After using soft water in the nitric acid absorption tower, the chloride content in the acid on the tower trays is high
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A few days ago, due to a shortage of desalinated water, the nitric acid plant was forced to switch from using desalinated water to soft water for feeding into the top of the absorption tower. It was found that the chloride content in the acid at layers 6–9 of the absorption tower was very high, exceeding 500 mg/l； When deionized water is used, the chloride content in the tray acid is only around 20 mg/l; I don’t know why
First, check the difference between the quality of the desalinated water and that of the soft water you are using; however, the chloride content in soft water should not be very high either. I guess the reason doesn’t necessarily lie in changing the water; perhaps it’s just that the timing of water change coincided with the time when the chlorine levels in those trays reached high values. You work in production, so you should know that in nitric acid absorption towers, the chloride level on the trays needs to be checked regularly; if it’s high, chlorine must be removed promptly.
When the chloride level is high on the trays of the nitric acid absorption tower, how is chlorine removed? If chloride ends up in the finished acid, it will have an impact on the subsequent production processes.
Reply to 3# Lcy0214: The CL roots are all routed to the finished acid, so as to prevent corrosion of the absorption tower; this is how it’s designed, and it won’t have any impact on the subsequent production systems.
It is recommended that the original poster analyze the chloride content in the soft water; if it is high, it is likely due to the use of soft water. If it is not high, other causes need to be investigated, such as whether there are leaks in the heat exchanger.
Due to the interruption in the supply of desalinated water, the water used for feeding at the top of the absorption tower was changed from desalinated water to soft water; it was found that the chloride concentration in the acid on the 6th to 9th trays of the absorption tower exceeded 500 mg/l； When desalinated water is used, the chloride content in the acid on the tower trays is only around 20 mg/l. This indicates that during the process of absorbing NO2 with water to form nitric acid, the increase in chloride ions in nitric acid occurs gradually. Chloride ions tend to accumulate more easily when the concentration of nitric acid is around 15%. In such cases, it is best to directly discharge the dilute nitric acid from that section of the tray system; if this is not possible, desalinated water can be used to gradually replace it. It should be noted that a high concentration of chloride ions in hydrochloric acid can easily cause corrosion of stainless steel.
I analyzed the chloride ions in soft water samples, and the level was 20–35 mg/l, while in desalinated water the chloride ion level was 2 mg/l； Therefore, the high chloride content in the acid of the 6th to 9th trays of the absorption tower is related to the use of soft water ； Chloride ions tend to accumulate in solutions with a nitric acid concentration of 20%, indicating that even when desalinated water is used continuously, long-term accumulation of chloride ions can result in concentrations exceeding 500 mg/l.
Reply to 7# Lcy0214: That’s right; CL ions tend to accumulate at nitric acid concentrations of around 20%–30% (I’ve read books on nitric acid before; it seems to be 17%, but I can’t remember exactly. This is because the concentration of nitric acid affects the equilibrium of several reactions, and at this concentration it facilitates the formation of CL ions). Therefore, chlorine removal pipelines are installed at those corresponding tray levels