1 [DESCRIPTION] [Invention Title] NATURAL GAS LIQUEFACTION PROCESS [Technical Field] 5 The present invention relates to a natural gas liquefaction process, and more particularly, to a natural gas liquefaction process capable of having excellent efficiency, decreasing the number of equipments, simplifying the structure of the liquefaction system, and 10 easily operating the liquefaction system by configuring a pre-cooling cycle so that the advantages of a pure refrigerant cycle and a mixed refrigerant cycle may be taken at the same time. [Background Art] 15 A thermodynamic process of liquefying natural gas to produce liquefied natural gas (LNG) has been developed since the 1970s in order to satisfy demands for higher efficiency, larger capacity, simpler equipment, and so on. In order to satisfy these demands, various attempts to 20 liquefy natural gas using different refrigerants or different cycles have been continuously conducted up to now. However, the number of liquefaction processes that are practically under current operation is very small. One of the liquefaction processes that are being 25 operated and have been most widely spread is a 'propane 2 pre-cooled mixed refrigerant process (or a C3/MR process)'. A basic structure of the C3/MR process is shown in FIG. 9. For reference, in FIG. 9, and the like, 'C3' indicates a propane refrigerant cycle, and 'MR' indicates a mixed 5 refrigerant cycle. Further, in FIG. 9, and the like, 'C' indicates a compressor, 'AC' indicates an after-cooler, 'V' indicates a valve, and 'HX' indicates a heat exchanger. As shown in FIG. 9, the feed gas is pre-cooled down to approximately 240 K by a multi-stage of propane (C3) 10 cooling cycle. The pre-cooled feed gas is condensed and sub-cooled down to approximately 113 K by a mixed refrigerant cycle, that is, by heat exchange with mixed refrigerant (MR) in a heat exchanger. Also in this C3/MR process, general features of a pure refrigerant cycle and a 15 mixed refrigerant cycle appear as they are. In pure refrigerant cycles, a structure is simple and an operation is easy, but a large number of refrigeration stages are required. On the contrary, in mixed refrigerant cycles, a structure is complicated and an operation is difficult, but 20 high efficiency may be obtained only with a small number of components. These features of the respective cycles appear as they are also in the C3/MR process. More specifically, in the mixed refrigerant cycle of the C3/MR process of liquefying (and sub-cooling) the pre 25 cooled feed gas, the mixed refrigerant composed of nitrogen, 3 methane, ethane, and propane, is generally used. In the mixed refrigerant cycle of the C3/MR process, high efficiency may be obtained using only a small number of equipments by appropriately selected compositions of these 5 components, appropriately separated mixed refrigerant into a gas-phase refrigerant portion and a liquid-phase refrigerant portion according to a difference in boiling points among the respective components, and then liquefying the natural gas through the respective refrigerant portions. 10 On the other hand, in the pure refrigerant cycle of the C3/MR process of pre-cooling the feed gas, since a pure refrigerant such as propane is used, the structure is simple and the operation is easy, but three or four pressure steps are required so that a large number of 15 compressors, and the like, are required. As a result, in the case of the C3/MR process, the pre-cooling cycle may be considered as being focused on simplicity (even though the number of equipments is many, the structure itself is simple), and the liquefying cycle may be considered as 20 being focused on efficiency (even though the number of equipments is small, the structure itself is complicated and efficiency is excellent). One of the other successful liquefaction processes that are being operated is a 'dual mixed refrigerant 25 process (or a DMR process)' . The basic structure of the 4 DMR process is shown in FIG. 10. As shown in FIG. 10, a liquefying (sub-cooling) cycle of the DMR is basically the same as the liquefying (sub-cooling) cycle of the C3/MR process. However, in the DRM process, another mixed 5 refrigerant cycle is used in order to pre-cool the feed gas unlike the C3/MR process. In the pre-cooling cycle of the DMR process, a gas-liquid separator is not generally present unlike the liquefying cycle in the DMR process. As a result, in the case of the DMR process, both of the pre 10 cooling cycle and the liquefying cycle may be considered as being focused on efficiency. However, it has been known that the efficiency of the DMR process is slightly smaller than that of the C3/MR process. As described above, it has been known that the 15 efficiency of the mixed refrigerant cycle is higher than that of the pure refrigerant cycle. However, the structure itself of the mixed refrigerant cycle is more complicated than that of the pure refrigerant cycle. In addition, many schemes of applying the mixed refrigerant cycle to a 20 cooling cycle for liquefying (sub-cooling) the pre-cooled natural gas to increase the efficiency of the entire liquefaction process have been suggested. Therefore, many studies on a pre-cooling cycle having a simple structure and excellent efficiency have been demanded. 