KR101615444B1

KR101615444B1 - Natural gas liquefaction process

Info

Publication number: KR101615444B1
Application number: KR1020140098911A
Authority: KR
Inventors: 이상규; 조병학; 이영범; 손영순; 최성희; 김석순; 차규상
Original assignee: 한국가스공사
Priority date: 2014-08-01
Filing date: 2014-08-01
Publication date: 2016-04-25
Also published as: KR20160015920A

Abstract

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 cooling step for cooling the natural gas in the first heat exchange zone using the first stream after the first separation step, A second cooling step of cooling the natural gas in the second heat exchange zone using the third stream after the second stage and a second cooling step of cooling the natural gas in the third heat exchange zone using the fourth stream after the second separation step, Cooling 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 7, 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. 8, 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. 9, 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 natural gas liquefaction process according to the present invention liquefies natural gas using one closed loop refrigeration cycle, so that the structure of the liquefaction process is simple and the operation of the liquefaction process is easy.

Further, in the natural gas liquefaction process according to the present invention, the mixed refrigerant is sequentially separated into three streams, and then the natural gas is sequentially cooled according to the cooling temperature in the three heat exchange zones by using them. There is an effect of being excellent.

1 is a flow chart illustrating a natural gas liquefaction process in accordance with an 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.
Fig. 4 is a flowchart showing a third modification of the natural gas liquefaction process according to Fig. 1
FIG. 5 is a flowchart showing a fourth modification of the natural gas liquefaction process according to FIG.
6 is a flowchart showing a fifth modification to the natural gas liquefaction process according to FIG.
7 is a flow chart conceptually illustrating a conventional C3 / MR process.
8 is a flow chart conceptually illustrating a conventional cascade process.
9 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.

1 is a flow chart illustrating a natural gas liquefaction process in accordance with one embodiment of the present invention. The liquefaction process according to an embodiment of the present invention uses a closed loop refrigeration cycle to cool natural gas (NG) to a liquefying temperature to produce liquefied natural gas (LNG ). &Lt; / 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 one embodiment of the present invention will be described in detail with reference to FIG. First, the mixed refrigerant (second 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 first stream cools the natural gas in the first heat exchange zone 121 after the separation (first cooling step). This allows natural gas to be pre-cooled. For example, the first stream is first introduced into the first heat exchange zone 121 via the conduit 211 after the separation (first inflow step). The first 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 first 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 temperature of the first stream is lowered by the 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 first stream introduced into the first heat exchange zone 121 through the conduit 213 is supplied to the first heat exchange zone 121 through the first heat exchange zone 121 via the conduit 211, The third 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 first 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 third stream cools the natural gas in the second heat exchange zone 122 after separation (second cooling step). This allows natural gas to be liquefied. For example, the third stream is first introduced into the first heat exchange zone 121 via the conduit 221 after the separation (second inflow step). The third stream discharged from the first heat exchange zone 121 then flows into the second heat exchange zone 122 through the conduit 222 (third inflow step). The third stream discharged from the second heat exchange region 122 then flows into the expansion means 132 via the conduit 223 and is expanded (a second expansion step).

The third stream is cooled down to 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 third stream introduced into the second heat exchange region 122 through the conduit 224 is connected to the third stream introduced into the second heat exchange region 122 through the conduit 222 and the third 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 third 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 third stream discharged from the second heat exchange region 122 after the heat exchange may be sent directly to the condensing stage 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 third stream can pre-cool the natural gas together with the first stream in the first heat exchange zone 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 first introduced into the first heat exchange zone 121 via the conduit 231 after the 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 zone 123 after heat exchange may be sent directly to the condensation stage through the conduit 238, but may be passed through the second heat exchange zone 122 through the conduit 236, And may be first sent to the first heat exchange zone 121 via conduit 237. [ In such a case, the fourth stream may liquefy the natural gas with the third stream in the second heat exchange zone 122 and pre-cool the natural gas with the first 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. The mixed refrigerant may be partially condensed through a series of compressions or a series of compression and cooling. After condensation, the mixed refrigerant can be separated into a liquid stream and a gaseous stream. Here, the streams may have different compositions and amounts depending on the pressure and temperature at the time of separation. However, it is preferable in terms of efficiency that the natural gas is cooled at a relatively high temperature in a stream containing a relatively heavy component.

