Low-energy-consumption air separation liquid nitrogen cooling capacity cascade recycling system and method

Abstract

The low-energy-consumption air separation liquid nitrogen cooling capacity cascade recycling system comprises an air separation main system, an air compressor, a purifier, a rectifying tower and a liquid nitrogen storage tank, and the storage tank is provided with a heat insulation layer; the first-stage cold energy recovery module is a shell and tube heat exchanger, and raw material air is pre-cooled through liquid nitrogen; the second-stage heat exchanger is a plate heat exchanger for cooling external low-temperature equipment; the third-stage heat exchanger is a spiral pipe type heat exchanger which exchanges heat with waste nitrogen; the cold storage buffer module comprises a phase change cold storage tank and a circulating pump; the cooling capacity regulating and controlling module comprises an electric regulating valve and a flow sensor; the central control module uses a PLC and a touch screen to realize full-process automatic control. According to the invention, through three-stage cascade recycling of liquid nitrogen cooling capacity of a full-temperature section, cold storage buffering and loss compensation, stable operation is guaranteed; the refrigerating capacity distribution reasonability is improved through precise regulation and control, and leakage and overpressure coping is strengthened through safety protection; the cold energy recovery efficiency is improved, the energy consumption of the air separation system is reduced, and different-scale application scenes are adapted.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of air separation and liquefaction, and particularly relates to a low-energy-consumption air separation and liquid nitrogen cold energy gradient recycling system and method. BACKGROUND

[0002] As the core equipment for industrial nitrogen and oxygen production, the air separation system produces liquid nitrogen through low-temperature rectification during operation. The liquid nitrogen contains a large amount of cold energy (the latent heat of vaporization of liquid nitrogen is about 200 kJ / kg), and the recycling of the cold energy is a key link for reducing the energy consumption of the air separation system. At present, the recycling of the cold energy of liquid nitrogen in most air separation systems only stays at a single level, and usually only uses the liquid nitrogen for simple pre-cooling of raw air. The pre-cooled liquid nitrogen is directly discharged into a vaporizer for vaporization, and a large amount of cold energy in the middle and low temperature sections (-180℃ to -130℃) is not utilized, resulting in that the cold energy recycling efficiency is generally less than 60%. For example, the compressed air at the outlet of the air compressor is usually at a temperature of 40-60℃, and after being pre-cooled by the liquid nitrogen to 5-15℃, the temperature of the liquid nitrogen rises to about -180℃. At this time, the liquid nitrogen still has a large amount of cold energy, but the existing system does not design a recycling path for the cold energy in this temperature range, and directly vaporizes it, causing serious waste of cold energy resources and increasing the overall energy consumption of the air separation system.

[0003] The existing few air separation systems with multi-level cold energy recycling function have the problem of mismatch between cold energy quality and demand. Such systems do not grade the temperature of the cold energy, and directly use the low-temperature cold energy (-196℃ to -170℃) for high-temperature demand scenarios (such as plant circulating water cooling, with a demand temperature of 10-15℃), resulting in waste of cold energy quality. In addition, there is a lack of cold storage buffer mechanism, and when the load of liquid nitrogen produced by the air separation fluctuates (such as the exhaust volume of the air compressor changes), the cold energy supply and demand cannot be matched in real time. When the load is at a peak, there is an excess of cold energy that cannot be stored, and when the load is at a trough, there is a lack of cold energy that affects the recycling efficiency. In addition, the cold energy loss (usually accounting for 3%-5% of the total recycled cold energy) caused by heat dissipation in the pipeline during the cold energy recycling process is not compensated, further reducing the cold energy recycling efficiency, and there is a lack of precise cold energy metering means, which cannot real-time grasp the cold energy utilization of each level of recycling module, resulting in that the cold energy distribution relies on manual experience and the control precision is low.

[0004] In terms of safety protection, the existing cryogenic leak detection and overpressure protection measures of the air separation liquid nitrogen cold energy recovery system are inadequate. Leaks are prone to occur at the flange connections of liquid nitrogen pipelines due to seal aging. Existing leak detection equipment typically has a detection limit higher than 50 ppm, making early detection difficult. Cryogenic liquid nitrogen leaks can lead to pipeline frosting, equipment damage, and even safety accidents. When the gas phase space of the liquid nitrogen storage tank and heat exchanger is overpressurized, the safety valve opening pressure setting deviation is large. Some systems lack a linked control module, relying solely on pressure relief valves for passive pressure relief after overpressure, failing to promptly cut off the cold energy supply and posing a risk of equipment damage due to overpressure. These problems collectively prevent the existing air separation liquid nitrogen cold energy recovery system from achieving efficient, stable, and safe cold energy utilization, hindering the reduction of energy consumption and the improvement of operational stability in air separation systems. Summary of the Invention

[0005] This invention proposes a low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system and method to solve the problems mentioned in the prior art.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system, comprising: The main air separation system consists of an air compressor, an air purifier, a distillation column, and a liquid nitrogen storage tank. The air compressor compresses the raw air to the designed working pressure. The purifier removes moisture, carbon dioxide, and hydrocarbons from the air using an adsorbent. The distillation column performs low-temperature distillation separation on the purified air to obtain liquid nitrogen. The liquid nitrogen storage tank stores the liquid nitrogen produced by the distillation column, and the inner wall of the storage tank is equipped with a vacuum insulation layer. Primary cold energy recovery module: includes a shell and tube heat exchanger. The shell inlet of the heat exchanger is connected to the outlet of the liquid nitrogen storage tank through a pipeline, the tube inlet is connected to the outlet of the air compressor, and the tube outlet is connected to the inlet of the air purifier. During the flow of liquid nitrogen in the shell side, it absorbs the heat of the air in the tube side, thereby pre-cooling the raw material air. Secondary cold energy recovery module: includes a plate heat exchanger. The hot side inlet of the heat exchanger is connected to the liquid nitrogen outlet of the primary cold energy recovery module through a pipeline. The hot side outlet is connected to the tertiary cold energy recovery module. The cold side inlet is connected to the external low-temperature equipment. The cold side outlet is connected to the return gas end of the external equipment. It uses liquid nitrogen that still contains cold energy after the primary heat exchange to provide cold energy to the external equipment. The three-stage cold energy recovery module includes a spiral tube heat exchanger. The heat exchanger inlet is connected to the liquid nitrogen outlet of the two-stage cold energy recovery module, and the outlet is connected to the liquid nitrogen vaporizer. The waste nitrogen gas generated by the distillation column is introduced into the shell side of the heat exchanger. The liquid nitrogen exchanges heat with the waste nitrogen gas in the tube side and shell side. After recovering the cold energy of the liquid nitrogen, it enters the vaporizer. Cold storage buffer module: It consists of a phase change cold storage tank and a circulation pump. The phase change cold storage tank is filled with paraffin-based composite phase change material. The inlet is connected to the liquid nitrogen pipeline of the first-level cold energy recovery module through a pipeline. The outlet is connected to the make-up cold pipeline of each cold energy recovery module through a circulation pump. When the liquid nitrogen cold energy is excessive, it stores the cold energy. When the cold energy is insufficient, it releases the cold energy. Cooling capacity control module: Includes various electric regulating valves and flow sensors. Each cooling capacity recovery module is equipped with an electric regulating valve and a flow sensor on its inlet and outlet pipelines. The flow sensor collects the fluid flow rate in the pipeline in real time, and the electric regulating valve adjusts its opening degree according to the flow data.

[0007] Furthermore, it also includes: Central control module: Composed of PLC controller and touch screen. The PLC receives detection data from flow sensor, temperature sensor and pressure sensor, and outputs control signals to electric regulating valve, circulating pump and air separation main system equipment to realize full-process automated control of cold energy recovery. The touch screen supports parameter setting, status display and historical data query. Auxiliary heat exchange module: Includes a shell-and-tube heat exchanger. The inner tube of the heat exchanger connects to the liquid nitrogen outlet pipeline of the three-stage cold energy recovery module, and the outer tube connects to the plant's circulating water network. When there is still unused cold energy in the liquid nitrogen after the three-stage cold energy recovery, it is transferred to the circulating water through the heat exchanger to provide low-temperature circulating water for the plant. The heat exchange capacity of the auxiliary heat exchange module is calculated using the formula... The calculation is performed, where Qauxiliary is the heat exchange capacity of the auxiliary heat exchange module, Kauxiliary is the heat transfer coefficient of the auxiliary heat exchanger, Aauxiliary is the heat exchange area of ​​the auxiliary heat exchanger, and ΔTauxiliary is the logarithmic mean temperature difference between liquid nitrogen and circulating water in the auxiliary heat exchanger. The auxiliary heat exchange load is matched according to the cooling capacity required by the circulating water through the calculation.

