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Everyone is familiar with all kinds of compressors and steam turbines, but do you really understand their role in air separation? An air separation workshop in a factory, do you know what it is like? Air separation, to put it simply, is used to separate the various components of the air gas, the production of oxygen, nitrogen and argon gas set of industrial equipment. There are also noble gases such as helium, neon, argon, krypton, xenon, radon, etc.

Air separation equipment in air as raw material, through the method of compression cycle deep freezing air into liquid, then after rectification and gradually from the liquid air separation to produce oxygen, nitrogen and argon in the equipment of inert gas, such as widely used conventional new coal chemical industry, metallurgy, professional, large nitrogenous fertilizer, gas supply, etc.

In a nutshell, the system process of air separation includes:

■ Compression system

■ Precooling 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 process of air separation system:

Compression systems

There are self-cleaning air filter, steam turbine, air compressor, supercharger, instrument compressor, etc.

(1) Self-cleaning filter generally increases with the increase of air volume, the number of filter cartridge increases, the number of layers is higher, generally more than 25,000 levels of double layer, more than 60,000 levels of three layer layout; Generally, a single compressor needs a separate filter arrangement, and at the same time, it is arranged in the upper tuyere.

(2) steam turbine is a high pressure steam expansion work, driving the coaxial impeller rotation, so as to achieve the type of work on the working medium. There are three commonly used forms of steam turbine: full coagulation, full back pressure and pumping, the more commonly used is pumping.

(4) the investment of air compressor general large air separation device is uniaxial isothermal centrifugal compressor, the energy consumption of imported is about 2% lower than that of domestic, and the investment is 80% higher; The air compressor adopts the outlet venting, does not set the backflow pipeline, generally has the minimum suction flow anti-surge requirements, the inlet guide vane is used for flow regulation, the imported domestic units are four grade compression three grade cooling (the final stage is not cooled). The main air compressor is equipped with a water washing system to wash sediments from the impeller and volute surfaces at all levels. The system is packaged with the main engine.

(5) the investment of the general large air separation device of supercharger adopts two kinds of uniaxial isothermal centrifugal compressor and gear centrifugal compressor, among which the gear type has a greater advantage in energy consumption, especially in the condition of relatively large pressure.

(6) Instrument gas compressor generally has three forms: oil-free screw machine, piston type and centrifugal type. Because the piston type and centrifugal type natural oil free, so do not need oil removal device, only need to support the drying device (water removal) and precision filter (in addition to solid particles) can be; The screw machine generally has two kinds of oil and no oil and oil removal, oil injection screw machine need to set the oil removal device, at the same time need to set a very high precision oil removal filter, in order to meet the requirements of the process, the advantage of this type is cheaper; Oil-free screw using dry rotor or water lubrication, this type of advantage is no oil, the disadvantage is that the price is more expensive. Gas capacity below 500NM ³/h is suitable for selecting piston type; Gas volume in the following 2000Nm³/h suitable for screw machine or piston machine; The gas volume is more than 2000Nm³/h, that is, three models can be selected. When the gas volume is large, the centrifugal compressor has the advantage of less wearing parts, and it is easy to maintain and cost-effective.

The instrument compressor is used when driving, and is extracted by the molecular sieve purifier after normal operation.

Precooling system

The air-cooled tower of the precooling system has two forms: closed cycle (the air-cooled tower is divided into upper and lower sections, and the frozen water circulates between the upper section of the air-cooled tower and the water-cooled tower) and open cycle (the inlet and circulating water system). The closed cycle is mainly used in chemical plants with poor water quality, and 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 precooling system also needs to consider the summer conditions.

The air cooling tower is generally designed for the bottom of the 1 m Φ76 stainless steel pall ring (high temperature), 3 m Φ76 enhanced polypropylene pall ring (large flux), 4 m Φ50 enhanced polypropylene pall ring.

