Low-temperature rectification system and low-temperature rectification method

Abstract

The invention discloses a low-temperature rectification system and a low-temperature rectification method, and belongs to the technical field of low-temperature rectification. The low-temperature rectification system comprises a rectification tower, a low-temperature circulation unit and a high-temperature circulation unit. The high-temperature circulation unit forms a circulation channel of high-temperature refrigerants, the low-temperature circulation unit forms a circulation channel of low-temperature mixed refrigerants, and the high-temperature refrigerants in the low-temperature state can exchange heat with the low-temperature refrigerants in the high-temperature state in the evaporative condenser to absorb heat of the low-temperature mixed refrigerants. At least two heat exchangers, at least two expansion devices and at least one gas-liquid separator are arranged in the low-temperature circulation unit, and the refrigerant with the high evaporation temperature in the low-temperature mixed refrigerant can be subjected to condensation, gas-liquid separation, throttling and cooling and can be subjected to heat exchange with refrigerant gas with the low evaporation temperature; gradient cooling of the refrigerant with the low evaporation temperature is achieved, and cooling capacity can be efficiently and accurately provided for the tower top condenser of the rectifying tower.

Description

Technical Field

[0001] This application relates to the field of cryogenic distillation technology, and more specifically, to a cryogenic distillation system and a cryogenic distillation method. Background Technology

[0002] The rapid development of nuclear energy technology and the semiconductor industry has created a huge demand for high-abundance boron-10 (used for neutron control) and boron-11 (used as dopants in the electronics industry) isotopes. Boron trifluoride (BF3) is the most commonly used precursor material for boron isotope separation, utilizing... 10 BF3 and 11 The slight differences in boiling points of BF3 molecules can be effectively separated using cryogenic distillation.

[0003] The core of this distillation process lies in maintaining the distillation column in an extremely stable and precise cryogenic environment, with an optimal operating temperature of approximately -100°C (about 173K). In existing technologies, liquid nitrogen (boiling point -196°C) is commonly used as the cold source. However, directly using liquid nitrogen has the following drawbacks: 1. Risk of material freezing: Liquid nitrogen temperature is much lower than the freezing point of BF3 (-127℃). Direct heat exchange can lead to localized supercooling within the distillation column, causing BF3 to liquefy and freeze, clogging trays, packing, and pipes, resulting in unit shutdown and even safety accidents.

[0004] 2. Low efficiency and unstable temperature control: To avoid freezing, liquid nitrogen is typically used to cool the intermediate refrigerant, which then indirectly cools the distillation column. This secondary heat exchange method results in significant heat transfer temperature differences and heat losses, leading to extremely low refrigeration efficiency and huge energy consumption. Furthermore, the high thermal inertia of the intermediate refrigerant system causes slow temperature control response, making it difficult to achieve the precise temperature control required for the distillation process.

[0005] Therefore, there is an urgent need in this field for a cryogenic distillation system and method that can efficiently and accurately maintain low-temperature cooling. Summary of the Invention

[0006] To address the aforementioned shortcomings, this application provides a cryogenic distillation system and method that can directly, efficiently, and accurately provide cryogenic cooling to the distillation column, thereby achieving cryogenic distillation.

[0007] This application is implemented as follows: In a first aspect, an example of this application provides a cryogenic distillation system, including a distillation column, a high-temperature circulation unit, and a cryogenic circulation unit. The distillation column is equipped with a top condenser. The high-temperature circulation unit includes a first compressor, a first condenser, and a first expansion device connected by pipes. The cryogenic circulation unit includes a first heat exchanger, a first gas-liquid separator, a second heat exchanger, a second expansion device, a third expansion device, and a second compressor connected by pipes.

[0008] The high-temperature circulation unit is equipped with the following configuration: the high-temperature refrigerant gas can flow sequentially through the first compressor, the first condenser and the first expansion device to the evaporator-condenser, where it exchanges heat with the low-temperature mixed refrigerant at the evaporator-condenser, absorbs heat and re-phases into high-temperature refrigerant gas, and thus circulates.

[0009] The cryogenic cycle unit is configured such that the cryogenic mixed refrigerant sequentially exchanges heat with the high-temperature refrigerant at the evaporator-condenser and the first working fluid at the first heat exchanger to form a first gas-liquid mixture. The first gas-liquid mixture is separated into refrigerant liquid and refrigerant gas by a first gas-liquid separator. The refrigerant gas, after condensing through heat exchange with the second working fluid at the second heat exchanger, flows into the top condenser via a third expansion device. The refrigerant liquid, via the second expansion device, mixes with the refrigerant discharged from the top condenser to form the second working fluid. The second working fluid, via the second heat exchanger, forms the first working fluid, and the first working fluid, via the first heat exchanger and the second compressor, forms the cryogenic mixed refrigerant, thus completing the cycle.

[0010] In the above implementation process, a high-temperature refrigerant is used in the high-temperature cycle, and a low-temperature mixed refrigerant is used in the low-temperature cycle. The evaporation temperature of the high-temperature refrigerant is higher than that of the low-temperature mixed refrigerant. Two separate refrigerant circulation units are set up. At the evaporator-condenser, the high-temperature refrigerant, in a low-temperature state, absorbs heat from the low-temperature mixed refrigerant, thus enabling the high-temperature circulation unit to perform the first cooling of the low-temperature mixed refrigerant. In the low-temperature circulation unit, at least two low-temperature refrigerants with different evaporation temperatures are mixed. After the first cooling, the mixed refrigerant flows to the first heat exchanger for heat exchange, absorbing the cold energy from the first working fluid for a second cooling. The low-temperature refrigerant with the higher evaporation temperature will condense first to form a refrigerant liquid, while the refrigerant with the lower evaporation temperature remains in a gaseous state, forming a first gas-liquid mixture. At this point, the first gas-liquid mixture is sent to the first gas-liquid separator for separation. The separated refrigerant gas is sent to the second heat exchanger, while the separated refrigerant liquid is sent to the second heat exchanger after being depressurized, throttled, and cooled by the second expansion device. The low-temperature refrigerant (with a higher evaporation temperature) is then used to perform a third cooling process on the aforementioned low-temperature refrigerant gas (with a lower evaporation temperature), causing the low-temperature refrigerant with the lower evaporation temperature to condense. The condensed low-temperature refrigerant with the lower evaporation temperature is then sent to the third expansion device for throttling, depressurization, and cooling, achieving a fourth cooling process to obtain an even lower-temperature refrigerant. This refrigerant is then used as the condensing medium in the top condenser of the distillation column, achieving cooling at the top of the column.

