Page 209 - Gas Purification 5E

Mechanical Design and operation of Alkamlamine Plants 1s

where corrosion increases with increasing pH. As reactions 3-7 and 3-1 1 indicate, the higher
the concentrations of H2S and CN-, the greater the rate of corrosion.
If C02 is trapped in the amine regenerator overhead system by ammonia, corrosion can
result by reaction 3-6.
Reactions 3-6 and 3-7 produce atomic hydrogen, @. Under normal circumstances, the
atomic hydrogen would combine at the metal surface to form molecular hydrogen. However,
steel surface poisoning agents, such as sulfde and cyanide anions, prevent this recombha-
tion, and a significant fraction of the atomic hydrogen may migrate into the metal lattice
(Fontana and Greene, 1967). Most of the atomic hydrogen passes completely through the
steel and forms molecular hydrogen on the opposite surface. However, if the atomic hydro-
gen encounters an inclusion or a subsurface discontinuity in the base metal, it becomes
trapped and recombines to form molecular hydrogen. As more and more molecular hydrogen
is trapped at these locations, the pressure builds up past the yield stcength of the steel. High
molecular hydrogen concentrations at these locations can lead to hydrogen blisteringhydro-
gen-induced cracking (HIC). In areas of high stress, such as weld heat affected zones, HIC
may propagate in a planar manner, through the metal wall thickness. This type of cracking is
referred to as stress oriented hydrogen induced cracking (SOHIC). Also, atomic hydrogen
dissolved in the steel lattice may embrittle areas of hard microstructures in carbon steel base
metal or weldments. High atomic hydrogen concentrations in carbon steel can lead to sulfide
stress cracking (SSC) of these hard areas. A detailed review of these cracking mechanisms is
presented later in this chapter.
Amine units treating gas from FCCUs are particularly susceptible to low temperature
hydrogen attack because the gas from these units can have a high cyanide content. Accord-
ing to Neumaier and Schillmoller (1953, hydrogen atrack should be expected whenever the
organic nitrogen compounds in the FCCU feed are greater than 0.05 wt%. Wash or reflux
water with a bIue color (Prussian blue) after oxidation by air indicates that cyanide-induced
corrosion is taking place (Ebmke, 1981A, B). A simple plant test to detect cyanide using a
dilute ferric chloride solution has been described by Nemaier and Schillmoller (1955) and
by Ehmke (1981A, B).

Prevention of Wet CO, and H# Corrosion

As shown in Figure 3-1, wet COz corrosion can occur in either the bottom of the amine
absorber or the amine regenerator overhead system. Wet CO, COIIosion can also occur in the
upper section of the absorber above the feed tray when the absorber is designed for selective
H2S removal. However, if the amine solution completely wets all the exposed carbon steel
surface, it greatly raises the pH of the condensate and reduces its corrosiveness as long as
cyanide is not present. In cases where the acid gas is 95% COz or greater, an amine spray in
the regenerator overhead has been recommended to minimize carbon steel corrosion (API,
1990; Gutzeit, 1986; Ballard, 1966). Sufficient amine should be injected so that the reflux
water contains 0.5 wt% amine (Gumit, 1986). In most rehery amine systems, wet C02 cor-
rosion of the amine regenemtor overhead system is not an issue because the acid gas is pre-
dominantly H2S, and the carbon steel in the amine regenerator overhead system is protected
by an iron sulfide film. However, an amine spray should be considered for amine regenera-
tors in hydrogen plants where COz is the only acid gas present-although an amine spray in
the regenerator overhead may be counterproductive if HCN is trapped in the regenerator
overhead system. See the prior discussion on the effect of HCN on wet acid gas corrosion for
more information.