All kinds of compressors and steam turbines are familiar to everyone, but do you really understand their role in air separation? The air separation workshop in the factory, do you know what it is like? Air separation, in simple terms, is a complete set of industrial equipment used to separate various components in the air to produce oxygen, nitrogen and argon. There are also noble gases such as helium, neon, argon, krypton, xenon, radon, etc.
Air separation equipment uses air as raw material, deeply freezes the air into liquid through the method of compression cycle, and then gradually generates inert gas such as oxygen, nitrogen and argon from liquid air separation after rectification. Metallurgy, professional, large-scale nitrogen fertilizer, gas supply, etc.
In short, the system process of air separation includes:
■ Compression system
■ Pre-cooling system
■ Purification system
■ heat exchange system
■ Product Delivery System
■ Expansion refrigeration system
■ Distillation column system
■ Liquid pump system
■ Product compression system
We introduce the equipment one by one according to the process flow of the air separation system:
compression system
There are self-cleaning air filters, steam turbines, air compressors, superchargers, instrument compressors, etc.
(1) The self-cleaning filter generally increases with the increase of air volume, the number of filter elements increases, and the number of layers is higher. Generally, the double-layer layout is above 25,000, and the three-layer layout is above 60,000. Generally, a single compressor needs a separate filter arrangement. , and at the same time arranged in the upper vent.
(2) The steam turbine is a type of high-pressure steam expansion that does work, and drives the coaxial impeller to rotate, thereby realizing work on the working medium. The common forms of steam turbines are full condensing, full back pressure and pumping, and the more commonly used is pumping.
(4) Air compressors are generally invested in single-shaft isothermal centrifugal compressors for large-scale air separation plants. The imported energy consumption is about 2% lower than that of domestic ones, and the investment is 80% higher. Generally, there is a minimum suction flow anti-surge requirement, the inlet guide vane is used for flow regulation, and the imported domestic unit is four-stage compression and three-stage cooling (the final stage is not cooled). The main air compressor is equipped with a water washing system, which is used to wash the deposits on the surfaces of the impellers and volutes of all stages. The system is packaged with the host.
(5) Supercharger Generally, the investment of large-scale air separation plants adopts two types of single-shaft isothermal centrifugal compressors and gear centrifugal compressors. Among them, the gear type has great advantages in energy consumption, especially in the case of high pressure.
(6) Instrument gas compressors generally have three forms: oil-free screw machine, piston type and centrifugal type. Since the piston type and centrifugal type are naturally oil-free, there is no need for a degreasing device, only a drying device (water removal) and a precision filter (solid particle removal) are required; screw machines generally have oil and oil-free and oil removal. Two, oil-injected screw machines need to be equipped with a degreasing device, and at the same time, a very high-precision de-oiling filter needs to be set up to meet the process. It is oil-free, the disadvantage is that it is more expensive. The piston type is suitable for the air volume below 500Nm³/h; the air volume below 2000Nm³/h is suitable for screw machine or piston machine; the air volume is greater than 2000Nm³/h, that is, three models are available. When the air volume is large, the advantages of centrifugal compressors are less wearing parts, convenient maintenance and high cost performance.
The instrument compressor is used when driving, and is extracted by the molecular sieve purifier after normal operation.
Pre-cooling system
The air cooling tower of the pre-cooling system has two forms: closed circulation (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 circulation (water inlet and circulating water system). Closed loops are mainly used in chemical plants with poor water quality, where fresh water and chemicals need to be added. Open circulation is widely used, but the circulating water system also needs to replenish fresh water regularly, and the pre-cooling system also needs to consider summer conditions.
The bottom of the air cooling tower is generally designed as 1m Φ76 stainless steel Pall ring (high temperature), 3m Φ76 reinforced polypropylene Pall ring (large flux), 4m Φ50 reinforced polypropylene Pall ring.
There are also two types of water cooling towers: two-stage type (no external cold source, sufficient nitrogen-cooled recovery of dry sewage, so that the pre-cooling system is guaranteed, but the resistance is doubled, (7 meters + 7 meters φ50 polypropylene Pall ring) and Section type (with external cooling source, 8 meters φ50 polypropylene Pall ring).