[0011] The cryogenic distillation system provided in this application utilizes multiple gradient cooling processes between a high-temperature circulation unit and a low-temperature circulation unit. Only two sets of refrigeration units are needed to achieve cryogenic cooling over a large range. During the cooling process, there is no need to perform multiple compressions of the refrigerant at a large compression ratio, which can reduce energy consumption. Appropriate high-temperature refrigerants and low-temperature mixed refrigerants can be selected according to the target condensation temperature of the distillation column, enabling more accurate and efficient adjustment of cooling capacity.

[0012] In one alternative embodiment, the cryogenic mixed refrigerant includes a first refrigerant and a second refrigerant, wherein the evaporation temperature of the first refrigerant is lower than that of the second refrigerant. A first gas-liquid separator is configured to separate the second refrigerant liquid and the first refrigerant gas. The first refrigerant gas undergoes heat exchange with a second working fluid at a second heat exchanger and condenses, then flows into the top condenser via a third expansion device. The second refrigerant liquid, via a second expansion device, mixes with the first refrigerant discharged from the top condenser to form the second working fluid.

[0013] In the above implementation process, a mixed refrigerant is formed using two low-temperature refrigerants with different evaporation temperatures. When this low-temperature refrigerant is circulated using the low-temperature circulation unit provided in this application embodiment, at the first heat exchanger, the second refrigerant in the low-temperature mixed refrigerant gas will condense first to form a second refrigerant liquid, while the first refrigerant absorbs cold energy but remains in a gaseous state. After gas-liquid separation by the first gas-liquid separator, the second refrigerant is throttled, depressurized, and cooled using the second expansion device to obtain a second refrigerant with an even lower temperature. The first refrigerant gas is then cooled again using this lower-temperature second refrigerant to obtain a first refrigerant liquid with an even lower temperature. Then, the first refrigerant liquid is depressurized, throttled, and cooled using the third expansion device to obtain a lower-temperature first refrigerant. This low-temperature first refrigerant is used as the cooling medium in the top condenser of the distillation column to condense the top product of the distillation column.

[0014] In one optional embodiment, the outlet of the first condenser is connected to the low-temperature inlet of the evaporative condenser via a first expansion device, and the high-temperature outlet of the evaporative condenser is connected to the inlet of the first compressor. The outlet of the second compressor is connected to the high-temperature inlet of the evaporative condenser, and the low-temperature outlet of the evaporative condenser is connected to the high-temperature inlet of the first heat exchanger. The high-temperature outlet of the first heat exchanger is connected to the inlet of the second compressor. The low-temperature outlet of the first heat exchanger is connected to the inlet of the first gas-liquid separator, the gas outlet of the first gas-liquid separator is connected to the high-temperature inlet of the second heat exchanger, and the liquid outlet of the first gas-liquid separator is connected to the low-temperature inlet of the second heat exchanger via a second expansion device. The high-temperature outlet of the second heat exchanger is connected to the low-temperature inlet of the first heat exchanger. The low-temperature outlet of the second heat exchanger is connected to the inlet of the top condenser via a third expansion device. The outlet of the top condenser is connected to the low-temperature inlet of the second heat exchanger.

[0015] In the above-described process, when using the aforementioned cryogenic distillation system, the high-temperature refrigerant liquid condensed in the first condenser under high pressure can be transported to the first expansion device for pressure reduction, throttling, and cooling, forming a high-temperature refrigerant in a low-temperature, gas-liquid two-phase state. This high-temperature refrigerant enters from the low-temperature inlet of the evaporator-condenser, exchanges heat with the low-temperature mixed refrigerant gas entering from the high-temperature inlet of the evaporator-condenser, absorbs heat and vaporizes, forming a high-temperature refrigerant gas, which is discharged from the high-temperature outlet of the evaporator-condenser, and then enters the first compressor to be compressed into a high-temperature, high-pressure refrigerant gas. The high-temperature, high-pressure refrigerant gas flows to the first condenser, exchanges heat with the refrigerant (e.g., air, water) at the first condenser, and condenses into a high-temperature refrigerant liquid, thus completing the cycle.

[0016] In the cryogenic cycle unit, the cryogenic mixed refrigerant gas discharged from the cryogenic outlet of the evaporator-condenser enters through the high-temperature inlet of the first heat exchanger, where it exchanges heat with the first working fluid entering through the cryogenic inlet of the first heat exchanger, absorbing cooling energy to form a first gas-liquid mixture. This first gas-liquid mixture flows to the first gas-liquid separator for gas-liquid separation. The first refrigerant gas discharged from the outlet of the first gas-liquid separator flows into the high-temperature inlet of the second heat exchanger, while the second refrigerant liquid discharged from the liquid outlet of the first gas-liquid separator is depressurized and cooled by the second expansion device before flowing into the cryogenic inlet of the second heat exchanger. At this point, the first refrigerant gas absorbs the cooling energy of the second refrigerant and then exits through the cryogenic outlet of the second heat exchanger. It is then depressurized and cooled by the third expansion device before entering the top condenser through the inlet. At the top condenser, the first refrigerant absorbs heat from the top layer and vaporizes, forming a high-temperature first refrigerant gas. This first refrigerant gas exits through the outlet of the top condenser and enters the second heat exchanger through the cryogenic inlet, where it mixes with the second refrigerant, which also enters through the cryogenic inlet, to form the second working fluid. The second working fluid exchanges heat with the first refrigerant gas discharged from the first gas-liquid separator at the second heat exchanger, absorbing heat, and is discharged from the high-temperature outlet of the second heat exchanger. The second working fluid discharged from the high-temperature outlet of the second heat exchanger forms the first working fluid, which enters through the low-temperature inlet of the first heat exchanger and exchanges heat with the low-temperature mixed refrigerant gas entering through the high-temperature inlet of the first heat exchanger, absorbing heat, and is discharged from the high-temperature outlet of the first heat exchanger. The low-temperature mixed refrigerant discharged from the high-temperature outlet of the first heat exchanger is in a high-temperature, low-pressure state. This high-temperature, low-pressure low-temperature mixed refrigerant is sent to the second compressor and compressed into a high-temperature, high-pressure low-temperature mixed refrigerant gas. Then, this high-temperature, high-pressure low-temperature mixed refrigerant gas enters the evaporator-condenser through the high-temperature inlet, exchanges heat with the high-temperature refrigerant entering through the low-temperature inlet of the evaporator-condenser, absorbing cooling capacity, and is then discharged from the low-temperature outlet of the evaporator-condenser, thus completing the cycle.

[0017] In one alternative embodiment, the cryogenic mixed refrigerant includes a second refrigerant, a third refrigerant, and a first refrigerant with gradually decreasing evaporation temperatures. The cryogenic cycle unit also includes a second gas-liquid separator, a fourth expansion device, and a third heat exchanger connected by piping.

