Process for preparing nitrogen and oxygen by high and low temperature dual-expansion air separation

Abstract

The invention relates to the technical field of gas separation, solves the problem of low separation efficiency of a process for preparing nitrogen and oxygen through air separation, and particularly discloses a process for preparing nitrogen and oxygen through high and low temperature double expansion air separation, which comprises the following steps: S100, filtering and compressing air, inputting the air into an air filter, inputting the air into an air compressor, cooling the air, and then pre-cooling the air in a pre-cooling system; s200, air cooling and purification, S300, high and low temperature double expansion: introducing the air into a high-temperature expansion machine, so that the air is adiabatically expanded, the distance between nitrogen and oxygen molecules in the air is increased, and primary separation is performed; one part of the air subjected to high-temperature expansion enters a low-temperature expansion machine for secondary expansion, and a low-temperature expansion process is adopted for further adiabatic expansion in a low-temperature environment; s400, air pressurization is carried out; and S500, performing air rectification, and performing component separation on the air according to the difference of boiling points of all components to obtain nitrogen-oxygen-argon products. The device is used for separating nitrogen, oxygen and argon components from air and obtaining a liquid product, the separation efficiency is high, and the product purity is high.

Description

Technical Field

[0001] The invention relates to the technical field of gas separation, in particular to a process for producing nitrogen and oxygen by high-low temperature double expansion air separation. Background Art

[0002] In the natural gas production process, pure oxygen (≥99.6%) and water are sometimes needed as raw materials and oxidants under a certain pressure to generate gases such as hydrogen, carbon monoxide and methane; at the same time, pure nitrogen (≥99.99%) is also needed as a protective gas, which requires a process that can prepare high-purity nitrogen and oxygen. High-purity nitrogen and oxygen products can be obtained by separating air using the cryogenic separation method. The principle is to use the difference in boiling points of different gases to cool and liquefy the mixed gas under high pressure, thereby achieving the effect of separating the mixed gas. This process is widely used to separate oxygen and other components in the air, as well as to separate cracking gas in the petrochemical industry. The main components of the system equipment to realize this process are: air compression system, impurity purification and heat exchange system, refrigeration system and liquefaction distillation system, etc. The purified pressurized or pressurized raw gas is cooled step by step to the condensation temperature of each separation component for condensation, or the raw gas is pressurized, cooled, liquefied, and then distilled for separation. Single-component gaseous and liquid products such as air, natural gas, coke oven gas, water gas, synthetic ammonia relaxation gas and various cracking gases can be separated from multi-component raw gas.

[0003] In the prior art, when air is separated by distillation, the gas is simply liquefied by cooling and then fractionated. This process has the problems of low separation efficiency and incomplete separation. Summary of the invention

[0004] In order to solve the problems of low separation efficiency and incomplete separation in the prior art of air separation nitrogen and oxygen production, the present invention provides a high-low temperature double expansion air separation nitrogen and oxygen production process.

[0005] The technical solution adopted by the present invention is:

[0006] A process for producing nitrogen and oxygen by high-low temperature double expansion air separation comprises the following steps:

[0007] S100, air filtration and compression, inputting air into the air filter, removing particulate impurities in the air filter, and then inputting the air into the air compressor, and after multi-stage compression and cooling in the interstage cooler, the air enters the precooling system for precooling;

[0008] S200, air cooling and purification, after the air is cooled by heat and mass exchange with water in the air cooling tower, it enters the molecular sieve adsorber for purification. The molecular sieve adsorber is used to remove water, carbon dioxide and hydrocarbons in the air;

[0009] S300, high and low temperature dual expansion, introduces air into the high temperature expander to expand the air adiabatically, reduce the internal pressure, increase the distance between nitrogen and oxygen molecules in the air, and achieve preliminary separation; a part of the air after high temperature expansion enters the low temperature expander for secondary expansion, and adopts the low temperature expansion process to further adiabatically expand in a low temperature environment;

[0010] S400, air boosting, air enters the boosting system, and the air pressure is increased to meet the conditions for entering the distillation tower operation;

[0011] S500, air distillation. Air enters the distillation tower for distillation. In the distillation tower, the air is separated into components through gas-liquid contact, heat and mass transfer according to the difference in boiling points of the components, and the liquid components are collected from various parts of the distillation tower.

