Abstract:
The invention concerns a gas scrubber ( 10 ) for scrubbing a gas of particles carried by the gas. The scrubber includes a duct ( 12 ) for conveying the flow of gas which is to be scrubbed. In the duct the gas, typically air, is mixed with a scrubbing liquid, typically clean water. The gas/liquid mixture is then subject to multiple scrubbing stages each designed to promote good contact between the gas-conveyed particles and the liquid. The first scrubbing stage includes an inductor ( 48 ) of reduced transverse dimension. The second scrubbing stage is provided by a transverse barrier ( 50.4 ) in the duct against which the gas/liquid mixture is caused to impact. The third scrubbing stage makes use of an impeller ( 50 ) which causes the gas/liquid mixture to swirl outwardly and impact against the duct ( 12 ). After the scrubbing stages, the gas/liquid mixture is subjected to centrifugal separation in a multi-stage centrifugal separator ( 49 ) which separates the liquid and gas components.

Description:
BACKGROUND TO THE INVENTION  
       [0001]     THIS invention relates to a gas scrubber.  
         [0002]     Gas scrubbers are used in many industrial processes and applications to clean gases, typically air. This may be for environmental purposes or to render the air suitable for recycling to the process by removing dust or other solid contaminants. Known scrubbers generally work on the principle of bringing a contaminated air flow into contact with water or other liquid droplets so that the particulate contaminants adhere to the droplets as a result of surface tension effects, thereafter separating the cleaned air component from the liquid component and, if necessary, removing the solid contaminants from the liquid to allow the liquid to be recycled.  
         [0003]     Known scrubbers for large industrial applications tend to be very large, bulky and expensive.  
         [0004]     U.S. Pat. No. 4,171,961 discloses an arrangement for intercepting entrained contaminants from a gaseous medium. A contaminated gaseous medium and droplets of a scrubbing liquid are admitted into a whirling chamber. The scrubbing liquid and gaseous medium become admixed such that the scrubbing liquid can intercept contaminants. U.S. Pat. No. 4,624,688 discloses a device for centrifugal separation on the basis of the specific gravity. The operation of the device is based on rotational motion in the same direction, the separation being carried out by changing angular velocities of the rotational motion. EP 1082989 discloses an air scrubbing method and apparatus comprising multiple purification stages and employing coarse and fine filtration devices and a solvent composition which breaks down paint particles.  
       SUMMARY OF THE INVENTION  
       [0005]     According to the present invention there is provided a gas scrubber for scrubbing a gas of particles carried by the gas, the scrubber comprising: 
        a duct for conveying a flow of gas to be scrubbed;     means for mixing the gas flow with a scrubbing liquid in the duct;     first, second and third scrubbing stages in the duct, each serving to promote contact between the liquid and the particles, the first scrubbing stage including an inductor of reduced transverse dimension, the second scrubbing stage comprising a transverse barrier in the duct against which the gas/liquid mixture is caused to impact, and the third scrubbing stage comprising an impeller for causing the gas/liquid mixture to swirl outwardly and impact against the duct; and     means in the duct downstream of the scrubbing stages to separate the gas from the liquid.        
 
         [0010]     According to another aspect of the invention there is provided a method of scrubbing a gas of particles carried by the gas, the method comprising the steps of: 
        causing a flow of gas which is to be scrubbed to flow through a duct,     in the duct, mixing the gas with a scrubbing liquid to form a gas/liquid mixture;     to promote contact between the liquid and the particles, subjecting the gas/liquid mixture, in the duct, to: 
            a first scrubbing stage in which the gas/liquid mixture is caused to flow through an inductor of reduced transverse dimension,     a second scrubbing stage in which the gas/liquid mixture is caused to impact against a transverse barrier in the duct,    
            a third scrubbing stage in which the gas/liquid mixture is caused by an impeller to swirl outwardly and impact against the duct; and     centrifugally separating gas and liquid components of the gas/liquid mixture in duct at a position downstream of the scrubbing stages.        
