Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 21

In 1964, E. I. du Pont de Nemours and Company, introduced 1,1,2-tr ichloro-1,2,2-tr il uoroethane,
also known as Freon 113, under the trade name Valclene™ (Johnson, 1973). Valclene™ is less
aggressive than perchloroethylene and evaporates from fabrics at lower temperatures and was thus
useful for cleaning delicate fabrics. Valclene was preferred over perchloroethylene for rubber,
painted fabrics, polystyrene, vinyl-coated fabrics, polyvinyl chloride i bers, some pigment colors,
furs, and leathers (Johnson, 1973). Although Freon 113 had favorable attributes for delicate clothing,
it was not widely used. It was banned in 1996 in accordance with the Montreal Protocol because of
its potential to deplete ozone and contribute to global warming (Johnson, 1973; CARB, 1996).
In the early 1980s, Dow Chemical introduced methyl chloroform as a dry-cleaning solvent (Linn
et al., 2004). It was not widely used in the dry-cleaning industry as a primary dry-cleaning solvent
because it damages delicate fabrics. Methyl chloroform is also somewhat unstable when mixed with
water, where it breaks down and forms acids that corrode dry-cleaning machines (CARB, 1996).
Methyl chloroform was also restricted from most uses following the passage of the Montreal
Protocol and Clean Air Act Amendments (Johnson, 1973; CARB, 1996).
1.1.1.4.9 Filtering Perchloroethylene
Three types of disposable cartridge i lters are used by 90% of dry cleaners: carbon i lters, paper
i lters, or combination paper and carbon i lters. Disposable cartridges are changed after cleaning
450-1000 pounds of clothing. Operators observe the solvent line pressure to determine when i lters
need to be changed, as indicated by increased pressure in the solvent line behind the i lter (Cantin,
1992; CARB, 1996). Operators typically drain used or spent cartridges for 24 h for paper i lters or
48 h for carbon cartridges prior to reuse or disposal at a facility that conducts hazardous waste treat-
ment. Spent cartridges can retain as much as 1 gallon of perchloroethylene before draining, and
from 0.13 to 0.25 gallon perchloroethylene remains in the cartridge after draining (CARB, 1996).
The i lter cartridge became prominent in the dry-cleaning industry in about 1961.
A smaller number of dry cleaners use spin-disk i lters. Spin-disk i lters are made up of multi-
ple high-surface-area polyester disks mounted in series to i lter the dirty solvent. Spin-disk i ltra-
tion systems may incorporate powdered activated carbon and diatomaceous earth. Filters are
regenerated by spinning the disks and directing the solvent and the “caked” impurities through a
drain valve into a distillation still. Spin-disk i lters last longer and produce less waste than the
cartridge systems, but skilled labor and frequent maintenance are required for proper operation
(Hygnstrom, 1995).
A third type of i lter is the regenerative or l ex-tube i lters, made of braided tubes coated with a
combination of diatomaceous earth and activated carbon. The i lter is cleaned by resuspending the
powder and dirt in solution by a countercurrent of air and backwashing; the resuspension creates a
mixture that is then redeposited back on the i lter to form a highly porous “cake” and uniform i lter
media for the proper removal of contaminants. The regeneration action is repeated with each clean-
ing cycle. Regenerative i lter systems are more expensive than spin-disk i lters and require skilled
labor to operate properly (CARB, 1996).
1.1.1.4.10 On-Site Distillation for Perchloroethylene Recovery
Approximately 90% of dry cleaners use distillation as a companion process to purify and recover
used solvent (Cantin, 1992). Distillation stills are built into most modern dry-cleaning machines;
stand-alone stills, called “muck cookers,” were used for earlier-generation machines. Steam is used
to heat the perchloroethylene to its boiling point (121°C/250°F). Solvent and water vaporize and rise
to the top of the still, where vapors are cooled back into a liquid that then goes to a water separator.
Sludge or “still bottoms”—composed of nonvolatile residues such as detergents, waxes, dyestuffs,
sizing, oils, and grease—are removed from the sump where they accumulate and are disposed of as
hazardous waste. The sludge or still bottoms can contain as much as 50-70% perchloroethylene.
A “muck cooker” is used to recover most of the perchloroethylene. A cooker or still can recover as
much as 95% of the solvent from the spent sludge (Morrison, 2002b).