GB2092909A - Process and system for elimination of paint solvent vapours - Google Patents

Abstract

A system for eliminating paint solvents released during paint spraying operations (10) includes a liquid spray solvent stripper (64) comprising a multi-stage absorber unit through which the exhaust air is passed. A heating arrangement (94, 100) is provided for regenerating the solvent absorber liquid and is arranged to recover heat from an incinerator (58) associated with a paint drying oven (54). Solvent vaporized during regeneration is passed over cooling coils (106) in a condensing chamber (104). The exhaust air is first treated (20) with water to remove paint solids, a proportion of the solvent vapours passing into solution with the treatment water. A water solvent stripper (130) is provided for applying a vacuum to the water circulated from a paint solid removal unit (44) to cause the solvent to be vaporized out of solution, the vapours being condensed by cooling coils (142) in a condensing chamber (134). The air and water stripper condensing chambers (104, 134) are purged by a vacuum pump (118) directing the uncondensed solvent vapours and non-condensables into the incinerator (58). <IMAGE>

Description

SPECIFICATION Process and system for elimination of paint solvent vapours The present invention generally relates to painting apparatus and processes, and deals more particularly with a system and process for removing and disposing of organic solvent vapours released from painting processes employing organic solvent based paints.

Recent trends have developed two major factors in the design of industrial facilities, i.e., the increasingly stringent governmental pollution control rquirements, and the cost and availability of energy. Particularly difficult problems in this regard are found in the design of paint spray booth facilities for the application of paint to automotive bodies and similar products, due to the enormous volumes of air flow required in the paint spray booths in industry.

The pollution problems are associated primarily with the organic liquid constituents, including solvents, thinners and diluents of paints. Such liquid constituents are vaporised during the paint application, which vapours pass into air circulated through the booth. Such air must be circulated at relatively high volume in order to enable the paint sprayers to work in a safe, healthful and clean environment.

The rigorous standards of emissions applied to industrial facilities precludes the discharge of such organic or solvent vapour laden air directly into the atmosphere, and such vapours in the discharged air are required to be reduced to very low levels.

A common approach in eliminating such vapours which are combustible is to incinerate the same by passing the air into an incineration chamber which is heated by a burner to raise the temperature of the air sufficiently to cause oxidation of, for example, the hydrocarbons into carbon dioxide and water vapour prior to being exhausted to the atmosphere.

However, for the enormous air flow rates exhausted from paint spray booths, the incineration process requires vast expenditures of energy, thus rendering this solution very costly to implement.

Another alternate approach which has been attempted and considered in this context is the use of adsorber beds such as of activated charcoal, over which the exhaust air is passed for direct adsorption of the solvent vapours, thus enabling their elimination from the exhaust air.

Again, the enormous airvolumes in automotive production and similar applications and the need to regenerate the adsorber beds render this approach extremely expensive.

Yet another approach which has been proposed involves the replacement of the organic solvent based paint with water based paint, the elimination of vapours thus enabling direct discharge of the exhaust air to the atmosphere, after filtration or other treatment of the air to remove the paint solids.

While effective in this regard, the water based paints require close control over the temperature and humidity of the air supplied to the booth, with air cooling and dehumidification during summertime, and heating and humidification during wintertime operation. This psychrometric control requires considerable capital investment and furthermore entails considerable energy to execute.

Certain advances have been made in reducing the solvent content of paints and also in the paint application process to reduce the quantity of organic solvent vapours escaping, but there has not been provided a relatively simple, inexpensive and reliable means for eliminating the organic emissions which does not entail excessive expenditure of energy when eliminating relatively low concentrations of solvents. Such low concentrations are a result of the working conditions in which large volumes of ventilating air are passed into the spray booth. These large volumes of fresh air flow must be warmed during wintertime operation and then exhausted to the atmosphere, representing a large energy loss.

It is an object of the present invention to provide a system and process for the elimination of organic vapours at relatively low concentrations, i.e., a few hundred parts per million, from the air exhausted from a paint spray facility in which the equipment required is relatively inexpensive to install and operate.

The present invention provides a system and process associated with a paint spray booth recovery system wherein the exhaust air is treated by being passed through a water scrubbing system such as to remove the paint solids from the air, and which water partially absorbs the organic vapours.

Solvent stripping is executed both on the water, after the filtration and removal of the paint solids, and also on the exhaust air after it has passed through the water scrubbing system to remove the paint solids.

The air stripping operation comprises directing the exhaust air through an absorber liquid spray scrubber in which the solvent is absorbed by an absorber liquid sprayed into the exhaust air stream. The scrubber includes a multi-stage absorber unit through which the exhaust air stream containing the solvent vapours is passed. Each stage consists of a vertical array of confined coalescer pad sections of knitted filaments of metal or similar extended surfaced over which the absorber liquid, typically an oil, is sprayed. The solvent laden air passes through the successive stages and 80-90% removal of the solvent is achieved so as to reduce the solvent concentration in the air to a level acceptable to be exhausted to the atmosphere.

The absorber liquid is collected by gravity draining from each of the coalescer pad sections, and iscircu- lated to a regeneration system for continuousi removing solvent from the absorber liquid and r Gcir- culating the same to enable recirculation for respraying onto the coalescer pads.

The regeneration process utilizes heat to drive off the solvents from the absorber liquid. The regenerator system is integrated into the paint spraying and drying operation, thereby substantially reducing energy requirements; the heat employed to vaporize the solvent is derived either from heat generated in a fume incinerator associated with the paint drying ovens or from the exhaust of such ovens.

The organic vapours driven off from the absorber liquid are condensed on cooling coils disposed in a condense g chamber and recovered in a suitable recover; vessel. The remaining uncondensed vapours are drawn out of the condensing chamber into the incinerator.

