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Alkclnohmines for Hydrogen Surfde and Carbon Dioxide Removal 133

1. Specification gas (either cO.25 or ~1.0 graid100 scfl can be met as long as the H2S con-
tent of the lean solution is less than 40 grains/gallon; however, the H2S content of the gas
does not appear to vary with solution H2S content below 40 grains/gallon.
2. Specification gas can be made with solution flow rates equivalent to a range from 1 to 2
moles amine per mole of acid gas as long as the solution is adequately regenerated.
3. DEA strips more readily than MEA.
a. At high HzS:C02 ratios regenerated DEA solutions contain 0.1 to 0.3 wt% C02 com-
pared to 1.1 to 1.3 wtQ COz in lean MEA solutions.
b. At low HzS:C02 ratios regenerated DEA solutions contain less than about 0.9 wt% C02
compared to 2.2 to 3.5 wt9 COz in lean MEA solutions.
4. Solution strengths from 20 to 30 wt% DEA can be used with no apparent change in treat-
ing ability.
5. Most of the plants use stripping steam rates of about 1.2 lb/gal.
Simplified Design Procedure

The simplified design procedure is based on the “approach to equilibrium” method
(described earlier) to determine the amine circulation rate, plus a series of enthalpy balances
to determine temperatures and heat duties. The calculation procedure involves four steps:

1. Estimating the lean amine loading. This is accomplished by the use of plant data and coda-
tions from plants operating with the same amine and similar acid gas compositions.
2. Determining the heat and material balance around the contactor for the case of equilibrium
at the contactor bottom. This is a trial and error calculation to establish the rich solution
loading and rich solution temperature corresponding to equilibrium between either one of
the acid gases (H2S or C02) in solution and the feed gas.
3. Determining the heat and material balance around the contactor for the case of a 75%
approach to equilibrium at the contactor bottom. This represents an adjustment of the step
2 results to provide a reasonable factor of safety and provides the basis for the actual plant
design.
4. Designing the regenerator system. These calculations consist of enthalpy balances around
regenerator system components based on flow rates and compositions developed in step 3
and empirical correlations of stripping column performance.
Estimating the Lean Amine Loading

The design procedure begins by establishing the lean amine temperature and the lean
amine loading (moles acid gas/mole amine). The lean amine temperature is dztermined by
the temperature of the available cooling medium and also by the requirement that the lean
amine be at least 10°F hotter than the feed gas hydrocarbon dew point. The lean amine acid
gas loading is more difficult to determine and is generally estimated on the basis of expen-
ence with similar systzms.
The stripping of H2S and COz from amine solutions is discussed in the previous section
entitled “Stripping System Performance.” For the case of H2S and C02 removal from natural
gas with MEA solutions, the Fitzgerald and Richardson correlations (Figures 2-90, 2-91,
and 2-92) can be used directly. Figure 2-92 is based on a regenerator bottom temperature of
252°F: equivalent to a pressure of about 15 psig. Values of the lean solution COz loading,
estimated from Figure 2-92, can be extrapolated to other stripper pressures (and tempera-