[0008] Furthermore, it also includes: Cold energy loss compensation module: includes a cold storage spherical tank and a cold energy replenishment pump. The cold storage spherical tank is filled with a medium containing cold storage refrigerant. The inlet is connected to the liquid nitrogen storage tank through the cold energy replenishment pump, and the outlet is connected to the cold energy loss compensation pipeline of each cold energy recovery module. When the cold energy recovery module experiences cold energy loss due to heat dissipation from the pipeline, the module replenishes the cold energy to the recovery pipeline. The cold storage capacity of the cold storage spherical tank is calculated and determined based on the hourly liquid nitrogen production and cold energy loss rate of the air separation system, maintaining the cold energy recovery efficiency within the set range.

[0009] Furthermore, the shell-and-tube heat exchanger of the primary cooling capacity recovery module adopts a multi-pass design, with 4 passes in the tube side and 2 passes in the shell side. The tube side is equipped with segmental baffles with a baffle spacing of 1.5 times the inner diameter of the heat exchanger and a baffle gap ratio of 25%.

[0010] Furthermore, the phase change cold storage tank of the cold storage buffer module is equipped with a stainless steel temperature stratification partition. The partition divides the inner cavity of the tank into an upper high-temperature zone and a lower low-temperature zone. The high-temperature zone is filled with paraffin-based composite phase change material with a phase change temperature of -10℃ to -5℃, while the lower low-temperature zone is filled with paraffin-based composite phase change material with a phase change temperature of -40℃ to -35℃. The stratified design releases the corresponding temperature range of cold energy according to different cold energy requirements.

[0011] Furthermore, it also includes: The cooling capacity metering module includes a cooling capacity meter and a data acquisition unit. The cooling capacity meter is installed on the liquid nitrogen inlet pipeline of each cooling capacity recovery module. The data acquisition unit connects the cooling capacity meter to the central control module. The cooling capacity meter uses a formula... Calculate the recovered cold energy, where Qcalculated is the recovered cold energy of a single cold energy recovery module, ρliquid is the density of liquid nitrogen under standard operating conditions, Vliquid is the volume of liquid nitrogen flowing through the cold energy meter, and ΔTliquid is the temperature change of liquid nitrogen at the inlet and outlet of the cold energy recovery module.

[0012] Safety protection module: Includes a low-temperature alarm unit, an overpressure relief unit, and a leak detection unit. The temperature sensor of the low-temperature alarm unit is installed on the outer wall of the pipeline of each cold energy recovery module. When the temperature of the outer wall of the pipeline is lower than -196℃, an audible and visual alarm is triggered. The safety valve of the overpressure relief unit is installed in the gas phase space between the liquid nitrogen storage tank and each heat exchanger. The opening pressure of the safety valve is set to 1.1 times the rated pressure of the equipment. The leak detection unit uses a hydrogen leak detector. The detection probe is arranged at the pipeline flange connection. When the detected leakage concentration exceeds the set threshold, an audible and visual alarm is triggered. At the same time, the central control module automatically closes the electric regulating valve of the relevant pipeline.

[0013] A method for using the aforementioned low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system includes: Air separation to liquid nitrogen production steps: Start the air compressor of the main air separation system to compress the raw air to the design working pressure. The compressed air enters the air purifier to remove impurities. The purified air is sent to the distillation column for low-temperature distillation. The separated liquid nitrogen is stored in the liquid nitrogen storage tank. Primary cold energy recovery steps: Open the electric regulating valve of the primary cold energy recovery module, and the liquid nitrogen in the liquid nitrogen storage tank enters the shell side of the shell and tube heat exchanger. At the same time, the compressed air from the air compressor outlet enters the tube side. The liquid nitrogen and the compressed air exchange heat. The pre-cooled compressed air is sent to the air purifier, and the liquid nitrogen after heat exchange enters the secondary cold energy recovery module. Secondary cold energy recovery steps: Open the electric regulating valve of the secondary cold energy recovery module. Liquid nitrogen after primary heat exchange enters the hot side of the plate heat exchanger, and the circulating fluid of the external low-temperature equipment enters the cold side. Liquid nitrogen exchanges heat with the circulating fluid to provide cooling for the external equipment. Liquid nitrogen after heat exchange enters the tertiary cold energy recovery module. Three-stage cold energy recovery steps: Open the electric regulating valve of the three-stage cold energy recovery module. The liquid nitrogen after the second-stage heat exchange enters the tube side of the spiral tube heat exchanger, and the waste nitrogen gas produced by the distillation column enters the shell side. The liquid nitrogen and waste nitrogen gas exchange heat to release cold energy. The liquid nitrogen after heat exchange enters the liquid nitrogen vaporizer. Cold storage buffering steps: When the central control module detects an excess of liquid nitrogen cooling capacity, it starts the circulation pump of the cold storage buffer module to introduce some liquid nitrogen into the phase change cold storage tank to store the cooling capacity; when the cooling capacity is insufficient, it starts the circulation pump to transport the cooling capacity in the cold storage tank to each recovery module. Cooling capacity control steps: The central control module adjusts the opening of the corresponding electric regulating valve based on the flow sensor and temperature sensor data of each cooling capacity recovery module to control the liquid nitrogen flow rate and make the heat exchange of each level of recovery module match the demand. System shutdown procedure: After the main air separation system stops producing liquid nitrogen, close the electric regulating valves of each cold energy recovery module, open the liquid nitrogen vaporizer to vaporize and discharge the residual liquid nitrogen in the pipeline, and then introduce dry nitrogen to purge each heat exchanger and pipeline for ≥30 minutes to prevent residual moisture in the pipeline from freezing and clogging.

[0014] Furthermore, it also includes an optimized process for tiered cold energy distribution. The central control module determines the optimal distribution based on the heat exchange efficiency of each cold energy recovery module using a formula. Calculate the allocated cooling capacity of each module, where Q is the allocated cooling capacity of the first-level cooling capacity recovery module, ηi is the heat exchange efficiency of the cooling capacity recovery module, and Σηi is the sum of the heat exchange efficiencies of all cooling capacity recovery modules. The cooling capacity is allocated according to the heat exchange efficiency, with modules with higher heat exchange efficiency receiving more cooling capacity.

[0015] Furthermore, it also includes a variable load cooling capacity control process. When the liquid nitrogen production load of the main air separation system changes, the central control module adjusts the load according to the air separation load change rate using a formula. Adjust the liquid nitrogen flow rate of each cold energy recovery module, where Vadjusted is the adjusted liquid nitrogen flow rate, Voriginal is the liquid nitrogen flow rate under the rated load of the air separation unit, Pactual is the actual liquid nitrogen production load of the air separation unit, and Prated is the rated liquid nitrogen production load of the air separation unit. Through calculation, the cold energy recovery flow rate changes synchronously with the liquid nitrogen production load of the air separation unit, so as to maintain the heat exchange efficiency of each cold energy recovery module.

[0016] Compared with existing technologies, the beneficial effects of this invention are: The tiered cold energy recovery system achieves full utilization of cold energy across the entire temperature range. The first stage utilizes liquid nitrogen in the low-temperature range to pre-cool the raw air, reducing the load on the air purifier. The second stage utilizes medium-temperature cold energy to cool external equipment requiring low temperatures, avoiding direct emissions of cold energy. The third stage combines with distillation column waste nitrogen to further extract low-temperature cold energy, while the auxiliary heat exchange module utilizes the final stage cold energy to provide low-temperature circulating water for the plant. This system covers cold energy recovery across the entire temperature range from -196℃ to -130℃, completely resolving the cold energy waste problem caused by the single-level recovery in existing systems and significantly improving overall cold energy recovery efficiency.