[0018] The first gas-liquid mixture is separated into a second refrigerant liquid and a refrigerant gas by a first gas-liquid separator. The refrigerant gas includes a first refrigerant gas and a third refrigerant gas. The refrigerant gas exchanges heat with the second working fluid at the second heat exchanger and condenses to form the second gas-liquid mixture. The second gas-liquid mixture is separated into a third refrigerant liquid and a first refrigerant gas by a second gas-liquid separator. The first refrigerant gas exchanges heat with the third working fluid at the third heat exchanger and condenses, then flows into the top condenser of the column through a third expansion device. The third refrigerant liquid passes through a fourth expansion device and mixes with the first refrigerant discharged from the top condenser to form the third working fluid. The third working fluid flows through the third heat exchanger to the second heat exchanger. The second refrigerant liquid flows through the second expansion device to the second heat exchanger and mixes to form the second working fluid.

[0019] In one optional embodiment, the low-temperature outlet of the second heat exchanger is connected to the inlet of the second gas-liquid separator, the gas outlet of the second gas-liquid separator is connected to the high-temperature inlet of the third heat exchanger, and the liquid outlet of the second gas-liquid separator is connected to the low-temperature inlet of the third heat exchanger via a fourth expansion device. The low-temperature outlet of the third heat exchanger is connected to the inlet of the top condenser via the second expansion device. The outlet of the top condenser is connected to the low-temperature inlet of the third heat exchanger, and the high-temperature outlet of the third heat exchanger is connected to the low-temperature inlet of the second heat exchanger.

[0020] In the above-described process, a second gas-liquid separator, a fourth expansion device, and a third heat exchanger are installed between the second heat exchanger and the third expansion device. This allows for a second gas-liquid separation of the refrigerant discharged from the low-temperature outlet of the second heat exchanger. The first refrigerant gas is discharged from the outlet of the second gas-liquid separator and transported to the high-temperature inlet of the third heat exchanger, while the third refrigerant liquid is discharged from the outlet of the second gas-liquid separator. After being throttled, depressurized, and cooled by the fourth expansion device, it is transported to the low-temperature inlet of the third heat exchanger, where it absorbs heat and cools down. Then, the first refrigerant liquid discharged from the low-temperature outlet of the third heat exchanger is transported to the third expansion device for throttling, depressurization, and cooling, before being delivered to the top condenser. Using this low-temperature distillation apparatus, the temperature gradient of the first refrigerant can be further increased, allowing for the delivery of a lower-capacity first refrigerant to the top condenser.

[0021] In one alternative embodiment, the cryogenic cycle unit further includes a fifth expansion device, the inlet of which is connected to the outlet of the second compressor, and the outlet of which is connected to the inlet of the top condenser.

[0022] In the above process, when the cooling capacity of the first refrigerant input to the top condenser is too low, a portion of the high-temperature and high-pressure mixed refrigerant gas discharged from the outlet of the second compressor can be sent to the fifth expansion device. After cooling and depressurization, it flows back to the top condenser, thereby regulating the temperature of the refrigerant at the top condenser.

[0023] In one alternative embodiment, the cryogenic distillation system further includes an emergency unit. The emergency unit includes a second condenser, the outlet of which is optionally connected to the inlet of the second condenser, and the outlet of which is optionally connected to the inlet of the top condenser.

[0024] In the above implementation process, an emergency unit is set up in the cryogenic distillation system. When the high-temperature cycle unit or the cryogenic cycle unit fails, the cryogenic mixed refrigerant in a high-temperature and high-pressure state discharged from the top condenser can be directly transported to the second condenser. The cryogenic mixed refrigerant is cooled by a refrigerant (such as liquid nitrogen or liquid oxygen) and then transported to the top condenser to condense the top product, thereby helping to stabilize the operation of the cryogenic distillation system.

[0025] In one alternative embodiment, the cryogenic cycle unit further includes a regenerator. The regenerator is provided with a heat exchange channel, and the high-temperature outlet of the first heat exchanger is connected to the inlet of the second compressor via the regenerator. The cryogenic distillation system also includes a feed delivery pipe, which passes through the heat exchange channel and connects to the feed inlet of the distillation column.

[0026] In the above implementation process, a regenerator is set between the high-temperature outlet of the first heat exchanger and the second compressor. The high-temperature gas discharged from the high-temperature outlet of the first heat exchanger can be used to preheat the raw material in the feed pipe of the distillation column. The preheated raw material is then transported into the distillation column to recover heat and further reduce energy consumption.

[0027] In one optional embodiment, a buffer tank is further provided between the outlet of the third expansion unit and the inlet of the overhead condenser. The cryogenic distillation system also includes a control unit. The control unit includes a controller, a delivery device, and a control device, as well as a temperature sensor located in the buffer tank. The delivery device includes a delivery pump and a flow control valve located at the pipe connecting the outlet of the buffer tank and the inlet of the overhead condenser. The control device includes a pressure control valve located at the pipe connecting the cryogenic inlet of the third heat exchanger and the outlet of the overhead condenser. The temperature sensor, delivery pump, flow control valve, and pressure control valve are all signal-connected to the controller. The controller is at least configured to: increase the flow rate of the flow control valve when the temperature sensor detects that the overhead temperature of the distillation column is higher than a set value; conversely, decrease the flow rate of the flow control valve when the temperature sensor detects that the temperature is lower than a set value.

[0028] In the above implementation process, a control unit is set up in the cryogenic distillation system, which can use a temperature sensor to monitor the temperature at the top of the distillation column in real time. When the temperature sensor detects that the temperature is higher than the set value, the controller can control the opening of the corresponding control valve to increase the refrigerant input flow rate, thereby reducing the temperature and pressure.

[0029] By using the aforementioned controller, control valve, temperature sensor, and delivery pump, the cooling capacity at the top of the distillation column can be adjusted more precisely, which helps maintain the stability of the distillation column.

[0030] In a second aspect, examples of this application provide a method for performing cryogenic distillation using the cryogenic distillation system provided in the first aspect, comprising: High-temperature refrigerant is injected into the high-temperature circulation unit, so that the high-temperature refrigerant gas flows sequentially through the first compressor, the first condenser and the first expansion device to the evaporator-condenser, where it exchanges heat with the low-temperature mixed refrigerant at the evaporator-condenser, absorbs heat and re-phases into high-temperature refrigerant gas, and so on.

[0031] A low-temperature mixed refrigerant is injected into the low-temperature circulation unit, where the gaseous low-temperature mixed refrigerant sequentially exchanges heat with the high-temperature refrigerant at the evaporator-condenser and the first working fluid at the first heat exchanger to form a first gas-liquid mixture. The first gas-liquid mixture is then separated into liquid and gaseous refrigerant using a first gas-liquid separator. The gaseous refrigerant is condensed after heat exchange with the second working fluid at the second heat exchanger, and then transported to the top condenser of the distillation column via a third expansion device, where it exchanges heat with the gas phase at the top of the distillation column to achieve low-temperature distillation. The liquid refrigerant is then mixed with the refrigerant discharged from the top condenser via the second expansion device to form the second working fluid. The second working fluid is then heated by the second heat exchanger to form the first working fluid, which is then passed through the first heat exchanger and the second compressor to form a low-temperature mixed refrigerant gas, and this cycle continues.

