Low-temperature nitrogen recycling process for medium-high pressure liquid nitrogen storage tank

Abstract

According to the low-temperature nitrogen recycling and reusing technology for the medium-high pressure liquid nitrogen storage tank, the temperature, pressure and flow design values of low-temperature nitrogen and the temperature, pressure and flow design values of an inlet of a first-stage intercooler of a nitrogen compressor are configured; low-temperature nitrogen diffused by a high-pressure liquid nitrogen storage tank in the air separation device is input into an inlet pipeline of a first-stage intercooler of a nitrogen compressor of the air separation device to be mixed and subjected to heat exchange with compressed nitrogen at an outlet of a first-stage working impeller of the nitrogen compressor, the low-temperature nitrogen is heated, and the mixed compressed nitrogen is input into the first-stage intercooler to be subjected to heat exchange with circulating cooling water; the gas enters a second-stage working impeller of the nitrogen compressor to be compressed; according to the technical scheme, implementation is easy, the implementation effect is remarkable, cooling capacity and materials of low-temperature nitrogen emptied by the middle-high pressure liquid nitrogen storage tank can be recycled at the same time, the effects of reducing energy consumption of an air separation device and reducing the low-temperature nitrogen diffusion rate are remarkable, and energy conservation, cost reduction and carbon emission reduction of enterprises are facilitated.

Description

Technical Field

[0001] This invention belongs to the field of deep cooling air separation and cryogenic liquid storage, and particularly relates to a cryogenic nitrogen recovery process for a liquid nitrogen storage tank in an air separation unit. Background Technology

[0002] Deep-cooled air separation equipment (hereinafter referred to as air separation unit) utilizes the principle of cryogenic refrigeration distillation. The process involves compressing the raw air using an air compressor, purifying it through an air pre-cooling system, adsorbing it through a molecular sieve purification system, refrigerating it with an expansion compressor, transferring heat through a heat exchanger, and separating it in a distillation column. Liquid nitrogen and gaseous nitrogen are produced at the top of the lower and upper columns, respectively. Liquid nitrogen at -180℃ and 400KPa is produced from the top of the lower column and transported through insulated pipelines to a liquid nitrogen storage tank for market sale or as feedstock for the liquid nitrogen vaporization unit of the air separation unit. Gas nitrogen at -193℃ and 20KPa is produced from the top of the upper column. In the main heat exchanger within the main cold box, the reflux cryogenic nitrogen exchanges heat with the forward flow ambient temperature raw air, reheating it to ambient temperature. It is then transported through an ambient temperature pipeline to the inlet of the nitrogen compressor, where it is compressed and pressurized before being distributed to the nitrogen pipeline network.

[0003] The air separation unit's distillation system mainly includes a main heat exchanger, main columns (upper and lower), a main condenser-evaporator, a crude argon column, and a refined argon column, all housed within a cold box. The cold box is filled with expanded perlite and purged with low-pressure dry nitrogen to suppress atmospheric heat transfer and maintain the operating temperature within the cold box at no higher than -170℃.

[0004] The nitrogen filling the cold box is drawn from the low-pressure nitrogen pipeline of the air separation unit. The low-pressure nitrogen pressure of the air separation unit is usually designed to be 500 kPa. It is reduced to 10 kPa through a pressure reducing valve and then introduced into the nitrogen filling pipeline of the cold box.

[0005] The air separation unit's circulating water system consists of a water storage tank, circulating water pumps, cooling fans, supply water pipelines, and return water pipelines. Circulating cooling water is supplied to the air compressor, oxygen compressor, and nitrogen compressor coolers, as well as the air cooling tower, through the supply water pipelines. After cooling the compressed air, oxygen, and nitrogen, the circulating water temperature rises to 30–40°C, then flows into the return water pipeline, is cooled and lowered by the cooling tower's cooling fans, and finally flows into the water storage tank.

[0006] Currently, there are two main types of liquid nitrogen storage tanks used in air separation units both domestically and internationally: atmospheric pressure flat-bottom liquid nitrogen storage tanks and vacuum powder-insulated liquid nitrogen storage tanks. Atmospheric pressure flat-bottom liquid nitrogen storage tanks are designed to operate at a pressure not exceeding 50 kPa, with a liquid nitrogen storage capacity of 200 m³ to 10,000 m³. Vacuum powder-insulated liquid nitrogen storage tanks are designed to operate at a pressure greater than 50 kPa, with a liquid nitrogen storage capacity of 5 m³ to 300 m³.

[0007] The vacuum powder insulated liquid nitrogen storage tank is designed with a double-layer structure, including an inner container and an outer container. Expanded perlite is arranged between the inner container and the outer container, and the space between the inner container and the outer container is evacuated to maintain the vacuum degree, shielding heat transfer and reducing the evaporation rate of liquid nitrogen in the inner container of the vacuum powder insulated liquid nitrogen storage tank.

