A natural gas BOG direct reliquefaction system and method using LNG cold energy and throttling refrigeration

Abstract

The application discloses a kind of natural gas BOG direct reliquefaction system using LNG cold energy and throttling refrigeration, belong to natural gas liquefaction technical field, including booster unit, regenerator, expansion valve, heat exchanger, BOG buffer tank, LNG storage tank, LNG buffer tank;Wherein, the outlet of booster unit is sequentially connected with the first channel of regenerator, expansion valve, the first channel of heat exchanger, the second channel of regenerator and the inlet of booster unit;The second channel of heat exchanger is sequentially connected with the inlet end of LNG storage tank;The liquid outlet of LNG buffer tank is connected with the third channel of primary heat exchanger.The application utilizes the cold energy of LNG and nitrogen throttling refrigeration technology, and low-temperature low-pressure BOG is directly liquefied, and the system has the characteristics of safety, simple process structure and good economy.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of natural gas liquefaction, and particularly to a natural gas BOG direct reliquefaction system and method using LNG cold energy and throttling refrigeration. BACKGROUND

[0002] In the natural gas industry chain, natural gas is often made into liquefied natural gas (LNG) (its volume is about 1 / 600 of the original gaseous volume) for storage and transportation. In the LNG production process, the final stage throttle valve is needed to reduce the liquefaction pressure to the storage tank pressure. The boiling points of various components in natural gas are different (at normal pressure, nitrogen: 77.36K, methane 111.7K, propane: 231.03K), and under the storage tank pressure, the throttling process is equivalent to a simple evaporation of LNG, and the gas molecules with lower boiling points are first released from the LNG, which is called flash gas (Boil Off Gas, BOG). In addition, the gas-liquid mixture formed after the LNG flows through the throttle valve is sent to the LNG tank for storage through a low-temperature pipeline, and during the storage process, the heat leakage from the environment to the storage tank will also produce BOG, the components of which are mainly nitrogen and methane. In order to avoid overpressure of the LNG tank, BOG needs to be removed, and the common processing methods are burning and reliquefaction, among which the latter has more economic and social benefits due to the recovery of methane.

[0003] Common BOG reliquefaction refrigeration processes mainly include: mixed refrigerant cycle and nitrogen reverse Brayton cycle. Among them, the nitrogen reverse Brayton cycle uses nitrogen as the working medium, and the nitrogen does not change phase during the whole process, which has the advantages of safety, good robustness, small occupation area, and low operation and maintenance cost, and is widely used in natural gas liquefaction processes. The principle is that high-pressure nitrogen is expanded by an expander to reduce its temperature, and then the low-temperature and low-pressure nitrogen provides the cold energy required for BOG reliquefaction through sensible heat change.

[0004] A BOG reliquefaction recovery process for LNG transport ships using BOG as refrigerant is disclosed in Chinese patent document CN108870866A. The process includes the following steps: the raw BOG is reheated by a heat exchanger, then enters a compressor and an expander compression end for compression and cooling, and then enters the heat exchanger for further cooling, is input into a cold box, is throttled after cooling and reduction, and then enters a gas-liquid separation tank, and the liquid phase LNG is stored in a storage tank; the refrigerant BOG and the reheated raw BOG enter the compressor and the expander compression end for compression and cooling, then enter the heat exchanger for further cooling, are input into the cold box for precooling, the pre-cooled refrigerant BOG is led out of the cold box, enters the expander expansion end, is cooled after expansion and throttling, and then returns to the cold box to provide cold energy for the cold box; the reheated refrigerant BOG and the reheated raw BOG repeat the above operation.

[0005] However, the existing BOG reliquefaction process is to reheat the BOG to normal temperature and then pressurize (increase the liquefaction temperature), and then secondary cooling liquefaction, and then throttling to normal pressure, which increases heat exchange loss, equipment investment (such as BOG compressor set, multi-stream heat exchanger, throttle valve) and system complexity (such as control of the flow of each stream in the multi-stream heat exchanger), and new BOG is inevitably generated after throttling, reducing the liquefaction yield. SUMMARY

[0006] To improve the shortcomings of the existing process, the present application provides a natural gas BOG direct reliquefaction system using LNG cold energy and throttling refrigeration, which directly liquefies the low-temperature and low-pressure BOG using the cold energy of LNG and nitrogen throttling refrigeration technology, and has the characteristics of safety, simple process structure and good economy.

[0007] The technical scheme of the present application is as follows:

[0008] A natural gas BOG direct reliquefaction system using LNG cold energy and throttling refrigeration, comprising a pressurizing unit, a regenerator, an expansion valve, a heat exchanger, a BOG buffer tank, an LNG storage tank and an LNG buffer tank.