The removal of the organic compounds in the water is achieved by circulation of the water downstream of the solids removal equipment through a region in which a vacuum is applied to reduce the pressure above the water flow to a point whereat the solvents are vaporized into the region. The vapours are then condensed in a condensing chamber by being passed over cooling coils while the chamber itself is purged of the uncondensed vapours and the collected vapours from the chamber are drawn into the incinerator.

The water is circulated through a double standpipe into the condensing chamber with the elevation of the standpipe enabling the vacuum to be applied to allow free flow of the water through the standpipe.

The double standpipe includes a central pipe disposed within a large diameter outer standpipe, with the water circulating up the annulus between the inner and outer standpipes and thence over and into the open end of the inner standpipe.

A refrigeration unit is employed to produce the necessary cooling liquid flow through the respective cooling coils, and the refrigeration unit also operates to dump the heat extracted during condensation of the vapours into the air exhaust during summertime conditions and to preheatthe incoming air during wintertime conditions, thus recovering heat and saving energy.

Embodiments of the present application will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: FIGURE 1 is a diagrammatic representation of a paint application facility including a system for eliminating paint solvent vapours from both air and water according to the preferred embodiment of the present invention, FIGURE 2 is a diagrammatic representation of a paint application facility incorporating a system for eliminating paint solvent vapours from the air according to an alternative embodiment of the present invention; FIGURE 3 is a longitudinal view of an absorption unit associated with the system depicted in FIGURE 2, parts being broken away in section for clarity; FIGURE 4 is a sectional view taken along the line 4-4 in FIGURE 5; FIGURE 5 is a plan view of the regenerator unit shown in FIGURES 3 and 4;; FIGURE 6A is a perspective view together with a phantom representation of a spray header withdrawal from the absorption unit depicted in FIG URES 3-5; FIGURE 6B is a reverse perspective view of the absorption unit shown in FIGURE 6Awith a phantom representation of a coalescer pad withdrawal; FIGURE 7 is an air entry view of a typical coalescer pad stage; FIGURE 8 is a side view of a typical coalescer and associated drain structure; FIGURE 9 is an enlarged detailed view depicting the area 8 of FIGURE 8; FIGURE 10 is an enlarged detailed view depicting the area 9 of FIGURE 11; FIGURE 11 is a plan view of a typical channel support for the coalescer pad; and, FIGURE 12 is a perspective view of an alternate form of an absorber unit which may be employed in the system depicted in FIGURE 1 or FIGURE 2.

The system disclosed herein has particular application to a paint spray booth of the type including the use of a water flooded floor beneath the paint spray area in which the paint spray operations are conducted and in which the paint and solvent laden air is drawn down through the floor grills and exits through particulate scrubbing means beneath the grilled floor of the spray booth working area.

The air and water passing with great turbulence and intimate contact through the tubes causes the solids in the paint, i.e., the pigments, resin and other components, to be removed from the air and become suspended within the water flow. Such an arrangement is disclosed in U.S. Patent No.

3,421,293.

In order to remove the paint solids from the circulated treatment water, arrangements have been provided for collecting the paint solids and removing them from the water flow, as by causing them to raft on the surface of a collecting tank. Such an arrangement is disclosed in U.S. Patent No. 4,100,066.

The particular composition in paint used in automotive production and similar processes varies with the particular requirements and with the particular manufacturerofthe paint. However, a number of organic liquds are usually included, as solvents, thinners and diluents, some of which are typically soluble in water and others of which are not.

Accordingly, in passing with the water in intimate contact therewith, the air exhausted from the spray booth tends to cause a proportion of the solvent vapours contained in the exhaust air to go into solution with the circulated treatment water. The remaining hydrophobic vapours pass out with the exhaust air.

Thus, if the composition of the paint is such that a significant proportion of the liquid constituents are water soluble, such water flow treatment utilized in the spray booth air exhaust system itself represents means for partially eliminating the organic vapours.

On the other hand, the recirculated water cannot continue indefinitely to pick up the organic compounds and soon becomes saturated with them, and some means must be provided fortheircontinuous removal from the circulated water.

In the past, such elimination has been difficult due to the presence of the paint solids. In addition, due to the high specific heat of water and the large volumes required, the conventional process of heating the water to drive off the vapours is rendered economically impractical, as a result of the enormous heat energy which would be required.

The system and process described hereinafter is advantageous for use in paint spray booths of the type having a water flow treatment of the air exhausted, with equipment of the general type described in the above mentioned US Patent No.

4,100,066 for removal of the paint solids, and for use with paint compositions having a significant proportion of the water soluble organic solvents.

Referring first to FIGURE 1, one embodiment of the invention is adapted for use with a paint spray booth 10, comprising an enclosure within which an auto body shell 12 or other component is painted during production. The air supplied for ventilation is received via ducting 14 and caused to be circulated by means of a supply blower 16.

Awintertime heat exchanger 17 is provided, for purposes to be described, and the inlet duct 19 receives fresh air from the building exterior. The air passes through a grill 18 disposed across the floor of the paint spray booth 10 and thence over a flooded subfloor 20 which receives water flow via supply line 22. The water over the subfloor 22 tends to partially collect the overspray during paint spraying operations, which settles into the water. The air with the balance of overspray, partially fine particles and solvent vapours, is circulated out through longitudinally spaced outlet tubes 24 through which the water from the floor 20 flows. An intense scrubbing action of the air occurs in the tubes and the paint is transferred to the water which flows into a collecting pan 26.

The exhausted air, after passing through the outlet tubes 24 and having the paint solids thereby substantially removed, passes out through an exhaust duct indicated at 28 and thence passes through an exhaust air organic vapour absorber unit or stripper 30, which will be described hereinafter in further detail, and is then directed by an exhaust blower 32 to an outside stack 34. A heat exchanger 36 is also provided, for purposes to be described.

The water entraining the paint solids and having a proportion of solvents dissolved therein passes into collecting flumes 38 which it is withdrawn through lines 40 and 42 to be directed to a paint solid removing apparatus 44. This apparatus 44 may be of the type described in the above mentioned US Patent No.4,100,066 and comprise a first collecting chamber46 which receives the paint solid containing water and causes collecting of the paint solids which are periodically removed.

