WO2012057505A2

WO2012057505A2 - Natural gas liquefaction process

Info

Publication number: WO2012057505A2
Application number: PCT/KR2011/007994
Authority: WO
Inventors: 이상규; 최건형; 차규상; 박창원; 이영범; 조용범; 이철구; 장호명
Original assignee: 한국가스공사연구개발원
Priority date: 2010-10-26
Filing date: 2011-10-25
Publication date: 2012-05-03
Also published as: US20130263623A1; AU2011321145B2; KR101037226B1; AU2011321145A1; CA2816047C; CA2816047A1; WO2012057505A3

Abstract

The present invention relates to a natural gas liquefaction process, which involves pre-cooling natural gas using a closed-loop pre-cooling cycle, and liquefying the pre-cooled natural gas using a closed-loop liquefying cycle. The closed-loop pre-cooling cycle includes first and second pre-cooling cycles, which cooperate with one another to pre-cool the supplied natural gas in the same first heat exchange region using pure refrigerants with which said pre-cooling cycles are respectively provided. The closed-loop liquefying cycle includes at least one liquefying cycle for liquefying the pre-cooled natural gas using a mixed refrigerant. The first and second pre-cooling cycles are closed-loop refrigerating cycles.

Description

Natural Gas Liquefaction Process

The present invention relates to a natural gas liquefaction process, and more particularly, to configure the pre-cooling cycle to take advantage of both the pure refrigerant cycle and the mixed refrigerant cycle, the efficiency of the liquefaction process, while the equipment of the liquefaction system is small and the structure is simple The present invention relates to a natural gas liquefaction process that is easy to operate.

Thermodynamic processes for liquefying natural gas to produce liquefied natural gas (LNG) have been developed since the 1970s to meet the demand for higher efficiency, greater capacity, and simpler equipment. Various attempts have been made to liquefy natural gas by using different refrigerants or using different cycles to satisfy these requirements, but 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). The basic structure of the C3 / MR process is as shown in FIG. For reference, in FIG. 9, 'C3' represents a propane refrigerant cycle, and 'MR' represents a mixed refrigerant cycle. In FIG. 9, 'C' represents a compressor, 'AC' represents an after-cooler, 'V' represents a valve, and 'HX' represents a heat exchanger.

As shown in FIG. 9, the feed gas is pre-cooled to approximately 240 K by a multi-stage propane (C3) refrigerant cycle. The precooled feed gas is condensed and sub-cooled to approximately 113 K by a mixed refrigerant cycle, ie through heat exchange with the mixed refrigerant (MR) in a heat exchanger. Even in such a C3 / MR process, the general characteristics of the pure refrigerant cycle and the mixed refrigerant cycle are maintained. That is, pure refrigerant cycles are simple in structure and easy to operate, but require a large number of refrigeration stages. Mixed refrigerant cycles are complex and difficult to operate, but can achieve high efficiency with only a few components. Each cycle features the same characteristics in the C3 / MR process.

In detail, the mixed refrigerant cycle of the C3 / MR process for liquefying (and subcooling) the pre-cooled feed gas is usually a mixture consisting of nitrogen, methane, ethane, and propane. Use refrigerant. By appropriately selecting the composition of these components, and depending on the difference in boiling points of the components, the mixed refrigerant is suitably separated from the gaseous refrigerant portion and the liquid refrigerant portion, and then liquefied natural gas through each refrigerant portion. In the mixed refrigerant cycle of the / MR process, only a small number of facilities can show high efficiency. On the other hand, the pure refrigerant cycle of the C3 / MR process for precooling the feed gas uses a pure refrigerant called propane refrigerant, which is simple in structure and easy to operate, but usually requires three or four pressure stages. Is required. After all, in the case of C3 / MR processes, the focus is on simplicity in the precooling cycle (large number of installations, but the structure itself is simple), and efficiency in liquefaction cycles (the number of installations, but the structure itself is complex and efficient). It can be seen that.

