Page 52 - Gas Purification 5E

42 Gas Purgcation

coworkers at Fluor Daniel (Frazier and Kohl, 1950; Kohl, 1951; Miller and Kohl, 1953) as a
selective absorbent for H2S in the presence of C02 as early as 1950, its use in industrial
processes has only become important in recent years. A somewhat different type of alka-
nolamine, 2-(2-aminoethoxy) ethanol, commercially known as Diglycolamine (DGA), was
first proposed by Blob and Riesenfeld (1955). This compound couples the stability and
reactivity of monoethanolamine with the low vapor pressure and hypscopicity of diethylene
glycol and, therefore, can be used in more concentrated solutions than monoethanolamine.
In addition to simple aqueous solutions of the previously mentioned alkanolamines, propri-
etary formulations comprising mixtures of the amines with various additives are widely used.
Formulated solvents are offered by: Dow Chemical Company (GASISPEC), UOP (andor
Union Carbide Corp.) (Amine Guard and UCARSOL), Huntsman Corporation (formerly Tex-
aco Chemical Company) (-AT), and BASF Aktiengesellschaft (Activated MDEA).
Some of Dow’s GAYSPEC and UOP’s Amine Guard formulations are basically corrosion
inhibited MEA and DEA solutions. However: the most significant development in formulated
solvents is the advent of tailored amine mixtures. These are usually based on MDEA, but con-
tain other amines as well as corrosion inhibitors, foam depressants, buffers, and promoters
blended for specific applications. They can be designed to provide selective H2S removal,
partial or complete CO? removal, high acid gas loading, COS removal, and other special fea-
tures (Manning and Thompson, 1991; Pearce and Wolcott, 1986; Thomas, 1988: Meissner
and Wagner. 1983; Meissner, 1983; Niswander et al., 1992).
A different class of acid gas absorbents, the sterically hindered amines, has recently been
disclosed by EXXON Research and Engineering Company (Anon., 1981; Goldstein, 1983;
Sartori and Savage, 1983). These absorbents, some of which are not alkanolamines, use
steric hinderance to control the C02/amine reaction. Several different solutions are offered
under the general name of Flexsorb solvents.
Typical ethanolamine gas-treating plants are shown in Figures 2-1, 2-2a, 22b, and 2-3.
Figure 2-1 is a photograph of a unit treating natural gas at high pressure to pipeline specifi-
cations using an aqueous diethanolamine solution (S.N.P.A.-DEA process). Figures 2-2a
and 2-2b depict a large gas treating complex (4 x 540 MMscfd Improved Econamine gas
treating trains) located in Saudi Arabia which uses Diglycolamine as the solvent. Figure 2-3
depicts another natural gas-treating plant using Diglycolamine.
BASIC CHEMISTRY

Structural formulas for the ahnolamines previously mentioned are presented in Figure
24. Each has at least one hydroxyl group and one amino group. In general, it can be consid-
ered that the hydroxyl group serves to reduce the vapor pressure and increase the water solu-
bility. while the amino group provides the necessary alkalinity in water solutions to cause the
absorption of acidic gases.
Amines which have two hydrogen atoms directly attached to a nitrogen atom, such as
monoethanolamine (MEA) and 2-(2-aminoethoxy) ethanol (DGA), are called primary
amines and are generally the most alkaline. Diethanolamine (DEA) and Diisopropanolamine
(DPA) have one hydrogen atom directly attached to the nitrogen atom and are called sec-
ondary amines. Triethanolamine (TEA) and Methyldiethanolamine (MDEA) represent com-
pletely substituted ammonia molecules with no hydrogen atoms attached to the nitrogen, and
are called tertiary amines.
The principal reactions occurring when solutions of a primary amine, such as
monoethanolamine, are used to absorb C02 and H2S may be represented as