25 [Disclosure] 5 [Technical Problem] Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art while advantages achieved by the prior art are 5 maintained intact. One objective to be achieved by the present invention is to provide a natural gas liquefaction process capable of having excellent efficiency, decreasing the number of equipments of a liquefaction system, simplifying a 10 structure of the liquefaction system, and easily operating the liquefaction system by configuring a pre-cooling cycle so that advantages of both pure refrigerant cycle and mixed refrigerant cycle may be taken. [Technical Solution] 15 In one aspect of the present invention, there is provided a natural gas liquefaction process of pre-cooling natural gas using a closed loop pre-cooling cycle and liquefying the pre-cooled natural gas using a closed loop liquefying cycle, wherein the closed loop pre-cooling cycle 20 includes two pre-cooling cycles for pre-cooling supplied natural gases together in the same first heat exchange region through the respective pure refrigerants, and the closed loop liquefying cycle includes at least one liquefying cycle for liquefying the pre-cooled natural gas 25 through a mixed refrigerant, the two pre-cooling cycles 6 being a closed circuit cooling cycle. The pure refrigerant of the first pre-cooling cycle may be ethane (C2), and the pure refrigerant of the second pre-cooling cycle may be butane (C4). Each pre-cooling 5 cycle may include a step of compressing the pure refrigerant, a step of cooling the compressed refrigerant, a step of additionally cooling the cooled refrigerant in the first heat exchange region, and a step of expanding the additionally cooled refrigerant. 10 The closed loop liquefying cycle may include a step of compressing the mixed refrigerant, a step of cooling the compressed refrigerant, a step of additionally cooling the cooled refrigerant in the first heat exchange region to partially condense the cooled refrigerant, a step of 15 separating the partially condensed refrigerant into a liquid-phase refrigerant portion and a gas-phase refrigerant portion according to a difference in boiling points, a step of primarily cooling the pre-cooled natural gas in a second heat exchange region using the liquid-phase 20 refrigerant portion, and a step of secondarily cooling the primarily cooled natural gas in a third heat-exchange region using the gas-phase refrigerant portion. The step of primarily cooling the pre-cooled natural gas may include a first step of cooling the liquid-phase 25 refrigerant portion through heat exchange in the second 7 heat exchange region, a second step of expanding the refrigerant portion cooled in the first step, and a third step of heat-exchanging the refrigerant portion expanded in the second step and the natural gas in with each other the 5 second heat exchange region to cool the natural gas. The step of secondarily cooling the primarily cooled natural gas may include a cooling step of cooling the gas-phase refrigerant portion through heat exchange in the second heat exchange region, a condensing step of condensing the 10 refrigerant portion cooled in the cooling step through heat exchange in the third heat exchange region, an expanding step of expanding the refrigerant portion condensed in the condensing step, and a step of heat-exchanging the refrigerant portion expanded in the expanding step and the 15 natural gas with each other in the third heat exchange region to cool the natural gas. [Advantageous Effects] In the natural gas liquefaction process according to an exemplary embodiment of the present invention, since the 20 pre-cooling cycles pre-cool the natural gas only with a single pressure step, they may be configured of only a relatively small number of equipments. In addition, in the natural gas liquefaction process according to an exemplary embodiment of the present 25 invention, since the respective pre-cooling cycles use the 8 pure refrigerant, the structure itself thereof is simple, and the operation is easy. Furthermore, in the natural gas liquefaction process according to an exemplary embodiment of the present 5 invention, since two pre-cooling cycles are disposed