Qualified operation of the air separation unit during the first eight hours of argon introduction in a cold state

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As the price of liquid argon in the liquid market continues to rise, it becomes particularly important to optimize the argon system in air separation units, as well as to carry out precise operations aimed at saving energy and increasing production. Optimizing the argon system not only lays a solid foundation for reducing the overall energy consumption of air separation units, but also creates more opportunities for selling liquid argon, thereby enhancing profitability. Therefore, as front-line production workers, it is essential to have a clear understanding of the precise operation of the entire air separation unit; innovative, optimized, energy-saving, and efficiency-enhancing methods of operation remain our constant challenge. Here is a summary of the experience gained from keeping the duration of each cold argon injection on April 22 and 24, 2014 – during which the oxygen and nitrogen contents in the liquid argon product were kept below 2 ppm – within eight hours, so that everyone can learn from one another and improve together. I. Precautions to be taken when shutting down the argon system 1. The liquid nitrogen feed valve V3 for the cooling source of the crude argon column condenser must be closed in a timely manner to prevent the column from being flooded or from drawing in air (if the argon vent valve of the crude argon column is not sealed properly, it may lead to the intake of moist air). In the case of a planned shutdown, it is recommended to stop the operation of the crude argon column condenser first, and then decide whether to start or stop the circulating argon pump based on the liquid level in the bottom of the crude argon column and the timing of restart. 2. Promptly close the valve of the argon removal storage tank for the product; close valve V706, which is used to remove liquid argon from the bottom of the pure argon tower, as well as the manual valve at its base. Drain any remaining liquid argon from the vacuum pipes to prevent contamination of the argon storage tank and to avoid explosions caused by the liquid argon in those pipes ； Switch off or disconnect the analysis instruments related to the argon system in a timely manner to prevent damage to the instrument components or contamination of the analysis channels. 3. The refined argon tower should be maintained at a slight positive pressure. This can be achieved by adjusting the heat source of the evaporator in the refined argon tower, or by regulating the flow of process argon through valve V9 that connects the crude argon tower to the refined argon tower. It is important to prevent contamination of the refined argon tower during shutdowns; in other words, during short-term shutdowns, efforts should be made to ensure that the tower is not contaminated by oxygen. To do this, first close the valve for supplying cold medium to the refined argon tower, thereby stopping the operation of its condenser, and also close the valve that allows process argon to enter the tower. The heat source of the evaporator in the refined argon tower should be adjusted according to the pressure within the tower, keeping it at a slight positive pressure. If the liquid level at the bottom of the refined argon tower drops too low, it is necessary to adjust the flow of process argon through the relevant valve to maintain a positive pressure in the tower. 4. Promptly and quickly start the circulating argon pump, which remains in a cold state after shutdown. Depending on the actual conditions on site (turning the pump → starting it → or draining liquid → heating → pre-cooling → turning the pump again → restarting), and based on past experience with starting such pumps, the time required from pre-cooling to successful startup can be kept within 10 minutes; almost no loss of liquid occurs. This approach helps to save time when introducing argon into the system. If the argon pump cannot be turned, it is necessary to heat the pump, close its inlet and outlet valves, and drain the liquid before heating it. The pressure of the heating gas should not exceed 60 kPa. As a general criterion for determining whether the heating is sufficient, the temperature should be above 0°C or the dew point should meet the requirements; in practice, however, it is sufficient as long as the pump can be turned. 5. For other operations, follow the operating procedures for the argon system for post-shutdown handling. II. The restart procedure for the argon system begins with the opening of the cold source injection valve V3 for the condenser in the crude argon column. It takes 6.5 hours to reduce the oxygen content in the argon used in the crude argon column process to ≤5 ppm, while it takes 1.5 hours to reduce the oxygen and nitrogen contents in the liquid argon product from the refined argon column to ≤2 ppm. 1. Adjust the operating conditions of the main column to ensure stability and to create the necessary conditions for argon introduction (main cooling load, cooling capacity, purity). 2. Start the circulating argon pump (or it is already running in circulation). 