Page 274 - Gas Purification 5E

258 Gas Purification

operating temperature and amine concentration in the kettle can be varied by diluting the lean
amine reclaimer feed with water. Water dilution is used to minimize MEA degradation and
improve recoveries at high still operating pressures. Use of amine still reflux as dilution water
has the advantage of maintaining the amine system water balance.
As the distillation continues, the temperature in the kettle increases because of the accumu-
lation of solids and high-boiling constituents. As the temperature rises, the steam flow control
valve opens, and the steam pressure in the kettle is gradually increased to a maximum of
about 100 psig. The addition of lean amine feed is stopped when the temperature in the kettle
reaches about 290°F. However, distillation is continued for a short time with the addition of
water as liquid feed or as steam to remove as much of the residual MEA as economically pos-
sible. Additional caustic soda can be added to the reclaimer if tests indicate that more amine
can be recovered from the still by neutralizing any remaining heat-stable salts. During this
phase of operation, the MEA content of the kettle vapor product should be monitored by sam-
pling, and steam or water injection stopped when the MEA content drops to 1 wt% @ow,
1962). The kettle is then cleaned and recharged, and the cycle is repeated. This simple system
can be used with aqueous monoethanolamine solutions because the vapor pressure of
monoethanolamine is sufficiently high to permit distillation at temperatures at which the
amine does not decompose thermally. High still operating pressures can lead to amine degra-
dation. Dow (1962) recommends atmospheric pressure reclaiming when still pressures exceed
10 psig; however, most commercial MEA reclaimers operate at 15 to 20 psig.

Thermal Reclajming of Diglycolamhe (DGA)

Thermal reclaiming of Diglycolamine from degraded solutions, as described by Dingman
(1977) and Kenney et al. (1994), is quite similar to reclaiming of MEA by semi-continuous
steam distillation. A diagram of a DGA reclaimer is shown in Figure 3-30. The reclaimer
should be sized for a sidestream of about 1 to 2% of the circulating solution, and distillation
is conducted at a kettle temperature of 360 to 380°F. High reclaiming temperatures maxi-
mize reclaimer throughput, while lower operating temperatures minimize solution degrada-
tion. When steam is available, it is usually sparged in below the tube bundle. As seen in Fig-
ure 3-31, at a temperature of 360°F and at a pressure of 20 psia, the vapor from the reclaimer
contains about 50 wt% Diglycolamine. This is returned directly to the stripping column.
DGA reacts with C02 to form bis(hydroxyethoxyethy1)urea (BHFiEU) and with CS2 to
form bis(hydroxyethoxyethy1)thiourea. While DGA reacts preferentially with COS to form
BHEEU, some bis-hydroxyethoxyethyl thiourea is also formed (Kenney et al., 1994). In
most circumstances, BHEEU is the predominant Diglycolamine solution degradation prod-
uct. Initially, as reported by Dingman and Moore (1968), DGA was reclaimed under vacuum
with caustic addition. However, Mason and Griffith (1969) discovered that the degradation
reactions to form BHEEU could be reversed without caustic addition by operation at higher
temperature. Later it was demonstrated that the reclaimer could be operated at the amine still
pressure and that the COS, CS2, and C02 degradation reactions could be reversed at reclaim-
ing temperatures without caustic addition.
DGA reclaimer control is based on reversing these reactions by maintaining reclaimer
operation at a fixed temperature. As shown in Figure 3-30, steam, typically 300 psig, is
added on flow control, lean DGA solution is added on level control, and condensate (or
DGA stripper reflux) is added to maintain temperature control. Since the predominant degra-
dation product is BHEEU, sodium carbonate or caustic soda is added to DGA reclaimers
only when solution analysis indicates that heat-stable salts are present. Normally, caustic