CA2179804C

CA2179804C - Method for voc abatement and paint spray booth incorporating such method

Info

Publication number: CA2179804C
Application number: CA002179804A
Authority: CA
Inventors: Thomas E. Grime; Andrew P. Gargac; Larry E. Campbell
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1995-07-31
Filing date: 1996-06-24
Publication date: 2000-08-22
Anticipated expiration: 2016-06-24
Also published as: CA2179804A1; TW360556B; KR100445013B1; KR970005365A; JPH0938484A; MX9602147A; EP0756885A2; US5968235A; US5814132A; EP0756885A3

Abstract

A VOC (volatile organic compounds) adsorbent material such as an alumino-silicate gel desiccant is treated with a masking agent to reduce its affinity for VOC's to a level that the material can be regenerated at a predetermined low temperature which will not desorb the masking agent. The treated adsorbent material is used to adsorb VOC's and is periodically regenerated with heated air. The treated adsorbent material may be used to adsorb VOC's from recirculated spray booth air while spraying a coating on a workpiece. Subsequent to spraying, the spray booth air is heated to above the VOC regeneration temperature to regenerate the adsorbent material and optionally to cure the applied coating. As the heated spray booth air is circulated through the adsorbent material, the previously adsorbed VOC's are desorbed. The desorbed VOC's may be destroyed by oxidizing, for example, with a catalyst.

Description

METHOD FOR VOC ABATEMENT A~D
PAI~T SPRAY BOOTH INCORPORATING SUCH METHOD
Technical Field The invention relates to VOC (volatile organic compounds) abatement 5 and more particularly to an improved method for abating VOC emissions from sources such as paint spray booths and to a paint spray booth incorporating such method.
Background Art Governments have become increasingly concerned with and have 10 increasingly regulated the emissions of VOC's such as organic paint and adhesive solvents and organic fabric dry cleaning solvents. Typical organic paint solvents include MEK, xylene, toluene, isopropyl alcohol and lacquer thinner. Two methods have been used in the past to abate the discharge into the atmosphere of VOC emissions from paint spray booths. According to one 15 method, at least a portion of the air circulated through a paint spray booth is withdrawn and passed through a burner which incinerates the VOC's. Direct incineration of the VOC's can consume a large amount of energy, since the e,xhausted air must be heated to approximately 1500F to 1700F (815C to 925C) during the entire time that the VOC's are emitted. Further, in colder 20 climates it may be necessary to heat a large amount of make up air which must be supplied to the spray booth to replace the exhausted air.
A second VOC abatement method involves collection of the VOC's in a filter bed, such as a charcoal filter bed. However, the filter bed either mustbe frequently replaced and properly disposed of or must be periodically 2 5 regenerated by desorbing the accumulated VOC's. A charcoal filter bed requires about 600F (about 31 5C) for regeneration. When a charcoal filter bed is used for collecting VOC's, there is a risk that an exothermic reaction can occur, creating sufficient heat to cause combustion. It also is known that certain adsorbent materials such as zeolite have a high molecular affinity for 30 VOC's and other vapors and gases, and may be ~lsed as a filter material for separating components of a gaseous mixture. A zeolite filter typically requires heating to about 300F to 350F (about 150C to 175C), or more, for regeneration. Because of the intense heat required for regeneration, it was necessary to transfer a carbon filter bed or a zeolite filter bed from the 35 -adsorption location in a spray booth to a separate furnace for regeneratiOn.