[0009] The outlet of the pressurizing unit is sequentially connected to the first channel of the regenerator, the expansion valve, the first channel of the heat exchanger, the second channel of the regenerator and the inlet of the pressurizing unit.

[0010] The BOG buffer tank is sequentially connected to the second channel of the heat exchanger and the inlet end of the LNG storage tank, and the outlet of the LNG buffer tank is connected to the third channel of the heat exchanger.

[0011] Further, the pressurizing unit adopts single-stage or multi-stage compression with inter-stage cooling. Preferably, water cooling is selected.

[0012] Further, the pressurized normal-temperature nitrogen gas from the pressurizing unit is pre-cooled by the cold nitrogen gas from the heat exchanger and the LNG from the LNG buffer tank in the first channel of the regenerator, the pre-cooled pressurized nitrogen gas is throttled and cooled by the expansion valve, the re-heated and gasified LNG is pressurized and metered for external use, and the re-heated nitrogen gas enters the pressurizing unit.

[0013] Furthermore, nitrogen throttling refrigeration technology is used to provide the cooling capacity required for BOG liquefaction. The working fluid after throttling through the expansion valve is in a gas-liquid two-phase state, and its temperature is lower than the saturation temperature of the liquefied BOG, preferably with a temperature difference of 1.5–3°C. This setting is because the pinch-point temperature difference of the two-stream heat exchangers generally occurs at the cold end of the heat exchanger. A small pinch-point temperature difference means a larger heat exchanger (affecting investment costs, floor space, etc.), but with higher heat transfer efficiency (according to the second law of thermodynamics, reducing the heat transfer temperature difference in the low-temperature section is more significant in improving system energy efficiency). Taking into account the heat exchange area of ​​the heat exchanger and the heat transfer temperature difference in the low-temperature section, a cold end temperature difference of 1.5–3°C is preferred.

[0014] Furthermore, the temperature of the nitrogen gas after being heated by the heat exchanger is lower than the temperature of the working fluid before expansion by the expansion valve, preferably with a temperature difference of 3-5℃. This setting is because the temperature of the nitrogen gas after being heated by the heat exchanger affects both the hot end temperature difference of the heat exchanger and the cold end temperature difference of the regenerator. To avoid the heat exchange area of ​​the regenerator being too large, and to take into account the heat transfer efficiency of both the heat exchanger and the regenerator, a temperature difference of 3-5℃ is preferred.

[0015] Furthermore, the BOG temperature in the BOG buffer tank is -120℃ to -140℃, and the gas pressure is 120kPa to 160kPa. The LNG in the LNG buffer tank is a saturated liquid with a gas pressure of 120kPa to 160kPa.

[0016] Furthermore, the nitrogen and natural gas at the outlets of the second and third channels of the regenerator are at the same temperature and above 0°C.

[0017] Furthermore, both the regenerator and the heat exchanger adopt a plate-fin structure.

[0018] The present invention also provides a method for direct reliquefaction of natural gas BOG, using the above-mentioned direct reliquefaction system for natural gas BOG, comprising the following steps:

[0019] Step 1: The circulating nitrogen is pressurized and cooled in the compression unit, and then enters the first channel of the regenerator to exchange heat with the cold nitrogen from the heat exchanger and the low-pressure saturated LNG from the LNG buffer tank, while being cooled itself.

[0020] Step 2: The low-pressure natural gas, which has been vaporized to near room temperature by the regenerator, is sent to the natural gas metering and pressurization device. The nitrogen gas, which has been cooled by the regenerator, is sent to the expansion valve for throttling and cooling, thus becoming a gas-liquid mixture.

[0021] Step 3: The nitrogen-liquid mixture obtained by depressurization through the expansion valve enters the first channel of the heat exchanger and exchanges heat with the low-pressure cryogenic BOG from the BOG buffer tank. At the same time, it is heated to a superheated state. The liquefied BOG enters the LNG storage tank for storage.

[0022] Step 4: The low-pressure superheated nitrogen gas, after being heated by the heat exchanger, enters the second channel of the regenerator and exchanges heat with the pressurized room-temperature nitrogen gas from the pressurization unit, while being heated and reheated itself.

[0023] Step 5: The nitrogen gas, after being reheated in the regenerator, enters the pressurization unit for pressurization and cooling, completing one cycle.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. This invention utilizes both the cold energy of LNG and nitrogen throttling refrigeration technology. The cold energy released from LNG vaporization provides most of the cooling required for pre-cooling nitrogen, thus eliminating the need for a turbine expander for nitrogen expansion and recovering valuable LNG cold energy. Furthermore, pressurizing the vaporized LNG eliminates the need for expensive cryogenic submersible pumps. The gas-liquid mixture of throttled nitrogen provides the cooling required for BOG reliquefaction, reducing nitrogen flow rate and heat transfer. This reduces energy loss (especially in the low-temperature range), improving the system's energy efficiency and economy.

