A cascade refrigeration system and a rectisol system

Abstract

This application discloses a cascade refrigeration system and a cryogenic methanol washing system. The cascade refrigeration system includes a cascade refrigeration unit, a second refrigerant storage tank, and an automatic refrigerant replenishment control valve. The cryogenic methanol washing system includes the aforementioned cascade refrigeration system, which is used to cool the methanol required for the cryogenic methanol washing process. Compared with related technologies, the cascade refrigeration system disclosed in this application, by adding a second refrigerant storage tank and an automatic refrigerant replenishment control valve, achieves automatic refrigerant replenishment of the cascade refrigeration unit, resulting in a high degree of automation, improved work efficiency, timely refrigerant replenishment, and avoidance of frequent manual refrigerant replenishment on-site, thus preventing the risk of asphyxiation associated with manual refrigerant replenishment.

Description

Technical Field

[0001] This application relates to the field of modern chemical technology, and more specifically, to a cascade refrigeration system and a low-temperature methanol washing system. Background Technology

[0002] Modern coal chemical processes, whether pulverized coal gasification or coal-water slurry gasification, are generally equipped with gas purification devices, namely low-temperature methanol washing.

[0003] CO2 / NH3 cascade refrigeration is commonly used in large-scale refrigeration units in the chemical industry. It not only provides deep cryogenic cooling capacity for low-temperature methanol washing but also liquefies and reuses CO2 gas desorbed from the low-temperature methanol washing regeneration system via the cascade refrigeration compressor unit. Furthermore, the cascade refrigeration unit uses NH3 (high efficiency in the mid-temperature range) in the high-temperature stage and CO2 (excellent low-temperature performance) in the low-temperature stage. Through a condenser-evaporator coupling two-stage cycle, it covers an ultra-wide temperature range of -55℃ to 0℃. By combining environmentally friendly working fluids, the amount of NH3 refrigerant required is significantly reduced, overcoming the dilemma of traditional single-medium refrigeration methods where safety, environmental protection, and high-efficiency refrigeration are difficult to achieve simultaneously.

[0004] Carbon Capture, Utilization and Storage (CCUS) is one of the key technologies for addressing climate change and reducing refrigerant emissions. CCUS systems separate and capture carbon dioxide from the exhaust gases of large emission sources (such as coal / gas-fired power plants, steel mills, cement plants, chemical plants, etc.), or capture carbon dioxide directly from the atmosphere. The captured refrigerant is utilized as a resource, while the unusable or surplus carbon dioxide is transported through pipelines to suitable locations for isolation and storage.

[0005] During operation, CO2 / NH3 cascade refrigeration units require manual replenishment of CO2 refrigerant. However, frequent manual replenishment of refrigerant on-site poses a risk of suffocation for workers, and the operation is cumbersome.

[0006] Therefore, how to efficiently and safely replenish refrigerant for cascade refrigeration units has become a technical problem that urgently needs to be solved by those skilled in the art. Utility Model Content

[0007] The purpose of this application is to disclose a cascade refrigeration system to improve the efficiency and safety of refrigerant replenishment in the cascade refrigeration system.

[0008] Another objective of this application is to disclose a low-temperature methanol washing system having the above-mentioned cascade refrigeration system.

[0009] A first aspect of this application provides a cascade cooling system, the cascade cooling system comprising: A cascade refrigeration unit is used to provide a first heat exchange medium for cooling methanol, wherein the methanol is methanol used in a low-temperature methanol washing process, and the cascade refrigeration unit is provided with a first refrigerant storage tank for storing the first heat exchange medium. The second refrigerant storage tank is used to store the first heat exchange medium for replenishment. The pressure in the second refrigerant storage tank is set to be greater than the pressure in the first refrigerant storage tank. The outlet of the second refrigerant storage tank is connected to the replenishment port of the first refrigerant storage tank. An automatic liquid replenishment control valve is installed between the first refrigerant storage tank and the second refrigerant storage tank. When the liquid level in the first refrigerant storage tank is lower than a set value, the automatic liquid replenishment control valve automatically opens; when the liquid level in the first refrigerant storage tank is equal to or higher than the set value, the automatic liquid replenishment control valve automatically closes.