2 1 79~J4 Separate heat sources ~vere required for heating the paint in the spray booth tocure the applied paint and for regenerating the filter.
It also is known that VOC's can be eliminated from a gas stream by oxidation ~vithout reaching the combustion temperature. The VOC vapor in a 5 gas stream can be oxidized by heating and contacting with a catalyst such as platinum. Typically, the catalyst is coated on a porous substrate, such as a ceramic honeycomb or foam, which provides a very large surface area for contact vith-the gas. Both catalytic oxidation and incineration are effective to reduce the emitted VOC's to carbon dioxide and ~vater before the exhaust gas 10 is released to the atmosphere.
Spray booths are generally used, for example, in commercial shops ~hich repaint vehicles, in order to confine paint overspray and to evacuate VOC's released during painting. Generally, the spray booth air has been exhausted to the atmosphere. Filtered spray booth air has been recirculated 15 back to the booth only ~hen the booth is unmanned. Preferably, a high flow of air is passed through the spray booth filtered during painting in order to maintain clean filtered air inside the spray booth to reduce the risk of paint o~erspray, dust and environmental contaminants from outside the booth from contacting and d~m~ging the wet finish. The exhausted booth air is easily 20 filtered to remove particulate overspray. Ho-vever, the highest level of VOC's are released during and immediately after the paint spraying operation. The booth air could not be recirculated during spraying in a manned booth since the painter would be subjected to an increasing VOC vapor level in the booth as painting progresses. If the spray booth air is exhausted to the atmosphere, a25 large quantity of clean, ~arm make up air is required. In colder climates, the make up air often needs to be heated. Preferably, the spray booth air is maintained at about 70F (21C) during painting and the booth temperature may be increased significantly during the drying and baking stage. For example, the painted surfaces and the booth air may be heated to reach about 30 140F to 180F (60C to 82C) during drying. A paint spray booth of a sufficient size for enclosing a vehicle may have an air flow rate on the order of 10,000 cubic feet per minute (283 cubic meters per minute). To exhaust this amount of air from the booth and replace it ~vith clean, ~ arm make up air can consume a large amount of energy in addition to the energy required for 35 removing the VOC's from the e~;hausted air This adds significantly to the - cost of painting a vehicle Ideally, anv particulates such as paint overspray 2 1 7980~

and the VOC's ~vould be filtered or other~vise separated from the warm spray booth air and the ~varm filtered air is then returned to the booth.
Disclosure Of Invention The present invention is directed to a method for VOC abatement and 5 to a paint spray booth incorporating the method. Desiccants have been used in the past for separating ~vater vapor from gases. It has been observed that one property of desiccant is that it has a molecular attraction both for water vaporand for VOC's. For at least some desiccants such as alumino-silicate gels, the attraction for VOC's is less than the attraction for water vapor. A higher 10 temperature is required during regeneration to drive off adsorbed water vaporthan to drive off adsorbed VOC's. Another property of these desiccants is that when they are saturated ~vith adsorbed materials, regeneration tal;es place overa range of temperatures. Some of the adsorbed material will be released at a lower temperature than the remainder of the adsorbed material. It has been 15 noted that as at least some desiccants adsorbs ~vater vapor, their VOC
aKraction is reduced. At the same time, the regeneration temperature required to release the molecular attraction for VOC's is reduced.
According to the invention, a VO~ adsorbent desiccant or other suitable material is treated ~ith a vapor ~vhich has a stronger molecular 20 attraction to the adsorbent material than the adsorbent material has to the VOC's in order to lower the VOC regeneration temperature to a desired temperature. Preferably, the filter material is a desiccant such as an alumino-silicate gel in the form of hard generally spherical beads treated ~vith water vapor to reduce its VOC regeneration temperature to about 140F to 180F
2~ (60C to 82C). The desiccant ~vill require heating to a higher temperature, such as within the range of about 2~0F to 400F (120C to 205C) to completely eliminate the adsorbed ~vater vapor. As a consequence of the ~vater vapor treatment, the desiccant may lose about 80% to 90% of its VOC
adsorption capability. The treated adsorbent material is used, for example, to 30 adsorb VOC's in air ~vhich is circulated from a spray booth through the adsorbent material and retumed to the spray booth during a painting operation All of the air ~vithdra~vn from the spray booth is filtered and passed through aVOC adsorption bed prior to returning the clean air to the spray booth. As a consequence of this process, an operator may remain in the recirculated air 3~ spray booth during spraying This differs from the prior art recirculated air spray bootlls \vllich co~lld not bc manned during sprayil1g 2 1 798~
~ .

Preferably, the spray booth also is used to hold a freshly sprayed workpiece while the applied coating is cured. Curing is enhanced by heating the air circulated through the spray booth to about 140F to 180F (60C to 82C). After spraying, the operator leaves the spray booth and the booth air is 5 heated by passing a portion of the filtered recirculated air throu~h a burner to heat such air portion to a high temperature. At least a portion of the heated air is mixed ~vith the ren~ining recirculated air and any makeup air and retumed to the booth. Mixing is controlled to achieve a desired booth temperature, e.g., about 140 F to 180F (60C to 82~C). During the heated paint curing cycle, the heated booth air is drawn through and regenerates the filter materialby causing the adsorbed VOC's to be released. The desorbed VOC's are then o~;idized either through incineration or, preferably, by contacting the hot gas with a catalyst immediately downstream from the burner. The hot effluent from the catalyst is returned to the booth, as needed, to maintain the booth temperature, and the remainder is discharged to atmosphere. Filtered make up air is provided to the booth, as needed, to compensate for the catalyst effluentdischarged to the atmosphere.
Accordingly, it is an ob~ect of the invention to provide a method and apparatus for removing VOC's from air.
Other objects and advantages ofthe invention will become apparent from the following detailed description of the invention and the accompanying drawings.
Brief Description Of The Drawings Fig. 1 is a fragmentary diagrammatic view of a portion of a filter material which has been treated with a vapor to reduce its affinity for VOC's;
Fig. 2 is a block flow diagram for a paint spraying system according to the invention;
Fig. 3 is a schematic diagram of a paint spraying system according to the invention;
Fig. 4 is a perspective view of the air handling portion of the system of Fig. 3 ; and Fig. 5 is a schematic diagram of a modified paint spraying system according to the invention.