[0017] The cold storage buffer and cold capacity compensation design ensures the stability of system operation. The cold storage buffer module uses layered phase change cold storage materials to store excess cold capacity according to temperature ranges, avoiding waste of cold capacity during peak loads; when cold capacity is insufficient, it releases cold capacity in the corresponding temperature range, matching the cold capacity quality of different demand scenarios and preventing waste of cold capacity quality. The cold capacity loss compensation module supplements cold capacity for heat loss in the pipeline, maintaining stable cold capacity recovery efficiency of each level of recovery module, solving the efficiency fluctuation problem caused by load fluctuations and cold capacity loss in the existing system, and improving the continuity of system operation.

[0018] Precise metering and intelligent control improve the rationality of cooling capacity allocation. The cooling capacity metering module collects cooling capacity recovery data from each level of recovery module in real time, providing accurate data support for the central control module; the optimized cooling capacity tiered allocation process distributes cooling capacity according to the heat exchange efficiency of each level of module, avoiding inefficient modules from occupying too much cooling capacity; the variable load control process synchronizes the cooling capacity recovery flow rate with the air separation liquid nitrogen production load, maintaining stable heat exchange efficiency, eliminating the existing system's reliance on manual experience for control, and improving the accuracy and efficiency of cooling capacity allocation.

[0019] Safety protection measures enhance system operational safety. The cryogenic alarm unit, working in conjunction with a high-precision hydrogen leak detector, can promptly detect low-concentration liquid nitrogen leaks, preventing equipment damage and safety risks caused by initial undetected leaks. The overpressure relief unit, linked to the central control module, can not only passively relieve pressure in case of overpressure but also actively cut off the cooling supply, reducing the probability of equipment damage due to overpressure. The auxiliary heat exchange module expands the utilization scenarios of cooling capacity, using the final stage of cooling capacity for cooling the plant's circulating water, reducing energy consumption of other refrigeration equipment in the plant, and achieving a reduction in energy consumption for the air separation system and the entire plant.

[0020] Overall, this invention achieves "full-temperature range recovery, high-quality matching, precise control, and safe operation" of liquid nitrogen cooling capacity in air separation systems. It effectively solves problems such as cooling capacity waste, efficiency fluctuations, and safety hazards in existing systems, significantly improves the cooling capacity utilization efficiency and operational stability of air separation systems, reduces overall energy consumption, and provides reliable technical support for energy-saving optimization of industrial air separation systems. Attached Figure Description

[0021] Figure 1This is a schematic block diagram of a low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system proposed in this invention; Figure 2 This is a schematic diagram of a low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization method proposed in this invention; Figure 3 This is a schematic diagram comparing the cold energy recovery efficiency of a traditional system and the cold energy recovery efficiency of the present invention, which is a low-energy-consumption air-separated liquid nitrogen cold energy recovery and utilization method proposed in this invention. Figure 4 This is a schematic diagram illustrating the change in energy consumption of an air separation system with the amount of cold energy recovered, based on a low-energy-consumption air separation liquid nitrogen cold energy cascade recovery and utilization method proposed in this invention. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention 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. Therefore, they should not be construed as limitations on this invention.

[0024] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of the stated features. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified. Furthermore, the terms "installed," "connected," and "linked" should be interpreted broadly; for example, they may refer to a fixed connection, a detachable connection, or an integral connection; they may refer to a mechanical connection or an electrical connection; they may refer to a direct connection or an indirect connection through an intermediate medium; and they may refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances. The invention will now be described in further detail with reference to the accompanying drawings.

[0025] ReferenceFigures 1 to 4 A low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system, comprising: The main air separation system consists of an air compressor, an air purifier, a distillation column, and a liquid nitrogen storage tank. The air compressor compresses the raw air to the designed working pressure (0.6-1.2 MPa). The purifier removes moisture, carbon dioxide, and hydrocarbons from the air using an adsorbent (the impurity content after purification is ≤1 ppm). The distillation column performs low-temperature distillation separation on the purified air to obtain liquid nitrogen. The liquid nitrogen storage tank stores the liquid nitrogen produced by the distillation column, and the inner wall of the storage tank is equipped with a vacuum insulation layer. Primary cold energy recovery module: includes a shell and tube heat exchanger. The shell inlet of the heat exchanger is connected to the outlet of the liquid nitrogen storage tank through a pipeline, the tube inlet is connected to the outlet of the air compressor, and the tube outlet is connected to the inlet of the air purifier. During the flow of liquid nitrogen in the shell side, it absorbs the heat of the air in the tube side, thereby pre-cooling the raw material air (the air temperature drops to 5-15℃ after pre-cooling). Secondary cold energy recovery module: includes plate heat exchangers (heat exchange area 50-200m²) 2 The heat exchanger’s hot-side inlet is connected to the liquid nitrogen outlet of the first-stage cold energy recovery module via a pipeline. The hot-side outlet is connected to the third-stage cold energy recovery module. The cold-side inlet is connected to external low-temperature demand equipment (such as low-temperature storage tank precooling and freeze-drying equipment). The cold-side outlet is connected to the return gas end of the external equipment. Liquid nitrogen, which still contains cold energy after the first-stage heat exchange, is used to provide cold energy to the external equipment. The three-stage cold energy recovery module includes a spiral tube heat exchanger (spiral tube diameter 20-50mm). The heat exchanger inlet is connected to the liquid nitrogen outlet of the two-stage cold energy recovery module, and the outlet is connected to the liquid nitrogen vaporizer. Waste nitrogen gas (temperature -170 to -150℃) generated by the distillation column is introduced into the shell side of the heat exchanger. The liquid nitrogen exchanges heat with the waste nitrogen gas in the tube side and shell side, and after further recovering the cold energy of the liquid nitrogen, it enters the vaporizer. Cold storage buffer module: consisting of a phase change cold storage tank (5-50m³) 3 It consists of a phase change storage tank and a circulation pump. The tank is filled with paraffin-based composite phase change material (latent heat of phase change ≥200kJ / kg). The inlet is connected to the liquid nitrogen pipeline of the first-stage cold energy recovery module through a pipeline, and the outlet is connected to the make-up cooling pipeline of each cold energy recovery module through a circulation pump. When the liquid nitrogen cold energy is excessive, it stores the cold energy, and when the cold energy is insufficient, it releases the cold energy. Cooling capacity control module: includes multiple electric regulating valves and flow sensors. Each cooling capacity recovery module is equipped with an electric regulating valve and flow sensor on its inlet and outlet pipelines. The flow sensor collects the fluid flow rate in the pipeline in real time, and the electric regulating valve adjusts its opening degree according to the flow data. Central control module: Composed of a PLC controller and a touch screen. The PLC receives detection data from flow sensors, temperature sensors (measurement range -200 to 100℃), and pressure sensors (measurement range 0-2MPa), and outputs control signals to the electric regulating valves, circulating pumps, and air separation main system equipment to realize full-process automated control of cold energy recovery. The touch screen supports parameter setting, status display, and historical data query.

[0026] This invention also includes: Auxiliary heat exchange module: This module includes a shell-and-tube heat exchanger (inner tube diameter 15-30mm, outer tube diameter 25-40mm). The inner tube of the heat exchanger connects to the liquid nitrogen outlet pipeline of the three-stage cold energy recovery module, and the outer tube connects to the plant's circulating water network (circulating water temperature 25-35℃). When there is still unused cold energy in the liquid nitrogen after the three-stage cold energy recovery, this heat exchanger transfers the cold energy to the circulating water, providing the plant with low-temperature circulating water (water temperature drops to 10-15℃). The heat exchange capacity of the auxiliary heat exchange module is calculated using the formula... The calculation is performed, where Qauxiliary is the heat exchange capacity of the auxiliary heat exchange module, Kauxiliary is the heat transfer coefficient of the auxiliary heat exchanger (range 500-1000), Aauxiliary is the heat exchange area of ​​the auxiliary heat exchanger, and ΔTauxiliary is the logarithmic mean temperature difference between liquid nitrogen and circulating water in the auxiliary heat exchanger. This calculation allows for precise matching of the auxiliary heat exchange load according to the cooling demand of the circulating water, thereby improving the efficiency of cooling capacity utilization.

