Air separation test questions

As a beginner who doesn’t know where to start, I’d like some recommendations – what kinds of books can be read and what practice tests are available.

Q&A on Air Separation Operations 1. What are the methods of air separation? Answer: There are three methods used in industrial production for air separation: (1) Deep freezing method: The air is first liquefied, and then, taking advantage of the difference in boiling points between oxygen and nitrogen, these gases are separated through a distillation process in specialized equipment (distillation towers). This method is the most economical for large-scale air separation plants. It can also produce oxygen and nitrogen products with very high purity. (2) Pressure swing adsorption method: The production of oxygen or nitrogen using the pressure swing adsorption method is carried out at room temperature. There are two mechanisms for this: one is to utilize the higher adsorption affinity of zeolite molecular sieves for nitrogen compared to oxygen, in order to separate oxygen from nitrogen ； Second, it takes advantage of the fact that oxygen diffuses through the micropores of molecular sieves faster than nitrogen. Separate oxygen and nitrogen under conditions far from equilibrium. Currently, the capacity and purity of the products produced by devices that use pressure swing adsorption to generate oxygen or nitrogen are subject to certain limitations. For example, devices that use this method to produce oxygen generally have a capacity of no more than 4000 Nm³/h, with a purity of no more than 95% ； Nitrogen production units generally have a capacity of less than 2000 Nm³/h, with a purity lower than 99.5%. (3) Membrane separation method: The method of separating the oxygen and nitrogen components in air by utilizing the permeation selectivity of polymer membrane films is called membrane separation. The devices used to produce oxygen or nitrogen by this method have limitations in terms of capacity and purity; they are generally used to produce nitrogen gas with a production rate of less than 800 Nm³/h and a purity of less than 99.5%. 2. What are the uses of oxygen? Answer: Oxygen is a substance upon which all living organisms on Earth depend for survival. It has very reactive chemical properties and easily combines with other substances to form oxides. Taking advantage of this physical property, oxygen is widely used in industries such as metallurgy, chemical engineering, and the defense industry. In the production of methanol synthesis, oxygen undergoes a partial oxidation reaction with water-coal slurry to produce effective feed gases: hydrogen and carbon monoxide. 3. What are the uses of nitrogen? Answer: (1) The molecular structure of nitrogen is very stable; it generally does not react easily with other substances, exhibiting high inertness. Therefore, it is commonly used as a protective gas in industry. (2) Storing fruits and vegetables under nitrogen is an advanced method for preservation; it allows these fruits and vegetables to slow down their metabolism in an environment with high nitrogen levels and low oxygen levels, entering a sort of dormant state and preventing further ripening, thereby enabling long-term preservation. (3) \"Vacuum nitrogen filling\" for storing rice and other grains helps to prevent infestations by insects, heat buildup, and mold growth. (4) Nitrogen is one of the essential nutrients for plant growth. Nitrogen in the air cannot be absorbed directly by plants; therefore, synthetic ammonia is produced, and this ammonia is then used as a raw material to manufacture various nitrogen fertilizers that can be absorbed by plants, such as urea. 4. What are the uses of argon? Answer: Argon is an inert element, accounting for 0.93% by volume in air. It has a wide range of applications; for example, argon is often used as a shielding gas in the welding of aluminum, aluminum alloys, and stainless steel ； It can also be used for the refining of semiconductor materials and rare metals, as well as in the manufacture of arc lamps ； Ultra-low carbon stainless steels and similar materials can be obtained using the \"argon-oxygen steelmaking method\". 5. What impurities are present in air? Why do they need to be removed? Answer: Air contains small amounts of water vapor, carbon dioxide, acetylene, hydrocarbons, as well as a little dust and other impurities. Allowing these impurities to enter air separation units is extremely dangerous; solid impurities can wear out the equipment, posing a threat to the safe operation of high-speed rotating devices. If they enter heat exchangers, they can also contaminate the heat transfer surfaces, reducing the efficiency of heat transfer ； Water vapor, CO2, and other substances will sequentially freeze and precipitate at low temperatures, thereby blocking the gas channels and the pores of the tray plates ； An excessive accumulation of acetylene and hydrocarbons in the distillation tower can easily lead to an explosion. Therefore, to improve the safety, reliability, and cost-effectiveness of the device’s operation, it is necessary to purify the air. 6. How does this device remove impurities from the air? Answer: The impurities in the air consist of both solid and gaseous substances; solid impurities include dust, mechanical particles, etc. These solid impurities are removed by the self-cleaning air filter K7600, which is located before the air compressor. Gas impurities include H2O, CO2, and hydrocarbon compounds such as CmHn. This device uses a pre-filter to purify these gas impurities; molecular sieve adsorbers MS711201/MS711202 are installed after AC711101. MS711201/MS711202 are double-layer horizontal adsorbers composed of alumina (Al2O3) and molecular sieve (13X). Alumina is used first to absorb moisture, while the molecular sieve is responsible for absorbing CO2 and CmHn. 7. How does the contaminated nitrogen in the water cooling tower (AC710002) cool the water? Answer: The nitrogen-water cooling tower in the second phase of our factory is a packed-type cooling tower. Circulating water at room temperature is sprayed downward from the top of the tower, while the contaminated nitrogen gas coming out of the heat exchanger (at a temperature of around 15°C) flows upward. The direct contact between the two enables both mass transfer and heat transfer; it is a rather complex process. On one hand, since the temperature of the water is higher than that of the contaminated nitrogen, heat is transferred directly from the water to the nitrogen, resulting in the cooling of the water. On the other hand, since polluted nitrogen is a dry gas, water molecules can continuously evaporate and diffuse into it. Evaporation of water requires the absorption of latent heat of vaporization, which removes heat from the water and causes its temperature to drop continuously; thus, the moisture-absorbing property of polluted nitrogen is the main reason for the cooling of water. Knowing this, one can understand why sometimes the outlet temperature of the cooling water is lower than the inlet temperature of the polluted nitrogen. 