Low-temperature nitrogen recycling process for normal-pressure liquid nitrogen storage tank

Abstract

According to the technology for recycling the low-temperature nitrogen of the normal-pressure liquid nitrogen storage tank, thermal load calculation is conducted according to the temperature, pressure and flow values of the low-temperature nitrogen in the normal-pressure flat-bottom liquid nitrogen storage tank and nitrogen filled in a cold box, the low-temperature nitrogen emptied by the normal-pressure flat-bottom liquid nitrogen storage tank is reheated, depressurized and input into a nitrogen filled pipeline of the cold box to serve as the nitrogen filled in the cold box, and the nitrogen filled in the cold box is recycled. And the matching rate of the low-temperature nitrogen discharge amount of the normal-pressure liquid nitrogen storage tank and the nitrogen filling amount of the cold box is calculated, the cooling capacity of the low-temperature nitrogen discharged from the atmosphere by the liquid nitrogen storage tank is recycled to the maximum extent, the emptied nitrogen product is fully utilized, and the purpose of reducing the energy consumption of the air separation device is achieved.

Description

Technical Field

[0001] This invention belongs to the field of deep-cooled 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 atmospheric pressure flat-bottom liquid nitrogen storage tank is designed with a double-layer structure, consisting of an inner container and an outer container. The inner container stores liquid nitrogen, and expanded perlite and foam glass bricks are placed between the inner and outer containers. Low-pressure dry nitrogen gas is filled between the inner and outer containers to suppress atmospheric heat transfer and reduce the evaporation of liquid nitrogen from the inner container of the atmospheric pressure flat-bottom liquid nitrogen storage tank.

[0008] The product liquid nitrogen from the air separation unit is fed into an atmospheric pressure flat-bottomed liquid nitrogen storage tank. Due to the large temperature difference between the ambient temperature and the cryogenic liquid nitrogen, heat transfer occurs, causing the cryogenic liquid nitrogen to evaporate and vaporize. The liquid nitrogen in the inner container continuously vaporizes into cryogenic nitrogen gas, resulting in a continuous increase in the pressure of the inner container of the liquid nitrogen storage tank. The atmospheric pressure flat-bottomed liquid nitrogen storage tank is equipped with a cryogenic nitrogen vent valve to release the cryogenic nitrogen gas in the inner container of the liquid nitrogen storage tank into the atmosphere, keeping the pressure of the inner container of the liquid nitrogen storage tank below the design value and ensuring the safe operation of the liquid nitrogen storage tank.

[0009] Currently, during the operation of liquid nitrogen storage tanks, when the cryogenic nitrogen vent valve is opened, the cryogenic nitrogen in the inner container of the liquid nitrogen storage tank is continuously discharged to the atmosphere. While maintaining the pressure of the liquid nitrogen storage tank at the set value, the following issues need to be improved.

[0010] Firstly, the purity of the cryogenic nitrogen in the liquid nitrogen storage tank is the same as that of the product liquid nitrogen, meeting national standards. The continuous release of cryogenic nitrogen at temperatures not exceeding -180℃ into the atmosphere causes nitrogen product flare-ups, resulting in losses in the air separation unit's cooling capacity, increased oxygen production energy consumption, and increased energy loss during air separation unit operation, thus affecting the economical operation of the air separation unit.

[0011] Secondly, the continuous emission of low-temperature nitrogen gas at a temperature not exceeding -180℃ into the atmosphere causes moisture in the air to condense and precipitate, resulting in frost and ice formation at the outlet of the low-temperature nitrogen vent valve, which affects the safe operation of the air separation unit.

[0012] Third, when cryogenic nitrogen is continuously released into the atmosphere, the oxygen content in the area at the outlet of the cryogenic nitrogen vent valve is low, which affects the safety of the air separation unit operators.

[0013] Taking the 1000m³ atmospheric pressure flat-bottom liquid nitrogen storage tank equipped in Maanshan Iron & Steel's 35000m³ / h air separation unit as an example, see... Figure 1 The inner container's designed working pressure is 18 kPa, and the designed daily liquid nitrogen evaporation rate is 0.32%. The 1000 m³ atmospheric pressure flat-bottom liquid nitrogen storage tank is equipped with one cryogenic pneumatic long-rod diaphragm regulating valve with a diameter of 100 mm, serving as a vent valve for the cryogenic nitrogen gas in the liquid nitrogen storage tank. The cryogenic pneumatic long-rod diaphragm regulating valve automatically adjusts its opening according to the working pressure of the inner container, allowing the cryogenic nitrogen gas in the liquid nitrogen storage tank to be discharged to the atmosphere, maintaining the working pressure of the inner container of the 1000 m³ atmospheric pressure flat-bottom liquid nitrogen storage tank at the designed value of 18 kPa.

