Environmental Engineering Reference
In-Depth Information
for inhibiting the breakdown of perchloroethylene from ultraviolet light is not discussed in these
patents. It is likely that these inhibitor compounds interrupt the free radical chain reaction, but do not
physically impede the action of the ultraviolet light catalyst on the carbon-carbon double bond.
1.2.6.4 Thermal Stabilizers
A few compounds are described as providing thermal stability to perchloroethylene and TCE. Such
stabilizers include diisobutylene, cyclohexene, amylene, pyrrole, and 1,2-epoxide (Stauffer, 1956;
Shepherd, 1962; Smallwood, 1993).
1.2.6.5 Acid Acceptors
Acid acceptors, also referred to as antacids in the older patent literature, react with and chemically
neutralize trace amounts of hydrochloric acid or acetic acid formed by hydrolysis or introduced dur-
ing degreasing operations. The hydrolysis reaction cannot be prevented; hence, acid acceptors—
which react with the acid to produce a benign alcohol—are added to solvents (DeGroot, 1998).
Acid-acceptor compounds are either neutral (e.g., epoxide compounds) or slightly basic (e.g., amine
compounds); they react with acid in the solvent and form an alcohol in the process (Archer, 1984;
Tarrer et al., 1989). If left unneutralized, hydrochloric acid and acetic acid can cause progressive
solvent degradation. Compounds commonly used to neutralize acids in chlorinated solvents are
summarized in
Table 1.23
.
Equation 1.14 (from Tarrer et al., 1989) demonstrates a typical acid acceptance reaction affected
by an acid-accepting epoxide stabilizer:
O
O H
C l
+ HCl
(1.14)
CH
2
CH
2
CH
2
CH
2
Ethylene oxide
1-Chloroethanol
Amines were among the more commonly used acid-acceptor compounds until about 1955, when
DuPont introduced a nonalkaline formulation based on a pyrrole and Westvaco introduced a neutral
formulation. These essentially replaced amines by 1961 (Doherty, 2000a). Used alone, amines can be
disadvantageous because they are dissipated by reaction with acids. Amines have the further disad-
vantage that amine salts (1) accelerate the corrosion of metals such as zinc in galvanized degreasers
and (2) can complex with copper and copper compounds (Stauffer, 1956; Copelin, 1959).
Certain combinations of acid-acceptor compounds can work together to regenerate the stabilizer by
returning it to its original state when the reaction terminates. An example of this is the synergistic
combination of an amine and an organic epoxide, which together are many times more effective than
either additive used alone. The regeneration of an amine acid inhibitor (such as triethylamine) or the
organic nitrogen-ring amine compound groups (pyridines and picolines) with an epoxide (such as
1,2-butylene oxide, propylene oxide, or cyclohexene oxide) may proceed as follows (Copelin, 1957):
Amine
+
HCl
→
Amine•HCl,
(1.15)
Amine•HCl
+
Epoxide
→
Chlorohydrin
+
Amine.
(1.16)
Because the amine compound is regenerated, less is needed; the mole ratio of epoxide to amine
can be 150:1. Eventually, epoxides will need to be replenished because they are consumed during
the acid acceptance reaction.
Early efforts to i nd good acid-acceptor compounds included additions of the following:
•
Caffeine, quinine, and limestone (calcium carbonate) to TCE (Dinley, 1937)
•
Resins such as gum mastic, sandarac, and rosin to protect carbon tetrachloride and perchlo-
roethylene for drum storage in contact with iron and copper (Stewart and DePree, 1933)
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