Page 210 - Gas Purification 5E

196 Gas Purijication

Wet COz corrosion can also be a problem in hydrogen plant amine absorbers. In the bottom
of the amine absorber, corrosion due to high CQ content of the acid gas can be prevented by
introducing the gas through a distributor immersed in the amine absorber sump liquid (Dupart
et al., 1993B). The distributor orifices should be immersed in the rich amine solution so that
the emerghg gas causes amine solution to splash and wet the carbon steel surface. If the dis-
tributor breaks or is installed above the solution level, wet CQ can rapidly attack the shell of
the absorber and its internal elements. An alternative solution for protecting the carbon steel
metallurgy in the bottom of the absorber from wet C@ corrosion is to drill a series of weep
holes around the perimeter of the bottom tray support ring. Amine flowing down through the
holes will wet the absorber walls and protect the carbon steel from attack by the CQ-rich gas.
An advantage of this technique in comparison to the use of a submerged sparger is that it min-
imizes gas entrainment in the rich amine leaving the absorber. If the absorber treating a CQ-
rich gas has carbon steel trays, the underside of the bottom tray may be attacked by acid gas
until the tray collapses, whereupon the second tray may be attacked, and fail in turn. As
shown in Figure 3-1, all the trays can be saved if the bottom tray is stainless steel. If the
mechanical design of either the bottom of the absorber or the amine regenerator overhead sys-
tem is such that amine does not wet all exposed carbon steel surfaces, piping should be stain-
less steel and all equipment should be either made of or lined with stainless steel.
Another factor that should be considered for high COz content sour gases is minimizing
high velocity acid gas vapor impingement on carbon steel surfaces. Dupart et al. (1993B)
report one instance where impingement of a COZ-rich gas led to corrosion in the bottom of
an amine contactor. This problem was corrected by modifying the feed gas distributor to
minimize impingement of the acid gas on the amine contactor wall.
Corrosion by wet H2S occurs primarily by sulfide stress cracking (SSC), hydrogen-
induced cracking (HIC), and stress oriented hydrogen-induced cracking (SOHIC). Control
measures to mitigate these types of corrosion are discussed in a separate section of this chap-
ter entitled “Cracking of Carbon Steel in Amine Service.”

Preventing NH3 and HCN-Based Corrosion

Several methods have been employed to reduce ammonia- and cyanide-induced corrosion
in the amine regenerator overhead system. These methods include

A reflux water purge to reduce the concentration of ammonium cyanide and bisulfide in the
amine regenerator overhead. See Figure 3-1.
A water wash upstream of the amine treating system to remove both ammonia and HCN
from the feed gas.
Injection of either ammonium or sodium polysulfide upstream of the amine regenerator or
into the amine regenerator overhead system to convert the cyanide iron to thiocyanate.
Stripping of corrosive components from the amine regenerator reflux.
Use of corrosion resistant materials in the amine regenerator overhead system.
Combinations of the above.

In general, the most economic method of reducing NH3- and HCN-based corrosion is to
modify the process conditions to permit the use of carbon steel. Elimination of HCN and
ammonia upstream of the amine treating unit is usually the most effective solution. A brief
review of each of these corrosion prevention methods follows: