Coal Oil,Chemical fertilizer Large air separation equipment, simply put, is used to separate the components of the air gas, the production of oxygen, nitrogen and argon a set of industrial equipment. There are also rare gases helium, neon, argon, krypton, xenon, radon and so on.

The application range of air separation equipment is very wide, not only used in chemical industry, but also widely used in petroleum, medicine, electronics, metallurgy, aerospace and other fields. In these fields, high-purity oxygen and nitrogen are indispensable raw materials or protective gases, which play an important role in ensuring the smooth progress of the production process.

At the same time, the application of air separation equipment has also contributed to the cause of environmental protection, for example, in the treatment of industrial waste gas, by separating nitrogen and oxygen, it can effectively reduce the emission of harmful gases and improve environmental quality.

Air separation equipment introduction

The working process of the air separation device is roughly as follows: First, the compressed air provided by the air compressor is treated by molecular sieve to remove the impurities such as water, carbon dioxide and hydrocarbons, and then divided into two parts, one part directly into the upper tower of the rectification tower, and the other part will be expanded and cooled by the expansion machine, and then sent to the lower tower. Inside the distillation tower, rising steam and falling liquid exchange heat, so that high purity nitrogen can be obtained at the top of the upper tower and high purity oxygen can be obtained at the bottom of the upper tower.

This process requires precise control and regulation to ensure that the separated gas meets the required purity standards. The design and operation of the whole system involves many physical and chemical principles, such as adsorption of molecular sieve, phase change in the distillation process, heat transfer and exchange.

The system flow of the empty branch plant includes

Compression system, pre-cooling system, purification system, heat exchange system, product delivery system, expansion refrigeration system, distillation tower system, liquid pump system, product compression system

We introduce the equipment one by one according to the air separation system process:

Compression system

There are self-cleaning air filters, steam turbines, air compressors, superchargers, instrument compressors, etc.

(1) Self cleaning filters generally have an increase in the number of filter cartridges and layers with the increase of air volume. They are generally arranged in double layers for grades above 25000 and three layers for grades above 60000; Generally, a single compressor needs to be equipped with a separate filter and placed at the air inlet.

(2) A steam turbine is a type of high-pressure steam that expands and does work, driving the coaxial impeller to rotate and perform work on the working fluid. There are three commonly used forms of steam turbines: full condensing, full back pressure, and extraction condensing, with extraction condensing being the most commonly used.

(4) The investment in large air separation units is generally single axis isothermal centrifugal compressors, which have an energy consumption about 2% lower and an investment 80% higher than domestic ones when imported; The air compressor adopts outlet venting and does not have a reflux pipeline. Generally, there is a minimum suction flow anti surge requirement, and inlet guide vanes are used for flow regulation. Imported 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 flush the sediment on the surface of the impellers and volutes at all levels. The system comes with the host as a complete set.

(5) Turbochargers are generally invested in large air separation units using two types: single axis isothermal centrifugal compressors and gear centrifugal compressors. Among them, gear compressors have a significant advantage in energy consumption, especially in working conditions with high pressure ratios.

(6) There are generally three forms of instrument air compressors: oil-free screw compressors, piston compressors, and centrifugal compressors. Due to the natural oil-free nature of piston and centrifugal systems, there is no need for an oil removal device. Only a drying device (for water removal) and a precision filter (for solid particles removal) are required; Screw machines generally have two types: oil and oil-free, followed by oil removal. Oil injected screw machines require oil removal devices and high-precision oil removal filters to meet process requirements. The advantage of this type of machine is that it is relatively inexpensive; The oil-free screw adopts dry rotor or water lubrication. The advantage of this model is that it is absolutely oil-free, but the disadvantage is that it is more expensive. Piston type is suitable for air flow below 500Nm ³/h; When the gas volume is below 2000Nm ³/h, it is suitable to choose a screw or piston machine; When the gas volume is above 2000Nm ³/h, all three models can be selected. Centrifugal compressors have advantages when the gas volume is large, with fewer vulnerable parts, easy maintenance, and high cost-effectiveness.

The instrument compressor is used during start-up and is extracted from the molecular sieve purifier after normal operation.

pre-cooling system

There are two forms of pre cooling system air cooling towers: closed cycle (the air cooling tower is divided into 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 cycle (entering the circulating water system). Closed cycle is mainly used in chemical plants with poor water quality, where fresh water and chemicals need to be replenished; Open cycle is widely used, but the circulating water system also needs to be regularly replenished with fresh water, and the pre cooling system also needs to consider summer working conditions.

