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Enclosed vapor degreasing systems
ENCLOSED VAPOR DEGREASING by Joseph Scapelliti Durr Automation
Inc.,
Davisburg,
SYSTEMS
Mich.
Historically, vapor degreasing has been the cleaning process of choice for the majority of manufacturers. It is primarily used for batch cleaning of parts in such industries as general metalworking, automotive components, aerospace, and electronics. Vapor degreasing traditionally used CFC- I 13 and I ,I, 1-trichloroethane solvents. Recent developments have forced both manufacturers and chemical companies producing solvent to reexamine current cleaning processes. It was the discovery that these solvents, commonly used in the cleaning process, were responsible for the depletion of the ozone layer that eventually lead to the Montreal Protocol. The Protocol called for a phased withdrawal of these solvents from the world marketplace. In the United States, taxes have been levied on continued use of these solvents and the U.S. EPA now requires warning labels on products that were manufactured using these solvents and placed into commerce. As a result, some solvent manufacturers have announced that they will cease production of these materials. These events have created many problems for manufacturers who rely on the vapor degreasing process. The “magic bullet” (a drop-in replacement solvent) that provides high threshold limit values (TLVs) and the good cleaning characteristics of 1, I, 1-trichloroethane and CFC- 113 without any ozone-depletion potential does not exist at present. Chemical manufacturers report that if replacements are developed, they will be more costly and perhaps have lower boiling points and lower TLVs. Most chemical companies won’t even speculate on when these replacement chemistries might be available. The consensus is, however, that new replacements will not be available before the existing chemistries are phased out. This has mandated industry to seek alternatives to the conventional vapor degreasing process and it must choose from technologies and chemistries that are currently available.
CONVENTIONAL
VAPOR
DEGREASING
Traditionally, a vapor degreasing system has been a open-top tank with a heater unit at the bottom to boil the solvent and a cool surface at the top of the unit to condense the vapors (Fig. I). Operation of these units is quite simple, but effective. Vapors from the boiling solvents condense on a cool part, which flushes off the oil, grease, and soil. When the temperature of the part reaches the temperature of the vapor, condensation ceases and clean, dry parts are then removed from the tank. This occurs because solvent vapors rise, displacing air in the tank. Vapors ate confined by condensing coils and a water jacket below the freeboard area in the upper part of the tank. As the solvent vapors reach the cool zone, they condense on the cooling coils. This process, while relatively efficient and cost effective, permits an extremely high percentage of the solvents to escape into the atmosphere. Chemical companies continue to research alternative solvents, but even the most optimistic forecasts indicate that regulatory deadlines will arrive before new chemical alternatives are available.
BENEFITS
OF ENCLOSED
VAPOR
DEGREASING
The benefits of vapor degreasing am well known. In a vapor degreaser, cleaning, rinsing, and drying all take place using one chemistry and, when compared with other cleaning processes, vapor degreasing uses less energy and requires less floor space per application. 144
Condensing
Coils
T
water Jacket
Fig. I. Conventional
open-top
vapor degreaser
with spray attachment.
Section 612 of the Clean Air Act empowered the EPA to develop a program for evaluating alternatives to ozone-depleting substances. The EPA refers to this program as the Significant New Alternatives Policy (SNAP) program. Under this program, trichloroethylene, perchloroethylene, and methylene chloride arc listed as accepted substitutes for 1 ,I, I-trichloroethane and CFC-I 13 when used for metal degreasing, precision cleaning, and electronics cleaning applications. One currently available technology that can effectively use the acceptable substitutes is the enclosed vapor degreaser (EVD). The advanced technology of EVD allows the continued use of the degreasing process safely and in compliance with federal regulations. It is the EVD design that allows such non-ozone-depleting solvents as trichloroethylene and perchloroethylene to be safely substituted for 1 , I,1 -trichloroethane and CFC- 113. Solvent emission levels from EVDs average less than 10 ppm at all times during the cleaning cycle and when work is being loaded and unloaded from the cleaning system. This is of particular importance, since both trichloroethylene and methylene chloride have TLVs of 50 ppm, with perchloroethylene at 25 ppm (Table I).
