Comparison between cryogenic nitrogen production and PSA nitrogen production machine

First, the preceding words

With the rapid development of industry, nitrogen has been widely used in chemical industry, electronics, metallurgy, food, machinery and other fields, and the demand for nitrogen in China is increased at a rate of more than 8% per year. The chemical properties of nitrogen are not vivid, and it is very inert under ordinary conditions, and it is not easy to react chemically with other substances. Therefore, nitrogen is widely used as maintenance gas and sealing gas in the metallurgical industry, electronics industry and chemical industry, and the purity of maintenance gas is usually 99.99%, and some demand more than 99.998% of high-purity nitrogen. Liquid nitrogen is a convenient cold source and has been widely used in food industry, medical operations and semen storage in animal husbandry. In the production of synthetic ammonia in the fertilizer industry, if the mixture of hydrogen and nitrogen of synthetic ammonia is purified by pure liquid nitrogen washing, the content of inert gas can be very small, and the content of sulfur monoxide and oxygen does not exceed 20ppm.

Pure nitrogen can not be directly obtained from nature, the first choice of air separation method. Air separation method includes: cryogenic method, pressure swing adsorption (PSA) method, membrane separation method.

PSA nitrogen machine technical process and equipment introduction:

1. Introduction to skill process

The air is removed by the air filter dust and mechanical impurities into the air compressor, compressed to the required pressure, after strict oil removal, water removal, dust removal purification treatment, the output of clean compressed air, the intention is to ensure the service life of the molecular sieve in the adsorption tower. There are two adsorption towers equipped with carbon molecular sieve, one tower is operated, the other tower is decompressed desorption. Clean air enters the adsorption tower, oxygen, carbon dioxide and water are adsorbed by the molecular sieve, and the gas flowing to the outlet is nitrogen and trace amounts of argon and oxygen. The other tower (desorption tower) removes adsorbed oxygen, carbon dioxide and water from the molecular sieve micropores into the atmosphere. In this way, the two towers take turns to complete the separation of nitrogen and oxygen, and successively output nitrogen, as shown in Figure -2. The purity of nitrogen produced by adsorption is 95%-99.9%. If nitrogen with higher purity is required, a nitrogen purification device must be added. The 95%-99.9% nitrogen output of the pressure shift adsorption nitrogen machine enters the nitrogen purification equipment, and the appropriate hydrogen is added by a flowmeter. The trace oxygen of hydrogen and nitrogen in the deaeration tower of the purification equipment is catalyzed to remove the oxygen and then cooled by the water condenser, the water separator dehydrates the water, and then passes through the deep boring machine (two adsorption boring towers are replaced by: One adsorbed dry water removal, the other heated desorption drainage), to obtain high purity nitrogen, nitrogen purity at this moment up to 99.9995%, see Figure -3. At present, the maximum production capacity of domestic pressure swing adsorption nitrogen production is 3000m3n/h.

Second, cryogenic nitrogen production skills process and equipment introduction

1, the typical skill process of cryogenic nitrogen production:

The whole process consists of air tightening and purification, air separation and liquid nitrogen vaporization.

(1) Air tightening and purification

Air through the air filter to remove dust and mechanical impurities into the air compressor, compressed to the required pressure, and then into the air cooler to reduce the air temperature. Then enter the air dry 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, is cooled to saturation temperature by the reflux gas (commercial nitrogen, waste 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 to the condensation evaporator after saving, and some nitrogen sent by the rectification tower is condensed together, and some of the condensed liquid nitrogen is used as the reflux liquid of the rectification tower. Another is used as a liquid nitrogen commodity out of the air separation tower.

The exhaust gas from the condensing evaporator is reheated by the main heat exchanger to about 130K. The expansion refrigeration of the expansion machine supplies cold capacity to the air separation tower. Some of the expanded gas is used as regeneration and blowing cold of the molecular sieve, and then discharged into the atmosphere by the silencer.

(3) Liquid nitrogen vaporization

The liquid nitrogen from the air separation tower is stored in the liquid nitrogen storage tank. When the air separation equipment is repaired, the liquid nitrogen in the storage tank is heated into the carburetor and sent into the commercial nitrogen pipeline.

Cryogenic nitrogen can produce nitrogen with purity of ≧99.999%.