Best Mode For Car~ing O~lt The Invention The invention is directed to a method for the abatement of VOC's through the use of a treated adsorbent material and to the adaptation of the method to the reduction of VOC's in a paint spray booth. It is ~ ell known that 5 certain adsorbent materials such as certain desiccants and zeolite have a strong affinity for certain vapors which render such materials useful as an adsorbent for separating such vapors from air. Desiccants such as alumino-silicate, for example, are known for adsorption of water vapor from air and other gases, while zeolite and charcoal are known for adsorbing VOC's from air.
Adsorbent materials may be regenerated by heating to a sufficiently high temperature to break the molecular attraction and drive off the adsorbed vapor. The actual temperature required for regeneration ~vill depend on the strength of the molecular attraction bet~veen the adsorbent material and the adsorbed vapor. An alumino-silicate gel desiccant, for example, is commonly 15 used as a drying agent for e~;tracting water vapor from gas. This desiccant has a strong attraction for water vapor and may require heating to about 250F to 400F (120C to 205C) to fully desorb water vapor. It has been observed that alumino-silicate gel desiccant also will adsorb VOC's and may require heating up to about 350F (about 175C) or more to fillly desorb typical 20 VOC's released in a paint spray booth. Lo-v boiling point hydrocarbons are desorbed at a lo~ver temperature than higher boiling point hydrocarbons. A
preferred desiccant for adsorbing VOC's is alumino-silicate gel containing about 3% alumna (Al~03) and 97% silica (SiO2) in the form of hard, generally spherical beads. Such a desiccant is commercially available, for example, 25 from Kali-Chemie Corporation of Greenwich, Connecticut under the trademark "Sorbead R". Other VOC adsorbent materials such as zeolite also may be acceptable alternatives.
Desorption takes place over a range of temperatures. The alumino-silicate gel desiccant's molecular attraction is stronger for water than for 30 VOC's. It has been found that as water vapor is adsorbed and the desiccant becomes loaded or masked with the adsorbed vater vapor, the molecular attraction for VOC's becomes weaker. Even when the desiccant has adsorbed as much water vapor as it will hold, it has a ~veak attraction for VOC's. The adsorbed VOC's are m~lch more easily desorbed than the adsorbed water 3~ vapor. The reason for this is not yet fully understood. A possible explanation is sho~vn in Fig. I \vllicl1 illustrates a fragmentary portion of a piece of desiccant 10. Tl~e desiccallt 10 has an irreglllar surt`ace I I \\ hich contail1s -crevasses or fissures 12. The fissures 12 and other irregularities in the surface I l may have the strongest molecular attraction for vapors. According to the invention, the desiccant 10 is treated by exposure to a vapor such as steam to partially fill the fissures 12 or other adsorption mechanism vith ~vater or 5 another suitable masking agent 13. The amount of water or other masking agent 13 adsorbed by the desiccant 10 will depend on the exposure to the steam. The water 13 is shown at the bottoms 14 ofthe fissures 12 where the molecular attraction is believed to be strongest. The desiccant l O must be heated to the upper end of the regeneration temperature range to release this 10 adsorbed water 13. When the desiccant 10 is subsequently exposed to air containing VOC's, it is possible that the VOC's are adsorbed as a layer 15 above the previously adsorbed ~vater 13.
Since the ~vater 13 fills the portions of the desiccant 10 ~hich create the strongest molecular attraction, the attraction for the subsequently adsorbed1~ VOC's is ~veakened. As more water 13 is adsorbed by the desiccant 10, the molecular attraction for the VOC's 15 becomes weaker. Ho~vever, the trade off is that the treated desiccant 10 is less efficient in adsorbing VOC's and, for example, may be perhaps 10% to 20% as efficient at adsorbing VOC's as untreated desiccant. The actual regeneration temperature vil-l depend on the 20 adsorbent material and both on the particular VOC's being adsorbed.
A particular paint may contain solvent having several different VOC's with different molecular attractions to the desiccant 10. If the desiccant 10 is .
treated so that the VOC having the strongest molecular attraction is released at150F (66C), other adsorbed VOC's may be released at lo~,ver temperatures, 25 for example, at only 120F (50C). Since the desiccant 10 has a stronger molecular attraction for the ~vater 13 than for the VOC's l S, the desorption temperature range for desorbing the ~vater 13 ~vill be significantly higher thanthe desorption temperature range for the VOC~s 15. Consequently, the desiccant 10 may be used to adsorb VOC's 15 and may be repeatedly 30 regenerated and reused ~vithout affecting the adsorbed ~,vater 13. It has been noted that the steam treated alumino-silicate gel desiccant does not appear to be sensitive to moisture in the spray booth air, ~vhile the desiccant still retains an affinity for VOC's. The treatment of the desiccant ~vith steam appears to sufficiently load the desiccant that it ~vill not adsorb additional moisture from 35 air at normal spray booth temperatures The treated desiccant 10 may be placed ill filter beds throuc~h vhich VOC containing air is circ~llated to se~arate the VOC's t`ronl tll air. In the broadest aspects of the invention, the filter bed may be used, for example, in either open front or closed paint spray booths, or for filtering VOC laden air from dry cleaning operations, manufacturing operations, etc. Depending on the application, the desiccant` 10 may be regenerated either in situ or by moving the filter beds to a separate furnace (not shown) which heats the desiccant 10 sufficiently to desorb the VOC's lS without desorbing the ~vater or other m~sking agent 1-3.
Figs. 2-4 illustrate a treated desiccant adsorbent bed 16 located to filter VOC's from air circulated through a closed paint spray booth 17. The 10 illustrated paint spray booth 17 is of a type suitable for use in an automobile paint refinishing operation ~herein paint is sprayed on a vehicle 18 and then, while the vehicle 18 remains in the booth 17, the painted surfaces are heated to about 140F to 1 80F (60C to 82C) to cure the applied paint. VOC's are released into the air in the booth primarily during the bake cycle.
Fig. 2 is a block flow diagram of a circuit for removing VOC's from the spray booth 17, ~vhich is illustrated in the schematic diagram of Fig. 3. The spray booth 17 is of the downdraft type in which a flow of air is discharged from ducts or a ceiling grid 19 at the top of the booth 17 and air with any entrained paint particles and VOC's is ~vithdrawn through a grid 20 in the 20 floor of the booth 17. The vehicle 18 is positioned over the floor grid 20. A pit 21 is located belo~v the floor grid 20. The VOC adsorbent bed 16 is mounted in the pit 21 and a particle filter 22 is mounted between the adsorbent bed 16 and the grid 20. Fig. 3 represents an end view ofthe spray booth 17 and of the floor grid 20. The particle filter 22 and the adsorbent bed 16 may 25 extend for substantially the full length of the spray booth 17. The spray booth air is drawn through the floor grid 20, through the particle filter 22 and through the adsorbent bed 16 by an air handling unit 23 and is returned to the spray booth 17 through the ceiling ~rid 19.
The minimum depth of the adsorbent beds 16 will depend on the 30 operating cycle. As the maximum amount of VOC's which must be adsorbed between regeneration cycles increases, the volume ofthe beds 16 must increase. It will be appreciated that there ~vill be a pressure drop as the booth air is dra~vn through the adsorbent bed 16. The adsorbent beds 16 may be a series of trays set on rails or steps (not sho~ n) on the sides of the pit 21. One 35 or more gaps 24 of a desired size mav be established between and in parallel ~-ith the aclsorbent becis 16 to control the air pressure in the spray booth 17.Since tl1e air is recirculatecl ti1ro~ h thc si~ray booth 1~, cln~ ~'OC's entrailled in air passing through the gaps 24 will be picl~ed up by the adsorbent bed 16 on a subsequent cycle through the spray booth. The gaps 24 function as a coarse adjustment for reducing the back pressure across the adsorbent beds 16 ` and may be necessary when a single blower is used to recirculate the spray booth air.
While the vehicle 18 is being sprayed in the spray booth 17, a high capacity blower 25 in the air handling unit 23 draws the booth air through the particulate filter 21 and the adsorbent bed 16 and returns the air through a damper 26 to the spray booth 17. For a spray booth of a size to hold a vehicle 18, the blower may, for example, recirculate on the order of about 10,000 cubic feet per minute (283 cubic meters per minute) of air through the spray booth. Since the air is being filtered to remo~e both particulates and VOC's and is recirculated, the opertion of the spray booth during spraying is energy efficient. It is not necessary to provide large quantities of heated and filtered ma~e up air to the spray booth 17. Air recirculation also reduces the risk of e,~posing the freshly painted surfaces to con~min~nts which could damage the finish. Since most VOC's are removed from the recirculated air, a spray gun operator with appropriate respiratory protection may be present in the spray booth during spraying.
After spraying is completed and the spray gun operator leaves the spray booth 17, a damper 27 is opened, a relatively lo-v capacity blo~ver 28 is turnedon. and a burner 29 is turrled on. The blo~ver 28 may only have a capacity of, for e~ample, about 1,000 cubic feet per minute (28.3 cubic meters per minute), or considerably less than the blo~ver 25. The burner 29, a catalyst 30,the blo~ver 28 and the damper 27 are arranged in a series flow path 31 which is connected in parallel ~vith a flow path 32 ~vhich comprises the blower 2 and the damper 26. When the damper 27 is opened and the blo~ver 28 is turned on, the air flow splits between the paths 31 and 32, with the majority ofthe recirculated air continuing to flo~v in the path 32. The burner 29 heats theair in the path 31 to a high temperature and the heated air is mixed with the air rrom the path 32 and returned to the spray booth 17. All of the heated air in the flo~v path 31 is initially returned to the spray booth 17 ~vhile the temperature of the spray booth is raised to the desired cure temperature. By modulating the dampers 26 and 27 after the spray booth air reaches the 3~ desired cure temperature, the air mi~ ratio may be controlled to maintain the telllperature in the spray booth 17. Prererably, the air hl the booth 17 is heated to a clesired tenlperat-lt-c cn the order or bet~ ee~ 0l .Itld I ~0l (60C to 217980~

