Page 277 - Gas Purification 5E

Mechanical Design and Operation of Alhnolamine Plants 261

should be designed with deflector plates so that the steam is directed away from the tube
bundle and reclaimer shell (Jefferson Chemicals, 1963). Block valves should be provided on
the vapor return line to the regenerator and on the reclaimer liquid feed lines so that the
reclaimer can be isolated from the still for cleaning.
Entrainment must be minimized, or contaminated solution will return to the still. Disen-
gagement space is provided inside the kettle to minimize entrainment. The rule of thumb is
that the top of the reclaimer bundle should be at the kettle centerline. Another rule of thumb is
that the reclaimer liquid residence time based on the flow of amine to the reclaimer should be
50 to 100 minutes (Blake and Rothert, 1962). Also, as shown in Figure 3-30, a short packed
column on the reclaimer vapor outlet can be used to further minimize entrainment. Jefferson
Chemicals (1963) recommends a 3- to 5-foot section of column filled with any common type
of packing as shown in Figure 3-30, while Blake (1963) recommends a separate KO drum. A
positive displacement type level transmitter should be used instead of a differential pressure
level transmitter since changes in liquid gravity during reclaiming can affect the latter type
and possibly expose reclaimer tubes and cause corrosion (Shaban et al., 1988).

Thermal Reclaiming of Secondary and Tertiaty Amines

DEA, DIPA, and MDEA can be thermally reclaimed by distillation under reduced pres-
sure. Operating conditions vary, but solution temperatures should be less than 400°F to pre-
vent degradation of the amines and maximize recovery (Simmons, 1991). For example, typi-
cal operating conditions for thermal reclamation of DIPA are 50 to 100 mm Hg absolute at
350°F (Butwell et al., 1982).
When they are reclaimed, secondary and tertiary amine solutions are usually handled on a
contract basis. Contract reclaiming can be either on-site, with a portable reclaimer, or off-
site. The advantage of contract reclaiming is usually the use of a more sophisticated reclaim-
ing unit operated by highly experienced operators. Contract reclaimers can be either batch or
continuous. Continuous operation usually has the advantage of higher amine recovery since,
with proper design, operating temperatures can be low. Millard and Beasley (1993) describe
the design of a commercial amine reclaiming unit. Key design features are 1) continuous
operation, 2) using waste liquid recycled through the tubes of a direct fired heater as the heat
source for vaporizing the products (amine and water), 3) feeding fresh amine plant solution
into the heated recycle liquid at a recycle-to-feed ratio of 40 1,4) flashing amine and water
vapor from the hot mixture, 5) discharging a portion of the remaining liquid as waste since
required to maintain a constant inventory, 6) limiting the temperature rise of the liquid in the
fired heater tubes to 25°F to minimize further decomposition, and 7) using a special fired
heater design which limits heat transfer to the convective section.
A simple vacuum reclaimer used for reclaiming monoethanolamine-diethylene glycol
solution is depicted in Figure 3-32. This installation consists of a small steam-heated or
direct-fired kettle. Proper heat densities to avoid excessive skin temperatures have to be used
in direct-fired kettles. The feed to the kettle is separated from the main solution stream at the
reboiler outlet; overhead vapors are condensed, collected in an accumulator, and returned to
the plant system. This vacuum reclaimer design, which was used in the now obsolete glycol-
amine process, may not be suitable for those tertiary and secondary amines where the amine
degradation products have nearly the same vapor pressure as the amine. In these instances,
the reclaimer design is more complex. In the case of diisopropanolamine (DIPA), the princi-
pal degradation product, 3-(2-hydroxypropyl)-5-methyl-2-oxazolidone (HPMO), has nearly
the same vapor pressure as DIPA. The reclaimer, which operates under vacuum (50 mm Hg,