Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 24

recyclers remove water by using fractional distillation, if available, or physical or mechanical
removal methods, such as i ltering the rei ned solvent through a desiccant. Anhydrous calcium chlo-
ride is very effective in the removal of trace water contamination. Rei ned solvents contaminated by
less than 0.5% moisture can be dried by calcium chloride down to 200 ppm moisture, but chloride
contamination of the rei ned solvent may occur, which can cause corrosion problems and solvent
breakdown. Ion-exchange resins are also used for the removal of trace water contamination. In sol-
vents with less than 0.5% moisture, water molecules attach to the resin, and dry solvent passes
through, reducing moisture levels to less than 100 ppm in the best possible case. Ion-exchange res-
ins used to remove water from the solvent can be regenerated. The best available technology for
moisture removal is the molecular sieve bed, which traps water molecules in the interstices of the
molecular sieve, allowing the dry solvent to pass through. Molecular sieves can dry solvents con-
taminated with 5% water to less than 1% and can dry solvents with trace moisture from 0.5% down
to less than 200 ppm. Molecular sieves can also be regenerated for continued use (Dawson, 1989).
1.2 ROLE OF SOLVENT STABILIZERS
Chlorinated solvents can become unstable when subjected to environmental stresses in the broad
variety of industrial applications for which they are used. Solvents are exposed to a wide range of
physical conditions and a great diversity of materials and substances that place unusual demands on
solvent performance. Chlorinated solvents must perform equally well in both liquid and vapor
phases and must remain inert to impurities. Solvents must remain stable under challenging physical
and chemical conditions, including high temperatures, ultraviolet light, moisture and water vapor,
exposure to alkali and acidic substances, and exposure to reactive metals.
As this section demonstrates, solvents often need to be fortii ed against the severe physical and
chemical conditions of the various environments in which they are used. “Solvent stabilizers” is the
term of art chosen for the group of chemicals added to chlorinated solvents to ensure that they
will not break down during their intended industrial applications. Unstabilized solvents will
deteriorate—some very quickly with dramatic and dangerous reactions, others slowly—over
repeated use cycles and in storage. In the most demanding applications, operators must regularly
replenish stabilizers to replace those consumed or lost during use. Stabilizer replenishment is com-
monly accomplished by adding fresh, stabilized solvent to make up for solvent losses during opera-
tion (USEPA, 1989b). The need to replenish spent stabilizers was documented as early as 1937
(Dinley, 1937). As detailed in Section 1.1 , solvents are lost from the operation when vapors escape,
when solvent dissolves into condensed water, and when solvent remains on parts or in textiles.
Improvements to machines that use solvents, such as vapor degreasers and dry-cleaning machines,
have minimized or completely eliminated solvent losses, as required by increasingly restrictive
emissions regulations. Stabilizers are nevertheless consumed in newer equipment. Stabilizer replen-
ishment is now accomplished with specialty stabilizer packages sold by the solvent manufacturer.
1.2.1 T YPES OF S OLVENT S TABILIZERS AND S OLVENT B REAKDOWN
Solvent stabilizers, also called inhibitors (and in the early literature, “anticatalysts”), are selected for
their ability to prevent the three main types of reactions that produce decomposition of chlorinated
solvents, which leads to acid formation: (1) hydrolysis; (2) oxidation, initiated by exposure to air, by
thermal breakdown, or by ultraviolet light; and (3) condensation reactions with alkali metal salts. *
The main categories of solvent stabilizers are acid acceptors, antioxidants, and metal inhibitors.
Ultimately, all stabilizers serve to mitigate the formation of acids, because each type of reaction
* A condensation reaction joins two molecules to create a complex molecule and a much simpler molecule as a by-product
of the reaction. The simple molecule eliminated from the two reactants can be water, ammonia, or alcohol. Each reactant
contributes to the eliminated molecule (Carey, 1987). The two combining molecules each contribute a single moiety
(a portion of a molecule having a characteristic chemical property) to the eliminated molecule.