KR20160015923A - Natural gas liquefaction process - Google Patents

Abstract

The present invention relates to a natural gas liquefaction process, wherein the liquefaction process according to the present invention is characterized in that by using a single closed-loop refrigeration cycle adapting a mixed refrigerant, natural gas is primarily cooled in the first heat exchange zone; natural gas is secondarily cooled in the second heat exchange zone; and natural gas is thirdly cooled in the third heat exchange zone. The closed-loop refrigeration cycle comprises: a condensation step for partially condensing a mixed refrigerant; a first separation step for separating the mixed refrigerant after the condensation step into a first stream and a gaseous second stream; a compression step for compressing the second stream after the first separation step; a second separation step for separating the second stream after the compression step into a third stream and a gaseous fourth stream, a first mixing step for mixing the first stem after the first and second separation steps into the third stream to form the first main stream; a third separation step for separating the first main steam after the first mixing step into liquid phase fifth stream and a gaseous sixth stream; a first cooling step for cooling natural gas in the first heat exchange zone by using the fifth stream after the third separation step; a second cooling step for cooling natural gas in the second heat exchange zone by using the sixth stream after the third separation step; and a third cooling step for cooling natural gas in the third heat exchange zone by using the fourth stream after the second separation step.

Description

NATURAL GAS LIQUEFACTION PROCESS

The present invention relates to a natural gas liquefaction process, and more particularly, to a natural gas liquefaction process having a simple structure of a liquefaction process, an easy operation of a liquefaction process, and an excellent liquefaction process.

Thermodynamic processes for liquefying natural gas and producing liquefied natural gas (LNG) have been developed since the 1970s to meet a variety of challenges, including higher efficiency and greater capacity requirements. Various attempts to liquefy natural gas using different refrigerants or using different cycles to meet these needs, i. E., To increase the efficiency and capacity of the liquefaction process, have continued to this day, but they have been used practically The number of liquefaction processes is very small.

One of the most prevalent and widely used liquefaction processes is the Propane Pre-cooled Mixed Refrigerant Process (or C3 / MR process). As shown in Figure 5, in the C3 / MR process, natural gas (NG) is first cooled to about 238 K through a Joule-Thomson cycle (or propane cycle) employing propane (C3) (pre-cooled). The natural gas is then liquefied and sub-cooled to approximately 123 K through a mixed refrigerant cycle using a mixed refrigerant (MR, Mixed Refrigerant or Multi-component Refrigerant). As described above, since the C3 / MR process uses a refrigeration cycle using a single refrigerant and a refrigeration cycle using a mixed refrigerant, the structure of the liquefaction process is complicated and the operation of the liquefaction process is difficult.

Another of the liquefaction processes in operation is the Cascade process by Conoco Phillips. As shown in Fig. 6, the cascade process by Conoco Phillips consists of three line-Thomson cycles using methane (C1), ethylene (C2) and propane (C3). Since the cascade process uses only a refrigeration cycle employing a single refrigerant, the operation of the liquefaction process is simple and the reliability of the liquefaction process is high. However, the cascade process has the disadvantage that the size of the liquefaction process can not be increased because each of the three refrigeration cycles requires a separate facility (e.g., a heat exchanger).

Another of the liquefaction processes in operation is the 'Single Mixed Refrigerant Process (or SMR process)'. As shown in FIG. 7, in the SMR process, natural gas is liquefied through one closed loop refrigeration cycle employing mixed refrigerant. This SMR process has the advantage that the structure of the liquefaction process is simple. However, the SMR process has a drawback that the efficiency of the liquefaction process is low.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a natural gas liquefaction process which is simple in the structure of a liquefaction process, easy in operation of a liquefaction process, .

The liquefaction process according to the present invention is characterized in that one closed loop refrigeration cycle employing mixed refrigerant is used to cool the natural gas in the first heat exchange zone and secondarily to cool the natural gas in the second heat exchange zone Wherein the closed loop refrigeration cycle comprises a condensing step of partially condensing the mixed refrigerant, a condensing step of partially condensing the mixed refrigerant, a step of condensing the mixed refrigerant in the first liquid phase A first separation step of separating the first stream into a first stream and a second stream of gaseous phase, a compression step of compressing the second stream after the first separation step, a second stream after the compression step into the third stream of liquid phase and the fourth stream of the vapor phase A first separating step of separating the first stream from the first stream and a second separating step of separating the first stream from the first stream, A third separation step of separating the first main stream into a fifth stream of liquid phase and a sixth stream of gaseous phase after the first introduction step, A second cooling step for cooling the natural gas in the second heat exchange zone using the sixth stream after the third separation step and a second cooling step for cooling the natural gas using the fourth stream after the second separation step, And a third cooling step for cooling the natural gas in the third heat exchange zone.

Since the natural gas liquefaction process according to the present invention liquefies natural gas using one closed loop refrigeration cycle, there is an effect that the structure of the liquefaction process is simple and operation of the liquefaction process is easy.

Further, in the natural gas liquefaction process according to the present invention, the mixed stream of the first stream and the third stream is separated into a liquid stream and a gaseous stream, and then each of them is used in a heat exchanger. .

1 is a flowchart showing a natural gas liquefaction process according to a first embodiment of the present invention;
Fig. 2 is a flowchart showing a first modification of the natural gas liquefaction process according to Fig. 1
3 is a flowchart showing a second modification of the natural gas liquefaction process according to FIG.
4 is a flowchart showing a natural gas liquefaction process according to the second embodiment of the present invention.
5 is a flow chart conceptually illustrating a conventional C3 / MR process.
6 is a flow chart conceptually illustrating a conventional cascade process.
7 is a flow chart conceptually illustrating a conventional SMR process.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited or limited by the following examples.

Example  One

1 is a flow chart showing a natural gas liquefaction process according to a first embodiment of the present invention. The liquefaction process according to the first embodiment of the present invention uses a closed loop refrigeration cycle as shown in FIG. 1 to cool natural gas (NG) to a liquefaction temperature to produce liquefied natural gas (LNG ). ≪ / RTI >

In particular, one closed loop refrigeration cycle employing a mixed refrigerant or a multi-component refrigerant is used to cool the natural gas in the first heat exchange zone and to cool the natural gas in the second heat exchange zone, And cooling the natural gas in the third heat exchange zone in the third heat exchange zone. For reference, the liquefaction process according to the present embodiment may further include a refrigeration cycle for cooling the mixed refrigerant or for cooling the natural gas.

Hereinafter, the liquefaction process according to the first embodiment of the present invention will be described in detail with reference to FIG. First, the mixed refrigerant (the third main stream to be described later) is partially condensed (condensation step). For example, the mixed refrigerant may be partially condensed through a series of compression, a series of compression and cooling, or a series of compression, cooling and mixing. By such condensation, the mixed refrigerant includes the liquid phase portion and the vapor phase portion. The mixed refrigerant is then separated into a first stream of liquid phase and a second stream of vapor phase by means of a separation means 111 (first separation step). The separating means 111 may be a conventional vapor-liquid separator. This also applies to other separation means to be described later. For reference, the composition and amount of the streams may vary depending on the temperature or pressure during gas-liquid separation.

The second stream flows into the compression means 140 through the conduit 201 after separation and is compressed (compression step). Here, the compression means 140 may be a conventional compressor, and may be a single compressor. This also applies to other compression means to be described later. The second stream then enters the cooling means 150 through conduit 202 and is cooled. Here, the cooling means 150 may be a water-cooled or air-cooled cooler. This also applies to other cooling means to be described later. The cooling means 150 may be provided when it is necessary to cool the compressed stream. This also applies to other cooling means. Then, the second stream is introduced into the separating means 112 through the conduit 203 to be separated into the liquid third stream and the gaseous fourth stream (second separation step).

