Answers to Practical Questions on Air Separation

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1. What is dew point? How is the water content in air expressed using dew point in operations? With the water content in the air remaining constant, lowering the temperature of the air can turn the previously unsaturated water vapor in it into saturated vapor. The temperature at which water vapor becomes saturated is called the dew point. By measuring the dew point temperature, the vapor content can be found using a table of saturated vapor contents. The lower the dew point, the less moisture is present in the air. 2. What preparations are required before starting the purification system? (1) The new adsorbents should be activated as specified, and a set of purifiers should be regenerated and ready for use before starting the air separation tower; (2) Check whether the containers of the purifiers, along with their safety valves, pressure gauges, thermometers, etc., are still within their service life and function properly; (3) Verify that all valves in the purifier system are operating correctly; (4) Coordinate communication between different positions involved in the process, as well as with departments responsible for power supply, water supply, steam supply, and gas analysis. 3. What are the signs of excessive carbon dioxide and water levels in the air after a molecular sieve adsorber? What is the reason? Signs that the limits will be exceeded are: (1) At the end of the service cycle of the molecular sieve adsorber, the levels of carbon dioxide and moisture increase rapidly; it is necessary to keep an eye on the alarm values indicated by the tester. (2) There is signs of freezing of carbon dioxide and moisture inside the main heat exchanger, along with an increase in the temperature difference at the hot end. The reasons are: (1) The molecular sieve has been in use for too long, resulting in a decline in its adsorption capacity; (2) Incomplete regeneration of the molecular sieve, leaks in the regeneration heater, as well as humid regeneration gas or excessive moisture in the air, all affect the adsorption of carbon dioxide; (3) An uneven bed layer in the horizontal molecular sieve adsorber leads to airflow short circuits; (4) Gaps in the bed layer of the vertical molecular sieve adsorber also cause airflow short circuits. 4. Why must the adsorbent be cooled after regeneration before it can be put into use? Heated regeneration takes advantage of the fact that the adsorption capacity of the adsorbent decreases at high temperatures, allowing regenerating gas to drive away the water and carbon dioxide absorbed by the adsorbent. Therefore, at the regeneration temperature, the adsorbent no longer has the ability to adsorb anything; it can only be prepared for further adsorption after being cooled down to the normal operating temperature. 5. What are the symptoms of liquid presence in a turbine expander? What are the harms? How to prevent it? Due to the very high rotational speed of the working wheel in the turbine expander, liquid droplets striking the blades can cause wear and damage to those blades. The droplets are flung to the gap between the outer edge of the impeller and the guide vane, raising the liquid temperature and causing it to vaporize rapidly, with its volume expanding suddenly. It can be seen from the gap pressure gauge that the pointer swings violently, and in some cases the gauge may even get damaged; therefore, the presence of liquid is not allowed. To prevent the formation of liquid, it is necessary to keep the temperature behind the machine above the corresponding liquefaction temperature, generally above 3°C. 6. How many methods are there for adjusting the cooling capacity of a turbine expander? What is it? The methods for regulating the cooling capacity of turbine expanders include: (1) inlet throttling, (2) regulation of the degree of air intake, (3) flow regulation by adjusting the angle of the nozzle vanes, (4) changing the speed of the fan to adjust the cooling capacity, and (5) increasing or decreasing the number of expanders in use to regulate the cooling capacity. 7. Why is it necessary to use a pressurized sealing gas in turbine expanders? How to operate it? Low-temperature gas will leak from the gap between the rotating impeller shaft and the stationary casing, increasing heat loss and causing the lubricating oil to freeze. Divide the labyrinth seal or graphite seal into two sections and introduce pressure-sealing gas in between; this helps to reduce the leakage of cold air, and it also prevents lubricating oil from entering the expander or from freezing. Before starting the oil pump of the expander, seal gas should be supplied first; when shutting down, the seal gas should be removed. 