82C) to decrease the paint cure time. However, the actual desired cure temperature may vary ~vith various factors including the properties of the applied finish and the ma~imum temperature to ~vhich the finished workpiece may be exposed.
The desiccant or other VOC adsorbent material in the adsorbent bed 16 is treated, as described above, to have a regeneration temperature for the VOC's at or belo~v the temperature to ~ hich the spray booth air is heated during the cure cycle. Consequently, ~ hen the spray booth 17 is heated to the desired cure temperature, the previously adsorbed VOC's are released from lO the adsorbent bed 16 into the recirculated air during paint cure. However, the released VOC's are confined within the closed spray booth system and the operator is not present in the spray booth at this time due to the high temperature. Preferably, the output of the burner 29 is sufficient to heat the air in the path 3 1 to about 650F (345C) to activate the catal~ st 30 to oxidize 15 the VOC's flowing in the path 31. For o~idizing VOC's from paint, the catalyst may be platinum coated on a porous substrate. The substrate may be, for example, a commercially available open celled ceramic foam having on the order of about 300 cells per cubic inch (about 18.3 cells per cubic centimeter). If the burner 29 heats the air flo~ing in the path 31 to about 20 650F (345C~ or more, the platinum catalyst 30 causes the VOC's to oxidize into primarily carbon dioxide and water. When heated sufficiently for activation, an exothermic reaction takes place at the catalyst 30, raising the temperature of the catalyst 30. A second damper 33 is located after the blo~ver 28, in addition to the damper 27. The damper 33 controls the venting 25 of gas in the flow path 3 1 through an exhaust duct 34 to atmosphere outside of the spray booth 17. After the spray booth 17 is heated to the desired bake temperature and while the VOC's are being oxidized by the catalyst, the damper 27 is closed and the damper 33 is opened to e~chaust to atmosphere the hot air flo~v in the path 31 ~vhich is not needed to maintain the spray booth air 30 temperature. As necessary, a small controlled amount of the heated air in thepath 31 may be provided by modulating the damper 27 to maintain the spray booth air temperature. A make up air damper 35 and a filter 36 are connected upstream from the high capacity blower 25 to provide an)~ needed malce up air from an intake duct 37 ~vhen the exhaust damper 33 is opened during the bake 3~ or cure cycle It ~vill be appreciated that onl)~ a small frac~ioll of tlle recirculated spray l~ooth air llo~vs throuoll lllc ~atl1 31. Consequelltly, VOC~ ill Ro~\~ in the 21 7~0~
`~ .