8. Why does the moisture content in air decrease after passing through a cooling tower? Answer: At a given pressure, the water content in saturated moist air decreases as the temperature drops. After being compressed by the air compressor, the process air enters the air water-cooling tower at a pressure of around 0.545 MPa(G), at which point its moisture content reaches saturation. The temperature continues to drop as it flows through the air cooling tower. As the temperature decreases, the saturated moisture content drops, meaning that some of the water evaporates from the air. This portion of water vapor condenses into water, releasing latent heat of condensation in the process. 9. What is an adsorbent? What are the requirements for adsorbents? Answer: A porous solid used for adsorption is called an adsorbent. The main requirements for adsorbents are: (1) The adsorbent must be a porous material. Since adsorption occurs only on the surface of solids, the more pores there are, the larger the surface area and thus the stronger the adsorption capacity. (2) The adsorbent must exhibit selective adsorption, that is, the adsorption process should be selective. It cannot adsorb all components of the mixture; it only adsorbs the impurities that need to be removed from the mixture. (3) The adsorbent should have a high capacity, that is, a large amount of substance can be adsorbed per kilogram of adsorbent. If the adsorption capacity is low, a larger amount of adsorbent is required. (4) It must have sufficient strength, be pressure and wear-resistant, and not easily break. (5) Easy to regenerate. (6) Low price. Among those that meet the above requirements, good adsorbents include commonly used silica gel, alumina gel, and molecular sieves. 10. What factors are related to the adsorption capacity? Answer: There are many factors that affect the adsorption capacity, the main ones being: (1) It is related to the temperature of the adsorption process and the partial pressure of the component being adsorbed. At the same partial pressure (or concentration) of the adsorbed component, the adsorption capacity decreases as the temperature rises ； At the same temperature, the adsorption capacity increases as the partial pressure (or concentration) of the adsorbed component rises, but there is a limit; once the partial pressure reaches a certain level, the adsorption capacity becomes essentially independent of it. . (2) It is related to the flow velocity of the gas (or liquid). The higher the flow rate, the worse the adsorption effect, as the contact time between the substance to be adsorbed and the adsorbent is short; a lower flow rate results in a better adsorption effect. (3) It is related to the degree of regeneration perfection of the adsorbent. The more thorough the regeneration desorption, the greater the adsorption capacity; conversely, the lower it is. (4) It is related to the thickness of the adsorbent. The greater the thickness of the adsorbent layer, the longer the contact time between the substance to be adsorbed and the adsorbent, and the greater the capacity. 11. What is the regeneration of an adsorbent? Answer: The adsorption process is a reversible physical process. All adsorbents have an adsorption capacity. When the adsorption reaches or exceeds this capacity, the adsorbent loses its ability to adsorb the adsorbate. At this point, it is necessary to desorb the adsorbent, thereby releasing the adsorbed substance from it and **reducing** the amount of adsorbate held by the adsorbent; once this is done, the adsorbent regains its adsorption capacity. The process of restoring the activity of an adsorbent that has become saturated with adsorbate, thereby regaining its adsorption capacity, is called regeneration. Regeneration is the reverse process of adsorption and is an endothermic reaction. There are two ways for regeneration: one is to increase the temperature in order to reduce the adsorption capacity and thereby cause the adsorbate to desorb ； Secondly, it reduces pressure to cause desorption of the adsorbate; this device uses low-pressure nitrogen gas heated to 150°C to regenerate the adsorbent. 12. Why is pressure equalization required when switching molecular sieve adsorbers? Answer: (1) Equalizing pressure before switching the molecular sieve adsorber is necessary to ensure the stable operation of the entire system. When switching molecular sieve adsorbers without equalizing the pressure, the air flow rate and pressure of the system experience sudden fluctuations as the internal space of the container is filled. (2) Additionally, if a molecular sieve adsorber that is about to be put into use is not subjected to pressure equalization first, it will suffer from strong impacts from the air during switching, which can cause the partitions to deform or even get damaged. Aluminum adhesive and molecular sieves also rub and compress against each other due to impact, generating dust particles that enter the channels of the adsorbent and cause blockages, thereby reducing the adsorption efficiency. Therefore, the molecular sieve adsorber must be pressure-equalized before switching. 13. Describe the programmed steps for the molecular sieve adsorbers. Answer: The two sets of molecular sieve adsorbers operate in alternation; when MS711201 is in the adsorption state, the other set, MS711202, is in the regeneration state. The program switching of each valve is controlled by the programs in the DCS system. The automatic control process of the molecular sieve adsorber is actually the self-control process for the regeneration of the molecular sieve adsorber; this self-control process consists of 18 steps, and the first step is initiated when the switching program is activated. Step 1: Prepare for pressure release: Turn off V1201 and V1203; confirm that they are turned off. Step 2: Pressure release: Turn on V1205, wait for 8 minutes; once PS961201 reaches the specified value and the time is up, turn off V1205. Step 3: Prepare for heating: Turn on V1213; confirm that it is turned on. Step 4: Heating: Turn off V1217 and V1210, turn on V1211 and V1218; wait for 100 minutes until the time is up. Step 5: Cooling: Turn off V1218, turn on V1217; wait for 100 minutes until the time is up. Step 6: Prepare for pressurization: Turn off V1211 and V1213, turn on V1210; confirm that V1211 is turned off. Step 7: Pressurization: Turn on V1207, wait for 22 minutes; once PDS961201 reaches the specified value and the time is up, turn off V1207. Step 8: Prepare for switching: Turn on V1203; confirm that it is turned on. Step 9: Switching: Turn on V1201; confirm that it is turned on, then wait for 10 minutes. Step 10: Prepare for pressure release: Turn off V1202 and V1204; confirm that they are turned off. Step 11: Pressure release: Turn on V1206, wait for 8 minutes; once PS961203 reaches the specified value and the time is up, turn off V1206. Step 12: Prepare for heating: Turn on V1214; confirm that it is turned on. Step 13: Heating: Turn off V1217 and V1210, turn on V1212 and V1218; wait for 100 minutes until the time is up. Step 14: Cooling: Turn off V1218, turn on V1217; wait for 100 minutes until the time is up. Step 15: Prepare for pressurization: Turn off V1212 and V1214, turn on V1210; confirm that V1212 is turned off. Step 16: Pressurization: Turn on V1207, wait for 22 minutes; once PDS961201 reaches the specified value and the time is up, turn off V1207. Step 17: Prepare for switching: Turn on V1204; confirm that it is turned on. Step 18: Switching: Turn on V1202; confirm that it is turned on, then wait for 10 minutes. 