaseous state to a liquefied state or may exist in both gaseous and liquefied states. The first vapor stream that has passed through the first expander  140   b  maintains the lowest temperature in the present embodiment. 
     On the other hand, the first vapor stream discharged from the first expander  140   b  flows into the second heat exchanger  170   b  along a ninth flow pass  125   b.    
     The first vapor stream that has passed through the second heat exchanger  170   b  flows into the distillation tower  160   b  along a tenth flow pass  127   b  at an upper position of the distillation tower  160   b  compared to the position of the second vapor stream that has passed through the sixth flow pass  124   b . In other words, the tenth flow pass  127   b  is connected to the distillation tower  160   b  at an upper position compared to the position of the sixth flow pass  124   b.    
     This is to reduce cold energy input to cool the overhead vapor stream by sequentially cooling the overhead vapor stream that is vaporized in the distillation tower  160   b  and then discharged from the top of the distillation tower  160   b  by using the second vapor stream that flows into the distillation tower  160   b  along the sixth flow pass  124   b  and the first vapor stream that flows into the distillation tower  160   b  along the tenth flow pass  127   b.    
     As such, the liquid stream, the first vapor stream, and the second vapor stream that flow into the distillation tower  160   b  are heated and vaporized by a reboiler  164   b  provided adjacent to the distillation tower  160   b  along a circulation flow pass provided in a lower portion of the distillation tower  160   b , and then flow again into the distillation tower  160   b.    
     The distillation tower  160   b  separates a raw natural gas into the methane-reinforced overhead vapor stream and the component-reinforced lower stream that is heavier than methane. 
     The overhead vapor stream is discharged from the top of the distillation tower  160   b  and cooled by passing through the first heat exchanger  130   b  along an eleventh flow pass  161   b , and is discharged from the first heat exchanger  130   b  along a twelfth flow pass  163   b  to flow into the second heat exchanger  170   b.    
     The second heat exchanger  170   b  exchanges heat among the raw natural gas that underwent the pre-treatment process, the overhead vapor stream that flows in along the twelfth flow pass  163   b , and the first vapor stream that flows in along the ninth flow pass  125   b . As described above, the natural gas that has passed through the second heat exchanger  170   b  is condensed and discharged along the second flow pass  102   b.    
     The first vapor stream that flows in along the ninth flow pass  125   b  functions as a coolant in the second heat exchanger  170   b . Accordingly, the natural gas that flows in along the first flow pass  101   b  and the overhead vapor stream that flows in along the twelfth flow pass  163   b  both are condensed and discharged from the second heat exchanger  170   b . The first vapor stream that flows in along the ninth flow pass  125   b  is heated and discharged from the second heat exchanger  170   b.    
     As such, since the overhead vapor stream that has passed through the second heat exchanger  170   b  is cooled and discharged from the second heat exchanger  170   b , the energy used by the compressor  180   b  to condense and cool the overhead vapor stream may be reduced. Also, since the first vapor stream that has sequentially passed through the first expander  140   b  and the second heat exchanger  170   b  has an increased temperature and then flows into the distillation tower  160   b , the energy used in the distillation tower  160   b  may be reduced. 
     The overhead vapor stream discharged from the second heat exchanger  170   b  flows into the compressor  180   b  along a thirteenth flow pass  165   b  to be compressed and condensed therein, and then flows along a fourteenth flow pass  167   b  and is stored in a reservoir (not shown). 
     On the other hand, the component-reinforced lower stream that is heavier than methane, which is discharged from the distillation tower  160   b , may flow along a discharge flow pass  162   b  connected to the lower portion of the distillation tower  160   b  and may be stored outside. 
     In the following description, a natural gas fractional distillation apparatus  100   c  according to another embodiment of the present invention is described. 
       FIG. 4  is a view illustrating a configuration of a natural gas fractional distillation apparatus  100   c  according to another embodiment of the present invention. 
