Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 57

concentrated in the vapor degreaser still bottoms owing to its much higher boiling point (121.4°C
versus 86.7°C for TCE). However, perchloroethylene is generally produced with the least impuri-
ties and requires the smallest quantities of stabilizers, as suggested by Table 1.14 .
Some investigators have concluded that the starting concentration of perchloroethylene as an
impurity of TCE is probably too small to manifest as a detectable constituent at TCE waste release
sites, even after iterative concentration in a vapor degreaser (Lane and Smith, 2006). Other investi-
gators have drawn the opposite conclusion: perchloroethylene—if initially present at 1% in TCE—
would reach a volume fraction of nearly 8% after multiple iterative cycles of boiling and condensation
in a vapor degreaser, as calculated from the Clausius-Clapeyron equation. The operation of this
process therefore allows enough perchloroethylene to be present in groundwater contaminated by
TCE waste solvent to give the appearance that perchloroethylene was used for degreasing (Morrison
et al., 2006). Moreover, direct chemical analysis of TCE of reagent grade, technical grade, and
pharmaceutical grade has reportedly detected the presence of perchloroethylene in all three. The
analysis was done to check for more toxic impurities in commercial grades of TCE, which are
currently used to anesthetize patients for surgery as a cost-saving measure in hospitals that cannot
afford more expensive agents of anesthesia (Totonidis, 2005). TCE produced in Europe had up to
0.03% perchloroethylene as an impurity (European Chemicals Bureau, 2004). Typical industrial
grade TCE has been known to include about 0.15% carbon tetrachloride as the major chlorinated
hydrocarbon impurity (Willis and Christian, 1957).
Since the 1980s, the major producers of chlorinated solvents have sold increasingly reliable
grades of TCE. In earlier decades, however, quality control may not have been as stringent, and
carryover of solvents from production led to impurities in the i nished product sold for vapor
degreasing. The patent literature from the mid-1950s notes that it was not then feasible to produce
high-purity perchloroethylene for commercial use. The presence of lower aliphatic hydrocarbons
introduces instability to perchloroethylene. Perchloroethylene produced through chlorination and
chlorinolysis * may contain trace impurities including dichloroethylene, TCE, trichloroethane, and
other compounds. Some patents from this era claim methods for restoring perchloroethylene to
purity (Skeeters, 1960b). Analysis has shown that perchloroethylene with a purity of less than
50 ppm of chlorinated ethanes is still susceptible to decomposition, leading to a maximum acidity
of 47 ppm (EPO, 1989).
The converse may also occur in which TCE is present in perchloroethylene as a degreasing blend
designed to extend the life of the perchloroethylene. Solvent producers typically recommend replac-
ing perchloroethylene in degreasing applications when the oil contamination reaches 25-30% or
when the specii c gravity drops to 1.4 from perchloroethylene's normal value of 1.6 (Mertens,
2000b). Producer guidance also advises that perchloroethylene should be replaced in degreasing
operations when the AAV drops into the range 0.02-0.06%. TCE is sometimes added to perchloro-
ethylene to allow continued use at higher acidity and higher oil content. The addition of up to 19%
TCE to perchloroethylene extended its operational life. By lowering the boiling point from 121°C
for perchloroethylene alone to 115°C for the perchloroethylene-TCE mixture, energy savings were
also achieved (Roehl, 1981).
Partitioning of stabilizers from their host solvents due to boiling-point differences may increase
as the boiling point of the solvent increases with continued use. A pure solvent has a dei ned boiling
point. When oil, grease, or other compounds with higher boiling points are added to solvents during
the vapor degreasing process, the boiling point of the mixture increases according to the molar
concentration and the vapor pressure of each component. Solvent boiling temperature increases
several degrees as the percentage of waste oil builds in the solvent sump. Changing out the solvent
is recommended when the waste oil content in the degreaser approaches 25%; boiling temperature
* In the chlorinolysis method for producing perchloroethylene, hydrocarbons are chlorinated at or near pyrolytic conditions
to produce a mixture of carbon tetrachloride and perchloroethylene.