14. What is enthalpy? What is entropy? Answer: Enthalpy is a concept used to represent the two forms of energy in a fluid—internal energy and kinetic energy. In other words, enthalpy represents the sum of the internal energy and kinetic energy of a fluid during flow; Enthalpy = Internal Energy + Kinetic Energy. Entropy is a state parameter of a gas; it has a certain value when the pressure and temperature of the gas are fixed. The unit of S is J/kg·K. When a gas absorbs heat, its entropy increases, and the amount of increase in entropy, ΔS, is equal to the heat absorbed, ΔQ, divided by the gas’s temperature T. That is, △s = △Q/T. Simply put, entropy is a quantity that measures the inequality between two states. 15. How does the energy of air change during isothermal compression? Answer: During isothermal compression, since the temperature remains constant, the kinetic energy of the gas molecules does not change. Meanwhile, as the distance between the molecules decreases, the potential energy resulting from the interactions between them also decreases. Therefore, the energy within the gas decreases. Moreover, when the pressure is not very high, compressed air can be treated as an ideal gas, and the work of flow for an ideal gas depends solely on temperature; with the same temperature, the work of flow remains the same. Therefore, from an energy perspective, the enthalpy of the gas after isothermal compression is lower than its enthalpy before compression. 16. What is cooling capacity? Answer: Cooling capacity is a term commonly used in refrigeration. Refrigeration involves achieving a temperature lower than that of the surrounding air; such a low temperature enables the substance in question to absorb heat from the surrounding air. The degree to which a substance at a lower temperature can absorb heat from the surrounding air is what is referred to as cooling capacity. The lower the temperature of an object (the greater the temperature difference with the surrounding air), and the greater its quantity, the greater its ability to absorb heat, that is, the greater its cooling capacity. 17. What is refrigeration? Answer: According to the second law of thermodynamics, heat can only spontaneously flow from a hotter object to a colder one. The reverse process cannot occur spontaneously. That is, without special methods, it is not possible to achieve a temperature lower than the ambient temperature. People have mastered methods of achieving low temperatures through practice. The process of achieving low temperatures by expending a certain amount of energy to compress a gas and then allowing it to expand is called refrigeration. A machine that produces low temperatures is called a “refrigerator”. For ordinary refrigerators, they can only achieve low temperatures in the low tens of degrees below zero; this is commonly referred to as standard freezing. Such as refrigerators, air conditioners, etc. In air separation units, it is necessary to achieve low temperatures of below –100°C, a condition commonly referred to as deep freezing, or simply “deep cryogenics”. 18. What is cooling capacity? Answer: Cooling capacity refers to the amount of energy (heat) that can be removed by an entire air separation unit. Depending on the method used to remove this energy, cooling capacity can be classified into cooling capacity resulting from throttling effects, cooling capacity resulting from expansion mechanisms, and cooling capacity from ammonia coolers. 19. What is throttling? Answer: When a gas or liquid flows through a constriction or valve in a pipe, its flow is hindered; vortices and collisions occur at the valve, as well as other forms of frictional resistance. For the fluid to pass through this valve, it must overcome these resistances, which results in the pressure behind the valve being much lower than the pressure before the valve. The process in which the pressure of a fluid decreases significantly due to local resistance encountered during flow is commonly referred to as throttling. 20. Why does the temperature generally decrease during gas throttling? Answer: From the perspective of the throttling process, it is a process of pressure reduction, and this pressure reduction is entirely used to overcome resistance; no work is outputted to the outside. At the same time, as the gas flows through the throttle valve, the time involved is very short, so it can be considered an adiabatic process. For the fluid itself, the total energy within it remains unchanged before and after throttling. As a fluid flows, the energy it possesses includes internal energy (the kinetic energy of molecular motion and the potential energy resulting from molecular interactions), as well as kinetic energy arising from the flow, where the fluid behind pushes the fluid ahead. The sum of these three forms of energy remains constant. And the magnitude of each form of energy changes before and after throttling. When throttling occurs, the pressure drops, causing the gas volume to expand and the distance between molecules to increase, which in turn raises the potential energy of the molecular interactions. Under normal circumstances, the change in kinetic energy is relatively small. Therefore, an increase in potential energy leads to a decrease in kinetic energy. The magnitude of the molecular kinetic energy reflects the temperature of the object. Kinetic energy decreases after throttling; therefore, under normal circumstances, the temperature of a gas always drops after throttling, and this is the case in air separation units. 21. What factors are related to the magnitude of the temperature drop during throttling? Answer: The purpose of throttling is generally to achieve a low temperature; therefore, the greater the temperature drop, the better. The factors affecting the throttling temperature drop effect are: (1) Temperature before throttling: The lower the temperature before throttling, the better the temperature drop effect. . (2) Pressure difference before and after throttling: The greater the pressure difference before and after throttling, the better the temperature reduction effect. 22. Why doesn’t the temperature drop when water flows through a valve for throttling, whereas the temperature of liquid air and contaminated nitrogen decreases when they flow through a throttle valve into the upper tower? Answer: When liquid air from the lower tower is throttled and sent to the upper tower, its temperature drops from –172°C to –190°C; the same phenomenon occurs with contaminated nitrogen, with its temperature dropping by more than ten degrees as well. This is because liquid air and contaminated nitrogen are saturated liquids before throttling, and when the pressure drops after throttling, some of the liquid molecules vaporize. Vaporization requires the absorption of heat, which leads to a decrease in the liquid’s temperature. Therefore, after throttling, it is a vapor-liquid mixture, with a temperature corresponding to the saturation temperature at the pressure after throttling. At environmental temperatures and pressures higher than atmospheric pressure, water is at a temperature far below its saturation point; it is therefore a \"supercooled\" liquid. Throttling does not cause this water to boil and vaporize, so no temperature drop due to throttling occurs. 23. What is the cooling capacity of expansion refrigeration? Answer: The cooling capacity of expansion refrigeration refers to the amount of energy removed from the expanding gas by means of the work done by that expanding gas. That is, the difference in enthalpy between the gas at the inlet and the gas at the outlet of the expander. 