[0014] Calculations show that the 35,000 m³ / h air separation unit operates for an average of 360 days per year. The average annual venting volume of cryogenic nitrogen from the liquid nitrogen storage tank is 1000 × 0.32% × 360 × 648 m³ = 746,496 m³. This loss of product nitrogen and cooling capacity increases the energy consumption for oxygen production in the 35,000 m³ / h air separation unit. Based on economic value, the current average annual market price for liquid nitrogen from Maanshan Iron & Steel is 535 yuan / m³. The estimated annual economic loss from the venting of the 1000 m³ atmospheric pressure flat-bottom liquid nitrogen storage tank is 1000 × 0.0032 × 360 × 535 yuan = 616,320 yuan. This only accounts for the loss of product nitrogen itself; the loss of liquid nitrogen cooling capacity is even more considerable. Summary of the Invention

[0015] (a) Technical problems to be solved

[0016] 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 atmospheric pressure liquid nitrogen storage tanks. During operation of the atmospheric pressure flat-bottomed liquid nitrogen storage tank, the cryogenic nitrogen emitted from the tank is recovered and input into other systems of the air separation unit for reuse, thereby reducing the energy consumption of the air separation unit.

[0017] (II) Technical Solution

[0018] To achieve the above objectives, the present invention provides the following technical solution:

[0019] This invention provides a process for the recovery and reuse of cryogenic nitrogen from an atmospheric pressure liquid nitrogen storage tank. The process includes: an atmospheric pressure flat-bottomed liquid nitrogen storage tank; a product liquid nitrogen delivery pipeline and a liquid nitrogen delivery valve; a cryogenic nitrogen discharge pipeline from the liquid nitrogen storage tank and cryogenic nitrogen pressure gauges and vent valves arranged sequentially along the cryogenic nitrogen output direction on the pipeline; a liquid nitrogen storage tank pressurization pipeline and a pneumatic pressurization valve and a booster installed on the pipeline; it also includes a cold box and a cold box filling nitrogen pipeline, wherein the cold box filling nitrogen pipeline is led out from the low-pressure nitrogen pipeline network and connected to the cold box, starting from the nitrogen input... The system is equipped with a low-pressure nitrogen regulating valve, a low-pressure nitrogen check valve, a pressure gauge, and a flow meter in sequence. This invention calculates the heat load based on the temperature, pressure, and flow rate of the low-temperature nitrogen in the atmospheric pressure flat-bottom liquid nitrogen storage tank and the nitrogen used for filling the cold box. The low-temperature nitrogen released from the atmospheric pressure flat-bottom liquid nitrogen storage tank is reheated and depressurized, and then input into the nitrogen filling pipeline of the cold box as the nitrogen used for filling the cold box. The matching rate between the low-temperature nitrogen discharge from the atmospheric pressure liquid nitrogen storage tank and the nitrogen filling amount of the cold box is calculated to maximize the recovery of the cold energy of the low-temperature nitrogen discharged from the liquid nitrogen storage tank.

[0020] When the atmospheric pressure flat-bottomed liquid nitrogen storage tank is operating normally, the liquid nitrogen storage tank stores the product liquid nitrogen. The cryogenic nitrogen vent valve is opened, and due to the huge temperature difference between the inside and outside of the storage tank, the cryogenic nitrogen in the inner container of the liquid nitrogen storage tank is continuously discharged to the atmosphere, maintaining the pressure of the liquid nitrogen storage tank at the set value. When filling liquid nitrogen tank trucks or when the vaporization pump is turned on due to supply needs, the pneumatic booster valve and booster are activated to increase the pressure in the storage tank and maintain the pressure of the liquid nitrogen storage tank at the set value.

[0021] A further technical solution of the present invention is to add a cryogenic nitrogen recovery device between the pipeline at the inlet end of the cryogenic nitrogen vent valve of the liquid nitrogen storage tank and the nitrogen filling pipeline of the cold box. The device includes a vent nitrogen recovery pipeline and components connected to the pipeline. The inlet end of the vent nitrogen recovery pipeline is connected to the inlet end of the cryogenic nitrogen vent valve. The vent nitrogen recovery pipeline is connected in sequence from the cryogenic nitrogen input direction to a cryogenic recovered nitrogen delivery valve, a cooling water heat exchanger, a normal temperature recovered nitrogen pressure regulating valve, a normal temperature recovered nitrogen check valve, and a normal temperature recovered nitrogen pressure gauge. The outlet end of the vent nitrogen recovery pipeline is connected to the outlet end of the low-pressure nitrogen check valve on the nitrogen filling pipeline of the cold box to deliver filling nitrogen to the cold box. The vented nitrogen recovery pipeline is divided into two parts. The pipeline before entering the cooling water heat exchanger is a low-temperature pipeline section. The diameter and pressure rating of this low-temperature vented nitrogen recovery pipeline are the same as those of the inlet pipeline of the low-temperature nitrogen vent valve. This pipeline is made of stainless steel. The part of the vented nitrogen recovery pipeline after entering the cooling water heat exchanger is a normal temperature pipeline section. The diameter and pressure rating of this normal temperature vented nitrogen recovery pipeline are the same as those of the nitrogen filling pipeline of the cold box. This pipeline is made of carbon steel.

