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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 convert 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 purification system are operating correctly; (4) Coordinate communication between different stations 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 limit 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 adsorbents be cold-blown after regeneration before they 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; it can only be prepared for re-adorption after being cooled down to the normal operating temperature. 5. What are the symptoms of liquid present in a turbine expander? What are the hazards? How to prevent it? Due to the very high rotational speed of the working wheel in the turbine expander, liquid droplets hitting 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 deflector, causing the liquid temperature to rise, leading to rapid vaporization and a sudden expansion in volume. It can be seen from the gap pressure gauge that the pointer swings violently, and it may even damage the gauge; 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 regulate the cooling capacity, and (5) increasing or decreasing the number of expanders in use to adjust 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. The labyrinth seal or graphite seal is divided into two sections, with pressure-sealing gas inserted 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 must be supplied first; upon shutdown, the seal gas should be cut off afterward. 8. What are the causes of excessive vibration in a turboexpander? The reasons for excessive vibration in the turbine expander are as follows. (1) Poor rotor dynamic balance. 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; its temperature rises, it vaporizes rapidly, and its volume expands suddenly, causing vibrations. (5) Surge of brake fans. 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 so on 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. When the reflux ratio is high, more cold liquid descends while less vapor rises; the temperature remains relatively low, and more oxygen components condense. As a result, the purity of the rising nitrogen is high, while that of oxygen is low. 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 a liquid nitrogen subcooler, gaseous nitrogen drawn 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 condensing 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 air level in air separation equipment be controlled and operated? The height of the liquid air level has no effect on the purity of liquid air and liquid nitrogen. However, if the liquid air level is too low, vapor will be entrained in the liquid air entering the upper column, thereby reducing the amount of rising vapor in the lower column. At the same time, this lowers the oxygen purity in the liquid air; in severe cases, it may even lead to liquid leakage. Conversely, if the liquid air level is too high, it will submerge the trays in the lower column, thus rendering the distillation process ineffective. 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 volume of air, thereby increasing oxygen production and reducing energy consumption. Additionally, distillation yields are better 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? In the lower column, liquid air and liquid nitrogen are both saturated liquids; 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 liquid 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 oxygen level in the upper column of an air separation unit be controlled and operated? The level of liquid air has no effect on the purity of either liquid air or liquid nitrogen. However, if the liquid air level is too low, it can cause vapor to be entrained in the liquid air entering the upper column, thereby reducing the amount of rising vapor in the lower column. This also decreases the oxygen purity in the liquid air; in severe cases, liquid leakage may occur. On the other hand, if the liquid air level is too high, it will submerge the trays in the lower column, thus rendering the distillation process ineffective. 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 decreasing the oxygen content in it; conversely, the liquid level drops, which reduces the purity of nitrogen in the upper part of the tower. 16. Why is it said that the rise or fall of the liquid oxygen level in the main cooler indicates whether there is sufficient cooling capacity? When there is an excess of cooling capacity, the humidity content of the air entering the lower tower increases. As a result, the amount of nitrogen that needs to be condensed in the condenser at the top of the lower tower decreases, and consequently, the evaporation rate of liquid oxygen also diminishes, causing the liquid oxygen level to rise. Conversely, when there is insufficient cooling capacity, the humidity content of the air entering the lower tower decreases. This leads to an increase in the amount of nitrogen requiring condensation in the condenser at the top of the lower tower; accordingly, the evaporation rate of liquid oxygen rises, causing its level to drop. Therefore, the rise or fall of the liquid oxygen level in the main cooler is an indication of whether there is sufficient cooling capacity. 17. When the system is temporarily shut down, why does the level of liquid air and liquid oxygen rise? 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 pores; this prevents the liquid from leaking out through those pores. The liquid can only flow along the tray before dropping into the overflow hopper. During temporary shutdowns, when vapor generation ceases, all the liquid on the trays flows downward. This causes the levels of liquid air and liquid oxygen to rise, potentially exceeding the levels at the outlets for expanded air and oxygen. Therefore, upon restarting the unit, one must pay attention to these liquid levels. If they are too high, some liquid should be drained off before restarting, in order to prevent any accidents. 18. How is the main cooler in a fully low-pressure air separation unit 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. When the cooling capacity exceeds the required amount and the level of the main refrigerant 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. Basically, it involves increasing or decreasing the expansion amount of the expander (or changing the pressure before the expander). 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 reduced in size to maintain stable liquid levels. When the level of the main cooling liquid is too high, some liquid oxygen can be drained to lower the level and to remove some impurities, which contributes to safety. 19. How to detect leaks in an air separation unit? The main indication of a leak in an air separation unit is a continuous drop in the main condenser liquid level. In the case of a large gas leak, there may be a rise in the pressure within the cold box, or cold air may escape from gaps in the cold box. 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 leaking 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.