KR101107437B1 - Natural gas liquefaction process - Google Patents

Abstract

A natural gas liquefaction process using three closed loop refrigeration cycles to liquefy natural gas comprises pre-cooling the natural gas supplied in the first heat exchange zone through vaporization of a first single refrigerant circulating in the first closed loop refrigeration cycle. Liquefying the pre-cooled natural gas in the second heat exchange zone through vaporization of the second single refrigerant circulating in the second closed loop refrigeration cycle, and the expanded nitrogen refrigerant in the process of circulating the third closed loop refrigeration cycle. Subcooling the liquefied natural gas through the indirect heat exchange of the third heat exchange zone.

Description

Natural Gas Liquefaction Process {NATURAL GAS LIQUEFACTION PROCESS}

The present invention relates to a natural gas liquefaction process, and more particularly, to a natural gas liquefaction process that can increase the efficiency and capacity of the entire liquefaction process by supercooling the liquefied natural gas through a nitrogen expansion cycle.

Thermodynamic processes for liquefying natural gas to produce liquefied natural gas (LNG) have been developed since the 1970s to meet a variety of challenges, including the need for higher efficiency and greater capacity. The liquefaction system consists of a heat exchanger, etc. to cool natural gas indirectly by heat exchange with a certain refrigerant to the LNG temperature. In order to reduce the power required for liquefaction in such a liquefaction system, it is important to reduce the entropy generation due to the temperature difference between the supply gas and the refrigerant in the heat exchanger. The natural gas provided to the liquefaction system is primarily a mixture of hydrocarbons. As a result, the specific heat is greatly changed during the liquefaction process. In order to cope with such a large change in specific heat, various attempts have been made to increase the efficiency of the liquefaction process. However, various attempts have been made to increase the efficiency and capacity of the liquefaction process using different refrigerants or different cycles. However, the number of liquefaction processes that are practically used is very small.

One of the most popular liquefaction processes in operation is the 'Propane Pre-cooled Mixed Refrigerant Process (or C3 / MR Process)' from Air Products and Chemicals Inc. The basic structure of the C3 / MR process is as shown in FIG. The feed gas is pre-cooled to approximately 238 K by a multi-stage propane (C3) Joule-Thomson (JT) cycle. The precooled feed gas is liquefied and sub-cooled to 123 K through heat exchange with mixed refrigerant (MR) in a large spiral wound heat exchanger. The proper composition of the mixed refrigerant allows liquefaction and subcooling of the feed gas over a wide temperature range with only one heat exchanger.

However, in the case of the C3 / MR process, not only the feed gas but also the mixed refrigerant must be cooled in the propane cycle, which increases the thermodynamic load on the propane cycle. Accordingly, due to technical limitations on the compressor applied to the propane cycle, the current C3 / MR process has only a capacity of about 5 MTPA (million tons per annum). In order to overcome these technical limitations, Air Products and Chemicals Inc. is based on the C3 / MR process, but adds a nitrogen Brayton cycle to the cold end of the C3 / MR process. X 'process was introduced. The AP-X process is currently known to have a capacity of about 8MTPA.

One of the other successful liquefaction processes in operation is by Conoco Phillips, which is based on the Cascade process. As conceptually illustrated in FIG. 10, the liquefaction process of 'Conoco Phillips' is a three JT cycle using methane (C1), ethylene (C2), and propane (C3) as single-component refrigerants. It is composed. Since the liquefaction process does not use a mixed refrigerant, there is an advantage that the operation of the liquefaction process is safe, simple and reliable.

As mentioned above, various liquefaction processes have been disclosed to meet the demand for higher efficiency and greater capacity. Some of these liquefaction processes are based on existing C3 / MR processes or cascade processes, but there are those that want to improve the efficiency of the liquefaction process by improving the process. Most of these attempts are based on C3 / MR processes. Accordingly, the development of a liquefaction process that can improve the efficiency and capacity while taking advantage of the safety, simplicity and reliability in operation based on the cascade process is currently strongly required.

Therefore, the present invention has been made to solve the above problems, one of the problems of the present invention in the liquefaction process for liquefying natural gas, while its operation is simple and reliable and can increase the efficiency and capacity of the liquefaction process It is to provide a liquefaction process.

