A direct reliquefaction system and method for natural gas BOG based on nitrogen expansion and throttling refrigeration

Abstract

The present invention discloses a natural gas BOG direct reliquefaction system and method based on nitrogen expansion and throttling refrigeration, wherein the outlet of the secondary boosting unit is connected to the first channel of the primary regenerator and then divided into two paths, one path is sequentially connected to the first channel of the secondary regenerator, the expansion valve, the first channel of the heat exchanger, the second channel of the secondary regenerator, and the inlet end of the second channel of the primary regenerator; the other path is sequentially connected to the expander unit, the third channel of the secondary regenerator, the third channel of the primary regenerator, and the inlet end of the primary boosting unit; the outlet of the primary boosting unit and the outlet of the second channel of the primary regenerator are respectively connected to the inlet end of the secondary boosting unit; the BOG buffer tank is sequentially connected to the second channel of the heat exchanger and the inlet end of the LNG storage tank. By using the present invention, the liquefaction yield of BOG can be increased and the liquefaction energy consumption can be reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas liquefaction, and in particular to a natural gas BOG direct reliquefaction system and method based on nitrogen expansion and throttling refrigeration. Background Art

[0002] In the natural gas industry chain, natural gas is often made into liquefied natural gas (LNG) (whose volume is about 1 / 600 of the original gaseous volume) for easy storage and transportation. In the LNG production process, the liquefaction pressure needs to be reduced to the tank pressure through the final throttle valve. The boiling points of the components in natural gas are different (under normal pressure, nitrogen: 77.36K, methane 111.7K, propane: 231.03K). Under the tank pressure, the throttling process is equivalent to a simple evaporation of LNG. The gas molecules with lower boiling points escape from LNG first, which is called flash gas (Boil Off Gas, BOG). In addition, the gas-liquid mixture formed after LNG flows through the throttle valve is sent to the LNG tank for storage through a low-temperature pipeline. During the storage process, BOG will also be generated due to heat leakage from the environment to the tank. Its components are mainly nitrogen and methane. In order to avoid overpressure in the LNG tank, BOG needs to be removed. Common treatment methods include combustion and reliquefaction. Among them, the latter has more economic and social benefits because of the recovery of methane.

[0003] The existing BOG reliquefaction process mostly adopts the method of reheating BOG to room temperature and then pressurizing (increasing the liquefaction temperature), then cooling and liquefying it for the second time, and then throttling it to normal pressure. For example, the Chinese patent document with the publication number CN108870866A discloses a BOG reliquefaction recovery process suitable for LNG transport ships using BOG as a refrigerant. The process includes the following steps: after the raw material BOG is reheated by the heat exchanger, it enters the compressor and the expansion end of the expander for compression and cooling, and then enters the heat exchanger for further cooling, and is input into the cold box for cooling and throttling, and then enters the gas-liquid separation tank, and the liquid phase LNG enters the storage tank for storage; the refrigerant BOG and the reheated raw material BOG enter the compressor and the expansion end of the expander for compression and cooling, and then enters the heat exchanger for further cooling, and is input into the cold box for pre-cooling. The pre-cooled refrigerant BOG is drawn out from the cold box, enters the expansion end of the expander, and after expansion and throttling, returns to the cold box to provide cooling for the cold box; the refrigerant BOG after reheating repeats the above operation with the reheated raw material BOG.

[0004] However, the above process increases heat exchange losses, equipment investment (such as BOG compressor unit, multi-stream heat exchanger, throttling valve) and system complexity (such as control of the flow rate of each stream in the multi-stream heat exchanger), and new BOG is inevitably generated after throttling, which reduces the liquefaction output.

