Page 280 - Gas Purification 5E

Ion Exchange for Amine Solution Purification

Ion exchange is the reversible exchange of ions between a liquid and a solid in which
there is no substantial change in the solid. Ion exchange removal of a typical heat-stable salt
anion, Cl-, from an amine by an anion exchange resin is illustrated in the following equation
where parentheses denote ions bound to the ion exchange resin:

(OH-) + R3NH+ + C1- = (Cl-) + R3NH + H20 (3-36)
The resin is then regenerated with caustic:
(Cl-) + OH-= (OH-) + C1- (3-37)

Similarly, undesirable cations (e.g., Na+) can be removed by exchange with H+ on a cation
exchange resin. One of the first commercial applications of ion exchange for amine reclaim-
ing was described by Morgan and Klare (1977). A side stream of the circulating amine solu-
tion, which was contaminated with sodium chloride, flowed downward through a bed of a
strong base anion exchange resin and the chloride ion was replaced with hydroxide ions. The
hydroxide ions reacted with C02 in the treating process, causing the precipitation of sodium
bicarbonate crystals which were collected in the amine filter. A similar application is
described by Bacon et al. (1986). In both of these applications, ion exchange was used as a
temporary measure to clean up contaminated amine solutions.
Keller et al. (1992), Yan (1993), and Cummings et al. (1991) describe further develop-
ments in the use of ion exchange for amine reclaiming. This commercial technology, called
the HSSX process, has a cation resin bed for removing sodium ions followed by two anion
resin beds, each containing a different resin. According to Keller et al., sodium is usually the
only cation requiring removal, while the most significant anions in amine reclaiming are
chloride, thiocyanate, acetate, formate, thiosulfate, and sulfate. After sodium removal in the
cation bed, the first anion resin bed removes anions which are easy to remove from solution,
but difficult to regenerate (possibly sulfate, thiosulfate, and thiocyanate). The second bed
removes anions which are difficult to remove, but easy to regenerate (possibly chloride,
acetate, and formate). Dividing anion removal into two separate resin beds duces chemical
consumption during regeneration. A similar regeneration technique was recommended by
F’rielipp and Pearce (1957) who investigated, but did not commercialize, the use of ion
exchange to reclaim amines.
Advantages of ion exchange are low energy usage, no further degradation of the amine
during reclaiming, and removal of only the solution contaminants. Disadvantages include
potentially high chemical and water use, the inability to remove non-ionized degradation
products such as those formed by the reaction of C02 with amines, and the disposal of sig-
nificant volumes of dilute sodium salts. Also, since ion selectivity varies, resin and regenera-
tion requirements vary from solution to solution, and each system must be custom designed
(Baconet al., 1988). See Table 3-6 for a further comparison of ion exchange versus vacuum
distillation or electrodialysis.
Electrodialysis for Amine Solution Purification

Electrodialysis uses a direct current and ion-selective membranes to move ions from one
solution chamber to another. Figure 334 is a schematic depiction of an electrodialysis cell.
The cell consists of a stack of alternating cation (C) and anion (A) selective membranes
located between two electrodes. Commercial electrodialysis cells can contain 100 or more