KR101153055B1 - Natural gas liquefaction system using plate fin heat exchanger - Google Patents

Abstract

PURPOSE: A natural gas liquefaction system using plate fin heat exchanger is provided to compactly constitute the liquefaction system by using a plate fin heat exchanger. CONSTITUTION: A natural gas liquefaction system using plate fin heat exchanger(111) comprises a first plate fin heat exchanger, a first storage tank, a first compressor(131), and a cooling device. The first plate pin cools natural gas with main refrigerant through heat change. The first storage tank stores sub refrigerant which is exhausted from the first plate pin row switch. The first storage tank comprises a weather vent and a liquid vent. In the weather vent, the weather part is exhausted among the sub refrigerant.

Description

Natural gas liquefaction system using plate fin heat exchanger {NATURAL GAS LIQUEFACTION SYSTEM USING PLATE FIN HEAT EXCHANGER}

The present invention relates to a natural gas liquefaction system, and more particularly to a natural gas liquefaction system that can be compactly configured liquefaction plant, also suitable for offshore plants.

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. Various attempts to liquefy natural gas using different refrigerants or using different cycles to satisfy these requirements, that is, to increase the efficiency and capacity of the liquefaction process, continue to be made. The number of liquefaction processes present is very small.

One of the most popular liquefaction processes in operation is the 'Propane Pre-cooled Mixed Refrigerant Process' (or C3 / MR process). As shown in FIG. 4, in the C3 / MR process, natural gas (NG) is first precooled to approximately 238 K via a Joule-Thomson cycle (or propane cycle) employing propane (C3) refrigerant. (pre-cooled) At this time, the mixed refrigerant MR for liquefying and subcooling the natural gas is also cooled together with the natural gas. Natural gas is then liquefied and sub-cooled to approximately 123 K through a mixed refrigerant cycle employing a mixed refrigerant (MR, Mixed Refrigerant or Multi-component Refrigerant).

Meanwhile, the propane cycle of the C3 / MR process typically uses a compressor in three stages to improve its efficiency (see FIG. 5). However, since the conventional C3 / MR process uses a shell & tube heat exchanger (see FIG. 6) in a propane cycle, the C3 / MR process compactly constitutes a liquefaction plant that implements the C3 / MR process. There is a problem that it is very difficult to construct a marine plant to implement the process.

More specifically in this regard, the shell tube heat exchanger allows only two flows (eg, one natural gas and one propane refrigerant), as shown in FIG. 6. Accordingly, in order to configure the compressor of the propane cycle in three stages in the C3 / MR process, as shown in FIG. 5, three heat exchangers must be used for natural gas and three heat exchangers for mixed refrigerant. Since six shell tube heat exchangers have to be used, a liquefaction plant that implements a conventional C3 / MR process has a considerable weight and volume (the weight and volume of the shell tube heat exchanger itself are considerable), As a result, it is very difficult to construct a liquefied plant compactly, and it is more difficult to construct an offshore plant with limited weight and volume.

In addition, when the C3 / MR process is implemented in an offshore plant, there is a problem that the heat exchanger constituting the propane cycle is very poor. In more detail, for example, when the liquefaction plant is implemented on a ship, a situation occurs in which the heat exchanger also tilts due to the shaking of the hull caused by the waves. When the heat exchanger is inclined as described above, in the case of the shell tube heat exchanger, a portion of the tube through which the natural gas (or mixed refrigerant) flows may be exposed out of the propane refrigerant. When a part of the tube is exposed out of the propane refrigerant in this way, since the heat exchange can not be achieved by that much, the performance of the heat exchanger is inevitably deteriorated.

The same problem can occur in the cascade process by Conoco Phillips. More specifically on this, the cascade process by Conoco Phillips consists of three Joule-Thomson cycles using methane (C1), ethylene (C2), and propane (C3) (see FIG. 7). Each of these cycles also typically uses a compressor in three stages to improve efficiency, and also uses a shell tube heat exchanger as the heat exchanger. As such, the same problems as in the propane cycle of the C3 / MR process can occur in the cascade process by Conoco Phillips.

