Method for volatile organic compound recycling

In the method of the instant invention, volatile organic compounds (VOCs) liberated in the course of solvent-based painting of a workpiece, are recovered. The atmosphere within a closed spray-booth is maintained at a fixed humidity such that vaporized water, supplied in the form of steam or nebulized water vapor, acts as a carrier for VOCs liberated in the course of spray painting or heat curing of a workpiece. The VOCs may be recovered continuously, including during the spray-painting or heat curing of the workpiece. The VOC laden, humidified air is circulated through a condenser such that VOCs dissolved in the water vapor condense and are directed to a recovery means. The water and solvent are separated, recovered and purified for re-use or placed in an appropriate container for disposal, thereby eliminating the usual practice of expelling VOC laden air into the atmosphere. The system is provided with a flame-free heat source to cure the painted workpiece and to guard against the danger of explosion.

BACKGROUND OF THE INVENTION 
This invention concerns a method and system for recycling volatile organic 
compounds (VOCs) used in solvent-based paints. The system and method is 
applicable whenever a closed chamber or booth is used for spray-painting a 
workpiece, such as a car, to eliminate venting of VOCs to the atmosphere 
while capturing the VOCs for recycling. 
Present day paint spray-booths need to be designed to provide a safe 
working environment, to prevent pollution of the atmosphere, and to 
enhance the quality of the product being sprayed. In general, paint 
spray-booths are equipped with a constant flow of air into and out of the 
spray-booth to maintain a reduced level of VOCs in the internal 
atmosphere. This arrangement has led to a number of attempts to reduce the 
level of VOCs in the exhaust air to minimize environmental pollution. 
One such attempt involves the use of aqueous-based, rather than 
solvent-based paints. Such efforts have generally produced inferior paint 
results and excessively long periods for drying of the workpiece. Limiting 
the amount of VOCs in solvent based paints, as reported by at least one 
manufacturer, does help to reduce VOC emissions. However, substantial VOC 
emissions still occur when using such paints, and a more comprehensive 
solution is required. 
Another attempt is found in U.S. Pat. No. 4,261,707, which discloses a 
complex process and system for elimination of paint solvent vapors from an 
automobile paint spray-booth and curing oven to maintain a healthful and 
clean environment for workers in the paint booth. The process and 
apparatus involves vacuum assisted vaporization of solvents from scrubber 
water for subsequent condensation and recovery of the solvent. 
In U.S. Pat. No. 4,444,573, a method was disclosed for using a hydrotropic 
substance to decrease the level of an organic solvent in an air stream 
from a paint spray-booth. A hydrotrope is defined as a chemical substance 
which includes an organic group chemically bonded to a polar group. The 
function of the hydrotrope is to increase the solubility of a volatile 
solvent in water. The method involves (a) contacting of solvent laden air 
from a paint spray-booth with a spray of aqueous hydrotrope, (b) 
extracting solvent from the aqueous hydrotrope using an organic oil, and 
(c) stripping the solvent from the oil using a steam stripper followed by 
condensation and recovery of the solvent. 
In U.S. Pat. No. 4,620,858, related to the U.S. Pat. No. 4,261,707 patent 
discussed above, a method and system is disclosed which achieves contact 
between solvent from paint spray and an organic solvent absorbing liquid, 
followed by regeneration of the solvent absorbing liquid. This contact is 
achieved in a chamber separate from the chamber in which the spraying 
operation is conducted. 
In U.S. Pat. No. 4,927,437, a method and apparatus for removing particles, 
including solvent droplets, from moving air in a paint spray-booth. The 
apparatus involves a cyclonic separator. 
In the instant invention, a closed system is provided such that, in 
general, air is not vented to the atmosphere during paint spray operation. 
Rather, a worker in the closed system is provided with an independent 
source of air, such as grade D fresh air supplied respirators, thus 
complying with health and safety requirements of the Occupational Safety 
and Health Administration (OSHA). The use of a supplied air respirator is 
already mandated by OSHA (29 CFR 1910.134d1). The instant system involves 
direct condensation of VOCs from the humidified paint booth atmosphere and 
direct condensation of solvent laden water vapor, rather than vacuum 
assisted vaporization and subsequent condensation. A controlled 
temperature and humidity environment for optimal painting conditions is 
thereby provided while at the same time reducing environmental pollution. 