[0008] In the vacuum powder insulated liquid nitrogen storage tank, due to the temperature difference between the atmospheric environment, the inner container, the outer container and the liquid nitrogen, the liquid nitrogen in the inner container is continuously vaporized into low-temperature nitrogen gas, causing the pressure in the inner container of the liquid nitrogen storage tank to continuously rise. Therefore, a low-temperature nitrogen gas vent valve is installed in the vacuum powder insulated liquid nitrogen storage tank to discharge the low-temperature nitrogen gas in the inner container of the liquid nitrogen storage tank to the atmosphere, keeping the pressure in the inner container of the liquid nitrogen storage tank not greater than the design value and ensuring the safe operation of the liquid nitrogen storage tank.

[0009] Currently, during the operation and storage of liquid nitrogen in the liquid nitrogen storage tank, when the low-temperature nitrogen gas vent valve is opened, the low-temperature nitrogen gas in the inner container of the liquid nitrogen storage tank is continuously discharged to the atmosphere. While keeping the pressure of the liquid nitrogen storage tank at the set value, there are the following problems to be improved.

[0010] First, the purity of the low-temperature nitrogen gas in the inner container of the liquid nitrogen storage tank is the same as that of the product liquid nitrogen, which is a nitrogen product meeting the national standard. The continuous discharge of low-temperature nitrogen gas not higher than -180°C to the atmosphere causes the nitrogen product to be dissipated, resulting in the loss of refrigeration capacity of the air separation unit, an increase in the oxygen production energy consumption of the air separation unit, and an increase in the non-effective power loss of the air separation unit, affecting the economic operation of the air separation unit.

[0011] Second, the continuous discharge of low-temperature nitrogen gas not higher than -180°C to the atmosphere causes the condensation and precipitation of moisture in the air, resulting in frosting and icing at the outlet of the low-temperature nitrogen gas vent valve, affecting the safe operation of the air separation unit.

[0012] Third, when the low-temperature nitrogen gas is continuously discharged to the atmosphere, the oxygen content in the area at the outlet of the low-temperature nitrogen gas vent valve is low, affecting the safety of the operating personnel of the air separation unit.

[0013] Taking the 100m³ vertical vacuum powder insulated liquid nitrogen storage tank supporting the 20000m³ / h air separation unit of Magang as an example, the designed working pressure value of its inner container is 200KPa, and the designed daily liquid nitrogen evaporation rate is 0.25%.

[0014] The 100m³ vertical vacuum powder insulated liquid nitrogen storage tank is equipped with 1 low-temperature pneumatic short-stem diaphragm regulating valve with a valve diameter of 20mm, which serves as the vent valve for the low-temperature nitrogen gas of the liquid nitrogen storage tank.

[0015] According to the set working pressure value of the inner container of the liquid nitrogen storage tank, the low-temperature nitrogen gas vent valve of the liquid nitrogen storage tank automatically adjusts the valve opening to discharge the low-temperature nitrogen gas in the liquid nitrogen storage tank to the atmosphere, keeping the working pressure value of the inner container of the 100m³ vertical vacuum powder insulated liquid nitrogen storage tank at the designed value of 200KPa.

[0016] Calculations show that with a 20,000 m³ / h air separation unit operating for an average of 360 days per year, the average annual venting volume of cryogenic nitrogen from the liquid nitrogen storage tank is 100 × 0.0025 × 360 × 648 m³ / h. 3 =58320m 3 The loss of nitrogen and cooling capacity in this part of the product increases the energy consumption of oxygen production in the 20,000 m3 / h air separation unit, resulting in a high low-temperature nitrogen release rate in the 20,000 m3 / h air separation unit.

[0017] Low-temperature nitrogen is the vaporization of liquid nitrogen, and its purity is consistent with that of the product nitrogen. Based on the current average annual market export price of liquid nitrogen of 535 yuan / m3, the liquid nitrogen in a 100m3 vertical vacuum powder-insulated liquid nitrogen storage tank vaporizes into low-temperature nitrogen. Venting the low-temperature nitrogen will result in an average annual economic loss of 100 × 0.0025 × 360 × 535 yuan = 48,150 yuan. This is only the loss of the product nitrogen itself; the loss of the cooling capacity of the liquid nitrogen is even more considerable.

[0018] See Figure 1 Schematic diagram of the cryogenic nitrogen venting process for a 100m³ vertical vacuum powder-insulated liquid nitrogen storage tank in a 20000m³ / h air separation unit. Summary of the Invention

[0019] (a) Technical problems to be solved

[0020] To address the shortcomings of existing technologies and solve the problem of increased refrigeration losses in air separation units due to the release of cryogenic nitrogen from liquid nitrogen storage tanks into the atmosphere, this invention provides a process for recovering and reusing cryogenic nitrogen from medium- and high-pressure liquid nitrogen storage tanks. This process recovers the cryogenic nitrogen discharged from the liquid nitrogen storage tanks and inputs it into other systems of the air separation unit for reuse, thereby reducing the energy consumption of the air separation unit.

