Self-recovering absorption pre-cooling mixed refrigerant natural gas liquefaction system and method

Abstract

The application discloses a kind of self-repeating cascade absorption type pre-cooling mixed refrigerant natural gas liquefaction system and method.The application uses the low-grade waste heat driven self-repeating cascade absorption refrigeration system of natural gas liquefaction factory as pre-cooling system, replaces typical propane vapor compression pre-cooling system, provides cold quantity for the pre-cooling heat exchanger of mixed refrigerant natural gas liquefaction system, effectively recycles and utilizes low-grade industrial waste heat, reduces the power consumption per unit mass of natural gas liquefaction, improves system operation stability, obtains better environmental protection and economic benefits.The application realizes the efficient recovery of low-grade industrial waste heat, meets the natural gas liquefaction deep pre-cooling process, reduces the specific power consumption of natural gas liquefaction process, and the system energy utilization is more economical and reasonable, and the carbon emission reduction benefit is remarkable.Meanwhile, since the pre-cooling system adopts heat-driven absorption refrigeration form, the system has fewer moving parts, so the stability and service life are further improved.

Description

Technical Field

[0001] This invention relates to the field of natural gas liquefaction technology, and more specifically to a mixed refrigerant natural gas liquefaction system and method with self-cascading absorption precooling. Background Technology

[0002] Liquefied natural gas (LNG) is an important emerging industrial sector. By converting gaseous natural gas into a liquid state, its volume is reduced by nearly 600 times, significantly improving transportation efficiency and lowering costs. Simultaneously, LNG has high energy density, is easy to store and use, and is suitable for various energy demand scenarios. Furthermore, natural gas is a clean, efficient, and environmentally friendly energy source. Compared to traditional energy sources such as coal and oil, the combustion products of natural gas are only water and carbon dioxide, producing no harmful substances such as sulfides and nitrogen oxides, thus having a smaller environmental impact. Therefore, with the transformation of the energy structure and the development of clean energy, the demand for natural gas is constantly increasing, and the LNG industry has experienced rapid development in recent years.

[0003] There are generally three types of natural gas liquefaction processes: cascade liquefaction, mixed refrigerant liquefaction, and expansion liquefaction. Mixed refrigerant liquefaction is widely used due to its low energy consumption. To further reduce energy consumption, a propane pre-cooling mixed refrigerant refrigeration liquefaction process has been developed. This process uses a gas compression propane refrigeration cycle for pre-cooling the mixed refrigerant natural gas liquefaction process. Due to its simplicity and efficiency, over 80% of baseload natural gas liquefaction plants worldwide currently use this process. However, because its pre-cooling cycle is a gas compression refrigeration cycle, the compressor in the booster unit consumes a relatively high amount of electricity, and the stability of the system operation needs further improvement.

[0004] Meanwhile, the industrial production process of natural gas liquefaction generates a large amount of low-temperature industrial waste heat below 350°C, such as exhaust gas from gas turbines. Currently, this low-grade heat energy is not effectively utilized, resulting in low energy efficiency. Therefore, how to recover low-grade heat energy and apply it to the production and processing of natural gas has become an urgent problem to be solved.

[0005] Absorption refrigeration systems primarily utilize low-grade heat sources such as industrial waste heat, solar energy, and geothermal energy to achieve refrigeration. Due to their ability to utilize low-grade heat and their environmentally friendly characteristics, they have attracted increasing attention. However, traditional absorption refrigeration cycles struggle to achieve low enough cooling temperatures for effective pre-cooling in natural gas liquefaction processes. For example, the LiBr / H2O absorption refrigeration cycle can only achieve cooling above 0°C, and the NH3 / H2O absorption refrigeration cycle typically only achieves cooling above -30°C.

[0006] The self-cascading absorption refrigeration cycle (ACAR) utilizes the same heat-driven unit and self-cascading principle as traditional absorption refrigeration systems to achieve deep cooling using low-grade heat energy. After the cooling temperature is lowered, it can provide pre-cooling capacity for second and even third pre-cooling heat exchangers.

[0007] Therefore, this invention proposes to use a self-cascade absorption refrigeration cycle as the precooling cycle for the mixed refrigerant natural gas liquefaction process, and to use a non-azeotropic mixed refrigerant as the working fluid of the precooling cycle, thereby improving the problems of high power consumption, unstable system operation and low-grade heat waste of propane gas compression precooling cycle, thereby improving the economics of liquefied natural gas production and reducing carbon dioxide emissions.

[0008] A search revealed no prior art related to the self-cascading absorption precooling mixed refrigerant natural gas liquefaction concept described in this invention in the field of natural gas liquefaction technology. Summary of the Invention

[0009] This invention provides a self-cascading absorption precooling mixed refrigerant natural gas liquefaction system and method.

[0010] This invention utilizes a self-cascade absorption refrigeration system driven by low-grade waste heat from natural gas liquefaction plants as a precooling system, replacing the typical propane vapor compression precooling system. This system provides cooling capacity to the precooling heat exchanger of the mixed refrigerant natural gas liquefaction system. This invention effectively recovers and utilizes low-grade industrial waste heat, reduces power consumption per unit mass of natural gas liquefaction, improves system operational stability, and achieves significant environmental and economic benefits.