[0010] In one possible implementation, the first refrigerant storage tank is equipped with a level transmitter, and the level of the first refrigerant storage tank is measured by the level transmitter.

[0011] In one possible implementation, the second refrigerant storage tank is equipped with a safety valve. When the pressure inside the second refrigerant storage tank exceeds a set safety threshold, the safety valve opens, and the first heat exchange medium inside the second refrigerant storage tank flows to the pressure relief system. When the pressure inside the second refrigerant storage tank is less than the set safety threshold, the safety valve closes.

[0012] In one possible implementation, the second refrigerant reservoir is equipped with a pressure indicator.

[0013] In one possible implementation, a check valve is provided between the first refrigerant storage tank and the second refrigerant storage tank, and the check valve is in a closed state in the direction from the first refrigerant storage tank to the second refrigerant storage tank.

[0014] In one possible implementation, when the first heat exchange medium is carbon dioxide, the inlet of the second refrigerant storage tank is connected to the carbon dioxide outlet of the CCUS system.

[0015] In one possible implementation, the cascade refrigeration unit includes a first heat exchange medium circuit and a second heat exchange medium circuit, wherein the first heat exchange medium circuit and the second heat exchange medium circuit are each independent circulation circuits, and the second heat exchange medium is evaporated into the first heat exchange medium for cooling through a condenser evaporator; the first refrigerant storage tank is disposed in the first heat exchange medium circuit.

[0016] In one possible implementation, the second heat exchange medium is ammonia, and the second heat exchange medium circuit includes: Ammonia compressor, the aforementioned ammonia compressor being driven by an electric motor; An ammonia condenser, the inlet of which is connected to the outlet of the ammonia compressor; A liquid ammonia storage tank, wherein the inlet of the liquid ammonia storage tank is connected to the outlet of the ammonia condenser, and the outlet of the liquid ammonia storage tank is connected to the ammonia inlet of the condenser evaporator. The ammonia outlet of the aforementioned condenser-evaporator is connected to the inlet of the aforementioned ammonia compressor.

[0017] In one possible implementation, the first heat exchange medium is carbon dioxide, and the first heat exchange medium circuit includes: A carbon dioxide compressor, wherein the aforementioned carbon dioxide compressor is driven by an electric motor; A carbon dioxide condenser, wherein the inlet of the carbon dioxide condenser is connected to the outlet of the carbon dioxide compressor. The first refrigerant storage tank, wherein the inlet of the first refrigerant storage tank is connected to the outlet of the carbon dioxide condenser. A liquid carbon dioxide transfer pump is connected to the liquid outlet of the first refrigerant storage tank and is used to transfer liquid carbon dioxide to the carbon dioxide inlet of the condenser-evaporator. The carbon dioxide outlet of the aforementioned condenser-evaporator is connected to the carbon dioxide inlet of the low-temperature methanol washing system, and the carbon dioxide outlet of the low-temperature methanol washing system is connected to the inlet of the aforementioned carbon dioxide compressor.

[0018] The cascade refrigeration system provided in this application automatically opens the automatic replenishment control valve when the liquid level of the first heat exchange medium in the first refrigerant storage tank drops below a set value. Because the pressure in the second refrigerant storage tank is greater than that in the first refrigerant storage tank, the first heat exchange medium in the second refrigerant storage tank flows into the first refrigerant storage tank under the pressure difference, causing the refrigerant level in the first refrigerant storage tank to rise. When the refrigerant level in the first refrigerant storage tank reaches the set value, the automatic replenishment control valve automatically closes, and the replenishment of the cascade refrigeration unit is completed.

[0019] Compared to related technologies, the cascade refrigeration system disclosed in this application achieves automatic refrigerant replenishment of the cascade refrigeration unit by adding a second refrigerant storage tank and an automatic refrigerant replenishment control valve. This results in a high degree of automation, improved work efficiency, timely refrigerant replenishment, and avoidance of frequent manual refrigerant replenishment on-site, thus preventing the risk of suffocation caused by manual refrigerant replenishment.

