Comparison of nitrogen production by cryogenic method and pressure swing adsorption (PSA)

Comparison of nitrogen production by cryogenic method and pressure swing adsorption (PSA)
With the rapid development of industry, nitrogen has been widely used in the fields of chemical industry, electronics, metallurgy, food, biology, medicine, etc. The demand for nitrogen has increased significantly year by year. Nitrogen is chemically inert and exhibits a large inertness under normal conditions and is not easily chemically reacted with other substances. Therefore, nitrogen is widely used as a shielding gas and a sealing gas in electronics, chemicals, food, biology, and medical treatment. The purity of the general shielding gas is 99.99%, and some require high purity nitrogen of 99.998% or more. Liquid nitrogen is a convenient source of cold and is increasingly used in food, medical, and life sciences.
Pure nitrogen cannot be extracted directly from nature, but it can be obtained by separating air. Existing air separation nitrogen methods include: cryogenic, pressure swing adsorption (PSA), membrane separation. This paper mainly discusses the cryogenic method and PSA method for nitrogen production.
principle
The PSA method is a method in which nitrogen is used as a raw material, carbon molecular sieve (CMS) is used as an adsorbent, and the principle of pressure swing adsorption is utilized, and carbon and molecular sieves are selectively adsorbed by oxygen and nitrogen to separate nitrogen and oxygen. After the air is removed by air filter to remove dust and mechanical impurities, it enters the air compressor and is compressed to the required pressure. After strict degreasing, water removal, dust removal and purification treatment, the clean compressed air is output to ensure the use of molecular sieve in the adsorption tower. life. There are two adsorption towers equipped with carbon molecular sieves. When one tower is working, the other tower is decompressed under reduced pressure. Clean air enters the working adsorption tower, and oxygen, carbon dioxide and water are adsorbed by the molecular sieve, and the gas flowing to the outlet end is nitrogen and a trace amount of argon and oxygen. Another column (desorption column) causes the adsorbed oxygen, carbon dioxide and water to escape from the molecular sieve pores to the atmosphere. In this way, the two towers are carried out in turn, the nitrogen and oxygen separation is completed, and the nitrogen gas is continuously output. The purity of nitrogen produced by PSA is 95%-99.9%. If it is treated by nitrogen purifying equipment, the purity of high-purity nitrogen can reach 99.9995%. At present, the maximum production capacity of PSA nitrogen is 3000 Nm 3 /h.
Cryogenic nitrogen production is a traditional method of nitrogen production that has been in recent decades. It uses air as the raw material, and uses liquid oxygen and liquid nitrogen to have different boiling points (the boiling point of O 2 is -183 ° C at 1 atm, and N 2 is -196 ° C), and they are separated by liquid air rectification. To obtain pure nitrogen, pure oxygen can also be obtained. The air is removed by air filters to remove dust and mechanical impurities, then enters the air compressor, is compressed to the required pressure, and then sent to the air cooler to reduce the air temperature. Then enter the air drying purifier to remove moisture, carbon dioxide, acetylene and other hydrocarbons from the air. The purified air enters the main heat exchanger in the air separation tower, is cooled to the saturation temperature by the reflux gas (product nitrogen, exhaust gas), is sent to the bottom of the rectification tower, and nitrogen is obtained at the top of the tower, and the liquid air is sent after throttling. The condensing evaporator is evaporated, and a part of the nitrogen gas sent from the rectification column is condensed, and a part of the condensed liquid nitrogen is used as a reflux liquid of the rectification column, and the other part is used as a liquid nitrogen product to separate the air separation column. The exhaust gas from the condensing evaporator is reheated by the main heat exchanger to about 130K into the expander, and the cooling is provided to provide cooling to the air separation tower. A part of the expanded gas is used for regeneration and blowing of the molecular sieve, and then discharged through the muffler. atmosphere. The liquid nitrogen from the air separation tower is stored in the liquid nitrogen storage tank. When the air separation unit is inspected, the liquid nitrogen in the storage tank is heated into the vaporizer and sent to the product nitrogen pipeline. Cryogenic nitrogen can produce nitrogen with a purity of 99.999%.
Membrane separation method uses air as a raw material, and under different pressure conditions, gases with different properties such as oxygen and nitrogen have different permeation rates in the membrane to separate oxygen and nitrogen.
Comparison
Process comparison
