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Description  |
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BACKGROUND OF THE INVENTION
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.
SUMMARY OF THE INVENTION
The apparatus of this invention comprises an enclosure having openings both
at its top and bottom, the enclosure being of sufficient size to
accommodate temporarily one batch of parts to be cleaned or degreased at a
time. The enclosure may be sealingly connected to the top opening of
cleaning, degreasing or paint stripping tanks. Alternatively, new
cleaning, degreasing or paint stripping tanks may built with the enclosure
as part of the original equipment.
The enclosure further provides a pair of movable doors at its top opening
and another pair of movable doors at its bottom opening, these doors being
capable of being opened or closed manually or electromechanically. The
doors, when closed, isolate the enclosure from the tank below and from the
outside environment above. They should be reasonably vapor-tight, though
they need not be capable of withstanding any substantial differential
pressure.
The movable doors preferably are a pair of sliding panels which slide from
opposing sides of the top and bottom openings of the enclosure and meet
half-way in the middle. The doors may be rigid panels, vapor-impervious
cloth rolled up along the sides of the enclosure in the manner of window
shades, hinged elongated sections of rigid material such as thin-gauge
sheet metal forming a type of curtain, hinged swinging doors, etc. The
term "doors" is used herein to denote all types of closures at the top and
bottom of the enclosure.
Connected to the enclosure is a pipe or duct leading to the suction of an
exhaust blower which conveys air containing solvent vapors to a system for
recovering such solvent vapor, e.g. a refrigerated condenser with a liquid
separator; an activated carbon adsorption systems capable of carbon
regeneration and liquid solvent recovery; or an absorption/stripping
system using an inert solvent medium such as a hydrocarbon fraction having
an boiling point substantially higher than that of the cleaning solvent,
to facilitate separation of the two solvents by vaporization or
distillation.
Ambient air, or air having been freed from solvent vapor, is admitted to
the enclosure to replace the solvent vapor mixture with air which is
removed from the enclosure by the exhaust blower.
The method of immersion or spray cleaning with liquid solvent, vapor
degreasing, and paint stripping by immersion using the improved apparatus
of this invention involves the lowering of the parts to be cleaned or
stripped of paint, suspended from a cable or wire rope connected to a
hoist, into the enclosure and then into the tank, and lifting the parts
out of the tank into the enclosure and then into the environment, while
the upper and lower pairs of doors are opened and closed so as to entrap
air containing solvent vapor, which is conveyed from the enclosure to the
system for recovering such solvent vapor for re-use.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a preferred embodiment of the
apparatus of this invention featuring the enclosure in combination with a
degreasing tank and a system for recovering solvent vapor.
FIGS. 2a through 2g illustrate the sequence of steps used in introducing
parts to be cleaned or stripped of paint via the enclosure into a tank and
subsequently removing them therefrom.
DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION
With reference to FIG. 1, an enclosure 1 has side walls 2 and a pair of
doors 3 and 4 provided near its top opening, and a second pair of doors 5
and 6 provided near its bottom opening. Panel doors sliding from opposite
sides of the top and bottom openings of the enclosure 1 and meeting
substantially half-way in the middle, at the center of the opening, are
preferred. Alternatively the doors may be made of vapor-impervious cloth
rolled up along the sides of the enclosure in the manner of window shades,
capable of being unrolled to cover the openings of the enclosure; hinged
elongated sections of rigid material such as thin-gauge sheet metal
forming a type of curtain; or hinged swinging doors. Sliding doors are
preferred, especially for the bottom opening of the enclosure, since
sliding doors while being opened and closed cause less air turbulence than
hinged doors. This minimizes the amount of solvent vapor swept up into the
enclosure during the opening and closing of the bottom pair of doors.
The doors 3, 4, 5, and 6 may be opened or closed manually or by remote
control by electromechanical means. The doors may be provided with strips
of pliable gasket material along their edges so that, when the doors close
on the cable or wire rope 21 supporting the parts to be cleaned, a
reasonably vapor tight seal is made both along the edges of the doors and
around the cable or wire rope.