[0005] In addition, for the BOG direct reliquefaction process using a single-stage nitrogen reverse Brayton cycle, the temperature rise curve of the low-temperature and low-pressure nitrogen (cold fluid) after expansion is a straight line with a slope (inversely proportional to the product of mass flow and constant-pressure specific heat capacity) greater than zero, while the temperature of the BOG (hot fluid) phase change process does not change much, and the constant-pressure specific heat capacity of nitrogen changes little with temperature (except for the critical point). Therefore, relying on the sensible heat of nitrogen to provide the latent heat required for BOG reliquefaction will result in a larger nitrogen flow rate and a higher heat transfer temperature difference in the low-temperature section, and the overall energy efficiency of the system is low. On the other hand, the low-temperature nitrogen after gasification is not enough to precool the high-pressure and normal-temperature nitrogen to the pre-throttling temperature. Summary of the invention

[0006] In order to improve the shortcomings of the existing process, the present invention provides a natural gas BOG direct reliquefaction system based on nitrogen expansion and throttling refrigeration, which adopts nitrogen expansion refrigeration technology and liquid nitrogen throttling refrigeration technology to directly liquefy the low-temperature and low-pressure BOG from the LNG storage tank. The system has the characteristics of safety, high thermodynamic efficiency, good economy and large product volume.

[0007] A natural gas BOG direct reliquefaction system based on nitrogen expansion and throttling refrigeration, comprising a primary booster unit, a secondary booster unit, a primary regenerator, a secondary regenerator, an expander unit, an expansion valve, a heat exchanger, a BOG buffer tank and an LNG storage tank;

[0008] The outlet of the secondary booster unit is connected to the first channel of the primary regenerator and then divided into two paths. One path is sequentially connected to the first channel of the secondary regenerator, the expansion valve, the first channel of the heat exchanger, the second channel of the secondary regenerator, and the inlet end of the second channel of the primary regenerator; the other path is sequentially connected to the expander unit, the third channel of the secondary regenerator, the third channel of the primary regenerator, and the inlet end of the primary booster unit.

[0009] The outlet of the first-stage boosting unit and the outlet of the second channel of the first-stage regenerator are respectively connected to the inlet end of the second-stage boosting 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.

[0011] Furthermore, the first-stage boost unit and the second-stage boost unit adopt a single-stage or multi-stage compression and inter-stage cooling method. Preferably, the first-stage boost unit adopts a two-stage compression and inter-stage water cooling method, and the second-stage boost unit adopts a first-stage compression and aftercooler water cooling method.

[0012] Furthermore, the expander unit adopts a multi-stage expansion refrigeration technology. Preferably, a two-stage gas bearing turbine expander is selected, which does not carry liquid after expansion.

[0013] Furthermore, the low-temperature nitrogen passing through the outlet of the first channel of the first-stage regenerator is divided into two paths, one path enters the expander unit for expansion and cooling; the other path enters the first channel of the second-stage regenerator, is cooled to be completely liquefied, and the temperature is not higher than the saturation temperature of nitrogen, and then is reduced in pressure through the expansion valve.

[0014] Furthermore, the temperature of the nitrogen-liquid mixture after throttling by the expansion valve is lower than the saturation temperature of the liquefied BOG, with a temperature difference of 1.5 to 3°C. This setting is because the pinch point temperature difference of the two-stream heat exchanger generally occurs at the cold end of the heat exchanger. A small pinch point temperature difference means a larger heat exchanger (affecting investment cost, floor space, etc.), but with higher heat transfer efficiency (according to the second law of heat, reducing the heat transfer temperature difference in the low temperature section is more significant in improving the energy efficiency of the system). Considering 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 to 3°C is preferably selected.

[0015] Furthermore, the temperature of the nitrogen after being heated by the heat exchanger is lower than the temperature of the nitrogen-liquid mixture before being expanded by the expansion valve, and the temperature difference is 3-5°C. This setting is because the temperature of the nitrogen after being heated by the heat exchanger affects both the temperature difference at the hot end of the heat exchanger and the temperature difference at the cold end of the secondary heat regenerator. In order to avoid an excessively large heat exchange area of ​​the secondary heat regenerator and to take into account the heat transfer efficiency of the heat exchanger and the secondary heat regenerator, the preferred temperature difference is 3-5°C.