[0011] This invention achieves efficient recovery of low-grade industrial waste heat, meeting the requirements of deep precooling processes in natural gas liquefaction, while simultaneously reducing the specific power consumption of the natural gas liquefaction process. The system's energy utilization is more economical and rational, resulting in significant carbon emission reduction benefits. Furthermore, because the precooling system employs a heat-driven absorption refrigeration method, the system has fewer moving parts, thus further improving stability and service life.

[0012] A self-cascading absorption precooling mixed refrigerant natural gas liquefaction system includes a mixed refrigerant I circulation loop, a mixed refrigerant II circulation loop, and a natural gas liquefaction pipeline;

[0013] The mixed refrigerant I circulation loop includes a generator; the mixed refrigerant I outlet of the generator is connected to a first gas-liquid separator via a first condenser; the liquid phase refrigerant I outlet of the first gas-liquid separator is connected to an absorber via a first throttling valve and a condenser-evaporator in sequence; the gas phase refrigerant I outlet of the first gas-liquid separator is connected to the absorber via a condenser-evaporator, a second throttling valve, a third precooling heat exchanger, and a first precooling heat exchanger in sequence; the dilute solution outlet of the generator is connected to the absorber via a solution heat exchanger and a third throttling valve in sequence; the concentrated solution outlet of the absorber is connected to the generator via a solution pump and a solution heat exchanger; both the concentrated solution and the dilute solution include the mixed refrigerant I and the absorbent, and the molar percentage of the mixed refrigerant I in the concentrated solution is higher than the molar percentage of the mixed refrigerant I in the dilute solution;

[0014] The mixed refrigerant II circulation loop includes a compressor; the mixed refrigerant II outlet of the compressor is connected to a second gas-liquid separator via a second condenser and a first precooling heat exchanger in sequence; the liquid phase outlet of the second gas-liquid separator is connected to a fourth throttle valve via a second precooling heat exchanger and a third precooling heat exchanger in sequence, and the other end of the fourth throttle valve is connected to the mixed refrigerant II inlet of the compressor via a third precooling heat exchanger and a second precooling heat exchanger in sequence; the gas phase outlet of the second gas-liquid separator is connected to a third gas-liquid separator via a second precooling heat exchanger and a third precooling heat exchanger in sequence. Separators; the liquid phase outlet of the third gas-liquid separator is connected to the fifth throttle valve through the fourth precooling heat exchanger, and the other end of the fifth throttle valve is connected to the mixed refrigerant II inlet of the compressor through the fourth precooling heat exchanger, the third precooling heat exchanger and the second precooling heat exchanger in sequence; the gas phase outlet of the third gas-liquid separator is connected to the sixth throttle valve through the fourth precooling heat exchanger and the subcooling heat exchanger in sequence, and the other end of the sixth throttle valve is connected to the mixed refrigerant II inlet of the compressor through the subcooling heat exchanger, the fourth precooling heat exchanger, the third precooling heat exchanger and the second precooling heat exchanger in sequence;

[0015] The natural gas liquefaction pipeline includes a first precooling heat exchanger, a second precooling heat exchanger, a third precooling heat exchanger, a fourth precooling heat exchanger, a subcooling heat exchanger, a seventh throttle valve, and a fourth gas-liquid separator connected in sequence.

[0016] The self-cascading absorption precooling mixed refrigerant natural gas liquefaction system of the present invention includes a self-cascading absorption precooling system and a mixed refrigerant natural gas liquefaction cycle system. The refrigeration stream of the precooling system flows sequentially through the third and first precooling heat exchangers, achieving energy coupling with the mixed refrigerant natural gas liquefaction side. By optimizing the composition and ratio of the mixed refrigerant I, the minimum refrigeration temperature of the precooling cycle can reach approximately -100 to -70°C (far lower than the traditional ammonia absorption refrigeration system), providing precooling capacity, reducing the heat exchange temperature difference, achieving cascade cooling of the two precooling heat exchangers, reducing the requirements for the heat exchangers, and also reducing the specific power consumption and cost of natural gas liquefaction.

[0017] In this invention, the self-cascading unit includes a first gas-liquid separator, a first throttling valve, and a condenser-evaporator, and the thermal drive unit includes a generator, a solution heat exchanger, a third throttling valve, a solution pump, and an absorber.

[0018] After the mixed refrigerant I is output from the circulating solution of the thermal drive unit, it is in a gas-liquid two-phase mixed state after passing through the first condenser. It then enters the self-cascading unit to form two refrigerant fluids with different compositions. The low-boiling-point component refrigerant is throttled and cooled by the second throttling valve and flows through two pre-cooling heat exchangers to provide cooling for the natural gas liquefaction system. It then mixes with the high-boiling-point component refrigerant and enters the thermal drive unit.

[0019] This invention utilizes unstable, low-grade thermal energy, overcoming the problems of low efficiency and high cooling temperatures associated with using such energy. This expands the application range of low-grade thermal energy and improves overall energy utilization. For example, the heat source for the generator may include exhaust gas from a gas turbine in a natural gas liquefaction plant. The temperature of the heat source can be 90–350°C. This invention uses low-grade thermal energy as the primary driving energy source. Because it lacks large rotating equipment, its maintenance costs are significantly lower than propane compression precooling systems. Furthermore, the specific power consumption of the entire system is lower than that of mixed refrigerant natural gas liquefaction systems using propane compression precooling.

[0020] The mixed refrigerant I is a non-azeotropic mixture of refrigerants, wherein the refrigerant may be selected from hydrofluoroalkanes, alkanes, alkenes, and hydrofluoroolefins. For example, the composition of the mixed refrigerant I may include ethylene (R1150) and other refrigerant components. Further, the other refrigerant components may include at least one of methane (R50), 2,3,3,3-tetrafluoropropylene (R1234yf), 1,3,3,3-tetrafluoropropylene (R1234ze), 1-chloro-3,3,3-trifluoropropylene (R1233zd), 1,1,1,4,4,4-hexafluoro-2-butene (R1336mzz), 1-chloro-2-,3,3,4-tetrafluoropropylene (R1224yd), trifluoromethane (R23), and 1,1,1,2-tetrafluoroethane (R134a). The mixed refrigerant I may also be composed of a mixture of other pure refrigerants with similar boiling points.

[0021] The absorbent may include at least one of N,N-dimethylformamide (DMF), N,N-dimethylacetamide, triethylene glycol dimethyl ether, and α-pyrrolidone.

[0022] Based on the total molar amount of all components in the mixed refrigerant I and the absorbent being 100%, the molar fraction of the absorbent can be 75% to 98%, the molar fraction of trifluoromethane can be 0% to 3.98% (more preferably 0.20% to 3.98%), the molar fraction of 1,1,1,2-tetrafluoroethane can be 0% to 12.45% (more preferably 1.03% to 12.45%), the molar fraction of ethylene can be 0.52% to 9.18%, and the molar fraction of methane can be 0% to 0.95%. % (further, it can be 0.001% to 0.95%), the molar fraction of 2,3,3,3-tetrafluoropropene can be 0 to 12.85%, the molar fraction of 1,3,3,3-tetrafluoropropene can be 0 to 12.75%, the molar fraction of 1-chloro-3,3,3-trifluoropropene can be 0 to 12.35%, the molar fraction of 1,1,1,4,4,4-hexafluoro-2-butene can be 0 to 12.45%, and the molar fraction of 1-chloro-2-,3,3,4-tetrafluoropropene can be 0 to 12.45%.

[0023] In the mixed refrigerant I circulation loop, the gaseous refrigerant I can provide a cooling temperature of -100 to -70°C in the third precooling heat exchanger and a cooling temperature of -70 to 28°C in the first precooling heat exchanger.

[0024] The mixed refrigerant II may include at least two of CH4, C2H6, C3H8, and N2. The mixed refrigerant II may also be composed of other pure refrigerants with similar boiling points.

[0025] Furthermore, the composition of the mixed refrigerant II may include CH4, C2H6, C3H8 and N2, wherein the molar fraction of CH4 may be 30.2% to 50.5%, the molar fraction of C2H6 may be 19.0% to 33.8%, the molar fraction of C3H8 may be 7.2% to 25.2%, and the molar fraction of N2 may be 8.5% to 21.0%.

[0026] In one embodiment, the cooling medium temperature of the first condenser, the second condenser, and the absorber is below 25°C, and may be, for example, cooling water.

[0027] In this invention, each throttle valve can be a manual valve or an automatic valve.

[0028] In this invention, the internal structure of each condenser can be of the floating head type, fixed tube sheet type, U-shaped tube sheet type, plate type, sleeve type, or shell and tube type, etc.

[0029] The absorber of the present invention can be a spray type, a packing type, a falling film type, or other types of absorbers.

[0030] The solution pump of this invention can raise the solution from a low pressure to a high pressure state, similar to the solution pump used in conventional absorption refrigeration systems.

[0031] The precooling heat exchangers and subcooling heat exchangers in this invention can be plate-fin multi-channel heat exchangers or other types of heat exchangers.

[0032] The condenser-evaporator and solution heat exchanger in this invention can be shell-and-tube type, plate type, or other types of heat exchangers.

[0033] A method for liquefying mixed refrigerant natural gas using a self-cascading absorption precooling system, comprising the aforementioned self-cascading absorption precooling mixed refrigerant natural gas liquefaction system;

[0034] The self-cascade absorption precooling mixed refrigerant natural gas liquefaction method includes a mixed refrigerant I cycle process, a mixed refrigerant II cycle process, and a natural gas liquefaction process.

[0035] The mixed refrigerant I circulation process includes: mixed refrigerant I overflows from the mixed refrigerant I outlet of the generator, is condensed by the first condenser, and then enters the first gas-liquid separator to be separated into liquid refrigerant I and gaseous refrigerant I. Liquid refrigerant I is throttled by the first throttle valve and enters the low-pressure side of the condenser-evaporator, while gaseous refrigerant I enters the high-pressure side of the condenser-evaporator. Liquid refrigerant I in the condenser-evaporator evaporates and cools gaseous refrigerant I before entering the absorber to be absorbed by a dilute solution. Gaseous refrigerant I cooled in the condenser-evaporator is throttled by the second throttle valve, and then passes through the third precooling heat exchanger and the first precooling heat exchanger in sequence to evaporate and cool before entering the absorber to be absorbed by a dilute solution. Driven by the solution pump, the concentrated solution in the absorber returns to the generator through the concentrated solution outlet via the low-temperature side of the solution heat exchanger to circulate the overflowing mixed refrigerant I. The dilute solution remaining after the overflowing mixed refrigerant I in the generator passes through the high-temperature side of the solution heat exchanger and the third throttle valve in sequence to enter the absorber to absorb liquid refrigerant I and gaseous refrigerant I to form a concentrated solution.