A level control tank 48 is also provided which causes the water to be regulated in the interior of the removing apparatus 44 in order to maintain the appropriate levels to conduct the removal process.

A return line 50 recirculates the water via pump 52 onto the spray booth subfloor to enable continuous execution of the process. Thus, the air exhausted through the exhaust duct 28 is substantially free from paint solids but does contain those organic solvent vapours which have not gone into solution with the water.

Similarly, the water in level control tank 48, while having the solids substantially removed therefrom, still contains the solvents in solution. The solvent must be continuously removed if the water is not to become saturated.

Also integrated into the system illustrated is a paint curing oven 54 in which air is heated and passed to maintain the temperature at a suitable high level for proper paint drying and curing. The air is withdrawn into a ceiling duct 56 and thence in part circulated through a fume incinerator 58. An exhaust blower 60 is provided to draw air upwardly through louvres 62 of the ceiling duct 56 and hence through an exhaust duct 64.

A proportion of the exhaust flow is recirculated by a supply fan 66 and a plenum 68 into the interior of the paint curing oven 54. A portion of the exhaust flow is drawn off by the exhaust blower 60, and passed through an air-to-air heat exchanger70 where it is preheated by incinerated air exiting from an incinerator chamber 72. The preheated air enters the inlet 126 of the incinerator chamber 72 via duct 128.

After passing through the incinerator chamber 72, a portion of the incinerated air is received by the inlet of supply fan 66 through duct 74.

The incinerator design is of a type as shown in U.S. Patent No.3,917,444.

A second air-to-air heat exchanger 76 provides an exchange of heat between fresh, make up air introduced through ducting 78 and the remaining incinerated air flow vented to the atmosphere through vent 80.

Tile process according to the present invention envisions the removal of solvent from the exhausted air after passing into the exhaust duct 28 and also the removal of the solvents from the water after solids removal in apparatus 44 and entry into the level control tank 48. As mentioned previously, the elimination of vapours from the air is achieved in an absorption type air stripper 30 in which absorbing liquid is circulated via a pump 82 through a bank of spray nozzles 84. The liquid may comprise an oil or oil base derivative or any other substance having an affinity for organic solvents.

The air passes through a matrix 86 which is sprayed with the liquid absorbent. The matrix 86 initially facilitates the development of a very large liquid absorbing surface to which the organic solvent vapours are attracted. Subsequently, it causes the now-solvent laden liquid to coalesce into larger droplets which may be separated from the air stream by gravity or as hereafter described.

After passing through the coalescing matrix 86, a series of baffles 88 cause the now-larger sized absorber liquid droplets to be eliminated from the air stream and to be collected in a collecting compartment 89 from whence the liquid is directed to the return side of the circulation pump p 82. The air is directed to a vent by exhaust blower 32 as indicated previously. An alternative embodiment of an absorbing unit will be described hereinbelow.

Since a given quantity of absorbing liquid Gar not absorb the solvent vapours indefinitely, rngercw#- tion apparatus is provided through which a portion of the absorbing liquid is circulated.

In the regeneration process, the hat required to drive off the solvent in the absorbing liquid is modest since the volume and specific heat of absorbing liquid is relatively low compared to other absorbing media.

The regeneration apparatus includes a liquid-toliquid heat exchanger 94. To maximize the efficiency of the regeneration process, the regeneration apparatus is integrated with the fume incinerator 58 which is associated with the paint curing oven 54.

The soln~-- ar: laden absorbing liquid is fed through the heat exchanger 94 and then through supply line 96 to a series of nozzles 98 arranged above high temperature heating coils 100. The nozzles 98 spray the liquid over the heating coils 100. The heating coils 100 receive incinerated air from the vent 80.

This incinerated air has passed from the incinerator chamber 72 and through the air-to-air heat exchangers 70 and 76. Thus, the heat of the incinerator is partially recovered to be utilized for regeneration heat A recirculating pump 102 causes recirculation of the regenerated absorbing liquid back to the supply side of the pump 82.

The absorbing liquid is heated and purged of some vents by contact with the hot surface of the heating coil 100 and releases the solvent vapours. These vapours are then condensed in a condensing chamber 104 by contact with a cooling coil 106 disposed in a collection compartment 107. The cooling coil 106 receives a flow of chilled liquid via line 108 which is connected to the expansion side 110 of a mechanical refrigeration unit generally indicated at 112. Collection compartment 107 is provided with a drain line 114 communicating with a solvent recov ery tank 116.

Condensing chamber 104 is maintained at a relatively low pressure to maximize vaporisation of solvents out of the absorbing liquid. The condensing chamber 104 is preferably put under a partial vacuum by means of a vacuum pump 118 having an inlet line 120 including a pressure responsive valve 122, which regulates the pressure maintained in the condensing chamber 104.

The vacuum pump 118 has an outlet 124 which communicates with the inlet 126 of the incinerator chamber 72 such that the organic vapours pass into the incinerator chamber 72 and are incinerated together with the vapour carrying air received from the outlet side of the air-to-air heat exchanger 70.

It will be appreciated that the system illustrated in Figure 1 enables organic vapours to be removed from the spray booth exhaust air with high efficiency. First of all, a substantial proportion of the vapours are passed into solution in the water and are later removed by vacuum distillation as is described hereinafter. The vapours which do remain in the air are removed by the absorbing liquid spray which is suited to handle relatively large quantities of air with relatively modest quantities of absorbing liquid.

Furthermore, the liquid is comparatively easily regenerated and thus a steady state process can be provided which is capable of dealing with the enormous volumes of air which are used in paint spray booth installations.

The regenerator apparatus itself is highly efficient as it utilizes both the liquid-to-liquid heat exchanger 94 and the other wise waste heat generated by the fume incinerator 58. By comparison to prior art approaches, the capital investment required is quite modest, as are the energy requirements in carrying out the process.

