Control of Solvent Contamination

The cleanness and chemical stability of the degreasing solvent are important influences on the efficiency of vapor degreasing. For example, an excess of contaminant oil raises the boiling point of the solvent and detracts from its effectiveness in cleaning.

Oils. The chlorinated solvents used in degreasers are stabilized or inhibited to resist the harmful effects of many contaminants. However, certain cutting oils with a high content of free fatty acid can overcome the effects of stabilization and may contribute to a sour, acidic condition. Oils with high contents of sulfur or chlorine as additives have the same effect. These oils and greases accumulate in the boiling or vapor chamber and cause foaming and a reduction in solvent evaporation. Baked sludge accumulates on the steam coils and other heated areas, thus reducing the efficiency of the degreaser.

When the oil content of the solvent reaches 25 vol%, the solvent should be replaced and the oily solvent reclaimed. The percentage of mineral oil in trichloroethylene, perchloroethylene, 1,1,1-trichloroethane, and methylene chloride can be determined from the boiling temperatures given in Table 5.

Table 5 Physical properties of mineral oil-in-solvent mixtures

Solvent

Boiling point for vol% oil loading:

Specific gravity at 25/25 °C for vol % oil loading:

0

10

20

30

°C

°F

°C

°F

°C

°F

°C

°F

0

10

20

30

Perchloroethylene

121

250

122

252

124

255

126

259

1.619

1.542

1.464

1.395

Trichloroethylene

87

189

88

190

89

192

90

194

1.457

1.406

1.345

1.288

1,1,1-trichloroethane

74

165

76

169

77

171

79

174

1.320

1.272

1.227

1.180

Paint Pigments. Pigments from painted surfaces that are washed into the degreaser should be filtered or removed by other mechanical means. The oils in pigment or paint dissolve in the degreasing solvent, but the remaining material is insoluble. This material usually floats on the surface of the degreaser solution and adheres to the work. In addition to reducing cleaning efficiency, these pigments may bake out on the heating coils and the work.

Chips washed from parts into the degreaser should be removed periodically, because they contaminate other parts entering the degreaser. Such contamination is possible even in ultrasonic degreasers when the solution is not filtered continuously. An excessive amount of chips in the vapor or boiling tank reduce heat transfer and evaporation rates. An accumulation of fine aluminum particles may also result in solvent breakdown.

Water can be present in degreasers as a result of the presence of water on parts being degreased or the accumulation of condensate on the cooling coil or jacket of the degreaser. Most chlorinated degreasing solvents are inhibited against the effects of hydrochloric acid formation in the presence of water; nevertheless, to avoid stains, spotting, and rusting of parts, all water must be removed from the degreaser. To accomplish this, degreasers should be equipped with one or more water separators that continuously remove free water from the circulating recondensed solvent (Fig. 3).

Fig. 3 Vapor degreasing unit designed specifically for a vapor-spray-vapor system

Other contaminants, such as silicones, should not be allowed to enter the degreaser, because they cause foaming at the surface of the liquid solvent. All acids, oxidizing agents, cyanides, or strong alkalis must be prevented from entering the degreasing solvent.

0 0

Post a comment