Page 281 - Gas Purification 5E

Mechanical Design and Operation of Alkanolarnine Plants 265

A C A C

Repeating Unit
A = anion exchange membrane
C = cation exchange membrane
Many repeating units are stacked between two electrodes.
Figure 3-34. Schematic of electrodialysis cell for amine reclaiming.

stacked membranes. When a current is applied across the two electrodes, the cations move
toward the negatively charged cathode, and the anions move toward the positively charged
anode. Anions, such as C1-, can penetrate the anion exchange membrane, A, but not the
cation exchange membrane, C. Cations, such as Na+, can penetrate the cation exchange
membrane, C, but not the anion exchange membrane. The net effect is to remove both
cations and anions from the impure feed stream and collect them in a concentrated brine
waste stream. Hydrogen and oxygen are produced at the anode and cathode, respectively.
Stacking the membranes increases the ratio of the ions separated to the oxygen and hydrogen
produced and lowers energy consumption.
Union Carbide has commercialized the use of electrodialysis to reclaim amines (Union
Carbide, 1994; Gregory and Cohen, 1988; Bums and Gregory; 1995). A mobile, commercial
unit is depicted in Figures 3-35a and 3-35b. Commercial units reportedly process a 5- to 15-
gprn slipstream of filtered (40 micron), lean, cool amine which has been neutralized with
caustic to liberate the amine tied up as heat-stable salts. It is claimed that a mobile unit can
remove over 3000 lb/day of heat-stable salts.
Bacon et al. (1988) claim that electrodialysis quires less capital than ion exchange and that
operating costs are competitive with ion exchange. Also, Bacon et al. state that electrodialysis
produces a small volume of concentrated brine for disposal while ion exchange produces a
high-volume, dilute salt solution as a purge stream. The disadvantage of electrodialysis (and
ion exchange) in comparison to vacuum distillation is that non-ionic contaminants, such as
amine-C02 degradation products, are not removed. In comparing ion exchange and electrodial-
ysis, Keller et al. (1992) claim that electrodialysis requires higher electric power consumption
to remove some anions, and that the salt purge stream may contain significant amounts of
amine, which can adversely impact the waste water treatment system. They also claim that
electrodialysis is most efficient when the heat-stable salt content of the amine is high and,
therefore, when the amine solution is more corrosive. Table 3-6 summarizes the advantages
ahd disadvantages of electrodialysis in comparison to vacuum distillation and ion exchange.
NONACIDIC-GAS ENTRAINMENT IN SOLUTION

In certain operations, especially when acid gas removal is carried out at high pressure,
appreciable amounts of nonacidic gases are carried by the solution from the contactor to the