Air cooling towers are generally designed with a 1-meter diameter 76 stainless steel ball ring (high temperature resistant) at the bottom, a 3-meter diameter 76 reinforced polypropylene ball ring (high flux), and a 4-meter diameter 50 reinforced polypropylene ball ring.

There are also two types of water-cooled towers: two-stage (when there is no external cooling source, the cold recovery of dry nitrogen gas is sufficient to ensure the pre cooling system, but the resistance is twice as high, (7 meters+7 meters with a diameter of 50 polypropylene ball ring) and one-stage (when there is an external cooling source, 8 meters with a diameter of 50 polypropylene ball ring).

In addition, all incoming water for the pre cooling system generally needs to be equipped with filters (usually 6: 4 water pumps, water cooling tower inlet, and evaporator side inlet of the chiller) to prevent impurities from entering the system. The effectiveness test of the pre cooling system shows that the outlet gas of the 4-meter packing section in the lower section is 1 ℃ lower than the inlet water; The outlet gas of the 8-meter packing section in the upper section is 1 ℃ higher than the water, and a thermometer is generally installed in the middle of the air cooling tower (extending into the interior).

purification system

There are three types of adsorbers used in the purification system: vertical axial flow, horizontal double-layer bed, and vertical radial flow.

Vertical axial flow is mainly used for supporting air separation equipment with a diameter of less than 10000 (up to 4.6m). The bed thickness is 1550-2300mm, and both double and single layers can be arranged. The vertical axial flow adsorber has the best airflow distribution.

The horizontal double-layer bed is mainly used for supporting large and medium-sized air separation equipment, with a bed thickness of 1150mm (molecular sieve)+350mm (aluminum gel).

The vertical radial flow adsorber can effectively utilize the internal space of the container, expanding the adsorption layer area by about 1.5 times for the same diameter. This can effectively reduce the height of the tower, while the vertical installation method occupies a smaller area. Due to uniform airflow distribution, unlike horizontal adsorbers with uneven airflow, the amount of molecular sieve used is reduced by 20%, and the regeneration energy consumption is also saved by 20%.

However, the disadvantage of vertical radial flow is that the airflow center is concentrated (fan-shaped area), which makes its penetration time faster than horizontal flow (CO2<0.5ppm required). The bed thickness is 1000mm+200mm, and the vertical radial flow can meet the configuration of air separation equipment with a capacity of over 20000 levels.

There are two types of regenerative heating: electric heaters and steam heaters.

There are horizontal (below 40000 level), vertical (above 40000 level), and vertical high-efficiency steam heaters (with high steam utilization and energy saving of 20%). The layout includes: one steam heater (with H2O leakage measuring point); Electric heaters (two in use and one backup or one in use and one backup) are connected in parallel (with high temperature and low flow interlocking stop settings to prevent burning out, and the heating tube material is 1Cr18Ni9Ti); Electric heater (suitable for activation regeneration, 250-300 ℃) is connected in parallel with steam heater; The electric heater is connected in series with the steam heater (when the steam temperature is low, but it causes significant regeneration resistance).

The purification system also needs to be equipped with a throttling regeneration pipeline to meet the needs of driving. In addition, safety valves are installed on the regeneration gas side and steam heater side to prevent equipment or valve pressure leakage or overpressure on the high side, as well as throttling overpressure.

The regeneration flow path is equipped with a manual butterfly valve to adjust the resistance, in order to ensure stable operation of the main tower (or not set, using a main pipe to adjust the timing of the regulating valve).

heat exchange system

Strictly speaking, the heat exchange system is designed with multiple mixed media streams in the same heat exchanger, allowing for automatic balance of heat transfer among different media and minimizing energy consumption. However, this will result in all heat exchangers being high-pressure heat exchangers for the internal compression process, leading to increased investment accumulation. Therefore, for internal compression heat exchangers with a rating of 20000 or above, the high and low pressure separation method is still adopted, which is more economical. For ratings below 20000, all high-pressure heat exchangers are configured.

product delivery system

Low pressure oxygen and nitrogen products are equipped with product regulating valves and venting flow paths, and vented into the muffler (with carbon steel for nitrogen and stainless steel for oxygen). The sewage nitrogen gas is set to be released from the water cooling tower (for the purpose of releasing sewage nitrogen gas, adjusting the regeneration gas, and regulating the upper tower pressure. The diameter of the water cooling tower should meet the release requirements, especially in situations where nitrogen gas is also introduced, and the upper tower pressure should not be increased. The resistance of the water cooling tower is 6kPa (8-meter high packing), the pipeline and valve are 4kPa, and the atmospheric pressure difference is 2kPa, for a total of 12kPa).

