Single-stage LNG cold energy recovery system with self-circulation

Abstract

The application discloses a single-stage LNG cold energy recovery system with self-circulation, which comprises an LNG supply main circuit, a main circulation loop, a self-circulation loop and a spiral pipe heat exchanger. The core feature of the system is that the self-circulation loop is connected in parallel to the main circulation loop through an inlet and outlet three-way valve, and a self-circulation pump that is always kept running is used to maintain the constant flow rate of the coolant in the heat exchanger, effectively solving the local freezing risk caused by insufficient heat exchange in the traditional single-stage circulation. The spiral pipe heat exchanger adopts a double-layer concentric pipe structure, LNG is sprayed from the jet hole of the inner pipe to realize atomization heat exchange, which significantly improves the heat transfer efficiency and reduces the heat exchange temperature difference. The system can adjust the bypass valve according to the downstream gas demand, and can be flexibly switched between different modes to balance the gas supply safety and cold energy recovery. The application has the advantages of compact structure, low cost, high heat recovery efficiency and safe and reliable operation, which significantly reduces the energy-saving modification cost of the existing LNG station.

Description

Technical Field

[0001] This invention relates to the field of cryogenic energy utilization and thermal management, specifically to a cold energy recovery system for the vaporization process of liquefied natural gas (LNG). Background Technology

[0002] With increasingly scarce energy resources and a growing emphasis on environmental protection, natural gas is gaining popularity compared to traditional fossil fuels due to its advantages such as lower pollution levels, lower carbon dioxide emissions, higher energy efficiency, and better economic cost-effectiveness. Currently, for convenient transportation, gaseous natural gas is often liquefied, and then vaporized before being supplied to users, releasing a significant amount of cold energy during this process.

[0003] In current industrial production, LNG is typically used as an energy reserve, and its vaporization process releases a significant amount of cold energy (approximately 860 kJ / kg). Traditional processes often use air vaporizers to directly release this cold energy into the atmosphere, resulting in substantial energy waste. Meanwhile, pharmaceutical and chemical production processes frequently require refrigerants with temperatures as low as -25°C (such as ethylene glycol solutions), currently relying heavily on high-power-consuming refrigeration units (COP≤1), resulting in enormous annual electricity consumption. Therefore, recovering the cold energy from LNG can not only significantly save energy and reduce consumption but also substantially lower plant operating costs.

[0004] Existing LNG cold energy recovery systems are generally divided into single-stage cycle (single-stage heat transfer) and two-stage cycle (two-stage heat transfer) systems: (1) In a single-stage circulation system, LNG directly exchanges heat with the terminal refrigerant (ethylene glycol) in a heat exchanger. The system has the simplest structure, low cost, high heat recovery efficiency, and wide usable temperature range. However, due to the large temperature difference between the two sides of the heat exchanger (LNG is -162°C, ethylene glycol is -20°C), it is very easy to cause insufficient heat exchange and local freezing of ethylene glycol, which poses a high safety risk.

[0005] (2) The two-stage circulation system transfers heat step by step by adding an intermediate refrigerant circulation stage, reducing the temperature difference of a single heat exchange. Although the system can reduce the risk of freezing, it is complex, has low cold energy utilization, low heat transfer efficiency, and is expensive.

[0006] Therefore, there is an urgent need for a cold energy recovery system that is simple, has high heat exchange efficiency, high cold energy utilization rate, and has no risk of freezing. Summary of the Invention

[0007] In existing LNG cold energy recovery systems: single-stage circulation systems are complex, costly, and require complex control, have low heat transfer efficiency, and have a small usable temperature range (-70℃); two-stage circulation systems have excessively large temperature differences in the heat exchangers, resulting in insufficient heat exchange and localized freezing, which poses safety risks.

[0008] To solve the above-mentioned technical problems, the present invention provides a single-stage LNG cold energy recovery system with self-circulation, characterized in that it includes: an LNG supply main line, a main circulation loop, a self-circulation loop, and a wound tube heat exchanger; the LNG supply main line includes an LNG storage tank, a first shut-off valve, a first pressure sensor, a bypass valve, an air-cooled vaporizer, a flow meter, a second shut-off valve, and a second pressure sensor, which are connected in sequence by a natural gas pipeline; the bypass valve is connected in parallel with the wound tube heat exchanger, and the natural gas pipeline is connected to the tube-side inlet and tube-side outlet of the wound tube heat exchanger respectively; the main circulation loop includes an outlet three-way valve, a main circulation loop, and a self-circulation loop, which are connected in sequence by a liquid pipeline. The system includes a ring pump, a storage tank, a filter, a main circulation loop flow meter, and an inlet three-way valve. The main circulation loop is connected to the shell-side inlet and shell-side outlet of the spiral wound tube heat exchanger between the outlet three-way valve and the inlet three-way valve. The self-circulation loop is connected to the main circulation loop through the inlet three-way valve and the outlet three-way valve. The self-circulation loop includes a self-circulation loop pump and a self-circulation loop flow meter, which are connected sequentially by liquid pipelines. The self-circulation loop and the main circulation loop are arranged in parallel. The spiral wound tube heat exchanger includes a tube-side inlet, a tube-side outlet, a shell-side inlet, and a shell-side outlet. Multiple parallel spiral wound tubes are connected between the tube-side inlet and the tube-side outlet.