 
         [0018]     Other features of the apparatus and method of the invention are defined in the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The invention will now be described in more detail, by way of example only, with reference to the accompanying diagrammatic drawings in which:  
         [0020]      FIG. 1  shows a side view of a gas scrubber according to this invention;  
         [0021]      FIG. 2  shows a partial, cross-sectional side view of the gas scrubber;  
         [0022]      FIG. 3  shows an axial view, taken in the downstream direction, of the centrifugal impeller used in the gas scrubber of the preceding Figures;  
         [0023]      FIG. 4  shows a side view of a preferred design of the sump and slurry collection components;  
         [0024]      FIG. 5  shows a plan view of the components seen in  FIG. 4 ;  
         [0025]      FIG. 6  shows a view of the same components, the view being in the direction of the arrow  6  in  FIG. 5 ; and  
         [0026]      FIG. 7  shows a view similar to that of  FIG. 4  of a preferred, multi-stage sump and slurry collection design. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]      FIGS. 1 and 2  diagrammatically illustrate a gas scrubber  10  according to this invention. In the example described below, the gas which is to be scrubbed is air, but it will be understood that the scrubber of the invention could be used to scrub other gases as well. In the example water is used as the scrubbing liquid but, once again, other scrubbing liquids are also within the scope of the invention.  
         [0028]     The air scrubber  10  includes a duct  12  having an axial inlet  14  at one end. The duct is supported, in the illustrated case with its axis horizontal, on a frame  15  having horizontal and vertical members  15 . 1  and  15 . 2  respectively. Located beneath the duct  12  is a sump system  18 .  
         [0029]     Connected to the opposite, outlet end  20  of the duct is a centrifugal fan  22  driven by an electrical motor  24 . The fan and motor are also mounted on the frame. The fan outlet is indicated by the numeral  26 . In use, the fan draws a flow of air through the duct from the inlet  14  to the outlet end  20  and exhausts it through the fan outlet  26 .  
         [0030]     An electrically powered centrifugal pump  28  receives clean water through a line  30  and pumps it through a line  32  to a nozzle  34 . The jet of water  36  which issues from the nozzle  34  impinges on a liquid spreader in the form of a dished member  38  supported centrally in the duct  12  with the concavity of the member facing the nozzle  14 . The shape of the member and the water flow parameters are such that the water is deflected outwardly, as an annular curtain  40  of water drops, into a shroud  42  mounted coaxially in the duct. Optimal setting of the water jet can be achieved by manipulation of a control valve  44 .  
         [0031]     The fan  22  draws air from the inlet  14  through the water curtain  40  in a direction towards the outlet  20 . The mixture of air and water passes through a frustonical section  46  of the shroud and from there moves through an inductor which includes a smaller diameter, throat section or vena contracta  48 . The air/water mixture is accelerated in this section to a velocity at which turbulent flow conditions prevail with the result that the air and water are thoroughly mixed with one another. The inductor forms a first scrubbing stage of the air scrubber  10 .  
         [0032]     The air/water mixture is directed by the throat section  48  towards a static impeller  50  which includes a series of helical impeller vanes  50 . 1  mounted between an upstream plate  50 . 2  formed with a central orifice  50 . 3  and a solid, downstream plate  50 . 4 . The upstream plate  50 . 2  spans transversely across the interior of the duct  12  and the orifice  50 . 3  therein has approximately the same diameter as the throat section  48 . The plate  50 . 4  is of smaller external diameter than the plate  50 . 2  as shown. The diameter of the plate  50 . 4  may, for instance, be approximately 80% of that of the plate  50 . 2 . A central, generally frustoconical member  50 . 5  extends downstream from the plate  50 . 4 .  
         [0033]     The air/water mixture impacts in a generally axial direction against the plate  50 . 4  which accordingly acts as a barrier to the flow of the air/water mixture. The barrier plate  50 . 4  forms a second scrubbing stage of the air scrubber  10 .  