Therefore, the present invention has been made to solve the above problems, the object of the present invention is to provide a natural gas liquefaction system that can be configured not only compactly liquefied plant, but also suitable for offshore plants.

According to a preferred embodiment of the present invention for achieving the above object of the present invention, at least two refrigeration cycles including one closed loop refrigeration cycle employing sub refrigerant and another closed loop refrigeration cycle employing main refrigerant The natural gas liquefaction system for liquefying and liquefying natural gas step by step, the first plate fin heat exchanger, the first plate fin heat exchanger for cooling the main refrigerant and natural gas together through heat exchange with the sub-coolant A first storage tank for storing the sub-coolant discharged from the heat exchanger, a first compressor for compressing the sub-coolant in the gas phase discharged from the first storage tank, and a cooler for cooling the sub-coolant discharged from the first compressor.

In the natural gas liquefaction system according to the present invention, by using a plate fin heat exchanger as a heat exchanger constituting a closed loop refrigeration cycle, the liquefaction plant implementing the liquefaction system can be compactly constructed, and the liquefaction plant can be easily converted into an offshore plant. Since the refrigerant discharged from the plate fin heat exchanger is stored in the storage tank prior to the compressor, various problems caused by using the plate fin heat exchanger can be solved by a simple structure of the storage tank.

1 is a flowchart showing a natural gas liquefaction system according to Embodiment 1 of the present invention.
FIG. 2 is a partial cutaway perspective view showing a plate fin heat exchanger applied to the natural gas liquefaction system of FIG.
3 is a flow chart illustrating a closed loop refrigeration cycle according to Embodiment 2 of the present invention.
4 conceptually illustrates a conventional C3 / MR process.
5 is a flow chart illustrating the propane cycle in more detail in the process of FIG. 4.
6 is a side cross-sectional view illustrating a shell tube heat exchanger applied to the propane cycle of the process of FIG. 4.
7 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

FIG. 1 is a flowchart illustrating a natural gas liquefaction system according to Embodiment 1 of the present invention, and FIG. 2 is a partially cutaway perspective view illustrating a plate fin heat exchanger applied to the natural gas liquefaction system of FIG. 1. The natural gas liquefaction system according to Embodiment 1 of the present invention gradually cools natural gas through at least two closed loop refrigeration cycles. For example, one refrigeration cycle (see propane cycle of C3 / MR process described above) may employ sub-coolant to precool natural gas, and another refrigeration cycle (mixed refrigerant cycle of C3 / MR process described above). The main refrigerant may be used to liquefy (or liquefy and supercool) natural gas.

However, the division between the sub coolant and the main coolant is not absolute. That is, the refrigerant of the refrigeration cycle for precooling natural gas is necessarily a sub refrigerant, and the refrigerant of the refrigeration cycle for liquefying natural gas is not necessarily the main refrigerant. In other words, the sub refrigerant and the main refrigerant are simply terms used to distinguish the refrigerant used in each cycle. For example, in the aforementioned cascade process, not only propane (C3) cycle for precooling natural gas but also ethylene (C2) cycle for liquefying natural gas may be a refrigeration cycle employing a sub refrigerant.

The natural gas liquefaction system according to the present embodiment may include two refrigeration cycles similar to the aforementioned C3 / MR process, and may include three refrigeration cycles similar to the above-described cascade process. However, for convenience of explanation, hereinafter, a closed loop refrigeration cycle for precooling natural gas by using a sub refrigerant (for example, propane refrigerant) and a main refrigerant (for example, a mixed refrigerant) may be used for natural gas. A liquefaction system including another closed loop refrigeration cycle of liquefaction will be described as an example.

As shown in FIG. 1, the liquefaction system according to the present embodiment includes a plate fin heat exchanger 111, a storage tank 121, a compressor 131, a cooler 141, and an expansion valve 151. Here, the plate fin heat exchanger 111 refers to a plate fin heat exchanger as its name. Plate fin heat exchanger 111, as shown in Figure 2, is one of the plate-type heat exchanger (plate-type heat exchanger), is made by stacking a layer of corrugated fin between the metal plate. The plate fin heat exchanger 111 may allow several flows in the heat exchanger. That is, as shown in FIG. 2, the plate fin heat exchanger 111 may allow three (see A, B, C) or more flow therein.

Accordingly, when the plate fin heat exchanger 111 is used, as shown in FIG. 1, the main refrigerant MR and the natural gas NG may be cooled together with only one flow of the sub refrigerant, and as a result, The number of heat exchangers to be installed in a natural gas liquefaction system can be reduced by half than when using a conventional shell tube heat exchanger. In addition, since the number of heat exchangers is reduced by half, the weight or volume of the liquefaction plant implementing the refrigeration cycle can also be significantly smaller than when using a conventional shell tube heat exchanger. Moreover, the weight and volume of the plate fin heat exchanger itself is very small compared to shell tube heat exchangers.

As a result, the liquefaction system according to the present embodiment can compactly configure the liquefaction plant that implements the liquefaction plant by using the plate fin heat exchanger 111 in the refrigeration cycle employing the sub-coolant. It can be easily configured as an offshore plant. This advantage is more pronounced when the compressor of the refrigerating cycle is configured in multiple stages as described in Example 2.

On the other hand, the sub coolant is partially changed to the gas phase through heat exchange in the plate fin heat exchanger (111). When the liquid refrigerant flows into the compressor 131, damage may occur to the compressor 131. Therefore, the sub refrigerant may be changed into the gas phase through heat exchange. In reality, however, only a part of the sub coolant changes to the gas phase. Therefore, even if the plate fin heat exchanger 111 has the above-described advantages, there is a problem that damage to the compressor 131 may occur if it is applied to the refrigeration cycle as it is. The liquefaction system according to the present embodiment solves this problem with the storage tank 121.

More specifically with respect to the storage tank 121, the storage tank 121 is a configuration for storing the sub-coolant discharged from the plate fin heat exchanger 111 described above. At this time, the liquid sub-coolant is stored in the storage tank 121 as it is, and the sub-coolant in the gas phase is discharged to the compressor 131 through a conduit. When the discharge port is formed in the upper portion of the storage tank 121, only the gaseous sub-coolant may be discharged to the compressor 131 through the above-described discharge port unless the liquid sub-coolant flows from the storage tank 121. As described above, the liquefaction system according to the present embodiment solves various problems that may occur because the plate fin heat exchanger 111 is used by a simple configuration of the storage tank 121.

That is, in the liquefaction system according to the present embodiment, since the sub-coolant passes through the plate fin heat exchanger 111 and then passes through the storage tank 121 having no weight or volume, the structure is not complicated. Without complicating or greatly increasing the weight or volume of the entire system, it is possible to enjoy the advantages of the plate fin heat exchanger 111 as well as to prevent damage to the compressor 131.

In addition, when the storage tank 121 is used as described above, even if the liquefaction system according to the present embodiment is implemented in an offshore plant, the performance of the plate fin heat exchanger 111 does not decrease. More specifically, in the case of the conventional shell tube heat exchanger, as described above, when the heat exchanger is inclined due to the shaking of the hull caused by waves, a part of the tube through which natural gas (or mixed refrigerant) flows may be exposed out of the propane refrigerant. As a result, the performance of the heat exchanger was inevitably deteriorated. That is, when a part of the tube is exposed out of the propane refrigerant, the heat exchanger area between the tube and the propane refrigerant is reduced, so the performance of the heat exchanger is inevitably deteriorated.