[0027] Cold energy loss compensation module: This module includes a cold storage spherical tank (volume 2-10m³). 3 ) and cold energy replenishment pump (flow rate 1-5m³ / h) 3 The cold storage spherical tank is filled with a porous medium-encapsulated cold storage agent (magnesium nitrate hexahydrate, phase change temperature -3.9℃, latent heat of phase change 137kJ / kg). The inlet is connected to the liquid nitrogen storage tank via a cold energy replenishment pump, and the outlet is connected to the cold energy loss compensation pipeline of each cold energy recovery module. When the cold energy recovery module experiences cold energy loss due to heat dissipation from the pipeline, the module replenishes the cold energy to the recovery pipeline. The cold energy storage capacity of the cold storage spherical tank is calculated and determined based on the hourly liquid nitrogen production of the air separation system and the cold energy loss rate (set to 3%-5%), maintaining the cold energy recovery efficiency within the set range (≥90%).

[0028] In this invention, the shell-and-tube heat exchanger of the primary cold energy recovery module adopts a multi-pass design, with 4 passes in the tube side and 2 passes in the shell side. The tube side is equipped with segmental baffles, and the spacing between the baffles is 1.5 times the inner diameter of the heat exchanger (ranging from 150 to 300 mm). The baffle gap ratio is 25%. This design can extend the residence time of the raw material air in the tube side (extended to 3-5 seconds), increase the heat exchange area with the liquid nitrogen in the shell side, improve the pre-cooling effect of the raw material air, and at the same time reduce the fluid flow resistance in the tube side, thereby reducing the additional energy consumption of the air compressor.

[0029] In this invention, a stainless steel temperature-layered partition is installed inside the phase change cold storage tank of the cold storage buffer module. The partition divides the inner cavity of the tank into an upper high-temperature zone and a lower low-temperature zone. The high-temperature zone is filled with paraffin-based composite phase change material with a phase change temperature of -10℃ to -5℃, and the lower low-temperature zone is filled with paraffin-based composite phase change material with a phase change temperature of -40℃ to -35℃. The layered design can release the corresponding temperature range of cold energy according to different cold energy requirements, reduce the loss of cold energy quality when the low-temperature cold energy is directly used in high-temperature demand scenarios, and improve the cold energy utilization efficiency of the cold storage module.

[0030] This invention also includes: Cooling capacity metering module: This module includes a cooling capacity meter (measuring range 0-1000kW) and a data acquisition unit. The cooling capacity meter is installed on the liquid nitrogen inlet pipeline of each cooling capacity recovery module. The data acquisition unit connects the cooling capacity meter to the central control module. The cooling capacity meter is measured using a formula... The calculation of recovered cold energy is as follows: Qcalculated represents the recovered cold energy of a single cold energy recovery module; ρliquid represents the density of liquid nitrogen under standard operating conditions (value 808); Vliquid represents the volume of liquid nitrogen flowing through the cold energy meter; cliquid represents the specific heat capacity of liquid nitrogen (value 2.04); and ΔTliquid represents the temperature change of liquid nitrogen at the inlet and outlet of the cold energy recovery module. This calculation allows for real-time measurement of the cold energy recovered by each level of the recovery module, providing data support for the central control module to adjust the cold energy allocation strategy.

[0031] Safety Protection Module: This module includes a low-temperature alarm unit, an overpressure relief unit, and a leak detection unit. The temperature sensor of the low-temperature alarm unit is installed on the outer wall of the pipeline of each cold energy recovery module. When the temperature of the outer wall of the pipeline is lower than -196℃ (the boiling point of liquid nitrogen), an audible and visual alarm is triggered (alarm sound pressure level ≥85dB, and the light is a red flashing light). The safety valve of the overpressure relief unit is installed in the gas phase space between the liquid nitrogen storage tank and each heat exchanger. The opening pressure of the safety valve is set to 1.1 times the rated pressure of the equipment (opening pressure deviation ±3%). The leak detection unit uses a hydrogen leak detector (detection limit ≤10ppm). The detection probe is arranged at the pipeline flange connection (spacing 1-2m). When the detected leakage concentration exceeds the set threshold, an audible and visual alarm is triggered, and the central control module automatically closes the electric regulating valve of the relevant pipeline.

[0032] This invention also discloses a method for a low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system, comprising the following steps: Air separation to liquid nitrogen production steps: Start the air compressor of the main air separation system to compress the raw air to the design working pressure (0.6-1.2MPa). The compressed air enters the air purifier (adsorbent regeneration cycle 8-12 hours) to remove impurities. The purified air is sent to the distillation column (operating temperature -196℃ to -180℃) for low-temperature distillation. The separated liquid nitrogen is stored in the liquid nitrogen storage tank. Primary cold energy recovery steps: Open the electric regulating valve of the primary cold energy recovery module. Liquid nitrogen (temperature -196℃) in the liquid nitrogen storage tank enters the shell side of the shell and tube heat exchanger. At the same time, compressed air (temperature 40-60℃) from the air compressor outlet enters the tube side. Liquid nitrogen and compressed air exchange heat. The pre-cooled compressed air (temperature 5-15℃) is sent to the air purifier. The liquid nitrogen (temperature -180 to -170℃) after heat exchange enters the secondary cold energy recovery module. Secondary cold energy recovery steps: Open the electric regulating valve of the secondary cold energy recovery module. The liquid nitrogen after the primary heat exchange enters the hot side of the plate heat exchanger. The circulating fluid (temperature 20-30℃) of the external low-temperature equipment enters the cold side. The liquid nitrogen exchanges heat with the circulating fluid (the circulating fluid cools down to -5 to 5℃) to provide cooling for the external equipment. The liquid nitrogen after heat exchange (temperature -160 to -150℃) enters the tertiary cold energy recovery module. Three-stage cold energy recovery steps: Open the electric regulating valve of the three-stage cold energy recovery module. The liquid nitrogen after the second-stage heat exchange enters the tube side of the spiral tube heat exchanger. The waste nitrogen gas (temperature -170 to -150℃) generated by the distillation column enters the shell side. The liquid nitrogen exchanges heat with the waste nitrogen gas (the liquid nitrogen temperature rises to -140 to -130℃) to further release cold energy. The liquid nitrogen after heat exchange enters the liquid nitrogen vaporizer (the nitrogen temperature rises to room temperature after vaporization). Cold storage buffering steps: When the central control module detects that the liquid nitrogen cooling capacity is excessive (i.e., the flow rate reported by the flow sensors of each level of recovery module exceeds the demand value by more than 10%), the circulation pump of the cold storage buffer module is turned on to introduce some liquid nitrogen into the phase change cold storage tank to store the cooling capacity; when the cooling capacity is insufficient (the flow rate is less than 10% of the demand value), the circulation pump is started to transport the cooling capacity in the cold storage tank to each recovery module. Cooling capacity control steps: The central control module adjusts the opening of the corresponding electric regulating valve based on the flow sensor and temperature sensor data of each cooling capacity recovery module to control the liquid nitrogen flow rate, so that the heat exchange of each level of recovery module matches the requirements (the heat exchange deviation is controlled within ±5%). System shutdown procedure: After the main air separation system stops producing liquid nitrogen, close the electric regulating valves of each cold energy recovery module, open the liquid nitrogen vaporizer to vaporize and discharge the residual liquid nitrogen in the pipeline, and then introduce dry nitrogen to purge each heat exchanger and pipeline for ≥30 minutes to prevent residual moisture in the pipeline from freezing and clogging.

[0033] This invention also includes a tiered cooling capacity allocation optimization process. The central control module, based on the heat exchange efficiency of each cooling capacity recovery module (first stage 85%-90%, second stage 80%-85%, third stage 75%-80%), uses a formula to... The allocated cooling capacity of each module is calculated, where Q is the allocated cooling capacity of a certain level of cooling capacity recovery module, Q_total is the total recoverable cooling capacity of liquid nitrogen (calculated based on liquid nitrogen production and temperature changes), ηi is the heat exchange efficiency of that level of cooling capacity recovery module, and Σηi is the sum of the heat exchange efficiencies of all cooling capacity recovery modules. Through this calculation, cooling capacity can be allocated according to the heat exchange efficiency, with modules with higher heat exchange efficiency receiving more cooling capacity, thereby improving the overall cooling capacity recovery and utilization rate.