3. Timing for opening the cold source injection valve V3 of the crude argon column condenser: When the crude argon column is shut down, the accumulation of large amounts of liquid, or high liquid levels and flooding due to delayed closure of valve V3, can result in high resistance readings, full-scale readings, or distorted indications in the column. As the argon pump continuously transfers the liquid from the crude argon column to the main column, the amount of liquid accumulated in the crude argon column decreases, the liquid level at the bottom of the column drops back within normal ranges, and the resistance values return to their normal static levels when there is no flooding. Therefore, the author believes that valve V3 should be opened when the temperature on the argon side of the crude argon column condenser is normal and the resistance begins to decrease. At this point, the argon content in the argon fraction also starts to decline gradually from its previous full-scale value, indicating that the gas flow path in the crude argon column is now accessible and the conditions for distillation are met. It is advisable to start supplying cold source to the crude argon column condenser at this time in order to establish the distillation conditions (resistance values) as quickly as possible. There is no need to wait until the liquid level at the bottom of the crude argon column starts to drop, as the rate of decline in this liquid level depends on the operating conditions of the main column, and this process is quite slow, which would delay the initiation of argon supply. 4. Gradually open the cold source injection valve V3 of the crude argon column condenser, and inject cold source into it in a timely and appropriate manner to gradually establish the refining conditions for crude argon. This allows the resistance in the long argon column to rise slowly above 5 kPa and remain stable (resistance is a reflection of the load; the load should correspond to that of the air separation unit. Considering the minimum operating load of 50% for the packed tower, the resistance should not be lower than 4 kPa). 5. The amount of process argon vented from the crude argon column in the initial stage should be determined based on the temperature TI702 on the argon side of the crude argon condenser and the concentration of the argon fraction. If the temperature TI702 remains low for whatever reason – whether due to nitrogen or argon accumulating when the argon column is shut down, or because unstable operating conditions in the main column result in a high content of argon and nitrogen in the argon fraction – it is necessary to increase the amount of process argon vented and to reduce as quickly as possible the nitrogen and argon contents in the argon fraction, so as to restore a normal heat transfer difference in the crude argon condenser. At the beginning of argon introduction, the argon fraction concentration should be kept between 10-12%. Once the temperature TI702 on the argon side of the crude argon condenser returns to normal and the argon fraction concentration stabilizes within the 10-12% range, the venting rate of process argon from the crude argon column can be reduced to below 100 m3/h. 6. During the initial and intermediate stages of argon injection, the main cooling load of the main tower must be at normal levels; the amount of oxygen extracted from the product should be reduced appropriately. The oxygen content in the waste nitrogen should not be kept too low, with 0.6–1% being an appropriate range. As a measure to prevent nitrogen blockages, the control valve V1 for cooling the liquid in the lower tower should be set to manual control in order to pre-store sufficient cooling capacity. 7. Adjust the liquid air injection valve V3 for the cooling source of the crude argon column condenser and the backflow valve V701 after the argon pump in a timely manner, based on the operating conditions of the main tower, in order to establish stable liquid levels as soon as possible: 240 mm for the crude argon column condenser and 1400 mm for the liquid level in the bottom of the crude argon column. Ultimately, the resistance PdI702 in the long argon column should be stabilized at 5.8–6 kPa, while the resistance in the short argon column should be 2–2.5 kPa. During this phase, it is essential to keep the argon content in the argon fraction between 10–12% in order to accelerate argon enrichment; too high an argon content in the fraction can lead to a nitrogen blockage (the temperature TI702 on the argon side of the crude argon column condenser must not drop below -184.17°C), while too low an argon content will slow down the decrease in the oxygen content AI703 in the argon used in the process. Both situations will delay the start of argon utilization. 8. Instruments related to the argon supply system, such as the AI703 for measuring oxygen content in process argon and the AI702 for measuring oxygen content in crude argon, are used. At the initial stage of argon introduction, the oxygen content in process argon as indicated by AI703 is relatively high, and it is not possible to measure it using the instrument designed for detecting trace amounts of oxygen; in such cases, the AI702 instrument, which measures higher levels of oxygen, can be used to determine the oxygen content in process argon. This allows for timely monitoring of changes in AI703 values and enables appropriate adjustments to be made. Once the oxygen content in process argon drops within the range that the instrument for detecting trace amounts of oxygen can measure, normal operation can be resumed (this approach prevents delays in obtaining an accurate indication of product purity due to prolonged replacement of the analysis pipelines). 