[0009] Optionally, the wound tube heat exchanger is also equipped with a third pressure sensor and a level gauge; a safety valve is connected to the bottom.

[0010] Optionally, the spiral wound pipe includes an outer pipe and an inner pipe, which are concentrically arranged, and the outer pipe is connected to the pipe-side inlet and pipe-side outlet respectively.

[0011] Optionally, jet holes are evenly arranged on the inner pipe, and the working fluid in the inner pipe of the heat exchanger is LNG, which is ejected from the jet holes while flowing in the inner pipe.

[0012] Optionally, the liquid working medium stored in the cold storage tank is an ethylene glycol solution.

[0013] Optionally, the liquid flow rate in the self-circulating loop is 5~10m / s. Optionally, the self-circulating pump in the self-circulating loop is always in operation to maintain a certain flow rate of the refrigerant in the heat exchanger and prevent local overcooling.

[0014] Optionally, this single-stage LNG cooling recovery system can operate in the following modes based on the supply and cooling demand of liquefied natural gas: (a) When the cooling demand is greater than the cooling capacity stored in the natural gas supply, the bypass valve is closed and LNG flows into the heat exchanger for full cooling capacity recovery; (b) When the cooling demand is less than the cooling capacity stored in the natural gas supply, the bypass valve is opened / partially opened to recover part of the cooling capacity / no cooling capacity recovery, and vaporization is carried out using an air-cooled vaporizer.

[0015] The beneficial effects of this invention are: (1) By adding a self-circulating loop and keeping it constantly open by a self-circulating pump, a stable flow rate of refrigerant in the heat exchanger is maintained. This active circulation mechanism effectively prevents local overcooling that is prone to occur in single-stage heat transfer, and fundamentally eliminates the safety risk of heat exchanger freezing.

[0016] (2) The system adopts a special spiral wound tube structure, combined with a concentric double-layer flow channel design within the tube. The LNG is atomized by jetting through uniformly arranged jet holes in the inner tube, which greatly enhances the gas-liquid heat exchange capacity and significantly improves the overall heat transfer efficiency while reducing the heat exchange temperature difference.

[0017] (3) Compared with the complex two-stage circulation, this solution simplifies the process flow and reduces equipment costs through modular skid design. During installation, it only needs to be connected to the existing pipelines without large-scale changes to the original system, providing LNG plants with a highly cost-effective energy-saving retrofit approach.

[0018] (4) The system realizes intelligent control of the cold energy recovery mode through the bypass valve. According to the needs of the downstream gas pipeline network, the system can flexibly switch between the "full recovery" and "partial recovery" modes, and maximize the recovery of energy under the premise of ensuring a stable supply of natural gas. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the liquefied natural gas cold energy recovery and utilization system of the present invention; Figure 2 This is a schematic diagram of the overall structure of the heat exchanger in the liquefied natural gas cold energy recovery and utilization system of the present invention; Figure 3 This is a schematic diagram of the tube-side structure of the heat exchanger in the liquefied natural gas cold energy recovery and utilization system of the present invention; The meanings of the specific components marked in the diagram are as follows: 1-LNG main supply line; 11-LNG storage tank; 12-First shut-off valve; 13-First pressure sensor; 14-Bypass valve; 15-Air-cooled vaporizer; 16-Flow meter; 17-Second shut-off valve; 18-Second pressure sensor; 2-Main circulation loop; 21-Outlet three-way valve; 22-Main circulation pump; 23-Cold storage tank; 24-Filter; 25-Main circulation loop flow meter; 26-Inlet three-way valve; 3-Self-circulation loop; 31-Self-circulation pump; 32-Self-circulation loop flow meter; 4-Spiked tube heat exchanger; 41-Pipe-side inlet; 42-Pipe-side outlet; 43-Shell-side inlet; 44-Shell-side outlet; 45-Spiked wound tube; 451-Outer layer pipe; 452-Inner layer pipe; 453-Jet orifice; 46-Third pressure sensor; 47-Level gauge; 48-Safety valve. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] As indicated in this application and claims, unless the context clearly indicates otherwise, the words "a," "an," "an," and / or "the" are not specifically singular and may include plural forms. Generally speaking, the terms "comprising" and "including" only indicate the inclusion of explicitly identified steps and elements, which do not constitute an exclusive list, and the method or apparatus may also include other steps or elements.

[0022] Unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps described in these embodiments do not limit the scope of this application. It should also be understood that, for ease of description, the dimensions of the various parts shown in the drawings are not drawn to actual scale. Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of the specification. In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters in the following drawings denote similar items; therefore, once an item is defined in one drawing, it need not be further discussed in subsequent drawings.

[0023] Furthermore, it should be noted that the use of terms such as "first" and "second" to define components is merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, these terms have no special meaning and therefore should not be construed as limiting the scope of protection of this application. In addition, although the terminology used in this application is selected from commonly known and used terms, some terms mentioned in this application's specification may have been chosen by the applicant according to his or her judgment, and their detailed meanings are explained in the relevant sections of this description. Moreover, this application should be understood not only through the actual terms used, but also through the meaning implied by each term.

[0024] like Figure 1 As shown, this embodiment of the invention provides a cold energy recovery system for single-stage LNG vaporization with self-circulation, comprising an LNG supply main line 1, a main circulation loop 2, a self-circulation loop 3, and a wound tube heat exchanger 4; wherein the LNG supply main line comprises, in sequence, an LNG storage tank 11, a first shut-off valve 12, a first pressure sensor 13, a bypass valve 14, an air-cooled vaporizer 15, a flow meter 16, a second shut-off valve 17, and a second pressure sensor 18, which are connected in sequence by a natural gas pipeline.

[0025] In this embodiment, the bypass valve 14 is connected in parallel with the wound tube heat exchanger 4 to switch the flow mode according to the downstream demand for natural gas; the natural gas pipeline is connected to the pipe-side inlet 41 and pipe-side outlet 42 of the wound tube heat exchanger 4 respectively; the main circulation loop 2 includes, in sequence, an outlet three-way valve 21, a main circulation pump 22, a cold storage tank 23, a filter 24, a main circulation loop flow meter 25, and an inlet three-way valve 26, which are connected through a liquid pipeline; the main circulation loop 2 is connected to the shell-side inlet 41 and shell-side outlet 42 of the wound tube heat exchanger 4 between the outlet three-way valve 21 and the inlet three-way valve 26.

[0026] In this embodiment, the self-circulation loop 3 is connected to the main circulation loop 2 through the inlet three-way valve 21 and the outlet three-way valve 26. The self-circulation loop 3 includes a self-circulation pump 31 and a self-circulation loop flow meter 32 in sequence. The self-circulation loop 3 and the main circulation loop 2 are connected in parallel and are connected through a liquid pipeline. The self-circulation pump 31 is always running to maintain a certain flow rate of the refrigerant in the heat exchanger to prevent insufficient heat exchange of the refrigerant from causing freezing in the pipeline.

[0027] like Figure 2 and Figure 3 As shown, the structure of the wound heat exchanger 4 has the following characteristics: The spiral wound tube heat exchanger 4 comprises a tube-side inlet 41, a tube-side outlet 42, a shell-side inlet 43, a shell-side outlet 44, a spiral wound tube 45, a third pressure sensor 46, a level gauge 47, and a safety valve 48. The spiral wound tube 45 consists of multiple parallel spiral wound tubes, which are connected to the tube-side inlet 41 and the tube-side outlet 42 respectively. The spiral wound tube 45 is a double-layer pipe with an outer pipe 451 and an inner pipe 452 arranged concentrically. The outer pipe is connected to the tube-side inlet 41 and the tube-side outlet 42 respectively. The inner pipe is 0.5m to 1m long and has jet holes 453 evenly arranged on it for injecting atomized natural gas to enhance the heat exchange capacity.

[0028] The self-circulating single-stage LNG cooling recovery system provided in this embodiment can generate the following operating modes through bypass valve 14, based on the supply of liquefied natural gas and cooling demand: (a) When the cooling demand exceeds the stored cooling capacity in the natural gas supply, the bypass valve (14) closes. At this time, LNG flows out from the storage tank 11, passes through the shut-off valve 12 and the pressure sensor 13, and then enters the heat exchanger 4. The stored cooling liquid is diverted through the outlet three-way valve 21 under the drive of the main circulation pump 22 and the self-circulation loop pump 31. One part flows into the main circulation loop 2, passing through the main circulation pump 22, the storage tank 23, the filter 24 and the main circulation loop flow meter 25 in sequence; the other part enters the self-circulation loop 3, passing through the self-circulation loop pump 31 and the self-circulation loop flow meter 32, using the self-circulation loop pump to maintain a certain flow rate of the liquid in the heat exchanger 4. Finally, the two parts of liquid converge through the inlet three-way valve 26 and enter the heat exchanger 4 to recover all the cooling capacity with the LNG.