[0016] Furthermore, the temperature of the nitrogen at the outlet of the second and third channels of the primary regenerator is equal and lower than the temperature of the nitrogen at the outlet of the secondary booster unit, with a temperature difference of 3-5°C. This temperature difference setting can not only recover as much cold nitrogen as possible, but also reduce the heat transfer temperature difference at the hot end of the primary regenerator, improve the system energy efficiency, and avoid excessive heat exchange area. Furthermore, the temperature of the nitrogen at the outlet of the second and third channels of the secondary regenerator is equal.

[0017] Furthermore, the nitrogen pressure from the primary booster unit is not lower than the nitrogen pressure from the outlet of the second channel of the primary regenerator. Preferably, the two pressures are selected to be equal.

[0018] Furthermore, the primary heat regenerator, the secondary heat regenerator and the heat exchanger adopt a plate-fin structure.

[0019] Furthermore, the BOG temperature in the BOG buffer tank is -120°C to -140°C, and the gas pressure is 120kPa to 160kPa.

[0020] The present invention also provides a method for reliquefying natural gas BOG, using the above system, comprising the following steps:

[0021] Step 1: The low-pressure circulating nitrogen is pressurized and cooled in the first-stage pressurizing unit, mixed with the low-pressure and room-temperature nitrogen from the second channel of the first-stage regenerator, and then enters the second-stage pressurizing unit for further pressurization;

[0022] Step 2, the high-pressure room-temperature nitrogen from the secondary booster unit enters the first channel of the primary regenerator for cooling, and the cooled high-pressure nitrogen is divided into two parts: one part enters the first channel of the secondary regenerator for heat exchange with the reflux and the low-pressure cold nitrogen from the expander unit, and is liquefied; the other part enters the expander unit for expansion and cooling;

[0023] Step 3, the liquid nitrogen from the first channel of the secondary regenerator is depressurized by an expansion valve to become a gas-liquid mixture;

[0024] Step 4, the nitrogen-liquid mixture obtained by throttling the expansion throttle valve enters the first channel of the heat exchanger, exchanges heat with the raw material BOG from the BOG buffer tank, and the liquefied BOG is sent to the LNG storage tank for storage;

[0025] Step 5, the low-pressure nitrogen heated by the heat exchanger and the low-pressure nitrogen expanded by the expander unit enter the second and third channels of the secondary regenerator respectively, providing the cooling capacity required for liquefying the high-pressure nitrogen, while being further heated;

[0026] Step 6, the heated low-pressure nitrogen from the second and third channels of the secondary regenerator enters the second and third channels of the primary regenerator respectively, exchanges heat with the high-pressure room-temperature nitrogen from the secondary booster unit, and is reheated at the same time;

[0027] Step 7, the reheated nitrogen from the third channel of the primary reheater enters the primary booster unit for boosting, and then is mixed with the reheated medium-pressure nitrogen from the second channel of the primary reheater, and then enters the secondary booster unit to complete a cycle.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. The present invention adopts nitrogen expansion refrigeration technology and liquid nitrogen throttling refrigeration technology at the same time, using the latent heat of liquid nitrogen generated by expansion valve throttling to provide the cooling capacity required for BOG reliquefaction, and using the cooling capacity generated by the expansion unit to liquefy nitrogen, thereby reducing nitrogen flow and heat transfer. The system reduces the loss (especially the low temperature section), thus improving the energy efficiency and economy of the system. Compared with the natural gas BOG reliquefaction system using a single-stage nitrogen reverse Brayton cycle, it can further increase the BOG liquefaction output and reduce the liquefaction energy consumption.