[0020] The second aspect of this application discloses a low-temperature methanol washing system, including a cascade refrigeration system, which is used to cool the methanol required for the low-temperature methanol washing process. The cascade refrigeration system is any of the cascade refrigeration systems described above.

[0021] The low-temperature methanol washing system disclosed in this application has all the technical effects of the aforementioned cascade refrigeration system, which will not be elaborated further here. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments or related technologies of this application, the accompanying drawings used in the description of the embodiments or related technologies will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the cascade cooling system disclosed in an embodiment of this application.

[0024] The attached figures are labeled as follows: 100. Second refrigerant storage tank; 110. Safety valve; 120. Level gauge for second refrigerant storage tank; 200. First refrigerant storage tank; 210. Level gauge for first refrigerant storage tank; 220. Level transmitter; 300. Automatic liquid replenishment control valve; 400. Pressure indicator; 500. Liquid carbon dioxide transfer pump; 600. Condenser / evaporator; 610. Liquid level indicator; 620. Condenser / evaporator level gauge; 700. Liquid ammonia storage tank; 710. Liquid ammonia storage tank level gauge; 720. Liquid ammonia storage tank level transmitter; 800. Ammonia condenser; 900, Ammonia Compressor; 1000, Electric motor; 1100. Carbon dioxide compressor; 1200. Carbon dioxide condenser; 1300, CO2 output of the CCUS system; 1400, Low-temperature methanol wash carbon dioxide inlet; 1500, Low-temperature methanol wash carbon dioxide outlet. Detailed Implementation

[0025] This application discloses a cascade refrigeration system to improve the efficiency and safety of refrigerant replenishment in the cascade refrigeration system.

[0026] This application also discloses a low-temperature methanol washing system having the above-described cascade refrigeration system.

[0027] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] See Figure 1 The cascade refrigeration system disclosed in this application includes a cascade refrigeration unit, a second refrigerant storage tank 100, and an automatic liquid replenishment control valve 300.

[0029] The cascade refrigeration unit is the primary cooling system, providing the first heat exchange medium for cooling methanol, specifically the methanol used in the low-temperature methanol washing process. The cascade refrigeration unit is equipped with a first refrigerant storage tank 200 for storing the first heat exchange medium. During operation, the first heat exchange medium inside the cascade refrigeration unit may experience chronic leakage and loss due to factors such as aging of seals.

[0030] The second refrigerant storage tank 100 is a pressure-resistant container that stores the same type of first heat exchange medium as the first refrigerant storage tank 200. The pressure inside the second refrigerant storage tank 100 is set to be greater than the pressure inside the first refrigerant storage tank 200. The liquid outlet of the second refrigerant storage tank 100 is connected to the liquid inlet of the first refrigerant storage tank 200. When the connecting pipeline between the second refrigerant storage tank 100 and the first refrigerant storage tank 200 is unobstructed, the first heat exchange medium in the second refrigerant storage tank 100 will flow to the first refrigerant storage tank 200 due to pressure.

[0031] An automatic refrigerant replenishment control valve 300 is installed on the connecting pipeline between the first refrigerant storage tank 200 and the second refrigerant storage tank 100, and is used to control the replenishment of the first heat exchange medium from the second refrigerant storage tank 100 to the first refrigerant storage tank 200. The automatic refrigerant replenishment control valve 300 may include a controller and an electric valve. The electric valve may be a solenoid valve or an electric ball valve. The controller may be a microcontroller or a relay control circuit, in which preset values ​​are stored. The controller receives signals from sensors, compares them with the preset values, performs logical judgments, and issues a switching command to the electric valve. When the liquid level in the first refrigerant storage tank 200 is lower than the preset value, the automatic refrigerant replenishment control valve 300 automatically opens; when the liquid level in the first refrigerant storage tank 200 is equal to or higher than the preset value, the automatic refrigerant replenishment control valve 300 automatically closes. The implementation of the automatic refrigerant replenishment control valve 300 includes, but is not limited to, the above-described embodiments, and can be any valve device in the prior art capable of automatic control.

