Page 202 - Gas Purification 5E

A4echanical Design and Operation of Alkanolamine Plants 189

figuration changes, process control strategies, selective use of corrosion resistant materials,
sidestream amine solution purification, and the choice of amine.

Relationship of Process Conditions to Corrosion

Figure 3-1 depicts a typical alkanolamine treating unit. The feed gas, containing either
C02 or HzS or a mixture of both acid gases, flows into the bottom of a trayed or packed col-
umn where it contacts an amine solution. Acid gas components are removed from the gas by
chemical reaction with the amine. The purified gas is the overhead product while the rich
amine solution flows from the bottom of the contactor on level control to a rich amine flash
drum. In the flash drum, the rich amine is flashed to a lower pressure to remove dissolved
and entrained hydrocarbons. The rich amine then flows on level control from the flash drum
through the ledrich amine heat exchanger and on to the amine regenerator. In the regenera-
tor, the acid gas components are stripped from the solution using heat supplied by the regen-
erator reboiler. Acid gas components are the amine regenerator overhead product, and lean
amine solution is the bottom product. The hot lean amine from the regenerator is heat
exchanged with the rich amine, pumped to the contactor operating pressure, and cooled
before entering the contactor.
As Figure 3-1 indicates, most of the equipment and piping in an alkanolamine plant is con-
structed of carbon steel. In fact, it is possible, in most cases, to build an alkanolamine plant
entirely of carbon steel by keeping amine regenemtor operating temperatures low and by min-
imizing the amine solution concentration and acid gas loading (moles acid gadmole amine).
However, as shown in Figure 3-1, it is common practice to construct certain sections of an
amine plant with stainless steel or other alloy. Selective use of corrosion resistant alloys per-
mits operation at higher amine concentrations and acid gas loadings, allows better stripping of
the lean amine solution and improved treating, reduces corrosion in susceptible areas, and
improves process economics. Locations marked in bold on Figure 3-1 show where carbon
steel is typically replaced by stainless steel. Areas marked with a dotted line show where
either process or mechanical design modifications, or corrosion resistant materials may be
required depending primarily upon the composition of the gas being treated. To comprehend
these choices, it is necessary to understand why carbon steel corrodes in amine plants.
In the absence of inhibitors, carbon steel corrodes in aqueous solutions by an electrochem-
ical mechanism. The anodic half reaction is the oxidation of iron to ferrous ion:

Fe = Fe" + 2e- (3-1)

The cathodic half reaction is the reduction of some form of hydrogen from the +1 oxida-
tion state to the element:

H+ + e-= H" (3-2)

Most of the hydrogen atoms, H", combine to form molecular hydrogen, H2. However, in
some circumstances, atomic hydrogen migrates into the metal lattice.
The net chemical reaction is the sum of the two half reactions:
Fe + 2H+ = 2H" + Fez+ (3-3)