In addition, all water inlets of the pre-cooling system should be equipped with filters (usually 6 units: 4 pumps, water inlets of water cooling towers, and water inlets on the evaporative side of the chiller) to prevent impurities from being brought into the system. The effect of the pre-cooling system is tested as follows: the outlet gas of the lower 4 m packing section is 1 °C lower than the inlet water; the outlet gas of the upper 8 m packing section is 1 °C higher than the water. Generally, a thermometer is set in the middle of the air-cooled tower (extending into the interior).
Purification system
There are three types of purification systems used in the adsorber: vertical axial flow, horizontal double bed and vertical radial flow.
Vertical axial flow is mainly used for supporting air separation equipment of grade 10,000 (diameter has reached 4.6m), the bed thickness is 1550∽2300mm, and both double and single layers can be arranged.
Horizontal bunk beds are mainly used to support large and medium-sized air separation plants. The bed thickness is 1150mm (molecular sieve) + 350mm (aluminum glue).
The vertical radial flow adsorber can effectively utilize the inner space of the container, expand the adsorption layer area of the same diameter by about 1.5 times, and can effectively reduce the tower height, while the vertical occupied area is small. Due to the uniform air distribution, different from the horizontal adsorber, the amount of molecular sieve is reduced by 20%, and the consumption of renewable energy is also saved by 20%.
However, the disadvantage of vertical radial flow is the central concentration (sector) of the airflow, which makes it faster than horizontal radial flow (CO2 < 0.5ppm). The bed thickness is 1000mm+200mm, and the vertical runoff can meet the configuration of air separation equipment above 20,000.
There are two types of regenerative heating: electric heaters and steam heaters.
Steam heaters include horizontal (below 40,000 grades), vertical (over 40,000 grades), and vertical high-efficiency steam heaters (high steam utilization rate, 20% energy saving) Layout: steam heater (with H2O leak detection point) ;Electric heaters (dual use and one standby or one use and one standby) in parallel (high temperature and low flow interlock stop setting to prevent burnout, heating tube material is 1Cr18Ni9Ti); electric heater (to meet activation and regeneration, 250∽300℃) and steam The heater is connected in parallel; the electric heater is connected in series with the steam heater (when the steam temperature is low, the regeneration resistance is large).
The purification system also needs to set up a throttling regeneration pipeline to meet the needs of startup. In addition, set a safety valve on the regeneration gas side and set a safety valve on the steam heater side to prevent leakage or overpressure on the high pressure side of the equipment or valve, as well as throttling overpressure.
The regenerative flow path is equipped with a manual butterfly valve to distribute the resistance, so that the main tower runs stably (or not used, using the timing adjustment of the main control valve).
So the heat exchange system
The heat exchange system is strictly designed with mixed media, flowing in the same heat exchanger, the heat transfer of each medium is automatically balanced, and the energy consumption is low, but this will cause all heat exchangers to be high pressure heat exchangers in the internal compression process, which will lead to increased investment. Accumulation, so the organization above 20000 level or high and low pressure compression heat exchanger shunt is more economical, and all below 20000 level use high pressure heat exchanger configuration.
Product has been shipped
For low-pressure oxygen and nitrogen products, set the product control valve and exhaust flow path, and the exhaust gas enters the silencer (carbon steel for nitrogen trim, stainless steel for oxygen trim). The rot nitrogen is set for the sewage of the water supply cooling tower (the rot nitrogen has the effect of sewage discharge, re-mixing, and adjusting the pressure, so that the tower diameter of the tower water cooling tower can meet the discharge requirements, especially when nitrogen can be introduced, so that the high pressure in the tower is not suppressed, and the water Cooling tower resistance 6 kpa (fill height 8 meters), pipes and valves 4 kpa, atmospheric exhaust port pressure difference 2 kpa, a total of 12 kpa).
For high pressure oxygen products, the exhaust is throttled in two stages. First, the gas nozzle of the high pressure product flows to 10barG, through the eccentric reducer, and the Monel noise reduction plate is set in the middle. Then, the diameter of the pipe is enlarged by the eccentric reducer, and the flow rate of the oxygen medium is controlled below 10m/s. High-pressure nitrogen products, the nitrogen products are first throttled to 10bar, pass through the stainless steel noise reduction plate, and then enter the throttle opening of the noise reduction tower, carbon steel noise reduction components; inside the blast wall).