The liquefaction process according to this embodiment sequentially separates the mixed refrigerant into a plurality of streams through the first separation step and the second separation step. Thus, in this embodiment, the first stream contains the heaviest component and the fourth stream contains the lightest component. And in this embodiment the first stream cools the natural gas at the highest temperature and the fourth stream cools the natural gas. As described above, in the liquefaction process according to the present embodiment, the mixed refrigerant is sequentially separated into three streams, and then the natural gas is sequentially cooled according to the cooling temperature in the three heat exchange regions using the separated refrigerant, 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.

The fourth stream is then mixed with the third stream. That is, the third stream is incorporated into the fourth stream (first mixing step). Such incorporation can be achieved by connecting one conduit 226 to another conduit 252. Or a separate configuration for incorporation may be employed. The third stream and the fourth 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 first main stream is formed. That is, the first main stream is a stream in which the third stream and the fourth stream are mixed.

The first main stream is compressed by the compression means 142 (second compression step). The first main stream then flows into the cooling means 152 through the conduit 253 and is cooled. The first main stream is then mixed with the first stream. That is, the first stream of conduit 214 is incorporated into the first main stream of conduit 254 (second entraining step). With this mixing, the second main stream is formed. The second main stream is compressed by the compression means 143 (third compression step). The second main stream then enters the cooling means 153 via conduit 255 and is cooled. The second main stream is then sent via conduit 256 to the first separation stage.

The liquefaction process according to this embodiment can be more efficient in the liquefaction process for the following reasons. In this embodiment, the fourth stream containing the lightest component is first compressed by the compression means 141. And the first stream containing the heaviest component is compressed by the compression means 143 a third time. If the streams are compressed in this way, the efficiency of the liquefaction process can be improved.

For reference, incorporation is a relative concept. Depending on the configuration of the conduit, the third stream may be considered to be incorporated into the fourth stream, and the fourth stream may be considered to be incorporated into the third stream. And the conduits discussed above may be different conduits or may be the same conduits according to the reference numerals. That is, even one conduit may be given two numerals for convenience of explanation. Alternatively, two conduits may be provided with one reference numeral for convenience of explanation.

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 third stream. That is, the third stream of conduit 226 is incorporated into the fourth stream of conduit 252 (first mixing step). With this mixing, the first main stream is formed. The first main stream is compressed by the compression means 142 (second compression step). The first main stream then flows into the cooling means 152 through the conduit 253 and is cooled.

The first stream flows in the heat exchanger 120 through the conduit 214 into the compression means 1431 and is compressed (third compression step). The first stream then enters the cooling means 1531 through conduit 215 and is cooled. The first stream of conduit 216 is then incorporated into the first main stream of conduit 254 (second entraining step). With this mixing, the second main stream is formed. The second main stream is then sent via conduit 254 to the first separation stage. For reference, the first stream and the first main stream may be introduced into the separating means 111 and then mixed in the separating means 111, respectively.

The liquefaction process according to the present modification may be more efficient in the liquefaction process for the following reasons. In this variant, the first stream is discharged from heat exchanger 120 and then compressed independently by compression means 1431. So that it is possible to optimize separately for the first 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 first stream. That is, the first stream of conduits 2141 is incorporated into the fourth stream of conduits 252 (first mixing step). With this mixing, the first main stream is formed.

The first main stream is compressed by the compression means 142 (second compression step). The first main stream then flows into the cooling means 152 through the conduit 253 and is cooled. The first main stream is then mixed with the third stream. That is, the third stream of conduits 2261 is incorporated into the first main stream of conduits 254 (second mixing step). With this mixing, the second main stream is formed. The second main stream is then sent via conduit 254 to the first separation stage.

The liquefaction process according to the present modification has the advantage that the number of compression means can be reduced compared to the liquefaction process according to FIG.

Meanwhile, the liquefaction process according to the present embodiment can be modified as shown in FIG. 4 is a flowchart showing a third modification of the natural gas liquefaction process according to FIG. As shown in Fig. 4, the liquefaction process according to the present modification is basically the same as the liquefaction process according to Fig. However, in this modification, the third stream is introduced into the compression means 1432 through the conduit 2261, compressed and then mixed into the first main stream of the conduit 254. Where the third stream can be cooled prior to incorporation. With this mixing, the second main stream is formed. The second main stream is then sent via conduit 254 to the first separation stage.