[0036] The mixed refrigerant II cycle process includes: under the drive of the compressor, mixed refrigerant II is pre-cooled by passing through the second condenser and the first pre-cooling heat exchanger in sequence, and then enters the second gas-liquid separator to be separated into first gas-phase refrigerant II and first liquid-phase refrigerant II. Both first gas-phase refrigerant II and first liquid-phase refrigerant II enter the second pre-cooling heat exchanger and the third pre-cooling heat exchanger in sequence for further pre-cooling. Then, first liquid-phase refrigerant II returns to the compressor after being throttled by the fourth throttle valve and provided with cooling capacity by the third and second pre-cooling heat exchangers. First gas-phase refrigerant II enters the third gas-liquid separator... The liquid separator separates the refrigerant into a second gaseous phase II and a second liquid phase II. Both the second gaseous phase II and the second liquid phase II enter the fourth precooling heat exchanger for precooling. Then, the second liquid phase II returns to the compressor after being throttled by the fifth throttle valve and supplied with cooling capacity by the fourth, third, and second precooling heat exchangers. The second gaseous phase II returns to the compressor after being precooled by the cold heat exchanger and throttled by the sixth throttle valve. After being throttled by the cold heat exchanger, the second gaseous phase II returns to the compressor after being supplied with cooling capacity by the cold heat exchanger, the fourth precooling heat exchanger, the third precooling heat exchanger, and the second precooling heat exchanger.

[0037] The natural gas liquefaction process includes: the feed natural gas is cooled sequentially by a first precooling heat exchanger, a second precooling heat exchanger, a third precooling heat exchanger, a fourth precooling heat exchanger, and a subcooling heat exchanger, and then throttled by a seventh throttle valve before entering a fourth gas-liquid separator to separate and obtain liquefied natural gas.

[0038] The self-cascade absorption precooling mixed refrigerant natural gas liquefaction system and method proposed in this invention can efficiently recover and utilize low-grade heat sources, providing sufficiently low temperatures and a wide temperature range for precooling. Through the rational arrangement of heat exchangers, the cascade utilization of this wide temperature range precooling capacity is achieved, improving the overall energy efficiency of the natural gas liquefaction plant. Compared to mixed refrigerant natural gas liquefaction systems using propane for precooling, this system and method can significantly reduce the high power consumption and maintenance costs associated with compressors in the precooling process, and reduce CO2 emissions. Compared to traditional ammonia absorption refrigeration cycles that only provide a precooling temperature of approximately -30°C, the rational formulation of the self-cascade unit and non-azeotropic mixed refrigerant allows for even lower cooling temperatures. Under different mixed refrigerant compositions and ratios, the minimum cooling temperature can reach approximately -100 to -70°C, thus providing effective and sufficient cooling capacity for the mixed refrigerant natural gas liquefaction process. Overall, this process offers better energy efficiency, environmental friendliness, and economy, and has significant practical application value in natural gas liquefaction plants.

[0039] Compared with the prior art, the beneficial effects of this invention are as follows:

[0040] 1. This invention proposes a mixed refrigerant natural gas liquefaction system based on absorption precooling. By utilizing thermally driven absorption refrigeration technology to replace the traditional propane compression refrigeration technology, the precooling process of mixed refrigerant natural gas liquefaction is realized. At the same time, addressing the problem that the cooling temperature (about -30°C) of typical ammonia absorption refrigeration technology is not low enough, a self-cascade absorption refrigeration technology is proposed as the precooling system. In the embodiment, DMF is used as the absorbent, and R23, R134a, R50 and R1150 are used as non-azeotropic mixed refrigerants. Under the conditions of generator generation temperature of 286°C and first condenser condensation temperature of 15°C, the system flow is established in Aspen Hysys. The calculation results show that the precooling system can achieve a cooling temperature of -76°C.

[0041] 2. Since the self-cascade absorption precooling technology proposed in this invention can achieve a cooling temperature of approximately -100°C, and can provide a wide temperature range of approximately -100°C to 28°C during the precooling process of mixed refrigerant and natural gas, in order to achieve cascade utilization of cooling capacity, i.e., optimal temperature matching in the precooling process, this invention, through analysis of the precooling process flow and related stream properties, proposes to use the first and third precooling heat exchangers as the precooling process. The first precooling heat exchanger provides a cooling temperature of approximately -70°C to 28°C, and the third precooling heat exchanger provides a cooling temperature of approximately -100°C to -70°C. The system achieves effective precooling of natural gas and mixed refrigerants, alleviating the problem of high compressor power consumption in vapor compression of mixed refrigerants. Calculation results from the examples show that, under similar natural gas liquefaction processes, compared to propane precooling mixed refrigerant liquefaction systems, the LNG specific power consumption of the self-cascade absorption precooling mixed refrigerant liquefaction system, which achieves cascade utilization of cooling capacity, is 1328 kJ / kg. This is approximately 32.46% lower than the 1966 kJ / kg specific power consumption of the propane precooling mixed refrigerant liquefaction system, significantly reducing power consumption and improving the economics of liquefied natural gas.