The muffler tower can also be combined with the air compressor system, the air compressor pressurization and noise reduction (calculated according to the amount of air compressors), through the muffler tower, and the decompression air of the purification system, the pressurization and return flow, and the discharge part.
Expansion refrigeration system
There are three types of expanders: low pressure expanders, medium pressure expanders and liquid expanders.
For a certain type of gas expander, the greater the volume flow of the working medium, the higher the efficiency. Generally, the efficiency of a low pressure expander with a flow rate greater than 8000Nm³ is 85∽88%, and the efficiency of a flow rate less than 3000∽8000Nm³ will be as low as 70∽80%.
The medium pressure expander generally adopts an imported one made in China (spare parts). The efficiency of the imported expander is 82∽91% (the pressure end is less than 4 points) with the air volume above 8000Nm³/h; the efficiency of the domestic expander is 78∽87% (the pressure end is less than 5 points).
Before starting the expander, it needs to be purged (to remove the impurities in the piping system and the impurities in the volute of the expander), and then the sealing gas (usually provided by the booster end) is introduced, and then the circulation and internal circulation of the external oil system are carried out. After the interlock test is completed, it can be started. After passing the cold test, it can be cold tensioned. Cold start requires the tank heater to start, not after normal operation. At this point, the heat and cold of the bearing have been balanced.
The essence of the liquid expander is to use the pressure head of the high-pressure liquid to do hydraulic work (at the same time, the enthalpy of the liquid is reduced, but it is far from the gas). Generally, the internal pressure air separation plant above 40,000 grades can use a liquid expander to replace the high-pressure liquid-air throttle valve. The advantage is that the liquid expansion mechanism is used to cool and expand power generation to achieve the purpose of energy saving, which can generally achieve energy saving of about 2%, but its investment is tens of millions of yuan.
Distillation Column System
1.5∽50000 class towers use more sieve tray towers, and the circulating plate tower diameter below 15000 class has more advantages (liquid convection is longer, but the production is complicated). The four overflow towers are dominated by more than 30,000 grades, and the energy consumption of the packed tower is low, but the tower height should be increased by 5 meters. The air separation of more than 50,000 grades is more advantageous, especially when the upper and lower towers are arranged in parallel.
Packed columns are used for upper column, crude argon column and fine argon column. The manufacturer is generally Sulzer or Tianda Beiyang. The cold source of the crude argon tower is generally oxygen-enriched liquid air, and the waste gas can be discharged into the dirty nitrogen pipeline, so the energy consumption is low when the argon system is stopped. The heat source of the argon tower is oxygen-enriched liquid air or nitrogen in the lower tower, and the cold source can be lean liquid air or liquid nitrogen. The feed can be in the liquid or gas phase. It should be noted that the sealing requirements of the plate-type crude argon column condenser are relatively high, otherwise the argon product will be unqualified.
The main cooling includes single-layer, vertical double-layer, horizontal double-layer, vertical three-layer and falling film main cooling (liquid oxygen and gas oxygen drop, with nitrogen flow).
There are 6 ways to arrange the distillation column system:
(1) The vertical arrangement of the upper and lower towers is a conventional arrangement. The height of the lower tower is low, and it is difficult for the lower tower liquid to enter the upper tower or the thick argon tower condenser without the lower tower (it can satisfy the upward back pressure of the whole liquid phase in the pipeline, and the pipe diameter cannot be small at this time);
(2) Vertical arrangement, regular arrangement up and down, medium height, it is difficult for the liquid to enter the column or the condenser of the crude argon column in the column adopts a stripping line to extract the liquid into the column (the outlet of the pipe meets rho nu squared > 3000, rho is the density , nu is the flow rate, the inlet position is 1% of the height of the vaporization pipe, a proper narrow pipe diameter is required, and the liquid subcooling degree is not large);
(3) The upper column is arranged in the argon distillation section. Two circulating oxygen pumps are used to connect the upper column. The lower height of the upper column can solve the problem that the liquid in the lower column cannot enter the upper column or the condenser of the crude argon column.