One other successful liquefaction process in operation is the 'Dual Mixed Refrigerant process' (or DMR process). The basic structure of the DMR process is as shown in FIG. As illustrated in FIG. 10, the liquefaction (supercooling) cycle of the DMR process is basically the same as the liquefaction (supercooling) cycle of the C3 / MR process, that is, the mixed refrigerant cycle. However, in the DMR process, unlike the C3 / MR process, another mixed refrigerant cycle is used to precool the feed gas. The precooling cycle in the DMR process, unlike the liquefaction cycle in the DMR process, usually does not have a gas-liquid separator. In the end, the DMR process focuses on efficiency in both precooling and liquefaction cycles. However, it is known that the efficiency of the actual DMR process is substantially lower than that of the C3 / MR process.

As described above, the mixed refrigerant cycle is known to be more efficient than the pure refrigerant cycle. However, the structure itself is that the mixed refrigerant cycle is more complicated than the pure refrigerant cycle. In addition, many proposals have been made to improve the efficiency of the entire liquefaction process by applying mixed refrigerant cycles to refrigeration cycles for liquefying (supercooling) the pre-cooled natural gas. Therefore, there is a great need for research on pre-cooling cycles having a simple structure and excellent efficiency.

Therefore, the present invention has been made to solve the above problems, the object of the present invention is to configure the pre-cooling cycle to take advantage of both the pure refrigerant cycle and the mixed refrigerant cycle, the efficiency of the liquefaction process, while the equipment of the liquefaction system It provides a natural gas liquefaction process that is low in cost, simple in structure, and easy to operate.

According to a preferred embodiment of the present invention for achieving the above object of the present invention, by using a closed loop pre-cooling cycle to pre-cool the natural gas and a closed loop liquefying cycle In a natural gas liquefaction process for liquefying precooled natural gas using a closed circuit precooling cycle, the first and second precooling cycles for precooling the natural gas supplied together in the same first heat exchange region through respective pure refrigerants are performed. The closed circuit liquefaction cycle includes at least one liquefaction cycle for liquefying natural gas precooled through the mixed refrigerant, and the first and second precooling cycles are closed circuit refrigeration cycles.

Here, the pure refrigerant of the first precooling cycle may be ethane (C2) refrigerant, and the pure refrigerant of the second precooling cycle may be butane (C4) refrigerant. The first and second precooling cycles may include compressing the pure refrigerant, cooling the compressed refrigerant, additionally cooling the cooled refrigerant in the first heat exchange area, and expanding the additionally cooled refrigerant. It may include a step.

The closed circuit liquefaction cycle may include: compressing the mixed refrigerant, cooling the compressed refrigerant, additionally cooling the cooled refrigerant in the first heat exchange area to partially condense, and boiling the partially condensed refrigerant. Separating the liquid refrigerant portion and the gaseous refrigerant portion according to the difference of, cooling the natural gas pre-cooled in the second heat exchange region by using the liquid refrigerant portion, and using the gaseous refrigerant portion by a third And secondarily cooling the naturally cooled natural gas in the heat exchange zone.

Wherein the primary cooling may include: a first step of cooling the liquid refrigerant part through heat exchange in the second heat exchange region, a second step of expanding the cooled refrigerant part through the first step, and the And a third step of cooling the natural gas by exchanging the refrigerant portion expanded through the second step with the natural gas in the second heat exchange region. The secondary cooling may include a cooling step of cooling the gaseous refrigerant part through heat exchange in the second heat exchange area, and a heat exchange of the refrigerant part cooled through the cooling step in the third heat exchange area. A condensation step of condensing through, an expansion step of expanding the refrigerant part condensed through the condensation step, and cooling the natural gas by heat-exchanging the expanded refrigerant part and the natural gas in the third heat exchange region through the expansion step. It may include the step.

Natural gas liquefaction process according to the present invention has the effect that the pre-cooling cycle can be configured with a relatively small number of facilities because the pre-cooling cycle pre-cools the natural gas in only one pressure step.