EVD-THE
ALTERNATIVE
CLEANING SYSTEM
The system design of an EVD is basically simple, but extremely effective (Fig. 2). It consists of the cleaning chamber where all the cleaning operations take place; a boil sump used to generate vapor for cleaning and drying the work, a condensing chamber used to condense solvent vapor into solvent liquid, a carbon adsorption chamber to remove solvent from saturated air; a spray reservoir, which supplies solvent for the spray cleaning cycle; and a water separation device to remove moisture from the condensed solvent. All of the equipment is housed in a cabinet to form the cleaning system. Pipe and ductwork join the various components of the system together. When a work load is placed into an EVD, it essentially is being placed into an empty vessel. There is no solvent or solvent vapor present in the work chamber of these systems when the cover is in the open position or when the degreaser is in the idle state. The entire cleaning chamber can be filled with work to be cleaned. In a conventional degreasing system, the work chamber always has solvent vapor present. Because this is the case, the length or the width of
146
Table I. Physical Properties
Chemical structure Molecular weight Boiling point, “F (“C) Specific gravity, 25”C/2S”C Density, lb/gal at 25°C Nonvolatile residue, max. ppm Acid acceptance (as NaOH). min. % wt Free halogens, ppm Acidity (as HCI). max. ppm Flammability limits LFL UFL OSHA (PEL) ACGIH (TLV )
CHzClz 84.94 104 (39.8) 1.316 IO.98 IO 0.23
CHCICCI, 131.40 189 (86.9) I.456 12.11 IO 0.17
NOIX IO 14.8% (25°C) 25% (50°C) 500 SO
ccI,ccI, 165.85 250 (121) I.613 13.47 25 0.10 None NOlIe
8.0 (2S”C) 10.5% (25°C) IOil 50
loo 25
the work load must be one-half the length or width of the degreaser to prevent the work load from acting like a piston and pumping out solvent vapor as it enters the vapor zone. The cleaning cycle begins by placmg the work load into the degreaser, closing the lid, and depressing the start button on the control console. Activating the start button seals the lid, isolating the work chamber (Fig. 2-area 1) from the workplace environment. Solvent from the distillate receiver is then sprayed onto the work surface to aid in the removal of insoluble contaminants. When the spray cycle is completed, solvent vapor flows into the chamber from the boil sump of the system (Fig. 2-area 2). As the vapor enters the chamber, it condenses on the part dissolving the oils and greases. Condensate cleaning continues until the temperature of the parts being cleaned and the vapor temperature reach equilibrium. When equilibrium is reached, the cleaning cycle is completed. At the completion of the vapor cleaning cycle, a refrigerated cooling coil is activated and the solvent vapor present in the cleaning chamber is condensed to a liquid. The liquid is pumped into a distillate receiver and remains there, ready to be used in the spray portion of the next cleaning cycle. The next step in the process requires the desorption of solvent from a carbon mat medium. This solvent being desorbed was accumulated in the previous cleaning cycle. Desorption takes place by circulating hot air through the carbon purification unit (Fig. 2%area 3), which causes the solvent on the surface of the carbon to release in concentrated form to the hot airstream. The saturated airstream then is directed through a cooling unit (Fig. 2%area 4) and the solvent is condensed out. The destxbed solvent is directed through the water separator and into the distillate receiver. When desorption of the carbon media is completed, the solvent-laden air volume of the work chamber, which is still sealed, is recirculated through the carbon mat filters to remove
Fig.2.