path 32 and the main blower 25 will return such VOC's to the spray booth.
The dampers may be adjusted so that, for example, about 90% of the air withdrawn from the spray booth 17 is returned to the spray booth 17 and about 10% of the air passes through the path 31 and is exhausted to atmosphere.
S However, because of the high spray booth air recirculation rate, the VOC's in the spray booth will be quickly reduced to an acceptable level during the heated cure time. Since only a small portion ofthe air is heated and only a small amount of make up air is needed, the air handling unit 23 has a high energy efficiency. It is more economical to heat a low volume air flow to a 10 higher temperature than to directly heat a high volume air flow to a lower temperature. Further, it is much easier to abate the VOC's in the relatively low air flow in the flow path 31 with a relatively small volume catalyst.
It should be appreciated that a single small high temperature burner serves two distinct functions. It serves to quickly heat and maintain the 15 temperature in the spray booth 17. It also serves to heat the VOC laden air passed through the catalyst to a sufficient temperature to activate the catalyst30. The hot air delivered to the spray booth also may serve two purposes. It may heat the painted workpiece to reduce the cure time. Also, it functions to regenerate the adsorbent bed 16 to release the adsorbed VOC's. However, it 20 also will be appreciated that it is not necessary to cure the painted workpiece in the spray booth 17. One or more ~orkpieces can be painted in the spray booth 17 and removed for curing elsewhere. Or, a painted vehicle can be left .
in the spray booth 17 overnight, for e~;ample, for curing the paint at room temperature. The temperature in the spray booth 17 may be raised for 25 regenerating the adsorbent bed 16 while no workpiece is present in the spray booth 17.
Fig. 5 shows a spray booth 40 having an air handling unit 41 according to a modifled embodiment ofthe invention. As with the previously described embodiment, air withdrawn from the spray booth 40 is drawn through a 30 particulate filter 42 and a VOC adsorbent bed 43 which pre~erably contains a treated desiccant or a similarly functioning VOC adsorbent material which has a lo-v regeneration temperature. The air is dra-vn from the spray booth 40 by a high capacity blo~ver 44 and is returned to the spray booth 40 through a flow path 45 including a damper 46 and a return air duct 47. The air discharged 35 from the blower 44 also may flow to the return air d~lct 47 through a path 4 w hich includes a burner 49, a catalyst ~0 and a damper 5 l . Alternately, air llo\v lrolll the catalyst 50 may pass throu~h a dalnpcl 5~ to an e.~;haust dLIct - ~ 2~7~8-~