     Referring to  FIG. 4 , a natural gas fractional distillation apparatus  100   c  according to the present embodiment includes a gas-liquid separator  110   c  into which a condensed natural gas flows and which separates the condensed natural gas into a vapor stream and a liquid stream, a gas separator  120   c  which separates the vapor stream separated in the gas-liquid separator  110   c  into a first vapor stream and a second vapor stream, a first heat exchanger  130   c  which condenses the first vapor stream separated in the gas separator  120   c , a first expander  140   c  which expands the first vapor stream condensed in the first heat exchanger  130   c , a second expander  150   c  which expands the second vapor separated in the gas separator  120   c , a distillation tower  160   c  into which the liquid stream separated in the gas-liquid separator  110   c , the first vapor stream expanded in the first expander  140   c , and the second vapor stream expanded in the second expander  150   c  flow in order to be divided into an overhead vapor stream containing methane and the component-reinforced lower stream that is heavier than methane, a second heat exchanger  170   c  which discharges a condensed natural gas by exchanging heat between the overhead vapor stream discharged from the first heat exchanger  130   c  after the heat exchange with the first vapor stream in the first heat exchanger  130   c , the second vapor stream expanded in the second expander  150   c , and the natural gas, and a compressor  180   c  which compresses the overhead vapor stream discharged after the heat exchange in the second heat exchanger  170   c.    
     In the natural gas fractional distillation apparatus  100   c  according to the present embodiment, the second vapor stream decompressed and cooled in the second expander  150   c  is used as a coolant of the second heat exchanger  170   c  and thus energy applied to the distillation tower  160   c  and energy used in the compressor  180   c  which condenses and cools the overhead vapor stream discharged from the distillation tower  160   c  may be reduced. 
     After having passed through a pre-treatment process, the natural gas flows into the second heat exchanger  170   c  along a first flow pass  101   c . The second heat exchanger  170   c  changes the state of natural gas into a condensed natural gas by condensing the natural gas in a gaseous state after having passed through the pre-treatment process. 
     The second heat exchanger  170   c  exchanges heat between the overhead vapor stream that is cooled after having sequentially passed through the distillation tower  160   c  and the first heat exchanger  130   c  and the second vapor stream that is decompressed and cooled by passing through the second expander  150   c.    
     The natural gas condensed in the second heat exchanger  170   c  is discharged along a second flow pass  102   c . The condensed natural gas flows into the gas-liquid separator  110   c  along the second flow pass  102   c . The gas-liquid separator  110   c  separates the condensed natural gas into a vapor stream in a gaseous state and a liquid stream in a liquefied state. 
     The liquid stream flows into a lower position of the distillation tower  160   c  along a third flow pass  113   c.    
     The vapor stream in the gaseous state separated from the gas-liquid separator  110   c  flows into the gas separator  120   c  along a fourth flow pass  111   c . The gas separator  120   c  separates the vapor stream into a first vapor stream and a second vapor stream according to a preset ratio. 
     The first vapor stream and the second vapor stream may be separated in a ratio of 2:8 to 1:9. This to reduce energy used by the compressor  180   c  to condense and cool the overhead vapor stream and improve energy efficiency of the whole natural gas fractional distillation apparatus  100   c  by separating the vapor stream flowing into the distillation tower  160   c  into the first vapor stream and the second vapor stream in the above ratio and cooling the overhead vapor stream discharged from the distillation tower  160   c  by using the second vapor stream. 
     The first vapor stream separated in the gas separator  120   c  flows into the first heat exchanger  130   c  along a seventh flow pass  121   c . The first vapor stream discharged from the first heat exchanger  130   c  flows into the first expander  140   c  along an eighth flow pass  123   c.    
     The first vapor stream rapidly expands and is decompressed in the first expander  140   c  and the temperature of the first vapor stream is rapidly lowered. Accordingly, the first vapor stream may be changed from a gaseous state to a liquefied state or may exist in both gaseous and liquefied states. The first vapor stream that has passed through the first expander  140   c  maintains the lowest temperature in the present embodiment. 