There are also two kinds of water cooling tower: two section type (no external cooling source, dry sewage nitrogen cold recovery is sufficient, so that the pre-cooling system is guaranteed, but the resistance is doubled, (7 meters +7 meters φ50 polypropylene pall ring) and a section type (with external cooling source, 8 meters φ50 polypropylene pall ring).

In addition, all water inlet of the precooling system should be set with filters (generally 6 sets: 4 pumps, water inlet of the water cooling tower, water inlet of the evaporation side of the water chiller) to prevent impurities from being brought into the system. The effect of the precooling system was detected as follows: the outlet gas of the lower 4 m packing section was 1℃ lower than the inlet water; The gas at the outlet of the 8 m packing section in the upper section is 1℃ higher than the water. Generally, a temperature gauge is set in the middle part of the air-cooled tower (extended into the interior).

Purification system

Purification system used by the adsorber has vertical axial flow, horizontal bunk bed and vertical radial flow three.

Vertical axial flow is mainly used for 10,000 grade (diameter has been to 4.6m) below the supporting air separation equipment, bed thickness 1550∽2300mm, double layer and single layer can be arranged, vertical axial flow adsorber airflow distribution is the best.

Horizontal bunk bed is mainly used for supporting large and medium-sized air separation equipment. The bed thickness is 1150mm (molecular sieve) +350mm (aluminum glue).

Vertical radial flow adsorber can effectively use the internal space of the container, so that the adsorption layer area of the same diameter is expanded by about 1.5 times, which can effectively reduce the height of the tower, while the vertical way occupies a small area. Because the air flow is evenly distributed, unlike the horizontal adsorber, the amount of molecular sieve is reduced by 20%, and the renewable energy consumption is also saved by 20%.

However, the disadvantage of vertical radial flow is that the center of air flow is concentrated (sector), which makes it faster than horizontal radial flow penetration time (CO2 < 0.5ppm). The bed thickness is 1000mm+200mm, and the vertical radial flow can meet the configuration of air separation equipment above 20,000 grade.

There are two ways of regenerative heating: electric heater and steam heater.

Steam heater has horizontal (below 40 thousand grade), vertical (above 40 thousand grade), vertical high efficiency steam heater (high steam utilization rate, energy saving 20%) layout: a steam heater (with H2O leakage detection point); Electric heater (dual use and a standby or one use and a standby) in parallel (high temperature and low flow interlock stop setting to prevent burning out, heating tube material is 1Cr18Ni9Ti); Electric heater (meet activation regeneration, 250∽300℃) and steam heater 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 the throttle regeneration pipeline to meet the needs of start-up. In addition, a safety valve is provided on the side of the regenerating gas, and a safety valve is provided on the side of the steam heater to prevent leakage or overpressure on the side of the high pressure of the equipment or valve, as well as throttling overpressure.

The regenerative flow path is equipped with manual butterfly valve to allocate resistance, so as to make the host tower run stably (or not, use the timing adjustment of main pipe regulating valve).

So the heat exchange system

Heat exchange system strictly hybrid medium design of the flow in the same heat exchanger, heat transfer automatic balance for each medium, the low energy consumption, but this can cause all heat exchanger for internal compression process of high pressure heat exchanger, will result in the accumulation of investment increase, so the above 20000 level organization or high-low voltage compression heat exchanger in separate way, more economical, below 20000 levels all adopt high pressure heat exchanger configuration.

The product is sent out

Low pressure oxygen and nitrogen products, set up product control valve and vent flow path, vent into the silencer (nitrogen internal parts for carbon steel, oxygen internal parts for stainless steel). Corrupt nitrogen Settings to water cooling tower blowdown (corrupt nitrogen blow-down role, mix again angry, and adjust the pressure, the effect of the tower water cooling tower tower diameter can meet discharge requirements, especially nitrogen can pass into the situation, does not make the tower high pressure suppress, water cooling tower resistance to 6 kpa (8 meters high packing), piping and valves 4 kpa, 2 kpa of atmospheric vent pressure difference, a total of 12 kpa).

