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Paper on air separation technology (2)
Paper on air separation technology II

Application of cryogenic technology in air separation plant design

Abstract: With the rapid development of national economy and the continuous improvement of scientific and technological level, various new types of air separation equipment appear constantly. As the earliest technology in air separation technology, cryogenic technology has been widely used in its development. This paper mainly analyzes and discusses the concept of cryogenic technology, the meaning of air separation equipment and the application of cryogenic technology in air separation equipment design.

Keywords: low temperature technology; Air separation equipment; Design; Application; concept

Classification number of China Library: TQ116.1Document ID: A document number:1006-8937 (2014) 23-0063-02.

Air separation technology originated from 1985 and 1903. The first set of air liquefaction equipment and 10 m3/h (oxygen) air separation equipment created by Professor Linde, in the development process of 100 years, with the continuous development of new technologies, air separation equipment and technology have been greatly developed. Cryogenic separation technology is the earliest technology in air separation. Through continuous research and innovation, it has become more and more mature in industrial production practice, equipment renewal and national economic development.

1 low temperature technology concept

Cryogenic technology refers to the use of refrigerant medium as cooling medium to continuously cool the quenched metal material to a certain temperature (-196℃) far below room temperature, so as to achieve the purpose of transferring the properties of metal materials. In recent years, with the continuous development of air separation equipment design, cryogenic technology, as a new technology, is the most effective, economical and practical metal workpiece delivery technology at this stage.

During deep cold working, a large amount of retained austenite in metal is transformed into martensite, especially during the process from-196℃ to room temperature, the supersaturation of supersaturated metastable martensite will be reduced and dispersed. Ultra-fine carbide maintains a lattice relationship with the matrix, and the current is only 20 ~ 60 A. This phenomenon can reduce the lattice distortion of martensite and reduce the micro-stress. In the plastic deformation of materials, fine and dispersed carbides can coexist, and ultrafine carbide particles should be evenly distributed on martensite matrix to effectively weaken the brittleness of grain boundaries. Refining the matrix structure can not only effectively weaken the segregation of impurity elements at the grain boundary, but also give full play to the grain boundary strengthening effect, thus greatly improving the performance of tools and dies, and improving their hardness, impact toughness and wear resistance. The application of low temperature technology is not only reflected in the working surface, but also infiltrated into the workpiece, showing the overall effect. On this basis, the workpiece can be reshaped and used many times. In the aspect of workpiece, the application of cryogenic technology can also effectively reduce quenching stress and enhance dimensional stability.

2 The meaning of air separation plant

With the rapid development of social economy and the continuous progress of science and technology, in the development of air separation research and other industries, the development of its equipment presents a trend of large-scale, and the requirements for supporting air separation equipment are getting higher and higher. At present, the level of air separation equipment in China has reached 60,000 units, which is very close to the advanced level in the world. In 2002, Hangyang's 30,000-class domestic air separation equipment was successfully operated in Baosteel Group, which greatly improved the competitiveness of China's air separation equipment and promoted the successful large-scale domestic air separation equipment to occupy the domestic market on a large scale.

Application of cryogenic technology in air separation plant design

The air cryogenic separation process is to produce high-purity oxygen, nitrogen and other products from compressed air through multi-tower low-temperature distillation. At present, the design of air separation equipment mainly has two forms. Firstly, the normal temperature air separation equipment is mainly carried out at normal temperature and non-low temperature. This normal temperature air separation plant can be divided into two different forms: pressure swing adsorption separation and membrane separation. What we want to analyze here is another design form of air separation equipment, deep cold air separation, which is mainly used in the case of very low temperature. In the 1950s, in order to enhance China's national defense strength and meet the national defense demand, China's deep cold air separation technology and equipment were imported and copied from the Soviet Union, and the imitation enterprise was Hangzhou Iron Works. Around 1953, China successfully copied its own cryogenic air separation equipment. Today, China's air separation technology and equipment manufacturing level have made great progress and contributed to the growth of the national economy. At present, the deep cold air separation technology is mainly used in the following aspects.

3. 1 compressed air purification module

The main components of the compressed air purification module are high-efficiency degreaser, freeze dryer, precision filter and movable filter. Firstly, air should be compressed in an air compressor, then run in an air buffer tank, and then most impurities, including oil, water and dust, can be effectively removed by an efficient oil remover. Freeze-dryer can be used for further dehumidification, precision filter can be used for oil and dust removal again, and finally activated carbon filter can be used for further oil removal.