53. An intake air duct 54 is connected through a damper 55, a low capacity blo-ver 56 and a filter 57 to supply make up air to the duct 47, as needed when air is exhausted to the duct 53.
The air handling unit 41 operates in a manner similar to the previously S described air handling unit 23. During the spraying cycle, the damper 46 is opened and the dampers 51, 52 and SS are closed. The blower ~4 is operated to draw spray booth air through the particulate filter 42 and the adsorbent bed 43 to remo~e particulates and VOC's and the air is returned to the spray booth 40. After spraying is completed and while an operator is not present in the 10 spray booth, the damper 51 is opened and the burner 49 is turned on to increase the air temperature in the spray booth. The dampers 46 and 51 may be modulated to provide a desired split in the air flow between the paths 45 and 48. The air flows in the paths 45 and 48 are mi~;ed to achieve a desired temperature and are then returned to the spray booth 40 in the d~lct 47. Once 15 the desired cure temperature is reached in the spray booth 40, VOC's will be quickly desorbed from the bed 43. At this time, the damper 51 is closed and the damper 52 is opened to exhaust the hot air from the catalyst ~0. Once the catalyst 50 is activated by the hot air, an exothermic reaction will take place at the catalyst 50 ~o oxidize the VOC's. The effluent from the catalyst is vented 20 to atmosphere through the damper 52 and the exhaust duct 53. At the same time, the damper 55 is opened and the blo~ver 56 is operated to supply any needed make up air. The dampers 46, 51, 52 and 55 may be modulated by a programmable controller (not shown) to provide a desired temperature and air pressure balance in the system. The damper 51 may be opened to return a 25 small portion of the hot gases from the catalyst 50 back to the spray booth 40 to maintain the spray booth cure temperature above the VOC desorbent temperature for the bed 43.
It will be appreciated that various modifications and changes may be made to the above described preferred embodiments of the invention without 30 departing from the spirit and the scope of the follo~,ving claims. For example, the specific catalyst used to o~;idize the VOC's may be of any ~nown type which will react with the particular VOC's being abated. Further, in the broadest aspects of the invention, the adsorbent material may be any VOC
adsorbent material ~ hich is capable of being regenerated at the relatively low 3~ temperatures to ~vllicll it may be heated in situ in the paint spray booth or in the air-recirculation passages for the spray booth or other process. Thc VOC
aclsorbent material sho-ll(l llave a VOC regenera~ioll tempclal~re ol`nc) greater 2 1 798~