For high-pressure oxygen products, two-stage throttling is adopted for venting. First, the high-pressure product’s gas nozzles flow to 10barG, through the eccentric reducer pipe, and the Monel noise reduction plate is set in the middle. Then, the pipe diameter is expanded through the eccentric reducer pipe, and the flow rate of oxygen medium is controlled below 10m/s. High pressure nitrogen products, nitrogen products first throttled to 10bar, through the stainless steel noise reduction plate, and then into the noise tower throttling vent, carbon steel noise reduction components; The oxygen valve shall not be operated by people (the regulating valve is forbidden to take the handwheel, and the manual valve is placed in the explosion-proof wall).

Anechoization tower can also be combined with the compressor system, air compressor booster noise reduction (calculated in accordance with the amount of air compressor), through the anechoization tower, as well as the purification system pressure relief air, booster play backflow, discharge part.

Expansion refrigeration system

There are three kinds of expander, that is, low pressure expander, medium pressure expander and liquid expander.

For a certain type of gas expander, the greater the volume flow of the working medium, the higher the efficiency. General flow of more than 8000Nm³ low pressure expander efficiency is 85∽88%, flow less than 3000∽8000Nm³ efficiency will be low to 70∽80%.

Medium pressure expander generally adopts an imported one domestic (spare). Air capacity 8000Nm³/h or more imported expander efficiency 82∽91% (pressurized end 4 points less); Domestic expander efficiency 78∽87% (pressurized end 5 points less).

Before the expansion machine starts, it is necessary to purge (remove the impurities in the pipe system and the impurities in the expansion machine volute), and then pass the sealing gas (normally provided by the pressurizing end), and then carry out the external circulation and internal circulation of the oil system. After finishing the interlocking test, it can be started. After passing the cold test, it can be cold tightened. Cold start needs to start the tank heater, which is not necessary after normal operation. At this time, the hot and cold of the bearing has been balanced.

The essence of liquid expander is to use the pressure head of high pressure liquid to do hydraulic work (at the same time, the enthalpy of liquid is reduced, but compared with gas, it is very far). Generally, more than 40,000 grade internal compression air separation equipment can use liquid expander to replace the high pressure liquid air throttle valve. Its advantage is to use liquid expansion mechanism cooling and expansion power generation to achieve the purpose of energy saving, generally can achieve energy saving of about 2%, but its investment of ten million yuan.

Distillation tower system

Tower 1.5 ∽ 50000 level using the sieve plate tower is more, circulation plate under 15000 grade diameter tower more advantages (liquid flow is convection is long, but to make complex), convection below 30000 level application more, more than 15000 grade is dominant, four overflow above the 30000 level tower is dominant, packed tower with low energy consumption, but the tower height to increase 5 meters. Air separation above 50 thousand grade is more advantageous, especially when the upper and lower towers are arranged in parallel.

Packing tower is used for upper column, coarse argon column and fine argon column. The manufacturer is generally Sulzer or Tianda Beiyang. The cold source of coarse argon column is generally oxygen-rich liquid air, and the waste gas can be released into the dirty nitrogen pipeline, so the energy consumption is low when the argon system is stopped. The heat source of the argon column is oxygen-rich liquid air or nitrogen in the lower column, and the cold source can be liquid-poor air or liquid nitrogen. The feed can be liquid phase or gas phase. It should be noted that the sealing requirements of the plate type of crude argon tower condenser are higher, otherwise it will lead to unqualified argon products.

The main cooling has a single layer, vertical double layer, horizontal double layer, vertical three layer and falling film main cooling (liquid oxygen and gas oxygen down, with nitrogen flow).

