Abstract:
A mist elimination device in the form of a hood for the removal of moisture from a gas stream treated by a WESP includes a series of gutters on the outer surface of the hood to trap liquid droplets and direct the moisture to channels to drain to the lower region of a housing. Gutters are also provided on the interior surface of the hood to catch liquid droplets carried by the gas stream into the interior of the hood.

Description:
REFERENCE TO RELATED APPLICATIONS 
   This application is a US National Phase filing under 35 USC 371 of International Patent Application No. PCT/CA2004/001037 filed Jul. 16, 2004 claiming priority under 35 USC 119(e) from U.S. Provisional Patent Application No. 60/487,929 filed Jul. 18, 2003. 

   FIELD OF INVENTION 
   The present invention is concerned with a mist elimination hood for a wet electrostatic precipitator system to remove moisture from a gas stream. 
   BACKGROUND OF THE INVENTION 
   Wet electrostatic precipitators (WESP) have been used for many years to remove dust, acid mist and other particulates from water-saturated air and other gases by electrostatic means. In a WESP, particulates and/or mist laden water-saturated air flows in a region of the precipitator between discharge and collecting electrodes, where the particulates and/or mist is electrically charged by corona emitted from the high voltage discharge electrodes. As the water-saturated gas flows further within the WESP, the charged particulates matter and/or mist is electrostatically attracted to grounded collecting plates or electrodes where it is collected. The accumulated materials are continuously washed off by both an irrigating film of water and periodic flushing. 
   This type of system is used to remove pollutants from the gas streams exhausting from various industrial sources, such as incinerators, coke ovens, glass furnaces, non-ferrous metallurgical plants, coal-fired generation plants, forest product facilities, food drying plants and petrochemical plants. 
   The elimination of free moisture (mist) from the gas stream discharging from the WESP is often provided by mesh pads or chevrons located at the outlet from the WESP. Both have problems associated with them. Mesh pads are best suited for the removal of entrained droplets from particulate-free stream, with high removals being achieved at low micron sizes. However, pads often suffer plugging problems where fiber, particulates and/or VOCs (tars and sublimates) are present in the gas stream. Chevrons provide a high efficiency entrainment separation with limit drop sizes of 15 to 25 microns, depending on gas velocity and blade spacing, but can also suffer from plugging problems. These problems, in turn, lead to regions of excessive velocity causing droplet re-entrainment and carry-over. 
   SUMMARY OF INVENTION 
   The present invention provides a novel mist elimination device which replaces conventional mesh pads and chevrons. 
   In the present invention, the gas exiting the WESP passes downwardly into an open-topped housing and over an outer hood surface within the housing and then upwardly to an outlet communicating with an upper region of the hood. Liquid droplets accumulate and are drained from a lower end of the housing. Such hood structures have previously been used to attempt to remove the liquid droplets but exhibit considerable moisture carry-over into the exhaust gas stream, especially under flushing conditions. 
   In accordance with the present invention, the problems of the prior art hood arrangement are decreased and exhaust gas stream can be provided with minimal or non-detectable carry-over by providing on the outer-hood surface a series of gutters which trap water droplets and direct the moisture to the channels to drain into the lower region of the housing. In addition, gutters are also provided on the interior surface of the hood to catch liquid droplets carried by the gas stream into the interior surface of the hood to drip down into lower region of the housing, to the moisture outlet from the housing. 
   The present invention, in addition to overcoming the problems associated with prior hood structures, overcomes the inherent limitations and drawbacks of mesh pads and chevrons. The mist elimination device of the invention prevents liquid droplet carryover, both during normal operation and during flushing of the WESP. No additional mist elimination device is required. The mist elimination device of the invention requires no maintenance, a considerable advantage over the mesh pads and chevrons. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a perspective view of WESP incorporating a mist eliminator according to one embodiment of the invention; 
       FIG. 2  is a close-up view of the mist eliminator of  FIG. 1 ; 
       FIG. 3  is an elevational view of the hood structure; 
       FIG. 4  is a perspective view from below of the mist eliminator of  FIG. 1 ; 
       FIG. 5  is a detail view of a ring channel; 
       FIG. 6  is a perspective view of a WESP incorporating a mist eliminator according to another embodiment of the invention; 
       FIG. 7  is an elevational view of the mist eliminator hood of  FIG. 6 ; and 
       FIG. 8  is a view from above of the mist eliminator hood of  FIG. 6 . 
   

