Nitrogen production by cryogenic air separation is a traditional method of nitrogen production, which has a history of recent decades. It takes air as raw material, after compression, purification, and then heat exchange to liquefy the air into liquid air. Liquid-air is mainly a mixture of liquid oxygen and liquid nitrogen, using the different boiling points of liquid oxygen and liquid nitrogen, through the distillation of liquid-air, so that they are separated to obtain nitrogen.

Cryogenic nitrogen production can produce nitrogen gas with a purity of ≥ 99.999%. The purity of nitrogen is limited by factors such as nitrogen load, number of trays, tray efficiency, and oxygen purity in the liquid air, and the adjustment range is very small. Therefore, for a set of cryogenic nitrogen production equipment, the product purity is basically certain and difficult to adjust. The purity of nitrogen produced by pressure swing adsorption is generally within the range of 95% -99.9%. If higher purity nitrogen is needed, nitrogen purification equipment needs to be added. The purity of nitrogen is only affected by the nitrogen load of the product. Under other constant conditions, the higher the nitrogen discharge, the lower the purity of nitrogen; Conversely, the higher it is. Therefore, for a set of pressure swing adsorption nitrogen production equipment, the product purity can be adjusted between 90-99.9% as long as the load is allowed.

The main equipment included in the cryogenic nitrogen generator unit

Air filtration

To reduce the wear and tear on the mechanical surfaces inside the air compressor and ensure air quality, the air must first pass through an air filter to remove dust and other impurities before entering the air compressor. Air compressors often use coarse or medium efficiency filters for intake.

Air compressor

According to the working principle, air compressors can be divided into two categories: volumetric and velocity compressors. Air compressors commonly use reciprocating piston air compressors, centrifugal air compressors, and screw air compressors.

Air cooler

It is used to reduce the temperature of the compressed air before entering the air dryer and air separation tower, avoid significant temperature fluctuations in the tower, and can release most of the moisture in the compressed air. Usually, nitrogen water coolers (consisting of a water cooling tower and an air cooling tower: the water cooling tower uses the exhaust gas from the air separation tower to cool the circulating water, and the air cooling tower uses the circulating water from the water cooling tower to cool the air) and Freon air coolers are used.

Air drying purifier

Compressed air still contains some moisture, carbon dioxide, acetylene, and other hydrocarbons after passing through an air cooler. The frozen moisture and carbon dioxide deposited in the air separation tower can block channels, pipelines, and valves. Acetylene accumulates in liquid oxygen and poses a risk of explosion. Dust can wear down operating machinery. In order to ensure the long-term safe operation of the air separation unit, specialized purification equipment must be installed to remove these impurities. The most commonly used methods for air purification are adsorption and freezing. Molecular sieve adsorption method is widely used in small and medium-sized nitrogen generator units in China.

Air separation tower

The air separation tower mainly includes a main heat exchanger, liquefier, distillation tower, condensing evaporator, etc. The main heat exchanger, condenser evaporator, and liquefier are plate heat exchangers, which are a new type of composite wall heat exchanger with an all aluminum metal structure. The average temperature difference is very small, and the heat transfer efficiency is as high as 98-99%. Distillation tower is an equipment for air separation, and the types of tower equipment are divided according to their internal components. A sieve tray tower is called a sieve tray tower, a bubble cover tower is called a bubble cover tower, and a packed tower is called a packed tower. The sieve plate has a simple structure, is easy to manufacture, and has high tray efficiency, so it is widely used in air separation distillation columns. The packed tower is mainly used for distillation towers with a diameter of less than 0.8m and a height of no more than 7m. Bubble cap towers are rarely used due to their complex structure and manufacturing difficulties.