24. How does a turbine expander work? Why is cooling produced? Answer: A turbine expander is a type of rotary refrigeration machine that consists of components such as a volute, guide vanes, and working wheels. When gas under certain pressure enters the volute, it is distributed to the diffusers, which are equipped with adjustable nozzle vanes. In the nozzle, the gas converts its internal energy into kinetic energy of the flow; as a result, pressure and enthalpy decrease, and the flow velocity can increase by about 200 m/s. When this high-speed airflow hits the blades of the impeller, it drives the impeller to rotate, converting kinetic energy into mechanical energy, which is then used through the shaft of the rotor to make the turbocharger do work. From the perspective of the entire process, the decrease in gas pressure is an expansion process, during which work is done on the outside. The delivery of external work is achieved by consuming internal energy, which results in a decrease in temperature and enthalpy; in other words, a portion of the energy within the gas is removed, and this is what is commonly referred to as the cooling capacity. 25. How do the vanes in a turbine expander increase the gas flow velocity? Can it generate cold energy? Answer: In daily life we can observe that the faster the water flow is in areas where the river channel is narrower. Inside the expander guide vanes, the cross-section of the flow channel on the nozzle blades is given a specific shape, with one section of this cross-section gradually decreasing; as the cross-section decreases, the speed of the air flow increases accordingly. When there is a certain pressure difference before and after the deflector, gas flows through the nozzle; the flow velocity increases, which raises the kinetic energy of the gas. This increase in kinetic energy results from the consumption of energy within the gas, manifesting as a decrease in gas temperature and enthalpy behind the deflector. The increase in its kinetic energy is equal to the decrease in its enthalpy; this represents two different types of energy conversion within the gas. No energy is released, and the total internal energy does not decrease; therefore, it cannot be concluded that cooling is generated as the airflow passes through the deflector. 26. What factors affect the cooling capacity of expansion refrigeration? Answer: The cooling capacity of an expander is related to the amount of expansion and the specific cooling capacity. The greater the expansion volume, the greater the cooling capacity. The specific cooling capacity is related to the pressure and temperature before expansion as well as the pressure after expansion, and the relationship is as follows: (1) When the inlet and outlet pressures are constant, the higher the temperature before the compressor, the greater the specific cooling capacity. Therefore, when the total cooling capacity required by the device remains constant, increasing the temperature before the compressor can reduce the amount of expansion. (2) When the inlet temperature is constant, it depends on the pressure difference between the inlet and outlet of the expander; the greater the pressure difference, the higher the unit cooling capacity. (3) It is related to the efficiency of the expander; higher efficiency results in a greater cooling capacity. 27. Why is a pressurized sealing gas used in turbine expanders? Answer: Turbine expanders require that all the gas entering the expander be able to expand through the guide vanes and the working wheel to generate cooling effect; however, the working wheel is a component that rotates at high speeds. The casing is a stationary component, and low-temperature gas may leak out through the gaps in the casing. This will reduce the total cooling capacity of the expander, while increasing cooling losses. Furthermore, the leakage of cold air can also cause the bearing lubricant to freeze, leading to mechanical failures. Therefore, a reliable seal must be used. Labyrinth seals are usually used. As the gas flows through the seal gap, the pressure gradually decreases, and the amount of leakage depends on the magnitude of the pressure difference. Therefore, by applying pressurized sealing gas to the outside of the sealing device, the pressure difference can be reduced, thereby decreasing the leakage of low-temperature gas. It also prevents bearing lubricant from seeping into the seal and entering the expansion turbine. 28. What is a pressurized turbine expander? Answer: A pressurized turbine expander uses the output power of the expander to directly increase the pressure of the gas entering the expander, thereby raising the pressure of the gas undergoing expansion. This increases the pressure difference before and after the expander, enhancing the amount of cooling capacity generated per unit of expanded fluid. Reducing the amount of expansion means less compression work is required in the cycle, which saves energy; it also avoids losses that occur when mechanical energy is converted into electrical energy, improving the efficiency of recovering expansion work. It can be said to be a more effective solution than the motors or fans traditionally used as braking devices for expanders. 29. Why can air turn into a liquid? Answer: For the same substance, under different conditions, it can exist in three different states: gas, liquid, and solid. Furthermore, they can transform into one another depending on the conditions, such as gas phase turning into liquid phase, and liquid phase turning into solid phase. This change in state is called a phase transition. The underlying reason for phase changes is that all substances are composed of molecules or atoms, and there are forces acting between these molecules and atoms. When the intermolecular forces increase to the point where the molecules can no longer move freely, the substance exists in a solid or liquid state. Therefore, the state of a substance depends on the energy level of its internal molecules. For air, as the temperature drops, the molecules lose their ability to move freely, the distance between them decreases, and the forces acting between them increase. Therefore, when the temperature drops to a certain level, air can turn into a liquid; this temperature is called the liquefaction temperature. At one atmosphere pressure, the liquefaction temperature of air is –191.3°C to –194.3°C. 30. Is the liquefaction temperature the same as the boiling temperature? Answer: The temperature at which a substance changes from a gas phase to a liquid phase is called the liquefaction temperature (also known as the dew point) ； The temperature at which a liquid turns into a gas is called the boiling temperature (also known as the boiling point). For the same substance, at a certain pressure, the liquefaction temperature of the gas is equal to the boiling temperature of the liquid; the two values are identical. So it is usually referred to as the saturation temperature of that substance at a certain pressure. In the case of mixtures such as air, during liquefaction, since the liquefaction temperature of oxygen is higher than that of nitrogen, more oxygen liquefies first. As the nitrogen concentration in the air increases, the liquefaction