8. What are the reasons for excessive vibration in the turbine expander? The reasons for excessive vibration in the turbine expander are as follows. (1) Poor dynamic balance of the rotor. The rotor undergoes dynamic balancing tests at the time of manufacture, but wear can occur during operation, or impurities inside and outside the impeller can freeze up, resulting in poor dynamic balance of the rotor and causing vibrations. (2) Rotor resonance. Rotor resonance generally occurs during commissioning operations, as a result of inadequate design, because the operating speed is close to the actual critical speed. (3) Oil film vibration. If the clearance between the journal and the bearing is not adjusted to the required range during installation, vibrations occur due to periodic changes in the thickness of the oil film during operation. (4) Liquid appears inside the expander. The presence of liquid can strike the blades, causing wear and breakage of those blades. The liquid is flung to the gap between the outer edge of the impeller and the diffuser; as its temperature rises, it vaporizes rapidly, causing its volume to expand suddenly and resulting in vibrations. (5) Surge in the brake fan. If the door covering the brake fan is closed too tightly, the fan will enter the surge zone and experience vibration. (6) Lubrication system failure. Low lubricant temperature, high viscosity, dirty oil, low oil pressure, and other factors can all cause vibration. 9. What is the reflux ratio? What is the impact on distillation? In air separation, the reflux ratio refers to the ratio of the amount of liquid flowing downward in the tower to the amount of vapor flowing upward. With a constant number of plates, the purity of the product depends on the reflux ratio; a higher reflux ratio results in higher purity of the gaseous nitrogen and lower purity of the liquid oxygen. Because when the reflux ratio is high, more cold liquid flows downward and less vapor rises upward, resulting in a relatively lower temperature; as a consequence, more oxygen condenses, and thus the purity of the rising nitrogen increases while the purity of oxygen decreases. When the reflux ratio is low, the situation is the opposite of what was described above. 10. Why can gaseous nitrogen be used as a cooling medium for liquid nitrogen in the liquid nitrogen subcooler? In this subcooler, gaseous nitrogen taken from the upper column is used to cool the liquid nitrogen that is supplied to the upper column. The pressure of nitrogen exiting the upper column is 0.15 MPa (absolute pressure), and it remains in gaseous state at -193°C; whereas the pressure of liquid nitrogen in the lower column is 0.6 MPa (absolute pressure), with a liquefaction temperature of -177°C. Therefore, the gaseous nitrogen from the upper column can be used to cool the liquid nitrogen in the lower column. 11. Why can liquid oxygen be used to condense gaseous nitrogen in a condensation evaporator? In the condensation evaporator, the liquid oxygen in the upper column absorbs heat and turns into gaseous oxygen, while the gaseous nitrogen in the lower column releases heat and condenses into liquid nitrogen. The boiling temperature of liquid oxygen in the upper column at 0.15 MPa (absolute pressure) is -180°C, while the liquefaction temperature of nitrogen in the lower column at 0.6 MPa (absolute pressure) is -177°C. Therefore, the boiling temperature of oxygen in the upper column is lower than the liquefaction temperature of nitrogen in the lower column, allowing liquid oxygen to be used to cool gaseous nitrogen. 12. How should the liquid level of liquid oxygen in air separation units be controlled and managed? The level of liquid oxygen has no effect on the purity of either liquid oxygen or liquid nitrogen; however, if the level is too low, vapor will be carried along with the liquid oxygen entering the upper column, which reduces the amount of vapor rising in the lower column and lowers the purity of oxygen in the liquid oxygen. In severe cases, this can lead to liquid leakage. If the level of liquid oxygen is too high and covers the trays in the lower column, distillation will no longer be possible. Therefore, the liquid level in Air Liquide should be maintained within a certain range. Closing the liquid nitrogen valve increases the amount of reflux liquid, causing the level of liquid air to rise and the oxygen content in the liquid air to decrease; conversely, the liquid level drops, and the purity of nitrogen in the upper part of the tower decreases. 13. What are the advantages of a low pressure in the upper tower? If the pressure in the upper column decreases by 0.001 MPa, the pressure in the lower column can be reduced by 0.03 MPa, while still maintaining the temperature difference in the main cooler. In this way, the air compressor can increase the amount of air, boost oxygen production, and reduce energy consumption. Additionally, distillation performance is good at low pressures. For example, at a pressure of 0.05 MPa (gauge pressure), the nitrogen concentration in the liquid is 50% while it is 83% in the gas phase; when the pressure rises to 0.1 MPa (gauge pressure), the nitrogen concentration in the liquid remains 50% but drops to 81% in the gas phase. This shows that the lower the pressure, the greater the concentration difference between the gas and liquid phases, and thus distillation is more effective at low pressures. 14. What is the function of liquid-air and liquid-nitrogen subcoolers? Liquefied air and liquid nitrogen are saturated liquids in the lower column; after pressure reduction through throttling, their original equilibrium is disrupted, resulting in a high vaporization rate of 17%–18%. This can affect the stability of the upper column, especially that of liquefied air. A high vaporization rate can affect the reflux ratio of the upper tower trays. If liquid air and liquid nitrogen are cooled with liquid nitrogen before entering the upper tower, thereby achieving a supercooling effect, the vaporization rate after throttling can be reduced to 11%–12%, which in turn stabilizes the operation of the upper tower. 