in parallel with each other to pre-cool the natural gas in the same heat exchange region, the efficiency of the liquefaction process is excellent. [Description of Drawings] 10 FIG. 1 is a flow chart showing the natural gas liquefaction process according to an exemplary embodiment of the present invention; FIG. 2 is a graph showing the temperature profile in a pre-cooling region of a C3/MR process according to the 15 prior art; FIG. 3 is a graph showing the temperature profile in a pre-cooling region of a DMR process according to the prior art; FIG. 4 is a graph showing the temperature profile in a 20 pre-cooling region of a liquefaction process according to the exemplary embodiment of the present invention; FIG. S is a temperature-entropy diagram of ethane and butane cycles in the liquefaction process according to the exemplary embodiment of the present invention; 25 FIGS. 6 to 8 are graphs showing exergy utilization and 9 irreversibility in a pre-cooling step of the C3/MR process according to the prior art, the DMR process according to the prior art, and the liquefaction process according to the exemplary embodiment of the present invention, 5 respectively; FIG. 9 is a flow chart showing the C3/MR process according to the prior art; and FIG. 10 is a flow chart conceptually showing the DMR process according to the prior art. 10 [Best Model Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to these exemplary embodiments. For reference, 15 the same reference numerals will be used to describe substantially the same components. Under this rule, a description may be provided while citing a content shown in other drawings. A content well-known to those skilled in the art to which the present invention pertains or a 20 repeated content may be omitted. FIG. 1 is a flow chart showing a natural gas liquefaction process according to an exemplary embodiment of the present invention. The liquefaction process according to the present embodiment may be applied to a 25 natural gas liquefaction process of pre-cooling natural gas 10 using a closed loop pre-cooling cycle and liquefying the pre-cooled natural gas using a closed loop liquefying cycle, as shown in FIG. 1. In addition, the liquefaction process according to the present embodiment may further include a 5 cooling cycle of further cooling a mixed refrigerant or further cooling the natural gas. Hereinafter, the liquefaction process according to the exemplary embodiment of the present invention will be described with reference to FIG. 1. The liquefaction 10 process according to the present embodiment basically includes a closed loop cooling cycle of pre-cooling supplied natural gas and a closed loop cooling cycle of liquefying (or liquefying and sub-cooling) the pre-cooled natural gas. Since both of the cooling cycles according to 15 the present embodiment are the closed loop cycles, the two respective cycles are independently performed as one closed cycle while having a compressing step, a condensing step, an expanding step, and an evaporating step. In addition, the closed loop cooling cycle for pre-cooling the supplied 20 natural gas, that is, the closed loop pre-cooling cycle includes two separate cycles. These two pre-cooling cycles are also closed loop cooling cycles. The liquefaction process according to the present embodiment will be described in detail. As shown in FIG. 1, 25 the supplied natural gas is pre-cooled by the first pre- 11 cooling cycle and the second pre-cooling cycle in the first heat exchange region 110 (as described below, in the liquefaction process according to the present embodiment, the first pre-cooling cycle is an ethane (C2) cycle and the 5 second pre-cooling cycle is a butane (C4) cycle). That is, the supplied natural gas is pre-cooled by a pure refrigerant of the first pre-cooling cycle and a pure refrigerant of the second pre-cooling cycle in the same heat exchange region 110. To this end, each of the first 10 pre-cooling cycle and the second pre-cooling cycle has a compressing step, a condensing step, an expanding step, and an evaporating step of a refrigerant. In the first and second pre-cooling cycle as described above, the pure refrigerants are first introduced into 15 compressors 151 and 161 through conduits 201 and 401 to thereby be compressed. Then, the pure refrigerants are introduced into coolers 152 and 162 through conduits 202 and 402 to thereby be cooled. The above-mentioned compressing and after-cooling process may be performed in a 20 multi-stage as shown in FIG. 1. That is, a plurality of compressors and after-coolers may be connected in series with each other. In this case, the cooled pure refrigerants may be again introduced into compressors 153 and 163 through conduits 203 and 