than 250F (120C) for applications in which paint is not being cured and in any event no greater than a safe temperature for the particular application, taking into account any temperatures w hich will result in a risk of damage or fire. Further, in the broadest aspect of one embodiment of the spray booth, the S catalyst may be eliminated and the VOC's may be oxidized by combustion, for example, with the burner 29 or the burner 49. The specified temperatures and air flow rates are intended to be exemplary and may vary ivith the types of VOC's being abated and with the specific application.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for removing volatile organic compounds (VOC's) from a gas mixture comprising the steps of:
a) treating VOC adsorbent material consisting of an alumino-silicate gel desiccant with a masking agent consisting of water vapour to reduce the attraction of said adsorbent material for VOC's and to reduce a regeneration temperature for desorbing VOC's from said VOC adsorbent material, said VOC adsorbent material having a stronger attraction for such masking agent than for VOC's;
b) passing a flow of gas containing VOC vapour through the VOC
adsorbent material treated with a masking agent to separate VOC's from the gas through adsorption of such VOC's in said treated VOC adsorbent material; and c) regenerating said treated VOC adsorbent material at a temperature sufficiently high to drive off adsorbed VOC's from said treated VOC adsorbent material while below a higher temperature required to drive off the masking agent.

2. A method for removing VOC's from a gas mixture, as set forth in claim 1 and wherein said treated VOC adsorbent material is treated with sufficient water vapour to reduce the VOC regeneration temperature of said treated VOC adsorbent material to no greater than 250°F.

3. A method for removing volatile organic compounds (VOC's) from air in a paint spray booth comprising the steps of:
a) treating a VOC adsorbent material with a masking agent to reduce the attraction of said VOC adsorbent material for VOC's and to reduce a regeneration temperature for desorbing VOC from said VOC adsorbent material, said VOC
adsorbent material having a stronger attraction for such masking agent than for VOC's whereby said treated VOC adsorbent material has a masking agent regeneration temperature above a VOC regeneration temperature;
b) circulating spray booth air through the VOC adsorbent material treated with a masking agent while a workpiece is painted in the spray booth to adsorb VOC's from such circulated spray booth air;
c) after painting a workpiece in the spray booth, heating the spray booth air while the workpiece remains in the spray booth to cure paint applied to the workpiece; and d) periodically regenerating said treated VOC adsorbent material with heated spray booth air at a temperature sufficient to remove adsorbed VOC's without removing the masking agent from said treated VOC adsorbent material.

4. A method for removing VOC's from air in a paint spray booth, as set forth in claim 3 and wherein when the spray booth air is heated the spray booth air is heated to at least the VOC regeneration temperature of the VOC adsorbent material treated with a masking agent and to less than the masking agent regeneration temperature and wherein said treated VOC adsorbent material is regenerated with the spray booth air which was heated to at least the VOC regeneration temperature.

5. A method for removing VOC's from air in a paint spray booth, as set forth in claim 4 and further including the step of oxidizing VOC's released into the heated spray booth air used to regenerate said VOC adsorbent material.

6. A method for removing VOC's from air in a paint spray booth, as set forth in claim 5, wherein said released VOC's are oxidized by heating at least a portion of the VOC containing spray booth air discharged from the adsorbent material during regeneration and contacting such heated VOC containing air with a catalyst.

7. A method for removing VOC's from air in a paint spray booth, as set forth in claim 4 and further including the step of incinerating VOC's released into the heated spray booth air used to regenerate said treated VOC adsorbent material.

8. A method for filtering VOC's from air in a paint spray booth, as set forth in claim 4 and wherein the VOC adsorbent material has a regeneration temperature no greater than 180° F. and wherein the spray booth air is heated to no greater than 180° F. to cure paint applied to the workpiece and to regenerate the VOC adsorbent material.