3.2 Air Buffer Tank

The main components of the air buffer tank assembly are the air buffer tank and the attached valve instruments. The main function of air buffer tank in the design of air separation plant is to buffer and effectively reduce airflow pulsation. In addition, the pressure fluctuation of the system is reduced, so that compressed air can smoothly pass through the compressed air purification assembly, and impurities such as water and water are effectively removed to the greatest extent. At the same time, it can also help the oxygen-nitrogen separation system to obtain a large amount of compressed air in a very short time when the adsorption tower is switched. The application of this technology is helpful for the pressure in the adsorption tower to rise rapidly and reach the working pressure, and can also ensure the reliability and stability of the operation of mechanical equipment.

3.3 oxygen and nitrogen separation system

The main components of oxygen-nitrogen separation system are adsorption tower, compaction device, attached valves and instruments. The adsorption tower in the composite bed structure design is selected, which is mainly divided into two towers, A tower and B tower, and the tower is filled with imported carbon molecular sieve (in order to improve the uniformity of carbon molecular sieve filling, the stretching, twisting and vibration filling mode can be selected for operation). Clean compressed air must first flow from the inlet end to the outlet end of tower A under the action of carbon molecular sieve. At this time, the main components it adsorbs are oxygen, carbon dioxide and water, and only the product nitrogen flows out from the outlet end of the adsorption tower. With the passage of time, when the adsorption of carbon molecular sieve in tower A reaches saturation, the phenomenon of automatic adsorption will stop. Clean compressed air will flow into tower B to absorb oxygen to produce nitrogen, and the molecular sieve of tower A will be regenerated. The regeneration of molecular sieve is realized by rapidly lowering the adsorption tower to atmospheric pressure to desorb oxygen and carbon dioxide. In the regeneration process of alternating adsorption towers A and B, not only oxygen and nitrogen can be separated, but also nitrogen can be continuously produced.

3.4 oxygen and nitrogen buffer system

The components of oxygen and nitrogen buffer system are mainly composed of nitrogen buffer tank, precision filter, flowmeter, pressure regulating valve, venting parts, etc. The nitrogen buffer tank mainly balances the pressure and purity of nitrogen separated from the nitrogen-oxygen separation system to ensure the stable and continuous supply of nitrogen. At the same time, after the switching of the adsorption tower, a part of its own gas is replenished to the adsorption tower, which can effectively increase the pressure of the adsorption tower, effectively protect the bed and greatly protect the air separation equipment at work. Finally, it is filtered again, mainly by precision filter, which can ensure the quality of nitrogen to the maximum extent.

As a traditional nitrogen production method, cryogenic air separation has been developed for decades. This method mainly uses air as raw material. In order to liquefy air into liquid air, strict compression, purification and heat exchange must be carried out. The mixture of liquid air is mainly composed of two parts: liquid oxygen and liquid nitrogen. Liquid air is rectified through their different boiling points, and nitrogen is obtained by effective separation between them. In the whole operation, the equipment and technology of nitrogen production by deep cold air separation are very complicated, which requires a lot of land, high capital construction cost, slow gas production speed, high installation technology requirements and long construction period. The factors such as deep cold air separation equipment, installation and grass-roots construction are comprehensively analyzed. When the equipment is less than 3500 N m3/h, the cost investment of the same specification PSA unit is 20% ~ 50% lower than that of the cryogenic air separation unit. In terms of economic adaptability, the cryogenic air separation nitrogen plant is not suitable for small and medium-sized industrial nitrogen production, but mainly suitable for large-scale industrial nitrogen production.

4 conclusion

To sum up, with the continuous improvement of science and technology, China's air separation equipment and technology are increasingly moving towards large-scale, professional and large-scale development trend. As the most important separation technology of air separation, cryogenic technology is the main task of air separation, which effectively reduces the energy consumption of cryogenic technology on the basis of ensuring the maximum benefit of enterprises.

References:

[1] Ye Binan. Hangyang applied new technology to develop a series of new products [J]. Low temperature technology, 1996, (4).

Jiang runmin Modular design and manufacture of air separation equipment and cryogenic liquefaction equipment [J]. Cryogenic technology, 1997, (4).

Helmut Chunman, Mao Jie. Present situation and prospect of air separation equipment [J]. Low temperature technology, 1974, (3).

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