[0042] 3. Because this invention employs heat-driven absorption refrigeration technology, it can efficiently recover and utilize a large amount of low-grade waste heat emitted by natural gas liquefaction plants. Based on the process and operating conditions described in Example 1, this technology is applied to a medium-sized LNG production plant with a production capacity of 200,000 tons / year, and the waste heat utilization can reach 1.929 billion MJ / year. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of a self-cascading absorption precooling mixed refrigerant natural gas liquefaction system and method, as shown in the embodiment. Detailed Implementation

[0044] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0045] See Figure 1 The self-cascading absorption precooling mixed refrigerant natural gas liquefaction system of this embodiment includes a mixed refrigerant I circulation loop, a mixed refrigerant II circulation loop, and a natural gas liquefaction pipeline.

[0046] The mixed refrigerant I circulation loop includes a generator 1. The mixed refrigerant I outlet of the generator 1 is connected to a first gas-liquid separator 3 via a first condenser 2. The liquid phase refrigerant I outlet of the first gas-liquid separator 3 is connected to an absorber 7 via a first throttle valve 4 and a condenser-evaporator 5. The gas phase refrigerant I outlet of the first gas-liquid separator 3 is connected to the absorber 7 via a condenser-evaporator 5, a second throttle valve 6, a third precooling heat exchanger 16, and a first precooling heat exchanger 13. The dilute solution outlet of the generator 1 is connected to the absorber 7 via a solution heat exchanger 10 and a third throttle valve 8. The concentrated solution outlet of the absorber 7 is connected to the generator 1 via a solution pump 9 and a solution heat exchanger 10. Both the concentrated solution and the dilute solution include the mixed refrigerant I and the absorbent, and the molar percentage of the mixed refrigerant I in the concentrated solution is higher than that in the dilute solution.

[0047] The mixed refrigerant II circulation loop includes a compressor 11. The mixed refrigerant II outlet of the compressor 11 is connected to a second gas-liquid separator 14 via a second condenser 12 and a first precooling heat exchanger 13. The liquid phase outlet of the second gas-liquid separator 14 is connected to a fourth throttle valve 17 via a second precooling heat exchanger 15 and a third precooling heat exchanger 16. The other end of the fourth throttle valve 17 is connected to the mixed refrigerant II inlet of the compressor 11 via the third precooling heat exchanger 16 and the second precooling heat exchanger 15. The gas phase outlet of the second gas-liquid separator 14 is connected to a third gas-liquid separator 18 via the second precooling heat exchanger 15 and the third precooling heat exchanger 16. The liquid phase outlet of the third gas-liquid separator 18 is connected to a fifth throttle valve 20 via a fourth precooling heat exchanger 19. The other end of the fifth throttle valve 20 is connected to the mixed refrigerant II inlet of the compressor 11 via the fourth precooling heat exchanger 19, the third precooling heat exchanger 16, and the second precooling heat exchanger 15. The gas phase outlet of the third gas-liquid separator 18 is connected to the sixth throttle valve 22 in sequence through the fourth precooling heat exchanger 19 and the subcooling heat exchanger 21. The other end of the sixth throttle valve 22 is connected to the mixed refrigerant II inlet of the compressor 11 in sequence through the subcooling heat exchanger 21, the fourth precooling heat exchanger 19, the third precooling heat exchanger 16 and the second precooling heat exchanger 15.

[0048] The natural gas liquefaction pipeline includes a first precooling heat exchanger 13, a second precooling heat exchanger 15, a third precooling heat exchanger 16, a fourth precooling heat exchanger 19, a subcooling heat exchanger 21, a seventh throttle valve 23, and a fourth gas-liquid separator 24 connected in sequence.

[0049] Combination Figure 1 The self-cascading absorption precooling mixed refrigerant natural gas liquefaction method of this embodiment adopts the above-mentioned self-cascading absorption precooling mixed refrigerant natural gas liquefaction system.

[0050] The self-cascading absorption precooling mixed refrigerant natural gas liquefaction method of this embodiment includes a mixed refrigerant I cycle process, a mixed refrigerant II cycle process, and a natural gas liquefaction process.

