Issues with the material of the exhaust gas preheater in the dilute nitric acid plant
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In nitric acid production facilities, titanium is used for components such as the heat exchange tubes in the exhaust gas preheater; the reason given is the production of condensed acid! However, components such as low-pressure reaction water condensers (which also have acid condensation functions) and certain nitric acid pipelines are made of 304 or 304L; why is titanium chosen only for this particular application? Seeking expert solutions and discussions!
Comprehensive treatment of corrosion in nitric acid exhaust gas preheaters http://wenku.baidu.com/view/e0024510f18583d049645932.html
The corrosive strength of nitric acid is related to its concentration and temperature; in this case, both the concentration and temperature of the condensed acid are relatively high
It should be made of titanium; it’s necessary for corrosion resistance!
In this area, nitric acid concentration, condensation, and reboiling occur. Therefore, 304 and 304L cannot withstand the corrosion of high-temperature concentrated acids.
Generally, in places where high-temperature concentrated acid is generated under normal nitric acid conditions, equipment made of titanium is used; both the low-pressure quick cooler and the exhaust gas preheater are made of titanium. The low-pressure quick cooler is required to cool the process gas from 150 degrees to 40 degrees, and this process inevitably generates condensed acidic heat; ordinary materials cannot withstand the corrosion caused by such acidic heat, which is why the low-pressure quick cooler is also made of titanium. In all areas that come into contact with NOX gases and may generate condensed acid, where the wall temperature is greater than 70°C, titanium material (or a titanium-lined surface) should be used. Both the low-pressure quencher and the exhaust gas preheater involve a process in which the conversion gas is cooled from 150 degrees to below 80 degrees; if titanium materials are not used, ordinary grades such as 304 and 304L cannot withstand corrosion.
Our factory also uses titanium materials, because of the high temperatures here, which increase the corrosiveness of dilute nitric acid
This post was last edited by ddfmy on 2012-1-18 at 17:52. Corrosion in exhaust gas preheaters mainly occurs in the lower plate; condensate acid forms here and becomes highly concentrated. The resulting concentration and temperature exceed the tolerance limits of 304L or 321L materials (321L is generally used in such applications). What’s worse, many manufacturers do not pay enough attention to the risk of crevice corrosion that arises from the gaps between the tubes and the plate during the production of these devices. The plate is not properly fitted during design and manufacturing, which exacerbates corrosion. This is the reason why exhaust gas preheaters from many manufacturers end up being scrapped; the upper plate generally suffers less severe corrosion. Using 321L material, satisfactory results can be achieved by welding the lower deck plate and applying expansion joints, or by using more reliable internal welding.