[0012] Furthermore, in S100, the pre-cooling system is entered to cool the temperature to 10°C-18°C.

[0013] Further, in S200, the temperature is lowered to 8°C-12°C in a direct contact air cooling tower.

[0014] Furthermore, in S200, two molecular sieve adsorbers are provided and used alternately, and when one molecular sieve adsorber is used to adsorb impurities, the other molecular sieve adsorber is heated by an electric heater to regenerate the molecular sieve.

[0015] Furthermore, during the high-temperature expansion process in S300, the temperature is controlled at 320°C-400°C, and the pressure is controlled at 350kPa-480kPa; during the low-temperature expansion process in S300, the temperature is controlled at 100°C-150°C, and the pressure is controlled at 50kPa-100kPa.

[0016] Furthermore, the air after two expansions in S400 is controlled at 650-750 kPa through supercharging.

[0017] Furthermore, the air in S500 first enters the buffer tank before entering the distillation tower. After being evenly mixed and the flow rate is stabilized in the buffer tank, the air enters the feed section of the distillation tower by adjusting the pressure through the throttle valve. The feed section of the distillation tower is provided with a temperature sensor and a pressure sensor to monitor the temperature and pressure of the air in real time.

[0018] Furthermore, the distillation process of air in S500 specifically includes:

[0019] S501, air enters the lower part of the distillation tower and exchanges heat with the reflux liquid rising from the bottom of the tower. The distillation tower is filled with fillers, and the oxygen, nitrogen and argon components in the air transfer mass and heat on the surface of the fillers; in the lower tower section, as the height increases, nitrogen is continuously enriched and moves upward, while oxygen remains in the lower liquid phase or is enriched in a specific area in the liquid phase; part of the liquid air is extracted from the lower tower section and sent to the reboiler for heating, and the heated liquid phase is partially vaporized and returned to the lower part of the distillation tower to form reflux liquid to maintain the distillation balance in the tower;

[0020] S502, when the air enters the fractionation section of the distillation tower upward, the heating device locally heats the air to adjust the relative volatility of different components; in the fractionation section, nitrogen is further enriched and moves upward, and components other than nitrogen and oxygen gradually begin to separate in this area, and part of the liquid air containing argon and other impurities is diverted to the argon purification branch and is treated separately in subsequent processes;

[0021] S503. An enriched nitrogen product is obtained at the top of the distillation tower. The nitrogen flows out from the top of the tower through a pipeline, enters a nitrogen storage tank after being cooled by a cooler, or is directly supplied to a use link. Liquid air enriched with oxygen is obtained at the bottom of the distillation tower, and is supplied for use after quality inspection and treatment.

[0022] By continuously adjusting the heating power of the reboiler, the proportion of the reflux liquid, and the pressure and temperature parameters of each section of the distillation tower, the multiple distillation equilibrium state in the distillation tower is maintained. The specific adjustment method is as follows:

[0023] The method for adjusting the heating power of the reboiler is to adjust it according to the purity requirements of the bottom product and the composition of the top distillate. The formula is:

[0024]

[0025] Where Q is the heating power, m is the mass flow rate of the liquid at the bottom of the tower, and ΔH vap is the molar latent heat of vaporization of the liquid, t is the time;

[0026] The method for adjusting the proportion of reflux liquid is to monitor the purity of the top product and the purity of the bottom product, and change the proportion of reflux liquid by adjusting the flow rate of the reflux pump or the opening of the reflux valve. The formula is:

[0027]

[0028] Among them, R is the reflux ratio, L is the reflux liquid volume, and D is the top extraction volume;

[0029] The method of adjusting the distillation tower pressure is to change the tower pressure by adjusting the cooling water volume or pressure control valve of the top condenser. The formula is:

[0030] ΔP=P top -P bottom

[0031] Where ΔP is the tower pressure difference, P top is the tower top pressure, P bottom is the tower bottom pressure;

[0032] The method for adjusting the tower temperature is to change the tower temperature by adjusting the heating power of the reboiler and the cooling effect of the top condenser.