(4) The upper column is arranged in sections of argon fractions and connected by a circulating pump. The upper part of the crude argon column is located in the upper part of the upper column, which can reduce the cold box space.
(5) The tower is arranged independently and is connected by a circulating pump, and the main cooling is at the top of the tower. The advantage is that the main cooling can be made large;
(6) The upper tower is independently arranged in a cold place and connected by a circulating pump. The top of the crude argon column is located in the upper part of the upper column. The advantage is that the main cooling can be made very large, and the space of the cold box can also be reduced.
Liquid Pump System
The horizontal pump is arranged horizontally under the drain pipe (the liquid enters the pipe), and it is necessary to set heating gas (installed in the pump, or filter before the pump to prevent impurities from entering), sealing air, drain and exhaust valve (lower drain, high exhaust ) and return pipe (liquid inlet), the rotating speed of the horizontal pump should not be too high, and the general pressure is below 30 barg. The horizontal pump has a better load on the cold shrink bearing due to the horizontal layout, but the dynamic balance of the high-speed rotor is not good enough.
The vertical pump adopts the bearing suspension arrangement (the water inlet pipe is higher than the drain pipe), which bears a large downward pulling force. The center of gravity of the rotor and the shaft is recombined, and the speed can be very high; generally above 30bar, it is necessary to set: the return air before the pump ( Note that there is no horizontal pump), heating gas (set before the pump filter, high intake), sealing gas, exhaust valve (low exhaust, high exhaust, check whether it is completely cold during pre-cooling) and return pipe (return liquid inlet stage). Vertical pumps are generally multi-stage, and the return pipeline must not be downward (flat, or inclined upward), otherwise the gas will not be discharged, which will easily lead to pump cavitation. also,
Liquid oxygen pump Liquid nitrogen pump is cold standby, the sealing gas pressure of the liquid nitrogen pump is greater than 7barG; the sealing gas pressure of the oxygen pump is 4barG (the pressure of the lower tower can be satisfied by nitrogen); Liquid argon is vaporized and sealed, and the flow rate is required to have a 20% margin. Generally, the return valve of the liquid argon pump itself is controlled by pressure bypass, and the flow level of the outlet valve is controlled by double-loop control.
Product Compression System
Nitrogen permeation can meet general compressed air, nitrogen turbo compressor has higher pressure, and gear type is more energy-efficient.
Oxygen is compressed to 30 bar through the row (8 stages) according to the pressure of one cylinder (low pressure) and two cylinders (high pressure and low pressure), generally below 30 barg, need to set a sealing gas of 5 barg (nitrogen pressure can be satisfied), and at the same time Because the oxygen medium is a high temperature and high pressure fire ring, all overcurrent parts are made of copper alloy, and safety nitrogen needs to be set up, which is usually considered by engineering design; the penetration price of imported oxygen is relatively high, about 2 times that of domestic products, and is generally not used. At present, oxygen penetration is generally used, the discharge pressure is 3∽30barG, and the flow rate is above 8000Nm³/h. However, the flow rate is small and the oxygen permeability efficiency is low, generally 8000Nm³/h (55%)∽80000Nm³/h (68%).
Generally suitable for the compression process of oxygen, starting from 3∽30 barg, but often use the internal compression process of the supercharger (generally the efficiency is above 70%, there are traffic restrictions, the efficiency is more than 10 points higher than oxygen, it can even offset The advantages of relatively less additional energy loss after compression, but the internal compression pressure of steel needs to be increased to avoid fluctuations in the heat exchange system) to compare and determine the energy consumption after the scheme.
What are the well-known companies in the industry?
Located in Hangzhou Economic and Technological Development Zone, Hangzhou Fuyang H Gas Zhejiang Technology Co., Ltd. is one of the enterprises specializing in the research, development, production and operation of industrial gas equipment. The company has a research and development center, a manufacturing and marketing service center, and high-level professional and technical personnel. Provide customers with technical consulting, program design, product manufacturing, personnel training, installation, commissioning and other services.
