I. Introduction to air separation unit
The so-called air separation unit, in simple terms, is a device that separates the main gas components in the air. It cools the air deeply to liquid state and uses the different boiling points of the components of liquid air to gradually separate oxygen, nitrogen and argon. The air also contains rare gases such as helium, neon, argon, krypton, xenon, radon, as well as impurities such as moisture, carbon dioxide, and dust, all of which need to be separated and purified. For most factories, the main task of the air separation unit is to separate and produce clean oxygen, nitrogen, argon, etc.
(I) Principle introduction
The main components of the air are nitrogen, oxygen and argon, so the main purpose of separation is to obtain these three pure gases.
The main impurities in the air are moisture, carbon dioxide and hydrocarbons.
The separation of air is to remove harmful impurities in the air and separate the mixed gas into "pure" gases such as nitrogen, oxygen and argon.
The basic principle of air separation is that nitrogen, oxygen and argon in the air have different bubble points (dew points, boiling points) and are separated by distillation. Their boiling points are: N₂: -195.8℃; O₂: -183.0℃; Ar: -185.7℃.
Distillation process: On the distillation plate, the saturated steam with higher temperature and the saturated liquid with lower temperature are in full contact. The saturated steam releases heat to the saturated liquid and partially condenses itself; the saturated liquid absorbs heat and partially evaporates. Oxygen is a high-boiling-point component and condenses more into the liquid phase; nitrogen is a low-boiling-point component and evaporates more into the gas phase.
II.Introduction to the process flow
The current process flow of air separation is roughly as follows: the atmosphere is first compressed by the compression system, then enters the pre-cooling system for preliminary cooling, then enters the purification system to remove some impurities such as moisture and carbon dioxide, and then further cooled by the heat exchange system, and then enters the distillation system (mainly including expanders, cold boxes, etc., and the cold box contains main towers, main cooling, liquid pumps and other equipment, and the distillation process is completed in this system), and finally enters the backup system such as product gasification, compression, and storage.
III. Introduction to the air separation unit system (or equipment)
(I) Compression system
There is an air filter at the beginning of the air compression system, which is used to filter mechanical impurities in the air. There are mainly self-cleaning air filters; the purpose of compression is to pre-compress the air, and the equipment mainly includes steam turbines, air compressors, superchargers, etc.
Self-cleaning filter: Generally, as the gas volume increases, the number of filter cartridges increases, and the number of layers is also higher. Double-layer for 25,000 grade and above, and three-layer for 60,000 grade and above; a single compressor needs to be equipped with a separate filter, and it is arranged at the upwind.
Steam turbine: The high-pressure steam expands to do work, driving the coaxial impeller to rotate, thereby achieving work on the working fluid. Common forms include full condensation, full back pressure and extraction condensation, and the more commonly used is extraction condensation.
Air compressor: Large air separation units mostly use single-shaft isothermal centrifugal compressors. The energy consumption of imported ones is about 2% lower than that of domestic ones, and the investment is 80% higher; the outlet is vented, and no return pipeline is set. There is a minimum suction flow anti-surge requirement, and the inlet guide vane is used for flow regulation. Imported and domestic units are all four-stage compression and three-stage cooling (the last stage is not cooled). The main air compressor is equipped with a water washing system to wash the deposits on the surface of the impeller and volute at each level, which is a complete set with the main engine.
Booster: Large air separation units mostly use single-shaft isothermal centrifugal compressors and gear centrifugal compressors. Among them, the gear type has a greater advantage in energy consumption, especially under conditions with relatively large pressure.
(II) Precooling system
The function of the precooling system is to preliminarily cool the air so that the water, carbon dioxide, dust and other impurities in the compressed air can be purified in the next step. The main equipment includes air cooling tower, water cooling tower, cooling water and chilled water pumps, etc.
Air cooling tower: There are two forms: closed circulation (the air cooling tower is divided into two sections, the upper and lower sections, and the chilled water circulates between the upper section of the air cooling tower and the water cooling tower) and open circulation (into the circulating water system). Closed circulation is mainly used in chemical plants with poor water quality, which need to be supplemented with fresh water and chemicals; open circulation is widely used, but the circulating water system also needs to be regularly supplemented with fresh water, and summer working conditions must be considered. Its design is generally 1 meter φ76 stainless steel ball ring (high temperature resistance) at the bottom, 3 meters φ76 reinforced polypropylene ball ring (large flux), and 4 meters φ50 reinforced polypropylene ball ring.
Water cooling tower: There are two types: two-stage type (when there is no external cold source, the cold capacity of dry dirty nitrogen is fully recovered to ensure the pre-cooling system, but the resistance is twice as large, 7 meters + 7 meters φ50 polypropylene ball ring) and one-stage type (when there is an external cold source, 8 meters φ50 polypropylene ball ring).
Others: Generally, all water inlets in the pre-cooling system must be equipped with filters (generally 6 units: 4 water pumps, water inlet to the water cooling tower, and water inlet to the evaporation side of the chiller) to prevent impurities from entering the system. The effect test is: the outlet gas of the lower 4-meter packing section is 1°C lower than the inlet water; the outlet gas of the upper 8-meter packing section is 1°C higher than the water. Generally, a thermometer is set in the middle of the air cooling tower (extending into the interior).
