A method and system for producing high-purity liquid CO2 using LNG cold energy

Abstract

This invention belongs to the technical field of direct utilization of LNG cold energy, specifically relating to a method and system for producing high-purity liquid CO2 using LNG cold energy. The system provided by this invention includes at least a CO2 pressure processing subsystem, a Rankine cycle subsystem, and a distillation and purification subsystem. The CO2 pressure processing subsystem uses CO2 effluent to exchange heat with the working fluid of the Rankine cycle subsystem to provide the energy required for the working fluid to generate electricity. The distillation and purification subsystem purifies the CO2 effluent after heat exchange. For high-pressure CO2 feed gas from pipelines, this invention converts part of the feed gas's pressure energy into electrical energy through diversion and expansion, increasing the energy conversion efficiency of the process. For lower-pressure CO2 feed gas from pipelines, it increases the pressure of part of the feed gas through diversion and compression to increase the CO2 liquefaction rate. Its distillation and purification subsystem constructs a refrigeration cycle between the condenser and reboiler, which utilizes LNG cold energy, significantly reducing refrigeration energy consumption.

Description

Technical Field

[0001] This invention belongs to the technical field of direct utilization of LNG cold energy, specifically relating to a method and system for producing high-purity liquid CO2 using LNG cold energy. Background Technology

[0002] Common technologies for utilizing the cold energy of liquefied natural gas (LNG) include LNG cold energy power generation, air separation, and the production of liquid CO2 or dry ice. Cold energy power generation technology plays an important role as one of the main methods for utilizing the cold energy of LNG.

[0003] The basic principle of LNG cold energy power generation is generally through a low-temperature power cycle process, using LNG as a low-temperature cold source, and using the mechanical work generated by the low-temperature power cycle to drive a generator set to produce electricity.

[0004] By using technological means to recover emitted CO2 gas and make full use of its waste heat resources, it is environmentally friendly in mitigating the greenhouse effect and can also generate economic value from the wasted waste heat resources.

[0005] Chinese patent application number 201710658357.2 proposes a carbon dioxide liquefaction transposition method. This method targets carbon dioxide from a flue gas carbon capture system, removes impurities, compresses it to a supercritical state, and achieves liquefaction through throttling expansion. The difference between this patent and the previous one is that the latter does not utilize LNG cold energy, the impurity removal process is relatively simple, the product liquid purity is low, and compressing the carbon dioxide to a supercritical state consumes a large amount of power.

[0006] Chinese patent application number 202310357469.X proposes a device for pressurizing and liquefying carbon dioxide using LNG cold energy. The difference between this patent and the present patent is that the present patent does not have a liquid carbon dioxide purification device and the cold energy conversion product is singular, consisting only of liquid carbon dioxide.

[0007] Chinese patent application number 201910049759.1 proposes a device for carbon dioxide liquefaction and purification. The difference between this patent and the device is that it does not utilize LNG cold energy, has no power output, and the purification part is more complex.

[0008] Chinese patent application number 201910342318.0 proposes a power generation method using carbon dioxide as the working fluid of the Rankine cycle, which recovers and utilizes the cold energy of LNG and utilizes the heat source of the first mass, but the system does not produce liquid carbon dioxide.

[0009] Chinese patent application number 202310563543.3 proposes an LNG cold energy utilization system that integrates energy storage, carbon capture and air conditioning. Through the combined application of various systems, it scientifically solves the problems of insufficient utilization of LNG cold energy and its matching with reality. However, the system is not equipped with a liquid carbon dioxide purification system, and the produced liquid products need to be purified before they can be used industrially.

[0010] Chinese patent application number 202220930402.1 proposes a carbon dioxide capture and transfer method for gas turbine units. The difference between this patent and the one in this patent is that the produced liquid carbon dioxide is not purified, and the cold energy conversion product is singular with no reactive power output.

[0011] Chinese patent application number 202120785147.1 proposes a carbon dioxide liquefaction and purification device utilizing LNG cold energy. The difference between this patent and the previous one lies in the fact that the cold energy conversion product is singular, with no reactive power output, and the production of liquid products cannot be effectively regulated.

[0012] Patent application number 202211285506.2 discloses a direct air carbon dioxide capture and utilization system and method utilizing LNG cold energy, including an LNG vaporization and cooling device, an air carbon dioxide capture device, and a carbon dioxide utilization device. However, this process utilizes only one type of cold energy product, generates no electricity, and the production of liquid carbon dioxide is not adjustable.

[0013] Patent No. 201510339362.8 discloses a method for capturing carbon dioxide from ore smelting waste gas using LNG cold energy, belonging to the environmental protection field. Specifically, it relates to a method for capturing carbon dioxide from waste gas generated during ore smelting using liquefied natural gas cold energy. This invention can concentrate and capture all fugitive carbon dioxide emissions, greatly reducing carbon emissions during magnesite smelting. It combines a two-stage Rankine cycle power generation system to achieve temperature matching and cascaded utilization. However, this process does not purify the liquefied carbon dioxide and cannot be used in industrial production.