BRIEF SUMMARY OF THE INVENTION 
In the method and system of the instant invention, volatile organic 
compounds (VOCs) liberated in the course of solvent-based painting of a 
workpiece, are recovered. The atmosphere within a closed spray-booth is 
controlled at a fixed humidity such that vaporized water, supplied in the 
form of steam, acts as a carrier for VOCs liberated in the course of spray 
painting or heat curing of a workpiece. The VOCs may be recovered 
continuously, including during the spray-painting or heat curing of the 
workpiece. The VOC laden, humidified air is circulated through a condenser 
such that VOCs dissolved in the water vapor condense and are directed to a 
recovery means. The water and solvent are separated, recovered and 
purified for re-use or placed in an appropriate container for disposal, 
thereby eliminating the usual practice of expelling VOC laden air into the 
atmosphere. The system is provided with a flame-free heat source to cure 
the painted workpiece and to guard against the danger of explosion. 
Thus, it will be appreciated by those skilled in the art that the instant 
disclosure has several objects. One object is to provide a paint 
spray-booth having a controlled temperature and humidity so that those 
working in the field will no longer be at the mercy of the elements when 
it comes to spray-painting a workpiece under optimal conditions of 
temperature and humidity. 
Another object of this invention is to provide a closed system paint 
spray-booth which substantially reduces emissions into the atmosphere of 
volatile organic compounds. 
Another object of this invention is to provide a paint spray-booth which 
will enable those working in the field to comply with local, state and 
federal clean air directives while at the same time providing an 
efficient, cost-effective and safe working environment for those working 
in the spray paint industry.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention involves a closed system spray-booth which provides a 
thermostatically and humidistatically controlled environment for painting 
a workpiece and which recovers volatile organic solvents released into the 
air within the spray-booth during spray-painting and curing of a 
workpiece. The invention comprises a chamber within which a workpiece is 
spray-painted and cured, and at least one associated housing containing 
air treatment components. The chamber comprises: 
(a) A sealable door through which a workpiece to be painted may be conveyed 
into and out of the chamber, a floor, a roof and walls; 
(b) at least one sealable door through which a person who will spray-paint 
the workpiece may enter and leave the chamber; 
(c) at least one source of supplied air for the person to use while present 
in the chamber; 
(d) a means for circulating air within the closed system spray-booth such 
that the atmosphere within the booth is constantly recirculated between 
the chamber and the at least one housing containing air treatment 
components; 
(e) a means for reducing or eliminating particulate matter from air as it 
passes from the chamber to the at least one housing containing air 
treatment components; 
(f) a humidistat for measuring the level of humidity in the chamber at any 
given time, wherein the humidistat is operationally linked with a source 
of water vapor located in the at least one housing containing air 
treatment components such that when the humidity in the chamber falls 
below a pre-set limit, additional water vapor is added to the air entering 
the chamber. 
The at least one housing containing air treatment components comprises: 
(g) a solvent recovery unit which comprises a condenser and a means for 
directing condensate to a means for recovering the condensed solvent; 
(h) a source of water vapor for maintaining the interior of the chamber in 
a humidified state; and 
(i) a thermostat regulated flame-free heat source to cure the painted 
workpiece. 
Air is constantly recirculated between the chamber and the at least one 
housing containing air treatment components which maintain the temperature 
and humidity of the air within the chamber within pre-set limits. The air 
treatment components constantly remove volatile organic compounds released 
into the air during spray-painting and subsequent curing of the painted 
workpiece. Only one housing containing all of the air treatment components 
(g), (h), and (i) need be provided while a second housing may provide all 
of these elements or only element (i) to ensure even heating of the 
workpiece. 
The operation of this system and its preferred embodiments may be best 
understood by referring now to FIG. 1. The paint spray-booth 10 of this 
invention is a sealed enclosure having four walls, a floor, a roof, at 
least one sealable door for a worker to enter and leave the booth, which 
may also be a double door with an interlock to minimize escape of internal 
air, and a sealable door 11 through which a workpiece to be painted is 
conveyed into the spray-booth. In operation, once the workpiece has been 
brought into the spray-booth 10, the door 11 is sealed and air 
recirculation within the booth is begun as described further below. 
Because the system is closed and fresh air from the outside is not 
supplied during operation, a person working in the booth must be equipped 
with a supplied air breathing system. Manufacturers of such units exist 
such that a variety of different units are available. For example, Martech 
Services Company produces a number of different supplied air breathing 
systems which comply with OSHA requirements and which supply grade D air 
to either one or several different supplied air respirators at the same 
time. Those units are equipped with alarm lights and audible horns. In 
addition, in using any such system in the spray-booth of the instant 
invention, in a preferred embodiment, an interlock may be provided such 
that unless the worker is using an operational supplied air respirator, 
the interlock prevents operation of the spray equipment such that no VOCs 
can be released into the work space until an appropriate breathing 
apparatus is employed. 