According to a preferred embodiment of the present invention for achieving the above object of the present invention, the natural gas liquefaction process using the three closed loop refrigeration cycle to liquefy the natural gas, the first circulating the first closed loop refrigeration cycle Precooling the natural gas supplied through the vaporization of a single refrigerant in the first heat exchange zone, and liquefying the precooled natural gas in the second heat exchange zone through the vaporization of the second single refrigerant circulating in the second closed loop refrigeration cycle. And supercooling the liquefied natural gas in the third heat exchange zone through indirect heat exchange with the expanded nitrogen refrigerant in the course of circulating the third closed loop refrigeration cycle.

The third closed loop refrigeration cycle may include: compressing the nitrogen refrigerant, precooling the compressed nitrogen refrigerant, and cooling the precooled nitrogen refrigerant through indirect heat exchange with the nitrogen refrigerant passing through the third heat exchange region. And expanding the cooled nitrogen refrigerant. The subcooling step may sequentially supercool the liquefied natural gas through the expanded nitrogen refrigerant in the second stage of the third heat exchange region.

In this case, the subcooling may indirectly exchange heat with the natural gas liquefied in the third heat exchange zone of the two stages, respectively, in which one expanded nitrogen refrigerant of two independent three closed loop refrigeration cycles is used. The two third closed loop refrigeration cycles may include compressing the nitrogen refrigerant, precooling the compressed nitrogen refrigerant, recompressing the precooled nitrogen refrigerant, and recooling the recompressed nitrogen refrigerant. The method may include cooling the recooled nitrogen refrigerant through indirect heat exchange with the nitrogen refrigerant passing through the third heat exchange region of each stage, and expanding the cooled nitrogen refrigerant.

Alternatively, the subcooling may indirectly exchange heat with the natural gas liquefied in the third heat exchange zone of the second stage through one third closed loop refrigeration cycle. Here, work generated through expansion means for expanding the nitrogen refrigerant of the third closed loop refrigeration cycle may drive compression means for compressing the second single refrigerant of the second closed loop refrigeration cycle.

And the one closed loop refrigeration cycle comprises: (a) compressing the nitrogen refrigerant, (b) precooling the compressed nitrogen refrigerant, (c) introducing the precooled nitrogen refrigerant into the first cooling zone, (d) distributing the nitrogen refrigerant having passed through the first cooling zone to the first partial refrigerant and the second partial refrigerant through step (c), and (e) distributing the distributed first partial refrigerant to the second cooling zone. Inflowing, (f) expanding the first partial refrigerant having passed through the second cooling region through the step (e), and (g) terminating the expanded first partial refrigerant in the second stage of the third heat exchange. Introducing into the zone, (h) introducing a first partial refrigerant that has passed through the third heat exchange zone of the second stage into the second cooling zone, (i) a second portion distributed through step (d) Expanding the refrigerant, (j) converting the expanded second partial refrigerant into the third heat exchange zone of the first stage; Introducing, and (k) cooling the second partial refrigerant that has passed through the third heat exchange zone of the first stage together with the first partial refrigerant that has passed through the second cooling zone through step (h). And introducing into the region, and compressing the first partial refrigerant and the second partial refrigerant through the step (k) through the step (a) to repeat the steps. Can be.

Meanwhile, the third closed loop refrigeration cycle may be based on a reverse Brayton cycle. And the third closed loop refrigeration cycle may be independent of the first and second closed loop refrigeration cycles. The first and second closed loop refrigeration cycles may also be based on Joule-Thomson cycles. And the second single refrigerant of the second closed loop refrigeration cycle may be condensed through indirect heat exchange with the first single refrigerant of the first closed loop refrigeration cycle.

The above pre-cooling step may sequentially pre-cool the supplied natural gas through indirect heat exchange with the first single refrigerant in two stages of first heat exchange zones, and the liquefaction may include three stages of pre-cooled natural gas. Liquefaction may be sequentially performed through indirect heat exchange with the second single refrigerant in the second heat exchange region. The first single refrigerant of the first closed loop refrigeration cycle may be a propane refrigerant, and the second single refrigerant of the second closed loop refrigeration cycle may be an ethylene refrigerant.

