Home >> Industry Knowledge >> Cryogenic nitrogen and compared with PSA Nitrogen Generator
I. Preface

     With the rapid development of industry, nitrogen in the chemical industry, electronics, metallurgy, food, machinery and other fields to obtain a wide range of applications, China's demand for nitrogen is greater than 8% annual rate of increase. Chemical properties of nitrogen inactive in the normal state showed a lot of inertia, and other substances is not easy chemical reaction. Therefore, the nitrogen in the metallurgical industry, electronic industry, chemical industry used a wide range of gas and sealed to protect the gas, general protection requirements for gas purity 99.99%, and some require more than 99.998% of high-purity nitrogen. Liquid nitrogen is a more convenient source of cooling in the food industry, medical profession, as well as animal husbandry and other aspects of semen storage applications become more and more common. The production of ammonia in the fertilizer industry, the raw material for synthetic ammonia gas - hydrogen, nitrogen gas if mixed with pure washing liquid nitrogen refined, will enable an insignificant content of inert gas, a sulfur and oxygen content of no more than 20ppm.

     Pure nitrogen can not be learned directly from nature, the main air separation. Air separation law include: cryogenic method, pressure swing adsorption (PSA), membrane separation.

Second, PSA Nitrogen Generator About the process and equipment

1, Introduction Process

     Air by the air filter to remove dust and mechanical impurities into the air compressor, compressed to the pressure required by the strict removal, in addition to water, dust purification treatment, the output of clean compressed air, designed to ensure that the use of molecular sieve adsorption tower life. Adsorption of carbon molecular sieve with a total of two towers, one tower work, and the other is vacuum desorption tower. Clean air into the adsorption tower work, after molecular sieve oxygen, carbon dioxide and water by their adsorption, flow to the exit of the gas is nitrogen and traces of argon and oxygen. Another tower (desorption tower) so as to enable the adsorption of oxygen, carbon dioxide and water from the row in microporous zeolite to the atmosphere. This rotation for the two towers to complete the nitrogen-oxygen separation, continuous output of nitrogen, see Figure -2. Preparation of PSA nitrogen purity of 95% -99.9%, If you need a higher nitrogen purity nitrogen purification equipment to be increased. PSA nitrogen output of 95% -99.9% nitrogen into the nitrogen gas purification equipment, through a flow meter at the same time to add the right amount of hydrogen, oxygen equipment in the purification of hydrogen and nitrogen Tazhong trace oxygen in the catalytic reaction carried out to remove oxygen and then through condenser cooling water, soft drinks in addition to the water separator, and then through the dryer drying depth (two adsorption drying tower used interchangeably: a dry addition to water absorption, and the other heating desorption drainage), and high-purity nitrogen, at this time of 99.9995% purity nitrogen, see figure -3. PSA currently the largest production capacity of nitrogen 3000m3n / h.

Third, nitrogen cryogenic processes and equipment Profile

1, the typical nitrogen cryogenic process:

     The entire process by the air compression and purification, air separation, the composition of liquid nitrogen evaporation.

Air compression and purification ⑴

     Air by the air filter to remove dust and mechanical impurities into the air compressor, compressed to the required pressure, and then into the air cooler, lower air temperature. Re-entering the dry air purifiers to remove moisture in the air, carbon dioxide, acetylene and other hydrocarbons.


⑵ air separation:

     After purification of the air entering the air separation column of the main heat exchanger, by reflux of gas (nitrogen products, emissions) cooled to saturation temperature, into the distillation column at the bottom, at the top of the tower to be nitrogen, after cutting by Air Liquide evaporation of condensate into the evaporator, condensate from the distillation column at the same time sent some nitrogen, liquid nitrogen after condensation as part of the distillation column as reflux liquid nitrogen as the other part of the air separation column products.
     By condensation from the evaporator air by the resumption of the main heat exchanger into the heat to about 130K expander expansion refrigeration cooling system for air separation amount of gas after the expansion as part of the regeneration of molecular sieve and cold winds, and then discharged into the muffler atmosphere.

