With the rapid development of industry, nitrogen has gained wide application in the fields of chemical industry, electronics, metallurgy, food, machinery, etc. China's demand for nitrogen is increasing at a rate of more than 8% per year. Nitrogen is chemically inactive, showing great inertness in unusual state and not easy to have chemical reaction with other substances. Therefore, nitrogen is widely used as protection gas and sealing gas in metallurgical industry, electronic industry and chemical industry. The general purity of protection gas is 99.99%, and some of them require 99.998% or more of high purity nitrogen. Liquid nitrogen is a convenient cold source, which is more and more commonly used in the food industry, medical business and semen storage in animal husbandry. In the fertilizer industry production of ammonia, ammonia raw gas - hydrogen, nitrogen mixture if the pure liquid nitrogen scrubbing and refining, can make the inert ** gas content is very small, sulfur monoxide and oxygen content of no more than 20 ppm.
Pure nitrogen can not be drawn directly from nature, the main use of air separation method. The air separation methods include: deep cooling method, pressure swing adsorption (PSA), and membrane separation method.
The process and equipment of PSA nitrogen generator
1, process introduction
Air enters into the air compressor after removing dust and mechanical impurities through the air filter, and is compressed to the required pressure, and then outputs clean compressed air after strict purification treatment of degreasing, de-watering, and de-dusting, which is aimed at ensuring the service life of molecular sieve in the adsorption tower. Fitted with carbon molecular sieve adsorption tower **** there are two, one tower work, the other tower is depressurized desorption. Clean air into the work of the adsorption tower, through the molecular sieve when oxygen, carbon dioxide and water are adsorbed, flow to the exit end of the gas is nitrogen and trace amounts of argon and oxygen. Another tower (desorption tower) so that the adsorbed oxygen, carbon dioxide and water from the molecular sieve microporous detachment discharged into the atmosphere. So the two towers take turns to complete the separation of nitrogen and oxygen, continuous output of nitrogen, see Figure-2. Variable pressure (_bian4 ya1) adsorption of nitrogen purity of 95% -99.9%, if you need a higher purity of nitrogen need to increase the nitrogen purification equipment. The 95%-99.9% nitrogen output from the variable pressure adsorption nitrogen generator enters into the nitrogen purification equipment, and at the same time, an appropriate amount of hydrogen is added through a flowmeter, and the catalytic reaction is carried out in the deoxidizing tower of the purification equipment between the hydrogen and the trace oxygen in the nitrogen in order to remove the oxygen, and then it will be cooled by the water condenser, and then the water is removed from the water by the vapor-water separator, and then it will be y dried through the desiccator (two adsorption desiccating towers are used alternately: one is used to dry the adsorption and remove the water and the other to heat the desorption and drainage), and the high quality is obtained. The high purity nitrogen is obtained by heating and desorption (the other one is heated and drained), and the purity of nitrogen at this time can reach 99.9995%, see Fig. 3. At present, the largest production capacity of nitrogen production by pressure change adsorption in the sea is 3,000m3n/h.
Three, the process and equipment of nitrogen production by deep-cooling
1. Typical process of nitrogen production by deep-cooling:
The whole process consists of compression of air and its purification, air separation, and liquid nitrogen vaporization.
1) Air compression and purification
Air enters the air compressor through air filter to remove dust and mechanical impurities, compresses it to the required pressure, and then sends it to the air cooler to reduce the air temperature. Then it enters the air drying purifier to remove water, carbon dioxide, acetylene and other hydrocarbons in the air.
(2) Air Separation:
The purified air enters the main heat exchanger in the air separation tower, and is cooled to the saturation temperature by the return gas (product nitrogen, exhaust gas), and is sent to the bottom of the distillation tower, and nitrogen is obtained at the top of the tower, and the liquid air is sent to the condensing evaporator by throttling to evaporate, and at the same time, condenses some of the nitrogen sent from the distillation tower, and part of the condensed liquid nitrogen serves as a reflux liquid of the distillation tower, and the other part serves as a liquid nitrogen product out of the air separation. The condensed liquid nitrogen is partly used as reflux liquid in the distillation tower and partly as liquid nitrogen product out of the air separation tower.
The exhaust gas from the condensing evaporator is reheated by the main heat exchanger to about 130K into the expander to expand the refrigeration to provide cooling for the air separation tower, and part of the expanded gas is used as the regeneration of the molecular sieve and blowing cold, and then it is discharged into the atmosphere through the silencer.
(3) Liquid nitrogen vaporization
Liquid nitrogen from the air separation tower is stored in the liquid nitrogen storage tank, and when the air separation equipment is overhauled, the liquid nitrogen in the tank enters the vaporizer to be heated up, and then sent to the product nitrogen pipeline.
Deep cooling nitrogen can produce nitrogen with purity≧99.999%.