However, when the sub-coolant passes through the plate fin heat exchanger 111 and passes through the storage tank 121 as in the liquefaction system according to the present embodiment, the heat exchanger 111 or the storage tank 121 is tilted due to the shaking of the hull. However, the inclination of the sub coolant occurs only in the storage tank 121 which is independent of the heat exchange (the sub coolant flows in a very small space in the plate fin heat exchanger). Accordingly, even if shaking occurs in the hull, the heat exchange area between the natural gas (or the main refrigerant) and the sub-coolant may be maintained as it is, and as a result, the performance of the plate fin heat exchanger 111 may be maintained regardless of the shaking. Can be.

Meanwhile, the sub-coolant in the gaseous phase discharged from the storage tank 121 flows into the compressor 131 and is compressed. Then, the sub refrigerant flows into the cooler 141 and is cooled. This cooler 141 may be a conventional air-cooled or water-cooled cooler. Finally, the sub refrigerant is expanded through the expansion valve 151 and then flows back into the plate fin heat exchanger 111. As such, the sub refrigerant passes through the plate fin heat exchanger 111, the storage tank 121, the compressor 131, the cooler 141, and the expansion valve 151 in sequence, and then returns to the plate fin heat exchanger 111. By entering it forms one closed loop. The main refrigerant also forms a closed loop. However, since the specific configuration may vary, only one box 150 is represented.

However, the use of the storage tank 121 also has the advantage that the control of the liquefaction system becomes easy. More specifically, when the sub coolant maintaining the equilibrium state in the storage tank 121 is maintained at a specific pressure, the temperature of the sub coolant is also determined in particular according to the pressure. Because the pressure is set, the equilibrium temperature is set. The liquefaction system according to the present embodiment can also easily adjust the temperature of the sub refrigerant by controlling the pressure of the sub refrigerant with the compressor 131 using this principle. Accordingly, the liquefaction system according to the present embodiment has an advantage of easy temperature control.

Despite this ease, however, if the amount of cold heat required by the heat exchanger 111 increases, the amount of sub coolant that changes to the gaseous phase in the heat exchanger 111 also increases, so that the liquid level of the sub coolant in the storage tank 121 increases. level) gradually lowers, and as a result, a problem arises that it is difficult to keep the performance of the plate fin heat exchanger 111 constant. In order to solve this problem, the liquefaction system according to the present embodiment uses the expansion valve 151. That is, when the flow rate of the sub coolant discharged from the cooler 141 through the expansion valve 151 and introduced into the first plate fin heat exchanger 111 is adjusted, as a result, the level of the sub coolant in the storage tank 121 is adjusted. Can be. Through such level control, the liquefaction system according to the present embodiment can maintain the performance of the heat exchanger constantly.

As a result, the liquefaction system according to the present embodiment not only can easily perform temperature control by using the storage tank 121, but also indirectly gives the liquid level control to the plate fin heat exchanger 111 to perform the performance of the heat exchanger. Can be kept constant. That is, the plate fin heat exchanger 111 does not have the concept of liquid level unlike the conventional shell tube heat exchanger, so in principle, the concept of liquid level control cannot be introduced, but the liquefaction system according to the present embodiment is stored as described above. By introducing the configuration of the tank 121, it is possible to maintain the performance of the heat exchanger 111 by controlling the liquid level indirectly through the storage tank 121.

On the other hand, if the sub coolant flows vertically upward (see FIG. 2C) inside the plate fin heat exchanger 111, there is an advantage that the flow of the sub coolant becomes smoother. Since the sub-coolant changes into the gas phase through heat exchange, the vertical coolant flows more smoothly than the vertical downward flow. On the other hand, the main refrigerant and the natural gas is preferably opposite to the sub refrigerant, that is, vertically flowing (see FIGS. 2A and 2B) for efficient heat exchange.