[0033] The beneficial effects of the present invention are:

[0034] The nitrogen and oxygen production process of the present invention adopts a high-low temperature dual expansion process. The high-temperature expansion utilizes the principle that the temperature of the gas decreases during the expansion process, that is, the gas temperature decreases during the adiabatic expansion process, causing the air to partially expand and reduce the temperature, thereby achieving preliminary separation of nitrogen and oxygen. Through high-temperature expansion, the air is initially cooled, causing the nitrogen and oxygen to begin to separate, while providing more favorable separation conditions for subsequent low-temperature expansion, thereby improving the overall separation efficiency; low-temperature expansion further reduces the gas pressure, and under low pressure, the difference in boiling points of the components in the air increases, making the separation process more efficient, further improving the separation efficiency, making the separation of nitrogen and oxygen more thorough, and improving the purity and yield of the product.

[0035] The present invention maintains the multiple distillation equilibrium state in the distillation tower by reasonably adjusting the reboiler heating power and the reflux liquid ratio during the distillation process. The precise control of the reboiler heating power can ensure that the vaporization phenomenon in the distillation process proceeds as expected, which can provide sufficient heat for the separation process and will not cause unnecessary consumption of energy due to overheating. The reasonable adjustment of the reflux liquid ratio ensures the separation effect while avoiding energy waste caused by too much or too little reflux. Through the precise control of key parameters, energy is used more efficiently in the entire distillation process. During the entire process operation, the present invention continuously monitors the parameters of each link, and makes timely adjustments based on the monitoring results, thereby ensuring the stability of the entire process, improving the consistency of product quality, effectively reducing the product quality fluctuations that may be caused by process fluctuations, and meeting the strict requirements of different users for the stable quality of nitrogen and oxygen products. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0037] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0038] Based on the high and low temperature double expansion air separation process of the present invention, please refer to Figure 1, and its specific implementation is as follows:

[0039] First, prepare and initially set up the equipment; for S100, select appropriate air filters for the air filtration and compression system to ensure that particulate impurities in the air can be effectively removed. Select a multi-stage air compressor of appropriate specifications and set up an interstage cooler so that the air after each stage of compression can be effectively cooled. The precooling system should have sufficient cooling capacity to precool the air to 10℃-18℃.

[0040] For S200, the air cooling and purification system uses a direct contact air cooling tower, and the parameters such as the cooling medium flow rate are set to cool the air to 8℃-12℃. Two molecular sieve adsorbers are installed, equipped with electric heaters for molecular sieve regeneration. A control system is set to enable the two molecular sieve adsorbers to work alternately.

[0041] For S300, the high and low temperature dual expansion system uses suitable high temperature expanders and low temperature expanders. Temperature and pressure monitoring devices are set at the inlet and outlet of the high temperature expander to control the temperature at 320℃-400℃ and the pressure at 350kPa-480kPa; corresponding monitoring devices are set at the inlet and outlet of the low temperature expander to control the temperature at 100℃-150℃ and the pressure at 50kPa-100kPa.

[0042] For S400, the air boost system is equipped with a boost device that can boost the air after two expansions to 650-750kPa.

[0043] For S500, the distillation system builds a distillation tower, sets temperature sensors and pressure sensors in the feed section, fills the tower with appropriate fillers, and selects a suitable reboiler to be installed at the bottom of the distillation tower.

[0044] After the equipment preparation and initial setup are completed, the operation during operation is adjusted; the operation of the distillation column section is as follows:

[0045] In the S501 stage: initially, the heating power of the reboiler is set according to theoretical calculations and experience, so that the air can obtain sufficient heat exchange with the reflux liquid in the lower part of the distillation tower.

[0046] The heating power of the reboiler is adjusted according to the detected temperature and pressure of the lower tower section, as well as the flow rate and quality requirements of the extracted liquid air. For example, if the temperature of the lower tower section is too low, the heating power is appropriately increased to increase the vaporization amount of the liquid phase, thereby increasing the amount of reflux liquid to increase the enrichment of nitrogen.

[0047] Observe the proportion of reflux liquid and make fine adjustments according to the requirements of distillation balance. If the initial separation effect of nitrogen and oxygen is not ideal, the proportion of reflux liquid can be appropriately increased.

[0048] In the S502 stage: according to the theoretical values ​​of relative volatility of different components and the gas and liquid composition of the fractionation section actually detected, the local heating power of the heating device is adjusted. For example, if the separation effect of nitrogen and other components such as argon is not good, the appropriate heating power adjustment value is determined through experiments and analysis to increase the relative volatility of nitrogen and other components and promote further enrichment of nitrogen.

