Environmental Engineering Reference
In-Depth Information
point, will concentrate in the liquid phase. Stabilizers that concentrate in the vapor phase will even-
t ua l ly be come deplete d i n op en-top vapor deg reasers (wh ich were com mon ly use d i n ea rl ier de cades)
because a substantial quantity of solvent vapor is lost to the atmosphere with evaporation. If a batch
of stabilized solvent is evaporated under these conditions, the stabilizer will leave before the solvent
is evaporated, resulting in lower stabilizer concentrations in the liquid solvent (DeGroot, 1998).
Stabilizers whose boiling points are higher or lower than the solvent to which they are added will
partition differentially when the solvent is boiled for vapor degreasing or other applications. * For
example, 1,4-dioxane boils at 101°C, whereas methyl chloroform boils at 78°C. Thus, 1,4-dioxane is
a “high-boiler” relative to methyl chloroform. Although enough 1,4-dioxane will vaporize to stabi-
lize methyl chloroform in the vapor phase, a larger proportion preferentially remains in the liquid
phase in the vapor degreaser boiling sump. The proportion has been determined: 27% of the 1,4-
dioxane will partition to the vapor phase, while 73% will remain in the liquid phase (Spencer and
Archer, 1981). Through continued use, a vapor degreaser iteratively partitions 1,4-dioxane such that
the proportion of 1,4-dioxane in the sump will increase over time. In several weeks of daily use, the
liquid solvent can take on a composition that is as much as 10-20% 1,4-dioxane. Because 1,4-diox-
ane is l ammable, this may pose a i re hazard (Archer et al., 1977).
The iterative degreasing cycle that partitions 1,4-dioxane in still bottoms presents a substantially
greater mass strength for sources of 1,4-dioxane groundwater contamination—far greater than what
might be inferred from the initial composition of stabilized methyl chloroform. As investigations of
dozens of 1,4-dioxane plumes mapped since 1995 have shown, the mass of 1,4-dioxane released in
methyl chloroform degreasing waste or solvent reclamation waste was indeed enough to sustain
large plumes.
Laboratory analysis of new and spent methyl chloroform sampled from a vapor degreaser at
Hayes International Corporation showed a 68% increase in 1,4-dioxane concentration (Tarrer et al.,
1989). Another vapor degreaser was operated under controlled conditions in a laboratory to measure
changes in stabilizer composition over time. The starting composition of the stabilized methyl chlo-
roform was 2.8% 1,4-dioxane. After operating the degreaser for 24 days, the 1,4-dioxane content
increased to 7.5%. Other stabilizers of methyl chloroform were also partitioned in this experiment.
tert -Amyl alcohol increased from 1.5% to 3.2%; nitromethane decreased from 0.5% to 0.25%; and
1,2-butylene oxide decreased from 0.75% to 0.5% (Spencer and Archer, 1981).
The decrease in nitromethane is due to its greater tendency to partition into the vapor phase. At
74°C, the boiling point of methyl chloroform, nitromethane partitions to 62% in the vapor phase and
38% in the liquid phase, even though its boiling point is 101°C (Spencer and Archer, 1981; Budavari,
1996). At 25°C, nitromethane has a vapor pressure of 35.8 mm Hg, whereas methyl chloroform has
a vapor pressure of 127 mm Hg (Daubert and Danner, 1989; Sunshine, 1969). The relative loss of
nitromethane to the vapor phase is compensated by including another nitroalkane as a stabilizer of
methyl chloroform. Nitroethane has a vapor pressure of 20.8 mm Hg and so is more favorably
* Partitioning stabilizers from boiling solvent may also depend on vapor pressure differences, vapor density contrasts,
vapor removal, and other factors such as “drag-out.” Vapor removal or drag-out is caused by retention of solvent on the
parts exiting the vapor degreaser and by moving parts or drafts that stir the vapors and cause them to escape the cooling
blanket. Although not accounting for uncontrolled factors such as drag-out, the tendency of a stabilizer compound to
partition out of a mixture into the vapor phase can be determined by combining the Clausius-Clapeyron equation,
Antoine's equation, and Raoult's law. This approach becomes difi cult when consideration is given to the waste oil com-
ponent of the mixture. Nevertheless, boiling-point difference is a good surrogate indicator of the potential for a stabilizer
compound to become concentrated in the liquid phase or to become depleted.
“Spent” solvent is a relative term; degreasing operator guidance calls for removing and concentrating the still bottoms
oily waste sludge when the oil content reaches 25-30% (Dow Chemical Company, 1999b). Temperature is a surrogate for
measuring oil content: Dow recommends that still bottoms be removed when the boiling temperature reaches 42°C for
dichloromethane (a 1°C increase from the normal boiling point for dichloromethane), 89°C for trichloroethylene (DOW
NEU-TRI Solvent; a 2.7°C increase), and 125°C for perchloroethylene (a 3.6°C increase) (Dow Chemical Company,
1999b). Still bottoms for methyl chloroform should be removed when the boiling temperature reaches 77°C (a 3°C
increase) (NZDL, 1981).
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