15. How should the liquid level of liquid oxygen in the upper column of air separation equipment be controlled and managed? The level of liquid oxygen has no impact on the purity of either liquid oxygen or liquid nitrogen. However, if the level of liquid oxygen is too low, vapor will be carried into the upper column, which reduces the amount of vapor rising from the lower column and lowers the purity of oxygen in the liquid oxygen. In severe cases, this can lead to liquid leakage. If the level of liquid oxygen is too high and covers the trays in the lower column, distillation ceases to function. Therefore, the liquid level in Air Liquide should be maintained within a certain range. Closing the liquid nitrogen valve increases the amount of reflux liquid, raising the level of the liquid-air mixture and reducing the oxygen content in it; conversely, the liquid level drops, which lowers the purity of nitrogen in the upper part of the tower. 16. Why is it said that an increase or decrease in the liquid oxygen level in the main cooler indicates whether there is sufficient cooling capacity? When there is too much cooling capacity, the air entering the lower tower contains too much moisture; as a result, less nitrogen needs to be condensed in the condenser at the top of the lower tower, and accordingly, less liquid oxygen evaporates, causing the liquid oxygen level to rise. On the other hand, when there is insufficient cooling capacity, the air entering the lower tower contains too little moisture; this results in more nitrogen needing to be condensed in the condenser at the top of the lower tower, and consequently, more liquid oxygen evaporates, leading to a decrease in the liquid oxygen level. Therefore, the rise or fall in the liquid oxygen level in the main cooler is an indicator of whether there is sufficient cooling capacity. 17. Why do the levels of liquid nitrogen and liquid oxygen rise during temporary shutdowns? What should I pay attention to when driving again? During normal operation, the liquid on the distillation tray is held in place by the rising vapor, which moves at a certain speed as it passes through the small holes; this prevents the liquid from leaking out through those holes. The liquid can only flow along the tray before dropping into the overflow basin. When parking temporarily and no vapor is rising any more, all the liquid on the tray flows downward, causing the levels of liquid nitrogen and liquid oxygen to rise; in some cases these levels may even exceed the positions of the expanded air and oxygen outlets. Therefore, it is important to pay attention to the liquid levels when restarting the system. If the levels are too high, some of the liquid should be drained first before restarting to avoid accidents. 18. How is the main cooling unit in a fully low-pressure air separation plant operated? During normal operation, the main function of the primary cooler is to maintain the oxygen level at a specified height. The amount of cooling capacity is required for the cooling balance of the entire air separation unit. If the cooling capacity is greater than the required amount and the level of the main coolant rises, the cooling capacity should be reduced. If the cooling capacity is less than the required amount, the level of the main coolant will drop, and it is necessary to increase the cooling capacity. To maintain a stable liquid level, increase or decrease the cooling capacity. It mainly involves increasing or decreasing the expansion amount of the expander (or changing the pressure before the machine). When the main cooling liquid level is too high or too low, it is also necessary to check whether the other liquid levels are appropriate. If the level of the main cooling liquid is too high while the level of the liquid in the lower column is too low, it may be due to an excessive amount of liquid-air mixture being fed into the upper column; therefore, the liquid-air valve should be closed to maintain stable liquid levels. When the level of the main cooling liquid is too high, some liquid oxygen can be drained, which helps to lower the liquid level and remove some impurities, thereby enhancing safety. 19. How to detect leaks in air separation units? The main sign of a leak in an air separation unit is the continuous drop in the level of the main coolant. In the event of a large gas leak, there will be an increase in pressure inside the cryogenic tank, or cold air will escape from the gaps in the tank. In the event of a liquid leak, a significant drop in the base temperature can be detected. It is further possible to measure the amount of leaked cold air; if more than 80% is nitrogen, it can be determined as a nitrogen leak; if more than 80% is oxygen, it can be determined as an oxygen leak. The location of the leak should be near the areas on the cold box where sweating or frosting occurs. The leak location can also be determined by switching valves and equipment. 20. During operation, what is the impact of internal leaks in the air separation unit on cooling losses? Leaks within the air separation unit, especially leaks of liquids, cause significant heat loss, which can disrupt the normal operation of the unit and even prevent production from continuing, forcing a shutdown. Taking an oxygen leak as an example, if the leak rate is 1 L/min, the expander will increase its expansion volume by 636 m3/h to compensate for it. Therefore, any leakage of gas must be completely prevented.