[0022] By adding a cooling water heat exchanger, the low-temperature nitrogen gas, which is no more than -180°C, vented from the atmospheric pressure flat-bottom liquid nitrogen storage tank, exchanges heat with the return water, which is more than 30°C, from the air separation unit's circulating water system. The low-temperature nitrogen gas is then warmed to room temperature and reduced to 10 kPa through a pressure regulating valve before being fed into the cold box filling nitrogen pipeline as the cold box filling nitrogen.

[0023] A further technical solution of the present invention is that the cryogenic nitrogen recovery delivery valve is a pneumatic aluminum angle valve, and the valve's pneumatic actuator is a diaphragm type. Its valve diameter and flow rate are the same as those of the cryogenic nitrogen vent valve. The cooling water heat exchanger is a shell-and-tube type cooler, in which cryogenic nitrogen and cooling water return water are arranged in counter-current flow, with cryogenic nitrogen flowing forward to heat up and cooling water returning backward to cool down. The tubes of the cooling water heat exchanger are arranged in a square pattern, and the shell side is E-type. The shell, tube box, heat exchange tubes, and tube sheet of the cooling water heat exchanger are all made of stainless steel. The ambient temperature nitrogen recovery pressure regulating valve is a pneumatic single-seat pressure regulating valve, and its pneumatic actuator is a diaphragm type. Its valve diameter and flow rate are the same as those of the nitrogen filling pipeline. The ambient temperature nitrogen recovery check valve is a double-disc wafer check valve, and the ambient temperature nitrogen recovery pressure gauge is a capacitive pressure transmitter.

[0024] The pressure gauge installed before the cold box inlet on the nitrogen filling pipeline of the cold box is changed from a mechanical type to a capacitive pressure transmitter, and its pressure setting value is set according to the pressure value of the nitrogen filling pipeline of the cold box.

[0025] The automated operation technical solution of the present invention is as follows: the low temperature nitrogen recovery delivery valve, the normal temperature nitrogen recovery pressure regulating valve, and the normal temperature nitrogen recovery pressure gauge configured on the venting nitrogen recovery pipeline, as well as the pressure gauge configured on the cold box filling nitrogen pipeline, are electrically connected to the air separation unit DCS control system. The pressure setting value of the normal temperature nitrogen recovery pressure gauge is set according to the pressure value of the cold box filling nitrogen pipeline.

[0026] In the DCS control system of the air separation unit, control logic is set for the automatic operation, regulation, and shutdown of the cryogenic nitrogen recovery and reuse process in the atmospheric pressure liquid nitrogen storage tank. First, the cryogenic nitrogen recovery valve on the venting nitrogen recovery pipeline is opened, allowing cryogenic nitrogen to enter the cryogenic section of the venting nitrogen recovery pipeline. After heat exchange through the cooling water heat exchanger, the nitrogen rises to room temperature and enters the room temperature section of the venting nitrogen recovery pipeline, triggering the opening of the room temperature nitrogen recovery pressure regulating valve. Simultaneously, the valve opening is automatically adjusted according to the pressure set value of the room temperature nitrogen recovery pressure gauge to maintain the nitrogen pressure in the venting nitrogen recovery pipeline at the set value. Second, when the room temperature nitrogen recovery pressure gauge shows that the nitrogen pressure in the venting nitrogen recovery pipeline has reached the pressure set value, the low-pressure nitrogen regulating valve on the cold box filling nitrogen pipeline automatically closes, and the cryogenic nitrogen venting pipeline... The cryogenic nitrogen vent valve automatically closes, and the nitrogen from the outlet of the vent nitrogen recovery pipeline enters the cold box filling nitrogen pipeline and is used as cold box filling nitrogen. When the recovered nitrogen amount is insufficient to meet the cold box filling amount, and the nitrogen pressure in the vent nitrogen recovery pipeline does not reach the pressure set value, the low-pressure nitrogen regulating valve on the cold box filling nitrogen pipeline automatically adjusts and supplements nitrogen until the pressure value of the pressure gauge installed before the cold box inlet on the cold box filling nitrogen pipeline reaches the set value. When the recovered nitrogen amount is greater than the cold box filling amount, in order to maintain the nitrogen pressure in the vent nitrogen recovery pipeline at the set value, the cryogenic nitrogen vent valve opens until the pressure value reaches the set value and then closes. Finally, when the cold box or storage tank is shut down, the cryogenic recovered nitrogen delivery valve closes, triggering the automatic closure of the ambient temperature recovered nitrogen pressure regulating valve, and the cryogenic nitrogen vent valve of the liquid nitrogen storage tank automatically opens.

