Environmental Engineering Reference
In-Depth Information
rising solvent costs and by solvent losses of up to 70% in open-top vapor degreasers. Another incentive
for solvent recovery from degreaser vapors was the introduction of standards for permissible worker
exposure and restrictions on air emissions. The most common method for recovering solvent from
vapors is carbon adsorption. Carbon cartridges designed to allow high air l ows adsorb most of the
solvent vapor. Periodically, steam is passed through the carbon bed to carry the vapors to a con-
denser. A water separator removes the water from the solvent, which is then recovered and returned
to storage tanks. As discussed in Section 1.2.7.5 , carbon adsorption also removes some solvent sta-
bilizers from the solvent, while introducing water contamination to the solvent. Carbon bed desorp-
tion is also accomplished by using hot air l ow on a daily 6-8-h cycle; solvent vapors are condensed
and returned to the solvent reservoir in the degreaser, and the carbon is regenerated for further use
(Reuter, 2002). Cryogenic solvent-recovery techniques are also used (Douthitt, 1990).
1.1.1.4 DryCleaning
Dry cleaning is the term of art for a number of textile-cleaning processes applied to clothing, fabric,
rugs, draperies, and other textile articles, which uses petroleum solvents and halogenated solvents.
The term “dry” refers to the absence of water as a primary means for laundering clothes. Clothes
obviously need to be cleaned regularly; laundering with water and detergent is insufi cient for some
fabrics and some soils and does not produce the same degree of cleanliness as can be obtained with
chlorinated solvents. The details of how perchloroethylene was used, i ltered, recovered, discharged,
and vented from dry-cleaning operations relate to the nature and volume of perchloroethylene
releases and to the partitioning and release of solvent stabilizers, as discussed in Section 1.2.7 .
Perchloroethylene became the solvent of choice for the dry-cleaning industry after surpassing
petroleum solvents approximately in 1960. Carbon tetrachloride, TCE, methyl chloroform, and
chlorol uorocarbons have also been used in the dry-cleaning industry. Today, more than 80% of dry
cleaners use perchloroethylene; some still use petroleum solvents, and a growing number of opera-
tors are switching over to machines that use carbon dioxide or other solvents or cleaning agents
deemed to be “green,” with fewer regulatory restrictions and environmental consequences.
The dry-cleaning industry is composed of two main sectors: the commercial sector and the
industrial sector. Previously, the coin-operated dry-cleaning sector was economically important,
but machine inefi ciency and regulation led to its elimination. Coin-operated dry-cleaning machines
were banned in December 1994. In the early 1990s, the U.S. dry-cleaning industry included about
30,500 commercial dry cleaners, 1400 industrial cleaners, and 3000 coin-operated dry cleaners
(USEPA, 1995). Industrial dry cleaners are large operations that clean uniforms, aprons, overalls,
shop towels, or other items. Approximately 50% of the industrial dry cleaners use petroleum sol-
vents. A typical industrial cleaner of clothing may have a 500-pound capacity washer-extractor and
3-6 dryers, each with 100-pound capacity. The annual solvent usage ranges from 200 to 1300
gallons per machine (USEPA, 1995).
1.1.1.4.1 Dry- CleaningProcess
The descriptions in this section apply primarily to dry-cleaning operations that used earlier-generation
machines from the 1950s through the early 1990s. Operations from this timeframe contributed sub-
stantially more perchloroethylene to the subsurface than current or recent operations. Since 1986,
there have been substantial improvements in solvent stewardship and dry-cleaner operator training,
adoption and enforcement of regulations, and improvements in equipment (Mohr et al., 2007).
The perchloroethylene dry-cleaning process consists of washing clothes in liquid solvent, extract-
ing solvent from the clothes, and drying the clothes. Two general categories of dry-cleaning machines
have been used: transfer machines and dry-to-dry machines. Transfer machines perform the wash-
ing and extraction steps; clothes are then manually transferred to a separate dryer. Dry-to-dry
machines perform all the three steps in the same machine (USEPA, 1980).
Dry cleaning involves two wash cycles, each followed by drying. In the i rst wash, soiled
garments are loaded into a rotating, perforated, stainless-steel cylindrical basket with a capacity
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