Four, deep cooling nitrogen and pressure adsorption nitrogen technology and economic comparison
1, process comparison
From the above discussion, we can find that: pressure adsorption nitrogen process is simple and simple, the number of equipment is small, the main equipment is only air compressor, air dryer, adsorption of nitrogen and storage tanks, etc. While the deep cooling nitrogen process is complex, the number of equipment is small, and the main equipment is only air compressor, air dryer, adsorption nitrogen and storage tanks. While the deep cooling nitrogen production process is complex, the number of equipment is large, the main equipment has an air compressor, air cooler, air purification dryer, heat exchanger, expander and fine flow tower.
Nitrogen generator
2, product type and purity comparison
Deep cooling nitrogen can not only produce nitrogen but also produce liquid nitrogen to satisfy the process requirements of liquid nitrogen, and can be stored in liquid nitrogen storage tank, when there is a nitrogen intermittent load or small repair of the air separation equipment, liquid nitrogen in the tank into the vaporizer is heated, and then sent to the product pipeline to meet the demand for nitrogen for the process device. Nitrogen demand of the process equipment is satisfied. The operation cycle of deep-cooling nitrogen generation (referring to the interval between two large heating) is generally more than one year, therefore, deep-cooling nitrogen generation is generally not considered as a backup. On the other hand, the pressure-variable adsorption nitrogen production can only produce nitrogen without backup means, and a single set of equipment can not guarantee the continuous operation of a long period.
Deep-cooling nitrogen generation can produce nitrogen with purity≧99.999%. The purity of nitrogen is limited by (de dan qi _dan qi chun du shou dao) nitrogen load, number of tower plates, tower plate efficiency and liquid oxygen purity in the air, etc., and the adjustment range is very small. Therefore, for a set of deep-cooling nitrogen equipment, the product purity is basically certain, and it is inconvenient to adjust. The purity of nitrogen produced by variable pressure adsorption nitrogen generation is generally in the range of 95%-99.9%, if you need higher purity of nitrogen, you need to increase the nitrogen purification equipment. Nitrogen purity is only affected by the nitrogen load of the product, under other conditions, the larger the amount of nitrogen discharged, the lower the purity of nitrogen; on the contrary, the higher the purity. Therefore, for a set of variable pressure adsorption nitrogen equipment, as long as the load promise its product purity can be adjusted between 90-99.9%.
3, operation control comparison
Deep-cooling method is carried out in very low temperature, the equipment must have a pre-cooling start-up process before putting into normal operation, and the start-up time from the start of the expander to the nitrogen purity to meet the requirements is generally not less than 12h; the equipment must have a period of thawing and warming before entering into the overhaul, which is generally 24h. Therefore, the deep-cooling method nitrogen equipment should not be often started, stopped, or stopped. Therefore, the nitrogen equipment of deep cooling method should not be often started and stopped, and should be operated continuously for a long time. When the variable pressure adsorption method is started, just press the button, and qualified nitrogen products can be obtained within 30 minutes of startup. If high-purity nitrogen is needed, then after the nitrogen purification device, 99.99%-99.9999% high-purity nitrogen can be obtained in about another 30 minutes. When stopping the machine, it is only necessary to press a button. Therefore, variable pressure adsorption nitrogen production is especially suitable for intermittent operation.
Nowadays, deep-cooling nitrogen production generally adopts advanced DCS (or PLC) computer control technology, which realizes the integration of central control, machine-side and on-site control, and can effectively monitor the production process of the whole set of equipment. Variable pressure adsorption nitrogen production adopts intelligent full-automatic control, and the button can be used for nitrogen production, without the need for special management.
V. Conclusion
For petrochemical plants, the purity of nitrogen is 99.9% in most of the plants, and from the above introduction and comparison of deep cooling nitrogen production and pressure adsorption nitrogen production, we can draw the following conclusions:
a) When the continuous load of nitrogen is more than 600 m3n/h, and the dosage of intermittent load is not too large, and it can be satisfied with the requirements through the vaporization of liquid nitrogen, deep cooling nitrogen production should be used. deep-cooling nitrogen production.
b)When the continuous load of nitrogen is more than 600 m3n/h, the intermittent load is large, and the liquid nitrogen vaporization can not satisfy the dosage, the deep-cooling nitrogen production can be used as the main method, and the variable pressure adsorption intermittent gas supply.
c)When the continuous load of nitrogen is less than 600 m3n/h, variable pressure adsorption can be used.
d)Variable pressure adsorption nitrogen production is especially suitable for nitrogen load less than 3000 m3n/h, nitrogen purity of 95%, and intermittent operation conditions.
e)When liquid nitrogen is required for the process device, unless there is a possibility of supplying liquid nitrogen from outside, deep-cooling nitrogen production should be used.