[0026] 2. This invention directly liquefies cryogenic, low-pressure boil-off gas (BOG) from LNG storage tanks, eliminating the need for BOG reheating, compression, and throttling equipment, thus simplifying the process structure and improving economic efficiency. Furthermore, it avoids the problems caused by the BOG reheating and secondary cooling processes. This reduces losses and improves the overall energy efficiency of the system. Furthermore, by not throttling BOG after liquefaction, the generation of new BOG is avoided, thus increasing the liquefaction yield of BOG. Attached Figure Description

[0027] Figure 1 This is a process flow diagram of a natural gas BOG direct reliquefaction system that utilizes LNG cold energy and throttling refrigeration according to the present invention.

[0028] In the diagram: 1. Pressurization unit; 2. Regenerator; 3. Expansion valve; 4. Heat exchanger; 5. BOG buffer tank; 6. LNG storage tank; 7. LNG buffer tank. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the embodiments described below are intended to facilitate the understanding of the present invention and do not constitute any limitation thereof.

[0030] like Figure 1 As shown, a natural gas BOG direct reliquefaction system utilizing LNG cold energy and throttling refrigeration includes a pressurization unit 1, a regenerator 2, an expansion valve 3, a heat exchanger 4, a BOG buffer tank 5, an LNG storage tank 6, and an LNG buffer tank 7.

[0031] The outlet of the pressurizing unit 1 is sequentially connected with the first channel of the regenerator 2, the expansion valve 3, the first channel of the heat exchanger 4, the second channel of the regenerator 2 and the inlet of the pressurizing unit 1.

[0032] The BOG buffer tank 5 is sequentially connected with the second channel of the heat exchanger 4 and the inlet end of the LNG buffer tank 6; the liquid outlet of the LNG buffer tank 7 is connected with the third channel of the regenerator 2.

[0033] In the embodiment, the pressurizing unit 1 adopts the single-stage compression and water cooling mode. The cold energy required for the pre-cooling of the nitrogen gas is provided by the cold energy of the LNG and the backflow of the cold nitrogen gas.

[0034] The application provides the cold energy required for the liquefaction of the BOG by adopting the nitrogen throttling refrigeration technology. The working medium throttled by the expansion valve 3 is in the gas-liquid two-phase state and the temperature is lower than the saturation temperature of the liquefied BOG, and the temperature difference is 3℃.

[0035] The temperature of the nitrogen gas heated by the heat exchanger 4 is -153.1℃, and the temperature of the working medium before expansion of the expansion valve is -150℃.

[0036] The temperatures of the nitrogen gas and the natural gas at the outlets of the second and third channels of the regenerator 2 are equal, which is 4.4℃.

[0037] The temperature and pressure of the BOG in the BOG buffer tank 8 are -140℃ and 160kPa respectively, and the pressure and temperature of the LNG in the LNG buffer tank 7 are 160kPa and -162℃ respectively.

[0038] The regenerator 2 and the heat exchanger 4 adopt the plate-fin structure.

[0039] The liquefaction method of the liquefaction system comprises the following steps:

[0040] Step 1: the circulating nitrogen gas with the flow, pressure and temperature of 1215kmol / h, 1296kPa and 4.4℃ respectively is pressurized and cooled in the pressurizing unit 1 to 3910kPa and 31.85℃, and then enters the first channel of the regenerator 2 to exchange heat with the cold nitrogen gas (-153.1℃) from the heat exchanger 4 and the LNG (365kmol / h) from the LNG buffer tank 7, and is cooled to -150℃ itself;

[0041] Step 2: the natural gas (150kPa, 4.4℃) after the gasification of the regenerator 2 is sent to the natural gas metering and pressurizing device, and the nitrogen gas (3890kPa, -150℃) after the cooling of the regenerator 2 is sent to the expansion valve 3 to be throttled and cooled to become a gas-liquid mixture;

[0042] Step 3, the nitrogen-liquid mixture (1300 kPa, -165℃) obtained by pressure reduction through the expansion valve 3 enters the first passage of the heat exchanger 4, exchanges heat with the BOG (446.1 kmol / h, 160 kPa, -140℃) from the BOG buffer tank 5, and is heated to -153.1℃ at the same time. The liquefied BOG (150 kPa, -162℃) enters the LNG storage tank 6 for storage.

[0043] Step 4, the nitrogen gas (1306 kPa, -153.1℃) heated through the heat exchanger 4 enters the second passage of the regenerator 2, exchanges heat with the pressurized normal temperature nitrogen gas (1215 kmol / h, 3900 kPa, 31.85℃) from the pressurizing unit 1, and is heated to 4.4℃ at the same time.

[0044] Step 5, the nitrogen gas (1296 kPa, 4.4℃) rewarmed in the regenerator 2 enters the pressurizing unit 1 for pressurization, and completes a cycle.