temperature continues to drop. Therefore, for mixtures, the liquefaction temperature and boiling temperature are different values, and at a certain pressure, the boiling temperature is lower than the liquefaction temperature. They are not constants. At 0.5 MPa, the liquefaction temperature of air is –174°C to –177.5°C. 31. How is the air in a distillation tower separated into oxygen and nitrogen? Answer: A distillation tower is a device that uses distillation to separate various components, thereby obtaining components of high purity. After being cooled to a temperature close to the liquefaction point, the air is fed into the lower section of the distillation tower. There, it comes into thorough contact with the cooler reflux liquid from bottom to top, allowing for mass and heat transfer, which results in part of the air condensing into a liquid. Since oxygen is a poorly volatile component while nitrogen is a readily volatile one, during the condensation process, more oxygen condenses than nitrogen, thereby increasing the purity of nitrogen in the gas. At the same time, when the gas condenses, latent heat of condensation is released, causing part of the returning liquid to vaporize. Since nitrogen is a volatile component, it evaporates more than oxygen, thereby increasing the oxygen purity in the liquid. In this way, the gas exchanges heat and mass with the reflux liquid on each tray, moving from bottom to top; with each tray passed, the purity of nitrogen in the gas increases. By the time the gas reaches the top of the tower, most of the oxygen has been condensed into the liquid, resulting in a nitrogen purity of 99.999% in the gas phase. A portion of the nitrogen enters the condensation evaporator, where it is condensed into liquid nitrogen, which serves as the reflux liquid. At the same time, the liquid oxygen at the bottom of the upper column vaporizes and, as rising gas in the upper column, participates in the distillation process there. The oxygen-enriched liquid with an oxygen content of 38–40% obtained at the bottom of the lower column is throttled and then fed into the upper column, where it serves as part of the reflux stream that comes into contact with the rising gas for mass and heat transfer; part of this oxygen-enriched liquid vaporizes. Since oxygen is a non-volatile component while nitrogen is a volatile one, more nitrogen evaporates than oxygen, thereby increasing the purity of liquid oxygen. The liquid undergoes multiple heat and mass transfer processes with the rising gas from top to bottom, resulting in an increasing oxygen purity in the liquid phase; by the time the liquid reaches the bottom of the upper tower, 99.6% liquid oxygen can be obtained. 32. Why are double-stage distillation columns generally used in air separation units? Is a single-stage distillation column sufficient? Answer: In typical air separation units, air is first distilled in the lower column (the pressure column) to separate it into oxygen-enriched liquid air and pure nitrogen. Then, in the upper column (the low-pressure column), the material obtained from the lower column is further distilled to produce high-purity oxygen and nitrogen. The link between the upper and lower towers is the condensation evaporator, which uses the nitrogen at pressure in the lower tower to heat the liquid oxygen in the upper tower, causing the liquid oxygen to evaporate while the gaseous nitrogen is condensed. The advantage of using a two-stage distillation tower is that it enables a high extraction rate of the product, while also allowing the production of both oxygen and nitrogen as high-value products, with low energy consumption for the production process. A single-stage distillation tower can also be used to produce oxygen and nitrogen products, but it is only possible to obtain one of these high-purity products at a time. Moreover, there are significant losses of oxygen and nitrogen, the recovery rate is low, and energy consumption is high; hence it is an inadequate device for air separation, which is why it is rarely used in practice. 33. Why can liquid oxygen be used to cool gaseous nitrogen in a condensation evaporator? Answer: In a condensation evaporator, the liquid oxygen in the upper column absorbs heat and evaporates into gaseous oxygen, while the gaseous nitrogen in the lower column releases heat and condenses into liquid nitrogen. In other words, the temperature of liquid oxygen is lower than that of gaseous nitrogen. We know that at 1 atmosphere of pressure, the liquefaction temperature of oxygen is about 13°C higher than that of nitrogen; under such conditions, it is impossible to use liquid oxygen to cool gaseous nitrogen and cause it to liquefy. Since the liquefaction temperature is related to pressure, it increases as pressure rises. For example, nitrogen has a liquefaction temperature of –195.8°C at 1 atmosphere pressure, while at 6 atmospheres the liquefaction temperature is –177°C, an increase of 18°C. This allows liquid oxygen to be used to cool gaseous nitrogen and thereby enable its liquefaction. 34. What factors influence the temperature difference of the main condensation evaporator? Answer: The temperature difference of the main condensation evaporator is affected by the purity of oxygen and nitrogen, as well as the pressures in the upper and lower columns. First, regarding the evaporation process of liquid oxygen: at constant oxygen purity, increasing the pressure raises its evaporation temperature and reduces the temperature difference in the main cooler ； At constant pressure, as purity increases, its evaporation temperature rises and the temperature difference across the main cooler decreases. Condensation process of nitrogen: With constant nitrogen purity, increasing the pressure in the lower column raises its condensation temperature, thereby increasing the temperature difference in the main cooler ； At constant pressure, increasing the nitrogen purity lowers its condensation temperature, thereby reducing the temperature difference in the main cooling system. It can be seen that, when the pressures in the upper and lower towers are constant, increasing the purity of liquid oxygen can reduce the temperature difference in the main cooler. Increasing the purity of gaseous nitrogen can reduce the temperature difference in the main cooler. If the pressure and purity of gaseous nitrogen remain constant, and the purity of liquid oxygen is fixed, increasing the pressure in the upper column can reduce the temperature difference in the main cooler. 35. What is distillation? Answer: Distillation is a process that takes advantage of the different boiling points of two substances, involving multiple cycles of partial condensation of the mixed vapor and partial evaporation of the mixed liquid, in order to achieve the separation of those substances. In a mixed liquid composed of two substances with different boiling points, the substance with the lower boiling point is referred to as the volatile component; that is, when part of the mixed liquid evaporates, more of this volatile component will evaporate as well. For example, in liquid air containing 61% nitrogen, the nitrogen content in its vapor at 0.5 MPa is 81% ； Conversely, substances with high boiling points are called non-volatile components, and when part of the mixed vapor condenses, more of these non-volatile components can be condensed out. In a mixed liquid composed of two substances with different boiling points, when it absorbs heat and partially evaporates, the more volatile component will evaporate to a greater extent; whereas when the mixed vapor releases heat and partially condenses, the less volatile component will condense to a greater extent. If saturated vapor at a higher temperature is brought into contact with saturated liquid at a lower temperature, the vapor releases heat to the saturated liquid, causing it to partially condense, while the liquid absorbs heat and partially evaporates. The aforementioned processes take place on the trays of a distillation column, through mass and heat transfer across numerous trays. This allows the two components to be separated, with the volatile component being obtained at the top of the tower and the less volatile component at the bottom. 