[0049] For the liquid air diverted to the argon purification branch, its flow rate and composition are monitored and adjusted according to the processing capacity of the argon purification branch.

[0050] In the S503 stage: according to the purity requirements of the nitrogen product, adjust the cooling efficiency of the cooler at the top of the distillation tower. If the nitrogen purity is not enough, it may be necessary to reduce the cooling efficiency to separate more impurity gases from the nitrogen.

[0051] For the liquid air enriched with oxygen at the bottom of the distillation tower, adjust the relevant parameters in the distillation process according to the oxygen quality test results. If the oxygen purity does not meet the standard, it may be necessary to adjust the parameters such as the reflux liquid ratio or heating power of the lower tower section. Specifically, the adjustment method can be as follows:

[0052] The method for adjusting the heating power of the reboiler is to adjust it according to the purity requirements of the bottom product and the composition of the top distillate. The formula is:

[0053]

[0054] Where Q is the heating power, m is the mass flow rate of the liquid at the bottom of the tower, and ΔH vap is the molar latent heat of vaporization of the liquid, t is the time;

[0055] The method for adjusting the proportion of reflux liquid is to monitor the purity of the top product and the purity of the bottom product, and change the proportion of reflux liquid by adjusting the flow rate of the reflux pump or the opening of the reflux valve. The formula is:

[0056]

[0057] Among them, R is the reflux ratio, L is the reflux liquid volume, and D is the top extraction volume;

[0058] The method of adjusting the distillation tower pressure is to change the tower pressure by adjusting the cooling water volume or pressure control valve of the top condenser. The formula is:

[0059] ΔP=P top -P bottom

[0060] Where ΔP is the tower pressure difference, P top is the tower top pressure, P bottom is the tower bottom pressure;

[0061] The method for adjusting the tower temperature is to change the tower temperature by adjusting the heating power of the reboiler and the cooling effect of the top condenser.

[0062] The overall system coordination adjustment is as follows:

[0063] Continuously monitor the pressure and temperature parameters of each link in the whole system. For example, if the air temperature at the outlet of the pre-cooling system deviates from the range of 10℃-18℃, check the operation of the cooling equipment and make adjustments.

[0064] For molecular sieve adsorbers, during the process of one adsorbing impurities, the heating temperature and time of the electric heater on the other molecular sieve adsorber are controlled to ensure that the molecular sieve is fully regenerated to ensure the next round of adsorption effect.

[0065] Data recording and optimization:

[0066] Establish a complete monitoring and data recording system to record various parameters at different operation stages and under different operating conditions, such as temperature, pressure, flow, product quality, etc. of each equipment.

[0067] The data is analyzed regularly and the operating parameters are further optimized based on the analysis results to continuously improve the efficiency and product purity of the high and low temperature double expansion air separation process for producing nitrogen, oxygen and argon.

[0068] The process of the present invention increases the distance between air molecules through high-temperature expansion, preliminarily separates nitrogen and oxygen, and creates more favorable conditions for subsequent low-temperature expansion and distillation. Low-temperature expansion further expands adiabatically in a low-temperature environment, more accurately adjusts the motion state of molecules, and enhances the separation effect. Compared with the single expansion method, the double expansion process can significantly improve the purity of nitrogen and oxygen, and enrich nitrogen and oxygen to a certain extent before distillation, reducing the energy consumption and processing volume in the distillation process. The high-temperature expander uses the heat energy of the air after compression to perform adiabatic expansion, and works externally during the expansion process, and part of the energy can be recycled. The low-temperature expander works at a lower temperature, and requires less energy than performing the same separation task at a higher temperature. The entire double expansion process reduces additional refrigeration energy consumption, and can save 15%-20% energy compared to the traditional air separation process. Due to the reduction in energy consumption, the energy consumed in the entire process is reduced, and the greenhouse gas emissions generated by energy production are correspondingly reduced, which is more environmentally friendly.

[0069] The present invention maintains the multiple distillation equilibrium state in the distillation tower by reasonably adjusting the heating power and reflux ratio of the reboiler. Accurate control enables efficient use of energy in the distillation process, avoids energy waste caused by excessive heating or reflux, and reduces the energy consumption of the distillation process. The process continuously monitors and adjusts the parameters of each link to make the entire process more stable. Stable operating conditions ensure the consistency of product quality, reduce product quality fluctuations caused by process fluctuations, and meet the requirements of different users for stable quality of nitrogen, oxygen and argon products.

[0070] Based on the above implementation, the following is one specific embodiment:

[0071] This embodiment further sets and explains the specific operation process and specific process parameters in the above process, specifically:

[0072] The air filter of the air filtration and compression system in S100 adopts a high-efficiency fiber filter with a filtration accuracy of 0.1μm, which can effectively remove dust, sand and other particulate impurities in the air.

[0073] The air compressor is a three-stage piston air compressor, and the compression ratios of each stage are 3:1, 2.5:1 and 2:1 respectively. The interstage cooler is cooled by circulating water, and the flow rate of cooling water is 10-20m / s depending on the operating status of the compressor. 3 The precooling system is an air-cooled precooler, which precools the air to 10℃-18℃ by adjusting the speed of the fan.

[0074] The direct contact air cooling tower of the air cooling and purification system in S200 adopts a tower structure. The cooling medium is cold water with a cold water flow rate of 20-30m 3 / h. The adsorbent of the molecular sieve adsorber is a mixture of 4A molecular sieve and 5A molecular sieve. The adsorption capacity of the two molecular sieve adsorbers is 100kg. The power of the electric heater is 50kW, and the heating temperature can reach 300℃.

[0075] The high-temperature expander of the S300 medium-high-low temperature dual expansion system adopts a radial turbine expander with a rotation speed of 30,000-40,000 rpm. The low-temperature expander is a reciprocating expander with a stroke of 200-300 mm. The temperature sensor accuracy at the inlet and outlet of the high-temperature expander is ±1°C, and the pressure sensor accuracy is ±0.1 kPa; the sensor accuracy of the inlet and outlet of the low-temperature expander is the same.

[0076] The air boost system in the S400 is a centrifugal supercharger with a boost ratio of 1.5-2.

[0077] The distillation tower of the distillation system in S500 is a packed tower, and the packing is a structured packing with a specific surface area of ​​300-500m2 / m 3 The reboiler is a shell and tube reboiler, the heating medium is steam, and the steam pressure is 0.5-1MPa. The volume of the buffer tank is 5-10m 3 The diameter of the throttle valve is selected according to the flow requirements.

[0078] Air filtration and compression process: the original air is at 10000m 3 / h flow enters the air filter, stays in the filter for 5-10s to remove particulate impurities, and then enters the air compressor. After three-stage compression, the air pressure increases from 101.325kPa to 3-4bar, and the temperature rises to 150-200℃. After being cooled by the interstage cooler, it enters the pre-cooling system and will be cooled to about 12℃.

[0079] Air cooling and purification process: The cooled air enters the air cooling tower, where it exchanges heat and mass with cold water and cools down to 10°C. It then enters the molecular sieve adsorber, one of which adsorbs water, carbon dioxide and hydrocarbons for 1-2 hours. The saturated molecular sieve adsorber is heated to 300°C by an electric heater and kept for 2-3 hours for regeneration, and then the two adsorbers work alternately.

[0080] High and low temperature double expansion process: The purified air enters the high temperature expander, the temperature is controlled at 350℃, the pressure is reduced from 3bar to about 300kPa, the air expands adiabatically, the distance between nitrogen and oxygen molecules increases, and the initial separation is achieved. A part of the air (about 60%) enters the low temperature expander, the temperature is 120℃, the pressure is reduced from 300kPa to 80kPa, and secondary expansion is carried out.

[0081] Air boosting process: After two expansions, the air enters the boosting system and is pressurized to 700kPa.

[0082] Air distillation process: The pressurized air enters the buffer tank, stays in the tank for 3-5 minutes to be evenly mixed and stabilize the flow, and then enters the feed section of the distillation tower by adjusting the pressure through the throttle valve. In the S501 stage, the heating power of the reboiler is initially set to 100kW, and the heating power is gradually increased to 120kW according to the temperature detected by the temperature sensor of the lower tower section (such as the temperature is lower than the expected 150℃). At the same time, observe the proportion of reflux liquid. If the proportion of reflux liquid is too low (such as lower than the normal range of 30%-40%), adjust the flow of the reflux pump to increase the proportion of reflux liquid.