[0032] In the cascade refrigeration system provided in this application, when the liquid level of the first heat exchange medium in the first refrigerant storage tank 200 drops below a set value, the automatic liquid replenishment control valve 300 automatically opens. Because the pressure in the second refrigerant storage tank 100 is greater than the pressure in the first refrigerant storage tank 200, the first heat exchange medium in the second refrigerant storage tank 100 flows into the first refrigerant storage tank 200 under the pressure difference, causing the refrigerant liquid level in the first refrigerant storage tank 200 to rise. When the refrigerant liquid level in the first refrigerant storage tank 200 reaches the set value, the automatic liquid replenishment control valve 300 automatically closes, and the liquid replenishment of the cascade refrigeration unit is completed.

[0033] Compared with related technologies, the cascade refrigeration system disclosed in this application achieves automatic refrigerant replenishment of the cascade refrigeration unit by adding a second refrigerant storage tank 100 and an automatic refrigerant replenishment control valve 300. It has a high degree of automation, improves working efficiency, ensures timely refrigerant replenishment, and avoids frequent manual refrigerant replenishment on site, thus preventing the risk of suffocation caused by manual refrigerant replenishment.

[0034] In one specific embodiment, the first refrigerant storage tank 200 may be equipped with a level transmitter 220, which measures the liquid level of the first refrigerant storage tank 200. The level transmitter 220 acts as a detection unit, continuously monitoring the liquid level in the first refrigerant storage tank 200 and feeding back a signal to the automatic liquid replenishment control valve 300. The automatic liquid replenishment control valve 300 receives the signal and quickly and accurately controls its own opening and closing. This design greatly improves the system's response speed; changes in liquid level are detected promptly by the level transmitter, and the automatic liquid replenishment control valve 300 immediately actuates, thus achieving faster and more accurate liquid level control.

[0035] In one specific embodiment, since the second refrigerant storage tank 100 needs to be maintained at a high pressure, a safety valve 110 can be installed in the second refrigerant storage tank 100 to ensure the safety of the refrigeration system. When the pressure inside the second refrigerant storage tank 100 exceeds a set safety threshold, the safety valve 110 opens, and the first heat exchange medium inside the second refrigerant storage tank 100 flows to the pressure relief system. When the pressure inside the second refrigerant storage tank 100 is less than the set safety threshold, the safety valve 110 closes. For example, if the operating pressure of the second refrigerant storage tank 100 is 3.3 MPa, the safety threshold can be set to 3.5 MPa. When the tank pressure rises to ≥3.5 MPa, the safety valve 110 activates, and the first heat exchange medium is rapidly and directionally released to the pressure relief system via the safety valve 110, ensuring equipment safety. This design helps ensure system integrity. The safety valve 110 also protects all pipes, valves, and instruments connected to the second refrigerant storage tank 100, preventing them from being damaged due to overpressure and avoiding larger cascading failures and more serious leaks.

[0036] In one specific embodiment, since the pressure inside the second refrigerant storage tank 100 is relatively high, a pressure indicator 400 can be installed in the second refrigerant storage tank 100 for easy monitoring. The pressure indicator provides the operator with the most direct pressure reading. Through the pressure indicator 400, it can be confirmed whether the pressure of the second refrigerant storage tank 100 is greater than the pressure of the first refrigerant storage tank 200, ensuring the normal operation of the system. It can also be confirmed whether the pressure of the second refrigerant storage tank 100 is within the safe range, avoiding the risks caused by operating due to excessive pressure.