The third stream is then mixed with the first stream. That is, the first stream is incorporated into the third stream (first mixing step). Such incorporation can be achieved by connecting one conduit 205 to another conduit 204. Or a separate configuration for incorporation may be employed. Further, the first stream and the third stream may be respectively introduced into the separating means 113, which will be described later, and then mixed in the separating means 113. With this mixing, the first main stream is formed. That is, the first main stream is a stream in which the first stream and the third stream are mixed. Where the third stream can be expanded by the expansion means 134 prior to mixing (expansion step). The expansion means will be described later. For reference, incorporation is a relative concept. That is, depending on the configuration of the conduit, the first stream may be considered to be incorporated into the third stream, and the third stream may be considered to be incorporated into the first stream. The first main stream is separated into a liquid fifth stream and a gaseous sixth stream by the separation means 113 (third separation step).

The fifth stream cools the natural gas in the first heat exchange zone 121 after separation (first cooling step). This allows natural gas to be pre-cooled. For example, the fifth stream is introduced into the first heat exchange zone 121 through the conduit 211 after separation (first inflow step). The fifth stream discharged from the first heat exchange region 121 then flows into the expansion means 131 through the conduit 212 and is expanded (first expansion step). Whereby the temperature of the fifth stream can be lowered. The expansion means may comprise a J-T (Joule-Thomson) valve. For example, the expansion means may comprise a conventional expansion valve. Alternatively, the expansion means may comprise an expander. This also applies to other expansion means to be described later. J-T valves can reduce both the pressure and temperature of the stream through the J-T effect.

The fifth stream is lowered in temperature by expansion and then flows back into the first heat exchange zone 121 through the conduit 213 to precool the natural gas in the first heat exchange zone 121 (precooling step). The fifth stream flowing into the first heat exchange zone 121 through the conduit 213 is passed through the conduit 211 to the fifth stream flowing into the first heat exchange zone 121, The sixth stream introduced into the region 121 and the fourth stream introduced into the first heat exchange region 121 through the conduit 231 may be cooled together with the natural gas. After such cooling, the fifth stream is sent via conduit 214 to the condensation stage. For reference, the conduits 212 and 213 may not pass through the second heat exchange region 122 and the third heat exchange region 123 unlike FIG.

The sixth stream cools the natural gas in the second heat exchange zone 122 after separation (second cooling stage). This allows natural gas to be liquefied. For example, the sixth stream is introduced into the first heat exchange zone 121 via conduit 221 after separation (second inflow step). The sixth stream discharged from the first heat exchange region 121 then flows into the second heat exchange region 122 through the conduit 222 (third inflow step). The sixth stream discharged from the second heat exchange zone 122 then flows into the expansion means 132 through the conduit 223 and is expanded (a second expansion step).

The sixth stream is cooled down to the expansion and then flows back to the second heat exchange zone 122 through the conduit 224 to liquefy the natural gas in the second heat exchange zone 122 (liquefaction step). The sixth stream introduced into the second heat exchange region 122 through the conduit 224 is connected to the sixth stream introduced into the second heat exchange region 122 through the conduit 222 and the sixth stream introduced into the second heat exchange region 122 through the conduit 232. [ The fourth stream entering the heat exchange zone 122 may also be cooled with natural gas. After such cooling, the sixth stream is sent via conduit 226 to the condensation stage.

For reference, the conduits 223 and 224 may not pass through the third heat exchange zone 123 differently from FIG. The sixth stream discharged from the second heat exchange region 122 after the heat exchange may be sent to the condensing stage directly through the conduit 226 but may be introduced into the first heat exchange region 121 through the conduit 225 as shown in FIG. . In this case, the sixth stream can cool the natural gas together with the fifth stream in the first heat exchange region 121. [

The fourth stream cools the natural gas in the third heat exchange zone 123 after separation (third cooling stage). This allows natural gas to be sub-cooled. For example, the fourth stream is introduced into the first heat exchange zone 121 through the conduit 231 after separation (fourth inflow step). The fourth stream discharged from the first heat exchange zone 121 then flows into the second heat exchange zone 122 through the conduit 232 (fifth inflow step). The fourth stream discharged from the second heat exchange zone 122 then flows into the third heat exchange zone 123 through the conduit 233 (sixth inflow step). The fourth stream discharged from the third heat exchange region 123 then flows into the expansion means 133 through the conduit 234 and is expanded (a third expansion step).