     The first vapor stream discharged from the first expander  140   c  flows into the distillation tower  160   c  along a ninth flow pass  125   c , at an upper position compared to the position of the second vapor stream that has passed through a tenth flow pass  126   c . In other words, the ninth flow pass  125   c  is connected to the distillation tower  160   c  at an upper position compared to the position of the tenth flow pass  126   c.    
     This is to reduce cold energy input to cool the overhead vapor stream by sequentially cooling the overhead vapor stream that is vaporized in the distillation tower  160   c  and then discharged from the top of the distillation tower  160   c  by using the second vapor stream that flows into the distillation tower  160   c  along the tenth flow pass  126   c  and the first vapor stream that flows into the distillation tower  160   c  along the ninth flow pass  125   c.    
     The second vapor stream separated in the gas separator  120   c  flows into the second expander  150   c  along the fifth flow pass  122   c . The second expander  150   c  expands the second vapor stream and lowers the temperature of the second vapor stream. As the temperature of the second vapor stream is lowered, the second vapor stream expanded in the second expander  150   c  may be changed from a gaseous state to a liquefied state or may exist in both gaseous and liquefied states. 
     On the other hand, the second vapor stream that has passed through the second expander  150   c  flows into the second heat exchanger  170   c  along a sixth flow pass  124   c.    
     The second vapor stream that has passed through the second heat exchanger  170   c  flows into the distillation tower  160   c  along the tenth flow pass  126   c  at a lower position of the distillation tower  160   c  compared to the position of the first vapor stream that has passed through the ninth flow pass  125   c . In other words, the tenth flow pass  126   c  is connected to the distillation tower  160   c  at a lower position compared to the position of the ninth flow pass  125   c.    
     This is to reduce cold energy input to cool the overhead vapor stream by sequentially cooling the overhead vapor stream that is vaporized in the distillation tower  160   c  and then discharged from the top of the distillation tower  160   c  by using the second vapor stream that flows into the distillation tower  160   c  along the tenth flow pass  126   c  and the first vapor stream that flows into the distillation tower  160   c  along the ninth flow pass  125   c.    
     As such, the liquid stream, the first vapor stream, and the second vapor stream that flow into the distillation tower  160   c  are heated and vaporized by a reboiler  164   c  provided adjacent to the distillation tower  160   c  along a circulation flow pass provided in a lower portion of the distillation tower  160   c , and then flow again into the distillation tower  160   c.    
     The distillation tower  160   c  separates a raw natural gas into the methane-reinforced overhead vapor stream and the component-reinforced lower stream that is heavier than methane. 
     The overhead vapor stream is discharged from the top of the distillation tower  160   c  and cooled by passing through the first heat exchanger  130   c  along an eleventh flow pass  161   c , and is discharged from the first heat exchanger  130   c  along a twelfth flow pass  163   c  to flow into the second heat exchanger  170   c.    
     The second heat exchanger  170   c  exchanges heat among the raw natural gas that underwent the pre-treatment process, the overhead vapor stream that flows in along the twelfth flow pass  163   c , and the second vapor stream that flows in along the sixth flow pass  124   c . As described above, the natural gas that has passed through the second heat exchanger  170   c  is condensed and discharged along the second flow pass  102   c.    
     The second vapor stream that flows in along the sixth flow pass  124   c  functions as a coolant in the second heat exchanger  170   c . Accordingly, the natural gas that flows in along the first flow pass  101   c  and the overhead vapor stream that flows in along the twelfth flow pass  163   c  both are condensed and discharged from the second heat exchanger  170   c . The second vapor stream that flows in along the sixth flow pass  124   c  is heated and discharged from the second heat exchanger  170   c.    