Means and methods for opening and closing the doors 3, 4, 5 and 6 are known
to the art and are not part of the present invention. Similarly, means of
reducing air and solvent vapor leakage around the edges of the doors,
between the pairs of doors when closed, and around the cable or wire rope
supporting the parts to be cleaned, also are known to the art. It will be
obvious to those skilled in the art that any gasket and packing materials
used on the lower set of doors 5 and 6 must be impervious to traces of
solvent vapors which, as is well known, penetrate and embrittle many
elastomeric materials. Viton is a generally acceptable gasket material.
The bottom opening of the enclosure communicates with the top opening of
the tank 7, the enclosure and the tank forming a unit. When an existing
tank is converted into the improved apparatus of this invention, an
enclosure of appropriate dimensions is mounted on top of the tank. FIG. 1
shows the enclosure in combination with a vapor degreasing tank. The
manner in which the enclosure is combined with a tank for immersion or
spray cleaning or for paint stripping is analogous.
Near the top of the tank 7, there is provided around the inside periphery
of the tank a set of pipe coils 9, which is cooled during operation with
cooling water or refrigerant. Optionally, below the bottom pair of doors 5
and 6 and above the pipe coils 9, a freeboard zone 8 may be provided,
whose function is similar to that of the freeboard zone in conventional
open-top tanks, i.e. to help reduce the upward transport solvent vapor, as
previously explained.
Below the pipe coils 9, there is a vapor zone 10 which, during operation,
is filled with pure solvent vapor. Below the solvent vapor zone 10, there
is a liquid solvent zone 11 containing liquid solvent maintained during
operation at its boiling point by a heating element 12.
Connected at one end to the enclosure 1 via its side wall 2 is a withdrawal
pipe 20, connected at its other end to the suction side of an exhaust
blower 21, which discharges through a pipe 22 to a system 23 for
recovering solvent vapor from air/solvent vapor mixtures and providing
recovered liquid solvent for re-use. This system may consist of a
refrigerated condenser with a liquid separator, in which the solvent vapor
is cooled, condensed, and separated as a liquid from air. The system may
consist of a carbon adsorption system, in which solvent vapor adsorbs on
activated carbon, which is later stripped of solvent by steam or hot
nitrogen, the stripped solvent then being cooled and condensed and the
liquid solvent condensate being collected and re-used; the stripping of
the solvent from the activated carbon and the recovery of liquid solvent
may take place in an integrated system having a plurality of activated
carbon vessels, or it may take place at a remote location, in which case
the carbon with adsorbed solvent is transported to the remote location in
a canister. The system may consist of an absorption/stripping systems
employing an inert solvent medium such as a hydrocarbon fraction having a
boiling point at least 30 degrees Celsius higher than that of the cleaning
solvent; the solvent being absorbed into the inert solvent medium
substantially at room temperature and subsequently stripped from the inert
solvent medium at elevated temperature, either in a batch or in a
continuous system. Such systems are known to the art. Solvent-free air is
discharged to the atmosphere via a pipe 24, and recovered liquid solvent
flows via a pipe 25 to a storage tank 26 for clean solvent or via a pipe
27 back into the pool of liquid solvent in the liquid zone 11 at the
bottom of the degreasing tank.
To replace the solvent vapor mixture with air removed during operation from
the enclosure 1 by the exhaust blower 11, fresh air is admitted into the
enclosure 1 via pipe 28, connected to the enclosure 1 opposite the
connection of pipe 20 to allow thorough purging of the enclosure.
Alternatively, solvent-vapor-free air from pipe 24 is recirculated and to
this end, pipe 24 may be connected to pipe 28 (interconnection not
illustrated).
Parts to be cleaned 30 are lowered into the apparatus and lifted out after
cleaning by a hoist system 31 employing a flexible cable or wire rope 32
to support the parts to be cleaned, either directly or by a basket, as is
known to the art.
The sequence of operations during the cleaning of parts with a solvent in
the apparatus of this invention is illustrated by FIGS. 2a through 2g.
FIG. 2a shows the parts 19 being lowered into the enclosure 1 via the top
opening, with doors 3 and 4 open. FIG. 2b shows the parts 30 suspended in
the enclosure 1 with both sets of doors closed. FIG. 2c shows the lower
set of doors 5 and 6 open and the parts 30 being lowered into the tank.
While the lower pair of doors is open, some solvent vapor is swept up into
the enclosure 1 by the turbulence created by the lowering of the parts 30.
