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Mechanical Design and Operation of Alkanolamine Plants 21 1

amines are not thermally reversible are called heat-stable acids, and products of their reac-
tions with amines are heat-stable salts. If heat-stable acids enter an amine unit or are generat-
ed in the amine solution by reaction with trace amounts of oxygen or by thermal degradation
of the amine, heat-stable salts can accumulate in the solution.
Heat-stable salts have several sources. In refineries, FCCU gases may contain traces of
formic, oxalic, and acetic acids. Traces of oxygen in various refinery gas streams (e.g..
FCCU, delayed coker, vacuum unit, vapor recovery system), air leaking into gas gathering
systems which are operated at subatmospheric pressure, and oxygen in unblanketed amine
storage tanks and sumps can react with the amine to form carboxylic acids and with H2S to
form elemental sulfur and thiosulfate. In refinery systems, elemental sulfur can then react
with cyanide to form thiocyanate.
Heat-stable salts reduce the acid gas removal capacity of the amine solution by tying up a
portion of the amine. The presence of heat-stable salts can also increase the corrosivity of
amine solutions (Dupart et al., 1993B). Such salts are corrosive because they lower the
amine solution pH, increase solution conductivity, and may also act as chelating agents, dis-
solving the protective film covering the base metal (Rooney et al., 1996). It is also possible
that some of the weaker heat-stable acids, such as formic acid, vaporize in the amine regen-
erator to release the free acid, which could then react with exposed carbon steel (McCul-
lough and Nielsen, 1996). Amine-C02 degradation products, some of which are strong
chelating agents, may also contribute to amine solution corrosion by removing protective
oxide or sulfide films (Polderman et al., 1955A, B; Chakma and Meisen, 1986). While it is
generally agreed that heat-stable salts and amine degradation products contribute to amine
solution corrosion, there is no definitive explanation of the corrosion mechanism. In fact, it
is likely that several factors, including lowering of the amine solution pH and chelating
effects, contribute to carbon steel corrosion by heat-stable salts and amine degradation prod-
ucts (Rooney et al., 1996). See Figure 3-5 and the discussion on amine-acid gas carbon steel
corrosion mechanisms for more information. Corrosion due to heat stable salts can be con-
trolled by amine reclaiming and/or the addition of soda ash or caustic soda to neutralize the
acids involved.

Amine RecZaiming. The operation of sidestream purification units (reclaimers) makes it
possible to maintain a constant concentration of active amine in the treating solution and pre-
vent the accumulation of corrosive heat-stable salts and amine degradation products. Com-
mercial techniques used to reclaim amine solutions include distillation under vacuum;
atmospheric or higher pressure distillation; ion exchange; and electrodialysis. Atmospheric
or higher pressure distillation can only be used for MEA and DGA, which are primary
amines. Secondary amines (DEA and DIPA) and MDEA, a tertiary amine, must be
reclaimed by vacuum distillation, ion exchange, or electrodialysis because these amines
decompose at atmospheric distillation temperatures. Design and operating guidelines for
MEA thermal reclaimers are provided in several references: Hall and Polderman (1960),
Blake and Rothert (1962), Blake (1963), Dow (1962), and Jefferson Chemicals (1963). DGA
reclaiming is reviewed by Kenney et al. (1994), ion exchange by Keller et al. (1992), and
electrodialysis by Union Carbide (1994) and Burns and Gregory (1995). Reclaiming of sec-
ondary and tertiary amines is usually on a contract basis, while primary amines are reclaimed
as a part of normal operation. Amine reclaiming should be considered when the heat stable
salt content is greater than 10% of the active amine concentration (Dupart et al., 1993B). A
detailed review of amine reclaiming techniques is presented later in this chapter.