After the temperature of the fourth stream is lowered by the expansion, it flows back into the third heat exchange zone 123 through the conduit 235 to subcool the natural gas in the third heat exchange zone 123 (supercooling step). The fourth stream introduced into the third heat exchange region 123 through the conduit 235 can also cool the fourth stream introduced into the third heat exchange region 123 through the conduit 233 together with the natural gas. After such cooling, the fourth stream is sent via conduit 238 to the condensation stage.

For reference, the fourth stream discharged from the third heat exchange region 122 after heat exchange may be sent directly to the condensation stage via the conduit 238, but may be passed through the second heat exchange region 122 through the conduit 236, And may be sent to the first heat exchange zone 121 through the conduit 237. [ In such a case, the fourth stream cools the natural gas together with the sixth stream in the second heat exchange zone 122, and cools the natural gas together with the fifth stream in the first heat exchange zone 121.

For reference, it is preferable that the first to third heat exchange regions 121, 122 and 123 are provided in one PFHE type heat exchanger 120. More specifically, in the case of a natural gas liquefaction process, a heat exchanger of a Plate Fin Heat Exchanger (PFHE) type or a spiral wound heat exchanger (SWHE) type is generally used for heat exchange. Since the PFHE type heat exchanger and the SWHE type heat exchanger have different structures, the liquefaction process based on the SWHE type heat exchanger may not be directly applicable to the liquefaction process using the PFHE type heat exchanger.

The PFHE type heat exchanger may have a plurality of streams to be cooled by other streams in one heat exchanger, and a plurality of streams to cool other streams may also be provided. That is, the PFHE type heat exchanger can provide a plurality of heat exchange areas in one heat exchanger. Where the heat exchange zone refers to the region where heat exchange takes place between two or more streams including the natural gas stream. The PFHE type heat exchanger is advantageous for miniaturization of the liquefaction system. However, the first to third heat exchange regions may be provided separately in a plurality of heat exchangers.

As described above, the liquefaction process according to the present embodiment liquefies natural gas using one closed loop refrigeration cycle. Therefore, the liquefaction process according to the present embodiment is advantageous in that the structure of the liquefaction process is simple and the liquefaction process is easy to operate.

In addition, the liquefaction process according to the present embodiment may be excellent in the efficiency of the liquefaction process for the following reasons. In this embodiment, the third stream is incorporated into the first stream. For such incorporation, the third stream may require a pressure drop prior to incorporation. However, such a pressure drop may partially cause a gas phase portion in the third stream. If such a vapor phase portion flows into the heat exchanger 120 together with the liquid phase portion, it may be difficult to efficiently cool the natural gas in the heat exchanger 120. However, in the liquefaction process according to the present embodiment, since the mixed stream of the first stream and the third stream is separated into the liquid stream and the gaseous stream, and then the mixed stream is utilized in the heat exchanger 120, It can be excellent.

On the other hand, the condensation step can be more specifically described as follows. The fourth stream flows into the compression means 141 through the conduit 238 in the heat exchanger 120 and is compressed (first compression step). The fourth stream then flows into the cooling means 151 through conduit 251 and is cooled.

And the sixth stream is discharged from the heat exchanger 120 and then incorporated into the fifth stream (second mixing step). That is, the sixth stream of conduits 226 is incorporated into the fifth stream of conduits 214. The fifth stream and the sixth stream may be respectively introduced into the compression means 142 to be described later and then mixed in the compression means 142. With this mixing, the second main stream is formed. The second main stream is compressed by the compression means 142 (second compression step). The second main stream then enters the cooling means 152 via conduit 215 and is cooled.

