Page 219 - Gas Purification 5E

Mechanical Design and Operation of Alhnolamine Plants 205

Table 3-3
Recommended Maximum Amine Solution Concentrations and Maximum
Rich and Lean Amine Solution Loadings

Mol Acid GaslMol Amine
Maximum Wt% Maximum Maximum
Amine Concentration Lean Loading Rich Loading Reference
MEA 15-20 0.10-0.15 0.30-0.35 Dupart et al., 1993B
MEA 15-20 0.08-0. 12 0.35-0.40 Dingman et al., 1966
MEA* 18 0.13 - Montrone and Long, 197 1
MEA 18 - 0.3-0.4 Ballard, 1966
MEA 10-20 - 0.25-0.45 Butwell et al., 1982
MEA e20 <o. 10* c0.4 Hall and Polderman, 1960
DEA 25-30 0.05-0.07 0.35-0.40 Dupart et al., 1993B
DEA 20-30 - 0.33-1.0 Smith and Younger, 1972
DEA 20-30 - 0.77-1 .O Wendt and Daily, 1967
DEA 2040 - 0.50-0.85 Butwell et al., 1982
MDEA 50-55 0.004-0.010 0.45-0.50 Dupart et al., 1993B
DGA 50-65 - 0.25-0.45 Butwell et al., 1982
DIPA 2040 - 0.50-0.85 Butwell et al., 1982
Notes:
*For C02 only.

(Wendt and Daily, 1967). The second control method uses the temperature between the top
regenerator tray and the regenerator overhead condenser to reset the reboiler heat medium
flow. See Figure 3-8. This controls the moles of reflux water per mole of acid gas leaving
the amine regenerator overhead because, at a fixed regenerator operating pressure, the tem-
perature above the top tray is directly related to the mol% water in the acid gas leaving the
top tray. Usually, a reflux ratio of 1.0-2.0 moles of water per mole acid gas is adequate to
strip most amines (Dupart et al., 1993B).
As Figure 3-6 illustrates, high rich amine loadings can also lead to excessive corrosion.
Figure 3-9 depicts a control strategy used to limit rich amine loadings and control corrosion
(Dingman and Moore, 1968). As shown in Figure 3-9, in a typical amine absorber most of
the heat of reaction is released in the bottom section of the tower. If there is too little amine
in relation to the amount of acid gas, the temperature bulge moves up the column. If there is
excessive amine, the temperature bulge moves to the bottom of the column and amine acid
gas loadings are low. Using the temperatm near the middle of the amine absorber to reset
the amine flow, as depicted, for example, in Figure 3-9, maintains a relatively constant rich
amine loading. This minimizes the chance of severe corrosion due to temporary overloading
of the rich amine solution and also minimizes lean amine pumping and rich amine stripping
costs. For this control scheme to work the temperature bulge must be at least 15 to 20"F, so
it is not suitable for an absorber treating a gas stream with a low acid gas content.
There is general agreement that concentrated amine solutions with low acid gas loadings are
less corrosive than less concentrated solutions with higher loadings when it is necessary to
absorb more acid gas. Butwell (1968), for example, recommends that amine concentrations be