Environmental Engineering Reference
In-Depth Information
TABLE 1.28
Boiling-Point Contrasts for Stabilizers with Dichloromethane
Maximum
Stabilizer (wt%)
Boiling-Point
Difference
a
(°C)
Stabilizer
Boiling Point (°C)
Rank
Concentrated in Still Bottoms
Thymol
0.5
233
193.25
1
Phenol
0.05
181.75
142
2
Ethanol
0.2
135
95.25
3
4-Cresol
0.5
129.5
89.75
5
Nitromethane
0.43
101.2
61.45
6
1,4-Dioxane
8
101.1
61.35
7
Cyclohexane
0.01
80.7
40.95
8
1,3-Dioxolane
1
78
38.25
9
Methanol
0.05
64.7
24.95
10
Lost to Vapor
tert
-Butyl amine
-
44.04
4.29
11
2-Methyl-2-butene
2.5
39
−0.75
12
Propylene oxide
-
34.23
−
5.52
13
Methanamine
1
−
6
−
45.75
14
Source:
See Appendix 2 for a tabulation of physical and chemical properties of solvent-stabilizer compounds with full
citations.
a
Dichloromethane's boiling point is 39.75°C.
solvent as a mixture of i ne free-phase water droplets. Dispersed water may be removed when
solvents are i ltered through membranes to remove waste in the solvent (Howell and Tarrer, 1994).
In dry-cleaning operations, on-site distillation of perchloroethylene was commonly performed to
remove impurities and water. Azeotropic reactions
*
typically allow for certain amounts of boil-over,
causing moisture and other impurities to recirculate through the water separator in the solvent/water
interphase and return to the clean tank. Distillation degrades and ultimately destroys solvent stabi-
lizers, which allows perchloroethylene to become acidic and corrosive (Laidlaw Company, 2004).
The cause of stabilizer loss is most likely related to differences in water solubility between the sta-
bilizers and perchloroethylene.
To minimize the effect of water on methyl chloroform, some solvent maintenance approaches
capitalized on the miscibility of water in alcohol by using compounds known as polyhydroxy alco-
hols (i.e., propylene glycol). Methyl chloroform is treated by drawing off water accumulated atop the
cooled liquid solvent and then adding a layer of propylene glycol, which also l oats atop the solvent.
The remaining water and dissolved acids preferentially partition into the alcohol, which can then be
easily removed because it remains as a free liquid phase atop the solvent (Cormany, 1977). Adding
alcohols and ethers to solvents can change the solubility of substances in the solvent, sometimes
with deleterious effects. For example, the addition of alcohols to TCE to stabilize metal reactions
was known to increase the solubility of hydrochloric acid in TCE, which led to corrosion problems
(Klabunde, 1949).
*
An azeotrope is a liquid mixture of two or more substances that retains the same composition in the vapor state as in the
liquid state when distilled or partially evaporated.
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