The second main stream of conduit 216 is then incorporated into the fourth stream of conduit 252 (third entraining step). With this mixing, the third main stream is formed. The third main stream is sent to the first separating step.

Meanwhile, the liquefaction process according to the present embodiment can be modified as shown in FIG. 2 is a flow chart showing a first modification of the natural gas liquefaction process according to FIG. As shown in FIG. 2, in the liquefaction process according to the present modification, the fourth stream flows in the heat exchanger 120 through the conduit 238 into the compression means 141 and is compressed (first compression step). The fourth stream then flows into the cooling means 151 through conduit 251 and is cooled.

The fourth stream is then mixed with the sixth stream. That is, the sixth stream of conduit 226 is incorporated into the fourth stream of conduit 252 (second mixing step). The fourth stream and the sixth stream may be respectively introduced into the compression means 143 to be described later, and then mixed in the compression means 143. With this mixing, the second main stream is formed. The second main stream is compressed by the compression means 143 (second compression step). The second main stream then flows into the cooling means 153 through the conduit 253 and is cooled.

And the fifth stream flows into the compression means 142 through the conduit 214 in the heat exchanger 120 and is compressed (third compression step). The fifth stream then flows into the cooling means 152 through conduit 215 and is cooled. The fifth stream is then incorporated into the second main stream. That is, the fifth stream of conduit 216 is incorporated into the second main stream of conduit 254 (third mixing step). With this mixing, the third main stream is formed. The third main stream is sent to the first separating step.

The liquefaction process according to the present modification may be more efficient in the liquefaction process for the following reasons. In this variant, the fifth stream is discharged from the heat exchanger 120 and then compressed independently by the compression means 142. So that it is possible to optimize separately for the fifth stream. This can improve the efficiency of the liquefaction process.

Meanwhile, the liquefaction process according to the present embodiment can be modified as shown in FIG. 3 is a flow chart showing a second modification of the natural gas liquefaction process according to FIG. As shown in FIG. 3, in the liquefaction process according to the present modification, the fourth stream flows in the heat exchanger 120 through the conduit 238 to the compression means 141 and is compressed (first compression step). The fourth stream then flows into the cooling means 151 through conduit 251 and is cooled.

The fourth stream is then mixed with the fifth stream. That is, the fifth stream of conduit 214 is incorporated into the fourth stream of conduit 252 (second mixing step). The fourth stream and the fifth stream may be respectively introduced into the compression means 143 to be described later, and then mixed in the compression means 143. With this mixing, the second main stream is formed. The second main stream is compressed by the compression means 143 (second compression step). The second main stream then flows into the cooling means 153 through the conduit 253 and is cooled.

Then, the sixth stream flows into the compression means 1421 through the conduit 226 in the heat exchanger 120 and is compressed (third compression step). The sixth stream then enters the cooling means 1521 via conduit 227 and is cooled. The sixth stream is then incorporated into the second main stream. That is, the sixth stream of the conduit 228 is incorporated into the second main stream of the conduit 254 (third mixing step). With this mixing, the third main stream is formed. The third main stream is sent to the first separating step.

The liquefaction process according to the present modification may be more efficient in the liquefaction process for the following reasons. In this variant, the sixth stream is discharged from the heat exchanger 120 and then compressed independently by the compression means 1421. Thus, it is possible to optimize separately for the sixth stream. This can improve the efficiency of the liquefaction process.