[0021] (II) Technical Solution

[0022] To achieve the above objectives, the present invention provides the following technical solution:

[0023] The technical solution of this invention includes a nitrogen compressor for an air separation unit (hereinafter referred to as a nitrogen compressor). The nitrogen compressor adopts the principle of multi-stage isothermal compression of nitrogen, compressing and cooling the low-pressure nitrogen product gas at room temperature in the cold box outlet pipe through multiple stages before delivering it to the nitrogen pipeline network. The nitrogen compressor consists of a rotor, a stator, and an intercooler, and is typically designed as a five-stage nitrogen compression unit with five intercoolers. The low-pressure nitrogen gas from the cold box first enters the first-stage impeller through the nitrogen inlet pipe of the first-stage intercooler of the nitrogen compressor, and then enters the first-stage intercooler. It is then successively compressed and pressurized by the five working impellers and cooled and cooled by the five intercoolers before being delivered to the nitrogen pipeline network. In the intercooler, room-temperature circulating cooling water exchanges heat with the high-temperature, pressurized compressed nitrogen gas. The intercooler cools and lowers the high-temperature, pressurized compressed nitrogen gas at the outlet of the working impeller to a design value slightly higher than the inlet temperature of the working impeller before inputting it into the next-stage working impeller and intercooler, thus achieving multi-stage isothermal compression of nitrogen gas.

[0024] The nitrogen inlet pipe thermometer at the inlet end of the first-stage intercooler of the nitrogen compressor is a resistance thermometer, which is suitable for high-precision continuous temperature measurement of nitrogen at low, normal, and high temperatures, and is electrically connected to the DCS control system of the air separation unit.

[0025] The technical solution of the present invention also includes a liquid nitrogen storage tank, a product liquid nitrogen conveying pipeline and a product liquid nitrogen conveying valve, a low-temperature nitrogen venting pipeline for the liquid nitrogen storage tank and a low-temperature nitrogen pressure gauge and a low-temperature nitrogen venting valve arranged sequentially in the direction of low-temperature nitrogen output on the pipeline, as well as a liquid nitrogen storage tank pressurization pipeline and a pneumatic pressurization valve and a pressurizer arranged on the pipeline.

[0026] The technical solution of this invention is as follows: Based on the design values ​​of low-temperature nitrogen temperature, pressure, and flow rate and the design values ​​of the inlet temperature, pressure, and flow rate of the first-stage intercooler of the nitrogen compressor, a process for recovering and reusing low-temperature nitrogen from a medium- and high-pressure liquid nitrogen storage tank is designed. The low-temperature nitrogen released from the high-pressure liquid nitrogen storage tank in the air separation unit is input into the nitrogen inlet pipe of the first-stage intercooler of the air separation unit's nitrogen compressor, where it mixes and exchanges heat with the compressed nitrogen at the outlet of the first-stage working impeller of the nitrogen compressor. The low-temperature nitrogen is heated up, and the mixed compressed nitrogen flows into the first-stage intercooler to exchange heat with the circulating cooling water. After that, it enters the second-stage working impeller of the nitrogen compressor for compression.

[0027] The technical solution described in this invention enables the complete recovery of the cold energy and materials of the low-temperature nitrogen gas released from the high-pressure liquid nitrogen storage tank in the air separation unit.

[0028] A further technical solution of the present invention is as follows: a cryogenic nitrogen recovery device is added between the inlet pipe of the cryogenic nitrogen vent valve of the liquid nitrogen storage tank and the inlet pipe of the nitrogen compressor. The device includes a vent nitrogen recovery pipe and components connected to the pipe. The inlet of the vent nitrogen recovery pipe is connected to the inlet pipe of the cryogenic nitrogen vent valve. The vent nitrogen recovery pipe is connected in sequence from the cryogenic nitrogen input direction to a cryogenic nitrogen recovery delivery valve, a cryogenic nitrogen recovery pressure regulating valve, a cryogenic nitrogen recovery check valve, and a cryogenic nitrogen recovery pressure gauge. The outlet of the vent nitrogen recovery pipe is connected to the nitrogen inlet pipe before the inlet thermometer of the first stage intercooler of the nitrogen compressor.

[0029] A further technical solution of the present invention is as follows: the diameter and pressure rating of the vented nitrogen recovery pipeline are the same as those of the cryogenic nitrogen vent pipeline of the liquid nitrogen storage tank, and it is made of stainless steel, suitable for -180℃ cryogenic nitrogen transportation; the valve diameter and flow rate of the cryogenic nitrogen recovery transportation valve are the same as those of the cryogenic nitrogen vent valve, and it is a cryogenic pneumatic aluminum angle valve with a diaphragm-type pneumatic actuator, electrically connected to the air separation unit's DCS control system; the cryogenic nitrogen recovery pressure regulating valve is a pneumatic single-seat pressure regulating valve with a diaphragm-type pneumatic actuator, and its valve diameter and flow rate are the same as those of the vented nitrogen recovery pipeline, and it is electrically connected to the air separation unit's DCS control system; the cryogenic nitrogen recovery check valve is a double-disc wafer-type check valve, suitable for low-pressure nitrogen transportation; the cryogenic nitrogen recovery pressure gauge is a capacitive pressure transmitter, suitable for high-precision continuous measurement of low-pressure nitrogen, and electrically connected to the air separation unit's DCS control system.