[0030] 2. The present invention directly liquefies low-temperature and low-pressure BOG from LNG storage tanks, eliminating the equipment required for BOG reheating, compression and throttling, simplifying the process structure and improving economic efficiency. Loss is reduced, thus improving the comprehensive energy utilization efficiency of the system. Thirdly, no throttling is performed after BOG liquefaction, thus avoiding the generation of new BOG and increasing the liquefaction yield of BOG. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The present invention is a process flow chart of a natural gas BOG direct reliquefaction system based on nitrogen expansion and throttling refrigeration.

[0032] In the figure: a primary booster unit 1, a secondary booster unit 2, a primary regenerator 3, a secondary regenerator 4, an expander unit 5, an expansion valve 6, a heat exchanger 7, a BOG buffer tank 8, and an LNG storage tank 9. DETAILED DESCRIPTION

[0033] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be pointed out that the embodiments described below are intended to facilitate the understanding of the present invention and do not have any limiting effect on the present invention.

[0034] like Figure 1 As shown, a natural gas BOG direct reliquefaction system based on nitrogen expansion and throttling refrigeration includes a primary boosting unit 1, a secondary boosting unit 2, a primary regenerator 3, a secondary regenerator 4, an expander unit 5, an expansion valve 6, a heat exchanger 7, a BOG buffer tank 8, and an LNG storage tank 9.

[0035] Among them, the outlet of the secondary booster unit 2 is sequentially connected to the first channel of the primary regenerator 3, the first channel of the secondary regenerator 4, the expansion valve 6, the first channel of the heat exchanger 7, the second channel of the secondary regenerator 4, and the inlet end of the second channel of the primary regenerator 3.

[0036] The outlet of the first channel of the primary reheater 3 is connected to the inlet end of the expander unit 5 and the inlet end of the first channel of the secondary reheater 4 respectively; the outlet of the expander unit 5 is connected to the third channel of the secondary reheater 4, the third channel of the primary reheater 3, and the inlet end of the primary booster unit 1 in sequence.

[0037] The outlet of the primary booster unit 1 and the outlet of the second channel of the primary regenerator 3 are respectively connected to the inlet of the secondary booster unit 2. The BOG buffer tank 8 is connected to the second channel of the heat exchanger 7 and the inlet of the LNG storage tank 9 in sequence.

[0038] In this embodiment, the first-stage booster unit 1 adopts a two-stage compression and inter-stage water cooling method, and the second-stage booster unit 2 adopts a one-stage compression and after-cooler water cooling method; the expander unit 5 uses two ordinary gas bearing turbine expanders, which do not carry liquid after expansion.

[0039] The low-temperature nitrogen at the outlet of the first channel of the primary regenerator 3 is divided into two streams, one of which enters the expander unit 5 for expansion and cooling, and the other enters the first channel of the secondary regenerator 4 to be cooled to be completely liquefied and the temperature is not lower than the saturation temperature of nitrogen, and then the pressure is reduced by the expansion valve 6. The temperature of the liquid nitrogen after throttling is lower than the saturation temperature of BOG, and the temperature difference is 3°C.

[0040] The nitrogen temperature at the outlet of the second and third channels of the primary regenerator 3 is equal and lower than the nitrogen temperature at the outlet of the secondary booster unit 2, with a temperature difference of 3.04° C. The nitrogen temperature at the outlet of the second and third channels of the secondary regenerator 4 is equal.

[0041] The nitrogen pressure of the primary booster unit 1 is equal to the nitrogen pressure from the second channel outlet of the primary regenerator 3. The two are mixed and enter the secondary booster unit 2 to complete a cycle. The primary regenerator 3, the secondary regenerator 4, and the heat exchanger 7 adopt a plate-fin structure. The temperature of the BOG in the BOG buffer tank 8 is -140°C and the pressure is 160kPa.