         [0034]     The vanes  50 . 1  of the static impeller  50  impart an outward, swirling motion to the air/water mixture. As indicated by the numeral  51 , the outwardly swirling mixture impacts against the inner surface of the duct  12 . Turbulence and thorough mixing of the air and water components is again promoted by the impeller and duct which form a third scrubbing stage of the air scrubber  10 .  
         [0035]     After impacting against the inner surface of the duct  12 , the air/water mixture enters a zone  53  surrounding the member  50 . 5  and thereafter moves longitudinally into a multi-stage centrifugal separator  49 . The centrifugal separator  49  includes a series of axially staggered cylindrical collector rings  52 . 1 ,  52 . 2 ,  52 . 3 ,  52 . 4  of progressively reducing diameter in the downstream direction. Beneath the rings  52 . 1  to  52 . 4  the lower wall of the duct  12  is formed with an outlet  54  leading via an inclined chute  56  to the sump system  18 .  
         [0036]     Water drops which are thrown outwardly through the greatest radial distance by the impeller  50  deposit on the outer surface of the first ring  52 . 1 . The water then flows around that surface and eventually drops off the ring to leave the duct through the outlet  54  from where it flows to the sump system  18 . Water drops which are not thrown out quite as far by the impeller are drawn further downstream by the action of the fan  22 , and deposit on the surface of the next ring  52 . 2  from where, once again, the collected water also finds its way to the sump system  18 . The process is repeated throughout the remaining length of the centrifugal separator  49  with those water drops thrown outwardly through the least distance by the impeller eventually being collected by the last ring  52 . 4  from where the water is once again directed to the sump system.  
         [0037]     The member  50 . 5  of the impeller maintains the spiral swirling motion of the water droplets and reduces the possibility of short-circuiting axially through the collector rings  52 . 1  to  52 . 4 . Accordingly, the impeller  50  and collector rings  52 . 1  to  52 . 4  act together as a multi-stage centrifugal separator. The air component of the original air/water mixture is drawn largely axially through the collector rings and is eventually exhausted by the fan  22  through the fan outlet  26 . The water droplets, on the other hand, are effectively separated from the air stream, and are collected by the collector rings which deliver them to the outlet  54 .  
         [0038]     In the embodiment of FIGS.  1  to  3 , the diagrammatically illustrated sump system  18  includes an open-topped sump box  60  and an open-topped collector  62  located within the outer tank. The upper edge of the collector  62  is lower than the upper edge of the sump box  60 . The collected liquid, carrying dust particles and other solid contaminants, is delivered by the chute  56  to the collector  62 .  
         [0039]     Dust and other solid particles which settle in the collector  62  are discharged into a slurry collection container  63  located beneath the sump box  60 . In order to achieve optimal settling of the solid contaminants, a suitable flocculant may be used. Clarified liquid, i.e. water, overflows the weir  64  of the collector  62  into the sump box from where it can either be disposed of through a drain line  66  or withdrawn through the line  30  by the pump  28  for recirculation to the nozzle  34 .  
         [0040]     The slurry collection container  63  is mounted movably on wheels  65  and has a vertical window  68  in its side wall through which the accumulation of solid material can be monitored visually. When a predetermined maximum level of solid material has collected in the container  63 , clean water is drained out of it through the drain line  66  and the container is detached from the sump  18  and removed. The collected solids, in the form of a thick slurry, are then removed from the container for disposal as necessary, and the container is re-installed for the scrubbing process to continue.  
         [0041]     FIGS.  4  to  6  illustrate an alternative, preferred design of the sump and slurry collection components. In this case, the sump box  60  accommodates an internal collector  62  bounded by porous baffle plates  82 . A funnel-shaped outlet  84  leads from the bottom of the collector  62  to the slurry collection container  63  via a conduit  86  controlled by a valve  88 . During operation of the scrubber  10 , the slurry which discharges from the duct  12  through the outlet  54  enters the collector  62 . The denser particles in the slurry precipitate through the outlet  84  into the container  63 . Cleaner liquid and less dense particles move outwardly through the baffle plates  82  into the surrounding sump box  60 . When the container  63  is full of dense slurry, as determined by visual inspection through the inspection window  68 , the valve  88  is closed to isolate the container.  