[0030] The automated operation scheme of this invention is as follows:

[0031] In the DCS control system of the air separation unit, the pressure setpoint of the pressure gauge is designed. Based on the pressure setpoint of the pressure gauge in the nitrogen pipeline, the valve opening of the pneumatic single-seat pressure regulating valve is automatically adjusted to maintain the nitrogen pressure in the nitrogen pipeline at the setpoint.

[0032] In the DCS control system of the air separation unit, a low temperature alarm value for the thermometer is designed. Based on the low temperature alarm value set by the thermometer at the end of the pipeline at the inlet of the first-stage intercooler of the nitrogen compressor, the pneumatic delivery valve of the nitrogen delivery pipeline is automatically selected to be fully open or fully closed.

[0033] In the DCS control system of the air separation unit, three control logics are set up for the automatic operation, regulation, and shutdown of the cryogenic nitrogen recovery and reuse device:

[0034] (1) Set the low alarm value of the inlet temperature gauge T1 of the first stage intercooler of the nitrogen compressor to 0°C. When the inlet nitrogen temperature of the first stage intercooler of the nitrogen compressor is greater than 0°C, the low temperature nitrogen recovery and reuse device is put into operation. When the low temperature nitrogen recovery delivery valve is fully open, the low temperature nitrogen recovery pressure regulating valve is opened to automatically adjust the low temperature nitrogen pressure. The low temperature nitrogen enters the inlet pipe L4 of the first stage intercooler of the nitrogen compressor, and the low temperature nitrogen recovery and reuse device is started.

[0035] (2) The pressure setting value of the cryogenic recovery nitrogen pressure gauge is the same as the pressure value of the air inlet pipe of the first stage intermediate cooler of the nitrogen compressor. According to the pressure setting value of the cryogenic recovery nitrogen pressure gauge, the cryogenic recovery nitrogen pressure regulating valve automatically adjusts the valve opening. According to the pressure setting value of the liquid nitrogen storage tank venting pipe pressure gauge, the cryogenic nitrogen venting valve automatically closes completely. The pressure boosting valve configured on the liquid nitrogen storage tank self-pressurization pipe automatically adjusts the opening. The cryogenic nitrogen evaporated in the liquid nitrogen storage tank is completely recovered and input into the nitrogen compressor through the cryogenic nitrogen venting recovery pipe.

[0036] (3) When the temperature value displayed by the inlet thermometer of the first stage intermediate cooler of the nitrogen compressor is not greater than 0℃, the low temperature recovery nitrogen delivery valve is fully closed, the low temperature recovery nitrogen pressure regulating valve is automatically fully closed, the low temperature nitrogen vent valve of the liquid nitrogen storage tank is automatically opened, the low temperature nitrogen in the liquid nitrogen storage tank is discharged into the atmosphere, and the low temperature nitrogen recovery and reuse device is shut down.

[0037] The technical solution of this invention is easy to implement and has significant effects. It can simultaneously recover the cold energy and materials of the low-temperature nitrogen released from the medium and high pressure liquid nitrogen storage tank, which has a significant effect on reducing the energy consumption of the air separation unit and reducing the low-temperature nitrogen release rate. It is beneficial for enterprises to save energy, reduce costs, and reduce carbon emissions. Attached Figure Description

[0038] Figure 1 Schematic diagram of the low-temperature nitrogen release process of the 100m³ vertical vacuum powder insulated liquid nitrogen storage tank in Maanshan Iron & Steel's 20000m³ / h air separation unit.

[0039] Figure 2 A schematic diagram of a low-temperature nitrogen recovery and reuse process for a medium- and high-pressure liquid nitrogen storage tank according to the present invention.

[0040] Figure Descriptions: B60, a medium-high pressure liquid nitrogen storage tank; L2, product liquid nitrogen input pipeline; V503, liquid nitrogen input valve; L1, cryogenic nitrogen vent pipeline; P5, vent pipeline pressure gauge; V504, cryogenic nitrogen vent valve; L3, liquid nitrogen storage tank self-pressurization pipeline; V513, pressure boosting valve; E3, pressure booster; L5, cryogenic nitrogen vent recovery pipeline; V510, cryogenic recovered nitrogen delivery valve; V511, cryogenic recovered nitrogen pressure regulating valve; V512, cryogenic recovered nitrogen check valve; P2, cryogenic recovered nitrogen pressure gauge; C90, nitrogen compressor; L4, nitrogen compressor first-stage intercooler inlet pipeline; C901, nitrogen compressor first-stage impeller; W901, nitrogen compressor first-stage intercooler; T1, nitrogen compressor first-stage intercooler inlet thermometer. Detailed Implementation

[0041] The following will be combined with the appendix of this invention. Figure 2 The technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0042] This invention takes the 100m³ vertical vacuum powder insulated liquid nitrogen storage tank B60 of the 20000m³ / h air separation unit of Maanshan Iron and Steel as an example to specifically describe the implementation process of the technical solution of this invention.

