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48 Gas Purijication

concentrations of primary or secondary amines (Polasek et al., 1990; Campbell and Weiland,
1989; Katti and Wolcott, 1987; Vickery et al., 1988).
The chemistry of acid gas reactions with sterically hindered amines is discussed in some
detail by Sartori and Savage (1983) and by Weinberg et al. (1983). A sterically hindered
amine is defined structurally as a primary amine in which the amino group is attached to a
tertiary carbon atom or a secondary amine in which the amino group is attached to a sec-
ondary or tertiary carbon atom. Typical sterically hindered amines are shown in Figure 2-7
(Sartori and Savage, 1983).
The key to the concept of C02 absorption by such amines is that, by control of the molec-
ular structure, amines can be synthesized which form either a stable carbamate ion, an unsta-
ble carbamate ion, or no carbamate ion. For example, by an appropriate molecular configura-
tion an unstable carbamate would be formed with C02 which is readily hydrolyzable,
resulting in the formation of bicarbonate as the end product. This would result in a theoreti-
cal ratio of one mole of C02 per mole of amine. For selective H2S absorption, a molecular
structure would be selected which suppresses carbamate formation and, consequently, the
rate of C02 absorption, without affecting the rate of H2S absorption. It is claimed that better
selectiviv can be obtained with sterically hindered amines than with the presently used ter-
tiary or secondary alkanolamines (Weinburg et al., 1983).
A hindered amine prepared from tertiary butylamine and diethylene glycol (N-tertiary
butyl diethylene glycolamine or TBGA) has been found to have advantages over MDEA
with regard to selectivity, acid gas loading, and product gas purity. However, the material is
still too expensive for general commercial use (Cai and Chang, 1992).

The choice of the process solution is determined by the pressure and temperature conditions
at which the gas to be treated is available, its composition with respect to major and minor con-
stituents, and the purity requirements of the treated gas. In addition, consideration must, of
course, be given to whether simultaneous H2S and C02 removal or selective H2S absorption is
desired. Although no ideal solution is available to give optimum operating conditions for each

0 \OH 2-pipidine ethanol (PE)
CHz,
CH2
I
H
Figure 2-7. Examples of sterically hindered amines. (Sarton' and Savage, 7983)