Meanwhile, the liquefaction process according to the present embodiment can be modified as shown in FIG. 5 is a flowchart showing a fourth modification of the natural gas liquefaction process according to FIG. As shown in FIG. 5, in the liquefaction process according to the present modification, 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. The first stream then flows into the compression means 1433 through the conduit 2146 in the heat exchanger 120 and is compressed (second compression step). The first stream then enters the cooling means 1533 via conduit 2147 and is cooled.

The first stream of conduit 2148 and the third stream of conduit 2266 are then incorporated into the fourth stream of conduit 252 (entraining step). With such mixing, the main stream is formed. The main stream is sent via conduit 252 to the first separation stage. It is noted that the first stream of conduit 2148 is incorporated into the fourth stream of conduit 252 and that the third stream of conduit 2266 is incorporated into the fourth stream of conduit 252 none. Or may be incorporated at the same time.

Meanwhile, the liquefaction process according to the present embodiment can be modified as shown in FIG. FIG. 6 is a flowchart showing a fifth modified example of the natural gas liquefaction process according to FIG. As shown in Fig. 6, the liquefaction process according to the present modification is basically the same as the liquefaction process according to Fig. In this variation, however, the third stream is introduced into the compression means 1434 through the conduit 2266, compressed and then mixed with the first stream of the conduit 2148 into the fourth stream of the conduit 252 . The third stream may be cooled prior to incorporation. With such mixing, the main stream is formed. The main stream is sent via conduit 252 to the first separation stage.

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

Claims

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 cooling step of cooling the natural gas in the first heat exchange zone using the first stream after the first separation step;
A second cooling step of cooling the natural gas in the second heat exchange zone using the third stream after the second 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,
Wherein the first, third and fourth streams cool the natural gas in the first, second and third heat exchange zones without mixing with each other after the first and second separation steps. .

The method according to claim 1,
Wherein the first cooling step comprises:
A first inflow step of introducing the first stream into the first heat exchange zone;
A first expansion step of expanding a first 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 by flowing the first stream back into the first heat exchange zone after the first expansion step,
And after the pre-cooling step, the first stream 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 third stream into the first heat exchange zone;
A third inflow step of introducing a third 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 third stream discharged from the second heat exchange zone after the third introduction step; And
And a liquefying step for introducing said third stream back into said second heat exchange zone after said second expansion step to liquefy said natural gas in said second heat exchange zone through said third stream,
And after said liquefaction step said third 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 first mixing step of mixing the third stream into the fourth stream after the first compression step to form a first main stream;
A second compression step of compressing the first main stream after the first mixing step;
A second mixing step of mixing the first stream into the first main stream after the second compression step to form a second main stream; And
And a third compression step of compressing the second main stream after the second incorporation step,
And the second main stream is sent to the first separating step after the third compressing step.

The method according to claim 1,
In the condensing step,
A first compression step of compressing the fourth stream;
A first mixing step of mixing the third stream into the fourth stream after the first compression step to form a first main stream;
A second compression step of compressing the first main stream after the first mixing step;
A third compression step of compressing the first stream; And
And a second mixing step of mixing the first stream into the first main stream to form a second main stream after the second compression step and the third compression step,
And the second main stream is sent to the first separation stage after the second mixing step.

The method according to claim 1,
In the condensing step,
A first compression step of compressing the fourth stream;
A first mixing step of mixing the first stream into the fourth stream after the first compression step to form a first main stream;
A second compression step of compressing the first main stream after the first mixing step; And
And a second mixing step of mixing the third stream into the first main stream after the second compression step to form a second main stream,
And the second main stream is sent to the first separation stage after the second mixing step.

The method according to claim 1,
In the condensing step,
A first compression step of compressing the fourth stream;
A first mixing step of mixing the first stream into the fourth stream after the first compression step to form a first main stream;
A second compression step of compressing the first main stream after the first mixing step;
A third compression step of compressing the third stream; And
And a second mixing step of mixing the third stream into the first main stream after the second compression step and the third compression step to form a second main stream,
And the second main stream is sent to the first separation stage after the second mixing step.

The method according to claim 1,
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 and the third stream after the second compression 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 1,
In the condensing step,
A first compression step of compressing the fourth stream;
A second compression step of compressing the third stream;
A third compression step of compressing the first stream; And
And a mixing step of mixing the third stream after the second compression step and the first stream after the third compression 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 any one of claims 1 to 10,
Wherein the first to third heat exchange areas are provided in one PFHE type heat exchanger.