[0032] Optionally, the low-temperature mixed refrigerant includes a first refrigerant and a second refrigerant, wherein the evaporation temperature of the first refrigerant is lower than the evaporation temperature of the second refrigerant, and the evaporation temperature of the second refrigerant is lower than the evaporation temperature of the high-temperature refrigerant.

[0033] Optionally, the high-temperature refrigerant includes at least one of R404A, R507, R414A, R454C, or R448A.

[0034] Optionally, the cryogenic mixed refrigerant includes at least two of R290, R170, R1150, R14, and R23.

[0035] Optionally, the cryogenic mixed refrigerant is R290, R14 and R23.

[0036] In the above-mentioned process, the cryogenic distillation system provided in the first aspect is used for cryogenic distillation production. High-temperature refrigerant is injected into the high-temperature circulation unit, and low-temperature mixed refrigerant is injected into the low-temperature circulation unit. At the evaporator-condenser, the high-temperature refrigerant cools the low-temperature mixed refrigerant. Then, the refrigerant with a higher evaporation temperature in the low-temperature mixed refrigerant is sequentially condensed, gas-liquid separated, and throttled for cooling, exchanging heat with the refrigerant gas with a lower evaporation temperature. This achieves gradient cooling of the refrigerant with the lower evaporation temperature, which helps to efficiently and accurately provide cooling capacity to the top condenser of the distillation column, realizing cryogenic distillation of the distillation column. During cryogenic distillation, multiple gradient coolings using the high-temperature and low-temperature circulation units require only two sets of refrigeration units to achieve a large span of cryogenic cooling. Multiple compressions of the refrigerant at a large compression ratio are unnecessary during the cooling process, reducing energy consumption. Appropriate high-temperature and low-temperature mixed refrigerants can be selected based on the target condensation temperature at the top of the distillation column, allowing for more accurate and efficient adjustment of the cooling capacity. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0038] Figure 1 This is a plan view of the cryogenic distillation system provided in the embodiments of this application; Figure 2 This is a schematic diagram showing the connection between the low-temperature cycling unit and the high-temperature cycling unit provided in an embodiment of this application; Figure 3 This is a connection diagram of the control unit provided in an embodiment of this application.

[0039] Icons: 100-Cryogenic Distillation System; 1-Distillation Column; 11-Top Condenser; 12-Raw Material Delivery Pipe; 2-High Temperature Circulation Unit; 21-First Compressor; 22-First Condenser; 23-First Expansion Unit; 3-Cryogenic Circulation Unit; 31-First Heat Exchanger; 32-First Gas-Liquid Separator; 33-Second Heat Exchanger; 34-Second Expansion Unit; 35-Third Expansion Unit; 36-Second Compressor; 37-Second Gas-Liquid Separator; 38-Fourth Expansion Unit; 39-Third Heat Exchanger; 40-Fifth Expansion Unit; 41-Regenerator; 4-Evaporator-Condenser; 5-Emergency Unit; 51-Second Condenser; 6-Control Unit; 61-Controller; 62-Flow Control Valve; 63-Temperature Sensor; 64-Pressure Control Valve; 65-Transfer Pump; 66-Buffer Tank. Detailed Implementation

[0040] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit this application; the terms “comprising” and “having”, and any variations thereof, in the specification and the foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0042] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0043] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0044] The core of cryogenic distillation lies in maintaining the distillation column in an extremely stable and precise cryogenic environment. For example, in cryogenic distillation processes using boron trifluoride for boron isotope separation, the optimal operating temperature is approximately -100°C (about 173 K). In existing technologies, liquid nitrogen (boiling point -196°C) is commonly used as the cold source. However, directly using liquid nitrogen has the following drawbacks: 1. Risk of material freezing: Liquid nitrogen temperature is much lower than the freezing point of BF3 (-127℃). Direct heat exchange can lead to localized supercooling within the distillation column, causing BF3 to liquefy and freeze, clogging trays, packing, and pipes, resulting in unit shutdown and even safety accidents.

[0045] 2. Low efficiency and unstable temperature control: To avoid freezing, liquid nitrogen is typically used to cool the intermediate refrigerant, which then indirectly cools the distillation column. This secondary heat exchange method results in significant heat transfer temperature differences and heat losses, leading to extremely low refrigeration efficiency and huge energy consumption. Furthermore, the high thermal inertia of the intermediate refrigerant system causes slow temperature control response, making it difficult to achieve the precise temperature control required for the distillation process.

[0046] Therefore, this application further improves the cryogenic distillation system, enabling it to provide cryogenic cooling capacity more directly, efficiently, and accurately. To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings.

[0047] Please see Figure 1 The cryogenic distillation system 100 provided in this application embodiment includes a distillation column 1, a high-temperature circulation unit 2, and a low-temperature circulation unit 3.

[0048] Please continue reading. Figure 1 The distillation column 1 is equipped with a top condenser 11. The high-temperature circulation unit 2 includes a first compressor 21, a first condenser 22, and a first expansion device 23 connected by pipes. The low-temperature circulation unit 3 includes a first heat exchanger 31, a first gas-liquid separator 32, a second heat exchanger 33, a second expansion device 34, a third expansion device 35, and a second compressor 36 connected by pipes.

[0049] Please continue reading for more details. Figure 2 The outlet of the first condenser 22 is connected to the low-temperature inlet of the evaporator-condenser 4 via the first expansion device 23. The high-temperature outlet of the evaporator-condenser 4 is connected to the inlet of the first compressor 21. The outlet of the second compressor 36 is connected to the high-temperature inlet of the evaporator-condenser 4, and the low-temperature outlet of the evaporator-condenser 4 is connected to the high-temperature inlet of the first heat exchanger 31. The high-temperature outlet of the first heat exchanger 31 is connected to the inlet of the second compressor 36. The low-temperature outlet of the first heat exchanger 31 is connected to the inlet of the first gas-liquid separator 32. The gas outlet of the first gas-liquid separator 32 is connected to the high-temperature inlet of the second heat exchanger 33, and the liquid outlet of the first gas-liquid separator 32 is connected to the low-temperature inlet of the second heat exchanger 33 via the second expansion device 34. The high-temperature outlet of the second heat exchanger 33 is connected to the low-temperature inlet of the first heat exchanger 31. The low-temperature outlet of the second heat exchanger 33 is connected to the inlet of the top condenser 11 via the third expansion device 35. The outlet of the top condenser 11 is connected to the low-temperature inlet of the second heat exchanger 33.