36. What is the reflux ratio? What impact does it have on distillation? Answer: The reflux ratio generally refers to the ratio of the amount of liquid flowing downward in the column to the amount of vapor flowing upward; it is also known as the liquid-to-vapor ratio. The purity of the distillate, with a constant number of tray levels, depends on the reflux ratio. When the reflux ratio is high, the purity of nitrogen in the gas phase is high while the purity of oxygen in the liquid phase is low; whereas when the reflux ratio is low, the purity of nitrogen in the gas phase is low and the purity of oxygen in the liquid phase is high. This is because, after the rising gas at a higher temperature mixes with the flowing liquid at a lower temperature on the tray to exchange heat and mass, their temperatures can, in an ideal situation, approach each other. That is, to reach the same temperature. This temperature lies between the gas and liquid temperatures. If the reflux ratio is high, that is, there is more liquid flowing downward or less vapor rising, then the temperature of the vapor-liquid mixture will lean towards that of the cooler liquid. As a result, the temperature drop of the rising vapor is greater, and more vapor condenses. Since oxygen is a component that is difficult to vaporize, more oxygen condenses, which in turn causes the nitrogen concentration in the rising gas to increase more rapidly. This is the case for each tray, which is why the nitrogen purity at the top of the tower is high. On the other hand, since the temperature of the gas-liquid mixture is lower than that of the liquid, the temperature rise of the liquid flowing downstream is small, and less liquid evaporates; as a result, less nitrogen from the liquid evaporates as well. This means that the oxygen concentration in the liquid flowing downstream increases slowly, and this holds true for each tray in the tower, which is why the oxygen concentration in the liquid at the bottom of the tower is low. When the reflux ratio is low, the situation is the opposite of what was mentioned above. Adjusting the distillation conditions essentially involves changing the reflux ratio in various sections of the tower; a high tower temperature indicates a low reflux ratio, while a low tower temperature suggests a high reflux ratio. 37. How to control the purity of liquid air and liquid oxygen? Answer: The liquid air from the lower column, along with contaminated liquid nitrogen, is supplied to the upper column as feed liquid. Therefore, bottom column distillation is the basis of top column distillation. The key to operating the lower tower lies in properly controlling the opening degree of the sewage nitrogen throttle valve (FV8513). Specifically, it means maximizing the throttle valve for waste liquid nitrogen, provided that the purity of the liquid nitrogen meets the required standards. This allows the reflux ratio in the distillation section of the upper column to increase, thereby reducing oxygen loss in the nitrogen-rich gas from the upper column and increasing the oxygen extraction rate of the unit. At the same time, as the purity of liquid air increases, the purity of liquid oxygen can also be improved. However, it’s not the case that the slurry nitrogen throttle valve should be opened as wide as possible. Because when the throttling valve for the contaminated nitrogen is opened to a certain extent, the oxygen content in the contaminated nitrogen increases; as a result, the oxygen content in the gas phase after throttling becomes higher than that in the nitrogen gas at the top of the upper tower. This leads to an increase in oxygen loss in the nitrogen gas discharged from the upper tower, thereby reducing the oxygen extraction efficiency of the plant. 38. What are the benefits of reducing pressure in distillation? Answer: Reducing pressure improves the distillation conditions in the column. Because the lower the pressure, the more the gas-liquid equilibrium approaches an ideal state; that is, at low pressures the difference between the concentration of a certain component in the liquid and its concentration in the vapor phase in equilibrium with it is larger, while at high pressures this difference is smaller. For example: at 0.05 MPa (G), when the nitrogen concentration in the liquid is 50%, the nitrogen concentration in the equilibrium vapor is 83% ； If the pressure is increased to 0.1 MPa (G) while the nitrogen concentration in the liquid remains at 50%, the nitrogen concentration in the equilibrium vapor decreases to 81%. The greater the difference in concentration between the gas and liquid phases, the better the separation of oxygen and nitrogen. In other words, with a constant number of tray levels, a lower pressure can improve the purity of oxygen and nitrogen. 39. Why are neon-helium purge tubes installed in the condensation evaporator? Answer: Although neon and helium account for a very small proportion in air. However, due to their very low liquefaction temperatures, the helium-neon gas rising from the lower column to the upper part of the condensing side of the main condenser does not condense. Along with the air entering the column, this helium-neon gas continues to enter the system and accumulates more and more on the condensing side, occupying part of the effective heat exchange space in the main condenser and thereby affecting condensation. This results in a reduced heat transfer efficiency, which ultimately impacts the load on the main cooler. Therefore, the non-condensable neon and helium gases must be regularly removed from the main condenser, which is why neon-helium purge tubes are installed in the main cooler. 40. Why is an undercooler E710002 installed? Answer: The oxygen-enriched liquid air, contaminated liquid nitrogen, and liquid nitrogen from the lower column are supplied to the upper column as reflux fluid after being throttled through a throttle valve. After throttling, the pressure of the saturated liquid decreases, and its corresponding saturated temperature also drops. As a result, the liquid after throttling partially vaporizes, which reduces the amount of reflux returning to the upper column – a situation that is unfavorable for distillation. Therefore, a subcooler E710002 is installed; the rich liquid, contaminated nitrogen, and liquid nitrogen use E710002 to recover the cooling capacity of the nitrogen gas from the upper column. It becomes a supercooled liquid; when this supercooled liquid passes through a throttle, its vaporization rate decreases, thereby ensuring an adequate supply of reflux liquid to the upper column and enhancing the distillation effect as well as improving the efficiency of oxygen recovery. Similarly, when liquid oxygen and liquid nitrogen are cooled before being fed into the storage tanks, their vaporization rate decreases, which reduces losses during tank filling. 