The natural gas liquefaction process according to the present invention maintains the basic framework of the cascade process, but since the closed loop refrigeration cycle in the subcooling stage consists of nitrogen expansion cycles, the refrigerants employed in each cycle are all single refrigerants, and as a result, the liquefaction process In this case, the operation is simple and reliable.

In addition, the natural gas liquefaction process according to the present invention can reduce the thermodynamic load applied to the first closed loop refrigeration cycle because the third closed loop refrigeration cycle is independent of the first and second closed loop refrigeration cycle, as a result As a result, the compressor having the same capacity can increase the overall capacity compared to the conventional liquefaction process.

Furthermore, since the natural gas liquefaction process according to the present invention employs a nitrogen expansion cycle based on the Brayton cycle in the subcooling step, there is an effect that the performance of the liquefaction process can be expected thermodynamically.

1 is a flowchart conceptually illustrating a liquefaction process according to Embodiment 1 of the present invention.
2 is a flow chart illustrating the flow of the liquefaction process of FIG.
3 is a flowchart showing the liquefaction process according to the first embodiment of the present invention in detail.
4 is a temperature-entropy diagram showing the thermodynamic properties of natural gas
5 is a temperature-entropy diagram in a propane cycle according to Example 1 of the present invention.
6 is a temperature-entropy diagram in an ethylene cycle according to Example 1 of the present invention.
7 is a temperature-entropy diagram in a nitrogen cycle according to Example 1 of the present invention.
8 is a flowchart showing the liquefaction process according to the second embodiment of the present invention in detail.
9 is a flowchart conceptually illustrating a conventional C3 / MR process.
10 is a flowchart conceptually illustrating a conventional cascade process.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings, but the present invention is not limited or limited by the embodiments. For reference, in the present description, the same numbers refer to substantially the same elements, and may be described by quoting contents described in other drawings under these rules. And it can be omitted that it is determined or repeated to those skilled in the art to which the present invention pertains.

Example 1

1 is a flowchart conceptually illustrating a liquefaction process according to Embodiment 1 of the present invention, FIG. 2 is a flowchart illustrating the flow of the liquefaction process of FIG. 1, and FIG. 3 is a flowchart of Embodiment 1 of the present invention. It is a flowchart which shows the liquefaction process in detail. As illustrated in FIG. 1, the liquefaction process according to the present embodiment basically cools natural gas to the liquefaction temperature by using three closed loop refrigeration cycles, thereby liquefying natural gas (LNG). It can be applied to the production process. In particular, the present invention may be applied to a cascade process of sequentially cooling natural gas using three closed loop refrigeration cycles each employing a single-component refrigerant having a different composition.

The natural gas liquefaction process based on this cascade process first pre-cools the natural gas through a first closed loop refrigeration cycle employing the first single refrigerant having the highest boiling point, and the second single refrigerant having an intermediate boiling point. Liquefying the pre-cooled natural gas through a second closed loop refrigeration cycle employing and subcooling the liquefied natural gas through a third closed loop refrigeration cycle employing a third single refrigerant having the lowest boiling point. -cooling). For this sequential cooling, propane (C3) refrigerant is generally used as the first single refrigerant, and ethane or ethylene refrigerant (C2) is used as the second single refrigerant. However, in the conventional cascade process, methane (C1) is used as the third single refrigerant, but in the liquefaction process according to the present embodiment, nitrogen (N2) is used as the third single refrigerant. The usefulness of this nitrogen cycle will be described later.

And the first and second closed loop refrigeration cycles according to this embodiment are based on the Joule-Thomson cycle in the same way as the conventional cascade process. In the Joule-Thompson cycle, the refrigerant causes a phase change between the liquid phase and the gas phase during the cycle of the cycle. For this cycle, the first and second closed loop refrigeration cycles basically include an evaporator (heat exchanger), a compressor, a condenser (cooler) and an expansion valve, similar to a normal refrigeration cycle. In contrast, a third closed loop refrigeration cycle employing a nitrogen refrigerant is based on a Reverse Brayton cycle. In the Brayton cycle, the cycle is cycled with the nitrogen refrigerant in the gas phase.