[0051] The mixed refrigerant I circulation process includes: mixed refrigerant I overflows from the mixed refrigerant I outlet of generator 1, is condensed by the first condenser 2, and then enters the first gas-liquid separator 3 to be separated into liquid refrigerant I and gaseous refrigerant I. Liquid refrigerant I is throttled by the first throttling valve 4 and enters the low-pressure side of condenser-evaporator 5, while gaseous refrigerant I enters the high-pressure side of condenser-evaporator 5. The liquid refrigerant I in condenser-evaporator 5 evaporates and cools the gaseous refrigerant I before entering absorber 7 to be absorbed by a dilute solution. The cooled gaseous refrigerant in condenser-evaporator 5... After being throttled by the second throttle valve 6, the refrigerant I passes through the third precooling heat exchanger 16 and the first precooling heat exchanger 13 in sequence for evaporation and cooling before entering the absorber 7 to be absorbed by the dilute solution. Driven by the solution pump 9, the concentrated solution in the absorber 7 returns to the generator 1 through the concentrated solution outlet via the low-temperature side of the solution heat exchanger 10 for circulation and overflow of mixed refrigerant I. The dilute solution remaining after the mixed refrigerant I overflows from the generator 1 passes through the high-temperature side of the solution heat exchanger 10 and the third throttle valve 8 in sequence to enter the absorber 7 to absorb liquid refrigerant I and gaseous refrigerant I to form a concentrated solution.

[0052] The mixed refrigerant II cycle process includes: driven by the compressor 11, the mixed refrigerant II is pre-cooled by passing through the second condenser 12 and the first pre-cooling heat exchanger 13 in sequence, and then enters the second gas-liquid separator 14 to be separated into a first gaseous refrigerant II and a first liquid refrigerant II. Both the first gaseous refrigerant II and the first liquid refrigerant II enter the second pre-cooling heat exchanger 15 and the third pre-cooling heat exchanger 16 in sequence for further pre-cooling. Then, the first liquid refrigerant II returns to the compressor 11 after being throttled by the fourth throttle valve 17 and receiving cooling from the third pre-cooling heat exchanger 16 and the second pre-cooling heat exchanger 15 in sequence. The first gaseous refrigerant II then enters the third gas-liquid separator. 18 is separated into a second gaseous refrigerant II and a second liquid refrigerant II. Both the second gaseous refrigerant II and the second liquid refrigerant II enter the fourth precooling heat exchanger 19 for precooling. Then, the second liquid refrigerant II is throttled by the fifth throttle valve 20 and then provides cooling capacity through the fourth precooling heat exchanger 19, the third precooling heat exchanger 16, and the second precooling heat exchanger 15 before returning to the compressor 11. The second gaseous refrigerant II is precooled by the cold heat exchanger 21 and then throttled by the sixth throttle valve 22. After returning, it is throttled by the cold heat exchanger 21, the fourth precooling heat exchanger 19, the third precooling heat exchanger 16, and the second precooling heat exchanger 15 before returning to the compressor 11.

[0053] The natural gas liquefaction process includes: the feed natural gas is cooled sequentially by the first precooling heat exchanger 13, the second precooling heat exchanger 15, the third precooling heat exchanger 16, the fourth precooling heat exchanger 19 and the subcooling heat exchanger 21, and then throttled by the seventh throttle valve 23 before entering the fourth gas-liquid separator 24 to separate and obtain liquefied natural gas.

[0054] In this embodiment, the driving heat source of generator 1 is low-grade industrial waste heat. Specifically, it utilizes industrial waste heat generated by a natural gas liquefaction plant to produce the pre-cooling capacity required for natural gas liquefaction. Compared with a mixed refrigerant natural gas liquefaction system that uses propane compression and pre-cooling, the specific power consumption (the amount of electricity consumed to produce one kilogram of LNG) can be further reduced.

[0055] In this embodiment, the first condenser 2 is a shell-and-tube heat exchanger, and the absorber 7 is a spray absorber.

[0056] In this embodiment, the mixed refrigerant I is a non-azeotropic mixed refrigerant, which is composed of trifluoromethane, 1,1,1,2-tetrafluoroethane, methane and ethylene, and the absorbent is N,N-dimethylformamide.

[0057] Based on the total molar amount of all components in the mixed refrigerant I and the absorbent being 100%, the molar fraction of N,N-dimethylformamide is 94%, the molar fraction of trifluoromethane is 0.6%, the molar fraction of 1,1,1,2-tetrafluoroethane is 3.21%, the molar fraction of methane is 0.01%, and the molar fraction of ethylene is 2.18%.

[0058] In this embodiment, the mixed refrigerant II is composed of CH4, C2H6, C3H8 and N2, wherein the molar fraction of CH4 is 35%, the molar fraction of C2H6 is 21%, the molar fraction of C3H8 is 24%, and the molar fraction of N2 is 20%.

[0059] The system flow was established in Aspen Hysys, and the calculation results showed that when the condensing temperature of the first condenser 2 was 15℃ and the generating temperature of the generator 1 was 286℃, the cooling temperature at the outlet of the second throttle valve 6 was -76℃, and the power consumption was reduced by 32.46% compared with the mixed refrigerant natural gas liquefaction system with propane compression and precooling.

[0060] The feed natural gas is cooled step by step from 35°C to -10°C, -40°C, -70°C, -120°C, and -158°C by passing through the first precooling heat exchanger 13, the second precooling heat exchanger 15, the third precooling heat exchanger 16, the fourth precooling heat exchanger 19, and the subcooling heat exchanger 21. After passing through the seventh throttling valve 23, it reaches -160°C and 110 kPa.

[0061] Furthermore, it should be understood that after reading the above description of the present invention, those skilled in the art can make various alterations or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims.

