Page 58 - Synthetic Fuels Handbook

46 CHAPTER TWO

absorbent is stripped of the gas components (regeneration) and recycled to the absorber.
The process design will vary and, in practice, may employ multiple absorption columns
and multiple regeneration columns.
Amine (olamine) washing of natural gas involves chemical reaction of the amine with
any acid gases with the liberation of an appreciable amount of heat and it is necessary to
compensate for the absorption of heat. Amine derivatives such as monoethanolamine or
ethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), methyldiethanol-
amine (MDEA), diisopropanolamine (DIPA), and diglycolamine (DGA) have been used in
commercial applications (Kohl and Riesenfeld, 1985; Speight, 1993; Polasek and Bullin,
1994; Mokhatab et al., 2006; Speight, 2007b).
Processes that use MDEA became popular with the natural gas industry because of its
high selectivity for hydrogen sulfide over carbon dioxide. This high selectivity allows for
a reduced solvent circulation rate, as well as a richer hydrogen sulfide feed to the sulfur
recovery unit. The reaction of MDEA with hydrogen sulfide is almost instantaneous.
However, the reaction of MDEA with carbon dioxide is much slower; the reaction rate of
MDEA with carbon dioxide is slower than that of carbon dioxide with MEA.
Depending upon the application, special solutions such as mixtures of amines; amines
with physical solvents such as sulfolane and piperazine; and amines that have been partially
neutralized with an acid such as phosphoric acid may also be used (Bullin, 2003).
The proper selection of the amine can have a major impact on the performance and cost
of a sweetening unit. However, many factors must be considered when selecting an amine
for a sweetening application (Polasek and Bullin, 1994). Considerations for evaluating an
amine type in gas treating systems are numerous. It is important to consider all aspects of
the amine chemistry and type since the omission of a single issue may lead to operational
issues. While studying each issue, it is important to understand the fundamentals of each
amine solution.

2(RNH ) + H S ↔ (RNH ) S
2 2 3 2
(RNH ) S + H S ↔ 2(RNH )HS
2
3 2
3
2(RNH ) + CO ↔ RNHCOONH R
2
2
3
These reactions are reversible by changing the system temperature. Ethanolamine also
reacts with carbonyl sulfide and carbon disulfide to form heat-stable salts that cannot be
regenerated.
Diethanolamine is a weaker base than ethanolamine and therefore the diethanolamine
system does not typically suffer the same corrosion problems but does reacts with hydrogen
sulfide and carbon dioxide:
2R NH + H S ↔ (R NH ) S
2 2
2
2
2
(R NH ) S + H S ↔ 2(R NH )HS
2
2
2 2
2
2
2R NH + CO ↔ R NCOONH R
2 2 2 2 2
Diethanolamine also removes carbonyl sulfide and carbon disulfide partially as its
regenerable compound with carbonyl sulfide and carbon disulfide without much solution
losses.
The general process flow diagram for an amine sweetening plant varies little, regardless
of the aqueous amine solution used as the sweetening agent (Fig. 2.7). The sour gas contain-
ing hydrogen sulfide and/or carbon dioxide will nearly always enter the plant through an
inlet separator (scrubber) to remove any free liquids and/or entrained solids. The sour gas
then enters the bottom of the absorber column and flows upward through the absorber in