Turbine expander

It is a rotary blade machinery used in nitrogen production equipment to generate cooling capacity, and is a gas turbine used under low-temperature conditions. Turboexpanders are divided into axial flow, radial flow, and radial axial flow according to the flow direction of gas in the impeller; According to whether the gas continues to expand in the impeller, it can be divided into counterattack type and impact type. Continuing expansion is called counterattack type, while not continuing expansion is called impact type. Single stage radial axial flow impact turbine expander is widely used in air separation equipment. The cryogenic air separation nitrogen generator equipment is complex, occupies a large area, has high infrastructure costs, requires a large one-time investment in equipment, has high operating costs, produces gas slowly, has high installation requirements, and has a long cycle. Taking into account factors such as comprehensive equipment, installation, and infrastructure, the investment scale of PSA units with the same specifications for equipment below 3500Nm3/h is 20% to 50% lower than that of cryogenic air separation units. The cryogenic nitrogen generator is suitable for large-scale industrial nitrogen production, while medium and small-scale nitrogen production appears uneconomical.

Cryogenic nitrogen equipment has the following characteristics

1. Low-temperature heat exchangers, rectification towers and other low-temperature containers and pipelines are placed in the cold box, and filled with insulation materials with low thermal conductivity to prevent heat from the surrounding, reduce cold loss, otherwise the equipment can not operate;
2. Materials used to manufacture low-temperature equipment require sufficient strength and toughness at low temperatures, as well as good welding and processing properties. Commonly used aluminum alloy, copper alloy, stainless steel and other materials;
3. Impurities with high boiling point in the air, such as water, carbon dioxide, etc., should be removed in advance at room temperature. Otherwise, the channel in the device will be blocked, so that the device can not work;
4. Acetylene and hydrocarbons in the air enter the air separation tower and accumulate to a certain extent, which will affect safe operation and even explosion accidents. Therefore, it is necessary to set up purification equipment to remove it;
5. Storage of low-temperature liquid closed container, when the outside heat is introduced, there will be some low-temperature liquid heat absorption and gasification, the pressure will automatically rise. In order to prevent overpressure, a reliable safety device must be set up;
6. Low-temperature liquid leaking into the foundation will freeze and crack the foundation, and the equipment will tilt. Therefore, it is necessary to ensure the tightness of equipment, pipes and valves, and consider the stress and deformation that may be caused by thermal expansion and cold contraction;
7. Porous organic substances such as wood and coke impregnated with liquid oxygen will explode violently when they contact the fire source or give a certain impact force. Therefore, no porous organic matter is allowed in the freezer. For the discharge of liquid oxygen, it should be considered in advance that there is a special liquid oxygen discharge pipeline and container, and it can not go to the ditch;
8. Long-term impact of low-temperature liquid on carbon steel plate will make the steel plate brittle. Therefore, the pipe and discharge tank of low-temperature liquid can not use carbon steel products;
9. Nitrogen and argon are asphyxiating gases, and their liquid discharge pipes should be led outdoors. The gas discharge pipe shall have a certain discharge height, and the discharge outlet shall not face the platform stairs;
10. Oxygen is a strong accelerant, and its discharge pipe can not be directly discharged in an unventilated plant.

Comparison of cryogenic nitrogen machine and PSA industrial nitrogen machine

With the rapid development of industry, nitrogen has been widely used in chemical industry, electronics, metallurgy, food, machinery and other fields, and the demand for nitrogen is increasing at a high speed every year. The chemical properties of nitrogen are not active, and it is very inert in its ordinary state, and it is not easy to react chemically with other substances. Therefore, nitrogen is widely used as a protective gas and sealing gas in the metallurgical industry, electronics industry and chemical industry, and the purity of the general protective gas is 99.99%, and some require 99.998% or more of high purity nitrogen. Liquid nitrogen is a convenient cold source and has been widely used in food industry, medical treatment and semen storage in animal husbandry. In the production of synthetic ammonia in the fertilizer industry, if the raw material gas of synthetic ammonia – hydrogen and nitrogen mixture is washed and refined with pure liquid nitrogen, the content of inert gas can be extremely small, and the content of sulfur monoxide and oxygen does not exceed 20ppm.

Pure nitrogen cannot be extracted directly from nature and is mainly separated by air. Air separation methods include: cryogenic method, pressure swing adsorption (PSA) method, membrane separation method.