   DESCRIPTION OF PREFERRED EMBODIMENT 
   In the drawings, preferred embodiments of the invention are illustrated by way of Example. It is to be expressly understood that the description and drawings are only for the purpose of illustration and as an aid to understanding, and are not intended to be a definition of the limits of the invention. 
   Referring first to  FIGS. 1 to 5  of the drawings,  FIG. 1  shows a WESP installation  10  modified to include a mist eliminator  12  constructed according to one embodiment of the invention. The WESP installation is of conventional construction comprising vertically arranged comprising discharge electrodes and collecting electrodes. Any desired arrangement of such elements may be employed, including square tube, round tube, hexagonal tube or plate. A moisture-laden gas stream to be treated is fed through an inlet header  14  to the upper inlet to the WESP down through the tubes containing the electrodes to the lower outlet  16 . 
   Connected to the lower outlet  16  is the mist eliminator device  12  provided in accordance with one embodiment of the present invention. The mist eliminator device  12  includes a chamber  18  having a sloped bottom wall  20  to a moisture outlet  22 . Inside the chamber  18  is a hood  24  comprising upwardly sloping walls  26 . 
   The mist eliminating device  12  accelerates the gas flow entering the device, causing droplets of free moisture to accelerate downward and then break free of the gas stream as the gas stream turns approximately 180 degrees into the interior of the hood  24  and from there to the gas outlet  28  from the chamber. 
   A problem of moisture moving down the outer wall  26  of the hood  24  and dripping off the edge of the hood  24  to be swept into the exiting gas stream, is eliminated by providing a series of drip rings or guides or gutters  30  on the external walls  26  of the hood  24 . The gutters  30  are downwardly sloping from an apex to guide the moisture on the hood to the sides of the walls and then to drains  32  from which the accumulated moisture drops into the lower portion of the chamber  18  to the moisture outlet  22 . 
   To capture any moisture which may be swept into the gas stream, further gutters  34  are provided on the interior wall of the hood  24 . In order to capture any moisture which may remain or accumulate on the inner walls of the gas outlet  28  and which is swept along by the gas stream, a ring channel  36  may be provided on the inner wall of the outlet  28  with any collected moisture draining through drain  38  to the moisture outlet. 
   The gutters  30  may also provide structural support to the hood  28 . 
   Turning now to  FIGS. 6 to 8 , there is shown thereon an embodiment of the invention in which the WESP is of circular cross-section. The same reference numerals are utilized to identify the equivalent parts. 
   EXAMPLES 
   Example 1 
   This Example illustrates the results obtained using a laboratory scale WESP structure. 
   A ⅛ scale model of a commercial rectangular cross-section WESP structure was set up as illustrated in  FIGS. 1 to 5 . The model extended from the WESP inlet, through the collection tubes and into the outlet hood and duct. The humidification spray nozzle was simulated using a single multi-orifice air atomized spray nozzle operated at 50 to 60 psi to ensure good atomization and fine droplet size. 
   In the absence of the gutters provided in the hood, there was strong visual evidence of a significant amount of water being carried into the outlet duct, particularly during a tube wash operation. However, with the addition of the water collection gutters to the outside of the hood, the amount of liquid observed to be carried over was significantly reduced. 
   The liquid carryover was determined by droplet counters and the average carryover was 0.000764 US gpm/ft 2 . With the wash spray on, the carryover was increased to only 0.00542 US gpm/ft 2 . 
   After several minutes of operation, liquid began to accumulate on the duct walls and ran along the surface, not being measured by the droplet counters in the outlet duct A ring channel was added to the interior of the hood to capture the accumulated liquid from the duct walls. 
   Example 2 
   This Example illustrates the results obtained using a further laboratory scale WESP structure. 
   A ⅙ scale model of a circular cross-section WESP structure was set up as illustrated in  FIGS. 6 to 8 , with the gutters in place. The model extended from the horizontal inlet duct through the WESP vessel to the outlet duct. The humidification spray nozzle was a single multiple-orifice air-atomized spray nozzle operated at 50 psig to ensure good atomization and fine droplet size with a mean of 27 microns. 
   The WESP flush sprays were simulated by a 8 Bete WL-1/4 60 degree hydraulic nozzles mounted on a ring header at equal spaces. During wash simulation, the header was operated at a flow rate of 6 usgpm at approximately 10 psig. 
   The liquid carryover to the outlet duct was determined by droplet counters. With only the inlet spray operating, the carryover was 4.43×10 −10  usgpm/ft 2  with an average droplet size in the outlet duct of 3.2 microns. With only the flush sprays operating, the carryover was 6.06×10 −8  usgpm/ft 2  with an average droplet size of 20.6 microns. With both sprays operating, the carryover was 9.07×10 −9  usgpm/ft 2  with an average droplet size of 12.1 microns. 
   There was little airborne water observed entering the outlet ductwork, as seen from these measurements. Any liquid running along the wall of the outlet duct, resulting from droplet impingement on the duct walls and condensation, was captured by an additional gutter or collection channel at the outlet. 
   Example 3 
   This Example illustrates the results obtained using a plant scale WESP structure. 
   A rectangular full scale mist eliminator hood structure as illustrated in  FIGS. 1 to 5  was installed to replace an existing mesh pad arrangement which was causing operational problems due to plugging and free moisture carryover from the WESP. 
   The installation reduced moisture carryover to values below detection as measured using EPA Method 5 and has been operating for more than eight months without maintenance or interfering with production. 
   SUMMARY OF DISCLOSURE 
   In summary of this disclosure, the present invention provides a novel mist elimination hood for a WESP system that removes specific amount of moisture from the gas stream while avoiding the problems of plugging and maintenance associated with the most commonly-employed mist elimination systems. Modifications are possible within the scope of the invention.