Page 56 - Gas Purification 5E

46 Gas PuniJication

(rem continuedfrom page 13)
Reactions 2-1, 2-3,24, and 2-5 account for the principal species present in aqueous alka-
nolamine treating solutions. These species are the unionized molecules H20, H2S, C02, and
RNHl and the ions H+, OH-, HS-, HC03-, RNH3+, and RNHCOO-. Alternative reaction
paths can, of course, be postulated which relate to the same species.
Additional reactions may occur to produce species other than those listed, but these are
not considered important in the basic absorptioddesorption operation. Examples of such
minor reactions are the dissociation of bisulfide to produce sulfide ions, the dissociation of
bicarbonate to produce carbonate ions, and the reaction of carbon dioxide with some amines
to produce nonregenerable compounds. Additional details with regard to chemical reactions
involved in the absorption of H2S and C02 are given in a subsequent section of this chapter
entitled “Acid Gas-Amine Solution Equilibrium Correlations.”
Although reactions 2-1 through 2-5 relate specifically to primary amines, such as MEA,
they can also be applied to secondary amines, such as DEA, by suitably modifying the amine
formula. Tertiary amine solutions undergo reactions 2-1 through 2-4, but cannot react direct-
ly with C02 to form carbamates by reaction 2-5.
The equilibrium concentrations of molecular HzS and C02 in solution are proportional to
their partial pressures in the gas phase (Le., Henry’s law applies) so reactions 2-2,2-3, and 2-
5 are driven to the right by increased acid gas partial pressure. The reaction equilibria are
also sensitive to temperature, causing the vapor pressures of absorbed acid gases to increase
rapidly as the temperature is increased. As a result it is possible to strip absorbed gases from
amine solutions by the application of heat.
If the reaction of equation 2-5 is predominant, as it is with primary amines, the carbamate
ion ties up an akanolammonium ion via equation 2-4 and the capacity of the solution for C02
is limited to approximately 0.5 mole of C02 per mole of amine, even at relatively high par-
tial pressures of C02 in the gas to be treated. The reason for this limitation is the high stabili-
ty of the carbamate and its low rate of hydrolysis to bicarbonate. With tertiary amines, which
are unable to form carbamates, a ratio of one mole of COz per mole of amine can theoretical-
ly be achieved. However, the C02 reactions which do not produce carbamate involve reac-
tion 2-3, which is very slow. In recently offered processes this problem is overcome (for
MDEA) by the addition of an activator, typically another amine, which increases the rate of
hydration of dissolved C02 (see following section).
The effectiveness of any amine for absorption of both acid gases is due primarily to its
alkalinity. The magnitude of this factor is illustrated in Figure 2-5, which shows pH values
on titration curves for approximately 2N solutions of several amines when thej7 are neutral-
ized with COP The curves were obtained by bubbling pure C02 through the various solu-
tions and periodically determining the concentration of the solution and pH. The curve for an
equivalent KOH solution is included for comparison. The relatively smooth curves for the
amines, as compared to the sharp breaks in the KOH curve, may be interpreted as an indica-
tion of the presence of non-ionized species during neutralization of the former compounds.
The curves for the tertiary amines, MDEA and TEA, are seen to cross the DEA and MEA
cun7es at a mole ratio near 0.5 indicating that the tertiary amines, while initially less alkaline,
may be expected to attain higher ultimate COz/amine ratios. Figure 2-6 shows a comparison
of pH values versus temperature curves of 20% solutions of monoethanolamine and
diethanolamine (Dow, 1962). The decreasing pH with increasing temperature is a factor in
the thermal regeneration process.
In view of the difference in the rates of reaction of HIS and Cot with tertiary amines, par-
tially selective H2S absorption would be expected with these compounds. The kinetics of