Example  2

4 is a flowchart showing a natural gas liquefaction process according to the second embodiment of the present invention. The liquefaction process according to the second embodiment of the present invention uses a closed loop refrigeration cycle as shown in FIG. 4 to cool natural gas (NG) to a liquefaction temperature to produce liquefied natural gas (LNG ). ≪ / RTI >

In particular, one closed loop refrigeration cycle employing a mixed refrigerant or a multi-component refrigerant is used to cool the natural gas in the first heat exchange zone and to cool the natural gas in the second heat exchange zone, To the natural gas liquefaction process. For reference, the liquefaction process according to the present embodiment may further include a refrigeration cycle for cooling the mixed refrigerant or for cooling the natural gas.

Hereinafter, the liquefaction process according to the second embodiment of the present invention will be described in detail with reference to FIG. First, the mixed refrigerant (main stream to be described later) is partially condensed (condensation step). By such condensation, the mixed refrigerant includes the liquid phase portion and the vapor phase portion. The mixed refrigerant is then separated into a first stream of liquid phase and a second stream of vapor phase by means of a separation means 111 (first separation step).

The second stream flows into the compression means 140 through the conduit 201 after separation and is compressed (compression step). The second stream then enters the cooling means 150 through conduit 202 and is cooled. Then, the second stream is introduced into the separating means 112 through the conduit 203 to be separated into the liquid third stream and the gaseous fourth stream (second separation step).

The first stream is introduced into the first heat exchange region 1201 via conduit 2111 after separation (first introduction step). The first stream discharged from the first heat exchange region 1201 then flows into the expansion means 1311 through the conduit 2112 and is expanded (first expansion step). The first stream is lowered in temperature due to expansion and then flows back to the first heat exchange region 1201 through the conduit 2113 to cool the natural gas in the first heat exchange region 1201 (first cooling step). This allows natural gas to be precooled. The first stream introduced into the first heat exchange region 1201 through the conduit 2113 is supplied to the first stream flowing into the first heat exchange region 1201 through the conduit 2111 and the second stream introduced into the first heat exchange region 1201 through the conduit 2311, The fourth stream introduced into the region 1201 can also be cooled together with the natural gas. After such cooling, the first stream is sent via conduit 2114 to the condensation stage. For reference, the conduits 2112 and 2113 may not pass through the second heat exchange region 1202 unlike FIG.

The third stream is sent to the condensation stage via conduit 204 after separation. Wherein the third stream can be expanded by the expansion means 134 before being sent to the condensation stage.

The fourth stream is introduced into the first heat exchange region 1201 through the conduit 2311 after separation (second inflow step). Then, the fourth stream discharged from the first heat exchange region 1201 flows into the second heat exchange region 1202 through the conduit 2312 (third inflow step). The fourth stream discharged from the second heat exchange region 1202 then flows into the expansion means 1321 via the conduit 2313 and is expanded (a second expansion step).

The fourth stream is cooled down to the expansion and then flows back into the second heat exchange zone 1202 through the conduit 2314 to cool the natural gas in the second heat exchange zone 1202 (second cooling step). This allows natural gas to be liquefied. Or liquefied and subcooled. The fourth stream introduced into the second heat exchange region 1202 through the conduit 2314 can also cool the fourth stream introduced into the second heat exchange region 1202 through the conduit 2312 together with the natural gas. After such cooling, the fourth stream is sent via conduit 2316 to the condensation stage.

For reference, the fourth stream discharged from the second heat exchange region 1202 after heat exchange may be directly sent to the condensing stage through the conduit 2316, but may be transferred through the conduit 2315 to the first heat exchange region 1201, May be sent first. In this case, the fourth stream can cool the natural gas together with the first stream in the first heat exchange region 1201.

As described above, the liquefaction process according to the present embodiment liquefies natural gas using one closed loop refrigeration cycle. Therefore, the liquefaction process according to the present embodiment is advantageous in that the structure of the liquefaction process is simple and the liquefaction process is easy to operate.

