Page 48 - Fluid Catalytic Cracking Handbook

30 Fluid Catalytic Cracking Handbook

NH 3 + H,S -> (NH 4) HS (1-7)
MW=17 " MW=34
Weight ratio: NH 3/H 2S = 0.5

Ammonia bisulfide is extremely corrosive to steel. The corrosion
product is hydrogen gas and iron sulfide. The reaction is normally self-
terminating because iron sulfide coats the metal surface with a
protective film that inhibits further corrosion. However, if cyanide is
present, the iron sulfide is removed and bisulfide corrosion is no longer
self-terminating. Hydrogen cyanide is formed in the riser from the
reaction of ammonia and CO. Ammonia cyanide is formed from the
reaction of hydrogen cyanide and ammonia. The ammonia cyanide will
dissolve in a wet environment and ionize into cyanide and ammonium
ions. The cyanide ion reacts with the insoluble iron sulfide to form a
soluble ferrocyonide complex. This destroys the iron sulfide protective
film and exposes fresh metal to further attack. As this corrosion
proceeds, it produces hydrogen atoms that penetrate into the metal
surfaces causing hydrogen blistering. This leads to stress corrosion
cracking (SCC).
The chemical reactions are:

1. Generation of hydrogen cyanide
CO + NH 3 -> HCN + H 2O (1-8)
2. Formation of ammonium cyanide

HCN + NH 3 -»NH 4CN (1-9)

3. lonization in water
(1-10)
4. Cyanide Corrosion

FeS + cyanide —> ferrocyanide + ammonium sulfide (1-11)

Ammonia can also react with hydrogen sulfide to form ammo-
nium sulfide:

2NH 3 + H 2S -> (NH 4) 2S (1-12)
MW~= 34, MW = 34 "
Weight ratio NH 3/H 2S = 1.0