[0083] In the S502 stage, when it is found that the separation effect of nitrogen and argon in the distillation section is not good, the local heating power of the heating device is increased from 50 kW to 60 kW.

[0084] In the S503 stage, if the purity of the nitrogen product is 95%, which is lower than the target of 99.9%, the cooling efficiency of the cooler at the top of the distillation tower is appropriately reduced to separate more impurity gases from the nitrogen. After adjustment, the nitrogen purity meets the requirements. For the liquid air at the bottom of the distillation tower, if the oxygen purity is insufficient, the reflux liquid ratio of the lower tower section is adjusted from 35% to 40%. After multiple adjustments, the oxygen purity meets the requirements.

[0085] In order to continuously optimize and improve the entire production process, data is recorded and parameters are optimized and adjusted throughout the entire production stage:

[0086] The key parameters of each device, such as temperature, pressure, flow, etc., as well as product quality data, such as nitrogen purity, oxygen purity, etc., are recorded every 15 minutes.

[0087] Analyze the data weekly and adjust the operating parameters based on the analysis results. For example, in winter when the ambient temperature is low, it is found that the pre-cooling effect of the pre-cooling system is better than in summer. At this time, the exhaust pressure of the air compressor can be appropriately increased to improve the overall production efficiency.

[0088] As a preferred embodiment, the process-related condition parameters of the present invention can also be set as follows:

[0089] Example 1

[0090] Temperature [℃]: -4.167~-96.38;

[0091] Pressure [bar]: 29.6~5.53;

[0092] Molar flow rate [Nm 3 / h(gas)]:1.420×10 4 ;

[0093] Mass flow rate [kg / h]: 1.835×10 4 ;

[0094] Standard ideal liquid flow rate [m 3 / h]: 21.1;

[0095] Molar enthalpy [kJ / kg mol]: -1076~-3593;

[0096] Molar entropy [kJ / kgmole-℃]: 124.2~126.4;

[0097] Heat flow [kW]: -189.4~-632.3.

[0098] Example 2

[0099] Temperature [℃]: -94.75~-173.7;

[0100] Pressure [bar]: 60.86~5.6;

[0101] Molar flow rate [Nm 3 / h(gas)]:1.920×10 4 ;

[0102] Mass flow rate [kg / h]: 2.481×10 4 ;

[0103] Standard ideal liquid flow rate [m 3 / h]: 28.53;

[0104] Molar enthalpy [kJ / kg mol]: -4667~-6347;

[0105] Molar entropy [kJ / kgmole-℃]: 102~104.8;

[0106] Heat flow [kW]: -1111 ~ -1510.

[0107] The above-mentioned embodiments only express the specific implementation of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A process for producing nitrogen and oxygen by high and low temperature double expansion air separation, characterized in that: The steps include: S100, air filtration and compression, inputting air into the air filter, removing particulate impurities in the air filter, and then inputting the air into the air compressor, and after multi-stage compression and cooling in the interstage cooler, the air enters the precooling system for precooling; S200, air cooling and purification, after the air is cooled by heat and mass exchange with water in the air cooling tower, it enters the molecular sieve adsorber for purification. The molecular sieve adsorber is used to remove water, carbon dioxide and hydrocarbons in the air; S300, high and low temperature dual expansion, introduces air into the high temperature expander to expand the air adiabatically, reduce the internal pressure, increase the distance between nitrogen and oxygen molecules in the air, and achieve preliminary separation; a part of the air after high temperature expansion enters the low temperature expander for secondary expansion, and adopts the low temperature expansion process to further adiabatically expand in a low temperature environment; S400, air boosting, air enters the boosting system, and the air pressure is increased to meet the conditions for entering the distillation tower operation; S500, air distillation. Air enters the distillation tower for distillation. In the distillation tower, the air is separated into components through gas-liquid contact, heat and mass transfer according to the difference in boiling points of the components, and the liquid components are collected from various parts of the distillation tower.

2. A process for producing nitrogen and oxygen by high and low temperature double expansion air separation according to claim 1, characterized in that: In S100, the pre-cooling system is entered and cooled to 10°C-18°C.

3. The process for producing nitrogen and oxygen by high-low temperature double expansion air separation according to claim 1, characterized in that: In S200, the temperature is reduced to 8°C-12°C in a direct contact air cooling tower.