[0027] The implementation of this invention has a significant effect on saving energy, reducing costs, and reducing carbon emissions for enterprises. Attached Figure Description

[0028] Figure 1 Schematic diagram of the low-temperature nitrogen venting process of the 1000m³ atmospheric pressure flat-bottom liquid nitrogen storage tank in Maanshan Iron & Steel's 35000m³ / h air separation unit.

[0029] Figure 2 A schematic diagram of a low-temperature nitrogen recovery and reuse process for an atmospheric pressure liquid nitrogen storage tank according to the present invention.

[0030] Figure 3 A schematic diagram of the cryogenic nitrogen recovery and reuse process of a 1000m³ atmospheric pressure flat-bottom liquid nitrogen storage tank in an embodiment of the present invention for a 35000m³ / h air separation unit.

[0031] Figure Descriptions: B50, Atmospheric pressure liquid nitrogen storage tank; L2, Self-pressurizing pipeline; E2, Atmospheric pressure liquid nitrogen storage tank booster; V512, Atmospheric pressure liquid nitrogen storage tank pneumatic booster valve; W1, Circulating water cooling tower; W2, Air cooling tower; A1, Cold box; C01, Air compressor; C40, Oxygen compressor; C60, Nitrogen compressor; L4, Product liquid nitrogen delivery pipeline; V501, Product liquid nitrogen delivery valve; L1, Vent pipeline; V502, Cryogenic nitrogen vent valve; P4, Cryogenic nitrogen pressure gauge; L3, Cold box nitrogen filling pipeline; V505, Low-pressure nitrogen regulating valve; V506, Low-pressure nitrogen check valve; P3, Cold box nitrogen filling pressure gauge; F1, Cold box nitrogen filling flow meter; L5, Vent nitrogen recovery pipeline; V507, Low-temperature nitrogen recovery delivery valve; E1, Cooling water heat exchanger; V508, Normal-temperature nitrogen recovery pressure regulating valve; V509, Normal-temperature nitrogen recovery check valve; P1, Normal-temperature nitrogen recovery pressure gauge. Detailed Implementation

[0032] The following will be combined with the appendix of this invention. Figure 3 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.

[0033] This invention discloses a process for low-temperature nitrogen recovery and reuse in an atmospheric pressure liquid nitrogen storage tank, implemented in the modification of the 1000m³ atmospheric pressure flat-bottom liquid nitrogen storage tank B50 in the 35000m³ / h air separation unit of Maanshan Iron and Steel. The relevant technical parameters and working principle of the air separation unit involved in this invention are as follows:

[0034] 1. The design working pressure of the 1000m³ atmospheric pressure flat-bottom liquid nitrogen storage tank B50 is 18kPa, the design daily liquid nitrogen evaporation rate is 0.32%, and the design values ​​for cryogenic nitrogen emission, pressure, and temperature are 86m³ / h, 18kPa, and -180℃, respectively. The original design pressure setting of the cryogenic nitrogen pressure gauge P4 inside the atmospheric pressure flat-bottom liquid nitrogen storage tank B50 is 18kPa. The valve opening of the pneumatic booster valve V512 of the atmospheric pressure flat-bottom liquid nitrogen storage tank B50 is automatically adjusted to maintain the pressure inside the container at the set value of 18kPa under the action of the booster originally installed in the atmospheric pressure flat-bottom liquid nitrogen storage tank B50.

[0035] 2. The design values ​​for the inlet pressure, flow rate, and temperature of nitrogen filling in cold box A1 are 10 kPa, 90 m3 / h, and 20℃, respectively.

[0036] 3. The design value of the return water flow rate of the circulating water system is 5000 m3 / h, and the design value of the return water temperature is 40℃.

[0037] The technical solution of this invention is as follows: -180°C low-temperature nitrogen gas evaporated from the atmospheric pressure flat-bottom liquid nitrogen storage tank B50 is drawn out and exchanged with 40°C return water in the circulating water system pipeline through a cooling water heat exchanger E1. The temperature of the low-temperature nitrogen gas rises to 20°C and the pressure drops to 10KPa. It is then fed into the cold box filling nitrogen pipeline L3 as the filling nitrogen for the cold box A1, and the low-temperature nitrogen gas discharged from the atmospheric pressure flat-bottom liquid nitrogen storage tank B50 is recovered.