     As such, since the overhead vapor stream that has passed through the second heat exchanger  170   c  is cooled and discharged from the second heat exchanger  170   c , the energy used by the compressor  180   c  to condense and cool the overhead vapor stream may be reduced. Also, since the second vapor stream that has sequentially passed through the second expander  150   c  and the second heat exchanger  170   c  has an increased temperature and then flows into the distillation tower  160   c , the energy used in the distillation tower  160   c  may be reduced. 
     The overhead vapor stream discharged from the second heat exchanger  170   c  flows into the compressor  180   c  along a thirteenth flow pass  165   c  to be compressed and condensed therein, and then flows along a fourteenth flow pass  167   c  and is stored in a reservoir (not shown). 
     On the other hand, the component-reinforced lower stream that is heavier than methane, which is discharged from the distillation tower  160   c , may flow along a discharge flow pass  162   c  connected to the lower portion of the distillation tower  160   c  and may be stored outside. 
     In the following description, a natural gas fractional distillation apparatus  100   d  according to another embodiment of the present invention is described. 
       FIG. 5  is a view illustrating a configuration of the natural gas fractional distillation apparatus  100   d  according to another embodiment of the present invention. 
     Referring to  FIG. 5 , the natural gas fractional distillation apparatus  100   d  according to the present embodiment includes a gas-liquid separator  110   d  into which a condensed natural gas flows and which separates the condensed natural gas into a vapor stream and a liquid stream, a gas separator  120   d  which separates the vapor stream separated in the gas-liquid separator  110   d  into a first vapor stream and a second vapor stream, a first heat exchanger  130   d  which condenses the first vapor stream separated in the gas separator  120   d , a first expander  140   d  which expands the first vapor stream condensed in the first heat exchanger  130   d , a third heat exchanger  190   d  which heats the first vapor stream expanded in the first expander  140   d , a second expander  150   d  which expands the second vapor separated in the gas separator  120   d , a distillation tower  160   d  into which the liquid stream separated in the gas-liquid separator  110   d , the first vapor stream heated in the third heat exchanger  190   d , and the second vapor stream expanded in the second expander  150   d  flow in order to be divided into an overhead vapor stream containing methane and the component-reinforced lower stream that is heavier than methane, a second heat exchanger  170   d  which discharges a condensed natural gas by exchanging heat between the overhead vapor stream discharged from the third heat exchanger  190   d  after the heat exchange in the third heat exchanger  190   d  and the natural gas, and a compressor  180   d  which compresses the overhead vapor stream discharged after the heat exchange in the second heat exchanger  170   d.    
     In the natural gas fractional distillation apparatus  100   d  according to the present embodiment, since the third heat exchanger  190   d  for exchanging heat between the overhead vapor stream discharged from the first heat exchanger  130   d  and the first vapor stream discharged from the first expander  140   d  is further provided, energy applied to the distillation tower  160   d  and energy used in the compressor  180   d  which condenses and cools the overhead vapor stream discharged from the distillation tower  160   d  may be reduced. 
     After having passed through a pre-treatment process, the natural gas flows into the second heat exchanger  170   d  along a first flow pass  101   d . The second heat exchanger  170   d  changes the state of natural gas into a condensed natural gas by condensing the natural gas in a gaseous state after having passed through the pre-treatment process. 
     The natural gas condensed in the second heat exchanger  170   d  is discharged along a second flow pass  102   d . In the second heat exchanger  170   d , the natural gas exchanges heat with the overhead vapor stream that has been cooled by sequentially passing through the distillation tower  160   d , the first heat exchanger  130   d , and the third heat exchanger  190   d.    
     The condensed natural gas flows into the gas-liquid separator  110   d  along the second flow pass  102   d . The gas-liquid separator  110   d  separates the condensed natural gas into a vapor stream in a gaseous state and a liquid stream in a liquefied state. 
     The liquid stream flows into a lower position of the distillation tower  160   d  along a third flow pass  113   d.    
     The vapor stream in the gaseous state separated from the gas-liquid separator  110   d  flows into the gas separator  120   d  along a fourth flow pass  111   d . The gas separator  120   d  separates the vapor stream into a first vapor stream and a second vapor stream according to a preset ratio. 