In addition, the natural gas liquefaction process according to the present invention has the effect that the structure itself is simple and easy operation of the liquefaction system because each pre-cooling cycle uses a pure refrigerant.

Furthermore, the natural gas liquefaction process according to the present invention has the effect that the efficiency of the liquefaction process is very excellent because two pre-cooling cycles are arranged in parallel to pre-cool the natural gas in the same heat exchange zone.

1 is a flowchart illustrating a natural gas liquefaction process according to an embodiment of the present invention.

2 is a graph showing the temperature profile in the precooling region of a conventional C3-MR process.

3 is a graph showing the temperature profile in the precooling region of a conventional DMR process

4 is a graph illustrating a temperature profile in a precooling region of a liquefaction process according to an embodiment of the present invention.

5 is a temperature-entropy diagram of ethane and propane cycles in a liquefaction process in accordance with one embodiment of the present invention.

6 to 8 are graphs showing exergy use and irreversibility of the conventional CR / MR process, the conventional DMR process, and the precooling step of the liquefaction process according to an embodiment of the present invention, respectively.

9 is a flow chart illustrating a conventional C3 / MR process.

10 is a flowchart conceptually illustrating a conventional DMR 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.

1 is a flowchart illustrating a natural gas liquefaction process according to an embodiment of the present invention. In the liquefaction process according to the present embodiment, as shown in FIG. 1, the natural gas is precooled using a closed loop pre-cooling cycle, and a closed loop liquefying cycle is used. It can be applied to the natural gas liquefaction process to liquefy the pre-cooled natural gas. In addition, the liquefaction process according to the present embodiment may further include a refrigeration cycle for cooling the mixed refrigerant or further cooling the natural gas.

Hereinafter, a liquefaction process according to an embodiment of the present invention will be described with reference to FIG. 1. Basically, the liquefaction process according to this embodiment includes a closed loop refrigeration cycle for precooling the supplied natural gas and a closed loop refrigeration cycle for liquefying (or liquefying and subcooling) the precooled natural gas. Since the refrigeration cycles according to the present embodiment are all closed loop cycles, each cycle undergoes one closed cycle while independently undergoing the steps of compression-condensation-expansion-evaporation. The closed circuit refrigeration cycle, that is, the closed circuit precooling cycle for precooling the supplied natural gas, includes two different precooling cycles. These two precooling cycles are of course also closed circuit refrigeration cycles.

Referring to the liquefaction process according to the present embodiment in more detail, the natural gas supplied as shown in FIG. 1 is precooled by the first precooling cycle and the second precooling cycle in the first heat exchange region 110 (described later). As described above, in the liquefaction process according to the present embodiment, the first precooling cycle is a ethane (C2) refrigerant cycle, and the second precooling cycle is a butane (C4) refrigerant cycle). That is, the supplied natural gas is precooled by the pure refrigerant of the first precooling cycle and the pure refrigerant of the second precooling cycle in the same heat exchange region of the first heat exchange region 110. To this end, the first precooling cycle and the second precooling cycle are each compressed, condensed, expanded, and evaporated.

In the above first and second precooling cycles, the pure refrigerant first flows into the

compressors

151 and 161 through the

conduits

201 and 401 and is compressed. Pure refrigerant then enters and cools the

coolers

152 and 162 through

conduits

202 and 402. This compression and cooling process can be made in multiple stages as shown in FIG. That is, a plurality of compressors and coolers can be connected in series. In this case, the cooled pure refrigerant flows back into the

compressors

153 and 163 through the

conduits

203 and 403, and the compressed pure refrigerant is again cooled through the

conduits

204 and 404. It may be introduced into and cooled. As such, when the compressor is configured in multiple stages and the pure refrigerant is compressed in multiple stages, the required power of the compressor can be reduced.