The internal air is recirculated by positive pressure and thereby induced 
to pass through a filter system 12 and into a housing 13 which contains 
air treatment components and which forms a duct system. Once a worker 
begins spray-painting the workpiece, such as a passenger vehicle, a piece 
of furniture or any other workpiece which requires a VOC solvated paint, 
spray particles in the air are trapped on the filters which comprise the 
filter system 12. According to an alternate embodiment, the particles may 
be removed from the air by a conventional air scrubber which forms a water 
and paint emulsion. This is less preferred because a large volume of 
contaminated water is thereby produced which also contains dissolved VOCs. 
Once the particulates are removed from the air, the solvent-laden air 
passing into the housing 13 experiences a different set of conditions 
depending on which side of the spray-booth is being considered. As shown 
in FIG. 1, there is a different set of components on the left side as 
opposed to the right side. This is a matter of choice and economy, and 
those skilled in the art will appreciate that the booth could operate with 
identical components on both sides of the booth. 
In the specific embodiment shown in FIG. 1, air passing through the filter 
system 12 on the right side of the booth first encounters condenser coils 
14 which condense any humidity and VOCs dissolved in the water vapor as 
the air passes over the coils. The condensate drains from the condenser 
coils 14 to a storage and solvent recovery unit 15 which is described 
further below. The condensation coils 14 are part of a condenser unit 
which is conventional in the art of air-conditioning and therefore 
requires no further description here except to say that it is preferred 
that the coils be disposed at an angle so that as condensate collects on 
the coils, it runs down the coils and drips into a receptacle for 
conveyance to the storage and solvent recovery unit 15. 
After dehumidification and removal of the VOCs entrained therein, the air 
is directed via the housing 13, which, as noted above, acts as a duct 
system for the air, into an area where a source of water vapor is added to 
the air by a humidistatically controlled water vapor generating device 16. 
Such devices are conventional in the art and need little further 
description here. Such devices as humidistatically controlled steam 
generators, sonic vapor generators or any like device which efficiently 
produces water vapor is acceptable. A commercially available 
humidistatically controlled steam generator is produced by AUTO-FLOW as a 
1400 watt, 120 volt unit (model ESU-14) and as a 2000 watt, 220 volt unit 
(model ESU-20). Incorporation of such a device into the paint spray-booth 
of this invention is therefore contemplated. Preferably, the humidistat 
provides for precise control of the relative humidity within the booth. 
During the spray-paint operation, it is desirable to maintain a relative 
humidity in the range of about 45% to 55% relative humidity. This allows 
for optimal performance of the VOC based paints and provides sufficient 
humidity in the air to allow for efficient condensation and removal of 
dissolved VOCs. Those skilled in the art will appreciate that water 
additives may be included in the water, such as the hydrotropic substances 
described in U.S. Pat. No. 4,444,573, herein incorporated by reference for 
this purpose. During the drying cycle of the workpiece, when the 
temperature in the paint spray-booth is elevated to cure the paint (see 
further description below), it is desirable to maintain a relative 
humidity in the range of about 20% to 45%. Thus, it is desirable to equip 
the water vapor generating device 16 with a humidistat that may be easily 
set to maintain the humidity within either of these preferred operating 
relative humidity ranges. Control of the humidity and removal of the 
entrained VOCs has several additional advantages. Proper regulation of the 
humidity avoids dye-back, solvent popping and reduced lustre, which are 
all problems encountered in conventional paint spray-booths due to 
excessive humidity. In addition, proper regulation of the humidity 
increases the efficiency of the cure cycle such that less heat needs to be 
supplied. Thus, in a conventional paint spray-booth for automobiles, 
typically heat is supplied by a 1.5 million BTU direct fired gas furnace. 
In the instant invention, adequate heating may be achieved with a 
residential, oil-fired water heater (see further description below). 
Once the air passes through the region in the housing 13 where water vapor 
is added, it is then forced into the a thermostatically controlled 
flame-free air-heating device 17. The device 17 may be any flame-free air 
heating device. However, since such devices were not readily commercially 
available, the instant disclosure provides for one such system which we 
installed for this purpose (see further description below). Air passing 
through the device 17 is heated to such an extent that the temperature 
within the paint spray-booth is maintained within desirable operating 
ranges both during the workpiece painting and curing cycles. During the 
painting cycle, the temperature is optimally maintained between about 
60.degree. F. and 90.degree. F., and most preferably at about 85.degree. 