[0037] In one specific embodiment, to prevent liquid in the first refrigerant storage tank 200 from flowing back into the second refrigerant storage tank 100, a check valve (not shown in the figure) is installed on the pipeline between the first refrigerant storage tank 200 and the second refrigerant storage tank 100. The check valve uses the weight of the valve core, the tension of the spring, or the kinetic energy of the fluid to prevent liquid or gas from flowing back in the pipeline. When the first heat exchange medium enters the check valve from the forward flow direction, that is, from the second refrigerant storage tank to the first refrigerant storage tank, the valve core is pushed open by the liquid, allowing the first heat exchange medium to flow smoothly. Once the flow of the first heat exchange medium decreases or stops, the valve core will automatically close under the action of spring tension or gravity, thereby preventing the first heat exchange medium from flowing back into the second refrigerant storage tank 100 and causing the first heat exchange medium in the cascade refrigeration unit to be lost.

[0038] In one specific embodiment, to make the refrigeration system more energy-efficient and environmentally friendly, when the first heat exchange medium is carbon dioxide, a CCUS (Carbon Capture Utilization and Storage) system can be used to provide replenishment carbon dioxide to the second refrigerant storage tank 100. The inlet of the second refrigerant storage tank 100 is connected to the carbon dioxide outlet 1300 of the CCUS system. The CCUS system captures carbon dioxide from industrial emission sources and compresses and transports it to the second refrigerant storage tank 100, realizing waste gas utilization, reducing dependence on traditional synthetic carbon dioxide, and improving the economics of the cascade refrigeration system.

[0039] In one specific embodiment, the cascade refrigeration unit may include a first heat exchange medium circuit and a second heat exchange medium circuit, each being an independent circulation loop. The second heat exchange medium is evaporated into the first heat exchange medium via a condenser-evaporator 600 for cooling. Specifically, the second heat exchange medium flows within the second heat exchange medium side pipe or shell of the condenser-evaporator 600 and absorbs external heat to completely evaporate, while the first heat exchange medium flows within the first heat exchange medium side pipe of the condenser-evaporator 600 and releases heat to liquefy. A first refrigerant storage tank 200 is disposed in the first heat exchange medium circuit.

[0040] In one specific embodiment, the second heat exchange medium can be ammonia, and the second heat exchange medium circuit may include an ammonia compressor 900, an ammonia condenser 800, and a liquid ammonia storage tank 700. The ammonia compressor 900 is driven by an electric motor 1000. The inlet of the ammonia condenser 800 is connected to the outlet of the ammonia compressor 900. The liquid inlet of the liquid ammonia storage tank 700 is connected to the outlet of the ammonia condenser 800, and the liquid outlet of the liquid ammonia storage tank 700 is connected to the ammonia inlet of the condenser-evaporator 600. The ammonia outlet of the condenser-evaporator 600 is connected to the inlet of the ammonia compressor 900, forming a complete circulation loop.

[0041] The circulation process of the second heat exchange medium loop is as follows: Motor 1000 starts, driving ammonia compressor 900. Ammonia compressor 900 draws low-temperature, low-pressure ammonia gas from the ammonia outlet of condenser-evaporator 600 and compresses it into high-temperature, high-pressure ammonia gas before discharging it. The high-temperature, high-pressure ammonia gas enters ammonia condenser 800, where it is cooled, releasing heat and condensing into room-temperature, high-pressure liquid ammonia. The liquid ammonia flows into liquid ammonia storage tank 700 for storage. When the system needs cooling, liquid ammonia flows out from the outlet of liquid ammonia storage tank 700. The liquid ammonia is throttled and depressurized by a throttling valve, causing a rapid temperature drop, becoming a low-temperature, low-pressure gas-liquid two-phase state. This gas-liquid two-phase mixture then enters the ammonia inlet of condenser-evaporator 600, where it absorbs heat from the first heat exchange medium and evaporates completely, lowering the temperature of the first heat exchange medium and achieving a cooling effect. The low-temperature, low-pressure ammonia gas produced after evaporation is discharged from the ammonia outlet of the condenser evaporator 600 and drawn back to the inlet of the ammonia compressor 900, thus starting a new cycle.