41. Why is the rise or fall of the liquid oxygen level in the main cooler considered a key indicator of whether there is sufficient cooling capacity? Answer: Under stable operating conditions, the cooling capacity generated by the air separation unit is balanced with the amount of cooling required; the operating conditions in various parts of the unit do not change over time, and the liquid oxygen level in the main cooler remains stable, with only minor fluctuations, showing no clear trend of rise or fall. When there is an excess of cooling capacity in the plant and the humidity level of the air entering the lower column is too high, the amount of gas that needs to be condensed in the condenser at the top of the lower column decreases; accordingly, the amount of liquid oxygen that evaporates also decreases, causing the liquid oxygen level to rise. Conversely, if the cooling capacity of the plant is insufficient, the amount of gas that needs to be condensed in the condenser at the top of the lower column increases, and as a result, the amount of liquid oxygen that evaporates increases, leading to a drop in the liquid oxygen level. Therefore, the change in the liquid oxygen level in the main cooler is the primary indicator to determine whether the cooling capacity of the device is sufficient. It is said to be the main symbol. This is because other factors can also affect changes in the liquid oxygen level; for example, the opening degree of the liquid air condenser control valve LICA960001, or the excessive opening degree of the liquid air separator level control valve LIC960004, can both cause the level of the main cooler liquid to rise. At the same time, however, the level of the liquid in the lower section of the distillation tower or in the liquid air separator will drop. This situation does not indicate an excess of cooling capacity; therefore, a comprehensive analysis should be conducted in actual operations in order to arrive at an accurate judgment. 42. Why are the temperatures different on various trays within a distillation column? What factors influence them? Answer: The temperature on a tray depends on the temperature of the gas and liquid above it, and the level of these temperatures is determined by the pressure in the column as well as their nitrogen content. At a certain pressure, the higher the oxygen concentration, the higher its temperature. As is well known, whether in the upper or lower tower, the oxygen concentration in the gas and liquid is higher at the bottom; the higher up one goes, the lower the oxygen concentration becomes, and consequently the temperature also decreases as one moves upward. Additionally, the higher the pressure, the higher the saturation temperature of the gas and liquid; when the system pressure increases, the temperatures of each tray also rise. In general, as the reflux ratio increases and the system operating pressure decreases, the column temperature drops; conversely, it rises. 43. What factors affect changes in tray resistance? What is the practical significance of observing tray resistance in operations? Answer: The factors that affect the plate resistance include the pore size of the sieve, the porosity of the plate, the specific gravity of the liquid, the surface tension of the liquid, the thickness of the liquid layer, the specific gravity of the vapor, and the velocity at which the vapor passes through the pores, among others. Among them, the specific gravities of steam and liquid, as well as the surface tension of the liquid, change very little during the production process. Although the pore size and porosity remain constant, they can change when the sieve pores become blocked, resulting in an increase in resistance. Furthermore, the liquid layer thickness and the speed at which steam passes through the sieve pores depend on the amount of liquid flowing downstream and the amount of steam rising upward; these values can change during operation, thereby affecting changes in the tray resistance. In particular, the effect of sieve pore velocity on resistance is quadratic, with a significant impact. Therefore, in practical operation, it is possible to determine whether the operating conditions inside the tower are normal by observing the changes in the resistance of various parts within the tower. If the resistance is normal, it indicates that the speed of the rising steam inside the tower and the amount of liquid flowing downstream are normal. If the resistance increases, it indicates that the amount of rising steam is too large or the amount of liquid flowing downstream is too large. 44. What is flooding? What phenomena occur when flooding takes place? Answer: In a distillation column, the gas and liquid phases flow in opposite directions, when the flow rates of both phases are low. The flow of one phase is not constrained by the other phase. As the two-phase flow velocity increases, some droplets can be carried by the upward airflow to the upper tray. Some bubbles can also be entrained into the downcomer in the liquid phase. This entrainment phenomenon intensifies as the flow rate increases, and in severe cases it can lead to flow blockage and flooding. Foaming is the limit that occurs when the gas and liquid phases flow in opposite directions. When flooding occurs, the pressure difference in the distillation column rises sharply, the liquid level drops, and then the pressure difference suddenly falls again, causing the liquid level to rise sharply. This process repeats itself, and at the same time, the pressure in the distillation column fluctuates, leading to a decrease in the purity of the product. 45. Why do the levels of liquid nitrogen and liquid oxygen rise during temporary parking? Answer: During normal operation, the liquid on the distillation tower trays is held in place by the steam that passes through the small holes at a certain speed. It prevents the liquid from leaking through the small holes; instead, it can only flow along the tray and then pass through the overflow hopper to the lower tray. After parking, due to the interruption of the rising steam, the liquid on the tray loses the support provided by that steam, and flows sequentially through the sieve pores of each tray toward the bottom, accumulating in the condensation evaporator and the lower part of the tower. Therefore, when parking temporarily, the liquid oxygen and liquid air levels will both rise. 46. What is the effect of an increased amount of air processed on the distillation process? Answer: As the amount of air processed increases, the amount of vapor rising in the distillation column increases as well; consequently, the amount of liquid condensed in the main condenser also increases. Therefore, it has little effect on the reflux ratio within the column, provided that the gas flow rate remains within a certain range. The oxygen-nitrogen purity remains essentially unchanged, while the output increases proportionally as the amount of air increases. However, as the amount of condensate in the main cooler increases, the thermal load on the main