As indicated above, the system includes vacuum distillation means for removing solvent from the circulated water. A vacuum is applied above the water as it circulates so as to cause the direct vaporization of the organic compounds. The water is elevated approximately9 metres to a condensing chamber to enable a low vapour pressure to be maintained in the chamber. The chamber is continuously purged to remove non-condensable gases such as air and the relatively low pressure facilitates the vaporization of the solvents and thus removal from the circulated water without heating of the water mass itself.

It can be shown that the vaporization rate produced by the pressure reduction is at a level corresponding to that achieved at a considerably elevated temperature which would be necessary if the space above the solvent laden water was at normal atmos pneric pressure.

The particular arrangement for achieving the pressure reduction over the top of the water includes an outer standpipe 130 of relatively large diameter, the lower end of which extends into level control tank 48. Within the interior of the large diameter outer standpipe 130 is a relatively smaller diameter inner standpipe 132 with a clearance space therebetween enabling the water to flow up the outer standpipe 130 and down the inner standpipe 132.

The large diameter outer standpipe 130 is of greater height than the inner standpipe 132 as illustrated such that the water is contained therein as it passes into the interior of the inner standpipe 132 while flowing exposed to the interior of a condensing chamber 134.

The condensing chamber 134 is connected via pressure regulation valve 140, branch connection 138 and line 136 to the vacuum pump 118 and is evacutated thereby. The pressure maintained in the chamber 134 is somewhat less than the vapour pressure of the water flowing through the standpipes 130 and 132.

The height of the standpipes 130 and 132 is selected to relate to the vacuum imposed on the condensing chamber 134, i.e., full vacuum of the corresponding pressure head would be approximately 98.6 KPa (33 feet of water) such that the water will be subjected to the low vacuum without causing it to be drawn into the interior of the condensing chamber 134.

This relatively low pressure in condensing chamber 134 produces a greatly enhanced tendency for the dissolved organics to vaporize, while the water, having much less tendency to vaporize, will flush off in the condensing chamber 134 to a much lesser extent so as to be present in relatively inconsequential quantities. The organicvapours are condensed by cooling coils 142 supplied with cooled heat transfer medium flowing via line 150 and which are disposed in the condensing chamber 134 to cause the vapour to be condensed into liquid form and collected in the compartment 144.

The collected liquid passes into drain line 146 and to a recovery vessel 148.

The continuous purging of the interior of the condensing chamber 134 via line 136 ensures the removal of non-condensable gases and the maintenance of the relatively low pressure at which the organic solvents freely evaporate out of the water.

The liquid level control tank 48 may also have a slight vacuum applied to the region above the water level by a condensing chamber 152 provided with a vent tube 154. A connection drain line 156 connects the chamber 152 to the recovery tank 148. A branch line including a pressure control valve 158 connects the chamber 152 to the vacuum pump 118. A cooling coil 153 is provided in the condensing chamber 152 and is supplied with a flow of cooled heat transfer medium via line 150.

A relatively slight vacuum is applied to the condensing chamber 152 to enable the free flow of water into and out of the level control tank 48 while collecting the solvent vapours which may accumulate above the water level in condensing chamber 152. The vacuum is set by the pressure control valve 158.

In order to maximize the efficiency of the use of the refrigeration unit 112, the heat exchanger 36 receives a flow of the heat transfer medium which is circulated through condenser tubes of a condenser 160 of the unit 112. This acts to dissipate heat transferred from the evaporator 110 by operation of a compressor 162 of the unit 112. The heat to the evaporator originates from the heat of condensation of the solvent vapours in the respective condensing chambers 1041 134 and 152. The heatexchanger36 rejects the heat into the exhaust air which is relatively cool during summer conditions.

During winter conditions, the heat is transferred via the heat exchanger unit 17 to prewarm the incoming air and thus enhance the efficiency of the process.

Attention is now directed to FIGURE 2, wherein an alternate form of the invention is depicted which is similar in many respects to that previously described. The alternate form of the invention is adapted for use with a paint spray booth 164 of the type utilized in conducting paint spray finishing of automotive truck and car bodies, which features an air supply system generally indicated at 166 that causes conditioned air to be entered into an upper plenum space 168 through a diffuser ceiling layer 170, before passing into the working area 172 of the booth.

The air is exhausted through an exhaust ducting 174 after passing through a paint solids removal system which preferably takes the form of a water washed series of tubes similar to that previously described with reference to FIGURE 1.

In this system, a series of exit tubes 176 pass out through the bottom of the floor 178 which is flooded with water and has a weir overflow (not shown) causing an outflow ofwaterthrough the exit tubes 176. Flow through the exit tubes 176 causes a thorough washing of the extract air passing out through a belowfloorspace 180 in communication with the exhaust ducting 1974.

Accordingly, the exhaust ducting 174 receives air which has been substantially completely freed of overspray paint solids but which contains a solvent vapour typically at 220 to 400 parts per million. While this constitutes a relatively dilute proportion of solvent vapours the concentration is still too high to simply be discharged to the atmosphere without potentially running afoul of pollution control standards.

An air exhaust fan 182 directstheexhaustflow through a multi-stage absorber unit 184 whereat the solvent vapours are absorbed prior to the air being passed through an exhaust stack 186. Such absorber unit 184 is typically of a special design as will be described hereinafter and essentially provides a contact of the air exhaust flow with a flowing film of solvent absorber liquid such as oil over which the air passes. The liquid is caused to be regenerated such that there is an inflow of relatively solvent free liquid received over a supply line 188 and a collection of solvent laden liquid over return line 190.

The supply and return lines 188 and 190, respectively, are in communication with the installation tank, pump and filter means generally indicated at 192, including a collection tank, liquid circulating pumps and filtration units; the latter remove the minor quantities of paint solids which may accumulate in the liquid which result from the small quantity of paint solids still present after the water scrubbing operation in the paint spray booth 164.