403 to thereby be 25 compressed, and the pure refrigerants compressed as 12 described above may be again introduced into coolers 154 and 164 through conduits 204 and 404 to thereby be cooled. When the compressor is configured as the multi-stage to compress the pure refrigerants in the multi-stage, required 5 input power of the compressor may be decreased. In addition, the pure refrigerants compressed and cooled as described above may be introduced into the first heat exchange region 110 through conduits 205 and 405 and heat-exchanged with a fed-back refrigerant to thereby be 10 further cooled. The pure refrigerants may be condensed through the above-mentioned process. However, as described below, according to boiling points of the pure refrigerant, the pure refrigerants may also be condensed by cooling by the above-mentioned cooler. In this case, the pure 15 refrigerants in the condensed state may be introduced into the first heat exchange region 110 to thereby be further cooled. The cooling of the refrigerants in the first heat exchange region 110 as described above may be performed by a refrigerant again introduced into the heat exchange 20 region 110 through conduits 207 and 407. That is, the pure refrigerants cooled through the heat exchange in the first heat exchange region 110 as described above may be introduced into expansion valves 155 and 165 through conduits 206 and 406 to thereby be expanded and cooled and 25 be then introduced again into the first heat exchange 13 region 110 through the conduits 207 and 407 to cool the natural gas and the refrigerant. The above-mentioned processes are similarly performed in the first pre-cooling cycle and the second pre-cooling 5 cycle. That is, as shown in FIG. 1, the first pre-cooling cycle and the second pre-cooling cycle are disposed in parallel with each other while forming a closed loop cycle, thereby complementarily pre-cooling the supplied natural gases in the first heat exchange region 110. As described 10 below, the most important feature in the liquefaction process according to the present embodiment is to dispose two closed loop cooling cycles using the pure refrigerant in parallel with each other as described above to pre-cool the supplied natural gases in the same heat exchange region. 15 In the present embodiment, ethane (C2) and butane (C4) are used as the pure refrigerants of the first and second pre-cooling cycles, respectively. In the C3/MR process described above, a pure refrigerant composed of propane (C3) is used in order to pre-cool the natural gas, and in 20 the DMR process described above, a mixed refrigerant composed of 45.5 mole % of ethane (C2), 4.9 mole % of propane (C3), and 49.6 mole % of butane (C4) is used in order to pre-cool the natural gas. That is, in the C3/MR process described above, since a single pure refrigerant is 25 used, a plurality of pressure levels are required, such 14 that a large number of equipments are required, but a structure itself is simple and an operation of a pre cooling cycle is easy. On the other hand, in the DMR process described above, since the mixed refrigerant is 5 used, a small number of equipments are required, but a structure itself is complicated and an operation of a pre cooling cycle is also difficult. In the liquefaction process according to the present embodiment, the two pure refrigerant cycles disposed in 10 parallel with each other as described above are used so that both of the advantages of two basic structures as described above, that is, the advantages in the case of using the pure refrigerant for the pre-cooling and the case of using the mixed refrigerant for the pre-cooling may be 15 taken. In addition, the two pure refrigerant cycles are configured of an ethane cycle and a butane cycle, thereby making it possible to optimize the entire efficiency of the liquefaction process. The mixed refrigerant in the pre cooling step in the DMR process described above is composed 20 of ethane, propane, and butane components, wherein an amount of propane component included in the mixed refrigerant is very small. In the liquefaction process according to the present embodiment, a pre-cooling effect due to the above-mentioned propane component may also be 25 considered as being replaced by a corresponding effect of 15 the ethane cycle and the butane cycle. The natural gas pre-cooled through the two pure refrigerant cycles as described above is liquefied (or liquefied and sub-cooled) through the mixed refrigerant 5 cycle. More specifically, the mixed refrigerant partially condensed through the heat exchange in the first heat exchange region 110 is introduced into a gas-liquid separator 171 through a conduit 301 to thereby be separated into a first refrigerant