[0014] Patent application number 202210978747.9 discloses a highly efficient integrated system and method for achieving LNG gasification and export, natural gas reforming for hydrogen production, hydrogen liquefaction, and CO2 liquefaction and recovery through comprehensive and optimized utilization of LNG cold energy within an LNG receiving terminal. The difference between this patent and the previous one is that it does not output electrical energy and cannot adjust the ratio of liquid carbon dioxide production to power generation.

[0015] Patent application number 202123015473.3 discloses a ship CO2 liquefaction and collection system utilizing LNG cold energy, including a liquefaction processing unit, a collection and unloading unit, and a venting unit. The difference between this patent and the one in question lies in their applicable scenarios. This patent is applicable to the cold energy utilization section within an LNG receiving terminal, and the latter does not output electricity and cannot effectively regulate the production of liquid carbon dioxide.

[0016] Patent No. 202120613476.8 discloses a system for producing dry ice using LNG cold energy, including an LNG storage tank, a liquid carbon dioxide storage tank, a first heat exchanger, a dry ice machine, and a compressor. The LNG storage tank provides liquefied natural gas. This system is used to produce dry ice, effectively reducing carbon dioxide loss and power consumption, and has significant environmental and economic benefits. However, this process utilizes only one cold energy product: dry ice, without generating electricity or liquid carbon dioxide.

[0017] The patented system with patent number 202110233126.3 solves the carbon dioxide emission problem in natural gas-to-hydrogen production by using the carbon dioxide generated in the reaction as a raw material, thus turning waste into treasure and achieving a simultaneous improvement in the material and energy utilization rates of the process system. However, the products of this process are hydrogen and liquid carbon dioxide, with no electricity generated, and this process is applicable to the natural gas-to-hydrogen part, which differs from the LNG cold energy utilization process proposed in this patent.

[0018] Patent No. 201721094836.8 discloses a process system for producing dry ice using LNG cold energy in liquefied natural gas (LNG) power generation. This system can effectively utilize combustion products, carbon dioxide, and LNG cold energy, achieving cascade utilization of cold energy and carbon emission reduction, which has significant economic, energy-saving, and emission-reduction implications. However, this process does not purify the carbon dioxide after liquefaction, and the generated electricity comes from the combustion of natural gas, rather than from the utilization of cold energy.

[0019] Patent No. 201510490793.4 discloses a system and method for capturing liquefied carbon dioxide using the cold energy of liquefied natural gas. However, this process is complex and only applicable to natural gas power plants, and the liquefied carbon dioxide is not purified.

[0020] Patent No. 201410425132.9 discloses a process and apparatus for preparing liquid carbon dioxide and dry ice using LNG cold energy. This process enables the cascaded utilization of LNG cold energy, achieving a high LNG cold energy utilization rate. By utilizing LNG cold energy to reduce the preparation pressure of liquid CO2 and dry ice to 0.6~1.0 MPa, and cooling the compressor inlet gas CO2 to -40~-30℃, it can ultimately achieve energy savings of over 58% compared to conventional low-pressure methods. However, this industrial product is singular, generates no electricity, and lacks a liquid carbon dioxide purification process, meaning the produced liquid carbon dioxide cannot be used for industrial applications.

[0021] Patent No. 201310618307.3 discloses a combined power cycle method and system for capturing carbon dioxide using the cold energy of liquefied natural gas. However, this process is complex, suitable for natural gas power plant applications, and the oxygen-enriched combustion requires the production of high-purity oxygen, leading to increased costs.

[0022] Patent No. 202310379259.0 discloses an LNG-powered ship energy comprehensive utilization system based on oxygen-enriched combustion and carbon capture, including an air separation oxygen production subsystem, a cold energy and waste heat utilization subsystem, and a cryogenic carbon capture subsystem. However, this patent is only applicable to LNG-powered ships, the process is relatively complex, and the need for high-purity oxygen production during oxygen-enriched combustion increases costs.

[0023] Patent No. 202220604871.4 proposes a device for deep recovery of carbon capture energy, which includes at least an organic Rankine cycle power generation system, a CO2 recovery system, and an LNG cold energy recuperation system. Embodiments of this invention can fully recover the low-grade heat from low-pressure steam and regenerated gas, obtaining a large amount of low-grade heat through an organic working fluid and converting it into electrical energy. However, this process uses an alkanolamine solution, which leads to environmental pollution, requires a large amount of heat to be supplied to the reboiler of the regeneration tower, and involves numerous pieces of equipment and a complex process.

[0024] Compared to the above patents, this invention combines a mixed working fluid Rankine cycle with a feed gas pre-expansion process and recovers the cold energy of the pressurized CO2, achieving a triple function of CO2 liquefaction, pressurization, and increased power generation. This allows for more efficient utilization of cold energy, significantly improving the efficiency of LNG cold energy utilization. Furthermore, by combining it with pre-expansion or compression processes, the ratio of power generation to liquefaction can be altered, producing high-purity liquid CO2 with high energy efficiency. The proposed integrated process design, with specific matching of the working fluid, components, and process parameters, enhances system efficiency. A refrigeration cycle is installed between the condenser and reboiler in the distillation column, utilizing LNG cold energy and significantly reducing the energy consumption of the refrigeration compressor. Through coupling with the Rankine cycle, high-purity liquid carbon dioxide can be produced. Summary of the Invention