After such filtration, the solvent laden liquid is circulated via line 194 to a heat exchanger 196. The heat exchanger 196 comprises a preheating means for the liquid contained in line 194 by a heat exchange relationship with the relatively hot liquid returning from a distillation column 198 in line 200, such that the relatively hot solvent free liquid causes an elevation in the temperature of the solvent laden liquid in line 200. The returning liquid, even after passing through the heat exchanger 196 while at a relatively reduced temperature in line 204 is still at an elevated temperature, i.e., of the order of 138 C.

The preheated liquid in line 202 is circulated through an incinerator heat exchanger 206. The heat exchanger 206 is associated with an exhaust paint drying oven 208 in which solvent laden air is exhausted through ducting 210 and passes through an incinerator section generally indicated at 212. The solvent vapors are incinerated such that a relatively hot solvent free exhaust gas may be exhausted through an exhaust stack indicated at 214. Such incinerator system may be similar to that previously described with reference to FIGURE 1.

This represents a clean source of heat which is employed to further heat the already preheated liquid flow in line 202 to a temperature at which a distillation regeneration process may be conducted.

Liquid in line 216 after passing through the second heat exchanger 206 is heated to a relatively elevated temperature level, i.e., typically of the order of 2 '#0C.

The heated solvent laden liquid in line 216 sr u rs the distillation column 198 at the upper reaches thereof and passes downwardly through trays disposed therein while giving up the solvent content by vaporization of the solvent content.

The distillation column 198 may be of conventional known construction peruse, as employed in various distillation processes in the oil refining industries.

Preferably, the operation of the distillation columr 198 is under a partial vacuum, i.e., 88KPa, in order to enable distillation to proceed at relatively modest temperatures, i.e., such as the aforementioned 249 C.

For this purpose, a vacuum pump 218 is employed having inlet line 220 applied to the lowest pressure area of the distillation column 198. The vacuum pump 218 also withdraws vapours from solvent condenser 224 and a condenser 226 such that the exhaust in vacuum pump 218 contains solvent and liquid vapours which are routed into the incinerator section 212 as shown to burn these vapours to allow exhaust into the atmosphere of the outflow from the vacuum pump 218.

The solvent vapours formed in the interior of the distillation column 198 are withdrawn to be collected in the solvent condenser 224 and with a collector line 232 leading to a solvent location facility. Any absorber liquid vapours which are formed are collected in the condenser 226, being withdrawn at a lower elevation in the distillation column 198.

Any condensed absorbent liquid vapours are returned via line 234 to the installation tank, pump and filter means 192 for recirculation to the absorber unit 184.

The hot solvent free unvaporized liquid removed from the lowersection of the distillation column 198 in line 200 is passed through the heat exchanger 196 where it is cooled to 1380C mentioned above.

In order to recover the heat energy in the liquid at about 1380C in return line 204, the liquid is circulated through a line 236 to a heat recovery means comprised of a series of air-to-liquid heat exchangers 238 as shown. The heat energy is thus utilized in the air supply system 166 in which the incoming air and ducting 240 are heated by passing through the airto-liquid heat exchanger 238. A series of such heat exchangers 238 are provided and each may be used to heat the air supplied to different spray booth sections.

Alternatively, the heat exchangers may be employed for supplying the heat for other low grade heat using processes, such as to heating of the water and phosphate solution in the pretreatment plant to the relatively modest temperatures required, for example, about 71 0C.

The liquid is cooled in return line 242 to the installation tank, pump and filter means 192 which then circulates the now cool, for example, 1 80C, solvent free liquid to the supply line 188 and thus enables continuous regeneration of the liquid to remove the solvent picked up in the booth exhaust air.

It can be appreciated that, in effect, the absorption process is carried out without the utilization of significant heat energy notwithstanding the utilization of a heat distillation process for regeneration. Thus, the absorption system is included in the heat recovery system and the heat energy recovered from the oven is utilized to precondition the supply air otherwise recovered. The absorption system is incorporated in this loop so as not to remove any net energy other than the negligible amount required for circulation of the liquids, pump, etc. At the same time, the system is relatively simple, reliable and trouble free as compared to other regeneration systems.

Also, the absorption unit itself is relatively lightweight and compact compared with other prior art structures such as to enable a great degree of flexi bility in the installation of such units.

The constructional details of the absorber unit 184 itself are illustrated in FIGURES 3 to 11.

The absorber unit 184 includes a sheet metal hous ing 244 having an inlet opening 246 at one end receiving ducting 248 connected to the exhaust from the air exhaust fan 182. Outlet openings 250 are connected to a cross ducting vent stack 252 with a plenum transition 254.

Suitable fire doors 256 and 258 are provided for closing off the absorber unit with a fusible link mechanism (not shown) enabling a closing off of the absorber unit in a manner known in the art.

The absorber unit 184 is of a multi-stage construction and it has been found that successive independent stages of air-to-liquid contact provide a very effective control or reduction of the solvent vapour content of the air such as to enable relatively complete solvent removal, i.e., 80% solvent content removed after passing therethrough.

Each stage consists of a coalescer pad assembly 260 which is sprayed with an absorber liquid such as oil. A final coalescer pad assembly 262 is provided at the exit end to remove any liquid droplets contained in the exhausting air. Each of the coalescer pad assemblies 260 and 262 consist of an array of pad assemblies 254,266,268 and 270 disposed to extend transversely across the interior of the sheet metal housing 244 so as to cause the entire flow to be intercepted in passing through the coalescer pad sections.

Each of the coalescer pad sections 264,266,268 and 270 includes an outer mesh 272 on either side of the pad 274 of knitted filament material, which may be either metal or plastics material. This material is of the type commercially available and is known as type H or equivalent, manufactured by Begg, Cousland & Co., Ltd. of the United Kingdom and is of a similar material to that employed in scouring pads.