     The first vapor stream and the second vapor stream may be separated in a ratio of 2:8 to 1:9. This to reduce energy used by the compressor  180   d  to condense and cool the overhead vapor stream and improve energy efficiency of the whole natural gas fractional distillation apparatus  100   d  by separating the vapor stream flowing into the distillation tower  160   d  into the first vapor stream and the second vapor stream in the above ratio and cooling the overhead vapor stream discharged from the distillation tower  160   d  by using the first vapor stream. 
     The second vapor stream separated in the gas separator  120   d  flows into the second heat exchanger  150   d  along a fifth flow pass  122   d . The second expander  150   d  expands the second vapor stream and lowers the temperature of the second vapor stream. 
     As the temperature of the second vapor stream that has passed through the second expander  150   d  is lowered, the second vapor stream may be changed from a gaseous state to a liquefied state or may exist in both gaseous and liquefied states. 
     After passing through the second expander  150   d , the second vapor stream flows into the distillation tower  160   d  along a sixth flow pass  124   d . The sixth flow pass  124   d  is connected to the distillation tower  160   d  at an upper position compared to the position of the third flow pass  113   d  such that the second vapor stream that has passed through the sixth flow pass  124   d  flows into the distillation tower  160   d  at an upper position compared to the position of the liquid stream that has passed through the third flow pass  113   d.    
     This is to reduce cold energy input to cool the overhead vapor stream by cooling the overhead vapor stream that is vaporized in the distillation tower  160   d  and then discharged from the top of the distillation tower  160   d  by using the second vapor stream that flows into the distillation tower  160   d  along the sixth flow pass  124   d.    
     On the other hand, the first vapor stream flows into the first heat exchanger  130   d  along a seventh flow pass  121   d , and then, the first vapor stream discharged from the first heat exchanger  130   d  flows into the first expander  140   d  along an eighth flow pass  123   d.    
     The first vapor stream rapidly expands and is decompressed in the first expander  140   d  and the temperature of the first vapor stream is rapidly lowered. Accordingly, the first vapor stream may be changed from a gaseous state to a liquefied state or may exist in both gaseous and liquefied states. The first vapor stream that has passed through the first expander  140   d  maintains the lowest temperature in the present embodiment. 
     The first vapor stream discharged from the first expander  140   d  flows into the third heat exchanger  190   d  along a ninth flow pass  125   d . The overhead vapor stream discharged from the top of the distillation tower  160   d  along an eleventh flow pass  161   d  and a twelfth flow pass  163   d  flows into the third heat exchanger  190   d.    
     The first heat exchanger  130   d  exchanges heat between the first vapor stream that flows in along the seventh flow pass  121   d  and the overhead vapor stream that flows in along the eleventh flow pass  162   d.    
     The first vapor stream that flows in along the seventh flow pass  121   d  functions as a coolant in the first heat exchanger  130   d . Accordingly, the first vapor stream that flows in along the seventh flow pass  121   d  is heated and discharged from the first heat exchanger  130   d , and the overhead vapor stream that flows in along the eleventh flow pass  161   d  is condensed and discharged from the first heat exchanger  130   d.    
     As such, since the overhead vapor stream that has passed through the first heat exchanger  130   d  is cooled and discharged from the first heat exchanger  130   d , the energy used by the compressor  180   d  to condense and cool the overhead vapor stream may be reduced. 
     The first vapor stream that has passed through the first heat exchanger  130   d  flows in the first expander  140   d  along the eighth flow pass  123   d . The first vapor stream rapidly expands and is decompressed in the first expander  140   d  and the temperature of the first vapor stream is rapidly lowered. Accordingly, the first vapor stream may be changed from a gaseous state to a liquefied state or may exist in both gaseous and liquefied states. The first vapor stream that has passed through the first expander  140   d  maintains the lowest temperature in the present embodiment. 