[0025] To address the shortcomings of existing technologies, this invention provides a method and system for producing high-purity liquid CO2 using LNG cold energy. The system includes a CO2 pressure processing subsystem, a Rankine cycle subsystem, and a distillation and purification subsystem. The CO2 pressure processing subsystem uses the CO2 output gas to exchange heat with the working fluid of the Rankine cycle subsystem, providing the energy needed for the working fluid to generate electricity. The distillation and purification subsystem purifies the heat-exchanged CO2 output gas. For high-pressure CO2 feed gas from pipelines, this invention converts a portion of the feed gas's pressure energy into electrical energy through diversion and expansion, increasing the process's energy conversion efficiency. For lower-pressure CO2 feed gas from pipelines, a portion of the feed gas's pressure is increased through diversion and compression, thereby increasing the CO2 liquefaction rate. The distillation and purification subsystem of this invention establishes a refrigeration cycle between the condenser and reboiler, which utilizes LNG cold energy, significantly reducing refrigeration energy consumption.

[0026] This invention utilizes expansion or pressurization to process CO2 gas transported from pipelines, depending on the pressure. In systems that utilize LNG cold energy to liquefy CO2, the production of liquid CO2 in the liquefaction separation system is increased through coupling methods such as inlet CO2 diversion expansion or compression, and targeted matching with the Rankine cycle working fluid and process parameters. Furthermore, the ratio between CO2 liquefaction and power generation can be adjusted according to market demand. By combining with pre-expansion or compression processes, the ratio of power generation to liquefaction can be altered, producing high-purity liquid CO2 with high energy efficiency. The proposed integrated process design, along with specific matching of the working fluid, components, and process parameters, further enhances system efficiency.

[0027] The technical solution provided by this invention is as follows: A high-purity liquid CO2 production system utilizing LNG cold energy, comprising at least: The first heat exchanger has a first flow channel, a second flow channel, a third flow channel and a fourth flow channel; The second heat exchanger has a first flow channel, a second flow channel and a third flow channel; The third heat exchanger has a first flow channel and a second flow channel; A first tee, a first expander, a second tee, and a first gas-liquid separator are sequentially connected, including a carbon dioxide pipeline, the first tee, the first expander, the second flow channel of the first heat exchanger, the first flow channel of the third heat exchanger, the second tee, and the first gas-liquid separator; or, a first tee, a first compressor, a first cooler, a second tee, and a first gas-liquid separator are sequentially connected, including a carbon dioxide pipeline, the first tee, the first compressor, the first cooler, the second flow channel of the first heat exchanger, the first flow channel of the third heat exchanger, the second tee, and the first gas-liquid separator. The working fluid expander and the working fluid pump are arranged in sequence, wherein the working fluid expander, the third flow channel of the second heat exchanger, the working fluid pump, the first flow channel of the second heat exchanger, the second flow channel of the third heat exchanger and the third flow channel of the first heat exchanger are connected in sequence, and the third flow channel of the first heat exchanger is also connected to the working fluid expander; The liquefied natural gas inlet pipe and the third tee are sequentially connected and arranged. The carbon dioxide-containing pipeline, the first tee, the first flow channel of the first heat exchanger, the throttle valve, and the second tee are sequentially connected.

[0028] The high-purity liquid CO2 production system utilizing LNG cold energy provided by the above technical solution can generate electricity and liquefy carbon dioxide by using a hydrocarbon mixture as the working fluid and recovering the cold energy of liquefied natural gas. This system uses low-temperature LNG as a low-temperature heat source and carbon dioxide as a high-temperature heat source, recovering the cold energy of LNG to generate mechanical energy and drive a generator to produce electricity.

[0029] The first expander, the heater, and the second expander constitute a CO2 expansion subsystem. After the high-pressure carbon dioxide gas is expanded, its temperature decreases, and then it undergoes heat exchange under a certain pressure. After passing through the first liquid phase separator, impurities can be removed. This can change the ratio of power generation to liquid CO2 production and improve the system efficiency. The first compressor, the first cooler, the second compressor, and the second cooler constitute a CO2 booster subsystem. The low-pressure carbon dioxide gas is compressed and its temperature rises. Then, it undergoes heat exchange under a certain pressure. After passing through the first liquid phase separator, impurities can be removed. This can change the ratio of power generation to liquid CO2 production and improve the system efficiency. The CO2 expansion subsystem and the CO2 pressurization subsystem constitute the CO2 pressure handling system, which is used to handle the pressure of CO2 gas delivered from the pipeline; The third flow channel of the first heat exchanger, the working fluid expander, the third flow channel of the second heat exchanger, the working fluid pump, the first flow channel of the second heat exchanger, and the second flow channel of the third heat exchanger constitute a Rankine cycle system. The circulating working fluid is pressurized and expanded to generate electricity in the Rankine cycle system, and exchanges heat with the CO2 gas. LNG is transported out in gaseous form after being heated by a heat exchanger.