portion and a second refrigerant 10 portion having a boiling point lower than that of the first refrigerant portion according to a difference in boiling points. That is, the partially condensed mixed refrigerant may be separated into the first refrigerant portion separated as a liquid-phase refrigerant portion due to a 15 high boiling point and the second refrigerant portion separated as a gas-phase refrigerant portion due to a low boiling point. The separated first refrigerant portion is introduced into a second heat exchange region 120 through a conduit 20 302 to thereby be cooled. The cooling of the refrigerant portion described above may be performed through heat exchange with a refrigerant introduced into the second heat exchange region 120 through a conduit 304. The cooled refrigerant portion is introduced into an expansion valve 25 172 through a conduit 303 to thereby be expanded. The 16 expanded refrigerant portion may be mixed with a second refrigerant portion to be described below and be then introduced again into the second heat exchange region 120 through the conduit 304 to cool other refrigerants and 5 liquefy the natural gas. Then, after the mixed refrigerant is subjected to a series of compressing and after-cooling processes, it may be introduced into the first heat exchange region 110 to thereby be cooled together with the supplied natural gas through the ethane cycle and the 10 butane cycle. In addition, the separated second refrigerant portion is introduced into the second heat exchange region 120 through a conduit 306 to thereby be cooled. The cooling of the refrigerant portion described above may be performed 15 through heat exchange with a refrigerant introduced into the second heat exchange region 120 through a conduit 304. The cooled refrigerant portion is introduced into a third heat exchange region 130 through a conduit 307 to thereby be condensed. The condensing of the refrigerant portion 20 described above may be performed through heat exchange with a refrigerant introduced into the third heat exchange region 130 through a conduit 309. The condensed refrigerant portion is introduced into an expansion valve 173 through a conduit 308 to thereby be expanded. The 25 expanded refrigerant portion is again introduced into the 17 third heat exchange region 130 through the conduit 309 to condense a refrigerant introduced into the third heat exchange region 130 and liquefy or sub-cool the natural gas through the heat exchange. For reference, the liquefied 5 natural gas may be expanded by the expansion valve 181 and be then introduced into a storing tank, or the like. The refrigerant portion of which the heat exchange in the third heat exchange region 130 is finished may be mixed with the above-mentioned first refrigerant portion and be 10 then introduced again into the second heat exchange region 120. For reference, the above-mentioned three heat exchange regions 110, 120, and 130 may be provided together in a single heat exchange unit as shown in FIG. 1 or be individually provided in three heat exchange units. In 15 addition, the heat exchange unit may be a general heat exchanger. An effect of the liquefaction process according to the present embodiment having the above-mentioned configuration will be described with reference to FIGS. 2 to 8. FIGS. 2 20 and 3 show temperature distributions in pre-cooling regions of the above-mentioned C3-MR process and DMR process, respectively. Since propane (C3) in the C3-MR process is a pure refrigerant and is subjected to several process steps, a temperature distribution has a stair shape as shown in 25 FIG. 2. On the other hand, the temperature distribution in 18 the pre-cooling region of the DMR process is gradually changed while showing a minimum difference (3K) in the middle of the heat exchange region. In addition, FIGS. 4 and 5 show a temperature distribution in a pre-cooling 5 region of a liquefaction process according to the present embodiment and temperature-entropy lines of ethane and propane cycles in the liquefaction process according to the present embodiment, respectively. Since the respective pure refrigerants in a pre 10 cooling step of the liquefaction process according to the present embodiment are introduced in a liquid phase into the heat exchange region (See a reference numeral 9 of FIGS. 4 and 5), the temperature of the cold stream has two horizontal regions (See reference numerals 9-10 and 11-12 15 of FIG. 4), corresponding to vaporization of the ethane refrigerant and the butane refrigerant. On the other hand, since the butane refrigerant after the natural gas is pre cooled is condensed while being subjected to a multi-stage of compressing and cooling processes through the 20 compressors and the coolers and is then introduced again in a liquid phase into the heat exchange region (See a reference numeral 5 of FIGS. 4 and 5), the temperature of the hot stream has only one horizontal region (See reference numeral 6-7 of FIG. 4), corresponding to the 25 condensation of the ethane refrigerant.