[0050] In this application, "high-temperature refrigerant" and "low-temperature refrigerant" refer to the difference in evaporation temperature of the refrigerant. That is, the evaporation temperature of the high-temperature refrigerant is higher than that of the low-temperature refrigerant. Generally, the evaporation temperature of the refrigerant directly determines the cooling effect. As an example, a high-temperature refrigerant L1 is used in the high-temperature cycle, and a low-temperature mixed refrigerant L2 is used in the low-temperature cycle. The evaporation temperature of the high-temperature refrigerant L1 is higher than that of the low-temperature mixed refrigerant L2. When the low-temperature mixed refrigerant includes a first refrigerant L21 and a second refrigerant L22 with an evaporation temperature higher than that of the first refrigerant L1, the cycle process is as follows: High-temperature refrigerant L1 gas is compressed by the first compressor 21 to form high-temperature, high-pressure refrigerant L1 gas. This high-temperature, high-pressure refrigerant L1 gas is then condensed by the first condenser 22 to form low-temperature, high-pressure refrigerant L1 gas. After being throttled and depressurized by the first expansion device 23, the low-temperature, high-pressure refrigerant L1 gas is in a low-pressure, two-phase state. The high-temperature refrigerant L1 gas discharged from the outlet of the first expansion device 23 is transported to the low-temperature inlet of the evaporator-condenser 4, where it exchanges heat with the low-temperature mixed refrigerant gas L2 flowing into the evaporator-condenser 4 from the high-temperature inlet. It absorbs heat from the low-temperature mixed refrigerant L2 gas, re-evaporates, and becomes low-temperature high-temperature refrigerant L1 gas again, flowing back into the first compressor 21, thus completing the cycle.

[0051] The second compressor 36 compresses the low-temperature mixed refrigerant L2 into a high-temperature, high-pressure state of low-temperature mixed refrigerant L2 gas. The low-temperature mixed refrigerant L2 gas flows into the high-temperature inlet of the evaporator-condenser 4, exchanges heat with the high-temperature refrigerant L1 at the evaporator-condenser 4, absorbs cold energy, and is discharged from the low-temperature outlet of the evaporator-condenser 4 after the temperature has decreased, thus achieving the first cooling of the low-temperature mixed refrigerant L2.

[0052] The temperature of the low-temperature mixed refrigerant L2 gas discharged from the low-temperature outlet of the evaporator-condenser 4 is recorded as the first temperature. The low-temperature mixed refrigerant L2 at the first temperature flows into the high-temperature inlet of the first heat exchanger 31, where it exchanges heat with the first working fluid, absorbs cold energy, and achieves a second cooling of the low-temperature mixed refrigerant L2. Since the evaporation temperature of the second refrigerant L22 in the low-temperature mixed refrigerant L2 gas is higher than that of the first refrigerant L21, the second refrigerant L22 will condense first, forming the second refrigerant L2 liquid, while the first refrigerant L21 remains in a gaseous state.

[0053] The temperature of the first gas-liquid mixture discharged from the low-temperature outlet of the first heat exchanger 31 is denoted as the second temperature. The second temperature is lower than the first temperature but higher than the evaporation temperature of the first refrigerant L21. The first gas-liquid mixture enters the first gas-liquid separator 32, where the first refrigerant L21 gas and the second refrigerant L22 liquid are separated. After the second refrigerant L22 liquid is throttled, depressurized, and cooled by the second expansion device 34, it forms a second refrigerant L22 in a gas-liquid two-phase state at the third temperature. It can be understood that the third temperature is lower than the second temperature. The first refrigerant L21 gas at the second temperature flows into the high-temperature inlet of the second heat exchanger 33, where it exchanges heat with the second working fluid. The first refrigerant L21 with a lower temperature is discharged from the low-temperature outlet of the second heat exchanger 33 (denoted as the fourth temperature), and the first working fluid with a higher temperature is discharged from the high-temperature outlet of the second heat exchanger 33 (denoted as the fifth temperature), thus achieving the third cooling of the first refrigerant L21. The first refrigerant L21 at the fourth temperature undergoes throttling, depressurization, and cooling (fourth cooling) in the third expansion device 35 before flowing into the top condenser 11. At the top condenser 11, the first refrigerant L21 exchanges heat with the top layer of material, absorbing heat, and is discharged from the top condenser 11 as a heated first refrigerant L2 gas (referred to as the sixth temperature). The sixth temperature is approximately maintained at the evaporation temperature of the first refrigerant L2. The first refrigerant L21 at the sixth temperature and the first refrigerant L22 at the third temperature mix to form the second working fluid at the seventh temperature, flowing into the low-temperature inlet of the second heat exchanger 33 to cool the first refrigerant L22 gas at the second temperature. The seventh temperature is lower than the second temperature.

[0054] After heat exchange at the second heat exchanger 33, the second working fluid, now at a higher temperature (fifth temperature), is discharged from the high-temperature outlet of the second heat exchanger 33. It is understood that the fifth temperature is lower than the second temperature, which is lower than the first temperature. Therefore, the first working fluid can cool the low-temperature mixed refrigerant L2 gas at the first temperature at the first heat exchanger 31. After heat exchange at the first heat exchanger 31, the first working fluid discharges the high-temperature, low-pressure low-temperature mixed refrigerant L2 gas from the high-temperature outlet of the first heat exchanger 31. The high-temperature, low-pressure low-temperature mixed refrigerant L2 gas is compressed by the second compressor 36 into a high-temperature, high-pressure low-temperature mixed refrigerant L2 gas, which is then transported to the high-temperature inlet of the evaporator-condenser 4, thus completing the cycle.

[0055] If a single refrigeration unit is used directly for cooling, high compression ratios are required to achieve a wide range of low temperatures, resulting in high energy consumption and difficulty in selecting a suitable refrigerant. To achieve gradient cooling, multiple independent refrigeration units, such as cascaded high-temperature, medium-temperature, and low-temperature cycles, would require compressors and condensers at each unit, necessitating multiple compressions and increasing energy consumption. The low-temperature distillation system 100 provided in this application utilizes multiple gradient cooling processes in the high-temperature cycle unit 2 and the low-temperature cycle unit 3, and employs multiple heat exchanges and gas-liquid exchanges using low-temperature mixed refrigerants with different evaporation temperatures. Only two refrigeration units are needed to achieve a wide range of low-temperature cooling. During the cooling process, high compression ratios of the refrigerant are not required, reducing energy consumption. Appropriate high-temperature and low-temperature mixed refrigerants can be selected based on the target condensation temperature of the distillation column 1, allowing for more accurate and efficient adjustment of cooling capacity.