148
Enclosed
vapor
degreaser.
solvent from the air. As the solvent/air mixture passes through the carbon medium, it is adsorbed onto its surface. This continues until the solvent concentration of the air in the work chamber reaches 1 gm/nG. When this level is reached, the entire cycle is completed, the seal on the cover is released, and the work can bc removed. The Battelle Institute was commissioned by the EPA to do a study on enclosed vapor degreasers. The study demonstrated that enclosed vapor degreasers reduce air emissions by 99% when compared with air emissions from typical open-top vapor degreasers. Battelle defined a typical vapor degreaser as having a 0.75 freeboard ratio, a primary cooling coil, electric hoist (to load and unload the degreaser), and no lip exhaust. Figure 3 (fugitive emissions) shows the relative concentration of perchloroethylene in the work chamber during the cleaning, desorption, and adsorption cycle versus the solvent concentration outside the degreaser at the sealed lid. The high point of concentration at the lip of the degreaser tank was 5 ppm, which occurred when the lid was open at the conclusion of the cycle. When the amount of oil or soluble contamination of the boil chamber reaches 30% by volume, the machine is placed into the distillation cycle. In this cycle, the solvent is heated in the boil chamber, creating a vapor and leaving the soluble contaminants (oil, waxes, etc.) behind. The vapor is directed through a refrigerated condenser coil and returns to a liquid state. The liquid is temporarily stored in a chamber of the degreaser until the distillation cycle is completed. When the distillation cycle is completed, the remaining contents of the boil chamber are allowed to cool and are then transferred to a suitable chamber and disposed of in accordance with federal, state, and local requirements. In addition to the EVD just discussed, there are versions that degrease by immersing the work to be cleaned in solvent. Immersion cleaning can be enhanced by adding ultrasonic and
injection flood wash agitation. Rotation of the work can also be employed to aid in cleaning and drying. Vacuum can also be employed to aid in the drying of the work.
SAFETY
AND
HANDLING
Chlorinated solvents have been used for many years with a low incidence of health-and-safety-related problems. The primary routes of exposure are inhalation and skin. Eye exposure is less common and oral ingestion is rare. Accidental swallowing of small amounts of any of the chlorinated solvents presents minimal hazard, but ingestion of substantial amounts can cause serious injury. To prevent accidental ingestion, always store solvents properly labeled and in containers that will not be mistaken for beverage use. If solvent is swallowed, do not induce vomiting, but obtain medical assistance. Prolonged and/or repeated skin contact with chlorinated solvents can extract fat from skin tissues and may cause dermatitis. Occasional contact should not produce adverse effects unless solvent is confined to the skin, for example under a ring or inside one’s clothing, and cannot evaporate, and which may result in a chemical bum. It is not likely that the solvents will be absorbed in acutely toxic amounts through the skin. Neoprene or polyvinyl alcohol gloves should be used for solvent handling. If skin contact occurs, flush skin thoroughly with water and wash contaminated clothing before reusing. Goggles should be worn when handling solvents. Splashing solvent into the eyes will produce pain and irritation, but no serious injury will result. In the case of eye contact, flush with running water for 15 minutes and then seek medical attention. The greatest potential hazard for exposure is from inhalation. Extremely high vapor levels can cause death. Inhaling excessive amounts of solvent vapor can produce anesthetic effects. These effects, such as lightheadness or dizziness, will occur at varying concentrations and durations of exposure, depending on the solvent used. Physiological injury may result from repeated or prolonged exposure at levels producing anesthetic effects. Excessive inhalation exposure can be prevented by proper engineering and/or ventilation in nearly all operations. Proper protection may require the use of respirators, hose masks, or self-contained breathing apparatus, depending on the operation being performed, when ventilation does not provide adequate protection. In the case of an inhalation exposure, remove the worker to fresh air and get medical attention. Apply mouth-to-mouth resuscitation of respiration stops. Do not administer stimulants, e.g., epinephrine. Exposure to the vapor of chlorinated solvents at the OSHA/ACGIH exposure limits or less should not cause adverse health effects.
A PROVEN
TECHNOLOGY
Enclosed vapor degreasing technology is proven. It has existed in Europe for more than a decade. EVDs are used to clean everything from aircraft parts to cosmetic jewelry containers. The good news it that for those manufacturers who wish to use vapor degreasing or for those who have no choice and must degrease, EVDs allow the process to be used in safety and in accordance with government mandates.