[0029] b. When the cooling demand is less than the cooling capacity stored in the natural gas supply, the bypass valve 14 is opened / partially opened, and the LNG partially / not entered the heat exchanger 4 for partial cooling capacity recovery / no cooling capacity recovery. The LNG that did not enter the heat exchanger 4 is vaporized using the air-cooled vaporizer 15.

[0030] Through the above design and operation, the present invention has the characteristics of simple and compact structure, low cost, simple control, stable and reliable heat transfer efficiency, moderate usable temperature range and no large temperature difference of heat exchanger. It can avoid local freezing and other safety problems caused by insufficient heat exchanger heat exchange, and is suitable for liquefied natural gas plants that perform cold energy recovery.

[0031] The above description is merely a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. For those skilled in the art, any obvious modifications, equivalent substitutions, or improvements made based on the technical teachings of the present invention without departing from the principles and concept of the present invention should be included within the scope of protection of the present invention. The scope of protection of the present invention should be defined by the appended claims.

Claims

1. A single-stage LNG cold energy recovery system with self-circulation, characterized in that, include: LNG supply main line (1), main circulation loop (2), self-circulation loop (3) and spiral tube heat exchanger (4); The LNG supply main line (1) includes an LNG storage tank (11), a first shut-off valve (12), a first pressure sensor (13), a bypass valve (14), an air-cooled vaporizer (15), a flow meter (16), a second shut-off valve (17), and a second pressure sensor (18) connected in sequence by a natural gas pipeline; the bypass valve (14) is connected in parallel with the spiral wound tube heat exchanger (4), and the natural gas pipeline is connected to the pipe-side inlet (41) and the pipe-side outlet (42) of the spiral wound tube heat exchanger (4) respectively; The main circulation loop (2) includes an outlet three-way valve (21), a main circulation pump (22), a liquid storage tank (23), a filter (24), a main circulation loop flow meter (25), and an inlet three-way valve (26) connected in sequence by liquid pipelines; the main circulation loop (2) is connected to the shell-side inlet (43) and shell-side outlet (44) of the wound tube heat exchanger (4) between the outlet three-way valve (26) and the inlet three-way valve (21); The self-circulating loop (3) is connected to the main circulation loop (1) through the inlet three-way valve (21) and the outlet three-way valve (26). The self-circulating loop (3) includes a self-circulating loop pump (31) and a self-circulating loop flow meter (32) connected in sequence by liquid pipelines. The self-circulating loop (3) is set in parallel with the main circulation loop (2). The spiral wound tube heat exchanger (4) includes a tube-side inlet (41), a tube-side outlet (42), a shell-side inlet (43), and a shell-side outlet (44); multiple parallel spiral wound tubes (45) are connected between the tube-side inlet (41) and the tube-side outlet (42).

2. The single-stage LNG cooling capacity recovery system according to claim 1, characterized in that: The spiral tube heat exchanger (4) is also connected to a third pressure sensor (46) and a level gauge (47); a safety valve (48) is connected to the bottom.

3. The single-stage LNG cooling capacity recovery system according to claim 1, characterized in that: The spiral wound pipe (45) includes an outer pipe (451) and an inner pipe (452), which are concentrically arranged. The outer pipe (451) is connected to the pipe-side inlet (41) and the pipe-side outlet (42) respectively.

4. The single-stage LNG cooling capacity recovery system according to claim 3, characterized in that: The inner pipe (452) is uniformly arranged with jet holes (453). The working fluid in the inner pipe of the heat exchanger (4) is LNG, which is ejected from the jet holes (453) during the flow of the inner pipe.

5. The single-stage LNG cooling capacity recovery system according to claim 1, characterized in that: The liquid working medium stored in the cold storage tank (23) is an ethylene glycol solution.

6. The single-stage LNG cooling capacity recovery system according to claim 1, characterized in that: The liquid flow rate in the self-circulating loop (3) is 5~10m / s.

7. The single-stage LNG cooling capacity recovery system according to claim 1, characterized in that: The self-circulating pump (31) in the self-circulating loop (3) is always in operation to maintain a certain flow rate of the refrigerant in the heat exchanger (4) and prevent local overcooling.

8. The single-stage LNG cold energy recovery system according to claim 1, characterized in that: This single-stage LNG cooling recovery system operates in the following modes based on the supply and cooling demands of liquefied natural gas: (a) When the cooling demand is greater than the cooling capacity stored in the natural gas supply, the bypass valve (14) is closed, and LNG flows into the heat exchanger (4) for full cooling capacity recovery; (b) When the cooling demand is less than the cooling capacity stored in the natural gas supply, the bypass valve (14) is opened / partially opened to recover part of the cooling capacity / no cooling capacity recovery, and vaporization is carried out using the air-cooled vaporizer (15).

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