cooler rises; this occurs when the heat transfer area is insufficient. The temperature difference in the main cooler will inevitably increase, causing the pressure in the lower column to rise accordingly. At the same time, as the gas flow velocity within the tower increases, the amount of liquid flowing downstream rises, causing the liquid layer on the tray to thicken. This increases the resistance of the tray, and the pressures at the top and bottom of the tower also rise accordingly. This has an adverse effect on the separation of oxygen and nitrogen, while also increasing energy consumption. When the gas flow rate is too high, both the plate resistance and the resistance encountered by the liquid flowing downward through the overflow tray increase significantly, resulting in an elevation of the liquid level in the overflow tray. Until flooding occurs, preventing the liquid from flowing downward. This will disrupt the normal operation of distillation. Furthermore, as the upward steam flow velocity increases, it is easier for liquid droplets to be carried to the upper tray, which affects the distillation efficiency, reduces nitrogen purity, and thus lowers the oxygen extraction rate. 47. What is the impact of insufficient processed air volume on distillation operations? Answer: According to the mass balance principle, as the amount of air used in processing decreases, the production of oxygen and nitrogen also has to decrease accordingly. When the volume of air flowing through the system decreases, the velocity of the steam stream reduces, and the amount of liquid on the tray levels also decreases; as a result, the liquid layer becomes thinner, which lowers the resistance at the tray level. Meanwhile, since the heat load on the main cooling system decreases, there is excess heat transfer area available, allowing the temperature difference for heat transfer to be reduced. All these factors contribute to a decrease in the pressure in both the upper and lower towers. When the gas flow rate decreases too much, it becomes too low to hold back the liquid on the sieve pores, resulting in liquid leakage that undermines the distillation process and affects the purity of oxygen and nitrogen. 48. What is cold loss? What are its types? Answer: The ability of a cold object to absorb heat is called coldness. If this amount of cooling capacity is not utilized, it is referred to as cooling loss. It includes the following aspects: (1) Incomplete heat exchange losses. When leaving the device, under ideal conditions, the low-temperature gas should be reheated to the same temperature as the air flowing into the device. In this way, all the cold energy can be recovered. In practice, the heat exchangers used cannot meet such ideal requirements; the low-temperature gas leaving the device is always at a lower temperature than the air flowing into the device in the forward direction. This results in heat loss, which can be reduced by increasing the heat exchange area. (2) Cold flow loss. Due to the very low operating temperature of the device, and even though a thermal insulation layer is in place, if the temperature of the surrounding air is higher than that inside the device, it is inevitable that some heat will be transferred into the device, raising the temperature of the cold object and thus consuming some of the cooling capacity. This amount of cold loss is called cold leakage loss. (3) Cooling loss of liquid products. When the device produces a certain amount of liquid product, the amount of heat carried away by this low-temperature liquid as it leaves the device is known as the heat loss of the liquid product. (4) Other cooling losses. When discharging liquid, or in the event of a leak in the device, additional cooling capacity is required, or some of the cryogenic liquid or gas is lost; such losses are considered part of other cooling losses. 49. Where is the cooling capacity consumption of air separation units? Answer: At the beginning of operation, all parts of the device are at room temperature; at this stage, cooling is required to gradually lower the temperature of these parts, and the cooling capacity is used for reducing the temperature of the equipment ； During the later stage of liquid accumulation after startup, the maximum amount of cooling capacity is required, as the cooling energy is used for the accumulation of liquid ； During normal operation, the cooling capacity is used for compensating for heat loss and for producing liquid products. In this case, if there is no heat loss and no liquid products are produced, that is, if all of the cooling capacity can be recovered, then cooling is not necessary. 50. What factors are related to the magnitude of cold flow loss? Answer: The cold flow loss depends on the heat transferred from the surroundings into the interior of the device. Its size is related to the following factors: (1) the capacity and type of the air handling unit. As the capacity of the unit increases, the cooling loss per 1 Nm3 of air processed decreases; for plate-fin air separation units, the compact structure of the heat exchangers results in a smaller volume, and thus lower cooling losses as well. (2) Operating environmental conditions: The heat transfer amount is proportional to the temperature difference for heat transfer; the higher the ambient temperature, and the greater the temperature difference within the device, the greater the cold loss will be. (3) Insulation measures: Generally, the cold box is filled with insulating materials with poor thermal conductivity, such as perlite or slag wool. Its heat insulation performance depends not only on factors such as the properties of the insulating material, the height and thickness of the filling layer, as well as the insulating measures for supports and hangers, but also on the condition of the filling. It is best to fill the dead corners of the insulated container with insulation material ； The insulating material used for filling must be dry ； The insulated box should be sealed and filled with a small amount of dry gas to maintain a slight positive pressure inside, thereby preventing outside humid air from entering. 51. What factors can cause an explosion in an oxygen generation device? Answer: There are three factors that can lead to an explosion: first, flammable materials, such as acetylene and hydrocarbons and other hazardous impurities ； The second is an oxidizer, oxygen ； Third is the ignition source; in oxygen generation devices, the ignition sources include (1) the friction between explosive impurity solid particles or their friction with the device walls. (2) Due to electrostatic discharge, when liquid oxygen contains a small amount of ice particles, solid CO2 generates static electricity; when the CO2 concentration is 20–30 ppm, the resulting electrostatic potential can reach 3000 volts. (3) Wave impacts cause a local increase in pressure, which in turn leads to an increase in temperature. (4) The presence of substances with particularly high chemical reactivity (such as ozone and nitrogen oxides) increases the explosive sensitivity of flammable substances in liquid oxygen. Oil must be strictly prohibited in air separation units; degreasing is necessary during installation, and no grease lubrication shall be used for any valves or valve stems. The content of harmful impurities is kept within the following range: total hydrocarbons