Page 124 - Gas Purification 5E
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1 14 Gas Purijcation
200
40
30
20
10 20 90 405060 80100
Ow Flowrate. MMscM
Figure 2-79. Approximate diameter required for baytype amine plant absoaers. (Iolan
and Manning, 1-
H2S in the treated gas leaving these trays. Therefore, in these circumstances, the H2S content
of the treated gas is independent of the absorber design and depends only on the lean amine
temperature and the amine regenemtor performance. Absorbers with 20 trays can usually
meet all common treated gas COz specifications; however, more than 20 trays may be
required if C02 in the treated gas is to be close to equilibrium with the lean amine. Therefore,
in applications such as synthesis gas treating, where it is advantageous to reduce the C@ con-
tent of the treated gas to very low levels, absorbers containing more than 20 trays or the
equivalent height of packing are often specified.
As previously noted, in typical 20-tray absorbers, the bulk of the acid gas is absorbed in the
bottom half of the column, while the top portion serves to remove the last traces of acid gas and
reduce its concentration to the required product gas specification. With sufficient trays and
amine, the ultimate purity of the product gas is limited by equilibrium with the lean solution at
the product gas temperature. When water washing is necessary to minimize amine loss (e.g.,
with low-pressure MEA absorbers), two to four additional trays are commonly installed above
the acid gas absorption section. A high efficiency mist eliminator is recommended for the very
top of the absorber to minimize carryover of amine solution or water.
Stripping columns commonly contain 12 to 20 trays below the feed point and two to six
trays above the feed to capture vaporized amine. Split stream plants obviously require more
trays. One unit, for example, employs a total of 33 trays in the stripping column with chim-
ney trays at the base and at the seventeenth tray from the bottom (Bellah et al., 1949). The
less volatile amines, such as DEA and MDEA, require fewer trays above the feed point to