[0030] Furthermore, high-purity liquid CO2 production systems utilizing LNG cold energy also include: The distillation column, condenser, and second gas-liquid separator are provided. The condenser has a first flow channel and a second flow channel. The gas phase outlet of the distillation column, the first flow channel of the condenser, and the second gas-liquid separator are sequentially connected. The liquid phase outlet of the second gas-liquid separator is also connected to the upper part of the distillation column. The distillation column includes a reboiler, a third gas-liquid separator, and a cooling water pipe. The reboiler has a first flow channel, a second flow channel, and a third flow channel. The liquid phase outlet of the distillation column, the first flow channel of the reboiler, and the third gas-liquid separator are sequentially connected. The gas phase outlet of the third gas-liquid separator is also connected to the lower part of the distillation column. The cooling water pipe is connected to the second flow channel of the reboiler. The system includes a refrigeration expander, a refrigeration heat exchanger, and a refrigeration compressor. The refrigeration heat exchanger has a first flow channel and a second flow channel. The refrigeration expander, the second flow channel of the refrigeration heat exchanger, the second flow channel of the condenser, the refrigeration compressor, and the third flow channel of the reboiler are sequentially connected. The third flow channel of the reboiler is also connected to the refrigeration expander. The liquefied natural gas inlet pipe, the third tee, and the first flow channel of the refrigeration heat exchanger are sequentially connected.

[0031] Based on the above technical solutions, the purity of carbon dioxide can be further improved through separation and distillation.

[0032] Furthermore, the system also includes a second expander, and the carbon dioxide pipeline, the first tee, the first expander, the second expander, and the second flow channel of the first heat exchanger are sequentially connected.

[0033] Based on the above technical solution, setting up a two-stage expansion can further increase the amount of liquefaction while increasing the power generation.

[0034] Furthermore, it also includes a second compressor and a second cooler, wherein the carbon dioxide pipeline, the first tee, the first compressor, the first cooler, the second compressor, the second cooler and the second flow channel of the first heat exchanger are sequentially connected.

[0035] Based on the above technical solution, setting up two-stage compression can further increase the liquefaction volume.

[0036] Specifically, the distillation column has 16 trays, with the inlet located at the eighth tray; the liquid phase outlet of the second gas-liquid separator is connected to the first tray of the distillation column; and the gas phase outlet of the third gas-liquid separator is connected to the sixteenth tray of the distillation column.

[0037] The above technical solutions can reduce energy consumption and improve production efficiency.

[0038] Specifically: The working fluid of the expander comprises ethane, propane, and ethylene in a molar percentage of (65-75)%:(15-25)%:(8-12)%. Preferably, the working fluid of the expander comprises ethane, propane, and ethylene in a molar percentage of 70%:20%:10%. The working fluid outlet conditions at the working fluid pump are: -63.62℃, 1.3 MPa, 130 t / h, and the working fluid outlet conditions at the working fluid expander are: -46.99℃, 0.3 MPa. One stream of carbon dioxide-containing feed gas is cooled to -21.37°C after passing through the first heat exchanger, and another stream of carbon dioxide-containing feed gas is cooled to -23.66°C after passing through the first and second heat exchangers. The combined streams have a temperature of -21.98°C. A stream of liquefied natural gas (LNG) passes through the second and first heat exchangers, and its temperature rises to -75.21°C. Another stream of LNG passes through a refrigeration heat exchanger, and its temperature rises to -4.86°C. The outlet pressure of the distillation column is 1.9 to 2 MPa at the bottom, preferably 1.95 MPa, and 1.8 to 1.9 MPa at the top, preferably 1.9 MPa.

[0039] The aforementioned process conditions constitute the boundary conditions for the operation of this system, and they interact and influence each other. Only by rationally adjusting the ratio of the mixed working fluid and coordinating it with process parameters such as the expansion pressure of the raw material gas can a high efficiency of cold energy utilization be ensured.

[0040] The ratio between liquefied products and power generation can be flexibly adjusted through the diversion and expansion of raw material carbon dioxide and the diversion and compression of the distillation tower. By changing the diversion ratio, the ratio of carbon dioxide liquefaction and power generation can be adjusted.

[0041] By splitting and expanding the feed gas and splitting and compressing the distillation column top, the heat exchange of hot and cold fluids in the heat exchanger can be divided into multiple stages. This, combined with the specific composition of the Rankine cycle working fluid and the specific range of process parameters, can improve the heat exchange in the heat exchanger, reduce irreversible losses, and improve the efficiency of the entire system.

[0042] This invention also provides a method for producing high-purity liquid CO2 using LNG cold energy. The high-purity liquid CO2 production system using LNG cold energy provided by this invention performs the following steps: high-purity liquid CO2 is prepared using liquefied natural gas as a cold source, and electricity is generated by working fluid expander. Under the matching of refrigeration, separation, working fluid, composition, process and parameters, the liquefaction and distillation sections achieve an efficiency of high-purity liquid CO2 production that exceeds the existing level, thus resulting in low energy consumption, low operating costs and good benefits.