Each of the coalescer pad sections 264,266,268 and 270 is disposed in a surrounding channel frame indicated at 276 consisting of bottom and top channels 278 and 280, and side channels 282 and 284 welded together to form the framework. The bottom and top channels 278 and 280 are joined with a deflector section 286 forming a V-shape transition into the coalescer pad sections such as to deflect the air flow smoothly into the coalescer pad sections. A drain tray 287 is disposed at the bottom of each pad section 264-270, on the air exit side thereof which captures absorbing fluid droplets that are drawn through the pad section and pass downwardly by gravity.

The bottom and top channels 278 and 280 are perforated as may be seen in FIGURE 11 and perforated with holes 288 in order to enable drainage of the liquid from each of the coalescer pad sections 264, 266,268 and 270.

The coalescer pads are retained at their outer edges with a suitable channel frame indicated at 290, the bottom of which is also perforated for this same purpose.

Each of the coalescer pads is fitted with an endcap 294 to which is bolted a suitable handle 296 provided to the outer channel frame 290 for this purpose.

As noted, each of the coalescer pad assemblies 260 is adapted to be sprayed with an absorber liquid such as oil in order to provide the saturation of the pad and enable the pad to be thoroughly wet with the absorber liquid to thus provide an intimate contact with the air flow through each of the pads.

A suitable arrangement is provided by a circulation pump 298 receiving inlet supply tube 300 positioned in a sump pan 302 positioned to be adapted to receive all of the drainage from each of the coalescer pad assemblies 260 via a collector pan 304 and a series of down tubes 306. The inclination of the collector pan 304 is such as to provide a selfscouring action to preclude accumulation of solids.

The output of the circulation pump 298 is connected with a header pipe 308 in turn in communication with a series of spray nozzle pipes 310 entering each of the housing stages immediately upstream of a respective coalescer pad having a plurality of nozzle openings 312 adapted to direct a spray of absorber liquid at each of the coalescer pad sections 264,266,268 and 270 to thoroughly wet these surfaces.

The collecting solvent laden liquid in the sump pan 302 is connected to the supply and return lines via openings 313 and 314, respectively, for continuous removal of the solvent content.

A series of access doors 316 are also provided which enable entry for cleanout and other maintenance purposes to each of the stages.

It has been found that this multi-stage approach operates efficiently to remove the solvent vapours to appropriate low levels in a relatively lightweight, simple and inexpensive construction, is highly reliable in operation and accordingly it is very well suited to automotive paint spraying installation applications.

Attention is now directed to FIGURE 12 wherein an alternate form of an absorber unit is depicted which may be advantageously employed with either of the systems shown in FIGURES 1 or 2. The alternate form of the absorber unit, generally indicated by the numeral 318, includes spray means 320, baffle means 322, coalescing means 324 and eliminator means 326. The spray means 320 preferably comprises a distribution tube connected to a source of absorber liquid and having a plurality of longitudinally spaced apertures 328 therein, preferably helically disposed about the longitudinal axis of the spray means 320.

Baffle means 322 comprises an inner cylindrically-shaped wall 330 having a plurality of perforations 334 therein and radially spaced from the spray means 320. Baffle means 322 further comprises an outer cylindrically-shaped wall 332 radially spaced from the inner wall 330 to define a longitudinally extending annular chamber 336 which is open at the outer end thereof.

Coalescing means 324 comprises a pervious pad of material, preferably formed of metal, disposed at the other end of baffle means 332 and in communication with the chamber 336. Coalescing means 324 is adapted to allow the passage of gas therethrough, but contacts and thereby coalesces finely atomized droplets of fluid suspended in such gas. The eliminator means 326 is of a construction well known in the art and is disposed at the downstream end of coalescing means 324.

In operation, the absorber unit 318 is positioned in the corresponding air stripper such that the chamber 336 is disposed toward the oncoming air flow. Solvent absorbing liquid discharged from the spray means discharge tube 320 emanates radially from the tube at relatively high velocity, and perpendicularto the air flow. The absorbing liquid entering the volume of space between the tube 320 and inner wall 330 becomes partially atomized in the air flow.

Due to the high velocity and positioning of the jets of absorbing liquid, the central core of the air flow inside inner wall 330 is directed to impinge on the surface of the inner wall 330 adjacent the tube 320.

The partially atomized mixture of fluid and air spreads upon impact over the surface of the inner wall 330 and eventually enters the perforations 334.

Air entering the chamber 336 and flowing over the perforations 334 acts to shear the atomized mixture of fluid and air passing radially outward through the perforations 334 thereby increasing the atomizing and mixing effect. The finely atomized mixture of fluid and air enters the coalescing means 324 which converts the mixture into larger droplets which then are drawn by the air flow into the eliminator means 326. Eliminator means 326 collects these larger droplets of the mixture and draws the collected, solvent laden liquid away from the absorber unit 318, as by gravity. It is to be noted that the particular dimensions of the various components of the absorber unit, e.g., the radius of the inner wall 330 and outer wall 332, will be governed by the particular application and type of absorbing liquid employed.It is also to be noted that both the baffle means 322 and coalescing means 324 may be rectangular in cross section if desired. Moreover, although a single baffle is depicted in the drawings, a plurality of concentrically disposed baffles may be employed if desired.

From the foregoing, it can be seen that a relatively efficient elimination of the organic vapours from the air developed as a result of the paint spraying operation is achieved by a simple arrangement for removing these compounds both from the air and from the paint solid collecting water circulation flow and which accommodates the high volume of air flow without excessively high energy consumption in so doing.

The operative components thereof operate in a highly reliable manner as well as optimizing ths util ization of such energy requirement as is needy d execution of the process. While the system is c > < own having particular application to particular com- ponentry associated with the paint spraying and drying operations specifically, it is of course understood that the arrangement could be applied to other systems.