[0042] The method for reliquefying natural gas BOG using the above system comprises the following steps:

[0043] Step 1, the circulating nitrogen with a temperature, pressure and flow rate of 28.81°C, 423 kPa and 2129 kmol / h respectively is pressurized and cooled to 1286 kPa, 31.85°C in the first-stage booster unit 1, and then mixed with the nitrogen with a flow rate of 1251 kmol / h (1286 kPa, 28.81°C) from the second channel of the first-stage regenerator 3, and enters the second-stage booster unit 2 for further pressurization.

[0044] Step 2, the pressurized room temperature nitrogen (4510 kPa, 31.85°C) with a flow rate of 3380 kmol / h from the secondary booster unit 2 enters the primary regenerator 3 for cooling. The temperature of the high-pressure nitrogen after cooling is -92°C, and then it is divided into two parts: one part (1251 kmol / h) enters the first channel of the secondary regenerator 4 to exchange heat with the reflux and the low-pressure cold nitrogen from the expander unit 5, and is liquefied itself; the other part (2129 kmol / h) enters the expander unit 5 for expansion and cooling.

[0045] Step 3, the liquid nitrogen (-147.5°C) from the secondary regenerator 4 is depressurized to 1316 kPa through the expansion valve 6 to become a gas-liquid mixture.

[0046] Step 4, the nitrogen-liquid mixture obtained by throttling the expansion valve 6 enters the first channel of the heat exchanger 7, exchanges heat with the raw material BOG (446.1 kmol / h) from the BOG buffer tank 8, and the liquefied BOG (-162°C) is sent to the LNG storage tank for storage.

[0047] In step 5, the nitrogen (-150.5°C) heated by the heat exchanger 7 and the nitrogen (-170.4°C) expanded by the expander unit 5 enter the second and third channels of the secondary regenerator 4 respectively, providing the cooling capacity required for liquefying the high-pressure nitrogen while being further heated.

[0048] In step 6, the heated nitrogen (-111.1°C) from the second and third channels of the secondary regenerator 4 enters the second and third channels of the primary regenerator 3 respectively, exchanges heat with the high-pressure room-temperature nitrogen (4500 kPa, 31.85°C) from the secondary booster unit 2, and is reheated at the same time.

[0049] Step 7, the reheated nitrogen (28.81°C) from the third channel of the primary reheater 3 enters the primary booster unit 1 for boosting, and then is mixed with the reheated nitrogen (28.81°C) from the second channel of the primary reheater 3, and then enters the secondary booster unit 2 to complete a cycle.

[0050] In this embodiment, the pinch point temperature difference of each heat exchanger is 3° C., the required refrigerant flow rate is smaller than that of a single-stage reverse Brayton cycle, and the unit liquefied product energy consumption is as low as 0.976 kW·h / kg (BOG).

