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58 Gas Purijcation

selectivity for H2S over COz can be adjusted, as required, by blending a primary or sec-
ondary amine with a tertiary amine, such as MDEA, in just the right proportions.

Sterically Hindered Amines

Although sterically hindered amines are not necessarily alkanolamines, their characteris-
tics as gas purification agents are sufficiently similar to those of the alkanolamines to be
included in this chapter. A family of solvents based on hindered amines is licensed by Exxon
Research and Engineering Co. under the broad designation of Flexsorb Solvents. The
processes have been described in some detail by Goldstein (1983): Weinberg et al. (1983),
and Chludzinski and Wiechart (1986). The hindered amines are used as promoters in hot
potassium carbonate systems (Flexsorb HP); as components of organic solventfamine sys-
tem with characteristics similar to Shell's Sulfinol process (Flexsorb PS); and as the princi-
pal agent in aqueous solutions for the selective absorption of HIS in the pre.sence of C02
(Flexsorb SE and SE+). Each system makes use of a different sterically hindered amine with
a specifically designed molecular configuration. On the basis of pilot and commercial plant
experience, substantial savings in capital and operating cost are claimed for this technology.
As of 1994, it was reported that 32 Flexsorb plants were operating or in design (Exxon
Research and Engineering Co., 1994).

Amine Concentration

The choice of amine concentration may be quite arbitrary and is usually made on the basis
of operating experience. Typical concentrations of monoethanolamine range from 12 wt% to
a maximum of 32 wt%. On the basis of operating experience in five plants, Feagan et al.
(1954) recommended the use of a design concentration of 15 wt8 monoethanolamine in
water. The same solution strength was recommended by Connors (1958). Dupart et al.
(1993A, 1993B) recommend a maximum MEA concentration of 20 wt%. However, it should
be noted that higher amine concentrations, up to 32 wt8 MEA, may be used when corrosion
inhibitors are added to the solution and when C02 is the only acid gas component.
Diethanolamine solutions that are used for treatment of refinery gases typically range in
concentration from 20 to 25 wt%? while concentrations of 25 to 30 wt% are commonly used
for natural gas purification. Diglycolamine solutions typically contain 40 to 60 wt% amine in
water, and MDEA solution concentrations may range from 35 to 55 wt%.
It should be noted that increasing the amine concentration will generally reduce the
required solution circulation rate. and therefore the plant cost. However, the effect is not as
great as might be expected, the principal reason being that the acid-gas vapor pressure is
higher over more concentrated solutions at equivalent acid-gaslamine mole ratios. In addi-
tion, when an attempt is made to absorb the same quantity of acid gas in a smaller volume of
solution, the heat of reaction results in a greater increase in tempexature and a consequently
increased acid-gas vapor pressure over the solution. The effect of increasing the amine con-
centration in a specific operating plant using DGA solution for the removal of about 158
acid gas from associated gas is shown in Figure 2-9. The authors of this study concluded
that the optimum DGA strength for this case is about 50 wt%. The effect of the increasing
amount of DGA at higher concentrations is almost nullified by the decreasing net acid gas
absorption per mole of DGA (Huval and van de Venne, 1981).