[0056] Furthermore, in some embodiments, please continue to refer to Figure 2 The cryogenic circulation unit 3 also includes a second gas-liquid separator 37, a fourth expansion device 38, and a third heat exchanger 39 connected by pipes. The cryogenic outlet of the second heat exchanger 33 is connected to the inlet of the second gas-liquid separator 37, the gas outlet of the second gas-liquid separator 37 is connected to the high-temperature inlet of the third heat exchanger 39, and the liquid outlet of the second gas-liquid separator 37 is connected to the cryogenic inlet of the third heat exchanger 39 via the fourth expansion device 38. The cryogenic outlet of the third heat exchanger 39 is connected to the inlet of the top condenser 11 via the second expansion device 34. The outlet of the top condenser 11 is connected to the cryogenic inlet of the third heat exchanger 39, and the high-temperature outlet of the third heat exchanger 39 is connected to the cryogenic inlet of the second heat exchanger 33.

[0057] In this way, three low-temperature refrigerants with different evaporation temperatures can be mixed to form a mixed low-temperature refrigerant. The refrigerant with the higher evaporation temperature is condensed and separated in sequence for throttling and cooling. It then exchanges heat with the refrigerant with the lower evaporation temperature at the heat exchanger, achieving a gradient cooling of the low-evaporation-temperature refrigerant. The energy consumption of the heat exchanger and expansion device is low, thus reducing the overall energy consumption of the low-temperature distillation system 100 and also contributing to the precise control of the refrigeration temperature.

[0058] Alternatively, in some other embodiments, the cryogenic refrigerant discharged from the second expansion device 34 can be used to cool the condensate contained in the top condenser 11, and after heat exchange, it flows back to the third heat exchanger 39. The condensate contained in the top condenser is used to exchange heat with the material at the top of the column.

[0059] To further precisely control the refrigeration temperature at the top condenser 11, in some embodiments, please refer to [the relevant documentation / reference needed]. Figure 2The cryogenic cycle unit 3 also includes a fifth expansion device 40. The inlet of the fifth expansion device 40 is optionally connected to the outlet of the second compressor, and the outlet of the fifth expansion device 40 is connected to the inlet of the top condenser 11. When the temperature at the top condenser 11 is too low, the opening of the fifth expansion device 40 can be controlled to supply an appropriate amount of cryogenic mixed refrigerant L2 at a high temperature to the top condenser 11, thereby increasing the temperature.

[0060] As an example, a check valve can be installed in the pipeline between the outlet of the fifth expansion unit 40 and the inlet of the top condenser 11.

[0061] To further precisely control the refrigeration temperature at the top condenser 11, in some embodiments, please refer to... Figure 3 A buffer tank 66 is also provided between the outlet 35 of the third expansion unit and the inlet of the top condenser 11. The cryogenic distillation system 100 is also provided with a control unit 6. The control unit 6 includes a controller 61, a conveying device, a control device, and a temperature sensor 63 installed in the buffer tank 66. The conveying device includes a conveying pump 65 and a flow control valve 62 installed in the pipeline connecting the outlet of the buffer tank 66 and the inlet of the top condenser 11. The control device includes a pressure control valve 64 installed in the pipeline connecting the cryogenic inlet of the third heat exchanger 39 and the outlet of the top condenser 11. The temperature sensor 63, the conveying pump 65, the flow control valve 62, and the pressure control valve 64 are all signal-connected to the controller 61.

[0062] The controller 61 is equipped with at least the following function: when the temperature sensor 63 detects that the top temperature of the distillation column 1 is higher than the set value, the flow rate of the flow control valve 62 is increased; conversely, the flow rate of the flow control valve 62 is decreased.

[0063] As an example, the flow control valve 62 can be a flow meter or a solenoid valve.

[0064] Furthermore, the distillation column needs to be controlled at a suitable column pressure. The controller 61 can adjust the opening of the pressure control valve 64 according to the detection results of the pressure sensor of the distillation column, adjust the discharge ratio of the refrigerant, and thus adjust the gas pressure.

[0065] Furthermore, the controller 61 can be connected to various expansion devices (e.g., automatic throttle valves) and the compressor via signal connection. Based on the detection data from the temperature sensor 63 and pressure sensor, the controller 61 can adjust the opening degree of each control valve, the compression ratio of the compressor, and the opening degree of the expansion devices, thereby regulating the temperature of the refrigerant delivered to the top condenser 11 and achieving dynamic and precise adjustment. The temperature sensor 63 can be selected from conventional sensors, and this application is not limited to any particular type. The controller 61 can be connected to the compressor, condenser, throttle valve, and temperature sensor 63 according to conventional control methods in the art. The signal connection can be wired or wireless. Figure 3 The dashed lines in the diagram represent the limited signal connection paths.

[0066] For further information, please refer to [link / reference]. Figure 2 The cryogenic distillation system 100 also includes an emergency unit 5. The emergency unit 5 includes a second condenser 51, with the outlet of the top condenser 11 optionally connected to the inlet of the second condenser 51. In the event of a failure in either the high-temperature cycle unit 2 or the low-temperature cycle unit 3, the high-temperature, high-pressure cryogenic mixed refrigerant discharged from the top condenser 11 can be directly supplied to the second condenser 51. A refrigerant (such as liquid nitrogen or liquid oxygen) is used to cool the cryogenic mixed refrigerant, which is then supplied to the top condenser 11 for condensation of the overhead material, thereby contributing to the stable operation of the cryogenic distillation system 100.

[0067] Alternatively, in some embodiments, the refrigerant (e.g., liquid nitrogen) of the emergency unit 5 can be directly delivered to the top condenser 11 to cool the condenser 11 and thus condense the material at the top of the tower.

[0068] Furthermore, in some embodiments, the cryogenic circulation unit 3 further includes a regenerator 41. The regenerator 41 is provided with a heat exchange channel, and the high-temperature outlet of the first heat exchanger 31 is connected to the inlet of the second compressor 36 through the regenerator 41. The cryogenic distillation system 100 is also provided with a raw material delivery pipe 12, which passes through the heat exchange channel and is connected to the feed inlet of the distillation column 1, which can preheat the raw material, recover heat, and reduce energy consumption.

[0069] The expansion device in this application can be a conventional throttling valve in the art, such as a manual throttling valve or an automatic throttling valve, as long as it can achieve the function of throttling and cooling. This application does not limit the specific type of heat exchanger; conventional heat exchangers in the art can be selected. As an example, the heat exchanger can be a co-tube heat exchanger, a plate heat exchanger, or a shell-and-tube heat exchanger. The compressor and condenser in this application are both conventional compressors and condensers in the art, and appropriate compressors and condensers can be selected according to compression and condensation requirements. The gas-liquid separator can be a commonly used gas-liquid separator in refrigeration systems; this application does not impose any restrictions. All components are connected by pipes, and the pipes are externally covered with insulation material.

[0070] This application does not limit the specific structure of the distillation column 1, and appropriate selections can be made as needed. In some embodiments, the distillation column 1 includes a lower column and six upper columns arranged in parallel, the bottom of each of the six upper columns being connected to the feed inlet of an intermediate reboiler. The bottom of the intermediate reboiler is connected to the top of the lower column. Each upper column is provided with a top condenser 11. The outlet of the third expansion device 35 is connected to a main pipe, and six branch pipes are connected to the main pipe, each of the six branch pipes corresponding to the inlet of one of the six top condensers 11.