Claims (48)

1. An air handling system for a paint applying enclosure wherein organic solvent base paint application processes are conducted, said air handling system comprising air supply means for introducing a flow of air into said enclosure, air exhaust means for causing an exhaust air flow from said enclosure, said exhaust means including paint solid removal means for removing paint solids feom said exhausted air flow to provide filtered exhaust air, vapour removal means for removing organic vapour from said filtered exhaust air, including means for producing contact between an organic vapour absorbing liquid and the filtered exhaust air, regenerator means coupled with said vapour removal means for removing absorbed organics from said absorber liquid to regenerate said absorber liquid, and means coupled with said regenerator means for recirculating regenerated absorber liquid to said means for producing contact.

2. An air handling system as claimed in Claim 1, further comprising vent means coupled with said vapour removal means for venting said filtered exhaust air from which vapour has been removed from said vapour removal means.

3. An air handling system as claimed in Claim 2, wherein said means for producing contact comprises an absorber liquid spray means directing absorber liquid as a spray into said filtered exhaust air.

4. An air handling system as claimed in Claim 3, wherein said means for producing organic absorber liquid contact further comprises a coalescing matrix receiving said filtered exhaust air absorber liquid spray mixture through small openings formed therein causing said absorber liquid spray droplets to coalesce and prevent re-release of said organic vapours by said absorber liquid.

5. An air handling system as claimed in Claim 4, wherein said means for producing absorber liquid contact further comprises a baffle array through which said filtered exhaust air passes after passing through said coalescing matrix, causing separation of said liquid droplets from said filtered exhaust air and further including a collecting compartment for collecting said separated liquid, and wherein said means for recirculating includes means for recirculating a portion of said collected liquid to said spray means and a portion of said regenerator means.

6. An air handling system as claimed in Claim 5, wherein said regenerator means comprises a condensing chamber, spray means disposed in said condensing chamber, and a high temperature coil disposed in said condensing chamber and positioned to be impinged by spray from said spray means, and means for heating said high temperature coil to cause vaporization of said absorbed organics therefrom upon being heated by passing into contact with said high temperature coil, said system further comprising condensing means disposed in said condensing chamber and including a cooling coil disposed therein and means for cooling said cooling coil and including a collection compartment adjacent said cooling coil, said system further including drain means for draining said condensed organics therefrom.

7. An air handling system as claimed in Claim 6, wherein said regeneration means further comprises means for purging said condensing chamber of non-condensable vapour driven out by heating of said spray and further including incinerntor means for incinerating said purged vapour.

8. An air handling system as claimed in Claim 7, wherein said means for heating further comprises said incinerator means.

9. An air handling system as claimed in Claim 8, wherein said means for cooling said cooling coil includes means circulating a heattransfermedium therethrough and means transferring heat out of said circulated medium, and wherein said air handling system further includes summertime heat exchanger means receiving said air vented from said means for producing organic absorber liquid contact and connected to said means for transferring heat so as to transfer a portion of the heat transferred away from said circulated medium during summertime operating conditions.

10. An air handling system as claimed in Claim 9, further including wintertime heat exchanger means receiving said air supply into said enclosure and wherein said wintertime heat exchanger means is connected to said means for transferring heat so as to transfer the heat transferred out of said circulated medium into said supply air during wintertime operation, whereby said air supply is preheated.

11. An air handling system as claimed in any preceeding claim, wherein said paint solid removal means comprises a water flow system washing said exhaust air flow, means for collecting said water flow after being passed in contact with said exhaust air flow, means for removing paint solids from said water and recirculating said water into said water flow system, water stripper means for removing dissolved organic compounds from said recirculated water prior to recirculation thereof.

12. An air handling system as claimed in Claim 11, wherein said water stripper means comprises means for applying a vacuum pressure to said collected water after removal thereof of said paint solids and further including means for condensing organic vapours created by said application of said vacuum pressure thereon.

13. An air handling system as claimed in Claim 12, wherein said water stripper means comprises a condensing chamber wherein said vacuum pressure is developed and means for recirculating said water flow through said chamber.

14. An air handling system as claimed in Claim 13, wherein said means for condensing said organic vapours comprises a cooling coil located in said condensing chamber, and a further including means for circulating a chilled heat transfer medium through said cooling coil, and further including a collecting compartment formed in said condensing chamber adjacent said cooling coil and enabling collection of said condensed vapours therefrom, and drain means collecting said condensed organics from said collecting compartment.

15. An air handling system as claimed in Claim 14, wherein said means for applying a vacuum pressure in said condensing chamber further includes vacuum pump means withdrawing vapours from said condensing chamber.

16. An air handling system as claimed in any of Claims 13 to 15, wherein said means for circulating said waterflowthrough said condensing chamber includes a large diameter outer standpipe of a predetermined height and means for passing said water flow into said standpipe, said upper end of said standpipe being opened and disposed within said condensing chamber, an inner standpipe of lesser height than said outer standpipe disposed within said outer standpipe with a clearance space therebetween, means for causing said circulated water to pass between said clearance space therebetween while being contained by said outer diameter standpipe, whereby said standpipe predetermined height enables said vacuum pressure to be applied to said water while enabling free flow thereof through said outer and inner standpipes.

17. An air handling system as claimed in Claim 16, further including incinerator means receiving vapours withdrawn from said condensing chamber by said means applying a vacuum pressure and said incinerator means including means for incinerating said withdrawn organic vapours.

18. An air handling system as claimed in Claim 15, wherein said cooling coil and means for circulating a chilled heat transfer medium comprise refrigeration means and means for transferring heat out of said medium to be chilled thereby and further including summertime heat exchanger means receiving said filtered exhaust air vented from said means for producing organic absorber liquid contact and wherein said refrigeration means includes means transferring heat from said medium circulated through said cooling coil into said summertime heat exchanger means during summertime conditions, whereby heat energy absorbed by said medium in condensing said organic vapours is dumped into said vented air during summertime conditions by said refrigeration means.