Vaporization of liquid nitrogen ⑶

     By the air separation column into liquid nitrogen from liquid nitrogen storage tank, when the air separation plant maintenance, the tank of liquid nitrogen into the carburetor is heated, the nitrogen into the product pipeline.
     Nitrogen cryogenic system to check ≧ 99.999% purity nitrogen.

IV nitrogen cryogenic nitrogen pressure swing adsorption and the technical and economic comparison

1, process comparison

     From the above discussion, we can find: nitrogen pressure swing adsorption process is simple, a small number of equipment, major equipment only air compressor, air dryers, adsorption, such as nitrogen and gas tank. The cryogenic nitrogen complex processes, equipment number, the main air compressor equipment, air coolers, air purification dryer, heat exchanger, expansion and refined flow tower.


2, the type and purity of product compared


     Cryogenic nitrogen and nitrogen not only to the production of liquid nitrogen can be produced to meet the needs of the technological requirements of liquid nitrogen, and can be stored in liquid nitrogen tank, when a load or intermittent nitrogen air separation unit for minor repairs, the tank of liquid nitrogen After entering the carburetor is heated, nitrogen gas into the product pipeline to meet the technology needs of devices on the nitrogen. The operation of cryogenic nitrogen cycle (referring to two large spacing between the heating period) is generally more than 1 year, therefore, nitrogen cryogenic not consider the general reserve. PSA nitrogen and the production of nitrogen only, no back-up means, a single set of equipment can not be guaranteed to run for long period.
     Nitrogen cryogenic system to check ≧ 99.999% purity nitrogen. Purity nitrogen by the nitrogen load, the number of tray, tray efficiency and the purity of liquid oxygen and other air restrictions, a very small range. Therefore, a cryogenic nitrogen plant essential purity of their products is a certain, not regulation. Preparation of PSA nitrogen nitrogen purity in 95% of the general range of -99.9%, if you need a higher nitrogen purity nitrogen purification equipment to be increased. Products only by the purity of nitrogen load of nitrogen, in the case of other conditions remain unchanged, the greater the output of nitrogen, the lower the purity of nitrogen; the other hand is higher. Therefore, a pressure swing adsorption nitrogen plant as long as the load to allow the purity of their products can be arbitrary regulation between 90-99.9%.

3, comparison of operational control

     Cryogenic method since it is carried out under very low temperature, the normal operation of equipment in operation before the start must have a pre-cooling process, start-up time to start from the expander to meet the required purity of nitrogen is generally not less than 12h; equipment Before entering the overhaul must be heated for some time to thaw, typically 24h. Therefore, nitrogen cryogenic equipment not always the rule of law, the stop should be a long time continuous operation. PSA started, simply click the button, to start within 30 minutes of nitrogen may be obtained on qualified products, if you need high-purity nitrogen, then nitrogen gas purification device after about 30 minutes and then was 99.99% -99.9999% The high-purity nitrogen. Shutdown when click button. Therefore, PSA Nitrogen is especially suitable for intermittent running.
     Cryogenic nitrogen is generally the rule of law are advanced DCS (or PLC) computer control technology, in control, machine next to the control of local integration, which can effectively control the entire production process equipment. PSA nitrogen using intelligent automatic control button for nitrogen production, no special management.

V. CONCLUSIONS

Chemical installations for the oil, most of the required nitrogen purity of 99.9%, from the above nitrogen cryogenic and pressure swing adsorption nitrogen profile and the comparison, we can draw the following conclusions:

     a) when the nitrogen load for more than 600 m3n / h, the amount of intermittent load is not too great, you can meet the requirements of vaporization of liquid nitrogen, cryogenic nitrogen should be used.
     b) when the nitrogen load for more than 600 m3n / h, the amount of continuous load, and vaporization of liquid nitrogen has been unable to meet its consumption, the use nitrogen in cryogenic mainly intermittent supply PSA approach.
     c) when the nitrogen load for less than 600 m3n / h, using nitrogen pressure swing adsorption.
     d) Nitrogen PSA Nitrogen is especially suitable for the load less than 3000 m3n / h, nitrogen purity of 95%, and intermittent operating conditions.
     e) When the installation process when the need for liquid nitrogen, unless there is the possibility of an external supply of liquid nitrogen or cryogenic nitrogen should be used.