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Mechanical Design and Operation of Alkanolarnine Plants 199

head system. In this method the amine regenerator reflux stream is directed to a dedicated
sour water stripper. The ammonia and HCN are then stripped from the reflux water in the
sour water stripper. The stripped reflux water is then returned to the amine regenerator. Since
cyanide and ammonia are not returned to the amine regenerator, there is no build-up of these
components in the amine regenerator overhead and the entire amine regenerator overhead
system can be made of carbon steel.
Amine Solution Carbon Steel Corrosion

Causes of Amine Solution Carbon Steel Corrosion

Amine solution corrosion of carbon steel can be influenced by a number of factors, including
High operating temperatures
High rich and lean amine loadings (moles acid gas/mole amine)
The ratio of C02 to H2S in the acid gas
Amine solution contaminants including amine degradation products and heat-stable salts
Amine solution concentration
Amine type

Amine solution corrosion is most significant in the hot bottom section of the regenerator.
It also occurs in the line connecting the contactor level control valve to the rich amine flash
drum, in the ledrich amine exchanger (rich solution side), and in the piping from the rich
amine flash drum level control valve to the regenerator. See Figure 3-1. Amine solution cor-
rosion in these areas can be minimized by selecting appropriate alloy materials of construc-
tion and by process modifications.
The general effects of process conditions and materials of construction on corrosion rates
are indicated in Table 3-2. Cases A and B represent laboratory tests. Case A was conducted
with pure MEA solution containing no acid gases, and all alloys performed well. For Case B,
carbon dioxide was bubbled through the solution during the test, and severe corrosion of car-
bon steel occurred. Cases C through F are based on corrosion coupons installed in operating
gas treating plants. In general, monel and the 300 series stainless steels performed well in all
plants, although some pitting occurred in the 304 and 316 stainless steel samples that were
exposed to DEA plant stripper conditions for 483 days. These conditions also proved too
severe for the carbon steel coupons, which were completely destroyed. Aluminum did not
prove suitable for any of the conditions tested.
Amine-Acid Gas Carbon Steel Corrosion Mechanisms

Pure amines and mixtures of only water and amines are not corrosive because they have
low conductivity and high pH (Dupart et al., 1993A, B). However, rich amine solutions,
which have high conductivity and a pH significantly lower than lean amine solutions, can be
quite corrosive.
The effects of acid gas loading and temperature on solution pH are illustrated in Figure 3-4.
The curves in this figure show that, for a 15% MEA solution, the pH decreases as either the
acid gas loading or the temperature increases. They also show that, at equal loading, CO2 has
a greater ability than H2S to reduce the pH; i.e., C02 is a stronger acid. It is interesting to