     The first vapor stream discharged from the first expander  140   d  flows into the third heat exchanger  190   d  along the ninth flow pass  125   d . The overhead vapor stream discharged from the first heat exchanger  130   d  flows into the third heat exchanger  190   d  along the twelfth flow pass  163   d.    
     The third head exchanger  190   d  exchanges heat between the first vapor stream that flows in along the ninth flow pass  125   d  and the overhead vapor stream that flows in along the twelfth flow pass  163   d.    
     The first vapor stream that flows in along the ninth flow pass  125   d  functions as a coolant in the third heat exchanger  190   d . Accordingly, the overhead vapor stream that flows in along the twelfth flow pass  163   d  is condensed and discharged from the third heat exchanger  190   d , and the first vapor stream that flows in along the ninth flow pass  125   d  is heated and discharged from the third heat exchanger  190   d.    
     As such, since the overhead vapor stream that has passed through the third heat exchanger  190   d  is cooled and discharged from the first heat exchanger  130   d , the energy used by the compressor  180   d  to condense and cool the overhead vapor stream may be reduced. Also, since the first vapor stream that has passed through the first expander  160   d  and the third heat exchanger  190   d  has an increase temperature and flows into the distillation tower  160   d , the energy used in the distillation tower  160   d  may be reduced. 
     The first vapor stream that has passed through the third heat exchanger  190   d  flows into the distillation tower  160   d  along a tenth flow pass  127   d , at an upper position compared to the position of the second vapor stream that has passed through the sixth flow pass  124   d . In other words, the tenth flow pass  127   d  is connected to the distillation tower  160   d  at an upper position compared to the position of the sixth flow pass  124   d.    
     This is to reduce cold energy input to cool the overhead vapor stream by sequentially cooling the overhead vapor stream that is vaporized in the distillation tower  160   d  and then discharged from the top of the distillation tower  160   d  by using the second vapor stream that flows into the distillation tower  160   d  along the sixth flow pass  124   d  and the first vapor stream that flows into the distillation tower  160   d  along the tenth flow pass  127   d.    
     As such, the liquid stream, the first vapor stream, and the second vapor stream that flow into the distillation tower  160   d  are heated and vaporized by a reboiler  164   d  provided adjacent to the distillation tower  160   d  along a circulation flow pass provided in a lower portion of the distillation tower  160   d , and then flow again into the distillation tower  160   d.    
     The distillation tower  160   d  separates a raw natural gas into the methane-reinforced overhead vapor stream and the component-reinforced lower stream that is heavier than methane. 
     The overhead vapor stream is discharged from the top of the distillation tower  160   d  and cooled by passing through the first heat exchanger  130   d  along the eleventh flow pass  161   d , and also cooled by passing through the third heat exchanger  190   d  along the twelfth flow pass  163   d , and then, flows into the second heat exchanger  170   d.    
     The second heat exchanger  170   d  exchanges heat between the raw natural gas that underwent the pre-treatment process and the overhead vapor stream that flows in along a fifteenth flow pass  165   d . As described above, the natural gas that has passed through the second heat exchanger  170   d  is condensed and discharged along the second flow pass  102   d.    
     The overhead vapor stream discharged from the second heat exchanger  170   d  flows into the compressor  180   d  along a thirteenth flow pass  167   d  to be compressed and condensed therein, and then flows along a fourteenth flow pass  169   d  and is stored in a reservoir (not shown). 
     On the other hand, the component-reinforced lower stream that is heavier than methane, which is discharged from the distillation tower  160   d , may flow along a discharge flow pass  162   d  connected to the lower portion of the distillation tower  160   d  and may be stored outside. 
     While the present invention has been particularly shown and described with reference to preferred embodiments using specific terminologies, the embodiments and terminologies should be considered in descriptive sense only and not for purposes of limitation. Therefore, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. 
     INDUSTRIAL APPLICABILITY 
     The present invention may improve energy efficiency by reducing energy consumed in dividing a natural gas that is a raw material.