Further, the liquefaction process according to the present embodiment can be simplified in structure of the liquefaction process for the following reasons. The liquefaction process according to the first embodiment described above has the separating means 113 for processing the gaseous portion that is partially generated in the third stream. Since the liquid phase portion and the vapor phase portion separated by the separation means 113 are both sent to the heat exchanger 120, the structure of the heat exchanger 120 becomes complicated. However, since the liquefaction process according to the present embodiment sends the third stream to the condensing stage, the structure of the liquefaction process can be simplified compared with the liquefaction process according to the first embodiment.

On the other hand, the condensation step can be more specifically described as follows. The fourth stream flows into the compression means 141 through the conduit 2316 in the heat exchanger 1200 and is compressed (first compression step). The fourth stream then flows into the cooling means 151 through conduit 251 and is cooled. The first stream then flows into the compression means 142 through the conduit 2114 in the heat exchanger 1200 and is compressed (second compression step). The first stream then flows into the cooling means 152 through conduit 2115 and is cooled.

The first stream of conduit 2116 and the third stream of conduit 204 are then incorporated into the fourth stream of conduit 252 (entraining step). With such mixing, the main stream is formed. The third stream may be expanded by expansion means 134 prior to incorporation. The main stream is sent via conduit 252 to the first separation stage. It is noted that the first stream of conduit 2116 is incorporated into the fourth stream of conduit 252 and that the third stream of conduit 204 is incorporated into the fourth stream of conduit 252 none. Or may be incorporated at the same time.

111, 112, 113: separation means
121, 122, 123: heat exchange zone
131, 132, 133, 134: expansion means
140, 141, 142, 143: compression means
150, 151, 152, 153: cooling means

Claims (12)