[0043] I. Comparison of various parameters and technical indicators of 100m3 vertical vacuum powder-insulated liquid nitrogen storage tank B60 and nitrogen compressor C90.

[0044] 1. The liquid nitrogen product of the 20,000 m3 / h air separation unit is produced from the top of the lower tower and stored in liquid nitrogen storage tank B60. The designed oxygen content of the liquid nitrogen product is 3 ppm.

[0045] The low-pressure nitrogen product of the 20,000 m3 / h air separation unit is produced from the top of the upper tower, compressed by a nitrogen compressor (referred to as: nitrogen compressor) C90, and transported to the medium-pressure nitrogen pipeline network. The oxygen content of the low-pressure nitrogen product is designed to be 3 ppm.

[0046] 2. The design working pressure of the 100m³ vertical vacuum powder-insulated liquid nitrogen storage tank B60 is 200KPa, the design daily liquid nitrogen evaporation rate is 0.25%, and the design values ​​for low-temperature nitrogen emission, pressure, temperature, and purity are 6.8m³ / h, 200KPa, -180℃, and 3PPm oxygen content, respectively.

[0047] 3. The C90 nitrogen compressor is a centrifugal compressor employing five-stage nitrogen compression. The design values ​​for the C90's exhaust flow rate and pressure are 14500 m³ / h and 2000 kPa, respectively. The design values ​​for the temperature, pressure, flow rate, and purity of the pressurized nitrogen at the outlet of the first-stage impeller C901 are 80℃, 180 kPa, 14500 m³ / h, and an oxygen content of 3 ppm, respectively. The design values ​​for the diameter and length of the inlet pipe L4 of the first-stage intercooler W901 are 150 mm and 6000 mm, respectively. Both the intercooler and the inlet pipe were originally designed to be made of 316 stainless steel.

[0048] The design value for the operating load range of nitrogen compressor C90 is 75% to 105%, and the design value for the compressed nitrogen flow rate of each stage of nitrogen compressor C90 is 10875 m3 / h to 15225 m3 / h.

[0049] 4. The design values ​​for the inlet nitrogen temperature and pressure of the first-stage intercooler of the nitrogen compressor are not less than 80℃ and not more than 200KPa.

[0050] The vacuum powder insulated liquid nitrogen storage tank is designed to operate at a pressure not exceeding 400 kPa. The pressure of the discharged low-temperature nitrogen gas from the liquid nitrogen storage tank is not exceeding 400 kPa and the temperature is not less than -180°C. The purity of the discharged low-temperature nitrogen gas from the liquid nitrogen storage tank is the same as that of the low-pressure product nitrogen gas at room temperature not exceeding 20 kPa from the outlet of the cold box of the air separation unit.

[0051] 5. The liquid nitrogen storage capacity of the vacuum powder-insulated liquid nitrogen storage tank is no more than 300 m³, and the designed daily (24-hour) liquid nitrogen evaporation rate is no more than 0.4%. Approximately 648 cubic meters of nitrogen gas are produced from the vaporization of 1 cubic meter of liquid nitrogen. Therefore, the calculated maximum cryogenic nitrogen emission from the vertical vacuum powder-insulated liquid nitrogen storage tank is 300 × 648 × 0.4% ÷ 24 = 32 m³. 3 / h,

[0052] II. Technical Principles of the Invention

[0053] The liquid nitrogen storage tank B60 recovers cryogenic nitrogen through the venting nitrogen recovery pipeline L5. After being regulated by the cryogenic nitrogen recovery pressure regulating valve V511, the pressure is reduced from 200 kPa to 180 kPa. It then enters the inlet pipeline L4 of the first-stage intercooler W901 of the nitrogen compressor C90. There, it mixes and exchanges heat with the pressurized nitrogen at 80°C, 180 kPa, 14500 m3 / h, and 3 ppm oxygen content at the outlet of the first-stage impeller C901 of the nitrogen compressor C90. The cryogenic nitrogen is heated up, and the compressed nitrogen after mixing enters the first-stage intercooler W901 to exchange heat with the circulating cooling water. Then, it enters the second-stage impeller of the nitrogen compressor for compression.