3. Skill and economic comparison between cryogenic nitrogen production and pressure swing adsorption nitrogen production

1. Process comparison

From the above discussion, we can find that the pressure swing adsorption nitrogen production process is simple, the number of equipment is small, and the main equipment is only air compressor, air boring machine, adsorption nitrogen machine and gas storage tank. The cryogenic nitrogen production process is messy, the number of equipment is large, and the main equipment is air compressor, air cooler, air purification boring machine, heat exchanger, expansion machine and fine flow tower.

Nitrogen making machine

2. Comparison of product variety and purity

Cryogenic nitrogen production can not only produce nitrogen and can produce liquid nitrogen, meet the technical needs of liquid nitrogen demand, and can be stored in the liquid nitrogen storage tank, when the nitrogen continuous load or air separation equipment minor repair, the liquid nitrogen in the storage tank into the carburetor is heated, into the commercial nitrogen pipeline to meet the technical equipment for nitrogen demand. The working cycle of cryogenic nitrogen production (refers to the distance period between two large heating) is usually more than 1 year, so cryogenic nitrogen production is usually not considered for backup. The pressure swing adsorption nitrogen production can only produce nitrogen, there is no backup method, a single set of equipment can not guarantee continuous long-term operation.

Cryogenic nitrogen can produce nitrogen with purity of ≧99.999%. Nitrogen purity is limited by (de dan qi _dan qi chun du shou dao) nitrogen load, plate number, plate power and liquid air oxygen purity, and the conditioning scale is very small. Therefore, the purity of a set of cryogenic nitrogen production equipment is basically necessary, and it is not convenient to adjust. The purity of nitrogen produced by pressure swing adsorption is usually in the 95%-99.9% scale, assuming that higher purity nitrogen is required to add nitrogen purification equipment. The purity of nitrogen is only affected by the load of commercial nitrogen. Under other conditions, the higher the nitrogen discharge, the lower the purity. Otherwise, the higher it is. Therefore, for a set of pressure swing adsorption nitrogen production equipment, only the load allows its commodity purity to be arbitrarily adjusted between 90-99.9%.

3, operation control comparison

The cryogenic method is carried out at very low temperatures. Before the equipment is put into normal operation, there must be a pre-cooling start process. The start time is usually not less than 12h from the expansion machine to the time when the nitrogen purity meets the demand. Before the equipment enters the overhaul, there must be a period of heating and freezing, usually 24h. Therefore, the cryogenic nitrogen equipment should not start and stop often, and it should run continuously for a long time. When the pressure swing adsorption method is started, just press the button, and qualified nitrogen products can be obtained within 30 minutes. Assuming that high-purity nitrogen is required, then after nitrogen purification equipment, 99.99%-99.9999% of high-purity nitrogen can be obtained in about 30 minutes. Just press the button to stop the machine. Therefore, pressure swing adsorption nitrogen production is particularly suitable for continuous operation.

Today, cryogenic nitrogen usually uses advanced DCS (or PLC) computer control skills to complete central control, machine side, and local integrated control, which can be useful to monitor the production process of the entire set of equipment. Pressure swing adsorption nitrogen production is controlled by intelligent automatic control, and nitrogen production can be carried out by button without special management.

V. Conclusion

For petrochemical equipment, most of the required nitrogen purity is 99.9%, from the above introduction and comparison of cryogenic nitrogen production and pressure swing adsorption nitrogen production, we can draw the following conclusions:

a) When the nitrogen continuous load is greater than 600 m3n/h, the continuous load amount is not too large, and the demand can be met by liquid nitrogen vaporization, cryogenic nitrogen production should be selected.

b) When the nitrogen continuous load is greater than 600 m3n/h, the continuous load is large, and the liquid nitrogen vaporization can not meet its dosage, the method of deep-cooling nitrogen production and pressure swing adsorption continuous gas supply can be selected.

c) When the nitrogen continuous load is less than 600 m3n/h, pressure swing adsorption can be used to produce nitrogen.

d) Pressure swing adsorption nitrogen production is especially suitable for nitrogen load less than 3000 m3n/h, nitrogen purity of 95%, and continuous operation conditions.

e) When the technical equipment needs liquid nitrogen, unless there is an external supply of liquid nitrogen, cryogenic nitrogen should be used.