This is excerpted from the MTI Material Selector MS-1. This is the most authoritative guide to material selection in acid that I have ever seen.
In short, 316 will not work.
The caution in the last sentence has to do with impurities in the acid.
Another issue is concentration ranges. If your acid never drops below 93.5-94% then all metal will work. If you ever see 92.5% you may be in a world of trouble.
Drying Towers
The gas feed to the converter is dried with 98% acid, producing 93% sulfuric in the
process. Traditionally, the drying tower was brick-lined steel with a fluorocarbon or
other membrane behind the brick. The packing was ceramic saddles supported on
ceramic piers and beams or ceramic domes. The distributors were generally
Lewmet® 66 or similar alloy cast-iron troughs. Mist eliminators were, and still are,
made from stainless steel or more resistant alloy, PTFE, glass, or ceramic fiber
depending on the design and operating parameters of the tower.
This type of tower design is still common, but all-metal towers are also now being
constructed. These are typically made from one of the 5% silicon austenitic stainless
steels, which are also being used to make acid distributors, either trough or pipe
design. The packing is usually still ceramic saddles, although the packing supports
may be metallic.
All metal towers are sensitive to excursions from the normal operating acid concentrations
and temperatures. Localized hot spots, non-wetted areas, and weak-acid
formation have all resulted in tower failures. The gas inlet area is subject to weak-acid
formation due to the lack of acid flow and the wet gas entering the tower. Special
attention must also be paid to the area under the packing support to ensure that acid
flows evenly across the surface of the metal to avoid dry areas.
The high-silicon stainless steels (SX®, SARAMET®, and ZeCor®) are sensitive to
weak-acid formation and the resulting higher corrosion rates. Agrade of SARAMET®
has now been produced that is more tolerant of somewhat lower acid strengths (see
Figure 8.16). A nickel-based alloy such as alloy C-22 (N08022) is more forgiving if
weak acid forms inside the tower, but it is also more expensive. All-metal towers are
generally not recommended for metallurgical or acid-regeneration plants.10
10. Anon, “Drying Towers,” DKL Engineering (2002),
acidmanual/materials_drytowers.htm.
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy
