1. Field of Invention
This invention refers to apparatus and a method for reducing the loss of solvent vapors to the atmosphere. More specifically, it refers to apparatus and a process for capturing and recovering for re-use most of the volatile solvent vapors which otherwise would be discharged to the atmosphere from cleaning, degreasing or paint stripping equipment in which the soiled parts to be cleaned or stripped of paint are immersed in liquid solvents, sprayed with liquid solvents, or suspended in solvent vapor for degreasing.
2. Description of Prior Art
Certain volatile halogenated hydrocarbon solvents have outstanding properties for dry-cleaning of fabrics, degreasing of metal parts and electronic circuit boards, and stripping of paints. Examples of such solvents are methylene chloride; 1,1,2-trichloro-1,2,2-trifluoroethane (chlorofluorocarbon 113, or CFC 113); 1,1,1-trichloroethane (TCA, or methyl chloroform); trichloroethylene; and tetrachloroethylene (perchlorethylene). In addition to their excellent cleaning, degreasing and paint stripping properties, these solvents have the great advantage over hydrocarbon, ester, ether and ketone solvents of being non-flammable. These solvents are frequently used in combination with minor amounts of other components added to enhance chemical stability.
The methods used for achieving a high degree of cleanliness in fabrics, metal parts, circuit boards, etc. (hereafter referred to as parts) include immersion of the parts to be cleaned in liquid solvent, spraying of the parts with liquid solvent while they are suspended in air or solvent vapor, or vapor degreasing. A combination of these cleaning techniques is sometimes used. In immersion cleaning, the parts to be cleaned are lowered into a tank containing a bath of liquid solvent and then raised above the liquid level to allow solvent adhering to the parts to drip back into the pool of liquid solvent at the bottom of the tank. Paint stripping is often carried out in this fashion also. In spray cleaning, liquid solvent is pumped through a pipe with connected spray nozzles, and the spray of liquid solvent is directed onto the parts to be cleaned. In vapor degreasing, the liquid solvent at the bottom of the tank is maintained at its boiling point by suitable heating means, e.g. a steam-heated pipe coil or an electrical resistance heating element, and there is provided a layer of solvent vapor in which the parts to be cleaned are suspended. Solvent vapor condenses on the surfaces of the parts until they reach the temperature of the vapor, and the solvent condensate drains by gravity into the pool of liquid solvent at the bottom of the tank. The rinsing action of the condensing solvent thoroughly cleans the surfaces of the parts.
The impurities removed from the parts, such as oil, grease, tar, resins, gums, and pigments from stripped paints, accumulate in the solvent at the bottom of the tank, and when the impurities level has built up to the point where effective cleaning ceases, the impurities-laden solvent is removed from the tank and most of the solvent is recovered by distillation for re-use.
In conventional cleaning, degreasing and paint stripping facilities using open-top tanks, large amounts of solvent vapors are discharged to the atmosphere. In recent years, it has become increasingly apparent that halogenated solvent vapors represent a potential health and environmental threat. Chlorinated hydrocarbon solvents pose a potential health threat to humans and animals through inhalation, skin contact and ingestion via contaminated water. Chlorofluorocarbons discharged into the atmosphere and accumulating in the stratosphere are believed to be responsible for depletion of the protective ozone layer which screens out much solar and cosmic ultraviolet radiation. Trichloroethane, too, is believed to have appreciable ozone depletion potential. Furthermore, volatile halogenated solvent vapors are believed to contribute to global warming due to the "greenhouse effect" caused primarily by the buildup of the carbon dioxide concentration in the Earth's atmosphere, which is feared to cause major climatic disruptions on our planet in the next century.
One approach for dealing with these health and environmental threats is to develop non-hazardous, non-toxic solvent substitutes for the halogenated solvents. Another approach is to develop partially halogenated chlorofluorocarbons whose vapors decompose in the lower atmosphere and which pose no threat to the ozone layer; whether such materials pose a health or greenhouse threat must also be determined. These approaches involve years of costly research and development and the investment in costly new equipment.
Another approach for dealing with the health and environmental problems caused by halogenated solvent vapors is to find means and methods for reducing the discharge of such vapors so the atmosphere to a minimum. Much progress has been made in this respect. In the prior art, dry cleaning solvent vapor is captured on charcoal filters, which are regenerated with steam or hot water; upon cooling, liquid solvent is recovered for re-use. U.S. Pat. No. 4,500,363 describes improved methods for carrying out such solvent recovery.
U.S. Pat. Nos. 4,101,340 and 4,844,743 teach a method whereby, following spray cleaning of parts with liquid solvent in a tank, solvent vapor contained in the air inside the tank may be removed by circulation of this air in a closed circuit through means for condensing and separating such vapors for re-use, whereby the amount of solvent vapor escaping to the atmosphere when the tank is opened for removal of the cleaned parts is substantially reduced.
In vapor degreasing apparatus, methods of reducing the discharge of solvent vapors include the installation of covers on normally open-top degreasing tanks; the installation of water-cooled or refrigerant-cooled pipe coils around the inside periphery of the tank to limit the extent of the solvent vapor layer; and the provision of a "freeboard zone," i.e. an enclosed space above the coils in which a solvent vapor/air mixture, which is heavier than pure air, settles down rather than spill over the top of the tank into the environment while parts are being lowered into and lifted out of the degreasing tank. In addition, workers are instructed to lower the parts to be cleaned into the tank slowly and to raise them slowly after cleaning so as to minimize turbulence, which causes the escape of solvent vapors via the top opening of the tank to the environment.
Despite these measures and precautions, tremendous amounts of solvent vapors continue to be discharged into the atmosphere daily at innumerable cleaning, degreasing and paint stripping installations. For example, vapor degreasers using trichloroethane typically lose 50-90 percent of the solvent used to the atmosphere. There is an urgent need therefore to provide apparatus and methods for reducing, if not eliminating, such solvent pollution of the air.
It is an object of this invention to provide apparatus, and a method of employing it, by which the discharge of solvent vapors from cleaning, degreasing and paint stripping equipment to the atmosphere is greatly reduced--to 1-2 percent of the solvent used. A further object of this invention is to enable manufactures to continue using halogenated solvents for cleaning, degreasing and paint stripping with minimal discharge of solvent vapors to the atmosphere. An advantage of this invention is that halogenated solvents can continue to be used without adverse health and environmental impact. A further advantage is that existing cleaning, degreasing, and paint stripping equipment can be retrofitted to achieve a large reduction in solvent vapor emissions to the atmosphere.