1 Preface

The molecular sieve purification system is the key equipment for air separation equipment to purify air and ensure safe operation. Understanding its performance and working principle， familiar with the various factors affecting its normal operation， the actual operation and management of air separation equipment is of great value.

2 Process Introduction and Important Role

2.1.1 Process Introduction

In the air separation equipment， the molecular sieve purification system is set up after the air pre-cooling system to remove water， carbon dioxide， acetylene and other impurities in the air. It consists of molecular sieve adsorber， regenerative heating equipment， as well as valves， pipelines， instrumentation and electrical control systems. Purification of air adsorbed impurities is done by the molecular sieve adsorber. The adsorber is filled with molecular sieves and other adsorbents (activated alumina is mainly used to adsorb water， molecular sieves are mainly used to adsorb carbon dioxide， acetylene and other hydrocarbon impurities).

The molecular sieve purification system generally uses two adsorbers to switch， one adsorption saturated， and the other regenerated into operation. After saturation， the adsorbent loses its ability to continue adsorption and can only be used after regeneration. Regeneration is the reverse process of adsorption - as the adsorbent loses its adsorption capacity after it has absorbed the adsorbed component， certain measures must be taken to drive the adsorbed component away from the adsorbent surface and restore the adsorption capacity of the adsorbent， which is called regeneration. The regeneration process is divided into four steps: pressure reduction， temperature increase， cooling， and pressure increase.

2.1.2 Adsorbent properties

In modern large air separation equipment， molecular sieves and activated alumina are commonly used to form a bunk bed. The characteristics of these two adsorbents are mainly introduced.

Molecular sieve: 1) has a strong adsorption capacity， selective adsorption performance; 2) very high dryness， high temperature and high speed airflow have good drying capacity. The lower the water vapor content， i.e.， the lower the relative humidity， the more significant the adsorption capacity. However， the adsorption capacity is smaller when the relative humidity is larger; 3) good stability， the adsorption capacity can still be maintained normally below 200℃. 4) The adsorption capacity of water is very strong， followed by acetylene and carbon dioxide.

Activated alumina: 1) high compressive strength， low wear rate， no pulverization and no bursting; 2) strong resistance to sudden changes in cold and heat; 3) good water absorption， low price and low resistance. It is generally placed in the lower layer for preliminary drying of the air entering the adsorber.

2.2 Important role

Air， the raw material for air separation production， contains small amounts of water， carbon dioxide， acetylene and other hydrocarbon gases， as well as solid impurities such as dust. Although the content of these impurities in each m3 of air is small， the total amount of air brought into the air separation equipment per hour is considerable because the air volume of large air separation equipment is tens of thousands or even hundreds of thousands of m3 per hour. As the air cools， the frozen water and carbon dioxide are deposited in the cryogenic heat exchanger， turbine expander or distillation tower， blocking passages， pipelines and valves; acetylene collects in the liquid oxygen and poses a risk of explosion. Remove and purify these impurities， to ensure the long-term safe operation of air separation equipment， is the important role of the molecular sieve purification system.

3 molecular sieve purification system common problems and corresponding measures

1) into the air temperature is too high: processing air by the air pre-cooling system after cooling into the molecular sieve， the higher the air temperature is not conducive to molecular sieve adsorption， must ensure the normal and effective work of the air pre-cooling system (can be appropriate to increase the amount of nitrogen and dirty nitrogen sent to the water cooling tower， to strengthen the cooling effect) generally control the air out of the air cooling system temperature is not higher than 15 ℃.

2) molecular sieve bed impact by airflow: maintain the uniformity of the molecular sieve bed， keep the molecular sieve system pressure stability (less than 10kPa)， with the air pressure post personnel work closely to prevent violent fluctuations in the amount of air.

3) warming and blowing cold air volume is insufficient: this problem is mostly caused by the failure of the valve switching of the molecular sieve system， the operation of the valve to closely monitor， timely adjustment to increase the amount of air， in serious cases， the molecular sieve system can be suspended in the running process， to extend the regeneration time.

4) electric heater failure: is a problem of equipment， in cooperation with maintenance personnel to check and maintain. Usually must pay attention to ensure that there is a good backup electric heater.

5) molecular sieve with water: this is a very serious problem， once with water will directly lead to the shutdown of the entire equipment， seriously affecting production. In the air-cooled tower is the exchange of air and water， keep the pressure in the air-cooled tower stable， to prevent its violent changes caused by water accidents (in the stop and start easy to happen); water-cooled tower level should also be strictly prevented from being too high， can be set up to prevent the overflow pipe to prevent the water level is too high， through the dirty nitrogen regeneration pipeline caused by water.

6) poor regeneration effect: in actual production， the highest peak value of the fraction sieve blowing cold as a criterion. To ensure that each regeneration cycle is not less than 90 ℃， the peak does not need to be too high， which can only cause the waste of regeneration gas.

7) Poor adsorption effect: From the characteristics of the molecular sieve， it is known that the adsorption effect of carbon dioxide is the lowest， as long as it is qualified， it means that the adsorption effect of other impurities is also guaranteed. The adsorption effect can be monitored by the parameter "CO2 content of air in the cold box" (generally less than 1.5PPm)， and when abnormalities are found， the stability of the data should be confirmed first， and the content should be too high.

4 Conclusion

In summary， the molecular sieve purification system is the basis for the safe production of air separation equipment， and must ensure normal and reliable operation.