The pure refrigerant compressed and cooled as described above may be further cooled through heat exchange with the refrigerant flowing into the

conduits

205 and 405 and the first heat exchange region 110, and through this process, the pure refrigerant may be condensed. have. However, as described below, depending on the boiling point of the pure refrigerant, the pure refrigerant may be condensed by cooling by the cooler described above. In this case, in the condensed state, the pure refrigerant may flow into the first heat exchange region 110 to be further cooled. Cooling of the refrigerant in the first heat exchange region 110 may be performed by the refrigerant flowing back into the first heat exchange region 110 through the

conduits

207 and 407. That is, the pure refrigerant cooled through the heat exchange in the first heat exchange region 110 as described above is introduced into the

expansion valves

155 and 165 through the

conduits

206 and 406, expanded and cooled, and then the

conduits

207 and 407. Into the first heat exchange region 110 again through) can cool the natural gas and the refrigerant.

The above processes are the same in the first precooling cycle and the second precooling cycle. That is, as shown in FIG. 1, the first precooling cycle and the second precooling cycle are arranged in parallel to form a closed circuit cycle to precool the natural gas complementarily supplied in the first heat exchange region 110. As will be described later, the most important feature in the liquefaction process according to the present embodiment is that two closed loop refrigeration cycles employing pure refrigerant are arranged in parallel to precool the natural gas supplied in the same heat exchange zone.

In this embodiment, ethane (C2) refrigerant and butane (C4) refrigerant are used as the pure refrigerant of the first and second precooling cycles. In the above-described C3 / MR process, a pure refrigerant composed of propane (C3) is used for precooling of natural gas, and in the aforementioned DMR process, 45.5 mole% of ethane (C2) and propane (C3) 4.9 for precooling of natural gas. A mixed refrigerant consisting of mole% and butane (C4) 49.6 mole% is used. That is, in the above-described C3 / MR process, since a single pure refrigerant is used, a plurality of pressure levels are required and a large number of facilities are required. However, the structure itself is simple and the operation of the precooling cycle is easy. In the case of using a mixed refrigerant, a small number of facilities are required, but the structure itself is complicated, and there is a difference that it is difficult to operate a precooling cycle.

The liquefaction process according to the present embodiment is arranged in parallel as described above to take advantage of the advantages of the two basic structures as described above, that is, the use of pure refrigerant and the use of mixed refrigerant for precooling. Two pure refrigerant cycles are used. In addition, the efficiency of the overall liquefaction process can be optimized by constituting two pure refrigerant cycles with an ethane cycle and a butane cycle. The mixed refrigerant in the pre-cooling step of the above-described DMR process is composed of ethane, propane and butane components, but contains very little propane components. In the liquefaction process according to the present embodiment, the precooling effect due to the above propane components is used. It can be said that it is replacing by the effect of the cycle.

Natural gas precooled through the two pure refrigerant cycles as described above is liquefied (or liquefied and supercooled) through the mixed refrigerant cycle. In detail, the mixed refrigerant partially condensed through the heat exchange in the first heat exchange region 110 is introduced into the gas-liquid separator 171 through the conduit 301, and according to the difference in boiling point, The second coolant part is separated into a lower boiling point than the coolant part. That is, the partially condensed mixed refrigerant may be divided into a first refrigerant portion separated into the liquid refrigerant portion due to the high boiling point through the gas-liquid separator 171 and a second refrigerant portion separated into the gaseous refrigerant portion due to the low boiling point. have.

The first refrigerant portion thus separated is introduced into the second heat exchange region 120 through the conduit 302 and cooled. Cooling of this refrigerant portion may be through heat exchange with the refrigerant entering the second heat exchange region 120 through the conduit 304. The cooled refrigerant portion enters and expands into expansion valve 172 through conduit 303. The expanded refrigerant portion may be mixed with the second refrigerant portion, which will be described later, and then introduced back into the second heat exchange region 120 through conduit 304 to cool other refrigerants and liquefy natural gas. Then, the mixed refrigerant may be cooled through a ethane cycle and a butane cycle together with the natural gas supplied to the first heat exchange region 110 after being subjected to a series of compression and cooling processes.