[0042] In one specific embodiment, the first heat exchange medium can be carbon dioxide, and the first heat exchange medium circuit may include a carbon dioxide compressor 1100, a carbon dioxide condenser 1200, a first refrigerant storage tank 200, and a liquid carbon dioxide transfer pump 500. The carbon dioxide compressor 1100 is driven by an electric motor. The inlet of the carbon dioxide condenser 1200 is connected to the outlet of the carbon dioxide compressor 1100. The liquid inlet of the first refrigerant storage tank 200 is connected to the outlet of the carbon dioxide condenser 1200. The liquid carbon dioxide transfer pump 500 is connected to the outlet of the first refrigerant storage tank 200 and is used to transfer liquid carbon dioxide to the carbon dioxide inlet of the condenser-evaporator 600. The carbon dioxide outlet of the condenser-evaporator 600 is connected to the carbon dioxide inlet of the low-temperature methanol washing system, and the carbon dioxide outlet of the low-temperature methanol washing system is connected to the inlet of the carbon dioxide compressor 1100.

[0043] The circulation process of the first heat exchange medium loop is as follows: Motor 1000 starts, driving carbon dioxide compressor 1100 to run. Carbon dioxide compressor 1100 draws low-temperature, low-pressure carbon dioxide gas from the carbon dioxide outlet of the low-temperature methanol wash system and compresses it into a supercritical, high-pressure, high-temperature fluid. This fluid enters carbon dioxide condenser 1200, is initially cooled, and flows into the first refrigerant storage tank 200 for storage. Subsequently, liquid carbon dioxide transfer pump 500 transports the carbon dioxide fluid to the carbon dioxide inlet of condenser-evaporator 600. Midway, it passes through a carbon dioxide throttling valve, where it is throttled and depressurized, becoming a gas-liquid two-phase mixture that enters the carbon dioxide side of condenser-evaporator 600. Inside condenser-evaporator 600, the carbon dioxide releases heat to the lower-temperature liquid ammonia on the other side, thus being completely condensed into high-pressure, low-temperature liquid carbon dioxide. The liquid carbon dioxide flows out from the carbon dioxide outlet of condenser-evaporator 600 and enters the low-temperature methanol wash system. In the low-temperature methanol wash system, the liquid carbon dioxide absorbs heat from methanol and evaporates violently, producing a cooling effect, and itself becomes low-temperature, low-pressure carbon dioxide gas. The low-temperature, low-pressure carbon dioxide gas ultimately returns to the inlet of the carbon dioxide compressor 1100 through the carbon dioxide outlet of the low-temperature methanol washing system, completing the cycle. This system uses carbon dioxide in the low-temperature stage and highly efficient ammonia in the high-temperature stage, significantly reducing the amount of ammonia refrigerant required. The entire system balances environmental protection and efficiency.

[0044] This application discloses a low-temperature methanol washing system including the aforementioned cascade refrigeration system. This low-temperature methanol washing system includes the cascade refrigeration system described in any of the possible implementations above. The cascade refrigeration system is used to cool the methanol required for the low-temperature methanol washing process. Because this low-temperature methanol washing system incorporates the aforementioned cascade refrigeration system, it possesses all the technical effects of such systems, which will not be elaborated upon further herein.

[0045] The terms "first" and "second," etc., used in the specification and claims of this application are used to distinguish different objects, not to describe a specific order, and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units may include steps or units not listed, but rather steps or units not listed. Additionally, in the description of embodiments in this application, "a plurality of" means two or more.

[0046] In the description of this application, it should be understood that the terms "height," "thickness," "upper," "lower," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. In the description of this application, "a plurality of" means two or more, and "at least one" can mean one, two, or more, unless otherwise expressly specified.