151
Inc.,
Davisburg,
SYSTEMS
Mich.
Historically, vapor degreasing has been the cleaning process of choice for the majority of manufacturers. It is primarily used for batch cleaning of parts in such industries as general metalworking, automotive components, aerospace, and electronics. Vapor degreasing traditionally used CFC- I 13 and I ,I, 1-trichloroethane solvents. Recent developments have forced both manufacturers and chemical companies producing solvent to reexamine current cleaning processes. It was the discovery that these solvents, commonly used in the cleaning process, were responsible for the depletion of the ozone layer that eventually lead to the Montreal Protocol. The Protocol called for a phased withdrawal of these solvents from the world marketplace. In the United States, taxes have been levied on continued use of these solvents and the U.S. EPA now requires warning labels on products that were manufactured using these solvents and placed into commerce. As a result, some solvent manufacturers have announced that they will cease production of these materials. These events have created many problems for manufacturers who rely on the vapor degreasing process. The “magic bullet” (a drop-in replacement solvent) that provides high threshold limit values (TLVs) and the good cleaning characteristics of 1, I, 1-trichloroethane and CFC- 113 without any ozone-depletion potential does not exist at present. Chemical manufacturers report that if replacements are developed, they will be more costly and perhaps have lower boiling points and lower TLVs. Most chemical companies won’t even speculate on when these replacement chemistries might be available. The consensus is, however, that new replacements will not be available before the existing chemistries are phased out. This has mandated industry to seek alternatives to the conventional vapor degreasing process and it must choose from technologies and chemistries that are currently available.
CONVENTIONAL
VAPOR
DEGREASING
Traditionally, a vapor degreasing system has been a open-top tank with a heater unit at the bottom to boil the solvent and a cool surface at the top of the unit to condense the vapors (Fig. I). Operation of these units is quite simple, but effective. Vapors from the boiling solvents condense on a cool part, which flushes off the oil, grease, and soil. When the temperature of the part reaches the temperature of the vapor, condensation ceases and clean, dry parts are then removed from the tank. This occurs because solvent vapors rise, displacing air in the tank. Vapors ate confined by condensing coils and a water jacket below the freeboard area in the upper part of the tank. As the solvent vapors reach the cool zone, they condense on the cooling coils. This process, while relatively efficient and cost effective, permits an extremely high percentage of the solvents to escape into the atmosphere. Chemical companies continue to research alternative solvents, but even the most optimistic forecasts indicate that regulatory deadlines will arrive before new chemical alternatives are available.
BENEFITS
OF ENCLOSED
VAPOR
DEGREASING
The benefits of vapor degreasing am well known. In a vapor degreaser, cleaning, rinsing, and drying all take place using one chemistry and, when compared with other cleaning processes, vapor degreasing uses less energy and requires less floor space per application. 144
Condensing
Coils
T
water Jacket
Fig. I. Conventional
open-top
vapor degreaser
with spray attachment.
Section 612 of the Clean Air Act empowered the EPA to develop a program for evaluating alternatives to ozone-depleting substances. The EPA refers to this program as the Significant New Alternatives Policy (SNAP) program. Under this program, trichloroethylene, perchloroethylene, and methylene chloride arc listed as accepted substitutes for 1 ,I, I-trichloroethane and CFC-I 13 when used for metal degreasing, precision cleaning, and electronics cleaning applications. One currently available technology that can effectively use the acceptable substitutes is the enclosed vapor degreaser (EVD). The advanced technology of EVD allows the continued use of the degreasing process safely and in compliance with federal regulations. It is the EVD design that allows such non-ozone-depleting solvents as trichloroethylene and perchloroethylene to be safely substituted for 1 , I,1 -trichloroethane and CFC- 113. Solvent emission levels from EVDs average less than 10 ppm at all times during the cleaning cycle and when work is being loaded and unloaded from the cleaning system. This is of particular importance, since both trichloroethylene and methylene chloride have TLVs of 50 ppm, with perchloroethylene at 25 ppm (Table I).
