Corrosion of pure titanium nitric acid distillation tower
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In one of our units’ nitric acid distillation towers (made of TA2 material), recent inspections have revealed severe corrosion and thinning of the tower body. In the areas where corrosion is most severe, the rate of corrosion has been measured at 2.16 mm/year; this occurs in the middle part of the tower, in the area where dilute nitric acid enters the system (30% dilute nitric acid at room temperature). Above this area, as well as on all the bubble generators and tray fasteners below it, corrosion has occurred resulting in their failure (these components were all replaced once 5 months ago). There is virtually no corrosion in the liquid phase section (at the bottom of the tower). Operating conditions: Nitric acid concentration at the bottom of the tower is 50–60%, temperature is 90–95°C, and pressure is -65 Kpa. The lower tube sheet of the distillation tower reboiler (TA2 + carbon steel, with an explosion-proof plate as the tube sheet) has been thinned by about 1.5 mm; however, the welding of the heat exchange tubes is in good condition, so it’s not certain whether corrosion is the cause. The lower tube box is intact, while the upper tube sheet and tube box show little signs of corrosion. It should be possible to replace the distillation tower with zirconium material, but it’s necessary to analyze the reasons first, as this equipment has not been in use for long and is quite valuable: Q
Corrosion under acidic gas atmosphere is different from that without it. It is recommended to consider these aspects.
Checking shows that the tube sheet is suffering from uniform corrosion, which indicates that there is an issue with the material selection. At this concentration and temperature, titanium does not offer very good corrosion resistance to nitric acid; zirconium and tantalum can be used, but the cost will increase significantly. If temperature and pressure permit, equipment with non-metallic linings can also be considered
If you want to save money, I’ve checked the information – you can use an enamel tower. Since your tower operates under negative pressure and at low temperatures, it can handle nitric acid within this concentration range. I’m not aware of the diameter of your device; I recommend choosing products from reputable manufacturers, otherwise there will be many risks later on. I’ve experienced this before: lol
Consider it from the perspective of galvanic corrosion
This device is made entirely of titanium; will there be galvanic corrosion?
Refer to domestic distillation towers of the same type
1. Do some internal anti-corrosion treatment? For example, silicone and plastics. 2. The feed of dilute nitric acid can be replaced with a hydrothermal feed to reduce stress caused by temperature fluctuations. 3. Does the feed pipe have a distributor? Avoid direct flushing. 4. Since the temperature is not high, a steel-lined PTFE option can be considered
It is likely gas-phase corrosion. Similar problems have been encountered during the maintenance of petrochemical equipment in the past; titanium materials are resistant to high-temperature corrosion, but their performance varies when it comes to erosion corrosion. This issue is particularly evident in areas where gas and liquid mix. If you are looking for solutions, please contact us. We are Dalian Jintuo Energy Engineering Company, and we can work together to analyze the situation and find appropriate solutions