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Mechanical Design and Operation of Alkanolamine Plants 233

Blanc et al. (1982A, B) also determined that thiosulfate is formed by the reaction of HzS
and Oz in the presence of amine. The experiment was performed with DEA solution in an
autoclave at 194°F (9OOC). The initial gas phase contained air at a partial pressure of 58 psia
and HIS at a partial pressure of 29 psia. After one hour the amine solution was found to con-
tain 1.4 wt% thiosulfate ion. When the same experiment was performed with pure water
instead of amine solution, only 200 ppm thiosulfate was formed. The same investigators ana-
lyzed solutions from operating DEA and MDEA plants for evidence of oxidation. They were
able to identify acetic, propionic, formic, and oxalic acids as well as thiosulfuric acid. The
concentrations of total amine salts of these acids in solution samples from nine plants ranged
from 0.24 to 3%.
Oxygen incursion into the amine unit and degradation of the amine can be minimized by
blanketing the amine sump and storage tank with nitrogen or fuel gas. It is also good operat-
ing practice to monitor the oxygen content of certain streams which can be contaminated
with oxygen (e.g., FCC, Delayed Coker, and Vacuum Column fuel gas streams).

Irreversible Reaction with CO,

Most of the commercial amines react in the presence of carbon dioxide to form degrada-
tion products. The degradation reactions of DEA involving COz are catalyzed by (but do not
consume) COz; while in other amine-COz degradation reactions, COz is consumed (Kim and
Sartori, 1984; Kim, 1988). Degradation products can reduce amine solution absorption
capacity, increase solution viscosity, increase solution foaming tendency, and in some cases
contribute to amine plant corrosion.
MEA reacts with COz to form a substituted imidazolidone that later hydrolyzes to produce
a diamine and release the C02 See equations 3-21 through 3-24. Both the imidazolidone and
the diamine degradation products can be removed from MEA solutions by thermal reclaim-
ing (Polderman et al., 1955). COz catalyzes a series of DEA degradation reactions that form
a variety of degradation products, including high molecular weight polymers (Polderman and
Steele, 1956; Kim and Sarbri, 1984; Hsu and Kim, 1985). The degradation products of DEA
and COz are best removed by vacuum distillation.
Kim (1988) has shown that diisopropanolamine (DIPA) reacts with COz to form an oxa-
zolidone which, due to steric interference, is the sole degradation product of DIPA and COz.
Diglycolamine (DGA) reacts with COz to form a urea (Dingman, 1977). The reaction of
DGA with COz can be reversed by thermal reclaiming. Vacuum distillation is required to
remove the degradation product of DIPA and COP MDEA, which does not form a carba-
mate, is not degraded by carbon dioxide (Blanc et al., 1982A, B). Since a new carbon-nitro-
gen bond appears in all the amine-COZ degradation products, it is likely that carbamate for-
mation is an essential step in these reactions. Those amines in which carbamate formation is
inhibited by steric effects are probably resistant to COz degradation.
The presence of C02-amine degradation products does not, in general, impair the absorp-
tion characteristics of the free amine contained in the solution as long as the concentration of
free amine is kept at a constant value. However, as previously noted, accumulation of large
amounts of amine degradation products results in increased treating solution viscosity and a
consequent decrease of absorption efficiency. Also, several of the degradation products may
be corrosive, particularly the polymeric degradation products of DEA, and could contribute
to the general corrosivity of the solution.
The amine-COZ degradation reactions are relatively slow, but do occur at a significant rate
under the conditions prevailing in the regeneration section of a purification plant. According