Page 285 - Petroleum and Gas Field Processing

and lower heat of reaction (easier to generate) than MEA. The basic
reactions with CO 2 and H 2 S are the same as MEA:

2R NH þ H S ,ðR NH Þ S ð7Þ
2 2
2
2
2
ðR 2 NH 2 Þ S þ H 2 S , 2R 2 NH 2 SH ð8Þ
2
2R 2 NH þ CO 2 , R 2 NCOONH 2 R 2 ð9Þ
Based on these reactions, 1.7 lbs of DEA can be circulated to react with the
same amount of acid gas as 1.0 lb of MEA. A higher strength of 35% (wt)
can be used because of its lower corrosivity. Loading up to 0.65 mol of acid
gas per mole of DEA can cause fewer operational problems than MEA
because the elimination of the degradation products and the absence of a
reclaimer. Corrosion is less because DEA is weaker than MEA. Foaming is
reduced probably due to absence of degradation and corrosion products.

Di-isopropanolamine Process
Di-isopropanolamine (DIPA) is a secondary amine and is used most
frequently in the ADIP process licensed by Shell. DIPA reacts with COS
and CS 2 and the products are easily regenerated. At low pressure, DIPA is
more selective to H 2 S, and at higher pressures, DIPA removes both CO 2
and H 2 S. It is noncorrosive and requires less heat for rich amine
regeneration.

Methyldiethanolamine
Methyldiethanolamine (MDEA) is commonly used in the 20–50% (wt)
range. Lower weight percent solutions are typically used in very low-pressure
and selectivity applications. Acid gas loading as high as 0.7–0.8 mol acid gas/
mol amine are practical in carbon steel equipment. Higher loading may be
possible with few problems. Corrosion is much reduced in this case even
under these high loadings. In the presence of oxygen, MDEA forms
corrosive acids which, if not removed from the system, can result in
buildup of iron sulfide. Other advantages include lower vapor pressure,
lower heat of reaction, higher resistance to degradation, and high selectivity
for H 2 S.
The overwhelming advantage of MDEA is its selectivity for H 2 Sin
the presence of CO 2 . At high CO 2 /H 2 S ratios, a major portion of CO 2 can
be slipped through the absorber and into the sales gas while removing
most of H 2 S. The enhanced selectivity of MDEA for H 2 S results from its
inability to form carbamate with CO 2 . Selectivity absorption of H 2 S can
be enhanced by controlling the residence time per tray to 1.5–3.0 s and

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.