[0038] The specific implementation steps are as follows:

[0039] 1. A venting nitrogen recovery pipeline L5 is led out from the inlet pipe of the cryogenic nitrogen vent valve V502 on the vent pipeline L1 of the atmospheric pressure flat-bottom liquid nitrogen storage tank B50. The venting nitrogen recovery pipeline L5 is connected in sequence from the cryogenic nitrogen input direction to the cryogenic recovery nitrogen delivery valve V507, the cooling water heat exchanger E1, the ambient temperature recovery nitrogen pressure regulating valve V508, the ambient temperature recovery nitrogen check valve V509, and the ambient temperature recovery nitrogen pressure gauge P1. The outlet end of the venting nitrogen recovery pipeline L5 is connected to the outlet pipe of the low-pressure nitrogen check valve V506 on the original filling nitrogen pipeline L3 of the cold box, and the filling nitrogen is delivered to the cold box A1.

[0040] 2. The vented nitrogen recovery pipeline L5 is divided into two parts. The part leading out from the low-temperature nitrogen vent valve V502 and before connecting to the cooling water heat exchanger E1 is the low-temperature pipeline section. The part after connecting to the cooling water heat exchanger E1 is the normal temperature pipeline section. The vented nitrogen recovery pipeline L5 has the same diameter and pressure rating as the vent pipeline L1, both being 100mm in diameter and 3.1MPa in pressure rating. The low-temperature pipeline section is made of 316 stainless steel and is suitable for transporting nitrogen at -180℃. The normal temperature pipeline section is made of carbon steel Q235B and is suitable for transporting nitrogen at normal temperature.

[0041] 3. The valve diameter and flow rate of the cryogenic nitrogen recovery delivery valve V507 are 100mm and 500m3 / h, respectively, which are the same as those of the cryogenic nitrogen vent valve V502. The cryogenic nitrogen recovery delivery valve V507 is a cryogenic pneumatic aluminum angle valve with a diaphragm-type pneumatic actuator and an air-to-open valve. It is electrically connected to the operator station of the DCS control system of the 35000m3 / h air separation unit.

[0042] 4. Cooling water heat exchanger E1 is a shell-and-tube cooler, with low-temperature nitrogen on the shell side and cooling water return water on the tube side. In cooling water heat exchanger E1, the low-temperature nitrogen and cooling water return water are arranged in counter-current flow. The temperature of the low-temperature nitrogen rises to 20℃, which reaches the design value of 20℃ for the nitrogen inlet temperature of cold box A1.

[0043] The technical parameters and calculations for cooling water heat exchanger E1 are as follows:

[0044] (1) Design parameters

[0045] Low-temperature nitrogen cooling water

[0046] Flow rate 86 m³ / h 5000 m³ / h

[0047] Inlet temperature -180℃ 40℃

[0048] Outlet temperature 20℃ 40℃

[0049] Pressure 18 kPa (absolute pressure 120 kPa) 2500 kPa

[0050] (2) Heat load calculation (nitrogen heat absorption)

[0051] Average specific heat capacity of nitrogen (-180℃→20℃): CpN2≈1.05kJ / (kg·K).

[0052] Nitrogen density (-180℃, 120 kPa): ρN2=P / RT=0.297×93120≈4.35kg / m³, (gas constant R=0.297kJ / (kg·K), temperature is taken as 93K absolute temperature).

[0053] Mass flow rate: mN2 = 86 m³ / h × 4.35 kg / m³ = 374.1 kg / h = 0.104 kg / s.

[0054] Heat load: Q = mN2 × CpN2 × ΔT = 0.104 × 1.05 × [20 − (−180)] = 21.84 kW.h.

[0055] (3) Calculation of cooling water temperature drop

[0056] 1kW.h=3600kJ, 21.84×3600=78624 kJ

[0057] Water density ρ = 1000 kg / m³, specific heat Cpw = 4.18 J / (kg·K).

[0058] Cooling water flow rate: mw = 5000 m³ / h × 1000 kg / m³ = 5 × 10⁶ kg / h = 1388.9 kg / s.

[0059] 1 kilojoule = 0.000278 degrees Celsius

[0060] Cooling water temperature drop: ΔTw = Q / (mw×Cpw)×0.000278 = 78624 / (1388.9×4.18) = 13.54×0.000278 = 0.0038℃

[0061] → The cooling water temperature remains basically constant (40℃).

[0062] (4) Logarithmic mean temperature difference (ΔTlm)

[0063] Countercurrent arrangement:

[0064] Nitrogen: -180℃ → 20℃. Cooling water: 40℃ ← 40℃ (approximate).

[0065] Temperature difference calculation: Counter-current logarithmic mean temperature difference formula. For a counter-current heat exchanger, the formula for calculating the logarithmic mean temperature difference is: ΔTlm = (ΔT1−ΔT2) / ln(ΔT1 / ΔT2), where:

[0066] ΔT1: Temperature difference at one end of the heat exchanger. ΔT2: Temperature difference at the other end of the heat exchanger.