[0071] Furthermore, embodiments of this application also provide a method for performing low-temperature distillation using the above-described low-temperature distillation system, comprising: High-temperature refrigerant is injected into the high-temperature circulation unit, so that the high-temperature refrigerant gas flows sequentially through the first compressor, the first condenser and the first expansion device to the evaporator-condenser, where it exchanges heat with the low-temperature mixed refrigerant at the evaporator-condenser, absorbs heat and re-phases into high-temperature refrigerant gas, and so on.

[0072] A low-temperature mixed refrigerant is injected into the low-temperature circulation unit, where the gaseous low-temperature mixed refrigerant sequentially exchanges heat with the high-temperature refrigerant at the evaporator-condenser and the first working fluid at the first heat exchanger to form a first gas-liquid mixture. The first gas-liquid mixture is then separated into liquid and gaseous refrigerant using a first gas-liquid separator. The gaseous refrigerant is condensed after heat exchange with the second working fluid at the second heat exchanger, and then transported to the top condenser of the distillation column via a third expansion device, where it exchanges heat with the gas phase at the top of the distillation column to achieve low-temperature distillation. The liquid refrigerant is then mixed with the refrigerant discharged from the top condenser via the second expansion device to form the second working fluid. The second working fluid is then heated by the second heat exchanger to form the first working fluid, which is then passed through the first heat exchanger and the second compressor to form a low-temperature mixed refrigerant gas, and this cycle continues.

[0073] It is understandable that the evaporation temperature of a high-temperature refrigerant is higher than that of a low-temperature mixed refrigerant. A low-temperature mixed refrigerant contains two refrigerants with different evaporation temperatures.

[0074] As an example, the low-temperature mixed refrigerant includes a first refrigerant and a second refrigerant, wherein the evaporation temperature of the first refrigerant is lower than the evaporation temperature of the second refrigerant, and the evaporation temperature of the second refrigerant is lower than the evaporation temperature of the high-temperature refrigerant.

[0075] As an example, high-temperature refrigerants include at least one of R404A, R507, R414A, R454C, or R448A.

[0076] As an example, the low-temperature mixed refrigerant includes at least two of R290, R170, R1150, R14, and R23.

[0077] As an example, low-temperature mixed refrigerants include R290, R14, and R23.

[0078] As an example, the high-temperature refrigerant is R404A, and the low-temperature mixed refrigerant includes R290, R14, and R23 configured in a certain proportion. During the low-temperature distillation process, the high-temperature refrigerant R404A gas flows sequentially through the first compressor, the first condenser, and the first expansion device to the evaporator-condenser, where it exchanges heat with the low-temperature mixed refrigerant gas of R290, R14, and R23 at the evaporator-condenser, absorbing heat and re-phaseing into high-temperature refrigerant R404A gas, thus completing the cycle. At the evaporator-condenser, the high-temperature refrigerant R404A can provide a cooling capacity of approximately -50°C to -30°C.

[0079] A mixture of low-temperature refrigerants R290, R14, and R23 sequentially exchanges heat with the high-temperature refrigerant R404A at the evaporator-condenser and the first working fluid at the first heat exchanger to form a first gas-liquid mixture. The first gas-liquid mixture is then separated into liquid refrigerant R290 and gases of refrigerants R14 and R23 using a first gas-liquid separator. The gases of refrigerants R14 and R23 are then condensed by heat exchange with the second working fluid at the second heat exchanger to form a second gas-liquid mixture. This second gas-liquid mixture is then separated into gaseous refrigerant R14 and liquid refrigerant R23 using a second gas-liquid separator. The gaseous refrigerant R14 is then exchanged with the third working fluid at the third heat exchanger, condensed, and then transported via a third expansion device to the top condenser of the distillation column. This condenser delivers approximately -100°C of cooling energy to the top of the distillation column, exchanging heat with the gas phase at the top of the column to achieve low-temperature distillation. Liquid refrigerant R23, after being throttled and cooled by the fourth expansion device, mixes with refrigerant R14 discharged from the top condenser to form the third working fluid. The third working fluid flows through the third heat exchanger to the second heat exchanger, while liquid refrigerant R290, after being throttled and cooled by the second expansion device, flows to the second heat exchanger and mixes to form the second working fluid (containing portions of R290, R14, and R23). The second working fluid is then heated by the second heat exchanger to form the first working fluid. The first working fluid then passes through the first heat exchanger and the second compressor to form a low-temperature mixed refrigerant gas, and this cycle continues.

[0080] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A low-temperature distillation system, characterized in that, include: A distillation column, wherein the distillation column is equipped with a top condenser; The high-temperature circulation unit includes a first compressor, a first condenser, and a first expansion device connected by a pipeline. The high-temperature circulation unit is configured such that high-temperature refrigerant gas can flow sequentially through the first compressor, the first condenser, and the first expansion device to the evaporator-condenser, where it exchanges heat with the low-temperature mixed refrigerant at the evaporator-condenser, absorbs heat, and re-phases into the high-temperature refrigerant gas, thus circulating in this manner. The cryogenic circulation unit includes a first heat exchanger, a first gas-liquid separator, a second heat exchanger, a second expansion device, a third expansion device, and a second compressor, all connected by pipes. The low-temperature circulation unit is configured such that: a low-temperature mixed refrigerant can sequentially exchange heat with the high-temperature refrigerant at the evaporator-condenser and the first working fluid at the first heat exchanger to form a first gas-liquid mixture; the first gas-liquid mixture is separated into refrigerant liquid and refrigerant gas by the first gas-liquid separator; the refrigerant gas is condensed after heat exchange with the second working fluid at the second heat exchanger and flows into the top condenser through the third expansion device; the refrigerant liquid is mixed with the refrigerant discharged from the top condenser through the second expansion device to form the second working fluid; the second working fluid is used to form the first working fluid through the second heat exchanger, and the first working fluid is used to form the low-temperature mixed refrigerant through the first heat exchanger and the second compressor, thus circulating in this manner.

2. The low-temperature distillation system according to claim 1, characterized in that, The low-temperature mixed refrigerant includes a first refrigerant and a second refrigerant, wherein the evaporation temperature of the first refrigerant is lower than the evaporation temperature of the second refrigerant; The first gas-liquid separator is configured to separate the second refrigerant liquid and the first refrigerant gas; the first refrigerant gas exchanges heat with the second working fluid at the second heat exchanger and condenses, then flows into the top condenser of the tower through the third expansion device; the second refrigerant liquid passes through the second expansion device and mixes with the first refrigerant discharged from the top condenser of the tower to form the second working fluid.