[0043] The beneficial effects of this invention are: The beneficial effects of this invention are that CO2 liquefaction, CO2 pressurization and power generation can be achieved simultaneously in the same process, and the process is simple and highly efficient. The technological innovation that achieves beneficial results lies in pre-depressurizing and expanding CO2 before cooling and heat exchange, followed by cooling using a mixed working fluid Rankine cycle. In both natural gas liquefaction and LNG cold energy utilization, the conventional approach is to first pressurize CO2 before liquefaction. This invention depressurizes and expands the feed gas before it enters the cryogenic heat exchanger, thereby effectively increasing power generation, achieving more efficient utilization of LNG cold energy, and further improving the efficiency of LNG cold energy utilization. This invention enables the full and efficient utilization of cold energy. It is achieved through the organic combination of multiple measures, including the Rankine cycle of the mixed working fluid, the CO2 expansion and cooling or pressurization and heating process, the specific composition and ratio of the mixed working fluid, and the specific value range of multiple process parameters. These measures are organically combined into a whole and cannot be separated. The present invention can also be used to distill the separated flue gas components to obtain relatively pure CO2 gas; This invention combines the advantages of compression or expansion, shallow low-temperature liquefaction, and distillation. It can separate impurities with boiling points similar to CO2, allowing low-concentration CO2 feed gas containing various impurities to obtain higher-concentration CO2. Further removal of other impurities yields high-purity industrial-grade liquid CO2 or food additive liquid CO2. The integrated system can rationally realize the liquefaction and separation of high-pressure or low-pressure carbon dioxide, exhibiting good adaptability and reliability. It also features pressure energy recovery and multi-stage heat exchange, and can generate electricity. The distillation section of this invention is designed to have low energy consumption and improve production efficiency. Attached Figure Description

[0044] Figure 1 This is a system diagram of the high-purity liquid CO2 production system using LNG cold energy for high-pressure carbon dioxide production provided by the present invention.

[0045] Figure 2 This is a system diagram of the high-purity liquid CO2 production system using LNG cold energy for low-pressure carbon dioxide provided by the present invention.

[0046] Appendix Figure 1 , 2 In the text, the meanings represented by each number are as follows: The gas supply lines are as follows: 1. First gas supply line; 2. Second gas supply line; 3. Third gas supply line; 5. Fourth gas supply line; 6. Fifth gas supply line; 7. Sixth gas supply line; 8. Seventh gas supply line; 9. Eighth gas supply line; 10. Ninth gas supply line; 11. Tenth gas supply line; 35. Eleventh gas supply line; 36. Twelfth gas supply line; 29. ​​Thirteenth gas supply line. The liquid CO2 states are as follows: 12, first liquid CO2; 13, second liquid CO2; 14, third liquid CO2; 15, fourth liquid CO2; 16, fifth liquid CO2; 17, sixth liquid CO2; 30, seventh liquid CO2; 31, eighth liquid CO2; 32, ninth liquid CO2; 33, tenth liquid CO2; 34, eleventh liquid CO2. The LNGs are: 18, First LNG; 19, Second LNG; 20, Third LNG; 21, Fourth LNG; 22, Fifth LNG; 37. Cooling water. Detailed Implementation

[0047] The principles and features of the present invention are described below. The embodiments given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0048] In the following embodiments, the isentropic efficiency of the compressor is taken as 0.75, the efficiency of the turbine is taken as 0.85, and the efficiency of the pump is taken as 0.75.

[0049] The LNG vaporization unit along the LNG flow path includes a first and a second heat exchanger and a first heat exchanger.

[0050] The carbon dioxide liquefaction unit passes through the first expander, the second expander, the first heat exchanger, and the third heat exchanger before entering the distillation tower for purification. The LNG is then transported out after passing through the second heat exchanger and the first heat exchanger in sequence.

[0051] The integrated utilization system provided in this embodiment also includes a heater, which is installed at the outlet of the first expander.

[0052] 200 t / h of LNG at 9.3 MPa and -138℃, delivered from the LNG receiving terminal, is fed into the second heat exchanger, where it exchanges heat with the mixed circulating working fluid. After the heat exchange, the temperature of the LNG rises to -64.17℃. The heated LNG enters the first heat exchanger, where it exchanges heat with the mixed circulating working fluid and carbon dioxide gas. After the heat exchange, the temperature of the LNG rises to -34.43℃ and is then transported out through the long-distance pipeline network. The mixed working fluid enters the second heat exchanger and exchanges heat with the LNG entering the second heat exchanger. The mixed working fluid is cooled down to -49.12℃ after heat exchange. The carbon dioxide liquefaction rate can reach 367.9 t / h, and the liquefaction rate decreases as the temperature of the inlet flue gas increases.

[0053] Example 1 The embodiment provides a process for producing high-purity liquid CO2 using LNG cold energy, the structure of which is as follows: Figure 1 As shown: The system includes a CO2 expansion power generation system, a Rankine cycle system, and a distillation and purification system. The CO2 expansion power generation system converts part of the pressure energy of the feed gas into electrical energy. The depressurized carbon dioxide gas enters the first heat exchanger of the Rankine cycle and liquefies with the circulating working fluid. Specifically, it may include a first expander, a heater, and a second expander connected in sequence. The circulating working fluid is pressurized and expanded to generate electricity in the Rankine cycle system. Specifically, it cycles through the third flow channel of the first heat exchanger, the working fluid expander, the third flow channel of the second heat exchanger, the working fluid pump, the first flow channel of the second heat exchanger, and the second flow channel of the third heat exchanger. LNG passes through a three-way valve and flows sequentially through the second heat exchanger. The second flow channel of the heat exchanger and the fourth flow channel of the first heat exchanger complete vaporization, while another stream completes vaporization after passing through a refrigeration heat exchanger. The expanded CO2 stream passes through the second flow channel of the first heat exchanger and the first flow channel of the third heat exchanger, then mixes with the unexpanded CO2 stream after passing through the first heat exchanger and a throttling valve, and is then introduced into a gas-liquid separator. The liquid CO2 flows out from the bottom of the gas-liquid separator and enters the distillation column for purification. The distillation purification system is responsible for removing impurities from the liquid CO2 to ensure its purity. The refrigeration cycle of the distillation purification system provides energy for the distillation column. Specifically, it may include a refrigeration heat exchanger, a distillation column condenser, a refrigeration compressor, a distillation column reboiler, and a refrigeration expander connected in sequence.