4. The process for producing nitrogen and oxygen by high-low temperature double expansion air separation according to claim 1, characterized in that: In S200, two molecular sieve adsorbers are provided and used alternately, and when one molecular sieve adsorber is used to adsorb impurities, the other molecular sieve adsorber is heated by an electric heater to regenerate the molecular sieve.

5. The process for producing nitrogen and oxygen by high-low temperature double expansion air separation according to claim 1, characterized in that: During the high-temperature expansion process in S300, the temperature is controlled at 320°C-400°C, and the pressure is controlled at 350kPa-480kPa; during the low-temperature expansion process in S300, the temperature is controlled at 100°C-150°C, and the pressure is controlled at 50kPa-100kPa.

6. The process for producing nitrogen and oxygen by high-low temperature double expansion air separation according to claim 1, characterized in that: The air in the S400 that has undergone two expansions is pressurized and controlled at 650-750 kPa.

7. The process for producing nitrogen and oxygen by high-low temperature double expansion air separation according to claim 1, characterized in that: In S500, the air first enters the buffer tank before entering the distillation tower. After being evenly mixed and the flow rate stabilized in the buffer tank, the air enters the feed section of the distillation tower by adjusting the pressure through the throttle valve. The feed section of the distillation tower is equipped with a temperature sensor and a pressure sensor to monitor the temperature and pressure of the air in real time.

8. The process for producing nitrogen and oxygen by high-low temperature double expansion air separation according to claim 1, characterized in that: The air distillation process in S500 specifically includes: S501, air enters the lower part of the distillation tower and exchanges heat with the reflux liquid rising from the bottom of the tower. The distillation tower is filled with fillers, and the oxygen, nitrogen and argon components in the air transfer mass and heat on the surface of the fillers; in the lower tower section, as the height increases, nitrogen is continuously enriched and moves upward, while oxygen remains in the lower liquid phase or is enriched in a specific area in the liquid phase; part of the liquid air is extracted from the lower tower section and sent to the reboiler for heating, and the heated liquid phase is partially vaporized and returned to the lower part of the distillation tower to form reflux liquid to maintain the distillation balance in the tower; S502, when the air enters the fractionation section of the distillation tower upward, the heating device locally heats the air to adjust the relative volatility of different components; in the fractionation section, nitrogen is further enriched and moves upward, and components other than nitrogen and oxygen gradually begin to separate in this area, and part of the liquid air containing argon and other impurities is diverted to the argon purification branch and is treated separately in subsequent processes; S503. An enriched nitrogen product is obtained at the top of the distillation tower. The nitrogen flows out from the top of the tower through a pipeline, enters a nitrogen storage tank after being cooled by a cooler, or is directly supplied to a use link. Liquid air enriched with oxygen is obtained at the bottom of the distillation tower, and is supplied for use after quality inspection and treatment.

9. A process for producing nitrogen and oxygen by high and low temperature double expansion air separation according to claim 8, characterized in that: By continuously adjusting the heating power of the reboiler, the proportion of the reflux liquid, and the pressure and temperature parameters of each section of the distillation tower, the multiple distillation equilibrium state in the distillation tower is maintained. The specific adjustment method is as follows: The method for adjusting the heating power of the reboiler is to adjust it according to the purity requirements of the bottom product and the composition of the top distillate. The formula is: Where Q is the heating power, m is the mass flow rate of the liquid at the bottom of the tower, and ΔH vap is the molar latent heat of vaporization of the liquid, t is the time; The method for adjusting the proportion of reflux liquid is to monitor the purity of the top product and the purity of the bottom product, and change the proportion of reflux liquid by adjusting the flow rate of the reflux pump or the opening of the reflux valve. The formula is: Among them, R is the reflux ratio, L is the reflux liquid volume, and D is the top extraction volume; The method for adjusting the distillation tower pressure is to change the tower pressure by adjusting the cooling water volume or pressure control valve of the top condenser. The formula is: ΔP=P top -P bottom ; Where ΔP is the tower pressure difference, P top is the tower top pressure, P bottom is the tower bottom pressure; The method for adjusting the tower temperature is to change the tower temperature by adjusting the heating power of the reboiler and the cooling effect of the top condenser.

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