High pressure oxygen products are vented using two-stage throttling. Firstly, the high-pressure product gas is throttled to 10barG and passes through an eccentric reducer with a Monel noise reduction plate in the middle. Then, the pipeline diameter is expanded through the eccentric reducer, and the oxygen medium flow rate is controlled below 10m/s. Finally, it is passed through a silencing tower for throttling and venting, with silencing components made of stainless steel; High pressure nitrogen product, the nitrogen product is first throttled to 10 bar, passed through a stainless steel noise reduction plate, and then passed into a silencing tower for throttling and venting. The silencing element is made of carbon steel; The oxygen valve requires that no one is allowed to operate it (the regulating valve is prohibited from carrying a handwheel, and the manual valve is placed inside an explosion-proof wall).

The silencing tower can also be combined with the compressor system for venting, and the air compressor booster for noise reduction (calculated according to the air compressor volume) can be introduced into the silencing tower, as well as the purification system for releasing compressed air, and the booster for reflux and release.

expansion refrigeration system

There are generally three types of expanders, namely low-pressure expanders, medium pressure expanders, and liquid expanders.

For a certain type of gas expander, the larger the volume flow rate of the working fluid, the higher the efficiency. The efficiency of a low-pressure expander with a general flow rate of over 8000Nm ³ is 85-88%, while the efficiency can be as low as 70-80% for flow rates less than 3000-8000Nm ³.

Generally, one imported and one domestically produced (spare) medium pressure expander is used. The efficiency of the imported expander with a gas volume of over 8000Nm ³/h is 82-91% (4 points less at the boosting end); The efficiency of domestic expansion machines is 78-87% (5 points less at the boosting end).

Before starting the expansion machine, it is necessary to blow (remove impurities from the piping system and the volute of the expansion machine), then pass the sealing gas (normally provided by the booster end), and then perform external and internal circulation of the oil system. After completing the interlock test, it can be started. After passing the cold test, it can be tightened; Cold start requires the activation of the fuel tank heater, which is not necessary after normal operation. At this point, the cold and heat of the bearings have been balanced.

The essence of a liquid expander is to use the pressure head of high-pressure liquid to perform hydraulic work (while the enthalpy of the liquid decreases, but it is far different from that of gas). Generally, liquid expanders can be used to replace high-pressure liquid air throttling valves in internal compression air separation equipment above 40000 levels. Its advantage is to use liquid expansion mechanism for cooling and expansion power generation to achieve energy-saving goals, which can generally achieve energy savings of about 2%, but its investment can reach tens of millions of yuan.

distillation tower system

There are more sieve plate towers used in the lower tower between 15000 and 50000 levels, and the circulating tray has an advantage in towers with diameters below 15000 levels (the liquid process is longer than convection, but the manufacturing is complex). Convection below 30000 levels is more commonly used, while towers above 15000 levels have an advantage. Large towers with overflow above 30000 levels have an advantage. The energy consumption of packed towers is lower, but the height of the lower tower needs to be increased by about 5 meters. Air separation systems with a rating of over 50000 have an advantage, especially when the upper and lower towers are arranged in parallel.

The upper tower, crude argon tower, and refined argon tower use packed towers, and the manufacturers are generally Sulzer or Tian Da Beiyang. The cold source configuration for the crude argon tower is generally oxygen rich liquid air, and the exhaust gas can be released into the sewage nitrogen pipeline. The energy consumption of the argon system is low when it is shut down; The heat source of the refined argon tower is oxygen rich liquid air or nitrogen gas from the lower tower, and the cold source can be lean liquid air or liquid nitrogen. There are two types of feed: liquid phase and gas phase. It should be noted that the sealing requirements for the condenser plate of the crude argon tower are high, otherwise it may result in unqualified argon products.

There are single-layer, vertical double-layer, horizontal double-layer, vertical three-layer, and falling film main coolers (with liquid oxygen and gaseous oxygen flowing downwards in the same direction as nitrogen).