For reference, the liquefaction system according to the present embodiment may include three or more refrigeration cycles. For example, the liquefaction system according to the present embodiment may include one refrigeration cycle employing the sub refrigerant and two refrigeration cycles employing the main refrigerant. In this case, each of the main refrigerants circulating in the refrigeration cycle may be introduced to the plate fin heat exchanger described above. Taking the above described cascade process (see FIG. 7) as an example, one refrigeration cycle employing propane (C3) refrigerant (sub refrigerant), an ethylene (C2) refrigerant (main refrigerant), and a methane (C1) refrigerant (main In each of the two refrigeration cycles employing the refrigerant), the methane (C1) refrigerant and the ethylene (C2) refrigerant may flow together into the same plate fin heat exchanger and be cooled by the propane (C3) refrigerant. As described above, when three or more refrigeration cycles are included, which one is regarded as a sub refrigerant and how many main refrigerants to be cooled by the sub refrigerant can be appropriately selected in consideration of the efficiency of the liquefaction process.

Example 2

3 is a flowchart illustrating a closed loop refrigeration cycle according to Embodiment 2 of the present invention. The refrigeration cycle shown in FIG. 3 represents one refrigeration cycle constituting the liquefaction system according to the second embodiment. Since the closed loop refrigeration cycle employing the main refrigerant is substantially the same as the closed loop refrigeration cycle employing the main refrigerant of Embodiment 1, its illustration is omitted in FIG. And the refrigeration cycle according to the present embodiment as shown in Figure 3 constitutes a compressor in three stages. In this regard, there is a difference from the refrigeration cycle according to the first embodiment of the compressor in one stage. For reference, the same (or equivalent) parts with the same (or equivalent) parts as those described above will be given the same reference numerals, and detailed description thereof will be omitted.

As shown in FIG. 3, the refrigeration cycle according to the present embodiment includes three heat exchangers 211, 212, and 213, storage tanks 221, 222, and 223, and three compressors 231, 232, and 233, respectively. . Each refrigerant flow is consequently all the same. That is, in the same flow, the sub coolant passes through the first plate fin heat exchanger 211, the first storage tank 221, and the first compressor 231 in the same flow. The fin heat exchanger 212, the second storage tank 222, and the second compressor 232 are sequentially passed. However, each flow constitutes one closed loop as a whole.

As described above, the storage tanks 221 and 222 according to the present exemplary embodiment have a gas outlet (not shown) and a liquid outlet (not shown) to make several flows with one sub-coolant. The gas phase outlet is a discharge port through which the gaseous portion of the sub refrigerants in the storage tanks 221 and 222 is discharged, and the liquid phase discharge port is a discharge port through which the liquid phase portion of the sub refrigerants in the storage tanks 221 and 222 is discharged. The sub-coolant in the gas phase discharged to the gas phase outlet is introduced into the compressors 231 and 232 as in the above-described embodiment. For reference, it can be seen that the third storage tank 223 has only a gas phase outlet without a liquid discharge outlet, and the sub-coolant in the gas phase discharged to the gas discharge outlet flows into the third compressor 233. And the liquid sub-coolant discharged to the liquid discharge port is introduced into the next plate fin heat exchanger (212, 213) to make one flow again.

That is, the sub-coolant in the gas phase discharged from the gas phase outlet of the first storage tank 221 flows into the first compressor 231 to maintain the existing refrigerant flow, and is discharged from the liquid outlet of the first storage tank 221. The liquid sub-coolant flows into the second plate fin heat exchanger 212 to form one refrigerant flow again. This branching of the refrigerant can be repeated continuously depending on the number of stages required. For example, when the three stage compressor is to be configured, the liquid sub-coolant may be introduced into the third plate fin heat exchanger 213 again from the liquid phase outlet of the second storage tank 222. It is the same that the sub coolant sequentially passes through the plate fin heat exchanger, the storage tank and the compressor in each flow. As a result, just defining two sets of plate fin heat exchangers, storage tanks, and compressors (i.e., first and second) can explain the liquefaction system in all cases.