9. A method for removing VOC's from air in a paint spray booth, as set forth in claim 3 and wherein said adsorbent material is an alumino-silicate gel desiccant and wherein the masking agent which is used to treat said adsorbent material is water vapour.

10. A method for removing VOC's from air in a paint spray booth, as set forth in claim 3 and wherein said treated VOC adsorbent material is a desiccant, wherein said masking agent is water vapour and wherein said desiccant is treated with sufficient water vapour to reduce its VOC regeneration temperature to no greater than 180° F.

11. A method for removing VOC's from air in a paint spray booth, as set forth in claim 3 and wherein the masking agent has a boiling point below the boiling point of some of the VOC's in the paint spray booth.

12. A method for removing volatile organic compounds (VOC's) from air in a spray booth comprising the steps of:
a) while spraying a coating on a workpiece in said spray booth, circulating spray booth air through a VOC adsorbent material to separate VOC's from said circulated spray booth air by adsorption;

b) returning the spray booth air to said spray booth after passing through said VOC adsorbent material;
c) after completion of spraying and while continuing to circulate spray booth air through said VOC adsorbent material, heating the air circulated through said spray booth to a temperature sufficient to regenerate said VOC adsorbent material with the circulated heated spray booth air to desorb VOC's whereby VOC's adsorbed by said VOC
adsorbent material are desorbed and released into heated spray booth air circulated through said VOC adsorbent material.

13. A method for removing VOC's from air in a spray booth, as set forth in claim 12 and wherein said spray booth air is heated after completion of spraying to a temperature which cures the paint sprayed on the workpiece and which also regenerates said VOC adsorbent material.

14. A method for removing VOC's from air in a spray booth, as set forth in claim 13 and wherein said circulated air is heated by heating a portion of the circulated air with a burner, mixing a controlled amount of the heated air from said burner with circulated air returned to said spray booth to establish a desired spray booth temperature and discharging any excess heated air from said burner to atmosphere and further including the step of passing said heated air from said burner through a catalyst to oxidize the VOC's in the heated air and wherein said burner heats such portion of the circulated air to at least an activation temperature for said catalyst.

15. A method for removing volatile organic compounds (VOC's) from a gas mixture comprising the steps of:
a) treating VOC adsorbent desiccant material with a sufficient quantity of a masking agent to reduce a regeneration temperature for desorbing VOC's from the VOC
adsorbent desiccant material treated with a masking agent to no greater than 250° F, said VOC adsorbent desiccant material treated with a masking agent having a stronger attraction for such masking agent than for VOC's, said masking agent reducing the attraction of said VOC adsorbent desiccant material for VOC's;
b) passing a flow of gas containing VOC vapour through the VOC
adsorbent desiccant material treated with a masking agent to separate VOC's from the gas through adsorption of such VOC's in the VOC adsorbent desiccant material treated with a masking agent; and c) regenerating the VOC adsorbent desiccant material treated with a masking agent at a temperature sufficiently high to drive off adsorbed VOC's from said treated VOC adsorbent material while below a higher temperature required to drive off the masking agent.

16. A method for removing VOC's from a gas mixture, as set forth in claim 15, wherein the VOC adsorbent desiccant material treated with a masking agent is regenerated by passing a gas heated to such sufficiently high temperature through the VOC
adsorbent desiccant material treated with a masking agent.

17. A method for removing VOC's from a gas mixture, as set forth in claim 15, wherein said masking agent is water vapour.

18. In combination with a spray booth for confining and collecting volatile organic compounds (VOC's) emitted during spraying, a filter having a VOC
adsorbent material which has a VOC regeneration temperature of not greater than 250° F, a blower adapted to circulate air from said spray booth through said VOC adsorbent material for separation of VOC's from such circulated air and to return the separated air to the spray booth while spraying a workpiece, a burner adapted for heating the air in the spray booth to at least said VOC regeneration temperature of said VOC adsorbent material while paint sprayed on a workpiece is cured, such circulated spray booth air desorbing VOC's from said VOC adsorbent material when heated to said regeneration temperature and means for oxidizing the desorbed VOC's from a portion of heated spray booth air passed through said VOC adsorbent material.

19. A spray booth, as set forth in claim 18 and wherein said means for oxidizing the desorbed VOC's from a portion of the heated spray booth air passed through said VOC adsorbent material comprises a precious metal catalyst.