[0054] The CO2 feedstock consists of 2.23% methane, 0.12% ethane, 96.98% carbon dioxide, and 0.67% nitrogen (molar percentage). In the CO2 gas expansion pipeline, after two stages of expansion, the pressure decreases from 7000 kPa to 2010 kPa, and the temperature drops to -19.98℃. It then enters the first heat exchanger for further cooling and liquefaction, decreasing to -21.23℃, followed by further cooling in the third heat exchanger to -21.37℃. In the unexpanded CO2 gas pipeline, after cooling in the first heat exchanger, the temperature drops to 0℃. Then, after passing through a throttling valve, the pressure decreases from 6995 kPa to 2000 kPa. The pressure is 1950 kPa, and the temperature drops to -23.66℃. After the two pipelines are mixed, they enter the gas-liquid separator. The liquid CO2 flows out from the bottom of the separator and enters the distillation column for purification. After purification, the high-purity liquid CO2 flows out from the bottom of the reboiler of the distillation column. The composition is 99.98% CO2 and 0.02% ethane (molar percentage). The pressure is 1950 kPa. The liquid carbon dioxide product is sent to the storage tank for storage.

[0055] The Rankine cycle system includes the following steps: The Rankine cycle uses a mixture of 70% ethane, 20% propane, and 10% ethylene (molar percentage).

[0056] The working fluid in the Rankine cycle is first pressurized to 1167 kPa by the working fluid pump, and then passes through the second heat exchanger, the third heat exchanger and the first heat exchanger in sequence. The temperature is increased to -49℃ to -51℃, -48℃ to -50℃ and -6℃ to -8℃ respectively. After becoming a pure gas phase, it enters the expander to do work and generate electricity. The pressure is reduced to 300 kPa. After the temperature decreases, it enters the second heat exchanger to absorb the cold energy of LNG and the working fluid of the cycle and liquefy it. After becoming a liquid, it enters the pump for pressurization and completes the cycle.

[0057] The refrigeration cycle of the distillation purification system includes the following steps: The working fluid for the refrigeration cycle of the distillation purification system is propane.

[0058] In the distillation and purification system, the working fluid for the refrigeration cycle is first pressurized to 720 kPa by the refrigeration compressor. Then, the high-pressure stream enters the reboiler of the distillation column for heat exchange. After heat exchange, the temperature drops to 38°C to 42°C. Subsequently, it enters the refrigeration expander to generate electricity, and the pressure drops to 150 kPa. After the temperature drops, it enters the refrigeration heat exchanger to absorb the cold energy of LNG and liquefy it. Finally, it enters the condenser of the distillation column for heat exchange, and the temperature increases to -28°C to 30°C. Then, it enters the refrigeration compressor to complete the cycle.

[0059] The physical property parameters of each logistics node in this embodiment are shown in Table 1.

[0060] Table 1 The performance achievable in this example is shown in Table 2.

[0061] Table 2 Example 2 This embodiment provides a high-purity liquid CO2 production process utilizing LNG cold energy. The difference from Example 1 is that the pipeline-transported carbon dioxide has a lower pressure, and its structure is as follows: Figure 2 As shown: The system includes: a CO2 booster system, a Rankine cycle system, and a distillation and purification system. The CO2 booster system increases the pressure of a portion of the feed gas, thereby increasing the amount of carbon dioxide liquefied. The boosted carbon dioxide gas enters the first heat exchanger of the Rankine cycle and liquefies with the circulating working fluid. Specifically, it may include a first compressor, a heater, and a second compressor connected in sequence. The circulating working fluid is boosted and expanded in the Rankine cycle system to generate electricity. Specifically, it passes through the third flow channel of the first heat exchanger, the working fluid expander, the third flow channel of the second heat exchanger, the working fluid pump, the first flow channel of the second heat exchanger, and the second flow channel of the third heat exchanger to complete the cycle. LNG passes through a three-way valve, and one stream passes through the first... The second flow channel of the second heat exchanger and the fourth flow channel of the first heat exchanger complete vaporization, while another stream completes vaporization after passing through a refrigeration heat exchanger. The pressurized CO2 stream passes through the second flow channel of the first heat exchanger and the first flow channel of the third heat exchanger, and then mixes with the uncompressed CO2 stream after passing through the first heat exchanger and a throttling valve. The mixture is then introduced into a gas-liquid separator, and the liquid CO2 flows out from the bottom of the gas-liquid separator and enters a distillation column for purification. The distillation purification system is responsible for removing impurities from the liquid CO2 to ensure its purity. The refrigeration cycle of the distillation purification system provides energy for the distillation column. Specifically, it may include a refrigeration heat exchanger, a distillation column condenser, a refrigeration compressor, a distillation column reboiler, and a refrigeration expander connected in sequence.