[0054] In the inlet pipe L4 of the W901 of the first-stage intercooler of the C90 nitrogen compressor, low-temperature nitrogen gas with a flow rate of 6.8 m³ / h, 180 kPa, -180°C, and an oxygen content of 3 ppm is mixed with pressurized nitrogen gas with a flow rate of 14500 m³ / h and an oxygen content of 3 ppm at 80°C. The flow rate and temperature of the compressed nitrogen gas after mixing are calculated as follows:

[0055] 1. Volumetric flow rate after mixing

[0056] The volumetric flow rate of cryogenic nitrogen, V1, is 6.8 m³ / h, and the volumetric flow rate of pressurized nitrogen, V2, is 14,500 m³ / h.

[0057] Qmix= V1+ V2=6.8+14500=14506.8m³ / h.

[0058] 2. Temperature calculation after mixing

[0059] (1) Mass flow rate: R = 0.297 kJ / (kg·K), temperature is calculated as absolute temperature scale T1 is -180℃ + 273 = 93 K, T2 is 80℃ + 273 = 353 K.

[0060] Density of nitrogen at low temperature (-180℃, 180kPa): ρ1=P / RT1=180 / 0.297×93≈6.52kg / m³.

[0061] The density of pressurized nitrogen (80℃, 180 kPa): ρ2 = P / RT2 = 180 / 0.297×353 ≈ 1.72 kg / m³.

[0062] Mass flow rate: Low-temperature nitrogen mass flow rate m1 = V1 × ρ1 = 6.8 × 6.52 = 44.3 kg / h.

[0063] The mass flow rate of pressurized nitrogen is m2 = V2 × ρ2 = 14500 × 1.72 = 24940 kg / h.

[0064] (2) Temperature after mixing:

[0065] Tmix=(m1T1+m2T2) / (m1+m2), substitute the data (T1 = 93 K, T2 = 353 K).

[0066] Tmix=44.3+24940[(44.3×93)+(24940×353) ] / (44.3+24940)≈352.6K.

[0067] Tmix = 352.6 − 273 = 79.6℃.

[0068] Based on the above calculations, the flow rate and temperature of the compressed nitrogen after mixing are 14506.8 m³ / h and 79.6℃, respectively, and the nitrogen compressor C90 is operating under the design value conditions. Specific Implementation

[0069] A cryogenic nitrogen recovery device is added between the vent pipe L1 at the inlet end of the cryogenic nitrogen vent valve V504 of the liquid nitrogen storage tank B60 and the air inlet pipe L4 of the first-stage intercooler W901 of the nitrogen compressor C90. The device includes a vent nitrogen recovery pipe L5 and components connected to the pipe. The inlet end of the vent nitrogen recovery pipe L5 is connected to the inlet end of the cryogenic nitrogen vent valve V504. The vent nitrogen recovery pipe is connected in sequence from the cryogenic nitrogen input direction to the cryogenic recovery nitrogen delivery valve V510, the cryogenic recovery nitrogen pressure regulating valve V511, the cryogenic recovery nitrogen check valve V512, and the cryogenic recovery nitrogen pressure gauge P2. The outlet end of the vent nitrogen recovery pipe L5 is connected to the air inlet pipe L4 of the first-stage intercooler W901 of the nitrogen compressor C90 and is connected to the air inlet side of the air inlet thermometer T1.

[0070] The L5 cryogenic venting nitrogen recovery pipeline has a diameter of 20mm and a pressure rating of 3.1MPa. It is made of 316 stainless steel and is used for cryogenic nitrogen transportation at -180℃.

[0071] The low-temperature nitrogen recovery gas delivery valve V510 has the same valve diameter and flow rate of 20mm and 100m3 / h as the low-temperature nitrogen vent valve V504. The low-temperature nitrogen recovery gas delivery valve V510 is a low-temperature pneumatic aluminum angle valve with a diaphragm-type pneumatic actuator, and is electrically connected to the DCS control system operator station of the 20000m3 / h air separation unit.

[0072] The cryogenic nitrogen recovery pressure regulating valve V511 is a pneumatic single-seat pressure regulating valve with a diaphragm-type pneumatic actuator. The valve diameter and flow rate of the cryogenic nitrogen recovery pressure regulating valve V511 are 20mm and 100m3 / h, respectively, which are the same as the diameter and flow rate of the cryogenic nitrogen venting and nitrogen recovery pipeline L5. It is electrically connected to the operating station of the DCS control system of the 20000m3 / h air separation unit.

[0073] The V512 cryogenic nitrogen recovery check valve is a double-disc wafer type check valve used for low-pressure nitrogen delivery.

[0074] The cryogenic nitrogen recovery pressure gauge P2 is a capacitive pressure transmitter, and the pressure gauge P2 is electrically connected to the operator station of the DCS control system of the 20000m3 / h air separation unit.