[0067] ΔT1=40−(−180)=220℃. ΔT2=40−20=20℃.

[0068] ΔTlm =(ΔT1−ΔT2) / ln(ΔT1 / ΔT2) =(220−20) / ln(220 / 20)≈88.6℃.

[0069] (5) Determination of the overall heat transfer coefficient (U)

[0070] Empirical range: for gas-water heat exchangers, U≈30−60W / (m²·K), and for low-temperature conditions, a conservative value of Uassumed=40W / (m²·K) is taken.

[0071] (6) Required heat transfer area (A)

[0072] A=Q / U×ΔTlm =21.84×1000 / 40×88.6≈6.16m².

[0073] (7) Structural design

[0074] Housing diameter: 300 mm (standard size).

[0075] Heat exchange tube: Material: 304 stainless steel (low temperature resistant). Specifications: Φ19 mm × 1.5 mm (outer diameter × wall thickness).

[0076] Length, 2 m (one way).

[0077] Number of pipes: Outer surface area of ​​a single pipe: Atube = π × 0.019 × 2 = 0.12 m².

[0078] Number of tubes: N = A / Atube = 6.16 / 0.12 ≈ 52 (rounded up to 54 tubes, arranged in a hexagonal pattern).

[0079] Shell-side flow path: single shell-side, arc-shaped baffle (25% cut).

[0080] (8) Design Results

[0081] Parameter values

[0082] Heat load 21.84kW

[0083] Heat transfer area: 6.16 m²

[0084] 54 heat exchange tubes × Φ19 × 2m

[0085] 300 mm in diameter

[0086] Fluid configuration type: single-shell / single-pipe counterflow

[0087] Overall heat transfer coefficient: 40 W / (m²·K)

[0088] 5. The ambient temperature nitrogen recovery pressure regulating valve V508 is a pneumatic single-seat pressure regulating valve with a diaphragm air-opening mechanism. The valve diameter and flow rate are 100mm and 500m³ / h, respectively. The ambient temperature nitrogen recovery check valve V509 is a double-disc wafer check valve used for low-pressure ambient temperature nitrogen transportation. The ambient temperature nitrogen recovery pressure gauge P1 is a capacitive pressure transmitter used for high-precision continuous pressure measurement of low-pressure ambient temperature nitrogen.

[0089] 6. In the DCS control system of the 35000m³ / h air separation unit, the cryogenic nitrogen recovery delivery valve V507, the ambient temperature nitrogen recovery pressure regulating valve V508, the ambient temperature nitrogen recovery pressure gauge P1, the cold box filling nitrogen pressure gauge P3, and the cold box filling nitrogen flow meter F1 configured on the venting nitrogen recovery pipeline L5 are electrically connected to the DCS control system of the air separation unit; the pressure setting value of the ambient temperature nitrogen recovery pressure gauge P1 is 10KPa, and the valve opening of the ambient temperature nitrogen recovery pressure regulating valve V508 is automatically adjusted to make the nitrogen pressure in the cold box A1 filling nitrogen pipeline reach the set value of 10KPa; the flow setting value of the cold box filling nitrogen flow meter F1 is 90m³ / h, and the valve opening of the cryogenic nitrogen recovery delivery valve V507 is automatically adjusted to maintain the nitrogen flow in the cold box A1 filling nitrogen pipeline at the set value of 90m³ / h.

[0090] 7. In the DCS control system of the 35000m3 / h air separation unit, design three-way control logic for the automatic supply, adjustment and shutdown of the 1000m3 atmospheric pressure flat bottom liquid nitrogen storage tank B50 cryogenic nitrogen recovery and reuse device.

[0091] The automatic operation procedures for the 1000m³ atmospheric pressure flat-bottom liquid nitrogen storage tank B50 cryogenic nitrogen recovery and reuse unit of the 35000m³ / h air separation unit are as follows:

[0092] First, the start switch of the cryogenic nitrogen recovery and reuse device is turned on. The cryogenic nitrogen recovery and reuse device is automatically put into operation. The cryogenic nitrogen recovery delivery valve V507 is automatically opened. Cryogenic nitrogen is input into the cryogenic pipeline section L5 of the vent nitrogen recovery pipeline. In the cooling water heat exchanger E1, after being heated to room temperature by cooling water, it enters the room temperature pipeline section L5 of the vent nitrogen recovery pipeline. The room temperature nitrogen recovery pressure regulating valve V508 is automatically opened.