Technical and economic comparison between cryogenic nitrogen production and pressure swing adsorption nitrogen production

1. Process comparison

The pressure swing adsorption nitrogen production process is relatively simple with a small number of equipment, mainly consisting of air compressors, air dryers, adsorption nitrogen generators, and gas storage tanks. The cryogenic nitrogen production process is complex with a large number of equipment, including air compressors, air coolers, air purification dryers, heat exchangers, expanders, and precision flow towers.

2. Comparison of product types and purity

Cryogenic nitrogen production can not only produce nitrogen but also produce liquid nitrogen, meeting the process requirements for liquid nitrogen. It can also be stored in liquid nitrogen storage tanks. When there is a nitrogen interruption load or minor maintenance of the air separation equipment, the liquid nitrogen in the storage tank enters the vaporizer and is heated before being sent into the product nitrogen pipeline to meet the nitrogen demand of the process equipment. The operating cycle of cryogenic nitrogen production (referring to the interval between two large heating cycles) is generally more than 1 year, therefore, cryogenic nitrogen production generally does not consider backup. However, pressure swing adsorption nitrogen production can only produce nitrogen gas without backup means, and a single set of equipment cannot guarantee continuous long-term operation.

Cryogenic nitrogen production can produce nitrogen gas with a purity of ≥ 99.999%. The purity of nitrogen is limited by factors such as nitrogen load, number of trays, tray efficiency, and oxygen purity in the liquid air, and the adjustment range is very small. Therefore, for a set of cryogenic nitrogen production equipment, the product purity is basically certain and difficult to adjust. The purity of nitrogen produced by pressure swing adsorption is generally within the range of 95% -99.9%. If higher purity nitrogen is needed, nitrogen purification equipment needs to be added. The purity of nitrogen is only affected by the nitrogen load of the product. Under other constant conditions, the higher the nitrogen discharge, the lower the purity of nitrogen; Conversely, the higher it is. Therefore, for a set of pressure swing adsorption nitrogen production equipment, the product purity can be adjusted between 90-99.9% as long as the load is allowed.

3. Comparison of operational control

The cryogenic method is carried out at extremely low temperatures, and there must be a pre cooling start-up process before the equipment is put into normal operation. The start-up time is generally not less than 12 hours from the start of the expander to the required nitrogen purity; Before the equipment enters major maintenance, there must be a period of heating and thawing, usually 24 hours. Therefore, the cryogenic nitrogen production equipment should not start and stop frequently, and should operate continuously for a long time. When starting the pressure swing adsorption method, just press the button and within 30 minutes, qualified nitrogen products can be obtained. If high-purity nitrogen is needed, it can be purified by a nitrogen purification device in about 30 minutes to obtain 99.99% -99.9999% high-purity nitrogen. Just press the button to stop the machine. Therefore, pressure swing adsorption nitrogen production is particularly suitable for intermittent operation.

At present, the cryogenic nitrogen method generally adopts advanced DCS (or PLC) computer control technology to achieve central control, machine-side, and local integrated control, which can effectively monitor the production process of the whole set of equipment. Pressure swing adsorption nitrogen production using intelligent automatic control, button can be nitrogen production, no need for special treatment.

For petroleum metallurgy and chemical plants, the required nitrogen purity is mostly 99.9%. From the introduction and comparison of cryogenic nitrogen production and pressure swing adsorption nitrogen production above, we can draw the following conclusions:

a) When the continuous nitrogen load is greater than 600 m3n/h and the intermittent load consumption is not too large, and the requirements can be met by liquid nitrogen gasification, cryogenic nitrogen production should be used.

b) When the continuous load of nitrogen is greater than 600 m3n/h and the intermittent load consumption is large, and the amount of liquid nitrogen vaporization is no longer satisfactory, the main method of nitrogen production is cryogenic, and the pressure swing adsorption intermittent gas supply can be used.

c) When the continuous nitrogen load is less than 600 m3n/h, pressure swing adsorption can be used for nitrogen production.

d) Pressure swing adsorption nitrogen production is particularly suitable for nitrogen loads less than 3000 m3n/h, nitrogen purity of 95%, and intermittent operation conditions.

e) When the process equipment requires liquid nitrogen, unless there is a possibility of external supply of liquid nitrogen, cryogenic nitrogen production should be used.