(III) Purification system
The function of the purification system is to remove impurities such as moisture, carbon monoxide, carbon dioxide, hydrogen and hydrocarbons in the air to ensure the purity of nitrogen, oxygen and argon in the air products. The adsorbers used are vertical axial flow, horizontal double-layer bed and vertical radial flow.
Vertical axial flow: mainly used for air separation equipment below 10,000 level (diameter has reached 4.6m), bed thickness 1550∽2300mm, double-layer or single-layer can be arranged, and air flow distribution is the best.
Horizontal double-layer bed: mainly used for large and medium-sized air separation equipment, bed thickness 1150mm (molecular sieve) + 350mm (aluminum gel).
Vertical radial flow: can effectively utilize the internal space of the container, so that the area of the adsorption layer with the same diameter is expanded by about 1.5 times, effectively reduce the height of the tower, and the vertical placement method occupies a smaller area. Because the airflow is evenly distributed, unlike the uneven airflow of the horizontal adsorber, the amount of molecular sieve is reduced by 20%, and the regeneration energy consumption is also saved by 20%. However, the disadvantage is that the airflow center is concentrated (fan-shaped area), and the penetration time is faster than the horizontal type (CO₂＜0.5ppm is required). The bed thickness is 1000mm+200mm, which can meet the configuration of air separation equipment above 20,000 levels.
IVHeat exchange system
The purpose of the heat exchange system is to further cool the air for the next distillation operation. The source of cold energy is the evaporation of a part of the air through the turbine expander. Its structure is a multi-layer plate-fin type. The logistics between adjacent channels are well heat exchanged through the fins. It is used to cool the compressed air that has been adsorbed by molecular sieves to remove water and CO₂. Each reflux gas (liquid) is heated to room temperature here.
Strictly speaking, the design of multiple streams of mixed media in the same heat exchanger can automatically balance the heat transfer of each medium and minimize energy consumption. However, for the internal compression process, all heat exchangers will be high-pressure heat exchangers, which will increase investment. Therefore, it is more economical to separate high and low pressure for internal compression heat exchangers above 20,000 levels, and all high-pressure heat exchangers are configured below 20,000 levels.
V Fractionation system
The purpose of the fractionation system is to fractionate the purified and deeply cooled compressed air into oxygen, nitrogen, argon, etc. step by step. The main equipment is the cold box (including the main tower, main cooling, subcooler, crude argon tower, liquid oxygen pump, liquid pump, etc.).
Cold box: It is a square or round metal structure, which is the highest symbol of the air separation workshop. It is filled with pearl sand to reduce the loss of cold. The air is separated in the cold box through the distillation effect on the tower plate and the packing.
Distillation tower: The tower body is cylindrical, with multiple layers of sieve plates in the lower tower, an overflow bucket is set on the sieve plate, an overflow baffle, and densely covered with small holes; the upper tower is equipped with regular packing and liquid distributor. During the distillation in the lower tower, the liquid flows through each sieve plate from top to bottom one by one. Due to the effect of the overflow weir, a certain liquid level height is formed on the pedal. When the gas passes through the small holes of the sieve plate from bottom to top, it contacts the liquid to produce bubbles, increasing the gas-liquid contact area, making the heat and mass exchange efficient. The low-boiling point components gradually evaporate, and the high-boiling point components gradually liquefy. At the top of the tower, pure nitrogen with low boiling point is obtained, and the bottom of the tower obtains oxygen-rich liquid air components with high boiling point. During the distillation in the upper tower, the gas passes through the distributor and rises along the packing plate. The liquid is evenly distributed on the packing plate from top to bottom through the water distributor. On the surface of the packing, the gas and liquid are fully in contact for efficient heat and mass exchange. The low-boiling point oxygen content in the rising gas continues to increase, and the high-boiling point component oxygen is washed down in large quantities to form reflux liquid, and finally low-boiling point pure nitrogen is obtained at the top of the tower, and high-boiling point liquid oxygen is obtained at the bottom of the tower.
VI Storage and vaporization backup system
Store, vaporize and fill the fractionated liquid oxygen, liquid nitrogen and liquid argon. The main equipment includes cryogenic liquid storage tank, vaporizer, bottle filling pump, filling platform, etc.
Low-pressure oxygen and nitrogen products: set product regulating valve and venting flow path, vent into silencer (nitrogen internal parts are carbon steel, oxygen internal parts are stainless steel).
Dirty nitrogen: set to vent to water cooling tower (to vent dirty nitrogen, allocate regenerated gas and adjust the upper tower pressure). The diameter of the water cooling tower is required to meet the discharge requirements. Especially when nitrogen is introduced, the upper tower pressure cannot be raised. The water cooling tower resistance is 6kPa (8-meter high packing), the pipeline and valve are 4kPa, and the pressure difference to the atmosphere is 2kPa, a total of 12kPa.
High-pressure oxygen products: two-stage throttling is used for venting. First, the high-pressure product gas is throttled to 10barG, passes through an eccentric reducer, a monel noise reduction plate is set in the middle, and then the pipeline diameter is expanded through an eccentric reducer. The oxygen medium flow rate is controlled below 10m/s, and then it is throttled and vented into the muffler tower. The muffler element is stainless steel.
High-pressure nitrogen products: Nitrogen products are first throttled to 10bar, pass through a stainless steel noise reduction plate, and then pass into the muffler tower for throttling and venting. The muffler element is carbon steel.
Oxygen valve: It is required not to be manually operated (the regulating valve is prohibited from having a handwheel, and the manual valve is placed inside the explosion-proof wall).