As a result, the conventional cascade process, as shown in Figure 10, while precooling, liquefaction, and supercooling natural gas based on three Joule-Thomson cycles employing propane, ethylene, and methane refrigerant respectively, according to the present embodiment The liquefaction process precools and liquefies natural gas based on two Joule-Thomson cycles employing propane and ethylene refrigerant, respectively, as shown in FIG. 1, but liquefied natural based on the Brayton cycle employing nitrogen refrigerant. There is a difference from the conventional cascade process in that the gas is supercooled, and this difference may be a main feature of the liquefaction process according to the present embodiment.

Hereinafter, the liquefaction process according to Embodiment 1 of the present invention applied to the natural gas liquefaction process employing the three refrigeration cycles as described above will be described based on the third closed loop refrigeration cycle. In the following description, a first refrigeration cycle employing propane refrigerant is referred to as a propane cycle, a second refrigeration cycle employing ethylene refrigerant is referred to as an ethylene cycle, and a third refrigeration cycle employing nitrogen refrigerant is referred to as a nitrogen cycle. For reference, in each of the drawings 'C2' is a configuration related to the ethylene cycle, 'C3' is a configuration related to the propane cycle, 'N2' represents a configuration related to the nitrogen cycle.

First, the basic concept of the liquefaction process according to the present embodiment will be described with reference to FIG. 1. Feed gas (ⓐ) is precooled through a propane cycle (S101). In this propane cycle, propane refrigerant is evaporated (vaporized) through indirect heat exchange with natural gas in the first heat exchange region 300 (see 3010 and 3020 in FIG. 3). The vaporized propane refrigerant is introduced into the compressor 330 (see 331 and 332 of FIG. 3) and compressed. In this case, the compression of the propane refrigerant may be performed through a plurality of compressors connected in series. This is because it is preferable to compress the refrigerant using a multi-stage compressor in terms of reducing the power required of the compressor.

The compressed propane refrigerant is cooled by the cooler 350 to condense. The cooler 350 may be either a water-cooled or air-cooled cooler using sea water or the like. Propane refrigerant cooled in the cooler 350 flows into the expansion valve 370 (see 371 to 373 of FIG. 3) and expands. The expanded propane refrigerant precools the natural gas and condenses the ethylene refrigerant through indirect heat exchange with the ethylene refrigerant of the natural gas and the ethylene cycle in the first heat exchange zone 300 (see 3010, 3020 of FIG. 3). For reference, the first heat exchange region 300 described above may be an evaporator in which propane refrigerant is evaporated. And such an evaporator can usually be a heat exchanger.

Natural gas ⓒ precooled through indirect heat exchange with the propane refrigerant of the propane cycle is liquefied through indirect heat exchange with the ethylene refrigerant of the ethylene cycle (S102). To this end, in the ethylene cycle, similarly to the propane cycle, the ethylene refrigerant is evaporated through indirect heat exchange with natural gas in the second heat exchange zone (200, see 2010, 2020, 2030 of FIG. 3). The vaporized ethylene refrigerant is introduced into the compressor 230 (see 231 to 233 of FIG. 3) and compressed. In this case, the compression of the ethylene refrigerant may also be performed through a plurality of compressors connected in series similarly to the compression of the propane refrigerant.

The compressed ethylene refrigerant flows into the first heat exchange region 300 of the propane cycle and condenses through heat exchange with the propane refrigerant. The condensed ethylene refrigerant flows into the expansion valve 270 and expands, thereby liquefying the natural gas through indirect heat exchange with the natural gas in the second heat exchange region 200. For reference, although FIG. 1 shows that the compressed ethylene refrigerant is condensed in the first heat exchange region 300 of the propane cycle, the condensation method of the ethylene refrigerant is not limited thereto. For example, as illustrated in FIG. 3 to be described later, the ethylene refrigerant may be condensed through heat exchange with the propane refrigerant in a separate cooling region 510. However, there is no difference in that the ethylene refrigerant of the second closed loop refrigeration cycle is indirectly exchanged with and condensed with the propane refrigerant of the first closed loop refrigeration cycle.

