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dlhmolaminesfir Hydrogen Sulfide and Carbon Dioxide Removal 61

Figure 2-12. Actfvated MDEA split flow configuration.

and cocmnt contactors can yield high removal efficiencies. With amines, however! the rich
solution usually has a significant acid gas vapor pressure, and a cocurrent contactor cannot
normally be used as the sole absorption device. In some cases, however, a cocurrent contac-
tor can be used in conjunction with a countercmnt unit to provide improved results.
Isom and Rogers (1994) evaluated several possible flow arrangements for incorporating
an SMV high efficiency cocurrent contactor into an existing gas treating system to increase
the H2S removal efficiency. They developed a ratebased modeling method to predict the
performance of combination systems that contain both a cocurrent contactor and a counter-
current unit. The existing countercurrent unit mated 10 MMscfd of gas containing 2.0% H2S
and 5.08 CG, and produced gas containing 1,OOO ppm H2S and 2.7% CO, using 66 gpm of
40% MDEA solution. The studies indicated that the optimum configuration would be that
shown in Figure 2-13 with the same 66 gpm of solution fed to the countercurrent absorber
and 74 gpm recycled through the cocurrent unit. With this arrangement the outlet gas com-
position was determinzd to be 608 ppm H2S and 2.5% C02.
A very similar configuration, in which the cocurrent contactor is in the form of a heat
exchanger, was proposed by Kohl and Bechtold (1952). This concept has found only limited
application to date. However with the trend toward more concentrated solutions, higher acid
gas loadings, and closer approach to equilibrium with the sour gas. it may be worth reconsid-
eration, particularly in areas such as the Middle East where cooling water is not available
and lean amine temperatures obtained by air cooling are relatively high. In this circumstance,
a substantial Won of the heat of reaction can be removed by heat exchans upsmam of
the amine contactor. The concept is illusnated in Figure 2-14. A related concept is the use of
a side cooler near the bottom of the absorber as previously described (see Basic Flow
Scheme section). In both of these designs, cooling reduces the rich amine temperahm, per-
mitting higher rich solution loadings.