Claims

1. A self-cascading absorption precooling mixed refrigerant natural gas liquefaction system, characterized in that, This includes a mixed refrigerant I loop, a mixed refrigerant II loop, and a natural gas liquefaction pipeline; The mixed refrigerant I circulation loop includes a generator (1); the mixed refrigerant I outlet of the generator (1) is connected to a first gas-liquid separator (3) through a first condenser (2); the liquid phase refrigerant I outlet of the first gas-liquid separator (3) is connected to an absorber (7) through a first throttle valve (4) and a condenser-evaporator (5) in sequence; the gas phase refrigerant I outlet of the first gas-liquid separator (3) is connected to the condenser-evaporator (5), a second throttle valve (6), a third precooling heat exchanger (16), and a first precooling heat exchanger in sequence. Heater (13) is connected to absorber (7); the dilute solution outlet of generator (1) is connected to absorber (7) in sequence through solution heat exchanger (10) and third throttle valve (8); the concentrated solution outlet of absorber (7) is connected to generator (1) through solution pump (9) and solution heat exchanger (10); both the concentrated solution and the dilute solution include the mixed refrigerant I and absorbent, and the molar percentage of mixed refrigerant I in the concentrated solution is higher than the molar percentage of mixed refrigerant I in the dilute solution; The mixed refrigerant II circulation loop includes a compressor (11); the mixed refrigerant II outlet of the compressor (11) is connected to a second gas-liquid separator (14) in sequence through a second condenser (12) and a first precooling heat exchanger (13); the liquid phase outlet of the second gas-liquid separator (14) is connected to a fourth throttle valve (17) in sequence through a second precooling heat exchanger (15) and a third precooling heat exchanger (16); the other end of the fourth throttle valve (17) is connected to the mixed refrigerant II inlet of the compressor (11) in sequence through a third precooling heat exchanger (16) and a second precooling heat exchanger (15); the gas phase outlet of the second gas-liquid separator (14) is connected to a third gas-liquid separator (18) in sequence through a second precooling heat exchanger (15) and a third precooling heat exchanger (16). The liquid phase outlet of the third gas-liquid separator (18) is connected to the fifth throttle valve (20) through the fourth precooling heat exchanger (19). The other end of the fifth throttle valve (20) is connected to the mixed refrigerant II inlet of the compressor (11) through the fourth precooling heat exchanger (19), the third precooling heat exchanger (16), and the second precooling heat exchanger (15) in sequence. The gas phase outlet of the third gas-liquid separator (18) is connected to the sixth throttle valve (22) through the fourth precooling heat exchanger (19) and the subcooling heat exchanger (21) in sequence. The other end of the sixth throttle valve (22) is connected to the mixed refrigerant II inlet of the compressor (11) through the subcooling heat exchanger (21), the fourth precooling heat exchanger (19), the third precooling heat exchanger (16), and the second precooling heat exchanger (15) in sequence. The natural gas liquefaction pipeline includes a first precooling heat exchanger (13), a second precooling heat exchanger (15), a third precooling heat exchanger (16), a fourth precooling heat exchanger (19), a subcooling heat exchanger (21), a seventh throttle valve (23), and a fourth gas-liquid separator (24) connected in sequence.

2. The self-cascading absorption precooling mixed refrigerant natural gas liquefaction system according to claim 1, characterized in that, The heat source of the generator (1) includes exhaust gas from the gas turbine of the natural gas liquefaction plant, and the temperature of the heat source is 90 to 350°C.

3. The self-cascading absorption precooling mixed refrigerant natural gas liquefaction system according to claim 1, characterized in that, The mixed refrigerant I is a non-azeotropic mixed refrigerant, wherein the refrigerant is selected from hydrofluoroalkanes, alkanes, alkenes, and hydrofluoroolefins; the composition of the mixed refrigerant I includes ethylene and other refrigerant components; the other refrigerant components include at least one of methane, 2,3,3,3-tetrafluoropropylene, 1,3,3,3-tetrafluoropropylene, 1-chloro-3,3,3-trifluoropropylene, 1,1,1,4,4,4-hexafluoro-2-butene, 1-chloro-2-,3,3,4-tetrafluoropropylene, trifluoromethane, and 1,1,1,2-tetrafluoroethane; The absorbent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, triethylene glycol dimethyl ether, and α-pyrrolidone.

4. The self-cascading absorption precooling mixed refrigerant natural gas liquefaction system according to claim 3, characterized in that, Based on the total molar amount of all components in the mixed refrigerant I and the absorbent being 100%, the molar fraction of the absorbent is 75%–98%, the molar fraction of trifluoromethane is 0%–3.98%, the molar fraction of 1,1,1,2-tetrafluoroethane is 0%–12.45%, the molar fraction of ethylene is 0.52%–9.18%, the molar fraction of methane is 0–0.95%, the molar fraction of 2,3,3,3-tetrafluoropropene is 0–12.85%, the molar fraction of 1,3,3,3-tetrafluoropropene is 0–12.75%, the molar fraction of 1-chloro-3,3,3-trifluoropropene is 0–12.35%, the molar fraction of 1,1,1,4,4,4-hexafluoro-2-butene is 0–12.45%, and the molar fraction of 1-chloro-2-,3,3,4-tetrafluoropropene is 0–12.45%.

