Reasons of molecular sieve inactivation in purification system of air separation unit

1, the effect of excessive water content on molecular sieve activity
The main function of the air separation unit purifier is to remove moisture and hydrocarbon content from the air to provide dry air for subsequent systems. The equipment structure is in the form of horizontal bunk bed, the lower activated alumina filling height is 590 mm, the upper 13X molecular sieve filling height is 962 mm, and the two purifiers are switched between each other. Among them, activated alumina mainly adsorbs water in the air, and molecular sieve uses its molecular selective adsorption principle to adsorb hydrocarbons. Based on the material composition and adsorption properties of molecular sieve, the adsorption order is: H2O> H2S> NH3> SO2 > CO2(the order of adsorption of alkaline gases). H2O> C3H6> C2H2> C2H4, CO2, C3H8> C2H6> CH4(order of adsorption of hydrocarbons). It can be seen that it has the strongest adsorption performance for water molecules. However, the water content of molecular sieve is too high, and free water will form water crystallization with molecular sieve. The temperature (220 °C) provided by 2.5MPa steam used for high-temperature regeneration still cannot remove this part of crystal water, and the pore size of molecular sieve is occupied by crystal water molecules, so it cannot continue to adsorb hydrocarbons. As a result, the molecular sieve is deactivated, the service life is shortened, and the water molecules enter the low-pressure plate heat exchanger of the rectification system, causing the flow channel of the heat exchanger to freeze and block, affecting the airflow channel and heat transfer effect of the heat exchanger, and in serious cases, the device cannot operate normally.
2. Effect of H2S and SO2 on molecular sieve activity
Due to the selective adsorption of molecular sieve, in addition to its high adsorption of water molecules, its affinity for H2S and SO2 is also better than its adsorption performance for CO2. H2S and SO2 occupy the active surface of the molecular sieve, and the acidic components react with the molecular sieve, which will cause the molecular sieve to be poisoned and deactivated, and the adsorption capacity of the molecular sieve will decrease. The service life of molecular sieve is shortened.
In summary, the excessive moisture content, H2S and SO2 gas content in the outlet air of the air separation air cooling tower is the main reason for the inactivation of molecular sieve and the shortening of service life. Through strict control of process indicators, the addition of purifier outlet moisture analyzer, reasonable selection of fungicide types, timely quantitative dosage of fungicide, water cooling tower to add raw water, regular sampling analysis of heat exchanger leakage and other measures, the safe and stable operation of the purifier can play a timely detection, timely warning, timely adjustment purposes, to a large extent to ensure the use of molecular sieve efficiency.