19 FIGS. 6 to 8 show exergy utilization and irreversibility in a pre-cooling step of the above mentioned three processes, that is, the C3/MR process, the DMR process, and the liquefaction process according to 5 present embodiment, respectively. Exergy efficiencies defined as a ratio of an increase in exergy to power input were 34.3%, 30.5%, and 31.5% in the respective liquefaction processes as shown in FIGS. 6 to 8, respectively. As described above, when considering that the C3/MR process 10 has a disadvantage that a large number of equipments are required since a plurality of pressure steps are required and the DRM process has a disadvantage that a structure itself is complicated and an operation of a liquefaction system is difficult since a mixed refrigerant is used, it 15 may be confirmed that an effect of the liquefaction process according to the present embodiment is excellent. That is, in the liquefaction process according to the present embodiment, since the respective pre-cooling cycles pre-cool the natural gas only with a single pressure step, 20 they may be configured only of a relatively small number of equipments, since the respective pre-cooling cycles use the pure refrigerant, the structure itself thereof is simple, and the operation of the liquefaction system is easy, and since the ethane refrigerant cycle and the butane 25 refrigerant cycle are disposed in parallel with each other 20 to pre-cool the natural gas in the same heat exchange region, the efficiency of the liquefaction process is high. As a result, the liquefaction process according to the present embodiment has not only all of the advantages of 5 the structure of pre-cooling the natural gas using the pure refrigerant and the structure of pre-cooling the natural gas using the mixed refrigerant but also has a high efficiency (the efficiency of the liquefaction process according to the present embodiment is very excellent when 10 considering that the C3/MR process among the liquefaction processes known up to now is one of the processes having very high efficiency). In addition, the irreversibility in the pre-cooling step of the respective liquefaction processes may be 15 represented by four groups of valve V, after-cooler (AC), compressor (C), and heat exchanger (HX) as shown in FIGS. 6 to 8. When comparing the liquefaction process according to the present embodiment with the C3/MR process, it may be confirmed that the irreversibility by the valve is 20 relatively larger in the C3/MR process than in the liquefaction process according to the present embodiment. In addition, when comparing the liquefaction process according to the present embodiment with the DMR process, it may be confirmed that the irreversibility by the after 25 cooler is relatively larger in the DMR process than in the 21 liquefaction process according to the present embodiment. As a result, it may be confirmed that although the irreversibility by the valve and the after-cooler is lower in the liquefaction process according to the present 5 embodiment than in the above-mentioned two liquefaction process, the irreversibility by the heat exchanger is high in the liquefaction process according to the present embodiment than in the above-mentioned two liquefaction processes. 10 Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the 15 invention as disclosed in the accompanying claims. Accordingly, such modifications, additions and substitutions should also be understood to fall within the scope of the present invention. Therefore, the scope and spirit of the present invention should be understood only 20 by the following claims, and all of equivalences and equivalent modifications to the claims are intended to fall within the scope and spirit of the present invention. [Industrial Applicability] The present invention relating to a natural gas 25 liquefaction process in which since the respective pre- 22 cooling cycles pre-cool the natural gas only with a single pressure step, they may be configured only of a relatively small number of equipments, since the respective pre cooling cycles use the pure refrigerant, the structure 5 itself thereof is simple, and the operation of the liquefaction system is easy, and since two pre-cooling cycles are disposed in parallel with each other to pre-cool the natural gas in the same heat exchange region, the efficiency of the liquefaction process is excellent has an 10 industrial applicability.