         [0042]     In  FIG. 4  the numeral  92  indicates a clean water supply line and the numeral  94  indicates an extension line going to drain. Valves  96  and  98  respectively control flow through these lines. A line  100  including a quick-coupler  102  extends to the container  63 . With the valve  96  closed and the valve  98  open, liquid above the slurry in the container can be drained off through the lines  100  and  94 . To allow this to happen a valve  104  is opened in an air vent  105  to permit air to enter the container. When the liquid has drained off, the quick-coupler  102  is released and bolts  106  are released to allow the container  63  to separated from the sump box  60  for removal of the slurry therein. The clean container  63  is then reconnected to the conduit  86 , and the quick-coupler  102  is connected again, whereafter the valve  98  is closed and the valve  96  is opened to admit clean water to the container. Sufficient replenishment of water is indicated by water flowing from the air vent  105 . The valves  96  and  104  are then closed again, and the valve  88  is reopened.  
         [0043]     Throughout the operation described above, the scrubber  10  continues to operate normally. While the valve  88  is closed, there is merely a build-up of slurry in the collector  62  which gravitates into the container  63  when normal slurry removal operations start again. The volume of clean water admitted to the container dilutes the accumulated, dense slurry which enters the container  63  at this stage.  
         [0044]      FIG. 7  shows a slightly more sophisticated arrangement in which there is a separation of heavy and light slurry components. In this case, lighter slurry which has passed outwardly through the baffle plates  82  into the main body of the sump  18  precipitates through an outlet  110  into a second slurry collection container  63 . 1 . This operates in exactly the same way as the primary container  63  and is drained and replenished in like manner. In  FIG. 7  the line  112  indicates that the quick-coupler in the line  100  can be coupled selectively to the container  63  or the container  63 . 1 , depending on which of these containers is to be separated and emptied of slurry.  
         [0045]     In  FIGS. 4, 5  and  7  the numeral  114  indicates an automatic, float-controlled make-up valve, connected to the clean water supply line  96 , through which make-up water can be added to the sump system  18  to replenish water losses automatically.  
         [0046]     Referring to  FIG. 1 , there is a line  70  to convey water from the duct  12 . In practice, this line can discharge such water into the sump  18 , again for make-up purposes.  
         [0047]     As described above there is a three stage scrubbing action resulting from the intimate mixing of the air and water components taking place firstly in the vicinity of the throat section  48 , secondly on impacting the plate  50 . 4  and thirdly on impacting the inner surface of the duct  12 . As a result of this multi-stage scrubbing action, intimate contact between the water drops and particles of dust and other solid contaminants is promoted. The solid particles adhere to the water droplets through surface tension effects and are accordingly removed from the air stream, along with the water, and with a high degree of efficiency.  
         [0048]     In trials scrubbers of the kind described above were able to remove in excess of 99% of an initial dust content of an air flow down to particle sizes as low as 0, 1 micron and even less. It is believed that such high efficiency levels are attributable to the three stage scrubbing action.  
         [0049]     Apart from its efficient operation a major advantage of a scrubber as described above is its relative compactness and lightness compared to prior art scrubbers capable of handling a comparable throughput. In practice, the scrubber could be utilised as a stand-alone scrubber, as a pre-scrubber or as a final filter scrubber at the end of a process, i.e. the duct  12  and associated apparatus can be added as a downstream addition to an existing scrubber. It is also possible to retrofit the scrubber  10  to existing ductwork.  
         [0050]     It should be noted that the fan  22  and motor  24  are optional and are provided to increase the flow rate through the scrubber and hence its capacity. In the absence of the fan, an air flow will be induced through the scrubber naturally by the action of the throat  48 .