Finally, the liquefied natural gas ⓕ is subcooled through indirect heat exchange with the nitrogen refrigerant of the nitrogen cycle (S103). In the nitrogen cycle described above, the gaseous nitrogen refrigerant is compressed by the compressor 130 and then precooled by the cooler 150. The precooled nitrogen refrigerant is cooled while passing through the cooling zone 170, and then flows into the expansion means 190 and is work expanded. Some work takes place in this expansion process, which can drive a compressor in the same or another cycle. In addition, the nitrogen refrigerant may be further cooled in the expansion process. For reference, the expansion means of the nitrogen cycle may be an expansion turbine or a turbo expander.

The expanded nitrogen refrigerant may indirectly heat exchange with the liquefied natural gas in the third heat exchange area 100 to supercool the liquefied natural gas. Thereafter, the nitrogen refrigerant flows back into the aforementioned cooling region 170. Since the nitrogen refrigerant flowing into the cooling zone 170 is still maintained at a low temperature, the nitrogen refrigerant flowing into the cooling zone 170 after being precooled by the cooler 150 may be cooled by indirect heat exchange. . Thereafter, the nitrogen refrigerant flows into the aforementioned compressor 130, and the above process is repeated. Finally, the nitrogen cycle subcools the liquefied natural gas based on the reverse Brayton cycle.

Based on the basic concept of the liquefaction process according to this embodiment with reference to Figure 3 will be described in more detail the liquefaction process according to the present embodiment. As shown in FIG. 3, the nitrogen cycle according to the present embodiment has two stages of third heat exchange zones 1010 and 1020. In addition, the ethylene cycle according to the present embodiment has three stages of second heat exchange zones 2010, 2020, and 2030 to effectively disperse the cooling load required for condensation. In addition, the propane cycle according to the present embodiment has two stages of first heat exchange zones 3010 and 3020, and has three stages of compressors 331, 332 and 333 for efficiently dissipating heat to the surroundings. As a result, the liquefaction process according to the present embodiment has seven cooling stages.

The nitrogen cycle among the three cycles according to the present embodiment will be described in detail. For reference, the propane cycle according to the present embodiment, the ethylene cycle is similar to the propane cycle according to the conventional cascade process, the ethylene cycle. That is, the propane cycle and the ethylene cycle according to the present embodiment are basically configured to continuously pass the compression-condensation-expansion-evaporation step according to the Joule-Thomson cycle as described above, and the natural gas and Natural gas is cooled by indirect heat exchange. And other information about the propane cycle and the ethylene cycle can be understood by those of ordinary skill in the art through the present invention, it will be omitted in the present specification.

The nitrogen cycle according to this embodiment has two stages of third heat exchange zones 1010 and 1020 as described above. The natural gas liquefied through the ethylene cycle may be supercooled by the expanded nitrogen refrigerant while sequentially passing through the third heat exchange zone 1010 of the first stage and the third heat exchange zone 1020 of the second stage. Herein, the expanded nitrogen refrigerant is introduced into the second heat exchange zones 1010 and 1020 of the second stage through two independent closed loop refrigeration cycles N2-1 and N2-2. Each closed loop refrigeration cycle is identical to the basic concept of the nitrogen cycle according to this embodiment described above. For reference, the fact that one refrigeration cycle is independent of another refrigeration cycle in this specification means that no heat exchange occurs between refrigerants of the two refrigeration cycles. For example, in this embodiment, no mutual heat exchange occurs between nitrogen refrigerants applied to two independent closed loop refrigeration cycles, and each completes one cycle within its own refrigeration cycle.