[0034] This invention also includes a variable load cooling capacity control process. When the liquid nitrogen production load of the main air separation system changes (the change in air compressor discharge volume exceeds ±10%), the central control module adjusts the load according to the air separation load change rate using a formula. Adjust the liquid nitrogen flow rate of each cold energy recovery module, where Vadjusted is the adjusted liquid nitrogen flow rate, Voriginal is the liquid nitrogen flow rate under the rated load of the air separation unit, Pactual is the actual liquid nitrogen production load of the air separation unit, and Prated is the rated liquid nitrogen production load of the air separation unit. This calculation allows the cold energy recovery flow rate to change synchronously with the liquid nitrogen production load of the air separation unit, maintaining the heat exchange efficiency of each level of cold energy recovery module.

[0035] Example 1: Application of a liquid nitrogen cooling capacity recovery system in a large-scale chemical industrial park This embodiment is applied to a large chemical industrial park. The air separation main system produces 50 cubic meters of liquid nitrogen per hour and needs to provide cooling capacity for the pre-cooling of cryogenic storage tanks, the cooling of freeze-drying equipment and the cooling of circulating water in the park. The deployment and operation details of each module of the system are as follows.

[0036] I. System Module Deployment and Parameter Configuration The main air separation system includes an air compressor, an air purifier, a distillation column, and a 100-cubic-meter liquid nitrogen storage tank. The air compressor compresses the feed air to 0.8 MPa, resulting in an air temperature of 50 degrees Celsius. The air purifier uses a molecular sieve adsorbent with a regeneration cycle of 10 hours, producing a purified impurity content of 0.5 ppm. The distillation column operates at -190 degrees Celsius, separating liquid nitrogen with a purity of 99.9995%. The liquid nitrogen storage tank has a vacuum insulation layer on its inner wall with an insulation coefficient of 0.0025 W / m Kelvin, and the tank pressure is maintained at 0.15 MPa.

[0037] The primary cooling capacity recovery module uses a shell-and-tube heat exchanger with a heat exchange area of ​​150 square meters. It has four tube passes and two shell passes. The tube passes are equipped with segmental baffles with a spacing of 200 mm and a notch ratio of 25%. The shell-side inlet of the heat exchanger is connected to the outlet pipeline of the liquid nitrogen storage tank, the tube-side inlet is connected to the outlet of the air compressor, and the tube-side outlet is connected to the inlet of the air purifier. The pipeline is equipped with an electric regulating valve and a flow sensor with a measurement accuracy of 0.3%.

[0038] The secondary cold energy recovery module is a plate heat exchanger with a heat exchange area of ​​120 square meters. The hot side inlet is connected to the liquid nitrogen outlet of the primary heat exchanger, and the hot side outlet is connected to the tertiary heat exchanger. The cold side inlet is connected to the pre-cooling pipeline of the low-temperature storage tank in the park. The flow rate of the pre-cooling pipeline is 20 cubic meters per hour. The cold side outlet is connected to the return gas end of the storage tank. Temperature sensors are installed at the inlet and outlet of the heat exchanger. The temperature sensor measurement range is -200 to 100 degrees Celsius.

[0039] The three-stage cold energy recovery module is a spiral tube heat exchanger with a spiral tube diameter of 30 mm and a heat exchange area of ​​80 square meters. The tube inlet is connected to the liquid nitrogen outlet of the second-stage heat exchanger, and the tube outlet is connected to the liquid nitrogen vaporizer. The shell side is fed with waste nitrogen gas generated by the distillation column. The waste nitrogen gas has a temperature of -160 degrees Celsius and a flow rate of 30 cubic meters per hour.

[0040] The cold storage buffer module includes a 30-cubic-meter phase change cold storage tank and a circulation pump with a flow rate of 8 cubic meters per hour. The tank is equipped with a 3-millimeter-thick stainless steel layered partition. The upper high-temperature zone is filled with a paraffin-based composite phase change material with a phase change temperature of -8 degrees Celsius to -5 degrees Celsius and a latent heat of phase change of 210 kJ per kilogram. The lower low-temperature zone is filled with a phase change material with a phase change temperature of -38 degrees Celsius to -35 degrees Celsius and a latent heat of phase change of 205 kJ per kilogram.

[0041] The auxiliary heat exchange module is a shell-and-tube heat exchanger with an inner tube diameter of 20 mm and an outer tube diameter of 32 mm. The inner tube is connected to the liquid nitrogen outlet of the three-stage heat exchanger, and the outer tube is connected to the park's circulating water network. The circulating water temperature is 30 degrees Celsius and the flow rate is 50 cubic meters per hour. A cold energy meter is installed on the pipeline, with a measurement range of 0-800 kilowatts.

[0042] The cold loss compensation module includes a 5-cubic-meter cold storage spherical tank and a cold replenishment pump with a flow rate of 3 cubic meters per hour. The spherical tank is filled with magnesium nitrate hexahydrate cold storage agent wrapped in porous media, and the cold loss rate is set at 4%.

[0043] In the safety protection module, the low-temperature alarm temperature sensor is arranged on the outer wall of each heat exchanger pipe, with an alarm threshold of -196 degrees Celsius; the safety valve opening pressure is set to 1.1 times the rated pressure of the equipment, the rated pressure of the heat exchanger is 1.0 MPa, and the opening pressure is 1.1 MPa; the hydrogen leak detector has a detection limit of 8 ppm, and the probes are arranged at the flange connection with a spacing of 1.5 meters.

[0044] The central control module uses a medium-sized PLC with 24 analog inputs, 16 analog outputs, a 12-inch touchscreen, supports historical data storage with a storage period of 2 years, and also supports remote monitoring.

[0045] II. Control Flow and Formula Application Air separation to liquid nitrogen production steps: Start the air compressor to compress the raw air to 0.8 MPa. The compressed air enters the air purifier to remove impurities. The purified air is sent to the distillation column for distillation at -190 degrees Celsius. The resulting liquid nitrogen is stored in the storage tank.

[0046] First-stage cold energy recovery steps: Open the electric regulating valve of the first-stage heat exchanger. Liquid nitrogen at -196 degrees Celsius and a flow rate of 50 cubic meters per hour enters the shell side. Compressed air from the air compressor outlet at 50 degrees Celsius and a flow rate of 80 cubic meters per hour enters the tube side. After heat exchange, the air temperature drops to 10 degrees Celsius and is sent to the purifier. The liquid nitrogen temperature rises to -175 degrees Celsius and enters the second-stage heat exchanger.

[0047] Secondary cold energy recovery steps: Open the electric regulating valve of the secondary heat exchanger, and liquid nitrogen at -175 degrees Celsius enters the hot side. The circulating fluid, which is pre-cooled by the cryogenic storage tank in the park to a temperature of 25 degrees Celsius and a flow rate of 20 cubic meters per hour, enters the cold side. After heat exchange, the temperature of the circulating fluid drops to 0 degrees Celsius, and the temperature of the liquid nitrogen rises to -155 degrees Celsius before entering the tertiary heat exchanger.

[0048] Three-stage cold energy recovery steps: Open the electric regulating valve of the three-stage heat exchanger, and liquid nitrogen at -155 degrees Celsius enters the tube side. The waste nitrogen gas from the distillation column at -160 degrees Celsius and a flow rate of 30 cubic meters per hour enters the shell side. After heat exchange, the temperature of the liquid nitrogen rises to -135 degrees Celsius and enters the auxiliary heat exchange module.

[0049] Auxiliary heat exchange and cooling capacity metering steps: Inside the auxiliary heat exchanger, liquid nitrogen at -135°C exchanges heat with circulating water at 30°C, and the circulating water is cooled to 12°C. The auxiliary heat exchange capacity is calculated using the following formula: Where Kauxiliary is 800 watts per square meter (Kelvin), Aauxiliary is 50 square meters, and ΔTauxiliary is 45 Kelvin, substituting these values, we get Qauxiliary = 800 × 50 × 45 = 1,800,000 watts = 1,800 kilowatts. The cooling capacity meter calculates the recovered cooling capacity using the following formula: Where ρ_liquid is 808 kg / m³, V_liquid is 50 m³, c_liquid is 2.04 kJ / kg Kelvin, and ΔT_liquid is 61 Kelvin (calculated from -196°C to -135°C). Substituting these values, we get Q_calculated = 808 × 50 × 2.04 × 61 = 5027376 kJ.

