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This post was last edited by wcq1010 on 2016-11-23 at 22:21. Water contamination in molecular sieves is one of the common accidents in air separation plants that use molecular sieves. In most cases, such incidents are related to operational and management issues; as a result, design and manufacturing companies often do not pay enough attention to this problem. It is assumed that no matter what mistakes the operators make, at most it will lead to a shutdown of the plant, without causing irreparable damage. The main causes of water contamination are as follows: 1. When the system is shut down, the check valve system for pressurized water returning to the air-cooling tower does not close properly, and the valves in the normal-temperature water pump system leak, allowing pressurized water to flow directly into the air-cooling tower. If the water discharge valve of the air-cooling tower is not opened in time, or if it isn’t opened at all due to the possibility of restart, then the circulating water will flow back through the return pipes, resulting in water contamination of the molecular sieve. 2. If the liquid level indicator of the air-cooled tower fails to function properly, it could be due to issues with the instrument itself, or it might be because the sampling tube is blocked by impurities or severe local scaling. In many cases, freezing inside the sampling tube during winter causes false signals, which in turn leads to the closing of the drainage valve of the air-cooled tower, resulting in an abnormally high resistance in that tower. 3. The design margin of the distributor is insufficient; the actual amount of air in use is high, or there are sudden pressure changes. For example, in cases where users using the centralized cooling method release air at too fast a rate, or when the molecular sieve is not properly pressurized before a switch is made, this can result in the speed of the air cooling tower exceeding the allowable value. 4. The water quality is poor; suspended impurities such as mud clog the water distributor, causing the water level in it to rise. The rising air carries a large amount of water with it, which then passes through the wire mesh mist eliminator. 5. There are too many water treatment chemicals for sterilization and algae removal, which causes a large amount of foam to form at the distributors on the air-cooling towers, increasing the air-cooling rate. 6. Excess water level in the water cooling tower enters the dirty nitrogen pipeline, and thus reaches the molecular sieve adsorber. 7. Leakage in the steam heater causes the regenerated gas to contain water. Preventive measures for molecular sieve water carryover accidents: 1. In air separation units with reduced capacity, it is necessary to drain the liquid level in the air cooling tower when shutting down the unit. In units that meet the requirements, the drain valve is equipped as a solenoid valve to enable automatic drainage upon shutdown of the pre-cooling system. To address the issue with the performance of check valves, it is possible to require that the manual valve on the automatic drainage path be closed during shutdown procedures. 2. First, units that meet the requirements should use a 3-out-of-2 level indication system. Second, capacitive level gauges or silicone oil sensors should be used for level measurement, in order to avoid using water as the medium for transmitting pressure. When there is only one transmitter, it is advisable to send the alarm signals to the control room, so that alarms can be triggered independently of the DCS. The inspection system strictly requires recording the error between the indications of the on-site level gauge and the transmitter. In cold regions, the heating treatment of sampling pipes must be carried out strictly in accordance with the established procedures. In the case of steam, measures should be taken to prevent freezing due to interruptions in steam supply; for electric heating, precautions must be taken to avoid failures in heating due to power outages or short circuits. 3. The design of the air-cooled tower should take into account a certain margin. The purification system switching procedure must include pressure and differential pressure verification. The low-pressure interlock at the outlet of the air-cooled tower must not be disabled; units that use centralized cooling methods must pay close attention to controlling the speed when releasing V11. 4. When the circulating water is quite dirty, it is strictly prohibited to reduce the filter screen in front of the water pump out of laziness and to avoid the hassle of frequently replacing the water pump. However, the power of the water pump can be chosen to be higher during the design phase, in order to extend the cycle time for switching and cleaning. If an inspection reveals water in the blow-off valve of the air inlet pipe for the molecular sieve, the plant should be stopped promptly to check the distributor and demister of the air cooling tower. 5. For users who inevitably need to control water quality, in addition to paying attention to the dosage of chemicals, they can opt for installing a separate spiral separation water separator. Kaiyuan resolved this issue by installing a water separator inside the air-cooling tower. In addition, users with the appropriate facilities can install a hydrophobic valve at the lowest point of the air duct, which can effectively prevent the molecular sieve from carrying large amounts of water. 6. The issue of water cooling with water flow can be resolved through the use of an overflow device. When installing an overflow device, attention must be paid to pressure balance to prevent siphoning effects that could drain the liquid level in the water cooling tower. At the same time, it is essential to ensure that the balanced air ducts are led to a safe outdoor discharge location; otherwise, the release of large amounts of contaminated nitrogen into the pre-cooling area could lead to casualties. 7. Steam heater leakage is also a common accident, and dew point analysis is usually carried out. Over-reliance on instruments, coupled with prolonged neglect, can lead to serious consequences in the event that those instruments fail; this may result in the fractionation tower being blocked by frozen carbon dioxide and water. 8. During normal operation, check for water presence by inspecting the purge valve on the air inlet main pipe of the molecular sieve. Regularly checking the regeneration temperature curve of the molecular sieve allows for timely assessment of the mist removal efficiency and water carryover in the air-cooled tower. 9. The pre-cooling system using the air compressor described earlier, along with end-cooling and a closed cycle, can fundamentally solve the problem of water presence, because even if all the water from the pre-cooling system enters the molecular sieve adsorber, it will not overwhelm the bed layer. At the same time, the water quality is no longer affected by the total circulating water.