3. The low-temperature distillation system according to claim 2, characterized in that, The outlet of the first condenser is connected to the low-temperature inlet of the evaporator-condenser through the first expansion device, and the high-temperature outlet of the evaporator-condenser is connected to the inlet of the first compressor; The outlet of the second compressor is connected to the high-temperature inlet of the evaporator-condenser, and the low-temperature outlet of the evaporator-condenser is connected to the high-temperature inlet of the first heat exchanger; the high-temperature outlet of the first heat exchanger is connected to the inlet of the second compressor. The low-temperature outlet of the first heat exchanger is connected to the inlet of the first gas-liquid separator; the gas outlet of the first gas-liquid separator is connected to the high-temperature inlet of the second heat exchanger; the liquid outlet of the first gas-liquid separator is connected to the low-temperature inlet of the second heat exchanger via the second expansion device; and the high-temperature outlet of the second heat exchanger is connected to the low-temperature inlet of the first heat exchanger. The low-temperature outlet of the second heat exchanger is connected to the inlet of the top condenser via the third expansion device; the outlet of the top condenser is connected to the low-temperature inlet of the second heat exchanger.

4. The low-temperature distillation system according to claim 1, characterized in that, The low-temperature mixed refrigerant includes a second refrigerant, a third refrigerant, and a first refrigerant, whose evaporation temperatures gradually decrease. The cryogenic circulation unit further includes a second gas-liquid separator, a fourth expansion device, and a third heat exchanger connected by pipes; the first gas-liquid mixture is separated into a second refrigerant liquid and a refrigerant gas by the first gas-liquid separator, the refrigerant gas including a first refrigerant gas and a third refrigerant gas; the refrigerant gas exchanges heat with the second working fluid at the second heat exchanger and condenses to form a second gas-liquid mixture; the second gas-liquid mixture is separated into a third refrigerant liquid and a first refrigerant gas by the second gas-liquid separator; the first refrigerant gas exchanges heat with the third working fluid at the third heat exchanger and condenses, flowing into the top condenser of the tower through the third expansion device; the third refrigerant liquid is mixed with the first refrigerant discharged from the top condenser of the tower through the fourth expansion device to form the third working fluid; the third working fluid flows through the third heat exchanger to the second heat exchanger, and the second refrigerant liquid flows through the second expansion device to the second heat exchanger, mixing to form the second working fluid.

5. The low-temperature distillation system according to claim 4, characterized in that, The low-temperature outlet of the second heat exchanger is connected to the inlet of the second gas-liquid separator; the gas outlet of the second gas-liquid separator is connected to the high-temperature inlet of the third heat exchanger; the liquid outlet of the second gas-liquid separator is connected to the low-temperature inlet of the third heat exchanger via the fourth expansion device; the low-temperature outlet of the third heat exchanger is connected to the inlet of the top condenser via the second expansion device; the outlet of the top condenser is connected to the low-temperature inlet of the third heat exchanger; and the high-temperature outlet of the third heat exchanger is connected to the low-temperature inlet of the second heat exchanger.

6. The low-temperature distillation system according to claim 1, characterized in that, The cryogenic cycle unit also includes a fifth expansion device, the inlet of which is optionally connected to the outlet of the second compressor, and the outlet of which is connected to the inlet of the top condenser.

7. The cryogenic distillation system according to claim 1, characterized in that, The cryogenic distillation system also includes an emergency unit, which includes a second condenser. The outlet of the top condenser can be selectively connected to the inlet of the second condenser, and the outlet of the second condenser can be selectively connected to the inlet of the top condenser.

8. The low-temperature distillation system according to claim 1, characterized in that, The low-temperature circulation unit also includes a regenerator, which is provided with a heat exchange channel. The high-temperature outlet of the first heat exchanger is connected to the inlet of the second compressor through the regenerator. The low-temperature distillation system is also provided with a raw material delivery pipe, which passes through the heat exchange channel and is connected to the feed inlet of the distillation column.

9. The low-temperature distillation system according to claim 4, characterized in that, A buffer tank is also provided between the outlet of the third expansion device and the inlet of the tower top condenser. The cryogenic distillation system is also equipped with a control unit, which includes a controller, a conveying device, and a control device, as well as a temperature sensor installed in the buffer tank; the conveying device includes a conveying pump and a flow control valve installed at the pipeline connecting the outlet of the buffer tank and the inlet of the top condenser. The control device includes a pressure control valve installed at the pipe connecting the low-temperature inlet of the third heat exchanger and the outlet of the top condenser. The temperature sensor, the delivery pump, the flow control valve, and the pressure control valve are all connected to the controller via signal connection. The controller is equipped with at least the following function: when the temperature sensor detects that the top temperature of the distillation column is higher than a set value, the flow rate of the flow control valve is increased; conversely, the flow rate of the flow control valve is decreased.

10. A low-temperature distillation method, characterized in that, Using the cryogenic distillation system according to any one of claims 1 to 9, the cryogenic distillation method includes: High-temperature refrigerant is injected into the high-temperature circulation unit, so that the high-temperature refrigerant gas flows sequentially through the first compressor, the first condenser and the first expansion device to the evaporator-condenser, where it exchanges heat with the low-temperature mixed refrigerant at the evaporator-condenser, absorbs heat and re-phases into the high-temperature refrigerant gas, and thus cycles. A low-temperature mixed refrigerant is injected into the low-temperature circulation unit, whereby the gaseous low-temperature mixed refrigerant sequentially exchanges heat with the high-temperature refrigerant at the evaporator-condenser and the first working fluid at the first heat exchanger to form a first gas-liquid mixture. The first gas-liquid mixture is then separated into liquid and gaseous refrigerant using the first gas-liquid separator. The gaseous refrigerant is condensed by heat exchange with the second working fluid at the second heat exchanger and then transported to the top condenser of the distillation column via the third expansion device, where it exchanges heat with the gas phase at the top of the distillation column to achieve low-temperature distillation. The liquid refrigerant is then mixed with the refrigerant discharged from the top condenser via the second expansion device to form the second working fluid. The second working fluid is then heated by the second heat exchanger to form the first working fluid, which is then passed through the first heat exchanger and the second compressor to form the gaseous low-temperature mixed refrigerant, and this cycle is repeated. Optionally, the low-temperature mixed refrigerant includes a first refrigerant and a second refrigerant, wherein the evaporation temperature of the first refrigerant is lower than the evaporation temperature of the second refrigerant, and the evaporation temperature of the second refrigerant is lower than the evaporation temperature of the high-temperature refrigerant; Optionally, the high-temperature refrigerant includes at least one of R404A, R507, R414A, R454C, or R448A; Optionally, the low-temperature mixed refrigerant includes at least two of R290, R170, R1150, R14, and R23; Optionally, the cryogenic mixed refrigerant includes R290, R14, and R23.

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