[0051] The embodiments described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for reliquefying natural gas BOG, characterized in that: A natural gas BOG direct reliquefaction system based on nitrogen expansion and throttling refrigeration is adopted, comprising a primary booster unit (1), a secondary booster unit (2), a primary regenerator (3), a secondary regenerator (4), an expander unit (5), an expansion valve (6), a heat exchanger (7), a BOG buffer tank (8) and an LNG storage tank (9); The outlet of the secondary booster unit (2) is connected to the first channel of the primary regenerator (3) and is then divided into two channels. One channel is connected in sequence to the first channel of the secondary regenerator (4), the expansion valve (6), the first channel of the heat exchanger (7), the second channel of the secondary regenerator (4), and the inlet end of the second channel of the primary regenerator (3); and the other channel is connected in sequence to the expansion unit (5), the third channel of the secondary regenerator (4), the third channel of the primary regenerator (3), and the inlet end of the primary booster unit (1). The low-temperature nitrogen gas at the outlet of the first channel of the first-stage regenerator (3) is divided into two paths, one of which enters the expansion unit (5) for expansion and cooling; the other enters the first channel of the second-stage regenerator (4) to be cooled to be completely liquefied and the temperature is not higher than the saturation temperature of the nitrogen gas, and then is reduced in pressure through the expansion valve (6); The outlet of the first-stage boosting unit (1) and the outlet of the second channel of the first-stage regenerator (3) are respectively connected to the inlet end of the second-stage boosting unit (2); The BOG buffer tank (8) is sequentially connected to the second channel of the heat exchanger (7) and the inlet end of the LNG storage tank (9); The method for reliquefying natural gas BOG comprises the following steps: Step 1, the low-pressure circulating nitrogen is pressurized and cooled in the first-stage pressurizing unit (1), mixed with the low-pressure and room-temperature nitrogen from the second channel of the first-stage regenerator (3), and then enters the second-stage pressurizing unit (2) for further pressurization; Step 2, the high-pressure and room-temperature nitrogen from the secondary booster unit (2) enters the first channel of the primary regenerator (3) for cooling, and the cooled high-pressure nitrogen is divided into two parts: one part enters the first channel of the secondary regenerator (4) for heat exchange with the reflux and the low-pressure cold nitrogen from the expander unit (5), and is liquefied; the other part enters the expander unit (5) for expansion and cooling; Step 3, the liquid nitrogen from the first channel of the secondary regenerator (4) is depressurized through an expansion valve (6) to become a gas-liquid mixture; Step 4, the nitrogen-liquid mixture obtained by throttling the expansion valve (6) enters the first channel of the heat exchanger (7), exchanges heat with the raw material BOG from the BOG buffer tank (8), and the liquefied BOG is sent to the LNG storage tank for storage; Step 5, the low-pressure nitrogen heated by the heat exchanger (7) and the low-pressure nitrogen expanded by the expander unit (5) enter the second and third channels of the secondary regenerator (4) respectively, providing the cooling capacity required for liquefying the high-pressure nitrogen, while being further heated; Step 6, the heated low-pressure nitrogen from the second and third channels of the secondary regenerator (4) enters the second and third channels of the primary regenerator (3) respectively, exchanges heat with the high-pressure room-temperature nitrogen from the secondary booster unit (2), and is reheated at the same time; In step 7, the reheated nitrogen from the third channel of the primary reheater (3) enters the primary booster unit (1) for boosting, and then is mixed with the reheated medium-pressure nitrogen from the second channel of the primary reheater (3) before entering the secondary booster unit (2), completing a cycle.

2. The method for reliquefying natural gas BOG according to claim 1, characterized in that: The first-stage booster unit (1) and the second-stage booster unit (2) adopt a single-stage or multi-stage compression and inter-stage cooling method.

3. The method for reliquefying natural gas BOG according to claim 1, characterized in that: The temperature of the nitrogen-liquid mixture after throttling by the expansion valve (6) is lower than the saturation temperature of the liquefied BOG, with a temperature difference of 1.5 to 3°C.

4. The method for reliquefying natural gas BOG according to claim 1, characterized in that: The temperature of the nitrogen after being heated by the heat exchanger (7) is lower than the temperature of the nitrogen-liquid mixture before being expanded by the expansion valve (6), with a temperature difference of 3-5°C.

5. The method for reliquefying natural gas BOG according to claim 1, characterized in that: The temperature of the nitrogen at the outlets of the second and third channels of the first-stage regenerator (3) is equal and lower than the temperature of the nitrogen at the outlet of the second-stage booster unit (2), with a temperature difference of 3-5°C.

6. The method for reliquefying natural gas BOG according to claim 1, characterized in that: The temperatures of the nitrogen at the outlets of the second and third channels of the secondary regenerator (4) are equal.

7. The method for reliquefying natural gas BOG according to claim 1, characterized in that: The pressure of the nitrogen from the first-stage booster unit (1) is not lower than the pressure of the nitrogen from the outlet of the second channel of the first-stage regenerator (3).

8. The method for reliquefying natural gas BOG according to claim 1, characterized in that: The BOG temperature in the BOG buffer tank (8) is -120°C to -140°C, and the gas pressure is 120 kPa to 160 kPa.

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