Hinged doors, while being opened and closed, generate more turbulence than
sliding doors. By generating less air turbulence, sliding doors cause less
solvent vapor to be swept up into the enclosure and allow the apparatus to
operate more efficiently. Sliding doors are therefore preferred to hinged
doors.
FIG. 2d shows the parts 30 suspended in the tank with both pairs of doors
closed. At this time, the parts are cleaned or stripped of paint by the
solvent. Optionally, at this time, the air in the enclosure 1, having been
previously contaminated with some solvent vapor, may be conveyed by the
exhaust blower 21 to the solvent recovery system 23 for recovering solvent
vapor as described above. Alternatively, the contaminated air in the
enclosure may be kept isolated at this time without such solvent removal
and recovery until later.
FIG. 2e shows the parts being lifted from the tank. At this time, more
solvent vapor air is swept up into the enclosure due to the turbulence
caused by the motion of the parts.
FIG. 2f shows the parts suspended in the enclosure 1 with both pairs of
doors closed. At this time, the solvent-contaminated air in the enclosure,
is conveyed by the exhaust blower 21 to the solvent recovery system 23 and
solvent is collected for re-use. Any droplets of liquid solvent adhering
to the parts vaporize and the parts are dried. The enclosure 1 at this
time is under a very slight vacuum due to the sucking action of the
exhaust blower 21 so that any small amount of leakage around the edges of
the doors 3, 4, 5 and 6 causes air and solvent vapor flow to the recovery
system 23, and no solvent vapor escapes from the apparatus to the
surrounding atmosphere.
FIG. 2g shows the parts being lifted out of the enclosure 1. Note that the
air in the enclosure, before the upper set of doors 3 and 4 is opened, has
been freed of solvent vapor so that, when the air from the enclosure mixes
with ambient air as the parts are lifted out of the enclosure, no solvent
vapor whatever is discharged to the environment.
It will be appreciated that, with the apparatus and method of this
invention, all the solvent vapor which would escape to the atmosphere from
conventional solvent cleaning, vapor degreasing or paint stripping
equipment is captured and conveyed to the system for recovering solvent. A
highly efficient system for recovering solvent vapor, such as carbon
adsorption system, may be expected to recover 98-99% of the solvent for
re-use. The net loss of solvent vapor thus is cut by a factor of 50-100
with respect to conventional cleaning, vapor degreasing or paint stripping
equipment. The following examples will illustrate the benefits of the
apparatus and method of this invention.
EXAMPLE 1
Parts are stripped of paint by immersion in liquid methylene chloride at 20
degrees Celsius. The vapor in the tank above the pool of liquid has a
partial pressure of methylene chloride of 340 mm Hg, corresponding to a
methylene chloride concentration of about 45 percent by volume, the rest
being air. The slightest disturbance in the air sends a large amount of
methylene chloride vapor via the top opening of the tank into the
atmosphere. 70-90 percent of the methylene chloride used for paint
stripping may be expected to end up in the atmosphere. By employing the
apparatus and method of this invention, the loss of methylene chloride
vapor is reduced to about 1 percent of methylene chloride fed to the tank.
EXAMPLE 2
Trichloroethane is used for vapor degreasing of metal parts. The liquid
solvent at the bottom of the tank, and the vapor zone above it, are at
74-78 degrees Celsius. A water-cooled pipe coil and a freeboard zone are
provided, the coil being at 20 degrees Celsius, and the freeboard zone at
25 degrees Celsius. The trichloroethane vapor concentration in the
freeboard zone, corresponding to the saturation vapor pressure of the
liquid solvent of 125 mm Hg, may be as high as 16 percent by volume. Fifty
percent of the trichloroethane used in the degreaser may be expected to
enter the atmosphere via the top opening. By using the apparatus and
method of this invention, the loss of trichloroethane vapor is reduced to
0.5-1 percent of the amount of trichloroethane used in the degreasing
tank.
Although the invention has been described relative to a specific embodiment
thereof, it is not to be limited to such and numerous variations and
modifications thereof will be readily apparent to those skilled in the art
in the light of the above teachings. It is therefore to be understood that
within the scope of the appended claims, the invention may be practiced
otherwise than as specifically described.
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