Referring to the two nitrogen cycles according to the present embodiment, as shown in Fig. 3, in each refrigeration cycle, the gaseous nitrogen refrigerants 115 and 125 are compressed by the compressors 131 and 133 and then cooled. It is precooled by 151,153. Thereafter, the precooled nitrogen refrigerants 111 and 121 flow into the cooling regions 171 and 173 and indirectly exchange heat with the nitrogen refrigerants 114 and 124 passing through the third heat exchange regions 1010 and 1020 of the stage. The nitrogen refrigerants 112 and 122 cooled by the heat exchange process are expanded by means of the expansion means 191 and 193 and then cooled to the third heat exchange zones 1010 and 1020 of the corresponding stage to supercool the liquefied natural gas. Let's do it. Thereafter, the nitrogen refrigerants 114 and 124 flow into the cooling zones 171 and 173 again, and the nitrogen refrigerants 115 and 125 passing through the cooling zones 171 and 173 flow into the compressors 131 and 133 again. Is compressed. In this refrigeration cycle, the step of compressing and precooling the nitrogen refrigerant may be repeatedly performed twice as shown in FIG. 3. That is, the nitrogen refrigerant is compressed by the compressors 131, 133 and precooled by the coolers 151, 153, then repeatedly recompressed by the separate compressors 132, 134 and the separate coolers 152, 154. Can be re-cooled by

In order to evaluate the performance of the liquefaction process according to the present embodiment having such a configuration, the temperature of the feed gas at points ⓐ to ⓗ in FIG. 3 is 300 K, ⓑ at the points ⓐ as shown in FIG. 4. 266 K at ⓒ, 245 K at ⓓ, 226 K at ⓓ, 204 K at ⓔ, 184 K at ⓕ, 147 K at ⓖ, 123 K at ⓖ, and temperature-entropy in each cycle. 5 and 7, respectively, are shown. 4 is a temperature-entropy diagram showing the thermodynamic properties of natural gas, and FIG. 5 is a temperature-entropy diagram in the propane cycle according to this embodiment. 6 is a temperature-entropy diagram in the ethylene cycle according to the present embodiment, and FIG. 7 is a temperature-entropy diagram in the nitrogen cycle according to the present embodiment. For reference, points indicated by numbers in the lines of FIGS. 5 to 7 correspond to points indicated by numbers in each cycle of FIG. 3.

In the liquefaction process according to this embodiment, the required power per unit LNG was estimated to be 1,494 KJ / Kg, and the figure of merit (FOM) was estimated to be about 30.9%. For reference, FOM is the ratio of actual work to minimum work. The highest reported liquefaction performance for the C3 / MR process is 1,050 to 1,100 KJ / Kg, and the highest reported liquefaction performance for the cascade process is 1,200 to 1,300 KJ / Kg. Considering the possibility of improvement of the liquefaction process according to the present embodiment, the liquefaction performance according to the present embodiment can be evaluated to a similar degree as the cascade process.

The liquefaction process according to the present embodiment basically maintains the basic framework of the cascade process, but since the closed loop refrigeration cycle in the subcooling stage is composed of nitrogen cycles, the refrigerants employed in each cycle are all single refrigerants. Accordingly, there is an advantage that the operation is simple and reliable in the liquefaction process. In addition, in the conventional C3 / MR process, not only natural gas is cooled in a propane cycle, but also a mixed refrigerant for liquefying and subcooling natural gas must be cooled. In the conventional cascade process, not only natural gas is cooled in the propane cycle, but also the refrigerant in the ethylene cycle and the refrigerant in the methane cycle must be cooled together.

Accordingly, in the conventional liquefaction process, the thermodynamic load applied to the propane cycle is inevitably increased, which leads to the enlargement of the compressor employed in the propane cycle. As a result, in the case of the conventional liquefaction process, the problem that the capacity (capacity) of the entire liquefaction process is limited depending on the compressor performance. However, in the liquefaction process according to the present embodiment, since the nitrogen cycle is independent of the propane cycle and the ethylene cycle, as shown in FIG. 3, the ethylene cycle refrigerant for liquefying natural gas in addition to cooling the natural gas in the propane cycle. Only need to cool. As a result, the thermodynamic load applied to the propane cycle can be reduced, and accordingly, the liquefaction process according to the present embodiment can increase the overall capacity compared to the conventional liquefaction process even with a compressor having the same capacity. This also means increasing the capacity of the entire liquefaction process.