The separated second refrigerant portion then enters the second heat exchange zone 120 through the conduit 306 and is cooled. Cooling of this refrigerant portion may be through heat exchange with the refrigerant entering the second heat exchange region 120 through the conduit 304. The cooled refrigerant portion enters and condenses the third heat exchange region 130 through conduit 307. Condensation of this refrigerant portion may be accomplished through heat exchange with the refrigerant entering the third heat exchange region 130 through the conduit 309. The condensed refrigerant portion enters and expands into expansion valve 173 through conduit 308. The expanded refrigerant portion enters the third heat exchange region 130 again through the conduit 309 to condense the refrigerant introduced into the third heat exchange region 130 through heat exchange and liquefy or supercool the natural gas. For reference, the liquefied natural gas may be expanded by the expansion valve 181 and then introduced into the storage tank.

The refrigerant part which has completed the heat exchange in the third heat exchange area 130 may be mixed with the aforementioned first refrigerant part and flowed back into the second heat exchange area 120. For reference, the three

heat exchange regions

110, 120, and 130 described above may be provided together in one heat exchange means as shown in FIG. 1, or may be provided in three heat exchange means, respectively. The heat exchange means may also be a conventional heat exchanger.

Effects of the liquefaction process according to the present embodiment having the above configuration will be described in detail with reference to FIGS. 2 to 8. 2 and 3 show the temperature distribution in the precooling region of the above-described C3-MR process and DMR process, respectively. Propane (C3) in the C3-MR process is a pure refrigerant and passes through several stages of pressure steps, so that the temperature distribution appears in a stepped manner as shown in FIG. In contrast, the temperature distribution in the precooling zone of the DMR process changes gradually, with a minimum difference (3K) in the middle of the heat exchange zone. 4 and 5 show the temperature distribution in the precooling region of the liquefaction process according to the present embodiment and the temperature-entropy diagrams of the ethane and butane cycles in the liquefaction process according to the present embodiment, respectively.

In the precooling step of the liquefaction process according to the present embodiment, since each pure refrigerant flows into the heat exchange zone in the liquid phase (see reference numeral 9 of FIGS. 4 and 5), the cold stream temperature is ethane refrigerant and butane. Corresponding to the vaporization of the refrigerant has two horizontal regions (see reference numerals 9 to 10 and 11 to 12 of FIG. 4). On the contrary, the butane refrigerant after pre-cooling the natural gas is condensed during the multi-stage compression and cooling process through the compressor and the cooler, and then flows back into the heat exchange area into the liquid phase (see reference numeral 5 of FIGS. 4 and 5). The temperature of the hot stream has only one horizontal region (see 6-7 in FIG. 4) in response to the condensation of the ethane refrigerant.

6 to 8 illustrate the exergy utilization and irreversibility of the three processes described above, namely, the CR / MR process, the DMR process, and the precooling step of the liquefaction process according to the present embodiment. Exergy efficiency, defined as the ratio of increase of exergy to power input, is 34.3%, 30.5%, and 31.5% in each liquefaction process, as shown in FIGS. appear. As described above, in the case of the C3 / MR process, a plurality of pressure stages are required and a large number of equipments are required. In the case of the DMR process, since the mixed refrigerant is used, the structure itself is complicated and operation of the liquefaction system In view of the difficulty of this difficulty, it can be seen that the effect of the liquefaction process according to the present embodiment is very excellent.

That is, in the liquefaction process according to the present embodiment, since each pre-cooling cycle pre-cools the natural gas with only one pressure step, the pre-cooling cycle can be configured with a relatively small number of facilities, and each pre-cooling cycle uses pure refrigerant. Since the structure itself is simple and the operation of the liquefaction system is easy, the liquefaction process is very excellent because the ethane refrigerant cycle and the butane refrigerant cycle are arranged in parallel to precool the natural gas in the same heat exchange area. As a result, the liquefaction process according to the present embodiment has both the advantages of the structure of pre-cooling natural gas by using pure refrigerant and the structure of pre-cooling natural gas by employing mixed refrigerant, and have very high efficiency (liquefaction known to date). The efficiency of the liquefaction process according to this embodiment is very good considering that the C3 / MR process is one of the very high efficiency processes.