[0045] In the embodiment of the liquefaction system, the complex and expensive low-temperature turbine expander is omitted, and the valuable LNG cold energy is recovered. The compression ratio of the compressor is not more than 3, and only one single-stage compressor is needed to meet the requirements. The highest operating pressure of the system is 3900 kPa, which reduces the strength requirements for equipment and pipelines. The energy consumption of the system per unit of liquefied product is as low as 0.17 kW·h / kg (BOG).

[0046] The above-described embodiments have described the technical solutions and beneficial effects of the present application in detail. It should be understood that the above-described embodiments are only specific embodiments of the present application, and are not used to limit the present application. Any modifications, supplements and equivalent replacements made within the principle range of the present application should be included in the protection range of the present application.

Claims

1. A method of direct reliquefaction of BOG from natural gas, characterized in that, The application discloses a natural gas BOG direct reliquefaction system using LNG cold energy and throttling refrigeration, which comprises a pressurizing unit (1), a regenerator (2), an expansion valve (3), a heat exchanger (4), a BOG buffer tank (5), an LNG storage tank (6) and an LNG buffer tank (7). The outlet of the pressurizing unit (1) is sequentially connected with the first channel of the regenerator (2), the expansion valve (3), the first channel of the heat exchanger (4), the second channel of the regenerator (2) and the inlet of the pressurizing unit (1). The BOG buffer tank (5) is sequentially connected with the second channel of the heat exchanger (4) and the inlet end of the LNG storage tank (6); and the outlet of the LNG buffer tank (7) is connected with the inlet of the third channel of the regenerator (2). The natural gas BOG direct reliquefaction method comprises the following steps. Step 1: circulating nitrogen is pressurized and cooled in the pressurizing unit (1), then enters the first channel of the regenerator (2) to exchange heat with cold nitrogen from the heat exchanger (4) and low-pressure saturated LNG from the LNG buffer tank (7), and is cooled at the same time. Step 2: low-pressure natural gas, which is gasified to near normal temperature through the regenerator (2), is sent to a natural gas metering pressurizing device; and the nitrogen cooled through the regenerator (2) is sent to the expansion valve (3) to be throttled and cooled to become a gas-liquid mixture; wherein the working medium throttled through the expansion valve (3) is in a gas-liquid two-phase state, and the temperature is lower than the saturated temperature of the liquefied BOG, and the temperature difference is 1.5-3 ℃. Step 3: the nitrogen-liquid mixture obtained by pressure reduction through the expansion valve (3) enters the first channel of the heat exchanger (4) to exchange heat with low-pressure low-temperature BOG from the BOG buffer tank (5), and is heated to a superheated state at the same time; and the liquefied BOG is stored in the LNG storage tank (6); wherein the temperature of the BOG in the BOG buffer tank (5) is-120 ℃ to-140 ℃, and the gas pressure is 120 kPa to 160 kPa. Step 4: the low-pressure superheated nitrogen heated through the heat exchanger (4) enters the second channel of the regenerator (2) to exchange heat with the pressurized normal-temperature nitrogen from the pressurizing unit (1), and is heated and recovered at the same time; wherein the temperature of the nitrogen heated through the heat exchanger (4) is lower than the temperature of the working medium before expansion of the expansion valve (3), and the temperature difference is 3-5 ℃. Step 5: the nitrogen recovered in the regenerator (2) enters the pressurizing unit (1) to be pressurized and cooled, and a cycle is completed.

2. The method of direct reliquefaction of natural gas BOG according to claim 1, characterized in that, The pressurizing unit (1) adopts single-stage or multi-stage compression and inter-stage cooling.

3. The method of direct reliquefaction of natural gas BOG according to claim 1, characterized in that, The pressurized normal-temperature nitrogen from the pressurizing unit (1) is pre-cooled in the first channel of the regenerator (2) by cold nitrogen from the heat exchanger (4) and LNG from the LNG buffer tank (7); the pre-cooled pressurized nitrogen is throttled and cooled through the expansion valve (3); the recovered and gasified LNG is pressurized, metered and then externally output for use; and the recovered nitrogen enters the pressurizing unit (1).

4. The method of direct reliquefaction of natural gas BOG according to claim 1, characterized in that, The LNG in the LNG buffer tank (7) is a saturated liquid with a gas pressure of 120 kPa to 160 kPa.

5. The method of direct reliquefaction of BOG from natural gas according to claim 1, characterized in that, The temperature of nitrogen and natural gas at the second and third passage outlets of the regenerator (2) is equal and higher than 0 DEG C.

6. The method of direct reliquefaction of BOG from natural gas according to claim 1, characterized in that, The regenerator (2) and the heat exchanger (4) are both of plate-fin structure.

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