In addition, thermodynamically, the more the temperature difference between the supply gas and the refrigerant is proportional to the absolute temperature, in other words, the smaller the temperature difference between the supply gas and the refrigerant is, the better the liquefaction process of liquefying natural gas is. Is increased. However, in the Joule-Thompson cycle, as shown in Figs. 5 and 6, since the refrigerant causes a phase change between the liquid phase and the gas phase during the cycle of the cycle (the same is true in the C3 / MR process or the cascade process), The change in temperature of the refrigerant is inevitably shown in the form of a step, which is not preferable in terms of thermodynamic performance since the temperature difference between the supply gas temperature and the refrigerant increases and decreases rapidly. However, in the liquefaction process according to the present embodiment, the above problems do not occur because the nitrogen cycle based on the Brayton cycle is employed in the subcooling step (see FIG. 7), and thus, the performance of the liquefaction process can be expected.

Example 2

8 is a flowchart showing the liquefaction process according to the second embodiment of the present invention in detail. The main difference between the nitrogen cycle according to the first embodiment and the nitrogen cycle according to this embodiment is shown in FIG. 8 in the case of the nitrogen cycle according to the present embodiment, as shown in FIG. The cycle is integrated into one nitrogen cycle. More specifically, in the nitrogen cycle according to the present embodiment, the nitrogen refrigerant completes one cycle through the following steps. First, the nitrogen refrigerant is compressed by the compressor 1131 and then precooled by the cooler 1151. Such compression and precooling may be repeatedly performed twice as shown in FIG. 8. That is, the nitrogen refrigerant may be compressed by the compressor 1131 and precooled by the cooler 1151, then repeatedly recompressed by the separate compressor 1132 and recooled by the separate cooler 1152.

The precooled nitrogen refrigerant is cooled by indirect heat exchange with the nitrogen refrigerant flowing into the first cooling region 1171 and refluxed. The cooled nitrogen refrigerant is then distributed into the first partial refrigerant and the second partial refrigerant. The distributed first partial refrigerant flows back into the second cooling region 1173 and is cooled by indirect heat exchange with the first partial refrigerant flowing back. The cooled first partial refrigerant is expanded through the first expansion means 1191 and then flows into the third heat exchange area 1020 of the second stage to supercool the natural gas through indirect heat exchange with the natural gas. Thereafter, the nitrogen refrigerant again flows into the compressor 1131 through the second cooling region 1171 and the first cooling region 1171.

The distributed second partial refrigerant is expanded through the second expansion means 1193 and then flows into the third heat exchange area 1010 of the first stage to supercool the natural gas through indirect heat exchange with the natural gas. Thereafter, the second partial refrigerant is mixed with the first partial refrigerant that has passed through the second cooling region 1171 and is introduced into the compressor 1131 through the first cooling region 1171 together with the first partial refrigerant. During this process, a certain work occurring in the first and second expansion means 1191 and 1193 can be used to drive the compressors 231 and 232 of the ethylene cycle. The liquefaction process as described above has the advantage that the expansion means of the nitrogen cycle and the compressor of the ethylene cycle can be integrated to reduce the number of configurations required for the entire process.

As described above, the present invention has been described with reference to preferred embodiments of the present invention, but a person of ordinary skill in the art does not depart from the spirit and scope of the present invention as set forth in the claims below. It will be understood that various modifications and variations can be made in the present invention. Therefore, the spirit of the present invention should be understood by the claims described below, and all equivalent or equivalent modifications thereof will belong to the scope of the present invention.

100: third heat exchange zone 200: second heat exchange zone
300: first heat exchange zone
131, 132, 133, 134, 231, 232, 233, 331, 332, 333: compressor
151, 152, 153, 154, 350: cooler
171, 173: cooling zone
271, 272, 273, 371, 372, 373: expansion valve
191, 193: expansion means

Claims (14)