[0062] The CO2 feedstock consists of 2.23% methane, 0.12% ethane, 96.98% carbon dioxide, and 0.67% nitrogen (molar percentage). In the CO2 gas pressurization pipeline, the pressure is reduced from 2000 kPa to 4000 kPa through two stages of compression, while the temperature increases to 62.07℃. It then enters the first heat exchanger for cooling and liquefaction, decreasing to -6.2℃. Next, it enters the third heat exchanger for further cooling, decreasing to -11.31℃. Finally, it passes through a throttling valve, reducing the pressure from 3985 kPa to 1995 kPa. The pressure is 1950 kPa, and the temperature drops to -25.57℃. In the unpressurized CO2 gas pipeline, after passing through the first heat exchanger, the temperature drops to -42.78℃. The two pipelines are mixed and enter the gas-liquid separator. The liquid CO2 flows out from the bottom of the separator and enters the distillation column for purification. After purification, the high-purity liquid CO2 flows out from the bottom of the reboiler of the distillation column, with a composition of 99.98% CO2 and 0.02% ethane (molar percentage) and a pressure of 1950 kPa. The liquid carbon dioxide product is sent to the storage tank for storage.

[0063] The Rankine cycle system includes the following steps: The Rankine cycle uses a mixture of 70% ethane, 20% propane, and 10% ethylene (molar percentage).

[0064] The working fluid in the Rankine cycle is first pressurized to 905 kPa by the working fluid pump, and then passes through the second heat exchanger, the third heat exchanger and the first heat exchanger in sequence. The temperature is increased to -60℃, -55℃ and -15.95℃ respectively. After becoming a pure gas phase, it enters the expander to do work and generate electricity. The pressure is reduced to 239 kPa. After the temperature decreases, it enters the second heat exchanger to absorb the cold energy of LNG and the working fluid of the cycle and liquefy it. After becoming a liquid, it enters the pump for pressurization and completes the cycle.

[0065] The refrigeration cycle of the distillation purification system includes the following steps: The working fluid for the refrigeration cycle of the distillation purification system is propane.

[0066] In the distillation and purification system, the working fluid for the refrigeration cycle is first pressurized to 1000 kPa by the refrigeration compressor. Then, the high-pressure stream enters the reboiler of the distillation column for heat exchange. After heat exchange, the temperature drops to 40°C. Subsequently, it enters the refrigeration expander to generate electricity, and the pressure drops to 150 kPa. After the temperature drops, it enters the refrigeration heat exchanger to absorb the cold energy of LNG and liquefy it. Finally, it enters the condenser of the distillation column for heat exchange, and the temperature increases to -27.70°C. Then, it enters the refrigeration compressor to complete the cycle.

[0067] The physical property parameters of each logistics node in this embodiment are shown in Table 3.

[0068] Table 3 The performance achievable in this example is shown in Table 4.