[0075] IV. Automation Setup and Operation

[0076] 1. In the DCS control system of the 20000m3 / h air separation unit, the pressure setting value of the cryogenic recovery nitrogen pressure gauge P2 is set to 180KPa. The valve opening of the cryogenic recovery nitrogen pressure regulating valve V511 is automatically adjusted so that the cryogenic nitrogen pressure at the inlet of the first stage intercooler W901 of the nitrogen compressor C90 reaches the set value of 180KPa, which is the same as the outlet pressure of the first stage impeller C901 of the nitrogen compressor C90, and the nitrogen compressor C90 operates under the design value condition.

[0077] 2. In the DCS control system of the 20000m3 / h air separation unit, the low alarm value of the inlet temperature gauge T1 of the first stage intercooler of the nitrogen compressor is set to 0℃. When the nitrogen temperature at the inlet end of the first stage intercooler W901 of the nitrogen compressor C90 drops to 0℃, the low temperature recovery nitrogen delivery valve V510 automatically closes completely.

[0078] 3. In the DCS control system of the 20000m3 / h air separation unit, three control logics are set for automatic feed, regulation, and shutdown.

[0079] (1) Set the low alarm value of the inlet temperature gauge T1 of the first stage intercooler of the nitrogen compressor to 0℃. When the nitrogen temperature at the inlet end of the first stage intercooler W901 of the nitrogen compressor C90 is greater than 0℃, the 100m3 vertical vacuum powder insulation liquid nitrogen storage tank B60 low temperature nitrogen recovery and reuse device is put into operation. When the low temperature recovery nitrogen delivery valve V510 is fully open, the low temperature recovery nitrogen pressure regulating valve V511 can be opened to automatically adjust the low temperature nitrogen pressure. The low temperature nitrogen enters the inlet pipe L4 of the first stage intercooler W901 of the nitrogen compressor C90 to heat up, and the liquid nitrogen storage tank B60 low temperature nitrogen recovery and reuse device is started.

[0080] (2) Based on the pressure setting value of 180 kPa for the cryogenic nitrogen recovery pressure gauge P2, the cryogenic nitrogen recovery pressure regulating valve V511 automatically adjusts the valve opening. Based on the pressure setting value of 200 kPa for the venting pipeline pressure gauge P5 of the liquid nitrogen storage tank B60, the cryogenic nitrogen venting valve V504 automatically closes completely, and the pneumatic booster valve V513 configured in the liquid nitrogen storage tank B60 automatically adjusts the opening, so that all the cryogenic nitrogen evaporated in the liquid nitrogen storage tank B60 is input into the nitrogen compressor C90.

[0081] (3) When the temperature displayed by the inlet thermometer T1 of the first stage intermediate cooler of the nitrogen compressor is not greater than 0℃, the low temperature recovery nitrogen delivery valve V510 is fully closed, the low temperature recovery nitrogen pressure regulating valve V511 is automatically fully closed, the low temperature nitrogen vent valve V504 of the liquid nitrogen storage tank B60 is automatically opened, the low temperature nitrogen in the liquid nitrogen storage tank B60 is discharged into the atmosphere, and the low temperature nitrogen recovery and reuse device is shut down.

[0082] The low-temperature nitrogen recovered from the liquid nitrogen storage tank B60 mixes with the low-pressure nitrogen in the first-stage intermediate cooling inlet pipe of the nitrogen compressor. This does not affect the normal operation of the nitrogen compressor, but reduces the temperature of the nitrogen entering the second-stage impeller of the nitrogen compressor, increases the amount of nitrogen in the product, reduces the energy consumption of the nitrogen compressor, and is beneficial to the operational stability of the nitrogen compressor.

Claims

1. A process for recovering and reusing cryogenic nitrogen from a medium- or high-pressure liquid nitrogen storage tank, the process comprising: The air separation unit's nitrogen compressor (hereinafter referred to as the nitrogen compressor) consists of a rotor, stator, and intercoolers. It typically involves five stages of nitrogen compression and is equipped with five intercoolers. Low-pressure nitrogen from the cold box enters the first-stage impeller through the inlet pipe of the first-stage intercooler, then enters the first-stage intercooler, and is successively compressed and pressurized by the five working impellers and cooled by the five intercoolers before being delivered to the nitrogen pipeline network. The nitrogen inlet pipe thermometer at the inlet end of the first-stage intercooler is a resistance thermometer and is electrically connected to the air separation unit's DCS control system. The unit also includes a liquid nitrogen storage tank, a product liquid nitrogen delivery pipeline and a product liquid nitrogen delivery valve, and a cryogenic nitrogen release valve for the liquid nitrogen storage tank. The air separation unit includes a cryogenic nitrogen pressure gauge and a cryogenic nitrogen vent valve arranged sequentially in the direction of cryogenic nitrogen output on the air pipeline, as well as a liquid nitrogen storage tank pressurization pipeline and a pneumatic pressurization valve and a pressurizer arranged on the pipeline. The key feature is that, based on the design values ​​of cryogenic nitrogen temperature, pressure, and flow rate and the design values ​​of the inlet temperature, pressure, and flow rate of the first-stage intercooler of the nitrogen compressor, the cryogenic nitrogen released from the high-pressure liquid nitrogen storage tank in the air separation unit is input into the inlet pipeline of the first-stage intercooler of the air separation unit's nitrogen compressor. There, it mixes and exchanges heat with the compressed nitrogen at the outlet of the first-stage working impeller of the nitrogen compressor, causing the cryogenic nitrogen to heat up. After mixing, the compressed nitrogen is input into the first-stage intercooler to exchange heat with circulating cooling water, and then enters the second-stage working impeller of the nitrogen compressor for compression.