Meanwhile, the sub refrigerant passing through the compressor of the next stage is introduced into the compressor of the previous stage and compressed again together with the sub refrigerant of the gas phase discharged from the storage tank of the previous stage. For example, the sub coolant discharged from the second compressor 232 flows into the first compressor 231 together with the sub coolant discharged from the gas phase outlet of the first storage tank 221. Expansion valves 252 and 253 are provided between the first storage tank 221 and the second plate fin heat exchanger 212, or between the second storage tank 222 and the third plate fin heat exchanger 213. . The expansion valves 252 and 253 also play the same role as the expansion valves 151 and 251 described in the above-described embodiment.

For reference, the refrigeration cycle according to the present embodiment has three coolers 241, 242, 243, as shown in FIG. 3. Accordingly, the sub refrigerant passing through the compressors 231, 232, and 233 is cooled while passing through the respective coolers 241, 242, and 243. However, the coolers 242 and 243 between the third compressor 233 and the second compressor 232 and between the second compressor 232 and the first compressor 231 are not essential and may be provided as necessary. May or may not.

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.

111: plate fin heat exchanger 121: storage tank
131: compressor 141: cooler
151: expansion valve

Claims (9)

A natural gas liquefaction system for cooling and liquefying natural gas in stages through at least two refrigeration cycles including one closed loop refrigeration cycle employing sub refrigerant and another closed loop refrigeration cycle employing main refrigerant.
A first plate fin heat exchanger for cooling the main refrigerant and the natural gas together through heat exchange with the sub refrigerant, and a first storage for storing the sub refrigerant discharged from the first plate fin heat exchanger And a tank, a first compressor for compressing the sub-coolant in the gaseous phase discharged from the first storage tank, and a cooler for cooling the sub-coolant discharged from the first compressor. The method according to claim 1,
The first storage tank includes a gas phase outlet through which the gaseous part of the sub refrigerant is discharged and a liquid phase outlet through which the liquid part of the sub refrigerant is discharged, and the sub refrigerant discharged from the gaseous outlet is introduced into the first compressor. The sub-coolant discharged from the liquid discharge port flows into a second plate fin heat exchanger provided separately, and the main refrigerant and the natural gas flow through the first plate fin heat exchanger and then flow into the second plate fin heat exchanger. Natural gas liquefaction system. The method according to claim 2,
Liquefied natural gas further comprises a second storage tank for storing the sub-coolant discharged from the second plate fin heat exchanger, and a second compressor for compressing the sub-coolant in the gas phase discharged from the second storage tank. system. The method according to claim 3,
The sub-coolant discharged from the second compressor is introduced into the first compressor together with the sub-coolant discharged from the gas phase outlet of the first storage tank. The method according to claim 1,
The natural gas liquefaction system further comprises a first expansion valve for controlling the liquid level of the sub-coolant in the first storage tank by adjusting the flow rate of the sub-coolant discharged from the cooler to the first plate fin heat exchanger . The method according to claim 3,
And a second expansion valve configured to control the liquid level of the sub coolant in the second storage tank by adjusting a flow rate of the sub coolant discharged from the liquid discharge port of the first storage tank and introduced into the second plate fin heat exchanger. Natural gas liquefaction system. The method according to claim 1,
The first plate fin heat exchanger is a natural gas liquefaction system, characterized in that the main refrigerant and the natural gas flows vertically downward, and the sub-refrigerant flows vertically upward. The method according to claim 2,
The second plate fin heat exchanger is a natural gas liquefaction system, characterized in that the main refrigerant and the natural gas flows vertically downwards, and the sub refrigerant flows vertically upward. The method according to claim 1,
A plurality of closed loop refrigeration cycles employing the main refrigerant is provided, and the main refrigerant of at least two refrigeration cycles of the plurality of closed loop refrigeration cycles performs heat exchange with the sub refrigerant in the first plate fin heat exchanger. Characterized in natural gas liquefaction system.

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