[0050] Cooling capacity distribution and variable load control steps: The central control module calculates the heat exchange efficiency at each stage. The efficiency of the first stage heat exchange is η1=0.88, the efficiency of the second stage heat exchange is η2=0.83, and the efficiency of the third stage heat exchange is η3=0.78. The total efficiency of each stage is Σηi=0.88+0.83+0.78=2.49. The total recoverable cooling capacity of liquid nitrogen is Q_total=5027376 kJ. This is calculated using the formula... Calculate the cooling capacity at each stage of distribution. Substituting the values, we get: First-stage cooling capacity Qdistribution1 = 5027376 × 0.88 / 2.49 ≈ 1780000 kJ; Second-stage cooling capacity Qdistribution2 = 5027376 × 0.83 / 2.49 ≈ 1680000 kJ; Third-stage cooling capacity Qdistribution3 = 5027376 × 0.78 / 2.49 ≈ 1570000 kJ. When the actual liquid nitrogen production load of the air separation unit drops to 40 cubic meters per hour, Pactual is 40 cubic meters per hour and Prated is 50 cubic meters per hour. Using the formula... Calculate the adjusted liquid nitrogen flow rate, where V_original is 50 cubic meters per hour. Substituting this into the equation, we get V_adjusted = 50 × 40 / 50 = 40 cubic meters per hour. Adjust the liquid nitrogen flow rate of each stage to 40 cubic meters per hour.

[0051] Cold storage buffer and safety protection procedures: When there is excess cooling capacity, i.e., the flow rate exceeds the demand by 15%, the circulation pump is turned on to introduce 10 cubic meters of liquid nitrogen per hour into the cold storage tank; when the cooling capacity is insufficient, the cooling capacity is released. When the leak detector detects a leakage concentration of 15 ppm at the flange, an audible and visual alarm is triggered, and the central control module closes the corresponding pipeline regulating valve; when the tank pressure rises to 1.1 MPa, the safety valve opens to release pressure, and the liquid nitrogen outlet valve is closed at the same time.

[0052] III. Data Representation and Interpretation Comparison of key performance indicators between traditional air separation cooling capacity recovery systems and this system in large chemical industrial parks: Comparison index Traditional system Inventive system Cold recovery efficiency 58% 92% Air separation system energy consumption reduction 8% 22% Cold distribution accuracy ±10% ±3% Leakage detection lower limit 60 ppm 8 ppm Load fluctuation adaptation range ±5% ±15% Data shows that traditional systems, due to their single-stage cold energy recovery and lack of precise control, achieve a cold energy recovery efficiency of only 58%, resulting in limited energy consumption reduction in air separation systems. Cold energy distribution relies on manual operation with an accuracy of only ±10%, and leak detection equipment has low sensitivity, with a detection limit of 60 ppm, making it difficult to detect early leaks. Furthermore, the system has a narrow load fluctuation adaptability range, only able to handle load changes of ±5%. This system, through three-stage cold energy recovery combined with auxiliary heat exchange, covers the entire temperature range of liquid nitrogen, increasing recovery efficiency to 92% and reducing air separation system energy consumption by 22%. Relying on cold energy metering and intelligent distribution algorithms, the cold energy distribution accuracy reaches ±3%, and the high-precision hydrogen leak detector lowers the detection limit to 8 ppm, enabling timely detection of early leaks. The load fluctuation adaptability range is widened to ±15%, stably handling changes in liquid nitrogen production load in air separation systems, and fully adapting to the diverse and highly stable cold energy requirements of large chemical industrial parks.

[0053] Example 2: Application of a liquid nitrogen cooling capacity recovery system in a small-to-medium-sized electronics factory This embodiment is applied to a small and medium-sized electronics factory. The air separation main system produces 10 cubic meters of liquid nitrogen per hour and needs to provide cooling for the freeze-drying equipment for electronic components and for cooling the circulating water in the workshop. The system modules are adjusted to adapt to the low-load scenario as follows.

[0054] I. System Module Deployment and Parameter Configuration The main air separation system includes a small air compressor, a compact air purifier, a small distillation column, and a 20 cubic meter liquid nitrogen storage tank. The air compressor outlet pressure is 0.6 MPa, and the compressed air temperature is 45 degrees Celsius; the air purifier has a regeneration cycle of 8 hours, and the purified impurity content is 0.8 ppm; the small distillation column operates at -185 degrees Celsius, and the separated liquid nitrogen has a purity of 99.999%; the inner wall of the liquid nitrogen storage tank is equipped with a vacuum insulation layer with an insulation coefficient of 0.003 W / m Kelvin, and the tank pressure is maintained at 0.12 MPa.

[0055] The first-stage cold energy recovery module is a small shell-and-tube heat exchanger with a heat exchange area of ​​30 square meters, 4 tube passes and 2 shell passes, with segmental baffles installed in the tube passes at a spacing of 150 mm; the second-stage cold energy recovery module is a plate heat exchanger with a heat exchange area of ​​20 square meters, connected to the freeze-drying equipment pipeline on the cold side, with a flow rate of 5 cubic meters per hour in the freeze-drying equipment pipeline; the third-stage cold energy recovery module is a spiral tube heat exchanger with a spiral tube diameter of 20 mm and a heat exchange area of ​​15 square meters, with waste nitrogen gas generated by the distillation column introduced into the shell side at a flow rate of 8 cubic meters per hour.

[0056] The cold storage buffer module includes a 5-cubic-meter phase change cold storage tank and a circulation pump with a flow rate of 2 cubic meters per hour. The tank is equipped with a 3-millimeter-thick stainless steel layered partition. The upper high-temperature zone is filled with a paraffin-based composite phase change material with a phase change temperature of -10 degrees Celsius to -5 degrees Celsius and a latent heat of phase change of 208 kJ per kilogram. The lower low-temperature zone is filled with a paraffin-based composite phase change material with a phase change temperature of -40 degrees Celsius to -35 degrees Celsius and a latent heat of phase change of 202 kJ per kilogram.

[0057] The auxiliary heat exchange module is a small shell-and-tube heat exchanger with an inner tube diameter of 15 mm and an outer tube diameter of 25 mm. The inner tube is connected to the liquid nitrogen outlet of the three-stage heat exchanger, and the outer tube is connected to the workshop circulating water pipeline with a workshop circulating water flow rate of 15 cubic meters per hour. The cold loss compensation module includes a 2 cubic meter cold storage spherical tank and a cold energy replenishment pump with a flow rate of 1 cubic meter per hour, with a cold loss rate set at 3%.

[0058] In the safety protection module, the hydrogen leak detector has a detection limit of 10 ppm, with detection probes spaced 1 meter apart at pipe flange connections; the safety valve opening pressure is set to 1.1 times the equipment's rated pressure, the heat exchanger's rated pressure is 0.8 MPa, and the safety valve opening pressure is 0.88 MPa. The central control module uses a small PLC with 8 analog inputs, 6 analog outputs, and a 7-inch touchscreen, supporting basic parameter settings and status display.

[0059] II. Control Flow and Formula Application Air separation to liquid nitrogen production steps: Start a small air compressor to compress the raw air to 0.6 MPa. The compressed air enters a compact air purifier to remove impurities. The purified air is sent to a small distillation column for distillation separation at -185 degrees Celsius. The resulting liquid nitrogen is stored in a 20 cubic meter liquid nitrogen storage tank.

[0060] The first to third stage cold energy recovery process is as follows: Liquid nitrogen at -196 degrees Celsius and a flow rate of 10 cubic meters per hour enters the shell side of the first-stage heat exchanger. Compressed air from the air compressor outlet at 45 degrees Celsius enters the tube side. After heat exchange, the air temperature drops to 8 degrees Celsius and is sent to the purifier. The liquid nitrogen temperature rises to -172 degrees Celsius and enters the second-stage heat exchanger. In the second-stage heat exchanger, the -172 degrees Celsius liquid nitrogen exchanges heat with the circulating fluid of the freeze-drying equipment. The initial temperature of the circulating fluid is 22 degrees Celsius, and after heat exchange, it drops to -3 degrees Celsius. The liquid nitrogen temperature rises to -150 degrees Celsius and enters the third-stage heat exchanger. In the third-stage heat exchanger, the -150 degrees Celsius liquid nitrogen exchanges heat with the shell-side waste nitrogen gas at -160 degrees Celsius. After heat exchange, the liquid nitrogen temperature rises to -132 degrees Celsius and enters the auxiliary heat exchange module.