2. The process for recovering and reusing cryogenic nitrogen from a medium-high pressure liquid nitrogen storage tank according to claim 1 is characterized in that: A cryogenic nitrogen recovery device is added between the inlet pipe of the cryogenic nitrogen vent valve of the liquid nitrogen storage tank and the nitrogen inlet pipe of the first-stage intercooler of the nitrogen compressor. The device includes a vent nitrogen recovery pipe and components connected to the pipe. The inlet end of the vent nitrogen recovery pipe is connected to the inlet pipe of the cryogenic nitrogen vent valve. The vent nitrogen recovery pipe is connected in sequence from the cryogenic nitrogen input direction to a cryogenic recovered nitrogen delivery valve, a cryogenic recovered nitrogen pressure regulating valve, a cryogenic recovered nitrogen check valve, and a cryogenic recovered nitrogen pressure gauge. The outlet end of the vent nitrogen recovery pipe is connected to the nitrogen inlet pipe before the thermometer at the inlet end of the first-stage intercooler of the nitrogen compressor.

3. The process for recovering and reusing cryogenic nitrogen from a medium-high pressure liquid nitrogen storage tank according to claim 2, characterized in that: The diameter and pressure rating of the vented nitrogen recovery pipeline are the same as those of the cryogenic nitrogen vent pipeline in the liquid nitrogen storage tank, and it is made of stainless steel. The valve diameter and flow rate of the cryogenic nitrogen recovery delivery valve are the same as those of the cryogenic nitrogen vent valve, and it is a cryogenic pneumatic aluminum angle valve with a diaphragm-type pneumatic actuator, electrically connected to the air separation unit's DCS control system. The cryogenic nitrogen recovery pressure regulating valve is a pneumatic single-seat pressure regulating valve with a diaphragm-type pneumatic actuator, and its valve diameter and flow rate are the same as those of the vented nitrogen recovery pipeline, and it is electrically connected to the air separation unit's DCS control system. The cryogenic nitrogen recovery check valve is a double-disc wafer check valve. The cryogenic nitrogen recovery pressure gauge is a capacitive pressure transmitter, electrically connected to the air separation unit's DCS control system.

4. The process for recovering and reusing cryogenic nitrogen from a medium-high pressure liquid nitrogen storage tank according to claim 3, characterized in that: In the DCS control system of the air separation unit, three control logics are set up for the automatic operation, regulation, and shutdown of the cryogenic nitrogen recovery and reuse device: (1) Set the low alarm value of the inlet temperature gauge T1 of the first stage intercooler of the nitrogen compressor to 0°C. When the inlet nitrogen temperature of the first stage intercooler of the nitrogen compressor is greater than 0°C, the low temperature nitrogen recovery and reuse device is put into operation. When the low temperature nitrogen recovery delivery valve is fully open, the low temperature nitrogen recovery pressure regulating valve is opened to automatically adjust the low temperature nitrogen pressure. The low temperature nitrogen enters the inlet pipe L4 of the first stage intercooler of the nitrogen compressor, and the low temperature nitrogen recovery and reuse device is started. (2) The pressure setting value of the cryogenic recovery nitrogen pressure gauge is the same as the pressure value of the air inlet pipe of the first stage intercooler of the nitrogen compressor. According to the pressure setting value of the cryogenic recovery nitrogen pressure gauge, the cryogenic recovery nitrogen pressure regulating valve automatically adjusts the valve opening. According to the pressure setting value of the liquid nitrogen storage tank venting pipe pressure gauge, the cryogenic nitrogen venting valve automatically closes completely. The pressure boosting valve configured on the liquid nitrogen storage tank self-pressurization pipe automatically adjusts the opening. The cryogenic nitrogen evaporated in the liquid nitrogen storage tank is completely recovered and input into the nitrogen compressor through the cryogenic nitrogen venting recovery pipe. (3) When the temperature value displayed by the inlet thermometer of the first stage intermediate cooler of the nitrogen compressor is not greater than 0℃, the low temperature recovery nitrogen delivery valve is fully closed, the low temperature recovery nitrogen pressure regulating valve is automatically fully closed, the low temperature nitrogen vent valve of the liquid nitrogen storage tank is automatically opened, the low temperature nitrogen in the liquid nitrogen storage tank is discharged into the atmosphere, and the low temperature nitrogen recovery and reuse device is shut down.

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