Abstract:
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 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.

Description:
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 of volatile organic compounds into the atmosphere. 
     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. 
     Other objects and advantages of this invention will become apparent from the full disclosure which follows. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a top view of the paint spray-booth of this invention. 
     FIG. 2 is a schematic representation of the water decontamination unit of this invention. 
     FIG. 2 a  is perspective view of an alternative representation of the water decontamination unit of this invention. 
     FIG. 3 is a schematic view of the housing containing air-treatment components as it would appear viewed through the wall of the paint spray-booth to which the housing is attached. 
     FIG. 4 is a side sectional view of the housing containing air-treatment components as it would appear viewed from the side of the paint spray-booth to which the housing is attached. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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 water decontamination unit which comprises a condenser and a means for directing condensate to a means for neutralizing 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 re-circulation 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 FIGS. 1,  3  and  4 , air passing through the filter system  12  on the right side of the booth. Upon exiting the filter system  12  the air flow is heated, via a steam generator  37 , to 290° F. The steam is piped from the steam generator  37  through insulated piping  44  to the first steam wand  45 . The 290° F. steam neutralizes many of the VOCs, transforming them into CO 2  and H 2 O. The air flow then 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 water decontamination 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 60% to 65%. 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° F. and 90° F., more preferably between about 70° F. and 80° F. and most preferably at about 72° F. During the curing cycle, the temperature is maintained at about 115° F. to about 145° F., and most preferably at about 120° F. to about 130° 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 . Additionally, to provide the least hostile environment to personnel working in the paint spray-booth  10 , a carbon or charcoal filter  46  is mounted in the duct system  42 . The filter  46  is to remove any remaining particulate matter from the air flow. The filter  46  is intended to be used only when personnel are working in the paint spray-booth  10 , and to be rotated out of the air flow when personnel are not in the paint spray-booth  10 . A rotation assembly  48  is mounted inside the duct system  42 . An external handle  51  may be provided to rotate the assembly, or the assembly may be designed to automatically rotate by a control unit which triggers rotation upon certain monitored events. The filter  46  is mounted to the rotation assembly  48 . A baffle  47  is mounted perpendicular to the filter  46  on the rotation assembly  48 , so that it is rotated into the air flow when the filter  47  is not in use. The baffle  47  maintains the same air flow rate as was present when the filter  46  was in the air flow. 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 over spray 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 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 woven 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 “meltingdown” with a small volume of lacquer or other available solvent. 
     The water decontamination unit  15  can be any sealed unit in which the various components of the condensate from the condenser  14  can be safely stored and the entrain solvent removed from the water. According to this embodiment of the invention, the water decontamination unit  15 , is a self contained free standing unit which is shown in some detail in FIG. 2, comprised of a three chambered system. The condensate from the condenser  14  is pumped  27  into the liquid level controlled wastewater tank  19  (e.g., 5 gallon), via an inlet tube  20 . While the wastewater tank  19  is filling with condensate the water and entrained solvent begin their natural process of separation, with the less dense solvent floating to the top. The water and entrained solvents remain in the wastewater tank  19  until the level rises to that of the UV chamber  28 . The UV chamber  28  preferably comprises a flat aluminum surface  23  impregnated with an ultraviolet (UV) reflective material, such as titanium dioxide, and a UV light source  24  situated above the flat surface  23 . The flat surface  23  and UV light source  24  span the width and length of the UV chamber  28 . Once the level in the wastewater tank  19  reaches the UV chamber  28 , the water and entrained solvents spill over into the UV chamber  28 . The water and entrained solvents flow over the impregnated flat surface  23  and are simultaneously subjected to a continuous level of UV light from the UV light source  24 . The UV light will tend to neutralize the entrained solvents in the water. Upon exiting the UV chamber  23  the treated water and any remaining solvents flow into the feed tank  22  (e.g., 15 gallons). The treated water will be stored in the feed tank  22  until it is reintroduced into the system. 
     In order to remove any remaining solvents present in the treated water, the feed tank  22  is lined with a disposable solvent absorbing urethane liner  29 . The treated water will remain in the feed tank  22  until the water level rises sufficiently to activate an outlet pump  26 , via a float activated switch  25 , for example. Once activated, the outlet pump  26  transfers the treated water via an exit tube  21  through a disposable filter  43 , preferably charcoal or carbon, to the upper fill compartment  36  for reintroduction into the system. The disposable filters  43  tend to remove any remaining particulate matter in treated water so as to maintain the operational integrity of the steam generators  37 . 
     In an alternative embodiment, the water decontamination unit  15  in a self contained free standing unit is depicted in FIG. 2 a.  The water and entrained solvent are pumped from the condenser  14  into the wastewater tank  19 . The water and entrained solvent will remain in the wastewater tank  19  until the waste and entrained solvent reach a sufficient level to activate the wastewater tank pump  49 . The wastewater tank pump  49  transfers the water and entrained solvents from the wastewater tank  19  into the spray manifold  50 . The spray manifold  50  uniformly distributes the water and entrained solvents over the flat aluminum surface  23 , where they are treated with UV light, from the UV light source  24 . The treated water flows into the carbon filter feed tank  22 . The treated water will remain in the feed tank  22  until the water level rises sufficiently to activate the outlet pump  26 . Once activated, the outlet pump  26  transfers the treated water via an exit tube  21  through a disposable filter  43 , preferably charcoal or carbon, to the upper fill compartment  36  for reintroduction into the system. The disposable filters  43  tend to remove any remaining particulate matter in treated water so as to maintain the operational integrity of the steam generators  37 . 
     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 . Upon exiting the filter system  12  the air flow is heated by the first steam wand  45  to 290° F. 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 is conducted via line  20  to the water decontamination 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 second 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 and atomized water. 
     Once the air passes the water vapor generator  16 , it passes through 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° 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.