From the principle, we can find that: PSA nitrogen production process is simple, the number of equipment is small, the main equipment is only air compressor, air dryer, adsorption nitrogen generator and gas storage tank. The process of cryogenic nitrogen production is complicated, and the number of equipment is large. The main equipments include air compressors, air coolers, air purification dryers, heat exchangers, expanders and precision flow towers.
Comparison of product type and purity
Cryogenic nitrogen can not only produce nitrogen but also produce liquid oxygen. Cryogenic nitrogen can not only meet the real-time process requirements of liquid nitrogen, but also store liquid nitrogen in the storage tank. When nitrogen load is interrupted or air separation equipment is overhauled, the liquid nitrogen in the storage tank is heated into the vaporizer and is satisfied. The need for nitrogen. The operation cycle of cryogenic nitrogen production (refer to the interval between two large heating) is generally more than one year, so cryogenic nitrogen is generally not considered for standby. However, PSA nitrogen production can only produce nitrogen, and there is no backup means. A single set of equipment cannot guarantee continuous long-term operation.
Cryogenic nitrogen can produce nitrogen with a purity of 99.999%. The purity of nitrogen is limited by the nitrogen load, the number of trays, the efficiency of the tray, and the oxygen purity in the liquid stream. The adjustment range is small. Therefore, for a set of cryogenic nitrogen-making equipment, the purity of its products is basically fixed, and it is inconvenient to adjust. The purity of nitrogen produced by PSA nitrogen production is generally in the range of 95% to 99.9%. If higher purity nitrogen is required, nitrogen purification equipment should be added. The purity of nitrogen is only affected by the nitrogen load of the product. Under other conditions, the larger the nitrogen output, the lower the purity of nitrogen; the higher the reverse. Therefore, for a set of PSA nitrogen production equipment, as long as the load allows, the purity of the product can be arbitrarily adjusted between 90-99.9%. There is no nitrogen storage device for PSA nitrogen production, the nitrogen system is stopped, and the nitrogen supply is terminated.
Comparison of operational controls
Since the cryogenic method is carried out at extremely low temperatures, the equipment must have a pre-cooling start-up process before it is put into normal operation. The start-up time is from the start of the expander until the nitrogen purity reaches the required time, generally not less than 12 h; Before entering the overhaul, there must be a period of warming and thawing, usually 24 hours. Therefore, the cryogenic nitrogen production equipment should not always start and stop, and should be operated continuously for a long time. When the PSA method is started, a qualified nitrogen product can be obtained by pressing the button for 30 minutes. If a high-purity nitrogen gas is required, the nitrogen purification device can obtain a high of 99.99%-99.9999% in about 30 minutes. Pure nitrogen. You can also press the button at the time of the shutdown. Therefore, PSA nitrogen is particularly suitable for intermittent operation. Although the cryogenic nitrogen production system generally adopts advanced DCS (or PLC) computer control technology to realize the control of central control, machine side and local integration, the process still needs real-time monitoring by professionals. PSA nitrogen production adopts intelligent automatic control, and the button can be used for nitrogen production without special management.
in conclusion
The cryogenic nitrogen production equipment is complex, covers a large area, has a high capital cost, has a large investment in equipment, high operating cost, low gas production (12-24 h), high installation requirements and long cycle. The cryogenic nitrogen generator is suitable for large-scale industrial nitrogen production, while the medium and small scale nitrogen production is uneconomical. Compared with the cryogenic method, the PSA method has the advantages of simple process flow, high degree of automation, fast gas production (15-30 min), low energy consumption, and the purity of the product can be adjusted according to user needs in a wide range, and the operation and maintenance are convenient and run. With low cost and strong adaptability to equipment, it is competitive in nitrogen production equipment below 1000 Nm 3 /h. It is more and more popular among small and medium-sized nitrogen users. PSA nitrogen has become a medium and small nitrogen user. The preferred method.

                                                                                  Central Gas Supply System

The medical central gas supply system plays a significant role in modern hosptial. The appropriate design and setup of medcal gas equipment are essential for the hospitals to ensure medical quality, as well as to save the lives and ease the pain of the gas station,Bed Head Unit,gas terminal, electrical system,nusre call system, medical pipelines and auxilliary equipment system.

Central Gas System

Central Gas Supply System

Central Gas Supply System,Hospital Central Gas Supply System,Hospital Gas Supply System ,Medical Gas Supply System

Hunan Eter Electronic Medical Project Stock Co., Ltd. , https://www.centralgas.be