In the natural gas liquefaction process using the three closed loop refrigeration cycle to liquefy natural gas,
Pre-cooling the natural gas supplied through vaporization of the first single refrigerant circulating in the first closed loop refrigeration cycle in the first heat exchange zone;
Liquefying the precooled natural gas in a second heat exchange zone through vaporization of a second single refrigerant circulating in a second closed loop refrigeration cycle; And
Sub-cooling the liquefied natural gas in a third heat exchange zone through indirect heat exchange with a work expanded nitrogen refrigerant in the course of circulating a third closed loop refrigeration cycle,
Wherein said third closed loop refrigeration cycle is based on a Reverse Brayton cycle. The method according to claim 1,
The third closed loop refrigeration cycle may include compressing the nitrogen refrigerant, precooling the compressed nitrogen refrigerant, and cooling the precooled nitrogen refrigerant through indirect heat exchange with the nitrogen refrigerant passing through the third heat exchange region. Natural gas liquefaction process comprising the steps of, and expanding the cooled nitrogen refrigerant. The method according to claim 1,
The subcooling step is characterized in that the liquefied natural gas is sequentially subcooled through a nitrogen refrigerant expanded in the second stage of the third heat exchange zone. The method according to claim 3,
The supercooling step includes the natural gas liquefaction process of one expanded nitrogen refrigerant of two independent three closed loop refrigeration cycle indirect heat exchange with the natural gas liquefied in the second heat exchange zone of the second stage. The method of claim 4,
The two third closed loop refrigeration cycles may include: compressing the nitrogen refrigerant, precooling the compressed nitrogen refrigerant, recompressing the precooled nitrogen refrigerant, recooling the recompressed nitrogen refrigerant, And cooling the re-cooled nitrogen refrigerant through indirect heat exchange with the nitrogen refrigerant passing through the third heat exchange zone of each stage, and expanding the cooled nitrogen refrigerant. The method according to claim 3,
The subcooling step is a natural gas liquefaction process, characterized in that the one expanded nitrogen refrigerant is indirect heat exchange with the natural gas liquefied in the second heat exchange zone of the second stage through one third closed loop refrigeration cycle. The method of claim 6,
Work generated through expansion means for expanding the nitrogen refrigerant of the third closed loop refrigeration cycle to drive the compression means for compressing the second single refrigerant of the second closed loop refrigeration cycle. Gas liquefaction process. The method of claim 6,
The one closed loop refrigeration cycle comprises: (a) compressing the nitrogen refrigerant, (b) precooling the compressed nitrogen refrigerant, (c) introducing the precooled nitrogen refrigerant into the first cooling zone, ( d) distributing the nitrogen refrigerant passing through the first cooling zone into the first partial refrigerant and the second partial refrigerant through step (c), and (e) introducing the distributed first partial refrigerant into the second cooling zone. (F) expanding the first partial refrigerant that has passed through the second cooling zone through step (e), and (g) the third heat exchange zone of the second stage having the expanded first partial refrigerant; (H) introducing the first partial refrigerant having passed through the third heat exchange region of the second stage into the second cooling region, (i) the second partial refrigerant distributed through the step (d) (J) introducing the expanded second partial refrigerant into the third heat exchange zone of the first stage; And (k) the second partial refrigerant having passed through the third heat exchange zone of the first stage together with the first partial refrigerant having passed through the second cooling zone through step (h); And a first partial coolant and a second partial coolant having passed through the first cooling region through step (k), and repeating the steps. Natural gas liquefaction process. delete The method according to claim 1,
Wherein said third closed loop refrigeration cycle is independent of said first and second closed loop refrigeration cycles. The method according to claim 1,
Wherein said first and second closed loop refrigeration cycles are based on a Joule-Thomson cycle. The method according to claim 1,
And the second single refrigerant of the second closed loop refrigeration cycle is condensed through indirect heat exchange with the first single refrigerant of the first closed loop refrigeration cycle. The method according to claim 1,
The precooling step sequentially precools the supplied natural gas through indirect heat exchange with the first single refrigerant in the first heat exchange zone of the second stage, and the liquefaction step includes the second stage of the third stage of the precooled natural gas. Natural gas liquefaction process, characterized in that to sequentially liquefy through indirect heat exchange with the second single refrigerant in the heat exchange zone. The method according to claim 1,
Wherein the first single refrigerant of the first closed loop refrigeration cycle is propane refrigerant and the second single refrigerant of the second closed loop refrigeration cycle is ethylene refrigerant.

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