[0069] Table 4 The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-purity liquid CO2 production system utilizing LNG cold energy, characterized in that, At least including: The first heat exchanger has a first flow channel, a second flow channel, a third flow channel and a fourth flow channel; The second heat exchanger has a first flow channel, a second flow channel and a third flow channel; The third heat exchanger has a first flow channel and a second flow channel; A first tee, a first expander, a second tee, and a first gas-liquid separator are sequentially connected, including a carbon dioxide pipeline, the first tee, the first expander, the second flow channel of the first heat exchanger, the first flow channel of the third heat exchanger, the second tee, and the first gas-liquid separator; or, a first tee, a first compressor, a first cooler, a second tee, and a first gas-liquid separator are sequentially connected, including a carbon dioxide pipeline, the first tee, the first compressor, the first cooler, the second flow channel of the first heat exchanger, the first flow channel of the third heat exchanger, the second tee, and the first gas-liquid separator. The working fluid expander and the working fluid pump are arranged in sequence, wherein the working fluid expander, the third flow channel of the second heat exchanger, the working fluid pump, the first flow channel of the second heat exchanger, the second flow channel of the third heat exchanger and the third flow channel of the first heat exchanger are connected in sequence, and the third flow channel of the first heat exchanger is also connected to the working fluid expander; The liquefied natural gas inlet pipe and the third tee are sequentially connected and arranged. The carbon dioxide-containing pipeline, the first tee, the first flow channel of the first heat exchanger, the throttle valve, and the second tee are sequentially connected and arranged. The system also includes a distillation column, a condenser, a second gas-liquid separator, a refrigeration expander, a refrigeration heat exchanger, and a refrigeration compressor. The condenser has a first flow channel and a second flow channel. The gas phase outlet of the distillation column, the first flow channel of the condenser, and the second gas-liquid separator are sequentially connected. The liquid phase outlet of the second gas-liquid separator is also connected to the upper part of the distillation column. The refrigeration heat exchanger has a first flow channel and a second flow channel. The refrigeration expander, the second flow channel of the refrigeration heat exchanger, the second flow channel of the condenser, and the refrigeration compressor are sequentially connected. The refrigeration compressor is also connected to the refrigeration expander. The liquefied natural gas inlet pipe, the third tee, and the first flow channel of the refrigeration heat exchanger are sequentially connected. The system also includes a reboiler, a third gas-liquid separator, and cooling water pipes. The reboiler has a first flow channel, a second flow channel, and a third flow channel. The liquid phase outlet of the distillation column, the first flow channel of the reboiler, and the third gas-liquid separator are sequentially connected. The gas phase outlet of the third gas-liquid separator is also connected to the lower part of the distillation column. The cooling water pipes are connected to the second flow channel of the reboiler. The refrigeration compressor, the third flow channel of the reboiler, and the refrigeration expander are sequentially connected. The distillation column has sixteen trays, with the inlet located at the eighth tray. The liquid phase outlet of the second gas-liquid separator is connected to the first tray of the distillation column. The gas phase outlet of the third gas-liquid separator is connected to the sixteenth tray of the distillation column. The system also includes a second expander, and the carbon dioxide pipeline, the first tee, the first expander, the second expander and the second flow channel of the first heat exchanger are sequentially connected; The working fluid of the working fluid expander comprises ethane, propane and ethylene in a molar percentage of (65-75)%:(15-25)%:(8-12)%; or, the working fluid of the working fluid expander comprises ethane, propane and ethylene in a molar percentage of 70%:20%:10%. The working fluid outlet conditions at the working fluid pump are: -63.62℃, 1.3 MPa, 130 t / h, and the working fluid outlet conditions at the working fluid expander are: -46.99℃, 0.3 MPa. One stream of carbon dioxide-containing feed gas is cooled to -21.37°C after passing through the first heat exchanger, and another stream of carbon dioxide-containing feed gas is cooled to -23.66°C after passing through the first and second heat exchangers. The combined streams have a temperature of -21.98°C. A stream of liquefied natural gas (LNG) passes through the second and first heat exchangers, and its temperature rises to -75.21°C. Another stream of LNG passes through a refrigeration heat exchanger, and its temperature rises to -4.86°C. The outlet pressure of the distillation column is 1.9 to 2 MPa at the bottom and 1.8 to 1.9 MPa at the top.

2. The high-purity liquid CO2 production system utilizing LNG cold energy according to claim 1, characterized in that: It also includes a second compressor and a second cooler, and the carbon dioxide pipeline, the first tee, the first compressor, the first cooler, the second compressor, the second cooler and the second flow channel of the first heat exchanger are sequentially connected.

3. A method for producing high-purity liquid CO2 using LNG cold energy, characterized in that, The high-purity liquid CO2 production system utilizing LNG cold energy as described in any one of claims 1 to 2 is used to perform the following steps: high-purity liquid CO2 is prepared using liquefied natural gas as a cold source, and power is generated by using a working fluid expander.

4. The method for producing high-purity liquid CO2 using LNG cold energy according to claim 3, characterized in that: In the CO2 gas expansion pipeline, after two stages of expansion, the pressure is reduced from 7000 kPa to 2010 kPa, and the temperature drops to -18℃ to -20℃. Then, it enters the first heat exchanger for cooling and liquefaction, with the temperature dropping to -20℃ to -21℃. Subsequently, it enters the third heat exchanger for further cooling, with the temperature dropping to -21℃ to -23℃. In the CO2 gas non-expansion pipeline, after being cooled by the first heat exchanger, the temperature drops to -1℃ to 1℃. After passing through the throttling valve, the pressure is reduced from 6995 kPa to 2000 kPa, and the temperature drops to -22℃ to -24℃. After the two pipelines are mixed, they enter the gas-liquid separator. The liquid CO2 flows out from the bottom of the gas-liquid separator and enters the distillation column for purification. After purification, the high-purity liquid CO2 flows out from the bottom of the reboiler of the distillation column. The composition is 99.98% CO2 and 0.02% ethane by molar percentage, and the pressure is 1950 kPa. The liquid carbon dioxide product is sent to the storage tank for storage. In the CO2 gas pressurization pipeline, the pressure increases from 2000 kPa to 4000 kPa and the temperature increases to 60℃ to 64℃ after two stages of compression. After initial cooling in the condenser, the gas then enters the first heat exchanger for further liquefaction, with the temperature decreasing to -5℃ to -8℃. It then enters the third heat exchanger for further cooling, with the temperature decreasing to -10℃ to -12℃. Finally, the pressure is reduced from 3985 kPa to 1995 kPa by the throttling valve. The pressure is 1950 kPa, and the temperature drops to -23℃ to -27℃. In the unpressurized CO2 gas pipeline, after passing through the first heat exchanger, the temperature drops to -40℃ to -43℃. The two pipelines are mixed and enter the gas-liquid separator. The liquid CO2 flows out from the bottom of the gas-liquid separator and enters the distillation column for purification. After purification, the high-purity liquid CO2 flows out from the bottom of the reboiler of the distillation column. The composition is 99.98% CO2 and 0.02% ethane by molar percentage, and the pressure is 1950 kPa. The liquid carbon dioxide product is sent to the storage tank for storage.

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