EVD-THE
ALTERNATIVE
CLEANING SYSTEM
The system design of an EVD is basically simple, but extremely effective (Fig. 2). It consists of the cleaning chamber where all the cleaning operations take place; a boil sump used to generate vapor for cleaning and drying the work, a condensing chamber used to condense solvent vapor into solvent liquid, a carbon adsorption chamber to remove solvent from saturated air; a spray reservoir, which supplies solvent for the spray cleaning cycle; and a water separation device to remove moisture from the condensed solvent. All of the equipment is housed in a cabinet to form the cleaning system. Pipe and ductwork join the various components of the system together. When a work load is placed into an EVD, it essentially is being placed into an empty vessel. There is no solvent or solvent vapor present in the work chamber of these systems when the cover is in the open position or when the degreaser is in the idle state. The entire cleaning chamber can be filled with work to be cleaned. In a conventional degreasing system, the work chamber always has solvent vapor present. Because this is the case, the length or the width of
146
Table I. Physical Properties
Chemical structure Molecular weight Boiling point, “F (“C) Specific gravity, 25”C/2S”C Density, lb/gal at 25°C Nonvolatile residue, max. ppm Acid acceptance (as NaOH). min. % wt Free halogens, ppm Acidity (as HCI). max. ppm Flammability limits LFL UFL OSHA (PEL) ACGIH (TLV )
CHzClz 84.94 104 (39.8) 1.316 IO.98 IO 0.23
CHCICCI, 131.40 189 (86.9) I.456 12.11 IO 0.17
NOIX IO 14.8% (25°C) 25% (50°C) 500 SO
ccI,ccI, 165.85 250 (121) I.613 13.47 25 0.10 None NOlIe
8.0 (2S”C) 10.5% (25°C) IOil 50
loo 25
the work load must be one-half the length or width of the degreaser to prevent the work load from acting like a piston and pumping out solvent vapor as it enters the vapor zone. The cleaning cycle begins by placmg the work load into the degreaser, closing the lid, and depressing the start button on the control console. Activating the start button seals the lid, isolating the work chamber (Fig. 2-area 1) from the workplace environment. Solvent from the distillate receiver is then sprayed onto the work surface to aid in the removal of insoluble contaminants. When the spray cycle is completed, solvent vapor flows into the chamber from the boil sump of the system (Fig. 2-area 2). As the vapor enters the chamber, it condenses on the part dissolving the oils and greases. Condensate cleaning continues until the temperature of the parts being cleaned and the vapor temperature reach equilibrium. When equilibrium is reached, the cleaning cycle is completed. At the completion of the vapor cleaning cycle, a refrigerated cooling coil is activated and the solvent vapor present in the cleaning chamber is condensed to a liquid. The liquid is pumped into a distillate receiver and remains there, ready to be used in the spray portion of the next cleaning cycle. The next step in the process requires the desorption of solvent from a carbon mat medium. This solvent being desorbed was accumulated in the previous cleaning cycle. Desorption takes place by circulating hot air through the carbon purification unit (Fig. 2%area 3), which causes the solvent on the surface of the carbon to release in concentrated form to the hot airstream. The saturated airstream then is directed through a cooling unit (Fig. 2%area 4) and the solvent is condensed out. The destxbed solvent is directed through the water separator and into the distillate receiver. When desorption of the carbon media is completed, the solvent-laden air volume of the work chamber, which is still sealed, is recirculated through the carbon mat filters to remove
Fig.2.