In addition, the irreversibility in the precooling step of each liquefaction process is represented by dividing into four groups of valve (V), aftercooler (AC), compressor (C), heat exchanger (HX), as shown in Figs. Can be. In comparison with the liquefaction process according to the present embodiment and the C3 / MR process, it can be seen that the C3 / MR process is relatively irreversible by the valve compared to the liquefaction process according to this embodiment. In comparison with the liquefaction process and the DMR process according to the present embodiment, it can be seen that the DMR process is relatively irreversible by the cooler compared to the liquefaction process according to the present embodiment. As a result, the liquefaction process according to the present embodiment has a lower irreversibility by the valve and the cooler than the two liquefaction processes described above, while the irreversibility by the heat exchanger is high.

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.

In the present invention, since the pre-cooling cycle pre-cools the natural gas in only one pressure step, the pre-cooling cycle can be constituted by a relatively small number of facilities, and the structure of each pre-cooling cycle uses pure refrigerant. It is a natural gas liquefaction process that has a very high efficiency of the liquefaction process because it is simple and easy to operate the liquefaction system, and two precooling cycles are arranged in parallel to precool the natural gas in the same heat exchange area. There is a possibility.

Claims

In a natural gas liquefaction process of pre-cooling natural gas using a closed loop pre-cooling cycle and liquefying the pre-cooled natural gas using a closed loop liquefying cycle,

The closed loop precooling cycle includes first and second precooling cycles for precooling together natural gas supplied in the same first heat exchange region through respective pure refrigerants, and the closed circuit liquefaction cycle is precooled through the mixed refrigerant. And at least one liquefaction cycle for liquefying natural gas, wherein the first and second precooling cycles are closed circuit refrigeration cycles.
The method according to claim 1,

The pure refrigerant of the first precooling cycle is an ethane (C2) refrigerant, the pure refrigerant of the second precooling cycle is a butane (C4) refrigerant.
The method according to claim 1,

The first and second precooling cycles may include: compressing the pure refrigerant, cooling the compressed refrigerant, further cooling the cooled refrigerant in the first heat exchange zone, and expanding the additionally cooled refrigerant. Natural gas liquefaction process comprising a.
The method according to claim 1,

The closed loop liquefaction cycle includes: compressing a mixed refrigerant, cooling the compressed refrigerant, additionally cooling the cooled refrigerant in the first heat exchange zone to partially condense, and partially boiling the partially condensed refrigerant to a boiling point. Separating the liquid refrigerant portion and the gaseous refrigerant portion according to the difference, first cooling the natural gas pre-cooled in the second heat exchange region using the liquid refrigerant portion, and using the gaseous refrigerant portion to perform a third heat exchange. Natural gas liquefaction process comprising the step of secondarily cooling the naturally cooled natural gas in the region.
The method according to claim 4,

The primary cooling may include a first step of cooling the liquid refrigerant part through heat exchange in the second heat exchange region, a second step of expanding the cooled refrigerant part through the first step, and the first step. And a third step of cooling the natural gas by exchanging the refrigerant portion expanded through the two steps with the natural gas in the second heat exchange region.
The method according to claim 4,

The secondary cooling may include a cooling step of cooling the gaseous refrigerant part through heat exchange in the second heat exchange region, and a refrigerant part cooled through the cooling step through heat exchange in the third heat exchange region. A condensation step of condensing, an expansion step of expanding the refrigerant part condensed through the condensation step, and cooling the natural gas by heat-exchanging the expanded refrigerant part and the natural gas in the third heat exchange area through the expansion step. Natural gas liquefaction process comprising the step.