Page 272 - Gas Purification 5E

258 Gas Pur$cation

in additional amine loss, coking of heat transfer surfaces, and the formation of volatile
decomposition products that can contaminate the amine vapm product. High salt concentra-
tion can also cause the precipitation of solid crystals that contribute to reclaimer fouling and
plugging problems.
These considerations generally limit the degree of concentration attainable in the
reclaimer bottoms product. Simmons (1991) recommends that the discharged residue contain
sufficient amine to be a viscous liquid on cooling-about as viscous as crude oil. Amine
remaining in the discharged mated represents an unavoidable loss.
Techniques that have been developed to minimize problems associated with high residue
concentration and high reboiler temperatures include 1) vacuum operation to reduce the
amine boiling point, 2) use of stripping vapor (such as steam or dilution water) in the
reclaimer kettle to reduce the amine partial pressure, and 3) addition of alkali to neutralize
strong acids and liberate the bound amine andor form inorganic salts that are molten at
reclaimer temperatures.
MEA and DGA thermal reclaimers normally operate at atmospheric or at the treating plant
still pressure and can easily be installed as permanent adjuncts to the treating unit. Thermal
reclaimers for higher molecular weight amines normally require vacuum, resulting in a more
complex and more costly installation. As a result, such units are frequently provided by a
contract organization, either as a mobile unit brought in as required, or as a central process-
ing plant- Contract plants, which process the amine from many treating units, can use a more
sophisticated design, including the capability for continuous rather than batch operation.

Thermal Reclaiming of MEA

In plants using aqueous monoethanolamine solutions, purification is effected by semi-con-
tinuous distillation as shown in Figure 3-28. Blake and ROW (1962). Blake (1963), Dow
(1962), Hall and Polderman (1960), and Jefferson Chemicals (1963) provide guidelines for
the operation and design of MEA reclaimers. In MEA reclaiming, sodium carbonate or
hydroxide is added, if necessary, to liberate the amine from the heat-stable acid salts and to
minimize corrosion. If carbon steel tubes are utilized in the reclaimer, Jefferson Chemicals
(1963) recommends adding sodium carbonate to the kettle before commissioning the
reclaimer to minimize corrosion. If the reclaimer has stainless tubes, lowchloride content
sodium carbonate should be used to minimize the possibility of chloride stress corrosion
cracking. A soda ash mix tank with a hard-piped connection to the reclaimer should be pro-
vided so that neutralizing solution can be added during the reclaiming cycle.
The amount of sodium carbonate or caustic required is based on the amine solution heat-
stable salts concentration. One mole of amine tied up as a heat-stable salt (usually reported
as the weight percent of bound amine based on the total solution) can be neutralized with one
mole of caustic or one-half mole of sodium carbonate. Neutralizing the heat-stable salts with
a strong base allows the bound amine to be liberated from the solution and raises the pH and
reduces the corrosivity of the solution in the reclaimer. The reaction of sodium carbonate
with heat-stable salts can be represented by equations 3-34 and 3-35 where RaH+ repre-
sents the heat-stable salt cation:

CO3*- + R&H+ = R3N + HC03- (3-34)

HCO3- + RaH+ = R3N + COZ + H20 (3-35)