Small Nitrogen Plant

PSA nitrogen production uses air as raw material and carbon molecular sieve as adsorbent. It uses the principle of pressure swing adsorption to make the carbon molecular sieve full of micropores selectively adsorb gas molecules, and obtains a new nitrogen production technology with 98.00% to 99.99% nitrogen.

Carbon molecular sieve is obtained by grinding hard coal, forming and sintering after a series of processing. The activated carbon molecular sieve has countless microporous holes distributed in its grains. Carbon molecular sieve is a speed separation adsorbent. Since the oxygen molecules with small molecular diameters diffuse 10 times faster than the nitrogen molecules with slightly larger molecular diameters in motion, when air enters the carbon molecular sieve bed, the oxygen with small molecular diameters enters the carbon molecular crystal micropores at a faster speed, and the oxygen is enriched in the solid phase of the carbon molecules, while the nitrogen is enriched in the gas phase, thereby separating oxygen and nitrogen from the air. The equilibrium adsorption amount of oxygen by carbon molecular sieve increases with the increase of adsorption pressure and decreases with the decrease of pressure, which is called pressure swing adsorption. During adsorption, the air pressure should be controlled within an appropriate pressure range to obtain a better adsorption effect. When the air pressure entering the carbon molecular sieve bed is reduced to atmospheric pressure (0.1 MPa), the oxygen molecules adsorbed in the micropores of the carbon molecular sieve are released, which is called atmospheric pressure analysis.

(1) The air compressor is generally a screw air compressor, which is the key equipment of the nitrogen production system. Providing sufficient gas volume and relatively constant input pressure (0.70-0.80 MPa) of raw gas is a necessary condition for the normal operation of the system. Therefore, the oil, water and dust content of the raw gas must meet the regulations. Otherwise, it will affect the efficiency of the carbon molecular sieve and cause the molecular sieve to be poisoned and fail.

(2) The cold dryer is a key equipment for removing water and oil in the air source purification process. It must be monitored and maintained in strict accordance with the use requirements to ensure that the water and oil content of the compressed air after drying by the cold dryer meets the standards.

(3) Filters at all levels are necessary supporting equipment to achieve water, oil and solid particle removal during the air source purification process, so that the quality of the air source entering the adsorption system meets the standards. When the equipment is operating normally, the working condition of the automatic drainer and the pressure drop at both ends of the filter inlet and outlet should be checked regularly, and the filter elements should be replaced regularly to ensure that the filters at all levels operate normally.

(4) The pressure swing adsorption separation system is the main body of the nitrogen production system. When the system is running, the factory operator observes whether the adsorption, equalization pressure, gas source pressure and nitrogen output pressure of the A and B adsorption towers are normal during the inspection.

(5) The electrical control system is the command center of the nitrogen production system. During normal production, the system is in automatic state. The operator can observe the working status of each program on the running equipment, and can query the running time of each program as needed

The company has a total of 6 pressure swing adsorption nitrogen generators, with production capacities of 600 m3/h and 300 m3/h respectively, and the nitrogen purity can reach more than 98%. After technical improvements, when the nitrogen pressure is above 0.60 MPa, the purity can reach more than 99%, which well meets the nitrogen pressure and purity required for normal production, as well as the nitrogen consumption during overhaul and shutdown peaks. Due to the strengthening of the maintenance and maintenance of the nitrogen generator and the scientific and reasonable operation of the equipment, the average molecular sieve replacement cycle is 6 to 7 years. The operating cost is 0.45 yuan/m3. Compared with the traditional deep cooling method, it has the advantages of low operating costs, low energy consumption, small equipment footprint, high operating safety factor, and high degree of automation.

Pressure swing adsorption nitrogen production technology is still a good method for chlor-alkali chemical enterprises to produce nitrogen. Although it is relatively mature, there are still some problems. For example, improper use in production operations can easily cause molecular sieve poisoning in the adsorption tower. In addition, molecular sieves are expensive and the pore size of molecular sieves cannot be effectively controlled. Therefore, the main research directions in the future are:

(1) Development of new adsorbents. In-depth research on adsorbents such as carbon molecular sieves and zeolite molecular sieves will be carried out to further improve the separation performance and strength of adsorbents;

(2) Development of integrated technology for multiple separation processes. Some domestic companies have started to develop integrated separation technologies, such as ultrafine hollow fiber membrane method, pressure swing adsorption deep cold combined process, etc.; (3) Improve the control level, develop towards intelligent control system, and finally realize fully automated operation;

(4) Carry out basic theoretical research, and make full use of computer technology to carry out basic theoretical research such as adsorption bed mathematical model and computer simulation, as well as the research on mass transfer and heat transfer laws in the adsorption and desorption process;

(5) While ensuring the purity of nitrogen, reduce the amount of adsorbent, improve equipment production capacity, increase nitrogen recovery rate and reduce energy consumption.

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