Page 137 - Gas Purification 5E

Alhnolamines for Hvdrogen Surfide and Carbon Dioxide Removal 127

Rnp.rahn+T
210 220 230 240290
I I I I I
10 20 30 40 50

0 0.1 0.2 oa OA 0.5
Solutlon Loading, Mole COJhbk MEA
Figure 2-87. Temperature and composition profiles in a packed column used for
stripping CO, from MEA solutions. Based on data from Sardar and Weiland (79853

Figure 2-90 shows the plant data collected by Fitzgedd and Richardson. The curves on
this fin- clearly indicate that for each feed gas H2S/C02 molar ratio, the lean solution HIS
content (graindgallon) approaches an asymptotic lower limit, independent of the amount of
stripping steam used (moles of reflux watedmole of acid gas). All amines exhibit this behav-
ior. See, for example, the MDEA solution data presented in Figure 286, which shows that
the H2S content of the treated gas (an indication of the lean amine H2S loading) reaches an
asymptotic lower limit independent of the amount of stripping steam.
Figure 2-91 is a crossplot of Figare 2-90. As Figure 2-91 indicates, the lean MEA solu-
tion loading (pins of HlSlper gallon solution) at any given stripping steam ratio is a strong
function of the feed gas H2S/C02 ratio. These lean amine data are for an MEA solution con-
taining 15.3 \vt% MEA and for an amine egenerator with a bottoms temperature of 252°F
(about 15 psig in the bottom of the regenerator). The data also assume that the leadrich
exchanger approach temperature (lean amine out minus rich amine in) is about 40°F. The
dashed line in Figure 2-91 indicates the stripper operating conditions required to produce X
grain treated gas (4 ppmv H2S j at 900 psig with lean amine at 1 lo'??. Less smpping steam is
required for feed gases with a low H2S/COl molar ratio, and more stripping steam is required
when the acid gas is predominantly H2S. The dashed line can be used to estimate the treated
gas H2S content at other operating pressures. For example, at 450 psig the dashed line would
denote an H2S content of H grain H2S or 8 ppmv.