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(Wuhan Petrochemical Air Separation Plant, Qingshan District, Wuhan 430082) Abstract: This paper introduces various types of water inlet faults in the high-purity nitrogen equipment of the new N2/O2 station, namely the KDN-2000 model, as well as in the molecular sieve purifiers of the air separation equipment of the KDON-150/2000 model. It analyzes the mechanisms behind these faults in detail and proposes solutions. Keywords: molecular sieve purifier ； Inlet water ； Fault ； Solutions to the Failures of the Sieve Purifier in Air Separation Equipment – Analysis and Solutions Li Yan (Air separation workshop of Wuhan Petrochemical Works, Qingshan District, Wuhan City 430082, Hubei Province) Abstract: This article describes various types of failures that occur in the sieve purifiers of the KDN-2000 high-purity nitrogen production equipment and KDON-150/2000 air separation equipment at the new nitrogen and oxygen production facility. The causes of these failures are analyzed in detail, and corresponding solutions are proposed. Keywords: Sieve Purifier; internal failures; Failures; Solutions 1 Introduction The new nitrogen and oxygen production facility in the air separation workshop is equipped with one set of KDN-2000 high-purity nitrogen production equipment manufactured by Kaifeng Air Separation Equipment Factory (referred to as Unit I), and one set of KDON-150/2000 air separation equipment manufactured by Harbin Hechang Oxygen Generator Factory (referred to as Unit II). Unit I was put into operation in 1997, while Unit II began trial operation in March 2005. Both units feature plate-fin heat exchangers, molecular sieve adsorption, turbine expanders, and a single-stage distillation process. Device I uses a PLC control system ； Unit II is equipped with an oxygen tower and utilizes a DCS control system. In terms of process layout, both units utilize air cooling towers that rely on direct contact between water and air for heat exchange; after exiting the air cooling tower, the air in Unit I is cooled by a chiller unit to around 8°C ； The air-cooling tower of Unit II has a two-stage structure; the spray water in the lower tower is normal-temperature circulating water, while the spray water in the upper tower is chilled water supplied by a chiller unit. Theoretically, direct heat exchange between water vapor and air can easily cause air to carry water into the molecular sieve purifier. If the water inflow exceeds the adsorption capacity of the molecular sieve, the excess gaseous water molecules carried in the air will condense within the channels of the main heat exchanger in the distillation system, blocking the inlet channels and preventing normal distillation operations from continuing. Since the commissioning of the two sets of equipment, water inlet failures in the molecular sieve purifiers have occurred frequently, resulting in multiple temporary shutdowns. This has posed a serious threat to the safe and normal operation of the air separation unit, as well as affecting its economic efficiency. Therefore, it is necessary to summarize various types of feedwater faults in molecular sieve purifiers, analyze the fault mechanisms in detail, and propose effective solutions. 2 Fault A: Foam entrainment leading to water ingress into the molecular sieve. On February 27, 2003, a bactericide was added to the circulating water without prior notification to our workshop. At 12:06 p.m., the operator on duty noticed a significant change in the air flow entering the distillation tower; the CO2 analyzer indicated that the CO2 concentration in the air at the outlet of the purifier exceeded the warning limit of 2 ppm, and the temperature of the air at the outlet of the refrigerator rose to 24°C. Upon on-site inspection, it was found that the drain valve of the chiller discharged large amounts of foamy water vapor, and the insulation layer of the outlet pipeline of the purifier was heating up and emitting steam. Immediately shut down the emergency shutdown device. The drainage valve at the bottom of the air-cooled tower was opened, and a large amount of foamy water was discharged. Irritating white foam can also be discharged from the bottom of the purifier, and at the same time the regeneration temperature does not reach the requirements specified in the process, indicating that the molecular sieve has become poisoned and is no longer effective. 2.1 Analysis of the cause of the fault: (1) The addition of disinfectant to the circulating water led to the accumulation of a large amount of foam in the air-cooled tower, which was then carried into the purifier by high-pressure air bubbles. (2) Failure cause identification is not timely. After the failure occurred, some process parameters showed abnormal values; for example, there were significant changes in the air temperature at the outlet of the freezer and the CO2 content in the air at the outlet of the purifier. However, it was not possible to accurately determine the cause of the failure and take appropriate measures. (3) No communication was made with my workshop before adding a bactericide to the circulating water, resulting in the failure to take timely measures to suspend operations temporarily. 2.2 Emergency measures and solutions for failures: Activate the emergency shutdown device, drain all liquid from the tower, and heat the distillation system to room temperature. Given that the on-site heating conditions were insufficient to activate the molecular sieve, we replaced all of the molecular sieve and carried out a 3-day heating purging of the distillation system. The consequences of mist entrainment in air-cooled towers leading to failures in the water supply to purifiers are severe, resulting in substantial losses. To prevent such accidents from occurring again, the following solutions are proposed: (1) Add a fresh water pipeline to the water pump of the pre-cooling system. (2) Before adding a bactericide to the circulating water, it is necessary to contact our workshop to specify the start and end times for adding the chemical. (3) Prepare an emergency plan for chemical dosing in the circulating water system; switch the cooling process from circulating water to fresh water in a timely manner before adding bactericides to the circulating water. (4) All positions are required to strengthen regular inspections in order to detect potential equipment issues as early as possible and prevent accidents from occurring. (5) Improve the analysis and alarm system, and establish a regular calibration system for analysis instruments. Ensure that the analytical instruments are in good working condition, so as to issue alerts in a timely manner to alert operators when there are deviations in production. Fault B: Sudden changes in air intake flow led to water ingress into the molecular sieve. On March 10, 2005, Unit II was started up for the first time; at 11:00, the pre-cooling system was functioning properly ； At 11:20, the operator started the purification system by directly opening the air inlet and outlet valves of the purification system ； At 11:40, it was observed that a large amount of water was discharged from the drain valve of the water separator, and a large quantity of milky white liquid flowed out from the bottom of the molecular sieve purifier; it was concluded that water had entered the molecular sieve. 