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Alkanolamines for Hydrogen Sulfide and Carbon Dioxide Removal 109

will be required in both the absorber and the stripper. The above “rules” are applicable only
if the unit is intended to provik- essentially complete removal of acid gas, the amine soh-
ticm concentmion is relatively low (12-20 wt%), and the partial pressure of the acid gas
relatively high (over about 1 psia). Pure rule of thumb designs are used only for “quickie“
hand calculation estimates of conventional systems.
2. Approach to Equilibrium Method: This technique works only for nonselective absorp-
tion. It is based on the premise that the theoretical maximum rich solution loading is rep-
resented by equilibrium with the feed gas at absorber bottom conditions. Since this theo-
retical maximum cannot be attained in practical equipment, it is customary to assume that
an approach of 75-804 will be attained. This approach can be considered in terms of
either a rich solution with an acid gas vapor pressure 75430% of the acid gas partial pres-
sure in the feed gas, or a rich solution with an acid gas loading 75-80% of the loading in
equilibrium with the feed gas. Both calculation methods are in use; however, the latter
method tends to be more conservative (i.e., results in a higher design solution flow rate).
When both H2S and CO, are present, they affext each other’s vapor pressure and this
must be taken into account. Normally, it is assumed that both acid gases are absorbed
completely from the feed gas. Carbon dioxide is a stronger acid in solution than hydro-
gen sulfide and is therefore held more tightly by the amine. As a result, the H2S/C02
ratio in the equilibrium vapor over the rich solution is usually higher than in the feed gas,
and the H2S vapor pressure is often the controlling factor in determining the 75-805
approach to equilibrium.
The approach to equilibrium method is normally used together with selected “rule of
thumb” correlations and rigorous thermal calculations in the design of nonselective treat-
ing systems. Details of this technique provided in subsequent sections of this chapter.
3. Equilibrium-Based Tray Eflciency Techniques: This is the standard technique for the
design of absorbem and smppers for nonreactive systems. A theoretical stage is consid-
ered in which the liquid and gas phases attain equilibrium, then the performance of this
theoretical stage is adjusted to represent a real tray by the use of a correction factor called
the tray efficiency. With nonreactive systems, the tray efficiency can be correlated by con-
sideration of gas and liquid properties, tray design, and flow dynamics. However, with
amine plants, where chemical reactions occur in the liquid, the correlation of tray efficien-
cy is much more complex because the dons affect both the equilibrium relationships
and the rate of absorption. Similar problems occur with the HEP concept for packed tow-
ers, which also uses an equilibrium-based theoretical plate. Nevertheless, equilibrium
based tray efficiency methods are sometimes used because they are convenient, adaptable
to graphical analysis, and amenable to either manual or computer calculations.
4. Rate-Based A4pproach: This approach is based on analyzing the mass and heat transfer
phenomena occurring on an actual tray (or section of packing) rather than on a “theoreti-
cal” tray or in a packing height equivalent to a theoretical plate. The basic procedure was
developed by Krishnamurthy and Taylor (1985A and B) for tray and packed towers,
respectively, operating with nonreactive systems. The method considers each actual tray
individually, and is based on separate mass and heat balances for each phase, which are
solved simultaneously with mass and energy rate equations on the tray. Because of the
large number of simultaneous equations to be solved, the rate-based approach is applica-
ble only to computer calculation.
The problem becomes even more complex when chemical reactions occur in the liquid
phase. The reactions affect both the vapor-liquid equilibrium and the rate of mass transfer.
However, correlations have been developed to predict vapor-liquid equilibria for amine-