F. During the curing cycle, the temperature is maintained at about 
125.degree. F. to about 145.degree. F., and most preferably at about 
135.degree. F. to about 145.degree. F. It is desirable that there be an 
air-heating unit on both sides of the paint spray-booth to maintain even 
heating of the workpiece, particularly during the curing cycle. As noted 
above, only one condenser and one source of water vapor need be used on 
one or the other side of the booth, although these elements may also be 
provided on both sides. 
Once the heated air leaves the flame-free air heating device 17, it is made 
to move into an air handling and fan assembly 18, mounted on top of the 
spray chamber, through a duct system 42. This assembly provides the source 
of positive pressure which causes the air trapped within the sealed paint 
spray-booth 10 to circulate through the filters and the other 
aforementioned components. The air, as it reaches the assembly, is in a 
heated and humidified state, having been stripped of VOCs. It is drawn 
back into the paint spray-booth to once again pick up VOCs as they are 
released during either the paint spray operation or the curing of the 
painted workpiece. For the purpose of the air handling system, a device 
such as a squirrel-cage fan operating at between about 9,000 to about 
12,000 rpm is adequate. The lowest possible air movement that achieves 
efficient removal of overspray and circulation of VOC laden water vapor is 
preferably used to optimize even paint application. In the instant system, 
only one fan for recirculating the air is needed, as an exhaust fan which 
drives VOC laden air into the atmosphere is not required. All that is 
needed is about a half of one pound of positive pressure to be produced to 
efficiently recirculate the air to achieve about 9,000 to about 12,000 
cu.ft/min. circulation (the internal dimensions of a typical automotive 
paint spray-booth are 23 ft by 14 ft by 9 ft=2898 cu.ft.). 
The above mentioned filter system 12 is preferably composed of a plurality 
of high-efficiency replaceable filter units. In one embodiment of this 
invention, the filter units are preferably comprised of an expanded 
polystyrene "pre-filter" and a woven DACRON filter. The polystyrene 
pre-filter is used to trap the bulk of the paint particles as they are 
drawn into the filter system 12. The relatively particulate-free air is 
then drawn into the DACRON filter material to remove any residual 
particulates. This particular arrangement is preferred because the 
polystyrene material is relatively inexpensive and can be disposed as a 
small volume of hazardous waste upon "melting-down" with a small volume of 
lacquer or other available solvent. 
The solvent storage and recovery device 15 can be any sealed unit in which 
the various components of the condensate from the condenser 14 can be 
safely stored. In the instant invention, a particular storage chamber 
which also functions as a solvent recovery system has been devised and 
implemented. According to this embodiment of the invention, the solvent 
recovery device 15, which is shown in some detail in FIG. 2, comprises a 
cylinder 19 having a volume of about seven gallons, having a first inlet 
tube 20 which conveys condensate from the condenser 14 into the solvent 
recovery device 15, a second exit tube 21 which runs from the solvent 
recovery device 15 to a storage vessel. The cylinder 19 is provided with a 
sealed lid 12 through the center of which is a gasketed aperture through 
which a rod 13 is slidably fixed. At the top end of the rod 23 on the 
outside of the cylinder 19 is provided a handle 24. At the bottom end of 
the rod 23 is provided a gasketed plunger 25. In operation of this solvent 
recovery device, the plunger 25 is initially depressed to the bottom of 
the cylinder 19. As condensate enters the cylinder 19 through the first 
inlet tube 20, the cylinder begins to fill. All the while that the 
cylinder is filling with condensate, water and entrained solvent begin 
their natural process of separation, with the less dense solvent floating 
to the top. After the cylinder has become quite full, the plunger 25 is 
pulled up through the gasketed aperture in the lid 22 which forces solvent 
out of the second exit tube 21 to be collected in a storage vessel. 
According to this embodiment of the invention, it is desirable for the 
cylinder 19 to be a transparent, solvent resistant material such as glass. 
In this way, as the solvent water interface is brought to the top of the 
cylinder through the upward motion of the plunger 25, the process may be 
halted and the lower, water phase may be returned to the water vapor 
generating system 16 to be reused to trap additional VOCs. To provide for 
efficient sealing, bolts 26 are provided on either side of the cylinder 
with butterfly nuts 27 holding the lid down. The bottom ends of the bolts 
26 are set in the base 28. To prevent backing up of solvent into the inlet 
tube, a closure device 29 is provided. Solvent recovered in this manner 
may be further purified in a conventional, commercially available solvent 
purification system such as that produced by Lenan Corporation under the 
trade name RECYCLIT, model SR-80 or like solvent recycling means. 