3.1 Fault Cause Analysis (1) During the design of Unit II, no gas guide valve for the purification system was considered; only a gas guide valve was installed at the inlet of the air-cooling tower. (2) Due to their old operating habits, the operators were unable to adapt to the new method of guiding air at the inlet of the air-cooled tower, and thus started the purification system directly. (3) During construction, the purge valve V1219 in front of the purifier was omitted, resulting in the lack of a means to adjust the gas flow rate. (4) The valves in the purification system are all pneumatic butterfly valves that can only be in either open or closed state. When the back pressure is low, opening large-diameter butterfly valves will inevitably cause a sharp change in the gas flow rate. The combined effect of these factors causes severe fluctuations in air pressure and flow rate within the air-cooled tower, and carries the water sprayed inside the tower into the molecular sieve purifier. 3.2 Emergency measures and solutions for failures: Perform an emergency unload of the air compressor; shut down the chilled water pump, as well as the low-temperature and normal-temperature water pumps, in sequence, to drain all water accumulated in the air-cooled tower. Due to the severe water ingress into the molecular sieves, it was decided to use dry air purified by the molecular sieve purifier in Unit I as the regeneration gas to regenerate the molecular sieves in Unit II separately. While ensuring the volume of regenerated gas, the regeneration temperature should be increased as much as possible to keep the temperature of the regenerated gas exiting the electric heating furnace above 200°C. Strict control is required over the outlet temperature of the regenerated gas during the heating and cooling phases of the molecular sieve; the peak value of this temperature should be above 100°C during heating, and around 130°C during cooling, in order to ensure thorough regeneration of the molecular sieve. To prevent such failures from occurring again, we have formulated the following solutions: (1) Add a DN150 butterfly valve in front of the purification system as a startup bleed valve. (2) Install the purifier pre-blowdown valve V1219 that was omitted during construction. (3) Strengthen employee training by teaching the key operational principles of the purification system. (4) Develop comprehensive operating procedures based on the existing problems. The basic principle is to fully open V1219 before starting gas flow in the molecular sieve purifier; subsequently, start the normal-temperature and low-temperature water pumps one after another. Once the liquid level in the air-cooling tower stabilizes, slowly open the gas flow valve to introduce gas into the purification system and distillation system, and then gradually close V1219. After confirming there are no faults, start the chiller unit last. Fault C: Instrumentation and valve issues in the purification system led to water entering the molecular sieve. In the early hours of March 30, 2005, the pressure equalization process in columns A and B of the molecular sieve purifier in Unit II was nearly complete, yet the pressure difference between the two adsorption columns remained above 0.2 MPa. Since a certain pressure difference has existed between the two adsorption cylinders since they were put into operation, the operators did not pay attention to it and continued to let the purification system program run. When the pressure equalization period came to an end, the two adsorption cylinders switched roles in operation; as a result, the air flow rate entering the fractionation tower experienced a huge fluctuation of up to 1500 m3/h, and the pressure inside the tower dropped sharply, with a pressure difference of around 0.1 MPa. After that, the operator spent nearly an hour readjusting the distillation conditions, and during a routine inspection it was found that water had entered the molecular sieve purifier again. 4.1 Fault Cause Analysis (1) All valves in the purification system are hard-sealed butterfly valves, which have poor airtightness; leaks were observed in valves such as V1211 and V1214 during operation. When the leakage becomes severe enough, it prevents the pressures in the two adsorption cylinders from being balanced during pressure equalization, resulting in excessive pressure fluctuations during the switching process. (2) Severe fluctuations in air pressure and flow rate can easily carry the water sprayed inside the air-cooled tower into the molecular sieve purifier. (3) The pressure difference self-protection interlock program of the purification system failed to function properly, failing to provide the necessary protection. 4.2 Emergency measures and solutions for failures: After a failure occurred, since the amount of water entering the molecular sieve was not severe, the workshop appropriately shortened the operating cycle of adsorption columns A and B based on the actual situation, and increased the temperature during heating. The operator was instructed to change the control mode of the purification system from automatic to manual just before switching between tanks A and B, to ensure that the pressures in tanks A and B were as balanced as possible before completing the switch, after which the control mode could be changed back to automatic. The malfunction on March 30 was simply a valve leak in the purification system; in fact, issues such as valve leaks, slow valve operation, failure of valves to open or close properly, or even sticking, as well as problems with the control programs, can all cause severe fluctuations in the inlet pressure and flow rate of the air separation unit. Such faults not only easily cause water to enter the molecular sieve purifier but also exert a significant impact on the molecular sieves, severely affecting safe production; therefore, such faults should be eliminated as soon as possible. To resolve this issue, we adopted the following solutions: (1) Inspected or replaced all faulty valves. (2) Improve the DCS control system, and redesign and verify the pressure differential automatic protection interlock program for the purification system. (3) Some valve failures can be resolved online without shutting down the equipment, in order to minimize losses. It is particularly important to note that when performing maintenance on the valves of the purification system without shutting down the system, the control mode of the switch valve must first be changed from automatic remote control to manual remote control. In the event that the maintenance period is extended beyond the specified time for the PLC system, the various switch valves will automatically switch over, disrupting the normal operation of the high-purity nitrogen equipment or posing a risk to the safety of the maintenance personnel.