Page 205 - Gas Purification 5E

Mechanical Design and Operation of Alkanolamine Plants 191

This is an irreversible reaction and, consequently, its rate is affected only by temperature
and the concentrations of the species on the left side of the equation. Therefore, the rate of
reaction 3-3 (and of corrosion) increases with increasing hydrogen ion concentration (i.e.,
decreasing pH), with increasing temperature, and, because the reaction is electrochemical,
with increasing conductivity of the liquid medium.
Figure 3-1 indicates the principal areas where corrosion can occur in alkanolamine gas
purification plants. As indicated in this figure, corrosion in amine plants can be divided into
two broad categories:

1. Wet Acid Gas Corrosion of carbon steel is the reaction of COz and HzS with iron in an
aqueous environment where little or no amine is present.
2. Amine Solution Corrosion is the corrosion of carbon steel in the presence of aqueous
amine.

Wet Acid Gas Corrosion

Aqueous acid gas solutions occur in the overhead section of the amine regenerator and in
the bottom of an amine contactor if the feed gas is water-saturated. Metal surfaces in these
sections of the amine plant may, therefore, be contacted with aqueous acid gas solutions con-
taining little or no amine. See Figure 3-1.

Mechanism of Wet CO, Corrosion

If there is a separate aqueous phase, and if the only acid gas present is carbon dioxide, the
COz will dissolve in the water and partially ionize to fom a weak acid according to mction 3-4

The increase in hydrogen ion concentration from reaction 3-4 accelerates corrosion by
reaction 3-3. As would be expected from reactions 3-3 and 3-4, the rate of corrosion increas-
es with increased COz concentration in the water (or increased COz partial pressure in the
gas phase).
As corrosion proceeds, ferrous ions produced at the corrosion site by reaction 3-3 react
with bicarbonate ions in solution to form ferrous carbonate, which is essentially insoluble
and precipitates as scale. Scale deposition reduces the rate of corrosion, but unfortunately the
ferrous carbonate formed is quite porous and provides only limited corrosion protection.
Hydrogen ions are removed from solution at the iron surface by reaction 3-3. This causes
the solution adjacent to the corroding surface to become less acid (more alkaline), thereby
reducing the rate of corrosion. However, the evolution of gases or other actions that enhance
liquid turbulence counter the build-up of alkalinity by bringing fresh acid to the metal surface.
With a weakly ionized acid, such as carbonic acid, the large concentration of dissolved,
but un-ionized carbon dioxide in the water provides a reservoir of reactive molecules, which
can produce additional hydrogen ions by reaction 3-4 to replace those used up by the corro-
sion reaction. With a strongly ionized acid, on the other hand, an extremely low acid concen-
tration is needed to produce the same pH as the carbonic acid solution, and depletion of
hydrogen ions at the corroding surface can reduce the corrosion rate. Such an effect may
explain the higher rates of corrosion observed for C02 solutions than for strong acid solu-
tions at the same bulk solution pH values (Berry, 1982).