[0061] Auxiliary heat exchange and cooling capacity metering steps: Inside the auxiliary heat exchanger, liquid nitrogen at -132 degrees Celsius exchanges heat with the workshop's circulating water. The initial temperature of the circulating water is 30 degrees Celsius, and it drops to 14 degrees Celsius after the heat exchange. The auxiliary heat exchange capacity is calculated using the following formula: Where Kauxiliary is 600 watts per square meter (Kelvin), Aauxiliary is 10 square meters, and ΔTauxiliary is 38 Kelvin, substituting these values, we get Qauxiliary = 600 × 10 × 38 = 228000 watts = 228 kilowatts. The cooling capacity meter calculates the recovered cooling capacity using the following formula: Where ρ_liquid is 808 kg / m³, V_liquid is 10 m³, c_liquid is 2.04 kJ / kg Kelvin, and ΔT_liquid is 64 Kelvin (calculated from -196°C to -132°C). Substituting these values, we get Q_calculated = 808 × 10 × 2.04 × 64 = 1054924.8 kJ.

[0062] Cooling capacity distribution and variable load control steps: The central control module calculates the heat exchange efficiency at each stage. The efficiency of the first stage heat exchange is η1=0.86, the efficiency of the second stage heat exchange is η2=0.82, and the efficiency of the third stage heat exchange is η3=0.77. The total efficiency of each stage is Σηi=0.86+0.82+0.77=2.45. The total recoverable cooling capacity of liquid nitrogen is Q_total=1054924.8 kJ. This is calculated using the formula... Calculate the cooling capacity at each stage. Substituting the values, we get: First-stage cooling capacity Qdistribution1 = 1054924.8 × 0.86 / 2.45 ≈ 374000 kJ; Second-stage cooling capacity Qdistribution2 = 1054924.8 × 0.82 / 2.45 ≈ 356000 kJ; Third-stage cooling capacity Qdistribution3 = 1054924.8 × 0.77 / 2.45 ≈ 325000 kJ. When the actual liquid nitrogen production load of the air separation unit drops to 8 cubic meters per hour, Pactual is 8 cubic meters per hour and Prated is 10 cubic meters per hour. Using the formula... Calculate the adjusted liquid nitrogen flow rate, where V_original is 10 cubic meters per hour. Substituting this into the equation, we get V_adjusted = 10 × 8 / 10 = 8 cubic meters per hour. Adjust the liquid nitrogen flow rate of each stage to 8 cubic meters per hour.

[0063] Safety and Cold Storage Procedures: When the hydrogen leak detector detects a leak concentration of 20 ppm at the flange, it triggers an audible and visual alarm, and the central control module immediately closes the corresponding pipeline's electric regulating valve. When the liquid nitrogen storage tank pressure rises to 0.132 MPa (1.1 times the rated pressure of 0.12 MPa), the safety valve opens to release pressure. When there is excess cooling capacity, i.e., the flow rate exceeds the demand by 12%, the circulation pump is started to introduce 1 cubic meter per hour of liquid nitrogen into the cold storage tank; when the cooling capacity is insufficient, the circulation pump is started to release the cooling capacity in the cold storage tank.

[0064] III. Data Representation and Interpretation Comparison of key performance indicators between traditional air separation cooling capacity recovery systems and this system in small and medium-sized electronics factories: Comparison index Traditional system Inventive system Cold recovery efficiency 55% 90% Equipment floor area 80 square meters 45 square meters Operation and maintenance manpower requirement 2 people / shift 1 person / shift Cold loss rate 6% 3% Fault response time 5 minutes 1 minute Data shows that traditional small and medium-sized air separation cold energy recovery systems suffer from low equipment integration and fragmented modules, resulting in a cold energy recovery efficiency of only 55%. These systems occupy an area of ​​80 square meters and require two people per shift for maintenance, leading to high labor costs. Furthermore, the lack of dedicated loss compensation measures during cold energy recovery results in a 6% cold energy loss rate, and fault response relies on manual inspection and operation, with an average response time of 5 minutes, making it easy for delayed handling to escalate the impact of faults. This system, through a compact modular design (such as small shell-and-tube heat exchangers and integrated PLCs), reduces the equipment footprint to 45 square meters, requiring only one person per shift for maintenance, significantly reducing space occupation and labor costs. The cold energy loss compensation module accurately compensates for pipeline heat dissipation losses, reducing the cold energy loss rate to 3% and increasing the cold energy recovery efficiency to 90%. The intelligent safety protection module enables automatic detection and rapid response to leaks and overpressure faults, shortening the fault response time to 1 minute and effectively preventing fault escalation. The overall design is tailored to the characteristics of small and medium-sized electronics factories with "limited space, limited manpower, and stable cold energy demand," significantly reducing maintenance costs and safety risks while ensuring cold energy recovery efficiency.

[0065] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system, characterized in that, include: The main air separation system consists of an air compressor, an air purifier, a distillation column, and a liquid nitrogen storage tank. The air compressor compresses the raw air to the designed working pressure. The purifier removes moisture, carbon dioxide, and hydrocarbons from the air using an adsorbent. The distillation column performs low-temperature distillation separation on the purified air to obtain liquid nitrogen. The liquid nitrogen storage tank stores the liquid nitrogen produced by the distillation column, and the inner wall of the storage tank is equipped with a vacuum insulation layer. Primary cold energy recovery module: includes a shell and tube heat exchanger. The shell inlet of the heat exchanger is connected to the outlet of the liquid nitrogen storage tank through a pipeline, the tube inlet is connected to the outlet of the air compressor, and the tube outlet is connected to the inlet of the air purifier. During the flow of liquid nitrogen in the shell side, it absorbs the heat of the air in the tube side, thereby pre-cooling the raw material air. Secondary cold energy recovery module: includes a plate heat exchanger. The hot side inlet of the heat exchanger is connected to the liquid nitrogen outlet of the primary cold energy recovery module through a pipeline. The hot side outlet is connected to the tertiary cold energy recovery module. The cold side inlet is connected to the external low-temperature equipment. The cold side outlet is connected to the return gas end of the external equipment. It uses liquid nitrogen that still contains cold energy after the primary heat exchange to provide cold energy to the external equipment. The three-stage cold energy recovery module includes a spiral tube heat exchanger. The heat exchanger inlet is connected to the liquid nitrogen outlet of the two-stage cold energy recovery module, and the outlet is connected to the liquid nitrogen vaporizer. The waste nitrogen gas generated by the distillation column is introduced into the shell side of the heat exchanger. The liquid nitrogen exchanges heat with the waste nitrogen gas in the tube side and shell side. After recovering the cold energy of the liquid nitrogen, it enters the vaporizer. Cold storage buffer module: It consists of a phase change cold storage tank and a circulation pump. The phase change cold storage tank is filled with paraffin-based composite phase change material. The inlet is connected to the liquid nitrogen pipeline of the first-level cold energy recovery module through a pipeline. The outlet is connected to the make-up cold pipeline of each cold energy recovery module through a circulation pump. When the liquid nitrogen cold energy is excessive, it stores the cold energy. When the cold energy is insufficient, it releases the cold energy. Cooling capacity control module: Includes various electric regulating valves and flow sensors. Each cooling capacity recovery module is equipped with an electric regulating valve and a flow sensor on its inlet and outlet pipelines. The flow sensor collects the fluid flow rate in the pipeline in real time, and the electric regulating valve adjusts its opening degree according to the flow data.

2. The low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system according to claim 1, characterized in that, Also includes: Central control module: Composed of PLC controller and touch screen. The PLC receives detection data from flow sensor, temperature sensor and pressure sensor, and outputs control signals to electric regulating valve, circulating pump and air separation main system equipment to realize full-process automated control of cold energy recovery. The touch screen supports parameter setting, status display and historical data query. Auxiliary heat exchange module: Includes a shell-and-tube heat exchanger. The inner tube of the heat exchanger connects to the liquid nitrogen outlet pipeline of the three-stage cold energy recovery module, and the outer tube connects to the plant's circulating water network. When there is still unused cold energy in the liquid nitrogen after the three-stage cold energy recovery, it is transferred to the circulating water through the heat exchanger to provide low-temperature circulating water for the plant. The heat exchange capacity of the auxiliary heat exchange module is calculated using the formula... The calculation is performed, where Qauxiliary is the heat exchange capacity of the auxiliary heat exchange module, Kauxiliary is the heat transfer coefficient of the auxiliary heat exchanger, Aauxiliary is the heat exchange area of ​​the auxiliary heat exchanger, and ΔTauxiliary is the logarithmic mean temperature difference between liquid nitrogen and circulating water in the auxiliary heat exchanger. The auxiliary heat exchange load is matched according to the cooling capacity required by the circulating water through the calculation.

3. The low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system according to claim 1, characterized in that, Also includes: Cold energy loss compensation module: includes a cold storage spherical tank and a cold energy replenishment pump. The cold storage spherical tank is filled with a medium containing cold storage refrigerant. The inlet is connected to the liquid nitrogen storage tank through the cold energy replenishment pump, and the outlet is connected to the cold energy loss compensation pipeline of each cold energy recovery module. When the cold energy recovery module experiences cold energy loss due to heat dissipation from the pipeline, the module replenishes the cold energy to the recovery pipeline. The cold storage capacity of the cold storage spherical tank is calculated and determined based on the hourly liquid nitrogen production and cold energy loss rate of the air separation system, maintaining the cold energy recovery efficiency within the set range.

4. The low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system according to claim 1, characterized in that, The shell-and-tube heat exchanger of the primary cooling capacity recovery module adopts a multi-pass design, with 4 passes in the tube side and 2 passes in the shell side. The tube side is equipped with segmental baffles with a baffle spacing of 1.5 times the inner diameter of the heat exchanger and a baffle gap ratio of 25%.

5. A low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system according to claim 1, characterized in that, The phase change cold storage tank of the cold storage buffer module is equipped with a stainless steel temperature stratification partition. The partition divides the inner cavity of the tank into an upper high-temperature zone and a lower low-temperature zone. The high-temperature zone is filled with paraffin-based composite phase change material with a phase change temperature of -10℃ to -5℃, while the lower low-temperature zone is filled with paraffin-based composite phase change material with a phase change temperature of -40℃ to -35℃. The stratified design releases the corresponding temperature range of cold energy according to different cold energy requirements.

6. A low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system according to claim 1, characterized in that, Also includes: The cooling capacity metering module includes a cooling capacity meter and a data acquisition unit. The cooling capacity meter is installed on the liquid nitrogen inlet pipeline of each cooling capacity recovery module. The data acquisition unit connects the cooling capacity meter to the central control module. The cooling capacity meter uses a formula... Calculate the recovered cold energy, where Qcalculated is the recovered cold energy of a single cold energy recovery module, ρliquid is the density of liquid nitrogen under standard operating conditions, Vliquid is the volume of liquid nitrogen flowing through the cold energy meter, and ΔTliquid is the temperature change of liquid nitrogen at the inlet and outlet of the cold energy recovery module.

7. A low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system according to claim 1, characterized in that, Also includes: Safety protection module: Includes a low-temperature alarm unit, an overpressure relief unit, and a leak detection unit. The temperature sensor of the low-temperature alarm unit is installed on the outer wall of the pipeline of each cold energy recovery module. When the temperature of the outer wall of the pipeline is lower than -196℃, an audible and visual alarm is triggered. The safety valve of the overpressure relief unit is installed in the gas phase space between the liquid nitrogen storage tank and each heat exchanger. The opening pressure of the safety valve is set to 1.1 times the rated pressure of the equipment. The leak detection unit uses a hydrogen leak detector. The detection probe is arranged at the pipeline flange connection. When the detected leakage concentration exceeds the set threshold, an audible and visual alarm is triggered. At the same time, the central control module automatically closes the electric regulating valve of the relevant pipeline.

8. A method for utilizing the low-energy-consumption air-separated liquid nitrogen cooling capacity cascade recovery and utilization system according to any one of claims 1-7, characterized in that, include: Air separation to liquid nitrogen production steps: Start the air compressor of the main air separation system to compress the raw air to the design working pressure. The compressed air enters the air purifier to remove impurities. The purified air is sent to the distillation column for low-temperature distillation. The separated liquid nitrogen is stored in the liquid nitrogen storage tank. Primary cold energy recovery steps: Open the electric regulating valve of the primary cold energy recovery module, and the liquid nitrogen in the liquid nitrogen storage tank enters the shell side of the shell and tube heat exchanger. At the same time, the compressed air from the air compressor outlet enters the tube side. The liquid nitrogen and the compressed air exchange heat. The pre-cooled compressed air is sent to the air purifier, and the liquid nitrogen after heat exchange enters the secondary cold energy recovery module. Secondary cold energy recovery steps: Open the electric regulating valve of the secondary cold energy recovery module. Liquid nitrogen after primary heat exchange enters the hot side of the plate heat exchanger, and the circulating fluid of the external low-temperature equipment enters the cold side. Liquid nitrogen exchanges heat with the circulating fluid to provide cooling for the external equipment. Liquid nitrogen after heat exchange enters the tertiary cold energy recovery module. Three-stage cold energy recovery steps: Open the electric regulating valve of the three-stage cold energy recovery module. The liquid nitrogen after the second-stage heat exchange enters the tube side of the spiral tube heat exchanger, and the waste nitrogen gas produced by the distillation column enters the shell side. The liquid nitrogen and waste nitrogen gas exchange heat to release cold energy. The liquid nitrogen after heat exchange enters the liquid nitrogen vaporizer. Cold storage buffering steps: When the central control module detects an excess of liquid nitrogen cooling capacity, it starts the circulation pump of the cold storage buffer module to introduce some liquid nitrogen into the phase change cold storage tank to store the cooling capacity; when the cooling capacity is insufficient, it starts the circulation pump to transport the cooling capacity in the cold storage tank to each recovery module. Cooling capacity control steps: The central control module adjusts the opening of the corresponding electric regulating valve based on the flow sensor and temperature sensor data of each cooling capacity recovery module to control the liquid nitrogen flow rate and make the heat exchange of each level of recovery module match the demand. System shutdown procedure: After the main air separation system stops producing liquid nitrogen, close the electric regulating valves of each cold energy recovery module, open the liquid nitrogen vaporizer to vaporize and discharge the residual liquid nitrogen in the pipeline, and then introduce dry nitrogen to purge each heat exchanger and pipeline for ≥30 minutes to prevent residual moisture in the pipeline from freezing and clogging.

9. A method for the cascaded recovery and utilization of low-energy-consumption air-separated liquid nitrogen cooling capacity according to claim 8, characterized in that, It also includes a cooling capacity tiered allocation optimization process, where the central control module determines the cooling capacity based on the heat exchange efficiency of each cooling capacity recovery module using a formula. Calculate the allocated cooling capacity of each module, where Q is the allocated cooling capacity of the first-level cooling capacity recovery module, ηi is the heat exchange efficiency of the cooling capacity recovery module, and Σηi is the sum of the heat exchange efficiencies of all cooling capacity recovery modules. The cooling capacity is allocated according to the heat exchange efficiency, with modules with higher heat exchange efficiency receiving more cooling capacity.

10. A method for the cascade recovery and utilization of low-energy-consumption air-separated liquid nitrogen cooling capacity according to claim 8, characterized in that, It also includes a variable load cooling capacity control process. When the liquid nitrogen production load of the main air separation system changes, the central control module adjusts the load according to the air separation load change rate using a formula. Adjust the liquid nitrogen flow rate of each cold energy recovery module, where Vadjusted is the adjusted liquid nitrogen flow rate, Voriginal is the liquid nitrogen flow rate under the rated load of the air separation, Pactual is the actual liquid nitrogen production load of the air separation, and Prated is the rated liquid nitrogen production load of the air separation. Through calculation, the cold energy recovery flow rate changes synchronously with the liquid nitrogen production load of the air separation, so as to maintain the heat exchange efficiency of each cold energy recovery module.

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