148
Enclosed
vapor
degreaser.
solvent from the air. As the solvent/air mixture passes through the carbon medium, it is adsorbed onto its surface. This continues until the solvent concentration of the air in the work chamber reaches 1 gm/nG. When this level is reached, the entire cycle is completed, the seal on the cover is released, and the work can bc removed. The Battelle Institute was commissioned by the EPA to do a study on enclosed vapor degreasers. The study demonstrated that enclosed vapor degreasers reduce air emissions by 99% when compared with air emissions from typical open-top vapor degreasers. Battelle defined a typical vapor degreaser as having a 0.75 freeboard ratio, a primary cooling coil, electric hoist (to load and unload the degreaser), and no lip exhaust. Figure 3 (fugitive emissions) shows the relative concentration of perchloroethylene in the work chamber during the cleaning, desorption, and adsorption cycle versus the solvent concentration outside the degreaser at the sealed lid. The high point of concentration at the lip of the degreaser tank was 5 ppm, which occurred when the lid was open at the conclusion of the cycle. When the amount of oil or soluble contamination of the boil chamber reaches 30% by volume, the machine is placed into the distillation cycle. In this cycle, the solvent is heated in the boil chamber, creating a vapor and leaving the soluble contaminants (oil, waxes, etc.) behind. The vapor is directed through a refrigerated condenser coil and returns to a liquid state. The liquid is temporarily stored in a chamber of the degreaser until the distillation cycle is completed. When the distillation cycle is completed, the remaining contents of the boil chamber are allowed to cool and are then transferred to a suitable chamber and disposed of in accordance with federal, state, and local requirements. In addition to the EVD just discussed, there are versions that degrease by immersing the work to be cleaned in solvent. Immersion cleaning can be enhanced by adding ultrasonic and
injection flood wash agitation. Rotation of the work can also be employed to aid in cleaning and drying. Vacuum can also be employed to aid in the drying of the work.
SAFETY
AND
HANDLING
Chlorinated solvents have been used for many years with a low incidence of health-and-safety-related problems. The primary routes of exposure are inhalation and skin. Eye exposure is less common and oral ingestion is rare. Accidental swallowing of small amounts of any of the chlorinated solvents presents minimal hazard, but ingestion of substantial amounts can cause serious injury. To prevent accidental ingestion, always store solvents properly labeled and in containers that will not be mistaken for beverage use. If solvent is swallowed, do not induce vomiting, but obtain medical assistance. Prolonged and/or repeated skin contact with chlorinated solvents can extract fat from skin tissues and may cause dermatitis. Occasional contact should not produce adverse effects unless solvent is confined to the skin, for example under a ring or inside one’s clothing, and cannot evaporate, and which may result in a chemical bum. It is not likely that the solvents will be absorbed in acutely toxic amounts through the skin. Neoprene or polyvinyl alcohol gloves should be used for solvent handling. If skin contact occurs, flush skin thoroughly with water and wash contaminated clothing before reusing. Goggles should be worn when handling solvents. Splashing solvent into the eyes will produce pain and irritation, but no serious injury will result. In the case of eye contact, flush with running water for 15 minutes and then seek medical attention. The greatest potential hazard for exposure is from inhalation. Extremely high vapor levels can cause death. Inhaling excessive amounts of solvent vapor can produce anesthetic effects. These effects, such as lightheadness or dizziness, will occur at varying concentrations and durations of exposure, depending on the solvent used. Physiological injury may result from repeated or prolonged exposure at levels producing anesthetic effects. Excessive inhalation exposure can be prevented by proper engineering and/or ventilation in nearly all operations. Proper protection may require the use of respirators, hose masks, or self-contained breathing apparatus, depending on the operation being performed, when ventilation does not provide adequate protection. In the case of an inhalation exposure, remove the worker to fresh air and get medical attention. Apply mouth-to-mouth resuscitation of respiration stops. Do not administer stimulants, e.g., epinephrine. Exposure to the vapor of chlorinated solvents at the OSHA/ACGIH exposure limits or less should not cause adverse health effects.
A PROVEN
TECHNOLOGY
Enclosed vapor degreasing technology is proven. It has existed in Europe for more than a decade. EVDs are used to clean everything from aircraft parts to cosmetic jewelry containers. The good news it that for those manufacturers who wish to use vapor degreasing or for those who have no choice and must degrease, EVDs allow the process to be used in safety and in accordance with government mandates.
151