There are six ways to arrange the distillation tower system:

(1) Vertical arrangement of the upper and lower towers is a conventional arrangement method, with a low height, making it difficult for liquid from the lower tower to enter the condenser of the upper tower or crude argon tower (the pipeline can meet the upward back pressure of the entire liquid phase, and the pipe diameter cannot be small at this time);
(2) Vertical arrangement of the upper and lower towers is a conventional arrangement method, with a moderate height. It is difficult for the liquid from the lower tower to enter the upper tower or the condenser of the crude argon tower. A stripping pipeline is used to transport the liquid to the upper tower (the pipeline outlet must meet the requirement of ρυ²>3000, where ρ is the density and υ is the flow rate, and the inlet position is at a height of 1% of the pipeline vaporization rate. At this time, the pipe diameter needs to be appropriately reduced, and the liquid subcooling cannot be too high);
(3) The upper tower is arranged in sections for argon distillation and connected by two circulating oxygen pumps. Lowering the height of the upper tower can solve the problem of the liquid in the lower tower being unable to enter the condenser of the upper tower or the crude argon tower;
(4) The upper tower is arranged in sections from argon distillation and connected by a circulating pump. The top section of the crude argon tower is located on the upper part of the upper tower, which can reduce the space of the cold box;
(5) The upper tower is independently arranged in a cold manner, connected by a circulating pump, and the main cooling is located at the top of the lower tower. The advantage is that the main cooling can be made very large;
(6) The upper tower is independently arranged in a cold manner, connected by a circulating pump. The top section of the crude argon tower is located on the upper part of the upper tower, which has the advantage that the main cooling can be made very large, and the cold box space can also be reduced.

liquid pump system

Horizontal arrangement of horizontal pumps (with the inlet pipe lower than the outlet pipe) requires the installation of heating gas (located behind the pump or in front of the pump filter to prevent impurities from entering), sealing gas, discharge and exhaust valves (for low position discharge and high position discharge), and return pipelines (for return liquid inlet phase). The speed of the horizontal pump should not be too high, generally with a discharge pressure below 30barG. Due to the horizontal arrangement, the cold contraction shaft of the horizontal pump has better bearing capacity, but the rotor dynamic balance is not easy to meet at high speeds.

The vertical pump adopts a bearing suspension arrangement (with the inlet pipe higher than the outlet pipe), which can withstand a large downward pulling force. The center of gravity of the rotor coincides with the shaft, and the speed can be very high; Generally above 30 bar, it is necessary to set up: return air in front of the pump (note that horizontal pumps do not have it), heating air (set in front of the pump filter, high altitude intake), sealing air, discharge and exhaust valves (low altitude discharge, high altitude exhaust, check if it is cold before pre cooling), and return pipeline (return liquid intake phase). Vertical pumps are generally multi-stage, and the return air pipeline must not be downward (flat or inclined upward), otherwise it will cause gas to not be discharged, which can easily lead to pump cavitation. In addition, the low-temperature pump motor needs to be equipped with a blowing pipeline to prevent overheating in summer and frosting in winter.

Liquid oxygen pump and liquid nitrogen pump are available for online cold standby, with the liquid nitrogen pump having a sealed gas pressure of over 7 barG; The sealing pressure of the oxygen pump is 4 bar G (the pressure of nitrogen in the lower tower can be satisfied); Circulating liquid argon pump, one in use and one as backup, generally uses liquid argon vaporization seal for sealing gas, requiring a 20% flow margin. The general liquid argon pump adopts dual loop control with reflux valve pressure bypass control and outlet valve flow level control.

product compression system

Nitrogen permeation can generally be satisfied by compressed air, and gear type is more energy-efficient for nitrogen turbine compressors with higher pressure.

Oxygen permeability can be divided into single cylinder (low pressure) and double cylinder (high pressure cylinder and low pressure cylinder) according to the discharge pressure (compressed to 30 bar in 8 stages). Generally, when the pressure is below 30 bar G, a sealing gas of 5 bar G is required (pressure nitrogen can meet the requirement). At the same time, due to the high pressure and high temperature fire hazard of oxygen medium, all overcurrent parts are made of copper alloy and require the installation of safety nitrogen gas, which is generally considered by the engineering design institute; The price of imported oxygen permeability is relatively high, about twice that of domestic products, and it is generally not used. Currently, most oxygen permeability products are available, with a discharge pressure of 3-30barG and a flow rate of 8000Nm ³/h or above. However, the flow rate is small and the oxygen permeability efficiency is low, generally ranging from 8000Nm ³/h (55%) to 80000Nm ³/h (68%).

Oxygen permeation is generally used in external compression processes, ranging from 3 to 30 barG, but it generally needs to be compared with internal compression processes with a booster (efficiency is generally above 70%, but there are also flow restrictions, and the efficiency is more than 10 points higher than oxygen permeation, which can even offset the advantage of external compression having less additional energy loss due to reheating compared to internal compression, but internal compression needs to be increased for steel plant pressure relief to avoid fluctuations in the heat exchange system), and the final solution is determined.