[0093] Simultaneously, the cryogenic nitrogen recovery device is adjusted, and the valve opening of the ambient temperature nitrogen recovery pressure regulating valve V508 is automatically adjusted according to the set value of pressure gauge P1, so that the nitrogen pressure in the vented nitrogen recovery pipeline L5 reaches the set value of 10 kPa; at the same time, the low-pressure nitrogen pressure regulating valve V505 set on the filling nitrogen pipeline L3 is automatically fully closed, and the cryogenic nitrogen vented from the ambient pressure flat-bottom liquid nitrogen storage tank B50 is input into the cold box filling nitrogen pipeline and enters the cold box A1 for use as cold box filling nitrogen; at the same time, according to the set value of pressure gauge P4 of 18 kPa, the cryogenic nitrogen vent valve V502 is automatically fully closed, and the pneumatic booster valve V512 is automatically adjusted, so that all the cryogenic nitrogen vented from the storage tank is input into the cold box.

[0094] In this embodiment, the amount of cryogenic nitrogen vented from storage tank B50 is matched with the amount of nitrogen filled into the cold box, so there is no need to vent the cryogenic nitrogen or replenish the cold box with nitrogen from the low-pressure nitrogen pipeline.

[0095] Finally, when the cryogenic nitrogen recovery and reuse device is shut down, the cryogenic nitrogen recovery delivery valve V507 is fully closed, the ambient temperature nitrogen recovery pressure regulating valve V508 is automatically fully closed, the cryogenic nitrogen vent valve V502 is automatically opened, and the cryogenic nitrogen is released into the atmosphere and does not enter the cold box. The cryogenic nitrogen recovery and reuse device in liquid nitrogen storage tank B50 is shut down. Simultaneously, the low-pressure nitrogen pressure regulating valve V505 is opened, and the ambient temperature nitrogen from the low-pressure nitrogen pipeline network enters the cold box filling nitrogen pipeline L3 and enters the cold box A1 for use as cold box filling nitrogen.

Claims

1. A process for recovering and reusing cryogenic nitrogen from an atmospheric pressure liquid nitrogen storage tank, the process comprising: The system includes an atmospheric pressure flat-bottomed liquid nitrogen storage tank, a product liquid nitrogen delivery pipeline and a product liquid nitrogen delivery valve, a liquid nitrogen storage tank cryogenic nitrogen discharge pipeline and a cryogenic nitrogen pressure gauge and a cryogenic nitrogen vent valve arranged sequentially in the direction of cryogenic nitrogen output, a liquid nitrogen storage tank pressurization pipeline and a pneumatic pressurization valve and a pressurizer arranged on the pipeline; it also includes a cold box and a cold box filling nitrogen pipeline, the cold box filling nitrogen pipeline being led out from the low-pressure nitrogen pipeline network and connected to the cold box, with a low-pressure nitrogen regulating valve and a low-pressure nitrogen... The system includes a one-way valve, a pressure gauge and flow meter for nitrogen filling in the cold box; its features are: based on the temperature, pressure and flow rate of the low-temperature nitrogen in the atmospheric pressure flat-bottom liquid nitrogen storage tank and the nitrogen filling in the cold box, the system calculates the heat load, reheats and depressurizes the low-temperature nitrogen released from the atmospheric pressure flat-bottom liquid nitrogen storage tank, and inputs it into the nitrogen filling pipeline in the cold box as the nitrogen filling in the cold box; it calculates the matching rate between the low-temperature nitrogen discharge from the atmospheric pressure liquid nitrogen storage tank and the nitrogen filling in the cold box, and maximizes the recovery of the cold energy of the low-temperature nitrogen discharged from the liquid nitrogen storage tank.

2. The process for low-temperature nitrogen recovery and reuse in an atmospheric pressure liquid nitrogen storage tank according to claim 1, characterized in that: in 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 filling pipe of the cold box. 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 cooling water heat exchanger, a normal temperature nitrogen recovery pressure regulating valve, a normal temperature nitrogen recovery check valve, and a normal temperature nitrogen recovery pressure gauge. The outlet of the vent nitrogen recovery pipe is connected to the outlet pipe of the low-pressure nitrogen check valve on the nitrogen filling pipe of the cold box to deliver filling nitrogen to the cold box.

3. The process for recovering and reusing cryogenic nitrogen from an atmospheric pressure liquid nitrogen storage tank according to claim 2, characterized in that: The vented nitrogen recovery pipeline is divided into two parts. The pipeline before entering the cooling water heat exchanger is a low-temperature pipeline section. The diameter and pressure rating of this low-temperature vented nitrogen recovery pipeline are the same as those of the inlet pipeline of the low-temperature nitrogen vent valve. This pipeline is made of stainless steel. The part of the vented nitrogen recovery pipeline after entering the cooling water heat exchanger is a normal temperature pipeline section. The diameter and pressure rating of this normal temperature vented nitrogen recovery pipeline are the same as those of the nitrogen filling pipeline of the cold box. This pipeline is made of carbon steel.