9. As the distillation operation in the crude argon column is gradually established and a concentration gradient of argon is formed within this column, the pressure after the circulating argon pump also increases. At this point, both the oxygen content in the process argon and that in the crude argon continue to decrease. When the pressure after the argon pump reaches 0.7 MPa, the oxygen content in the process argon drops to 5 ppm, and that in the crude argon drops to 35%, the refined argon system can be put into use. 10. When feeding argon for purification, first close the waste gas vent valve V760 of the pure argon tower. Set the control valve on the nitrogen side of the pure argon tower condenser at 45 kPa for automatic control, and then gradually open the cold source supply valve V5 of the pure argon tower condenser to supply cooling capacity to it in a timely and appropriate manner. 11. As the cooling source for the condenser of the refined argon column is continuously supplied, the pressure in this column will drop rapidly and experience significant fluctuations. It is necessary to promptly reduce the opening of the process argon vent valve V762, and increase the opening of the valve that sends process argon to the refined argon column, V9, to prevent a negative pressure from developing in the column. The best approach at this stage is to activate a portion of the heat source for the evaporator at the bottom of the pure argon column in advance; in other words, the establishment of resistance in the pure argon column should follow the principle of \"starting from the bottom and moving upward.\" This will accelerate the increase in the purity of the argon product. It also prevents large fluctuations in the pressure within the pure argon column due to increased cooling capacity in its condenser, which could otherwise lead to fluctuations in the amount of argon extracted for use in the process. Excessive extraction of argon from the crude argon column can cause the oxygen content in that argon to either rise or fall more slowly, thereby slowing down the purification process. However, the amount of argon fed into the pure argon column should be no less than 450 m3/h – firstly to ensure the minimum required operating load for distillation in the pure argon column, and secondly because failing to extract enough argon from the crude argon column can result in nitrogen buildup there. As the cooling capacity of the condenser and the heating capacity of the evaporator increase, the pressure in the pure argon column can be maintained stably (with the waste gas vent valve V76 set at 7–8 kPa for automatic control). Ultimately, the normal distillation conditions in the pure argon column should be maintained based on its resistance level, which should be between 1.25–1.4 kPa. 12. Stabilize the liquid argon level in the bottom of the refined argon tower, and promptly install the relevant analysis instruments for the liquid argon product from this tower, namely the analyzers for oxygen and nitrogen content in refined argon (AI704-1, AI704-2). Ensuring that the oxygen content in refined argon is less than 2 ppm and the nitrogen content is less than 3 ppm, the liquid argon should be pre-cooled in a timely manner and sent through the vacuum pipeline to the argon storage tank. 13. Towards the end of argon injection, as the operating conditions of the main tower continue to improve and the argon system reaches normal operation, the argon content in the argon fraction should be increased to between 13.5–14.5% Ar. The amount of argon extracted in the process should also be raised to 710 m3/h, as specified in the design, in order to increase the argon extraction efficiency. III. Several key operational points for reducing argon injection time 1. Proper operations after shutting down the argon system ； 2. Rapid start-up of the argon pump (zero cold loss) ； 3. Timely establishment of the heat transfer temperature difference in the crude argon column condenser, the resistance of the crude argon column, and the argon gradient concentration in the crude argon column ； 4. Rational extraction of argon flow rate in the crude argon column (at different stages) ； 5. Synchronous startup of the pure argon column at the end of the washing process in the crude argon column ； 6. Timely and proper deployment of relevant analysis instruments ； 7. Early activation of the heat source for the evaporator in the refined argon tower. IV. Economic Benefit Analysis: Based on the operation status of the air separation units over the past three years and the production trends at Southeast Steel, it is estimated that there will be at least eight planned and unplanned shutdowns per year. In the past, it took on average around 14 hours to restart the system after each shutdown. Using an normal production capacity of 710 m3/h for the argon system, zero loss of cooling capacity from this system, and a selling price of liquid argon equivalent to 1000 yuan/h, the economic benefit resulting from the time saved through faster restarts can be calculated as follows: 1000 yuan/h × 6 hours × 8 times/year = 48,000 yuan.