5. The self-cascading absorption precooling mixed refrigerant natural gas liquefaction system according to claim 3 or 4, characterized in that, In the mixed refrigerant I circulation loop, the gaseous refrigerant I provides a cooling temperature of -100 to -70°C in the third precooling heat exchanger (16) and a cooling temperature of -70 to 28°C in the first precooling heat exchanger (13).

6. The self-cascading absorption precooling mixed refrigerant natural gas liquefaction system according to claim 1, characterized in that, The mixed refrigerant II comprises at least two of CH4, C2H6, C3H8, and N2.

7. The self-cascading absorption precooling mixed refrigerant natural gas liquefaction system according to claim 6, characterized in that, The mixed refrigerant II comprises CH4, C2H6, C3H8 and N2, wherein the molar fraction of CH4 is 30.2% to 50.5%, the molar fraction of C2H6 is 19.0% to 33.8%, the molar fraction of C3H8 is 7.2% to 25.2%, and the molar fraction of N2 is 8.5% to 21.0%.

8. The self-cascading absorption precooling mixed refrigerant natural gas liquefaction system according to claim 1, characterized in that, The cooling medium temperature of the first condenser (2), the second condenser (12) and the absorber (7) is below 25°C.

9. A method for liquefying mixed refrigerant natural gas using self-cascade absorption precooling, characterized in that, The mixed refrigerant natural gas liquefaction system with self-cascade absorption precooling as described in any one of claims 1 to 8 is adopted; The self-cascade absorption precooling mixed refrigerant natural gas liquefaction method includes a mixed refrigerant I cycle process, a mixed refrigerant II cycle process, and a natural gas liquefaction process. The mixed refrigerant I circulation process includes: mixed refrigerant I overflows from the mixed refrigerant I outlet of the generator (1), is condensed by the first condenser (2), and then enters the first gas-liquid separator (3) to be separated into liquid refrigerant I and gas refrigerant I. Liquid refrigerant I is throttled by the first throttle valve (4) and enters the low-pressure side of the condenser-evaporator (5). Gas refrigerant I enters the high-pressure side of the condenser-evaporator (5). Liquid refrigerant I in the condenser-evaporator (5) evaporates and cools gas refrigerant I before entering the absorber (7) to be absorbed by a dilute solution. Gas refrigerant I cooled in the condenser-evaporator (5) is then separated by the first gas-liquid separator (3). The solution is throttled by the second throttle valve (6), and then passes through the third precooling heat exchanger (16) and the first precooling heat exchanger (13) for evaporation and cooling before entering the absorber (7) to be absorbed by the dilute solution. Driven by the solution pump (9), the concentrated solution in the absorber (7) returns to the generator (1) through the concentrated solution outlet via the low temperature side of the solution heat exchanger (10) for circulation and overflow of mixed refrigerant I. The dilute solution left after the mixed refrigerant I overflows in the generator (1) passes through the high temperature side of the solution heat exchanger (10) and the third throttle valve (8) in sequence to enter the absorber (7) for absorbing liquid refrigerant I and gaseous refrigerant I to form a concentrated solution. The mixed refrigerant II cycle process includes: under the drive of the compressor (11), the mixed refrigerant II is pre-cooled by passing through the second condenser (12) and the first pre-cooling heat exchanger (13) in sequence, and then enters the second gas-liquid separator (14) to be separated into the first gas phase refrigerant II and the first liquid phase refrigerant II. The first gas phase refrigerant II and the first liquid phase refrigerant II are both pre-cooled by passing through the second pre-cooling heat exchanger (15) and the third pre-cooling heat exchanger (16) in sequence. Then, the first liquid phase refrigerant II is throttled by the fourth throttle valve (17) in sequence and is provided with cooling capacity by the third pre-cooling heat exchanger (16) and the second pre-cooling heat exchanger (15) before returning to the compressor (11). The first gas phase refrigerant II enters the third gas-liquid separator (18). The refrigerant is separated into a second gaseous refrigerant II and a second liquid refrigerant II. Both the second gaseous refrigerant II and the second liquid refrigerant II enter the fourth precooling heat exchanger (19) for precooling. Then, the second liquid refrigerant II is throttled by the fifth throttle valve (20) and provided with cooling capacity by the fourth precooling heat exchanger (19), the third precooling heat exchanger (16) and the second precooling heat exchanger (15) before returning to the compressor (11). The second gaseous refrigerant II is precooled by the cold heat exchanger (21) and throttled by the sixth throttle valve (22). After returning, it is throttled by the cold heat exchanger (21), the fourth precooling heat exchanger (19), the third precooling heat exchanger (16) and the second precooling heat exchanger (15) before returning to the compressor (11). The natural gas liquefaction process includes: the feed natural gas is cooled sequentially by the first precooling heat exchanger (13), the second precooling heat exchanger (15), the third precooling heat exchanger (16), the fourth precooling heat exchanger (19) and the subcooling heat exchanger (21), and then throttled by the seventh throttle valve (23) before entering the fourth gas-liquid separator (24) to separate and obtain liquefied natural gas.

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