Based on the analysis of the process characteristics of air separation units, this paper proposes a reasonable transformation of the process from the perspectives of pressure swing adsorption, membrane separation, and cryogenic deep cooling.
Small and Medium Size Industrial Air Separation Unit Manufacturer
1 Brief description of air separation process
1.1 Pressure swing adsorption process
1.2 Membrane separation process
1.3 Cryogenic distillation process
2 Selection of process flow of air separation unit
2.1 Process selection for liquid product production
1 Brief description of air separation process
my country's air separation technology has a long history of development. In the early days, the commonly used process was the cryogenic deep cold separation process. However, with the development of technology and the current status of domestic air separation devices, there are currently three main air separation processes: pressure swing adsorption process, membrane separation process, and cryogenic distillation process.
1.1 Pressure swing adsorption process
The pressure swing adsorption process mainly uses molecular sieves as adsorbents. Through the change of pressure, the nitrogen and oxygen molecules in the air raw material form adsorption force differences under the action of the molecular sieve. After a period of time, the separation of nitrogen and oxygen molecules can be achieved (adsorption phase enrichment and gas phase enrichment), and then the nitrogen and oxygen molecules are unloaded (pressure swing process). The molecular sieve can be recycled after unloading the nitrogen and oxygen molecules. This process of adsorbing and releasing nitrogen and oxygen molecules by molecular sieves and changing the pressure is called "pressure swing adsorption process". In the actual production process, air purification is also required to remove impurities in the air and send it to the buffer tank. The manufacturing process of the pressure swing adsorption process is not complicated, but the quality of the nitrogen and oxygen products obtained is low.
1.2 Membrane separation process
In principle, the membrane separation technology applies the diffusion principle, that is, according to the difference in the coefficient of gas dissolution and diffusion in the membrane, different permeation rates are achieved to achieve the separation of air components. It is not difficult to see that the performance of the separation membrane determines the level of the air separation process. The air raw material acts on both sides of the membrane, and the part with fast permeability and the part with slow permeability are naturally separated, but in order to ensure the yield, a catalyst is generally used: in the actual production process, it still needs to pass through the purification pretreatment system first. The membrane separation process is the most flexible of all process types. By replacing different fiber material membranes, gas products of different types and purities can be obtained. Practical research has shown that the greater the pressure faced by the membrane separation process, the greater the output, so the development of fiber membrane technology is the core issue of this process.
1.3 Cryogenic distillation process
The cryogenic distillation process is an upgraded version of the cryogenic deep cold air separation process. Its principle is to use the different physical properties of nitrogen and oxygen molecules in the air. Oxygen molecules and nitrogen molecules have different boiling points. The air is first liquefied under high pressure and low temperature, and then distilled. The heat transfer separation has been converted into liquid air, thereby separating oxygen molecules and nitrogen molecules.
This process retains a large number of traditional means in actual applications, such as air compressors, purification devices, heat exchange systems, and distillation systems. Compared with the first two processes, the cryogenic distillation process has a very obvious characteristic, that is, it can achieve high purity of products. The cryogenic distillation process is impacted by new processes, mainly due to its own complex process and defects such as long device startup time. In particular, the inconvenience caused by the device itself, including air compressors, purifiers, heat exchange systems, etc., the initial investment cost is much higher than that of molecular sieve and membrane separation processes.
However, it is undeniable that it is impossible to obtain high-purity nitrogen and oxygen products (especially liquid products) at room temperature, and the cryogenic deep cold process is still indispensable.
2 Selection of process flow of air separation unit
Air separation units have a wide range of applications and are of various types. Different industries should make their own selections based on their own needs. After understanding the main process principles of air separation units, it is also necessary to determine what form of product (liquid or gas) the user wants to produce. NEWTEK has made the following selection recommendations.
2.1 Process selection for liquid product production
The raw materials corresponding to the air separation unit and process are in gas form, and high pressure, low temperature and other conditions are required to convert them into liquid form. Based on the characteristics of the above processes, it is not difficult to judge that the low-temperature distillation process is the best choice to obtain liquid products. This is because, under non-low-temperature conditions, although molecular sieves or separation membranes can be used to obtain a collection of nitrogen and oxygen molecules, under an atmospheric pressure, the boiling point of oxygen (pure oxygen) is 90.17K (-182.98℃). The boiling point of nitrogen (pure nitrogen) is 77.35K (-195.80C). It is impossible to form a liquid form.
Therefore, to obtain liquid products, temperature and pressure must be considered. At the same time, the storage process requirements of liquid products are also very strict, which requires strict conditions to achieve docking with the production process.
2.2 Process selection for gaseous product production
The production process of gaseous products has a wide range of options. In addition to non-cryogenic distillation processes, molecular sieves and separation membranes are more commonly used. Both processes can achieve cyclic production and save costs. However, the disadvantage is that products with higher purity cannot be obtained. For some gaseous products that require higher purity, the full low-pressure air separation low-temperature double-tower distillation process is mainly used. The reason is that although adsorption, membrane separation and other processes
can achieve the purpose of air separation, there are natural defects (process limitations) in the extraction of high purity. The full low-pressure air separation low-temperature double-tower distillation process can achieve a process level of 99.99% purity.
3 Conclusion
Through the above introduction, NEWTEK has analyzed the main air separation processes. On the one hand, the emergence of new processes has greatly increased the output of air separation process, saved costs and improved efficiency; but on the other hand, new processes cannot completely replace traditional processes. In order to obtain high-purity products, traditional processes still have development prospects. NEWTEK Group will start with the process system for the future development of air separation process to meet the needs of economic development and environmental ecology.
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