There are 6 ways in which the rectifying tower system can be arranged:

(1) The vertical arrangement of the upper and lower towers is a conventional arrangement. The height is low, and the liquid in the lower tower is difficult to enter the upper tower or the condenser of the coarse argon tower without the lower tower (the upward back pressure of the whole liquid phase in the pipeline can be satisfied, and the pipe diameter can not be small at this time);

(2) vertical layout, up and down as the regular arrangement, medium height, liquid is difficult to enter the tower or the tower crude argon column condenser using set stripping line take liquid to the tower (pipe exports meet rho nu squared > 3000, rho for density, nu as flow velocity, inlet position in vaporizing tube height at a rate of 1%, need appropriate narrow diameter, at the same time, the liquid super-cooling degree not big);

(3) The upper column is arranged in the section of argon fraction. 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 coarse argon column.

(4) The upper column is arranged in sections of argon fraction and connected by circulating pump. The upper section of the coarse argon column is located in the upper part of the upper column, which can reduce the space of the cold box.

(5) the tower independent cold layout, the use of circulating pump connection, the main cooling in the top of the tower, the advantage is that the main cooling can be done very large;

(6) The upper tower is independently arranged in a cold place and connected by circulating pump. The top section of the coarse argon tower is located at the upper part of the upper tower. 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

Horizontal pump horizontal arrangement under drainage tube (liquid into the tube), you need to set up heating gas (installed in the pump, or pump filter before, and prevent impurities from entering the), sealed air, drainage exhaust valve (lower drainage, high exhaust) and return line (the liquid inlet), horizontal pump speed cannot too tall, general pressure under 30 barg, horizontal pump due to the horizontal layout, cold contraction bearing load is better, but high speed rotor dynamic balancing is bad enough.

The vertical pump adopts bearing suspension type arrangement (the inlet pipe is higher than the drain pipe), bears the downward tension is larger, the center of gravity of the rotor and the shaft are re-combined, and the speed can be very high; Generally above 30bar, it is necessary to set: return air in front of the pump (note that there is no horizontal pump), heating gas (set in front of the pump filter, high air intake), sealing gas, discharge valve (low discharge, high exhaust, check whether it is completely cold when precooling) and return pipe (return liquid intake phase). Vertical pump are generally multi-stage, return pipe road requirements shall not be down (flat, or inclined upward), otherwise it will cause gas can not be discharged, easy to lead to pump cavitation. In addition, the low temperature pump motor needs to set the blowing pipeline to prevent overheating in summer and frosting in winter.

Liquid oxygen pump liquid nitrogen pump standby in cold state, in which the sealing gas pressure of liquid nitrogen pump is more than 7barG; The sealing gas pressure of the oxygen pump is 4barG(the pressure of the lower tower can be met by nitrogen); Circulating liquid argon pump, one use and one standby, sealing gas generally adopts liquid argon vaporization seal, the flow is required to have 20% margin. General liquid argon pump itself reflux valve pressure-by-pass control, outlet valve flow-level control, using double circuit control.

Product compression system

Nitrogen penetration can meet the general compressed air, nitrogen turbine compressor pressure is higher, the gear type is more energy saving.

Oxygen through according to the row of a single cylinder pressure (low pressure) and two cylinder (high pressure and low pressure cylinder) (8 level compression to 30 bar), generally below 30 barg, you need to set up 5 barg sealing gas (nitrogen pressure can meet), at the same time, due to oxygen medium for high temperature high pressure HuoHuan reasons, all flow part adopt copper alloy, you need to set up security nitrogen, usually by the engineering design consideration; Imported oxygen penetration price is higher, about 2 times as much as domestic, generally not used, currently generally all hang oxygen oxygen penetration, discharge pressure 3∽30barG, flow of 8000Nm³/h above can be met. However, the flow rate is small and the oxygen permeability efficiency is low, generally 8000Nm³/h (55%) ∽80000Nm³/h (68%).

General applied to the compression process of oxygen, from 3 ∽ 30 barg were, but often with internal compression process of booster (generally more than 70% efficiency, also has the traffic restrictions, efficiency is higher than oxygen through more than 10 points, it can even offset the compression relatively less in compression after heat the advantage of the additional energy losses, but the internal compression for steel pressure need to improve, so as to avoid heat exchange system fluctuations) are compared, and energy consumption after the plan is determined.

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