19. An air handling system as claimed in Claim 18, further including wintertime heat exchanger means receiving said supply air and receiving heat transferred out of said medium circulated through said cooling coil in said refrigeration means into said supply air during wintertime conditions.

20. An air handling system as claimed in any of Claims 11 to 19, wherein said paint solid removing means includes a water level control tank and further including means for collecting organic vapours developed in said water level control tank.

21. An air handling system as claimed in Claim 1, wherein said contact producing means comprises spray means coupled with said source for producing a spray of said liquid, baffle means at least partially circumscribing said spray means for increasing atomization of said liquid in said filtered exhaust air, said baffle means having a plurality of apertures therein opposing said spray means for receiving liquid delivered by said spray means therethrough and an opening on one end thereof for receiving said filtered exhaust airtherewithin, coalescing means on the opposite end of said baffle means for collecting atomized drops of said liquid, and eliminator means communicating with said coalescing means for carrying collected liquid away from said contact producing means.

22. An air handling system as claimed in Claim 21, wherein said spray means comprises a liquid distribution tube having 3 plurality of longitudinally spaced apenures therein and said baffle means includes an inner longitudinally extending baffle wall surrounding said tube and radially spaced therefrom.

23. An air handling system as claimed in Claim 22, wherein said baffle means comprises an outer baffle wall radially spaced from said inner wall defining a longitudinally extending annular conduit, and said coalescing means comprises a pad pervious to the passage of airtherethrough.

24. A method for removing organic vapours from the exhaust air of a paint spray booth comprising the steps of (A) removing the majority of paint solids from the exhaust air; (B) passing the exhaust air through an atmosphere of distributed substance having an affinity for organic vapours thereby removing the vapours from the exhaust air; (C) removing the organics from the distributed substance; and, (D) regenerating the distributed substance for use in step (B).

25. A method as claimed in Claim 24, wherein step (C) is performed by heating said substance and said substance exhibits a relatively large surface area to volume ratio.

26. A method as claimed in Claim 25, wherein step (B) is performed by spraying said substance into said exhaust air.

27. A method as claimed in Claim 25, wherein said heating is carried out in an environment of reduced vapour pressure.

28. A method of removing organic vapours from the exhaust air of a paint spray booth, comprising the steps of: (A) substantially removing paint solids from the exhaust air; (B) mixing said exhaust air with a liquid having an affinity for organic vapours; (C) removing the organics from the liquid to regenerate said liquid; and, (D) repeating step (B) using liquid regenerated in step (C).

29. A method as claimed in Claim 28, wherein step (B) is performed by spraying said liquid into said exhaust air.

30. A method as claimed in Claim 28, wherein step (C) is performed by heating said liquid.

31. A method as claimed in Claim 30, wherein the heating of said liquid is performed under reduced vapour pressure.

32. Apparatus for removing organic solvents from a solution including water and organic solvent based paints solids, comprising: paint solids removing means for substantially removing said paint solids from said solution; and, first solvent removing means coupled with said paint solid removing means for removing said solvents from said water.

33. Apparatus as claimed in Claim 32, including second solvent removing means coupled with said paint solids removing means for removing said sol tents from said paint solids.

?4. Apparatus as c3aimed in Claims 32 or 33, whereir. a aed first solvent r amoving means includes means uorvaporizing said soLvents in said liquid and means for condensing at lecst a portion of the vaporised solvents.

35. Apparatus as claimed in Claim 34, wherein said first solvent removing means includes means accumulating the condensed solvents.

36. Apparatus as claimed in Claims 34 or 35, wherein said condensing means comprises a condensing chamber into which said liquid may be introduced and said vaporizing means includes means for reducing the pressure within said chamber.

37. Apparatus as claimed in Claim 36, wherein said pressure reducing means comprises inner and outer concentric radially spaced standpipes of differeing diameters between which water may be pumped upwardly and into said condensing chamber, the inner standpipe being adapted for drawing water away from said condensing chamber.

38. Apparatus as claimed in Claims 36 or 37, wherein said condensing means includes means for cooling the environment within said condensing chamber.

39. Any apparatus as claimed in any of Claims 34 to 38, further comprising vent means for removing another portion of said vaporized solvents from vaporizing means and means coupled with said vent means for combusting said another portion of said vaporized solvents.

40. Apparatus for removing organic solvent vapours from air, comprising a source or organic vapour absorbing liquid; means coupled with said source for intimately contacting said air with said liquid to remove said vapours from said air, regenerator means coupled with said contacting means for removing absorbed organics from said liquid to regenerate said liquid, and recirculation means coupled with said regenerator means for recirculating the regenerated liquid from said regenerator means to said source.

41. Apparatus as claimed in Claim 40, wherein said contacting means comprises means for atomizing said liquid in said air.

42. Apparatus as claimed in Claim 41, including means for coalescing said atomized liquid and wherein said regenerator means comprises means for heating said coalesced liquid, means for vaporizing the organic solvents in heated liquid and means for collecting at least a portion of said vaporized organic solvents.

43. Apparatus as claimed in Claim 42, wherein said vaporizing means includes means for spraying said heated liquid into a volume of gas and said collecting means comprises means for cooling said vaporized organic solvents whereby to condense the latter.

44. Apparatus as claimed in Claims 42, or 43, including conduit means for coupling said source, said contacting means, said heating means, said collecting means and said recirculation means in a fluid flow loop.

45. Apparatus as claimed in Claims 42,43 or 44, including vent means for carrying another portion of said vaporized organic solvents away from said atomizeing means and means coupled with said vent means for combusting said another portion.

46. An air handling system for a paint applying enclosure substantially as hereinbefore described with reference to and as illustrated in the accom panyingdrawings.

47. A method for removing organic vapours from the exhaust air of a paint spr3y booth substantially as hereinbefore described with reference to the accompanying drawings.

48. Apparatus for removing organic vapours from the exhaust air of a paint spray booth substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.