4. The process for recovering and reusing cryogenic nitrogen from an atmospheric pressure liquid nitrogen storage tank according to claim 2, characterized in that: The cryogenic nitrogen recovery delivery valve is a pneumatic aluminum angle valve, and the valve's pneumatic actuator is a diaphragm type. Its valve diameter and flow rate are the same as those of the cryogenic nitrogen vent valve.

5. The process for recovering and reusing cryogenic nitrogen from an atmospheric pressure liquid nitrogen storage tank according to claim 2, characterized in that: The cooling water heat exchanger is a shell-and-tube cooler. Low-temperature nitrogen and cooling water return water are arranged in counter-current flow in the cooling water heat exchanger. Low-temperature nitrogen flows forward to heat up, and cooling water returns backward to cool down. The tubes of the cooling water heat exchanger are arranged in a square pattern, and the shell is of type E. The shell, tube box, heat exchange tubes, and tube sheet of the cooling water heat exchanger are all made of stainless steel.

6. The process for recovering and reusing cryogenic nitrogen from an atmospheric pressure liquid nitrogen storage tank according to claim 2, characterized in that: The ambient temperature nitrogen recovery pressure regulating valve is a pneumatic single-seat pressure regulating valve, and the valve's pneumatic actuator is a diaphragm type. Its valve diameter and flow rate are the same as those of the nitrogen filling pipeline. The ambient temperature nitrogen recovery check valve is a double-disc wafer check valve, and the ambient temperature nitrogen recovery pressure gauge is a capacitive pressure transmitter.

7. The process for recovering and reusing cryogenic nitrogen from an atmospheric pressure liquid nitrogen storage tank according to claim 2, characterized in that: The nitrogen pressure gauge for the cold box filling is a capacitive pressure transmitter, and its pressure setpoint is set according to the pressure value of the nitrogen filling pipeline.

8. The process for low-temperature nitrogen recovery and reuse in an atmospheric pressure liquid nitrogen storage tank according to claims 4, 6, and 7, characterized in that: The cryogenic nitrogen recovery delivery valve, ambient temperature nitrogen recovery pressure regulating valve, ambient temperature nitrogen recovery pressure gauge, as well as the cold box filling nitrogen pressure gauge and cold box filling nitrogen flow meter configured on the venting nitrogen recovery pipeline are electrically connected to the air separation unit DCS control system. The pressure set value of the ambient temperature nitrogen recovery pressure gauge is set according to the pressure value of the cold box filling nitrogen pipeline.

9. The process for recovering and reusing cryogenic nitrogen from an atmospheric pressure liquid nitrogen storage tank according to claim 8, characterized in that: In the DCS control system of the air separation unit, control logic is set for the automatic operation, regulation, and shutdown of the cryogenic nitrogen recovery and reuse process in the atmospheric pressure liquid nitrogen storage tank. First, the cryogenic nitrogen recovery valve on the venting nitrogen recovery pipeline is opened, allowing cryogenic nitrogen to enter the cryogenic section of the venting nitrogen recovery pipeline. After heat exchange through the cooling water heat exchanger, the nitrogen rises to room temperature and enters the room temperature section of the venting nitrogen recovery pipeline, triggering the opening of the room temperature nitrogen recovery pressure regulating valve. Simultaneously, the valve opening is automatically adjusted according to the pressure set value of the room temperature nitrogen recovery pressure gauge to maintain the nitrogen pressure in the venting nitrogen recovery pipeline at the set value. Second, when the room temperature nitrogen recovery pressure gauge shows that the nitrogen pressure in the venting nitrogen recovery pipeline has reached the pressure set value, the low-pressure nitrogen regulating valve on the cold box filling nitrogen pipeline automatically closes, and the cryogenic nitrogen venting pipeline... The cryogenic nitrogen vent valve automatically closes, and the nitrogen from the outlet of the vent nitrogen recovery pipeline enters the cold box filling nitrogen pipeline and is used as cold box filling nitrogen. When the recovered nitrogen amount is insufficient to meet the cold box filling amount, and the nitrogen pressure in the vent nitrogen recovery pipeline does not reach the pressure set value, the low-pressure nitrogen regulating valve on the cold box filling nitrogen pipeline automatically adjusts and supplements nitrogen until the pressure value of the pressure gauge installed before the cold box inlet on the cold box filling nitrogen pipeline reaches the set value. When the recovered nitrogen amount is greater than the cold box filling amount, in order to maintain the nitrogen pressure in the vent nitrogen recovery pipeline at the set value, the cryogenic nitrogen vent valve opens until the pressure value reaches the set value and then closes. Finally, when the cold box or storage tank is shut down, the cryogenic recovered nitrogen delivery valve closes, triggering the automatic closure of the ambient temperature recovered nitrogen pressure regulating valve, and the cryogenic nitrogen vent valve of the liquid nitrogen storage tank automatically opens.

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