One closed loop refrigeration cycle employing a mixed refrigerant is used to cool the natural gas primarily in the first heat exchange zone, to cool the natural gas secondarily in the second heat exchange zone, to cool the natural gas in the third heat exchange zone, In a natural gas liquefaction process for cooling a natural gas,
The closed-loop refrigeration cycle includes:
A condensing step of partially condensing the mixed refrigerant;
A first separation step of separating the mixed refrigerant into a liquid first stream and a gaseous second stream after the condensing step;
A compressing step of compressing the second stream after the first separating step;
A second separation step of separating the second stream into a liquid third stream and a gaseous fourth stream after the compression step;
A first mixing step of mixing the first stream into the third stream after the first separation step and the second separation step to form a first main stream;
A third separation step of separating the first main stream into a liquid fifth stream and a gaseous sixth stream after the first introduction step;
A first cooling step of cooling the natural gas in the first heat exchange zone using the fifth stream after the third separation step;
A second cooling step of cooling the natural gas in the second heat exchange zone using the sixth stream after the third separation step; And
And a third cooling step of cooling the natural gas in the third heat exchange zone using the fourth stream after the second separation step. The method according to claim 1,
Further comprising an expansion step of expanding the third stream between the second separation step and the first mixing step. The method according to claim 1,
Wherein the first cooling step comprises:
A first inflow step of introducing the fifth stream into the first heat exchange zone;
A first expansion step of expanding the fifth stream discharged from the first heat exchange zone after the first inflow step; And
Cooling the natural gas in the first heat exchange zone to pre-cool the natural gas in the first heat exchange zone via the fifth stream, after the first expansion step, into the first heat exchange zone again,
And the fifth stream after the pre-cooling step is sent to the condensing step. The method according to claim 1,
The second cooling step may include:
A second inflow step of introducing the sixth stream into the first heat exchange zone;
A third inflow step of introducing the sixth stream discharged from the first heat exchange area after the second inflow step into the second heat exchange area;
A second expansion step of expanding the sixth stream discharged from the second heat exchange zone after the third introduction step; And
And a liquefying step for introducing the sixth stream back into the second heat exchange zone after the second expansion step to liquefy the natural gas in the second heat exchange zone through the sixth stream,
And after said liquefaction step said sixth stream is sent to said condensing step. The method according to claim 1,
The third cooling step may include:
A fourth inflow step of introducing the fourth stream into the first heat exchange zone;
A fifth inflow step of introducing a fourth stream discharged from the first heat exchange area after the fourth inflow step into the second heat exchange area;
A sixth inflow step of introducing a fourth stream discharged from the second heat exchange area after the fifth inflow step into the third heat exchange area;
A third expansion step of expanding the fourth stream discharged from the third heat exchange zone after the sixth introduction step; And
And subcooling said natural gas in said third heat exchange zone through said fourth stream by introducing said fourth stream back into said third heat exchange zone after said third expansion step,
And the fourth stream is sent to the condensing stage after the subcooling step. The method according to claim 1,
In the condensing step,
A first compression step of compressing the fourth stream;
A second mixing step of mixing the sixth stream into the fifth stream to form a second main stream;
A second compression step of compressing the second main stream after a second mixing step; And
And a third mixing step of mixing the second main stream into the fourth stream after the first compression step and the second compression step to form a third main stream,
And the third main stream is sent to the first separation step after the third mixing step. The method according to claim 1,
In the condensing step,
A first compression step of compressing the fourth stream;
A second mixing step of mixing the sixth stream into the fourth stream after the first compression step to form a second main stream;
A second compression step of compressing the second main stream after the second mixing step;
A third compression step of compressing the fifth stream; And
And a third mixing step of mixing the fifth stream into the second main stream after the second compression step and the third compression step to form a third main stream,
And the third main stream is sent to the first separation step after the third mixing step. The method according to claim 1,
In the condensing step,
A first compression step of compressing the fourth stream;
A second mixing step of mixing the fifth stream into the fourth stream after the first compression step to form a second main stream;
A second compression step of compressing the second main stream after the second mixing step;
A third compression step of compressing the sixth stream; And
And a third mixing step of mixing the sixth stream into the second main stream to form a third main stream after the second compression step and the third compression step,
And the third main stream is sent to the first separation step after the third mixing step. The method according to any one of claims 1 to 8,
Wherein the first to third heat exchange areas are provided in one PFHE type heat exchanger. In a natural gas liquefaction process in which natural gas is primarily cooled in a first heat exchange zone and secondarily natural gas is cooled in a second heat exchange zone using one closed loop refrigeration cycle employing a mixed refrigerant,
The closed-loop refrigeration cycle includes:
A condensing step of partially condensing the mixed refrigerant;
A first separation step of separating the mixed refrigerant into a liquid first stream and a gaseous second stream after the condensing step;
A compressing step of compressing the second stream after the first separating step;
A second separation step of separating the second stream into a liquid third stream and a gaseous fourth stream after the compression step;
A first inflow step of introducing the first stream into the first heat exchange zone after the first separation step;
A first expansion step of expanding a first stream discharged from the first heat exchange zone after the first inflow step;
A first cooling step of flowing the first stream back into the first heat exchange zone after the first expansion step to cool the natural gas in the first heat exchange zone through the first stream;
A second inflow step of introducing the fourth stream into the first heat exchange zone after the second separation step;
A third inflow step of introducing a fourth stream discharged from the first heat exchange area after the second inflow step into the second heat exchange area;
A second expansion step of expanding a fourth stream discharged from the second heat exchange zone after the third introduction step; And
And a second cooling step for introducing said fourth stream back into said second heat exchange zone after said second expansion step to cool said natural gas in said second heat exchange zone through said fourth stream,
Characterized in that said first stream is sent after said first cooling step, said third stream after said second separating step and said fourth stream is sent to said condensing step after said second cooling step. Gas liquefaction process. The method of claim 10,
In the condensing step,
A first compression step of compressing the fourth stream;
A second compression step of compressing the first stream; And
And a mixing step of mixing the first stream after the second compression step and the third stream after the second separation step into the fourth stream after the first compression step to form a main stream,
Wherein the mainstream is sent to the first separation stage after the incorporation step. The method according to claim 10 or 11,
Wherein the first and second heat exchange areas are provided in one PFHE type heat exchanger.

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