The disposition of several of the above mentioned air-treatment components 
within the housing 13 is shown in greater detail in FIGS. 3 and 4. These 
figures show air entering the housing through the filter system 12. The 
air is forced to move upward through the condenser 14 by the suction 
created by the fan assembly 18 and the seal created by baffles 30. As 
moisture containing VOCs condenses on the condenser 14, it drips into the 
reservoir 31, from which it is conducted via line 20 to the solvent 
recovery unit 15. Lines 32 and 33, for conveying coolant to and from a 
compressor 34 are shown. 
Once beyond the condenser 14, the air passes a source of water vapor 16 
which distributes the water vapor into the moving air via a steam wand 35 
or like means. In one embodiment, the source of water vapor 16 is a steam 
generator having an upper fill compartment 36, which can receive water 
retrieved from the solvent recovery unit 15. The upper fill compartment 36 
is connected to a lower, steam generator 37 via a hose 38. The steam 
generator 37 may, in addition, be equipped with a water line and a 
constant water level maintained by a float shutoff valve or electronically 
according to methods standard in the art. It will be appreciated by those 
skilled in the art that the steam generator may be, instead, a nebulizer 
or an ultrasonic generator for production of atomized water. 
Once the air passes the water vapor generator 16, it passes thorough the 
flame-free air heating device 17, which may be comprised of a series of 
coils 39 through the inside of which heated liquid is passed from a remote 
liquid heating unit via inlet and exit pipes 40 and 41. Within the housing 
13, as the recirculating air is passed through the heated coils, the 
humidified air becomes heated and enters the paint spray-booth as 
described above. In a particular embodiment of this invention, the liquid 
heating unit is a remote oil-powered 185,000 BTU water heater. Use of an 
oil fired unit is advantageous as it allows for better per day 
calculations of fuel use, although other fuel sources such as natural gas, 
propane or wood could be employed in an appropriately adapted boiler. To 
the circulating water, an appropriate concentration of polyethylene glycol 
or like material is added to elevate the boiling point and depress the 
freezing point of the water. The liquid is heated to about 238.degree. F. 
and then conveyed through insulated 1.25 inch copper pipes to the coils 
internal to the flame-free heating device 17 and from there, back to the 
remote heating unit. Once the air has passed through the flame-free heater 
17, it is conducted back into the sealed chamber through duct work 42 
which connects the housing with the chamber, via the fan assembly 18. 
The housing 13, and indeed the entire booth 10, may be insulated with about 
1.5 inch thick high-density foam, with an R rating of about 18 to reduce 
heat loss and thereby decrease the cost of heating. 
It will be appreciated from the foregoing description of this invention 
that a novel, sealed paint spray-booth has been provided which 
substantially reduces the level of VOCs emitted into the atmosphere as a 
result of the use of solvent based spray paints. In addition, it will be 
appreciated that in a simple and cost effective fashion, a conventional 
paint spray-booth may be equipped with the various aforedescribed 
components so as to operate in an optimally thermostated and humidistated 
environment for optimal painting performance. Thus, both newly 
manufactured and retrofitted pre-manufactured or pre-fabricated paint 
booths comprising the elements herein described and claimed naturally come 
within the scope of this invention. While the invention has been described 
with respect to its preferred embodiments, and while specific details have 
been provided to enable those of ordinary skill in the art to comprehend 
the involved principles, it would be obvious to those skilled in the art 
from the foregoing disclosure that alternate embodiments may be operated 
in equivalent fashion to that described herein. Thus, the invention is not 
to be construed as being limited to the disclosed specifics herein. The 
invention as herein described is only limited by the appended claims and 
equivalents thereof. 
References 
Bradshaw, Norman F., and Ivan Bloomer, U.S. Pat. No. 4,261,707, issued Apr. 
14, 1981. 
Cosper, David R., and Gretchen L. McKay, U.S. Pat. No. 4,444,573, issued 
Apr. 24, 1984. 
Bradshaw, Norman F., and Ivan Bloomer, U.S. Pat. No. 4,620,858, issued Nov. 
4, 1986. 
Richerson, Ben M., U.S. Pat. No. 4,927,437, issued May 22, 1990.