[0047] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Specific technical means in some embodiments may be incorporated, in whole or in part, into another embodiment unless explicitly excluded by another embodiment. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A cascade cooling system, characterized in that, include: A cascade refrigeration unit is used to provide a first heat exchange medium for cooling methanol, wherein the methanol is methanol used in a low-temperature methanol washing process, and the cascade refrigeration unit is provided with a first refrigerant storage tank (200) for storing the first heat exchange medium. The second refrigerant storage tank (100) is used to store the first heat exchange medium for replenishment. The pressure in the second refrigerant storage tank (100) is set to be greater than the pressure in the first refrigerant storage tank (200). The outlet of the second refrigerant storage tank (100) is connected to the replenishment port of the first refrigerant storage tank (200). An automatic liquid replenishment control valve (300) is installed between the first refrigerant storage tank (200) and the second refrigerant storage tank (100). When the liquid level in the first refrigerant storage tank (200) is lower than a set value, the automatic liquid replenishment control valve (300) automatically opens; when the liquid level in the first refrigerant storage tank (200) is equal to or higher than the set value, the automatic liquid replenishment control valve (300) automatically closes.

2. The cascade cooling system as described in claim 1, characterized in that, The first refrigerant storage tank (200) is equipped with a level transmitter (220), and the level of the first refrigerant storage tank (200) is measured by the level transmitter (220).

3. The cascade cooling system as described in claim 1, characterized in that, The second refrigerant storage tank (100) is equipped with a safety valve (110). When the pressure in the second refrigerant storage tank (100) is greater than the set safety threshold, the safety valve (110) opens and the first heat exchange medium in the second refrigerant storage tank (100) flows to the pressure relief system. When the pressure in the second refrigerant storage tank (100) is less than the set safety threshold, the safety valve (110) closes.

4. The cascade cooling system as described in claim 1, characterized in that, The second refrigerant storage tank (100) is equipped with a pressure indicator (400).

5. The cascade cooling system as described in claim 1, characterized in that, A check valve is provided between the first refrigerant storage tank (200) and the second refrigerant storage tank (100). The check valve is in the closed state in the direction from the first refrigerant storage tank (200) to the second refrigerant storage tank (100).

6. The cascade cooling system as described in claim 1, characterized in that, When the first heat exchange medium is carbon dioxide, the inlet of the second refrigerant storage tank (100) is connected to the carbon dioxide outlet (1300) of the CCUS system.

7. The cascade cooling system as described in claim 1, characterized in that, The cascade refrigeration unit includes a first heat exchange medium circuit and a second heat exchange medium circuit. The first heat exchange medium circuit and the second heat exchange medium circuit are each independent circulation circuits. The second heat exchange medium is evaporated into the first heat exchange medium through the condenser evaporator (600) for cooling. The first refrigerant storage tank (200) is located in the first heat exchange medium circuit.

8. The cascade cooling system as described in claim 7, characterized in that, The second heat exchange medium is ammonia, and the second heat exchange medium circuit includes: An ammonia compressor (900) is driven by an electric motor; An ammonia condenser (800) has its inlet connected to the outlet of the ammonia compressor (900); A liquid ammonia storage tank (700) is provided, with its inlet connected to the outlet of the ammonia condenser (800) and its outlet connected to the ammonia inlet of the condenser evaporator (600). The ammonia outlet of the condenser-evaporator (600) is connected to the inlet of the ammonia compressor (900).

9. The cascade cooling system as described in claim 7, characterized in that, The first heat exchange medium is carbon dioxide, and the first heat exchange medium circuit includes: A carbon dioxide compressor (1100) is driven by an electric motor; A carbon dioxide condenser (1200) has its inlet connected to the outlet of the carbon dioxide compressor (1100). A first refrigerant storage tank (200) has its inlet connected to the outlet of the carbon dioxide condenser (1200). A liquid carbon dioxide transfer pump (500) is connected to the liquid outlet of the first refrigerant storage tank (200) and is used to transfer liquid carbon dioxide to the carbon dioxide inlet of the condenser evaporator (600). The carbon dioxide outlet of the condenser evaporator (600) is connected to the carbon dioxide inlet of the low-temperature methanol washing system, and the carbon dioxide outlet of the low-temperature methanol washing system is connected to the inlet of the carbon dioxide compressor (1100).

10. A low-temperature methanol washing system, characterized in that, The system includes a cascade refrigeration system for cooling methanol required for a low-temperature methanol washing process, wherein the cascade refrigeration system is the cascade refrigeration system as described in any one of claims 1-9.

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